Key Engineering Materials Vol. 697

Abstract: MoO₂ has been widely used in many fields such as catalyst, gas-senor, super capacitor and Li-ion battery electrode. In this paper, MoO₂ nanoplates were synthesized in high density and large scale on silicon substrates via simple thermal evaporation of MoO₃ and S powders at 950 °C in a tube furnace. The morphology, composition and structure of the nanoplates were characterized by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The results indicate that the as-synthesized nanoplates are of well crystalline structure, and the thickness of these nanoplates is in the range of 100-300 nm. The growth mechanism of the nanoparticles was proposed as a vapor-solid process.

Abstract: Transparent aluminum oxynitride (AlON) ceramics have been prepared through a method based on direct reaction sintering of alumina and aluminum nitride powders using MgO and Y₂O₃ as co-additives. The sintering additives could cause the formation of liquid phase during sintering, which would greatly promote the densification and eliminate pores. The grain size of AlON is about 50-100μm. The influence of different component of Al₂O₃ and AlN as well as sintering temperature on microstructure and optical properties of AlON have been studied. High transparent AlON ceramics with the in-line transmittance of 80.3% at 2000 nm wavelength have been prepared when the concentration of sintering additives was 0.16wt% Y₂O₃ and 0.02wt% MgO.

Abstract: This paper reports a method for producing α-Al₂O₃ at low temperature from aluminum alkoxide using a combination of seeding of α-Al₂O₃ nanocryatallites and adding of inorganic alumina sol. An alkoxide alumina sol was obtained by hydrolyzing aluminum isopropoxide in water at 80°C and then peptizing the hydrolyzed aluminum isopropoxide using acetic acid at 80°C. An inorganic alumina sol was obtained by producing aluminum compound with a homogeneous precipitation method using aluminum nitrate and urea in aqueous solution and then peptizing the aluminum compound using acetic acid at room temperature. α-Al₂O₃ nanocrystallites were added to the alkoxide alumina sol containing the inorganic alumina sol. The addition of inorganic alumina sol provided successful fabrication of a crack-free α-Al₂O₃-seded alumina film by a spin-coating technique. The sol containing α-Al₂O₃ nanocrystallites was transformed to an α-Al₂O₃-seeded alumina gel by drying the sol at room temperature. The non-seeded alumina gel was crystallized into γ-Al₂O₃ at a temperature below 900°C. In contrast, the alumina seeded at 1% α-Al₂O₃ nanocrystallites content began to be transformed to α-Al₂O₃ by annealing at the temperature. The seeding and the adding promoted crystallization of the alumina gel into α-Al₂O₃. The promotion of crystallization was significant with an increase in α-Al₂O₃ nanocrystallites content by weight in the final seeded alumina gel.

Abstract: Nanocrystalline yttria powders were successfully synthesized by microwave-induced solution combustion method using a binary yttrium salt system with yttrium nitrate as oxidant and yttrium acetate as reductant. The process involved the redox reaction between the two yttrium salt under the heat generated by absorbing microwaves. The prepared powders were characterized by X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) to study the structure and powder morphology. The prepared powders were indicated to exhibit single-phase cubic crystalline yttria structure. The oxidant/reductant ratios and the calcination temperatures had an effect upon the particle size and powder morphology. The size of the crystallites varied in the range of 16 nm~27 nm with different reductant proportion. The powders were observed to show loosely agglomerated fractals.

Abstract: A precursor was obtained after dissolution, drying and cooling using MgSO₄·7H₂O and NH₄Al (SO₄)₂·12H₂O as raw materials. Then the high-purity magnesium aluminate (MgAl₂O₄) spinel powder was synthesized via the thermal decomposition process of the precursor calcined at different temperatures. The phase, morphology and particle size of the powder obtained at different calcining temperatures were characterized by X-ray diffraction, scanning electron microscopy and laser particle size analyzer. And also the purity of the powder was tested by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The results show that the powder exhibits better crystalline shape and bigger crystalline size with the calcining temperature increasing. The calculated grain size is below 30 nm according to Scherrer formula. The particle size of the powder is below 35μm, the particle size distribution is relatively wide and some particles reunite to be bigger ones with the calcining temperature rising. The powder appears to be plate-shaped and the morphology of the grain is irregular particle. The purity of the powder is relatively high. Especially, the purity of the powder obtained at 1150 °C is 98.88%.

Abstract: SiOC ceramic as anode material for lithium ion batteries has received extensive attention recently. In this work, polycarbosilane and polymethylhydrosiloxane were used as precursor polymers through a curing, pyrolysis and ball-milling progress to synthesize three kinds of ceramic powders with different components noted as SCO1, SCO2 and SCO3, respectively. The pyrolysis process of precursor polymers were investigated by the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). And the effect of ball milling progress on the particle size of SiOC ceramic powders was researched. The microstructure of ceramic powders was investigated by scanning electron microscopy (SEM), phase and element composition was analyzed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), and the tap density of ceramic powders was measured. Results show that when the curing temperature was 190 °C and the pyrolysis temperature was 900 ^oC , the ceramic powders with the particle size between 70 nm-2 μm can be obtained through the ball-milling method at the milling speed of 350 r∙min^-1 and ball-milling time of 25 h. The SiOC ceramic powders pyrolyzed at 900°C were amorphous, containing Si, C and O elements. The addition of polymethylhydrosiloxane can adjust contents of C and O in SiOC ceramics. The tap density of the powders was increased from 0.65g∙cm^-3 to 0.76 g∙cm^-3 with the addition of polymethylhydrosiloxane.

Abstract: Zirconium diboride nanopowders were synthesized by sol–gel method using different zirconium sources of zirconium carbonate and zirconium nitrate based on different gelling processing. Both of zirconium source can be applied in the synthesis of good performance ZrB₂ powders while the in-suit sol-gel using zirconium carbonate tend to form the spherical ZrB₂ powders about 50nm and the traditional sol-gel using zirconium nitrate prefer to form worm ZrB₂ powders about 200nm. The influences of B/Zr molar ration of zirconium carbonate and zirconium nitrate of sol-gel method on the phase constitution was investigated. And the gel mechanism was discussed to explain the different phase constitution, morphology of final products.

Abstract: The ZrB₂ powders with different morphology were prepared by pressureless reactions, using ZrO₂, B₂O₃, B₄C, and graphite as raw materials. Three kinds of chemical reaction system were employed. The ratio of raw materials and reaction temperature were adjusted to prepare ZrB₂ powders of different morphology and particle size. The phase composition and purity of the as-prepared powders were analyzed by XRD, while the morphology and particle size were analyzed by SEM. The ZrB₂ powders were purified by removing impurities at 600 °C in a muffle furnace in air atmosphere. The results showed that in the reaction systems of ZrO₂-B₂O₃-C and ZrO₂-B₄C-C, the ZrB₂ could be generated at 1500 °C. The morphology of the as-prepared ZrB₂ powders were particles, rod-like or near spherical for ZrO₂-B₂O₃-C system and particles for ZrO₂-B₄C-C system. In the reaction system of ZrO₂, B₂O₃, B₄C, and C with a mole ratio of 3:2:1:8, the ZrB₂ powders with high purity could be produced at 1700 °C. The ZrB₂ powder was near spherical. After heat treatment, the particle size and morphology changed to some extent.

Abstract: Zirconium diboride (ZrB₂) powders with no impurities were prepared using zirconium oxide (ZrO₂), boron carbide (B₄C) and carbon black as raw material by carbon thermal reduction. The phase composition of ZrB₂ powders was characterized by X-ray diffraction (XRD) and the micro-morphology was observed by scanning electron microscope (SEM). The effects of B4C content, calcination temperature and holding time were studied on the formation of ZrB2 powders. The results indicated that ZrB₂ powders could be successfully synthesized while holding for 1.5 h at 1600 °C in Ar atmosphere. When the content of B₄C was 17.7 wt% , there existed the crystal phase of ZrB₂ only. The impurities could be easily led in with the increase of content of B₄C. ZrB₂ powders with the size from 1-5 μm could be obtained eventually and the particle size grew obviously with increase of the holding time.

Abstract: Zirconium diboride (ZrB₂) nanoparticles were synthesized by polymer template method using zirconyl (ZrOCl₂∙8H₂O), boric acid (H₃BO₃), Chitosan and Mannitol (C₆H₁₄O₆) with microwave sintering method. SEM and other analytic methods were used to study the phase composition and the microstructure of the nanoparticles.The influence of the ratio of zirconium to boron and carbon on the purity of ZrB₂ powders, as well as the synthesis temperature and the holding time were studied. Pure ZrB₂ at the molar ratio of C/Zr/B=10/5.2/1 can be obtained. The optimum parameter for microwave sintering is 1320 °C for 1h, which is about 300°C lower than conventional carbonization reduction to obtain ZrB₂ powders. The synthesized powders have small particle sizes (50nm) with nearly spherical morphology.