GB2190929A

GB2190929A - Refractory composite bodies

Info

Publication number: GB2190929A
Application number: GB08710317A
Authority: GB
Inventors: Graham Partridge; Andrew Richard Hyde; John Kenneth Haines
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1986-04-30
Filing date: 1987-04-30
Publication date: 1987-12-02
Also published as: GB2190929B; GB8710317D0; GB8610552D0

Abstract

A method of making refractory composite bodies consisting of glass-ceramic, glass, ceramic or carbon fibre embedded in a refractory matrix. The matrix is formed by coating the fibre with a sol, winding the coated fibre on to a mandrel and heat treating the composite mass to produce a sol-gel and finally a refractory matrix. The component so made may be used as an insulator or a mechanical member, for example a cylinder liner. The sol is prepared by hydrolyser of a metal salt, metal alkoxide, silica forms or silane. By use of a square section mandrel flat laminae can be produced which are plied up to form a multilayer multiangle or unidirectionally reinforced composite.

Description

SPECIFICATION Refractory composite bodies This invention relates to refractory composite bodies for use as electrical or mechanical components which have to withstand or may occasionally be subjected to elevated temperatures.

A major limit to the application of conventional organic polymers is their relatively low stability at elevated temperatures. Sol-gel technology has been described as the science of inorganic polymerisation and as such offers a method of substituting inorganic materials into the organic polymer range of products to provide 'high-temperature-withstand', i.e. refractory, capabiiities.

Sol-gel technology involves the use of organic compounds of glass, glass ceramic or other inorganic materials dissolved in an organic solvent, evaporated to form a gel and heat treated to form a refractory material.

The inclusion of fibres in a simple glass or ceramic matrix is now a well established means of increasing strength and fracture toughness of the matrix materials. Essentialiy the fibre acts to take the major part of a tensile stress applied and to cause a fracture front to deviate, thus requiring a greater work of fracture to be required to cause the fracture to propagate through the material. Furthermore, by mismatch of thermal expansion between fibre and matrix, compressive stresses can be introduced into the matrix, again leading to a strengthening and toughening effect.

Compatibility of fibre and matrix is obviously of considerable importance at all stages of manufacture and during service. The use of sol-gel compositions in this field offers the possibility of new combinations and methods of approach.

It is to be noted that other workers have combined refractory fibres with sol-gel matrix technology but the fibre lengths have been relatively short, and laminating procedures have been necessary (e.g. see US Patent 4,460,639).

According to the present invention, a method of making a refractory body comprises winding refractory fibre in one or more layers, coating the fibre, before or after winding, with a sol material and heat treating the composite body to convert the sol to a solgel and then to a refractory material.

Thus a feature of the present invention is the use of continuous fibres of considerable length, i.e., many times the dimensions of the resultant refractory body and up to several kilometres. The use of such fibres in a winding technique facilitates the forming of the body and permits considerable choice of the axis of reinforcing strength.

The fibre may comprise an individual fibre or a tow of individual fibres. The fibre may be coated with sol by drawing the fibre through the sol or by impregnation of the wound fibre with sol.

The fibre may be wound on a mandrel to produce a cross-over pattern in successive layers, the winding angle being, suitably, approximately 70".

The fibre may be wound on a rectangular section mandrel, the composite body so formed being sectioned along the corners to provide flat laminate. A plurality of the flat laminae may be superimposed, re-impreganted with sol and heat treated. The laminae may be superimposed with different orientation of the fibres to produce reinforcement in different directions.

The refractory fibre may consist of carbon, silicon carbide, E-glass, or a combination of these fibres. The sol-gel may be prepared by: hydrolysis of a metal salt, glass or ceramic oxide, a chloride, or any combination of these; hydrolysis of a metal salt of an organic acid; hydrolysis of one or more of the alkoxides of the metals aluminium, titanium, zirconium, yttrium, lithium, sodium, potassium, magnesium, calcium, strontium, barium, boron and niobium; hydrolysis of salts of one or more of the metals aluminium, titanium, zirconium, yttrium, lithium, sodium, potassium, magnesium, calcium, strontium, barium, boron and niobium; organic compounds of alumina and/or yttria; silica fume.

a silane, which may be a chlorinated silane, e.g. trichlorophenylsilane. Alternatively the silane may be tetraethoxysilane, triethoxyethylsilane or a combination of these.

The sol may contain a material producing small quantities of SiO2, Awl203, SiO2/TiO2, SiOJA203 or Nb205. Such material could be an organic compound such as an alkoxide, a lactate or an oxalate. Alternatively a small quantity of SiO2, Al203, SiO2/TiO2, SiO2/A203 or Nb205 may be added to the sol in powder form.

According to another aspect the invention comprises a composite refractory body made by a method as aforesaid.

According to a further aspect of the invention, a composite refractory body comprises a multiple layer winding of glass, glass/ceramic or ceramic fibre embedded in a refractory matrix of sol-gel derived inorganic material.

A number of methods of making a refractory body in accordance with the invention will now be described, by way of example.

The body is to consist of a refractory fibre of glass, glass/ceramic, or ceramic embedded in a matrix of glass, glass/ceramic or ceramic.

Some refractory metal fibres, e.g. tungsten, molybdenum, of say 10-15 microns diameter can also be employed. The matrix is composed of a sol-gel derived material which may also contain certain glass or ceramic powders produced by more conventional processing procedures.

A sol-gel coating mixture is prepared, typically by hydrolysis of suitable alkoxides, e.g.

silica (six2) from tetraethoxysilane Si(OEt)4.

Other sources include soluble salts and fumes.

A typical example of the derivation of a solgel is based on a combination of tetraethoxysilane and triethoxyethylsilane dissolved in isopropanol. Water and acetic acid is added to assist the conversion to a gel.

One suitable sol composition is 100ml isopropanol 25 ml tetraethoxysilane 75 ml triethoxyethylsilane 40ml water 3ml acetic acid Evaporation and heat treatment produces a gel which cures with time and temperature.

The sol can be applied to a fibre by drawing the fibre through the sol, or, where the fibre is in the form of a tow, i.e. a multiplicity of fibres, it can be impregnated by dipping in the sol.

The fibre tow may be a mixture of different fibres e.g., carbon and silicon carbide.

The coated fibre is wound upon a mandrel of dimensions and cross section according to the form of the required final component. It may be circular, rectangular, square or of any other suitable section.

While it is generally easier and more satisfactory to coat the fibre before winding it may in some circumstances be convenient to wind the fibre first and then impregnate the wound fibre.

The fibre is wound as on a bobbin, traversing the length of the mandrel repeatedly in alternate directions to produce a crossover pattern in multiple layers and a desired ratio of wall thickness to bore.

The winding angle, i.e., the angle off the direction of rotation, is conveniently about 70" but may be chosen to suit the component and may be varied from layer to layer.

The resulting wound composite bodies are cured in air followed by a heat treatment stage typically up to 1200C but within a range of 60"C to 200"C. At this stage the composite is workable and can be cut to a required section if required. Alternatively machining can be performed on the finished composite. In the case of rectangular or square section mandrel, after removal from the mandrel the composite body may be sectioned along the corners to provide flat laminae. These may be used individually or superimposed, re-impregnated with sol and heat treated to provide a monolithic body.

Orientation of the laminae so that their fibres lie in different directions provides reinforcement in corresponding directions.

After air curing the composite can be reimpregnated by sol under vacuum to reduce the porosity.

Final heart treatment involves firing at elevated temperatures generally in the region of 1000"C but within a range between 350"C and 15500C. Again, sol impregnation under vacuum may be performed to remove as far as possible any porosity of the matrix. Successive impregnations will generally improve the composite in this respect.

Refractory composite bodies formed as described above find application in various electrical and mechanical equipment. They may provide, for example, electrical stand-off insulators or cylinder liners for engines. Although a cylindrical form is the most convenient one to make, other shapes are possible. The winding mandrel may be square or tapered or otherwise than cylindrical. Similarly the outer surface may be barrel-shaped or otherwise contoured. Completely solid components can be made by machining pieces or sections out of the basic hollow cylinder.

A number of examples of sol-gel and fibre composites will now be given.

EXAMPLE 1 A tow of continuous SiC (Nicalon) fibres was dip infiltrated with a 1:3 tetraethoxysilane, triethoxyethylsilane based sol. This tow of fibres was wound onto a 40cmx6cm rotating mandrel with a 70" winding angle pattern. By continued winding a structure was produced four patterns thick. The structure was allowed to air cure for three days. Subsequent heat-treatment to 150"C was made over five days with an increase in temperature of about 30"C per day. The filament wound structure was cooled to room temperature and removed from the supporting mandrel. 15mm long rings were machined from the low tem- perature heat-treated structure. There followed heat-treatment of machined rings to 1200"C in nitrogen for one hour. Reimpregnation coupled with heat-treatment to 1 2000C in nitrogen yielded structures with apparent tensile strengths of 20MPa after four treatments.

EXAMPLE 2 A filament wound composite structure was prepared from a 3:1 tetraethoxysilane, triethoxyethylsiiane based sol and high modulus carbon fibres as described in Example 1. Heattreatment of machined 15mm rings to 12000C in nitrogen yielded structures with apparent tensile strengths of about 0.5MPa under NOL ring testing. Vacuum reimpregnation coupled with heat-treatment to 1 2000C in nitrogen yielded structures with apparent tensile strenths of 91MPa after 10 treatments.

EXAMPLE 3 A filament wound composite structure prepared from an alumina based sol and SiC fibres as described in Example 1.

EXAMPLE 4 A filament wound composite structure prepared from a tetraethoxysilane based sol and carbon and E-glass fibres (a standard, commercially available fibre) as described in Example 1 and heat-treated to 950"C.

EXAMPLE 5 A filament wound composite structure prepared from a tetraethoxysilane based sol, lead glass slurry and E-glass fibres as described in Example 1 and heat-treated up to 950"C.

EXAMPLE 6 A filament wound composite structure prepared from a tetraethoxysilane based sol and E-glass fibres. Heat-treatment of machined bars (25mmx4mmx4mm) to 500"C for 1 hour yielded modulus of rupture of about 0.5 MPa. Subsequent heat-treatment to 800"C for 1 hour yielded modulus of rupture of 21 MPa.

Instead of producing composites with a winding angle of 70 to yield a cross-over pattern the winding of tows or fibres can be altered so as to produce different angles of cross-over or structures with fibre bundles aligned side by side. By the use of a square section mandrel flat laminae can be produced which can be 'plied' up to yield a multilayer multiangle or unidirectionally reinforced composite.

EXAMPLE 7 A unidirectional reinforced composite structure prepared from a 1:3 tetraethoxysilane and triethoxyethylsilane containing sol and SiC- TiO2 (Tyranno) fibres which yielded a modulus of rupture of 368 MPa after heat-treatment to 1 1000C in nitrogen for one hour.

EXAMPLE 8 As Example 7 but coated with a 3:1 tetraethoxysilane and triethoxyethylsilane containing soi.

EXAMPLE 9 As Example 7 but coated with an alumina sol.

EXAMPLE 10 As Example 7 but with SiC (Nicalon) fibres.

EXAMPLE 11 As Example 10 but with a 1:3 tetraethoxysilane, triethoxyethylsilane containing sol.

EXAMPLE 12 As Example 11 but with a 1:1 tetraethoxysilane, triethoxyethylsilane containing sol.

EXAMPLE 13 As Example 7 but with carbon fibres.

EXAMPLE 14 As Example 13 but with a 3:1 tetraethoxysilane, triethoxyethylsilane containing sol.

EXAMPLE 15 As Example 10 but heat-treated to 120"C in air.

EXAMPLE 16 As Example 10 but coated with a 3:1 tetraethoxysilane, diethoxydiethylsilane containing sol.

EXAMPLE 17 As Example 10 but coated with a 1:1 tetraethoxysilane, diethoxydiethylsilane containing sol.

EXAMPLE 18 As Example 10 but coated with a 1:3 tetraethoxysilane, diethoxydiethylsilane containing sol.

EXAMPLE 19 As Example 17 but heat-treated to 1200C.

EXAMPLE 20 As Example 1 impregnated with a 24:72:1 tetraethoxysilane, triethoxyethylsilane, niobium sol.

Claims

1. A method of making a refractory body by winding refractory fibre in one or more layers, coating the fibre, before o after winding, with a sol material and heat treating the composite body to convert the sol to a solgel and then to a refractory material.

2. A method according to Claim 1, wherein said fibre comprises a tow of individual fibres.

3. A method according to Claim 1 or Claim 2, wherein said fibre is coated with sol by drawing the fibre through the scl.

4. A method according to Claim 1 or Claim 2, wherein said fibre is coated with sol by impregnation of the wound fibre with sol.

5. A method according to any preceding claim, wherein the fibre is wound on a mandrel to produce a cross-over pattern in successive layers.

6. A method according to Claim 5, wherein the winding angle is approximately 70 .

7. A method according to any preceding claim, wherein the fibre is wound on a rectangular section mandrel, the composite body so formed being sectioned along the corners to provide flat laminae.

8. A method according to Claim 7 wherein a plurality of said flat laminae are superimposed, re-impregnated with sol and heat treated.

9. A method according to Claim 8 wherein the laminae are superimposed with different orientation of the fibres to produce reinforcement in different directions.

10. A method according to any preceding claim, wherein the refractory fibre consists of carbon, silicon carbide, E-glass, or a combination of these fibres.

11. A method according to any preceding claim, wherein said sol-gel is prepared by hydrolysis of a metal salt, glass or ceramic oxide, a chloride, or any combination of these.

12. A method according to Claim 11 wherein said sol-gel is prepared by hydrolysis of a metal salt of an organic acid.

13. A method according to any of Claims 1 to 10, wherein said sol-gel is prepared by hydrolysis of one or more of the alkoxides of the metals aluminium, titanium, zirconium, yttrium, lithium, sodium, potassium, magnesium, calcium, strontium, barium, boron and niobium.

14. A method according to any of Claims 1 to 10, wherein said sol-gel is prepared by hydrolysis of salts of one or more of the metals aluminium, titanium, zirconium, yttrium, lithium, sodium, potassium, magnesium, calcium, strontium, barium, boron and niobium.

15. A method according to Claim 14, wherein said salts are salts of organic acids.

16. A method according to any of Claims 1 to 10, wherein said sol-gel is prepared from organic compounds of alumina and/or yttria.

17. A method according to any of Claims 1 to 10, wherein said sol-gel is prepared from a silica fume.

18. A method according to any of Claims 1 to 10, wherein said sol-gel is prepared from a silane.

19. A method according to Claim 18, wherein said silane is a chlorinated silane.

20. A method according to Claim 18, wherein said silane is trichlorophenylsilane.

21. A method according to Claim 18, wherein said silane is tetraethoxysilane, triethoxyethylsilane or a combination of these.

22. A method according to Claim 21, wherein the sol-gel is derived from a sol containing tetraethoxysilane and triethoxyethylsilane in a proportion 1:3.

23. A method according to Claim 21, wherein the sol-gel is derived from a sol containing tetraethoxysilane and triethoxyethylsilane in a proportion 3:1.

24. A method according to any of Claims 20, 21 & 22, as appendent to Claim 5, wherein the structure is cured in air for several days, and said heat-treating comprises increasing the temperature to about 150"C by about 30"C per day, cooling to room temperature, removing the mandrel, machining the aircured body to a desired form, and firing the air-cured body.

25. A method according to Claim 24, wherein said firing is performed at about 1200"C in an inert atmosphere for about one hour.

26. A method according to Claim 24 or Claim 25, including the steps of re-impregnating the fired body with sol under vacuum and then re-firing the body.

27. A method according to Claim 26 wherein the re-impregnating and firing steps are performed several times.

28. A method according to any preceding claim ;wherein said heat treatment comprises the steps of curing the composite body in air, heat treating it at a temperature between 60"C and 200"C and further heat treating at a temperature between 350"C and 1550"C.

29. A method according to any preceding claim, wherein said sol contains a material producing small quantities of a powder of SiO2, A1203, SiO2/TiO2, SiO2/A203 or Nb205.

30. A method according to any of Claims 1 to 28, wherein a small quantity of SiO2, A1203, SiO2/TiO2, SiOJA203 or Nb205. is added to the sol in powder form.

31. A method according to any preceding claim using a sol-gel such that the material of the inorganic matrix in the refractory body has a thermal coefficient of expansion slightly greater than that of the fibre material so as to maintain the matrix in compression and the fibres in tension at elevated temperatures.

32. A method of making a refractory body comprising a fibre-reinforced sol-gel matrix in accordance with any example hereinbefore described.

33. A composite refractory body made by a method according to any preceding claim.

34. A composite refractory body comprising a multiple layer winding of glass, glass/ ceramic of ceramic fibre embedded in a refractory matrix of sol-gel derived inorganic material.