Self-propagating High-temperature Reaction Research Articles

Mechanochemical ignition of self-propagating reactions in equimolar Al-Ni powder mixtures and multilayers.

This work addresses a long standing question in the field of mechanochemistry, namely the role of mesostructure in the initiation of self-propagating high-temperature reactions in exothermic chemical systems, commonly referred to as ignition. In an attempt to find robust evidence in this regard, we compare the ignition behaviour of equimolar Al-Ni powder mixtures and equimolar Al-Ni multilayers. To achieve the best possible control of experimental conditions and allowing high reproducibility, we used elemental powders sieved in the range between 20 μm and 44 μm, and multilayers with bi-layer thickness between 10 nm and 800 nm. We carried out systematic ball milling experiments involving pristine powder mixtures and multilayers as well as a mix of pristine material and material prone to ignition suitably prepared. Experimental findings suggest that pristine powder mixtures and multilayers with bi-layer thickness of 240 nm have analogous ignition behaviour. Along the same lines, data suggest that pristine powder mixtures undergo ignition when they attain a mesostructure similar to that of multilayers with bi-layer thickness of 10 nm.

Ultrafast formation of single phase B2 AlCoCrFeNi high entropy alloy films by reactive Ni/Al multilayers as heat source

High entropy alloy films of AlCoCrFeNi B2-ordered structure are formed during an ultrafast heating process by reactive Ni/Al multilayers. The self-propagating high-temperature reaction occurring in reactive Ni/Al multilayers after ignition represents an ultrafast heat source which is used for the transformation of a thin films Al/CoFe/CrNi multilayer structure into a single-phase high entropy alloy film. The materials design of the combined multilayers thus determines the phase formation.Conventional rapid thermal annealing transforms the multilayer into a film with multiple equilibrium phases. Ultrafast combustion synthesis produces films with ultrafine-grained single-phase B2-ordered compound alloy. The heating rates during the combustion synthesis are in the order of one million K/s, much higher than those of the rapid thermal annealing, which is about 7 K/s. The results are compared with differential scanning calorimetry experiments with heating rates ranging from about 100 K/s up to 25000 K/s. It is shown that the heating rate clearly determines the phase formation in the multilayers. The rapid kinetics of the combustion prevents long-range diffusion and promotes the run-away transformation. Thus, multilayer combustion synthesis using reactive Ni/Al multilayers as heat source represents a new pathway for the fabrication of single phase high-entropy alloy films.

Aluminum–nickel combustion for joining lunar regolith ceramic tiles

Combustion-based methods are attractive for space manufacturing because the use of chemical energy stored in reactants dramatically decreases the required external energy input. Recently, a sintering technique has been developed for converting lunar/Martian regolith into ceramic tiles, but it is unclear how to build a reliable launch/landing pad from these tiles with small amounts of energy and materials. Here we explored the feasibility of joining the regolith tiles using self-propagating high-temperature reactions between two metal powders. Combustion of an aluminum/nickel mixture placed in a gap between two tiles, made of JSC-1A lunar regolith simulant, was studied in an argon environment at 1 kPa pressure. Stable propagation of the combustion front was observed over the tested range of distances between the tiles, 2–8 mm. The front velocity increases with increasing the distance between the tiles. Joining of the tiles was achieved in several experiments and improvement with increasing the tile thickness was observed. Thermophysical properties of the tiles, the reactive mixture, and the reaction product were determined using differential scanning calorimetry and laser flash analysis. A model for steady propagation of the combustion wave over a condensed substance layer placed between two inert media was applied for analysis of the investigated system. Testing the model has resulted in reasonable agreement between the experimental and modeling dependencies. Both experimental and modeling results indicate a narrow quenching distance in the investigated system, which implies that a small amount of the reactive mixture would be required for sintering regolith tiles on the Moon.

Kinetics of SHS reactions: A review

The current state of chemical kinetics for self-propagating high-temperature non-catalytic reactions has been reviewed for results over the past 50 years. Five different characterization techniques are primarily considered: differential thermal analysis (DTA), electrothermal explosion (ETE), electrothermography (ET), combustion velocity/temperature analyses (Merzhanov–Khaikin and Boddington–Laye approaches), and other advanced in-situ diagnostics, including time-resolved X-ray diffraction (TRXRD). Based on the summary of results thus far, recommendations are given for the future of SHS kinetic research.

Refractory phases synthesis at the surface microalloying using a wide aperture electron beam

The experimental results prove the ability to realize technology of chemical heat treatment of some materials by surface microalloying using a wide-aperture low-energy high-current electron beam. Such layers were produced due to initiating exothermic chemical self-propagating high-temperature reactions in the thermal explosion mode between the base and the thin film covered on the base. New phase compounds in reaction products were found.

Inverse Model for the Solute Micro-Field Formation during Self-Propagating High Temperature Reaction

AbstractA new thermodynamic description for the self-propagating high temperature synthesis (SHS - reaction) is presented in the “inverse” version. This description is worked out for the diffusion barrier, thickness of which is at the limit, i.e. its value is infinitesimally small. The solution to the diffusion equation delivered in the description can be easily extended for the diffusion barrier of a greater thickness. The Ni/Al multi-layers system is treated as a virtual eutectic alloy solidifying with the rate equal to that involved by the self-propagating reaction. It is suggested to inverse the curves obtained for solidification in order to characterize the melting completed by the formation of the AlNi - intermetallic phase required in the self-propagating synthesis.

Self-propagating high-temperature synthesis of boron phosphide

A new method of producing boron phosphide (BP) submicron powders by a self-propagating high-temperature reaction between boron phosphate and magnesium in the presence of an inert diluent (sodium chloride) has been proposed. Bulk polycrystalline BP with microhardness of H V = 28(2) GPa has been prepared by sintering the above powders at 7.7 GPa and 2600 K.

A Systematic Investigation to Obtain Physical Assets on the Moon through Self-propagating High-temperature Reactions

Future space missions on the Moon, Mars and near Earth asteroids, etc., are expected to be strongly facilitated and time extended by the possibility of “In-Situ Fabrication and Repair” (ISFR) the required equipments and infrastructures. In addition, the combination of the latter approach with the “In-Situ Resources Utilization” (ISRU) paradigm contributes to overcome drawbacks related to the transportation of the needed material from the Earth. In this regard, various technologies have been recently proposed with the aim of developing suitable structures to be placed on the Moon surface for the protection against cosmic rays, solar wind, meteoroids, etc. Specifically, the possibility of exploiting combustion synthesistype reactions using Lunar resources for the fabrication of ceramic-based products was considered. Along these lines, by taking advantage of the fact that Lunar soil contains up to 20 wt.% of ilmenite (FeTiO3), the highly exothermic thermite reduction of the latter oxide with Al is systematically investigated in this work. A self-propagating (SHS) behavior is displayed only above a certain Al/FeTiO3 molar ratio (0.9). In addition, as the amount of Al in the mixture is increased, the reactive process proceeds faster and the combustion temperature becomes higher, due to the increased system exothermicity. Correspondingly, the maximum amount of Lunar regolith to be possibly reacted with additional FeTiO3 and Al is identified. The obtained products (a mixture of Al-, Ti-, Mg-, and Ca-oxides, and metallic phases) exhibit satisfactory compressive strength properties (≥ 25.8 MPa) that make them promising as construction materials. Parabolic flight experiments evidenced that SHS process dynamics and product characteristics are barely affected by gravity. The obtained findings allow us to conclude that the optimal conditions identified during terrestrial experiments are still valid for in-situ applications in Lunar environment.

In situ observation of self-propagating high temperature syntheses of Ta5Si3, Ti5Si3 and TiB2 by proton and X-ray radiography

Self-propagating high temperature reactions of tantalum and titanium with silicon and titanium with boron were studied using proton and X-ray radiography, small-angle neutron scattering, neutron time-of-flight, X-ray and neutron diffraction, dilatometry and video recording. We show that radiography allows the observation of the propagation of the flame front in all investigated systems and the determination of the widths of the burning zones. X-ray and neutron diffraction showed that the reaction products consisted of ≈90 wt% of the main phase and one or two secondary phases. For the reaction 5Ti + 3Si → Ti5Si3 flame front velocities of 7.1(3)–34.2(4) mm/s were determined depending on the concentration of a retardant added to the starting material, the geometry and the green density of the samples. The flame front width was determined to be 1.17(4)–1.82(8) mm and depends exponentially on the flame front velocity. Similarly, for the reaction Ti + 2B → TiB2 flame front velocities of 15(2)–26.6(4) mm/s were determined, while for a 5Ta + 3Si → Ta5Si3 reaction the flame front velocity was 7.05(4) mm/s. The micro structure of the product phase Ta5Si3 shows no texture. From SANS measurements the dependence of the specific surface of the product phase on the particle sizes of the starting materials was studied.

Self-propagating high-temperature reactions for the fabrication of Lunar and Martian physical assets

The development of a new process potentially useful for future manned Lunar and/or Martian space missions in the framework of the so-called ISRU (In-Situ Resource Utilization) and ISFR (In-Situ Fabrication and Repair) concepts is described and discussed in this work. This process involves the fabrication of physical assets by self-propagating high temperature synthesis (SHS) for construction applications in Lunar and Martian environments starting from different Lunar or Martian regolith simulants and aluminum, as reducing agent. In addition, although Moon and Mars already contain ilmenite (FeTiO 3) and iron oxides, respectively, the latter ones are also added to the initial mixtures to promote suitable SHS reactions. A complete scheme for the fabrication of physical assets to be used as protection against solar rays, solar wind and meteoroids, where all required stages are indicated, is finally proposed in the framework of a recently filed patent.

Self-propagating high-temperature synthesis of nanostructured titanium aluminide alloys with varying porosity

Metallic foams were synthesized by means of a self-propagating high-temperature reaction producing a highly porous solid metal alloy with customizable material properties. Nanoscale aluminum (nAl) and nanoscale titanium (nTi) particles were mixed with either nanoscale aluminum passivated with a gasifying agent such as perfluoroalkyl carboxylic acid (C 13F 27COOH) or polytetrafluoroethylene (Teflon) (C 2F 4) n particles and pressed into pellets. These samples were then ignited with a laser producing a reaction product composed of an AlTi alloy that has a highly porous structure. Objectives of this study were to use combustion synthesis to create a functionally graded porous AlTi alloy and identify correlations between the product microstructure and parameters such as type and amount of gasifying agent present in the reactants. Photographic data allowed interpretation of the reaction propagation while characterization of the final product indicated porosity and morphology. Results from this study may have applications in biomaterial development by tailoring porosity throughout the matrix.

The optimization of the procedure for the preparation of nanotubes under self-propagating high-temperature synthesis conditions: The influence of catalysts and reagents

The conditions of self-propagating high-temperature synthesis in a powdered sodium carbonate-magnesium mixture optimum for the preparation of the largest amount of carbon nanotubes (CNTs) were studied. The yield of nanotubes and nanofibers was weakly sensitive to the selection of the amount of a catalyst. The yield of nanotubes ceased to increase noticeably at a relative catalyst content higher than 10 wt %. For the first time, the self-propagating high-temperature reaction was performed with an iron-nickel catalyst in a limestone-magnesium mixture, that is, with the cheapest powdered reagent containing carbon. The reaction produced a small number of CNTs and nanofibers; cubic crystals, predominantly of MgO, were also observed.

Fabrication of Metal/Intermetallic Compound Laminate composites by Thin Foil Hot Press Process

Thin foil hot press process was used to fabricate metal/intermetallic compound laminate composites to induce self-propagating high-temperature (SHS) reaction between different pure metal sheets. In the present study, Ni/Ni-aluminide and Ti/Ti-aluminide laminate composites were fabricated through diffusion bonding, reaction synthesis and post-heat treatment of alternatively stacked pure Ni/Al and Ti/Al foils, respectively. Thick intermetallic layers of NiAl and TiAl3 were formed by a self-propagating high-temperature synthesis (SHS) reaction, and thin continuous layers of Ni3Al and TiAl were formed by a solid-state diffusion. Also, Ni/Ni-aluminide//Ti/Ti-aluminide laminate composite, considered as a functionally gradient material, was manufactured from stacked foils of pure Ni, Ti and Al in order of Ni/Al/Ni/.../Ni/Al /Ti/.../Ti/Al/Ti. Nb/Nb-aluminide laminate composite was manufactured with pure Nb and Al multilayered foils, consisting of fine Nb/Nb3Al/Nb2Al/NbAl3 layer structure.

Interaction of a ceramic material based on hematite with molten iron produced by an aluminothermic reaction

The problems regarding the use of hematite-based ceramics as sacrificial ceramic materials are considered. The interaction of hematite with an iron melt produced by a self-propagating high-temperature reaction is investigated. It is revealed that the hematite-based ceramic material containing activating additives of 3d element oxides is wet by the iron melt. This leads to an intense redox reaction at the iron-ceramic interface.

Laser-induced combustion synthesis of Zr–Ti–Al–Ni amorphous-based alloys

Bulk metallic glasses, like many crystalline intermetallics, have large negative enthalpy of mixing among the major constituent elements, and hence are potential candidates of self-propagating high-temperature reaction systems. Based on this characteristic, the laser-induced combustion synthesis (LCS) technique has been applied to fabricate amorphous-containing alloys. In the present paper, we report the LCS of the Zr–Ti–Al–Ni alloys. A series of Zr–Ti–Al–Ni alloys is designed and synthesized by LCS. The LCS products mainly consist of intermetallic phases, but in Zr55Ti10.8Al17.1Ni17.1 and Zr50Ti21.6Al14.2Ni14.2 amorphous phases are found. The hardness and tribology characteristics are closely related to the phase contents. The amorphous phases, ductile and soft, lower the hardness and increase the friction coefficient of the LCS samples.

Microstructural correlations between reaction medium and combustion wave propagation in heterogeneous systems

The combustion synthesis (CS) of materials is an advanced approach in powder metallurgy. Our previous studies of various CS systems have shown that at small time scales ( 10 - 3 – 10 - 4 s ), the behavior of self-propagating high-temperature reaction waves in heterogeneous mixtures follows two modes, classified according to their microstructures: quasihomogeneous and scintillating reaction waves. The latter is a novel phenomenon where the reaction propagates in a form of short and high-temperature flashes, so-called scintillations, which randomly arise in vicinity of the combustion front and promote its local movement. Using Ti–Si mixture as an example, this work is aimed to find quantitative relationship between reactant particle size and microstructural characteristics of the reaction front, including scintillation size and its distribution, as well as lifetime. It was shown that, for coarse powders ( > 40 μ m ) , specific number of Ti particles approximately equals the number of hot spots observed during the combustion process. This means that essentially every Ti particle forms a hot spot. However, for finer Ti particles, average hot spot is formed by several (3–5) such particles. Since size of scintillation decreases to a lesser extent than Ti particle size, the scintillation regime does not vanish when using finer Ti powders. These results confirm the general assumption of the relay-race model that the limiting factor of combustion wave propagation is heat transfer between reaction cells rather than reaction within the cell itself.

Reaction mechanism in an Al-TiO2-C system for producing in situ Al/(TiC + Al2O3) composite

In the last decade, production of particulate reinforced metal matrix composites (MMCs) through in situ reaction has attracted much attention due to its many advantages, e.g., low production costs, clean particle-matrix interfaces, over conventional methods [1–4]. Particulates used for reinforcement mainly include TiC [2–4], Al2O3 [1], TiB2 [1], etc. In a typical in situ reaction, one or more particles is formed through direct chemical reaction in the melt of the metallic matrix. The preparation of Al2O3/TiC ceramic composites through SHS (self-propagating high-temperature reaction) in the TiO2-C-Al system has been extensively studied recently [5–7], but little work has been conducted to prepare TiC and Al2O3 particle reinforced Al matrix composites through in situ reaction of the three starting materials. Aiming to providing a basis for the production of Al/(TiC + Al2O3) composites, this letter mainly deals with the reaction process in an Al-TiO2-C system by using DSC, XRD and TEM. A mixture of 15 g Al, 5 g TiO2, and 0.75 g C powders (99.0% purity or better), i.e., 72.3 wt%Al-14.1 wt% TiO2-3.6 wt% C, was sealed in a vacuum stainless steel vial together with steel balls ten times the weight of the powder mixture, and then ball milled for 9 h using a QM-1SP2 planetary-type ball mill with a planetary rotation speed of 450 rev min−1. About 0.35 g of ballmilled powders were heated under argon in a Netzsch 404 differential scanning calorimeter (DSC) at a rate of 20 ◦C/min, from 25 ◦C to the desired temperatures and then cooled to the ambient temperature. The heated powders were analyzed using a Rigaku D/max-rB Xray diffractometer (XRD) with Cu Kα radiation or a H800 transmission electron microscope (TEM) to detect phases formed during heating. Fig. 1 presents the DSC traces of the powder mixture heated to 1300 ◦C and cooled to ambient temperature, both at a rate of 20 ◦C/min. An endothermic peak during heating, with peak temperature at 667.8 ◦C, and an exothermic peak during cooling are visible. They are obviously caused by the melting and solidification of Al, respectively. No other thermal effects could be observed during cooling besides the solidification of Al, indicating that phases formed during heating are thermodynamically stable. In the heating curve, another endothermic peak with peak temperature at 886 ◦C and

Combination of mechanochemical activation and self-propagating behaviour for the synthesis of Ti aluminides

The synthesis of TiAl 3 is not easy, since conventional techniques based on thermal treatments provide mixtures of intermetallics, due to the particular thermodynamic stability of the TiAl and Ti 3Al phases. The synthesis of the aluminium-rich intermetallic TiAl 3 becomes, however, feasible when non-equilibrium processing techniques are properly combined. The methodology we set up for the preparation of the TiAl 3 consists of two different stages. During the first stage, reactant powders at the correct composition undergo mechanochemical activation inside a ball mill under inert atmosphere. A self-propagating high-temperature reaction is then induced on the pre-treated powders through thermal ignition. At the end of the reaction, the TiAl 3 equilibrium compound is obtained with impurities below 5 wt.%.

A Review on Combustion Synthesis of Novel Materials: Recent Experimental and Modeling Results

AbstractFor Abstract see ChemInform Abstract in Full Text.

Self-Propagating High-Temperature Synthesis of Intermetallic Compounds: A Computer Simulation Approach to the Chemical Mechanisms

The self-propagating high-temperature reaction in nickel−aluminum powder mixtures has been investigated by computer simulated experiments using a monodimensional model. The model describes the overall reaction mechanism considering several elemental steps such as melting of the reactant Al, diffusion-controlled dissolution of solid nickel into the molten pool, (possible) melting of the Ni reactant, precipitation and melting of the compound, and precipitation of the eutectic mixture of Al + AlNi. Melting of pure aluminum and nickel and eutectic crystallization are considered in terms of thermal balance only, whereas an explicit diffusion controlled kinetic for the dissolution of nickel in molten aluminum was accounted for. The results show that the reaction propagates under almost steady conditions with constant wave velocity and peak temperature, close to experimentally reported values, when suitable values for the thermal conductivity of the green compact were set.