Outer Sheath Material Research Articles

Magnetization AC losses of MgB2 wires with thin filaments and resistive sheath

Magnetization AC losses of fine-filamentary MgB2 wires with resistive CuNi sheaths were measured. The effects of varying the number of filaments (114–342, corresponding to effective filament diameters of 14–20 μm), twist pitch (10–30 mm) and outer sheath material on the total AC loss were studied. For a better understanding of individual loss contributions, the effects of varying applied temperature, magnetic field, and frequencies were examined. It is found that hysteresis loss per volume decreases with the reduced filament size and that coupling current losses play a dominant role. The effect of decoupling by twisting was clearly observed for the smallest twist pitches. Considering the possible degradation of transport currents by twisting, AC losses were also normalized by the critical currents of the same wires. While twisting to short pitch decreases losses significantly, it apparently does not reduce the transport current. Consequently, the fine-filamentary MgB2 wires with resistive CuNi sheath examined in this paper are excellent candidates for future low loss applications. Unlike ReBCO tapes, round MgB2 wires enable easy single strand twisting, and the braiding or cabling, of wires into a variety of specific shapes and diameters.

Research on External Insulation Characteristics of Composite Cross-Arm of 10 kV Distribution Network Based on Multi-Factor Aging.

With the application of the composite cross-arm in power systems, comprehensive anti-aging performance is a key factor to determine whether it can operate safely. In order to study the influence of the operating environment on the external insulation characteristics of composite cross-arms of distribution networks, various aging conditions such as voltage, rain, temperature, humidity, salt fog and ultraviolet light were simulated in a climate chamber based on the real operation conditions of the 10-kV composite cross-arm. A multi-factor aging test of composite cross-arms with two kinds of cross-section shapes (T-shaped and square) was carried out for 5000 h. The change trends of leakage current and flashover voltage of the composite cross-arms before and after aging were analyzed. Finally, the aging mechanism of the silicone rubber sheaths was analyzed to further explain the reasons for the change of external insulation performance of composite cross-arms. The results show that the leakage current rising rate of T-shaped and square composite cross-arms after aging increases significantly, and the minimum flashover voltage decreases to 58.3 kV and 49.502 kV, respectively. The results of FTIR, SEM and hydrophobic angle tests show that, after aging, the performance of the silicone rubber outer sheath material decreases in varying degrees. In general, UV aging has the greatest influence on the external insulation characteristics of composite cross arms. Generally speaking, after 5000 h of multi factor aging, although the external insulation characteristics of the 10-kV composite cross-arm decreases to a certain extent, there is still enough margin to meet the normal operation.

Processing Parameters that Affect the Tolerable Bending Diameter of Reacted MgB2 Wires

In this study, tolerable bending diameters of reacted MgB2 wires fabricated to produce varying outer sheath materials, numbers of MgB2 filaments, and sheath layer thickness were investigated. The result showed that the tolerable bending diameters of the MgB2 wires with Cu, Cu–Ni, and Monel sheaths were the same. This implies that the sheath material strength of these wires does not affect the tolerable bending diameter. In addition, mono-filament wires were more vulnerable to bending strain than multi-filament wires due to their porous microstructure. An increase in the sheath layer thickness of the MgB2 wires resulted in less degradation of the wires at high bending strains because the strain acting on the MgB2 filaments was reduced by the thick sheath layer. This study generally confirmed that the sheath layer thickness of the MgB2 wire is a key factor to increase the bending tolerance of the wire.

HITEMAL-an outer sheath material for MgB2 superconductor wires: The effect of annealing at 595–655 °C on the microstructure and properties

An aluminum material stabilized with a 1.6 vol% of nanometric Al2O3 dispersoids, named HITEMAL, was fabricated by hot extrusion of a fine atomized Al powder. The Al2O3 dispersoids stemmed from a native Al2O3 films on an aluminum powder surface. An extruded rod of the material was then cold-deformed out into a thin wire by rotatory swaging, hydro extrusion and rolling. Such severely deformed HITEMAL was studied as a prospective outer sheath stabilizer material for MgB2 core superconductor wires. An emphasis was put on the effect of annealing realized at various temperatures close to the melting point of Al, which is typically applied to form an in-situ MgB2 superconductive core, on the mechanical and electrical properties of the HITEMAL wire. A systematic investigation was performed of changes to a severely deformed Al grain structure and Al2O3 dispersoids upon annealing the as-deformed wire from 595 to 655 °C for 30 min. Mechanical and electrical properties of the wire were measured at room and cryogenic temperatures. The results showed that the ultrafine-grained Al + 1.6 vol% Al2O3 wire exhibited advantageous properties, and surface integrity after annealing up to 642 °C. It is thus a lightweight material of choice for outer sheaths of MgB2 superconductors.

Fire behavior of halogen-free flame retardant electrical cables with the cone calorimeter

Fires involving electrical cables are one of the main hazards in Nuclear Power Plants (NPPs). Cables are complex assemblies including several polymeric parts (insulation, bedding, sheath) constituting fuel sources. This study provides an in-depth characterization of the fire behavior of two halogen-free flame retardant cables used in NPPs using the cone calorimeter. The influence of two key parameters, namely the external heat flux and the spacing between cables, on the cable fire characteristics is especially investigated. The prominent role of the outer sheath material on the ignition and the burning at early times was highlighted. A parameter of utmost importance called transition heat flux, was identified and depends on the composition and the structure of the cable. Below this heat flux, the decomposition is limited and concerns only the sheath. Above it, fire hazard is greatly enhanced because most often non-flame retarded insulation part contributes to heat release. The influence of spacing appears complex, and depends on the considered fire property.

An Effort Toward Development of <inline-formula> <tex-math notation="TeX">$\hbox{MgB}_{2}$</tex-math></inline-formula>-Based Current Leads With 2000-A Rating

In this paper, we report an effort toward the development of general-purpose conduction-cooled current leads based on MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , which can carry 2000 A at 25 K. The leads have an overall length of 40 cm and a diameter of 5.8 mm, with Cu end caps with a length of 5 cm each for anchoring them to the superconducting system and the current source. Fe, Cu, and Ni are used as the barrier, the stabilizer, and the outer sheath materials, respectively. The critical parameters of the current leads, such as rated current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , thermal cycling stability, and heat leak, were tested and evaluated using a cryocooler-integrated cryostat test facility.

Bi2Sr2CaCu2O8 + x round wires with Ag/Al oxide dispersion strengthened sheaths: microstructure–properties relationships, enhanced mechanical behavior and reduced Cu depletion

Ag/Al alloys with various Al content (0.50 wt%, 0.75 wt%, 1.00 wt%, and 1.25 wt%) are made by powder metallurgy and used as the outer sheath material for Bi2Sr2CaCu2O8 + x (Bi2212)/Ag/AgAl multifilamentary round wires (RW). Bi2212/Ag/AgAl RW microstructural, mechanical and electrical properties are studied in various conditions, including as-drawn, after internal oxidation, and after partial melt processing (PMP). The results are compared with the behavior of a Bi2212/Ag/Ag0.20Mg wire of the same geometry. The grains in as-drawn Ag/Al alloys are found to be ∼25% smaller than those in the corresponding Ag/0.20 wt%Mg, but after PMP, the Ag/Al and Ag/0.20 wt%Mg grain sizes are comparable. Tensile tests show that Bi2212/Ag/AgAl green wires have yield strength (YS) of ∼115 MPa, nearly 65% higher than that of Bi2212/Ag/Ag0.20Mg. After PMP, the Bi2212/Ag/AgAl YS is about 35% greater than that of Bi2212/Ag/Ag0.20Mg. Furthermore, Bi2212/Ag/AgAl wires exhibit higher ultimate tensile strength and modulus and twice the elongation-to-failure. Atomic resolution high-angle annular dark-field scanning transmission electron microscopy, high resolution transmission electron microscopy and energy dispersive spectroscopy demonstrate the formation of nanosize MgO and Al2O3 precipitates via internal oxidation. Large spherical MgO precipitates are observed on the Ag grain boundaries of Ag/0.20 wt%Mg alloy, whereas the Al2O3 precipitates are distributed homogenously in the dispersion-strengthened (DS) Ag/Al alloy. Furthermore, it is found that less Cu diffused from the Bi2212 filaments in the Bi2212/Ag/Ag0.75Al wire during PMP than from the filaments in the Bi2212/Ag/Ag0.20Mg wire. These results show that DS Ag/Al alloy is a strong candidate for improved Bi2212 wire.

Considerations for Designing a Conduction-Cooled Sample Holder

Usually, superconductive strands are characterized and quality controlled by performing short sample tests. These tests are often carried out using liquid cryogen or cold gas flow as coolant. However, in some measurements, the use of a more adiabatic approach is beneficial. This can be achieved with a conduction-cooled system. The advantages and the disadvantages of using a conduction-cooled measurement are presented. The thermal and electrical aspects of a typical conduction-cooled sample holder are discussed by studying the steady-state and transient operation of such a setup with a 3-D finite-element method model. Also, improving the electrical contact resistance of a clamped current/cooling contact facilitating fast sample changeover was experimentally analyzed. The obtained results indicated that an inadequate sample holder can cause excessive heat generation, can slow down quench recovery, and can complicate burnout protection. Finally, the electrical and thermal properties of typical MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> samples under test were modeled to have a significant effect on the operation of a conduction-cooled measurement system. An order of magnitude increase was observed in both ohmic losses and quench recovery time, when the outer sheath material of the simulated superconductor was changed from copper to Monel.

Influence of Strand Design, Boron Type, and Carbon Doping Method on the Transport Properties of Powder-in-Tube $\hbox{MgB}_{2-{\rm X}}\hbox{C}_{\rm X}$ Strands

The transport properties of a number of MgB2 strands have been investigated in terms of their response to strand design, starting B powder choice, and the approach to C doping used. The strands had various designs, specifically; (i) several chemical barriers were introduced, viz: Fe and Nb, (ii) the strands were encased in various outer-sheath materials, viz.: monel, Cu+monel, monel+glidcop, Nb+monel, (iii) the filament counts were varied (1, 18, and 36), and (iv) the final strand diameter was varied. In addition, for a subset of the strand designs several B powder and C-dopant types were investigated. Specifically, two types of amorphous B powder were used: (i) Moissan based "Tangshan boron" (ii) "SMI-boron" which is produced in a plasma torch by the reduction-by-hydrogen of BCl3. Two approaches to C doping were taken: (i) "malic-acid treatment" in which C is introduced into the B powder precursor by the moderate temperature drying out a slurry of B mixed in with a malic-acid-toluene solution (during which the malic acid decomposes leaving C as the only solid residue) before the Mg powder is mixed in; (ii) direct C doping of the SMI-produced B by introducing a known percentage of CH4 into the plasma flame. Critical current densities, Jc, were measured on 1.5 m long samples at 4.2 K in fields of up to 14 T; of all the strands measured, that doped with SMI-C at a nominal 4 mol% C (in relation B) yielded the highest Jc values e.g 1.1x105 A/cm2 at 7 T, 4.5x104 at 10 T, and 2.2x104 A/cm2 at 12 T. The n-values are given for all strands at 5 and 10 T, and for a certain set of strands the magnetic field dependencies of the n-values and the influence of C-doping is presented. Finally we demonstrate that, over a wide range of B, log(Jc) decreases linearly with B with a slope -{\alpha} such that the Jc(B) of any strand can be parameterized in terms of {\alpha} and its zero-field intercept Jc(B=0).

Contact resistance simulations and measurements of MgB 2–MgB 2 lap joints

Resistive joints are found in many systems using superconductors. Joints are used to connect the superconductor to a normal conducting current terminal or to connect two superconductors, for example in pancake type windings. Knowing the resistance between the contacts is important in studying the heat balance of a superconducting system. We performed several experiments with MgB 2–MgB 2 lap joints to determine the relation between the contact resistance and solder joint length. Also, the effect of the outer sheath material on the contact resistance was studied. To support the experiments, a computational model using Finite Element Method was created. The measured and computed results showed adequate correlation. In the experiments, the soldered joint length was varied from 3 to 20 mm resulting in contact areas between 2.5 and 16.6 mm 2. The results indicated that the outer sheath material has significant effect on the contact resistance. For a Monel sheathed conductor the measured contact resistances varied between 4 and 16 μΩ and if a copper sheath was used, the resistances were an order of magnitude smaller.

Calculation of water ingress in a HV subsea XLPE cable with a layered water barrier sheath system

AbstractThe main aging mechanism of electrical cables with polymeric electrical insulation is the growth of water trees. Water trees are initiated if the relative humidity (RH) in the electrical insulation is above a critical level. Delaying the water ingress into the electrical insulation system delays the water tree initiation and reduces water tree growth, thus extending the service life of the cable. For a cable without any metallic water barrier, the water ingress can be significantly delayed by the use of an outer sheath material with low water permeability. An even greater delay in the water ingress into the electrical insulation can be achieved using a layered sheath system. To explore the possibilities of a layered sheath system, calculations of water ingress into a typical cable cross section has been performed using a finite element method. The water diffusion and sorption data used in the calculation has been measured for typical cable materials. Calculations have been performed for uniform temperature conditions and for a temperature gradient due to resistive current heating. The time to reach critical humidity levels and stationary humidity levels in the insulation system has been determined for several different arrangements of the sheath system. A sheath system with an outer layer of a material with low water permeability and an inner layer of a material with a high water absorption capacity is shown to give a significant delay of the water ingress into the electrical insulation. For the sheath materials used in this study, there is an optimum distribution of thickness of each layer. The calculations also show that a temperature gradient across the insulation system of a cable in operation gives an advantageous RH profile. With a temperature gradient the equilibrium RH level in parts of the electrical insulation can be lower than the critical value for water tree initiation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Self-field quench behavior of multifilamentary [formula omitted] wires in liquid helium

Self-field quench behavior of two types of multifilamentary MgB 2 wires with different numbers of filaments and outer sheath materials are tested. One sample type has a Glidcop outer sheath and the other has a Monel outer sheath. Samples are bath cooled in liquid helium at 4.2 K. A normal zone is initiated by a heater wound around the sample. Voltage and temperature along the samples are monitored and results from two sample types are compared. For the same transport current and end-to-end voltage, the normal zone length is shorter in the Monel-sheathed sample than that in the Glidcop-sheathed sample, which leads to higher electrical field and temperature gradient in the Monel-sheathed sample. The relationship between the voltage and hot-spot temperature is discussed for both samples. Quench energy and normal zone propagation velocity are measured at different transport currents and compared with the existing data for similar samples measured in an adiabatic environment.

High Critical Current Density Multifilamentary ${\rm MgB}_{2}$ Strands

Magnesium diboride multifilamentary strands consisting of 7, 19, 37, and 61 filaments were fabricated using a Nb chemical barrier, 101 Cu as an internal stabilizer, and either monel or oxide-dispersion-strengthened Cu (ODS Cu) as an outer sheath. The various strands were given single-step heat treatments of 700degC for 20-40 minutes. Transport critical current density, Jc, measurements were performed at 4.2 K on 1-meter lengths of strand on ITER barrels in fields of up to 15 T. Higher temperature transport values at 0-12 T were subsequently measured on short samples. Typical Jc values were at 1times105 A cm-2 at 5 T and 4.2 K and similar values at 2 T and 20 K. The use of ODS Cu as the outer sheath material improved the ductility and stability of the strand without much sacrifice in transport Jc. Optimization of transport properties through variation of the Mg/B stoichiometry was also studied. Pinning force curves at various temperatures were analysed.

Effect of alloying additions to Ag on thermal conductivity, electrical and mechanical properties of Ag-sheathed Bi-2223 tapes

Ag-sheathed Bi-2223 tapes were fabricated by a powder-in-tube technique with different configurations of the inner and outer sheath materials: Ag, Ag–Mg–Ni, Ag–Sb and Ag–Au. Characterization of thermal conductivity at 4.2 K to 100 K showed that the addition of Au decreased the thermal conductivity remarkably. The value at 40 K was 53.8 W/(mK) for AgAu–AgAu tape, five times lower than that of Ag–AgMgNi tape. The addition of Sb reduced AC losses much more than other alloyed element, partly due to its high electrical resistivity at 77 K. The measurement results of the normalized Ic dependence on magnetic field of the tapes indicated, however, no conclusive correlation between alloy type and Ic performance in field was highlighted. The mechanical property of alloy-sheathed tape was also evaluated. The sequence of the tapes’ tensile strength from highest to lowest was: Ag–Mg–Ni, Ag–Sb, Ag–Au, Ag.

Molecular pathogenesis of the cell surface proteins and lipids from Treponema denticola.

Treponema denticola, frequently isolated from the human oral cavity, is thought to be a major pathogen of human periodontal disease. Recent developments in molecular analysis have clarified the surface structure of this microorganism and the characteristics of its pathogenic factors. Structural analysis of the outer sheath showed T. denticola to have a new type of outer membrane lipid. Limited exposure of the major outer sheath protein is suggested by electron-microscopic analysis. A protease-deficient mutant has revealed the roles of the protease in the organization of the outer sheath material and in T. denticola pathogenicity. The surface features that contribute to the pathogenicity of T. denticola in periodontal disease are gradually being elucidated, and are reviewed.

Molecular pathogenesis of the cell surface proteins and lipids from Treponema denticola

Treponema denticola, frequently isolated from the human oral cavity, is thought to be a major pathogen of human periodontal disease. Recent developments in molecular analysis have clarified the surface structure of this microorganism and the characteristics of its pathogenic factors. Structural analysis of the outer sheath showed T. denticola to have a new type of outer membrane lipid. Limited exposure of the major outer sheath protein is suggested by electron-microscopic analysis. A protease-deficient mutant has revealed the roles of the protease in the organization of the outer sheath material and in T. denticola pathogenicity. The surface features that contribute to the pathogenicity of T. denticola in periodontal disease are gradually being elucidated, and are reviewed.