A solid oxide fuel cell (SOFC) is an energy conversion device that has the potential to efficiently generate electricity in an environmentally-friendly manner. In general, a SOFC operates between 750&deg;C and 1000&deg;C utilizing hydrogen or hydrocarbons as fuel and air as an oxidant. The three major components comprising a fuel cell are the electrolyte, the cathode, and the anode. At present, the state-of-the-art SOFC is made from a dense yttria-stabilized zirconia (YSZ) electrolyte, a porous lanthanum manganite cathode, and a porous nickel/YSZ composite anode. With the advent of the anode-supported SOFC and the increased interest in using a wider range of fuels, such as those containing sulphur, knowledge of the anode properties is becoming more important. <br /> The properties of the current anodes are limited due to the narrow range of nickel loadings imposed by the minimum nickel content for electrical conductivity and the maximum allowable nickel loading to avoid thermal mismatch with the YSZ electrolyte. In addition, there is little research presented in the literature regarding the use of nickel metal as a starting anode material, rather than the traditional nickel oxide powder, and how porosity may affect the anode properties. <br /> The purpose of this investigation is to determine the influence nickel morphology and porosity distribution have on the processing and properties of tape cast Ni/YSZ composites. Specifically, the sintering characteristics, electrical conductivity, and thermal expansion behaviour of tape cast composites created from YSZ, nickel, nickel oxide (NiO), nickel coated graphite (NiGr), and/or graphite (Gr) powders are investigated. In addition to samples made from 100% YSZ, 100% Ni, and 100% NiO powders, five composite types were created for this investigation: NiO/YSZ, NiO&Gr/YSZ, Ni/YSZ, NiGr/YSZ, and Ni&Gr/YSZ each with nickel loadings varying between 4 vol% Ni of total solids and 77 vol% Ni of total solids. Another set of composites with a fixed nickel loading of 27 vol% Ni and 47 vol% Ni of total solids and varying graphite loadings were also created. <br /> During the burnout stage, the composites made from nickel oxide powder shrink slightly while the composites made from nickel metal expand due to nickel oxidation. Graphite additions below 20 vol% of the green volume do not alter the dimensional changes of the composites during burnout, but graphite loadings greater than 25 vol% of the green volume cause significant expansion in the thickness of the composites. <br /> After sintering, the amount of volumetric sintering shrinkage decreases with higher nickel loadings and is greater for the composites made with nickel oxide compared to the composites made from nickel metal. The porosity generated in the composites containing graphite is slightly higher than the volume of graphite added to the composite and is much greater than the porosity contained in the graphite-free composites. <br /> Dimensional changes of the porous Ni/YSZ and NiO/YSZ composites during both burnout and sintering were analysed based on concepts of constrained sintering of composite powder mixtures. In some cases constrained sintering was evident, while in others, a more simple rule of mixtures behaviour for shrinkage as a function of YSZ content was observed. <br /> When nickel oxide is reduced to nickel metal during the reduction stage there is essentially no change in the composite volume for the composites containing YSZ because the YSZ prevents the composites from shrinking. After reduction the additional porosity generated in the composites is equivalent to the change in volume due to the reduction of nickel oxide to nickel metal. <br /> When measuring the electrical conductivity, each composite type demonstrated classic percolation behaviour. The NiGr/YSZ composites had the lowest percolation threshold, followed by the Ni/YSZ and NiO/YSZ composites. When graphite was added with a nickel coating, the added porosity did not disrupt the nickel percolation network and allowed the nickel to occupy a larger effective volume compared to a composite made with similar sized solid nickel particles. When graphite was added to the composites, the electrical conductivity was reduced and the percolation threshold increased. <br /> Generally, the coefficient of thermal expansion (CTE) for Ni/YSZ composites are expected to follow the rule of mixtures prediction since the elastic properties for nickel and YSZ are similar. However when porosity is distributed unevenly between the YSZ and nickel phases, the CTE prediction will deviate from the rule of mixtures. When cornstarch was added to the NiGr/YSZ composites, the CTE increased as the amount of porosity in the YSZ phase increased. The CTE of the NiGr/YSZ composites followed the rule of mixtures indicating that the porosity was evenly distributed between the nickel and YSZ phases. For the other composite types, the measured CTE was higher than the rule of mixtures prediction suggesting that more porosity was contained within the YSZ phase.

A solid oxide fuel cell (SOFC) is an energy conversion device that has the potential to efficiently generate electricity in an environmentally-friendly manner. In general, a SOFC operates between 750&deg;C and 1000&deg;C utilizing hydrogen or hydrocarbons as fuel and air as an oxidant. The three major components comprising a fuel cell are the electrolyte, the cathode, and the anode. At present, the state-of-the-art SOFC is made from a dense yttria-stabilized zirconia (YSZ) electrolyte, a porous lanthanum manganite cathode, and a porous nickel/YSZ composite anode. With the advent of the anode-supported SOFC and the increased interest in using a wider range of fuels, such as those containing sulphur, knowledge of the anode properties is becoming more important. <br /> The properties of the current anodes are limited due to the narrow range of nickel loadings imposed by the minimum nickel content for electrical conductivity and the maximum allowable nickel loading to avoid thermal mismatch with the YSZ electrolyte. In addition, there is little research presented in the literature regarding the use of nickel metal as a starting anode material, rather than the traditional nickel oxide powder, and how porosity may affect the anode properties. <br /> The purpose of this investigation is to determine the influence nickel morphology and porosity distribution have on the processing and properties of tape cast Ni/YSZ composites. Specifically, the sintering characteristics, electrical conductivity, and thermal expansion behaviour of tape cast composites created from YSZ, nickel, nickel oxide (NiO), nickel coated graphite (NiGr), and/or graphite (Gr) powders are investigated. In addition to samples made from 100% YSZ, 100% Ni, and 100% NiO powders, five composite types were created for this investigation: NiO/YSZ, NiO&Gr/YSZ, Ni/YSZ, NiGr/YSZ, and Ni&Gr/YSZ each with nickel loadings varying between 4 vol% Ni of total solids and 77 vol% Ni of total solids. Another set of composites with a fixed nickel loading of 27 vol% Ni and 47 vol% Ni of total solids and varying graphite loadings were also created. <br /> During the burnout stage, the composites made from nickel oxide powder shrink slightly while the composites made from nickel metal expand due to nickel oxidation. Graphite additions below 20 vol% of the green volume do not alter the dimensional changes of the composites during burnout, but graphite loadings greater than 25 vol% of the green volume cause significant expansion in the thickness of the composites. <br /> After sintering, the amount of volumetric sintering shrinkage decreases with higher nickel loadings and is greater for the composites made with nickel oxide compared to the composites made from nickel metal. The porosity generated in the composites containing graphite is slightly higher than the volume of graphite added to the composite and is much greater than the porosity contained in the graphite-free composites. <br /> Dimensional changes of the porous Ni/YSZ and NiO/YSZ composites during both burnout and sintering were analysed based on concepts of constrained sintering of composite powder mixtures. In some cases constrained sintering was evident, while in others, a more simple rule of mixtures behaviour for shrinkage as a function of YSZ content was observed. <br /> When nickel oxide is reduced to nickel metal during the reduction stage there is essentially no change in the composite volume for the composites containing YSZ because the YSZ prevents the composites from shrinking. After reduction the additional porosity generated in the composites is equivalent to the change in volume due to the reduction of nickel oxide to nickel metal. <br /> When measuring the electrical conductivity, each composite type demonstrated classic percolation behaviour. The NiGr/YSZ composites had the lowest percolation threshold, followed by the Ni/YSZ and NiO/YSZ composites. When graphite was added with a nickel coating, the added porosity did not disrupt the nickel percolation network and allowed the nickel to occupy a larger effective volume compared to a composite made with similar sized solid nickel particles. When graphite was added to the composites, the electrical conductivity was reduced and the percolation threshold increased. <br /> Generally, the coefficient of thermal expansion (CTE) for Ni/YSZ composites are expected to follow the rule of mixtures prediction since the elastic properties for nickel and YSZ are similar. However when porosity is distributed unevenly between the YSZ and nickel phases, the CTE prediction will deviate from the rule of mixtures. When cornstarch was added to the NiGr/YSZ composites, the CTE increased as the amount of porosity in the YSZ phase increased. The CTE of the NiGr/YSZ composites followed the rule of mixtures indicating that the porosity was evenly distributed between the nickel and YSZ phases. For the other composite types, the measured CTE was higher than the rule of mixtures prediction suggesting that more porosity was contained within the YSZ phase.

abstract: Nanoporous electrically conducting materials can be prepared with high specific pore volumes and surface areas which make them well-suited for a wide variety of technologies including separation, catalysis and owing to their conductivity, energy related applications like solar cells, batteries and capacitors. General synthetic methods for nanoporous conducting materials that exhibit fine property control as well as facility and efficiency in their implementation continue to be highly sought after. Here, general methods for the synthesis of nanoporous conducting materials and their characterization are presented. Antimony-doped tin oxide (ATO), a transparent conducting oxide (TCO), and nanoporous conducting carbon can be prepared through the step-wise synthesis of interpenetrating inorganic/organic networks using well-established sol-gel methodology. The one-pot method produces an inorganic gel first that encompasses a solution of organic precursors. The surface of the inorganic gel subsequently catalyzes the formation of an organic gel network that interpenetrates throughout the inorganic gel network. These mutually supporting gel networks strengthen one another and allow for the use of evaporative drying methods and heat treatments that would usually destroy the porosity of an unsupported gel network. The composite gel is then selectively treated to either remove the organic network to provide a porous inorganic network, as is the case for antimony-doped tin oxide, or the inorganic network can be removed to generate a porous carbon material. The method exhibits flexibility in that the pore structure of the final porous material can be modified through the variation of the synthetic conditions. Additionally, porous carbons of hierarchical pore size distributions can be prepared by using wet alumina gel as a template dispersion medium and as a template itself. Alumina gels exhibit thixotropy, which is the ability of a solid to be sheared to a liquid state and upon removal of the shear force, return to a solid gel state. Alumina gels were prepared and blended with monomer solutions and sacrificial template particles to produce wet gel composites. These composites could then be treated to remove the alumina and template particles to generate hierarchically porous carbon. / Dissertation/Thesis / Ph.D. Chemistry 2012

A porous silica –silica composite was processed with varying fiber diameters using the slurry moulding technique. The advantage of the process was that the density of the composite could be processed to the required levels. The reinforcements used were fibers obtained by leaching E-glass cloth, imported silica fibers with diameter <1.8µ and hollow silica fibers processed using sol-gel method. All the properties depend on the density of the composite. The compressive strength was measured in the perpendicular and parallel directions. Strength was high when the load axis was along the fiber direction. The composite with fine fibers (< 1.8 µ pure silica fibers) showed higher strength compared to the leached silica fibers. The thermal conductivity measurement on these composites showed an increase with temperature owing to the domination of radiation at high temperatures. As the vacuum level was approached, the thermal conductivity showed a decrease due to the absence of the convective part of the thermal transfer process. For use as a thermal protection system, it is important to measure the thermal response of these tiles in a simulated re-entry environment. Tests were done to measure this response for a given heat flux conditions at 38W/cm2 to 75W/cm2 and the backwall temperature was measured for various types of silica -silica composites. The role of impurities like sodium and B2O3 was also studied with respect to the conversion from amorphous to crystalline forms of SiO2. The severe increase in the coefficient of thermal expansion when SiO2 converted from amorphous to α– crystoballite was also measured. CSZP CSZP which belongs to the NZP family was processed using the co-precipitation technique. The influence of substituting the ‘P’ site with ‘Si’ atom was studied for its influence on thermal expansion – both at the bulk level by dilatometry and at the intrinsic level using high temperature XRD. For many anisotropic materials micro-cracking is a serious issue while cooling from the sintering temperature. It has been previously proved that this extent of micro-cracking depends on the particle size. Smaller the particle size is therefore preferred. One of the significant results obtained in this study was the successful use of microwaves to process crack free CSZP with fine grain size. CSZP with 95% density having a grain size as small as 1µ have been processed using microwave sintering. Dielectric property evaluation namely dielectric constant, dielectric loss and temperature coefficient of resonant frequency which are vital parameters required if this material is to be used as a candidate TPS have also been measured. The thermal conductivity of the sample was measured using Laser flash apparatus and was found to be 0.9 W/mk which provides an indication that this material can be used as a successful material for TPS. Finally a composite consisting of silica fiber with CSZP as matrix was processed and tested for heat flux. The low back wall temperature indicates that this material is a potential replacement for silica tile.