Johney Isaac

Stability of the ferroelectric phase in polycrystalline KNO3 investigated by photoacoustic and calorimetric techniques

The successive phase transitions in a polycrystalline KNO3 sample have been studied using photacoustic and calorimetric techniques. The measured temperature dependence of the photoacoustic amplitude carries signatures of the transitions undergone by the sample. The results indicate that the intermediate ferroelectric phase in KNO3 is only a metastable one. This raises doubts regarding its applicability in fabricating ferroelectric memory devices.

Characterization of ZnO plasma in a radio frequency sputtering system

Plasma generated by the radio frequency (rf) magnetron sputtering of gallium doped zinc oxide (ZnO) has been studied using cylindrical Langmuir probe and optical emission spectroscopy. The electron density and electron temperature were calculated for various rf powers at different spatial positions along the propagation direction of the plume. The electron density was found to increase with argon gas pressure whereas electron temperature decreased. The focusing of the plasma due to the magnetic field of the magnetron was observed at 3cm distance from the target at low rf powers, while shift in the focusing of the plume was observed for high sputtering powers. Plasma potential of the plume was also evaluated at all sputtering conditions and its spatial variations were studied. Optical emission spectra of the plume were recorded at various spatial positions of the plume to get qualitative estimate of species in the plasma at various sputtering conditions. ZnO thin films were deposited on glass substrates pl...

Thermal diffusivity of solids by photoacoustic cell rotation and phase lag measurement

A single‐beam photoacoustic phase lag measurement is demonstrated for determining the thermal diffusivity α of solid samples. The method involves measurement of the phase difference of the photoacoustic signal between front and rear surface illuminations at a single chopping frequency with the same light beam. Results on representative samples, with widely different thermal properties, show very good agreement with literature values. Temperature variation of α can also be measured with the setup.

Magneto-electric characterization of x (Na0.5K0.5)0.94Li0.06NbO3- (1-x) NiFe2O4 composite ceramics

Lead free magneto-electric composites of Sodium Potassium Lithium Niobate-Nickel Ferrite (NKLN-NFO) with different molar weight percent of NKLN are prepared by solid state reaction involving co-precipitation and sintering. Four different compositions of x NKLN-(1-x) NFO have been selected to perform systematic studies on their magneto-electric properties. The dielectric constant of dense samples is measured as functions of frequency, temperature and external magnetic field. Dielectric constant is found to decrease after poling, and show resonance/relaxation peaks in dielectric properties at certain frequencies. AC conductivity is determined to understand the mechanism of conductivity. Changes in dielectric constant with applied magnetic field (or magneto-dielectric effect) are also reported. The magneto-electric coefficient for the composition with x = 0.8 is found to be 2.32 × 10−2 V/cm-Oe while with x = 0.85 it is 2.43 × 10−2 V/cm-Oe. These are the highest values reported so far for a lead free composite material prepared by normal solid state reaction cum sintering route. We find that x NKLN- (1-x) NFO exhibits sufficiently high room temperature magneto-electric coefficient suitable for applications compared to other lead free composites prepared by solid state route.

Thermal diffusivity of Bi-Sr-Ca-Cu-O superconductor: Effect of Pb doping

Abstract We have measured the temperature dependence of the thermal diffusivity of the high-temperature superconductor Bi-Sr-Ca-Cu-O with varying Pb doping levels using a photoacoustic technique. Pb doping stabilizes the superconducting phase around 110 K. It is found that above T c thermal diffusivity decreases as the Pb concentration increases whereas below T c it tends to saturate at low Pb concentrations.

Abrupt increase in thermal diffusion belowTc in YBa2Cu3O7−δ

We report the variation of the thermal diffusivity in the high temperature superconductor YBa2Cu3O7−δ below the transition temperature measured using photo-acoustic technique α is found to increase sharply belowTc. Qualitative explanations are given.

Anomalous change in the photoacoustic phase near superconducting transition in YBa2Cu3O7

Photoacoustic amplitude and phase measurements have been carried out near the superconducting transition temperature in the highTcsuperconductor YBa2Cu3O7. In both the samples investigated, the photoacoustic phase undergoes an anomalous step-like decrease aroundTc≈ 96 K and increases at lower temperatures. A clear temperature hysteresis for the phase change has been noticed. No anomalous variation has been observed in the amplitude values. Neither phase nor amplitude shows any anomaly in a non-superconducting sample of the same composition.

General-purpose high performance temperature controller for elevated temperatures

A high performance temperature controller working in the range 50-1200°C has been designed and fabricated. It maintains the temperature constant to within ±0-57deg;C over the entire range. It has a sensitive thermocouple amplifier with active cold junction compensation. The proportional plus integral mode of control is adopted and the control signal is pulse width modulated. The unit can be built cheaply and works as a general purpose temperature controller.