An improved method of manufacturing high-temperature superconductors of general composition YBa2Cu3O7— x and various grain structures has been devised. This method involves mixing, pressing, and heating of Y2O3, CuO, and Ba(NO3)2 powders. This method is easier than an older method that involved mixing and sintering of Y2O3, CuO, and BaCO3 powders followed by repeated grinding and sintering, followed by pressing to final size and shape.
In the present method, the first step is to mix commercial grade powders of Y2O3, CuO, and Ba(NO3)2 by percent by weight of the final mixture. Optionally, the powder mixture can be doped with a small proportion of silver (such doping results in, among other things, a higher-density final product). The powder mixture is placed in a die and pressed at either room temperature or an elevated temperature. Pressing densifies the powder mixture and forms a piece of approximately final size and shape.
Both cold and hot pressing yield similar results, though hot pressing is ordinarily preferable because it results in a piece that is denser and less delicate and thus easier to handle. The denser hot pressed samples also maintain closer to the final desired shape. In the case of hot pressing, care must be taken not to exceed the peritectic-transformation temperature-and-pressure range of the powders because, if such a transformation occurs, then the powders react prematurely with each other and/or react with the die.
The pressed piece is taken from the die and placed on a substrate of densified aluminum oxide or other suitable material in a furnace. The temperature-vs.-time schedule of the furnace can be altered to obtain desired results. The best results occur when the temperature of the pressed piece is heated slowly to 550 °C and held there for one hour before raising the temperature slowly to 650 °C. Thereafter, to obtain the desired grain structure and quality, the pressed piece can be treated by almost any temperature-vs.-time profile or post-sintering procedure (for example, annealing in oxygen) cited in any of the numerous patents and research papers that address the manufacture of YBa2Cu3O7 —x high-temperature superconductors.
A piece of bulk superconductor made according to this method can be used as is, provided that it has the desired final size and shape. Alternatively, it can be processed further; for example, it could be ground into a YBa2Cu3O7 —x that could be further pressed and sintered to final size and shape.
This work was done by Glen A. Robertson of Marshall Space Flight Center.