Synthesis, characterization and thermal behaviour of new copper and rare-earth metal tungstates

Three series of new copper and rare-earth metal tungstates with the formulas: CuRE ₂W₂O₁₀ (RE = Nd, Sm, Eu) and Cu₃ RE ₂W₄O₁₈ (RE = Sm, Eu or RE = Dy, Ho, Er) were synthesized by the solid-state reaction method. The CuRE ₂W₂O₁₀ and Cu₃ RE ₂W₄O₁₈ (RE = Dy, Ho, Er) compounds crystallize in the monoclinic system. The Cu₃ RE ₂W₄O₁₈ phases with the large rare-earth ions crystallize in the triclinic system. All obtained compounds melt incongruently below 1273 K. The anion lattice of the Cu₃ RE ₂W₄O₁₈ phases is built from isolated groups of octahedra (W₄O₁₆)^8?, while CuRE ₂W₂O₁₀ from WO₆ octahedra forming structural elements [(W₂O₉)^6?]^∞. The EPR spectra of analyzed compounds consisted of an intense line originating generally from the rare-earth ions and a weak, narrow line from Cu²⁺ separate centers appearing only on the surface of the grains. The absence of bulk copper in the EPR spectrum is probably due to a very short relaxation time of the Cu²⁺ subsystem.     

Synthesis and thermal stability of rare-earths molybdates and tungstates with fluorite- and scheelite-type structure

Polycrystalline samples of rare-earths molybdates and tungstates, i.e., CdRE₄Mo₃O₁₆ (RE = Eu, Gd, Y, Ho) and Pb_1–3x□_xEu_2xWO₄ (0 < x ≤ 0.1296 and □ denotes cationic vacancies) have been successfully prepared by high-temperature annealing of adequate CdMoO₄/RE₂MoO₆ and PbWO₄/Eu₂(WO₄)₃ mixtures, respectively. According to the X-ray diffraction analysis, the CdRE₄Mo₃O₁₆ compounds crystallize in a cubic, fluorite-related-type structure with space group \(Pn\bar{3}n\). In turn, new Pb_1–3x□_xEu_2xWO₄ phases crystallize in the scheelite-type, tetragonal symmetry, space group I4₁/a. Cadmium and rare-earth molybdates decompose in the solid state and the solid products of their decomposition are two RE molybdates, i.e., RE₂MoO₆ and RE₂(MoO₄)₃. Thermal stability of CdRE₄Mo₃O₁₆ decreases with decreasing of RE³⁺ radius. The melting point of each sample of Pb_1–3x□_xEu_2xWO₄ solid solution is lower than melting point of pure matrix, i.e., PbWO₄ (1116 °C), and it decreases with increasing in Eu content. Both CdRE₄Mo₃O₁₆ as well as Pb_1–3x□_xEu_2xWO₄ samples are insulators, and their optical band gap (E _g) is bigger than 3 eV.     

Application of neural networks in analysis of thermal decomposition of CoSO4·7H2O

Thermal decomposition of CoSO₄·7H₂O was investigated by simultaneous DTA-TG techniques and XRD method. Neural networks were used for DTA-TG curves analysis. Additionally, the network architecture (GRNN - Generalized Regression Neural Networks) and its statistical parameters were calculated. This method permits to generate DTA-TG curves without using kinetic models. This revised version was published online in July 2006 with corrections to the Cover Date.     

On weakly dispersive multiple-trapping transport

Equations for multiple-trapping carrier transport, corresponding to the time-of-flight method are approximately solved under the assumption that the majority of carriers are in a thermal quasi-equilibrium. The solutions show a Gaussian shape of the carrier packet. The mean velocity of the carrier sheet for the dispersive transport regime decreases in time and its dispersion grows faster than the square root of time. The accuracy of the obtained formulas is verified by Monte Carlo calculations for exponential and Gaussian trap distributions. A satisfactory agreement is obtained up to the effective carrier transit time, provided that the trap density falls-off sufficiently fast in the energy gap. A new method of determining energetic trap profiles in disordered solids from the time-of-flight measurements is proposed.     

Reactivity in the solid-state between ZnWO<Subscript>4</Subscript> and some rare-earth metal molybdates RE<Subscript>2</Subscript>MoO<Subscript>6</Subscript> (<Emphasis Type="Italic">RE</Emphasis>=Y,Sm, Eu,Gd, Dy,Ho, Er and Lu)

Organic peroxides (OPs) have caused many momentous explosions and runaway reactions, resulting from thermal instability, chemical pollutants, and even mechanical shock. In Taiwan, dicumyl peroxide (DCPO), due to its unstable reactive nature, has caused two thermal explosions and runaway reaction incidents in the manufacturing process. To evaluate thermal hazards of DCPO in a batch reactor, we studied thermokinetic parameters, such as heat of decomposition (†H _d), exothermic onset temperature (T ₀), maximum temperature rise ((dT/dt)_max), maximum pressure rise ((dP/dt)_max), self-heating rate (dT/dt), etc., via differential scanning calorimetry (DSC) and vent sizing package 2 (VSP2).     

Working Model for Explanation of Polytype formation Based on Superlattice Ordering of Point Defects in Close Packing Structures

The model is an attempt to give an explanation of polytype orderings in the crystals. Basic assumptions of the model are: the point defects play the important part in the polytype formation; these defects influence the configuration (hexagonal or cubic) of individual layers. The defects have a tendency to ordering themselves into superlattices. It acts selectively on particular stacking of the layer sequences. The results of model operation are shown in examples of experimental data of polytypic materials; they are SiC and A^IIB_VI compounds.     

Thermally stimulated carrier transport due to stepwise sample heating

The article concerns non-isothermal dispersive carrier transport in an insulating solid with traps. The approximate solutions of transport equations derived previously are extended to the case of stepwise sample heating. The specific features of thermally stimulated currents (TSCs) can be attributed to non-linear dependence of the demarcation level on temperature. In particular, the initial TSC rise has a thermally activated character where activation energy equals to the demarcation energy at the end of previous heating cycle. The accuracy of the formulae describing TSCs is verified by Monte Carlo calculations for Gaussian trap distribution.