Graphite crystal defects Structural analysis of HOPG

At Keio Techno-Mall in December 2009, there was an exhibit on graphite crystal defects, by Assistant Professor Chiba from the Tsuji Laboratory in the Department of Physics.

Graphite, also known as black lead, is a form of carbon, like diamond but with a different structure. Graphite is familiar from its use in pencils, but its also used as a moderator in nuclear reactors.

Q. In nuclear reactors, fast-moving neutrons are produced, and graphite is used to stop them. Recently, most reactors have been the light-water type, using water as the moderator. But its said that from now on, graphite-moderated reactors may become the norm. When a particle beam hits graphite, it creates defects in the graphite. These defects cause problems, such as changes in the size of the moderator.

Accordingly, Professor Chibas group are studying defects in crystalline graphite, using a variety of methods. In particular, theyve succeeded in measuring the state of angstrom- or nano-scale defects, by using the very low irradiance of 0.0001 dpa.

Q. Companies that make graphite manufacture a variety of products. Were investigating which ones are suitable as moderators, or how to make them suitable for use as moderators. So we hope our results can be reflected in graphite products.

The exhibit included an experimental demo of magnetic levitation using graphite.

Magnetic levitation using graphite is sometimes demonstrated using special graphite called HOPG (highly ordered pyrolytic graphite), in which the crystal axes are aligned. But actually, it can be done with an ordinary pencil lead, like this, if the shape of the graphite is carefully controlled.

Q. Were using a pencil lead, which is a very familiar object, and neodymium magnets, which are quite strong. This poster shows how the levitation experiment can be done at room temperature. It uses a property called diamagnetism. In fact, as water is also diamagnetic, its even possible to levitate a frog in a powerful magnetic field.

The absolute value of magnetic susceptibility in graphite is small, so in practice, its difficult to use diamagnetic materials for levitation directly. But they can be used to stabilize magnetic levitation.

According to Earnshaws theorem, the force varies inversely with the square of the distance. So it isnt possible to levitate objects stably. But by placing an object between two diamagnetic bodies, a point of stability is created, making it possible to achieve stable magnetic levitation without intentional feedback.

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Science & Technology

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• chiba
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[AVAMagneticlev]

Hello have you looked at a Pyrolytic Graphite for testing as a moderator its uniformed structure may help reduce size deformation.