| 1. |
- Fu, Yifeng, 1984, et al.
(författare)
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Selective growth of double-walled carbon nanotubes on gold films
- 2012
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Ingår i: Materials Letters. - : Elsevier BV. - 1873-4979 .- 0167-577X. ; 72, s. 78-80
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Tidskriftsartikel (refereegranskat)abstract
- Growth of high-quality vertical aligned carbon nanotube (CNT) structures on silicon supported gold (Au) films by thermal chemical vapor deposition (TCVD) is presented. Transmission electron microscopy (TEM) images show that the growth is highly selective. Statistical study reveals that 79.4% of the as-grown CNTs are double-walled. The CNTs synthesized on Au films are more porous than that synthesized on silicon substrates under the same conditions. Raman spectroscopy and electrical characterization performed on the as-grown double-walled CNTs (DWNTs) indicate that they are competitive with those CNTs grown on silicon substrates. Field emission tests show that closed-ended DWNTs have lower threshold field than those open-ended.
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| 2. |
- Zhang, Zhfei, et al.
(författare)
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Characterization of CNT Enhanced Conductive Adhesives in Terms of Thermal Conductivity
- 2012
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-5862 .- 1938-6737. - 9781607683186 ; 44:1, s. 1011-1017
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Konferensbidrag (refereegranskat)abstract
- CNTs have excellent thermal, electrical and mechanical properties. They can be used in various ways. One researched field of application is CNT-polymer composites which combine common technologies with advanced materials. This paper will focus on the thermal property of CNT-Ag-filled adhesives and compares the new materials with conventional, electrical Ag-filled conductive adhesives. Several analytical methods, FTIR, Raman analysis, SEM and TEM have been carried out to examine the different surface conditions after physicval and chemical modification of CNTs. The thermal conductivities of composites containing different types of CNTs were investigated. The incorporation of CNTs into polymers resulted in enhancement of the thermal conductivity compared to Ag-filler. The increase of thermal conductivity with addition of CNT filler is obvious, especially for the purified CNT. The value of thermal conductivity is about two times higher than the Ag-filled conductive adhesive. However, improvement on thermal conductivity of the surface modified CNT-filled conductive adhesives is not so obvious. Since the thermal conduction in CNT is by phonon transfer, the nanometric size and the huge interface lead to strong phonon-scattering at the interface. Thus, a relatively low interfacial area, weak interfacial adhesion promotes the conduction of phonons and minimizes coupling losses. According to this, the non-treated MWCNTs seem to have the highest potential to improve the thermal conductivity of epoxies.
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| 3. |
- Granath, Mats, 1972, et al.
(författare)
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Anharmonic softening of Raman active phonons in iron-pnictides: Estimating the Fe isotope effect due to anharmonic expansion
- 2009
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Ingår i: Physical Review B. - 1098-0121. ; 79:23, s. 235103-
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Tidskriftsartikel (refereegranskat)abstract
- We present Raman measurements on the iron-pnictide superconductors CeFeAsO1-xFx and NdFeAsO1-xFx. Modeling the Fe-As plane in terms of harmonic and a cubic anharmonic Fe-As interactions, we calculate the temperature dependence of the energy and lifetime of the Raman active Fe B-1g mode and fit to the observed energy shift. The shifts and lifetimes are in good agreement with those measured also in other Raman studies which demonstrate that the phonon spectrum, at least at small wave numbers, is well represented by phonon-phonon interactions without any significant electronic contribution. Even at zero temperature there is a non-negligent effect of interactions on the phonon energy, which for the Fe B-1g mode corresponds to 6 cm(-1) or 3% of the total energy of the mode. We also estimate the anharmonic expansion from Fe (56 -> 54) isotope substitution to Delta a approximate to 5.1 x 10(-4) angstrom and Delta d(Fe-As) approximate to 2.5 x 10(-4) angstrom and the shift of harmonic zero-point fluctuations of bond lengths less than or similar to 3 x 10(-5) angstrom(2), giving a total relative average decrease in electronic hopping integrals of vertical bar delta vertical bar/t less than or similar to 2.0 x 10(-4). For a nonphonon-mediated weak-coupling superconductor this gives an isotope exponent alpha similar to 10(-2). The results pose a serious challenge for any theory of superconductivity in the pnictides that does not include electron-phonon interactions to produce a sizable Fe isotope effect.
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| 4. |
- Bielecki, Johan, 1982, et al.
(författare)
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Structural and magnetic properties of isovalently substituted multiferroic BiFeO3 : Insights from Raman spectroscopy
- 2012
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 86:184422
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Tidskriftsartikel (refereegranskat)abstract
- Raman spectra, supplemented by powder x-ray diffraction and magnetization data of isovalently A- and B-site substituted BiFeO3 in the Bi1−xLaxFeO3 (0≤x≤1), Bi1−xTbxFeO3 (0≤x≤0.2), and Bi0.9Sm0.1Fe1−xMnxO3 (0≤x≤0.3) series, are presented. A good agreement between the structural transitions observed by x-ray diffraction and the vibrational modes observed in the Raman spectra is found over the whole substitutional ranges, and in particular we find spectroscopic signatures of a PbZrO3-type structure for Bi0.8La0.2FeO3. Mode assignments in the substituted materials are made based on Raman spectra of the end-members BiFeO3 and LaFeO3. Moreover, by comparing spectra from all samples with R3c structure, the phonon assignment in BiFeO3 is revisited. A close connection between the degree of octahedral tilt and the Raman shift of the A1 oxygen a−a−a− tilt mode is established. An explanation for the strong second-order scattering observed in Bi1−xLaxFeO3 and Bi1−xTbxFeO3 is suggested, including the assignment of the previously mysterious BiFeO3 mode at 620 cm−1. Finally, the magnetization data indicates a transition from a cycloidal modulated state towards a canted antiferromagnet with increasing A-site substitution, while Bi0.9Sm0.1Fe1−xMnxO3 with x=0 and 0.15 exhibit an anomalous closing of the hysteresis loop at low temperatures. For low A-site substitution levels (x≤0.1) the decreasing Raman intensity of the Fe derived modes correlates with the partial destruction of the spin cycloid as the substitution level increases.
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| 5. |
- He, Wenxiao, 1985, et al.
(författare)
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Formation of Bone-like Nanocrystalline Apatite Using Self-Assembled Liquid Crystals
- 2012
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 24:5, s. 892-902
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Tidskriftsartikel (refereegranskat)abstract
- A two-step process using liquid crystalline phases combined with controlled postcrystallization for the preparation of bone-like apatite has been developed. First, amorphous calcium phosphate (ACP) spherules with a diameter of 10.8 +/- 1.4 nm and specific surface area (SSA) in the range of 150-170 m(2)/g were synthesized within a reverse hexagonal liquid crystalline (LC) phase. Second, the ACP spherules were dispersed and aged in Milli-Qwater, where they crystallized into poorly crystalline apatite (PCA). The addition of heparin during aging was explored, which was shown to retard the ACP - PCA conversion. The particle formation within the LC phase was monitored using synchrotron small-angle X-ray scattering, and the formed materials were characterized by X-ray diffraction, conventional and high-resolution transmission electron microscopy, nitrogen adsorption, thermogravimetry with infrared-coupled analysis, and Raman spectroscopy. The PCA formed using the LC aging route presented bone-resembling features, such as,Ca2+ and OH- deficiency, CO32- substitution, poor crystallinity; and ultrahigh SSA of 356 m(2)/g. The resulting particles were compared to hydroxyapatite synthesized via a conventional water-based precipitation method. The LC-aging route exhibited excellent controllability over the CaP crystallization, which enabled facile tailoring of the resulting material properties for different types of application.
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| 6. |
- Wiedorn, Max O., et al.
(författare)
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Megahertz serial crystallography
- 2018
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Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The new European X-ray Free-Electron Laser is the first X-ray free-electron laser capable of delivering X-ray pulses with a megahertz inter-pulse spacing, more than four orders of magnitude higher than previously possible. However, to date, it has been unclear whether it would indeed be possible to measure high-quality diffraction data at megahertz pulse repetition rates. Here, we show that high-quality structures can indeed be obtained using currently available operating conditions at the European XFEL. We present two complete data sets, one from the well-known model system lysozyme and the other from a so far unknown complex of a beta-lactamase from K. pneumoniae involved in antibiotic resistance. This result opens up megahertz serial femtosecond crystallography (SFX) as a tool for reliable structure determination, substrate screening and the efficient measurement of the evolution and dynamics of molecular structures using megahertz repetition rate pulses available at this new class of X-ray laser source.
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