Self-propagating Combustion Synthesis Research Articles

The use of an electric field as a processing parameter in the combustion synthesis of ceramics and composites

The imposition of an electric field is shown to activate self-propagating combustion reactions and thus makes possible the synthesis of a variety of ceramic and composite phases. Experimental observations and modeling studies indicated that activation is accomplished by the localized effect of the current. The relationship between wave propagation and the direction of the applied field was investigated. The synthesis of composites by field-activated combustion synthesis (FACS) was demonstrated. It was shown that the imposition of a field during the combustion synthesis of MoSi2 results in a decrease in the product particle size. The results suggest that the field can be used as a processing parameter in self-propagating combustion synthesis.

A micromechanistic model of the combined combustion synthesis-densification process

A series of computer experiments has been conducted in order to study the combined combustion synthesis-densification process, in which a mechanical load is applied to a sample as it undergoes a combustion synthesis process. The current work is an extension of a theoretical model of the combustion synthesis process that was developed previously.1,2In this work, the appropriate constitutive equations for sample deformation have been incorporated, in order to account for the pore-volume change that may take place when the mechanical load is applied, thus densifying the sample. It was shown that the brief appearance of a liquid phase in the combustion wave front provides an important opportunity for densification when the self-propagating combustion synthesis process is conducted in conjunction with an applied mechanical load. That is, the concomitant decrease in the (local) total volume fraction of the solid phases—due to the elementary melting and dissolution processes that also occur (locally)—effectively lowered the (local) apparent yield strength of the sample, thus allowing for the compaction and densification of the sample (i.e., locally). Results indicated that the mechanical load should be applied at the instant at which the sample is ignited, in order to ensure that articles whose density is uniform throughout the sample can be fabricated. This work provided a more detailed and quantitative understanding of this unique process for preparing dense articles by the self-propagating combustion synthesis process, that is, when it is conducted in conjunction with an applied mechanical load.

Thermal conductivity of Fe-Si alloys prepared by the SHS process

FeSi2 is one of the most interesting high-temperature thermoelectric transition materials due to its high transition efficiency, low cost and high oxidation resistance at working temperatures [1]. FeSi2 thermoelectric devices have been prepared conventionally through melting raw metals, crushing, sintering and annealing. In this study, a novel processing technique, i.e. self-propagating high-temperature synthesis (SHS) or combustion synthesis, was used to prepare Fe-Si alloys. The SHS process is superior to other conventional methods in terms of saving energy and processing time. The SHS-products are usually highly pure because at extremely high reaction temperatures the volatile contaminants in the samples mostly vaporize. Owing to the very high cooling rate following SHS reaction, high defect concentrations and non-equilibrium structures generally exist in the SHS-produced materials, rendering the products more reactive and metastable [2]. In this Fe-Si alloy system, direct formation of /3 phase from the melt can be expected during SHS reaction when the cooling rate is high enough. The purpose of this study is to measure the thermal conductivity of Fe-Si alloys synthesized by SHS and to investigate the applicability as a thermoelectric generator material. To obtain /3-FeSi2 by the SHS process, iron (>99%) and silicon (98%) powders were mixed according to the composition shown in Table I, corresponding to /3-FeSi 2 and a little excess Si, by considering the loss of Si during combustion reaction because Si is more volatile than Fe. KNO 3 (>99.9%) was also added to the Fe-Si powder mixture in the weight ratio of KNO3/(Fe + Si) = 0.2 to effectively activate ignition of the mixture by arc discharge at room temperature [3]. The powder mixture was compacted to pellets at 100MPa pressure by a uniaxial press. Washing, crushing and screening processes followed the combustion reaction. The produced powder (<325 mesh) was analysed by Xray diffractometer, and shown to be composed of eFeSi, o~-Fe2Si5 and a trace of unreacted Si. This powder was uniaxially pressed into pellets at 100MPa pressure together with the binder PVA, which was removed by heating at 500 °C for 2 h in air, and the pellets were sintered at 1155 °C for 3 h in Ar. X-ray diffraction analysis showed that the sintered samples consisted of a-Fe2Si5 and e-FeSi. The sintered specimens were annealed at 840 °C for 12 h in Ar to transform these two metallic phases to semiconducting ¢l-FeSi2 phase, and compared with those sintered and annealed under the same conditions using a FeSi 2 commercial powder (CERAC, USA). Fig. 1 shows X-ray diffraction patterns for the annealed samples. The main phase /3-FeSi2 can be observed for all samples and the intensity of the eFeSi peaks decreases with increasing Si content. It is considered that the composition of specimens changed to an Fe-richer type compared with the starting

A micromechanistic model of the combustion synthesis process: Part II. Numerical simulation

A series of computer experiments was conducted for the self-propagating combustion synthesis process in the Nb-C system, based on the general theoretical model that was developed previously.1A detailed and quantitative description was given for the various physical and chemical processes that take place during the combustion synthesis process. The results are presented at various length scales in order to provide an insight into understanding the mechanisms that are responsible for the self-propagating behavior. It was shown that a fundamental understanding and precise control of the process require a strong emphasis on the joint contributions of theratesof the various mass and energy redistribution processes that occur during the combustion synthesis process. A proper balance of each of the elementary process rates must be achieved to give rise to self-propagating behavior. This paper illustrates some of the capabilities of the general theoretical model in quantitatively describing the self-propagating combustion synthesis process.

Mechanism of combustion synthesis of silicon carbide

The mechanism of self-propagating high-temperature synthesis (SHS) or combustion synthesis of SiC has been investigated using pellets consisting of silicon and carbon powders. The combustion reaction was initiated by rapidly heating the pellet on a graphite strip. The reaction products were analyzed using scanning and transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. The results show that it is possible to produce β-SiC without any residual silicon and carbon. Occasionally, a very small number density of α-SiC precipitates embedded in the β-SiC matrix was observed. Based upon the microstructural features, it is proposed that the formation of SiC involves the dissolution of carbon into liquid silicon, diffusion of C into liquid silicon, and subsequent precipitation of SiC. The size of the SiC crystallites is determined by the diffusion coefficient of carbon in liquid silicon and the time available for SiC precipitation. The activation enthalpy for the SHS process is estimated to be 59±3 kcal/mol.

Ignition criteria for self-propagating combustion synthesis

Ignition criteria for gasless self-propagating combustion synthesis have been investigated through an ignition temperature analysis. The calculations were based on the dimensionless energy and mass continuity equations where the dimensionless parameters associated with the rate of local heat generation (β), activation energy (γ), the rate of surface heat loss by convection (ω), the rate of surface heat loss by radiation (δ), and the rate of reaction (λ) were incorporated. The relative significance of each of these parameters on the ignition of the self-propagating combustion reaction was evaluated to be γ &gt; β &gt; δ &gt; ω. The ignition region, transition region, and nonignition region were identified for selected conditions. The correlations between ignition behavior and the material properties, the thermodynamic and kinetic properties, as well as the experimental conditions were discussed. The calculations indicated that only those systems with δH/Cp &gt; 1.5 × 103 (K) will give rise to a self-propagating combustion reaction without external energy input. Thus, this value can be used as an approximate guide for the existence of self-sustaining combustion. The calculations provide a sound basis toward interpreting experimental observations and developing a fundamental understanding of the process.

Preparation of [formula omitted] composites from elemental powders by reactive co-synthesis

The intermetallic compound MoSi[sub 2] has potential for use as an elevated temperature structural material. Desirable properties include high melting temperature (2030[degree]C), low density (6.25 g/cm[sup 3]) and excellent oxidation resistance. However, the fracture toughness of MoSi[sub 2] is about 3 to 5 MPa[radical]m and the elevated temperature strength of MoSi[sub 2] is inadequate. The addition of SiC as particles or whiskers has improved the elevated temperature strength and creep resistance of MoSi[sub 2]. The fracture toughness of MoSi[sub 2] reinforced with SiC whiskers has been reported to be 8.2 MPa[radical]m. Thus, MoSi[sub 2]/SiC is emerging as a promising high temperature composite. It was first reported in the late 1940's and early 1950's that MoSi[sub 2] could be synthesized by the direct reaction of elemental Mo and Si powders (9). This technique involves intimately mixing elemental powders to stoichiometric proportions. The reaction is initiated between the powders by either externally heating the mixture (9) or by passing current through an electrode that was buried in the mixture (10). The reaction product is commonly milled into prealloyed powder. The reaction is exothermic in nature due to the high thermal stability of the forming intermetallic compound. As a consequence, heat is liberatedmore » and the reaction becomes self-sustaining and self-propagating. This type of materials synthesis approach has been described variously in the literature as self-propagating high-temperature synthesis (SHS), combustion synthesis or reactive synthesis. SiC is among the many compounds, besides MoSi[sub 2], that have been produced by the SHS technique (13-15). This research effort was undertaken in an attempt to synthesize MoSi[sub 2]/Si[sub C] composites in situ from elemental Mo, Si and C powders using a SHS type reaction that the authors have named reactive co-synthesis.« less

Processing of compact materials by the use of self-propagating high-temperature synthesis and pseudo-hot isostatic pressing

The processing of compact materials by self-propagating combustion synthesis and pseudo-hot isostatic pressing is studied for the case of the intermetallic NiAl and functional gradient composite material TiB 2-Cu with copper content 0–30 at%. The singlephase NiAl compact with a grain size of ≈ 35 μm was obtained by initiating self-propagating combustion synthesis with a current of 50–90 A and by the action of an external pressure of 40 MPa under conditions of pseudo-hot isostatic pressing. Under similar conditions functional gradient TiB 2-Cu was synthesized although the pressure of 40 MPa was not sufficient to obtain a highdensity product.

Numerical modeling of solidification combustion synthesis

A numerical study of self-propagating combustion synthesis is carried out to determine the effect of the heat of reaction(Q), the activation energy (E), the frequency factor (K0), thermal conductivity(K*), and initial temperature(T 0 on the combustion velocity and combustion temperature in the presence of cooling at one end. The numerical procedure allows for the formation and solidification of nonstoichiometric combustion products. This includes the phase change of the product through its solidification and eutectic range. Calculations are carried out for the Ti-C system with an objective of predicting solutions which are comparable to previously reported, experimentally determined values. Calculations are also compared with the thin zone analytical solution to the combustion problem. The use of lowK 0 values to bring the solutions close to the experimentally determined numbers is discussed. Solutions are presented to elucidate the effect of relevant parameters on the thickness of the combustion and preheat zones. Conditions where extinction is expected to occur are identified. The effect of the thermal conductivity on the velocity may be to increase or decrease the velocity, depending on the value ofK 0. At lowK 0 values, an increase in the thermal conductivity may lead to a decrease in the combustion velocity. The effect of the initial temperature on the combustion velocity and temperature is to increase both; however, the increase in the combustion temperature may not be proportional to the increase in the initial temperature. The activation energyE has a pronounced effect on reducing the combustion velocity while not influencing the combustion temperature. The time rate of the solidification process which determines the final microstructure is discussed.

Temperature-enthalpy approach to the modelling of self-propagating combustion synthesis of materials

A new temperature-enthalpy approach has been proposed to model self-propagating combustion synthesis of advanced materials. This approach includes the effect of phase change which might take place during a combustion process. The effect of compact porosity is also modelled based on the conduction, convection and radiation in the local scale. Various parametric studies are made to analyse numerically the effects of activation energy, non-reacting phase content, porosity, Biot number, etc. The model predictions of the combustion pattern are in close agreement with those observed in experiments.