Microstructural Characterization Techniques Research Articles

Insight into Al–Si interface of PERC by Kelvin probe force microscopy

Passivated emitter and rear cell (PERC) has the advantage of higher short circuit current and open circuit voltage, which are generally claimed to be related to the reduction of rear side recombination and the increase of rear surface reflection. However, few works have focused on exploring the internal conducting mechanism about it. Herein, the influence of PERC technique on improving the short circuit current is investigated by comparing a PERC with a single crystalline silicon (sc-Si) solar cell. The surface potential results measured by Kelvin probe force microscopy show a higher surface potential step at the Al–Si interface of PERC than that of sc-Si cell, indicating a severe energy band variation and a better carrier collecting ability of PERC. Moreover, by using advanced microstructure characterization techniques, the relationship among the surface potential step, morphology and element distribution is fully studied, which proposes a new viewpoint to explain the enhanced performance of PERC.

Microstructural Characterization of Calcite Mineral Precipitation in Bacteria Incorporated Concrete

Metabolic activity of alkali-philic calcite (CaCO3) mineral precipitating bacteria when introduced into concrete heals the cracks and improves the microstructure of the concrete. This process of bio-mineralization by mineral producing bacteria can be characterized and quantified by using microstructure characterization techniques. The present paper is focused on characterizing the mineral precipitation in concrete by Sporosarcina pasteurii as calcite using SEM, XRD and TGA nano-characterization procedures to validate that cracks and pores in bacteria incorporated concrete were closed with the mineral precipitates produced due to urealotic activity of bacteria by hydrolyzing the urea based nutrients.

Development of High-Fidelity Imaging Procedures to Establish the Local Material Behavior in Friction Stir Welded Stainless Steel Joints

Friction stir welded (FSW) 304 austenitic stainless steel (SS) joints are studied using a range of microstructural characterization techniques to identify various sub-regions across the weld. A high-resolution (HR) 2D-digital image correlation (DIC) methodology is developed to assess the local strain response across the weld surface and cross-section in the elastic regime. The HR-DIC methodology includes the stitching of multiple images, as it is only possible to partially cover the FSW region using a single camera with the high-resolution optical set-up. An image processing procedure is described to stitch the strain maps as well as strain data sets that allow full-field strain to be visualized and interrogated over the entire FSW region. It is demonstrated that the strains derived from the DIC can be associated with the local weld geometry and the material microstructure in the region of the FSW. The procedure is validated in the material elastic range and provides an important first step in enabling detailed mechanical assessments of the local effects in the FSW process.

Advanced Technique Development for Li-Ion Electrode Characterisation Using X-Ray Computed Tomography

Porous electrodes are the fundamental building block of most modern battery designs. Historically, 1-dimensional models have been used to predict battery performance at low cycling rates, but these fail to account for degradation after extended cycling due to their oversight of the 3-dimensional nature of electrode microstructures. The advancement of X-ray computed tomography (CT) has allowed for imaging of battery materials in a non-destructive way with a range of ex-situ, in-situ and operando techniques.1–3 We present the development of a suite of techniques devised to further the understanding of these complex microstructures, using Lithium Nickel Manganese Cobalt Oxide (NMC) as a model electrode and characterising it from the nano- to the micro-scale using different approaches. In the first instance ex-situ nano- and micro-CT were applied to compare how the microstructural evolution of an electrode calendered with a pellet press differs from that of one compressed with a calendering roller. Particle shape analysis and the characterisation of the porous networks and their associated transport properties were carried out and a novel laser micro-machining technique used to manufacture geometrically optimised samples for nano-CT is also discussed. This allows creating 80 μm electrode pillars that maintain their structure and directionality, whilst providing excellent resulting image quality.4 In a second instance, to overcome the limitations of X-ray CT in imaging the carbon and binder domain (CBD), a novel correlative approach combining multi-length scale CT and focused ion beam-scanning electron microscope (FIB-SEM) is presented.5 FIB-SEM and nano-CT were used to calculate the effective diffusivity of the CBD porous network by using the aforementioned analytical tools and sample preparation techniques. The CBD diffusivity is turn incorporated into a larger NMC framework obtained via micro-CT. The specific contribution of the CBD is hence discussed in the context of overall ionic transport of a full electrode. Finally, a technique to compress electrodes whilst imaging them in-situ with nano-CT is presented. This was developed with the intent of emulating calendering and allows the tracking of active material particles as a function of applied load. Strain hot-spots as a result of the movement of secondary particles were identified in the 3-D volume by using a digital volume correlation algorithm. A suite of microstructural characterisation and quantification techniques examining the solid volume fraction, inter-particle separation and general morphology of the electrode were also applied. Further application of this technique will allow for the development of complex multi-phase models that will be able to accurately predict the electrochemical performance of calendered electrodes as a function of load. D. P. Finegan et al., Nat. Commun., 6, 6924 (2015) http://www.ncbi.nlm.nih.gov/pubmed/25919582.O. O. Taiwo et al., J. Power Sources, 342, 904–912 (2017).A. Yermukhambetova et al., Sci. Rep., 6, 35291 (2016) http://www.nature.com/articles/srep35291.J. J. Bailey et al., J. Microsc., 0, 1–13 (2017) http://doi.wiley.com/10.1111/jmi.12577.S. R. Daemi et al., ACS Appl. Energy Mater., 1, acsaem.8b00501 (2018) http://pubs.acs.org/doi/10.1021/acsaem.8b00501.

Influence of Nb Content on Sensitization and Pitting Corrosion of Welded AISI 430 Ferritic Stainless Steel

Abstract In this work, the effect of Nb addition on sensitization and pitting corrosion of welded AISI 430 ferritic stainless steel subjected to autogenous GTAW was investigated by microstructural characterization and electrochemical techniques. The three distinct regions of welding process, weld metal, heat affected zone and base metal of AISI 430 steel with and without Nb were evaluated. The degree of sensitization was measured by double loop electrochemical potentiodynamic reactivation and pitting corrosion was studied by potentiostatic polarization test. The microstructural analyses reported martensite network in the weld metal and heat affected zone of AISI 430 non stabilized. The electrochemical studies revealed that the highest degree of sensitization and the lowest pitting potential are in the weld metal of AISI 430 without Nb.

A Kinetic Study on the Evolution of Martensitic Transformation Behavior and Microstructures in Ti–Ta High-Temperature Shape-Memory Alloys During Aging

Ti–Ta alloys represent candidate materials for high-temperature shape-memory alloys (HTSMAs). They outperform several other types of HTSMAs in terms of cost, ductility, and cold workability. However, Ti–Ta alloys are characterized by a relatively fast microstructural degradation during exposure to elevated temperatures, which gives rise to functional fatigue. In the present study, we investigate how isothermal aging affects the martensitic transformation behavior and microstructures in Ti70Ta30 HTSMAs. Ti–Ta sheets with fully recrystallized grain structures were obtained from a processing route involving arc melting, heat treatments, and rolling. The final Ti–Ta sheets were subjected to an extensive aging heat treatment program. Differential scanning calorimetry and various microstructural characterization techniques such as scanning electron microscopy, transmission electron microscopy, conventional X-ray, and synchrotron diffraction were used for the characterization of resulting material states. We identify different types of microstructural evolution processes and their effects on the martensitic and reverse transformation. Based on these results, an isothermal time temperature transformation (TTT) diagram for Ti70Ta30 was established. This TTT plot rationalizes the dominating microstructural evolution processes and related kinetics. In the present work, we also discuss possible options to slow down microstructural and functional degradation in Ti–Ta HTSMAs.

Microstructural Materials Design Via Deep Adversarial Learning Methodology

Identifying the key microstructure representations is crucial for computational materials design (CMD). However, existing microstructure characterization and reconstruction (MCR) techniques have limitations to be applied for microstructural materials design. Some MCR approaches are not applicable for microstructural materials design because no parameters are available to serve as design variables, while others introduce significant information loss in either microstructure representation and/or dimensionality reduction. In this work, we present a deep adversarial learning methodology that overcomes the limitations of existing MCR techniques. In the proposed methodology, generative adversarial networks (GAN) are trained to learn the mapping between latent variables and microstructures. Thereafter, the low-dimensional latent variables serve as design variables, and a Bayesian optimization framework is applied to obtain microstructures with desired material property. Due to the special design of the network architecture, the proposed methodology is able to identify the latent (design) variables with desired dimensionality, as well as capturing complex material microstructural characteristics. The validity of the proposed methodology is tested numerically on a synthetic microstructure dataset and its effectiveness for microstructural materials design is evaluated through a case study of optimizing optical performance for energy absorption. Additional features, such as scalability and transferability, are also demonstrated in this work. In essence, the proposed methodology provides an end-to-end solution for microstructural materials design, in which GAN reduces information loss and preserves more microstructural characteristics, and the GP-Hedge optimization improves the efficiency of design exploration.

Comparative Study of Two Aging Treatments on Microstructure and Mechanical Properties of an Ultra-Fine Grained Mg-10Y-6Gd-1.5Zn-0.5Zr Alloy

Developing high strength and high ductility magnesium alloys is an important issue for weight-reduction applications. In this work, we explored the feasibility of manipulating nanosized precipitates on LPSO-contained (long period stacking ordered phase) ultra-fine grained (UFG) magnesium alloy to obtain simultaneously improved strength and ductility. The effect of two aging treatments on microstructures and mechanical properties of an UFG Mg-10Y-6Gd-1.5Zn-0.5Zr alloy was systematically investigated and compared by a series of microstructure characterization techniques and tensile test. The results showed that nano γ’’ precipitates were successfully introduced in T5 peak aged alloy with no obvious increase in grain size. While T6 peak aging treatment stimulated the growth of α-Mg grains to 4.3 μm (fine grained, FG), together with the precipitation of γ’’ precipitates. Tensile tests revealed that both aging treatments remarkably improved the strengths but impaired the ductility slightly. The T5 peak aged alloy exhibited the optimum mechanical properties with ultimate strength of 431 MPa and elongation of 13.5%. This work provided a novel strategy to simultaneously improve the strength and ductility of magnesium alloys by integrating the intense precipitation strengthening with ductile LPSO-contained UFG/FG microstructure.

Effect of heat treatments on microstructural evolution of additively manufactured and wrought 17-4PH stainless steel

Additively manufactured (AM) components usually have nonequilibrium microstructures. Post-built heat treatments are recommended for AM components to achieve homogenous microstructures. In this study, the effects were investigated of conventional solutionizing and precipitation hardening (H-900) heat treatments on the microstructure evolution of 17-4PH AM and wrought components. Microstructural characterization techniques including SEM, TEM and EBSD analysis were used on 17-4PH AM and wrought components to obtain quantitative information about the microstructure and phase evolution during these heat treatments. These microstructural studies demonstrate that 17-4PH AM components can achieve microstructures and hardnesses similar to those of wrought samples by post-built heat treatments.

The effect of high-temperature water chemistry and dissolved zinc on the cobalt incorporation on type 316 stainless steel oxide

Oxidation tests on Type 316 stainless steel were performed under hydrogen water chemistry and normal water chemistry for 500 h with continuous injection of a 59Co solution with and without 5 ppb of Zn injection. The present paper identifies the resultant oxides, analysed using analytical electron microscopy and complementary surface microstructural characterisation techniques. Zn injection has been shown to reduce Co incorporation in the inner oxide layer under both water chemistry conditions. The secondary effects of Zn injection on the oxide film growth have also been investigated and discussed.

Dealloyed Pt3Co nanoparticles with higher geometric strain for superior hydrogen evolution reaction

In the present work, the effect of surface strain in the carbon supported Pt3Co dealloy catalyst towards hydrogen evolution reaction (HER) has been reported. Dealloying process is adopted to generate the geometric strain in Pt3Co/C alloy by preferential dissolution of non-noble metal (Co) from the alloy. The developed geometric strain has been estimated by different microstructural characterization techniques. Electrochemical studies showed that the highest current density for HER was obtained for Pt3Co/C dealloy catalyst and it was nearly 2 and 5 times higher than Pt3Co/C alloy and Pt/C respectively. Tafel slope for HER was improved from 49 (Pt/C) to 34 mV dec−1 (Pt3Co/C dealloy), indicating that the surface strain plays important role in the improvement of the catalytic activity of Pt3Co catalyst. The chronoamperometry data, LSV curves and ECSA values before and after chronoamperometry confirmed that Pt3Co/C dealloy catalyst was a stable as well as a durable electrocatalyst for HER.

An electrochemical study of silver recovery in thiosulfate solutions. A window towards the development of a simultaneous electroleaching-electrodeposition process

The simultaneous electroleaching-electrodeposition of silver was studied systematically to develop an alternate process for the recovery of metallic silver. The system was assessed employing electrochemical and microstructural characterization techniques in a thiosulfate-ammonia medium. The electrochemical studies revealed the dependence of silver electroleaching rate in thiosulfate-ammonia solutions on different processes controlled by anodic potential. At the anodic potential below 200mVSHE, the silver is electro-oxidized and complexed by the applied potential and thiosulfate, respectively. Increasing the potential up to 445mVSHE, the thiosulfate is oxidized and the silver leaching current is decreased due to the diffusional control of the leaching. It was also found that the sulfite species formed from the thiosulfate decomposition can leach small amounts of metallic silver. Finally, at potentials from 695 to 795mVSHE, the thiosulfate is further decomposed and a silver sulfide film is generated which inhibits the silver dissolution. The simultaneous electroleaching-electrodeposition of silver was also studied in an electrolytic cell with graphite electrodes at room temperature. The results showed that a 74% of silver can be recovered at a low applied current density of 0.066mA/cm2 with a negligible thiosulfate decomposition and a current efficiency of 76.54%. On the other hand, when the current is increased the silver recovery and current efficiency is decreased, due to the oxidative decomposition of thiosulfate which promotes the formation of silver sulfide. These results are corroborated by the X-ray diffraction and SEM-EDS analyses.

Exploring the Microstructure of Ice

Abstract Characterizing ice and snow is important not only for building accurate climate models, but also for activities such as relating the mechanical properties of sea ice to its microstructure so that the interaction of ice with ships and structures can be better understood. This article describes the microstructural characterization of ice. Many microstructural characterization techniques that can be applied to other materials also can be used to examine ice.

Microstructural characterization of catalysis product of nanocement based materials: A review

Cement as an essential element for cement-based products contributed to negative environmental issues due to its high energy consumption and carbon dioxide emission during its production. These issues create the need to find alternative materials as partial cement replacement where studies on the potential of utilizing silica based materials as partial cement replacement come into picture. This review highlights the effectiveness of microstructural characterization techniques that have been used in the studies that focus on characterization of calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) formation during hydration process of cement-based product incorporating nano reactive silica based materials as partial cement replacement. Understanding the effect of these materials as cement replacement in cement based product focusing on the microstructural development will lead to a higher confidence in the use of industrial waste as a new non-conventional material in construction industry that can catalyse rapid and innovative advances in green technology.

Experiments on surface hardening of aluminum components by high-energy centrifugal milling

This work investigates a surface-hardening technique applied to pure aluminum rods via dry centrifugal milling. The intrinsically hard binary Fe-Cu material powder, also produced by high-energy ball milling, is incorporated into the surface of the samples to enhance their mechanical properties. The process has the rods and Cu-Fe powder placed in the grinding bowl so the deposition takes place under high centrifugal forces. These forces cause the particles integrate uniformly with the surface of the rods according to the process parameters, such as rotational speed, filling ratio, and time. Various microstructural characterization techniques (X-ray diffraction, scanning electron microscopy, and glow discharge analysis) are applied to investigate the deposition quality, as well as standard tests conducted to assess the improvement of mechanical properties, such as the compression and the micro hardness tests. The results show tangible benefits to the mechanical properties and a significant increase in hardness using this facile process.

Preventing damage and redeposition during focused ion beam milling: The “umbrella” method

Focused ion beam (FIB) milling has enabled the development of key microstructure characterization techniques (e.g. 3D electron backscatter diffraction (EBSD), 3D scanning electron microscopy imaging, site-specific sample preparation for transmission electron microscopy, site-specific atom probe tomography), and micro-mechanical testing techniques (e.g. micro-pillar compression, micro-beam bending, in-situ TEM nanoindentation). Yet, in most milling conditions, some degree of FIB damage is introduced via material redeposition, Ga+ ion implantation or another mechanism. The level of damage and its influence vary strongly with milling conditions and materials characteristics, and cannot always be minimized. Here, a masking technique is introduced, that employs standard FIB-SEM equipment to protect specific surfaces from redeposition and ion implantation. To investigate the efficiency of this technique, high angular resolution EBSD (HR-EBSD) has been used to monitor the quality of the top surface of several micro-pillars, as they were created by milling a ringcore hole in a stress-free silicon wafer, with or without protection due to an “umbrella”. HR-EBSD provides a high-sensitivity estimation of the amount of FIB damage on the surface. Without the umbrella, EBSD patterns are severely influenced, especially within 5 µm of the milled region. With an optimized umbrella, sharp diffraction patterns are obtained near the hole, as revealed by average cross correlation factors greater than 0.9 and equivalent phantom strains of the order 2 × 10−4. Thus, the umbrella method is an efficient and versatile tool to support a variety of FIB based techniques.

Corrosion fatigue and microstructural characterisation of Type 316 austenitic stainless steels tested in PWR primary water

Fatigue crack growth experiments have been performed on high and low sulphur Type 316 austenitic stainless steel specimens in simulated Pressurised Water Reactor primary coolant environments and evaluated via microstructural characterisation techniques to further the understanding of the mechanistic behaviour. At relatively high loading frequencies, the enhanced crack growth for both specimens appeared to be crystallographic and associated with slip localization. However, when the loading frequency was decreased, the crack growth rates for the low S specimen increased, whereas the high S specimen exhibited retarded crack growth and the crack path was no longer crystallographic.

Use of silica fume and natural volcanic ash as a replacement to Portland cement: Micro and pore structural investigation using NMR, XRD, FTIR and X-ray microtomography

This work investigates the effectiveness of the use of volcanic ash along with silica fume as a partial replacement for Portland cement. Multiple mix combinations of volcanic ash, silica fume and Portland cement were examined using various pore and microstructure characterization techniques. Hardened cement pastes were cured for 28days and their pore and microstructures were examined using X-ray Microtomography, Magic Angle Spinning (MAS) Nuclear magnetic resonance (NMR) for 27Al and 29Si, X-ray Diffraction (XRD) and Fourier transform infra-red spectroscopy (FTIR). Microstructure examination of mixes prepared with volcanic ash, silica fume and Portland cement revealed the co-existance of calcium silicate hydrate (C-S-H) and calcium–alumino-silicate-hydrate (C-A-S-H) gels, along with other hydration products that led to a reduction in porosity and densification of the cement matrix. These findings indicate that volcanic ash along with silica fume is a viable substitute for Portland cement up to 40% and provides a sustainable, cost effective and environmentally friendly solution to volcanic ash disposal.

Highly active Pt decorated Pd/C nanocatalysts for oxygen reduction reaction

This article describes findings of the correlation between the atomic scale surface structure and the electrocatalytic performance of nanoengineered Pt-Pd/C catalysts for oxygen reduction reaction (ORR), aiming at providing a new fundamental insight into the role of the detailed atomic decorated structure of the catalysts in fuel cell reactions. Carbon-supported Pt decorated Pd nanoparticles (donated as Pt-Pd/C), with Pt coverage close to a monolayer, were prepared from a simple galvanic replacement reaction between Pd/C particles and PtCl42− at room temperature. The decorated architecture was confirmed by extensive microstructural characterization techniques, including TEM, XRD, XPS, HAADF-STEM, ICP and HS-LEIS. The catalysts were also examined for their intrinsic kinetic activities towards oxygen reduction reaction. The results have shown that the Pt-Pd/C catalysts are highly active towards molecular oxygen electrocatalytic reduction. These findings have profound implications to the design and nanoengineering of decorated surfaces of catalysts for oxygen reduction reaction.

Precipitation in simultaneously nitrided and aged Mo-containing maraging steel

The excellent mechanical properties of maraging steels are ascribed to nanometer-sized intermetallics which precipitate during aging in the ductile very low carbon Ni-martensite. Their wear and fatigue properties can be improved by nitriding. The non-equilibrium precipitation reactions in Fe-Ni-Co-Mo maraging steels are studied during an aging heat treatment executed in a nitriding atmosphere. The precipitates formed during the initial stages of precipitation are characterized with transmission electron microscopy and atom probe tomography. Spherical intermetallic precipitates having a diameter of around 3nm were detected in the aged, bulk material. These ω-type precipitates formed during the early stages of aging, have a trigonal crystal lattice and their chemical composition is close to (Fe,Ni)7Mo2. In the nitrided layer, Mo-N disc-shaped nitrides on the {100} martensitic lattice having a diameter of 3 to 4nm were found but their exact crystal structure could not be determined with microstructural characterization techniques. Density functional theory calculations confirmed that a single layer of Mo atoms, substituting Fe on the {100} plane of the Fe-matrix, is stable and showed that the N atoms prefer to be in the Mo-layer, on the octahedral sites with Fe as nearest neighbors.