Kaoru Yamafuji

Theory of Flux Motion in Non-Ideal Type-II Superconductors

A phenomenological expression of the pinning force density is proposed, and the equation of force balance including this expression is shown to describe well the motion of magnetic fluxoids in non-ideal type-II super-conductors in cooperation with the equation describing the continuity of magnetic flux flow. The parameters related to an individual pinning force and pin distribution are included in the present expression which contains those from the models of Bean-London, Silcox-Rollins and Yasukōchi as its special cases. The calculated magnetizations agree well with observed data by adjusting the parameters. The hysteresis losses in the course of magnetization are calculated and the field dependence of the a.c. losses are explained. The formulas of the a.c. dynamic losses are also shown. The field dependence of the critical current in the resistive state and the relaxation time of flux penetration are calculated and compared with observed data.

Multichannel taste sensor using lipid membranes

Abstract We have developed a taste sensor consisting of a multichannel electrode with transducers composed of lipid membranes immobilized with a polymer, i.e., eight different kinds of lipid analogs mixed with poly(vinyl chloride) and plasticizer. This multichannel sensor responds to five basic taste substances in five different ways; the taste patterns composed of eight electric-potential responses could be clearly distinguished from each other for the five basic taste substances. Moreover, similar but distinguishable patterns were obtained for substances eliciting a similar taste in humans. The reproducibility is noticeably improved from those of lipid membranes which were previously investigated for application in a taste sensor. The present results imply that the multichannel lipid membrane device has the ability to sense tastes like a human being.

A NEW INTERPRETATION OF THE GLASS-LIQUID TRANSITION OF PINNED FLUXOIDS IN HIGH-TC SUPERCONDUCTORS

Abstract The present paper proposes a possibility that the transitions between the vortex glass state and the vortex liquid state, which have been observed by the resistive measurements in high- T c cuprate superconductors, can be interpreted as the thermal-fluctuation-induced percolative transitions of pinned fluxoids. Since weaker flux-pinning centers become successively ineffective due to thermal agitation as the temperature increases, this kind of percolative transition occurs when a narrow channel of unpinned fluxoids penetrates throughout the whole specimen, and hence the flux flow begins to occur through the channel of unpinned fluxoids, where the long range order among fluxoids recovers. The results of computer simulation on the motion of pinned fluxoids support strongly the present mechanism. The present theoretical expression for the scaled master curves of the induced electric field versus the transport current density shows good agreement with existing observed data. The relations of the transition temperature and the scaling exponents to the flux-pinning characteristics are also discussed.

Flux creep and irreversibility line in high‐temperature oxide superconductors

The irreversibility line in high‐temperature oxide superconductors is theoretically investigated from a viewpoint of dependence on the flux‐pinning strength and a general relation between the effective pinning potential and the critical current density is derived. It is shown that the irreversibility magnetic field at 77 K in strongly pinned oxide superconductors is sufficiently high for application.

Current-voltage characteristics near the glass-liquid transition in high-Tc superconductors

Abstract Theoretical expressions for the induced DC electric field, E , are derived as a function of the applied current density, J , near the glass-liquid transition temperature of T = T GL as E(J) = E (J)[1 + ( δE(J) E (J) )] , where E (J) is the scalable part and δE(j) E (j) is the unscalable part given by a power series of | T GL − T | nv ( z + 2 − D ) wi Then the condition of δE E ⪡1 gives a measure of the width of the critical regime, in which E = E ( J ) is well scalable. The general scaling characteristics of E= E (J) are the same as those predicted by Fisher and coworkers based on the conventional theories of phase transitions with the scaling hypothesis, while the present theory provides a concrete expression for E (J) . Furthermore, the expressions for the scaled master curves of the AC impedance are derived. The reason, why the observed E vs. J characteristics are scalable over wider ranges of the temperature and flux density than the critical regime, is also discussed.

Multichannel taste sensor using electric potential changes in lipid membranes

A taste sensor with a multichannel electrode was developed by using lipid membranes as a transducer of taste substances. The sensor can detect the taste in a similar manner to the human gustatory sensation by response patterns of electric potential to taste substances. The sensitivity, reproducibility and durability were superior to those of humans. The same taste as that elicited by some commercial aqueous drink was reproduced by making aqueous solution mixed from four kinds of basic taste substances, the concentrations of which were determined so that the electric-potential pattern of this mixed solution could agree well with the pattern by the drink. Different brands of beer were easily distinguished by the electric-potential patterns.

Dielectric behaviors of some polyvinyl esters

SummaryThe effects of the bulky side groups on the dielectric high and low temperature absorptions are discussed with respect to the shapes, the magnitudes and the apparent activation energies of the absorptions. To this purpose, the properties of the dielectric absorptions of polyvinyl acetate and polyvinyl benzoate are compared in detail.The asymmetrical shapes of the absorption curves of these polymers can be explained by the characteristic natures of the absorptions in the amorphous linear high polymers, that is, by the fact that the dipoles are combined with each other by the chains in the amorphous solid state. The agreement between the theoretical and the observed curves seems to be satisfactory. The effects of the bulky side groups on the shapes of the absorptions are remarkable in the high frequency side of the absorption curve.The magnitudes of the absorptions of PVAc are larger than those of PVBz, which can be explained by taking account of the difference of the dipole concentrations between these polymers.The attachment of the bulky side groups makes the values of the activation energies increase. The difference of the chemical structures seems to affect more sensitively to the high temperature absorption.ZusammenfassungDie Effekte voluminöser Seitengruppen zu den Hoch- und Tieftemperaturabsorptionsmaxima werden hinsichtlich ihrer Form, ihrer Größe und der scheinbaren Aktivierungsenergien diskutiert. Dies geschieht durch einen Vergleich der dielektrischen Eigenschaften von Polyvinylacetat und Polyvinylbenzoat.Die asymmetrische Gestalt der Absorptionsgruppen dieser Polymeren kann durch die charakteristische Art der Absorption für amorphe lineare Hochpolymere erklärt werden, nämlich damit, daß die Dipole durch die Hauptketten im amorphen festen Zustand miteinander verkoppelt sind. Die übereinstimmung zwischen theoretischen und beobachteten Kurven erscheint befriedigend. Die Effekte der großen Seitengruppen auf die Form der Absorption ist auf der Hochfrequenzseite der Absorptionskurve beträchtlich.Die Größen der Absorption für Polyvinylacetat sind stärker als die für Polyvinylbenzoat, erklärbar aus dem Unterschied der Dipolkonzentrationen in beiden Polymeren.Die Festlegung der voluminösen Seitengruppen läßt die Aktivierungsenergie-Werte ansteigen. Unterschiede in den chemischen Strukturen scheinen die Hochtemperaturabsorption empfindlicher zu beeinflussen.

Preliminary tests of a 500 kVA-class oxide superconducting transformer cooled by subcooled nitrogen

The authors have designed and fabricated a 500 kVA-class oxide superconducting power transformer operated in sub-cooled nitrogen. The primary and secondary windings are three- and six-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. In the parallel conductors, the strands are transposed several times for uniform current distribution among them. A transformer, cooled by liquid nitrogen of 77 K, was steadily operated with a 500 kVA secondary inductive load. The efficiency in full operation at 77 K was 99.1 %, even with the refrigeration penalty of liquid nitrogen, 20, for the thermal load to the coolant. They installed the transformer in a continuous flow system of sub-cooled nitrogen as a fundamental step for compact superconducting transformers operating in sub-cooled nitrogen with a single-stage refrigerator. Short-circuit tests of the transformer were also performed in a region of temperature below 70 K. The transformer was operated with no quenching up to a level of critical current at 66 K, that is equivalent to 800 kVA. The efficiency estimated was improved to 99.3 % in the sub-cooled nitrogen.

Abnormal quench process with very fast elongation of normal zone in multi-strand superconducting cables

Abstract An abnormal quench process with apparent high propagation velocity of the normal zone, ·fast quench’, was studied in relation to electromagnetic and thermal couplings among strands for multi-strand superconducting cables. The quench process was numerically evaluated by simultaneous equations of heat balance and equivalent electrical circuit of the multi-strand cables. Experimental results are well explained by the numerical simulation concerning threshold current of the fast quench for a two-strand cable. The quench process of a six-strand cable was also calculated numerically and the relation between the fast quench and current commutation among strands is discussed.

Anisotropy and hysteresis of transport critical currents in high temperature Ln-Y-Ba-Cu-O superconductors

Abstract Following the measurements of anisotropy and hysteresis in transport critical currents with changing temperature and polarity of magnetic field, a.c. magnetic measurements by the Campbell method were performed for sintered Ln-Y-Ba-Cu-O superconductors. The information derived from them indicated that hysteresis does not occur in the intragrain current but in the intergrain one. The results are interpreted in terms of flux pinning, where the anisotropy originates from the texture structure and hysteresis is associated with the superconducting weak links of the sintered oxide pellets.