Patent GB-790672-A (GB)
Title: Process of making recrystallized silicon carbide articles
IPC: C01B31/36
Filed: 1956-03-28
Abstract: A process of making recrystallized silicon carbide articles comprises preparing a free-flowing slip of water and silicon carbide crystals, which are at least 50 per cent fines no larger than 40 microns and 95 per cent pure silicon carbide, casting the slip in a porous mould (see Group V) and thereby removing enough water to coalesce the crystals to form an article which can be handled without breaking, drying the article and then firing and recrystallizing it in a reducing atmosphere at a temperature of 2100 DEG  C.-2450 DEG  C., all percentages being by weight.  A wetting agent may be added to the slip before it is poured, or a deflocculating agent, a dispersing agent or an electrolyte, some suitable substances mentioned being alkaline sulphonated alcohols or aromatics and soluble inorganic substances such as the alkali or alkaline earth salts of the halogens.  The particular use of such agents is exemplified in five examples setting forth conditions of percentages of water and silicon carbide in the slip, grain size of silicon carbide, and drying and firing temperatures to make refractory articles such as laboratory crucibles, turbine blades, rocket nozzles, filters and resistors. The porosity of the article can be controlled by varying the grain sizes and the conditions of the recrystallizing step.  Since the slip is cast and fired without a permanent bond the product is as refractory as silicon carbide itself.  In the example relating to a rocket nozzle the recrystallization takes place in an atmosphere of silica vapour with the result that the silicon carbide crystals are enlarged and the article has a harder surface and increased flame resistance.  Crucibles for melting materials which must be protected from contamination, e.g. synthetic mica, should be wholly of silicon carbide but for some purposes one or more other refractory hard carbides can be mixed with the silicon carbide to make the slip.  If a solution of sodium silicate is used as a wetting agent it acts as a dispersing agent and during the firing the silicon units with the carbonaceous vapour and forms more silicon carbide, and if the proportion of silicon carbide in the slip is already high and the firing temperature is high the resulting product can be used for filtering hydrofluoric acid and other liquids and gases which attack porcelain but not silicon carbide.  Specification 716,083, [Group V], is referred to.ALSO:Refractory filter tubes, filter plates and the like especially for liquids which attack porcelain, e.g. hydrofluoric acid, are made by a process (see Group V) in which a silicon carbide slip is cast and then fired in a reducing atmosphere, whereby the silicon carbide is recrystallized and the crystals grow together to form a porous refractory body.  The porosity can be controlled by varying the recrystallization conditions (temperature, time, nature of reducing atmosphere &c.) as well as by varying the size of the silicon carbide particles with which the slip is made and the composition of the slip, numerical examples being given.  It is stated that the nearer a condition of complete p  recrystallization is approached the more pronounced is the anastomosis of the pores. Other carbides may be mixed with the silicon carbides: titanium carbide, vanadium carbide, chromium carbide, zirconium carbide, hafnium carbide, niobium carbide, molybdenum carbide, tantalum carbide, tungsten carbide and quadriboron carbide are referred to. Specification 716,083 [Group V] is referred to.ALSO:A process of making recrystallized silicon carbide articles comprises preparing a free flowing slip of water and silicon carbide crystals, the slip having at least 10 per cent water and at least 45 per cent silicon carbide crystals which are at least 50 per cent fines no larger than 40 microns and 95 per cent pure silicon carbide, casting the slip in a porous mould and thereby removing enough water to coalesce the crystals to form an article which can be handled without breaking, drying the article and then firing and recrystallizing it in a reducing atmosphere at a temperature of 2100 DEG -2450 DEG C., all percentages being by weight.  A suitable material for the mould is plaster of Paris, which removes the water and all but a trace of material other than silicon carbide crystals.  The slip is poured in to the appropriate thickness, the clear liquid poured out of the mould, and the material dried either wholly in the mould or initially in the mould and finally outside it.  The mould may be in sections and dismantled to remove the casting, or for complicated articles the drying and firing are completed while the material is still in the mould, which is thus destroyed.  A wetting agent may be added to the slip before it is poured, or a deflocculating agent, a dispersing agent or an electrolyte, some suitable substances mentioned being alkaline sulphonated alcohols or aromatics and soluble inorganic substances such as the alkali or alkaline earth salts of the halogens.  The particular use of such agents is exemplified in five examples setting forth conditions of percentages of water and silicon carbide in the slip, grain size of silicon carbide, and drying and firing temperatures to make refractory articles such as laboratory crucibles, turbine blades, rocket nozzles, filters and resistors. The porosity of the article can be controlled by varying the grain sizes and the conditions of the recrystallizing step.  Since the slip is cast and fired without a permanent bond the product is as refractory as silicon carbide itself.  In the example relating to a rocket nozzle the recrystallization takes place in an atmosphere of silica vapour with the result that the silicon carbide crystals are enlarged and the article has a harder surface and increased flame resistance.  Crucibles for melting materials which must be protected from contamination, e.g., synthetic mica, should be wholly of silicon carbide but for some purposes one or more other refractory hard carbides can be mixed with the silicon carbide to make the slip.  A small portion of hard refractory oxides may also be added, e.g., alumina and zirconia, which are changed into carbides in the reducing atmosphere of the recrystallizing step. If a solution of sodium silicate is used as a wetting agent it acts as a dispersing agent and during the firing the silicon unites with the carbonaceous vapour and forms more silicon carbide, and if the proportion of silicon carbide in the slip is already high and the firing temperature is high the resulting product can be used for filtering hydrofluoric acid and other liquids and gases which attack porcelain but not silicon carbide. Specification 716,083 is referred to.