Hybrid composite pipes reinforced with high-strength polyethylene fiber (DF) and glass fiber (GF) were prepared to develop the coil bobbin for stable superconducting coils. The bobbin that in which the circumferential thermal strain expands with cooling and in which the circumferential Young's modulus is large would be effective for stable coils. The unidirectional DGFRP showed negative thermal expansion coefficient, and its Young's modulus was similar to DFRP in fiber direction; Young's modulus perpendicular to fiber direction was larger than that of DFRP. The circumferential Young's modulus of DGFRP pipe did not decrease with the filament winding (FW) angle to compare with DFRP. The calculated thermal strain with a cooling of the pipe showed good agreement with the average of observed inner and outer thermal strains. The circumferential thermal strain showed an expansion with FW angle of 50-90 deg and the absolute value was smaller than that of DFRP. The inner and outer circumferential thermal strains were different. The difference decreased with increasing inner diameter/thickness, and the differences were smaller than those of DFRP. The experimental data were obtained to make it possible to devise a coil bobbin with negative thermal expansion coefficient by DGFRP.
Alumina ceramics (Al2O3) is well known as an excellent material for insulation equipment, such as insulation breaks and feed-through terminals, at room temperature. A mechanical test of the ceramics was carried out for the application of an insulation break at low temperature. The insulation break consisted of the ceramics and the transition metal with a joint between them. The brazing was applied to the joint and its mechanical strength was measured. A mechanical test of the ceramics itself was also carried out. The testing temperatures were 4.2K, 77K and room temperature. The tensile strength of the alumina was slightly increased at the liquid helium temperature comparison with room temperature. The maximum of the tensile strength on the brazing joint was achieved as 270MPa at 4.2K testing, which consists of copper and alumina of 95% purity. The stress distribution at the joint was evaluated by the finite element analysis to discuss the tensile strength of the brazing joint. A trial fabrication of the insulation break was carried out by using alumina. The results of the trial showed that the insulation break made from alumina had sufficient enough mechanical and electric properties for a low temperature application.