Chapter 2: Methods
2.1 Synthetic Methods
A major undertaking in this study was developing synthetic methods that could produce large amounts of a material in a high purity that were appropriate for further study. Descriptions of the optimization of each method are given in further detail in the chapters corresponding to each specific material.
2.1.1 Solid State Synthesis
The basic solid state synthesis method was applied to iterations of materials presented in Chapters 3, 4, 5 and 7. This method was used for its relative simplicity, enabling bulk synthesis of functional ceramics. This method refers to reacting (sintering) solid raw materials at elevated temperature after milling and pressing them together, in order to form a homogenous product with a single crystal structure.
2.1.1.1 Raw Materials
The raw materials were commercially sourced (Table 2.1). Prior to use the raw materials were placed and stored in an oven at 150 °C to remove moisture. Materials were weighed in stoichiometric amounts to make batches with masses in the range of 15 – 20 g in total.
Table 2.1. Reagent details for solid state syntheses.
Reagents/Solvents Brand Purity (%) Chapters
Bi2O3 Alfa Aesar 99 3, 4, 5
Fe2O3 Sigma-Aldrich ≥99 3, 4, 5 Cr2O3 Koch-Light 99.999 3 TiO2 Sigma-Aldrich ≥99 4, 5, 7 MgO Hudson 99.999 4 Mg(CH3COO)2.4H2O Analar ≥98 4 3MgCO3.Mg(OH)2.3H2O M&B ≥97 4
NiO Alfa Aesar 99 4
Ni(CH3COO)2.4H2O Unilab 98 4
CuO Univar ≥99.9 4
ZnO Univar 99.5 4
V2O5 Research Organic Chemicals 99.8 7
C2H5OH (ethanol) Merck ≥99.9 3, 4, 5
C3H6O (acetone) Merck ≥99.8 7
Fe* Puratronic Grade II 5
*powder for production of FeO, after mixing with Sigma-Aldrich Fe2O3
and sealing inside a quartz tube for heat treatment. 2.1.1.2 Milling and Pressing
Milling and mixing of the reagents was conducted via either hand or ball milling (specified within relevant chapters). Hand milling involved adding the raw materials to an agate mortar and pestle, covering with ~ 50 mL of suitable solvent for containment (ethanol for all but Chapter 7 in which acetone was used) and grinding. The grinding process was continued until the solvent evaporated three times, then the mixture was allowed to fully dry in air before further processing.
Ball milling was conducted by adding the materials to Teflon canisters filled with yttrium- stabilized zirconia balls of sizes 1 to 12 mm and around 100 mL of suitable solvent. The sealed canisters were rotated in a planetary ball mill at 20 Hz for at least 16 hours. The milled mixture was then sieved through a mesh sieve into a shallow glass dish to remove the balls, and then allowed to evaporate in air until totally dry (~ 12 hours for ethanol). The dried mixtures were then briefly milled by hand using a mortar and pestle to prepare for pressing.
In some samples the dried, milled powder was further processed, including sieving for the VTO50 (60 – 120 μm grade) and BFNTO samples (~ 300 μm), where the smallest grains were retained for this pressing, and PVA solution was added to other samples (noted in text). When suitably prepared, milled powders were loaded into the cavity of a 12 mm stainless steel die, and a weight of ~ 5 tonnes (unless otherwise noted in text) was applied to the ram using a hydraulic press to result in a pressure of ~ 60 MPa, and pellets ~ 3 mm thick.
2.1.1.3 Sintering
The resulting pellets were placed directly onto/into alumina crucibles with lids in Chapters 3 to 5. In Chapter 3 samples were also placed into quartz crucibles constructed using glass blowing techniques. Samples were placed into alumina foil crucibles/boats in Chapter 7. The samples were reacted (sintered) in furnaces, while inside/on their specified vessels (Figure 2.1). A KSL 1200X muffle furnace was used for Chapter 3 and parts of Chapter 4, 5, 6, and 7. This furnace had a gas supply option to ‘blanket’ the chamber in N2 gas. A Daihan Scientific Ceber muffle furnace (max. 1200 °C) was also used in Chapter 5, which had a temperature offset of 40°C in comparison to the KSL furnace, and the quoted operation temperatures in text are adjusted as such. High temperature iterations in Chapter 7 were conducted using a KSL1700X muffle furnace, with blanketing gas supply option. Also in Chapter 7, a Labec horizontal tube furnace was used to achieve a low oxygen atmosphere. These samples were placed on platinum inside a quartz tube, where nitrogen was passed through and vented into a fume hood (Figure 2.1 (b) below).
Heating rates were typically set to 5 °C/minute unless otherwise specified in the body of the text. Natural cooling of samples referred to leaving the sample in the closed furnace chamber until it was at room temperature. Quenching of samples from high temperature was done by extracting the vessel with tongs at high temperature and placing on a heat proof mat in open air to cool. Specific detail on temperatures and dwell times used in the sintering steps are given in the body of each chapter.
Figure 2.1. Schematic illustration of (a) the muffle furnace and (b) the tube furnace setups for sample sintering.
2.1.2 Metal Organic Decomposition
The metal organic decomposition (MOD) method was used in Chapters 3, 4 and 5. This involves starting with metal organic and nitrate precursors which are decomposed to oxides in
a ‘calcining’ step. This ‘precursor’ is then pressed and sintered in place of the milled oxide solid state synthesis method addressed above. The MOD method was used to promote homogenous mixing of metal ions and preclude some types of impurity formation that the solid state method suffered from.
2.1.2.1 Raw Materials
The raw materials (Table 2.2) were weighed in stoichiometric quantities for a targeted 4 g of oxide product. These were added to a beaker. Approximately 40 mL of solvent (ethylene glycol, ‘MOD-EG’ or reverse osmosis water, ‘MOD-H2O’) was added and magnetically stirred. Liquid reagents were delivered by syringe where necessary.
Table 2.2.Reagent details for metal organic decomposition syntheses.
Reagents/Solvents Brand Purity (%) Chapters Bi(NO3)3.5H2O Sigma-Aldrich ≥98 3, 4, 5 Fe(NO3)3.9H2O Sigma-Aldrich ≥98 3, 4, 5
Ti(OC4H9)4 Aldrich 97 3, 4, 5
Cr(NO3)3. 9H2O HW ~98 3
Mg(NO3)2.6H2O Merck ≥99 4
Ni(NO3)2.6H2O Alfa Aesar 98 4
C2H4(OH)2 (ethylene glycol) Fluka AG ≥99.5 3, 4, 5
2.1.2.2 Volume Reduction and Calcination
Chromium containing MOD-EG solutions in Chapter 3 began as a dark green/black colour, while magnesium and nickel solutions were orange and green respectively in Chapter 4. Bismuth iron and titanium only solutions in Chapter 5 were yellow to red depending on the iron content, and when using the MOD-H2O approach, contained white particulate with yellow supernatant.
These mixtures were then transferred into shallow glass dishes and the ethylene glycol (or water) was evaporated slowly into a fume hood at 100 – 200 °C using a hotplate. This was conducted with continual stirring for approximately 3 hours until the volume was reduced to ~ 10 mL. During this volume reduction, typically MOD-EG solutions would darken in colour, then lighten back to a pale yellow or pale green (Figure 2.2 (a) – (c) illustrates this process for preparation of BFTO-623 in Chapter 5). If the solution is over reduced, a precipitation may occur resulting in an opaque paste.
The resulting cooled viscous solutions were removed from heat and transferred to an alumina crucible (Figure 2.2 (d)) and cooled to room temperature. These were then placed in a secondary containment crucible and into a Kilnwest furnace that vents into a fume cupboard for calcination (note: only 10 mL could be calcined at one time using this method). The 32
solutions were heated to 600 °C at 5 °/min then held for two hours as the solvent and metal organic/nitrate species decomposed. These were then cooled naturally to room temperature. The final dried products resembled a porous mass, often orange in colour (Figure 2.2 (e)), which was ground with a mortar and pestle for pressing as outlined in the previous section.
Figure 2.2. Example MOD-EG process for BFTO-623 in Chapter 5. A dissolved bismuth nitrate, iron nitrate, titanium butoxide and ethylene glycol solution at room temperature is shown in (a), (b) shows the solution after transfer to a shallow dish and commencement of heating, (c) depicts the solution lightening after 3 hours when near minimum volume, (d) shows the solution cooling in a crucible and (e) shows the decomposed product after calcining at 600 °C.
2.1.3 Solvothermal Synthesis
The solvothermal method was used in to produce samples presented in Chapter 6. This method involved performing reactions under pressure to crystallize the desired phase from solution. This method was used for its ability to produce nanostructured materials.
2.1.3.1 Raw Materials
To generate solutions for the solvothermal reaction, first titanium tetrachloride was delivered to 240 mL of ethanol via a syringe. The vanadium source was then added (in a fixed V:Ti ratio ~ 0.05:1 between trials) and the solution magnetically stirred for an hour until the solution became transparent green (vandyl acetylacetonate), or yellow (vanadium oxychloride). The nitric acid was then added (in a different amount per individual trial ranging from 4 – 16 mL) with due care and stirred for a further 30 minutes, during which the solutions turned ‘neon’ green or yellow. These solutions were then split equally among four autoclaves, filling to 60% capacity. Details of reactants are outlined in Table 2.3.
Table 2.3. Reagent details for solvothermal syntheses.
Reagents/ Solvent Brand Purity (%) Chapter
TiCl4 Sigma-Aldrich 99.9 6
VOCl3 Sigma-Aldrich 99 6
VO(C5H7O2)2 Fluka AG ≥96 6
C2H5O5 (ethanol) Merck ≥99.8 6 HNO3 (nitric acid) Univar 70 6 2.1.3.2 Reaction and Processing
The Teflon lined autoclaves were tightly sealed, then transferred to a low temperature oven in a specially designed lab for high pressure experiments. The vessels were heated for 17 hours at 200 °C. After reaction the vessels were removed from the oven and cooled to room temperature before being carefully opened. The reaction mixture was decanted into plastic containers with screw seal lids for use in a Sigma 3-30KS centrifuge. The products were centrifuged for 20 minutes, decanted, and then washed with H2O. They were then centrifuged and washed a further two times before decanting and placing the whole tube in an oven to dry at 100 °C. After 24 hours of drying, the powders were collected into containers for use in the experiments outlined in Chapter 6.
Throughout each of the synthetic approaches addressed in this section, the phase purity and crystal structure of the synthesized samples had to be monitored. The next section outlines the diffraction and microscopy methods used and how in-depth characterisations of their crystalline natures were performed.