Publications
Effect of high temperature sintering schedule for Enhanced CMR properties of La0.67Ca0.33MnO3 close to room temperature
 
 
ABSTRACT: Magnitude of [D r (T,H)/r (0)] and insulator to metal transition temperature (TIM ) in the colossal magnetoresistance (CMR) bulk ceramics response of La0.67Ca0.33MnO3 (LCMO) manganites are observed to be changed significantly as a function of higher sintering temperature (close to melting) and long hours sintering schedule. Magnetoresistance and magnetization (upto 5 Tesla) of LCMO are found to be decreased considerably as the sintering temperature is increased from 1400° C to 1600° C. Further reduction of CMR response is also observed for long time (a few tens of hours in O2) at higher temperature. However, on the other hand the value of TIM is significantly enhanced and reaching close to room temperature under the same sintering schedule to achieve drastic reduction of CMR response. Microstructural analysis revealed the presence of clean and clear inter grain strong connectivity at 1600° C sintering temperature for period of 17 hours. This process enhances the physical densification of the bulk and promotes the creation of new conduction channels and reduction of inter grain effective contact area for transport current under magnetic field. Hence, drastic reduction of resistance associated with the CMR response is due to this reason. The disruption in the crystalline order observed by TEM at the grain (formulation of anti phase) boundaries promotes local spin disorder which may alter the conductivity of polarized electrons. It is also observed that short-range local magnetic order is disrupted by the presence of grain boundaries which may be responsible for the reduction of magnetoresistance and increase of TIM point towards room temperature. © 2000 Elsevier Science S.A. All rights reserved. B. K. Roul1, D. R. Sahu1, S. Mohanty1 & A. K. Pradhan2 1Institute of Materials Science, 68/1, Laxmi Vihar, Bhubaneswar, India 2International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan

Sintering of Al-Zr based oxide ceramics using Thermal Plasma

ABSTRACT: A novel and low cost extended arc thermal plasma heating (EATPH) reactor [1] has been used to sinter Al-Zr oxide ceramics within a few minutes in Ar atmosphere instead of few tens of hour by conventional furnace heating. Critical experimental parameters such as plasma power (kW), plasmagen gas flow rate, sintering time and electrode spacing distance are optimised to achieve high density sintered materials. It is noted that sintering time is approximately reciprocal to plasma power. Higher plasma power with less sintering time can generate high-density homogeneous material without significant grain growth. XRD and SEM studies were carried out to characterize and evaluate the sintered materials. Surface morphology revealed uniform particle size distribution in long-range order, with no runway grain growth during this EATPH sintering process. This simple EATPH method provides an alternative and quick technique for tailoring high temperature ceramic materials over conventional sintering method due to fast heat and mass transfer kinetics inside thermal plasma reactor. © 2000 Elsevier Science S.A. All rights reserved.   B. K. Roul, D. R. Sahu, S. Mohanty, B. C. Mohanty* and S. K. Singh* Institute of Materials Science 68/1, Laxmi Vihar, Bhubaneswar - 751 005, India and
*Regional Research Laboratory, Bhubaneswar, India

Diamond and Diamond-like-Carbon Growth on Si (100) by Hot Filament-Assisted RF Plasma CVD

ABSTRACT: Diamond and diamond-like carbon (DLC) crystallites can be grown on a Si (100) substrate by hot filament-assisted RF plasma CVD in a low-vacuum (2-Torr) environment from C2H2 without using hydrogen. The deposition was carried out at 900° C onto the pre cleaned diamond polished Si (100) substrate. XRD and Raman spectra revealed the presence of diamond, graphite, lonsdaleite, and high-order polymeric hydrocarbon phases. The morphology observed by SEM shows typical habit and facets of the diamond crystallites grown on silicon. An attempt was made to explain some discrepancies observed in the XRD and Raman spectra of our multiphasic DLC films.   B. K. Roul 1, B. B. B. Nayak 2, P. K. Mishra 2 and B. C. Mohanty 2 1.Institute of Materials Science 68/1, Laxmi Vihar, Bhubaneswar - 751 005, India
 2. Regional Research Laboratory, Bhubaneswar, India
Enhanced room-temperature magnetoresistance in partially melted La0.67Ca0.33MnO3 manganites

ABSTRACT We have performed magnetic and transport measurements on La0.67Ca0.33MnO3 polycrystalline and partially melted samples. The magnetization and resistance decrease as sintering temperature is increased. The insulator-to-metal transition temperature is remarkably enhanced in the partially melted sample, enabling its possible applications at room temperature. The microscopic studies in melt samples show an excellent connectivity between grains, suggesting the enhancement of percolative transport by opening new conduction channels and the disappearance of magnetic phase boundary at elevated temperature by the ordering of Mn spins blocked at the grain boundary of sintered samples. © 2000 American Institute of Physics. [S0003-6951(00)03905-X]  
A.K. Pradhan, a) B. K. Roul, J. G. Wen, b) Z. F. Ren, b) M. Muralidhar,) P. Dutta c), D.R. Sahu, S. Mohanty and P. K. Patro

Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India

(Received 30 August 1999; accepted for publication 8 December 1999)

Laser Excitation Effect on the Optical and Dielectric Properties of Ferroelectric KNbO3 Single Crystals

ABSTRACT
The effects of laser irradiation on the optical absorption (200 to 800-nm range) and dielectric properties (102 - to 107 - Hz range) of KNbO3 single crystals in ferroelectric phase have been studied. All these properties are found to be reduced significantly at lower frequencies with an increase in laser irradiation time. However, the phase transition temperature remains unchanged. It is observed that the dielectric constant (K) and loss (tan d ) values of KNbO3 single crystals are decreased significantly at room temperature or at Curie temperature by laser excitation. Appreciable changes in the band gap (eV) and activation energy are also observed after lasr excitation.

Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India

Effect of DC Biasing Field on the Dielectric Properties of High Treated SrTiO3 Single Crystals

ABSTRACT
Dielectric constant (K) and loss (Tand ) of strontium titanate single crystals subjected to high electric fields (10 KV/cm, AC or DC) were measured as a function of frequency (102 - to 107 - Hz). These properties were also studies as a function of time at 103 Hz immediately after the high field treatment. The value of K and tan d of the field-treated samples attained a steady value after 20 hours. Measurements of K and Tan d as a function of time and frequency under small DC biasing fields (200 V/cm) were taken. The values of K and Tan d of the high field treated samples decreased with time and frequencies; however under DC biasing field, K increases slightly up to 10 minutes and then decreases slowly with time to steady value. The inversion temperature (Ti) at different frequencies get reduced with high field treatment. However, the application of small DC-biasing field increased the Ti value with the increase of biasing field strength. On the other hand, activation energy increased with high field and decreased with biasing field. The effect of high DC field treatment on SrTiO3 single crystal exhibits more prominent changes in K, Tan d , Ti and activation energy that than of high AC field treated samples.


Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India

Gamma Ray Irradiation Effects on Electrical Transport and Microwave Properties of BISCO Superconducting Films

ABSTRACT High-Tc Superconducting films of Bi Sr Ca Cu O (2:2:1:2) have been synthesized by spray pyrolysis of nitrate precursors onto yttria stabilized zirconia (YSZ). g -ray irradiation of the sample, was carried out using a 60Co source of 103 Ci strength for several hundreds of hours; the dose received by the samples was 80 K rad / hr. Superconducting properties such as critical transition temperature (Ti), resistivity ®, critical current density (Ju). Voltage time relaxation (Vt) and microwave induced de voltage were investigated as a function of temperature down to 77K after g -ray irradiation. g -ray irradiation was found to have practically no effect on its structural modification and on the critical transition temperature. However, transport critical current density (Ju) increased. The increase of pinning energies with irradiation suggests that these changes in properties are dominated by radiation-induced randomly distributed mobile oxygen defects in the films. An appreciable decrease in the microwave induced de voltage at 77 K was also observed after irradiation which suggests that the mobile defects are clustered at the major defect region and reduce the total number of weak links. These results suggests that oxygen defects induced by g -ray irradiation of BISCO films act as important and major pinning centres which is responsible for the enhancement of Ju and reduction of microwave induced dc voltage.   B. K. Roul
Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India


Thermometry Between 10-300 K Using GaAlAs Diode

Abstract
The forward characteristics of a cryogenic GaAlAs temperature sensor diode ( Lake Shore Cryogenic , Inc) have been presented in the temperature range 10-300K and for fixed current values between 10nA and 500 m A . The upturn in the forward characteristic below 35 K was found to shift to lower temperature by reduce the current . T he sensitivity at low temperature was found to reducewith the current (I )according to a powr law. The sensitivity at high temperature was found to increase linearly with decrease in lg(I)
S.B.OTA & SMITA OTA* Institute of Physic , Bhubaneswar , Sachivalaya Marg, 751005, India
*Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India

A Study of Forward Characteristics of a GaAlAs Temperature Sensor Diode

Abstract The forward characteristics of a cryogenic GaAlAs temperature sensor diode ( Lake Shore Cryogenic , Inc) in the temperature range 10-300K and for fixed current values between 10nA and 500 m A have been presented. . The upturn in the forward characteristic below 35 K was found to shift to lower temperature on reducing the current . The sensitivity at high temperature was found to increase linearly with decreasing log (I) . We suggest the use of a current of less than 10m A , which has the advantage of reducing Joule power dissipation and improving high temperature sensitivity and linearity.   S.B.OTA & SMITA OTA* Institute of Physic , Bhubaneswar , Sachivalaya Marg, 751005, India
*Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India

Microcanonical Monte Carlo Simulations of the First-order Trasition in the two-dimensional Potts Model

 

Abstract:
Microcanonical Monte Carlo Simulations have been implemented in the two-dimensional ( 2D) q-state Potts model. The ergodicity of this simulation technique for the Potts model is studied. It does not seem to depend on the value of q. A lack of ergodicity for small values of the system energy is reported and discussed . It has been found that the temperature dependences of the physical quantity exhibit an 'S' - shaped nature at the first-order transition. The degree of 'S' - shaped nature was enhanced by increasing q and reducing the system size. We believe on the basis of our computer simulation that the 'S' shape represents the equilibrium behavior of a finite isolated system.
S.B.OTA & SMITA OTA* Institute of Physic , Bhubaneswar , Sachivalaya Marg, 751005, India
*Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India

Measurement of Temperature between 0.05-1 K using GaAlAs Diode

Abstract We have measured the forward characteristics of a cryogenic GaAlAs temperature sensor diode ( Lake Shore Cryogenic , Inc) in the temperature range 10-70K and for fixed current values between 10nA and 500 m A .On the basis of certain extrapolations , we suggest that these diodes can be used for temperature measurement at ultra low temperatures.   Snehadri.Bihari .Ota , Juan Bascunan * & Smita Ota** Institute of Physic , Bhubaneswar , Sachivalaya Marg, 751005, India
* CRYO Industries of America Inc., 11124 S. Willo St., Manchester , NH 03103, U.S.A.
** Institute of Materials Science , 68/1 , Laxmi Vihar , Bhubaneswar , 751005 , India