Surface free energy of various carbon fibers was determined by tensiometric method in three different procedures. The dispersive ys a and the nondispersive y~ components were separately determined by contact angle measurements in two-phase system, formamide-n-alkanes. The yw increased by oxidation and decreased by hydrogen reduction, while the ys a remained almost constant. The y~ value determined from contact angle of water/ethyleneglycol solution tended to become higher. The critical surface flee energy was in harmony with the total free energy determined from formamide-n-alkanes system as long as liquids were properly selected. The formation of oxygen-containing functional groups was evidenced by ESCA, and the surface [O]/[C] ratio was related linearly to the surface polarity defined by yw a + y~]. Oxidation in liquid phase resulted in the formation of both hydroxyls and carboxyls, while only hydroxyls were formed in gas phase. The interracial shear strength of the fiber-epoxy resin could be closely correlated with thermodynamical work of adhesion calculated on the basis of surface free energy and its component.

We fabricated screen-printed Ag-(Bi,Pb)2Sr2Ca2Cu3Ox monofilament and multifilament tapes with filament number 1≤N≤5. As N increases, the critical current Ic increases monotonically until it reaches 54 A (77 K, 0 T) for N=5, while the critical current density Jc is around 1.6×104 A/cm2 (77 K, 0 T) nearly independent of N. The bend strain tolerance is found to decrease with increasing N. For monofilament tapes the strain tolerance is 2%–3% strain while for multifilament tapes it decreases to ∼1% at N=3 and ∼0.4% at N=5.

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