On the effect of impurities on the metastable zone width of phosphoric acid

The experimental data published in the literature on the metastable zone width, as determined by the maximum supercooling ΔT_max using the conventional polythermal method, of phosphoric acid aqueous solutions containing impurities were analyzed to understand an increase in ΔT_max/T₀ with an increase in saturation temperature T₀ of solute–solvent system and the effect of impurities on the metastable zone width. For the analysis the following relations were used: ln(ΔT_max/T₀)=Φ+βln R (K. Sangwal, Cryst. Res. Technol. 44, 2009, 231−247) and (T₀/ΔT_max)²=F(1−Zln R) (K. Sangwal, Cryst. Growth Des. 9, 2009, 942−950; J. Cryst. Growth 311, 2009, 4050−4061), where Φ, β, F and Z are constants. Analysis of the experimental data revealed that: (1) the parameters Φ and F strongly depend on saturation temperature T₀ and concentration c_i of impurities, but the constants β and Z are independent of T₀ and depend on c_i, (2) the dependence of the parameters Φ and F on T₀ follows an Arrhenius-type equation with activation energy E_sat, (3) the activation energy E_sat for diffusion of ions/molecules of phosphoric acid containing impurity ions is equal to the differential heat of adsorption Q_diff for these impurities, (4) the effectiveness of an impurity is directly connected with the values of their differential heat of adsorption Q_diff; the lower the values of Q_diff for an impurity, the lower is its effectiveness in promoting nucleation, (5) the activation energy E_sat is not related with its heat of dissolution ΔH_s and (6) the increase in ΔT_max/T₀ with an increase in T₀ for phosphoric acid is associated with the activation energy E_sat for diffusion of solute molecules in the solution such that E_sat<0.     

Energy transfer up-conversion in Tm-doped silica fiber

A study of the mechanisms responsible for the infra-red to near infra-red up-conversion in Tm³⁺-doped silica fibers is presented. Up-conversion luminescence was observed from the ³H₄ level of Tm³⁺ under 1586 nm pumping into the ³F₄ level. The quadratic dependence of the up-conversion luminescence at 800 nm on the 1800 nm luminescence from the ³F₄ level confirms that the ³H₄ level is populated by a two photon process. Two possible processes are proposed as mechanisms responsible for the up-conversion: excited state absorption and energy transfer up-conversion. The decay characteristics of the luminescence from the ³H₄ level were studied under direct and indirect pumping at 786 and 1586 nm, respectively. By comparing the decay waveforms to the solution of a simple set of rate equations, the energy transfer up-conversion process (³F₄, ³F₄ → ³H₄, ³H₆) was established at Tm₂O₃ concentrations greater than 200 ppm.     

Effect of impurities on the metastable zone width of solute–solvent systems

The novel approach to interpret the metastable zone width obtained by the polythermal method using the classical theory of three-dimensional nucleation proposed recently [K. Sangwal, Cryst. Growth Des. 9 (2009) 942] is extended to describe the metastable zone width of solute–solvent systems in the presence of impurities. It is considered that impurity particles present in the solution can change the nucleation rate J by affecting both the kinetic factor A and the term B related with the solute–solvent interfacial energy γ. An expression relating metastable zone width, as defined by the maximum supercooling ΔT_max of a solution saturated at temperature T₀, with cooling rate R is proposed in the form: (T₀/ΔT_max)²=F(1−Z ln R), where F and Z are constants. The above relation can also be applied to describe the experimental data on maximum supercooling ΔT_max obtained at a given constant R as a function of impurity concentration c_i by the polythermal method and on maximum supersaturation σ_max as a function of impurity concentration c_i by the isothermal method. Experimental data on ΔT_max obtained as a function of cooling rate R for solutions containing various concentrations c_i of different impurities and as a function of concentration c_i of impurities at constant R by the polythermal method and on σ_max as a function of impurity concentration c_i by the isothermal method are analyzed satisfactorily using the above approach. The experimental data are also analyzed using the expression of the self-consistent Nývlt-like approach [K. Sangwal, Cryst. Res. Technol. 44 (2009) 231]: ln(ΔT_max/T₀)=Φ+β ln R, where Φ and β are constants. It was found that the trends of the dependences of Φ and β on impurity concentration c_i are similar to those observed in the trends of the dependences of constants F and Z on c_i predicted by the approach based on the classical nucleation theory.     

Nucleation velocity of NaPO3 melts

The time τ for visible crystallization of NaPO₃ melts is measured in the whole range of undercoolings from the melting temperature T_m to the temperature of vitrification T_g. It is shown that the time τ, needed for the appearance of a visible crystallization at high supersaturations is determined by the linear growth rate, whereas the rate of heterogeneous nucleation is the main factor, determining the experimentally measured time τ at low supersaturations. The specific surface energy σ at the melt-crystal interface is calculated from the data on nucleation velocity in the vicinity of T_m and the temperature dependences of both heterogeneous and homogeneous nucleation velocity for NaPO₃ melts are constructed. It is concluded that at normal cooling rates only nucleation, initiated by active crystallized cores could be observed for NaPO₃.     

Energy transfer between Tm and Er ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in Tm³⁺/Er³⁺ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er³⁺ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm³⁺ ions. This is evidence of the relevant contribution of the route Er₁(⁴I_11/2) + Er₂(⁴I_13/2) → Er₁(⁴I_15/2) + Er₂(⁴F_9/2) to the process. Energy migration among pumped ⁴I_9/2 level reducing the efficiency of the upconversion emission rate (³H_11/2, ⁴S_3/2, and ⁴F_9/2) is observed for Er³⁺ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.     

Thermo-optic properties of B2O3 doped Li2O-Al2O3-SiO2 glass-ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li₂O-Al₂O₃-SiO₂ glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The dS/dT value of 2.6 mol% B₂O₃-doped glass-ceramic was 12.5  × 10⁻⁶/°C, which was 0.9 ×  10⁻⁶/°C smaller than that of B₂O₃-free glass-ceramic. On the other hand, reduction in dS/dT through B₂O₃ doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass-ceramic through B₂O₃ doping is caused by the reduction in ?. The reduction in ? from B₂O₃ doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B₂O₃-doped glass-ceramic crystallized under 196 MPa was 11.7 ×  10⁻⁶/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass.     

Growth and optical properties of high-density InN nanodots

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10¹⁰ cm⁻² at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH₃ flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V_In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E_a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.     

Electrical properties of TiO2 thin films

The electrical properties of n-type titanium dioxide thin films deposited by magnetron-sputtering method have been investigated by temperature-dependent conductivity. We observed that the temperature dependence of the electrical conductivity of titanium dioxide films exhibits a crossover from T^?1/4 to T^?1/2 dependence in the temperature range between 80 and 110 K. Characteristic parameters describing conductivity, such as the characteristic temperature (T₀), hopping distance (R_hop), average hopping energy (Δ_hop), Coulomb gap (Δ_C), localization length (ξ) and density of states (N(E_F)), were determined, and their values were discussed within the models describing conductivity in TiO₂ thin films.     

Electrical properties of a-Ge-Se-In thin films

Steady state and transient photoconductivity has been measured on Ge₂₀Se_80−xIn_x (x = 0, 5, 10, 15, 20) vacuum evaporated thin films. Study of temperature dependent dark conductivity σ_d and photoconductivity σ_ph measurements in the temperature range 303-375 K, shows that the conduction in this glass is through an activated process having single activation energy. The activation energy value of photoconduction is smaller in comparison to activation energy in dark. The photosensitivity shows a maximum value at 10 at.% of In concentration. This is attributed to the decrease in the density of defect states of Ge-Se alloy with increase of In content. The results of intensity dependent steady state photoconductivity σ_ph follow a power law with intensity (F), i.e. σ_ph α F^γ where the value of power γ lies between 0.5 and 1.0, suggesting bimolecular recombination. Rise and decay of photocurrent for different concentration of In shows that photocurrent rises monotonically to the steady state value and the decay of photocurrent is also very fast. An attempt has been made to explain the results on the basis of defects and density of states.     

Molecular dynamics in glass-forming poly(phenyl methyl siloxane) as investigated by broadband thermal,dielectric and neutron spectroscopy

The molecular dynamics of glass-forming poly(methyl phenyl siloxane) (PMPS) is studied by thermal (10⁻³–5 × 10² Hz), dielectric (10⁻³–10⁹ Hz) and neutron (5 × 10⁸–10¹² Hz) spectroscopy. Because of the broad frequency range of 15 orders of magnitude the study provides a precise determination of glassy dynamics in a wide temperature range using different probes. The relaxation rates extracted from the different methods agree quantitatively in both their absolute values and in their temperature dependencies. A detailed analysis of the temperature dependence of the relaxation rate f_p by a derivative technique shows that the α-relaxation of PMPS has to be characterized by a high and a low temperature branch separated by a crossover temperature T_B = 250 K. In both temperature ranges the temperature dependence of f_p has to be described by Vogel/Fulcher/Tammann laws with different Vogel temperatures. Also the analysis of the dielectric strength in its temperature dependence gives a crossover behavior from a low to a high temperature region with a similar value of T_B. T_B can be interpreted as onset of cooperative fluctuations and the formation of dynamical heterogeneities. The dependence of the relaxation rate on the scattering vector Q extracted from neutron scattering obeys a power law τ ∼ Q^−Slope, where the power Slope varies between Slope = 2 and Slope = 3.5 with increasing temperature. This anomalous dependence of the relaxation time on the momentum transfer is discussed in terms of dynamic heterogeneities in the underlying motional processes even at temperatures above T_B. Besides the segmental dynamics the fast Methyl group rotation is considered as well. The relaxation rates of this process have an activated temperature dependence with an activation energy of 8.3 kJ/mol. The data were discussed in the framework of the threefold jump model were the incoherent elastic scattering from ‘fixed’ atoms which are frozen on the time scale of the Methyl group rotation was taken into account.     

Erbium doped fluoride glass-ceramics waveguides fabricated by PVD

Transparent amorphous and glass-ceramics waveguides in the system ZrF₄-LaF₃-ErF₃-AlF₃ (ZELA) have been fabricated by physical vapor deposition (PVD). The ceramming process was studied by means of X-ray diffraction and transmission electron microscopy for different deposition temperatures. With increasing deposition temperature, formation of La_xEr_1−xF₃ nanocrystals with x ∼ 0.3 was observed. The decay curves of the ⁴I_13/2 level in the glass-ceramics with 14.5 mol% Er³⁺ gave evidence of the presence of erbium both in the amorphous matrix (τ = 8.6 ms) and in the crystal phase (τ = 2.2 ms). The decrease of lifetime was due to clustering of erbium incorporated in LaF₃ crystal lattice. No significant increase of attenuation loss was detected after waveguide cerammization (1.3 dB/cm at 1304 nm).     

Electrical and dielectric properties of glass system NaPO3–KHSO4

Glass samples have been prepared in the NaPO₃–KHSO₄ binary system with the classical melting, casting and annealing steps. Electrical and dielectrical properties of glass samples were studied. Measurements of DC and AC conductivity and complex electrical permittivity of xNaPO₃–(100 ? x)KHSO₄ glass system were carried out at temperatures ranging from room temperature to temperature located 15 °C below glass transition temperature T_g. Results showed that changes of NaPO₃ concentration considerably affect values of observed parameters. DC conductivity of glass increases as NaPO₃ concentration grows until concentration x = 60. However, beyond this value a sharp decrease of DC conductivity was observed. In addition relaxation times showed abrupt changes at concentration x = 60, corresponding to the lowest relaxation times at the temperature 90 °C.     

Pressure and temperature dependence of the electrical resistivity of bulk Al23 Te77 glass

Electrical resistivity of bulk amorphous Al₂₃T₇₇ samples has been studied as a function of pressure (up to 80 kbar) and temperature (down to 77 K). At atmospheric pressure the temperature dependence of resistivity obeys the relation ? = π₀ exp(δE/RT) with two activation energies. In the temperature range 300 K ? T > 234 K the activation energy is 0.58 eV and for 234 >T ? 185 K the value is δE = 0.30 ev. The activation energy has been measured as a function of pressure. The electrical resistivity decreases exponentially with the increase of pressure and at 70 kbar pressure the electrical behaviour of the sample shows a metallic nature with a positive temperature coefficient. The high pressure phase of the sample is found to be a crystalline hexagonal phase.     

Increase in the Fluorine-Ion Conductivity of Single Crystals of Tysonite-type CeF<Subscript>3</Subscript> Superionic Conductor by Substituting Polarized Cd<Superscript>2+</Superscript> Ions for Ce<Superscript>3+</Superscript> Ions

The fluorine-ion conductivity of single crystals with a tysonite (LaF₃) structure with heterovalent isomorphic substitutions of highly polarizable Cd²⁺ cations with a 18-electron shell for rare earth ions Ce³⁺ have been studied for the first time. Ce_0.995Cd_0.005F_2.995 single crystals have been grown from melt by the Bridgman technique in a fluorinating atmosphere. The fluorine-ion conductivity of single crystal is measured by impedance spectroscopy in the temperature range from 153 to 1073 K, where it increases by a factor of 10⁹, approaching the value σ_dc = 5 × 10^–2 S/cm at 1073 K. At T₀ = 450 ± 20 K, the dependence σ_dc(T) is split into two portions with the ion-transport activation enthalpy ΔH_σ = 0.39 ± 0.01 eV (T < T₀) and ΔH_σ = 0.23 ± 0.02 eV (T > T0). It is found that at T = 293 K the conductivity σ_dc = 3 × 10^–5 S/cm of Ce_0.995Cd_0.005F_2.995 crystal is higher by a factor of 10 than the conductivity of the tysonite matrix CeF₃ and close to the σ_dc value for Ce_0.995Sr_0.005F_2.995 crystal. This finding indicates a significant effect of the substitutions of Cd²⁺ ions for Ce³⁺ on the σdc value and the advantage of Cd²⁺ ions over Ca²⁺ and Ba²⁺ from the viewpoint of increasing σ_dc.     

Electrical conductivity studies in single and mixed alkali doped cobalt-borate glasses

The glasses of the type (Li₂O)_x-(CoO)_0.2-(B₂O₃)_0.8−x and (Li₂O)_0.2-(K₂O)_x-(CoO)_0.2-(B₂O₃)_0.6−x were prepared by melt quench technique and their non-crystallinity has been established by XRD studies. The glasses were investigated for room temperature density and dc electrical conductivity in the temperature range 300-550 K. Molar volumes were estimated from density data. Composition dependence of density and molar volume in both the sets of glasses has been discussed. Conductivity data has been analyzed in the light of Mott’s Small Polaron Hopping (SPH) Model and activation energies were determined. Variation of conductivity and activation energy with Li₂O content in single alkali glasses indicated change over conduction mechanism from predominantly electronic to ionic, at 0.4 mole fraction of Li₂O. In mixed alkali glasses, the conductivity has passed through minimum and activation energy has passed through maximum at x = 0.2. This has been attributed to the mixed alkali effect. It is for the first time that a change over of predominant conduction mechanism in lithium-cobalt-borate glasses and mixed alkali effect in lithium-potassium-cobalt-borate glasses has been observed. Various physical and polaron hopping parameters such as polaron hopping distance, polaron radius, polaron binding energy, polaron band width, polaron coupling constant, effective dielectric constant, density of states at Fermi level have been determined and discussed.     

In situ gravimetric monitoring of surface reactions between sapphire and NH3

Surface reactions between a (0 0 0 1) C-plane sapphire and NH₃, with He as an inert carrier gas, were investigated at high temperatures over 1200 °C using the in situ gravimetric monitoring method. Although the sapphire substrate was stable up to 1400 °C under a He atmosphere, decomposition started to occur at 1300 °C under a 0.1 atm NH₃+He and the decomposition rates were found to be lower than those in 0.1 atm H₂+He at each temperature. These results imply that sapphire can be decomposed by NH₃ and/or hydrogen generated by the decomposition of NH₃ over 1300 °C. The decomposition rate in NH₃+He was decreased with increase in NH₃ flow time, and the decomposition rate became constant after 60 min of NH₃ flow. Moreover, the activation energy for sapphire decomposition before 60 min of NH₃ flow was different from that after 60 min of NH₃ flow time, which indicates that the surface reaction between sapphire and NH₃ and/or hydrogen generated from NH₃ changes depending on the time of NH₃ flow. The dependence of the surface reactions and rate-limiting reactions between sapphire and NH₃ on the time of NH₃ flow is discussed.     

Crystallization of monoclinic WO3 in tungstate fluorophosphate glasses

Tungstate fluorophosphate glass compositions with high WO₃ concentration were prepared in the ternary system (80−0.8x)NaPO₃-(20−0.2x)BaF₂-xWO₃ with x = 40,50 and 60 mol%. Transparency decreases as WO₃ concentration increases but can be improved by addition of oxidizing systems such as CeO₂ or Sb₂O₃/NaNO₃. Characterizations by thermal analysis (DSC) point out that an increase in the amount of WO₃ results in a higher glass transition temperature. In addition, such compositions are very stable against devitrification since samples containing 40% and 50% of WO₃ donot even exhibit the expected crystallization event. In these samples, the stability against crystallization decreases with the WO₃ content and vitreous sample containing 60% of WO₃ exhibits an endothermic event around 620 °C due to crystallization of monoclinic WO₃ phase. In these glasses, it was shown that the nucleation stage can be induced by thermal-treatment when external nucleating agents such as Ti or Sb are used. Finally, gold-doped samples exhibit a higher crystallization tendency and monoclinic WO₃ phase can be grown in such glasses.     

Fabrication and properties of novel low-melting glasses in the ternary system ZnO-Sb2O3-P2O5

Glasses in the ternary system ZnO-Sb₂O₃-P₂O₅ were investigated as potential alternatives to lead based glasses for low temperature applications. The glass-forming region of ZnO-Sb₂O₃-P₂O₅ system has been determined. Structure and properties of the glasses with the composition (60 − x)ZnO-xSb₂O₃-40P₂O₅ were characterized by infrared spectra (IR), differential thermal analysis (DTA) and X-ray diffraction (XRD). The results of IR indicated the role of Sb³⁺ as participant in glass network structure, which was supported by the monotonic and remarkable increase of density (ρ) and molar volume (V_M) with increasing Sb₂O₃ content. Glass transition temperature (T_g) and thermal stability decreased, and coefficient of thermal expansion (α) increased with the substitution of Sb₂O₃ for ZnO in the range of 0-50 mol%. XRD pattern of the heat treated glass containing 30 mol% Sb₂O₃ indicated that the structure of antimony-phosphate becomes dominant. The improved water durability of these glasses is consistent with the replacement of easily hydrated phosphate chains by corrosion resistant P-O-Sb bonds. The glasses containing ?30 mol% Sb₂O₃ possess lower T_g (<400 °C) and better water durability, which could be alternatives to lead based glasses for practical applications with further composition improvement.