GB2131410A - Beta-alumina material - Google Patents

Abstract

For toughening or strengthening ceramic material, particularly beta alumina, by the addition of zirconia, the zirconia is incorporated in the ceramic precursor materials before sintering in the form of a zirconium compound which reacts with a constituent of the precursor materials to dissociate or to form zirconium oxide and an oxide constituent of the ceramic whereby the zirconium oxide is formed simultaneously or substantially simultaneously with formation of the ceramic material and with the zirconium oxide dispersed through the ceramic. In making beta alumina, conveniently the zirconia is introduced in the form of sodium zirconate which dissociates to provide some or all of the required soda content as well as zirconia.

Description

SPECIFICATION Ceramic material, particularly beta alumina electrolyte material incorporating zirconia This invention relates to ceramic material incorporating alumina and particularly to material incorporating zirconia.

It is known to incorporate zirconia in ceramic material to increase the strength and toughness of the ceramic. Zirconia is a polymorphic material; the monoclinic form which is stable at room temperatures, transforms on heating (at about 10000 C) to a tetragonal form and, at much higher temperatures, to a cubic form. The transformations are reversible but, provided the zirconia is finely divided, the cubic and/or tetragonal forms can be stabilised or partly stabilised using Y203 or CeO2 or a number of other rare earth oxides. The transformation, on cooling, from the tetragonal and/or cubic form to the monoclinic results in a substantial increase in volume, about 5%.If finely divided zirconia is dispersed in other material for forming a ceramic, on sintering, the zirconia is transformed into the tetragonal (or possibly cubic) form and, particularly if this zirconia is partially stabilised, it remains in the tetragonal form on cooling. The tetragonal form zirconia is locked in the matrix of ceramic material and cannot expand. When the ceramic is subjected to stress, microcracks develop permitting transformation to the monoclinic form and energy is absorbed from the crack. This gives increased strength and toughness.

This property of finely divided zirconia for toughening ceramic material has been applied to many different ceramics; see, for example "Zirconia: Second Phase Particle Transformation Toughening of Ceramics" by R. Stevens, Trans.

and J. Brit. Ceram. Soc. 80 (1981) pages 81-85.

The present invention is concerned more particularly with the method of incorporating the zirconia in a ceramic, particularly in beta alumina.

The zirconia must be finely divided and dispersed in the ceramic if it is to be effective for increasing toughness and strength.

Heretofore the common way of incorporating it has been to add finely powdered zirconium oxide (ZrO2), with or without a stabiliser, to the powdered oxide material (for forming the ceramic) before firing. The sintering process forms the tetragonal phase zirconia which is dispersed in a matrix of the ceramic. One of the problems with this technique is that very fine zirconia particles tend to agglomerate and form large lumps. These lumps form monoclinic zirconia on cooling and serve only to toughen without any increase in strength. Even if the zirconia is partially stabilised, the large agglomerations are undesirable; for example beta alumina electrolyte material having high conductivity has to have a fine grain structure and, moreover, the stabilising material may be detrimental to the electrical properties.

An alternative technique that has been proposed in U.K. Patent Specification No.

2067177, which describes the formation of mullite, is to obtain a fine dispersion of zirconia by including in the powdered material, for forming the ceramic, a suitable zirconium compound, such as zirconium silicate, which, after the sintering operation, can be reacted with another material, e.g. alumina, to give zirconium dioxide in the required ceramic matrix. This technique requires a dense mixture of zirconium silicate and alumina to be formed first by firing at a sintering temperature lower than the reaction temperature. Then a subsequent heat treatment at a higher temperature than the sintering temperature is required to change the zirconium compound to zirconia and to form the mullite matrix. Such heat treatment subsequent to sintering is highly undesirable in forming many ceramic materials.

For example, in forming beta alumina, it could lead to undesirable grain growth.

According to one aspect of the present invention, a method of making a ceramic material incorporating finely divided zirconia dispersed in a matrix of the ceramic comprises the steps of mixing, with precursor materials for the ceramic, a zirconium compound which, at a temperature below the sintering temperature of the ceramic, reacts with a constituent of said precursor materials to dissociate or to form zirconium oxide and an oxide constituted of the ceramic, whereby, on heating, the zirconium oxide is formed simultaneously or substantially simultaneously with formation of the ceramic material with the zirconium oxide dispersed through the ceramic.

In this technique, the zirconia is incorporated by using a zirconium compound which reacts to provide zirconia and also a suitable precursor for the matrix material, this reaction occurring prior to sintering. The material is already mixed and hence the zirconia is dispersed amongst the precursor materials which form a ceramic matrix around the fine particles of zirconia.

For producing a zirconia-containing sodium beta alumina, the zirconium compound conveniently is sodium zirconate and the precursor materials comprise alumina, with or without lithia and/or magnesia and with or without soda or a material or materials (e.g. a carbonate or carbonates) which, on heating, produce an aforesaid precursor material or materials. The sodium zirconate provides both soda and zirconia. Further soda may be required to give the desired proportion of soda in the beta alumina. The lithia and/or magnesia are required as stabilisers if the p" form of beta alumina is to be produced.

Other ceramic materials may be produced in a similar way; for example, for making zirconiacontaining barium titanate, the zirconium compound may be barium zirconate and the precursor materials barium zirconate and titanium dioxide. For making zirconia-containing magnesium aluminate, the zirconium compound may be magnesium zirconate and the precursor materials magnesium zirconate and alumina. For making zirconia-containing strontium ferrite, the zirconium compound may be strontium zirconate and the precursor materials strontium zirconate and iron oxide.

The invention finds particular application however in making beta alumina solid electrolyte material. The zirconia may be used for toughening the ceramic, in which case the zirconia content of the sintered product might be 2 to 25% by weight.

For electrolyte material, where high ionic conductivity is important, it will generally be undesirable to dilute the beta alumina with large amounts of non-conductive zirconia and more typically the zirconia content might be 4 to 1 5% by weight. The technique of the present invention however also finds particular application in incorporating small quantities of zirconia (0.1 to 3.0% by weight) in sodium beta alumina containing ss and p" alumina in order to increase the proportion of the ,B" form as explained in the co-pending Patent Application No. filed concurrently herewith (Reference BA4268).

The invention includes within its scope a method of making beta alumina solid electrolyte material comprising the steps of mixing finely divided sodium zirconate with alpha alumina and with lithium oxide and/or magnesium oxide, and with or without further soda, heating the mixture to cause the sodium zirconate to react with other constituents to form zirconium oxide and beta alumina with the beta alumina as fine particles dispersed in a matrix of beta alumina, and then sintering the material to form a solid electrolyte.

Conveniently the sodium zirconate is added to an aqueous slurry of the precursor materials whereby the sodium zirconate dissociates forming hydrated zirconia and sodium hydroxide dispersed in the precursor materials. The slurry is dried and the resulting powder is compacted and sintered to produce zirconia particles in a matrix of beta alumina. The sodium zirconate is an unstable material which dissociates in the presence of water. The hydrated zirconia which is produced forms fine particles of zirconium oxide on heating.

The sodium hydroxide and the precursor materials form beta alumina on heating to a calcining temperature which is below the sintering temperature. Thus the beta alumina with dispersed zirconia particles is formed before the material is sintered and the technique produces finely dispersed zirconia particles in a matrix of beta alumina.

The quantities of the constituents are chosen to give a beta alumina of the required composition.

For electrolyte material, where a high proportion of the ,5" form is desirable, the amount of soda must be sufficient, typically 8 to 10% by weight of the beta alumina, and stabilisers, MgO and/or Li2O, have to be used, typically 0.2 to 2.5% by weight. The balance of the beta alumina is AI2O3.

In the following description, Example 1 is an example of the use of the technique of the present invention in the manufacture of zirconiatoughened beta alumina ceramic electrolyte material and Example 2, for comparison purposes, relates to a material of the same chemical composition as that of Example 1 but in which the zirconia is incorporated by mechanical mixing not using the technique of the present invention.

EXAMPLE 1 In this example, zirconia-toughened beta alumina was made which incorporated 13% by weight of zirconia in a matrix of beta alumina. The beta alumina had a composition, by weight, of 8% Na2O, 2% MgO and the balance Al2O3. The amounts of the various ingredients empioyed were chosen to give this composition, taking into account the introduction of soda and zirconia in the form of sodium zirconate.

An aqueous slurry was formed of finely powdered alpha alumina, magnesia and sodium hydroxide. To this was added sodium zirconate derived by calcining a mixture of zirconia (Type Z1 4) and sodium hydroxide. The sodium zirconate, when added to the aqueous slurry, dissociated forming hydrated zirconia and sodium hydroxide. The slurry was dried and the resulting powder compacted to form an electrolyte element which was sintered by firing in a pass-through furnace. This pass-through firing technique is described in U.K. Specifications Nos. 1297373, 1375167, 1458221 and 1458222.

The resultant material had the zirconia in the form of finely dispersed zirconia particles in a matrix of beta alumina. Examination showed that about 50% of the zirconia remained in the tetragonal form and thus contributed to toughening of the ceramic. The measured critical stress intensity factor (kit) was 4.60 MN/m'5.

EXAMPLE 2 A material of the same composition as that of Example 1 was made but the zirconia, in the form of finely milled zirconia powder (Type Z1 4) was mechanically mixed directly with the powdered precursor material for the beta alumina. The material was compared and sintered as in Example 1. In this case however the amount of tetragonal form zirconia in the final product was < 1 %. The measured critical stress intensity factor (kit) was 3.70 MN/m15.

For comparison purposes, the critical stress intensity factor of beta alumina of the same composition as the matrix material but without zirconia (i.e. by weight 8% Na2O, 2% MgO, balance Al2O3) was measured and found to be 2.5 MN/m15.

Claims (12)

1. A method of making a ceramic material incorporating finely divided zirconia dispersed in a matrix of the ceramic which method comprises the steps of mixing, with precursor materials for the ceramic, a zirconium compound which, at a temperature below the sintering temperature of the ceramic, reacts with a constituent of said precursor materials, to dissociate or to form zirconium oxide and an oxide constituent of the ceramic, whereby, on heating, the zirconium oxide is formed simultaneously or substantially simultaneously with formation of the ceramic material with the zirconium oxide dispersed through the ceramic.

2. A method as claimed in claim 1 and for making zirconia-containing sodium beta alumina wherein the zirconium compound is sodium zirconate and wherein the precursor materials comprise alumina with or without lithia and/or magnesia and with or without soda or a material or materials which, on heating, produce an aforesaid precursor material or materials.

3. A method as claimed in claim 1 and for making zirconia-containing barium titanate, wherein said zirconium compound is barium zirconate and said precursor materials are barium hydroxide and titanium dioxide.

4. A method as claimed in claim 1 and for making zirconia-containing magnesium aluminate wherein said zirconium compound is magnesium zirconate and said precursor materials are magnesium hydroxide and alumina.

5. A method of making beta alumina solid electrolyte material comprising the steps of mixing finely divided sodium zirconate with alpha alumina and with lithium oxide and/or magnesium oxide, and with or without further soda, heating the mixture to cause the sodium zirconate to react with other constituents to form zirconium oxide and beta alumina with the beta alumina as fine particles dispersed in a matrix of beta alumina, and then sintering the material to form a solid electrolyte.

6. A method as claimed in claim 5 wherein the sodium zirconate is added to an aqueous slurry of the precursor materials whereby the sodium zirconate dissociates forming hydrated zirconia and sodium hydroxide dispersed in the precursor materials and wherein the slurry is dried and the resulting powder is compacted and sintered to produce zirconia particles in a matrix of beta alumina.

7. A method as claimed in either claim 5 or claim 6 wherein the amount of sodium zirconate is such that the zirconia content of the sintered product is 2 to 25% by weight.

8. A method as claimed in claim 7 wherein the amount of sodium zirconate is such that the zirconia content of the sintered product is 4 to 15% by weight.

9. A method as claimed in either claim 5 or claim 6 wherein the amount of sodium zirconate is such that the zirconia content of the sintered product is 0.1 to 3.0% by weight.

10. A method as claimed in any of claims 5 to 8 wherein the amounts of the precursor materials are such that the sintered sodium beta alumina matrix in which the zirconia particles are dispersed has a composition by weight of 8 to 10% Na2O, 0;2 to 2.5% by weight MgO and/or Li2O, balance alumina.

11. A ceramic material produced by the method of any of claims 1 to 4.

12. A sodium beta alumina electrolyte element produced by the method of any of claims 5 to 10.