Lee Joon Hon Alvin

55 Pages·2015·6.1 MB·English

Characterisation of Thermoelectric Properties
in R Ca MnO Perovskites (R = La, Pr, Nd,
0.65 0.35 3
Sm, Y, Bi) and Construction of High
Temperature Measurement Setup

by Lee Joon Hon Alvin
(A0086862R)

A thesis submitted for the degree of
Bachelors of Science (Honours)
at the National University of Singapore

4 April 2015
Abstract

R Ca MnO Perovskites is a class of metal oxide materials that exhibit various interesting
0.65 0.35 3
properties. Their ability to remain stable at high temperatures makes them good candidates
for feasibility studies into their potential as thermoelectric materials while their phase
transitions at low temperatures allows us to gain an insight into their electronic transport
properties.
In this article, we will look at the thermopower and resistivity properties of the perovskites
across a wide temperature range of 100K to 700K as well as the interpretation of these
properties. We will also look at the properties of Nd Ca MnO within a strong magnetic
0.65 0.35 3
field environment to explore the possible phase transitions in the low temperature range.
An unprecedented 1000K High Temperature Measurement setup will also be constructed in
order to study the thermopower, resistivity as well as thermal conductivity properties of
future materials. The progress and challenges of the construction process will be mentioned
for future references in the construction of similar high temperature setups.
Acknowledgements

I would like to thank my project supervisor, Professor Mahendiran Ramanathan, for the
opportunity to work in his lab as well as for the guidance and directions for the project. His
great insights in the results interpretation taught me a lot throughout the course of this project.
I would also like to thank Dr. Pawan Kumar for his countless support in the experimental
methodologies and experimental discussions, especially in the construction of the 1000K
High Temperature Setup. My thanks also go to fellow honours student, Ivan Lee, for the
numerous discussions in measurement results as well as the planning of the project pace and
scope. Last but not least, I would like to thank the fellow lab members, Rubi, Maheswar
Repaka, Amit Chanda and Himadri Roy for the endless support they gave for the success of
this project.
Chapter 1

Introduction and Theory

1. Background of Thermoelectric Effect
Thermoelectric effect mainly consist of the Seebeck and Peltier effects.
The Seebeck effect was discovered in 1821 when Thomas Johann Seebeck discovered that a
coil wound around a compass would deflect the compass when the coil ends were closed with
a dissimilar metal and a temperature difference applied. In other words, a temperature
difference applied across two dissimilar-metal junctions creates an electromotive force.
In 1834, Jean Charles Athanase Peltier discovered the reverse of the Seebeck effect when a
potential difference applied across the junctions gave rise to a temperature difference.

The thermoelectric effect first found commercial use as electronic thermometers in the early
1900s, at a time when voltmeters became increasing more precise. These thermometers have
the advantage of having a wide range of operating temperatures as well as high precision
compared to other thermometers at that time.

Later, at the height of the space race, thermoelectric devices filled an extremely niche role as
the power source of long haul space exploration probes. Today, one such exploration probe,
the Voyager 1, became famous for being the first manmade object to cross the heliopause in
September 2012. These Radioisotope Thermoelectric Devices have the advantage of
operating in high temperatures as well as having no moving parts, allowing it to work with
1
high temperature radioisotope pellets while having high durability.

An increasingly common application of thermoelectric effect today can be found in
refrigeration as Peltier Modules. The high durability and small construct allow these modules
to be used in consumer products such as portable fridge, CPU coolers and even dehumidifiers.

In light of these developments, it is important to characterise and determine the factors that
will allow us to construct increasingly efficient thermoelectric devices and enable us to utilise
the technology more effectively.
1.1 Thermoelectric Basics
The thermoelectric phenomenon of interest in this article can be broken down into two
thermodynamically opposite effects. As mentioned above, they are the Seebeck and Peltier
effects. The Seebeck effect is the emergence of an electromotive force when a temperature
gradient is present on the two ends of a thermoelectric material. On the other hand, the Peltier
effect is the emergence of a temperature gradient when an electric current is passed through a
thermoelectric junction. The Seebeck and Peltier effects can be summarised in the equations
below:
(cid:3004)(cid:3042)(cid:3041)(cid:3046)(cid:3047)(cid:3028)(cid:3041)(cid:3047) (cid:3020) ∆(cid:1848) (cid:1848)(cid:3035)(cid:3042)(cid:3047) −(cid:1848)(cid:3030)(cid:3042)(cid:3039)(cid:3031)
(cid:1845)(cid:1857)(cid:1857)(cid:1854)(cid:1857)(cid:1855)(cid:1863) (cid:1831)(cid:1858)(cid:1858)(cid:1857)(cid:1855)(cid:1872): ∇(cid:1848) = −(cid:1845)∇(cid:1846) (cid:4657)(cid:1755)(cid:1755)(cid:1755)(cid:1755)(cid:1755)(cid:1755)(cid:4654) (cid:1845) = − = −
∆(cid:1846) (cid:1846) −(cid:1846)
(cid:3035)(cid:3042)(cid:3047) (cid:3030)(cid:3042)(cid:3039)(cid:3031)
Where S is the Seebeck Coefficient and ∇(cid:1846) (∆(cid:1846)) is the temperature gradient (difference)
between the two junctions and ∇(cid:1848) (∆(cid:1848)) is the potential gradient (difference) generated
between the two ends of a thermoelectric material. The negative sign is present so that a
positive carrier mediated material gives a positive Seebeck Coefficient.
(cid:1842)(cid:1857)(cid:1864)(cid:1872)(cid:1861)(cid:1857)(cid:1870) (cid:1831)(cid:1858)(cid:1858)(cid:1857)(cid:1855)(cid:1872): (cid:1843)(cid:4662) = (cid:4666)Π −Π (cid:4667)(cid:1835)
(cid:3002) (cid:3003)
Where (cid:1843)(cid:4662) is the rate of heat pumped from the cold to hot junction, Π and Π are the Peltier
(cid:3002) (cid:3003)
Coefficients of materials A and B respectively and I is the current passing through the
thermoelectric materials.
2
When the Seebeck Coefficient varies with temperature, a variation of the Peltier effect arises.
This is described by the following Thomson effect:
(cid:1846)(cid:1860)(cid:1867)(cid:1865)(cid:1871)(cid:1867)(cid:1866) (cid:1831)(cid:1858)(cid:1858)(cid:1857)(cid:1855)(cid:1872): (cid:1869)(cid:4662) = −Κ∙J∙∇T
(cid:3031)(cid:3020)
Where the Thomson Coefficient, Κ, is related to the Seebeck Coefficient via Κ = (cid:1846) .
(cid:3031)(cid:3021)
The coefficients are then related via the two Thomson relations which are given by:
(cid:3031)(cid:2952)
Κ = −(cid:1845) and Π = (cid:1846)(cid:1845)
(cid:3031)(cid:3021)
Since the experiment to be described in this article focuses specifically on the Seebeck effect,
we shall discuss no further on the Peltier and Thomson effects.
1.2 Thermoelectric Unit
To visualise the mechanism that give rise to the thermoelectric effect, it is useful to consider
a simplified conduction model such as the Drude Model. In the simple Drude model [See
Figure 1], the conduction electrons of a metal (as a first consideration) moves around as an
electron gas within the bulk of the material. The electrons’ speed and therefore rate of
diffusion will then be proportional to the temperature of the region of material that the
electrons are in.
When a temperature difference is applied across two ends of a metal [See Figure 2], the
electrons at the hotter end will diffuse away from that end at a higher rate compared to the
electrons at the colder end diffusing away from the cold end. As a result, the colder end will
have a higher density of conduction electron with respect to that of the hotter end and a
potential difference arises. The potential difference will grow until it sufficiently oppose the
effect of electron diffusion as a result of the temperature difference.
Figure 1: Diagram illustrating the random motion and distribution of electrons in a metal, modelled as an
electron gas according to the Drude Model. The charge distribution is uniform in the absence of an electric
field or temperature gradient.
3
Figure 2: Applying a temperature difference to the material results in the accumulation of charge carriers at the
cold end. This give rise to a potential difference that opposes further diffusion and accumulation of charges.
The same visualisation can be made for materials that conducts primarily through holes, with
the effect of reversing the sign of the potential difference. When these two different types of
thermoelectric materials are arranged as in Figure 3[1], a Thermoelectric Unit is formed.
Figure 3: Diagrams showing the inner workings of a Thermoelectric Unit. The diagram on the left shows an
electric current being generated as a result of a temperature difference while the diagram on the right shows heat
being pumped due to the direction of movement of charges caused by the electromotive force.
In Figure 3, when a temperature difference is applied across the junctions, the charge carriers
flow from the hot end to the cold end. Due to the way the thermoelectric materials are
arranged, an electrical current flows with which a load can be powered. On the other hand,
when an electromotive force is applied to the ends of the device, the electrons and holes flow
in the same spatial direction and the device acts as a thermoelectric heat pump, removing heat
from one side and depositing it on another.
The Thermoelectric Unit is the basic component of any thermoelectric device and its
efficiency is directly related to the performance of its constituent thermoelectric materials.
4
Therefore, it is imperative to choose the right combination of thermoelectric materials for the
specific application.
1.3 Figure of Merit
The figure of merit for accessing the performance of thermoelectric materials is the
dimensionless quantity ZT. Its definition is given by:
(cid:1842) (cid:1846) (cid:2026)(cid:1845)(cid:2870)(cid:1846) (cid:2026)(cid:1845)(cid:2870)(cid:1846)
(cid:3033)
(cid:1852)(cid:1846) = = =
(cid:2018) (cid:2018) (cid:2018) +(cid:2018)
(cid:3032)(cid:3039)(cid:3032)(cid:3030) (cid:3043)(cid:3035)
Where (cid:1842) = (cid:2026)(cid:1845)(cid:2870) is the Power Factor of the material which describes the absolute power per
(cid:3033)
squared-temperature attainable regardless of efficiency, (cid:2026) is the electrical conductivity of the
material and (cid:2018) = (cid:2018) +(cid:2018) is the thermal conductivity of the material and can be broken
(cid:3032)(cid:3039)(cid:3032)(cid:3030) (cid:3043)(cid:3035)
down into the electrically-mediated thermal conductivity and phonon-mediated thermal
conductivity respectively.
It is important to note here that the temperature, T, is present to make the figure of merit
dimensionless, where Z would otherwise have a dimension of K-1.
According to the Wiedemann-Franz law (cid:3428)(cid:2018) = (cid:1838)(cid:1846)(cid:2026) = (cid:3436)(cid:3095)(cid:3118)(cid:4672)(cid:3038)(cid:3251)(cid:4673)(cid:2870)(cid:3440)(cid:1846)(cid:2026)(cid:3432), the electrical
(cid:3032)(cid:3039)(cid:3032)(cid:3030)
(cid:2871) (cid:3032)
conductivity, (cid:2026), is directly proportional to the electrically-mediated thermal conductivity,
(cid:2018) . Hence, the figure of merit can only be improved by increasing S, which is an intrinsic
(cid:3032)(cid:3039)(cid:3032)(cid:3030)
property of the class of thermoelectric material, increasing carrier mobility, which have the
effect of increasing (cid:2026), or by reducing (cid:2018) .
(cid:3043)(cid:3035)
2. Focus on R Ca MnO Perovskites
0.65 0.35 3
Perovskites are a class of metal oxide materials that has the Perovskite Structure [See
Figure 4][2] and the chemical formula ABO . In this article, the A atoms are the R atoms and
3
Calcium in stoichiometric ratios and the B atoms are Manganese atoms. The R atoms in this
article consists of either La, Pr, Nd, Sm, Y* or Bi.

* The Y Ca MnO sample was prepared but eventually unused due to the unattainably high temperature
0.65 0.35 3
(1380°C) needed to produce a strong bulk sample for measurement.
5
Figure 4: Diagram of Perovskite Structure. Each B atoms are surrounded by 6 oxygen atoms arranged in an
Octahedral manner. Each BO group can then be illustrated as being caged by 8 A atoms (a) or 8 BO groups
6 6
caging an A atom (b).
In pure CaMnO perovskite, the Calcium, Manganese and Oxygen ions have oxidation states
3
of +2, +4 and -2 respectively. The substitution of Calcium atoms for the R atoms in this
article (R = La, Pr, Nd, Sm, Bi, Y) have the effect of reducing the oxidation state of a
proportion of manganese ions from +4 to +3 as the R atoms commonly form ions in the +3
oxidation state. In this case, the proportion of manganese ions would theoretically be 65% in
the +3 state and 35% in the +4 state. The substitution of Calcium atoms also have the effect
of distorting the Perovskite structure as the replacement ions have different ionic radius
compared to the Calcium ions in the compound.
The R Ca MnO Perovskites are of interest in this research due to the following factors:
0.65 0.35 3
1) While many different types of thermoelectric materials have been studied extensively,
little focus have been placed on metal oxides as they were thought to be poor
thermoelectric candidates due to their low carrier mobility and high phonon
velocity[3]. This changed in the mid 1990s when the Na CoO oxide was discovered
x 2
to have anomalously high thermoelectric performance relative to the oxides known at
that time. We intend to study the above R Ca MnO Perovskites because their
0.65 0.35 3
thermoelectric properties are not well studied.

2) The R Ca MnO Perovskites exhibit Charge Ordering (CO) and Magnetic phase
0.65 0.35 3
transitions at various temperatures. It is hoped that we can gain an insight to the
electronic transport properties of the thermoelectric materials at these transitions and
perhaps use this knowledge to increase the thermoelectric performance of future
6
materials.

3) The Perovskites, being metal oxides, have the greatest tolerance for high temperature
environments. They are, after all, synthesised using the ceramic-sintering process
which can reach temperatures upwards of 1500 Kelvins. The high temperature
operation not only increase the Power Factor but also increase the thermodynamic
efficiency. This allows metal oxide thermoelectric devices to be powered by high
temperature radioactive sources in niche applications such as deep space explorations.
2.1 About the R Atoms
The R atoms that were chosen for this experiment consists of La, Pr, Nd, Sm, Y and Bi.
These atoms commonly form ions in the +3 oxidation state and, as mentioned above, would
cause the nominal valence of the Mn ions in the Perovskites to be present in a mix of +4 and
+3 when replacing the Calcium ions. In this case, the theoretical proportion of Mn ions would
be fixed.
On the contrary, the ionic radius of these R atoms changes as one moves along the series. The
properties of the R atoms are summarised in Table 1[4][5][6][7][8].
Table 1: Table summarising the R3+ionic radii and their associated perovskite structures.
For comparison, the ionic radius of Ca2+ ion is 1.14Å and that of Mn3+ and Mn4+ ions are
0.72Å and 0.67Å respectively. O2- ions have a relatively large ionic radius of 1.26Å.
Strictly speaking, the true Perovskite Structure can only exist when the A, B and O ions have
bond lengths that are in the specific ratio of √2:1 (A-O: B-O) at 0K. Only then will the
7