Use of barium peroxide in superconducting Y1Ba2Cu3Ox and related materials.

FIELD OF THE INVENTION

This invention relates to the preparation of superconducting ceramics and more particularly to an improvement in preparing the metal oxide system generically represented by L.M.A.O. where O is oxygen and (e.g.) L may be yttrium, M may be barium, and A, copper.

BACKGROUND OF THE INVENTION

It has long been known that the effective resistivity of certain metals was sometimes substantially eliminated when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4° on the Kelvin scale (°K), and certain niobium alloys are known to be superconductive at about 15°K, some as high as about 23°K. More recently, an oxide containing lanthanum, barium, and copper was discovered which became superconductive at temperatures of about 30°K, and in some circumstances at temperatures of about 20° higher. Current advances have identified materials which become superconductive at temperatures near 100°K, such that liquid nitrogen cooling could be used. Of special interest are ceramic materials which have reduced electrical resistance properties that are stable over time such that they could be developed for use in practical applications. While the phenomena of reduced electrical resistance and even superconductivity have now been observed at liquid nitrogen temperatures or above, these properties are still considered to be achieved primarily at low temperatures when compared to ambient conditions. However, there is some indication that ceramic materials might be formulated which can reliably exhibit reduced electrical resistance and perhaps superconductivity at ambient conditions.

A composition having an approximate unit cell formula of Y.Ba₂.Cu₃.O_z, where z is typically about 7, and various related materials, represents a particularly promising group of ceramics for superconducting applications. The compositions are typically formulated from precursors which can be mixed to provide the desired ceramic. In one formulation for these ceramic materials, for example, carbonate and/or oxide powders of the solid elements are mixed and raised to a temperature of about 1,000°C, driving off volatile materials, such as carbon dioxide. The mixture is reground and reheated, ordinarily several times, to improve the intimacy of the mixture, and then can be pelletized, sintered for several hours, and then gradually cooled to below 250°C.

Pellets have proven convenient to research applications properly involving ceramic superconductive materials since they can be readily formed by pressing together the powdered materials and binding them by a sintering process. These ceramic materials are typically brittle such that they are also more readily handled in pellet form. However, commercial applications of superconductors are likely to require substantial quantities of such materials in useful shapes such as tubes, rods, wires, or sheets, and other techniques for conveniently and reliably shaping these ceramic materials, while maintaining their ability to conduct electricity with reduced resistance, are being sought.

Reportedly, one procedure has been developed in which the ceramic powder is encased in a thin tube of metal such as silver, and then drawing the filled tube to form a wire. Evaporative techniques have also been reportedly used to produce films of superconducting materials from multiphase material comprising yttrium, barium, copper, and oxygen. In still another procedure, the ceramic powder, or even its ingredients, are mixed into an organic binder such as polyethylene glycol which is then extruded to form a plastic wire. After the wire is formed into the desired shape, the binder is burnt off and the residual powders are sintered to form the product filament. Tapes have also been produced by embedding ceramic particles in organic material to produce a flexible tape which can be shaped and then sintered. The conductive performance of the final ceramic material is known to be dependent upon the uniformity of element distribution throughout the composition. A common objective in any of the techniques for formulating and processing superconductive materials is to assure intimate mixing of precursor materials to provide a relatively homogeneous ceramic product.

Wu et al, Superconductivity at 93K in a New Mixed Phase Y-Ba-Cu-O Compound System at Ambient Pressure, Physical Review Letters, <u style="single">58</u>, 908-910 (2 March 1987), discloses making the title compounds by solid state reaction of Y₂O₃, BaCO₃, and CuO.

Engler et al, Superconductivity above Liquid Nitrogen Temperature: Preparation and Properties of a Family of Perovskite-Based Superconductors, J. Am. Chem. Soc. <u style="single">109</u>, 2848-2849 (1987), mixes Y₂O₃, BaCO₃, and CuO in a ball mill to give a 1:2:3 ratio of Y, Ba, Cu. The powder was heated in an alumina boat at 950°C, and the resulting black powder was reground and heated again.

Wang et al, Comparison of Carbonate, Citrate, and Oxalate Chemical Routes to the High-T_c Metal Oxide Superconductors La_2-xSr_xCuO₄, Wu et al, Inorg. Chem. <u style="single">26</u>, 1474-1476 (1987). This reference uses a carbonate precipitation technique. The precipitant was K₂CO₃. According to the paper, it was necessary to wash the precipitate repeatedly, an obvious disadvantage in production work. Washing was necessary because potassium adversely affects superconductivity properties of the finished material.

SUMMARY OF THE INVENTION

A mixture of L.M.A oxides is shaped and heat-treated to make a superconductor. In this mixture the M oxide is a peroxide, which confers several benefits, including holding the shape together before and during firing and, supplying free oxygen during heating. Barium peroxide is particularly noteworthy in that it facilitates sintering because of its relatively low melting point, thereby reducing time in the furnace. M is at least one metal of the group Ba, Mg, Ca, and Sr and is preferably Ba; L is at least one metal of the rare earth group (elements with atomic numbers 57-71), including scandium and yttrium and is preferably yttrium; A is at least one metal of the group Cu, Ag, and Au, preferably Cu.

It has been reported that BaO₂ has been used as a Ba source to make a Y₁Ba₂Cu₃O_x powder, but so far as we are aware, no one previously made such BaO₂-containing pellet or other shape and fired the shape directly to make a superconductor. We differ from prior practice in that we heat the BaO₂-containing shape and by that one step make a superconductor, whereas the prior art (a) mixed the powders, (b) calcined the mixed powders, (c) shaped the calcined powders, and (d) heated the shape to achieve superconductivity. Note that in the prior art process several steps were required to make a superconducting shape. Our invention makes a superconducting shape in essentially one step, i.e., simply by heating the BaO₂-containing shape. Note further that in the prior art process the peroxide oxygen is lost in the first calcination and is unavailable to boost oxygen content for the final superconducting composition. In our composition, on the contrary, the peroxide oxygen is available during the step (i.e., the first and only firing) in which superconductivity is developed.

DETAILED DESCRIPTION OF THE INVENTION

In a generic sense we intimately mix together a peroxide, MO₂; an L oxide; and an A oxide, where M is at least one member of the group Ba, Mg, Ca, and Sr, preferably Ba; L is at least one metal of the rare earth group (elements with atomic numbers 57-71), including scandium and yttrium, and is preferably yttrium; and A is at least one metal of the group Cu, Ag, and Au, preferably Cu. The oxides should provide an atomic ratio in the range of L_a:M_b:A_c where a is about 0.8 - 2.2, preferably about 1; b is about 1.8 - 2.2, preferably about 2; and c is about 2.7 - 3.3, preferably about 3. This results in an amount of oxygen in the heat-treated product equal to about 7 (or a little less) atoms per atom of L.

Our powders are suitably about 100-mesh (i.e., about 90% will pass a 100-mesh U. S. Screen). The reagents may usually be purchased in this form as reagent-grade oxides from various commercial suppliers. These materials contain at most only trace impurities, which do not affect properties of the finished superconductor.

Next, we form the powder mixture into a pellet or other shape by compression or other conventional techniques.

The pellets in the examples were made with a Carver laboratory hydraulic press, and were about ½-1" in diameter and ¼" in height. These dimensions are of course not critical.

This peroxide-containing shape is the crux of our invention. When such shape is heat-treated, it ipso facto becomes superconducting. Further processing for superconductivity is unnecessary.

The heat-treated shape has substantially the metal ratios of the powder mixture. The oxygen content is lower, since the peroxide oxygen is diminished. Thus the heat-treated shape has the composition L_aM_bA_cO_x where L, M, and A and a, b, and c are as above defined, and x is about 7.

By shape we mean products made when intimate mixtures of the above powders are pressed or adhered together. This is commonly done by compressing the powdered mixture into pellets, bars, briquets, and the like. Also, shapes can be made by extrusion as, e.g., rods, tubes, etc.

In a preferred aspect of our peroxide-containing shape, the L oxide/A oxide can be furnished as a precalcined powder (Cf. Example 2).

As we have mentioned, barium peroxide has reportedly been admixed with Y₂O₃ and CuO powders, and then the powdered mixture first fired, then the fired mixture shaped, the shape re-fired with or without regrinding, and so on, until superconductivity is achieved. So far as we are aware, no one has previously shaped a powdered mixture of BaO₂, Y₂O₃, and CuO. Yet, this is the simplest and most efficient way of utilizing the unique contributions of BaO₂. Summarily stated, when such shape is sintered, it becomes superconducting. Further processing is unnecessary. While we do not wish to be bound by any particular mechanism, we believe that BaO₂ accomplishes this by (a) decomposing to provide free oxygen within the sample body during sintering, thereby helping maintain oxygen requirements in the final composition Y₁Ba₂Cu₃O_x; (b) melting at a relatively low temperature (around 450°C) thereby aiding sintering and thus assisting homogeneity; (c) reducing furnace time in that byproduct gases (e.g., CO₂, nitrogen oxides, water, etc.) need not be driven off.

After the pellet or other shape is made, it is heat-treated (i.e., calcined, fired, or sintered) in a furnace, in air or oxygen at 925 - 1,025°C, preferably about 950-975°C, and held there for about 1 - 2 hours, suitably about 1.5 hours. Longer times can be used, but are a waste of energy. Following this heat treatment, the shape is allowed to cool, e.g., to room temperature. At this point a test of the shape will show that it is superconducting.

For the aforesaid heat treatment the shape is preferably placed in an inert container, e.g., an alumina boat.

From the foregoing description it will be evident that our process introduces no extraneous substances into the L.M.A oxide mix; viz., no cations other than L.M.A enter the system, and no anions whatever are used. The process thus results in an oxide mix of extraordinary purity at all stages, from initial powder mixture to finished fired shape.

The following examples illustrate without limiting the invention.

Example 1

Powders: Y₂O₃, 0.8184 g BaO₂, 2.4537 g CuO, 1.7293 g

The powders, proportioned in the proper ratio to make Y₁Ba₂Cu₃O_x, were mixed, then pressed into a pellet. The pellet was placed in a furnace for sintering at 950-975°C for 1 hour, 28 minutes, in air. The heat-treated pellet was allowed to cool in the furnace. The solidified body was cooled in liquid nitrogen and was able to float a magnet above its surface.

Example 2

Powders: BaO₂,      0.7363 g Y₁Cu₃O_y,      0.7640 g

The Y₁Cu₃O_y powder was previously prepared by calcining together Y₂O₃ and CuO in a molar ratio of 1:6, at 950-975°C in air for 1.5 hours.

The above materials provided the desired ratio, Y₁Ba₂Cu₃O_x; the components were mixed as in Example 1, pressed into a pellet, and the pellet was heated at 950-975°C for 1 hour 28 minutes, during which time it sintered. When the body was cooled in liquid nitrogen, it floated a magnet above its surface.

Example 3

A run was made following the procedure of Example 1 except that sufficient Y was used to provide a Y₂Ba₂Cu₃O_x ratio in the powdered mixture, viz., Y₂O₃,      5.46 g BaO₂      8.20 g CuO,      5.77 g

Following sintering and cooling, the pellet (19.4360 g) showed superconductivity by the Levitation Test below.

Example 4

(Control)

A run was made following the procedure of Example 1 except that BaO was used instead of BaO₂. It was not possible to make a suitable pellet owing to the hydroscopicity of the BaO, which caused the attempted pellet to crumble before and during heat treatment. Further, even before it disintegrated, the pellet absorbed water from the atmosphere so rapidly that it could not be accurately weighed.

Extraneous Materials

Prior art processes conventionally enhance homo­geneity by grinding the calcined intended superconductor, followed by recalcining. In some instances this sequence may be repeated several times. It is known that improved homogeneity in the general case enhances super­conductivity. The problem here is that effective grinding inevitably and inherently introduces impurities into the ceramic, simply by impacting the ceramic between the balls and walls (or other grinding surfaces) of the grinding mill. It is known, for example, that silica or stainless steel balls in a ball mill lose significant mass over a period of use. This mass, of course, disappears into whatever was being milled. Mills that comminute by particle self-impact lose metal by wall-scouring, particularly in the area of stream entry. If the product is ground in a ball mill using quartz or silica balls, some of the impurity is silica.

Thus, the firing-grinding-refiring technique rapidly achieves a balance: improvement in homogeneity tends to be matched by contamination build-up that cancels part or all of the improvement.

Our process avoids the grinding problem in the general case. Our heat-treated product, can, of course, be subjected to the conventional grinding-shaping-refiring cycle, but this is superfluous.

Levitation Test for Superconductivity

Various tests are available for the determination of superconductivity. One of these tests is conventional, simple, and is accepted in the art as definitive. We used this, the so-called levitation test, in our determinations, and we describe it below.

The pellet (e.g., the heat-treated shape of Example 1) is placed in the center of a glass dish, and then liquid nitrogen (77°K) is poured into the dish. The pellet bubbles a bit at first, as the nitrogen boils on contact, and as surface air is flushed from the pellet. In a few minutes gas evolution diminishes to nearly zero, and the pellet may be assumed to be chilled to approximately the temperature of liquid nitrogen. A chip of a rare earth magnet is now dropped gently over the pellet. If the magnet levitates, i.e., hovers in the air over the pellet - the so-called "Meissner Effect" - the pellet is superconducting.