Forming carbon nanotube composites by directly coating forests with inorganic materials using low pressure chemical vapor deposition

Low pressure chemical vapor deposition has been used to fill carbon nanotube (CNT) forests with inorganic materials (polysilicon and silicon nitride). Forest filling proceeds by deposition around individual CNTs. As the coating thickness around each CNT increases, the free volume between adjacent nanotubes is filled and finally results in a contiguous composite film. The process maintains the forest height and alignment; however, the coating thickness around the CNTs is in general smaller at the base of the forest than it is at the top. This can cause a contiguous solid film to form at the top of the forest while the forest is only partially filled at the base. Once the top of the forest becomes filled, it prevents growth from occurring at the base. Consequently, the growth process can cap the top of the forest and leave voids between thinly coated CNTs at the base. Such composites have reduced hardness (4 GPa or less). Depositing at reduced temperatures and/or decreased precursor gas flow rates reduces the void fraction through improving the step coverage modulus. This allows one to produce thick (> 50 μm) polysilicon-CNT composite films having hardness approximately equal to that of polysilicon thin films (12.4 GPa).