Microstructural engineering of piezoelectric composites

该项目将创造新的制造方法，使用冷冻铸造法结合滑动和胶带铸造，生产具有微结构的压电复合材料，其压电性能超过现成材料的性能，同时具有传统制造方法的优势。2019年，压电陶瓷的全球市场价值为196亿美元，预计未来十年，该市场将在能源收集、物联网相关传感器和压电复合材料领域增长。压电复合材料对英国的国防（声纳）和公共卫生（医疗超声）部门至关重要，同时在运输和能源工业中得到广泛应用。开发生产高性能压电复合材料的新方法在材料成本和制造以及设备性能方面都具有显著的优势，可以实现低成本制造定制的压电材料，并根据所需的应用进行性能调整。冷冻铸造是一种控制多孔材料微观结构的有效方法，通过在溶剂晶体上模板孔隙，其生长和形态取决于加工过程中的温度梯度和冻结行为。这些多孔微结构，例如多孔压电陶瓷，可以用聚合物第二相渗透，以改善机械和电气性能。压电复合材料的性能在很大程度上取决于电场和机械场之间的局部相互作用以及材料结构在一定长度范围内的相互作用，从铁电畴（亚微米）到复合材料的宏观结构（毫米级）。在这个项目中，目的是增加对压电复合材料中机电场/材料相互作用的理解，并设计微结构来开发相应的有益效果。这将通过开发先进的数值模型来支持，以帮助微观结构/制造工艺设计，并提供对项目期间制造的材料性能的实验观察的见解。与目前用于生产商用压电复合材料的技术相比，将要研究的方法具有几个优点。首先，材料可以以接近净的形状生产，减少了加工后的工艺或手工纤维铺设，这是通过dice-/排列和填充工艺制造的宏观纤维复合材料的常见工艺。其次，虽然还没有实现，但理论上可行的控制水平，通过利用冻结过程来模板微结构，提供了制造具有定制特性的材料的潜力，可以针对特定应用进行调整，从而产生压电、介电和机械性能的优化组合，以促进有源压电和更宽器件之间的机电耦合。第三，使用冷冻铸造引入的微观结构特征的长度尺度缩小，例如，与掷块填充复合材料相比，可能为工程设计压电陶瓷基体的固有特性提供了一条途径。使用水作为冷冻剂意味着这些工艺对环境的影响很小，并且具有与致密压电陶瓷相当性能的近净形状、优化的复合微结构将首先减少所需原材料的体积。

This project will create novel fabrication approaches, using the freeze-casting method combined with slip- and tape-casting, to produce piezoelectric composites with microstructures tailored to yield piezoelectric properties that exceed the performance of off-the-shelf materials, whilst providing advantages over traditional manufacturing methods. The global market for piezoelectric ceramics was valued at $19.6 billion in 2019 and is expected to grow in the areas of energy harvesting, IoT-related sensors and piezoelectric composites in the next decade. Piezoelectric composites are critical to the UK's defence (SONAR), and public health (medical ultrasound) sectors, as well as being used widely in the transport and energy industries. Developing new methods for producing high performance piezoelectric composites represents a significant benefit in terms of materials cost and manufacture, as well as device performance, by enabling low-cost fabrication of bespoke piezoelectric materials with properties tuned depending on the desired application.Freeze casting is an effective method for controlling the microstructures of porous materials, whereby pores are templated on solvent crystals whose growth and morphology depends on temperature gradients and freezing behaviour during processing. These porous microstructures, e.g. porous piezoelectric ceramics, can then be infiltrated with polymer second phases to improve mechanical and electrical properties. The properties of piezoelectric composites depend strongly on local interactions between electric- and mechanical fields and the material structure over a range of length scales, from ferroelectric domains (sub-micron) through to macro-structure (on the order of millimetres) of the composites. In this project, the aim is to increase the understanding of these electromechanical field/material interactions in piezoelectric composites and design microstructures to exploit beneficial effects accordingly. This will be underpinned by developing advanced numerical models to both aid with microstructural/fabrication process design, and provide insight into experimental observations of the properties of materials fabricated during the project. The methods that will be investigated offer several advantages over current techniques used to produce commerically available piezoelectric composites. Firstly, the materials can be produced at near-net shape, reducing post-machining processes or manual fibre lay up common for macro-fibre composites fabricated by dice-/arrange-and-fill processes. Secondly, the level of control that is theoretically possible, although not yet realised, by utilising freezing processes to template microstructures, provides the potential to fabricate materials with bespoke properties tuned to specific applications, yielding an optimised combination of piezoelectric, dielectric and mechanical properties to promote enhanced electromechanical coupling between the active piezoelectric and the wider device. Thirdly, the reduced length scale of microstructural features introduced using freeze casting, compared to dice-and-fill composites for example, may provide a route to engineering the inherent properties of the piezoelectric ceramic matrix. Using water as a freezing agent means these processes have a low environmental impact, and near-net shape, optimised composite microstructures with comparable performance to dense piezoceramics will reduce the volume of raw material required in the first place.