Patent GB-767899-A (GB)
Title: Improvements in and relating to electric heating elements and processes
IPC: H05B3/14
Filed: 1955-03-15
Abstract: An electric heating element for use in vacuum metallizing apparatus comprises a carbon base with a surface layer comprised substantially of a silicide of a metal of the group tungsten, molybdenum, tantalum, niobium, thorium, hafnium and zirconium or a combination of said silicides.  A layer of a metal of the said group may first be provided on the base.  The carbon base, which may be a shallow container of elongated or circular shape or a rod of round, flat or square cross section, can be heated directly by passage of current or by induced current.  According to one example, a mixture of the selected metal and silicon in powder or granulated form is piled on a horizontal surface of the carbon base or, as a thick suspension in acetone or alcohol, is applied with a brush.  Suggested ratios of the constituents of some of the mixtures are:-60 per cent tungsten and 40 per cent silicon, 58 per cent molybdenum and 42 per cent silicon, 60 per cent tantalum and 40 per cent silicon or 50 per cent niobium and 50 per cent silicon.  The carbon base with the mixture upon it is then heated gradually in vacuum or in inert gas such as argon. the metal and the silicon reacting at about 1500 DEG C., after which the temperature is raised to the melting point of the silicides, e.g. between 2000 DEG C. and 2500 DEG C. to cause the silicide layer to flow freely.  The heating is then stopped and the element is ready for use.  In a modification, copper silicide or a mixture of copper and silicon is mixed with a metal of the said group, the copper being evaporated during the firing.  In a further modification, a mixture of the oxide powder of a metal of the said group, silicon dioxide powder and aluminium or magnesium powder is fired on the base, the coating then containing aluminium or magnesium oxides. In applying a first metallic layer to the base, a binder such as a suspension of silicon powder in acetone or alcohol is applied to the base and a metal sheet not less than 0.05 mm. thick is secured on the base by mechanical means e.g. copper wire.  Silicon, copper silicide or a mixture of copper silicide and silicon is then applied to the metal sheet and the whole fired in vacuum to a temperature exceeding 2400 DEG C.  The metal to be evaporated and deposited, e.g. aluminium, titanium, zirconium, chromium is fed to the element at a rate appropriate to the rate of evaporation and so as to maintain a covering of metal upon the evaporating area of the element.ALSO:An electric heating element for use in vacuum metallizing apparatus comprises a carbon base with a surface layer comprised substantially of a silicide of a metal of the group tungsten, molybdenum, tantalum, niobium, thorium, hafnium and zirconium or a combination of said silicides.  A layer of a metal of the said group may first be provided on the base.  The carbon base, which may be a shallow container of elongated or circular shape or a rod of round, flat or square cross section can be heated directly by passage of current or by induced current.  According to one example, a mixture of the selected metal and silicon in powder or granulated form is piled on a horizontal surface of the carbon base or, as a thick suspension in acetone or alcohol, is applied with a brush.  Suggested ratios of the constituents of some of the mixtures are: 60 per cent tungsten and 40 per cent silicon, 58 per cent molybdenum and 42 per cent silicon, 60 per cent tantalum and 40 per cent silicon or 50 per cent niobium and 50 per cent silicon.  The carbon base with the mixture upon it is then heated gradually in vacuum or in inert gas such as argon, the metal and the silicon reacting at about 1500 DEG C., after which the temperature is raised to the melting point of the silicides, e.g. between 2000 DEG C. and 2500 DEG C. to cause the silicide layer to flow freely. The heating is then stopped and the element is ready for use.  In a modification, copper silicide or a mixture of copper and silicon is mixed with a metal of the said group, the copper being evaporated during the firing.  In a further modification, a mixture of the oxide powder of a metal of the said group, silicon dioxide powder and aluminium or magnesium powder is fired on the base, the coating then containing aluminium or magnesium oxides. In applying a first metallic layer to the base, a binder such as a suspension of silicon powder in acetone or alcohol is applied to the base and a metal sheet not less than 0,05 mm. thick is secured on the base by mechanical means e.g. copper wire. Silicon, copper silicide or a mixture of copper silicide and silicon is then applied to the metal sheet and the whole fired in vacuum to a temperature exceeding 2,400 DEG C.  The metal to be evaporated and deposited, e.g. aluminium, titanium, zirconium, chromium, is fed to the element at a rate appropriate to the rate of evaporation and so as to maintain a covering of metal upon the evaporating area of the element.