FR2742001A1 - SUBSTRATE FOR A HIGH TEMPERATURE REMOVER CONDUCTIVE AND METHOD OF MANUFACTURE - Google Patents

Abstract

As a substrate for the application of thin films and thick films of high temperature superconductors are suitable single crystals of Mg0, which however are expensive to manufacture for large areas and require a sintering temperature> = 1650 deg. C. If Cu0 is added to the starting fine powder of Mg0 as a sintering aid, so that a substrate of composition (Mg1-x Cux) 0 results, with 0.06 = <x = <0.08, a starting powder is obtained, which can be sintered at a sintering temperature of between 1050 deg.C to <1350 deg.C and which does not worsen the properties of superconductors of the type Bia Srb Cac Cud 0, which are manufactured in as thin layers or thick layers depending on the fusion process. The average particle size of the crystallites of the substrate thus manufactured is between 1 μm and 3 μm. As starting materials an aqueous solution of Cu (NO 3) 2 and a suspension of fine powder Mg (OH) 2 in water are used. The desired shape of the substrate can be achieved by strip casting or slip casting or injection casting or by uniaxial and isostatic pressing.

Description

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  Substrate for a high S superconductor .. u .................

  .................................................. .. h .... t..e e.mPé. The invention relates to a substrate for a high temperature superconductor, which contains magnesium oxide. The invention also relates to a method of manufacturing such a substrate with a DTD: first powder containing magnesium and oxygen.

  With the substrate and the method of the aforementioned type the invention refers to a state of the art, as known from J. R. Spann et al., Oriented BSCCO

  thick film coatings on polycrystalline MgO, J. Mater.

  Res., Vol. 5, No. 6, June 1990, pages 1163-1168. It is indicated as substrates for the production of thin films and strongly oriented thick films of BSCCO (Bi.SrbCa.CudOx) polycrystalline MgO, MgO single crystals, Ag and Ni foils, a copper plate, SrTiO3 and stabilized zirconium dioxide, the monocrystalline MgO substrate providing the best BSCCO films. The following manufacturing processes are indicated: crystallisation from melting, spraying, in particular plasma spraying, vapor separation, epitaxy from a liquid phase and thermal decomposition of complex salts . The layers of Bi (2212) of fusion have on these substrates, for 64 K, a critical current density of = 2 kA / cm 2. Substrates in A1203

wet poorly and are irregular.

  It is disadvantageous in this case that large surface MgO single crystals are very expensive to manufacture. In order to produce pure MgO with a desired relative density of 98%, a sintering temperature of 650.degree. C. is required. This requires costly furnaces. Hot pressing at = 1200 C is also expensive. ZrOz and A12z03 partially destroy

  the superconductor at high temperature to apply.

  About the relevant state of the art it

  is returned in addition to a publication of X.L.

  Wang et al., The preparation and electrical properties of Bi2SrzCaCu20O thick films with high Th on (100) MgO substrate, Supercond. Sci. Technol. 8 (1995), pages 229 to 233. With the process which is described a critical temperature of 92 K. has been reached. At a desired service temperature of 77 K (liquid nitrogen), one has with layers of Bi (2212) of fusion the critical current density which is of the order of i kA / cm2 to

2 kA / cm2.

  By a publication by Theodore P. Hyatt, Electronics: Tape Casting, Roll Compaction, in: The American Ceramic Society Bulletin, Volume 74, No. 10, October 1995, pp. 56-59, a forming process for A1203 and others is known. materials, which can be

  also applied in the present invention.

  High temperature superconductors react at higher temperatures with most ceramic materials. The preferred manufacturing processes for high temperature superconductors, for which they partially or totally melt, only succeed on few ceramic materials, but at best on MgO, which does not react chemically up to 900 C. in addition to the MgO ceramic has about the same thermal expansion as the superconducting phase BizSr2Ca1Cu2O0., (Bi2212), so it is particularly suitable as a substrate material. The manufacture of MgO ceramics that can be used is, however, very complex, since it takes almost 700 C to sinter in a totally dense manner.

Pure MgO.

  The object of the invention is to develop a substrate for a high temperature superconductor of the aforementioned type and a process for its manufacture.

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  way that the substrate is made on a large

surface and at an advantageous cost.

  An advantage of the invention lies in the fact that it is possible to manufacture substrate surfaces> 10 cm. 10 cm by a usual dry pressing,

  cost-effective or by strip casting.

  In the case of adding CuO as a sintering aid to give MgO, no noxious gas is released; at the same time low sintering temperatures of the order of 1100 ° C. to 1200 ° C. are possible, so that it is possible to use ovens at a more advantageous price, which also have a lower

relatively long life.

  To a first powdery starting material, which contains magnesium and oxygen, a second powdery starting material which contains copper and oxygen is added as a sintering aid. A third powdery starting material is thus formed, which is sintered at a sintering temperature <1350 C. The proportion of the second starting material in the third starting material is 1 mole% to 25 mole%, preferably 4 moles% to 12

moles%.

  It has surprisingly been found that copper oxide CuO is a particularly active and non-degrading sintering aid for an MgO substrate, which substantially decreases the MgO sintering temperature. In doing so no aggressive gas that could shorten the life of an oven used for sintering, is released. For the complete sintering at maximum density of a fine starting powder of MgO with addition of CuO, temperatures lower than 1350 ° C are already sufficient. The addition of moles% to 10 mole% CuO in the MgO powder with a particle diameter <i pm leads to

best results.

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  The finished substrate has the chemical composition (Mg, -xCu) 0 with 0.01 <x 0.01, preferably with

0.06 x 0.08.

  For the production of the desired substrate, a suspension of an Mg (OH) 2 powder in water is added to an aqueous solution of Cu (NO 3) 3. In this case, the following reaction occurs:

  Cu (NO 3) 2 + Mg (OH) 2 Cu (OH) 2 + Mg (NO 3) 2.

  Because of the large excess of Mg (OH) 2, the reaction

  10 goes quantitatively to the right.

  The suspension is then dried and the powder

  thus obtained is heated to a temperature of 600 C.

  Mg (OH) 2 and Mg (NO3) 2 is decomposed into MgO; H20 and NOX escape as a gas. Cu (OH) 2 is decomposed into CuO + H20; the latter escapes in the form of gas. The CuO formed is very fine; the average particle diameter is 1 μm. This very fine powder of MgO / CuO can be compacted by uniaxial pressing and is sintered already at a temperature of 1 150 C in one

  substrate with> 95% theoretical density.

  With aprotic liquids (ethanol, ketone, octane) and appropriate auxiliaries, for example triethanolamine or menhaden fish oil, it is possible to prepare directly from this powder good slips and by injection casting, slip casting or by sheet-casting, it is possible to manufacture, if necessary, shaped pieces, for example plates, crucibles, tubes, etc., which are sintered at maximum density also between 150 ° C. and 1350 ° C., depending on the green density obtained during forming. The resulting substrate wafers have been successfully tested as a substrate for high temperature fusion superconductor layers Bi (2212) with critical current density values> 1 kA / cm 2. The substrate according to the invention is particularly suitable for the manufacture of

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  Thin films and thick films of superconductors to

high temperature.

  With ceramic substrates of MgO + 6 mole% CuO, sintered at maximum density at 1200 ° C., the following values for the critical current density were obtained with Bi (2212) of fusion, which contained additions different from Ag. : Layer N Material critical superconducting current density in kA / cm2 i Bi (2212) 1.15 2 Bi (2212) + i M% Ag 1.3 3 Bi (2212) + 5 M% Ag 1.6 4 Bi (2212) + 10M% Ag 1.45 By comparison, a non-silver-addling superconducting material Bi (2212) provided on a substrate made of a monocrystalline MgO surface (100) a critical current density of 2 15 kA / cm 2. Bi (2212) superconducting material without silver addition to a commercial grade MgO substrate with composition

  silicate was not superconducting.

  Embodiment: 227 g of Mg (OH) 2 powder was mixed with 300 ml of distilled water. In addition, 72.3 g of Cu (NO3) z were dissolved. 6 H 2 O crystalline in 300 ml of distilled water. The two preparations were then combined and mixed for 60 minutes at 80 ° C. Then the product was filtered, washed three times with distilled water, dried in the C 4 vacuum drying oven and then sieved through a filter. sieve opening

  mesh of 100 pm and calcined for 3 hours at 650 C.

  The powder thus obtained was suspended for disagglomeration with isopropanol without water in a polyethylene bottle, by addition of 2 kg of grinding balls with a diameter of 5 mm ceramic ZrO2, stabilized with water. yttrium and rolled for 24 hours on a roller bench. It was added to the

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  suspending, as a fluidifier, an amine. From the deagglomerated powder, the isopropanol was then extracted under partial vacuum. The dry powder thus obtained was then sieved through a sieve with a mesh width of 50 μm and, after adding a pressing aid, it was was pressed by means of a steel tool, uniaxially, in discs with a diameter of 25 mm and a height of 3 mm. These discs were then air-sintered in a muffle furnace for 3 hours at 1200.degree. C. After cooling, the ceramic thus obtained had a density of 3.71 g / cm.sup.3, that is to say 98%. the theoretical density of MgO with 6 mole% CuO,

that is, 3.79 g / cm3.

  With this manufacturing process, a starting powder is obtained with which ceramic pieces can be sintered at maximum density, even between 1100.degree. C. and 1200.degree. C.; so that there is no more open porosity (94% to 98% theoretical density). The ceramic material therefore has an extremely fine structure 0; the average particle size of

  crystallites is between 1 pm and 3 pm.

  By uniaxial dry pressing, we can realize

  layers with an area of 15 cm. 15 cm.

  Particularly advantageous is the production of (Mg, Cu) 0 ceramic substrates with the forming process which is referred to as strip casting or sheet casting and which is customary among other things for

the substrates of A1203.

  It is important that the finished magnesium oxide substrate, containing copper oxide, has a composition (Mg-xCu) O with 0.01-x 0.25. Particularly suitable for manufacture is a MgO powder with a grain diameter of 5 mm, which is mixed with Cu (II) oxalate powder, CuC24O. 1/2 H 2 O, in a mixing ratio of from 1 mole% to 25 mole%, in a ball mill. The mixture is then calcined at a

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  temperature at 600 C then brought to the desired shape by dry pressing or slip casting or strip casting.

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Claims (10)

  1. - Substrate for a superconductor at high temperature, a) which contains magnesium oxide, characterized b) in that the substrate additionally contains copper oxide of composition (M9_, Cu) O with 0.01 x 0.25.   2. - Substrate according to claim 1,   characterized in that 0.06 x x 0.08.   3. - Substrate according to any of   claims i or 2, characterized in that the   average particle size of these crystallites is the order i pm at 3 pm.   4. - Method of manufacturing a substrate for a superconductor at high temperature according to one   any of claims i to 3,   a) with a first powder containing magnesium and oxygen, characterized b) in that this first powder is mixed with a second powder containing copper and oxygen to give a powder mixture, c) in that the proportion of the second powder in the powder mixture is from 1 mol% to 25 mol%, d) in that the powder mixture is then calcined at a temperature of 600 ° C., in which the calcined powder is then molded or compacted and f) is sintered in the temperature range   between 1 050 C and <i 350 C.   5. - Process according to claim 4, characterized in that 4 mol% to 12 mol% of the second powder are used. 6. Method according to claim 4, characterized in that the first powder contains MgO powder with a diameter grains 5 mm.   7. - Process according to any one of   Claims 4 to 6, characterized in that the first   powder contains Mg (OH) 2 powder with a particle size <i pm, which is suspended in water.   8. - Process according to any one of   Claims 4 to 7, characterized   a) in that the second powder contains Cu (NO 3) z powder, which is dissolved in water and / or b) in that the second powder contains copper oxalate with a particle size of 5 1 pm 9. - Process according to any one of   Claims 4 to 8, characterized   a) in that the mixture is dried and calcined, b) is then compacted by uniaxial pressing and c) is then sintered.   10. - Process according to any one of   Claims 4 to 9, characterized   a) in that the mixture of powders is prepared before sintering with an aprotic liquid to give a slurry and b) in that its forming is then carried out by strip casting or c) by slip casting or d) by injection casting or   e) by uniaxial and isostatic pressing.