Patent GB-1417791-A (GB)
Title: Fibres
Inventor: GREEN JAMES RALPH
IPC: C04B35/00
Filed: 1973-01-22
Abstract: 1417791 Vitrified coated refractory oxide fibre E I DU PONT DE NEMOURS & CO 22 Jan 1973 3090/73 Heading C1M A fibre consists of (a) a polycrystalline refractory oxide core having a diameter of 3-250 microns and consisting of grains having a mediam grain diameter of less than 3 microns and less than 10% of the diameter of the fibre core; and (b) a vitrified coating in the form of an optically uniform layer adhered to the fiber core, said coating comprising at least one glassforming oxide and having a calculated thickness of less than 1 micron and less than 5% of the diameter of the fiber core. An optically uniform layer is defined as a layer exhibiting continuous phase contrast when viewed by phase contrast microscopy and two methods of instigating optical uniformity are described. The calculated thickness of the coating is obtained from the equation: thickness in Á = G<SP>1</SP> Î 1/d c , where d c  is the density (gm./cm.<SP>3</SP>) of the coating material and G<SP>1</SP> = DW c /4V where D is the fiber substrate diameter (m), V is the volume (m<SP>3</SP>) of the fiber substrate sample and W c  is the weight of the coating (gms.). Suitable refractory oxides forming the fiber core are Al 2 O 3 , MgO, ThO 2 , ZrO 2 , Cr 2 O 3 , Fe 2 O 3 , NiO, CoO, Ce 2 O 3 , VO 2 , BeO, HfO 2 , TiO 2 , La 2 O 3  and mixtures of oxides such as 3Al 2 O 3 . 2SiO 2 , Al 2 O 3 . AlPO 4 , Al 2 O 3 . MgO, Al 2 O 3 . SiO 2  and mixed oxides of ZrO 2  and CaO or MgO. Preferred fiber cores contain at least 60% by weight of a single simple refractory oxide, preferably Al 2 O 3 . Other oxdies which may be present are B 2 O 3 , P 2 O 5 , As 2 O 3 , TeO 2  and SiO 2 . Most preferably the fiber core is substantially pure aluminia or alumina containing up to 5% by weight of oxides of Co, Mg, La, Ni, Cu or Cd. The porosity of the fiber cores is preferably below 20%. The vitrified coating is formed by heating a fiber core which has been coated with one or more glass-forming oxides or precursors therefor, optionally together with one or more intermediates and/or modifiers and/or precursors therefor. Suitable "glass formers" are oxides of Si, B, Ge and P; suitable "intermediates" are TiO 2 , ZnO, PbO and BeO; and suitable "modifiers" Ii 2 O, MgO, CaO, CdO, BaO and SrO. The coating prior to vitrification may be amorphous or partially or totally crystalline. Precursors of glass-forming oxides include SiCl 4  and organo-silicon compounds which are convertible to SiO 2 , such as organopolysiloxanes e.g. poly dimethyl siloxane. Other suitable precursors are BCl 3 , BBr 3 , PCl 3 , POCl, and GeCl 4 . The vitrified coating preferably contains at least 50% by weight of silica based on the total weight of the coating, e.g. substantially pure SiO 2 . The glass-forming oxide or precursor therefor may be applied in solid, liquid or vapour form, e.g. a colloidal aqueous dispersion of the oxide. Precursors are more usually applied as a liquid or vapour and this method is applied to the coating if bundles of fibers, the individual fibers not sticking together. One method for coating fibres with a water-reactive precursor of silica, e.g. SiCl 4 , comprises exposing the fibers to an atmosphere of steam and then passing the moist fibers through SiCl 4 , which reacts with water on the fiber surface to produce hydrated silica. Common solvents or diluents may be included in the liquid coating composition. Liquids may be applied by means of a bath, by passage over a surface wet with the composition or by spraying. The oxides or precursors that have been applied are #itrified by heating the fiber at a temperature above the melting or sintering temperature of the glass-forming oxide, e.g. greater than 1100‹ C. for SiO 2 . Amorphous silica may be vitrified by heating the coated fiber at a temperature of at least 1350‹ C. for less than 30 seconds, e.g. at 1500-1900‹ C. for 0À1-5 seconds. The specific examples describe the coating of alumina fibres with: (1) silicon tetrachloride vapour; (2a) a solution of sodium silicate (Na 2 O/SiO 2  = 1/3À25) which theoreticaly produces a vitrified coating of 76% by weight SiO 2  and 24% by weight Na 2 O; (2b) an aqueous solution of guaridinium silicate and lithium chloride (Li 2 O/SiO 2  = 1/19) which theoretically produces 95 % by weight SiO 2  and 5 % by weight Li 2 O; (3) liquid silicon tetrachloride; (4) three different dimethyl polysiloxanes and a chlorophenyl methyl silicone; and (5) a colloidal silica dispersion.