Elektroschmelzwerk Kempten Gmbh

Klaus Hunold

Alfred Lipp

Klaus Reinmuth

Peter Arnold

Karl A Schwetz

Johannes Napholcz

Karl-Alexander Schwetz

Jochen Kriegesmann

Peter Matje

Franz Isemann

Peter Sindlhauser

Max Mohr

Wolfgang Grellner

The invention relates to the reprocessing of used evaporation boats. This is done by a method which comprises comminuting used, aluminum-contaminated evaporation boats and roasting them in a nitrogen-containing atmosphere.

Reprocessing of used evaporation boats

Pressure-sintered polycrystalline mixed materials, with a base of boron nitride, oxides and carbides, are provided in which the hexagonal boron nitride fraction is about 30 to about 85% by weight, the oxide fraction (ZrO.sub.2, MgO) is about 10 to about 50% by weight, and the carbide fraction (SiC, TiC, ZrC) is about 5 to about 20% by weight. These mixed materials have a density of at least 94% of the theoretical density and can be produced by hot-pressing powder mixtures composed of hexagonal boron nitride, the selected oxides and carbides at temperatures in the range of from 1500.degree. C. to 1800.degree. C. under a die pressure of 7 to 20 MPa or by isostatically hot-pressing, in a vacuum tight closed case, at temperatures in the range from about 1400.degree. C. to about 1700.degree. C. under a pressure of about 100 to about 300 MPa in a high-pressure autoclave using an inert gas as the pressure transfer medium. These mixed materials are resistant to liquid metals, wear resistant and thermal shock resistant and, owing to these properties, are suitable for use as detachment or break rings in the horizontal continuous casting of steel and non-ferrous metals.

Pressure-sintered polycpystalline mixed materials with a base of

The invention is a molded body of polycrystalline aluminum nitride having a density of at least 99.8% TD and comprising:

Dense molded bodies of polycrystalline aluminum nitride and process for

The invention is an improved theroelement which comprises a graphite/boron carbide thermocouple, the arms of which are constructed as a tube and a rod, arranged concentrically inside the tube, which are electrically connected at one end. One of the arms of the thermocouple consists essentially of boron carbide formed from self bonding boron carbide by pressureless sintering. Prefabricated sleeves of high-purity, boron-oxide-free hexagonal boron nitride are used between the arms of the thermocouple as spacers. Only the end of the arm of the thermocouple constructed as a rod is secured in an electrically conductive connecting piece and the arm is freely mobile in the direction towards the cold junction. As the material for the arm of the thermocouple constructed as a rod, there can be used, for example, boron carbide having a coarse-grained structure and a density of from 60 to 80% TD, which has been manufactured from boron carbide powder having bimodal particle size distribution by means of extrusion, followed by pressureless sintering at from 2000.degree. to 2250.degree. C. The thermoelement can also be manufactured in one operation by means of pressureless sintering of the boron carbide powder in situ. Such a thermoelement comprises, for example, a flexible graphite cord for the arm of the thermocouple constructed as a rod, a boron-nitride envelope and a boron-carbide casing, the envelope and casing having been formed from powders sintered in situ without pressure.

Thermoelement comprising a graphite/boron carbide thermocouple

The invention is a process for the manufacture of shaped articles of reaction-bonded silicon nitride in which pre-shaped articles of silicon powder are heated in a first stage under a nitrogen gas pressure of at least 6 MPa at a heating rate of not more than 50.degree. C. per hour to a reaction temperature below the melting point of silicon, the article is maintained at the reaction temperature for at least 0.5 hours and subsequently, in a second stage, heating the article at a heating rate of at least 500.degree. C. per hour to a temperature above the melting point of silicon and maintaining the article at that temperature for from 1 to 7 hours. The process can nitridate both pre-shaped articles consisting of silicon powder and, optionally, up to 15% by weight Si.sub.3 N.sub.4 powder and those containing up to 10% by weight of a sintering aid such as Y.sub.2 O.sub.3. In the former case, porous shaped articles having a density of from 87 to 91% TD and, in the latter case, dense shaped articles having a density of more than 98% TD, are obtained. In all cases, in a total nitridation time of less than 15 hours, a conversion of at least 95% of the silicon originally present into silicon nitride is achieved, which, in the finished shaped article, is substantially in the .beta.-modification.

Process for the manufacture of shaped articles from reaction-bonded

The invention is a shaped article of boron nitride having a density of at least 95% of the theoretical density which consists of polycrystalline hexagonal boron nitride in the form of a homogeneous isotropic microstructure and which has been manufactured from pure boron nitride powder without the concomitant use of sintering aids by isostatic hot-pressing in a vacuum-tight casing at temperatures of from about 1200.degree. to 1500.degree. C. under a pressure of from about 50 to 300 MPa. Boron nitride powders having a free boric oxide content of not more than 1.0% by weight and having a specific surface area of from 5 to 30 m.sup.2 /g are used as starting material. The powders either are filled into prefabricated casings consisting of steel or glass and densified by vibration or are preshaped to form green bodies having pores open to the surface and then placed in prefabricated casings or coated with a material which forms a vacuum-tight casing. The powders or the preshaped green bodies in the vacuum-tight casings are then subjected to an isostatic hot-pressing process in a high-pressure zone using an inert gas such as argon or nitrogen, as the pressure-transfer medium.

Dense shaped articles consisting of polycrystalline hexagonal boron

The invention is substantially pore-free shaped articles which consist essentially of polycrystalline .alpha.- or .beta.-silicon carbide in the form of a single-phase homogeneous microstructure having grain sizes not exceeding 8 .mu.m, and which are manufactured from SiC powder, without the concomitant use of sintering aides, by isostatic hot-pressing in a vacuum-tight casing at temperatures of from about 1900.degree. C. to 2300.degree. C. and at pressures of from about 100 to 400 MPa. The SiC starting materials have .alpha.- and/or .beta.-SiC powders having a total content of metallic impurities not exceeding 0.1% by weight and a particle size of 4 .mu.m and finer.

Shaped polycrystalline silicon carbide articles and isostatic

The invention is substantially pore-free shaped articles consisting essentially of polycrystalline silicon nitride and polycrystalline silicon carbide in the form of a homogeneous microstructure having grain sizes of not more than 10 .mu.m manufactured from mixtures of Si.sub.3 N.sub.4 powder and SiC powder in a weight ratio of from 5:95 to 95:5 without the concomitant use of sintering aids by isostatic hot pressing in a vacuum-sealed casing at temperatures of from 1800.degree. to 2200.degree. C. and a pressure of from 100 to 400 MPa.

Substantially pore-free shaped articles of polycrystalline silicon

The invention is refractory, electrically conductive, mixed materials based on hexagonal boron nitride and at least one electrically conductive component such as titanium boride and zirconium boride, and having a density of at least about 95% of the theoretical density and nondirection-dependent properties, which have been manufactured from powder mixtures that are practically free of oxygen and metal and comprise

Refractory, electrically conductive, mixed materials containing boron

The invention provides substantially pore-free sintered polycrystalline articles comprising .alpha.-silicon carbide, boron carbide and free carbon, the quantitative proportions of which, in percent by weight, are defined by the trapezoidal area having, in the ternary system B/Si/C of FIG. 1 the corner points

Substantially pore-free sintered polycrystalline articles of

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. As pulverulent materials, there may be used titanium boride, graphite, SiC or boron nitride, which do not react with the glass casing, or glass powder or silica, which react with the glass casing to form a glass melt of at least the same viscosity as, or of a higher viscosity than, the glass casing.

Process for the manufacture of substantially pore-free shaped

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.

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Kempten, Germany

Klaus Hunold