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  • 1.
    Andersson, B.M.
    et al.
    Department of Physics, Umeå University.
    Sundqvist, B.
    Department of Physics, Umeå University.
    Niska, John
    Luleå tekniska universitet.
    Loberg, Bengt
    Easterling, K.
    Luleå tekniska universitet.
    Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing1990In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 170, no 5-6, p. 521-531Article in journal (Refereed)
    Abstract [en]

    Highly dense sintered YBa2Cu4O8 has been produced by hot isostatic pressing (HIP). The electrical resistivity ρ of this material has been measured as a function of temperature T and pressure p in the range 40-650 K and 0-0.7 GPa. Both the temperature dependence and the pressure dependence of ρ are found to be well described by a model based on the standard Bloch-Grueneisen theory. It is pointed out that ρ is liner in T only under isobaric conditions, while ρ is strongly nonlinear in all high-Tc superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm2 at 4 K, while the resistivity is 630 μΩ cm at 294 K.

  • 2.
    Andersson, Britt
    et al.
    Umeå university.
    Sundqvist, Bertil
    Umeå university.
    Niska, John
    Luleå tekniska universitet.
    Loberg, Bengt
    Thermal conductivity of polycrystalline YBa2Cu4O81994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 49, no 6, p. 4189-4198Article in journal (Refereed)
    Abstract [en]

    We have measured the thermal conductivity κ and the thermal diffusivity a of a dense bulk ceramic polycrystalline sample of YBa2Cu4O8 (1:2:4) in the temperature range 30-300 K. We find κ≊10 W m-1 K-1 at 100 K, significantly higher than in ceramic YBa2Cu3O7-δ (1:2:3) and approaching the in-plane value for single-crystal 1:2:3, and decreasing to 7.6 W m-1 K-1 at 300 K. The data for this sample can be described by standard theories for phonon thermal conductivity of crystalline materials with boundary, phonon, and electron scattering. The higher κ in 1:2:4 as compared to 1:2:3 is, in this model, due to the smaller point defect scattering in the former. The fitted parameters for the three scattering mechanisms all agree with independent estimates based on simple models; inserting data for electric resistivity, grain size, carrier density, and lattice properties we can predict κ and its T dependence to within about 20%. We also discuss models for the phonon and electron thermal conductivities in some detail, including some second-order effects such as inelastic electron scattering and a T-dependent carrier density.

  • 3. Billy, J.
    et al.
    Johansson, T.
    Loberg, Bengt
    Easterling, Kenneth
    Luleå tekniska universitet.
    Stress-relief heat treatment of submerged-arc-welded microalloyed steels1980In: Metals Technology, ISSN 0307-1693, Vol. 7, no 2, p. 67-78Article in journal (Refereed)
    Abstract [en]

    A wide range of microalloyed steels containing various combinations of Al, Ti, V, Nb, and submerged-arc welded using wires of either high or low Mo additions, were studied in both the as-welded and stress-relieved conditions, with particular reference to impact toughness and microstructure. It was found that, irrespective of thermal treatment, the heat-affected base metal always has a much lower impact transition temperature than the weld metal. In the as-welded condition, the most important constituent with respect to toughness is the amount of pro-eutectoid ferrite present

  • 4.
    Dou, S.X.
    et al.
    School of Materials Science and Engineering, University of New South Wales.
    Liu, H.K.
    School of Materials Science and Engineering, University of New South Wales.
    Wang, J.
    School of Materials Science and Engineering, University of New South Wales.
    Apperley, M.H.
    School of Materials Science and Engineering, University of New South Wales.
    Sorrell, C.C.
    School of Materials Science and Engineering, University of New South Wales.
    Guo, S.J.
    Luleå tekniska universitet.
    Loberg, Bengt
    Easterling, Kenneth E.
    Luleå tekniska universitet.
    Microstructure and flux pinning in superconducting Bi-Pb-Sr-Ca-Cu-O wires1990In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 172, no 1-2, p. 63-70Article in journal (Refereed)
    Abstract [en]

    The critical current density (Jc) of Ag-clad Bi-Pb-Sr-Ca-Cu-O wire has been measured to be 1.2×104 A/cm2 at 77 K in zero field. The high Jc is attributed to a combination of elimination of the poisoning effect of Ag on superconductivity, grain alignment, and enhancement of flux pinning. Jc-H dependence was significantly improved in the Ag-clad tape, which has a Jc of 1.0×103 A /cm2 at 77 K and 4000 Oe, while the Jc of the sintered pellet drops two orders of magnitude at only 100 Oe. A pronounced anisotropy in Jc under high magnetic field is attributed to the grain alignment. Planar defects, such as heavy stacking faults parallel to the a-b plane in the rolled tape, are considered to be effective pinning centres.

  • 5.
    Grishin, A.
    et al.
    Kungliga tekniska högskolan, KTH.
    Niska, John
    Luleå tekniska universitet.
    Loberg, Bengt
    Weber, Hans
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Straightened voltage effect in high-Tc superconductors1994In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 76, no 10, p. 6947-6949Article in journal (Refereed)
    Abstract [en]

    A new effect called ``ac-current straightening'' has been observed in ceramic (Bi,Pb)-2223 slabs carrying ac current Idc+Iac cos(ωt). The current-voltage (I-V) characteristics of the ceramic were measured at 77 K at frequencies ranging from 50 to 20 000 Hz. A spectrum analyzer showed a series of high harmonics in the voltage signal as well as a constant voltage drop. The full set of experimental data has been explained theoretically using the Bean-Kim critical state model with a magnetic field dependent critical current jc(H)=jc(0)/(1+H/H0). A low transport ac current gives a voltage linearly proportional to the frequency and quadratically proportional to the ac-current amplitude Iac. It consists of odd harmonics only. If a bias dc current is switched on, then even harmonics and a dc-voltage drop appear. Their amplitudes are proportional to the small parameter Iac/cH0 and depend on the Idc/Iac ratio.

  • 6.
    Guo, S.J.
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Dou, S.X.
    University of New South Wales.
    Liu, H.K.
    University of New South Wales.
    Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors1992In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 27, no 11, p. 3043-3049Article in journal (Refereed)
    Abstract [en]

    Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J c, drops 80% from 124 (OT) to 29 A cm-2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm-2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm-2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ c=(α/B 0)/[(1+B/B 0)] n can be used to describe the field dependence of the critical current density, Jc, in the field range 0.2-1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB c2 for bismuth-based bulk highT c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.

  • 7.
    Johansen, T.H.
    et al.
    Olso Universitet.
    Bratsberg, H.
    Oslo universitet.
    Yang, Z.J.
    Oslo universitet.
    Guo, S.J,
    Luleå tekniska universitet.
    Loberg, Bengt
    Transition from elastic to dissipative motion in a magnet--high-T superconductor system1991In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 70, no 12, p. 7496-7499Article in journal (Refereed)
    Abstract [en]

    Using quasistatic harmonic excitations, we have measured the lateral force between a permanent magnet and a Bi-based ceramic high-Tc superconductor (HTSC) for lateral displacement amplitudes down to 1 µm. We find clear evidence for a transition from elastic (nondissipative) to inelastic interaction, and attribute the effect to flux pinning. The crossover amplitude can easily reach several micrometers, with the consequence that the lateral disturbance of a levitated magnet will decay to this amplitude, whereas further damping will be extremely slow. For applications of HTSCs in magnetic bearing systems this can be a very relevant aspect of the interaction, and it can set the limit for precision positioning of levitated objects

  • 8.
    Larker, R.
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Diffusion reactions between silicon oxynitride and silicon nitride during HIP-synthesis1988In: Journal de Physique. Colloque, ISSN 0449-1947, Vol. 49, no C-5, p. 219-225Article in journal (Refereed)
    Abstract [en]

    Solid state bonding by diffusion reactions is an attractive route to achieve joints that retain strength at high temperature. Intimate physical contact across the joint during bonding is important for the diffusion process. Hot isostatic pressing (HIP) can enhance the contact and thus produce a stronger joint at a lower bonding temperature than other methods. Silicon oxynitride synthesized from a stoichiometric mixture of silicon nitride and silicon oxide by HIP has some interesting properties that might be useful as an intermediate layer in bonding silicon nitride to metals. This work is a part of an assessment to develop solid state bonding methods for the joining of silicon nitride to superalloys by HIP. It presents the first steps in the development, namely the optimization of the synthesis of pure silicon oxynitride during densification by HIP and joining of the oxynitride to silicon nitride during synthesis and densification by HIP. The achievements in these areas and the direction of future work are discussed

  • 9.
    Larker, Richard
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Diffusional bonding reactions between silicon nitride, silicon oxynitride and Incoloy 909 by hot isostatic pressing1987In: High technology joining: 5th International conference : Papers., BNF Metals Technology Centre , 1987, p. 11.1-11.8Conference paper (Refereed)
    Abstract [en]

    Recent advances in production processes of high performance ceramics have made it possible to produce fully dense net shape turbocharger and gas turbine rotors made of silicon nitride. These rotors must usually be attached to a metal shaft by some joining technique. Solid state bonding by diffusion reactions is an attractive route to achieve joints that retain strength at high temperature. Intimate mechanical contact across the joint during bonding is important for the diffusion process. Hot isostatic pressing (HIP) can enhance the contact area and might produce a stronger joint at a lower bonding temperature than other methods. Silicon oxynitride synthesized from a stoichiometric mixture of silicon nitride and silicon oxide by HIP has some interesting properties that might be usesful as an intermediate layer in bonding silicon nitride to metals. This work is a part of an assessment to develop solid state bonding methods for the joining of silicon nitride to superalloys by HIP. It presents the first steps in the development, such as synthesis of pure silicon oxynitride, joining of the oxynitride to silicon nitride and the first attempt at a comparative study of joining reactions between silicon nitride and the superalloy vs. silicon oxynitride and the superalloy. Graphs, Photomicrographs, Diffraction patterns.

  • 10.
    Larker, Richard
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Johansson, Thomas
    Luleå tekniska universitet.
    Diffusion bonding reaction between silicon nitride, silicon oxynitride and Incoloy 909 by hot isostatic pressing1989In: Ceramic materials and components for engines: proceedings of the third international symposium, Las Vegas, Nevada, U.S.A., November 27-30, 1988 / [ed] Victor Joseph Tennery, Columbus, Ohio: American Ceramic Society, 1989, p. 503-512Conference paper (Refereed)
    Abstract [en]

    Solid state bonding diffusion reaction is an attractive route to achieve joints that retain strength at high temperature. Hot isostatic pressing (HIP) can enhance the interfacial area and thereby produce a stronger joint at a lower bonding temperature than other methods. Silicon oxynitride, Si2N2O, is often formed on the surface of silicon nitride components during sintering and has several properties that might be beneficial as an interfacial layer against metals. This work concerns the development of solid state bonding methods for the joining of silicon nitride to superalloys by HIP. It presents the first two steps in the development: the synthesis of pure dense Si2N2O for joining purposes and the first comparative study of the joining reactions Si3N4/superalloy vs Si2N2O/superalloy

  • 11.
    Larker, Richard
    et al.
    Luleå tekniska universitet.
    Nissen, A.
    IVF.
    Pejryd, L.
    Volvo Flygmotor.
    Loberg, Bengt
    Diffusion bonding reactions between a SiC/SiC composite and two superalloys during joining by hot isostatic pressing1992In: Acta Metallurgica et Materialia, ISSN 0956-7151, Vol. 40, no 11, p. 3129-3139Article in journal (Refereed)
    Abstract [en]

    Diffusion reactions during solid state joining of a ceramic SiC/SiC composite to two superalloys, Hastelloy X and Incoloy 909, by hot isostatic pressing (HIP) have been investigated. The HIP pressure was 200 MPa in all joining cycles, and the temperature/dwell time were either 800°C/15 min, 900°C/1 h or 1000°C/1 h. The reaction zones formed consisted of a thin layer of carbides surrounded by several layers containing silicides and free carbon. The thicknesses of the reaction layers increased with increasing temperature, but were more affected by the composition of the alloy. With more carbide formers in the alloy, the thickness of the reaction layer decreased. The SiC composite was found to be considerably more prone to reactions with these superalloys during HIP as compared to Si3N4 under similar conditions

  • 12.
    Larker, Richard
    et al.
    Luleå tekniska universitet.
    Wei, Liu-Ying
    Luleå tekniska universitet.
    Olsson, Mikael
    Depertment of Materials Sciences, Uppsala university.
    Loberg, Bengt
    Diffusion bonding of Si3N4/TiN and Si3N4/TiB2 composites to Incoloy 9091992In: 4th International Symposium on Ceramic Materials and Components for Engines: [proceedings ... held at Göteborg, Sweden, 10 - 12 June 1991] / [ed] Roger Carlsson; Thomas Johansson; Lars Kahlman, London: Elsevier, 1992, p. 340-347Conference paper (Refereed)
    Abstract [en]

    Hot isostatic pressed (HIPed) mixtures of Si3N4/60%TiN and Si3N4 /50%TiB2 were joined to Incoloy 909 (42%Fe, 38%Ni, 12.9%Cr, 4.7%Nb, 1.5%Ti, 0.4%Si) by hot isostatic pressing at 927 deg.C and 200 MPa. Microstructure (TEM and STEM) and diffusion profiles are reported and discussed. The work is relevant to the use of graded ceramics to reduce thermomechanical stresses in silicon nitride /superalloy joints

  • 13.
    Larker, Rickard
    et al.
    Luleå tekniska universitet.
    Anevik, K.
    Kristiansson, S.
    Loberg, Bengt
    Heat treatments of the low-expansion superalloy incoloy 909 for application in ceramic/metal joints and in metal matrix composites1992In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 13, no 1, p. 11-15Article in journal (Refereed)
    Abstract [en]

    Increasing efficiency demands on gas turbines have promoted the development of superalloys with low thermal expansion up to intermediate temperatures, to reduce the need of cooling for preservation of efficient clearances between rotating and stationary parts. These materials are also of prime interest for joins to engineering ceramics such as silicon nitride and silicon carbide, and as a matrix with W or SiC fibres in metal matrix composites (MMC). The paper deals with the thermomechanical and microstructural stability of Incology 909 during possible joining and densification treatments. Thermal expansion behaviour and joining/densification temperatures suitable for retaining the desired structure of the superalloy are presented.

  • 14.
    Larker, Rickard
    et al.
    Luleå tekniska universitet.
    Wei, L.-Y
    Luleå tekniska universitet.
    Loberg, Bengt
    Olsson, M.
    Department of Materials Science, Uppsala University.
    Johansson, S.
    Department of Materials Science, Uppsala University.
    AEM investigation of ceramic/incology 909 diffusional reactions after joining by HIP1994In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 29, no 16, p. 4404-4414Article in journal (Refereed)
    Abstract [en]

    Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si3N4 and either 60 vol% TiN or 50 vol% TiB2, while three were monolithic materials, namely Si3N4 with 2.5 wt% Y2O3 as a sintering additive, Si3N4 without additives, and Si2 N2O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 μm in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (≤100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.

  • 15. Loberg, Bengt
    et al.
    Easterling, Kenneth E.
    Luleå tekniska universitet.
    Metallographical study of hot isostatically pressed copper encapsulation of nuclear fuel elements1986In: Scientific basis for nuclear waste management IX: symposium held September 9 - 11, 1985, Stockholm, Sweden / [ed] Lars O. Werme, Pittsburgh, Pa: Materials Research Society, 1986, p. 453-459Conference paper (Refereed)
    Abstract [en]

    Light optical and electron microscopy have been used to study both laboratory-scale and full-scale hot isostatically pressed (HIPed) copper as employed for encapsulation of spent nuclear fuel elements. This work includes the development of computing models for predicting the optimal conditions for HIPing copper and other possible encapsulation materials such as lead and alumina. Mechanical testing of HIPed samples has also been carried out. The current method of using hydrogen to reduce oxides at powder surfaces prior to HIPing is questioned on the basis that metallography reveals definite signs of hydrogen embrittlement. An alternative method of reduction is to use CO/CO//2 mixtures instead of hydrogen, and it is suggested that this approach could be strongly advantageous in future

  • 16. Loberg, Bengt
    et al.
    Johansson, T.
    Luleå tekniska universitet.
    Easterling, K.E.
    Luleå tekniska universitet.
    Improvement in toughness of welded constructional steels through titanium additions1980In: Philosophical Transactions of the Royal Society of London. Series A, ISSN 0080-4614, Vol. 295, no 1413, p. 306-Article in journal (Refereed)
    Abstract [en]

    Summary form only given. The authors present an interim report on the effect of various alloying additions in the weld deposit and base material on the precipitate dispersion and stability, the microstructure and toughness of microalloyed constructional steels, in both the as-welded as well as welded and stress-relieved conditions. Materials investigated include welded AlTi, AlTiV, AlTiNb and AlV steels with the use of electrodes either with or without Mo additions

  • 17. Loberg, Bengt
    et al.
    Niska, John
    Luleå tekniska universitet.
    Rubin, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Measurements of the self-field limitations of the critical current in Bi-2223 superconductive bars1993In: Proceedings of the 3rd international conference and exhibition: World Congress on Superconductivity : 15 - 18 September 1992, Munich, Germany, Oxford: Pergamon Press, 1993, p. 1757-1762Conference paper (Refereed)
  • 18. Loberg, Bengt
    et al.
    Niska, John
    Luleå tekniska universitet.
    Rubin, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gustafsson, R.
    ABB Corporate Research, 721 78, Västerås.
    High critical currents in Bi-2223 phase bars1993In: Cryogenics (Guildford), ISSN 0011-2275, E-ISSN 1879-2235, Vol. 33, no 5, p. 475-476Article in journal (Refereed)
    Abstract [en]

    The critical current Ic and the critical current density Jc of Bi-2223 phase bars have been found to depend on the mass density of the bars. Bars densified using a cold isostatic pressing (CIPing) process have been obtained with transport critical currents of gt;150 A with Jc levels up to approximately equals 103 A cm-2 at 77 K in the absence of an applied magnetic field. Bending or kinking of the grains may promote densification during CIPing of a sintered bar.

  • 19. Loberg, Bengt
    et al.
    Nordgren, A.
    Luleå tekniska universitet.
    Strid, J.
    Easterling, Kenneth
    Luleå tekniska universitet.
    Role of alloy composition on the stability of nitrides in Ti-microalloyed steels during weld thermal cycles1984In: Metallurgical Transactions. A, Physical Metallurgy and Materials Science, ISSN 0360-2133, Vol. 15A, no 1, p. 33-41Article in journal (Refereed)
    Abstract [en]

    The effect of cycling on titanium nitride stability in a range of normalized steels containing additions of V, Al, and N has been investigated. Nitride dispersions and chemical analysis of individual particles are studied using transmission electron microscopy and a quantitative 200 kV STEM-EDX-EELS microanalysis system. Whereas normalized material contains nitrides of mixed compositions, only nitrides based on TiN survive high energy simulated weld cycles. Grain growth is dependent on stability, and this depends on the original composition of the nitrides. The presence of aluminum is particularly detrimental. The best gain growth control and highest toughness are found in steels based on optimum ratios of Ti/V/N and on low Al levels

  • 20. Loberg, Bengt
    et al.
    Rubin, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    HIPing of rapidly solidified high-carbon steel powder1988In: HIP: proceedings of the International Conference on Hot Isostatic Pressing, Luleå, 15-17 June 1987 / [ed] Tore Garvare, Luleå: CENTEK , 1988, p. 205-208Conference paper (Refereed)
    Abstract [en]

    The investigated alloy had a nominal composition of Fe--1.6C--1Cr. HIP-parameteres used: T = 750, 650, 550 deg C, p = 200 MPa, t = 2 h. Even though densification is relatively easily achieved due to the small grain size, the surface contaminants (especially oxides) play a key roll in the bulk properties and create problems which increase as the HIP temperature decreases. The oxide solubility is too low for dissolution mechanisms to work, with the result that the fragmented oxide layer stays in the vicinity of the prior particle boundaries, influences the area of metallic bonding and thus decreases material performance (e.g. ductility). Photomicrographs

  • 21.
    Niska, John
    et al.
    Luleå tekniska universitet.
    Andersson, B.M.
    Umeå universitet.
    Loberg, Bengt
    Easterling, Kenneth
    Luleå tekniska universitet.
    Sundqvist, B.
    Umeå universitet.
    Formation of the 124-phase superconductor YBa2Cu4O8 by retaining oxygen in a reaction HIP sintering process1990In: Journal of materials science letters, ISSN 0261-8028, E-ISSN 1573-4811, Vol. 9, no 7, p. 770-771Article in journal (Refereed)
    Abstract [en]

    The sample was hot isostatically pressed at about 1005°C and 200 MPa using a hold time of 2h. The HIP was slowly cooled to about 400°C while retaining high pressure to avoid decomposition. The zero resistance Tc is about 70 K, which is lower than expected. The microhardness can be related to the yield strength, with the maximum strength approximately one-third the Vickers hardness. The use of HIP to form the 124-phase superconductor by reaction sintering gives a dense bulk superconducting material suitable for mechanical and electrical tests requiring sintered bodies. The superconductor has high hardness and therefore a high compressive strength, but it was also brittle. This brittle behaviour could lead to difficulties is fabricating electrical devices from the bulk material

  • 22.
    Niska, John
    et al.
    Luleå tekniska universitet.
    Babushki, O.
    Luleå tekniska universitet.
    Loberg, Bengt
    High-temperature X-ray diffraction of the PbBi-2223 phase superconductor in different atmospheres1993In: Journal of materials science letters, ISSN 0261-8028, E-ISSN 1573-4811, Vol. 12, no 17, p. 1337-1339Article in journal (Refereed)
  • 23.
    Niska, John
    et al.
    Luleå tekniska universitet.
    Easterling, K.E.
    Luleå tekniska universitet.
    Loberg, Bengt
    Thermal expansivity of HIP synthesized YBa2Cu4O81991In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 44, no 1, p. 93-94Article in journal (Refereed)
    Abstract [en]

    The thermal expansion coefficient of a sample of the YBa2Cu4O8 phase superconductor was measured using a low temperature X-ray diffractometer. The thermal expansivity was found to be highly anisotropic. The average coefficient of thermal expansion over the temperature range of 150 to 450 K was approximately equal for the a and b axes at 9×10-6/°C, vs a coefficient of about 15×10-6/°C for the c axis. This would indicate that the bonding is weaker along the c-axis than in the ab-plane and that the specific heat of single crystals of the high Tc superconductors can depend on their geometry. The low thermal expansivity in the ab-plane relative to typical metal conductors as copper and silver could lead to compressive stresses in the superconductor when cooling a composite metal-superconductor wire having good alignment of the ab-plane along the wire axis

  • 24.
    Niska, John
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Evidence for unpinned magnetic flux vortices above the Ic transition in the 2223-phase high-Tc superconductor1993In: Journal of Superconductivity, ISSN 0896-1107, E-ISSN 1572-9605, Vol. 6, no 4, p. 255-259Article in journal (Refereed)
    Abstract [en]

    similarity in the dc voltage-current (V I) curves for both direct and alternating transport currents is used to propose that unpinned flux vortices are generated above the Ic transition for dc transport currents, when Abrikosov flux vortices begin to penetrate the superconductor. Two methods can be used to give a dc voltage drop for an ac transport current: (1) if there is a slight dc voltage in the ac current which favors vortex loop collapse as in a traditional dc Ic test, or (2) if an asymmetric transverse magnetic field is present which favors vortex loop collapse for current in one direction over the reverse direction.

  • 25.
    Niska, John
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Hot isostatic pressing of high temperature superconducting ceramics1994In: Materials technology (New York, N.Y.), ISSN 1066-7857, E-ISSN 1753-5557, Vol. 9, no 9-10, p. 211-215Article in journal (Refereed)
    Abstract [en]

    Large specimens of ceramic superconductors are difficult to solid-state sinter to full density, while maintaining superconductivity, without the high temperatures and pressures found in a hot isostatic press (HIP). HIPing can give highly dense, superconducting Bi2Sr2CaCu2O8 (Bi2212) and Bi2Sr2Ca2Cu3O10 (Bi2223) with only slight decomposition. If Y123 is HIPed to full density in a metal capsule, then the nonsuperconducting tetragonal phase forms as a result of decomposition. HIPing of CuO plus YBa2Cu3O7 gives highly dense, superconducting YBa2Cu4O8 (Y124), which is stable at high temperatures and oxygen pressures.

  • 26.
    Niska, John
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Retaining oxygen when hiping the YBa2Cu3O7 superconductor1989In: Hot isostatic pressing: theory and applications : International conference : Papers / [ed] R.J. Schaefer; M. Linzer, ASM International, 1989, p. 305-313Conference paper (Refereed)
    Abstract [en]

    series of HIP cycles were made on powder samples of the high Tc superconductor YBa2Cu3O7-x in the range of 600 to 1000 C, and pressures of 150 to 200 MPa. The high Tc superconducting orthorhombic phase is difficult to fully densify, because the material easily decomposes and releases oxygen. Two types of encapsulation materials have been used, mild steel and borosilicate glass, of which, glass has a lower oxygen diffusivity and reactivity than steel. Two decomposition regimes were observed. The first regime is from about 600 to 850 C when there is a partial decomposition leading to oxygen backpressure in glass and a transformation of the orthorhombic phase into the tetragonal phase in steel. The second stage of decomposition is above about 900 C, when the superconductor powder in a steel encapsulation decomposes into simpler oxides, and in glass nearly full density results, but without the desired orthorhombic YBa2Cu3O7 structure. Addition of excess CuO has been found to yield the 124 phase (YBa2Cu4O8) superconductor. The 124 phase is the thermodynamically favoured phase at high temperatures and pressures. The 124 phase samples reached nearly full density at the highest temperatures used

  • 27.
    Niska, John
    et al.
    Luleå tekniska universitet.
    Loberg, Bengt
    Easterling, K.E.
    Luleå tekniska universitet.
    Effect of oxygen loss on densification when hot isostatic pressing YBa2Cu3O7-δ1989In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 72, no 8, p. 1508-1510Article in journal (Refereed)
    Abstract [en]

    Hot isostatic pressing of the high-Tc superconductor YBa2Cu3O7-δ can lead to loss of oxygen and transformation of the material from the high-Tc orthorhombic phase to the nonsuperconducting tetragonal phase. It is shown that glass encapsulation helps retain the orthorhombic structure, whereas steel encapsulation resulted in formation of the tetragonal phase. Reasons for this phenomenon are discussed. The equilibrium oxygen gas pressure for the oxygen decomposition reaction in YBa2Cu3O7, however, prevents full densification of this material in glass when employing hot isostatic pressing conditions of 200 MPa and 845°C.

  • 28.
    Wei, L.Y.
    et al.
    Luleå tekniska universitet.
    Guo, S.J.
    Luleå tekniska universitet.
    Loberg, Bengt
    TEM studies of single- and poly-crystals in Bi (Pb)-Sr-Ca-Cu-O system1991In: Micron and Microscopica Acta, ISSN 0739-6260, E-ISSN 1878-111X, Vol. 22, no 1-2, p. 191-192Article in journal (Refereed)
  • 29.
    Yang, Z.J.
    et al.
    University of Oslo.
    Johansen, T.H.
    University of Oslo.
    Bratsberg, H.
    University of Oslo.
    Skjeltorp, A.T.
    Institute for Energy Technology, Kjeller.
    Guo, S.J.
    Luleå tekniska universitet.
    Loberg, Bengt
    Study of flux motion in Bi-based superconductors by a pendulum1992In: Journal of Superconductivity, ISSN 0896-1107, E-ISSN 1572-9605, Vol. 5, no 2, p. 127-135Article in journal (Refereed)
    Abstract [en]

    Using a computerized mechanical pendulum, we have studied the lateral magnetic flux motion of the Bi-based high-T c superconductors in the quasi-static magnetic field (0.1 Hz) for the range 1 μm to 0.35 mm. The effect of d.c. and a.c. transport currents on the flux motion has also been investigated. We argue that the lateral magnetic stiffness is a measure of the pinning potential of the fluxoids in the superconductors. The pinning energy barrier ε is estimated to be about 0.3 eV based on the stiffness measurements, which is comparable with the data from the YBa2Cu3O7-χ superconductors.

1 - 29 of 29
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