Introduction to MEng Thesis Project
Problem Statement
There are a number of experiments that require high pressures to be exerted on relatively large samples. To do experiments like these diamond anvils are forbiddingly expensive. Thus, it has been proposed that sapphire crystals (Al2O3) could be an appropriate gem-quality substitute as they are sufficiently strong and transparent. Spherical sapphire ball bearings are commercially produced and readily available, thus much less expensive, solving the cost problem posed by diamonds. Moreover, there could be advantages of sapphire’s material properties for anvil use over diamonds. The spherical shape of the ball bearings could also potentially provide structural benefits. The use of sapphire ball bearings in high-pressure research has never been property explored. This project will discuss the advantages and disadvantages of sapphire ball bearings as anvils and provide Finite Element Analysis (FEA) to show their suitability. This analysis will also consider methods for optimisation of sapphire anvil geometry and support conditions.
AimsPrimary Aims:
Secondary Aims:
|
ObjectivesPrimary Objectives:
Secondary Objectives:
|
Anvil AssemblyThe diagrams opposite show the proposed spherical sapphire anvil assembly. The sapphires are of 5mm diameter with 1mm diameter machined face on each. Due to the anisotorpic nature of sapphire, the working face will be machined perpendicular to sapphire's strongest axis - its so-called c-axis. The tungsten carbide supports are of dimensions shown and a viewing hole of 1mm diameter at the back of each anvil which extends at an angle of 30 degrees outwards on either side. The gasket is of diameter 2mm with a 0.5mm hole in its centre for the sample, ruby and sample medium.
|
Material Properties
- Sapphire ball bearings, as mentioned previously, are commercially and readily available as well as relatively cheap. The sapphires used for this study are based on those manufactured and sold by the global-known company Kyocera who specialise in speciality materials for use in solar panels, technological equipment and kitchenware.
- The support must be very strong as stresses will propagate through the assembly and into the support. Tungsten Carbide has been chosen for use as the support material due to its very high strength and stiffness allowing it to provide sufficient suport for the sapphire anvil.
- The gasket is a thin ductile material, usually a metal, with a hole drilled through the middle. The principal purpose of the gasket is to hold in place the pressure medium containing the sample and test ruby. However, it has a secondary purpose; on loading the gasket must partially flow out from between the anvil faces and slightly around them to act almost like a passive girdle. It makes a so-called cup around the working face of each anvil which provides it with a small amount of lateral, or flank, support allowing the anvil to withstand even higher pressures. It is very important to choose the correct gasket material in order to fulfill these two purposes. Beryllium Copper, supplied by NGK Berylco is softened by heat treatment making it an ideal material for sapphire anvil gaskets. It will under go sufficient bilinear hardening to allow it to flow outwith the anvil faces under working conditions.
- Salt is an appropriate pressure medium as it is able to hold the sample being tested while deforming plastically around it as pressure is exerted.