Lossy Titanite-Based Ceramics with Nominal Compositions CaTi<sub>1-</sub><i><sub>x</sub></i>M<i><sub>y</sub></i>SiO <sub>5</sub> (0 ≦ <i>X</i>, <i>Y </i>≦ 1, M = Mn, Sn, Zr, Nb) Applicable to Millimeter Electromagnetic Wave Absorber

The titanite-based ceramics with nominal composition CaTi 1- x M y SiO 5 (0≦ x ≦1, M = Mn, Sn, Zr ( y = x ), and M = Nb ( y = 4 x /5)) in which part x of Ti sites are replaced by several kinds of atoms had remarkable increase in both the real and imaginary parts of complex relative permittivity around x = 0.0125~0.1 compared with those of pure titanite CaTiSiO 5 ( x = 0) at 70 GHz. Real part varied from 3 to 43, and the imaginary part from 0 to 12 (tangent delta from 0 to 0.28). No reflection condition is fulfilled for M = Zr when x = 0.05, d / λ 0 =0.042, and for M = Nb in both cases when 0 < x < 0.0125, d / λ 0 = 0.05 and 0.1 < x < 0.2, d / λ 0 =0.042, where d is thickness of the plate sample and λ 0 is the wavelength of incident electromagnetic wave. The dominant dielectric dispersion may occur due to difference of ionic polarization between Ti 4+ ions and Mn 4+ , Sn 4+ , Zr 4+ , or Nb 5+ ions relative to O 2- ions, which becomes inactive and saturates around x = 0.0125~0.05. From the measurement of the lattice parameters, a , b , c , and the angle β , characterizing monoclinic crystal structure, this saturation may have close correlation with some structural rearrangement of constituting atoms, Ti and substituted M atoms in CaTi 1- x M y SiO 5