temperature is undoubtably one of the most important parameters to know in various fields of science and technology , and in everyday life .
major contactless techniques include optical pyrometry , ir thermography , and phosphor thermometry . among them
, optical pyrometry is only suitable for measurements at high temperatures ( > 600 c ) . on the other hand ,
for example , precise temperature determination is essential in marine research , geochemical studies,(3 ) medicine , and aeronautics,(6 ) to mention only a few areas .
moreover , temperature is a key parameter in optical sensing since the response of virtually all such sensors is temperature dependent .
ir thermography has become very widespread but is still rather expensive and may suffer from various artifacts.(7 ) luminescent temperature - sensitive materials represent a promising alternative here .
thermographic phosphors are well established and have been in use for decades.(8 ) some of them find application in commercial devices ( e.g. , fiber - optic thermometers from lumasense technologies , www.lumasenseinc.com ) .
however , most of the thermographic phosphors are intended for sensing high temperatures and become practically useless at ambient conditions.(9 ) the suitable phosphors for rt sensing include manganese(iv)-doped magnesium fluorogermanate mg4fgeo6 , chromium(iii)-doped yttriumaluminum garnet y3al5o12,(12 ) aluminum oxide ( ruby ) , spinel mgal2o4,(15 ) and yttriumaluminum perovskite yalo3,(16 ) as well as europium(iii)-doped lanthanum oxysulphide la2o2s .
some other phosphors also have been proposed.(19 ) the limitations of these phosphors include rather low temperature sensitivity and/or insufficient luminescence brightness .
apart from thermographic phosphors , organometallic complexes which possess temperature - dependent luminescence were found to be very useful probes .
recently , they became increasingly popular and were applied in dually sensing materials and pressure - sensitive paints .
such materials usually rely on the use of ruthenium(ii ) polypyridyl complexes and europium(iii ) tris--diketonates .
generally , these temperature probes suffer from photostability issues , decreased luminescence intensity at higher temperatures , and pronounced cross - sensitivity to oxygen so that their immobilization in gas - blocking polymers such as polyacrylonitrile is usually essential.(32 ) some temperature - sensitive fluorophores also were reported but they are less convenient due to measurements of fluorescence intensity and not the decay time .
evidently , state of the art thermographic phosphors and luminophores have their limitations and more advanced materials are highly desired .
an  ideal  luminescent material for sensing temperature should fulfill the following requirements : ( i ) have high brightness ; ( ii ) possess excitation in the visible or nir part of the spectrum to minimize autofluorescence interferences ; ( iii ) have high - temperature sensitivity in the desired range ; ( iv ) be photostable ; ( v ) have negligible cross - sensitivity to other analytes such as , e.g. oxygen ; and ( vi ) be commercially available or simple to manufacture .
very often relatively long luminescence decay times ( > 1 s ) are desirable in order to enable lifetime interrogation . in this contribution
we will demonstrate that chromium(iii)-doped yttrium aluminum borate ( yab ) fulfills all these requirements and is , therefore , a thermographic phosphor of choice for sensing and imaging at ambient temperatures .
yttrium oxide ( 99.99% ) , gadolinium nitrate hexahydrate ( 99.9% ) , chromium(iii ) nitrate nonahydrate ( 99% ) , and molybdenum(vi ) oxide ( 99.5+% ) were obtained from aldrich ( www.sigmaaldrich.com ) , aluminum nitrate nonahydrate ( 99+% ) , yttrium nitrate hexahydrate ( 99.9% ) , urea ( 99.5% ) , and chromium(iii ) oxide ( 99,997% ) were from fischer scientific ( www.fishersci.com ) , aluminum oxide ( > 99% ) was from merck ( www.merck.de ) , hydrogel d4 was from advansourse biomaterials ( www.advbiomaterials.com ) , boron trioxide ( > 99% ) , potassium hydroxide , and potassium sulfate ( 99+% ) were from fluka ( www.sigmaaldrich.com ) , and boric acid ( 99.99% ) was from abcr ( www.abcr.de ) .
microcrystalline powders of chromium(iii)-doped yag y3al4.8cr0.2o12 , spinel mgal1.96cr0.04o4 , and ruby al1.96cr0.04o3 were obtained from the nitrates and urea with the solution combustion technique .
the luminescent beads of ruthenium(ii ) tris-1,10-phenanthroline in poly(acrylonitrile ) ( = ru - phen / pan ) and europium(iii ) tris(2-thenoyltrifluoroacetonato)-4-(4,6-di(indazol-1-yl)-1,3,5-triazin-2-yl)-n , n - diethylbenzenamine in poly(styrene - block - vinylpyrrolidone ) ( eu(tta)3deadit / ps - pvp ) were prepared as described elsewhere .
the cr(iii)-doped yab crystals were obtained by high - temperature flux growth technique the appropriate amounts of the yttrium oxide , aluminum oxide , boron trioxide , and chromium(iii ) oxide were homogenized in a mortar and mixed with a flux system consisting of potassium sulfate and molybdenum(vi ) oxide .
the mixture was loaded in platinum crucibles and melted in a front - loaded electric resistance furnace , equipped with a programmable temperature controller . the temperature program consisted of heating to 1120 c at 300 deg / h , keeping that temperature for 3 h , and then slowly cooling to 900 c at a rate of 1 deg / h . upon removing the flux by boiling in concentrated potassium hydroxide ( c = 8.0 m ) yab crystals ( 100 m10 mm )
microcrystalline yab powders were synthesized employing the solution combustion method with urea as a fuel .
typically , 5 mmol ( 1.915 g ) of yttrium nitrate hexahydrate , 13.514.85 mmol ( 5.0645.570 g ) of aluminum nitrate nonahydrate , 0.251.5 mmol ( 0.1000.600 g ) of chromium nitrate nonahydrate , 20 mmol ( 1.236 g ) of boric acid , and 100 mmol ( 6 g ) of urea were dissolved in 10 ml of deionized water .
the solution was heated in a resistance furnace to 500 c ( heating rate 20 deg / min ) and this temperature was maintained for 15 min .
the obtained raw material was milled in an agate mortar and sintered in a porcelain crucible for 24 h at 1100 c in air .
the powder was ground in a ball mill to result in 2.6  0.5 m microcrystalls .
microcrystalline cr(iii)-doped gadolinium aluminum borate ( gab ) powder was obtained in a similar manner , where gadolinium(iii ) nitrate was used instead of yttrium(iii ) nitrate .
emission and excitation spectra were acquired on a hitachi f-7000 fluorescence spectrometer ( www.hitachi.com ) equipped with a red - sensitive photomultiplier r 928 from hamamazu ( www.hamamatsu.com ) .
the emission spectra were corrected for the sensitivity of the pmt as described previously.(38 ) the emission and excitation spectra were acquired for the phosphor powders fixed in a homemade cell positioned at 60 toward the excitation light .
luminescence phase shifts were measured with a two - phase lock - in amplifier ( sr830 , standford research inc . , www.thinksrs.com ) .
the phosphors were ( crystals or microcrystalline powders ) placed in a homemade cuvette and excited with the light of a 405 nm led ( www.roithner-laser.com ) filtered through a bg-12 glass filter ( www.schott.com ) .
the luminescence was detected with a photomultiplier tube ( h5701 - 02 , hamamatsu , www.sales.hamamatsu.com ) after passing an rg 630 filter from schott .
a bifurcated fiber bundle was used to guide the excitation light to the probe and to guide back the luminescence .
modulation frequencies of 1 and 2 khz were used for cr(iii)-doped yab and cr(iii)-doped gab , respectively .
the luminescence decay profiles were acquired on a bmg labtech fluostar optima microplate reader ( www.bmg-labtechnologies.com ) .
the powder was excited at 405 nm and a long - pass rg 9 filter was used for the emission .
imaging in the time domain was performed with a gated sensicam camera ( cooled ccd - chip , 640  480 pix , black and white , www.pco.de ) .
a high - power 405-nm led ( www.roithner-laser.com ) was used as an excitation source .
the x - ray powder diffraction data were collected with a laboratory powder diffractometer ( philips x - pert ) from a flat sample ( braggbrentano geometry ) with use of cu k radiation .
the measurements were carried out from 10 to 95 2 , with a step width of 0.02 and a constant counting time of 5 s per step .
was taken from icsd , fiz karlsruhe ( yal3(bo3)4 , 91962,(41 ) ybo3 , 84653).(42 ) a pseudo - voigt profile shape function was used .
yttrium aluminum borate ( yab ) is isostructural with the mineral huntite and has a trigonal crystal structure of r32 space group.(43 ) both trivalent rare - earth and transition metal ions ( e.g. , chromium(iii ) ) can be doped into the material , where rare - earth ions then occupy yttrium sites and transition metal ions replace aluminum(iii ) on sites of octahedral symmetry.(44 ) it is known that concentration quenching of luminescence in this structure is usually lower than in other solid hosts , thus allowing higher dopant levels.(45 ) two different techniques were used to obtain cr(iii)-doped yab , namely flux growth and the solution combustion method . in the first method , cr(iii)-doped yab crystals are grown from the solution in high - temperature flux of potassium molybdate .
the yab crystals obtained by this method have high purity as demonstrated by powder diffraction patterns ( figure 1 ) .
first , the method is time - consuming due to slow cooling required during crystal growth ( 1 deg / h ) and has a laborious extraction procedure ( dissolving of the flux in concentrated potassium hydroxide ) .
second , micrometer - sized powders are strongly preferred for practical applications and grinding of the crystals becomes necessary .
third , the amount of dopant can not be controlled precisely since some of the chromium(iii ) ions may remain in the flux . therefore , the solution combustion method represents a promising alternative to the flux growth technique .
microcrystalline powders are easily obtained by combustion of the solutions containing respective metal nitrates , boric acid , and urea ( used as a fuel ) with subsequent 24 h sintering at 1100 c . the powder diffraction patterns ( figure 1 )
the impurity is attributed to the presence of about 4% yttrium borate(48 ) quantified by rietveld refinement .
in fact , maia et al.(49 ) have shown that yabs obtained by the solgel and the polymeric precursor methods contain ybo3 phase as an impurity .
the authors have demonstrated that it is possible to obtain pure microcrystalline powders using the polymeric precursor method with subsequent sintering at 1150 c .
powder diffraction patterns for yabs : ( a ) obtained by the combustion method with subsequent 24 h sintering at 1100 c , impurity of about 4% ybo3 ; ( b ) flux growth crystals ( 5% cr(iii ) in the oxide mixture ) , powder pattern shows preferred orientation ; ( c ) simulated powder diffraction pattern of single phase yab .
the emission and luminescence excitation spectra for the cr(iii)-doped yab are shown in figures 2 and 3 , respectively .
the phosphor has an intense broad luminescence in the nir region associated with the t2  a2 transition and an additional sharp r - line associated with the e  a2 transition .
the schematic configuration coordinate diagram ( see figure 4b ) highlights the zero - phonon nature of the sharp transition from the e state as compared with the broad vibronic transition from the t2 state , which is placed toward lower energy . in the tanabesugano diagram for cr(iii ) in octahedral crystal fields ( see figure 4a ) the 10 dq / b values for all investigated cr(iii ) containing phosphors are indicated .
all hosts provide 10 dq / b ratios which have the e state as the lowest lying emitting state .
it is evident that for the yab the energy difference between the two emitting states ( e and t2 ) is the smallest .
excitation of the cr(iii)-doped yab is attributed to the a2  t2 and a2  t1 transitions with band maxima at 421 and 599 nm , respectively .
therefore , the cr(iii)-doped yab can be excited both in the blue and in the red part of the spectrum , which is nicely compatible with the emission of the 635 nm red laser diode .
notably , among thermographic phosphors only cr(iii)-doped y3al5o12 shows similar spectral features ( figure 3 ) .
it is evident that other thermographic phosphors such as cr(iii)-doped mgal2o4 and al2o3 as well as mn(iv)-doped mg4fgeo6 can not be efficiently excited in the red part of the spectrum ( figure 3 ) .
the 10 dq / b values of yal3(bo3)4:cr ( solid ) , y3al5o12:cr(dotted ) , mgal2o4:cr ( dot and dash ) , and al2o3:cr ( dashed ) are indicated .
( b ) configuration coordinate diagram ( not to scale ) of cr(iii ) in octahedral environments .
figure 5 shows the dependences of the luminescence decay time and the luminescence intensity on the amount of cr(iii ) ions which substitute al(iii ) in yab .
evidently , concentration quenching is observed even at rather low cr(iii ) doping levels , which is indicated by the decrease in luminescence decay time .
however , the luminescence intensity does increase with increasing of cr(iii ) doping since more light can be absorbed by the phosphor . at higher levels of the activator concentration quenching becomes predominant and the intensity decreases after more than 4% of aluminum ions are substituted by chromium ( x = 0.12 ) .
thus , yabs where 24% of al(iii ) is substituted by cr(iii ) can be a good compromise between luminescence brightness and the degree of concentration quenching .
the luminescence decay time of cr(iii)-doped yab was found to be highly temperature - dependent , which is the basis for its use as a thermographic phosphor ( figure 6 ) . generally , the crystals obtained by top - seeded solution growth show the longest decay times and the highest temperature sensitivity approaching 1% of the lifetime change per 1 deg . for all cr(iii)-doped yabs ,
temperature sensitivity is the highest at 0 c and becomes lower if the temperature increases .
the decay time becomes slightly shorter if the crystals are ground into powder ( figure 6 ) .
this common phenomenon originates from inner defects in the crystal structure which accumulate during grinding .
notably , the lifetime for the powders obtained via the combustion route is generally lower than that for the crystals and is less temperature - dependent , which is attributed to the impurity of yttrium borate and inner defects in the crystal structure .
as demonstrated by maia et al.,(49 ) yabs of higher purity can be obtained using the polymer precursor method .
such yabs are likely to show improved photophysical performance ( longer decay times , higher brightness , and temperature sensitivity ) .
temperature dependence of the luminescence decay time for cr(iii)-doped yab phosphors : ( 1 ) crystals obtained by high - temperature solution growth ( 5% cr(iii ) in the oxide mixture ) ; ( 2 ) ground crystals ; and ( 3 , 4 , 5 ) microcrystalline powders obtained via combustion with 1% , 2.5% , and 4% of cr(iii ) , respectively .
evidently , temperature significantly affects the intensity of the r - line associated with the e  a2 transition , which decreases with temperature . on the other hand , broad nir luminescence associated with the t2
the isosbestic point at 738 nm is explained by the thermal population of the t2 emitting state from the metastable e state . as has been shown in figure 4 for the yab host these two states are closest in energy . as demonstrated by dominiak - dzik et al.,(50 ) at lower temperatures ( 77 k )
the broad nir emission further decreases in intensity accompanied by the increase of intensity for the r - line .
notably , the overall decrease in luminescence intensity on going from 1 to 70 c is negligible and does not exceed 0.2% .
thus , the s / n ratio for cr(iii)-doped yab remains constant in this temperature range . on the contrary ,
if luminescent metalligand complexes are used as temperature probes the decrease in luminescence intensity is usually very pronounced and is often as high as several percent per degree.(34 ) this results in much lower s / n ratios at higher temperatures
. despite the fact that cr(iii)-doped yab emits from both e and t2 states , the luminescence decay profiles are virtually monoexponential ( correlation coefficient > 0.9996 ) , figure 8 .
the decay times obtained in the time domain and in the frequency domain are virtually identical , e.g. 154 and 155 s for the yab containing 2.5% cr(iii ) , for the time domain and frequency domain measurements , respectively .
temperature dependence of the luminescence intensity for cr(iii)-doped yab ( 2.5% cr(iii ) , combustion method ) .
time dependence of the emissions for cr(iii)-doped yab ( 2.5% cr(iii ) , combustion method ) .
lines represent a fit via monoexponential decay model ( main graph ) and a linear fit ( the insert ) .
the luminescence brightness of the cr(iii)-doped yab microcrystalline powder ( 2.5% cr(iii ) ) was estimated from the corrected emission spectrum under 405 nm excitation .
it was compared to the brightness of other thermographic phosphors which are also excitable at this wavelength ( figure 3 ) .
for all the phosphors , 20 mg of a microcrystalline powder ( 25 m ) was dispersed in 1 ml of poly(ethylene glycole ) and the suspension was interrogated in a fluorescence spectrometer .
the relative brightnesses were determined to be 1 , 0.61 , 0.69 , 0.42 , and 0.16 for mn(iv)-doped mg4fgeo6 , cr(iii)-doped yal3(bo3)4 , al2o3 , mgal2o4 , and y3al5o12 , respectively . in another experiment ,
the microcrystalline phosphor powders were positioned in a homemade cell and excited in the fluorescence spectrometer in a 60 angle , at two different temperatures , 1 and 60 c .
these results are summarized in table 1 . as can be seen , they are similar to those obtained for the phosphor suspensions . evidently , cr(iii)-doped yab shows excellent brightness , which is comparable to that of mn(iv)-doped mg4fgeo6 ( lamp phosphor ) .
unfortunately , comparison with the established temperature - sensitive organometallic complexes is more complicated due to scattering of microcrystalline powders .
therefore , freeze - dried polymeric beads of ru - phen in polyacrylonitrile and eu(tta)3deadit in poly(styrene - block - vinylpyrrolidone ) were used for comparison .
at low temperature ( 1 c ) the brightness of these materials is comparable to that of the cr(iii)-doped yal3(bo3)4 .
however , in contrast to thermographic phosphors , the brightness of the organometallic complexes deteriorates dramatically if the temperature is increased to 60 c ( table 1 ) . for ru - phen
as is clearly visible from figure 9 , cr(iii)-doped yabs shows much higher temperature sensitivity compared to other visible light - excitable thermographic phosphors such as cr(iii)-doped mgal2o4,(15 ) cr(iii)-doped al2o3,(14 ) and mn(iv)-doped mg4fgeo6.(11 ) it should be emphasized that although other thermographic phosphors such as ruby- and mn(iv)-doped mg4fgeo6 are rather established in thermometry , they provide 35 times poorer resolution that cr(iii)-doped yab . at higher temperatures cr(iii)-doped
y3al5o12 could compete with yabs in terms of temperature sensitivity , but its performance is compromised by low brightness .
as can be seen , ruthenium(ii ) tris-1,10-phenanthroline complex ( ru - phen ) has lower temperature sensitivity than cr(iii)-doped yab .
generally , only luminescent europium(iii ) complexes such as a visible - light excitable eu(tta)3deadit(33 ) possess higher temperature sensitivity exceeding 1% per deg .
moreover , the organic metalligand complexes need to be embedded into gas blocking polymers such as polyacrylonitrile , poly(vinylidene chloride - co - acrylonitrile ) , etc . in order to avoid quenching by oxygen .
this is not necessary for the phosphors which additionally benefit from excellent photostability and can be continuously excited with leds for many days and even weeks without showing any deterioration . on the contrary ,
all metal complexes are prone to photobleaching , which is often very pronounced for the europium(iii ) luminophores .
table 1 summarizes the properties of the most common probes used in room temperature thermometry .
it is evident that the cr(iii)-doped yab is an excellent alternative to the state of the art thermographic phosphors and luminescent organometallic complexes .
temperature sensitivities of the visible light - excitable thermographic phosphors and luminescent metalorganic complexes : ( 1 ) eu(tta)3deadit in polystyrene ; ( 2)yal3(bo3)4:cr crystals obtained by high - temperature solution growth ( 5% cr(iii ) in the oxide mixture ) ; ( 3 ) yal3(bo3)4:cr microcrystalline powder ( 2.5% cr ) ; ( 4 ) y3al5o12:cr ; ( 5 ) ru - phen in polyacrylonitrile ; ( 6 ) mgal2o4:cr ; ( 7 ) al2o3:cr ; and ( 8) mg4fgeo6:mn .
pressure - sensitive paints , particularly , rely on luminescent indicators which are used to monitor the total pressure via oxygen partial pressure.(31 ) unfortunately , temperature affects both the luminescence of an oxygen indicator and gas permeability coefficients of a polymer where the indicator is dissolved . to obtain correct
it is usually achieved by addition of temperature - sensitive metal complexes ( commonly immobilized in beads ) or thermographic phosphor powders to the pressure - sensitive paint .
figure 10a shows a photographic image of the sensor coating ( cr(iii)-doped yab dispersed in hydrogel d4 ) , which reveals a very homogeneous layer .
this is of course not surprising considering the size of the microcrystalline powder ( 2.6  0.5 nm ) .
the luminescence decay times ( figure 10bd ) calculated from the images obtained with a gated ccd camera clearly demonstrate the suitability of the material for imaging of temperature distribution on surface .
however , it should be considered that the sensitivity of the ccd cameras decreases in the nir region , which decreases the s / n ratio in the case of cr(iii)-doped yab .
( a ) a photographic image of the 10 m thick coating ( 50% w / w of cr(iii)-doped yab , 2.5% doping , in hydrogel d4 ) . ( bd ) lifetime distribution for the same coating at different temperatures . apart from yttrium aluminum borate several similar hosts were reported .
it is well - known that such trivalent ions as lanthanum , gadolinium , and lutetium can occupy the yttrium sites and scandium(iii ) or gallium(iii ) can substitute aluminum .
thus , fine - tuning of photophysical properties of the cr(iii)-doped borate phosphors might be possible .
for example , it was demonstrated(44 ) that cr(iii)-doped yttrium scandium borate is excitable at longer wavelength than cr(iii)-doped yab and has bathochromically shifted emission .
both excitation and emission spectra are bathochromically shifted if aluminum is partly substituted by gallium.(47 ) for both phosphors the intensity of the sharp r - line associated with the e  a2 transition is very low .
our preliminary investigations indicate that cr(iii)-doped ysc3(bo3)4 and yga3(bo3)4 possess lower brightness than the respective cr(iii)-doped yal3(bo3)4 with the same doping level . on the other hand ,
the brightness of cr(iii)-doped gadolinium aluminum borate ( gab ) is comparable to that of cr(iii)-doped yab .
the excitation spectra of the two phosphors are very similar ( exc 423 and 591 nm for gab and 421 and 600 nm for yab ) .
however , the luminescence decay time of cr(iii)-doped gab is significantly lower than that of cr(iii)-doped yab ( figure 11 ) .
thus , this phosphor may be suitable for application in dual lifetime referenced ( dlr ) optical sensors(51 ) since inertness to chemical and physical parameters ( including temperature ) is highly desirable in this method .
temperature dependence of the luminescence decay time for cr(iii)-doped yal3(bo3)4 and gdal3(bo3)4 microcrystalline powders ( 2.5% cr(iii ) doping ) . in conclusion , cr(iii)-doped yab represents a new type of thermographic phosphor which provides uncompromised solution for sensing and imaging of ambient temperatures .
the new material is distinguishable for the following features : ( i ) excitation in the blue and the red part of the spectrum and compatibility with red laser diodes ; ( ii ) nir emission ; ( iii ) excellent luminescence brightness ; ( iv ) large temperature coefficients of the decay time at ambient temperatures ; ( v ) uniform brightness in the whole temperature range of interest ; and ( vi ) simplicity in preparation via the combustion route .
compared to luminescent metalligand complexes , cr(iii)-doped yab additionally benefits from chemical and photochemical inertness and the absence of the cross - sensitivity to oxygen .
potential applications include sensing and imaging of temperature in those fields where invasiveness is undesirable and combination of the phosphor with other indicators in temperature - compensated optical chemosensors and pressure - sensitive paints .
 a new thermographic phosphor based on chromium(iii)-doped yttrium aluminum borate ( yab ) is obtained as single crystals by high temperature flux growth and as a microcrystalline powder via solution combustion synthesis . 
 the phosphor is excitable both in the blue ( max 422 nm ) and in the red part of the spectrum ( max 600 nm ) and shows bright nir emission . 
 the brightness of the phosphor is comparable to that of a well - known lamp phosphor mn(iv)-doped magnesium fluorogermanate . at ambient temperatures , 
 the cr(iii)-doped yab shows high temperature dependence of the luminescence decay time , which approaches 1% per deg . 
 the material shows no decrease in luminescence intensity at higher temperatures . 
 the new phosphor is particularly promising for applications in temperature - compensated optical chemosensors ( including those based on nir - emitting indicators ) and in pressure - sensitive paints . 