Densification of glasses

Figure 8.3 Densification of glass powders occurs by viscous flow. The matter transport paths are not clearly defined as for crystalline powders, (a) Two possible flow fields for viscous sintering of spheres are illustrated. (From Ref. 1.) (b) Simulations by finite-element analysis indicate that the flow field on the right-hand side is more realistic. (From Ref. 31, with permission.)...

Yoshida and coworkers discuss densification of glasses caused by indentation [8]. Now consider the finding Bhushan e.a. [9] that microhardness measurements of worn metal samples show a 10-80 % increase of hardness in the worn layer. While behavior of the metals is different from that of polymers since the latter are viscoelastic, a possible connection between the characteristics of groove profiles we have obtained with hardness determination seemed worth pursuing. 

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

The concept of a single structural transition in amorphous material, i.e., an amorphous-amorphous transition, was coined in 1985 by Mishima et al. [25] on the example of water. Today, in many respects the nature of pressure- and/or temperature-induced transformations in glasses and amorphous solids remains unclear. In most cases, the pressure treatment of glasses and amorphous solids results in residual densification. In the process, the densified glasses and... 

The behavior of the Si02 glass under pressure has been intensely studied as well [39, 53, 64—83]. It was established long ago that the high-pressure, high-temperature treatment causes a significant residual densification of silica glass... 

The maximum densification of 18-20% is achieved after a pressure of 16-20 GPa—room temperature treatment or after 5-8 GPa—800-1000 K treatment. The densified glasses have predominantly tetrahedral coordination of silica atoms, Z 4-4.5 [14, 71, 88], Elastic moduli and optical characteristics of densified glasses are distinctly different from those of pristine silica glasses. 

The earlier data did not shed light on how closely this coordination transformation is related to residual densification of the glass. Another unanswered question was whether glass densification after 20 GPa pressure at room-temperatures treatment and the one at 5-8 GPa and high-temperatures treatment are of a similar nature. 

Thermal mechanical analysis was utilized by Ophir 174) to study the densification of Bisphenol-A-based epoxies. The glass transition temperature can easily be characterized by a slope change as the resin transits from the glassy state to the rubbery state (see Fig. 25). Hence, in glassy material, it is typically represented by two thermal expansivity parameters, one below T (glassy thermal expansivity) and one... 

J. Arndt, U. Homemann, and W.F. Muller, Shock-wave densification of silica glass. Physics and Chemistry of Glasses 12, pp. 1—7 (1971). 

B. Reynard, M. Okuno, Y. Shimada, Y. Syono, and C. Willame, A Raman spectroscopic study of shock-wave densification of anorthite (CaAhSizOs) glass. Phys. Chem. Mineral. 26, pp. 432—436 (1999). 

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