Three-Dimensional Calcium Copper Titanate/Epoxy-Boron Nitride Nanosheets Dielectrics toward High Dielectric Constant and Low Loss

Epoxy dielectric materials with high permittivity and low loss were fabricated via inversely introducing epoxy-boron nitride nanosheets (EP-BNNS) mixtures into a continuous three-dimensional CaCu3Ti4O12 (3D CCTO) ceramic network constructed by template method. The obtained 3D CCTO/EP-1.0%BNNS composite exhibited an enhanced permittivity of 25.8 and suppressed low loss of 0.032 (@ 1 kHz), 170%, and 89% of conventional CCTO/EP system, respectively. In addition, the activation energy derived from the isothermal dielectric relaxation spectra increases with the addition of BNNS, indicating that the presence of BNNS restrains the dipolar polarization of EP composites.


Introduction
High dielectric constant polymers play a crucial role in all aspects of our social lives, such as aerospace, electronics, and hybrid electric vehicles, because of their rapid charge and discharge rates, lightweight, and high-power density [1].However, the dielectric constant of a single polymer is very low, which cannot meet the current needs of high-performance electronic components.Inorganic ceramics are commonly incorporated into polymers due to their large permittivity and relatively low dielectric loss to enhance their dielectric performance [2].However, even if the fillers have ideal dispersion in the polymer matrix, the permittivity of the binary composite is often lower than that predicted by the logarithmic model, i.e., failing to achieve the ''Big Missing Area'', as mentioned in [2,3].In this regard, Luo et al. [4] prepared a three-dimensional barium titanate (3DBT) network using cellulose as a template.Subsequently, the 3DBT/epoxy (3DBT/EP) composites were prepared by reverse infiltration of EP into the 3DBT network.The composite's permittivity and energy storage capacity containing 16 vol% 3DBT is 34.5 at 1 kHz and 8.3×10 -3 Jꞏcm -3 under 10 kVꞏmm -1 , which is 9.6 times and 16.6 times of pure epoxy, respectively.Moreover, the composite achieves a high dielectric constant at a much lower BT loading than conventional BT nanoparticles due to the continuous network structure of 3DBT, which provides continuous polarization tunnels.In this paper, the template method easily fabricated a continuous three-dimensional CaCu 3 Ti 4 O 12 (3D CCTO) ceramic network.Subsequently, 3D CCTO/epoxy-boron nitride nanosheets (EP-BNNS) composite dielectric composites were prepared by reverse infiltration of the EP-BNNS mixtures into the 3D CCTO network.The 3D CCTO/EP-BNNS composite exhibited an enhanced permittivity and suppressed dielectric loss than the corresponding CCTO/EP composite.

Construction of 3D CCTO network and 3D CCTO/EP-BNNS composites
Briefly, 21.74 g of copper nitrate trihydrate, 7.08 g of calcium nitrate tetrahydrate, 3.00 g of citric acid monohydrate, and 41 ml tetrabutyl titanate were dissolved in ethyl alcohol (59 ml).Subsequently, a small amount of acetyl acetone is added into the solution as a stabilizer and subjected to 2 h stirring at room temperature.The cleanroom wiper with a dimension of 40×40×0.1 mm 3 was immersed into the CCTO precursor solution until saturation aided by ultrasonication.The cleanroom wiper template with the absorbed CCTO sol was placed in a vacuum drying oven at 60 o C for 24 hours to get the CCTO gel.The cleanroom wiper gel was then calcined in an oven at 1000 o C for 3 hours under an air atmosphere.Finally, the 3D CCTO ceramic network was obtained.In addition, the h-BN was exfoliated into BNNS using the ball milling method in an ethanol solvent.Different amount of BNNS was dispersed in EP resin.The EP-BNNS solutions with 0 wt%, 0.5 wt%, and 1.0 wt% of BNNS were then reversely introduced into the above 3D CCTO network aided by suction filtration, named as 3D CCTO/EP, 3D CCTO/EP-0.5%BNNS,and 3D CCTO/EP-1.0%BNNS.The content of 3D CCTO in the saturated 3D CCTO/EP composites is ~34 wt% as determined from TGA analysis (Fig. 1(f)).The obtained 3D CCTO was subjected to ball milling for 12 h.34 wt% CCTO/EP and 50.0 wt% CCTO/EP composite were prepared by traditional solution mixing method.1(e) provides additional evidence of the continuous network structure of 3D CCTO within the EP matrix and the homogeneous distribution of BNNS throughout the EP phase.The thermal stability of representative EP, 3D CCTO/EP, and 3D CCTO/EP-1.0%BNNScomposites are illustrated in Fig. 1(f).The weight percentage of 3D CCTO in 3D CCTO/EP-BNNS composites was ~34 wt%.In addition, the final decomposition temperature (T df ) of 3D CCTO/EP-1.0%BNNS(T df =485 o C) was 9 o C higher than that of 3D CCTO/EP without BNNS, showing enhanced thermal stability [6].Fig. 2 shows the dielectric properties spectrum and composites' polarization schematic.The permittivity of 3D CCTO/EP-BNNS composites increases more significantly than that of filling CCTO particles at the same frequency (Fig. 2(a)).The dielectric constant of 3D CCTO/EP reaches 31.6 (@1 kHz), which is 2.0 times that of the composite containing 34 wt% CCTO (ε'=15.5).With the addition of BNNS, there is a slight decrease in permittivity due to the broad bandgap of BNNS, which hinders the dipole polarization of composites.Fig. 2(b) shows the dielectric loss of 3D CCTO/EP-BNNS composites.It can be observed that the dielectric loss of the 3D CCTO/EP-BNNS system decreases distinctly with the introduction of BNNS.The loss tangent of 3D CCTO/EP-1.0%BNNScomposite is reduced to 0.032 at 1 kHz, 66.7% of 3D CCTO/EP (tan δ=0.048).The primary reason for the reduced loss is the inherent low-loss BNNS, which effectively restricts the movement of free charges.Fig. 2(c) displays theoretical predictions of dielectric constant using Logarithmic, parallel, and Maxwell Garnett models.It is observed that the permittivity of binary CCTO/EP composites is lower than the predicted logarithmic model.In contrast, the permittivity of 3D CCTO/EP-BNNS composites lies in the ''Missing area'', indicating much-enhanced polarization at low CCTO loadings.Fig. 2(d) displays a schematic illustration of 3D CCTO/EP-BNNS composites, where the continuous 3D CCTO provides a continuous polarization channel within the composites, thus leading to a large dielectric constant [7].Furthermore, BNNS hinders the movement of charge carriers, resulting in reduced dielectric loss.Fig. 3 shows the isothermal relaxation spectra of 3D CCTO/EP-BNNS composites at different temperatures (90 o C, 120 o C, 150 o C, and 180 o C, respectively).The relaxation peak of the 3D CCTO/EP-BNNS composite shifts to a higher frequency with elevated temperature (Fig. 3(a')-3(c')).Furthermore, the relaxation peaks move to a higher frequency with the addition of BNNS at the same temperature.The Arrhenius equation can be used to describe the isothermal dielectric relaxation phenomenon of the composites:

Results and discussions
where f is the maximum peak frequency under T temperature, and E b corresponds to the activation energy [8].Fig. 4 displays the E b of 3D CCTO/EP-BNNS composites, and the inset of Fig. 4 illustrates the linear fitting of Arrhenius plots of the composites.The results conform well to the Arrhenius plot, and an increase in E b was observed with the increase of BNNS.The activation energy is associated with the reorientation of dipolar entities of composites.Hence, incorporating BNNS restricts the polarization of CCTO and EP dipoles.

Fig. 1
illustrates the structure and morphology of 3D CCTO and 3D CCTO/EP-BNNS composites.The XRD patterns of BNNS, CCTO, and 3D CCTO/EP-BNNS are shown in Fig. 1(a).The distinct peaks located at 2θ=29.6 o , 34.3 o , 38.5 o , 42.2 o , 46.0 o , 49.3 o , 61.4 o , and 72.2 o correspond to characteristic diffractions of cubic CCTO [5].The emergence of a new peak at 2θ=26.8 o corresponds to the distinctive diffraction peak of BNNS.The cleanroom wiper seen from the SEM image of Fig. 1(b) and the corresponding optical microscope (OM) image of Fig. 1(b′) displays an interwoven fibric network structure, which provides a continuous 3D backbone for the fabrication of 3D CCTO.SEM images in Fig. 1(c) show the interwoven 3D CCTO network with continuous CCTO nanoparticles (Fig. 1(c′)).Fig. 1(d) displays a low-magnification SEM image of 3D CCTO/EP-BNNS composites, confirming the successful infiltration of EP-BNNS into the 3D CCTO network.The OM image depicted in Fig.

Fig. 4
Fig. 4 E b of 3D CCTO/EP-BNNS composites derived by isothermal relaxation (The inset is the linear plot of the Arrhenius equation) 4. Conclusions EP-BNNS mixed solution was successfully introduced into the 3D CCTO network.The obtained 3D CCTO/EP-1.0%BNNScomposite exhibited the best dielectric performance, showing an enhanced permittivity of 25.8 and a suppressed loss tangent of 0.032 at 1 kHz, which was 170% and 89% of 34 wt% CCTO/EP composite.The synergistic effects of the 3D CCTO network and BNNS provide a novel method for preparing high-performance polymer dielectrics.What's more, it has been verified from the activation energy of polarization that the addition of BNNS inhibits the dipole relaxation of 3D CCTO/EP composites.