4.1 Characterization of BT nws and Dopa@BT nws
4.1.1 Morphology analysis of BT nws and Dopa@BT nws
The morphology of nanowires is closely related to the reaction temperature, time and concentration. Generally, the higher the reaction temperature, the greater the concentration of reactants and the longer the reaction time results in the larger diameter of the nanowire.
Fig. 2 shows microtopography of nanowires. It can be seen from Fig. 2(a) that a large number of uniform BT nanowire are formed, and the average length of these 1D BT nanostructures is greater than 5 microns. Combined with the local high-resolution SEM image, it can be seen that the maximum diameter of the nanowires is less than 200 nm, and it is conservatively estimated that the aspect ratio of the BT nws is above 30.Moreover, the element mapping of BT nws (Fig. 2(b), 2c) shows that the ratio of Ba and Ti is 1:1. To sum up, BT nws was effectively synthesized by two-step hydrothermal reaction.
Furthermore, the characteristics and morphological of the Dopa@BT nws were also determined by high-resolution TEM. Dopa is used as a surface modifier to improve the compatibility between BT nws filler and ArPTU matrix. Dopa can be deposited on nanowires with the evidence that a distinct thin layer is formed on the surface of BT nws. As shown in Fig. 3(a), which helps to disperse the nanowires well in the polymer matrix. It also can be seen from the Fig. 3(a), the diameter of a single BT nws is about 120 nm, and the smooth surface of the nanowire is formed. After the surface modification of Dopa, a uniformly amorphous layer of several nanometers thick can be clearly observed on the surface of the nanowire, indicating the successful modification of Dopa on BT nws. In addition, the element mapping of the Dopa@BT nws (Fig. 3(b), 3c) proved the content, coexistence of C and N elements in the ternary Dopa@BT nws .
4.1.2 Contact angle analysis of BT nws
It is well known that, surface contact angle shows great relation to the surface energy of the materials, the greater the surface energy, the smaller the contact angle with water would be presented. In order to further confirm the successful modification of the BT nws with Dopa, a surface contact angle test was characterized as shown in Fig. 4.
It can be seen from the Fig. 4 that the contact angle of BT nws and dopamine-modified BT nws is less than 90°. This phenomenon indicates that both nws are hydrophilic. The surface contact angle of pure BT nws is about 63°, indicating a relatively hydrophobic surface. However, the surface contact angle of modified BT nws is about 22°, which is more hydrophilic than unmodified BT nws. This result indicates that the C-N bonds modification of dopamine improves the hydrophilicity of the BT nws surface. The surface modification, on the one hand, changes the surface energy of BT nws and results in well dispersion of nws in polymer matrix accordingly. More importantly, it also constructs ultrathin layer to enhance synergistic effect between the BT nws and ArPTU polymer matrix.
4.2 Characterization of Dopa@BT nws/ArPTU composite film
4.2.1 SEM images of composite film
Fig. 5 shows the surface SEM images of pure ArPTU (Fig. 5(a)), 10 wt.% BT nws composite film (Fig. 5(b)) and 10 wt.% dopamine modified BT nws composite film(Fig. 5(c)). It can be seen from the figure, after the incorporation of the unmodified nanowires into ArPTU, the composite film shows a morphology with pinholes distributed in films. The reason for this phenomenon may be the incompatibility between ArPTU and BT nws [19,20]. However, the Dopa surface-modified BT nws is uniformly dispersed in the ArPTU matrix to form a uniform and high-quality nanocomposite film. The addition of dopamine effectively improves the compatibility of ArPTU and BT nws.
In addition, Fig.5 shows the cross-section SEM images of composite films (Fig. 5(d)). It can be seen from Fig. 5(d) that the BT nws with Dopa modification was well distributed inside the polymer matrix and the nanowires are tightly fixed in the matrix, indicating that the polydopamine-coated nanowires exhibit excellent compatibility with the ArPTU matrix.
4.2.2 FTIR spectrum of composite film
In order to confirm the modification of Dopa on BT nws, the FTIR analysis was performed on the nws before and after modification, as shown in Fig. 6(a). From the infrared spectrum, the new absorption peaks at 1445 cm-1, 1288 cm-1 and 3758 cm-1 rising from Dopa@BT nws are observed. They correspond to the C-C skeleton vibration of the aromatic ring, the stretching vibration of the C-N bond of the aromatic amine, the stretching vibration absorption peaks of the O-H and N-H bonds respectively [20]. This spectrum further confirms the successful modification of Dopa on the BT nws. The FTIR spectrum of Dopa@BT nws/ArPTU-based nanocomposite film with given weight fraction of nws (1 wt.%, 3 wt.%, 5 wt.%, 7 wt.% and 10 wt.%) were also characterized, which are shown in Fig. 6(b). The peak positions of pure ArPTU and dopa@BT nws filled composite film with different volume fractions are all at the same position. With the continuous increase of BT nws quantity, the intensity of some peaks in the range of 500 cm-1-1500 cm-1 change. This result also confirms that the inorganic ceramic BT nanowires were successfully incorporated with ArPTU.
4.2.3 XRD analysis of composite film
In order to study the crystal structure of the composite material, the XRD spectra of BT nws, Dopa@BT nws and the composite films with different mass fractions were characterized, as shown in Fig. 7. The synthesized BT nws was a perovskite structure with the evidence that the appearance of (100), (110), (111), (200), (210), (211), (220) crystal peaks and its crystal structure shows no change after the addition of Dopa (As shown in the Fig. 7(a)). In Fig. 7(b), the pure ArPTU shows no obvious characteristic peaks, indicating that the amorphous structure of ArPTU. After the addition of Dopa@BT nws, the composite film gradually transformed from an amorphous structure to a crystalline structure partly. Moreover, the intensity of the diffraction peak increases with the increase of Dopa@BT nanowire content, which means that the crystallinity of the composite film increases with the increase of Dopa@BT nanowire content [21,22].
4.3 Energy storage characteristics of composite films
4.3.1 Dielectric properties of composite films
Compared with the 0 D nanofiller, the 1 D nanofiller has a larger dipole moment, which can increase the dielectric constant of the nanocomposite at a relatively low concentration of the nanofiller. In addition, the 1 D ceramic filler has a small specific surface area, resulting in minimal agglomeration inside the polymer matrix. It can be seen from Fig. 8(a) that, with the weight fraction of BT nws increases, the effective dielectric constant of the nanocomposite can be tuned effectively. When the content of Dopa@BT nws is 10 wt.%, the maximum dielectric constant of the nanocomposite is about 10.6 at 1 kHz. The increase in the dielectric constant of the nanocomposite is due to the electric field enhancement caused by the high-k nanowire in the polymer matrix [23,24]. The introduction of nanowires also causes the accumulation of charge carriers at the interface of the heterogeneous system, resulting in interface polarization for improved dielectric constant. From the Fig. 8(a) , we also find that with the frequency increasing in the range of 1 kHz - 5 MHz, the dielectric constant of the polymer matrix and nanocomposite materials gradually decreases. This behavior can be attributed to the limited dipole mobility of the polymer matrix and the reduction of interface polarization at high frequencies. The interface polarization increases with the increase of the nanowire/polymer interface area, resulting in a strong frequency-dependent dielectric constant in the nanocomposite with a high load of nanowires.
For the modified BT nanowire composite film, the loss tangent values of different films show small changes, and are close to 0.03 at 1 kHz (Fig. 8(b)). The polar group in dopamine promotes a stronger dipole interaction with the ArPTU matrix, thereby reducing interface polarization and increasing dispersion of nanowires. We also conclude that the Dopa modifier acts as a charge trap, reducing space charge polarization and minimizing conduction paths in the polymer film, which results in reduced dielectric loss especially rising form electrical conducting. In addition, the dopamine-modified nanowires show good interface compatibility with the polymer matrix, reducing the existence of voids in composite [25,26]. With the increase of filler loading, the loss tangent increase from 0.02 to 0.035. This is attributed to the presence of free residual substances in the composite caused by the physically adsorbed organic modifier, which leads to an increase in leakage current and an increase in dielectric loss.
4.3.2 Electrical breakdown of composite materials
The breakdown strength of the composite film is a key parameter for dielectric materials especial for device applications. Moreover, in this composite films, the breakdown strength largely depends on the dielectric constant of the single filler and the polymer matrix.
![](https://myfiles.space/user_files/58894_9946feeafa4c1df7/58894_custom_files/img1612850147.JPG)
The Eq. 1 and Eq. 2 are the expression of the electric field in the filler and the polymer matrix. In these equation, φm is the volume fraction of the filler, εm and εf are the dielectric constants of the polymer matrix and the nanofiller respectively, and Ef, Em are the internal electric field of the polymer matrix and the nanofiller respectively. The large difference between the dielectric constants will cause extremely distortion of electric field distribution inside the entire film.
Furthermore, a finite element simulation was utilized to simulate the local electric field concentration of nanocomposites with high dielectric constant. As shown in the Fig. 9, consider the electric field distribution of BT nanoparticles and nanowires filled with the same mass fraction, it can be shown that when nanoparticles are added to the polymers, there is an electric field around the nanoparticles, which causes the distortion of internal electric field. For composite films, the breakdown phase tends to grow at the nanofiller/matrix interface, and then penetrate the particles near the breakdown path. However, when breakdown encounters in composite containing nanofiber, it tends to penetrate the fiber rather than bypass the fiber. Therefore, the polymer nanowires composite material can withstand a higher breakdown field than the polymer nanoparticle composite material. Moreover, considering the orientation of nws, It can also be seen from the simulation that vertically distributed nanowires can obstruct the external electric field more than parallel distributed nanowires. Therefore, nanocomposites with vertically arranged and randomly arranged nws in polymer matrix can successfully block the external electric field. On the other hand, 1D filled nanocomposites provide the highest polarizability, so the increase in dielectric constant is much larger than that of nanoparticles [6,27,28]. Therefore, according to the stimulation, the addition of dopamine modified BT nws will improved dielectric constant and breakdown voltage of composite effectively.
Weber distribution is used to characterize electrical breakdown performance of composite films, which are shown in Fig. 10. It can be seen from the figure, for the composite modified with dopamine, when the high dielectric constant filler is added to the polymer matrix, the total breakdown strength of the nanocomposite enhances and then reduces. The maximum breakdown voltage is 538 kVmm-1, which is comparable for pure ArPTU. This high breakdown voltage performance indicates that, since the amount of filler added is lower than its threshold volume fraction, composite film can successfully prevent conducting path from forming, such as electric field treeing, at high electrical field. Compared with pure polymer, the ordered and dense structure of Dapa@BT nanocomposite makes it highly insulating, which leads to a slightly increase in leakage current density in nanocomposite. During the solution casting of nanocomposite films, medium and high aspect ratio nanowires tend to be oriented in the in-plane direction, resulting in anisotropic polymer nanocomposite dielectrics. When an electric field is applied in an out-of-plane direction, these nanowires will cause the magnetic susceptibility anisotropy of the nanocomposite film, which leads to a decrease in the electric field concentration in the polymer matrix. In addition, the oriented nanowires in the nanocomposite can inhibit the growth of electrical conducting path, such as electric field treeing, by establishing a twisted tree-like path and acting as a scattering center for the charge carriers in the nanocomposite [28].
4.3.3 D-E electric hysteresis loop
The Fig. 11 shows the D-E loops of nanocomposite films with different weight fractions of nanowires under a varying electric field of 100 Hz at room temperature. It can be observed that the electrical displacement of the nanocomposite increases with the volume fraction of the nanofiller. This can be attributed to the higher dielectric constant of the BT nws. When the content of BT nws is 10 wt.% in terms of volume fraction, the electrical displacement of the nanocomposite shows the most significant enhancement. At the same time, the composite film exhibits a wider D-E loop, which deviates from the linear relationship between the electric displacement of the linear dielectric and the electric field. This phenomenon should be attributed to conductivity and dielectric loss, which further reduces the energy storage density of the nanocomposite. So, proper addition of BT nws would result in less hysteresis in composite film, achieving high dielectric constant and high energy storage efficiency accordingly.
4.3.4 Energy density and efficiency
Fig. 12 shows the energy storage density (Fig. 12(a).) and charge-discharge efficiency (Fig. 12(b).) of composite films. It has been found that an 7.5 Jcc-1 energy storage density is achieved, this is a comparably high energy storage density based on HDMLP and their composites. Furthermore, at a load of 3 wt.%, the discharge efficiency more than 90 % was achieved at 250 MVm-1. This could produce high energy density polymer dielectrics with high efficiency for practical capacitor applications. The efficiency of nanocomposites gradually decreases with the increase of nanowire load due to the increased loading of Dopa@BT nws, which results in increased hysteresis in composite. In conclusion, by combination of high dielectric constants BT nws with linear polymer dielectric ArPTU, a high performance composite dielectrics is constructed for practical high energy storage density capacitor applications.