Somnath Bhattacharyya

Transition metal-based carbo-nitride solid solution phases have wide application in electrical, electronic, automotive, and refractory industries due to the excellent combination of metallic and ceramic properties. Mainly, carbo-nitride phases were synthesized by high-temperature solid-state diffusion, carbothermal reduction-nitridation, HIP, SHS, pressureless sintering at high temperature, etc. All the methods required considerable investment and production costs, limiting their application in the larger area of research. To overcome all the problems related to the synthesis, one simple, cost-effective, energy-efficient, and environmentally friendly method is required where synthesis will be carried out at a lower temperature with a shorter soaking time. Molten salt-assisted synthesis method fulfils these requirements by accelerating the diffusion rate for phase formation through a chemically inert liquid medium. In this manuscript, carbo-nitride TiC0.5N0.5 and MAX phases Ti2AlC0.5N0.5 and Ti3AlCN have been synthesized using this method with a different atmosphere (Ar, N2, and Air) at a relatively lower temperature with a shorter period of time and characterized by XRD, SEM, particle size analyzer, and HRTEM. The relative density was measured by Archimedes’ principle in case of all the samples and electrical conductivity of the respective samples was determined by the four-probe method.

Bimetallic solid solution MAX phases have attracted significant interest because of their tunable properties. Easy machinability, relatively low densities, high elastic constants, catalytic activity, etc., make (Ti,V)2AlC MAX phase an appreciable candidate for different mechanical, electrical, and electrochemical based applications. Earlier reported synthesis methods of bimetallic MAX phases are not economical due to high temperature, high pressure, an inert atmosphere, etc. Herein we present an alternative route, modified molten salt shielded synthesis, to produce (TixV1−x)2AlC MAX phases with varying Ti/V ratio. X-ray diffraction shows the shift in main peak positions with varying Ti/V ratios. Post oxidation XRD of the MAX phase constituents shows that increasing Ti / V ratio increases the peak intensity of TiO2 and reduces the intensity of V2O5. UV–vis spectroscopy data illustrate that increasing the Ti/V ratio enhances the optical band gap. Calculated thermodynamic parameters like activation energy, entropy, enthalpy, and Gibb's free energy manifest the reaction kinetics of the oxidation behavior of the MAX phases. Electrical resistivity measured at room temperature decreases with increasing Ti/V values. The outcome of this present systematic and meticulous work provides greater insight into the synthesis of the solid solution based (TixV1−x)2AlC MAX phase and its thermal, optical, and electrical behavior with varying Ti/V ratios.

SmCo5/Co exchange-coupled films with high saturation magnetization, and hence high energy product were fabricated on MgO substrates by sequential deposition of elemental Sm and Co layers at 450 °C followed by subsequent annealing at the same temperature for 1 h. Formation of defects induced multi-layered structures and the composition of individual layers were confirmed through high resolution Scanning Transmission Electron Microscopy (HRSTEM) imaging acquired on high angle annular dark field (HAADF) detector and Energy Dispersive X-ray (EDX) spectroscopy. Micromagnetic simulations and magnetic measurements further confirmed that the observed structure is coupled in rigid phase between soft and hard magnetic layers and the enhancement in coercivity results due to the domain wall pinning at the defects sites. This leads to the magnetic energy density in the film of the order of 22.2 MGOe which is close to the bulk value and have potential applications towards Micro-Electro-Mechanical Systems.

Dynamical behavior of intergranular glassy films (IGFs) in undoped and Lu2O3– MgO doped Si3N4 ceramics was studied by reconstructing complex-valued exit wave using focal series bright field images acquired at different temperatures during heating in-situ within transmission electron microscope (TEM). Width and value of retrieved phase change of the complex-valued exit wave across the IGFs were used to study changes that occur during in–situ heating until 950 °C. Results revealed that the IGF of an undoped sample maintains its equilibrium configuration. In the case of doped one, the phase difference between adjacent grains with IGF decreases but at the same time width of IGF increases during in-situ heating. Collectively, these two trends are indicative of the change in the mean inner potential of IGF during heating which in turn reveals that IGF in Lu2O3– MgO doped Si3N4 does not maintain equilibrium configuration. Mean inner potential differences between IGF and adjacent grain of doped samples at room temperature exhibit a higher value than that of the undoped one due to rare earth element segregation within IGF. This might be the cause of the difference in the dynamical behavior of IGFs observed in the present study.

Biocompatible β-type Ti alloys with high ultimate tensile strength (UTS) and yield strength are potential candidates for certain orthopedic and cardiovascular implants. Aiming for these applications, Ti alloy with 14 wt % Mn (Ti-14 Mn) as β-stabilizer was processed through thermomechanical treatment along with solutionizing and quenching, followed by 95% cold rolling, which resulted in ultrahigh UTS and yield strength. 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolimbromide assay with different cell lines suggests efficient cell growth on alloy surface without compromising biocompatibility. Cell adhesion and spreading assay show that cells are not only able to attach to the alloy surface but also able to spread and grow with normal morphology, which projects this material as a potential candidate for biomedical application. Previous studies on binary β-type Ti alloy systems treated with the above-mentioned processing route confirm the presence of nanoscale phase separation, which enhances its mechanical properties. To discover the same phenomena in the alloy of the present study, bright-field and high-resolution transmission electron microscopy (HRTEM) imaging experiments were performed and nanoscale contrast-modulated lamella regions were observed. Geometrical phase analysis on complex-valued exit wave, reconstructed using focal series HRTEM images, demonstrates that the lamella is a result of d-spacing modulation. Ab initio calculation indicates that d-spacing modulation with the same crystal structure occurs due to composition modulation and was proved by scanning transmission electron microscopy imaging coupled with quantitative energy-dispersive X-ray spectroscopy. Correlating contrast, strain, and composition modulation confirms nanoscale phase separation, which is the first report of this phenomenon in Ti-Mn alloy system.

Nowadays, transition metal-based boride-like layered MAB phases are getting more attention due to their excellent combination of metal-ceramic properties, like good thermal and electrical conductivity, oxidation and thermal shock resistance, excellent wear resistance etc. So far, WAlB was synthesized at high temperature (1550 °C) with 24 hr soaking time by aluminium flux technique with nominal single-phase purity, which requires huge investment and production costs. However, Mn2AlB2 was recently synthesized using KBr salt via molten salt shielded synthesis (MS3) at a relatively low temperature 1000 °C for a longer soaking time (12 hr) which may enhance the chances of oxidation. Therefore, the motivation of this work is to synthesize WAlB and Mn2AlB2 MAB phase at lower temperatures with high purity via a newly developed simple, cost-effective, environmental-friendly method named modified molten salt shielded synthesis method. This paper synthesized both these phases with high yield at lower temperatures and much shorter soaking time than earlier reported literature.

Boron doped nanocrystalline cubic silicon carbide (3C-SiC) films have been prepared by HW-CVD using silane (SiH4)/methane (CH4)/diborane (B2H6) gas mixture. The influence of boron doping on structural, optical, morphological and electrical properties have been investigated. The formation of 3C-SiC films have been confirmed by low angle XRD, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infra-red (FTIR) spectroscopy and high resolution-transmission electron microscopy (HR-TEM) analysis whereas effective boron doping in nc-3C-SiC have been confirmed by conductivity, charge carrier activation energy, and Hall measurements. Raman spectroscopy and HR-TEM analysis revealed that introduction of boron into the SiC matrix retards the crystallanity in the film structure. The field emission scanning electron microscopy (FE-SEM) and non contact atomic force microscopy (NC-AFM) results signify that 3C-SiC film contain well resolved, large number of silicon carbide (SiC) nanocrystallites embedded in the a-Si matrix having rms surface roughness ∼1.64 nm. Hydrogen content in doped films are found smaller than that of un-doped films. Optical band gap values, ETauc and E04 decreases with increase in B2H6 flow rate.

We have measured the mean inner potential depth at a Σ5 grain boundary in a SrTiO3 bicrystal by reconstructing the exit-face wave function from an image focal series collected by transmission electron microscopy. We find that, as the annealing temperature increases, the potential depth at the grain boundary exponentially increases. We interpret the temperature dependence of the potential depth as the signature of a grain-boundary thermal roughening transition.

Heterostructure interfaces play a major role in defining the performance of thin-film devices. High-k dielectric oxide-semiconductor heterostructures are being extensively investigated as promising candidates for future integrated circuits, thus it becomes important to precisely probe the interfaces at the atomic scale for technological advancements. In this work, a high-k dielectric oxide (Gd2O3)-semiconductor (Ge) interface was characterized at the atomic scale using complex-valued exit wave reconstructed from a set of focal series high-resolution transmission electron microscopy (HRTEM) images acquired without objective lens spherical aberration correction. The complexity of this characterization lies in removing image artefacts produced by amorphous layer deposited on the imaged region during ion milling which was successfully solved using an algorithm to remove amorphous background developed recently. The final result reveals that the interface of the present study is atomically sharp and flat. The thickness of the imaged region along viewing direction was estimated from channelling map. Comparing reconstructed amplitude of experimental data with that of simulated one generated using Density Functional Theory (DFT) optimized interface structure, it was found that the Gd2O3 layers were terminated at the Gd atoms in the interface.

Influence of filament temperature (TFil) on the structural, morphology, optical and electrical properties of silicon carbide (SiC) films deposited by using hot wire chemical vapor deposition technique has been investigated. Characterization of these films by low angle XRD, Raman scattering, XPS and TEM revealed the multiphase structure SiC films consisting of 3C–SiC and graphide oxide embedded in amorphous matrix. FTIR spectroscopy analysis show an increase in Si–C, Si–H, and C–H bond densities and decrease in hydrogen content with increase in TFil. The C–H bond density was found higher than the of Si–H and Si–C bond densities suggesting that H preferably get attached to C than Si. AFM investigations show decrease in rms surface roughness and grain size with increase in TFil. SEM studies show that films deposited at low TFil has spherulites-like morphology while at high TFil has cauliflower-like structure. Band gap values ETauc and E04 increases from 1.76 to 2.10 eV and from 1.80 to 2.21 eV respectively, when TFil was increased from 1500 to 2000 °C. These result show increase in band tail width (E04–ETauc) of multiphase SiC films. Electrical properties revealed that σDark increases from ~7.87 × 10−10 to 1.54 × 10−5 S/cm and Eact decreases from 0.67 to 0.41 eV, which implies possible increase in unintentional doping of oxygen or nitrogen due to improved crystallinity and Si–C bond density with increase in TFil. The deposition rate for the films was found moderately high (21 < rdep < 30 Å/s) over the entire range of TFil studied.