Bi系2223高温超导带材力学性质

对4种Bi系2223高温超导带材77 K下的力学性质进行了实验研究.通过选择合适的合金包套替代传统的银包套,带材的机械性能能有所提高,但电学性质有所降低,尤其是内包套采用合金、外包套采用银,其机械性能反而低于纯银包套带材.通过X射线衍射图谱发现,当包套采用合金材料,尤其是内包套采用合金、外包套仍采用纯银,会大大降低样品中2223相的体积含量.相比较临界电流密度,n值对材料的破坏更为敏感.利用X光同步辐射技术初步探讨了银包套带材在室温和77 K下的损伤破坏过程.     

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

Fabrication of a working Bi-2223 superconducting magnet cooled by liquid nitrogen

A practical Bi-2223 superconducting magnet, working in liquid nitrogen (L.N₂), was designed and fabricated. Bi-2223 tape with a critical current of 147 A was prepared by a controlled overpressure (CT-OP) process at 77.3 K in self-field. Ten double-pancake coils were resistively connected by copper terminals. The bore diameter was 54 mm?, the magnet outer diameter was 122 mm?, the height of the magnet was 124 mm, and the weight of the magnet was about 3 kg. The maximum magnetic field at the center of the bore was 0.48 T with an operating current of 50 A. The experimental results agree well with design predictions calculated by finite element method. AC operation was also performed, and no distortion of the voltage waveform was observed. Therefore, this Bi-2223 superconducting magnet is a suitable replacement for copper magnets designed for applications in science and technology.     

弯曲应变对Bi2223/Ag带材临界电流的影响

本文主要研究了Bi223/Ag带材的弯曲应力-应变特征及弯曲疲劳对其在77K自场下临界电流的影响。分析临界电流Ic降低的原因是应变和热循环引起的超导陶瓷芯内部的微裂纹。实验研究发现当带材的弯曲应变超过0.3%以后,Ic显著降低;当带材受到多次弯曲时,前四次弯曲会使Ic急剧降低,然后Ic降低非常缓慢。因此,在实际应用过程中,应使Bi223/Ag带材的弯曲应变不超过0.3%,且在Bi223/Ag带材的生产和使用过程中,均应尽量减少其弯曲的次数。     

Microstructure of Ag/Bi(Pb)-2223 Tapes Prepared by Solid-State Reaction

The processing of Ag/Bi(Pb)-2223 tapes via the prevailing solid state reaction was investigated. A precursor powder of the composition Bi_1.86Pb_0.26Sr_1.96Ca_1.95Cu_2.97O₁₀₊ containing mainly Bi-2223 phase was employed. Particular attention was devoted to the microstructural properties of the tapes subjected to rolling and subsequent heat treatment in one, two, and three steps, respectively. A sharp texturing gradient exists in the superconducting cores, and the repeated treatment leads to deterioration of the superconducting properties. The latter effect is ascribed to the formation of periodic defects arising during the repeated rolling of the sintered superconducting core.     

Intrinsic critical current of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Oz tapes

True zero-field critical current densityJ _c of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm² at 77 K) agree well with the transportJ _c of the same tape. Because of a very thin BPSCCO, the coreJ _c determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ _c 's follow the same (1-T/T _c )ⁿ withn=1.45 dependence.J _c shows an approximateH ^–0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ _c withT andH seem to be determined by the flux creep.     

Development of high-Tc superconducting Bi-2223 tape joints

High-T _c superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T _c superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape,I _cj/I_c, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77 K,I _cj/I_c=99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be 4×10^–13 between the decay time of 120 and 3600 sec.     

Mechanical and transport properties of IBAD/EDDC–SmBCO coated conductor tapes during fatigue loading

In electrical devices like superconducting motor, generator and SMES, HTS coated conductor (CC) tapes will be subjected to alternating stress or strain during manufacturing and operation. The repeated loading will affect the mechanical integrity and eventually the electrical transport property of CC tapes. Therefore in such applications, electro-mechanical property of CC tapes should be evaluated. In this study, the endurance of an IBAD/EDDC–SmBCO CC tape under high-cycle fatigue loading has been evaluated. Applied maximum stress and fatigue life (S–N) relation was obtained at 77 K. The mechanical properties and the critical current, I_c, of the sample under fatigue loading were investigated at 77 K. Considering the practical operating environment, the effect of the stress ratio R, on the degradation behavior of I_c under fatigue loading was also examined.     

Orientation Distribution of Grain Planes in Polycrystal Ag/Bi-2223 Tape

For polycrystalline Ag/Bi-2223 tapes, the preferential orientation of grains is a very important issue. The platelike grains in the tapes are generally believed in a high-order alignment. However, microstructural observation by SEM shows that the grain alignment is far from perfect. Theoretically, upper critical field, H _c2, for H parallel to ab plane and c axis was calculated form the relation of M(H). However, the ratio of H _{c2 (H//ab)} to H _c2(H//c) depends on the angle () between the ab plane of grains and the broad surface of the tapes. Based on the ratio, the orientation distribution of grains is obtained. The results show that grains in Ag/Bi-2223 tapes can grow with ab plans at any angle between 75° > > –75°, but no grain can grow with its ab plane perpendicular to the tape broad surface. The overall distribution is that: >90% grains orient in the angle range of || < ± 75°, about 5% grains at ±75° || ±85°, and no grains at || > ±85°.     

Preparation and characterization of silver clad (Bi,Pb)-Sr-Ca-Cu-O 2223 superconducting tapes with high critical current density

Silver clad Bi-2223 tapes with consistently high critical current densities of over 30,000 A/cm² at 77 K and zero field were prepared by powder-in-tube (PIT) technique. Powder XRD, electron microscopy, a.c. susceptibility and critical current measurements were used to study the phase assemblage, microstructure and transport properties of these tapes at various stages of processing. The precursor powder for PIT process was prepared by a sol-gel route by acrylate method using freshly prepared nitrates of Bi, Pb, Sr, Ca and Cu. The carbon content in the powder was minimized by subjecting it under dynamic vacuum calcination followed by heating in free flow of oxygen for long durations with intermittent grindings. The choice of initial stoichiometry, high reactivity of the precursor, effective removal of carbon, choice of phase assemblage at the filling stage and the multistage thermomechanical processing at optimized conditions were found to be responsible for the high critical current density. The work was done under the National Superconductivity Programme funded by the Department of Science and Technology (DST), New Delhi.     

The Study on AC Susceptibility and Magnetization of High-Tc Superconductor Bi-2223 Using a Critical-State Model

The Anderson–Kim model for a granular superconductor was employed to calculate both temperature and magnetic field dependencies of the AC susceptibility of a Bi-2223 superconductor. Moreover, similar calculations were performed for the magnetization. The prediction of the model, including the intergranular and intragranular contribution, for susceptibilities and magnetizations was consistent with experimental data very well by considering the temperature-dependent effective volume fraction. The temperature-dependence of fitting parameters was shown to obey almost quadratic power relation (1–T/T _c) with 2.     

Upper Critical Field Hc2 on c Axis of Ag/Bi-2223 Polycrystal Tape

The upper critical field H _c2, based on Ginzburg–Landau theory at medial magnetic fields, can be calculated from the relationship of the magnetization intensity M(H) versus magnetic field H if M is linear with ln₀ H. For Ag/Bi-2223 tapes, the measured M(H) was found to be linear with ln₀ H. In this case, the values of H _c2(T) may in principle be determined. To do this, we will meet another problem in that the obliquity of crystal planes is not equal for different grains in tapes, and values of grain-oblique angle will appear in the calculated H _c2. Obviously, for Ag/Bi-2223 tapes is an uncertain parameter and hard to determine. To avoid the effects of , we only study the H _c2 in the c axis direction and the projections of H onto the c axis is taken as the actual applied fields. Thus, the effects of grain-oblique angle may be counteracted when measuring critical current density J, if the external magnetic fields are applied to the tape along both the parallel to and perpendicular to the c axes directions (on the narrow surfaces of the tape). On medial fields (H = 0 – 3 T), the upper critical fields H _c2(T) on c axis for the Ag/Bi-2223 tape are obtained and fitted as ₀ H _c2(T) = 830 e ^–0.07T . The average slope d[₀ H _c2(T)]/dT – 8T/K is found to be as large as that of Bi-2212. On extrapolating the relation to T = 0 K, the value of ₀ H _c2(0) 800T. The coherence length _ab is determined from H _c2(c), and _ab (0) = 0.63 nm at T = 0 K.     

Thermal stability in high-Tc coil and magnet design by process control of Bi(Pb)-2223/Ag multifilamentary tapes

It has been found that the degree of thermal stability of Bi(Pb)-2223/Ag pancake-shaped coils at 77 K can be determined by controlling the amount of matrix and superconducting materials during processing. The intermediate deformation step between sintering stages has been found to be crucial in optimizing the performance of the processed composite tapes as well as governing the thermal stability of the subsequently made pancake-shaped coils. Results obtained from numerical analysis of the finite element method has shown that monolayer coils produced from Bi(Pb)-2223/Ag composite tapes are thermally very stable with high values of the fill factor. However, increasing the number of co-wound tapes would require either a reduction in the fill factor or an increase in cooling rate for thermal stability to be sustained as would otherwise be achieved with the metallurgically same single tape.     

Temperature dependence of ac-losses in multifilamentary Bi-2223 tapes between 4.2 K and Tc

The ac-losses of twisted multifilamentary Bi-2223/Ag tapes were measured in the temperature range between 4.2 K and the critical temperature T_c∼110 K. Stacks of tapes in perpendicular and in parallel field were investigated. The loss measurement is performed by the standard magnetisation technique, the absolute loss value is obtained by a calorimetric calibration measurement. For a fixed ac-field amplitude the energy loss per cycle Q is determined, at a given temperature, as a function of frequency f. The extrapolation to f→0 gives the hysteresis loss. Coupling losses are obtained from the slope of the loss curve Q(f) in the low frequency range ωτ?1. The measured total losses as a function of temperature show, at small field amplitudes, a minimum around 50 K. This is explained by the different temperature dependence of hysteresis and coupling losses. While coupling losses decrease with increasing temperature, hysteresis losses increase for field amplitudes below the penetration field. Coupling losses show a much weaker temperature dependence than the silver conductivity, which is explained by the existence of an interfacial resistance between filaments and silver matrix. Measured hysteresis losses were compared with available theoretical models and good agreement was found in parallel field.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.     

The Jc Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Damage evolution and real-time non-destructive evaluation of 2D carbon-fiber/SiC-matrix composites under fatigue loading

Electrical resistance acquisition, acoustic emission (AE) monitoring and infrared thermography were employed to evaluate damage evolution of 2D carbon-fiber/SiC-matrix composite under fatigue loading. Damage evolution was discussed on the basis of the calculation results of the modulus and mechanical hysteresis variation. At lower stress levels, the majority of damage was produced in the first few cycles and then the rate of damage accumulation gradually approached a steady value as the cycles proceeded. When the applied stress exceeded the endurance fatigue limit, extensive damage took place and led to failure of the composite. Changes of composite electrical resistance, AE activity and surface temperature had fairly well agreement with the modulus and hysteresis responses. It can be concluded that it is possible to employ these real-time non-destructive evaluation methods as in-situ damage evolution indicators for this kind of composites under fatigue loading.     

On the giga cycle fatigue behaviour of two-phase (α2+γ) TiAl alloy

Fatigue crack initiation behaviour is investigated at room temperature in the (α₂-Ti₃Al and γ-TiAl) alloy. High cycle fatigue tests ranging up to 10¹⁰ cycles are carried out on the powder metallurgy (P/M) bar specimens under different loading conditions with a stress ratio of R=0.1 and R=0.5. Microstructural characterization and fracture surface analysis are also investigated by optical (OM) and scanning electron microscopy (SEM). Ti–Al alloy studied here shows two phases in microstructure (nearly refined lamellar thickness) composed of α₂-Ti₃Al and γ-TiAl (hereafter called γ+α₂ alloys) and fracture mechanism is explained with different plastic incompatibilities between the two phases.     

Stress/strain characteristics of Cu-alloy sheath MgB2 superconducting wires

The mechanical properties of Cu and Cu-alloy (Cu-Zr, Cu-Be and Cu-Cr) sheath in situ PIT-processed MgB₂ superconducting wires were studied at room temperature (RT) and 4.2 K. The effects of stress/strain on the critical current (I_c) of the wires have also been studied at 4.2 K and in magnetic fields up to 5 T. Alloying the Cu sheath significantly increased the yield stress of the wires. The 0.5% flow stresses of the Cu-alloy sheath wires were 147-237 MPa, whereas that of Cu was 55 MPa. At RT, the serration in the stress-strain curves corresponding to the multiple cracking was observed around a strain of 0.4% and the curve almost saturated beyond that point. The strain dependence of I_c prior to the critical strain (ε_irr) was different depending on the magnetic field; being almost constant at 2 T and increased with strain at 5 T. The I_c decreased beyond ε_irr, which was much larger for Cu-alloy sheath wires as compared with Cu sheath wire. The magnitude of ε_irr is due to the difference in the thermal compressive strain in the MgB₂ core, which was relaxed by yielding in the sheath materials. The transverse compression tests revealed that the I_c of the Cu-alloy sheath wire did not degrade up to about 95 MPa, which is also higher than that of Cu sheath wire.