Batteries: thermoelectric and photoelectric – Thermoelectric – Having particular thermoelectric composition
Reexamination Certificate
2000-08-25
2002-09-03
Bell, Bruce F. (Department: 1741)
Batteries: thermoelectric and photoelectric
Thermoelectric
Having particular thermoelectric composition
C136S203000, C136S205000, C136S238000, C136S240000
Reexamination Certificate
active
06444896
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to thermoelectric materials and more particularly to quantum dot superlattice structures.
2. Description of the Related Art
As is known in the art, there exists a class of materials referred to as thermoelectric materials. A thermoelectric material is a type of material which can directly convert thermal energy into electrical energy or vice versa.
Although certain thermoelectric materials have been known in the art for a number of years (e.g.—bulk semiconductors), it has only recently been found that thermoelectric materials having a superlattice structure can possess thermoelectric properties which are better than the corresponding thermoelectric properties of other thermoelectric materials.
A superlattice structure denotes a composite structure made of alternating ultrathin layers of different component materials. A superlattice structure typically has an energy band structure which is different than, but related to, the energy band structures of its component materials. The selection of the component materials of a superlattice structure, and the addition of relative amounts of those component materials, will primarily determine resulting properties of a superlattice structure as well as whether, and by how much, those properties will differ from those of the superlattice structure's component material antecedents.
It is generally known that thermoelectric materials and thermoelectric materials having a superlattice structure find application in the fields of power generation systems, and the heating and/or cooling of materials. One problem, however, is that although these fields place ever-increasing demands on thermoelectric materials to possess ever-improving thermoelectric performance characteristics, the thermoelectric materials and thermoelectric materials having a superlattice structure known in the art have, as of yet, not been able to keep pace with such performance demands.
One way to predict the thermoelectric behavior of thermoelectric materials or thermoelectric materials having a superlattice structure in the fields of power generations systems, and the heating and/or cooling of materials is to calculate a thermoelectric figure of merit for the materials. The thermoelectric figure of merit, ZT, is a dimensionless material parameter in which T corresponds to temperature and Z is the figure of merit. ZT is a measure of the utility of a given thermoelectric material or thermoelectric materials having a superlattice structure in power generation systems, and heating and/or cooling applications at a temperature T.
The relationship of ZT to the material properties of thermoelectric materials and thermoelectric materials having a superlattice structure is shown by the following equation:
ZT=S
2
&sgr;T/&kgr;=S
2
ne&mgr;T
/(&kgr;
1
+&kgr;
e
)=
P
F
T/K=S
2
GT/K
in which S, &sgr;, T and &kgr; are, respectively, the Seebeck coefficient, the electrical conductivity, the temperature, and the thermal conductivity and where n, e, &mgr;, &kgr;
1
and &kgr;
e
are, respectively, the carrier density, the electronic charge, the carrier mobility, the lattice part of the thermal conductivity and the electronic part of the thermal conductivity, and where P
F
is the power factor, and where G and K are, respectively, the electrical conductance and the thermal conductance.
Generally, it is known in the art that it is desirable for thermoelectric materials to have a relatively high value for their thermoelectric figure of merit (ZT) in order for those thermoelectric materials to perform well in the fields of power generation systems and the heating and/or cooling of materials. From inspection of the above equation, it appears that to provide a thermoelectric material having a high ZT, one need only fabricate on it a superlattice structure having relatively high values for its Seebeck coefficient, its electrical conductivity, and its temperature while, at the same time, having a relatively low value for its thermal conductivity.
It has proven difficult in practice to provide a thermoelectric material or a thermoelectric material having a superlattice structure that has a high thermoelectric figure of merit (ZT) value. Past findings in the art have suggested that the inherent interrelationships between the material properties included in the above equation for ZT such as carrier mobility, lattice thermal conductivity, power factor and Seebeck coefficient may limit, or place a ceiling upon, the ZT values of thermoelectric materials or thermoelectric materials having a superlattice structure.
As is also known in the art, multilayer systems prepared by molecular beam epitaxy (MBE) can provide materials having improved thermoelectric properties. Superlattice systems having reduced dimensionality have been proposed as a means to greatly enhance the thermoelectric figure of merit (ZT) as a result of the effects of confinement on the electronic density of states. It has also been shown that additional effects need to be included in order to obtain a more complete understanding of these complex structures.
The above discoveries have led to increasing interest in quantum-well and quantum-wire superlattice structures in the search to find improved thermoelectric materials for applications in cooling and power generation. Investigation of Pb
1−x
Eu
x
Te/PbTe quantum-well superlattices grown by MBE yielded an enhanced ZT due to the quantum confinement of electrons in the well part of the superlattice structure have been conducted.
Quantum wells have two-dimensional carrier confinement whereas quantum wires have one-dimensional confinement of the carrier. Quantum wires have been calculated to have much higher ZTs than quantum wells due to improved confinement. And, it has been recognized that quantum dots (QDs) may have even higher ZT values than quantum wires.
Quantum dots have zero-dimensional confinement and represent the ultimate in reduced dimensionality, i.e. zero dimensionality. The energy of an electron confined in a small volume by a potential barrier as in a QD is strongly quantized, i.e., the energy spectrum is discrete. For QDs, the conduction band offset and/or strain between the QD and the surrounding material act as the confining potential. The quantization of energy, or alternatively, the reduction of the dimensionality is directly reflected in the dependence of the density of states on energy. For a zero-dimensional system (e.g. a QD superlattice), the density of states (dN/dE) of the confined electrons has the shape of a delta-like function
dN
/
dE
α
∑
ϵ
i
δ
(
E
-
ϵ
i
)
where &egr;
1
are discrete energy levels and &dgr; is the Dirac function. Thus, an enhanced density of states is a possibility even in partially confined QD superlattice (QDSL) structures.
It would, therefore, be desirable to provide a thermoelectric material or materials having a superlattice structure which have a relatively high thermoelectric figure of merit and which are suitable for usage in power generation systems, and in heating and/or cooling applications.
SUMMARY OF THE INVENTION
In view of the above, it has been recognized that an enhancement in the Seebeck coefficient, S, and the thermoelectric power factor, P=S
2
&sgr; may occur for a suitable quantum dot (QD) superlattice (SL) structure in which the chemical potential lies within a few kTs of the delta-like function of the ground state or one of the excited states of the partially confined QDs. In addition, the chemical potential should lie near a suitable band edge of a good thermoelectric material. In real materials, tunneling, thermal and inhomogeneous broadening as well as a weak potential barrier surrounding the QD may contribute to reducing the confinement effect. An enhancement of the Seebeck coefficient and the power factor in the PbSeTe/PbTe QDSL system have been found.
In addition to the possibility of an enhancement in
Harman Theodore C.
Taylor Patrick J.
Walsh Michael P.
Bell Bruce F.
Daly, Crowley & Mofford LLP
Massachusetts Institute of Technology
Parsons Thomas H.
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