GB2314076A

GB2314076A - Superconducting materials

Info

Publication number: GB2314076A
Application number: GB9711677A
Authority: GB
Inventors: Peter Schaetzle; Gernot Krabbes; Gudrun Stoever
Original assignee: Institut fuer Festkoerper und Werkstofforschung Dresden eV
Current assignee: Institut fuer Festkoerper und Werkstofforschung Dresden eV
Priority date: 1996-06-10
Filing date: 1997-06-05
Publication date: 1997-12-17
Anticipated expiration: 2017-06-05
Also published as: FR2749580B1; GB2314076B; DE19623050C2; FR2749580A1; DE19623050A1; GB9711677D0

Description

2314076 A PROCESS FOR PRODUCING A HIGH-TEMPERATURE SUPERCONDUCTING SOLID

MATERIAL The invention relates to a process for producing a high-temperature superconducting, melt-textured solid material which, in the form of blocks, moulded bodies, strips and the like, may be used, for example, in power, drive and transport engineering.

it is already known to produce solid high-temperature superconducting materials through peritectic crystallisation a-superconductor phase (solid material) - melt + a-solid phase, which takes place as an a-phase in air at 10200C for YBa2CU307-, (YBCO). Non-reacted g-solid phase Y,Ba,Cu,o, (211) remains as an inclusion in the structure in the case of YBCO. The crystallisation on which this process is based, together with a temperature gradient, is called melt texturing.

In this case it is common practice to firstly compress pulverulent starting material into moulded bodies. These are heated in a melting step to beyond the peritectic melting temperature. The melt is then homogenised by means of a holding step far above this temperature. The temperature is afterwards lowered rapidly to just above the peritectic melting temperature to cause crystal 1 lsat ion, which takes 2 place with low cooling rates in temperature ranges below that at which peritectic decomposition occurs and results in the textured structure.

If the starting powder only consists of high-temperature superconducting material, a high proportion of melt escapes from the moulded body with disadvantageous effects when the peritectic temperature is exceeded. This entails a deterioration in or even a loss of superconducting properties. Moreover, the melt which has escaped may react with the crucible material, resulting in a further deterioration in the superconductor properties.

It is already known to use starting powder containing additives to prevent melt from escaping in this way. For example, Y2Ba,Cu,O, andy.03 are added for YBCO to lower the reaction temperature and reduce the 211 inclusions to a certain degree (G. Krabbes et al Physica C 244 (1995) 145152). However the disadvantage in this case lies in the occurrence of an inhomogeneous grain size distribution of the inclusions and cracking.

A certain improvement is achieved by carrying out the homogenisation step in a platinum crucible and at a temperature of 12000C, afterwards quenching the melt to ambient temperature and then processing it into powder. The starting powder which is thus formed is extremely sensitive to air and moisture and, because of the additional expenditure which this method entails, its production requires high process costs. A further disadvantage lies in the fact that the 211 inclusions are still distributed inhomogeneously in the superconducting solid materials produced from this powder. These disadvantages cannot be eliminated even by adding Pt or PtO, and limiting the temperature to 11000C. Although other non-specific additives such as BaTio,, BaCeO, and BaSn03 (Ch.- J. Kim et al J. Mater. Sci. Lett. 11 (1992) 831; Ch-- J. K1m, P.J. McGinn, 3 Physica C 222 (1994) 177; W. Gawalek et al Cryogenics 33 (1993) 65) enable the process costs to be reduced, these cannot prevent the 211 inclusions from being inhomogeneously distributed or prevent cracking. One reason for this lies in the fact that these additives react with the peritectic melt and lead to the formation of secondary phases which disturb the equilibrium. This results in the melt texturing process being severely disturbed and becoming difficult to control and reproduce.

The object of the invention is to create a method for producing hightemperature superconducting, melt-textured solid materials such that it entails low process costs and Such that 211 precipitations which develop in the material are distributed as homogeneously as possible and have a small grain size, and that cracking is largely prevented.

The invention lies in the f= that a starting powder of the formula SEJYI,Cii,Oxy where SE = Y, Nd, Sm, La, Dy, Er or Gd or a mixture of two or more of the above elements M = Ba, Sr or Ca or a mixture of two or more of the above elements U = 0.8 to 2 4 v = 1 to 2 W = 3 to 4 and X = 1.5 U + v + (0.5... 1.5)W, is mixed with a spwffic platinum-containing compound of the formula SEkPtIM.Cu.0., where 3 k 0 to 2 1 1 to 2 m 2 to 4 n 1 to 2 and X k + m + (n + 1)z, where (z - 0.5 2), in a mole fraction of 0.2 to 7 mol %, and that a high-temperature superconducting material is produced from this mixture by comminution and compression, in addition to beat treatment, which, preferably, includes a melt-texturing process without a crucible.

According to a preferred embodiment of the invention, YBa2Cu3C.8 or Ndj.. Ba2_.Cu301. 6, where y - 0 to 0.06 and 6 - 0 to 1, may be used as the starting material and PtBaCu20,9 added as the specific platinumcontaining compound in a mole fraction of 2 mol The specific platinum-containing compound is preferably produced by means of a powder technology synthesis process.

The proposed method advantageously results in dimensionally stable hightemperature superconductor materials with high frozen-in residual fields and critical current densities, an required in 'power and transport engineering.

The addition according to the invention of a platinumcontaining compound to the starting powder before partial melting does not change the peritectic crystallisation on which the melt texturing process is based. The usual process temperatures and parameters, such as heating rates, holding times and cooling rates, can be maintained, although the method is advantageously less sensitive to fluctuations in the process parameters.

The specific additive according to the invention increases viscosity and lowers interfacial tension in a manner similar to that of a non-specific additive, although has the additional advantage of helping to establish and maintain the equilibrium of the melt. There are no reactions giving 6 rise to unwanted secondary phases which impair the properties. As a result, the melt texturing process and the development of the superconductor phase proceed in a more stable and reproducible manner. One advantage lies in the fact that the amount of melt which escapes and thus the possibility of a reaction occurring with the crucible material remain slight. This means that there is no appreciable contamination or degradation, which is linked with this, of the superconducting properties in the melttextured material.

The 211 precipitations which develop are characterised by a very small grain size and by a homogeneous distribution over the entire sample volume. The development of cracks is greatly reduced or prevented entirely by the nearequilibrium crystallisation.

The invention is illustrated in detail in the following on the basis of embodiments.

Example 1

2 mol k of a pulverulent specific compound of the composition PtBaCu.O. and 24 mol %. of pulverulent Y203 are mixed with a commercially available starting powder of the high-temperature superconducting compound YBa2CU301-6. The above-mentioned specific compound was produced beforehand from Y203. BaCO,, Cuo and Pt by means of an adapted powder technology synthesis process.

The powder mixture obtained is firstly comminuted and then compressed uniaxially to form cylindrical moulded bodies of a length of 15 mm and a diameter of 30 mm or cuboid moulded bodies of the dimensions 27 x 27 x 10 mml. The moulded bodies are then heated at 300 K/h to 11500C and maintained at this temperature for 20 minutes, after which the temperature is 7 lowered to 11200C. Finally, melt texturing is carried out by cooling at 6 K/h with a temperature gradient of 20 K/cm.

The structure of the high-temperature superconducting moulded bodies which are thus produced consists of crystals which are several cubic centimetres large and have different orientations. The 211 precipitations are distributed very finely and homogeneously. The melt loss is insignificant, in spite of the temperatures employed, which are distinctly higher than the temperature at which the peritectic melt forms (- 1020OC).

Following the final, necessary step of charging with oxygen at 3700C, the critical current density of the moulded bodies is greater than 104 A/cm' at 77 K and 0 T.

Example 2

Cylindrical moulded bodies of a length of 15 mm and a diameter of 30 mm are produced by compression from the powder mixture described in Example 1, consisting of starting powder, specific compound and Y.03. with a Sm123nucleus being added to each of the moulded bodies before compression. The moulded bodies, which are compressed as in Example 1, are heated at 300 K/h to 10500C and textured at a cooling rate of O.SOC/h. The melt loss is slight in this case as well, in spite of the low rates of cooling and the long process times which these entail.

The structure of the moulded bodies thus produced consists of one to two crystals which exhibit the preferred orientation of the Sm-123-nucleus.

Following a final step of charging with oxygen at 3700C over 72 h in a flowing oxygen atmosphere, the high-temperature superconducting moulded bodies thus produced have a frozenin residual field of 550 mT and a levitation force of 50 N

8 at 77 K. The critical current density of the moulded bodies is greater than 10' A/cm2 at 77 K and 0 T.

Example 3

2 mol k of a pulverulent specific compound of the composition PtBa,Cu.0. and 40 mol %. of pulverulent Nd2BaO, are mixed with a starting powder of the composition NdBa2CU.07-.. The above-mentioned specific compound was produced beforehand from Y203. BaC03. Cuo and Pt by means of an adapted powder technology synthesis process.

After comminution, the powder mixture is compressed uniaxially to form cylindrical moulded bodies of a length of 6 mm and a diameter of 14 mm. The moulded bodies are then melt-textured in a nitrogen/oxygen atmosphere with only 1 V by volume 02. For this purpose the moulded bodies are heated to 11000C and, starting from 10400C, cooled to ambient temperature by cooling at 1 K/h with a temperature gradient of 2 K/cm. No appreciable melt lone occurs during the melt texturing.

The high-temperature superconducting moulded bodies thus produced have a structure which is characterised by a plurality of crystals and very small, homogeneously distributed Nd-422 inclusions of a size of < 5 gm.

Following the final oxygen charging step, the critical current densities of the moulded bodies are > 104 A/cm2 at 77 K and 0 T.

-I-

Claims

1. A process for producing a high-temperature superconducting, melt- 3 textured solid material, using a superconducting starting material comprising an additive, characterised in that a starting material of the formula SEffl,Cu,.O., where SE Y, Nd, Sm, La, Dy, Er or Gd, or a mixture of two or more of these elements m Ba, Sr or Ca, or a mixture of two or more of these elements U 0.8 to 2 v 1 to 2 W 3 to 4 13 and X 1.5 u + v + (0.5 1.5)w, is mixed with a specific platinum-containing compound of the formula SEkPtIM.Cu.0., where k - 0 to 2 1 - 1 to 2 m - 2 to 4 n - 1 to 2 and X - 1.5 k + m + (n + 1)z, where (z - 0.5 2)3 in a mole fraction of 0.2 to 7 mol %, and a high-temperature superconducting solid material is produced from this mixture by comminution and compression, in addition to heat treatment.

2. A process as claimed in claim 1, wherein said heat treatment includes a melt-texturing step without a crucible.

3. A process as claimed in claims 1 or 2, wherein YBa2CU301-8 or Ndj+.Ba2. yCU307,:61where y - 0 to 0.06 and 6 - 0 to 1, is used as the starting material and PtBaCu,09 is added as the specific platinum-containing compound in a mole fraction of 2 mol

4. A process as claimed in claims 1, 2 or 3, wherein the specific platinumcontaining compound is produced by means of a powder technology synthesis process.

5. A process for producing a high-temperature superconducting, melttextured solid material substantially as hereinbefore described in the foregoing examples.

6. A high-temperature superconducting, melt-textured solid material 13 obtained or obtainable by a process as claimed in any one of the preceding claims.

7. A high-temperature superconducting melt-textured solid material substantially as hereinbefore described in the foregoing examples.

8. An article comprising a high-temperature superconducting, melttextured solid material as claimed in claims 6 or 7.

9. A specific platinum-containing compound of the formula SEkPtIM Cu,,0., where SE - Y, Nd, Sm, La, Dy, Er or Gd, or a mixture of two or more of these elements and m m Ba, Sr or Ca, or a mixture of two or more of these elements.

k 0 to 2 1 m 1 to 2 m - 2 to 4 n m 1 to 2 and X - 1.5 k + m + (n + 1)z, where (z - 0.5... 2),

10. Use of a specific platinum-containing compound as claimed in claim 9 for producing a high-temperature superconducting, melt-textured solid materiaL