Kinetics of the exchanges at the solid–liquid interface during the dissolution process of gallium orthophosphate

Gallium orthophosphate is a piezoelectric material with an α-quartz structure. In order to manufacture bulk acoustic wave devices (BAW), controlled chemical dissolution is often used to reduce the thickness of the piezoelectric membranes. This paper presents the kinetics of the chemical exchanges, which occur at the solid–liquid interface during the chemical dissolution of GaPO₄ in phosphoric acid. Based on chemical composition of phosphoric acid solvent, the pure dissolution rate is determined. A strong anisotropy of chemical reactivity is formed. The dissolution rate is the lowest for the crystallographic z-plane (0 0 0 1) but this orientation is the most sensitive with respect to the proton concentration and the temperature. In accordance with the crystal growth rates, the nucleation at the interface for the (1 0 2 0) plane, named X-plane, is also the most rapidly dissolved. Assuming the activation energies corresponding to dissolution and to nucleation are like standard activation energies, the different values of the standard enthalpy variation are calculated. The most important variation is obtained for the z-plane (Δ_rH=−14.3 kJ/mol) and the lowest for the X-plane (Δ_rH=−5.4 kJ/mol).     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.     

Influence of thermal and thermoelectric treatments on structure and electric properties of B2O3–Li2O–Nb2O5 glasses

A transparent glass with the composition 60B₂O₃–30Li₂O–10Nb₂O₅ (mol%) was prepared by the melt quenching technique. The glass was heat-treated with and without the application of an external electric field. The as-prepared sample was heat-treated (HT) at 450, 500 and 550 °C and thermoelectric treated (TET) at 500 °C. The following electric fields were used: 50 kV/m and 100 kV/m. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman, dc and ac conductivity, as a function of temperature, were used to investigate the glass and glass-ceramics properties. LiNbO₃ crystals were detected, by XRD, in the 500 °C HT, 550 °C HT and 500 °C TET samples. The presence of an external electric field, during the heat-treatment process, improves the formation of LiNbO₃ nanocrystals at lower temperatures. However, in the 550 °C HT and in the TET samples, Li₂B₄O₇ was also detected. The value of the σ_dc decreases with the rise of the applied field, during the heat-treatment. This behavior can indicate an increase in the fraction of the LiNbO₃ crystallites present in these glass samples. The dc and ac conduction processes show dependence on the number of the ions inserted in the glass as network modifiers.The Raman analysis suggests that the niobium ions are, probably, inserted in the glass matrix as network formers.These results reflect the decisive effect of temperature and electric field applied during the thermoelectric treatment in the structure and electric properties of glass-ceramics.     

Experimental determination of the solid–liquid equilibrium,metastable zone,and nucleation parameters of the flunixin meglumine–ethanol system

Measurements of the metastable zone and solubility for flunixin meglumine–ethanol system were obtained. The solubility was measured within the temperature range from 288.15 to 328.15 K. The mole fraction solubility was correlated satisfactorily with the temperature by the equation: x_eq=2.35×10^?12e^0.07121T. The value of enthalpy of dissolution, enthalpy of fusion and enthalpy of mixing were determined to be 49.04, 64.03 and ?14.99 kJ mol^?1 respectively. The metastable zone width of flunixin meglumine was measured by an electric conductivity method. A comparison of the nucleation temperatures from electric conductivity measurement and from focused beam reflectance measurement (FBRM) shows that both detection techniques give almost the same results for flunixin meglumine. The nucleation parameters of flunixin meglumine in ethanol were determined from the metastable zone data. Over the equilibrium temperature range from 312.28 to 325.55 K, the nucleation rate constant was varied from 0.00001 to 0.00120 #/m² min, whereas the nucleation order was varied from 2.23022 to 3.39299. The obtained high values of nucleation order indicated a high rate of nucleation.     

Theoretical and Practical Studies on Effects of External Electrostatic Electric Field on Nucleation and Growth Kinetics of Protein Crystals

The crystallization technique where an electric field is applied is an extremely powerful tool to control the crystallization processes of various materials. In particular, the method with application of an external electrostatic electric field can have a significant effect on the phase equilibrium of the liquid and solid phases. This review demonstrates that the crystallization processes of proteins are significantly impacted by the application of an external electrostatic electric field: (1) Control of both the increase and decrease in the nucleation rate can be achieved by changing the applied frequency of the external electrostatic electric field. (2) The effect of the external electrostatic electric field on the nucleation rate can be controlled by regulating the thickness of the electric double layer (EDL) formed at the interface. (3) The quality of the grown crystals can be improved by an increase in the step free energy under application of an external electrostatic electric field at 1 MHz. The effect of the external electrostatic electric field on nucleation and growth kinetics during crystal growth of proteins is also discussed based on a thermodynamic perspective.     

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO₂ films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5 nA, and an acceleration voltage of 25 kV for 25 s, a Ge-doped H:SiO₂ film (3.8 at.% Ge) showed a maximum second-order nonlinearity of d₃₃ = 0.0005 pm/V. In contrast, a H:SiO₂ film with a smaller Ge content (1.0 at.% Ge), showed a large SHG: d₃₃ = 0.06 pm/V when irradiated for 15 s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.     

Solid–liquid interface energies in the succinonitrile and succinonitrile–carbon tetrabromide eutectic system

The equilibrated grain boundary groove shapes for the commercial purity succinonitrile (SCN) and succinonitrile–carbon tetrabromide (CTB) eutectic system were directly observed. From the observed grain boundary groove shapes, the Gibbs–Thomson coefficients for the solid SCN–liquid SCN and solid SCN–liquid SCN CTB have been determined to be (5.43±0.27)×10⁻⁸ Km and (5.56±0.28)×10⁻⁸ Km, respectively, with numerical method. The solid–liquid interface energies for the solid SCN–liquid SCN and solid SCN–liquid SCN CTB have been obtained to be (7.86±0.79)×10⁻³ J m⁻² and (8.80±0.88)×10⁻³ J m⁻², respectively from the Gibbs–Thomson equation. The grain boundary energies in the SCN and SCN rich phase of the SCN–CTB system have been calculated to be (15.03±1.95)×10⁻³ J m⁻² and (16.51±2.15)×10⁻³ J m⁻², respectively, from the observed grain boundary groove shapes. The thermal conductivity ratios of the liquid phase to the solid phase for SCN and SCN–4 mol% CTB alloy have also been measured.     

Crystallization-in-emulsion process of a melted organic compound: In situ optical monitoring and simultaneous droplet and particle size measurements

The crystallization-in-emulsion process allows the production of solid particles exhibiting specific features. Here, the batch crystallization process carried out by cooling a melted oil dispersed as an oil-in-water emulsion was studied. Two experimental set-ups allowing the in situ visualization of the nucleation and growth phenomena occurring in the dispersed liquid phase were developed. Observations in quiescent medium of motionless droplets having a diameter of few tens of micrometers showed that primary nucleation started on the inner surface of the droplets. The fast growth of the crystals consumed all the liquid contained within each droplet and was confined within each droplet by the oil–water interface. Solid polycrystalline particles similar in size to the parent droplets were produced. Dynamic tracking of the transient evolution of the size distributions of the two populations of droplets and solid particles during the cooling process in a stirred vessel was carried out using an in situ optical probe. It was shown that the droplets crystallized very progressively during cooling, starting with the largest droplets and ending with the smaller size droplets since the induction time of primary nucleation was dependent on droplet volume. In dilute conditions (1% wt% of dispersed phase) each droplet was converted into a single solid particle. Secondary nucleation based on inter-droplet collisions was not observed in these conditions.     

Self-catalyzed vapor–liquid–solid growth of InP/InAsP core–shell nanopillars

Indium phosphide/indium arsenide phosphide core–shell nanopillars have been prepared by the vapor–liquid–solid method using liquid indium droplets as the catalyst. The indium droplets were generated in situ in the deposition reactor. The hexagonal nanopillars exhibited hexagonal shaped sidewalls with average width and height of 150 and 250 nm, respectively. Cross-section transmission electron microscopy with selected area electron diffraction and X-ray dispersion energy analysis verified that an InAsP layer, approximately 10 nm thick, coated the pillars. Photoluminescence spectra at 77 K yielded an extremely intense band at 0.76 eV (1.63 μm), which was due to the InAsP shell on the pillars.     

Mechanisms controlling primary crystallisation of Ga20Te80 glasses

The kinetic study of the crystallisation process of Ga₂₀Te₈₀ glass from isothermal and continuous heating calorimetric data have been performed applying a recently developed procedure. The kinetic information was complemented with X-ray diffraction measurements. With this scope, three crystallisation patterns, with three-dimensional isotropic growth have been analysed: (i) site saturation and interface controlled growth. (ii) homogeneous nucleation with interface controlled growth and (iii) homogeneous nucleation with two simultaneous modes of crystal growth (interface- and diffusion-controlled). A complex model with two simultaneous modes of three-dimensional isotropic crystal growth with decreasing homogeneous nucleation and soft impingement has been applied for modelling primary crystallisation of the Ga₂₀Te₈₀ glass. The model goes beyond the isokinetic hypothesis when coupling isothermal and continuous heating kinetic data. The apparent activation energy E_a = (2.06 ± 0.03) eV/at obtained for the primary crystallisation of the phase Te is shown to correspond to an activation energy for nucleation E_I = (2.85 ± 0.03) eV/at and an interface controlled activation energy for growth E_u = (1.90 ± 0.03) eV/at at the crystallisation onset.     

Continuous compositional changes of crystal and liquid during crystallization of a sodium calcium silicate glass

This paper deals with a systematic study of crystal nucleation and growth kinetics in a 14.6Na₂O–34.0CaO–51.4SiO₂ mol% glass, which is close to the CaO · SiO₂–Na₂O · SiO₂ pseudo-binary section, just left of the stoichiometric Na₂O · 2CaO · 3SiO₂ (N₁C₂S₃) compound. We show that crystallization begins with nucleation of a Na_4+2xCa_4−x[Si₆O₁₈] (0 < x < 1) solid solution that is enriched in sodium as compared with both parent glass and the N₁C₂S₃ compound; while a fully crystallized sample is composed only by a solid solution that is stable at very high temperatures, but is metastable in the temperatures under investigation. We thus confirm a continuous compositional change of the crystals during the course of crystallization.     

The preferential orientation of the directionally solidified Si–TaSi2 eutectic in situ composite

The Si–TaSi₂ eutectic in situ composite is a favorable field emission material due to relatively low work function, good electron conductivity, and three-dimensional array of Schottky junctions grown in the composite spontaneously. The preferential orientation during directional solidification is determined by the growth anisotropy. In order to obtain the preferential direction of the steady-state crystal growth, the transmission electron microscopy (TEM) is used for analysis. It is found that the preferential orientation of the Si-TaSi₂ eutectic in situ composite prepared by Czochralski (CZ) technique is [3 3¯ 2¯] Si∥[0 0 0 1] TaSi₂, (2 2 0)Si∥(2 2¯ 0 0) TaSi₂. Whereas the preferential orientation of the Si–TaSi₂ eutectic in situ composite prepared by electron beam floating zone melting (EBFZM) technique is [0 1¯ 1¯ ]] Si∥[0 0 0 1] TaSi₂,(0 1¯ 1) Si∥(0 1¯ 1 1)TaSi₂. The preferential directions of the Si-TaSi₂ eutectic in situ composites prepared by two kinds of crystal growth techniques are distinctly different from each other, which results from different solid–liquid interface temperatures on account of the different crystal growth conditions, e.g. different solidification rate, different temperature gradient, different solid–liquid interface curvature and different kinetic undercooling.     

The comparisons of growth mechanisms for Si thin films with different deposition rates in CVD process by the description of the roughness evolution

The roughness evolutions of micro-crystalline silicon thin films (μc-Si:H) with different growth rates prepared by chemical vapor depositions have been investigated by atomic force microscopy. The growth exponent β was measured as 0.8 ± 0.03, 1.1 ± 0.07 and 0.75 ± 0.02 for three sets of samples prepared by PECVD with and without hydrogen dilution ratio modulation and by HWCVD, respectively, and does not correlated with the deposition rate in a set. However, the root-mean-square roughness and lateral correlation length decrease with increasing the deposition rate for both PECVD and HWCVD process. We suggested that the nonstationary growth with large β is correlated with the shadowing effect. The influence of the deposition rate on the surface roughness could be related to the diminishing of the shadowing effect by surface species diffusion with higher mobility on an H-covered surface. The initial surface and nucleation condition play an important role in the surface roughness evolution.     

Solid–liquid interfacial energy of neopentylglycol solid solution in equilibrium with neopentylglycol–(D) camphor eutectic liquid

The grain boundary groove shapes for equilibrated solid neopentylglycol (NPG) solution (NPG–3 mol% D-camphor) in equilibrium with the NPG–DC eutectic liquid (NPG–36.1 mol% D-camphor) have been directly observed using a horizontal linear temperature gradient apparatus. From the observed grain boundary groove shapes, the Gibbs–Thomson coefficient (Г), solid–liquid interfacial energy (σ_SL) of NPG solid solution have been determined to be (7.5±0.7)×10^?8 K m and (8.1±1.2)×10^?3 J m^?2, respectively. The Gibbs–Thomson coefficient versus T_mΩ^1/3, where Ω is the volume per atom was also plotted by linear regression for some organic transparent materials and the average value of coefficient (τ) for nonmetallic materials was obtained to be 0.32 from graph of the Gibbs–Thomson coefficient versus T_mΩ^1/3. The grain boundary energy of solid NPG solution phase has been determined to be (14.6±2.3)×10^?3 J m^?2 from the observed grain boundary groove shapes. The ratio of thermal conductivity of equilibrated eutectic liquid to thermal conductivity of solid NPG solution was also measured to be 0.80.     

Extended free radical network from condensed cyanuric chloride with p-phenylenediamine: An EPR/FMR study

A covalent layered network was obtained by condensation of cyanuric chloride with bridging paraphenylenediamine. The local chemical environment of the layered solid can be changed by a redox reaction to obtain new reconstructed derivatives. A blue product was obtained by treating an alcoholic dispersion of the layered solid with ferric nitrate or potassium persulfate, indicating the possible formation of an extended free radical. When iron nitrate was used as oxidant, the temperature-dependent magnetic resonance spectra were measured in the 290–4 K region. The magnetic resonance measurements showed the coexistence at room temperature of two spectra arising from two different magnetic centers: a narrow line centered at g = 2.0038(1) with linewidth of ΔH = 7.42(2) G (free radical) and a broad line centered at g = 2.254(1) with linewidth of ΔH = 1300(5) G (magnetic iron-oxide cluster). A new sample was prepared so that the broader line was more intense. The temperature dependence of the magnetic resonance lines was subject to intense changes in both cases. The integrated intensities decreased with decreasing temperatures in both spectra in the high temperature region. This type of behavior is similar to that of magnetic nanoparticles in non-magnetic matrices. Upon reducing the temperature with the gradient of ΔH_r/ΔT = 1.5(1) G/K, the resonance field of the broad line was shifted towards lower magnetic fields, while the narrow line was shifted towards higher magnetic fields with ΔH_r/ΔT = 0.020(1) G/K. The linewidth of the broader line increased with decreasing temperature, while the narrow line remained almost constant. The magnetic iron-oxide clusters could produce an internal magnetic field acting on free radicals. This field could compel free radicals to form a magnetic ordered state at high temperatures.     

Dynamics of low temperature PECVD growth of microcrystalline silicon thin films: Impact of substrate surface treatments

Microcrystalline silicon (μc-Si) films have been deposited on polyimide, Corning glass and c-Si(0 0 1) by rf plasma-enhanced chemical vapour deposition (PECVD) using both SiF₄–H₂ and SiH₄–H₂ plasmas. The effect of substrate pre-treatment using SiF₄–He and H₂ plasmas on the nucleation of crystallites is investigated. Real-time laser reflectance interferometry monitoring (LRI) revealed the existence of a ‘crystalline seeding time’ that strongly impacts on the crystallite nucleation, on the structural quality of the substrate/μc-Si interface and on film microstructure. It is found that SiF₄–He pre-treatment of substrates is effective in suppressing porous and amorphous interface layer at the early nucleation stage of crystallites, resulting in direct deposition of μc-Si films also on polyimide at the temperature of 120 °C.     

Vitrification of wastes and preparation of chemically stable sintered glass-ceramic products

The present work illustrates the conversion of wastes, such as panel glass from dismantled cathode ray tubes, mining residues from feldspar excavation and lime from fume abatement systems in the glass industry, into two glasses which were subjected to a sinter-crystallization process. The surface mechanism of nucleation, starting from finely ground powders (<37 μm), allowed a rapid crystallization at a relatively low temperature (2 h at 880 °C). This resulted in with the preparation of glass-ceramics possessing a mechanical properties (bending strength and microhardness exceeding 60 MPa and 6.3 GPa, respectively) comparable to that of commercial glass-ceramics and natural stones for building applications, coupled with a good chemical durability. This feature, together with the type of crystals developed, was correlated to the fluorine content of the parent glasses.     

Studies of crystal phase formations in high-strength lithium disilicate glass–ceramics

The objective of the study was to analyze the nucleation, primary phase formation and solid state reaction to form lithium disilicate glass–ceramics derived from the SiO₂–Li₂O–Al₂O₃–K₂O–ZrO₂–P₂O₅ system. The concentration of P₂O₅ was increased from zero up to 3.2 wt%. Thermal analysis, scanning electron microscopy and X-ray diffraction were used to characterize the microstructure formation, the nucleation process and the solid state reaction of the crystal phase precipitation in the glass–ceramics. Additives of P₂O₅ allowed the control of bulk crystallization. Nucleation was catalyzed by nano-scaled Li₃PO₄ phases, visualized by HR-SEM. Li₃PO₄ reacts most probably as the heterogeneous catalyst, acting by epitaxy, of both Li₂SiO₃ and Li₂Si₂O₅ crystals. Based on the discussion of the main results, the authors deduced a four-step reaction mechanism. This mechanism demonstrated that after nucleation of lithium metasilicate and lithium disilicate, the latter phase grows as agglomerated nanocrystals, but remained in a relative small amount. By contrast, lithium metasilicate grows rapidly and decomposes at 780–820 °C with the result of a drastic increase of lithium disilicate phase. This was a result of a solid state reaction with the SiO₂-rich glassy phase. In a parallel reaction, cristobalite was formed as a preliminary phase. The final product of a glass–ceramic with 3.2 wt% P₂O₅ shows a highly crystalline interlocking microstructure demonstrating a high-strength of 726 ± 63 MPa and translucency.     

Crystallization behavior and microstructure of (CaO·ZrO2·SiO2)–Cr2O3 based glasses

The effects of chromium oxide on the crystallization behavior of glass compositions in the calcium, zirconium and silicon oxides system were investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopic. Results indicate that crystallization is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 1.0 mol% of doping oxide. The effect of heating rate on differential thermal analysis curves was studied in order to investigate nucleation mechanisms and to extract the corresponding crystal growth activation energies E_c for the different crystalline phases. Activation energy (E_c) was found to be 490 ± 5 kJ/mol for 5.0 mol% chromium oxide in glasses. The most suitable nucleation temperature was determined as 810 °C for the above mentioned glass. The results of this study have highlighted that a small percentage of chromium oxide strongly affects the crystal formation thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallization of calcium, zirconium and silicon oxides glasses.     

Microwave power absorption analysis in the devitrification process of Co-based amorphous ribbons

Melt-spun Co₆₆Fe₄B₁₂Si₁₃Nb₄Cu soft magnetic ribbons were devitrified at low annealing temperatures (623 K), for times 5–20 min. Microwave power absorption measurements at 9.4 GHz (X-band) were carried out in two geometries. In geometry 1, the ribbon’s plane was oriented parallel to AC magnetic field. For the orientation 2, the ribbon’s plane was normal to the AC magnetic field. In both cases, the ribbon’s axis was parallel to the DC magnetic field. For both orientations, two absorptions were observed: the first corresponds to a low field microwave absorption (LFA) centered in zero dc magnetic field, and a higher field absorption corresponding to the ferromagnetic resonance (FMR). In the geometry 1, a single FMR spectrum was observed for all the samples, with a shift in resonant field as annealing increased. For geometry 2, evidence of the superposition of two FMR signals was observed. FMR spectra are therefore due to a combination of two different magnetic phases corresponding to the amorphous matrix and nanocrystallites. Deconvolution calculations were carried out on FMR spectra to separate the contributions. Their behavior as a function of annealing time was in good agreement with the magnetic softening, also obtained with LFA results. The differences in microwave absorption, for both geometries, can be explained by differences in the electromagnetic wave propagation volume.