材料专业外文资料翻译--热处理对三种不同途径生产的纳米粉氧化锆晶体结构和形态的影响-材料科学(编辑修改稿)

材料专业外文资料翻译--热处理对三种不同途径生产的纳米粉氧化锆晶体结构和形态的影响-材料科学(编辑修改稿)内容摘要：

ystallite sizes of formed singlephase tZrO2 nanopowders as calcu lated from XRD analyses using DebyeScherrer formula of the most intense peaks (1 1 1) plane were in the range of , and nm at 700 ◦ C for CP, CGC and MRP methods, respectively. The presence of tetragonal phase in as prepared ZrO2 and the powder formed at low temperature is attributed to the fact that the specific surface free enthalpy of tetragonal ( = J/m2 ) is smaller than that of mono clinic ( = J/m2 ). The large surface area of assynthesized nanopowders bees a thermodynamic barrier for tZrO2 to mZrO2 phase transformation. Consequently, tetragonal phase is remained. Liang et al. (Liang et al., 2020) explained the formation of tetragonal phase at low temperature is attributed to that the structure of zirconia precursor is regarded as hydrous zirconia (ZrO2 nH2 O) and the schematic structure unit has 16 zirconium atoms, 20 nonbridging hydroxogroups, 22 bridging oxide bond and 20 coordinated water and based on this model, the following equations is obtained by increased the temperature up to 700 ◦ C: [Zr16 O22 (OH)20 (H2 O)20 ] xH2 O Naturally dried −→ [Zr16 O22 (OH)20 (H2 O)20 ] + xH2 O (2) Fig. 1 – XRD patterns of the produced ZrO2 powders by CP method at 120, 500, 700, 1000 and at 1200 ◦ C for 1 and 3 h. [Zr16 O22 (OH)20 (H2 O)20 ]−→ 16ZrO2 + 30H2 O heat (3) 5 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 178–185 181 Fig. 3 – XRD patterns of the produced ZrO2 powders by MRP method at 120, 500, 700, 1000 and at 1200 ◦ C for 1 and 3 h. When ZrO2 nH2 O is heated up to about 300 ◦ C, the metastable tetragonal zirconia is observed pared to its stable temperature around 1100–2370 ◦ C. The amorphous to tetragonal phase transformation is attributed to the loss of water from the amorphous hydrous zirconia resulting from the release of water of hydration and the production of water via oblation. Both processes lead to a reduction in the BET sur face area of the calcined solid and a consequent increase in the average particle size (Jung and Bell, 2020). Table 1 showed that the relation between the crystallite size and the surface area of the obtained ZrO2 nanopowders produced by differ ent techniques. For CP method, when the crystallite sizes of the produced powders increased from 7 nm for the precursor thermally treated at 500 ◦ C to nm at 700 ◦ C. The surface area of amorphous zirconia produced at 120 ◦ C was 250 m2 /g which was decreased to 230 m2 /g for the precursor thermally treated at 500 ◦ C and decreased again to 180 and 20 m2 /g for the precursor thermally treated at 700 and 1000 ◦ C, respectively. For CGC method, the BET specific surface area of amorphous zirconia was 280 m2 /g then decreased to 210 m2 /g (crystallite size was nm) for the precursor treated at 700 ◦ C then to 60 m2 /g for the sample treated at 1000 ◦ C (crystallite size was nm). Moreover, for MRP method, the BET specific surface area was also 280 m2 /g then decreased to 200 m2 /g (crystallite size was 21 nm) and 45 m2 /g (crystallite size was nm) for the precursors annealing at 700 and 1000 ◦ C, respectively. The tetragonal phase then inverted to pure monoclinic phase (JCPDS 371484) by increasing the temperature up to 1000–1200 ◦ C for 1 h in case of CP and CGC techniques. Trans formation from the tetragonal to monoclinic phase have been attributed to the relative stability of these two phases depend on the sum of the free energies from particle surface, bulk and strain contribution (Jung and Bell, 2020). Because of the lower bulk free energy of mZrO2 and the lower surface free energy of tZrO2 , the latter phase is stabilized below a critical particle size for a given temperature. This critical size is esti mated to be 10 nm at 298 K. In the absence of particle strain, this thermodynamic description has been found to give the correct temperature for the tetragonal to monoclinic phase transformation for the particles ranging from 9 nm to 10 m. The phase transformation occurred when the size of zirconia particles is equal to or greater than the critical size determined from an analysis of the thermodynamic stability of small parti cles of t and mZrO2 . The validity of a purely thermodynamic explanation has been questioned since several investigations have observed tZrO2 that is larger than the critical particle size by talking into account factors such as domain boundary stresses, nucleation embryos, anionic vacancies and adsorbed cations and anions, all of which contribute to the stabiliza tion of tZrO2 . On these factors, the effects of external strain and the adsorbed ionic species on the surface free energy of zirconia can be acmodated within the thermodynamic theory for the tetragonal to monoclinic phase transforma tion. In addition, the phase transformation of zirconia starts from its surface region and then gradually develops into the bulk. The tetragonal phase in the surface region is difficult to stabilize. Treatment at progressively higher temperature in absence of strain is acpanied by loss of surface area. Moreover, when hydrous zirconia inverted to tZrO2 , zirconia Table 1 – Crystallite size (Cs) and the specific surface area SBET values of zirconia nanopowders synthesized by CP, CGC and MRP methods at different calcination temperatures Temperature (◦ C) Cs (nm) 120 500 700 1000 – 7 CP method SBET (m2 /g) 250 230 180 20 CGC method Cs (nm) – MRP method Cs (nm) – SBET (m2 /g) 280 235 210 60 SBET (m2 /g) 260 240 200 45 6 182 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 5 ( 2 0 0 8 ) 178–185 Fig. 4 – SEM micrographs of the produced ZrO2 powders by CP (a and b), CGC。