Solid State Growth of Piezoelectric Single Crystals

Whilst most piezoelectric applications rely on polycrystalline ceramic forms of piezoelectric oxides, single crystals, with dimensions of order 1 to 10 cm, are gaining market share. Typical of these are crystals based upon Pb(Mg1/3Nb2/3)O3-PbTiO3, (PMN-PT). In medical imaging and sonar applications, their enhanced piezoelectric properties of the crystals provide advantages in terms of greater bandwidth and resolution, higher sensitivity and reduced size and power consumption. The main growth technique relies on directional freezing from the melt, known as Bridgman growth. However, such crystals suffer two major shortcomings: (i) they are expensive due to the slow growth rates and use of significant quantities of platinum as a crucible and (ii) they are inherently non-uniform due to the partitioning of some constituents between the melt and solid forms, resulting in a composition gradient in the crystal.
An alternative growth technique is that of solid state crystal growth (SSCG), in which a small seed crystal is placed in contact with a polycrystalline body and annealed. The grains of the polycrystalline body that are both in contact with, and crystallographically aligned with the seed crystal, grow at the expense of the other grains and eventually coalesce to form a large single crystal. The process has been demonstrated for several materials including barium titanate (BaTiO3) and the PMN-PT family of crystals. The technique has a number of potential advantages over Bridgman growth: (i) it avoids the concentration gradients inherent in Bridgman growth; (ii) it is potentially cheaper than Bridgman growth in terms of both capital and consumables expenditure; (iii) a wider range of compositions can be grown, including systems which are incongruently melting that are difficult to grow from the melt; (iv) net-shaped crystals can be grown through the use of ceramic pre-forms.
Most of the research into this process has previously been undertaken by one academic group in South Korea, of which the principle investigator is also the owner of a company that has commercialised the process. Consequently, the published technical details are insufficient to replicate the process and may even be unreliable. There is no generally accepted model of the growth process, which would be invaluable if the technique is to be successfully applied to a range of materials. Hence the project will focus initially on understanding the growth process, before proposing and testing methods for optimum growth of selected compositions.
Although a key aim of the project is to demonstrate reproducible SSCG of PMN-PT based crystals, such materials are to complex for the purposes of understanding the growth mechanism. The volatility of PbO is an issue during the extended annealing times required for SSCG, creating a time varying parameter. Hence the initial work aimed at understanding the process will focus BaTiO3. As a small quantity of transient liquid phase at the interface between the growing crystal and polycrystalline matrix is known to play a role in SSCG, this can be investigated in BaTiO3, as the BaO-TiO2 eutectic at 1320 degrees celcius allows a liquid phase to be introduced by controlling the TiO2 excess. In addition, the influence of the following parameters will be investigated: annealing temperature, temperature gradient in the matrix, grain size of the matrix and additional interfacial layers at the crystal matrix interface. Electron microscopy techniques including scanning electron microscopy, transmission electron microscopy and electron back-scatter diffraction will be employed to measure the rate of crystal growth and understand the changing nature of the crystal-matrix interface.
Once a sound understanding of the process is achieved, the model will be tested by applying it to other materials, such as modified PbTiO3 (e.g. PMN-PT and Bi(Zn1/2Ti1/2)O3-PbTiO3) and experimental lead-free materials in the modified KNbO3 family.