Solid State Growth of Piezoelectric Single Crystals

虽然大多数压电应用依赖于多晶陶瓷形式的压电氧化物，但尺寸在1到10厘米的单晶正在获得市场份额。其中典型的是基于PbO_3-PbTiO_3(PMN-PT)的晶体。在医学成像和声纳应用中，它们增强的晶体的压电性能提供了更高的带宽和分辨率、更高的灵敏度以及更小的尺寸和功耗。主要的生长技术依赖于熔体的定向冻结，即所谓的布里奇曼生长。然而，这类晶体有两个主要缺点：(I)由于生长速度慢和大量使用铂作为坩埚，它们价格昂贵；(Ii)由于某些成分在熔体和固体之间的分配，它们固有的不均匀，导致晶体中的组成梯度。另一种可选的生长技术是固态晶体生长(SSCG)，即将一个小的种子晶体与多晶体接触并进行热处理。多晶体的颗粒既与种子晶体接触，又在结晶学上与种子晶体对准，以牺牲其他颗粒为代价生长，最终结合形成大的单晶。这一过程已经在几种材料上得到了验证，包括钛酸钡(BaTiO3)和PMN-PT系列晶体。与布里奇曼法相比，这项技术有许多潜在的优势：(I)它避免了布里奇曼法所固有的浓度梯度；(Ii)就资本和消耗品支出而言，它可能比布里奇曼法更便宜；(Iii)可以生长更广泛的成分，包括难以从熔体中生长的不一致熔化的系统；(Iv)通过使用陶瓷预制件可以生长网状晶体。对这一过程的大部分研究以前是由韩国的一个学术小组进行的，该小组的首席研究员也是一家已将该过程商业化的公司的所有者。因此，公布的技术细节不足以复制这一过程，甚至可能是不可靠的。目前还没有被普遍接受的生长过程模型，如果这项技术要成功地应用于一系列材料，这将是无价的。因此，该项目最初将专注于了解生长过程，然后提出和测试优化生长所选成分的方法。尽管该项目的一个关键目标是展示PMN-PT基晶体的可重复性单晶生长，但为了了解生长机理，这种材料过于复杂。在SSCG所需的延长退火时间期间，PBO的挥发性是一个问题，这产生了一个时变参数。因此，旨在了解这一过程的最初工作将集中在BaTiO3上。由于在生长的晶体和多晶基质之间的界面上有少量的瞬时液相在SSCG中起作用，这可以在BaTiO_3中进行研究，因为在1320摄氏度时，BaO-TiO_2共晶允许通过控制TiO_2的过剩引入液相。此外，还将研究以下参数的影响：退火温度、基质中的温度梯度、基质的晶粒尺寸以及在晶体基质界面处的附加界面层。电子显微镜技术，包括扫描电子显微镜、透射电子显微镜和电子背散射衍射将被用来测量晶体的生长速度和了解晶体-基质界面的变化性质。一旦对该过程有了充分的理解，该模型将被应用于其他材料，如改性PbTiO3(例如，PMN-PT和Bi(Zn1/2Ti1/2)O3-PbTiO3)和实验的无铅材料。

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.