Madhumita Goswami

Effect of B2O3 addition on microhardness and structural features of 40Na2O-10BaO-xB2O3-(50-x)P2O5 glass system

Phosphate glasses having composition, 40Na2O-10BaO-xB2O3-(50-x)P2O5, wherex = 0–20 mol% were prepared using conventional melt quench technique. Density of these glasses was measured using Archimedes principle. Microhardness (MH) was measured by Vicker’s indentation technique. Structural studies were carried out using IR spectroscopy and31P and11B MAS NMR. Density was found to vary between 2.62 and 2.77 g/cc. MH was found to increase with the increase in boron content.31P MAS NMR spectra showed the presence of middle Q2 groups and end Q1 and Q0 groups with P-O-B linkages. FTIR studies showed the presence of BO3 and BO4 structural units along with the depolymerization of phosphate chains in conformity with31P MAS NMR.11B NMR spectra showed increase in BO4 structural units with increasing boron content. The increase in MH with B2O3 content is due to the increase of P-O-B linkages and BO4 structural units as observed from MAS NMR studies resulting in a more rigid borophosphate glass networks.

Preparation and characterization of magnesium-aluminium-silicate glass ceramics

Synthesis of machinable quality magnesium aluminium silicate (MgO-Al2O3-SiO2) for fabrication of insulators/spacers usable in high voltage applications under high vacuum conditions has been carried out following two different routes i.e. (i) sintering route, and (ii) glass route. A three-stage heating schedule involving calcination, nucleation and crystallization, has been evolved for the preparation of magnesium aluminium silicate (MAS) glass ceramic with MgF2 as a nucleating agent. The effect of sintering temperature on the density of compacted material was studied. Microstructure and machinability of samples obtained from both routes were investigated. They were also characterized for microhardness. Initial studies on material obtained by glass route reveal that these samples are superior to those obtained from sintered route in respect of their high voltage breakdown strength and outgassing behaviour. Outgassing rate of 10−9 Torr l·s−1 cm−2 and breakdown strength of 160 kV/cm were obtained. Different types of spacers, lugs, nuts and bolts have been prepared by direct machining of the indigenously developed glass ceramic.

Preparation and studies of some thermal, mechanical and optical properties ofxAl2O3(1 −x)NaPO3 glass system

Sodium aluminophosphate glasses having compositions of xAl2O3(1-x)NaPO3 (x = 0.05-0.2) were prepared using conventional melt-quench technique. Density, glass transition temperature, microhardness (MH), thermal expansion coefficient (TEC) and transmission characteristics were measured as a function of alumina content for different samples. They were found to depend on O/P ratio with pronounced changes taking place for O/P ratio ≥3.5. Density, glass transition temperature and microhardness were found to increase up to 15 mol% of alumina and then they showed a decreasing trend. Thermal expansion coefficient decreased continuously with alumina content. Optical gaps for different glass samples as measured from transmission characteristics were found to be in the range 3.13–3.51 eV. It initially decreased with alumina content up to 15 mol% and then increased. The behaviour was explained on the basis of change in the average aluminum coordination number from six Al(6) to four Al(4) (i.e. Al(OP)6/Al(OP)4 ratio) along with the changes in polyhedra linkages in the glass network due to change in O/P ratio.

Some recent studies on glass/glass-ceramics for use as sealants with special emphasis for high temperature applications

Glass-ceramics owing to a combination of useful properties such as tuneable thermal expansion coefficient (TEC), good mechanical durability and chemical inertness find widespread uses in a variety of applications including seals and coatings. Glass-ceramic-to-metal seals have been fabricated with various silicate, phosphate and borate based oxide glasses depending upon the intended application. In this article, we review our studies on various glass and glass-ceramics materials development with a view to understand bonding behaviour with metals/alloys at ambient and high temperatures through a comprehensive structure property correlation investigations. Detail studies on BaO-CaO-Al2O3-B2O3-SiO2 (BCABS), barium strontium alumino-silicate, and strontium alumino-silicate with different additives (like Nd2O3, La2O3, NiO, TiO2, V2O5, ZrO2, Cr2O3, and P2O5) and barium/strontiun zinc silicate (B/SZS) glass-ceramics for high temperature sealing. We shall illustrate the role of various thermo-physical and structural characterization techniques that allowed optimum selection of materials and processing parameters. We particularly highlight the complementary role of NMR and XRD in studying the material at the short range and long range length scales.

Growth, optical transmission and X-ray photoemission studies of BaB2O4 single crystals

Beta barium borate (β-BBO) crystals have been grown by the top seeded solution growth technique (TSSG) using Na2O as a flux. The crystals exhibited high transparency and the absence of inclusions and found to have sodium contamination in the range 150 to 230 p.p.m. The effect of this contamination on some crystal properties of interest has been investigated. The presence of impurities causes optical absorption below 550 nm in BBO crystals of both α and β-phases. X-ray photo emission spectroscopy (XPS) measurements performed on these crystals show that sodium gives rise to a measurable shift in the binding energies of the constituent ions. Further, the results show that Na+ ions enter into the lattice substitutionally and provide charge trapping sites close to the band edge.

White light emitting Ho3+-doped CdS nanocrystal ingrained glass nanocomposites

We report the generation of white light from Ho3+ ion doped CdS nanocrystal ingrained borosilicate glass nanocomposites prepared by the conventional melt-quench method. Near visible 405 nm diode laser excited white light emission is produced by tuning the blue emission from the Ho3+ ions, green band edge, and orange-red surface-state emissions of the nanocrystalline CdS, which are further controlled by the size of the nanocrystals. The absorption and emission spectra evidenced the excitation of Ho3+ ions by absorption of photons emitted by the CdS nanocrystals. The high color rendering index (CRI = 84–89) and befitting chromaticity coordinates (x = 0.308–0.309, y = 0.326–0.338) of white light emission, near visible harmless excitation wavelength (405 nm), and high absorbance values at excitation wavelength point out that these glass nanocomposites may serve as a prominent candidate for resin free high power white light emitting diodes.

Single-step in-situ synthesis and optical properties of ZnSe nanostructured dielectric nanocomposites

This work provides the evidence of visible red photoluminescent light emission from ZnSe nanocrystals (NCs) grown within a dielectric (borosilicate glass) matrix synthesized by a single step in-situ technique for the first time and the NC sizes were controlled by varying only the concentration of ZnSe in glass matrix. The ZnSe NCs were investigated by UV-Vis optical absorption spectroscopy, Raman spectroscopy, and transmission electron microscopy (TEM). The sizes of the ZnSe NCs estimated from the TEM images are found to alter in the range of 2–53 nm. Their smaller sizes of the NCs were also calculated by using the optical absorption spectra and the effective mass approximation model. The band gap enlargements both for carrier and exciton confinements were evaluated and found to be changed in the range of 0–1.0 eV. The Raman spectroscopic studies showed blue shifted Raman peaks of ZnSe at 295 and 315 cm−1 indicating phonon confinement effect as well as compressive stress effect on the surface atoms of the...

Dielectric measurement on magnesium aluminum silicate glass–ceramics prepared by different routes

Abstract Synthesis of machinable quality magnesium aluminum silicate (MgO-Al2O3-SiO2) for fabrication of insulators/spacers usable in high-voltage applications under high vacuum conditions has been carried out following two different routes, i.e., (a) sintering route and (b) glass route. The dielectric constant (e′) and the dielectric loss (D=tanδ) have been measured in MAS glass–ceramics (GC) at different temperatures for frequencies up to 12 MHz. The analyses for these samples indicate that dielectric constant variation with frequency exhibits an initial drop in the lower frequency region followed by slight increase at higher frequency. The influence of temperature on dielectric constant is discussed.

CdSe nanocrystals ingrained dielectric nanocomposites: synthesis and photoluminescence properties

Cadmium selenide (CdSe) nanocrystals ingrained dielectric nanocomposites in a B2O3–SiO2–Al2O3–Na2O–K2O borosilicate glass system were synthesized by a single step in situ melt quenching technique. The sizes of the nanocrystals as well as the band gap of the nanocomposites were controlled by both concentration of CdSe and post thermal treatment duration. The nanocomposites were characterized by different instrumental techniques including detailed photoluminescence studies. The sizes of the CdSe nanocrystals were found to alter in the range 4–16 nm as estimated from the effective mass approximation model and optical absorption spectroscopy. However, the TEM analysis revealed the generation of two different size ranges, 3–4 and 23–45 nm, of the particles within the dielectric matrix. Selected area diffraction (SAED) and x-ray diffraction (XRD) patterns authenticate the formation of hexagonal nanostructures of CdSe. These nanocomposites were found to be capable of exhibiting strong visible red luminescence around 715 nm on excitation at 446 nm. This has originated from the electron–hole recombination of CdSe nanocrystal and defects or traps related transitions. The properties of these nanocomposites advocate their significant applications as semiconductor based luminescent materials.

Glasses and Glass-Ceramics for Vacuum and High-Temperature Applications

This chapter presents a brief review of glass and glass-ceramics and their applications in the form of hermetic seals, especially for use in extreme condition of pressure and temperature. The beginning of this chapter is an overview of glass and glass-ceramics and their related properties. This section is followed by a general discussion on nucleation and growth phenomena and their importance in preparation of tailor-made glass-ceramics. Various aspects related to hermetic seal fabrications with various glass/glass-ceramics systems for high vacuum and high temperature are highlighted. A short discussion on various important parameters, i.e., thermal expansion behavior, different bonding mechanisms at the interfaces, calculation of stresses at the joints, and seal fabrication process parameter are also included in this chapter. Different techniques employed for glass-to-metal and glass-ceramics-to-metal seal fabrication are outlined, and a few glass-ceramics systems, i.e., lithium aluminum silicate, lithium zinc silicate, and barium calcium/strontium borosilicate, are discussed in detail with related thermomechanical, structural, and interface studies. Effects of nucleants and heat-treatment schedules in controlling the nature/concentration of crystalline phase formation on the sealing are reviewed for some selected systems. Some techniques used for characterization of glasses/glass ceramics/seals are described wherever necessary. In addition, glass systems used for high-temperature sealing applications are discussed with emphasis on their long-term stability. The chapter also covers an overview of graded seals, which make hermetic sealing possible when a gradual change in thermal expansion behavior is needed from glass to metal, thereby reducing the chances for cracking under thermal stress. The interesting and emerging area of machinable glass-ceramics is also reviewed in view of its growing relevance. Structure–property correlations are discussed to understand the role of various additive/constituents is various glass/glass-ceramics systems. The chapter concludes with a note on possible future directions of glasses and glass-ceramics in the field of application in extreme conditions of temperature and pressure along with an up-to-date bibliography.