Processing and transport properties of high-Jc silver-clad Bi-2223 tapes and coils

The powder-in-tube process has been used to fabricate long lengths of flexible, high-J_c, silver-clad Bi-2223 HTS conductors. By improving thermomechanical processing and precursor powder preparation, we have succeeded in achieving J_c values of≥4×10⁴ A/cm² at liquid nitrogen (77K) temperature and >10⁵ A/cm² at liquid helium (4.2K) and liquid neon (27K) temperatures in short tape samples. Detailed measurements with high applied magnetic fields are reported. Several long tapes up to 10 m in length have also been fabricated and cowound into small superconducting pancake coils by the “wind-and react” approach. Transport measurements at 77 and 4.2K for these coils are also reported.     

双粒径组分YSZ陶瓷的烧结致密化研究

为了在低于1400℃的烧结温度下获得相对密度高于95%的氧化钇稳定氧化锆(YSZ)陶瓷,将粒径不同的YSZ粉体球磨混合成双粒径组分粉体,采用流延成型工艺制得生坯并分别在1 350,1 400,1 450℃下进行烧结。研究了双粒径粉体的组分对所制YSZ陶瓷性能的影响。结果表明:粗细粉组成的双粒径粉体试样烧结密度较之单一原料粉体有所提高,微米级(中径1.46μm)粉体与造粒后的纳米级(中径90 nm)粉体进行级配(质量比7∶3)后在1 350℃烧结所制的YSZ陶瓷相对密度达到97%。     

Aspects of the current understanding of the supercurrent transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed Tapes—the “railway-switch” model

The “railway-switch” model describes the superconducting current transport in (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes under the basic assumption that small-angle c axis tilt grain boundaries (“railway-switch”) constitute strong intergrain links for the supercurrent in the textured filament [B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, and R. Flükiger,Physica C 205, 329 (1993)]. We give an overview of the model and some recent experimental results with the objective to identify the mechanisms that limit the critical current density. The measurements have been performed on monofilamentary “powderin-tube” samples [J_c(T < 77K, B < 0 T) < 20..30 kA/cm²] that were prepared in long lengths by rolling as the only tape-forming process. We conclude that the low intragrain critical current density j_c ^c along the c axis (or the even lower critical current density j_c ^t across twist boundaries or intergrowths) is the dominant limitation for the transport critical current in high-quality tapes. We discuss possible starting points for a performance improvement of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes for applications. On leave from Consorzio INFM, Universitá di Genova, Italy.     

Crystallization and stability of the Bi2Sr2CaCu2Ox superconductor

A basic study of the stability of the Bi₂Sr₂CaCu₂O_x phase has been carried out in order to identify the composition and processing conditions for optimum superconducting properties. Analytical electron microscopy has been used to follow the crystallization of this phase during annealing in the solid state as well as from the melt. The crystal chemistry and phase purity is found to depend strongly on the processing conditions. Significant differences in crystallization behavior may be related to kinetic limitations of oxygen transport. The Bi₂Sr₂CaCu₂O_x phase has also been subjected to thermomechanical processing in order to improve our understanding of the deformation processes involved during fabrication of wires and tapes. It has been found that mechanical deformation can have significant effects on the microstructure of the material which in turn result in changes in superconducting properties.     

Influences of the precursor powder on the microstructure of HTSC-tapes

Wires and tapes are the most promising application of high temperature superconductor material for the future. For their preparation both the doctor-blade process (DBP) and the powder-in-tube((PIT) method with silver as a matrix material are mainly used. To obtain good electrical properties, the preparation had to start from a precursor material instead of fully reacted, phase pure superconducting powder. Tapes of Bi(Pb)−Sr−Ca−Cu−O 2212 and 2223 were fabricated by the DBP or PIT methods, respectively. We compared the suitability of precalcined precursor powders prepared by different routes and investigated their microstructure in the reacted wire or tape. Homogeneity, phase purity, and carbon content were investigated. The precursor powder prepared by a modified coprecipitation exhibited improved properties in comparison to other routes.     

Recent status on high temperature superconducting Bi2Sr2CaCu2O8+x wire development at NYSIS: 1-90 meter length Jcs and 3 meter diameter ring furnace design

The status of long length, Bi₂Sr₂CaCu₂O_8+x (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport J_cs (4.2K, 0 T) of Bi-2212/Ag oxide powder-in-tube singlefilamentary tapes have reached 70,000-80,000, 50,000-60,000, and 30,000–40,000 A/cm² for 1, 4–15, and 40–90 meter length tapes, respectively. The decrease in J_c as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of J_c to temperature nonuniformities (±3‡C) in the box-type furnace used for annealing. To reduce this problem, a ringtype high-temperature furnace (∼3 meter diameter) was designed and constructed which provides a large-volume (∼13^w × 10^h × 1000¹ cm) processing zone with expected excellent temperature uniformity (±0.5‡C). The advantages of the ring-type furnace for processing of kilometer-length conductors are described.     

Development of needle-shaped grains from Bi2Sr2CaCu2O8+x with high critical current density suitable for making superconducting wires

We present a grain microstructure for Bi(2212) consisting of only giant needle-shaped grains of around 1.5 mm length and 100 μm diameter. We study the structural and chemical changes suffered by a conventional ceramic Bi(2212) sample in the course of the thermal treatment used to obtain those giant needle-shaped grains. For that, different samples of the same batch were treated with incomplete thermal treatments, and the resulting samples were analysed by using scanning electron microscopy (SEM), optical microscopy, energy dispersed spectroscopy (EDS), inductively coupled plasma (ICP) and X-ray diffraction (XRD). To verify the superconducting nature of the needle-shaped grains, we have performed magnetization, resistivity, and critical current measurements on the original ceramic sample, and on that formed as giant needle-like grains. The critical temperature of these last grains is nearly the same as that of the ceramic sample (T_c∼90 K), which is a high value for the Bi(2212) compound. The critical current density (J_c) of the needle-shaped grains is around 2500 A/cm² at 77 K and in absence of applied magnetic field, a value comparable with that presented for the best wires and thick films. Not only are the shape and the size of these grains very suitable for making superconducting wires, but also the superconducting properties, T_c and J_c, are both high enough to be confident about the possibility of improving the actual Bi(2212) superconducting wires for high current applications.     

Chemical synthesis and charge transport mechanism in solution processed flexible polypyrrole films

Polypyrrole (PPy) has been synthesized by a chemical oxidation method using ammonium persulfate to obtain a solution processable PPy powder. The resultant PPy powder was then solution processed to deposit flexible thin films of PPy over flexible substrates. PPy film samples were then characterized using UV–vis spectroscopy, FTIR spectroscopy and X-ray diffraction. It was found that conductivity of PPy (σ=2.4×10⁻² S/cm) reduces by an order of magnitude after solution processing in the form of films. The temperature dependent conductivity of PPy pellet and flexible films of PPy were measured in the temperature range of 80–300 K. It was observed that PPy films show stronger temperature dependence than pelletized samples. Charge transport in PPy samples has been investigated using Kivelson׳s and Mott׳s variable range hopping models. Mott׳s parameters such as density of states at the Fermi level N(E_F), average hopping distance (R) and average hopping energy (W) have been estimated for PPy samples. The results showed that at room temperature average hopping distance for PPy film was about 22.3 Å and average hopping energy was 128.6 meV.     

Effects of Sm<Superscript>3+</Superscript> Doping on Dielectric Properties of Anatase TiO<Subscript>2</Subscript> Nanoparticles Synthesized by a Low-Temperature Hydrothermal Method

Samarium-doped thermally stable TiO₂ nanoparticles in the anatase phase have been synthesized by a low-temperature hydrothermal method. The formation of the anatase phase has been investigated by x-ray diffraction. Thermogravimetry and differential thermal analysis have been used for thermal studies. The morphology and composition of synthesized powders have been studied using scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectroscopy. Surface areas were studied by the Brunauer–Emmett–Teller method. Dielectric properties were studied for dopant levels of 0.2 mol% and 0.5 mol% at 300 K in the frequency range of 42 Hz to 5 MHz. At low frequency, charge carriers at the grain boundary produce interfacial polarization giving rise to a high dielectric constant (ε′), which is significantly reduced by doping with samarium ions (Sm³⁺). Strong frequency dependence of the dielectric loss was also observed for each concentration. Conductivity studies showed that the reduction in conductivity is due to the decrease in particle size with the increase in Sm³⁺ dopant level.     

Quantitative Magneto-Optical Imaging for Superconducting Thick Films and Tapes

Practical superconducting thick films and tapes, manufactured in an industrial process, have microscopic inhomogeneities. Quantitative magneto-optical imaging (MOI) is one of the most desirable techniques and provides both local and global information on defects, flux pinning, critical current density J _c, and current distribution. We present, herein, a comparative magneto-optical imaging study of the flux profile in YBa₂Cu₃O_7−δ (YBCO) thick films prepared in two different processes, the laboratory versus the industrial scalable process. The remarkably different flux propagation patterns are the manifestation of their different defect landscapes within these films. A method of determining J _c near zero applied field is also given for locally nonuniform superconducting films, using the quantitative MOI technique.     

Internal strain and dislocations in Ga1−xAs crystals grown by liquid phase epitaxy/electroepitaxy

Strain relaxed, low dislocation density In_xGa_1−xAs crystals, 0 < x <0.2, have been successfully grown by liquid phase electroepitaxy on the GaAs substrate, despite the crystal/substrate lattice mismatch. Residual strain in these novel substrates is below 10⁻⁴, at least an order of magnitude lower than in the molecular beam epitaxially (MBE) or metalorganic chemical vapor deposition-grown ternary buffer layers of similar composition. Threading dislocation density induced by both the crystal/substrate lattice mismatch and unavoidable composition variations has been reduced from the low 10⁶ cm⁻² range, while growing directly on GaAs, to the mid 10⁴ cm^-2 by employing both the MBE grown ternary buffer layer and selective lateral overgrowth of an SiO₂ mask which, prior to the crystal growth, was deposited on the buffer layer and patterned by photolithography with 10 μm wide, oxide free seeding windows. The full width at half maximum of the rocking curves measured for In_xGa_1−xAs crystals grown by liquid phase epitaxy/electroepitaxy on patterned, closely lattice matched buffer layers was in the 20–23 arc s range. Further reduction of the dislocation density and a more uniform dislocation distribution is expected by modifying the initial growth conditions, improving substrate preparation, and optimizing the seeding window geometry.     

Comparison of spatial compositional uniformity and dislocation density for organometallic vapor phase epitaxial Hg1−x CdxTe grown by the direct alloy and interdiffused growth processes

Etch pit density and spatial compositional uniformity data are presented for organometallic vapor phase epitaxial Hg_1−x Cd_x Te grown by the direct alloy and interdiffused growth methods. For alloy growth, composition variation is as low as Δx=0.004 and 0.02 over 2- and 3-in diam areas, respectively; while for growth on CdZnTe substrates, etch pit density values lower than 2×10⁵ cm⁻² have been achieved. For interdiffused growth on CdZnTe, etch pit density values lower than 5×10⁵ cm⁻² have been obtained, while the composition variation is usually Δx≤0.004 and 0.014 over 2- and 3-in diam areas, respectively. Data demonstrate that the choice of particular CdZnTe substrate strongly affects the subsequent etch pit density measured in the layer. Reasonably uniform n-type doping over 3-in diam area using the source triethylgallium is also reported for both growth methods.     

Material properties and RF applications of high k and ferrite LTCC ceramics

The continuous trend in modern electronic applications towards smaller size, higher integration density and enhanced functionality requires new materials, which allow embedding passive functions into the substrate. In this paper an LTCC material system with specialized dielectric and magnetic LTCC tapes cofireable in a low-shrinkage process for RF-passive integration is reported.An LTCC dielectric with a dielectric constant of up to 80 is presented. The material is successfully used in a cofired multi-material stack to realize a fully integrated band-pass filter for Bluetooth applications of 1.3 mm³ volume.A ferroelectric LTCC ceramic with a maximum dielectric constant of 3000 is presented and the reduction of the dielectric constant to a maximum value of 1100 under constrained-sintering is discussed.Magnetic permeabilities of 14 for a NiZnCo-ferrite and 3.5 for a Ba₃Co₂Fe₂₄O₄₁ (Co₂Z) ferrite have been realized under LTCC processing conditions, with gyromagnetic resonance frequencies above 1 GHz and 3 GHz respectively. The permeability of these materials is determined for constrained and unconstrained sintering conditions. A maximum absorption of 27 dB/cm and 30 dB/cm is measured for an embedded stripline in NiZnCo- and Co₂Z ferrite respectively. Two-winding planar RF-chokes in different multi-layer stacks are compared. A maximum of 14.3 nH inductance is realized for a 1.8 mm × 2 mm coil in an LTCC research pilot line.     

Solution-Processed High-<Emphasis Type="Italic">k</Emphasis> Dielectric,ZrO<Subscript>2</Subscript>, and Integration in Thin-Film Transistors

We report a sol–gel method to deposit a high-k dielectric, zirconium oxide (ZrO₂). This solution-based approach has advantages of easy processing and low fabrication cost. Effects of annealing temperatures on dielectric properties, such as tunneling current density and capacitance density, are reported. Morphological and chemical characterizations suggest that the process temperature can be kept at or below 300°C. We have employed the solution-processed ZrO₂ dielectric in a zinc tin oxide thin-film transistor. Saturation mobility of 4.0 cm²/V s at operating voltage of 2 V has been observed. The measured subthreshold swing is 74 mV/decade, which is the result of the combination of an electronically clean dielectric/semiconductor interface and high insulator capacitance.     

Investigation of aluminum and gallium co-doped ZnO powders and their effects on the properties of targets

Aluminum and gallium co-doped ZnO (AGZ) powders were synthesized by the chemical co-precipitation method with the same Al doping concentration (3 at%) and different Ga doping concentrations (0–0.5 at%). The microstructure, lattice distortion and surface morphology of AGZ powders were investigated because the properties of targets were related to corresponding powders. Both AZO and AGZ targets were prepared by molding and atmospheric pressure sintering. The microstructure, morphologies, electrical properties and densification of sintered targets were also studied. The measured results showed that the grain sizes of AGZ powders are smaller than AZO powder, the former has a larger specific area, and the distribution of AGZ particles are more homogeneous, which are good for preparation of a high-density target. Besides, the extent of ZnO lattice distortion exhibits a downward trend with the increase of the Ga doping concentration. The AGZ target with appropriate concentration of Ga (0.3 at%) has the lowest resistivity of 2.518×10⁻³ Ω cm and the highest relative density of 99.2%. In general, the moderate Al–Ga co-doping proportion leads to finer grain size, lower resistivity, higher Hall mobility and higher sintered density.     

High-temperature superconducting magnetic shields formed by deepdrawing

A new method for the construction of high-temperature superconducting magnetic shielding structures has been demonstrated. With this process, a ceramic laminate of high-temperature superconducting powder and silver metal sheets is formed and then shaped into a cylindrical magnetic shield by deep drawing before being sintered. Two types of superconducting powders were used in this experiment, YBa₂Cu₃O₇ and Bi_1.8 Pb_0.4Sr₂Ca₂Cu₃O_x , which exhibited shielding factors of 1100 and 330, respectively     

Sandwich Roiling,Jc, and pinning energy in Ag-sheathed BPSCCO superconducting tapes

A “sandwich rolling” process was developed to prevent the formation of sausaging and cracks in the longitudinal direction. The stress-strain state of the tape in “sandwich” rolling is the same as that of uniaxial pressed tape because the deformation of steel sheets is negligible in comparison to that of Ag-clad tape. Critical current densities of 3.2 × 10⁴ A/cm² at 77K and 2.7 × 105 A/cm² at 4.2K and zero field Ag-sheathed Bi-based 2212 tapes have been achieved using a melt and atmosphere-controlled process. The comparison of pinning potential U₀(B) < U(T-0, B) for Bi-2212 tape and Bi-2223 tapes consisting of a different fraction of 2212 phase as well as Bi-2212 and Bi-2223 thin films shows that for the same fields, the U_o for good quality 2223 tapes is at least 1.3 times that for the best 2212 tape and epitaxial thin films after taking into account the difference of the T_c between 2223 tape and 2212 tape, indicating that in BSCCO compounds, in addition to anisotropy, the specific pinning centers such as dislocations, introduced during processing, affect the flux motion at lower B.     

Synthesis of Nanometric-Sized Barium Titanate Powders Using Acetylacetone as the Chelating Agent in a Sol-Precipitation Process

Nanometric-sized barium titanate powders were prepared by using titanium isopropoxid as the raw material and acetylacetone as a chelating agent, in a strong alkaline solution (pH > 13) through the sol-precipitation method. The preparatory variables affect the extent of cross-linking in the structure, change the mode of condensation of the gels, and even control the particle size of the powder. The reaction rate of forming powder, at a higher temperature such as 100°C and more water content (the molar ratio of water to titanium isopropoxide is 25) or fewer acetylacetone (the molar ratio of acetylacetone to titanium isopropoxide is 1), is rapid and the particle size formed is finer at 60–80 nm. On the contrary, that of forming powder, at lower temperature (40°C) and less water content (molar ratio of water/titanium isopropoxide = 5) or higher acetylacetone (acetylacetone/titanium isopropoxide = 7), is slow and the particle size of the powder is larger. The optimal preparatory conditions were obtained by using the experimental statistical method; as a result, nanometric-sized BaTiO₃ powder with an average particle size of about 50 nm was prepared.     

Growth of device quality GaAs by chemical beam epitaxy

The growth of high purity GaAS with excellent uniformity and very low defect density by chemical beam epitaxy using triethylgallium and arsine is described. The residual background impurity is mostly carbon. A mobility of 518 cm²/Vs with a hole density of 3.6 x 10¹⁴ cm⁻³ has been obtained for a growth temperature of 500° C. The electrical quality is further evaluated by fabricating a Si doped epilayer into MESFET device using 1 μm gate length. A transconductance of 177 mS/mm has been measured. The results indicate that chemical beam epitaxy is a very attractive growth technique for GaAs integrated circuits.     

Explosive Consolidation of (Bi,Pb)-Sr-Ca-Cu-0 Superconductor Powders

Superconducting (2212 phase) Bi-Sr-Ca-Cu-0 (BSCCO) and BSCCO-Ag composites were explosively consolidated in silver tubing and then drawn and rolled into tapes. These silver-sheathed tapes were then subjected to repeated cycles of pressing and heat treatment, which resulted in enhanced texturing and grain growth and a subsequent increase in critical current density (J_c). The effect of Ag powder additions to the superconducting powder further increased texturing and J_c, with optimal properties occurring with 10 vol.% silver powder additions. Scanning electron microscopy indicated that grain alignment increased and grain structure became refined after the thermomechanical treatment. X-ray diffraction studies indicated that grain orientation and conversion of 2212 phase to 2223 phase are improved when explosive consolidation is introduced before the drawing step in the powder-in-tube process.