FRANKFURT, July 11 (Reuters) - China's planned export controls on gallium have caused frantic stockpiling of speciality semiconductor wafers made from the metal, according to the world's largest gallium buyer. Freiberger Compound Materials relies almost entirely on Chinese suppliers for its gallium needs to make wafers that go into mobile phone radio signal amplifiers and optical electronics. Consuming an estimated 10% of global gallium output, the company has found itself at the centre of turmoil following China's surprise announcement to control exports of gallium and germanium products from Aug. 1. "My clients are not relaxed about this at all. There's...