GB1580041A

GB1580041A - Refractory materials

Info

Publication number: GB1580041A
Application number: GB13123/76A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1976-03-31
Filing date: 1976-03-31
Publication date: 1980-11-26
Also published as: SE7703703L; FR2346303A1; CA1096892A; DE2714139A1; IT1116726B; JPS52125516A; FR2346303B1; SE421783B

Description

(54) IMPROVEMENTS IN OR RELATING TO REFRACTORY MATERIALS (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to refractory materials and finds one application in the production of nuclear fuels (e.g. ceramic particles for nuclear fuel applications).

The term "refractory materials" as used in this specification includes ceramic materials and metals.

Our British Patents Nos. 1175834, 1231385, 1253807, 1313750 and 1363532 relate to gel-precipitation processes and reference should be made to these for information regarding such processes. British Patent No. 1,313,750 discloses "modifying agents" which can be used in gelprecipitation processes (e.g. in the preparation of actinide metal oxide ceramic particles containing uranium oxide and/or plutonium oxide).

Briefly, in the production of a refractory material body by the gel-precipitation process a feed solution containing a compound of an element (typically of a metal) and an organic gelling agent (gelating agent), or agents, is introduced into a precipitating agent to give a gel containing the element and the gelling agent, or a derivative thereof.

(It will be understood that the element will generally be present in the form of a compound, rather than as the free element.) The feed solution may also contain other constituents such as a modifying agent as hereinbefore mentioned. The gel precipitated gel is conveniently subsequently washed with water, treated to debond it and heated to give a refractory material.

The organic gelling agent enables the feed solution to gel in a coherent manner in the presence of a precipitating agent. Such gelling agents are usually water soluble high molecular weight polymeric compounds as disclosed in our British Patent specifications hereinbefore mentioned.

As disclosed in our British Patent Specification Nos. 1,363,532 and 1,313,750 carbon may be added to the feed solution for example for use in a reducing step to give a carbide, oxycarbide or metal, or for introducing porosity into the refractory product (i.e. in this last case carbon is acting as a fugitive additive) .

According to one aspect of the present invention there is provided a process for the production of a refractory material which includes heating an intermediate material containing carbon to cause a thermally induced reaction involving carbon in the intermediate material, wherein the intermediate material has been produced by heating a shaped gel precipitated gel, and the carbon in the intermediate material for participating in the thermally induced reaction has been produced from a gelling agent, or a derivative thereof, incorporated in the gel during gel precipitation.

In one embodiment the process of the present invention also includes an additional step of heating a gel precipitated gel to produce the intermediate material containing carbon.

In another embodiment the quantity of carbon in the intermediate material is such that, and the heating of the intermediate material containing carbon is carried out under conditions such that, the thermally induced reaction involving carbon is the carbothermic reduction of a chemical substance in the intermediate material.

The present invention can be applied to the production of porous refractory materials. Thus, carbon in the intermediate material for subsequent removal to give porosity can be produced from gelling agent, or a derivative thereof, incorporated in the gel during gel precipitation.

Thus, in a further embodiment of the process of the present invention the quantity of carbon in the intermediate material is such that, and the heating of the intermediate material containing carbon is carried out under conditions such that, the thermally induced reaction involving carbon results in the removal of carbon from the intermediate material to give porosity.

(It will be appreciated that to produce porosity the intermediate material can be heated in the presence of a gas which can react with the carbon to give a volatile product. Such a gas can be, for example, oxygen so that carbon is removed mainly as carbon dioxide (with some carbon monoxide), or the gas may be carbon dioxide in which case the carbon is removed as carbon monoxide.) It is understood that where a carbothermic reduction is to be effected the gel precipitated gel is heated first to convert the gelling agent, or derivative thereof, to carbon by thermal degradation and subsequently to effect carbothermic reduction. Also it is to be understood that where carbon is to be used in a carbothermic reduction step (and is therefore not to be removed to produce porosity) the carbonising step and subsequent heating to achieve carbothermic reduction should be garried out in a substantially non-oxidising atmosphere (e.g. argon).

Where the refractory material is required in the form of particles (e.g. ceramic nuclear fuel microspheres or tungsten carbide microspheres) the gel precipitation process may be carried out by forming the feed solution into droplets and contacting the droplets with a precipitating agent to give gel particles.

In carrying out the process of the present invention to produce a refractory material we prefer that the gel precipitated gel to be heated is prepared from a feed solution containing an inorganic salt or a sol. In one embodiment of the invention the inorganic salt or sol and the precipitating agent are chosen such that the gel precipitated gel to be heated to produce a refractory material in accordance with the present invention con tains, in addition to the gelling agent or derivative thereof, a hydrous oxide or hyd roxide. For example the inorganic salt may be a metal nitrate and the precipitating agent a base (e.g. ammonium hydroxide) so that the gel precipitated gel contains gelling agent or derivative thereof and a hydrous metal oxide or hydroxide.

Also in preparing a refractory material in accordance with the present invention the composition of the refractory material can be further influenced by optionally incorporat ing a an additive (e.g. tungsten carbide or cobalt) in the feed solution.

In addition gel precipitated gels for the production of refractory materials in accordance with the present invention may be produced from feed solutions containing modifying agents are hereinabove mentioned. The term "modifying agent as used in this Specification is defined as having the meaning disclosed in BP 1313750 page 1 lines 27 to 36.

In producing gels by the gel precipitation process if is generally the practice to incorporate as little gelling agent in the feed solution as is consistent with obtaining satisfactory properties in the precipitated gel (e.g. satisfactory gel microsphere formation where ceramic particles are to be produced. A common concentration in such cases is 1 to 2% by weight).

However in carrying the present invention into effect it will be understood it is necessary to incorporate enough gelling agent, or derivative thereof, in the gel to produce sufficient carbon to effect the desired reaction involving carbon (e.g. reduction) on heating of the gel. (For example in the case of forming a U/Pu carbide the gelling agent (or derivative) content in the gel can be of the order of 12%).

It has been observed that in certain circumstances it is not possible to retain a sufficiently large amount of gelling agent or derivative thereof in the gel since that proportion of the gelling agent, or derivative thereof, which is in excess of that chemically required by other constituents of the feed solution tends to pass into solution during contact with the precipitating agent and in the subsequent washing step.

Also, as hereinafter mentioned, some gelling agents have lower carbon yields than others.

Thus, two main factors in choosing a gelling agent for a particular application are: a) the gelling agent should generally be one which interacts, or gives rise to a derivative which interacts, with other chemical constituents (e.g. metal compounds) in the feed solution sufficiently strongly to form a gel precipitate which retains gelling agent, or derivative thereof, in the gel precipitate during contact with the chosen precipitating agent and during subsequent washing.

b) to keep to a minimum the amount of gelling agent, or derivative thereof, needed to provide a desired amount of carbon in the intermediate material the gelling agent should be chosen from those suitable for a particular application on the basis of highest carbon yield.

Concerning a), to enable sufficient gelling agent to be retained in the gel during the precipitation to give sufficient carbon for the reduction step the gel precipitation feed solution should generally contain at least one element (e.g. metal species) capable of interacting with the gelling agent to form a gelatinous precipitate in which the gelling agent or derivative thereof is retained during the precipitation and subsequent washing and drying steps.

The ability to interact may be affected by the reactive nature of the element and in certain circumstances the choice of the gelling agent.

Tetravalent metal species such as thorium and plutonium are examples of elements capable of interacting with a gelling agent to form a gelatinous precipitate as hereinbefore mentioned.

Where the gel precipitation feed solution is intended to produce gel precipitated gel particles as an intermediate in the production of refractory particles, the viscosity of the gel precipitation feed solution must be sufficiently low to facilitate the formation of droplets of solution for contacting with a precipitating agent. This means that a gelling agent used in the production of gels for the production of refractory materials in accordance with the present invention will normally be of a lower molecular weight than that generally used in gel precipitation processes. This of course arises since in accordance with the present invention it is generally necessary to use a higher concentration of gelling agent, but still meet the viscosity requirement hereinbefore mentioned.

In carrying out the process of the present invention a wide variety of known gelling agents may be used to form the gel precipitate gel.

However we have found that polyacrylamide is generally a satisfactory gelling agent for use in the production of refractory materials for use in the nuclear and nonnuclear fields. Thus polyacrylamide has been used successfully in accordance with the process of the present invention to produce particulate refractory particles or uranium/ a plutonium oxide, uranium/pluronium carbide, uranium/thorium carbide, tungsten carbide and tungsten carbide/cobalt metal by the gel precipitation route from feed solutions containing compounds of plutonium, thorium or uranium and mixtures thereof, and also from feed solutions containing tungsten compounds and optionally cobalt metal.

A particular polyacrylamide we have used as a gelling agent is Superfloc 16 (Registered Trade Mark) which is a polyacrylamide of average molecular weight 4 x 106 and marketed by Cyanamid of Great Britain Limited.

This polyacrylamide may be degreaded to a lower molecular weight for use in accordance with the present invention.

The quantity of gelling agent, or derivative thereof, incorporated in the gel precipitated gel, and hence the amount of carbon available in the intermediate material, can be controlled to produce various refractory material products.

For example it is possible to include only sufficient gelling agent to give enough carbon to effect a desired change in an oxygen to metal ratio in an oxide (e.g. uranium/a plutonium dioxide). Alternatively further gelling agent can be incorporated in the gel in order to give sufficient carbon to produce metal by reduction. Yet further gelling agent can be used to give sufficient carbon to produce a carbide or even a carbide plus some free carbon in the final refractory material.

In some cases free energy considerations favour the production of an oxide or a carbide rather than a metal (e.g. in the case of plutonium). In these circumstances oxycarbides rather then metal will be produced as the carbon content is increased from that necessary to reduce oxide to metal ratio to that necessary to produce carbides.

We have found that a plot of the amount of polyacrylamide gelling agent in the feed solution against the weight So carbon in the intermediate material of the present invention shows a good linear relationship. Different gelling agents can lead to different proportions of carbon in the intermediate material. In general gelling atents of relatively high oxygen content (e.g. dextran) will have low carbon residues whereas gelling agents of low oxygen content (e.g. polyacrylamide) will have high carbon residues.

It will be appreciated that the present invention may be utilised substantially to avoid the necessity of adding additional carbon in the feed material in order to carry out carbothermic reduction.

Using the gelling agent as a precursoe for carbon in the present invention has the advantages: (i) that the carbon may be evenly dis tributed throughout the intermedi ate material, (ii) the carbon may be more reactive since it is more finely divided than carbon added in the feed solution stage, and (iii) the preparation of the feed solution may be simplified.

(In connection with i) above, it will be understood that the carbon may be evenly distributed in the sense of homogeneously distributed throughout a given portion of intermediate material and, where the intermediate material is particulate, in the sense of evenly distributed between the particles.

In known methods where additional carbon is added there is the disadvantage that it may not be distributed evenly between the particles when the intermediate material is particulate. In view of the foregoing comments it will be appreciated that the present invention may be used to substantially avoid this disadvantage).

Carbides produced in accordance with the present invention can be further treated by nitriding to give nitrides.

According to a further aspect the present invention provides a refractory material produced by a process in accordance with the invention.

The invention will now be further described by reference to the following examples: Example 1 In this example a carbide was produced in accordance with the present invention.

30 mls of a solution containing 350 grams heavy metal perlitre of plutonium and uranyl nitrate (with a U:Pu ratio of 70:30) in 2.2 M nitric acid was mixed with 52.6 grams of degraded Superfloc S16 (Registered Trade Mark) in the form of 10.15 weight %solution in 92.5 %formamide water mixture (viscosity 5.5 poise) to form a gel precipitation feed solution.

The carbon available from the Superfloc S 16 gelling agent was calculated as being equivalent to that required by the reaction: MO2 + 3C > MC + 2CO The above feed solution was thoroughly stirred and then formed into spherical droplets which were subsequently contacted with ammonia gas and ammonium hydroxide to produce gel precipitated gel spheres. The gel spheres were washed with cold water and then with hot water (90"C) prior to drying in air at 22"C. The dried gel spheres were heated in argon to a maximum temperature of 500"C to give an intermediate material which had a mercury density of 4.16 grams per cc and contained 11.75% carbon.

The intermediate material was subsequently heated in argon at 16500C. The refractory ceramic product thus produced was found to have a mercury density of 11.67 grams per cc (i.e. 87.1% of theory for (U/Pc)C) and to contain 5.21% carbon, 0.11% oxygen and 0.07% nitrogen.

Example 2 In this example the oxygen to metal ratio in a plutonium/uranium oxide system was reduced in accordance with the present invention.

A gel precipitation feed solution was prepared by thoroughly mixing 30 mls of a plutonium nitrate and uranyl nitrate (with a U:Pu ratio of 70:30) solution (350 grams heavy metal per litre concentration) in 2.2 M nitric acid with 30 mls of degraded Superfloc S 16 (Registered Trade Mark) 4%solution in 95% formamide water mixture).

The gel precipitation feed solution thus prepared was formed into spherical droplets and contacted with ammonia and with ammonium hydroxide (containing 0.1% TOT surfactant; "TOT" is the trade name of a surfactant marketed by DEB Chemical Proprietaries Ltd) to form gel precipitated gel spheres. These gel spheres were washed in cold water and hot water (900 C) and subsequently dried by contacting with hexanol.

The gel spheres were heated to a maximum of 700"C in a mixture of 5% hydrogen/argon. After attaining this temperature CO2 was passed over the spheres for 15 minutes and argon subsequently passed over the spheres for 5 minutes prior to allowing the spheres to cool in a 5% hydrogen/argon mixture to give an intermediate material having a carbon content of 1.09%.

This intermediate material was sintered under vacuum at a maximum temperature to 16500C. The refractory product contained 25 ppm carbon, 15 ppm hydrogen, had an oxygen to metal ratio of 1.911 and a mercury density of 10.7 grams per cc. (i.e. 98.2% of theoretical density).

Example 3 In this example an intermediate material suitable for the preparation of a thorium/uranium carbide was prepared.

A gel precipitation feed solution was prepared by thoroughly mixing 30 mls of thorium and uranium nitrates (with a U:Th ratio of 70:30) in 2.2 M nitric acid (metal concentration equivalent to 350 grams per litre) and 30 mls of 10.15% solution of degraded Superfloc S 16 (Registered Trade Mark) in 92.5% formamide water mixture.

The gel precipitation feed solution thus prepared was formed into spherical droplets and contacted with ammonia and ammonium hydroxide containing 0.1 % TOT surfactant.

The gel spheres thus produced were washed in cold and hot (900C) water and air dried overnight. Subsequently the gel spheres were heated to 500"C under argon to give an intermediate material which contained 7.4% carbon and was suitable for further treatment to produce a carbide.

Example 4 In this example tungsten carbide was prepared in accordance with the present invention.

A gel precipitation feed solution was prepared by thoroughly mixing together 10 mls of aqueous sodium tungstate solution (350 grams tungsten metal per litre concentration), 5.05 grams degraded Superfloc S 16 solution (Registered Trade Mark) (24.48% in 92.5%formamide/water mixture) and 0.1 ml of the surfactant Nonidet P42 (Registered Trade Mark).

This feed solution was dropped in the form of spherical droplets through HCI gas, through an HCI foam and into conc. HCI solution containing a surfactant (Nonidet P42) to form gel spheres in a manner as disclosed in our British Patent No.

1,401,962.

The gel spheres were washed in hot water and air dried and subsequently heated at a maximum temperature of 600"C in argon to produce an intermediate product having 17.7% carbon content.

The intermediate product was heated for 1 hour at 1 1000C under argon to produce tungsten carbide spheres having less than 2% impurities.

Example 5 In this example tungsten carbide spheres were produced using ammonium tungstae.

Thus a gel precipitation feed solution was prepared by thoroughly mixing together 18.1 mls ammonium tungstate solution 115.6 gm per litre concentration), 8.405 grms degraded Superfloc S 16 (Registered Trade Mark) solution (24.48% in 92.5% formamide water mixture) and 0.05 mls of the surfactant Nonidet P42 (Registered Trade Mark).

The gel spheres were prepared by contacting droplets of the feed solution with hydrochloric acid.

The gel spheres were dried and heated to 1200"C under an argon atmosphere to produce an intermediate material having 17.2% carbon content. Subsequently the intermediate material was heated to 1700"C under argon to produce partially sintered tungsten carbide spheres.

Example 6 In this example tungsten carbide was prepared using a feed solution to which a suspension of tungsten carbide had been added in order to increase the amount of tungsten carbide in the product.

Thus a gel precipitation feed solution was prepeared by thoroughly mixing together 8.36 grms of sodium tungstate in solution, 33.62 gm of degreaded Superfloc S 16 (Registered Trade Mark) solution (24.48% in 95% formamide/water mixture), 25 mls water, 25 grms of suspended tungsten carbide and 0.2 mls of Nonidet P42 (Registered Trade Mark) surfactant.

The feed solution was formed into droplets and the droplets contacted with a layer of hexanol saturated with HC1 and subsequently with concentrated HC1 solution to give coherent, uniform, regularly sized gel spheres.

The gel spheres were heated to 12000C to give an intermediate material having 17.2% carbon content (ignoring added tungsten carbide). The intermediate material was then heated to 1700"C to give tungsten carbide containing less than 1 % weight free metal.

Example 7 In this example a refractory product containing tungsten carbide and cobalt metal was prepared.

A gel precipitation feed solution was prepared by thoroughly mixing 23.8 mls of sodium tungstate solution (350 gm tungsten per litre concentration), 33.6 gm of degraded Superfloc S 16 (Registered Trade Mark) solution (24.48 wt% in 92.5% formamide/water) o.94 gm of finely divided cobalt metal and 10 mls of water containing 0.2 mls of the surfactant Nonidet P42 (Registered Trade Mark).

The feed solution was formed into droplets and contacted with HCl foam and HCl solution containing 2 mls of Nonidet P42/1 to form gel spheres of approximately 2 mm diameter. The gel spheres were washed with hot water and air dried for 16 hours at 220C.

Subsequently the dried gel spheres were heated at 600"C in argon to give an intermediate material having 16% carbon content.

The intermediate material was then taken up to 16500C and held at that temperature for 100 minutes to give a spherical refractory product containing tungsten carbide and cobtalt metal.

Example 8 In this example tungsten carbide was produced with cobalt metal additive.

A feed solution was prepared by mixing 23.8 mls of sodium tungstate solution (350 grms per litre concentration), degraded Superfloc S 16 (Registered Trade Mark) (33.62 grms), 0.94 grms of cobalt metal, 10 mls of water and 0.2 mls of Nonidet P42 (Registered Trade Mark).

The feed solution was formed into droplets and the droplets were passed through a layer of hexanol saturated with HC1 and subsequently through a concentrated HC1 solution to produce gel spheres.

After washing with hot water and distilled water the particles were superficially dried with acetone and subsequently air dried.

The gel particles were heated under argon at 600"C to give an intermediate material having a carbon content of 16%, and this was subsequently heated to 11000C to give a product containing tungsten carbide with a crystallite size of 450 A and cobalt in the form of metal.

Example 9 The procedure of Example 6 was repeated with the addition of 7 gm of finely divided cobalt metal to the gel precipitation feed solution.

The refractory product produced had the composition: > 99% tungsten carbide (ignoring cobalt).

WHAT WE CLAIM IS: 1. A process for the production of a refractory material which includes heating an intermediate material containing carbon to cause a thermally induced reaction involving carbon in the intermediate material, wherein the intermediate material has been produced by heating a shaped gel precipitated gel, and the carbon in the intermediate material for participating in the thermally induced reaction has been produced from a gelling agent, or a derivative thereof, incorporated in the gel during gel precipitation.

2. A process as claimed in Claim 1

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 17.7% carbon content. The intermediate product was heated for 1 hour at 1 1000C under argon to produce tungsten carbide spheres having less than 2% impurities. Example 5 In this example tungsten carbide spheres were produced using ammonium tungstae. Thus a gel precipitation feed solution was prepared by thoroughly mixing together 18.1 mls ammonium tungstate solution 115.6 gm per litre concentration), 8.405 grms degraded Superfloc S 16 (Registered Trade Mark) solution (24.48% in 92.5% formamide water mixture) and 0.05 mls of the surfactant Nonidet P42 (Registered Trade Mark). The gel spheres were prepared by contacting droplets of the feed solution with hydrochloric acid. The gel spheres were dried and heated to 1200"C under an argon atmosphere to produce an intermediate material having 17.2% carbon content. Subsequently the intermediate material was heated to 1700"C under argon to produce partially sintered tungsten carbide spheres. Example 6 In this example tungsten carbide was prepared using a feed solution to which a suspension of tungsten carbide had been added in order to increase the amount of tungsten carbide in the product. Thus a gel precipitation feed solution was prepeared by thoroughly mixing together 8.36 grms of sodium tungstate in solution, 33.62 gm of degreaded Superfloc S 16 (Registered Trade Mark) solution (24.48% in 95% formamide/water mixture), 25 mls water, 25 grms of suspended tungsten carbide and 0.2 mls of Nonidet P42 (Registered Trade Mark) surfactant. The feed solution was formed into droplets and the droplets contacted with a layer of hexanol saturated with HC1 and subsequently with concentrated HC1 solution to give coherent, uniform, regularly sized gel spheres. The gel spheres were heated to 12000C to give an intermediate material having 17.2% carbon content (ignoring added tungsten carbide). The intermediate material was then heated to 1700"C to give tungsten carbide containing less than 1 % weight free metal. Example 7 In this example a refractory product containing tungsten carbide and cobalt metal was prepared. A gel precipitation feed solution was prepared by thoroughly mixing 23.8 mls of sodium tungstate solution (350 gm tungsten per litre concentration), 33.6 gm of degraded Superfloc S 16 (Registered Trade Mark) solution (24.48 wt% in 92.5% formamide/water) o.94 gm of finely divided cobalt metal and 10 mls of water containing 0.2 mls of the surfactant Nonidet P42 (Registered Trade Mark). The feed solution was formed into droplets and contacted with HCl foam and HCl solution containing 2 mls of Nonidet P42/1 to form gel spheres of approximately 2 mm diameter. The gel spheres were washed with hot water and air dried for 16 hours at 220C. Subsequently the dried gel spheres were heated at 600"C in argon to give an intermediate material having 16% carbon content. The intermediate material was then taken up to 16500C and held at that temperature for 100 minutes to give a spherical refractory product containing tungsten carbide and cobtalt metal. Example 8 In this example tungsten carbide was produced with cobalt metal additive. A feed solution was prepared by mixing 23.8 mls of sodium tungstate solution (350 grms per litre concentration), degraded Superfloc S 16 (Registered Trade Mark) (33.62 grms), 0.94 grms of cobalt metal, 10 mls of water and 0.2 mls of Nonidet P42 (Registered Trade Mark). The feed solution was formed into droplets and the droplets were passed through a layer of hexanol saturated with HC1 and subsequently through a concentrated HC1 solution to produce gel spheres. After washing with hot water and distilled water the particles were superficially dried with acetone and subsequently air dried. The gel particles were heated under argon at 600"C to give an intermediate material having a carbon content of 16%, and this was subsequently heated to 11000C to give a product containing tungsten carbide with a crystallite size of 450 A and cobalt in the form of metal. Example 9 The procedure of Example 6 was repeated with the addition of 7 gm of finely divided cobalt metal to the gel precipitation feed solution. The refractory product produced had the composition: > 99% tungsten carbide (ignoring cobalt). WHAT WE CLAIM IS:

1. A process for the production of a refractory material which includes heating an intermediate material containing carbon to cause a thermally induced reaction involving carbon in the intermediate material, wherein the intermediate material has been produced by heating a shaped gel precipitated gel, and the carbon in the intermediate material for participating in the thermally induced reaction has been produced from a gelling agent, or a derivative thereof, incorporated in the gel during gel precipitation.

2. A process as claimed in Claim 1

wherein the quantity of carbon in the intermediate material is such that, and the heating of the intermediate material containing carbon is carried out under conditions such that, the thermally induced reaction involving carbon is the carbothermic reduction of a chemical substance in the intermediate material.

3. A process as claimed in Claim 1 and for the production of a porous refractory material wherein the quantity of carbon in the intermediate material is such that, and the heating of the intermediate material containing carbon is carried out under conditions such that, the thermally induced reaction involving carbon results in the removal of carbon from the intermediate material to give porosity.

4. A process as claimed in any one of Claims 1,2 and 3 including an additional step of heating a shaped gel precipitated gel to produce the intermediate material containing carbon.

5. A process as claimed in claim 4 wherein the shaped gel precipitated gel is prepared from a feed solution containing an inorganic salt or sol.

6. A process as claimed in claim 5 wherein the inorganic salt or sol and the precipitating agent are chosen such that the shaped gel precipitated gel contains, in addition to the gelling agent, or derivative thereof, a hydrous oxide or hydroxide.

7. A process as claimed in claim 6 wherein the inorganic salt is a metal nitrate.

8. A process as claimed in claim 6 or claim 7 wherein the precipitating agent is a base.

9. A process as claimed in any preceding claim wherein the gel precipitated gel is produced from a feed solution containing a modifying agent (as hereinbefore defined).

10. A process as claimed in any preceding claim wherein the composition of the refractory material is further influenced by incorporating an additive in the feed solution from which the gel precipitated gel is prepared.

11. A process as claimed in claim 10 wherein the additive is tungsten carbide or cobalt.

12. A process as claimed in any preceding claim wherein the intermediate material is in the form of particles so as to give particles of refractory material on heating, the intermediate material having been produced by forming a gel precipitation feed solution into droplets, contacting the droplets with a precipitating agent to give gel particles and heating the gel particles.

13. A process as claimed in any one of claims 1.2 and 4 to 12 wherein the quantity of gelling agent, or derivative thereof, in the gel precipitated gel is chosen to effect a given change in an oxygen to metal ratio of an oxide refractory material.

14. A process as claimed in any one of claims 1, 2 and 4 to 12 wherein the quantity of gelling agent, or derivative thereof, in the gel precipitated gel is chosen to effect production of metal in the refractory material by reduction.

15. A process as claimed in any one of claims 1, 2 and 4 to 12 wherein the quantity of gelling agent, or derivative thereof, in the gel precipitated gel is chosen to give sufficient carbon to produce a carbide, or a carbide and free carbon in a refractory material.

16. A process as claimed in any preceding claim wherein the gelling agent is polyacrylamide.

17. A process as claimed in claim 16 wherein the average molecular weight of the polyacrylamide is 4 x 106.

18. A process as claimed in any preceding claim wherein the refractory material comprises uranium/plutonium oxide, or uranium / plutonium carbide, or thorium/uranium carbide, or tungsten carbide, or tungsten carbide/cobalt metal.

19. A process as claimed in any preceding claim wherein the gel precipitated gel is produced from a feed solution containing a tetravalent metal species.

20. A process as claimed in claim 19 wherein the tetravalent metal species is of thorium of plutonium.

21. A process wherein a carbide prepared by a process as claimed in any one of the preceding claims is further treated by nitriding to give a nitride.

22. A process as claimed in any preceding claim wherein the refractory material is in the form of ceramic nicrospheres.

23. A refractory material produced by a process as claimed in any one of the preceding claims.

24. A refractory material as claimed in claim 23 in the form of ceramic microspheres.

25. A process for the production of a refractory material substantially as hereinbefore described with reference to any one of Examples 1, 2 and 4 to 9.

26. A refractory material substantially as hereinbefore described with reference to any one of Examples 1, 2 and 4 to 9.