Electromagnetic field controlled domain wall displacement for induced strain tailoring in BaTiO3-epoxy nanocomposite

Abstract

Failure in an epoxy polymer composite material is prone to initiate by the coalescence of microcracks in its polymer matrix. As such, matrix toughening via addition of a second phase as rigid or/and rubber nano/micro-particles is one of the most popular approaches to improve the fracture toughness across multiple scales in a polymer composite, which dissipates fracture energy via deformation mechanisms and microcracks arrest. Few studies have focused on tailorable and variable toughening, so-called 'active toughening', mainly suggesting thermally induced strains which offer slow and irreversible toughening due to polymer's poor thermal conductivity. The research presented in the current article has developed an instantaneous, reversible extrinsic strain field via remote electromagnetic radiation. Quantification of the extrinsic strain evolving in the composite with the microwave energy has been conducted using in-situ real-time fibre optic sensing. A theoretical constitutive equation correlating the exposure energy to micro-strains has been developed, with its solution validating the experimental data and describing their underlying physics. The research has utilised functionalised dielectric ferroelectric nanomaterials, barium titanate (BaTiO₃), as a second phase dispersed in an epoxy matrix, able to introduce microscopic electro-strains to their surrounding rigid epoxy subjected to an external electric field (microwaves, herein), as result of their domain walls dipole displacements. Epoxy Araldite LY1564, a diglycidyl ether of bisphenol A associated with the curing agent Aradur 3487 were embedded with the BaTiO₃ nanoparticles. The silane coupling agent for the nanoparticles' surface functionalisation was 3-glycidoxypropyl trimethoxysilane (3-GPS). Hydrogen peroxide (H₂O₂, 30%) and acetic acid (C₂H₄O₂, 99.9%) used as functionalisation aids, and the ethanol (C₂H₆O, 99.9%) used for BaTiO₃ dispersion. Firstly, the crystal microstructure of the functionalised nanoparticles and the thermal and dielectric properties of the achieved epoxy composite materials have been characterised. It has been observed that the addition of the dielectric nanoparticles has a slight impact on the curing extent of the epoxy. Secondly, the surface-bonded fibre Bragg grating (FBG) sensors have been employed to investigate the real-time variation of strain and temperature in the epoxy composites exposed to microwaves at 2.45 GHz and at different exposure energy. The strains developed due to the in-situ exposure at composite, adhesive and their holding fixture material were evaluated using the FBG. The domain wall induced extrinsic strains were distinguished from the thermally induced strains, and found that the increasing exposure energy has an instantaneously increasing effect on the development of such strains. Post-exposure Raman spectra showed no residual field in the composite indicating no remnant strain field examined under microwave powers < 1000 W, thus suggesting a reversible strain introduction mechanism, i.e. the composite retaining its nominal properties post exposure. The dielectric composite development and quantifications presented in this article proposes a novel active toughening technology for high-performance composite applications in numerous sectors.

Figure 1

Schematic diagram of the extrinsic strain and intrinsic strain that contributes to macroscopic strain in a tetragonal BaTiO₃ crystal embedded in epoxy (red, green and blue arrows represent electric field induced intrinsic strain, extrinsic strain and macroscopic strain, respectively).

Figure 2

Schematic illustration of a spherical nanoparticle (simplified as a crystallite) with a laminar 90° domain structure with spontaneous polarisation ${\mathit{P}}_0$ and domain size $\mathit{d}$, recreated from.

Figure 3

Schematic diagram of Eshelby's inclusion utilised in our analysis.

Figure 4

Schematic diagram of the principle of equivalent eigenstrain of transforming inhomogeneous problem to a virtual homogeneous problem.

Figure 5

Schematic view of a BaTiO₃ nanoparticle with radius ($\mathit{r}$) embedded in the epoxy matrix with eigenstrain ($\mathrm{\Delta }{\mathit{l}}_{\mathit{m}}$).

Figure 6

X-ray diffraction patterns of purchased BaTiO₃ powders. The inset shows the split peaks of (002) and (200) indicating the tetragonal phase.

Figure 7

Schematic diagram of the functionalisation process of BaTiO₃ with 3-GPS.

Figure 8

FTIR spectra of Si-BaTiO₃ (black) and untreated (red) BaTiO₃ powders.

Figure 9

TGA curves of untreated and Si-BaTiO₃ powders. Green lines indicate the weight percentage of the powders versus temperature, and blue lines the rate of change of the weight percentage versus temperature.

Figure 10

Glass mould used for nanocomposite fabrication and the cured nanocomposite samples.

Figure 11

Schematic illustration of the layout of sample for microstrip line methods.

Figure 12

Schematic illustration of the sample of epoxy nanocomposite with BaTiO₃ on the PTFE holder in the microwave oven with surface bonded FBG arrays connected to an interrogator.

Figure 13

SEM images of fracture cross of epoxy nanocomposites with BaTiO₃ at different weight loading, (a)–(d) images of 1 wt.%, 5 wt.%, 10 wt.% and 15 wt.% Si-BaTiO₃/Epoxy nanocomposite samples. (e) and (f) 15 wt.% untreated-BaTiO₃-epoxy nanocomposite at lower (670x) and higher (6620x) magnification.

Figure 14

DSC spectra of neat epoxy and nanocomposites with 10 wt.% and 15 wt.% of untreated BaTiO₃ (pink and green) and Si-BaTiO₃ (red and blue). Weak endothermic peak around Tg and weak exothermic peak from 160 to 220 °C are identified.

Figure 15

(a) DSC spectra of the neat epoxy (black) and Si-BaTiO₃-Epoxy nanocomposite samples at 5 wt.% (Red), 10 wt.% (Blue), and 15 wt.% (Pink), (b) Tg temperature as a function of weight loading of Si-BaTiO₃ (Red) and untreated BaTiO₃ (Black) of the epoxy nanocomposites samples.

Figure 16

Glass transition temperatures of silane treated and untreated BaTiO₃-Epoxy nanocomposites.

Figure 17

Dielectric measurements (a) real permittivity $\mathit{\epsilon }{}^{\mathbf{\prime }}$, (b) loss tangent $\mathit{t}\mathit{a}\mathit{n}(\mathit{\delta })$ from 0 to 4.5 GHz at room temperature of neat epoxy and BaTiO₃-Epoxy nanocomposites at different weight loading.

Figure 18

Schematic illustration of surface-bonded FBG arrays measuring in-situ temperature and strain.

Figure 19

Strain and temperature change measurements of 15 wt.% silane-treated BaTiO₃-Epoxy samples under 100 W for 600 s.

Figure 20

Strain and temperature change measurements of 15 wt.% silane-treated BaTiO₃-Epoxy samples under 440 W for 148 s.

Figure 21

Schematic illustration of (above) the location of FBGs on the sa mple surface, and (below) standing waves formed in a microwave oven showing hot spots (antinodes) and cold spots (nodes).

Figure 22

Strain and temperature variations measured by sensor 2 before and after the ‘sudden drop’ at 100 W (a, b) and 440 W (c, d).

Figure 23

Strain and temperature measurements of FBG sensor 1 of (a) and (c) 15 wt.% BaTiO₃/Epoxy nanocomposite sample under 100 W for 600 s, (b) and (d) 15 wt.% BaTiO₃-Epoxy nanocomposite sample under 440 W for 148 s.

Figure 24

Strain and Temperature evolution measurements via FBG sensors in adhesive (Permabond 50) bonded on the surface of the PTFE holder under 100 W for 650 s and 440 W for 110 s.

Figure 25

Strain and temperature evolution of the neat epoxy sample under 100 W and 440 W.

Figure 26

Schematic illustration of the location of FBG sensors on the neat epoxy surface, 20 mm apart, and standing waves formed in a microwave oven showing hot spots (antinodes) and cold spots (nodes).

Figure 27

Raman spectra of (a) neat epoxy, (b) three random points of 15 wt.% epoxy nanocomposites with Si-BaTiO₃ before microwave exposure, (c) 15 wt.% epoxy nanocomposites with Si-BaTiO₃ before and after microwave exposure of 440 W for 150 s (d) 15 wt.% epoxy nanocomposites with Si-BaTiO₃ before and after microwave exposure of 1000 W for 60 s.

Figure 28

Evolution of micro-strain with temperature rise—theoretically calculated thermal expansion of adhesive (black), actual strain data from sensor 5 of adhesive bonded onto neat epoxy (red), and actual strain data from sensor 1 of adhesive bonded to 15 wt.% BaTiO₃-epoxy nanocomposite (blue) under 100 W at 5 °C, 10 °C, 15 °C, 20 °C, 25 °C, and 30 °C.

Figure 29

Evolution of micro-strain with temperature rise—theoretically calculated thermal expansion of adhesive (black), actual strain data from sensor 5 of adhesive bonded onto neat epoxy (red), and actual strain data from sensor 1 of adhesive bonded to 15 wt.% BaTiO₃-epoxy nanocomposite (blue) under 440 W at 5 °C, 10 °C, 15 °C, and 21 °C.

Figure 30

Schematic of the 15 wt.% BaTiO₃-epoxy nanocomposite under exposure at different scales.

Figure 31

Nanocrystalline structure of BaTiO₃ nanoparticles well-dispersed within the area over which the extrinsic strains are measured by the FBG sensors.

Figure 32

Schematic diagram of adhesively bonded FBG arrays to the sample’s surface and non-uniform thermal expansion due to mismatch in CTEs.

Figure 33

Temperature change measurements (micro-strain) by FBG sensors (left), and energy absorbed (right) versus time (s) under 100 W.

Figure 34

Temperature change measurements (micro-strain) by FBG sensors (left), and energy absorbed (right) versus time (s) under 440 W.