A theta-matrix compositor was a component of a Starfleet vessels warp drive. Dilithium was recrystalized in the compositor. (TNG episode: "Family")
The crew of the USS Voyager experienced a problem with the theta-matrix compositor in 2372, when the devices slowed recrystalizing. (VOY novel: Bless the Beasts)
In 2386, the USS Robinson's theta-matrix compositor was damaged in an attack by the Glant. Commander Relkdahz reported to Benjamin Sisko that new parts would be required to make repairs. (DS9 novel: Original Sin)