DC electrical conductivity of semiconducting cobalt-phosphate glasses

The dc conductivity of semiconducting cobalt-phosphate glasses has been measured at temperatures ranging from 213 to 530 K. Four bulk samples of CoO-P₂O₅ glasses of different compositions were produced by melting dry mixtures of analytical reagent grades of CoO and P₂O₅ at temperatures between 1200-1250 °C for 2 h using a press-quenching method from glass melt. Samples were annealed at 400 °C for 1 h. The dc conductivity was found to be dependent on the CoO content in the glass. At temperatures from 213 to 444 K, however, both Mott's variable-range hopping (VRH) and the Greaves' intermediate range hopping models are found to be applicable. VRH at this range of temperatures is attributed to large values of the disorder energy of these glasses.     

Structural transformation on Se0.8Te0.2 chalcogenide glass

Using X-ray diffraction and differential scanning calorimetry (DSC), the structure and the crystallization mechanism of Se_0.8Te_0.2 chalcogenide glass has been studied. The structure of the crystalline phase has been refined using the Rietveld technique. The crystal structure is hexagonal with lattice parameter a = 0.443 nm and c = 0.511 nm. The average crystallite size obtained using Scherrer equation is equal 16.2 nm, so it lies in the nano-range. From the radial distribution function, the short range order (SRO) of the amorphous phase has been discussed. The structure unit of the SRO is regular tetrahedron with (r₂/r₁) = 1.61. The Se_0.8Te_0.2 glassy sample obeys the chemical order network model, CONM. Some amorphous structural parameters have been deduced. The crystallization mechanism of the amorphous phase is one-dimensional growth. The calculated value of the glass transition activation energy (E_g) and the crystallization activation energy (E_c) are 159.8 ± 0.3 and 104.3 ± 0.51 kJ/mol, respectively.     

Lu5Ir4Si10 whiskers: Morphology,crystal structure,superconducting and charge density wave transition studies

Highly perfect single crystal whiskers of Lu₅Ir₄Si₁₀ were successfully grown out of the melt. Details of the surface and morphology of the whiskers are presented. X-ray diffraction data confirmed that the whisker structure has the same tetragonal P4/mbm space group symmetry as bulk single crystals with lattice parameters a=12.484(1) and c=4.190(2) Å. By means of field emission scanning electron microscopy, the morphology of the whiskers has been studied. Using a 4-circle X-ray diffractometer we found that whiskers grow along the c-axis direction and all side faces are oriented along the [1 1 0] direction. The mosaicity has been measured and is found to be almost perfect: below 0.15° along the c-axis. According to our transport measurements performed along the c-axis, the whiskers present a sharp superconducting transition at T_c=4.1 K and show a charge density wave (CDW) transition at 77 K. From the hysteresis of the temperature dependance of the electrical resistivity study, the CDW transition is found to be of first order.     

Structural analysis of Cd-Te-O films prepared by RF reactive sputtering

Crystalline and microcrystalline Cd-Te-O samples have been obtained by RF reactive sputtering from a CdTe target using N₂O as oxidant. The growth conditions were substrate temperatures of 323 K, 573 K and 773 K and cathode voltage of −400 V, corresponding to 30 W of forward power. The samples were studied by micro-Raman spectroscopy, X-ray diffraction and optical transmittance. The films are remarkably transparent in the visible range, with transmittances about 88% at 400 nm and band gap energies above the absorption edge of the glass substrates. Although only the samples prepared at 773 K present defined diffraction peaks, the analysis of the Raman spectra indicate that samples prepared at 323 K and 573 K have a defined microstructure indeed. The spectra fitting performed by comparison with pattern compounds demonstrate that Cd-Te-O films are formed of Te-O units similar to those present in metal oxide-doped tellurite glasses, such as TeO₃ and TeO_3 + 1 linked through Cd-O bonds. As the substrate temperature increases the microstructure evolves from a γ-TeO₂ richer state to Cd_xTe_yO_z. In the crystalline sample the main phase identified was CdTeO₃ even though evidence of other phases was observed.     

Rheological and thermodynamic behaviors of different calcium aluminosilicate melts with the same non-bridging oxygen content

The relationships between the chemical composition and the derivative rheological and thermodynamic values have been determined for two melt series in the anorthite-wollastonite-gehlenite (An-Wo-Geh) compatibility triangle. The melt series have 0.5 and 1 non-bridging oxygens per tetrahedrally coordinated cation (NBO/T), respectively. The influences of the ratio Si/(Si + AlCa_1/2) and NBO/T on the fragility and the configurational entropy at T_g are evaluated. Linear dependencies of the viscosity, the glass transition temperature and the fragility on the ratio Si/(Si + AlCa_1/2) are found for the two melt series. A crossover in the viscosity-temperature relationship is observed for both series, i.e. an inverse compositional dependence of viscosity in the high and low viscous range. The crossover presumably reflects different responses of the adjustment of melt structure to the substitution of Al³⁺ + 1/2Ca²⁺ for Si⁴⁺ in the low versus the high viscous ranges. The crossover shifts to higher temperature with increasing NBO/T.     

Luminescence of GeO2 glass, rutile-like and α-quartz-like crystals

The luminescence of GeO₂ rutile-like crystals was studied. Crystals were grown from a melt of germanium dioxide and sodium bicarbonate mixture. Luminescence of the crystal was compared with that of sodium germanate glasses produced in reduced and oxidized conditions. A luminescence band at 2.3 eV was observed under N₂ laser (337 nm). At higher excitation photon energies and X-ray excitation an additional band at 3 eV appears in luminescence. The band at 2.3 eV possesses intra-center decay time constant about 100 μs at 290 K and about 200 μs at low temperature. Analogous luminescence was obtained in reduced sodium germanate glasses. No luminescence was observed in oxidized glasses under nitrogen laser, therefore the luminescence of rutile-like crystal and reduced sodium germanate glass was ascribed to oxygen-deficient luminescence center modified by sodium. The band at 2.3 eV could be ascribed to triplet-singlet transition of this center, whereas the band at 3 eV, possessing decay about 0.2 μs, could be ascribed to singlet-singlet transitions. Both bands could be excited in recombination process with decay kinetics determined by traps, when excitation realized by ArF laser or ionizing irradiation with X-ray or electron beam. Another luminescence band at 3.9 eV in GeO₂ rutile-like crystal was obtained under ArF laser in the range 100-15 K. Damaging e-beam irradiation of GeO₂ crystal with α-quartz structure induces similar luminescence band.     

Glass formation and structure of glasses in the ZnO―Bi2O3―WO3―MoO3 system

The glass formation region in the ternary ZnO―Bi₂O₃―WO₃ system is determined by melt quenching technique (cooling rates 10¹-10² K/s). New original glasses are obtained in a narrow concentration range with high WO₃ content (60-75 mol%). Homogeneous glasses of the composition (100 − x)[0.2ZnO·0.3Bi₂O₃·0.5WO₃]xMoO₃, were obtained between 20 and 60 mol% MoO₃. Characterization of the amorphous samples was made by x-ray diffraction (XRD), differential thermal analysis (DTA) and infrared spectroscopy (IR). The thermal stability of glasses decreases with the increasing of MoO₃ content. The glass transition temperature, T_g, varies between 340-480 °C, while the crystallization temperature, T_x, varies between 388-531 °C. The tungstate glasses possess higher crystallization temperature (T_x over 500 °C) in comparison with the other vanadate and molybdate non-traditional glasses. The glass network is realized by transformation of three-dimensional structure of WO₃ into a layered one, consisting mainly of WO₆ units. We supposed that the network of quaternary glasses is built up by MoO₄, MoO₆ and WO₆. At low concentration ZnO and Bi₂O₃ facilitate the disorder in the supercooled melts, while at high concentration stimulate crystallization processes. These oxides belong to the intermediate ones.     

Crystallization behavior and microstructure of powdered and bulk ZnO-Al2O3-SiO2 glass-ceramics

The crystallization behavior of glass with the composition: 55.6 mol% SiO₂, 22.8 mol% Al₂O₃, 17.7 mol% ZnO and 3.84 mol% of TiO₂ as nucleating agent and with different particle sizes has been studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and tranmission electron microscopy (TEM). In glass powders two crystalline phases: zinc-aluminosilicate s.s. with high-quartz structure, Zn_x/2Al_xSi_3−xO₆, (x varies dependent on heat-treatment temperature) and gahnite are formed. The ratio of these phases depends on particle sizes. In bulk glass, however, gahnite is the sole crystalline phase. The composition of initially formed zinc-aluminosilicate s.s. was determined by Rietveld refinement of XRD patterns to be Zn_0.69Al_1.38Si_1.62O₆. With temperature increase, the amount of zinc-aluminosilicate s.s decreased with simultaneous reduce of zinc and aluminum incorporated in the structure. Eventually at 1423 K almost pure high-quartz structure was formed. The activation energies of zinc-aluminosilicate s.s. and gahnite crystallization were determined by non-isothermal method to be 510 ± 18 and 344 ± 17 kJ mol⁻¹, respectively. The latter value matches well with those cited in literature for crystal growth of gahnite in similar glasses. That is attributed to the fact that the high-quartz structure acts as a precursor for gahnite crystallization.     

Electrical transport properties of 0.5Li2O-0.5M2O-2B2O3 (M = Li, Na and K) glasses

Transparent glasses in the system 0.5Li₂O-0.5M₂O-2B₂O₃ (M = Li, Na and K) were fabricated via the conventional melt quenching technique. The amorphous and glassy nature of the samples was confirmed via the X-ray powder diffraction and the differential scanning calorimetry, respectively. The frequency and temperature dependent characteristics of the dielectric relaxation and the electrical conductivity were investigated in the 100 Hz-10 MHz frequency range. The imaginary part of the electric modulus spectra was modeled using an approximate solution of Kohrausch-Williams-Watts relation. The stretching exponent, β, was found to be temperature independent for 0.5Li₂O-0.5Na₂O-2B₂O₃ (LNBO) glasses. The activation energy associated with DC conduction was found to be higher (1.25 eV) for 0.5Li₂O-0.5K₂O-2B₂O₃ (LKBO) glasses than that of the other glass systems under study. This is attributed to the mixed cation effect.     

Synthesis and characterization of calcium phosphate based bioactive quaternary P2O5-CaO-Na2O-K2O glasses

Calcium phosphate based bioactive quaternary glass systems P₂O₅-CaO-Na₂O-K₂O were prepared by melt growth technique. Glasses were prepared in five different compositions by fixing P₂O₅ at 47 mol% and CaO at 30.5 mol% and by varying the K₂O and Na₂O concentrations. The structural properties of the glasses are analyzed using X-ray diffraction (XRD) studies and scanning electron microscopy (SEM) studies; and the composition of the glasses are studied using energy dispersive X-ray spectrum (EDS). The microhardness of the glass systems are studied by Vickers hardness measurements and the bioactivity of the glasses are studied using in vitro study. The thermal properties have been examined by means of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The ultrasonic velocity measurements show that the addition of K₂O contents produces non-bridging oxygen ion and hence weaken of the glass structure. The weakening of the glass structure reduces the ultrasonic velocity and hence an increase in attenuation.     

Synthesis and physico-chemical characterization of metal free, sodium and potassium phthalocyanine complexes

Sodium phthalocyanine (Na₂Pc), potassium phthalocyanine (K₂Pc) and hydrogen phthalocyanine (H₂Pc) are synthesized in pure state. These complexes are characterized using physico-chemical methods like electronic, IR spectral, magnetic susceptibility measurements, thermogravimetric analysis and X-ray powder diffraction studies. Kinetic and thermodynamic parameters associated with the thermal decomposition were calculated using thermogravimetric analytical data. X-ray diffraction studies revealed that these complexes are having monoclinic crystal lattice structure. Electrical conductivity studies are carried out using two-probe technique in the temperature range 298-473 K for each complex. Sodium phthalocyanine showed semiconducting behavior, whereas potassium phthalocyanine showed first metallic behavior in the temperature range 300-378 K and then semiconducting property in the temperature range 383-473 K. But hydrogen phthalocyanine showed semiconducting behavior in the temperature range 300-390 K and metallic conduction behavior in the temperature range 395-473 K.     

Study of sodium tellurite glass using the thermally stimulated depolarization current technique (TSDC)

In order to have a better understanding of the role of the structure and the defects involved in the polarization processes in an 85TeO₂-15Na₂O mol% glass, we used the thermally stimulated depolarization currents (TSDC technique). The TSDC of the non-irradiated sample presented a strong negative peak of current at the temperature of 340 K, preceded by a relatively weak positive peak at about 300 K, after different d.c. voltages of 1200, 1500 and 2000 V were applied. No response was obtained with 1000 V, but the peak intensity increased considerably for voltages above 1200 V. After γ-irradiation of 25 and 50 KGy doses, a depolarization of the negative peak was observed in the sample submitted to25 KGy, whereas for the sample irradiated with 50 KGy, six TSDC peaks appeared at regular intervals of 5 KGy, in the temperature range of 100 and 300 K.     

Non-isothermal crystallization kinetics study on new amorphous Ga20Sb5S75 and Ga20Sb40S40 chalcogenide glasses

Bulk glasses of the system Ga₂₀Sb_xS_80−x (x = 5 and 40) were prepared for the first time by the known melt quenching technique. Non-isothermal differential scanning calorimetric (DSC) measurements of as-quenched Ga₂₀Sb_xS_80−x (x = 5 and 40) chalcogenide glasses reveal that the characteristic temperatures e.g. the glass transition temperature (T_g), the temperature corresponding to the maximum crystallization rate (T_p) recorded in the temperature range 400-650 K for x = 5 and 480-660 K for x = 40 are strongly dependent on heating rate and Sb content. Upon heating, these glasses show a single glass transition temperature (T_g) and double crystallization temperatures (T_p1 and T_p2) for x = 5 which overlapped and appear as a single crystallization peak (T_p) for x = 40. The activation energies of crystallization E_c were evaluated by three different methods. The crystallization data were examined in terms of recent analysis developed for non-isothermal conditions. The crystalline phases resulting from (DSC) have been identified using X-ray diffraction.     

The glass transition temperature and infrared absorption spectra of: (70−x)TeO2 + 15B2O3 + 15P2O5 + xLi2O glasses

Glasses of the system: (70−x) TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by melt quench technique. Dependencies of their glass transition temperatures (T_g) and infrared (IR) absorption spectra on composition were investigated. It is found that the gradual replacement of oxides, TeO₂ by Li₂O, decreases the glass transition temperature and increases the fragility of the glasses. Also, IR spectra revealed broad weak and strong absorption bands in the investigated range of wave numbers from 4000 to 400 cm⁻¹. These bands were assigned to their corresponding bond modes of vibration with relation to the glass structure.     

Thermodynamical miscibility in 0.8[xB2O3-(1 − x)P2O5]-0.2K2O glasses

The thermodynamical miscibility in 0.8[xB₂O₃-(1 − x)P₂O₅]-0.2K₂O glasses has been studied by measuring the glass transition temperature and the mixing enthalpy. Measurements have been performed by differential scanning calorimeter and hydrofluoric acid solution calorimetry at 298 K. The enthalpies of mixing are significantly negative over the whole range of composition. The results are discussed in terms of intermolecular associations.     

High pressure x-ray diffraction measurements on Mg2SiO4 glass

The structure factors of Mg₂SiO₄ glass have been measured using high energy x-ray diffraction up to pressures of 30.2 GPa, and the equation of state measured up to 12.8 GPa. The average Mg-O coordination numbers were extracted from the experimental pair distribution functions assuming two cases (i) there is no change in Si-O coordination number with pressure and (ii) the average Si-O coordination number increases the same as for pure SiO₂ glass. Both analyses give similar results and show a gradual increase in the average Mg-O coordination number from 5.0 at ambient pressure to ~ 6.6(6) at 30.2 GPa. There is good qualitative agreement between the experimental structure and equation of state data for the glass compared to several recent molecular dynamics simulations carried out on liquid Mg₂SiO₄.     

Thermal Properties and Phase Transformation of 2 mol% Y2O3-PSZ Nanopowders Prepared by a Co-precipitation Process

Two mol% Y₂O₃-PSZ precursor powders for dental applications were synthesized using ZrOCl₂·8H₂O and Y(NO₃)₃·6H₂O by a co-precipitation process at pH 7 and 348 K for 2 h. The thermal properties and phase transformation of 2Y-PSZ nanocrystallite powder have been investigated using a thermogravimetric and difference scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy (TEM), selected area electron diffraction and dilatometric analysis. Two weaker broad exothermic peaks appear at around 618 and 718 K were explored in DSC results. The TG analysis shows that minor weight loss occurs from 298 to 348 K, followed by three major weight losses at 610, 630 and 773 K. Calcinated at 773 and 1273 K, the crystallized phases are composed of the major phase of tetragonal ZrO₂ and minor phase of monoclinic ZrO₂. TEM reveals that the tetragonal ZrO₂ with an average size of less than 20.0 nm is mainly aggregated into the secondary aggregates with a size of small than 30.0 nm. The sintering curve of the compact pellet has a significant shrinkage with a linear rate of 18.5% at about 1341 K. Maximum densification rate happened at 1473 K, demonstrating the good low temperature sinterability for dental applications.     

Crystallization behavior of (GeS2)0.1(Sb2S3)0.9 glass

The crystal growth kinetics of antimony trisulfide in (GeS₂)_0.1(Sb₂S₃)_0.9 glass has been studied by microscopy and DSC. The linear crystal growth kinetics has been confirmed in the temperature range 492 ? T ? 515 K (E_G = 405 ± 7 kJ mol⁻¹). The applicability of standard growth models has been assessed. From the crystal growth rate corrected for viscosity plotted as a function of undercooling it has been found that the most probable mechanism is interface controlled 2D nucleated growth. The non-isothermal DSC data, corresponding to the bulk sample, can be described by the Johnson-Mehl-Avrami equation.     

Growth,thermal and optical properties of a new nonlinear optical crystal: ZnBi2B2O7

Single crystals of ZnBi₂B₂O₇ (ZBBO) have been successfully grown by the top-seeded growth method from a high-temperature melt. The crystal was colorless and transparent with size of 15×10×5 mm³. The orientation of ZBBO crystal has been discussed. The melting point, molar enthalpy of fusion, and molar entropy of fusion of the crystal were determined to be 964.02 K, 110680.36 J mol⁻¹, and 113.92 J K⁻¹ mol⁻¹, respectively. The transparency range of the crystal extends from 370 to 2100 nm.     

Structural changes during the crystallization of the Bi4Ge3O12 glasses

Bismuth-germanate glass ceramics with the composition 40% Bi₂O₃-60% GeO₂ (in molar percents) were prepared through controlled crystallization of melt-quenched glass. The Raman and FTIR spectra recorded in the as-made glasses show broad bands at 240, 400, 780 cm^− 1 and 400, 745 cm^− 1 have been assigned Ge-O bonds which appear right after preparation. X-ray diffraction has shown that the as-made glasses are amorphous, but after annealing above the crystallization temperature at 558 °C, BGO nano-crystallites with a size of about 50 nm precipitate in the glass matrix. The Raman and FTIR spectra reveal sharp peaks associated to the “internal” and “external vibrations” of GeO₄ tetrahedral groups inside the BGO nano-crystallites. In the glass ceramic sample the transparency region is shifted at longer wavelengths compared to as-made glass, due to the Rayleigh scattering on the BGO nano-crystallites.