SHS Compaction Research Articles

Synthesis of high-temperature Mo5SiB2-based ceramics in the combustion mode

This study is devoted to the synthesis and investigation of the Mo5SiB2-based ceramic material (T 2 phase). The influence of the initial temperature on basic combustion parameters is shown. It is found that preheating the reaction mixture enables the initiation of the combustion in the self-oscillatory mode and the initial temperature dependences of the combustion temperature and rate have a linear character. The effective activation energy of the SHS process is calculated. Some variants of chemical reactions between Mo, Si, and B are proposed to explain the combustion mechanism in the investigated ternary system. Compacted samples are obtained using the forced SHS compaction. The phase composition, structure, and properties of synthesized ceramic materials, in which Mo5SiB2 grains are the main component with an average size of 10–20 μm, are studied. The lines of Mo3Si and Mo intermediate phases, the total fraction of which does not exceed 4%, are identified as well. The produced T 2 phase-based material has a high specific density and hardness.

Mechanically activated combustion synthesis of molybdenum silicides and borosilicides for ultrahigh-temperature structural applications

Molybdenum silicides and borosilicides are promising structural materials for gas-turbine power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo5Si3 (called T1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2−T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5SiB2 (called T2) phase is considered as an attractive material. In the present paper, MoSi2−T1 composites and materials based on T2 phase are obtained by mechanically activated SHS. Use of SHS compaction (quasi-isostatic pressing) significantly improves oxidation resistance of the obtained MoSi2−T1 composites. Combustion of Mo−Si−B mixtures for the formation of T2 phase becomes possible if the composition is designed for the addition of more exothermic reactions leading to the formation of molybdenum boride. These mixtures exhibit spin combustion, the characteristics of which are in good agreement with the spin combustion theory. Oxidation resistance of the obtained Mo−Si−B materials is independent on the concentration of Mo phase in the products so that the materials with a higher Mo content are preferable because of better mechanical properties.

Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides

ABSTRACTMolybdenum silicides and borosilicides are promising structural materials for advanced power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo5Si3 (called T1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2−T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5SiB2 (called T2) phase and alloys based on this phase are promising materials.In the present paper, MoSi2−T1 composites and materials based on T2 phase are obtained by mechanically activated self-propagating high-temperature synthesis (MASHS). To obtain denser products, the so-called SHS compaction (quasi-isostatic pressing of hot combustion products) has been employed. Thermal analysis has shown that SHS compaction significantly improves the oxidation resistance. Self-sustained combustion of Mo/Si/B mixtures for the formation of T2 phase becomes possible if the composition is designed for adding a more exothermic reaction of MoB formation. These mixtures exhibit spin combustion. Oxidation resistance of the obtained multi-phase Mo−Si−B materials is independent on the concentration of Mo phase in the products. The “chemical oven” technique has been used to obtain a single Mo5SiB2 phase and an alloy consisting of α-Mo, Mo5SiB2, and Mo3Si phases.

Self-propagating high-temperature synthesis of advanced ceramics in the Mo–Si–B system: Kinetics and mechanism of combustion and structure formation

The goal of this work is to investigate the combustion mechanisms of reactions in the Mo–Si–B system and to obtain ceramic materials using the SHS method. It is concluded that the following processes are defined by the SHS for Si-rich Mo–Si–B compositions: silicon melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo3Si-phase film. The subsequent diffusion of silicon into molybdenum results in the formation of MoSi2 grains and molybdenum boride phase forms due to the diffusion of molybdenum into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO3 gaseous species to boron particles. The stages of chemical interaction in the combustion wave are also investigated. The obtained results indicate the possibility of both parallel and consecutive reactions to form molybdenum silicide and molybdenum boride phases. Thus the progression of combustion process may occur through the merging reaction fronts regime and splitting reaction fronts regime. Molybdenum silicide formation leads the combustion wave propagation during the splitting regime, while the molybdenum boride phase appears later. Finally, targets for magnetron sputtering of promising multi-phase Mo–Si–B coatings are synthesised by forced SHS compaction method.

SHS of fine-grained ceramics containing carbides, nitrides, and borides

Fine-grained composites with high impact resistance (intentionally armor plates) were prepared from inexpensive raw materials by forced SHS compaction in the Ti-B-N-Cu (I) and Ti-B-N-C-Cu (II) systems ([Cu] = 0–10 wt %). The materials synthesized under optimized conditions were practically pore-free compacts with the following parameters: porosity 0.4–1.2%; hardness 91.5–92.5 and 92.3–93.3 HRA; density 4.3–4,4 and 4.5–4.7 g/cm3, for systems I and II respectively. The materials were found to withstand impacts with an energy of 18000–20000 J (for materials with d = 6.8–6.5 g/cm2).

Towards better combustion of lunar regolith with magnesium

It has been shown recently that lunar regolith forms combustible, thermite-type mixtures with magnesium. Combustion of these mixtures could be used for the production of construction materials on the Moon. The present paper focuses on decreasing magnesium content in mixtures based on JSC-1A lunar regolith simulant and increasing the density and strength of the combustion products of these mixtures. Combustion of JSC-1A/Mg mixture pellets was studied in argon gas and in silica powder. Preheating to 100°C decreased the minimum concentration of magnesium, required for combustion, from 10wt.% to 8wt.%. Submerging the pellets into silica increased the product strength. SHS compaction experiments were conducted, which involved quasi-isostatic pressing of the products immediately after the end of the combustion process. This resulted in a significant increase in the product density. The compressive strength exceeded 10MPa, i.e., was higher than that of common bricks (5MPa).

Producing multicomponent SHS-compacted cathodes based on refractory titanium compounds for vacuum-arc coatings

The technological features of and a device for manufacturing multicomponent cathodes of arc evaporators using SHS compaction are considered. Physically, an SHS-compacted cathode is a four-layer function-gradient product and consists of a metallic base in the form of a water-cooled bottom part of a regular cathode, an evaporating operating layer, an intermediate layer, and the SHS solder layer required for joining the synthesized material with the base. Compositions and weight characteristics of layers that allow us to produce defect-free multicomponent SHS-compacted cathodes based on stainless steel are presented.

Mineral-like ceramics for immobilization of nuclear wastes by forced SHS compaction

Dense cylindrical samples based on Y2Ti2O7 ceramics with a pyrochlore structure were produced by forced SHS compaction and characterized as a matrix for immobilization of actinide-containing nuclear wastes. Synthesized materials were found to exhibit high hydrolytic resistance and mechanical strength.

Self-propagating high-temperature synthesis of Al2O3/TiC-based ceramic materials

Dense Al2O3/TiC composite ceramic materials are synthesized using the SHS compaction of mixtures based on TiO2 + Al + C. Mineralizing and heating additives are introduced into compounds. The phase composition and microstructure of combustion products are investigated by x-ray phase analysis, electron microscopy, and microprobe techniques. Two Al2O3 modifications are revealed. Special attention is devoted to the presence of residual graphite. The mechanisms of phase formation and formation of the microstructure of combustion products are considered.

Combustion and structure formation in the mechanoactivated Cr-B system

For Cr + B mixtures, we investigated the influence of mechanoactivation (MA) on the thermal effect of SHS reaction, burning velocity, and combustion temperature, and optimized MA conditions. Using the technique of forced SHS compaction, we manufactured the CrB2-based compacts up to 125 mm in diameter (residual porosity 3–5%) from green mixtures subjected to MA under optimized conditions.

Mechanism of compacting and forming in the course of SHS compaction with a granular shell

By a calculational and experimental method, the mechanism of compacting and forming of flat blanks produced by the method of SHS compaction with the use of a granular shell is investigated. The features of the kinetics of compacting hot solid-liquid products of synthesis are considered. A mathematical model of forming flat blanks in the course of SHS compaction is proposed. The mechanism of deforming and compacting the granular shell is investigated. The effect of size and mechanical properties of the shell on the mechanism of compacting and forming of SHS blanks is evaluated. The technical procedures for improvement of the standard of SHS-compacted flat blanks are presented.

Self-propagating high-temperature synthesis of target cathodes in the system Ti-Ta-C-Ca3(PO4)2 for the ion-plasma deposition of multifunctional tissue-compatible nanostructured coatings

We present the results of investigations of synthesis in a burning mode of tissue-compatible compact composition materials of tantalum-containing target cathodes (Ti, Ta)Cx + Ca3(PO4)2 obtained by forced SHS compaction from a Ti + Ta + C + Ca3(PO4)2 mixture and assigned to the ion-plasma deposition of nanostructured tissue-compatible coatings. The effect of the amount of additives of the tantalum-carbon mixture with respect to the weight of the starting charge is investigated. Regularities of phase formation and structural formation in synthetic products are analyzed. A new and important feature is that there is no dependence of the burning velocity and temperature on the relative fraction of the Ta + C mixture. In the temperature profiles, two characteristic peaks of heat release were detected. These peaks indicate that parallel and sequential chemical reactions of the formation of titanium and tantalum carbides proceed. The growth of the relative fraction of the Ta + C mixture led to modification of the carbide structure, to an increase in the residual porosity, and to a considerable increase in hardness.

Combustion and structure formation in the Ti-Ta-C-Ca3(PO4)2 system

Biocompatible composites (Ti, Ta)Cx + Ca3(PO4)2 for deposition of nanofilms onto load-bearing implants by ion-plasma sputtering were prepared from Ti + Ta + C + Ca3(PO4)2 mixtures by forced SHS compaction. The effect of Ta + C addition to green mixtures (characterized by parameter z) on the structure/phase formation in combustion products was explored. The addition of tantalum and carbon was found to have little or no influence on the burning velocity U and combustion temperature Tc. Two thermal spikes exhibited by thermograms were associated with the occurrence of two consecutive reactions leading to formation of titanium and tantalum carbides. With increasing z, the grain size of (Ti, Ta)C was found to diminish, its relative density to decrease, while the hardness to markedly grow.

Microstructure Evolution and Isothermal Compaction in TiO 2 -Al-C Combustion Reaction

Combustion reaction in the TiO2-Al-C system was investigated by the combustion wave freezing technique with a wedge-shaped copper-made quenching block. The combustion reaction was a combined process in which the aluminothermic reduction of TiO2 (3TiO2 + 4Al → 2Al2O3 + 3Ti) and TiC formation reaction (Ti + C → TiC) occur in series. First, the aluminothermic reduction was activated by wet spreading of molten Al into interspaces between TiO2 particles to produce rounded Al2O3 grains embedded in the Ti-rich liquid phase. In the later combustion stage, the Ti-rich phase reacted with the reactant C to produce TiC grains in the Ti-rich liquid phase. The three-dimensionally interconnected Al2O3 structure typically shown in this system mainly originated from interconnection between the rounded Al2O3 grains due to the high combustion temperature from the high exothermic TiC formation reaction. With decreasing the combustion temperature and controlling the C and TiC content, the interconnected Al2O3 structure changed to isolated. The isolated Al2O3 structure showed superior isothermal compaction behavior to the interconnected. Finally, it is suggested that the microstructure of combustion reaction should be one of the important factors in the SHS compaction process.

Self-Propagating High-Temperature Synthesis of Functionally Graded PVD Targets with a Ceramic Working Layer of TiB 2 -TiN or Ti 5 Si 3 -TiN

Two- and three-layer functionally graded PVD targets with a “working” layer (sputtering surface) based on TiN-TiB2 or TiN-Ti5Si3 formation via the technology of forced SHS compaction were considered. It was shown that rather wide (up to 1 mm) transition diffusional zones providing firm adhesion of the layers were formed between the working, intermediate, and third layers. The compositions TiN + TiB2 + 20%Cu and TiB2 + 50%Cu were found to be optimum for the intermediate (“damping”) and third layers of the three-layer PVD targets. The specific features of phase and structure formation of the two- and three-layer products with the working layer based on TiN-TiB2 and TiN-Ti5Si3 were elucidated. Variation of the “chemical oven” mass exerted a noticeable effect of reduction of the residual porosity and enlarging of the grains of the product phases owing to prolongation of the time for secondary structure formation. The relationship of the hardness of the layers in the functionally graded target on the chemical oven mass allow prediction of the properties of the resultant graded material systems.

SHS Immobilization of Radioactive Wastes

Abstract SHS technology for immobilization of high-level radioactive wastes (HLWs) is based on combustion-assisted embedding of HLWs into chemically inert analogs of natural minerals used as preserving matrices of high chemical and radiation resistance. A pilot-scale production unit for SHS compaction of blocks up to 1.75 kg in their weight was designed. Although the studies on immobilization of nuclear wastes are currently conducted in many countries, the problem of convenient and reliable preserving matrices is yet to be resolved.

Thermal Conditions in the SHS Compaction of a Powder

Thermal Conditions in the SHS Compaction of a Powder

Macrorheological theory of high-temperature deformation of powder materials under SHS compaction

The present work surveys the results of mathematical modelling of deformation and thermal processes of SHS compaction of advanced ceramics. Various regimes of SHS compaction have been studied based on a rheodynamic model. The model takes into account the initial density distribution throughout the material and the nonsteady state of the process. The wave regime and the regime having a zone of decreased density in the centre of the specimen are discussed. The influence of delay time on the process of densification is described. It is shown how the model developed facilitates solution of practical problems of that process.

Self-propagating high-temperature synthesis of composite materials. The high-melting compounds (TiC, TiB2)-intermetallides, their structure and properties

Regularities of the SHS of four-component powder mixtures of quasibinary sections (titanium carbide or diboride-intermetallide Ni3Al or NiTi) are studied. Combustion rates and temperatures are determined. Specific features of the combustion as a function of the composition are revealed. The character of structure formation and the properties of the synthesized materials, including those produced in SHS compaction, are considered.

Mathematicla modeling of thermal regimes of SHS compaction

Mathematicla modeling of thermal regimes of SHS compaction