1. 1) H. Schweickert, K. Reuschel and H. Gutsche: U.S. Patent, US3,011,877 (1961).
2. 2) Y. Yatsurugi, A. Yusa and N. Takahashi: Japanese Patent Applications, JPA1978–106626 (1978).
3. 3) P. D. Maycock (eds): PV NEWS, (PV Energy Systems, Inc., Warrenton).
4. 4) S. Nakamura (eds): Industrial Rare Metals, Annual Review 2008, Vol. 124 (Arumu Publish Co., Tokyo, 2008).
5. 5) S. Nakamura (eds): Rare Metal News, (Arumu Publish Co., Tokyo).