Surface Layer Of Crystal Research Articles

Grain Refining Mechanism and Fatigue Properties of Bogie Welded Cruciform Joints Treated by Ultrasonic Impact

Surface treatment was carried out on the welded cruciform joint of 16MnR steel by means of ultrasonic impact treatment. The effect of ultrasonic impact on the fatigue properties of welded cruciform joint of 16MnR steel was studied. For comparison, fatigue experiments were carried out with both treated and un-treated specimens. The ultrasonic impact current was 1.2A, the impact amplitude was 30 microns. The grain refining mechanism was investigated using high resolution transmission electron microscope. The experimental results show that the fatigue performance of the welded cruciform joint of 16MnR steel is significantly improved through the ultrasonic impact treatment. The fatigue limit (2x106) of un-treated and treated joint is about 157MPa and 235MPa, respectively. Compared to the un-treated joint, the conditions of fatigue limit of the treated joint increased by 50% and the fatigue life increased 45-52 times. The equiaxed crystal nanostructured surface layers were fabricated on the surface of welded cruciform joint of 16MnR steel by ultrasonic impact, with the grain size being less than 100nm. The main reason for improving the fatigue performance is that the ultrasonic impact can reduce the stress concentration in the weld toe, decrease the tensile stress, and even change to compressive stress in the weldment, with the grain size in the welded joint being greatly refined. It indicates that ultrasonic impact treatment has remarkable effects on the fatigue life of 16MnR steel. Keywords: Fatigue strength, ultrasonic impact, grain refining, cruciform joint, 16MnR steel, transmission electron microscope, fatigue limit, crystal nanostructured surface

T–T–T behaviour of bioactive glasses 1–98 and 13–93

Abstract In this work crystallization kinetics of bioactive glasses 1–98 and 13–93 are discussed. Within a certain temperature–time window these glasses can be hot worked into various products without interfering with crystallization. The crystallization was studied isothermally by heating glass plates at different temperatures for different times. Phases in the samples were studied through XRD and SEM analyses. The nucleation-like curves and crystallization characteristics were measured with DTA. The temperature of maximum nucleation was measured for glass 1–98 at 725 °C and for 13–93 at 700 °C. The activation energy of crystallization of both glasses was 280 kJ/mol. The Johnson–Mehl–Avrami exponent and the SEM micrographs of the samples suggested surface crystallization. The primary crystalline phase was wollastonite. The growth rate of the crystallized surface layer was 1 order of magnitude higher in the plates of 1–98 than in 13–93. The results can be utilized to optimize the parameters in hot-working of the glasses.

Preparation of high-capacity substrates from polycrystalline silver chloride for the selective detection of tyrosine by surface-enhanced infrared absorption (SEIRA) measurements

In this work, stacked but isolated silver nanoparticles (AgNPs) on silver chloride were prepared in order to create sensitive substrates that could be used to determine analytes in aqueous solution by performing surface-enhanced infrared absorption (SEIRA) measurements. These substrates have a high loading capacity, which improves sensitivity, thus allowing the detection of biospecies such as weak infrared absorbers (including amino acids) in aqueous solution. AgNPs were obtained by converting the surface layers of the silver chloride crystals into stacked but isolated AgNPs using reducing agents. To optimize the conditions used to prepare the SEIRA substrates, the roles of four common reducing agents-hydrazine, sodium borohydride, glucose, and formaldehyde-were explored by probing the finished substrates with para-nitrobenzoic acid (PNBA). Factors that influence the morphologies of the AgNPs were examined systemically. These factors include the pH of the reducing solution, the reaction time, and the concentration of the reducing agent. Results indicate that the concentration of the reducing agent and the pH of the solution strongly influence the AgNP morphology and hence the SEIRA signals. Under optimized conditions, the resulting substrates showed intense SEIRA spectra from PNBA, with enhancement factors of around two orders of magnitude compared to conventional transmission methods. The new substrates offer a high loading capacity and good heat tolerance, allowing the direct infrared detection of tyrosine in aqueous solution.

Emergent Multistability in Assembled Nanostructures

Scanning tunneling microscopy (STM) at 5 K reveals that native atoms in the surface layer of a semiconductor crystal become bistable in vertical height when a nanostructure is assembled nearby. The binary switching of surface atoms, driven by the STM tip, changes their charge state. Coupling is facilitated by assembling adatom chains, allowing us to explore the emergence of complex multiple switching. Density-functional theory calculations rationalize the observations and a lattice-gas model predicts the cooperative behavior from first principles.

IR Studies of H/D Exchange of Water, Hydroxyl, and Carboxylic Groups Reveal Slowly Diffusing Lattice Defects in Sub-Nanometer Pores

We have studied the properties of a series of solid hydrated organic porous networks with pore diameters ranging from approximately 0.4 to 0.9 nm using the attenuated total reflection infrared (ATR-IR) technique. These subnanometer organic pores are composed of water and of organic racemic bicyclic monomers containing carboxylic, alcoholic, ether functions and different appendices. In particular, the doubly hydrated hydroxyl acids 1 2H2O and 2 2H2O form cylindrical pores in which half of the water molecules are part of the walls and the other half are located inside the pores. In a first step, by a comparison of the spectra of a family of related compounds, the COOH, COH as well as the wall and pore water stretches were assigned. The COOH bands are broad and red-shifted as compared to carboxylic acid dimers, and exhibit a substructure assigned to Fermi resonances. The OH stretches fulfill well Novak’s correlation with the corresponding crystallographic O...O distances. In a second step, we have followed the deuteration of the different functional groups of solid 2 2H2O by ATR-IR by heavy water vapor. Surprisingly, we observe that the rates of deuteration are the same for all functional groups although exhibiting biexponential time dependence, in contrast to the liquid state where COOH groups exchange protons with water much faster than with alcohols. This result is rationalized in terms of slowly diffusing lattice defects resembling a local liquid or glass in the subnano scale in which the different exchange reactions take place. The nonexponential deuteration is explained in terms of a faster deuteration of crystal surface layers.

Mesoscopic phenomena in oxide nanoparticles systems: processes of growth

The process of nanoparticles growth has been investigated and discussed in terms of mesoscopic approach on example of ZrO2–3 mol%Y2O3 system. Growth process of nanoparticles synthesized by co-precipitation has three stages: cooperative-oriented crystallization of ordered areas in xerogel polymer matrix and disintegration of crystallized areas (350–400 °C); oriented attachment of particles into single crystal caused by electrostatic interaction (400–600 °C); attachment of particles to single and poly-crystals by oxygen diffusion through vacancies in surface layers of joining crystals (600–1,000 °C). Proposed conception on mesoscopic processes of nanoparticles formation make the understanding and theoretical description of significant amount of experimental data possible and open the way for purposeful governing by oxide powder system on the stages of obtaining, compaction, and sintering.

XPS Study of Changes in the Chemical Composition of Langasite Crystal Thin Surface Layers during Vacuum Annealing

La3Ga5SiO14 langasite (LGS) crystals have been studied before and after 1000–1050°C, 650°C vacuum annealing. XPS studies have shown that the relative intensity of the gallium bands in the survey spectra of the as-annealed LGS crystals at elevated temperatures is one order of magnitude lower compared to the source specimen. To quantify the changes in the chemical composition of the crystal thin surface layers, we have measured depth profiles with Ar+ sputtering. It was shown that crystal surface is depleted of gallium during annealing at elevated temperatures. We suggest a model in which we attribute the changes in the chemical composition of the LGS crystal thin surface layers during vacuum annealing to the formation of the volatile Ga2O oxide. Vacuum annealing of the LGS crystals at 650°C did not cause any significant changes in the surface chemical composition of the crystals.

Influence of nanosecond laser irradiation on optical properties of surface layers of CdTe crystals

The effect of surface laser processing on the optical properties and structure of the surface region of (111) oriented CdTe single crystals have been studied by multiple-angle-of-incidence single-wavelength ellipsometry. The measurements were performed both on Cd and Te surfaces of CdTe(111) crystals and morphology was monitored by AFM. CdTe crystals were subjected to various treatments including chemical and laser etching. The ellipsometric parameters Δ and Ψ were obtained at several light incidence angles and data were interpreted based on the model of an absorbing surface layer located on an absorbing substrate. Surface roughness was taken into account and the surface layer was considered as an equivalent film with flat boundaries and effective optical parameters. The refraction n and absorption k indexes, and thickness d of the modified surface layer were calculated and features of nanosecond laser irradiation on CdTe(111) polar surfaces were discussed.

Nonlinear surface waves on the boundary of a photorefractive crystal

Excitation of nonlinear surface waves is studied at the SBN-75 crystal — air interface. The SBN-75 crystal is characterised by the diffusion-type photorefractive nonlinearity, the surface wave being excited in the surface layer and the depth of its penetration into the crystal being determined by the nonlinear crystal parameters. Because of a large refraction coefficient in SBN-75, the penetration depth of the surface wave into the air is small; therefore, the nonlinear surface wave is localised in the surface crystal layer several micrometers in thickness. The oscillating intensity distribution of the surface wave at the output end of the crystal is measured. The oscillation period is determined by the angle of incidence of the exciting beam. The nonlinear wave is excited not only at the crystal — air interface but also in the case when the active surface of the crystal is covered by an electrode, for example, an aquadag layer. This circumstance opens up a possibility of studying new properties of the surface waves when an external electric field is applied to the crystal.

First-principles investigation of the structure and electronic properties of CdS/CdSe/CdS and CdS/CdTe/CdS quantum wells using a slab approximation

The structure and electronic properties of CdS, CdSe, and CdTe thin films and multilayer CdS/CdSe/CdS and CdS/CdTe/CdS heterostructures simulating quantum wells of these compositions are performed using density functional theory and a slab approximation. Various crystal modifications and various crystallographic surfaces of the specified compounds are considered. In the calculations, the GGA approximation for the exchange-correlation functional (PW91) and ultrasoft pseudopotentials are used. For the systems in study, the work functions are calculated for various surfaces. The calculated structural data are in good agreement with experiments. The calculated surface reconstruction can be qualitatively described as an inward displacement of cadmium atoms into the surface crystal layer and an outward displacement of chalcogen atoms. The calculated surface energies indicate that the (110) surface formation energy is lower than the (100) surface formation energy. The calculated work functions decrease in the sequence CdS > CdSe > CdTe, while the calculated work functions for multilayer structures are between the corresponding values for the pure components of the structure.

Fine structure of faces and imperfection of surface layers of shaped sapphire crystals

The development of faces on cylindrical and ribbon-shaped sapphire single crystals grown from a melt using the Stepanov method is investigated by optical and electronic microscopy. The crystallographic indices on the detected faces are established, and the microstructure of the growth layers is studied. The X-ray spectral microanalysis of foreign particles on the sapphire surface confirms the presence of molybdenum single crystals on it. Cathodoluminescence spectra of surface layers of sapphire single crystals are obtained.

Freezing in the bulk controlled by prefreezing at a surface

We use Monte Carlo simulations of the Lennard-Jones model to study the nucleation of a crystal phase at a flat surface. Our motivation is the observation that crystal phases almost always nucleate at a surface. We find that a surface phase transition (prefreezing) can control nucleation of the bulk crystal. This finding should be general and so surface phase behavior should be considered whenever nucleation of bulk phases at surfaces is considered. Also, nucleation of the bulk crystal transforms smoothly into the nucleation of a surface crystal layer as the bulk transition is crossed.

Nano-scale chemical and structural segregation induced in surface layer of NaNbO3 crystals with thermal treatment at oxidising conditions studied by XPS, AFM, XRD, and electric properties tests

The process of ionic segregation in the surface layer induced by the thermal treatment of the NaNbO3 crystal was studied. The study of the as grown crystal and the crystal heated at 970 K in ambient air was carried out for comparison. The changes in the concentration of the elements within the surface layer were determined using XPS measurements. The AFM technique was applied to determine the surface morphology related to nano-scale transformation. The possibility of appearance of the Ruddlesden–Popper phases was detected by the XRD test. The change in the features of the electric permittivity, the loss tangent, and the electric conductivity was ascribed to the marked participation of the chemically modified surface layer in the measured effective electrical properties.

Effect of a temperature gradient on the polarization distribution in the surface layer of TGS crystals

Effect of variable temperature gradients on the polarization of the surface layers of triglycine-sulphate (TGS) crystals has been analyzed. A fundamental difference in the impact of these gradients on the sample sides corresponding to the positive (“+” Ps) and negative (“−” Ps) yield of the polarization vector has been discovered. In particular, it has been found that repolarization of the surface layer under the impact of thermal radiation occurs only on the “−” Ps side.

The principle of continuity of phase formation at mineral surfaces

One of the main principles of physicochemical analysis—the principle of continuity—says that “continuous change of parameters of the state of the system results in continuous change of properties of individual phases; the properties of the whole system also show continuous changes if no new phases appear and no existing phases disappear” [1]. This formulation does not specify which phases are implied: surface, nonautonomous, or traditional autonomous phases. This problem becomes significant for studying mineral surfaces. Nonautonomous phases (NAP) are nanosized (submicron) surface phases, which are the products of chemical modification and structural reconstruction of surface layers of mineral crystals [2, 3]. They are formed in the crystal surface layer by means of its interaction with components of the growth medium or contacting autonomous phases. Unlike the latter, the nonautonomous phases cannot be extracted from the system without a change in their composition and structures and characteristics of the associated phases. At the same time, their reaction to changes in the state parameters and system composition is different from the reaction of traditional phases, owing to which they can incorporate trace elements, which are incompatible with the structure of the corresponding bulk phase [2]. The aim of this work is to clarify whether the principle of continuity is applicable to the nonautonomous phases and which mineralogical sequences it causes. The study objects are sulfide minerals ascribed mainly to the systems Pb‐S and Fe‐Zn‐S: galena PbS, pyrite, FeS 2 , pyrrhotite Fe 1 – x S , and sphalerite (Zn,Fe)S. Minerals were synthesized by the conventional technique of hydrothermal thermal-gradient synthesis in Ti inserts [3] at temperatures of 400, 450, and 500°C and pressure of 0.5 and 1 kbar both individually and as phase associations containing up to three phases (Fesphalerite + pyrite + pyrrhotite). The mineralizers were either 10% NH 4 Cl, or those with added Na 2 S or HCl (1 wt %) (pyrite synthesis). In some experiments with pyrite, the starting material was enhanced with As (up to 5 wt %), while the starting material for the synthesis of sphalerite and galena contained Cd and Hg sulfides (1 wt % each). Inserts equipped with a self-sealing closure were placed in autoclaves of stainless steel and held for 3 days in an isothermal regime to homogenize the starting material, and then for 7, 9, or 12 days at 500, 450, and 400°C , respectively, at a temperature drop ( 15°C along the outer wall of the autoclave). The experiments were terminated by quenching the autoclave in cold running water. Data previously obtained on the system PbS‐(HgS,CdS) (Cd and Hg sulfides as admixtures) [4] were also used in this work.

Influence of composition of NiPd single crystal surface layers on sputtering process

NiPd disordered crystal sputtering properties are studied via molecular dynamics simulation taking into account the segregation of upper layers. Computation is performed for a polycrystal and the (001), (110), and (111) faces of a single crystal under conditions of bombardment by argon ions with energies of 0.1 to 5 keV for normal and oblique incidence of ions. The energy and angular dependences of the sputtering process, as well as the spatial distribution of emitted particles, are analyzed.

Nanocrystalline SnO 2 from thermal decomposition of tin citrate crystal: Luminescence and Raman studies

Nanocrystalline SnO 2 particles are prepared by thermal decomposition of tin citrate crystal. Distinction between the surface traps and lattice defects is carried out by the photoluminescence emission study, which gives two characteristic peaks at ∼400 and 470 nm respectively. The peak at ∼400 nm shifts toward the lower wavelength with increase of heat treatment because of decrease of defects. The intensity of peak at 470 nm due to the surface traps decreases with increase in particle size. Raman spectra exhibited new peaks, which are related to a surface layer of small crystals, and shift in peaks are found with different particle sizes.

Influence of a structurally damaged surface layer of the hexagonal crystal on the dispersion and attenuation of Rayleigh surface acoustic waves

Analytical expressions for the dispersion of the phase velocity and the inverse attenuation length of Rayleigh waves are derived with allowance made for a thin (as compared to the length of the surface wave) isotropic damaged surface layer that is contiguous with vacuum and located on the surface of a hexagonal crystal with the sixfold axis perpendicular to the surface. It is demonstrated that, in the limit of long wavelengths (as compared to the characteristic inhomogeneity size), which is of greatest interest for experimenters, the change in the dispersion of the phase velocity of Rayleigh waves is proportional to the second power of the frequency, whereas the inverse attenuation length of Rayleigh waves is proportional to the fifth power of the frequency. The inverse attenuation length of the Rayleigh wave is calculated numerically. The calculation method previously proposed by one of the authors (Kosachev, 1998) is generalized to the case of an isotropic damaged layer on an anisotropic (hexagonal) substrate.

Investigation of temperature dependence of surface anisotropy field and constant in the system of BaFe 12O 19 nanocrystals

We show a possibility to determine the parameters characterizing surface magnetic anisotropy of platelet BaFe 12O 19 nanocrystals with thickness comparable to the magnetoperturbed surface layer of high-anisotropic hexagonal crystal. Taking into account the above-mentioned specific character of the particles, we introduce the constant K S as the surface anisotropy energy per unit volume of near-surface layer. Temperature dependence of the surface anisotropy field as one of the components of effective anisotropy was corrected taking into account the thermal fluctuations. Discovered anomaly initiated calculation of the surface anisotropy constant with different approaches for two different temperature ranges. Analyzing the K S sign changes with temperature, we conclude about the transformation of surface anisotropy from plane to axis type.

MICRO-GRATING OF PSEUDOMORPHIC AMORPHOUS PHASE AND MICRO-CUBIC CRYSTALLINE PHASE OBTAINED BY ALKALI DISSOLUITON OF ISOTHERMAL TREATED NH4-ZEOLITE A

Ammonium exchanged zeolites A were isothermally treated at 400-1000°C for durations up to 100 min in the air, and then were leached in 1 M NaOH solution at 70°C. The products were characterized by XRD, SEM, ICP-AES and ATR-FTIR. The results from isothermal treatments confirmed that the pseudomorphic transformations took place through the formations of an amorphous phase with the tweed-like patterns on the surface of the cubic solid and a subsequent recrystallization to a cubic-shaped composite of mullite and amorphous material. These characterizations supported the existence of a thin surface layer of zeolite crystals acting as a microvessel, which was consistent with a previous hypothesis. After leaching by hot alkali solution, micro-grating patterns appeared on the surfaces of the pseudomorphic amorphous phase which exhibited the tweed-like patterns. A cubic solid with fine voids were obtained from a composite of mullite and an amorphous phase. The observed differences in the surface morphology are closely related to the investigated solids.