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Robert K. Williams

Other affiliations: AMIT
Bio: Robert K. Williams is an academic researcher from Oak Ridge National Laboratory. The author has contributed to research in topics: Thermal conductivity & Electrical resistivity and conductivity. The author has an hindex of 21, co-authored 56 publications receiving 1417 citations. Previous affiliations of Robert K. Williams include AMIT.


Papers
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Journal ArticleDOI
TL;DR: In this article, measurements of thermal conductivity, electrical resistivity, and Seebeck coefficient for single-crystal and large-grained polycrystalline specimens of 99.99+% pure silicon were presented.
Abstract: Results are presented of measurements of the thermal conductivity (90-1328\ifmmode^\circ\else\textdegree\fi{}K), the electrical resistivity (300-1273\ifmmode^\circ\else\textdegree\fi{}K), and the Seebeck coefficient (350-1273\ifmmode^\circ\else\textdegree\fi{}K) for single-crystal and large-grained polycrystalline specimens of 99.99+% pure silicon. The thermal conductivity above 387\ifmmode^\circ\else\textdegree\fi{}K was measured by an absolute radial-heat-flow technique; below 350\ifmmode^\circ\else\textdegree\fi{}K, by an absolute longitudinal technique. Some intermediate thermal-conductivity measurements from 300-400\ifmmode^\circ\else\textdegree\fi{}K were made on the polycrystalline material using a comparative longitudinal-heat-flow apparatus. The estimated errors of these three thermal-conductivity methods were \ifmmode\pm\else\textpm\fi{}2%\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}K, \ifmmode\pm\else\textpm\fi{}1.2%\ifmmode\pm\else\textpm\fi{}0.1\ifmmode^\circ\else\textdegree\fi{}K, and \ifmmode\pm\else\textpm\fi{}4.0%\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}K, respectively. The estimated error for the electrical-resistivity measurements was \ifmmode\pm\else\textpm\fi{}1.4%\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}K, and for the Seebeck measurements \ifmmode\pm\else\textpm\fi{}1.6%\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{}K. The thermal-conductivity values were compared with conflicting data from the literature, and they corroborate the higher-temperature results obtained by Glassbrenner and Slack. Therefore, we agree with their conclusion that the electronic contribution is reasonably close to theoretical estimates which include a large ambipolar-diffusion term. The temperature dependence of the lattice thermal resistance has been compared to various theoretical models but no approach seems to explain the data in detail. An abrupt slope change in the thermal resistivity at about 670\ifmmode^\circ\else\textdegree\fi{}K is a major cause of the difficulty.

181 citations

Journal ArticleDOI
TL;DR: In this paper, the role of meso-scale defects such as grain boundaries on long-range current flow of HTS conductors made using the rolling assisted biaxially textured (RABiTS) approach is reported.
Abstract: 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.

113 citations

Journal ArticleDOI
TL;DR: In this article, 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 was investigated.
Abstract: 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.

94 citations

Journal ArticleDOI
TL;DR: A solution process was used to grow epitaxial La 2 Zr 2 O 7 (LZO) buffer layers on roll-textured Ni (100) substrates to produce YBa 2 Cu 3 O 7− δ (YBCO)-coated conductors.
Abstract: A solution process was used to grow epitaxial La 2 Zr 2 O 7 (LZO) buffer layers on roll-textured Ni (100) substrates to produce YBa 2 Cu 3 O 7− δ (YBCO)-coated conductors. The LZO precursor solution was prepared by an all alkoxide sol–gel route using mixed metal methoxyethoxides in 2-methoxyethanol. The partially hydrolyzed solution was either spin-coated or dip-coated onto the textured Ni substrates. The amorphous thin film was then heat treated at 1150°C under (96%)Ar/(4%)H 2 atmosphere for 1 h. X-ray diffraction (XRD) of the buffer layer indicated a strong c -axis orientation on the Ni (100) substrate. The LZO (222) pole figure revealed a single cube-on-cube texture. SEM images of the LZO buffer layer showed a dense microstructure without cracks. The YBCO deposited on the sol–gel LZO-buffered Ni substrates with sputtered YSZ and CeO 2 top layers had a critical current density of 480,000 A/cm 2 at 77 K and self-field.

82 citations

Journal ArticleDOI
TL;DR: In this article, a comparison of Auger electron spectra obtained from intergranular and transgranular areas exposed on fracture surfaces of sintered YBa2Cu3O7−x specimens indicates that most grain boundary surfaces are deficient in oxygen and rich in copper compared to the bulk.
Abstract: Comparison of Auger electron spectra obtained from intergranular and transgranular areas exposed on fracture surfaces of sintered YBa2Cu3O7−x specimens indicates that most grain‐boundary surfaces are deficient in oxygen and rich in copper compared to the bulk. The thickness of this region of altered composition is estimated to be in the range of 15–50 A. No evidence of segregation of impurities to grain boundaries was seen. The results suggest that the grain‐boundary layer is nonsuperconducting and a likely contributor to the problem of low critical current densities in these materials. It is believed that carbon at grain boundaries which has been reported results from incomplete calcination in material which is slightly off stoichiometry.

82 citations


Cited by
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Book ChapterDOI
Glen A. Slack1
TL;DR: In this article, the authors studied the thermal conductivity of non-metallic crystals at temperatures comparable to or higher than the Debye temperature, where the dominant carriers of thermal energy are phonons and the dominant scattering mechanism is the intrinsic phonon-phonon scattering.
Abstract: Publisher Summary This chapter reviews the thermal conductivity of nonmetallic crystals at temperatures comparable to or higher than the Debye temperature. It deals with the intrinsic behavior of such pure crystals at high temperatures. In such crystals, the dominant carriers of thermal energy are phonons and the dominant scattering mechanism to be considered is the intrinsic phonon–phonon scattering. This is a small section of the much larger problem of the thermal conductivity of nonmetallic solids and clearly it neglects possible heat transport by photons, charge carriers, polarons, and magnons. It also neglects other possible phonon scattering mechanisms such as isotopes, impurities, vacancies, charge carriers, dislocations, grain boundaries, and crystal boundaries. It presents the absolute value of the thermal conductivity, K, as determined by phonon–phonon scattering, the temperature dependence of K, the volume dependence of K, the change in K upon melting, and the minimum value of K. The chapter discusses a composite curve for the thermal conductivity versus temperature of pure KCl measured at a constant pressure of, say, one atmosphere.

734 citations

Journal ArticleDOI
19 Aug 2004-Nature
TL;DR: A method for achieving a dispersion of ∼8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current by a factor of two to three for high magnetic fields.
Abstract: Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length ( approximately 2-4 nm for YBCO at temperatures < or =77 K) and when the areal number density of defects is of the order of (H/2) x 10(11) cm(-2), where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of approximately 8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77 K) by a factor of two to three for high magnetic fields.

694 citations

Journal ArticleDOI
TL;DR: In this paper, an inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature.
Abstract: An inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature. As a result of this assessment, experimentally based values for the valence‐band ‘‘densities‐of‐states’’ effective mass are determined in the 300–500 K range and are shown to be in good agreement with recent theoretical calculations. At 300 K, experimentally based values of 3.1×1019 cm−3 for the valence‐band effective densities of states and 1.08×1010 cm−3 for the intrinsic carrier concentration are determined. Although in good agreement with theoretical calculations, these are significantly higher and lower, respectively, than commonly used values in the past. These results have important implications in the calculation of other silicon material and device parameters.

600 citations

Journal ArticleDOI
TL;DR: In this article, the microstructural and superconducting properties of light rare earth elements (LREs) are reviewed and the flux pinning mechanism is also discussed, on the basis of their study over the last several years, the melt processes for LRE - Ba - Cu - O are described.
Abstract: Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

550 citations

Journal ArticleDOI
TL;DR: The phonon dispersion relation in the phonon glass-electron crystal material Ba(8)Ga(16)Ge(30) using neutron triple-axis spectroscopy is investigated, disclosing unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches.
Abstract: Engineering of materials with specific physical properties has recently focused on the effect of nano-sized ‘guest domains’ in a ‘host matrix’ that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without simultaneously reducing the charge transport. This is named the ‘phonon glass–electron crystal’ concept and may be realized in host–guest systems. The guest entities are believed to have independent oscillations, so-called rattlermodes,which scatter the acoustic phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass–electron crystal material Ba8Ga16Ge30 using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected, and a new explanation for the low L is provided.

517 citations