Natural Single Crystals Research Articles

Structural examinations of synthetic and natural single crystals of mullite

Structural examinations of synthetic and natural single crystals of mullite

Synthesis of In<sub>2</sub>O<sub>3</sub>/BiOCl Composite Photocatalyst and its Photocatalytic Activity for the Degradation of Rhodamine B under Visible Light Irradiation

We report on the synthesis and photocatalytic activity of 10 wt% In2O3/BiOCl composite photocatalyst. The 10 wt% In2O3/BiOCl composite photocatalyst was synthesized by a simple chemical coprecipitation method at room temperature followed by thermal treatment at 400 °C for 2 h. During thermal treatment at 400 °C for 2 h, the amorphous-to-crystalline phase transformation occurs in In2O3. Pure BiOCl possesses flower-like nanostructures composed of nanoplates with single-crystal nature. With the incorporation of 10 wt% In2O3, the morphology of the composite photocatalyst is mainly constituted of particles with two different forms: nanoflowers of BiOCl and nanoparticles of In2O3. The effect of 10 wt% In2O3 on the photocatalytic activity of the composite photocatalyst was evaluated for the degradation of Rhodamine B (RhB) under visible light irradiation. The 10 wt% In2O3/BiOCl composite photocatalyst shows enhanced photocatalytic activity compared to pure BiOCl. Based on the results obtained in this study, the mechanism for the enhanced photocatalytic activity of 10 wt% In2O3/BiOCl composite photocatalyst is discussed.

Crystalline–Crystalline Phase Transformation in Two-Dimensional In2Se3 Thin Layers

We report, for the first time, the fabrication of single-crystal In2Se3 thin layers using mechanical exfoliation and studies of crystalline-crystalline (α → β) phase transformations as well as the corresponding changes of the electrical properties in these thin layers. Particularly, using electron microscopy and correlative in situ micro-Raman and electrical measurements, we show that, in contrast to bulk single crystals, the β phase can persist in single-crystal thin layers at room temperature (RT). The single-crystal nature of the layers before and after the phase transition allows for unambiguous determination of changes in the electrical resistivity. Specifically, the β phase has an electrical resistivity about 1-2 orders of magnitude lower than the α phase. Furthermore, we find that the temperature of the α → β phase transformation increases by as much as 130 K with the layer thickness decreasing from ~87 nm to ~4 nm. These single-crystal thin layers are ideal for studying the scaling behavior of the phase transformations and associated changes of the electrical properties. For these In2Se3 thin layers, the accessibility of the β phase at RT, with distinct electrical properties than the α phase, provides the basis for multilevel phase-change memories in a single material system.

Retraction of Nanoporous PtCo Surface Alloy Architecture with Enhanced Properties for Methanol Electrooxidation

By selectively dealloying a PtCoAl ternary alloy, a novel nanoporous PtCo (np-PtCo) alloy with a three-dimensional bicontinuous pore-ligament structure is successfully fabricated. X-ray diffraction and electron microscopic characterizations demonstrate the single-crystal nature of the alloy ligament with a ligament size down to ∼3 nm. After a mild electrochemical dealloying process, a nanoporous near-surface alloy structure with a Pt-rich surface and a PtCo alloy core is obtained. Electrochemical measurements show that the np-PtCo surface alloy has greatly enhanced catalytic activity and durability toward methanol electrooxidation compared with a state-of-the-art Pt/C catalyst. The peak current density of methanol electrooxidation on a np-PtCo surface alloy is more than 5 times of that on Pt/C. More importantly, continuous potential cycling from 0.6 to 0.9 V (vs RHE) in a 0.5 M H2SO4 aqueous solution demonstrates that a np-PtCo surface alloy has excellent structure stability, with more than 90% of the ini...

Crystal Inclusions in Diamonds and Other Crystals

Single crystal inclusions in natural single crystals bear a lot of information on the genesis and phase stabilities between host and guest crystals. Not only parameters of the natural environment of crystallization may be elucidated, also the history of up-rise within the earth to the surface and the development during procedures, eventually applied by mankind to enhance the physical properties of crystals, may be detectable. Structural reactions resulting in phase separation due to chemical stress during change of physical parameters shed light on the borders of solid solution and exsolution domains. Natural crystals of proper quality allow us to evaluate a time factor in phase stabilities.Crystal structures are able to react by epitaxial growth, by chemical induced twinning, by miscibility reactions, exsolution, and by solid solutions. Some of the phenomena we are meanwhile able to detect by means of (sub-) micro analytical methods. But many problems are left undiscovered between chemical analysis and phase detection due to the local behavior of atoms or ions within a given architecture, which locally provides stable and/or stabilizing crystallization.

Submodeling Simulations in Fusion Welds: Part II

In part I, three-dimensional transient non-linear sub modeling heat transfer simulations were performed to study the thermal histories and thermal cycles that occur during the welding process at the macro, meso and micro scales. In the present work, the corresponding non-uniform temperature changes were imposed as load conditions on structural calculations to study the evolution of localized plastic strains and residual stresses at these sub-level scales. To reach the goal, a three-dimensional finite element elastic-plastic model (ABAQUS code) was developed. The sub-modeling technique proposed to be used in coupling phase-field (and/or digital microstructures) codes with finite element codes, was used to mesh a local part of the model with a refined mesh based on interpolation of the solution from an initial, relatively coarse, macro global model. The meso-sub-model is the global model for the subsequent micro sub-model. The strategy used to calculate temperatures, strains and residual stresses at the macro, meso and micro scale level, is very flexible to be used to any number of levels. The objective of this research was to initiate the development of microstructural models to identify fusion welding process parameters for preserving the single crystal nature of gas turbine blades during repair procedures. The multi-scale submodeling approach can be used to capture weld pool features at the macro-meso scale level, and micro residual stress and secondary dendrite arm spacing features at the micro scale level.

WO3 nanoplates, hierarchical flower-like assemblies and their photocatalytic properties

Monoclinic WO3 was prepared by a hydrothermal reaction of PbWO4 in the presence of HNO3. WO3 rectangular nanoplates with a side length of 50–150nm and a thickness of about 25nm were obtained at 4M HNO3 solution. And the single crystal nature was confirmed by the selected area electron diffraction. Whereas WO3 hierarchical flower-like assemblies with 3–5μm in diameter were self-organized by nanoplates in the presence of 15M HNO3 solution. Compared with commercial WO3 particles, our products showed an enhancement of photocatalytic properties for the degradation of rhodamine B under visible light irradiation.

Investigations on influence of coloration on growth of high quality hydrazonium l-tartrate single crystal

Bulk single crystals of undeuterated Hydrazonium l-tartrate (HLT) an organic crystal have been grown by the isothermal solvent evaporation method. The occurrence of coloration of HLT solution which is reported to have hindered the growth of HLT crystals under normal ambient conditions has been deferred to a considerable duration by modifying the experimental conditions to facilitate the crystal growth. Studies such as powder X-ray diffraction (XRD) and Carbon, Hydrogen and Nitrogen (CHN) analysis confirmed that the crystals grown using modified growth conditions are indeed HLT. The powder second harmonic generation (SHG) efficiency of the HLT analyzed using Nd:YAG laser was about 1.5 times that of potassium dihydrogen phosphate (KDP), which is higher than the reported value. Dielectric tensor measurements exhibited the anisotropic nature of HLT single crystal. Optical transmission studies performed in the range 190–1100nm revealed that the undeuterated HLT crystal could be suitable for optical applications.

Solvent-Induced Structural Dynamics in Noninterpenetrating Porous Coordination Polymeric Networks

Three novel soft porous coordination polymer (PCP) or metal-organic framework (MOF) compounds have been synthesized with a new rigid ligand N-(4-pyridyl)-1,4,5,8-naphathalenetetracarboxymonoimide (PNMI) by partial hydrolysis of N,N'-di-(4-pyridyl)-1,4,5,8-naphthalenete-tracarboxydiimide (DPNI) during solvothermal reactions with Zn(II), Cd(II), and Mn(II) salts, and they are [Zn(PNMI)]·2DMA (1·2DMA, 1a), [Cd(PNMI)]·0.5DMA·5H2O (2·0.5DMA·5H2O), and [Mn(PNMI)]·0.75DMF (3·0.75DMF). The structure of 1 is based on paddle-wheel secondary building unit (SBU) with a 3,6-connected rtl net topology, whereas 2 and 3 are isotypical but the M(O2C-C)2 fragments aggregate in one-dimension and the overall connectivity is the same rtl net topology. All these three MOFs have one-dimensional rhombic channels filled with guest molecules. The guest molecules in 1a can be exchanged with EtOH in a single-crystal to single-crystal (SCSC) manner to 1·1.25EtOH·0.375H2O (1b). Further, the guest molecules in 1b can be replaced with ethylene glycol, triethylene glycol and allyl alcohol without destroying its single crystal nature. These guest exchanges are accompanied by reduction in volume of the unit cell up to 16%, as well as the void volume up to 33.1%. Similarly, triethylene glycol (TEGly) selectively exchanges EtOH in a mixture of the above solvents, which might be the result of correct fit of the hydrogen-bonded TEGly dimer in the channel of 1. While activated 1 and 3 exhibit no uptake of N2 and H2 at 1 bar and 77 K and very low uptake of CO2 gas at 1 bar and 196 K, activated 2 shows selective CO2 uptake, 278 cm(2)·g(-1), over N2 and H2 at 1 bar and 196 K, which corresponds to 5.87 molecules of CO2 per formula unit of 2.

Microwave-assisted chemical synthesis of copper nanotadpoles

Novel and uniform one-dimensional (1D) tadpole-shaped Cu nanostructures with single-crystal nature have been successfully fabricated on a large scale by using a convenient microwave-assisted chemical reductive process in the absence of any surfactants. The as-synthesized products are characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDX), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electronic diffraction (SAED) and UV–visible spectroscopy. FESEM and TEM images indicate that the prepared Cu nanotadpoles have hexagonal plate-like heads with edge size of 600–800 nm and 4–6 μm long wire-like tails with the middle part diameters of 100–200 nm. The purity and morphology of Cu crystals are greatly affected by the concentration of hexamethylenetetramine and ascorbic acid, which are realized by carefully controlling the experimental conditions. The possible mechanism for the formation of tadpole-shaped Cu nanostructures is discussed.

Controlled Preparation and Surface Structure Characterization of Carbon-Coated Lithium Iron Phosphate and Electrochemical Studies as Cathode Materials for Lithium Ion Battery

Amo rphous carbon-coated lithiu m iron phosphate (C-LiFePO4) particles have been mass synthesized at the commercial scale by a controlled solid-state reaction method. Particles morphologies , o livine-type phase structures and the carbon coating features were investigated in details by various techniques as X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) imaging, and transmission electron microscopy (TEM, HRTEM ) imag ing, selected-area electron diffraction (SAED), X-ray energy dispersive spectroscopy microanalysis (XED S), and X-ray photoelectron spectroscopy (XPS). Single-crystal nature of the olivine type LiFePO4 structures was revealed by XRD and SA ED analyses. TEM imaging showed rough nanoparticles spherical features with average size range of 50-200 n m. Ho mogenous coating features of carbon layers on the particles surface and olivine-LiFePO4 phase are clearly observed in HR-TEM imaging and confirmed by the corresponding SAED pattern. Elemental b inding energy fro m XPS analysis also confirmed that an amo rphous sp2 carbon coating layer and olivine type LiFePO4 structures. It is indicated that the characteristics of sp2 type conducting-coating layer on the particles surfaces gave rise to improved electrical conductivity by reducing the diffusion path of the electron and lithiu m ions, as directly evidenced fro m our charge-discharge cycling testing as the cathode in the Lithiu m ion battery cell.

Single-crystal β-MnO2 hollow bipyramids: synthesis and application in lithium ion batteries

Single-crystal β-MnO2 hollow bipyramids (HB-β-MnO2) were synthesized via a template-free hydrothermal method. The as-synthesized hollow bipyramids with 100–300 nm pores along the axis direction of the β-MnO2 bipyramids are formed through self-assembly and phase transformation processes from α-MnO2 nanowires to β-MnO2 bipyramids followed by chemical etching of their metastable crystal faces. Compared to the commercial bulk β-MnO2 (c-β-MnO2), the as-synthesized HB-β-MnO2 exhibits a better electrochemical performance with an initial discharge capacity of 269 mA h g−1, which equates to up to 0.87 Li+ intercalation per β-MnO2 unit, while only 0.2 Li+ intercalation per β-MnO2 unit is observed for the commercial c-β-MnO2. The excellent electrochemical activity of the as-synthesized HB-β-MnO2 can be attributed to its hollow structure and single crystal nature. The former can provide higher contact area with the electrolyte and is able to act as a buffer against volume change during the charge/discharge processes, while the latter contributes to good electronic conductivity and stable structural integrity. Moreover, the initial HB-β-MnO2 bipyramids undergo an irreversible phase transformation to orthorhombic LixMnO2 after the first discharging process, which shows good structural stability and capacity (up to 150 mA h g−1) after 50 charge/discharge cycles.

Millimeter-sized single crystalline ZnO networks composed of nanostructures

Feather-like ZnO single-crystalline networks are synthesized via a facile one-step catalyst-free chemical vapor deposition method. Their axial size extends to millimeter-scale along their backbone while their construction unit is submicron, naturally constructing a bridge between microcosm and macrocosm. The whole feather-like network is an interconnection of a single crystal ZnO without any phase interface or crystal boundary in the junctions. The millimeter-scale ZnO networks have single-crystal nature, parallel connectivity, plenty of holes and large specific surface area, with which potential applications for carrier transporters, filters, sensors, wave-absorbing materials and interconnected computing units are expected.

Holistic approach to dissolution kinetics: linking direction-specific microscopic fluxes, local mass transport effects and global macroscopic rates from gypsum etch pit analysis

Dissolution processes at single crystal surfaces often involve the initial formation and expansion of localized, characteristic (faceted) etch-pits at defects, in an otherwise comparatively unreactive surface. Using natural gypsum single crystal as an example, a simple but powerful morphological analysis of these characteristic etch pit features is proposed that allows important questions concerning dissolution kinetics to be addressed. Significantly, quantitative mass transport associated with reactive microscale interfaces in quiescent solution (well known in the field of electrochemistry at ultramicroelectrodes) allows the relative importance of diffusion compared to surface kinetics to be assessed. Furthermore, because such mass transport rates are high, much faster surface kinetics can be determined than with existing dissolution methods. For the case of gypsum, surface processes are found to dominate the kinetics at early stages of the dissolution process (small etch pits) on the cleaved (010) surface. However, the contribution from mass transport becomes more important with time due to the increased area of the reactive zones and associated decrease in mass transport rate. Significantly, spatial heterogeneities in both surface kinetics and mass transport effects are identified, and the morphology of the characteristic etch features reveal direction-dependent dissolution kinetics that can be quantified. Effective dissolution velocities normal to the main basal (010) face are determined, along with velocities for the movement of [001] and [100] oriented steps. Inert electrolyte enhances dissolution velocities in all directions (salting in), but a striking new observation is that the effect is direction-dependent. Studies of common ion effects reveal that Ca(2+) has a much greater impact in reducing dissolution rates compared to SO(4)(2-). With this approach, the new microscopic observations can be further analysed to obtain macroscopic dissolution rates, which are found to be wholly consistent with previous bulk measurements. The studies are thus important in bridging the gap between microscopic phenomena and macroscopic measurements.

SUBMODELING SIMULATIONS IN FUSION WELDS

The technological properties of fusion welds are formed due to the simultaneous effects of different physical phenomena which occur on different length scales. Correspondingly, the simulation of technological properties of fusion welds requires the multiscale approach. By performing three-dimensional transient nonlinear sub-modeling heat transfer simulations, thermal histories and thermal cycles that occur during the welding process were numerically investigated at the meso and microscales. The sub-modeling technique (a separate analysis that can be used to any number of levels) proposed to be used in coupling phase-field codes with finite element codes, was used to mesh a local part of the model with a refined mesh based on interpolation of the solution from an initial, relatively coarse, macro global model. The meso-sub-model is the global model for the subsequent micro sub-model. Microstructure evolution using the phase field method can be easily predicted at these sub-level scales by using the calculated numerical thermal results. The objective of this research was to initiate the development of microstructural models (ABAQUS code) to identify fusion welding process parameters for preserving the single crystal nature of gas turbine blades during repair procedures. The multiscale submodeling approach can be used to capture weld pool features at the macro-meso scale level, and secondary dendrite arm spacing features at the microscale level. The strategy is very flexible to be used to any number of levels.

Bow Free 4'' Diameter 3C-SiC Epilayers Formed upon Wafer-Bonded Si/SiC Substrates

The physical and electrical properties of 4-inch 3C-SiC epitaxial layers, deposited on Si (111) wafers, which were wafer bonded to polycrystalline silicon carbide carrier wafers, are presented. We show that this novel Si/SiC wafer bonding process leads to reduced wafer bow, confirmed by imaging, in the form of optical microscopy (×100 objective lens) together with a digital camera. All 3C-SiC layers grown above Si/SiC structures by conventional chemical vapor deposition techniques are shown to be single crystal in nature. 3C-SiC metal oxide semiconductor capacitors have been fabricated via thermal oxidation at 1100°C in pure oxygen for 90 minutes, with a density of interface states measured at ∼2 × 1011 cm−2 eV−1 at 0.2 eV beneath the conduction band edge. These structures have the potential to realize thick, bow-free 3C-SiC layers suitable for power device fabrication.

Almandine: Lattice and non-lattice heat capacity behavior and standard thermodynamic properties

The heat capacity of three synthetic polycrystalline almandine garnets (ideal formula Fe3Al2Si3O12) and one natural almandine-rich single crystal was measured. The samples were characterized by optical microscopy, electron microprobe analysis, X-ray powder diffraction, and Mossbauer spectroscopy. Measurements were performed in the temperature range 3 to 300 K using relaxation calorimetry and between 282 and 764 K using DSC methods. All garnets show a prominent λ-type heat-capacity anomaly at low temperatures resulting from a paramagnetic-antiferromagnetic phase transition. For two Fe3+-free or nearly Fe3+-free synthetic almandines, the phase transition is sharp and occurs at 9.2 K. Almandine samples that have ~3% Fe3+ show a λ-type peak that is less sharp and that occurs at 8.0 ± 0.2 K. The low- T C P data were adjusted slightly using the DSC results to improve the experimental accuracy. Integration of the low- T C P data yields calorimetric standard entropy, S∘ , values between 336.7 ± 0.8 and 337.8 ± 0.8 J/(mol·K). The smaller value is recommended as the best S∘ for end-member stoichiometric almandine, because it derives from the “best” Fe3+-free synthetic sample. The lattice (vibrational) heat capacity of almandine was calculated using the single-parameter phonon dispersion model of Komada and Westrum (1997), which allows the non-lattice heat capacity ( C ex) behavior to be modeled. An analysis shows the presence of an electronic heat-capacity contribution ( C el, Schottky anomaly) superimposed on a larger magnetic heat-capacity effect ( C mag) around 17 K. The calculated lattice entropy at 298.15 K is S vib = 303.3 J/(mol·K) and it contributes about 90% to the total standard entropy at 298 K. The non-lattice entropy is S ex = 33.4 J/(mol·K) and consists of S mag = 32.1 J/(mol·K) and S el = 1.3 J/(mol·K) contributions. The C P behavior for almandine above 298 K is given by the polynomial [in J/(mol·K)]: C P = 649.06 ( ± 4 ) - 3837.57 ( ± 122 ) · T - 0.5 - 1.44682 ( ± 0.06 ) · 10 7 · T - 2 + 1.94834 ( ± 0.09 ) · 10 9 · T - 3 which is calculated using the measured DSC data together with one published heat-content datum determined by transposed-drop calorimetry along with a new determination in this work that gives H 1181K − H 302K = 415.0 ± 3.2 kJ/mol. Using our S∘ value and the C P polynomial for almandine, we derived the enthalpy of formation, Δ H °f, from an analysis of experimental phase equilibrium results on the reactions almandine + 3rutile = 3ilmenite + sillimanite + 2quartz and 2ilmenite = 2Fe + 2rutile + O2. A Δ H °f = −5269.63 kJ/mol was obtained.

Pseudodielectric function and critical-point energies of iron pyrite

Spectroscopic ellipsometry was used to determine the pseudodielectric function � e �=� e1 �+ i� e2� spectrum of a natural single crystal of iron pyrite (cubic FeS2) from 0.5 to 4.5 eV with the sample at 77 K. The � e� spectrum exhibits several pronounced optical features associated with the interband critical points (CPs). Accurate CP energies are obtained by fitting standard line shapes to second energy derivatives of the � e� data. The electronic origins of the six observed CP features are identified through density functional theory calculations and momentum matrix analyses.

One-step solution synthesis of urchin-like ZnO superstructures from ZnO rods

The urchin-like ZnO superstructures have been directly prepared by the assistance of poly (acrylic acid) (PAA, Mw 5000) under a one-step solution-based process. X-ray diffraction (XRD) patterns indicate that the crystal structure of the special ZnO urchins is hexagonal. The results of Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) tests show that the urchins are composed of rods and the average aspect ratio of them is about 10 with a length of about 1.5μm. Selected area electron diffraction (SAED) pattern reveals that the rods are single crystal in nature, which preferentially grow up along the 〈0001〉 direction. Furthermore, the sizes and aspect ratios of the rods can be easily controlled by regulating the concentration of ZnSO4 solution. It is believed that the process of crystallization, including nucleation and crystal growth, happens along PAA chains resulting in the production of rods and assembly of them into superstructures.

Structural, Thermal and Magnetic Characterization of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

Ni-Mn-Ga Ferromagnetic Shape Memory Alloys (FSMA) are highly interested in industrial actuators and energy harvesters because of its uniqueness in microstructure and multiple phases of these materials at different temperatures. The type of crystal structure, lattice parameters and magnetic properties depend on the alloy composition. The transformation and crystallographic properties are usually mapped to average number of valence electrons per atom. The present work deals with the fundamental characterization of single crystal and polycrystalline nature of these alloys. Though, the polycrystal shows low magnetic field induced stain (MFIS), the easy preparation techniques and cost effective aspects compared with the single crystals makes an impressive interest in dealing with that. The structural, thermal and magnetic parameters are observed and compared for these single and polycrystals.