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There is a synergy between welding and additive manufacturing with reference to spatial and temporal variations of heat transfer. In this research, in-situ measurements of heat transfer conditions ...

Additive manufacturing (AM) of functional alloys has become a promising area of research for the development of novel devices with complex geometric features that cannot be manufactured using conventional methods. In this work, we investigate the additive manufacturing of Fe 3Si and Fe 6Si benchtop scale transformer cores. A novel design inspired by a Hilbert curve was developed to exploit the geometric complexity of AM, and cores of each alloy were successfully printed, heat treated, machined, pickled, and assembled. The microstructure and magnetic performance of the cores were characterized and compared to additively manufactured components with simpler square cross-sections as well as to conventionally laminated non-oriented electrical steel sheet. The AM cores showed performance roughly comparable or better than the conventional non-oriented sheet, but higher power losses than Goss oriented steel. The increased Si content of the Fe 6Si alloy resulted in a significant reduction in core losses. The transformer cores had higher losses than the similarly manufactured simple cross-sections, which was attributed to defects in fabrication and assembly that resulted in air gaps between the transformer legs. The performance was also rationalized relative to nanoscale carbide and oxide inclusions.

Additive manufacturing (AM) is being considered as a primary manufacturing process for heat pipes. The Savannah River National Laboratory is researching AM to fabricate a hydrogen isotope separation unit for a thermal cycling absorption process that cycles from cryogenic to moderate temperatures and is a pressure boundary. AM is being explored as a replacement technology to improve heat transfer. Simple test samples with three internal geometries were designed and built from type 316L stainless steel using the powder bed fusion laser process. Nine test article geometries were prepared and subjected to tensile and burst testing and were interrogated using x-ray computed tomography and metallography. The parameters selected for processing the tubes produced a consistent product with acceptable tensile properties and microstructures; however, the process parameters used did not produce full density and minor modifications are needed to achieve full density.

In laser powder bed additive manufacturing processes, feedstock materials are often recycled after each build. Currently, a knowledge gap exists regarding powder reuse effects on powder size distribution, morphology, and chemistry as a function of part geometry and processing conditions. It was found during selective laser melting (SLM) of 316 stainless steel that a significant amount of (0.100 wt pct) oxygen pickup can occur in molten material (spatter) ejected from the powder bed surface. This value was significantly larger than the oxygen content of the as-received powder feedstock (0.033 wt pct). Furthermore, the powders in the heat-affected-zone regions, adjacent to molten pool, also exhibit oxygen pickup (≥ 0.043 wt pct). The oxygen content in unmelted 316L powder was found to vary as a function of its spatial position in the powder bed, relative to the heat source. Interestingly, the volume of melted material (i.e., thin vs thick walls) did not correlate well with the extent of oxygen pickup. Possible mechanisms for oxygen pickup in the powder during SLM, such as adsorption and breakdown of water, oxygen solubility, spatter re-introduction, and solid-state oxide growth, are discussed.

Components made by laser powder bed fusion (L-PBF) additive processes require extensive trial and error optimization to minimize defects and arrive at targeted microstructure and properties. In this work, in situ infrared thermography and ex situ surface roughness measurements were explored as methodologies to ensure Inconel® 718-part quality. For a given laser energy of 200 Watts, prismatic samples were produced with different exposure times (80 to 110 µs) and point spacings (80 to 110 µm). The infrared intensities from laser–material interaction zones were measured spatially and temporally. The conditions leading to higher IR intensity and lowest surface roughness values correlated well with less porosity and coarse solidification grain structure. The transition from highly columnar to misoriented growth is attributed to changes in thermal gradients and liquid–solid interface velocities. Hardness measurements and electron microscopy of the as-processed and post-processed heat-treated samples show complex transitions in microstructural states including the heavily dislocated FCC matrix, reduction of dislocation density, and copious precipitation, respectively. These results show that the geometry-process-structure-property correlations are dynamic, and they cascade depending on the transitions of phase states from powder to liquid to solid, as well as phase decompositions and deformations within the solid FCC phase. Validity of using analytical weld process models to describe the above phenomena is also highlighted.

This research paper evaluated three pathways for qualification of 316 L stainless steel components made by laser powder bed fusion additive manufacturing (AM). Comprehensive and consistent process flows with computational modeling, in-situ measurements, ex-situ characterization and mechanical testing with simple- and complex- geometries were explored. The role of post-process hot isostatic pressing (HIP), and solution anneal treatment were evaluated. By using HIP, the scatter in 316 L steel AM properties within single and complex components was minimized to meet the requirement of existing industry standards. For applications where HIP may not be feasible and with some extent of defect tolerance, alternative qualification methodologies of deploying l -PBF AM parts were also explored with samples made with and without engineered porosities. The data generated in this research will be relevant to deployment of AM components for emerging nuclear energy applications.

Fabrication of nuclear reactor components using additive manufacturing (AM) methods is now a practical option since the AM technologies have advanced to allow for building of complex parts with high quality materials. To assess the mechanical performance of printed components in reactor-relevant conditions and to build a property database for the AM 316L stainless steel (SS), mechanical testing and characterization were performed before and after neutron irradiation. Miniature tensile specimens were irradiated at the High Flux Isotope Reactor (HFIR) to 0.2 and 2 displacements per atom (dpa) at 300 and 600°C. The AM 316L SS was tested in the as-built, stress-relieved, and solution-annealed conditions, and the wrought (WT) 316L SS in solution-annealed condition as a reference alloy. The baseline test result showed that the AM 316L SS, regardless of the post-build heat treatment, had higher strength than the WT 316L SS, but similar ductility. Post-irradiation tensile testing was conducted at RT, 300°C, and 500°C for selected irradiation conditions. Neutron irradiation induced significant changes in the mechanical behavior of the AM stainless steels, including both hardening and softening. Although the as-built 316L steel after 300°C irradiation showed necking just after yielding, the overall property changes of the as-printed alloy became less significant after 600°C irradiation. Irradiation-induced ductilization was also observed after the higher temperature irradiation. In general, the strength change was smaller in the relatively stronger as-built and stress-relieved AM SSs than in the solution-annealed AM and WT SSs. These relatively lower strength 316L SSs overall retained higher ductility in the irradiation conditions tested, but the stronger 316L SSs demonstrated a similar level of ductility after the higher temperature (600°C) irradiation. It is a positive assessment for the AM 316L materials that no embrittlement was observed within the test and irradiation conditions of the experiment.

We report the magnetic field and temperature dependence of the critical current density {Jc(H) and Jc(T)} flowing in a 5 μm thick sample of SmBa2Cu3O7−δ (SmBCO) film. The film is a coated conductor (CC) deposited on an IBAD-MgO textured metallic template. For a range of intermediate fields, we find J c α H - α with values of α between 0.44 and 0.49. These values are lower than those reported for other CC in the literature. The sample has a Jc of 13.6 MA/cm2 at 5 K and self-field. Such high-Jc wires should find applications in large-scale energy applications of high-temperature superconductors. The temperature dependence of Jc at self-field and different applied fields is also analyzed.

Effects of low dose irradiation on the electrical transport current properties of commercially available high-temperature superconducting, coated-conductor tapes were investigated, in view of potential applications in the irradiative environment of fusion reactors. Three different tapes, each with unique as-grown flux-pinning structures, were irradiated with Au and Ni ions at energies that provide a range of damage effects, with accumulated damage levels near that expected for conductors in a fusion reactor environment. Measurements using transport current determined the pre- and post-irradiation resistivity, critical current density, and pinning force density, yielding critical temperatures, irreversibility lines, and inferred vortex creep rates. Results show that at the irradiation damage levels tested, any detriment to as-grown pre-irradiation properties is modest; indeed in one case already-superior pinning forces are enhanced, leading to higher critical currents.

Following a review of heating induced chemical and structural changes in RBa2Cu3O7 (RBCO) fluoride precursors (R = rare earth of Y) deposited by metalorganic deposition (MOD) and physical vapor deposition (PVD), a modular process comprising successive, functionally distinct, brief annealing steps (modules) is introduced. By decoupling events that otherwise occur simultaneously, the modular process provides a framework for addressing the complex kinetics associated with the temperature ramp of the ex situ conversion anneal. Modules for modifying the F concentration, porosity and microstructure, and RBCO nucleation are described.

While continuous stabilization in filamentary Y-Ba-Cu-O (YBCO) coated conductors can generate additional heating due to coupling loss, a balance between low ac loss and conductor stability can be found through the introduction of interfacial resistance between the stabilizer and the filaments. The impact of interfacial resistance with respect to ac loss as a function of field was studied for YBCO filaments with a continuous silver cap layer. Starting with uncoated YBCO filaments on standard ion beam assisted deposition (IBAD) templates, an interfacial resistance between filaments was introduced through the exposure of filaments to air at room temperature and followed by the deposition of a continuous, 3 mum thick silver cap layer. The interfacial resistance between silver and the YBCO was controlled by oxygen annealing at different temperatures between 200degC and 450degC. Measurements of the ac loss as a function of applied perpendicular field and frequency and the interfacial resistance revealed a correlation between the coupling loss and the oxygen annealing temperature, where the coupling loss was lowest at an oxygen annealing temperature of 200degC.

Raman microprobe spectroscopy and scanning electron microscopy were used to study the initial nucleation and growth of YBCO in thick precursors by the BaF2 ex situ process. For quenched films of 2 mum thickness, the data indicate a low density of c-axis nuclei near the substrate, apparently due to a reduced oxygen concentration deep inside the precursor layer. Significant non c-axis growth was also observed; the majority of this material nucleates away from the substrate. Measurement of the conversion rate by in situ XRD for films in the range 0.2-2 mum suggest a weak thickness dependence.

Epitaxial YBa2Cu3O7-delta was formed by processing of laser-ablated, fluorine-free precursor films. The depositions were conducted at room temperature in low oxygen pressure on LaAlO3(LAO) single crystal substrates. Processing was done in the same deposition chamber by heating the precursor film to reaction temperatures of 750-850degC in a reducing gas ambient, and then raising the oxygen pressure to the conversion point. Typical processing times are a few minutes, corresponding to minimum YBCO growth rates of 1 nm/s. XRD analysis shows epitaxial growth and high crystallinity, although measured Tc values are somewhat suppressed at 88 K, with resulting critical current density, Jc of about 1 MA/cm2 at 77 K, as determined by magnetic hysteresis. Properties indicate that the materials lack appropriate level of defects needed both for rapid oxygenation and flux pinning.

To achieve low ac losses in applied ac fields, YBa2Cu3Ox (YBCO) filaments were created on a RABiTS buffered substrate through solution inkjet deposition. A metal organic decomposition (MOD) solution was placed into an inkjet dispenser and filaments of widths of 100 mum and 0.8 mm were deposited on the substrate at a spacing of 50 to 100 mum. Each sample, which had a width of 1 cm and a nominal length of 4 cm, was placed in a perpendicular ac field and the ac losses were measured thermally as a function of the field strengths up to 100 mT and at frequencies between 60 Hz and 120 Hz. Samples with inkjet filaments had a high coupling loss. This coupling between filaments may extend along the entire sample length because removal of the conductor ends did not reduce the coupling loss contribution. Reduction in ac loss was observed in samples with laser-scribed filaments that were made from the same MOD solution.

Low-cost coated-conductor fabrication methods are essential for various electric-power applications. Metal-organic-deposition (MOD) approach to grow both YBa2Cu3O7-delta (YBCO) and buffer layers on textured metal substrates is very promising towards fabrication of lower-cost second generation wires. YBCO coated conductors (CC) are being developed with high critical currents that should be sufficient for their extensive use in power applications. However, the present CC has high energy losses in ac magnetic field that are unacceptable. We have developed a modified MOD precursor route to deposit ~ 0.8 mum thick YBCO films in a single coat that requires less than one-fifth of the pyrolysis time compared to the traditional MOD approach. We have also developed a filamentization technique of CC using ink-jet printing to reduce ac losses due to applied ac fields. The preliminary results of YBCO films deposited on standard RABiTS template yielded an of 140 A/cm at 77 K and self- field. A modest reduction of ac loss was observed for the solution ink-jet printed filamentary conductor.

Inkjet printing is a potentially low cost, high rate method for depositing precursors for filamentary YBCO coated conductors. The method offers considerable flexibility of filament pattern, width, and thickness. Using standard solution precursors and RABiTS substrates, the printing, processing, and properties of some inkjet-derived filamentary YBCO coated conductors for second generation (2G) wire are demonstrated on a laboratory scale. Some systematic variations of growth rate and critical transport current with filament width are observed and discussed.

The development of biaxially textured, second-generation, high-temperature superconducting (HTS) wires is expected to enable most large-scale applications of HTS materials, in particular electric-power applications. For many potential applications, high critical currents in applied magnetic fields are required. It is well known that columnar defects generated by irradiating high-temperature superconducting materials with heavy ions significantly enhance the in-field critical current density. Hence, for over a decade scientists world-wide have sought means to produce such columnar defects in HTS materials without the expense and complexity of ionizing radiation. Using a simple and practically scalable technique, we have succeeded in producing long, nearly continuous vortex pins along the c-axis in YBa2Cu3O7?? (YBCO), in the form of self-assembled stacks of BaZrO3 (BZO) nanodots and nanorods. The nanodots and nanorods have a diameter of ~2?3?nm and an areal density ('matching field') of 8?10?T for 2?vol.% incorporation of BaZrO3. In addition, four misfit dislocations around each nanodot or nanorod are aligned and act as extended columnar defects. YBCO films with such defects exhibit significantly enhanced pinning with less sensitivity to magnetic fields H. In particular, at intermediate field values, the current density, Jc, varies as Jc~H??, with ?~0.3 rather than the usual values 0.5?0.65. Similar results were also obtained for CaZrO3 (CZO) and YSZ incorporation in the form of nanodots and nanorods within YBCO, indicating the broad applicability of the developed process. The process could also be used to incorporate self-assembled nanodots and nanorods within matrices of other materials for different applications, such as magnetic materials.

Pulsed electron deposition (PED), an ablation-based film growth technique similar to pulsed-laser deposition, is a relatively new method for the physical vapor deposition (PVD) of films. In the area of PVD ex situ superconductor precursor deposition, PED is potentially simpler and more versatile than its traditional e-beam co-evaporation counterpart. We have deposited near-stoichiometric YF 3 –BaF 2 –CuO PED precursors of various thicknesses in moving configuration on RABiTS substrates. Critical current densities ( J c ) as high as 1.6 MA/cm 2 have been achieved on these precursors converted at standard slow rates of roughly 1 A/s, which has previously been necessary for high performance PVD precursors. More importantly, these as-deposited PED precursors were found to be able to tolerate aggressive conversion conditions. Consequently, simultaneous attainment of high conversion rate of nearly 8 A/s and high J c of 1 MA/cm 2 has been realized.

Implementation of improved processing for BaF/sub 2/ ex situ YBCO coatings from e-beam evaporated precursors enables faster conversion with rates up to 12 /spl Aring//s and critical currents I/sub c/ at 77 K greater than 350 A/cm-width on a RABiTS template. Details of the faster processing are described and compared to an earlier slower process. A linear relation between I/sub c/ and YBCO layer thickness provides evidence of new opportunities to further improve I/sub c/.

Adhesion of thin films of epitaxial oxides to nickel-based metallic substrates is important for the successful development of high-temperature superconductor coated conductors. Detachment of epitaxial oxide buffer layers at the oxide/metal interface during either oxide growth or subsequent processing renders the conductor useless. In this study, thermal desorption spectroscopy (TDS) has been used to identify and understand one of the causes of buffer layer detachment, oxidation of carbon at the oxide–metal interface to form carbon monoxide. Results of TDS indicate that on the surface of a bare nickel-based alloy substrate, the rate of carbon oxidation depends on both the supply of carbon from the substrate and the supply of oxygen from the vapor. Sulfur at the surface of the alloy substrate reduces the rate of carbon oxidation. The effectiveness of various treatments of the bare substrate to eliminate CO formation and epitaxial oxide detachment has been demonstrated. TDS provides both a means to evaluate the kinetics of the oxidation reaction and a tool to assess the need and effectiveness of a substrate oxidation treatment.

The growth of 0.9–1.0 µm thick Y Ba2Cu3O7−δ (YBCO) films on biaxially textured Ni–3 at.% W (NiW) substrates using the BaF2 ex situ process was investigated at reduced pressures. By varying the water vapour pressure (PH2O), Y–BaF2–Cu–O (YBFCO) precursor films deposited by e-beam co-evaporation were converted at a reduced total pressure (Ptotal) of 50–55 Torr and conversion temperature (TS) of 740 °C for a wet conversion time (tW) of 60 min. Critical current density (JC) values greater than 1 MA cm−2 for the thick YBCO films were obtained under the condition of varying PH2O from low pressure to 10 Torr. The transition temperatures (TC) of the samples were over 90 K with ΔTC = 1.8–2.5 K. Pre-heat treatment of the precursor films on CeO2/Y SZ/Y2O3/Ni/NiW substrates under an O2 atmosphere condition before the conversion resulted in smooth surfaces without large secondary phase particles embedded in the films.

The effects of conversion parameters on transport properties of YBa2Cu3O7-δ (YBCO) films on rolling assisted biaxially textured substrates (RABiTS) in the BaF2 ex situ process were investigated for total pressures Ptotal between 0.1 and 1.3 atm, water vapor pressures PH2O between approximately 7 and 70 Torr and processing temperatures TS between 700 and 790 °C. For this study, a 0.3-μm-thick Y–BaF2–Cu–O precursor film was deposited on a 1-cm-wide Ni=3 at.% W RABiTS with a buffer layer architecture of CeO2/YSZ/Y2O3/Ni deposited in single passes in various reel-to-reel systems for each layer. Under the conditions of Ptotal = 0.1 atm, TS = 740 °C and PO2 approximately 150 mTorr, JC > 2 MA/cm2 was obtained at 77 K and self field for PH2O ≤ 20 Torr. At higher PH2O (=70 Torr), however, the maximum attainable JC decreased. In addition, the JC at these higher PH2O dropped rapidly with increased dwell time. The highest JC, 2.5 MA/cm2, was achieved at 730 °C with Ptotal = 0.1 atm and PH2O approximately 7 Torr. Finally, the variation of IC with wet conversion time was performed at each processing temperature.

Use of many Ni-based and Cu-based alloys requires deposition of a Ni overlayer prior to the deposition of the seed layer in order to obtain better epitaxy. In this work, Ni overlayers have been deposited by DC sputtering on biaxially textured NiW and Cu substrates. Films were characterized by x-ray diffraction and atomic force microscopy as a function of temperature, film thickness and deposition rate. The optimum temperature range for deposition was found to be around 400?500??C, which results in relatively smooth films with a good cube texture on both substrates.

A reel-to-reel furnace equipped with seven transverse flow modules has been developed to continuously process ex situ BaF2 precursor coated conductors. Experimental results have revealed that as the precursor thickness increases, the processing parameters need to be adjusted in order to obtain high quality YBCO films with Jc' s greater than 1 MA cm−2 at 77 K and self-field. For thin precursors, a relatively aggressive conversion environment of high P(H2O) can be used to obtain epitaxial YBCO with an extremely small amount of undesirable random or a-axis grains. Thin 0.3 µm YBCO was converted on a 2.3 m long by 1 cm wide Ni RABiTS substrate, yielding an end-to-end Ic of 18.9 A (Jc = 0.63 MA cm−2). 16 cm sectional measurements showed that the tape performance is uniform, with a mean Ic of 19.1 A (0.64 MA cm−2) and a 5.4% SD. In thick precursors, however, a less aggressive environment in the form of low P(H2O) is better suited for epitaxial YBCO nucleation during the incubation period. By using a low initial P(H2O) with progressively higher values during later stages of conversion, an end-to-end Ic of 92 A (1.02 MA cm−2) was obtained in 1.1 m long by 1 cm wide 0.9 µm YBCO on better-textured Ni–W RABiTS. This conductor possesses uniform Ic according to 1 cm sectional measurements with a mean Ic of 94.4 A (1.05 MA cm−2) and only 4.3% SD. Further adjustment in the P(H2O) schedule resulted in a 45 cm long sample with end-to-end Ic of 111.8 A (1.24 MA cm−2), a mean Ic of 114.8 A (1.28 MA cm−2) and 4.8% SD. The highest Ic of 138 A (1.53 MA cm−2) was obtained in a short 0.9 µm YBCO sample on a pre-treated Ni–W RABiTS substrate.

In situ X-ray diffraction has been used to study the conversion of ∼3000 A thick, evaporated barium fluoride precursor films to YBCO on buffered metal substrates for a range of conversion time, ramp rate, water vapor pressure, oxygen pressure, and temperature. All films were taken from the same continuously processed tape. Critical current densities as high as 1.34 MA/cm 2 have been obtained for a precursor reaction rate of 2.0 A/s. Higher reaction rates (up to 17 A/s) lead to lower critical current densities and incompletely developed YBCO. The reaction rate increases with increasing P H 2 O , increasing T , and decreasing P O 2 . The reaction rate increases linearly with increasing P H 2 O for the range of pressures used here (0.0125–5.0 mTorr). The formation of non-epitaxial YBCO is generally associated with lower T and higher reaction rate, whereas the formation of barium cerate, which results from a reaction of the precursor and the CeO 2 -capped substrate, is generally associated with higher T and lower reaction rate. General trends in the formation of crystalline phases during conversion may serve as a baseline for development of higher rate conversion processing for thicker, higher performance YBCO.

The current carrying capabilities of RABiTS are connected to the crystalline quality of the seed buffer layer and the stability of the metal/seed layer interface. Our study shows that the epitaxial growth of commonly used seed layers on textured Ni is mediated by a sulfur superstructure present on the metal surface. Many structural defects generated during seed layer growth (secondary orientations, in-plane rotation, incomplete cube texture) can be related to the S surface concentration and superstructure coverage. More generally, our results indicate that the epitaxial deposition of several classes of oxides (fluorite, perovskite, RE/sub 2/O/sub 3/) on several {100} fcc metals depends, in addition to chemical stability and lattice match, on the existence and optimization of S superstructures on the metal surface. On these bases, we discuss issues related to the growth of different oxides on Ni, Ni-alloys, and Pd surfaces having different chemical and structural properties.

Fabrication of a biaxially textured, strengthened Ni substrate with small alloying additions of W and Fe is reported. The substrates have significantly improved mechanical properties compared to 99.99% Ni and surface characteristics which are similar to that of 99.99% Ni substrates. High quality oxide buffer layers can be deposited on these substrates without the need for any additional surface modification steps. Grain boundary misorientation distributions obtained from the substrate show a predominant fraction of low-angle grain boundaries. A high critical current density, J c , of 1.9 MA/cm 2 at 77 K, self-field is demonstrated on this substrate using a multilayer configuration of YBCO/CeO 2 /YSZ/Y 2 O 3 / Ni–3at.%W–1.7at.%Fe. This translates to a I c /width of 59 A/cm at 77 K and self-field. J c at 0.5 T is reduced by only 21% indicating strongly-linked grain boundaries in the YBCO film on this substrate.

A reel-to-reel tape handler mounted on a four-circle diffractometer is used to provide a characterization by X-ray diffraction of the entire 1 m length of a YBa2Cu3O7/CeO2/YxZr1−xO2/CeO2/Pd/Ni (YBCO/CeO2/YSZ/CeO2/Pd/Ni) tape. Cube-textured Ni was formed by rolling and annealing; epitaxial CeO2/YSZ/CeO2/Pd buffer layers were deposited by reactive sputtering; YBCO was converted ex situ from Y, BaF2, and Cu codeposited by e-beam evaporation. Rocking curve FWHM (mean±standard deviation) for 95 segments of 1 cm length are: YBCO (0 0 5)=6.2±0.5° , YSZ (0 0 2)=10.4±0.4° , and Ni (0 0 2)=7.6±0.3° . φ scan FWHM are: YBCO (1 1 3)=9.6±0.4° , YSZ (1 1 1)=13.0±0.4° , and Ni (1 1 1)=10.6±0.4° . Greater than 95% of the tape at each point is cube textured from Ni to YBCO. The critical current density Jc is 0.36±0.04 MA/cm2 at 77 K and is inversely correlated with the rocking curve FWHM. Calculations suggest that Jc might be increased by a factor of 2.1 by producing a sharper texture and that the uniformity of the texture will support scaling to kilometer lengths.

We investigated the influence of a chemisorbed S template with c(2 × 2) structure on the epitaxial growth of different oxide buffer layers on {100}〈100〉 Ni. The sulfur superstructure spontaneously forms on the Ni surface during the texturing anneal as a consequence of segregation. However, depending on the initial S bulk concentration and/or specific annealing conditions, the S layer can cover less than the entire substrate's surface. We show that an incomplete c(2 × 2) coverage causes degradation of the seed buffer layer texture as compared to the substrate texture. A simple step consisting of an H2S predeposition anneal can be used to control the superstructure coverage and optimize the seed layer texture.

Transmission electron microscopy was used to study the microstructure of pulsed laser deposited YBa 2 Cu 3 O 7− δ (YBCO) layers on four different biaxially textured Ni-substrates with epitaxial oxide buffer layers. The oxide layer sequence, YBCO/YSZ/CeO 2 /Ni (YSZ signifies yttria-stabilized zirconia) was the same in each sample, but different deposition techniques were used to deposit the buffer layers. The thicknesses of all three oxide layers varied from sample to sample. A columnar sub-grain structure composed of equiaxed sub-grains in the plane of the film was observed in the YBCO layer of each sample. The sub-grain shape and local mosaic spread (∼3°) appeared to be largely insensitive to the surface topography, density, or method of depositing the YSZ layer immediately below. The sub-grain size (0.2–1.2 μm) increased with film thickness (0.2–2.3 μm), but showed no obvious correlation with the microstructure of the underlying YSZ. The measured misorientation angles ( θ ) at individual sub-grain boundaries suggested that a large fraction of the boundaries in the sub-grain boundary network have θ in the range of 2–3°. For most boundaries, the measured θ was below the 5° “threshold” angle for weak link behavior. Thus, a representative picture of the sub-grain boundary network above a single grain of nickel in a film of sub-micron thickness might be a honeycomb shaped arrangement of 2.5° [0 0 1] tilt boundaries spaced 0.25 μm apart and containing dislocations at a spacing of 10 nm. This structure probably has consequences for the structure of grain boundaries in the YBCO that are replicated from the Ni template below. The superposition of the Ni grain boundary network with the grain boundary network due to mosaic spread almost certainly introduces substantial variations of θ along such boundaries on the sub-micron to micron length scale.

The main objective of this research is to find a suitable alternate solution based seed layer for the standard RABiTS three-layer architecture of physical vapor deposited CeO2 cap/YSZ barrier/Y2O3 seed on Ni-5%W metal tape. In the present work, we have identified CeO2 buffer layer as a potential replacement for Y2O3 seeds. Using a metal-organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of CeO2 (pure and Zr, Cu and Gd-doped) directly on biaxially textured Ni-5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial CeO2 phase with slightly improved out-of-plane texture compared to the texture of the underlying Ni-W substrates can be achieved in pure, undoped CeO2 samples. We have also demonstrated the growth of YSZ barrier layers on pure CeO2 seeds using sputtering. Both sputtered CeO2 cap layers and MOD-YBCO films were grown epitaxially on these YSZ-buffered MOD-CeO2/Ni-5W substrates. High critical currents per unit width, Ic of 264 A/cm (critical current density, Jc of 3.3 MA/cm2) at 77 K and 0.01 T was achieved for 0.8 μm thick MOD-YBCO films grown on MOD-CeO2 seeds. These results indicate that CeO2 films can be grown directly on Ni-5W substrates and still support high performance YBCO coated conductors. This work holds promise for a route for producing low-cost buffer architecture for RABiTS based YBCO coated conductors.

Progress made in the fabrication of rolling assisted biaxially textured substrates (RABiTS) and epitaxial deposition or formation of HTS on such substrates is reported. Significant progress has been made in understanding the role of meso-scale defects such as grain boundaries on long-range current flow of HTS conductors made using the RABiTS approach. Both experimental and theoretical calculations suggest that in well-textured samples these commonly present defects do not provide an intrinsic barrier to current flow in long-length conductors. Significant progress has also been made in the reel-to-reel deposition of oxide buffer layers and in the fabrication of long-length superconductors using the ex situ BaF 2 technique. Finally, non-magnetic, mechanically strengthened, biaxially textured metal templates have been fabricated with high quality oxide buffer layers. Epitaxial formation of YBCO on such substrates yields critical current densities over 1 MA/cm 2 at 77 K, 0 T.

Y-BaF2-Cu precursor films for Y-Ba2Cu3O7-y (YBCO) coated conductors have been deposited on both uncoated nickel and Rolling Assisted Biaxially Textured Substrate (RABiTSTM) tapes by electron beam coevaporation. The water partial pressure was kept constant at 1×10-5 Torr during the deposition in order to control the oxygen content in the precursor films. Rutherford backscattering spectroscopy measurements indicate an average composition of Y:Ba:Cu of 1.05:2.10:3.0 with a standard deviation of less than 2.0%. This result was obtained for both moving and stationary tapes. The thickness variation of such precursor films is less than 2.5% over lengths of up to 1 m. Films up to 5 µm thick have been deposited on a stationary tape. An end-to-end Jc of 100 000 A cm-2 from a 1 m length of RABiTSTM sample has been obtained with the highest Jc of 774 000 A cm-2 in a short section within the tape. The present research demonstrates a possible route for industrial scale-up using electron beam coevaporation of YBCO precursors.

A low-cost, non-vacuum, solution precursor route has been developed to produce epitaxial Gd/sub 2/O/sub 3/ and Eu/sub 2/O/sub 3/ buffer layers and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) superconductors on biaxially textured metal substrates. On sol-gel Eu/sub 2/O/sub 3/ seed layers with sputtered YSZ and CeO/sub 2/ top layers, a YBCO film with a J/sub c/ of over 1 MA/cm/sup 2/ at 77 K was obtained. On all solution buffer layers (CeO/sub 2//Eu/sub 2/O/sub 3//Ni), YBCO film with a J/sub c/ of 200,000 A/cm/sup 2/ at 77 K was grown using pulsed laser deposition (PLD). Meter lengths of epitaxial and crack-free Gd/sub 2/O/sub 3/ buffer layers were fabricated on cube textured Ni-W (3 at.%) substrates for the first time. High quality YBCO films were deposited on Rolling-Assisted Biaxially Textured Substrates (RABiTS) using a trifluoroacetate (TFA) precursor approach. The precursors were either spin-coated or dip-coated and decomposed in a newly developed fast 3-hour burn-out step followed by post-annealing. In a stationary burn-out route, we have produced 40 cm long crack-free YBCO TFA precursors on RABiTS. On short segments, YBCO films with a J/sub c/ of over 500,000 A/cm/sup 2/ at 77 K were grown on all PLD buffered-Ni substrates (CeO/sub 2//YSZ/CeO/sub 2//Ni).

Solution-based techniques have been examined as potential low-cost processes for manufacturing YBCO coated conductors. YBCO films prepared from metal trifluoroacetate precursors have achieved performance levels equaling or exceeding that of vapor deposited films with the same thickness on CeO/sub 2//YSZ(sc) substrates. J/sub c/'s of 4.5 MA/cm/sup 2/ and 2 MA/cm/sup 2/ have been achieved in 0.4 /spl mu/m thick YBCO films on CeO/sub 2//YSZ(sc) and CeO/sub 2//YSZ/CeO/sub 2//Ni substrates, respectively. Textured Gd/sub 2/O/sub 3/ buffer layers have been deposited on deformation textured Ni substrates in a reel-to-reel process. The performance of YBCO films deposited on substrates containing the Gd/sub 2/O/sub 3/ seed layers is comparable in performance to YBCO films grown on all vacuum deposited buffer layers.

A reel-to-reel electron beam evaporation system has been developed to continuously deposit Y–BaF2–Cu precursor for ex situ YBa2Cu3O7−x (YBCO) processed films. The quality of a YBCO film on biaxially textured, oxide buffered, Ni tape strongly depends on the condition of the precursor deposit. For continuous precursor deposition, a uniform cation stoichiometry (i.e., Y:Ba:Cu=1:2:3) over the entire length of precursor film is essential. Rutherford backscattering spectrometry studies indicated that the partial pressure of water vapor during the deposition has a significant effect on cation deposition rates and the oxygen content in the precursor film. The oxygen content of the precursor is increased and the precursor stability upon exposure to air is improved by introduction of water vapor during precursor deposition. High quality precursor films with a thickness of 300 nm have been deposited on lengths of rolling-assisted, biaxially textured substrates in the reel-to-reel electron beam evaporation system. In this paper, long length precursor deposition processing will be discussed. Properties of some post-annealed YBCO films will also be presented.

The electromagnetic connectivity and microstructure of three YBa 2 Cu 3 O 7− δ (YBCO) films grown on biaxially textured substrates were investigated by magneto optic (MO) imaging and scanning electron microscopy (SEM). The films were deposited by pulsed laser deposition (PLD) on yttria-stabilized zirconia (YSZ) and CeO 2 -buffered, biaxially textured Ni tapes. The transport critical current density ( J c ) values of the films were 0.3, 0.6 and 0.7 MA/cm 2 (77 K, 0 T). MO imaging revealed clearly granular electromagnetic behavior in the lowest J c and one of the higher J c samples, but considerably better connectivity in the sample with a J c value of 0.6 MA/cm 2 . High resolution SEM showed a dense and rather featureless microstructure in the YBCO of the most highly electromagnetically connected sample, whereas pores and/or second phase particles cluttered the YBCO layers of the granular samples. Thus, the granular behavior in these samples appears to be caused by pores and second phase particles that locally obstruct the superconducting current in the YBCO layer. Control of these types of defects clearly is important for raising the J c value.

In order for many large-scale bulk applications of high-temperature superconducting materials to be realized, the cost/performance of the superconductors needs to be optimized. From a performance standpoint, a long, flexible, single-crystal-like wire is required; from a cost-and-fabrication standpoint, an industrially scalable, low-cost process is required. Both of these critical requirements are met by rolling-assisted biaxially textured substrates, a conductor-fabrication technique that employs simple, scalable, thermomechanical processing techniques to obtain a near-single-crystal-like, flexible metal substrate in arbitrary lengths on which epitaxial oxide buffer layers and superconductors are then deposited.

A reel-to-reel, electron beam evaporation system has been developed to continuously deposit epitaxial CeO/sub 2/ and other oxide buffer layers on meter-long lengths of biaxially textured Ni tapes. The deposition system includes two interconnected electron beam evaporation chambers and a chamber in which as-rolled Ni tape is in situ annealed to develop biaxial texture. An integral reel-to-reel system with tension control enables motion of the tape with little or no plastic deformation. When depositing epitaxial oxides on Ni, the formation of unfavorably oriented NiO is difficult to avoid. Oxide free, {100} oriented Ni tapes are prepared by control of the partial pressures of H/sub 2/, H/sub 2/O and O/sub 2/ during Ni annealing. X-ray /spl phi/-scans have been performed as a function of length to determine the crystallographic consistency of the epitaxial CeO/sub 2/ over length. Results of SEM examinations of the CeO/sub 2/ buffer layer microstructure are presented. Results for YBCO films deposited on short segments of these buffered substrates are summarized.

The biaxial textures created in metals by rolling and annealing make them useful substrates for the growth of long lengths of biaxially textured material. The growth of overlayers such as high-temperature superconductors requires flat substrates with a single, sharp texture. A sharp cube texture is produced in high-purity Ni by rolling and annealing. We report the effect of rolling reduction and annealing conditions on the sharpness of the cube texture, the incidence of other orientations, the grain size and the surface topography. A combination of high reduction and high-temperature annealing in a reducing atmosphere leads to >99% cube texture, with a mosaic of about the rolling direction, about the transverse direction, and about the normal direction.

Sputtered Co/Cu films were deposited on silicon either directly, using an argon plasma, or through an intermediate step involving the deposition of CoN/Cu3N by reactive sputtering in a nitrogen plasma. The nitride composite was pyrolyzed to release the nitrogen. The effect of preparation route and different annealing treatments on the resistivity of these films as a function of magnetic field and temperature was established. As in previous studies particle size is of primary importance in determining the giant magnetoresistance (GMR) characteristics of a film; thus optimizing the magnitude of the GMR effect requires close control of the duration and temperature of annealing treatments. The microstructure of these films was examined by means of atomic force microscopy. The intermediate nitride step produced a film in which the GMR effect was degraded apparently through the inclusion of voids that are not eliminated in the pyrolysis stage.

React-before-wind surface-coated BSCCO-2212 is being established as a relatively low cost HTS conductor for practical applications. Quality tape is presently being manufactured in lengths of 450 to 500 m at a cost estimated to be 1/3 to 1/5 of the industry costs of BSCCO-2223 powder-in-tube tape. Robust, mechanically sound coils for applications ranging from NMR insert magnets to transformer windings are being made from this BSCCO-2212 tape. The coils have performed consistently through test and thermal cycling without degradation and operate as projected from short sample measurements. A hybrid approach, which uses mainly BSCCO-2212 augmented by BSCCO-2223 conductor in the high radial field end regions, is expected to halve the overall superconductor cost for magnet systems.

The transport critical current ( I c ) of polycrystalline (Tl 0.5 Pb 0.5 )(Sr 0.8 Ba 0.2 ) 2 Ca 2 Cu 3 O x (Tl1223) and Bi 2 Sr 2 CaCu 2 O x (Bi2212) conductors has been determined for a range of temperature (25–77.3 K) and a range of externally applied magnetic field ( H app = 0–1.5 T). Hysteresis of the I c versus H app behavior has been observed for the Tl1223 conductor and has been interpreted using Bean's critical state model [1]. For the Tl1223 conductor, the remnant critical current measured for H app = 0 after exposure to 1.5 T was found to be independent of temperature. This behavior has been shown to be consistent with the critical state model. No hysteresis of I c versus H app was detectable for the Bi2212 conductor.

The rate at which thin film YBCO precursor converts to high J/sub c/ superconductor depends on many factors including the method by which the precursor is prepared, the precursor film thickness, and the conditions during precursor deposition and conversion. Using in-situ X-ray diffraction and reduced pressures, we have examined and compared conversion rates for precursors prepared by physical vapor deposition (PVD) and metal-organic deposition (MOD). For conversion conditions found to be optimal for obtaining high J/sub c/ and high precursor conversion rate (G/sub p/) for PVD precursors, the G/sub p/ is nearly identical (/spl sim/0.2 nm/sec) for the both PVD and MOD precursors. The development of crystalline BaF/sub 2/ is, however, distinctly different for the two precursors. This may suggest that conversions of these PVD and MOD precursors follow different chemical pathways. For both precursors, higher G/sub p/ is realized using higher water pressure and ramp rate. Although G/sub p/ is higher, YBCO is randomly oriented and J/sub c/ is zero for the PVD precursor. The MOD precursor seems to be generally more tolerant of higher reaction rates than the PVD precursor. The reaction rate versus P/sub H2O/ determined for both precursor types shows a square root of P/sub H2O/ dependence of G/sub p/. This behavior is consistent with a reaction rate limited by gas-phase HF removal.

Studies have been undertaken using thermal analysis, in conjunction with high-temperature and room temperature X-ray diffraction, fraction, to elucidate phase relationships during thermal processing of thick films of initially phase pure Bi2Sr2CaCu2Oy (2212) on silver substrates in various oxygen-containing atmospheres (0.001 to 100% O2). Exothermic events on cooling at 10°C/min from a partially liquid state vary with oxygen partial pressure and can be grouped into three sets (I–III). Set I is prominent for 0.001% and 0.1% O2 in the range of 740–775°C and is believed to be associated with the crystallization of a Cu-free ∼ Bi5Sr3Ca1 oxide phase. Set II results from the crystallization of 2212; it is observed for p(O2)≥1.0% in the temperature range 800–870°C. Set III appears for 21% and 100% O2 in the temperature range 880–910°C, and its origin is not clear from the results of this study. Subsequent room temperature X-ray diffraction from these samples suggests that in general high oxygen partial pressures (100% O2) tend to favor the formation of Bi2Sr2CuO6 (2201), whereas low oxygen partial pressures (0.001-0.1% O2) lead to the formation of a Cu-free, Bi-Sr-Ca oxide phase. The 2212 phase forms at this cooling rate predominantly for intermediate oxygen partial pressures (7.6–21% O2). High-temperature X-ray diffraction during cooling (2°C/h) from the partially liquid state shows a pronounced dependence of the order of evolution of crystalline 2212 and 2201 phases on p(O2). For an oxygen partial pressure of 1.0% the formation of 2212 precedes that of 2201, whereas for 0.01% O2 2201 crystallizes at a higher temperature than 2212. The implications of these results pertaining to thermal processing of thick 2212 films are discussed.

Results are reported for short samples of BSCCO-2212 conductor made by a dip-coat process. Samples were characterized by XRD, optical microscopy, SEM and transport critical current measurements. Critical currents of up to 14 amperes were measured in 5 mm wide tapes immersed in liquid nitrogen in zero applied magnetic field, with corresponding overall critical current densities at 77K, 0T of up to 2,700 A/cm/sup 2/. >

It is well established that high efficiency (20%) solar cells can be routinely fabricated using single crystal photovoltaic (PV) materials with low defect densities. Polycrystalline materials with small grain sizes and no crystallographic texture typically result in reduced efficiencies. This has been ascribed primarily to the presence of grain boundaries and their effect on recombination processes. Furthermore, lack of crystallographic texture can result in a large variation in dopant concentrations which critically control the electronic properties of the material. Hence in order to reproducibly fabricate high efficiency solar cells a method which results in near single crystal material is desirable. Bulk single crystal growth of PV materials is cumbersome, expensive and difficult to scale up. We present here a possible route to achieve this if epitaxial growth of photovoltaic materials on rolling-assisted-biaxially-textured-substrates (RABiTS) can be achieved. The RABiTS process uses well-established, industrially scaleable, thermomechanical processing to produce a biaxially textured or single-crystal-like metal substrate with large grains (50–100 μm). This is followed by epitaxial growth of suitable buffer layers to yield chemically and structurally compatible surfaces for epitaxial growth of device materials. Using the RABiTS process it should be possible to economically fabricate single-crystal-like substrates of desired sizes. Epitaxial growth of photovoltaic devices on such substrates presents a possible route to obtaining low-cost, high performance solar cells.