SHS Reaction Research Articles

SHS of ultrafine and nanosized MoSi2 powders

Ultrafine MoSi2 powders were prepared by SHS reaction from the elements (under an Ar pressure of 5 atm) followed by optimized diminution process and chemical dispersion (hot alkali etching). Raw SHS products (cakes) were found to contain tetragonal MoSi2 and an admixture of Mo4.8Si3C0.6. Chemical dispersion of ground product yielded MoSi2 powders in the form of porous agglomerates. The precipitates containing 50% MoSi2 and 50% K2MoO4 were separated from etching solutions. The size of MoSi2 particles was below 100 nm.

Aluminothermic SHS: The effect of silica sol added as a binder

Aluminothermic combustion of “Al-metal oxide” blends compacted in the presence of silica sol added as a binder was found to proceed at lower combustion temperatures and burning velocities as compared to those in the absence of sol. The results show that the addition of silica sols to green mixtures can be used as an effective approach to performing aluminothermic SHS reactions yielding new refractory composite materials at moderate temperatures of 1000–1600dgC.

Thermally coupled SHS reactions

Thermally coupled SHS reactions

In-situ formation of cast granules in thermit-type SHS reactions

Addressed is the phenomenology of combustion in thin layers of thermit-type SHS compositions. The combustion was found to yield cast granules of combustion products whose size could be affected by variation in the green layer thickness and amount of added neutral diluent. The process looks promising for fabrication of cast granules with a particle size of 0.2–4.0 mm.

Fabrication of Ni-Al Intermetallic Compound Layer on Spheroidal Graphite Cast Iron Substrate by SHS Reaction

Fabrication of Ni-Al Intermetallic Compound Layer on Spheroidal Graphite Cast Iron Substrate by SHS Reaction

Effect of composition of starting materials of Mo–Si on self-propagating high temperature SHS reaction

An experimental study of the self-propagating high temperature synthesis of Mo–Si alloys was conducted from elemental powder compacts. Test specimens with seven compositions, including Mo/Si = 1∶1·25, 1∶1·50, 1∶1·75, 1∶2·00, 1∶2·25, 1∶2·50 and 1∶2·75 respectively, were employed. Experimental evidence showed that a combustion wave featuring a spinning reaction zone can be observed. When the powder compacts are from Mo∶1·25Si to Mo∶1·75Si, the combustion temperature and the propagation velocity of combustion wave increase with increasing silicon in the sample, and the combustion products are composed of MoSi2, Mo5Si3 and Mo. However, when the powder compacts are from Mo∶2·25Si to Mo∶2·75Si, the combustion temperature and the propagation velocity decrease rapidly as the silicon in the compact increases, and the combustion products are composed of MoSi2 and Si. The sample with Mo/Si = 1∶2·00 possesses the highest combustion temperature (1628·9 K) and propagation velocity (3·13 mm s−1). A single-phase MoSi2 is synthesised by the Mo/Si = 1∶2·00 sample.

SHS in the Al-SiO2-C system: The effect of additives

Explored was the effect of various additives on SHS reactions in the Al-SiO2-C system. Dilution of basic green mixtures with alumina-containing additives (bentonite, kaolin, and industrial aluminum oxide) was found to decelerate SHS reactions and affect the composition/properties of combustion products. It has been shown that SHS reactions in the system under study can be activated upon addition of heat-generating additives such as carbonates, oxides, and boron- or fluorine-containing compounds. The best results were obtained for fluorine-containing additives. Discussed was also the effect of additives on phase compositions of synthesized materials.

SHS reactions in nanosized multilayers: Analytical model versus numerical one

SHS reactions in solid nanosized multilayers yielding a single-phase product were modeled numerically and analytically. Scaling behavior of the product “burning velocity × multilayer period” was predicted. The effect of mean vacancy free pathlength and initial vacancy supersaturation on the SHS rate was investigated numerically. Within a wide range of process parameters, the front velocity appears to be inversely proportional to the square root of mean vacancy free pathlength.

The mechanism of combustion synthesis of (TiC xN y-TiB 2)/Ni from a Ni-Ti-C-BN system

The combustion reaction mechanism of the Ni-Ti-C-BN system was investigated by a combustion front quenching method. The results showed that the SHS reaction in the Ni-Ti-C-BN system starts with the formation of TiN x and TiB from the solid-state reaction between Ti and BN. Meanwhile, the solid-state reaction between Ni and Ti occurs at some locations to form Ni 3Ti. Subsequently, the resultant TiN x reacts with Ni to form the Ni-Ti compounds such as NiTi and Ni 3Ti. As the temperature increases further, Ni-Ti liquid appears between NiTi and Ni 3Ti, and some B atoms that diffused away from BN could dissolve into the Ni-Ti liquid to form TiB 2. The heat generated from these reactions promotes the further dissolution of the C, B and N atoms in the Ni-Ti liquid to form the Ni-Ti-B-N-C liquid. As the B, N and C atoms in the liquid become sufficiently supersaturated, precipitation of plenty of TiB 2 and TiC x N y grains occurs. In addition, the reasons for the prior formation of TiB 2 over TiC x N y and separated residence of the TiB 2 and TiC x N y grains in the microstructure were discussed.

Formation of porous structure in SHS products

224 1 Ever growing attention to the mechanism of pore formation in SHS reactions stems from the fact that SHS produced porous materials find their practical implementation in various filters, catalysts, and heat shields. According to the model suggested in [1, 2] for combustion in solid–gas systems, product porosity arises largely owing to a volume change during SHS reaction. Upon addition of blowing agents to green mixtures, SHS reactions can produce cermet foams in yields of up to 90% [3, 4].

Deposition of composite metallic coating onto Al through mechanical impregnation followed by thermal treatment

Investigated were the processes taking place in thermally treated Al plates mechanically impregnated with a mixture of metallic particles. Vibratory treatment of an Al surface in a powder mixture of metals was found to result in destruction of the surface Al2O3 film and formation of extensive physical contacts between the Al matrix and the metallic particles mechanically impregnated into the near-surface layer of Al. Subsequent thermal treatment was then used to launch chemical reaction yielding intermetallides within the impregnated layer. Performing SHS reactions yielding melted intermetallides in the coatings (or pellets) deposited (or placed) onto the surface of thus treated Al plates, one can obtain strong weld joining between solidified SHS products and Al. This approach can also be used for deposition of coatings with a desired composition onto Al substrates.

Advanced experimental facilities for SHS Studies and physico-chemical characterization of inorganic materials: An autoreview

Overviewed are new experimental methods for investigating the processes of phase formation during SHS. The first experiments using penetrating synchrotron radiation and energy dispersive detectors for different classes of complex inorganic materials were carried out in ESRF (Grenoble, France), LURE (Orsay, France) and Daresbury (UK). A new and very sensitive thermal imaging method (Thermal Imaging Technique, TIT) is based on continuous registration of the whole combustion process by using a highly sensitive IR camera and software developed by MIKRON Instruments Co. (USA) was also used for precise registration of combustion parameters. SHS was performed on different types of pure on doped complex inorganic materials in pellet and powder form in a range of dc magnetic field induction up to 20 T and electrical field strength up to ±220 kV/m. The dc magnetic field was applied during the reaction, supplied either by permanent magnet (transverse, 1.1 T) or by an electromagnet (longitudinal, up to 20 T). The dc electrical field was applied along direction of the combustion wave propagation. The combined processes of SHS and SLS (Selective Laser Sintering) of 3D articles for different powdered compositions was realized with optimal parameters of laser irradiation, under what SHS reactions proceeded in controlled regime.

Fe–Al phase formation around SHS reactions under isothermal conditions

This study investigates the phenomena preceding and accompanying the SHS reaction between Fe and Al elemental powders during sintering. SEM and XRD analysis were used to observe the mechanisms of formation of Fe–Al intermetallic phases. The analysis of sintered material just before the SHS reaction demonstrates that in addition to the well-known Fe 2Al 5 phase and the low-aluminum solid solution of iron, the high-aluminum phases FeAl 2 and FeAl 3 are formed. The kinetics of phase transformations under isothermal conditions were investigated by DSC using the JMA (Johnson–Mehl–Avrami) model. This approach allowed us to calculate Avrami coefficients, which characterize the speed and the manner of particular phase transformations.

Dense Mosi 2 produced by reactive flash sintering: Control of Mo/Si agglomerates prepared by high-energy ball milling

The objective of this work is to determine the influence of the agglomeration state of the MA mixture on the microstructure and the chemical composition of SPS end-products. In order to produce MoSi 2 with a microstructure and a density perfectly controlled via reactive sintering implying an SHS reaction, the characteristics of Mo/Si mechanically activated (MA) powder mixtures were investigated. Indeed, the MA powders have been characterized in terms of their surface specific area, size, phase composition and microstructure. The high-energy milling allows the formation of agglomerates (0.8 to 800 μm) composed of nanometric crystallites of molybdenum and silicon, as a consequence of a continuous fracture-welding process. This enhances the reactivity of powder mixtures due to an increase of interface number. The simultaneous synthesis–consolidation in one step could be performed using the spark plasma sintering (SPS) process. This study was performed on three agglomerate sizes: ∅ < 125 μm, ≤125 μm < 250 μm and ≥ 250 μm.

Structural and dielectric behavior of yNi1 − x Cd x Fe2O4 + (1 − y)Ba0.8Sr0.2TiO3 magnetoelectric composites prepared through SHS route

The structural and dielectric properties of SHS-produced yNi1 − xCdxFe2O4 + (1 − y)Ba0.8Sr0.2TiO3 (x = 0.2, 0.4, 0.6; y = 15, 30, 45%) magnetoelectric composites were characterized by XRD, SEM, and resistivity/dielectric measurements. SEM images reveal that SHS reaction can produce two pure phases simultaneously. The grown Cd-substituted nickel ferrite grains were well dispersed in a BST matrix. A decrease in resistivity with temperature shows the semiconducting nature of synthesized samples. The dielectric results demonstrated an attractive response of dielectric constant to frequency and temperature. The Curie temperature of about 480°C was observed in the Ni0.4Cd0.6Fe2O4 + Ba0.8Sr0.2TiO3 composite.

Interaction of SHS-produced melt with a Ti surface in microgravity conditions

In this paper, we report on the SHS reaction 3NiO + 5Al → 3NiAl + Al2O3 carried out aboard International Space Station (Flight Mission 18) with special emphasis on the effect of the composition of reaction products on the composition of coatings formed on a Ti substrate.

Effect of Sn addition on porosity and microstructure of products of self-propagating high temperature synthesis reaction of Al–Ti–C system

Effect of Sn addition on the self-propagating high temperature synthesis reaction has been investigated. The results show that Sn serves not only as a diluent but also as a reactant and reacts with Ti to form Ti6Sn5 phase. As the Sn content ranges from 6 to 15 wt-%, the combustion temperature of the system decreases and the porosity of the products is reduced, however, the size of TiC particulate decreases and the shape of TiC particulate has significantly changed in the SHS reaction products. Specially, when Sn content is over 12 wt-%, the shape of TiC particles gradually transforms from the multi-angular shape to the spherical shape. The in situ formed master alloy containing Al and Sn, as well as a large volume fraction of TiC particulates is successfully synthesised.

Synthesis of transition metal hydrides and a new process for production of refractory metal alloys: An autoreview

Briefly overviewed are the SHS-based processes developed at our lab for synthesis of metal hydrides and their alloys. More than 200 binary and complex hydrides have been synthesized by SHS reactions of metals in hydrogen. Radiation-induced synthesis of transition metal hydrides, termed Thermal-Radiation Synthesis (TRS), was suggested and used to prepare more than 50 different binary and complex hydrides. A phenomenon of Cold Synthesis (CS) of hydrides at room temperature from irradiated metal powders has been observed. Advance in these studies has led to elaboration of a new technique, termed hydride-cycle process, for fabrication of compact blocks of hydrides and refractory metal alloys. A wide range of binary and multicomponent alloys and their hydrides have been synthesized by this route. The developed processes are offered for industrial-scale implementation.

Kinetics of reactions in FeAl synthesis studied by the DTA technique and JMA model

The phenomena of Fe–Al intermetallic phase formations preceding the SHS reaction were investigated. The XRD analysis and SEM observations have shown that not only the Fe 2Al 5 phase and the solid solution of aluminum in iron, but also the FeAl 3 and FeAl 2 phases are formed before the start of the SHS reaction. The kinetics of the sintering process of elemental Fe and Al powders was investigated by the DTA technique. The kinetics of the phase transformations was determined at different temperatures with heating rates of 0.5–10 °C/min for non-isothermal conditions using the Johnson–Mehl–Avrami model (JMA). The activation energy of specific phase formations was determined by the Kissinger method. Moreover, the Avrami coefficients, describing the rate and the character of specific phase transformations were calculated. The influence of the heating rate of the powder mixture on the rate of phase transformation was shown. It was found that a heating rate lower than 2.5 °C/min yields the full sequence of phase transformations: Fe + Al → FeAl 3 → Fe 2Al 5 → FeAl 2 → FeAl, compatible with the binary phase diagram of Fe–Al.

Effect of batch pelletizing on a course of SHS reactions: An overview

Such a simple technological trick as pelletizing (granulating) green powder mixtures (popular in conventional powder technology) has seldom been used in SHS technology. We have demonstrated that pelletizing green powder mixtures can markedly affect the parameters of various SHS reactions (SHS in a mode of infiltration-mediated combustion, SHS with a reduction stage, combustion of thermit mixtures, SHS in aluminum melt) and properties of their products.