Special features of manufacturing highly porous materials from silicon nitride,silicon oxynitride,and sialons

Results obtained in a study of factors that affect shaping and sintering of highly porous materials are presented. A technology providing highly porous silicon nitride materials that is based on shaping preliminarily fabricated pieces of thermoplastic slip makes it possible to use initial silicon powders of different dispersities. Highly porous materials can also be produced on the basis of sialons and exhibit a diminished thermal conductivity and a heat resistance at the level of similar silicon nitride materials.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 11, pp. 8 – 13, November, 1996.     

Interaction of Aluminum with Detonation Products

A method of electrical conductivity and an analysis of recovered explosion products are used to study interaction of aluminum with detonation products of condensed high explosives. The electrical conductivity of HMX/Al and RDX/Al mixtures is inhomogeneous; a region with the maximum electrical conductivity is adjacent to the detonation front, whereas the electrical conductivity decreases with distance from the front. If the wave is incident onto a wall, the electrical resistance of the composite high explosive increases, which indicates that the high-conducting zone disappears. The electrical conductivity, resistance of the conducting zone, and the time of resistance growth are found as functions of the particle size of the additive. The results obtained confirm the reaction of the metal additive with detonation products in a microsecond range of time. An analysis of condensed explosion products shows that the reaction of aluminum with detonation products proceeds on the particle surface. The amount of reacted aluminum and the oxide-layer thickness are estimated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 1, pp. 120–129, Jnuary–February, 2006.     

Role of bulk and mass effects of reactions in reaction sintering processes

Peculiarities of the fabrication of ceramic porous and dense composite materials based on compounds of the Si–C–O–N system with the participation of chemical reactions and the formation of new phases are discussed. An analysis of comparatively new technologies is attempted in terms developed in earlier studies on the reaction sintering of silicon nitride, carbide, and oxynitride. It is shown that the approach to reaction sintering that includes the selection of promising reaction systems allowing for the bulk effect of reactions accompanying material formation can be extended to the fabrication of porous and highly porous materials. In contrast to the fabrication of dense materials, when reaction systems with positive bulk effects are used, the reaction systems with negative bulk effects can be used in the fabrication of highly porous materials.     

Electrical resistance of copper under shock compression: Experimental data

The electrical resistance of copper foil under shock compression is measured. The electrical resistance and electrical conductivity are plotted as functions of the shock pressure in the interval up to 20 GPa. These dependences are monotonic and have no visible inflections or singularities. A qualitative dependence of the electrical resistance of the metal on the shock impedance of the material of the block containing the sample is found. A comparison of the data obtained in this study with results of other authors shows that it is important to take into account the block material, the shape and thickness of the sample, and the procedure of determining the state of the sample.     

Phase Formation in the Synthesis of Ti2AlN by Spark Plasma Sintering in the Ti/AlN System

Refractories and Industrial Ceramics - The synthesis of a Ti2AlN-based material by mechanical activation (MA) of a Ti–AlN powder mixture in a planetary ball mill followed by vacuum spark...     

Reaction Sintering with Negative Volume Changes

The preparation of high-porosity sialon and SiC materials by reaction sintering accompanied by weight loss and shrinkage is considered. -Sialon ceramics with a 60% porosity, pore size of 0.7 m, and gas permeability K = 0.04 m² are obtained via carbothermal reduction and simultaneous nitriding of kaolin in kaolin + graphite green compacts. The SiC materials prepared from mixtures of SiO₂ and graphite have a porosity of 83% and a pore size of 1 m. The most promising technique is elemental synthesis. In the SiC materials prepared by milling silicon + graphite mixtures to a specific surface area of 80 m²/g, pressing at 100–200 MPa, and sintering in argon at 1600°C for 15 min, the pore size is as small as 0.2–0.3 m.     

Structural Changes during Milling of Silicon Carbide

The kinetics of milling and structural changes during milling of commercial silicon carbide powder in a ball mill lined with hard alloy are studied. Data are provided for the specific surface, x-ray patterns, and infrared spectra. It is shown that milling kinetics are described best of all by an exponential relationship and that milling of silicon carbide powder is accompanied by changes in polytype composition. The x-ray patterns and infrared spectra obtained make it possible to assume that during milling the 15R polytype content in the powder composition decreases while the content of structures with less layering (the 4H polytype and -SiC) increases; in this case the number of absorption bands in the infrared spectra decreases.     

Electromagnetic Field Formed by Shock Compression of a Conducting Magnetic

The structure of the electromagnetic field in a conducting magnetic compressed in a shock wave is analyzed. It is shown that compression of a magnetic material in an external magnetic field leads to origination of a system of two currents identical in magnitude but opposite in direction. One of them passes ahead of the shock front in the undisturbed substance, and the oppositely directed current passes over the shockcompressed substance. As the shock wave moves further, the absolute value of current monotonically increases. The parameters determining the global electromagnetic pattern in the shockcompressed magnetic are found. These parameters can be considered as the generalization of the governing parameters found previously by the authors for a nonmagnetic conductor. The formulated model offers a qualitative explanation for the results of dynamic experiments with an 80NKhS magnetic soft alloy. The voltage record on the specimen surface indicates effective shockinduced demagnetization of the material.     

Current Waves Generated by Detonation of an Explosive in a Magnetic Field

The structure of the electromagnetic field in detonation of a condensed explosive in a magnetic field is analyzed qualitatively. Propagation of a detonation wave in a magnetic field leads to generation of an electric current in explosion products. The physical reason for current generation is the freezing of the magnetic field into the conducting substance at the detonation front and subsequent extension of the substance and the field in the unloading wave. The structure of the current layer depends on the character of the boundary magnetic fields and conditions on the surface of initiation of the explosive. Detonation of the explosive in an external magnetic field B₀ generates a system of two currents identical in magnitude but opposite in direction. The structure of the arising current and its absolute value are determined by the parameter R₁ = _{0 0}D²t (₀ is the magnetic permeability of vacuum, ₀ is the electrical conductivity of detonation products, D is the detonationfront velocity, and t is the time). The value of the current increases with the detonationwave motion, and the linear current density is limited from above by 2B₀/₀. For R₁ 1, the electric field in the conducting layer is significantly nonuniform; for detonation products with a polytropic equation of state, a region of a constantdensity current is adjacent to thedetonation front. The results of this analysis are important for interpretation of experiments performed and development of new methods for studying the state of the substance in the detonation wave.     

Measurement of electrical conductivity of condensed substances in shock waves (Review)

Available experimental techniques of electrical conductivity measurements under strong shock compression are analyzed. Dielectric-semiconductor, dielectric (semiconductor)-metal, and metal-metal (semiconductor) transitions are considered. Methods and schemes of contact and contactless measurements in inert and electrically active media, implemented by various authors, are discussed. In-depth analysis of measurement circuits, two-dimensional and three-dimensional modeling of currents, fields, and hydrodynamic flows, passing from the electric engineering model to the field electromagnetic model, and allowance for transitional electrodynamic processes have contributed to the significant recent improvement of the time resolution and to extending the range of conductivity registration under shock compression. A typical feature of new techniques is solving a differential equation for the electrical circuit or finding electrical conductivity by solving an inverse boundary-value problem for the magnetic diffusion equation. In particular, the problem of electrical conductivity registration on dielectric (semiconductor) — metal transitions, which has been known since the 1950s, is solved in this manner. Difficulties, constraints, and unsolved problems of experimental techniques are discussed.