This invention relates to compositions for use as thermoelectric
cooling and power generation elements and to methods for their preparation.
The field of thermoelectric cooling is an important one,
having obvious commercial applications, such as in refrigerators. Semiconductor
devices, which utilise the Peltier effect, are known, the primary example being
Bi Te based devices. Such devices, which are in widespread commercial use, employ
suitably doped Bi Te alloys as both p and n type semiconductor elements. Sb and/or
Se may be used as additives in order to modify the physical (primarily thermal)
characteristics of the elements.
The efficiency of such thermoelectric cooling devices (i.e.
the heat absorbed per unit of consumed power) is generally represented by a figure
of merit or performance factor Z, which is given by:
where S is the thermopower coefficient or Seebeck voltage (V/°C), &sgr; is
the electrical conductivity (&OHgr;-1cm-1) and &kgr; is
the thermal conductivity (Wcm-2 or °Ccm-1).
It is generally accepted that with the Bi Te based systems
further improvements in Z are unlikely. Thus there is a need to provide new materials
having improved values of Z. United States patent US 5 275 001 describes alternative
materials for thermoelectric cooling which are based around various complex Sr Ti
oxides. These materials are n type semiconductors.
The present invention addresses the abovementioned need
by providing thermoelectric cooling elements having p type semiconductors exhibiting
high values of figure of merit Z to be optimised, by optimising the combination
of thermo power and electrical conductivity.
Gonçalves et al (A P Gonçalves, I C Santos,
E B Lopes,. R T Henriques, M Almeida and M O Figueiredo, Phys. Rev. B
37 (1988) 7476) discloses compositions of the general formula Y1-xPrxBa2Cu3O7-&dgr;,
where 0 ≤ x ≤ 1. Measurements of thermopower and resistivity are made,
for the purpose of explaining the presence - or absence - of superconductivity in
these materials. Furthermore, the precise oxygen deficiencies &dgr; of the reported
compositions are not established. Further still, the distribution of Y and Pr with
the composition is homogeneous.
Macklin and Moseley (W J Macklin and P T Moseley, Materials
Science and Engineering B7 (1990) 111) described the thermoelectric data
available at the time concerning a number of complex copper oxides, and commented
generally upon the prospects of using such oxides in thermoelectric applications.
It is well known that it is possible to generate electrical
power by "reversing" the above described process, i.e., by applying heat to materials
of the type described above it is possible to generate electrical power.
According to a first aspect of the invention there is provided
a p-type semiconductor composition described by the formula
wherein: the composition comprises granules of RBa2Cu3O7-&dgr;
and granules of PrBa2Cu3O7-&dgr;;
R comprises Y, Ce, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or Lu;
&dgr; is such that the RBa2Cu3O7-&dgr; component
of the composition is in a metallic phase.
Compositions as defined above may be used as thermoelectric
cooling elements. These mixed oxide compositions exhibit large values of Z, since
the presence of the Pr oxide (which is a semiconductor irrespective of the oxygen
deficiency) results in a high thermopower value, whilst the presence of the Y oxide
in a metallic phase results in high values of electrical conductivity.
Preferably, x is less than 0.4, most preferably x is in
the range 0.10 ≤ x ≤ 0.25.
The invention also provides thermoelectric cooling devices
in which at least one cooling element comprises a composition as defined above,
and thermoelectric power generation devices in which at least one power generation
element comprises a composition as defined above.
According to an alternative aspect of the invention there
is provided a thermoelectric cooling element or a thermoelectric power generation
element comprising a p-type semiconductor composition described by the formula
- wherein: R comprises Y, Ce, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or
- and &dgr; is such that the composition is in a transitional state between
semiconducting and metallic phases.
Oxygen deficiencies corresponding to the phase transition
from metallic to semiconductor behaviour (with a corresponding transition from orthorhombic
to tetragonal lattice structures) - result in an optimal combination of electrical
conductivity and thermopower.
The composition may be YBa2Cu3O7-&dgr;,
Eu Ba2 Cu3O7-&dgr; or SmBa2 Cu3O7-&dgr;
The invention also provides a thermoelectric cooling device
in which at least one cooling element comprises a composition as defined with regard
to this alternative aspect of the invention, and a thermoelectric power generation
device in which at least one power generation element comprises a composition as
defined with regard to this aspect of the invention.
The quenching may comprise quenching of the composition
on an aluminia plate, a copper plate or in liquid nitrogen.
A method for preparing compositions as previously defined,
the method comprising the step of sintering granules of a predetermined range of
grain sizes. The predetermined grain sizes may be in the range 0.1 to 100 µm,
preferably 0.1 to 30 µm, most preferably 0.1 to 2 µm.
A method for preparing compositions as previously defined
in which ultrasonic treatment is employed.
Typically, the at least one cooling element is in the form
of a thin film. Typically, the at least one power generation element is in the form
of a thin film.
The films are convenient to prepare and provide easily
controllable and uniform ceramic structures. Furthermore, it is possible to provide
films having highly anisotropic electrical properties and pre-defined grain size
and structure, thereby permitting optimisation of the figure of merit. Still further,
the heat generated by thermocooling is efficiently dissipated from thin films because
of the inherently high surface area to volume ratios.
Preferably, the thickness of the film is less than 5 µm,
most preferably less than 1 µm.
The semiconductor material may be texturised.
Embodiments of compositions and methods according to the
invention will now be described with reference to the accompanying drawings, in
- Figure 1
- is a schematic diagram of apparatus for measuring thermopower;
- Figure 2
- shows log &sgr; against S/(µV)K-1 for a number of ceramics;
- Figure 3
- shows grain structure in a monocrystalline film of Y Ba2Cu3O7-&dgr;;
- Figure 4
- shows &rgr;/µ&OHgr;cm against oxygen content for a number of thin film
samples of YBa2Cu3O7-&dgr;; and
- Figure 5
- shows S/(µV)K-1 against &rgr;&dgr;/&rgr;6.93
for a number of thin film samples.
A composition is described by the formula
+ (Pr Ba2Cu3O7-&dgr;)1-x
- wherein: R comprises Y, Ce, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and/or
- &dgr; is such that the R Ba2Cu3O7-&dgr;
component of the composition is in a metallic phase.
Such materials are particularly promising as cooling elements
in thermoelectric devices, because it is possible to produce very large values of
Z by independently controlling thermopower and electrical conductivity. Both oxide
components of the composition - RBa2Cu3O7-&dgr;
and PrBa2Cu3O7-&dgr; - adopt a distorted Perovskite
lattice structure. The tetravalent Pr is present as an oxide which acts as a semiconductor
for all values of &dgr;, and as such exhibits large thermopower but relatively
poor electrical conductivity. However, the properties of the R oxide vary as a function
of &dgr;. At room temperature, high oxygen deficiencies (approximately 0.75 <
&dgr; ≤ 1 for the Y oxide) result in a pseudotetragonal lattice structure
exhibiting properties characteristic of a semiconductor, i.e. large thermopower
and relatively poor electrical conductivity. With increasing oxygen content (approximately
0.5 < &dgr; < 0.7 for the Y oxide) there is a transition from a tetragonal
to an orthorhombic structure. At low oxygen deficiencies (approximately 0 ≤
&dgr; ≤ 0.5 for the Y oxide) the R oxide is metallic in behaviour, exhibiting
large electrical conductivities but poor thermopowers.
The mixed oxide composition of the type described above
is characterised (i) by a low oxygen deficiency, with the consequence that the R
oxide component is present in a "metallic" phase. The Pr oxide component is present
in a semiconductor phase even at low oxygen deficiency. Furthermore, (ii) the composition
is not homogeneous, it comprising physically distinct granules of the Pr oxide and
granules of the R oxide. As a result, the mixed oxide composition exhibits both
a large thermopower (from the Pr oxide component)
high electrical conductivity (from the R oxide), producing a very large value
of Z. In effect, a material having a large value of Z is produced by
controlling thermopower S and electrical conductivity &sgr;.
The ratio of R to Pr is defined by the variable x. Preferably,
x is less than 0.4, most preferably x is in the range 0.10 to 0.25, i.e. close to
the percolation value. For YBa2Cu3O7-&dgr;, the
percolation value is ca. 0.17.
The manufacture of thermoelectric cooling elements from
suitable semiconducting material is well-known in the art - details can be found,
for example in US 5 275 001. It will be apparent to the skilled reader that the
compositions described herein may also be used as thermoelectric power generation
elements, in which electrical power is produced from heat applied to the element.
It is noted that Gonçalves et al discloses
the mixed oxide Y1-x Prx Ba2Cu3O7-&dgr;
(0 ≤ x ≤ 1). However, the precise oxygen deficiencies of the prepared
mixed oxide materials are not discussed or determined. Macklin and Moseley refer
to Gonçlaves et al in their speculative discussion of possible uses
of copper oxides in thermoelectric cooling. However, this article does little more
than précis the reported measurements of resistivity and thermopower: no teaching
is provided on optimal oxygen deficiencies, and, indeed, it is not suggested that
the compositions of Gonçlaves et al would be suitable as thermoelectric
Furthermore, and very importantly, the Y and Pr component
are homogeneously distributed within the composition of Gonçlaves et al. In
the present invention, the material is a composite comprising granules of R Ba2Cu3O7-&dgr;
and granules of PrBa2Cu3O7-&dgr;. This permits
control of factors such as grain boundary density which, as discussed in more detail
below, have profound effects on S and &sgr;.
+ (PrBa2Cu3O7-&dgr;)1-x may be prepared
by solid state reaction of RBa2Cu3O7-&dgr; and
PrBa2Cu3O7-&dgr;, the amounts of these compounds
used being calculated such that the correct value of xresults. The individual R
and Pr oxides are prepared by known means (see, for example, Gonçlaves et al
and references therein, the references of Mackin and Moseley and the preparations
devised herein). The oxides are ground, preferably to a grain size less than 1 to
2 µm, mixed and pressed into pellet form. The pellet is heated at
ca. 920° C for ca. 10 hours, and then slowly cooled. It is important
that "chemical" reaction does not occur - i.e, the distribution of R and Pr remains
A single phase composition described by the formula
can be used as a thermoelectric cooling element or a thermoelectric power generation
- wherein: R comprises Y, Ce, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and/or
- and &dgr; is such that the composition is in a transitional state between
semiconducting and metallic phases;
As discussed above, the properties of these compositions
are dependent on the oxygen deficiency. Using Y Ba2Cu3O7-&dgr;,
as an example, when 0.7 < &dgr; ≤ 1, the composition behaves as a semiconductor,
with the consequence that the electrical conductivity is too low for a significant
value of Z to be achieved. When 0 ≤ &dgr; ≤ 0.5, these compositions
exhibit metallic behaviour, and the electrical resistivity increases more rapidly
than the Seebeck voltage, with the consequence that an insufficiently large value
of Z is obtained. In the oxygen deficiency range 0.5 ≤ &dgr; ≤ 0.7
there is orthorhombic to tetragonal phase transition with an associated transition
from metallic to semiconductor type behaviour. In this transition range, the thermopower
and electrical conductivites are such that large values of Z may be obtained.
The distorted Perovskite metal oxide compositions RBa2Cu3O7-&dgr;
are extremely well known as high temperature superconductors, with the consequence
that the main thrust of the literature concerning these materials has focused on
properties contributing to the understanding of the observed superconductivities.
Macklin and Moseley reviewed the available thermoelectric data on such complex copper
oxides and concluded that the figures of merit associated with these materials were
too low for commercialisation as thermoelectric elements. Macklin and Moseley tentatively
suggested that improvements in the figure of merit might be achieved by varying
the oxygen stoichiometry and doping levels. However, Macklin and Moseley disclose
no strategy or rationale regarding such the attainment of such improvements : indeed,
the use of samples having a very high oxygen deficiency is implied.
Furthermore, as described in more detail below, the present
invention provides methods and strategies which enable the figure of merit Z to
be optimised. Such methods and strategies are a result of careful consideration
of sample preparation conditions, and of the factors which affect electropower and
Three groups of samples ofEuBa2Cu3O7-&dgr;
and SmBa2Cu3O7-&dgr; were prepared by solid state
Group "A" was prepared from powders of Eu2/O3/Sm2O3,
BaCO3 and CuO, these powders being mixed and ground, and then sintered
for 20 hours at 900°C. The resultant composition was reground and pressed into
pellets, which were then sintered at 920°C for 10 hours.
Group "B" was prepared from identical reagents to group
"A", the preparation now comprising two cycles of mixing, grinding, re-grinding
and sintering for 10 hours at 900°C. The resulting composition was pressed
into pellet form, and then sintered at 920°C for 30 hours.
Group "C" samples were prepared by further treatment of
group "B" samples, this treatment comprising re-grinding, screening grains between
20 and 100 µm, pressing them into pellets and sintering at 920°C for 20
A variety of cooling techniques were employed, namely natural
furnace cooling, and quenching on a aluminia plate, on a copper plate and in liquid
Resistances were measured by a standard four-probe method.
Thermopower was measured using the differential method against lead. The apparatus
for performing the thermopower measurements is shown in Figure 1, and comprises
the peltier device 10, thermocouples 12, 14, and temperature sensors 16, 18 connected
to a differential thermometer 20 thereby providing the temperature differential
between the warm and the cold side. The apparatus further comprises a direct current
source 22 and current switching means 24, the latter permitting measurement of the
thermopower at either polarity of the applied current.
Table 1 shows room temperature resistivities and absolute
Seebeck voltage, obtained thereby, together with a quantity of
where Z* is a "partial" figure of merit, S is the Seebeck voltage and &rgr; the
resistivity. It should be noted that calculation of a complete figure of merit 2,
requires a knowledge of the thermal conductivity, &kgr;; However, &kgr; does
not greatly depend on the rare earth metal present or the oxygen deficiency, it
being typically in the range 2 to 4 Wm-1K-1 Table 1 demonstrates
that : i) huge improvements in Z* are possible if preparation conditions are correctly
selected and ii) resistivity varies much more strongly with preparation conditions
than thermopower. This is because conductivity (resistivity) is mainly determined
by weak inter-granular media links, whilst thermopower is determined by granular
conditions. Therefore, it is possible to almost independently control the conductivity
and thermopower in these ceramic semiconductors.
Figure 2 shows the results of precision measurements of
conductivity and Seebeck voltage against oxygen deficiency in a ceramic of high
quality. The data are not explained by the simple thermoactivation mechanism (shown
on the line 1g (e/k) in Figure 2) which describes usual semiconductor behaviour.
This indicates that other mechanisms - probably a hopping mechanism - are operating.
The deviation from thermoactivation type behaviour corresponds to the transition
region between semiconducting to metallic behaviour.
Another factor influencing Z is the existence of anisotropic
resistivities in samples from group "C", where &rgr;∥ = 1.45 x 10-4
&OHgr; cm, whilst &rgr;⊥ = 1.25 x 10-3 &OHgr; cm, &rgr;∥
and &rgr;⊥ representing the resistivities parallel and perpendicular to
the pressure axis, respectively. Upon regrinding samples from group "B", the granules
were found to be monocrystals with highly anisotropic resistivities (&rgr;ab/&rgr;c
~ 100-1000). After screening, the larger sized granules were uniaxially compressed
into a pellet which exhibited anistrophy. Similar results were obtained for a Bi
Te ceramic after sintering from small monocrystals.
Thin films of three types of YBa2Cu3O7-&dgr;
were prepared, namely single crystalline, texturised and polycrystalline (non-oriented).
The monocrystalline thin films were deposited onto a single
crystal plane (100) of LaAlO substrates by a rf magnetron sputtering method, producing
a film thickness of about 0.5 to 0.6 µm. The surface of these films was interrogated
by optical scanning microscopy (SEM), which revealed that the films consist of large
(ca. 200 µm) longitudinally regular grains (see Figure 3). Samples of
YBa2Cu3O6.93 yielded a resistivity &rgr; of 300
µ &OHgr; cm, a superconducting transition temperature, Tco, of
93K, with a transition width &Dgr;Tco of 0.2K, and a critical current
density Jc (77K) of 1.6 x 107Acm-2
For quenched samples, Figure 4 shows &rgr; vs oxygen
content. These values are obtained indirectly from the relationships between Tco
and &dgr;, and &rgr; and Tco.
Texturised polycrystalline films of YBa2Cu3O7-&dgr;
were grown on Yttrium stabilised ZrO2 substrates by pulsed laser deposition
from pellets of the same stoichiometry. Films of thickness 0.3 µm were produced
using the (100) plane of the substrate for deposition thereon. Non-oriented films
were produced by a similar process using non-oriented substrate surfaces. SEM morphological
analysis revealed the presence of irregularly shaped grains of size 2 to 3 µm.
In the texturised films of prevalance of c axis oriented grains was observed. The
following typical parameters were observed : for texturised films, &rgr;(300K)
- 0.9 m &OHgr; cm; Tco =91K; &Dgr; Tco = 2K; Jc = 106Acm-2
; for non-oriented polycrystalline films, &rgr;(300K) = 2.4 m &OHgr; cm; Tco
= 81K; &Dgr;Tco = 10K; Jc = 105Acm-2.
Resistivities were measured using a four probe method.
Contacts were made by firing gold paste, resulting in a typical resistance of 100
m &OHgr; cm-2. Thermopower measurements were made using the apparatus
depicted in Figure 1. The direction of the dc current flow through the Peltier device
10 was switched using switching means 24, thereby reversing the temperature gradient.
The measurements made in each direction of the applied current, S+ and
S-, yield an averaged thermopower S = (S+ - S.)/2. The measurements
include some parasitic influence from the thermocouples 12, 14.
The measurements of thermopower verses &rgr;&dgr;/&rgr;6.93
are shown in Figure 5, where &rgr;&dgr;/&rgr;6.93 is
the ratio of the measured resistivity to the resistivity obtained from the sample
having &dgr; = 0.07. The legend shown in Figure 5 relates to the following examples
: (1) corresponds to a monocrystalline film with measurements made along the grain
length; (2) as (1) but with measurements made perpendicular to the grain length;
(3) corresponds to the texturised film: and (4) corresponds to the polycrystalline
Figure 5 demonstrates that there is a significant difference
in thermopower when the temperature gradient is parallel to and perpendicular to
the monocrystalline film. Even in the parallel case there is a relatively small
value of the thermopower. These phenomena can be explained by a 2D layer model,
which indicates that grain boundaries determine the thermopower when the temperature
gradient is perpendicular to the grain.
Smaller grain size, as occurs with the texturised film,
results in an increase in thermopower. There is no thermopower anisotrophy with
the texturised film surface because the grains are oriented in all directions.
Texturised film gives rise to increased thermopowers, which
is due to the smaller grain size compared to the monocrystalline film. The measured
resistivities are also higher, this being due to the increase of grain boundary
density. However, it should be noted that resistivity is also increased by microcracking.
The grain size threshold for microcracking in YBa2Cu3O7-&dgr;
is estimated to be of the order of 1 µm.
A bulk YBa2Cu3O7-&dgr;
ceramic was prepared using ultrasonic treatment at 30kHz on granulated powder followed
by uniaxial pressing into a pellet. The final value of &dgr; was ca. 0.6.
X-ray reflection spectroscopy and SEM of sections taken perpendicular and parallel
to the press axis indicate a prevalence of (ab) plane layered grains. This is desirable
because thermopower is highly anistropic, with Sc >> Sab.
The prepared sample exhibited a thermopower S of 130 µVK-1 and resistivity
&rgr; of 4 x 10-3 &OHgr; cm, giving a partial figure of merit Z* (as
previously defined) of 4225000 (µV)2 &OHgr;-1 cm-1
The typical grain size in the sample was 5 to 30 µm.
Further improvements are to be anticipated if smaller grains are employed, firstly
due to the microcracking phenomena described above, and secondly because, in the
bulk, three dimensional, case electroconductivity increases as grain size decreases.
Smaller grains may be prepared by mechanical means, i.e. grinding, or by a cryogenic
liquid phase preparation of the oxide.
(&rgr;/&OHgr; cm, S/µVK-1 Z*(µV)2&OHgr;-1cm-1K-2
Quenching on alumina plate
Quenching on copper plate
Quenching in liquid nitrogen
5 x 10-3
4.1 x 10-3
1.3 x 10-3
5 x 10-2
1.7 x 10-3
1.45 x 10-4
7.5 x 10-3
1.75 x 10-2
1.8 x 10-2
0.8 x 10-3
1.8 x 10-2
1.8 x 10-2
3 x 10-2