Nucleation time-lag from nucleation and growth experiments in deeply undercooled glass-forming liquids

A new approach is proposed to explain the strong difference between the induction periods (nucleation time-lags) obtained from nucleation rate measurements and from crystal growth experiments for lithium silicate glasses; and their similar magnitude for a Na₂O · 2CaO · 3SiO₂ glass. For these two glass families, the time-lags for nucleation estimated from crystal growth kinetics were compared with those directly obtained from nucleation experiments. A theoretical analysis was performed employing analytical solutions of the Frenkel-Zeldovich equation. In such analysis, the frequently assumed condition of size-independence of the thermodynamic properties of the crystallites was used. Provided this assumption is correct, time-lag data obtained in the two above mentioned ways should coincide. Consequently the significant difference between the values of nucleation time-lag for lithium silicate glasses from nucleation and growth data gives a strong indirect evidence for the deviation of the properties of critical nuclei from the respective parameters characterizing the state of the newly evolving macrophase. For Na₂O · 2CaO · 3SiO₂ glass at intermediate stages of crystallization we show that the average composition of the growing crystals is close to that of the near-critical nuclei. The fact that the nucleation and growth rates of this soda-lime-silica glass refer to the same phase provides an explanation for the similarity of the induction periods estimated from nucleation and growth experiments.     

Electrochemical determination of reduction potentials in glass-forming melts

Square-wave voltammetry (SWV) has been used to determine the reduction potentials of several multivalent elements in situ in an alkali borosilicate melt. Such reduction potentials measured at melt temperature were found to be proportional to the relative reduction potentials obtained from analyses of the quenched glasses. Accordingly, the redox state of the glass typically reflects the redox state of the melt. Although SWV is a sensitive tool for monitoring the total concentration of specific multivalent elements in the melt, the method does not directly monitor the redox state of the melt.     

Thermodynamic properties and structure of ternary silicate glass-forming melts: Experimental studies and modeling

Thermodynamic properties and vaporization processes of ternary silicate glass-forming melts containing Cs₂O, MgO, CaO, BaO, B₂O₃, Al₂O₃ and TiO₂ obtained by high temperature mass spectrometric method at the temperatures up to 1900 K were discussed. These data were used for modeling thermodynamic properties and structure of these melts based on the vacancy model of the general lattice theory of associated solutions.     

Initial stages of nucleation of carbide phases in oxide melts

The initial stages of nucleation of Mo₂C and W₂C crystals in the tungstate-molybdate-carbonate melts of the known compositions on Ag, Au, Cu, Pt, and Ni substrates are studied by the method of electrodeposition depending on the electrocrystallization conditions—temperature, deposition time, initial current pulse, and current density.     

Induction periods and mechanism and kinetics of nucleation of barium tungstate crystal growth from sodium tungstate melts in platinum crucibles

Induction periods (t) and mechanism and kinetics of nucleation in barium tungstate crystallization from sodium tungstate melts in platinum crucibles were studied. A theoretical relation has been developed to express the dependence of t on the cooling rates (R_T) and the rate (R_c) of development of excess solute concentration. At any crystallization temperature the average rate-constant (k_n) for heterogeneous nucleation was related to R_c by (k_n/p)^1/(p+1) = 1/(tR_c^γ), where γ is a constant and p is the average number of particles in the critical nuclei. The critical temperature (T), critical supersaturation (S) and γ values were estimated.     

Crystal growth of Mg- and Ti-substituted hexagonal ferrites from PbO-B2O3 based melts

Crystals of Mg and Ti-substituted hexaferrites have been prepared as perspective materials for the photoelectrochemical conversion of solar energy by means of anodes. The melts on the basis of PbO B₂O₃ were used to grow both bulk crystals and thin films. In the chosen system the primary crystallization field of hexaferrites was found. The crystals were grown by conventional unseeded crystal growth using (ACRT) accelerated crucible rotation technique to ensure their good quality and homogeneity. The size of the crystals varied from 10 to 23 mm in diameter. Maximum total content of Mg²⁺ and Ti⁴⁺ amounted to 2 atoms per formula unit of hexaferrite.     

Effect of low-frequency vibration on nucleation of crystals in supercooled melts

Nucleation of transparent organic substances in supercooled melts under vibration (50 Hz, 1–5 mm) has been studied experimentally. In the absence of a seeding crystal, vibration did not affect nucleation within the whole range of melt supercoolings studied, ΔT= 0.1–10 K. The introduction of a seeding crystal provides a threshold supercooling ΔT* below which no nucleation occurs at all, whereas above it, an explosion-like nucleation is observed. It is shown that mass growth of small crystals at ΔT>ΔT ^* is not associated with the detachment of growing-crystal fragments and seems to be explained by three-dimensional nucleation on the surface of a seeding crystal.     

A test of classical nucleation theory: crystal nucleation of lithium disilicate glass

For several inorganic glasses predictions have been made of the homogeneous crystal nucleation rate using classical nucleation theory. However, in none of these cases were comparisons made with experiment, due primarily to the inability of being able to guarantee homogeneous conditions. Evidence has been provided that crystalline formation in Li₂O · 2 SiO₂ glass may initiate by a homogeneous mechanism. Thus, we have computed the nucleation rate curve of lithium disilicate crystals in this glass. It is found that not only do all forms of the classical theory predict nucleation rates many orders of magnitude smaller than those observed, but also that the temperature dependence of the theoretical rate is quite different from that observed.     

Crystal growth from the vapour phase: Confrontation of theory with experiment

Starting with the BCF theory recent crystal growth models based on computer simulations are discussed. These models are confronted with experiments for growth from the vapour concerning rates of growth and the direct observations of step patterns, growth spirals and etch pits.     

Thermophysical properties of Ni-Nb and Ni-Nb-Sn bulk metallic glass-forming melts by containerless electrostatic levitation processing

The containerless high-temperature high vacuum electrostatic levitation (ESL) technique was used for the determination of thermo-physical properties of the binary eutectic alloy, Ni_59.5Nb_40.5, and the ternary alloy Ni₆₀Nb_34.8Sn_5.2. The thermo-physical properties measured were density, specific heat over hemispherical total emissivity, surface tension and viscosity. The density for both the melts was found to be higher than that predicted by the rule of mixtures. This was attributed to strong attractive interaction between the constituents. The temperature dependent specific heat capacity of the binary alloy was determined by fitting with TEMPUS results. The hemispherical total emissivity was estimated to be 0.27. Using the specific heat data and the hemispherical total emissivity, the enthalpy and entropy of fusion were calculated for the binary alloy. The viscosities for both the alloys were fit with the Vogel-Fulcher-Tammann equation to obtain the fragility parameter. The surface tension for the binary alloy showed an anomalous positive gradient while that of the ternary alloy showed a small negative gradient with temperature.     

Grain size distribution in the Kolmogorov model for nucleation and growth: Heterogeneous versus homogeneous nucleation

A theory of grain size distribution in nucleation and growth reactions described by the KOLMOGOROV model is developed. The distribution of agglomerates of growing and impinging nuclei is explicitely calculated for the two cases of heterogeneous and homogeneous nucleation. The results are compared and discussed with respect to the investigation of the nucleation mechanism in crystallization processes.     

Curvature- and temperature-dependent interfacial tension in classical nucleation theory

Curvature- and temperature-dependent interfacial tension is introduced in classical nucleation theory (CT-CNT). In this case, the nucleation rate depends on temperature, T, viscosity, η, the reduced surface tension, , the difference between the chemical functions of the liquid and solid phase, Δμ, and unlike with the common classical nucleation theory (CNT), also on the derivative of this function, ∂Δμ/∂T. Because of this latter dependency, the nucleation rate is highly sensitive to Δμ and no approximation formula can be used for this function. Kauzmann's catastrophe can be introduced artificially by approximation formulas that are commonly used in CNT. The latter theory is not sensitive to this problem.     

Crystal growth in bile

Crystal growth frequently occurs in bile, resulting usually in the formation of gallstones. From patient to patient, these vary in number, size, colour and shape, as well as in composition. Cholesterol is the most common constituent, but certain calcium salts, notably calcium carbonate, are frequently present.Factors which can contribute to the crystallisation of bile are discussed, together with those resulting in the transferrence of tiny nuclei into the complex structures of gallstones. Some of these may be crystal aggregation, overgrowth and epitaxy which may or may not take part in a gel which is the matrix of gallstones.     

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si₂C and SiC₂ transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.