The thermal expansion of a silicate glass system containing four types of modifiers at temperatures above the glass transition can be correlated with changes in medium range order (viz., the ring size distribution) without significant modifications in short range order such as Si coordination and Q‐speciation. An empirical linear relation between the network densities of the two groups of silicate glass‐forming systems and their characteristic ring size has been found using the topological pruning picture proposed by Stixrude and Bukowinski (Am Mineral, 1990, 75:1159‐69). Although variation in thermal expansion among different samples exists, for individual composition of the glass‐forming liquids, a strong linear correlation exists between the ring size distribution and the thermal expansion contributed by both configurational change and thermal vibration. The ring evolution of the glass‐forming liquid driven by temperature has been modeled by assuming an Arrhenius‐like activation picture for small member rings and treated as constant numbers for large rings. The compositional dependence of the ring structure can be reflected in the mixed‐alkali or alkaline‐earth effect on the small ring activation energy.
ASilicate glass coatings on Vitallium Ò , a Co-Cr alloy were prepared using a simple enameling technique. The composition of the glasses in the SiO 2 -Na 2 O-K 2 O-MgO-CaO-P 2 O 5 system has been tailored to match the thermal expansion of the alloys. The optimum glass composition and firing conditions (temperature and time) needed to fabricate homogeneous coatings with good adhesion to the alloy were determined. The final coating thickness ranged between 40 and 60 mm. Coatings fired under the optimum conditions do not delaminate during indentation tests of adhesion. Excellent adhesion to the alloy has been achieved through the formation of 150 nm thick interfacial layers (CrOx). The silicate glass can be successfully coated to Co-Cr alloy and formed hydroxyapatite on the glass surface when immersed in a simulated body fluid (SBF) for 30 days.
We investigate the structural origin of the large thermal expansion coefficient of hotcompressed silica glass upon heating to a threshold temperature using molecular dynamics (MD) simulations. While the simulated thermal expansion at low temperature correlates well with the elongation of the average interatomic separation distance of the Si-O or Si-Si pair, the excess thermal expansion due to hot compression mainly results from change in medium range order, including a decrease in the population of large rings upon heating, without significant modification of the small ring population and without changing the short range ordering, such as the Si-O coordination number. The reduction in the characteristic ring size can be connected to the high thermal expansion through the topological pruning mechanism. Such large thermal expansion is suppressed when the silica glasses are held at their respective quench pressures. The suppression of this excess thermal expansion is consistent with the pressure stabilization of the large ring structures.
15,17,24,25) shear modulus, 12,14) Young's modulus 12,14,16) and density 14,15,17) ) of rare-earth containing oxide and/or oxynitride glasses that are the grain boundary phase of Si 3 N 4 ceramics sintered with rare-earth oxide additives, and thus, can also affect grain growth of b-Si 3 N 4 itself. However, to our best knowledge, there are no available data on the physical properties of rare-earth containing high temperature melts. Here, we intend to measure the viscosity (bulk property) and the surface tension (surface property) of RE 2 O 3 -MgO-SiO 2 (REϭY, Gd, Nd and La) melts, and to make clear the effect of rare-earth oxide additions on these physical properties.
Experimental
Sample PreparationSamples for viscosity and surface tension measurements were prepared from reagent grade Y 2 O 3 , Gd 2 O 3 , Nd 2 O 3 , La 2 O 3 , MgO and SiO 2 powders (Sigma-Aldrich Japan). These reagents were precisely weighed to form given compositions (cf.
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