Hansen, Karin Vels

Jacobsen, Torben

Mogensen, Mogens Bjerg

Nørgaard, Anne-Mette

Ohmer, N.

The controlled atmosphere high temperature scanning probe microscope was designed to study the electrical properties of surfaces at elevated temperatures by using the probe as an electrode. The capability of a simultaneous acquisition of topographical and electrical data for the same surface area at 650°C is demonstrated on several samples

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Scanning Probe Microscopy at 650 °C in AirHansen, Karin Vels; Jacobsen, Torben; Nørgaard, Anne-Mette; Ohmer, N.; Mogensen, Mogens BjergPublished in:Electrochemical and Solid-State LettersLink to article, DOI:10.1149/1.3183881Publication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Hansen, K. V., Jacobsen, T., Nørgaard, A-M., Ohmer, N., & Mogensen, M. B. (2009). Scanning ProbeMicroscopy at 650 °C in Air. Electrochemical and Solid-State Letters, 12(10), B144-B145. DOI:10.1149/1.3183881Scanning Probe Microscopy at 650°C in AirK. Vels Hansen,a,z T. Jacobsen,b,* A.-M. Nørgaard,a N. Ohmer,c andM. Mogensena,*aFuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy,Technical University of Denmark, DK-4000 Roskilde, DenmarkbDepartment of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, DenmarkcInstitute of Pure and Applied Chemistry, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg,GermanyThe controlled atmosphere high temperature scanning probe microscope was designed to study the electrical properties of surfacesat elevated temperatures by using the probe as an electrode. The capability of a simultaneous acquisition of topographical andelectrical data for the same surface area at 650°C is demonstrated on several samples.© 2009 The Electrochemical Society. DOI: 10.1149/1.3183881 All rights reserved.Manuscript submitted March 31, 2009; revised manuscript received June 25, 2009. Published July 24, 2009.The controlled atmosphere high temperature scanning probe mi-croscope CAHT-SPM was designed for in situ study of electrical/electrochemical properties of surfaces at elevated temperatures in acontrolled atmosphere especially with regard to solid oxide fuel cellSOFC electrodes.1 The main purpose of this instrument is to en-able a study of chemical and physical properties of surfaces down toa scale corresponding to single particles of the porous compositeelectrodes. These are usually of submicrometer size. It is intended toperform a variety of electrical/electrochemical experiments, and thevery first results are reported here. Recent advancements have madeit possible to study surfaces at temperatures at least up to 650°C inair at a resolution below 500 nm for contact mode measurements.High temperature surface imaging with atomic force microscopyAFM usually involves temperatures up to 130°C on polymermaterials.2-4 Only a few references are found on AFM at highertemperatures, e.g., Ref. 5, where 400°C was reached, and the veryrecent results on AFM at 500°C in air, where atomic step resolutionwas reported by Broekmaat et al.6Several techniques for the characterization of the electrical prop-erties of surfaces in combination with a scanning probe microscopeSPM exist.7 The method used for the generation of electrical prop-erty images in the present article corresponds to the nanoimpedancemicroscopy technique described in Ref. 7.The results reported here are the imaging capability at 650°C andthe simultaneous acquisition of topographical and electrical datafrom the same surface area.ExperimentalThe CAHT-SPM, which was developed by DME Danish MicroEngineering A/S, is described in detail in Ref. 1. It includes a fur-nace with a control system for heating of the sample and a water-based cooling system, where gravity drives the water through thecooling channels. Electrical fittings allow leads to be connected tothe probe and to electrical contacts placed on the sample. TheCAHT-SPM works in contact mode.The custom-made probes used for the acquisition of images wereprepared from a 0.1 mm Pt–Ir wire. The wire was electrochemicallyetched in a solution of CaCl2/HCl, which led to the formation of asharp tip. To obtain a reflecting surface, the wire was pressed be-tween gauge blocks in a hydraulic press. To ensure the quality of theprobes, the tip was inspected with a digital microscope. Furtherquality control was achieved through calibration against a standardgrid before and after usage of the probe.The first sample in the present work was a polished and sinteredstabilized zirconia SZ disk with a 250 nm thick rectangular goldpattern. The second sample was a disk of sintered and polished SZthat was heat-treated for 10 h at 1600°C to produce grain boundarygrooving by thermal etching. The SZ pellets were attached to theheating plate with a pair of springs. The third sample was a sym-metrical cell with SOFC electrodes prepared by tape casting andsintering. It consisted of a 10 m thick SZ electrolyte with a15 m thick NiO/SZ SOFC anode precursor and a 190 mNiO/SZ support on both sides. The dimensions of the cell were5.0  5.0  0.5 mm. The symmetrical cell was mounted in a spe-cial stainless steel sample holder where the cell was positioned in aslit. The cell was polished on end such that a cross section of the cellcould be analyzed in the microscope. The sample holder was thenmounted in the furnace.Conductivity imaging was performed with a setup similar to thatused in Ref. 8. The conductivity was determined by an ac signal inthe range of 1–10 kHz with a suitable amplitude. The ac signal waspassed from the generator output of the lock-in amplifier StanfordResearch Systems, SR 830 through the probe and the spring-loadedplatinum counter electrode into the current input of the lock-in am-plifier.With a scan rate of 256 pixels/s, the time per pixel was 4–40 fullperiods, and a sufficient signal to noise ratio could be achieved witha filter time constant of a few milliseconds without reduction in theimage quality.The current amplifier analog output giving the real part of theprobe current was fed into an auxiliary input of the CAHT control-ler, and a conductivity image supplementing the topography imagewas obtained.The voltage read from the color scale in the images was propor-tional to the current through the probe. Thus, the color scale couldread as a conductivity scale. Due to differences in tip dimensions,conductivity values could only be compared qualitatively from im-age to image.ResultsFigure 1 shows conductivity images of the sample with the goldpattern. At 225°C, the conductivity image shows straight edges Fig.* Electrochemical Society Active Member.z E-mail: karin.vels@risoe.dkFigure 1. Conductivity images a at 225°C, b 650°C first scan, and c650°C second scan 30 min later. Image parameters: Scan speed of 25 m/s,resolution of 256  256, and frequency of 10 kHz.Electrochemical and Solid-State Letters, 12 10 B144-B145 20091099-0062/2009/1210/B144/2/$25.00 © The Electrochemical SocietyB144Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp1a. As time and temperature increase, the edges become wavy. Fig-ure 1b shows the first scan at 650°C, and Fig. 1c shows the secondscan 30 min later. Shortly after this, the gold pattern breaks up intoirregular areas.Figure 2 shows images of the thermally etched SZ sample ac-quired at 650°C. Comparing the topographic and conductivity im-ages, the grain boundary pattern is reproduced precisely. The grainboundary region consists of two shoulders and a groove in which theactual grain boundary is situated. In the conductivity image, thegrain boundary regions are light colored, whereas the grain interiorspredominantly have a darker color, indicating that the grain bound-ary regions conduct poorly compared to the grain interiors.Topographical and conductivity images acquired at 650°C of apolished cross section of a symmetrical cell are shown in Fig. 3.Even though the sample has been polished, the difference in hard-ness of SZ and NiO/SZ induces difference in the surface structure.The electrolyte and electrode can thus be recognized in the topo-graphical image. There is, however, no direct indication of the elec-trode microstructure. The conductivity image clearly illustrates thedifferences in conductivity of the electrolyte and the anode. Thenonconducting electrolyte shows up as the white bar, and the anodeis characterized by the difference in conductivity between the lightSZ grains and the darker NiO grains. A magnification of Fig. 3bshows that the resolution is below 500 nm.DiscussionScanning probe microscopy at high temperatures requires con-sideration of some issues about thermal stability, drift, and reso-lution that are different from room-temperature SPM. Significantdrifting is not observed through a scan because features are sharpand straight throughout the images.Control of the tip size and shape is a determining factor in thesuccess of the experiments because contact mode scanning wearsthe tip. As the tip is not coated but consists of solid Pt–Ir, mechani-cal wear increases the contact area but not the chemical compositionof the contact.The images of the gold pattern demonstrate that the setup isworking and returning the expected result that the conductivity ofgold is high compared to that of the SZ. Furthermore, in situ mea-surements of a very mobile surface can be depicted with nanoimped-ance microscopy at 650°C.The conductivity images of the thermally etched SZ presentedshow that grain interiors have a higher conductivity than grainboundary regions. During heat-treatment of SZ impurities accumu-late at surfaces and in grain boundaries,9-11 and grain boundary con-ductivity depends on the amount of impurities.9 The surface distri-bution of impurities on SZ heat-treated at high temperatures hasbeen imaged with time-of-flight secondary-ion mass spectrometry,12and depth profiling shows that the grain boundaries are rich in im-purities. This distribution of impurities may be a determining factorfor electrical properties found with the CAHT-SPM.Symmetrical cells are used in the SOFC research for electro-chemical and microstructural studies on porous electrodes as theyresemble, in composition and structure, the electrodes on full cells.The conductivity image of the symmetrical cell shows that theCAHT-SPM can visualize electrical features that cannot be distin-guished in the topographical image. The complex microstructure ofthe electrodes and support layer is reproduced, and features below500 nm can be distinguished.ConclusionThe results presented above demonstrate the capability of theCAHT-SPM to image surface topography at 650°C. The results alsoshow that the CAHT-SPM can be used for electrical measurementswith the probe and can scan the surface simultaneously for topo-graphical and electrical data. The resolution is currently below 500nm.AcknowledgmentThis work was supported financially by the Programme Commis-sion on Sustainable Energy and Environment, The Danish Councilfor Strategic Research, via the SERC project, contract no. 2104-06-0011. Colleagues at The Fuel Cell and Solid State Chemistry Divi-sion are acknowledged for their continuous support, and KarenBrodersen and Tânia Ramos especially are thanked for supplying thesymmetrical cell. N.O. acknowledges the Ernest-Solvay Foundationfor financial support during the work.Risø National Laboratory for Sustainable Energy assisted in meeting thepublication costs of this article.References1. K. V. Hansen, C. Sander, S. Koch, and M. Mogensen, J. Phys.: Conf. Ser., 61, 3892007.2. I. Musevic, G. Slak, and R. Blinc, Rev. Sci. Instrum., 67, 2554 1996.3. H. D. Sikes and D. K. Schwartz, Science, 278, 1604 1997.4. S. G. Prilliman, A. M. Kavanagh, E. C. Scher, S. T. Robertson, K. S. Hwang, andV. L. Colvin, Rev. Sci. Instrum., 69, 3245 1998.5. M. DiBattista, S. V. Patel, J. F. Mansfield, and J. W. Schwank, Appl. Surf. Sci.,141, 119 1999.6. J. Broekmaat, A. Brinkman, D. H. A. Blank, and G. Rinders, Appl. Phys. Lett., 92,043102 2008.7. S. V. Kalinin, R. Shao, and D. A. Bonnell, J. Am. Ceram. Soc., 88, 1077 2005.8. L. S. C. Pingree, E. F. Martin, K. R. Shull, and M. C. Hersam, IEEE Trans.Nanotechnol., 4, 255 2005.9. S. P. S. Badwal and A. E. Hughes, J. Eur. Ceram. Soc., 10, 115 1992.10. A. E. Hughes and B. A. Sexton, J. Mater. Sci., 24, 1057 1989.11. M. de Ridder, A. G. J. Vervoort, R. G. van Welzenis, and H. H. Brongersma, SolidState Ionics, 156, 255 2003.12. K. V. Hansen, K. Norrman, and M. Mogensen, Surf. Interface Anal., 38, 9112006.Figure 2. a Topography and b conductivity images of a 50  50 marea at a surface temperature of 650°C. The grain boundary pattern is visiblein both images. The light-colored grain boundary regions indicate that theconductivity here is lower than that in the grain interiors. Image parameters:Scan speed of 50 m/s, resolution of 128  128, and frequency of 10 kHz.Figure 3. a Topography and conductivity images of an 80  80 m areaof a cross section of an unreduced symmetrical cell with NiO/stabilizedzirconia composite electrode precursors at a surface temperature of 650°C.The electrolyte, the anode precursor fine structure, and the anode supportprecursor coarser structure can be distinguished. The light-colored areasindicate that the conductivity here is lower than that in the darker areas.Image parameters: Scan speed of 189 m/s, resolution of 1024  1024, andfrequency of 10 kHz.B145Electrochemical and Solid-State Letters, 12 10 B144-B145 2009Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

Scanning Probe Microscopy at 650 °C in Air

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Scanning Probe Microscopy at 650 °C in Air

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