WASHINGTON China 's retaliatory export controls could take a toll on the growing U.S. clean energy sector and its defense industry , analysts say, as a trade tussle escalates between the world's two biggest economies. Beijing announced last week it would ban exports of gallium, germanium and antimony to the United States, targeting materials used for everything from semiconductors to solar cells. China also tightened restrictions on graphite, which is key to the electric vehicle industry. The