A Nivel Local

The thermal behaviour of all precursor gels was investigated by TG and DTA/DSC using a Netzsch STA 449C Jupiter instrument, which is schematically presented in Figure 3.1. The

heat flow and weight change of samples placed in crucibles were measured in air (50 ml∙min-1₎

from 25ºC to 800ºC at a heating rate of 5ºC∙min-1_{. In addition, these experiments were}

repeated in flowing oxygen to allow simultaneous analysis of off-gases by mass spectrometry (MS). A correction run with empty crucibles was performed before analysis of a sample. All data was analysed using Netzsch Proteus-Thermal Analysis software package, version 4.8.5.

Figure 3.1(a) Cross-section of the Netzsch STA 449C Jupiter, (b) TG and DTA crucibles.118

3.2.2 Dilatometry

The sintering properties of compacted samples were studied using a Netzsch DIL 402C push-rod dilatometer, which is presented in Figure 3.2. Due to the different characteristics of the materials tested – YSZ and SDC – different temperature programs were applied, the details of which are provided in Sections 4.3.3 and 5.3.7, respectively. Air (50 ml/min) was employed as the purge gas in all experiments.

Figure 3.2Schematic of the DIL 402C dilatometer: (a) cross section and (b) sample holder.119

3.3 Brunauer-Emmett-Teller method

Brunauer-Emmett-Teller analysis (BET) by nitrogen adsorption-desorption were performed on powders using a Micrometrics ASAP 2020 instrument in order to obtain values of specific surface area (SSA). All samples were pre-treated at 120°C for 6 h. The Micrometrics TriStar software V1.01 was used to obtain automatically results of specific surface area.

3.4 Inductively Coupled Plasma Mass Spectroscopy

Elemental analysis was performed for all materials investigated in this thesis using an AGILENT 7500 Series ICP-MS instrument (New Wave Research Products), equipped with a UP-213 Laser Ablation System. The powder samples were blended with a Teflon standard in order to form a pellet prior to analysis supported by laser ablation. All sintered pellets were analysed using this method without any additional preparation procedures.

3.5 Electron microscopy

3.5.1 Scanning Electron Microscopy

SEM images, which are presented in this thesis, were obtained with two microscopes. A Philips XL30 ESEM instrument with a field emission gun was employed for microstructure studies of YSZ pellets and SDC nanopowders. The details of these analyses, including sputter coating and thermal etching aspects of surface preparation of the samples, are specified in the relevant results chapters. The other micrographs were obtained using a JEOL JSM-5600 instrument and this was intensively used for ceria samples. Conductive carbon sticky pads were used to attach all specimens to the sample holder in both microscopes. Grain size analyses were performed using either the Scion Image (version Alpha 4.0.3.2) or ImageJ (v. 1.42d) software packages and further details are given in the respective results sections.

3.5.2 Transmission Electron Microscopy

All materials investigated in this work were analysed in a JEOL JEM-2011 HRTEM instrument operated at 200 kV. Elemental analysis was performed using an Oxford Instruments X-ray analysis detector (ISIS 300 EDS) in order to obtain the chemical

composition of each sample and assess for potential impurities. The

DigitalMicrograph v.3.3.4 (Gatan Software Team, USA) and the ImageJ v.1.42d (National Institutes of Health, USA) software packages were used for analysis of images and for Fast Fourier Transform (FFT) procedures. Grain size analyses were performed using the

ImageJ(v.1.42d)software package.

An increased dispersion of particles of each specimen was of interest in order to obtain reliable TEM results. This was usually obtained by placing a small amount of each powder in vials with n-hexane, which were subjected to 5 min ultrasonication. Differences in preparation procedure were applied only for YSZ nanopowders and these details are provided in Chapter 4. All samples were deposited on 3 mm Cu TEM grids coated with holey carbon film by repeatedly dipping the grid into an ultrasonicated dispersion of the relevant sample powder. The grids with deposited sample were dried overnight using a lamp as a heat source before being loaded into the TEM instrument for study.

3.6 Impedance Spectroscopy

All electrolyte materials in the form of sintered discs were analysed by two-electrode IS conductivity measurements using a Solartron 1260 Frequency Response Analyser (FRA). The main physical differences between the pellets were associated with the pellet thickness and the area of the electrodes for each sample. These parameters were taken into account in conductivity calculations, according to Equation 2.17. In addition, two different techniques of electrodes preparation were employed in this experiment.

In order to prepare cells for impedance measurements, electrodes were deposited on both sides of the YSZ pellets by sputtering from a platinum target onto the sample surface in areas defined by aluminium foil templates. These were prepared with significantly smaller diameters than the sintered pellets themselves in order to avoid transfer of electrons between the two electrodes by surface conduction. The templates were also helpful to avoid displacing the electrodes on the two opposing surfaces of the pellet with respect to each other. Each exposed area on either side of the pellet was covered with a 120 nm layer using 0.1 mm Pt targets mounted in a Cressington 208 HR Sputter Coater. A current of 80 mA and gas pressure of 0.01 mbar were necessary to carry out this deposition process.

The surface of each SDC pellet was polished with a diamond paste prior to IS measurements. An inorganic-free Pt ink (Engelhardt) was used to deposit electrodes on both sides of the pellet using a screen printer. Special care was taken to apply a similar amount of the Pt paint. It was thought that this technique would allow symmetrical electrodes of 12 mm diameter with approximately the same thickness to be obtained. The position of a pellet in the screen printer was fixed by a template, so that deposited electrodes were placed in the same positions on both sides of each pellet.

Platinum wires (Alfa Aesar, 0.25 mm dia, 99.9%) in a spiral shape were attached to the electrodes using the Engelhardt Pt ink. The paint was lightly dabbed onto the spiral, which rested on the top of sputtered or screen-printed electrode. Due to the surface tension from the paint, the wires were permanently bonded, when the whole assembly was exposed to thermal treatment. In the first step, samples were heated at 2°C/min rate to 200°C and kept for 2 h at this temperature. In the second step, the temperature was increased to 1000°C at 4°C/min and maintained at this temperature for 1 h. An example of an SDC sample used in the IS measurements is presented in Figure 3.3.

Figure 3.3SDC pellet Pt electrodes with attached wires on representative.

The resulting pellets were placed onto the sample support inside the quartz reactor, which is presented in Figure 3.4. Electrical connections between the Pt wires attached to the electrodes and the Pt wires connected to the leads of the FRA were made by welding. The

resistance of the Pt wires (Rw), which varied in the range 2-5Ω, was measured using a

multimeter (Metrix MX 56C) before each set of impedance measurements and was subtracted from the impedance data.

All complex impedance measurements were carried out under an atmosphere of flowing pre-dried, synthetic air (50 ml/min) at a number of temperatures over the range 150 – 800°C. The rig including moisture trap, control valves and flow meter was built to ensure identical conditions across all of the samples tested. The temperature of the furnace was controlled by a

chromel-alumel (Type K) thermocouple connected to a computer programme via a

PICO 8 channel Thermocouple Data Logger. An a.c. voltage of 0.1 V amplitude was applied and the frequency was swept from 20 MHz to 1 Hz. At each test temperature, spectra were recorded repeatedly until no change was noted between them. Once the final spectrum was taken, the temperature of the furnace was increased to the next test temperature. Before starting measurements at each temperature, the system was left for a period of at least 1 h in order to allow the system to attain chemical and thermal equilibrium. All impedance spectra recorded in order of increasing temperature were repeated over the same range of temperature in decreasing order.

Figure 3.4Schematic of a quartz reactor used in IS measurements.

The resistance, capacitance and inductance values were extracted from each spectrum using ZView software (Scribner Associates, Inc). Each Nyquist plot was first examined with reference to the number of processes occurring within the sample. Usually arcs were easy to identify. Bulk and grain boundary contribution arcs were of interest in the studies of both electrolyte materials. In some impedance plots, two small arcs were represented as one elongated feature in the lower frequency range, therefore additional precautions had to be taken to ensure that numerous minor processes were not overlooked. Inductive effects as a result of the current and its magnetic field were taken into account by including a corresponding inductance element in the equivalent electronic circuit. For example, the

proposed model at low temperatures included: Rw as the resistance of the wires, L as the

contribution. Single out-lying points at 50 Hz were occasionally seen on some spectra. These corresponded to interference from the mains power and were deleted as otherwise they would affect the fitting procedure.

Care was taken to select appropriate points on the Nyquist plot in order to obtain preliminary data values for each arc by using the ‘fit circle’ option. When the circle was fitted, the program provided visual representation of the fitted circle and gave the estimated values of the resistance (R) and (where possible) the capacitance (C). Next, these rough values were entered into the equivalent circuit tool and a least squares fitting method was run to optimise the results. The different circuit values were manipulated ensuring that the resistance and capacitance data were realistic until the best fit model for the arcs of interest was obtained. Additionally, it was ensured that the circuit values were only being fitted over the relevant frequency range. This produced much more reliable values for the equivalent circuit than if all the points were fitted simultaneously. Figure 3.5 demonstrates the final fitted impedance data (green line) of the Nyquist plot for a representative YSZ sample.

Figure 3.5Final fitting and experimental data in Nyquist plot for YSZ pellet recorded at 250°C depicting bulk and grain boundary contributions.

3.7 X-ray Photoelectron Spectroscopy

XPS measurements were carried out on SDC and Pd/SDC samples at the University of Málaga (Spain) using a Physical Electronics 5700C Multitechnique spectrometer, which

operated with Mg K radiation (h= 1253.6 eV). In order to determine all the elements

present on the catalyst surface, general spectra were recorded for the samples by scanning up

to a binding energy (E_b) of 1200 eV. The E_b of the Pd 3d, Sm 3d, Ce 3d and O 1s core level

and full width at half maximum (FWHM) values were used to determine the chemical state of the elements. Correction for binding energies due to sample charging was done by taking the

C 1s peak (284.6 eV) as an internal standard. The accuracy of the Eb scale was ±0.1 eV. The

data analysis procedure involved smoothing, a Shirley background subtraction and curve fitting using mixed Gaussian-Lorentzian functions by a least-squares method. The atomic ratios of the elements were calculated from the relative peak areas of the respective core level lines using Wagner sensitivity factors.