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Han N.R.,National University of Singapore | Chen Z.C.,National University of Singapore | Chen Z.C.,Data Storage Institute Singapore | Lim C.S.,Data Storage Institute Singapore | And 4 more authors.
Optics Express | Year: 2011

Microscopic split-ring-resonator (SRR) arrays are fabricated on 100 μm thick polyethylene naphthalate (PEN) films by femtosecond laser micro-lens array (MLA) lithography. The transmission properties of these metamaterials are characterized by THz Time Domain Spectroscopy (THz-TDS). Tunable resonance responses can be achieved by changing SRR structural design parameters. By stacking 2D PEN metamaterial films with different frequency responses together, a broadband THz filter with full width at half maximum (FWHM) of 0.38 THz is constructed. The bandwidth of the resonance response increases up to 4.2 times as compared to the bandwidths of single layer metamaterials. Numerical simulation reveals that SRR layers inside the multi-layer metamaterials are selectively excited towards specific frequencies within the broadband response. Meanwhile, more than one SRR layers respond to the chosen frequencies, resulting in the enhancement of the resonance properties. The multi-layer metamaterials provide a promising way to extend SRR based metamaterial operating region from narrowband to broadband with a tunable feature. © 2011 Optical Society of America.

Kashyap R.,University of Delhi | Thakur O.P.,Solid State Group | Tandon R.P.,University of Delhi
Ceramics International | Year: 2012

Effect of Gd on microstructural, dielectric and electrical properties has been studied over wide temperature (300-500 K) and frequency range (100 Hz-1 MHz). Gd substitution in CCTO system results in decrease in the grain size and increase of Schottky potential barrier which causes lower value of dielectric constant. The dielectric constant remains nearly constant in temperature range 300-350 K. Doped samples show lower dielectric loss in middle frequency range (∼10 kHz-1 MHz) at room temperature. The AC conductivity (σ ac) obeys a power law, σ ac = Af n, where n is temperature dependent frequency exponent. The AC conductivity behaviour can be divided into three regions depending on conduction processes and the relevant charge transport mechanisms have been discussed. © 2011 Elsevier Ltd and Techna Group S.r.l.

Pal V.,Jaypee Institute of Information Technology | Dwivedi R.K.,Jaypee Institute of Information Technology | Thakur O.P.,Solid State Group
Current Applied Physics | Year: 2014

In the present work, polycrystalline (Bi1-xGdx) 0.5Na0.5TiO3 (BGNT) ceramics with low amount of rare earth ion Gd3+ (x = 0, 0.02, 0.03, 0.04) have been synthesized by a semi-wet technique. XRD patterns show single phase formation for all the samples with a rhombohedral structure at room temperature. FE-SEM images show decrease in grain size with Gd concentration. The temperature dependence of dielectric constant has revealed that the depolarization temperature 'T d' decreases with increasing x and the temperature 'Tm' of maximum dielectric constant increases initially for x = 0.02 thereafter decreases. All the samples have shown saturated hysteresis (P-E) loop at room temperature. The BGNT ceramic system for composition, x = 0.02 exhibits improved piezoelectric properties and strong ferroelectricity. With increasing temperature, polarization has been found to be reduced and deformed P-E loops are observed around 'Td'.© 2013 Elsevier B.V. All rights reserved.

Ashiri R.,Islamic Azad University at Dezful | Nemati A.,Sharif University of Technology | Sasani Ghamsari M.,Solid State Group
Ceramics International | Year: 2014

In this report, crack-free nanostructured barium titanate thin films are prepared by a sol-gel processing method. Glacial acetic acid, barium acetate, titanium tetraisopropyl alkoxide, 2-propanol and deionized water are used as precursors for preparing stable precursor solution. Then very thin films (thickness=26 nm) are deposited on fused silica substrates. In order to prepare crack-free thin films of BaTiO3, much attention is given to solvent evaporation process and a controlled pre-drying process is developed. It is found that the control of the process, especially in the initial stage of drying process is crucial and important for preparation of the crack-free ultrathin nanostructured BaTiO3 films. In our proposed method, as-deposited thin films undergo a slow drying process in saturated vapor pressure of the precursor solution. The above process leads to the formation of crack-free nanostructured BaTiO3 thin films. Structure and morphology of the thin films are characterized by XRD, SEM and AFM characterization methods. Results indicate that the thin films are crack-free with a dense nanogranular structure. © 2014 Elsevier Ltd and Techna Group S.r.l.

Pal V.,Jaypee Institute of Information Technology | Dwivedi R.K.,Jaypee Institute of Information Technology | Thakur O.P.,Solid State Group
Materials Research Bulletin | Year: 2014

Polycrystalline specimens of (Bi1-xNdx) 0.5Na0.5TiO3 (BNNT) ceramic system with compositions x ≤ 0.04 were synthesized by semi-wet technique using ethylene glycol precursor. Structural and electrical properties were investigated in detail to observe the effect of neodymium (Nd) substitution in BNT system. XRD patterns for all the specimens showed single phase formation with rhombohedral structure. Field emission scanning electron micrographs (FE-SEM) revealed that the grain growth was inhibited significantly with Nd content. The temperature dependence behaviour of dielectric constant revealed that the depolarisation temperature 'Td' decreases whereas temperature of maximum dielectric constant 'Tm' increases with Nd concentration. The piezoelectric charge coefficient (d33) showed maxima at x = 0.02 and well defined ferroelectric behaviour was observed for all the samples. © 2013 Elsevier Ltd.

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