Aluminium boron carbides

Inoue Z, Tanaka H, Inomata Y, Synthesis and X-ray crystallography of aluminium boron carbide, AI8B4C7 , J Mater Sci, 1980 15 3036-3040. 

The carbides and nitrides are well known for their hardness and strength, and this section will briefly compare a number of these properties with those of the pure metals. Concentration will be placed here on the first row compounds, since these constitute a complete series, and Mo and W, since these are the most commonly studied metals. As will be shown, the physical and mechanical properties of carbides and nitrides resemble those of ceramics not those of metals. Comparisons will be made with boron carbide (B4C), silicon carbide (SiC), aluminium nitride (AIN), silicon nitride (Si3N4), aluminium oxide (A1203), and diamond, as representative ceramic materials. 

Acceptor dopants are introduced in the crucible either in elemental form or in the form of carbides. If a dopant is introduced in elemental form, it is placed in a special internal crucible with carbon or silicon carbide powder. This is required to prevent the dissolution of the crucible, in the case of aluminium doping, and to reduce the boron vapour pressure to the equilibrium value for the SiC-C system, in the case of boron doping. If elemental boron is placed in the vicinity of the substrate, this results in the formation of boron carbide on the crystal faces of SiC [46]. For moderate doping of crystals, grown at high temperatures, doped SiC sources also can be employed. 

Reactivity is controlled by rods consisting of articulated absorber elements formed from hollow cylindrical sections of boron carbide (65 mm diameter x7-5 mm thick) sheathed in the annulus between two aluminium alloy tubes of 70 mmx2 mm and 50 mmx2 mm respectively. They are inserted or removed from the core at a rate of 0-4 m/s (the 12 local automatic control rods are withdrawn at 0-2 m/s) by individual servomotors installed at the top of the control rod channels. With the exception of the automatic rods, all the rods are fitted with graphite followers so that, as they are withdrawn, they are not replaced by water. The square lattice of 211 control rods and 12 vertical power profile sensors has a pitch of 700 mm and is angled at 45 to the fuel lattice. The channels are made... 

Sensitivity of NM is also increased by additives that are inert to it and which can represent active EMs (RDX, ammonium nitrate) [137], or by non-active hard powders (aluminium, copper, fumed silica, boron carbide etc.) [131,137]. The amounts of the additives are 3 and 40% w/w in [131] and [136], respectively. The sensitivity of the mixtures is higher than that of neat NM [131,137] however, for example, the sensitivity increase achieved by addition of 3% w/w of boron carbide is less efficient than the effect of the same amount of added diethylenetriamine [131]. The increase can be due [131] to difference in the number density of each in the mixture (difference in the number of available hot spots) or to an impedance mismatch difference, or to a fimction of both variables. 

The conclusion that can be drawn from these experiments is that the use of highly priced fillers such as diamond powder does not improve the thermal conductivity better than less expensive materials such as aluminium nitride, boron nitride, boron carbide, or sdicon carbide. Within certain limits, the higher the X value of the filler particles, the higher the thermal conductivity of the adhesives with respect to the X/Xp ratio that exhibits a favourable optimized value at about 100. This means that fillers with a thermal conductivity in the range... 

Pan] Panasiuk, A.D., Oreshkin, V.D., Maslennikova, V.R., Investigation of Kineties of Boron Carbide Interaction with Liquid Aluminium, Silicon, Nickel and Iron (in Russian), Poroshkov. Metall. (Kiev), 199(7), 79-83 (1979) (Experimental, Kinetics, Morphology, Meehan. Prop., 9)... 

Finely divided boron, tetraboron carbide, and boron-aluminium mixtures will... 

When a composite is subjected to external forces, the energy of the matrix is only transferred to the fibres when there is question of a proper attachment. For that reason fibres are some-times provided with a layer of another material. An example boron fibres in an aluminium matrix are provided with a silicon carbide coating and as a result the fibres are called borsic fibres. The thermal expansion coefficient of a fibre and its matrix must correspond. Figure 14.9 is a representation of what takes place when a crack in a fibre-reinforced matrix grows. 

Traditional fibres used as reinforcement in polymer composites are generally either polymers or ceramics the polymer aramids, glass, carbon, boron, aluminium oxide and silicon carbide. Carbon is a high-performance fibre material that is the most commonly used reinforcement in advanced polymer-matrix composites. Glass fibre is readily available and may be fabricated into a glass-reinforced plastic economically using a wide variety of composite-manufacturing techniques. 

Natural fibres such as flax, hemp, silk, jute, sisal, kenaf, cotton, etc are being used to reinforce matrices mainly thermoplastics and thermosets by many researchers. The principal synthetic fibres in commercial use are various types of glass, carbon, or aramid although other fibres, such as boron, silicon carbide, and aluminium oxide, are used in limited quantities. All these fibres can be incorporated into a matrix either in continuous lengths or in discontinuous (short) lengths. Both these fibres have some advantages and disadvantages. 

It is possible to make electrolithically deposited coatings in special cells and in the cells under operation [123, 124]. Thermodynamically, it is possible to receive the deposited coatings, combining the addition of titanium compounds in electrolyte, and boron oxides to the carbon anode material. Metal oxides dissolve in electrolyte the ions of metals discharge at the cathode and deposit on the cathode as titanium boride and titanium carbide. The problem involves the poor controllability of the process and the need to fulfill the required purity of aluminium (in titanium and boron content). Once small amounts of boron oxide and titanium (in the form of oxide of salt) are added, it is possible to obtain the metal of required purity and quality, but the coating process lasts for a long time and is poorly ccmtroUed. 

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