S. C. Gadkari

Enhanced Field-Emission from SnO2:WO2.72 Nanowire Heterostructures

The field-emission properties of SnO(2):WO(2.72) hierarchical nanowire heterostructure have been investigated. Nanoheterostructure consisting of SnO(2) nanowires as stem and WO(2.72) nanothorns as branches are synthesized in two steps by physical vapor deposition technique. Their field emission properties were recorded. A low turn-on field of ~0.82 V/μm (to draw an emission current density ~10 μA/cm(2)) is achieved along with stable emission for 4 h duration. The emission characteristic shows the SnO(2):WO(2.72) nanoheterostructures are extremely suitable for field-emission applications.

Role of ambient air on photoluminescence and electrical conductivity of assembly of ZnO nanoparticles

Effect of ambient gases present in air on photoluminescence (PL) and electrical conductivity of films prepared using ZnO nanoparticles (NPs) have been investigated. It is observed that NPs of size below 20 nm kept inside a chamber exhibit complete reduction in their visible PL when oxygen partial pressure of the surrounding gases is decreased by evacuation. However, the visible PL from ZnO NPs is insensitive to other major gases present in the ambient air. The rate of change of PL intensity with pressure is inversely proportional to the ambient air pressure and increases when particle size decreases due to the enhanced surface to volume ratio. On the other hand, an assembly of ZnO NPs behaves as a complete insulator in the presence of dry air and its major components like N2, O2, and CO2. Electrical conduction having resistivity ∼102–103 Ωm is observed in the presence of humid air. The depletion layer formed at the NP surface after acquiring donor electrons of ZnO by the adsorbed oxygen has been found to ...

Redox reaction based negative differential resistance and bistability in nanoparticulate ZnO films

Negative differential resistance (NDR) and bistability in the current-voltage (I-V) characteristics have been demonstrated for a simple device fabricated from ZnO nanoparticles (NPs). The Au/ZnO/Au device (ZnO nanoparticles size: 5–15 nm) attains ON and OFF states during forward and reverse voltage scans, respectively, akin to a cyclic voltammogram. The origin of the observed NDR and bistability is traced to the protonic conduction resulting from the dissociation of adsorbed moisture on the surface of ZnO nanoparticles. Water layers adsorbed on the ZnO surface dissociate into hydroxyl ions at the oxygen vacancy (Vo) site and transfer a proton to the nearby water molecule. Signature of NDR is created when the rate of redox reactions at the electrode is faster than the generation and transport of reactants. Oxygen vacancies considered as active sites for water dissociation reside predominantly at the surface due to their low formation energy. So, the degree of bistability and NDR are found to decrease with ...

An instrument for automatic measurements of critical current of superconductors in pulse mode

An instrument has been developed for direct measurement of critical current and plotting of current–voltage (I–V) characteristics of superconductors in pulse mode. For these measurements, current pulses of increasing height are applied to sample and the voltage developed across the sample is monitored using a low noise voltage measuring circuit. Different sources of error responsible for limiting accuracy of pulse measurements techniques are discussed. A compensation network has been developed to reduce inductively induced voltages generated during pulse measurements, to negligible values. The instrument enables automatic critical current measurement with 1–10 μV/cm criteria in pulse mode without a computer and has voltage sensitivity of 10 nV. Critical current as a function of temperature/magnetic field and I–V characteristics of Bi-2212 single crystal and Y-123 thin films have been plotted using the circuit and the results have been compared with those obtained using direct current measurements.

A circuit and five probe resistivity method for measuring pulse I–V characteristics of superconductors

An electronic circuit has been developed to study the pulse I–V characteristics of superconductors. A special feature of the circuit is its low rms noise voltage of less than 10 nV, which allows measurements over a broad voltage range of 50 nV to 500 mV for the first time. In order to overcome the limitations of a four probe technique at high common mode voltages, a novel five probe geometry is developed. The measurements on single crystals of the Bi2Sr2CaCu2Ox superconductor are carried out to demonstrate the circuit performance.

Development of X-ray CCD camera based X-ray micro-CT system

Availability of microfocus X-ray sources and high resolution X-ray area detectors has made it possible for high resolution microtomography studies to be performed outside the purview of synchrotron. In this paper, we present the work towards the use of an external shutter on a high resolution microtomography system using X-ray CCD camera as a detector. During micro computed tomography experiments, the X-ray source is continuously ON and owing to the readout mechanism of the CCD detector electronics, the detector registers photons reaching it during the read-out period too. This introduces a shadow like pattern in the image known as smear whose direction is defined by the vertical shift register. To resolve this issue, the developed system has been incorporated with a synchronized shutter just in front of the X-ray source. This is positioned in the X-ray beam path during the image readout period and out of the beam path during the image acquisition period. This technique has resulted in improved data quality and hence the same is reflected in the reconstructed images.

Effect of Co-doping On the Radiation Hardness of

Single crystals of Gd3Ga3Al2O12:Ce with Ca, B and Ba codopants were successfully grown using the Czochralski technique. The samples of each composition were irradiated to 10 kGy and 100 kGy gamma dose to determine the radiation induced absorption. The Ca co-doped crystal was found to have maximum induced absorption, while B and Ba co-doped were found to be more radiation hard. The reduction in transmission could be partially restored at room temperature without any annealing treatment of the crystals. The additional absorption was also measured after annealing the crystals in reducing environment and compared with the radiation induced absorption. Thermoluminescence measurements were carried out to explain the defect structure and recovery of the transmission reduction at room temperature.

{{\rm Gd}_3}{{\rm Ga}_3}{{\rm Al}_2}{{\rm O}_{12}}

Abstract The use of accelerator-driven systems for incineration of nuclear waste and energy production requires monitoring of different parameters that govern reactor safety. One of the most important parameters is the multiplication factor keff. The present paper describes the results of experiments carried out on a subcritical system (BRAHMMA) using a pulsed neutron source. The value of the multiplication factor keff obtained from time responses of the core that were measured in situ using neutron detectors after insertion of a neutron pulse matches well with the calculated value.

: Ce Scintillators

Partially transparent ceramic of 2 at.% Eu doped CaF2 have been grown preferentially towards [111] direction. For this purpose, Eu doped CaF2 nanoparticles (size∼12 nm) obtained by a low temperature solution growth method has been pressed at 1000°C under vacuum. The preferentially grown ceramic shows 15% transparency within the visible range of spectrum. As confirmed by the X-ray diffraction result, the hot pressed ceramic exhibits reduced lattice volume than the nanopowder. It indicates Eu3+ as the dominant substituting ions at the Ca2+ sites of CaF2 lattice in the hot pressed ceramic material. It is corroborated by the photoluminescence results of hot pressed ceramic which shows strong red emission corresponding to Eu3+ sites. However, photoluminescence of nanopowder exhibits intense peak in the blue region of the spectrum which is characteristics of Eu2+ sites.

Pulsed Neutron Source Measurements in the BRAHMMA Accelerator-Driven Subcritical System

Glass having composition 40SrO−10ZnO−40SiO2−2B2O3−2Al2O3−2TiO2−2Cr2O3−2Y2O3, (mol %) was prepared by melt-quench technique and converted into glass-ceramics by subjecting it to varying heat treatments. Thermal properties were measured by thermo-mechanical analyzer and differential thermal analyzer. The XRD revealed that initially Sr2ZnSi2O7 phase at lower temperature and later SrSiO3/Sr3Si3O9 phase crystallized. The structural elucidation by Raman spectroscopy shows the presence of mainly Q1 structural units along with Q2 and Q0 units in the base glass. Raman spectra revealed that during crystallization initially crystalline phase having Q1 structural units (corresponding to Sr2ZnSi2O7 phase) are formed and later crystalline phase having Q2 structural units with 3 member ring type structure crystallizes. Thus, Raman spectroscopy and XRD together confirm that in early stage of crystallization, Sr2ZnSi2O7 phase and later Sr3Si3O9 phase formed in the glass-ceramics.