Forming glass-ceramics

Oxynitride glasses may be heat treated to form glass-ceramics, effectively multi-phase composites. The process involves heat treatment at two different temperatures, firstly to induce nucleation, then to allow crystal growth of the nuclei. The crystalline phases formed depend on both the composition of the parent glass and the temperatures used for heat treatment. The extent of their formation and growth, the relative amounts and distributions of different phases (including residual glass) and their characteristics will determine the overall properties of the particular composite. The formation of these types of materials and their properties is outlined below. 

Crystallisation of glasses in the Nd-Mg-Si-O-N system to form glass-ceramic composites has been investigated. Morrissey et al. (1990) showed that heat treatment at a single temperature resulted in only a small increase in hardness for a 12 24 64 Nd Mg Si composition, but two-stage heat treatments resulted in a much higher increase. They found that the optimum nucleation temperature was related to the glass transition temperature of the materials (usually 7g... 

Since glasses are thermodynamically unstable, they will eventually crystallize to form glass-ceramics at high temperatures. It is therefore advantageous to control the crystallization process in order to adjust glass... 

Many other methods of strengthening are based on formation of composites by inclusion of fibers or whiskers or by crystallization to form glass-ceramics. Phase separation may also affect strength by altering crack propagation mechanisms. Transformation toughening has also been attained by formation of a small concentration of zirconia crystals in glasses. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

Glass-ceramics are a family of materials that are polycrystalline in nature and are formed from the liquid or glassy state. A glass-ceramic article is made by the heat treatment of a vitreous body in two stages ... 

Kokubo, T., Ito, S., Huang, Z.T., Hayashi, T., Sakka, S., Kitsugi, T. and Yamamuro, T. (1990) Ca, P-rich layer formed on high-strength bioactive glass-ceramic A-W. Journal of Biomedical Materials Research, 24, 331-343. 

---