Understanding the Giant Seebeck Coefficient of MnO 2 Nanoparticles Costel Constantin James Madison University James Madison University, October 2012

- Crystal structures and semiconductor properties. - Materials characterization methods. - Giant Seebeck Coefficient Observed in Manganese Oxide Nanostructures? Outline

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

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, H 2, O 2. HOW ABOUT THE ENERGY LEVELS IN A CRYSTAL?

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

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

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

Materials Characterization Principles and Techniques

X-ray Diffraction (XRD) 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

[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.

How Do We Measure Seebeck Effect

Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) Fig. 1 Scanning Electron MicroscopeFig. 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, (2012).

Einstein Prediction For Lowest Thermal Conductivity

XRD of our as-received MnO 2 powder Fig. 1 X-ray diffraction of as-received MnO 2 powderFig. 2 NIST MnO 2 standard

Crystal Structure of our Manganese Oxide powder  Rutile structure.  Gray atoms are Mn.  Red Atoms are O.

SEM and TEM images of MnO 2 powder 1. FangFang Song, Liming Wu and S Liang, Nanotechnology 23 (2012) (4pp) Fig. 1 Scanning electron microscope image of MnO 2 [ref. 1] Fig. 2 Transmission electron microscopy image of MnO 2.

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

Preliminary Results ZT vs. MnO 2 particle stacking density

Preliminary Results Seebeck vs. MnO 2 particle stacking density

Preliminary Results Thermal conductivity vs. MnO 2 particle stacking density

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

THANK YOU

Giant Seebeck Coefficient Thermoelectric Device of MnO 2 powder

FangFang Song, Liming Wu and S Liang, Nanotechnology 23, (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

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.