Ultramicrotomy of Powder Material for TEM/STEM StudyLIU-YINGWEI1* AND TONG LI21Department of Engineering Materials, Luleå University of Technology, 971 87 Luleå, Sweden2 Eric Erlandsson AB, Gårdsvagen 15, S-951 56 Luleå, Sweden

KEY WORDS industrial dust powder; epoxy impregnation; glass/diamond knife; ultramicrotom-ing; TEM/STEM

ABSTRACT This paper summarizes methods conventionally used to prepare thin foil samples ofpowder materials for transmission electron microscopy (TEM) and introduces another variant,ultramicrotomy, for the preparation of TEM samples of industrial dust powder. The choice ofultramicrotoming in the present work was based on two features of this technique: (1) it can producethin-sectioned specimens with a uniform thickness; (2) it can retain the original elementaldistribution in phases of the sample during sectioning. Dust powder preparation and the sectioningprocedure are described in this paper. The results of the method are illustrated by examples ofTEM/STEMmicrographs of industrial dust.Microsc. Res. Tech. 36:380–381, 1997. r 1997 Wiley-Liss, Inc.

INTRODUCTIONThere is an increasing interest in the TEM study of

powder materials, e.g., ceramic powder, metallic pow-der, industrial dust particles, and dustfall (solid par-ticles in the atmosphere). Although this is an importantarea of research, there are few TEM investigationsreported in the literature. One of the reasons is thedifficulty in TEM specimen preparation of small par-ticles. TEM specimens need to be sufficiently thin to betransparent to the electron beam. They should beparallel-sided, and the preparation methods should notintroduce any artefacts.Several methods have been reported as techniques

for TEM specimen preparation of powder materials.For very small particles that do not require thinning, itis common to place a monolayer of the powders ontocarbon film supported on a 3-mm grid, so that the fineparticles can be studied directly by TEM. For mostpowder material problems, however, the particles arelarge and not electron transparent. The particles mustbe thinned, and there are different methods that can beused for this purpose. For example, the particles can bedeposited onto a layer of electrically conductive sub-strate first. The particles are then plated with nickeluntil the particles are covered completely. After remov-ing the substrate layer, conventional electrochemicalpolishing (formetallic powder or other conductivemate-rials) or ion beam thinning methods can be used toobtain a thin foil sample (Field and Fraser, 1978).Gatan (Alani, 1994) has developed a newmethod of thinfoil preparation for powders, fibers, and other smallobjects. The powder is first mixed with epoxy, and thenthe mixture is transferred into a metal tube with a3-mm diameter. After curing for 10 minutes at 130°C,the tube can be sliced into discs of 250–400 µm inthickness. Finally, the discs will be ground, dimpled,and ion beam thinned.Acompactingmethod (Kaufman,1988) can also be used for powder materials. The disk ofcompacted powder can be thinned by using a standardmethod such as electropolishing.Ultramicrotomy or diamond/glass knife sectioning

has been the main technique for TEM specimen prepa-

ration in the life sciences. However, material scientistshave increasingly begun to consider ultramicrotomy asa complimentary technique for TEM specimen prepara-tion. In our laboratory, ultramicrotoming has beenfound to be an efficient and effective technique toproduce thin foil samples of powder materials for TEMstudies.An ultramicrotome is a slicing instrument that can

be applied to many types of samples. For powdermaterials, the particles need to be embedded in epoxyresin before sectioning, in order to provide supportduring cutting. The method of epoxy impregnation incombination with ultramicrotoming can produce thinsection specimens with a uniform thickness, controlledby a thermal advance system. During cutting, theoriginal elemental distributions of the phases of thesample are generally retained. These two features, i.e.,uniform thickness and retained elemental distribution,are very important for analytical work on materialscontaining phases with widely differing chemistry andfor the requirements of accurate chemical microanaly-sis.TEM/STEM investigation of industrial dust has been

carried out in our laboratory. The dust is in the form offine powder and is usually a complicated mixture ofdifferent compounds with a wide range of particle sizedistributions. Direct examination of the tiny particlesin the TEM was not possible because they were gener-ally too thick to be electron transparent. In the investi-gation, an ultramicrotoming technique was developedas described below and a compositional and structuralcharacterization of individual dust particles was per-formed on the microtomed specimens.

EXPERIMENTAL PROCEDURESThe powder was infiltrated with Spurr low-viscosity

epoxy resin which is an embedding medium suitable for

*Correspondence to: Liu-Ying Wei, Dept. of Engineering Materials, LuleåUniversity of Technology, S-971 87 Luleå, Sweden.Received 1 December 1994; Accepted in revised form 16 March 1995.

MICROSCOPY RESEARCH AND TECHNIQUE 36:380–381 (1997)

r 1997 WILEY-LISS, INC.

TEM samples (Spurr, 1969). Before the infiltration withresin, the dust samples were first dehydrated in anoven at 100°C for 12 hours, since Spurr epoxy is notmiscible with water. The mixture was poured into amould and stirred in order to obtain a reasonablyuniform distribution of particles. The resin in themould was polymerised at 60°C for 12 hours to give asolid rigid block. The end of the block had a truncatedpyramid shape with a square top of about 1-mmedge-length. Sectioning was performed using an LKB2088 Ultramicrotome with freshly made glass knives ata speed of 1 mm.s21. Knives with different angles, e.g.,35°, 45°, and 55°, were made and assessed for thisapplication. It was found that the knife with a 45° angleresulted in the best performance. For the sections withparticles infiltrated with large areas of resin, thethickness could be estimated according to the interfer-ence colour between the resin and the water. Feeding ofthe specimen had to be adjusted very carefully until agolden colour was obtained, indicating that the thick-ness was in the range of 90–150 nm (Robinson et al.,1987). As shown in Figure 1, sections were cut and

floated onto water in the groove of the knife, and werethen collected on 400 mesh copper grids. The specimenswere examined in a JEOL 2000 EX TEM/STEM instru-ment with an attached LinkAN 10000 EDX system.

RESULTS AND DISCUSSIONUltramicrotoming of metallurgical dusts was per-

formed successfully, and made it possible to study thecrystallography and chemical composition of individualparticles. The particles shown in Figure 2 were micro-tomed uniformly and were sufficiently thin for electrondiffraction work. This dust powder contained a largeamount of zinc oxide, but also some phases containingtrace amounts of elements (e.g., ,0.05%) of Cl andother elements. The recycling processes for this particu-lar dust are very sensitive to such trace elements,therefore identification of phases containing them isvery important in order to choose methods for theirremoval. The identification of suchminor phases is verydifficult and TEM/STEM proved to be very effective inthis respect. Almost all particles present in the speci-menwere examined. The Cl-containing phase, as shownin Figure 3, was at last found and was identified byusing both SAED and EDX techniques.

REFERENCESAlani, R. (1994) Gatan Technical Note 9203 and (1994) TEM samplepreparation of materials by dimpling and inert ion milling/chemicalion milling. Report No. R-209, Linkoping University, S-581 83Linkoping, Sweden, p. 35.

Field, R.D., and Fraser, H.L. (1978) Microstructure observation atmetal powders using analytical electron microscopy. Metall. Trans.,9A:131–134.

Kaufman, M.J. (1988) Analytical electron microscopy of fine powders.J. Met., 40:15–17.

Robinson, D.G., Ehlers, U., Herken, R., Herrmann, B., Mayer, F., andSchurmann, F.-W. (1987) Method of preparation for electron micros-copy. In:An Introduction for the Biomedical Sciences. Spring-Verlag,NewYork, pp. 43–76.

Spurr, A.R. (1969) A low viscosity embedding medium for electronmicroscopy. J. Ultrastruct. Res., 26:31–43.

Fig. 1. The principle of ultramicrotomy. (From an LKB informationbrochure.) The specimen block is moved steadily to pass a fixed knife ofglass or diamond. The thin sections are cut and then floated onto waterin the groove of the knife.

Fig. 2. a: A TEM micrograph of particles in dust powder; micro-tomed section through dust particles infiltrated with Spurr’s epoxyresin. b: SAED pattern taken from a zinc oxide particle marked withan arrow in c, indicating the [010] zone axis.

Fig. 3. STEM micrograph of particles in dust powder; particleswith lower contrast and marked with arrows are the Cl-containingphase.

381ULTRAMICROTOMY OF POWDER MATERIAL