Preparation of ultrafine alumina powders by plasma evaporation

It has been shown that the centrifugal liquid-wall furnace is a convenient and economically attractive tool for the vaporisation of alumina. A reducing environment, conveniently supplied by using composite cores of alumina and particulate carbon, can greatly increase the evaporation rate and, on reoxidation of the vapour produced, an ultrafine oxide of high and controllable surface area is obtained.The product is comparable with fumed aluminas made commercially by the oxidation of aluminium halides in the oxyhydrogen flame. Potential applications include flow control of liquids and powders, reinforcement of elastomers and adhesives, ceramic fabrication and heterogeneous catalysis.     

煅烧法制备亚微米氧化铝粉体

以氢氧化铝为原料,将其酸洗预处理在1 200、1 250、1 300、1 350℃煅烧,球磨获得α氧化铝。研究发现:酸洗预处理可以减小氢氧化铝原料中的氧化钠和产物中氧化钠杂质的质量分数,提高最终α相的转化率。煅烧温度影响产物的比表面积,煅烧温度为1 300℃时,制备的α氧化铝具有较高的反应活性,相转化率高达95%。产物球磨6 h后的粒径能达到0.73μm,属于亚微米范围。     

Preparation and characterization of polyaniline-copper composites by electrical explosion of wire

Polyaniline-copper composites with a polyacrylic acid (PAA) were synthesized by electrical explosion of wire. Polyaniline (PANI) and PAA were put into the explosion medium, deionized water (DIW) and ethanol, stirred for 24 hrs and sonicated for 2 hrs. These solutions were used as base liquids for explosion process to fabricate Cu nanoparticle. Optical absorption in the UV-visible region of PANI and PANI/PAA-Cu composites was measured in a range of 200-900 nm. X-ray diffraction was used to analyze the phase of the composites. XRD pattern showed the PANI was amorphous and copper was polycrystalline. Two phases of Cu and Cu2O were formed in aqueous solution while single Cu phase was obtained in ethanol solution. Field emission scanning electron microscope was used to observe the microstructure of the composites. The synthesized composites were extensively characterized by Fourier Transform Infrared (FTIR) spectroscopy and electrical measurements.     

Preparation and characterization of alumina–iron cermets by hot-pressing of nanocomposite powders

Alumina composites with 0–36 vol% iron metal dispersed inclusions have been prepared by a new processing route. Nanostructured powders were synthesized by a high energy dry ball-milling process and then consolidated by sintering at 1700 K under a pressure of 30 MPa. The materials consist of micrometre-size Al₂O₃ with metallic Fe in isolated regions from 10 μm down to nanometre size. From low to high metallic contents, the same type of interwoven microstructure is observed. Hardness, elastic and shear moduli, fracture toughness and the size of the inclusions vary regularly with the metal content. Addition of metal increased the fracture toughness from 3.0 to 8 MPa m^1/2. This revised version was published online in November 2006 with corrections to the Cover Date.     

Studies on the electric discharge compaction of metal powders

Consolidation and densification of metal powder are usually attained by compaction followed by prolonged high-temperature sintering or by hot pressing. However, these methods are not suitable where grain growth is to be avoided. So electric discharge compaction is being studied so as to establish a fast, cheap, and reproducible process. In this process, high current pulse for a few millisecond duration is passed through metal powder compacts held under pressure in a die. This paper gives the details of the investigation being carried out to establish the process parameters for the densification of titanium, tin and zinc powders. Remarkable improvements in the density and microstructure are noticed after subjection to the electric discharge.     

Pressure waves generated by a nanosecond electric explosion of a tungsten wire in water

We have studied pressure waves generated during a nanosecond electric explosion of 70-μm-thick tungsten wires in water. The measurements were performed at a distance of 3–8 mm from the exploding conductor. In order to reduce the error caused by two-dimensional effects, we used pressure sensors with dimensions not exceeding 3 mm. The results of measurements revealed a two-component structure of the electric-explosion-induced shock wave.     

Preparation and stability of gold colloid by electrical explosion of wire in various media

Gold colloids were prepared by electrical explosion of wire in various media: cold water (0 degrees C), room temperature water (25 degrees C), hot water (80 degrees C), 0.01 M polysorbate surfactant 20 (TW 20) solution, mixture of 0.01 M TW 20 and 0.01 M ascorbic acid. The size distribution of nanoparticles measured by transmission electron microscope was found to shift to a smaller size with a decrease of temperature and a presence of TW 20 surfactant. The multiple light scattering results showed that medium temperature and ambient medium of explosion process is much influence on the stability of colloid. The gold colloid prepared in cold water is unstable in comparison with one prepared in warm and hot water. The best stability of gold colloid obtained with explosion medium of TW 20 and ascorbic acid solution.     

Silicone-coated alumina powders by the emulsion technique

Alumina powders were prepared using an emulsion technique to chemically bond silanol-terminated poly dimethylsiloxane (PDMS or silicone) to the surface of alumina powders. Stable aqueous emulsions of silicone were prepared using Tween-80 surfactant. Transmission optical microscopy was used to measure the oil-droplet and coated-particle sizes. Fourier transform-infrared spectroscopy was used to quantify bonding between the silicone and the Al₂O₃. Coated powders were dry-pressed, and PDMS was compared to paraffin for dry-pressing behaviour and green strength. Densification studies were made on powders. X-ray diffraction and scanning electron microscopy were used to characterize the sintered ceramic.     

Bonding of alumina ceramics with nanoscaled alumina powders

Nanoscaled Al₂O₃-powders can be employed for diffusion bonding of alumina ceramics. In order to accomplish bonding of the ceramics, Al₂O₃-nanopowder with a median particle size of 14 nm in diameter is sandwiched between two commercial microcrystalline corund discs, followed by uniaxially hot compressing of the assembly in vacuum at 80 MPa and 1100 °C for 2 h. Scanning electron microscopic investigations reveal a nanocrystalline structure of the joint with a mean grain size of about 50 nm in diameter and extensive consolidation of the powder without substantial shrinkage void formation. Microhardness measurements across the interface yield a value of 200 HV. In order to achieve complete densification and strength enhancement of the joint material, the sample is subsequently sintered at 1500 °C and 1600 °C for several hours in air. It was found that the hardness of the joint depends strongly on the porosity content and/or grain size and that a hardness of 1700 HV is obtained when both a mean particle size of about 1 μm and complete densification of the joint is achieved. The results show for the first time that Al₂O₃-nanopowders are suitable for diffusion bonding of alumina ceramics. Possible mechanisms are discussed.     

Preparation and sintering behaviour of nanostructured alumina/titania composite powders modified with nano-dopants

Nanostructured alumina/titania composite powders were prepared by doping with small amounts of nanosized zirconia and ceria. The nanosized raw materials powders were reconstituted into nanostructured particles by ball milling, spray drying and heat treating. Then, the nanostructured reconstituted powders were cool-isostatic pressed and pressureless-sintered into bulk ceramics. The phase composition and microstructures of reconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS). The sintering behaviour of the nanostructured ceramic composite powders and the effects of nano-dopants and sintering temperatures on the microstructures of the ceramic composites were investigated and discussed. It was found that nano-dopants could lower the sintering temperature and accelerate densification of ceramic composites.     

Synthesis of alumina powders by the glycine-nitrate combustion process

The combustion synthesis technique using glycine as fuel and aluminum nitrate as an oxidizer is able to produce alumina powders. Thermodynamic modeling of the combustion reaction shows that as the fuel-to-oxidant ratio increases, the amount of gases produced and adiabatic flame temperatures also increases. X-ray diffractions showed the amorphous structure for as-synthesized powder and presence of well-crystallized α-Al₂O₃ after calcination at 1100 °C during soaking time of 1 h. Alumina's largest measured specific surface area was 15 m²/g with BET method and 0.51 glycine-to-nitrate ratio.     

Nano-sized particles formed by pulsed discharge of powders

Exploding wire and pulsed wire discharge are effective methods to form nano-sized particles. They are based on evaporation of thin metal wires by applied large electric current as the principle to form particles. Many kinds of wires have been used to form nano-sized particles so far. However, materials which are not sufficiently ductile to form thin wires are unsuitable for the methods. We have developed a method for powders to be held between electrodes by using heat-shrinkable tubes and discharged for evaporation. Nano-sized intermetallic NiAl compound particles were synthesized from mixed powders of Al and Ni. Si powder could be discharged by sealing with a thin Al wire, and nano-sized Si particles were formed. Utilizing this technique, various metal powders can be used for raw materials in pulsed discharge.     

Processing high-purity and liquid-phase-sintered alumina ceramics using locally synthesized alumina powders

The feasibility of using locally synthesized powders in the development of high-purity and liquid-phase-sintered (LPS) aluminas, for potential use as ceramics in erosive wear environments, was explored. The principal purpose of the project was the development of processing philosophies for alumina-based ceramics that meet the dual, and usually conflicting needs, for improved properties at lower cost, with special attention to the Western Australian mining industry. The incorporation of glassy grain-boundary phases via sintering additives makes the achievement of low-cost LPS aluminas possible, but also threatens to compromise properties. Sintering behaviour, phase assemblage, microstructural evolution and mechanical properties were explored as key variables in this goal. The results indicate that the locally sourced alumina and mineral additives can be combined to produce LPS aluminas which are comparable in mechanical performance to currently-available commercial LPS alumina ceramics. © 1998 Chapman & Hall     

Microwave synthesis of mesoporous and microporous alumina powders

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of ceria-based polishing powders from carbonates

The preparation of Ce(IV) oxide-based polishing powders by thermal decomposition of carbonates was studied and the effect of the calcination process conditions on the properties of product was examined. High-grade polishing powders can be produced at calcination temperatures 700–1200°C, the maximum polishing efficiency is achieved by calcining at 1000–1100°C. The observed dependences were interpreted from the standpoint of the present mechanochemical theory of glass polishing.     

Preparation of porous scaffold from hydroxyapatite powders

Hydroxyapatite (HA) powder was prepared by wet chemical method. The hydroxyapatite phase was stable up to 1250 °C without decomposition to beta-tricalcium phosphate. Interconnected porous hydroxyapatite scaffold resembling trabecular bone structure was developed from polymeric replica sponge method. The prepared scaffold has 60 vol.% porosity having a major fraction of ~ 50–125 μm pore diameter. The pore content, pore morphology, pore interconnectivity of scaffold and their compressive strength were dependent on the solid loading and binder content. In-vitro bioactivity and bioresorbability confirmed the feasibility of the developed scaffolds.     

Pressure generated by the electric explosion of metal foils

We have measured the pressure generated by the electric explosion of various metal (Cu, Al, Fe, Ta, Ti, Pb, W, and brass) foils. The pressure amplitude as a function of the specific energy supplied to the exploding foil is well approximated by a linear dependence.