The fringes alignment of diamonds anvil faces is a highly delicate procedure, decisive in reach high temperature and in order to avoid the premature failure of diamonds under high pressures. Its aim is to position the diamond faces parallel to each other.
Looking with a microscope through one of the DAC diamonds, the operator moves the discs in which the diamonds are set to achieve a coarse translational alignment. Then, after a first rough parallelism achieved through a digital caliper, the flat faces of the anvils are delicately moved towards each other and put into contact. At this point, parallelism can be tested by looking at the interference fringes generated by white light coaxial illumination. If the light source is large and the distance between the diamonds small, fringes are visible also with non-monochromatic light. The fringes show the regions of the diamond faces which are separated by a distance multiple of
If the two diamonds prove perfectly parallel, no interference fringe is present. In order to achieve this condition, the orientation of the hemisphere has to be adjusted with the aid of referred screws. The alignment is considered perfect when the fringe width is greater than the anvil flat diameter and no fringe is visible.
For a culet of 500 µm in diameter, the wedge angle measures less than 1 mrad. At this point, the fine adjustment of the translation of the two discs holding the diamonds can be performed without compromising the tilting procedure, and the alignment procedure is at an end. The mechanical stability of the diamond anvils’ alignment is maintained under pressure during all operation phases, including load application. The alignment of the anvils’ working surfaces is central to the achievement of ultra-high pressures. Also the concentricity of flat diamond surface are fundamental to reach very high pressure.