Pengaruh variasi komposisi ukuran partikel carbon black dan lama pencampuran grafit dan carbon black terhadap distribusi sifat-sifat pelat bipolar PEMFC berbasis komposit grafit/epoksi = The influence of variations in composition of carbon black particle size and mixing time of graphite and carbon black to the distribution of properties of PEMFC bipolar plates based on graphite epoxy composite