H. C. Park

Reaction sintering and microstructural development in the system Al2O3-AlN

Abstract The reaction sintering and microstructural development of alumina with additions of 1–25 mol% AlN have been investigated by heating under 1-atm nitrogen gas at 1600–1800°C. Sintering Al 2 O 3 with 1 mol% AlN addition at 1750°C, resulted in a ceramic with close to theoretical density of α-Al 2 O 3 (3.98 g/cm 3 ). For the different compositions, the sintered densities decreased with increasing AlN content. This trend was attributed to the presence of secondary phases such as AlON(9Al 2 O 3 ·5AlN) and φ(5Al 2 O 3 ·AlN) formed by reacting Al 2 O 3 with AlN. For a given AlN addition, densification increased with sintering temperature, due to Al 2 O 3 and/or AlON grain growth control. The stability of the individual crystalline phases depends on both sintering temperature and batch composition.

Sintering and microstructure development in the system MgO–TiO2

The sintering and microstructure development of magnesia containing 0–10 wt% TiO2 at temperatures in the range 1300–1600°C have been investigated. The addition of TiO2 markedly promoted densification at relatively low temperature, and grain growth. Excess TiO2 over the solid solubility limit of TiO2 (0.3 wt%) reacted with magnesia to form inter- and intra-granular magnesium titanate (Mg2TiO4) above 1300°C. The grain size of MgO increased with increasing TiO2 content, and densification was mainly governed by MgO grain growth.

Self-propagating high-temperature synthesis of ZrB2 in the system ZrO2-B2O3-Fe2O3-Al

Self-propagating high-temperature synthesis (SHS) has been used to prepare many refractory materials such as carbides, borides, silicides, oxides, hydrides, intermetallics and complex composites [1, 2]. Zirconium diboride (ZrB2), because of its high melting point, superior hardness, low electrical resistance, and inertness to many molten metals and slags, is a strong candidate material for wear parts, cutting tools, nozzles and electrodes [3, 4]. Previous work on SHS production of ZrB2 has been based on the synthesis directly from its elements [5, 6]. However, the main disadvantage of SHS is that the products are porous because of the large pores in the original mixture of reactants and because of the large volume change resulting from the transformation of the reactants to the products. Also, volatilization of impurities at elevated combustion temperature can increase the degree of porosity of the final products. Synthesis concurrent with densification can improve the situation some extent. Recently, Feng et al. [7] prepared ZrB2-Al2O3-Al ceramic metal composites with an improved density and toughness by combustion synthesis. They used an excess amount of Al in a mixture of ZrO2, B2O3 and Al in order to infiltrate simultaneously molten Al into the pores of the ZrB2-Al2O3 matrix formed by the SHS reaction. Dense ceramic alloy or casting materials may be obtained when the processing temperature exceeds the melting points of the final products. Combustion synthesis is expected to melt at least a portion of constituents due to the highly exothermic reaction and with selection of an appropriate reaction system [8]. The objective of this study was to investigate the possibility of producing molten phases of ZrB2, during SHS reaction by adding Fe2O3 and excess Al to a stoichiometric mixture of ZrO2, B2O3 and Al. The reactant powders of ZrO2 (∼1 μm), B2O3 (∼5μm), Fe2O3 (∼5μm) and Al (∼7μm) were mixed and homogenized by ball milling in ethyl alcohol for 10 h using a polyethylene bottle with zirconia ball media. After drying, the mixed powders were shaped into compacts (55 mm in diameter× 30 mm in height). The compacts were charged in a graphite crucible, and then placed into a reaction chamber. The reaction chamber was vacuum treated and preheated to the ignition temperature (800–900 ◦C) with a heating rate of 10 ◦C/min under 2 atm. Ar atmosphere. After the combustion reaction was completed, samples were cooled in the chamber to room temperature. The synthesized products were investigated using X-ray diffraction (XRD) for phase analysis, scanning electron microscopy (SEM) for microstructure observation, and a pycnometer was employed for density measurement. The calculated variations of adiabatic combustion temperature with initial temperature for two reactant systems of ZrO2-B2O3-Al and ZrO2-B2O3-Fe2O3-Al are shown in Fig. 1. During the combustion, the adiabatic temperature (Tad) in the latter system reaches a higher level than that of the former system with ignition temperature. This result indicates that in coupling the forming reactions of ZrB2 and Al2O3 with another, i.e., thermite reaction of iron(III) oxide and aluminum, a higher adiabatic temperature is achieved. Also, in assuming that the ignition temperature (Tig) is 1000 K and the system is operating under adiabatic conditions, the adiabatic combustion temperatures of ZrO2-B2O3Al and ZrO2-B2O3-Fe2O3-Al systems were 2840 K and 3320 K, respectively. At these temperatures, in the former case the adiabatic temperature is lower than the melting point of ZrB2 (3300 K), but in the latter case it is slightly above. Thus, in latter case the final products may at least be partially molten. However, it is expected that the actual reaction system will deviate

Preparation of beta-alumina powder from kaolin-derived aluminium sulphate solution

The synthesis of beta-alumina powder from kaolin-derived aluminium sulphate solution is described. A homogeneous mixture of Al2(SO4)3·18H2O and Na2SO4·10H2O was precipitated by dropping the mixed solution of the kaolin-derived aluminium sulphate and sodium hydroxide into ethanol with agitation by stirring. The preparation conditions - mixing mole ratio of Na2O/Al2O3 in the solution and calcination temperature of the resultant precipitate — were experimentally determined for obtaining the beta-alumina powder with a structure consisting of β- and β″-alumina and a well crystallized state.

Sintering temperature and dielectric constant of glass-alumina composite with NaF addition

The effect of NaF addition on the dielectric properties of the glass-alumina composite was examined. The glass-alumina composite was prepared from sodium borosilicate glass powder containing NaF and Al 2 O 3 powder

Martensite and spin-glass transition of a Cu-Zn-Al-Fe shape memory alloy single crystal

Summary form only given. In spite of many researches carried out on Cu-Zn-Al shape-memory alloys, only a few studies have been reported on the magnetic properties. We report that a quarternary Cu-Zn-A1-Fe single crystal simultaneously shows shape memory effect and spin glass behaviour. All the measurements have been made on a Cu-17.25 Zn-15.00 Al-1.00 Fe (at%) single crystal grown by Bridgman method with element purity more than 99.99% in vacuum-sealed quartz capsules. DC magnetization and magnetic hysteresis measurements were carried out using a commercial superconducting quantum interference device (SQUID) magnetometer in temperature range 8 /spl tilde/ 300 K and in applied magnetic fields ranging from 0 Oe to 20 kOe. The spontaneous measurements of electrical resistivity of complex impedance versus temperature was performed to determine the martensite starting temperature (M/sub s/). X-ray diffraction, SEM, EPMA and optical microscope were used to analyze the microstructure and the prepicitates.