Toshihiro Kashima

Thermal Conductivity and Diffusivity of High-Strength Polymer Fibers.

The thermal conductivity κ and diffusivity α of high-strength and high-modulus crystalline polymer fibers (polybenzobisoxazole (Zylon\circR) and polyethylene (Dyneema\circR)) and their fiber-reinforced plastics (FRPs) were measured in directions parallel and perpendicular to the molecular chain axis of the fibers. The main contribution to thermal conductivity was from phonon conduction along the molecular chains in both fibers and the phonon conduction was limited by boundary-like scattering over the temperature range of 10–260 K. From the analyses using a phenomenological model, the thermal conductivity anisotropy ratio (κ// fiber/κ⊥ fiber) of Zylon fiber was estimated to be 80 at 100 K, which was about two or three times larger than that of Dyneema fiber.

Thermal Strain of Pipes Composed with High Strength Polyethylene Fiber Reinforced Plastics at Cryogenic Temperatures

High strength polyethylene fiber (Dyneema® fiber; hereinafter abbreviated to DF) has a large negative thermal expansion coefficient. Several kinds of pipes were prepared by means of filament winding or sheet winding method. The thermal strain or residual stress of those pipes were measured at liquid nitrogen temperature. The thermal strain was also calculated and was compared with the measured values. The circumferential thermal strain of the inner surface was found to be much different from that of outer surface. The circumferential strain changed with the ratio of inner diameter to thickness of pipes. The mean thermal strain of inner and outer surface was found to agree well with that of calculated value. It was confirmed that the negative thermal expansion can be realized even in the pipes. The design methodology of the pipes with negative thermal expansion was discussed.

Drawing Effect on Thermal Properties of High-Strength Polyethylene Fibers

The thermal conductivity κ and thermal diffusivity α of high-strength polyethylene fibers, DyneemaTM, with various draw ratios were measured from 10 K to 260 K. κ and α increased with increasing draw ratio and α was nearly proportional to the square root of the tensile modulus E of the fiber. The estimated phonon mean free path l (40–60 A) was almost temperature independent. Boundarylike scattering due to characteristic microstructures with comparable size to l may effectively limit the thermal conduction in these fibers.

Coil Bobbin for Stable Superconducting Coils

The coil bobbin for a.c. coils have been prepared with the high strength polyethylene fiber (DF) reinforced plastics (DFRP) or with hybrid composites reinforced by DF and glass fiber (GF). The coils with the bobbin were found to be markedly stable. The DF has a large negative thermal expansion coefficient and hence the circumferential thermal strain of bobbin can be designed by changing the ratio of DF to GF layer thickness (DF/GF). It was found that the thermal expansion coefficient in the circumferential direction of the outer surface changed from negative to positive with increasing DF/GF and became nearly zero at a DF/GF of approximately 5. 1 kArms class a.c. coils having a bobbin with a negative thermal expansion coefficient or small thermal contraction in the circumferential direction were fabricated and were confirmed to show higher quench current than that with a GFRP bobbin.

Frictional Properties on Surfaces of High Strength Polymer Fiber Reinforced Plastics

High strength polymer fiber reinforced plastics (ZFRPs) have a remarkable characteristic of a negative thermal expansion coefficient, that is, ZFRPs expand during cooling down from room temperature to cryogenic temperature. Hence, it is expected that ZFRPs are suitable for bobbins of superconducting magnets from a view point of stability against mechanical disturbance due to abrupt conductor motions. A frictional property of contacting surfaces between magnet bobbins and superconductors is one of important parameters for characterizing the stability of the superconducting magnets.

Frictional properties of ceramics, MoS2 coated films and polyethylene fibre reinforced plastics at 4.2 K in liquid helium

Abstract Frictional problems in superconducting magnets are related to the stability and reliability of the magnets, because frictional heating causes quench of the magnet and the frictional resistance determines the limit of the shear force at the contacting surfaces. To find those materials possessing low friction, the frictional properties of ceramics of ZrO2 and Si3N4, three different kinds of MoS2 coated films and polyethylene fibre reinforced plastics (DFRPs) were examined at 4 K in liquid helium under reciprocating sliding conditions. The two ceramics showed similar tendencies in terms of friction. The organically bonded MoS2 film exhibited the lowest coefficient of friction of 0.06, while the coefficient of friction of DFRP was lower than that of glass fibre reinforced epoxy (GFRP).

Dependence on winding tensions for stability of a superconducting coil

Abstract The purpose of this study is to manufacture a high performance superconducting pulse coil by using high strength polyethylene fiber (DF; Dyneema ® fiber) reinforced plastic (DFRP) or Dyneema–glass hybrid composite fiber reinforced plastic (DGFRP) as material of a coil bobbin, which has negative thermal expansion, low frictional coefficient and high thermal conductivity. Description in this paper is as follows. First, thermal strains of several kinds of FRP pipes made by filament-winding (FW) method are measured, and the measured results well agreed with calculated ones by our proposed calculation method about thermal strains of a FW-pipe form. This shows that thermal expansion can be controlled by the proposed design technique of a DGFRP FW-pipe. Moreover, frictional coefficients of FRP plates using as coil structural material are measured and frictional heats are calculated for respective material when contact forces are changed. From these results, we find that the lower winding tension of a coil generates the smaller frictional heat when the frictional coefficient of the coil structural material is low. Furthermore, we systematically measure quench characteristics of many specimens of small superconducting coils using DFRP or DGFRP bobbins with different thermal expansions. From the results of quench tests, we find that the higher winding tension’s coils tend to decrease quench current when the coil’s bobbin expanded to a direction of circumference during the cooling down to cryogenic temperature, and suitable values of winding tension in a coil are located in region of 2–4 kg/mm 2 . Finally, we design and manufacture 100 kJ class superconducting pulse coil by using a DGFRP bobbin, which wound in winding tension of 4 kg/mm 2 . In addition, we prepare another 100 kJ class coil with the higher winding tensions of 8 kg/mm 2 , and the quench characteristics of the coils are compared. The quench currents in the coils exceed the 95% rating on the load line for critical current value of a conductor, and a coil with winding tension of 4 kg/mm 2 is more stable.

Coil bobbin composed of high-strength polyethylene fiber reinforced plastics for a stable high-field superconducting magnet

High-field superconducting solenoid magnets sometimes quench by wire motion induced by electromagnetic force. Fiber reinforced plastic [Dyneema fiber reinforced plastic (DFRP)] pipes composed of high-strength polyethylene fiber by filament winding method could be constructed so as to expand in the circumferential direction when cooled to low temperature with an appropriate selection of winding angle and shape of the pipes. In the case of a superconducting coil fabricated with a DFRP bobbin, it is expected that wire motions in high field are decreased by expansion of the coil bobbin. In this paper, tap voltage between both ends of the coils fabricated with DFRP bobbin and stainless steel (SUS) bobbin were measured with increasing current. The coil using SUS bobbin showed many sharp peaks in tap voltage induced by quick wire motions. In contrast, those using DFRP bobbin showed only a few small peaks. These results suggest that wire motions were constrained by DFRP bobbin. The training effects were observed in both cases.

Frictional coefficients of structural materials in AC superconducting coils

Abstract Dyneema ® and glass fiber reinforced plastics (DGFRPs) expand when they are cooled down to cryogenic temperature. Therefore, they may have applications as structural materials in superconducting magnets to increase stability against abrupt motion of superconductors in the magnets. To obtain the mechanical properties of DGFRPs, we measured coefficients of friction on surfaces of DGFRPs under various experimental conditions at three temperatures, four different forces, and two types of DGFRPs. In all of the experimental conditions, the measured coefficients of friction were quite low. The range of measured coefficients was 0.07–0.14.

Thermal Expansion Coefficient of Unidirectional High-Strength Polyethylene Fiber Reinforced Plastics at Low Temperature

High-strength polyethylene fiber (Dyneema®, DF) has a negative linear thermal expansion coefficient in the direction of the fiber. Thermal expansion coefficients of fiber-reinforced plastics are of important applications in cryogenic use. The purpose of this study is to construct the engineering technology for the thermal strain of DF reinforced plastics (DFRP) for cryogenic engineering. In this study, we investigate the thermal expansion coefficient of unidirectional high-strength DF reinforced plastics (UD-DFRP) in which DFs with different tensile moduli (15–134 GPa) and thermal expansion coefficients are used. Furthermore, using the thermal expansion coefficient and tensile modulus of each DF fiber and the matrix resin, the thermal expansion coefficients of the DFRPs are estimated by the rule of mixtures and are compared with the measured values. The results indicate that the thermal expansion coefficient of UD-DFRP behaves in a similar way to that expected from calculations based on the rule of mixtures. In addition, relationships concerning the difference between the calculated and measured values, the difference in the thermal expansion coefficient of the fiber and the resin, and the surface treatment of the fiber (adhesivity), as well as the diameter of the fiber (the area of the interfacial bonding) are studied. The results show that all of these influence the thermal expansion coefficient of UD-DFRP. The design of negative thermal expansion coefficient of UD-DFRP is established.