• Krishna

      Articles written in Bulletin of Materials Science

    • Anomalous photovoltaic effect and disorder in ZnS crystals

      M T Sebastian P Krishna

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      Some crystals of ZnS are known to produce an anomalously high photovoltage, up to several hundred volts per cm, when illuminated by uv light in the absorption edge region. This has been attributed to the presence of alternate regions of hexagonal and cubic packing with charged dislocations at the interfaces producing built-in electric fields. Differential absorption of the incident light in the hexagonal and cubic regions is believed to create the necessary asymmetry in the built-in fields, causing an addition of tiny photovoltages at a series of interfaces which finally results in the abnormally high photovoltages observed.

      This paper investigates the possible mechanism by which disordered ZnS crystals containing alternating regions of cubic and hexagonal packing can result. X-ray diffraction studies show that such a disordered configuration results during the 2H to 3C phase transformation in ZnS. It is suggested that the transformation occurs by the non-random nucleation of deformation faults wherein the probability (α) of random nucleation of the faults is much less than the probability (β) for the faults to occur at two-layer separations.

    • Structural disorder and solid state transformations in single crystals of ZnxCd1−xS and ZnxMn1−xS

      M T Sebastian P Krishna

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      Single crystals of ZnxCd1−xS and ZnxMn1−xS were grown from the vapour phase at 1100°C in the rangex=0·9 to 1. X-ray characterization shows that polytypes and disordered structures occur in ZnxCd1−xS forx ≥ 0·94, whereas ZnxMn1−xS displays disordered and polytype structures in the entire rangex=0·9 to 1. It is observed that ZnxCd1−xS and ZnxMn1−xS undergo a 2H-6H solid state transformation on annealing in vacuum around 600°C. Experimental analysis of the intensity distribution along the 10·L reciprocal lattice row as recorded on a single crystal diffractometer from partially transformed crystals shows that the mechanism of the transformation cannot be explained in terms of the one-parameter models of non-random faulting reported earlier. A two-parameter theoretical model with α representing the probability of random insertion of a fault in the 2H structure and β representing the probability of the growth of the 6H nucleus, is developed both for a deformation mechanism and a layer displacement mechanism. It is found that the theoretical model of non-random deformation faulting with β ≫ α approximates the actual mechanism of transformation in these crystals.

    • A novel method ofrf powder sputtering

      K Solomon Harshavardhan K N Krishna

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      A new method ofrf sputtering by which thin films of metals, semiconductors and insulators can be sputtered from their respective powders, has been successfully demonstrated. The films have been characterized for their surface and crystal structure using conventional methods ofsem andtem. All the films are amorphous with a relatively smooth surface topography. The relative merits and demerits of the technique have been briefly discussed.

    • Superconductivity in Na-doped Bi2Sr2CaCu2Oy system

      S V Suryanarayana B Gopala Krishna Ravichandra Gundakaram S Venkat Reddy

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      Samples with the nominal composition Bi2Sr2Ca1−xNaxCu2Oy (x=0, 0·1, 0·2 and 0·3) were prepared by solid-state reaction of the individual compounds. X-ray diffraction patterns indicate that the samples have a majority 2212 phase with 2223 also being present. From the DC four-probe resistance data, we have observed that the furnace-cooled samples show metallic behaviour while the quenched samples show superconductivity up to 97 K.

    • Effect of annealing and quenching on superconductivity in Pb and Sb-doped Bi2Sr2Ca2Cu3Ox

      S V Suryanarayana Ravichandra Gundakaram B Gopala Krishna S Venkat Reddy

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      Samples of the series Bi1·9−xPbxSb0·1Sr2Ca2Cu3Oy withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withTc=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.

    • On the tensile creep behaviour of a directionally-solidified Ni3Al-based alloy

      M S Gopala Krishna M C Pandey

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      High temperature tensile creep behaviour of a directionally-solidified Ni3Al-based alloy is presented. The study involved selection of nine alloy systems based on Ni3Al. The alloys contained varying amounts of Cr and Ta, fixed amounts of 1·5 at.% Hf and 0·5 at.% Zr and doped with 0·2 at.% each of C and B. The alloys were vacuum arc-melted into buttons and homogenized at 1050°C for 68 h. The test pieces of the alloys were hot compression tested at 600, 700, 800 and 900°C. The yield strength data of some of the alloys were superior to conventionally cast Mar-M 200, a cast nickel-base superalloy widely used in gas turbine structural applications. The best alloy system was chosen based on consistent performance in the hot compression studies. The alloy so chosen was directionally solidified and vacuum-homogenization-treated for 20 h at various selected temperatures. Optimum creep properties were observed at 1120°C, 20 h treatment. The minimum creep rate data of the DS alloy showed relatively higher values even at lower temperatures and stress levels as compared to Mar-M 200. Hence, the alloy is less promising in replacing nickel-based superalloys used as structural materials in gas turbine applications.

    • Critical analysis of driving force for pure-step migration by vacancy supersaturation and crystal anisotropy

      V Hari Krishna

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      The gist of the paper is to introduce the other compelling factors for the boundary displacement which are dominant in the lower temperature regime such as non equilibrium vacancy concentration and anisotropy and to prove that the driving force due to the above factors is sufficient to induce grain boundary migration.

    • Synthesis and properties of zirconia thin films

      M H Suhail M Ghanashyam Krishna G Mohan Rao S Mohan

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      Thin films of zirconia have been synthesized using reactive DC magnetron sputtering. It has been found that films with good optical constants, high refractive index (1·9 at 600 nm) and low extinction coefficient can be prepared at ambient temperatures. The optical constants and band gap and hence the composition are dependent on the deposition parameters such as target power, rate of deposition and oxygen background pressure. Thermal annealing of the films revealed that the films showed optical and crystalline inhomogeneity and also large variations in optical constants.

    • Structural characterization of gel-grown neodymium copper oxalate single crystals

      K S Raju K N Krishna Jayakumari Issac M A Ittyachen

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      Sparingly soluble neodymium copper oxalate (NCO) single crystals were grown by gel method, by the diffusion of a mixture of neodymium nitrate and cupric nitrate into the set gel containing oxalic acid. Tabular crystal, revealing well-defined dissolution figures has been recorded. X-ray diffraction studies of the powdered sample reveal that NCO is crystalline. Infrared absorption spectrum confirmed the formation of oxalato complex with water of crystallization, while energy dispersive X-ray analysis established the presence of neodymium dominant over copper in the sample. X-ray photoelectron spectroscopic studies established the presence of Nd and Cu in oxide states besides (C2O4)2− oxalate group. The intensities of Nd (3d5/2) and Cu (2p3/2) peaks measured in terms of maximum photoelectron count rates also revealed the presence of Nd in predominance. The inductively coupled plasma analysis supports the EDAX and XPS data by the estimation of neodymium percentage by weight to that of copper present in the NCO sample. On the basis of these findings, an empirical structure for NCO has been proposed. The implications are discussed.

    • Chemically modified clays as recyclable adsorbents for iodine

      B S Krishna S Selvaraj B V Mohan D S R Murty B S Jai Prakash

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      The adsorption behaviour of iodine on chemically modified swelling type of clays has been studied. Chemical modification was brought about by interacting the clay with surfactants such as tween-80 and polyethylene glycol. Adsorption of iodine was found to increase by several orders of magnitude on chemical modification which remained constant between pH 1 and 10. The adsorption isotherms were non-linear and fitted the Freundlich equation for swelling clays. Scatchard analysis of the data indicated minimum two types of active sites with the tween-80 modified clay and one type with the polyethylene glycol modified one. The iodine sorbed on the surface was found to get desorbed almost completely on leaching with water. Modification of the clay surface with surfactant thus offers a method of designing a recyclable adsorbent for iodine.

    • Impact toughness of ternary Al-Zn-Mg alloys in as cast and homogenized condition measured in the temperature range 263–673 K

      Harish Kundar Krishna K Srinivasan

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      Impact toughness of six Al-Zn-Mg ternary alloys are compared with two Al-Zn binary alloys and one CP aluminium metal at eight different temperatures of 263, 268, 273, 300, 373, 473, 573 and 673 K. The effects of alloying and temperature on toughness have been compared and analysed. The influence of alloying is more pronounced than that of temperature in reducing the toughness.

    • Systematic hardness measurements on single crystals and polycrystalline blanks of cesium halides

      D B Sirdeshmukh P Geeta Krishna K G Subhadra

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      Vickers and knoop hardness measurements were carried out on CsBr and CsI single crystals. Polycrystalline blanks of CsCl, CsBr and CsI were prepared by melting and characterized by X-ray diffraction. Vickers hardness measurements were carried out on these blanks. The hardness values were correlated with the lattice constant and the Schottky defect formation energy.

    • Systematic hardness measurements on mixed and doped crystals of rubidium halides

      D B Sirdeshmukh T Kumara Swamy P Geeta Krishna K G Subhadra

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      Efforts are made to improve the hardness of rubidium halide crystals by

      solid solution hardening and

      impurity hardening.

      Systematic microhardness measurements have been made on rubidium halide mixed crystals (RbBr–RbI and KI–RbI) and rubidium halide crystals doped with Sr2+ ions. The composition dependence of the hardness of mixed crystals follows the law $\Delta H_V$ = $K\ x$ (1–𝑥), where $\Delta H_V$ is the enhancement in hardness, 𝐾 a constant and 𝑥 and (1 – 𝑥) the concentrations of the first and second component of the mixed crystals, respectively. The hardness of doped crystals increases with the concentration 𝐶 of the dopant according to the law, $\Delta H_V$ = $k\ C^m$, where 𝑘 and 𝑚 are constants. The relative efficacy of the two methods of hardening is discussed.

    • Optical, structural and electrical properties of Mn doped tin oxide thin films

      Rajeeb Brahma M Ghanashyam Krishna A K Bhatnagar

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      Mn doped SnO𝑥 thin films have been fabricated by extended annealing of Mn/SnO2 bilayers at 200°C in air for 110 h. The dopant concentration was varied by controlling the thickness of the metal layer. The overall thickness of the film was 115 nm with dopant concentration between 0 and 30 wt% of Mn. The films exhibit nanocrystalline size (10–20 nm) and presence of both SnO and SnO2. The highest transmission observed in the films was 75% and the band gap varied between 2.7 and 3.4 eV. Significantly, it was observed that at a dopant concentration of ∼ 4 wt% the transmission in the films reached a minimum accompanied by a decrease in the optical band gap. At the same value of dopant concentration the resistivity also reached a peak. This behaviour appears to be a consequence of valence fluctuation in Sn between the 2+ and 4+ states. The transparent conductivity behaviour fits into a model that attributes it to the presence of Sn interstitials rather than oxygen vacancies alone in the presence of Sn2+.

    • Effect of microstructural evolution on magnetic properties of Ni thin films

      Prashant Kumar M Ghanashyam Krishna A K Bhattacharya

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      The magnetic properties of Ni thin films, in the range 20–500 nm, at the crystalline–nanocrystalline interface are reported. The effect of thickness, substrate and substrate temperature has been studied. For the films deposited at ambient temperatures on borosilicate glass substrates, the crystallite size, coercive field and magnetization energy density first increase and achieve a maximum at a critical value of thickness and decrease thereafter. At a thickness of 50 nm, the films deposited at ambient temperature onto borosilicate glass, MgO and silicon do not exhibit long-range order but are magnetic as is evident from the non-zero coercive field and magnetization energy. Phase contrast microscopy revealed that the grain sizes increase from a value of 30–50 nm at ambient temperature to 120–150 nm at 503 K and remain approximately constant in this range up to 593 K. The existence of grain boundary walls of width 30–50 nm is demonstrated using phase contrast images. The grain boundary area also stagnates at higher substrate temperature. There is pronounced shape anisotropy as evidenced by the increased aspect ratio of the grains as a function of substrate temperature. Nickel thin films of 50 nm show the absence of long-range crystalline order at ambient temperature growth conditions and a preferred [111] orientation at higher substrate temperatures. Thin films are found to be thermally relaxed at elevated deposition temperature and having large compressive strain at ambient temperature. This transition from nanocrystalline to crystalline order causes a peak in the coercive field in the region of transition as a function of thickness and substrate temperature. The saturation magnetization on the other hand increases with increase in substrate temperature.

    • Interface controlled growth of nanostructures in discontinuous Ag and Au thin films fabricated by ion beam sputter deposition for plasmonic applications

      R Brahma M Ghanashyam Krishna

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      The growth of discontinuous thin films of Ag and Au by low energy ion beam sputter deposition is reported. The study focuses on the role of the film–substrate in determining the shape and size of nanostructures achieved in such films. Ag films were deposited using Ar ion energy of 150 eV while the Au films were deposited with Ar ion energies of 250–450 eV. Three types of interfaces were investigated in this study. The first set of film–substrate interfaces consisted of Ag and Au films grown on borosilicate glass and carbon coated Cu grids used as substrates. The second set of films was metallic bilayers in which one of the metals (Ag or Au) was grown on a continuous film of the other metal (Au or Ag). The third set of interfaces comprised of discontinuous Ag and Au films deposited on different dielectrics such as SiO2, TiO2 and ZrO2. In each case, a rich variety of nanostructures including self organized arrays of nanoparticles, nanoclusters and nanoneedles have been achieved. The role of the film–substrate interface is discussed within the framework of existing theories of thin film nucleation and growth. Interfacial nanostructuring of thin films is demonstrated to be a viable technique to realize a variety of nanostructures. The use of interfacial nanostructuring for plasmonic applications is demonstrated. It is shown that the surface Plasmon resonance of the metal nanostructures can be tuned over a wide range of wavelengths from 400 to 700 nm by controlling the film–substrate interface.

    • Effect of T6 heat treatment on damping characteristics of Al/RHA composites

      D Siva Prasad A Rama Krishna

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      In the present work, effect of T6 heat treatment on the damping behaviour of aluminum/rice husk ash (RHA) composites fabricated by vortex method was studied using dynamic mechanical analyser (DMA) at frequencies ranging from 1 Hz to 25 Hz at room temperature under three-point bending test mode. The matrix material for the present work was A356.2 and reinforced with different weight % of 4, 6 and 8 rice husk ash particles. It was observed that composite exhibits high damping capacities than unreinforced alloy and increases with increase in weight % and the storage modulus increases with the addition of RHA particles but decreases with the increase in weight %. The heat treated composites exhibit higher damping capacity than the composites without heat treatment and increases with the increase in weight % of the reinforcement and loss in the storage modulus was observed and further decreases with the increase in the weight %of reinforcement. The related mechanisms were also discussed.

    • X-ray determination of crystallite size and effect of lattice strain on Debye–Waller factors of platinum nano powders

      E Purushotham N Gopi Krishna

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      In the present study, nano platinum particles were produced by ball milling process. The lattice strains in platinum (Pt) powders produced by milling have been analysed by X-ray powder diffraction. The lattice strain (𝜀) and Debye-Waller factor (𝐵) are determined from the half-widths and integrated intensities of the Bragg reflections. In Pt, the Debye–Waller factor is found to increase with lattice strain. From the correlation between the strain and effective Debye–Waller factor, the Debye–Waller factors for zero strain have been estimated for Pt. The variation of energy of vacancy formation as a function of lattice strain has been studied.

    • Effect of particle size and lattice strain on Debye–Waller factors of Fe3C nanoparticles

      E Purushotham N Gopi Krishna

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      Fe3C nanoparticle powders have been prepared by a high energized ball mill. The resulting nano-particle powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. The high-energy ball milling of Fe3C after 10 h resulted in crystalline size of about 5 nm. The Debye temperature, mean-square amplitudes of vibration, Debye–Waller factor, lattice parameters, particle size, lattice strain and vacancy formation of energies of Fe3C nanoparticles prepared by ball mill have been obtained from X-ray integrated intensities. The integrated intensities have been measured with a Philips CWU 3710 X-ray powder diffractometer fitted with a scintillation counter using filtered CuK𝛼 radiation at room temperature and have been corrected for thermal diffuse scattering. The X-ray Debye temperatures obtained in the present investigation has been used to estimate the vacancy formation energies for Fe3C nanoparticles.

    • Effects of metal doping on photoinduced hydrophilicity of SnO2 thin films

      Debarun Dhar Purkayastha Rajeeb Brahma M Ghanashyam Krishna V Madhurima

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      The influence of metal dopants (Mn2+, Al3+ and Cu2+) on the wetting properties of SnO2 thin films deposited by thermal evaporation is reported. The undoped and doped SnO2 films crystallize into the orthorhombic structure upon annealing at 200°C for 110 h. It is shown that wettability behaviour, before and after ultraviolet (UV) irradiation, is dependent on the ionic radius of the dopant. The contact angle of un-irradiated samples increases with increase in ionic radius of the dopant and also in comparison with the undoped sample. It is 54° for pure SnO2 and increases to 77.5°, 92.3° and 95.9° for the Al3+, Mn2+ and Cu2+ doped samples, respectively. After UV irradiation, the value is 5.4° for the undoped sample. This increases to 21.2° for Al doping reaching a minimum of 6.4° for the Mn-doped sample increasing thereafter to 63.3° for the Cu-doped sample. It is observed that pre-irradiation contact angle behaviour can be correlated with the change in roughness of the films with increasing ionic radius. In contrast, photoinduced hydrophilicity of the films correlates with their optical bandgap. The contact angle is lowest for the lowest bandgap material, i.e., Mn-doped SnO2, with a bandgap of 2.48 eV. Thus, the band structure of SnO2 that can be controlled by dopant ionic radius can in turn be employed to manipulate the wettability of these surfaces.

    • Biological synthesis and characterization of silver nanoparticles using Eclipta alba leaf extract and evaluation of its cytotoxic and antimicrobial potential

      Paramasivam Premasudha Mudili Venkataramana Marriappan Abirami Periyasamy Vanathi Kadirvelu Krishna Ramasamy Rajendran

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      With increasing global competitions there is a growing need to develop environmentally benevolent nanoparticles without the use of toxic chemicals. The biosynthesis of silver nanoparticles (AgNPs) using plant extracts became one of the potential areas of research. The bioreduction of metal ion is quite rapid, readily perform at room temperature and easily scale up. The present study describes a rapid and eco-friendly synthesis of AgNPs using Eclipta alba plant extract in a single pot process. The efficiency and the influence of various process variables in the biosynthesis of AgNPs analysed include redundant concentration, temperature and time. AgNPs were rapidly synthesized using aqueous leaf extract of E. alba and was observed when the medium turned to brown colour with the addition of silver ion. Biosynthesized AgNPs were characterized by the help of UV–visible spectroscopy for their stability and physicochemical parameters were studied by dynamic light scattering, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy. The obtained results confirmed that recorded UV spectra show the characteristic surface plasmon resonance band for AgNPs in the range of 400–440 nm and physiochemical structural analysis shown that obtained AgNPs were crystalline in nature. Further, cytotoxic and antimicrobial activities of biosynthesized AgNPs against RAW 254.7, MCF-7 and Caco-2 cells as well as Gram positive and Gram negative bacteria were assessed. In-vitro cytotoxicity activity of characterized AgNPs against tested cell lines showed significant anti-cell-proliferation effect in nanomolar concentrations. The antibacterial activity of synthesized AgNPs showed effective inhibitory activity against human pathogens, including, Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa. Thus, the significant outcome of this study would help to formulate value added herbal-based nano-materials in biomedical and nanotechnology industries.

    • Efficient strategy to Cu/Si catalyst into vertically aligned carbon nanotubes with bamboo shape by CVD technique

      V MOHANA KRISHNA T SOMANATHAN

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      Bamboo-shaped vertically aligned carbon nanotubes (bs-VACNTs) were fabricated on Cu/Si catalyst by chemical vapour deposition (CVD) technique under the atmospheric pressure. The catalytic material (Cu/Si) playeda vital role in attaining bs-VACNTs, which is synthesized by drop cast method in a cost-effective manner. Using this catalytic support, we have achieved the tip growth bs-VACNTs at low temperature with well graphitization. The as-grown carbon material was then characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) analyzer, high-resolution transmission electron microscope (HRTEM) and Raman spectroscopy. XRD technique confirms the formation of hexagonal graphitic carbon planes of carbon nanotubes (CNTs). The surface morphology of the material was characterized by SEM, which clearly infervertically aligned CNTs. The nature, diameter and crystallinity were noticed by HRTEM and Raman spectroscopy, respectively. Further, we have also studied the electrochemical properties of the bs-VACNTs and it seems to be proved as highly electroconductive when compared to multi-walled carbon nanotubes (MWCNTs).

    • Effect of pre-annealing on the structural evolution and optical response of Ag films exposed to iodine vapours

      R PRAVEENA V SRAVANI SAMEERA N V PRABHAKARA RAO THIRUMALA C S SUNANDANA M GHANASHYAM KRISHNA

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      Ag thin films of 5nm thickness were deposited on glass substrates by thermal evaporation. The films were divided into two sets, out of which, one set was not annealed and the other set was subjected to pre-annealing at 300$^{\circ}$C for2 h in air. The un-annealed and pre-annealed films were exposed to iodine vapours at room temperature for the durations from 5min to 10 h. The un-annealed films were crystallized into the $\beta$ phase of AgI after exposure for 5 h. In contrast, for the pre-annealed films, crystallization into the $\beta$ phase occurred within the first 5 min. Both sets of films, however, exhibit astrong preferential c-axis orientation in the $\beta$-AgI phase. Optical absorption studies reveal that the un-annealed films exhibit a localized surface plasmon resonance (LSPR) with a peak at 545 nm and a long wavelength shoulder at 620 nm, which shifts to 516nm after iodization for a few minutes. This peak position does not change with further iodization. The LSPR for the pre-annealed films has a single peak at 538 nm. After iodization for a few minutes, this peak shifts to 525 nm. Iodization for 3 h results in a further blue-shift of this resonance to 475 nm. The photoluminescence spectrum reveals two peaks, oneat 368 nm and the other at 712 nm. The first one is assigned to the excitons of AgI, whereas the long wavelength peak is attributed to the presence of disorder in the films. The reasons for the difference in behaviour of the un-annealed andpre-annealed films are discussed.

    • Energy-harvesting enhancement in composites of microwave-exfoliated KNN and multiwall carbon nanotubes

      NANDINI R NADAR M KRISHNA A V SURESH

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      The objective of this research work is to investigate the effect of multiwall carbon nanotube (MWCNT) content (0.3–1.2 wt%) on a potassium sodium niobate (KNN)-based piezoelectric unimorph harvester for enhancing the energy generation capacity. KNN–MWCNT composites were fabricated by using a microwave solid state technique. The energy-harvesting performance of the KNN–MWCNT composite was determined by the base excitation method and sized to resonate between 20 and 100 Hz at 1 MΩ load resistance. The energy performance of the KNN composite at percolation threshold (0.6 wt% MWCNT) showed a maximum power generation of 2.94 μW, the power density of 7.15 μWm⁻₃ and overall efficiency of 83.75% at an input acceleration of 0.5 g and a load resistance of 1 MΩ. Improvements observed in the power generation by percolation phenomena and ionic flow over the nanotube surface of KNN composites prove to be a boon for low-power sensing devices.

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