• A Sarkar

      Articles written in Bulletin of Materials Science

    • An experimental investigation of electrical conductivities in biopolymers

      H Mallick A Sarkar

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      Gum arabica obtained from acacia plant is a conducting biopolymer. Experiments are carried out on this natural gum arabica. In the present study TGA, ion transference number, transient ionic current, thermal analysis, frequency and temperature variation of a.c. conductivity, Arrhenius plot and volt-ampere characteristics of specimens are carried out. The total electrical conductivity of these biopolymers are comparable to that of synthetic polymers doped with inorganic salts. The ion transference number of these biopolymers show their superionic nature of electrical conduction. The overall conduction mechanism seems to be protonic in nature rather than electronic one.

    • Preparation and characterization of magnesium-aluminium-silicate glass ceramics

      Madhumita Goswami T Mirza A Sarkar Shobha Manikandan Sangeeta S L Verma K R Gurumurthy V K Shrikhande G P Kothiyal

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      Synthesis of machinable quality magnesium aluminium silicate (MgO-Al2O3-SiO2) for fabrication of insulators/spacers usable in high voltage applications under high vacuum conditions has been carried out following two different routes i.e. (i) sintering route, and (ii) glass route. A three-stage heating schedule involving calcination, nucleation and crystallization, has been evolved for the preparation of magnesium aluminium silicate (MAS) glass ceramic with MgF2 as a nucleating agent. The effect of sintering temperature on the density of compacted material was studied. Microstructure and machinability of samples obtained from both routes were investigated. They were also characterized for microhardness. Initial studies on material obtained by glass route reveal that these samples are superior to those obtained from sintered route in respect of their high voltage breakdown strength and outgassing behaviour. Outgassing rate of 10−9 Torr l·s−1 cm−2 and breakdown strength of 160 kV/cm were obtained. Different types of spacers, lugs, nuts and bolts have been prepared by direct machining of the indigenously developed glass ceramic.

    • Preparation and studies of some thermal, mechanical and optical properties of 𝑥Al2O3(1 – 𝑥)NaPO3 glass system

      K V Shah V Sudarsan M Goswami A Sarkar S Manikandan Rakesh Kumar B I Sharma V K Shrikhande G P Kothiyal

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      Sodium aluminophosphate glasses having compositions of 𝑥Al2O3(1 – 𝑥)NaPO3 (𝑥 = 0.05–0.2) were prepared using conventional melt-quench technique. Density, glass transition temperature, microhardness (MH), thermal expansion coefficient (TEC) and transmission characteristics were measured as a function of alumina content for different samples. They were found to depend on O/P ratio with pronounced changes taking place for O/P ratio ≥ 3.5. Density, glass transition temperature and microhardness were found to increase up to 15 mol% of alumina and then they showed a decreasing trend. Thermal expansion coefficient decreased continuously with alumina content. Optical gaps for different glass samples as measured from transmission characteristics were found to be in the range 3.13–3.51 eV. It initially decreased with alumina content up to 15 mol% and then increased. The behaviour was explained on the basis of change in the average aluminum coordination number from six Al(6) to four Al(4) (i.e. Al(OP)6/Al(OP)4 ratio) along with the changes in polyhedra linkages in the glass network due to change in O/P ratio.

    • Phase formation of superconducting MgB2 at ambient pressure

      A Talapatra S K Bandyopadhyay Pintu Sen A Sarkar P Barat

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      MgB2 superconductor has been synthesized using a simple technique at ambient pressure. The synthesis was carried out in helium atmosphere over a wide range of temperatures. Magnesium was employed in excess to the stoichiometry to prevent the decomposition of MgB2. Samples of MgB2 thus prepared have been almost free from MgO as compared to other methods. Resistivities of the samples are quite low with residual resistivity ratio (RRR) of around 3. 𝑇c (𝑅 = 0) is 38.2–38.5 K with 𝛥𝑇c of 0.6–1.0 K. Comparative studies of various methods of low pressure synthesis have been presented.

    • Effect of B2O3 addition on microhardness and structural features of 40Na2O–10BaO–𝑥B2O3–(50–𝑥)P2O5 glass system

      K V Shah M Goswami M N Deo A Sarkar S Manikandan V K Shrikhande G P Kothiyal

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      Phosphate glasses having composition, 40Na2O–10BaO–𝑥B2O3–(50–𝑥)P2O5, where 𝑥 = 0–20 mol% were prepared using conventional melt quench technique. Density of these glasses was measured using Archimedes principle. Microhardness (MH) was measured by Vicker’s indentation technique. Structural studies were carried out using IR spectroscopy and 31P and 11B MAS NMR. Density was found to vary between 2.62 and 2.77 g/cc. MH was found to increase with the increase in boron content. 31P MAS NMR spectra showed the presence of middle 𝑄2 groups and end 𝑄1 and 𝑄0 groups with P–O–B linkages. FTIR studies showed the presence of BO3 and BO4 structural units along with the depolymerization of phosphate chains in conformity with 31P MAS NMR. 11B NMR spectra showed increase in BO4 structural units with increasing boron content. The increase in MH with B2O3 content is due to the increase of P–O–B linkages and BO4 structural units as observed from MAS NMR studies resulting in a more rigid borophosphate glass networks.

    • Effect of transient change in strain rate on plastic flow behaviour of low carbon steel

      A Ray P Barat P Mukherjee A Sarkar S K Bandyopadhyay

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      Plastic flow behaviour of low carbon steel has been studied at room temperature during tensile deformation by varying the initial strain rate of 3.3 × 10-4 s-1 to a final strain rate ranging from 1.33 × 10-3 s-1 to 2 × 10-3 s-1 at a fixed engineering strain of 12%. Haasen plot revealed that the mobile dislocation density remained almost invariant at the juncture where there was a sudden increase in stress with a change in strain rate and the plastic flow was solely dependent on the velocity of mobile dislocations. In that critical regime, the variation of stress with time was fitted with a Boltzmann type Sigmoid function. The increase in stress was found to increase with final strain rate and the time elapsed in attaining these stress values showed a decreasing trend. Both of these parameters saturated asymptotically at a higher final strain rate.

    • On statistical behaviour of stress drops in Portevin–Le Chatelier effect

      A Chatterjee P Mukherjee N Gayathri P Barat Arnab Barat A Sarkar

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      The Portevin–Le Chatelier (PLC) effect is a kind of plastic instability observed in many dilute alloys when deformed at certain ranges of strain rate and temperature. In this paper we present a comprehensive statistical analysis of the observed experimental data obtained during PLC effect and establish that the occurrence probability of the stress drops in the dynamical process responsible for PLC effect is Poisson in nature.

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