Hardev Singh Virk

Recent Advances in the Synthesis and Characterization of Chalcogenide Nanoparticles

Chalcogenide semiconductor nanoparticles and their self-assembly structures have become the most explored group of semiconductor nanomaterials due to the interesting physics involved in quantum confinement, surface chemistry and variety of applications. In the last couple of decades, facile routes for their synthesis and strategies for controlling the size, shape and morphology have been reported. In the present review, synthesis strategies of size and shape controlled nanoparticles belonging to II-VI group of semiconductor chalcogenides are presented and each method for preparation of nanoparticles is critically analysed. Role of various factors that affect the nucleation and growth of nanoparticles is discussed at length. Nanoparticles and self-assemblies of CdSe, CdTe, HgTe and ZnSe are synthesized using new and facile single molecular precursor based noble route by our group that uses non-pyrophoric, low temperature and non-toxic chemicals, their properties and synthesis scheme are discussed as future development in this field. Some recent applications of chalcogenides QDs in the fields of solar cell, optical fibre amplifiers, biosensing and bo-imaging are discussed and reviewed.

Recent Advances and Opportunities in TLD Materials: A Review

Thermoluminescence (TL) is the thermally stimulated emission of light from an insulator or a semiconductor following the previous absorption of energy from ionizing radiation. TL dosimetry is used in many scientific and applied fields such as radiation protection, radiotherapy, industry, and environmental and space research, using many different materials. The basic demands of a thermoluminescent dosimeter (TLD) are good reproducibility, low hygroscopicity, and high sensitivity for very low dose measurements and good response at high doses in radiotherapy and in mixed radiation fields. In this review, we have discussed the past developments and the future opportunities in TLD materials and our efforts to make better future use of low cost materials in TLD applications. For this we have studied and discussed two efficient TLD phosphors with low cost and simple method of preparation on large scale for TLD materials. One of the phosphors is LiF:Mg,Cu,P (LiF: MCP), and another one is LiCaAlF6:Eu, which has the potential to replace conventionally used CaSO4:Dy TL dosimeter. LiF: MCP and LiCaAlF6: Eu phosphors are potential candidates for TL dosimetry and could be good replacement for commercially available phosphors. Apart from this, we have also studied thermoluminescence in Aluminate and Borate materials. We have discussed in detail all three types of TLD materials. First, our study includes complete detail of material properties, methods and dosimetric characterizations of LiF: MCP Phosphor; second, it includes a new TL Dosimeter, LiCaAlF6: Eu and its dosimetric characterizations; and lastly on some TL properties of Li5AlO4: Mn and MgB4O7: Dy,Na. In this review, we discus some recent developments in radiation dosimetry with regards to the measurement techniques and material preparations. Although many materials have been and are currently being studied for TLD, still there is a scope for the improvement in the material properties useful for the TLD, and the synthesis of new, more suitable materials. Contents of Paper

Structural and Magnetic Properties of Mn-Zn Ferrites Synthesized by Microwave-Hydrothermal Process

Nanocrystalline Mn1-xZnxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) ferrites have been successfully synthesised using microwave–hydrothermal method for high frequency applications. The nanopowders were characterised using X-ray diffraction (XRD) and sintered using microwave furnace at 900°C and the total time taken for sintering is 30 min. The frequency dependence of real and imaginary part of permeability were measured in the range 1 MHz to 1.8 GHz. The saturation magnetisation and coercive force were obtained using a vibration sample magnetometer (VSM) in the field of 1.5 T. The temperature dependence of initial permeability (μi) was measured in the temperature range of 300K to 600K at 10 kHz. The high values of permeability and saturation magnetization enables these materials to be the potential candidates for a number of applications, for example, in transformers, choke coils, noise filters and recording heads.

Luminescence Phenomena: An Introduction

Luminescence is cold light, light from other sources of energy, which can take place at normal and lower temperatures. There are several varieties of luminescence, each named according to what the source of energy is, or what the trigger for the luminescence is. Luminescence is a collective term for different phenomena where a substance emits light without being strongly heated, i.e., the emission is not simply thermal radiation. This definition is also reflected by the term cold light.

Y-Type Hexaferrites: Structural, Dielectric and Magnetic Properties

This paper attempts to provide a historical survey of structure of various types of hexaferrites. It provides information about synthesis, characterization, structural, magnetic and dielectric properties of Y-type hexagonal ferrites using various chemical routes. We have prepared a series of cobalt doped Sr2Cu2-xCoxFe12O22 (x = 0.0 to 1.0) hexaferrites using a wet chemical co-precipitation technique. The prepared hexaferrite precursors were calcined at 950 °C for 4 hours in a furnace and slowly cooled to room temperature. The crystal structure of Y-type hexaferrites is rather complicated. The chemical and structural changes were examined in detail by X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Scanning electron microscopy (SEM), and Fourier transform infra-red (FTIR) spectroscopy. X-ray diffraction studies showed that sintering temperature as low as 950°C was sufficient to produce a single-phase Y-type hexaferrite material. The dielectric measurements were carried out over the frequency range of 100 Hz to 2 MHz at room temperature using an LCR meter to study the variation of dielectric constant and loss tangent with frequency. The magnetic properties of hexaferrite samples were investigated using a vibration sample magnetometer (VSM), and a superconducting quantum interference device (SQUID) magnetometer in the temperature range 30K to 200K. A change from ferromagnetic state to super paramagnetic state has been observed in Co doped Sr2Cu2-xCoxFe12O22 (x= 0.6 to 1.0) hexaferrite. The novel applications of all types of hexaferrite materials have been described.

SSNTD Technique in Photo-Neutron Applications

Passive Solid State Nuclear Track Detectors (SSNTDs) are a versatile tool for neutron studies as has been shown long ago and several good quality materials are commercially available. They are useful for charged particle detection in the linear energy transfer (LET) range above the threshold value of ~10 keV μm-1. Linacs, operating above 6 MeV up to the energy region where radiotherapy is applied usually up to ~25MeV, induce unwanted photo-neutron field; their spectra shows two components due to reaction dynamics based on evaporation and knock-on mechanisms. Neutrons produced by Linacs are often neglected in health application; however, today it has become necessary to assess the effect on patient, staff and radiation workers. Radiation studies using SSNTDs play a major role in this case. Other fields also take advantage of the passive detectors properties; in fact they are employed with success to measure neutron signals relevant for plasma diagnostics as it was demonstrated at the RFX facility as part of the ITER project. The PADC-NTD techniques provide information on external neutron field values around the RFX-installation during pulsed operation. In any case, converter materials, as charged particles from (n, p) and (n, α) reactions, are required to produce neutron fingerprints through latent tracks. These once etched provide information on neutron fluence spatial values. Track histograms are then employed to determine photo-neutron induced damage in materials as well as radiation dose to both patient and professionally exposed workers. The estimated neutron fluence that can be determined by NTM covers a large range of values, the largest being above 1010 (± 12%) neutrons/cm2.

Modgil-Virk Formulation of Single Activation Energy Model of Radiation Damage Annealing in SSNTDs: A Critical Appraisal

Passage of heavy ions produces radiation-damage trails known as latent tracks in a variety of solid-state nuclear-track detectors (SSNTDs). These tracks are made visible in an optical microscope by a simple process known as chemical etching. It is a well-known fact that latent tracks are radiation damage trails in SSNTDs, which can be annealed by thermal heating. Modgil-Virk formulation of single-activation-energy model of radiation damage annealing was proposed as an empirical approach for explaining the thermal fading of nuclear tracks in SSNTDs. The empirical formulation of this model is based on track annealing data collected from both isothermal and isochronal experiments performed on different types of SSNTDs using a variety of heavy ion beams and fission fragments. The main objective of this empirical model was to resolve some contradictions of variable activation energy derived by using Arrhenius plots to study annealing in mineral SSNTDs. Some equivalent versions of the Modgil-Virk model have been proposed but the concept of single activation energy is vindicated in all empirical formulations. The model always yields a unique value of activation energy independent of the nature of the ion beam used and the degree of annealing. The anisotropy of the mineral SSNTDs is revealed by variation in activation energy along different crystal planes and even with different orientations of the ion beam on the same plane. Some recent experiments are a pointer to the successful exploitation of this model for future cosmic-rays studies using SSNTDs.

History of Luminescence from Ancient to Modern Times

Luminescent phenomena have fascinated mankind since the earliest times. The light from the aurora borealis, glow worms, luminescent wood, rotting fish and meat are all examples of naturally occurring luminescence. E. Newton Harvey’s 770 page volume “A History of Luminescence: From the Earliest Times until 1900” is a classic which narrates interesting stories from ancient cultures to modern times. The earliest written account of a solid state luminescent material comes from a Chinese text published in the Song dynasty (960–1279 A.D.). The Buddhist sacred jewel, called hashi-no-tama in Japan, is alleged to be self-luminous and to shed a brilliant light on its surroundings. In the Svetasvatara Upanishad, probably recorded at some time before the sixth century BC, we find a mention of fire-flies as one of the manifestations of Brahma.

Applications of Nanostructured Materials as Gas Sensors

Gas detection instruments are increasingly needed for industrial health and safety, environmental monitoring, and process control. To meet this demand, considerable research into new sensors is underway, including efforts to enhance the performance of traditional devices, such as resistive metal oxide sensors, through nanoengineering. The resistance of semiconductors is affected by the gaseous ambient. The semiconducting metal oxides based gas sensors exploit this phenomenon. Physical chemistry of solid metal surfaces plays a dominant role in controlling the gas sensing characteristics. Metal oxide sensors have been utilized for several decades for low-cost detection of combustible and toxic gases. Recent advances in nanomaterials provide the opportunity to dramatically increase the response of these materials, as their performance is directly related to exposed surface volume. Proper control of grain size remains a key challenge for high sensor performance. Nanoparticles of SnO2 have been synthesized through chemical route at 5, 25 and 50°C. The synthesized particles were sintered at 400, 600 and 800°C and their structural and morphological analysis was carried out using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The reaction temperature is found to be playing a critical role in controlling nanostructure sizes as well as agglomeration. It has been observed that particle synthesized at 5 and 50°C are smaller and less agglomerated as compared to the particles prepared at 25°C. The studies revealed that particle size and agglomeration increases with increase in sintering temperature. Thick films gas sensors were fabricated using synthesized tin dioxide powder and sensing response of all the sensors to ethanol vapors was investigated at different temperatures and concentrations. The investigations revealed that sensing response of SnO2 nanoparticles is size dependent and smaller particles display higher sensitivity. Table of Contents

Studies on Internal Friction and Curie Temperature of NiMgCuZn Spinel Ferrites for Micro-Inductor Applications

Three series of NiMgCuZn ferrites were prepared by conventional double sintering ceramic process. The formation of single phase in these ferrites was confirmed by X-ray diffraction. A brief review of the important investigations carried out on the internal friction behaviour of NiMgCuZnFe2O4, in the temperature range 40oC to 360oC, has been reported. In the present investigation, the composite piezoelectric resonator method has been used. The effect of compositional changes of ferrites on Curie temperature and internal friction are reported. Results and discussions on the temperature variation of internal friction of the three series of NiMgCuZnFe2O4 samples are discussed. In all the series studied, only single stress induced relaxation peaks are observed. These studies were carried out to develop a ferrite composition for their use as core materials for microinductor applications. The results are explained in the light of structural phase transitions.