Using ferroelectric domain walls for active control of heat flow at the nanoscale

为了满足社会对微电子器件性能的持续改进的需求，晶体管小型化是一种持续的驱动力，从而可以最大化空间封装密度。然而，相关工作功率密度的增加会导致发热量增加，芯片上温度升高，从而影响器件的可靠性能。这是一个巨大的技术挑战，显然需要识别和表征具有新的热性能的材料，使其能够在纳米级进行卓越的热能管理。特别是，利用外部刺激(如电压)主动控制热流的能力可能会对下一代微电子产品的热管理需求和寿命产生重大影响。在这一点上，氧化物铁电材料提供了一个令人兴奋的机会。在铁电材料中，存在着原子上尖锐的结构界面，称为磁畴壁(DWS)，它通过破坏热振动来阻碍热流。DWS的独特之处在于，通过施加电压或压力，它们能够以完全可逆的方式在材料内部创建、擦除或重新定位。这一特性提供了一种前所未有的方法来主动控制热流，因为它能够在给定时间改变材料中存在的DW的数量和它们的排列方式。然而，要实现利用DWS的热流控制，首先必须确定DWS在不同材料中提供的界面热阻的最终估计。因此，该项目的主要目标之一是通过直接测量导热系数来量化DW热阻。然后将识别具有有效抑制热流的DW的铁电材料系统。在此之后，将制造原型热设备，其中通过使用施加的电压来可逆地改变DW图案来改变通过材料的相对容易的热流。这也将为创造一种更具异国情调的纳米结构热镜设备的长期研究愿景奠定基础。在这种情况下，可以设想DWS可以被设计成充当热波的周期性反射器，以便最大限度地拒绝热能，就像光如何被介质镜中的多层高效反射一样。在过去的十年中，可以清楚地看到，DWS可以被认为是一种新型的片状功能材料，其性能可以与块状材料显著不同。例如，当主体相对绝缘时，DW中的导电可以是金属的，甚至可以是超导的。已经制作了原型有源器件，其中功能完全来自于导电DWS的部署。然而，狭窄的DW区域可能完全具有其自身的热性质的补充想法是全新的和未被探索的。在进行DWS时，由于额外的热载体(如移动电子)的存在，热流很可能会得到加强，并将进行热导率测量以证实这一点。由于最近的预测表明，与块体相比，导电DWS中的热电功率可以提高高达100%，因此还将探索将废热转化为电能的导电DWS。总体而言，铁电DWS是作为热设备中的有源元件使用的令人兴奋的候选材料，因为DWS可以在功能上增强或限制热流。然而，这两起案件目前都没有得到很好的描述和理解。这些DW与生俱来的可重构性意味着，基于铁电材料设计和制造新型有源热敏器件具有真正的潜力，但尚未得到充分利用。

In order to satisfy societal demand for continual improvements in microelectronic device performance, there is an ongoing drive for transistor miniaturisation so that spatial packing densities can be maximised. However, the associated increases in operational power density leads to increased heat generation and rises in on-chip temperature that can prevent reliable device performance. This represents a tremendous technological challenge and there is a clear need to identify and characterise materials with novel thermal properties that will enable superior thermal energy management at the nanoscale. In particular, the ability to actively control heat flow with an external stimulus (e.g. voltage) could have dramatic implications for the thermal management demands and lifetimes of next generation microelectronics. In this regard, oxide ferroelectric materials present an exciting opportunity.In ferroelectric materials, there exist atomically sharp structural interfaces called 'domain walls' (DWs) that are known to impede heat-flow by disrupting thermal vibrations. What is unique about DWs is their remarkable ability to be created, erased or repositioned inside the material in a fully reversible way by using applied voltages or pressure. This property provides an unprecedented means to actively control heat flow by being able to alter the number of DWs present in the material at a given time and the way in which they are arranged. However, to realise heat flow control using DWs, definitive estimates for the thermal interfacial resistance presented by DWs in different materials must first be determined. Therefore, one of the main goals of this project is to quantify DW thermal resistances through direct thermal conductivity measurements. Ferroelectric material systems having DWs that effectively inhibit heat flow will then be identified. Following this, prototype thermal devices will be fabricated where the relative ease of heat flow through the material will be changed by using applied voltages to reversibly alter the DW pattern. This will also provide the foundation for a longer-term research vision to create a more exotic nanostructured 'thermal mirror' device. In this case, it is envisaged that DWs can be engineered to behave as periodic reflectors of thermal waves in order to maximise the rejection of thermal energy, much like how light is reflected with high efficiency by the multiple layers in a dielectric mirror. Over the last decade, it has become clear that DWs can be considered as a new type of sheet-like functional material with properties that can be remarkably different than bulk. For example, electrical conduction within DWs can be metallic, or even superconducting, when the bulk is comparatively insulating. Prototype active devices have been fabricated where functionality is derived entirely from deployment of electrically conducting DWs. However, the complementary idea that the narrow DW region may have thermal properties entirely of its own is completely new and unexplored. Within conducting DWs, it is likely that heat flow will be enhanced, due to the availability of extra heat carriers (e.g. mobile electrons), and thermal conductivity measurements will be carried out to confirm this. Conducting DWs will also be explored for conversion of waste heat into electricity since recent predictions indicate that the thermoelectric power can be enhanced by up to 100% within DWs, compared to bulk.Overall, ferroelectric DWs are exciting candidates for use as the active elements in thermal devices since the DWs may behave functionally to either enhance or restrict heat-flow. However, neither case is currently well characterised nor understood. The innate reconfigurability of these DWs means there is real potential to design and build new types of active thermal devices based on ferroelectric materials that has yet to be capitalised upon.