Department of Physics

Vellore Institute of Technology (VIT) is a private university located in&nbsp;Tamil Nadu, India. Founded in 1984, as Vellore Engineering College, the institution offers 20 undergraduate, 34 postgraduate, four integrated and four research programs. It has campuses in Vellore, Amravati, Bhopal and Chennai.

VIT is one of the top ranked private universities in India according to NIRF, THE and QS Rankings.&nbsp;Govt. of India has recognized&nbsp;VIT, Vellore as an&nbsp;Institution of Eminence. This has allowed VIT to take independent quality initiatives and move up in world ranking.

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VIT University

Physics

The chiral smectic phases of calamitic liquid crystals, SmC∗ and SmCA∗, are characterized by the synclinic ferroelectric F ordering and the anticlinic antiferroelectric A ordering in adjacent layers. Various states with mixed A and F orderings are degenerate at the frustrated phase-transition point. The degeneracy lifting is commonly caused by the long-range interlayer interactions (LRILIs), producing a series of biaxial subphases specified by a relative ratio of both orderings, qT=[F]/([A]+[F]). Sandhya et al. [Phys. Rev. E 87, 012502 (2013)PLEEE81539-375510.1103/PhysRevE.87.012502] established, however, the importance of thermal fluctuations in the degeneracy lifting in some binary mixtures of MC881 and MC452. They observed the most intriguing interplay of thermal fluctuations and LRILIs in the stabilization of an apparently single subphase. Since no other detailed experimental study of the subphase has so far been made, we carry out its dielectric investigations and clarify the following five points: (1) the subphase is surely a single phase from ≈80°C down to room temperature; (2) the imaginary part of complex permittivity ϵ″ shows the weak antiphase mode and hence it must be antiferroelectric qT=1/2; (3) ϵ″ becomes much stronger above ≈80°C, indicating the emergence of ferroelectric and/or ferrielectric states; (4) the dielectric amplitude gradually increases at least just above the 1/2 subphase, suggesting it be due to a continuous increase of qT; and (5) at low temperatures the antiphase relaxation mode shows irregularities that indicate the important role played by the cooperative motion of the layer undulation as well as of the director tilting. © 2020 American Physical Society.

World University root node

School of Advanced Sciences

Chennai Campus

Physical Review E

Dielectric study of a subphase stabilized in an exceptionally wide temperature range by a delicate balance of interlayer interactions and thermal fluctuations

Thermoelectric materials that can work at operating temperatures of T ≥ 900 K are highly desirable since the key thermoelectric factors of most thermoelectric materials degrade at high temperatures. In this work, we investigate the high temperature thermoelectric performance of EuFeO3 using a combination of first-principles methods and semi-classical Boltzmann transport theory. High temperature thermoelectric performance is achieved owing to the presence of corrugated flatbands in the valence band region and extremely flatbands in the conduction band region. The lowest energetic structure of EuFeO3 lies within a G-type antiferromagnetic configuration, and the effect of compressive and tensile strains (-7% to +7%) along the (a, b) axes on thermoelectric performance is systematically analyzed. An extremely high value of the Seebeck coefficient (more than 1000 μV/K) is consistently recorded in the high temperature region between 900 K and 1400 K in this material. Furthermore, electrical conductivities and power factors are high and electronic thermal conductivities are low in the considered range of temperatures. The calculated theoretical minimum lattice conductivity is small, estimated at around 1.47-1.54 W m-1K-1. A compressive strain of -3% is revealed to be the optimum level of strain for enhancing the key thermoelectric factors. Overall, p-type doping shows better thermoelectric performance than n-type doping in EuFeO3. © 2020 Author(s).

Chemistry

Fulltext

Journal of Applied Physics

High thermopower and power factors in EuFeO3for high temperature thermoelectric applications: A first-principles approach

We report the growth process and linear and nonlinear optical characterization of high quality improvised semiorganic 2A5NPDP single crystal using the modified SR method for the first time. The major difficulties for the growth of 2A5NPDP single crystal have been resolved with the aid of a temperature lowering method. After optimization of the growth conditions, a transparent 2A5NPDP single crystal was harvested with a size of 85 mm length and 10 mm diameter over a period of 60 days. The growth rate of the modified SR method grown crystal was found to be 10 times higher than that of the conventional slow evaporation solution technique (SEST) grown crystal. The unit cell parameters of the grown crystal were ascertained by the single crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) techniques. The high crystalline perfection of the crystal was ascertained by using a high-resolution X-ray diffraction (HRXRD) technique. The presence of multifarious functional groups of the 2A5NPDP crystal were confirmed by Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectral analyses. The thermal property was assessed by the thermogravimetric (TG) and differential thermal analysis (DTA). It shows that the grown crystal withstands temperature up to 175 °C. UV-vis-NIR analysis was used to measure the linear optical property while third-order nonlinear optical properties were ascertained using Z-scan and optical limiting (OL) techniques. The optical limiting threshold of conventional method grown crystal is 5.7 mW/cm2, whereas the modified SR method grown crystal is 4.7 mW/cm2. Low value of limiting threshold indicates that the grown crystal has minimum defects. Finally, 800 nm wavelength of 140 fs at 80 MHz repetition rate obtained from a Ti:sapphire oscillator was used for the terahertz (THz) generation between 0.1 and 2.1 THz range. The efficiency of the generated THz signal was measured using another 800 nm wavelength of 40 fs, at 1 kHz repetition rate obtained from a Ti:sapphire amplifier. The generated THz signal is separated out from unconverted 800 nm using Teflon filters, and power is measured using a pyrometer. The maximum conversion efficiency of generated THz signal is on the order of 0.63 × 10-5%. Copyright © 2019 American Chemical Society.

Civil

School of Civil Engineering

Crystal Growth & Design

Evaluation of Linear and Nonlinear Optical Properties of D−π–A Type 2-Amino-5-Nitropyridinium Dihydrogen Phosphate (2A5NPDP) Single Crystal Grown by the Modified Sankaranarayanan–Ramasamy (SR) Method for Terahertz Generation

Materials Science for Energy Technologies

Synthesis, growth and physicochemical properties of new organic nonlinear optical crystal l-threoninium tartrate (LTT) for frequency conversion

Bertrand’s theorem is one of the landmark results in the context of the central force problem in mechanics. It leads to the conditions for closed bound orbits. This pedagogical article intends to motivate the teacher as well as the student to engage in a formal proof of the theorem. We overview the basics of central forces and the Abel integral. Thereafter, we obtain the condition for closed orbits. We provide the reader with a broad panorama of areas/topics where Bertrand’s theorem arises, including interesting scenarios in the regime of Einstein’s general theory of relativity. © 2019, Indian Academy of Sciences.

VIT Unitversity

Resonance

A Pedagogical Relook at Bertrand’s Theorem

Ni2+ doped CuO (Cu1-xNixO for 0 ≤ x ≤ 0.3) nanocrystalline powders were synthesized by combustion technique. The doped samples were found single phasic from powder XRD for up to 10 mol%, above which precipitation of NiO phase was observed. Raman study however indicated the presence of Cu2O phase in both undoped and doped samples up to 1 mol% Ni2+ doping. At and above 2 mol% doping, precipitation of the NiO phase was observed. Morphology of powders got modified drastically upon doping. Band gap showed decreasing trend with the increase of doping concentration in CuO, attributed to the formation of intra-band defect states. XPS revealed the presence of adsorbed oxygen or carbonate species on the surface. For the first time, Ni2+ doped CuO is reported as catalyst (active electrode) for ppm level NH3 sensing in amperometric mode with high sensitivity of 135.5 μA/decade at 550 °C using LaGaO3 as solid electrolyte. © 2019 Elsevier Ltd

Department of Digital Communications

School of Information Technology and Engineering

Vellore Campus

Materials Research Bulletin

Ni doping in CuO: A highly sensitive electrode for sensing ammonia in ppm level using lanthanum gallate based electrolyte

Ca2+ substituted La2CuO4 (LCC) perovskite nanostructures have been prepared by microwave combustion. X-ray diffraction analysis indicates that the undoped La2CuO4 (LCC0) crystallizes within a single perovskite-type phase with an orthorhombic crystal structure. For smaller Ca2+ doping of 0.1 (LCC1), a minor impurity was observed, while for concentrations 0.2 (LCC2) and 0.3 (LCC3), orthorhombic-to-tetragonal phase transformation occurs. In contrast, the tetragonal phase disappears at 0.4 and 0.5 (LCC4 and LCC5) as confirmed by XRD and Rietveld refinements. The orthorhombic and tetragonal perovskite phases have an average crystallite size in range 34.2–54.3 nm and 39.8–39.4 nm, respectively. Fourier transform infrared spectroscopy study established the characteristic absorption bands of La2CuO4 perovskite orthorhombic structure; the correlated bands are observed at 683 cm−1 for La-O and 516 cm−1 for Cu-O stretching modes. The optical band gap as determined by diffuse reflectance spectroscopy (DRS) increases then decreases with increasing Ca2+ content due to the quantum confinement effect. The surface morphology observations using scanning electron microscopy display nano-sized pores and pore walls are fused grain boundaries. Magnetization-field curves exhibit a ferro-/paramagnetic behavior for undoped La2CuO4 while Ca-doped system has only a paramagnetic behavior due to the exchange of A and B sites within La2CuO4 host lattice. © 2019 Elsevier Ltd

Vacuum

Structural, optical, and magnetic properties of Ca2+ doped La2CuO4 perovskite nanoparticles

Single crystal growth of anhydrous citric acid (ACA) crystal from slow evaporation technique is reported. It crystallizes in monoclinic system with P21/a space group. Its transmission bandwidth extends from 200 nm to 1100 nm. The different functional band frequencies associated with the crystal, were found out from FTIR and FT Raman spectroscopic analyses. The molecular structure was established by NMR studies. It is thermally stable upto 162 °C. The third order nonlinear optical (NLO) susceptibility χ3 is found to be 1.77 × 10−7 esu and it is better than some reported crystals. © 2019 Elsevier GmbH

Optik

Structural, spectral, thermal and nonlinear optical analysis of anhydrous citric acid crystal

Visible light driven photocatalytic property of nickel doped BiFeO3 nanoparticles were explored from the degradation of Acid red-85 dye. Fuel (citric acid) assisted auto-combustion method was adopted for the synthesis of BiFe(1-x)NixO3 (x = 0.25, 0.5, 0.75 and 0.10) nanoparticles. The synthesized particles were characterized by x-ray diffraction, from which reduction in crystallite size has been observed from 57 nm to 28 nm. Lattice parameters also have been calculated for the same which shows an increase in lattice parameters confirming the presence of nickel in the BiFeO3 lattice. Morphological studies were carried out using high resolution scanning electron microscopy on undoped and 10% nickel doped BiFeO3 samples which show a decrease in particle size in the doped sample compared to the bare BiFeO3 similar results have been observed in Transmission Electron Microscopy images. Reduction in the optical band gap from 2.2 eV to 2.02 eV of BiFeO3 compared to 10% Nickel doped bismuth ferrite is observed. Vibrating sample magnetometer studies performed at room temperature indicates that addition of nickel increases both remnant and saturation magnetization to the bismuth ferrite sample thus helping in the removal of photocatalyst after treatment. Photocatalytic study on the Acid red-85 dye by visible light irradiation showed the complete degradation of dye in 40 min. The 10% Nickel doped BiFeO3 showed the best photocatalytic activity. The enhancement in the photocatalytic activity of the doped BiFeO3 is because of efficient charge separation which has been confirmed by photoluminescence studies. © 2019 IOP Publishing Ltd.

Mechanical

School of Mechanical Engineering

Materials Research Express

Photocatalytic degradation of acid red-85 dye by nickel substituted bismuth ferrite nanoparticles

With high power density and promising possibilities for high energy density, the electrochemical capacitor has become an indispensable energy storage device to satisfy the future energy demands. Here in, we report on synthesis of graphene nanosheets (GNS) supported manganese sulfide (MnS) hybrid (GNS-MnS) nanocomposite by a simple, facile hydrothermal process. The resultant GNS-MnS hybrid nanocomposite with robust electronic amalgamation facilitates swift transfer of both ions and electrons across the interface between electrode surface and electrolyte ions than pristine MnS. Benefiting from the better conduction along with improved active sites of the MnS in GNS-MnS hybrid nanocomposite, a high ratio surface/near-surface reactions is dominated by high specific capacity even at high current rate. As such, the GNS-MnS hybrid nanocomposite exhibited a maximum specific capacitance of 792 F g −1 at 2 A g −1 along with better retention rate of 58% at 15 A g −1 , is significantly higher than that of pristine MnS (423 F g −1 at 2 A g −1 and 37% retention at 15 A g −1 ). In addition, the fabricated symmetric GNS-MnS hybrid nanocomposite cell delivered a high energy density of 25 W h kg −1 and power density of 7160 W kg −1 . Moreover, specific capacitance of 91.1% is retained after 15000 th cycles with columbic efficiency of ∼100% at 20 A g −1 . The remarkable electrochemical performance of GNS-MnS hybrid nanocomposite electrode demonstrated its potential as a key material for developing high energy supercapacitors. © 2019 Elsevier B.V.