Study on anisotropy of growth rate of ice crystal in supercooled water

The purpose of this research was to investigate the ice growth of a single crystal in three dimensions. Three-dimensional pattern of ice crystal growth in supercooled water was observed using Mach–Zehnder spectro-interferometer. Temperature was varied from −0.3 to −1.6 °C. It was found that the ice crystal began to grow as a single crystal at the tip of the capillary tube and propagated freely in supercooled water. Time variation of the shape of dendrite on a–c plane was obtained. It was found that half parabola fits the shape very closely, and the coefficient of squared term, a, of a quadratic function was calculated. The coefficient, a varied in time but at quasi steady state it was found to be depending mostly upon the degree of supercooling. Furthermore, the growth velocity in c-axis at the flat surface was calculated from the thickness measured. It was found that the velocity in c-axis is independent of the degree of supercooling but depends upon time, in other words, the thickness in c-axis.     

Synthesis of size-tunable ZnO nanorod arrays from NH3·H2O/ZnNO3 solutions

Well-aligned crystalline ZnO nanorod arrays were fabricated via an aqueous solution route with zinc nitrate and ammonia as precursors. Dip-coating was firstly utilized to form a ZnO film on ITO substrate as a seed layer for subsequent growth of ZnO nanorods. The effects of NH₃·H₂O/ZnNO₃ molar ratio, ZnNO₃ concentration, growth temperature and time on nanorod morphology were respectively investigated. It was found that the size of nanorod is mainly determined by the molar ratio and concentration. XRD demonstrates that ZnO nanorods are wurtzite crystal structures preferentially orienting in the direction of the c-axis. SEM confirms that ZnO nanorods grew up perpendicular to the substrate. The diameter and length were tunable in a broad range from 80 nm to 500 nm and 250 nm up to 8 μm, respectively. The aspect ratio changed from 3 to 17 mainly dependent on composition of the aqueous solution.     

Structural and electrical properties of β-FeSi2 single crystals grown using Sb solvent

β-FeSi₂ bulk single crystals were grown using a temperature gradient solution growth (TGSG) process in Sb solvent. The microstructural and electrical properties of the β-FeSi₂ were then characterized. Rectangular-shaped single crystals were basically obtained with facetted crystallographic planes having the predominant growth direction of β-FeSi₂ [011]. No evidence for the formation of 90° order domains around the a-axis, domain boundaries and long period structures was observed by transmission electron microscopy (TEM). It was also found that the crystal shows n-type behavior, and that the donor ionization energy deduced from the temperature dependence of the electron concentration is approximately 0.12-0.15 eV. The nature of defects will be discussed along with the photoconductivity and the electron spin resonant (ESR) measurements.     

Mechanical properties of Gd123 bulk superconductors at room temperature

In order to investigate the mechanical properties of Gd123 single-grain bulk superconductors fabricated using a modified quench and melt growth method, tensile tests in the direction parallel and perpendicular to the c-axis have been carried out at 293 K by using the small specimens cut from bulk superconductors. As for the mechanical properties perpendicular to the c-axis, there was no significant difference between those in the crystal growth direction and those perpendicular to it. While the average value of the Young’s modulus of the bulk sample with 33.0 mol%-Gd211 secondary phase particles, 118 GPa, was higher than that of the bulk sample with 28.7 mol%-Gd211, 111 GPa, the average value of the tensile strength of the former, 36 MPa, was lower than that of the latter, 40 MPa. The tensile strength and the Young’s modulus in the c-axis, 10 MPa and 37 GPa, were quite low compared with those mentioned above. Poisson’s ratio based on the transverse strain in the c-axis, 0.15, was significantly smaller than that perpendicular to it, 0.30. In the specimens with higher length, however, the difference was decreased to some extent. With regard to the anisotropy of the Poisson’s ratio, the effect of a pre-existing micro-crack opening in the c-axis direction was discussed coupled with the constraints at the interfaces between the specimen and the sample holder.     

溶液类样品冻干过程中搁板预冻特性分析

搁板预冻式冻干机在对溶液类样品预冻过程中,样品内存在较大的温度梯度,溶质自下而上的迁移量较大,易形成高浓度表层,在冻干过程中形成相对致密的"硬壳".三步预冻法较适用于溶液类样品搁板预冻过程,三步法中的第二步温度扯平有效消除了冷却过程的温度梯度,相当于降低了预冻初温,加快了溶液上表面冰晶的形成与冰晶生长的速度,致使冰晶的参差生长方式,降低了溶质自下而上的迁移速度,防止了高浓度表层问题的产生.     

Structural dependence of ionic motion at interfaces between NaCl crystal surfaces and supersaturated solutions in crystallization process

The effects of surface structure and solution concentration on crystal morphology have been investigated through molecular dynamics (MD) simulation in the NaCl crystallization process. Four types of crystal surface structures were prepared for simulation. The probability of ion existence suggests that the solute ions were hardly taken into flat surfaces, while they were easily taken into roughs with some kinks and steps. This is because solute ions are more stable at a kink than at a terrace. The present simulated results suggest that the mechanism of crystal growth is dependent on the solution concentration. It was demonstrated that some ions are slotted on the crystal surface at low supersaturation after moving to the crystal surface. Under high supersaturation, the clusters are formed and adsorbed on the crystal surface. When comparing the crystal growth rates of Na⁺ and Cl⁻, the attractive force of Na⁺ from the crystal surface was found to be larger than that of Cl⁻. Moreover, the repulsive force of Na⁺ from the crystal surface was smaller than that of Cl⁻ in approaching the crystal surface. The adsorption of Na+ was also found to occur before that of Cl⁻.     

Anisotropic thermal expansion of stoichiometric lithium niobate crystals grown along the normal direction of facets

In this paper, a stoichiometric lithium niobate (SLN) crystal with the size up to 20 × 20 × 18 mm³ was grown along the normal direction of the (0 1 2) facet from the 16 mol% K₂O fluxed melt by the top-seeded solution growth method. The anisotropic thermal expansion of the SLN crystal and congruent lithium niobate (CLN) crystal was measured along different directions by using a Shimadzu thermomechanical analyzer. As compared with CLN, the SLN crystal exhibited slightly larger thermal expansion along the Z-axis and slightly smaller expansion along the X-axis. Both the SLN and CLN crystals showed strong anisotropy in the thermal expansion. The thermal expansion coefficient of SLN along the X-axis (16.7 × 10⁻⁶ °C⁻¹ at 300 °C) is much larger than that along the Z-axis (2.5 × 10⁻⁶ °C⁻¹ at 300 °C). Based on the experimental data and polynomial fitting results, we calculated the thermal expansion coefficients for different directions. In the case of growing the SLN crystal along the normal direction of (0 1 2) facet, we studied the radial anisotropic thermal expansion and discussed the cracking problem of the crystal according to its actual growth morphology. It is found that the cracks of SLN can be suppressed by growing the crystal along the W-axis due to its reduced radial anisotropy in the thermal expansion.     

Single-crystal growth and anisotropic electrical properties of (La1− xCa x )2CaCu2O6

Large single crystals of (La_1?x Ca _x )₂CaCu₂O₆ were synthesized by the travelling solvent floating zone (TSFZ) technique. The dimension of the grown boules was typically 4 mmφ×30 mm long with thec-axis perpendicular to the growth direction. After oxygen loading at 1080°C in 400 atm of O₂, they became superconductive with a sharp transition at around 50 K. The resistivity was metallic both along thec-axis and within theab-plane, with an anisotropy ratio ofγ ²≡(ρ _c/ρ _ab ) ～ 50 which is almost independent of temperature. Details of the crystal growth, heat treatment procedures, and the results of electrical and magnetic property measurements are presented.     

Determination of butanol distribution on salol-butanol crystals using Micro Raman Spectroscopy

We carried out several analytical studies in order to determine the butanol distribution on a salol-butanol crystal. This work is required for a research using microgravity condition focused upon the in-situ observation with an interferometer of the temperature and concentration field for the organic transparent crystal (salol-butanol). In order to evaluate the concentration field for the liquid phase with an interferometer in case of crystal growth from solution in space, it is significant to determine the distribution of solute (butanol) on starting crystal before sample launching using a non-destructive analytical method. The Micro Raman Spectroscopy (MRS) was choice as the most appropriate analytical method among several analytical methods. We prepared a salol-butanol crystal enclosed into the 1mm thick quartz glass ampoule in order to verify the propriety for MRS. Obtained Raman spectrums for salol, butanol and salolbutanol crystal show that the butanol 2D-distribution on salolbutanol crystal can be determined by MRS. These results also demonstrate that there are no influences of thick glass cell upon measurements and that 3D-measurement is possible. In conclusion, we argue that MRS is the most appropriate method for determination of the 2D- and/or 3D- distribution of solute on the crystal among several non-destructive analytical methods.     

Reflection electron diffraction study of oblique textures in CdS thin films produced by electron bombardment evaporation

Reflection electron diffraction studies have been made of obliquely deposited CdS films. The films were produced by electron bombardment evaporation on polished corundum single-crystal substrates at room temperature. The orientation of the uppermost layer of the film was determined for different growth conditions from reflection electron diffraction patterns and the progress in the inclination of the c-axis was studied. It was seen that for the deposition conditions used the c-axis tilted increasingly towards the vapour beam direction for thicknesses ranging from 700 to 10 000 Å. A growth orientation occured, predominantly a two degree orientation in which there was also an a-axis alignment. The influence of the deposition rate and of the angle of incidence were studied.     

温度梯度和生长速率对CdZnTe-VBM生长晶体的影响

计算模拟了半导体材料CdZnTe垂直布里奇曼法(CdZnTe-VBM)单晶体生长过程，分析了炉膛温度梯度和坩埚移动速率对结晶界面形态和晶体内组份偏析的影响。计算结果表明炉膛温度梯度和生长速率的变化明显影响固-液界面前沿对流场的形态和强度。界面凹陷深度随着炉膛温度梯度的增加和生长速率减小而减小。炉膛温度梯度的增加和生长速率的减小虽然均能有效的减小径向偏析，但却增加轴向偏析，减小轴向等浓度区的长度。     

Intrinsic effect of anionic surfactant on the morphology of hydroxyapatite nanoparticles and its structural and biological properties

This paper describes the potential effect of anionic surfactant on the morphology of hydroxyapatite (HA) nanostructures during hydrothermal synthesis. Sodium dodecyl sulfate (SDS) was used as an anionic surfactant. Various concentrations of SDS were used to study morphological changes in HA due to the presence of the surfactant. The final morphology, after treatment by a hydrothermal method, revealed that the anionic surfactant induced growth in one direction (a-axis) and inhibited growth in the other (c-axis) based on the charge distribution on the crystal faces of HA. Further structural analysis (by X-ray diffraction) confirmed this growth along the a-axis. In-vitro cellular analysis revealed that the plate-like nanoparticles possess better bioactivity than their bulk counterparts. Therefore, HA nanoplates could be used for applications that include controlled drug delivery and bone mineralization.     

Ice crystal growth in supercooled solution

Study on the crystal growth of ice in water-ethyleneglycol solution was carried out, experimentally. The ice crystal, which was seeded on top of the capillary tube, propagated inside the tube slowly and began to grow freely at the tip of the tube in subcooled solution. The outer diameter of the tip of the capillary tube was less than 0.1 mm, which was much smaller than that of other researchers. Hence, considerable reduction of the influence of the existence of a capillary was accomplished and the initial growth of a single crystal was observed, precisely. Under the condition of subcooling of less than 8 K, the shape of the crystal was observed to be different from that of the one in pure water. The velocity of dendrite ice growth and the radius of the curvature of the tip were measured. It was found that after the dendrite ice developed to a certain size, the velocity of the dendritic growth and the radius of curvature were kept steady, and its values were dependent on the degree of subcooling and the concentration of the solution.     

Interfacial Microstructure Evolution of (B, Al)N Films Grown on Diamond Substrates

This study reports on the deposition of c-axis oriented boron-aluminum nitride ((B, Al)N) layers on polycrystalline and single crystal (111) diamond substrates using a magnetron co-sputtering system. In this study, the lattice mismatch between (B, Al)N and diamond appears to be an important factor in the mechanism behind the growth of (B, Al)N on diamond. Diamond has a significantly tighter lattice than (B, Al)N, and the single (111) diamond substrate has the same lattice symmetry as that of aluminum nitride (AlN) (0002). X-ray diffractometry and transmission electron microscopy were employed to analyze the microstructure of (B, Al)N films on both substrates, and the results showed a continuous variation in structure, from randomly oriented nano-grains to c-axis oriented columns. In addition, the (B, Al)N film exhibited a thinner, randomly oriented nano-grain layer on single crystal (111) diamond than on polycrystalline diamond.     

Reconstruction of the concentration field around a growing KDP crystal with schlieren tomography

Salt concentration distribution around a potassium dihydrogen phosphate (KDP) crystal growing from its aqueous solution has been experimentally determined using a laser schlieren technique. The growth process is initiated by inserting a KDP seed into its supersaturated solution, followed by slow cooling of the solution. Fluid convection leads to a distribution of concentration around the growing crystal. The pattern and strength of convection are important factors for the determination of the crystal growth rate and quality. Experiments have been conducted in a beaker with a diameter of 16.5 cm and a height of 23 cm. A monochrome schlieren technique has been employed to image the concentration field from four view angles, namely, 0 degrees, 45 degrees, 90 degrees, and 135 degrees. By interpreting the schlieren images as projection data of the solute concentration, the three-dimensional concentration field around the crystal has been determined using the convolution backprojection algorithm. The suitability of the overall approach has been validated using a simulated convective field in a circular differentially heated fluid layer, where full as well as partial data are available. Experiments have been conducted in the convection-dominated regime of crystal growth. The noncircular shape of the crystal is seen to affect axisymmetry of the concentration field close to the crystal surface. The reconstructed concentration fields reveal symmetry of the flow field away from the growing crystal. The solute concentration contours show large growth rates of the side faces of the crystal in comparison with the horizontal faces. In this respect, the concentration profiles are seen to correlate with the crystal geometry.     

Synthesis and ultrastructure of plate-like apatite single crystals as a model for tooth enamel

Hydroxyapatite (HAp) is an inorganic constituent compound of human bones and teeth, with superior biocompatibility and bioactivity characteristics. Its crystal structure is hexagonal, characterized by a(b)- and c-planes. In vertebrate long bones, HAp crystals have a c-axis orientation, while in tooth enamel, they have an a(b)-axis orientation. Many methods can be used to synthesize c-axis oriented HAp single crystals; however, to the best of our knowledge, there have been no reports on a synthesis method for a(b)-axis oriented HAp single crystals. In this study, we successfully synthesized plate-like HAp crystals at the air–liquid interface of a starting solution via an enzyme reaction of urea with urease. Crystal phase analysis and ultrastructure observations were carried out, and the results indicated that the particles were single crystals, with almost the same a(b)-axis orientation as tooth enamel. It is hoped that by utilizing their unique surface charge and atomic arrangement, the resulting particles can be used as a high-performance biomaterial, capable of adsorbing bio-related substances and a model for tooth enamel.     

Fresnoite crystal structure in glass-ceramics

A glass-ceramic including a crystallite of fresnoite crystal phase was prepared by crystallization of glass with a composition of 2BaOTiO₂3SiO₂. The length of the a-axis of fresnoite in the glass-ceramic was shorter than that of the single crystal, and that of the c-axis was longer. Parameters of the crystal structure were refined using the Rietveld method, and the refinement was terminated to a profile reliability factor of 0.080. It was concluded that atomic displacements of the single crystal depended on the crystallization-induced stress formed in the crystallization process and the thermal contraction stress created in the cooling process after crystallization, and that these stresses were responsible for the expansion and contraction of the crystal lattice.     

Investigating Metastable hcp Solid Helium Below Its Melting Pressure

We report a first attempt to produce metastable hcp solid helium below its melting pressure. A focused sound pulse is emitted along the c-axis of a mono-domain hcp helium-4 crystal starting from a static pressure just above the melting pressure. The sound pulse is made as simple as possible with one negative and one positive swing only. Density at focus is monitored by an optical interferometric method. Performed numerical simulations show that the crystal anisotropy splits the focused wave into two separate pulses, corresponding to a longitudinal wave along the c-axis and a radial one perpendicular to it. The amplification factor due to focusing remains nevertheless important. Negative pressure swings up to 0.9 bar have been produced, crossing the static melting pressure limit. Improvements in the detection method and in the focusing amplification are proposed.     

Textured crystal growth of Si3N4 ceramics in high magnetic field

A systematic investigation has been made on the textured crystal growth of Si₃N₄ ceramics in high magnetic field with and without rotating of the gypsum mold during the slip casting. According to X-ray diffraction and microstructure analysis, a 10 T magnetic field is found to have a tendency to orient Si₃N₄ grains with their a- and b-axis parallel to the magnetic field. If the gypsum mold is rotated during the slip casting in the magnetic field, the crystal growth of Si₃N₄ grains has a tendency to align on a certain direction.