High sensitivity gravimetric sensor made of unidirectional carbon fiber epoxy composite on (1-x)Pb(Zn_1/3Nb_2/3)O₃- xPbTiO₃ single crystal substrate

We have derived a general formula for sensitivity optimization of gravimetric sensors and have used it to design a high sensitivity gravimetric sensor using unidirectional carbon fiber epoxy composite (CFEC) waveguide layer on (1-x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃ (PZN-xPT) single crystal substrate with the carbon fibers parallel to the x₁ and x₂ axes, respectively. The normalized maximum sensitivity (|S_m^f|λ)_max exhibits an increasing tendency with the decrease of (h/λ ight)_opt and the maximum sensitivity (|S_m^f|λ)_max increases with the elastic constant c₆₆^E of the piezoelectric substrate material. For the CFEC/011_c poled PZN-7%PT single crystal sensor configuration, with the carbon fibers parallel to the x₁ axis at λ = 24 μm, the maximum sensitivity |S_m^f|_max can reach as high as 1156 cm²/g, which is about three times that of a traditional SiO₂/ST quartz structure gravimetric sensor. The better design selection is to have the carbon fibers parallel to the direction of propagation of Love wave in order to obtain the best sensitivity.