Initially, the morphology of samples post cure (modified and unmodified BaTiO3) was studied to quantify the distribution of BaTiO3 in the epoxy matrix.
2.3.1. Scanning Electron Microscope (SEM)
The BaTiO3 particles were modified using the silane coupling agent 3-GPS to produce a finer dispersion in the epoxy resin. The SEM was used to characterise the dispersion and distribution of the nanoparticles at different wt.% in the epoxy. There were several levels of aggregation events due to higher particle weight fractions and improved dispersion due to surface functionalisation: SEM images of the cracked cross-section of functionalised BaTiO3 epoxy composites at 1,5,10 wt.% are shown in Fig. 7(a)-(c). SEM image of the cracked cross-section of non-functionalised BaTiO3 epoxy composites at 10 wt. % is shown in Fig. 7(d). A positive of the use of coupling agent for uniform dispersion is observed, comparing Fig. 7(c) and (d). The silane-treated particles size is also nearly 200 nm. Moreover, there is no apparent aggregation of Si-BaTiO3 particles. The effectiveness of the silane functional group is consistent with previous research [18, 19].
2.3.2. Electric field Induced Tensile Testing
The in-situ tensile testing was performed at 0V,6V,9V,24V, showing the tensile strength, elongation and modulus changing with the content of BaTiO3 and the voltage. The overall tensile strength and elongation of the 1 wt.% BaTiO3- epoxy nanocomposites were relatively smaller compared to those of the 5 wt.% and 10 wt.% BaTiO3- epoxy nanocomposites under the applied voltage conditions. The tensile strength and elongation of the nanocomposites increased with increasing the voltage, as shown in Fig. 8(a-c). To assess the repeatability of the data, 2–3 tests per category were conducted at 1wt.%, 5wt.% and 10wt.%, shown in Fig. 9 for the extreme cases of 0V (no field) and 24V applications. Si-BaTiO3-epoxy nanocomposite samples. Generally, an increasing tensile strength and its corresponding elongation with the increasing content of BaTiO3 and voltage are observed. The results demonstrated that the voltage has a profound effect on the elongation and tensile strength of the nanocomposites due to the ferroelectric polarisation of BaTiO3 molecules at intrinsic strain levels, which collectively attributes to the instantaneous (almost) application of extrinsic strains on their surrounding polymer, therefore providing an interfacial compressive strain.
It is well known that the elastic modulus is strongly dependent on the nanofillers content and matrix. The in-situ tensile tests were performed at different voltage, and the tensile modulus was measured both when varying the content of BaTiO3 and the voltage. Generally, the results show an increasing elongation at failure. Specifically, the results show that in the case of 1wt% BaTiO3 the tensile strength significantly increased with the increasing voltage with the slight change in the tensile modulus, as shown in Fig. 10(a). In the cases of 5wt% and 10wt% BaTi03 the tensile strength slightly changes with the increasing voltage, as shown in Figs. 10(b,c) while there is an apparent reduction in the tensile modulus for 5wt.% and 10 wt.% in the presence of the field when compared to 0 V application. The increase in elongation and strength is attributed to the compressive field introduction observed via fibre optic sensing in the research conducted in [18]. The reduction in modulus in the cases of 5 and 10 wt.% due to the presence of an electric field has not been further explored, and would require further experimentations, however it can be observed that the electric field application at higher BaTiO3 content has a softening effect on the nanocomposite material. Such effect was not observed in the 1wt.% content, therefore the softening effect at higher content would require further experimentation prior to the failure point of the nanocomposites (e.g. using in-situ Raman measurements).
2.2.3. Electric field Induced Raman Spectroscopy
Raman spectroscopy is one of the effective methods to investigate local residual strains in crystalline materials by examining a shift of scattering frequency with respect to the variation in the inter-atomic spacing. Nevertheless, this technique is also established for polymer matrix composites. Raman spectroscopy is additionally employed for the validation of variation in the crystal structure of Si-BaTiO3. Such characterisations are initially conducted on Si-BaTiO3/Epoxy nanocomposite samples with 1, 5, 10 wt.% under the electric field voltages of 0, 6, 12, 24 V, as shown in Fig. 11. The Raman sharp peaks under no electric field (Fig. 11a-c) are identified as 515, 641, 826 and 1369 cm− 1 representing the characteristic peaks of BaTiO3, Si-BaTiO3, Si- epoxy, amorphous epoxy, respectively. It is observed that the intensity of peak has been slightly increased by increasing the content of BaTiO3. The intensity of Raman peaks represents the polarizability of the molecule and the concentration of the active molecular groups, therefore the slight increase in intensity indicates sight increase in polarizability of Si-BaTiO3-epoxy nanocomposites and increasing active species with the increasing content of Si-BaTiO3. Raman spectra of Si-BaTiO3-epoxy nanocomposites at 1, 5, 10 wt.% of Si-BaTiO3 under 6V field (Fig. 11d-f) also shows an increase in the intensity when compared to the no-field peaks. Such trend of increase in the intensity repeats itself in higher field strengths of 12V and 24V (Fig. 11(g-i) and (j-l) respectively). Moreover, it is observed that the higher the voltage the broader the peaks. Such data ratifies the fact that the polarizability is not only dependent on the content of BaTiO3 but also dependent upon the applied voltage. The broadening of peak and increasing the intensity of peak indicate the stretching of bond and structural change via field induced dipolar displacement at intrinsic level and polarization under such field. When taking this intrinsic straining/stretching effect into account for Fig. 9 data, it can be implied that such stretch at interatomic crystalline level (under a simplified assumption of full crystalline BaTiO3 structure) would contribute to the softening effect and thus reduction in the tensile modulus at larger extrinsic scales.