Field-activated Combustion Synthesis Research Articles

Synthesis of Cr–Si intermetallic compounds by field-activated combustion synthesis

The use of an electric field to activate the combustion synthesis of chromium silicides was investigated. Despite their relatively low adiabatic temperatures, all four silicides were synthesized by field-activated combustion synthesis. However, although self-propagating synthesis reactions were initiated, the products were not pure but contained other silicides and reactant phases. The purity of the samples increased with increasing field strength, and under the highest field, the products contained the desired silicide as the major phase with minor amounts of other stoichiometries. Observation of microstructural evolution in quenched reactions revealed the key role played by the liquid phases in the propagation of the combustion front. The phase Cr5Si3 was the first product of the interaction between the reactants when either solid–solid or solid–liquid processes were involved. These results were confirmed by isothermal solid–solid and solid–liquid diffusion couple experiments.

Modeling and experimental studies on the effect of thermophysical properties on field-activated combustion synthesis reactions

Abstract The influence of thermal and electrical conductivities and relative density on the dynamics of self-sustaining reactions activated by an electric field was investigated by modeling. Under a given field, the variation of the wave velocity and maximum temperature with normalized electrical and thermal conductivities and density exhibited a maximum at an optimum value of each of the three parameters. The results are discussed in terms of a Fourier relationship modified by the Joule heat contribution of the imposed field. A simulation was also made to investigate the configuration of the reaction front in systems that are ignited and sustained by a current. A transition from a volume (uniform) combustion reaction to a wavelike form was found to occur as the sample size is increased. These changes are also attributed to the interrelation between the thermophysical parameters.

Mechanistic investigation of the field-activated combustion synthesis (FACS) of titanium aluminides

The reaction mechanism of the field-activated combustion synthesis (FACS) of the phase Ti 3Al from elemental powders is investigated. Intermediates of reaction leading to final product are identified by performing X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) of quenched samples obtained by turning off the applied field during wave propagation. The combustion reaction is preceded by the melting of aluminum which then spreads over the titanium particles. Hence, the liquid aluminum reacts with titanium giving rise to a layer of TiAl 3 which grows around the original Ti particles until the aluminum disappears completely. Subsequently, the unreacted Ti interacts with TiAl 3. The resulting grains consist of Ti cores surrounded by a layer of T 3Al which in turn is surrounded by another layer of TiAl. This interaction further proceeds leading to a shrinkage of Ti cores, the gradual disappearance of the TiAl layer, and the simultaneous increase of the Ti 3Al content. A single Ti 3Al phase product is finally obtained.

Modeling studies of the effect of thermal and electrical conductivities and relative density of field-activated self-propagating combustion synthesis

The role of the electrical conductivity of the product and of the thermal conductivities of the reactants on self-propagating combustion synthesis was investigated through modeling studies. Similar studies were made to investigate the role of the relative density of the reactants. The effect of an imposed electric field on the results of the modeling analysis was considered. For any given imposed field, the wave velocity exhibited a maximum at a given normalized thermal conductivity, electrical conductivity, and relative density. The results are discussed in terms of the Joule heat contribution of the field and are compared with experimental observations.

Field-activated combustion synthesis of titanium aluminides

The feasibility of synthesizing the titanium aluminides Ti3Al and TiAl through field-activated, self-propagating combustion synthesis is demonstrated. A self-sustaining combustion wave can be initiated only when the imposed field is above a threshold value for each of these two aluminides. At the threshold values, wave propagation resulted in an incomplete reaction between the metals and the products, which contained several phases in addition to the desired one. As the field strength was increased, the reaction approached completion and the amounts of the secondary phases decreased. At a sufficiently high field, a single-phase product was obtained in the case of Ti3Al, but, in the case of TiAl, the product contained Ti3Al as a secondary phase even with the highest imposed field. The effect of reactant compact density was investigated for the case of Ti3Al synthesis. At a fixed value of imposed field, the degree of reaction completion and the conversion to the desired phase increased as the relative density decreased. These observations are discussed in light of the role of the electric field in activating the self-propagating combustion synthesis reactions and the effect of relative density on this activation. The results show that the synthesis by self-propagating high-temperature synthesis (SHS) can be optimized by the combination of field strength and relative density.

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.

An investigation of the combustion synthesis of MoSi 2β-SiC composites through electric-field activation

Using field-activated combustion synthesis (FACS), composites of (1− x) MoSi 2− xSiC were prepared directly from elemental powders. Systems with 0< x≤0.18 can sustain a combustion wave but the product contained unreacted Si and C in addition to MoSi 2 and β-SiC. Systems with x>0.18 do not sustain a wave without the application of an external electric field. Composites of MoSi 2 and β-SiC can be synthesized for 0≤ x≤1.0 in the presence of fields above threshold field values. The threshold value decreased with increasing x value. Both combustion wave velocity and temperature increased nearly linearly with an increase in field above the threshold value for any given composition ( x value). The influence of carbon particle size on the dynamics of the synthesis and the effect of the magnitude of the field on the nature and microstructure of the product were also investigated.

Effect of Product Conductivity on Field‐Activated Combustion Synthesis

Experimental and modeling results on the field‐activated combustion synthesis (FACS) of Nb5Si3 are reported. In the absence of an electric field and without reactant preheating, Nb5Si3 cannot be prepared by self‐propagating high‐temperature synthesis (SHS). Under the influence of a field a self‐sustaining combustion wave is established whose rate of propagation decreases with traveled distance. For relatively low field values, the wave propagation mode changes from a continuous (smooth) to a spin mode. The product of synthesis depends on the mode of propagation. Synthesis during continuous wave propagation results in the formation of Nb5Si3, primarily in the α‐modification. In contrast, when the wave propagates in a spin mode, the product is NbSi2 with unreacted niobium. The present observations demonstrate a case where the field effect is not localized, as was the case in previous studies. The difference in behavior is attributed to differences in the electrical conductivities of the product phases.

Electric field-activated combustion synthesis of Ti5Si3-Nb and Ti5Si3-ZrO2 composites

The field-activated combustion synthesis of the composites Ti5Si3−x Nb (0≤x≤0.35) and Ti5Si3−y ZrO2 (0≤y≤0.3) has been investigated. Composites with x≥0.35 and y≥0.2 can only be synthesized in the presence of an electric field. Although in the absence of a field the systems with x=0.35, y=0.2 and y=0.3 can sustain a non-steady combustion wave, the reaction is not complete. An unstable wave propagates to the middle of the sample and then becomes extinguished. The wave velocity of the Ti5Si3-Nb and Ti5Si3-ZrO2 composites increased slightly with the application of a field.

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.

Relationship between Field Direction and Wave Propagation in Activated Combustion Synthesis

Using experimental and modeling studies, the effect of field direction on the field‐activated combustion synthesis of SiC was investigated. Field application imposed in a direction perpendicular to wave propagation resulted in enhancement: the wave velocity increased linearly with an increase in field strength. For field application parallel to wave propagation, two cases were considered: confined and nonconfined areas of impacted energy from an ignition source. In the confined case both experimental (using laser ignition) and simulation studies were made. Both showed the establishment of a two‐front combustion with waves moving to opposite sides in a direction perpendicular to the field. In the nonconfined case, initial waves propagated in a direction parallel to the field but were independent of field strength. The results are interpreted in terms of a model in which the current is confined to the combustion zone, adding another source of heat to the combustion process, at a rate of σE2 (J·cm‐3·s‐1).

Field-activated combustion synthesis in the Nb-Si system

The activation of self-propagating combustion reactions in the Nb-Si system was investigated using an electric field. Self-sustaining combustion in elemental reactants with compositions corresponding to Nb 5Si 3 can be initiated only when the imposed field is above a threshold value of 7.4 V cm −1. The mode of wave propogation and the nature of the product depended on the magnitude of the field. Under steady-state wave propagation, the product contained the high temperature (β) and low temperature (α) modifications of Nb 5Si 3. The relative concentration of these phases showed a dependence on the applied field and on the relative density of the reactant compacts. The effect of Nb particle size on the synthesis of Nb 5Si 3 was investigated and the feasibility of the synthesis of Nb/silicide composites was demonstrated.

The influence of an electric field on the mechanism of combustion synthesis of tungsten silicides

The synthesis of tungsten silicides by self-propagating combustion has been successfully accomplished under the influence of an electric field. Materials with starting composition ranging from 6 to 30 wt. % Si were investigated by the method of field-activated combustion synthesis (FACS). A threshold field value was required to initiate a self-sustaining wave; the threshold value depended on composition. It was shown that the level of the applied field can influence the mechanism of silicide formation. The silicide W5Si3 could be formed only at relatively high field values while WSi2 can be formed at any field. The effect of the field on the silicide formation is discussed in terms of its role in liquid phase formation.