G. P. Kothiyal

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.

Structural aspects of B2O3-substituted (PbO)0.5(SiO2)0.5 glasses

Lead borosilicate glasses having general formulae (PbO)0.5 −x(SiO2)0.5(B2O3)x with 0.0 ≤ x ≤ 0.4 and (PbO)0.5(SiO2)0.5 −y(B2O3)y with 0.0 ≤ y ≤ 0.5 have been prepared by a conventional melt–quench method and characterized by 29 Si, 11 B magic angle spinning (MAS) NMR techniques and infrared spectroscopy, as regards their structural features. From 29 Si NMR results, it has been inferred that with increasing concentration of boron oxide, (PbO)0.5 −x(SiO2)0.5(B2O3)x glasses exhibit a systematic increase in the number of Q4 structural units of Si at the expense of Q2 structural units, along with the formation of Si–O–B linkages. On the other hand, for (PbO)0.5(SiO2)0.5−y(B2O3)y glasses, there is no direct interaction between SiO2and B2O3 in the glass network, as revealed by the 29 Si MAS NMR studies. Boron exists in both trigonal and tetrahedral configurations for these two series of glasses and for the (PbO)0.5(SiO2)0.5−y(B2O3)y series of glasses; the relative concentration of these two structural units remains almost constant with increasing B2O3 concentration. In contrast, for (PbO)0.5−x(SiO2)0.5(B2O3)x glasses, there is a slight increase in the number of BO3 structural units above x = 0.2, as there is a competition between SiO2 and B2O3 for interaction with Pb2+, thereby leading to the formation of BO3 structural units. For both series of glasses, the thermal expansion coefficient is found to decrease with increasing B2O3 concentration, the effect being more pronounced for the (PbO)0.5 −x(SiO2)0.5(B2O3)x series of glasses due to the increased concentration of Q4structural units of silicon and better cross-linking as a result of the formation of Si–O–B-type linkages.

29Si MAS NMR and microhardness studies of some lead silicate glasses with and without modifiers

Abstract Lead silicate glasses having different mole ratios of SiO2 to PbO and containing different amounts of alkali/alkaline earth metal oxides were prepared by conventional melt-quench method. The different structural units of Si present in these glasses were identified by 29Si MAS NMR. Addition of PbO above 50 mol% to SiO2 and Na2O up to 15 mol% in (SiO2)0.5(PbO)0.5 glasses has been found to give similar 29Si MAS NMR patterns, characteristic of depolymerised silicon structural units (Q2 and Q1), which interconnect the extended Pb–O–Pb network in the glass. The microhardness values of lead silicate glasses show systematic decrease with increase in lead oxide contents. The microhardness values of lead silicate glasses having SiO2 to PbO mole ratio 6.9 and containing different amounts of alkali/alkaline earth metal oxides, have been found to vary in a complex manner. For higher concentrations of alkali/alkaline earth metal oxides, microhardness values have been found to decrease, probably due to conversion of more rigid covalent Q4, and Q3 structural units to less rigid Q2 and Q1 structural units.

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.

Structure–property correlations in lead silicate glasses and crystalline phases

Lead silicate glasses containing 40–65 mol% of PbO were prepared at two melt-quenching rates and characterized by X-ray diffraction, UV-Visible absorption spectroscopy, density, microhardness, thermo-mechanical analysis, differential scanning calorimetry and Raman scattering studies. On increasing the PbO concentration, density increases, glass transition temperature decreases and the optical absorption edge shifts towards longer wavelength. An intense optical absorption band was observed just below the absorption edge in glasses with 55 mol% and higher concentration of PbO. Dilatometric measurements show an unusual property that glasses do not show any abrupt increase in volume near the glass transition temperature but transform directly into the liquid state. Raman spectroscopy confirmed that the concentration of SiO4 tetrahedra containing one or more NBOs increase with PbO mol%. Devitrification studies on lead silicate glasses found that samples with 40–45 mol% of PbO do not crystallize, whereas samples with higher PbO concentration produce multiple crystalline phases like PbSiO3, Pb33Si24O81, Pb2SiO4 and Pb3Si2O7 on heat treatment.

Influence of phosphate precursors on the structure, crystallization behaviour and bioactivity of sol–gel derived 45S5 bioglass

Obtaining bioglass composition 45S5® in completely amorphous form without crystalline inclusion by the sol–gel route has remained a challenge so far. Here, we demonstrate that with appropriate phosphate precursor and by controlling the polymerization and gelation reactions by adjusting the pH of the reactants, one can overcome this challenge. As-synthesized glass was heat treated to understand the devitrification behaviour of the sol–gel derived glass and to obtain bioglass–ceramics. As-synthesized and heat treated glasses were characterized using powder X-ray diffraction, transmission electron microscope, differential scanning calorimeter and differential thermal analyzer. For assessing the in vitro bioactivity of sol–gel derived glass and glass–ceramic powders, we tested the apatite forming ability on their surface upon immersion in simulated body fluid. Properties of sol-derived 45S5 glass are then compared with the bulk counterpart obtained by conventional melt quenching method. This study reveals a procedure to prepare completely amorphous sol–gel derived 45S5 glass which can be used as a bioactive material for bone implant, tooth coating, bone tissue engineering and drug delivery applications.

A Comparative Overview of Glass-Ceramic Characterization by MAS-NMR and XRD

Glass-ceramics are polycrystalline materials formed by controlled crystallization of parent glasses, which can be studied complementarily by XRD and NMR. XRD probes long-range order, while NMR is more element specific and probes the immediate environment around the atoms. These techniques allow considerable insights into the crystalline and amorphous constituents of glass-ceramics. In this article, we highlight some key issues in the characterization of glass-ceramics: phase identification and quantification, and structural characterization of the residual glass. The application of both NMR and XRD techniques are considered in these areas and their advantages and limitations discussed using our recent work and examples from the literature.

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...