High-purity Crystals Research Articles

Ultrasound-assisted crystallization of high purity of 2,4-dinitrotoluene from spent acid

The high purity crystal of 2,4-dinitrotoluene (DNT) could be successfully recovered from spent acid in a short period through diluting method assisted with ultrasonic irradiation. The sonication tests were carried out to elucidate the influence of various ultrasonic powers on the performance of crystallization of 2,4-DNT. It is remarkable that under the supersaturated condition ultrasonic irradiation could significantly accelerate the crystal formation, in which the purity of 2,4-DNT reaches to approximate 98.9wt%. Based on the particle size distribution of crystals, it is proposed that the enhancement on recovery rate of 2,4-DNT crystals by ultrasound is mainly ascribed to the elevation of mass transfer rate for crystal growth. According to the spectra examined by field emission scanning electron microscope (FESEM), the above hypothesis is further verified by the appearance of smoother surface of crystals. Furthermore, the results of metallurgical microscope measurements show that ultrasound has also an abrasive effect on 2,4-DNT crystals under high ultrasonic power inputs.

High Mobility Organic Single-Crystal Transistors

We report a series of our experiments using organic single crystals to reach the maximum performance intrinsic to the materials. A consequence of the experiments is that a good prescription for realizing high-mobility devices is to induce carriers in inner crystals to avoid scattering at the surfaces. Intrinsic-semiconductor character of the high-purity organic crystals favors thermal diffusion of the carriers into the crystals in the presence of weak gate-electric fields. [DOI: 10.1380/ejssnt.2008.21]

In-crystal Carrier Transport in Organic Single Crystal Transistors

AbstractWe report a series of our experiments using organic single crystals to reach the maximum performance intrinsic to the materials. A consequence of the experiments is that a prescription for realizing high-mobility devices is to induce carriers in inner crystals to avoid scattering at the surfaces. Intrinsic-semiconductor character of the high-purity organic crystals favors thermal diffusion of the carriers into the crystals in the presence of weak gate-electric fields. Furthermore, it is demonstrated that the high-mobility transport of the in-crystal carriers are highlighted in double-gate single-crystal transistors with the two gate electric field balanced with each other.

Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure

Materials emitting light in the deep ultraviolet region around 200 nanometers are essential in a wide-range of applications, such as information storage technology, environmental protection, and medical treatment. Hexagonal boron nitride (hBN), which was recently found to be a promising deep ultraviolet light emitter, has traditionally been synthesized under high pressure and at high temperature. We successfully synthesized high-purity hBN crystals at atmospheric pressure by using a nickel-molybdenum solvent. The obtained hBN crystals emitted intense 215-nanometer luminescence at room temperature. This study demonstrates an easier way to grow high-quality hBN crystals, through their liquid-phase deposition on a substrate at atmospheric pressure.

In-Crystal and Surface Charge Transport of Electric-Field-Induced Carriers in Organic Single-Crystal Semiconductors

Gate-voltage dependence of carrier mobility is measured in high-performance field-effect transistors of rubrene single crystals by simultaneous detection of the longitudinal conductivity sigma(square) and Hall coefficient R(H). The Hall mobility mu(H) (identical with sigma(square)R(H)) reaches nearly 10 cm(2)/V s when relatively low-density carriers (<10(11) cm(-2)) distribute into the crystal. mu(H) rapidly decreases with higher-density carriers as they are essentially confined to the surface and are subjected to randomness of the amorphous gate insulators. The mechanism to realize high carrier mobility in the organic transistor devices involves intrinsic-semiconductor character of the high-purity organic crystals and diffusive bandlike carrier transport in the bulk.

Synthesis of (Hg,Re)Ba 2Ca n−1 Cu nO 2+2 n+ δ ( n = 2, 3, 4) single crystals and their magnetization properties

Single crystals of (Hg,Re)Ba 2Ca n−1 Cu n O 2+2 n+ δ ( n = 2, 3, 4) with size up to 1 × 1.1 × 0.1 mm 3 were grown by the flux method in quartz ampoules from flux compositions with excess Ba and Cu. In order to synthesize high purity crystals without contamination from crucible materials, the BaZrO 3 crucibles which are stable against molten Ba–Cu–O, were used. The obtained single crystals were of nearly carrier optimally doped state with T c on = 123 , 131 and 125 K for crystals with n = 2, 3, 4, respectively. Broad second peaks were observed in their magnetization hysteresis loops in a wide temperature range, and therefore, the existence of effective field-induced pinning sites probably due to inhomogeneous distribution of rhenium was indicated. The behaviors of irreversibility lines as a function of reduced temperatures by T c, T/ T c, of these crystals were almost identical independent of n and their electromagnetic anisotropy parameter γ 2 ( = m c ∗ / m ab ∗ ) were estimated to be 500–700.

The effects of iron as a dopant on the dielectric properties of ferroelectric potassium tantalate niobate (KTaxNb1−xO3)

Presented here is a preliminary study on the effects of iron as a dopant in potassium tantalate niobate (KTaxNb1−xO3) (KTN). Crystals of high purity were grown using a top-seeded isothermal method with starting materials that were purified ensuring that the impurity content was controlled down to less than 10ppm. The study included introducing iron to KTN single crystals of high purity both during crystal growth and through thermal diffusion after growth. Dielectric measurements versus temperature were used to reveal the origins of the ferroelectric phase transition in the KTN:Fe system. Large shifts in the Curie temperature have been shown to be a direct consequence of the presence of iron impurities in KTN. The conventional theories of ferroelectricity do not explain this effect; however, a model is proposed to explain the observed behavior. The model is based on the concept of “polar microregions” proposed previously by Bidault and Maglione [J. Phys. I France 7, 543 (1997)] and others to explain ferroelectricity in potassium tantalate lightly doped with niobium.

Rapid densification of nanocrystalline hydroxyapatite for biomedical applications

The effect of rapid sintering by microwave heating on the properties of nanocrystalline hydroxyapatite (HA) bioceramic powder was studied. The starting high-purity nano HA crystals was synthesized using a wet chemical precipitation technique. The HA compacts were microwave-sintered over the temperature range 1000–1300 °C, using a rapid sintering schedule of 30 min for each temperature. Shrinkage of all compacts was uniform and the sintered material did not exhibit any cracks. XRD phase analysis indicated that the HA phase stability was not disrupted throughout the sintering regime employed. A relative density of 90–91% was attained for samples sintered at 1000 °C, whereas a higher relative density of above 96% and ∼98% was measured for HA sintered at 1100 °C and 1300 °C, respectively. The present work revealed that microwave heating was beneficial in preventing grain growth, particularly when sintered at temperatures <1200 °C, and coupled with improved mechanical properties, i.e., high fracture toughness of 1.45 MPa m 1/2 and hardness of 6.38 GPa were measured for HA compacts sintered at 1050 °C and 1150 °C, respectively.

Influence of the thermoelastic effect on the acoustic properties of pure metals at low temperatures

Dedicated to Professor Dr. Gernot Kostorz on the occasion of his 65th birthday The influence of the thermoelastic effect on the sound propagation in metals is discussed. A strict description of the effect based on a combined solution of the equations of elasticity and heat conductivity is presented. It is shown that acoustic relaxation resonance caused by the thermoelastic effect can be observed when metal single crystals of high purity are studied by pulse-echo spectroscopy at about 10 K.

Residual impurities and native defects in 6H-SiC bulk crystals grown by halide chemical-vapor deposition

A variety of defect-sensitive techniques have been employed to detect, identify, and quantify the residual impurities and native defects in high-purity (undoped) 6H-SiC crystals grown by halide chemical-vapor deposition technique. The incorporation efficiencies of N and B are determined by the site-competition effect. Most notably, material with low residual N levels (∼1014cm−3) can be produced. In addition, the nitrogen concentrations obtained from Hall-effect measurements and low-temperature photoluminescence are systematically lower than those determined from secondary-ion-mass spectrometry. The difference is ascribed to nitrogen forming complexes with native defects. The energy level of this complex is approximately 0.27eV below the conduction band. Four major electron traps with activation energies of 0.4, 0.5, 0.65, and 1eV and five hole traps with activation energies of 0.3, 0.4, 0.55, 0.65, and 0.85eV were observed by deep-level transient spectroscopy. The concentration of all traps decreased strongly with increasing C∕Si ratio during growth. Increasing the C∕Si flow ratio also led to increased resistivity of the crystals and change of conductivity from conductive n type to high-resistivity p type. The Fermi level in p-type material is pinned either to highly compensated shallow B acceptors or to deep B-related center at 0.6eV above the valence band. Electron paramagnetic resonance shows the presence of positively charged carbon vacancies in such high-resistivity material.

HPT-Deformation of Copper and Nickel Single Crystals

Copper and nickel single crystals of high purity with a crystallographic orientation, (001) and (111) respectively, were deformed by applying high pressure torsion (HPT) at room temperature. Special interest was devoted to the structural evolution of the material, which was characterized by electron backscatter diffraction (EBSD) and X-ray texture analysis as well. In addition back scatter electron investigations were applied to characterize shape and size of the new formed structure. Furthermore the study is focused on the micro structural and micro textural evolution that lead to the increase of missorientation angle with increasing plastic deformation. We observed an increasing fragmentation of the structure with increasing plastic equivalent strain up to a level where the grain size is saturated. The saturation could be traced back to dynamical recovery and recrystallisation during the deformation process that is depending on the purity of the material.

Fluxon pinning in the nodeless pairing state of superconducting YBa2Cu3O7

Positive-muon spin rotation (μ+SR) spectroscopy and magnetic moment measurements were used to probe fluxon (or vortex) formation in the superconducting mixed state of a high-purity YBa2Cu3O7 crystal. Random potentials caused by crystal-lattice defects pin fluxons. A fluxon lattice forms in an external magnetic field, and changes of thermal activation lead to fluxon pinning and depinning. The root second moment of the local magnetic field distribution (σ) determined by μ+SR contains information on the magnetic penetration depth and the pinning. Fluxon pinning leads to temperature-dependent transverse displacements of the fluxons that decrease σ and also fluctuations in the separation between fluxons that tend to increase σ. By accounting for the field-dependent and temperature-activated fluxon disorder, it is found that the experimental results for the penetration depth are consistent with a supercon-ducting order parameter of a strong-coupling two-fluid model, confirming that the superconductivity is nodeless with s-wave superconducting pairing. Quantitative results for fluxon displacements are discussed within the context of the fluxon field-temperature phase diagram.

Advances in Silicon Carbide Electronics

AbstractAfter substantial investment in research and development over the last decade, silicon carbide materials and devices are coming of age. The concerted efforts that made this possible have resulted in breakthroughs in our understanding of materials issues such as compensation mechanisms in high-purity crystals, dislocation properties, and the formation of SiC/SiO2 interfaces, as well as device design and processing. The progress accomplished over the last eight years in SiC-based electronic materials is summarized in this issue of MRS Bulletin.

Optimal recovery of high-purity rutin crystals from the whole plant of Fagopyrum esculentum Moench (buckwheat) by extraction, fractionation, and recrystallization

Rutin, one of the flavonoids derived from plants, is increasingly in demand in the food, pharmaceutical, and cosmetic industries due to its various biological and physiological activities including antioxidation, anti-inflammation, and anti-hypertension. The whole plant of buckwheat ( Fagopyrum esculentum Moench) is a major source of natural rutin. This study developed a low-cost process encompassing the efficient extraction, fractionation, and recrystallization to obtain high-purity rutin from buckwheat, and it could improve the economic utilization of this abundant low-value agricultural product. The sequential separation and purification procedures established in this study involved extraction with 50% (v/v) aqueous ethanol at 80 °C for 1 h followed by elution with water and aqueous ethanols at 20% and 30% (v/v) on a styrene-based resin column, and recrystallization at 4 °C for 12 h. These conditions resulted in the recovery of 92% of total rutin with over 95% purity. In the present study, high-purity rutin was obtained from whole buckwheat through low-cost processes, the separation and purification strategy established in this study could provide valuable information to the relevant industries.

Super-hard diamond indenter prepared from high-purity synthetic diamond crystal

The Knoop hardness in (001)⟨110⟩ of high-purity synthetic type IIa diamond crystal synthesized under high pressure and high temperature is known to be extremely high. Knoop indenters were prepared from the synthetic type IIa diamonds by taking the tip orientation to the hard crystal orientation of the diamonds, i.e., the z axis and the longitudinal direction of the indenter tip are parallel to the ⟨001⟩ and ⟨110⟩ direction of the crystal, respectively. Numerous indentation tests on diamond crystals revealed that the indenter is substantially rigid and has very high performance. The lifetime of the indenter was more than ten times longer than that of the conventional natural diamond indenter.

High-quality CdS nanoribbons with lasing cavity

High-density and high-quality CdS nanoribbons were synthesized by a thermal evaporation process. The nanoribbons were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy. CdS nanoribbons were found to be single crystals of high phase purity and low defect density. Lasing was observed in the CdS nanoribbons upon optical pumping. The growth of CdS nanoribbons was explained by the vapor–liquid–solid mechanism.

A novel process for enzymatic synthesis of N-lauroyl-β-amino propionitrile using packed bed reactor coupled with on-line separation

N-Lauroyl-β-amino propionitrile is an intermediate for synthesis of sodium N-lauroyl-β-alanine, an antimicrobial surfactant. We provide a novel process for enzymatic synthesis of N-lauroyl-β-amino propionitrile, using a cascade connection of an enzyme packed bed reactor (EPBR) with a crystallization separator for on-line separation. The substrate solution was fed to the reactor inlet. High-purity crystal product was obtained from the separator outlet with a yield of 91.7% under the optimum conditions. The immobilized lipase can be utilized repeatedly. The solvent and unreacted substrates were recovered and reused on-line.

Ab initio theory of charge-carrier conduction in ultrapure organic crystals

We present an ab initio description of charge-carrier mobilities in organic molecular crystals of high purity. Our approach is based on Holstein’s original concept of small-polaron bands but generalized with respect to the inclusion of nonlocal electron–phonon coupling. By means of an explicit expression for the mobilities as a function of temperature in combination with ab initio calculations of the material parameters, we demonstrate the predictive power of our theory by applying it to naphthalene. The results show a good qualitative agreement with experiment and provide insight into the difference between electron and hole mobilities as well as their peculiar algebraic and anisotropic temperature dependencies.

VUV spectroscopy of pure LiCaAlF6 crystals

Reflection, excitation and luminescence spectra of as-grown and X-ray irradiated high-purity LiCaAlF6 crystals were studied in the temperature range of 10–300K using synchrotron radiation in VUV. The intrinsic luminescence of samples at 10K consists of a non-elementary broad band with maximum at 4.4eV under excitation at 11.45eV. It is ascribed to the radiative decay of self-trapped excitons. The energy gap is estimated to be 12.65eV in LiCaAlF6. Under interband excitation a red shift of luminescence was observed. The electron–hole recombination leads to the emission peaking at 3.7eV. The excitation processes and origin of overlapping emissions of LiCaAlF6 are discussed.

Evaluation of the calibration errors on cooling of a differential scanning calorimeter using different sets of standard metals

The temperature calibration on cooling of thermal analysis instruments is important for the accurate study of the non-isothermal crystallization kinetics of semi-crystalline polymers. In previous works, a methodology was proposed for performing the calibration on cooling of differential scanning calorimeters (DSCs) with standard metals, and the calibration errors were checked using transitions of high-purity liquid crystals. In this work, alternative, physically meaningful, procedures for carrying out the calibration on cooling are analyzed and validated. The calibration errors are evaluated also with liquid crystalline transitions, when the calibration is performed with standard metals, in a wide temperature range, and when due account is taken for the need of isothermal corrections to the temperature measurements. It is shown that any pair of standard metals may be used to calibrate on cooling, that the calibration errors increase for wider working temperature ranges and that, providing that certain conditions are fulfilled, both calibration procedures yield similar results.