tailieunhanh - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 1 Part 10

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 218 13 Direct Diffusion Studies The best way to determine the resulting concentration-depth profile is serial sectioning of the sample and subsequent determination of the amount of tracer per section. To understand sectioning the reader should think in terms of isoconcentration contours. For lattice diffusion these are parallel to the original surface on which the thin layer is deposited and perpendicular to the diffusion direction. The most important criterion of sectioning is the parallelness of sections to the isococentration contours. For radioactive tracers the specific activity per section A x is proportional to the tracer concentration A x kC x . Here k is a constant which depends on the nature and energy of the nuclear radiation and on the efficiency of the counting device. The specific activity is obtained from the section mass and the count rate. The latter can be measured in nuclear counting facilities such as Y- or -counting devices. Usually the count-rate must be corrected for the background count-rate of the counting device. For short-lived radioisotopes half-life corrections are also necessary. According to Eq. a diagram of the logarithm of the specific activity versus the penetration distance squared is linear. From its slope 4Dt -1 and the diffusion time the tracer diffusivity D is obtained. In an ordinary thin-layer sectioning experiment one wishes to measure diffusion over a drop of about three orders of magnitude in concentration. About twenty sections suffice to define a penetration profile. The section thickness Ax required to get a concentration decrease of three orders of magnitude over 20 sections is Ax tt yfDt . Thicker sections should be avoided for the following reason in a diffusion penetration profile the average concentrations specific activities per section are plotted versus the position of the distance of the center of each section from the surface. Errors caused by this procedure are only negligible if the sections .