Antimony-121 Mössbauer spectral study of the Eu14MnSb11 and Yb14MnSb11 Zintl compounds

The antimony-121 M?ssbauer spectra of the Yb14MnSb11 and Eu14MnSb11 Zintl compounds have been measured between 2 or 5 and 300 K. The resulting three-dimensional arrays of the spectral counts, velocity, and temperature have been simultaneously fit with a minimum number of free parameters. These fits yield a 0 Kelvin transferred hyperfine field of 2.9(2) T, a Curie temperature of 57(3) K, and a M?ssbauer temperature of 182(2) K for Yb14MnSb11; in this case the transferred field arises solely from the ordering of Mn2+. Because Eu14MnSb11 has both Eu2+ and Mn2+ ions that are magnetically ordered, its antimony-121 M?ssbauer spectra are more complex and reveal two magnetic transitions, the first at 92(1) K resulting from the ordering of the Mn2+ ions and the second at 9.5(1.0) K resulting from the ordering of the Eu2+ ions; the corresponding 0 Kelvin transferred hyperfine fields are 1.3(1) and 3.7(1) T. The antimony-121 isomer shifts yield electronic configurations of 5s1.745p4.28 and 5s1.745p4.19 for the average antimony anion in Yb14MnSb11 and Eu14MnSb11, respectively.     

Longitudinal conductivity of thin films of La1 − x Sr x F3 − x solid solutions on glass ceramics

The longitudinal conductivity of La_{1 ? x} Sr _x F_{3 ? x} solid solution films (x = 0–0.24) with thicknesses of 40–260 nm grown on glass ceramics at temperatures from room temperature to 300°C and frequencies of 10^?1–10⁶ Hz was studied by impedance spectroscopy. The concentration dependence of film conductivity on the SrF₂ content had a maximum near x = 0.05. An equivalent circuit was constructed on the basis of the impedance plots to describe migration processes. The DC conductivity was evaluated for all samples under study. The activation energies were estimated from the temperature dependences of the DC conductivities of the films. The resulting dependences of electrophysical parameters were compared with those for bulk materials in terms of the relaxation conductivity model.     

Thermal studies of potassium tetrahydroborate−sodium tetrafluoroborate mixtures

The KBH₄?NaBF₄ mixture was studied by thermal analysis (differential scanning calorimetry). Chemical analysis, X-ray powder diffraction analysis, IR spectroscopy, and ¹¹B and ¹⁹F MAS NMR spectroscopy showed that the primary stage of the complex pyrolysis process is a metathesis reaction between components to form a new mixture, NaBH₄?KBF₄, the decomposition of which with the release of gaseous products and the formation of polyhedral borohydride compounds (mainly B₁₂H₁₂ ^2-) in the solid residue begins at a temperature of about 563 K. At a certain ratio between reactants in the initial mixture KBH₄?NaBF₄, the B₁₂H₁₂ ^2- anion can form with the material participation of the BF₄ ^- anion.     

A study on the adsorption-exchange of UO2 2+,137Cs,169Yb and HPO4 2− on modified peat

The adsorption-exchange equilibrium time and the adsorption isotherms of UO₂ ²⁺,¹³⁷Cs,¹⁶⁹Yb and HPO₄ ²⁻ on modified peat have been investigated by batch experiments. The effect of pH on the adsorption-exchange percentage (E) and the distribution coefficients (K _d) was also examined. It was found that the adsorption-exchange of UO₂ ²⁺ and¹⁶⁹Yb on the modified peat was described well by Freundlich isotherm, whereas the adsorption-exchange of¹³⁷Cs and HPO₄ ²⁻ on modified peat corresponded to a Langmuir isotherm and the maximum adsorption capacities of the modified peat for¹³⁷Cs and HPO₄ ²⁻ ions were 4.4 and 4.1 μg/g respectively. The optimum pH for the adsorption-exchange of UO₂ ²⁺,¹³⁷Cs,¹⁶⁹Yb and HPO₄ ²⁻ on the modified peat was 7.0 at 25°C.     

Chromatography of Anions on Alumina Thin Layers: Effect of Transition Metals on CI−-Br−-I− and No− 2-NO− 3 Separations

Abstract

Chromatographic behaviour of eighteen anions on thin layers of alumina and alumina mixed with silica gel (1:1, 1:2 and 2:1) has been studied using mixed acidic organic solvent systems containing formic acid. Though the addition of silica gel to alumina enhances the mobility and clarity of detection of anions, but it causes the increased tailing for Fe(CN)^3- ₆, Cr0₄ ^2- and Cr₂O₄ ^2-. Formic acid is responsible for the differential migration of anions. All the anions remained at the starting line (R_F = 0) in pure organic solvents. Formic acid-Ketone systems gave better results compared to formic acid-alcohol systems. Development time increases with the increase of viscosity/mol. wt. of organic solvents. The mutual separation of C1, Br^?, Br^?, I^? and NO^? ₂ and NO^? ₃ were achieved on pure alumina using formic acid-acetone solvent systems. The effect of transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) on C1^?-Br^?-I^? and NO^? ₂-NO^? ₃ separations has been studied.     

Thermal and dielectric characterization of multi-walled carbon nanotubes−thermoplastic polyurethanes composites

Multi-walled carbon nanotubes–thermoplastic polyurethanes composites were characterized by means of differential scanning calorimetry and dielectric relaxation spectroscopy. The composite is characterized by two glass transition temperatures T _g . The T _g associated with the soft segment decreases by increasing of carbon nanotubes content, while carbon nanotubes content has practically no effect on the value of the T _g associated with the hard segments. It was observed that rising the temperature and carbon nanotubes content resulted in the increased of both the dielectric permittivity and the loss factor. The presence of carbon nanotubes produces an enhancement of charge carriers trapping, increasing the electrical conductivity. The electrical conductivity of the composite was found to exhibit an insulator to conductor transition at a carbon nanotubes critical content, i.e., the percolation threshold, near 6 wt %.     

Synthesis, electrical and thermal properties of Bi4V2−xMexO11 (x = 0.0 and 0.02) ceramics

Polycrystalline ceramic samples of Bi₄V_2?xMe_xO₁₁ (Me = Nb, Zr, Y and Cu and x = 0.0 and 0.02) have been synthesized by standard solid state reaction method using high purity oxides. The formation of the compounds have been analysed by X-ray diffraction method. The dielectric constant, dielectric loss and AC conductivity as a function of frequency and temperature have been measured. The dielectric studies indicate that the material is highly lossy and hence its AC conductivity increases with the increase of temperature. The DC conductivity of material has been measured as a function of temperature from room temperature to 380 °C and its activation energy was calculated using the relation σ = σ ₀exp (?E _a/kT). The modulated differential scanning calorimetry has been used to investigate the effect of substitution on the heat capacity and heat flow of the compounds. The results are discussed in detail.     

Solvent‐Free Thermal Dehydration of Pentitols on Zeolites

Abstract

Dehydration of D‐arabinitol, ribitol, and xylitol at high temperature in the presence of molecular sieves without solvent in an argon atmosphere is described. Products arising after the dehydration–cyclization (cyclodehydration) reaction with retention or inversion of the configuration of asymmetric carbon atoms, were observed. Complete analytical separations of exhaustively O‐acetylated reaction products were achieved by means of GC. The chemical structures of the compounds obtained were assigned using co‐injection with standards.     

Sol–gel titania coating on unmodified and surface-modified polyimide films

Sol–gel coating of metal oxides on polymer substrates is a useful process to fabricate various organic–inorganic hybrid materials under mild conditions. However, this process is hardly applicable to pristine polyimide (PI) films because their surfaces do not display effective functional groups for metal oxide coatings. In this study, we firstly examined direct sol–gel coating of titania thin layers on unmodified PI film surfaces. The results confirmed homogeneous, ultrathin titania layer coating and showed that the thickness and microscopic morphology of the titania layers were affected by titanium alkoxide concentrations in the spin coating solutions. We next investigated titania layer coating on surface-modified PI films that prepared using alkaline hydrolysis, which generated carboxylic acid groups on the film surfaces. Optimal hydrolysis time was determined using FT-IR spectroscopy and contact angle measurements. After sol–gel titania coating on the hydrolyzed PI film surfaces, the Scotch tape test was conducted to evaluate adhesion strength between the titania layers and PI film surfaces. Morphological observations of the sample surfaces after the tests clearly showed that surface modification of PI films increased titania layer adhesions. Effect of hydrothermal treatments on film formability and adhesion strength between titania-PI film interfaces was also evaluated.     

Sintering, microstructures and dielectric properties of Ba1−xPbx(Ti1−xLix)O3−3xF3x ferroelectric ceramics

Various compositions (1−x)BaTiO₃ + xPbF2 + xLiF were prepared, shaped to pellets then sintered at 900°C for 2 h in gold sealed tubes. The purity and the symmetry of the obtained samples were checked by X-ray diffraction. A new solid solution with Ba_1−x Pb _x (Ti_1−x Li _x ) O_3−3x F_3x formula occurs in the composition range 0 ≤ x ≤ 0.20. SEM observations were performed on polished and fractured ceramics. The complex permittivity was measured as a function of temperature (−120°C ≤ T ≤ 250°C) and frequency (50 Hz ≤ f ≤ 4 × 10⁷ Hz). The dielectric performances are the best for ceramic Ba_0.97Pb_0.03(Ti_0.97Li_0.03)O_2.91F_0.09. The real component ε′, exhibits a maximum of approximately 7500 at the ferroelectric Curie temperature T _C ≈ -18°C, the dielectric losses tan δ value being 0.012. At room temperature, the relaxation frequency f _r is around 40 MHz for this ceramic. This novel ferroelectric oxifluoride is a promising material for applications, in particular in the field of Z5U multilayer capacitors.      

Modulated DSC studies of Pb1−X Ca X TiO3 ferroelectric ceramics

The polycrystalline ceramic samples of general formula Pb_1?X Ca _X TiO₃ with X = 0.00, 0.1, 0.2 and 0.3 have been synthesized by standard high temperature solid state reaction method using high purity oxide and carbonates. The formation of the single phase compounds have been checked by X-ray diffraction technique. The Modulated Differential Scanning Calorimetry has been used to investigate the effect of substitution on the phase transition temperature and the corresponding change on the enthalpy and other thermal parameters of the substituted compound/solid solutions. It was observed that the phase transition temperature (T _c ) decreases linearly with the increase of substitution concentration. The linear decrease in T _c with increase of substitution concentration may be useful for the eventual functionality of the materials for different ferroelectric devices. The results are discussed in detail.     

Molecular Dynamics Studies on Thermal Behavior of a FAU-type Zeolite

IntroductionZeolitesaregenerallycrystallinealuminosilicates.TheirframeworksarecomposedofinterconnectedTO.(T=Si.Al)tetrahetralnetworkswhichhavenumerousmoleculardimensionalmicropores.Itisknownthatzeoliteframeworkscanvibratethermallyorbedistortedbychangeoftemperatureorthepresenceofsorbedmolecules.Thecatalyticpropertiesofthezeolitesandthestabilityoftheframeworkaregreatlyaffectedbythetemperatures.Itisthusmeaningfultomodeltheflexibleframeworksinasimulationinordertounderstandthepropertiesofthezeol…     

Thermal behaviour of chemically deintercalated Li1−1Ni1+xO2

The thermal behaviour of chemically deintercalated samples obtained by HCl treatment of non-stoichiometric Li/Ni mixed oxides is studied. A multi-step reduction of Ni with O₂ evolution is observed between room temperature and 800C. Two cation redistribution processes are shown by the exothermal occurrence of a spinel phase at 300C and the endothermal reduction to a rock-salt phase with random distribution of Li and Ni cations in octahedral sites at 800C.The authors wish to express their acknowledgement to CICYT and PFPI for financial support.     

Controllable thermal expansion and phase transition in Yb2−xCrxMo3O12

The thermal expansion and phase transition of solid solutions Yb_2?xCr_xMo₃O₁₂ have been investigated by X-ray powder diffraction and differential thermal analysis. The XRD patterns and the results of Rietveld refinement of Yb_2?xCr_xMo₃O₁₂ indicate that the solid solution limit was in the composition range of 0.0 ≤ x ≤ 0.4 and 1.7 ≤ x ≤ 2.0. Yb_2?xCr_xMo₃O₁₂ (0.0 ≤ x ≤ 0.4) has an orthorhombic structure and exhibits negative thermal expansion between 200 °C and 800 °C. Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) crystallizes in monoclinic below the phase transition and above, transforms to orthorhombic. Both monoclinic and orthorhombic compounds Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) present positive thermal expansion. Orthorhombic Yb_2?xCr_xMo₃O₁₂ exhibit anisotropic thermal expansion with the contraction of a and c axes, and the linear thermal expansion coefficients range from negative to positive with increasing chromium content. Partial substitution of Yb³⁺ for Cr³⁺ exhibits depressed monoclinic to orthorhombic phase transition.     

INDO Studies on the Electronic Structure of(2,4-C_7H_(11))_2Yb(DME)

ＩＮＤＯＳｔｕｄｉｅｓｏｎｔｈｅＥｌｅｃｔｒｏｎｉｃｓＳｔｒｕｃｔｕｒｅｏｆ（２,４－Ｃ７Ｈ１１）２Ｙｂ（ＤＭＥ）ＦＥＮＧＪｉａｎ－ｎａｎ,ＷＡＮＧＺｈｉ－ｚｈｏｎｇ,ＺＨＡＮＧＳｏ－ｂｏ,ＬＩＵＪｕ－ｚｈｅｎｇ（ＩｎｓｔｉｔｕｔｅｏｆＴｈｅｏｒｅｔ...     

The Study on the Thermal Stability of CLA in Egg Yolk

Conjugated linoleic acids (CLA) have been a subject of extensive investigation for their anticarcinogenic, hypolipidemic, antiatherosclerotic and immune-enhancing activities. Stability of CLA in foods has not received much attention by both academics and industry. Although CLA has shown many beneficial effects, its decomposition must be prevented when CLA in foods is processed, stored and transported. However, no study to date has addressed the stability of CLA in foods. The present stud…     

Thermal degradation studies on PMMA–HET acid based oligoesters blends

Imparting thermal stability to polymethyl methacrylate (PMMA) without affecting its optical clarity is attempted by incorporating HET acid based oligoesters. Pure PMMA and PMMA containing five and 20 wt% of four different oligoesters are separately prepared using bulk polymerization. The thermal properties of the materials studied using DSC, TG, TG–FTIR and Pyr–GC–MS are presented. The main volatile degradation products identified are CO, HCl, CO₂, H₂O, hexachlorocyclopentadiene, hexachloroendomethylene tetrahydrophthalic acid/anhydride and methyl methacrylate. A detailed mechanism for the influence of the degradation products of HET acid based oligoesters on the thermal degradation of PMMA is also presented.     

Miniaturization of GPS patch antennas based on novel dielectric ceramics Zn(1−x)MgxAl2O4 by sol–gel method

The need for miniaturization and weight reduction of GPS patch antennas has prompted the search for new microwave dielectric materials. In this study, a sol–gel method was used to prepare Zn_(1?x)Mg_xAl₂O₄ thin films and fabricate GPS patch antennas at a low annealing temperature (700 °C). X-ray diffraction (XRD) patterns, field emission scanning electron microscopy images, Fourier transform infrared spectra, and optical band gap analyses confirmed the nanostructure of (Mg/Zn)Al₂O₄. The XRD patterns displayed the characteristic peaks of (Mg/Zn)Al₂O₄ with a face-centered cubic structure. Mg addition decreased the crystallite size, surface morphology, and lattice parameters of the resultant films, evidently affecting their density and dielectric constant (? _r ). Based on the material investigated and microwave antenna theory, GPS patch antennas were fabricated using Zn_(1?x)Mg_xAl₂O₄ and then studied using a PNA series network analyzer. The fabricated patch antennas with different ? _r ceramics decreased in size from 12.5 to 10.8 cm². The patch antennas resonated at a frequency of 1.570 GHz and provided a return loss bandwidth between ?16.6 and ?20.0 dB; their bandwidth also improved from 90 to 255 MHz. The GPS patch antenna fabricated from Zn_0.70Mg_0.30Al₂O₄ showed an excellent combination of return loss (?20.0 dB), small size (10.8 cm²), and wide bandwidth (255 MHz). Therefore, addition of Mg improves antenna performance and decreases the dimensions of the device.