**3.2. Electrical properties**

The dielectric constant was computed from the experimental results by using the capacitance equation for a parallel plate capacitor (Eq. (1)) at different frequencies. In a previous study, the addition of cellulose was detrimental to the capacitance of the nanocomposites while larger amounts of STO nanoparticles raised it [11]. Since the capacitance and the dielectric constant are directly proportional, higher amount of cellulose decreased the dielectric constant while the addition of the STO nanoparticles raised the dielectric constant of the nanocomposites (**Figure 5**). In addition, the dielectric constant of the nanocomposites diminished at high frequencies [25]. We expect that this finding will not only increase the capacity of energy storage devices but also control their capacitance values at different frequencies.

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**Figure 5.** Calculated dielectric constant of the nanocomposites as a function of frequency.

is apparent on the polymer surface. However, a significant dispersion of STO aggregates

**Figure 4.** Secondary electron images obtained from the biocomposite with 20 wt% STO particles: (a) low magnification;

The dielectric constant was computed from the experimental results by using the capacitance equation for a parallel plate capacitor (Eq. (1)) at different frequencies. In a previous study, the addition of cellulose was detrimental to the capacitance of the nanocomposites while larger amounts of STO nanoparticles raised it [11]. Since the capacitance and the dielectric constant are directly proportional, higher amount of cellulose decreased the dielectric constant while the addition of the STO nanoparticles raised the dielectric constant of the nanocomposites (**Figure 5**). In addition, the dielectric constant of the nanocomposites diminished at high frequencies [25]. We expect that this finding will not only increase the capacity of energy storage

becomes visible on the polymer surface at high magnification.

**Figure 3.** Particle size of STO nanoparticles at different milling times.

devices but also control their capacitance values at different frequencies.

**3.2. Electrical properties**

(b) high magnification.

80 Ferroelectrics and Their Applications

As aforementioned, chitosan was dissolved in a water/acetic acid solution, enhancing the interaction between cellulose and water molecules in the solution. Such molecular interactions can be: (a) water molecules linked to cellulose hydroxyl (–OH) groups, or (b) water molecules confined between the polymer chains due to intermolecular hydrogen bonds. These interactions heightened the water content in the nanocomposites and, hence, raised the current flow through the capacitor [11, 26].

Moreover, higher amounts of STO nanoparticles increased the dielectric constant of the material [11]. The main characteristic of these ferroelectric nanoparticles consists of their ability to raise the material's stored energy. However, these ferroelectric nanoparticles are difficult to disperse in a polymeric matrix due to their high internal energy. It has also been reported that particle size reduction, by expanding the polymer-particles interface, enhanced the electrical properties of nanocomposites [11, 14].

Furthermore, the literature demonstrates how frequency alters the dipoles orientation of the STO nanoparticles [4–7]. When those nanoparticles are aligned with the applied electrical field, the dielectric material becomes polarized. However, the dipoles cannot remain aligned to the electrical field at higher frequencies [11]. In as much as the dielectric constant dwindles at those high frequencies, the polarization mechanism cannot contribute effectively to the dielectric properties. This represents a limitation because for higher frequencies applications this will cause decrease in the dielectric properties.

After analyzing the dielectric constant of the composites, we gaged the current density, i.e., charge transported through the cross-sectional area (**Figure 6**). Thus, we discovered that the current density raised for higher amounts of cellulose in the composites, as well as for voltages from 5 to 60 V. As explained previously, the water content of in the nanocomposites swells with the addition of cellulose, resulting in higher current densities through the dielectric material. Moreover, the STO nanoparticles addition improves the capacitor's ability to store more energy by lowering the current flow through the dielectric material. As shown in **Figure 6**, the current density for the composites with STO nanoparticles increased

**3.3. Thermal and tensile analysis**

nanoparticles, i.e., 0, 10 and 20 wt%.

mentation operating in tropical regions.

out being mechanically ruptured upon service.

strontium titanate nanoparticles.

In this section, the thermal and mechanical properties of the composites were studied as a function of the cellulose percent, i.e., 15 and 25 v%, and the amount of strontium titanate

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As presented previously, the degradation temperature of the nanocomposites was analyzed via thermogravimetric analysis. Our results suggest that higher degradation temperature resulted from increasing the amount of cellulose in the composites bearing STO nanoparticles, which is opposite to the observed behavior for the composites without STO nanoparticles (**Figure 8**). In other words, the higher stability of the STO phase prevents early degradation (low Tdeg) of the biocomposites. This outcome suggests that higher levels of both cellulose and STO could render these composites suitable for high temperature applications as in instru-

In terms of mechanical behavior, the ultimate tensile strength (UTS) was determined using a uniaxial testing machine, as mentioned before. **Figure 9** reveals that higher UTS values were obtained in composites bearing more cellulose: from 15 to 25 v% for the composites containing STO nanoparticles, which is contrary to the behavior observed in composites without nanoparticles. When the UTS values for the composites are compared, the UTS values increased as the percentages of STO nanoparticles decreased from 20 to 0 wt%. We deem this an important finding as it enables the design of devices that can withstand minor loads with-

The addition of cellulose lowered slightly the Tdeg and UTS for the composites without STO nanoparticles, an outcome attributed to the pH value of the solution. As shown in **Table 1**, the pH value for the composites without STO nanoparticles increased from 4.78 to 5.08. At higher pH values, the amino groups are protonated causing electrostatic repulsion between the polymer's chains [27]. Therefore, the electrostatic repulsions better the swelling degree of the polymer, as the water content heightens in the polymer [29]. Because of this apparent

**Figure 8.** TGA analysis for composites made of 1.5v% chitosan/0.5v% cellulose considering 20, 10, and 0 wt% of

**Figure 6.** Effects of the applied electrical field, cellulose and STO nanoparticles content in the current density passing through the capacitors.

slightly [27]. Additionally, due to the fact that for safety reasons our instrumentation could only apply 60 V, we could not observe any breakdown in the composites up to that voltage. Not finding that breakdown voltage proves to be one limitation of the present research that could be overcome with a more energetic testing system. Yet, we strongly believe that the capacitors could withstand higher voltages with the addition of STO nanoparticles.

Furthermore, the electrical conductivity of the dielectric material was computed from the current density. For this calculation, a linear regression analysis was applied to the curves of current density as a function of voltage. The slope of the curve represented the conductivity divided by the thickness of the capacitor (σ<sup>t</sup> ). According to the results, higher content of cellulose heightened the conductivity of the dielectric material while the addition of STO nanoparticles lowered it (**Figure 7**). This is an important finding, since one can design capacitors by tuning the levels of cellulose and STO particles present in the biocomposite which are also in agreement with a recent study by Wang et al. [28].

**Figure 7.** Effects of the cellulose and STO nanoparticles content in the electrical conductivity of the capacitors.
