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.     

On the metal and oxygen content determination in Bi-2212 and Bi-2223 HTSCs

The oxygen content in Bi₂Sr₂Ca₁Cu₂O_8+δ and Bi₂Sr₂Cu₃O_10+δ superconductor samples is determined using a iodometric titration technique and using a thermogravimetric reduction technique. The reliability of the iodometric oxygen determination was exemplified by the reproducibility of at least 0.01 in δ and the excellent agreement between results obtained separately from iodometry and thermal analysis on a series of samples. Compared to similar determinations in YBa₂Cu₃O_7-δ superconductors, the determination of the metal content is an extra analytical requirement in the case of Bi and Pb containing ceramic materials. For the determination of the metal content, a reproducible and rapid method, based on differential pulse polarography, was optimized. Using EDTA-HCl as supporting electrolyte, we were able to determine the Pb, Bi and Cu content in a single experiment with a relative standard deviation (RSD) < 1 %. Using the techniques mentioned above, we were able to investigate the relationship between T_c and δ in a series of Bi-2212 and Bi-2223 HTSC's. There appears to be a maximum in the T_cvs δ curves for both phases.     

Bi-2223/Ag-sheathed tape processing studies

The production of high critical current density J_c Bi-2223/Ag sheathed conductors is a complex process involving interactions among many different parameters. The effects of three factors: 1) powder production path, 2) the first sinter temperature, and 3) the subsequent sinter temperatures were investigated. Statistical methods were used to design the experiment and interpret the results. Transport J_c was the main response for the analysis, but microstructural results were also used to assess the physical basis for the differences in performance. The powder variable had the largest main effect with only very weak main effects for the other factors.     

Relationship between grain structures and texture of damascene Cu lines

The anisotropic spread of the central peak in a (111) pole figure by x-ray diffraction (XRD) was observed for damascene Cu lines of 0.18–2 μm in width and 0.5 μm in depth. The spread originates from the existence of slightly tilted (111) grains because of inclined sidewalls. The tilted (111) orientation is favorable only for polygranular clusters whose sidewall energies can be minimized simultaneously. Consequently, bamboo grains have an exact 〈111〉 orientation, while the polygranular clusters have a tilted 〈111〉 orientation. Using this concept, the volume fraction of the bamboo grains and polygranular clusters in the damascene Cu lines were quantified using the XRD pole plots.     

Texture and microstructure of thin copper films

Microstructure is an important factor influencing the reliability of thin film interconnects. The microstructure of copper films is of particular interest because of its use in numerous electronic applications. Pole figure x-ray diffraction and transmission electron microcopy were conducted on copper films deposited by several techniques: sputtering, partially ionized beam deposition, chemical vapor deposition, evaporation, and electroplating. Quantitative texture data are determined from fiber texture plots. A typical copper film consists of three texture components: (111), (200), and random. (220) and (511) texture components are possible under some deposition conditions. Compared to aluminum films, the fraction of the random texture component and the distribution of the (hkl) components in copper films are relatively large. Bimodal grain size distributions are observed in some films.     

Current-voltage characteristics and transport current distributions in Ag/Bi-2223 monocore tapes

Current-voltage characteristics of Ag-sheathed Bi-2223 monocore tapes have been measured at 77 K in low d.c. magnetic fields. Results have been obtained with the c-axis of the sample both parallel and perpendicular to the applied field, together with the force-free orientation. The current–voltage characteristics have been analysed in terms of their n-value (V∝Iⁿ), which characterizes the sharpness of the resistive transition and the second differential (d²V/dI²), which shows the distribution in local critical current density values. The second differential has been fitted and analysed using the phenomenological Weibull distribution function, which is based on a weak-link model. The four parameters of this function have been used to investigate the microstructural nature of the sample, in particular, the factors influencing the critical current density and the transport current pathways. The results are consistent with the view that at low fields the critical current density is limited by weak-link grain boundaries and at high fields by intra-granular flux pinning.     

Effect of lead content on Bi-2223 formation by a two-powder process

Decreasing the total lead content from Pb_0.4 to Pb_0.3 significantly stunts Bi-2223 phase development and greatly reduces the critical current density (J_c) of powder-in-tube tapes made by a two-powder process. This effect can be explained on the basis of the solubility limit for lead in 2212. Pb_0.4 samples exceed the solubility limit for lead in 2212, so lead is rejected to create a lead-rich liquid that enhances the kinetics of 2223 formation during heat treatment. By contrast, a liquid does not form in the Pb_0.3 sample because its lead content is below the solubility limit. As a result, 2223 formation is much slower and J_c is much lower in the Pb_0.3 sample.     

Self-field and geometry effects in transport current applications of multifilamentary Bi-2223/Ag conductors

This paper describes quantitatively the influence of the self-field and the cross-sectional geometry on the effective critical current and the ac losses in transport current applications of nontwisted multifilamentary Bi-2223/Ag conductors. The results are obtained with finite-element method simulations. The numerical implementation includes an anisotropic model for the dependence of the critical current density J/sub c/ and the power index n on the local parallel and perpendicular magnetic field components. The relation is given between the intrinsic critical current density and the effective critical current for different multifilamentary conductors. Shown are examples of the current and magnetic flux density distributions in order to demonstrate their effect on the ac losses in self-field.     

Fabrication and characteristics of tapes and test magnets made from Ag-Clad Bi-2223 superconductors

Pb_0.4Bi_1.8Sr₂Ca_2.2Cu₃O_x (Bi-2223) precursor powder was prepared by a solid-state reaction of carbonates and oxides of lead, bismuth, strontium, calcium, and copper, and the powder was then used to fabricate silver-clad tapes by the powder-in-tube technique. Transport critical current density (J_c) values>4×10⁴ A/cm² at 77K and 2×10⁵ A/cm² at 4.2 and 27K have been achieved in short tape samples. Long lengths of tape were tested by winding them into pancake coils. Recently, we fabricated a test magnet by stacking ten pancake coils, each containing three 16m lengths of rolled tape, and tested it at 4.2, 27 and 77K. A maximum generated field of 2.6 T was measured in zero applied field at 4.2K and the test magnet generated significant self-field in background fields up to 20 T. The results are discussed in this paper.     

High-resolution X-ray diffraction studies of molecular beam epitaxy-grown HgCdTe heterostructures and CdZnTe substrates

Lattice mismatch between substrates and epitaxial layers of different molefractions can create a variety of distortions and defects in Hg_(1−x)Cd_(x)Te epilayers, thus degrading the performance of infrared detectors fabricated from this material. X-ray diffraction is a sensitive nondestructive technique, which allows in-depth characterization of the crystal lattice prior to detector fabrication. We present results of triple-axis diffractometry performed on single- and double-layer HgCdTe films grown on (211)B CdZnTe substrates by molecular beam epitaxy (MBE). In this study, both the ω and 2θ diffraction angles have been recorded absolutely so that the diffraction peaks in the RSMs can be positioned directly in reciprocal space, without requiring reference to a substrate peak. The positions of both surface-symmetric and asymmetric diffraction peaks have been used to extract lattice spacings parallel and perpendicular to the (211) growth direction. The relaxed lattice parameter of each epilayer has been calculated assuming that the layers are elastically strained. The low symmetry of the (211) growth direction, coupled with the anisotropic elasticity of zinc-blende semiconductors, results in monoclinic distortion of the lattice, as observed in these samples. In double-layer samples, the mosaicity of both layers is greater than that observed in single epilayers. Layers subjected to a Hg-saturated anneal show greater lattice distortion than as-grown samples.     

The role of lattice mismatch in the deposition of CdTe thin films

Cadmium telluride (CdTe) is the most well-established II–VI compound largely due to its use as a photonic material. Existing applications, as well as those under consideration, are demanding increasingly stringent control of the material properties. The deposition of high-quality thin films is of utmost importance to such applications. In this regard, we present a report detailing the role of lattice mismatch in determining the film quality. Thin films were deposited on a wide variety of substrate materials using the pulsed laser deposition (PLD) technique. Common to all substrates was the strong tendency toward the preferential alignment of CdTe’s (111) planes parallel to the substrate’s surface. X-ray diffraction analysis, however, revealed that the crystalline quality varied dramatically depending upon the substrate used with the best results yielding a single-crystal film. This tendency also manifested itself in the surface morphology with higher structural perfection yielding smoother surfaces. The film quality showed a strong correlation with lattice mismatch. Texture analysis using the [111] pole figure confirmed that improvements in the lattice mismatch led to a higher degree of in-plane alignment of the (111) grains.     

Crystallographic texture of C54 titanium disilicide as a function of deep submicron structure geometry

Detailed analysis of the crystallographic texture of C54 TiSi₂ was performed and showed a strong correlation between the geometry of the silicide structures and preferential crystallographic orientation. The study was undertaken on blanket and patterned TiSi₂ films formed in the reaction between 32 nm of Ti and undoped polycrystalline silicon using both in situ x-ray diffraction during heating and post-anneal four-circle pole figure reflection geometry measurements. Full pole figures were taken to determine the distribution of C54 TiSi₂ grain orientations in narrow (0.2 to 1.1 μm) lines which was compared with similar results obtained from unpatterned (blanket) films. While in blanket films we found the presence of weak <311> C54 TiSi₂ crystallographic orientation perpendicular to the sample surface, the <040> preferential orientation dominated in patterned submicron line structures and increased with decreasing linewidth. Using pole figure analysis, we observed strong <040> fiber texture in narrow lines with a slight variation in the tilt of the (040) planes normal in the direction perpendicular to the line (full width at half maximum [FWHM] ≈6°), but very little along the length of the line (FWHM ≈2°). In addition, a preferred in-plane (azimuthal) orientation of <040> crystals was found which showed that most of the <040> grains had their (004) plane normals oriented parallel with the line direction. These findings support a model of the C49 to C54 TiSi₂ transformation involving rapid growth of certain orientations favored by the one-dimensional geometry imposed by narrow lines.     

冷轧铝板微织构演变的原位加热电子背散射衍射分析

采用原位加热电子背散射衍射(EBSD)技术,对冷轧铝板在回复和再结晶转变过程中的微织构演变规律进行了动态研究。轧制态铝板以铜型织构{112}〈111〉为主。再结晶转变后,以立方织构{001}〈100〉和R织构{124}〈211〉为主。在回复和再结晶过程中,{001}〈100〉晶粒和{124}〈211〉晶粒优先在形变带边缘形成和长大。晶粒长大通过晶界迁移并吞噬周围晶粒而得以发展。本研究对于理解轧制铝板的再结晶过程和机制,并有效控制轧制铝板的织构具有重要的参考价值。     

Fabrication of long length Bi-2223 superconductor tape using continuous electrophoretic deposition on round and flat substrates

We have developed a continuous fabrication process for producing long lengths of Bi-2223 superconductor tapes. The process involves sequentially electrophoretically depositing and sintering superconductor and then silver layers on a substrate, followed by rolling and thermal processing. Both round and flat silver substrates have been used. Bi-2223 tapes made using flat silver substrates require only a few processing steps. Transport critical current densities at 77K in zero applied magnetic field exceeding 20000 A/cm² have been obtained.     

X-Ray diffraction observation of surface damage in chemical-mechanical polished gallium arsenide

Two novel x-ray diffraction techniques with enhanced surface sensitivity, grazing incidence x-ray diffraction (GIXD) and inclined Bragg plane x-ray diffraction (IBXD), have been used to study surface damage in gallium arsenide (GaAs) due to bromine/methanol (Br₂/MeOH) chemical mechanical (CM) polishing. A factorial design was implemented to determine the effects of four polishing variables on the surface structure of GaAs. Precise lattice parameter measurements were made in both the surface regions using GIXD and deeper into subsurface regions using IBXD after the various CM polishing treatments. Bromine concentration was found to primarily affect the surface lattice parameter, while the total polish time influenced both the surface and subsurface lattice parameters in GaAs samples that were heavily damaged prior to CM polishing. The combined effect of polishing pad rotation speed and the force exerted on the sample was found to have a much greater effect on the surface lattice parameter than either variable had alone.     

The growth and characterization of GaN on sapphire and silicon

MOCVD (metalorganic chemical vapor deposition) of GaN on both silicon and sapphire substrates was studied over the temperature range of 370 to 1050° C. The crystallinity and surface morphology of the films varied with the deposition temperatures. By first depositing an AlN buffer layer, the crystallinity of GaN was improved for low temperature depositions, but little improvement in the surface morphology was observed. On sapphire (0001) substrates, epitaxial layers were produced at a deposition temperature as low as 500° C. With silicon substrates, polycrystalline films were produced which were randomly oriented on the (111) plane and highly oriented on the (100) plane. The surfaces of the films were smooth and specular at low deposition temperatures, but degraded at higher temperatures. The energy band gaps of these films are in the vicinity of 3.4 eV, close to where they are expected. Elemental analysis by Auger electron spectroscopy (AES) showed the films to be stoichiometric with low residual impurity concentrations.     

Interrelations between defects in the Hg1−xCdxTe epilayers and their measured lattice parameters and composition

Absolute values of lattice parameters in Hg_1−xCd_xTe epilayers were precisely measured by high-resolution x-ray diffraction (Bond method), and then compared with those calculated using Cd contents, x, which were derived from Fourier transform infrared transmission spectra. A part of the samples revealed significant discrepancies between measured and calculated lattice parameters, the differences being both of positive and negative signs. The obtained results are discussed in the framework of clusters of the point defects that were visualized by high-resolution scanning electron microscopy.     

The relationship between lattice matching and crosshatch in liquid phase epitaxy HgCdTe on CdZnTe substrates

X-ray topography provides a very sensitive map of lattice mismatch between a HgCdTe LPE epitaxial layer and its (111) CdZnTe substrate. A well-defined Crosshatch pattern in the three «110» directions indicates a positive room-temperature lattice mismatch. For conditions of near-perfect lattice matching (±0.003% mismatch), the Crosshatch pattern disappears, presumably because there are few or no misfit dislocations present near the interface, and a region free of topographic contrast is observed. The crosshatch-free region occurs for a small positive room-temperature mismatch (about 0.02%); this is attributed to differences in the lattice matching condition at room temperature and the growth temperature. For negative mismatches, where the film is in tension, a mosaic pattern, rather than a crystallographically oriented Crosshatch, is observed in the topograph. Rocking curve full width at half maximum of the epitaxial layer is minimized in the crosshatch-free zone at a value nearly equal to that of the substrate. Etch pit density of the HgCdTe layer shows a strong minimum for perfect room temperature lattice matching, with values as low as 1 x 10⁴ cm^?2. For nearly lattice matched layers, Crosshatch is present throughout the thickness of the epitaxial layer except for a narrow graded-composition region near the substrate interface. Crosshatch contrast appears to result from long-range strain fields associated with a misfit dislocation network near the substrate interface. Spatial variations in topographic features and mismatch across relatively small lateral distances are caused by variations in substrate alloy composition. For truly lattice-matched substrates, better control over the substrate lattice parameter is required.     

The structural changes in CdS-CdTe thin films due to annealing

Thin film solar cells based upon CdS-CdTe heterojunctions have become an important alternative to silicon based devices. The film structures formed during fabrication are critical to cell efficiency and thus their study is fundamental to improving device performance. We have used synchrotron x-ray diffraction to investigate the effect of a post deposition anneal upon the film structures and, in particular, have examined the dynamic formation of intermixed regions adjacent to the original, metallurgical interface. Our results have enabled us to produce a dynamic model for the structural changes which includes the extent of interdiffusion. We show that, for a 400 nm CdTe film in the presence of chlorine, the original CdS and CdTe layers are completely transformed into layers with average compositions CdS_0.93Te_0.07 and CdTe_0.94S_0.06, respectively. We present evidence that the interdiffusion occurs during or following a recrystallization and that, to a limited extent, these changes also occur without chlorine.     

Substrate orientation and processing effects on GaAs/Si misorientation in GaAs-on-Si grown by MBE

The angular misorientation of GaAs epitaxial layers grown on silicon substrates by molecular beam epitaxy has been measured using x-ray diffraction. A significant misorientation, or tilt, between the epitaxial layer and the substrate has been observed. The magnitude of the tilt depends on the initial substrate orientation, the silicon substrate type (float zone or Czochralski), postgrowth annealing, and epitaxial layer thickness. In almost all cases, the sense of the tilt is such that the GaAs 〈001〉 lies between the surface normal and the silicon 〈001〉. While the presence of interfacial dislocations with Burgers vectors that are approximately parallel to the heterointerface does predict a tilt between the substrate and the epitaxial layer, the sense of the tilt that arises from these dislocations is opposite to that observed experimentally. A model, based on the relief of misfit by dislocations inclined approximately 45 degrees to the interface, is proposed that correctly describes the observed tilt.