Straining of a Bi-2223/Ag ribbon studied by the method of acoustic emission

The process of cooling of a Bi-2223/Ag ribbon after secondary sintering at 1000 K is studied by the method of acoustic emission (AE). The acoustic response is observed below 500 K and increases in intensity with decreasing temperature. It is established that the AE signal variation reflects the plastic flow in the silver coating caused by a difference between the thermal expansion coefficients of silver and the ceramic core of the Bi-2223/Ag ribbon.     

The J c 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.     

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.     

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°.     

Experimental Investigations on the Critical Current of the Jointed Bi-2223/Ag HTS Tapes

This paper summarizes the experimental investigations on the critical current of two jointed Bi-2223/Ag superconducting tapes connected by Sn63Ag2Pb solder. Different lap lengths of contact surface were studied. The joint resistance was measured to be in the range of 0.059??0.76????, and the critical current of the jointed Bi-2223/Ag HTS tapes was measured with different charging rates using standard four-point-method in a zero-applied magnetic field applying 1 ??V/cm criterion. The experimental results showed that the longer the lap length was, the smaller the joint resistance was and the nearer the critical current approached that of Bi-2223/Ag superconducting short-sample. On the other hand, the critical current decreased with the increasing of the charging rate.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

The influence of electroplastic rolling on the mechanical deformation and phase evolution of Bi-2223/Ag tapes

Electroplastic rolling (EPR) of Bi-2223/Ag superconducting wires was performed, where pulse currents were applied during rolling to introduce an electroplastic effect. It was found that the rolling force decreased significantly compared with the traditional rolling process. Furthermore, EPR favorably minimized the sausage effect. It is revealed that the electroplastic effect can facilitate the mechanical deformation of Bi-2223/Ag composites. Segments of the Bi-2223/Ag tapes were heat treated at 830 °C for different time periods. The phase assemblies of these samples suggest that current pulses contribute to faster transformation kinetics from the Bi-2212 phase to the Bi-2223 phase. In addition, a preliminary improvement of 28% of critical current density has been achieved in a fully processed tape with EPR.     

Secondary phase formation during the Bi-2223 phase calcination process

In this work, we report the secondary phase evolution that occurs during calcination of Bi-2223 precursor powder prepared by the polymer-matrix method. The effect of the calcination conditions (time, temperature, and intermediate milling) on the formation of secondary phases has been studied. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. The calcined powder phase assemblage is sensitive to the calcination conditions, and control of the calcination parameters is necessary to form the adequate reactive secondary phases (Bi-2212, Ca₂PbO₄, Ca₂CuO₃, CuO). These phases are necessary for the proper synthesis of Bi-2223 during the sintering step. The optimized calcination conditions necessary to obtain these phases are 24 h at 820 °C.     

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.     

Phase and Texture Evolution of Ag/Ni Composite-Sheathed Bi-2223 Tapes with Different Thickness

The influence of green tape thickness on the Bi-2223 phase formation and texture evolution in Ag/Ni composite-sheathed tapes fabricated by the “powder-in-tube” technique has been studied. Microstructural observations by SEM as well as critical current density (J _c) measurements at 77 K, 0 T have been performed to analyze the performance of the tapes. The results show an important influence of the green tape thickness on the critical current depending on the content and texture of Bi-2223 phase. The J _c increases with decreasing thickness. Moreover, texture measured by omega scans shows that the texture of the Bi-2223 phase is significantly influenced by the thickness of the green tape after the first and final sintering processes. Alignment of Bi-2223 grains in the thin tapes is much better. Higher performance of Ag/Ni composite-sheathed Bi-2223 tapes can be obtained by controlling the thickness of the green tapes.     

Effect of Various Intermediate Deformation Techniques on Jc and Grain Texture for Ag/Bi-2223 Tapes

Three Ag sheathed Bi-2223 multifilamentary tapes were produced by a processing method that consists of two sintering treatments with an intermediate deformation, involving sandwich rolling (SR), pressing (P), or normal rolling (NR). The magnetic field dependence of the critical current density J _c was measured with the magnetic field H applied parallel to both the ab plane (H ab) and the c-axis (H c) of the Bi-2223 grains. Experimental results show that J _c of the pressed (P) tape (J _cP) for both H ab and H c is about 1.5–1.8 times higher than that for the NR tape (J _cNR) and the SR tape, although J _cSR is always larger than J _cP. The ratio of J _cSR/J _cNR for H c increases rapidly with the applied magnetic field and reaches a maximum of about 12 at ₀ H 900 T. The calculated density of the pinning force F as a function of magnetic field shows that curves of F for SR, NR, and P tapes all have their maximum F _max at different magnetic fields and the magnitudes of F _max are also different from each other. The SR tape has the largest value of F _max, while NR has the smallest. XRD analysis shows that an intermediate deformation can destroy the grain alignment, and the larger the deformation, the worse the grain texture will be. Our experimental results, however, clearly show that J _c for Bi-2223 multifilamentary tapes is independent of grain alignment. The significant differences in J _c for tapes processed using the three different intermediate deformation procedures are dependent on the density of the pinning force and cannot be attributed to the grain alignment. Our experimental results support the view that SR processing is the best method for fabricating Ag/Bi-2223 tapes of high quality.     

The Mechanical Properties of Y-Doped Bi-2223 Superconductors

In this study the mechanical properties of Bi_1.7?x Y _x Pb_0.3Sr₂Ca₂Cu₃O _y (x=0, 0.05, 0.1, and 0.25) samples with doped Yttrium are investigated. The load-independent Vickers hardness (H ₀), Young’s modulus, and yield strength of samples are calculated. The possible reasons for the observed changes in mechanical properties are investigated by using different approaches such as the Hays–Kendall approach. These mechanical properties of the samples were found to be load and Yttrium content dependent.     

The effects of yttrium substitution in Bi-2223 superconductors

Bi_1.7?xY_xPb_0.3Sr₂Ca₂Cu₃O_y (x = 0, 0.05, 0.1 and 0.25) samples were prepared by a conventional solid state reaction method. The influence of Y on structural and superconducting properties has been investigated by XRD measurements, scanning electron microscopy (SEM), dc electrical resistivity, Vickers hardness and magnetic-hysteresis loop measurements. SEM micrographs show that the best surface morphology and largest grain size are observed for undoped samples. The critical transition temperatures of the samples decrease with the increase in the Y content. J_c values of the samples, obtained from Bean’s model, show that J_c decreases with increasing Y content. The mechanical properties of the samples, measured by Vickers hardness (H_v), were found to be load dependent.     

The Effects of Mg Substitution in Bi-2223 Superconductors

The effects of Mg substitution in Bi-2223 superconductor system has been studied for the Bi_1.7Pb_0.3Sr₂Ca₂Cu_3−x Mg _x O _y nominal composition (x=0.00, 0.05, 0.10, 0.15 and 0.20) which was prepared by the conventional solid-state reaction. The properties of these compounds have been investigated by measuring the electrical resistivity, X-ray diffraction (XRD) and density. Also, scanning electron microscopy (SEM) was employed to investigate the surface microstructure of the samples. It has been found that the effects of Mg substitution support the development of both the Bi-2212 and Bi-2223 phases. These measurements and analyses enable us to discuss the effects of Mg dopant on superconducting properties. We found that onset critical temperatures (T _c, onset) decrease with addition x>0.10 in resistivity measurements. The presence of Mg influenced the microstructure of the samples and decreased the mean grain size of Bi-2223 grains up to x=0.10.     

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.     

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.     

Characterization of the damage process in short-fibre/thermoplastic composites by acoustic emission

Fracture processes of short glass fibre-reinforced thermoplastic tensile specimens have been investigated in relation to acoustic emission (AE) characteristics. Two fibre diameters (d=10, 23 m) were adopted for this study. Frequency analysis employing band-pass filters suggested that AE with higher amplitude was due to fibre breakages. Fibre–matrix interfacial failure and matrix fracture emitted lower amplitude AE waves. Reflected and transmitted optical microscopy, in combination with scanning electron microscopy, revealed that fibre breakage ahead of the artificial notch tip led to the initiation of a macroscopic fracture. This coincided with the results that AE peak amplitudes, Vp, showed a minimum at fracture initiation. When the macroscopic fracture propagated, Vp again increased rapidly, indicating additional fibre breakage. These fracture mechanism worked consistently for both d=10 and 23 m. Based on the above findings, AE source characterization was proposed for the stable fracture process of short fibre-reinforced plastics. © 1998 Chapman & Hall     

Fractographic and acoustic emission of mullite-alumina-zirconia composites prepared by reaction sintering

The fracture behaviour of a mullite-alumina-zirconia material prepared by reaction sintering has been studied by fractography and acoustic emission during indentation tests. The acoustic emission characteristics of the mullite composite are compared with those obtained from tetragonal zirconia polycrystal and aluminium oxide tested under the same conditions.     

Characterization of the Elastic Properties of Bi2223/Ag/Ag Alloy Composite Tapes by the Generalized Self-Consistent Approach

The characterization of the elastic properties of whole Bi2223/Ag/Ag alloy composite tapes is a very important issue, even when these properties of each constituent are known. In this paper, the generalized self-consistent approach is used for predicting the elastic properties of inner core (Bi2223 filaments combined with Ag matrix), and the mixture rule for those of the whole composite tape (the inner core sheathed with Ag alloy). Compared with the simple estimation of the mixture rule, the prediction of the generalized self-consistent approach is not only physically rational, but also is much more accurate. Further, the theoretical prediction agreed well with the measured data. Hopefully, the generalized self-consistent approach can be adopted to predict the elastic properties of other superconducting composite structures.     

The Effect of Barium Doping on the Selective Structure of Bi-2223 Phase

The partial substitution of Sr by Ba in the two nominal compositions of Bi_1.8Pb_0.4Sr_2−x Ba _x Ca_2.2Cu₃O _y [x=0.0, 0.1, 0.2, and 0.3 (A group)] and Bi_1.66Pb_0.34Sr_2−x Ba _x Ca₂Cu₃O _y [x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 (B group)] have been investigated by resistivity, ac susceptibility measurements and by XRD and SEM analysis. In general, the nature of the temperature dependence of resistivity and susceptibility measurements indicate the presence of a superconducting transition between grains coupled by weak links. However, the XRD and SEM analyses show that the relative composition of initial elements used in Bi-(2223) is essential to the site that is selected by the Ba ions. In the A group, Ba doping up to x=0.1 will improve the phase formation of Bi-2223, and improve the superconductivity properties of the samples. In the B group, although Ba doping up to x=0.1 will enhance the phase formation of Bi-2223, it will decrease the coupling between the grain and the superconductivity properties of these systems. The presence of lower Tc phases will begin to appear for x>0.1, in both of these systems. The superconductivity properties and the phase formation of Bi-(2223) will decrease as the Ba concentration increases.