GB2187178A - A method of improving the oxidation resistance of graphites - Google Patents

Abstract

A method wherein a graphite body is impregnated with a precursor of a refractory material, and this precursor is converted by hydrolysis, by thermal decomposition or by chemical reaction with the graphite to form a protective film of refractory material covering the surfaces of the internal pores of the graphite. Examples of the refractory material that may be deposited include alumina, silicon carbide, refractory metal, refractory metal oxide, borides, other carbides and silicides. Vacuum impregnation and reduction of a concentrated solution of aluminium nitrate is exemplified.

Description

SPECIFICATION A method of improving the oxidation resistance of graphites Technical field The present invention relates to a method of improving the oxidation resistance of graphites.

Graphite is, in many respects, an excellent constructional material for high temperature applications. It is inert to a wide range of chemical reagents and it has a high sublimation temperature and a high strength-to-weight ratio, which increases with temperature. However, its usefulness is limited by its reactivity towards oxidising gases.

Background art Previous approaches to the problem of improving the oxidation resistance of graphites include: The production of new forms of carbon eg. pyrolytic graphite (PG) and vitreous carbon (VC): these sufferfrom various disadvantages, including dimensional limitations, comparatively high cost and poor machinability, and (in the case of PG) anisotropy; lmprovementsofthe pitch/coke process, ie.the "Acheson " process used for the production of "synthetic" graphites - These have included the development oftechniques for producing denser, finergrained material having improved oxidation resistance; The incorporation of inhibitors - Certain metal phosphates and borates have been proposed as in hibitorsforthe oxidation of graphite at comparatively low temperatures; however, even at 600"C the effect decreases with time, while at the much highertemperatures such inhibitors are not effective (See Earp F.K., et al., "Industrial carbon and graphite", Proc. S.C.I. Conf., London (1957)) p.326; The incorporation of additives such as zirconium carbide and zirconium diboride in the graphite raw mix to yield a composite material with improved oxidation resistance; here, the function of the additive is (strictly speaking) not as an inhibitor but as a more reactive second phase which oxidises preferentially to yield a protective metal oxide film.The quantities of additives required are large (typically 10-15% by weight) so that significant increases in density and decreases in machinabiiity occur (See Goldstein E.M., 8th Int.Conf. on Carbon, Buffalo (1967)); and, the use of coatings of four example refractory metals, carbides, etc, which material also may be diffused into the bulk of the body. However, the presence of flaws in the coating can lead to catastrophic oxidation ofthe underlying carbon; Coatings must be applied to each individual shape after machining, in order to protectthe whole ofthe exposed surface; and, In addition, properties of graphite which are important in some applications (eg. lubricating properties, thermal shock resistance thermal conductivity) can be adversely affected by coating.

Disclosure ofthe invention The present invention is intended to provide a method of producing readily machinable graphites having improved oxidation resistance.

In accordance with the invention thus there is provided a method of improving the oxidation resistance of graphites, this method including the following steps of: providing a graphite body; impregnating the body with a substance capable of being converted into a refractory material; forming a film of this substance over substantially all accessible internal pore surfaces of the graphite body; and, converting the film of said substance into a coherent, oxidation-resistant refractory film.

The conversion, for example, can be effected by hydrolysis, thermal decomposition, or other suitable methods.

Materials that can be utilised for the impregnation step aforesaid include materials capable of conversion into alumina, silicon carbide, refractory metals, refractory metal oxides, borides, carbides and silicides.

The invention also provides a graphite body produced bythe method aforesaid.

In this way, a quantity of refractory additive, too small to affectthe physical and engineering properties of the graphite to any significant extent, is introduced and provides bulk protection against oxidation. This protection would not be destroyed by machining blocks of graphite after treatment, and any faults which developed in the protective film would not have a catastrophic effect (as would a fault in an external coating) since instead of a single protective film there would be a network, following the open pore network of the graphite itself.

Description ofpreferred embodiments The foregoing and otherfeatures of the invention will be better understood from the following description of embodiments of the invention. This description is given by way of example only.

The production of graphite bodies, which are readily machinable and which have improved oxidation resistance can be achieved now by impregnating a suitable commercial graphite body with a substance or solution of a substance that is capable of being converted readily into a refractory material, for example, by either hydrolysis or thermal decomposition. This step of impregnation substantially fills the accessible internal pore structure of the body with the substance. After this the impregnated body is dried, a film of the substance is formed over substan tially all of the accessible internal pore surfaces of the graphite body and thereafter the film is converted into a coherent, oxidation-resistant refractory film, for example of metal oxide or carbide, thereby improving the oxidation resistance properties of the graphite body.

It is preferable that the impregnation techniques employed in this method are based on the use of liquids eg. nitrate solutions and isopropoxides. However, by way of an alternative, vapourimpreg nation could be utilized, but it is in general relatively slow, expensive, comparatively difficult to control and less suitable for relatively thick graphite bodies.

The quantity ofthe substance which is used to provide the protective refractory film is not large enough to affectthe physical and engineering properties of the graphite to any significant extent but the protect ive film does provide bulk protection forthe graphite body against oxidation. The bulk protection is not destroyed by machining the graphite body after the impregnation and conversion process because any faults which are developed in the protective film after the machining would, due to the open pore network of the graphite body, be readily rectified by the action ofthe oxidising gas since the penetration of one bar rierfilm ofthe open pore network by the oxidising gaswould resultinonlyasmall amountofoxidation of the graphite body before another barrier film is exposed.

In one particular example of the application of this method a graphite body having afine-grained structure is vacuum impregnated with a concentrated solution of aluminium nitratetosubstantiallyfillthein- ternal open pore structure of the body with the aluminium nitrate solution. Typically, approximately 4 mg. of aluminium nitrate can be deposited overthe internal surfaces of a 1.6 g. graphite body ie. 0.25% by weight.

The impregnated graphite body is dried in an oven to produceafilm of aluminium nitrateoverthe internal surfaces ofthe body and it is then heated in an inert atmosphere (for example argon) to a temperature of the order of 900"C to convert the aluminium nitrate film into a coherent film of alumina over substantially all the accessible pore surfaces of the graphite body.

Oxidation tests on the alumina coated graphite body in CO2 have demonstrated that the reaction rate of the graphite body at a temperature of 1 000"C can be reduced to as little as 1/15 ofthat of an untreated body of the same graphite. This observed effect however is found to decrease with increase in the oxidation temperature, becoming negligible at 1 300"C, probably duetothe conversion of"- alumina to a- alumina at a temperature of the order of 1 200"C and associated cracking of the film. However, the pronounced reduction in the oxidation rate observed at a temperature of 1 000"C is significant particularly in view ofthe small amount of alumina that is required to be deposited to achieve this result.

As an example ofthe hydrolysis method a graphite body is vacuum impregnated with a suitable quantity of a solution of 0.5 gm Yttrium Isopropoxide, 5 gm of Zirconium n - propoxide, 4 drops (~2my) of nitric acid in 10 ml ethoxyethanol,to which has been added with stirring a second solution of 0.3 gm water in 10 ml ethoxyethanol. This mixed solution is stable at room temperature for a period of several hours, but on heating to 50"C reacts by hydrolysis ofthe propoxides to yield a coating of mixed oxide/ propoxide. The body is then heated at 500"C in an inert atmosphere to remove moisture and organic residues and then at 1200 Ctoyield afilm of Yttriastabilised Crystalline Zirconia.

Other materials that could be utiiized forthe impregnating substance to give the improved oxidation resistance, in some instances attemperatures higherthan 1 000"C, are materials capable of being converted in sftu into alumina, silicon carbide, refractory metals, other refractory metal oxides, borides, othercarbides and silicides.

Field of application Graphite bodies treated by this inventive method have applications, for example, to the production of high-temperaturefurnaceware, glass-working tools and metal casting equipment.

Claims (13)

CLAIMS Whatweclaim is:

1. A method of improving the oxidation resistance ofgraphites, this method including the following steps of: providing a graphite body; impregnating the body with a substance capable of being converted into a refractory material; forming a film of this substance over substantially all accessible internal surfaces ofthe graphite body; and, converting the film of said substance into a coherent, oxidation-resistant refractory film.

2. A method, as claimed in claim 1, wherein said substance is a precursor of one ofthe following refractory materials: alumina, silicon or other carbide, a refractory metal, a refractory metal oxide, a boride, or a silicide.

3. A method, as claimed in either claims 1 or2, wherein the substance is converted to refractory material by hydrolysis.

4. Amethod,asclaimed ineitherclaimsl or2, wherein the substance is convertedto refractory material by thermal decomposition.

5. A method, as claimed in any one of the pre- ceeding claims, wherein said substance is impregnated whilst in a liquid form.

6. A method, as claimed in claim 5,wherein the substance is introduced into the pores by vacuum impregnation.

7. A method, as claimed in claim 6, wherein the graphite body is vacuum impregnated with a sol utionofaluminium nitrate, dried to form a nitrate film oversubstantially all accessible internal pore surfaces, and the nitrate converted by heat to form a refractory alumina film.

8. A method, as claimed in claim 7, wherein the nitrate is converted by heating to a temperature of 900"C in an inert atmosphere.

9. A method, as claimed in claim 6, wherein the graphite body is vacuum impregnated with a mixed solution of Yttrium isopropoxide and Zirconium n propoxide and a water solution, oxides produced by hydrolysis and a refractory film formed over the accessible internal pore surfaces ofthe body.

10. A method, as claimed in claim 9, wherein the hydrolysis is conducted at a temperature of 50"C or thereabouts.

11. A method, as claimed in either claims 9 or 10, wherein following hydrolysis the body is heated to 500"C, or thereabouts, in an inert atmosphere and finaliy heated to 1 200into yield a film of Yttr-astabilised Zirconia.

12. A method of improving the oxidation resistance of graphites, when performed substantially as described hereinbefore.

13. A graphite body when treated by any method as claimed herein, and any article machined there from.