Pembuatan dan Karakterisasi Bi1-xYxFeO3/C, BiFe1-yZnyO3/C, dan Bi0.88Y0.12Fe1-yZnyO3/C sebagai Absorber Gelombang Mikro pada Frekuensi X Band = Synthesis and Characterization of Bi1-xYxFeO3/C, BiFe1-yZnyO3/C, and Bi0.88Y0.12Fe1-yZnyO3/C as Microwave Absorber at the X-Band Frequency