Understanding the Giant Seebeck Coefficient of MnO 2
Nanoparticles Costel Constantin James Madison University James
Madison University, October 2012
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- Crystal structures and semiconductor properties. - Materials
characterization methods. - Giant Seebeck Coefficient Observed in
Manganese Oxide Nanostructures? Outline
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Crystal Structures WHAT KEEPS THE ATOMS TOGETHER INSIDE OF A
CRYSTAL? unique arrangement of atoms in a crystal. composed of a
unit cell, which is periodically repeated in three dimensions on a
lattice. a = lattice constant
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Types of Crystal Structures
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Forces Between Atoms in a Crystal IONIC BONDS - electrostatic
forces between two oppositely-charged ions, e.g. alkali halogenides
METALLIC BONDS - electrostatic attraction between the metal atoms
or ions and the FREE electrons, also called CONDUCTION electrons.
e.g. Metals. COVALENT BONDS - sharing of pairs of electrons between
atoms, e.g. Semiconductors, Organic Molecules; C, Si, InSb. VAN DER
WAALS BONDS arises from the polarization of molecules into dipoles.
e.g. Noble Gas crystals, H 2, O 2. HOW ABOUT THE ENERGY LEVELS IN A
CRYSTAL?
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Electronic Band Structure in Solids Electrons live in ENERGY
ORBITALS = ENERGY LEVELS. ENERGY LEVELS in a crystal, where ions
bond, form ENERGY BANDS. HOW CAN WE UNDERSTAND THE DIFFERENCE
BETWEEN METALS, INSULATORS, AND SEMICONDUCTORS IN TERMS OF ENERGY
BANDS? E 1s 2s 2p 3s 3d 3p
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Metals, Insulators, Semiconductors WHAT IS THE MECHANISM FOR
SOME MATERIALS TO CONDUCT ELECTRICITY? Define E F as the level
below which all electrons fill up the states (little cups). METALS
- Fermi energy level falls at the middle of the allowed band.
INSULATORS and SEMICONDUCTORS - Fermi energy level falls at the
middle of the forbidden gap. 1s 2s 2p 3s 3d 3p
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Conduction Bands, Valence Bands, and Band Gaps VALENCE BAND -
created by the outer shell electrons, and most of the states (cups)
are occupied by electrons. CONDUCTION BAND - free electrons coming
from VB and able to conduct electricity. BAND GAP - the width of
the forbidden band. 1s 2s 2p 3s 3d 3p
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Materials Characterization Principles and Techniques
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X-ray Diffraction (XRD) n = integer number for constructive
interference. = the wavelength of the incoming and outgoing X-ray.
= the diffraction angle. Great technique for identifying crystal
structures
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Seebeck Effect
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[a] Before Thermal Excitation [b] After Thermal Excitation
Seebeck coefficient, S = - V/ T. Typical values in the order of V/(
o ). It can give an easy carrier type determination for
semiconductor substrates.
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How Do We Measure Seebeck Effect
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Scanning Electron Microscope (SEM) and Transmission Electron
Microscope (TEM) Fig. 1 Scanning Electron MicroscopeFig. 2 Scanning
Electron Microscope
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Giant Seebeck Coefficient Observed in Manganese Oxide
Nanostructures
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Why Manganese Oxide Nanoparticles? FangFang Song, Liming Wu and
S Liang, Nanotechnology 23, 085401 (2012).
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Einstein Prediction For Lowest Thermal Conductivity
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XRD of our as-received MnO 2 powder Fig. 1 X-ray diffraction of
as-received MnO 2 powderFig. 2 NIST MnO 2 standard
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Crystal Structure of our Manganese Oxide powder Rutile
structure. Gray atoms are Mn. Red Atoms are O.
http://en.wikipedia.org/wiki/File:Rutile-unit-cell-3D-balls.png
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SEM and TEM images of MnO 2 powder 1. FangFang Song, Liming Wu
and S Liang, Nanotechnology 23 (2012) 085401 (4pp) Fig. 1 Scanning
electron microscope image of MnO 2 [ref. 1] Fig. 2 Transmission
electron microscopy image of MnO 2.
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Figure of Merit and Harman Transient Method Thermoelectric
materials are characterized by the figure of merit ZT. Sigma ( )
electrical conductivity. S Seebeck coefficient. Kappa (k) thermal
conductivity. ZT = V DC /V AC - 1
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Preliminary Results ZT vs. MnO 2 particle stacking density
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Preliminary Results Seebeck vs. MnO 2 particle stacking
density
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Preliminary Results Thermal conductivity vs. MnO 2 particle
stacking density
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Conclusions MnO 2 nanoparicles are promising for creating
devices: Seebeck coefficient can be improved, conductivity can be
improved, and they exhibit very low thermal conductivity. MnO
2
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THANK YOU
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Giant Seebeck Coefficient Thermoelectric Device of MnO 2
powder
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FangFang Song, Liming Wu and S Liang, Nanotechnology 23, 085401
(2012). How does the Figure of Merit behave as a function of
temperature, particule size, and particule density? By applying the
transient Harman method we can find an answer to all these
questions. Important Problems to be Studied
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Doping Semiconductors INTRINSIC do not conduct electricity
because electrons are tightly bonded to the nucleus. N-type doping
with Phosphorous atoms introduce an extra electron in the
conduction band. P-type doping with Boron atoms introduce an extra
hole in the conduction band.