Thermal conductivity and heat capacity of a ZnWO<Subscript>4</Subscript> crystal

The thermal conductivity of a ZnWO₄ single crystal in the principal crystallographic directions has been studied experimentally in the temperature range of 50–573 K, and the heat capacity of the single crystal has been measured in the range of 81–301 K.     

Determination of the supercooling dependence of the (211) facet of BGO crystal on growth rate from the temporal dependence of supercooling at a prescribed melt-cooling rate

The results of determination of the supercooling dependence at the interphase boundary on the rate of growth from melt of the monocrystal Bi₄Ge₃O₁₂ in the [211] direction are presented. Determination of supercooling was based on optical pyrometer measurements of the intensity of heat emission from the interphase boundary through the growing crystal. The crystal was grown by the AHF method (axial heat flux near the front of crystallization) by cooling the AHF heater at a specified rate. The growth rate corresponding to the measured supercooling value was determined by calculations. The study has shown that the supercooling is high and varies nonlinearly with the growth rate.     

Comparative analysis of the heat transfer processes during growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> crystals by the low-thermal-gradient Czochralski technique

The heat transfer processes occurring in the solid and liquid phases during growth of Bi₁₂GeO₂₀ and Bi₄Ge₃O₁₂ crystals by the low-thermal gradient Czochralski method are analyzed and compared. It is experimentally found that, under similar growth conditions, the deflection of the crystallization front for the Bi₁₂GeO₂₀ crystal is considerably smaller than the deflection of the crystallization front for the Bi₄Ge₃O₁₂ crystal and the faceting of the former front is observed at the earlier stage of pulling. The results of the numerical simulation demonstrate that the different behavior of the crystallization fronts is associated with the difference between the coefficients of thermal absorption in the crystals.     

Specific features of the phase formation,synthesis, and growth of ZnMoO4 crystals

The ZnO-MoO₃ phase diagram in the range of ZnO compositions from 0.95 to 1.05 mol % is refined by differential scanning calorimetry. Data on crystal stoichiometry are obtained using the weighting method by reducing ZnMoO₄ in a hydrogen atmosphere. The specific features of solid-phase synthesis of ZnMoO₄ are studied, and its heat of fusion is measured. The modes of solid-phase synthesis and growth of ZnMoO₄ crystals are optimized. Some experimental data on the ZnMoO₄ crystal growth in the [001] direction by the low-thermal gradient Czochralski method are presented. Crystals with a cross section of ～50 × 50 mm², a length of 160 mm, and a weight up to 1 kg have been obtained.     

GROWTH AND SPECTROSCOPIC INVESTIGATIONS OF Na2W2O7 SINGLE CRYSTALS DOPED WITH CERIUM AND CHROMIUM IONS

Journal of Structural Chemistry - Alkaline tungstate crystals are promising scintillation materials for rare events search, especially for WIMP investigations. Na2W2O7 single crystals doped with...     

Collisions of Two-Phase Liquid Droplets in a Heated Gas Medium

The paper presents the experimental research findings for the integral characteristics of processes developing when two-phase liquid droplets collide in a heated gas medium. The experiments were conducted in a closed heat exchange chamber space filled with air. The gas medium was heated to 400–500 °C by an induction system. In the experiments, the size of initial droplets, their velocities and impact angles were varied in the ranges typical of industrial applications. The main varied parameter was the percentage of vapor (volume of bubbles) in the droplet (up to 90% of the liquid volume). The droplet collision regimes (coalescence, bounce, breakup, disruption), size and number of secondary fragments, as well as the relative volume fraction of vapor bubbles in them were recorded. Differences in the collision regimes and in the distribution of secondary fragments by size were identified. The areas of liquid surface before and after the initial droplet breakup were determined. Conditions were outlined in which vapor bubbles had a significant and, on the contrary, fairly weak effect on the interaction regimes of two-phase droplets.     

Control of multi-zone resistive heater in low temperature gradient BGO Czochralski growth with a weighing feedback,based on the global dynamic heat transfer model

The global heat transfer in a crystallization setup has been optimized to develop a strategy of control over a three-zone heater in the BGO Czochralski process, in order to provide invariable thermal conditions near the solid–liquid interface in the stage of a constant-diameter crystal growth. The functional related to the exactness of the heat balance condition at the crystallization front, i.e., the Stefan problem, was chosen as the target function. The optimization yielded unexpected results. The temperature of the lower heater should be lowered, relative to that of the middle heater, with increasing crystal length, whereas the temperature of the upper heater is to be raised. These recommendations were incorporated into a dynamic model of the oxide Czochralski process with a weighing control and into the control loop of the temperature regulators of a crystallization setup. A comparison of results of the time-dependent simulation with the real growth process confirmed that the new control strategy minimizes the deviation of the solid–liquid interface from the prescribed one, significantly decreases variations of interface shape during the process, and enables growth of high-quality crystals.     

The growth of ZnWO4 and CdWO4 single crystals from melt by the low thermal gradient Czochralski technique

Crystallography Reports - The features of growth of single crystals of cadmium and zinc tungstates by the low thermal gradient Czochralski technique have been investigated. The effect of change in...     

EPR of copper ions in Pb<Subscript>2</Subscript>MoO<Subscript>5</Subscript> crystals

The article presents the results of EPR studies of Pb₂MoO₅ crystals containing a copper impurity. Based on the analysis of angular dependences of the EPR spectra, it is found that copper ions incorporate into the structure of the Pb₂MoO₅ crystal in the Cu²⁺ state and occupy the molybdenum site with the formation of a linear extended Cu(II)–V(O)–Pb(IV) defect along the а axis of the crystal. An oxygen vacancy appears in the structure of the defect to compensate the charge and the lead ion acquires the Pb⁴⁺ charge state. According to the structure of this center, one magnetically non-equivalent position with the direction of main values of А and g of A(Cu)_zz and g_zz tensors parallel to the а axis is observed in the EPR spectra. Moreover, the EPR spectra exhibit an addition hyperfine structure from one lead atom on which the unpaired electron density is 0.061%. The obtained data on the structure of the defect formed when the copper impurity incorporates into the Pb₂MoO₅ crystal provided the assumption that the observed light scattering when the light beam is directed perpendicular to the а axis may be due to the cooperative effect of the presence of di- and tetravalent ions substituting for molybdenum in the linear configuration of Pb–O–Mo bonds.     

Effect of the porosity and particle size of materials on sound-wave velocity

Methods were developed to measure longitudinal and transverse sound velocities in porous materials — various Zn-S mixtures and KBr samples. It is shown that, on exposure to ultrasound with a wavelength far exceeding the pore size in pressed samples, a porous body behaves as a continuous medium. Sound velocity in a porous material was found to depend on the quantitative ratio of vacuum, air, and toluene in the pores. Bulk sound velocities estimated using an additive method agree with experimental data within an error not more than 10%. It was found that removal of moisture traces from porous samples led to significant absorption of sound waves. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 4, pp. 121–127, July–August, 2009.