Isotope diffusion, SIMS

A shortcoming of both NRA and SIMS is that the spatial resolution is not high in all three directions. While depth resolutions on the order of a few hundred nanometers or less are routine, the profiles are measured over surface areas of a few hundred square micrometers or even millimeters. Thus, the concentration profile can be an average from various localized reaction or fast diffusion (e.g. pipe diffusion). The consequences of dual-diffusion mechanisms on the geometry of isotopic profiles is discussed in more detail below. There is no available method to by pass these problems for isotopic diffusion, although for elemental diffusion, alytical Transmission Electron Microscopy (ATEM) offers an alternative (Meissner et al. 1997). 

Fig. 4.31 (a) AES composition depth profile (b) SIMS isotope diffusion profile and (c) schematic layered oxide structure for a NisAl alloy oxidized in 02 and then 02 environment at 600 °C. Vertical dashed lines indicate oxide interfaces and vertical dotted lines indicate interfaces between isotope oxides. Reprinted with permission from Haasch RT, Venezia AM, Loxton, CM (1992) J Mater Res 7 1341-49. Copyright 1992, Materials Research Society... 

Layne GD (2003) Advantages of secondary ion mass spectrometry (SIMS) for stable isotope microanalysis of the trace light elements. EOS Trans, Am Geophys Union 84 F1635 Lundstrom CC, Chaussidon M, Kelemen P (2001) A Li isotope profile in a dunite to Iherzolite transed within the Trinity Ophiolite evidence for isotopic fractionation by diffusion. EOS Trans, Am Geophys Union 82 991... 

In recent years, there have been several attempts to determine diffusion coefficients, mostly utilizing secondary ion mass spectrometry (SIMS), where isotope compositions have been measured as a function of depth below a crystal surface after exposing the crystal to solutions or gases greatly enriched with the heavy isotopic species. 

Milke R., Wiedenbeck M., and Heinrich W. (2001) Crain boundary diffusion of Si, Mg, and O in enstatite reaction rims a SIMS study using isotopically doped reactants. Contrib. Mineral. Petrol. 142, 15-26. 

Both the ability to detect hydrogen and the ability of SIMS to differentiate between isotopes were exploited in a study of deuterium diffusion in hydrogenated amorphous Si (14). Layered samples of hydrogenated and deuterated films were depth profiled before and after various heat treatments. The diffusion coefficient for deuterium obtained from these experiments implied that degradation of these films due to hydrogen out-di fusion at 100 C would not be significant until after more than 10 years. 

Imaging of elemental and isotopic distributions on surfaces has proved useful for diffusion measurements, corrosion and reactivity studies, contamination identification, and many other applications. SIMS imaging is particularly useful because of its high sensitivity. An example was the application to a problem in which a U-... 

SIMS has become one of the most important tools for the characterization of experimental products because of its minimal sample requirements, high spatial resolution, excellent sensitivity, and unsurpassed ability for depth-profile measurements. Most of the experimental work can be split into two different areas. The first consists of studies examining diffusion rates of different elements in minerals or melts under a variety of pressure, temperature, and fluid conditions, typically by using an isotopically enriched tracer. These analyses are done either by cutting a surface parallel to the diffusion direction and taking a traverse of spot analyses (for conditions in which profiles in the tens to hundreds of micrometers are expected) or by depth-profiling in from the mineral surface to depths of as much as 5-10 micrometers. In the latter mode, depth resolution on the tens of nanometer scale is possible (see Chapter 4). The second area is focused on determining partition coefficients for trace elements between different minerals and fluids/melts at specific temperatures, pressures, and fluid conditions, to provide the data needed to interpret trace element contents measured in natural minerals. This type of analysis typically involves spot analysis of mineral run products. 

In this review the various modes of SIMS and examples of their applications are discussed. SIMS depth profiles are widely used to study dopant profiles and Intermetallic diffusion. The extreme surface sensitivity and low concentration detection limits of SIMS make It useful for Investigation of substrate and metallization cleaning processes. SIMS elemental Imaging Is also used In contamination studies. The ability of SIMS to provide Isotopic Information has allowed elegant mechanistic studies. The Identification and determination of the relative abundance of various molecular or elemental species by SIMS Is applicable to the development characterization and understanding of microelectronic processing. The capability of SIMS In the area of quantitative analyses Is also discussed. 

In order to gain a direct measurement of the oxide ion diffusion and surface exchange coefficients in ceramic materials an isotopic exchange and SIMS analysis technique has to be used. This technique is detailed elsewhere [9] and has been successfully used for a number of SOFC component materials, most notably with electrolytes [10,11] and cathodes... 

Applications of ISS are similar to the applications of static SIMS, described subsequently. ISS is used to study surface reaction mechanisms, catalyst behavior, and adsorption-desorption processes at surfaces. Because of its ability to discriminate among isotopes of an element, ISS can be used to study diffusion or any other reactions involving the replacement of one isotope for another. 

Isotope exchange diffusion profiles can also be measured ex situ by SIMS, and can, in principle, reveal surface kinetics in addition to bulk transport... 

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