WO1990011968A2 - COPPER OXIDE OF MIXED VALENCY DERIVED FROM THE STRUCTURE OF PERVSKITE OF TYPE (ACuO3-x)2(A'O)¿2? - Google Patents

Abstract

The invention concerns a copper of mixed valency derived from the structure of perovskite and of formula: Pb1-x Alt2+y+x Tr1-z Et2 O7-ε with z greater than or equal to 0, lesser than or equal to 0.6, preferably greater than 0.1; x greater than or equal to 0 and lesser than 0.9, preferably greater than 0.1; y greater than or equal to 0 and lesser than or equal to 1, preferably greater than 0.1 and lesser than 0.9; with epsilon varying between -0.1 and 2/3 preferably 1/2; Alt represents the alkaline-earths with the exception of magnesium, Tr represents the rare-earths, preferably yttrium, lanthanum and the lanthanides; and Et represents the transition elements from d5 to d10, copper representing at least seven out of eight atoms of said transition elements. Application: electrical and electronic industry.

Description

COPPER OXIDE WITH MIXED VALENCE DERIVED FROM THE STRUCTURE OF THE PEROWSKITE TYPE (ACuO _j -x ^ CA'O ^

The present invention relates to new copper oxides with mixed valence of structure derived from perows ite.

It relates more particularly to superconductive compositions having at least partially a structure derived from perowskite.

During the last five years these oxides, which result from an intergrowth of a structure of the perowskite type and of a structure of the sodium chloride type, we have been the subject of very numerous studies, and all the more so their electrical and more particularly superconductive properties have been demonstrated at temperatures. greater than

100K.

These copper oxides have structures generally qualified as oxygen deficient, which means that there is not enough oxygen to fill all the available sites of the perowskite. In fact, they have superstoichiometric zones of oxygen with respect to copper at degree of oxidation II. This overstoichiometry is undoubtedly linked to their characteristic of being superconductive at relatively high temperatures.

The structures of these copper oxide compounds are described in numerous publications. Among these, we should mention:

A) Article by B. RAVEAU "Many oxide structures can be constructed from the basic type ReO," - Proc. Indian Natn. Sci Acad., 52, A, n ° 1, 1986, pages 67-101;

B) Article by L. ER-RAKHO, C. MICHEL, 3. PROVOST and B. RAVEAU "Perowskite series in the absence of oxygen containing CuII, CuIII" -

Journal of Solid State Chemistry 37, 151-156 (1981);

C) Article by C. MICHEL and B. RAVEAU "Oxidation of oxygen in mixed valence copper oxides related to perovskites" - Revue de Chimie Minérale, t 29, 1984, page 407; D) Article by C. MICHEL, F. DESLANDES, 3. PROVOST, P.

LE3AY, R. TOURNIER, M. HERVIEU, B. RAVEAU, C.R. Academy of Sciences, Tome 304, II, N ° 17, p.1059 (1987);

E) Article by C. MICHEL, F. DESLANDES, J. PROVOST, P. LEJAY, R. TOURNIER, M. HERVIEU and B. RAVEAU, report to the Académie des Sciences, t 304, II, n ° 19, p. 1,169 (1987);

F) The review article, which in addition to the structural properties

LOCATION describes electrical and magnetic properties and which is published in Research No. 195 in January 1988, vol. 19, pages 53-60, entitled "The discovery of high temperature superconductivity" by K. Alex MULLER and J. Georg BEDNORZ. Furthermore, such structures and their industrial applications have been, for example, described in patent applications whose objects are inventions made at the CRISMAT Laboratory in Caen, attached to the CNRS, in particular those under the numbers S7 03717 and 87 03975.

Over the past year, numerous sub-families of mixed valence copper oxide with a structure derived from perowskite have been studied, in particular with a view to obtaining a range of superconductors with specific and better adapted characteristics. than others to certain applications.

The number of elements used in these structures and the variability of proportions lead to an almost infinite number of combinations. Only a very small fraction of these compounds has interesting electrical properties.

Among the mixed valence copper oxides obtained by the intergrowth of a perowskite type network and a sodium chloride type network, which has been symbolized by the formula (ACuO,) (A'O), the members having superconductive properties where n = 2 and m = 2 are relatively few. The most interesting being those with thallium in their network. In the formula above the group ACuO.- represents the perowskite layers, m indicating the number of layers, while the group A'O denotes the sodium chloride layers, n by marking the number, A and A 'represent metals , for example A represents divaients such as alkaline earth and A 'trivalents such as bismuth. There is however no precise rule, each new perowskite being a particular case. Only the perowskite network and the sodium chloride networks are maintained.

The number of elements used in these structures and the variability of the proportions lead to an almost infinite number of combinations. Only a very small fraction of these compounds has interesting electrical properties.

More particularly, we sought to obtain perowskites whose point of zero resistance (TcR = 0) is greater than the temperature boiling liquid nitrogen. Considerable progress was made when mixed valence copper oxides containing thallium were

R = 0 discovered, their Te sometimes being greater than 100K. However, compounds comprising thallium have a major drawback, namely the toxicity and volatility of thallium.

Therefore, one of the aims of the present invention is to provide a new composition having properties similar to thallium compounds, such as TlBa_CaCu_0 ₇ , but which do not contain thallium.

This object and others which will appear subsequently are achieved by means of a mixed valence copper oxide derived from perowskite and of formula:

Pb 1, -x Alt- 2 + y + z Tr 1, -z Et 2- O, 7- £ _ with z greater than or equal to 0, less than or equal to 0.9, preferably greater than 0.1; x greater than or equal to 0 and less than 0.6, preferably greater than 0.1; y greater than or equal to 0 and less than or equal to 1, preferably greater than 0.1 and less than 0.9; with epsilon which can vary between -0.1 and 2/3, preferably 1/2;

Alt represents the alkaline earths with the exception of magnesium, Tr represents the rare earths, preferably yttrium, lanthanum and lanthanides; and Et represents the transition elements from d_- to d. ₀ , copper representing at least seven atoms out of eight of said transition elements; advantageously y is less than or equal to x.

Although Alt could in theory be barium, the presence of this element makes the synthesis of copper oxides according to the invention particularly delicate due to the formation of barium compound with other elements which are more stable than the composition according to the present invention. This is why it is preferable that there is very little barium, that is to say that Alt is a mixture of strontium and calcium with possibly the other alkaline earths, strontium representing at least the 2/3 of the Alt atoms. The preferred rare earths are yttrium and yttria.

Advantageously, the value of x is equal to the value of y plus or minus 0.2, preferably plus or minus 0.1. The value of z is advantageously between 0.3 and 0.6. A particularly interesting subfamily of the compounds according to the present invention consists of the neighboring compounds of the following average formula: ^Pb 0.5 ^Sr 2.5 ^Ca 0.5 ^Y 0.5 ^Cu 2 ° 7- £ -

The structure shown in Figure 1 shows the typical limit structure of copper oxides according to the invention, when the network is perfect.

To simplify the reading of the figure, the structure of the oxides according to the invention has been shown here for the above formula, where Tr is yttrium. This figure is only a paradigm of structure, that is to say an example of reference.

This structure comprises in succession:

. a plan (1) (Pb _w AIt ..) 0

. a plane (2) < ^AI V ^Pb v

. a plan (3) CuG ₂

. a plane (4) ^Y l- ₂ ^Alt z

. a plan (5) Cu0 ₂

. a plane (l ¹ ) (Pb _y Alt _u ) 0 with v + 2v '= 1 - x; u + 2u '= 2 + y with the above-mentioned bounds for x and y and v + u ≤- 1, v' + u '^ 1. Planes 1, 2 and 6 constitute an insulating sodium chloride bilayer. The planes 2, 3, 4, 5, 6 constitute a double layer of perowskite type of formula Pb, Alt, Y, Alt Cu-O _é _,. It should be noted that planes 2 and 6 participate in both the double layer of sodium chloride type and the double perowskite layer.

Referring to Figure 1, the oxygen atoms are symbolized by a white circle, while in planes 1, 2 and 6, lead and alkaline earths are symbolized by a black disc; in planes 3 and 5, copper is symbolized by a small black disc; in the plan

4 rare earth and alkaline earths are symbolized by a large white disc struck with a black star. It should also be noted that if there are metallic gaps in the network, these gaps essentially take the place of the metals represented in the networks of sodium chloride type. The synthesis of these compounds is similar to the compounds of the same family, but requires precautions during the heating step or the partial pressure of oxygen is particularly critical to avoid oxidizing the elements having two valence stages and the lowest valence of which is sought, in particular in the case of lead. This is why the process for synthesizing mixed valence copper oxides according to the invention has been developed, comprising the following steps: a) grinding and mixing of the oxides to obtain the desired proportion of different elements; b) compression of the mixtures obtained in step a) in the form of pellets; c) subjecting these pellets to heating between 800 and 1000 ° C, preferably between 850 and 900 ° C, under an atmosphere having a partial oxygen pressure of less than 10 Pa, preferably

3 less than 10 for a period varying from 1/10 to a day; d) rapid cooling to room temperature. The compounds thus formed may have little or no superconducting properties. In these cases, a simple storage under ordinary atmospheric conditions makes it possible to gradually give them a significantly improved superconductivity. However, if it is desired to accelerate this phenomenon, it is possible to carry out this activation by simple heating under ordinary atmospheric conditions or under an oxygen-enriched atmosphere, that is to say one whose partial pressure is greater than the atmospheric partial pressure, at temperatures substantially lower than the temperature in step c) above, that is to say at temperatures which can vary from ambient to 500 ° C. The duration of this activation heating is not very critical but as an indication, can vary from one hour to one day. The temperature rises for carrying out the heatings of step c) can be done quickly and are only limited by the questions of inertia and mechanical resistance of the oven and of the containers in which the heating is carried out.

The cooling is preferably carried out quickly, that is to say that the copper oxide pellets according to the invention are removed quickly and allowed to cool in their container at room temperature.

A good way to regulate the partial pressure of oxygen during step c) is to mix according to the following wording:

2AltEt0 ₂ + ySr0 ₂ + (1-z) Tr ₂ 0 ₃ + zAltO + (lx). (I / 2 PbO + l / 2Pb0 ₂ ) and place the mixture obtained after step b) in a closed enclosure of which the volume is between 1 to 100 cm ³ / g of mixture, preferably from 5 to 10 cm ³ / g, and to create a partial vacuum in said enclosure, so that the pressure in said enclosure is at most equal to 100 Pa, preferably at most equal to 10 Pa, the heating of said pellets being carried out through this enclosure.

In general, the vacuum is achieved by means of a vane pump. It is therefore a primary vacuum which corresponds to a value between 10 - ^" 2 and 10- ^" 3 mm of mercury, that is to say a residual pressure of the order of 0.1 to 1 Pa.

The grinding is carried out so as to obtain a usual particle size in the material such as a particle size having a dr, _Q less than 20 micrometers, preferably 10 micrometers, generally around 5 micrometers. It is possible to work with even weaker grinding meshes by carrying out wet grinding, which is relatively favorable to the final compound but which requires a drying operation before step b) or during step a). This is why dry grinding is preferred. The compression of step b) is carried out under usual pressure in the material, such as compressions greater than 0.5 tonnes / cm ² (50.10 Pa), in general between 0.1 and 1.10 Pa.

Thus the compounds according to the present invention are either superconductive ceramics or precursors of superconductive ceramics. These copper oxides can be used as constituents of electrical or electronic components.

The following nonlimiting example illustrates the new superconductors according to the present invention.

A mixture corresponding to the general formula was prepared from SrCuO-, Sr0 ₂ , Y ₂ ° 3 'CO, PbO and PbO-. After grinding to a particle size of less than about 10 μm, the mixture is put into the form of pellets by compression under a pressure of the order of 5 tonnes / cm ²

9 (0.5.10 Pa). The pellets thus obtained are brought to a temperature of between approximately 850 and 900 ° C. in quartz tube sealed under primary vacuum for approximately 1/4 of a day and then are cooled to room temperature by natural cooling of the tube exposed to the outside temperature.

The pellets are ground to a particle size between about 10 and about 20 micrometers. The samples used for the physical characteristics are prepared in the form of bars of 12 x 2 x 1 mm ³ , the powders with a particle size between 10 and 20 micrometers, are pressed under 4 g tonnes / cm ² (0.4.10 Pa) and sintered for 4 to 5 hours under the same atmosphere as that used for the synthesis and at the same temperature. Any activation takes place after the sintering step.

Resistive transition measurement

The method used is that of the four points: 1 contact at each end of the bar, 2 contacts on one of the faces. The contacts are made either by silver lacquer or by diffusion of indium in the frit by means of ultrasound.

A current generator (model 103A - Adret Electronique) sends a direct current (5.10 ~) mA ^ I ___ 109.99 m A through the external points. The voltage between the center points is measured using an eithley digital multimeter with a resolution of 1 μV.

The compound of formula Pb-,. Sr- _. Y _n , - Ca _n , - Cu-O-, gives the R / R curve, - _π „as a function of the temperature expressed in K and reported in Figure 2. We see that the resistive transition takes place between 50 and 100 K.

Claims

1. Mixed valence copper oxide derived from the structure of the perowskite and of formula: Pb 1, -x Alt- 2 + y + z Tr I. -z Et- 2 O- 7, - * c- with z greater than or equal to 0, less than or equal to 0.9, preferably greater than 0, 1; x greater than or equal to 0 and less than 0.6, preferably greater than 0.1; y greater than or equal to 0 and less than or equal to 1, preferably greater than 0.1 and less than 0.9; with epsilon which can vary between - 0.1 and + 2/3 preferably 1/2; Alt represents the alkaline earths and their mixtures with the exception of magnesium, Tr represents rare earths and their mixtures, preferably yttrium, lanthanum and lanthanides; and Et represents the transition elements from d- to d. _n and their mixtures, copper representing at least seven atoms out of eight of said transition elements. 2. Copper oxide according to claim 1, characterized in that Alt is strontium or a mixture of strontium with the other alkaline earths, the strontium representing at least 2/3 of the Alt atoms. 3.. Copper oxide according to either of Claims 1 and 2, characterized in that Alt is a strontium-calcium mixture. 4. Copper oxide according to one of claims I to 3, characterized in that x = y plus or minus 0.2, preferably x = y plus or minus 0.1. 5. Copper oxide according to one of claims 1 to 4, characterized in that z is between 0.3 and 0.6. 6. Compound according to one of claims l a. 5, characterized by the fact that x is equal to 0.5, y is equal to 0.5, z is equal to 0.5, that Et is copper, by the fact that Alt is strontium and calcium and that Tr is yttrium. 7. Process for the synthesis of copper oxides according to claims 1 to 6, characterized in that it comprises the following stages: a) grinding and mixing of the oxides to obtain the desired proportion of different elements; b) compression of the mixtures obtained in step a) in the form of lozenges ; c) subjecting these pellets to heating between 800 and 1000 ° C, preferably between 850 and 900 ° C, under an atmosphere having a partial oxygen pressure of less than 10 Pa, preferably 5 less than 10 Pa for a period varying from 1/10 to a day; d) rapid cooling to. ambient temperature. 8. Method according to claim 7, characterized in that it further comprises a step e) of activation of the copper oxide by heating in the presence of oxygen at a temperature varying from ambient to 10 500 ° C approximately. 9. Method according to one of claims 7 and 8, characterized in that Alt is at least partly of strontium and in that the mixture of step a) is carried out according to the following formulation: 2 AltEtO ₂ + ySr0 ₂ + (1-z) Tr ₂ 0 ₃ + AltO + (lx). (L / 2 PbO + l / 2PbO-). 15 10. Method according to claim 9, characterized in that the control of the partial pressure of oxygen according to step c) is carried out by the fact that the mixture obtained after step b) is placed in a sealed enclosure whose volume is between 1 to 100 cm ³ / g of mixture, preferably 5 to 10 cm ³ / g, and by the fact that a vacuum is created 20 partial in said enclosure, so that the pressure in said enclosure is at most equal to 100 Pa, preferably at most equal to 10 Pa, the heating of said pellets being carried out after closing through this enclosure. 1 1. Electric or electronic component characterized by the fact 25 that it consists at least in part of the oxide according to claims 1 to 6. *