*2.3.2. Electrical analysis*

To measure the capacitance of the composites, we utilized a QuadTech 2200 automatic transformer system (with an AC supply). The frequency range was set from 1 to 10 kHz. Once the capacitance readings as a function of the frequency were recorded, the dielectric constant was determined for the biocomposites using Eq. (1).

$$K = \frac{C.d}{A.\varepsilon\_0} \tag{1}$$

where K is the dielectric constant; C, the capacitance of the dielectric material; A, the area of the plates; d, the distance between the plates; and ε<sup>o</sup> , the permittivity of vacuum [16].

To study dielectric breakdown in the composite, a GW INSTEK GPS-3303 DC power supply provided the current passing through the material when the applied voltage changed from 5 to 60 V. A GW-INSTEK GDM-8246 power meter connected in series was used to register the current values by following the guidelines of the ASTM D-149 standard [17]. According to this standard, the current flow must be recorded at equal increments of voltage and after a specific time. For our research, 5 V increments were applied to the dielectric material and then, after 10 s, the current flow was recorded at each of those increments. The current allowed us to determine the conductivity of the dielectric material.

To construct the capacitor, an AJA ATC Orion magnetron sputtering unit permitted to apply a titanium coating on both sides of the biocomposite film for 20 min at 200 W. All these capacitors were characterized using a Caframo™ stirrer clamp made of cast zinc-aluminum alloy and coated with epoxy as shown in **Figure 1**. The stirrer clamp also included a hold chuck key to ensure a close circuit between the capacitor and the copper electrodes. One electrode was placed at the base of the chuck key while another electrode was placed at the base of the Calframo™ stirrer clamp. In addition, a small weight of 1.27 N was placed on top of the chuck key to ensure proper contact between the capacitor and the electrodes. The ASTM D-150 standard provided guidelines to select the dimensions of the capacitors, i.e., 10 mm in length and 10 mm in width [18, 19].

Furthermore, reducing the size of the particles can create impurities upon ball milling and to confirm that the crystalline structure of the strontium titanate remained without changes, the powder was analyzed by x-ray diffraction. **Figure 2** displays the resulting diffractogram that

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

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

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As aforementioned, from the diffractograms, Scherrer equation allowed estimating the particle size of the STO powder at different milling times. The equation takes into consideration the width of the largest XRD peak in the diffractogram [21, 22], which in our case (**Figure 2**) lies at θ = 32.5°. The Miller indexes were identified by comparison with a prior publication by

**Figure 3** presents the milling time effect on the particle size. Two stages can be observed in the reduction (by fragmentation) of the particle size. The first stage occurs between 0 to 5 h of milling and presents a rapid reduction in the particle size from 46 to 18 nm. The rapid reduction in the particle size may be attributed to the fragmentation of large agglomerates into individual aggregates and therefore, the fracture of aggregates into individual primary particles and small aggregates [24]. In the second stage (5–20 h) of milling, the change in size was negligible. This was attributed to the increasing agglomeration of the fractured particles that impedes further fragmentation and, hence, size reduction; further, formation of surface cracks between the small aggregates or at the surface of individual particles could also have

As indicated previously, the relevance of adjusting the particle size lies on its effect on the electrical and mechanical properties of the nanocomposites. According to the literature, the nanoparticles agglomeration could adversely affect the electrical and mechanical properties of the nanocomposites [21, 22]. As the particle size decreases, the dispersion of the nanoparticles increases in the polymeric matrix and, therefore, improves the electrical and mechanical

Scanning electron microscopy (SEM) images in **Figure 4** allow observing the dispersion of the nanoparticles in the polymer matrix. At low magnification, the presence of STO aggregates

**Figure 2.** X-ray diffraction pattern of STO after 10 h of milling and powder diffraction line pattern.

matched with the expected STO pattern; the presence of impurities was not detected.

Trepakov et al. [23].

occurred [24].

properties.

**Figure 1.** Calframo™ stirrer clamp utilized in the measurement of the dielectric properties.

### *2.3.3. Thermal analysis*

A Mettler Toledo TGA/SDTA thermogravimetric / differential thermal analyzer (operated at a 5°C/min. Temperature ramp from 25 to 500°C) allowed determining the degradation temperature (Tdeg) of the composites in a nitrogen atmosphere. The samples were placed on a weight scale, which detects the mass loss of the samples as a function of temperature. To determine the said Tdeg, the first derivate was applied to the heating rate curve obtained from the thermal analyzer.

### *2.3.4. Mechanical characterization*

The samples' ultimate tensile strength was measured at room temperature (25°C) using a low force Instron® model 5944 universal testing machine. The deformation velocity was set at 1 mm/min. Also, the dimensions of the samples followed the ASTM D-1708, which specifies a 22 mm gauge length, a 5 mm width, and a 5 mm radius fillet.
