WO1997031878A1

WO1997031878A1 - Ceramic member, method for producing such a member and use of such a member

Info

Publication number: WO1997031878A1
Application number: PCT/IB1997/000178
Authority: WO
Inventors: Andreas Erb; Erich G. Walker; René L. J. FLÜKIGER
Original assignee: Universite De Geneve
Priority date: 1996-02-28
Filing date: 1997-02-27
Publication date: 1997-09-04

Abstract

The ceramic member consists substantially exclusively of pure BaZrO3 and/or SrZrO3. The ceramic member may constitute for instance a receptacle (1) for melting a material (15) and/or for processing a molten material (15) such as a superalloy or another metallic material or a material for forming an oxide superconductor. The ceramic member may also be adapted for shaping a molten or at least softened metallic material. The ceramic member may moreover constitute a part of an electric heating resistor or at least a part of a thermal insulation. The ceramic member may further constitute a substrate for supporting a thin epitaxial layer of an oxide superconductor. The ceramic member has a high melting temperature, is not submitted to crystallographic phase transitions between room temperature and the melting temperature, is mechanically and chemically stable up to high temperatures and is able to resist strong thermal shocks.

Description

Ceramic member, method for producing such a member and use of such a member

1. FIELD OF THE INVENTION

The invention relates to a ceramic member.

More in particular, the ceramic member may for instance constitute a receptacle - e.g. crucible - for melting a material and/or processing a molten material, particularly a metallic material having a high melting temperature, such as a superalloy, or a material which comprises at least one oxide and serves for instance for forming an oxide superconductor. The superconductor may for instance consist of YBa₂Cu₃O_x or PrBa₂Cu₃O_x, x being 6 to 7. Superconductors containing yttrium oxide are designated also as Y₁₂₃ superconductors . The receptacle or crucible may then be used for instance for growing single crystals.

The member may also constitute an extrusion die or a mold or the like for shaping a metallic material - particularly a superalloy - which was previously molten or at least sufficiently softened by heating that it can more or less flow.

The ceramic member may also be used as thermal insulating material and/or as heating resistance for a high temperature furnace. The ceramic member may further constitute a substrate or support - such as a dice - for supporting a layer of a material which may be layered onto the member for instance by deposition from a molten state or by deposition from a vapor or by sputtering.

DESCRIPTION OF THE PRIOR ART

Known crucibles consist for instance of alumina, i.e. Al₂0₃. However, alumina is not suited for melting metallic materials having high melting temperatures - such as superalloys - which have often melting temperatures of approximately 1500° C or more. Alumina is also chemically reactive at high temperatures so that an alumina crucible may negatively affect the purity and composition of the molten material.

Other known crucibles comprise zirconium oxide, - i.e. Zr0₂ - as major component and about 5% to 15% of yttrium oxide - i.e. Y₂0₃_ The latter serves as stabilizer for suppressing the crystallographic phase transition of Zr0₂. However, such crucibles tend to be corroded by melts which have high temperatures and/or are highly reactive. Hence, such crucibles may also negatively affect the purity of a molten superalloy or other metallic material. Zirconium oxide stabilized with yttrium oxide has moreover the drawback that it is sensitive to thermal shocks so that strong and rapid temperature changes cause often damages of ceramic members which are formed of this material.

Similar problems arise with extrusion dies or other members serving for shaping superalloys or other metallic materials with high melting temperatures. Presently known crucibles for melting oxide superconductors consist also of Al₂0₃ or of Zr0₂ stabilized with Y₂0₃. Such crucibles have partly similar drawbacks as in use for melting superalloys. As superconductors are extremely sensitive to impurities, these drawbacks may even be more serious for superconductors. If a crucible of Zr0₂ stabilized with Y₂0₃ is used for growing crystals of YBa₂Cu₃O_x, a corrosion product of the crucible walls may overgrow already grown superconductor crystals and form a solid layer on these crystals which get thereby lost, i.e. defective.

The US 3 523 915 A discloses ceramic mixtures for making electrodes for use in magnetohydrodynamic generators . The mixture comprises zirconia and at least one zirconate such as strontium zirconate, calcium zirconate and barium zirconate. The SU 489 743 A reveals ceramic members obtained from a mixture comprising zirconia, barium zirconate, barium aluminate and a binder. As the ceramic materials disclosed by these documents comprise a large fraction of zirconia, they are likely to be corroded if they get into contact with high temperature and/or high temperature melts similarly as the crucible on Zr0₂ base already described. These known ceramic materials would therefore not be suitable for uses where they come into contact with molten materials such as superalloys or oxide superconductors. It is moreover doubtful if the materials disclosed by said two documents can withstand strong thermal shocks without being damaged.

There are known substrates - e.g. dices - supporting a layer of an oxide superconductor such as Y₁₂₃. The substrates consist for instance of magnesia or lanthan aluminate or strontium titanate. The superconductor is often layered onto the substrates in molten state or in vapor state. In both cases, the substrates are temporarily heated to temperatures in the range of 900° C to 1000° C or more. The known substrates are submitted to undesired chemical reactions at these temperatures. These reactions negatively affect the composition of the superconductors. Moreover, the structure of the formed layers deviate often strongly from the desired epitaxial structure.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a ceramic member that permits to avoid drawbacks of known ceramic members and can be used for instance for melting a material and/or processing a molten material or for shaping a material which is molten or at least softened by heat or for supporting a layer of material which is layered onto the hot member or for forming a heat insulation or a heating resistor. More particularly, the ceramic member shall be mechanically stable up to high temperatures and resistant against thermal shocks. The ceramic member shall also be chemically stable up to high temperatures and not affect the chemical composition of a molten metallic material or of a molten oxide super conductor material which gets into contact with the ceramic member.

According to the invention, this object is achieved by means of a ceramic member consisting substantially exclusively of barium zirconium oxide or strontium zirconium oxide.

The invention relates further to a method for producing a ceramic member as specified in claim 9..

The invention concerns also a use of a ceramic member as specified in claim 10.

Preferred features of the invention are specified in the dependent claims. If the ceramic member consists substantially exclusively of barium zirconium oxide, the ratio between the number of barium atoms and the number of zirconium atoms present in the ceramic member is preferably close to 1 and shall deviate preferably not more than 1%, better not more than 0.5%, even better not more than 0.2% and for instance at most or approximately 0.1% from 1. The ceramic member can then comprise numbers of barium, zirconium and oxygen atoms which are substantially in stoichiometric proportions corresponding to the formula BaZr0₃_

The ceramic member has a degree of purity that is preferably at least 95% and better at least 99%, i.e. contains at least 95% or at least 99% of BaZr0₃. The percentages relate thereby to the contents of atoms forming BaZr0₃.

BaZr0₃ has a cubic structure with a lattice constant of 0.419 nm at room temperature. BaZr0₃ has a theoretical density or X-ray density of 6.242 g/cm³ at room temperature. The theoretical density or X-ray density is the density present if BaZr0₃ forms an ideal single crystal having all lattice points occupied by atoms. The true density of a ceramic member consisting substantially of BaZr0₃ is preferably at least 95%, better at least 97% and for instance approximately 98.5% of the theoretical density.

The melting temperature of a ceramic member consisting of BaZr0₃ is approximately 2600° C. Thereby it is a great advantage for many applications that no crystallographic phase transitions occur if the temperature is varied between room temperature and the melting temperature. A ceramic member consisting of BaZr0₃ remains mechanically stable and intact up to high temperatures. The thermal expansion coefficient of BaZr0₃ is about 7.8 x 10^"6/K at temperatures from 0° C to 1000° C. This low thermal expansion coefficient makes a ceramic member consisting of BaZr0₃ resistant against thermal shocks, i.e. against rapid and strong temperature changes. Hence, a ceramic member consisting of BaZr0₃ can normally withstand much stronger thermal shocks than Zr0₂ without being damaged by crack formation or in other ways.

BaZr0₃ is also chemically stable and resists reactive materials up to very high temperatures.

Hence, a ceramic member consisting of substantially pure BaZr0₃ is well suited for use at high temperatures - for instance at temperatures of at least 800° C or at least

1000° C or even above 1500° C. A ceramic member consisting of BaZr0₃ is also well suited for contacting practically all molten metallic materials and particularly superalloys.

It may be remarked here that superalloys are alloys having high melting temperatures which are often between 1500° C and 1600° C. Such superalloys remain firm and tough up to at least 600° C, possibly up to 1000° C or even up to 1300° C. Superalloys comprise normally a relative great number of components, wherein the major or basic component may consist for instance of cobalt or nickel. Although such superalloys have often a strong chemical reactivity in the molten state, it was found that ceramics of BaZr0₃ can contact molten superalloys without being corroded or submitted to other chemical reactions and without affecting the composition and purity of the superalloy.

Hence, a ceramic member consisting substantially exclusively of pure BaZr0₃ may constitute a receptacle - for instance crucible - and is then well suited for melting materials having a high melting temperature. The ceramic receptacle or crucible may then be disposed for its use for instance removably within a high temperature furnace. It is also possible to equip the ceramic receptacle or crucible itself with a heating device and a thermal insulation so that the ceramic receptacle or crucible forms then a part of a high temperature melting device and/or high temperature melting furnace.

The ceramic receptacle or crucible consisting substantially exclusively of BaZr0₃ can for instance be used for melting metallic materials and particularly superalloys and/or for processing molten metallic materials. The ceramic member may also constitute another part or device which must contact a metallic material - such as superalloy - which is molten or at least partly softened by heating. The ceramic member may for instance be configurated as shaping member for shaping a metallic material which was previously molten or at least softened by heating so much that it can be well deformed by plastic deformation and can more or less easily flow. The ceramic member may constitute for instance an extrusion die for continuous casting or possibly a mould for ordinary casting or the like.

The ceramic member may also constitute a receptacle - e.g. a crucible - for melting a material serving to form oxide superconductors and for growing single crystals from the molten material. There were made tests with molten materials for forming Y₁₂₃ superconductors. The melts had temperatures of approximately 900° C to 1000° C. These tests have shown that the receptacles of BaZr0₃ did not corrode and did not affect the composition, purity and crystal lattice structure of the grown superconductor single crystals. Accordingly, no superconductive material was covered by corrosion products. BaZr0₃ is not only resistant against high temperatures, but has further a low heat conductivity at all temperatures. A ceramic member consisting of BaZr0₃ may therefore be used as thermal insulating material . A ceramic member of BaZr0₃ may constitute for instance at least a part of a thermal insulation of a high temperature furnace or other high temperature device.

Pure BaZr0₃ is electrically insulating at room temperature. However, BaZr0₃ gets electrically conductive at high temperatures. Thus, a ceramic member of BaZr0₃ may constitute an electrical heating member, i.e. the high-resistivity material or part of an electrical heating resistor for a heating device of a high temperature furnace or the like. The ceramic member may then be provided with an electric connection means comprising for instance metallic electrodes. There may also be provided an additional preheating means in order to preheat the BaZr0₃ member to a temperature where the latter gets sufficiently electrically conductive.

A ceramic member of BaZr0₃ may also serve as a substrate or support - e.g. a dice - for supporting a thin epitaxial layer of an oxide superconductor such as an Y₁₂₃ superconductor. For producing such a thin layer, one may for instance dissolve an amount of superconductive material in a high temperature solution and dip the substrate into the solution so that dissolved superconductive material is crystallized on the substrate . The substrate is thereby temporarily heated by the hot solution. Instead of that, one may heat the solid substrate to a temperature being a little below the melting temperature of the superconductive material and then layer superconductive material onto the substrate by directing a beam of vaporized material toward the substrate. The super conductive material may also by layered onto the cold or hot substrate by sputtering or the like. It was found that a ceramic substrate consisting of pure BaZr0₃ is particularly well suited for supporting a thin pure epitaxial layer of an Y₁₂₃ superconductor because BaZr0₃ and an Y₁₂₃ superconductor have similar crystall lattice structures and grid constants. This is also true for other oxide superconductors, particularly for those comprising at least one oxide of a rare earth such as the already mentioned superconductor comprising praseodymium (Pr) oxide.

The ceramic member may consist substantially exclusively of substantially pure strontium zirconium oxide, i.e. of SrZr0₃ instead of BaZr0₃. The ceramic member may also consist of a mixture of BaZr0₃ and SrZr0₃. SrZr0₃ is isomorphic to BaZr0₃ and has similar properties as the latter. Hence, many of the previous statements made with regard to BaZr0₃ are in an analogous manner valid also for SrZr0₃ and for combinations of BaZr0₃ and SrZr0₃.

The invention as well as its objects and advantages will become more apparent in the detailed description of the preferred embodiments subsequently presented.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention is illustrated with reference to embodiments shown in the drawings, in which

Fig. 1 shows a section of a device for shaping a ceramic member constituting a receptacle,

Fig. 2 shows a schematic section of a melting furnace comprising the shaped receptacle, Fig. 3 shows an apparatus for continuous casting, the apparatus being to the largest part represented in section,

Fig. 4 a section of a ceramic member constituted as substrate supporting a layer of superconductive material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the production of the ceramic member 1 shown in different states in Figs 1 and 2, one provides at first two particulate starting substances, namely barium carbonate, i.e. BaC0₃, and zirconium oxide, i.e. Zr0₂. The starting substances should have a degree of purity of at least 95% and better of at least 99%, wherein the percents relate to the number of molecules and/or atoms. Such starting substances are commercially available - for instance from Fluka Chemie AG, Buchs in Switzerland - and have for instance particle sizes of the order of 1 μm. One weights then a portion or amount of each of these starting substances so that the two portions comprise as precisely as possible the same number of barium atoms and zirconium atoms, respectively. The ratio between the number of barium atoms and the number of zirconium atoms shall deviate not more than 1%, preferably not more than 0.5%, better at most 0.2% and still better at most 0.1% from the ideal value of 1. The two portions are then mixed so that a homogeneous mixture is produced.

The mixture is then at least one time calcinated in air at a temperature being for instance in the range of 1200° C to 1300° C. In this calcination, the mixture reacts and forms particulate BaZr0₃. During the calcination, there are formed particles which have greater sizes than the original particles. There may be formed particles having for instance sizes of the order of 20 μm. The intermediate product resulting from the calcination is then milled. Preferably, the intermediate product is calcinated at least one further time and afterwards milled at least one further time. One may for instance perform alternately three calcinations and three milling operations. Each calcination may take for instance approximately 10 hours. The milling can be performed by means of ball mill containing milling balls of Zr0₂. There is added preferably some organic milling aids at least to the last milling operation. These organic milling aids consist for instance of stearic acid and are then removed from the calcinated, particulate intermediate product before the latter is shaped and sintered. The particles shall have after the last calcination and the last milling operation an average size which is at most 1 μm, preferably at most 0.5 μm, better at most 0.35 μm and for instance approximately 0.3 μm. The maximum particle size is then preferably at most 2.5 times to 3 times the average particle size.

The intermediate particulate product consisting of BaZr0₃ particles is then shaped for forming the already mentioned ceramic member 1. The latter may constitute a receptacle 1 or, more in particular, a crucible 1. The receptacle or crucible has a wall with a bottom and with a jacket. The latter is conically narrowing toward the bottom.

The shaping operation is performed for instance by means of the shaping device 3 represented schematically in Fig. 1. This device comprises a hollow cylinder 4, a piston 5 and a shaping member 6. The latter can be removably attached inside the cylinder and comprises a core which projects upward and is complementary to the interior of the ceramic member or receptacle 1 to be produced. For shaping a ceramic member or receptacle, one packs a predetermined amount of particulate BaZr0₃ around and onto the core of the shaping member 6 by means of a weak, flexible bag or sleeve 7 consisting for instance of plastics. The bag or sleeve is attached gas- tightly to the shaping member, evacuated through a connector section and then closed tightly. One fills then a liquid 8 - for instance water - into the cylinder 4 so that the liquid encompasses and covers that section of the bag or sleeve which contains the particulate BaZr0₃. The liquid 8 is then pressurized by means of the piston 5. The liquid exerts then a substantially isostatic pressure onto the package of particulate BaZr0₃. This pressure is preferably at least 100 MPa and for instance about 400 MPa. The pressurized liquid compresses, densities and shapes the package of particulate BaZr0₃. This shaping is performed at normal room temperature.

The shaping operation could also be done by a slip casting method.

The shaped and more or less solidified ceramic member or receptacle 1 is then sintered without sintering additives. The sintering process is carried out in air at about 1700° C during approximately 48 hours.

The ceramic member 1 constituting a receptacle 1, i.e. crucible 1 may be integrated into a melting device such as the melting furnace 11 shown in Fig 2 and form the inner wall of the melting furnace. The melting furnace 11 comprises further an outer wall 12 and heating device 13 which is for instance disposed outside the ceramic member or crucible 1 between the latter and the outer wall 12. The receptacle or crucible 1 may be connected for instance in such a detachable manner with the other parts of the melting device or furnace 11 that it can be easily separated from these other parts, removed and - if necessary - exchanged.

The outer wall 12 is made for instance at least in part of one or more BaZr0₃ members serving as thermal insulation. Furthermore, the heating device 13 may possibly comprise at least one electric heating resistor formed by a BaZr0₃ member provided with two electrodes serving as electric connections. The heating device 13 may then comprise additional auxiliary heating means for preheating the at least one BaZr0₃ heating resistor to a temperature where the or each BaZr0₃ heating resistor gets sufficiently electrically conductive.

The melting furnace may serve for melting a material 15 disposed within the ceramic member 1, i.e. crucible 1. The latter and the material 15 can be heated by means of the heating device 13, so that the material 15 melts. The molten material 15 contacts then at least one surface of the crucible 1, namely the inner, upper surface of the bottom and the inner surface of the jacket.

The material 15 may be metallic and consist of a superalloy. This superalloy may then serve for instance for forming a part of a gas turbine, a jet engine, a rocket, a nuclear reactor plant or the like.

However, the material 15 may comprise instead at least one oxide, e.g. an oxide compound for forming a superconductor, such as a compound comprising yttrium oxide, e.g. Y₁₂₃. The molten material can then be further processed for growing single crystals of the material.

Fig. 3 shows a apparatus 21 for continuous casting. The apparatus comprises a melting furnace 22 with an inner wall formed by a receptacle 23 or crucible 23, an outer wall 24 and a heating device 25. The melting furnace is provided with an outlet 26. The latter is connected to a ceramic member 28 constituting an elongate extrusion die 28 limiting a passage. A cooling device 29 is disposed on the outer side of the extrusion die 28. The ceramic member or extrusion die 28 consists of BaZr0₃. The receptacle 23 and/or the outlet 26 may possibly also be formed by a ceramic member of BaZr0₃. The outer wall 24 comprises and/or forms a thermal insulation and may consist at least in part of one or more ceramic member(s) of BaZr0₃.

The melting furnace 22 can be used for melting a material 31 which is metallic and consists for instance of a superalloy. The molten material can then flow under the influence of gravity and/or of a not shown pressurizing and conveying device into and through the passage of the ceramic member, i.e. die 28. The inner surface of the latter contacts the molten material passing through said passage . The material is then cooled within and/or after the die, so that the material is first made pasty and then completely solidified. The ceramic member, i.e. die shapes the passing material 31. The latter may then form a rod or wire 32 or the like.

Fig. 4 shows a ceramic member 41 consisting of BaZr0₃. The ceramic member 41 forms a substrate 41 or a support, e.g. a dice. The substrate 41 has for instance the shape of a plane disk. The ceramic member or substrate 41 may be shaped during a shaping operation for instance by applying an uniaxial pressure perpendicular to the plane surfaces of the substrate to be formed.

The upper surface of the substrate 41 supports a thin layer of a material 42 consisting of a superconductor such as Y₁₂₃. The material may have been layered onto the substrate 41 by one of the processes mentioned in the introduction. The material 42 may then form a layer which is practically perfectly epitaxial and very pure.

The ceramic members may of course constitute receptacles or shaping members with a variety of other shapes. The substrate 41 may possibly support a layer which consists of a material different from an oxide superconductor, for instance a layer of a metallic material and/or a semiconductor.

Claims

1. Ceramic member, characterised in that it consists substantially exclusively of barium zirconium oxide and/or strontium zirconium oxide.

2. Ceramic member as claimed in claim 1, characterised in that said oxide has a degree of purity of at least 95%.

3. Ceramic member as claimed in claim 1 or 2, characterised in that said oxide has a degree of purity of at least 99%

4. Ceramic member as claimed in any of claims 1 to 3, characterised in that it comprises a number of barium and/or strontium atoms and a number of zirconium atoms, that the ratio between these two numbers deviates not more than 1% from 1 and that it has a composition corresponding substantially to the formula BaZr0₃ and/or SrZr0₃.

5. Ceramic member as claimed in claim 4, characterised in that said ratio deviates not more than 0.5% from 1.

6. Ceramic member as claimed in any of claims 1 to 5, characterised in that it is configurated as receptacle (1, 23) for melting a material and or for processing a molten material and/or that it is configurated as shaping means, for instance a die (2) or mould, for shaping a material which is softened by heating and/or molten and/or that it is adapted to constitute at least a part of a thermal insulation for a high temperature device, e.g. furnace (11, 22) .

7. Ceramic member as claimed in any of claims 1 to 5, characterised in that it is configurated as a part of an electric heating resistor.

8. Ceramic member as claimed in any of claims 1 to 5, characterised in hat it is configurated as substrate (41) for supporting a layer of a superconductive oxide material (42) , for instance material for an Y₁₂₃ superconductor or material comprising at least one rare earth oxide.

9. Method for producing a ceramic member, characterised in that an intermediate product is produced which consists substantially exclusively of barium zirconium oxide or of strontium zirconium oxide and that the ceramic member is formed of said intermediate product .

10. Use of a ceramic member as claimed in any of claims 1 to 8, wherein the ceramic member is heated during its use at least temporarily and/or wherein a surface of the ceramic member is brought into contact with a metallic or superconductive oxide material which is softened by heating and/or molten and/or in vapour state and/or sputtered onto the surface and/or wherein the ceramic member serves to constitute at least a part of a thermal insulation and/or wherein the ceramic member serves to form at least a part of an electric heating resistor.