In recent years there has been a steady increase in the maximum static pressures attainable, from -35 GPa in the 1970’s up to a maximum reported pressure of 550 GPa in 1986.1m3 These incredible records have been achieved
through refinements of diamond-anvil-cell (DAC) technology.z3 All DACs generate pressures by compressing samples between the faces of two diamond anvils. Generally, a gasket formed out of a hard metal is used to contain the
samples on the diamond tip (culet), the region of maximum pressure. A typical gasket is produced by first preindenting a sheet of rhenium to a thickness of 20-50 pm.4 A small hoIe is then drilled that is centered on the culet imprint in the deformed sheet. A general trend leading to higher pressures has been the use of smaller culet faces in the diamond anvils. For instance, diamonds with culet flats of 50 pm are routinely used in multimegabar experiments, requiring gasket hole diameters as small as 25 pm.5 Complicating the gasket making process are sample stability considerations, specially crucial for soft systems such as HZ.’ Unstable samples will migrate under load from the center to the edge of the culet, limiting the highest pressures of the experiment and often causing premature failure of the anvils. One key factor to enhancing stability of soft samples is the precise centering of the hole relative to the culet face. We note that making a small (-25 pm) hole within 3 pm of the culet center is extremely difficult and can require several attempts even under ideal circumstances.
Commonly used techniques for drilling small holes have significant drawbacks. The most common method suited for large holes (greater than -100 pm) is mechanical drilling, but this process becomes increasingly difficult to implement with decreasing hole diameter. A zoom microscope (80X magnification), drill press, and small-diameter tungsten carbide bits are the required equipment. The microscope and the drill press currently cost about $3200 (U.S.). A reasonable supply of bits can also be very costly; for example, 25-w-diam tungsten carbide bits cost -$llO each.7 Because of their fragility, one may expect to break about 4 bits in machining one small diameter (25 m) hole. Thin shafts also flex markedly during drilling, severely complicating the centering procedure.
Laser drilling is another method for preparing holes but, of course, is dependent on the availability of a high-power laser with appropriate focusing optics. A typical high-power Nd:YAG or CO, laser used in these applications costs -$20 000 and a typical optical microscope setup is -$lO 000. Despite its ease of use, this technique is not a viable alternative for many laboratories because of its high initial cost. Also, there are important safety issues related to the use of invisible radiation at extremely high powers.
The third alternative that we discuss here is the electric discharge machine (EDM). The EDM is well suited for cutting arbitrary shapes in metals that are difficult to machine due to hardness and/or brittleness.8p9 The EDM process was described in the 1940’s by the Lazarenko’s,” and since then numerous refinements have been discussed in the Iiterature with regard to particular applications.“-‘4 In this process, a tool and a sample of different voltage polarity are brought into close proximity initiating a spark across the gap. With enough available energy, this spark can melt a small portion of the sample. By iterating this erosion step, intricate shapes can be machined into metals. The process is in general slower than either mechanical or laser drilling. Since the process can be applied to either soft or hard metals, the EDM is ideally suited for drilling high-pressure gasket materials. Unfortunately, commercial machines having the required high tolerances needed for diamond anvil cell applications can cost upwards of $10 000. Motivated by a desire to drill out small holes efficiently and inexpensively, we developed an EDM that is well suited for this task and can be easily built in any laboratory.
We have designed and built our system with specific abilities in mind: to drill holes, small or large, which have precisely defined geometries, to center holes with 2-3 ,um tolerances reproducibly, and to handle any metal, specifically hard-to-machine metals such as rhenium and Inconel. We will describe an apparatus that not only meets these goals but that can also be built for at most $1700.
