SEraMic: A semi-automatic method for the segmentation of grain boundaries

The SEraMic method, implemented in the SEraMic plugin for Fiji or ImageJ software, was developed to calculate a segmented image of a ceramic cross section that shows the grain boundaries. This method was used to accurately and automatically determine grain boundary positions and further assess the grain size distribution of monophasic ceramics, metals, and alloys. The only required sample preparation is polishing the cross section to a mirror-like finish. The SEraMic method is based on at least six backscattered electron scanning electron microscopy images of a unique region of interest with various tilt angles ranging from -5° to +5°, which emphasises the orientation contrasts of the grains. Because the orientation contrast varies with the incident beam angle on the sample, the set of images contains information related to all the grain boundaries. The SEraMic plugin automatically calculates and builds a segmented image of the grain boundaries from the set of tilted images. The SEraMic method was compared with classical thermal etching methods, and it was applied to determine the grain boundaries in various types of materials (oxides, phosphates, carbides, and alloys). The method remains easy to use and accurate when the average grain diameter is greater than or equal to 0.25 μm.     

A reaction model for the electrochemical production of p-anisidine

The work examines the possibility of a simple reaction model describing a complex organic electrosynthesis, such as the formation of p-anisidine. The experimental results obey the linear relationships of the model and in consequence the kinetic constants obtained in this way define reaction behaviour. The paper demonstrates how such a model can play a useful role in the design of pilot plant experimentation. Results from a parallel plate cell fit prediction from the model.Nomenclature [X] Concentration of species X (kmol m^–3) - b Slope of Tafel plot (mV^–1) - E Electrode potential (mV) - F Faraday (C g-equiv^–1) - F Faraday based on k-equiv = 10³F (C k-equiv^–1) - i _A Partial current density for the primary reaction (A m^–2) - i _B Partial current density for the consecutive secondary reaction (A m^–2) - i _H Partial current density for the parallel secondary reaction (A m^–2) - i Total current density=i _A+i _B+i _H (A m^–2) - k Reaction rate constant (A m^–2 per kmolm^–3) - k _H Rate constant for the parallel secondary electrode reaction (A m^–2) - k _I Individual mass transfer coefficient (m s^–1) - N Flux (kmol m^–2 s^–1) - r Reaction rate (kmol m^–2 s^–1) Sufixes A Appertaining to primary electrode reaction or species A - B Appertaining to consecutive secondary electrode reaction or species B - b In the bulk of the electrolyte - H Parallel secondary electrode reaction - s Near the electrode surface     

A reaction model for the electrochemical synthesis of adipontrile

A theoretical and experimental study of the electrohydrodimerization process to produce adiponitrile is used to determine an appropriate reaction model. From numerical simulations of five proposed reaction schemes and subsequent comparison with experimental data, the most favoured route is via an anion, intermediate of acrylonitrile. This route, a five step reaction involving electrochemical and chemical reaction in a diffusion/reaction layer, gives good predictions of product distributions over a wide range of current densities and acrylonitrile concentrations of product distributions over a wide range of current densities and acrylonitrile concentrations.     

Microstructures and grain boundaries of cubic boron nitrides

Two types of chemically pristine polycrystalline cubic boron nitrides (c-BN) are sintered at different conditions using the starting cubic and hexagonal BN powders and their microstructures and grain boundaries are investigated systematically by transmission electron microscopy. The two c-BN samples are found to exhibit a number of twins inside their grains and have a similar grain size, despite their huge differences in the grain size of the starting powders and sintering conditions. Twin width for the c-BN sintered from hexagonal BN is significantly smaller than that for the c-BN sintered from c-BN. Grain boundaries in the two samples can be atomically abrupt without any amorphous or secondary layers and oxygen is detected merely at the grain boundaries of the c-BN sintered from the c-BN powders. Such microstructural differences have a direct impact on mechanical behaviors of the c-BN, as the Vickers hardness of the c-BN sintered from hexagonal BN powders is found to be higher than that of the c-BN sintered from the c-BN powders.     

The impedance of ceramics with highly resistive grain boundaries: validity and limits of the brick layer model

Impedance spectroscopy is an important tool to investigate the electrical properties of grain boundaries. For the analysis of the impedance spectra cubic grains, laterally homogeneous grain boundaries and identical properties of all grain boundaries are usually assumed (brick layer model). However, in real ceramics these assumptions are generally violated. Using the finite element method we calculated the impedance of several polycrystals exhibiting highly resistive grain boundaries with microstructures and grain boundary properties deviating from the simple brick layer model. Detours around highly resistive regions (e.g. due to high grain boundary density or enhanced grain boundary resistivity) can play an important role and lead to grain-boundary semicircles depending on bulk properties and even to additional semicircles. Conditions are discussed within which the brick layer model allows for a reasonable evaluation of the spectra.     

Influence of grain interior and grain boundaries on transport properties of scandium-doped calcium zirconate

Calcium zirconate-based protonic conductors are currently the most promising electrolyte for high-temperature hydrogen sensors, however, protonic conductors exhibit mixed protons, oxygen vacancies and electron-holes conduction above 700°C, and the protons transport number is significantly influenced by the atmosphere. Therefore, the relationship between protons transport number and oxygen/water vapor partial pressure should be established to improve the veracity of the hydrogen sensor. Herein, CaZr_0.9Sc_0.1O_3-α perovskite oxides are prepared and the influence of grain interior and grain boundaries on transport properties is systematically investigated by using with defect chemistry theory. And the relationship between protons transport number and oxygen/water vapor partial pressure should be obtained. The results indicate that the dominant conduction carriers of CaZr_0.9Sc_0.1O_3-α were protons in Ar and reductive atmospheres at 500°C-800°C, and the conductivity () and transport number () of holes are remarkably increased with increasing oxygen partial pressure. In addition, protons, oxygen vacancies and electron-holes transport properties of grain interior and grain boundaries in scandium-doped calcium zirconate reveal that grains can effectively block oxygen vacancies transport at 550°C-800°C, but grains cannot block the holes transport. Therefore, the oxygen vacancies trend to transport through grain boundaries.     

A model for the electrochemical reduction of 2-ethylpicolinate under galvanostatic control

A model is presented for the variation in reactant concentrations during the electrochemical reduction of 2-ethylpicolinate on a lead cathod in sulphuric acid solutions under galvanostatic conditions. The electrolyses were performed in a laboratory filter-press reactor. Successive or parallel electrochemical reactions coupled with chemical reactions are taken into account, according to a reaction scheme in agreement with the experimental results. Analytical expressions are used to describe the progress of the various reactions, taking into account both chemical and electrochemical kinetics and transfer properties. All reactions are assumed to be first or pseudo-first order. The variations in charge-transfer rate constants are considered as functions of reactant conversion. The effects of acidity, current efficiency, initial concentration of 2-ethylpicolinate and temperature are presented. The model aims at estimating the yield of electrolysis products under the experimental conditions necessary for obtaining 2-hydroxymethylpyridine in optimum quantities.     

A general probabilistic model of electrochemical nucleation

A general probabilistic model is developed for the random time at which crystals appear in electrochemical nucleation. Using the model, formulae are found for the distribution functions, the density functions, and the moments of crystal radii during nucleation and growth. Similar formulae are found for crystal volumes and for electrical currents.The subsampling of crystals using small elements of electrode surface is also considered. General formulae are obtained for the moments of electrical currents observed on microelectrodes, and a special case is considered, namely, nucletion consisting of a Poisson point process along the time axis.In the final section of the paper, a model of electrochemical nucleation is developed which takes into account the fact that active sites on electrode surfaces may have a distribution of activities towards electrochemical nucleation.     

Change in the structural state of grain boundaries under high-rate mechanical loading

The molecular-dynamic modeling of the behavior of a three-dimensional crystallite that contains a specific-type grain boundary under shear loading is performed. It is found that the accommodation of displacements of the material grains can be realized owing to structural changes in the intergrain boundaries. The crystal-like structure of the grains can be restored after the external action terminates. The results obtained give deeper insight into the nature of the structural response of the material under mechanical loading at the atomic level. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 112–114, November–December 1999.     

A theoretical evaluation of the temperature and strain-rate dependent fracture strength of tilt grain boundaries in graphene

Based on molecular dynamic simulations, we investigate the effects of temperature and strain rate on the strength of single layer graphene with tilt grain boundaries under tension. The simulation results show that temperature plays an important role in the strength of graphene with grain boundaries. The strength of graphene with grain boundaries decreases significantly as temperature increases. In particular, we confirm a previous report that graphene with large angle tilt boundaries (which has a high density of defects) may be much stronger than that with low angle boundaries. This finding holds true for temperatures from 10 to 1800 K and strain rates from 0.0001 to 0.01 ps⁻¹.     

Mechanical strength characteristics of asymmetric tilt grain boundaries in graphene

We investigate the ultimate strengths and failure types of asymmetric tilt grain boundaries (GBs) with various tilt angles with the aid of molecular dynamics simulations and analytic theory. The tensile strength of armchair-oriented graphene GBs shows a tendency to increase as the misorientation angle rises, while that of zigzag-oriented graphene GBs non-monotonically increases. The overall strength enhancement and weakening behaviors can be explained by a continuum stress analysis of pentagon–heptagon defects along GBs. Nevertheless, full atomistic analyses are required to predict the exact bonds from which mechanical failure initiate. Two different fracture types are observed in our studies; one with cracks growing along the GB and another with cracks growing away from the GB. Detailed understanding of the atomic arrangement along the GB, in addition to defect density, is necessary to ascertain the ultimate strength and rupture process of GBs.     

Review of grain boundary complexion engineering: Know your boundaries

Grain boundary structure-property relationships influence bulk performance and, therefore, are an important criterion in materials design. Materials scientists can generate different grain boundary structures by changes in temperature, pressure, and chemical potential because interfaces attain their own equilibrium states, known as complexions. Complexions undergo first-order transitions by changes in thermodynamic variables, which results in discontinuous changes in properties. Grain boundary complexion engineering is introduced in this paper as a method for controlling complexion transitions to improve material performance. This International Conference on Sintering 2017 lecture describes the tools for grain boundary complexion engineering: complexion equilibrium and time-temperature-transformation (TTT) diagrams. These tools can be implemented in processing design to tailor grain boundary properties, including grain boundary mobility. While impactful, these diagrams are often limited in scope because they are currently empirically derived. This article discusses how measurement techniques can be combined with data analytical methods to build mechanistically derived complexion equilibrium and TTT diagrams.     

The mechanical responses of tilted and non-tilted grain boundaries in graphene

Various mechanical characteristics of tilted and non-tilted grain boundaries in graphene were investigated under tension and compression in directions perpendicular and parallel to the grain boundaries using molecular dynamics simulation. In contrast to the non-tilted grain boundary and the pristine graphene, the mechanical response of tilted grain boundary was observed to be quite unique under perpendicular tension, exhibiting distinct crack propagation prior to tensile failure and the subsequent pattern of incomplete fracture. These features are manifested as a remarkable decrease in the slope and a rugged pattern in the stress–strain curves. The characteristic of incomplete fracture was striking especially for large misorientation angles with formation of long monoatomic carbon chains, suggesting a methodology for feasible production of the monoatomic carbon chains that have been difficult to synthesize and extract. Under perpendicular compression, the folding of the sheet occurred consistently along grain boundaries during the entire process, indicating a tunable folding, while the folding line wandered extensively for pristine graphene. Under parallel compression, we found that folding along grain boundaries disturbed the bending of the graphene substantially for intrinsic reinforcement.     

Ionic conductivities and high resolution microscopic evaluation of grain and grain boundaries of cerium-based codoped solid electrolytes

Doped CeGdO and codoped CeGdOSmO compositions were synthesized, giving rise to nanoparticulate powders. Ionic conductivities at bulk and grain boundaries of the sintered samples were determined, exhibiting increased conductivity in the samaria-codoped samples. Scanning electron microscopy (SEM) showed a significant reduction in the grain size of samaria-codoped electrolytes. This reduced grain size of the codoped samples caused a reduction in Schottky barrier height, increasing oxygen vacancy concentration in the space-charge layer of the grain boundary and culminating in greater ionic conductivity in the boundary region. For the gadolinium doped samples, high resolution transmission electron microscopy images at grains showed the presence of large cluster of defects (nanodomains), hindering the movement of charge carriers and reducing ionic conductivity. However, the samaria-codoped system displayed better homogeneity at atomic level, resulting in reduced oxygen vacancy ordering and, consequently, smaller nanodomains and higher bulk (grain) conductivity. The reduced grain sizes and smaller nanodomains caused by codoping favor the ionic conductivity of ceria-based ceramics, doped with gadolinia and codoped with samaria.     

Local field electron emission from grain boundaries of CVD diamond film

In this article, we have investigated local field electron emission from grain boundaries of diamond films with (100) preferential orientation by double-probe scanning electron microscopy (SEM). Compared with the field emission from the plane area of diamond film, local field emission from grain boundary area is greatly enhanced at the same applied field, and further increased with the increasing of grain boundary number density. This result provides a direct evidence that grain boundary plays an important role in field emission from diamond film because a great deal of sp² graphitic carbon phases exists in grain boundary areas as electron transport channels for the surface field emission process.     

Structures and electronic properties of symmetric and nonsymmetric graphene grain boundaries

The graphene grain boundaries with periodic length up to 18 Å have been studied using density functional theory. Atomic structures, thermodynamic stabilities and electronic properties of 40 grain boundaries with symmetric and nonsymmetric structures were investigated. According to the arrangements of pentagons and heptagons on the boundary, grain boundaries were cataloged into four classes. Some nonsymmetric grain boundaries constructed here have identical misorientation angles to the experimentally observed ones. The formation energies of grain boundaries can be correlated with the misorientation angle and inflection angle. Nonsymmetric grain boundaries possess comparable formation energies to their symmetric counterparts when the periodic length along the defect line is larger than 1 nm. Analysis of electronic density of states shows that the existence of a grain boundary usually increases the density of states near the Fermi level, whereas some symmetric grain boundaries can open a small band gap due to local sp²-to-sp³ rehybridization.     

A model of the electrochemical behaviour within a stress corrosion crack

A mathematical model of the electrochemical behaviour within a stress corrosion crack is proposed. Polarization field, crack geometry, surface condition inside the crack, electrochemical kinetics, solution properties and applied stress can be represented by the polarization potential and current, the electrochemical reactive equivalent resistance of the electrode, the change in electrolyte specific resistance and surface film equivalent resistance, respectively. The theoretical calculated results show that (i) when anodic polarization potential is applied, the change in the crack tip potential is small; (ii) when cathodic polarization potential is applied, the crack tip potential changes greatly with the applied potential; (iii) the longer the crack, the smaller the effect of the applied potential on the crack tip potential in both anodic polarization and cathodic polarization conditions. The calculated results are in good agreement with previous experimental results.Notation coordinate, from crack mouth (on the metal surface) to crack tip (cm) - y y = s _L L/(s ₀ – s _L) + L – , function of (cm) - y ₀ y ₀ = s _L L/(s ₀ – s _L) + L (cm) - V polarization potential (V) - galvanic potential of electrode (V) - ₁ galvanic potential of electrolyte (V) - t sample thickness (cm) - w sample width (cm) - S _L crack tip width (cm) - S _o crack mouth width (cm) - L crack length (cm) - s() crack width at position (cm) - _lo specific resistance of electrolyte, as a constant ( cm) - _s specific resistance of metal ( cm) - (, y) specific resistance of electrolyte, varies with potential and crack depth ( cm) - R _b (, y) electrochemical reactive equivalent resistance of electrode, varies with potential and crack depth () - R ₁ electrolyte resistance () - R _s metal resistance () - r(, y) surface film equivalent resistance, varies with potential and crack depth () - r _o surface film equivalent resistance, as a constant () - I _o total polarization current (A) - I net polarization current from integrating 0 to in Fig. 2 (A) - polarization overpotential (V) - _a anodic polarization overpotential (V) - _c cathodic polarization overpotential (V) - Euler's constant