RU2234167C1 - Method for manufacturing substrate for mgo-ceramic and silver base high-temperature superconductor coatings - Google Patents

Abstract

FIELD: cryogenic electronics.

SUBSTANCE: proposed method designed for use in manufacturing film components possessing high-temperature superconductivity includes following procedures: 30 - 70 mass percent of silver is introduced in source MgO powder and then this powder is compressed. Newly introduced procedures are pre-baking at about 1000°C, surface compacting with aid of high-melting material, and post-baking grinding of working surface to obtain desired surface properties. These procedures enlarge surface concentration of silver due to increasing source concentration within 30 - 70 mass percent and holding it inside substrate.

EFFECT: enhanced quality of high-temperature superconductor coatings.

1 cl, 4 dwg

Description

The invention relates to cryoelectronics and can be used in the manufacture of film elements having high-temperature superconductivity (HTSC).

Known methods for the manufacture of substrates for HTSC coatings, in which a layer of silver is applied to MgO ceramic [1]. HTSC coatings on such substrates have higher characteristics, however, at a high temperature of the HTSC coating, the silver coatings melt and coalesce, exposing ceramics to contact with the HTSC coating.

A known ceramic method of manufacturing substrates for HTSC coatings, in which a mixture of MgO and silver powders is prepared, then this mixture is pressed, after which the pressing is calcined [2].

In this case, silver is well connected with ceramics, however, the substrate, due to the high firing temperature (more than 1600 ° C), is depleted in silver, and an increase in the initial silver concentration of more than 20% leads to the formation of connecting pores and the leakage of silver from them when the substrate is fired [3] .

The technical result of the invention is to improve the quality of HTSC coatings due to an increase in the surface concentration of silver in the ceramic substrate by increasing its initial volume concentration from 30 to 70 wt.% And retaining it in the substrate.

The technical result is achieved by the fact that additional operations are introduced into the manufacturing process of the substrate: intermediate firing; compaction of the surfaces of the substrate and grinding of the working surface. The required amount of silver is added to the initial charge, then it is pressed into the desired shape and fired. The firing temperature of MgO ceramics reaches 1700 ° C, the melting point of silver is 961 ° C. Depending on the concentration of the starting silver, two options are possible for sintering the substrate during firing. If the silver content is less than 20 wt.%, It forms isolated inclusions from each other. If the silver concentration is more than 30 wt.%, These inclusions combine and form a branched, spatial network. At a temperature higher than the melting point, silver goes into a liquid state and is retained inside the ceramic by capillary forces. Since the surface tension of most metals is greater than the surface tension of oxides, wetting in the system is incomplete. With increasing temperature, the surface tension and viscosity of silver decrease. During sintering, the liquid phase can exit [3]. If the silver content is more than 70 wt.%, Then the formation of a solid matrix of silver with inclusions of ceramics is possible. Such a system does not belong to cermets and has a low melting point close to the melting point of silver [3]. For the MgO-ceramic-silver system, silver is released at a temperature of 1100 ° C. After sintering (T = 1700 ° C), no silver remains in the ceramic. The capillaries are empty. To keep silver inside the ceramics, it is proposed to stop the rise in the firing temperature at 1000 ° C, when the ceramic strength increases (preliminary firing), to seal the substrate surfaces by introducing MgO or other refractory material into the surface of the powder. After sealing the surfaces, the substrate is usually fired, and the densified surfaces retain silver inside the capillaries. After cooling, grinding of the working surface of the substrate is carried out, removing the surface layer to a silver-containing region. A comparative analysis of the claimed technical solution with the prototype shows that the claimed method is characterized in that the initial silver concentration is 30-70 wt.%, After pressing, preliminary firing is carried out at a temperature of about 1000 ° C, then the surfaces are densified by the introduction of refractory material and grinding is carried out after firing work surface, which in this set of features in the known technical solution is absent, thereby causing the novelty of the technical solution. The technical solutions, which carry out preliminary firing, compact the surface of the substrate and grind the surface to the required depth after firing, are unknown to the authors, in addition, the set of features consisting of preliminary firing of the substrate, subsequent compaction of the surfaces and grinding of the working surface after firing will allow us to detect the proposed method the manufacture of substrates for HTSC coatings are other, in contrast to the known, properties, which include:

- the ability to change the surface concentration of silver by adjusting the depth of removal of the surface layer, which allows you to influence the characteristics of HTSC coatings;

- the ability to avoid losses of silver in the furnace when it is released during firing;

- the possibility of forming curved (for example, cylindrical) substrates with a uniform surface concentration of silver;

- the ability to adjust the surface resistance of the substrate, changing the initial concentration of silver, firing time and the depth of removal of the surface layer. Thus, other, in contrast to the known technical solutions, properties inherent in the proposed method, prove the presence of significant differences aimed at achieving a technical result.

Figure 1 shows a section of a section of a substrate with isolated inclusions of silver 1. Figure 2 shows a section of a section of a substrate with through pores 2 after removing silver during firing. Figure 3 presents a section of a portion of the substrate after compaction of the surface layer 3 with silver-filled pores 4. Figure 4 shows a section of a portion of the substrate after removal of the surface layer.

The proposed method of manufacturing a substrate for HTSC coatings was implemented as follows. A mixture was prepared containing MgO powders, silver (≈30 wt.%) And an organic binder (3-5%). The mixture was homogenized, dried and pressed under a pressure of 4-6 GPa. The compacts were in the form of tablets (D = 40 mm, h = 4 mm). Pressings were divided into two parties. The samples of the first batch were fired according to the standard scheme: heating at a rate of 2 ° C / min to 450 ° C; excerpt - 2 hours; heating at a rate of 4 ° C / min to 1700 ° C; excerpt - 10 hours; cooling with the oven turned off. Samples of the second batch were fired taking into account this technical solution: after pressing, intermediate firing was performed: heating at a speed of 2 ° C / min to 450 ° C; excerpt - 2 hours; heating at a rate of 4 ° C / min to 1000 ° C; excerpt - 10 hours; cooling with the oven turned off. Then, mechanical incorporation of MgO powder into the surface of the samples was carried out. After firing, the work surface was ground until silver appeared. A paste consisting of HTSC Bi powder (2212) and an organic binder was applied to all substrates by thick-film technology. Samples were annealed according to the standard scheme (maximum temperature - 1000 ° С, exposure time - 24 hours at 870 ° С). After cooling, the HTSC coatings were examined for the critical current density. The research results showed that this value is 50-70% higher for samples from the second batch. Visual studies of the sections of the substrates indicate the absence of silver in the substrates from the first batch.

Using the proposed method of manufacturing a substrate for HTSC coatings provides the following advantages:

- the ability to produce HTSC coatings with higher characteristics;

- the ability to change the surface conductivity of the substrates.

Sources of information

1. Technology of thick and thin films / Ed. A. Reisman, C. Rose. - M .: Mir, 1972, - p. 9-13; 83-87.

2. Vyaznikov N.F., Yermanov S.S. Ceramic-metal materials and products. - L .: Matgiz, 1967, - 224 p.

3. Cermets / Ed. Tinclio J.R., Crandall W.B. - M .: IIL, 1962, p. 19-27.

Claims (1)

A method of manufacturing a substrate for HTSC coatings based on MgO-ceramic and silver, in which a mixture of MgO and silver powders is prepared, then it is pressed and fired, characterized in that the mixture contains from 30 to 70 wt.% Silver, preliminary firing is carried out before firing at at a temperature of about 1000 ° C, then the surfaces are compacted by the introduction of a refractory material, and after firing, the surface is grinded to obtain the desired surface properties.

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