Xiao-xian Huang

Preparation and characteristics of nanocrystalline NiO by organic solvent method

Nanocrystalline nickel oxide (NiO) powder was prepared by thermal decomposition of the precursor of NiC2O4·2H2O obtained via the reaction between Ni(NO3)·6H2O and H2C2O4·2H2O in ethanol solvent. The products were characterized using X-ray diffraction, infrared absorption spectroscopy, thermogravimetric analysis_differential thermal analysis, and transmission electron microscopy. The particle sizes of the as-calcined NiO increased with increase in temperature. The as-calcined NiO at 400°C for 1 h was of narrow distribution, weak agglomeration and small particle size (average=8 nm). The reason for weak agglomeration was explained.

Preparation of platelike nano alpha alumina particles

Abstract A novel synthesis process has been developed for producing high purity nonagglomerate nano platelike α-Al 2 O 3 particles. The process mainly utilizes a seed-effect of fine α-Al 2 O 3 grains, worn from the milling mediums and uniformly mixed with the hydrous alumina during grinding, and also utilizes ZnF 2 additive to reduce the transformation temperature and modify the alumina particle shape. The aspect ratio and the average size of Al 2 O 3 particles prepared at 900°C for 1xa0h is 2–4 and 40xa0nm, respectively.

Microstructure and mechanical properties of ZTA fabricated by liquid phase sintering

Abstract Zirconia-toughened alumina (ZTA) ceramics with ZrO2 volume fraction from 0 to 50 vol.% were pressurelessly prepared with the addition of 1 wt.% TiO2–1 wt.% MnO2 and 2 wt.% CaO–Al2O3–SiO2 (CAS) as sintering aids. The influence of ZrO2 content and sintering temperature on microstructure and mechanical properties of the composites were investigated. All samples can be fully densified at as low as 1400xa0°C. For pure Al2O3 sample, platelike grains appeared. With the addition of ZrO2, platelike grains of Al2O3 conversed to equiaxed grains and their sizes decreased gradually. Mechanical properties increased with ZrO2 content. When 50 vol.% was added, strength and toughness of composite reached 619±36 MPa and 7.02±0.06 MPam1/2, respectively. The toughening mechanism was phase transformation based on analyzing the transformable ZrO2 content.

Fabrication of hot-pressed zircon ceramics: Mechanical properties and microstructure

Abstract Zircon ceramics were fabricated by the hot-pressing process starting from synthesized sub-micron zircon powder with high purity. The effects of hot-pressing conditions on the density of the specimens were investigated. The relative density of zircon hot-pressed at 1600 °C for 1 h reached 99.1%, with a flexural strength and fracture toughness of 320 ± 20 MPa and 3.0 ± 0.4 MPa · m 1 2 , respectively. The flexural strength can be maintained up to 1000 °C, followed by a slight degradation occurring at 1200 °C, while the retention of room temperature strength at 1400 °C was still at a level of about 70%. The fracture behaviour of zircon ceramics was studied on the basis of microstructural observations carried out by SEM, TEM and HREM, indicating that the presence of a glassy phase at some of the triple junctions was responsible for the decrease of flexural strength at high temperatures.

Fabrication of Ni-coated Al2O3 powders by the heterogeneous precipitation method

Abstract Ni-coated Al 2 O 3 powders were prepared by the heterogeneous precipitation method using Al 2 O 3, Ni(NO 3 ) 2 ·6H 2 O and NH 4 HCO 3 as the starting materials. The amorphous NiCO 3 ·2Ni(OH) 2 ·2H 2 O was uniformly coated on the surface of Al 2 O 3 particles with the thickness of 20 nm. The amorphous coating layer was crystallized to NiO with the size of about 15 nm at 500°C for 2 hours in air, meantime, the dispersant was eliminated. NiO was completely reduced to Ni at 700°C for 4 hours in hydrogen atmosphere. Ni with the size of about 20 nm in the coating layer was spherical and weakly agglomerated. The continuous coating layer became discontinuous during heating treatment.

Mullite formation from reaction sintering of ZrSiO4/α-Al2O3 mixtures

Abstract An investigation was carried out concerning mullite formation in reaction sintering of ZrSiO 4 /α-Al 2 O 3 mixtures using XRD, TEM and SEM techniques. It was demonstrated that mullite might nucleate within the Al 2 O 3 -rich amorphous matrix, which was formed from the solid state reaction between α-Al 2 O 3 and ZrSiO 4 . The formation of Al 2 O 3 -rich mullite and amorphous phase may imply that dissolution of Al 2 O 3 is easier than dissociation of ZrSiO 4 in the present system, and that dissociation of ZrSiO 4 is a rate-limiting step in mullite grain growth at low temperatures, although the exact mechanism is still unclear. Available evidences indicate that, at final stage of mullization, Al 3+ or Si 4+ diffusion process through mullite layer might control mullite grain growth.

Fabrication and mechanical properties of Al2O3–Ni composite from two different powder mixtures

Abstract Al 2 O 3 –4vol.%Ni composites were prepared by hydrogen reduction and hot pressing of powder mixtures of NiO and Al 2 O 3 obtained by two processes, namely, the ball-milled mixing process (process A) and the coating process (process B). The microstructural features and sintering behavior of the powder mixtures, and mechanical properties of composites were investigated. The samples obtained by hot pressing powder mixtures synthesized through process A and B at 1450 and 1400xa0°C reached >98% theoretical densities. The microstructural homogeneity of the samples was in agreement with characteristics of two different powder mixtures. Compared with that by process A, the samples fabricated by process B exhibited an increase in toughness and a decrease in strength. The toughening was mainly attributed to crack deflection and crack branching. The difference of strength was discussed through the observed microstructure and fracture model of the composites. The standard deviations (S.D.) of the strength and toughness indicated the samples prepared by process B had better reliability and stability.

Microstructural development and mechanical properties of self-reinforced alumina with CAS addition

Abstract In-situ development of platelike alumina grains were obtained by adding CAS glass powder (CaO–Al2O3–SiO2) into fine alpha alumina powder sintered in the range from 1550 to 1600°C. The sintering behavior, microstructural development and mechanical properties of self-reinforced alumina ceramics with different amount of additives were investigated. The theoretical densities of the samples sintered at 1550°C reached >97% which appeared to be the optimal sintering temperature and the addition of CAS slightly decreased the densities of samples sintered from 1300 to 1600°C, compared with the samples without CAS additive. The platelike grains were not formed in alumina ceramics without adding CAS, even sintered at 1600°C. When the 0.1∼0.5 wt.% CAS glass powder was added to the starting composition, the platelike grains were formed completely at 1550°C. The formation of platelike grains increased the fracture toughness but decreased the bending strength, the combined action of crack deflection and crack bridging accounted for the observed increase of fracture toughness. The mechanical properties decreased with increasing the sintering temperatures from 1550 to 1660°C. Adding nano γ-Al2O3 can enhance the mechanical properties of the samples sintered at 1550 and 1600°C.

Sinterability of ZrSiO4/α-Al2O3 mixed powders

Abstract The sinterability of ZrSiO 4 /α-Al 2 O 3 mixed powders with three different compositions was investigated with respect to phase development during firing. The study reveals that the 65/35 sample (low-Al 2 O 3 ) has good sinterability below the temperature of 1450xa0°C because it contains more easily sintered ZrSiO 4 /α-Al 2 O 3 particle interfaces. With increasing temperature and the reaction proceeding, ZrSiO 4 /α-Al 2 O 3 particle interfaces disappeared and the sinterability of samples was changed greatly by the phase developments. As a result, the 80/20 sample (high-Al 2 O 3 ) became more sinterable at high temperatures. SEM observations show a smaller ZrO 2 grain size in the 80/20 sample than in the 65/35 sample, because ZrO 2 grain growth in the 80/20 sample involved longer diffusion paths.

Wet chemical synthesis of ZrO2-SiO2 composite powders

Abstract Gels of composition xZrO2(1−x)SiO2, with x = 10, 20, 30 and 40 vol%, have been prepared by a wet chemical method using fumed silica and zirconyl chloride as precursors. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) show that weight loss is caused by release of the absorbed water and decomposition of the Zr(OH)4 gels. Gels were heat-treated for 2 h at 500, 700, 900, 1100 and 1350 °C, and the products examined using infra-red (IR) spectroscopy. The increasing intensity of the peak at 800 cm−1 in the IR spectra with increasing temperature is attributed to the formation of Siue5f8Oue5f8Si bonds among different SiO2 particles, which means that the SiO2 particles grow bigger with increasing temperature. The DTA exothermic peak as well as the IR results reveal that the crystallization of tetragonal zirconia (t-ZrO2) begins at about 900 °C, which is confirmed by X-ray diffraction (XRD). XRD curves also suggest that the silica matrix contributes to the thermal stability of t-ZrO2. The stability of t-ZrO2 is interpreted by the particle-size effect.