2.2.1 Optical microscopy
An optical ML 2000 Microscope (Meiji Techno Co. Ltd.) was used to examine LSM tubes produced, and particularly to identify any cracks in dip-coated YSZ electrolytes. The microscope has an adapter to fit a Canon Powershot G6, 7.1 megapixel digital camera, which was used to take some optical micrographs, at magnifications of 100x, 200x, 500x, and 1000x.
2.2.2 Scanning electron microscopy (SEM)
A scanning electron microscope (SEM, JSM 5600, Jeol) was used to examine samples at the small scale.
2.2. CHARACTERISATION METHODS 85
Powder examination
The powders examined in the SEM were sprinkled onto sticky conductive carbon paper, and held upside down so the excess fell off. Alternatively, suspensions containing powders were placed on carbon paper and allowed to dry before SEM examination.
Samples mounted in resin
The majority of samples examined by SEM were mounted in resin. The sample was placed in a mould greased with high vacuum grease (Dow Corning). EpoFix resin and hardener (Struers) were mixed in a 6:1 ratio by volume, poured over the sample, and placed in a glass vacuum chamber, or Citovac vacuum chamber (Struers). The air was evacuated for 2-3 min until all bubbles of air inside the sample had expanded out, and then the samples were taken out, and left overnight to cure.
The disc of resin with the sample was then sectioned with a Minitom diamond saw (Struers) and polished on a Metaserv 2000 grinder/polisher (Buehler) according to the procedure in table 2.2.1. They were then gold coated, by an high vacuum gold evaporation method, using equipment made by the University of St Andrews.
Polishing media Duration Lubricant 600 grit SiC paper 2 min water 1200 grit SiC paper 3 min water
Cloth with 6 µm diamond paste 10 min Metadi fluid (Buehler)
Cloth with 3 µm diamond paste 5 min Metadi fluid (Buehler)
Cloth with 1 µm diamond paste 3 min Metadi fluid (Buehler)
Table 2.2.1: Polishing process for resin mounted SEM samples. In between each step, the resin discs were ultrasonicated in an ultrasonic bath (Ultrawave Ltd.) in water for 2 min to clean them.
Fractured and conductive samples
Some samples such as YSZ coated LSM tubes were fractured, and the fracture surface was examined in the SEM. The less conductive YSZ contrasted with the more conductive LSM under the electron beam. These samples not mounted in epoxy, because the thin solid YSZ layers are easily damaged. The vacuum that is applied to draw air out of all the pores and infiltrate liquid epoxy resin, puts pressure on the YSZ layer, causing it to shatter. Then, as the epoxy cures, it shrinks, which also puts pressure on the YSZ layer. Therefore, the samples are best examined without being mounted in epoxy resin.
2.2.3 Energy dispersive X-Ray spectroscopy (EDX)
A scanning electron microscope (JSM-5600, Jeol) was used to carry out energy dispers- ive X-ray spectroscopy. Various samples were examined, and elemental composition was determined over a set area (map), along a line, (line scan) or a single point (spectrum).
Furthermore, by combining data from two different elements, maps of particular phases were produced.
2.2.4 Thermal gravimetric analysis
Thermal gravimetric analysis is a technique which accurately measures mass changes in a sample during a heating profile, and can be carried out in either a reducing or oxidising atmosphere, with a variety of gases. It was used in this work to measure the temperatures at which organic components of the extrusion recipes combusted. This is important to understand, as organic components that persist to hotter temperatures will have a greater impact on pore formation in the LSM substrate. Typically, 10 mg - 20 mg of the organic component was placed in a platinum crucible in a thermal gravimetric analyser (TG-1000- M, Rheotherm), and was heated at 5 ◦C min−1 to 900◦C or 950 ◦C. The mass loss as a
function of temperature was plotted, which showed the temperatures that the components burned out at, and allows comparison between them.
2.2.5 Particle size distribution analysis
Particle size distribution measurement may be carried out by several methods, but the one used in this work is laser diffraction. The particles to be examined are suspended in a solvent, and flow past a laser. When the laser hits the particles, the light is scattered. Smaller particles create a greater scattering angle, but weaker intensity of light, and larger particles are the opposite. In this way, a particle size distribution measurement can be made. In-situ ultrasonication can also be used to break up agglomerates in the slurry to be examined.
LSM used for extrusion
A Hydro 2000S Mastersizer (Malvern) was used to make particle size distribution measure- ments on the LSM used for extrusion. 20 mg - 30 mg of LSM was dispersed in a solution of 2 % by mass Triton QS-44 dispersant in isopropanol (99.8%, BDH), using an Ultrawave ultrasonic bath (Ultrawave Ltd.) for 2 min, and then the particle size distribution was recorded three times.
YSZ dip-coating slurries
A selection of YSZ dip-coating slurries were measured in the particle size analyser. The typical procedure was similar to that used for the LSM, but the samples were not ultra- sonicated. They were shaken for 1 min by hand, allowed to settle for 24 h, and then the suspended layer was sampled. The slurries were then shaken for a further 1 min, and the mix of the settled and suspended layers was sampled. The suspended layer is that which is coated onto the tubes, and the mix of the two layers shows how well the YSZ is dispersed in the slurry.