- Email: [email protected]
MeV Si ions bombardment effects on the thermoelectric properties of Co0.1SbxGey thin films
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1588–1591
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
MeV Si ions bombardment effects on the thermoelectric properties of Co0.1SbxGey thin films S. Güner a,b,*, S. Budak c, R. Amaral Minamisawa a, C.I. Muntele a, D. Ila a a
Center for Irradiation Materials, Department of Physics, Alabama A&M University, Normal, AL 35762, USA _ Department of Physics, Fatih University, 34500 Büyükçekmece/Istanbul, Turkey c Department of Electrical Engineering, Alabama A&M University, Normal, AL 35762, USA b
a r t i c l e
i n f o
Article history: Available online 8 February 2009 PACS: 81.15.Jj 72.20.Pa 07.81.+a Keywords: Ion beam deposition Thermoelectric properties Rutherford backscattering Figure of merit
a b s t r a c t We have grown three different monolayer Co0.1SbxGey (x = 2, 4, 11 and y = 15, 7, 15) thin films on silica substrates with varying thickness between 100 and 200 nm using electron beam deposition. The highenergy (in the order of 5 MeV) Si ion bombardments have been performed on samples with varying fluencies of 1 1012, 1 1013, 1 1014 and 1 1015 ions/cm2. The thermopower, electrical and thermal conductivity measurements were carried out before and after the bombardment on samples to calculate the figure of merit, ZT. The Si ions bombardment caused changes on the thermoelectric properties of films. The fluence and temperature dependence of cross plane thermoelectric parameters were also reported. Rutherford backscattering spectrometry (RBS) was used to analyze the elemental composition of the deposited materials and to determine the layer thickness of each film. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Skutterudites are the promising intermetallic compounds for the thermoelectric applications and investigated since 1990s in detail. All these materials derive from the archetypal mineral skutterudite, CoAs3 [1]. Binary skutterudites with the formula TmX3 are based on transition metals Tm = Co, Rh, Ir from the ninth group of the periodic system and the pnictide elements such as X = P, As and Sb. Ternary semiconducting skutterudites are based on transition metals from the Fe group and need for stabilization of the structural motif electropositive metal cations RE leading to the chemical formula (RE)Tm4X12. Here, RE usually is trivalent rare-earth ions that donate electrons to the [Co4Sb12] polyanionic host structure [2]. These compounds are called ‘‘filled skutterudites’’ since the stabilizing atoms reside in large voids within the cubic transition metal-pnictogen framework. Recently, a new series of filled skutterudites with monovalent cations Na, K and Tl has been discovered [3,4]. The main interest originates in large part to the ability to greatly vary of lattice thermal conductivity, which is facilitated by filling of the voids within the structure with small diameter, large mass interstitials such as trivalent Re ions [5].
Nolas et al. have shown in an investigation of thermal conductivity of ReCo4Sb9Ge3 that the smaller more massive ions that are incorporated within the skutterudite voids result in the lowest lattice thermal conductivity [6]. In our study, we investigated thermoelectric properties of different composition of Co0.1SbxGey mixtures and the high-energy Si ions bombardment effects. The recent studies indicated that unfilled skutterudites exhibit better thermoelectric properties than filled structures [7]. So any composition ratios of Co:Sb:Ge are interest of study for thermoelectric applications. The high-energy ion bombardment can originate the mass difference of constituent elements in the structure. The mass difference scattering in the alloys reduces the lattice thermal conductivity significantly without much changing degradation to the electrical conductivity [8]. The same effect is expected for our structures. The performance of any thermoelectric substance is evaluated by the dimensionless figure of merit expression ZT = S2rT/j, where S, r, j and T are the Seebeck coefficient (V/K), electrical conductivity (X 1 m 1), thermal conductivity (W/mK) and absolute temperature (K), respectively [9–11]. 2. Experimental
* Corresponding author. Address: Department of Physics, Fatih University, 34500 _ Büyükçekmece/Istanbul, Turkey. E-mail address: [email protected] (S. Güner). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.157
Three different compositions of Co0.1SbxGey (x = 2, 4, 11 and y = 15, 7, 15) single layer thin films on silica substrates were deposited between 100 and 200 nm thickness by Ion Beam As-
1589
S. Güner et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1588–1591
300
Ge
a Co0.1Sb2Ge15
250
Counts
200 150 Co Sb
100
C Sb
50 0 700
Sb
b Co0.1Sb4Ge7
600 500
Counts
sisted Deposition (IBAD) technique. The three electron-gun evaporators for evaporating the powder targets in different crucibles were turned on. The base pressure obtained in IBAD chamber was 5 10 6 torr. An Inficon quartz crystal monitor indicated the growth rates. The substrates were not heated before or during the deposition process, so temperature inside chamber was about 20 °C when deposition started. The films are in amorphous structure due to deposition conditions in the IBAD chamber. The geometries are for deposition and thermal conductivity, Seebeck and electrical conductivity measurements in Fig. 1(a)– (c), respectively. The electrical conductivity was measured by the Van der Pauw system and Seebeck coefficient was measured by Seebeck system. We used a home made system to measure the thermal conductivity that is based on the 3x technique. The Ag strip deposited on the film has 50 lm width, 200 nm thickness and contact places for silver paste. The electrical conductivity, thermal conductivity and Seebeck coefficient measurements have been performed at room temperature. Detailed information about the 3x technique may be found in [12–14]. The 5 MeV Si ion bombardments were performed with the Pelletron ion beam accelerator at the Alabama A&M University Center for Irradiation of Materials (AAMU-CIM). SRIM simulation software (SRIM) was used to determine the energy level of the bombarding Si ions. The fluences used for the bombardment were 1 1012, 1 1013, 1 1014 and 1 1015 ions/cm2. Rutherford backscattering spectrometry (RBS) was performed using 2.1 MeV He+ ions with the particle detector placed at 170 degrees from the incident beam to monitor the film thickness and stoichiometry before and after 5 MeV Si ion bombardment [15,16].
Sb
400 Ge
300 200 C
Co
100 0 500
3. Experimental analysis and discussion
Sb
c Co0.1Sb11Ge15
400
300
Counts
Fig. 2(a)–(c) shows the RBS spectra of Co0.1Sb2Ge15, Co0.1Sb4Ge7 and Co0.1Sb11Ge15 single layer films, respectively, on a glassy polymeric carbon (GPC) substrate when the sample is at the 90° of angle. Each element that was used in the deposition is revealed in the RBS spectrum. The Sb deposition exhibited better uniformity with the higher concentration. The RUMP software program was used to determine the composition rates and the film thickness of each element. The film thickness for each film has been found as 192, 132 and 155 nm. Fig. 3(a) shows the fluence dependence of square of Seebeck coefficients. The S is a rate of average thermal energy carried by charge carriers. Between the unbombarded samples, Co0.1Sb2Ge15
Ge
200 C
100
Co
0 0
a
Ag strip Co0.1Sb xGe y SiO2, 100 nm coating
Silicon substrate
b
Au
Au Co0.1Sb xGe y
Au
Fused Silica (Suprasil)
c
Co0.1Sb xGe y SiO2, 100 nm coating
Silicon substrate Fig. 1. Geometry of samples from the cross-section.
200
400
600
800
Channel Fig. 2. He RBS spectra and RUMP simulations for Co0.1SbxGey monolayer thin films on GPC substrate.
film has the greatest amount of S = 72.25 lV/K which is a significant range for room temperature n-type ternary skutterudites. All compositions exhibit negative thermopower that is, the electrons are the main carriers. The effects of bombardment with different fluences are observed as different on the thermopower properties. A remarkable increase is observed for Co0.1Sb4Ge7 film especially at the fluence of 1 1013 ions/cm2 while sharp decreases are seen for the other compositions. The changes in electrical conductivity values due bombardment at increasing fluence are given in Fig. 3(b). The r values are in the order of 105 X 1 m 1 for the unbombarded compositions Co0.1Sb2Ge15 and Co0.1Sb4Ge7. In general, iron group based ternary filled skutterudites have electrical conductivity in the order of 104 X 1 m 1 [17,18]. So our partially filled films exhibit almost
1590
S. Güner et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1588–1591
6000
a
Square of Seebeck coefficients
5000 Co0.1Sb2Ge15 Co0.1Sb4Ge7 Co0.1Sb11Ge15
S ( μ V/K)
2
4000
2
3000 2000 1000 0 20 18
electrical conductivity Co 0.1Sb 2Ge 15 Co 0.1Sb 4Ge 7 Co 0.1Sb 11 Ge15
b
-1
σ ( Ω m) x10
4
16 14 12 10 8 6 4 2 10 9
c
thermal conductivity Co0.1Sb 2Ge15 Co Sb 4Ge7
8
0.1
Co0.1Sb 11Ge15
κ (W/mK)
7 6
4. Conclusion
5 4
All compositions have n-type of thermopower. Seebeck coefficient increased more or less than the Seebeck coefficient of asdeposited sample with increasing fluence just for Co0.1Sb4Ge7 composition. A permanent increase was observed at the electrical conductivity of Co0.1Sb11Ge15 samples as in reverse case of decrement for the other compositions. Thermal conductivity decreased more with the fluence for the higher composition ratios of Sb to Ge. The smallest ratio of Sb to Ge is at the film of Co0.1Sb2Ge15 and thermal conductivity increases with an undesired sharp rate. Dimensionless figure of merit increased for the higher composition ratios of Sb to Ge with the increasing amount of fluence.
3 2 1 0.16 0.14
d
dimensionless figure of merit Co 0.1 Sb 2 Ge 15 Co 0.1 Sb 4 Ge 7 Co 0.1 Sb 11 Ge 15
0.12 0.10
ZT
and final values of the electrical conductivity. The r values increased with the bombardment permanently just for Co0.1Sb11Ge15 films. The increase in electrical conductivity is most likely a simple consequence of increased hopping conduction in the irradiated films. Fig. 3(c) shows the thermal conductivity and fluence dependence of thermal conductivity. Thermal conductivity values are high for the virgin samples and vary between 1 and 3 W/mK. The bombardment causes to significant decrease in thermal conductivity Co0.1Sb4Ge7 and Co0.1Sb11Ge15 films with increasing fluence. The composition, Co0.1Sb2Ge15 exhibits a reverse behavior and j increases rapidly with the fluence. When the Co0.1SbxGey films are bombarded with high-energy Si ions, the created heat energy causes some interface scattering and absorption of phonons. Scattering and absorption of phonons cause decrease in thermal conductivity. If bombardment continues on the samples, the amount of the thermal conductivity may increase due to the increase of the discrete energy levels in between the conduction and valence bands [19]. The room temperature figure of merit of unbombarded films and their fluence dependence for single layer Co0.1SbxGey (x = 2, 5, 1 and y = 15, 7, 15) films are given in Fig. 3(d). The virgin samples have a magnitude of figure of merit in the range of 0–0.2. This range is common for iron group based ternary skutterudites [5]. If we look at Fig. 3(a) and (d), the unbombarded films are detected to have same order of the S coefficient and ZT from lower to high. This is mainly due the dominant effect of S2 for the calculation of ZT. The effect of bombardment on the ZT is observed as positive and it slightly increases for the compositions Co0.1Sb4Ge7 and Co0.1Sb11Ge15 with the increasing fluence. The composition Co0.1Sb2Ge15 exhibits a reverse behavior and ZT drops from 0.14 to 0.014 at the fluence of 1 1015 ions/cm2. The reason for this case is the great negative effect of bombardment on the thermal conductivity values with the increasing amount of fluence.
0.08 0.06
Acknowledgements
0.04
This research was sponsored by the Center for Irradiation of Materials, Alabama A&M University and by the AAMURI Center for Advanced Propulsion Materials under the contract number NNM06AA12A from NASA, and by National Science Foundation under Grant No. EPS-0447675.
0.02 0.00 0
1E12
1E13
1E14
1E15
2
Fluence (ions/cm ) Fig. 3. Thermoelectric properties of Co0.1SbxGey thin films.
2–3 times higher conductivity. There is a more or less continuous decrease in electrical conductivity only for one composition (Co0.1Sb2Ge15). It decreases for Co0.1Sb4Ge7 after irradiation with the fluence of 1 1013 ions/cm2, but then it increases with increasing the fluence, so that there is no difference between the initial
References [1] C. Recknagel, N. Reinfried, P. Hohn, W. Schnelle, H. Rosner, Yu Grin, A. LeitheJasper, Sci. Technol. Adv. Mater. 8 (2007) 357. [2] B.C. Sales, in: K.A. Gschneidner Jr., J.C.G. Bünzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 33, Elsevier, Amsterdam, 2003, p. 1 (Chapter 211). [3] A. Leithe-Jasper, W. Schnelle, H. Rosner, N. Senthilkumaran, A. Rabis, M. Baenitz, A.A. Gippius, E.N. Morozova, J.A. Mydosh, Yu Grin, Phys. Rev. Lett. 91 (2003) 037208.
S. Güner et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1588–1591 [4] A. Leithe-Jasper, W. Schnelle, H. Rosner, M. Baenitz, A. Rabis, A.A. Gippius, E.N. Morozova, H. Borrmann, U. Burkhardt, R. Ramlau, U. Schwarz, J.A. Mydosh, Yu Grin, Phys. Rev. B 70 (2004) 214418. [5] T.M. Tritt, M.A. Subramanian, MRS Bull. 31 (2006) 190. [6] G.S. Nolas, D.T. Morelli, T.M. Tritt, Annu. Rev. Mater. Sci. 29 (1999) 89. [7] J.P. Fleurial, T. Caillat, A. Borshchevsky, in: Proceedings of the XVI International Conference on the Thermoelectrics, Dresden, Germany, 26–29 August 1997. [8] A.F. Ioffe, Semiconductor Thermoelements and Thermoelectric Cooling, Infosearch Ltd., 1957. [9] S. Budak, C. Muntele, B. Zheng, D. Ila, Nucl. Instr. and Meth. B 261 (2007) 1167. [10] S. Budak, C.I. Muntele, R.A. Minamisawa, B. Chhay, D. Ila, Nucl. Instr. and Meth. B 261 (2007) 608.
1591
[11] B. Zheng, S. Budak, R.L. Zimmerman, C. Muntele, B. Chhay, D. Ila, Surf. Coat. Technol. 201 (2007) 8531. [12] Z. Xiao, R.L. Zimmerman, L.R. Holland, B. Zheng, C.I. Muntele, D. Ila, Nucl. Instr. and Meth. B 242 (2006) 201. [13] D.G. Cahill, M. Katiyar, J.R. Abelson, Phys. Rev. B 50 (1994) 6077. [14] T.B. Tasciuc, A.R. Kumar, G. Chen, Rev. Sci. Instr. 72 (2001) 2139. [15] L. Lu, W. Yi, D.L. Zhang, Rev. Sci. Instr. 72 (2001) 2996. [16] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping Range of Ions in Solids, Pergamon Press, New York, 1985. [17] W.K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978. [18] D.T. Morelli, G.P. Meisner, J. Appl. Phys. 77 (1995) 3777. [19] D.A. Gajewski, N.R. Dilley, E.D. Bauer, E.J. Freeman, R. Chau, M.B. Maple, D. Mandrus, B.C. Sales, A.H. Lacerda, J. Phys.: Condens. Matter 10 (1998) 6973.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
MeV Si ions bombardment effects on the thermoelectric properties of Co0.1SbxGey thin films S. Güner a,b,*, S. Budak c, R. Amaral Minamisawa a, C.I. Muntele a, D. Ila a a
Center for Irradiation Materials, Department of Physics, Alabama A&M University, Normal, AL 35762, USA _ Department of Physics, Fatih University, 34500 Büyükçekmece/Istanbul, Turkey c Department of Electrical Engineering, Alabama A&M University, Normal, AL 35762, USA b
a r t i c l e
i n f o
Article history: Available online 8 February 2009 PACS: 81.15.Jj 72.20.Pa 07.81.+a Keywords: Ion beam deposition Thermoelectric properties Rutherford backscattering Figure of merit
a b s t r a c t We have grown three different monolayer Co0.1SbxGey (x = 2, 4, 11 and y = 15, 7, 15) thin films on silica substrates with varying thickness between 100 and 200 nm using electron beam deposition. The highenergy (in the order of 5 MeV) Si ion bombardments have been performed on samples with varying fluencies of 1 1012, 1 1013, 1 1014 and 1 1015 ions/cm2. The thermopower, electrical and thermal conductivity measurements were carried out before and after the bombardment on samples to calculate the figure of merit, ZT. The Si ions bombardment caused changes on the thermoelectric properties of films. The fluence and temperature dependence of cross plane thermoelectric parameters were also reported. Rutherford backscattering spectrometry (RBS) was used to analyze the elemental composition of the deposited materials and to determine the layer thickness of each film. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Skutterudites are the promising intermetallic compounds for the thermoelectric applications and investigated since 1990s in detail. All these materials derive from the archetypal mineral skutterudite, CoAs3 [1]. Binary skutterudites with the formula TmX3 are based on transition metals Tm = Co, Rh, Ir from the ninth group of the periodic system and the pnictide elements such as X = P, As and Sb. Ternary semiconducting skutterudites are based on transition metals from the Fe group and need for stabilization of the structural motif electropositive metal cations RE leading to the chemical formula (RE)Tm4X12. Here, RE usually is trivalent rare-earth ions that donate electrons to the [Co4Sb12] polyanionic host structure [2]. These compounds are called ‘‘filled skutterudites’’ since the stabilizing atoms reside in large voids within the cubic transition metal-pnictogen framework. Recently, a new series of filled skutterudites with monovalent cations Na, K and Tl has been discovered [3,4]. The main interest originates in large part to the ability to greatly vary of lattice thermal conductivity, which is facilitated by filling of the voids within the structure with small diameter, large mass interstitials such as trivalent Re ions [5].
Nolas et al. have shown in an investigation of thermal conductivity of ReCo4Sb9Ge3 that the smaller more massive ions that are incorporated within the skutterudite voids result in the lowest lattice thermal conductivity [6]. In our study, we investigated thermoelectric properties of different composition of Co0.1SbxGey mixtures and the high-energy Si ions bombardment effects. The recent studies indicated that unfilled skutterudites exhibit better thermoelectric properties than filled structures [7]. So any composition ratios of Co:Sb:Ge are interest of study for thermoelectric applications. The high-energy ion bombardment can originate the mass difference of constituent elements in the structure. The mass difference scattering in the alloys reduces the lattice thermal conductivity significantly without much changing degradation to the electrical conductivity [8]. The same effect is expected for our structures. The performance of any thermoelectric substance is evaluated by the dimensionless figure of merit expression ZT = S2rT/j, where S, r, j and T are the Seebeck coefficient (V/K), electrical conductivity (X 1 m 1), thermal conductivity (W/mK) and absolute temperature (K), respectively [9–11]. 2. Experimental
* Corresponding author. Address: Department of Physics, Fatih University, 34500 _ Büyükçekmece/Istanbul, Turkey. E-mail address: [email protected] (S. Güner). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.157
Three different compositions of Co0.1SbxGey (x = 2, 4, 11 and y = 15, 7, 15) single layer thin films on silica substrates were deposited between 100 and 200 nm thickness by Ion Beam As-
1589
S. Güner et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1588–1591
300
Ge
a Co0.1Sb2Ge15
250
Counts
200 150 Co Sb
100
C Sb
50 0 700
Sb
b Co0.1Sb4Ge7
600 500
Counts
sisted Deposition (IBAD) technique. The three electron-gun evaporators for evaporating the powder targets in different crucibles were turned on. The base pressure obtained in IBAD chamber was 5 10 6 torr. An Inficon quartz crystal monitor indicated the growth rates. The substrates were not heated before or during the deposition process, so temperature inside chamber was about 20 °C when deposition started. The films are in amorphous structure due to deposition conditions in the IBAD chamber. The geometries are for deposition and thermal conductivity, Seebeck and electrical conductivity measurements in Fig. 1(a)– (c), respectively. The electrical conductivity was measured by the Van der Pauw system and Seebeck coefficient was measured by Seebeck system. We used a home made system to measure the thermal conductivity that is based on the 3x technique. The Ag strip deposited on the film has 50 lm width, 200 nm thickness and contact places for silver paste. The electrical conductivity, thermal conductivity and Seebeck coefficient measurements have been performed at room temperature. Detailed information about the 3x technique may be found in [12–14]. The 5 MeV Si ion bombardments were performed with the Pelletron ion beam accelerator at the Alabama A&M University Center for Irradiation of Materials (AAMU-CIM). SRIM simulation software (SRIM) was used to determine the energy level of the bombarding Si ions. The fluences used for the bombardment were 1 1012, 1 1013, 1 1014 and 1 1015 ions/cm2. Rutherford backscattering spectrometry (RBS) was performed using 2.1 MeV He+ ions with the particle detector placed at 170 degrees from the incident beam to monitor the film thickness and stoichiometry before and after 5 MeV Si ion bombardment [15,16].
Sb
400 Ge
300 200 C
Co
100 0 500
3. Experimental analysis and discussion
Sb
c Co0.1Sb11Ge15
400
300
Counts
Fig. 2(a)–(c) shows the RBS spectra of Co0.1Sb2Ge15, Co0.1Sb4Ge7 and Co0.1Sb11Ge15 single layer films, respectively, on a glassy polymeric carbon (GPC) substrate when the sample is at the 90° of angle. Each element that was used in the deposition is revealed in the RBS spectrum. The Sb deposition exhibited better uniformity with the higher concentration. The RUMP software program was used to determine the composition rates and the film thickness of each element. The film thickness for each film has been found as 192, 132 and 155 nm. Fig. 3(a) shows the fluence dependence of square of Seebeck coefficients. The S is a rate of average thermal energy carried by charge carriers. Between the unbombarded samples, Co0.1Sb2Ge15
Ge
200 C
100
Co
0 0
a
Ag strip Co0.1Sb xGe y SiO2, 100 nm coating
Silicon substrate
b
Au
Au Co0.1Sb xGe y
Au
Fused Silica (Suprasil)
c
Co0.1Sb xGe y SiO2, 100 nm coating
Silicon substrate Fig. 1. Geometry of samples from the cross-section.
200
400
600
800
Channel Fig. 2. He RBS spectra and RUMP simulations for Co0.1SbxGey monolayer thin films on GPC substrate.
film has the greatest amount of S = 72.25 lV/K which is a significant range for room temperature n-type ternary skutterudites. All compositions exhibit negative thermopower that is, the electrons are the main carriers. The effects of bombardment with different fluences are observed as different on the thermopower properties. A remarkable increase is observed for Co0.1Sb4Ge7 film especially at the fluence of 1 1013 ions/cm2 while sharp decreases are seen for the other compositions. The changes in electrical conductivity values due bombardment at increasing fluence are given in Fig. 3(b). The r values are in the order of 105 X 1 m 1 for the unbombarded compositions Co0.1Sb2Ge15 and Co0.1Sb4Ge7. In general, iron group based ternary filled skutterudites have electrical conductivity in the order of 104 X 1 m 1 [17,18]. So our partially filled films exhibit almost
1590
S. Güner et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1588–1591
6000
a
Square of Seebeck coefficients
5000 Co0.1Sb2Ge15 Co0.1Sb4Ge7 Co0.1Sb11Ge15
S ( μ V/K)
2
4000
2
3000 2000 1000 0 20 18
electrical conductivity Co 0.1Sb 2Ge 15 Co 0.1Sb 4Ge 7 Co 0.1Sb 11 Ge15
b
-1
σ ( Ω m) x10
4
16 14 12 10 8 6 4 2 10 9
c
thermal conductivity Co0.1Sb 2Ge15 Co Sb 4Ge7
8
0.1
Co0.1Sb 11Ge15
κ (W/mK)
7 6
4. Conclusion
5 4
All compositions have n-type of thermopower. Seebeck coefficient increased more or less than the Seebeck coefficient of asdeposited sample with increasing fluence just for Co0.1Sb4Ge7 composition. A permanent increase was observed at the electrical conductivity of Co0.1Sb11Ge15 samples as in reverse case of decrement for the other compositions. Thermal conductivity decreased more with the fluence for the higher composition ratios of Sb to Ge. The smallest ratio of Sb to Ge is at the film of Co0.1Sb2Ge15 and thermal conductivity increases with an undesired sharp rate. Dimensionless figure of merit increased for the higher composition ratios of Sb to Ge with the increasing amount of fluence.
3 2 1 0.16 0.14
d
dimensionless figure of merit Co 0.1 Sb 2 Ge 15 Co 0.1 Sb 4 Ge 7 Co 0.1 Sb 11 Ge 15
0.12 0.10
ZT
and final values of the electrical conductivity. The r values increased with the bombardment permanently just for Co0.1Sb11Ge15 films. The increase in electrical conductivity is most likely a simple consequence of increased hopping conduction in the irradiated films. Fig. 3(c) shows the thermal conductivity and fluence dependence of thermal conductivity. Thermal conductivity values are high for the virgin samples and vary between 1 and 3 W/mK. The bombardment causes to significant decrease in thermal conductivity Co0.1Sb4Ge7 and Co0.1Sb11Ge15 films with increasing fluence. The composition, Co0.1Sb2Ge15 exhibits a reverse behavior and j increases rapidly with the fluence. When the Co0.1SbxGey films are bombarded with high-energy Si ions, the created heat energy causes some interface scattering and absorption of phonons. Scattering and absorption of phonons cause decrease in thermal conductivity. If bombardment continues on the samples, the amount of the thermal conductivity may increase due to the increase of the discrete energy levels in between the conduction and valence bands [19]. The room temperature figure of merit of unbombarded films and their fluence dependence for single layer Co0.1SbxGey (x = 2, 5, 1 and y = 15, 7, 15) films are given in Fig. 3(d). The virgin samples have a magnitude of figure of merit in the range of 0–0.2. This range is common for iron group based ternary skutterudites [5]. If we look at Fig. 3(a) and (d), the unbombarded films are detected to have same order of the S coefficient and ZT from lower to high. This is mainly due the dominant effect of S2 for the calculation of ZT. The effect of bombardment on the ZT is observed as positive and it slightly increases for the compositions Co0.1Sb4Ge7 and Co0.1Sb11Ge15 with the increasing fluence. The composition Co0.1Sb2Ge15 exhibits a reverse behavior and ZT drops from 0.14 to 0.014 at the fluence of 1 1015 ions/cm2. The reason for this case is the great negative effect of bombardment on the thermal conductivity values with the increasing amount of fluence.
0.08 0.06
Acknowledgements
0.04
This research was sponsored by the Center for Irradiation of Materials, Alabama A&M University and by the AAMURI Center for Advanced Propulsion Materials under the contract number NNM06AA12A from NASA, and by National Science Foundation under Grant No. EPS-0447675.
0.02 0.00 0
1E12
1E13
1E14
1E15
2
Fluence (ions/cm ) Fig. 3. Thermoelectric properties of Co0.1SbxGey thin films.
2–3 times higher conductivity. There is a more or less continuous decrease in electrical conductivity only for one composition (Co0.1Sb2Ge15). It decreases for Co0.1Sb4Ge7 after irradiation with the fluence of 1 1013 ions/cm2, but then it increases with increasing the fluence, so that there is no difference between the initial
References [1] C. Recknagel, N. Reinfried, P. Hohn, W. Schnelle, H. Rosner, Yu Grin, A. LeitheJasper, Sci. Technol. Adv. Mater. 8 (2007) 357. [2] B.C. Sales, in: K.A. Gschneidner Jr., J.C.G. Bünzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 33, Elsevier, Amsterdam, 2003, p. 1 (Chapter 211). [3] A. Leithe-Jasper, W. Schnelle, H. Rosner, N. Senthilkumaran, A. Rabis, M. Baenitz, A.A. Gippius, E.N. Morozova, J.A. Mydosh, Yu Grin, Phys. Rev. Lett. 91 (2003) 037208.
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