CHAPTER 8 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK

Transcription

1 CHAPTER 8 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 8.1. Conclusions Referring to the aims of the research project in Chapter 4, the following conclusions can be drawn on the basis of the present work: Hydrothermal synthesis has been used successfully to produce PZT powders having a submicron particle size and a narrow particle size distribution. It has been found that the base mineraliser (type and concentration), hydrothermal environment, temperature and time are the most important parameters in determining the phase formation, particle size and morphology of PZT powders during their hydrothermal synthesis. Nucleation appears proceed mostly via in-situ transformation from the amorphous zirconia-titania gel and hydrated lead oxide precursors in the presence of the critical mineraliser concentration (specific to the mineraliser used) required for the formation of cubic morphology perovskite PZT particles. Only minor dissolution/recrystallisation occurs, accounting for the formation of intermediate phases such as PbO, TiO 2, and PbO-TiO 2 solid solutions. This mechanism becomes more obvious when the mineraliser concentration increases and/or during the later stages of crystal growth. The base mineraliser type and its concentration play important roles in promoting the solubility and rearrangement of the network by functioning as a template. The smaller the cation radius of the base, the more efficient it is as a mineraliser. The mineraliser concentration should be above a critical level in terms of providing both the necessary soluble Pb species and enough template ions 199

2 to cause disruption of the amorphous zirconia-titania gel network; thereby facilitating the diffusion of the Pb species into the network, and hence, the in-situ nucleation of PZT. However, the minimum mineraliser concentration is strongly dependent on both the hydrothermal temperature and environment. Increasing the synthesis temperature and using the two-step derived feedstock, which minimises the interference of the acetate ions, reduce the minimum mineraliser concentration (of a specific mineraliser) required for perovskite PZT formation. Consequently, the tendency of the primary PZT nuclei to coagulate during the particle growth stage is decreased because the ionic strength of the hydrothermal solution is reduced. The particle size and morphology of the PZT powders from hydrothermal synthesis can, therefore, be controlled by choosing carefully the mineraliser type and concentration to be used with a given synthesis temperature and for a chosen synthesis time. This will simultaneously ensure only perovskite PZT phase formation and prevent overly rapid PZT particle growth. Perovskite PZT powders with a mean particle size of 0.2 to 0.3 µm and a narrow particle size distribution have thus been synthesised hydrothermally at 300 C under an autogeneous pressure of 8 MPa for 2 hours using either 0.4 M KOH or 0.3 M NaOH as the mineraliser, together with the two-step derived feedstock. Colloidal processing in conjunction with hydrothermal synthesised PZT powders has shown distinct advantages over the conventional process involving the dry powder pressing of mixed-oxide PZT powders. The sintering temperature for hydrothermal PZT powders is about 200 C to 300 C lower than that for the mixed-oxide PZT powders due to the finer particle size and narrower particle distribution and the apparent defect-activated 200

3 sintering mechanism of the hydrothermal PZT powders. Consequently, the best electrical properties for both doped and undoped PZT ceramics, which are in agreement with the best data in literature, have been achieved at lower sintering temperatures for the hydrothermal PZT ceramics. The electrical properties of PZT ceramics seem more sensitive to the doping level rather than to the microstructure. For example, in the present work, the addition of 1 mol % Nb or La as a dopant increases K p and ε r by about 50%, while the presence of 3 mol% K increases Q m significantly in comparison with the undoped PZT ceramics, even though their microstructures are similar. The feasibility of processing PZT thick films via electrophoretic deposition (EPD) directly from the aqueous hydrothermal PZT suspension has been demonstrated. Unlike the conventional screen printing technique, the EPD thick films can be deposited on any conducting substrate, not being restricted to a flat surface. Increasing the suspension concentration can increase the thickness of the films. However, owing to the relatively low applied electrical field strength due to the low decomposition voltage of the water, the maximum film thickness achievable was about 5 µm in this work. Increasing the deposition time resulted in the increased particle agglomeration within the films. The greatest problem in the processing of PZT thick films is the high vapour pressure and diffusivity of lead oxide which causes the loss of lead and diffusion / reaction with the substrate when the sintering temperature is above 1000 C. Therefore, careful selection of the substrate material and control of the sintering atmosphere are equally important in making fully dense PZT films. The electrical properties of the PZT films have not been measured in this work due to lack of fully dense films. Further optimisation is needed to get dense, pore-free films from both the processing routes investigated. 201

4 8.2. Suggestions for Future Work Concerning the hydrothermal synthesis of PZT powders, it has been shown that perovskite PZT powders with a mean particle size of 200 nm can be synthesised in the present work. It seems, however, difficult to reduce the particle size further to below 100 nm because of the nucleation and growth mechanism of the PZT particles observed under the present synthesis conditions, using a strong inorganic base as the mineraliser. Although the mineraliser assists in the nucleation of PZT perovskite powders, particle growth is also accelerated. Therefore, the minimum use of a base mineraliser is desirable in order to limit the particle size. One way is to choose a more efficient mineraliser, which can decrease the critical mineraliser concentration required for perovskite PZT formation. According to the function of the mineraliser proposed in this work, choosing the mineraliser with a cation and/or anion of smaller radius (e.g. lithium and fluorine) is expected to improve the efficiency. However, a systematic investigation is needed to ascertain which is the most efficient mineraliser. It should also be noted that lithium and fluorine are reported to be retained selectively as impurities in PZT and that they also increased the level of retention of the associated alkali or halide [Beal, 1987]. Another way is to minimise the interference of other ions with the mineraliser. As shown in this work, the critical mineraliser concentration can be effectively reduced by using the two-step derived feedstock. However, there was still some interference from the precursor 202

5 lead acetate used because the acetate group not only consumes the base mineraliser but also binds strongly to the Ti and/or Zr atoms, which retards the homogeneous crystallisation of the PZT powders. Therefore, further work is needed to optimise the precursors used; thus, ensuring the effective functioning of the base mineraliser. Another possible way to synthesis PZT powders with a nanometer particle size (<100 nm) is to abandon the use of mineraliser completely while increasing the hydrothermal temperature and pressure. It was reported that the simultaneous control of particle size, particle size distribution, morphology, and crystal structure can be achieved for singlecation ceramic powders at higher temperatures of 400 to 500 C and higher pressures of 30 to 35 MPa in supercritical water without the use of mineralisers [Adschiri et al., 1992]. The reaction time for the hydrothermal synthesis of ultrafine ceramic powders was less than 2 minutes. The dehydration of metal (hydrous) oxide precursors took place rapidly before the hydrous oxide particles grew too large because of the high reaction temperature and the high diffusivity in supercritical water. It should be possible to synthesis nano-sized PZT powders under such conditions assuming homogeneous mixing of the multi-cation precursors can be achieved fully. This should be investigated further. Colloidal processing in conjunction with hydrothermal synthesis has shown some distinct advantages over the conventional ceramic powder processing for PZT ceramics in this work. The reduction of sintering temperatures by as much as 300 C and a sintering time as short as 5 minutes has been observed for the hydrothermally synthesised PZT powders. While this is attributed to the proposed defect-activated sintering mechanism of the hydrothermal PZT powders, the actual details of the mechanism have yet to be determined. 203

6 Thus, the lattice defect structure formed during the hydrothermal synthesis of PZT powders and its effect on the sintering and the subsequent electrical properties needs to be investigated further. The novel combination of hydrothermal colloidal processing should also be applicable to other ceramic systems; in particular, for those where high-temperature sintering is restricted (e.g. LaCrO 3 used for the interconnector of Solid Oxide Fuel Cells (SOFC)), as in the PZT system, or excessive grain growth is undesirable (e.g. Al 2 O 3 /ZrO 2 nanocomposite ceramics). This combination of processing techniques has also shown promise for the fabrication of ceramic films or coatings (< 5 µm), which can be formed directly from aqueous solution; thereby employing the full advantages of both processing techniques. The applicability of these techniques to other materials systems (e.g. ZrO 2 or BaTiO 3 ) should also be investigated. Finally, the electrical and mechanical properties of the PZT ceramics processed by the novel processing routes developed in this work could be improved by further optimising the processing conditions, since homogeneous, fine-grained and defect-free PZT ceramics are achievable in principle by these wet chemical routes. In this work, only the electrical properties were considered. The future work, however, should be extended to investigate the mechanical properties of the PZT ceramics processed by this novel processing since PZT ceramics have also been used in some high stress situations, such as high-power actuators, where high fracture strength and toughness are required. 204