N.K. Karan

High energy density metal-insulator-metal capacitors with Ba[(Ni1∕2,W1∕2)0.1Ti0.9]O3 thin films

Metal-insulator-metal capacitors with high-k Ba[(Ni1∕2,W1∕2)0.1Ti0.9]O3 thin film dielectrics were fabricated by chemical solution deposition technique. High dielectric constant (85), low dielectric loss (0.007), and high breakdown field (∼3.0MV∕cm) at room temperature were achieved. The temperature and frequency dependences of capacitance and loss tangent were small around room temperature (300±25K). At room temperature, high capacitance density (3.1fF∕μm2) along with high energy density (34J∕cm3) and low leakage current (7.7×10−6A∕cm2 at 20V) were obtained, indicating high potential for this material in the integrated circuits and power electronic applications.

New cryogenic phase transitions in SrSnO3

Strontium stannate is under study as an ultra-stable dielectric material for microelectronic applications at low temperatures. It is known to have a remarkably temperature-independent dielectric constant from 27 K to room temperature. However, we report anomalies in the Raman spectra, dielectric response, and differential thermal analysis of strontium stannate compatible with a structural phase transition at 160 K. Further anomalies are seen in calorimetric and Raman data (but not dielectric data) that suggest another phase transition at 270 K. A preliminary x-ray powder diffraction study confirms a small change in the pseudo-cubic lattice constant a(T) at the lower transition.

Structural and Electrochemical Characterization of Pure LiFePO 4 and Nanocomposite C- LiFePO 4 Cathodes for Lithium Ion Rechargeable Batteries

Pure lithium iron phosphate () and carbon-coated (C-) cathode materials were synthesized for Li-ion batteries. Structural and electrochemical properties of these materials were compared. X-ray diffraction revealed orthorhombic olivine structure. Micro-Raman scattering analysis indicates amorphous carbon, and TEM micrographs show carbon coating on particles. Ex situ Raman spectrum of C- at various stages of charging and discharging showed reversibility upon electrochemical cycling. The cyclic voltammograms of and C- showed only a pair of peaks corresponding to the anodic and cathodic reactions. The first discharge capacities were 63, 43, and 13 mAh/g for C/5, C/3, and C/2, respectively for where as in case of C- that were 163, 144, 118, and 70 mAh/g for C/5, C/3, C/2, and 1C, respectively. The capacity retention of pure was 69% after 25 cycles where as that of C- was around 97% after 50 cycles. These results indicate that the capacity and the rate capability improved significantly upon carbon coating.

Structural and Electrochemical Characterization of Pure and Nanocomposite C- Cathodes for Lithium Ion Rechargeable Batteries

Pure lithium iron phosphate () and carbon-coated (C-) cathode materials were synthesized for Li-ion batteries. Structural and electrochemical properties of these materials were compared. X-ray diffraction revealed orthorhombic olivine structure. Micro-Raman scattering analysis indicates amorphous carbon, and TEM micrographs show carbon coating on particles. Ex situ Raman spectrum of C- at various stages of charging and discharging showed reversibility upon electrochemical cycling. The cyclic voltammograms of and C- showed only a pair of peaks corresponding to the anodic and cathodic reactions. The first discharge capacities were 63, 43, and 13 mAh/g for C/5, C/3, and C/2, respectively for where as in case of C- that were 163, 144, 118, and 70 mAh/g for C/5, C/3, C/2, and 1C, respectively. The capacity retention of pure was 69% after 25 cycles where as that of C- was around 97% after 50 cycles. These results indicate that the capacity and the rate capability improved significantly upon carbon coating.

Structural and Electrochemical Characterization of PureLiFePO4and Nanocomposite C-LiFePO4Cathodes for Lithium Ion Rechargeable Batteries

Pure lithium iron phosphate () and carbon-coated (C-) cathode materials were synthesized for Li-ion batteries. Structural and electrochemical properties of these materials were compared. X-ray diffraction revealed orthorhombic olivine structure. Micro-Raman scattering analysis indicates amorphous carbon, and TEM micrographs show carbon coating on particles. Ex situ Raman spectrum of C- at various stages of charging and discharging showed reversibility upon electrochemical cycling. The cyclic voltammograms of and C- showed only a pair of peaks corresponding to the anodic and cathodic reactions. The first discharge capacities were 63, 43, and 13 mAh/g for C/5, C/3, and C/2, respectively for where as in case of C- that were 163, 144, 118, and 70 mAh/g for C/5, C/3, C/2, and 1C, respectively. The capacity retention of pure was 69% after 25 cycles where as that of C- was around 97% after 50 cycles. These results indicate that the capacity and the rate capability improved significantly upon carbon coating.

Effect of Sr substitution on tunability and dielectric properties in Pb(Zr0.50Ti0.50)O3 thin films fabricated by chemical solution deposition

(Pb1¿xSrx)(Zr0.5Ti0.5)O3 ( x= 0 to 0.65) thin films were grown on Pt/TiO2/SiO2/Si substrates by chemical solution deposition. Films were crystallized in the single phase perovskite structure in the temperature range ( 550¿700°C). Dielectric constant and loss tangent at room temperature reduced as the percentage of Sr substitution increased at the A-site of this perovskite structure. The phase transition temperature (Tc) lowered with Sr-substitution and Tc below the room temperature was obtained for composition with Sr ¿ 40%. The room temperature tunability reduced with Sr substitution, however, the k-factor slightly improved.

Phase Transition and Dielectric Tunability of Chemical Solution Deposited (Pb 0.35 Sr 0.65 )(Zr 0.5 Ti 0.5 )O 3 Thin Films on Pt/ZrO 2 /SiO 2 /Si Substrates

(Pb 0.35 Sr 0.65 )(Zr 0.5 Ti 0.5 )O 3 thin films were grown on Pt/ZrO 2 /SiO 2 /Si substrates by chemical solution deposition. As-deposited (pyrolysed at 500°C) films were amorphous and single phase films were obtained at temperature as low as 550°C with a 30 nm SrTiO 3 seed layer. Dielectric constant and loss tangent at room temperature were 210 and 0.022, respectively at 100 kHz for the film annealed at 700°C. Frequency dispersion of the dielectric properties was low. The phase transition temperature (ferroelectric to paraelectric) was well below the room temperature and was around 220 K. The room temperature tunability and the k-factor at 500 kV/cm was around 45% and 16, respectively.

Electrochemical Properties of Solution Synthesized Undoped and Aluminum Doped Lithium Manganate Thin Films

The spinel structured lithium manganate (LMO) is a promising cathode material for lithium ion rechargeable micro-batteries due to its higher energy density, environmentally benign nature, and low cost. To date, self-discharge and capacity fading (4 and 3V range), especially at elevated temperatures, still remains major research issues of LMO based cathodes. In the present work we have successfully synthesized lithium manganate thin films by a cost effective solution growth technique. These films exhibited excellent reversible lithium ion intercalation behavior with a discharge capacity of about 55.08:Ahcm −2 :m −1 at a load of 20:Acm −2 . The Li + diffusivity was found to increase by substituting a part of manganese with aluminum (Al) in LMO lattice. Al substituted LMO films exhibited better cycleability as compared to the undoped LMO films. Further studies are in progress to investigate the effect of Al substitution on the cycleability of the films.