Let's Make Diamonds

This project was part of URECA 187 for the spring 1997 semester at the State University of New York at Stony Brook. The experiment was performed at the laboratories of the Center for High Pressure Research. Our mentors were Janet Niebling and Glenn Richard. From left to right in the picture are: Suzanne Wilkens, Valerie Joe, Stephanie Goren, Jessica Vassallo, Marilyn Macuha

Goal and Background

Carbon is polymorphic, meaning it can have more than one form. Carbon atoms can bond together to form either graphite or diamonds, or something else, depending on the organization of its atomic structure. Under extremely high temperatures and pressures the graphite can be converted to diamonds. The bonds between the carbon atoms are shorter in diamonds than the long bonds in graphite. The extreme pressure causes the compression, or shortening or the bonds, and then the rearrangement of atoms necessary to convert graphite to diamonds. This creates a more efficient packing of the atoms.

It is true that according to the laws of thermodynamics a diamond spontaneously changes to graphite under atmospheric pressure. The reason that diamonds can be found on the Earth's surface is because they are metastable. The change from diamonds to graphite takes place so slowly that it cannot be detected.

In this experiment, our goal was to prove that if graphite is placed under extreme conditions of high temperature and pressure, it would change to diamond.

The size of the Earth is about 12,750 kilometers in diameter; and is made up of three layers: crust, mantle, and core. The crust, the outermost layer, is rigid and very thin compared with the other two. The thickness of the crust beneath the continents average about 30 km; although, under large mountain ranges, the base of the crust can be as deep as 100 km. The Earth’s crust is brittle and can break.

Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. It is hotter and denser because temperature and pressure inside the Earth increase with depth. At the center of the Earth lie the core. Its composition is metallic rather than stony, making it nearly twice as dense as the mantle. The Earth’s core is actually made up of two distinct parts: the liquid outer core (2,200 km-thick) and solid inner core (1,250 km-thick).

Diamonds are one of the crystalline forms of carbon. They are formed deep within the Earth: between 100 km and 200 km below the surface, under conditions of high pressure (more than 30 kilobars), high temperature (more than 400 degrees Celsius), and in the absence of oxygen. They are carried to the surface in volcanic features called kimberlite pipes. To ensure that they are not converted to graphite, diamonds must be transported rapidly to Earth’s surface. Kimberlite magmas carrying diamonds erupt between 10 and 30/hr. Eruption velocity probably increases to several hundred km/hr with the last few kilometers toward the Earth’s surface.

Methods

First we must understand that diamonds are formed at a high temperature and high pressure. In order to create such conditions we use a machine called the Kennedy Press. The diamonds form in these conditions from graphite disks, a weaker structure of carbon atoms. Our sample had to be placed inside eight tungsten carbide cubes, to which we attached pyrophyllite gaskets that helped to contain our sample. We also added teflon tape to some of the sides of the odd-numbered cubes in order to control the electric current which is used to heat the sample.

To maximize the conditions of pressure we placed our sample inside of an octahedron. Inside the Kennedy Press the pressure is applied from above and below the sample to the first-stage anvils, which in turn, apply the pressure to the tungsten crabide cubes. The octahedron has a hole cut out, in which we placed our sample cup. Inside the sample cup we layered graphite disks and nickel-manganese catalyst. Around the sample cup we placed a graphite furnace to help heat the sample.

The octahedron containing our sample was then placed between the eight tungsten carbide cubes. We fastened the cubes together with six pieces of plastic on all sides. Two of them contained copper electrodes that allowed the electric current to pass through the sample. Finally the sample was placed in the Kennedy Press.

Results

The octahedron we retrieved was smooth and gray on the outside, and shiny with what we hoped to be diamonds on the inside. Since the shiny material was so small, it was necessary that we put it underneath a microscope and onto a TV screen in order to look closely at the shiny substance. We "snapped" it onto the computer and came to the conclusion that the shiny substance looked like diamond. The diamonds were extremely small, and what we saw was mostly graphite, with a layer of the shiny substance mixed with it.

We tested the hardness of the substance that we thought to be diamonds on the silicon carbide slate and found that it made a scratch. This means that the shiny substance is harder that silicon carbide, so it is almost certain that the shiny substance was diamonds.

Discussion

In the Let's Make Diamonds experiment, we were able to successfully make a shiny, hard material that resembled diamonds. We are pretty sure that the material made was diamonds. There were difficulties in determining whether the material we produced was actually made up of diamonds. One of these tests was the hardness test, which involved trying to scratch a piece of silicon carbide with the material we made. Ordinarily, diamonds would be able to scratch the silicon carbide, but in our situation the diamond-like material and graphite were combined in one solid material. So as we tried to scratch our material on the silicon carbide, the graphite would crumble into pieces. It was hard to tell if there were scratches in the silicon carbide because we were not able to obtain large continuous scratches. Although there was some evidence of scratches in the silicon carbide. The appearance of the material we produced resembled diamonds when observed under the microscope. Even though there were difficulties in determining whether the material produced were diamonds, we believe that there is enough evidence to conclude that the material we produced contained some diamonds.