Self-propagating High-temperature Synthesis System Research Articles

Self-propagating rapid synthesis and characterization of LaCrO3 powder

Lanthanum chromite (LaCrO3) is a promising functional material because of its high melting point, high thermal conductivity and excellent fire resistance. In this study, LaCrO3 was successfully prepared by self-propagating high-temperature synthesis (SHS) method using CrO3 and Cr as the oxidant and reductant. The optimum mole ratio of La2O3, Cr and CrO3 was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The result shows that the LaCrO3 powder with acceptable purity and good morphology was obtained when Cr and CrO3 are 10% excess over La2O3. The real temperature of SHS system reaction was recorded by paperless recorder. The particle size distribution of the SHS prepared LaCrO3 powder demonstrates median diameter of 1040 nm according to laser particle size spectrometer.

In-situ synthesis of TiC-Al2O3 coating on copper surface

In-situ TiC-Al2O3 strengthening coating was applied on copper, using the vacuum-expendable pattern casting (VEPC) in combination with the self-propagating high-temperature synthesis (SHS) technology. Due to the high heat consumption, highly exothermic CuO-Al (CA) reaction was employed to guarantee 100% completion of the TiC reaction. Consequently, TiC- Al2O3 coating was obtained after the ignition of Ti-C-CuO-Al SHS system by molten copper. Here, the optimal CA content was 10 wt%. During the casting process, molten copper infiltrated into the SHS coating, resulting in the achievement of dense coating microstructure. The hardness and wear resistance were also significantly improved. The hardness value of copper matrix was only 40 HB, while that of the composite coating was up to 195 HB. The mass loss reduced from 7.98 g to 0.44 g at 40 N load. Besides, metallurgical bonding was obtained with an ideal bond strength of 293 MPa.

Surface strengthening aluminum alloy by in-situ TiC-TiB2 composite coating

An integration technology was employed to prepare TiC-TiB2 strengthening coating on aluminum alloys, with the combination of Self-propagating high-temperature synthesis (SHS) and Vacuum-expendable pattern casting (VEPC). During the VEPC process, the Ti-C-B4C-TP SHS reaction was ignited by molten Al alloy, resulting in the simultaneous obtainment of TiC-TiB2 SHS coating with the cast Al alloy. Specifically, Teflon (PTFE) as reaction promoter was introduced into the SHS system to guarantee the reaction to be ignited successfully. With 3.8 wt% PTFE addition, homogeneous and dense TiC-TiB2 coating microstructure was obtained. Compared to the matrix, the hardness of the surface coating increased from 80 HB to 284 HB. And, the weight loss decreased from 533 mg to 52 mg at load of 20 N, indicating the significant improvement of wear resistance. In addition, a comprehensive bonding strength of 160 MPa was achieved. The proposed method for preparing hard coating on Al alloys broadens their industry application, where higher hardness and better wear resistance are required.

Controllable SHS Synthesis of ZnO Nanostructures

Synthesis of high-purity ZnO nanostructures with different micro-morphologies was achieved using self-propagating high-temperature synthesis (SHS) method. The morphologies of products can be controlled by simply tuning the mass fraction of ZnO. Scanning electron microscope was used to study the particle size and morphology. The ZnO nanopowders were characterized for crystal structure using X-ray diffractometry. The influence of the amount of energy obtained by SHS systems on the morphology of ZnO nanostructures was studied.

PREDICTION OF REACTION KINETIC IN MECHANICALLY ACTIVATED SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS PROCESS

In this paper we have tried to develop a semi-empirical formula for estimation of starting time of reactions during mechanical alloying process according to self-propagating high temperature synthesis (SHS) mechanism. For this purpose, three SHS systems containing Ti – C , Mo – Si and Si – C were selected and their behaviors were observed. Aforementioned systems were milled in a planetary ball mill equipped with temperature sensor detector of cups. Samplings were done at different times of discontinuously milling. To change mills' energy, stainless steel and tungsten carbide balls were used. In order to detect the phases and characterizations of milled powder, XRD instrument was utilized. Results showed that all productions were synthesized after sudden increase in temperature. Maximum measured temperature and critical time had up and downtrends for production of TiC , MoSi 2 and SiC , respectively. Crystalline size of milled powder had nano-meter scale. By using experimental data along with theoretical equations, a semi-empirical formula between critical time for transformation of raw materials to productions, type of milled system and ball mill parameter can be presented with high accuracy. According to calculated formula, critical time was related to ball mill energy and Gibbs free energy of milled system with direct and inverse proportionality, respectively.

Effect of Ni–Al SHS reaction on diamond grit for fabrication of diamond tool material

Abstract Diamond grits were added to Ni–Al self-propagating high-temperature synthesis system for fabrication of diamond tool materials. The propagating processes and the microstructure of the reacted composite were investigated. The surface characteristics of diamond grits after Ni–Al SHS reaction were examined by scanning electron microscopy (SEM) and Raman spectroscopic analysis. Owing to the presence of diamond grits, the flame-front propagation velocity of Ni–Al/diamond was reduced by 33% comparing to that of Ni–Al. Intermetallics such as NiAl, Ni 2 Al 3 , Ni 3 Al and NiAl 3 were identified in the reacted Ni–Al/diamond composite. Due to the high temperature of Ni–Al SHS, some of the diamond grits’ surface was graphitised. There were a large number of craters on the surface of graphitised diamond grits, in which the spherical and rod-like graphite particles were found. The mean compressive strength of the SHS-treated diamond grits decreased about 20% comparing to that of the initial diamond grits.

Study of structure formation in TiC–TiB 2–Me xO y ceramics fabricated by SHS and densification

Bulk ceramic materials based on TiC–TiB 2–Me x O y systems have been produced by means of pressure-assisted self-propagating high-temperature synthesis (SHS). The selection of the SHS systems was based on thermodynamic analyses in order to obtain regimes of synthesis with formation of liquid phase allowing a proper densification. The liquid phase formed in the products consisted of either melted oxides or TiC–TiB 2 eutectic depending on the combustion temperature. The mechanisms of structure formation and the effect of grain growth inhibition due to the metal oxide particles have been evaluated. The microstructural investigations have confirmed the role of the metal oxides as promoter of the final products densification and of the grain refinement in the SHS ceramic materials. A core–shell model is proposed to explain the structure formation of the SHS composites through a chemical interaction involving two stages. The good agreement between calculated and experimental results confirmed the validity of the core–shell mechanism.

Microstructure of Ni-Al-Diamond Composite Fabricated by Self-Propagating High Temperature Synthesis

Diamond tool based on Ni-Al self-propagating high temperature synthesis (SHS) was introduced in this paper. Different heating methods such as muffle furnace, vacuum furnace and induction were used to ignite the Ni-Al-diamond SHS system. The morphology and microstructure of the Ni-Al-diamond composite were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM). It was found that SHS Ni-Al-diamond composite ignited by induction heating owned the best morphology and microstructure to work as diamond tool.

Burning velocities in catalytically assisted self-propagating high-temperature combustion synthesis systems

The current work presents an experimental study of catalytically assisted self-propagating high-temperature synthesis (SHS) of the tantalum/carbon material system. The effects of controlled amounts of two gas-phase transport agents (carbon dioxide and vaporized iodine) were examined. The gas-phase transport agents resulted in burning velocities 2 to 4 times faster than burning velocities observed in comparable catalytically assisted and unassisted Ta/C SHS systems. The gas-phase transport agents clearly augment the combustion synthesis process. The burning velocities of the catalytically assisted, non-melting SHS systems were found to be strong functions of particle size, pressure and the amount of gas-transport agent present. The results are compared with previous studies of both catalytically assisted and unassisted Ta/C SHS, and discussed in the context of proposed transport mechanisms.

Combustion synthesis and structure formation in a model Cr-CrO3 self-propagating high-temperature synthesis system

The combustion synthesis (self-propagating high-temperature synthesis, SHS) of a model Cr-CrO3 system is studied both experimentally and theoretically. The system is chemically simple, i.e. it is composed of only two elements, and convenient for combustion and phase formation studies. A new combustion model for a “metal-metal oxide” system and a mathematical model of phase and structure formation are developed. A number of dependencies are determined in the experiments and are described in the combustion model. It is shown that the leading combustion stage is the gas phase oxidation reaction. Different types of microstructures formed from homogeneous melt are revealed. A qualitative agreement between experimental data, theoretical analysis and calculation results is obtained.

Direct determination of the enthalpy of formation of MoB in synthesis from simple substances in an SHS system

In a calorimetric bomb equipped with an electric microfurnace for heating a sample, self-propagating high-temperature synthesis (SHS) of molybdenum boride is realized in accordance with the reaction Mo (c) +B (amorph.)=MoB (c); the enthalpy of this reaction is measured, and the standard enthalpy of formation of molybdenum boride from metal molybdenum and crystaline boron is calculated: Δf H 0 (MoB,c,δ,β)=103.9α1.2kJ/mole.

Thermal explosion in self-propagating high-temperature synthesis systems with allowance for chemical gas transport

On the basis of a model of the reaction of components, taking the change in the contact surface into account, we analyze how the transport of one component to the surface of the other affects the ignition temperature. We have found that chemical gas transport converts the reaction process from ignition at the center of the space to ignition from the reactor wall. In the reactor are two regions, delimited by a curve of equilibrium concentrations of the carrier compound transporting the other component. The carrier compound forms and builds up in one region and in the other region separates out on the surface of the other. The fact that the reactor is closed and that diffusion occurs from one region to the other make it possible to produce superequilibrium concentrations of the carrier compounds in the region of deposition.

Mechanism and macrokinetics of reactions accompanying the combustion of SHS systems

Self-propagating high-temperature synthesis (SHS) processes are used primarily to prepare materials with fixed properties. The mechanism of reaction and formation of structure in SHS waves can be determined only based on the results of special experiments that take into account the real structure of samples and the significant nonisothermality using not one but many independent methods that mutually supplement one another. This paper evaluates methods and instruments for studying the mechanisms of SHS and conducts a theoretical study of these mechanisms using results from the visualization of the interaction process at the particle level, enabled by electron microscopy, and by the use of x-ray phase analysis on a synchrotron radiation diffractometer, which enabled following the dynamics of the phase transformations in the combustion wave.