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.     

Simulation of metastable zone width and induction time for a seeded aqueous solution of potassium sulfate

The metastable zone width (MSZW, ΔT_m) and induction time (t_ind) were determined with computer simulation for seeded batch crystallization of potassium sulfate from aqueous solution. The MSZW and induction time determined with simulation showed the same behavior as experimental values reported in the literature; log (ΔT_m) increased linearly with an increase in log R (R: cooling rate) and t_ind decreases in proportion to (ΔT)⁻ⁿ (ΔT: supercooling, n: nucleation order in the secondary rate expression of B=k_n(ΔT)ⁿ). The secondary nucleation parameters (k_n and n) were deduced both from the simulated MSZW and induction times by using the previously proposed model [J. Cryst. Growth, 2010, 312, 548–554]. The secondary nucleation rate calculated with the deduced parameters was in agreement with that calculated with the parameters input for simulation.     

The influences of O/Zn ratio and growth temperature on carbon impurity incorporation in ZnO grown by metal-organic chemical vapor deposition

The strong correlations between the O/Zn ratio and carbon impurity incorporation have been observed on the ZnO films grown using N₂O or O₂ as oxygen source in metal-organic chemical vapor deposition (MOCVD). From in-situ mass spectrometric measurements, the O/Zn ratio in the MOCVD reactor is found to decrease to a minimum value as the growth temperature increased till a critical growth temperature T_c, and then increased above T_c due to different dissociation rates of the oxygen and Zn sources. The strongest D and G modes, which are ascribed to carbon clusters sp² related modes, have been observed in Raman scattering spectroscopy for the ZnO samples grown at T_c, indicating the highest incorporation rate of carbon impurity in the samples grown at T_c. Compared with O₂, N₂O has a low dissociation rate and that leads to a lower value of O/Zn ratio, resulting in much stronger D and G modes and higher incorporation rate of carbon impurities in the samples grown at T_c. It is interesting to note that the lowest specific resistances from Hall effect measurements were also obtained on the samples grown at T_c, indicating possible electrical contributions from the formation of carbon clusters, which should be highly conductive regions in ZnO. Furthermore, ionization or addition of H₂ in the case of N₂O can significantly enhance the dissociation of N₂O, with film quality improved significantly. This study shows that a high O/Zn ratio is critical to suppress carbon impurity incorporation and to grow high quality ZnO by MOCVD, especially at low growth temperature.     

Crystal growth and luminescent properties of Pr-doped K(Y,Lu)3F10 single crystal for scintillator application

Pr1%:K(Y_1−xLu_x)₃F₁₀ (x=0, 0.2, 0.4) single crystals were grown by the μ-PD method. All the grown crystals were greenish and perfectly transparent without any inclusions or cracks. Radioluminescence spectra and decay kinetics of the Pr1%:K(Y,Lu)₃F₁₀ crystals were measured. Emission from the Pr³⁺ 5d–4f transition, peaking around 260 nm and of the decay time of around 22 ns were observed. The 5d–4f emission intensities of the Pr1%:K(Y,Lu)₃F₁₀ crystals were higher than that of the standard BGO scintillator.     

A novel self‐consistent Nývlt‐like equation for metastable zone width determined by the polythermal method

Using a power‐law relation between three‐dimensional nucleation rate J and dimensionless supersaturation ratio S, and the theory of regular solutions to describe the temperature dependence of solubility, a novel Nývlt‐like equation of metastable zone width of solution relating maximum supercooling ΔT_max with cooling rate R is proposed in the form: ln(ΔT_max/T₀) = Φ + β lnR, with intercept Φ = {(1–m)/m }ln(ΔH_s/R_GT_lim) + (1/m)ln(f/KT₀) and slope β = 1/m. Here T₀ is the initial saturation temperature of solution in a cooling experiment, ΔH_s is the heat of dissolution, R_G is the gas constant, T_lim is the temperature of appearance of first nuclei, m is the nucleation order, and K is a new nucleation constant connected with the factor f defined as the number of particles per unit volume. It was found that the value of the term Φ for a system at saturation temperature T₀ is essentially determined by the constant m and the factor f. The value of the factor f for a solute–solvent system at initial saturation temperature T₀ is determined by solute concentration c₀. Analysis of the experiment data for four different solute‐water systems according to the above equation revealed that: (1) the values of Φ and m for a system at a given temperature depend on the method of detection of metstable zone width, and (2) the value of slope β = 1/m for a system is practically a temperature‐independent constant characteristic of the system, but the value of Φ increases with an increase in saturation temperature T₀, following an Arrhenius‐type equation with an activation energy E_sat. The results showed, among others, that solubility of a solute is an important factor that determines the value of the nucleation order m and the activation energy E_sat for diffusion. In general, the lower the solubility of a solute in a given solvent, the higher is the value of m and lower is the value of E_sat. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Local structural,magnetic, and optical properties of Zn1−xFexO thin films

High quality Zn_1−xFe_xO thin films were deposited on α-sapphire$\alpha \text{-sapphire}$ substrates by RF magnetron sputtering. X-ray absorption fine structure measurements showed that the chemical valence of Fe ions in the films was a mixture of 2+ and 3+ states, and Fe ions substituted mainly for the Zn sites in the films. DC-magnetization measurements revealed ferromagnetic properties from 5 to 300 K. The photoluminescence measurements at 15 K showed a sharp main transition peak at 3.35 eV along with a broad impurity peak at 2.45 eV. The structural and magnetization analyses of the Zn_1−xFe_xO films strongly suggested that the ferromagnetism was the intrinsic properties of the films.     

Thermodynamics of the Al–Ga–N2 system

The thermodynamic properties of the Al–Ga–N₂ system under high N₂ pressure up to 10 kbar and 1800 °C are investigated. On the basis of the experimental p–T growth conditions for (Al,Ga)N crystals, the standard Gibbs free energy as well as the standard enthalpy and entropy of formation of the Al_xGa_1−xN crystals as a function of composition x were calculated. The a_N2–T and x–T phase diagrams for (Al,Ga)N are presented.     

Influence of TM and RE elements on glass formation of the ternary Al-TM-RE systems

The effects of TM and RE elements on glass formation ability (GFA) of the Al-TM-RE systems are studied systematically. The TM elements show distinct differences: critical sizes of the Al-TM-Ce systems are in the order of Ni > Co > Fe > Cu. However, the RE elements show similar effect on glass formation. These results are discussed in terms of the GFA-related factors, i.e., reduced glass transition temperature (T_rg = T_g (T_x)/T_L, where T_g, T_x and T_L are the glass transition temperature, onset crystallization temperature and liquidus temperature, respectively) and the mixing enthalpy ΔH_Al-TM. A new parameter of T_L − T_g (T_x) is potentially proposed and used to evaluate the GFA of the Al-based alloys, as well as bulk glass-formers.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

Thermal stability of thin InGaN films on GaN

The thermal stability of ∼200-nm-thick InGaN thin films on GaN was investigated using isothermal and isochronal post-growth anneals. The In_xGa_1−xN films (x=0.08–0.18) were annealed in N₂ at 600–1000 °C for 15–60 min, and the resulting film degradation was monitored using X-ray diffraction (XRD) and photoluminescence (PL) measurements. As expected, films with higher indium concentration showed more evidence for decomposition than the samples with lower indium concentration. Also for each alloy composition, decreases in the PL intensity were observed starting at much lower temperatures compared to decreases in the XRD intensity. This difference in sensitivity of the PL and XRD techniques to the InGaN decomposition suggest that defects that quench luminescence are generated prior to the onset of structural decomposition. For the higher indium concentration films, the bulk decomposition proceeds by forming metallic indium and gallium regions as observed by XRD. For the 18% indium concentration film, measurement of the temperature-dependent InGaN decomposition yields an activation energy, E_A, of 0.87±0.07 eV, which is similar to the E_A for bulk InN decomposition. The InGaN integrated XRD signal of the 18% film displays an exponential decrease vs. time, implying InGaN decomposition proceeds via a first-order reaction mechanism.     

Effect of antimony addition on the thermal and electrical-switching behavior of bulk Se-Te glasses

The thermal properties and electrical-switching behavior of semiconducting chalcogenide Sb_xSe_55−xTe₄₅ (2 ? x ? 9) glasses have been investigated by alternating differential scanning calorimetry and electrical-switching experiments, respectively. The addition of Sb is found to enhance the glass forming tendency and stability as revealed by the decrease in non-reversing enthalpy ΔH_nr, and an increase in the glass-transition width ΔT_g. Further, the glass-transition temperature of Sb_xSe_55−xTe₄₅ glasses, which is a measure of network connectivity, exhibits a subtle increase, suggesting a meager network growth with the addition of Sb. The crystallization temperature is also observed to increase with Sb content. The Sb_xSe_55−xTe₄₅ glasses (2 ? x ? 9) are found to exhibit memory type of electrical switching, which can be attributed to the polymeric nature of network and high devitrifying ability. The metallicity factor has been found to dominate over the network connectivity and rigidity in the compositional dependence of switching voltage, which shows a profound decrease with the addition of Sb.     

Controlling metal–insulator transition in the hetero-epitaxial VO2/TiO2 bilayer grown on Al2O3

Hetero-epitaxial VO₂/TiO₂ bilayers were synthesized on Al₂O₃ substrates by using pulsed laser deposition, and their physical properties with the changes of oxygen pressure and the substrate orientation were investigated. A metal–insulator transition of the VO₂ was observed only in a narrow oxygen pressure range of 5–20 mTorr. As the oxygen pressure increased, X-ray diffraction peak for the (2 0 0)VO₂ shifted to a lower 2θ position, while the metal–insulator transition temperature (T_MI) decreased by ∼7 K. On the other hand, T_MI was largely varied with substrate orientation. The (2 0 0)VO₂/TiO₂ on the c-plane sapphire showed the highest T_MI of about 350 K, while the (0 0 2)VO₂/TiO₂ on the m-plane sapphire displayed the lowest T_MI of about 310 K. The (1 0 1)VO₂/TiO₂ on the r-plane and the a-plane exhibited T_MI∼340 and 330 K, respectively. The observed variations of T_MI with the oxygen pressure and substrate orientation were presumably due to the change in oxygen content of the VO₂ layer and/or in lattice strain.     

The iso-structural viscosity,configurational entropy and fragility of oxide liquids

This paper describes how the fragility of a liquid is linked to the ratio between the energy barrier (E_eq) for the equilibrium viscous behavior and that (E_iso) for the non-equilibrium iso-structural viscous behavior. Using the concept of iso-structural viscosity, two functions describing the variation of the configurational entropy (S_c) with temperature (T) are obtained from the Avramov-Milchev (AM) and the Vogel-Fulcher-Tammann (VFT) viscosity equations, respectively. The two S_c(T) functions exhibit different relations to the liquid fragility. The AM S_c(T) function is a power function with the exponent of F ? 1, where F is the AM fragility index. In this case, S_c vanishes at T = 0 K. For the VFT function, S_c vanishes as T is lowered to a finite temperature T₀, whereas it reaches the maximum value S_c,max at infinitively high T. S_c,max is proportional to the VFT fragility index. Thus, the VFT equation is not only a dynamical, but also a thermodynamic model. It is proved that for oxide liquids, the VFT equation describes viscosity data better than the AM equation, provided the pre-exponential factor η₀ is fixed to a generally accepted value, e.g., 10^?3.5 Pa s.     

Generation and annealing of defects in virgin fused silica (type III) upon ArF laser irradiation: Transmission measurements and kinetic model

The transmission of ArF laser pulses in virgin fused silica (type III) samples changes during N = 10⁶ pulses at an incoming fluence H_in = 5 mJ cm⁻² pulse⁻¹. The related absorption is determined by the pulse energy absorption coefficient α(N, H_in) using a modified Beer’s law, yielding initial values α_ini around 0.005 cm⁻¹, a maximum α_max ? 0.02 cm⁻¹ at N = 10³-10⁴ and stationary values 0.0045 cm⁻¹ ? α_end ? 0.0094 cm⁻¹ after N ≈ 6 × 10⁵ pulses. The development α(N, H_in = const.) is simulated by a rate equation model assuming a pulse number dependent E′ center density E′(N). E′(N) is determined by a dynamic equilibrium between E′ center generation and annealing. Generation occurs photolytically from the precursors ODC II and unstable SiH structures upon single photon absorption and from strained SiO bonds via two-photon excitation. Annealing in the dark periods between the laser pulses is dominated by the reaction of E′ with H₂ present in the SiO₂ network. The development α(N, H_in = const.) is observed for the very first sample irradiation only (virgin state). The values α_end are not accessible by simple spectrophotometer measurements.     

Characterization of MOVPE-grown p-InGaAs/n-InP interfaces

The low-pressure MOVPE-grown p-InGaAs-on-n-InP and n-InP-on-p-InGaAs diodes were characterized by I–V and C–V measurements to study the effects of the growth conditions on the heterointerface. The obtained band discontinuity in the conduction band ΔE_c ranged from 0.19 to 0.32 eV. It was found that ΔE_c was very sensitive to the growth interruption at the InP/InGaAs heterointerface. The n-InP-on-p-InGaAs diodes tend to show higher ΔE_c than the p-InGaAs-on-n-InP diodes. The decreased ΔE_c at InGaAs-on-InP heterointerface might be attributed to the graded layer of InGaAsP formed by intermixing at the interface. It is concluded from the estimated ΔE_c that the InP-on-InGaAs heterointerface is more abrupt than the InGaAs-on-InP heterointerface. An improvement of the InGaAs-on-InP heterointerface is mandatory for fabrication of high-performance double heterojunction bipolar transistors with InP collector layers.     

Characterization of bulk GaN crystals grown from solution at near atmospheric pressure

The properties of GaN crystals grown from solution at temperatures ranging from 780 to 810 °C and near atmospheric pressure ∼0.14 MPa, have been investigated using low temperature X-band (∼9.5 GHz) electron paramagnetic resonance spectroscopy, micro-Raman spectroscopy, photoluminescense spectroscopy, and photoluminescence imaging. Our samples are spontaneously nucleated thin platelets of approximate dimensions of 2×2×0.025 mm³, or samples grown on both polycrystalline and single crystal HVPE large-area (∼3×8×0.5 mm³) seeds. Electron paramagnetic resonance spectra consists of a single Lorentzian line with axial symmetry about the c-axis, with approximate g-values, g_∥=1.951 and g_⊥=1.948 and a peak-to-peak linewidth of∼4.0 G. This resonance has been previously assigned to shallow impurity donors/conduction electrons in GaN and attributed to Si- and/or O impurities. Room temperature photoluminescence and photoluminescence imaging data from both Ga- and N-faces show different dominant emission bands, suggesting different incorporation of impurities and/or native defects. Raman scattering and X-ray diffraction show moderate to good crystalline quality.     

Studies of some mechanical and optical properties of: (70 − x)TeO2 + 15B2O3 + 15P2O5 + xLi2O glasses

Specimens of the glassy system: (70 − x)TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by the melt-quenching. An ultrasonic pulse-echo technique was employed, at 5 MHz, for measuring: the ultrasonic attenuation, longitudinal and shear wave velocities, elastic moduli, Poisson ratio, Debye temperature and hardness of the present glasses. It is found that the gradual replacement of TeO₂ by Li₂O in the glass matrix up to 30 mol% leads to decrease the average crosslink density and rigidity of prepared samples which affects the properties, i.e., the hardness, ultrasonic wave velocities and elastic moduli are decreased, while the Poisson ratio and the ultrasonic attenuation are increased. Also, optical absorption spectra were recorded in the range, 200-800 nm for these glasses. The obtained results showed that a gradual shift in the fundamental absorption edge toward longer wavelengths occurred. Values of both of the optical energy gap, E_opt, and width tails, ΔE, are determined. It is observed that E_opt is decreased and ΔE increased with the increase of Li₂O in the glass matrix up to 30 mol%. The compositional dependences of the above properties are discussed and correlated to the structure of tested glasses.     

Thermal and some physical properties of glasses in the La2O3−CaO−B2O3 ternary system

Glasses in the La₂O₃−CaO−B₂O₃ ternary system were studied. The glass forming range as determined by the appearance of the annealed cast was found to match previously published findings. Clear glasses were formed in the composition range of 5.7−19.1 mol% La₂O₃ with constant B₂O₃ content of 71.4 mol%, and in glasses of constant La₂O₃:CaO ratio of 1:4 with B₂O₃ content in the range of 71.4-55.0 mol%. The non-linear optical crystalline phase La₂Ca₂B₁₀O₁₉ was crystallized from the clear glasses after heat treatments, as determined by powder XRD. Two types of the LaBO₃ crystalline phases were detected in the partially and the fully crystallized glass compositions outside the glass forming range. Data are reported for the glass transition temperature (T_g), dilatometric softening point (T_d), linear coefficient of expansion (α), onset crystallization temperature (T_x), exothermal peak temperature (T_P), density (ρ) and index of refraction (n_D) in the clear glasses.     

Interpretation of the critical length scale determining the conductivity in ionically conducting silicate glasses

Implications of basic percolation theories based on a random-barrier model for hopping conductivity have been explored for the interpretation of length scales determining dc conduction in ionically conducting alumina-silicate glasses. Independent measurements of activation energy E_c, conductivity spectra and Mott-Schottky capacitance-voltage characteristics indicate a common typical size for the average extent of displacement at the frequency of onset of ac conduction, L_c∼10 nm, which is interpreted as the effective correlation length of the infinite cluster determined by the percolation threshold ξ_c=E_c/k_BT. The localization of carriers in clusters of sites between critical barriers causes a reduction of the effective mobile ion concentration (≈4 × 10¹⁶ cm⁻³) by many orders of magnitude below the nominal alkali concentration.     

Model-free method for analysis of non-isothermal kinetics of a bulk sample of selenium

In this work, the activation energies of crystallization of amorphous Se were studied under non-isothermal conditions using a differential scanning calorimetric (DSC) technique. The Johnson-Mehl-Avrami and isoconversional models were used to describe the DSC crystallization data. The isoconversional methods of Friedman, Kissinger-Akahira-Sunose (KAS) and Vyazovkin were used to determine the variation of the activation energy for crystallization with temperature, E_α (T). The KAS and Vyazovkin methods gave identical values, and the range of E_α (T) was found to vary in the range 110.4-44.2 kJ mol⁻¹, while the Friedman method gave lower values with E_α (T) in the range 100.5-28.3 kJ mol⁻¹. The effects of annealing were revealed by studying the morphology of the samples with a scanning electron microscope.