High-temperature Synthesis Reaction Research Articles

Modeling solution for electric field-activated combustion synthesis

A computer model of the effect of an electric field on self-propagating high-temperature synthesis (SHS) reactions is presented. Using the synthesis of SiC as a model, the analysis showed that, in addition to the chemical heat release, the combustion zone also includes heat release from an electric source, a value equivalent to σE 2, where σ is the conductivity and E is the field. Using a two-dimensional Fourier heat balance equation and accounting for the field contribution in terms of Joule heating, a set of equations were developed. Solutions of these were made by a finite difference scheme, coupling the nonlinear partial differential equations. The imposition of this electric field results in a highly localized distribution of the current density. The current is primarily located at the reaction front where the temperature is the highest. This is consistent with the fact that the electrical conductivity increases with increasing temperature. From the dependence of the degree of conversion to the product on the applied voltage, it is shown that the velocity of the combustion wave is linearly proportional to the field.

Study of the synthesis of sialon phases from silica–aluminium powder mixture using microwave energy

Using a domestic microwave oven, pellets of a silica–aluminium powder were ignitedand a self-propagating high-temperature synthesis (SHS) reaction occurred to produce Si+AlN+Al2O3 as the resultant phases. Silicon AlN and Al2O3 were subsequently nitrated to synthesize sialon phases under a nitrogen atmosphere without cooling. Thus both the SHS process and the nitration were finished within one-step process, which could save processing time and energy. © 1998 Kluwer Academic Publishers

Mechanically Activated SHS Reaction in the Fe-Al System: In Situ Time Resolved Diffraction Using Synchrotron Radiation

The Mechanical Activation Self propagating High temperature Synthesis (M.A.S.H.S.) processing is a new way to produce nanocrystalline iron aluminide intermetallic compounds. This process is maily the combination of two steps ; in the one hand, a mechanical activation where the Fe - Al powder mixture was milled during a short time at given energy and frequency of shocks and in the other hand, a Self propagating High temperature Synthesis (S.H.S.) reaction, for which the exothermicity of the Fe + Al reaction is used. This fast propagated MASHS reaction has been in -situ investigated using the Time Resolved X - Ray Diffraction (TRXRD) using a X - ray synchrotron beam and an infrared thermography camera, allowing the coupling of the materials structure and the temperature field. The effects of the initial mean compositions, of the milling conditions as well as of the compaction parameters on the MASHS reaction are reported.

Dimensional change of PM premixes : K.K.Orban et al. (Technical University of Cluj-Napocca, Romania.)

Multilayered Ti-Al based intermetallic sheets were fabricated by sintering alternately layered titanium and aluminum foils. The microstructure and phase formation of the obtained sheets under different sintering conditions were evaluated by various techniques. The results reveal that when the sintering temperature is above the melting point of aluminum, the self-propagating high-temperature synthesis reaction occurs between Ti and Al, and forms various phases of Ti-based solid solutions including α-Ti Ti3Al, TiAl, TiAl2 and α-Ti including TiAl3, etc. When the sintering time increased, Ti-based solid solution, TiAl2 and TiAl3 disappeared gradually, and the sheet containing Ti3Al and TiAl phases in a multilayered structure formed finally. A lot of voids were also observed in the sintered structures, which were caused by the melting Al, Kirkendall effect and the difference of molar volumes between reactants and products. The voids were eliminated and a dense sample was obtained by the following hot press.

Preparation of ReB 2 single crystals by the floating zone method

Single crystals, 0.9 cm in diameter and 3 cm long, were prepared at a rate of 1 cm/h, using starting materials obtained by a self-propagating high-temperature synthesis reaction. The floating zone growth revealed that the ReB 2 phase is a stoichiometric compound and melts congruently.

減圧プラズマ溶射によるＮｉＣｒＡｌＹ／ＮｉＡｌ多層膜の生成

NiCrAlY/NiAl multi-layered coating was produced on SUS310S steel by means of mutual low pressure plasma spraying of NiCrAlY and Al powders which was accompanied with self propagating high-temperature synthesis (SHS) reaction of metal deposits. The NiAl layer contained Ni3Al particles and Cr2Al phase along the fine grain boundary. Also, Ni3Al was detected in the NiCrAlY layer with a small amount of NiAl particles. As the result, high hardness was obtained in both the layers, i.e., 650HV in NiAl layer and 450HV in NiCrAlY one at 673K. The structure of the multilayered coating changed hardly during annealing lower than 973K because enriched Cr at NiCrAlY/NiAl interface suppressed NiAl+Ni3Al→Ni5Al3 peritectoid reaction. The SHS reaction time of an compressed Al droplet in diameter of 50μm was calculated as 4.17×10-3 second.

Densification process of TiCx–Ni composites formed by self-propagating high-temperature synthesis reaction

Densification behaviour of the TiC-Ni system formed by self-propagating high-temperature synthesis (SHS) were studied to develop the structural metal-matrix composites. The composites were prepared by two routes: (1) consolidation during the SHS reaction, and (2) consolidation process after the SHS reaction. The final phases of the stoichiometric reactant mixture of titanium and graphite with 50 wt% nickel produced by simultaneous combustion reactions, were titanium carbide in a nickel-rich solid solution containing carbon and titanium. The density of the products was relatively low, with a value of 90% theoretical density. In the second approach, liquid infiltration and liquid-phase sintering were applied for the titanium carbide-nickel mixture. Densification rates were reduced due to the excess carbon in the combustion products of titanium carbide. The densities of the liquid-phase sintered samples were more than 97% theoretical density.

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.

Indication of complete delubrication: H.S. Nayar, G. White (BOC Gases, USA)

TiC particles were prepared in situ by self-propagating high-temperature synthesis (SHS) reaction in the Fe–Ti–C elemental powder mixture. The reaction behavior and formation route of synthesizing TiC in the Fe–Ti–C system were investigated. With Fe contents increasing, the adiabatic temperatures, reaction temperatures and TiC particles sizes obviously decreased. The addition of Fe promotes the reaction between Ti and C through a “bridging” effect. Fe plays an important role in controlling the reaction behavior and morphology of products, serving not only as a diluent to inhibit the TiC particles from growing, but also as an intermediate reactant to promote the reaction.

A lumped parameter analysis of powder injection moulding: M.F. Aksit, D.Lee (Rensselear Polytechnic Inst., Troy, New York, USA). Int. J. Powder Metall., vol 31, no 4, 1995, 351–361

TiC particles were prepared in situ by self-propagating high-temperature synthesis (SHS) reaction in the Fe–Ti–C elemental powder mixture. The reaction behavior and formation route of synthesizing TiC in the Fe–Ti–C system were investigated. With Fe contents increasing, the adiabatic temperatures, reaction temperatures and TiC particles sizes obviously decreased. The addition of Fe promotes the reaction between Ti and C through a “bridging” effect. Fe plays an important role in controlling the reaction behavior and morphology of products, serving not only as a diluent to inhibit the TiC particles from growing, but also as an intermediate reactant to promote the reaction.

Influence of Al additions on the reactive synthesis of MoSi 2

The influence of small additions of Mg, Ti, and their hydrides on the kinetics of the Self-propagating High-temperature Synthesis (SHS) reaction during sintering of iron and aluminum elemental powders was investigated using the DSC technique and XRD analysis. It was observed that the additives have a significant effect on the SHS reaction parameters. The start of the SHS reaction in a Fe–Al system containing additives (especially with Mg and MgH2) is shifted to lower temperatures as compared with that of an undoped Fe–Al system. The influence of these additives is also visible from changes in the Avrami exponent and the activation energy value of specific phase formation from the Fe–Al equilibrium system. Furthermore, additives cause a change of intensity of the SHS reaction, which should allow control of the degree of sintering porosity.

TG-DTA-MS study of self-ignition in self-propagating high-temperature synthesis of mechanically activated Al-C powder mixtures

When mechanically activated Al-C powder mixtures were heated in flowing nitrogen containing a small amount of oxygen, or in static air, the exothermic reaction due to the oxidation of the disordered carbon, formed by grinding, was initiated at around 100°C, and was followed by the oxidation of Al metal. This result verified that the disordered carbon served as an igniter for the self-propagating high-temperature synthesis reaction induced by mechanical activation of Al-C powder mixtures.

Production and properties of steel–TiC composites for Wear applications

Fe–(WTi)C composite granules containing up to 80 wt-% carbide have been produced by a selfpropagating high temperature synthesis reaction. These can be readily distributed in conventional steel melts. Additions up to 17 wt-% carbide have been made to a 0·4 wt-%C steel which was subsequently cast and hot rolled to plate. The microstructures of cast, rolled, and heat treated. samples display a homogeneous distribution of carbides which do not significantly affect the rolling performance of the steels. The carbides and grain refinement in heat treated samples result in a marked improvement in mechanical properties. The most significant improvement as a fraction of carbide additions is seen in abrasive wear performance.MST/3196

The effect of an electric field on self-sustaining combustion synthesis: Part I. modeling studies

An analysis of the effect of an electric field on self-propagating high-temperature synthesis (SHS) reactions is presented. Using the synthesis of SiC as a model, the analysis showed that the imposition of a field results in a highly localized distribution of the current density. It was shown that the current is primarily restricted to the region just ahead of the combustion zone. Thus, in addition to the chemical heat release, this zone also includes heat release from an electric source, a value equivalent to σE2 where σ is the conductivity andE is the field. From the dependence of the degree of conversion to the product on the applied voltage, it is shown that the velocity of the combustion wave is linearly proportional to the field.

Microstructural and failure characteristics of metal-lntermetallic layered sheet composites

A processing technique for the fabrication of layered metal-intermetallic composites is presented, in which a self-propagating, high-temperature synthesis reaction (SHS) was initiated at the interface between dissimilar elemental metal foils. The resultant composite microstructure consisted of a fully dense, well-bonded metal-intermetallic layered composite. In this United States Bureau of Mines study, metal (Fe, Ni, or Ti) foils were reacted with Al foils to produce metal-metal aluminide layered composites. Tensile tests conducted at room temperature revealed that composites could be designed to behave in a high-strength and high-toughness manner by altering the thicknesses of the starting elemental foils. Failure characteristics revealed that the processes that govern ductilevs brittle behavior of the composites occur early in the fracture.

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the Ti+C system

The ignition phenomena and the reaction mechanism of the self-propagating high-temperature synthesis reaction of titanium and carbon powders were experimentally investigated. When using coarse graphite powders (<325 mesh) as the carbon source, the ignition temperature ranged from 1650–1720‡C and was independent of the C/Ti ratio. The ignition temperature could be significantly lowered by using finer graphite powders (e.g. 1400‡C for <1 Μm powder). When using carbon black as the carbon source, the ignition temperature ranged from 1050–1475‡C and was dependent on the C/Ti ratio. The ignition was confirmed in this study to be controlled by the rate of the surface reaction between titanium and carbon which, in turn, was determined by the contact surface area between them. The fractured surfaces of the products showed two different types of morphology, i.e. groups of grains similar to sintered bodies and agglomerated fine particles. The relative quantities of the two types of morphology depended on the type of carbon used, the C/Ti ratio, the particle size of graphite and the density of the reactant pellet. Possible reaction mechanisms have been proposed on the basis of the experimental observations of the ignition phenomena and the product morphology.

Processing, structure and properties of metal-intermetallic layered composites

In this US Bureau of Mines study, metal-intermetallic laminar composites were produced by induction of a self-propagating, high-temperature synthesis (SHS) reaction at the interface between dissimilar metal foils. The reaction between Ni and Al foils produced a composite that consisted of alternating, well-bonded Ni and Ni 2Al 3 layers. The initial metallic foil thicknesses (composites were produced from various elemental foils ranging in thickness from 0.05 to 0.25 mm) only affected the volume fraction of the resultant Ni and Ni 2Al 3 layers and not the actual phases formed. The tensile behavior of these composites were found to be independent of the volume fraction of the layers present in the resultant metal-intermetallic laminar composites. In addition, the fracture behavior of the composites was similar from room temperature to 600 °C, with the Ni layers rupturing in a ductile manner after bridging many cracks in the Ni 2Al 3 layers. Heat treatment of the composite at 950 °C for 50 h resulted in a microstructure consisting of Ni and Ni 2Al 3 layers, but with a thick NiAl interphase layer. Heat treatment at 1150 °C resulted in a composite comprising Ni and Ni 3Al layers. The microstructure and properties of similarly processed TiAl-based composites are discussed. The room-temperature tensile behavior of the TiAl composites was sensitive to the volume fraction of the resultant Ti and Al 3Ti layers. The feasibility of the production of near-net-shaped metal-intermetallic layer composites by this technique was demonstrated.

Fracture characteristics of metal/intermetallic laminar composites produced by reaction sintering and hot pressing

Metal/intermetallic layered composites were formed by a process recently developed in which a self-propagating, high-temperature synthesis reaction was initiated at the interface between dissimilar metal foils. After the reaction, one of the metal foils was entirely consumed, resulting in a metal/intermetallic laminar composite. This study details the tensile fracture characteristics of these unique composites. Fracture mechanism and failure energy were controlled by varying the intermetallic-to-metal volume ratio. Failure initiated with the formation of cracks in the intermetallic layer. For high intermetallic-to-metal ratios, the intermetallic crack release energy was too great to prevent cracks from propagating through the metal layer and propagating the crack into the adjacent intermetallic layers, leading to a fast, low energy fracture. For lower intermetallic-to-metal ratios, the metal layers adsorbed the intermetallic crack release energy and blunted the propagating crack. Final failure resulted by ductile fracture of the metal layer after extensive intermetallic cracking.

Joining of the Intermetallic Compound TiAl Using Self-propagating High-temperature Synthesis Reaction

The intermetallic compound TiAl was bonded by using self-propagating high-temperature synthesis (SHS) reaction of a mixture of elemental Al and Ti powders used as a filler metal. The structure of SHS reacted filler metal was inhomogeneous and consisted of Ti, {alpha}{sub 2} (Ti{sub 3}Al) and TiAl{sub 3}. On the bonding interface between base metal and filler metal, TiAl{sub 3} was formed as a reaction layer. By the subsequent heat treatment of the SHS joint at 1573 K, the structure of the filler metal changed to a fine lamellar structure consisting of {alpha}{sub 2} and {gamma} (TiAl) phases. The TiAl{sub 3} reaction layer, however, did not transform to a lamellar structure but only to a single-phase {gamma}. Joint strength at room temperature and at 873 K exhibited about 220 MPa which was about the same as that of the base metal. (orig.)

Differential thermal analysis of ignition temperatures in a self-propagating high-temperature synthesis reaction

Differential thermal analysis was carried out on the self-propagating high-temperature synthesis reaction 3TiO2+4Al+3C→3TiC+2Al2O3. The results allow the ignition temperature of the reaction to be estimated and the reaction mechanism to be identified. The ignition temperature was 900°C and the results suggest that the reaction proceeds by an initial reaction between titania and aluminium (3TiO2+4Al→3Ti+2Al2O3) and the titanium formed reacts with the carbon (Ti+C→TiC).