GB2219998A

GB2219998A - Mixed phase ceramic compounds

Info

Publication number: GB2219998A
Application number: GB8914663A
Authority: GB
Inventors: David Anthony Cardwell
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1988-06-24
Filing date: 1989-06-26
Publication date: 1989-12-28
Also published as: GB8815102D0; GB8914663D0

Abstract

A mixed phase ceramic compound is described in which the elements of Strontium, Barium and Copper together with one or more of the elements of Calcium, Berylium, Aluminium and Scandium, are chemically combined with one or more of the elements Oxygen, Fluorine, Nitrogen and Chlorine. The compound exhibits electrical properties indicative of mixed phases. The compound may have at least one phase which becomes superconducting between 90K and 700K. A compound containing Strontium, Barium, Copper, Calcium and Oxygen is particularly described.

Description

MIXED PHASE CERAMIC COMPOUNDS This invention relates to the formulation of ceramic "high temperature" superconducting compounds, for example, ceramic oxides. It is well known that a number of complex oxides exhibit the phenomenon of superconductivity (the ability to conduct electric current without the loss of power due to ohmic resistance). Recently it has been shown that several of these compounds exhibit the phenomenum at temperatures above the boiling point of liquid nitrogen (77K). Some of the oxides which show this effect are: yttrium barium copper oxide (YBa2Cu307) with a transition temperature of 94K, bismuth strontium calcium copper oxides (formulations currently under investigation) with transition temperatures of 85K and 105K, and thallium calcium barium copper oxide with a transition temperature of 125K.The present application is directed to the formulation of some new compounds which can be made to be either semiconducting or metallic at room temperature and which show evidence for the existence of a superconducting transition at a temperature considerably higher than any of the above temperatures.

According to the present invention, there is provided a ceramic compound having, as cations, Strontium, Barium, Copper and one or more of the elements Calcium, Berylium, Aluminium and Scandium, and as anions one or more of the elements Oxygen, Fluorine, Nitrogen and Chlorine.

Where one of the anions is Chlorine, it constitutes less than 50% of the anions.

The invention also includes a preferred method of manufacturing a ceramic calcium strontium barium copper oxide comprising the steps of reacting together CaCO3, SrCO#, BACON and CuO, and then firing in air Advantageously, CaCO3, SrC03 and BaC03 are firstly heated to form calcium strontium barium oxide and then heated in the presence of CuO, prior to firing in air.

Where the method of forming the compound comprises calcination, calcium, strontium, and barium carbonates may be heated between 8000C and 10000C to form calcium strontium barium oxide.

In one embodiment, calcium strontium barium oxide is heated in the presence of copper which is conveniently in the form of copper oxide.

Alternatively, the component metal compounds may be prepared from solution. In such instance, the invention comprises the steps of starting with the acetate or nitrate of barium, copper, calcium and strontium as precursors, in an aqueous solution, and preferably drying by spraying the solution into an environment maintained at an elevated temperature.

It is possible to make the ceramic compound of the invention by a method in which layers of calcium oxide, strontium oxide, barium oxide and copper oxide or mixed layers of calcium, strontium, barium and copper oxides are formed using chemical vapour deposition techniques. This method advantageously includes passing oxides or oxidisable compounds of calcium strontium, barium and copper over a substrate whereon they produce the complex oxides.

In an alternative method, thin films of complex calcium strontium barium copper oxide are formed by evaporation of calcium, strontium, barium and copper, in the presence of oxygen, or their oxides, onto a substrate.

Thin films can also be manufactured by electron beam sputtering of a calcium strontium barium copper oxide target, onto a substrate.

The ceramic calcium strontium barium copper oxide compound of the invention preferably has at least one phase which becomes superconducting between 90K and 700K.

Advantageously at least one of the product phase of the ceramic compound is ferromagnetic at 77K and at least one of the product phases may also be ferromagnetic at 300K.

The product may exhibit at least two anomalies in electrical resistivity between 1 00K and 200K.

The invention will be described further, by way of example, with reference to the formation of a ceramic compound having as cations, Calcium, Barium, Strontium and Copper and having Oxygen as the sole anion of the compound.

A formulation which could be expected on valency considerations is as follows: CaBa2#xSrxCu3On where x=O to 0.5, and n=6 (1) It will be appreciated that the actual level (defined by n in the above formulation (1)) of oxygen in the compound of the present invention is determined by the oxides used and the processing, to form the compound. It is not controlled specifically during the compounding of the oxides.

A method used for the fabrication of oxide ceramics with the above formulation (1) is as follows: 1. Calcium, barium and strontium carbonates and copper oxide were weighed-out in the appropriate molar proportions and sieved into a mortar made of alumina where they were mixed by hand, using a pestle, for 2 minutes.

2. The mixed carbonates and oxide were placed into an alumina crucible and heated to a temperature of 9000C for 10 hours.

This process is called "calcination".

3. The calcined powder was broken down by hand in an alumina mortar again using a pestle and passed through a 50 micron sieve.

4. Step 2 was repeated.

5. Step 3, was repeated. At this stage all of the carbonates had been broken down to form mixed oxides (as inferred by the loss of carbon dioxide, according to the theoretically calculated weight loss).

6. The fine mixed oxides so obtained were pressed in a steel die at 15750 tonnes/square metre into pellets 3 mm thick and 20 mm diameter.

7. The pellets were sintered into ceramics by heating to 9500C at a rate of 200 C/Hr.; holding at 9500C for 16 Hrs.; cooling at 50 C/Hr. to 4500C; holding at 4500C for 6 Hrs.; and cooling to room temperature at 50 C/Hr.

After Step 7, it was noted that the Samples had partially melted some fusion having occurred, but were nevertheless usable as ceramic bodies. Microscopic examination of the specimens has shown that they are multiphase, with at least 3 crystallographically distinct phases present. The compositions of these phases have been determined (using the technique of energy dispersive X-ray analysis in a scanning electron microscope) and have been identified to lie within the following stoicheiometric constraints.

CaxSryBazCuk(x+y+z)O(k+i)(x+y+z) where A, x+y+z=l, k=l B, x+y+z=l, k=4 C, x+y+z=2, k=l.5.

Examples of specific compositions detected within these respective crystallographic phases are listed below.

A, Ca0.22Sr0.06Ba0.72Cu02 CaO.04Sr04B a0.92Cu02 B, Cao.gSro.lBa0.lcu4os C Cao,gBal.2Cu3os Ca0.13Sr0.09Bal 78Cu305 In addition, samples within the above systems but doped with the following species to the following concentrations would be expected to enhance the properties of the product ceramic: Dopant For Atomic % 1. Fluorine Oxygen 0 to 100 2. Chlorine Oxygen 0 to 50 3. Nitrogen Oxygen 0 to 100 4. Berylium Calcium 0 to 100 5. Aluminium Calcium 0 to 100 6. Scandium Calcium 0 to 100 A measurement of resistance as a function of temperature.

conducted using a standard four-point method at a frequency of 80Hz. using the circuit of Fig. la on the multiphase sample described above gave the result shown in Figure lb. It can be seen that although the resistance does not fall to zero at any temperature down to 77K, there is evidence which indicates that there may well be superconducting phases present in the specimen. Firstly, the linear drop in resistance from room temperature down to 190K is indicative of metallic behaviour. This is similar to the behaviour of the other high temperature superconducting oxides listed above and is unusual in an oxide material. Secondly, there is a sharp drop in the resistance of the material, starting at 190K and falling by a factor of about 5 to 153K. This thought to be evidence that one of the phases in the sample is undergoing a superconducting transition.There is also a second enhanced decrease in resistivity starting at 130K and ending at 100K. This may be evidence for a second phase in the compound undergoing a superconducting phase transition.

It was observed that the oxide ceramics made in this way exhibited ferromagnetic behaviour at room temperature and that they were still ferromagnetic at 77K. This is in itself a very unusual phenomenon which would not be expected in an oxide with this formulation.

It is considered that these oxides are technologically useful for a number of reasons: Firstly, the resistance anomalies are evidence that one or more of the phases present is exhibiting superconducting behaviour at a higher temperature than any other material reported to date. These phases, when used in isolation are likely to have use in a wide range of electrical machines such as motors, generators, power transmission lines etc. and electronic devices such as signal transmission lines, aerials, computing elements etc..

Secondly, the resistance anomalies themselves are technologically useful because they can be used for sensing temperature changes. Also, materials which show such effects can be used for self-stabilising heating elements, as when biased with a constant voltage supply, they will tend to maintain themselves at the resistance anomaly through Joule heating.

Thirdly, the oxides exhibit ferromagnetic behaviour and constitute a new class of oxides so to do. Such behaviour offers the possibility of use in permanent magnets, for which there is a very wide range of applications, such as motors, generators, television cathode ray tube beam focussing, microphones, etc..

It will be appreciated by those skilled in the art of making oxide ceramics that there are many difference ways of making a ceramic with a formulation as defined in (1) above which provide variations on the basic method outlined in steps 1 to 7 above.

Firstly, the starting materials do not necessary have to be those defined in step 1. For example, in place of the calcium, strontium and barium carbonates, the acetates, oxalates, acetyl acetonates, hydroxides, nitrates or other inorganic or organic compounds can be used which decompose to give the oxides when heated. Similarly, the copper oxide can be replaced by a compound such as copper nitrate, carbonate, acetate, oxalate, acetyl acetonate or another inorganic or organic compound which decomposes to give the oxide on heating.

The mixing of these compounds does not necessarily have to be conducted using a mortar and pestle, but can be carried out in a number of ways. For example, the compounds can be placed in a container with hard spheres or cylinders (milling elements) made from a material such as zirconia or alumina and tumbled for a period of time. This mixes and also grinds the compounds together, reducing their particle sizes.

In another variation, the metal ions can be obtained from soluble compounds such as nitrates (which can be dissolved in water) or alkoxides (which can be dissolved in alchols). The soluble compounds are taken up into solution and mixed there. The metal ions can then be brought out of solution in a variety of ways. For example, the solutions can be freeze dried or spray dried (techniques well known to those skilled in the art). Alternatively, the metal ions can be precipitated out of solution as insoluble compounds. For example, if a stirred, mixed nitrate solution is used, the addition of an alkali such as ammonium hydroxide will precipitate insoluble hydroxides in an intimately mixed form. The powders which are produced by the alternative mechanical or chemical mixing methods can then be treated as in the Steps 2 to 7 defined above.

Secondly, the mixed oxides do not need to be placed into an alumina crucible for calcination. For example a platinum or other heat resistant, non-reactive crucible may be used. In addition, particle decomposition of the starting materials may be achieved with calcination temperatures and times other than those described above (Step 2). For example, partial reaction of the starting materials at sub-melting point temperatures would occur to varying degrees using calcination temperatures in the range 7000C to 9400C. The extent to which the starting materials decomposed would then be determined by the period of time over which the calcination was performed, with extended periods at a given temperature yielding the greatest percentage of reacted powder. Furthermore, it would be possible to obtain complete decomposition with only one calcination step.This could be achieved, for example, by calcining the starting materials at 9000C for 20 hours or longer.

Thirdly, the reacted powder could be broken down by hand in any hard, non-reactive container suitable for confining the loose powder product such as an agate pestle and mortar. The requirement to sieve before a second calcination and/or compaction is not essential. Any of the materials the composition of which was listed under A to C above could be produced directly at this stage by pressing and sintering the coarse powder according to Steps 6, and 7, above. Sieving enhances the homogeneity of the powder and may be performed by the use of any size mesh sieve. Alternatively, after breaking down the reacted or partially reacted starting materials into a powder, their size could be further reduced by tumbling them in a container with hard spheres such as zirconia or alumina.

Fourthly, the object of the second calcination is to achieve full decomposition of the starting materials. The temperature and times relating to the second calcination, therefore, depend entirely upon the degree to which the starting materials were decomposed during the initial calcination. For example if a lower calcination temperature or for a shorter time than quoted in Step 2 above, were to be employed during the initial calcination, an extended calcination period at 9000 C, above that quoted in Step 4, would be required.

Breaking down the fully reacted starting materials after the second calcination may be performed by any suitable method such as those applied to the powder after the first calcination as described above.

Fifthly, the object of sintering in this case is to produce samples which are usable ceramic bodies. Consequently the fully decomposed starting materials may be formed into a range of sample geometries, prior to sintering, by a range of different techniques. For example the reacted powder may be compacted in a pressing die by the application of a uni-axial pressure. There is no necessity for the die to be composed of steel (as those skilled in the art will appreciate).

For example silicon nitride or any other hard non-reactive, pressure stable material of suitable geometry could be used. As the fundamental properties of the material, after sintering, are independent of sample geometry, any method of compaction or moulding the fully decomposed starting materials may be employed for this process.Other variants include: (1) isostatic pressing, in which the reacted powder is encompassed in a flexible mould of a desired geometry and pressure is applied simultaneously at all points to its surface through a gaseous or liquid medium: (2) extrusion. in which the fully decomposed starting materials are mixed with a polymer (for example polyethylene, polypropylene etc.) binder or other viscous composite binder and then pressed through an aperture of appropriate diameter to produce a wire: (3) tape casting, in which a suspension of the fully decomposed starting materials in a suitable liquid medium (e.g. a solution of polyethylene glycol, polyvinyl butyl, tolulene, dioctyl phthalate and an organic dispersant is cast over a given area and dried to form a polymer/powder composite thick film.

Sixthly, the heating rate to the sintering temperature described in Step 7, above may be varied considerably as those skilled in the art will appreciate. The lower heating rate is limited only by time constraints and the upper by furnace capability. The time for which the sample is held at the sintering temperature described in Step 7, above may be as short as 2 hours. There is no limit on the upper temperature hold time. The sintering temperature may be reduced to 9000C without subsequent loss of the ferromagnetic properties in the resultant sample. For sintering temperatures below 950 C.

however, the magnitude of the anomaly in the electrical resistivity of the sample particularly over the range 136K to 166K becomes reduced significantly whereas the magnitude of the lower temperature anomaly becomes increased. Lower cooling rates than those described in Step 7, above may be used when the sample is cooled to 4500C. The upper cooling rate, however, is preferably constrained to be a maximum of approximately 3000C/Hr. to minimise problems associated with thermal gradients in the sample such as cracking etc.. The hold at 4500C is not essential and is included to ensure an equilibrium state is obtained should there be any diffusion of gaseous species (oxygen, for example) into the sample. This could also be achieved, however, for hold temperatures in the range 4000C to 6000C for periods of time up to 10 hours.

Alternatively, a slow cool rate to room temperature may be employed (for example cool rates up to and including 50 C/Hr. would be appropriate) in which case the requirement to hold at any temperature on cooling would be avoided.

Claims

1. A ceramic compound having, as cations, Strontium, Barium, Copper and one or more of the elements Calcium, Berylium, Aluminium and Scandium, and, as anions, one or more of the elements Oxygen, Fluorine, Nitrogen and Chlorine.

2. A compound as claimed in claim 1 wherein one of the anions is Chlorine, and it constitutes less than 50% of the anions.

3. A method of manufacturing a ceramic calcium strontium barium copper oxide comprising the steps of reacting CaCO3, SrCO3, BACON and CuO together and thereafter firing in air.

4. A method as claimed in claim 3 wherein the CaCO3, SrCO3 and BaCO3 are first heated to form calcium strontium barium oxide which is then heated in the presence of CuO prior to firing in air.

5. A method as claimed in claim 4 wherein the calcium, strontium, and barium carbonates are heated to between 8000C and 1000 C to form the calcium strontium barium oxide.

6. A method as claimed in claim 4 or 5 wherein the calcium strontium barium oxide is heated in the presence of a copper compound which, on heating forms copper oxide.

7. A method of forming a ceramic compound as claimed in claim 1 wherein the component metal compounds are prepared from acqueous solutions comprising the acetate or nitrate of barium, copper, calcium and strontium as precursors and thereafter removing the solvent

8. A method as claimed in claim 7 wherein the solvent is removed by spray drying the solution into an environment maintained at an elevated temperature.

9. A method of making a ceramic compound as claimed in claim 1 comprising the steps of forming layers of calcium oxide, strontium oxide, barium oxide and copper oxide or mixed layers of calcium, strontium, barium and copper oxides using chemical vapour deposition techniques.

10 A method as claimed in claim 9 including the step of passing oxides or oxidisable compounds of calcium, strontium, barium and copper over a substrate whereon they produced the complex oxides.

11. A method of making a ceramic compound a claimed in claim 1 comprising the steps of forming strontium, barium, copper oxide by evaporation of calcium, strontium, barium and copper, in the presence of oxygen, or their oxides, onto a substrate.

12. A method of making a ceramic compound as claimed in claim 1 comprising the steps of sputtering of a calcium strontium barium copper oxide target, onto a substrate.

13. A ceramic calcium strontium barium copper oxide compound made by the method of claim 3 and having at least one phase which becomes superconducting between 90K and 700K.

14. A ceramic calcium strontium, barium, copper oxide made by the method of claim 3 and having at least one phase which is ferromagnetic at 77K.

15. A compound as claimed in claim 13 or 14 wherein at least one phase is ferromagnetic at 300K.

16. A compound as claimed in any of claims 14, 14 or 15 which exhibits at least two anomalies in electrical resistivity between 100K and 200K.

Amendments to the claims have been filed as follows 1. A ceramic compound having, as cations, in the absence of Titanium, Yttrium and/or a rare earth element, Strontium, Barium, Copper and one or more of the elements Calcium, Berylium, Aluminium and Scandium, and, as anions, one or more of the elements Oxygen, Fluorine, Nitrogen and Chlorine.

3. A method of manufacturing a ceramic calcium strontium barium copper oxide comprising the steps of reacting, in the absence of Titanium, Yttrium and/or a rare earth element, CaCO3, SrCO#, BaCO3 and CuO together and thereafter firing in air.

4. A method as claimed in claim 3 wherein the CaCO3, SrCO3 and BACON are first heated to form calcium strontium barium oxide which is then heated in the presence of CuO prior to firing in air.

5. A method as claimed in claim 4 wherein the calcium, strontium, and barium carbonates are heated to between 8000C and 10000C to form the calcium strontium barium oxide.

7. A method of forming a ceramic compound as claimed in claim 1 wherein the component metal compounds are prepared from acqueous solutions comprising the acetate or nitrate of barium, copper, calcium and strontium as precursors and thereafter removing the solvent 8. A method as claimed in claim 7 wherein the solvent is removed by spray drying the solution into an environment maintained at an elevated temperature.