Formation and properties of some Nb-doped SrTiO3-based solid solutions

It is well known that SrTiO₃ is one of the widely used materials in the electronic ceramic industry. Donor-type dopants are often added in order to improve the electrical properties of SrTiO₃-based ceramics. In the present study, the formation and characteristics of undoped and Nb-doped SrTiO₃ were studied as a function of both the niobium dopant concentration and the Sr/Ti ratio (at the same Nb content). The formation mechanism of the mixed Sr(Ti,Nb)O₃ crystals was investigated by XRD and thermal analysis. The niobium solubility in the SrTiO₃ perovskite lattice was estimated by means of unit cell parameter data. The influence of Sr concentration on the microstructure of the ceramics obtained was investigated by SEM. Ceramic properties pointed out the influence of the dopant concentration and stoichiometry on the sintering behavior of these materials. This revised version was published online in July 2006 with corrections to the Cover Date.     

Combustion characteristics of lignite and oil shale samples by thermal analysis techniques

In this research thermal analysis and kinetics of ten lignite's and two oil shale samples of different origin were performed using a TA 2960 thermal analysis system with thermogravimetry (TG/DTG) and differential al analysis (DTA) modules. Experiments were performed with a sample size of ~10 mg, heating rate of 10°C min^-1. Flow rate was kept constant (10 L h^-1) in the temperature range of 20-900°C. Mainly three different reaction regions were observed in most of the samples studied. The first region was due to the evaporation of moisture in the sample. The second region was due to the release of volatile matter and burning of carbon and called as primary reaction region. Third region was due to the decomposition of mineral matter in samples studied. In kinetic calculations, oxidation of lignite and oil shale is described by first-order kinetics. Depending on the characteristics of the samples, the activation energy values are varied and the results are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evolved gas analysis of dichlorobis(thiourea)zinc(II) by coupled TG-FTIR and TG/DTA-MS techniques

Identification and monitoring of gaseous species released during thermal decomposition of the title compound 1, Zn(tu)₂Cl₂, (tu=thiourea, (NH₂)₂C=S) have been carried out in flowing air atmosphere up to 800°C by both online coupled TG-EGA-FTIR and simultaneous TG/DTA-EGA-MS. The first gaseous products of 1, between 200 and 240°C, are carbon disulfide (CS2) and ammonia (NH₃). At 240°C, an exothermic oxidation of CS₂ vapors occurs resulting in a sudden release of sulphur dioxide (SO₂) and carbonyl sulphide (COS). An intense evolution of hydrogen cyanide (HCN) and beginning of the evolution of cyanamide (H₂NCN) and isothiocyanic acid (HNCS) are also observed just above 240°C. Probably because of condensation and/or polymerization of cyanamide vapors on the windows and mirrors of the FTIR gas cell optics, some strange baseline shape changes are also occurring above 330°C. Above 500°C the oxidation process of organic residues appears to accelerate which is indicated by the increasing concentration of CO₂, while above 600°C zinc sulfide starts to oxidize resulting in the evolution of SO₂. All species identified by FTIR gas cell were also confirmed by mass spectrometry, except for HNCS. This revised version was published online in July 2006 with corrections to the Cover Date.     

TRANS-1,4,5,8-tetraazodecalin crystals occurring in ethylenediamine

Composition and structure of crystals of unknown origin, crystallizing spontaneously from ethylenediamine on standing, has been determined by elemental analysis, FTIR, ¹H and ¹³C NMR spectroscopy and X-ray diffraction. The crystal with molecular formula C6H14N4 has been found to be a highly symmetric saturated imino compound with double-ring structure, and unambiguously identified as trans-1,4,5,8-tetraazodecalin by ¹H NMR and powder X-ray diffraction based on both its specific AA'BB' spin coupling system and simulated XRD pattern calculated from available data of previous single crystal structure determination, respectively. Simultaneous TG/DTA measurement shows one-step degradation of this compound. The volatile decomposition products have been followed by both TG/DTA-MS and TG-FTIR. Group of the largest fragments (m/z=80, 81 and 82) observed by TG/DTA-MS corresponds to an aromatic 1,4-diazine (pyrazine). In the EGA-FTIR spectrum of released gaseous species measured at the highest evolution rate by TG-FTIR, ethylenediamine can be identified as another decomposition product. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal behaviour study of some sol–gel iron-silica nanocomposites

Two series of nanocomposites from the Fe_xO_y–SiO₂ system, containing 20 mass% iron oxide were prepared by the alkoxide route of the sol–gel method, in the absence and presence of catalyst. The silica gel has been obtained using tetraethoxysilane. The iron(III) nitrate nonahydrate has been used as iron oxides source. The samples have been prepared in identical conditions, differing only by the gelation times, induced by different surface of evaporation/volume (S/V) ratios of sol let to gelify. Thermal analysis data have established the thermal treatments conditions of the prepared samples and were correlated with X-ray diffraction, IR spectroscopy and TEM results, in order to accomplish a complete structural characterization. The correlation between the structural modifications of the Fe_xO_y–SiO₂ nanocomposites and different conditions of drying has been established.     

Static and dynamic properties of R(Rh1−x Ru x )2Si2 probed by μSR (R=U,Nd,Ce)

Muon spin relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of URh₂Si₂, U(Rh_0.35Ru_0.65)₂Si₂, NdRh₂Si₂ and CeRh₂Si₂. In order to obtain information on the localisation and diffusion properties of the muon, some measurements have been performed also on isostructural diamagnetic compounds. From our measurements. information on the magneto-crystalline anisotropy of the samples has been obtained. Depending on the compound, we found a static field distribution below the Néel temperature. We discuss the implications of this result on the magnetic properties of the various materials.     

On Five- vs Six-membered Diacetal Formation from Threitol and the Intermediacy of Unusually Stable Protonated Species(1)

The long known, but hitherto poorly understood, thermodynamically controlled diacetalation of rac-threitol with alkylaldehydes provided bicyclic, cis-tetraoxadecalin (TOD) ("66") and bi(dioxolanyl) (BDO) ("55") products, shown to be formed in acid-concentration and temperature-dependent ratio. The configurational and conformational isomeric diacetals obtained in four such reactions of substituted aldehydes (RCHO, R = CH(3), CH(2)Cl, CH(2)Br, CO(2)CH(3)) with rac-threitol were isolated and characterized. A variable acid-concentration analysis of the equilibrium mixture of products in one such case (R = CH(2)Br) was performed and provided equilibrium constants and, hence, free-energy differences among these products and their relatively stable protonated intermediates. The latter were rationalized by the unusually high proton-affinity calculated for the cis-TOD ("66") form.     

Structure and evolved gas analyses (TG/DTA-MS and TG-FTIR) of <Emphasis Type="Italic">mer</Emphasis>-trichlorotris(thiourea)-indium(III), a precursor for indium sulfide thin films

The indium complex, mer-trichlorotris(thiourea)-indium(III) (In(tu)₃Cl₃, 1), crystallized from aqueous solution of InCl₃ and SC(NH₂)₂ (tu) with molar ratio of 1:3, is a single-source precursor for In₂S₃ films by chemical spray pyrolysis. The structural model of the triclinic crystal 1 (space group P-1 with a = 8.4842(2) Å, b = 10.5174(2) Å, c = 13.1767(2) Å, α = 111.1870(10)°, β = 98.0870(10)°, γ = 97.889(2)°) has been improved by single crystal X-ray diffraction analysis through successful separation of the disordered positions of the asymmetric complex molecule situated on the inversion centre into two spatial arrangements. Thermal decomposition of 1 occurs with very similar mass loss courses till 400 °C in both nitrogen and air, anyhow the DTA curve indicates a gas-phase oxidation with an additional exothermic heat effect at 255 °C in air. Partial or more advanced oxidation of the initially evolved CS₂ has taken place in both atmospheres, as its oxidation products, SO₂, COS, CO₂ are accompanied by the release of NH₃, HCl in temperature range of 205–275 °C, while H₂NCN and HCN evolve in air. In the third mass loss step, in the temperature interval of 405–750 °C in nitrogen and 405–700 °C in air, two processes, evaporation and oxidation of the solid residues are competing with each other, resulting in final decomposition product of 1 in air In₂O₃, while also some In₂O₃ in inert atmosphere beyond the main phase of In₂S₃ where, in addition considerable extent of loss of indium occurs, probably through volatile dimeric indium chloride species, which could not be detected either by EGA-MS or EGA-FTIR systems of ours. Nevertheless, evolution of HNCS is confirmed by EGA-FTIR, and release of CO₂, H₂NCN, SO₂, and a little HCl is detected at temperatures above 450 °C in both atmospheres.