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  • 201.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Electrical resistance of YBa2Cu3O7-delta in the range 300-500 K and 0-1.5 GPa1988In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 66, no 6, p. 623-627Article in journal (Refereed)
    Abstract [en]

    The electrical resistance R of single-phase, sintered YBa2Cu3O7−δ has been studied as a function of pressure p and temperature T in the range 300 to 500 K and 0 to 1.5 GPa (15 kbar). R is linear in T and has an initial pressure coefficient of about −13 %/GPa. The pressure dependence of Tc has also been measured up to 1 GPa, with the result dTc/dp = 0.45 K/GPa. The results are discussed in terms of electron-electron scattering, electron-phonom scattering, and the resonating valence bond theory, but none of these can fully explain the results.

  • 202.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Fullerenes under high pressure2000In: Fullerenes: Chemistry, Physics and Technology / [ed] Karl M. Kadish and Rodney S. Ruoff, New York: John Wiley & Sons , 2000, p. 611-690Chapter in book (Other academic)
    Abstract [en]

    This Chapter deals with the structures and physical properties of solid fullerenes under high pressures. Subjects discussed include the interaction between fullerenes and pressure media, the phase transformations and phase diagrams of C60 and C70, and the structural, electric, and optical properties of fullerenes. Special emphasis is put on the compressibility and bulk moduli of fullerenes. The review also treats the polymerization of fullerenes under pressure, and the reaction phase diagrams and the high pressure structural phases are discussed in detail.

  • 203.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fullerenes under high pressures1999In: Advances in Physics, ISSN 0001-8732, E-ISSN 1460-6976, Vol. 48, no 1, p. 1-134Article, review/survey (Other academic)
    Abstract [en]

    This paper reviews the properties and phases of fullerenes and their derivatives and compounds under high pressures. For obvious reasons most of the paper deals with C60 but the materials reviewed also include C70, simple derivatives of C60, carbon nanotubes, and intercalation compounds of C60 with both acceptors and donors, mainly alkali metals. After a brief overview of high-pressure techniques and the structures and properties of C60 at atmospheric pressure, the structural phase diagram of C60 from atmospheric pressure to above 40GPa (400kbar) is reviewed. The evolution with pressure of the orientational and translational structure of 'normal' molecular C60 in the range up to 1-5GPa (depending on temperature) is discussed in some detail, as is the appearance of a large number of polymeric phases at higher pressures and temperatures, some of them known to have extreme mechanical properties. At very high static (or shock) pressures or temperatures, C60 transforms into ordered or disordered forms of diamond or graphite. The phase diagram is reasonably well investigated up to near 10GPa, but at higher pressures there are still large gaps in our knowledge. Available experimental data for the physical properties of both monomeric and polymeric C60 under high pressures are reviewed as far as possible. The compression behaviour of C60 has been well investigated and is discussed in detail because of its basic importance, but optical, electrical and lattice properties have also been studied for several of the many structural phases of C60. Whenever possible, experimental data are compared with the results of theoretical calculations. The phase diagram and properties of C70 are much less known because of the larger complexity caused by the anisotropy of the molecule, and very little is known about most compounds of C60. However, noble-gas intercalation in C60 has been reasonably well investigated. Finally, the high-pressure properties of superconducting alkali-metalintercalated C60 are briefly reviewed.

  • 204.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Fullerites and hard carbons2001In: Encyclopedia of Materials: Science and Technology, Oxford: Pergamon/Elsevier , 2001, p. 3387-3395Chapter in book (Other (popular science, discussion, etc.))
  • 205.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    High metallic resistivities and resistivity saturation: High-pressure and thermal expansion effects 1993In: Modern physics letters B, ISSN 0217-9849, Vol. 7, no 8, p. 491-499Article, review/survey (Other academic)
    Abstract [en]

    Some recent developments in the field of resistivity saturation in metallic materials are discussed, concentrating on effects of thermal expansion and high pressures. It is shown that thermal expansion effects can significantly modify the measured temperature dependence of the resistivity, and that high pressure studies are an important, but little used, source of information. Examples are shown for transition metals and alloys, high transition temperature superconductors, and graphite intercalation compounds.

  • 206.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    High Pressure Phases and Properties of C601998In: High Pressure in Food Science, Bioscience and Chemistry: Proceedings of the 35th Joint Meeting of the European High Pressure Research Group and the Food Chemistry Group of the Royal Society of Chemistry on High Pressure Food Science, Bioscience and Chemistry, Reading 1997, Cambridge: The Royal Society of Chemistry , 1998, p. 12-17Conference paper (Refereed)
    Abstract [en]

    Fullerenes are molecular solids with very strong intramolecular bonds but very weak intermolecular ones. However, a large number of reactive intramolecular double bonds on each molecule also enables C60 to form covalent intermolecular bonds under pressure. This implies that a) pressure has a large effect on the physical properties of C60 in its pristine zero-pressure forms, and b) treatment at high pressure and high temperature leads to the formation of new polymeric phases with covalent intermolecular bonds, phases which have new, interesting properties and which may have practical applications in the future. I discuss here some recent experimental results on the structure and properties of C60 under high pressure.

  • 207.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Improving thermal insulation in high pressure experiments1998In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 69, no 9, p. 3433-3434Article in journal (Refereed)
    Abstract [en]

    The use of internal ovens is common in high-temperature–high-pressure experiments. Improving the thermal insulation means better efficiency and lower thermal strain on pressure vessels and feedthroughs, but is difficult when using solid media. The addition of a layer of amorphous solid (glass) in the form of a mixed powder is reported to give a significant reduction (20%–50%) in the power dissipated in a high pressure oven.

  • 208.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Interaction between C60 and gases under high pressure2003In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 29, no 5, p. 440-444Article, review/survey (Other academic)
    Abstract [en]

    A brief review is given of the interaction between fullerite C60 and various gases under elevated pressure. Subjects discussed include the formation of ordered interstitial gas-fullerene compounds, reactions between intercalated gases and fullerene molecules to form new endohedral and exohedral compounds, and changes in the structure and properties of C60 because of intercalated gas atoms or molecules.

  • 209.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics. Jilin Univ, State Key Lab Superhard Mat, Changchun 130012, Jilin, Peoples R China.
    Intermolecular bonding in C70 at high pressure and temperature2017In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 125, p. 258-268Article in journal (Refereed)
    Abstract [en]

    Near-infrared (NIR) Raman spectroscopy has been used to analyze intermolecular bonding in solid C70, treated for two hours under pressures in the range 0.8 to 2 GPa at temperatures between 350 and 600 K. The spectra obtained are analyzed in detail with the aim of finding spectral lines characteristic for the monomer C70, the dimer C140, and possibly for higher oligomers. In agreement with earlier studies, several new modes are observed in the NIR Raman spectra, in sharp contrast to what is found in Raman studies using visible light excitation. From the relative intensities of the characteristic dimer stretch line at 88 cm-1 and a mode at 457 cm-1, characteristic for the monomer, a semi-quantitative estimate for the dimer fraction in the material can be found as a function of pressure and temperature. From these results a reaction map, similar to that recently derived for C60, can be drawn. The data obtained indicate that C70 is almost completely polymerized into large, random oligomers (or polymers) after long-time treatment at elevated temperatures (> 500 K) and pressures (>1.2 GPa). The phase diagram of C70 is updated and briefly discussed in the light of this and other recent high pressure studies.

    The full text will be freely available from 2019-11-04 09:18
  • 210.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Low T hydrostatic limits of n-pentane/iso-pentane mixture measured by a self-supporting Manganin pressure gauge1987In: Journal of Physics. E, Scientific Instruments, ISSN 0022-3735, Vol. 20, no 8, p. 984-986Article in journal (Refereed)
    Abstract [en]

    A simple design for a self-supporting Manganin pressure gauge is described. The shear-strain sensitivity of the gauge has been used to determine the hydrostatic pressure limit below 150 K of the 50/50 mixture by volume of n- and isopentane that is commonly used as a pressure-transmitting medium in high-pressure experiments.

  • 211.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mapping intermolecular bonding in C602014In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, p. 06171-Article in journal (Refereed)
    Abstract [en]

    The formation of intermolecular bonds in C60 has been investigated in detail at pressures below 2.2 GPa and up to 750 K. Fullerene samples were heated in a temperature gradient to obtain data on the formation of dimers and low-dimensional polymers along isobars. Intermolecular bonding was analyzed ex situ by Raman scattering, using both intramolecular modes and intermolecular stretching modes. Semi-quantitative reaction maps are given for the formation of dimers and chains. The activation energy for dimer formation decreases by 0.2 meV pm-1 when intermolecular distances decrease and dimer formation is noticeably affected by the rotational state of molecules. Above 400-450 K larger oligomers are formed; below 1.4 GPa most of these are disordered, with small domains of linear chains, but above this the appearance of stretching modes indicates the existence of ordered one-dimensional polymers. At the highest pressures and temperatures two-dimensional polymers are also observed.

  • 212.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Point defects and thermal conductivity of C601993In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 48, no 19, p. 14712-14713Article in journal (Refereed)
    Abstract [en]

    Recent literature data for the thermal conductivity κ of single-crystal C60 are analyzed using a standard model. It is shown that in the intermediate range 100–250 K, below the rotational transition, κ can be described extremely well by a model taking into account phonon and point defect scattering only. The magnitude derived for the phonon-phonon scattering term agrees very well with an estimate from a simple model.

  • 213.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Polymeric phases of fullerenes2004In: Fullerene-Based Materials: Structures and Properties, Berlin/Heidelberg: Springer , 2004, p. 85-106Chapter in book (Other academic)
    Abstract [en]

    Fullerenes, in particular C60, can form covalent intermolecular bonds ("polymerize") when treated at high temperature under high pressure. This contribution briefly reviews some recent advances in this field. Short overviews are given of the pressure-temperature phase diagrams of pristine C60 and C70 and of their physical properties under high pressure. The paper has its main emphasis on the structures and properties of the one- and two-dimensional (chains and planes) polymeric phases of C60 created in the range 1–9 GPa (10–90 kbar) and up to 1100 K. Their structures, as obtained from studies on polymerized single crystals, the treatment conditions used to obtain well-defined structures, and some selected physical properties, including the surprising discovery of ferromagnetism, are discussed. The paper presents the results of both ex situ and in situ structural studies. Three-dimensional polymers, and the possibility to create well-defined three-dimensional structures by high-pressure treatment of low-dimensional polymers, are also discussed. Finally, a short section devoted to higher fullerenes discusses the polymeric forms of C70, including the ordered zigzag chain phase recently created under hydrostatic pressure conditions.

  • 214.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Polymerization of C60 under pressure revisited2012In: Extended Abstracts of Carbon 2012, Krakow: Polish Carbon Society , 2012, p. 608-Conference paper (Other academic)
    Abstract [en]

    The polymerization of C60 by covalent bonding has been studied as a function of pressure and temperature using a new method, in which a rod-shaped sample is heated at one end. After quenching a reaction record is obtained, documenting the results of different reaction conditions along the rod. The results are analyzed using Raman spectroscopy. It is shown that dimers dominate the reaction results over a narrow temperature rane from 0.8 to 1.8 GPa, and that pure polymeric phases are not obtained at any reaction coordinate below 2 GPa and 500 K.

  • 215.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pressure-temperature phase relations in complex hydrides2009In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 150, p. 175-195Article, review/survey (Refereed)
    Abstract [en]

    Interest in hydrogen as a future energy carrier in mobile applications has led to a strong increase in research on the structural properties of complex alkali metal and alkaline earth hydrides, with the aim to find structural phases with higher hydrogen densities. This contribution reviews recent work on the structural properties and phase diagrams of these complex hydrides under elevated pressures, an area where rapid progress has been made over the last few years. The materials discussed in greatest detail are LiAlH4, NaAlH4, Li3AlH6, Na3AlH6, LiBH4, NaBH4, and KBH4. All of these have been studied under high pressure by various methods such as X-ray or neutron scattering, Raman spectroscopy, differential thermal analysis or thermal conductivity measurements in order to find information on their structural phase diagrams. Based mainly on experimental studies, preliminary or partial phase diagrams are also given for six of these materials. In addition to this information, data are provided also on experimental results for a number of other complex hydrides, and theoretical predictions of new phases and structures under high pressures are reviewed for several materials not yet studied experimentally under high pressure.

  • 216.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Proceedings of the 11th European Conference on Thermophysical Properties (ECTP-11), Umeå, Sweden, 13-16 June 19881989Conference proceedings (editor) (Other (popular science, discussion, etc.))
  • 217.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China.
    Raman identification of C70 monomers and dimers2017In: Diamond and related materials, ISSN 0925-9635, E-ISSN 1879-0062, Vol. 73, p. 143-147Article in journal (Refereed)
    Abstract [en]

    Fullerenes easily polymerize under high pressure by forming covalent intermolecular bonds. For C60 this reaction is easily monitored by Raman scattering, but for C70 no simple method to determine its bonding state is known. In this work C70 has been heated over a wide range of temperatures at a pressure of 1.6 GPa and the treated material has been studied by Raman spectroscopy, using 830 nm laser excitation to avoid photo-induced polymerization. By comparing the present data with earlier results for pure dimers, zig-zag chain polymers and pressure treated powders, characteristic fingerprint peaks can be found for the C70 monomer and the C140 dimer. The molecular stretching mode near 88 cm-1 is a clear fingerprint for dimers while the strong A1′ peak near 455 cm-1 clearly shows the presence of monomers. Several other new peaks appear in pressure-treated material and the relative intensities of many peaks change in a systematic way, but it is not clear whether these changes indicate the presence of dimers or of small oligomers in general. Simple strategies for semi-quantitative structural analysis of pressure-treated C70 material by Raman spectroscopy are briefly discussed.

  • 218.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    RC oscillator linearizes thermistor output1983In: Electronics, ISSN 0883-4989, Vol. 56, no 1, p. 169-171Article in journal (Other (popular science, discussion, etc.))
  • 219.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Resistivity saturation in fcc La under high pressure1992In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 69, no 18, p. 2693-2696Article in journal (Refereed)
    Abstract [en]

    Data are presented for the electrical resistivity ρ of fcc La as a function of T and p in the range 70 to 700 K and 0 to 1.25 GPa. The data are analyzed using the phenomenological ‘‘parallel resistivity’’ formula for resistivity saturation, which describes very well the T dependences of both ρ and its pressure coefficient. The pressure coefficients of the electron-phonon interaction parameter and Tc are calculated from the p dependence of the electron-phonon component, and the p dependence of the parallel saturation resistivity is obtained from band-structure calculations using a simple Bloch-Boltzmann model. In both cases excellent agreement with experiment is found.

  • 220.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Senaste nytt om fullerener2001In: Fysikaktuellt, ISSN 0283-9148, no 4, p. 19-22Article in journal (Other (popular science, discussion, etc.))
  • 221.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Simple electronic resistance bridge with µOhm resolution at low current1985In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 56, no 11, p. 2166-2167Article in journal (Refereed)
    Abstract [en]

    A simple electronic resistance bridge is described. The bridge compares an unknown four-terminal resistor in the mOmega range with a precision resistance decade in the kOmega range. A lock-in amplifier is used as zero detector. With 10-mA exciting current a resolution better than one part in 3000 is obtained for unknown resistors in the range 3–100 mOmega. Experimental results are shown for a gold sample under pressure in the range 0–35° C and 0–1.3 GPa.

  • 222.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Simple, wide range linear temperature-to-frequency converters using standard thermistors1983In: Journal of Physics E: Scientific Instruments, ISSN 0022-3735, Vol. 16, no 4, p. 261-264Article in journal (Refereed)
    Abstract [en]

    The exponential voltage-time relationship of a charging RC circuit is used to linearise the approximately exponential resistance (R) against temperature (T) behaviour of a thermistor. The resulting output signal is a frequency, linear in and directly proportional to absolute temperature. Using standard, inexpensive thermistors a temperature error of less than 0.5K over a 75K range is obtained, the error being due to deviations from the thermistor's nominally exponential R(T) relationship. The circuit can be used to linearise any transducer having an exponential relationship between input and output signals, or to obtain an output signal proportional to the logarithm of any input voltage signal (that is, proportional to the input signal in dB).

  • 223.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    The electrical resistivity of La under pressure between 70 and 300 K1991In: High Pressure Research, volume 7 and volume 8, issues 1-3: High Pressure and Materials; Proceedings of the 28th EHPRG Conference and 1st European Technology Forum, Bordeaux 1990., Reading: Gordon and Breach , 1991, p. 250-252Conference paper (Refereed)
    Abstract [en]

    The electrical resistivity of fcc La has been measured as a function of temperature and pressure between 70 and 300 K. The pressure coefficient of resistance dlnR/dp is found to decrease from +0. 022 GPa-1 at 75 K to -0. 014 GPa-1 at 300 K.

  • 224.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    The structures and properties of C60 under pressure1999In: Physica B, vol 265: Proceedings of the 36th meeting of the European High Pressure Research Group (EHPRG 98), Catania 1998, Elsevier B.V. , 1999, p. 208-213Conference paper (Refereed)
    Abstract [en]

    The strong intramolecular bonds and very weak intermolecular bonds in fullerenes such as C60 lead to a strong pressure dependence in many of their physical properties and to subtle structural changes under pressure. Compression of the lattice in combination with orientational ordering also enables rather easy formation of covalent intermolecular bonds under pressure, such that treatment at high pressure and high temperature leads to the formation of several different polymeric phases. Some recent experimental results on the structure and properties of C60 under high pressure are discussed, with particular emphasis on the effects of changes in the orientational structure on the bulk modulus and the effects of polymer dimensionality on the macroscopic physical properties of C60 polymers, respectively.

  • 225.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    The thermal conductivity of metals under pressure1984In: High Pressure in Science and Technology, Part I: Collective Phenomena and Transport Properties: Proceedings of 9th AIRAPT International High Pressure Conference, Albany, NY, 1983. Materials Research Society Symposia Proceedings vol. 22., New York: North-Holland, Elsevier Science Publishing Co., Inc. , 1984, p. 261-268Conference paper (Refereed)
    Abstract [en]

    Available experimental data for the pressure dependence of the thermal conductivity of metals are reviewed and compared with the predictions of simple theoretical models. Good agreement is found for simple metals, while experimental results for ferromagnetic transition metals re found to be anomalous, in that Wiedemann-Franz law is not even approximately obeyed under pressure.

  • 226.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity and Lorenz number of nickel under pressure1981In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 37, no 3, p. 289-291Article in journal (Refereed)
    Abstract [en]

    The thermal and electrical conductivities of nickel have been studied under hydrostatic pressures up to 1.6 GPa at room temperature. Allowing for the pressure dependence of the lattice thermal conductivity, the electronic Lorentz function increases with pressure with an approximate pressure coefficient of 2 × 10-2 GPa-1. This is in strong disagreement with the standard Bloch-Grüneisen theory which predicts a volume independent Lorentz number

  • 227.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Thermal conductivity of some metals under pressure1978Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Although measurements of a wide variety of physical properties have been carried out under pressure during the last decades, data for thermal conductivity under pressure are sparse. This is especially so in the case of metals, where the experimental difficulties are very large. In this work a new method to make such measurements is presented, together with experimental results for copper, silver, gold, and aluminium at room temperature and pressures up to 2.5 GPa.The first part of the work describes in some detail the method used.This method is a variation of the well known Ångström method. The measured property is thus not the thermal conductivity, but instead the thermal diffusivity. Both the Ångström method, and the variation of this used in this work, are described in detail. Several further simplifications of the method are also investigated. The experimental equipment is described in detail, as is also the construction of samples and pressure cells.After a description of the experimental procedure, experimental results are presented for the thermal diffusivity of the metals studied, at pressures up to 2.5 GPa. These data are converted to thermal conductivity data by the use of available data on density and specific heat capacity for these metals under pressure. In a few cases, theoretically calculated values for the latter quantity are used. After a brief look at the theory for the thermal conductivity of metals, the experimental results are compared to simple theoretical predictions. The agreement is found to be good. Contrary to theoretical predictions, however, the electronic Lorentz function is found to increase with increasing pressure for the noble metals.

  • 228.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of some metals under pressure1980In: High Pressure Science and Technology: Proceedings of the 7th International AIRAPT High Pressure Conference, Le Creusot 1979, Oxford: Pergamon Press , 1980, p. 483-485Conference paper (Refereed)
  • 229.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal diffusivity and thermal conductivity of Chromel, Alumel, and Constantan in the range 100-450 K1992In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 72, no 2, p. 539-545Article in journal (Refereed)
    Abstract [en]

    Data are presented for the thermal diffusivity and the thermal conductivity of commercially available Chromel, Alumel, and Constantan alloys, and the specific heat capacity of Alumel, over the temperature range 100 to 450 K. Over this range, the thermal conductivity of all materials increases by more than 50%; room-temperature values are 17, 29, and 23 W m−1 K−1, respectively. The thermal conductivity data are discussed in terms of simple theory.

  • 230.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal diffusivity measurements by Ångström's method in a fluid environment1991In: International journal of thermophysics, ISSN 0195-928X, E-ISSN 1572-9567, Vol. 12, no 1, p. 191-206Article in journal (Refereed)
    Abstract [en]

    Ångström's method has been used for measuring the thermal diffusivity, a, and the radial heat loss coefficients of a thin constantan wire, both in a vacuum and with the wire immersed in air and in four different liquids, and using temperature wave periods of 1 to 1000 s. The presence of a surrounding fluid medium causes errors of up to 90% in the measured values of a. It is shown that both the experimental errors and the radial heat loss coefficients can be accurately calculated using simple models, both at high and at low frequencies, and that a previously developed two-frequency model can be used to obtain accurate data for a even under these conditions, provided the frequency is high enough. We also present a new variety of Ångström's model, valid only at very low frequencies and with purely convective heat loss.

  • 231.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal diffusivity measurements under hydrostatic pressure1981In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 52, no 7, p. 1061-1063Article in journal (Refereed)
    Abstract [en]

    The original Ångström method has been modified to allow, for the first time, measurements of the thermal diffusivity of metals and other very good conductors under high hydrostatic pressure. The method can also be used to obtain at the same time the pressure dependence of the thermal properties of the pressure transmitting medium, although with reduced accuracy. Some other modifications of the Ångström method are commented on and improved.

  • 232.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Transport properties of iron and nickel under pressure1982In: High Pressure in Research and Industry, vol. 1: Proceedings of the 8th International AIRAPT and 19th EHPRG High Pressure Conferences, Uppsala 1981, Uppsala, 1982, p. 432-433Conference paper (Refereed)
  • 233.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Transport properties of liquid mercury under high pressure1986In: High Temperatures - High Pressures Volume 18: Proceedings of the 10th European conference on Thermophysical Properties (ETPC 10), Frascati 1986., London: Pion Ltd , 1986, p. 655-663Conference paper (Refereed)
    Abstract [en]

    The thermal diffusivity, electric resistivity, and Seebeck coefficient of liquid mercury have been measured under pressures up to >600 MPa (6 kbar) near room temperature. The thermal diffusivity was measured by a modified two-frequency Ångström method, capable of taking into account radial heat loss by conduction. The pressure dependence of the thermal conductivity was calculated, and the Lorenz ratio was found to be independent of volume. In the same experiment, the compressibilities of two epoxy resins (Araldite) were measured.

  • 234.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Britt M.
    Umeå University, Faculty of Science and Technology, Physics.
    Resistivity of high-Tc superconductors: Linear in T at constant p, non-linear at constant V1990In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 76, no 8, p. 1019-1022Article in journal (Refereed)
    Abstract [en]

    The observed constant-pressure resistivity varrhop of most high-Tc superconductors is linear in temperature T over large ranges. We show that this is not true for the constant-volume resistivity varrhov; for 1–2–3 YBCO the correction from varrhop to varrhov is -32% at 500 K relative to the low-T value. Since theory predicts varrhov rather than varrhop, the observed linearity of varrho probably cannot be used as a tool to choose between theories for high-Tc superconductivity.

  • 235.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Britt M.
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of bulk YBa2Cu4O8 and the phonon transport model1994In: Physica C, volume 235-240: Proceedings of the 4th International Conference on Materials and Mechanisms of Superconductivity: High-Temperature Superconductors (M2S HTSC IV), Grenoble 1994, part II, Elsevier Science B.V. , 1994, p. 1377-1378Conference paper (Refereed)
    Abstract [en]

    We have measured the thermal conductivity varkappa of bulk YBa2Cu4O8 (1-2-4) between 30 and 310 K and at pressures p up to 1 GPa. At p = 0, varkappa = 10 Wm−1K−1 near 100 K, twice that of sintered YBa2Cu3O−δ (1-2-3). varkappa also decreases with increasing T more rapidly than in 1-2-3, to 7.4 Wm−1K−1 at 300 K. Under pressure, varkappa increases slowly with p above 150 K, but dvarkappa/dp decreases at lower T. Our results are in excellent agreement with a semi-classical model for phonon thermal conductivity, provided we assume significant charge transfer with pressure. The calculated phonon mean free path is much larger than the lattice spacing at all T.

  • 236.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Odd E.
    Umeå University, Faculty of Science and Technology, Physics.
    McRae, Edward
    Lelaurain, Michelle
    Marêché, Jean-Francois
    A study of temperature and pressure-induced structural and electronic changes in SbCl5 intercalated graphite: Part III. Analysis of the T and p dependence of the c-axis resistivity1995In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 10, no 2, p. 436-446Article in journal (Refereed)
    Abstract [en]

    We present experimental data for the c-axis electrical resistivity of SbCl5 intercalated graphite between 20 and 300 K. The data are analyzed together with our previous results for these and other samples [O. E. Andersson et al., J. Mater. Res. 7, 2989 (1992)]. Before the analysis, we correct the experimental data to constant volume, as assumed by theorists. We show that the correction factor is much larger for these materials than for normal metals. Although the original data showed significant nonlinearities with T, the corrected data are linear in T to within the experimental accuracy for low-stage compounds below the intercalate crystallization temperature. We compare our results with several models and conclude that both the temperature dependence, the pressure dependence, and the relative changes in the in-plane and c-axis resistivities associated with intercalate crystallization can be best described by a band conduction model.

  • 237.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Low-temperature phase transformation in NaBH4 under pressure2006In: Physical Review B, ISSN 1098-0121, Vol. 73, no 9, p. 092102-Article in journal (Refereed)
    Abstract [en]

    The pressure-temperature structural phase diagram of NaBH4 has been mapped by thermal conductivity

    measurements below room temperature and up to 2 GPa. The critical transition temperature for the transformation

    from the low-temperature tetragonal structure to the room temperature cubic one increases from near

    190 K at zero pressure to about 235 K at 2 GPa. The thermal conductivity data are consistent with the

    order-disorder character of the transition.

  • 238.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Thermal conductivity and phase diagrams of some potential hydrogen storage materials under pressure2009In: International Journal of Thermophysics, vol. 30, issue 4: Proceedings of the 18th European Conference on Thermophysical Properties (ECTP 18), Pau, France, 2008, 2009, p. 1118-1129Conference paper (Refereed)
    Abstract [en]

    Experimental data for the thermal conductivity of the complex hydrides NaAlH4, LiBH4, NaBH4, and KBH4, in dense, solid form over wide ranges in temperature and pressure are presented. These materials contain high volume and mass fractions of hydrogen and are considered possible candidates as future hydrogen storage materials for mobile applications. The pressure–temperature phase diagrams of several materials as obtained from thermal-conductivity studies are briefly discussed, and the temperature and pressure dependencies of the thermal conductivity of the bulk materials are also discussed using simple theoretical models.

  • 239.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edlund, Ulf
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Fransson, Åke
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Inaba, Akira
    Jacobsson, Per
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johnels, Dan
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Launois, Pascale
    Meingast, Christoph
    Moret, Roger
    Moritz, Thomas
    Persson, Per-Axel
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Umeå University, Faculty of Science and Technology, Department of Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Physical properties of pressure polymerized C601996In: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, volume 3: Proceedings of the 189th ECS Meeting, Symposium Fullerenes: Chemistry, Physics, and New Directions VIII, Los Angeles 1996, Pennington, NJ: The Electrochemical Society , 1996, p. 1014-1028Conference paper (Refereed)
    Abstract [en]

    We present in this paper an overview of the physical properties of the high pressure polymerized C60 phase commonly known as "soft fcc". This phase has been studied by several methods over wide ranges in temperature T and/or pressure, p. We present here experimental information about the specific heat capacity, the thermal expansion coefficient, the lattice structure, and the thermal conductivity, and we also show results obtained by NMR and Raman spectroscopy. All data presented agree with the accepted model that the individual molecules in this phase are covalently bound to form linear molecular chains. In particular, the NMR data show clearly the presence of covalent bonds, and the Raman data exhibit several new lines at very low energies connected with chain vibrations. Thermal conductivity data obtained during polymerization show both the time dependence of the process and that polymerization occurs at lower p and T than observed previously for this phase.

  • 240.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gong, Chen
    State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China.
    Liu, Bingbing
    State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China.
    Tonpheng, Bounphanh
    Umeå University, Faculty of Science and Technology, Department of Physics. Faculty of Natural Sciences, National University of Laos (NUOL), Vientiane, Laos.
    Yu, Junchun
    Umeå University, Faculty of Science and Technology, Department of Physics. Borås, Sweden.
    Yao, Mingguang
    State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China.
    AC impedance of A4C60 fullerides under pressure2015In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 17, no 2, article id 023010Article in journal (Refereed)
    Abstract [en]

    Three A4C60 compounds, with A = Li, Na and K, have been studied by impedance spectroscopy between 100 K and 293 K at pressures up to 2 GPa. The results are in very good agreement with earlier DC resistance studies and with data from the literature. For all three materials the measured conductivity can be fitted by a sum of at least two Arrhenius terms. The band gaps derived from the resistance data, 0.3 eV for Na4C60 and 0.5 eV for K4C60, are in excellent agreement with data measured by other methods. For Li4C60, our results disagree with a recent suggestion that the conductivity is dominated by ionic conduction. Although a certain ionic component probably exists we suggest that electronic transport dominates in our samples at and below room temperature because the derived “activation energy” decreases under pressure, the derived “activation volume” is negative, and we observe neither a significant electrode blocking capacitance nor any significant metal transport under DC conditions.

  • 241.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Lundin, Anders
    Umeå University, Faculty of Science and Technology, Physics.
    Persson, Per-Axel
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Fullerenes under pressure: Structure, order, and disorder1996In: High Pressure Science and Technology: Proceedings of the Joint XV AIRAPT and XXXIII EHPRG International Conference, Warsaw 1995, Singapore: World Scientific , 1996, p. 697-701Conference paper (Refereed)
    Abstract [en]

    The weak intermolecular interactions in solid C60 and other fullerenes make crystal structures and other properties very sensitive to applied pressure. We review recent results on the properties and phases of fullerenes under pressure, concentrating on the low-p range up to about 1 GPa. Subjects discussed include compression and transport studies, orientational and rotational disorder, the glassy crystal transition, and pressure-induced polymerization.

  • 242.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Lundin, Anders
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Phase diagram, structure, and disorder in C60 below 300 K and 1 GPa1995In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 93, no 2, p. 109-112Article in journal (Refereed)
    Abstract [en]

    Earlier structural studies have shown that the pentagon-to-hexagon orientation ratio in the orientationally ordered simple cubic phase of C60 decreases under pressure. From anomalies observed in the compressibility and thermal conductivity of C60 under pressure we have deduced a pressure-temperature phase diagram for this substance in the range below 300 K and 1 GPa (10 kbar). We conclude that C60 forms a new, completely “hexagon” ordered structural phase above about 0.6 GPa at 150 K (1.2 GPa at 300 K), and that the glass transition shifts upwards in T under pressure by 54 K GPa−1. However, above 0.1 GPa, pentagon-to-hexagon orientation relaxation seems to occur on heating at an almost pressure independent temperature near 100 K.

  • 243.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Lundin, Anders
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Structure, disorder, and phase diagram of C60 up to 1 GPa and below 300 K1994In: High Pressure in Material Science and Geoscience: Proceedings of the XXXII Annual Meeting of the European High Pressure Research Group, Brno 1994, Praha, Czech Republic: Prometheus , 1994, p. 109-112Conference paper (Refereed)
    Abstract [en]

    We have measured the compressibility and the thermal conductivity of C60 in the range below 300 K and up to 1 GPa (10 kbar). From the data obtained we have deduced the p-T phase diagram of C60. Literature data show that the ratio of pentagon-to-hexagon oriented molecules in the orientationally ordered phase decreases with increasing pressure, and from our data we conclude that C60 forms a new, completely "hexagon oriented" phase above 0.6 GPa at 150 K, or 1.2 GPa at 300 K. The glass transition temperature (90 K at atmospheric pressure) is found to increase by 54 K/GPa.

  • 244.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quwar, Issam
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Talyzin, Alexandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low temperature phase diagram of NH3BH32011In: Materials Research Society Symposium Proceedings: Proceedings of Symposium EE: Solid-State Chemistry of Inorganic Materials VIII / [ed] P.S. Halasyamani, D.G. Mandrus, K.-S. Choi and S.J. Clarke, Cambridge: Cambridge University Press , 2011, p. mrsf10-1309-ee06-25-Conference paper (Refereed)
    Abstract [en]

    The pressure-temperature (p-T) phase diagram of NH3BH3 has been investigated by thermal conductivity measurements up to 1.5 GPa at temperatures between 100 and 300 K, and the phase boundaries between the three known structural phases have been identified. The transformation between the room temperature tetragonal I4mm phase and the low temperature orthorhombic Pmn21 phase (Tc = 218 K at p = 0) shows only a small hysteresis. The transformation into the high pressure orthorhombic Cmc21 phase (at 1.0 GPa near 292 K) has a very strong hysteresis, up to Δp = 0.5 GPa, and below 230 K a fraction of this phase is metastable even at atmospheric pressure.

  • 245.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Comment on “Pressure-Induced Structural Metastability in Crystalline C601995In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 75, no 15, p. 2906-Article in journal (Refereed)
  • 246.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Physics.
    Phase transitions in hydrogen storage compounds under pressure2007In: Journal of Physics Condensed Matter, vol. 19, issue 42: Proceedings of the 3rd Asian Conference on High Pressure, Lijiang City 2006, Bristol: Intitute of Physics , 2007, p. 425201-Conference paper (Refereed)
    Abstract [en]

    Solid, hydrogen-rich compounds, such as alkali metal hydrides, MAH4, where

    M is an alkali metal and A is boron or aluminium, may be used for hydrogen

    storage. We briefly review recent high-pressure work in this field aimed at

    exploring the phase behaviour, and especially the possibility to find highly

    dense new structures. In particular we present experimental data on the

    structure, lattice dynamics, phase diagrams, and thermal properties obtained by

    us and others by Raman scattering, x-ray diffraction, and thermal conductivity

    measurements under pressure between 100 and 400 K. From these data and the

    results of theoretical calculations we map observed structural phases and phase

    transitions in the pressure–temperature plane for the materials that have so far

    been investigated under pressure.

  • 247.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bäckström, Gunnar
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Low noise and drift by parallel amplifiers1975In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 46, no 7, p. 928-929Article in journal (Refereed)
    Abstract [en]

    A low noise and low drift amplifier has been constructed using several monolithic operational amplifiers in parallel. It is shown that each amplifier may be connected by only three resistors. An input noise of 0.15 µV (p–p) in the band 0.1–10 Hz was obtained with six amplifiers, and the noise is expected to decrease as N−1/2 as the number of units is increased.

  • 248.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bäckström, Gunnar
    Umeå University, Faculty of Science and Technology, Physics.
    Pressure dependence of the thermal conductivity of aluminium1977In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 23, no 10, p. 773-775Article in journal (Refereed)
    Abstract [en]

    The thermal diffusivity, a, of aluminium has been measured at pressures up to 2.5 GPa at room temperature, and from these results the pressure dependence of the thermal conductivity, λ, has been calculated. Both quantities increase with pressure. The increase in a amounts to 4.6% to 1 GPa and 10.4% to 2.5 GPa. The initial pressure coefficient of the electronic thermal conductivity λe is found to be [λe]-1λe/P = 3.7 × 10-2GPa-1, which agrees very well with a recent theoretical calculation.

  • 249.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bäckström, Gunnar
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conduction of metals under pressure1976In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 47, no 2, p. 177-182Article in journal (Refereed)
    Abstract [en]

    It is shown that the original Ångström method of determining the thermal diffusivity of metals is not valid in the case of a specimen surrounded by a solid medium, and an appropriate modification is described. Several simplifications of this method are also presented and criteria for their validity given. An electronic system has been developed for automatic sampling and analysis of the temperature data. The new method has been applied to Cu under pressure, and the results show that the thermal conductivity rises by 6.4% up to 2.5 GPa (25 kilobar).

  • 250.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bäckström, Gunnar
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of copper under high pressure1977In: High Temperatures-High Pressures, ISSN 0018-1544, E-ISSN 1472-3441, Vol. 9, no 1, p. 41-48Article in journal (Refereed)
    Abstract [en]

    The thermal diffusivity, a, of copper has been measured at room temperature up to a pressure of 2.5 GPa (25 kbar) by means of a method recently developed. In this pressure range the diffusivity increases linearly with a slope of (1/a)da/dp = 2.7x10-2 GPa-1. As the density and specific heat of copper are known as functions of pressure, this result can be used to obtain the pressure dependence of the thermal conductivity, lambda, with the result (1/lambda)d(lambda)/dp = 3.1x10-2 GPa-1. This value is shown to agree well with theoretical calculations of this slope. However, a comparison between this result and the pressure dependence of the electrical conductivity shows deviations from the Wiedemann-Franz law.

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