554

Nuclear Instruments and Methods in Physics Research A238 (1985) 554-557North-Holland, Amsterdam

POWDER SPECIMEN PREPARATION FOR TRANSMISSION EXAFS MEASUREMENTS

Joe WONGGeneral Electric Corporate Research and Development, P.O. Box 8, Schenectady, NY 12301, USA

Received 13 December 1984 and in revised form 14 March 1985

A simple, reproducible method for preparing permanent films of powder specimens for transmission extended X-ray absorptionfine structure (EXAFS) measurments is described. Film microstructure of some selected powder materials were determined by opticalmicroscopy, and their EXAFS spectra were compared with those of the corresponding foils, whenever possible.

1. Introduction

X-ray absorption spectroscopy using both the EX-AFS effect [1] and near-edge structure [2] has now beenrecognized to be a powerful, and sometimes unique, toolin studying the structure and bonding of constituentatoms in a material. For bulk specimens, the spectra aremost conveniently obtained with the transmission tech-nique. The technique consists of measuring the incidentand transmitted intensities, Io and I, of an X-ray beamthrough a sample of thickness t, and the resultantexperimental spectrum is plotted as ln(I0/I) vs energy.For K-edge measurements of pure metals, absorptionthicknesses varying from acouple of micrometers for Tito - 20 Itm for Pd are required [3] . For compounds,such as metal oxides, sample thickness can be larger dueto a lower effective density of the X-ray absorbing metalatoms.

Ideally, metal foils or thin films of uniform thicknessshould be used for such transmission measurements .However, not all metals are ductile and thin film pre-paration by either polishing a thick slab or vapor de-position are usually difficult, if not impossible . Conse-quently, most of the transmission X-ray measurementson bulk solid materials reported to date are made onpowder specimens prepared in a variety of ways [4] .

In this paper, we report a simple and reproduciblemethod of preparing permanent films of powder speci-mens cast in Ducô cement [5]. The microstructure ofthese Ducô films have been examined using opticalmicroscopy and their EXAFS spectra measured andcompared with those of the corresponding foil, wheneverpossible .

2. Method

The starting powder must be ground down to at least-400 mesh . (i) About 0.1 g of the fine powder is placed

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on a 2" X 3" microscope slide and about 2 g of Ducôcement is added to cover the powder. A small stainlesssteel spatula is used to disperse the powder in theDucô cement to form a homogeneous mull. The mixingshould take only 1 to 2min. (ii) Ablob of the well-mixed"putty-like" mull is now quickly transferred to a new2" X 3" microscope slide at about 1" from the short

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edge. A second clean 2" x 3" slide is now applied ontop of the transferred blob and squeezed gently, spread-ing the blob to form a circular disk as shown in fig . 1(a) .A light box may be used to visually gauge uniformityand thickness required . (iii) The configuration in fig.1(a) is now glided apart (fig . 1(b)), using the indexfinger and thumb of one hand to hold still and tomaintain a constant gap between the slides while glidingthe other slide with the other hand . This operationresults in the preparation of two films of the samesamples shown in fig. 1(c) . Steps (ii) and (iii) aboveshould be completed within 1 min (since Ducô cementtakes about 5 min to set in air) . The configuration in fig.1(c) is allowed to dry in air for 30 min. The cast film

J. Wong /Powder specimen preparation

can then be removed from the glass slide with a 'sharprazor blade.

3. Results and discussion

555

To illustrate the nature of the Ducô films preparedwith this method, we have selected the following powdersand determined their microstructure using optical mi-croscopy : (a) amorphous Fe75 Si 15B1o particles, 10-20Am in size and produced by a spark erosion technique[6] ; (b) -400 mesh Ni 2B crystalline powder ; (c) -400mesh Ge powder which was subjected to a furtherjet-milling, and (d) -400 mesh V205 powder . The

Fig. 2 . Cross sectional optical micrographs of cast Duco® films of (a) spark eroded glassy Fe 75 Si15B1o particles, 10-20 Am indiameter ; (b) glassy Fe75Si15B1o ribbon by chill block melt-spinning ; (c) -400 mesh Ni2B powder; (d) jet-milled -400 mesh Gepowder ; (e) -400 mesh V205 powder; and (f) pin-hole detected in another V205 cast film: this micrograph was taken normal to thefilm surface.

556

J. Wong / Powder specimen preparation

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Fig . 3 . Experimental EXAFS scans of (a) 10-20 pin spark-eroded Fe75 Si 15 010 particles cast in Ducô cement ; (b) 12 .5 pm thickFe 75 Si 15010 ribbon ; (c) -400 mesh N' 2 B powder cast in Ducô ; (d) 10 pm Ni2 B foil crystallized from the glass of the samecomposition ; (e) jet-milled -400 mesh Ge powder cast in Duco®; and (f) -400 mesh Vz05 powder cast in Ducô .

transmission EXAFS spectra were recorded with theEXAFS 1-5 spectrometer at SSRL (Stanford synchro-tron radiation laboratory) during a parasitic run ofSPEAR(Stanford positron and electron accelerator ring)operating with an electron energy of 2.4 GeV and storedcurrent of - 15 mA. The synchrotron X-ray beam fromSPEAR was monochromatized with a channel-cutSi(220) crystal. The spectrometer was calibrated wühthe corresponding pure metals. The Berkeley software[7] was used to collect the EXAFS data. All spectrawere recorded at room temperature .

The cross sectional micrograph of the spark erodedFe75 Si 15B1O glassy particles is shown in fig . 2(a) . Theparticles are spherical in shape and have a size ranging10 to 20 jAm in diameter . Dark holes within Ducecement band are due to pullouts during polishing . Aribbon of the same material made by chill block melt-spinning [81 is also shown in fig. 2(b) . The respectiveEXAFS scans are shown in figs. 3(a) and 3(b) .

The cross sectional micrograph of -400 mesh N'2Bcrystalline powder is shown in fig. 2(c) . The particles arequite irregular in shape due to the crushing in a tungs-ten carbide shatter box [9] . The transmission spectrumis shown in fig. 3(c) and is comparable to that of aribbon crystallized from a glassy ribbon of the samecomposition shown in fig . 3(d).

Like Ni 2B, the Ge particles are also irregular inshape (fig 2(d)), but have a finer particle size distribu-tion due to an extra jet-milling step . The EXAFS spec-trum shown in fig. 3(e) compares well with those re-ported in the literature [1,101 and exhibits a characteris-tic intense white line above the absorption edge.

Finally, the cross sectional micrograph of a -400mesh V205 film is shown in fig. 2(e) . The correspondingEXAFS and near-edge spectrum is shown in fig . 3(f)and exhibits a strong is - 3d pre-edge peak at 5.6 eVcharacteristic of V5+ in a square pyramidal coordina-tion [111 .A major disadvantage of this method is the occur-

rence of pin holes in the cast film shown in fig . 2(f) .These pin holes usually arise from gas bubbles trappedin the Duce cement and act as leakage points for theincident beam in a transmission measurement, which inturn will lead to amplitude distortion of the EXAFSsignal [121 . Hence care must be taken to produce pinhole free films for quantitative measurement of theamplitude function. Pin holes may usually be eliminatedby more thorough mixing of the powder and Ducecement using acetone added to lower the viscosity.Powder aggregation such as that of ZnO and Ti02 willalso result in streaking in the film during the glidingprocedure. Ultrasonically dispersing the powder in

J. Wong / Powder specimenpreparation

acetone and air-drying before mixing with Duce ce-ment will help to obviate this problem. When pin holesare present, the spectrum can at best be regarded asqualitative .

The advantage of this method is obvious. It is simpleand quick. The cast film is permanent and can beretained as a reference or to be cross checked by adifferent worker in another laboratory. In addition,costly materials can be recovered or re-cast simply bydissolving the film in acetone. It is usually advisable toprepare a range of thicknesses of the same sample, sothat thickness effect if suspected may be ascertainedexperimentally.

Finally, other dispersive media such as Elmer's orFranklin glue and epoxy may be used, depending on thechemical nature of the powder specimen. In general,there should be no chemical reaction between the powderspecimen and the film material. For example, stronglyoxidizing materials like KMnO4 and Cr03 cannot becast into films with Duce cement .

Discussions with F.W . Lytle in the course of thiswork and experimental opportunity at SSRL, which issupported by the US Department of Energy are grate-fully appreciated . The optical micrographs were takenwith the able assistance of C.R. Rodd, and the spark-eroded specimen was supplied by A.E. Berkowitz.

References

P.A . Lee, P.H . Citrin, P. Eisenberger and R.M . Kincaid,Rev. Mod. Rhys. 53 (1981) 769.

[2] Articles in ; EXAFS and Near-Edge Structure vol . 27, ed.,A. Bianconi, L. Incoccia and S. Stipaich (Springer, Heidel-berg, Berlin, G6ttingen, 1983) .W.H. McMaster, N. Nerr del Grande, J.H. Hallet and J.H .Hubbell, Compilation of X-ray Cross Sections, LawrenceRadiation Laboratory UCRL-50/74, sect . 2, rev. (1969) .

[41 G.B . Bunker, PhD thesis, University of Washington (1984)unpublished.A trademark of the DuPont Company.A.E . Berkowitz, J.L . Walter and K.F . Wall, Phys . Rev.Lett . 46 (1981) 1484 .J.A . Kirby, Manual for Data Collection Program (Univer-sity of California, Berkeley, 1978) unpublished.

[81 H.H . Liebermann, Mater. Sci. Eng. 43 (1980) 203.[9] Manufactured by Spex Industries, Inc.

[10] F.W . Lytle, D.E . Sayers and E.A . Stern, Phys. Rev. Bll(1975) 4825 .

[11] J. Wong, F.W . Lytle, R.P . Messmer and D.H. Maylotte,Phys . Rev. B30 (1984) 5596 .

[121 E.A . Stem and K. Kim, Phys. Rev. B23 (1981) 3781 .

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