Status of the development of ceramic regenerator materials

This paper describes the status of recent developments of ceramic regenerator materials. First, improvement of the thermal and mechanical properties of Gd2O2S(GOS) regenerator material has been achieved. Since the heat capacity of GOS is very high (> 1 J/cm(3)), with a k type peak at 5.2 K, the thermal penetration depth is about 30% smaller than that of HoCu2 below 6 K. Thus, a small particle size (less than 0.3 mm) with a high homogeneity is important for optimization of cooling efficiency. The recent introduction of Hot Isostatic Pressing (HIP) for fabricating GOS particles has been shown to be very effective in increasing the quality. Higher thermal conductivity and a withstanding pressure higher than 5 MPa have been achieved. Secondly, new concept regenerators using a variety of ceramic materials have been considered. Multicomponent magnetic materials that consist of (GdxTb1-x)(2)O2S have been fabricated for cooling temperatures above 6 K. Magnetic materials consisting of two magnetic elements show a broad heat capacity peak involving the combination of the two different heat capacity curves. By adjusting the heat-treatment conditions with catalyst materials there is a high potential for controlling the heat capacity curve using multicomponent magnetic materials. In order to freeze the movement of the spherical magnetic materials in the regenerator, a 'solid' 4 K regenerator consisting of sintered GOS particles has been fabricated. Cooling-test results with the solid GOS regenerator show a certain level of decrease in the cooling capacity at 4.2 K. For a particle size of 0.4 mm in diameter, the cooling capacity at 4.2 K decreases by 20% compared with a regular regenerator with unsintered particles. However, this value is comparable to the cooling capacity achieved using only HoCu2 for the lowest temperature portion of the 2nd regenerator.