Molecular dynamics (MD) simulation of uniaxial tension of some single-crystal cubic metals at nanolevel

Molecular Dynamics (MD) simulations of uniaxial tension at nanolevel have been carried out at a constant rate of loading (500 ms⁻¹) on some single-crystal cubic metals, both FCC (Al, Cu, and Ni) and BCC (Fe, Cr, and W) to investigate the nature of deformation and fracture. Failure of the workmaterials due to void formation, their coalescence into nanocracks, and subsequent fracture or separation were observed similar to their behavior at macroscale. The engineering stress–strain diagrams obtained by the MD simulations of the tensile specimens of various materials show a rapid increase in stress up to a maximum followed by a gradual drop to zero when the specimen fails by ductile fracture. The radius of the neck is found to increase with an increase in the deformation of the specimen and to decrease as the ductility of the material decreases. In this investigation, the strain to fracture is observed to be lower with the BCC materials than FCC materials. In the case of BCC crystals, no distinct linear trend in the engineering stress–strain characteristics is observed. Instead, rapid fluctuations in the force values were observed. If the drop in the force curves can be attributed to the rearrangement of atoms to a new or modified crystalline structure, it appears that BCC materials undergo a significant change in their structure and subsequent realignment relative to the FCC materials, as previously reported in the literature. While good correlation is found between the D- and α-parameters of the Morse potential with the ultimate strength and the strain to failure for the FCC metals, no such correlation is found for the BCC metals. From this, it appears that Morse potentials may not represent the deformation behavior of BCC metals as accurately as FCC metals and alternate potentials may need to be considered.     

Thermal analysis of laser surface transformation hardening—optimization of process parameters

This paper deals with the optimization of process parameters for maximum productivity (given by the product of scanning velocity and cross feed) in laser transformation hardening. The process parameters considered are laser beam power, P; laser beam diameter, D_b; and the heat intensity distribution, namely, normal, bimodal, or uniform. A thermal analysis of the laser surface transformation hardening of gears was conducted (based on Jaeger’s classical moving heat source method) by considering the laser beam as a moving plane (disc) heat source to establish the temperature rise distribution in the workpiece (gear) of finite width. In a recent investigation [Int. J. Heat Mass Transfer 44 (2001) 2845], the authors considered the case of a heat source with a pseudo-Gaussian (or normal) distribution of heat intensity. The analytical results were compared with the experimental results published in the literature. In laser heat treatment of steel, it is generally considered preferable to use a wider heat intensity distribution, such as uniform or bimodal, for it enables more uniform case hardening depth. In this paper, this model is extended to cover bimodal and uniform distributions and compared with the normal distribution. Scanning velocities for no surface melting and for a case hardening depth of 0.1 mm were determined for surface transformation hardening of AISI 1036 (EN 8) steel for a range of laser beam powers, P, laser beam diameters, D_b, and various heat intensity distributions. Since diffusion during the heat treatment (surface transformation hardening) process is a time dependent phenomenon, based on the literature review, an interaction time of 15 ms was taken as a basis. It is hoped that laser industry with adequate facilities available can validate the thermal analysis and subsequent optimization presented in this paper.     

Thermal post-buckling analysis of slender columns using the concept of coupled displacement field

Thermal post-buckling analysis of columns with an axially immovable ends is studied using the Rayleigh-Ritz (R-R) method, where the admissible displacement functions are chosen based on the concept of coupled displacement field (CDF) criteria. Geometric non-linearity is considered using the von-Karman strain displacement relations of the beam. Furthermore, the displacement fields derived from CDF criteria are used in an intuitive formulation, where the thermal post-buckling behavior can be predicted by using two parameters namely tension developed in the column and linear buckling load. An exhaustive set of column boundary conditions are considered namely classical such as hinged-hinged, clamped-clamped, clamped-hinged and non-classical such as clamped-guided and hinged-guided. Post-buckling analysis results are presented in the form of closed form expressions, where the ratio of post-buckling load to linear buckling load parameter is expressed as a function of central amplitude of the column for all the boundary conditions considered. The amount of non-linearity predicted using the present formulations (R-R method and intuitive method) based on the concept of coupled displacement field (CDF) criteria shows an excellent agreement with the available literature results for both classical and non-classical boundary conditions.     

Influence of Synthesis Conditions and Cerium Incorporation on the Properties of Zr-Pillared Clays

Zr-pillared clays were prepared from ZrOCl₂ pillaring solutions by adopting different preparative conditions. Ce³⁺ ions are introduced to Zr-pillared clays by co-intercalation method. The resulting samples were characterized by XRD, TGA, N₂ sorption and UV-VIS-Diffuse reflectance spectroscopy techniques. Basal spacings in the range of 18–21 Å were observed depending upon the preparative condition. TG analysis shows three weight loss regions corresponding to removal of various types of water molecules. All pillared clays show Type-I sorption isotherm typical of microporous materials. Pillaring under refluxing condition is found to have beneficial effect on the surface area and pore volume of the Zr-pillared clay. The chemical environment and location of Ce³⁺ ions is studied by UV-VIS-DRS. The Ce³⁺ ions are found to be present in the micropores of the Zr-pillared clays. However heat treatment at higher temperature may result in peripheral interaction between Ce³⁺ ions and Zr-pillars. Catalytic activity of these pillared clays was evaluated for cyclohexanol dehydration which correlates well with the Brønsted acidity of these materials. The Zr-Pillared clay containing Ce³⁺ ions show good catalytic activity and stability with reaction time which has been ascribed to the stabilazition of the Brønsted acidic centers.     

A mechanism for the accelerated wear of high speed sliding surfaces in the presence of spherical particles

A mechanism for the accelerated wear of bearing surfaces due to the cutting action of the spherical particles between bearing surfaces in sliding contact is proposed and verified experimentally.     

Micro and nano-architectures of Co3O4 on Ni foam for electro-oxidation of methanol

Spinel Co₃O₄ material with different morphologies is directly grown on Ni foam by simple hydrothermal method and subsequent calcination processes. The direct growth of binder free active phase of Co₃O₄ on Ni foam is an effective approach to enhance the electrocatalytic activity of the material. The morphologies of Co₃O₄ strongly depend on the anion of the precursor salt used. Microflowers, microspheres and nanograss morphologies of Co₃O₄ are obtained using chloride, sulfate and acetate salts of cobalt, respectively. The BET surface areas of these cobalt oxide materials are found to increase in the order of microflower-Co₃O₄ (53 m² g^?1) < nanograss-Co₃O₄ (65 m² g^?1) < microsphere-Co₃O₄ (100 m² g^?1). The electrocatalytic activity of these Co₃O₄ materials has been tested for methanol oxidation by cyclic voltammetry and chronoamperometry. All three samples show low onset potentials (0.32–0.34 V) for methanol oxidation. The vanodic peak current of methanol oxidation is found to increase in the order of microflower-Co₃O₄ (28 A g^?1) < nanograss-Co₃O₄ (34.9 A g^?1) < microsphere-Co₃O₄ (36.2 A g^?1) at 0.6 V. This study highlights the significance of the morphology of cobalt oxide in the development of oxide based non-precious electrocatalysts for methanol oxidation.     

Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells

Nitrogen doped mesoporous carbons are employed as supports for efficient electrocatalysts for oxygen reduction reaction. Heteroatom doped carbons favour the adsorption and reduction of molecular oxygen on Pt sites. In the present work, nitrogen doped mesoporous carbons (NMCs) with variable nitrogen content were synthesized via colloidal silica assisted sol-gel process with Ludox-AS40 (40 wt% SiO₂) as hard template using melamine and phenol as nitrogen and carbon precursors, respectively. The NMC were used as supports to prepare Pt/NMC electrocatalysts. The physicochemical properties of these materials were studied by SEM, TEM, XRD, BET, TGA, Raman, XPS and FTIR. The surface areas of 11 wt% (NMC-1) and 6 wt% (NMC-2) nitrogen doped mesoporous carbons are 609 m² g^?1 and 736 m² g^?1, respectively. The estimated electrochemical surface areas for Pt/NMC-1 and Pt/NMC-2 are 73 m² g^?1 and 59 m² g^?1, respectively. It is found that Pt/NMC-1 has higher ORR activity with higher limiting current and 44 mV positive onset potential shift compared to Pt/NMC-2. Further, the fuel cell assembled with Pt/NMC-1 as cathode catalyst delivered 1.8 times higher power density than Pt/NMC-2. It is proposed that higher nitrogen content and large pyridinic nitrogen sites present in NMC-1 support are responsible for higher ORR activity of Pt/NMC-1 and high power density of the fuel cell using Pt/NMC-1 cathode electrocatalyst. The carbon support material with high pyridinic content promotes the Pt dispersion with particle size less than 2 nm.     

Bismuth oxycarbonate grafted NiFe-LDH supported on g-C3N4 as bifunctional catalyst for photoelectrochemical water splitting

In the present study, we report the synthesis of photoactive bismuth oxycarbonate (BOC, Bi₂O₂CO₃) grafted NiFe layered double hydroxide (LDH) supported on g-C₃N₄ (15 wt% of g-C₃N₄) by coprecipitation method. The band gap of this photoactive material is determined to be 1.7 eV. The Bi₂O₂CO₃ agglomerates are anchored on NiFe-LDH plates and g-C₃N₄ nanosheets intercalated between the LDH plates. This architecture helps in expediting electron transfer for hydrogen and oxygen evolution reactions. The pristine NiFe-LDH photoanode acquires bifunctional character because of Bi₂O₂CO₃ agglomerates and g-C₃N₄ embedded in the architecture of BOC/NiFe-LDH@g-C₃N₄. This is found to be an efficient photoanode for oxygen evolution and photocathode for hydrogen evolution reactions. The water splitting process occurs along the heterojunction formed between g-C₃N₄ nanosheets and Bi₂O₂CO₃ grafted NiFe-LDH. Further, an additional interfacial charge transfer aided by Bi₂O₂CO₃ results in S-scheme mechanism, which enhances the rate of photoelectrochemical hydrogen and oxygen evolution reactions.     

Waste-to-wealth approach in water economy: The case of beneficiation of mercury-contaminated water in hydrogen production

Heavy metals (Hg, Cd, Pb, etc.) are micro-pollutants and result in water contamination. Significant bio-concentration of heavy metal like Hg can lead to fatal disease such as Minamata. Given this context, heavy metal removal from wastewater is essential before discharge. The wastewater treatment process requires considerable amount of energy which is being met by the conventional carbon-based fuels. This contributes to the global carbon dioxide emission, and hence global warming. Therefore, if clean energy sourcing is enabled during the treatment of the wastewater; it would offer obvious advantages. If the energy production is ‘clean’ and achieved via the process itself, it would serve two outcomes: (a) meeting the energy demand for wastewater treatment, and (b) getting rid of the need for external ‘carbon-based’ energy. Recently a few research articles have reported simultaneous clean energy production from wastewater during its treatment. Thus, the energy demand of the wastewater treatment process can be potentially met with the clean energy produced during the process. In this review, we will discuss mercury-contaminated wastewater treatment with simultaneous hydrogen production. We will provide a brief overview of waste-to-wealth approaches currently prevailing in water economy, recent mercury removal processes, and discuss future possibilities of self-sustained Hg-contaminated wastewater treatment.