Role of nanoscale AlN and InN for the microwave characteristics of AlGaN/(Al,In)N/GaN-based HEMT

A new AlGaN/GaN-based high electron mobility transistor (HEMT) is proposed and its micro-wave characteristics are discussed by introducing a nanoscale AlN or InN layer to study the potential improvement in their high frequency performance. The 2DEG transport mechanism including various sub-band calculations for both (Al,In) N-based HEMTs are also discussed in the paper. Apart from direct current characteristics of the proposed HEMT, various microwave parameters such as transconductance, unit current gain (h ₂₁ = 1) cut-off frequency (f _t ), high power-gain frequency (f _max). Masons available/stable gain and masons unilateral gain are also discussed for both devices to understand its suitable deployment in microwave frequency range.     

Investigation of the structure of an amorphous As-Se semiconductor system by relaxation methods

The results of a complex investigation into dark-current relaxation in the long-time region of an MIM structure based on an As-Se thin-film chalcogenide system are presented. The values of parameters describing the electronic processes ocurring in the contact layers of the investigated compounds are estimated. The coincidence of the nature of conductivity and charge-accumulation mechanisms is revealed. The relaxation-time distribution function is calculated, and its structural sensitivity to such technological factors as the change in the composition stoichiometry and the method for manufacturing experimental samples is established.     

Modification of electrical and optical properties of ZnO films under ultraviolet irradiation

The effect of ultraviolet radiation of polycrystalline zinc oxide films (with a thickness of 200 nm) on their resistivity, transparency, and luminescence in the visible and violet spectral regions is studied. It is shown that, under irradiation of the films in air and vacuum, the conductivity, transmittance, and edge luminescence intensity increase with characteristic times of about 100 min. It is established that the corresponding processes controlled by desorption of oxygen atoms and molecules from the surface of nanocrytals in the ZnO films are reversible.     

Size effects in foams: Experiments and modeling

Mechanical properties of cellular solids depend on the ratio of the sample size to the cell size at length scales where the two are of the same order of magnitude. Considering that the cell size of many cellular solids used in engineering applications is between 1 and 10 mm, it is not uncommon to have components with dimensions of only a few cell sizes. Therefore, both for mechanical testing and for design, it is important to understand the link between the cellular morphology and size effects, which is the aim of this study. In order to represent random foams, two-dimensional (2D) Voronoi tessellations are used, and four representative boundary value problems - compression, shear, indentation, and bending - are solved by the finite element (FE) method. Effective elastic and plastic mechanical properties of Voronoi samples are calculated as a function of the sample size, and deformation mechanisms triggering the size effects are traced through strain maps. The modeling results are systematically compared with experimental results from the literature. As a rule, with decreasing sample size, the effective macroscopic stiffness and strength of Voronoi samples decrease under compression and bending, and increase under shear and indentation. The physical mechanisms responsible for these trends are identified.     

Processing and electromechanical properties of lanthanum-doped Pb(Zr,Ti)O<Subscript>3</Subscript> extruded piezoelectric fibres

This article describes the processing and characterisation of lanthanum-doped lead zirconate titanate (PLZT)-based ferroelectric fibres for composite transducer applications. X-ray diffraction of the extruded and sintered fibres indicated some lead loss during sintering; however, the fibres exhibited low porosity (1.54%), high maximum piezoelectric strain (4041 ppm) and relatively low coercive field (0.77 kV/mm). The low coercive field of the lanthanum-doped fibres may be advantageous in terms of facilitating polarization of the fibres in composite architectures.     

Determination of Se,As, Sb,Te, and Bi in wastewaters of mining-and-processing integrated works with the use of AAS-ETA

The procedure of analysis of wastewaters of mining-and-processing integrated works on the content of Se, As, Sb, Te, and Bi with the use of an atomic absorption method with electrothermal atomizer, which involves the preliminary sample preparation and choice of temperature program, modifier, and corrector of nonselective absorption, was described. The conditions were matched for the determination of low concentrations of these elements in natural waters.     

High-pressure volume magnetostriction in the diluted magnetic semiconductor Cd1 ? xMnxGeAs2 (x = 0.06–0.3)

The specific volume of the diluted magnetic semiconductor Cd_{1 − x} Mn _x GeAs₂ (x = 0.06–0.3) has been determined for the first time by strain measurements at pressures of up to 7 GPa. From the pressure dependences of the relative specific volume, we evaluated the volume magnetostriction (spontaneous magnetization coefficient). A scaling relation was used to estimate the bulk modulus of the magnetically ordered and disordered phases.     

Optical study of poly(vinyl alcohol)/hydroxypropyl methylcellulose blends

The proposal in this study was to evaluate the optical properties of different biopolymers films. The materials used were poly(vinyl alcohol) (PVA) and hydroxypropyl methylcellulose (HPMC). PVA/HPMC blends were prepared by casting technique. Variations in the group coordination in the infrared region were followed. The effects of HPMC concentrations on the optical properties of the PVA films were studied by near infrared, ultraviolet/visible, transmittance, and reflectance in the spectral region 200–2500 nm. Absorption, transmittance, and reflectance spectra were used for the determination of the optical constants. The study has been also extended to include the changes in the optical parameters including the band tail width and band gap energies and extinction coefficient for the investigated films. The results indicate that the optical band gap was derived from Tauc’s extrapolation and decreases with the HPMC contents. The obtained optical parameters were found to be strongly affected by HPMC contents.     

Study on conductivity and redox stability of iron orthovanadate

FeVO₄ was synthesised by conventional solid state technique. Impedance measurements using a silver electrode were unsuccessful due to a solid state reaction between FeVO₄ and Ag, forming α-AgVO₃ and α-Fe₂O₃ at the interface. Impedance measurements, with a platinum electrode, reaffirmed that FeVO₄ exhibits semiconductor behaviour in air. In a reducing atmosphere, 5% H₂/Ar, high electronic conductivity, from 1 S cm⁻¹ at 300 °C to 2 S cm⁻¹ at 700 °C, was observed with an activation energy of 0.13(1) eV. X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry data determined that the change in electronic conductivity was due to the degradation of the material into FeV₂O₄ and α-Fe₂O₃. It is believed that the conduction was due to electron hopping between vanadium d-orbitals. Neither FeVO₄ nor FeV₂O₄ are deemed suitable as anode materials for solid oxide fuel cells, due to redox instability.