Intergrain Boundaries Research Articles

Geometry of polycrystals and microstructure

We investigate the geometry of polycrystals, showing that for polycrystals formed of convex grains the interior grains are polyhedral, while for polycrystals with general grain geometry the set of triple points is small. Then we investigate possible martensitic morphologies resulting from intergrain contact. For cubic-totetragonal transformations we show that homogeneous zero-energy microstructures matching a pure dilatation on a grain boundary necessarily involve more than four deformation gradients. We discuss the relevance of this result for observations of microstructures involving second and third-order laminates in various materials. Finally we consider the more specialized situation of bicrystals formed from materials having two martensitic energy wells (such as for orthorhombic to monoclinic transformations), but without any restrictions on the possible microstructure, showing how a generalization of the Hadamard jump condition can be applied at the intergrain boundary to show that a pure phase in either grain is impossible at minimum energy.

Metallographic study of construction materials with atomic force microscopy method

The results of atomic force microscopy (AFM) investigation of the micro- and nanostructure of 09G2S steel are presented. The method was developed for investigation of construction steel with AFM which allowed improving the quality and reliability of metallographic studies. Identification of the phase state of a material and determination of such microstructure parameters as grain size, pearlite fraction, the size of pearlite colonies, pearlite structural characteristics, and the width of intergrain boundary areas were performed on the basis of AFM images of 09G2S steel.

Interrelation between catalytic activity for oxygen electroreduction and structure of supported platinum

Electrocatalytic activity for the oxygen reduction reaction (ORR) of the carbon-supported Pt catalysts obtained by chemical and electrochemical deposition has been studied in 0.10M HClO4 at 293K in the expectation to gain new insights into the influence of the substructure of platinum on its catalytic properties for the ORR. Electron microscopy, XRD and electrochemical techniques have been employed to demonstrate that: (i) the ‘chemically’ synthesised Pt/C catalysts contain highly-dispersed isolated metal particles at intermediate metal loadings and both isolated crystallites and crystallite aggregates at higher loading; (ii) the electrodeposited Pt/C materials are ‘nanostructured’, i.e. contain large (up to ca 350nm) aggregates with the deposition potential dependent morphology composed of comparatively small (<20nm) platinum crystallites. No structural dependence of the ORR catalytic properties for the ‘chemically’ synthesised Pt/C catalysts has been revealed, except for the well-known negative size effect. On the contrary, platinum electrodeposited on glassy carbon or furnace black demonstrates atypically high ORR catalytic activity, especially, when electrodeposition is undertaken at 0.10V. Presumably, easily oxidisable defect sites, intergrain boundaries in particular, present at comparatively high concentrations in the electrodeposited materials provide efficient adsorption of O2, which further reduction occurs at the highly catalytically active Pt(111) facets. Structural defects present at an optimal concentration are concluded to promote the ORR catalytic activity of platinum, but excessive defectiveness of the Pt catalyst might hinder the kinetics of the oxygen electroreduction.

New possibilities to obtain ceramic nanoheterostructures with enhanced ionic conductivity

New possibilities to improve the transport characteristics of ceramic electrode materials, which are important for improvement high-energy electrochemical devices, lithium-ion batteries (LIBs), and solid oxide fuel cells (SOFCs), have been determined. Since the full-scale search for new electrochemically active inorganic structures does not promise substantial progress, it is topical to use the possibilities associated with optimizing the structural parameters of already known materials. It is shown that an enhancement in the specific power of these devices associated with an increase in the ionic conductivity of electrode materials can be achieved at the expense of a combination of size effects and experimentally proven effects of amorphization, i.e., the effects of nonautonomic phases formed on the interfaces of composites from incommensurate structural elements. They can provide a substantial increase in the diffusion mobility of lithium and oxygen ions at intergrain and interphase boundaries. Simple experimental methods to obtain such multi-structured nanomaterials based on electrochemically active phases for LIB positive electrodes are proposed.

Plasma sputtering of polycrystalline Pb1−xSnxTe thin films grown on glass substrates using hot wall deposition

In this work the investigations of the argon inductively coupled plasma sputtering of the Pb1−xSnxTe thin films with the composition variation of x = 0.16–0.95 grown by hot wall deposition technique on glass substrates were carried out. As-grown films had a columnar polycrystalline structure with the grain lateral dimensions of 0.2–5.0 μm, and the dependence of the lattice constant on composition x had a linear behaviour described by the Vegard’s law. Energy dispersive x-ray microanalysis showed the presence of 5–8 at.% of oxygen in the films, which can be accumulated from the ambient air or from the substrate in the inter-grain boundaries. A phenomenon of a sputtering rate decrease for the polycrystalline lead tin telluride films in comparison to the single-crystal films is discussed. A novel important phenomenon of the formation of nanostructure arrays on the surface of the Pb1−xSnxTe thin films with the dependence on the sputtering rate during plasma treatment is reported.

Correlation Between Magnetoresistance and Magnetization Hysteresis in a Granular High-T C Superconductor: Impact of Flux Compression in the Intergrain Medium

The correlation between experimental magnetic field dependences of magnetoresistance and magnetization hysteresis in granular YBa2Cu3O7 is established. Within the proposed approach, magnetoresistance is assumed to be determined by the effective field in the intergrain boundaries the ensemble of which is considered to be a Josephson medium. The effective field in the intergrain medium can be written in the form Beff(H)=H−4πM(H)×α, where α is the parameter of averaged demagnetizing factors of grains and the degree of flux compression. A comparison of experimental magnetoresistance R(H) and magnetization M(H) hysteresis dependences obtained at different external magnetic field sweep rates yields the value α∼10, which is caused by the flux compression between grains. The proposed model describes well most of the features of the magnetoresistance hysteresis in granular high-TC superconductors.

Microscopic Model of Intergrain Boundary Junction

The statistical mechanics differential-difference equations for the ions concentration distribution that account for the diffusion, electrical conductivity and Poisson contributions are derived. They represent differential-difference analogues of the phenomenological continuity equations for the ions. Spatial inhomogeneities (grain interiors, grain boundaries, intergrain regions, etc.) can easily be taken into account by proper adjusting the system material parameters (diffusion coefficients or particle transition rates, electric conductivities, thermodynamic factors or chemical capacitances). The solution of the equations allows investigating impedance spectra of inhomogeneous systems, e. g. electro-conducting ceramics. The results can be used for interpretation of experimental impedance spectra and evaluation of the medium transport characteristics. A simple example of the intergrain boundary junction is considered.

Influence of Crystallite Size on Cation Conductivity in Faujasitic Zeolites

The influence of particle size on the ionic conductivity of ceramic materials is an active area of research, and novel effects are observed as particles approach the nanoscale in size. Zeolites are crystalline aluminosilicates with ion-exchangeable cations that are responsible for ionic conductivity at high temperatures. In this paper, we present systematic results for the first time of ionic conductivity in alkali metal ion-exchanged faujasitic zeolites with morphologies ranging from a zeolite membrane, micrometer-sized, submicrometer, and nanoparticles of zeolite. Using impedance spectroscopy in the range of 10 MHz to 0.1 Hz, we have obtained the activation energy (E(act)) of cation motion with these various morphologies in the temperature range of 525-625 °C. Overall, the E(act) decreases with Si/Al ratio. Surface modification of the zeolite particles was carried out with a silylating agent, which upon high temperature calcination should lead to the formation of a monolayer Si-O-Si film on the particle surface. This surface modification had minimal influence on the E(act) of micrometer-sized zeolites. However, E(act) increased rapidly as the zeolite particle approached the nanoscale. These observations led us to propose that, for the high-temperature, low-frequency (10(4)-10(5) Hz), long-range ionic conduction in zeolites, cation hopping across grain boundaries is relevant to ion transport, especially as the size of the crystallite approaches the nanoscale. Intergrain boundaries are more defective in the nanosized zeolite and contribute to the higher E(act).

Features of the crystal structure and electrical properties of sodium chloride at pressure 20–50 GPa

The electrical properties of polycrystalline samples of sodium chloride are studied at direct and alternating current in a wide range of frequencies at high pressure and room temperature. Graphic analysis of the experimental data [1–3] in the view of equivalent circuits allowed us to separate the contributions to conductivity caused by grains and intergrain boundaries. Features of impedance at pressure up to 37 GPa are in good agreement with earlier data and structural changes. It is shown that in the studied materials the electrical resistance of grains is much greater than the resistance of intergrain boundaries.

Phonon spectroscopy of the nano- and microstructured ceramics

A relation of the diffusion coefficient of the subterahertz thermal phonons with the system of intergrain boundaries, grain size and structure, and the technological conditions for the synthesis of ceramics is established. It is demonstrated that the temperature dependence of the phonon diffusion coefficient at liquid-helium temperatures is determined by the spectral properties of the intergrain layer, which makes it possible to estimate the sample-averaged thickness and acoustic impedance. The conditions for and properties of the gap in the natural phonon spectrum of ceramics are studied. Specific features of the transport of sub-terahertz phonons in ceramic samples based on ferroelectric materials, solid solutions of electrolytes, and cermets are analyzed.

PtSn/C catalysts for ethanol oxidation: The effect of stabilizers on the morphology and particle distribution

PtSn/C catalysts are synthesized by using four stabilizers, i.e., ethylene diamine tetra-acetic acid (EDTA), sodium citrate (NaCitrate), glycine and P-aminobenzoate (PABA) with the same reducing process and reaction parameters. XRD characterization shows all PtSn/C catalysts possess face-centered cubic structure with different alloying degree. TEM results show that stabilizers have a significant impact on the morphology and particle size distribution. PtSn/C EDTA and glycine have good particle dispersion with sphere and short nanowire morphology, while obvious particle agglomeration occurred on PtSn/C NaCitrate and PtSn/C PABA. Electrocatalytic activities of these PtSn/C catalysts for ethanol electrooxidation are also measured by cyclic voltammetry. Enhancement of electrocatalytic activity is ascribed to the high particle distribution of PtSn nanoparticles on the carbon supports, not the alloying degree of PtSn nanoparticles. The difference of glycine activities between PtSn/C EDTA and PtSn/C glycine could be ascribed to the variation in the amount of Sn oxides on the surface and density of inter-grain boundary regions in PtSn nanoparticles.

Zeolite Imidazolate Framework–Ionic Liquid Hybrid Membranes for Highly Selective CO2 Separation

Zeolitic imidazolate framework ZIF-69 membranes were grown on porous α-alumina substrates via seeded secondary growth and further functionalized by a CO2-selective tricyanomethanide anion/alkylmethylimidazolium cation-based ionic liquid (IL) to plug the gaps between the ZIF crystals yet leave the framework pores open for gas diffusion. In this configuration, ZIF intergrain boundaries and defects were repaired by a medium that exhibits high selectivity for CO2. As a result, the selectivity of the hybrid membrane was significantly higher than that of as-grown ZIF membranes and, because of the existence of the ZIF channels, the permeability was higher than that corresponding to bulk IL. Specifically, CO2 permeated 20 times faster than N2 through the intact ZIF pores and 65 times faster than through the bulk IL phase. The developed membranes at room temperature and under a 2 bar transmembrane pressure exhibited CO2 permeance of 5.6 × 10–11 and 3.7 × 10–11 mol m–2 s–1 Pa–1 and real CO2/N2 selectivities of 44 a...

Internal (SiH)X groups, X = 1–4, in microcrystalline hydrogenated silicon and their IR spectra on the basis of periodic DFT modelling

Vibrational Si–H frequencies were calculated on the basis of density functional theory (DFT) using periodic boundary conditions for N-Si voids, N < 8, in microcrystalline hydrogenated silicon (MHS) and (100), (110), and (111) slabs of 8, 5, and 8 layers, respectively, with the dangling bonds being saturated with hydrogen atoms. The slabs are considered as the models of inter-grain boundaries (IGB) in MHS. The N-Si voids of different shapes have been obtained via random deleting N silicon atoms. It was shown that the high stretching modes (HSM) of Si–H vibrations, which are usually assigned to SiHX, appear also due to (SiH)X groups, X = 2–4, in the N-Si voids. No such (SiH)X groups were formed with X > 1 at the IGB. The low stretching modes (LSM) are thus assigned to Si–H groups presented at both N-Si voids and IGB. Similar relative stability of the voids is obtained with two different DFT approaches, i.e., B3LYP with atomic basis set and Perdew-Burke-Ernzerhof (PBE) with plane wave basis set. This result allows a simple interpretation of usually small IHSM/(ILSM + IHSM) intensity ratio as a consequence of minor concentration of any voids in device quality MHS.

Temperature effects in carbon monoxide and methanol electrooxidation on platinum–ruthenium: influence of grain boundaries

The temperature dependence of methanol and CO monolayer oxidation is studied on carbon-supported PtRu (1:1 atomic ratio) electrodes with different metal percentages (5, 30, and 60 wt.%) in an aqueous H2SO4 electrolyte. High-resolution transmission microscopy confirms that at high (30 or 60 wt.%) metal percentage PtRu nanostructures with a high concentration of intercrystalline boundaries are formed. These nanostructures comprise multiple-twinned particles, particles with intersecting randomly oriented intergrain boundaries, or particles with parallel intergrain boundaries. Formation of such nanostructures leads to a decrease of the apparent activation energy of the methanol and CO monolayer oxidation, while the Tafel slope and the reaction order in methanol show minor dependence on the type of nanostructure. Materials with a high concentration of grain boundary regions may be of interest for practical applications in direct methanol or proton exchange fuel cells fed with reformate.

Magnetovoltage Measurements and Hysteresis Effects in Polycrystalline Superconducting Y1Ba2Cu3O7−x /Ag in Weak Magnetic Fields

The magnetovoltage measurements (V–H curves) with different sweeping rates (dH/dt) of the external magnetic field in Ag-added polycrystalline YBa2Cu3O7−x sample (YBCO/Ag) were investigated. The measurements of V–H curves were carried out as functions of the transport current (I) and temperature (T). Upon cycling H, all V–H curves measured for different values of I exhibit a clockwise hysteresis effects in forward region. The hysteresis effects in the V–H curve were interpreted in terms of two-level magnetic system, which considers the superposition of the external magnetic field and the local magnetic fields in the intergrain boundaries induced by magnetic dipole moment of neighbor superconducting grains. The analysis of magnetovoltage data showed that the flux trapping in the junction network has a negligible effect on the evolution of the V–H curves and the irreversibilities arise mainly from the flux trapping inside the grains. It is shown that the width of V–H curves shows a universal scaling behavior with respect to the applied current below the critical temperature T c . The comparison of V–H curves of the YBCO/Ag sample with those of YBCO shows that adding Ag to the superconducting structure weakens the pinning properties of Josephson medium and provokes the instabilities in measured dissipation. The presence of Ag in the superconducting matrix causes marked decrease in hysteresis effects and makes the V–H curves dH/dt dependent. At high values of dH/dt, the instabilities and plateau regions in V–H curves increase significantly as compared to those of observed in YBCO. In addition, the interrelation between the evolution of V–H curves obtained for different values of I and the critical current I c was demonstrated experimentally.

Experimental study of the built-in field in intergrain channels of thin ferroelectric Pb(Zr,Ti)O3 films

The experimentally measured dependence of the photo-emf on the remanent polarization in thin-film M/Pb(Zr,Ti)O3(PZT)/M capacitors correlates well with the model developed by us for the intergrain photovoltaic effect for films with columnar structure of PZT grains and heterophase intergrain boundaries. In this case, the photo-emf is determined by the depolarization field generated by the uncompensated polarization charge at PZT grain boundaries. It is shown that the magnitude and orientation of the built-in field in an intergrain channel of such PZT films can be derived from measurements of the photo-emf at zero polarization with a sensitivity on the order of a few millivolts.

Superconductivity of individual grains and inter-grain boundaries for polycrystalline [formula omitted

Polycrystalline FeSr2YCu2O6+y was synthesized and its transport and magnetic properties were studied. Diamagnetism was observed below 60K. Zero resistivity was observed below 38K under zero magnetic field and below 10K under 160kOe. A two-step transition was observed in resistivity measurement due to the superconductivity in individual grains and across inter-grain boundaries. The critical current density in individual grains, Jcintra, at 2K under 1kOe was deduced 3.4×105A/cm2 from the Bean model. In contrast, the critical current density in inter-grain boundaries, Jcinter, at 2K was 1.7A/cm2 in voltage–current measurement. The two-step transition seems to result from the large difference between Jcintra and Jcinter.

Peculiarities of aluminium interaction with Ga85In15 eutectics as evidenced by X-ray synchrotron diagnostics

A set of X-ray synchrotron techniques, viz., diffraction, EXAFS/XANES spectroscopy and microtomography, is applied to elucidate microstructural changes in a technical aluminium alloy treated with GaIn eutectics. Such a treatment gives rise simultaneously to a prominent enbrittlement of the material and its activation towards reaction with water with the hydrogen evolution. The latter fact makes the activated aluminium a promising energy carrier for the small-scale hydrogen energetics. It is demonstrated that both phenomena are caused by the fast diffusion of the eutectics along intergrain boundaries and microcracks throughout the bulk of polycrystalline Al. The diffusion is promoted by the formation of (Al-Ga-In) solid solution in near-surface regions of Al crystalline grains. The progressive loss of activity of aluminium treated with GaIn eutectics upon a prolonged storage in humid air is due to the decomposition of the eutectics accompanied by the segregation of indium metal and partial gallium oxidation.

Structural analysis of the (BiScO3)1−x -(PbTiO3) x ceramic materials using the resonance and phonon spectroscopy

The resonance acoustic spectroscopy shows significant structural ordering of the (BiScO3)1 − x -(PbTiO3) x ceramic material in the course of polarization, which leads to an additional increase in the velocity of acoustic waves. The decay rate of acoustic waves increases with increasing frequency and parameter x. The analysis of the ceramic material using the method of thermal pulses at liquidhelium temperatures yields sufficiently high quality of the intergrain boundaries.

Charnockite microstructures: From magmatic to metamorphic

Charnockites sensu lato (charnockite-enderbite series) are lower crustal felsic rocks typically characterised by the presence of anhydrous minerals including orthopyroxene and garnet. They either represent dry (H2O-poor) felsic magmas that are emplaced in the lower crust or granitic intrusions that have been dehydrated during a subsequent granulite facies metamorphic event. In the first case, post-magmatic high-temperature recrystallisation may result in widespread metamorphic granulite microstructures, superimposed or replacing the magmatic microstructures. Despite recrystallisation, magmatic remnants may still be found, notably in the form of melt-related microstructures such as melt inclusions. For both magmatic charnockites and dehydrated granites, subsequent fluid-mineral interaction at intergrain boundaries during retrogradation are documented by microstructures including K-feldspar microveins and myrmekites. They indicate that a large quantity of low-H2O activity salt-rich brines, were present (together with CO2 under immiscible conditions) in the lower crust.