CA2015168A1

CA2015168A1 - Process for producing a high-temperature superconductor

Info

Publication number: CA2015168A1
Application number: CA002015168A
Authority: CA
Inventors: Thomas Baumann; Peter Unternahrer
Original assignee: Asea Brown Boveri AG Switzerland
Current assignee: ABB Schweiz Holding AG
Priority date: 1989-04-24
Filing date: 1990-04-23
Publication date: 1990-10-24
Also published as: EP0394799A1; DE3913397A1; JPH02296767A

Abstract

ABSTRACT OF THE DISCLOSURE
Conventionally produced sintered ceramics of high-temperature superconductors having critical tempera-tures up to 125 K based on An1Bn2-.epsilon.1Can3+.epsilon.1Cun4On5+.epsilon.2, where A - Tl or Bi or Pb or a mixture of these three metals, B = Be or Mg or Sr or Ba or Ra or a mixture of these elements, n1 = 1 or 2, n2 = 1 or 2 or 3, n3 = 2, n4 = 2 or 3 or 4, n5 - 8 or 9 or 10 or 11, ¦.epsilon.1¦ ? 1, ¦.epsilon.2¦ < 1 have a critical current density of up to approximately 250 A/cm2 which is too low for applications of superconductors, for example, for high-power magnetic coils. In order to achieve a critical current density of > 1750 A/cm2 in such superconductors, the superconductor synthesis is carried out in a main process at a temperature in the region of 650 °C - 900 °C for a time in the region of 1 h - 100 h.
Prior to this, a presynthesis heat treatment can be carried out at a temperature in the region of 650 °C -900 °C for a time in the region of 0.5 h - 10 h for the purpose of degassing and a grain melting can be carried out at >900 °C for 1 min - 1 h for the purpose of material compaction. To establish the desired oxygen stoichiometry, a posttreatment at 300 °C - 750 °C for 0.5 h - 5 h in an oxygen atmosphere of ? 0.1 bar partial pressure can be provided after the main process.

Description

201~168 , 21.4.89 Rz/sm 89/058 TITLE OF THE INVENTION

Process for producing a high-temperature superconductor BACKGROUND OF THE INVENTION

Field of the Invention The invention is based on a process for producing a high-temperature superconductor as defined by the pre-amble of patent claims 1 and 3.

Discussion of Backaround The preambles of patent claims 1 and 3 relate the invention to a prior art such as is known from Physica C 156 (1988~, pages 822-826, North Holland, Amsterdam.
The latter describes the production of superconductors of the composition Bi2Sr2Ca3Cu4Os and Bi2Sr2Ca4Cu5Os with a critical temperature of 110 K. For this purpose a powder mixture composed of Bi2O3, SrCO3, CaCO3 and CuO was cal-cined for 12 h at 800 C. Dense, comminuted mineral chips of material pretreated in this way were melted in a platinum crucible and quenched with a copper surface heated to 200 C. Very dense glass plates approximately 0.5 mm thicX were formed therefrom and annealed for 7 days at 850 DC and 870 C in air.
The production of a superconductor having a critical temperature of 125 R and a critical current density of 250 A/cm2 composed of Tl2Ca2Ba2Cu3O~ is specified in the journal entitled Appl. Phys. Lett. 53(5), 1988, pages 414-416. In view of the high ~olatility of Tl2O3, CaBaCu2O~ is first synthesized by reacting CaCO3, BaCO3 and CuO in a ratio of 1 : 1: 2 at 1195 K for a period of about 72 h. Tl2O3 and CaBaCu2O~ are then mixed in a stoichio-metric ratio, ground and pressed into tablets of 12 mm diameter and 1 mm - 2 mm height. A tablet in an aluminum oxide boat was rapidly placed in a furnace heated to ~ . ~ . . . ~ ..................................... . .: .

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1200 K and kept there in a stream of oxygen for 5 min -8 min. It was then quenched to room temperature and after S min was again placed in flowing oxygen in the furnace whose heating had been switched off so that the tablet slowly cooled down to room temperature. A higher furnace temperature of 1210 X and a longer tablet dwell time of 10 min - 15 min at 1200 R resulted in the critical temperature being reduced to 114 R or <105 R. When the tablet was wrapped in a palladium-silver foil before the heat treatment, the optimum heating time was 15 min.
The disadvantage of this process is that the critical current density of the superconductor thereby achievable is relatively low, with the result that it can virtually not be used in practice.
For the relevant prior art, reference is made to the ~ournal entitled Science 240 (1988), pages 631-633.
The production of a single-phase superconductor having a critical temperature of 125 K and composed of Tl2Ba2Ca2Cu3O1~
is described therein, a stoichiometric powder mixture composed of Tl2O3, CaO2, BaO2 and CuO being pressed to form ~ -tablets having a diameter of 1.3 cm and a height of 10 cm and being kept at a temperature in the region of 850 C -925 C for varying lengths of time. If the mixture was heated for > 2 h at 890 C or for a shorter time at > 900 ~C, the main phase produced was Tl2Ba2Ca2Cu3O~ and to a lesser extent Tl2Ba2Ca2Cu3O10. The heating was carried out in sealed gold tubes. ~ -Compositions of high-temperature superconductors of the type (AO)mM2CanlCunO2n+2 are to be found in the MRS
Bulletin, vol. XIV, No. 1 (1989), pages 45 - 48. The A
cation may be one of the elements Tl, Pb, Bi or a mixture of said elements. m may be 1 or 2, but only 2 for Bi. The M cation may be Ba or Sr. m denotes the number of CuO2 layers lying on top of each other. Ca can be replaced by Sr.

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SUMM~RY OF THE INVENTION

- Accordingly, one object of this invention, as defined in patent claims 1 and 3, is to provide a novel process for producing a high-temperature superconductor of the type specified in the preamble which has a com-paratively high critical current density, in particular higher than 300 A/cm2.
An advantage of the invention is that said super-conductors reach a critical current density which is higher by at least one power of ten than in known super-conductors of the type mentioned in the preamble. As a consequence of the relatively low processing temperature or processing time, any reaction between the substance mixture of the superconductor and the material of the melting crucible is avoided. -~
According to an advantageous development of the invention, a marked increase in the critical current density can be achieved by briefly melting the grain boundaries of the substance mixture before it is syn-thesized to form a superconductor.
A presynthesis heat treatment has the advantagethat the oxides are mixed in the atomic range and undesir-able gases ( CO2 ~ 2 ) escape.
A simple posttreatment of the superconductor in an oxygen atmosphere results in more oxygen in the super-conductor and consequently in a further improvement of -its conductivity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To produce, for example, a superconductor having the composition Tl2Ba2Ca2Cu3Ol0 with a critical temperature of 124 K, a powder mixture of Tl2O3, BaO2, CaO and CuO, corresponding to a stoichiometric ratio of Tl : Ba : Ca:
Cu of 2 t 2 : 2 : 3. The tablets are surrounded with a vapor barrier, that is to say, they are wrapped in platinum foil which permits gas exchange with the sur-roundings but, at high temperatures, insures a reduced ^. ~ ~ - : : ~ :

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201~168 - thallium loss in the tablet. Said tablet~ are sub~ected sequentially to the following 4 temperature treatments:
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1. Presynthesis heat treatment at a temperature in the region of 660 C - 900 C, preferably in the region of 820 C - 880 C, for a time in the region of 0.5 h - 10 h, preferably in the region of 0.5 h -2 h;

2. Melting the grain boundaries of the mixture at a temperature of > 900 C, in particular in the region of 980 C - 1350 C, preferably in the region of 1000 C - 1200 C, for a time in the region of 1 min - 1 h, preferably in the region of 3 min - 20 min;
' 3. Main process for superconductor synthesis at a temperature in the region of 660 C - 900 C, preferably in the region of 680 C - 860 C, for a time of > 2 h, in particular in the region of 2 h -100 h, preferably in the region of 12 h - 48 h in an oxygen atmosphere of s 1 bar;

4. Posttreatment for the stoichiometric adjustment of the oxygen in the superconductor at a temperature in the region of 300 C - 750 C, preferably in the region of 450 C - 600 C, for a time in the region of 0.5 h - 5 h, preferably in the region of 1 h - 2 h, in a pure oxygen atmosphere or at an oxygen partial pressure of > 0.1 bar.

The main process with a relatively low synthesis temperature, i.e. without the abovementioned temperature treatments 1, 2 and 4 achieves the formation of a super-conductor. During the superconductor synthesis, fairly large platelets or crystallites grow which absorb smaller ones. With regard to their crystal structure, such superconductors are up to about 50 % vacant, i.e. are provided with large interstices between the crystalli~es.
~he current density inside the superconducting crystal-... .: : ~ . -.; :. - . . , :

201~168 lites is in the order of magnitude of Io6 A/cm2. Grain boundaries, i.e. links between the crystallites, are the reason for the total current density of the ceramic being reduced by orders of magnitude (<1000 A/cm2 without magnetic field and about 10 A/cm2 with a field strength of lOZT).
The current-carrying capacity of the supercon-ductor can be improved if the current-carrying junctions between the crystallites are improved, i.e. two-dimen-sional junctions are provided instead of punc~iform ones.
This can be achieved: a) by hot pressing, b) extrusion, c) by high-isostatic pressure, d) crystallizat~on pro-moters, e) by melting the grains of the substance mix-ture, in which process superconductor material densities of 90 %, corresponding to 10 % vacant space between the crystallites can be achieved. Particularly advantageous, since it is relatively easy to carry out, is the melting according to point 2 of the abovementioned temperature treatments. This enables critical superconductor current densities of 2 kA/cm2 to be achieved.
To produce such a superconductor, tablets wrapped in platinum foil on a carrier or boat or crucible made of aluminum oxide ceramic or platinum foil or another chemically resistant material having a high melting point are rapidly placed in a preheated furnace. The tablet temperature is monitored with 2 thermocouples which are disposed at opposite ends of the tablets. In an exemplary embodiment, temperature equilibrium prevailed about 7 min after introducing the tablets into the furnace, the temperature reading of the 2 thermocouples differing by less than 2 K from the temperature of a temperature-control element.
In producing the superconductor without a preced-ing melting process, the temperature was kept for 2 h between 780 C and 850 C or for 16 h at 700 C in the main process. The superconductors thereby produced have a high content of superconducting material and floated above a magnet at a temperature of 77 K.
With a temperature of 830 C and reduced oxygen ~,~, ': ' :' 201~8 . .
partial pressure during the main process, a superconduc-tor was predominantly produced which had the stoichio-metric ratio of Tl : Ba : Ca : Cu of 2 : 2 : 2 : 3, hereinafter abbreviated to the 2223 phase. No chemical reaction ever occurred between the powder mixture and the aluminum carrier or the platinum foil respectively.
Four exemplary embodiments are specified below in which only the main process, without the abovementioned temperature treatments 1, 2 and 4, were used.

Example 1:
Tablets made of a mixture containing th~e above-mentioned oxides are exposed for 14 h to a temperature of 710 C in a gas atmosphere containing 7 % oxygen. The reaction product chiefly contains the 2212 phase with impurities consisting of Ca2CuO3 and unknown phases. The critical temperature is 100 K. The crystal sizes are in the region of 0.5 ~m - 3 ~m. At 77 X, the magnetic hysteresis curve looks quite different to that of YBC0 or thallium superconductors which were synthesized at about 900 C. The hysteresis curve corresponds more to a classical superconductor than to a granular one, which indicates the presence of strong bonds between the crystallites in addition to weak bonds in the super-conductor.
The grain boundaries present a higher resistance to the penetration of the magnetic field than is other-wise usual, with the result that the critical current density decreases to a lesser extent in this region.
~aterial which predominantly contains the 2212 phase but which was synthesized at 880 C does not exhibit this behavior. A similar result is obtained with a synthesis temperature of 690 C in a 100 ~ oxygen atmosphere, whereas, with an oxygen partial pressure of 2 %, another, unknown phase was produced which exhibited no super-conduction above 77 R.

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::; . ' ' Example 2:
Tablets containing mixed oxides were kept for 2 h at 780 C in atmospheres containing 2 % or 7 % or 20 %
or 100 % oxygen, respectively. In all cases superconduct-ing material containing 2212 and 2223 phase in the ratio 2 : 1 and having a critical temperature of 100 K was produced. From magnetic measurements it may be concluded that less strong bonding exists than in the case of a synthesis temperature of 700 C.

Example 3:
Tablets composed of a mixture of the individual oxides were kept for 2 h at 830 C in atmospheres con-taining 2 % or 20 % oxygen, respectively. With 2 ~
oxygen, the 2223 phase is dominant in the superconductor, while with 20 % oxygen, the 2212 phase is in a ratio to the 2223 phase of 2 : 1. The critical temperature is s 100 R.

Example 4:
Tablets containing mixed oxides were kept for 2 h at 850 C in an atmosphere containing 7 % oxygen, in which process 2223 material was predominantly produced. With a purely nitro~en atmosphere no superconducting material was produced. The mean size of the cry~tallites is S ~m -10 ~m. The magnetic hysteresis is similar to that of good YBaCu material with weak bonding between the crystallites in thallium-containing superconductors.
Two exemplary embodiments with all four tempera-ture treatments are specified below.

Example 5:
Tl2O3, BaO2, CaO and CuO were mixed in a stoichio-metric ratio of Tl : Ba : Ca : Cu of 2 : 2 : 2 : 3 and heated in air for 2 h at 874 C tpresynthesis). The resulting material was ground and pressed into tablets of 13 mm diameter. Said tablets were inserted in a platinum tube having an inside diameter of 13 mm and a length of 100 mm and sealed by pressing. The platinum tube was now placed ~, ~. .. :
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for 5 min in a furnace preheated to 960 C (melting) and then quenched to room temperature. The ingot so produced was a thick black body which could easily be removed from the platinum tube. It contained various crystal phases S with a very weak superconductor component.
Tablets separated off by cutting and having a t.hickness or height of 1 mm were wrapped in platinum foil and kept for 60 h at 870 C (main process) in order to obtain the superconductor. This wa~ followed by a post treatment of unpacked material for lh at 480 C with oxygen flowing in order to adjust the oxygen content of the superconductor to stoichiometry. The superconductor thus produced had a density of 90 ~ and floated above a magnet at the temperature of liquid nitrogen.
Althoughthestoichiometryofthestartingmaterial corresponded to the compound Tl2Ba2Ca2 Cu3Ol0, the compound Tl2Ba2CaCu20~, which is evidsntly found more frequently in fused bodies, was predominantly found in the superconduc-tor. The critical temperature was 110 K and the critical current density was 1.6 kA/cm2, i.e. about 1 power of ten higher than in standard thallium-containing superconduc-tors. Alternating current magnetization even yields a critical current density of approximately 7 kA/cm2, the hysteresis loop corresponding to that of a superconductor with strong bonding between the crystallites.

Example 6:
The oxide powders mentioned in connection with Example 5 were mixed in accordance with the stoichiometry of Tl2Ba2CaCu2O8. An aluminum oxide tube was used instead of the abovementioned platinum tube for melting. The melting temperature was 1300 C. In other respects, the procedure was as in Example 5.
A specimen having a cross section of 1.5 mm2 con-ducted a current of 26 A without loss and then bscame norm-ally conducting as a consequence of a considerable develop-ment of heat at the contacts. The critical current density was not reached in this experiment and was > 1.8 kA/cm2.
The family of thallium-containing superconductor~

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was in this case chosen as a working example since it yields superconductors with a critical temperature well above 77 K. Depending on heat treatment they exhibit good or poor grain boundary properties; i.e. good grain boun-daries (and consequently high critical current densities)are not the result of selecting a particular chemical substance group but a result of correct process engineer-ing. In general, the process according to the invention is suitable for producing superconductors composed of 10A~lB~2-e1C~3+elCU~4~s+e2~ where A = Tl or Bi or Pb or a mixture of these three metals, B = Be or Mg or Sr or Ba or Ra or a mixture of these elements, nl = 1 or,2, n2 =
1 or 2 or 3, n3 = 2, n4 = 2 or 3 or 4, n5 = 8 or 9 or 10 or 11, ¦el¦ s 1, ¦e2¦ < 1.
15Obviously, numerous modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

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Claims

1. A process for producing a high-temperature super-conductor a) composed of a substance mixture which contains oxygen compounds of the alkaline earth elements calcium and barium and also of copper and at least one further metallic component, it being possible for barium to be replaced entirely or partially by strontium, b) the substance mixture being exposed in a main process to a first processing temperature in the region of 660 °C - 790 °C, which comprises c) exposing the substance mixture, before the main process, to a melting process for a second process-ing time in the region of 1 min - 1 h, and d) to a second processing temperature above 900 °C.

2. The process as claimed in claim 1, wherein the second processing time is in the region of 3 min - 20 min.

3. A process for producing a high-temperature super-conductor a) from a substance mixture which contains oxygen com-pounds of the elements calcium and copper and also at least a third metallic component, b) the substance mixture being exposed to a melting process at a second processing temperature above 900 °C and then, c) in a main process for superconductor synthesis, to a first processing temperature above 660 °C, wherein d) the first processing temperature is in the region of 600 °C - 900 °C, and e) wherein the second processing temperature is in the region of 980 °C - 1350 °C, and f) is applied for a second processing time in the region of 3 min - 20 min.

4. The process as claimed in claim 2 or 3, wherein the second processing temperature is in the region of 1000 °C - 1200 °C.

5. The process as claimed in one of claims 1 to 4, wherein a) the substance mixture is exposed, before the main process, to a presynthesis heat treatment at a third processing temperature in the region of 820 °C -880 °C
b) for a third processing time in the region of 0.5 h -10 h.

6. The process as claimed in claim 5, wherein the third processing time is in the region of 0.5 h - 2 h.

7. The process as claimed in one of claims 3 to 6, wherein a) the first processing time is in the region of 12 h -100 h, b) in particular in the region of 12 h - 48 h, and c) wherein the first processing temperature is in the region of 680 °C - 860 °C.

8. The process as claimed in one of claims 1 to 7, wherein the substance mixture contains as further metal-lic component bismuth and/or thallium, it being possible for bismuth and thallium to be partially replaced by lead.