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Schematics of (a) thermal conductivity measurement system and (b) sample holder, sample, diode, heater, and their connections.
Source publication
We evaluated the effect of alloying additions to Ag on thermal conductivity and mechanical properties of Ag-alloy sheathed Bi-2223 (BSCCO) superconductor tape. The tapes were made with combinations of Ag alloys such as Ag-Mg, Ag-Sb, and Ag-Au for inner and outer sheath. Thermal conductivity of the tapes was evaluated by using thermal integral metho...
Context in source publication
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... where, is the heat flux (W), is the cross-sectional area (m ), is the length of the specimen (m), and is the thermal conductivity (W m K ). In the measurement, a cryocooler (CTI model 350) was used to vary the temperature from 10 to 120 K, and the specimen was loaded onto a copper holder in the chamber as shown in Fig. 1. To reduce the heat flow through ambient atmosphere and conduction through the setup, the at- mosphere was set to 10 torr and epoxy glass was inserted be- tween the heater and the copper holder. Once the chamber was cooled down to 10 K, a predetermined current was applied to the heater to induce a temperature gradient ( ) of ...
Citations
... Alloying is a simple and effective method for improving the performance of pure metals [6,7]. Among the numerous available alloys [8][9][10], Ag-Cu alloys [11,12] are a commonly used silver alloy for electrical contacts and decoration purposes, due to its improved mechanical performance, good electrical and thermal conductivity, and moderate cost. However, the introduction of the Cu element reduces the Ag's chemical stability, such that the Ag-Cu alloy is more likely to react with the element sulfur to generate black sulfides, in comparison to pure silver [13]. ...
Ag and its alloys, when prepared by a selective laser melting (SLM) process, have a low density and poor overall performance due to their high reflectivity when the most commonly used laser (λ = 1060 nm) is used, and they have exorbitant thermal conductivity. These characteristics lead to the insufficient melting of the powders and severely limit the applications of additive manufactured silver alloys. To improve the absorption of the laser, as well as for better mechanical properties and higher resistance to sulfidation, Ag–Cu alloys with different La2O3 contents were prepared in this work using the SLM process, via the mechanical mixing of La2O3 nanoparticles with Ag–Cu alloy powders. A series of analyses and tests were conducted to study the effects of La2O3 in Ag–Cu alloys on their density, microstructure, mechanical properties, and corrosion resistance. The results revealed that the addition of La2O3 particles to Ag–Cu alloy powders improved the laser absorptivity and reduced defects during the SLM process, leading to a significant rise from 7.76 g/cm3 to 9.16 g/cm3 in the density of the Ag–Cu alloys. The phase composition of the Ag–Cu alloys prepared by SLM was Silver-3C. La2O3 addition had no influence on the phase composition, but refined the grains of the Ag–Cu alloys by inhibiting the growth of columnar grains during the SLM process. No remarkable preferred orientation existed in all the samples prepared with or without La2O3. An upwards trend was achieved in the hardness of the Ag–Cu alloy by increasing the contents of La2O3 from 0 to 1.2%, and the average hardness was enhanced significantly, from 0.97 GPa to 2.88 GPa when the alloy contained 1.2% La2O3 due to the reduced pore defects and the refined grains resulting from the effects of the La2O3. EIS and PD tests of the samples in 1% Na2S solution proved that La2O3 addition improved the corrosion resistance of the Ag–Cu alloys practically and efficaciously. The samples containing La2O3 exhibited higher impedance values and lower corrosion current densities.
... Most often gold is used with the atomic fraction in the alloy that may be as high as 10 % [3]. It is also known from the reported data, that the thermal conductivity of pure Ag can be lowered by the addition of such elements as Sb, Mg or Pd [4,5]. In the case of Ag-Mg [6] or Ag-Sb [4] alloys, there have been reports of even lower thermal conductivities than for the Ag-Au alloys. ...
... The Ag-6.2Sn is the most promising material because its heat conductivity is an order of magnitude lower compared to those of the other alloys. The drawback of the alloy is a lower critical temperature and lower critical current in comparison with other Ag-based alloys -see FIGURE 5. ...
The thermal conductivity of BSCCO tapes is practically wholly determined by the thermal conductivity of the sheath material, as the conductivity of superconducting filaments can be treated as negligible. The reported data concerning the thermal conductivity of Ag
alloys are very rare and frequently it is not possible to relate them to the properties of sheaths for BSCCO tape, because the thermal conductivity of any metal or alloy is significantly dependent on the parameters of mechanical and thermal treatment processes. This paper gives the theoretical analysis of the influence of alloying additions in Ag
alloy on the thermal conductivity of sheaths for BSCCO tape. The results of the theoretical analysis of the thermal conductivity in Ag‐Sn, Ag‐Cu‐Zr and Ag‐Cu‐Ni alloys are presented in the paper. The materials have been chosen as potential sheaths for BSCCO tape with the low thermal conductivity, suitable for HTS current leads. The applicability of the alloys for production of the monocore BSCCO tapes has been successfully demonstrated.
... In applications, tapes sheathed with Ag-alloy have an added advantage because of their better strength compared to pure Ag [8]–[10]. Pure Ag is too soft compared to the BSCCO superconductor core. ...
... It is thus necessary to develop an understanding of the relationship between the mechanical properties, microstructure, and the transport properties of tapes. The mechanical properties [9] and thermal conductivities [10] of Bi2223 tapes fabricated by a powder-in-tube technique with different configurations of Ag, AgAu7wt% AgSb0.6wt% and AgMg0.2wt% as the precursor and restack sheath materials have been reported. ...
... Since the Bi2223 material is a brittle ceramic , the metal sheathing is needed to impart mechanical, electrical and cryogenic stability to the superconductor [1]–[14]. In applications, tapes sheathed with Ag-alloy have an added advantage because of their better strength compared to pure Ag [8]–[10] . Pure Ag is too soft compared to the BSCCO superconductor core. ...
37 filament Bi2223 tapes with different configurations of Ag, AgAu7wt% AgSb0.6wt% and AgMg0.2wt% as the precursor and restack sheaths were fabricated using commercial Bi2223 precursor material and powder-in-tube techniques. Short length samples were heat treated at a temperature in the range of 832degC to 846degC for the first stage (HT 1), followed by a second stage (HT 2) at 825degC for 40 h and slow cooled to 785degC. An intermediate roll pass was performed between the heat treatment stages. The critical current (I<sub>c</sub>) of the tapes was measured at 77 K in self-field and in fields up to 1 T. The microstructure of the alloy sheaths was examined using optical microscopy, and the Bi2223 filaments after HT 1 and HT 2 were examined using scanning electron microscopy (SEM). It was observed that the sequence of hardness, tensile strength, and critical bend strains from higher to lower levels is very much related to the grain sizes in the restack sheath. J<sub>c</sub> of the tapes in zero field and in applied field was influenced by the phase composition, core density, grain connectivity, grain alignment and the interface between the Bi2223 filament and sheath.
Thermal conductivity of superconductor tapes is an indispensable parameter for engineering design of high temperature superconductor current leads. To meet the requirement of determining the thermal conductivity of the superconductor tapes used in the high temperature superconductor leads projects carried out at IPP, CAS, a test apparatus is being developed to measure the thermal conductivity using the longitudinal steady-state heat flow method in the working temperature range of 5-80 K. In this paper, detailed description of the apparatus' design, instrumentation, data acquisition and control are given, and the proof-test results from this apparatus are presented as well.
Free-standing and flexible graphene-silver (GE-Ag) composite paper has been successfully fabricated through an evaporation of graphene oxide/AgNO3 aqueous aerosol followed by a chemical reduction. The size, thickness and shape of GE-Ag paper can be tuned accordingly the Teflon substrate used. The GE-Ag paper presents the advantages of fine flexibility, high electrical conductivity (159 Ωsq-1) and good thermal conductivity in the vertical direction (3.3 W/mK), which are more optimal than pure GE paper.
Sumitomo Electric has been developing the silver-sheathed Bi2223 multi-filamentary wires since the discovery of Bi-based superconductors. DI-BSCCO (Dynamically-Innovative BSCCO) is the high performance wires produced with the controlled-overpressure (CT-OP) sintering technique. The recent R&D activities have enabled to produce the 1000 m-long wires with Ic of 200 A and the maximum Ic reached 250 A at 77 K by short sample. Besides, the fine control of the carrier density with the non-stoichiometric oxygen enhanced the in-field Ic performances at lower temperature. At a magnetic field of 3 T applied normal to wire surface, the Ic of 280 A at 30 K, and 420 A at 20 K were successfully achieved. To meet the growing needs for various high temperature superconducting applications, Type HT wire with high mechanical strength and Type G wire with low thermal conductivity have been developed.
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/(m K) 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.
