Papers by Keyword: Combustion Synthesis (CS)

Abstract: TiC- and TiB2-FeAl composites have been produced using the Self-propagating High-temperature Synthesis (SHS) method under Pseudo Hot Isostatic Pressing (PHIP). When mixtures of the elemental powders were heated to a temperature near the melting point of Al under a PHIP of 150 MPa, the powder mixtures exothermically reacted and produced TiC particle dispersed and TiBB2 particle dispersed FeAl-matrix composites. As the volume factions of TiC and TiB2 particles increased from 0.3 to 0.8, the average particle size increased from approximately 1 to 10 μm and the average Vickers hardness increased from approximately 600 to 1600 in both the TiC-FeAl and TiB2-FeAl systems. The application of the PHIP remarkably reduced the porosity of the SHS products. Preheating of the elemental powder mixtures at 773 K for 30 minutes also reduced the porosity. Moreover, the preheating reduced the particle size in the SHS products. It was suggested that degassing of the powder surfaces and mutual diffusion between the different powders should have occurred during the preheating, which led to reduction in pore formation and reduction in heat generation at the SHS reactions, respectively.

Abstract: Gas-solid systems are among a large class of systems in which self-propagating hightemperature synthesis (SHS) is observed. Powders of metals and non-metals burn in the atmosphere of gaseous reagent and the products of the reaction are solid. A variety of nitrides and different composites were synthesized by combustion of porous powder compacts in nitrogen atmosphere. The conversion, achieved during the combustion process, as well as the composition and structure of the products, significantly depends on the nitrogen pressure. Most of the published experimental results on this topic were obtained under nitrogen pressure of less 10 MPa and only few systems were investigated under higher pressures. In this work we present the results of the studies on the combustion process in the range of high nitrogen pressures of 40-400 MPa for two systems, such as Nb-N2 and B-C-N2. Dependences of combustion characteristics, phase composition and microstructure of the synthesized products on the nitrogen pressure were studied.

Abstract: This invited talk reviews recent advances in synthesis of nanopowders in selfpropagating high-temperature regime with emphasis on chemically-assisted combustion synthesis of nonoxide ceramic nanopowders, specifically SiC, Si3N4, and AlN-BN. Recent results of in-situ densification of intermetallic-ceramic composites formed from nanosized reactants in a combustion regime are also presented. Examples of combustion synthesized TiAl3-Al2O3 and NiAl-Al2O3-CNT nanocomposites are included and discussed.

Abstract: A combustion synthesis reaction has been developed for the α-Si3N4 powder under low nitrogen pressure. Si and solid state nitrogen source were used as the reactants, and ammonium halide powders were added as a catalytic agent. These powders were mixed and pressed into a cylindrical compact and wrapped up with an ignition agent (i.e., Ti + C). The synthesis reaction was triggered by combustion of the igniting agent. The product as synthesized is composed mostly of α-phase and in the form of agglomerated fine particles (0.1-1 μm in diameter). The effects of the additives and other experimental parameters on the process were investigated and a possible reaction mechanism was also proposed.

Abstract: Aluminum foam is a class of porous materials; in which closed pores are produced by a gas generation in liquid (or semi-liquid) aluminum. Aluminum foams are, generally, fabricated by heating a foamable precursor (a powder compact consisting of aluminum and TiH2 powders). Decomposition of TiH2, which is followed by a hydrogen gas release, produces bubbles in molten aluminum. In this research, aluminum foam was fabricated with the help of a chemical exothermic reaction. Titanium and boron carbide (B4C) powders were blended in the Al-TiH2 precursor as reactive powder elements. When one end of the precursor was heated, a strong exothermic reaction between titanium and B4C took place (3Ti + B4C 􀃆 2TiB2 +TiC + 761KJ), and the neighboring part of the precursor was heated by the heat of reaction. Hence, once the reaction happens at the end of the precursor, it propagates spontaneously throughout the precursor. The blowing process takes place at the same time as the reaction because aluminum melts and TiH2 decomposes by the heat of reaction. The advantage of this process is that the energy to make aluminum foam is not necessarily supplied form the external source, but generated form inside of the precursor. Therefore the blowing process is self sustainable (Self-Blowing Process). In this work, the effect of processing parameters on the Self-Blowing Process was observed. The processing parameters we focused on were blending ratio of the starting powders (aluminum, TiH2, titanium, B4C) and heating methods.

Abstract: This work aims at characterising an alumina powder obtained by combustion synthesis, and at evaluating the technical properties of sintered bodies. Particle size distribution and XRD were used to characterise the alumina powder; electrophoresis and rheological measurements were used to select the most suitable processing conditions. Sintered density, flexural strength, Vickers hardness and SEM analysis were used to characterise the bodies sintered at 1600°C. The experimental results showed that a density of 3.60 g/cm3 could be obtained in absence of sintering aids and that the sintered bodies exhibited a flexural strength of 220 MPa and a Vickers hardness of 1100.

Abstract: Transition-metal spinels are efficient catalysts in a number of heterogeneous processes, such as CO oxidation, catalytic combustion of hydrocarbons and oxychlorination of methane. The properties of catalytic materials are highly dependent on the synthesis route. Spinels are often produced at high temperatures by the calcination of precursors such as powder mixtures, slurries or resins. Combustion synthesis is a cost-efficient method used to produce homogeneous and fine particles with high reproducibility. Cu0.8Ni0.2Cr2O4 spinel was obtained by the combustion of metallic nitrates using urea as fuel. The resulting powders were calcinated at different temperatures and characterized by thermogravimetric and particle size analyses, X ray diffraction, and scanning electron microscopy. The effect of urea on the control of the process and particle morphology was investigated. The results revealed the formation of porous powders with increasing crystallinity as the calcination temperature increased. Crystallization of spinel started at 700 oC.

Abstract: Nanosized spinel nickel ferrite particles have attracted considerable attention and efforts continue to investigate them for their technological importance to the microwave industries, high speed digital tap or disk recording, repulsive suspension for use in levitated railway systems, ferrofluids, catalysis and magnetic refrigeration systems. Nanosize nickel ferrite powders (NiFe2O4) have been prepared by combustion reaction using nitrates and urea as fuel. The resulting powders were characterized by X-ray diffraction (XRD), BET, and transmission electron microscopy (TEM). The results showed nanosize nickel ferrite powders with high specific surface area (55.21 m2/g). The powders showed extensive XRD line broadening and the crystallite size calculated from the XRD line broadening was 18.0 nm. The samples were uniaxially compacted by dry pressing, sintered at 1200°C/2h and characterized by bulk density, SEM and magnetic properties measurements. The samples showed uniform microstructures with grain size of 4.45 μm, maximum flux density of 0.18T, field coercive of the 488 A/m, and hysteresis loss of 47.58 W/kg.

Abstract: Experimental results on the preparation of functionally graded TiC-based cermets obtained by combustion synthesis (also known as Self-Propagating High-Temperature Synthesis, SHS) followed by quasi-isostatic (QIP) pressing in a granulate medium are presented. Pellets of TiC-Fe graded cermets are produced by stacking layers of Ti and C powder mixtures in which the content of a NiFe alloy (50 wt% Ni and 50 wt% Fe) is varied from 5 up to 25 vol %. X-ray diffraction showed that the NiFe alloy did not react with the TiC, thus preserving its special properties. Scanning electron microscopy results show a graded material with pores increasing in size towards the side with the highest ceramic fraction.

Abstract: In this study, Ti3SiC2 was successfully synthesized by the combustion of green samples with a composition formula: (3-x)Ti/Si/(2-x)C/xTiC, where x ranged from 1.0 to 1.8. First, the Ti3SiC2 yield monotonically increased with x, but the maximal yield was less than 50%. The most important factor to increase the Ti3SiC2 yield was found to be the post-combustion heating. The yield dramatically increased to about 90% when a 150-second post-combustion heating time was provided. Furthermore, the yield was proportional to the post-heating time, and linearly increased with the amount of applied post-combustion heating energy. Next, excess Si in the starting reactant powders can further increase the extent of Ti3SiC2. When the mole of Si increased from 1.0 to 1.1, the resulted yield was more than 99%. For the study of reaction mechanism, analyses of sequential layers of quenched samples resulted in the following proposed mechanism. First, TiC, TiSi2, Ti5Si3 were formed from the reactions of Ti, Si and C powders. Next, the eutectic liquids (Ti-Ti5Si3 and Si-TiSi2) covered the TiC particles at the elevated temperature. In the final stage, product Ti3SiC2 was resulted from heating the eutectics and TiC. It is noted that the last two steps required large energy, indicating the post-combustion heating was crucial to produce high-purity Ti3SiC2 product.