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Thermoelectricity of p-CCO and n-ZAO Thin Films
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 61 (2014) 795 – 798
The 6th International Conference on Applied Energy – ICAE2014
Thermoelectricity of p-CCO and n-ZAO thin films Weerasak Somkhunthota*, Nuwat Pimpabutea, Arthorn Vora-udb, Tosawat Seetawanb, Thanusit Burinprakhonc b
a Program in Physics, Faculty of Science and Technology, Loei Rajabhat University 42000, Thailand Program in Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University 47000, Thailand c Department of Physics, Faculty of Science, Khon Kaen University 40000, Thailand
Abstract Thin films of calcium cobalt oxide and zinc aluminium oxide were prepared onto ceramic substrates by a bipolar pulsed-dc magnetron sputtering system using a Ca3Co4O9 target and a ZnAlO target, respectively, under an argon atmosphere. Energy dispersive spectroscopy analysis revealed that the as-deposited films from the Ca3Co4O9 target comprised Ca, Co, O elements, while those from the ZnAlO target contained Zn, Al, O elements. Cross-sectional view estimation by the scanning electron microscope indicated that the as-deposited Ca-Co-O (CCO) and Zn-Al-O (ZAO) films had the thickness of 0.55 μm and 0.58 μm, respectively. X-ray diffraction analysis showed that the CCO thin film was grown in amorphous phase while the ZAO thin film exhibited hexagonal structure. From measurement of the properties, the p-CCO and n-ZAO films were found to exhibit the power factor of 0.13 and 14.39 μW/m⋅K2, respectively. A thermoelectric module made from three pairs of the p-n thin film stripes provided the open circuit voltage and short circuit current up to 55.6 mV and 0.04 μA for a temperature difference of 101.40 K, respectively. © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014 Keywords: Thermoelectricity ; Thermoelectric module ; Thin films of p-CCO and n-ZAO
1. Introduction Thermoelectric materials can directly convert thermal energy into electricity or vice versa, through the so called Seebeck effect and Peltier effect, respectively. The phenomena are the basis of the operation of thermoelectric generators (TEG) and thermoelectric refrigerators (TER). Practical TEG or TER employs one or more thermoelectric modules, in which thermoelectric materials are used as thermoelectric elements. A thermoelectric module is composed of several pairs of p-type and n-type thermoelectric elements connected electrically in series but thermally in parallel. The performance of a thermoelectric element can be evaluated from the material's electrical power factor, P = S2/ρ, or the dimensionless figure of merit, ZT = S2T/ρκ, where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and T is the absolute temperature [1,2,3].
* Corresponding author. Tel.: +66-0-4283-5238 ; fax: +66-0-4283-5238 E-mail address: [email protected].
1876-6102 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.11.967
796
Weerasak Somkhunthot et al. / Energy Procedia 61 (2014) 795 – 798
The search for thermoelectric materials with high efficiency has paid attention on several metal oxide compounds, including Ca3Co4O9 and ZnAlO. The former exhibits p-type conduction while the latter is ntype. There have been numerous reports on the preparation of the two compounds using different techniques, in the form of powders, bulks, and thin films [4,5,6,7]. However, employing these thin films as the p-n elements of a thermoelectric module has not been reported. This is of interest for our study. In this paper, the investigation of thin film deposition from the p-Ca3Co4O9 and n-ZnAlO targets using a bipolar pulsed-dc magnetron sputtering system is reported. The results of crystal structure, elemental composition and thermoelectric property measurements of the deposited thin films are present. Fabrication and testing of a thermoelectric module made from the thin films are reported and discussed. 2. Experimental Procedures The experimental procedures including thin films deposition, characterizations and thermoelectric properties measurement, and fabrication and testing a thermoelectric module are described as follows. 2.1. Thin films deposition Fig 1 shows the schematic diagram of a bipolar pulsed-dc magnetron sputtering system used for thin film preparations. Details of a system have been given elsewhere [8]. The depositions were carried out under an argon atmosphere (Ar 99.999%, at the gas flow rate of 23.3±0.1 sccm and total pressure of 70 mTorr) using the p-Ca3Co4O9 and n-ZnAlO targets of 60 mm diameter and 3.0 mm thickness. The substrates were Al2O3:clay ceramics (25.0×25.0×1.0 mm3) placed at the distance of 5.0 cm above the target without additional heating facility. The pulsed-dc power applied to the targets was operated at approximately 17 kHz, with the sputtering on-time (−ton) of 20 μs, the reverse sputtering on-time (+ton) of 10 μs, and the pulse off-time (toff) of 14 μs. The negative voltage pulse/current at the target were 244±15 V/120 mA, and 600±15 V/120 mA for p-Ca3Co4O9 and n-ZnAlO targets, respectively. The positive voltage pulse (+V) was set at 100±10 V for both targets. The deposition time was 60 min for each sample. The optical emissions from plasma during sputter deposition of films were observed in the wavelength range of 360-800 nm using a high resolution spectrometer (the getSpec-2048 spectrometer, Sentronic GmbH). The spectral lines were indexed to the Atomic Spectra Database (ASD) of the National Institute of Standards and Technology (NIST) [9]. Oscilloscope ×100 High Voltage Probe Vacuum Chamber Anode Substrate Bipolar DetectorHigh Resolution toff ton Pulsed-DC V Plasma t t Spectrometer Power off on Time V Supply Cathode Magnetron Sputtering Gun, Target Fig. 1. Experimental setup of a bipolar pulsed-dc magnetron sputtering system
2.2. Characterizations and thermoelectric properties measurement The thin films of 20.0×20.0 mm2 were prepared for crystal structure, elemental composition, thickness, and thermoelectric property determinations. Crystal structure of the as-deposited films was investigated by the x-ray diffractometer (XRD-6100 Shimadzu). Elemental composition analysis was carried out by the energy dispersive spectroscopy (EDS, JEOL JSM-5410). Film thickness determination was estimated from the cross-sectional images from the scanning electron microscope (SEM, JEOL JSM-5410). Charge carrier types and Seebeck coefficient were determined by the hot-probe experiment and the standard steady state measurement. Electrical resistivity was measured by the standard four-probe technique of theVan der Pauw method.
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Weerasak Somkhunthot et al. / Energy Procedia 61 (2014) 795 – 798
2.3. Fabrication and testing of a thermoelectric module A thermoelectric module was fabricated from three pairs of the p-type and n-type thin film stripes (each of 2.5 mm in width and 20.0 mm in length) deposited in parallel next to each other on a ceramic substrate and connected in series by using sputtered copper thin films as connectors. The module patterning was achieved by mean of masking with copper masks during the thin film depositions. For preliminary test at room temperature, the module was used for electrical power generation. A hot plate was placed on a module to provide a heat source at temperature TH, while a cold plate (heat sink) at temperature TC was opened to air. The open circuit voltage (VOC) and the short circuit current (ISC) of the module were measured as a function of the temperature difference (ΔT = TH−TC). 3. Results and Discussion The optical emission spectra from the plasma during thin film depositions (not shown) showed that the Ca3Co4O9 and ZnAlO targets were successfully sputtered. EDS analysis (results are not given) revealed that the as-deposited films from a Ca3Co4O9 target comprised Ca, Co, O elements, while those from a ZnAlO target contained Zn, Al, O elements. From this point onward, the as-deposited films will be referred to as CCO for the Ca-Co-O film and ZAO for the Zn-Al-O film. Fig 2 show the XRD patterns of the as-deposited CCO and ZAO thin films on the ceramic substrate, respectively, with respected to that of a bare substrate. It can be seen that the majority of the peaks in the XRD patterns are the substrate contributions, including Al2O3, SiC, SiO2 phases. In case of the CCO film, none of the peaks in the XRD pattern (Fig 2(a)) can berelated to those of the crystalline Ca3Co4O9. This suggests that the CCO film were mainly grown in amorphous phase, as a result of the low substrate temperature (< 150 °C). On the contrary, there are several peaks of the ZAO film that are corresponding to the diffractions from the (100), (002), and (101) planes of the crystalline ZnO [JCPDS 36-1451], as shown in the Fig 2(b). Furthermore, no trace of crystalline aluminum is detected in the XRD pattern. Thus, it appears that polycrystalline ZnO doped with Al can be grown at the low substrate temperature. This implies that only the energy provided by the pulsed-dc plasma to the depositing atomic species is sufficient to promote the Al-doped ZnO crystal formation. Cross-sectional SEM views indicated that the as-deposited CCO and ZAO films thickness of 0.55 μm and 0.58 μm were obtained, respectively. The CCO film exhibited ptype conduction with S = 103.84 μV/K, ρ = 8.29 Ω⋅cm, P = 0.13 μW/m⋅K2, while the ZAO film was ntype with S = −34.33 μV/K, ρ = 0.01 Ω⋅cm, P = 14.39 μW/m⋅K2. It should be noted that the electrical resistivity of the CCO film is about 2 order of magnitude as high as that of the ZAO film. This is not desirable for thermoelectric module applications. (a) Ca-Co-O film on ceramic substrate
(b) Zn-Al-O film on ceramic substrate
25
Ceramic Substrate 30 35 40 45 50 2θ (degree)
55
60
65
25
(002) (101)
(100)
Intensity (counts)
Intensity (counts)
Zn-Al-O
Ceramic Substrate 30 35 40 45 50 2θ (degree)
55
60
65
Fig. 2. XRD patterns of the as-deposited films/substrate samples in comparison to that of the ceramic substrates
The plots of the open circuit voltage and the short circuit current of the tested thermoelectric module of p-CCO and n-ZAO thin films are shown in Fig 3. It is apparent that the open circuit voltage and short
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Weerasak Somkhunthot et al. / Energy Procedia 61 (2014) 795 – 798
circuit current increased with increasing temperature difference up to 55.6 mV and 0.04 μA for a temperature difference of 101.40 K (TH = 477.55 K and TC = 376.15 K), respectively. The low short circuit current is likely a result of the high series resistance contributed by the p-CCO leg.
40
0.05 Short Circuit Current, ISC (μA)
0.04
(b) The short circuit current as a function of temperature difference Cu Connectors
0.03
30
0.02
20
p-CCO n-ZAO p-CCO n-ZAO p-CCO n-ZAO
Open Circuit Voltage, VOC (mV)
60 (a) The open circuit voltage as a function of temperature difference 50
0.01
10 0 0 10 20 30 40 50 60 70 80 90 100110 Temperature Difference, ΔT (K)
0.00
Ceramic Substrate 0 10 20 30 40 50 60 70 80 90 100110 Temperature Difference, ΔT (K)
Fig. 3. Test results on a thermoelectric module of p-CCO and n-ZAO thin films (a) the open circuit voltage and (b) the short circuit current as a function of temperature difference
4. Conclusions Thin films of p-CCO and n-ZAO prepared on ceramic substrate by the pulsed-dc magnetron sputtering method were used to make a thermoelectric module. The built module can generate the open circuit voltage and short circuit current up to 55.6 mV and 0.04 μA at a temperature difference of 101.40 K. This test demonstrated that the thermoelectric module containing these thin films is considered as an interesting platform for further work. The investigation on the improvement of the thermoelectric module performance by improving the electrical conductivity of the p-CCO film to match that of the n-ZAO film is of particular interest. Acknowledgements This work was financially supported by the Research and Development Administration Division, Electricity Generating Authority of Thailand (EGAT). The authors would like to gratefully thank the Thermoelectric Research Center (TRC), Sakon Nakhon Rajabhat University for XRD and thermoelectric property measurement facilities, and the National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (ASTDA), Thailand for EDS and SEM analysis. References [1] Rowe DM. Thermoelectrics handbook macro to nano, Boca Raton, FL: CRC Press; 2006, p. 1-14. [2] Nolas GS, Sharp J, Goldsmid HJ. Thermoelectrics: Basic principles and new materials developments, Verlag, Berlin, Heidelberg: Springer; 2001, p. 1–14. [3] Wood C. Materials for thermoelectric energy conversion. Rep Prog Phys 1988;51:459-539. [4] Jood P, Peleckis G, Wang X, Dou SX. Thermoelectric properties of Ca3Co4O9 and Ca2.8Bi0.2Co4O9 thin films in their island formation mode. J Mater Res 2013;28(14) :1932-1939. [5] Somkhunthot W, Pimpabute N, Seetawan T. (2013). Thermoelectric module of p-type Ca-Co-O/n-type ZnO thin films. Adv Mater Res 2013;622-623:726-733. [6] Chitra M, Uthayarani K, Rajasekaran N, Girija E.K. Preparation and characterisation of Al doped ZnO nanopowders. Phys Procedia 2013;49:177-182. [7] Ohtaki M, Araki, K. Aluminum-containing zinc oxide-based n-type thermoelectric conversion material, Japan Science and Technology Agency; 2013, Information on http://all-patents.com/us-patent/8454860. [8] Somkhunthot W, Burinprakhon T, Thomas I, Amornkitbamrung V, Seetawan T. Bipolar pulsed-dc power supply for magnetron sputtering and thin films synthesis. ELEKTRIKA J Elec Eng 2007;9(2):20-26. [9] Ralchenko Y. Atomic spectra database, National Institute of Standards and Technology; 2013, Information on http://www.nist.gov/pml/data/asd.cfm.
ScienceDirect Energy Procedia 61 (2014) 795 – 798
The 6th International Conference on Applied Energy – ICAE2014
Thermoelectricity of p-CCO and n-ZAO thin films Weerasak Somkhunthota*, Nuwat Pimpabutea, Arthorn Vora-udb, Tosawat Seetawanb, Thanusit Burinprakhonc b
a Program in Physics, Faculty of Science and Technology, Loei Rajabhat University 42000, Thailand Program in Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University 47000, Thailand c Department of Physics, Faculty of Science, Khon Kaen University 40000, Thailand
Abstract Thin films of calcium cobalt oxide and zinc aluminium oxide were prepared onto ceramic substrates by a bipolar pulsed-dc magnetron sputtering system using a Ca3Co4O9 target and a ZnAlO target, respectively, under an argon atmosphere. Energy dispersive spectroscopy analysis revealed that the as-deposited films from the Ca3Co4O9 target comprised Ca, Co, O elements, while those from the ZnAlO target contained Zn, Al, O elements. Cross-sectional view estimation by the scanning electron microscope indicated that the as-deposited Ca-Co-O (CCO) and Zn-Al-O (ZAO) films had the thickness of 0.55 μm and 0.58 μm, respectively. X-ray diffraction analysis showed that the CCO thin film was grown in amorphous phase while the ZAO thin film exhibited hexagonal structure. From measurement of the properties, the p-CCO and n-ZAO films were found to exhibit the power factor of 0.13 and 14.39 μW/m⋅K2, respectively. A thermoelectric module made from three pairs of the p-n thin film stripes provided the open circuit voltage and short circuit current up to 55.6 mV and 0.04 μA for a temperature difference of 101.40 K, respectively. © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014 Keywords: Thermoelectricity ; Thermoelectric module ; Thin films of p-CCO and n-ZAO
1. Introduction Thermoelectric materials can directly convert thermal energy into electricity or vice versa, through the so called Seebeck effect and Peltier effect, respectively. The phenomena are the basis of the operation of thermoelectric generators (TEG) and thermoelectric refrigerators (TER). Practical TEG or TER employs one or more thermoelectric modules, in which thermoelectric materials are used as thermoelectric elements. A thermoelectric module is composed of several pairs of p-type and n-type thermoelectric elements connected electrically in series but thermally in parallel. The performance of a thermoelectric element can be evaluated from the material's electrical power factor, P = S2/ρ, or the dimensionless figure of merit, ZT = S2T/ρκ, where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and T is the absolute temperature [1,2,3].
* Corresponding author. Tel.: +66-0-4283-5238 ; fax: +66-0-4283-5238 E-mail address: [email protected].
1876-6102 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.11.967
796
Weerasak Somkhunthot et al. / Energy Procedia 61 (2014) 795 – 798
The search for thermoelectric materials with high efficiency has paid attention on several metal oxide compounds, including Ca3Co4O9 and ZnAlO. The former exhibits p-type conduction while the latter is ntype. There have been numerous reports on the preparation of the two compounds using different techniques, in the form of powders, bulks, and thin films [4,5,6,7]. However, employing these thin films as the p-n elements of a thermoelectric module has not been reported. This is of interest for our study. In this paper, the investigation of thin film deposition from the p-Ca3Co4O9 and n-ZnAlO targets using a bipolar pulsed-dc magnetron sputtering system is reported. The results of crystal structure, elemental composition and thermoelectric property measurements of the deposited thin films are present. Fabrication and testing of a thermoelectric module made from the thin films are reported and discussed. 2. Experimental Procedures The experimental procedures including thin films deposition, characterizations and thermoelectric properties measurement, and fabrication and testing a thermoelectric module are described as follows. 2.1. Thin films deposition Fig 1 shows the schematic diagram of a bipolar pulsed-dc magnetron sputtering system used for thin film preparations. Details of a system have been given elsewhere [8]. The depositions were carried out under an argon atmosphere (Ar 99.999%, at the gas flow rate of 23.3±0.1 sccm and total pressure of 70 mTorr) using the p-Ca3Co4O9 and n-ZnAlO targets of 60 mm diameter and 3.0 mm thickness. The substrates were Al2O3:clay ceramics (25.0×25.0×1.0 mm3) placed at the distance of 5.0 cm above the target without additional heating facility. The pulsed-dc power applied to the targets was operated at approximately 17 kHz, with the sputtering on-time (−ton) of 20 μs, the reverse sputtering on-time (+ton) of 10 μs, and the pulse off-time (toff) of 14 μs. The negative voltage pulse/current at the target were 244±15 V/120 mA, and 600±15 V/120 mA for p-Ca3Co4O9 and n-ZnAlO targets, respectively. The positive voltage pulse (+V) was set at 100±10 V for both targets. The deposition time was 60 min for each sample. The optical emissions from plasma during sputter deposition of films were observed in the wavelength range of 360-800 nm using a high resolution spectrometer (the getSpec-2048 spectrometer, Sentronic GmbH). The spectral lines were indexed to the Atomic Spectra Database (ASD) of the National Institute of Standards and Technology (NIST) [9]. Oscilloscope ×100 High Voltage Probe Vacuum Chamber Anode Substrate Bipolar DetectorHigh Resolution toff ton Pulsed-DC V Plasma t t Spectrometer Power off on Time V Supply Cathode Magnetron Sputtering Gun, Target Fig. 1. Experimental setup of a bipolar pulsed-dc magnetron sputtering system
2.2. Characterizations and thermoelectric properties measurement The thin films of 20.0×20.0 mm2 were prepared for crystal structure, elemental composition, thickness, and thermoelectric property determinations. Crystal structure of the as-deposited films was investigated by the x-ray diffractometer (XRD-6100 Shimadzu). Elemental composition analysis was carried out by the energy dispersive spectroscopy (EDS, JEOL JSM-5410). Film thickness determination was estimated from the cross-sectional images from the scanning electron microscope (SEM, JEOL JSM-5410). Charge carrier types and Seebeck coefficient were determined by the hot-probe experiment and the standard steady state measurement. Electrical resistivity was measured by the standard four-probe technique of theVan der Pauw method.
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Weerasak Somkhunthot et al. / Energy Procedia 61 (2014) 795 – 798
2.3. Fabrication and testing of a thermoelectric module A thermoelectric module was fabricated from three pairs of the p-type and n-type thin film stripes (each of 2.5 mm in width and 20.0 mm in length) deposited in parallel next to each other on a ceramic substrate and connected in series by using sputtered copper thin films as connectors. The module patterning was achieved by mean of masking with copper masks during the thin film depositions. For preliminary test at room temperature, the module was used for electrical power generation. A hot plate was placed on a module to provide a heat source at temperature TH, while a cold plate (heat sink) at temperature TC was opened to air. The open circuit voltage (VOC) and the short circuit current (ISC) of the module were measured as a function of the temperature difference (ΔT = TH−TC). 3. Results and Discussion The optical emission spectra from the plasma during thin film depositions (not shown) showed that the Ca3Co4O9 and ZnAlO targets were successfully sputtered. EDS analysis (results are not given) revealed that the as-deposited films from a Ca3Co4O9 target comprised Ca, Co, O elements, while those from a ZnAlO target contained Zn, Al, O elements. From this point onward, the as-deposited films will be referred to as CCO for the Ca-Co-O film and ZAO for the Zn-Al-O film. Fig 2 show the XRD patterns of the as-deposited CCO and ZAO thin films on the ceramic substrate, respectively, with respected to that of a bare substrate. It can be seen that the majority of the peaks in the XRD patterns are the substrate contributions, including Al2O3, SiC, SiO2 phases. In case of the CCO film, none of the peaks in the XRD pattern (Fig 2(a)) can berelated to those of the crystalline Ca3Co4O9. This suggests that the CCO film were mainly grown in amorphous phase, as a result of the low substrate temperature (< 150 °C). On the contrary, there are several peaks of the ZAO film that are corresponding to the diffractions from the (100), (002), and (101) planes of the crystalline ZnO [JCPDS 36-1451], as shown in the Fig 2(b). Furthermore, no trace of crystalline aluminum is detected in the XRD pattern. Thus, it appears that polycrystalline ZnO doped with Al can be grown at the low substrate temperature. This implies that only the energy provided by the pulsed-dc plasma to the depositing atomic species is sufficient to promote the Al-doped ZnO crystal formation. Cross-sectional SEM views indicated that the as-deposited CCO and ZAO films thickness of 0.55 μm and 0.58 μm were obtained, respectively. The CCO film exhibited ptype conduction with S = 103.84 μV/K, ρ = 8.29 Ω⋅cm, P = 0.13 μW/m⋅K2, while the ZAO film was ntype with S = −34.33 μV/K, ρ = 0.01 Ω⋅cm, P = 14.39 μW/m⋅K2. It should be noted that the electrical resistivity of the CCO film is about 2 order of magnitude as high as that of the ZAO film. This is not desirable for thermoelectric module applications. (a) Ca-Co-O film on ceramic substrate
(b) Zn-Al-O film on ceramic substrate
25
Ceramic Substrate 30 35 40 45 50 2θ (degree)
55
60
65
25
(002) (101)
(100)
Intensity (counts)
Intensity (counts)
Zn-Al-O
Ceramic Substrate 30 35 40 45 50 2θ (degree)
55
60
65
Fig. 2. XRD patterns of the as-deposited films/substrate samples in comparison to that of the ceramic substrates
The plots of the open circuit voltage and the short circuit current of the tested thermoelectric module of p-CCO and n-ZAO thin films are shown in Fig 3. It is apparent that the open circuit voltage and short
798
Weerasak Somkhunthot et al. / Energy Procedia 61 (2014) 795 – 798
circuit current increased with increasing temperature difference up to 55.6 mV and 0.04 μA for a temperature difference of 101.40 K (TH = 477.55 K and TC = 376.15 K), respectively. The low short circuit current is likely a result of the high series resistance contributed by the p-CCO leg.
40
0.05 Short Circuit Current, ISC (μA)
0.04
(b) The short circuit current as a function of temperature difference Cu Connectors
0.03
30
0.02
20
p-CCO n-ZAO p-CCO n-ZAO p-CCO n-ZAO
Open Circuit Voltage, VOC (mV)
60 (a) The open circuit voltage as a function of temperature difference 50
0.01
10 0 0 10 20 30 40 50 60 70 80 90 100110 Temperature Difference, ΔT (K)
0.00
Ceramic Substrate 0 10 20 30 40 50 60 70 80 90 100110 Temperature Difference, ΔT (K)
Fig. 3. Test results on a thermoelectric module of p-CCO and n-ZAO thin films (a) the open circuit voltage and (b) the short circuit current as a function of temperature difference
4. Conclusions Thin films of p-CCO and n-ZAO prepared on ceramic substrate by the pulsed-dc magnetron sputtering method were used to make a thermoelectric module. The built module can generate the open circuit voltage and short circuit current up to 55.6 mV and 0.04 μA at a temperature difference of 101.40 K. This test demonstrated that the thermoelectric module containing these thin films is considered as an interesting platform for further work. The investigation on the improvement of the thermoelectric module performance by improving the electrical conductivity of the p-CCO film to match that of the n-ZAO film is of particular interest. Acknowledgements This work was financially supported by the Research and Development Administration Division, Electricity Generating Authority of Thailand (EGAT). The authors would like to gratefully thank the Thermoelectric Research Center (TRC), Sakon Nakhon Rajabhat University for XRD and thermoelectric property measurement facilities, and the National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (ASTDA), Thailand for EDS and SEM analysis. References [1] Rowe DM. Thermoelectrics handbook macro to nano, Boca Raton, FL: CRC Press; 2006, p. 1-14. [2] Nolas GS, Sharp J, Goldsmid HJ. Thermoelectrics: Basic principles and new materials developments, Verlag, Berlin, Heidelberg: Springer; 2001, p. 1–14. [3] Wood C. Materials for thermoelectric energy conversion. Rep Prog Phys 1988;51:459-539. [4] Jood P, Peleckis G, Wang X, Dou SX. Thermoelectric properties of Ca3Co4O9 and Ca2.8Bi0.2Co4O9 thin films in their island formation mode. J Mater Res 2013;28(14) :1932-1939. [5] Somkhunthot W, Pimpabute N, Seetawan T. (2013). Thermoelectric module of p-type Ca-Co-O/n-type ZnO thin films. Adv Mater Res 2013;622-623:726-733. [6] Chitra M, Uthayarani K, Rajasekaran N, Girija E.K. Preparation and characterisation of Al doped ZnO nanopowders. Phys Procedia 2013;49:177-182. [7] Ohtaki M, Araki, K. Aluminum-containing zinc oxide-based n-type thermoelectric conversion material, Japan Science and Technology Agency; 2013, Information on http://all-patents.com/us-patent/8454860. [8] Somkhunthot W, Burinprakhon T, Thomas I, Amornkitbamrung V, Seetawan T. Bipolar pulsed-dc power supply for magnetron sputtering and thin films synthesis. ELEKTRIKA J Elec Eng 2007;9(2):20-26. [9] Ralchenko Y. Atomic spectra database, National Institute of Standards and Technology; 2013, Information on http://www.nist.gov/pml/data/asd.cfm.