BaTiO<sub>3</sub> (BT) nanocrystals doped polycarbonate polymer composite thin films (BT/PC) with different BT concentrations were prepared by spin coating method. Ultra-fine BT (~40-50 nm) nanocrystals with pure perovskite tetragonal phase were synthesized by hydrothermal method. The structure of BT nanocrystals and composite films were studied by means of XRD and TEM. The composite films were poled with a high electric field at a suitable temperature to yield a non-centrosymmetric arrangement and get better electro-optic properties. The Electro-optic (E-O) coefficients of composite films with various BT concentration were also evaluated. The average effective linear E-O coefficient and figure of merit of 20 wt% BT doped composite films were estimated to be 63.1 pm/V and 103.1 pm/V, respectively. The E-O properties of BT/PC composite films were further enhanced by increasing the concentration of the doped BT nanocrystals when the concentration of BT nanocrystals in composite films was low. The orientation degree of BT nanocrystals in PC polymer was detected by measuring the transparency spectra of unpoled and poled composite thin films.
An interferometric fiber optic sensor is used to monitor the vibration of a bar under tension. An analytical formula is derived to relate the density. Young modulus and the physical dimensions of the bar to its frequency of vibration, which is measured through FFT of the monitored signal from the fiber optic sensor. Bars made up of homogeneous materials are tested for the validity of the method, which is then applied to composite materials. In the latter application, an optical fiber, which form an integral part of the sensor, can be embedded into the bar during its fabrication process. For materials of stable moduli, the sensor can be used to measure the tension of the bar. For materials with unstable moduli, during its curing process, the aging period or otherwise, the sensor can be used to monitor the change. The method can also be used as an alternative to measure the moduli of materials and is especially suitable for composite materials. Due to the complexity of a composite material, the modulus measured by this method reflects an average effect of the bulk, whereas a conventional tensile test depends more on localized weaknesses of the composite.
The sensitivity, flexibility, compactness and lightness of an optical fiber are utilized to monitor the acoustic waves propagating in a film. In this work, the wave is initiated by the irradiation of a pulsed Nd<SUP>-</SUP>-YAG laser line focused onto the film surface through a cylindrical lens. The sensing arm of a Mach-Zehnder fiber optic interferometer sensor is attached to the film at a known distance from the focus to pick up the acoustic signal. The laser pulse also provides the essential synchronizing trigger to measure the arrival time of the acoustic wave. The speed of the wave through the film can be evaluated from the wave propagation time and the position of attachment of the sensing fiber. The modulus of the film can then be deduced from the speed of the wave. The sensor is stabilized against thermo-drift with a thermal feed back circuit and enhanced in visibility with a polarization controller. The moduli of several metal, polymer and diamond- like carbon films have been obtained by this method.