Investigations of LiNb1−xTaxO3 nanopowders obtained with mechanochemical method

Nanocrystalline compounds LiNb1−xTaxO3 of various compositions (x = 0, 0.25, 0.5, 0.75, 1) were synthesized by high-energy ball milling of the initial materials (Li2CO3, Nb2O5, Ta2O5) and subsequent high-temperature annealing of the resulting powders. Data on the phase composition of the nanopowders were obtained by X-ray diffraction methods, and the dependence of the structural parameters of LiNb1−xTaxO3 compounds on the value of x was established. As a result of the experiments, the optimal parameters of the milling and annealing runs were determined, which made it possible to obtain single-phase compounds. The Raman scattering spectra of LiNb1−xTaxO3 compounds (x = 0, 0.25, 0.5, 0.75, 1) have been investigated. Preliminary experiments have been carried out to study the temperature dependences of their electrical conductivit

Calcium Tungstate is a Perspective Acousto-Optic Material. Photoelastic Properties

Abstract -Acousto-optic modulators are main component of communication fibre-optic systems. From the other side the studies of the absolute piezo-optic effect (POE) are one of the important steps to estimate the acousto-optic efficiency of optical materials. In this article the investigation results of POE in calcium tungstate crystals (CaWO 4 ) has been represented. It is also proved that this crystal is essentially better acousto-optic material in comparison with lithium niobate (LiNbO 3 ) widely used in acousto-optic devices. Keywords -acousto-optic efficiency, elastic constants, piezo-and elastooptic coefficients, interferometric method The absolute piezo-optic coefficients (POCs) have been studied using the interferometric method taking into account the real lack of parallelism of sample faces. eliminates the influence of inconsiderable sample wedgeshaping on the error, indices m, k, i denote the directions of one axis pressure, light propagation and is polarization accordingly, is the wave length. Experimental values fort the main geometries of the experiment (i, m = 1, 2, 3) has been given in the table 1. The sign "+" or "-" at values of control stresses denote the increasing or decreasing of the natural optical light beam path induced by the change of the light path through the one axis. Besides that is necessary to take into account, that to the mechanical compression stresses assigned the "minus" sign. The estimated values of the main coefficients im are given in the tabl. 2. + ) and is ± 10% (is verified by multiple measurements), and the error of the second summand is caused of the elastic coefficient S km (± 5%). We see in the tabl. 2, that POE value of the calcium tungstate is greater as POE of lithium niobate: the corresponding POC of CaWO 4 crystals are more then ~ 2-7 times higher, and the sum of absolute values of im is 3 times higher. Nevertheless for estimating the acousto-optic efficiency of calcium tungstate it is necessary to determine the elastooptic coefficients (EOCs) p in , using the determined im and known tensor relation p in = im mn , where mn denote the elastic stiffness constants

A Tunable and Electrically Small Antenna for Compact GNSS Receivers

The electronic receivers of global navigation satellite systems (GNSSs) are implemented in various handheld electronic devices such as laptops, smartphones, and smartwatches; therefore, their dimensions are of critical importance. Achieving a GNSS terminal of a small size is difficult due to its relatively low operational frequency (L-band), which is equivalent to a wavelength of approximately 24 cm. As an efficient half-wavelength antenna is too large for compact devices, in this paper, an electrically small antenna (ESA) for GNSS terminals is presented. The antenna was miniaturized by using a dielectric block with relatively high permittivity, making some parts virtual, and optimizing its geometry. The operational frequency of the ESA is tunable by means of metallic rods of variable heights inserted into a cylindrical cavity drilled inside the dielectric block. The results confirm the feasibility of the concept and the usability of the ESA for compact GNSS terminals