Porous Ni-Al intermetallics are attractive materials for high-temperature applications due to their excellent oxidation resistance and thermal stability. However, the high cost of nickel motivates the development of more economical porous intermetallics produced by combustion synthesis. This work establishes the processing window for the combustion synthesis of macroporous Ni–Fe–Al alloys by systematically investigating the effect of replacing Ni with Fe while maintaining a constant aluminum content. Stable, self-propagating combustion, producing homogeneous porous alloys, was achieved for Fe substitution levels up to 80%. Without preheating, self-propagating combustion was possible only up to 9% Fe substitution. Increasing Fe content beyond this required progressively higher initial temperatures to compensate for reduced reaction exothermicity and maintain stable combustion. X-ray diffraction revealed a gradual transition from Ni–Al to Fe–Al intermetallic phases with increasing Fe substitution, and no ternary phases were detected. The average scaffold strut size ranged from 0.25 to 2.21 mm, depending on alloy composition and synthesis conditions. For a fixed alloy composition, the scaffold strut size could be tailored from 0.26 to 0.57 mm by adjusting the initial reaction temperature. These results define the compositional and thermal conditions required for the combustion synthesis of porous Ni–Fe–Al intermetallics and demonstrate a practical strategy for producing cost-effective porous materials with controllable structures for high-temperature applications.

This study reports on the fabrication of porous conductive TiFeSi2/Al2O3 ceramic-matrix composites via selfpropagating high-temperature synthesis (SHS, also known as combustion synthesis) using low-cost natural ilmenite and quartz minerals. Investigation of the reaction kinetics and phase formation revealed that silicon (Si) and carbon (C) concentrations are critical parameters for tailoring the microstructure and functional performance. We demonstrate that the incorporation of C suppresses the coalescence of the intermetallic phase during combustion, leading to structural refinement; the average particle size of the TiFeSi2 phase is reduced from 4.1 to 1.3 mu m, promoting the formation of a dense, highly connected percolation network. This refinement results in a tunable electrical resistivity (0.15-1.3 Omega & sdot;cm) and enhanced compressive strength (up to 23.4 MPa). Furthermore, the application of chemical-furnace-assisted combustion synthesis (CFACS) that combines SHS with a specialized chemical furnace effectively mitigates thermal gradients during synthesis (reducing the central-to-peripheral temperature difference from 320 to 70 degrees C), thereby ensuring structural homogeneity and minimizing residual stresses. The preliminary operational stability of electrical heating elements made from the synthesized composites was demonstrated for 4 h at temperatures up to 800 degrees C. These findings provide a fundamental basis for engineering high-performance, cost-effective porous materials for Joule-heated catalytic systems and advanced heating applications.

Spherical powders based on Ni-Al intermetallic compounds are of significant importance for the additive manufacturing of high-temperature structures used in the energy and aerospace industries. This article presents a novel, energy-efficient approach to producing finely dispersed (<= 63 mu m) rounded granules of Ni-(13.6-17.0)wt%Al alloys. These alloys consist of L1(2)Ni(3)Al, B2NiAl, and L1(0)NiAl phases, and are synthesized via combustion synthesis in a Ni-Al reaction mixture with CaCO3 and Ca(OH)(2) as modifying additives. The synthesis and spheroidization of the materials occur simultaneously within the reaction wave. We have demonstrated that the formation of the granules is a consequence of the capillary fusion of the Ni-Al melts into isolated droplets, followed by rapid crystallization within the pulsating hot-spot combustion. This process occurs over a period of approximately 10-100 ms. The effect of the additives on the reaction temperature and product morphology was investigated. Compared to combustion products without additives, rounded granules synthesized with CaCO3 and Ca(OH)(2) exhibit increased flowability and a reduced concentration of O and N impurities. Their characteristics are similar to those of spherical NiAl-Cr-Co-Hf powder, which was previously produced through a two-step process: combustion synthesis and plasma spheroidization.

The widespread use of flat electric heaters in equipment and household appliances requires the search for simpler and cheaper technologies for their production. This paper proposes a method for producing an electrically conductive coating (NiAl) and an electrically insulating layer (glass ceramics) in one stage: self-propagating high-temperature synthesis in a thermally coupled (Ni + Al)/(PbO2 + B + Al2O3 + glass) powder mixture. The combustion wave front propagation process is investigated, and the effect of layer thickness along with the ratio of the PbO2 + B + Al2O3 + glass powder mixture components on the wave front propagation velocity and the wave front temperature is described. It is revealed that the wave front of the exothermic process expands when a PbO2 + B mixture is added to the bottom layer. The addition of this mixture makes it possible to reduce the NiAl layer thickness and ensure the formation of a uniform dielectric coating. The phase composition and microstructure of the coating are studied. Optimal ratios are determined for layer thicknesses and the composition of the powder mixture of the layers. The fundamental possibility of forming an electrically conductive and dielectric coating by the method of self-propagating high-temperature synthesis in one stage is shown.

Magnesium aluminate spinel (MAS) containing a carbon impurity was prepared by self-propagating high-temperature synthesis (SHS). Periclase MgO, alumina Al2O3, fuel Al powder, 2 wt% boron, and oxidizer Mg (NO3)2 & sdot;6H2O were used as starting reagents for MAS synthesis. The phase composition of original boron and synthesized products, the structure and surface morphology of the samples were investigated. EDS analysis and elemental mapping confirmed that the impurity detected as crystals formed during the synthesis was carbon. IR spectroscopy showed that the carbon detected in the synthesized product has a diamond-like lattice similar to that of detonation diamonds. The analysis of gases evolved during combustion was performed. A proton-boron reaction is shown to take place during fast self-propagating high-temperature synthesis of spinel. The probable mechanisms of carbon formation in the synthesis products are proposed using the experimental data.

The composite material 2(CrxTi1-x)AlC was obtained using the method of self-propagating high-temperature synthesis. The material can be used to produce heat-resistant products and coatings. The synthesis is organized as a layered structure from a mixture of powders Ti + C/Crx + Ti1−x + Al + C/Ti + C. Where the mixture Ti + C is a heat donor and the mixture Crx + Ti1−x + Al + C is acceptor. By changing the ratio of layer thicknesses, it is possible to control the temperature, the propagation velocity of the front, the phase composition and the microstructure of the material. A material containing a large amount of the target phase Cr2AlC has been synthesized. The thermal-oxidative stability of such a material is maximal. Adding titanium to the Cr + Al + C mixture reduces it. By this method, two products can be synthesized at once: TiC and Cr2AlC.

The passage of a high-temperature synthesis wave through a perforated metal plate mounted inside a cylindrical sample of a Ni + Al powder mixture was studied experimentally and theoretically. Copper and steel plates of different thickness were used. The propagation of the exothermic reaction front through a hole in the barrier was investigated for different thermophysical characteristics of the plate and different geometric dimensions of the hole. The minimum critical diameter of the hole required for the propagation of the combustion wave in the sample was determined as a function of plate parameters.

A wide range of capillary processes participates in the structural transformation of the reacting metal powders during the combustion synthesis of intermetallics. Using additives that decompose in the com-bustion wave allows controlling the formation and interaction of melts that open new opportunities for direct manufacturing of materials with desired shape and structure, which are ready for applications without additional processing steps. This paper studies the combustion mechanism of Ni + Al powder mix-tures (average size of particles 10 mu m) doped with CaCO3. The methods of high-speed imaging, pyromet-ric temperature measurements, and reaction quenching were used. It was found that CaCO3 decomposes in the preheat zone of the combustion wave resulting in the fluidization of the powder mixture. In the reaction zone of the combustion wave, an ensemble of moving microdroplets with a diameter of 0.1- 0.2 mm forms. Some microdroplets drift in a preheat zone of the combustion wave, which stimulates the fluidization process. The microdroplets ultimately form larger droplets with a diameter of up to 2 mm that crystallize and form spherical struts of the gas-permeable scaffold of the combustion products. Syn-thesis of intermetallic compounds couples with the formation of calcium aluminates, which affects the processes of reaction coalescence and the structure of combustion products. For the CaCO3 concentra-tion below 5-8%, the combustion products represent strong porous scaffolds consisting of tightly fused spherical-like intermetallic struts forming a net of mm-sized interconnected pore channels. Brittle scaf-folds comprised of detached intermetallic granules with a diameter of 25-200 mu m separated by oxide layers form if CaCO3 concentration exceeds 14%. The washing-out of the oxide phases with 5% hydrochlo-ric acid allows for obtaining the powder of the remaining granules. The effect of self-fluidization on the structure of the synthesized alloys has been discussed in detail.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Исследован механизм структурирования газопроницаемых Ni-Al сплавов при самораспространяющемся высокотемпературном синтезе (СВС) в порошковых смесях Ni + Al с добавкой CaCO3, которая позволяет формировать псевдоожиженное состояние реакционной смеси в зоне волны горения. С использованием методов скоростной видеосъемки, динамической термометрии и закалки реакциипоказано, что трансформация реакционной среды в зоне волны горения происходит с участием высокоподвижных микрокапель реагирующих расплавов Ni и Al (диаметром ∼ 0,1-0,2 мм) и сопровождается широким комплексом капиллярных процессов: (1) формирование капель в процессе реакционной коалесценции расплавов; (2) поглощение частиц порошковой смеси движущимися каплями; (3) движение расплавов на поверхности формирующихся капель; (4) термокапиллярный дрейф капель в псевдоожиженной порошковой среде. Проведен анализ причин псевдоожижения реакционной смеси при горении и их влияние на структуру конечного продукта реакции.

Carbon was detected in magnesium aluminate spinel (MAS) prepared by self-propagating high-temperature synthesis (SHS). Periclase MgO, alumina Al2O3, fuel Al powder, 2 wt% boron, and oxidizer Mg(NO3)2·6H2O were used as starting reactants for MAS synthesis. The phase composition, structure, and surface morphology of the samples were studied. Gases evolved during combustion were analyzed. IR spectroscopy shows that the carbon in the synthesized product has a diamond-like structure that is similar to detonation diamonds. Cascade parallel and consecutive exothermic reactions cause an increase in temperature. The initial components are impregnated with melts of boron oxide and metallic aluminum and saturated with the evolved gases during fast synthesis. The rapid growth of spinel crystals and cavitation occurring in a dynamically changing system create high pressure. All this triggers the proton-boron reaction with the formation of carbon. Possible mechanisms for the formation of carbon in the synthesized products are suggested on the basis of the experimental data.

Исследован процесс распространения фронта волны горения в двухслойной порошковой системе (Ni + Al)/(PbO2 + B + Al2O3+ стекло). Обнаружен эффект расширения волны экзотермического процесса при добавлении смеси PbO2 + B в нижний слой. Добавление смеси PbO2 + B в нижний слой позволяет снизить толщину слоя NiAl и обеспечить формирование однородного диэлектрического покрытия. Установлены оптимальные соотношения толщин слоев и состава порошковой смеси слоев.

Development of new energy-efficient high-temperature sintering furnaces is one of the key factors for cost reduction of refractory ceramic manufacturing. In this paper, a sintering furnace in which the volume of the furnace chamber is completely filled with a packed bed of ceramic spheres, and the energy source is the premixed combustion of natural gas with atmospheric air and oxygen has been experimentally studied. The furnace design allows the use of two combustion modes: (i) filtration combustion, when the narrow reaction front (combustion wave) freely propagates through the packed bed, (ii) jet-stabilized combustion when the reaction front is stabilized near gas inlet nozzles used for supplying the fresh mixture into the packed bed. Experimental analysis of both combustion modes was performed in the ranges of specific fuel flow rate from 7.10(-3) to 107.10(-3) Nm(3).s(-1)/ m(2), firing rate from 244 to 3724 kW/m(2), fuel-to-oxidizer ratio from 0.40 to 3.30, oxygen content in the oxidizer from 21 to 30 vol%. In these ranges, the temperature of the packed bed can be controlled within 1230-2220 K. Test sintering of samples pressed from powders of MgO and Al2O3 was carried out at 2170 K. It was found that the filtration combustion mode is effective for sintering parts with a characteristic size of up to 10 mm, and the packed bed can be completely formed of samples to be sintered. Jet-stabilized combustion mode is effective for sintering large pieces. In such a case, submerging samples within the packed bed enables double fuel saving compared to the stacking of samples in a furnace chamber of a conventional free-volume design. The new furnace based on the principles of premixed combustion in porous inert media can be in demand for small-scale production of ceramic parts.

The synthesis of an intermetallic compound from a powder mixture of Ni + Al + Al2O3 with an inner metal plate oriented in the direction of combustion wave propagation has been studied by experiments and theoretical calculations as applied to the problem of self-propagating high-temperature synthesis of functionally graded materials. Copper, titanium, and steel plates are considered. The propagation of the exothermic reaction front is analyzed for various thermophysical characteristics of the inner metal layer. In samples with a copper plate, the combustion front near the contact boundary is extended in the direction of the combustion wave. An increase in the average burning rate of the sample with the heat recovery effect is observed when using copper plates 1–3 mm thick. During combustion of samples with titanium and steel plates, there is a deceleration of combustion wave propagation along the contact boundary.

The research revealed the carbon micro traces in the synthesis products of magnesium aluminate spinel produced by the method of self-propagating high-temperature synthesis (SHS) in the MgO-Al2O3-Mg(NO3)2·6H2O-Al system with boron additives. The phase composition, structure, and morphological features of the surface of the samples were examined. The gases evolving during combustion were analyzed. IR spectroscopic analysis showed that carbon has a diamond-like lattice similar to the lattice of detonation nanodiamonds. It was shown that low-energy nuclear reactions (boron-proton reaction) proceed in the combustion wave during high-speed SHS processes under certain conditions. Based on the experimental data, the most probable mechanisms of carbon formation in the synthesized products were formulated and proposed.

The paper focuses on the theoretical and experimental study of the mechanisms of reaction mixture combustion in the ≪chemical oven≫ mode in a three-layer Ni–Al/Ti–Co/Ni–Al sample. Experimental studies were carried out in a reactor in an argon atmosphere at atmospheric pressure and an ambient temperature of 298 K on rectangular samples pressed from Ni–Al and Ti–Co powder mixtures in the form of a three-layer package. The Ti–Co acceptor layer was in the middle of the sample, and the Ni–Al donor layer was outside. The acceptor layer thickness was varied from 4.3 to 13 mm, while the donor layer thickness (4.7 mm) remained constant. It was found that as the acceptor layer thickness increases, the combustion wave front propagation velocity and reaction initiation temperature decrease, and the maximum temperature in the front remains constant and equal to the melting point of the final product. The time of acceptor layer heating before the reaction increases. The acceptor mixture reaction proceeds in the thermal explosion mode when the thickness of the acceptor layer exceeds that of the donor one. Maximum temperature in this case is higher than the melting point of the final product. The inner layer synthesis modes change with an increase in the acceptor layer thickness: stationary – pulsating – extinction. The mathematical model of the three-layer sample high-temperature synthesis in dimensional variables is constructed taking into account heat transfer with the environment. As a result of experimental studies and numerical calculations, the critical thickness of the inner layer was found to be 15 mm, at which the inner layer combustion becomes impossible at fixed sizes of donor layers. Critical conditions for the combustion wave propagation along the acceptor layer are weakly dependent on the external heating source. The experimental technique and mathematical model of the layered system combustion can be used to assess the critical conditions for the metal composite synthesis in the frontal combustion mode.

This study is devoted to the technology of producing cermet coatings using self-propagating high-temperature synthesis (SHS). The importance of the study is related to the universal usage of flat electric heaters and protective coatings intended for various purposes. A technique is proposed to obtain electrically conductive coatings by the SHS in powdered Ni + Al and Ti + Al + C mixtures. Specific features of the evolution of the autowave SHS in these mixtures are investigated. The mixture is deposited on a ceramic base as a layer of a suspension in isopropyl alcohol (0.2–2.0) × 10 –3 m in thickness using a stencil. The effect of the thickness of the powdered mixture layer on the velocity of front propagation and maximum temperature is studied. These parameters are shown to naturally increase with an increase in thickness. The coating based on the Ni + Al mixture is found to consist of NiAl and Ni 3 Al intermetallic compounds, while that based on Ti + Al + C is found to consist of TiC and MAX phases of Ti 2 AlC and Ti 3 AlC 2 . The coating based on intermetallic compounds consists of rounded particles that are fused together and contains NiAl and Ni 3 Al phases. The coatings obtained from the Ti + Al + C mixture contain needle-shaped crystals of MAX phases and interspersed rounded particles of titanium carbide. The content of the NiAl and Ti 2 ALC target phases increases with the increasing layer thickness. Coatings based on NiAl, Ni 3 Al, and Ti 2 AlC, Ti 3 AlC 2 heat-resistant phases (0.2–1.2) × 10 –3 m in thickness with a specific electrical resistance of 0.1–0.6 μOhm m are obtained.

Experimental studies have been carried out with theoretical calculations of wave synthesis in the Ni–Al–Cu system using a mathematical model. Approximate analytical formulas are obtained for synthesis performance evaluation. The inverse problem method was used to get kinetic constants that determine process dynamics based on the experimental data and analytical relationships. It is shown that the combustion front propagation velocity increases monotonically with an increase in the reaction sample relative density in the range of relative density values of 0.4 to 0.6. The depth of copper-melt penetration from the center of the sample into the nickel–aluminum matrix depends on the relative density of the sample and diameter of the copper wire: higher densities and larger diameters lead to an increase in the liquid-phase impregnation area. The rate of nickel and aluminum powder frame wetting with copper melt is limited by the synthesis wave speed. Based on the experimental data and analytical ratios, we estimate the effective kinetic constants describing the high-temperature synthesis of the Ni + Al reaction mixture in the presence of copper additives. The thermal effect of the NiAl intermetallic formation reaction and the preexponential factor in the chemical transformation equation are calculated, the exponent value in the ratio for the mixture thermal conductivity is established, and a constant determining the process of nickel–aluminum matrix impregnation with copper melt is found. The macroscopic approach used to analyze the NiAl intermetallic synthesis makes it possible to determine all the desired physicochemical characteristics and model parameters. The mathematical model is suitable for predictive estimates and experimental data analysis in the macroscopic approximation. Approximate analytical formulas are obtained for calculating the NiAl intermetallic synthesis characteristics. They allow for calculating the through channel characteristics and can be used in the design of NiAl products.

SHS of layered NiAl/Cu/NiAl composite was studied experimentally and by mathematical modeling with special emphasis on critical conditions for combustion synthesis in (Ni–Al)/Cu/(Ni–Al) compacts. Our results may turn interesting to those engaged in fabrication of functionally graded materials.