The formation of TiC/Al2O3 microstructures by a self-propagating high-temperature synthesis reaction

The effect of processing variables on reaction rate and ceramic microstructure are examined for the self-propagating high-temperature synthesis reaction 3TiO₂+4Al+3C 3TiC+2Al₂O₃. Reaction controlling methods used are reactant particle size, the use of diluents (to lower the combustion temperature) and the use of reactant preheating (to increase the combustion temperature). The ceramic microstructure has an unusual laminar structure which is generally only observed during unstable combustion wave propagation.     

Self-propagating high-temperature synthesis of Al2O3-TiC-Al composites by aluminothermic reactions

Al₂O₃-TiC-Al composites were fabricated by self-propagating high-temperature synthesis process using aluminothermic reactions with titania, aluminum, and graphite powders. As the molar ratio x of the excessive aluminum in the reactants increases, the adiabatic temperature of the reaction and the melting rate of alumina in the products obviously decrease according to thermodynamics. This reaction is theoretically presumed to be ignited at preheat temperature of 900 K even though x is up to 13 mole. The experimental results revealed that the critical molar ratio of excessive Al, which the combustion reaction can self-sustain, is 7.66 mole with a preheat temperature of 400–500 K. The excessive aluminum favors to fill in the pores of the products, and a cylindrical Al₂O₃-TiC-Al composite with a relative density of 70% can be obtained, and its tensile strength is higher ten times than that of the Al₂O₃-TiC composite. Moreover, TiC and Al₂O₃ grains in the composites are fined as the excessive aluminum increases. Although the excessive aluminum does not take part in the combustion reaction, it strongly affects combustion process and microstructures of the products.     

Preparation of Y-TZP/AI2O3 whisker preform by an in situ method

In situ growth of AI₂O₃ whiskers into the matrix of Y-TZP (yttria-doped tetragonal zirconia polycrystals) was examined in order to prepare Y-TZP/AI₂O₃ whisker preform for the composites. Various shapes of AI₂O₃ particles were grown by the reaction of AI₂O₃ and AIF₃ powders with moist nitrogen or oxygen gases at high temperature. They showed a trend to change the particle shapes from massive rhombohedron whisker platelet as the processing temperature was increased. These particles, however, grew only on the surface and not inside the pellets It was found necessary to introduce the carrier gas inside the pellets for particle growth to occur internally. AI₂O₃ whiskers can be synthesized inside the pellets by mixing with an organic space-forming agent having a relatively large particle size.     

Mullite formation from xerogels of (0.84–2.2) Al2O3·1SiO2

Xerogels of (0.84–2.2) Al₂O₃·1SiO₂ prepared by chemical coprecipitation of Al(NO₃)₃·9H₂O and Si(OC₂H₅)₄ experience three thermal reaction paths for mullite formation. Those with pseudoboehmite are found to form mullite via the paths of either Al-Si spinel mullite transformation or -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite, depending upon the ratios of Al₂O₃/SiO₂. Higher SiO₂ content may prefer the former reaction. Xerogels composed of bayerite form mullite via the route -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite. Mullite thus formed exhibits a different crystal size, being 20–25 nm for that obtained from pseudoboehmite and around 37 nm for bayerite. The highest yield of mullite formation may be achieved with xerogels of pseudoboehmite with the stoichiometric mullite compositions, 3Al₂O₃·SiO₂.     

Interface reaction between oxide glasses and Fe-Al-Si magnetic alloy

The interface reactions between oxide glasses and magnetic alloy, Fe-Al-Si (so-called Sendust), were analysed. The oxide glasses used were SiO₂-PbO, SiO₂-Na₂O, SiO₂-Li₂O and B₂O₃-Na₂O binary glasses. It was observed that the lattice constant of the alloy decreases and the saturation magnetic-flux density of the alloy increases on reaction with the glasses. It was found that the aluminium atoms in the alloy diffuse to the interface and dissolve into the glass melt as Al³⁺ ions, leading to the iron-rich composition of the alloy. On the other hand, Pb²⁺, Na⁺ and H⁺ ions in the molten glasses were reduced at the interface. Metallic lead particles about 20 m in diameter were found to be dispersed in the SiO₂-PbO melt. Reduced sodium was thought to evaporate from the SiO₂-Na₂O melt, and H₂ gas bubbles were observed at the interface between B₂O₃-Na₂O melt and the alloy. These reactions were analysed based on the standard free energy diagrams of oxidation-reduction reaction, and expressed as 3Pb _glass ²⁺ +2Al_alloy 3Pb+2Al _glass ³⁺ 3Na _glass ⁺ +Al_alloy 3Na+Al _glass ³⁺ 6H _glass ⁺ +2Al_alloy 3H₂+2Al _glass ³⁺     

Mechanochemical effects for some Al2O3 powders of dry grinding

Three kinds of Al₂O₃ powders, i.e. two kinds of low-soda Al₂O₃ with average particle sizes of 3.9 and 0.6 m and an electrofused Al₂O₃ with an average particle size of 21.8 m, were ground for up to 300 h in a dry vibration ball mill. Variations in particle-size distribution, specific surface area, crystallite size, lattice strain, effective temperature factor and lattice constant were examined against milling time. The mechanism of grinding was found to differ between low-soda Al₂O₃ and electrofused Al₂O₃. The mechanochemical effects on these Al₂O₃ powders occurred in the order decrease of crystallite size increase of effective temperature factor increase of lattice strain. The length of the a-axis was clearly increased by a prolonged grinding. The difference in the ground state of three specimens was attributed to differences in the physical state of particles originating from the preparation methods, and also to particle size.     

Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.     

Reactive processing of nickel-aluminide intermetallic compounds

NiAl have been fabricated by reactive sintering compacts of ball-milled powder mixtures containing Ni and Al. The reaction mechanism, as well as phase and microstructural development, were investigated by analyzing compacts quenched from different temperatures during reactive hot compaction. It was found that the reaction process was strongly affected by pressure, heating rates, heat loss from the sample to the environment. The application of 50 MPa prior to the reaction resulted in the intermetallic-formation reaction initiating at a temperature (480°C) much lower than that (550°C) when no pressure was applied. At high heating rate (50°C/min), when the heat loss is small, the formation of NiAl occurs rapidly via combustion reaction. On the other hand, if the heat loss is significant as in slow heating rate (10°C/min), the reaction process is controlled by solid-state diffusion. The phase formation sequence for the slow solid-state reaction was determined to be: NiAl₃ Ni₂Al₃ NiAl NiAl (Al-rich) + Ni₃Al NiAl.     

Self-propagating high-temperature synthesis of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiC-based ceramic materials

Dense Al₂O₃/TiC composite ceramic materials are synthesized using the SHS compaction of mixtures based on TiO₂ + Al + C. Mineralizing and heating additives are introduced into compounds. The phase composition and microstructure of combustion products are investigated by x-ray phase analysis, electron microscopy, and microprobe techniques. Two Al₂O₃ modifications are revealed. Special attention is devoted to the presence of residual graphite. The mechanisms of phase formation and formation of the microstructure of combustion products are considered.     

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Two types of Al₂O₃/Mo composites were fabricated by hot-pressing a mixture of - or -Al₂O₃ powder and a fine molybdenum powder. For Al₂O₃/5 vol% Mo composite using -Al₂O₃ as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al₂O₃ grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m^1/2). In the system using -Al₂O₃ as a starting powder, nanometre sized molybdenum particles were dispersed within the Al₂O₃ grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al₂O₃/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al₂O₃ grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m^1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (tm) occurring in ZrO₂ precipitates in a Ca-PSZ alloy and particles in Al₂O₃-ZrO₂ composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al₂O₃-ZrO₂ alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.     

Anisotropic grain growth of R-α-sialon (R = Nd and Er)

Two R--sialon (R_0.6Si_9.3Al_2.7O_0.9N_15.1, R=Nd and Er) compositions were first fired at 1750°C/25 min and 1650°C/2 h respectively for completion of the phase transformation. Elongated -sialon grain morphology was developed in both samples after being re-fired at 1800°C for different periods of time. The growth in width of R--sialon grains is controlled by diffusion in the liquid, while the length growth tends to be interfacial reaction controlled. The anisotropic growth of R--sialon is attributed to the large difference in the growth rate constant between the length and the width directions of the grain.     

Mechanochemical carboaluminothermic reduction of rutile to produce TiC–Al2O3 nanocomposite

This investigation aims to produce TiC–Al₂O₃ nanocomposite by reducing rutile with aluminum and graphite powder via a mechanochemical process. The effect of milling time on this process was investigated. The characterization of phase formation was carried out by XRD and SEM. Results showed that after a 10 h milling, the combustion reaction between Al, TiO₂ and C was started and promoted by a self-propagation high temperature synthesis. Extending the milling time to 20 h, the reaction was completed. The XRD study illustrated after a 20 h milling, the width of TiC and Al₂O₃ peaks increased while the crystallite sizes of these phases decreased to less than 28 nm. After annealing at 800 °C for 1 h in a tube furnace, TiC and Al₂O₃ crystallite sizes remained constant. However, raising the annealing temperature to 1200 °C caused TiC and Al₂O₃ crystallite size to increase to 49 nm and 63 nm, respectively. No new phase was detected after the heat treatment of the synthesised TiC–Al₂O₃ nanocomposite.     

Vinylphosphonic acid-modified calcium aluminate and calcium silicate cements

Cementitious materials in terms of calcium phosphate cements (CPC) were prepared through the acid-base reaction between vinylphosphonic acid (VPA) and calcium aluminate cement (CAC) reactants or calcium silicate cement (CSC) reactants at 25 °C. Using CAC, two factors were responsible for the development of strength in the cements: one is the formation of an amorphous calcium-complexed vinylphosphonate (CCVP) salt phase as the reaction product, and the other was the high exothermic reaction energy. Because the formation of CCVP depletes the calcium in the CAC reactants, Al₂O₃·xH₂O gel was precipitated as a by-product. CCVP amorphous calcium pyrophosphate hydrate (CPPH) and Al₂O₃·xH₂O -AlOOH phase transitions occurred in the CPC body autoclaved at 100 °C. Increasing the temperature to 200 °C promoted the transformation of CPPH into crystalline hydroxyapatite (HOAp). In the VPA-CSC system, the strong alkalinity of CSC reactant with its high CaO content served in forming the CPPH reaction product which led to a quick setting of the CPC at 25 °C. Hydrothermal treatment at 100 °C resulted in the CPPH HOAp phase transition, which was completed at 300 °C for both the VPA-CAC and VPA-CSC systems, and also precipitated the silica gel as by-product. Although the porosity of the specimens was one of the important factors governing the improvement of strength, a moderately mixed phase of amorphous CPPH and crystalline HOAp as the matrix layers contributed significantly to strengthening of the CPC specimens.     

Pressureless sintering of SiC-TiC composites with improved fracture toughness

Composites of SiC-TiC containing up to 45 wt% of dispersed TiC particles were pressureless sintered to 97% of theoretical density at temperatures between 1850°C and 1950°C with Al₂O₃ and Y₂O₃ additions. An in situ-toughened microstructure, consisted of uniformly distributed elongated -SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase, was developed via pressureless sintering route in the composites sintered at 1900°C. The fracture toughness of SiC-30 wt% TiC composites sintered at 1900°C for 2 h was as high as 7.8 MPa·m^1/2, owing to the bridging and crack deflection by the elongated -SiC grains.     

Studies of Ta,Al, and Carbon Sources on Combustion Synthesis of Alumina–Tantalum Carbide Composites

Alumina–tantalum carbide (Al₂O₃–TaC and Al₂O₃–Ta₂C) composites were synthesized by incorporating aluminothermic reduction into a self-propagating combustion process. The test specimens adopted were composed of Ta₂O₅, Al, Al₄C₃, and carbon powders. Experimental evidence showed that the use of Al₄C₃ to provide Al and carbon decreased combustion exothermicity and hindered Ta₂O₅ from being fully reduced. In contrast, for the formation of Al₂O₃–TaC and Al₂O₃–Ta₂C composites, Al₄C₃-free samples with molar ratios of Ta₂O₅:Al:C = 3:10:6 and 3:10:3, respectively, exhibited the highest combustion temperature and reaction rate in the self-propagating high-temperature synthesis process and yielded products with very few minor phases.     

Characteristics of translucent alumina produced by slip casting method using gypsum mold

Translucent Al₂O₃ ceramics were successfully produced by slip casting using a gypsum mold, provided that CaSO₄ impurities, which had penetrated into the green bodies from the gypsum mold, were removed by the wash of HCl aqueous solution. Some of the calcined Al₂O₃ compacts were washed with HCl aqueous solution before sintering the compacts and the others were not washed with HCl aqueous solution. The relative densities of the sintered Al₂O₃ ceramics with HCl treatment were higher than those of the untreated samples. Grains in the HCl-treated samples, which sintered at 1350°C, grew homogeneously with about 1 m in diameter. When the sintering temperature was higher, the grains grew homogeneously. The sintered Al₂O₃ ceramics with the HCl treatment were translucent. The transmittance value increased from 0 to 12% with increasing wavelength from 300 to 900 nm. The Al₂O₃ ceramics with the HCl treatment did not have the transmittance when the solid contents of slurry were low. The transmittance was influenced by the solid contents of slurry. On the other hand, grains in the HCl-untreated samples, which sintered at 1350°C, grew heterogeneously with the range from 0.2 to 2 m. The Al₂O₃ ceramics did not have the transmittance.     

Conductivity threshold and kinetics of the phase transition in Fe2O3-Fe3O4 thin films made by chemical vapour deposition

Iron oxide films were made by chemical vapour deposition and annealing post-treatment. Optical and d.c. electrical measurements probed the Fe₂O₃ Fe₃O₄ transition. It could be understood as a thermally activated process, with an activation energy equal to the band-gap of Fe₂O₃. A.c. electrical data gave evidence against the transition being percolative.     

The structure of plasma-prepared Al2O3-TiO2 powders

The morphology and phase constitution of sub-micron Al₂O₃-TiO₂ powders prepared by oxidation of mixtures of Al₂Br₆ and TiCl₄ in an oxygen-argon high-frequency plasma have been studied. The particle size and distribution were consistent with formation of liquid particles by rapid nucleation and surface reaction followed by growth by coalescence of droplets. The particle size of the powders is related to the concentration of reactants in the gas stream and the temperature difference between condensation and solidification. A metastable solution of TiO₂ in -Al₂O₃ was formed in the range 0 to 7 wt% TiO₂, at higher TiO₂ concentrations particles consisted of a dispersion of rutile particles (~ 10 nm) within single crystals of -Al₂O₃. A metastable phase identified as 3Al₂O₃·TiO₂ was also formed in powder with compositions in the range 14 to 40 wt% TiO₂. Over the composition range 40 to 80 wt% TiO₂ the powder consisted predominantly of crystals with a two-phase Al₂TiO₅-rutile structure. Pure TiO₂ consisted largely of anatase and the addition of Al₂O₃ resulted in the formation of rutile as the major phase. The phase constitution of the powders is interpreted in terms of the nucleation kinetics of the various phases.