Awadesh Kumar Mallik

Large area deposition of polycrystalline diamond coatings by microwave plasma CVD

Polycrystalline diamond (PCD) films have been grown over 100 mm diameter silicon (100) substrate, using microwave plasma chemical vapour deposition (MPCVD) technique. The deposition was carried out inside a 15 cm diameter quartz chamber with microwave power of 15 kW at 915 MHz frequency. Uniform substrate surface temperature of 1050°C with plasma heating was maintained with simultaneous cooling arrangement. The pressure was 110 Torr and the microwave incident power was 8.5 kW. Temperature uniformity and plasma geometry over the substrate are the key parameters for producing uniformly thick MPCVD diamond films of high quality. Thickness uniformity of as-deposited films is ±10% across 100 mm diameters with a growth rate of 1 µm.h–1. The grown PCD was characterized by X-ray diffractometry (XRD), Raman spectrometry, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and bright field imaging technique. Experimental results indicate columnar growth of a very densely crystalline PCD with (111) facets of high quality morphology.

CeO2@C derived from benzene carboxylate bridged metal–organic frameworks: ligand induced morphology evolution and influence on the electrochemical properties as a lithium-ion battery anode

We report herein a facile metal–organic framework (MOF) derived route for the synthesis of carbon embedded CeO2 (CeO2@C) with a pre-designed shape-specific morphology by varying the organic linker and by using PVP as the structure directing agent. It is found that the general morphological features of the parent MOF are mimicked by the derived oxide. Four different linkers have been used to prepare CeO2@C particles with three different shapes – spherical, bar-shaped and thin plate-like. A probable formation mechanism is discussed based on metal–ligand coordination. Influence of the morphology on the electrochemical properties as a lithium-ion battery (LIB) anode has been studied in coin cells vs. Li/Li+. The spherically shaped CeO2@C-14 shows a superior performance with a maximum specific capacity of 715 mA h g−1 at 0.05 mA cm−2, good rate performance (413 mA h g−1 at 0.5 mA cm−2) and cycling stability (∼94% capacity retention after 100 cycles). The present results demonstrate that the major limitations of metal oxide anodes – volume expansion during lithiation/delithiation, rate performance and capacity fading upon cycling – could be overcome to a great extent by adopting the two-way approach of morphology design through the MOF route and in situ embedded carbon matrix.

ROS mediated high anti-bacterial efficacy of strain tolerant layered phase pure nano-calcium hydroxide

The present work provides the first ever report on extraordinarily high antibacterial efficacy of phase pure micro-layered calcium hydroxide nanoparticles (LCHNPs) even under dark condition. The LCHNPs synthesized especially in aqueous medium by a simple, inexpensive method show adequate mechanical properties along with the presence of a unique strain tolerant behaviour. The LCHNPs are characterized by FTIR, Raman spectroscopy, XRD, Rietveld analysis, FE-SEM, TEM, TG-DTA, surface area, particle size distribution, zeta potential analysis and nanoindentation techniques. The LCHNPs have 98.1% phase pure hexagonal Ca(OH)2 as the major phase having micro-layered architecture made up of about ~100-200nm thick individual nano-layers. The nanomechanical properties e.g., nanohardness (H) and Youngs modulus (E) of the LCHNPs are found to have a unique load independent behavior. The dielectric responses (e.g., dielectric constant and dielectric loss) and antibacterial properties are evaluated for such LCHNPs. Further, the LCHNPs show much better antibacterial potency against both gram-positive e.g., Staphylococcus aureus (S. aureus) and gram-negative e.g., Pseudomonas putida (P. putida) bacteria even in dark especially, with the lowest ever reported MIC value (e.g., 1 μg ml-1) against the P. putida bacterial strain and exhibit ROS mediated antibacterial proficiency. Finally, such LCHNPs has almost ~8-16% inhibition efficacy towards the development of biofilm of these microorganisms quantified by colorimetric detection process. So, such LCHNPs may find potential applications in the areas of healthcare industry and environmental engineering.

Property mapping of polycrystalline diamond coatings over large area

Large-area polycrystalline diamond (PCD) coatings are important for fields such as thermal management, optical windows, tribological moving mechanical assemblies, harsh chemical environments, biological sensors, etc. Microwave plasma chemical vapor deposition (MPCVD) is a standard technique to grow high-quality PCD films over large area due to the absence of contact between the reactive species and the filament or the chamber wall. However, the existence of temperature gradients during growth may compromise the desired uniformity of the final diamond coatings. In the present work, a thick PCD coating was deposited on a 100-mm silicon substrate inside a 915-MHz reactor; the temperature gradient resulted in a non-uniform diamond coating. An attempt was made to relate the local temperature variation during deposition and the different properties of the final coating. It was found that there was large instability inside the system, in terms of substrate temperature (as high as ΔT = 212 °C), that resulted in a large dispersion of the diamond coating’s final properties: residual stress (∼15.8 GPa to +6.2 GPa), surface morphology (octahedral pyramids with (111) planes to cubo-octahedrals with (100) flat top surfaces), thickness (190 μm to 245 μm), columnar growth of diamond (with appearance of variety of nanostructures), nucleation side hardness (17 GPa to 48 GPa), quality (Raman peak FWHM varying from 5.1 cm−1 to 12.4 cm−1 with occasional splitting). This random variation in properties over large-area PCD coating may hamper reproducible diamond growth for any meaningful technological application.

Influence of growth conditions on microstructure and defects in diamond coatings grown by microwave plasma enhanced CVD

Diamond coatings were grown on SiO2/Si substrate under various process conditions by microwave plasma chemical vapour deposition (MPCVD) using CH4/H2 gas mixture. In this paper, we present a microstructural study to elucidate on the growth mechanism and evolution of defects, viz., strain, dislocations, stacking faults, twins and non-diamond impurities in diamond coatings grown under different process conditions. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the diamond coatings. It has been shown that our new approach of prolonged substrate pre-treatment under hydrogen plasma yielded a new growth sequence that the SiO2 layer on the Si substrate was first reduced to yield Si layer of ∼150 nm thickness before diamond was allowed to grow under CH4–H2 plasma, created subsequently. It has also been shown that Si and O as impurity from the substrate hinders the initial diamond growth to yield non-diamond phases. It is being suggested that the crystal defects like twins, stacking faults, dislocations in the diamond grains and dislocations in the intermediate Si layer are generated due to the development of non-uniform stresses during diamond growth at high temperature.

Microwave Plasma CVD Grown Single Crystal Diamonds – A Review

Diamond offers a range of unique properties, including wide band of optical transmission, highest thermal conductivity, stiffness, wear resistance and superior electronic properties. Such high-end properties are not found in any other material, so theoretically it can be used in many technological applications. But the shortcoming has been the synthesis of the diamond material in the laboratory for any meaningful use. Although microwave plasma chemical vapour deposited (MPCVD) has been in practice since 1980s for the diamond growth but it is in the past 7-8 years that its potential has been realised by the industry due to capability of MPCVD to deposit diamond, pure and fast, for commercial uses. There are many CVD techniques for growing diamond but among them MPCVD can only make single crystal diamond (SCD) effectively. SCD grown by MPCVD is also superior to other forms of diamond produced in the laboratory. For example, SCD is necessary for the best electronic properties - often outperforming the polycrystalline diamond (PCD), the high pressure high temperature (HPHT) diamond and the natural diamond. In many applications the short lateral dimensions of the lab-grown diamond available is a substantial limitation. Polycrystalline CVD diamond layers grown by hot filament CVD solved this problem of large area growth, but the presence of grain boundaries are not appropriate for many uses. On the other hand, there is still limitation in the area over which SCDs are grown by MPCVD, only upto 10-15 mm lateral sizes could have been achieved so far, while there are recipes which rapidly grow several mm thick bulk SCDs. This lateral size limitation of SCDs is primarily because of the small seed substrate dimension. Although natural and HPHT diamonds may not be suitable for the intended application, still they are routinely used as substrates on which SCD is deposited. But the problem lies in the availability of large area natural SCD seeds which are extremely rare and expensive. Moreover, large diamond substrate plates suitable for CVD diamond growth have not been demonstrated by HPHT because of the associated high economic risk in their fabrication and use. Other than lateral dimension, purity of SCD is also very important for technological use. Natural diamond is often strained and defective, and this causes twins and other problems in the CVD overgrowth or fracture during synthesis. In addition, dislocations which are prevalent in the natural diamond substrate are replicated in the CVD layer, also degrading its electronic properties. HPHT synthetic diamond is also limited in size, and generally is of poorer quality in the larger stones, with inclusions being a major problem. There will be much research interest in the next 10 years for the MPCVD growth of SCD. Purer and bigger SCDs will be tried to grow with faster and reproducible MPCVD recipes. Here the MPCVD growth of SCD is being reviewed keeping in mind its huge technological significance in the next decades or so. Discoveries of the commercial productions of silicon, steel, cement different materials have built modern societies but higher scales will be achieved with the advent of lab-grown diamond.

Severe wear behaviour of alumina balls sliding against diamond ceramic coatings

At present alumina is the most widely used bio-ceramic material for implants. However, diamond surface offers very good solid lubricant for different machinery, equipment including biomedical implants (hip implants, knee implants, etc.), since the coefficient of friction (COF) of diamond is lower than alumina. In this tribological study, alumina ball was chosen as the counter body material to show better performance of the polycrystalline diamond (PCD) coatings in biomedical load-bearing applications. Wear and friction data were recorded for microwave plasma chemical vapour deposition (MWCVD) grown PCD coatings of four different types, out of which two samples were as-deposited coatings, one was chemo-mechanically polished and the other diamond sample was made free standing by wet-chemical etching of the silicon wafer. The coefficient of friction of the MWCVD grown PCD against Al2O3 ball under dry ambient condition was found in the range of 0.29–0.7, but in the presence of simulated body fluid, the COF reduces significantly, in the range of 0.03–0.36. The samples were then characterized by Raman spectroscopy for their quality, by coherence scanning profilometer for surface roughness and by electron microscopy for their microstructural properties. Alumina balls worn out (14.2 × 10−1 mm3) very rapidly with zero wear for diamond ceramic coatings. Since the generation of wear particle is the main problem for load-bearing prosthetic joints, it was concluded that the PCD material can potentially replace existing alumina bio-ceramic for their better tribological properties.

Phase pure, high hardness, biocompatible calcium silicates with excellent anti-bacterial and biofilm inhibition efficacies for endodontic and orthopaedic applications

Here we report for the very first time the synthesis of 100% phase pure calcium silicate nanoparticles (CSNPs) of the α-wollastonite phase without using any surfactant or peptizer at the lowest ever reported calcination temperature of 850 °C. Further, the phase purity is confirmed by quantitative phase analysis. The nano-network like microstructure of the CSNPs is characterized by FTIR, Raman, XRD, FESEM, TEM, TGA, DSC etc. techniques to derive the structure property correlations. The performance efficacies of the CSNPs against gram-positive e.g., S. pyogenes and S. aureus (NCIM2127) and gram-negative e.g., E. coli (NCIM2065) bacterial strains are studied. The biocompatibility of the CSNPs is established by using the conventional mouse embryonic osteoblast cell line (MC3T3). In addition, the biofilm inhibition efficacies of two varieties of CSNPs e.g., CSNPs(W) and CSNPs(WC) are investigated. Further, the interconnection between ROS e.g., superoxide (O2.-) and hydroxyl radical (.OH) generation capabilities of CSNPs and their biofilm inhibition efficacies is clearly established for the very first time. Finally, the mechanical responses of the CSNPs at the microstructural length scale are investigated by nanoindentation. The results confirm that the α-wollastonite phases present in CSNPs(W) and CSNPs(WC) possess extraordinarily high nanohardness and Youngs moduli values. Therefore, these materials are well suited for orthopaedic and endodontic applications.

Evaluation of temperature-dependent microstructural and nanomechanical properties of phase pure V 2 O 5

AbstractPhase pure, mesoporous, and crystalline V2O5 is synthesized by acid hydrolysis technique and subsequently heat treatment is carried out at 450, 500, 550, and 600 °C in air. The as-synthesized and heat-treated powders are thoroughly studied by X-ray diffraction, electron microscopy, dynamic light scattering, and spectroscopic techniques. A unique morphological tuning of V2O5 powders from as small as ~80 nm tiny nanorod to as large as a ~2.5 μm hexagonal grain as microstructural unit blocks is observed. A qualitative mechanism is suggested for particle growth. Further, the powders are pelletized and subsequently sintered in air at the same temperatures of 450, 500, 550, and 600 °C at which the powders were heat treated. Finally, nanomechanical properties of bulk pelletized V2O5 such as nanohardness and Young’s modulus are also evaluated by nanoindentation technique at nine different loads e.g., 10, 30, 50, 70, 100, 300, 500, 700, and 1000 mN. HighlightsPhase pure, mesoporous, and crystalline V2O5 powder synthesized by acid hydrolysis.V2O5 powders thoroughly studied as a function of various heat treatment temperatures.Morphology tuned from nanorod to hexagonal micron sized grain.Qualitative model suggested for particle growth mechanism.Load-dependent nanoindentation studied on various sintered V2O5 pellets.

Microscopic properties of MPCVD diamond coatings studied by micro-Raman and micro-photoluminescence spectroscopy

Diamond coatings were deposited on silicon (100) substrate using the microwave plasma chemical vapour deposition (MPCVD) technique at different process conditions. Process parameters such as CH4–H 2 gas mixture concentration, microwave power, chamber pressure and substrate temperature were varied. The diamond coatings were characterized by micro-Raman and micro-photoluminescence (PL) spectroscopy techniques. In this paper we report a comparison of the overall quality of MPCVD polycrystalline diamond coatings grown under different processing conditions in terms of stress distribution, thickness uniformity and surface roughness. Micro-Raman spectroscopy studies over various points on the deposited coating showed that the Raman line widths of diamond peak varied from 3.2 to 18.3 cm−1 with the variation of CH4 and H2 gas concentration. The micro-PL spectra suggested the presence of impurity concentration and defects within the diamond coating synthesized at different processing conditions. Transmission electron microscopy (TEM) images provide the direct evidence of the presence of crystal defects which corroborates the Raman and PL results. The coherence scanning interferometry (CSI) showed that surface roughness of diamond coating varied from 0.43 to 11 μm with thickness at different positions of the three coating samples. It has been concluded that Raman line-width broadening and Raman-shift are due to the presence of crystal defects as well as non-uniform distribution of stresses present in the diamond crystals of the coating, due to the incorporation of Si as impurity element and non-uniform temperature distribution during growth. Defect density gets reduced at higher processing temperatures. It is also being proposed that better thickness uniformity and lower surface roughness can be achieved for coatings deposited at low methane concentration under optimized process conditions.