Autoradiographic Investigations on the Incorporation of Ca Ions in KCl Crystals during Kyropoulos Growth

Ca doped KCl crystals were grown by the Kyropoulos method. 10⁻⁵ to 10⁻³ mole fractions of Ca (as CaCl₂ with a small amount of Ca-45 (β-emitter)) were added to the melt. By autoradiography of suitable cleavage planes the incorporation of the Ca ions was found to take place preferentially in a periodic manner parallel to the solid liquid interface. In the case of a rotating seed there is a segregation process in which only one zone of maximum Ca content is built up like a helix throughout the whole crystal, the axis being identical with the growth axis of the crystal and the slope of the helix corresponding to the growth rate per seed rotation, this leading to striations of long range periodicity on vertical cleavage planes. In the case of a non-rotating seed there is a different segregation process including a periodical incorporation too, but in discrete layers of relatively small separation parallel to the solid liquid interface in short range periodicity. Explanations for the segregation phenomena are given in terms of periodically varying effective growth rates related to periodic temperature fluctuations in the neighbourhood of the solid liquid interface.     

A diffusion-controlled kinetic model for growth of Au-catalyzed ZnO nanorods: Theory and experiment

A kinetic model for growth of ZnO nanorods via vapor–liquid–solid (VLS) mechanism based on the bulk diffusion of Zn atoms through the Au–Zn droplet is presented. The dependences of the growth rate on size are given quantitatively. A general expression for the growth rate of nanorods during VLS process is derived. The derived formula shows the dependences of growth rate on lateral size of nanorods, concentration and supersaturation of Zn atoms in the liquid droplet. Based on the presented kinetic model the smaller nanorods have faster growth rate. Au-catalyzed ZnO nanorods are grown by chemical vapor transport and condensation (CVTC) process experimentally. Theoretical and experimental rate/radius curves are compared to each other. Theoretical predictions are in good agreement with the experimental results.     

The structure of rapidly solidified ribbons of Al-12 at. % Zn alloy

The structure of melt spun rapidly solidified ribbons of the Al-12 at.% Zn alloy has been investigated by scanning electron microscopy and by X-ray diffraction. Four solidification processes differing in the extent of associated segregation of zinc have been found to be operative during the solidification of the ribbon. These successive processes beginning from above the sites of good contact of the melt with the rotating plane of the copper substrate are as follows: massive planar growth of grains, arrayed dendritic growth, unaligned growth of dendrites, and star-like dendritic growth out from fragments of dendrites. Smaller grains resulting from the last two mentioned processes have been also found within the regions located above the sites of poor contact of the melt with the substrate (lift-off regions). All indicated successive growth modes starting with the epitaxial regrowth of grains have been also found to proceed during the solidification of the melt flowed over the already solidified alloy ribbon. Grain boundary precipitation of equilibrium β (≈︁ Zn) phase has been detected in all stages of solidification. High density of lattice defects built-up during solidification is leading to the immediate intragranular heterogeneous precipitation of β phase at elevated temperatures within the two-phase region without the formation of intermediate metastable precipitates.     

Preparation and growth mechanism of clustered one-dimensional SiOx amorphous nanowires by catalytic pyrolysis of a polymer precursor

Large-scale synthesis of clustered one-dimensional amorphous silica nanowires was achieved by simple thermal pyrolysis of an amorphous preceramic powder from perhydropolysilazane on alumina wafers coated with catalyst FeCl₂. Scanning electron microscopy and transmission electron microscopy observations showed that the silica nanowires had smooth surface, and lengths of hundreds of micrometers and diameters in the range of 30–40 nm. Energy dispersive X-ray spectroscopy revealed that these nanowires consisted of Si and O elements in an atomic ratio of approximately 1:2, consistent with the stoichiometric formula SiO₂. The two amorphous bulges in Raman spectrum at the centers of around 260 cm^?1 and 800 cm^?1 were identified to be those of amorphous silica. The growth mechanism of the as-produced silica nanowires could be attributed to vapor–liquid–solid mechanism. These results provide an alternative and simple preparation procedure for nanostructures with controlled morphology, and it will be helpful to understand the growth mechanism of one-dimensional SiO₂ nanostructures.     

Optimization of control parameters of cadmium zinc telluride Bridgman single crystal growth

The temperature gradient within a furnace chamber and the crucible pull rate are the key control parameters for cadmium zinc telluride Bridgman single crystal growth. Their effects on the heat and mass transfer in front of the solid‐liquid interface and the solute segregation in the grown crystal were investigated with numerical modeling. With an increase of the temperature gradient, the convection intensity in the melt in front of the solid‐liquid interface increases almost proportionally to the temperature gradient. The interface concavity decreases rapidly at faster crucible pull rates, while it increases at slow pull rates. Moreover, the solute concentration gradient in the melt in front of the solid‐liquid interface decreases significantly, as does the radial solute segregation in the grown crystal. In general, a decrease of the pull rate leads to a strong decrease of the concavity of the solid‐liquid interface and of the radial solute segregation in the grown crystal, while the axial solute segregation in the grown crystal increases slightly. A combination of a low crucible pull rate with a medium temperature gradient within the furnace chamber will make the radial solute segregation of the grown crystal vanish. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification

The purpose of this work was to investigate bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification. The experiments were performed in succinonitrile-based transparent alloys (SCN-1.5 wt%ACE). While the solid–liquid interface is cellular, the solid–liquid interface is separated into two layers by the bubble. Experimental results show that a cellular–planar–cellular transition for solid–liquid interface occurs on the lower layer and the stability of the tubular bubble is determined by local microstructure on the upper layer. When the interface is dendritic, morphological instability occurs on the solid thin film attached to the bubble after the solid–liquid interface hits the bubble. We analyzed the evolution of such cells (some cells become dendrites with time) as a function of the angle between the opposite growth direction of dendritic array and the small cell growth direction. It is demonstrated that the relative position between the existing bubble and the dendritic tip influences on the local growth pattern of dendritic array.     

Determination of mechanical, electrical and thermal properties of the Sn―Bi―Zn ternary alloy

The development of lead-free solders has emerged as one of the key issues in the electronics packaging industries. Sn―Zn―Bi eutectic alloy has been considered as one of the lead-free solder materials that can replace the toxic Pb―Sn eutectic solder without increasing soldering temperature. This study investigates the effect of temperature gradient and growth rate on the mechanical, electrical and thermal properties of the Sn―Zn―Bi eutectic alloy. Sn-23 wt.% Bi-5 wt.% Zn alloy was directionally solidified upward with different growth rates (V = 8.3-478.6 μm/s) at a constant temperature gradient (G = 3.99 K/mm) and with different temperature gradients (G = 1.78-3.99 K/mm) at a constant growth rate (V = 8.3 μm/s) in the Bridgman-type growth apparatus. The microhardness (HV), tensile stress (σ_t) and compressive stress (σ_c) were measured from directionally solidified samples. The dependency of the HV, σ_t and σ_c for directionally solidified Sn-23 wt.% Bi-5 wt.% Zn alloy on the solidification parameters (G, V) were investigated and the relationships between them were obtained by using regression analysis. According to present results, HV, σ_t and σ_c of directionally solidified Sn-23 wt.% Bi-5 wt.% Zn alloy increase with increasing G and V. Variations of electrical resistivity (ρ) for cast samples with the temperature in the range of 300-420 K were also measured by using a standard dc four-point probe technique. The enthalpy of fusion (ΔH) and specific heat (C_p) for same alloy was also determined by means of differential scanning calorimeter (DSC) from heating trace during the transformation from eutectic liquid to eutectic solid.     

On the formation and technical importance of the microsegregation in austenitic chromium-nickel alloyed steels

The austenitic chromium nickel alloyed steels are showing different types of concentration-distributions of the elements chromium and nickel in the austenite in dependance on the content of alloys. As a consequence of the crystallization of the heat to austenite the “equal directed” type of segregation forms. The ferritic solidification of a heat and the following ferrite to austenite transformation leads to “unequal directed” segregation in the austenite. Heats having both the types of segregation solidify by way a 3-phase reaction. For heats ferritically solidified better hot plasticity, lower susceptibility to hot cracking in the weld and better polishability is observed compared with heats solidified austenitically. The advantages are accounted to the lower separation during the ferritic solidification as consequence of the higher diffusivity of the alloying and accompanying elements in the ferrite compared with the austenite.     

A two-dimensional Zn(II) coordination polymer with a three-dimensional supramolecular architecture comprising 5-dimethylamino-isophthalic acid and 1,3-bis(4-pyridyl)propane

A new zinc coordination polymer, [Zn(L)(bpp)·H₂O]_n, was synthesized using 5-dimethylamino-isophthalic acid (H₂L) and 1,3-bis(4-pyridyl)propane (bpp) as organic linkers. In the complex, adjacent Zn²⁺ ions are linked by L^2? anions to form a one-dimensional (1D) [ZnL]_n chain. Then bpp ligands expand the 1D chains into a corrugated two-dimensional (2D) layer network by linking neighboring Zn²⁺ ions. These adjacent layers are further stacked together by direct C?H···π supramolecular interactions, generating a three-dimensional (3D) supramolecular structure. From the viewpoint of topology, the 2D network can be rationalized to a uninodal four-connected non-interpenetrated sql/Shubnikov tetragonal plane net with {4⁴.6²} topology. Moreover, the solid state properties such as thermogravimetric analysis and luminescence were also investigated.     

TEM analysis of the container effect of Au‐based catalyst droplets during vapour‐liquid‐solid growth of axial ZnTe/CdTe nanowires

Axial heterostructure nanowires (NWs) of ZnTe/CdTe were grown by vapour‐liquid‐solid growth realized in a molecular beam epitaxial chamber. By alternative supply of Zn or Cd and constant Te the heterostructure was generated. The liquid phase is provided by a Au‐based eutectic droplet which stays at the tip of the NW during the entire growth. For structural and chemical characterization by TEM the NWs were harvested from the substrate and transferred to a holey carbon film. The NWs exhibit an expansion of the diameter correlated with the interface region between ZnTe and CdTe. Idiomorphic growth of the CdTe is evident from electron diffraction experiments. The growth rate of CdTe appears to be smaller compared to that of ZnTe at the same temperature. Both, quantitative high‐resolution TEM and energy dispersive X‐ray spectroscopy line scans reveal a smeared ZnTe/CdTe interface along about 200 nm. The smearing is due to both, the liquid catalyst which buffers the supply of Cd instead of Zn at the liquid/solid interface and to the strain which is induced by the lattice mismatch. It forces the system to consume the remnant Zn for the NW growth in favour of Cd. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solvothermal synthesis and growth mechanism of asymmetric ZnO hierarchical structures in diethylene glycol with ammonia solution

Several novel asymmetric ZnO hierarchical structures were synthesized in diethylene glycol (DEG) with different amount of ammonia solution via solvothermal process. The submicron‐rods or nanorods as the building units are distributed on the two sides in an asymmetric manner, resulting in formation of the shiitake‐like or bouquet‐like morphology. A possible formation mechanism was proposed on the basis of the experimental result. The consumption of the precursor could lead to a two‐step nucleation and growth process. The relative content of the [Zn(OH)₄]²⁻ and the [Zn(NH₃)₄]²⁺ precursors varies with the amount of ammonia solution, which affect the size and morphology of the asymmetric structures. The [Zn(OH)₄]²⁻ complex and the [Zn(NH₃)₄]²⁺ complex are absorbed on the positive (0001)‐Zn polar surface and the negative (000–1)‐O polar surface respectively, which lead to the alteration of growth rate of these polar surfaces.     

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.     

ACRT forced convection and its effects on solute segregation and heat and mass transfer during single crystal growth

A numerical simulation study was carried out for CdZnTe vertical Bridgman method crystal growth with the accelerated crucible rotation technique (ACRT). The convection, heat and mass transfer in front of the solid‐liquid interface, and their effects on the solute segregation of the grown crystal can be characterized with the following. ACRT brings about a periodic forced convection in the melt, of which the intensity and the incidence are far above the ones of the natural convection without ACRT. This forced convection is of multiformity due to the changes of the ACRT parameters. It can result in the increases of both the solid‐liquid interface concavity and the temperature gradient of the melt in front of the solid‐liquid interface, of which magnitudes vary from a little to many times as the ACRT wave parameters change. It also enhances the mass transfer in the melt in a great deal, almost results in the complete uniformity of the solute distribution in the melt. With suitable wave parameters, ACRT forced convection decreases the radial solute segregation of the crystal in a great deal, even makes it disappear completely. However, it increases both the axial solute segregation and the radial one notably with bad wave parameters. An excellent single crystal could be gotten, of which the most part is with no segregation, by adjusting both the ACRT wave parameters and the crystal growth control parameters, e.g. the initial temperature of the melt, the temperature gradient, and the crucible withdrawal rate. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and structure of a 2D Zn complex with mixed ligands stacked in offset ABAB manner

The title complex, {[Zn(ODIB)_1/2(bpdc)]·2DMF}_n was prepared under hydrothermal conditions (dimethylformamide and water) based on two ligands, namely, 1,1′-oxy-bis[3,5-diimidazolyl-benzene] (ODIB) and biphenyldicarboxylic acid (H₂bpdc). ODIB ligands link Zn cations to give layers in crystal. bpdc^2– anions coordinate to Zn atoms, however, their introduction does not increase the dimension of the structure. Each layer is partially passes through the adjacent layers in the offset ABAB manner.     

Numerical simulation of the solidification processes of copper during vacuum continuous casting

A numerical simulation method is used to analyze the microstructure evolution of 8-mm-diameter copper rods during the vacuum continuous casting (VCC) process. The macro–microscopic coupling method is adopted to develop a temperature field model and a microstructure prediction model. The effects of casting parameters, including casting speed, pouring temperature, cooling rate, and casting dimension on the location and shape of the solid–liquid (S/L) interface and solidified microstructure are considered. Simulation results show that the casting speed has a large effect on the position and shape of the S/L interface and grain morphology. With an increase of casting speed, the shape of the S/L interface changes from a planar shape into an elliptical shape or a narrow, pear shape, and the grain morphology indicates a change from axial growth to axial–radial growth or completely radial growth. The simulation predictions agree well with the microstructure observations of cast specimens. Further analysis of the effects of other casting parameters on the position and shape of the S/L interface reveals that the casting dimension has more influence on the position and shape of the S/L interface and grain morphology than do pouring temperature and cooling rate. The simulation results can be summarized to obtain a discriminant of shape factor (η), which defines the shape of the S/L interface and grain morphology.     

Zum Wachstum von AIIIBV-Halbleitern aus nichtstöchiometrischen Schmelzen (II) Dotierung von GaAs und Ga1−xAlxAs mit Zink

Doping of GaAs and Ga_1−xAl_xAs with Zn in the LPE process was studied by a radioanalytical method. It is found that Zn diffuses from the Ga-(Al)-As-Zn-solution into the n-GaAs substrate before epitaxial growth starts. This “pre-diffusion” and the following diffusion of Zn out of the epitaxial layer into the substrate results in three regions with distinct Zn-graduation in the vicinity of the pn-junction. There are no striking differences for GaAs/GaAs and Ga_1−xAl_xAs/GaAs structures. The Zn concentration in GaAs epitaxial layers decreases exponentially from the substrate up to the layer surface. From this profile the temperature dependence of the Zn segregation coefficient is calculated. At 900°C a value k_eff = 5,2 · 10⁻² is found. The doping profile in Ga_1−xAl/As layers is more complex. It is influenced by the changings of temperature during the growth of the layer and by the nonuniform Aldistribution over the layer thickness.     

Inverted SiC nanoneedles grown on carbon fibers by a two-crucible method without catalyst

Inverted single crystalline SiC nanoneedles with hexagonal cross-sections were grown on the surface of carbon fibers by high-frequency induction heating two-crucibles without using any catalysts. we employ a carbothermal reduction method of silicon monoxide with coke fibers to synthesize SiC nanoneedles within 5 min. The as-grown SiC nanoneedles shows bright blue color on carbon fibers in the [1 1 1] orientation of 3C-SiC structure. The needle-like structures grew on the substrate while the spindle portion was sticked into the carbon fibers which were different from other nanoneedles. Finally, the growth mechanism of SiC nanoneedles is proposed to be an axial direction growth with a driving force of screw dislocation and a radial direction growth with vapor–solid mechanism meanwhile.     

Synthesis,characterization and X‐ray crystal structure of guanidinuim (pyridine‐2,6‐dicarboxylato)(pyridine‐2,6‐monocarboxylato) zincate(II) mono pyridine‐2,6‐dicarboxylic acid tetrahydrate

A new complex of Zinc(II) and guanidinium pyridine–2,6–dicarboxylate proton transfer compound, (GnH)₂ (pydc), was synthesized and characterized using IR, ¹H NMR and ¹³C NMR spectroscopy and single crystal X–ray diffraction. The chemical formula and space group of the resulting complex is (GnH)[Zn(pydc)(pydcH)] · (pydcH₂) · 4H₂O, P 1 where the final R value is 0.0310 for 13287 reflections collected. The [Zn(pydc)(pydcH)]^– anions and the (GnH)⁺ counter‐cations form a three‐dimensional solid‐state structure by a variety of noncovalent interactions such as ion pairing, hydrogen bonding and π–π stacking which are going to be discussed. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation process in a cast alloy Al-3.5 wt.pct Zn-4 wt.pct Mg

The structural changes during ageing of an alloy containing 3.5 wt.pct Zn and 4 wt.pct Mg were studied by means of transmission electron microscopy. It was shown that the precipitation process in this alloy differs from that in the alloys having high Zn/Mg ratios. The following phases were observed: hexagonal transition T′ phase, cubic stable T phase, hexagonal transition β′ phase, cubic stable β phase and hexagonal transition β′ phase. The precipitation process proposed the formation of two kinds of clusters in the beginning, i.e. Zn-rich and Mg-rich clusters. The former gradually transforms into GP zones and then into the T′ and T phases. The latter transforms into the β′ and β phases. The existence of the β′ phase seems to be a result of the segregation of the solute elements in the cast alloy.