J.K. Guo

Preparation and characterization of nanosized TiO2 powders from aqueous TiCl4 solution

Abstract A solution-based processing method has been used to synthesize nanocrystalline TiO2 powders by controlling the hydrolysis of TiCl4 in aqueous solution. As-prepared powder was characterized by TEM, HREM, XRD, BET and ED techniques. In the presence of a small amount sulphate ions, the powder was pure anatase and its primary particle size is finer than that of alkoxide-derived powder, moreover the transformation of anatase to rutile was retarded. Both hydrolysis temperature and amount of sulfate ions had effects on the morphology and crystallization of the powder. In an optimum process, TiO2 powder in the anatase phase was fabricated without calcination and its primary size was 4nm, BET surface area was 290m2/g.

Sintering kinetics of α-Al2O3 powder

The sintering kinetics of α-Al2O3 powder are reviewed in this paper. The initial sintering of α-Al2O3 micropowder and α-Al2O3 nanopowder is all controlled by grain boundary diffusion. The sintering kinetics dominate up to a relative density of 0.77, where the coarsening kinetics dominate during further densification. Herrings scaling law can be used to predict the approximate sintering temperature of α-Al2O3 powder and demonstrates that if the particle size can be reduced to <20 nm, sintering below 1000°C may be possible. ©

A new sol-gel route using inorganic salt for synthesizing Al2O3 nanopowders

Abstract Boehmite (γ-AlOOH) powder was first prepared by sol-freeze drying method using inexpensive AlCl 3 .6H 2 O as raw material, and then γ-Al 2 O 3 nanopowder with average diameter of 6 nm and α-Al 2 O 3 nanopowder with mean diameter of 30 nm were obtained by the calcination of the above γ-AlOOH powder at 500 °C and 1100 °C, respectively. Potential-pH diagrams of Al-H 2 0 system at 25 °C and 90 °C were obtained, separately. Thermodynamic analysis of the main reactions in the process of thermal decomposition of Al(OH) 3 was also made using Temkin-Schwarzmans method and by means of regression analysis. Investigations of the potential-pH diagrams of Al-HO system and the thermodynamic analysis of thermal decomposition of Al(OH) 3 provide a guide to the formation of boehmite sol and heat treatment of boehmite powder, respectively.

Preparation and properties of intragranular Al2O3-SiC nanocomposites

Abstract Nanoscale precursor Al2O3-5vol.% SiC composite powder was prepared by a precipitation method. SiC particles were randomly dispersed into Al2O3, and SiC particles were coated with Al2O3. Dense Al2O3-SiC nanocomposites were then made by HP sintering. The nano SiC particles were mainly located within the Al2O3 grains because of the transformation of Al2O3 from the γ-phase to the α-phase. This intragranular microstructure was effective in improving mechanical properties. The strength of Al2O3-SiC nanocomposites was 523 MPa, and the toughness was 4.7 MPa.m 1 2 .

Synthesis and characterization of nanocrystalline zinc ferrite

Abstract Nanocrystalline zinc ferrite powders with a partially inverted spinel structure were synthesized by high-energy ball milling in a closed container at ambient temperature from a mixture of α-Fe2O3 and ZnO crystalline powders in equimolar ratio. From low-temperature and in-field Mossbauer measurements it is revealed that ZnFe2O4 particles prepared are in superparamagnetic state at ambient temperature. A doublet with an average quadrupole splitting of 0.8 mm/s is observed for the as-milled sample at 295 K, which is much larger than that for bulk ZnFe2O4 prepared by traditional ceramic method and that for ultrafine ZnFe2O4 particles prepared by the co-precipitation method. This indicates larger structural defects in the nanometer-sized ZnFe2O4 particles prepared by high-energy ball milling.

Synthesis and characterization of yttria-stabilized tetragonal zirconia polycrystalline powder coated with silica layers

Abstract Tetragonal zirconia polycrystalline (TZP) powders stabilized with 3 mol% yttria (Y-TZP) coated with amorphous silica were synthesized by a sol-gel method using ZrOCl 2 YCl 3 solutions and tetraethoxysilane (TEOS) as starting materials. Differential thermal analysis (DTA) and thermal gravimetric analysis (TG) showed that with increasing temperature up to 750 °C, the silica gel transformed successively to a three-dimensional network by a polycondensation reaction. The powders were examined by spectroscopy (IR) and by high resolution electron microscopy (HREM).

Morphological evolution of ZrO2-SiO2 composite gel and stability of tetragonal ZrO2

Abstract Composite ZrO 2 -SiO 2 gels were prepared using fumed silica and ZrOCl 2 solution as starting materials. Morphological evolution of ZrO 2 particles formed during heat treatment, and the particle sizes of the composites were investigated by transmission electron microscopy. The morphology of the ZrO 2 particles experiences several changes during heat treatment, i.e. the initially loose sponge-like body changes to dense sponge-like agglomerates, then to spherical particles which are embedded in the silica matrix, and finally to larger spherical particles. The stability of tetragonal zirconia in the gels is discussed in the light of the particle-size effect. The martensitic transformation of ZrO 2 (to the monoclinic phase) was found to occur in situ, when irradiated by a high-energy electron beam.

Synthesis and sintered properties of mullite powder from seeded diphasic Al2O3-SiO2 gel

Abstract Diphasic Al2O3-SiO2 gel was prepared by using AlCl3 and fumed silica as the two starting materials through coprecipitation. The seeded gel was produced by incorporating a small amount of crystalline mullite particles into the diphasic Al2O3-SiO2 gel. Phase evolution as a function of heat treatment in the unseeded and seeded diphasic gels were characterized by differential thermal analysis (DTA) and X-ray diffraction (XRD). The seeding was found to promote the formation of crystals, but, in spite of that, the crystallization temperature could not be lowered. A diphasic Al2O3-SiO2 gel with 4 wt% seeds could yield a well crystallized mullite without any other residual phases after calcination at 1300 °C for 2 h, while the unseeded gel did not show complete crystallization even though the calcination temperature was raised further. The mullite powder from seeded diphasic gel is of high purity and high activity, and can be sintered at 1650 °C for 4 h to over 99.0% of the theoretical density, showing a high flexural strength of 310 MPa.

Toughening mechanisms and properties of mullite matrix composites reinforced by the addition of SiC particles and Y-TZP

Mullite matrix composites reinforced by SiC particles and Y-TZP, were fabricated by hot-pressing. The effects of adding SiC particles and Y-TZP to mullite or mullite-based materials on properties and toughening mechanisms in the composites were investigated. Crack deflection is proposed as the principal toughening mechanism, produced by the addition of SiC particles. Transformation and microcrack toughening are the two main toughening mechanisms caused by Y-TZP addition. However, the magnitude of their contribution varied with increasing Y-TZP addition. With low Y-TZP addition, the transformation toughening dominated, while at a higher Y-TZP content, the microcrack toughening was dominant. The simultaneous addition of SiC particles and Y-TZP to mullite resulted in higher increases in both flexural strength and fracture toughness, than the simple sum of those obtained by the separate processes. It appears that the two toughening processes were coupled, thereby leading to synergistic toughening and strengthening effects in the mullite composites.

Synthesis and thermal expansion of M12Sr14Zr2P3O12(M = Li, Na, K) compounds

Abstract Thermal expansion and phase transition of M 1 2 Sr 1 4 Zr 2 P 3 O 12 (M = Li, Na, K ) compounds have been studied using high-temperature X-ray diffraction (XRD) and dilatometry measurements. The composition dependence of the thermal expansion has been discussed in terms of the ionic raddi and crystallographic sites of the interstitial ions. Dense ceramics with high strength and low thermal expansion were synthesized which might be useful as thermal shock-resistant materials.