The fundamental absorption edge and optical constants of some charge transfer compounds

A series of the charge-transfer compounds determined as 1:1 stochiometric ratio by job method has been prepared with the interaction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and anthracene, 9-methylanthracene, 9-bromanthracene in dichloromethane at room temperature. The values of the optical band gap E_g, and Urbach energy E₀ were determined from the optical absorption. The optical absorption measurements indicate that the absorption mechanism is due to allowed direct transitions for the compounds and it is evaluated that the optical band gap and Urbach energy values changes with incorporating R group in the compounds. The optical constants such as refractive index n, and extinction coefficient k and real and imaginary part of dielectric constant and optical conductivity of the compounds were calculated. E_g and E₀ reflect the influence of different types of disorder on the absorption spectra processes. Thus, a correlation between E_g and E₀ was made. Form this correlation, G value that is proportional to the second-order deformation potential and, E_f value that depend on local coordination, parameters are found to be 0.29 and 2.37 eV, respectively.     

Some properties of r.f.-sputtered hafnium nitride coatings

Hafnium nitride coatings were deposited by reactive r.f. sputtering from a hafnium target in nitrogen and argon gas mixtures. The rate of deposition, the composition, the electrical resistivity and the complex index of refraction were investigated as functions of the target-to-substrate distance and the nitrogen fraction f_N2 in the sputtering atmosphere. It was found that the relative composition of the coatings is independent of f_N2 for values above 0.1. The electrical resistivity of the hafnium nitride films changes over 8 orders of magnitude when f_N2 changes from 0.10 to 0.85. The index of refraction is almost constant at 2.8(1?0.3i) up to f_N=0.40 then decreases to 2.1(1?0.1i) for higher values of f_N2.     

First-principles calculations of crystal structure,electronic structure and optical properties of Ba<Subscript>2</Subscript>RETaO<Subscript>6</Subscript> (RE = Y,La, Pr,Sm, Gd)

The crystal structure, electronic structure and optical properties of Ba₂RETaO₆ (RE?=?Y, La, Pr, Sm, Gd) have been studied by first-principles calculation. The calculated lattice parameters are in good agreement with the previously reported values. With increasing the atomic number of RE (i.e., the number of 4f electrons), the energy level of RE 4f bands becomes lower continuously. The relationship between the electronic structure and optical properties is explored based on first-principles calculation. The electron transitions between O 2p states, RE 4f states and Ta 5d states have a key effect on optical properties such as dielectric function, refractive index, absorption coefficient and reflectivity. The phase structures have great influence on the optical properties of Ba₂SmTaO₆ and Ba₂GdTaO₆, and the big variation of reflectivity induced from phase transition makes them have potential applications in the infrared radiation protection area.     

Transport Properties of Two-dimensional Snowballs Below the Surface of 4He-II Under Rotation

Kelvin-waves play an important role for the dissipation of quantum turbulence at low temperatures. Here the plasma resonance of two-dimensional (2D) snowballs trapped below the surface of rotating superfluid ⁴He are measured for the first time in order to examine whether 2D snowballs could be a new probe for study of vortex dynamics. Below 200 mK, a positive shift of the resonant frequency f ₁ and linewidth broadening are observed as small variations in the absorption spectra under rotation. Both f ₁ and the linewidth Δf increase linearly with the rotation speed, and the slopes of f ₁ and Δf against the rotation speed have no temperature dependence. The increase of Δf suggests that an additional dissipation is caused by the coupling between the snowballs and vortices. We provide a qualitative explanation for the linear increase of Δf in the context of Kelvin-waves excited by the motion of snowballs.     

The synthesis and photoluminescent properties of one-dimensional ZnMoO4:Eu nanocrystals

ZnMoO₄:Eu³⁺ nanocrystals were synthesized by a mild and simple hydrothermal method. The results indicate that the pH value of the precursor solution plays a crucial role in controlling the morphology, size and structures. The leaflike, nanorods and nanowires were obtained by modulating the pH value of the precursor solution. The crystalline structure of ZnMoO₄:Eu³⁺ nanocrystals are affected by the precursor solution. The excitation and emission spectra were studied. The results indicate that relative intensity of f-f transitions to charge transfer (CT) absorption and ⁵D₀-⁷F₂ to ⁵D₀-⁷F₁ transitions greatly change in different samples.     

Progress in low-frequency microwave absorbing materials

Electromagnetic wave equipment and devices working at low frequency of 0.5–8 GHz have been extensively used in wireless data communication systems, local area network, household appliances and so on. It is found that the extensive use of such devices have a terrible pollution to their surroundings and moreover threaten the health of human being by weakening biological immune systems, breaking DNA strands, promoting cancers. A key solution to this problem is to develop materials that are able to attenuate the harmful electromagnetic waves pollution. This review aims at summarizing the progresses obtained in conventional materials and new emerging structures for microwave absorption at low frequency. The ultimate aim of these materials is to realize a wider effective absorption frequency bandwidth (f_E) at a thinner coating thickness (d). Typical and well-received component and construction of composite, synthesis methods, and f_E are summarized in several tables in detail. The different characteristics of different types of absorbing materials are given much attention in this review.     

Transport Properties of Topological Insulator-Based Ferromagnet/f-Wave Superconductor Junction

We investigate effect of magnetically-induced relativistic mass and also anisotropic f-wave pair coupling on the tunneling conductance on the surface of a 3D topological insulator ferromagnet/superconductor junction, which two types of pairing for superconductivity are possible. A topological insulator as a new state of condensed-matter caused by spin–orbit interaction and time-reversal symmetry has a bulk band gap and gapless surface states. We use the BTK formalism to find the charge carriers behavior. Due to two different nature of order parameters of the f-wave superconductivity, the tunneling conductance is found to be linearly dependent on the magnetic gap in terms of f ₁ and the exponential for f ₂. It is shown that the conversion of the conductance peak from ZBCP to ZBCD occurs in the f ₁ case with increasing m, while this is not observed in f ₂. Also, we find that the conductance behaves as a unit step function for the superconductor electrostatic potential in f ₁, and this should be usable in nanoelectronic switch devices. In addition, we illustrate how the magnetic gap affects the transmission coefficient in quite different behaviors for order parameters.     

SiGe heterojunction bipolar transistor issues towards high cryogenic performances

The performances increase at low temperature make the SiGe HBT a masterpiece for cryogenic circuits. The time-progressive enhancement of f_T and f_MAX toward the THz frequency at room and at cryogenic temperatures is presented along with STMicroelectronics and IBM successive HBTs generations. The influence of the Ge content and graduality into the base is discussed, highlighting the keys for best high-frequency cryogenic operation. This is shown with eight different cases and addressed on f_T, f_MAX, the transit time, the minimum noise figure and the equivalent noise resistance.     

Influence of strain amplitude on damping property of aluminum foams reinforced with copper-coated carbon fibers

Aluminum foams containing 0.35, 1.0, 1.7 vol.% copper-coated carbon fibers were fabricated by a melt route. The room temperature damping property of Al/C_f foam was studied at different strain amplitude in two directions. The experimental results show that the critical strain amplitude decreases and the damping capacity of Al/C_f foam increases with the copper-coated carbon fibers contents. It can be attributed to the interfacial micro-slip increasing with the C_f contents and the microplasticity deformation arises from the micro-crack among the C_f–Al interface. Moreover, the damping property in the transverse direction is higher than that in the longitudinal direction. The ratio of longitudinal loss factor to transverse loss factor is almost independent of the C_f contents and strain amplitude.     

Temperature-dependent dielectric and microwave absorption properties of silicon carbide fiber-reinforced oxide matrices composite

Simultaneous improvement of mechanical and microwave absorption properties of the composites at high temperatures still undergoes considerable challenges. We have investigated the high-temperature microwave absorbing properties of the silicon carbide fiber-reinforced oxide matrices (SiC_f/mullite–SiO₂) composite on the basis of our previous work. Results indicate that the complex permittivity increases from 8.19 ? j5.09 to 16.39 ? j9.83 at 10 GHz with the temperature rising from 200 to 600 °C. The SiC_f/mullite–SiO₂ composite has relatively high tanδ values indicating superior microwave attenuation ability. The reflection loss (RL) values of the composite increase with rising thickness. It can be noticed that the RL response curves of different thicknesses are basically consistent at 200 and 400 °C. In addition, the RL value of the composite is less than ??5 dB in the whole X band when the thickness is under 2.9 mm and the temperature is below 400 °C. The hybrid oxide matrices of mullite and SiO₂ are beneficial to improve the dielectric properties, especially high-temperature microwave absorption properties of the SiC fiber-reinforced ceramic matrix composite. The superior microwave absorption properties indicate that the SiC_f/mullite–SiO₂ composite is a promising candidate in aircraft engine nozzle and aerodynamic heating parts of aircrafts at high temperatures.     

Growth of silicon carbide thin films by hot-wire chemical vapor deposition from SiH4/CH4/H2

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and their structural properties were investigated by X-ray diffraction, Fourier transform infrared absorption and Raman scattering spectroscopies. At 2 Torr, Si-crystallite-embedded amorphous SiC (a-Si_1 − xC_x:H) grew at filament temperatures (T_f) below 1600 °C and nanocrystalline cubic SiC (nc-3C-SiC:H) grew above T_f = 1700 °C. On the other hand, At 4 Torr, a-Si_1 − xC_x:H grew at T_f = 1400 °C and nc-3C-SiC grew above T_f = 1600 °C. When the intakes of Si and C atoms into the film per unit time are almost the same and H radicals with a high density are generated, which takes place at high T_f, nc-3C-SiC grows. On the other hand, at low T_f the intake of Si atoms is larger than that of C atoms and, consequently, Si-rich a-Si_1 − xC_x:H or Si-crystallite-embedded a-Si_1 − xC_x:H grow.     

Experimental investigation of dimension effect on electrical oscillation in planar device based on VO2 thin film

In this paper, we report on an experimental analysis of dimension effect on a room-temperature electrical oscillation in a planar device using vanadium dioxide (VO₂) thin film. We investigate the variation of the oscillation current (I_O) and frequency (f_O) due to the variation of the dimension of the VO₂ devices, i.e., the length and width of the current channel of the device. For five different VO₂ devices with different dimensions, I_O and f_O are observed at room temperature in a simple circuit composed of a dc voltage source and a standard resistor including one of the VO₂ devices. From the experimental investigation, it is concluded that the peak-to-peak amplitude of I_O and f_O decrease with the increase of the length and width of the current channel. This indicates that f_O depends on not only the external source voltage but also the device dimension.     

First-Principles Study of the Electronic Structure of Heavy Fermion YbNi2

We investigate the electronic properties of YbNi₂ by means of band structure calculations based on the density functional theory within LDA (local density approximation), fully relativistic, and LDA+U schemes. The 4f derived bands are studied within a relativistic framework which yields flat and spin-orbit split bands, and a correlated band method (LDA+U) that includes correlation corrections. In both cases, the 4f bands, which is located roughly 200 meV below the Fermi level (E _F ), hybridize weakly with the dispersive Ni-3d bands. When the fully relativistic scheme is applied, the 4f derived bands split into lower and higher bands due to spin-orbit coupling effects. The 4f electrons are delocalized through the hybridization with conduction electrons, and the hybridization between f and conduction d electrons also plays a important role in YbNi₂. The on-site Coulomb potential is added to the Yb-derived 4f orbitals, the degeneracy between the 4f orbitals would be lifted partially and they are split into three manifolds bands. The Fermi surface splits into three different sheets which are from main the Yb-4f derived bands and Ni-3d bands. Band structure calculations reveal a saddle points existence at the L point in the energy dispersion curve closed to E _F , whereby, we think YbNi₂ might have a superconducting properties. In addition, the quasiparticle mass enhancement inferred by comparing γ to the density of states (DOS) at the Fermi level indicates the effective mass of YbNi₂ enhanced with the fully relativistic results.     

Predicting breakage behavior and particle size of bronze and cast iron machining chips pulverized by jet milling

It has been proposed that the breakage behavior of particulate materials can be described by two material parameters f_mat and W_min. f_mat describes the resistance of the material to fracture in impact pulverization and W_min characterizes the specific energy which a particle can absorb without fracture. It is shown in this study that this concept can be used to quantify breakage behavior of bronze and cast iron chips in jet milling process and also to predict particle size of the jet milled product. Different tin bronze and cast iron chips with varying initial size were pulverized in a target plate jet mill with different velocity. f_mat was found to be in the range of 0.06–0.09 and 0.18–0.25 for bronze and cast iron alloys, respectively. For the cast iron alloys f_mat increased with increasing content of carbon and silicon. Similarly, for the bronze alloys, f_mat increased with increasing tin content. An equation was developed to predict mean particle size of the jet milled chips as a function of the kinetic energy, initial chip size and material parameters. The experimental results of various alloys confirmed that the mean particle size after single and multiple impacts were accurately predicted.     

Structural and optical properties of thermally evaporated zinc phthalocyanine thin films

Thermally evaporated zinc phthalocyanine (ZnPc) films in the as deposited condition were identified to be as-amorphous. It undergoes structural transformation upon annealing up to 613 K. The optical properties and spectral behavior of as deposited and annealed thin films of ZnPc were studied using spectrophotometric measurements of the transmissivity and reflectivity at normal incidence of light in the wavelength range 200–2500 nm. The refractive index, n, and absorption index, k, were calculated and it was found that they are independent of film thickness in the thickness range 205–530 nm. Annealing at 613 K increases absorbance of films by 5–6 times in comparison with absorbance of as deposited ones and shifts peak positions of all bands towards low energy side of spectra except the peak position of N-band is shifted towards high energy side of spectra. The absorption spectra in the UV–VIS. region has been analyzed in terms of both molecular orbital and band theories. Indirect allowed transitions near the onset and fundamental absorption edges were observed. The energy at the onset was obtained and equals to 1.45 and 1.51 eV for as deposited and annealed films, respectively. The fundamental energy gap was obtained and equals to 2.94 and 2.88 eV for as deposited and annealed films, respectively. The absorption spectra shows four absorption bands. The oscillator strength, f, the electric dipole strength, q², the molar extinction coefficient, ζ_molar, were calculated for as deposited and annealed ZnPc thin films.     

Preparation and properties of multi-walled carbon nanotube/poly(organophosphazene) composites

Carbon nanotubes (CNTs) are promising materials because of their unique properties. However, the poor solubility in solvents limits the function of CNTs and hinders their applications in many fields. Surface modification of CNTs with polymers is an efficient method to solve this problem. Several polymers were tested for the preparation of CNT dispersions. In comparison with organic polymers, poly(organophosphazenes) are highly stable macromolecules with adjustable properties which depend on the side groups. This article is to describe the synthesis of thermally stable and soluble multi-walled CNT/poly(organophosphazene) composites. The poly(organophosphazene)s substituted with (a) 100 % quaternary protonated pyridinoxy (PPY), (b) 50 % quaternary protonated pyridinoxy and 50 % a long aliphatic chain alcohol (1-dodecanol) (PDK), and (c) 50 % quaternary protonated pyridinoxy and 50 % a glycol ether [(2-(2-methoxyethoxy)ethanol] (PET) have been synthesized. f-MWCNT/poly(organophosphazene) composites have been prepared by the treatment of the functionalized multi-walled carbon nanotubes (f-MWCNT) with the protonated polyphosphazenes (PPY, PDK, and PET) using different feed ratios [R _feed = 1:1, 1:3, 1:5, 1:10 (w:w)]. The thermal stability of prepared composites (f-MWCNT/PPY, f-MWCNT/PDK, and f-MWCNT/PET) have been investigated by TGA. By considering thermal stabilities and solubility of all prepared composites, f-MWCNT/PPY_1:5, f-MWCNT/PDK_1:5, and f-MWCNT/PET_1:5 have been chosen as optimum composite composition and characterized by ³¹P NMR, ¹H NMR, XRD, Raman spectroscopy, and EDX analysis. The morphologic characterizations of the f-MWCNT/PPY_1:5, f-MWCNT/PDK_1:5, f-MWCNT/PET_1:5 nanocomposites have been carried out by HRTEM. Excellent dispersions of the nanocomposites in water and common organic solvents have been achieved. The solubility and thermal stability of nanocomposites depend on the side groups on poly(organophosphazene).     

Effects of loading rate on the local cleavage fracture stress σf in notched specimens

Four point bending (4PB) notched specimens with different notch sizes are tested at various loading rates at a temperature of −110 °C for a C-Mn steel. An elastic-plastic finite element method (FEM) is used to determine the stress and strain distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ_f is measured. The results show that the local cleavage fracture stress σ_f does not essentially change with loading rate V and notch size. The reason for this is that the cleavage micromechanism does not change in the different specimens at various loading rates. The cleavage micromechanism involves competition of two critical events of crack propagation and crack nucleation in the high stress and strain volume ahead of notch root. The large scatter of σ_f and notch toughness are mainly caused by the different critical events in different specimens.     

Mechanical and microstructural properties of waste andesite dust-based geopolymer mortars

Waste andesite dust (WAD) occurs during sawing and other stone dressing processes of andesite stone. The disposal of WAD may cause storage and environmental pollution problems. The use of WAD in geopolymer production may be a solution to these problems. The mechanical and microstructural properties of geopolymer mortars synthesized from WAD were investigated in this work. To investigate these properties, the geopolymer mortars were manufactured using WAD with different molarity variations of NaOH as alkali-activator. The produced fresh WAD-based geopolymer mortars were cured at 100 °C in three different periods (12, 24, and 48 h). The results revealed that the ultrasonic pulse velocity (U_pv), flexural strength (f_fs), compressive strength (f_cs) of WAD-based geopolymer mortars increased as increasing NaOH molarity until reaching an optimum concentration (12 M for this work). Curing periods also significantly affected the mechanical and microstructural properties of WAD-based geopolymer mortars. The results showed that the higher f_fs and f_cs values can be obtained with a longer curing periods. Moreover, considering overall performance analysis, geopolymer synthesis with WAD promises a solution for sustainable mortar production and waste elimination.     

Dielectric and optical properties of TlGa1 − x Er x S2 (x = 0, 0.001, 0.005, 0.01) single crystals

TlGa_{1 ? x} Er _x S₂ (x = 0, 0.001, 0.005, 0.01) solid solutions, based on the layered compound TlGaS₂, have been prepared by direct elemental synthesis. The effect of Er concentration on the dielectric and optical properties of the TlGa_{1 ? x} Er _x S₂ solid solutions has been studied. The results demonstrate that increasing the Er content of the TlGa_{1 ? x} Er _x S₂ solid solutions decreases the real part of their complex dielectric permittivity and increases their dielectric loss tangent. The conductivity (σ) of the TlGa_{1 ? x} Er _x S₂ solid solutions in the frequency range f = 1 to 35 MHz exhibits σ ～ f ^0.8 behavior, indicative of hopping charge transport through their band gap. We have evaluated the key parameters of this charge transport mechanism. We have studied temperature-dependent optical properties of the TlGa_{1 ? x} Er _x S₂ solid solutions. At temperatures in the range T = 77–200 K, the TlGa_0.999Er_0.001S₂ solid solution has an absorption band near its fundamental absorption edge, which is due to transitions to a direct exciton state.     

Ultrafast pump-probe spectroscopy studies of CeO2 thin film deposited on Ni-W substrate by RF magnetron sputtering

This study presents the first investigation of rapid dynamical processes that occur in pure CeO₂ thin film, using ultra fast pump-probe spectroscopy at room temperature. For this purpose we have used a single (200) oriented CeO₂ film deposited on biaxially textured Ni-W substrate by RF magnetron sputtering technique. The ultrafast transient spectra show initial sharp rise transition followed by an exponential photon decay. This rise time is about 10 ps irrespective of the probe wavelengths range 500–800 nm. The initial decay constant (τ) at 500 nm probe wavelength is found to be 171 ps, while at 800 nm probe wavelength it is 107.5 ps. The ultrafast absorption spectra show two absorption peaks at 745 and 800 nm, and are attributed to the electronic transitions from ²F_7/2–²F_5/2 and ¹S₀–¹F₃ respectively. The relatively high intensity absorption peak at 745 nm indicates dominant f–f electronic transition. Further, the absorption peak at 745 nm splits into two distinct peaks with respect to delay time, and is attributed to the charge transfer in between Ce⁴⁺ and Ce³⁺ ions. These results indicate that CeO₂ itself is a potential candidate and can be used for optical applications.