Development of barocaloric materials for next generation refrigerants

自1994年淘汰氟氯化碳以来，氢氟碳化物(HFCs)已成为氯氟烃(CFCs)事实上的替代品，主要用于供暖、通风和空调设备。由于自身的毒性问题和对环境的破坏性影响，美国和欧盟现在寻求逐步淘汰HFC的使用。除了这些崇高的理由外，制冷业目前占世界电力消耗量的17%；因此，在经济和环境意义上，任何效率的提高都将是受欢迎的。寻找氢氟碳化合物的替代品带来了重大的技术和科学挑战。理想情况下，任何新技术都应该来自可持续的来源，并提供比当前做法更高的效率和环境证书。近年来，开发具有热效应的固体材料成为研究的热点。固体材料的制冷是由外加电场引起的大等温熵变和大的绝热(孤立系统)温度变化引起的。外场可以采取磁场(磁热)、电场(电热)或流体静压(气压)的形式。虽然磁和电热效应需要很大的磁场或电场，而磁场或电场的产生依赖于稀土元素，但压力的产生不适用于此。因此，原则上，基于大气压(BC)效应的应用对商业实现的限制较小。采用BCS比现有制冷系统潜在节省的能源为1260太瓦时。材料中的BC效应是通过对材料施加外部压力来释放的。这会导致结构变化，并伴随着温度的上升，就像你拉伸弹性橡皮筋使其升温一样。这种固体-固体相变的过程可以像现有的蒸汽压缩技术一样循环使用，作为制冷剂使用。到目前为止，几乎没有发现具有BC效应的材料，那些确实因类型而异的材料，从金属合金到聚合物和塑料晶体。这意味着，尽管发表的BC材料很少，但它们肯定比最初认为的更广泛。该奖学金的范围是使用计算和实验相结合的方法来搜索、理解和控制多态材料的BC响应。我有将材料化学的计算方法和实验方法结合起来的经验，并发现这种互补性对于充分理解结构变化以及这些变化的能量学是必不可少的。该项目将扩展我们的固态材料库，建立在我们对如何最大化BC效应的新理解基础上。具体地说，我将设计能够调整其工作温度的材料，因为工业需要广泛的温度控制环境。最终目标是汇编在不同温度下具有BC响应的材料组合，这些材料可以在固定温度下作为制冷剂和冷却剂进行商业应用。这些材料将是无毒的，易于处置，而且比当今技术的现状更有效。固态BC材料作为制冷剂的发展将：(1)减少与制冷行业相关的温室气体排放。(2)创造出比目前基于气体/液体的技术更容易处置/回收的固态材料。(3)提高换热效率，减少制冷能源需求。(4)提高对通过相变控制材料性能的设计原理的认识，该设计原理适用于许多领域，包括制药、热电池和热致变色材料。

Hydrofluorocarbons (HFCs) have become the de facto alternative to chloroflurocarbons (CFCs), since CFC phasing out in 1994, and are used primarily in heating, ventilation and air-conditioning equipment (HVAC). The US and EU now seek to phase-down HFC use due to their own toxicity issues and damaging environmental impact. In addition to these noble reasons, the refrigeration industry currently accounts for 17 % of the world's electricity consumption; any increase in efficiency would therefore be welcomed in both an economic and environmental sense. Finding alternatives to HFCs has created a major technological and scientific challenge. Ideally, any new technology should be made from sustainable sources and offer increased efficiencies and environmental credentials over current practices. Recently, there has been a strong focus on developing solid state materials which demonstrate caloric effects, where refrigeration is caused by an external field which induces a large isothermal entropy change and large adiabatic (isolated system) temperature changes. The external field can take the form of a magnetic field (magnetocaloric), electric field (electrocaloric) or hydrostatic pressure (barocaloric). While, magneto- and electrocaloric effects require large magnetic or electric fields, which are reliant on rare-earth elements for their generation, the same does not apply to the generation of pressure. Thus, in principle, applications based on the barocaloric (BC) effect will have less limitations for commercial realisation.The potential energy savings through the adoption of BCs over current refrigeration systems has been calculated to be 1260 terawatt-hours. The BC effect in materials is unlocked via the application of external pressure to the material. This causes a structural transformation which is coupled with an increase in temperature, much like a when you stretch an elastic rubber band causing it to heat up. This process of a solid-solid phase transition can be cycled like the established vapour-compression technology to work as a refrigerant. To date few materials have been found to have the BC effect, and those that do vary wildly by type, ranging from metal alloys, to polymers and plastic crystals. This means that although there are few published BC materials, they must be more widespread than first thought.The scope of this fellowship is to use a combined computational and experimental approach to search, understand and control the BC response of polymorphic materials. I have experience of combining both computational and experimental methods in materials chemistry and have found that this complementarity is essential in order to fully understand structural changes as well as the energetics of those changes. The project will extend our library of solid-state materials built from our new understanding of how to maximise BC effects. Specifically, I will design materials to be able to tune their working temperatures, as industry requires a wide range of temperature-controlled environments. The ultimate goal is to compile a portfolio of materials which have BC responses at different temperatures which can be explored for commercial application as refrigerants and coolants at fixed temperatures. These materials will be non-toxic, easy to dispose of and more efficient than the status-quo of today's technology. The development of solid-state BC materials as refrigerants will:(1) Reduce the greenhouse gases emissions associated with the refrigeration industry.(2) Create solid-state materials which can be disposed/recycled more easily than current technologies based on gases/liquids.(3) Improve efficiency of the heat transfer, reducing refrigeration energy demands.(4) Improve the knowledge of design principles for controlling materials properties via phase changes which is applicable to many areas including pharmaceuticals, heat batteries and thermo/piezochromic materials.