Qualification: 
Ph.D, MPhil, MSc
Email: 
a_umamaheswari@cb.amrita.edu

Dr. Umamaheswari A. currently serves as Assistant Professor in Physics, Department of Sciences, School of Engineering, Coimbatore Campus. Her areas of research include Theoretical Physics.

Dr. A. Uma Maheswari currently serves as Assistant Professor (Selection Grade) at the Department of Sciences (Physics), School of Engineering, Coimbatore.

Her research interest includes perovskites/metal oxides for energy harvesting applications and High-temperature wear and corrosion-resistant nanocomposite coatings.

Affiliation(s)

  • Assistant Professor, Department of Sciences

Qualification

  • May 2011: Ph. D. ( Physics)
    Amrita Vishwa Vidyapeetham, Coimbatore, TamilNadu, India
  • April 1999: M.Phil (Physics)
    PSG College of Arts and Science, Coimbatore, TamilNadu, India
  • May 1997: M. Sc. (Physics)
    PSG College of Arts and Science, Coimbatore, TamilNadu, India
  • April 1995: B. Sc. (Physics)
    AvinasilingamDeemed University, Coimbatore, TamilNadu, India

Awards, Certificates, Honors and Societies

  • “Excellence in Research” (yr: 2017-18) by Amrita Vishwa Vidyapeetham

Research Interest

  1. Area of Interest
    • Metal oxides for energy harvesting applications
    • High temperature wear and corrosion resistant nanocomposite coatings

Research Group

Present Team

A. Kathirvel
Research interest: Metal oxide thin films for photovoltaic applications

Funded Project

Year Sponsor Title of the project Grant sanctioned Investigators Status
March 2017- August 2020 SERB Nanostructured iron-based mixed metal oxide photovoltaic thin films 31.05 Lakhs A. Uma Maheswari, M.Sivakumar, Sudip Kumar Batabyal Completed
July 2012 - July 2015 DRDO Nanostructured Thin Films of Composites Synthesized by Spray Coating Technique for Corrosion and Wear Resistance Applications 27.14 Lakhs M. Sviakumar, A.Uma Maheswari, S. Saravanakumar Completed

Teaching

  1. UG Theory:
    • Modern Physics
    • Solid state physics
    • Concepts of Nanoscience and nanotechnology
    • Mechanics
    • Electricity and Magnetism
    • Engineering physics
    • Electrical Engineering Materials
    • Quantum Physics and its applications
    • Laser physics and its applications

Publications

Publication Type: Journal Article

Year of Publication Title

2020

K. K. Anjali, Dr. Umamaheswari A., and Sivakumar, M., “Size Dependent Dielectric Properties of TiO2 Nanoparticles”, Material and Manufacturing Technology X, 2020.[Abstract]


In this study, influence of nanoparticles size on optical and dielectric properties of TiO2 nanoparticles is investigated through thermal treatment of hydrous amorphous titania synthesized by chemical precipitation method at temperatures 300 °C and 600 °C. The average sizes of nanoparticles estimated respectively are 8 nm and 22 nm. Although the optical bandgap energy of both samples remains the same the Eg Raman mode observed at 144 cm-1 for bulk TiO2 is shifted to 150 cm-1 only for nanoparticles calcined at 300 °C. The shift is ascribed to the size as well as higher density of surface defects. Moreover, the presence of surface defects like oxygen vacancies which provide effective sites for catalytic reaction are confirmed by EPR and photoluminescence studies. The oxygen vacancies enhances space charge polarization and consequently results in higher dielectric constant. In addition, the peak shift of loss tangent which determines the mobility of charge carriers is found to be size dependent. Hence calcination temperature has significant influence on defect levels which in turn determine the optical and dielectric properties of TiO2 nanoparticles.

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2020

R. Anjitha, Kathirvel, A., Dr. Umamaheswari A., and Sivakumar, M., “Influence of B-site doping on structural, optical and dielectric properties of bismuth ferrite”, Materials Today: Proceedings, vol. 33, pp. 1293-1297, 2020.[Abstract]


The present work discusses the nature of electrical defects that influence the dielectric response of BiFe1-xZrxO3 nanostructures fabricated by hydrothermal method for x = 0.0, 0.005, 0.01, 0.015, 0.02 and 0.025. X-ray diffraction studies reveal that a higher concentration of Zr4+ ions can lead to the formation of an impure phase like Bi2Fe4O9 which increases the leakage current. Besides, the suppression of E-modes in a 1.5% doped sample confirms the structural distortion in FeO6 octahedra. A decrease in the optical band gap of Zr-doped samples indicates the presence of oxygen vacancies as donor levels within the forbidden gap. The dielectric constant of a 1.5% doped sample is quite higher than pure BiFeO3 sample at a frequency below 1 kHz. However, at higher frequency region (>1 kHz) the difference in dielectric constant between 1.5% Zr doped and a pure sample is quite less. Further, the broadened peak of M“(ω) of the doped sample indicates the non-Debye relaxation of charge carriers with different time constants.

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2019

S. Suresh, Kathirvel, A., Dr. Umamaheswari A., and Sivakumar, M., “Frequency dependent dielectric relaxation of Ba-doped BiFeO3 nanoparticles”, Materials Research Express, vol. 6, p. 115057, 2019.[Abstract]


In this work, dielectric properties of barium doped bismuth ferrite (BFO) nanoparticles [Bi1-xBaxFeO3 (x = 0.005–0.025)] synthesized by hydrothermal method are investigated. X-ray diffractograms of pure and doped samples compared with rhombohedral R3c structure without the trace of any impurity phase. The average grain size of doped nanoparticles decreases with increase of Ba content. All the samples exhibited broad absorption in the region extending from 200–650 nm. The optical bandgap of doped samples reduces with an increase of Ba content and a significant reduction in bandgap is observed for 1.5% and 2.5% doping. Samples Bi1-xBaxFeO3 (x = 0.0 and 0.015) exhibits anomalous dielectric response attributed to difference in frequency dependence relaxation of charge carriers within the grain and grain boundaries. The grain (Rg) and grain boundary (Rgb) resistances of these samples are evaluated by fitting the measured complex impedance data with an equivalent circuit. The dielectric constant of Bi1-xBaxFeO3 (x = 0.015) is higher than that of pure BFO in the frequency range of 100 Hz–1 kHz. This is attributed to smaller grains and higher density of defects like oxygen vacancies leading to large space charge polarization. However, at higher frequencies (>1 kHz), the dielectric constant of pure BFO is higher than that of Bi1-xBaxFeO3 (x = 0.015) and hence high dielectric loss. Besides, 1.5% Ba doping has made the ions rigid, which shifts the relaxation peak of tanδ to lower frequency side and reduces the leakage current.

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2019

K. N. Akhil, Dr. Umamaheswari A., and Sivakumar, M., “Influence of Annealing Temperature on the Optical Properties of TiO2Thin Films”, Materials Today: Proceedings, vol. 18, pp. 1416-1421, 2019.[Abstract]


The optical band gap of TiO2 films greatly determines its electro-optical and photocatalytic properties. By optimizing the annealing temperature of titania film the above said properties can be tuned. The present study investigates the influence of annealing temperature on optical properties of nanostructured titania films fabricated on quartz substrate by spray technique. The white anatasetitania nanoparticles needed for film fabrication is synthesized by precipitation method using titanium isopropoxide. Further, to have fine dispersion of TiO2 nanoparticles in spay solvent, its surface is modified using (3-Glycidyloxypropyltrimethoxysilane(GPTMS). The fabricated films are air annealed at two different temperatures 150 and 300 oC and their optical properties are studied. There is significant reduction in optical bandgap of titania film annealed at 300 oC to 1.5 eV. However, the optical band gap of the titania film annealed at 150 oC is 2.9 eV. The difference is attributed to surface defects and its presence is analyzed from PL spectra. Both the films exhibited emission attributed to oxygen vacancies. Nevertheless the emission intensity for film annealed at 300 oC is very low, which indicates the recombination rate of photogenerated electrons and holes are highly reduced. This is also confirmed from time resolved PL decay recorded for excitation 300 nm.

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2019

P. Srilakshmi, Dr. Umamaheswari A., Sajeev, V., and Sivakumar, M., “Tuning the optical bandgap of V2O5 nanoparticles by doping transition metal ions”, Materials Today: Proceedings, vol. 18, pp. 1375-1379, 2019.[Abstract]


In this study the influence of transition metal ions doping on optical properties of V2O5 nanoparticles is investigated. The shift in absorption edge caused by the dopant is analyzed from the UV-visible absorption spectra. The absorption edge of Ti doped V2O5 nanocrystals is shifted to longer wavelength region, while that of Zr doped V2O5 showed a blue shift. Ti doped V2O5 nanocrystal in spite of its large crystallite size showed improved optical properties and its optical bandgap (1.96 eV) is less than un-doped V2O5 (2.2 eV). This is attributed to lattice expansion by Ti ion and formation of oxygen vacancies within the bandgap. The oxygen vacancies acts as intermediate energy states and reduce the optical bandgap. However, the PL spectra of Ti-doped V2O5 did not exhibited emission peak corresponding to oxygen vacancies due to high crystallinity. The life time of excitons in excited state is investigated from time resolved PL decay. The excitons in doped sample showed tri-exponential decay whereas in pure sample they exhibited bi-exponential decay. The average lifetime of excitons in doped V2O5 is higher than pure V2O5 sample. This study reveals that by optimizing the concentration of Ti and Zr dopant ions in V2O5 lattice, its energy band structure can be tailored for photovoltaic, catalytic and photoluminescence applications. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i-2017).

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2019

M. Shajudheen P., S., S. Kumar, V., S. Kumar, Dr. Umamaheswari A., M., S., and Mohan, S. R., “Study of anticorrosive nature of spray coated titanium dioxide and nickel oxide composite thin films on stainless steel in briny surroundings”, AIP Conference Proceedings, vol. 2162, p. 020040, 2019.

2019

Dr. Umamaheswari A., Anjali, K. K., and Sivakumar, M., “Optical absorption enhancement of PVP capped TiO 2 nanostructures in the visible region”, Solid State Ionics, vol. 337, pp. 33-41, 2019.[Abstract]


In the present study, the role of polymeric surfactant polyvinylpyrrolidone (PVP) in enhancing optical absorption of PVP capped TiO 2 nanoparticles in the visible region is investigated. When the PVP capped hydrous amorphous TiO 2 is calcined at temperature less than thermal degradation temperature of PVP, the polymeric surfactant alters the band structure of TiO 2 nanoparticles resulting a significant change in optical absorption by introducing Ti 3+ ions and oxygen vacancies in the band gap. The induced surface modifications of TiO 2 nanoparticles by PVP were studied using micro Raman, XPS, UV–Vis., EPR and photoluminescence spectroscopy. The Raman active mode at 144 cm −1 which is ascribed to anatase TiO 2 is shifted and broadened in PVP capped samples (TP 1 ) due to oxygen related stoichiometry defects. The excess oxygen vacancies in TiO 2 lattice leads to the formation of Ti 3+ ions to maintain the electrostatic balance. The existence of Ti 3+ ions is also confirmed from EPR spectra. The observed red shift of optical absorption spectra is due to reduction in the band gap of PVP capped nanoparticles. Furthermore the deconvoluted Ti2p 3/2 and Ti2p 1/2 XPS peaks shows prominent shoulder peaks at 457.7 and 463.5 eV corresponding to Ti 3+ . The formation of Ti 3+ upper shifts the O1s line of TP1 due to transfer of charges from oxygen atoms of PVP to Ti 4+ ions. Besides PL emission at 481 nm confirms the formation of Ti 3+ due to surface oxygen vacancies induced by PVP. The surface defects Ti 3+ and oxygen vacancies act as trap centers and extend the lifetime of charge carriers. © 2019

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2019

S. R. Mohan, Dr. Umamaheswari A., and Sivakumar, M., “Structure dependent dielectric response of spray coated nanostructured zirconia thin films”, Materials Research Express, vol. 6, 2019.[Abstract]


In this work the structural dependent dielectric property of ZrO 2 nanoparticles and films prepared using these nanoparticles is studied. The tetragonal and monoclinic dominant zirconia nanoparticles were obtained through thermal treatment of PVP/CTAB capped hydrous amorphous zirconia (HAZ) at 500 °C. The grain (R g ) and grain boundary (R gb ) resistances of both types of nanoparticles along with films are estimated by fitting the respective complex impedance (Nyquist) plots with an equivalent electrical circuit. The nanostructured tetragonal dominant zirconia films are highly conductive as compared to monoclinic films due to large amount of Zr 3+ ions present at tetragonal sites. Consequently the film exhibits a high dielectric constant at lower frequency range due to Maxwell-Wagner polarization. Moreover the EPR signals observed at 1.946 (g || ) and 1.967 (g) reveals the presence of greater Zr 3+ ions tetragonal films. The R gb of monoclinic dominant film is increased by an order of 10 3 and consequently results in significant reduction of dielectric constant. Further the shift of loss tangent peak to higher frequency side of tetragonal dominant films endorses the higher mobility of charge carriers as that of monoclinic films. The enhancement of dielectric properties shows the potential application of tetragonal dominant ZrO 2 films for future electronic devices. © 2019 IOP Publishing Ltd.

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2018

M. V. P, Rani, A., Kumar, V. Senthil, Dr. Umamaheswari A., Manickam, S., and Kumar, S., “Optical and Corrosion Studies of Spray Pyrolysis Coated Titanium Dioxide Thin Films”, Advanced Science Letters, vol. 24, pp. 5836-5842, 2018.[Abstract]


In the present work, nanostructured thin films of titanium dioxide (TiO 2 ) have been coated on the stainless steel (SS 304L) substrate by spray pyrolysis coating technique. The surface morphology and chemical constituents of the thin film have recorded using Field Effect Scanning Electron Microscopy (FESEM) and Energy Dispersive Analysis of X-rays (EDAX) respectively. The structural and optical properties of the films of as deposited were examined by Micro Raman, Photoluminescence Spectroscopy (PL) and UV-Vis absorption method. The FESEM micrograph showed the microporous nature of the film. EDAX spectrum illustrated the presence of Ti and O on the coated surface of the steel substrate. The peaks in the micro Raman spectrum indicated that the TiO 2 samples of present study are in rutile phase of titanium dioxide. A strong emission peak around 350 nm was observed in the Photoluminescence spectrum of the samples. The anti-corrosion properties of the TiO 2 coated samples were investigated by neutral salt spray test for 390 h. Electrochemical Impedance Spectroscopy (EIS) analysis and Tafel analysis were performed before and after salt spray test and the results suggested an increase of corrosion resistance of the titanium dioxide thin film in a corrosive environment. The positive shift of equilibrium corrosion potential (E corr ) of bare stainless steel to thin film coated stainless steel (from −0.96 V to −0.38 V) in the electrochemical Tafel analysis implied the significant increase of corrosion resistance.

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2017

Dr. Umamaheswari A., Mohan, S., and Dr. Sivakumar M., “Effect of Surfactant and Mineralizer on the Dielectric Properties of Zirconia Nanocrsytals”, Applied Surface Science, vol. 427, pp. 1174-1182, 2017.[Abstract]


The combined effect of surfactants (PVP/CTAB) and alkaline mineralizers (NaOH/NH4OH) on dielectric properties of zirconia nanocrystals is analyzed. It is found that, the stabilization of zirconia tetramers by surfactants and the rate of hydroxyl ions released by alkaline mineralizers have significant impact on the dielectric properties of nanocrystals. The PVP capped tetramers form highly conducting grains with insulating boundaries, whereas the grains of CTAB capped tetramers are highly insulating with conducting grain boundaries, as revealed by Nyquist plots. Consequently, the space charge polarization would be quite large in highly conducting grains resulting in higher dielectric constant values at lower frequencies. The higher dielectric constant of PVP capped nanocrystals is due to greater tetragonal coordination of 3d5/2 and 3d3/2 electrons of Zr4+ ions than that of CTAB capped nanocrystals. Further, the surface oxygen vacancies of PVP samples are higher, resulting in a high space charge polarization. The ESR signal corresponding to F+ centers appears stronger for PVP/NH4OH nanocrystals. Moreover, the larger ESR line width of PVP/NH4OH nanocrystals corresponding to more oxygen vacancies is in accordance with the inference attained from the XPS analysis.

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2017

M. V. P, S, S. Kumar, Kumar, V. Senthil, Dr. Umamaheswari A., Dr. Sivakumar M., and Rani, A., “Investigation of anticorrosion properties of nanocomposites of spray coated zinc oxide and titanium dioxide thin films on stainless steel (304L SS) in saline environment”, Materials Research Express, vol. 5, 2017.[Abstract]


The present study reports the anticorrosive nature of nanocomposite thin films of zinc oxide and titanium dioxide on steel substrate (304L SS) using spray coating method. The morphology and chemical constituents of the nanocomposite thin film were characterized by field effect scanning electron microscopy and energy dispersive analysis of x-ray (EDAX) studies. From the EDAX studies, it was observed that nanocomposite coatings of desired stoichiometry can be synthesized using present coating technique. The cyclic voltametric techniques such as Tafel analysis and electrochemical impedance spectroscopy (EIS) analysis were conducted to study the anticorrosion properties of the coatings. The E corr values obtained from Tafel polarization curves of the sample coated with nanocomposites of ZnO and TiO2 in different ratios (5:1, 1:1 and 1:5) indicated that the corrosion resistance was improved compared to bare steel. The coating resistance values obtained from the Nyquist plot after fitting with equivalent circuit confirmed the improved anticorrosion performance of the coated samples. The sample coated with ZnO: TiO2 in the ratio 1:5 showed better corrosion resistance compared to other ratios. The Tafel and EIS studies were repeated after exposure to 5% NaCl for 390 h and the results indicated the anticorrosive nature of the coating in the aggressive environment. The root mean square deviation of surface roughness values calculated from the AFM images before and after salt spray indicated the stability of coating in the saline environment.

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2017

S. R. Mohan, Dr. Umamaheswari A., and Dr. Sivakumar M., “Structural transformation of polymeric and cationic capped zirconia nanoparticles with temperature”, International Journal of Nanoparticles, vol. 9, pp. 88-100, 2017.[Abstract]


The phase transformation mechanisms of zirconia nano crystals obtained on thermal treatment of polyvinyl pyrrolidone (PVP) and cetyltrimethylammoniumbromide (CTAB) capped hydrous amorphous zirconia (HAZ) on thermal treatment are investigated. In PVP assisted synthesis, tetragonal zirconia nanoparticles formation is the predominant one as compared to CTAB assisted synthesis. The decomposition of surfactants capped on HAZ with temperature (526°C for PVP and 466°C for CTAB) and subsequent interaction of the by-products with zirconia nanoparticles are responsible for tetragonal to monoclinic phase transformation. The decomposition temperature of capped surfactants is observed from thermogravimetric analysis (TGA) and subsequent interaction of by-products with zirconia nanoparticles at different temperatures is investigated from FTIR spectra. Further the structure of nanoparticles annealed at different temperatures is investigated using XRD, micro Raman and photoluminescence (PL) analysis. Copyright © 2017 Inderscience Enterprises Ltd.

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2016

V. P. Muhamed Shajudheen, Viswanathan, K., K Rani, A., Dr. Umamaheswari A., and S Kumar, S., “A Simple Chemical Precipitation Method of Titanium Dioxide Nanoparticles Using Polyvinyl Pyrrolidone as a Capping Agent and Their Characterization”, World Academy of Science, Engineering and Technology, International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering, vol. 10, pp. 552–555, 2016.

2016

Dr. Umamaheswari A., Sivakumar, M., Indhumathi, N., and Mohan, S. R., “Corrosion Resistance of 304L SS Spray Coated with Zirconia Nanoparticles”, IOP Conference Series: Materials Science and Engineering, vol. 149, p. 012068, 2016.[Abstract]


Influence of substrate temperature on corrosion (in 3.5% NaCl) and wear resistance of nanostructured zirconia thin film coated 304L SS substrates are studied by electrochemical and nano-indentation methods. This analysis shows 304L SS substrate spray coated with nanostructured zirconia at substrate temperature closer to the boiling point of the spray solvent ethanol exhibited good corrosion and wear resistance behaviour. This is because of the compressive stress developed during film fabrication at lower substrate temperature (∼50 °C) and hence constrains the indentation plasticity, which leads to higher indentation load than the bare 304L SS. More »»

2015

Dr. Umamaheswari A., Mohan, S. R., Dr. Sivakumar M., and S Kumar, S., “Tweaking Electrical and Dielectric Properties of Nickel Oxide Nanocrystals by Varying the Surfactant”, Journal of nanoscience and nanotechnology, vol. 15, pp. 9423–9430, 2015.[Abstract]


The influence of cationic cetyltrimethylammonium bromide (CTAB) and neutral polymeric polyvinylpyrrolidone (PVP) surfactants on electrical and dielectric properties of NiO nanocrystals is investigated. It is demonstrated that, compressive strain of nanocrystals is higher with PVP than that of CTAB. Consequently surfactant type has significant influence on intrinsic defects of nanocrystals. This is attributed to the difference in stabilization of metallic ions against agglomeration that leads to variation in rate of hydrolysis. Particularly, in the case of PVP assisted synthesis, higher stabilization leads to slow nucleation rate with lower defect density. As a result the hopping time of charge carriers decreases which in turn enhances the conductivity of nanocrystals as evidenced from the shifting of dielectric loss peak to higher frequency.

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2014

Dr. Umamaheswari A., Mohan, S. R., S. Kumar, S., and Dr. Sivakumar M., “Phase tuning of zirconia nanocrystals by varying the surfactant and alkaline mineralizer”, Ceramics International, vol. 40, pp. 6561 - 6568, 2014.[Abstract]


The influence of cationic (CTAB)/neutral polymeric (PVP) surfactants and strong (NaOH)/weak (NH4OH) alkaline mineralizers on phase stabilization of zirconia nanocrystals synthesized by chemical precipitation is investigated. X-ray diffraction and micro-Raman analysis of the as-prepared samples show that tetragonal zirconia is predominant as compared to monoclinic using \{PVP\} with NH4OH. The phases are also evident from lattice fringes of \{TEM\} images and the corresponding \{SAED\} pattern. Photoluminescence spectra of samples reveal oxygen vacancies present in the zirconia nanocrystals. The group H Raman vibration modes identified are attributed to surface defects and quantum size effects of nanocrystals. The phase stabilization of zirconia nanocrystals is explained using the polymerization rate of tetramers during synthesis. The rate can be varied by proper selection of the surfactant and the mineralizer. A slow polymerization rate with \{PVP\} and \{NH4OH\} favors the formation of tetragonal zirconia. Thus, a simple method for phase stabilization of zirconia nanocrystals at room temperature using chemical precipitation by varying the surfactant and the mineralizer is demonstrated.

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2013

Dr. Umamaheswari A., S Kumar, S., and Dr. Sivakumar M., “Influence of Alkaline Mineralizer on Structural and Optical Properties of ZrO2 Nanoparticles”, Journal of nanoscience and nanotechnology, vol. 13, pp. 4409-14, 2013.[Abstract]


In this work, the influence of different alkaline mineralizers on structural and optical properties of zirconia nanoparticles synthesized by chemical co-precipitation technique using zirconium oxychloride octahydrate (ZrOCI2 x 8H2O) as precursor is studied. The mineralizers used for the synthesis of nanoparticles are NaOH and NH4OH. X-ray diffraction, Fourier Transform Infrared (FTIR) spectroscopy, UV-visible absorption spectroscopy and Photoluminescence (PL) spectroscopy were used for characterizing the nanoparticles. Structural analysis of the sample synthesized using NaOH shows monoclinic phase as predominant one, however when NH4OH is used the major phases are cubic and tetragonal. The difference is attributed to the number of hydroxyl ions produced and their rate of release during the reaction process. The presence of these phases in both samples is further confirmed by vibrational bands of FT-IR spectra. Also, the low energy bands due to the presence of defects in nanoparticles are also explicitly observed in the photoluminescence spectra. Further the defects lead to a red shift in the band gap of ZrO2 which is observed when the samples are subjected to UV-absorption spectroscopy analysis. It has been demonstrated that zirconia nanoparticles with desired structural properties can be synthesized by changing the type of mineralizer without the necessity of doping.

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2011

Dr. Umamaheswari A., “Study of resonances and absorption in one dimensional quantal scattering”, 2011.[Abstract]


Ever since the formulation of quantum laws governing the microworld, the study of quantum scattering in one dimension (1D) and three dimension (3D) has been indispensable in understanding the physics and interactions governing the world of molecules, atoms, nuclei and sub-atomic particles. Recently there is a renewed interest in the study of 1D quantum transmission and tunneling across heterostructures like barrier, well and its combination. This is because these structures are quite important for the fabrication of short-wavelength light-emitting diodes and diode lasers, and for other optoelectronic applications based on resonant tunneling. High quality heterostructures is possible by growth techniques like molecular beam epitaxy and metalorganic chemical vapor deposition. They posses predesigned potential profiles and impurity dis- tributions with dimensional control close to interatomic spacing. If the energy of the incident particle coincides with the resonance states generated by the heterostructure then the transmission gets enhanced and hence they play quite important role in the transport of charges in electronic devices. In view of this a systemic study of 1D scattering which is of pedagogical and academic interest is undertaken in this thesis. Analytical S matrix theory of 3D potential scattering provides a unified framework for the identification of bound and resonance states generated by the potential. Both these types of states are represented by the poles of S -matrix in complex energy or momentum planes. Bound states generated by the scattering potential are normalizable negative energy states with zero width. On the other hand resonance states are complex eigenstates, the real part corresponds to the resonance energy Er and imaginary part corresponds to its width Γr. If the resonance is very sharp, it corresponds to long lived quasibound (QB) state and within the interaction domain the QB state wave function behaves like a bound state wave function. More »»

2010

Dr. Umamaheswari A., Prema, P., and Shastry, C. S., “Resonant states and transmission coefficient oscillations for potential wells and barriers”, American Journal of Physics, vol. 78, pp. 412–417, 2010.[Abstract]


The oscillatory behavior of the transmission coefficient TT as a function of energy is examined for an attractive square well and a rectangular barrier. We calculate TT using resonant state boundary conditions and demonstrate that the maxima in TT are correlated with the broad resonances generated by these potentials. For barrier potentials the maxima signify resonances occurring at energies above the barrier height. It is shown that the resonance position and width can also be generated from the complex poles of the amplitude of the transmitted plane wave. We also explain the relation between the positions of the resonances generated by the square well and the rectangular barrier to the energy eigenvalues of the corresponding rigid box with the same range. We show for a potential with an attractive well and a repulsive barrier that TT exhibits oscillations when the particle energy is below the barrier, implying that in many cases the simple WKB type barrier penetration expression for TT is not adequate. These features of TT are likely to hold for most attractive potentials and flat repulsive barriers. We also discuss the attractive modified Poschl–Teller type potential for which TT does not show oscillations as a function of energy. More »»

2009

Dr. Umamaheswari A., Prema, P., Dr. Mahadevan S., and Shastry, C. S., “Quasi-bound states, resonance tunnelling, and tunnelling times generated by twin symmetric barriers”, Pramana, vol. 73, p. 969, 2009.[Abstract]


In analogy with the definition of resonant or quasi-bound states used in three-dimensional quantal scattering, we define the quasi-bound states that occur in one-dimensional transmission generated by twin symmetric potential barriers and evaluate their energies and widths using two typical examples: (i) twin rectangular barrier and (ii) twin Gaussian-type barrier. The energies at which reflectionless transmission occurs correspond to these states and the widths of the transmission peaks are also the same as those of quasi-bound states. We compare the behaviour of the magnitude of wave functions of quasi-bound states with those for bound states and with the above-barrier state wave function. We deduce a Breit-Wigner-type resonance formula which neatly describes the variation of transmission coefficient as a function of energy at below-barrier energies. Similar formula with additional empirical term explains approximately the peaks of transmission coefficients at above-barrier energies as well. Further, we study the variation of tunnelling time as a function of energy and compare the same with transmission, reflection time and Breit-Wigner delay time around a quasi-bound state energy. We also find that tunnelling time is of the same order of magnitude as lifetime of the quasi-bound state, but somewhat larger. More »»

2007

Dr. Umamaheswari A., Dr. Mahadevan S., Prema, Pa, Shastry, C. Sa, and Agarwalla, S. Kb, “Transmission and scattering by an absorptive potential”, American Journal of Physics, vol. 75, pp. 245-253, 2007.[Abstract]


Transmission and scattering problems involving complex potentials are important in physics, in particular in describing nuclear collisions. We describe many pedagogical features of transmission in one dimension and scattering and absorption cross sections in three dimensions for a rectangular absorptive potential and compare the results with scattering from real barriers and a hard sphere. For a given energy in one dimension we show that the absorption reaches a maximum for a critical value of the absorption strength and then monotonically decreases with an increase in the absorption strength. An infinitely absorptive well becomes almost fully reflective and thus is similar to an infinitely high barrier. Similar results are found in three dimensions. We show that the absorption cross section in three dimensions diverges at a threshold energy, which is of critical importance for exothermic reactions. © 2007 American Association of Physics Teachers. More »»

2006

Dr. Mahadevan S., Dr. Umamaheswari A., Prema, P., Shastry, C. S., and , “Quantum mechanical transmission with absorption”, Phys. Educ, vol. 23, pp. 13–21, 2006.[Abstract]


Transmission and reflection across a rectangular barrier at energies below and above barrier is the most commonly studied topic in nonrelativistic quantum mechanics. However the subtle inter-relationship between the barrier problem and the corresponding well problem is not widely known, in particular when absorption is present. In this article we show that when a particle traverses an absorptive medium, at any given energy the absorption peaks for a particular value of absorption potential strength W0. Similarly we study the corresponding cases when incident energy E is increased keeping W0 constant. Further, we show that for a given E when W0. is made very large absorption gradually decreases and reflection overtakes it and tends towards unity. We also study the case of transmission across a potential barrier and well and interpret physically the behavior of absorption, transmission and reflection. More »»

Publication Type: Conference Proceedings

Year of Publication Title

2018

V. P. Muhamed Shajudheen, K. Rani, A., V. Senthil Kumar, Dr. Umamaheswari A., Sivakumar, M., and S. Kumar, S., “Comparison of Anticorrosion Studies of Titanium Dioxide andNickel Oxide Thin Films Fabricated by Spray Coating Technique”, Materials Today: Proceedings, vol. 5. pp. 8889-8898, 2018.[Abstract]


The titanium dioxide and nickel oxide nanoparticles are synthesized by chemical precipitation method. The as prepared nanoparticles of titanium dioxide and nickel oxide are coated on steel substrate (304L SS) using spray coating technique. The corrosion resistance of 304L SS substrates spray coated with nanostructured titanium dioxide (TiO2) and Nickel oxide (NiO) in 3.5% NaCl is studied from Tafel polarization curve and electrochemical impedance spectra. The polarization curves of coated 304L SS substrates showed positive shift in corrosion potential with respect to that of bare. The Nyquist plot of the coated films confirmed that the coatings have improved the corrosion resistance of 304L SS substrate. The variation of impedance with frequency of the coated 304L SS before and after salt spray test shows that NiO and TiO2 films are quite stable in salt environment. There is no much change in surface morphology and surface roughness of films after corrosion test and this further confirm the film stability in corrosive environment. The absence of crack in AFM images of films subjected to load shows that the films are quite tough. The non-existence of rust in coated substrate exposed to salt environment (5% NaCl at 30°C) for 390 hrs confirms that the films can act as anticorrosive coating for 304L SS substrate in adverse salt environment.

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2018

S. Mohan, Dr. Umamaheswari A., and Dr. Sivakumar M., “Effect of Sintering Temperature on Dielectric Properties of Hydrous Amorphous Zirconia”, Materials Today: Proceedings, vol. 5, no. 8. pp. 16598-16609, 2018.[Abstract]


The effect of sintering temperature on structural and dielectric properties of Hydrous Amorphous Zirconia (HAZ) synthesized through surfactant assisted chemical precipitation method is investigated. X-ray diffraction studies reveal that sintering temperatures greater than lowest crystallization temperature (500 °C) of HAZ caused an increase in lattice spacing resulting in monoclinic phase dominant ZrO2 nanocrystals. The room temperature impedance measurement of pelletized hydrous amorphous zirconia powder sintered at 500 and 800 °C is carried out in the frequency range of 40 HZ to 5 MHZ. From the impedance data, the dielectric constant, dielectric loss, loss tangent and AC conductivity of HAZ sintered at these temperatures are calculated. It is found that dielectric constant is higher for sample sintered at lower crystallization temperature due to higher tetragonal content. This is attributed to higher concentration of oxygen vacancies which results in large space charge polarization and high dielectric constant. Further the loss tangent curve indicates that mobility of charge carriers is decreased in HAZ sintered at higher temperature and hence the conductivity also decreases. However at higher frequency region, the dielectric constant of monoclinic zirconia is greater than tetragonal zirconia.

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2018

M. V. P, Kumar, V. Senthil, Dr. Umamaheswari A., Dr. Sivakumar M., S Kumar, S., and Rani, A., “Characterization and anticorrosion studies of spray coated nickel oxide (NiO) thin films”, Materials Today: Proceedings, vol. 5, no. 2. pp. 8577-8586, 2018.[Abstract]


In the present work, nanostructured thin films of NiO were synthesized using spray coating technique. The morphology and composition of the film were analyzed. The UV Visible absorption, micro Raman, and Photoluminescence spectrum of the NiO thin films were recorded and analyzed. The electrochemical behaviour of the nickel oxide thin film was examined by means of Electrochemical Impedance Spectroscopy (EIS) analysis, Tafel analysis and Open Circuit Potential (OCP) measurements. Salt spray corrosion test was applied to assess the stability of the NiO thin film over the steel substrate in the corrosive salt environment. The electrochemical impedance spectroscopy (EIS) analysis, Open Circuit Potential measurements (OCP), Tafel analysis revealed the enhanced protection of the substrate by nickel oxide thin film. The salt spray test carried out for 390 h, the thin film exposed to salt spray confirmed that the NiO films provides effective protection against corrosion of the stainless steel SS 304L compared with the substrate without nanoparticle deposition. Electrochemical Impedance Spectroscopy (EIS) analysis, Tafel analysis, Open Circuit Potential measurements (OCP) were repeated after the salt spray corrosion test and the result were again recorded, analyzed and compared with previous results which infers the effective protection of the nickel oxide thin film against corrosion in a corrosive environment..

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2013

Dr. Umamaheswari A., Mohan, S. R., and S. Kumar, S., “Effect of strong base on the crystalline phase of zirconia nanoparticles synthesized by arrested chemical precipitation method”, National Conference on Recent Advances in Surface Science(RASS 2013). Gandhigram Rural Institute - Deemed University, Gandhigram - 624 302, Dindigul, 2013.

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