Glasses alteration layers

A second set of examples deals with the analysis of near-surface regions of glasses which normally have so-called altered or leached layers. The altered layer is found for soda-lime glasses and for many glasses used for optical applications. The chan-... 

Fig. 4.52. SIMS and IBSCA depth profiles of the altered layer region of a lithium aluminosilicate (LAS) glass ceramic (conditions SkeVAr" ).

Fig. 2. Alteration layers observed on the surface of basaltic and nuclear glasses, comprising a porous zone and phyllosilicates (a) tholeiitic basalt glass altered 120 days at 50 C (Crovisier et al. 1987) (b) MW nuclear glass altered 5.7 years at 90 CC (Curti et al. in preparation).

Abdelouas, A., Crovisier, J. L., Lutze, W., Muller, R. Bernotat, W. 1995. Structure and chemical properties of surface layers developed on R7T7 simulated nuclear waste glass altered in brine at 190°C. European Journal of Mineralogy, 7, 1101-1113. 

Clark, D. E., Schulz, R. L., Wicks, G. G. Lodihng, A. R. 1994. Waste glass alteration processes, surface layer evolution and rate limiting steps. In Murakami, T. Ewing, R. C. (eds)... 

Feng, X., Cunnane, J. C. Bates, J. K. 1994. A literature review of surface alteration layer effects on waste glass behavior. Ceramics Transactions, 39, 341-352. 

Murakami, T., Banba, T., Jercinovic, M. J. Ewing, R. C. 1989. Foimation and evolution of alteration layers on borosilicate and basalt glasses Initial stage. In Lutze, W. Ewing, R. C. (eds) Scientific Basis for Nuclear Waste Management XII. Materials Research Society Symposia Proceedings, 127, 65-72. 

The surfaces of multicomponent glasses are significantly altered when exposed to aqueous solutions the extent of the alteration layer is dependent on a variety of parameters, including the glass composition, the duration of exposure, the temperature and pH of the solution, and the ratio of the glass surface area to the solution volume. This section is not intended to be a discussion of the aqueous corrosion of glass rather, several different studies are mentioned to provide an overview of the analytical tools that characterize such interactions. 

As far as sodic or potassic glasses are concerned, either in an aqueous environment or hidden in the ground, it is well known that the initial composition of glass is the essential factor with regard to the alteration, to the evolution the alteration layer (thickness, structure, global chemical composition, etc.), and to the role of certain chemical elements such as Mg, Ca, P, or Si in the development of this layer. 

It is beyond the scope of the present chapter to enter into the details of all the available investigation methods, the results, and the conclusions. However, one must keep in mind that here, again, much research remains to be done after the pioneering and founding work of Macquet [43], who has been able to elaborate a model which accounts for the development of the alteration layer under various physicochemical circumstances. Macquet was also able to elaborate a systematic scientific approach of glass restoration for different kinds of glasses and different kinds of environmental aggressions, Her results are remarkable, as illustrated in Fig. 41. 

Depth profiling glasses with ion beam techniques is also difficult due to surface charging problems. Pulsed ion beams in ToF-SIMS with relatively long interpulse periods have been used successfully, however. ARXPS can be used to study variation in composition and chemical states over the outer 10 nm. Nuclear techniques, such as PIXE, NRA, RBS and ERDA, are particularly useful in studying the composition of deeper lying altered layers to depths of pm. The latter techniques generally have depth resolutions of the order of 10 nm which is sufficient in most cases [93]. 

The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Electron Beam Lithography. LB PMMA films with thicknesses of 6.3 nm (7 layers) are sufficient for patterning a Cr film suitable for photomask fabrication. For ultrathin PMMA films the resolution (see Fig. 1) is limited by the smallest spot diameter available on MEBES I (1/8 pm). However, it is not possible to obtain this resolution if a thicker resist (>100 nm) is used under the same exposure and development conditions, which demonstrates that ultrathin resists are able to minimize the proximity effect. Also, since the radius of gyration of 188,100 Mw PMMA is about 10 nm in the bulk, and the thickness of the 7 layer film (6.3 nm) is less than 10 nm, it is reasonable to assume there must be an alteration of chain configuration in the ultrathin films. This will be particularly true when the post-deposition baking temperature of the multilayer films is less than the glass transition temperature (115°C), as is the case for the present experiments. In such a case, interdiffusion of PMMA chains between the deposited layers may not result in chain configurations characteristic of the bulk. 

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