WO1992000930A1

WO1992000930A1 - Sintering aid

Info

Publication number: WO1992000930A1
Application number: PCT/GB1991/001148
Authority: WO
Inventors: Timothy Paul Beales; John Bultitude
Original assignee: Cookson Group Plc
Priority date: 1990-07-13
Filing date: 1991-07-11
Publication date: 1992-01-23

Abstract

A method of sintering a ceramic body which comprises the use as a sintering aid of a mixed metal oxide of the formula AxCuOy, or a precursor therefor, where A is a Group IIA metal, or a mixture of Group IIA metals, x is a number in the range of from 0.1 to2.0 and y is a number in the range of from 1.1 to 3.0.

Description

SINTERING AID

The present invention relates to a sintering aid for ceramic materials and, in particular, to a sintering aid for ceramic material intended for use as substrates, barrier layers on substrates or container materials for ceramic superconductors.

Previously, substrates or container materials for ceramic superconductors have usually been made from ceramic materials such as aluminium oxide, stabilised zirconium oxide or magnesium oxide. At typical superconductor processing temperatures, these materials exhibit chemical interaction and/or interdiffusion with the ceramic superconductor material, usually resulting in a deterioration of the properties of the ceramic superconductor. From the point of view of chemical interaction, materials consisting of highly stable alkaline earth metal compounds are likely to be more suitable. In particular, barium zirconate, strontium titanate and calcium silicate have been reported to exhibit little or no chemical interaction with ceramic superconductors at temperatures up to around 950°C.

One characteristic of such materials, however, is the fact that they are generally difficult to sinter to high density. High densities are desirable in substrates or container materials for ceramic superconductors because of the tendency of such ceramic superconductors to form a quantity of liquid phase during processing at temperatures close to or above their melting points, and the fact that this liquid would tend to be drawn into the pores of a porous substrate or container material, leading to phase inhomogeneity in the superconductor after processing. The use of a conventional sintering aid such as a glass to improve the sintering characteristics of the substrate or container material is generally undesirable, due to the increased chemical reactivity with respect to the ceramic superconductor which such an addition would impart upon the substrate or container material.

We have now developed a novel sintering aid for ceramic materials which forms a liquidus phase during sintering and which thereby acts to transport material within the ceramic body.

Accordingly, the present invention provides a method of sintering a ceramic body which comprises the use as a sintering aid of a mixed metal oxide of the formula A_xCu0y, or a precursor therefor, where A is a Group IIA metal, or a mixture of Group IIA metals, x is a number in the range of from 0.1 to 2.0 and y is a number in the range of from 1.1 to 3.0.

Preferred Group IIA metals for use in the present invention are calcium, strontium and barium, or a mixture thereof. A preferred sintering aid for use in the present invention is BaCuθ2«

It will be understood by those skilled in the art that precursors for the sintering aid of the formula A_xCu0_y may also be used. For example the precursor YBa₂Cu₃07._x may be used to provide BaCuθ2, or a stoichiometric mixture of BaO and CuO may be used to provide BaCuθ2.

The sintering aid is preferably used in the present invention in an amount of from 0.5 to 10% by weight, more preferably in an amount of 1% to 5% by weight.

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Examples of ceramic materials which can be sintered advantageously using the sintering aid in accordance with the invention are barium titanate, barium zirconate, strontium titanate, strontium zirconate, calcium titante and calcium zirconate, or mixtures thereof.

In one embodiment of the present invention an intimate mixture of the sintering aid and the ceramic material is prepared by milling with a milling medium. The milling medium may be, for example, water, an organic medium such as 1,1,1-trichloro- ethane, or a mixture thereof. The milling step produces an intimately mixed slurry which is preferably dried prior to calcination. The dried mixture may optionally be sieved to separate large particles therefrom prior to calcination. The mixture is then formed into a ceramic body, for example by pressing. The ceramic body is then sintered, using conventional techniques and firing schedules.

The method of the present invention enables dense sintered ceramic bodies to be obtained at sintering temperatures below those required to produce bodies of the same density from the same ceramic material without sintering aids.

The invention also includes within its scope sintered ceramic bodies produced by the aforesaid method and substrates, barrier layers on substrates or containers for superconductors which comprise such a sintered ceramic body.

The present invention will be further described with reference to the following non-limiting Examples.

Example j

A mixed barium copper oxide, BaCuθ2, was prepared by mixing together quimolar proportions of barium peroxide, Ba0₂, and copper oxide, CuO, and calcining the mixture at 850°C. The mixed barium copper oxide, BaCu0₂, was formed as confirmed by X-ray diffraction studies. - 4 -

log of the barium copper oxide and 90g of barium titanate (HPB grade from Tarn Ceramics, Inc.) were milled together for five hours in sufficient water to form an appropriate slurry. Some decomposition occurred which was believed to be the reaction of the barium copper oxide with water to form copper hydroxide. The milled mixture was then dried at 100°C and sieved to separate the fraction above 150 micrometres particle size. lOg of the sieved mixture was pressed into pellets 19mm in diameter at a pressure of 130 lb/in². The pellets were than placed in a furnace and sintered by raising the temperature of the furnace from ambient temperature to 1000°C over a period of 4 hours, holding the temperature at 1000°C for 2 hours and then allowing the furnace to cool to room temperature over a period of 4 hours.

The density of the pressed and sintered pellets was 5.50g/cm³.

Example 2

lOg of a mixed barium copper oxide, B Cuθ2, and 90g of barium titante (HPB grade from Tarn Ceramics, Inc.) were milled together for 16 hours with sufficient 1,1,1-trichloroethane to form an appropriate slurry. No decomposition occurred. The milled mixture was dried, sieved and pressed into pellets in the manner as described in Example 1. The pellets were sintered in accordance with the firing regime described in Example 1. The sintered pellets had a density of 5.68g/cm³.

In comparison, barium titanate (HPB grade from Tarn Ceramics, Inc.) only attained a density of 5.57g/cm³ when it was fired at 1360°C for 4 hours. Example 3

The procedure of Example 1 was repeated, except that the pellets formed were sintered at 950°C for 2 hours, the temperature being raised to 950°C using a 5°C/minute ramp and cooled from this temperature using at 5°C/minute ramp.

The density of the sintered pellets was 5.30g/cm².

Example 4

The procedure of Example 2 was repeated, except that the pellets formed were sintered at 950°C for 2 hours, the temperature being raised to 950°C using a 5°C/minute ramp and cooled from this temperature using at 5°C/minute ramp.

The density of the sintered pellets was 5.62g/cm³.

Example 5

6g of a mixture of BaCuθ2 and I94g of barium titanate (HPB grade, TAM Ceramics Inc.) was milled overnight for 17 hours with sufficient

1,1,1-trichloroethane to form a slurry. No decomposition occurred. The milled mixture was dried at 100°C, sieved to separate the fraction above 150 micrometres particle size and pressed into a pellet 19mm in diameter at a pressure of 130 lb/in². The pellet was sintered at 1000°C for 2 hours (ramp rate = 5°C/min from ambient) . The furnace was then cooled to room temperature over a period of 4 hours. The density of the pressed and sintered pellet was 5.79g/cm³. Example 6

A mixture of barium titanate (76.8g) (HPB, TAM Ceramics Inc.), strontium titanate (20.15g), (TILCON 55, TAM Ceramics Inc.) and 3.00g of a mixed BaCuθ2 was milled for 17 hours overnight with an amount of isopropanol sufficient to form a slurry. The milled mixture was dired, sieved and pressed into a pellet in the manner described in Example 1. The pellet was sintered at 1100°C for 4 hours using a 5°C/min ramp rate from ambient. The furnace was then cooled to room temperature over a period of 4 hours. The sintered pellet had a density of 5.35g/cm³.

In comparison a pellet made from a mixture of barium titanate (76.85g) and strontium titanate

(20.15g) milled for 17 hours overnight with sufficient isopropanol to form a slurry only attained a density of 5.35g/cm³ when fired at 1360°C for 4 hours using at 5°C/min ramp rate from ambient.

Example 7

A range of mixtures of barium zirconate, BaZrθ₃ (TAM Ceramics, Inc.), and barium copper oxide, BaCuθ2, was prepared by dry mixing approximately 20g of each formulation for two hours. After mixing, quantities of approximately 4g of each mixture were pressed into pellets 20mm in diameter at a pressure of 200 lb/in². The pellets were than placed in a furnace together with a pellet of pure barium zirconate pressed in the same way, and sintered by raising the temperature of the furnace from ambient temperature to 1600°C at a ramp rate of 3°C per minute, holding the temperature at 1600°C for two hours and then allowing the furnace to cool to room temperature at a ramp rate of 5°C per minute. The following table lists the weight percentage BaCuC>2 in each pellet and the density values obtained after sintering.

Weight percentage BaCuθ2 Density (g/cm³)

The procedure of Example 7 was repeated with a smaller number of compositions, except that the sintering treatment was at 1500°C for six hours. After sintering, the density of the pellets was as follows:

Weight percentage BaCu0₂ Density (g/cm³)

0 5.10

3 5.60

10 5.64

Claims

- 8 -CLAIMS :

1. A method of sintering a ceramic body which comprises the use as a sintering aid of a mixed metal oxide of the formula A_xCuOy, or a precursor therefor, where A is a Group IIA metal, or a mixture of Group IIA metals, x is a number in the range of from 0.1 to 2.0 and y is a number in the range of from 1.1 to 3.0.

2. A method as claimed in claim 1 wherein the Group IIA metal is calcium, strontium, barium or a mixture thereof.

3. A method as claimed in claim 1 or claim 2 wherein the mixed metal oxide is BaCuθ2«

4. A method as claimed in any one of the preceding claims wherein the sintering aid is used in an amount of from 0.5 to 10% by weight.

5. A method as claimed in any one of the preceding claims wherein the ceramic body is formed from barium titanate, barium zirconate, strontium titanate, strontium zirconate, calcium titanate, calcium zirconate, or mixtures thereof.

6. The use as a sintering aid for ceramic materials of a mixed metal oxide of the formula A_xCuOy, where A is a Group IIA metal, or a mixture of Group IIA metals, x is a number in the range of from 0.1 to 2.0 and y is a number in the range of from 1.1 to 3.0.