Atsuhiko Yamanaka

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.

Thermal Conductivity of High-Strength Polyethylene Fiber and Applications for Cryogenic Use

The local temperature rise of the tape is one of instabilities of the conduction-cooled high temperature superconducting (HTS) coils. To prevent the HTS tape from locally raising a temperature, high thermal conductive fiber reinforced plastic was applied to coil bobbin or spacer for heat drain from HTS tape. The thermal conductivity of ramie fibers increases by increasing orientation of molecular chains with drawing in water, and decreases by chain scission with γ-rays irradiation or by bridge points in molecular chains with vapor-phase-formaldehyde treatments. Thermal conductivity of high strength ultra-high-molecular-weight (UHMW) polyethylene (PE) fiber increases lineally in proportion to tensile modulus and decreases by molecular chain scissions with γ-rays irradiation. This result suggested the contribution of the long extended molecular chains due to high molecular weight on the high thermal conductivity of high strength UHMW PE fiber. Thermal conductivity of high strength UHMW PE fiber reinforced plastics in parallel to fiber direction is proportional to the cross sectional ratio of reinforcement oriented in the conduction direction. Heat drain effect of high strength UHMW PE fiber reinforced plastic from HTS tape is higher than that of glass fiber reinforced plastic (GFRP) and lower than that of aluminum nitride (AlN). In the case of HTS coil, the thermal stability wound on coil bobbin made of high strength UHMW PE fiber reinforced plastic is good as that of AlN, and better than that of GFRP.

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.

Influence of thermal expansion of coil bobbins made of DFRP and GFRP on quench characteristics of superconducting coils

We have excited two types of superconducting coils having same specifications. One is made of DFRP (Dyneema(R) fiber reinforced plastic) bobbins which expands during cooling process from room temperature to liquid helium temperature. The other is made of GFRP (glass fiber reinforced plastic) bobbins which contract during the cooling process. Firstly, we have numerically estimated changes of winding tensions of the two types of the coils by the cooling down. And then, training behaviors of the DFRP and GFRP coils wound with the same winding tension at room temperature were measured. Influence of quench currents in the training quenches and the number of quenches to reach to the maximum currents on the winding tensions at liquid helium temperature is systematically discussed.

Relation between frictional loss and combination of thermal coefficient of bobbins and winding tension in AC superconducting coils

Frictional losses in windings are one of the loss types in AC superconducting coils. When an AC current is supplied to the coil, a superconducting wire in the winding start to vibrate. And frictional heat generates at contact segments between the wire and a coil bobbin. And hence, the losses are not electromagnetic losses such as coupling losses but mechanical losses in superconducting coils. We prepared four types of bobbin materials. Two bobbins expanded during a cooling process from room temperature to liquid helium temperature. The other two bobbins contracted during the cooling down. Winding tensions for the four kinds of coils were 0.5, 3.5, and 5.0 N. And then, the AC losses of the twelve coils were measured. When the coils whose bobbins have thermal expansion were used, the AC losses increased with the gain of the winding tensions, in spite that the experimental conditions such as coil currents and background magnetic field were same. On the contrary, in case of the contraction bobbins coils were used, the losses decreased with the tensions increased. To analyze the experimental cases of the winding tensions of the sample coils at the liquid helium temperature were calculated, and the relation between the tensions at the cryogenic temperature and the frictional losses was discussed.

Temperature Dependence of Sound Velocity in High-Strength Fiber-Reinforced Plastics

Longitudinal sound velocity in unidirectional hybrid composites or high-strength fiber-reinforced plastics (FRPs) was measured along the fiber axis over a wide temperature range (from 77 K to 420 K). We investigated two kinds of high-strength crystalline polymer fibers, polyethylene (Dyneema) and polybenzobisoxazole (Zylon), which are known to have negative thermal expansion coefficients and high thermal conductivities along the fiber axis. Both FRPs had very high sound velocities of about 9000 m/s at low temperatures and their temperature dependences were very strong. Sound velocity monotonically decreased with increasing temperature. The temperature dependence of sound velocity was much stronger in Dyneema-FRP than in Zylon-FRP.

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.

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.