Study On The Low-temperature Combustion Synthesis And Properties Of CeO₂ Based Electrolyte

Doped ceria is a promising intermediate temperature solid electrolyte because of its higher oxygen ion conductivity and good compatibility with cathode and anode. But CeO2-based materials have some drawbacks. Firstly, it is difficult to densify CeO2 based solid electrolyte materials and a sintering temperature above 1600℃is required to prepare dense CeO2 based solid electrolyte materials by the conventional solid-state reaction method;Secondly, doped ceria are brittle materials and have weak mechanical strength, which would be susceptible to fracture due to thermal stresses and mechanical stresses during cell fabrication and operation; Thirdly, electronic conduction will be introduced because of the reduction of Ce4+ under low oxygen partial pressure, which would reduce the output voltage and output power of the cells. The cell power loss arising from electronic conductivity in the electrolyte due to reduction at the fuel electrode would not be detrimental to their utility under typical operating conditions in the range 500 to 600℃, and the electronic conductivity will be restrained if enhancing the ionic conductivity of the CeO2 based electrolyte. So it is essential to improve the oxygen ionic conductivity of doped CeO2 solid electrolytes in the range of intermediate temperature or low temperature to meet the application requirement. In this paper, extensive research was conducted in the following respects:Ce_0.8Y_0.2O_1.9 nanopowders were prepared using a sol-gel low-temperature combustion process, utilizing citric acid as reducing agent and complexing agent and metal nitrates as oxidizing agent. The effect of the pH value of the solutions and the molar ratio of metal ions (nCA/nMn+) to citric acid on the complex of the metal ions was studied. The effect of the ratio value (φ) of nitrates to citric acid on the combustion fashions and combustion reaction time of the gels, the properties of the as-synthesized powders, the sintering behavior of the burnt powders and the conductivity of the sintered pellets were also investigated. It was found that when the pH value of the solutions was 6⁸ and nCA/nMn+ was bigger than 1.25, the ring complexes RE(Hcit)+, [RE(Hcit)2]- and [RE2cit3]3- were formed between citric acid and the rare earth metal ions(RE3+). RE(Hcit)+, [RE(Hcit)2]- and [RE2cit3]3- are very stable. When the fuel was deficient (φ= 1.5), the burnt nanopowders with the uniform size distribution and good dispersion were obtained. The powders had good compressibility and sinteriability and the relative density of the sintered sample was over 95% and the average grain size was 0.46μm at 1350℃for 4h. Compared with that of the CeO2 based electrolyte powders prepared by the conventional solid state reaction process, the sintering temperature of the doped CeO2 synthesized by the low-temperature combustion synthesis method was about 200℃lower. The conductivity of the sintered specimen was 0.034S·cm-1 at 700℃`and the activation energy was 0.85eV.Ce1-xYxO2-x/2 nanopowders were prepared by a low-temperature combustion synthesis method. The influence of the Y3+-doped concentration on the sintering properties, ionic conductivity and flexural strength of CeO2 was studied. Raman result showed that the solid solubility limit of Y3+ in CeO2 was between 30mol% and 35mol%. The lattice parameter of the Ce1-xYxO2-x/2 did not decrease linearly with increasing the dopant content of Y3+. A composition with higher Y3+ ions content densified with greater difficulty than compositions with lower Y3+ ions dopant concentration. The relative density and the average grain size of Ce1-xYxO2-x/2 decreased with increasing the Y3+ ions dopant concentration and the relative densification rate/grain growth rate ratio increased with increasing the Y3+-doped concentration. The grain conductivity decreased with the increase of the Y3+-doped concentration, but the grain boundary conductivity firstly increased with the increase of the Y3+-doped content and then decreased. The maximum grain boundary conductivity was obtained when the Y3+-doped content was 20mol%. The total conductivity of the Ce1-xYxO2-x/2 was mainly determined by the grain boundary conductivity, and the total ionic conductivity had the same change tendency as the grain boundary conductivity with the increase of the Y3+-doped content. The highest total conductivity and lowest activation energy were achieved when the Y3+-doped content was 20mol%.The Al₂O₃/Ce_0.8Y_0.2O_1.9 composite powders with 0¹0mol% Al₂O₃ were prepared by a sol-gel low-temperature combustion process. The influences of Al₂O₃ content on sintering, behavior and flexural strength of Al₂O₃/Ce_0.8Y_0.2O_1.9 composites were investigated. The effect of Al₂O₃ additions on the total conductivity, the bulk conductivity and the grain boundary conductivity of Al₂O₃/Ce_0.8Y_0.2O_1.9 composites were also discussed and the mechanism was analyzed. The backscattered SEM analysis showed the solid solubility limit of Al₂O₃ in Ce_0.8Y_0.2O_1.9 was between o.5mol% and 1mol%. Below the solid solubility limit, Al₂O₃ could promote the sintering of Ce_0.8Y_0.2O_1.9 due to the small size effect of Al3+. When the doped concentration of Al₂O₃ in Ce_0.8Y_0.2O_1.9 was over its solid solubility limit, Al₂O₃ particles gather at grain boundaries and they restrained the grain growth and increased the activation energy of the grain growth because of the pinning effect of Al₂O₃ particles at grain boundaries. The activation energy for the grain growth process of undoped Ce_0.8Y_0.2O_1.9 and 10mol% Al₂O₃ doped Ce_0.8Y_0.2O_1.9 are 386KJ/mol and 490KJ/mol respectively. Because of the compressive stress caused by the mismatching of lattice parameters and the thermal expansion coefficients between matrix and Al₂O₃ particles and the pinning crack effect caused by the Al₂O₃ particles, Al₂O₃ could significantly enhance the flexural strength of Al₂O₃/Ce_0.8Y_0.2O_1.9 composites. The flexural strength was 270MPa for 10mol% Al₂O₃ doped Ce_0.8Y_0.2O_1.9 while the flexural strength was 121MPa for the undoped Ce_0.8Y_0.2O_1.9 specimen. The bulk resistance and the grain boundary resistance increased when the Al₂O₃ additions were below its solid solubility limit in Ce_0.8Y_0.2O_1.9, so the total ionic conductivity of Ce_0.8Y_0.2O_1.9 electrolyte decreased. When the Al₂O₃ additions were above its solid solubility limit in Ce_0.8Y_0.2O_1.9, the bulk resistance was constant. Because of the"scavenging"effect of Al₂O₃ particles, the grain boundary resistance decreased and the total oxygen ionic conductivity of the electrolyte increased. The maximum oxygen ionic conductivity was obtained when the Al₂O₃ concentration was 2mol%. Because the distribution of Al₂O₃ particles at the grain boundaries was homogeneous and the particle was only hundreds of nanometer, the grain boundary resistance almost unchanged and the total ionic conductivity of Ce_0.8Y_0.2O_1.9 is constant when the Al₂O₃ addition was between 2mol%⁵mol%. When the doping concentration of Al₂O₃ was over 5mol%, the grain boundary resistance began to reduce rapidly due to the"blocking"effect of Al₂O₃ particles, so the total ionic conductivity of Ce_0.8Y_0.2O_1.9 also reduce rapidly. Al₂O₃ made the activation energies of doped ceria for the bulk conductivity and the grain boundary conductivity increase when the Al₂O₃ additions were below its solid solubility limit in Ce_0.8Y_0.2O_1.9. The activation energy for the bulk conductivity was constant because Al₂O₃ particles gathered at the grain boundaries when the Al₂O₃ additions were above its solid solubility limit in Ce_0.8Y_0.2O_1.9. The Al₂O₃ particles gathering at the grain boundaries contributed to the grain-boundary blocking effect by decreasing the conduction path width and constricting current lines. The activation energy of the grain boundary was insensitive to the doped concentration of Al₂O₃ when the doped concentration of Al₂O₃ was above its solid solubility limit in Ce_0.8Y_0.2O_1.9.Ceria-based electrolyte co-doped with Y2O3 and CaO were prepared by a low-temperature combustion method. The conductivity behavior of the Y2O3 and CaO doubly doped electrolytes was studied and compared with that of singly doped ceria electrolytes. The influence of the co-doped effect on the conductivity and activation energy of the bulk and grain boundary was also discussed. The Ce_0.8+xY_0.2-2xCa_xO_1.9 powders had good sinterability, and the relative density was over 95% at 1350℃for 4h. The relative density of the electrolyte increased with increasing the Ca2+ doped concentration. The average grain sizes firstly increased and then decreased with increasing the Ca2+ doped concentration. When the Ca2+ doped concentration was 5mol%, the maximum average grain size was obtained. The average grain size was 4.25μm when the Ca2+ doped concentration was 5mol% and it was 0.8μm when the Ca2+ doped concentration was 10mol%. Compared with the singly doped electrolytes, the doubly doped electrolytes have higher ionic conductivity and lower activation energy. Ce_0.85Y_0.1Ca_0.05O_1.9 has the highest ionic conductivity and the lowest activation energy. The ionic conductivity is 0.0192 S?cm-1 at 600℃and the activation energy is 0.8eV. The doubly effect is more prominent on the conductivity and activation energy of the grain boundary than on that of the bulk. Ce0.85Y0.1Ca0.05O1.9 has the highest grain boundary conductivity and the lowest grain boundary activation energy.