The present invention relates to a method for the preparation of superconducting products on the basis of superconducting compounds having the general formula (Me.sup.1).sub.p (Me.sup.2 ).sub.q Cu.sub.3 O.sub.x F.sub.y, where Me.sup.1 is one or more metals selected from yttrium, scandium and the rare earth metals, Me.sup.2 is barium and/or strontium, p a number 0.8-2.5, q a number 0.8-3, x a number 5-8.5 and y a number 0-6, in which process oxides and optionally fluorides of Me.sup.1, Me.sup.2 and Cu are reacted with each other at an elevated temperature.
It is known that metal-oxide ceramic compounds of this kind, which are frequently called Y--Ba--Cu--O and in which the metals are present in the oxidic form, possess superconducting properties at a substantially higher temperature than that known for classic superconductors (up to about 23 K), frequently at a little above the boiling point of liquid nitrogen at atmospheric pressure. It is also known that a partial replacement of oxygen by fluorine may improve the superconducting properties.
Although the metals in compounds of this kind typically are yttrium, barium and copper, yttrium may be entirely or partly replaced by scandium and/or one or more lanthanides, and barium entirely or partly by strontium. In the materials of the kind so far known, copper is compulsory; it is known that the superconducting properties are improved concurrently with the degree to which copper is present at a higher oxidation state than 2, preferably predominantly or entirely in oxidation state 3. Known materials of the kind in question have, for example, the composition YBa.sub.2 Cu.sub.3 O.sub.z, where z is a number 6 to 8, or La.sub.1.85 Ba.sub.0.15 CuO.sub.4.
Surveys of superconducting substances of the kind in question are, i.a., an article by Ron Dagani in Chemical & Engineering News, May 11, 1987, p.p. 7-16, and by Ib Johannsen and Thomas Bj rnholm in Dansk Kemi 5, May,1987, p.p. 172-175. There is a bibliography in the latter.
It is known that such compounds can be prepared by a solid phase reaction between suitable compounds, e.g. oxides, carbonates or oxalates, in powder form under an oxygen-containing atmosphere; that the reaction must be conducted under a controlled temperature profile; and that the composition of the surrounding atmosphere is important, notably the partial pressure of oxygen. Hitherto one has always used barium oxide, BaO, and possibly strontium oxide, SrO, as the source of Me.sup.2 and conducted the reaction in an oxygen-containing atmosphere, frequently in pure oxygen.
It may hereby be difficult to control the temperature profile and the oxygen pressure of the surrounding atmosphere so closely as to obtain a well-defined product, especially a compound in which Cu is present in a state of oxidation above 2. It might be important to provide another source of oxygen than elemental oxygen from the atmosphere or in an industrailly commonly available pure form.
The known processes for the preparation of Y--Ba--Cu--O compounds have the well-known drawbacks of solid phase reactions, including a slow reaction, difficulties in obtaining the needful contact between the reactants, difficulties in obtaining uniform products having high contents of desired phases and in obtaining a desired crystalline form of desired purity and dimensions, notably in obtaining perfect crystals, monocrystals.
The superconducting crystals prepared by the known processes must frequently be employed not as they are, but must be worked up and shaped to intermediate products or final products, e.g. superconductor cables. Such working up frequently involves complicated and difficult methods.