Preparation of pure and chemically substituted XBa2Cu4O8 via freeze-dried nitrates

Samples of single phase YBa2Cu4O8 (124) and Y(SrxBa1−x)2Cu4O8 (x⩽0.4) and primary phase Y1−xC xBa2Cu4O8 (0.1⩽x⩽0.2) have been prepared below 800°C at 1 atm oxygen pressure and without the aid of catalysts via freeze-dried nitrate precursors. Samples were characterized by X-ray powder diffraction, thermogravimetric analysis, and microwave absorption/ susceptibility techniques. Below 675°C, the stable phase assemblage was determined to be Y2O3+CuO+Ba2Cu3O6−x. Pure and Sr-substituted 124 exhibited a transition temperature onset of 80 K while 10% Ca substitution increased Tc to 85 K. Ca subsitutions greater than 10% were achieved, but they failed to increase Tc above 85 K. Powders were produced with particle sizes as low as 0.3 μm which were ideally suited for sintering.