Solid State Growth of Piezoelectric Single Crystals

虽然大多数压电应用依赖于压电氧化物的多晶陶瓷形式，但尺寸为1至10 cm的单晶正在获得市场份额。其中典型的是基于Pb（Mg 1/3 Nb 2/3）O3-PbTiO 3（PMN-PT）的晶体。在医学成像和声纳应用中，晶体的增强压电特性在更大的带宽和分辨率、更高的灵敏度以及更小的尺寸和功耗方面提供了优势。主要的生长技术依赖于熔体的定向凝固，称为布里奇曼生长。然而，这种晶体具有两个主要缺点：（i）由于生长速率慢和使用大量铂作为坩埚，它们是昂贵的，和（ii）由于某些成分在熔体和固体形式之间的分配，它们是固有的不均匀的，导致晶体中的组成梯度。其中将小籽晶放置成与多晶体接触并退火。与晶种接触并在晶体学上与晶种对准的多晶体的晶粒以其他晶粒为代价生长，并最终聚结以形成大的单晶。该过程已被证明为几种材料，包括钛酸钡（BaTiO 3）和PMN-PT家族的晶体。该技术与布里奇曼生长相比具有许多潜在的优点：（i）它避免了布里奇曼生长中固有的浓度梯度;（ii）就资本和消耗品支出而言，它可能比布里奇曼生长便宜;（iii）可以生长更宽范围的组合物，包括难以从熔体生长的不一致熔化的系统;（iv）利用陶瓷预制坯体可生长网状晶体。有关这方面的研究，大部分由南韩的一个学术团体进行，而该学术团体的主要研究者亦是一间公司的拥有人，该公司已将这一过程商业化。因此，公布的技术细节不足以复制这一过程，甚至可能不可靠。目前还没有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，这将是非常宝贵的。因此，该项目将首先关注于了解生长过程，然后提出和测试所选成分的最佳生长方法。尽管该项目的主要目的是展示PMN-PT基晶体的可重复SSCG，但这些材料对于了解生长机制来说过于复杂。PbO的挥发性在SSCG所需的延长退火时间期间是一个问题，产生了随时间变化的参数。因此，旨在了解该过程的初步工作将集中在BaTiO 3上。由于在生长晶体和多晶基质之间的界面处的少量瞬态液相已知在SSCG中起作用，这可以在BaTiO 3中进行研究，因为在1320摄氏度下的BaO-TiO 2共晶允许通过控制TiO 2过量来引入液相。此外，以下参数的影响将被调查：退火温度，温度梯度的矩阵，晶粒尺寸的矩阵和附加的界面层在晶体基质界面。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，将用于测量晶体生长的速率，并了解晶体-基体界面的变化性质。一旦对该过程有了充分的了解，将通过将其应用于其他材料来测试模型，例如改性PbTiO 3（例如PMN-PT和Bi（Zn 1/2 Ti 1/2）O3-PbTiO 3）和改性KNbO 3族中的实验无铅材料。

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.