according to one embodiment of the present invention , a gdc - coated nano - composite is created through chemical synthesis . the term nano - sized as used herein means a size on the order of about 100 nm or less . first an alumina sol is created . then the manganese and cobalt oxide precursors are added to create the p - type phase at the alumina surface . finally the composite is formed by coating with gdc . when the electrolyte is formed , the alumina is chemically created by mixing al ( oc 4 h 9 ) 3 with water , stirring vigorously for 30 minutes , and adding hno 3 . after five days of aging at 95 ° c ., an alumina sol , which will be the nano - composite &# 39 ; s substrate , forms . to form the first coating layer , a manganese compound [ mn ( no 3 ) 2 . 6h 2 o + h 2 o ] and a cobalt compound [ co ( no 3 ) 2 . 6h 2 o + h 2 o ] are added to the alumina sol . finally nh 4 oh + h 2 o is stirred into the solution at 90 ° c . for 4 hours and aged for 24 hours , creating the manganese oxide and cobalt oxide coated alumina sol . in the final steps , the resulting composite is coated with a 20 % gadolinia - doped ceria . this is done by adding gadolinium nitrate [ gd ( no 3 ) 3 . h 2 o ] and cerium nitrate [ ce ( no 3 ) 3 . 6h 2 o ] and stirring vigorously at 93 ° c . for six hours . then nh 4 oh + h 2 o is added to the solution , and it is aged at room temperature for 24 hours . this will yield a second coating of gdc on the alumina particles . finally , the sol is dried . in the case of this specific embodiment , the manganese - cobalt oxide coated alumina nano - particle creates an electron trapping mechanism at the interface between alumina and ceria . the manganese and cobalt oxides in the composite structure has electron trapping acceptor states at the interfaces between the insulating grains ( e . g ., alumina ) and semi - conducting ( gdc ) grain boundary regions . this enhances the ionic transference number of the gdc , which helps the gdc nano - composite overcome the electronic conductivity problems of conventional gdc , especially under low po 2 . fig1 a shows a schematic design of such a composite structure . the energy band diagram for electron trapping across the interface is shown in fig1 b . electron trapping mechanisms at interface states are commonly observed for electro - ceramic devices such as varistors and thermistors , but have not heretofore been applied to electrolyte ion conductors for sofcs . the estimated thickness of the ceo 2 grain boundary depletion regions is approximately , though not exactly 30 - 50 nm . this requires the use of nano - coatings of gdc on the surface of an insulating grain or seed particle ( inclusion ) and these layers have to densify during sintering in order to form a percolation path for oxygen ion conduction as shown in fig2 a . fig2 b shows the results of testing the electrical conductivity of gdc and the nano - composite according to this embodiment of the present invention at 600 ° c . it can be seen that the nano - composite material did not have the dramatic increase in electronic conductivity of the conventional gdc at oxygen partial pressures at and below 10 − 15 . this is not attributed to the elimination of the reducing atmosphere effect on gdc , but to the electron trapping mechanism at the interface between insulating grains and semiconducting grain boundaries of the nano - composite . also integral to creating the boundary - layer mechanism is the sintering process used to convert the nano - composite particles into a dense layer ( electrolyte membrane ). in conventional sintering , long cycle times (& gt ; 24 hours ) are required to densify the ceramic , but this long cycle time lead to grain growth . however , such grain growth can undesirably reduce the effectiveness of the electron trapping mechanism due to diffusion of gdc layers away from the interfacial region . grain growth may also alter the ionic conductivity of the electrolyte , as some particles may fuse into a larger particle within the insulating non - continuous phase . to better control the densification process , microwave sintering is preferably used . the microwave process has demonstrated a significant reduction in the cycle time necessary to densify the material . fig3 a shows the time - temperature profiles used to sinter the test specimens . fig3 b and 3c show scanning electron microscope photomicrographs of the nano - composite sintered the conventional and microwave processes . the grain sizes seen in the conventionally sintered material are larger compared to the grains resulting from the microwave sintering process . over the last half century , electromagnetic processing of materials is a technology which has been developing into a viable and cost effective means of realizing changes , in materials , which would have otherwise only been possible by way of an externally generated thermal heating . microwave frequency heating technology has expanded its breadth of applications from the food service industries to drying unfired ceramics , vulcanizing rubber , laminating plywood , and curing composites . electromagnetic materials processing technologies are inherently faster , environmentally cleaner and uniquely more uniform than conventional thermal processing technologies . thermal technologies require combustible fossil fuels or electrical resistance heating elements . fossil fuel based thermal heating technologies have large environmental impact footprints . although electrical resistance heating is considerably cleaner , it is expensive and lacks energy efficiency , particularly in that it is still a thermal heating process . there are clear mechanistic distinctions that can be observed between electromagnetic and conventional heating technologies . for example , materials are heated from the outside inward from the infrared heat energy generated in conventional processes . on the other hand , in electromagnetic heating , the field interacts directly and internally with materials being processed , thereby generating heat from within the material outward . furthermore , a high frequency molecular vibratory coupling occurs between the electromagnetic field and the materials being processed allowing a more direct relationship to be achieved between the electromagnetic energy and the matter being heated . this generates heat internally , volumetrically and uniformly within the materials being electromagnetically processed . as a consequence , this process is physically more energy efficient . refinements in both the equipment and methods used in electromagnetic processing of materials have made it possible to use this technology for firing advanced ceramics and powdered metals at very high temperatures . additionally , the processing times have been dramatically reduced , as compared with conventional thermal processing , thereby minimizing the total energy expenditure required to process identical lots of material . the mechanical properties of materials processed by this method have also been shown to be superior to those fired by conventional means . widespread use of this technology for more than the simple process of melting may now bring about the realization of materials with higher quality and lower energy costs from a per item perspective and delivered in less time . microwave sintering of materials over a range of temperatures from 400 ° c . to 2000 ° c . has been examined during the last quarter century . a variety of approaches have been used , to this end , to exploit those virtues offered through the use of microwave energy . there are several reasons why interest , in processing materials with microwave energy , is being renewed . for example , microwave materials processing offers the most significant prospect for reductions in manufacturing costs by virtue of reductions in energy usage and reduction of processing cycle times . microwave materials processing has also shown an improvement in both product uniformity and yield quantity . in addition , microwave processing has yielded improvements in materials in the form of unique and controlled microstructure with finer grain size than is achievable by way of conventional sintering methods , retention of both nano - sized grains and nano - structures are possible , superior and near theoretical material properties has been demonstrated , synthesis of new materials has been made possible and decrystallization of materials has been shown to be possible , thereby eliminating the need to form glasses from completely molten material . microwave sintering may be classified into two distinct types . direct microwave sintering can be achieved in specific materials as a consequence of the dielectric constant and the dielectric loss of the material at a specific microwave frequency . the specific frequency , that is the optimal frequency at which a given material will most effectively couple directly with microwave energy , is dictated by the complex permittivity of that material . that is to say , if the dielectric constant and the dielectric loss factor are such that when irradiated at a specific microwave frequency , the material will absorb , store , and transform into thermal energy the microwave energy to which it has been subjected . this behavioral phenomenon in materials is often referred to as susceptibility . the susceptibility of a given material will become greater as the temperature of the material is increased . however , this phenomenal behavior diminishes in some materials as the dielectric loss of the materials increases to the point whereby the same material will become reflective to microwave energy . room temperature susceptibility is ideal for materials to be sintered by microwave energy . however , at 2 . 45 ghz , the most commonly used microwave frequency for materials processing , most materials are not readily susceptible at room temperature . therefore , materials with high susceptibility at room temperature are required in concert with materials having low susceptibility at room temperature in order to achieve microwave sintering of low susceptibility materials . this method of microwave materials processing defines hybrid sintering . the material with high susceptibility absorbs the microwave energy and transforms this energy into infrared energy , which is emitted , thereby heating the low susceptibility material . as the temperature of this secondary material increases , it becomes more susceptible to direct absorption of microwave radiation and subsequently couples directly with the microwave energy . in hybrid heating , the primary susceptor material responds to microwave energy to become an infrared radiant heater . the secondary material responds to the radiant energy of the primary susceptor material until sufficient temperature has been achieved for the secondary materials to couple directly to the microwave radiation . as mentioned above , the microwave process is innately more energy efficient , thermally uniform and proceeds much faster without significant grain growth , which is essential for the preservation of nano - structures , especially at the grain boundary region . in addition , the short sintering cycle time saves a significant amount of energy , which translates into a lower cost of the finished product . another advantage of the microwave sintering process was seen in the form of increased densification of the nano - composites compared to the densities achieved using conventional sintering under comparable sintering conditions . fig4 a and 4b show the bulk density and vickers hardness of the various samples when both the conventional and microwave process were optimized to achieve the best possible densification . sample 1 is 100 % gdc and samples 2 and 3 are different nano - composite formulations . conventional gdc materials , for example , are generally lacking in strong physical properties , and consequently , conventional gdc cells are fragile . however , the increase in bulk density and vickers hardness , as well as the observed reduction in handling damage with the microwave sintered parts shows that the combination of nano - composites and microwave sintering improves the strength of the final electrolyte layer . the examples below demonstrate that electronic conductivity of the electrolyte material can be suppressed by the presence of electron trapping interfaces distributed throughout the electrolyte material , for example , by manganese oxide and / or cobalt oxide doped / coated alumina seed particles or inclusions which create the electron trapping sites . as mentioned above , the electron trapping mechanism works over a certain distance ( i . e ., the electron depletion layer ), which must be carefully controlled in order to provide and maximize the positive effects of suppressing the electronic conductivity . the present invention is further described in detail herein below by way of examples , but is in no way limited to the specific examples herein . according to one embodiment of the present invention , gadolinium is added to ceria to create oxygen vacancies , which in effect enhances the oxygen conductivity of the material . however , as noted above , cerium ions can also under go a reduction from the + iv to + iii oxidation state , which leads to electronic conductivity , and which , in effect , reduces the usefulness of the material as an oxygen ion conductor membrane , since it can lead to recombination effects in the sofc . reagent grade al ( oc 4 h 9 ) 3 , mn ( no 3 ) 2 2 . h 2 o , co ( no 3 ) 2 . 6h 2 o , ce ( no 3 ) 2 . 6h 2 o , gd ( no 3 ) 2 . 6h 2 o , and nh 4 oh solution were obtained from aldrich and used without further purification . all salts were pre - dissolved as 0 . 2 mol dm − 3 stock - solutions . alumina sols were prepared by adding 123 g aluminum alkoxide under vigorous stirring into 900 g water at 75 ° c . after stirring for 30 minutes , 0 . 830 g conc . hno 3 was added and the solution aged for an additional 5 days at 100 ° c . at that point 125 g of the alumina sol was used for the further coating process . in about 100 to 200 ml of water 0 . 090 g of mn ( no 3 ) 2 - 2 h 2 o , 0 . 091 g of co ( no 3 ) 2 . 6h 2 o , and approximately 1 g of a 28 wt % ammonia solution were slowly added at 90 ° c ., stirred for 4 hours and further aged for an additional 12 hours . finally , in about 600 to 700 ml of water 20 . 74 g of ce ( no 3 ) 2 . 6h 2 o , 5 . 388 g of gd ( no 3 ) 2 . 6h 2 o , and approximately 4 . 5 g of a 28 wt % ammonia solution were slowly added at 90 ° c ., stirred for 4 hours and further aged for an additional 12 hours . water was slowly evaporated at 100 ° c . and the powder was pre - calcined at 900 ° c . for 4 hours in air . the above described process yielded in an overall ( theoretical ) composition of 49 . 84 wt % al , 0 . 68 % mn , 0 . 68 % co , 39 . 05 % ce , and 9 . 76 % gd ( sample coat - b ). in addition , a composition with half the concentration for mn and co ( sample coat - c ) was prepared as well as a ‘ simple ’ gd - doped ceria with a 20 / 80 ratio ( sample gdc ). after calcination , the powders were ground and sieved using a 75 μm mesh . 2 to 3 g pellets were uniaxially die - pressed at 15 mpa pressure and then cold isostatically pressed at 200 mpa to form cylindrical green pellets having a height of 3 mm and a diameter of 11 mm . the pellets were sintered in one of ( a ) a ‘ conventional ’ furnace at 2 ° c ./ min heating rate to 1350 ° c . ( 5 hours hold ) followed by 2 ° c ./ min cool - down , and ( b ) a hybrid microwave furnace at 10 ° c ./ min heating rate to 1350 ° c . ( 30 minutes hold ) followed by 10 ° c ./ min cool - down . x - ray diffraction experiments were performed using a philips , xrg 3100 x - ray generator . the x - ray generator was set to 40 kv and 20 ma current , utilizing cu — kα radiation with a wavelength of 1 . 54 å , from 20 ° to 70 ° ( 2θ ), with a step size of 0 . 04 ° and count times of 4 . 0 seconds . sem analysis was performed using an feg 200 ( fei company , hillsboro , oreg .) environmental sem ( esem ) with a field emission gun ( feg ) operating at 10 kv . the energy - dispersive - x - ray spectrometer ( eds ) attached to the esem was used for chemical composition analysis . powder samples were prepared by drying suspensions and in the case of sintered pellets , fracture surfaces were observed . before analyzing all samples were coated with a 60 : 40 au : pd conductive layer in order to prevent charge build - up . impedance spectroscopy was used in a frequency range of 1 hz to 10 mhz using a solartron 1260 impedance / gain analyzer along with a centurion qex furnace . sintered cylindrical specimens were coated on both faces with platinum ink a3788a ( engelhard — lot # m34831 ) and cured at 900 ° c . ( 30 minutes hold ). correction files were used to account for and eliminate any resistance due to the leads . all of the data was collected while sweeping from high to low frequency in order to avoid polarization in the sample . nyquist plots were then generated at each temperature and analyzed in the z - view software ( version 2 . 6 scribner associates , inc , southern pines , n . c ., 2002 ). a kepco power supply ( 55v - 2 a max . set at 20v ) was used in combination with a keithley multimeter to measure 4 - point dc conductivity . sintered pellets were shaped into rectangular bars and electroded with platinum ink a3788a ( engelhard — lot # m34831 ). measurements were performed between 200 ° c . to 1000 ° c . ( in air ) and 500 ° c . to 1200 ° c . ( controlled atmosphere ). the oxygen pressure ( 10 − 30 to 2 atm .) was controlled by using ar — o 2 or co — co 2 mixtures . for each data point temperature and po 2 - equilibrium were established before making the final measurement ( indicated by a steady value of conductivity ). tga / dta experiments indicated an approximate weight - loss between 35 and 40 wt % at a temperature of about 250 ° c . and some minor continued weight - loss until about 500 ° c . the as prepared powders were essentially amorphous . however , even at temperatures of 300 ° c ., the gd 0 . 2 ce 0 . 8 o 1 . 9 phase was clearly noticed in xrd tests ( see fig5 ). the xrd - peaks indicated only a minor increase in crystallite size as the calcination temperature was increased to 900 ° c . density measurements , as shown in fig6 a and 6b , demonstrated an advantage of microwave sintering , whereby the coated samples reached densities above 5 . 0 g / cm 3 at 1250 ° c . in the microwave sintering experiment . on the other hand , a higher temperature of 1450 ° c . was needed in order to achieve comparable density measurements using conventional sintering . in addition , microwave sintering lead to higher hardness values as shown in table 1 . this effect can be explained by the smaller grain sizes , which are beneficially encountered in microwave sintering due to the relatively ‘ lower ’ sintering temperatures needed to reach a certain density , and the faster sintering cycle ( 30 minutes versus 5 hours hold time at temperature ). fig7 a and 7b shows micrographs of fracture surfaces for samples sintered at different temperatures . the average grain size is clearly below 1 μm and it is also evident that microwave sintering requires lower indicated temperatures in order to achieve comparable ( or higher ) densities than conventional firing . it should also be noted that gdc samples without the second phase showed micro - cracks . as mentioned above , while adding gadolinium to ceria creates oxygen vacancies , which in effect will enhance the oxygen conductivity , the cerium ions undergo a reduction from the + iv to + iii oxidation state . this leads to electronic conductivity , which , in effect , reduces the usefulness of the composite material as an oxygen ion conductor membrane , since it can lead to undesirable recombination effects in the sofc . measuring the conductivity in air ( fig8 ) is essentially due to ion conductivity only . it is not surprising that , compared with the ‘ pure ’ gadolinium doped ceria ( gdc ), the conductivity of the nano - composite samples are lower since 50 wt % of the material is ‘ insulating ’ alumina nano - seeds . however , when the conductivity was measured at lower oxygen partial pressures ( where electronic conductivity becomes more significant as seen by the increased conductivity below oxygen pressures of 10 − 15 for the pure gdc ), the nano - composite suppressed the electronic conductivity which remained low at around 10 − 3 to 10 − 4 s / cm , and no electronic conductivity due to the ce + iv to ce + iii conversion was detected ( see fig9 a and 9b ). in addition to the desire to reduce the influence of the electronic conductivity characteristics of the composite electrolyte material under low temperature and low po 2 conditions , it is also desirable to improve the mechanical strength of the solid electrolyte material to improve performance characteristics and increase the useful life of the sofc . as explained above , using microwave sintering offers several advantages over conventional sintering in this regard . the present inventors found that the novel composite electrolyte material of the present invention can be further structurally fortified to meet desire for improved mechanical characteristics in the following manner . of course , it is also desirable for the electrolyte membrane or structure to have zero porosity for optimal ion conductivity , which is achieved during the sintering step . however , in order to give added strength to the composite , the composite material is formed into a green body structure such as a tape , pellet or other suitable structure depending on the purpose , and green body is bisque fired at temperatures in a range of 850 - 900 ° c . for 2 - 4 hours , before sintering , in order to intentionally create an interconnected pore network within the continuous composite electrolyte structure . the continuous and interconnected pore structure of the composite electrolyte is then infiltrated with a strengthening composition , such as but not limited to 3 ysz , and then sintered at a temp of 1475 - 1525 ° c . for 1 - 2 hours to provide a final sintered composite electrolyte material having essentially zero porosity , improved mechanical properties , such as strength and flexibility , along with improved ionic conductivity characteristics . the strengthening composition is not limited to 3ysz , and examples of other suitable strengthening compositions include , but are not limited to 8ysz ( when used to strengthen an ion conducting phase of gdc when the seed particles are rare earth doped 3ysz seed particles , for example ). again , microwave sintering is preferred from the standpoint of controlling grain growth in the composite electrolyte so as to retain the benefits of the suppressed electronic conductivity provided by the electron depletion zones in connection with the inclusions . conventional sintering techniques can also be employed , however , the sintering time and temperatures must be carefully tailored and controlled in order to prevent excessive grain growth , and microwave sintering remains the preferred method .