Science: Heat, Lengthwise

Graphite, the substance in lead pencils, is a form of carbon that has long been one of the most useful minerals in the scientific laboratory and in industry. It is soft enough to be a good dry lubricant; its high heat resistance makes it a good material for crucibles and as a moderator in nuclear reactors. In the new age of rocketry, scientists have eyed it for use in rocket nozzles or in nose cones, which must resist the heat of reentry. But ordinary graphite has two faults: it is permeable to gases and is structurally so weak that it crumbles when subjected to high-velocity rocket exhaust.

Three years ago, Raytheon Co. of Waltham, Mass, set out to see what it could do to cure these shortcomings. Its scientists started with the knowledge that when carbon-rich gases are put in a lab furnace and decomposed by high heat, they sometimes deposit carbon in the form of a peculiarly dense graphite. At first this stuff was only a laboratory curiosity, and for a long time no one made it in quantity or thoroughly tested its properties. But after considerable experimentation, Raytheon's furnaces yielded a hard, impermeable, layered material that looks like black porcelain. Called Pyrographite, it proved to be five times as strong as ordinary graphite, keeps its strength at temperatures up to 6,700° F., also has the extraordinary property of conducting heat 100 times better along its main surfaces than perpendicular to them.

This peculiar behavior is explained by the structure of graphite crystals, whose carbon atoms are arranged in sheets one atom thick. When the sheets are stacked up in a crystal, the distance between the atoms in adjoining layers is more than twice as great (3.35 angstroms*) as the distance between the atoms in the individual sheets (1.42 angstroms). In ordinary commercial graphite, microscopic crystals are jumbled almost at random, but in Pyrographite they are mostly aligned with their sheets parallel (see diagram). This builds up a layered structure that resists the motion of heat across the layers but permits easy passage along them.

Pyrographite can be deposited in sheets up to ½ in-thick, can be shaped to form rocket nozzles and caps for nose cones. Both these parts get punishing heat concentrated on rather small areas. The beauty of Pyrographite is that it conducts heat away from these danger points as fast as copper can, but it does not permit nearly as much heat to pass through it. A Pyrographite nose cone, for instance, spreads the heat of air friction over a large area and permits it to be radiated harmlessly away, but it does not let heat strike through the cone and damage the sensitive instruments or warhead inside.

Pyrographite is in limited production now, mostly for military purposes, but Raytheon sees many commercial possibilities, e.g., as lightweight insulation against extreme heat.