Electronic Materials Letters, Vol. 9, No. 4 (2013), pp. 445-449 DOI: 10.1007/s13391-013-0025-1
A Study of Electrodes for Thermoelectric Oxides Chang-Hyun Lim,1,2 Soon-Mok Choi,3,* Won-Seon Seo,1,* Myung-Hyun Lee,1 Kyu Hyoung Lee,4 and Hyung-Ho Park2 1
Energy & Environmental Division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Korea 2 Dept. of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea 3 School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan 330-708, Korea 4 Functional Material Group, Samsung Advanced Institute of Technology, Yong-In 446-712, Korea (received date: 25 October 2012 / accepted date: 9 January 2013 / published date: 10 July 2013) Oxide module studies for thermoelectric energy harvesting at a high temperature were performed in this study. Considering the degradation of the electrode properties by oxidation at normal operating temperatures in air, a silver electrode (Ag) was selected for use in this study. Two types of contact problems causing a severe loss in electrical power generation were investigated: potential barriers at the interfaces, and inter-diffusion problems. Also, two types of thermoelectric modules were fabricated using Ag electrodes: a module with n-type CaMnO3 and p-type Ca3Co4O9 oxides and another module with n-type (ZnO)7In2O3 and p-type Ca3Co4O9 oxides. The measured power density values were compared with the values calculated from the materials properties. Keywords: thermoelectric, oxide, electrode, silver
1. INTRODUCTION Thermoelectric power generators which convert waste heat into electricity have recently received much attention due to their ability to improve the efficiency of a conventional power generation system.[1,2] In addition to notable Bi2Te3 systems for thermoelectric power generation at room temperature, silicides and oxides have been increasingly reported for high-temperature applications, including industrial waste heat applications.[3-7] Currently, module studies for thermoelectric energy harvesting at high temperatures, as well as developments of material properties, are being published.[8-10] Electrode properties are among the key factors to consider when fabricating thermoelectric modules for practical applications. The electrode needs to have high electrical and thermal conductivity, but it is also important to reduce both the electrical and thermal resistance at the interface between the thermoelectric material and the electrode for the high efficiency of the module. Especially for thermoelectric oxides, the electrode's conducting properties at a high temperature as well as at room temperature are important. Another significant factor affecting long-term use is the thermal *Corresponding author:
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stability of the electrode at high temperatures.[8-10] Although Cu, Fe, and Ni have been reported as common electrodes for thermoelectric modules in the mid-high temperature region,[10] there are few reports on electrodes for thermoelectric oxides.[11,12] In this study, Ag-metal as a hightemperature electrode was investigated for thermoelectric modules based on oxide thermoelectrics. Ca3Co4O9 specimens were prepared as a p-type thermoelectric oxide. Both CaMnO3 and (ZnO)7In2O3 specimens were also used as ntype thermoelectrics for tests to characterize the contact properties of the electrodes. As a result, thermoelectric modules were fabricated with the developed Ag electrode, and the measured power output values were compared to the expected electrical power as calculated from the material properties of the thermoelectric oxide.
2. EXPERIMENTAL PROCEDURE The thermoelectric (TE) materials (Ca3Co4O9, Ca0.9Nd0.1MnO3, (ZnO)7In2O3) used in this work were synthesized by a solidstate reaction. Synthesized Ca3Co4O9 powders were sintered by the spark-plasma sintering technique (SPS) at 1213 K for 5 min under 50 MPa in a vacuum. Ca0.9Nd0.1MnO3 and (ZnO)7In2O3 powders were sintered at 1573 K for 15 h and at 1823 K for 2 h in air, respectively, after cold isostatic pressing (CIP) at 200 MPa. The process conditions are detailed in the literature.[8,9]
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Commercial pastes of silver and nickel (Ni) were screenprinted on TE bulk materials and were heated to 1123 K for 12 h in air to obtain the energy dispersive spectroscopy (EDS) data. After a polishing, the samples were analyzed by a field emission scanning electron microscope (FE-SEM 6700F, JEOL, Japan) linked to EDS. The TE bar specimens were bonded with Ag paste, and after a drying, were heated to 1123 K for 10 min in air to metalize them for the electrical properties measurement. In the (ZnO)7In2O3 sample especially, an indium tin oxide (ITO) interlayer was inserted between the Ag electrode and the (ZnO)7In2O3 oxide as a buffer layer. ITO paste was made from commercial nano powders and organic additives. Measurements of the internal resistance of the samples were carried out using a standard DC fourterminal method at room temperature, and electrical conductivity values were measured in the direction vertical to the interface at a high temperature, specifically, through the interface.
3. RESULTS AND DISCUSSION The main loss of electrical power associated with electrodes here was thought to be due to the high electrical resistance of the electrodes and their interfaces. Considering the degradation of the electrode properties by oxidation at the operating temperature in air, a silver electrode (Ag) was selected in this study. As a result, the Ag electrodes on thermoelectric oxides (Ca3Co4O9, CaMnO3, and (ZnO)7In2O3) showed good oxidation-resistance properties at an operation temperature of about 1150 K, unlike other electrodes, including those consisting of Ni. Another difficulty besides oxidation when fabricating electrodes is the need to mitigate contact problems at the interfaces between the electrodes and the thermoelectric oxides. Contact problems, which can be divided into two difficulty levels, cause severe losses of electrical power generation from thermoelectric modules. The first consideration with contact problems is with potential barriers at the contacts between the metallic electrodes and the thermoelectric oxides. In this study, potential barriers were investigated at the interfaces between the Ag electrodes and three different thermoelectric oxides, (p-type Ca3Co4O9 and n-type CaMnO3 and (ZnO)7In2O3). The height of the potential barrier at the interfaces between the Ag electrodes and the thermoelectric oxides were indirectly checked by measuring the conductivity of an oxide specimen having interfaces with the Ag electrode in the middle of the specimen. Figure 1 shows the test results. The conductivity of the specimens with such interfaces in the middle of the specimen was measured by means of the four-point probe method in a direction vertical to the interface, specifically through the interface, as mentioned above in the ‘‘EXPERIMENTAL’’ section. If the work function of an n-type semiconductor is
Fig. 1. Electrical contact characteristics at the interfaces: (left column) a comparison of the resistivity values of specimens with an Ag interface and without this interface, (right column) the linearity of the voltage variation in accordance with the change in the sample size at room temperature. (a, b) Ca3Co4O9 (CCO), (c, d) Ca0.9Nd0.1MnO3 (CMO), (e, f) (ZnO)7In2O3 (IZO).
greater than that of the metal, electrons flow from the metal to the semiconductor and make the surface of the semiconductor more of an n-type surface. This is a typical example of ohmic contact, as no barrier exists for the flow of electrons in either direction for n-type materials. The reverse is true for a p-type semiconductor when its work function is smaller than that of the metal. When an ohmic contact is formed between the p-type semiconductor and the metal, no barrier exists for the flow of electronic holes.[13] Therefore, if a potential barrier is not formed or if it is very small, the conductivity value of the specimen having such an
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interface should be equal to a specimen without this type of interface, confirming the formation of an ohmic contact at the interface with the Ag electrode.[13] As shown in Figs. 1(a) and 1(c) for the Ca3Co4O9 and CaMnO3 specimens, the conductivity differences between interfaces with and without Ag were not great, showing nearly the same level between the two results. These results are indirect evidence of the small potential barriers at the interfaces between Ag electrodes and these thermoelectric oxides. However, the conductivity difference for the (ZnO)7In2O3 specimen with and without the Ag interfaces was much greater than that of the other oxide cases, as shown in Fig. 1(e), showing indirect evidence of the formation of a higher potential barrier at the interface between the Ag electrode and the semiconducting (ZnO)7In2O3 oxide. This higher potential barrier at the interface of the Ag electrode with the (ZnO)7In2O3 oxide as compared to other oxide systems could also be verified by checking the linearity of the resistance variation according to a change in the length of the specimens, including the Ag electrode in them. As shown in Figs. 1(b), (d), and (f), the linearity of the voltage variation was also checked for different measuring points on all oxide specimens while keeping the current density constant at room temperature. In the Ca3Co4O9 and CaMnO3 specimens, the relationship between the voltage and the measuring point on the specimens showed good linearity. Moreover, the linear fitting line went through the origin of the coordinate at room temperature, as shown in Figs. 1(b) and (d). These results reconfirm that the potential barriers at the interfaces between the Ag electrodes and these thermoelectric oxides are negligible. However, in the (ZnO)7In2O3 case, the relationship between the voltage and the measuring point on the specimens could not be linearly fitted. As shown in Fig. 1(f), the tendencies of the conductivity variation with a change in the dimensions of the specimen were quite different from the case without an Ag electrode, and from the other case with an Ag electrode in the specimen. This result is also evidence of the formation of a higher potential barrier at the interface between the Ag electrode and the semiconducting (ZnO)7In2O3 oxide. Therefore, an operation to lower the potential barrier at the interface between them is necessary to apply an Ag electrode to an (ZnO)7In2O3 oxide. In this study, an attempt to improve the potential barrier problem was to insert an ITO interlayer as a buffer layer between the Ag electrode and the (ZnO)7In2O3 oxide. A photograph of the structure of the interface is shown in Fig. 2(c), showing the ITO interlayer, with a thickness of 5 µm, between the Ag electrode and the (ZnO)7In2O3 oxide. After inserting the ITO interlayer between the Ag electrode and the (ZnO)7In2O3 oxide, the conductivity of the specimen with the Ag electrode in the middle of the specimen was fairly high compared to the case without an ITO interlayer,
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Fig. 2. EDS analysis of the reactivity of the interface between the TE oxides and the electrodes: (a) Ca3Co4O9, (b) Ca0.9Nd0.1MnO3, (c) (ZnO)7In2O3.
Fig. 3. Difference in the electrical conductivity of the samples with an ITO interlayer and without this interface between the Ag electrode and (ZnO)7In2O3.
as shown in Fig. 3. Although this conductivity value was not as high as that of the specimen without any electrode materials in the middle of the specimen, this improvement could serve as a guideline for solving potential barrier problems at the interfaces between metallic electrodes and thermoelectric oxides. In addition, it is often observed
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experimentally that the height of the barrier does not depend on the work function of the metal, as surface states on the semiconductor may be more important in determining the size of the potential barrier. The other consideration next to the potential barrier at the interface is an interdiffusion problem between the electrode and the thermoelectric materials, which could result in a secondary phase in the interface, or the degradation of the material’s properties. Fortunately, the Ag electrode on the all three oxides of p-type Ca3Co4O9, n-type CaMnO3 and (ZnO)7In2O3) in this study did not show interdiffusion, even after firing at a high temperature of 1123 K, as shown in Fig. 2. In the results of the Ag-(ZnO)7In2O3 system, it should be noted that the ITO interface (about 5 µm) was inserted between the Ag electrode and the (ZnO)7In2O3 thermoelectric oxide. From these results, the Ag electrode is confirmed as a proper electrode for n-type CaMnO3 and p-type Ca3Co4O9 oxide-based modules. For the n-type (ZnO)7In2O3 oxide, it was concluded that an ITO interlayer was effective at reducing the potential barrier at the interface between the Ag electrode and the (ZnO)7In2O3 oxide. Therefore, in this study, two types of thermoelectric modules were fabricated using Ag electrodes: a module with the n-type CaMnO3 and p-type Ca3Co4O9 oxides, and another module with the n-type (ZnO)7In2O3 and the p-type Ca3Co4O9 oxides. The measured power density values were then compared with the values calculated from the materials properties. First, from the open circuit voltage, the temperature of the cold zone can be calculated using the equation below:[14] Vmeas. = ( αp − αn ) × ( Thot, meas. − Tcold ) .
(1)
For both types of modules, the calculated temperatures were different from the actual measured values. This inaccuracy could originate from many factors, including inhomogeneous temperatures and high thermal resistances at the many interfaces.[14] With these temperature conditions (Thot_measured and Tcold_calculated), the measured power density values can be compared with the values calculated from the electrical conductivities of the oxides. The power output from a thermoelectric module can be calculated by the equation:[14]
much higher (≈500%) than the value calculated from the electrical conductivities of oxides and the dimension of the module. Further research should be conducted to reduce the internal resistance (dry joints, pores, other defects owing to the thermal expansion differences and poor mechanical affinity between the legs and the electrodes). In addition, for the module made of the n-type (ZnO)7In2O3 and the p-type Ca3Co4O9 oxides, a much smaller power output value (
