S.D. Goren

Performance of an inductive fault current limiter employing BSCCO superconducting cylinders

Inductive fault current limiters operating at high levels of short-circuit currents are plagued by appearance of overheated thermal domains in active superconducting elements. Excessive growth of thermal domains may lead to a fatal mechanical destruction of the superconducting element during a fault event. It has been determined that employment of superconductors with gradual dissipation onset controlled by flux relaxation processes can efficiently prevent local overheating. Operation of such elements, fabricated by melt cast technique, has been investigated experimentally in a small-scale open-core model of an inductive fault current limiter. The results of the experiments demonstrate the feasibility of application of superconducting cylinders having properties dominated by flux relaxation processes in inductive current limiters. The most important parameter of a superconducting element designated to operate in such devices is the rate of flux relaxation and its dependence on ac current amplitude. It has been found that ac losses associated with flux relaxation in the investigated cylinders allow for a reliable limiter operation at the nominal current level. Projection of the parameters of the investigated small-scale model to the full-scale device has been performed using the concept of physical modeling. The obtained results indicate that it is possible to build a full-scale device based on flux creep dissipation mechanisms for distribution networks.

High-Tc superconducting inductive current limiter for 1 kV/25A performance

The results of investigations of an inductive current limiting device prototype based on superconducting to normal state transition in 0.2 m o/d. BSCCO rings are discussed. Thermal processes in the ring were found to have an important influence on transient response characteristics of the limiter. In a marked difference to small scale devices, the quenching process in a medium scale current limiter is accompanied by an intense heating of the HTSC ring. Because of high losses and large thermal inertia, the superconducting ring remains in the normal state during entire limitation time and at least a few AC cycles pass before the superconducting state in the rings is resumed after a fault occurrence.<<ETX>>

INFLUENCE OF HYDROGEN ON THE 89Y NMR IN HXYBA2CU3O7

The ambient-temperature {sup 89}Y magnetic resonance line in H{sub {ital x}}YBa{sub 2}Cu{sub 3}O{sub 7} was measured at 14.707 MHz as a function of hydrogen concentration {ital x}. The Knight shift relative to an aqueous solution of YCl{sub 3} increases from {minus}100 ppm for {ital x}=0 to +115 ppm for {ital x}=1.82. The linewidth at half-amplitude is 1.2 kHz and increases slowly with {ital x} in the superconducting samples (0{lt}{ital x}{lt}0.74), then increases sharply in the normal samples. It was seen that, for a number of parameters, increasing the hydrogen content gives similar behavior to decreasing the oxygen concentration. Our {sup 89}Y NMR data and those of other investigators are analyzed and are consistent with previous evidence for local antiferromagnetic correlations in the superconducting compound.

NMR multiple-quantum dynamics with various initial conditions

An approach to creating the highest-order multiple-quantum coherences is developed. The method is based on using the multiple-quantum NMR technique and various initial conditions and can be considered as supplemental to earlier developed methods. Using the dipolar ordered state as the initial one, it is demonstrated that in a cluster of four and eight dipolar-coupled nuclear spins the highest-order multiple-quantum coherences can be created.

19F NMR study of CF bonding and localization effects in fluorine-intercalated graphite

Abstract The CF bonding and localization effects in fluorine-intercalated graphite C x F with 3.8 ⩽ x ⩽ 6.5 have been studied by 19 F NMR. An analysis of 19 F shielding measured in our experiment and of data taken from the literature shows that the changing character of the CF bond occurs for x x in the range from 3 to 6. We suggest that the drop in conductivity for fluorine concentrations above that corresponding to σ max is caused by a percolation mechanism rather than by the change in bond length.

Nuclear magnetic resonance study of fluorine-graphite intercalation compounds

To study the origin of semimetal-metal and metal-insulator transformations, localization effects and C-F bonding in fluorine-intercalated graphite and NMR investigations have been carried out for a wide range of fluorine content, . Fluorine spectra for small fluorine content, x > 8, are attributed to mobile fluorine acceptor species which are responsible for the increase of electric conductivity in the dilute compound. When increasing the fluorine content to corresponding to the maximum electric conductivity, covalent C-F bonds start to occur. The number of these bonds grows with fluorine content resulting in a decrease in conductivity which is caused by a percolation mechanism rather than by a change in bond length. A difference in chemical shift for fluorine-intercalated graphite and covalent graphite fluoride has been observed and is attributed to different C-F bonding in these compounds.

Neutron diffraction study of hydrogen in α titanium

Abstract It has been determined, using neutron diffraction, that hydrogen dissolved in the α (h.c.p.) phase of titanium is distributed randomly among the tetrahedral sites.

Analytical approach to AC loss calculation in high-Tc superconductors

Abstract Using a linear spline approximation for the E – J characteristic of a superconductor and representing the solution in form of series, analytical expressions for AC losses have been obtained. The expression explains experimentally observed frequency and magnetic field dependencies of AC losses. Cases of complete and incomplete magnetic field penetration have been distinguished. AC losses per cycle decrease with increasing frequency in the case of incomplete penetration, the case relevant to thick slabs and low amplitude magnetic fields, while in thin slabs and large magnetic fields they increase with increasing frequency, the case of complete penetration. The analysis of the analytical solutions obtained has given a simple criterion for the applicability of the critical state model (CSM) to the calculations. This criterion involves characteristics of both superconductor and applied magnetic field. The physical meaning of the criterion in terms of ratios between the characteristic decay times of magnetic field energy in a superconductor and the period of applied magnetic field has been established. It has been shown that the analytical solutions can be applied for various forms of E – J characteristics by means of defining effective critical current density and flux flow resistivity.

Evidence of local magnetic order in the superconducting phase of hydrogen doped YBa2Cu3O7 using nuclear magnetic resonance

Abstract Nuclear magnetic resonance absorption traces were obtained for the 1H line in H0.11YBa2Cu3O7 as a function of temperature. The superconducting temperature Tc is 93K. A spectral splitting was observed which we attribute to local antiferromagnetism. The Neel temperature is approximately 320K.

Size dependence of 13C nuclear spin-lattice relaxation in micro- and nanodiamonds.

Size dependence of physical properties of nanodiamond particles is of crucial importance for various applications in which defect density and location as well as relaxation processes play a significant role. In this work, the impact of defects induced by milling of micron-sized synthetic diamonds was studied by magnetic resonance techniques as a function of the particle size. EPR and (13)C NMR studies of highly purified commercial synthetic micro- and nanodiamonds were done for various fractions separated by sizes. Noticeable acceleration of (13)C nuclear spin-lattice relaxation with decreasing particle size was found. We showed that this effect is caused by the contribution to relaxation coming from the surface paramagnetic centers induced by sample milling. The developed theory of the spin-lattice relaxation for such a case shows good compliance with the experiment.