MICROSCOPY FOR MATERIALS CHARACTERIZATION continued


diffraction technique in the TEM is convergent beam electron diffraction (CBED), which yields much more detailed information: the unit cell and associated lattice parameters, crystal system identification, and three-dimensional crystal symmetry, including the point group and space group. CBED and observation of interference fig- ures in the light microscope are both conoscopic techniques, utilizing converging beams (cones) of illumination.


Elemental and chemical identification and response


to conditions As noted above, elemental analysis is possible in electron optical instruments equipped with detectors for EDS. Interaction of primary beam electrons with core shell electrons in the atoms of the sample leads to emission of X-rays with energies characteristic of the elements pres- ent. A spectrum of peaks corresponding to specific X-ray energies is generated, and the peak intensities reflect relative elemental con- centrations. In the SEM and TEM, EDS data can be gathered from selected areas or features to determine composition associated with different textures, morphologies, or crystal- line phases. Areas analyzed may range from several micrometers to sub-nanometer in size.


In the light microscope, a variety of mi- crochemical tests and reactions to various conditions can assist in the identification of materials. Techniques include solubility tests, replacement reactions, spot tests, and recrystallizations. Observation of reactions to heat and flame can include burning, melting, sublimation, explosion, decomposition, loss of hydration, or no reaction. These and other techniques done on a micro-scale using the light microscope foreshadowed the develop- ment of in situ techniques in electron optical instruments such as the environmental SEM, or the TEM equipped with environmental holders or with modifications to the micro- scope that allow for viewing of samples in reactive environments.


Inclusion of techniques for compositional analysis strengthens the power of any mi- croscopy technique, confirming preliminary identifications based on imaging and crystal- lographic results. When observations are taken


to the next level and carried out in controlled environments that reflect conditions of use in the real world, an even better understanding is gained between a material’s structure and properties, with an improved ability to predict and control performance.


Quality of results and an


integrated approach Regardless of the type of microscope used, calibration, performance verification, and good maintenance practices are vital for en- suring quality of results. However, no matter how good the instrument or the analyst, the nature of the specimen will drive the charac- terization process and ultimately determine the quality of the results obtained. Sample preparation techniques will be more or less rigorous depending on the type of micros- copy used, but representative sampling and minimization of artifacts introduced by prepa- ration must be considered, regardless of the scale at which one is working. Also, the higher the magnification, the smaller the field of view, necessitating more time to examine enough areas to ensure that results are representative. While 15 minutes may be sufficient for identi- fication of a material using light microscopy, analysis times of at least an hour per sample, and often much longer, are common for TEM analysis. Because small volumes of material are examined by any microscopic technique, using microscopy at different scales and com- paring results with those obtained from bulk analyses should always be considered. This integrated approach will provide the most accurate and comprehensive insights into the nature of a material.


Summary Two of the most enjoyable aspects of any form


of microscopy are the interactive nature of the analysis and the fact that multiple techniques can be employed in a single instrument. Analysis in the microscope is a decision- making process that begins with some amount of knowledge about the material, and usually with some specific questions to be answered using microscopy. As the nature of the sample is revealed by examination of a suc- cession of areas and features, the preliminary analysis plan may change, and the analyst may


AMERICAN LABORATORY • 26 • SEPTEMBER 2014


Figure 4 – Polarized light microscope (Olympus BX51) and TEM (JEOL JEM-3010): more similar than they appear.


select from among the multitude of imaging, crystallographic, and compositional tech- niques that microscopy provides, to gather the most relevant and representative data.


This brief discussion of microscopes at two ends of the resolution scale does not even begin to touch on the vast array of special- ized microscopes and associated techniques available today for materials characterization. However, even this simple overview of simi- larities between the light microscope and the TEM, shown together in Figure 4, illustrates the wealth of information that can be gathered using microscopy, and the vital role it plays in characterization and development of today’s high-performance materials.


Elaine F. Schumacher is a Senior Research Scientist at McCrone Associates, Inc., and co-teaches a TEM course at Hooke College of Applied Sciences, LLC, 850 Pasquinelli Dr., Westmont, IL 60559, U.S.A.; tel.: 630-887-7100; e-mail: eschumacher@ mccrone.com; www.mccrone.com


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