Synthesis and characterization of strip-shape diamonds from Fe-based alloy and graphite system under high pressure and high temperature

The unusual strip-shape diamond crystals were successfully synthesized in the system of Fe-based powder alloy and graphite under pressures of 5.6–5.8 GPa and temperatures of 1600–1650 K in a cubic anvil high pressure high temperature (HPHT) apparatus (SPD-6×1200). The synthetic diamond crystals have a strip-shape with light green color. The lengths of the crystals are 0.6–0.8 mm with length to diameter ratios around 2.0–2.5. SEM photographs reveal that the crystals have two main kinds of shapes: one elongated along the (1 0 0) and the other along the (1 1 1) directions. The FTIR spectra of the diamond synthesized indicate that its nitrogen concentration is higher than the normal diamond crystal.     

Growth of ZnO crystal by self-flux method using Zn solvent

Zinc oxide (ZnO) single crystal was grown, for the first time, by the self-flux method using a high-purity zinc solvent saturated with oxygen. ZnO crystals were successfully grown in the solvent by moving a growing crystal slowly, at about 17 mm/day along a temperature gradient (1.5 °C/mm) in a furnace, from 1050 to 500 °C. Hexagonal single crystals 1–3 mm in length and 0.5–1 mm in thickness were obtained. Energy-dispersive X-ray diffraction, secondary ion mass spectrometry, and photoluminescence confirmed that the purity and crystallinity of the ZnO crystals grown in this work were high.     

On the Bridgman growth of lead–tin selenide crystals with uniform tin distribution

The application of T–x–y phase diagram for the control of the crystal composition of alloy crystals during Bridgman growth is illustrated using (Pb_1−xSn_x)_1−δSe_1+δ solid solutions as an example. Within the framework of the proposed approach, the axial segregation can be minimised if the feed composition is chosen in the regions of phase diagram where the iso-mole-fraction lines are perpendicular to isotherms. For the system Pb–Sn–Se the component distribution in crystals grown from a melt with δ=0, a Se-rich melt (δ>0) and a metal-rich melt (δ<0) are discussed. Uniform crystals with x=(0.03–0.15)±0.01 can be obtained from Se-rich melt (Se enrichment is 12–5 at%Se, T=1280–1200 K). The use of metal-rich melt allows obtaining crystals with moderate segregation of 3–5 mol% SnSe for x=0.13–0.23 at 3–9 at% of metal enrichment and T=1270–1200 K and for x=0.35–0.45 at 25–35 at% of metal enrichment and T=1120–1050 K.     

Growth of dl-malic acid-doped ammonium dihydrogen phosphate crystal and its characterization

Single crystals of dl-malic acid-doped ammonium dihydrogen phosphate have been grown using slow evaporation method and also by Sankaranarayanan–Ramasamy (SR) method with the vision to improve the properties of the ADP crystals. The characterization of grown crystals was made by single-crystal X-ray diffraction, powder X-ray diffraction, UV–vis. spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DTA), Vicker's microhardness, dielectric measurements, high-resolution X-ray diffraction (HRXRD) and second-harmonic studies. Structural difference between pure and doped crystal has been studied by XRD analysis. Functional groups were identified by FTIR spectroscopy. The grown crystals were found to be transparent in the entire visible region. Decomposition temperatures of the grown crystals were measured by DTA. Vicker's hardness study carried out on (0 0 1) face at room temperature shows increased hardness of the doped crystals and SR-method-grown crystals. Dielectric measurements reveal that SR-method-grown DLM-doped ADP crystals have low dielectric loss. Crystalline perfection of the grown crystals is analyzed using HRXRD. Preliminary measurements indicate that the second harmonic generation efficiency of the doped crystals at a fundamental wavelength of 1064 nm is roughly 1.5 times greater than that of pure ADP.     

Growth,thermal and optical properties of a new nonlinear optical crystal: ZnBi2B2O7

Single crystals of ZnBi₂B₂O₇ (ZBBO) have been successfully grown by the top-seeded growth method from a high-temperature melt. The crystal was colorless and transparent with size of 15×10×5 mm³. The orientation of ZBBO crystal has been discussed. The melting point, molar enthalpy of fusion, and molar entropy of fusion of the crystal were determined to be 964.02 K, 110680.36 J mol⁻¹, and 113.92 J K⁻¹ mol⁻¹, respectively. The transparency range of the crystal extends from 370 to 2100 nm.     

Misoriented domain formation in 6H-SiC single crystal

6H-SiC single crystals were grown by the physical vapor transport (PVT) technique. Misoriented domains (MDs) were observed in as-grown crystals. Raman spectra and X-ray diffraction indicated that the MDs are 4H polytype with either (1 0 1¯ 2) or (1 0 1¯ 6) growth plane. Formation probability of MDs increased continuously as the thermal insulator had been repeatedly used. Simulations based on heat transfer demonstrated that the changes of the temperature and the temperature axial gradient at the center of the growth front were responsible for the phenomenon. The formation mechanism was put forward in terms of atomic structure of various crystal planes.     

Growth rate of α-calcium sulfate hemihydrate in K–Ca–Mg–Cl–H2O systems at elevated temperature

Kinetics of calcium sulfate hemihydrate (HH) crystal growth plays an important role in mineralization of calcium sulfate phases in nature. HH crystal growth and the conversion of calcium sulfate phases form the basis for the production and application of gypsum based building material. α-HH crystals have been grown in 3.74 M CaCl₂ solutions at a fixed initial ratio of calcium to sulfate under atmospheric pressure. The variations of sulfate ions were determined to obtain the α-HH crystal growth kinetics information. Effects of Mg²⁺ and K⁺ ions on α-HH growth were investigated to find an optimal composition of solution for α-HH preparation. The orders of α-HH growing in the CaCl₂ solution were found, in most cases, to be near 2.0 in presence or in absence of Mg²⁺ and K⁺ ions. Mg²⁺ ions enhance the growth of α-HH in CaCl₂ solution mainly due to initial supersaturation enhancing effects. K⁺ ions also improve the growth rate, which has been attributed to the reduction of interfacial energy. In a Ca (3.74 M)–Mg (0.20 M)–K (0.09 M) chlorides solution, the growth rate of α-HH increases with temperature from 80 to 100 °C, and the activation energy was calculated to be 40 kJ/mol.     

Single-crystal growth of Ti–Nb–Ta–Zr–O alloys and measurement of elastic properties

Single crystals of β-type Ti alloy system Ti–Nb–Ta–Zr–O have been grown successfully in an Ar gas flow by a floating zone method. The growth orientations were determined approximately by using seed crystals with the desired orientations. The various growth conditions were realized by choosing the gas purity, the gas flow rate, and the growth rate as variables. Composition analysis of the grown crystals was done to check any variation from the values of the raw material along with the bulk homogeneity, followed by measurements of the lattice parameter and the hardness, which provides the following results: (1) the composition of oxygen varies with respect to the flow rate, or is increased as the purity is degraded, (2) the lattice parameter is increased with increasing composition of oxygen, (3) which is also the case with the hardness. Measurements of Young's moduli were performed to investigate the elastic properties. The results indicate that the crystals exhibit the anisotropy which was expected previously. The elastic constants were estimated from the moduli, giving the ideal stress 1.7–1.9 GPa which is on a level with the real strength. Additionally, the tensile stress–strain curve for the crystallographic direction 〈1 1 0〉 exhibited nonlinear elasticity and hysteresis.     

Characterization of a newly synthesized organic nonlinear optical crystal: Urea ninhydrin monohydrate

Urea ninhydrin monohydrate (UNM) was synthesized and grown for the first time from aqueous solution employing the slow evaporation method. Single crystal structure was determined by X-ray diffraction data and it reveals that the crystal belongs to centrosymmetric with space group of P2₁/c. The grown crystals were characterized by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and UV–vis–NIR spectroscopy. Preliminary Z-scan measurement indicates that nonlinear refractive index of this crystal is −4.1×10⁻⁸ cm²/W. The etching study was performed to assess the growth pattern of the crystal. Dielectric response of the crystal was analyzed for different frequencies and temperatures.     

Three-dimensional visualization and characterization of morphology and internal microstructural features of primary silicon crystals in a cast Al–Si base alloy

Primary Si crystals are usually present in the cast microstructures of near-eutectic, eutectic, and hyper-eutectic Al–Si base alloys. Three-dimensional digital images of individual primary Si crystals present in a permanent mold cast unmodified Al-12 wt% Si-1 wt% Ni base alloy are reconstructed using a combination of montage serial sectioning and three-dimensional digital image processing techniques. Octahedral, prismatic, and plate-like three-dimensional morphologies of the primary Si crystals are present in the microstructure. Some of the primary Si crystals contain interior regions/islands of Al-alloy that are completely enclosed in the corresponding Si crystals indicating certain variations in the crystal growth velocities during the evolution of these crystals. The boundaries of these interior regions/islands are non-faceted smooth and curved indicating re-melting of the Al-rich islands and re-dissolution of some Si near these internal boundaries in the Al-alloy as a result of the heat generated by liquid-to-solid transformation of Si away from the islands.     

Growth of large-sized Ce:Y3Al5O12 (Ce:YAG) scintillation crystal by the temperature gradient technique (TGT)

In this work, Ce:YAG crystal with the size of ?4 in was successfully grown by the TGT method. The optical and scintillation properties of as-grown Ce:YAG crystals were investigated. Three obvious absorption bands at 223, 340 and 460 nm and two weak color-center absorption bands at 296 and 370 nm are observed in as-grown Ce:YAG crystal. Fluorescence with an emission peak at 398 nm is observed due to the color centers, and absorption bands of the color centers can be eliminated by annealing in O₂ or H₂ atmosphere at 1673 K for 24 h. Yellow-green fluorescence centered at 530 nm is found when the crystal was excited at 460 nm and the 530 nm excitation spectrum shows two peaks at 340 and 460 nm. X-ray fluorescence spectrum of as-grown crystal shows three emission peaks at 300, 360 and 530 nm. An average light output of 1360 phe/MeV and a single exponential decay with the decay time constant of 62.97 ns are found in as-grown Ce:YAG crystal.     

Bulk GaN crystals grown by HVPE

We succeeded in preparing very thick c-plane bulk gallium nitride (GaN) crystals grown by hydride vapor phase epitaxy. Growth of the bulk GaN crystals was performed on templates with 3 μm GaN layer grown by metal organic chemical vapor deposition on (0 0 0 1) sapphire substrates. Colorless freestanding bulk GaN crystals were obtained through self-separation processes. The crystal's diameter and thickness were about 52 and 5.8 mm, respectively. No surface pits were observed within an area of 46 mm diameter of the bulk GaN crystal. The dislocation density decreased with growth direction (from N-face side to Ga-face side) and ranged from 5.1×10⁶ cm⁻² near the N-face surface to 1.2×10⁶ cm⁻² near the Ga-face. A major impurity was Si, and other impurities (O, C, Cl, H, Fe, Ni and Cr) were near or below the detection limits by SIMS measurements.     

Bulk single-crystal growth of ternary AlxGa1−xN from solution in gallium under high pressure

We have successfully grown bulk, single crystals of Al_xGa_1−xN with the Al content x ranging from 0.5 to 0.9. Samples were grown from Ga melt under high nitrogen pressure (up to 10 kbar) and at high temperature (up to 1800 °C) using a gas pressure system. The homogeneity and Al content of the crystals were investigated by X-ray diffraction and laser ablation mass spectrometry. On the basis of the high-pressure experiments, the corresponding pressure–temperature (p–T) phase diagram of Al–Ga–N was derived. The bandgap of the material was determined by the femtosecond two-photon absorption autocorrelation method and is equal to 5.81±0.01 eV for the Al_0.86Ga_0.14N crystals.     

Formation of twinned lamellas with the application of static magnetic fields during semi-continuous casting of Al–0.24 wt%Fe alloy

Al–0.24 wt%Fe was semi-continuously cast with the application of static magnetic field of 0.1 and 0.2 T. The magnetic field transfers the microstructure from the columnar grains growing in the 〈1 0 0〉 direction into twinned lamellar crystals growing in the 〈1 1 0〉 direction. The lamellas possess the morphology and the crystallographic growth direction of the “feathery crystals”. However, the “feathers” (secondary arms) that branch off the twinning plane only start to form at 0.2 T. Under the two strengths of the magnetic field applied, no side arms in the twinning planes were observed. The increase of the lamellar spacing with the magnetic field is due to the initiation of the secondary arms.     

Reduction of grown-in dislocation density in Ge Czochralski-grown from the B2O3-partially-covered melt

Germanium (Ge) single crystals with an extremely low density of grown-in dislocations were grown by the Czochralski (CZ) technique with boron oxide (B₂O₃) liquid. Because attachment of particles floating on the melt surface to a growing Ge crystal leads to generation of dislocations during the growth, partial covering of the Ge melt surface with B₂O₃ liquid was attempted. Such attachment of particles was drastically suppressed or the particles were caught by the introduction of B₂O₃ liquid, and a particle-free Ge melt was realized in the central region of the melt surface. Ge single crystals were successfully grown from such melt, the grown-in dislocation density being 0–1×10³ cm⁻², which was remarkably lower than that in Ge crystals grown by a conventional CZ technique. The contaminations by B and O atoms of the grown crystal detected by SIMS analysis were very low. These Ge crystals have the potential for application to be applied as high-quality, dislocation-free substrates of GaAs solar cells for various usages including in space.     

Growth of cadmium mercury thiocyanate single crystals using acetone–water mixed solvent and their characterization studies

Cadmium mercury tetrathiocyanate single crystals were grown from acetone–water (4:1) mixed solvent by slow evaporation solution technique. The structure of the grown crystal was confirmed by single-crystal X-ray diffraction analysis. The crystalline perfection of cadmium mercury thiocyanate (CMTC) was ascertained by high-resolution X-ray diffraction (HRXRD) analysis. The presence of functional groups and the coordination of thiocyanate ion in the CMTC compound were confirmed by laser Raman spectroscopy. The optical transparency of CMTC single crystal was studied by UV–vis spectroscopy. Single-shot surface laser damage threshold of CMTC estimated using an Nd-YAG laser of wavelength 1064 nm was found to be 4.59 GW/cm² and second harmonic generation (SHG) output power was estimated through the Kurtz powder method. Dielectric constant and dielectric loss for various frequencies and temperatures were performed on CMTC single crystal.     

Specific surface free energy and morphology of chlorapatite crystals studied by modified Wilhelmy method

Barium chlorapatite [Ba₅Cl(PO₄)₃] and strontium chlorapatite [Sr₅Cl(PO₄)₃] crystals were grown from a sodium chloride flux. The aspect ratios for these crystals were distributed in the range 1–9, and maximum number of crystals was observed in the range 3–4 for both chlorapatite crystals. The contact angle of water on (1 0 1? 0) face of each chlorapatite crystal was observed using a modified Wilhelmy method, where the change of liquid weight was measured instead of the crystal weight. The contact angle depends on the aspect ratio of the crystal. Both the advancing and the receding contact angles showed maximum value when the aspect ratios of the crystals were approximately 4 for both Ba₅Cl(PO₄)₃ and Sr₅Cl(PO₄)₃ crystals. The specific surface free energy of (1 0 1? 0) face was calculated using Neumann’s equation. The (1 0 1? 0) face of the crystals of the aspect ratio 4 has a minimum specific surface free energy, indicating that they have the most stable (1 0 1? 0) face when the aspect ratio is 4.     

Studies on the crystal growth,crystal structure,optical and thermal properties of an organic crystal: Benzophenone hydrazone

Single crystals of benzophenone hydrazone (BH) were successfully grown by slow evaporation method at constant temperature, 30 °C. The crystal structure of BH has been determined, and it belongs to the noncentrosymmetric space group P2₁. The grown crystal has been characterized by FT-IR). The optical transition and the lower cutoff wavelength of the BH have been identified by UV–vis–NIR studies. Thermo gravimetric analysis and differential thermal analysis have been carried out; the BH was found to be thermally stable up to 104 °C. Powder second harmonic generation (SHG) was investigated to explore its nonlinear optical (NLO) properties. The high SHG is due to one dimensional charge transfer between amine and phenyl group. Both the real ε_r and imaginary ε_i components of the dielectric constant have been calculated as functions of photon energy.     

Synthesis of GaN crystal by the reaction of Ga with Li3N in NH3 atmosphere

A novel method to synthesize GaN crystals was studied by the reaction of Ga with Li₃N under NH₃ atmosphere. We have already reported the synthesis technique of GaN by the reaction of Ga₂O₃ with Li₃N. However, the size of GaN crystals obtained by this method was limited to be smaller than several micrometers because of the solid phase reaction. In order to increase the size of GaN crystals, the method using liquid Ga as gallium source was studied for solid–liquid phase reaction. We found that the GaN crystals with the size of more than 100 μm were synthesized at 750 °C for 24 h under NH₃ atmosphere. We propose the possible reaction mechanism as follows. Lithium amide (LiNH₂) is synthesized by the reaction of Li₃N with NH₃ gas and then the crystal growth of GaN occurs by the reaction of Ga with LiNH₂. We found that LiNH₂ is a useful nitrogen source for the GaN synthesis method.     

Floating-zone growth of large high-quality CoAl2O4 single crystals

We report the first successful floating-zone growth of high-quality CoAl₂O₄ single crystals with volume up to 1 cm³ free from inclusions and sub-grains. The neutron rocking curves of the CoAl₂O₄ crystal have the width of about 0.30 degree proving the excellent quality of the grown samples. X-ray synchrotron experiments show that crystals have spinel structure with the lattice constant a₀=8.09853(1) Å. Magnetization measurements give the effective magnetic moment μ_eff=4.63 μ_B per Co⁺² ion in a good agreement with previous measurements on ceramic samples.