Growth Of Large Crystals Research Articles

Growth of GaN Single Crystals by Li3N Flux with Mn as Addition

Metal Mn was introduced into Li3N flux to grow GaN single crystals from Ga melt. Colorless, transparent GaN single crystals with an average size of 2−3 mm were obtained at 800 °C under N2 pressure of about 2 atm. The effects of Mn on the quality, size distribution, morphologies, and formation mechanism of GaN crystals were investigated. It was found that the proper addition of Mn favored GaN growth, impeded the homogeneous nucleation to some extent, and resulted in a more uniform size distribution and high quality of GaN crystals as compared with that of GaN grown using Li3N as flux only. The formation mechanism was enhanced by Mn addition. These results suggested a promising new route for the growth of large size and high quality GaN single crystals from a Li−Ga−N system in the future by optimizing the molar ratio of Mn to Li3N.

Concentrating colloids with electric field gradients. I. Particle transport and growth mechanism of hard-sphere-like crystals in an electric bottle

This work concerns the use of electric field gradients to manipulate the local particle concentration in a hard-sphere-like suspension. Inside a specially designed "electric bottle," we observed our colloids to collect in the regions of lowest field strength ("negative dielectrophoresis"). This allows for the use of larger field gradients and stronger dielectrophoretic forces than in the original electric bottle design, which was based on positive dielectrophoresis [M. T. Sullivan et al., Phys. Rev. Lett. 96, 015703 (2006)]. We used confocal scanning laser microscopy to quantitatively follow the time-dependent change in the particle density and the suspension structure. Within a few days, the dielectrophoretic compression was seen to initiate a heterogeneouslike growth of large single crystals, which took place far out-of-equilibrium. The crystals had a random hexagonal close-packed structure and displayed an intriguing growth mechanism, during which the entire crystal was continuously transported, while growing both on the "high-field" and the "low-field" sides, although at different rates. After switching off the electric field, the compressed crystals were found to relax to a lower packing fraction and melt, at a much slower rate than the crystal growth. Besides revealing the particular (far out-of-equilibrium) crystal growth mechanism in these electric bottles, our observations also shed light on the role of the different particle transport processes in the cell and some of the relevant tuning parameters. This is useful for different types of experiments, for instance, focusing more on melting, homogeneous crystallization, or the glass transition.

Control for oriented growth of large size KCl crystals by the competition between spontaneous and induced nucleation/growth on a Langmuir–Blodgett film

Langmuir–Blodgett (LB) film of stearic acid was used as template to induce the nucleation and growth of KCl crystals when the KCl solution was cooled from 50 to 25°C. When the LB film template was vertically dipped into the solution, only induced crystals with (110) orientation were formed. However, if the template was horizontally placed into solutions, both the induced nuclei at the solution/film interface and spontaneous nuclei formed in solution were simultaneously absorbed onto the LB film, and then grew further to form crystals. X-ray diffraction (XRD) patterns and optical microscopy images showed that the orientation and morphology of the crystals were controlled properly by changing the orientation and position of the LB films in the solutions.

Scintillation response of CaF2 to H and He over a continuous energy range

Recent demands for new radiation detector materials with improved γ-ray detection performance at room temperature have prompted research efforts on both accelerated material discovery and efficient techniques that can be used to identify material properties relevant to detector performance. New material discovery has been limited due to the difficulties of large crystal growth to completely absorb γ-energies; whereas high-quality thin films or small crystals of candidate materials can be readily produced by various modern growth techniques. In this work, an ion-scintillator technique is demonstrated that can be applied to study scintillation properties of thin films and small crystals. The scintillation response of a benchmark scintillator, europium-doped calcium fluoride (CaF2:Eu), to energetic proton and helium ions is studied using the ion-scintillator approach based on a time-of-flight (TOF) telescope. Excellent energy resolution and fast response of the TOF telescope allow quantitative measurement of light yield, nonlinearity and energy resolution over an energy range from a few tens to a few thousands of keV.

Formation of large (≃100 μm) ice crystals near the tropical tropopause

Abstract. Recent high-altitude aircraft measurements with in situ imaging instruments indicated the presence of relatively large (≃100 μm length), thin (aspect ratios of ≃6:1 or larger) hexagonal plate ice crystals near the tropical tropopause in very low concentrations (<0.01 L−1). These crystals were not produced by deep convection or aggregation. We use simple growth-sedimentation calculations as well as detailed cloud simulations to evaluate the conditions required to grow the large crystals. Uncertainties in crystal aspect ratio leave a range of possibilities, which could be constrained by knowledge of the water vapor concentration in the air where the crystal growth occurred. Unfortunately, water vapor measurements made in the cloud formation region near the tropopause with different instruments ranged from <2 ppmv to ≃3.5 ppmv. The higher water vapor concentrations correspond to very large ice supersaturations (relative humidities with respect to ice of about 200%). If the aspect ratios of the hexagonal plate crystals are as small as the image analysis suggests (6:1, see companion paper (Lawson et al., 2008)) then growth of the large crystals before they sediment out of the supersaturated layer would only be possible if the water vapor concentration were on the high end of the range indicated by the different measurements (>3 ppmv). On the other hand, if the crystal aspect ratios are quite a bit larger (≃10:1), then H2O concentrations toward the low end of the measurement range (≃2–2.5 ppmv) would suffice to grow the large crystals. Gravity-wave driven temperature and vertical wind perturbations only slightly modify the H2O concentrations needed to grow the crystals. We find that it would not be possible to grow the large crystals with water concentrations less than 2 ppmv, even with assumptions of a very high aspect ratio of 15 and steady upward motion of 2 cm s−1 to loft the crystals in the tropopause region. These calculations would seem to imply that the measurements indicating water vapor concentrations less than 2 ppmv are implausible, but we cannot rule out the possibility that higher humidity prevailed upstream of the aircraft measurements and the air was dehydrated by the cloud formation. Simulations of the cloud formation with a detailed model indicate that homogeneous freezing should generate ice concentrations larger than the observed concencentrations (20 L−1), and even concentrations as low as 20 L−1 should have depleted the vapor in excess of saturation and prevented growth of large crystals. It seems likely that the large crystals resulted from ice nucleation on effective heterogeneous nuclei at low ice supersaturations. Improvements in our understanding of detailed cloud microphysical processes require resolution of the water vapor measurement discrepancies in these very cold, dry regions of the atmosphere.

Neutron protein crystallography: beyond the folding structure of biological macromolecules

Neutron diffraction provides an experimental method of directly locating H atoms in proteins, a technique complementary to ultra-high-resolution X-ray diffraction. Three different types of neutron diffractometers for biological macromolecules have been constructed in Japan, France and the USA, and they have been used to determine the crystal structures of proteins up to resolution limits of 1.5-2.5 A. Results relating to H-atom positions and hydration patterns in proteins have been obtained from these studies. Examples include the geometrical details of hydrogen bonds, the role of H atoms in enzymatic activity, CH3 configuration, H/D exchange in proteins and oligonucleotides, and the dynamical behavior of hydration structures, all of which have been extracted from these structural results and reviewed. Other techniques, such as the growth of large single crystals and a database of hydrogen and hydration in proteins, are described.

Investigations on the nucleation kinetics of methyl-para-hydroxy benzoate single crystals

The solubility, metastable zone width and induction period have been determined for the solution grown methyl-para-hydroxy benzoate (MHB) single crystals. Nucleation parameters such as Gibbs free energy, radius of critical nucleus, critical free energy barrier and number of molecules in the critical nucleus and nucleation rate have also been investigated. Interfacial energy has been estimated using the experimentally determined induction period values. It is observed that the nucleation rate increases with the increase of supersaturation. The evaluated interfacial tension values are found to be comparable with the theoretical values. The growth parameters of MHB were optimized for the growth of large size crystals.

Influence of UV-exposure on the crystallization and optical properties of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass. Photoinduced crystalline phase precipitation results in refractive index variations in UV exposed areas of PTR glass. The precipitation of silver containing particles which occurs during photo-thermo-refractive process increases the optical absorption of the samples in the range 350nm to NIR wavelengths and the growth of sodium fluoride crystals and their aggregation increases light scattering in visible and NIR regions. We show that one effect of the UV-exposure is a decrease in the crystallization temperature by ∼50°C compared to the unexposed areas as measured by differential scanning calorimetry, which we attribute to an increase in nucleation rate. Using spectro-photometric measurements, a linear function is fitted to the changes in the amplitude of the absorption band of the silver containing particles versus the UV-dosage. The root mean square scatter of the data from the linear function is better than 0.99 and the slope of the function is 0.32±0.01cm/J. The IR absorption of PTR sample, measured by laser calorimetry shows that the increase of the absorption in infrared region at 1.1μm, is due to the tail of the absorption band of silver containing particles having maximum at 465nm. We finally show that after hyper-development, one effect of UV-exposure at 325nm on the crystallization kinetics of PTR glasses is a decrease in particle sizes from micron size to nanometers size. But additional investigations demonstrate that smaller dosage UV-exposures (a few tens of milliwatts) increase the optical scattering by one order of magnitude. Optical micrographs taken after UV-exposure and hyper-development reveal the use of smaller dosages enhances nucleation rate without preventing the growth of large crystals and therefore induces higher scattering.

Epitaxial Growth of Large Pentacene Crystals on Si(001) Surfaces Functionalized with Molecular Monolayers

The nucleation and growth of pentacene thin films are controlled largely by the energies associated with the interfaces. We have used low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) to investigate the nucleation and growth of pentacene thin films on Si(001) surfaces modified with two different molecular monolayers. Clean Si(001)-(2 × 1) surfaces were modified with either 1,5-cyclooctadiene or 1-dodecene prior to pentacene growth to study the effects of exposed π bonds at the interface, orientation of those π bonds relative to each other, and rigidity of the molecular layer on pentacene nucleation, growth, and crystalline orientation. Both molecular monolayers weaken the substrate−pentacene interaction sufficiently to allow for low pentacene nucleation density and good pentacene diffusion, leading to the growth of pentacene grains as large as 100 μm. Pentacene grows epitaxially on both functionalized surfaces, adopting an orthorhombic unit cell that follows the orientatio...

High-Resolution Dynamic Analysis of the Phase Transformation in Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> Alloy

Phase transformation and crystal growth behavior of Ge2Sb2Te5 were investigated systematically by means of in situ heating (from room temperature to 500 oC) of amorphous Ge2Sb2Te5 alloy in a high voltage electron microscope with real-time monitoring. Large-scale crystallization occurred to amorphous Ge2Sb2Te5 around 200 oC. Large crystal growth developed on heating from 200 oC to 400 oC, and single crystalline grains grew up to 150 nm. Eventually the onset of partial melting of thin Ge2Sb2Te5 foil was at 500 oC and liquid Ge2Sb2Te5 was observed for the first time by high-resolution transmission electron microscopy. Hexagonal Ge2Sb2Te5 phase remains after a subsequent cooling.

Growth of large (1 mm) MgSiO3 perovskite single crystals: A thermal gradient method at ultrahigh pressure

Large single crystals of MgSiO 3 perovskite were successfully synthesized by a thermal gradient method at 24 GPa and 1500 °C. This was achieved by an improvement of high-pressure synthesis technique that allowed us to grow single crystals under such ultrahigh-pressure conditions in relatively large volumes (e.g., 10 mm 3 ). Since crystal growth is hindered by neighboring crystals, the nucleation density was suppressed by reducing the thermal gradient to 20 °C/mm, permitting an increase in free space for large crystal growth. KHCO 3 -Mg(OH) 2 solvent can be used to grow perovskite crystals. However, the carbonate solvent produces melt inclusions. Silicate sources with MgSiO 3 composition produce stishovite inclusions, which in turn cause splitting of perovskite crystals. The formation of these inclusions is avoided by using H 2 O as a solvent and 85MgSiO 3 -15Mg 2 SiO 4 as a silicate source. The H 2 O also allows homogeneous crystal growth, probably because of its low viscosity and high silicate solubility. High-quality single crystals larger than 1 mm were successfully synthesized through these technical developments.

Large Bi-2212 single crystal growth by the floating-zone technique

Effects of growth velocity on the crystal-growth behavior of Bi 2Sr 2Ca 1Cu 2O x (Bi-2212) have been studied by the floating-zone technique. The results show that a necessary condition for obtaining large single crystals along the c-axis is that the solid–liquid interface of a growing rod maintains a stable planar growth front. The planar liquid–solid growth interface tends to break down into a cellular interface when the growth velocity is higher than 0.25 mm/h. Single crystals of up to 50×7.2×7 mm 3 along the a-, b- and c-axes, respectively, have been cut from a 7.2 mm diameter rod obtained with optimum growth conditions. T c onset is 91 K as determined from magnetization measurements on as-grown crystals. Optical polarization microscopy and neutron diffraction show that the quality of the large single crystals is good.

Salt weathering in dual-porosity building dolostones

The influence of rock fabric on physical weathering due to the salt crystallization of selected brecciated dolostones is discussed. These dual-porosity dolostones are representative of heterogeneous and anisotropic building rocks, and present highly complex and heterogeneous rock fabric features. The pore structure of the matrix and clasts is described in terms of porosity and pore size distribution, whereas the relative strength for each textural component is assessed using the Knoop hardness test. The whole characterisation process was carried out using the same samples as those used in the standard salt durability test (EN-12370), including connected porosity, the water saturation coefficient, fissure density, compressional wave velocity and waveform energy. Results show the most important rock fabric elements to be considered are the matrix and clast properties and the nature of fissures. Firstly, a relatively weak matrix was the focus of major granular disintegration as it presents high porosity, low pore radius and reduced strength. Secondly, narrow micro-fissures appear to be important in the decay process due to the effectiveness of crystallization pressure generated by salt growth. On the contrary, macro-fissures do not contribute greatly to rock decay since they act as sinks to consume the high supersaturations caused by growth of large crystals. Additionally, an analysis of stress generated by crystallization was carried out based on the general situation of a lenticular crystal geometry. Finally, the relationships between whole petrophysical properties and durability were established using a principal component analysis. This analysis has clearly established that the durability of rocks affected by salt crystallization mechanisms diminishes in weaker and anisotropic rocks with high porosity and fissure density.

Temperature field design, process analysis and control of SAPMAC method for the growth of large size sapphire crystals

AbstractIn this paper, the relationship between quality of sapphire crystal and growing parameters of SAPMAC (Sapphire growth technique with micro‐pulling and shoulder‐expanding at cooled center) method was discussed. Optimized temperature distribution and technique control were proposed by theoretical analysis, numerical simulation computation and experimental validation to obtain large size sapphire crystals. For a‐axis crystallized direction, with 1.0‐5.0mm/h growth velocity and 10‐30K/h temperature decreasing speed, large sapphire single crystal (∅︁240mm×210mm, 27.5kg) having high optical quality was successfully grown. The absorption spectrum of standard samples was measured as well. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure and crystallization behavior of Nylon 66/multi-walled carbon nanotube nanocomposites at low carbon nanotube contents

Multi-walled carbon nanotubes (MWNTs) were modified with poly(hexamethylene adipamide) (also known as Nylon 66) via a controlled polymer solution crystallization method. A “nanohybrid shish kebab” (NHSK) structure was found wherein the MWNT resembled the shish while Nylon 66 lamellar crystals formed the kebabs. These Nylon 66-functionalized MWNTs were used as precursors to prepare polymer/MWNT nanocomposites. Excellent dispersion was revealed by optical and electron microscopies. Nitric acid etching of the nanocomposites showed that MWNT formed a robust network in Nylon 66. Non-isothermal DSC results showed multiple melting peaks, which can be attributed to lamellar thickness changes upon heating. The crystallite sizes L 100 and L 010 of Nylon 66, determined by WAXD, decreased with increasing MWNT contents. Isothermal DSC results showed that crystallization kinetics increased first and then decreased with increasing MWNT contents in Nylon 66. This study showed that the effect of MWNTs on Nylon 66 crystallization is twofold: MWNTs provide heterogeneous nucleation sites for Nylon 66 crystallization while the tube network structure hinders large crystal growth.

Single crystal growth of Nd(Ce)214 by laser floating zone furnace

(Nd 0.925Ce 0.075) 2CuO 4 single crystals were grown by the newly developed laser floating zone (FZ) furnace. The extremely sharp temperature gradient by focused laser light enabled to maintain the small molten zone, resulting in successful growth of large single crystals with high crystallinity. In particular, a crystal grown in oxygen atmosphere showed a sharp peak in a rocking curve for the (0 0 6) reflection with a FWHM of 0.10°. The grown crystals exhibited superconductivity with T c(onset) ∼ 20 K after annealing in evacuated quartz tubes.

Progresses of the Na Flux Method as a New Technology for the Growth of Large GaN Single Crystals

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Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature

Epitaxial growth on nickel-plated diamond seeds at high pressure and high temperature (HPHT) was observed with graphite as carbon source. The thickness of the electroplating nickel film which acts as a catalyst/solvent ranges from 54.6 μm to 255.6 μm. The relationship between the Ni film thickness and diamond growth rate is investigated. When the nickel film thickness is from 90 μm to 129 μm, diamond crystals can nearly grow up to three times as large as the original seeds at ∼ 5.8 GPa and ∼ 1460 °C within 14 min. The mechanism of the crystal growth with nickel-plated diamond seeds under HPHT is discussed. The results and techniques might be useful for high quality saw-grade diamonds production and large diamond single crystal growth.

Growth of plate-type β-FeSi 2 single crystals by optimization of composition ratio of source materials

The importance of the β-FeSi 2 bulk single crystals has increased not only to investigate the intrinsic properties of β-FeSi 2 but also to use it as substrate of β-FeSi 2 thin films for optical devices. Though single crystals of β-FeSi 2 are grown by chemical vapor transport (CVT) method, most of those crystals are needle-like and widths of those flat surfaces are 0.5 mm or less. In order to understand the mechanism of the growth process of β-FeSi 2 by the CVT method and to obtain the conditions for large size crystal growth, we have carried out in-situ observations of the crystal growth by using a transparent electric furnace. Based on the experimental data, we have proposed the most likely reaction process, FeI 2(g) + 2SiI 4(g)→FeSi 2(s) + 5I 2(g), and we found that the crystal growth progresses under the environment where the FeI 2 gas is insufficient compared with a suitable ratio of FeI 2/SiI 4. Then, to raise the partial pressure of FeI 2 gas, the composition ratio of Fe to Si for the source material was increased and we have obtained the plate-type β-FeSi 2 crystals that exceeded a few square millimeters in size.

A methodology and an instrument for the temperature-controlled optimization of crystal growth

A method and a device for the promotion of crystal growth by keeping the crystallization solution metastable during the growth process are described. This is achieved by controlled temperature variation of the crystallization solution using parameters determined in situ during the growth process. The technique finds application in the growth of large high-quality crystals for neutron crystallography. Thus, it has been applied to grow large crystals of several proteins of interest such as human gamma-crystallin E, PA-IIL lectin from Pseudomonas aeruginosa, yeast inorganic pyrophosphatase, urate oxidase from Aspergillus flavus and human carbonic anhydrase II.