Abstract: The invention relates to a process for the manufacture of substantially pore-free shaped articles of polycrystalline material such as aluminium oxide, titanium boride, silicon nitride or silicon carbide, in which pulverulent materials or preformed articles having pores open towards the surface, in a vacuum-tight sealed glass casing that has a viscosity of less than 10.sup.6 Pas at the pressing temperature employed, are isostatically hot-pressed in a high-pressure autoclave using an inert gas as pressure-transfer medium. By being embedded in a flowable pile or by having a layer applied to them in the form of a suspension, the glass-encased powders or articles are brought into contact with a heat-resistant pulverulent material that, upon softening of the glass casing, penetrates into the surface of the glass of reacts with it, with the result that the glass casing is reinforced from the outside at the pressing temperature.

Abstract: The invention is a process for the manufacture of substantially pore-free shaped articles from polycrystalline material, such as tungsten carbide, silicon nitride, titanium boride or silicon carbide, in which encased articles preformed from pulverulent materials and having open pores are isostatically hot-pressed in a vacuum-sealed casing of vitreous or ceramic-like material in a high-pressure autoclave using an inert gas as pressure-transfer medium. Before the application of the casing material, a first layer comprising material that can be decomposed or melted, such as an organic wax or a resin-like product, or of a compressible heat-resistant material such as inorganic fibers or felts, is applied to the preformed articles. Then, the encased articles are thermally treated until the glass or ceramic layer is fused or sintered gas-tight. The articles pretreated in this manner are then isostatically hot-pressed.

Abstract: The invention relates to dense shaped articles of polycrystalline .beta.-silicon carbide containing small amounts of aluminum and optionally, nitrogen and/or phosphorus, having a virtually single-phase microstructure since the additional constituents are substantially in the form of a solid solution in the .beta.-SiC lattice. The shaped articles are characterized by high temperature flexural strengths of at least 600 N/mm.sup.2 at temperatures up to 1450.degree. C. The fracture mechanism is transgranular up to at least 1450.degree. C. The shaped articles are manufactured by hot-pressing and simultaneously shaping pulverulent .beta.-silicon carbide with small amounts of an aluminum-containing additive, such as aluminum powder or aluminum nitride and/or aluminum phosphide, at temperatures between 1750.degree. C. and 2200.degree. C. under a pressure of at least 100 bar (10 MPa).

Abstract: The subject of the invention is thin large-area neutron-absorber plates having a volume composition of from 40 to 60% and preferably from 45 to 60% by volume of boron carbide, from 25 to 5% by volume and preferably 15 to 5% by volume of free carbon, the remainder being pores, a density of from 1.4 to 1.8 g/cm.sup.3, a flexural strength at room temperature of from 15 to 45 N/mm.sup.2, a compressive strength at room temperature of from 25 to 60 N/mm.sup.2, a modulus of elasticity at room temperature of from 10,000 to 20,000 N/mm.sup.2, and a resistance to ionizing radiation of at least 10.sup.11 rad, which plates may be produced by mixing boron carbide powder, containing at least 75% by weight of boron and a proportion of boron oxide of less than 0.5% by weight, and having a particle size distribution of at least 95% finer than 50 .mu.

Abstract: There are provided shaped articles of polycrystalline silicon carbide having densities of at least 97% and improved mechanical properties particularly at high temperatures, said articles consisting ofat least 95.4% by weight of .alpha.-silicon carbide,about 0.1 to 2.0% by weight of additional carbon,about 0.2 to 2.0% by weight of aluminium,about 0 to 0.5% by weight of nitrogen, andabout 0 to 0.1% by weight of oxygen,the .alpha.-silicon carbide being in the form of a homogeneous microstructure with an average grain size of less than 10 .mu.m.There is also provided a process for the preparation of such articles by pressureless sintering submicron powder compacts consisting of .alpha.-silicon carbide and aluminium- and carbon-containing additives as sintering aids.

Abstract: A method of producing dense shaped articles of pure boron carbide that optionally contain 0.1 to 8% by weight of free carbon in the form of graphite is disclosed involving the steps of homogeneously mixing boron carbide in submicron powder form with small quantities of a carbon containing additive, forming the powder mixture into a shaped green body and then sintering the body in a controlled atmosphere and in the absence of external pressure at a temperature of about 2100.degree.-2200.degree. C. The shaped articles thus obtained are polycrystalline and, insofar as they correspond to the stoichiometric composition B.sub.4 C, single-phase; they have an average grain size of not more than 10 .mu.m, a density of at least 90% of the theoretical density of boron carbide and a flexural strength of at least 300 N/mm.sup.2. The fracture mode of the sintered articles is completely transcrystalline.

Abstract: An electric resistance furnace for the production of silicon carbide from silicon dioxide and carbon has a U-shaped carbonaceous resistance core embedded in the burden of coke, quartz and other additives. The electrodes are disposed adjacent to each other and are connected to the resistance core at the ends of the legs of the U. The ratio of the length of the resistance core to the electrode spacing is at least 1.6.

Abstract: Process for the manufacture of hexagonal boron nitride which comprises causing a starting material containing an alkali metal boride or an alkaline earth metal boride, preferably a hexa- or dodecaboride, to react with nitrogen at temperatures of at least 900.degree. C and, if desired, up to 2500.degree. C, in the presence of impurities or additives of other boron compounds or compounds of the alkaline earth metals or of carbon compounds. Boron nitride has many uses in various fields, among them, electronics and crucible melting for most non-metallic melts.

Abstract: An electrical resistance furnace installation, especially for the production of silicon carbide, has no side walls. In the one arrangement, the installation has two end walls, each provided with an electrode, with the resistive core for heating reacting valves extending between the electrodes at the end wall. In another arrangement, the installation has only one end wall, with the resistive core extending between an electrode in the one end wall, and a connection to a further electrode arranged in the bottom of the furnace. In a still further arrangement, the furnace has neither end walls nor side walls, and the resistive core is horizontally supported above the furnace floor and electrically connected through electrodes in the bottom of the furnace. The reacting ballast, in each embodiment, is poured onto the furnace floor to surround the resistive core for heating of the ballast.

Abstract: An electrical resistance furnace, especially for the production of silicon carbide, in which the furnace has a porous bed on its floor, and a gas flow is provided for withdrawing gaseous reaction products from the furnace. Alternatively, or in addition to the porous bed, a covering sheet is provided over the burden. In addition, ducts may be provided within the furnace, beneath the cover and/or in communication with the porous bed, for the removal of gaseous products. The furnace may be of the open mound type, without side walls, in which case the cover may be a flexible cover that covers the top and sides of the burden.

Abstract: Process for preparing hydrophobic SiO.sub.2 by reacting solid SiO.sub.2 of finest particle size with organosilicon compounds while subjecting the reaction mixture to a mechanical impact treatment which will at the same time result in a comminution of the SiO.sub.2 particles as well as of their aggregates, said process comprising incorporation to the reaction mixture, in addition to the water adsorbed by the SiO.sub.2, an amount of 0.3 to 3% by weight of water calculated on the weight of the SiO.sub.2.

Abstract: In a furnace installation for producing silicon carbide in an intermittent operation, carbon electrodes arranged in the bottom of the furnace. A horizontally arranged resistance core is electrically connected to the carbon electrodes by vertically extending elements of an electrically conductive material having a higher specific conductivity than the core material. The resistivity of the connection between the electrodes and the core and the current passing through the connection produces a temperature lower than that necessary to form silicon carbide, and thereby prevents the destruction of the connections and electrodes normally caused by the downward movement of the silicon carbide roller.

Abstract: The present invention relates to powdered boriding compositions for the production of single-phase boride layers on metal articles being substantially free from cracks in the case of mechanical stress, which comprise an activator and a mixture of a boron-releasing agent and an unreactive refractory material, such as silicon carbide.