| 1. |
- Andersson, Ove, et al.
(författare)
-
Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH3BH3
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:2, s. 024115-
-
Tidskriftsartikel (refereegranskat)abstract
- The potential hydrogen storage compound NH3BH3 has three known structural phases in the temperature and pressure ranges 110–300 K and 0–1.5 GPa, respectively. We report here the boundaries between, and the ranges of stability of, these phases. The phase boundaries were located by in situ measurements of the thermal conductivity, while the actual structures in selected areas were identified by in situ Raman spectroscopy and x-ray diffraction. Below 0.6 GPa, reversible transitions involving only small hysteresis effects occur between the room-temperature tetragonal plastic crystal I4mm phase and the low-temperature orthorhombic Pmn21 phase. Transformations of the I4mm phase into the high-pressure orthorhombic Cmc21 phase, occurring above 0.8 GPa, are associated with very large hysteresis effects, such that the reverse transition may occur at up to 0.5 GPa lower pressures. Below 230 K, a fraction of the Cmc21 phase is metastable to atmospheric pressure, suggesting the possibility that dense structural phases of NH3BH3, stable at room temperature, could possibly be created and stabilized by alloying or by other methods. Mixed orthorhombic Pmn21/Cmc21 phases were observed in an intermediate pressure-temperature range, but a fourth structural phase predicted by Filinchuk et al. [ Phys. Rev. B 79 214111 (2009)] was not observed in the pressure-temperature ranges of this experiment. The thermal conductivity of the plastic crystal I4mm phase is about 0.6 W m−1 K−1 and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.
|
|
| 2. |
- Bagatskii, M. I., et al.
(författare)
-
Low-temperature heat capacity of fullerite C-60 doped with deuteromethane
- 2012
-
Ingår i: Low temperature physics (Woodbury, N.Y., Print). - Melville, NY : American Institute of Physics (AIP). - 1063-777X .- 1090-6517. ; 38:1, s. 67-73
-
Tidskriftsartikel (refereegranskat)abstract
- The heat capacity C of fullerite doped with deuteromethane (CD4)(0.4)(C-60) has been investigated in the temperature interval 1.2-120K. The contribution Delta C-CD4 of the CD4 molecules to the heat capacity C has been isolated. It is shown that at T approximate to 120K the rotational motion of CD4 molecules in the octahedral voids of the C-60 lattice is weakly hindered. When the temperature is lowered to 80K, the rotational motion of the CD4 molecules changes from weakly hindered rotation to libration. In the range T = 1.2-30 K, Delta C-CD4 is described quite accurately by the sum of contributions from the translational and librational vibrations and tunneling rotation of CD4 molecules. The contribution of tunneling rotation to the heat capacity Delta C-CD4(T) is dominant below 5K. The effect of nuclear-spin conversion of the CD4 molecules on the heat capacity has been observed and the characteristic times for nuclear spin conversion between the lowest levels of the A- and T-species of CD4 molecules at T < 5K have been estimated. A feature observed in Delta C-CD4(T) near T = 5.5K is most likely a manifestation of a first-order phase transition in the orientational glass form of the solution. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3677237]
|
|
| 3. |
- Bagatskii, M.I., et al.
(författare)
-
The specific heat and the radial thermal expansion of bundles of single-walled carbon nanotubes
- 2012
-
Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : American Institute of Physics (AIP). - 1063-777X .- 1090-6517. ; 38:6, s. 523-528
-
Tidskriftsartikel (refereegranskat)abstract
- The specific heat at constant pressure C(T) of bundles of single-walled carbon nanotubes (SWNTs) closed at their ends has been investigated in the temperature interval of 2–120 K. It is found that the curve C(T) has features near 5, 36, 80, and 100 K. The experimental results on the C(T) and the radial thermal expansion coefficient αR(T) of bundles of SWNTs oriented perpendicular to the sample axis have been compared. It is found that the curves C(T) and αR(T) exhibit a similar temperature behavior at T > 10 K. The temperature dependence of the Grüneisen coefficient γ(T) has been calculated. The curve γ(T) also has a feature near 36 K. Above 36 K the Grüneisen coefficient is practically independent of temperature (γ ≈ 4). Below 36 K, γ(T) decreases monotonically with lowering temperature and becomes negative at T < 6 K.
|
|
| 4. |
- Cui, Wen, et al.
(författare)
-
Reversible pressure-induced polymerization of Fe(C5H5)(2) doped C-70
- 2013
-
Ingår i: Carbon. - : Pergamon-Elsevier Science. - 0008-6223 .- 1873-3891. ; 62, s. 447-454
-
Tidskriftsartikel (refereegranskat)abstract
- High pressure Raman, IR and X-ray diffraction (XRD) studies have been carried out on C-70(Fe(C5H5)(2))(2) (hereafter, "C-70(Fc)(2)") sheets. Theoretical calculation is further used to analyze the Electron Localization Function (ELF) and charge transfer in the crystal and thus to understand the transformation of C-70(Fc)(2) under pressure. Our results show that even at room temperature dimeric phase and one dimensional (1D) polymer phase of C-70 molecules can be formed at about 3 and 8 GPa, respectively. The polymerization is found to be reversible Upon decompression and the reversibility is related to the pressure-tuned charge transfer, as well as the overridden steric repulsion of counter ions. According to the layered structure of the intercalated ferrocene molecules formed in the crystal, we suggest that ferrocene acts as not only a spacer to restrict the polymerization of C-70 molecules within a layer, but also as charge reservoir to tune the polymerization process. This supplies a possible way for us to design the polymerization of fullerenes at suitable conditions.
|
|
| 5. |
- Cui, Wen, et al.
(författare)
-
Synthesis of alkali-metal-doped C60 nanotubes
- 2011
-
Ingår i: Diamond and Related Materials. - : Elsevier BV. ; , s. 93-96
-
Konferensbidrag (refereegranskat)abstract
- C60 nanotubes have been synthesized by a solution–solution method. After degassing in a dynamic vacuum, the C60 nanotubes were doped with alkali metals by means of vapor evaporation method. Different temperatures have been studied to evaporate the alkali metals for the doping experiments. Raman spectrum was further employed to analyze the doping concentration of the obtained samples. It was found that all three alkali metals (Li, Na and K) used can be efficiently doped into the C60 nanotubes, forming AxC60 nanotubes. The doping concentration of Li, Na changed from low to high level, depending on the experiment temperatures, while K doping always gave saturated doping. The melt points, the ionic sizes and vapor pressures of alkali metals were thought to affect the final doping results.
|
|
| 6. |
- Danilchenko, Boris A., et al.
(författare)
-
1/f noise and mechanisms of the conductivity in carbon nanotube bundles
- 2011
-
Ingår i: Carbon. - : Elsevier Ltd. - 0008-6223 .- 1873-3891. ; 49:15, s. 5201-5206
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental results are reported of the investigation of conductivity mechanisms in metallic single-wall carbon nanotube (SWCNT) bundles in a wide temperature range from 4.2 K to 300 K. The temperature dependence of the resistance and noise parameters – the logarithmic slope of the current dependence of noise as well as the normalized current noise – are compared. Remarkable changes in noise characteristics are registered at temperatures typical of the transition from hopping conductivity to Luttinger liquid conductivity and the transition from Luttinger liquid conductivity to diffusion conductivity. In the first transition region, the slope of the normalized noise level of the current changes significantly as a function of temperature. In the region of diffusion conductivity, a stronger variation of the normalized noise level is revealed. These changes in noise properties are correlated with changes in the transport characteristics of SWCNT bundles that allow us to adequately explain the mechanisms of conductivity in the system.
|
|
| 7. |
- Danilchenko, Boris A., et al.
(författare)
-
High temperature Luttinger liquid conductivity in carbon nanotube bundles
- 2010
-
Ingår i: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 97:7, s. 072106-
-
Tidskriftsartikel (refereegranskat)abstract
- The conductance and the current-voltage characteristics of metallic single wall carbon nanotube bundles have been measured between 4.2 and 330 K using 10–30 ns electric pulses to avoid overheating. The current-voltage characteristics at different temperatures collapse to a single curve when plotted in the specific coordinates following from the Tomonaga–Luttinger (T–L) liquid concept. Direct evidence is obtained for the existence of a T–L liquid phase up to 190 K and the system shows a transition between the T–L liquid state and a Mott insulating phase below 25 K.
|
|
| 8. |
- Danilchenko, Boris A, et al.
(författare)
-
Self-heating of metallic carbon nanotube bundles in the regime of the Luttinger-liquid conductivity
- 2011
-
Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 37:8, s. 710-717
-
Tidskriftsartikel (refereegranskat)abstract
- The conductivity of bundles of carbon single-walled nanotubes with metallic conductivity (metallic nanotubes) is investigated over the wide temperature range 4.2-330 K and electrical fields up to 50 V. The usage of short electrical pulses of the duration of 10 ns allowed to avoid an influence of a self-heating of the investigated structures on current-voltage characteristics. It is shown that the temperature dependence of conductivity is described by the power function G proportional to T(alpha). At helium temperatures the asymptotic dependence of current on applied voltage is close to J proportional to V(1+alpha) with alpha = 0.45. From comparison of the obtained results of measurements with calculations, it is shown that the conductivity of nanotube bundles is well described within the theory of the Luttinger-liquid conductivity for one-dimensional conductors. The self-heating of the carbon nanotube bundles was observed in the case of measurements in the regime of dc current. A method for determination of the self-heating temperature of nanotube bundles as a function of an applied electrical field is proposed. The power dependence of the self-heating temperature on voltage T proportional to V(p) with the exponent p = 2.1 was observed above some threshold voltage in the temperature range 4.2-200 K. Above 200 K the exponent decreased down to p = 1.35. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3662331]
|
|
| 9. |
- Danilchenko, Boris A., et al.
(författare)
-
Stability of the Tomonaga-Luttinger liquid state in gamma-irradiated carbon nanotube bundles
- 2013
-
Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 25:47, s. 475302-
-
Tidskriftsartikel (refereegranskat)abstract
- We report experimental results for the changes in conductivity of single-wall carbon nanotube bundles when irradiated by Co-60 gamma-rays in various environments. In the current study the samples investigated were irradiated in hermetic cells, either evacuated (0.1 Pa) or filled with hydrogen or deuterium at atmospheric pressure. In situ measurements of the resistance change as a function of irradiation dose at room temperature are presented. It was found that, for all irradiation conditions, the normalized resistance versus irradiation dose demonstrates a logarithmic behaviour. A phenomenological model for the observed dependence is derived. The current-voltage characteristics of the irradiated samples were measured in the temperature range from 4.5 to 300 K using short (10 ns) electric pulses, and the results demonstrate a scaling behaviour. This scaling occurs in the universal coordinates that correspond to the Tomonaga-Luttinger liquid concept. Our results confirm the existence of the Tomonaga-Luttinger liquid phase up to room temperature in carbon nanotubes after gamma-irradiation to a dose of 5 x10(7) rad in vacuum, 1 : 7 x 10(7) rad in hydrogen and 1 : 24 x 10(8) rad in deuterium.
|
|
| 10. |
- Dolbin, Alexander V., et al.
(författare)
-
Low-temperature radial thermal expansion of single-walled carbon nanotube bundles saturated with nitrogen
- 2010
-
Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 36:5, s. 365-369
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of a N2 impurity on the radial thermal expansion coefficient αr of single-walled carbon nanotube bundles has been investigated in the temperature interval 2.2–43 K by the dilatometric method. Saturation of nanotube bundles with N2 caused a sharp increase in the positive magnitudes of αr in the whole temperature range used and a very high and wide maximum in the thermal expansion coefficient αr(T) at T ~ 28 K. The low temperature desorption of the impurity from the N2-saturated powder of bundles of single-walled carbon nanotubes with open and closed ends has been investigated.
|
|
