High-pressure cells for low-temperature neutron scattering at ISIS

我们建议将高压、低温和中子衍射技术结合起来研究材料，使ISIS能够实现高压和低温的新极限。在高压下研究材料的能力使研究人员能够修改它们的许多特性，并重现存在于地球深处或其他行星上的条件。研究固体中的磁性和其他量子现象需要非常低的温度。中子散射是研究磁性和许多结构有序现象的理想工具。提出的研究重点是通过开发适合于中子衍射研究的高压电池，将这三个组成部分结合起来，工作温度低于绝对零度的十分之一度，同时达到10 GPa或更高的压力。压力单元的设计将基于反砧原理，其中所研究的样品被挤压在两个硬砧之间，同时由其侧面的金属垫圈支撑。对于中子散射来说，在中子束中尽可能多的取样是很重要的，因此这些压力电池将需要大的砧。已知最硬的材料是钻石，但大钻石非常昂贵，而且可能无法获得。因此，我们建议在初始阶段使用相对便宜的大型碳化硅（莫桑石）砧。碳化硅的莫氏硬度为9.25（金刚石的莫氏硬度为10），据报道莫桑石压力电池的压力高达60 GPa（约600,000 atm）。为了实现这一点，重要的是要优化砧的形状，以便在不破裂的情况下获得最大可能的压力，并且它们彼此之间的对齐和压力单元是完美的。因此，部分拟议的研究将集中在使用强大的有限元分析（FEA）技术来优化砧的形状。使用有限元分析不仅有助于加快项目速度，找到合适的砧形，而且可以在试验中最大限度地减少砧的破损。FEA与计算机辅助设计（CAD）相结合，也将有助于设计压力单元本身，并确保它们适合低温设备，并与探测器对齐。所提出的压力电池的另一个重要特征是，当电池在低温恒温器内时，操作人员能够改变压力，同时通过光纤电缆测量电池内的压力。这对于在恒定低温下改变压力是非常有用的，这是目前技术无法做到的。这节省了时间，因为不需要将细胞加热到室温来改变压力，然后在每个新压力下重新调整细胞内的样品。该电池最初将用于研究模型系统中的有序跃迁，如多铁性材料、表现出巨大磁电阻的锰氧化物和具有复杂自旋结构的磁性稀土氧盐。

We propose to combine high-pressure, low-temperature and neutron diffraction techniques for studies of materials, enabling ISIS to achieve new extremes of high-pressure and low-temperature. The ability to study materials at elevated pressures allows researchers to modify many of their properties and to recreate the conditions existing in deep earth or on other planets. Very low temperatures are needed to study magnetic and other quantum phenomena in solids. Neutron scattering is the ideal tool for magnetic and many structural ordering phenomena. The proposed research is focused on combining these three components by developing high-pressure cells suitable for neutron diffraction studies working at temperatures down to a few tenths of a degree above absolute zero while achieving pressures of 10 GPa or more. The design of the pressure cells will be based on the opposed-anvil principle, in which the studied sample is squeezed between two hard anvils while supported by a metal gasket on its sides. It is important for neutron scattering to have as much sample as possible in the neutron beam and so large anvils will be needed for these pressure cells. The hardest known material is diamond, but large diamonds are forbiddingly expensive and may not be available. We therefore propose to use in the initial stages relatively inexpensive large silicon carbide (moissanite) anvils. Silicon carbide has a Mohs hardness of 9.25 (diamond has 10 on the same scale) and moissanite pressure cells are reported to achieve pressures of up to 60 GPa (approximately 600,000 atm). To accomplish this, it is important that the shape of the anvils is optimised in order to achieve maximum possible pressures without breaking and that their alignment with respect to each other and the pressure cell is perfect. Therefore, part of the proposed research will be focused on using the powerful technique of finite element analysis (FEA) for optimising the shape of the anvils. Using FEA will not only help to speed up the project and find the right shape for the anvils but will minimise anvil breakages in trial experiments. FEA, combined with computer aided design (CAD), will also help to design the pressure cells themselves and make sure that they will fit into the cryogenic equipment and are aligned with the detectors. Another important feature of the proposed pressure cells is the ability of the operator to change the pressure while the cell is inside the cryostat and measure the pressure inside the cell at the same time via a fibre optic cable. This will be very useful for changing pressure at constant low temperature, which is not possible with present technologies. This saves time in not having to warm the cell up to room temperature to change pressure and, then having to realign the sample inside the cell at each new pressure. The cells will initially be used to study ordering transitions in model systems such as multiferroic materials, manganese oxides exhibiting colossal magnetoresistance and magnetic rare earth oxosalts with complex spin structures arising from frustration.