1756.1 Introduction

Many of the advances in modern technologies are traced back to the outcomes of material research. By understanding the different physical and chemical properties of different compounds; scientists can produce materials that end up advancing and improving many different aspects of our daily lives. Advances in semiconductors, for example, that led to improving data processing, telecommunications, electronics and many other applications all have originated from the better understanding of how matter behaves and what affect its different properties. Through material science or material research, scientists are continuously finding better techniques in making alternative materials that ultimately make things more efficient, reliable, robust and finally to reduce the cost of the end product. Thus, by linking how the structure of a material affects its properties and how this structure can be controlled through different synthesis and processing methods; this scientific discipline is quite important and indeed beneficial on a large scale.

There are many techniques that material scientists rely on that to expand their understanding of materials from the atomic and molecular stage to the macroscopic behaviour and properties. X-ray and neutron diffraction are widely used to characterise the atomic arrangement of different materials as well as to check the purity of synthesised samples. Other techniques include scanning and transmission electron microscopy, electron diffraction, spectroscopy and thermal analysis.

Thermal analysis is a wide discipline in material science that covers a wide range of techniques. They include differential scanning calorimetry(DSC), differential thermal analysis (DTA), dielectric thermal analysis (DETA), dilatometry, dynamic

176

mechanical analysis (DMA), Thermogravimetric analysis (TGA) and many more.[1,2] Of the many techniques, TGA is an important method when it comes to studying materials behaviour and changes with temperature.

One of the most technologies that rely on thermal analysis is the search for alternative materials for solid oxide fuel cells (SOFCs). Since these cells are usually operated at temperatures higher than 600o_{C, this puts a huge strain on the components}

of a typical fuel cell.[3] Elevated temperatures can affect the lifetime of a fuel cell through degradation of its components as well as chemical and physical instability. A ceramic based anode material must be electrically conductive at high temperatures and many of these undergo structural and chemical changes that affect their conductivity. TGA can give a better insight into the physical and chemical stability of an anode material at high temperatures as well as to the electrochemical processes that govern its conductivity and overall catalytic activity. Thus it is obvious that both properties; i.e. the mass change and electrical conductivity; are important in determining the suitability of a material to be a component of a fuel cell. In most cases, TGA and electrical conductivity results are reported separately. Almost all TGA studies are conducted on samples in powder form within a crucible. On the other hand, electrical conductivity of different materials is usually measured on bars or pellets of the same material. To some extent this is tolerable depending on the level of care in conducting the experiments and the analysis of the different data to correlate them.

Many researchers are targeting materials that show both ionic and electronic conductivity; i.e. mixed ionic electronic conductor (MIEC); to employ them usually as electrodes to extend the triple phase boundary (TPB) of the electrochemical reaction in a fuel cell. It is usually difficult to determine the contribution of the ionic conductivity

177

to the total conductivity.[4,23] A very important parameter is used to compare between different MIEC materials, this parameter is the chemical diffusion coefficient.[5,6,7] It provides a quantitative measure of the type of defects that control both types of conductivity and their mobility and concentrations. There are many methods that can be used to measure the oxygen diffusion namely oxygen permeation, oxygen ionic relaxation, isotope exchange using secondary ion mass spectrometry (SIMS), electronic blocking electrodes and electrical conductivity relaxation.[8,9] The electrical conductivity relaxation method is widely used to measure the diffusion coefficient of oxygen which determines the ionic conductivity and other parameters as explained in the theory section of this chapter. Usually this technique is accompanied by a TGA analysis to determine the oxygen stoichiometry in MIEC materials. Thus it is very obvious that there is a great potential of combining conductivity relaxation measurement with a simultaneous thermogravimetric analysis (TGA) in one setup to better understand the electrochemistry of such materials.

Hence, we are presenting a novel analysis technique that combines TGA with a simultaneous measurement of DC conductivity of solid materials in controlled temperature and oxygen partial pressure settings.

178

6.2 - Theory

6.2.1 - Thermogravimetric Analysis

Among the many techniques employed by chemists to quantify materials properties is the thermogravimetric analysis (TGA). It is defined as the monitoring of mass change of a substance measured as a function of temperature at a controlled temperature programme.[10] A typical TGA device records the mass change of a sample as a function of temperature as the sample is heated or cooled in a controlled atmosphere usually provided with certain gas purging. Also, mass changes can occur when the environment surrounding the sample is changed; i.e. by switching between different purge gasses at a fixed temperature; and mass is recorded as a function of time. TGA is very useful in studying the volatility, stability, water content, oxidation and lifetime of different materials.

6.2.2 DC Conductivity Measurement

Measuring the conductivity of a material is very important in many scientific and industrial fields. The resistance of a sample (R) is given by Ohm’s law:

𝑅 =

𝐼 (6.1)

Where V is the voltage measured when the sample is subjected to a current I. While the resistance of a sample can be easily measured, it does not present a specific property for the material under test. The resistivity of a material is on the other hand specific and depends on the structure and composition of a sample. Resistivity can be obtained from the resistance of a sample under a constant current; however it requires a correction factor for the geometry of the sample.

179

The resistivity of a wire for example is given by:

𝜌 =

𝑙 (6.2)

Where A is the cross section area of the wire and l is its length as shown in figure 6.1.

Figure 6.1: The geometry of a wire. Conductivity is simply the inverse of the resistivity of a substance:

𝜎 =

𝜌 (6.3)

The most common techniques that are used to measure the conductivity of solids are the 4-point probe technique and the van der Pauw technique which are explained in the next few pages.

6.2.2.1 - The 4-point Probe DC Conductivity Technique

This technique is based on using four finite probes to measure the resistance across the surface of a sample. The typical arrangement of the technique is shown in figure 6.2. Four probes are equally separated from each other, where two probes supply the current and the other two are used to measure the voltage generated across the sample.