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Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics
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CERAMICS INTERNATIONAL
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Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics A.N. Bondarchuka,n, J.A. Aguilar-Martinezb,c, M.I. Pech-Canuld a Universidad Tecnológica de la Mixteca, Huajuapan de León, Oaxaca 69000, México Centro de Investigación en Materiales Avanzados, S.C. (CIMAV), Alianza Norte No. 202, Parque de Investigación e Innovación Tecnológica (PIIT), Nueva Carr. Aeropuerto km. 10 Apodaca N.L. 66600, México c Universidad Autónoma de Nuevo León, Facultad de Ingeniería Mecánica y Eléctrica, Centro de Investigación e Innovación en Ingeniería Aeronáutica (CIIIA); Carretera a Salinas Victoria km. 2.3, C.P. 66600 Apodaca, N.L., México d Cinvestav Saltillo, Av. Industria Metalúrgica No. 1062, Parque Industrial Saltillo-Ramos Arizpe. Ramos Arizpe, Coahuila 25900, México b
Received 18 December 2013; received in revised form 7 March 2014; accepted 19 March 2014
Abstract Electrical properties of indium oxide ceramics doped by Sr and Zr and sintered at 1473 and 1573 K in air were studied and are discussed. In2O3–SrO ceramics exhibit nonlinear current–voltage behavior with the current limiting effect: current rises more weakly than voltage, is saturated and even decreased. The Zr dopant changes the type of current–voltage dependency in such materials. In2O3–SrO–ZrO2 ceramics with low Zr content (5–10 wt% ZrO2 in mixture) possess linear current–voltage characteristic and very high conductivity. For ceramics with high content of Zr (60 wt% ZrO2 in mixture) the current–voltage dependency is superlinear (current is increased stronger than voltage) but the current limiting effect is not observed. Based on the data of dc and ac measurements, temperature dependence of conductivity, X-ray diffraction, and scanning electron microscopy results, the conductivity features of obtained materials are discussed. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Grain boundaries; Indium oxide; Grain-boundary potential barriers; Current limiting; Adsorption processes
1. Introduction Indium oxide In2O3 is a transparent wide-gap semiconductor which has a high n-type electrical conductivity owing to oxygen vacancies acting as donors. Its direct energy gap equals 3.6 eV [1,2] and a minimum concentration of conduction electrons reaches the order of 1017 cm 3 at room temperature [3]. Doped In2O3 possesses high sensitivity to different gases. These properties make indium oxide a widely used material in optoelectronics [4,5] and in sensor applications [6,7]. Additionally, In2O3-based ceramics are of interest as a material exhibiting non-Ohmic current–voltage behavior. The electrical current in In2O3–SrO ceramics exhibits a tendency toward saturation and can even decrease with an increase in n
Corresponding author. Tel.: þ52 9535320214x305. E-mail address: [email protected] (A.N. Bondarchuk).
voltage [8–11]. This current limiting effect is explained by the increase of potential barriers at the boundaries of ceramic grains which control a charge carrier transfer in polycrystalline semiconductors [9]. The rise of barrier height after voltage application is a result of additional oxygen absorption stimulated by an electric field [9]. The current limiting effect is observed in In2O3–SrO ceramics sintered at relatively low temperature, between 1373 and 1473 K in air. Recently it was shown that such In2O3–SrO ceramics sintered at 1473 K contain a metastable hexagonal R-phase having the form of a network made up of mesoscopic clusters (60–180 Å in size) [12]. The R-phase is formed by strontiumenriched regions near grain boundaries in the main cubic structure of indium oxide. Such clusters are directly involved in the formation of the grain-boundary potential barriers because a grain boundary with clusters accommodates a much larger number of localized states than a “pure” grain boundary,
http://dx.doi.org/10.1016/j.ceramint.2014.03.096 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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e.g., in undoped In2O3 [12]. The capture of electrons at these localized states leads to an increase of barrier height at the grain boundaries. In addition, such clusters of the R-phase may favor charge carrier transfer and diffusion of adsorbed oxygen along grain boundaries [12]. Materials which possess a capability to conduct an electron current and ions of oxygen are of interest for the creation of new oxygen sensors and as cathode material for solid oxide fuel cells (SOFC). It is known that In2O3–ZrO2 compound sintered around 1773 K presents an electronic and/or ionic conductivity according to the proportions of both oxides [13]. The structures exhibiting the mixed electronic and ionic conductivity are used as cathode material for SOFC. The ionic radii of indium (0.62 Å for In3 þ ) and zirconium (0.59 Å for Zr4 þ ) are close [14] which enables them to easily penetrate into the crystal lattice of each other oxides. In2O3-doped ZrO2, with relatively low dopant ratios ( o 40%), stabilizes solid solutions of cubic or tetragonal ZrO2 phases exhibiting high ionic conductivities at elevated temperatures 1073–1273 K [13]. However, high operating temperatures of the SOFC based on In2O3–ZrO2 compound complicate their application. Therefore, a search for new materials exhibiting the mixed high electronic and ionic conductivity at reduced temperature (o 1073 K) is of great interest. The absorption of oxygen by In2O3–SrO ceramics from gas atmosphere, the process which is controlled by electric field [9,11], can be a useful capability of material under review for SOFC applications. There is a possibility that In2O3–SrO ceramics with current limiting effect can be a promising basic structure to create a cathode material for SOFC. To improve the diffusion of oxygen along grain boundaries in In2O3–SrO ceramics and to form regions with ionic conductivity near grain boundaries, the Zr dopant can be used. In this study – the first of a series – the effect of Zr dopant and sintering temperature on the electrical properties of In2O3– SrO ceramics at room temperature with current limiting effect was investigated. In2O3–SrO–ZrO2 ceramics with different Zr contents (from 5 to 60 wt% ZrO2 in the mixture) were obtained at the temperatures of 1473 K and 1573 K. Subsequently, after evaluating their electrical properties, the influence of Zr content and sintering temperature on the current–voltage response was thoroughly discussed. 2. Material and methods The In2O3–SrO and In2O3–SrO–ZrO2 ceramics were prepared by the conventional mixed oxides method with sintering ALDRICH Indium (III) oxide (99.99% trace metals basis), ALDRICH Strontium carbonate (Z 99:9% trace metals basis), and ALDRICH Zirconium (IV) oxide nanopowder (o100 nm particle size) at a temperature of 1473 K and 1573 K for 2 h. The In2O3–SrO ceramics (basic composition) were obtained by mixing 90 wt% In2O3 and 10 wt% SrCO3 as they had been used previously to obtain materials with the current limiting effect [9–12]. The In2O3– SrO–ZrO2 ceramics were synthesized from powders of ZrO2 and the basic composition mixed in the following proportion (wt%): 5:95, 10:90, and 60:40 respectively.
Mixed powders were compressed into the tablets 10 mm in diameter and 1–2 mm in thickness under a pressure of 100 MPa. The Pt-electrodes of samples were fired at 1073 K (1 h) and Ag- electrodes at 873 K (1 h) in air by heating and cooling of a sample with a rate of 3 K/min. The electrical properties of the obtained materials were evaluated at the temperature of 298 K in air with a relative humidity near 50%. The current–voltage characteristics were studied utilizing a Keithley-2410 unit under computer control. Voltage was applied for 200 ms (current was measured at the end of this interval). Small-signal capacitance and active conductance were registered using an INSTEK LCR-8101G instrument with the test fixture LCR-13. Measurements were performed using ac voltage amplitude 10 mV. The relative dielectric permittivity ε and the ac conductivity g were calculated from the expression C ¼ εε0 S=d and G 1 ¼ g 1 d=S , where C is the capacitance, G is the active conductance of sample, ε0 is the permittivity of vacuum, d and S are the thickness and the cross-section of a sample, respectively. For measurements in air at 75% relative humidity, the samples were placed in a closed chamber at a height which was above the surface of a NaCl salt saturated water solution. Before the measurements, the samples were stored in the chamber for 48 h. The record of electrical current was performed at a fixed voltage, in the Ohmic region. The density of the sintered samples was determined by the Archimedes' principle method using an analytical balance Ohaus AV64C. The temperature dependence of dc electrical conductivity was registered in the Ohmic region using Keithley 6517B unit in the range of 290–1000 K in air by heating and cooling of a sample with a rate of 3 K/min. The dc conductivity s was calculated from the expression D 1 ¼ s 1 d=S, where D is the dc conductance of sample. The activation energy of electrical conduction Ea was found from sðTÞ using the expression: sðTÞ ¼ s0 expð E a =ðkTÞÞ where s0 is a constant, k is the Boltzmann's constant, and T is the absolute temperature. The X-ray diffraction (XRD) patterns were recorded using a Panalytical Empyrean diffractometer employing Cu Kα1 radiation (λ=1.5406 Å) operated at 45 kV and 40 mA and an X'Celerator detector in a Bragg–Brentano geometry. The scans were done in the 2Θ range from 10 to 1201 with a step scan of 0.0161 and 80 s per step in a continuous mode. Phase identification was performed using the X'Pert HighScore Plus software (version 3.0d) and ICDD PDF-4 plus database (ICDDInternational Centre for Diffraction Data, Newtown Square, PA). Examination of the samples by scanning electron microscopy (SEM) was performed in the Nova NanoSEM200 microscope (FEI Company, USA). 3. Results and discussion 3.1. Structure analysis Representative X-ray diffraction patterns of In2O3–SrO and In2O3–SrO–ZrO2 ceramics are presented in Fig. 1. In2O3–SrO
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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30000 20000 10000 0 90000 60000 30000 0 40000 20000 0
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0 100000 50000 0 30000 20000 10000 0
30
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2 (degrees) Fig. 1. X-ray diffraction patterns for In2O3–SrO (a, d) and In2O3–SrO–ZrO2 ceramics with 10 wt% (b, e) and 60 wt% (c, f) ZrO2 in mixture. Materials were sintered at 1473 K (a, b, c) and 1573 K (d, e, f). Detected phases: 1 – In2O3; 2 – In2SrO4; 3 – ZrO2 (monoclinic); 4 – ZrSrO3; and 5 – ZrO2 (tetragonal).
ceramics sintered at 1473 K and 1573 K exhibit cubic In2O3 and orthorhombic In2SrO4 phases (Table 1). The cubic In2O3 phase is registered as well in all obtained In2O3–SrO–ZrO2 ceramics. However, the In2SrO4 phase is observed only for materials with low Zr content (10 wt% ZrO2 in mixture) and not detected if the content of zirconium is high (60 wt% ZrO2 in mixture). In addition to cubic In2O3 phase, the ceramics with high Zr content contain tetragonal ZrO2, monoclinic ZrO2, and ZrSrO3 phases (Table 1). In all obtained In2O3– SrO–ZrO2 ceramics the dopants of zirconium and strontium form the ZrSrO3 phase. The ionic radius of zirconium is less than that of indium, respectively 0.59 Å (Zr4 þ ) and 0.62 Å (In3 þ ) according to [14]. Therefore, zirconium can easily penetrate into the main cubic structure of indium oxide at high temperature near 1773 K [13]. However, the lattices parameters of In2O3 phases in obtained In2O3–SrO and In2O3–SrO–ZrO2 ceramics are close and do not change substantially (Table 1). This can be related to low sintering temperatures (1473 and 1573 K) at which the diffusion of atom impurity into indium oxide is weak. Therefore, Zr does not penetrate deeply into the crystal lattice of In2O3 and forms zirconium-enriched regions mainly near grain boundaries. In ceramics obtained at this sintering temperature (1473 K or 1573 K) the strontium cannot penetrate deeply into In2O3-grains as well because the ionic radius of strontium (1.18 Å for Sr2 þ ) is
3
substantially bigger than that of indium (0.62 Å for In3 þ ) [14]. Thereby, it can be expected that the strontium and zirconium enrich mainly the regions of cubic structure of indium oxide near grain boundaries in the obtained ceramics. Illustrative SEM micrographs of obtained materials are presented in Fig. 2. For In2O3–SrO ceramics the grain size increases with sintering temperature; for example, in the sample sintered at 1473 K (Fig. 2a) grains in the range 0:1–1 μm are observed, in contrast to the sample sintered at 1573 K (Fig. 2b), where the grain size is 0:5–2:5 μm. Both materials are sufficiently porous, which is a condition favorable for adsorption processes at grain boundaries. The density of samples varies with sintering temperature and is 5.27 g/cm3 (1473 K) and 5.52 g/cm3 (1573 K), respectively. For In2O3–SrO–ZrO2 ceramics of the same composition, the grain size and the porosity are not changed noticeably if the sintering temperature is varied from 1473 to 1573 K. However, the grain sizes and the porosity are decreased with the increase of Zr content (Fig. 2c and d). For example, in ceramics with low Zr content (Fig. 2c), the grain sizes are in the range 0:3–1:2 μm and the density of sample is 5.93 g/cm3. For In2O3–SrO ceramics with high Zr content (Fig. 2d), the grain size does not exceed 0:8 μm, and there are a lot of small grains with size less than 0:1 μm (Fig. 2d); the density of this sample (Fig. 2d) reaches 5.81 g/cm3 and is higher than that of In2O3–SrO ceramics (5.52 g/cm3). This suggests that the high percentage of ZrO2 dopant in mixture (60 wt%) slows down grain growth in ceramics sintered at relatively low temperatures (1473 and 1573 K). 3.2. Current–voltage characteristics Graphs of current (I)–voltage (U) dependencies for In2O3– SrO and In2O3–SrO–ZrO2 ceramics are shown in Fig. 3. In In2O3–SrO ceramics sintered at 1473 K the current limiting effect is observed: current rises more weakly than voltage, is saturated and even decreased (the region CD on curve 1, Fig. 3a). This effect is registered after linear (AB) and superlinear (BC) rise of current with voltage (curve 1, Fig. 3a). In higher electric fields, after current-limiting behavior (CD) the superlinear rise of current with voltage (DE) is observed (curve 1, Fig. 3a). In In2O3–SrO ceramics obtained at 1573 K the I(U) dependence does not contain a clearly expressed region where current is saturated and even decreased with voltage increase, as was observed for the material sintered at 1473 K. Instead, only a region of I(U) dependency, where current is increased weaker than voltage, is observed (Fig. 3a, curve 2). The current limiting effect is not observed as well for In2O3–SrO ceramics if sintering temperature of this material is higher than 1573 K. For such ceramics, after Ohmic region of I(U) dependency only superlinear rise of current with voltage can be registered. The dopant of Zr changes current–voltage behavior in indium oxide ceramics. For In2O3–SrO–ZrO2 material with high Zr content (60 wt% ZrO2 in mixture) sintered at 1473 K, the I(U) dependency contains a region of the superlinear rise of
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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Table 1 The results of XRD analysis of In2O3-SrO and In2O3–SrO–ZrO2 ceramics. Material
Phase present
Crystal system
Sintering ZrO2 in temperature, mixture, wt K % 1473
0 10
60
1573
0 10 60
Lattice parameter (Å) a
b
c
α
β
γ
wt (%)
Weighted value, Rwp (%)
Expected value, Rexp (%)
9.48
4.43
6.22
4.58
6.61
3.47
4.82
3.62
6.71
3.09
4.82
4.70
7.02
3.96
5.21
3.13
6.41
3.13
4.76
4.18
5.36
3.22
3.86
2.74
In2O3 In2SrO4 In2O3 In2SrO4 ZrSrO3 In2O3 ZrSrO3 ZrO2 ZrO2
Cubic Orthorhombic Cubic Orthorhombic Cubic Cubic Cubic Tetragonal Monoclinic
10.1254 9.8458 10.12042 9.839 8.2103 10.1205 8.1928 3.6020 5.1594
10.1254 3.2686 10.12042 3.2653 8.2103 10.1205 8.1928 3.6020 5.2071
10.1254 11.4963 10.12042 11.493 8.2103 10.1205 8.1928 5.183 5.3111
90 90 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 99.204
90 90 90 90 90 90 90 90 90
71.5 28.5 87.5 3.9 8.6 27.8 4.9 10.4 56.9
In2O3 In2SrO4 In2O3 In2SrO4 ZrSrO3 In2O3 ZrSrO3 ZrO2 ZrO2
Cubic Orthorhombic Cubic Orthorhombic Cubic Cubic Cubic Tetragonal Monoclinic
10.121 9.8368 10.1211 9.8394 8.2105 10.1208 8.1929 3.5954 5.1601
10.121 3.2664 10.1211 3.2658 8.2105 10.1208 8.1929 3.5954 5.2039
10.121 11.4922 10.1211 11.493 8.2105 10.1208 8.1929 5.192 5.3157
90 90 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 98.948
90 90 90 90 90 90 90 90 90
72.8 27.2 86.3 5.3 8.4 30.7 3.6 8.7 57
Profile Goodness of value, Rp fit value, χ2 (%)
Fig. 2. SEM micrographs of In2O3–SrO ceramics sintered at 1473 K (a) and 1573 K (b); In2O3–SrO–ZrO2 ceramics sintered at 1573 K from mixture with 10 wt% (c) and 60 wt% and (d) ZrO2. Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
A.N. Bondarchuk et al. / Ceramics International ] (]]]]) ]]]–]]]
Current density (A/cm2)
10-1 E
10-2 C
10-3 10-4
1 2 3
10-5 B
10-6
A
1
Current density (A/cm2)
D
10 100 Electric field (V/cm)
1000
8x10-3 6x10-3 4x10-3
4 5 6 7
2x10-3 0 0.0
0.4 Electric field (V/cm)
0.8
Fig. 3. Current–voltage dependency for (a) In2O3–SrO material (curves 1 and 2) and In2O3–SrO–ZrO2 ceramics (curve 3) with high content of Zr (60 wt% ZrO2 in mixture); (b) low Zr-doped In2O3–SrO–ZrO2 ceramics obtained from mixture with 5 wt% ZrO2 (curves 4 and 6) and 10 wt% ZrO2 (curves 5 and 7). Materials were sintered at 1473 K (curves 1, 3, 4, and 5) and 1573 (curves 2, 6, and 7).
current with voltage but the current limiting effect is not observed (curve 3, Fig. 3a). For In2O3–SrO–ZrO2 ceramics with low Zr content (5 and 10 wt% ZrO2 in mixture) sintered at 1473 K, the I(U) dependencies are linear up to 1 mA/cm2 (curves 4 and 5, Fig. 3b). For low Zr doped In2O3–SrO–ZrO2 ceramics obtained at 1573 K (curves 6, 7 in Fig. 3b) the type of current–voltage behavior is the same as in this material sintered at 1473 K (Fig. 3b, curves 4, 5). However, the conductivity of ceramics sintered at 1573 K is higher. The I(U) dependency in obtained In2O3–SrO ceramics with the current limiting effect was explained in terms of a modified barrier model [9] based on the results of measurements in different gas environments and computer simulation [9–11]. In this model grain-boundary states which are occupied by electrons (filled grain-boundary states) are associated with adsorbed oxygen in charged form. Grain-boundary states which are not occupied by electrons (empty grain-boundary states) are associated with adsorbed oxygen in neutral form. The density of empty and filled grain-boundary states is assumed to be equal to the density of adsorbed oxygen in
5
neutral and charged forms, respectively. The capture of electrons at the grain-boundary states leads to the transition of adsorbed oxygen from neutral to charged form. The adsorption equilibrium at the grain surface is maintained at the expense of oxygen adsorption and desorption only in the neutral form [9]. Therefore, if the amount of adsorbed oxygen in neutral form is decreased (as a result of capture of electrons at the grain-boundary states), the adsorption–desorption equilibrium at the grain boundary is broken and additional oxygen adsorption starts. In the equilibrium state at grain boundary (when voltage bias is zero), the capture of electrons at the grain-boundary states is balanced by their emission and the barrier height is not changed. However, under applied voltage the barrier height is decreased and the capture of electrons at the grain-boundary states is increased. This results in the transition of adsorbed oxygen from neutral to charged form, and the adsorption– desorption equilibrium at the grain boundary is broken. Additional oxygen adsorption appears and leads to a rise in the number of empty states at grain boundary. Therefore, under applied voltage the capture of electrons dominates over their emission until adsorption equilibrium is reached. Thus, the barrier height is increased and as a result of this, the current limiting behavior takes place (Fig. 3a, curve 1, the region CD). For these processes of adsorption, access of oxygen to grain boundary is needed. In In2O3–SrO ceramics the important role in oxygen diffusion along grain boundary can be played by the metastable hexagonal R-phase formed by strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide [12]. It is reasonable to suppose that the contact between grains is closer in In2O3–SrO ceramics sintered at 1573 K than in material obtained at 1473 K. Therefore, access of oxygen to grain boundary is hindered in ceramics sintered at 1573 K or at higher temperatures, and as a consequence the current limiting effect is less pronounced (Fig. 3a, curve 2) or is absent. The In2O3–SrO–ZrO2 ceramics with low Zr content possess very high electrical conductivity (curve 7 in Fig. 3b). To be sure that current–voltage behavior in this material is defined by sample's volume but not Pt-contacts, the I(U) dependencies at different thickness of the sample were recorded. In each case only one side of the sample with Pt-contact was abraded. The experiment shows that the calculated conductance of the sample per unit of contact substantially depends on the thickness of material between electrodes. In addition, the same I(U) dependencies were obtained on the samples with Agcontacts. This lets us assert that the results of dc measurements are related to the material behavior. As it was shown in [11], In2O3–SrO ceramics possess a weak sensitivity to humidity in consequence of underdeveloped system of transport pores (open macro- and mesopores from nm to hundreds nm) which are needed for effective development humidity-absorption processes. The experiment shows that the sensitivity of all obtained In2O3–SrO–ZrO2 ceramics to humidity is low as well. After a transfer of samples from air at 50% relative humidity to air at 75% humidity, the sample's conductance is increased but not more than 6% up
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from its initial resistance recorded in air at 50% relative humidity. Observed changes in the conductance can be related to the lowering of the barrier height at grain boundary. Also some contribution of surface leakage in humid air cannot be excluded completely. The high conductivity of In2O3–SrO–ZrO2 ceramics with low Zr content (5 and 10 wt% ZrO2 in mixture) can be related to low heights of grain-boundary barriers. To corroborate this, for obtained ceramics, the temperature dependence of conductivity and frequency dependence of capacitance and ac conductance were studied.
3.3. Temperature dependence of conductivity Temperature dependences of dc conductivity for In2O3–SrO and In2O3–SrO–ZrO2 ceramics are presented in Fig. 4. The comparison of dependences for ceramics of the same composition but sintered at different temperature shows that material obtained at 1473 K has lower conductivity at room temperature and higher activation energy of electrical conduction in the interval of 450–520 K (Table 2) than ceramics sintered at 1573 K. This conforms to our previous supposition that the contact between grains is better in ceramics sintered at 1573 K than in material obtained at 1473 K. At room temperature, the conductivity of low Zr doped In2O3–SrO–ZrO2 materials is four orders of magnitude higher than that of In2O3–SrO ceramics sintered at the same temperature (Fig. 4, curves 2 and 6). However, the conductivity of material with high Zr content (60 wt% ZrO2 in mixture) remains at the same order as that of In2O3–SrO ceramics (Fig. 4, curves 2 and 4). The conductivity increase of indium oxide ceramics caused by Zr dopant was expected, as zirconium is known to act as a donor in In2O3 and increases electron concentration and electronic conductivity [15,16]. Therefore, the height of grainboundary barrier is decreased in accordance with the known
Conductivity (Ohm-1 cm-1)
102 101 100 10-1 10-2
1 2 3 4 5 6
10-3 10-4 10-5 10-6 0.5
1.0
1.5
2.0
2.5
3.0
3.5
1000/T(K-1) Fig. 4. Conductivity as a function of the temperature recorded at heating for In2O3–SrO (curves 1 and 2) and In2O3–SrO–ZrO2 (curves 3–6) ceramics sintered at 1473 K (curves 1,3,5) and 1573 K (curves 2,4,6). In2O3–SrO–ZrO2 materials were obtained from mixture with 10 wt% (curves 5 and 6) and 60 wt % (curves 3 and 4) ZrO2.
Table 2 The activation energy of electrical conduction for ceramics sintered from ð1 xÞ (In2O3–SrCO3)–x ZrO2 composition. x
0 0 5 5 10 10 60 60
Sintering temperature of ceramics, K
1473 1573 1473 1573 1473 1573 1473 1573
Activation energy Ea (eV) 7 0.01 eV in the interval: 450–520 K
750–950 K
0.50 0.29 0.24 0.23 0.23 0.15 0.30 0.16
1.12 0.83 0.31 0.25 0.30 0.29 0.78 0.77
expression [17,18]: φ0 ¼
q2 m20 8εε0 N D
ð1Þ
where q is the elementary charge, m0 is the surface density of electrons captured at the grain-boundary states at U=0 , ND is the concentration of fully ionized donors in the grains, ε and ε0 are the relative permittivity and the permittivity of vacuum, respectively. This decreasing of grain-boundary barriers after the doping by Zr is confirmed by lower magnitude of Ea for In2O3–SrO–ZrO2 ceramics than for In2O3–SrO materials in the interval of 450–520 K (Table 2). In addition to barrier lowering, the enriching of grainboundary regions by donor dopant of Zr leads to narrowing of the space-charge region in the Schottky barriers. Therefore, these barriers in low Zr-doped materials can be sufficiently thin to be transparent for tunneling electrons at room temperature. As a result of this, I(U) dependencies observed for low Zrdoped In2O3–SrO materials are linear (Fig. 3b). The supposition about electron tunneling through grain boundaries is confirmed by weak temperature dependence of the conductivity registered for low Zr-doped ceramics in the interval of 300–350 K (Fig. 4, curves 5 and 6). At heating above 350 K, the conductivity of ceramics starts to increase more strongly with temperature than between 300 and 350 K (Fig. 4, curves 5 and 6). This is explained by the increase of thermionic emission over barrier which gives a bigger contribution to electrical current through grain boundary at higher temperatures. The decrease of conductivity registered at heating between 500 and 660 K (Fig. 4, curves 1–6) is related to the transformation of adsorbed oxygen O to O2 (O þ e-O2 ) and O2 to 2O (O2 þ e-2O ) which takes place at high temperatures [19,20]. This leads to increasing of negative electrical charge localized on grain boundary and so the grain-boundary barriers are increased. Respectively, the conductivity of materials becomes lower. Such behavior of conductivity versus temperature for obtained In2O3–SrO ceramics under different gas environments was studied in detail in the work [21]. The high-temperature increase of conductivity above 700 K can be related to the generation of new oxygen vacancy in grains and oxygen desorption from grain boundaries. These processes
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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3.4. Frequency dependence of capacitance and ac conductance The plots of relative dielectric permittivity (ε) and ac conductivity (g) versus frequency (f) for In2O3–SrO material and In2O3–SrO–ZrO2 ceramics with high Zr content are presented in Fig. 5a and b. For these materials, the decrease of ε and the weak increase of g are observed with the rise of frequency from 100 Hz to 1 MHz. Curves of εðf Þ and g(f) dependencies are reproducible at the increase and the decrease of frequency. To explain these εðf Þ and g(f) dependences, let us consider the admittance YðjϖÞ of two adjacent grains with the potential barrier at the grain boundary (Fig. 5c). In this case YðjϖÞ is expressed as YðjωÞ ¼ G þ jωC
ð2Þ
where G¼
GG GB ðGG þ GB Þ þ ðωCB Þ2 GG ðGG þ GB Þ2 þ ðωC B Þ2
ð3Þ
CB G2G ðGG þ GB Þ2 þ ðωCB Þ2
ð4Þ
and C¼
Here CB is the barrier capacitance, ω is the angular frequency, GG and GB are the conductance of “grain volume” and grain boundary, respectively. If the conductance of the grain volume is much more than the conductance of the grain boundary (jGG j b jGB þ jωC B j ¼ ½ðGB Þ2 þ ðωCB Þ2 1=2 ), we obtain from (3) and (4): C C B and G GB þ ðGG Þ 1 ðωC B Þ2 . In this case, the weak frequency dependence of capacitance C and sharp increase of the ac conductance G are expected. However, instead of this, in the
104 103 102 101 g (Ohm-1 cm-1)
lead to the increasing of the electron concentration in grains. Therefore the height of grain-boundary barrier is decreased, and so conductivity of ceramics is increased at heating above 700 K (Fig. 4). The lower conductivity of In2O3–SrO–ZrO2 ceramics with high Zr content compared with that of low Zr-doped material (Fig. 4, curves 3 and 5) can be the result of the presence of monoclinic ZrO2 phase in high amount, around 57 wt% (Table 1). The ZrO2 phase can form high-Ohmic layers between grains of indium oxide which leads to the decrease of conductivity of the material. In the case of low Zr content in material, the ZrO2 phase is absent or its concentration is too low to be detected by the diffractometer (Table 1) and so this phase cannot isolate In2O3 grains. In addition, the In2O3–SrO– ZrO2 ceramics with high Zr content have a lot of small grains (less than 0:1 μm) located between groups of sintered grains (Fig. 2d). In comparison with material with low content of Zr where grain size is in the range 0:3–1:2 μm (Fig. 2c), this leads to a bigger number of grain-boundary barriers per unit of length, at least three times. Therefore, the conductivity of In2O3–SrO–ZrO2 ceramics with high Zr content is lower than for low Zr-doped material.
7
10-3 10-4 10-5 10-6 10-7 10-8
1 2
3 4
104
103
102
1 2
105
106
3 4
Frequency (Hz)
Fig. 5. (a) The relative dielectric permittivity ε and (b) ac conductivity g versus frequency recorded for In2O3–SrO ceramics (curves 1 and 2) and for In2O3–SrO–ZrO2 material (curves 3 and 4) with high Zr content (60 wt% ZrO2 in mixture). Ceramics were sintered at 1473 K (curves 1 and 3) and at 1573 K (curves 2 and 4). (c) The equivalent circuit of two adjacent grains: CB is the barrier capacitance and GB and GG are the conductance of the grain boundary and the grain volume, respectively.
experiment the decrease of ε and the weak increase of g were registered at the rise of frequency (Fig. 5). This is possible if jGG j and jGB þ jωCB j are comparable, and the registered values of G and C are determined by (3) and (4). In this case, the voltage of ac measuring signal is redistributed between the grain volume and the grain-boundary region. As a result, in ceramics the calculated value of the relative dielectric permittivity ε is decreased to the value of the relative dielectric permittivity in pure In2O3. However, sufficiently high values of ε at low-frequencies (Fig. 5a, curves 1–4) show that the inequality jGG j b jGB þ jωCB j is performed at least at 100 Hz for these ceramics. The typical frequency dependencies of capacitance and ac conductance for In2O3–SrO–ZrO2 ceramics with low Zr content (5 and 10 wt% ZrO2 in mixture) are presented in Fig. 6. At first the capacitance is decreased until negative values at the rise of frequency from 100 Hz to 4 kH (Fig. 6a, curve 1). In this frequency range, the decreasing of capacitance is accompanied by the decrease of ac conductance (Fig. 6b, curve 1). With the frequency increase from 4 kHz to 20 kHz, the capacitance starts to increase sharply but ac conductance continues to decrease. However, above 20 kHz and up to 1 MHz the capacitance starts to decrease again (Fig. 6a, curve 1) and is accompanied by the increasing of conductance g (Fig. 6b, curve 1). This behavior of capacitance and conductivity versus frequency is repeated at successive measurements (curves 1–3 in Fig. 6) and at higher voltage (10–50 mV) of signal amplitude. However, during these successive measurements the curves of ac conductance versus frequency are shifted to the area of lower conductivity (Fig. 6b, curves 1–3). The calculated ε is near 48 000 at 100 Hz and 250 at 1 MHz respectively. The sharp increase of capacitance in the frequency range 4–20 kHz which is accompanied by the sharp decrease of ac conductance can be a result of charge trapping at the grain boundary. However, discussion of such processes requires
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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1.0x10-8
1.5x10-10 1.0x10-10
Capacitance (F)
8.0x10-9
5.0x10-11 0.0
6.0x10-9
-5.0x10-11 -1.0x10-10
4.0x10-9 2.0x10-9 0.0 101
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Acknowledgments This study was supported by the National Science and Technology Council of Mexico (CONACYT) under the project “Convocatoria Ciencia Basica 2010 (the project number 154625-Y)”. Authors thank Ms. Nayely Pineda Aguilar and Mr. Cesar Leyva Porras for their assistance in the microstructure characterization by SEM (Cimav-Monterrey).
1.7x10-2
Conductance (Ohm-1)
is related to the presence of high-resistance ZrO2 phase between grains of indium oxide in materials with high content of Zr. For low Zr-doped In2O3–SrO–ZrO2 ceramics (5 and 10 wt% ZrO2 in mixture) the ZrO2 phase is not detected. Frequency dependences of capacitance and ac conductance for low Zr-doped In2O3–SrO–ZrO2 ceramics contain a region between 4 kH and 20 kHz where the sharp increase of conductance which is accompanied by the decrease of ac conductance is observed. The obtained In2O3–SrO–ZrO2 ceramics are very porous, which is favorable for adsorption processes and diffusion of oxygen through the material.
1.7x10-2
References 1 2 3
1.7x10-2 1 10
102
103 104 Frequency (Hz)
105
106
Fig. 6. (a) The capacitance and (b) ac conductance versus frequency recorded at three successive measurements (curves 1–3) of the sample of In2O3–SrO– ZrO2 ceramics with low Zr content (10 wt% ZrO2 in mixture) sintered at 1473 K. Each curve was recorded at increasing and subsequent decreasing frequency.
more detailed experimental investigation of this effect which will be presented in the next paper. 4. Conclusions The effects of Zr dopant and sintering temperature on the electrical properties of In2O3–SrO based ceramics have been investigated. The electrical current in In2O3–SrO ceramics sintered at 1473 K exhibits a tendency toward saturation and can even decrease with an increase in voltage. However, this current-limiting effect is not registered if sintering temperature of ceramics is above 1573 K. Observed current limiting is explained in terms of the modified grain boundary model which considers a relationship between the electronic and adsorption processes. The dopant of zirconium in low concentrations acts as a donor in In2O3–SrO–ZrO2 ceramics sintered at relatively low temperature (1473 and 1573 K). Such materials possess high conductivity and linear current–voltage dependencies. However, if the concentration of zirconium is high (60 wt% ZrO2 in mixture), the conductivity of In2O3–SrO–ZrO2 ceramics lowers and nonlinear current–voltage behavior is observed. This
[1] P. Erhart, A. Klein, R.G. Egdell, K. Albe, Band structure of indium oxide: indirect versus direct band gap, Phys. Rev. B: Condens. Matter 75 (2007) 153205–153209. [2] H. Baqiah, N.B. Ibrahim, M.H. Abdi, S.A. Halim, Electrical transport, microstructure and optical properties of Cr-doped In2O3 thin film prepared by sol–gel method, J. Alloys Compd. 575 (2013) 198–206. [3] Y. Ohya, T. Yamamoto, T. Ban, Equilibrium dependence of the conductivity of pure and tin-doped indium oxide on oxygen partial pressure and formation of an intrinsic defect, J. Am. Ceram. Soc. 91 (2008) 240–245. [4] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, Bristol, 1995. [5] Y. Kuo, Characterization of indium tin oxide and reactive ion etched indium tin oxide surfaces, Jpn. J. Appl. Phys. 29 (1990) 2243–2246. [6] N.E. Trofimenko, S.V. Baran, N.P. Masherova, K.A. Lesnikovich, Production and use of new materials-properties of ceramics and sensors based on modified indium oxide, Russ. J. Appl. Chem. 68 (1995) 397–399. [7] A.P. Belyaev, V.P. Rubets, I.P. Kalinkin, Current transport processes in In2O3–ZnSe–(Zn1 x Cdx Te)1 y (In2Te3)y heterostructures, Thin Solid Films 256 (1995) 23–30. [8] A. Glot, G. Behr, J. Werner, Current limiting effect in In2O3 ceramics based structures, Key Eng. Mater. 206–213 (2002) 1441–1444. [9] A. Bondarchuk, A. Glot, G. Behr, J. Werner, Current saturation in indium oxide based ceramics, Eur. Phys. J: Appl. Phys. 39 (2007) 211–217. [10] A.B. Glot, S.V. Mazurik, B.J. Jones, A.N. Bondarchuk, R. Bulpett, N. Verma, Current limiting and negative differential resistance in indium oxide based ceramics, J. Eur. Ceram. Soc. 30 (2010) 539–544. [11] A. Bondarchuk, O. Shpotyuk, A. Glot, H. Klym, Current saturation in In2O3–SrO ceramics: a role of oxidizing atmosphere, Rev. Mex. Fis. 58 (2012) 313–316. [12] Z.A. Samoilenko, V.D. Okunev, S.J. Lewandowski, P. Aleshkevych, Yu.M. Nikolaenko, E.I. Pushenko, O. Abal'oshev, P. Gierlowski, A.N. Bondarchuk, A.B. Glot, Influence of a strontium-enriched intergranular cluster network on the electrical conductivity of In2O3–SrO ceramics, Tech. Phys. 58 (2013) 1144–1151. [13] A. Ringuedé, P. Mourot, C. Alvarez Lugano, J-C. Badot, M. Cassir, Elaboration, characterisation and electrical properties of ZrO2–In2O3 compounds with different compositions for intermediate temperature SOFC, J. New Mater. Electrochem. Syst. 9 (2006) 201–208.
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[19] S.C. Chang, Oxygen chemisorption on tin oxide: correlation between electrical conductivity and EPR measurements, J. Vac. Sci. Technol. 17 (1980) 366–369. [20] J.F. McAleer, P.T. Moseley, B.C. Tofield, D.E. Williams, Factors affecting the performance of SnO2 as a gas sensor, Proc. Br. Ceram. Soc. 36 (1985) 89–105. [21] A. Bondarchuk, A. Glot, G. Behr, J. Werner, High temperature anomaly of electrical conduction in In2O3–SrO ceramics, J. Phys. Stud. 11 (2007) 408–414.
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics A.N. Bondarchuka,n, J.A. Aguilar-Martinezb,c, M.I. Pech-Canuld a Universidad Tecnológica de la Mixteca, Huajuapan de León, Oaxaca 69000, México Centro de Investigación en Materiales Avanzados, S.C. (CIMAV), Alianza Norte No. 202, Parque de Investigación e Innovación Tecnológica (PIIT), Nueva Carr. Aeropuerto km. 10 Apodaca N.L. 66600, México c Universidad Autónoma de Nuevo León, Facultad de Ingeniería Mecánica y Eléctrica, Centro de Investigación e Innovación en Ingeniería Aeronáutica (CIIIA); Carretera a Salinas Victoria km. 2.3, C.P. 66600 Apodaca, N.L., México d Cinvestav Saltillo, Av. Industria Metalúrgica No. 1062, Parque Industrial Saltillo-Ramos Arizpe. Ramos Arizpe, Coahuila 25900, México b
Received 18 December 2013; received in revised form 7 March 2014; accepted 19 March 2014
Abstract Electrical properties of indium oxide ceramics doped by Sr and Zr and sintered at 1473 and 1573 K in air were studied and are discussed. In2O3–SrO ceramics exhibit nonlinear current–voltage behavior with the current limiting effect: current rises more weakly than voltage, is saturated and even decreased. The Zr dopant changes the type of current–voltage dependency in such materials. In2O3–SrO–ZrO2 ceramics with low Zr content (5–10 wt% ZrO2 in mixture) possess linear current–voltage characteristic and very high conductivity. For ceramics with high content of Zr (60 wt% ZrO2 in mixture) the current–voltage dependency is superlinear (current is increased stronger than voltage) but the current limiting effect is not observed. Based on the data of dc and ac measurements, temperature dependence of conductivity, X-ray diffraction, and scanning electron microscopy results, the conductivity features of obtained materials are discussed. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Grain boundaries; Indium oxide; Grain-boundary potential barriers; Current limiting; Adsorption processes
1. Introduction Indium oxide In2O3 is a transparent wide-gap semiconductor which has a high n-type electrical conductivity owing to oxygen vacancies acting as donors. Its direct energy gap equals 3.6 eV [1,2] and a minimum concentration of conduction electrons reaches the order of 1017 cm 3 at room temperature [3]. Doped In2O3 possesses high sensitivity to different gases. These properties make indium oxide a widely used material in optoelectronics [4,5] and in sensor applications [6,7]. Additionally, In2O3-based ceramics are of interest as a material exhibiting non-Ohmic current–voltage behavior. The electrical current in In2O3–SrO ceramics exhibits a tendency toward saturation and can even decrease with an increase in n
Corresponding author. Tel.: þ52 9535320214x305. E-mail address: [email protected] (A.N. Bondarchuk).
voltage [8–11]. This current limiting effect is explained by the increase of potential barriers at the boundaries of ceramic grains which control a charge carrier transfer in polycrystalline semiconductors [9]. The rise of barrier height after voltage application is a result of additional oxygen absorption stimulated by an electric field [9]. The current limiting effect is observed in In2O3–SrO ceramics sintered at relatively low temperature, between 1373 and 1473 K in air. Recently it was shown that such In2O3–SrO ceramics sintered at 1473 K contain a metastable hexagonal R-phase having the form of a network made up of mesoscopic clusters (60–180 Å in size) [12]. The R-phase is formed by strontiumenriched regions near grain boundaries in the main cubic structure of indium oxide. Such clusters are directly involved in the formation of the grain-boundary potential barriers because a grain boundary with clusters accommodates a much larger number of localized states than a “pure” grain boundary,
http://dx.doi.org/10.1016/j.ceramint.2014.03.096 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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2
e.g., in undoped In2O3 [12]. The capture of electrons at these localized states leads to an increase of barrier height at the grain boundaries. In addition, such clusters of the R-phase may favor charge carrier transfer and diffusion of adsorbed oxygen along grain boundaries [12]. Materials which possess a capability to conduct an electron current and ions of oxygen are of interest for the creation of new oxygen sensors and as cathode material for solid oxide fuel cells (SOFC). It is known that In2O3–ZrO2 compound sintered around 1773 K presents an electronic and/or ionic conductivity according to the proportions of both oxides [13]. The structures exhibiting the mixed electronic and ionic conductivity are used as cathode material for SOFC. The ionic radii of indium (0.62 Å for In3 þ ) and zirconium (0.59 Å for Zr4 þ ) are close [14] which enables them to easily penetrate into the crystal lattice of each other oxides. In2O3-doped ZrO2, with relatively low dopant ratios ( o 40%), stabilizes solid solutions of cubic or tetragonal ZrO2 phases exhibiting high ionic conductivities at elevated temperatures 1073–1273 K [13]. However, high operating temperatures of the SOFC based on In2O3–ZrO2 compound complicate their application. Therefore, a search for new materials exhibiting the mixed high electronic and ionic conductivity at reduced temperature (o 1073 K) is of great interest. The absorption of oxygen by In2O3–SrO ceramics from gas atmosphere, the process which is controlled by electric field [9,11], can be a useful capability of material under review for SOFC applications. There is a possibility that In2O3–SrO ceramics with current limiting effect can be a promising basic structure to create a cathode material for SOFC. To improve the diffusion of oxygen along grain boundaries in In2O3–SrO ceramics and to form regions with ionic conductivity near grain boundaries, the Zr dopant can be used. In this study – the first of a series – the effect of Zr dopant and sintering temperature on the electrical properties of In2O3– SrO ceramics at room temperature with current limiting effect was investigated. In2O3–SrO–ZrO2 ceramics with different Zr contents (from 5 to 60 wt% ZrO2 in the mixture) were obtained at the temperatures of 1473 K and 1573 K. Subsequently, after evaluating their electrical properties, the influence of Zr content and sintering temperature on the current–voltage response was thoroughly discussed. 2. Material and methods The In2O3–SrO and In2O3–SrO–ZrO2 ceramics were prepared by the conventional mixed oxides method with sintering ALDRICH Indium (III) oxide (99.99% trace metals basis), ALDRICH Strontium carbonate (Z 99:9% trace metals basis), and ALDRICH Zirconium (IV) oxide nanopowder (o100 nm particle size) at a temperature of 1473 K and 1573 K for 2 h. The In2O3–SrO ceramics (basic composition) were obtained by mixing 90 wt% In2O3 and 10 wt% SrCO3 as they had been used previously to obtain materials with the current limiting effect [9–12]. The In2O3– SrO–ZrO2 ceramics were synthesized from powders of ZrO2 and the basic composition mixed in the following proportion (wt%): 5:95, 10:90, and 60:40 respectively.
Mixed powders were compressed into the tablets 10 mm in diameter and 1–2 mm in thickness under a pressure of 100 MPa. The Pt-electrodes of samples were fired at 1073 K (1 h) and Ag- electrodes at 873 K (1 h) in air by heating and cooling of a sample with a rate of 3 K/min. The electrical properties of the obtained materials were evaluated at the temperature of 298 K in air with a relative humidity near 50%. The current–voltage characteristics were studied utilizing a Keithley-2410 unit under computer control. Voltage was applied for 200 ms (current was measured at the end of this interval). Small-signal capacitance and active conductance were registered using an INSTEK LCR-8101G instrument with the test fixture LCR-13. Measurements were performed using ac voltage amplitude 10 mV. The relative dielectric permittivity ε and the ac conductivity g were calculated from the expression C ¼ εε0 S=d and G 1 ¼ g 1 d=S , where C is the capacitance, G is the active conductance of sample, ε0 is the permittivity of vacuum, d and S are the thickness and the cross-section of a sample, respectively. For measurements in air at 75% relative humidity, the samples were placed in a closed chamber at a height which was above the surface of a NaCl salt saturated water solution. Before the measurements, the samples were stored in the chamber for 48 h. The record of electrical current was performed at a fixed voltage, in the Ohmic region. The density of the sintered samples was determined by the Archimedes' principle method using an analytical balance Ohaus AV64C. The temperature dependence of dc electrical conductivity was registered in the Ohmic region using Keithley 6517B unit in the range of 290–1000 K in air by heating and cooling of a sample with a rate of 3 K/min. The dc conductivity s was calculated from the expression D 1 ¼ s 1 d=S, where D is the dc conductance of sample. The activation energy of electrical conduction Ea was found from sðTÞ using the expression: sðTÞ ¼ s0 expð E a =ðkTÞÞ where s0 is a constant, k is the Boltzmann's constant, and T is the absolute temperature. The X-ray diffraction (XRD) patterns were recorded using a Panalytical Empyrean diffractometer employing Cu Kα1 radiation (λ=1.5406 Å) operated at 45 kV and 40 mA and an X'Celerator detector in a Bragg–Brentano geometry. The scans were done in the 2Θ range from 10 to 1201 with a step scan of 0.0161 and 80 s per step in a continuous mode. Phase identification was performed using the X'Pert HighScore Plus software (version 3.0d) and ICDD PDF-4 plus database (ICDDInternational Centre for Diffraction Data, Newtown Square, PA). Examination of the samples by scanning electron microscopy (SEM) was performed in the Nova NanoSEM200 microscope (FEI Company, USA). 3. Results and discussion 3.1. Structure analysis Representative X-ray diffraction patterns of In2O3–SrO and In2O3–SrO–ZrO2 ceramics are presented in Fig. 1. In2O3–SrO
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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2 (degrees) Fig. 1. X-ray diffraction patterns for In2O3–SrO (a, d) and In2O3–SrO–ZrO2 ceramics with 10 wt% (b, e) and 60 wt% (c, f) ZrO2 in mixture. Materials were sintered at 1473 K (a, b, c) and 1573 K (d, e, f). Detected phases: 1 – In2O3; 2 – In2SrO4; 3 – ZrO2 (monoclinic); 4 – ZrSrO3; and 5 – ZrO2 (tetragonal).
ceramics sintered at 1473 K and 1573 K exhibit cubic In2O3 and orthorhombic In2SrO4 phases (Table 1). The cubic In2O3 phase is registered as well in all obtained In2O3–SrO–ZrO2 ceramics. However, the In2SrO4 phase is observed only for materials with low Zr content (10 wt% ZrO2 in mixture) and not detected if the content of zirconium is high (60 wt% ZrO2 in mixture). In addition to cubic In2O3 phase, the ceramics with high Zr content contain tetragonal ZrO2, monoclinic ZrO2, and ZrSrO3 phases (Table 1). In all obtained In2O3– SrO–ZrO2 ceramics the dopants of zirconium and strontium form the ZrSrO3 phase. The ionic radius of zirconium is less than that of indium, respectively 0.59 Å (Zr4 þ ) and 0.62 Å (In3 þ ) according to [14]. Therefore, zirconium can easily penetrate into the main cubic structure of indium oxide at high temperature near 1773 K [13]. However, the lattices parameters of In2O3 phases in obtained In2O3–SrO and In2O3–SrO–ZrO2 ceramics are close and do not change substantially (Table 1). This can be related to low sintering temperatures (1473 and 1573 K) at which the diffusion of atom impurity into indium oxide is weak. Therefore, Zr does not penetrate deeply into the crystal lattice of In2O3 and forms zirconium-enriched regions mainly near grain boundaries. In ceramics obtained at this sintering temperature (1473 K or 1573 K) the strontium cannot penetrate deeply into In2O3-grains as well because the ionic radius of strontium (1.18 Å for Sr2 þ ) is
3
substantially bigger than that of indium (0.62 Å for In3 þ ) [14]. Thereby, it can be expected that the strontium and zirconium enrich mainly the regions of cubic structure of indium oxide near grain boundaries in the obtained ceramics. Illustrative SEM micrographs of obtained materials are presented in Fig. 2. For In2O3–SrO ceramics the grain size increases with sintering temperature; for example, in the sample sintered at 1473 K (Fig. 2a) grains in the range 0:1–1 μm are observed, in contrast to the sample sintered at 1573 K (Fig. 2b), where the grain size is 0:5–2:5 μm. Both materials are sufficiently porous, which is a condition favorable for adsorption processes at grain boundaries. The density of samples varies with sintering temperature and is 5.27 g/cm3 (1473 K) and 5.52 g/cm3 (1573 K), respectively. For In2O3–SrO–ZrO2 ceramics of the same composition, the grain size and the porosity are not changed noticeably if the sintering temperature is varied from 1473 to 1573 K. However, the grain sizes and the porosity are decreased with the increase of Zr content (Fig. 2c and d). For example, in ceramics with low Zr content (Fig. 2c), the grain sizes are in the range 0:3–1:2 μm and the density of sample is 5.93 g/cm3. For In2O3–SrO ceramics with high Zr content (Fig. 2d), the grain size does not exceed 0:8 μm, and there are a lot of small grains with size less than 0:1 μm (Fig. 2d); the density of this sample (Fig. 2d) reaches 5.81 g/cm3 and is higher than that of In2O3–SrO ceramics (5.52 g/cm3). This suggests that the high percentage of ZrO2 dopant in mixture (60 wt%) slows down grain growth in ceramics sintered at relatively low temperatures (1473 and 1573 K). 3.2. Current–voltage characteristics Graphs of current (I)–voltage (U) dependencies for In2O3– SrO and In2O3–SrO–ZrO2 ceramics are shown in Fig. 3. In In2O3–SrO ceramics sintered at 1473 K the current limiting effect is observed: current rises more weakly than voltage, is saturated and even decreased (the region CD on curve 1, Fig. 3a). This effect is registered after linear (AB) and superlinear (BC) rise of current with voltage (curve 1, Fig. 3a). In higher electric fields, after current-limiting behavior (CD) the superlinear rise of current with voltage (DE) is observed (curve 1, Fig. 3a). In In2O3–SrO ceramics obtained at 1573 K the I(U) dependence does not contain a clearly expressed region where current is saturated and even decreased with voltage increase, as was observed for the material sintered at 1473 K. Instead, only a region of I(U) dependency, where current is increased weaker than voltage, is observed (Fig. 3a, curve 2). The current limiting effect is not observed as well for In2O3–SrO ceramics if sintering temperature of this material is higher than 1573 K. For such ceramics, after Ohmic region of I(U) dependency only superlinear rise of current with voltage can be registered. The dopant of Zr changes current–voltage behavior in indium oxide ceramics. For In2O3–SrO–ZrO2 material with high Zr content (60 wt% ZrO2 in mixture) sintered at 1473 K, the I(U) dependency contains a region of the superlinear rise of
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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4
Table 1 The results of XRD analysis of In2O3-SrO and In2O3–SrO–ZrO2 ceramics. Material
Phase present
Crystal system
Sintering ZrO2 in temperature, mixture, wt K % 1473
0 10
60
1573
0 10 60
Lattice parameter (Å) a
b
c
α
β
γ
wt (%)
Weighted value, Rwp (%)
Expected value, Rexp (%)
9.48
4.43
6.22
4.58
6.61
3.47
4.82
3.62
6.71
3.09
4.82
4.70
7.02
3.96
5.21
3.13
6.41
3.13
4.76
4.18
5.36
3.22
3.86
2.74
In2O3 In2SrO4 In2O3 In2SrO4 ZrSrO3 In2O3 ZrSrO3 ZrO2 ZrO2
Cubic Orthorhombic Cubic Orthorhombic Cubic Cubic Cubic Tetragonal Monoclinic
10.1254 9.8458 10.12042 9.839 8.2103 10.1205 8.1928 3.6020 5.1594
10.1254 3.2686 10.12042 3.2653 8.2103 10.1205 8.1928 3.6020 5.2071
10.1254 11.4963 10.12042 11.493 8.2103 10.1205 8.1928 5.183 5.3111
90 90 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 99.204
90 90 90 90 90 90 90 90 90
71.5 28.5 87.5 3.9 8.6 27.8 4.9 10.4 56.9
In2O3 In2SrO4 In2O3 In2SrO4 ZrSrO3 In2O3 ZrSrO3 ZrO2 ZrO2
Cubic Orthorhombic Cubic Orthorhombic Cubic Cubic Cubic Tetragonal Monoclinic
10.121 9.8368 10.1211 9.8394 8.2105 10.1208 8.1929 3.5954 5.1601
10.121 3.2664 10.1211 3.2658 8.2105 10.1208 8.1929 3.5954 5.2039
10.121 11.4922 10.1211 11.493 8.2105 10.1208 8.1929 5.192 5.3157
90 90 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 98.948
90 90 90 90 90 90 90 90 90
72.8 27.2 86.3 5.3 8.4 30.7 3.6 8.7 57
Profile Goodness of value, Rp fit value, χ2 (%)
Fig. 2. SEM micrographs of In2O3–SrO ceramics sintered at 1473 K (a) and 1573 K (b); In2O3–SrO–ZrO2 ceramics sintered at 1573 K from mixture with 10 wt% (c) and 60 wt% and (d) ZrO2. Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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Current density (A/cm2)
10-1 E
10-2 C
10-3 10-4
1 2 3
10-5 B
10-6
A
1
Current density (A/cm2)
D
10 100 Electric field (V/cm)
1000
8x10-3 6x10-3 4x10-3
4 5 6 7
2x10-3 0 0.0
0.4 Electric field (V/cm)
0.8
Fig. 3. Current–voltage dependency for (a) In2O3–SrO material (curves 1 and 2) and In2O3–SrO–ZrO2 ceramics (curve 3) with high content of Zr (60 wt% ZrO2 in mixture); (b) low Zr-doped In2O3–SrO–ZrO2 ceramics obtained from mixture with 5 wt% ZrO2 (curves 4 and 6) and 10 wt% ZrO2 (curves 5 and 7). Materials were sintered at 1473 K (curves 1, 3, 4, and 5) and 1573 (curves 2, 6, and 7).
current with voltage but the current limiting effect is not observed (curve 3, Fig. 3a). For In2O3–SrO–ZrO2 ceramics with low Zr content (5 and 10 wt% ZrO2 in mixture) sintered at 1473 K, the I(U) dependencies are linear up to 1 mA/cm2 (curves 4 and 5, Fig. 3b). For low Zr doped In2O3–SrO–ZrO2 ceramics obtained at 1573 K (curves 6, 7 in Fig. 3b) the type of current–voltage behavior is the same as in this material sintered at 1473 K (Fig. 3b, curves 4, 5). However, the conductivity of ceramics sintered at 1573 K is higher. The I(U) dependency in obtained In2O3–SrO ceramics with the current limiting effect was explained in terms of a modified barrier model [9] based on the results of measurements in different gas environments and computer simulation [9–11]. In this model grain-boundary states which are occupied by electrons (filled grain-boundary states) are associated with adsorbed oxygen in charged form. Grain-boundary states which are not occupied by electrons (empty grain-boundary states) are associated with adsorbed oxygen in neutral form. The density of empty and filled grain-boundary states is assumed to be equal to the density of adsorbed oxygen in
5
neutral and charged forms, respectively. The capture of electrons at the grain-boundary states leads to the transition of adsorbed oxygen from neutral to charged form. The adsorption equilibrium at the grain surface is maintained at the expense of oxygen adsorption and desorption only in the neutral form [9]. Therefore, if the amount of adsorbed oxygen in neutral form is decreased (as a result of capture of electrons at the grain-boundary states), the adsorption–desorption equilibrium at the grain boundary is broken and additional oxygen adsorption starts. In the equilibrium state at grain boundary (when voltage bias is zero), the capture of electrons at the grain-boundary states is balanced by their emission and the barrier height is not changed. However, under applied voltage the barrier height is decreased and the capture of electrons at the grain-boundary states is increased. This results in the transition of adsorbed oxygen from neutral to charged form, and the adsorption– desorption equilibrium at the grain boundary is broken. Additional oxygen adsorption appears and leads to a rise in the number of empty states at grain boundary. Therefore, under applied voltage the capture of electrons dominates over their emission until adsorption equilibrium is reached. Thus, the barrier height is increased and as a result of this, the current limiting behavior takes place (Fig. 3a, curve 1, the region CD). For these processes of adsorption, access of oxygen to grain boundary is needed. In In2O3–SrO ceramics the important role in oxygen diffusion along grain boundary can be played by the metastable hexagonal R-phase formed by strontium-enriched regions near grain boundaries in the main cubic structure of indium oxide [12]. It is reasonable to suppose that the contact between grains is closer in In2O3–SrO ceramics sintered at 1573 K than in material obtained at 1473 K. Therefore, access of oxygen to grain boundary is hindered in ceramics sintered at 1573 K or at higher temperatures, and as a consequence the current limiting effect is less pronounced (Fig. 3a, curve 2) or is absent. The In2O3–SrO–ZrO2 ceramics with low Zr content possess very high electrical conductivity (curve 7 in Fig. 3b). To be sure that current–voltage behavior in this material is defined by sample's volume but not Pt-contacts, the I(U) dependencies at different thickness of the sample were recorded. In each case only one side of the sample with Pt-contact was abraded. The experiment shows that the calculated conductance of the sample per unit of contact substantially depends on the thickness of material between electrodes. In addition, the same I(U) dependencies were obtained on the samples with Agcontacts. This lets us assert that the results of dc measurements are related to the material behavior. As it was shown in [11], In2O3–SrO ceramics possess a weak sensitivity to humidity in consequence of underdeveloped system of transport pores (open macro- and mesopores from nm to hundreds nm) which are needed for effective development humidity-absorption processes. The experiment shows that the sensitivity of all obtained In2O3–SrO–ZrO2 ceramics to humidity is low as well. After a transfer of samples from air at 50% relative humidity to air at 75% humidity, the sample's conductance is increased but not more than 6% up
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6
from its initial resistance recorded in air at 50% relative humidity. Observed changes in the conductance can be related to the lowering of the barrier height at grain boundary. Also some contribution of surface leakage in humid air cannot be excluded completely. The high conductivity of In2O3–SrO–ZrO2 ceramics with low Zr content (5 and 10 wt% ZrO2 in mixture) can be related to low heights of grain-boundary barriers. To corroborate this, for obtained ceramics, the temperature dependence of conductivity and frequency dependence of capacitance and ac conductance were studied.
3.3. Temperature dependence of conductivity Temperature dependences of dc conductivity for In2O3–SrO and In2O3–SrO–ZrO2 ceramics are presented in Fig. 4. The comparison of dependences for ceramics of the same composition but sintered at different temperature shows that material obtained at 1473 K has lower conductivity at room temperature and higher activation energy of electrical conduction in the interval of 450–520 K (Table 2) than ceramics sintered at 1573 K. This conforms to our previous supposition that the contact between grains is better in ceramics sintered at 1573 K than in material obtained at 1473 K. At room temperature, the conductivity of low Zr doped In2O3–SrO–ZrO2 materials is four orders of magnitude higher than that of In2O3–SrO ceramics sintered at the same temperature (Fig. 4, curves 2 and 6). However, the conductivity of material with high Zr content (60 wt% ZrO2 in mixture) remains at the same order as that of In2O3–SrO ceramics (Fig. 4, curves 2 and 4). The conductivity increase of indium oxide ceramics caused by Zr dopant was expected, as zirconium is known to act as a donor in In2O3 and increases electron concentration and electronic conductivity [15,16]. Therefore, the height of grainboundary barrier is decreased in accordance with the known
Conductivity (Ohm-1 cm-1)
102 101 100 10-1 10-2
1 2 3 4 5 6
10-3 10-4 10-5 10-6 0.5
1.0
1.5
2.0
2.5
3.0
3.5
1000/T(K-1) Fig. 4. Conductivity as a function of the temperature recorded at heating for In2O3–SrO (curves 1 and 2) and In2O3–SrO–ZrO2 (curves 3–6) ceramics sintered at 1473 K (curves 1,3,5) and 1573 K (curves 2,4,6). In2O3–SrO–ZrO2 materials were obtained from mixture with 10 wt% (curves 5 and 6) and 60 wt % (curves 3 and 4) ZrO2.
Table 2 The activation energy of electrical conduction for ceramics sintered from ð1 xÞ (In2O3–SrCO3)–x ZrO2 composition. x
0 0 5 5 10 10 60 60
Sintering temperature of ceramics, K
1473 1573 1473 1573 1473 1573 1473 1573
Activation energy Ea (eV) 7 0.01 eV in the interval: 450–520 K
750–950 K
0.50 0.29 0.24 0.23 0.23 0.15 0.30 0.16
1.12 0.83 0.31 0.25 0.30 0.29 0.78 0.77
expression [17,18]: φ0 ¼
q2 m20 8εε0 N D
ð1Þ
where q is the elementary charge, m0 is the surface density of electrons captured at the grain-boundary states at U=0 , ND is the concentration of fully ionized donors in the grains, ε and ε0 are the relative permittivity and the permittivity of vacuum, respectively. This decreasing of grain-boundary barriers after the doping by Zr is confirmed by lower magnitude of Ea for In2O3–SrO–ZrO2 ceramics than for In2O3–SrO materials in the interval of 450–520 K (Table 2). In addition to barrier lowering, the enriching of grainboundary regions by donor dopant of Zr leads to narrowing of the space-charge region in the Schottky barriers. Therefore, these barriers in low Zr-doped materials can be sufficiently thin to be transparent for tunneling electrons at room temperature. As a result of this, I(U) dependencies observed for low Zrdoped In2O3–SrO materials are linear (Fig. 3b). The supposition about electron tunneling through grain boundaries is confirmed by weak temperature dependence of the conductivity registered for low Zr-doped ceramics in the interval of 300–350 K (Fig. 4, curves 5 and 6). At heating above 350 K, the conductivity of ceramics starts to increase more strongly with temperature than between 300 and 350 K (Fig. 4, curves 5 and 6). This is explained by the increase of thermionic emission over barrier which gives a bigger contribution to electrical current through grain boundary at higher temperatures. The decrease of conductivity registered at heating between 500 and 660 K (Fig. 4, curves 1–6) is related to the transformation of adsorbed oxygen O to O2 (O þ e-O2 ) and O2 to 2O (O2 þ e-2O ) which takes place at high temperatures [19,20]. This leads to increasing of negative electrical charge localized on grain boundary and so the grain-boundary barriers are increased. Respectively, the conductivity of materials becomes lower. Such behavior of conductivity versus temperature for obtained In2O3–SrO ceramics under different gas environments was studied in detail in the work [21]. The high-temperature increase of conductivity above 700 K can be related to the generation of new oxygen vacancy in grains and oxygen desorption from grain boundaries. These processes
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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3.4. Frequency dependence of capacitance and ac conductance The plots of relative dielectric permittivity (ε) and ac conductivity (g) versus frequency (f) for In2O3–SrO material and In2O3–SrO–ZrO2 ceramics with high Zr content are presented in Fig. 5a and b. For these materials, the decrease of ε and the weak increase of g are observed with the rise of frequency from 100 Hz to 1 MHz. Curves of εðf Þ and g(f) dependencies are reproducible at the increase and the decrease of frequency. To explain these εðf Þ and g(f) dependences, let us consider the admittance YðjϖÞ of two adjacent grains with the potential barrier at the grain boundary (Fig. 5c). In this case YðjϖÞ is expressed as YðjωÞ ¼ G þ jωC
ð2Þ
where G¼
GG GB ðGG þ GB Þ þ ðωCB Þ2 GG ðGG þ GB Þ2 þ ðωC B Þ2
ð3Þ
CB G2G ðGG þ GB Þ2 þ ðωCB Þ2
ð4Þ
and C¼
Here CB is the barrier capacitance, ω is the angular frequency, GG and GB are the conductance of “grain volume” and grain boundary, respectively. If the conductance of the grain volume is much more than the conductance of the grain boundary (jGG j b jGB þ jωC B j ¼ ½ðGB Þ2 þ ðωCB Þ2 1=2 ), we obtain from (3) and (4): C C B and G GB þ ðGG Þ 1 ðωC B Þ2 . In this case, the weak frequency dependence of capacitance C and sharp increase of the ac conductance G are expected. However, instead of this, in the
104 103 102 101 g (Ohm-1 cm-1)
lead to the increasing of the electron concentration in grains. Therefore the height of grain-boundary barrier is decreased, and so conductivity of ceramics is increased at heating above 700 K (Fig. 4). The lower conductivity of In2O3–SrO–ZrO2 ceramics with high Zr content compared with that of low Zr-doped material (Fig. 4, curves 3 and 5) can be the result of the presence of monoclinic ZrO2 phase in high amount, around 57 wt% (Table 1). The ZrO2 phase can form high-Ohmic layers between grains of indium oxide which leads to the decrease of conductivity of the material. In the case of low Zr content in material, the ZrO2 phase is absent or its concentration is too low to be detected by the diffractometer (Table 1) and so this phase cannot isolate In2O3 grains. In addition, the In2O3–SrO– ZrO2 ceramics with high Zr content have a lot of small grains (less than 0:1 μm) located between groups of sintered grains (Fig. 2d). In comparison with material with low content of Zr where grain size is in the range 0:3–1:2 μm (Fig. 2c), this leads to a bigger number of grain-boundary barriers per unit of length, at least three times. Therefore, the conductivity of In2O3–SrO–ZrO2 ceramics with high Zr content is lower than for low Zr-doped material.
7
10-3 10-4 10-5 10-6 10-7 10-8
1 2
3 4
104
103
102
1 2
105
106
3 4
Frequency (Hz)
Fig. 5. (a) The relative dielectric permittivity ε and (b) ac conductivity g versus frequency recorded for In2O3–SrO ceramics (curves 1 and 2) and for In2O3–SrO–ZrO2 material (curves 3 and 4) with high Zr content (60 wt% ZrO2 in mixture). Ceramics were sintered at 1473 K (curves 1 and 3) and at 1573 K (curves 2 and 4). (c) The equivalent circuit of two adjacent grains: CB is the barrier capacitance and GB and GG are the conductance of the grain boundary and the grain volume, respectively.
experiment the decrease of ε and the weak increase of g were registered at the rise of frequency (Fig. 5). This is possible if jGG j and jGB þ jωCB j are comparable, and the registered values of G and C are determined by (3) and (4). In this case, the voltage of ac measuring signal is redistributed between the grain volume and the grain-boundary region. As a result, in ceramics the calculated value of the relative dielectric permittivity ε is decreased to the value of the relative dielectric permittivity in pure In2O3. However, sufficiently high values of ε at low-frequencies (Fig. 5a, curves 1–4) show that the inequality jGG j b jGB þ jωCB j is performed at least at 100 Hz for these ceramics. The typical frequency dependencies of capacitance and ac conductance for In2O3–SrO–ZrO2 ceramics with low Zr content (5 and 10 wt% ZrO2 in mixture) are presented in Fig. 6. At first the capacitance is decreased until negative values at the rise of frequency from 100 Hz to 4 kH (Fig. 6a, curve 1). In this frequency range, the decreasing of capacitance is accompanied by the decrease of ac conductance (Fig. 6b, curve 1). With the frequency increase from 4 kHz to 20 kHz, the capacitance starts to increase sharply but ac conductance continues to decrease. However, above 20 kHz and up to 1 MHz the capacitance starts to decrease again (Fig. 6a, curve 1) and is accompanied by the increasing of conductance g (Fig. 6b, curve 1). This behavior of capacitance and conductivity versus frequency is repeated at successive measurements (curves 1–3 in Fig. 6) and at higher voltage (10–50 mV) of signal amplitude. However, during these successive measurements the curves of ac conductance versus frequency are shifted to the area of lower conductivity (Fig. 6b, curves 1–3). The calculated ε is near 48 000 at 100 Hz and 250 at 1 MHz respectively. The sharp increase of capacitance in the frequency range 4–20 kHz which is accompanied by the sharp decrease of ac conductance can be a result of charge trapping at the grain boundary. However, discussion of such processes requires
Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096
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8
1.0x10-8
1.5x10-10 1.0x10-10
Capacitance (F)
8.0x10-9
5.0x10-11 0.0
6.0x10-9
-5.0x10-11 -1.0x10-10
4.0x10-9 2.0x10-9 0.0 101
-1.5x10-10
1 2 3 102
-2.0x10-10 3 10
103 104 Frequency (Hz)
104
105
105
106
Acknowledgments This study was supported by the National Science and Technology Council of Mexico (CONACYT) under the project “Convocatoria Ciencia Basica 2010 (the project number 154625-Y)”. Authors thank Ms. Nayely Pineda Aguilar and Mr. Cesar Leyva Porras for their assistance in the microstructure characterization by SEM (Cimav-Monterrey).
1.7x10-2
Conductance (Ohm-1)
is related to the presence of high-resistance ZrO2 phase between grains of indium oxide in materials with high content of Zr. For low Zr-doped In2O3–SrO–ZrO2 ceramics (5 and 10 wt% ZrO2 in mixture) the ZrO2 phase is not detected. Frequency dependences of capacitance and ac conductance for low Zr-doped In2O3–SrO–ZrO2 ceramics contain a region between 4 kH and 20 kHz where the sharp increase of conductance which is accompanied by the decrease of ac conductance is observed. The obtained In2O3–SrO–ZrO2 ceramics are very porous, which is favorable for adsorption processes and diffusion of oxygen through the material.
1.7x10-2
References 1 2 3
1.7x10-2 1 10
102
103 104 Frequency (Hz)
105
106
Fig. 6. (a) The capacitance and (b) ac conductance versus frequency recorded at three successive measurements (curves 1–3) of the sample of In2O3–SrO– ZrO2 ceramics with low Zr content (10 wt% ZrO2 in mixture) sintered at 1473 K. Each curve was recorded at increasing and subsequent decreasing frequency.
more detailed experimental investigation of this effect which will be presented in the next paper. 4. Conclusions The effects of Zr dopant and sintering temperature on the electrical properties of In2O3–SrO based ceramics have been investigated. The electrical current in In2O3–SrO ceramics sintered at 1473 K exhibits a tendency toward saturation and can even decrease with an increase in voltage. However, this current-limiting effect is not registered if sintering temperature of ceramics is above 1573 K. Observed current limiting is explained in terms of the modified grain boundary model which considers a relationship between the electronic and adsorption processes. The dopant of zirconium in low concentrations acts as a donor in In2O3–SrO–ZrO2 ceramics sintered at relatively low temperature (1473 and 1573 K). Such materials possess high conductivity and linear current–voltage dependencies. However, if the concentration of zirconium is high (60 wt% ZrO2 in mixture), the conductivity of In2O3–SrO–ZrO2 ceramics lowers and nonlinear current–voltage behavior is observed. This
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Please cite this article as: A.N. Bondarchuk, et al., Effects of Zr dopant and sintering temperature on electrical properties of In2O3–SrO based ceramics, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.096