| The high-temperature structural ceramics, as the key materials, can be used as turbine blades for the shuttle in the future. The alumina doped with the rare earth having the melting point higher than 1700℃attracted much attention widely. However, there were still many problems about its preparation through pressureless sintering process, such as the formation of uniform structure, the increase of the density and the strength, et al. To prepare this kind of material with uniform structure and excellent strength through pressureless sintering process, it is necessary to investigate its sintering behavior.The pressureless sintering behaviors, thermal expansion behaviors, phase transformation process and the working mechanism at constant heating rates were investigated forα-Al2O3 and nanometerγ-Al2O3 doped with the rare earth (RE= La, Ce, Nd, Gd, Eu, Y, Dy). Otherwise, the master sintering curves (MSC) were constructed at low heating rates as well as the thermal behaviors were investigated for pureα-Al2O3.The MSC of 200-500nm and 2-3μmα-Al2O3 were constructed based on the combined-stage sintering model only at low constant heating rates lower than 5℃/min. In this condition, the sintering activation energies evaluated based on the MSC theory were 1035 and 1148 kJ/mol for 200-500nm and 2-3μm samples respectively, which were greater than that obtained at higher heating rates reported in other works. This phenomenan may be induced by the surface diffusion in the low temperature stage. The densities measured by Archimedes method for the samples undergoing different heating history agreed with that determined by the MSC. The results showed that the master sintering curve, in which the sintered density uniquely relies on the integral of a temperature function versus time, is insensitive to the heating path. Quantitative image analysis was used to characterize the sintered microstructure as a function of the time-temperature sintering conditions, and to verify the linkage between sintered density and microstructure. The results demonstrated how MSC theory can be applied to design a reproducible process to fabricate controlled density and microstructure ceramics undergoing the heating history with low heating rates.To study the thermal behaviors during the sintering process forα-Al2O3 with the particle size of 200-500nm, the thermodynamics functions of original particles and granulated particles with 10-20 mesh were investigated by DSC, respectively. It was found that the whole sintering process is an endothermic process. At the same heating rates, the enthalpy of the original particles was higher than that of granulated grains. For both of the two kinds of particles, the enthalpy during the sintering at low heating rates was greater than that of high heating rates, which complied with changing trend of the sintering activation energy. The specific heat capacity (cp, J g-1 K-1) of two samples obtained using the direct method increased with the temperature increasing. In the temperature range of room temperature-1273.5K, the relationship of cp and the temperature can be expressed as certain multinomials: 200-500nm (cp = 2.70177-0.01187×T+1.35929×10-5×T2, goodness of fitting was 0.99), 10-20 mesh (cp = 1.34048 -0.00354×T + 3.2239×10-6×T2, goodness of fitting was 0.98). The absolute values of enthalpy, entropy and Gibbs energy rised with temperature increasing, which showed that the sintering process was anon-reversible process and could be expressed as dS>0 or dG<0.Three kinds of structural compounds including garnet-structure RE3Al5O12(RE=Y, Dy), magnetoplumite-structure REAl11O18(RE=Nd, La, Ce) and perovskite-structure AIREO3 (RE=Eu, Gd) came into being due to the solid reaction ofα-Al2O3 with the corresponding rare earth oxide during the sintering process. The initial shrinking temperatures for the doped samples were enhanced compared with the pure alumina, which for the Nd-doped sample was about 200℃higher than that of the un-doped specimen. The rare-earth dopants (La excluded) promoted the overall densification and inhibited the particle coarsing (La showing the stronger effect) without abnomal grain growth. The shrinkage of the Eu-doped sample was 3% higher than that of the un-doped specimen. There were some lathlike alumina grains in Nd- and La- doped specimens, which could be attributed to the preferential segregation of ions to some basal planes due to the higher mobility of some dissociative boundaries. In addition, it was found that the secondary phase precipitates occurred due to the supersaturation of Nd3+ at grain boundaries, which located predominantly at grain triple points. The fracture mode of intergranular dominated in the RE-doped samples, which could be ascribed to a weakened interface bonding as a result of the coexistence of rare-earth oxides and other oxides. However, the reduced grain boundary cohesion in RE-doped alumina is expected to have only a limited effect on fracture toughness.The average line expansion coefficients of the green compacts (αg) before its shrinking and the sintered body (as) were enhanced by RE doping. For the green compact, theαG, of Nd-doped specimen was 8.59×10-6/℃, whereas, which was 6.78×10-6 /℃for un-doped sample, it was found that the light RE was more strongly than the heavy RE did for enhancingαG For the sintered body, theαS of Eu-doped specimen was 10.1×10-6/℃, nevertheless, which was 7.54×10-6 /℃for the un-doped sample, it was found that the differences of light RE and heavy RE for enhancingαS were little. The main factors causing the enhancedαS were the formation of new compounds and the increasing vacancies due to the doping rare earth. According to the rule of the expansion coefficients for composites, it could be concluded that theαS of REAl11O18(RE=La, Ce, Nd), REAlO3(RE= Gd, Eu) and RE3A15O12(RE=Y, Dy) were higher than that of the corundum-structure alumina.The sintering curves could be divided into two regimes of R1 (associated with the phase transition ofγ→αand R2 (densification ofα-Al2O3) for both RE-doped and un-doped nanometerγ-Al2O3. It was found that the RE doping inhibited the overall densification. An enhanced Rl relative density change, over and above that expected for the phase transitionγ→α, was brought about by particle re-arrangement (influenced by the doped RE) during the transformation. It was found that the time needed for performing the transformation of oneθ-crystallite of critical size to oneα-nucleus was lengthened significantly by the RE doping compared with the pure sample. The oxygen sublattice and the occupying interstitial sites of Al3+ for different alumina polymorphs were different, which would influence the vibration of Al-O bonds and further affected the IR spectra. The adsorption bands in the range of 400-1000cm-1 (characteristic bands of A-O vibration) for La-doped samples differenated greatly from that of un-doped specimens undergoing the same sintering process, which proved that the ionicity of Al3+ and O2- were changed significantly due to the La dopant. The presence of LaAlO3 precipitates (in the temperature range of 900-950℃) would block diffusion of O2- and Al3+, which would retard the formation of the transition alumina effectively during the phase transitionγ→αand then enhancing phase transition temperature. It was found that the effect of doped RE for enhancing the ending temperature of phase transition was different, which could be attributed to the differences of them on influencing the migration rates of Al3+ from tetrahedral interstitial sites to octahedral interstitial sites.
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