C.P. Sharma

Total-Reflection Switch Using High-Tilt-Angle Cholesteric-Smectic C* Phase Transition Ferroelectric Liquid Crystal

A total internal reflection (TIR) microoptic switch using high-tilt-angle materials showing cholesteric-smectic C* phase transition for multimode fiber is described. The device shows excellent temperature-independent electrooptic characteristics and exploits the maximum contribution of ferroelectric liquid crystal (FLC) birefringence. The switch shows asymmetric response and unusual nonlinear behaviour, which is analysed by polarisation reversal current. The nonlinear response is due to the polar surface interaction which behaves as if it were positively charged and has been analysed by the transmission characteristics and by current analysis on applying square and triangular waves of various frequencies. The switch has a switching response of ~20 ms limited due to monostable surface conditions, and shows a beam attenuation of ~5×107 in the off state.

Micro-optic switch using chiral nematic-smectic C* phase transition ferroelectric liquid crystals

Abstract A micro-optic switch based on total-internal reflection using ferroelectric liquid crystal is described. It uses a chiral nematic (N*)-smectic C* phase transition ferroelectric liquid crystal (FLC) in which the optic axis rotates by 90° during switching. The device exploits the maximum of birefringence in minimizing its size. The material inherent advantage of tilt angle stability with temperature provides the switching characteristics which are more stable over a wide range of temperature. An exceptionally high attenuation of 5x107 in the off state has been demonstrated.

Dielectric and structural properties of iron doped titanate nano-composites

TiO2 has been used for gas sensing devices (Mohammadi et al., 2007) photo catalytic devices (Litter and Navio, 1996) and photoelectric devices (Levy, 1997). Recently the dielectric properties of TiO2 have been of great interest for applications in the telecommunications industry due to its unusual high dielectric constant and low dielectric loss. The use of layers of high dielectric constant materials in small scale metal insulator semiconductor devices has been considered in numerous recent publications these materials unable to maintain the same capacitance density as SiO2 films but provide a smaller leakage current density. Examples of such materials include tantalum and yttrium oxides (Moon et al., 1999) and titanium dioxide which has the largest dielectric constant value varying in the range of 25-100 (Tang et al, 1997).

Effect of thallium concentration on Tl-2201 superconducting phase by flash sintering process

Abstract Samples with nominal composition TlxBa2Cu2O6 where x = 0.5, 1.0, 1.5, 1.75, 2.0, 2.5 and 3.0 were calcined at 890°C for 10 h and sintered at various temperatures for 3 1 2 min. X-ray powder diffraction patterns of all the samples show the characteristics of the 2201 phase which suggests the transformation of all the nominal compositions to 2201 phase. In some cases extra peaks due to BaCuO2 and Tl2O3 are also present. The samples having Tl contents 1.5, 1.75 and 2.0 are monophase material with unit cell parameters a = 3.86 A , c = 23.20±0.02 A . The overall symmetry is tetragonal. All the lines are indexible on the I4/mmm space group. Samples with Tl content 1.5-2.0, sintered at 930–950°C, show the best superconducting properties. The highest Tc is obtained in Tl2.0 and the highest Meissner fraction in Tl1.5. Chemical analysis indicates Tl deficiency in all the samples, which suggests substitution of Tl3+ by Cu1+ or Cu2+ and oxidation of copper to Cu3+, which increases monotonically with Tc. Tl contents in optimum samples lie in the range 1.14-1.82 and oxygen content varies from 5.6-6.1

Optimization of synthetic parameters for Tl-2201 superconducting phase by flash sintering process

Abstract Samples with nominal composition Tl 2 Ba 2 Cu 1.2 (copper-rich Tl-2201) were calcined at various temperature, i.e. from 865 °C to 910°C and for various durations (10–20 h). The samples were sintered for 3 1 2 min at various temperatures. The X-ray powder diffraction pattern indicates that most of the Tl-Ba-Cu samples (Tl-2201) are single-phase materials. All the lines in the X-ray pattern are indexable on a tetragonal unit cell showing tetragonal symmetry having space group 14/mmm with a=3.86 A c=23.11 ± 0.06 A as unit cell parameters. R−T and AC susceptibility measurements show T on c , from 90 to 104 K. There is an elongation of the c -axis ( c=23.24 A ) for the sample showing the highest T c . Wet chemical analysis confirms the deficiency of thallium. This suggests some substitution of Cu in place of thallium which leads to an increase in the average oxidation state of copper, responsible for the creation of holes (charge carriers).

The effect of high temperature sintering on the Tc of Tl1Ba2Can−1CunOx system

Abstract The effect of high temperature sintering ( T ≅ 970° C ) has been studied on the Tl 1 Ba 2 Ca n −1 Cu n O x systems where n varies from 1 to 5. It has been found that under optimised conditions of preparation the final phase composition obtained can vary considerably from the starting or nominal composition. XRD studies have revealed that in the case of n = 1 and 2, the Tl 1 nominal composition 1201 and 1212 have converted into 2201 and 2212 with a = 3.85 A and c = 23.23 A for 2201 and a = 3.86 A and c = 29.04 A for 2212, with an enhanced T c for 2201 ( T c ∼ 97.2K) over previously reported T c s. The other nominal compositions of n = 3, 4 and 5 (1223, 1234 and 1245) were showing only the Tl 1 series behaviour with slightly enhanced T zero c and T onset c . The XRD studies showed that in all the above phases a = 3.86 A and c = 15.5 A. The low angle peak appears at 2 π = 5.25° indicating the formation of n = 3 or Tl 1 Ba 2 Ca 2 Cu 3 O x compound.

The preparation of an enhanced-Tc superconducting Tl2Ba2CuOx phase by using low Tl concentrations

Calcium-free high-Tc superconducting compounds of the Tl2Ba2Can-1CunOx series have been fabricated with an enhanced onset critical temperature, Tcon approximately=97.2 K. The optimisation of parameters has revealed that the choice of nominal composition plays an important role in the formation of the final phase. While a nominal 2201 composition did not yield a high-Tc phase, compounds with low thallium concentration (1201) gave a 2201 high-Tc phase. XRD studies confirm the formation of a 2201 phase from 1201 and 1/2201 phases with a approximately=3.86 AA and c approximately=23.23 AA. It has also been found that a high sintering temperature of 97 degrees C maintained for a short duration of three to five minutes is favourable for the enhancement of Tc.

The Enhancement of Tc in Calcium Free Tl-Bilayer Superconducting System (Tl2Ba2Cu1Ox)

The observation of enhanced Tc in calcium free Tl compounds Tl2Ba2Cu1Ox (2201) with varying concentration of Tl and Cu has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under the optimised conditions: sintering temperature 970°C and duration 3–10 minutes the highest Tc (onset) ~115 K and Tc (zero) ~95 K was found. The XRD studies showed the transformation of all the three nominal compositions into 2201 phase with different Tcs.

Lead-doped electron-beam-deposited BiSrCaCuO superconducting thin films☆

Abstract Superconducting thin films of the lead-doped BiSrCaCuO system have been prepared on (100) single-crystal SrTiO 3 substrates by an electron beam deposition technique using a single sintered pellet as the evaporation source. As-deposited films are amorphous and non-superconducting; post-deposition annealing at an optimized temperature in air has been found to result in crystalline and superconducting films. The superconducting characteristics of the films have been observed to be sensitive not only to the duration and temperature of post-deposition annealing but also to the lead content and the sintering parameters for the pellet to be used as the evaporation source. A pellet with nominal composition Bi 3 Pb 1 Sr 3 Ca 3 Cu 4 O y that had been sintered for 200 h zero resistivity at T c 0 = 112 K . However, films deposited using such a pellet as the evaporation source had T c 0 ≈ 73–78 K, as had the films deposited from a pellet without any lead. We investigated systemativally films deposited from pellets with more lead and sintered for different durations. It is evident from these investigations that pellets with nominal composition Bi 3 Pb 2 Sr 3 Ca 3 Cu 4 O y , i.e. with an excess of lead, and sintered for about 75 h when used as the evaporation source yield films with T c 0 ≈ 100 K when annealed between 835 and 840 °C for an optimized long duration. The films are characterized by X-ray diffraction and energy-dispersive spectroscopy techniques and have been found to be highly c axis oriented. The effect of lead in promoting a high T c 0 = 110 K phase seems to be similar to that in bulk ceramics.

Enhancement ofT c (∼ 103 K) in calcium-free Tl2Ba2CuO x (2201) compound

EnhancedTc in calcium-free Tl compounds of the series TlmBa2Can−1CunOx (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestTc(onset) ranges from 103 K to ∼ 115 K andTc (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentTcs.