This work presents a power equation for the conductivity of graphene-based polymer composites by the tunneling length, interphase deepness and filler size. The impressions of these factors on the effective concentration and percolation beginning of graphene nano-sheets in nanocomposites are also expressed. The developed equations for percolation beginning and conductivity are examined by the experimented data of some examples, which can guesstimate the interphase depth, tunneling size and percolation exponent. Besides, the impacts of numerous factors on the percolation beginning and conductivity are designed. The developed equation for percolation beginning shows the formation of thick interphase and large tunnels in the reported samples. So, disregarding of tunneling and interphase spaces in polymer graphene nanocomposites overpredicts the percolation beginning. Additionally, the developed model presents the acceptable calculations for the conductivity of samples. Among the mentioned parameters, the concentration and graphene conductivity in addition to the interphase depth induce the strongest effects on the conductivity of composites. Graphene as one-atom planar sheet with nanoscale and remarkable aspects such as high electrical conductivity, significant stiffness and large specific surface area can replace the common fillers for fabrication of polymer nanocomposites28. Single graphene nano-sheets show the best intrinsic properties, but, it is problematic to gain them at high-quality, large-scale and by low-cost. Also, the tendency of graphene to rolling, scrolling or wrinkling is an important challenge, which deteriorates the aspect ratio (ratio of diameter to thickness) and morphology of graphene30.Many types of nanoparticles were reported in literature32. It is known that above a determinate filler amount in nanocomposites as percolation beginning, the nets are formed and a significant conductivity is obtained. The percolation beginning links to the dimensions of graphene layers in addition to the dispersion quality33. The low percolation beginning and high conductivity of polymer graphene nanocomposites are qualified by the big aspect ratio, big specific superficial zone and homogenous spreading of graphene layers34, although some undesirable phenomena such as aggregation, crimping and difficult networking of graphene weaken their efficiency for conductivity35.The thin and large layers of graphene produce the conductive nets in polymer nanocomposites at low filler contents38. They attempted to show a poor percolation beginning and great conductivity by little filler contents in different nanocomposites. However, the effects of different factors on the percolation beginning and conductivity of graphene systems were not studied. The previous articles mostly applied the power-law percolation theory to approximate the percolation beginning and to interpret the conductivity39. In fact, the former studies only focused on the percolation beginning in these nanocomposites, while the main effects of some important factors like interphase parts on the conductivity were neglected.There are many experimental studies in literature on the conductivity of graphene-filled samples46. The interphase includes the altered configuration of polymer chains near the nanoparticles, because the large surface area of nanoparticles as well as the strong interactions between polymer and nanofiller mainly affects the polymer chains near the nanofiller. So, the interphase has a higher stiffness and conductivity compared to bulk polymer chains. Figure 48. Additionally, many models have been developed to calculate the interphase properties by tensile modulus and strength51. Importantly, it was shown that the interphase part contiguous nanoparticles can participate in the filler nets quickening the percolation beginning in the samples, because the interphase reduces the spaces between two nearby nanoparticles53. The networking efficiency of interphase was also studied in the toughness of CNTproducts54, but its effect on the conductivity has not been reported. Generally, there is no model, which can show the impact of interphase on the conductivity of graphene products. Moreover, the tunneling effect acts a chief role in the conductivity of nanocomposites57, but this mechanism cannot be considered by conventional theories. Figure Polymer nanocomposites include a third phase around nanoparticles as interphase regionsIn this work, a developed form of the power-law model is presented for approximation of conductivity in graphene systems assuming interphase depth, tunneling size and particle size. The stimuli of these factors on the percolation beginning of graphene are also expressed by a developed equation. The developed equations for percolation beginning and conductivity are evaluated by the experimented data for some examples from previous reports. Moreover, the impressions of different factors on the percolation beginning and conductivity are explored using the developed equations.36 as:f” is filler conduction, “58. The values of “b” were reported as 1.6–2 for a three-dimensional (3D) system and 1–1.3 for 2D one58, but more “b” range was also suggested for polymer graphene nanocomposites.A power model based on percolation idea was advised for the conductivity of composites59 is obtained by:i” show the thickness of nano-sheets and interphase, in that order.The interphase regions commonly form in polymer nanocomposites, which grow the efficiency of nanoparticles. The volume portion of interphase in the system comprising nano-sheetsThe actual level of filler volume portion in nanocomposites can be suggested by total contents of interphase and filler as:entclass1pt{minima60 as:Also, the percolating of 3D randomly organized graphite sheets in nanocomposites was expressedHowever, Eq.  cannot cThe effects of tunneling and interphase regions can be expected in the latter equation as:When “rom Eqs.  and 7)  \documenmentclass2pt{minimi) and tunneling length (d) are calculated. The different levels of these parameters are also represented in Table i” and “d” reveal that the interphase regions and tunneling effect play main roles in the percolation beginning and it is not possible to predict the percolation level in absence of these parameters. In other words, disregarding of interphase area and tunneling effect overpredicts the percolation beginning. The level of “ti” links to the interfacial interactions61. The polystyrene (PS)/graphene sample containing the thickest interphase (ti = 8 nm) shows the strongest interfacial interactions among the reported samples. On the other hand, samples with ti = 2 nm show the thinnest interphase as the poorest interfacial adhesion between polymer and graphene.The developed equations for percolation beginning and conductivity are valued by the experimental measurements from previous articles. Table 64. It can be suggested that the calculated “d” for the examples changes in a sensible range to provide the tunneling effect. The developed model in Eq.  (PET)/graphene34 and epoxy/graphene63 nanocomposites. The calculations of developed model illustrate the respectable arrangements with the tested data, but the original model underestimates the conductivity. Accordingly, the developed model assuming tunneling length and interphase regions can predict the conductivity of graphene system, while the original model is inappropriate.The average tunneling length “d” also varies from 3 to 8 nm in the examples. The maximum tunneling length, which can transfer the electrons was reported as 10 nm58, but it can be suggested that “b” relates to interphase depth and tunneling length in graphene products. Further studies in this area are necessary to show the dependency of “b” exponent to the actual parameters in polymer nanocomposites.The values of “b” exponent Table  change fThe impacts of model’s variables on the percolation beginning and conductivity are studied by the advanced equations. The parametric studies give the calculated results from the developed model. Actually, the parametric examinations confirm the roles of all parameters in the conductivity of nanocomposites using the developed model. Certainly, obtaining the calculated levels by experiment needs much time and cost, but 3D and contour plots only demonstrate and validate the reasonable effects of all parameters on the conductivity.i = 2 nm and d = 5 nm. The maximum percolation beginning as 0.022 is calculated by t = 5 nm and D = 1 μm, while t = 1 nm and D = 3 μm produce the smallest percolation level. Actually, small “t” and high “D” yield the desirable level for percolation beginning in polymer graphene nanocomposites, while high “t” and low “D” negatively increase the percolation point. Accordingly, the percolation of thin and large graphene nano-sheets more quickly occurs in nanocomposites compared to thick and small ones.Figure 65. They indicated that a low percolation beginning is gotten by a big aspect ratio of graphene, i.e. thin and large nano-sheets. So, their predictions for percolation beginning agree with the developed equation in this study. In fact, the thin and big nano-sheets can occupy large regions in the nanocomposite, which facilitates the formation of nets by low filler concentration.The skinny and large nano-sheets harvest a big aspect ratio and big surface zone, which decrease the spaces between nano-sheets and grow the possibility of percolating and networking. Conversely, dense and small nano-sheets cannot introduce numerous inter-particle contacts, which weaken their percolating efficiency in nanocomposites. As a result, thin and large nano-sheets provide desirable percolation beginning by little amount of graphene. The previous articles reported an opposite correlation among percolation level and aspect ratioi” and “d” are also illustrated in Fig. i = 10 nm and d = 10 nm, although ti = 2 nm and d = 2 nm raise the percolation level to 0.007. Accordingly, the highest values of “ti” and “d” can create the lowest percolation beginning in the samples. Instead, a high amount of nanoparticles is compulsory to get the percolation beginning in the case of thin interphase and short tunneling length.The variations of percolation beginning at different series of “t67, but the interphase stimuli on the percolation beginning and conductivity of graphene- filled system are studied in this paper for the first time.The interphase regions around nano-sheets can generate the nets before the physical bonding of nano-sheets. So, a thick interphase about the large nano-sheets can cause the percolation beginning at very low volume portion of nanoparticles. Accordingly, the huge interphase regions in nanocomposites positively affect the percolating and networking of nanoparticles, because they form a high portion of polymer nanocomposites. However, the positive effect of interphase regions depends on their thickness and particle size according to Eq. . UndoubtThe positive role of a large tunneling length in the percolation beginning of graphene is also reasonable. The graphene can form the nets when a short distance between adjacent nano-sheets exists as tunneling length. Actually, the neighboring nano-sheets separated by tunneling length can create the nets in the samples. Consequently, a large tunneling length can create the nets by few numbers of nano-sheets demonstrating the fast percolation by low filler concentrations. In conclusion, the developed equation suitably shows the powers of interphase depth and tunneling length on the percolation beginning.i = 2 nm, d = 5 nm and b = 4. Obviously, these parameters directly affect the conductivity, because the graphene nanofiller is more conductive than polymer matrices. When f = 2.5*106 S/m, the conductivity of 120 S/m is attained for nanocomposite, but the nanocomposite remains approximately insulated when Figure  = 0.01 and σf = 105 S/m. The maximum conductivity as 2 S/m is obtained by the slightest amounts of “Figure A poor percolation beginning produces the conductive nets of graphene nano-sheets at low filler contents. So, high conductivity is shown by low filler concentrations in this condition and the addition of nano-sheets above percolation beginning increases the net density and dimensions. Obviously, the dense and big nets can enlarge the transferring of electron in the nanocomposites. However, a high percolation beginning shows the networking of graphene nano-sheets at high filler concentration. In this state, the nanocomposite can show the conductivity only by great filler concentration. In addition, a high value of “b” considerably decreases the conductivity, but the exact definition for this parameter is not available. It seems that the “b” links to filler size, net dimensionality, interphase scale and tunneling length. It can be thought that the big values of “b” demonstrate the inappropriate conditions in nanocomposite for conductivity.i = 2 nm, f = 105 S/m, d = 5 nm and b = 4. The uppermost conductivity is witnessed by the thinnest and the biggest nano-sheets. The supreme conductivity of 0.55 S/m is attained at t = 1 nm and D = 3 μm. However, thick graphene nano-sheets (t > 2 nm) significantly weaken the conductivity. As a result, the dimensions of graphene nano-sheets control the conductivity. It can be suggested that the poor dispersion of nanoparticles thickening the graphene nano-sheets negatively affects the conductivity.The conductivity linking to “t” and “D” are designed in Fig. The thick and small nano-sheets show small surface area, which decreases the contacts between nanoparticles in nanocomposite. So, the percolating and networking of thick and small nano-sheets at low concentrations is difficult, which results in poor conductivity. Also, the thick and small nano-sheets produce small and light nets after percolation beginning, which are not efficient for electron transportation and conductivity. Therefore, thick and short nano-sheets cannot produce proper conditions for percolating of nanoparticles and achieving a desirable conductivity. In summary, the undesirable characters of thick and small graphene nano-sheets in the conductivity can be interpreted by their contribution to the aspect ratio and surface area of conducting filler. Hence, the established model accurately exhibits the stimuli of these factors on the conductivity.i” and “d” as interphase deepness and tunneling size in the conductivity based on Eq.  produces the best conductivity of 12 S/m. Therefore, the conductivity directly changes by interphase depth, but the role of tunneling length is negligible.Figure d on Eq.  at σf = The significant aspect ratio of graphene nano-sheets intensifies the effect of interphase on the common features of system, because the interphase regions can occupy a high part of nanocomposite. The thick interphase areas harvest the nets at lower filler portions, because they form the net structures before the real attachment of fillers. So, thick interphase regions attach to the nets and improve the net properties and conductivity.69.The contributions of interphase regions to the real filler portion and percolation beginning are assumed by Eqs.  and 7),,7), respParametric examinations validated the developed model by a simple methodology using 3D and contour plots. The effects of all parameters on the conductivity were revealed by the plots, but the analysis of correlation among the parameters was not given, because the proper roles of all factors in the conductivity of samples are sufficient for the validation of developed model. Actually, 3D and contour plots demonstrate the roles of parameters in the conductivity and the trends are not different by dissimilar couples of parameters.The power-law model was progressed for graphene-filled composites by the impacts of tunneling length, interphase deepness and filler size on the actual concentration and percolation beginning of graphene. The developed equations were examined by the experimental measurements and the influences of several factors on the percolation beginning and conductivity were discussed. The experimental levels of percolation beginning show the development of dense interphase and long tunnels in the examples and thus, the nonattendance of these factors overestimates the percolation beginning. Also, the novel model can successfully foretell the conductivity for the examples. A poor percolation beginning is gotten by thin and large graphene nano-sheets as well as by thick interphase and long tunneling length. However, filler dimensions more significantly change the percolation beginning compared to other parameters. In addition, the high values of filler concentration, filler conduction, filler diameter and interphase depth cause good conductivity, whereas a higher conductivity is attained by lower percolation beginning, minor “b” exponent and thinner nano-sheets. Also, the tunneling length shows negligible influence on the nanocomposite conductivity. The most influences on the conductivity are introduced by the conduction and concentration of graphene and interphase depth.


Since its isolation in 2004, monolayer graphene has attracted enormous attention within the scientific community, the industry, and the general public owing to its exceptional properties  and prospects ,2. A fulGraphene-based composites, often containing multilayer types of graphene of different sizes and several defects, is also a bustling field of research due to the boosting properties that this filler offers to matrix materials such as polymer, metal, or ceramics ,8,9. ThiIn particular, it offers two review papers, one from Zamri and Haseeb on the application of graphene/conductive polymer composites such as chemiresistive sensors focusing on the preparation methods and sensing performance of these composites , and theTwo articles deal with the effect of graphene nanoplatelets over the friction and wear properties of materials; both are key characteristics from the industrial point of view. The work of Omrani et al.  highligh2O3 fibers that were coated by CVD methods, evidencing the notable increase in the thermopower factor for specific compositions and, accordingly, their potential use for waste heat conversion.The article by Saffar Shamshirgar  studies 2O3 materials, in particular, γ- and α-Al2O3 lattices prepared by additive manufacturing methods, stressing the crucial reinforcing effect of GO nanoribbons over other fillers like CNT.Finally, the paper of Ramírez et al.  deals wi


Graphene can effectively improve the mechanical and electrical properties of cement-based materials due to its excellent tensile strength, thermal conductivity and electrical conductivity. In this paper, the effects of freeze–thaw on the conductivity and sensing properties of graphene-based cement materials were investigated. After the preparation of graphene-based cement materials, they were subjected to different times of freeze–thaw action. The experiments were designed to analyze the influence of freeze–thaw on the electrical conductivity, humidity sensitivity, thermosensitivity and pressure sensitivity of graphene-based cement composites. The results show that the influence of freeze–thaw on the electrical conductivity of graphene is mainly manifested in the influence on the resistivity and the extension of the polarization time, and the influence on the percolation transition zone is small. After freeze–thaw, the polarization time of the specimen decreases with the increase of the relative water content. The temperature has a great influence on the polarization effect of graphene-based cement composites and the composites with graphene content of the zone B still show satisfactory pressure-sensitive property after freeze–thaw. As one of the most important building materials in civil engineering, cement-based materials are widely used in infrastructure because of their wide range of sources and low prices. With the rapid development of science and technology, engineering structures are becoming larger and more complex, such as large sea crossing bridges, super high-rise buildings, large water conservancy projects, etc. The design service life of these major infrastructures is required to be high . During the service period, under the combined action of long-term environmental erosion, material aging, long-term load effect, fatigue effect and other factors, common concrete damage such as cracking, deterioration and surface peeling is inevitable, and further reduce the service life of concrete structures. If not dealt with in time, it may lead to sudden structural damage, serious loss of life and property. Therefore, it has become a consensus to continuously monitor the performance of these cement composites in terms of stress, strain and damage indication, and to find and deal with the damage in time and establish the corresponding response mechanism ,3,4,5.Cement-based conductive composite can be used as a sensor, which can be poured synchronously with concrete, has good compatibility with concrete structure and deteriorates synchronously ,7,8. It Much research has been done on the conductive and sensing properties of graphene-based cement materials ,22,23,24−8 S/m. However, the incorporation of 1% graphene increases the conductivity by three orders of magnitude. The increase in graphene content seemed to be accompanied by changes in electrical properties from insulation to semiconductor behavior. Du et al. [Bai et al.  studied u et al.  investigu et al.  studied u et al. ,30 foundu et al.  believedCui et al.  reportedSaafi  indicateAlthough the conductive and sensing properties of graphene-based cement composites have been systematically studied, they mainly focus on the properties of new composites themselves, without considering the influence of freeze–thaw action on their conductive and sensing properties. In the long-term service process of cement-based materials, they are inevitably affected by freeze–thaw damage, which will change their composition, structure and performance. It can be predicted that freeze–thaw action will have an important impact on the conductivity and sensing properties of graphene-based cement materials. In this paper, graphene-based cement composites were taken as the research object, and experiments were designed to investigate the effects of freeze–thaw on the electrical conductivity, humidity sensitivity, thermosensitivity and pressure sensitivity ofgraphene-based cement composites. The influence mechanism of freeze–thaw on the electrical conductivity of composites was analyzed by SEM test method. The influence of freeze–thaw action, relative water content and temperature on the resistivity of composites were also explored.The cement was P. II 42.5 Portland cement. All performance indexes of cement meet the requirements of Chinese Standard GB175-2007, and its chemical composition and physical properties are shown in 2 content of more than 85%, an average particle size of about 0.2 μm, and a specific surface area of 26.4 m2/g. Its chemical composition is listed in The silica fume used in this study had a SiOThe graphene powder was purchased from Shenzhen Guosen Linghang Technology Co., Ltd. (China), and prepared by physical exfoliation method. The micromorphology is presented in The mixing water was the tap water of the laboratory, which met the requirements of the Chinese standard of water for concrete.Admixtures included graphene dispersant (sodium dodecyl benzene sulfonate), superplasticizer (polycarboxylic acid type) and defoamer (silicone type).The mix proportions of graphene-based cement composites are shown in Graphene dispersion was pre-prepared according to the mix proportions in According to the mix proportions in After the specimens were cured, they were put into a water tank, and tap water was injected to cover the specimens by more than 20 mm. The water tank was then put into a low-temperature freezer for freezing, and the temperature was set to −20 °C. When the water tank was fully frozen, it was taken out and thawed at room temperature. Thus, a freeze–thaw cycle was completed. The specimens were taken out for other tests while 25, 50, 75 and 100 cycles were accomplished.Since freeze–thaw is easy to cause the specimen to break along the stainless-steel gauze used in direct current (DC) four-probe method, DC two-probe method was employed to measure the electrical resistivity of specimen after freeze–thaw in this study ,40.ρ can be obtained as follows:R, U and I are the resistance, voltage and current at both ends of the specimen, respectively, S0 represents the sectional area of the specimen and L is the length between the two ends.After curing and freeze–thaw tests, conductive copper foil paper was pasted on the two ends of the specimen, and the wire was led to measure the voltage and current at two ends of the specimen. The electrical resistivity S is the relative water content of the specimen, ms is the mass of the specimen when its relative water content is S, m0 and m0′ are the mass when the specimen is saturated and dried, respectively.The humidity sensitivity of the specimen is the relationship between the resistivity and the internal moisture content of the specimen. The relative water content of the specimen is calculated as follows:In order to prepare the specimen with the desired relative water content, the mass of the specimen was measured first when it was dried and saturated, respectively. Then the specimen was wrapped with plastic wrap to prevent the increase or loss of water content while the desired relative water content was reached and left to stand for one month to make the moisture inside the specimen relatively uniform and stable.The thermosensitivity of the specimen is the relationship between the resistivity and the temperature of the specimen. The specimen was dried and cooled to room temperature and wrapped with plastic wrap to prevent the specimen from absorbing water. If the ambient temperature was above zero, a water bath was used to set the test temperature. A low temperature freezer was adopted to set the test temperature if the ambient temperature was below zero. The specimen was placed in the environment at the set temperature for more than five hours to achieve the same internal and external temperature. The resistivity of the specimen was measured after reaching the set temperature.The specimen with the desired temperature and humidity was placed vertically on the universal testing machine, and the place in contact with the indenter was separated by an insulating gasket to prevent leakage. Then the resistivity measurement equipment was connected, and the ammeter and voltmeter (Fluke multimeter) were connected to the computer to record the whole process of current and voltage changes under compression. Before the universal testing machine started, the resistivity of the specimen was measured first to complete the polarization. When the resistivity was stabilized, the universal testing machine was set to load at a rate of 0.2 kN/s to crush the specimen and the pressure-time curve was recorded in real time. If the resistivity under cyclic loading was measured, different maximum loads and loading rates were set: For the load rate was 0.2 kN/s, the maximum loads were 1, 2 and 3 kN, respectively, and for the maximum load was 2 kN, the corresponding loading rates were 0.2, 0.5 and 1 kN/s, respectively.The polarization time obtained from The change of the electrical resistivity of the specimen not subjected to freeze–thaw action with the content of graphene is plotted in The freeze–thaw damage goes from the surface to the inside. Literature  had inveThe effect of relative water content on the electrical resistivity of graphene-based cement composites before and after 50 freeze–thaw cycles is displayed in As presented in For the specimens with graphene content of zone A and relative water content of 50 and 100%, respectively, it is mainly ionic conduction, and the conductive path is composed of a large amount of pore solution in the connected pores. The microcracks caused by freeze–thaw increase the porosity, enhance the conductive network of pore solution, and reduce the resistance of pore solution, resulting in a negative relative change rate of resistivity. At the same time, due to the freezing and melting of water in the pores, some ions are free to the outside of the matrix, and the content of conductive ions in the pores decreases. Under the combined effect of the two, the relative change rates of the resistivity of the specimens are similar when the relative water content is 50 and 100%, respectively. In the case of the specimens containing graphene content of zone B, the ionic conduction of the pore solution and tunneling conduction play a role together. With the increase of graphene content, tunneling conduction gradually plays a leading role. Freeze–thaw produces more pores or microcracks, which increases the spacing of graphene particles, resulting in a decrease in the conductivity of tunneling conduction. The relative change rates of resistivity of specimens increase rapidly with the increase of graphene content. For the specimens incorporating graphene content of zone C, contacting conduction is the main conductive form. The concentration of graphene particles is greater than the percolation threshold. And the particles overlap each other to form a complete conductive network, by which the conductivity is carried out. The cracks caused by freeze–thaw destroy the contact of graphene particles, which can reduce the conductivity of graphene network. Especially when the number of freeze–thaw cycle is more, the specimen is seriously damaged, and the electrical resistivity of the specimen is increased ,44.+, K+, Ca2+ and OH− to form ion conduction, which can reduce the resistivity of the matrix by two orders of magnitude. At the same time, due to the huge specific surface area of graphene, water will be adsorbed on the graphene surface to form a layer of water film. When the water film is on graphene which is close to each other, it can reduce the resistance of electron transition and enhance the tunneling conduction effect. In the complete graphene contact conductive network, however, the water film will increase the resistance of free electrons to directional movement under the action of electric field, resulting in an increase in resistivity. After freezing and thawing, as shown in the right figure of −6/K, while the thermal expansion coefficient of graphene is about −4 × 10−6/K [For graphene-based cement composites, there are two main reasons for the influence of temperature on its resistivity. On the one hand, the increase of temperature can increase the concentration and migration rate of carriers in the material and decrease the resistivity. On the other hand, the material volume changes due to temperature rise. The thermal expansion coefficient of cement matrix is about 15 × 10× 10−6/K  within t× 10−6/K ,44.ρ is the electrical resistivity, mρ and b are the fitting coefficients, and T is the absolute temperature.According to literatures ,47 and tThe fitting results are shown in The electrical resistivity of graphene-based cement composites after freeze–thaw has good regularity with temperature in the process of monotonic heating. In order to better reflect the stability of the law between electrical resistivity and temperature, the cement-based composites with graphene content of 1, 2 and 3% were subjected to multiple temperature rise and fall between 253 and 333 K after different freeze–thaw cycles. The resistivity values were measured and recorded at different temperatures and compared with the relationship between the resistivity and temperature fitted in the s is the sensitivity (%/kN); ρ0 is the resistivity of the composites without load.Here, the pressure sensitivity of graphene-based cement composites is defined as the quotient of the change rate of electrical resistivity under a certain stress and the resistivity of the composites without load, and the expression is:Due to uniform loading in this study, the relationship between load and time is linear, and the sensitivity can also be written as:The pressure sensitivity of the uniform decline period before freeze–thaw is −3.5%/kN according to Equation (5), and it is −10.2%/kN after freeze–thaw, which is nearly two times higher, and the duration of the platform stability period is shortened by about half.The cement-based composite will undergo volume deformation or even failure under pressure, which will affect the internal porosity and produce microcracks. In addition, the contact between conductive phases will also change. These factors will eventually cause changes in resistivity.In the uniform decline period, the cement-based composite material produces strain under pressure. When it is in the range of linear elastic deformation, the strain changes monotonically but unevenly. It can be seen from During the stable platform period, the pressure exceeds the proportional limit, and with the increase of pressure, new microcracks occur in the composite. The germination of fresh microcracks leads to destruction and reconstruction of the conductive network, so that the resistivity does not increase significantly. The curve maintains horizontal in a small range or oscillates up and down on the horizontal line.In the rapid rise period, the pressure continues to increase and approaches the failure load. The cracks propagate and increase obviously, expand the distance between graphene, and destroy the conductive path, making the resistivity rises rapidly. After freeze–thaw, the duration of the stable platform period is shorter than that before freeze–thaw, which may be due to the larger porosity of the specimen after freezing and thawing, and the cracks are more likely to expand and increase ,48.(1).The influence of freeze–thaw on the electrical conductivity of graphene is mainly manifested in the influence on the electrical resistivity and the extension of the polarization time, and the influence on the percolation transition zone is small. Freeze–thaw leads to an increase in electrical resistivity, which is positively correlated with the number of freeze–thaw cycles.(2).With the increase of freeze–thaw cycles, the cracks in the matrix increase, the crack width becomes larger, and the gel becomes loose. The water in the pores freezes and expands, making the pore size larger, generating new pores, and increasing the matrix porosity. More pores are generated in the matrix, which reduces the ionic conductivity of the composite with graphene content in zone A, increases the spacing of graphene particles in the composite with graphene content in zone B, resulting in a decrease in its tunneling conductivity, destroys the contact continuity of graphene particles in the composite with graphene content in zone C, and increases the resistivity of the specimens.(3).After freeze–thaw, the polarization time of the specimen decreases with the increase of the relative water content, and the freeze–thaw action does not change the influence trend of relative water content on electrical resistivity. The resistivities of specimens incorporating graphene content of zone A and zone B decrease with the increase of water content after freeze–thaw, while the resistivity of specimens containing graphene content of zone C increases slightly with the increase of water content. For the specimens with graphene content of zone A or zone B and relative water content of 50 and 100%, respectively, ionic conduction in pore solution is an important form of their conduction. The microcracks caused by freeze–thaw increase the porosity, enhance the conductive network of pore solution, and reduce the resistance of specimens. The water content has little effect on the contacting conduction of the zone C specimen. The relationship between resistivity and water content of graphene-based cement composites under freeze–thaw action can be expressed by the improved Logistic function.(4).The temperature has a great influence on the polarization effect of graphene-based cement composites after freeze–thaw, and the polarization time gradually shortens with the increase of temperature. In general, the electrical resistivity of the specimen decreases with the increase of temperature. Nevertheless, there is a temperature inflection point for specimen incorporating graphene content of zone B or zone C, and the resistivity increases with the increase of temperature after the inflection point. The reason is that before the inflection point, the thermal motion is dominant, which makes the resistivity decrease. After the inflection point, the change in material volume caused by temperature rise is the main factor, resulting in an increase in resistivity. The relationship between resistivity and temperature of graphene-based cement composites under freeze–thaw action can be expressed by the exponential function. In the temperature range of 273 to 333 K, the response of resistivity to temperature of composites after freeze–thaw is stable, reversible and repeatable, with good thermosensitivity and decreased sensitivity.(5).After freeze–thaw, the composites incorporating graphene content of the zone B still show satisfactory pressure-sensitive property, and the sensitivity increases significantly with the degree of freeze–thaw. The compressive stress shortens the distance between the graphene particles in the specimens with graphene content of zone B, enhances the tunneling conduction, and leads to a decrease in resistivity. Freeze–thaw causes more microcracks and larger porosity of the composites, and the change in the compaction process is greater than that before freeze–thaw.The freeze–thaw degree of graphene-based cement composites specimens after freeze–thaw treatment was characterized by freeze–thaw times. The influence of freeze–thaw on the electrical conductivity of graphene-based cement composites was investigated from two aspects of polarization effect and percolation threshold, and the mechanism of freeze–thaw effect on its electrical conductivity was analyzed. The effects of different humidity and temperature on the humidity sensitivity and thermosensitivity of the composites after freeze–thaw action were also studied. In addition, the effects of monotonic loading and cyclic loading on the pressure-sensitive property of composites after freeze–thaw were analyzed. Through the above research, the main conclusions are as follows:


What is the geographic variation in the capacity of military and civilian psychiatrists within a 30-minute driving time of TRICARE  beneficiaries’ communities, and how does a community’s likelihood of having a psychiatrist shortage differ between historically underserved and other communities?In this cohort study of 39 487 zip code communities between 2016 and 2020, 35% of TRICARE beneficiaries lived in communities with a shortage of both military and civilian psychiatrists and 6% of beneficiaries had no psychiatrists available within a 30-minute driving time. Beneficiaries in low-income communities with high income inequality and rural communities had the highest likelihood of experiencing a shortage of psychiatrists.The study suggests that rural and economically disadvantaged communities need targeted strategies as the US Department of Defense considers realignment of military psychiatric capacity, because they cannot rely on civilian psychiatrists to fulfill the need gap for these shortage areas. Military service members and their families have greater mental health care needs compared with their civilian counterparts. Some communities have inadequate access to psychiatrists for this population.To identify geographic variations in the availability of military and civilian psychiatrists within a 30-minute driving time of TRICARE  beneficiaries’ communities and compare the likelihood of living in areas with inadequate access to psychiatrists for historically underserved and other communities.This retrospective cohort study of all zip code communities in the continental US, Hawaii, and Alaska with at least one TRICARE beneficiary between January 1, 2016, and September 30, 2020, combines data from the Defense Health Agency, the National Plan and Provider Enumeration System, and the US Census to estimate a logistic regression to compare differences between communities with and without a psychiatrist shortage. Data were analyzed from September 2021 to November 2022.A community’s likelihood of having a shortage of military and civilian psychiatrists within a 30-minute driving time and a community’s likelihood of having no psychiatrists. Odds ratios were calculated to estimate likelihood of the outcomes.This study includes 39 487 unique communities where 13% of the population is Black and 14% of the population is Hispanic. During the study period, 35% of TRICARE beneficiaries lived in communities with a shortage of both military and civilian psychiatrists, and 6% lived in communities with no access to military or civilian psychiatrists. Low-income communities with high income inequality were 1.64  times more likely to have inadequate access to psychiatrists and 2.59  times more likely to have no access to psychiatrists, compared with reference communities ; low-income communities without high income inequality were 1.37  times more likely to have inadequate access to psychiatrists and 1.93  times more likely to have no access to psychiatrists. Rural communities were 6.65  times more likely to have inadequate access to psychiatrists than urban communities.In this cohort study of US communities, 35% of TRICARE beneficiaries lived in communities with inadequate access to psychiatrists. Psychiatric capacity was structurally inequitable along 2 separate dimensions: the income gradient and rurality. Developing targeted strategies for these shortage areas could alleviate disparities. This cohort study identifies geographic variations in the availability of military and civilian psychiatrists within a 30-minute driving time of TRICARE beneficiaries’ communities and compares the likelihood of living in areas with inadequate access to psychiatrists for underserved and other communities. Military service members and their families generally have higher mental health care needs than the civilian population, and they also have unique challenges to access mental health care services due to military deployments, other high-risk and stressful activities associated with their jobs, frequent relocations, and family member absences.3 Although many active duty service members and some military families live on or near military installations and use military treatment facilities (MTFs) for medical care, many military families rely on civilian medical professionals.3Structural barriers to care for the US population are particularly salient in mental health settings, including long waiting lists, clinicians not accepting new patients, and a lack of insurance coverage.8 Although some geographic variation in health care spending and use might be due to individual clinician or patient behaviors, the built environment of health care supply also contributes to access and treatment inequities for historically underserved populations.10 The per-capita supply of behavioral health clinicians varies substantially across Census divisions and between urban and rural areas,11 and specialized services such as percutaneous coronary intervention facilities or stroke care are underprovided in historically underserved  communities while oversupplied in other communities.13 The military health system, despite being centrally managed, can still experience unexplained geographic variation in spending and use of selective procedures.15 Beneficiaries of TRICARE  have similar geographic variations in health care use as the colocated civilian population for primary care, but less variation for specialty care.16 Military treatment facilities provide an alternative to civilian health care resources (and vice versa), which could reduce disparities for beneficiaries who live in historically underserved communities. However, to our knowledge, little research exists examining the simultaneous psychiatric capacity of MTFs and civilian facilities for the military population.Geographic variation in health care spending, use, and quality has been well documented in the civilian market, and there is continued debate about whether resources are efficiently allocated across space.Our study has 2 objectives. First, we analyze geographic variations in psychiatric capacity within 30-minute driving times of TRICARE beneficiaries’ communities, focusing on availability of both MTF and civilian psychiatrists. Second, we compare the likelihood of living in areas with psychiatrist shortages between beneficiaries in historically underserved communities  and other communities. We hypothesize that historically underserved communities are more likely to have psychiatrist shortages.STROBE) reporting guideline. Because the data were administratively collected and deidentified, the study did not require informed consent.The study population includes 39 487 zip code communities in the United States (excluding Puerto Rico and US territories) that contained at least 1 TRICARE beneficiary between January 1, 2016, and September 30, 2020. Communities are defined by beneficiary mailing zip code. This study is approved through the Naval Postgraduate School institutional review board and follows the Strengthening the Reporting of Observational Studies in Epidemiology (17 We used a web-based query18 to derive a database of driving time between centers of each zip code community and (1) the practice location of the MTFs’ psychiatrists and (2) the zip code centers of civilian psychiatrists’ practice location. See eAppendix 1 in We combined monthly data to form our analysis to measure beneficiary population size, demographic, and military branch of service characteristics of TRICARE beneficiaries; civilian and MTF psychiatrist capacity; and community characteristics. Data sources include data from the Defense Health Agency, the National Plan and Provider Enumeration System National Provider Identifier data, the US Census, American Community Survey, and the Social Determinants of Health Database.We used zip codes to define community because it is the smallest geographic unit we can capture across data sources. We used travel time from the patient’s perspective to define the relevant access area, rather than political boundaries  or a fixed radius approach . Each mental health episode can require multiple follow-up visits, so we chose a 30-minute car driving time (1-hour round trip) as a reasonable threshold for TRICARE beneficiaries to travel on a repeated basis. Although driving time accounts for a large percentage of travel time, actual travel time would be longer than 30 minutes . We provide additional rationale for our geographic definition in eAppendix 1 in 19 We used the 20 000:1 threshold because the military population has higher exposure to stress and risks. We created separate shortage area designators for MTF-provided care and care from civilian facilities because the relevant populations are different. For MTF-provided care, the relevant population is TRICARE beneficiaries because non-TRICARE patients cannot use the MTF, and the ratio was computed as the number of full-time equivalent psychiatrists in the MTF within a 30-minute driving time of a given community divided by the number of TRICARE beneficiaries within a 30-minute driving time. For civilian psychiatrists, the relevant patient population is all residents, regardless of insurance status. We computed the ratio as the number of civilian psychiatrists practicing in zip codes within a 30-minute driving time of a given community divided by the number of residents within the same set of zip codes. A community was considered a shortage area for both military and civilian psychiatrist capacity if both ratios were smaller than 1:20 000. Our secondary outcome is a subset of a shortage area that has no military or civilian psychiatrists available within a 30-minute driving time.Our primary outcome is whether a community is a psychiatrist shortage area. The Health Resources and Services Administration (HRSA) defines shortage areas as those where the population-to-psychiatrist ratio exceeds 20 000:1 for populations with unusually high needs of mental health care and 30 000:1 for populations with normal needs.20 with robust SEs that accounted for 2 levels of correlations: intracommunity correlation among observations from different months of the same community and intercommunity correlations among communities from the same county, as those communities might be subject to similar local health regulations. All regressions were weighted by the beneficiary population size of each community to obtain TRICARE beneficiary-representative estimates.21 All models included time dummies to control for macro trends. All P values were from 2-sided tests and results were deemed statistically significant at P < .05.Our unit of analysis was the community-month. Because our outcomes are binary, we implemented logistic regressions in Stata, version 17 (StataCorp LLC),22 Model 1 focused on socioeconomic conditions based on income level (as measured by median family income) and income inequality .23 Model 3 focused on racial and ethnic distribution of the communities and included 2 sets of categorical variables based on their Black and Hispanic share of population: whether a community is in the lowest (reference group), middle, or highest tertile of Black residents and likewise for Hispanic distribution. Race and ethnicity information were self-identified by respondents to the US Census.Model 2 focused on regional variations and included indicators for geographic regions  and whether the zip code community was defined as rural by the Federal Office of Rural Health Policy.24 Other included nonactive duty family members such as spouses of the active duty beneficiaries.25Model 4 focused on service branch distribution for a given community. A community was classified as an Army community if at least 50% of beneficiaries in that community were affiliated with the Army, and likewise for the Navy, Air Force, and Marine Corps. Communities where no service exceeded 50% were classified as Joint (reference group). Model 5 focused on size and type of TRICARE beneficiaries in each community. We categorized a community as having low, medium, or high presence of TRICARE based on tertile of the share of the community population that are TRICARE beneficiaries. We further captured the presence of different types of TRICARE beneficiaries based on tertile of the share of TRICARE beneficiaries who were dependent children, retirees, or other nonactive duty population. Dependent children are children of active duty primary beneficiaries, while retirees have retired from a military career, often after at least 20 years of service.Finally, model 6 was a fully adjusted model that included the complete set of factors from models 1 to 5 to estimate the association between a community’s psychiatrist shortage status and a single factor, after accounting for all other factors included in the model.There were larger disparities across income gradiant when the outcome is a community have no access to a psychiatrist instead of having a shortage of psychiatrists. In the fully adjusted model, communities with low income and high income inequality had the highest odds of having no access to psychiatrists , followed by low-income communities without high income inequality  . High-in29 rural communities were 6.65  times more likely than urban communities to have a shortage of both MTF and civilian psychiatrists and 5.48  times as likely to have no psychiatrists. The patterns demonstrated that the South was most likely to have a shortage compared with the other areas, but Alaska  and Hawaii  were most likely to have no psychiatrists.The second panel of The third panel of After accounting for other factors, service affiliation was not associated with being in a shortage area. Communities with a high share of TRICARE retirees were 1.92  times more likely to be in shortage areas and 3.83  times more likely to have no access to psychiatrists compared with communities with a low presence of military retirees .shortage area as a 30 000:1 population-to-psychiatrist ratio. Under this definition, 24% of TRICARE beneficiaries lived in communities with a psychiatrist shortage. Second, we expanded drive time to 40 minutes. Under this definition, 27% of TRICARE beneficiaries lived in communities with a psychiatrist shortage. Finally, less than 40% of behavioral health professionals (including psychiatrists) accepted new TRICARE patients between 2008 and 2020.31 Using fiscal year 2021 as the benchmark, if we assume that only 37% of civilian psychiatrists were accessible to TRICARE beneficiaries, 60% of TRICARE beneficiaries lived in communities with a psychiatrist shortage. Logistic regression results of community characteristics remained similar across all 3 alternative assumptions  have the lowest likelihood of experiencing psychiatrist shortage.33 However, our findings show that military retirees are more likely to be in communities with too few or no psychiatrists. Although the access gap for this subgroup might be filled partially by care provided by the Veterans Administration (VA), only 60% of veterans are eligible for VA health care under the current policy and only 50% of eligible veterans use VA health benefits.35Our findings reveal important disparities in access to psychiatrists among military retirees. Military retirees are especially vulnerable to mental health problems.37 our results highlight that market forces incentivize civilian mental health professionals to locate in nonrural and affluent communities, but they also represent opportunities where the public sector can play a role in reducing disparities. As the military health system considers realignment of military psychiatric capacity, it could develop strategies to mitigate disparities between affluent and underserved communities. Such strategies could include optimally placing satellite clinics or coordinating telemedicine capacity supported by MTF psychiatrists. The military health system could alter its reimbursement rates for care purchased from civilian mental health care professionals to mitigate shortage problems in economically underserved and rural communities. However, any solutions must work within the broader context of a general psychiatrist shortage.38 The HRSA projects that by 2030 the US will have 12 500 fewer adult psychiatrists than needed.39 The HRSA also projects an increase in nurse practitioners, who can also prescribe medication. Future studies should examine whether increasing nurse practitioners in communities with a psychiatrist shortage would effectively improve mental health care needs for the military population.Providing mental health care to the military population and their dependents involves multiple health care systems—TRICARE, VA, and the private sector. The fact that the military population, despite having access to care in multiple systems, is not immune to structural inequity in psychiatric care provision is perhaps not surprising; there are fewer than 500 MTFs that provide psychiatric care, while the military population is distributed widely over 39 487 zip codes. The military population must rely on civilian psychiatric care in many areas. Consistent with prior studies,42 by quantifying geographically based shortages in access to psychiatrists in historically underserved civilian communities.44 Civilian populations within psychiatrist shortage areas share similar experiences with their military counterparts. Improving the psychiatrist capacity provided by the military health system could reduce shortage problems experienced by civilians, if initiatives free up the resources of civilian psychiatrists. State or federal governments could also decrease disparities within both civilian and military populations by, for instance, providing geographically based incentives in loan repayment programs.Although military populations are important in their own right, our analysis also answers the call for increased research regarding structural discrimination and inequity more broadlyshortage. Data on psychiatrists are also more reliable than the broader set of mental health care professionals, especially in the civilian data. The focus on psychiatrists reduces estimation errors.This study includes several limitations. First, while we recognize that psychiatric nurse practitioners and primary care physicians can also prescribe medications and serve in similar roles as psychiatrists, we did not include them in measuring psychiatric capacity, because we needed to be consistent with the HRSA population-to-psychiatrist definition of Second, our capacity metrics are measured with error if there are registry processing delays when a practitioner moves or closes a clinic. If these errors do not systematically vary by community characteristics, it should not confound our findings. Driving time was also measured with error because the query assumes normal traffic. Both sources of measurement error create attenuation bias, making our estimated results conservative or lower than they should be.45Third, we focused on access barriers of the built environment. A military-affiliated individual with psychiatrists available within a 30-minute travel time can still face many other barriers to accessing psychiatric care, including difficulty in getting a referral, lack of coordination among primary care and specialists, office visit wait times, and waiting lists for new patients.31), the percentage of TRICARE beneficiaries living in communities with a psychiatrist shortage increased to 60%. However, if acceptance rates are lower in historically underserved communities, we would underestimate the true disparity between historically underserved and other communities.Fourth, not all civilian psychiatrists accept TRICARE patients due to low reimbursements. When we assumed a uniform 37% acceptance rate across all communities (per fiscal year 2021Fifth, many communities have a shortage of MTF psychiatrists but adequate access to civilian psychiatrists. Although psychiatrists working in MTFs and civilian practices underwent similar training, substituting a psychiatrist well acquainted with military issues with a psychiatrist in civilian practice may not be an equal swap.Sixth, we used a 30-minute driving time as the geographic access area to identify shortages, following Centers for Medicare & Medicaid Services and HRSA guidelines. This definition might result in overestimating the population-to-psychiatrist ratio in urbanized areas where both population and psychiatrists are dense. This might exacerbate the estimated access disparity between urban and rural regions, where population and psychiatrists are dispersed. However, sensitivity analyses showed that results were similar when we changed our shortage threshold from a ratio of 20 000:1 to 30 000:1, changed drive time to 40 minutes, or specified as outcome having no psychiatrist within the drivable area. Driving is also only part of travel time—actual travel time would be longer, making our estimates conservative. Overall, the purpose of this article is not to estimate the precise number of psychiatrists the TRICARE population have access to, but rather to identify characteristics associated with shortage areas so that decision makers can target resources accordingly.In this cohort study of US zip code communities, 35% of TRICARE beneficiaries lived in communities with a shortage of psychiatrists from both MTF and civilian sectors, and 6% had no psychiatrists available within a 30-minute driving time. Beneficiaries living in low-income communities with high income inequality and rural communities had the highest likelihood of experiencing a shortage of psychiatrists from both the military and civilian sectors. Communities with a higher presence of military retirees also had higher risks of experiencing a shortage of or no access to psychiatrists. Our findings suggest that psychiatric capacity was structurally inequitable along the income gradient and rurality and that certain types of TRICARE beneficiaries had worse access than others. As the military health system considers realignment of its psychiatric capacity, it would be important to develop targeted strategies for shortage areas, since it cannot rely on civilian mental health care professionals to care for the military population in many communities.


The novel paper-based Bi-Material Cantilever (B-MaC) valve allows the autonomous loading and control of multiple fluid reagents which contributes to the accurate operation of paper-based microfluidic devices utilized for biological and chemical sensing applications. In this paper, an extensive parametric study is presented to evaluate the effects of key geometric parameters of the valve, such as paper direction, cantilever width, paper type, tape type, and sample volume, in addition to the effects of relative humidity and temperature on the functionality of the B-MaC and to provide a better understanding of the rate of fluid flow and resulting deflection of the cantilever. Machine direction, cantilever width, paper type, and tape type were found to be important parameters that affect the B-MAC’s activation time. It was also observed that the rate of fluid imbibition in the B-MaC is considerably affected by change in humidity for high (55 °C) and low (25 °C) temperatures, while humidity levels have no significant effect during imbibition in the B-MaC at an ambient temperature of 45 °C. It was also found that a minimum distance of 4 mm is required between the B-MaC and the stationary component to prevent accidental activation of the B-MaC prior to sample insertion when relative humidity is higher than 90% and temperature is lower than 35 °C. The rate of fluid imbibition that determines the wetted length of the B-MaC and the final deflection of the cantilever are critical in designing and fabricating point-of-care microfluidic paper-based devices. The B-MaC valve can be utilized in a fluidic circuit to sequentially load several reagents, in addition to the sample to the detection area. Microfluidic Paper-based Analytical Devices (μPADs) provide many advantages due to their usage of paper as a substrate. Paper is an inexpensive material, can be easily transported, and is biodegradable. This helps in developing inexpensive devices that use small volumes of sample and reagents. They are also easy to dispose of, are user friendly, are easy to deploy, and are compatible with biological and chemical samples. Since reagents and samples flow by capillary force through the paper substrate, a μPAD does not require pumps for fluid transport, making the paper-based device a compact self-contained system. Therefore, μPADs can be used for Point-Of-Care Testing (POCT) or On- Site Testing (OST), for immediate results. The use of these devices has been growing over the past several years due to their suitability for a variety of applications, such as infectious disease diagnosis , paper material , and environmental conditions , generally influence the behavior of a fluid flowing in a paper-based microfluidic device ,34. To aIn this paper, we comprehensively investigate the effect of various material and geometrical parameters, along with the influence of surrounding conditions, on the functionality of B-MaCs. An experimental setup was designed to study B-MaC behavior as a function of cantilever width, direction of paper cut (machine or cross-machine direction), sample volume, paper type, and tape type. In addition, an environmental chamber was also designed and fabricated to study the effect of relative humidity and temperature on the rate of fluid flow of filter paper, as well as that of the B-MaC. The final deflection of cantilevers in various humidity conditions is reported on. Paper-based devices utilizing B-MaCs can benefit from this analysis of critical parameters when tests are conducted in different ambient conditions and when the sample volumes are relatively small. Besides introducing and providing two examples of the application of B-MaC as a novel valve technology in paper-based devices, proving itself to be quick to respond, easy to manufacture, and needing only small sample amounts to function, as a consequence of taking into account the interaction between temperature and humidity on the imbibition rate in filter paper, which, to the best of our knowledge, has not been fully examined in prior research investigations, the results reported in this study are beneficial to developers of paper-based devices.® Tape 600 , Scotch® Heavy Duty Shipping Packaging Tape , and Duck Brand HD Clear High-Performance Packaging Tape . The following items were used in preparing and running the experiments presented in this paper. The paper-based valve used one of the following paper types, Whatman grade 1 chromatography paper , Whatman filter papers grade 1 , grade 4 , or grade 41 . Tape is also an integral part of the B-MaC and one of the following tapes were used in creating the valve, ScotchASTM Type 1 deionized water  was used in all experiments. A few drops of food coloring (Wilton Icing Colors) were added to this water to aid in visualizing fluid flow.The layout of the paper-based valve was designed using vector graphics software  and then cut out using a laser engraver (Epilog mini 40 W) in the paper direction of interest. An 8-megapixel video camera with 30 frames per second capability was used to capture the actuation of the B-MaC. These video recordings were then replayed using a media player (Avidemux) to collect the data.A strip of filter paper (4 mm × 30 mm), partially laminated with tape (4 mm × 19 mm) on one side, leaving a 2 mm distance from the end of the paper, functioned as a cantilever actuator. While the tape layer’s length did not change, the cellulose fibers in the paper experienced hygro-expansion when exposed to fluid flow. This resulted in the free end of the cantilever bending toward the tape layer side, while the other end of the cantilever was fixed and attached to a solid support. The cantilever bent because of the difference in reaction of the paper and tape components to fluid flow, and, thus, opened or closed a fluidic channel at the stationary component (SC). The stationary component was a 1 cm × 1 cm piece of chromatography paper. The two orientations of the bi-material cantilever are shown in Fabricating a B-MaC consisted of first attaching tape to the paper. Separately, using vector graphics software (CorelDraw X6), the cantilevers’ dimensions were drawn. Then, the laser engraver was used to cut off the cantilevers from the tape and paper. The cuts were made in the cross-machine direction. The cross-machine direction was 90 degrees from the direction the pulp flowed during the paper’s fabrication.An environmental chamber was designed and built at the Microfluidic Laboratory at URI to study the effects of varying ambient conditions on the performance of the B-MaC. This chamber consisted of a test space, an electric heater, an ultrasonic atomization humidifier, humidity and temperature sensors and controllers, a cooling fan, a dehumidifying fan, a drain valve, and a vent valve. The cooling fan was controlled in such a way as to push room air into the chamber to reduce the temperature inside; whereas the dehumidifying fan would suck air out of the chamber to reduce the humidity inside. The temperature and humidity in the lab were typically controlled at a comfortable indoor condition (Temp. 23 ± 1 °C and RH 50 ± 5%).The chamber was made up of Plexi glass (3 mm thick) joined together by a few steel L-brackets and screws. A sealant  was used along all of the inside edges to reduce air infiltration into the chamber. A mini electric heater (YOUCIDI 200 watts) was used to provide the heat required to raise the temperature inside the chamber. The humidifier used was a COOSPIDER Reptile Fogger Terrariums Humidifier to increase the relative humidity inside the chamber. The temperature sensor and controller used was the Inkbird ITC-308 Digital Temperature Controller, with a resolution of 0.1 °C and an accuracy of 1 °C. This controller was used to activate the operation of the electric heater and cooling fan in the chamber. The humidity sensor and controller used was the Inkbird Humidity Controller IHC200 with a resolution of 1% and an accuracy of 3%. This controller was used to activate the operation of the humidifier and the dehumidifying fan in the chamber. The cooling and dehumidifying fans each used a 4″ AC-RXS4. Two main experiments were conducted in this study and are summarized in It has been previously demonstrated that cellulose fibers line up in a certain preferential orientation during the manufacturing process. This orientation is referred to as the Machine Direction (MD) and is the direction in which paper sheets are stretched in the direction of machine pull while they are still wet and, then, later compacted to dry ,43. The t(8) = 4.445, two-tailed p-value = 0.002) faster activation time  compared to those cut in the machine direction .Whatman filter paper strips, 4 mm wide and grade 41, were used in both machine and cross machine directions along with scotch tape to make the cantilevers. The time it took for the free end of the cantilever to deflect and connect to the stationary component, referred to as the activation time, was then evaluated using a 16 μL fluid sample, and the results are displayed in Considering a cantilever as a beam, one possible explanation as to why the activation time was faster in the CMD might be attributed to the spring stiffness that depended on the geometry of the beam. Therefore, the cantilever made in cross machine direction had lower stiffness and tended to deflect more easily, since fiber orientations did not hinder its deflection.p-value < 0.01) effect on the response time of the cantilever. As the width of the cantilever increased, the end of the cantilever connected to the stationary component in a shorter period of time. Therefore, it is necessary to take into account how width affects the cantilever activation time in designing paper-based devices using B-MaCs. However, the activation time of the 4 mm wide cantilevers was not statistically significantly different than the activation time of the 5 mm wide ones. We chose the 4 mm wide cantilevers for further investigation in this study, since their activation time demonstrated a lower standard deviation than those of 5 mm.The impact of cantilever width on activation time was also investigated by creating cantilevers in the cross-machine direction with four different widths of 2, 3, 4, and 5 mm. We chose this range of widths since most lateral flow microfluidic devices are being developed with widths up to 5 mm . Wider pThe influence of width on the rate of fluid flow in μPADs has been previously studied and results show that wicking time decreases with increasing width, i.e., the fluid flows faster in the strip with the larger width; however, this dependence gets weaker as width gets wider ,41,47. IF = 3.24, F-value = 0.75, p-value = 0.54). The p-value showed that there was no significant effect due to the sample volume. Therefore, 12 μL was selected for further investigations, since it had the lowest standard deviation.Chen et al.  and GerbAs μPADs are increasingly finding use in many different applications and more complex chemical and biological tests and assays, a proper choice of the paper type to be used is crucial. It has been experimentally shown that paper-based devices utilizing thicker papers as substrates result in slower fluid flow ,50. Seve® Tape 600 , Scotch® Heavy Duty Shipping Packaging Tape , and Duck Brand HD Clear High-Performance Packaging Tape  were utilized to produce cantilevers and examine the effect of different tapes on the B-MaC activation time. The cantilevers were made from Whatman filter paper grade 41 in cross-machine direction and then cut in 4 mm widths. The tests were run using 12 μL of sample. As indicated in The commonly available ScotchThe effect of temperature and humidity on the wetted length of filter paper and B-MaC was extensively investigated. Graphical results are provided to study the relationship between the dependent variable and independent variables. The wetted length of the filter paper on the B-MaC was set to be the dependent variable and temperature and humidity were set to be the independent variables. Results for the study were obtained for four temperature settings, 25 °C, 35 °C, 45 °C, and 55 °C, with a corresponding relative humidity value ranging from 10% to 90%, with an incremental step of 20%.In summary, the fluid imbibition in filter paper at low temperature was not affected by the humidity level, but for higher temperatures, the time to reach a wetted length of 40 mm decreased. To the best of our knowledge, the effect of relative humidity and temperature on fluid flow behavior in paper strips has so far only been characterized by two research groups. Nevertheless, their results appeared to contradict each other on the effect of humidity on the imbibition flow in filter paper strips. While Walji and MacDonald  observed2 = 0.9988. Considering this fact, the wetted lengths of B-MaCs were calculated by a line integral using the following formula:Since the cantilever bends with the flow of fluid, it is more difficult to experimentally measure the wetted length of cantilevers over time. For this purpose, the B-MaC was set against the graduated wall with a resolution of 1 mm by 1 mm. The experiment was conducted by placing the B-MaC on a fixed component and loading fluid onto the B-MaC with a capillary tube, as was presented in In summary, the fluid imbibition in B-MaC at 45 °C temperature was not affected by the humidity level. However, on an increase in temperature, the average time to achieve a wetted length of 25 mm increased with a decrease in humidity, and on a decrease in temperature, the average time to achieve a wetted length of 25 mm increased with an increase in humidity. It can be concluded from the graphical results that the wetted length for B-MaC was drastically affected by a change in humidity for high temperatures.Activation of the B-MaC is based on the difference between the elongation of paper and that of tape when paper gets wetted. However, the paper part of the B-MaC can also absorb moisture from the ambient surroundings and cause the B-MaC to start bending even without introducing any sample to it. Thus, the effect of temperature and relative humidity should be carefully studied to know the behavior of B-MaCs as a component in microfluidic paper-based devices. We recorded the initial deflection of B-MaC while in the equilibrium state within the environmental chamber after 10 min and before the introduction of the fluid sample. The B-MaC’s final deflections were also recorded when fully wetted by the fluid sample. The difference between the B-MaC’s final and initial deflections was referred to as the net deflection. The B-MaC offers an inexpensive and simple-to-fabricate actuator system that can be easily integrated in 2D or 3D microfluidics paper-based devices. The bi-material actuator may also be used in complicated systems, like the enzyme-linked immunosorbent assay (ELISA) protocols, where many reagent fluids are involved, and sequential loading or timing delays are necessary. In this work, the effect of various material and geometrical parameters used in the B-MaC, such as direction of paper to be cut, paper width, sample volume, tape type, and paper type, on its activation time were studied experimentally. Test results showed that all of these parameters, aside from sample volume, play a significant role in the B-MaC’s activation time and an optimized value for each parameter was provided to lower its activation time. These optimized values were then used in the design of an experiment to study the combined effect of relative humidity and temperature on the rate of fluid flow in filter paper, as well as their effect on the B-MaC in an environmental chamber specifically built for that purpose.Results showed that the fluid imbibition in filter paper at a temperature of 25 °C was not affected by the different humidity levels examined in the study. This was not the case for higher temperatures where humidity had an effect on fluid flow and the average time to achieve a wetted length of 40 mm decreased with increasing temperature. The level of humidity had an effect on the imbibition rate in the B-MaC at different temperatures and this could be attributed to the difference in response of the paper and tape layers in the B-Mac. The study also found the net deflection of the B-MaC in various environmental conditions. Finally, a recommendation was made on the minimum distance required between the cantilever and the stationary component to prevent the inadvertent activation of the B-MaC in various environmental conditions.The results presented in this paper are of value to designers of paper-based devices, since the results take into account the interaction of temperature and humidity on the imbibition rate in filter paper which, to the best of our knowledge, has not been exhaustively looked at in previous research studies. Designers of paper-based devices utilizing B-MaCs in their architecture will also benefit from this study as it provides optimized values for the different geometric parameters of interest when designing the B-MaC. Several reagents and the sample can be loaded successively into the detecting region using the B-MaC valve in a fluidic circuit.


The energy density of NSG@GF reaches 3.40 μWh cm−2 after nitrogen doping, which is 2.59 times of that of as-prepared one. Moreover, Nitrogen-doped graphene fiber-based supercapacitor (NSG@GF FSSC) exhibits good conductivity (155 S cm−1) and cycle stability (98.2% capacitance retention after 5000 cycles at 0.1 mA cm−2).In this study, a strategy to fabricate nitrogen-doped porous core-sheath graphene fibers with the incorporation of polypyrrole-induced nitrogen doping and graphene oxide for porous architecture in sheath is reported. Polypyrrole/graphene oxide were introduced onto wet-spun graphene oxide fibers by dip-coating. Nitrogen-doped core-sheath graphene-based fibers (NSG@GFs) were obtained with subsequently thermally carbonized polypyrrole/small-sized graphene oxide and graphene oxide fiber slurry (PPY/SGO@GOF). Both nitrogen doping and small-sized graphene sheets can improve the utilization of graphene layers in graphene-based fiber electrode by preventing stacking of the graphene sheets. Enhanced electrochemical performance is achieved due to the introduced pseudo-capacitance and enhanced electrical double-layered capacitance. The specific capacitance (38.3 mF cm With the emerging electronic textiles, the demand for rechargeable energy storage technologies is urgently needed ,2. FiberOwing to the remarkable electrical conductivity, higher tensile strength, and controllable structure ,9,10, gr−2) and 159.5% enhancement of energy density 3.40 μWh cm−2), compared with that of the pure graphene fibers. NSG@GF FSSC also demonstrated ultralong cycling life and good conductivity.In this work, nitrogen-doped graphene fiber electrodes by dip-coating wet-spun graphene oxide fibers with slurry containing small-sized graphene oxide and pyrrole monomers as nitrogen sources and thermal carbonization are designed. The influence of mass ratio of different components on the electrochemical performance of graphene fiber electrodes was systematically investigated. The graphene fibers were successfully doped with nitrogen atoms. Moreover, the nitrogen doping resulted in 159.7% enhancement of area specific capacitance , and sulfuric acid  were purchased from Chemical Reagent Co., Ltd., Shanghai, China. Deionized (DI) water was made with Master-Q15 (resistivity~18.3 MΩ cm), Shanghai, China.The graphite oxide powder was bought from the Sixth Element Materials Technology Co., Ltd., Changzhou, China. Polyvinyl alcohol  was purchased from Sigma-Aldrich, Shanghai, China. Pyrrole (PY), ferric chloride (FeClA certain amount of graphite oxide was dispersed in abundant DI water and subsequently ultrasonicated by a Biosafer with a power of 312 W for 1 h to form graphene oxide (GO) solution. To remove the unexfoliated graphite oxide, the above dispersion was centrifuged at 3000 rpm for 15 min. Then the supernatant was centrifugated at 8000 rpm for 15 min to remove the small-sized graphene oxide, and condensed to serve as the solution for wet spinning graphene oxide fibers.−1) for further wet spinning.The small-sized graphene oxide (SGO) (~150 nm in diameter) was prepared by breaking graphene oxide into smaller pieces with high-power ultrasonication for 20 h. Then the SGO dispersion was centrifuged at 3000 rpm for 30 min to obtain SGO with suitable sizes. Lastly the SGO dispersion was thermally concentrated to a suitable concentration (15 mg mL−1 GO dispersion was injected into an ethanol/DI water (1:3 v/v) coagulation bath containing 5 wt.% CaCl2 through a spinneret (inner diameter of 0.3 mm) at room temperature. By mounting onto a syringe pump (LSP02-1B), the continuous wet-spun GOFs were drawn out from the coagulation and washed by DI water and ethanol. The GOFs were then wound onto a winder and dried under tension by infrared drying at 100 °C for 4 h.The graphene oxide fiber (GOF) was wet spun by the method described in our earlier work . A totalSmall-sized graphene-based fibers (SG@GFs) were fabricated by the method as described below. First, the as-prepared GOFs were immersed in the SGO solution, and dried and solidified at 80 °C. The resultant fibers are denoted as SGO@GOF. SG@GF was prepared by pre-oxidized SGO@GOFs with multiple temperature steps at 120 °C, 150 °C, and 180 °C for 1 h. Then, they were thermally annealed at 800 °C for 3 h under nitrogen protection to form SG@GFs.3 solution to polymerize pyrrole monomers in the SGO sheath. Then, PPY/SGO@GOF was finally obtained after drying at 60 °C for 1 h.Core-sheath nitrogen-doped graphene-based fibers (NSG@GFs) were prepared by the following two-step method. The core fiber is GOF, the sheath was fabricated of polypyrrole in SGO. The PPY/SGO@GOFs were fabricated by the “dip-coating” method. First, a certain amount of pyrrole was mixed into SGO slurry. Then, the mixture was homogenized with a high shear dispersion device  at 10,000 rpm for 15 min and cooled in an ice bath for 3 h. Then GOF were dipped in the above mixture for sheath coating, and then immersed in FeCl2) purge with a heating rate of 2.5 °C/min. Then the core-sheath NSG@GF was produced at 800 °C for 3 h, towards the final required NSG@GFs.NSG@GFs were prepared by pre-oxidizing PPY/SGO@GOFs with multiple temperature steps at 120 °C, 150 °C, and 180 °C for 1 h, respectively. The pre-oxided PPY/SGO@GOFs were thermally annealed by a high-temperature vacuum tubular furnace (OTF-1200X) under nitrogen (N2SO4 (98 wt.%) was added and stirred at room temperature to prepare PVA/H2SO4 gel electrolyte. To fabricate quasi-solid-state fiber-shaped supercapacitor, two fiber electrodes with length of ~7 mm were placed in parallel on a flexible polyethylene terephthalate (PET) substrate with an electrode spacing of ~2 mm. Each end of the fibers was connected to polished copper foils by silver paste. Then assembled NSG@GF FSSCs were solidified for 24 h at room temperature to make the electrolyte sufficiently infiltrate into fiber electrodes.The NSG@GF-based FSSCs in aqueous electrolyte were prepared with the following method. A total of 1 g PVA powder was dissolved in 10 mL DI water at 90 °C. The PVA solution was vigorously stirred for 2 h until fully dissolved. Subsequently, 0.98 g H−1, the scanning wave number range is 4000–400 cm−1, to characterize the chemical structure of GOF, GF, SG@GF, and NSG@GF, where KBr was used to mix with samples to prepare thin films for FTIR measurements.Scanning electron microscopy  was used to characterize the morphological feature of GF, SG@GF, and NSG@GF samples. The diameters of GF, SG@GF, and NSG@GF were measured with polarizing optical microscope (NIKON ECLIPSE LV100POL). The microstructure and compositional element distribution of the NSG@GF were further investigated by using transmission electron microscopy  coupled with energy dispersive spectrometer (EDS) mapping. Element valence states and contents were preceded on an X-ray photoelectron spectroscopy . Fourier transform infrared spectroscopic (FTIR) measurements were conducted on a Nicolet NEXUS-670; where the resolution is 4 cmElectrochemical tests were carried out on an electrochemical workstation  with a two-electrode configuration for analysis of cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy . The electrical conductivity (σ) was measured with four-wire resistivity measurement method at room temperature.3 solution was then carefully dropped into the above mixture to produce PPy@SGO slurry. The slurry was then dip-coated onto GOF due to the hydrophilic characteristics between them. Finally, thermal carbonization was conducted to form the designed core-sheath porous graphene-based composite fiber, NSG@GF. For electrochemical measurements, NSG@GF FSSC was assembled by immersing two pieces of NSG@GF into PVA/H2SO4 gel electrolyte and aligning them in parallel and dried under ambient condition. SG@GF was fabricated by the as-prepared GOF immersed in the SGO solution with subsequent thermal carbonization.In comparison, the surface of SG@GF shows less wrinkled morphology along the fiber axis as seen in It can be clearly seen from To further identify the nitrogen doping, EDS was conducted to characterize the element distribution of NSG@GF. As seen in To quantitively characterize the valency and nitrogen doping level, we conducted XPS characterization for NSG@GF. −1, 1714 cm−1, and 1400 cm−1, which originate from -OH, C=O, and the C-O bond, respectively. this indicates that the surfaces of GF, GOF, and SG@GF contain abundant oxygen-contained groups [−1 (-CH), 1632 cm−1 (C=C), and 1115 cm−1 (C-N), confirming that NSG@GF is heavily reduced and doped with nitrogen atoms, which has good agreement with XPS results.Furthermore, d groups . However−1, 47.96 S cm−1, and 155 S cm−1, respectively. The decrease in the conductivity of the SG@GF is due to the increase in the diameter and the internal resistance of the fibers, due to the stacking of the GO layer. In contrast, the increase in the conductivity of the fiber after the nitrogen doping coating is due to the improvement of the electrical conductivity structure of the fiber [It is seen from he fiber .−1 for GF and SG@GF, of which the shapes are nearly rectangular [The electrochemical properties of GF, SG@GF, and NSG@GF were summarized in tangular , indicattangular . The cur−2 , indicating the existence of pseudo-capacitance from redox reactions on the shell surface with nitrogen-doped areas in graphene. Compared with the data for GF and SG@GF , lower than that of GF (1.31 μWh cm−2), but the EA of NSG@GF was improved to 3.40 μWh cm−2 after nitrogen doping.Based on these GCD curves, we calculated their specific capacitances with respect to the current density from 0.1 to 1 mA cm−2 f. It shoAs observed in In summary, we report a novel strategy to fabricate high-performance FSSCs assembled by nitrogen-doped core-sheath graphene fibers incorporated by slurry containing small-sized graphene oxide and pyrrole monomers serve as nitrogen sources. NSG@GF FSSCs display high electrical conductivity and excellent electrochemical performance. Moreover, the as-assembled FSSCs exhibit good capacitance and cyclic stability. The facile nitrogen-doped method, unique core-sheath graphene-shaped structure, and superior electrochemical properties endow a new avenue in the fields of fiber-shaped energy storage devices.