Field-activated Combustion Synthesis Research Articles

Combustion synthesis and the electric field: A review

This brief review discusses principles and some latest developments in electrically activated combustion synthesis. Processes discussed include field-activated combustion synthesis (FACS), field-activated pressure-assisted synthesis (FAPAS), reactive spark plasma sintering (R-SPS), electrothermal explosion (ETE), and electrostatic-field-activated combustion synthesis (EFACS). These processes have demonstrated clear benefits to the process of combustion synthesis through the application of electric field. Although a significant amount of works have been published in the area, there still remain some directions within the field where sustained research may provide even more scientific reward.

Electric field activated combustion synthesis in the Ti + Al system under terrestrial and reduced gravity conditions

The electric field-activated combustion reaction between Ti and Al was investigated under terrestrial and microgravity conditions with different ignition modes. Samples were reacted with and without an igniter layer and the effects of pre-ignition heating and convective heat transfer on combustion wave velocity were investigated for both cases. The contribution of these effects relative to the effect of the electric field was found to depend on the strength of the field, being less significant at high applied fields. The threshold level of the electric field for the formation of stable combustion waves was higher under reduced gravity conditions and increased further in the top ignition experiments; these observations were attributed to the decrease of preheating to the sample due to the reduction of gravity driven convection effects. Observations on the distribution of the molten reactant (aluminum) in quenched and self-extinguished samples were analyzed in terms of the contribution of forces of gravity, surface tension, and viscosity through calculations of the Bond (Bo) and Capillary (Ca) numbers. The results showed the dominance of the surface tension contribution even under microgravity conditions. Marangoni flows are influenced by concentration gradients due to differences in the Ti content in the molten aluminum.

Numerical modeling of field-activated combustion synthesis process of the B4C system

Abstract A field-activation combustion synthesis process of the 4B + C reactive system was numerically simulated to investigate the effect of external field and porosity on the combustion reaction by an implicit difference method and a Gauss–Seidel iteration procedure. The new features of the model include a consideration of the melting of each constituent of the reactants and product and the inclusion of considerations involving porosity and dilution. The results show that the self-sustaining reactions are not possible until field-activated temperature is more than 1100 K, which agree with the theoretic calculation. As the reactant porosity values are decreased from 60% to 20%, the combustion velocity first increases because of an increase in the thermal conductivity. The combustion velocity, after reaching a maximum, decreases with a further decrease in the porosity because of the high value of the thermal conductivity of the reactants.

Synthesis and characterization of TiB 2–Ni–Ni 3Al–CrNi alloy graded material by field-activated combustion

The formation of TiB 2–Ni/Ni 3Al/CrNi alloy functionally graded materials (FGMs) by field-activated combustion synthesis was investigated. Four-layered FGM samples of 50TiB 2–50Ni(wt%)/30TiB 2–70Ni/Ni 3Al/CrNi alloy were produced in which the intermetallic Ni 3Al was reactively formed during the process. Concentration profiles of elements across interfaces between layers showed significant inter-diffusion with indication of formation of diffusion bonding. Measured micro-hardness values increased smoothly from the steel substrate to the outer composite layer of 50TiB 2–50Ni. The values ranged from about 370 to over 3100 HK, over a distance of less than 2 mm. The results demonstrate the feasibility of the field-activated and pressure-assisted combustion synthesis to rapidly form FGMs with good diffusion bonding between the layers. Thermal shock and wear resistance of the FGM were investigated, the wear resistance of the FGM was shown to be higher than the base steel.

Field-activated pressure-assisted combustion synthesis of MoSi2

The simultaneous synthesis and densification of MoSi2 starting from elemental Mo and Si was carried out through a process of field-activated pressure-assisted combustion synthesis (FAPACS). After heating up to 922°C, the synthesis reaction occurs in the mode of thermal explosion but is ceased subsequently due to a sudden shock. The product consists of MoSi2, residual Mo and amorphous Si with precipitation of very small MoSi2 primary crystals containing. There is no transition phase layer between Mo and MoSi2 as well as Si and MoSi2. This indicates that metallic Mo reacts with molten Si directly forming MoSi2 in the FAPACS process, without any intermediate reaction steps involved. The formation of MoSi2 occurs via dissolution of Mo into Si melt followed by MoSi2 precipitation from the melt.

Microstructures and densification of MoSi2–SiC composite by field-activated and pressure-assisted combustion synthesis

Abstract Using Mo, Si, and C element powders as starting materials, a field-activated and pressure-assisted combustion synthesis (FAPACS) was employed to synthesize high-density bulk MoSi2–SiC composite in one step. Effects of sintering temperatures (1100, 1200, 1400 and 1600 °C) on microstructures and densification were investigated. With the exception of 1100 °C, at which synthesis was incomplete, the FAPACS was characterized by the four stages, i.e. initial expansion, slight shrinkage, abrupt shrinkage and slight expansion. The study shows that the sintering ability at the higher temperature is better than that at the relatively lower temperature. When the sintering temperature reaches 1400 or 1600 °C with holding time of 10 min, the high-temperature transitional phases, i.e. Mo4.8Si3C0.6, were presented and the produced SiC particles are agglomerate. The relative density of the products (holding time: 10 min) was improved tremendously from 59% to 96.0% over the range 1100–1400 °C, and a smaller enhancement in densification up to 98.5% was achieved between 1400 and 1600 °C. Moreover, an extended holding time (up to 20 min) at 1600 °C resulted in nearly Mo4.8Si3C0.6-free and dense MoSi2–SiC products with the relative density of 99.0%. Simultaneously, a locally uniform distribution of SiC in the synthesized MoSi2 matrix was achieved. And the interfaces between the synthesized phases are clean without impurities (e.g. SiO2) and porosities. Thereby, in order to pursue an equilibrium among densification, microstructural uniformity and formation of transitional phases, the most suitable sintering temperature and holding time for fabrication of MoSi2–SiC by FAPACS are recommended to be 1600 °C and 20 min in the present work.

Microstructure and Mechanical Properties of Graded Materials Prepared by Field-Activated and Pressure-Assisted Combustion Synthesis

Using the processes of field-activated and pressure-assisted combustion synthesis (FAPACS), FGM materials (FGMs) were prepared under the conditions of field-assisted and the hot-press. The microstructure and the phase composition of the interface of the graded materials were investigated and the results showed that the metallurgical joining layer was formed in the interfaces of the (TiB2)pNi/Ni3Al/405 steel. The mechanical characterization of the gradient materials showed that the composition and the micro-hardness of the gradient material were gradient distributed, and its surface Rockwell hardness and wear resistance are better than that of hardened 20Cr steel.

Microstructures, densification and mechanical properties of TiC–Al 2O 3–Al composite by field-activated combustion synthesis

Dense TiC–Al 2O 3–Al composite was prepared with Al, C and TiO 2 powders by means of electric field-activated combustion synthesis and infiltration of the molten metal (here Al) into the synthesized TiC–Al 2O 3 ceramic. An external electric field can effectively improve the adiabatic combustion temperature of the reactive system and overcome the thermodynamic limitation of reaction with x < 10 mol. Thereby, it can induce a self-sustaining combustion synthesis process. During the formation of Al 2O 3–TiC–Al composite, Al is molten first, and reacted with TiO 2 to form Al 2O 3, followed by the formation of TiC through the reaction between the displaced Ti and C. Highly dense TiC–Al 2O 3–Al with relative density of up to 92.5% was directly fabricated with the application of a 14 mol excess Al content and a 25 V cm −1 field strength, in which TiC and Al 2O 3 particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al. The hardness, bending strength and fracture toughness of the synthesized TiC–Al 2O 3–Al composite are 56.5 GPa, 531 MPa and 10.96 MPa m 1/2, respectively.

Hydrogen storage Mg 2Ni alloy produced by induction field activated combustion synthesis

It is difficult to obtain pure Mg 2Ni alloy by conventional melting method because of the large difference in vapor pressure and melting point between Mg and Ni. However, Mg 2Ni with high purity and fine properties as a hydrogen storage material was successfully synthesized from elemental Mg and Ni powders by Induction Field Activated Combustion Synthesis (IFACS). The IFACSed Mg 2Ni showed convenient hydrogen properties as same as the reference. The effect of Mechanical Grinding (MG) on the hydrogen storage behavior in Mg 2Ni is also described. The MG process developed the applicability of Mg 2Ni as a hydrogen storage material by decreasing the absorption pressures and enhancing the reaction kinetics.

Influence of an electric field on combustion synthesis process and microstructures of TiC–Al 2O 3–Al composites

In order to achieve combustion synthesis (CS) of high-density and fine-structured TiC–Al 2O 3–Al composites, the activation of combustion reactions of 3TiO 2–3C–(4 + x) Al system with x ≥ 10 mol was conducted by means of external electric field. During the field-activated combustion synthesis (FACS), liquid Al in excess stoichiometric amount was generated and it infiltrated into the synthesized TiC–Al 2O 3 ceramic matrix with porosities, which directly results in a dense TiC–Al 2O 3–Al composites. The influence of electric field on CS process of the reactive system and the resulting microstructures of TiC–Al 2O 3–Al composites was investigated. The results show that an external electric field can effectively improve the adiabatic combustion temperature of the reactive system. Therefore a self-sustaining combustion of the system can be induced thermodynamically. In the experiments, both practical combustion temperature and propagation velocity of combustion wave were obviously enhanced with increasing field strength, whereas sizes of the synthesized TiC and Al 2O 3 particles were significantly decreased at the same time. Under an electric field of 25 V cm −1, TiC–Al 2O 3–Al composites with a relative density of 92.5% was successfully fabricated through the combustion reaction in the system with x = 14 mol, in which TiC and Al 2O 3 particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al.

Microwave activated combustion synthesis of titanium aluminides

TiAl and Ti 3Al titanium aluminide intermetallic compounds were synthesized using a microwave activated combustion synthesis (MACS) method and compared to thermal activation (TACS) and conventional combustion (CS) methods. MACS represents an example of our “synergistic processing” philosophy, wherein the useful attributes of microwave processing and combustion synthesis are used synergistically. Thermodynamic calculations indicated activation was required for combustion synthesis to be successful in the Ti–Al system. MACS and TACS yielded Ti–Al products, while no reaction occurred with CS. Peak temperatures for TiAl during combustion were measured at 1500 °C for TiAl and 1450 °C for Ti 3Al in MACS, compared to 1400 °C for TiAl and <1000 °C for Ti 3Al in TACS. The change in peak temperature between the activation routes most likely corresponds to absorption of microwave energy at the reaction front. In MACS, product Ti 3Al was produced in single-phase using an off-stoichiometric reacted mixture of 86:14 Ti:Al (w/w), while TiAl with a minimized Ti 3Al secondary phase was obtained using a reactant mixture of 60:40 Ti:Al (w/w). The level of Ti 3Al present in TiAl was minimized in MACS and could be controlled by changing the reactant mixture. Ti 3Al content in TiAl was higher and consistent for different reactant mixtures in TACS samples. The present results obtained using the simple MACS process are comparable to those obtained using the TACS (at high heating rates) and the field activated combustion synthesis (FACS). An increase in volume and porosity was noted in all samples by all processing routes and was attributed to the intrinsic volume change associated with the reaction and the vaporization of persistent volatile impurities.

Synthesis of tungsten carbide–cobalt composites by the field-activated combustion synthesis method

The activation of self-propagating combustion reactions in the tungsten carbide–cobalt (WC-Co) system was achieved by using an electric field. Self-sustaining combustion in elemental reactants with composition corresponding to WC- x wt.% Co (0 ≤ x ≤ 10) composites can be activated only when the imposed field is above a threshold value. The nature of combustion products depended on the magnitude of the field. The relative strength of W 2C diffraction peaks in XRD diagram decreased when moving from the reactants region to the final products one or increasing electric field on the reactants. The effect of the relative density of the reactant compacts on the synthesis of WC-Co composites was also investigated. Finally, the reaction mechanism of WC-Co composites was proposed. X-ray and microscopic analysis of the quenched combustion front suggests that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. Molten Co redounds the transformation from W 2C to WC phase.

Mechanistic investigation of the field-activated combustion synthesis of tungsten carbide with or without cobalt added

The activation of self-propagating combustion reactions in the system of tungsten and carbon and its composites with cobalt as additive was achieved using an electric field. The reaction mechanism of Field-Activated Combustion Synthesis (FACS) of tungsten carbide and its composites has been investigated using the quenching sample method. By turning off the electric field during FACS, a series of combustion products with different phase compositions has been obtained. Layer to layer X-ray and microscopic analyses of these combustion products across the quenched combustion front suggest that the synthesis of WC involves the solid-phase diffusion of carbon into a carbide layer. W 2C is the intermediate phase between WC and the reactants (W and C). A metallic additive produces liquid phase and accelerates the diffusion of the solid reactants (W and C); this facilitates the formation of W 2C and the transformation of W 2C to WC. Moreover, molten Co reacts with W and W 2C to form mixed compounds of type W xC yCo z.

Combustion synthesis of tungsten carbides under electric fieldI. Field activated combustion synthesis

The activation of self-propagating combustion reactions in the W–C system was achieved by using an electric field. Self-sustaining combustion in elemental reactants with the composition corresponding to WC can be activated only when the imposed field is above the threshold value of 1V cm −1. The nature of the combustion products depended on the magnitude of the field. The product consisted of two kinds of tungsten carbides, WC and W 2C. The effects of tungsten particle sizes and the relative densities of the reactant compacts on the synthesis of tungsten carbide were also investigated. Finally, the reaction mechanism of tungsten carbide was proposed. X-ray and microscopic analysis of the quenched combustion front suggested that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. W 2C is the intermediate phase between WC and reactants (W and C).

Effect of Ni and Co Additives on Phase Decomposition in TiB2–WB2 Solid Solutions Formed by Induction Field Activated Combustion Synthesis

Solid solutions of TiB2–WB2 were densified and annealed simultaneously to cause the decomposition into the phases (Ti,W)B2 and (W,Ti)B2. Ni and Co were added to solid solutions formed by induction field activated combustion synthesis. The presence of these metals as additives markedly enhanced the kinetics of the subsequent decomposition process. With these additives, decomposition to the two phases occurred within minutes (360 s) in contrast to hours when the solutions did not include the additives. The phases resulting from decomposition, (Ti,W)B2 and (W,Ti)B2, were identified by X‐ray to have the hexagonal AlB2 and W2B5 structures, respectively. The precipitated phase, (W,Ti)B2, occurred as elongated grains with aspect ratios of as high as about 10 in samples containing Ni as the additive.

Mechanistic investigation of the field-activated combustion synthesis (FACS) of tungsten carbide with or without nickel additive

The activation of self-propagating combustion reactions in the W-C system and its composite with Ni as additive was achieved by using an electric field. The reaction mechanisms of Field-Activated Combustion Synthesis (FACS) of tungsten carbide and its composite have been investigated by using sample-quenched method. Through turning off electric field during FACS process, a series of combustion products with different phase compositions have been obtained. Layer to layer X-ray and microscopic analyses of these combustion products across quenched combustion front suggested that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. W2C is the intermediate phase between WC and reactants (W and C). Metal additive produces liquid phase and accelerates the diffusion between solid reactants (W and C), which facilitates the formation of W2C and the transformation from W2C to WC phase. Moreover, melted Ni reacts with W and W2C to form mixed compounds of type W x C y M z .

Field activated combustion synthesis of the silicides of vanadium

The synthesis of vanadium silicides was investigated using the field-activated combustion synthesis technique. For all V–Si compounds, self-sustaining combustion reactions could be obtained when fields above a threshold value were imposed. Monophasic products were obtained only for the starting compositions V:Si=l:2 and V:Si=5:3. For all other compositions the reaction produced a polyphasic mixture. No significant variation of phase composition was observed with an increase in field strength. In contrast with other systems, the field was seen to have a weak effect on the combustion macrokinetic parameters. This was interpreted on the basis of the large electrical conductivity of the reaction products, driving a large part of the electric flux away from the reaction front. The reaction mechanism was investigated through the use of quenching experiments. Only the VSi 2 and V 5Si 3 phases were observed in the leading edge of the combustion front, with the other phases forming from solid–solid interactions in the afterburn. These results have been compared with observations relative to the mechanism of silicides formation in isothermal solid–solid and solid–liquid diffusion couples.

Field-activated combustion synthesis of Ta–Si intermetallic compounds

The synthesis of silicides of tantalum (TaSi2, Ta5Si3, Ta2Si, and Ta3Si) by field-activated combustion synthesis was investigated. The phases TaSi2, Ta5Si3, and Ta2Si can be synthesized in relatively pure form through field activation. The first compound requires a threshold field of 6 V cm−1 to initiate a combustion wave. The second and third compounds can be synthesized with all field values (0 &lt; E &lt; 20 V cm−1), but require considerable ignition time when E = 0. The stoichiometry Ta3Si does not sustain a combusion front unless E ≥ 20 V cm−1. However, the product is not the desired single phase. Wave velocities in the synthesis of the Si-rich phase, TaSi2, are considerably lower than those of the other two and showed a much weaker dependence on the field. The mechanism of phase formation for TaSi2 and Ta5Si3 was investigated from frozen wave analysis. No intermediate phase forms in the TaSi2 case, while in the case of Ta5Si3, TaSi2 is present as an intermediate phase. Additional investigations on the mechanism of silicide formation included isothermal solid–solid diffusion-couple and solid–liquid experiments.

Mechanistic investigation of electric field-activated self-propagating reactions: experimental and modeling studies

The mechanism of electric field-activated self-propagating reactions is investigated using the combustion front quenching technique. In particular, previously published experimental results obtained through the field-activated combustion synthesis (FACS) of β-SiC, TaC, Ti 3Al and B 4C–TiB 2 are re-examined and compared. Pre-combustion and combustion stages involved during synthesis wave propagation are postulated for all systems. Subsequently, modeling results aimed at simulating the process where an electric field-activated, self-propagating reaction takes place are presented. In particular, a one-dimensional model of FACS technique is developed to simulate the rapid quenching of the reaction during its progress as the applied field is turned off. A rate expression which accounts for the influence of temperature, particle size, compaction density, reactant stoichiometry, and inert content is included in the model.

Effect of current application methods on the preparation of Fe–Al intermetallic compounds by field-activated combustion synthesis

Pure FeAl intermetallic compound was prepared by the field-activated combustion synthesis (FACS) process. Effects of compaction pressure (150, 250, and 350 MPa) and particle size (−25, −80, and −325 mesh) on the characteristics of products were investigated. As the compaction pressure and electric field increased, the combustion temperature and velocity increased. The influences of different current application methods on the reaction in this system were investigated. In the absence of an electric field, the reaction could not sustain a steady combustion wave and was not completed. The reaction products were characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy to determine the structures and compositions.