Self-propagating high-temperature synthesis of high-temperature light refractories

Data on the synthesis of light high-temperature refractories of various compositions in the SHS regime are presented. Optimum conditions for the synthesis are determined; the physicochemical properties of the refractories produced are studied.Institute for Combustion Problems, Almaty. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 4, pp. 490–491, October, 1993.     

Self-propagating high-temperature synthesis of titanium borides

The application of self-propagating high-temperature synthesis (SHS) to prepare a few borides of titanium was investigated. Using the plane wave propagation mode, the synthesis of titanium borides in the cold-pressed cylindrical specimens of the component powder mixtures was effected and was studied as a function of boron content in the initial mix and the specimen size. SHS reaction in compacts having diam. of 6 mm or less and high bulk density could not be initiated and/or sustained and was considered to be a result of rapid heat dissipation.     

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.     

TiB/Ti复合材料自蔓延高温燃烧合成的研究

采用自蔓延高温燃烧合成－准热等静压工艺（SHS／PHIP）制备了TiB-Ti体系复合材料,理论计算了该体系的绝热温度,测量了燃烧温度和燃烧速度。结果表明,绝热温度、燃烧温度和燃烧速度均随Ti含量的增加而降低。对合成产物的分析发现：反应产物主要由TiB和Ti两组组成,TiB相分布均匀,主要有棒状和块状两种形态,并且随Ti含量的增加,TiB尺寸减小;部分产物中还有少量TiB2相存在。合成产物具有高的致密度和硬度,其相对密度超过94％,硬度HRA＞82。     

Oxidation of molybdenum disilicide

The oxidation kinetics of molybdenum disilicide over the temperature interval, 1328 to 1765° K, has been studied using a thermal conductivity method. The rates of oxidation were found to follow a parabolic relationship once a protective coating was formed. From the temperature dependence of the parabolic rate constants, an activation energy of 20 kcal/mole was obtained.     

Processing of molybdenum disilicide

Inspection of the scientific literature reveals that intermetallic compounds have, in recent years, attracted considerable interest as a result of their unique elevated temperature characteristics. Among the wide range of intermetallic compounds that are actively being studied, MoSi₂ has been singled out as a result of its unique combination of properties, which include an excellent oxidation resistance, a high modulus of elasticity, and an elevated melting point (2030°C). In view of this interest, the present work was undertaken with the objective of providing the reader with a comprehensive review of the mechanical and oxidation behaviour of MoSi₂, paying particular attention to the synergism between processing and microstructure. Accordingly, synthesis techniques, including powder metallurgy, self-propagating hightemperature synthesis, spray processing, solid-state displacement reactions, and exothermic dispersion, are critically reviewed and discussed. In addition, recent efforts aimed at using MoSi₂ as a matrix material in metal-matrix composites are also critically reviewed and discussed.     

Self-propagating high-temperature synthesis of ultrafine and nanometer-sized TiC particles

The feasibility of preparing ultrafine and nanometer-sized titanium carbide particles by self-propagating high-temperature synthesis (SHS) has been studied. Data are presented on the structure formation of TiC powders during SHS with a reduction step. Basic to this process is an exothermic reaction between titanium dioxide, magnesium metal, and carbon. The effects of the composition of the starting mixture, relationship between its components, and the morphology and particle size of the starting TiO₂ powder on the particle size of the forming material have been investigated. The TiC powder was recovered from the sinter cake by chemical dispersion, a chemothermal treatment of the synthesis product in different solutions. The results demonstrate that treatment of the sinter cake with appropriate solutions removes impurities and causes imperfect intergranular layers to dissolve. As a result, the cake breaks down into homogeneous single-crystal particles. Subsequent treatment in different solutions further reduces the particle size of the powder. The effect of the composition of the dispersing solution on the particle size of the TiC powder has been studied. Our results made it possible to identify conditions for the preparation of titanium carbide powders containing up to 70% of particles less than 0.3 μm in size by SHS followed by chemical dispersion.     

Self-propagating high-temperature synthesis of composite material TiB2-Fe

Results of an investigation of the microstructure and phase composition of materials of the Ti-B-Fe system, obtained by self-propagating high-temperature synthesis (SHS) are presented.Investigations were conducted for two types of powder mixtures; the first was a mixture of elemental powders Ti, B, Fe and the second was a mixture of ferroboron alloys, FeB_n, with titanium. The possibility of using commercial ferroalloys to obtain of boron-containing MMC_s is shown. The combination of SHS with force effect rooling leads to a dense product with high operational properties. More efficient fields of the application of SHS composite to produce TiB₂-Fe are shown.     

Self-propagating high-temperature synthesis of boron subphosphide B12P2

Two new methods to produce nanopowders of B₁₂P₂ boron subphosphide by self-propagating high-temperature synthesis have been proposed. Bulk polycrystalline B₁₂P₂ with microhardness of H _V = 35(3) GPa and stability in air up to 1300 K has been prepared by sintering these powders at 5.2 GPa and 2500 K.     

自蔓延高温合成氮化硅镁粉体

研究了不同起始反应物体系(Mg+Si,Mg2Si和Mg+Si3N4)对氮气中燃烧合成氮化硅镁(MgSiN2)粉体的影响,结果显示,不同反应物体系的燃烧产物不同.Mg2Si和Mg+Si3N4能获得单一相的MgSiN2,而且氧含量低至0.356%(质量分数),而Mg+Si体系燃烧产物除主相MgSiN2外,还含有少量Si和MgO及一些未知相.     

Self-propagating high-temperature synthesis of ceramic matrices for immobilization of actinide-containing wastes

Self-propagating high-temperature synthesis (SHS) of mineral-like matrices based on pyrochlore (Y₂Ti₂O₇) and zirconolite (CaZrTi₂O₇) phases for immobilization of actinide-containing wastes (HLW) was studied. Thermodynamic analysis of the systems Ti-ZrO₂-CaO-Y₂O₃-Me _n O _m (Me = Mo, Fe, Ni, Cr, Mn, Cu) was performed. The possibility of preparing matrices based on the structure of titanium pyrochlore Y₂Ti₂O₇ enriched in zirconium was demonstrated. The dependences of the formation and ratio of crystalline phases in SHS products on the charge composition and HLW concentration were revealed. The HLW components do not form intrinsic crystalline phases and are incorporated as isomorphous impurities in the crystal lattices of pyrochlore, zirconolite, and perovskite. The advantages of using Fe₂O₃ as oxidant were demonstrated. It prevents possible loss of HLW elements, associated with high-temperature evaporation of the oxidant. A matrix material consisting of two phases: (1) titanate pyrochlore Y₂Ti₂O₇ containing HLW elements and enriched in zirconium and (2) Fe metal was prepared. The total content of lanthanides in separate pyrochlore crystals varies from 0.9 to 1.6 at. %, with 27 to 31% replacement of Ti atoms by Zr atoms. The study was performed with simulated HLW.     

The synthesis of nanostructured molybdenum under self-propagating high-temperature synthesis mode

The formation of nanostructured metallic molybdenum from the MoO₃–NaBH₄–kNaCl system (where k is mole-number of NaCl) is analyzed thermodynamically and investigated experimentally under self-propagating high-temperature synthesis (SHS) mode. The laws of combustion and phase formation and an effect of sodium chloride on the characteristics of Mo powder are discussed. The result shows that mean particle size of Mo produced depends greatly on the amount of sodium chloride, especially the increasing of NaCl concentration leads to a small size Mo powder formation. Mo powder with the particle size at least 20 ∼ 100 nm was synthesized at k = 2. As synthesized Mo powder have been compacted and successfully consolidated to densities exceeding 90% at the temperature of 1700 °C.     

Self-propagating high-temperature synthesis of titanium nitride with the participation of ammonium chloride

We have studied the combustion of titanium in nitrogen in the presence of ammonium chloride. It has been shown that the use of NH₄Cl in the self-propagating high-temperature synthesis of TiN considerably reduces the combustion temperature, prevents sintering of the synthesized titanium nitride particles, and increases their specific surface area. The synthesis products have the form of nanostructured titanium nitride particles which reproduce the shape of the starting titanium particles but consist of equiaxed titanium nitride grains ranging in size from 50 to 500 nm. We have obtained nanostructured titanium nitride powders ranging in specific surface area up to 80 m²/g.     

Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials

Self-propagating high-temperature synthesis (SHS) of powder compacts is a novel processing technique currently being developed as a route for the production of engineering ceramics and other advanced materials. The process, which is also referred to as combustion synthesis, provides energy- and cost-saving advantages over the more conventional processing routes for these materials. At the same time, the rapid heating and cooling rates provide a potential for the production of metastable materials with new and, perhaps, unique properties. This paper reviews the research that has been, and is being, undertaken in this exciting new processing route for high-technology materials and examines the underlying theoretical explanations which will, eventually, lead to improved control over processing parameters and product quality.     

Combustion synthesis and mechanical properties of molybdenum disilicide composites reinforced with SiC particulate

Intermetallic composites of molybdenum disilicide reinforced with silicon carbide were produced by combustion synthesis of the elemental powders. The combustion reaction was initiated near 700°C and completed within a few seconds. The end product was a porous composite which was subsequently hot pressed to >97% theoretical density. The grains of the matrix were 8–14 m in size surrounded by SiC particulate reinforcement of 1–5 m. The mechanical properties of the composites improved with increasing SiC reinforcement. The hardness of the materials increased from 10.1 GPa to 12.7 GPa with the addition of 20 vol% SiC reinforcement, while the strength increased from 195 MPa to 299 MPa. The fracture toughness also increased from 2.79 MPa m1/2 to 4.08 MPa m1/2 with 20 vol% SiC. © 1998 Chapman & Hall     

Self-propagating high-temperature synthesis of Al2O3-TiC-Al composites by aluminothermic reactions

Al₂O₃-TiC-Al composites were fabricated by self-propagating high-temperature synthesis process using aluminothermic reactions with titania, aluminum, and graphite powders. As the molar ratio x of the excessive aluminum in the reactants increases, the adiabatic temperature of the reaction and the melting rate of alumina in the products obviously decrease according to thermodynamics. This reaction is theoretically presumed to be ignited at preheat temperature of 900 K even though x is up to 13 mole. The experimental results revealed that the critical molar ratio of excessive Al, which the combustion reaction can self-sustain, is 7.66 mole with a preheat temperature of 400–500 K. The excessive aluminum favors to fill in the pores of the products, and a cylindrical Al₂O₃-TiC-Al composite with a relative density of 70% can be obtained, and its tensile strength is higher ten times than that of the Al₂O₃-TiC composite. Moreover, TiC and Al₂O₃ grains in the composites are fined as the excessive aluminum increases. Although the excessive aluminum does not take part in the combustion reaction, it strongly affects combustion process and microstructures of the products.