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1
1
2
4
12-2x
x
x
2
• Harnessing electricity from heat is the concept of thermoelectricity.
• Examples of thermoelectric applications include power plant generators and refrigerating systems.
• Future applications for thermoelectric materials include implementing in motor vehicles to improve efficiency in gasoline consumption which is only at around 14-30% for conventional vehicles. [1]
Figure 1. Electricity will be generated from the thermoelectric device due to the temperature gradient between cool air from the environment and waste heat from the hot exhaust through the
Seebeck Effect [2].
• However, implementing thermoelectric devices into automobiles requires better and more improved thermoelectric materials still in development.
• To develop the best thermoelectric materials that can conduct a maximum amount of electricity from small temperature gradients as well as withstand extremely high ones, a study is performed to investigate skutterudite series Co
4
Sb
12-2x
Te x
Ge x
.
1) Before performing any complex simulations, the atomic structure of skutterudite series, Co
4
Sb
12-
2x
Te x
Ge x
, needed to be optimized.
• First Principle Calculations of crystal structure were performed using the program Vienna
Simulation Package (VASP) based on the
Density functional theory (DFT). [4]
Figure 3. SSH Secure Shell Client and
File Transfer access the server to run and receive VASP calculations. VMD and p4v are both used for visual outputs from VASP.
• Input files contained:
 XC- Exchange
Correlation Energy
Functional
 K-point coordinates for size of sampling the Brillouin Zone.
 Lattice parameters & atomic coordinates
 PP-pseudopotentials for each atomic species
• Output files included:
 CONTCAR- relaxed atomic structure
 CHGCAR- optimized electronic density
 WAVECARoptimized wave density
Figure 4. Optimized Co
4
Sb
12
structure
• Both lattice constant and volumes were calculated using DFT and DFT+U.
Table 1. Average optimized lattice constant for atomic structure of Co
4
Sb
12-2x where x= 0, 0.5, 1, 2, & 3. Each column after the ‘x’ column represents data calculated from each concentration three different ways.
Te x
Ge x
,
0 9.03662(4) 9.110858907
9.097107787
0.5
9.01633(4)
1 8.99555(5)
2 8.94781(7)
3 8.89584(5)
9.081218599
9.061538786
8.887053776
8.970463368
9.589741047
9.262686611
9.491395559
9.470189676
Table 2. Optimized lattice volume for atomic structure of Co each concentration three different ways.
4
Sb
12-2x
Te x
Ge x
, where x=
0, 0.5, 1, 2, & 3. Each column after the ‘x’ column represents data calculated from
3
0 737.936(11) 756.27
752.85
0.5
732.975(10)
1 727.918(12)
2 716.39(2)
3 703.981(12)
748.91
744.03
701.81
721.7
881.88
794.33
855
847.01
• Experimental data validated computation with DFT and DFT+U for this skutterudite series.
• With DFT having a better representation for the atomic cell unit, the output files were used for the
Electronic Density of States calculation to estimate a band gap at each concentration.
Figure 5. Electronic density of states (DOS) given as a function of energy for
Co
4
Sb
12
. Vertical dotted line represents the Fermi energy, 𝐸
𝐹
. Core electrons are on the left with the Valence Band being the closest part on the left side of the Fermi line.
Conduction band represent the right side.
• Compound with cage-type crystal structure formed by group 15 elements, featuring loosely bound guest
(filler) atoms inside.
• Typically exhibit good intrinsic electrical transport properties and have a thermal conductivity that can be minimized by coordinating the filler atoms. [3]
• General Formula: M
– A = Fe, Ru, Co x
A
4
B
12
– B = P, As, Sb (group
15 elements)
– M x
= filler atom
• Replace Sb with other species: Co
4
Sb
12-2x
Te x
Ge x
Figure 2. A toms form the cage structure that is filled with filler atoms M and B atoms that form nearly-square planar rings.
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2) The calculated structural data was compared with experiment.
3) Based on the validation with experiment, the method, i.e. DFT or DFT+U, would be selected for further use to determine the electrical conductivity properties of the compound.
Figure 6. Electronic density of states (DOS) given as a function of energy for
Co
4
Sb
11
Te
0.5
Ge
0.5
. Vertical dotted line represents the Fermi energy, 𝐸
𝐹 .
• In both Figure 5 and 6, the conduction and valence bands are very close.
• This small range band gap indicates that the skutterudite is semi-conducting.
• The band gap can be minimized by increasing the concentration of Te and Ge substitutions.
• Using computational modeling, the ground state structures of skutterudite series Co were successfully predicted.
4
Sb
12-2x
Te x
Ge x
• In comparison with experimental data, the DFT calculations for the lattice constant and volume were supported over the DFT+U method.
• The Co
4
Sb
12-2x
Te x
Ge x skutterudite is semiconducting, and its electronic band gap decreases as Te and Ge substitutions increase.
• Future work: further prediction of thermal conductivity properties of skutterudite series Co
4
Sb
12-2x
Te x
Ge x
.
[ 1] U.S. Department of Energy, “Where the Energy Goes: Gasoline Vehicles”,
Energy requirement estimates are based on analysis of over 100 vehicles by
Oak Ridge National Laboratory using EPA Test Car List Data Files, http://www.fueleconomy.gov/feg/atv.shtml
[2] The Nanometer Structure Consortium at Lund University , http://www.nano.lth.se/research/nano-energy/thermoelectrics?layoutmode=print
[3] Y. G. Yan, W. Wong-Ng, L. Li, I. Levin, J. A. Kaduk, M. R. Suchomel, X.
Sun, G. J. Tan, X. F. Tang, ”Structures and thermoelectric properties of double-filled(Ca x
Ce
1-x
)Fe
4
Sb
Chemistry, in press (2014).
12 skutterudites”
, Journal of Solid State
[4] June Gunn Lee,
Computational Materials Science An Introduction
, CRC
Press Taylor & Francis Group, Boca Raton, FL, USA, pp 180-200 (2012).
A special thanks to graduate student, Izaak Williamson, for his helpful discussions and aid with computational modeling setup for the calculations. Many thanks to, Dr.
Lan Li, for her supervision and supportive guidance throughout the entire project. These research and educational activities were supported by the National
Science Foundation, Office of Special Programs, and
Division of Materials Research under Grant No. DMR
1359344.
Q U I C K S TA RT ( c o n t . )
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