Surface crystallization of rare-earth aluminosilicate glasses

Surface crystallization in a rare-earth aluminosilicate glass (Nd₂O₃–Al₂O₃–SiO₂–TiO₂) was studied using an isothermal method and the crystal growth rate of the glasses was evaluated as a function of the composition. For measuring the surface crystal growth rate, two different methods: measurement of the crystal layer in the longitudinal and lateral growth direction. It was found that crystallization proceeded by surface crystallization only and TiO₂ did not act as a nucleating agent. The growth rate was strongly dependent on the viscosity of glass and agreed with prediction from the Preston model using the known viscosity and melting temperature. As the Si/Nd and Si/Al ratios decreased, the crystal growth rate increased. TiO₂ and Nd₂O₃ played the role of network modifier, which decreased the viscosity of the glass, facilitating crystallization of the rare-earth aluminosilicate glass.     

Viscous behaviour of undercooled metallic melts

Viscosity data for non-crystalline Au_0.77Ge_0.136Si_0.094 alloys in the region of the glass transition temperature and above the melting point, are fitted to a single expression of the type proposed by Doolittle. This expression also leads to reasonable values for the activation energy for hole formation in the liquid and for the liquid free volume at the glass transition temperature. A similar procedure was applied to the viscosity data of a Pd_0.775Cu_0.06Si_0.165 non-crystalline alloy. By assuming values for the liquid free volume at both the glass transition temperature and the melting point, the analysis is also extended to Pd_0.82Si_0.18 alloys for which no viscosity data are available. Here, time-temperature-transformation (T-T-T) curves for crystallization are calculated for each of the three alloys, and used to determine the critical cooling rates for the formation of an amorphous solid.     

Crystallization kinetics of a Se–Ge–Sb alloy glass

The glass-forming ability of a Se_0.615Ge_0.154Sb_0.231 alloy is analysed by computing the critical cooling rate and the time-temperature-transformation curve, at various temperatures. The calculation is based on some experimentally determined portions of the time-temperature-transformation curve, from which the rate of increase of viscosity with falling temperature below the melting point is deduced, using classical crystallization kinetics treatment.     

循环流化床粉煤灰可控制备高纯F型八面沸石研究

利用粉煤灰制备沸石分子筛是其高值化利用的重要方向之一.以循环流化床粉煤灰为原料,采用酸浸预处理-氢氧化钠碱熔活化-水热晶化法制备F型八面沸石,并用于吸附亚甲基蓝.考察酸浸温度、碱熔温度及碱灰质量比对粉煤灰结构的影响及碱熔温度、碱灰质量比、液固比及晶化时间对沸石的结构和形貌的影响.通过XRD和SEM对粉煤灰沸石的晶体结构和形貌进行表征.结果表明,利用循环流化床粉煤灰制备高纯F型八面沸石适宜条件为碱熔温度550℃,碱灰质量比1.5:1,液固比12 mL/g,晶体导向剂用量10;,晶化温度100℃,晶化时间20 h.其比表面积高达357 m2/g,且对亚甲基蓝的饱和吸附量高达178 mg/g.     

Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement

The crystal nucleation effect in lithium aluminosilicate glasses was investigated by the viscosity measurement with aid of the fiber elongation method. The abrupt increase of viscosity due to the crystallization of glass was observed in viscosity-temperature curve but the minimum viscosity temperature (T_η) related with crystallization showed a strong dependence on the nucleation state such as nucleation temperature, nucleation time and heating rate. The results by viscosity agreed well with those of DTA. The nucleation effect on the microstructure of glass-ceramics was also discussed. Finally, the nucleation effect on the crystallization kinetics was approached quantitatively by calculating the crystal volume from viscosity value.     

Thickness dependence of crystallization and melting kinetics of eutectic Sb70Te30 phase change recording film

As the thickness of eutectic Sb₇₀Te₃₀ phase change recording film was reduced, the crystallization temperature and activation energy for crystallization would increase, while the melting temperature and activation energy for melting would decrease. Accordingly, the archival stability and recording sensitivity can be improved. However, the recording speed for direct over write will be slowed down. Based upon the Johnson-Mehl-Avrami equation, it was found that the increase of film thickness would increase the nucleation rate of eutectic Sb₇₀Te₃₀ recording film and make the crystallization process become more nucleation-dominated.     

Morphology of sic epitaxial layers grown by temperature gradient zone melting (I). Dependence of the crystallization boundary smoothness on the epitaxial layers growth conditions

The dependence of the smoothness of the crystallization boundary on the temperature gradient zone melting conditions is investigated. The observed depth of the crystallization boundary is compared with the depth, calculated by the method proposed by D. E. TEMKIN.     

Calorimetric studies of Se80−xTe20Bix bulk samples

The crystallization kinetics of the bulk Se_80−xTe₂₀Bi_x chalcogenide glasses were studied by using differential scanning calorimetry with different heating rates (5,10,15 and 20 K/min) under non-isothermal conditions. The values of glass transition temperature, peak crystallization temperature and melting temperature are found to increase with increase in heating rate as well as with bismuth content. The activation energy for glass transition and that for crystallization have been determined using the Kissinger equation and Matusita equation. The thermal stability and glass-forming tendency have also been studied.     

The thermal evolution of metakaolin geopolymers: Part 2 – Phase stability and structural development

The structural evolution of a systematic series of geopolymers derived from metakaolin is explored. The onset temperature of crystallization, phase composition and densification of the specimens in the current work are determined by quantitative XRD, FTIR and dilatometric analyses. Alkali cation is observed to have no significant effect on the onset temperature of crystallization and quantity of crystalline phase developed in geopolymers, though the phase composition of NaK- and K-geopolymers is more complex than Na-geopolymers, as is expected from phase diagrams. The amount of crystalline phase decreases in all systems with increasing Si/Al ratio, which is proportionally related to the extent of densification observed in Part 1 of the work. Despite similar phase composition of all geopolymers, the onset temperature of densification is observed to be higher in all K-geopolymers compared to the NaK- and Na-specimens of analogous Si/Al ratio. It is proposed that the increase in densification temperature is due to the increase in gel viscosity of K-geopolymer compared to Na- and NaK-geopolymers. The effect of heating rate on the densification mechanisms of geopolymers is analyzed, showing that the amount of crystalline phase and gel viscosity of geopolymers are critical in determining the extent of shrinkage during densification.     

Comparative molecular dynamics study of melting and crystallization of Ni and Au nanoclusters

The melting and crystallization hysteresis for nanoclusters of two transition metals (nickel and gold) have been comparatively studied using the molecular dynamics method and the tight-binding potential. It is established that the onset and end temperatures of the corresponding first kind of phase transition (i.e., melting and crystallization temperatures) should be considered for Ni nanoclusters, whereas Au nanoclusters are melted and crystallized at certain temperatures. The size dependence of the melting temperature of Au nanoclusters determined using the computation results is compared with the molecular dynamics data from other researchers and the results of two independent laboratory experiments. It is shown that our molecular dynamics results are in much better agreement with the experimental data than those obtained by other researchers.     

Melting and Crystallization Processes of EBBA I. Thermal Analysis

Melting and crystallization processes of EBBA (N-p-Ethoxybenzylidene-p'-butylaniline) have been studied by the method of thermal analysis and measurement of light transmittance. When the sample is cooled down from the nematic phase at a rapid cooling rate, solid phase (solid S) is formed directly. Solid S contains two solid modifications (solid S₁ and solid S₂. By heating the solid S, some amount of Solid S₁ is transformed into the nematic phase through the process of (melting of solid S₁ → crystallization to solid S₂ → melting of solid S₂). After the melting of solid S₁, some liquid crystalline state appears transiently. Solid S₁ is stabilized by the heat treatment at low temperature. The quantity of solid S₁ in solid S increases with the heat treatment time and/or with cooling at a low temperature.     

Thermal characterization, melting behaviors, dynamic mechanical properties, and morphology in the polymorphism of syndiotactic polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide)

The thermal characterization, melting behaviors, dynamic mechanical properties, and morphology of syndiotactic polystyrene and its blends with poly(2,6-dimethyl-1,4-diphenylene oxide) were examined by differential scanning calorimetry, dynamical mechanical analyzer, and scanning electron microscope. With differential scanning calorimetry and scanning electron microscope results, two melting endothermic peaks and blending morphology were observed. We suggest that the lower melting endothermic peak is due to the melt of syndiotactic polystyrene, however, the higher melting endothermic peak comes from the melt of recrystallized syndiotactic polystyrene crystals during the differential scanning calorimetry scan. With dynamical mechanical analyzer results, we find that addition of poly(2,6-dimethyl-1,4-diphenylene oxide) into syndiotactic polystyrene can increase the soft temperature and toughness. Moreover, the experimental results also reveal that crystallization of syndiotactic polystyrene in the syndiotactic polystyrene/poly(2,6-dimethyl-1,4-diphenylene oxide) blend is impeded in the presence of poly(2,6-dimethyl-1,4-diphenylene oxide) and the melting behaviors of syndiotactic polystyrene/poly(2,6-dimethyl-1,4-diphenylene oxide) blends are homogeneous.     

Viscous flow behavior of selenium

The viscosity of 99.99% pure selenium has been determined over the available range of temperature where crystallization could be avoided. A bending beam viscosimeter was used to obtain data in the high viscosity region, and a rotating cylinder instrument was employed for the low viscosity range. Also determined was the stress dependence of the viscosity at temperatures of 31.5 and 29.6°C. The critical stress for non-Newtonian flow was found to lie in the range of hundreds of psi, which is appreciably smaller than that for oxide liquids in a similar range of viscosity.     

Crystallization behavior during cooling and glass-forming ability of Al2O3-poor Li2O–Al2O3–SiO2 melts

The influence of K₂O/(MgO + K₂O) on some melt properties, including crystallization during cooling of melts and glass-forming ability, was investigated in the Li₂O–Al₂O₃–SiO₂ system with low Al₂O₃ content. The dependence of viscosity on K₂O/(MgO + K₂O) above 1000 °C showed a monotonic decrease due to the reduction of [MgO₄] concentration and the conductivity also decreased due to the larger ion radius of K. The temperature dependence of conductivity for all melts showed an abrupt change at one temperature due to crystallization in which temperature of crystallization decreases with increase of K₂O. The crystallization behavior near liquidus temperature was studied quantitatively by calculating the crystal volume fraction from apparent viscosity value. The glass-forming ability of the melts was discussed by using data related with viscosity and crystallization. Finally, it was suggested that the melts with K₂O/(MgO + K₂O) ? 0.75 have a good glass-forming ability.     

Magnetic properties of nanoparticles glass–ceramic rich with copper ions

The current study aimed at the assessment of CuO gradual addition on the crystallization behavior and magnetic properties of ferromagnetic glass ceramic in the system Fe₂O₃·CaO·ZnO·SiO₂. Ferromagnetic glass–ceramics with a high quantity of magnetite were designed to be crystallized in the system Fe₂O₃·CaO·ZnO·SiO₂. The influences of gradual addition of CuO and melting temperature on the sequence of crystallization and magnetic properties were studied. The X-ray diffraction patterns showed the presence of nanometric magnetite crystals in a glassy matrix after direct cooling from the melt without any additional heat treatments. Increasing the melting temperature resulted in an increase in the magnetite crystallization. The addition of up to 20 g CuO/100 g batch composition revealed a decreasing effects in both endo- and exothermic values, whereas, the same values were inversely increased with increasing the CuO addition to 30 g. In general, increasing the CuO amounts greatly enhanced the crystallization of magnetite. A significant amount of delaffosite (CuFeO₂) was unexpectedly detected and increased by increasing both the added amount of CuO and the treatment temperature. The XRD results detected some traces of cuprite (Cu₂O) in the samples of high CuO content. The TEM results reflected the precipitation of nano-magnetite crystals in the 2–10 nm size range. Magnetic hysteresis cycles were analyzed using a vibrating sample magnetometer with a maximum applied field of 20 kOe at room temperature in quasi-static conditions. From the obtained hysteresis loops, the saturation magnetization (Ms), remanence magnetization (Mr) and coercivity (Hc) were determined.     

Molecular dynamics simulations of the crystal–melt interface mobility in HCP Mg and BCC Fe

The temperature profile around the moving solid–liquid interface during non-equilibrium molecular dynamics (MD) simulations of crystallization and melting is examined for HCP Mg and BCC Fe. An evident spike (valley) is found around the solid–liquid interface during solidification (melting). Considering the effect of a non-uniform temperature distribution, it is found that, if the actual interface temperature is adopted to compute the interface mobility, rather than the thermostat temperature (or the mean temperature of the whole system), the kinetic coefficient is approximately a factor of two larger than previous estimates. Although the magnitude of the kinetic coefficient is larger than the previous estimates, the crystalline anisotropies derived in the current work are consistent with earlier calculations.     

AgGa1-xInxSe2晶体的生长习性研究

采用DTA对不同铟含量(x)的AgGa1-xInxSe2多晶熔化和结晶温度进行了测试.结果表明:随着x值的增加其过冷度增大.采用改进的垂直布里奇曼法和实时补温技术,对AgGa1-xInxSe2晶体生长过程中的结晶特性和生长温度场关系进行了研究,并对其结晶形态进行了观测.发现:随着晶体生长过程的进行,熔体结晶温度呈下降趋势,固-液界面发生移动;生长晶体表面存在外形规则、形状相同的半球状小孔,有取向一致的台阶反光面,小孔底部为{112}面.研究结果为大尺寸、高质量的AgGa1-xInxSe2单晶体生长奠定了基础,生长出了尺寸达20 mm×60 mm的完整AgGa1-xInxSe2单晶体.     

Catalyzed Nucleation of Poly (Vinyl Chloride)

Microscopically visible crystallization in a typical non-crystallizing organic polymer - poly(vinyl chloride) (PVC) is achieved by dissoloving it in an appropriate solvent (cyclohexanone) and inducing the nucleation by introducing various insolube crystallization cores - microcrystals of noble metals (Au, Pd, Pt, Rh, Ir, Ru, Os). It is established that the higher the melting temperature of the metal the higher its activity as a crystallization core in the heterogeneous nucleation of PVC. The reverse process-oriented crystallization of a foreign substance - NaCl crystals on PVC films is also demonstrated. This result gives an indication of precrystalline structures in seemingly amorphous PVC films.     

Formation and structure of titanate glasses

The formation of high titanium oxide (30–60 wt%) containing glasses was studied. Stable titanate glasses with high content of TiO₂ and BaO could be obtained even without other glass-forming oxides. It is demonstrated that the coloration of titanate glasses with high content of TiO₂ is due to oxygen loss during the melting. A systematic study of controlling melting and heat treatment conditions led to the successful decoloration of titanium oxide containing glasses. Infrared spectra and X-ray diffraction studies showed that Ti⁴⁺ in titanate glasses is in sixfold coordination. Phase separation was observed by electron microscopy when the titanate glasses were heat-treated at the temperature above T_g. The crystallization of titanate glasses is generally preceded by phase separation. The obtained crystalline phases are mainly different titanates.     

Kinetics of non-isothermal crystallization and glass transition phenomena in Ga10Se87Pb3 and Ga10Se84Pb6 chalcogenide glasses by DSC

The crystallization process affects solid properties through the crystal structure and morphology established during the transition process. An important aspect of the crystallization process is its kinetics, both from the fundamental point of view of amorphous material as well as the modeling and phase transition. In the present research work, non-isothermal crystallization data in the form of heat flow vs. temperature curves has been studied by using some well known models for amorphous Ga₁₀Se₈₇Pb₃ and Ga₁₀Se₈₄Pb₆ chalcogenide glasses, prepared by the melt quenching technique. The glass transition phenomena and crystallization of these glasses have been studied by using non-isothermal differential scanning calorimetery (DSC) measurements at constant heating rates of 5, 10, 15, 20, 25 and 30 K/min. The glass transition temperature (T_g), crystallization temperature (T_c), and melting temperature (T_m) were determined from DSC thermograms. The dependence of T_g and T_c on the heating rate was used to determine different crystallization parameters such as the order parameter (n), the glass transition energy (ΔE_g) and the crystallization activation energy (ΔE_c). The results of crystallization were discussed on the basis of different models such as Kissinger's approach and the modification for non-isothermal crystallization in addition to Johnson, Mehl, Ozawa and Avrami.