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Sample preparation for electron probe microanalysis - Pushing the limits
No full textThere are two fundamental considerations in preparing samples for electron probe microanalysis (EPMA). The first one may seem obvious, but we often find it is overlooked. That is, the sample analyzed should be representative of the population from which it comes. The second is a direct result of the assumptions in the calculations used to convert x-ray intensity ratios, between the sample and standard, to concentrations. Samples originate from a wide range of sources. During their journey to being excited under the electron beam for the production of x rays there are many possibilities for sample alteration. Handling can contaminate samples by adding extraneous matter. In preparation, the various abrasives used in sizing the sample by sawing, grinding and polishing can embed themselves. The most accurate composition of a contaminated sample is, at best, not representative of the original sample; it is misleading. Our laboratory performs EPMA analysis on customer submitted samples and prepares over 250 different calibration standards including pure elements, compounds, alloys, glasses and minerals. This large variety of samples does not lend itself to mass production techniques, including automatic polishing. Our manual preparation techniques are designed individually for each sample. The use of automated preparation equipment does not lend itself to this environment, and is not included in this manuscript. The final step in quantitative electron probe microanalysis is the conversion of x-ray intensities ratios, known as the "k-ratios," to composition (in mass fraction or atomic percent) and/or film thickness. Of the many assumptions made in the ZAF (where these letters stand for atomic number, absorption and fluorescence) corrections the localized geometry between the sample and electron beam, or takeoff angle, must be accurately known. Small angular errors can lead to significant errors in the final results. The sample preparation technique then becomes very important, and, under certain conditions, may even be the limiting factor in the analytical uncertainty budget. This paper considers preparing samples to get known geometries. It will not address the analysis of samples with irregular, unprepared surfaces or unknown geometries
Copper oxide precipitates in NBS Standard Reference Material 482
No full textCopper oxide has been detected in the copper containing alloys of NBS Standard Reference Material (SRM) 482. This occurrence is significant because it represents heterogeneity within a standard reference material that was certified to be homogeneous on a micrometer scale. Oxide occurs as elliptically to spherically shaped precipitates whose size differs with alloy composition. The largest precipitates occur in the Au20-Cu80 alloy and range in size from submicrometer up to 2 micrometers in diameter. Precipitates are observed using light microscopy, electron microscopy, and secondary ion mass spectrometry (SIMS). SIMS has demonstrated that the precipitates are present within all the SRM 482 wires that contain copper. Only the pure gold wire is precipitate free. Initial results from the analysis of the Au20-Cu80 alloy indicate that the percentage of precipitates is less than 1 % by area. Electron probe microanalysis (EPMA) of large (2 micrometers) precipitates in this same alloy indicates that precipitates are detectable by EPMA and that their composition differs significantly from the certified alloy composition. The small size and low percentage of these oxide precipitates minimizes the impact that they have upon the intended use of this standard for electron probe microanalysis. Heterogeneity caused by these oxide precipitates may however preclude the use of this standard for automated EPMA analyses and other microanalysis techniques
John Todd
No full textJOHN TODD
Inducted: 2002
Citation:
For pioneering in the development of numerical analysis for computers and playing a key role in the development of some of the first large computers
Tenure: 1947-1957
Birth: 1911, Carnacally, Cy. Down, Northern Ireland
Death: 2007
Education:
Queen’s University (Belfast), BSc, 1931
(1st Class Honors in Maths and Mathematical Physics)
St. Johns College, Cambridge, 1931-33
Positions held:
Mathematician, Institute for Numerical Analysis
Chief, Computation Laboratory
Chief, Numerical Analysis Section, Applied Mathematics Division
Honors:
Emeritus Professor of Mathematics, California Institute of Technology, 1981
Fulbright Professor, University of Vienna, 1966
Memberships:
American Mathematical Society
British Association for the Advancement of Science
Mathematical Association of America, Governor, 1966
Mathematical Association of England
Society for Industrial and Applied Mathematics, National Lecturer
Publications:
Convergence of Cauchy-Riemann sums to Cauchy-Riemann integrals (with O. Szász), J. Research Nat. Bur. Standards, 47, (1951).
Table of Rational Arctangents, Applied Mathematics Series 11, NBS, 1951
Generation and testing of pseudo-random numbers (with O. Taussky- Todd), Symposium on Monte Carlo Methods, University of Florida, (1954.)
Evaluation of the exponential integral for large complex arguments,. J. Research Nat. Bur. Standards 52, (1954).
The Condition of Certain Matrices. II, Arch. Math. 5, (1954).
Motivation for Working in Numerical Analysis, Comm. Pure Appl. Math. 8, (1955)
A Direct Approach to the Problem of Stability in the Numerical Solution of Partial Differential Equations, Comm. Pure Appl. Math. 9 (1956)
The Prehistory and Early History of Computing at the NBS, in: A History of Scientific Computing, Stephen C. Nash, ed., Addison- Wesley, (1990)
NBS/INA-The Institute for Numerical Analysis at UCLA 1947-1954, (with M. R. Hestenes), NIST Spec. Pub. 730, (1991
