| Ceria powders were prepared by different ways and compared with the particle size,dipersion and morphology. Results showed that spherical ceria particles with the averagediameter of 9.5 nm and nicer dispersion were obtained using cerium nitrate hexahydrateand ammonia as raw meterials. The sol-gel method assisted with microwave can not onlyreduce the time but save energy on the gel formation and drying, by which ceria powderwith the average diameter of 15nm and narrow distribution was prepared after calcinationonly at 300℃. Rod-like nanocrystallines ceria with 90nm in length and 15nm in diameterwere prepared using NH4HCO3 as precipitant and had an inherited relationship onmorphology with precuesor Ce2O(CO3)3·H2O.The composite powders of Al2O3-ZrO2, Al2O3-CeO2 and ZrO2-CeO2 were synthesizedby co-precipitation method and characterized by XRD, DTA, TG and TEM, respectively.The reason why t-ZrO2 can be stabilized at room temperature after the ZrO2-Al2O3powders were calcined at 1200℃was that Al2O3 with high modulus elasticicy producedstress on ZrO2 during cooling process, which hindered the t→m transformation. As forAl2O3, it directly transformed into stable, phaseα-Al2O3 at high temperatures fromamorphous structure, but not formed solid solution with ZrO2. The ZrO2-CeO2 powdersprepared by ammonia precipitation method transformed into well crystallized cubic solidsolution after calcined at 700℃and tetragonal after 1200℃, respectively. In Al2O3-CeO2system, only one crystallized phase of CeO2 with cubic fluorite structure formed aftercalcined at 700℃, and the otherγ-Al2O3 phase generated at 950℃but then completelytransformed intoα-Al2O3 at 1200℃.The transformations during calcination and the phase composition at differenttemperatures were studied in ZrO2-Al2O3-CeO2 system synthesized with ammoniaprecipitation method. Results revealed that there wasn't CeAlO3 or any chemicalcompounds formed by Al2O3 and CeO2 and CeO2 was prior to form solid solution withZrO2. Moreover, amorphous Al2O3 also directly transformed into weakly crystallizedα-Al2O3 until 1300℃. The transformation during calcination process from roomtemperature to 1300℃proceeded as follows: amorphous→c-(Zr, Ce)O2→t-(Zr, Ce)O2→t-(Zr, Ce)O2+α-Al2O3. However, when prepared by NH4HCO3 co-precipitation method,c-(Ce,Zr)O2, t-(Zr, Ce)O2, m-ZrO2 andα-Al2O3 coexisted after calcination at 1200℃for1h. The inhomogeneity of precursor caused that at the Ce-rich area c-(Ce,Zr)O2 formedand at the Zr-rich area t-(Zr, Ce)O2 formed. Furthermore, t→m transformation of ZrO2happened after 1000℃calcination due to the lack of stabilization by CeO2 and ihibitingeffect by Al2O3.
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