Surface Layer Of Crystal Research Articles

Study of damage mechanism on single crystal 4H-SiC surface layer by picosecond laser modification (PLM)

The stable structure, high hardness, and brittleness of single-crystal 4H-SiC present challenges in achieving efficient and damage-free polishing processing. Ultra-short pulsed laser-induced surface modification provides a new solution to enhance the manufacturability of 4H-SiC. This paper aims to investigate the underlying mechanism of the picosecond laser-induced surface structural changes in 4H-SiC and the process of material removal from solid to vapour, elucidating the interaction mechanism between picosecond laser and 4H-SiC. A temperature gradient distribution model for picosecond laser irradiation on 4H-SiC was built to reveal the formation mechanism of subsurface crack damage. The results show that a combination of phase explosion and thermal effects controlled picosecond laser-modified 4H-SiC surfaces deposited spherical SiO2 particles and the process. The study demonstrates that the cracking behaviour in the subsurface of picosecond laser-modified SiC predominantly occurs in the recast region. The fundamental cause of cracking is attributed to the tensile stresses generated by thermal effects and the volumetric changes in the molten material resulting from the overlapping of neighbouring spots in alternating hot–cold cycles. The nanoindentation demonstrated that laser modification can effectively enhance the machinability of SiC. The findings of this study can provide a theoretical basis for efficient non-destructive machining of hard and brittle materials.

Preparation and photo-responsive behavior of azobenzene decorated PEG single crystal

In this work, we prepared the azobenzene decorated PEG single crystal for the first time. The azobenzene terminated PEG was prepared by the nucleophilic substitution reaction between the tosylate ended PEG and 4-hydroxy-4′-methoxy azobenzene. Uniform square-shaped crystals were obtained by a self-seeding approach. Due to the relatively large size of azobenzene, the azobenzene groups could only exist in the surface layer of the lamella crystals. The photo-responsive behavior of the azobenzene was studied by using UV–vis spectra. The photo-induced trans-cis isomerization of azobenzene could take place very well. The trans-cis isomerization rate was even similar with that in solution, which proved that the azobenzene groups attached on the single crystal had relatively loose surroundings.

Local crystallization in amorphous Fe80Si11B9 ribbon by pulsed laser interference heating

Pulsed laser (Q-switched Nd:YAG) interference heating was used to create periodically distributed, crystallized, micro-areas, ∼10 μm in diameter within the amorphous matrix on Fe80Si11B9 alloy. Laser energy and a number of consecutive pulses were varied as controlled parameters of this study. The influence of such interference heating, on the resulting microstructure, magnetic domain structure and magnetic field was examined. TEM analysis showed that crystallized surface layer of the sample after pulsed laser heating consists of α-Fe(Si) nanocrystals embedded in the amorphous matrix. Research, using the Fresnel imaging (defocus mode) showed that the nanocrystallization of the amorphous alloy caused a significant refinement of the magnetic domain structure, as compared to amorphous state. The electron holography images show that the α - Fe(Si) crystallites introduce additional distortion of the magnetic field lines in the sample. The effects of an external magnetic field were studied by examining the configuration of the phase shift contours as a function of its magnitude.

The Influence of Internal Stress on the Nanocrystal Formation of Amorphous Fe73.8Si13B9.1Cu1Nb3.1 Microwires and Ribbons

The early stages of nanocrystallization in amorphous Fe73.8Si13B9.1Cu1Nb3.1 ribbons and microwires were compared in terms of their internal stress effects. The microstructure was investigated by the X-ray diffraction method. Classical expressions of crystal nucleation and growth were modified for microwires while accounting for the internal stress distribution, in order to justify the XRD data. It was assumed that, due to the strong compressive stresses on the surface part and tensile stresses on the central part, crystallization on the surface part of the microwire proceeded faster than in the central part. The results revealed more rapid nanocrystallization in microwires compared to that in ribbons. During the initial period of annealing, the compressive surface stress of a microwire caused the formation of a predominantly crystallized surface layer. The results obtained open up new possibilities for varying the high-frequency properties of microwires and their application in modern sensorics.

Molten Chlorides as the Precursors to Modify the Ionic Composition and Properties of LiNbO3 Single Crystal and Fine Powders.

Modifying lithium niobate cation composition improves not only the functional properties of the acousto- and optoelectronic materials as well as ferroelectrics but elevates the protonic transfer in LiNbO3-based electrolytes of the solid oxide electrochemical devices. Molten chlorides and other thermally stable salts are not considered practically as the precursors to synthesize and modify oxide compounds. This article presents and discusses the results of an experimental study of the full or partial heterovalent substitution of lithium ion in nanosized LiNbO3 powders and in the surface layer of LiNbO3 single crystal using molten salt mixtures containing calcium, lead, and rare-earth metals (REM) chlorides as the precursors. The special features of heterovalent ion exchange in chloride melts are revealed such as hetero-epitaxial cation exchange at the interface PbCl2-containing melt/lithium niobate single crystal; the formation of solid solutions with cation vacancies as an intermediate product of the reaction of heterovalent substitution of lithium ion by calcium in LiNbO3 powders; the formation of lanthanide orthoniobates with a tetragonal crystal structure such as scheelite as the result of lithium niobate interaction with trichlorides of rare-earth elements. It is shown that the fundamental properties of ion-modifiers (ion radius, nominal charge), temperature, and duration of isothermal treatment determine the products’ chemical composition and the rate of heterovalent substitution of Li+-ion in lithium niobate.

Effect of pH on the Surface Layer of Molecular Crystals at the Solid-Liquid Interface.

Dissolution of solid matter into aqueous solution is one of the most challenging physicochemical aspects related to drug development. While influenced by several parameters, the effect of pH remains the most important one to be fully understood. The dissolution process is essentially controlled by activity at the surface of the molecular crystals, which is difficult to characterize experimentally. To address this, a combination of in situ atomic force microscopy (AFM) with molecular dynamics (MD) simulation is reported. AFM allows for direct visualization of the crystal surface of basic and acidic model compounds (carvedilol and ibuprofen) in contact with an aqueous medium with varying pH. A dramatic increase in surface mobility in the solid-liquid interface could be observed experimentally as a function of pH. The in situ AFM approach opens up for a more detailed understanding of the behavior of particulate matter in solution with importance at different levels, ranging from engineering aspects related to crystallization, and biological considerations related to bioavailability of the final drug product.

High entropic coatings FeCrNiTiZrAl and their properties

In our proposed empirical model, the anisotropy of the surface energy and the thickness of the surface layer of the high-entropy FeCrNiTiZrAl alloy are calculated. The thickness of the surface layer of this alloy is about 2 nm, which is an order of magnitude greater than the thickness of the surface layer of complex crystals, but is of the same order of magnitude as that of metallic glasses. The hardness and other properties of the high-entropy alloy are the same as for metallic glasses, but are 2-3 times higher than the hardness of stainless steels. The surface energy of the high-entropy FeCrNiTiZrAl alloy is about 2 J/m2, which corresponds to the surface energy of magnesium oxide and other crystals with a high melting point. However, unlike these crystals, the friction coefficients of a high-entropy alloy (~ 0.06) are much lower than that of ordinary steels (~0.8). We have theoretically shown that the friction coefficient is proportionally dependent on the surface energy and inversely proportional to the Gibbs energy, which significantly decreases for a high-entropy alloy, leading to low friction. The high hardness and low coefficient of friction of the high-entropy alloy facilitates the deposition of coatings from them on structural metal products, which contributes to their widespread use.

SPECIATION FEATURES OF GOLD IN ORES AND MINERALS OF THE NATALKINSKOE DEPOSIT (NORTH-EAST RUSSIA)

The distribution and speciation features of gold in ores and minerals of the Natalkinskoe gold deposit (North-East Russia) are studied using light microscopy (LM), scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX), X-ray electron probe microanalysis (EPMA), "phase" chemical analysis with atomic absorption spectrometry (PCA-AAS) and atomic absorption spectrometry with analytical data selections for single crystals (AAS-ADSSC). The vein and streaky-vein ores are high-grade ores, whereas veinlet-disseminated ores are less rich and disseminated ores are poor in gold. Up to 85 % of the gold in the ores is in a free native state, associated with quartz and sulfide minerals. LM, SEM-EDX and EPMA reveal that the predominant gold grains are 0.01 to 2.00 mm in size and at a fineness of 720 to 900 ‰. The finely dispersed and submicron elemental gold particles (Au0) amounted to 20 % and are mainly enclosed into arsenopyrite and pyrite. According to PCA-AAS data, the highest Au concentrations (up to 1383 ppm) are recorded in arsenopyrite; lower contents are typical of pyrite (up to 158.2 ppm). In these sulfides, two non-mineral species of "invisible" Au are the structurally bound and surface-bound species recognized by AAS-ADSSC. The structural Au is included in the mineral structure. The surface-bounded Au prevails and is confined to nano-sized, non-autonomous phases (NAPs) on the sulfide surface. In common with "invisible" Au, the micro-sized particles of native gold are often observed on the surface and within the surface layers of sulfide crystals. This is consistent with the model of post-growth transformations of nano-sized NAPs, resulting in the formation of nano and micro-sized Au0 particles. It is expected that the major part of gold contained in arsenopyrite and pyrite as finely dispersed and submicron particles, as well as the surface-bound gold in NAPs, can be won with modified current schemes of gold concentration, which enhances the value of the gold ore mining.

Gold in pyrites and sulfides by scintillation analysis

The method of scintillation atomic emission spectrometry (SAES) allows determination of the form of the element present in the sample and the size of inclusions in addition to the total content of the element in the direct analysis of powder samples. The aim of the work is to verify the expedience of using the direct method of analysis of individual gold particles in studying the possible forms of gold appearance in pyrites and sulfides, including the so-called «invisible» finely divided gold. The measurements were carried out both on individual pyrite single crystals, and on the samples of sulfide gold and brown coal deposits of Kazakhstan. The studies were carried out on a Grand-Potok complex (VMK Optoelektronika, Ltd.) equipped with a laser ablation system and a system for injection of aqueous solutions and aerosols samples. The samples of pyrite crystals of various sizes were analyzed to determine the content and speciation of gold in the samples. No dependence of the gold content on the crystal size was noted for all the studied pyrites. It is shown that gold and other precious metals are concentrated in the surface layers of pyrite crystals and in crystal lattice defects in the bulk of the crystal (according to SAES data during laser ablation of the sample in an arc discharge). The total content of gold in the bulk of pyrite crystals is about 2 g/ton, platinum and silver is less than 0.02 g/ton, while the content of Au, Ag, Pt on the surface ranges from 2 to 5 g/ton. The detection limit of gold determination by the SAES method is 0.01 g/ton. The SAES method can be used for determination of the gold speciation in pyrites and sulfides (finely dispersed, individual particles up to 1 μm or even less).

Investigation of crystal structure formation by electropulse treatment of amorphous Ti50Ni25Cu25 alloy ribbons

The study of the effect of electropulse treatment with a variable duration on the crystallization processes and the structure of a amorphous TiNiCu alloy with 25 at.% Cu in comparison with isothermal annealing and heating at a constant speed was carried out. The alloy was fabricated by rapid-quenching from the liquid state (melt spinning technique) at the cooling rate of the melt of about 106 °C/s in the form of a ribbon with a thickness of 28 μm with a surface crystal layer with a thickness of about 2-3 μm. To remove the crystal layer, the method of double-sided electrochemical polishing was used. The studies were carried out by methods of differential scanning calorimetry, metallography and scanning electron microscopy. It was established that the formation of the crystalline phase in the electropulse treatment of the amorphous ribbon occurs from the surface to the inner part due to the predominant formation and growth of columnar crystals with subsequent nucleation and growth of crystals in the rest of the ribbon.

Distribution of Molybdenum Atoms over Depth of the Surface Layer of a Niobium Single Crystal Produced by Ion Bombardment

Distribution of Molybdenum Atoms over Depth of the Surface Layer of a Niobium Single Crystal Produced by Ion Bombardment

The influence of single and binary mixtures of collagen amino acids on the structure of synthetic calcium hydroxyapatite as a nanobiomaterial.

The modifications carried out as part of this study were aimed at examining the effect of the addition of collagen amino acids: glycine, proline and hydroxyproline (used separately and in binary mixtures) on the physicochemical properties of hydroxyapatite obtained in their presence in vitro. The influence of mixtures of amino acids on these properties is an important element of scientific novelty. The obtained samples were tested with the use of instrumental methods: FT-IR, TEM, EDXMA, PXRD and UV spectrophotometry. The results showed the influence of the amino acids used on changes in the relative content of the labile phosphate groups constituting the structure of the hydrated surface layer of crystals. As a consequence, there were differences in some physicochemical properties of the obtained hydroxyapatites (degree of crystallinity, molar Ca/P ratio). It was also determined how the ability of the used amino acids to bind to hydroxyapatite changes.

High entropic coatings FeCrNiTiZrAl and their properties

In our proposed empirical model, the anisotropy of the surface energy and the thickness of the surface layer of the high-entropy FeCrNiTiZrAl alloy are calculated. The thickness of the surface layer of this alloy is about 2 nm, which is an order of magnitude greater than the thickness of the surface layer of complex crystals, but is of the same order of magnitude as that of metallic glasses. The hardness and other properties of the high-entropy alloy are the same as for metallic glasses, but are 2-3 times higher than the hardness of stainless steels. The surface energy of the high-entropy FeCrNiTiZrAl alloy is about 2 J/m2, which corresponds to the surface energy of magnesium oxide and other crystals with a high melting point. However, unlike these crystals, the friction coefficients of a high-entropy alloy (~ 0.06) are much lower than that of ordinary steels (~ 0.8). We have theoretically shown that the friction coefficient is proportionally dependent on the surface energy and inversely proportional to the Gibbs energy, which significantly decreases for a high-entropy alloy, leading to low friction. The high hardness and low coefficient of friction of the high-entropy alloy facilitates the deposition of coatings from them on structural metal products, which contributes to their widespread use.

ANISOTROPY OF THE SURFACE OF CARBON MATERIALS

In this work, a model of the surface layer of perfect single crystals is used and the role of surface energy in physical processes occurring in the region of nanosized carbon materials is clarified. Of these, diamond, graphite, carbyne and fullerenes have been investigated. The thickness of the surface layer of diamond with cubic symmetry is 8.2 nm and is a nanostructure. The average size of the synthesized nanodiamond is of the order of ~ 8 nm. The value of the surface energy σhkl calculated by us along the diamond planes (100), (110), and (111) is in good agreement with experiment and other calculations. The thickness of the surface layer of graphite along the a axis is equal to R(I)a = 8.0 nm and also represents a nanostructure. But along the c axis we have a layer thickness of about 1.5 nm and the number of monolayers is only 2. On this c axis, graphite can be created a monolayer by turning it into graphene. The σhkl value calculated by us along the a and c planes of graphite is 25957 and 5515 mJ/m2 , respectively. Carbines represent a polymeric polyyne or cumulene chain composed of sp-hybridized carbon atoms. If we imagine that the thickness of the surface layer of carbyne is stretched into a one-dimensional chain along the c axis, then the length of this chain is up to 200 nm for α-carbyne. The thickness of the surface layer of fullerenes significantly exceeds the thickness of the surface layer of pure metals. The surface energy of fullerenes σhkl increases with an increase in the number of carbon atoms С36 → С96. It also changes in the series (111) → (100) → (110).

Blue-Shifting Mechanofluorochromic Luminescent Behavior of Polymer Composite Films Using Gelable Mechanoresponsive Compound

Mechanochromic luminescent dyes change their luminescence color upon exposure to external mechanical stimuli. In this study, we synthesized a liquid crystalline mechanochromic luminescent dye containing a terminal cholesterol molecule. The dissolution of the dye in 1,4-dioxane resulted in the formation of a gel. The luminescence of the xerogel obtained from the dioxane solution changed from green to blue upon grinding, indicating mechanochromic luminescence behavior. The anisotropic patterning of short-wavelength-shifted luminescence color change by directional handwriting on surface layer of liquid crystal was successfully demonstrated. Furthermore, blue-shifting mechanoresponsive polymer composite surface was fabricated by using the luminophore.

The study of spontaneous domain nucleation in the interelectrode gap of phase modulator based on titanium indiffused waveguides in lithium niobate crystals

The paper presents the analysis of nucleating kinetics and growing of switched domains in the surface layer of monodomain lithium niobate X-cut crystal in the interelectrode gap of integrated optical phase modulators. The work proposes the morphology model of domains growing along the boundary of surface electrodes in X-cut phase modulators. The mechanism of spontaneous needle-like domain growing as a result of the electric field induced by the pyroelectric effect at temperature changing of the crystal was theoretically substantiated. The Comsol Multiphysics cross-platform was used for the numerical estimation of the pyroelectric field in the interelectrode gap. The needle-like domain structures were studied experimentally at industrial samples of integrated optical phase modulators based on Ti:LiNbO3 waveguides. The experimental research of the form and size of domains was performed with the anisotropic etching method by HF solution and followed by visual analysis. For non-destructive testing, the authors used scanning electron microscopy and piezo-response force microscopy. For the first time, the morphology of needle-like domains occurring in the interelectrode gap of phase modulators based on lithium niobate was experimentally studied. The results showed the theoretical and numerical model of domain growing that involves the pyroelectric nature of the electric field. It was demonstrated that along the electrode boundary, the needle-like domains grow up to 20 μm long at normal conditions and achieve 30 μm after the thermal shock by cooling at ∆T = – 125 °С. The discovered switched domains in the interelectrode gap can affect electro-optical characteristics of integrated optical phase modulators with the lithium niobate base and should be taken into account in the future design of electrode topology and modulator usage.

Pressure-induced large increase of Curie temperature of the van der Waals ferromagnet VI3

Evolution of magnetism in single crystals of the van der Waals compound VI3 in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at T1 = 54.5 K, T2 = 53 K, TC = 49.5 K, and TFM = 26 K, respectively have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature TC and the transition between two different ferromagnetic phases TFM, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. TC was found to be almost unaffected by pressures up to 0.6 GPa whereas TFM increases rapidly with increasing pressure and reaches TC at a triple point at ~ 0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is dramatically reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. We discussed these results in light of recent theoretical studies to analyze exchange interactions and provide how to increase the Curie temperature of VI3.

Investigation of thermal-structural characteristics at two-way shape memory effect of rapid quenched laminated amorphous-crystalline ribbons of Ti50Ni25Cu25 alloy

The work is devoted to the study of processes of structural changes leading to the realization of the two-way shape memory effect (TWSME) in the rapid quenched amorphous-crystalline alloy Ti50Ni25Cu25. The object of the study was a rapid quenched laminated amorphous-crystalline Ti50Ni25Cu25 alloy ribbon with a thickness of about 39 um and with a thickness of surface crystal layer about 10 μm was present on the non-contact side relative to the quenching wheel. In the ribbon under study, without any additional heat treatments the TWSME with bending deformation from a rectilinear shape during heating and cooling in the range of martensitic transformation is manifested. The samples were characterized by means: scanning and transmission electron microscopy; X-ray diffraction analysis; differential scanning calorimetry; measuring the characteristics of the crystalline layer during the implementation of TWSME and the temperature dependence of the form change. It was shown that the formation of a laminated amorphous-crystalline composite occurs due to the martensitic transformation of B2∼B19 in the crystal layer and the accompanying TWSME as a result of which the crystal layer is reduced.

Investigation of the effect of electric pulse heat treatment of rapid-quenched Ti50Ni25Cu25 alloy ribbons with a thin surface crystal layer on the structure formation from an amorphous state

This work is devoted to the study of the influence of electric pulse heat treatment of rapid-quenched Ti50Ni25Cu25 alloy ribbons (at. %) with a surface crystal layer. The object of the study was a rapid-quenched layered amorphous-crystal ribbon with a thickness of about 30 µm and a width of about 1.5 mm, in which a thin surface crystal layer (2.5 µm) was present on the non-contact side relative to the quenching wheel. The alloy samples were subjected to electric pulse treatment by passing a single pulse of electric current through the sample with a variable duration. For comparison, samples of the alloy were obtained, crystallized by standard isothermal heat treatment. The samples were characterized by means of scanning electron microscopy techniques, energy dispersive X-ray analysis and differential scanning calorimetry. It was shown that electric pulse treatment with an exposure time of less than 1 second leads to a significant change in the formed crystalline structure in comparison with the structure obtained by isothermal treatment. The microstructure of such samples in cross section is characterized by an uneven distribution of crystals over the thickness of the ribbon: columnar crystals are observed from the contact and non-contact sides of the ribbon, and large crystals are present in the inner part of the ribbon.

Verification of physical parameters of insity p-type HPGe detector by scan method using collimated low energy photon beam and MCNP simulation

In this work, the physical dimensions and the actual position of germanium crystal within a detector housing, the homogeneity of the crystal surface and outer dead layer thickness for a p-type HPGe detector were confirmed by the scan method using the collimated low energy photon beams combined with Monte Carlo simulation. The length and the diameter of the crystal were found to match with the values supplied by the manufacturer in discrepancy of about 3%. Only one mounting strap (Typical) for holding the crystal inside the mounting cup instead of two which is indicated in the detector drawing supplied by manufacturer was revealed by scanning along the lateral face of detector. Scanning on the front surface and around the lateral face of detector by the collimated 59.5 keV photon beam verified the outer dead layer thicknesses at the front surface and lateral face of the crystal averagely increases about 6.5% and 12% respectively. Adjusting the detector parameters for MCNP simulation by these verified values, the simulated peak efficiencies for different photon energies become being in accordance with the experimental peak efficiencies.