Advisor(s)

Welville B. Nowak

Contributor(s)

Teichi Ando, Gregory J. Kowalski

Date of Award

2008

Date Accepted

12-2008

Degree Grantor

Northeastern University

Degree Level

Ph.D.

Degree Name

Doctor of Philosophy

Department or Academic Unit

College of Engineering. Deparment of Mechanical and Industrial Engineering

Keywords

ZT, semiconductors

Subject Categories

Semiconductors--Thermal conductivity, Thermoelectric materials

Disciplines

Mechanical Engineering

Abstract

When the electronic component of the thermal conductivity is much greater than the lattice or phonon component thereof, the dimensionless thermoelectric figure of merit, ZT, may be expressed as ZT=S2/L, where S is the Seebeck coefficient or thermoelectric power, L is the Lorenz number, T is the absolute temperature, and Z=S2σ/κ. σ is the electrical conductivity and κ is the thermal conductivity. The starting point of this dissertation was to pick values of ZT and find corresponding values of S, using the foregoing equation. On this basis, a major investigation was carried out to find the upper limit that would be reasonably expected on ZT. We were able to show that such a limit does exist and amounts to 23.9 for degenerate semiconductors and 24.5 for nondegenerate semiconductors. Before reaching this conclusion, several intermediate paths were pursued. We investigated Mott's equation for the Seebeck coefficient and found it to be inappropriate for nondegenerate semiconductors and partly correct for degenerate ones, although the S values it yields for the latter still need to be multiplied by a correction factor of 2. This should come as no surprise, since we already know that Mott's equation was derived for metals. Combining the Wiedemann-Franz and Eucken laws, we were able to derive a number of theoretical correlations for the ratio of the electronic to the lattice thermal conductivity. These were used to help verify the validity of the equation ZT=S2/L under particular or extreme conditions. We ended up deriving equations expressing ZT as a function of the free charge carrier concentration, n, and the absolute temperature,T, for both degenerate and nondegenerate semiconductors. Numerical results obtained on the basis of these equations are presented in tables. These were eventually used to specify the conditions under which the limit on ZT would be achieved for degenerate and nondegenerate semiconductors.

Document Type

Dissertation

Rights Holder

Michael Constantine Nicolaou


View Full Text

Click button above to open, or right-click to save.

Share

COinS