Flexible films or sheets for forming high-breakdown strength, high-temperature capacitors are disclosed. Amorphous metal oxides and nitrides, preferably SiO.sub.2 or HfO.sub.2, with a dielectric constant (k) greater than 2 and stacks of oxides and nitrides formed over conducting substrates may be formed. The dielectrics may be formed by reactive sputter deposition of the amorphous materials onto cooled substrates. The cooled substrate allows the films to be amorphous or nanocrystalline and results in films that can be flexed and that can be rolled into cylindrical shapes. An important application for these dielectrics is in high energy-density wound capacitors.