High throughput screening of novel oxide conductors using SIMS

Applied Surface Science 252 (2006) 7159–7162 www.elsevier.com/locate/apsusc

High throughput screening of novel oxide conductors using SIMS S. Fearn a,*, J.C.H. Rossiny a, J.A. Kilner a, Y. Zhang b, L. Chen b b

a Department of Materials, Imperial College London, Exhibition Rd, London SW7 2AZ, UK Department of Materials, Queen Mary, University of London, Mile End Rd, London E1 4NS, UK

Received 12 September 2005; accepted 15 February 2006 Available online 15 May 2006

Abstract Conventional synthesis of ceramic oxide compositions is time consuming and consequently limits the compositions that can be studied. A way round this is the use of combinatorial methods to explore much wider ranges of compositions. Using an inkjet based robot system; London University Search Instrument (LUSI) combinatorial arrays of ceramic dot samples can be produced. The first part of the study consists of the characterisation of a printed array of La0.8Sr0.2CoO3 d to ensure the reproducibility and quality of the ceramic dots and suitability for isotopic exchange experiments. # 2006 Elsevier B.V. All rights reserved. Keywords: Combinatorial methods; Mixed conductors; SOFC cathodes; Isotopic exchange

1. Introduction Mixed electronic ionic conducting ceramics (MEIC’s) are used in electrochemical devices, such as oxygen separation membranes, SOFC cathodes, or in syngas reactors according to their ionic conduction properties. One of the most promising class of materials for the search for new MEICs is the perovskite oxides with the general formula ABO3, A and B are rare earth/ alkaline earth ions and transition metal cations, respectively. By doping both the A- and B-sites, the composition of these materials can be broadened to encompass a very large number of possible combinations. Dopant species and compositions can have a major effect on both the ionic and electronic components of the conductivity and thus the possibilities of finding new and optimal materials presents a very large challenge. The electrical behaviour of perovskites has been investigated in much greater detail than their oxygen ion transport properties although the use of isotopic exchange and SIMS to determine the oxygen surface exchange and diffusion in ceramic ionic conductors has become well established over the last 20 years [1–4]. However, to obtain accurate diffusion profiles the ceramic sample must have a density greater than 95%, to ensure that the ceramic is gas tight and bulk oxygen diffusion is being measured and not diffusion through pores [5]. * Corresponding author. Fax: +44 207 594 6757. E-mail address: [email protected] (S. Fearn). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.177

Therefore, the number of compositions being studied has been limited due to the traditional method of synthesising dense ceramic pellets, which is lengthy and time-consuming. For example, the La1 xSrxMnyCo1 yO3 (LSMC) binary system has been studied by several authors and the range of ionic conductivities for this system shown to vary by five orders of magnitude [6–8]. Yet there are still regions within this system which have not been studied at all and little is known of their properties. The use of combinatorial methods to explore much wider ranges of compositions based upon rational choices of starting compositions could be one way to increase sample production. Combinatorial methods have been extensively deployed by the drug industry [9,10], and now the methodology is being applied to inorganic materials, especially ceramics [11]. The measurement of oxygen transport in such samples would be complex; and a much more practical method would be to produce small but discrete samples as libraries. A simple and fast approach to creating an oxygen transport library is a discontinuous inkjet printing, which has been demonstrated to successfully produced cathode materials for SOFCs [12]. The overall aim of the work is to thoroughly investigate the O18 isotopic exchange properties for the extended range of La1 xSrxMnyCo1 yO3 compositions, with x and y varying by as little as a few percentage. In order to produce such a large compositional spread the London University Systems Investigation (LUSI) robot has been used to investigate the

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possibility of fast inkjet printing of ceramic dots [13]. The printed dot arrays will then undergo isotopic exchange, and the O18 diffusion profiles will be measured via SIMS on the Focused Ion Beam workstation. The flexibility of the FIBSIMS instrument to extract accurate isotope tracer measurements from complex ceramic compositions has recently been highlighted [14]. To asses the feasibility of this approach, ceramic printing work has started with the well know (La0.8Sr0.2)CoO3 d (LSC) ceramic. In this paper, work on the initial material libraries is presented highlighting the reproducibility and quality of ceramic dot production. Importantly, for the future isotopicexchange experiments, modelling of the oxygen diffusion has been carried out to identify the changes that may occur due to the geometry changing from a large pellet to a small dot, and if measuring the diffusion profiles is still possible, particularly for fast diffusing materials. 2. Experimental Commercially produced (La0.8Sr0.2)CoO3 d (LSC) was used as a start material, and supplied by Praxair Ceramics Inc. To print the array of ceramic dots, an ink was made with and without the dispersant Dispex A40. For the inks with the dispersant the following weight percent ratio was used: 30.5% LSC:7.5% DispexA40:62.0% Distilled water. The ink dispensing system combines a high-resolution displacement syringe pump with a high-speed microsolenoid valve leading to the jetting tip. The movement of the printing system is controlled by a Devices Cartesian Technologies ProSysTM 4510 system with an overhead XYZ platform, and an 8-channel dispenser to transfer liquid to different positions. Initially, the syringe and the valve are primed with enough isopropanol until they are fully filled this controls the volume and jetting behaviour of the ink held in the tip. The dispenser is then moved to the ink well to aspirate 8 mL of the ink (a small volume is aspirated to avoid contamination of the valve). The dispenser is then moved to the wash station and the syringe pushed while the valve is closed so as to set up a pre-pressure in the liquid. A small volume of the ink (10 nL) is then pre-dispensed 20 times to achieve the correct steady state pressure (SSP). Finally, the dispenser is moved to the Pt coated alumina slide for the printing of the ceramic dots. A volume of 10 nL, is dispersed 100 times to build up a 1 mm ink drop, the volume of the dot is easier to control this way, rather than dispersing one large volume in a single deposition.

The final drop is then left to dry. A total of 24 by 12 dots were printed on an alumina substrate 25 mm  50 mm. The printed dot arrays are then finally moved to a furnace for sintering. In this case, the LSC dot array was sintered at 11008C for 2 h. The dots were studied using a Focused Ion Beam workstation 200 with SIMS from FEI. As well as imaging capabilities this system can also perform secondary ion mass spectrometry (SIMS) of the sputtered material. The FIB microscope images through bombardment of the sample surface with ions from a liquid Ga ion source. These ions are accelerated at 30 keV and focused to a beam size of 5 nm. A beam current of between 50 and 500 nA was used to produce ion images of the dots. SIMS mass spectra were taken on the dots to identify any contamination on the sample surface. Areas 10 mm  10 mm were analysed and both positive and negative spectra were taken. The data was collected continuously and saved at intervals of 1, 5, 15, and 30 scans to identify if contamination was just occurring in the near surface. X-ray diffraction was also carried out to ensure the dots produced had remained single phase. A Phillips PW17010 automated powder diffractometer equipped with a secondary graphite crystal monochromator and using Cu Ka radiation ˚ ) was used. Measurements were made relative (l = 1.54178 A to an external silicon standard. For standard phase identification analysis, samples were scanned in the region 2u = 20–908 using a step size of 0.048 s 1. 3. Results and discussion To ensure a single phase perovskite had been retained after sintering and no reactions had taken place with the substrate, some dots were removed from the array plate and ground to a fine powder for the purposes of XRD. A LSC pellet which had been sintered at the same temperature and time was also analysed for comparison. The XRD results showed that the dots had retained a single phase perovskite, with no evidence of interaction. Over the printed dot array the morphology and the size of the dots varied, with some dots having formed a ‘sunken dome’ shape while others have a more regular hemi-spherical form. Despite the variation in size and morphology, the dots are well defined, and adhered well to the substrate, whereas the dots dispensed without dispersant all fell off the alumina substrate. The dome shaped LSC dots have been closely examined using the FIB-SIMS microscope. A typical example of a hemispherical printed dot is shown in the positive ion image of Fig. 1(a). The image shows that the dot has shrunk from the

Fig. 1. The positive ion images of (a) a LSC dot printed with dispersant and (b) the SIMS craters in the contaminated and clean region of the dot.

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Fig. 2. (a) Positive, (b) negative mass spectra on the ‘contaminated’ region, (c) the positive, and (d) negative mass spectra taken on the ‘clean’ region of the dot, after 30 scans.

original area covered by the ink dot, as indicated by the outer ring around the dot. The dot also appears to be lifting off the Pt coated alumina slide. This is due to the difference in expansion coefficients between the LSC and the Pt coated substrate. On the top of the dots, there appear to be regions of contamination indicated by the darker patches, this is highlighted in the positive ion image of Fig. 1(b) above. Both positive and negative SIMS mass spectra have been taken in the ‘contaminated’ and ‘clean’ regions highlighted in Fig. 1(b). The mass spectra were collected after 1, 5, 15, and 30 scans in order to identify if the contamination was occurring just on the surface of the dot or was also present in the bulk of the ceramic. Comparison of the mass spectra indicated no difference from the 1 scan to the 30th scan. This suggests that the contamination is not solely on the surface of the dot but is also present in the bulk of the material. The positive and negative spectra obtained from the ‘contaminated’ region are shown in Fig. 2(a) and (b), respectively, and the positive and negative spectra obtained from the ‘clean’ region are shown in Fig. 2(c) and (d), respectively, after 30 scans corresponding to a depth of 300 nm. On Fig. 2(a–c) all of the peaks for the main elements of the dot La(139), Sr(86, 88), and Co(59) can be clearly identified, along with the molecular ions of LaO+ and SrO+. The primary Ga+ ions at mass 69 and 71 are also detected after becoming implanted during the sputtering process. However, on Fig. 2(a) there is also a strong Al+ signal at mass 27, along with Na+, K+, and Ca+ contamination. None of these signals were observed on the mass spectra of the ‘clean’ area of the dot. On the negative mass spectra from the ‘contaminated’ region, Fig. 2(b), there also appears to be extraneous signals arising from the presence of C , F , Cl , and possibly S , along with CN and C2H which may come from the dispersant. Again, the mass spectra from the ‘clean’ area, Fig. 2(d) did not show

the presence of any of these signals. Similar regions of contamination were also observed on the dots where no dispersant was used. After carrying out SIMS mass spectra on the dots, again regions of contamination containing Al and Na were identified, however, the organic fragments CN and C2H5 were not detected. The mass spectra strongly indicate that there is contamination occurring in the dot production. One of the main reasons could be that during the deposition too much ink is entering the valve and picking up residue creating local areas of contamination. Although the nozzles are cleaned and efforts are being made not to aspirate large quantities of ink to avoid this, problems are still occurring. The contamination may also occur during the sintering process, particularly as the Al and Na are identified in the bulk of the dot. The presence of contamination will have consequences for any isotopic exchange experiments, and therefore needs to be eradicated beforehand. 4. Conclusion Combinatorial methods to determine oxygen transport in MEIC materials, such as perovskites presents a difficult challenge. Our initial studies show that the inkjet method of producing looks promising. From the characterisation of the printed LSC dots, reasonably good dot morphology can be achieved, however problems related to the difference in expansion coefficients of the ceramic and substrate has resulted in the dots spalling away from the substrate. Focussed ion imaging of a milled crosssection of the dot indicated that the dots have some porosity. SIMS mass spectra taken on the dot indicated that regions were highly contaminated with Al+, C , F , Cl , and CN . Contamination could have occurred from the printing process, along with residue from the dispersant, and/or during the

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sintering. Other dispersants not previously tested are being investigated as well as alterations to the printing process. It is hoped that with further work, regular dense dots will be readily printable, and isotopic exchange experiments will be performed on the arrays. References [1] J.A. Kilner, B.C.H. Steele, L. Ilkov, Solid State Ionics 12 (1984) 89. [2] B.C.H. Steele, J.A. Kilner, P.F. Dennis, A.E. McHale, Solid State Ionics 18–19 (1986) 1038. [3] R.J. Chater, S. Carter, J.A. Kilner, B.C.H. Steele, Solid State Ionics 53–56 (1992) 859.

[4] A. Atkinson, R.J. Chater, R. Rudkin, Solid State Ionics 139 (2001) 233. [5] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, John Wiley and Sons, 1975. [6] R.A. De Souza, J.A. Kilner, Solid State Ionics 106 (1998) 175. [7] R.A. De Souza, J.A. Kilner, Solid State Ionics 126 (1999) 153. [8] A. Berenov, J.L. MacManus, J.A. Kilner, Solid State Ionics 122 (1999) 41. [9] J.J. Hanak, J. Mater. Sci. 5 (1970) 964. [10] P.M. Doyle, J. Chem. Technol. Biotechnol. 64 (1995) 317. [11] H. Koinuma, I. Takeuchi, Nat. Mater. 3 (2004) 429. [12] J.P. Lemmon, V. Manivannan, T. Jordan, L. Hassib, O. Siclovan, M. Othon, M. Pilliod, Mater. Res. Soc. Symp. Proc. 804 (2004). [13] J.R.G. Evans, M.J. Edirisinghe, P.V. Coveney, J. Eames, J. Eur. Soc. 21 (2001) 2291. [14] R.J. Chater, D.S. McPhail, Appl. Surf. Sci. 231–232 (2004) 834.