GB2142343A - Composite silicon carbide sintered shapes and their manufacture - Google Patents

Abstract

A composite silicon carbide sintered shape is provided in two forms, one of which includes rare earth oxides as a sintering assist, and the other of which includes rare earth oxides and/or aluminium oxide or boron oxide as a sintering assist, characterised in that both forms have a surface layer abundant in rare earth oxides. Methods for manufacturing such sintered shapes are also provided. The preferred compositions are:- (1) One or more rare earth oxides 11.3-65 atomic % S,C Balance @ and (2) One or more rare earth oxides 0.021-65.0 atomic % At least one of aluminium or boron oxide 0.006-79.984 atomic % S,C Balance where the total of rare earth oxide, plus aluminium and/or boron oxide is 11.306-80.0 atomic %.

Description

SPECIFICATION Composite silicon carbide sintered shapes and their manufacture The present invention relates to composite sintered shapes of silicon carbide having a surface layer abundant in rare earth oxides, i.e. a surface layer containing more quantity of rare earth oxides than the interior, and method of producing such shapes. More particularly the present invention relates to composite sintered shapes of silicon carbide having a surface layer abundant in rare earth oxides of insulating property and an interior structure abundant in SiC and having a high bending strength and semi-conductive property, and method of producing such shapes.

Silicon carbide has been used for heat resisting shapes or high temperature heating elements due to its excellent high temperature strength, high thermal shock resistance, acid resistance, anit-wearing property and creep resistance, and recently extended its usage to the field of high temperature construction material or anti-wearing material.

However, silicon carbide (hereinafter sometimes referred to as SiC) is generally difficult to sinter, and it is infeasible to produce dense sintered shapes of high strength using SiC alone. Therefore, there have been practised the hot pressing process wherein a sintering assist such as Awl203, iron ozide, or AIN is added to SiC powder and mixed together, and the reaction sintering process wherein a shape of mixed powder of SiC and C is melted or reacted with gaseous silicon. However, with the conventional processes it is difficult to form shapes of complicated configuration and they are not suitable for mass production.

Recently, a more suitable process, namely pressureless sintering has been adopted for the manufacture of SiC shapes. This process involves, as disclosed in U. S. Patent Specification No. 4,090,735, the steps of adding C and B as sintering assists to SiC powder, mixing the combined powders, forming the mixed powder into green shapes and sintering the same without an application of pressure.

We have previously proposed manufacturing silicon carbide sintered shapes with or without an application of pressure using specific sintering agents or assists of oxides as disclosed in Japanese Patent Application No.

56-044109, and U. S. Patent Specification No. 3,208,896 In the above-mentioned proposal, sintering assists selected from rare earth oxides, and further containing one or more of carbon, aluminium oxide and boron oxide are used as sintering assist. The amount of the assist contained in a sintered shape is 11.300 atomic % maximum for rare earth oxides, or 11.500 atomic % maximum rare earth oxides plus aluminium and/or boron oxide. Silicon carbide sintered shapes produced by the above-mentioned method contain a rather small amount of sintering assist existing in the most part in the grain boundary of the shape which assists feasible sintering.

The object of the present invention is to provide composite silicon carbide sintered shapes especially suitable for substrates of a high dielectric breakdown strength for electronic circuits having a surface layer abundant in rare earth oxides.

It has now been found that when a larger amount of sintering assist is added to SiC and mixed therewith, then the mixture is sintered into shape in which the sintering assist diffuses not only into the grain boundary of SiC polycrystal but also into the surface layer abundant in rare earth oxides having a high dielectric strength.

In other words, the composite sintered shape produced by the present invention consists of two phases, i.e. the interior abundant in silicon carbide and the surface layer abundant in rare earth oxides.

According to a first aspect, the present invention provides a composite silicon carbide sintered shape containing 11.300-65.000 atomic % of at least one rare earth oxide (hereinafter referred to as R oxides) and the remainder substantially of SiC.

R oxides contained in the composite silicon carbide sintered shape formed by the present invention is or are one or more of the oxides of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

If the content of R oxide in a sintered shape is less than 11.300 atomic %, the formation of surface layer of oxide is insufficient, whereas when the content is greater than 65.000 atomic %, silicon carbide is dispersed or reacted with oxide reducing the remaining SiC amount.

The surface layer formed on the sintered shape manufactured according to the first aspect of the invention consists of R oxide and a small amount of SiO2, or R oxide having SiC dispersed therein. The interior of the shape is comprised mainly of polycrystal silicon carbide comprising R oxide in the grain boundary. The surface layer of the shape can have a high specific resistance of more than 10" ohm.cm or a high insulating property, while the interior exhibits a similar level of specific resistance as that of silicon carbide.

Further the interior exhibits the semi-conductive property. Since the inside silicon carbide is protected by the surfce layer from the environment, when the shape is exposed at an elevated temperature, e.g. 1300-1500 C in air, oxidation is reduced to less than one tenth compared with a shape without the surface layer. The thermal conductivity of the layer is less than one tenth that of a shape without such layer and, for example, a shape produced in accordance with the present inven tion having a layer thickness of 50 elm is about one third of a shape without such layer.

The bonding strength between the surface layer and the interior of the shape is sufficiently high, and the thermal shock of alternate heating and cooling several times does not cause any separation of the bonding.

According to a second aspect of the invention composite silicon carbide sintered shapes comprise 0.021-65.000 atomic % of R oxide, 0.006-79.984 atomic % of either aluminium oxide (hereinafter referred to as Al oxide) or boron oxide (hereinafter referred to as B oxide), the total amount of said Al and B oxides being 11.306-80,000 atomic %, and the remainder substantially of SiC. R oxide of more than 65.000 atomic %, Al oxide or B oxide of more than 79.984 atomic %, and R oxide plus Al oxide or B oxide of more than 80.000 atomic % cannot produce a desired shape because silicon carbide is dispersed in these oxides or reacted with them and is exhausted. Also combined R oxide plus Al oxide or B oxide of less than 11.306 atomic % cannot produce a desired surface layer.

The surface layer of a composite silicon carbide sintered shape of the second aspect of the invention comprises a mixture of three kinds of combinations of R oxide plus Al oxide; R oxide plus B oxide; R oxide plus Al oxide and B oxide; and a small amount of SiC and/or SiO2 dispersed in the oxides. The interior of the shape comprises polycrystal silicon carbide including at least one oxide selected from R oxide, Al oxide and B oxide in the grain boundary. Specific resistance of the surface layer of these shapes is more than 1011 ohm.cm providing a high insulating property while specific resistance of the interior is of similar level to that of normal silicon carbide shapes providing the semi-conductive property. Since the oxides in the layer protect the inside silicon carbide from the oxidation of the shape under an elevated temperature, e.g.

1300-1500 C in air, the resistance is reduced less than one tenth in comparison with that of shapes without the surface layer. A larger amount of B oxide in the layer tends to flow out of the surface with the other R and Al oxides, and makes formation of a stable oxide layer difficult. Therefore, the amount is preferably less than 40% by weight. Thermal conductivity of the surface layer is less than one tenth of that of the normal SiC sintered shapes. For example, shapes in accordance with the present invention having a surface layer of 100 cm thickness exhibit about 12.5 of that of normal SiC shapes without the surface layer. Bonding between the surface layer and the interior is of sufficient strength to resist separation thereof after an alternate thermal shock test including successive heating and cooling.Now manufacture of composite silicon carbide sintered shapes by the present invention will be described.

In accordance with the present invention, at least one of alpha-SiC, beta-SiC and amorphous SiC may be used. It has been found that the material comprising beta-SiC or amorphous SiC fine powder with addition of 0.1- 10% by weight of alpha-SiC controls nonuniform growth of SiC grain providing high hot strength and creep resistance. It is preferable to use SiC powder which is free of impurities to form the sintered shapes of the present invention.

As SiC material, SiC fines or compounds including silicon-carbon bond, for example, organic silicon compounds or organic high molecular silicon compounds, or mixtures thereof may be used.

In accordance with the present invention, the following groups of sintering assists may be used: (a) rare earth elements and rare earth compounds; (b) less than 99% by weight of at least one member selected from aluminium, carbon, boron and compounds thereof and the balance substantially being at least one member selected from rare earth elements and rare earth compounds.

The above-mentioned R compounds, aluminium compounds and boron compounds include the respective oxides or composite oxides, hydroxides, acid adducts of hydroxides, phosphates, carbonates, basic carbonates, nitrates, sulphates, orgainic acid salts, halides, organic metal compounds, chelate compounds and alcoholates.

The acid adduct of hydroxides in the abovelisted sintering assists are synthesized by reaction of the hydroxides with acids. When lesser equivalent of the acid than metal element equivalent in the hydroxide is reacted with said hydroxide, the acid reacts with a part of the metal element to form an acid adduct which is dissolved in water. These acids include hydrochloric, sulphuric, nitric, hydrofluoric, phosphoric, perchloric, carbonic acids, organic acids (formic, acetic, propionic, tartaric, fumaric, lactic, oxalic, stearic, maleic, binzoic, malic, malonic, citric and lactic acids) and others.

In accordance with the present invention, a mixture of silicon carbide fines and sintering assists may be prepared by the following four methods. The first method is mixing of sintering assists (oxides, hydroxides and metal elements) insoluble in a solvent and silicon carbide fines. In this method, a dry mixing process is performed with a mixer for a sufficient period of time, while a wet mixing process is performed with a solvent, such as an alcohol. The second method is adopted when a sintering assist (e.g. acid adducts of hydroxides, nitrates, sulphates, organic acid salts, basic carbonates, carbonates, phosphates, perchlorates, halides, organic metal compounds, alcoholates, chelate compounds etc.) is soluble in a solvent.The sintering assist is dissolved in solvent such as water, alcohols, ethers, ketones, hydrocarbons, DMSO, DMF and others and mixed with silicon carbide fines in a blender for a sufficient period of time. In the mixing operation silicon carbide fines are covered by a membrane of the assist, and a rather small quantity of the assist provides a sufficient sintering effect. Examples of the sintering assists soluble in solvents are as follows: acid adducts of hydroxide are soluble in water; some alcoholates are soluble in ethers and aromatic hydrocarbons; some chelate compounds are soluble in water, alcohols, ethers, and hydrocarbons; organic metal compounds are soluble in organic solvents such as hydrocarbons and ethers; and some of nitrates, sulphates, organic acid salts and halides are soluble in water.

The third method is adopted when the sintering assists are in liquid form at the ambient temperature or may be melted under heating (e.g. some of organic metal compounds, chelate compounds and organic acid salts). The sintering assist and silicon carbide fines are mixed with or without heating and blended together for a sufficient period of time.

The fourth method is a combination of the above-mentioned first through third methods, for example, when the sintering assist includes two or more kinds of compounds, a solution of the assist is mixed in silicon carbide fines.

In manufacturing the sintered shapes including a rather large quantity of oxides in accordance with the present invention, they tend to crack during the sintering operation.

The fourth method mentioned above is advantageous to prevent the cracking failure. The amount of the assist necessary to sinter SiC is several per cent of the oxide. To perform an effective blending of the assists and SiC powder, a solvent soluble assist is dissolved in a solvent, then mixed with SiC powder. When an assist (e.g. oxides) insoluble in the solvent is added to the mixture, it is advantageous to minimise the occurrence of cracking failure in the sintered products.

Mixing operation may be performed with conventional powder mixers or kneaders.

The atmosphere for the mixing operation may be an oxidizing one such as air, carbon dioxide gas and oxygen gas, or non-oxidizing one such as nitrogen, argon, helium, hydrogen, neon, carbon monoxide and hydrocarbon gases, or a vacuum. The mixing operation may be generally performed in air. During mixing in air, a part of the compounds including organic metal compound, alcoholates and complexes, chelate compounds, and halides react with oxygen, carbon dioxide gas or water to form oxides, hydroxides or carbonates. The resulting compounds in microparti cie form adhere to SiC powder surface to promote sintering.

The mixed material is compacted into a desired configuration to form a green shape.

The compacting operation may be performed with convention techniques of powder metallurgy.

When the mixed material comprises SiC fines and sintering assist in powder form, about one per cent by weight of a lubricant such as stearic acid or its salt may be advantageously used in the mould pressing.

Application of pressure may be by a singleacting or double-acting press or a hydrostatic press etc. A pressed shape, or a compact of a rather simple configuration -may be subjected to the subsequent process, but a compact having a complex configuration requires reforming operation with a grinding or milling machine. When a high mechanical strength shape is required for machining, the shape may be pre-heated in a temperature range of 300-1 600 'C under an oxidizing or non-oxidizing atmosphere, or in vacuum. Also, the ma teriai may be slip-cast. In the slip-casting without any solvent, dispersing medium, preferably water, added with an anti-coagulating agent is mixed with the material.On the other hand, when a solvent is used for the mixing a rather large amount of the solvent is added to form the slip-casting material. The material is poured into a mould of calcined plaster to form a green shape. SiC mixed material in paste form may be compacted by an injection moulding process. In shaping the paste, a bonding agent is advantageously used in addition of the solvent. Suitable bonding agents include polyvinyl alcohol, polyethylene glycol and wax which evaporate during the sintering.

When a solution of the assist, e.g. acid adduct of hydroxide, is viscous, a suitable paste may be formed without any bonding agent. In this case the assist serves as a sintering and bonding agent.

A green shape thus formed is then sintered in a furnace which preferably is changeable to an oxidizing or non-oxidizing atmosphere, or to vacuum. The sintering temperature is preferably from 1 600 to 2300"C. A lower temperature than 1600"C does not cause sintering reaction, while a higher temperature than 2300"C would disintegrate SiC. The sintering of a composite SiC shape or compact may be completed in this temperature range, and the sintering at a relatively low temperature is completed with a large amount of the sintering assist. With a shape of large size or complicated configuration or a sintering assist which may produce a gas during the sintering, such sintering process may be advantageously performed in two stages, i.e. in low and high temperature operations.An oxidizing atmosphere or vacuum is desired for the high temperature sintering. For the non-oxidizing atmosphere, nitrogen, argon, helium, neon or carbon monoxide is used. A high pressure is desirable but generally normal pressures give a good result. Low temperature sintering does not necessarily require a non-oxidizing atmosphere or vacuum. Sintering in the air at a temperature less than 1300"C does not cause oxidation of silicon carbide. In this case sintering assists other than oxides will oxidize during the sintering, but such assists in the form of oxide do not have any adverse effect on the sintering operation. For example, sintering assists other than rare earth elements, aluminium metal, boron and their oxides are partly or wholly oxidized, but no disadvantageous effects occur.

Temperature raising rate varies depending on the size of a shape. The larger the size the slower is the appropriate rate. The rate up to 1 600"C for one hour may be applied because the sintering does not proceed to a large extent, but when the assists other than oxides and metals are used they cause reaction and produce even a small amount of gas therefore the rate up to 1 600'C for over three hours are desired. A slow rate, e.g. 7"C/min. higher than 1600'C will give good results to avoid shrinkage of the shape.

A composite product sintered at a high temperature including aluminium, boron in its initial composition loses a part of these assists and the compounds. Specifically, aluminium or boron compound reacts with rare earth compounds during the sintering and then decomposes at a high temperature, and a part of aluminium or boron react with silicon compound (mainly oxide) on the SiC particle surface to evaporate. The remaining part of the oxide existing in the grain boundary of SiC polycrystal promotes the sintering, and further additional oxide diffuses to form the surface layer of the sintered shape. The oxide component in the layer consist of; mixtures of a small amount of SiC and/or SiO2 plus R oxide; R oxide plus Al oxide; R oxide plus B oxides; and R oxide plus Al and B oxides.In other words, the oxide component includes a small amount of SiO2 and a small amount of SiC dispersed therein. When a large amount of B oxide is used it will melt at a relatively lower temperature during the sintering operation and flow out of the surface. Therefore a lesser amount than 40% by weight of B oxide in total oxides gives a desired result.

Carbon is added together with oxide or similar compounds. When a large amount of carbon is used it remains in the composite sintered shape to deteriorate its oxidation resistance at a high temperature. Therefore, a lesser amount than 10% by weight of carbon reacts with sintering assists and SiO2 and is exhausted.

Carbon and carbon compounds to be mixed in the green shape include acetylene black, carbon black, graphite powder, coal fines; active carbon, high molecular aromatic compounds (e.g. tar and pitch) and organic compounds leaving carbon after sintering (e.g.

phenol resin, aniline formaldehyde resin, cresol formaldehyde resin and furan resin).

It is already known that carbon or carbonaceous compounds mixed in a green shape react with SiO2 membrane around SiC particle and promote bonding to SiC particles due to the existence of boron. It is now believed that the bonding between the particles is strengthened by the existence of rare earth oxides in accordance with the present invention.

Combined use of sintering assists, aluminium, aluminium compound plus carbon, carbonaceous compounds plus boron, boron compounds, with rare earth elements and rare earth compounds is believed to promote the respective sintering effects of aluminium, boron and carbon.

The present invention may be practised with sintering under either pressurised or pressureless conditions. Specifically, composite sintered shapes having high density and high bending strength may be obtained by the pressureless sintering. A part of rare earth elements, boron and aluminium mixed in SiC powder as sintering assist, remains in oxide form within SiC boundary, and the remainder disperses outwards to form the surface layer.

Such sintering under pressure causes-similar reaction, and provides further advantage of better control on uniform quality. The pressure sintering may be performed with hot pressing, hot isostatic pressing or sintering under pressurised atmosphere.

Reasons for limiting the composition range in the present invention are explained in the following.

In a composite SiC sintered shape of the first aspect of the present invention, if the content of rare earth oxide is less than 11.300 atomic %, the formation of the surface oxide layer is insufficient, while if the content is larger than 65.000 atomic %, SiC in the interior of the shape disperses in the oxides or becomes unreactive to make the formation of a desired shape difficult. Therefore, the content of rare earth oxide should be limited in a range of 11.300-65.000 atomic %.

In a composite SiC sintered shape of the second aspect of the present invention, if the content of rare earth oxide is less than 0.021 atomic %, since an action to promote the sintering is less effective it is difficult to increase the density of the shape, whilst if the content is larger than 65.000 atomic %, SiC in the interior of the shape disperses in the oxides or becomes unreactive. Therefore, the content of rare earth oxide should be limited in a range of 0.021-,65.000 atomic %.

When the content of oxides of aluminium and boron is less than 0.006 atomic %, since an action to promote the sintering is less effective, it is difficult to increase the density of the shape, while when the content is larger than 79.984 atomic %, SiC in the interior disperses in oxides and becomes unreactive, the content of oxides of aluminium and boron should be limited in a range of 0.006-79.984 atomic %. Further, when the total amount of the above-mentioned rare earth oxides and aluminium oxide and/or boron oxide is less than 11.306 atomic %, the formation of the surface layer of oxide is insufficient, while the amount is larger than 80.000 atomic %, SiC in the interior of the shape disperses in the oxides and becomes unreactive to make the formation of the desired shape difficult.Therefore, the total amount of the above-mentioned two kinds of oxides should be limited in a range of 11.306-80.000 atomic %.

In the manufacture of sintered shape by the present invention, comprising the steps of mixing silicon carbide powder with sintering assist comprising substantially at least one of rare earth elements and rare earth compounds, the amount of sintering assist on the rare earth oxide basis should be limited in a range of 11.300-65.000 atomic % (sum of SiC plus assist being 100 atomic %). When the amount is less than 11.300 atomic %, the formation of the surface layer of a sintered shape is insufficient, while when the amount is larger than 65.000 atomic %, SiC in the interior of the shape disperses in the oxides or becomes unreactive, and it is difficult to form a desired sintered shape.

On the other hand, in the method of the invention comprising mixing silicon carbide powder with sintering assist comprising less than 99 per cent by weight of at least one of aluminium, carbon, boron and their compounds and the balance being at least one of rare earth elements and rare earth compounds, the amount of rare earth elements, aluminium, carbon, boron, and their compounds should be limited in a range of 0.02165.000 atomic % on the rare earth oxide basis (sum of SiC and oxides being 100 atomic %), because with a lesser amount than 0.021 atomic %, an action to promote the sintering is less effective to make the formation of a desired shape of high density, while a larger amount than 65.000 atomic % within the interior of the shape disperses and becomes unreactive, and the formation of a desired composite sintered shape cannot be performed.

The amount of aluminium, carbon, boron and their compounds should be limited in a range of 0.006-85.000 atomic % on the total oxide basis, because a lesser amount than 0.006 atomic % is less effective to promote the sintering to make the formation of a desired composite shape difficult, while with a larger amount than 85.000 atomic % SiC within the interior of the shape disperses in the oxides- or becomes unreactive, and the formation of a desired shape cannot be performed.

Though not described impurities in SiC material used in the present invention, e.g. SiO2, Fe, Co, Al, Ca, free carbon and other trace elements, are generally existing in the material. Therefore, the sintered shapes produced from such material fall within the scope of the present invention.

The invention will now be further described by means of the folowing Examples.

Example 1 1 g of scandium oxide is dissolved in 20 ml of hot 2 N-hydrochloric acid solution, then 5 ml of aqueous ammonia is added to precipitate scandium hydroxide. The precipitate is separated by filtering and washed with distilled water several times and 20 ml of 1 Nhydrochloric acid solution is added to the precipitate. 1 2 g of beta-SiC and 6 g of La203 are mixed with the reaction solution, and water is removed by evaporation. The dried powder is initially pre-shaped by a singleacting press then compacted with a hydrostatic press under 2 x 102 MPa to form a green shape.The green shape is burnt in the primary firing in the air at a rate of 100"C/h up to 500"C. The burnt shape is sintered in a Tammann furnace under argon atmosphere in a temperature range of 500"-1900"C at a rate of 200"C/h and held at 1900"C for one hour to obtain a composite SiC sintered shape. The surface layer of the shape is composed of mixed oxides of Sc2O3 and La2O3, and the interior comprises silicon carbide. The shape exhibits 70 kg/mm2 of bending strength and oxidizing rate at 1 400'C is one-fifteenth of a shape produced with sintering assists of B and C. The specific reistance of the shape is 3 x 1012 ohm-cm.

Example 2 9 g of yttrium oxide is dissolved in 180 ml of N-hydrochloric acid, and 30 ml of aqueous ammonia is added to precipite yttrium hydroxide which is separated by filtering. The resulting yttrium hydroxide is mixed with 260 ml of formic acid solution of pH 2, and agitated for four hours at room temperature to cause a reaction. The reacting solution is condensed under a reduced pressure and dried in vacuum to obtain 18.5 g of acid adduct of yttrium hydroxide. On the other hand, 30 g of aluminium isopropoxide is dissolved in 1 20 ml of benzene, and 200 ml of 1 N-hydrochloric acid solution is added. The resulting aluminium hydroxide immediately reacts with hydrochloric acid, and the reaction is completed within several hours.The mixed solution is condensed under a reduced pressure and dried in vacuum to obtain 20 g of acid adduct of aluminium hydroxide. 1.5 g of the acid adduct of yttrium hydroxide and 1.5 g of aluminium hydroxide are dissolved in about 10 ml of water. 1 6.5 g of beta-SiC (containing 5% of alpha-SiC) particles having average size 0.27 cm, and 10.5 g of Y203 are added to the solution, and mixed, then the solution is dried. The dried powder is placed in a metal mould, and pre-shaped with a single-acting press, then compacted with a hydrostatic press at a pressure of 2 x 102 MPa to form a green shape.The green shape is burnt in the air to a temperature of 500"C at a rate of 100"C/h. Then the shape is sintered in a temperature range of 500 -1 950 C at a rate of 200"C/h and held at this temperature 1 950 C for 30 minutes to obtain a composite sintered shape. The shape exhibits the bending strength of 80 kg/mm2 and the oxidizing rate is 1/12 of a sintered SiC shape produced with sintering assist of B and C, and the thermal conductivity is four times higher than that of the latter shape. The specific resistance is 1 x 10t2 ohm-cm.

Example 3 10 g of a composite oxide AI2Y4O9 is ground into particles having a size less than 1 ym, and 30 g of alpha-SiC powder having an average size of 0.40 pm and 2 g of boric acid are added to the oxide. The mixture is broken down in a pulverizer for three hours. The mixed fines added with a small quantity of water is placed in a metal mould and preshaped in a single-acting press then compacted in a hydrostatic press under a pressure of 2 x 102 MPa to form a green shape.The green shape is burnt in a Siliconit furnace under argon atmosphere in a temperature range from room temperature to 1300"C at a rate of 100'C/h, then in Tammann furnace under argon atmosphere in a temperature range of 1300 -1 850 C at a rate of 100"C/h and held at this temperature for 30 minutes to obtain a composite SiC sintered shape. The shape exhibits specific resistance of 1 x 1014 ohm-cm and bending strength of 60 kg/mm2.

Oxidizing rate at 1300"C of the composite shape is reduced to 1 /1 2 of that of a similar SiC sintered shape produced with a hot press using an assist of Al2O3.

Example 4 5 g of cerium nitrate is dissolved in about 10 ml of water, and 25 g of Ce2O3, 2g of boron, 2 g of active carbon and 50 g of beta SiC are added and mixed together. The mixture is dried into powder form, and preshaped in a metal mould with a single-acting press, then a hydrostatic press at a pressure of 1 x 102 MPa to form a green compact or shape. The shape is burnt in a Siliconit furnace under argon atmosphere to 1400"C at a rate of 100 C/h. The burnt shape is placed in a graphite mould and sintered in a range of 1400"-2000"C at a rate of 200"C/h in an induction furnace, and held at this temperature 2000"C for 30 minutes to obtain a composite SiC sintered shape.The shape exhibits bending strength of 62 kg/mm2 and specific resistance of 1 x 1014 ohm-cm. The oxidizing rate of the shape is reduced to 1/12 of that of a similar shape produced with sintering assists of B and C.

Example 5 3 g of neodymium acetyl acetonate and 3 g of aluminium isopropoxide are dissolved in about 10 ml of benzene, and 30 g of beta-SiC and 5 g of Al203 are added and mixed together. After the evaporation of benzene there is obtained dried powder which is left in the air for one week. The powder is placed in a metal mould and preshaped in a single-acting press then compacted in a hydrostatic press at a pressure of 2 x 102 MPa to form a green shape. The shape is burnt in a temperature range from room temperature to 500"C at a rate of 50"C/h, then sintered in a Tammann furnace under argon atmosphere in a temperature range of 500"-1900"C at a rate of 100"C/h and held at this temperature of 1900"C for 30 minutes to obtain a composite SiC sintered shape.The shape exhibits bending strength of 90 kg/mm2 and specific resistance of 1 x 1012 ohm-cm. Thermal conductivity of the shape is 3.5 times greater than that of a similar shape without the surface layer.

Example 6 3 g of samarium acetyl acetonate are dissolved in about 10 ml of benzene, and 25 g of beta-SiC and 1 5 g of Sm203 are added and mixed together. After evaporation of benzene there is obtained dried powder. The powder is mixed with an aqueous solution of 3 g of boric acid in about 10 ml of water and mixed together. After evaporation of water there is obtained dried powder. The powder after leaving in the air for four days is placed in a metal mould and pre-shaped in a single-acting press then in a hydrostatic press to obtain a green shape.The shape is burnt in the air in a temperature range from room temperature to 500"C at a rate of 100"C/h, then sintered in a Tammann furnace under argon atmosphere in a temperature range of 500"-1900"C at a rate of 200"C/h and held at this temperature 1900"C for 30 minutes to obtain a composite sintered SiC shape. The shape exhibits specific resistance of 1 x 1014 ohm-cm and bending strength of 74 kg/mm2. The oxidizing rate of the shape at 1400"C is 1/11 of a similar shape produced in pressureless sintering using sintering assists of B and C.

Example 7 5 g of praseodymium propionate and 2 g of aluminium nitrate are dissolved in about 10 ml of water, and 20 g of beta-SiC, 25 g of Pr2O3 and 1 g of active carbon are added and mixed together. After evaporation of water there is obtained dried powder. The powder is placed in a metal mould and compacted in a single-acting press and then in a hydrostatic press at a pressure of 2 x 102 MPa to form a green shape. The shape is burnt in the air in a temperature range from room temperature to 500"C at a rate of 50"C/h then sintered in a Tammann furnace under nitrogen atmosphere in a temperature range of 500"-2000"C at a rate of 200"C/h and held at this temperature 2000"C for 30 minutes to obtain a composite SiC sintered shape.The shape exhibits specific resistance of 1 x 1013 ohm-cm and bending strength of 80 kg/mm2. The oxidizing rate at 1 500 C in the air is 1/13 of that of a similar shape produced with sintering assists of B and C.

Example 8 2 g of HCOOH adduct of yttrium hydroxide produced by Example 2, and 2 g of HCI adduct of aluminium hydroxide are dissolved in about 10 ml of water, and 20 g of beta-SiC and 1 8 g of Eu203 are added and mixed together. After evaporation of water there is obtained dried powder. The powder is placed in a graphite mould and sintered in a hot press under argon atmosphere at a pressure of 100 kg/cm2 and a temperature of 1800"C for two hours to obtain a composite sintered SiC shape. The shape exhibits specific resistance of 1 x 1012 ohm-cm and bending strength of 84 kg/mm2.

As stated above, the composite sintered SiC shapes produced in accordance with the present invention have high density and bending strength, and excellent oxidation resistance, wear resistance, creep resistance, and thermal shock resistance. The shapes have insulating property having specific reistance more than 10' ohm-cm and having lesser thermal conductivity than that of silicon carbide but larger than that of oxides. The composite material from which the shapes may be made are unknown in prior arts and also may be processed with pressure or pressureless sintering into various parts having complex configurations, hollow parts or thin belt-like form.

Therefore, the composite sintered SiC shapes may be applicable broadly to manufacture of gas turbine blades, gas turbine parts, parts in apparatus for corrosive gases, crucibles, lining of ball mills, heat exchanger in high temperature furnace and refractory material, heating elements, burning tube, die-cast pump, thinwalled tubings, nuclear fusion reactor material, atomic reactor material, solar furnace material, tools and parts thereof, grinding material, thermal insulator, single crystal substrates for electronic devices, substrates for electronic circuits and insulating material and others.

Claims (7)

1. A composite silicon carbide sintered shape having its interior abundant in silicon carbide and its surface layer abundant in rare earth oxides.

2. A composition silicon carbide sintered shape as claimed in claim 1 comprising 11.300-65.000 atomic per cents of one or more of rare earth oxides and balance substantially of SiC, said shape having a surface layer abundant in rare earth oxide.

3. A composite silicon carbide sintered shape as claimed in claim 1 comprising 0.021-65.000 atomic per cents of one or more of rare earth oxides, 0.006-79.984 atomic per cents of at least either of aluminium oxide or boron oxide, total sum of said rare earth oxide plus aluminium oxide or boron oxide being 11.306-80.000 atomic per cents, the balance substantially being of SiC, said shape having a surface layer abundant in rare earth oxide.

4. A method of manufacturing a silicon carbide sintered shape comprising the steps of mixing silicon carbide power with sintering assist comprising essentially of at least one of rare earth elements and rare earth compounds, the amount of said assist being in a range of 11.300-65.000 atomic per cents on rare earth oxide basis, 100 atomic per cents being the sum of said assist and SiC, and subsequent compacting and sintering processes, said shape having a surface layer abundant in rare earth oxide.

5. A method of manufacturing a silicon carbide sintered shape comprising the steps of mixing silicon carbide powder with sintering assist comprising less than 99 per cents by weight of at least one of aluminium, carbon, boron and their compounds, and the balance being of at least one of rare earth elements and rare earth compounds, the amount of said rare earth elements or their compounds being 0.021-65.000 atomic per cents on rare earth oxide basis, the amount of said aluminium, carbon, boron and their compounds being 0.006-85.000 atomic per cents on their oxide basis, 100 atomic per cents being the sum of said assist and SiC, and subsequent compacting and sintering processes, said shape having a surface layer abundant in rare earth oxide.

6. A composite silicon carbide sintered shape substantially as hereinbefore described with reference to any one of the foregoing Examples.

7. A method of manufacturing a composite silicon carbide sintered shape substantially as hereinbefore described with reference to any one of the foregoing Examples.