Jingkun Guo

Synthesis and characterization of nanocrystalline CoAl2O4 spinel powder by low temperature combustion

Abstract Single-phase nanocrystalline CoAl 2 O 4 spinel powder with a high surface area has been directly prepared by an auto-ignited gel combustion process using citric acid as a reductant/fuel and metal nitrates as oxidants. The molar ratio of fuel to nitrates in the redox mixture and the subsequent calcination upon the burnt powder were found to have a significant influence on the characteristics of the as-synthesized CoAl 2 O 4 crystallites. By controlling the CA/NO 3 − ratio and calcination temperature, homogeneous crystalline CoAl 2 O 4 powders with different particle sizes from 9 to 110 nm have been obtained. Gel formation, combustion behavior, microstructures and properties of the powders were characterized using DTA/TGA, FT-IR, XRD, FESEM, TEM, BET and UV/vis/IR.

Microwave attenuation of multiwalled carbon nanotube-fused silica composites

Multiwalled carbon nanotubes (MWCNTs) were used to convert radome materials to microwave absorbing materials. Dense MWCNT-fused silica composites were prepared by hot-pressing technique. The composites exhibit high complex permittivities at X-band frequencies, depending on the content of MWCNTs. The value of the loss tangent increases three orders over pure fused silica only by incorporating 2.5vol% MWCNTs into the composites. The average magnitude of microwave transmission reaches −33dB at 11–12GHz in the 10vol% MWCNT-fused silica composites, which indicates the composites have excellent microwave attenuation properties. The attenuation properties mainly originate from the electric loss of MWCNTs by the motion of conducting electrons.

Effect of hydrolysis conditions on morphology and crystallization of nanosized TiO2 powder

Abstract Nanosized titania powders were prepared by controlling the hydrolysis of TiCl4 in aqueous solution. The powders were characterised by TEM, HREM, XRD, ED, and BET techniques. In the presence of a small amount sulphate ions, when TiCl4 solution hydrolysed at 70°C, the obtained powder was pure anatase and its primary particle size was 3.5 nm, which is finer than that of alkoxide-derived powders, moreover, its anatase-rutile transformation was retarded. However, at the same temperature, in the absence of SO42− the synthetic powder was a mixture of the anatase and rutile, the primary particle size in the rutile phase was 4.3 nm. When TiCl4 solution hydrolysed at 20°C, the prepared TiO2 powder was amorphous and its BET surface area was as high as 501 m2/g. The results of UV–Vis absorption spectra indicate that the presence of sulphate ions accelerated the growth of TiO2 clusters to anatase.

Size effect on dielectric properties of fine-grained BaTiO3 ceramics

The effect of grain size on the dielectric behavior of high-purity, fine-grained BaTiO3 ceramic has been investigated. It was found that the dielectric constant and dissipation factor changed much with the decreasing of average grain size. The specimen with grain size of 280 nm had a high dielectric constant at room temperature, and the er-T and tanδ-T curves remarkably changed with the grain size. Part of the grains remaining ferroelectric structure was ascribed to the high value of dielectric constant. ©

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.

Surfactants assisted processing of carbon nanotube-reinforced SiO2 matrix composites

The dispersion homogeneity of carbon nanotubes (CNTs) is one of the most critical problems in carbon nanotube matrix composites. The dispersive role to CNTs of three kinds of surfactant including cetyltrimethyl ammonium bromide (C16TMAB cationic), polyacrylic acid (PAA anionic) and C16EO (CH3(CH2)14CH2(OC2H5)10OH nonionic) was investigated. The dispersion mechanism may include steric repulsive effect and static electric effect. CNT/SiO2 (5 vol.%) composites with and without C16TMAB were fabricated by sol–gel process. The average bending strength and fracture toughness of sample prepared with C16TMAB, compared with monolithic SiO2 glass, were enhanced 88 and 146% respectively. The properties of sample fabricated without C16TMAB were enhanced only 48 and 118%.

Mechanical properties and electrical conductivity of TiN–Al2O3 nanocomposites

Abstract TiN–Al2O3 nanocomposites with various additions of TiN were prepared by hot pressing of nano TiN–Al2O3 powder that was produced by in-situ nitridation of nano TiO2–Al2O3 powder. TiO2–Al2O3 powder mixtures are prepared by a heterogeneous precipitation method. The effects of nano TiN particles on the mechanical properties and electrical conductivity of TiN–Al2O3 nanocomposites were studied. Experimental results show that the addition of 20 vol.% TiN could increase the bending strength of hot-pressed Al2O3 from 370 to 725 MPa and the fracture toughness from 3.40 to 5.27 MPa m1/2, respectively. The resistivity of nanocomposites decreases with increasing amount of TiN and reaches to a minimum value of 5.5×10−3 Ω cm for the nanocomposite with 25 vol.% TiN.

Fabrication of composite YAG/Nd:YAG/YAG transparent ceramics for planar waveguide laser

Composite YAG/Nd:YAG/YAG transparent ceramics for planar waveguide laser were fabricated by non-aqueous tape casting and solid-state reactive sintering. The slurry made from the oxide powder mixtures shows a shear thinning behavior. The morphologies of the tapes were homogeneous in structure, and the tapes had appropriate strength and toughness. After calcining at 600°C for 10h in air, the samples contained less than 0.05wt.% of carbon. No gaps were found between the layers on the fracture surface of the green body compacted by cold isostatic pressing. The composite YAG/Nd:YAG/YAG transparent ceramics with in-line transmittance of 82.5% at 1064nm were obtained by vacuum-sintering at 1760°C for 30h, whose average grain size is 36.8μm. The diffusion distance of the Nd3+ ions was about 150μm along the thickness direction of the ceramics.

Preparation of BaTiO3 by the hydrothermal method

Abstract The results of hydrothermal preparation of BaTiO3 fine powders are reported. The effects of the reaction temperature, the molar ratio of Ba Ti in the precursors, the chemical form of the precursors on the phase composition, the size and the morphology of the products are presented. The higher the temperature, the higher the basicity and the greater the molar ratio of Ba Ti in the precursor, the easier the formation of the perovskite type BaTiO3 crystallite will be. It was found that BaTiO3 microcrystals(150–300 nm) could be synthesized through the hydrothermal reaction of commercial TiO2 with Ba(OH)2 aqueous solution, while the hydrothermal reaction of the newly prepared Ti(OH)4 gel with the Ba(OH)2 solution produced highly crystallized, well dispersed perovskite type BaTiO3 crystallites with very fine (

Effect of nickel content on the sintering behavior, mechanical and dielectric properties of Al2O3/Ni composites from coated powders

Abstract Al 2 O 3 /Ni composites were fabricated by hot pressing coated powders, in which nanosized Ni particles were coated with layers of alumina precursor by the heterogeneous precipitation method. The calcined powders could be sintered to above 96% of the theoretical density at as low temperature as 1350°C. The average alumina grain size in the composites decreased with increasing Ni content. Compared with that of monolithic alumina, the fracture toughness and the fracture strength of alumina could be increased by 28 and 43% by adding 2 and 5.5 vol.% Ni, respectively. The enhancement in mechanical properties can be explained from the unique microstructure of sintered bodies. The dielectric constant of Al 2 O 3 /Ni composites was found to increase with Ni content, which should be primarily attributed to the size effect of alumina grains.