A vertical experimental set-up for simultaneous recording transmission and scattering Mössbauer spectra is described. Application of the apparatus for the study of model powder mixture of two components with different grain sizes shows differences in the relative intensity ratio of these fractions in absorption and conversion electron Mössbauer spectroscopy (CEMS) spectra. This effect, explained as a result of the size selectivity of CEMS technique, is parameterized by the grain radius ratio of components. The possible application for investigation of isolated surface nano- and microobjects is discussed

Fornal, Piotr

Stanek, Jan

Nukleonika

English

Jagiellonian Univeristy Repository

NUKLEONIKA 2007;52(Supplement 1):S13−S16 PROCEEDINGSIntroductionTransmission Mössbauer spectroscopy applied for thestudy of a nano- and microobject usually deals with bulkaggregations of interacting particles. However, recentlya new field of nanotechnology, related to the thin filmsas matrix for individual nanoparticles, such as ferromag-netic and paramagnetic precipitates in diamagneticlayers [2, 4] or hard carbides crystallites in mechanicallyprotecting coatings [3], became especially interesting.For studying such problems, the CEMS techniqueseems to be more adequate or, sometimes, the onlypossible.In our previous paper [1], it has been pointed outthat CEMS is not only surface sensitive but also a sizeselective method, enhancing contribution from smallerparticles. Now, we show that the comparison oftransmission and CEMS spectra of powder samplescontaining two different fractions of different diametersmay supply new information on the grain sizes. If onlyCEMS spectra are available, the concentration ofdifferent fractions cannot be determined from the areaunder the corresponding subspectra.ApparatusThe reproducible preparation of a mixed powdersample for CEMS measurements is complicatedbecause of segregation of fractions due to electrostaticand magnetic properties, differences in grain sizes andspecific weights. These problems are especially criticalfor samples placed vertically when horizontally orienteddrives are used. Thus, we constructed a vertical set-upSimultaneous transmissionand scattering Mössbauer measurementsof powder samplesPiotr Fornal,Jan StanekP. FornalInstitute of Physics,Cracow University of Technology,1 Podchorążych Str., 30-084 Kraków, Poland,Tel.: +48 12 6370666 ext. 29,E-mail: pufornal@kinga.cyf-kr.edu.plJ. StanekThe Marian Smoluchowski Institute of Physics,Jagiellonian University,4 Reymonta Str., 30-059 Kraków, PolandReceived: 20 June 2006Accepted: 5 September 2006Abstract. A vertical experimental set-up for simultaneous recording transmission and scattering Mössbauer spectra isdescribed. Application of the apparatus for the study of model powder mixture of two components with different grainsizes shows differences in the relative intensity ratio of these fractions in absorption and conversion electron Mössbauerspectroscopy (CEMS) spectra. This effect, explained as a result of the size selectivity of CEMS technique, is parameterizedby the grain radius ratio of components. The possible application for investigation of isolated surface nano- andmicroobjects is discussed.Key words: methodology of Mössbauer spectroscopy •  CEMS •  micro-powdersS14 P. Fornal, J. Stanekfor simultaneous registration of CEMS and transmissionspectra, see Fig. 1. Two special aspects of the systemshould be underlined. (a) The system has a modulestructure with fixed geometry. The exchange of thesample, attached to the CEMS counter, does not requiredemounting of the system and removing the sourcewhich is feed through a central tube of the drive. (b)The CEMS proportional, gas-flow counter is an originalconstruction. The sample is negatively polarized(~ −1400 V) and the signal is taken from tungsten wiresat a virtually zero potential directly to the operatio-nal amplifier. Such a solution reduces the electricalnoises and improves the energy resolution. In conse-quence, the counter becomes sensitive to the low energyelectrons emitted from the deeper layers of the samples[2] and may be applied for qualitative depth selectiveMössbauer spectroscopy.ExperimentalThe composite samples of a mixture of siftedFeSO4· 4H2O powders (with radii R = 0.043(4),0.034(2), 0.025(4), 0.015(4) mm; the numbers inbrackets are radius distributions) and a Fe2O3 powderwith grains of radius r = ~0.02 mm were prepared byvarious methods. At first, the mixed powder was simplydistributed or glued to an Al foil; later, a single layer ofcrystallites was placed on an adhesive tape. The CEMSand transmission spectra were recorded simultaneouslyat room temperature.ResultsThe quadrupole Fe2+ doublet of FeSO4·4H2O andZeeman sextet of Fe3+ in Fe2O3 are clearly distinguishedin Mössbauer spectra, see Fig. 2. The intensity ratio ofthese two subspectra, determined from transmissionmeasurement, is proportional to the amount of iron inthese two compounds. It differs from the correspondingintensity ratio obtained from CEMS spectra; the diffe-rence depends on the ratio of grain radii and thecomposition of the specimen. For thick samples madeby simple mixing, such differences were poorly repro-ducible, being influenced by the process of samplepreparation. On the contrary, the results for the samplesconsisting of single layer of mixed FeSO4·4H2O andFe2O3 on the scotch tape exhibited a clear trend. The Tto C ratio, where T and C are the Fe2+/Fe3+ intensityratios in transmission (ITR/ITr) and CEMS spectra(ICR/ICr), respectively, is a linear function of theFeSO4·4H2O grain radius, R, see Fig. 3.DiscussionThe results obtained for thick samples were notreproducible, which reflects the fact that the process ofthe sample preparation was not fully controlled. Probably,in these samples big and small grains overlappeddifferently, being more or less exposed to CEMS.Moreover, in this case, the transmission spectra maybe significantly influenced by the thickness effect; theestimated effective thickness of a single FeSO4·4H2Ograin with R = 0.04 mm is about 1. In any case,comparison of the transmission and CEMS spectra maygive same information on the morphology and grainsegregation in granular composites. In case of a singlelayer of isolated grains, the linear T/C dependence onthe R radius may be explained in simple terms.The intensity ratio of Mössbauer lines of the twocomponents, in the form of spherical particles with radiiR and r in transmission experiment, T, and CEMSspectrum, C, are:(1)(2)where N are the number of particles per cm2, ρ, g and fare the density, mass concentration of iron and recoilfree fraction, respectively, and λ is the thickness of thesurface layer sensitive to CEMS technique. Subscriptsr and R refer to the components with grain radius r andR, respectively. Here, the influence of thickness effecton the intensity of transmission spectra was neglected.Consequently, for the T/C ratio one obtains:Fig. 1. A scheme of the experimental set-up. (a) – Drive invertical position, (b) – source, (c) – source fixing point, (d) –collimator, (e) – CEMS counter module, (f) – sample holdermodule, (g) – proportional counter, (i) – gas-in, (k) – anodewires at zero potential, 4 mm from the sample, (l) – o-ring,(m) – sample at high voltage potential (~ –1440 V).334343R R R RTRTr r r r rR g f NITI r g f Nπ ρ= =π ρ22CR R R R R RCr r r r r rI R g f NCI r g f Nπ ρ λ= =π ρ λS15Simultaneous transmission and scattering Mössbauer measurements of powder samplesFig. 2. Transmission (left) and CEMS (right) spectra of the FeSO4 and Fe2O3 powder mixture. The shadowed subspectracorrespond to the FeSO4 fraction with different grain radius: (a) – 0.015(4) mm, (b) – 0.025(4) mm, (c) – 0.034(2) mm,(d) – 0.043(4) mm.               -10                 0                 10       -10                   0                 10                                                Velocity (m/s)   100.0     99.5   100.0    99.6   100.0    99.6   100.0    99.0(d)(c) (b)(a)S16 P. Fornal, J. Stanek(3) T/C = (1/r) ⋅RThis result shows that the difference in the intensityratio of any two powder components in the transmissionand CEMS spectra may be explained by the differencein the grain of radii. Our data confirmed fully thesecalculations: the solid line in Fig. 3 is a linear fit of T/Cin function of R; its slope determines the radius of Fe2O3equal to 0.020(5) mm, in agreement with the observa-tion under optical microscope.ConclussionsThe described vertical set-up makes possible thesimultaneous recording of the transmission and CEMSMössbauer spectra, which may eliminate artifactsrelated to the sample preparation. Such experimentsmay lead to the determination of the radius ratio ofany two types of isolated in surface layers nano- ormicroobjects. It should be underlined, however, thatthe derived formula applies strictly only if: (a) the grainsdo not overlap, and (b) the radius of grains is biggerthan the penetration depths (~100 nm) of 7.3 keVconversion electrons. In any case, the presented resultand discussion clearly show that when determining thecomposition of powder composites from CEMS spectra,one should be aware of the fact that this technique is“size selective”.References  1. Fornal P, Stanek J, Średniawa B, Zach R, Fruchart D(2003) Local magnetic properties of (Fe0.975Ni0.025)2Ppowders. In: Görlich EA, Królas K, Pędziwiatr AT (eds)Proc on XXXVIII Zakopane School of Physics, Zakopane,Poland, 14−19 May 2003. Institute of Physics, JagiellonianUniversity, Kraków, pp 187−193  2. Stanek J, Rajchel B, Fedotova J, Fornal P, Binczycka H(1998) Interactions of iron with TiN and DLC coatings.Hyperfine Interact 112:55−58  3. Weissmantel S, Reisse G, Rost D (2004) Preparation ofsuper-hard amorphous carbon films with low internalstress. Surf Coat Technol 268:188−189  4. Zalewski A, Fedotova J, Ilyuschenko A et al. (2006)Mössbauer, XRD, and SEM study of FeAl-based powderalloys with nanoinclusions. Acta Phys Pol A 109:441−444Fig. 3. Comparison of phase population determined fromtransmission and CEMS experiments: the ratio of the FeSO4to Fe2O3 fractions measured in transmission (T) and in CEMSexperiment (C) vs. the radius of FeSO4 grains, R.RT/C (mm)

Simultaneous  transmission and scattering Mössbauer measurements of powder samples

Fornal, P.

Stanek, J.

Biblioteka Nauki - repozytorium artykuÅÃ³w

Simultaneous transmission and scattering Mössbauer measurements of powder samples

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Simultaneous transmission and scattering Mössbauer measurements of powder samples

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Simultaneous transmission and scattering Mössbauer measurements of powder samples

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