The effect of impurities on the β → α transformation process in tin

This article presents data on the impurities influencing the β → α transformation of high purity tin (99.9995%): temperature of the α → β transition-T₀, nucleation and α-crystal growth.     

In situ observation of β‐BaB2O4 microcrystal growth in glass matrix

The morphological evolution and growth mechanism of β‐BaB₂O₄ microcrystal in Li₂B₄O₇‐BaB₂O₄ glass (Li₂O‐B₂O₃‐BaO) matrix were investigated by optical in situ observation method. And the crystallization temperature T_c has been examined by differential thermal analysis (DTA). It demonstrates that homogeneous distribution of hexagonal shaped BBO microcrystals with size up to several tens of microns is typical when temperature is much higher than T_c, however, heterogeneous nucleation occurs when annealing temperature is close to T_c. For the latter case, crystal clusters that consist of several microcrystal grains are obvious. When the crystals in one specific cluster grows larger, crystal motion occurs in glass matrix while their orientation and symmetrical shape keep nearly no changes. Additionally, the BBO microcrystal has been determined to grow nearly in linear with time, which suggests a mechanism of interface‐controlled growth. Furthermore, the activation energy of BBO crystal growth in glass matrix is calculated which is around 2.4 eV. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of dynamic recrystallization on the growth velocity of new phase crystals at polymorphic transformation of tin and thallium

The analysis of dynamic recrystallization influencing the crystal growth of the new phase in high purity tin and thallium (99.9995%) is carried out. It is shown that in tin, in which the β → α transition proceeds on the dislocation or the normal mechanisms (depending upon a supercooling degree, ΔT) a superposition of dynamic recrystallization results in an increase of the growth velocity of α-Sn crystals. The most distinct of acceleration is observed at −21 ÷ − 23 °C, when the growth velocity of recrystallization centers and the maximal growth velocity, V_max, of the crystals are comparable values. The dynamic recrystallization superposition in thallium offers to transform the transformation mechanism. Thereby an erase of the features of the martensite α ⇄ β transition mechanism and an appearance of the features of the normal crystal growth mechanism of the new phase are observed.     

Spherulitic crystallization of β‐In2Te3 by physical vapour deposition

Different morphologies of indium telluride (In₂Te₃) including novel spherulites were crystallized using the physical vapour deposition (PVD) method, by varying the difference in the growth and source zone temperature (ΔT) of a dual zone horizontal furnace assembled indigenously. Whiskers and kinked needles of In₂Te₃were grown at ΔT = 250 K and 300 K respectively, maintaining the growth zone at 500 °C. At high supersaturation (Δ T = 400 K), spherulitic crystals were obtained. The stoichiometric composition of these crystals has been confirmed using energy dispersive analysis by x‐rays (EDAX). The structure of β‐In₂Te₃ spherulitic crystals is identified as zinc blende with lattice parameter a = 6.159 Å, from x‐ray diffraction (XRD) studies. The scanning electron microscope (SEM) images revealed the radial structure of the grown spherulites. The growth mechanism for the spherulitic crystallization of β‐In₂Te₃ crystals has been discussed based on the theoretical models. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Terminal Groups on Formation of Crystal Structure in α,γ-Disubstituted n-Alkanes

The crystal and molecular structure of 1,16-dibromohexadecane have been determined by X-ray diffraction. Crystal data; empirical formula, C₁₆H₃₂Br₂, formula weight M_r = 384.24, monoclinic, P2₁/a, a = 31.00(2), b = 5.38(1), c = 5.48(1) Å, β = 90.8(2)°, V = 914(3) ų, Z = 2, D_x = 1.396 g/cm³, δ(CuKα) = 1.5418 Å, μ(CuKα) = 55.41 cm⁻¹, F(000) = 396.00, T = 293 ± 1 K, wR = 0.082, R = 0.054 for 940 unique reflections [I > 3.00σ(I)]. The molecular skeleton has an all trans zigzag hydrocarbon chain. The crystal has a layer structure, but the directions of molecular axes in neighbouring layers alternate. This molecular packing resembles to that of 1,16-hexadecanediol, whereas n-alkanes or α-monosubstituted n-alkanes do not appreciably have these molecular arrangement. The effect of terminal groups at both ends of α,ω-disubstituted n-alkanes on the crystal structure is discussed.     

Crystal structure,thermal and compositional deformations of β‐CsB5O8

The crystal structure of β‐CsB₅O₈ has been determined from X‐ray powder diffraction data using synchrotron radiation: Pbca, a = 7.8131(3) Å, b = 12.0652(4) Å, c = 14.9582(4) Å, Z = 8, ρ_calc = 2.967 g/cm³, R‐p = 0.076, R‐wp = 0.094. β‐CsB₅O₈ was found to be isostructural with β‐KB₅O₈ and β‐RbB₅O₈. The crystal structure consists of a double interlocking framework built up from B‐O pentaborate groups. The crystal structure exhibits a highly anisotropic thermal expansion: α_a = 53, α_b = 16, α_c = 14 · 10^‐6/K; the anisotropy may be caused by partial straightening of the screw chains of the pentaborate groups. The similarity of the thermal and compositional (Cs‐Rb‐K substitution) deformations of CsB₅O₈ is revealed: increasing the radius of the metal by 0.01 Å leads to the same deformations of the crystal structure as increasing the temperature by 35°C. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrothermal synthesis of ultralong single‐crystalline α‐Ni(OH)2 nanobelts and corresponding porous NiO nanobelts

Ultralong α‐Ni(OH)₂ nanobelts with uniform size have been prepared on large scale via a facile template‐free hydrothermal method. The as‐prepared nanobelts were single crystals, with several tens of microns in length and about 100 nm in width. For the whole process, a novel nucleation–aggregation–dissolution–seed‐directed growth mechanism was proposed based on the experimental results. The roles of aqueous ammonia and hydrothermal temperature were also discussed. Furthermore, porous NiO nanobelts were obtained by annealing the as‐prepared Ni(OH)₂ nanobelts. This facile, template‐free, and low cost method might feasibly be scaled up for industrial production. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Effect of the Temperature Fluctuation of the Melt on β‐BaB2O4 (BBO) Crystal Growth

In this paper the effect of the growth temperature fluctuation, for instance, the transient furnace temperature variation due to a short‐term electric power supply interruption on BBO crystal growth was investigated based on the theory of temperature wave transmitting in melt and the boundary layer theory of melt. It was found that the critical width of the temperature pulse to avoid the temperature wave penetrating through the boundary layer and reaching to the growth interface at a constant rotation speed (9∼4 r/min) is 69∼150 s and the corresponding amplitude of the temperature pulse is high more than 60 °C due to the large thickness of the velocity boundary layer of the melt. This result indicates that a small transient temperature fluctuation has no significant effect on the crystal quality, and therefore implies that not only transport processes but interface growth kinetics, a two‐dimensional nucleation growth mode at the interface may also dominate the crystal growth.     

Cd‐Doping Effects on The Properties of Polycrystalline α‐In2Se3 Thin Films

The X‐ray diffraction has revealed that the polycrystalline hexagonal structured α‐In₂Se₃ thin films grown at substrate temperature of 200 °C with the unit cell parameters a = 4.03 Å and c = 19.23 Å becomes polycrystalline hexagonal structured InSe with a unit cell parameters of a = 4.00 Å and c = 16.63 Å by Cd‐doping. The analysis of the conductivity temperature dependence in the range 300‐40 K revealed that the thermionic emission of charged carriers and the variable range hopping are the predominant conduction mechanism above and below 100 K, respectively. Hall measurements revealed that the mobility is limited by the scattering of charged carriers through the grain boundaries above 200 K and 120 K for the undoped and Cd‐doped samples, respectively. The photocurrent (I_ph) increases with increasing illumination intensity (F) and decreasing temperature up to a maximum temperature of ∼100 K, below which I_ph is temperature invariant. It is found to have the monomolecular and bimolecular recombination characters at low and high illumination intensities, respectively. The Cd‐doping increases the density of trapping states that changes the position of the dark Fermi level leading to the deviation from linearity in the dependence of I_ph on F at low illumination intensities.     

Zur Kristallographie der martensitischen ε → α-Gitterumwandlung

Many iron and cobalt alloys transform to the martensitic reaction γ → ε → α. The platelets of the hex. ε-martensite are very thin. Thus the growth of the crystals is limited in the direction [0001]_ε and the transformation ε → α can be treated as a plane problem. Metallographic investigations demonstrated the size, form and orientation of the α-crystals after cooling without and with external tension. On the basis of connection of the hex. and the bcc. lattice formulae are discussed of the directions of the lattices, and the changes of the volume and the shape, resp.     

‘Intra-cluster rearrangement' model for the α-process in supercooled liquids, as opposed to ‘cooperative rearrangement of whole molecules within a cluster'

A new model for the α-relaxation process in supercooled liquids and glasses is proposed which distinguishes between a structurally correlated region (cluster) of molecules and a unit of molecules for rather independent, correlated rearrangement motion. The essential aspects of the model are that the α-process is due to rearrangement of one or a few molecules within the cluster, while essentially the same motion in the space between the clusters is responsible for the β-process. The model leads to the following expectations: (i) absence of divergent behavior of α-relaxation times at non-zero temperature (e.g., Kauzmann temperature), (ii) close agreement between the glass transition temperatures, T_g, for the α-relaxation in liquid and crystalline phases of the same composition and (iii) possibility of crystal nucleation proceeding much below the T_g, and evidence for the latter two is presented.     

Investigation of nucleation kinetics and crystal defects of HMX

The nucleation kinetics of HMX (cyclotetramethylene tetranitramine, C₄H₈N₈O₈) in γ‐butyrolactone was studied in cooling process by induction time method. The laser scattering method was used to measure the solubility data and metastable region of HMX in γ‐butyrolactone. The induction time was measured over a range of supersaturation at different temperatures. Then, the nucleation mechanism of HMX in γ‐butyrolactone was investigated by analysis the relationships between induction time and supersaturation. The results indicated homogeneous nucleation dominated at high supersaturation of S >1.35, while the heterogeneous nucleation dominated at low supersaturation of S < 1.35. The values of interfacial tension at different final temperatures were calculated to indicate the ability of HMX to be crystallized. The growth mechanism of HMX was investigated by the data fitting applying different growth mechanism models and identified as two‐dimensional nucleation‐mediated (2D) growth. Finally, the effects of supersaturation and temperature on the crystal defects were analyzed based on the nucleation kinetics. When the temperature is below 303.15K, homogeneous nucleation dominated the nucleation process at higher supersaturation. Fine HMX crystals with more defects were produced. On the contrary, heterogeneous nucleation mechanism dominated at lower supersaturation. large regular HMX crystals with fewer defects were formed when the temperature is above 318.15K.     

Growth Habit and Habit Variation of γ‐CuI Crystallites under Hydrothermal Conditions

The octahedra were observed in the γ CuI crystallites synthesized by hydrothermal method using 1.6g (C₂H₅OO)₂Cu and 2.66g KI as precursors at 200 °C for 12 hours in the de‐ionized water. The effect of additives on the morphology of γ‐CuI crystallites was investigated. Results show that the morphology of γ‐CuI crystallites prepared in the mixed solution of de‐ionized water and alcohol at 200 °C for 12 hours is the tetrahedron. In order to disclose the effect of the additive on the growth habit of γ‐CuI crystallites, the microcosmic growth mechanism of γ‐CuI crystal is investigated from the complex of I^‐ and Cu⁺ ions to each other. It is concluded that the effect of alcohol on the morphology of γ‐CuI crystallites is carried out through changing the relative rate of complex of anion and cation to each other at the interface. Based on the above analysis, the growth habit of γ‐CuI crystallites and the habit variation under hydrothermal conditions are explained reasonably.     

Influence of tempereature on Struvite precipitation by CO2‐deagassing method

The present study explores struvite precipitation by degassing method under different temperature (14.5‐35 °C). It is shown that an increase of temperature does not affect the nucleation kinetics but strongly influences the growth rate and the efficiency of the phosphate removal. This was attributed to the effect of temperature on the struvite solubility and then the reached supersaturation coefficient. It was also shown, by using degassing method, that the determined solubility constant (pK_s) values of struvite, at 14.5, 20, 25 and 35 °C are 13.00, 12.82, 12.63 and 13.20, respectively, are close to those presented in the literature. The results show that temperature influences the kinetics of precipitation. First‐order kinetics were found to be sufficient to describe the rate data. The rates decreased with increasing temperature and the apparent rate constants for the reaction were determined. Arrhenius plots yielded a relatively apparent high activation energy E_a = 31.0 kJ·mol^–1. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strukturelle Umklapp-Vorgänge in metastabilen β1-Kupfer-Zink-Mischkristallen bei tiefen und höheren Temperaturen (I). Gitterdynamische Skizze1)

The quantity of the shear-modulus G′ = (c₁₁ – c₁₂)/2 is a measure for the probability of structural Umklapp-processes to occur in metastable β₁-Cu-Zn-solid solutions. Such processes take place, if G′ = G′(T, x) would fall below a critical limit, G′_crit, e.g. by lowering of temperature T or/and Zn-concentration x, because the Fermi-contribution to G′will sink in the 1^st and with him additional the Coulomb-contribution in te 2^nd case. Both ones are the authoritative stabilizing factors for β₁ and therefore specific fo its lattice-dynamical behaviour, especially in the longwavy range of thermoacoustic lattice-vibrations, by which the cooperative Umklapp-motion will escape. Umklapping can also be initiated by favourably oriented dislocations, is G′ would approach to G′_crit in their neighbourhood: G′_critG′(xM8), with xM8  Umklapp-concentration. This is possible by variation of x during isothermal reactions, in the course of whicht he β₁-matrix will heterogenize itself into Zn-poorer and -richer districts β and β (pre-diffusion by means of quenchy vacancies). Both kinds of districts, among which the latter ones will enhance their degree of stability, are joint together coherently. They build up a so called β /β -parquet. The parquet-bricks can reach a critical size, which is necessary that sufficient large atom-collectives can simultaneously be caused to an Umklapp-motion and to occupate new equilibrium positions by thermo-acoustic shear-waves (comparison with a sin-wave beeing changed to a zigzag-line). Only at higher temperatures the bricks come up to the critical size. Umklapping comes about only in the β -bricks, which turn by it to a transition lattice (β₂) with a structure similary to that of the lowtemperature-martensite β′′. After that β₂ changes to α-phase. The way β ⇒ α is marked by the following steps: prediffusion, Umklapping + dislocation, leading to β₂, and a further dislocation dissoziation, leading the atoms to the equilibrium positions in the α-lattice. The so stepped mechanism acts an nucleationmechanism of the α-phase. After the nucleation the α-nuclei grow at the cost of too much formed β (postdiffusion). By isothermal reactions at too low temperatures a mini-herterogenized state of the β₁-matrix will be caused comparable with coldhardening states of other alloys (Guinier-Preston-zones). A β₁-matrix in such a state is unable to isothermal Umklapping, so that α-crystals can be formed – provided that the mini-heterogenities are resolved by increasing of the reaction temperature.     

On the Influence of Short R.T. preageing on the formation of the η′-phase in an Al-Zn(4.5 at.%)-Mg(2.5 at.%) alloy

The influence of short R.T. preageing (1 min) after quenching from 480 °C into RT-water on the formation of the η′-phase in an Al-Zn(4.5)-Mg(2.5) alloys has been investigated by means of TEM, XSAS, resistivity, and hardness measurements in the ageing temperature range between T_a = 120 °C comparing samples directly quenched (DQ) to T_a and those quenched into RT-water, kept there for 1 min, and subsequently stored in an oil-bath of T_a (IQ). At T_a ≦ 170 °C both heat treatments result in finely distributed precipitates of the η′-phase accompanied by a fairly high hardness value. But at T_a ≧ 180 °C the DQ leads only to few coarse heterogeneously formed η′- and η-precipitates, whilst the IQ yields qualitatively the same precipitation structure as already described at T_a ≦ 170 °C even till 280 °C. These results are interpreted assuming that. A1) the limitation temperature of homogeneous formation of η′ is located at about T_h = 175°c and A2) during the pre-ageing at RT “nucleation sites” are formed as already suggested by RYUM, which are able to act as sites for heterogeneous formation of η′ even at T_a > T_h. Hints are obtained that the formation of these “nucleation sites” is not so strongly governed by the quenched-in excess vacancies as discussed in the literature.     

AlxGa1−xAs LPE growth

Al_xGa_1−xAs LPE growth was studied within the temperature range of 930–900°C with Al concentrations in solutions from 0.04 to 2.4 at.%. AlAs concentration in layers has been shown to grow with the cooling rate increase of solution. Interface and volume nucleation parameter dependence of K_i and K_v and formation time t_f on Al concentration in Ga solution have been found. Addition of Al to Ga solution increases critical values of As supersaturation (supercooling) and, as a result, increase in thickness of Al_xGa_1−xAs layers compared with GaAs layers have been determined in spite of As concentration lowering in Ga solution.     

The Morphology of Some Hydrothermally Synthesized Li-Minerals: A-Zeolite, α-Eucryptite, β-Spodumene and β-Eucryptite

The hydrothermal treatment of glass with the composition 2 LiF, Al₂O₃, 3 SiO₂ at 1 kbar in the range of 150 °C to 850 °C gave rise to the formation of A-zeolite with orthorhombic unit cell with å = 10.31 Å/b̊ = 8.18 Å/c̊ = 5.0 Å (space group Pna2₁), α-eucryptite, trigonal, å = 13.4 Å/bº = 13.4/Å/cº = 9.0 Å (space group R 3 ), β-spodumene, pseudocubic/tetragonal, å = 7.53 Å/b̊ = 7.53 Å/c̊ = 9.15 Å (space group P4₃2₁2), β-eucryptite, monoclinic, å = 7.75 Å/b̊ = 5.2 Å/c̊ = 11.16 Å (space group P6₂22). The morphology and development of the mineral phases depend on the temperature of formation.     

Effect of supersaturation on the crystallization of phenylbutazone polymorphs

The article describes the effect of degree of supersaturation, σ, on the crystallization of specific polymorphs of phenylbutazone from its methanolic solution at 20 °C. At low initial supersaturation, σ ≤ 2.0, the fraction of the metastable α polymorph in the crystallized product exceeds that of the δ polymorph, while at σ ≥ 5.0, the fraction of the stable δ polymorph increases in the crystallized product. The results are explained by the effect of supersaturation on the relative rates of nucleation and crystal growth of the polymorphs. Furthermore, the mechanism of nucleation and crystal growth also change with supersaturation. Supersaturated methanolic solutions of phenylbutazone exhibit a critical temperature at which the nucleation rates of the polymorphs decrease drastically. This effect is partly explained by the decreased mobility of phenylbutazone molecules at lower temperatures. Nucleation is most rapid when the crystallization temperature is close to the transition temperature, T_t(α ⟷ δ), between the polymorphs, α and δ. The nucleation rate decreases as the temperature difference between T_t(α ⟷ δ) and the crystallization temperature increases. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)