Preparation of glass articles by sintering of glass and amorphous powders is considered. The classification of sintering processes includes: (a) sintering of premelted and pulverized glasses; (b) sintering of premelted chemically treated glasses; (c) sintering without melting; (d) sintering with melting. The most interesting class of processes is sintering without, or simultaneously with, melting and it includes preparation of preforms for optical fibres and the sol-gel route for glass preparation. This last route is reviewed in detail. Two main versions are considered: preparation of gels by hydrolysis and polymerization of alkoxides, and sintering of amorphous colloidal powder compacts. The sol-gel processes represent an embryo of a new technology for the production of high-melting glasses (including quartz glass) at relatively low temperatures.

Preparation of glass by sintering

Past, present, and future studies of glass structure by nuclear magnetic resonance (NMR) are surveyed. Quadrupole or chemical shift effects for various nuclei (10B, 11B, 17O, 29Si, 125Te) are analyzed to produce structural models for glasses. Temperature-dependent properties of NMR spectra are used to monitor ion hopping in glasses and secure the activation energies. High-field NMR with sample spinning is exemplified with 29Si, 27Al and 23Na nuclei. Future NMR studies of glass structure will involve many additional nuclei including those of Ge, Mg, Sc, La, Nb, and Zr.

Nuclear magnetic resonance studies of glass structure

Presently described are retroreflective articles, such as pavement markings, that comprise transparent microspheres partially embedded in a (e.g. polymeric) binder. Also described are (e.g. glass-ceramic) microspheres, methods of making microspheres, as well as compositions of glass materials and compositions of glass-ceramic materials. The microspheres generally comprise lanthanide series oxide(s), titanium oxide (TiO2), and optionally zirconium oxide (ZrO2).

Pavement marking, reflective elements, and methods of making microspheres

The detection, acceptance and further investigation of glass-glass phase separation was closely connected with the development of methods making detection possible, such as electron microscope technique pioneered and exploited by Werner Vogel, to whom this paper is dedicated on the occasion of his 65th birthday. Structures are not always found in agreement with rules derived from thermodynamic principles, and also depend on thermal history, as well as mode of preparation. The phenomenon is exemplified in many glass-forming systems. Among more recent applications are the control of crystallization by preceding phase separation as in glass-ceramics and semiconductor-doped glasses for non-linear optics.

Phase separation in glasses

Porous glass-ceramics of the TiO2-SiO2 system of high titania content have been prepared by heat treatment and subsequent acid leaching of phase-separated glasses of the TiO2-SiO2-Al2O3-B2O3-CaO-MgO (or -Na2O) system. The porous glass-ceramics obtained in the study had a surface area of 100 to 300 m2g−1, with an average pore radius of 3 to 9 nm. The ceramics which contained a large amount of anatase and rutile were expected to be applied in the field of photocatalysts.

Preparation of porous glass-ceramics of the TiO2-SiO2 system

Substitution of the end-member oxides in the ternary sodium borosilicate system diagram was studied. Replacing SiO2 in this ternary system with a combination of CeO2 and Nb2O5 was found to produce glasses which, after heat treatment, decomposed into two immiscible microphases of which one is water-soluble. The mode of phase decomposition is sensitive to impurities. The importance of crucible selection, e.g. Al2O3 or Pt/Rh, is demonstrated by differences in pore size, surface area, heat resistance, and structure of the insoluble phase for a series of Ce2O3-Nb2O5 and Ce2O3−Nb2O5−Al2O3 glass-ceramics. These new materials have a specific surface area of up to 217 m2/g and a pore radius of between 1.1 and 54.5 nm.

A Ce-Nb-(Al) Oxide Porous Glass-Ceramic

Studies on the substitution of the end-member oxides in the ternary sodium borosilicate system have included work on the replacement of SiO2 with combinations of La2O3 and TiO2 which after heat treatment resulted in leachable sodium borate and insoluble La2O3-TiO2 ceramic phases. Sensitivity to impurities during this phase decomposition is illustrated by the differences in physical properties, chemical resistance properties and structures of the insoluble phase when results obtained from melts from Pt/Rh or Al2O3 crucibles are compared. These materials exhibit after leaching interconnected pores with surface area of up to 217 m2 g−1 and pore radii ranging between 1.2 and 10.25nm with noteworthy alkali resistance of up to 0.08×10−2 mg dm−2 Na2O mg calculated on the whole surface area of the samples.

A lanthanum titanium porous glass ceramic

The substitution of the end-member oxides in the ternary glass forming sodium borosilicate system was studied. The replacement of SiO2 with combinations of La2O3 and Ta2O52 was found to produce glasses which, after heat treatment decomposed to form a leachable sodium borate phase and an insoluble ceramic phase. The mode of phase decomposition is sensitive to impurities: the importance of crucible selection, e.g. Al2O3 or Pt/Rh is demonstrated by resulting differences in pore size, surface area, chemical resistance and structure of the insoluble phase for a series of La2O3-Ta2O5 and La2O3-Ta2O3-Al2O3 glass ceramic. These new materials have a specific surface area of up to 145 m2 g−1 and average pore radii ranging between 0.7 and 28.6 nm and show a varying degree of chemical resistance.

A lanthanum-tantalum (Al) oxide porous glass ceramic

Substitution of the end-member oxides in the ternary sodium borosilicate system was studied. Replacing SO2 with a combination of beat-resistant oxides Ce, Hf, Th, and Zr (with or without La2O3 and/or Y2O3) was found to produce glasses (droplet like phases present), which after heat treatment decomposed into two immiscible microphases (coarsening process) of which one is water-soluble. The structure of the leached materials or the materials sintered after leaching (CeO2-HfO2-La2O3-ThO2-Y2O3-ZrO2), (CeO2-HfO2-La2O3-ThO2-ZrO3), (CeO2-HfO2-ThO2-YZO3-ZrO2), and (CeO2-HfO2-ThO2-ZrO2) is predominantly glasslike. Some Na2O-B2O3 may be expected to remain unleached in the pores, as has been observed for silica-based materials. However, no evidence for this was found during X-ray diffraction analysis and therefore any such presence if crystalline must be below 1%. The new glasslike materials have a higher sintering temperature (1520°C) than porous silica glass (900° to 1200°C). The specific surface area of the leached glasslike materials is 93.7 to 295.8 m2/g and an average pore radius of 0.4 to 7.3 nm was calculated, A favorable alkali resistance of 5.12 × 10−3 to 1.36 × 10−2 mg/dm2 as well as a water resistance of 5.9 to 13.8 Na2O in mg/g was measured.

Heat‐Resistant Nonsilicate Porous Glasslike Material

Substitution of SiO2 with CeO2 in the ternary sodium borosilicate system was found to produce phase-separable glasses. Heat treatment of these glasses resulted in separation into two different phases. The one phase enriched in sodium borate was then leached out leaving a CeO2-rich framework. The structure of the leached material was crystalline (Pt/Rh crucible melt) which changed to a rather net-like appearance if Al2O3 resulting from erosion of alumina crucibles was added. B2O3 remained partially in the insoluble CeO2-skeleton. X-ray diffraction analysis of leached material proved the presence of crystalline cubic CeO2 and cerium borate (metaborate of the aragonite type) in Pt/Rh crucible melts, whereas cubic CeO2, 2Al2O3 · B2O3 and traces of sodium borate were detected in Al2O3 containing melts. The specific surface areas of the leached materials ranged between 25 and 120m2g−1 while the main radii of interconnected pores were calculated to be between 0.5 and 17nm. A sintering temperature of about 1500° C was estimated from void volume and bulk density measurements.

M. A. Res

Papers

A Ce-Nb-(Al) Oxide Porous Glass-Ceramic

A lanthanum titanium porous glass ceramic

A lanthanum-tantalum (Al) oxide porous glass ceramic

Heat‐Resistant Nonsilicate Porous Glasslike Material

Transformation of Na2O−CeO2−B2O3 glass into a material with interconnected pores