High Tc superconductors are well known nowadays. Technical developments of this kind of superconductors are so attractive to be applied to practical use. Among various materials, YBa₂Cu₃O_7–d (YBCO) has been significantly well studied in the wide range of the field on superconducting in a bulk [1] or porous [2] ceramics as well as in a thin-film form. This material is surely supposed to be the most reliable high Tc superconductor. Nowadays, sophisticated study has been performed: the YBCO/metal interface and its application [3,4], YBCO composites with Ag [5] or polymer [6]. Comprehensive survey of current knowledge on this material is described in a book [7].

The bulk ceramic superconductor of YBCO is usually produced by means of sintering process. Since this process is based on a solid chemical reaction, it requires long time and high temperature maintained with a large amount of energy. Baghurst et al. reported microwave syntheses for the formation of YBCO ceramics [8] for the merit of low cost of energy.

Kato et al. reported [9] that a domestic microwave oven was capable to grow YBCO ceramics. This report symbolized that the weak microwave could be applied to form YBCO ceramics effectively. The advantage of this method is surely low energy cost for the processing. One different thing is that BaO was required as a starting material instead of BaCO₃.

The interesting method was applied to a continuous growth process by Marinel et al. [10] in such a way that a starting material was passed through a microwave cavity. This is similar to floating zone method for the growth of single crystal of silicon. We expect that the microwave processing will be widely used as a rapid and lowcost method for superconductors.

For the production of YBCO bulk ceramics, a press procedure to form a pellet is known to be one of key points. It is because YBCO is formed by solid phase reactions. We investigated the effect of the pressure on the obtained YBCO characteristics in the microwave procedure.

Commercially available Y₂O₃, BaO₂ and CuO powder at the atomic ratio of Y:Ba:Cu = 1:2:3 (totally 12 g) were mixed and milled in a mortar for 20 min. A fraction of the mixed powder (3 g) was pressed mechanically to be a pellet with the dimensions of 20 mm in diameter and 3 mm in thickness. The pressure was varied between 80 and 2000 kgf/cm². We did not use any binder materials through the experiment.

The coin-formed pellet was surrounded by an additional mixed powder of 5.5 g and wrapped by a quartz glass wool (2.18 g) and put in a crucible. These surroundings were important to suppress heat dissipation during heating and to absorb enough oxygen into the pellet. The pellet was heated for 25 min. at the microwave power of 200 W and then cooled down naturally. The domestic microwave oven we used was a EM-B1(W) type manufactured by Sanyo Co. Ltd. This oven was useful because the output power of microwave was continuously varied between 50 and 500 W. The frequency was 2.45 GHz.

The samples were investigated by an XRD measurement with RAD-IIA diffractometer (Rigaku, Japan). The x-ray from Cu target whose wavelength was 0.15405 nm was used. The pellet was put on the diffractometer without any process. In the measurements, the θ-2θ method was employed.

Figure 1 shows XRD pattern when the pellet was pressed at 160 kgf/cm². The Miller indices are assigned to those of YBCO. The other non-labeled structures may derived from the existence of impurities such as starting materials that did not react enough or other phase. A relatively strong impurity structure situated around 30 degrees corresponds to three reflections by (5 3 0), (6 0 0) and (6 1 1) planes in BaCuO₂, assigned with JPCDS card of x-ray diffraction data.