Non-uniform Crystal Research Articles

Controllably Constructing morphology and structure of potassium chloride crystal by green supersaturation modulation

Controllable and green construction of morphology and structure in potassium chloride (KCl) crystals is necessary in the fields of pharmaceuticals, catalysis, minerals, and jewels design. Here, the work presented a lucid way for preparing KCl crystal with various morphology including hopper-, sphere- and hollow cube-like structure through green supersaturation modulation. Parameters including stirring speed, cooling range, cooling rate, and excess inorganic composing ions influenced the growth kinetics of hopper KCl crystal as well as the surficial morphology and structure: (1) Metastable region and stable conditions were beneficial for uniform growth of crystal; (2) Small size of crystal particles were obtained with high shear force; (3) Corner angle of crystal would be abraded in slow cooling rate; (4) Non-uniform crystal appeared in improper cooling range condition; (5) Additional composing ions might introduce deformed structure according to the basic crystal repeating unit with corresponding elements. Based on the above study of growth kinetics, hopper-, sphere- and hollow cube-like crystal were obtained. The morphology and structure properties of various crystal were researched by three-dimensional visualization, X-ray diffraction (XRD), and scanning electron microscope (SEM) for optimized application such as higher exposing surface area of hopper-like crystal than original cubic crystal. The main exposed faces for smooth cubic, hollow cubic, and hopper structure exhibited were (200), (220), and (400) facet, respectively, which could be a consequence of the inner beveling. Surface of spherical structure was a circular arc with relative high roughness, being ascribed to the formation mechanism of spherical structure from cubic structure. Therefore, the work presented a strategy to controllably design KCl crystal with different structure and morphology in a green way.

Эффект модификации вклада термоэмиссии локализованных зарядов в релаксацию фотоотклика монокристалла TlGaSe-=SUB=-2-=/SUB=-

The studing of thermoemission process peculiarities from the of charge localization centers in electrically non-uniform TlGaSe2 crystal are presented. Heterogeneity was formed by preliminary polarization of a sample at low temperature of ferroelectric state of the crystal. The results are good agreement with peculiarities of the localized charges thermoemission registration on TlInS2 in the temperature region of ferroelectric state of the crystal.

Small non-uniform basal crystal fields in HVPE free-standing GaN:Mg as evidenced by angular dependent and frequency-dependent EPR

We studied thin-film and free-standing Mg-doped GaN using multi-frequency electron paramagnetic resonance (EPR) at 3–3.5 K and 9.4–130 GHz. Free-standing samples exhibit a highly anisotropic intensity, varying by a factor of 20 from 0° to 60°. In contrast, the intensity of the thin-film samples is significantly more isotropic, varying by no more than 10% over the same range of angles. The angular dependent intensity can be modeled in both free-standing and thin-film samples similarly to the g-factor anisotropy reported for thin films, supporting the theoretical predictions that the hole is on a basal site around the Mg acceptor. In addition, frequency-dependent transmission EPR measurements reveal a distribution of in free-standing samples, indicating that the local basal crystal field is non-uniform.

Self-assembled vortex crystals induced by inhomogeneous magnetic textures

We investigate the self-assembly of vortices in a type-II superconducting disk subjected to highly nonuniform confining potentials produced by inhomogeneous magnetic textures. Using a series of numerical experiments performed within the Ginzburg–Landau theory, we show that vortices can arrange spontaneously in highly nonuniform, defect-free crystals, reminiscent of conformal lattices, even though the strict conditions for the conformal crystal are not fulfilled. These results contradict continuum-limit theory, which predicts that the order of a nonuniform crystal is unavoidably frustrated by the presence of topological defects. By testing different cooling routes of the superconductor, we observed several different self-assembled configurations, each of which corresponding to one in a set of allowed conformal transformations, which depends on the magnetic and thermal histories of the system.

Grain fragmentation in ultrasonic-assisted TIG weld of pure aluminum

Under the action of acoustic waves during an ultrasonic-assisted tungsten inert gas (TIG) welding process, a grain of a TIG weld of aluminum alloy is refined by nucleation and grain fragmentation. Herein, effects of ultrasound on grain fragmentation in the TIG weld of aluminum alloy are investigated via systematic welding experiments of pure aluminum. First, experiments involving continuous and fixed-position welding are performed, which demonstrate that ultrasound can break the grain of the TIG weld of pure aluminum. The microstructural characteristics of an ultrasonic-assisted TIG weld fabricated by fixed-position welding are analyzed. The microstructure is found to transform from plane crystal, columnar crystal, and uniform equiaxed crystal into plane crystal, deformed columnar crystal, and nonuniform equiaxed crystal after application of ultrasound. Second, factors influencing ultrasonic grain fragmentation are investigated. The ultrasonic amplitude and welding current are found to have a considerable effect on grain fragmentation. The degree of fragmentation first increases and then decreases with an increase in ultrasonic amplitude, and it increases with an increase in welding current. Measurement results of the vibration of the weld pool show that the degree of grain fragmentation is related to the intensity of acoustic nonlinearity in the weld pool. The greater the intensity of acoustic nonlinearity, the greater is the degree of grain fragmentation. Finally, the mechanism of ultrasonic grain fragmentation in the TIG weld of pure aluminum is discussed. A finite element simulation is used to simulate the acoustic pressure and flow in the weld pool. The acoustic pressure in the weld pool exceeds the cavitation threshold, and cavitation bubbles are generated. The flow velocity in the weld pool does not change noticeably after application of ultrasound. It is concluded that the high-pressure conditions induced during the occurrence of cavitation, lead to grain fragmentation in a pure aluminum TIG weld during an ultrasonic-assisted TIG welding process.

Theoretical study on depolarization of the light transmitted through a non-uniform liquid crystal cell

We proposed a model approach to investigate the depolarization of the light transmitted through non-uniform Liquid Crystal (LC) cell. For this aim, we assumed that the cell consists of N local areas of which thicknesses change from one area to another. We showed that the spot size as well as the polarization direction of the incoming light play an important role on depolarization. Our theoretical results were found to be in a good agreement with experimental study in which light depolarization was observed in the LC cell illuminated with different beam spot size.

Crystallographic and Microstructural Studies of Lath Martensitic Steel During Tensile Deformation

Significant crystallographic refinement was observed in the as-quenched lath martensitic steel during tensile deformation up to a strain of 20 pct. Crystallographic investigation by electron backscattering diffraction analysis coupled with the use of crystal plasticity model confirmed that the activation behavior of slip systems is inhomogeneous even within each martensite block. In particular, the activation behavior of slip systems is initially affected by the initial crystal orientation; however, the effect becomes negligible with increasing deformation. In addition, slip systems parallel to the lath plane are preferentially activated during elongation. Our results indicate that the nonuniform crystal rotation within individual blocks is induced not only by grain interaction phenomena but also by the anisotropic activity of slip systems. These are key factor that determines the grain refinement behavior in the as-quenched lath martensitic steel during deformation.

Influence of pre-eruptive degassing and crystallization on the juvenile products of laterally directed volcanic explosions

Strikingly similar examples of edifice collapse and directed blast are the 18 May 1980 eruption of Mount St. Helens (MSH), Washington, USA, and the 30 March 1956 eruption of Bezymianny Volcano (BZ), Kamchatka, Russia. In these cases, flank failures led to near-instantaneous decompression and fragmentation of intra-edifice cryptodome magma, which produced catastrophic, laterally directed blasts. In both instances, the blast products consisted of juvenile material with bimodal density/vesicularity distributions: low- and high-density modes at 1900 and 2400 kg m − 3 for BZ, 1600 and 2300 kg m − 3 for MSH, although the proportion of high-density material is greater at BZ. Blast materials also exhibit striking variety in groundmass crystallinity (< 40 to > 90 vol.%) despite having fairly uniform pheno-crystallinities, suggesting that degassing-driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes. New bulk-rock H 2O and ∂D measurements confirm that progressive open-system outgassing occurred prior to both blasts. The correlations between crystallinity, clast density, and bulk H 2O contents suggest that syn-blast magma expansion was modulated both by non-uniform volatile distribution within the cryptodome and rheological controls associated with non-uniform crystal content. Spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin, which probably correlates with timing of magma injection. The greater proportion of high-density material at BZ is speculatively related to lower blast energy compared with MSH.

Photonic Quantum Information Applications of Patterned Liquid Crystals

In this paper we review recent results we obtained in the field of photonic quantum information that were made possible by the introduction of patterned non-uniform liquid crystal cells known as “q-plates”: (i) the generation of entangled states of polarization and orbital angular momentum of a photon; (ii) the transfer of a qubit of quantum information from the spin to the orbital angular momentum of photons and vice versa; (iii) the Hong-Ou-Mandel coalescence in the same outgoing mode of a beam-splitter of two photons having nonzero orbital angular momentum; (iv) the universal optimal quantum cloning of orbital-angular-momentum-encoded qubits.

Flow banding in obsidian: A record of evolving textural heterogeneity during magma deformation

We perform a quantitative textural analysis of banding in obsidian from Big Glass Mountain (BGM), Medicine Lake Volcano, California and from Mayor Island (MI), New Zealand. The samples are compositionally homogeneous, with banding defined by variable microlite content (BGM) or vesicularity (MI). The spatial distribution of alternating light and dark bands has a 1/wavenumber power-spectral density, as well as multifractal characteristics. Through the use of simple mathematical models, we demonstrate that formation of the observed banding structure is consistent with a continuous deformational reworking of magma. Our results indicate that changes in crystallinity (vesicularity) were concurrent with magma deformation and are random multiples of the total amount of crystallinity (vesicularity) that was already present at any given time. Banded obsidian from BGM and MI can be found in close spatial proximity to brittle deformational textures. We therefore propose that a key element of banding formation in our samples may be magma brecciation or fragmentation. Fragmentation can result in variable rates of degassing of fragments, which in turn can lead to nonuniform crystal (vesicle) nucleation and growth rates. Repeated fragmentation, welding and subsequent viscous deformation can thus lead to banding formation.

Modification of CdTe–HgTe defect substructure and carrier concentration by laser irradiation

Studies of the structure defects arising in the HgTe, (Cd,Hg)Te and CdTe crystals under both ruby laser pulse irradiation of ms duration and change carriers concentrations profiles of the (Cd,Hg)Te near-surface layers disturbed by laser were conducted. The threshold density of laser irradiation energy is defined, when it exceeds the structural defects are formed in a near-surface layer. The dislocation generation is connected with thermomechanical stresses caused by large temperature gradients (up to 10 K/μm) and nonuniform thermal crystal extension in the zone of laser pulse action. The change carrier concentration profiles are studied by the field effect method in electrolytes used in combination with layer-by-layer etching. The carrier concentration has its maximum for a depth h that increases when irradiation energy density increases. The possibility to form reproducibly in the near-surface region of CdTe crystals a layer with high dislocation density not changing the volume properties opens new prospects in using such layers for gettering undesirable dopants and structure defects.

Anisotropic surface free energy and the roughening transition of the diffuse crystal-liquid interface.

An explicit expression for the surface free energy of a nonplanar diffuse crystal-liquid interface is derived in terms of microscopic parameters starting from a free-energy functional for a nonuniform crystal. The broken symmetries of the crystal lattice manifest themselves in this effective surface Hamiltonian as an anisotropic surface tension, plus its derivatives with respect to orientation, and as contributions that vary periodically upon translation of the interface. The amplitudes of these latter components, expressed in terms of the bulk Debye-Waller factors and the intrinsic interfacial width, are related to the roughening temperature. A variational treatment is carried out upon a discrete Gaussian model involving two periodic potentials and, hence, two competing length scales, which displays families of commensurate arrays of ledges.

Molecular theory of liquid crystals: Application to the nematic phase

A density-functional description of liquid crystals is developed. Formally exact expansions for thermodynamic functions in terms of a direct-correlation function are given. Approximations to the direct-correlation function lead to different versions of the molecular-field theories of liquid crystals. The properties of the uniform nematic phase including the isotropic-nematic phase transition at constant pressure are discussed. Expressions are given for the change in density and order parameters at the transition in terms of the direct-correlation function of the isotropic phase. Methods for calculating the direct-correlation functions of the isotropic phase are also discussed. A formal theory for the statistical mechanics of a nonuniform liquid crystal is developed and used to derive expressions for the Frank elastic constants. This approach provides a starting point for the investigation of the phase transitions in the liquid crystals and the properties of inhomogeneous systems. A microscopic description of the Landau---de Gennes theory is also given.