Fluorene–fullerene dyads: Modulation of interaction

Two new fullerene[C₆₀]–fluorene dyads, N-methyl-2-(9-fluorenyl)-3,4-fulleropyrrolydine and N-methyl-2-(2-fluorenyl)-3,4-fulleropyrrolydine, were synthesized and investigated. Cyclic voltammetry, fluorescence experiments, electronic spectra and Density Functional Theory (DFT) calculations, all indicate ground and excited state interactions when fluorene is linked at carbon 9, whereas linkage through position 2 does not affect the ground and excited state of the fulleropyrrolidine moiety. As a consequence, only changing the position of the fluorene chromophore linkage, opens the way to modulate interactions, choosing between a simple spacer and an active unit.     

Photoelectrochemical cells based on LB films of fullerene–thiophene derived dyads

Photoelectrochemical properties of fullerene with three oligothiophenes (1T-F, 2T-F, 3T-F), ethylenedioxythiophenyl (EDOT-F) and hexylthiophenyl (HXT-F) groups were investigated with the photocurrent measurements. Electron paramagnetic resonance (EPR) spectroscopy has been used for study the fullerene–thiophene derived systems in wide temperature range. For electrochemical examination of these systems Langmuir–Blodgett (LB) layers deposited on a gold substrate were prepared. The light-induced photocurrent study shows that the structure of the dyad has an influence on both photovoltage and photocurrent, although these two variables are not correlated with each other. The role of the structure of the dyads is also seen in electron paramagnetic resonance investigations. All these results supported by previous IR and UV–vis spectral studies suggest, that the photoinduced electron transfer from oligothiophenes or EDOT and HXT to C₆₀ is a primary process of the photocurrent generation in the fullerene–thiophene-derived dyads.     

Relationship between structure and optoelectrical properties of organic–inorganic hybrid materials containing fullerene derivatives

Novel lead iodide-based layered perovskite compounds, which contain fullerene derivatives, N-methyl-2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (AmPF) and N-(n-dodecyl)-2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (C12AmPF), in their organic layers were fabricated as thin solid films by spin-coating. The XRD profiles showed that (AmPF)PbI₄ molecules were arranged in a closer-packing form, compared with (C12AmPF)PbI₄. The photoluminescence spectra of thin films suggested the presence of energy transfer between C₆₀ moiety and lead(II) iodide layers, which led to the disappearance of fluorescence peak at 517 nm and the appearance of a new fluorescence peak at 780 nm. (AmPF)PbI₄ and (C12AmPF)PbI₄ films exhibited photoconductivity when ultraviolet light was irradiated, and the photocurrent values with applying 1.0 V bias voltage were 1.77 μA and 1.48 × 10^?2 μA, respectively.     

Elastic properties of silver–phospho–vanadate glasses

The elastic properties of xAg₂O–(50 ? x) P₂O₅–50V₂O₅ glasses are investigated using ultrasonic pulse-echo measurements and their elastic properties have been characterized at room temperature. Results from the studies show that both longitudinal and transverse velocities decrease with increase of Ag₂O concentration. The elastic constants C₁₁, C₄₄ and Young's modulus show decreasing trend while constant C₁₂, bulk modulus and Poisson's ratio show an increasing trend as the fraction of Ag₂O increases. Another notable observation is that the glass with 15 and 40 mol% of Ag₂O concentrations exhibits the low velocities and low elastic moduli. This behavior of the elastic properties is related to the change in the structure of glasses as well as the interatomic bonding.     

Thermophysical properties of liquid Co–Cu–Ni alloys

Abstract

Within the Coolcop project, funded by ESA, the thermophysical properties of selected liquid Cu–Co–Ni alloys were investigated. Using the new high-temperature oscillating cup viscometer at DLR, the viscosity of this alloy along two perpendicular cuts in the ternary phase diagram was measured as a function of temperature. Along the same cuts also surface tension and density were determined by using electromagnetic levitation and image processing.     

High-temperature mechanical properties of Ir–Al alloys

To design an alloy with high strength around 1773 K and good ductility at room temperature, the microstructure, the compression strength and the creep properties at 1773 K of the Ir–Al alloys with an fcc and B2 two-phase structure were investigated. High-temperature mechanical properties are discussed in terms of microstructure.     

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg_24+xY₅, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH₂ and YH₃. After hydrogen desorption pure Mg and YH₂ were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH₂ decomposition enthalpy was determined to ?68 kJ/mol H₂, and the calculated activation energy of hydrogen desorption of MgH₂ was 150(±10) kJ/mol.     

Composition-dependent electronic properties of indium–zinc–oxide elongated microstructures

Microrods and hierarchical structures of indium–zinc–oxide (IZO) with different compositions were grown by thermal treatments of mixtures of InN and ZnO powders. In long rods, an increase in Zn content along the growth axis is revealed by energy dispersive spectroscopy. The structures obtained range from Zn-doped indium oxide with a few atomic per cent of Zn, to IZO compounds of the type Zn_kIn₂O_k+3. X-ray photoelectron spectroscopy measurements with spatial resolution show that IZO microstructures degenerate at room temperature, with carrier concentration of the order of 10²⁰ cm^?3. Electron accumulation has been found for undoped (1 0 0) and (1 1 1) surfaces, whereas depletion of carriers at the surface is observed in IZO samples. The Fermi level position correlates with the Zn concentration at the surface which, taking into account the surface dependence of the ionization potentials, work functions and band gaps, could lead to tunable material properties for device applications. Cathodoluminescence emission intensity is enhanced by the presence of Zn, which induces spectral changes and broadening of the emission band compared with undoped material. The results are discussed in terms of the charge neutrality level and the band structure of the material.     

Critical properties of symmetric nanoscale metal–ferroelectric–metal capacitors

The size, surface and interface effects on the magnitude and stability of spontaneous polarization in a symmetric nanoscale ferroelectric capacitor were studied by analyzing its evolutionary trajectory based on a thermodynamic model. Analytic expressions of the Curie temperature, spontaneous polarization, critical thickness and the Curie–Weiss relation were derived, taking into account the effects of the depolarization field, built-in electric field, interfaces and surfaces. Our results show that the critical properties are not only functions of the ambient temperature, misfit strain and electromechanical boundary conditions, but also depend on the characteristics of electrodes, surfaces and interfaces, through the incomplete charge compensation, near-surface variation of polarization and work function steps of ferroelectric–electrode interfaces, which are adjustable.     

Shape memory properties of Ti–Nb–Mo biomedical alloys

Mo is added to Ti–Nb alloys in order to enhance their superelasticity. The shape memory properties of Ti–(12–28)Nb–(0–4)Mo alloys are investigated in this paper. The Ti–27Nb, Ti–24Nb–1Mo, Ti–21Nb–2Mo and Ti–18Nb–3Mo alloys exhibit the most stable superelasticity with a narrow stress hysteresis among Ti–Nb–Mo alloys with Mo contents of 0, 1, 2 and 3 at.%, respectively. The ternary alloys reveal better superelasticity due to a higher critical stress for slip deformation and a larger transformation strain. A Ti–15Nb–4Mo alloy heat-treated at 973 K undergoes (2 1 1)〈1 1 1〉-type twinning during tensile testing. Twinning is suppressed in the alloy heat-treated at 923 K due to the precipitation of the α phase, allowing the alloy to deform via a martensitic transformation process. The Ti–15Nb–4Mo alloy exhibits stable superelasticity with a critical stress for slip deformation of 582 MPa and a total recovery strain of 3.5%.     

Microstructure and properties of aging Cu–Cr–Zr alloy

The crystallography and morphology of precipitate particles in a Cu matrix were studied using an aged Cu–Cr–Zr alloy by transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM). The tensile strength and electrical conductivity of this alloy after various aging processes were tested. The results show that two kinds of crystallographic structure associated with chromium-rich phases, fcc and bcc structure, exist in the peak-aging of the alloy. The orientation relationship between bcc Cr precipitate and the matrix exhibits Nishiyama–Wasserman orientation relationship. Two kinds of Zr-rich phases(Cu4Zr and Cu5Zr)can be identified and the habit plane is parallel to {111}Cu plane during the aging. The increase in strength is ascribed to the precipitation of Cr- and Zr-rich phase.     

The electronic properties of polyacetylene–polymethineimine block copolymers

Ionization energies (IE), electron affinities (EA) and electronic transition energies (ΔE(S₀–T₁) and ΔE(S₀–S₁)) of a series of polyacetylenes (PA), H(HCCH)_nH (n≤30), polymethineimines (PMI), H(HCN)_nH, and their block copolymers (PA–PMI), H(HCCH)_n/2(HCN)_n/2H, were investigated using the MP2/6-31G(d) (IE, EA, ΔE(S₀–T₁)) and CIS/6-31G(d) (ΔE(S₀–T₁) and ΔE(S₀–S₁)) quantum mechanical methods.IEs and EAs of PA–PMI approached values found in PA and PMI, respectively, although small residual differences remained in the asymptotic limit. The results were explained on the basis of orbital visualization, calculated charge distributions and fragment molecular orbital (FMO) analysis, which revealed that, whereas the PA–PMI HOMO is localized in the PA block, the copolymer’s LUMO contains significant electron density in both blocks, resulting from interactions of the PA LUMO with several of the low-lying PMI fragment’s virtual orbitals.Electronic transition energies, ΔE(S₀–T₁) and ΔE(S₀–S₁), of the copolymer approached values found in pure PA, which is not consistent with the above picture. These results were explained on the basis of excited state orbital relaxation, in which the cation-like “hole”, localized in the PA block, exerts a greater pull on the electron than does the more electronegative PMI portion of the molecule.     

Thermal properties of pressureless melt infiltrated AlN–Si–Al composites

Thermal properties of AlN-Si-Al composites produced by pressureless melt infiltration of Al/Al alloys into porous α-Si₃N₄ preforms were investigated in a temperature range of 50-300 °C. SEM and TEM investigations revealed that the grain size of AlN particles was less than 1 μm. In spite of sub-micron grain size, composites showed relatively high thermal conductivity (TC), 55-107 W/(m.K). The thermal expansion coefficient (CTE) of the composite produced with commercial Al source, which has the highest TC of 107 W/(m.K), was 6.5×10^?6 K^?1. Despite the high CTE of Al (23.6×10^?6 K^?1), composites revealed significantly low CTE through the formation of Si and AlN phases during the infiltration process.     

Preparation and properties of nickel–zirconia nanocomposite coatings

The possibility of preparing and properties (surface morphology, microhardness, corrosion resistance) of nickel–zirconia composite coatings electrodeposited from nickel acetate solutions containing a dispersed phase in the form of a conventional polydisperse crystalline micropowder and a sol with nanoscale particles have been discussed. The effect of the particle size and concentration and the electrolysis conditions on the properties of the coatings has been determined.     

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys have been studied. In order to optimize ductility and strength of the orthorhombic alloys with the nominal compositions of Ti–22Al–23Nb–3Ta and Ti–22Al–20Nb–7Ta, various thermomechanical processing steps were implemented as part of the processing route. With a special heat treatment before rolling to obtain a fine and homogeneous rolled microstructure, the rolled microstructure resulted in a good combination of high tensile yield strength and good ductility of the alloys through available solution and age treatments. The duplex microstructure with equiaxed α₂/O particles and fine O phase laths in a B2 matrix, deforming in α₂+B2+O phase field and treating in O+B2 phase field, possesses the highest tensile properties. The R.T. yield strength and ductility of the Ti–22Al–20Nb–7Ta alloy are 1200 MPa, and 9.8% respectively. The yield strength and ductility values of 970 MPa and 14% were also maintained at elevated temperature (650°C).     

Microstructure and properties of Cu–2.8Ni–0.6Si alloy

The phase transformation behavior and heat treatment response of Cu-2.8Ni-0.6Si (wt%) alloy subjected to different heat treatments were studied by X-ray diffraction, transmission electron microscopy observation, and measurement of hardness and electrical conductivity. The variation of hardness and electrical conductivity of the alloy was measured as a function of aging time. On aging at the temperature below TR (500-550°C) in Cu-2.8Ni-0.6Si alloy, the transformation undergoes spinodal decomposition, DO22 ordering, and d-Ni2Si phase. On aging at the temperature above TR (500-550 °C), the transformation products were precipitations of d-Ni2Si. The free energy versus composition curves were employed to explain the microstructure observations.     

Extrusion and selected engineering properties of Mo–Si–B intermetallics

In this study, an extrusion process has been developed to produce defect free, high-density rods of Mo–Si–B material. An initial powder composition (53.5 vol.%, 91 wt.%) of 66 vol.% Mo₅Si₃B_x (T1)–16 vol.% MoB–18 vol.% MoSi₂ was mixed with a paraffin-wax based binder (46.5 vol.%, 9 wt.%) and extruded using a twin-screw extruder. Following binder removal by a combination process of wicking and thermal degradation, the material was sintered at 1800°C. The bulk density of the sintered material was 90–92% of theoretical. Thorough binder removal was evidenced by low impurity levels: 258±6 ppm carbon and 772±10 ppm oxygen. The material demonstrated excellent high temperature oxidation resistance. The calculated parabolic rate constant is 1.1×10⁻² mg²/cm⁴/h at 1600°C. The extruded material was also successfully tested as a resistance heating element. These materials show promise for the development of heating elements with enhanced performance compared to current MoSi₂-based heating elements.