**3.3. Thermal and tensile analysis**

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

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

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

lose 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

). According to the results, higher content of cellu-

capacitors could withstand higher voltages with the addition of STO nanoparticles.

divided by the thickness of the capacitor (σ<sup>t</sup>

through the capacitors.

82 Ferroelectrics and Their Applications

agreement with a recent study by Wang et al. [28].

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

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 instrumentation operating in tropical regions.

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 without being mechanically ruptured upon service.

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 strontium titanate nanoparticles.

All in all, we were able to establish a characterization baseline that can serve as design platform for capacitors intended to sustain high load at relatively elevated temperatures by adjusting the cellulose level and nanoparticles content. In closing, we are confident that this research would lead to the creation of organic and inexpensive tunable capacitors for RF applications

On the Mechanical and Dielectric Properties of Biocomposites Containing Strontium Titanate Particles

http://dx.doi.org/10.5772/intechopen.76858

85

The present work aimed at studying the electrical, thermal, and mechanical properties of composites made of chitosan, cellulose and strontium titanate nanoparticles. The compositional variables considered encompassed: cellulose content (15 and 25 v%) and amount of

The chitosan-cellulose and polymer-nanoparticles composites were successfully fabricated via sol gel casting method. Strontium titanate nanoparticles were dispersed in the polymeric matrix using a double-layered technique. To achieve a better dispersion of the nanoparticles, the particles size was reduced from 43 to 18 nm using the high ball mill technique. As a result,

With respect to the measured electrical properties, the addition of the strontium titanate nanoparticles raised the dielectric constant, capacitance, and electrical resistivity of the composites, as expected from a dispersed dielectric material. Similarly, the addition of the nanoparticles decreased the current density passing through the biocomposite. In addition, the dielectric rupture of the composites was not observed up to a maximum applied voltage

Furthermore, mechanical and thermal analysis tests of the composites revealed that the addition of cellulose adversely affected the ultimate tensile strength and the degradation temperature of the composites without strontium titanate due to the high content of water. However, an opposing behavior was observed on the composites with strontium titanate nanoparticles, in which higher content of cellulose raised both the ultimate tensile strength and the degrada-

Preliminary work on strontium titanate-containing biocomposites was published in an international journal [11]. This work is supported by the National Science Foundation under research Grant Nos. 0833112 and 1345156 (CREST program) and under instrumentation Grant Nos. 0619349 and 0922994. The authors would like to thank the invaluable assistance of the undergraduate student Javier Martínez and the technical personnel of the Materials Laboratories and Power Electronics Laboratories of the University of Puerto Rico-Mayagüez.: Dr. Eduardo I. Ortiz Rivera, and the students Daniel A. Merced Cirino and Alexander Collazo

a high dispersion of the nanoparticles in the polymeric matrix was achieved.

tion temperature while the addition of titanate nanoparticles lowered it.

like antennas or MEMS-based tunable filters.

strontium titanate nanoparticles (10 and 20 wt%).

**4. Conclusions**

of 60 V.

Irizarry.

**Acknowledgements**

**Figure 9.** Tensile analysis for composites made of 1.5 v% chitosan/0.5 v% cellulose considering 20, 10 and 0 wt% of STO nanoparticles.


**Table 1.** Measurements of pH value for the solutions containing different percentages of cellulose and STO nanoparticles.

shortcoming, when designing a device with these composites, one must consider this finding. Further testing of these composites under high humidity environment could shed light on the potential water absorption leading to swelling.

A different behavior was observed in the composites with STO nanoparticles where the Tdeg and UTS increased. The polymer-polymer interaction and the polymer-particle interaction can be responsible of these results. The addition of STO nanoparticles could have furthered the entanglement of the polymer's chains and, consequently, the free volume. However, the water present in these free spaces can be removed during the drying process because the molecules of water are not confined between the polymer's chains.

In addition, the presence of more nanoparticles heightens the polymer-nanoparticles interfacial energy, which could cause cracks through the nanoparticles in the polymeric matrix. Besides, any agglomeration of nanoparticles can be detrimental for the polymer-nanoparticle interfacial bonding. Such agglomerates can favor the presence of pores and nucleate microspaces through the nanoparticles, raising the brittleness of the composites. All these are factors that require further experimentation, which falls beyond the scope of the present research.

All in all, we were able to establish a characterization baseline that can serve as design platform for capacitors intended to sustain high load at relatively elevated temperatures by adjusting the cellulose level and nanoparticles content. In closing, we are confident that this research would lead to the creation of organic and inexpensive tunable capacitors for RF applications like antennas or MEMS-based tunable filters.
