News

Pyrolytic Graphite Grids

Click to enlarge

Pyrolytic Graphite (PG) is a unique form of graphite with interesting mechanical and electrical characteristics, which makes it a prime material choice for gridded tubes. PG is grown on a mandrel by pyrolysis or hydrocarbon decomposition as opposed to standard graphite that is mixed with binder and hot pressed. This Chemical Vapor Deposition CVD technique greatly enhances physical properties along the plane of the grid, which is the outline of the mandrel. Thermal conductivity is 320 to 700 Watts/meter deg. C, greater than that of silver. Heat generated at the grid is readily transferred to the copper grid support. Electrical resistivity is comparable to that of copper, .5 X 10 _ 3 ohm_cm, which minimizes IR losses. And Thermal expansion is .5 X 10_6 cm/cm/deg. C, less than that of ceramics, so the grid maintains its geometry at all RF powers, and filament levels. The grids are grown on the mandrels at 2000 C at approximately 1 Torr methane. At this temperature only carbon is stable and hence metallic impurity is low <5ppm. The growth rate is .001 inches per hour. As the deposition furnace cools the graphite mandrels contracts away from the PG grid. The grid is then conventionally machined to .006" thick. The screen or web pattern is then laser machined. The molecular crystalline growth of the carbon is analogous to stacked layer of cards. Special techniques are employed to destroy the periodic repetition of lattice positions. This randomness significantly increases tensile strength, flexural strength and impact resistance of the L-3 Electron Devices grids.