Charge carrier dynamics in nanocrystalline Dy ...

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May 12, 2016 - Citation: AIP Conference Proceedings 1728, 020070 (2016); doi: ... http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov.
Charge carrier dynamics in nanocrystalline Dy substituted ceria based oxygen ion conductors Sk. Anirban and A. Dutta Citation: AIP Conference Proceedings 1728, 020070 (2016); doi: 10.1063/1.4946121 View online: http://dx.doi.org/10.1063/1.4946121 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nanoscale mapping of oxygen vacancy kinetics in nanocrystalline Samarium doped ceria thin films Appl. Phys. Lett. 103, 171605 (2013); 10.1063/1.4826685 Chemical Synthesis of Nanocrystalline Ceria AIP Conf. Proc. 1349, 401 (2011); 10.1063/1.3605904 Ultrafast dynamics of photoexcited charge carriers in nanocrystalline diamond Appl. Phys. Lett. 93, 083102 (2008); 10.1063/1.2970962 Positional disorder of oxygen ions in ceria at high temperatures Appl. Phys. Lett. 84, 526 (2004); 10.1063/1.1644053 Charge carrier interactions in ionic conductors: A classical molecular-dynamics and Monte Carlo study on AgI J. Chem. Phys. 112, 6416 (2000); 10.1063/1.481205

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Charge Carrier Dynamics in Nanocrystalline Dy Substituted Ceria Based Oxygen Ion Conductors Sk. Anirban1, 2, a) and A. Dutta 1,b) 1

Department of Physics, The University of Burdwan, Burdwan-713104, India Department of Physics, Govt. General Degree College, Singur, Hooghly-712409, India

2

a)

b)

[email protected] Corresponding author: [email protected]

Abstract. Nano-crystalline Ce1-xDyxO2-δ (x = 0.1-0.5) materials were prepared using the low temperature citrate autoignition method. The Rietveld analysis of the XRD data confirmed the single phase cubic fluorite structure. The particle sizes of the sintered samples are in nano range and lattice parameter increases with Dy concentration. Polydispersed and agglomerated particles are observed by SEM. The EDAX spectra show good stoichiometry of the different atoms in the samples. The conductivity is found to have both grain and grain boundary contribution and shows highest value at x= 0.2. The frequency dependence of dielectric permittivity has been analyzed using Havrilliak-Negami formalism. The variation in different electrical properties has been explained by formation defect associates and their interaction with charge carriers.

INTRODUCTION In the recent years rare earth doped ceria has received much attention due to their high ionic conductivity compared to the traditional yttria-stabilized zirconia solid electrolytes [1]. Several studies of literature [1-2] reveals that ionic conductivity of rare earth ceria is approximately an order of magnitude greater than that of YSZ especially when they are in nanocrystalline form. So they are considered as good electrolyte materials for IT-SOFCs. Among the various trivalent rare earth dopants Gd+3 and Sm+3 show highest ionic conductivity at intermediate temperatures. The rare earth doped ceria exhibits high oxide ionic conductivity because of its small association enthalpy between the dopant cation and the associated oxygen vacancies in cubic fluorite lattice [3]. According to V. Butler et. al [4] association enthalpy of Dy-doped ceria is comparable to that of the samarium or gadolinium doped ceria. Therefore, it is expected that Dy-doped ceria would be a good solid electrolyte in intermediate temperature SOFCs. In this work, we report the structural and ionic transport properties of Dy-doped nano ceria prepared through citrate auto-ignition method.

EXPERIMENT The nanocrystalline Ce1-xDyxO2-δ (x = 0.1-0.5) were prepared through the low temperature citrate auto ignition method. The starting materials were Ce(NO3)3.6H2O (99.9%), Dy2O3 (99.9%) and anhydrous citric acid which were taken in proper stoichiometric proportion. The detail of sample preparation process is discussed in our earlier work

International Conference on Condensed Matter and Applied Physics (ICC 2015) AIP Conf. Proc. 1728, 020070-1–020070-4; doi: 10.1063/1.4946121 Published by AIP Publishing. 978-0-7354-1375-7/$30.00

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[5]. The as prepared material was calcined at 400°C for 3 hours and then sintered for 12 hours at temperature 800°C. Yellowish powder was formed which were made into pellets. Graphite paste was applied to both faces of the pellet to make the electrode. The electrical measurements were performed in a tube furnace using two probe methods. An LCR meter (Hioki, Model 3532-50) interfaced with the personal computer was used to collect the electrical data in the frequency range 42Hz to 5MHz and in temperature range 250 -550oC. The microstructures of the sintered materials were observed by XRD, SEM and EDAX.

RESULTS AND DISCUSSIONS Structural analysis Fig.1 shows the Rietveld refined XRD patterns of the powdered samples of Ce1-xDyxO2-δ (x=0.1-0.5). All the diffraction peaks as displayed in Fig.1, are well indexed to the single phase cubic fluorite structure. No additional phase corresponding to Dy is observed which ensures the complete dissolution of the dopant into ceria matrix. The particle sizes of samples were evaluated from Rietveld analysis and were found in the range 12-19 nm. The lattice parameter was found to increase with x from 5.36049-5.39051Å. It is because the ionic radius of Dy (1.027Å) is greater than the Ce4+(0.97Å). The micro-strain of the samples were also calculated and was found minimum for the sample x = 0.2.

FIGURE 1. The Rietveld refined XRD patterns of the powdered samples

Figure 2 shows the SEM image of sintered sample x = 0.2. From this image, it can be seen that the particles are polydispersed and agglomerated. The compositional distribution of the samples has been examined by EDAX analysis (Fig. 3). The EDAX analytical data on atomic and weight % of Ce, Dy and O were found agreeable with their corresponding expected values. The EDAX spectrum confirms that Dy enters into the ceria lattice.

FIGURE 2. The SEM image of sintered sample x= 0.2.

FIGURE 3. EDAX of the sample x = 0.2.

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Electrical properties Fig. 4 shows the complex impedance plots (Nyquist plot) for the composition Ce 0.8Dy0.2O2-δ at different temperatures. This plot shows two successive depressed semicircular arcs. The high and low frequency semicircular arcs correspond to grain and grain boundary contribution respectively. Therefore, the total electrical conductivity has got both the grain and grain boundary contributions. The typical equivalent electrical circuit for such impedance plot is given in the inset of Fig. 4.

FIGURE 5.The variation of activation energy and conductivity at 550Ԩ for (a) grain and (b) grain boundary with the doping concentration of Dy3+ ions.

FIGURE 4. The complex impedance plot for the composition Ce0.8Dy0.2O2-δ at different temperatures

The variation of activation energy and conductivity at 550Ԩ for both the grain and grain boundary with the doping concentration of Dy3+ ions (x) are shown in Fig. 5. It can be observed that, the activation energy decreases with x and shows a minimum value for the composition Ce 0.8Dy0.2O2-δ. After that, activation energy again increases with x while the conductivity shows an opposite behavior. Both the grain and specific grain boundary conductivity increases with x reaching a maximum value at x = 0.2 and then decreases as dopant concentration increases. When Dy3+ ions are doped into ceria then one oxygen vacancy is formed for every two Dy 3+ ions for charge neutrality which may be represented by the Krᠰger – Vink notation: Ԣ ൅͵‘‫ ݔ‬൅‘ȈȈ  ›ʹ͵൅ʹ‡ʹ՜ʹ›‡



ሺͳሻ 

Ԣ indicates one Ce4+ site occupied by Dy3+ ion and ‘ȈȈ  is the oxygen vacancy. Generally, oxygen Here ›‡ vacancies (ͲȈȈ ) increases with doping concentration of Dy3+ ions. As the free oxygen vacancies are mobile and can contribute significantly to oxygen ion transport, thus with the increase of Dy 3+ ions concentration the conductivity increases and reaches a maximum value with minimum activation energy at x = 0.2. After that, the conductivity decreases with the concentration of Dy3+ ions which may be due to the increasing interactions between Dy3+ ions and oxygen vacancies and also formation of local defect structures which lowers the mobile oxygen vacancies. We have analyzed the frequency dependence of the complex dielectric spectra using the generalized HavriliakെNegami (HN) formalism [6]. The generalized dielectric function is given by,

ߝ ‫ כ‬ሺ߱ሻൌߝ Ԣ ሺ߱ሻ െ ݅ߝ ԢԢ ሺ߱ሻൌߝλ ൅

ߝ ‫ ݏ‬െߝ λ ሾͳ൅ሺ݅߱ ߬ ‫ ܰܪ‬ሻߙ ሿߚ

൅

ܵ ݅߱ ‫݌‬



ሺʹሻ

 Where ߝ‫ ݏ‬and ߝλ are the relaxed and unrelaxed permittivity respectively. ߙ and ߚare the shape parameters satisfying the condition 0൑ ߙ ൑ ͳ and 0൑ ߙߚ ൑ 1 and ߬‫ ܰܪ‬is the characteristic relaxation time. Figs 6(a) and (b) represents the frequency variation of ߝ Ԣ ሺ߱ሻ at different temperatures for the sample Ce0.8Dy0.2O2-δ and for all the compositions at a specific temperature. ߝ Ԣ (߱ሻ exhibits a step (indicated by arrow mark in 6(a)) in the intermediate frequency range that may be due to the grain boundary relaxation which leads to the formation of plateau. With the rise in temperature the plateau shifts toward the high frequency region i.e. the grain boundary relaxation is thermally activated process. Fig. 6(b) shows that, for the compositions x = 0.40 and x = 0.50 there are no such plateau i.e. grain boundary relaxation is suppressed in these two samples. This is in good agreement with the complex impedance spectra. It also reveals that, the plateau shifts towards the high frequency

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region with the doping concentration of Dy3+ ions and reaches a maximum shift for x = 0.20 and reverses the direction towards lower frequency region with further increase of doping concentration. This is similar to the variation of grain boundary conductivity. Fig. 6(c) shows the frequency variation of ߝ ԢԢ (߱ሻ for the composition x = 0.20 at different temperatures. The blue straight line represents conduction part in the ߝ ԢԢ (߱ሻ spectra. A single Havriliak-Negami dielectric loss peak is shown in Fig. 6(c) by red dotted line. Fig. 6(d) shows the variation of ߝ ԢԢ with doping concentration at temperature 500oC and at frequency 7.19 KHz which exhibits a highest value of ߝ ԢԢ for the composition x = 0.20.

FIGURE. 6(a) The frequency variation of the real part of the complex dielectric function at different temperatures for the sample Ce0.8Dy0.2O2-δ, (b) for all compositions at temperature 525Ԩ, (c) the variation of ߝ ԢԢ (߱ሻ with frequency for the sample sample x = 0.20 at different temperatures. The blue straight line represents conduction part and HN dielectric loss peak is shown by red dotted line. (d) The variation of ߝ ԢԢ with doping concentration.

CONCLUSIONS Dy doped nano ceria was prepared through citrate auto ignition process. The XRD pattern of the sintered samples indicates the cubic fluorite structure of the materials. The EDAX data showed a good agreement of constituents with their corresponding expected values. The impedance spectra showed that conductivity comprises of both the grain and grain boundary. The analysis of dielectric and conductivity data showed a maximum for sample x= 0.2. The variation in conductivity and dielectric properties were correlated with oxygen vacancies and defect associates.

ACKNOWLEDGEMENTS The authors thankfully acknowledges the financial assistance from DST (Govt. of India) (Grant no: SR/FTP/PS141-2010 and Grant no: SR/FST/PS-II-001/2011 (FIST)) and UGC for departmental CAS scheme.

REFERENCES 1. 2. 3. 4. 5. 6.

B.C.H. Steele, Solid State Ionics 129, 95-110 (2000). J. Hormes, M. Pantelouris, G. B. Balazs, and B. Rambabu, Solid State Ionics 136, 945–954 (2000). J. Kilner, Solid State Ionics 8, 201–207 (1983). V. Butler, C. R. A. Callow, B. E. F. Fender, and J. H. Harding, Solid State Ionics 8, 109–113 (1983). Sk. Anirban, A. Dutta, J. Phys. Chem. Solids 76, 178-183 (2015). S. Havriliak, S. Negami, Polymer 8, 161-210 (1967).

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