Transcript Understanding the Giant Seebeck Coefficient of MnO2 Nanoparticles Costel Constantin James Madison University James Madison University, October 2012

Understanding the Giant Seebeck
Coefficient of MnO2 Nanoparticles
Costel Constantin
James Madison University
James Madison University, October 2012
Outline
- Crystal structures and semiconductor properties.
- Materials characterization methods.
- Giant Seebeck Coefficient Observed in Manganese Oxide
Nanostructures?
Crystal Structures
a = lattice constant
unique arrangement of atoms in a crystal.
composed of a unit cell, which is periodically repeated in three dimensions on a lattice.
WHAT KEEPS THE ATOMS TOGETHER INSIDE OF A CRYSTAL?
Types of Crystal Structures
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, H2, O2.
HOW ABOUT THE ENERGY LEVELS
IN A CRYSTAL?
Electronic Band Structure in Solids
E
3s 3d 3p
2s 2p
1s
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?
Metals, Insulators, Semiconductors
3s 3d 3p
2s 2p
1s
Define EF 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.
WHAT IS THE MECHANISM FOR SOME MATERIALS TO CONDUCT ELECTRICITY?
Conduction Bands, Valence Bands, and Band Gaps
3s 3d 3p
2s 2p
1s
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.
Materials Characterization Principles and Techniques
X-ray Diffraction (XRD)
n
d
2sin( )
n = integer number for constructive interference.
λ = the wavelength of the incoming and outgoing X-ray.
 = the diffraction angle.
Great technique for identifying crystal structures
Seebeck Effect
Seebeck Effect
Seebeck Effect
[a] Before Thermal Excitation
[b] After Thermal Excitation
Seebeck coefficient, S = - DV/ DT.
Typical values in the order of mV/(o).
It can give an easy carrier type determination for semiconductor substrates.
How Do We Measure Seebeck Effect
Scanning Electron Microscope (SEM) and Transmission
Electron Microscope (TEM)
Fig. 1 Scanning Electron Microscope
Fig. 2 Scanning Electron Microscope
Giant Seebeck Coefficient Observed in Manganese Oxide
Nanostructures
Why Manganese Oxide Nanoparticles?
FangFang Song, Liming Wu and S Liang, Nanotechnology 23, 085401 (2012).
Einstein Prediction For Lowest Thermal
Conductivity
XRD of our as-received MnO2 powder
Fig. 1 X-ray diffraction of as-received MnO2 powder
Fig. 2 NIST MnO2 standard
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
SEM and TEM images of MnO2 powder
Fig. 1 Scanning electron microscope
image of MnO2 [ref. 1]
Fig. 2 Transmission electron microscopy
image of MnO2.
1. FangFang Song, Liming Wu and S Liang, Nanotechnology 23 (2012) 085401 (4pp)
Figure of Merit and Harman Transient Method
Thermoelectric materials are characterized by the figure of merit “ZT”.
Sigma (s) – electrical conductivity.
S – Seebeck coefficient.
Kappa (k) – thermal conductivity.
ZT = VDC/VAC - 1
Preliminary Results
ZT vs. MnO2 particle stacking density
Preliminary Results
Seebeck vs. MnO2 particle stacking density
Preliminary Results
Thermal conductivity vs. MnO2 particle stacking density
Conclusions
 MnO2 nanoparicles are promising for creating devices: Seebeck coefficient can be improved,
conductivity can be improved, and they exhibit very low thermal conductivity.
MnO2
THANK YOU
Giant Seebeck Coefficient Thermoelectric
Device of MnO2 powder
Important Problems to be Studied
 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.
FangFang Song, Liming Wu and S Liang, Nanotechnology 23, 085401 (2012).
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.