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

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Engineering

Abstract

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.

Publications

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Bocian A (2010) Gas loading apparatus for the Paris-Edinburgh press. in The Review of scientific instruments

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Fang J (2010) A rotator for single-crystal neutron diffraction at high pressure. in The Review of scientific instruments

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Oka K (2010) Pressure-induced spin-state transition in BiCoO3. in Journal of the American Chemical Society

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Ridley C (2014) High pressure neutron and X-ray diffraction at low temperatures in Zeitschrift für Kristallographie - Crystalline Materials

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Wang W (2011) Large volume high-pressure cell for inelastic neutron scattering. in The Review of scientific instruments

 
Description We developed a novel device for neutron scattering experiments at low temperatures. This pressure cell has a unique combination of features which allows it to be used efficiently for experiments at large scale facilities.

The key component of the cell is a custom-designed bellow built based on the technology used in the "joints" of the Mars Rover. The bellow can withstand applied gas pressures of up to one kilobar expanding by several millimetres which is essential for load generation on the sample to be studied. Along the way there we explored a number of various options related to the ways the pressure is generated and transmitted to the cell placed in cryogenic environment. We explored mechanical means of generating load are unsuitable as they tend to create large thermal linking between the ambient conditions and the working volume of the cryostat. We have also explored use of piston-cylinder hydraulic designs for generating pressure in the cell using compressed gas but found that the seals on the piston work unreliably at low temperatures due to differences in thermal expansion.

Another feature developed and integrated into the pressure cell is the fibre optic cable and focusing optics which allow pressure to be measured in situ during the experiment. This is a great improvement compared to the existing clamped designs in which pressure cells need to be warmed up and taken outside cryostats for pressure to be changed. Such pressure changes lead to loss of beamtime during the neutron experiments.

The pressure cell uses large sapphire anvils for generating pressure on samples and we performed finite element analysis of stresses and deformations of these anvils in order to optimise their geometry and make them withstand higher loads.
Exploitation Route The pressure cell has generated interest at international large scale facilities and can be used universally with a number of neutron scattering instruments.

Our work on the design of the bellow would be on interest in dynamic applications involving moving parts in extreme environments (high pressure and low temperature).

Finite element modelling of hard brittle sapphires will be of interest to manufacturers of specialised windows for high-pressure applications and developers of tools made of hard ceramic materials.
Sectors Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology,Other

 
Description The major impact of our research was in reducing waste of time on conducting high-pressure experiments at the ISIS neutron facility by removing the need to interrupt experiments in order to change pressure. The pressure cell has also been also used for studies of properties of novel functional materials.
First Year Of Impact 2013
Sector Education
Impact Types Economic

 
Description Instrument development grant
Amount £52,609 (GBP)
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 09/2010 
End 09/2013
 
Description Standard grant
Amount £434,080 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2009 
End 09/2012
 
Description Standard grant
Amount £897,965 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2009 
End 02/2012