The influences of cations on the properties of Y-Mg-Si-Al-O-N glasses

Two series (A and B series) of oxynitride glasses were prepared by melting batches at 1580 °C for 3 h under local CO reducing atmosphere in a Si-Mo-heated resistance furnace. Nominal compositions of A and B series glasses in equivalent percent (eq.%) are (28−x)Y:xMg:48Si:24Al:83O:17N and (28−x)Y:xMg:56Si:16Al:83O:17N (x = 0, 7, 14, 21), respectively. The influences of Mg/Y and Al/Si ratios on the properties such as glass transition temperature (T_g), crystallization temperature (T_C), knoop hardness (H), three-point bending strength (σ) and chemical durability in 20%HF were investigated. At the same time, the relationship between these properties and the structures of the glasses were discussed. At constant ratio Si-Al-O-N, T_g decreases nonlinearly but glass leaching ratio increases linearly with increasing Mg/Y ratio. However, H and σ increase first and then decrease as the Mg/Y ratio increases. When the Y/Mg/O/N ratio is constant, T_g decreases slightly but H and σ increase slightly as the Al/Si ratio increases.     

Topology of a Surface of the Phase Diagram of the Cholesteric Lyotropic Mesophase: Potassium Laurate/1-Decanol/Water/I-N-Lauroyl Potassium Alaninate

A surface of the phase diagram of the lyotropic cholesteric mesophase of potassium laurate/1-decanol/H₂O/l-N-lauroyl potassium alaninate (l-LAK) is studied by optical microscopy and X-ray diffraction as a function of the relative molar concentration of l-LAK and temperature. Cholesteric discotic and calamitic phases are observed and also a large domain of biaxial cholesteric phase. The inverse pitch as a function of the l-LAK relative molar concentration presents a linear behavior in the doping range studied. The l-LAK doping and its influence in favoring cholesteric calamitic phase formation are discussed in terms of the packing of the l-LAK amphiphile in the bilayer and the orientational fluctuations of the intrinsically biaxial correlation volumes.     

Relaxation of glasses: The Kohlrausch exponent

Relaxation of glasses is analyzed in the framework of the generalized Vogel-Fulcher-Tamann law. In this model the relaxation times of the cooperative and individual motions (α and β) verify the relation log τ_α(T,T′) = 1/n(T,T′) log τ_β(Τ) . In the glass state the equivalent temperature T′ is the temperature of the liquid (at equilibrium) which has the volume of the glass, T′ is then the function of the aging time and of the experiment time. In the liquid state (T = T′), n(T) ~ T − T₀ is the Kohlrausch exponent (T₀ is the Vogel temperature). In the glass state the parameter, n(T,T′), is a function of the aging time and temperature, of the rheological parameters of the liquid (WLF constants C₁ and C₂) and of the expansion coefficients of the liquid and glass states. The Kohlrausch exponents deduced from the properties below Tg (volume, creep, stress relaxation) are deduced from the generalized VFT model.     

Nanocrystallization effects on the specific heat of Fe–Co–Nb–B amorphous alloy

Specific heat at constant pressure, C_P, was measured on amorphous, nanocrystalline and fully crystalline samples of Fe₆₀Co₁₈Nb₆B₁₆ alloy. Magnetic and calorimetric measurements agree, describing a continuously decreasing Curie temperature of the amorphous phase. A clear enhancement of C_P over the Dulong–Pettit limit has been observed (from 14% to 25 %). Part of the enhancement is due to magnetic (mainly amorphous phase) and electronic contributions, although an excess volume can be inferred from the high value of the slope of C_P versus temperature.     

Poisson’s ratio of orientationally disordered hard dumbbell crystal in three dimensions

Elastic properties of a rotator phase of hard homonuclear dumbbells are determined by constant pressure Monte Carlo simulations. Simple approximations expressing the influence of molecular anisotropy on the elastic constants and Poisson’s ratio are found. The maximum density at which the dumbbells with their mass centers fixed at the lattice sites can freely rotate is observed to be strongly correlated with the density at which C₁₂ = C₄₄. It is also shown that Poisson’s ratio measured along any direction and averaged with respect to directions transverse to it is always positive. At any fixed pressure, the averaged Poisson ratio increases with increasing dumbbell anisotropy.     

Specific heat of vitreous systems: Temperature dependence near absolute zero

Unlike the low temperature heat capacities (L.T. C_V) of crystals, those of vitreous substances near T = 0 K require the formula C_V/3R = C₁T+C₂T²+C₃T³+C₅T⁵+…, where the constants C₁ and C₂ represent the anomalous excess characteristic of vitreous systems. The C₃ and C₅ represent, respectively, Debye and dispersion terms as for crystals. Recent literature confirms C₁ ≠ 0 for various glasses and offers diverse theoretical justifications for this. The C₂T² term has been proposed but not evaluated by Leadbetter. As shown here, it is required to represent the experimental C_V of vitreous silica at T < 2 K, is inferred from its Raman spectrum and is supported by theoretical calculations.     

MOCVD and characterization of Hf-silicate thin films using HTB and TEMAS

Hafnium silicate (HfSi_xO_y) films were deposited by metal-organic chemical vapor deposition (MOCVD) using a combination of precursors: hafnium tetra-tert-butoxide [Hf(OC(CH₃)₃)₄, HTB] and tetrakis-ethylmethylamino silane [Si(N(C₂H₅)(CH₃))₄, TEMAS]. The activation energy was independent on the ratio of precursor amounts in the surface reaction regime. The grown films showed Hf-rich characteristics and the impurity concentrations were less than 1 at.% (below detection limits). Hafnium silicate films were amorphous up to 700 °C annealing. Hf/(Hf + Si) composition ratio and dielectric constant (k) of the Hf-silicate films decreased by increasing the growth temperature above 270 °C.     

Theory of Systems of RodIike Particles: II. Thermotropic systems with orientation-dependent interactions

Abstract

A system of rigid, inpenetrable, rodlike molecules subject to orientation-dependent mutual attractions is treated by extension of the theory presented in the preceding paper. This energy is formulated, for a system at constant volume, by considering interactions between pairs of segments in contact, rather than in terms of interactions between entire molecules. The orientation-dependent energy between a pair of segments is taken to be proportional to cos² Ψ_ij, where Ψ_ij is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis. A characteristic temperature T? measures the intensity of these interactions. The orientational energy of the system as a whole is of the form derived by Maier and Saupe. The orientation distribution with respect to the domain axis and the partition function are formulated with T? and the axial ratio x as parameters, Steric effects of molecular shape asymmetry, embodied in x, are of foremost importance. The reduced temperature [Ttilde]_ni = T_ni/xT? at which the nematic-isotropic transition takes place in the neat liquid decreases with decrease in x below its athermal limit x_erit = 6.417 for [Ttilde]^–1 _ni = 0. Both the entropy difference between isotropic and nematic phases and the orientational heat capacity C^p, are monotonic through the transition; C_p, diverges at a temperature appreciably above T_nl, where the metastable anisotropic state becomes unstable. Comparison of theory with experiments on those nematogens whose molecules can be approximated by rigid rods lends encouragement to the prospect of relating characteristics of the nematic-isotropic transition to molecular structure.     

Structure and properties of niobium-doped potassium titanyl phosphate crystals

A series of potassium titanyl phosphate crystals, KTiOPO₄, with various concentrations of niobium dopant has been grown, and some of their physical properties and structural characteristics have been studied. The incorporation of a small amount of niobium results in considerable changes in the electrical conductivity of KTP: Nb crystals and the temperature of the ferroelectric phase transition. Thus, the presence of 3–4 at. % of niobium results in an increase of conductivity by more than an order of magnitude, whereas T _C decreases from 930 to 620°C. The X-ray diffraction study of the crystals has been performed at room temperature; the neutron diffraction analysis was made at temperatures of 20, 330, and 730°C. It was revealed that two crystallographically independent positions are statistically (by 90%) occupied by potassium cations, which results in the concentration of potassium atoms in the structure higher than it was expected from the condition of preservation of crystal electroneutrality. At high niobium concentrations, the monoclinic compound of the composition K₂TiNb₂P₂O₃ is formed.     

Sequence of phase transitions in PbHfO<Subscript>3</Subscript>

The temperature changes in the PbHfO₃ structure in the temperature range of 20 < t < 400°C have been studied by X-ray powder diffraction. The sequence of phase changes with an increase in temperature was found to be as follows: orthorhombic phase Pbam (O1), orthorhombic phase C2mm (O2), tetragonal phase P4mm (T), and cubic phase Pm3m (C). The C2mm and P4mm phases are ferroelectric, which is confirmed by measuring the dependences ɛ(t). The similarity of the transition pattern obtained with the known transition sequences for ferroelectric (barium titanate, potassium niobate, and lead titanate) and antiferroelectric (lead zirconate) oxides is analyzed.     

Synthesis and crystal structure of low ferrialuminosilicate sanidine

Iron-containing potassium feldspar crystals are prepared using the hydrothermal synthesis in an alkaline medium at temperatures ranging from 500 to 52°C. The crystal structure of the synthetic potassium feldspar is refined [Ital Structures diffractometer, MoKαradiation, 1327 unique reflections with Fs>σ(F), anisotropic approximation, R(F) = 0.044]. It is established that, under the given preparation conditions, the synthesis leads to the formation of the monoclinic modification with the following unit-cell parameters: a = 8.655(7) Å, b = 13.101(9) Å, c = 7.250(8) Å, β = 116.02(2)°, space group C2/m, and Z = 4. The cation distribution over crystallographically inequivalent tetrahedral positions T(1) and T(2) is determined and justified using X-ray diffraction data. According to this distribution, the iron-containing potassium feldspar is assigned to the low ferrialuminosilicate sanidine. The proposed structural formula K _{A = 0.99}(Si_1.2Fe_0.5Al_0.3)_{ΣT(1) = 2}(Si_1.81Al_0.19)_{ΣT(2) = 2}O₈ agrees well with the data of the electron microprobe analysis. It is revealed that iron occupies the T(1) position and manifests itself as a majority rather than minority impurity element with respect to aluminum.     

Synthesis of nonstoichiometric zirconium carbide whiskers by chemical vapor deposition

Nonstoichiometric zirconium carbide crystals with various compositions were prepared by chemical vapor deposition. Two gaseous mixtures, zirconium tetrachloride and argon, toluene and hydrogen, were introduced to the reaction zone where a graphite substrate was heated between 1200 and 1400°C. The deposition rate was proportional to the partial pressure of toluene. The compositional ratio of n_C/n_Zr in the gaseous mixture from 2.0 to 6.0 was found to be optimum for producing needle-like crystals. Needle-like crystal with smaller size were formed when the ratio of n_C/n_Zr was smaller than 2.0, and less needle-like crystals accompanied with more carbon were also produced when the ratio of n_C/n_Zr was larger than 6.0. The temperature of the substrate suitable for the growth of needle crystals was in the range from 1250 to 1300°C. The lattice constants of the products varied as a function of the ratio of n_C/n_Zr in the gaseous mixtures.     

Electrical characterisation of SrTiO3/Si interfaces

Deposition of SrTiO₃ (STO) thin films by ultra-high vacuum rf magnetron sputtering was performed in order to produce high-quality STO/p-Si (1 0 0) interfaces and STO insulator layers with high dielectric constants. The deposition temperatures were in the range from room temperature to 550 °C. Capacitance-voltage (C-V) and conductance-frequency measurements showed that the dielectric constant of the films ranges from 55 to 120. C-V measurements on Al/STO/p-Si structures clearly revealed the creation of metal-insulator-semiconductor diodes. The interface state densities (D_it) at the STO/p-Si interfaces were obtained from admittance spectroscopy measurements. The samples deposited at lower temperatures revealed values of D_it between 2×10¹¹ and 3.5×10¹² eV⁻¹ cm⁻² while the higher temperature deposited samples had a higher D_it ranging between 1×10¹¹ and 1×10¹³ eV⁻¹ cm⁻². The above results were also well correlated to X-ray diffraction measurements, Rutherford backscattering spectroscopy, and spectroscopic ellipsometry.     

Glass transition and fragility of Na2O-TeO2 glasses

The heat capacity (C_p) change in the glass transition region for the xNa₂O ·(100−x)TeO₂, mol%, glass forming melts with x=7.5, 11.1, 15.0, 20.0 and 25.0 was measured as a function of heating rate (2, 4, 6, 10 and 15 °C/min) using differential scanning calorimetry. The glass transition properties that have been measured and reported in this paper include the glass transition temperature (T_g), glass transition width (ΔT_g), heat capacities in the glassy and liquid state (C_pg and C_pl) and the activation enthalpy for glass transition (). T_g for these sodium tellurite melts decreased and increased with increasing Na₂O. Values of the ratio C_pl/C_pg ranged between 1.28 and 2.47, and the fragility parameter ranged between 100 and 130, suggesting that these glass forming melts may be classified as intermediate between typical strong and fragile liquids. The viscosity, η, calculated at a few selected temperatures near the glass transition region decreased with increasing Na₂O at any given temperature, which is also expected.     

The Johari-Goldstein β-relaxation of glass-forming binary mixtures

The present paper shows, by means of broadband dielectric measurements, that the primary α- and the secondary Johari-Goldstein (JG) β-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range.We found that the coupling parameter n = 1 − β_KWW and the ratio between α- and β-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T-P combinations but with an invariant α-relaxation time are well superposed in both the α- and β-frequency ranges. Experimental results can be rationalized by Coupling Model equation.     

Temperature variation of the mobility gap in non-polar amorphous semiconductors

For a non-polar amorphous semiconductor such as a-Si, we derive an explixit formula for (?E_g/?T)_V, the derivative with temperature of the mobility gap E_g at constant volume V. Within the framework of second-order perturbation theory for the electron-phonon (eφ) interaction, many of our physical assumptions are fundamentally different from those that apply to the crystal phase. The principal ingredients of our model are: (1) the random-phase-model (RPM); (2) the principle of non-conservation of particle momentum in the eφ interaction; and (3) the deformation potential approximation. Narrowing of E_g is found with increasing values of the temperature T. At very low T, we have $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{<mtext>V</mtext>}}\text{≌ ? ¢A · c}{}_{\mathrm{V}}\text{(T)}$, where c_V(T) is the average lattice specific heat per mode at constant volume and ¢A is a positive dimensionless quantity in the model. By contrast with low-temperature behavior of the crystal, this result implies that the mobility gap at constant volume dynamically responds to the phonomic “gas” of the disordered lattice. The high-T limit yields behavior quite similar to that of the crystal phase. We find $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{V}}\text{≌ ? x ¢A · k}{}_{\mathrm{<mtext>B</mtext>}}$, where k_B is Boltzmann's constant and the parameter x, expected to be confined to the interval $\text{1}\text{2}\text{? x ? 1}$, measures the admixture of the optical-phonon and acoustical-phonon coupling strengths.     

The small anomaly in the temperature dependence of the electrical conductivity of iron containing phosphate glasses

The dc conductivities, σ,of 2 CaO·K₂O·x Fe₂O₃·(5 - x)P₂O₅ glasses (1.4 ?x?2.2) [Fe³⁺/(Fe²⁺ + Fe³⁺)= constant (0.88±0.015)] have been measured in the temperature range from room temperature to 300°C. It has been found that the glass transition temperature increases rapidly with increasing Fe/P ratio and that mixed-valence hopping type electronic conduction takes place in these glasses and every log σ versus 1/T curve obtained shows a change in slope near 100°C, which produces a small increase, 0.14 (for x=2.2)?0.18 eV (for x=1.4), in activation energy at higher temperatures. Both the activation energy and pre-exponential factor for conduction display different dependences on the Fe/P ratio in the different temperature ranges on both sides of the anomaly temperature. These results are discussed in terms of the Mott's equation for small polaron hopping.     

X-Ray Studies of Some Europium Compounds at Elevated Temperatures

The temperature variation of the lattice constants of europium iron garnet (Eu₃Fe₅O₁₂), europium sulphide (EuS) and europium fluoride (EuF₂) has been studied using an X-ray powder diffractometer. The lattice constant of Eu₃Fe₅O₁₂ increases linearly upto 800 °C with an expansion coefficient of 10.4 × 10⁻⁶ °C⁻¹. In the case of EuS, the lattice constant increases non-linearly with temperature. At room temperature the expansion coefficient has a value of 14.3 × 10⁻⁶ °C⁻¹. In EuF₂ the lattice constant increases non-linearly upto 140 °C. At higher temperatures, the lattice constant decreases with increasing temperature with a negative expansion coefficient of −29 × 10⁻⁶ °C⁻¹ over the range 170–235 °C. The cause of the anomalous behaviour observed in EuF₂ is yet to be understood.     

Spectroscopy,oxidation-reduction behaviour and DC conductivity of a vanadium-containing barium aluminoborate glass

The vanadium(IV)-vanadium(V) equilibrium in a 37.5BaO, 5.0Al₂O₃, 57.5B₂O3 mol% + X mol% V₂O₅ (where X = 0.25?32.5) glass system has been studied as functions of temperature, partial pressure of oxygen and total vanadium concentration of the melt. The vanadium(V)/vanadium(IV) ratio in the melt increased with increasing partial pressure of oxygen, lowering temperature of melting, and with increasing total vanadium content of the melt. With X ? 10, the vanadium(V)/vanadium(IV) ratio became almost independent of the total vanadium content of the melt.With this knowledge of oxidation-reduction behaviour, a series of glasses containing 2.8?32.5 mol% V₂O₅ (at about 4 mol% intervals) and having a constant vanadium(IV)/vanadium(V) ratio (0.17) were prepared. Density, electronic absorption spectrum (both d-d and charge transfer transitions), and ESR of these glasses were measured. Optical and ESR spectra of these glasses indicated the vanadium(IV) to be present as vanadyl ion, VO²⁺; g| decreased monotonically with increasing vanadium content of these glasses, whereas gβ remained unchanged. The charge transfer transition energy due to vanadium(V) decreased, and the extinction coefficient increased by orders of magnitude with increasing vanadium content of the glass; the most striking changes occurred at X ≈ 10 mol%. DC conductivity of these glasses was measured at different temperatures; a plot of log (?/T) versus 1/T produced straight lines. The slope of these lines remained almost constant (39 ± 1 kcal/mol) for the glasses containing up to about 10 mol% V₂O₅; with further increase of V₂O₅ the slope decreased sharply.It has been concluded that the abrupt changes in properties like partial molar volume of V₂O₅, charge transfer spectrum of vanadium(V), activation energy of polaron hopping — all of which occurred around X ≈ 10 mol% — is due to a major change in the nature of vanadate groups rather than vanadium(IV) in these glasses.     

The effect of pressure on fast ion conductivity in glasses

We have measured the effect of pressure and temperature on the ionic conductivity of glasses in the system B₂O₃/Li₂O(LiX)₂ (X = F, Cl, Br, I), where X_B2O3 ? 0.7 and X/O ratio was varied while maintaining a constant Li⁺ content of X_(LiX)2 + Li₂O ? 0.3. All glasses exhibited a very small pressure coefficient of conductivity, i.e. ΔV ? 1 cm³ mol^?1, which decreased very slightly as the Cl/O ratio increased or as Cl was replaced by Br at constant X/O ratio. The results were compared with the ELectrostatic Strain Energy theory; the results of this comparison lead to the conclusion that for Li⁺ conduction the electrostatic term is dominant in determining the activation energy for conduction.