DD281591A5 - METHOD FOR PRODUCING CERAMIC HIGH TEMPERATURE SUPERSHIPERS - Google Patents

Abstract

The invention relates to a method for the production of ceramic high-temperature superconductors. The aim of the invention is to reduce the time required for pre-firing and sintering of the materials used and thus contribute to a less energy-consuming production of high-temperature superconductors. This goal is achieved by applying a reducing atmosphere during pre-firing which accelerates the chemical reaction of the starting materials. High temperature superconductors; ceramics; preliminary firing}

Description

For this 4 pages drawings

Field of application of the invention

The invention relates to a process for the production of ceramic high-temperature superconductors based on Y 2 O ₃ , CuO, BaCO ₃ .

Characteristic of the known state of the art

Methods for the preparation of high temperature superconductors can be divided into:

- conventional ceramic method

Mixed precipitation method for producing the substance in stoichiometric composition for subsequent sintering,

Methods for producing thin layers (e.g., sputtering, evaporation, etc.).

In the conventional ceramic technique, the starting materials (Y 2 O ₃ , CuO, BaCO ₃ ) are mixed in the desired ratio and then subjected to pre-firing (temperature range: 850-1OCO ⁰ C, atmosphere: air or oxygen, time: 6 h and more). During the pre-firing, the starting materials used should react with one another and, for the most part, already form the desired chemical compound. Thereafter, the material is homogenized (ball mill), pressed, subjected to sintering (temperature: 900-1000 ° C, atmosphere: oxygen, time: 6h and more) and slowly cooled in oxygen. In these methods described in the literature (eg: C. Allgeier, JSSchilling, E.Amberger Phys.Rev.B35 116] 1987 S.8791; RJCava, B.Batlogg, RBvan Dover, DW Murphy, S.Sunshine, T. Siegrist, JPRemeika, EARietman, S. Zaruhak and GPEspinosa Phys.Rev.Lett.58 [16] 1987 p. 1676), a relatively large amount of time is required as a result of the multiple pre-firing in order to achieve as complete as possible a conversion of the starting materials used.

Object of the invention

The aim of the invention is to reduce the time duration for the complete implementation of the starting materials used during pre-firing and sintering and thus to produce Y-Ba-Cu-O-high temperature superconductor less energy consuming.

Explanation of the essence of the invention

The object of the invention is to accelerate the reaction of the starting materials used in the conventional <eramischen manufacturing technology (for example, Y2O3, CuO, BaCO _3), particularly the reaction of the carbonate input to achieve a single phase end product with the highest possible T _c.

According to the invention, this object is achieved in that the pre-firing is carried out under reducing conditions. In this case, all inert gases are in agreement, preferably N ₂ and Ar. In contrast to pre-fires in an oxidizing atmosphere (eg air, O ₂ ), the powders assume the desired black coloration already after pre-firing, which is typical for high-temperature superconductor samples based on Y-Ba-Cu-O. By X-ray diffraction, it was demonstrated that the conversion of the starting materials used during the pre-firing in a reducing atmosphere is more advanced than in pre-firing in the usual atmosphere of air or O ₂ . An advanced variant of the erfindungsgmäßen method for accelerated implementation of the starting materials used is to carry out the pre-firing in an atmosphere in which several times reducing and oxidizing conditions change. The phase analysis by means of X-ray diffraction revealed that a phase mixture formed during the three-hour pre-firing in a reducing atmosphere in which in addition to the unreacted starting materials superconducting and non-superconducting phases are contained in approximately equal parts (Figure 1, curve a). The extended variant of the pre-firing, in which reducing and oxidizing conditions change over time several times, yields a phase mixture which consists predominantly of the superconducting phase (FIG. 1, curve b). The treatment in a reducing atmosphere obviously accelerates the conversion of the starting materials and the intermediate treatment in a pure oxygen atmosphere promotes the further conversion of the resulting intermediates to the superconducting Y-Ba-CU-O phase. After Vorbrand the material is homogenized in a conventional manner, pressed and subjected to sintering. The times of sintering of the powders prefired in a reducing atmosphere can be shortened compared to those prefired in an oxidizing atmosphere under otherwise identical conditions.

Export Example

In the exemplary embodiment, a comparison is made between 2 samples, one of which was prefired under otherwise identical conditions in reducing and dioxygenating atmospheres.

The starting materials Y! O ₃ , CuO ₁ BaCO ₁ were weighed into the ratio corresponding to the formation of the substance Y ₁ Ba ₂ Cu ₃ O _x and mixed. A part (hereinafter referred to as Sample I) of the powder was subjected to a preliminary firing under conditions of: temperature: 875 ° C, atmosphere: N ₂ , atmospheric pressure, time: 3 hours, and another part (designated Sample II) with a precombustion the conditions: temperature: 875 ⁰ C, atmosphere: O ₂ , atmospheric pressure, time duration: 3d. After preburn, sample I has a black color and sample Il a gray color. Both powders were homogenized in a ball mill for 1 h and then pressed into tablets at a pressure of 1000 kg / cm ² . Subsequently, both samples were simultaneously subjected to a sintering treatment under the following conditions: temperature: 925 ^° C., atmosphere: O ₂ , atmospheric pressure, time: 3 h.

The cooling of the samples was carried out under flowing oxygen atmosphere with a cooling rate <5 K / min. The tablet of Pre-fired Pulver I gave a solid black body. The tablet of material II pre-fired under oxidizing conditions shows a black-green color and is less well sintered together.

To characterize the resulting ceramic with respect to their superconducting properties, electrical resistance and magnetic susceptibility as a function of temperature were measured. The temperature dependence of the electrical resistance of samples I and II is shown in Fig.2. Sample I (pre-firing under reducing conditions) has a phase transition to the superconducting state at 84K, and for sample II (pre-firing under oxidizing conditions) the electrical resistance remains finite in the investigated temperature range. Figures 3 and 4 show the temperature dependence of the magnetic susceptibility for both samples. Only with sample 1 one can speak of a superconducting transition (T _c = 83K, ΔΤ - 14 K). In sample II, the diamagnetism typical for the superconducting state changes almost linearly between 15K and 92 K, which is based on a strong multiphase nature of the sample. The advantages of the method according to the invention are that, with otherwise identical production conditions, the use of a reducing atmosphere in the manner described permits the production of samples with better superconducting properties.

Claims (1)

Process for the production of ceramic high-temperature superconductors based on conventional ceramic sintering technology, characterized in that - That the necessary precombustion of the mixed starting materials (Y2O3, CuO, BaCOa) takes place in a reducing atmosphere; or - That the necessary precombustion of the mixed starting materials (Y2O3, CuO, BaCOa) is carried out so that alternately pure oxygen and reducing atmosphere is used.