Animesh Jha

Formation of silicon carbide whiskers and their microstructure

Thermodynamic and kinetic conditions for the formation of SiC whiskers are established. The mechanism of their nucleation and growth are studied and, on this basis, the magnitude of the thermally activated barrier is determined from the rate of reduction data. The microstructures of whiskers are analysed and the role of interfacial tension between the nuclei and impurities, and the metallic iron catalyst is studied in relation to the formation of SiC whiskers. A possible reason for polytypism in SiC whiskers is also proposed.

1G4 lifetimes, optical and thermal characteristics of Pr-doped GeS2-chalcohalide glasses

Abstract The low phonon-energy of GeS 2 glass ( 350 cm − ) makes it an ideal host for designing Pr-doped glasses for 1300 nm fibre amplifier. In the present investigation, a number of chalcohalide and chalcogenide glasses based on GeS 2 as a major constituent were studied. The effect of the removal of OH − ions from the starting powders by vacuum drying on prolonging the 1 G 4 lifetimes and on increasing the 1 G 4 absorption intensity in Pr-doped GeS 2 -based glasses is discussed. The structural role of network modifiers in improving the spectroscopic ( 1 G 4 ) and thermal characteristics is also reported. The longest measured metastable lifetime is 200 μs in a 2000 ppm by weight (ppmw) Pr-ion doped glass. In the light of the Johnson-Mehl-Avrami transformation kinetics equation, the computed critical cooling rate for glass formation has been compared with the measured cooling rate from the melt homogenization temperature.

Kinetics of crystallization of rapidly quenched FeNdB alloy and its application in the processing of permanent magnets

Abstract A differential scanning calorimetric technique has been used to study the kinetics of devitrification of overquenched FeNdB ribbons using a modified Kissinger method. The activation energy, E c , for low (4.8 at% B), intermediate (6.1 at% B) and high (7.8 at% B) boron ribbons was determined and the effect of varying ribbon thickness on the activation energy was also investigated. The relative stabilities of the glassy phases with varying boron content are discussed qualitatively in relation to the phase constitution in the FeNdB system and the structures of the crystalline phases. The glass-forming ability of FeNdB melts is considered by plotting a time-temperature-transformation diagram, calculated on the basis of theories of homogeneous nucleation and growth and of the kinetics of phase transformations. The effect of the kinetic on the processing conditions and on the microstructure required for high coercivity in rapid quenched and hot pressed magnets is also discussed.

Improved fluoride glasses for 1.3 μm optical amplifiers

Abstract Preliminary materials results are reported of a study to develop novel fluoride glasses exhibiting maximum phonon energies lower than those of ZBLAN with the objective of realising an efficient optical amplifier for the coveted 1.3 μm wavelength domain. Pertinent material attributes including peak phonon energy, 1 G 4 lifetimes and thermal properties are reported for each glass host.

Spectroscopic and Thermal Properties of GeS2-based chalcohalide glasses

Abstract The low phonon energy of germanium sulphide glasses makes them ideal candidates as hosts for 1·3 μm fibre amplifier applications. However, the GeS2 glass host suffers from a major drawback of poor rare-earth ion solubility. In an efficient device, the solubility of Pr ions has to be enhanced, without adversely affecting either the thermal or the spectroscopic properties of the glass. In the present investigation, we report the synthesis and optical properties of modified GeS2-based chalcohalide glasses with excellent thermal characteristics suitable for drawing low-loss optical fibres.

Phase equilibria in the Si-C-N-O system and the kinetic analysis of silicon carbide whisker growth

The significant binary, ternary and quaternary phase equilibria in the Si-C-N-O system are reviewed particularly with regard to the relative stabilities of α and β forms of silicon carbide and nitride. The standard Gibbs free energies of formation of the relevant high-temperature phases are compared, and for the α-form of SiC and Si3N4 the values are derived from the empirical results. From these free energy values, the phase boundaries in the Si-C-N-O system have been calculated and are plotted as typical RTInPO2 versus 1/T plots. The significance of various phase fields in relation to the processing and fabrication of carbide, nitride and oxynitride ceramics is discussed. The rate of chemical reaction has been analysed and the mechanism of reduction of silica to SiC whisker has been proposed. The energetics of whisker growth is also discussed and the derived value of activation energy is compared with the surface self-diffusivity of carbon in pure β-SiC crystals.

Cadmium mixed halide glass for optical amplification at 1.3 μm

Optical and spectroscopic data and calculations in support of an efficient 1.3 µm amplifier in Pr3+-doped cadmium mixed halide glass host are presented. We find that the dominant parameter affecting the amplifier gain is the lifetime (325 µm) of the Pr3+-1G4 state. The long lifetime is the direct consequence of a decreased multiphonon decay rate in this low-phonon-energy glass host. We predict gain in excess of 30 dB for 100 mW pumping in low loss fibres, and for background loss of 1 dB/m, gain figures are as high as 15-20 dB.

Kinetics of glass formation of heavy metal fluoride melts

Abstract The stability of flouride glasses against crystallisation is studied by computing the time-temperature-transformation curve. Based on the homogeneous nucleation and simultaneous crystal growth, and the Johnson-Mehl-Avrami equation for transformation kinetics, the calculated crystral growth rate and nucleation frequencies are compared with the experimental results for fluorozirconate glasses. The estimated values of the average size of growing crystals of barium fluorozirconate (β-BZ) indicate that the extrinsic scattering loss will increase significantly above 615 K.

Design and fabrication of Pr3+-doped fluoride glass optical fibres for efficient 1.3 mu m amplifiers

Silica optical fibre technology for communication networks has grown rapidly in the last 15 years, and its development has made possible broadband telecommunications. The bandwidth of silica-based passive optical network (PON) is many orders of magnitude larger than that of copper coaxial cables (~20 MHz over 3-5 km). As a result, optical fibres have revolutionised telephone networks. Current figures suggest that approximately 80% of British telephone traffic is carried by optical fibres. By the year 2015, the entire Japanese telephone network will be optical fibre-based, enabling it to carry data and video via telephone cables into every home. A large component of the world-wide optical fibre system constitute the terrestrial networks which utilise the second transmission window of silica at 1.3µm. At this wavelength, no optical amplifier is as yet commercially available, necessitating the use of electronic repeaters. By contrast, for the lowest-loss window at 1.5µm, efficient high gain Er doped optical amplifiers have been developed and are widely available. Compared with optical amplifiers, electronic repeaters are more expensive, slower and less flexible in operation. The major motivation for the current project is the need for an efficient optical fibre amplifier operating in the 1.3µm window. The purpose of the project is to design and develop such amplifier based on Pr doped fluoride fibres.

Spectroscopy of Pr3+-doped low-phonon-energy glasses based on halides and sulfides

The optical properties of praseodymium-doped glasses have attracted considerable attention recently for their potential application as a 1.3 micron optical amplifier. We report here on our spectroscopic evaluation of a series of low-phonon-energy glasses based on halides and sulphides. These results, though driven by the desire for a practical amplifier, provide insight into the application of these glasses not only for telecommunications applications, but also an understanding of the overall optical properties of a low-phonon-energy glass. Using Raman spectroscopy, the vibrational characteristics of the glass host are determined. Absorption measurements across the visible and infrared allow evaluation of the intrinsic loss of these glasses when in fiber form, as well as providing an indication of glass purity. Fluorescence of Pr3+-doped glasses, through excitation of the 3P0, 1D2 and 1G4 levels, is measured along with the fluorescence lifetimes. These radiative properties are compared to those predicted by a Judd-Ofelt analysis, which has been performed on all glasses. In this way, this work provides an overall spectroscopic evaluation of the optical properties of low-phonon-energy glasses, leading the way towards a practical device.