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Non-equilibrium oxide solid solutions through thermal decomposition
Abstracts
Effect of Iron Impurities in Yttrium-Partially Stabilized Zirconia lavier Sofia, Jos~ C. Conesa~ Jos~ S. Moya and Rodrigo Moreno, C.S.I.C., Spain. The effect of the iron impurities concentration on the color change produced u p o n reduction in tetragonal Y-PSZ has been studied by ESR. The compari3on of the spectra of the original and the reduced samples indicates a direct relationship between the reduction of Fe 3+ ions isolated in the zirconia lattice and the formation of a segregated magnetite phase. The concentration on the zirconia surface of small amounts of this black comp o u n d can justify the darkening of the sample. An increasing concentration of iron is accommodated in zirconia with the for~..ation of a separated Fe203 phase, instead of increasing the number of isolated Fe 3÷ ions. In this case, the magnetite signal contributes less to the ESR spectrum of the reduced sample, b u t the signal of isolated Fe2+-Fe 3÷ pairs, which constitute the nuclei of the magnetite: shows a larger contribution. Non-Equilibrium Oxide Solid Solutions through Thermal Decomposition Gior~io Svinola and Umberto Anselmi-Tamburini, University of Pavia, Italy. In situ X-ray diffraction shows that a 1:1 (Ca,Mg)O
solid solution can be obtained by thermal decomposition of [CaMg(CO3)2]. This is a new method for preparing oxide solid solutions with regular crystal structure b u t with compositions outside the thermodynamic stability field. It can be applied to a wide range of starting materials and c o m p a r e s favorably w i t h high t e m p e r a t u r e quenching or isothermal composition variation techniques. The t h e r m o d y a n a m i c and kinetic r e q u i r e m e n t s n e e d e d to p r o d u c e out-ofequilibrium oxide solutions are discussed with reference to the characteristics of the binary system of the starting materials and of the oxides. Mechanism of Low Temperature Calcite Decomposition Giorgio Spinola and Umber , Anselmi-Tamburini, University of Pavia, Italy. The microstructure of CaO produced by decomposing calcite powders at 820 and 870 K and at various CO2 pressures has been investigated with in situ wide angle X-scattering and nitrogen adsorption
175
isotherm determinations and SEM. The combined analysis of crystalline microstrains, sizes and size distribution, of surface areas, and of pore shapes ans size distributions shows that the microstructure is constant during each isothermal-isobaric decomposition run, if sufficiently low reaction fluxes are obtained. The microstructural parameters, however heavily depend on the external P,T variables. This conflicting evidence is explained with a mechanism which takes into account both transport and shear-transformation processes. Reactivity of Molybdenum Oxide Thin Films Epitaxied on Cobalt Oxide: A Surface Study A. Steinbrunn and M. Bordignon, University of Bourgogne, France. The reactivity of molybdenum oxide thin films epitaxied on fresh cleaved COO(100) surface was studied by RHEED and AUGER spectrometry. The uppermost !aye_~ of the molybdenum-cobalt composite oxide films possess a great multiplicity of chemical compositions and subsequent structures which strongly depend on the annealing temperature and on the gaseous phase comvosition. After a 300°C annealing under U.H.V., metallic cobalt islands nucleate. The overgrowth of epitaxial cobalt molybdate a-CoMoO4 is observed at 700°C. ;-~at two-dimensional crystallites with various structures (p(2x2), p(2xl), etc.., are detected after a 900°C treatment. The sulfurization by H2S (p=0.1-15 Pa) of these superficial microcrystallites (300-400°C) is much slower than that of the underlying substrate. The reaction products are epitaxied crystallites of Co9S8 an0 islands of amorphous MoS2. Stabilization of SrlYCU3Oy by Aluminum S. A. Sunshine, L. F. Schneemeyer and T. Siegrist, AT&T Bell Laboratories, USA. Unlike Lal.85M0.15CuO~. (M=Ba, Sr, or Ca), the high Tc c o m p o u n d Ba2YCu307 is not stable when barium is completely replaced with strontium or calcium. We have found, however, that complete replacement with strontium can be accomplished if some of the copper is replaced with aluminum. The resulting phase, Sr2YCu3.xAlxOy, is stable for x>0.7. Crystallographic, thermogravimetric, and transport properties of this system have been measured and will be reported.
Effect of Iron Impurities in Yttrium-Partially Stabilized Zirconia lavier Sofia, Jos~ C. Conesa~ Jos~ S. Moya and Rodrigo Moreno, C.S.I.C., Spain. The effect of the iron impurities concentration on the color change produced u p o n reduction in tetragonal Y-PSZ has been studied by ESR. The compari3on of the spectra of the original and the reduced samples indicates a direct relationship between the reduction of Fe 3+ ions isolated in the zirconia lattice and the formation of a segregated magnetite phase. The concentration on the zirconia surface of small amounts of this black comp o u n d can justify the darkening of the sample. An increasing concentration of iron is accommodated in zirconia with the for~..ation of a separated Fe203 phase, instead of increasing the number of isolated Fe 3÷ ions. In this case, the magnetite signal contributes less to the ESR spectrum of the reduced sample, b u t the signal of isolated Fe2+-Fe 3÷ pairs, which constitute the nuclei of the magnetite: shows a larger contribution. Non-Equilibrium Oxide Solid Solutions through Thermal Decomposition Gior~io Svinola and Umberto Anselmi-Tamburini, University of Pavia, Italy. In situ X-ray diffraction shows that a 1:1 (Ca,Mg)O
solid solution can be obtained by thermal decomposition of [CaMg(CO3)2]. This is a new method for preparing oxide solid solutions with regular crystal structure b u t with compositions outside the thermodynamic stability field. It can be applied to a wide range of starting materials and c o m p a r e s favorably w i t h high t e m p e r a t u r e quenching or isothermal composition variation techniques. The t h e r m o d y a n a m i c and kinetic r e q u i r e m e n t s n e e d e d to p r o d u c e out-ofequilibrium oxide solutions are discussed with reference to the characteristics of the binary system of the starting materials and of the oxides. Mechanism of Low Temperature Calcite Decomposition Giorgio Spinola and Umber , Anselmi-Tamburini, University of Pavia, Italy. The microstructure of CaO produced by decomposing calcite powders at 820 and 870 K and at various CO2 pressures has been investigated with in situ wide angle X-scattering and nitrogen adsorption
175
isotherm determinations and SEM. The combined analysis of crystalline microstrains, sizes and size distribution, of surface areas, and of pore shapes ans size distributions shows that the microstructure is constant during each isothermal-isobaric decomposition run, if sufficiently low reaction fluxes are obtained. The microstructural parameters, however heavily depend on the external P,T variables. This conflicting evidence is explained with a mechanism which takes into account both transport and shear-transformation processes. Reactivity of Molybdenum Oxide Thin Films Epitaxied on Cobalt Oxide: A Surface Study A. Steinbrunn and M. Bordignon, University of Bourgogne, France. The reactivity of molybdenum oxide thin films epitaxied on fresh cleaved COO(100) surface was studied by RHEED and AUGER spectrometry. The uppermost !aye_~ of the molybdenum-cobalt composite oxide films possess a great multiplicity of chemical compositions and subsequent structures which strongly depend on the annealing temperature and on the gaseous phase comvosition. After a 300°C annealing under U.H.V., metallic cobalt islands nucleate. The overgrowth of epitaxial cobalt molybdate a-CoMoO4 is observed at 700°C. ;-~at two-dimensional crystallites with various structures (p(2x2), p(2xl), etc.., are detected after a 900°C treatment. The sulfurization by H2S (p=0.1-15 Pa) of these superficial microcrystallites (300-400°C) is much slower than that of the underlying substrate. The reaction products are epitaxied crystallites of Co9S8 an0 islands of amorphous MoS2. Stabilization of SrlYCU3Oy by Aluminum S. A. Sunshine, L. F. Schneemeyer and T. Siegrist, AT&T Bell Laboratories, USA. Unlike Lal.85M0.15CuO~. (M=Ba, Sr, or Ca), the high Tc c o m p o u n d Ba2YCu307 is not stable when barium is completely replaced with strontium or calcium. We have found, however, that complete replacement with strontium can be accomplished if some of the copper is replaced with aluminum. The resulting phase, Sr2YCu3.xAlxOy, is stable for x>0.7. Crystallographic, thermogravimetric, and transport properties of this system have been measured and will be reported.