3.2. Physico-chemical characterization
Fig. 3 (a) shows XRD diffraction patterns of prepared materials. All samples exhibited low-crystalline hexagonal structure with broad reflections characteristic for HAp with nanocrystalline nature. Crystallite size in [002] crystallographic direction, d(002), was calculated and compared for different materials, as shown in Fig. 3 (b). Regardless of the investigated synthesis temperature application of AC electric field influenced the growth of crystallites with their size increasing almost 10% compared to that obtained in the absence of electric field. Crystallite size for the HAp powder prepared at 30 °C under AC field was similar to that of HAp prepared at 50 °C without the applied field, which implies that the electrical energy contributes to the crystal growth. The growth of crystallite size with processing temperature was already reported for HAp ceramics [28]. Moreover, reflection (202) was not observed for the materials synthesized at 30 °C, regardless of the AC field presence, while it was clearly defined for the material synthesized at 50 °C in the presence of AC electric field. That can also be observed for (102) and (210) reflections. These observations indicate that the presence of AC electric field contributed to the crystallization of HAp crystal lattice in a similar manner as the usually applied temperature increase. This is an important finding, since the synthesis of HAp at lower temperatures results in a low-crystalline material with an amorphous HAp matrix. After 10 h in simulated body fluid conditions at 37 °C, prepared HAp exhibited crystal structure and chemical functionality [29] similar to that of HAp prepared at 30 °C under the action of AC field, but with low thermal stability. The obtained values of crystallite size are comparable to those reported in the literature for the same pH value and synthesis temperature [30]. Table 3 shows the changes in d(002) and in the unit cell parameters. Calculation of unit cell parameters revealed that the parameter a = b is decreased by the application of AC field. Parameter c is similar for both HAp materials prepared at 30 and 50 °C without AC field, while it is decreased after AC field application at 30 °C, and increased when synthesis temperature is raised at 50 °C. The observed changes in unit cell parameters are similar to those reported for low-temperature sol-gel synthesis of HAp, but with longer reaction times [31]. Local movement of reactant ions can be aligned and preferably driven by the imposed AC electric field. Under such conditions, mutual interaction of reactant ions could be improved by the applied electrical energy and lead to enhanced crystal growth. This is corroborated by the XRD measurements showing increased crystallite size along the crystallographic direction [002]. Although the results of only two electrical conditions are presented, it should be noted that modulation of crystallite size also happened for the syntheses in the presence of AC field even for lower amplitudes of electric excitation with extended reaction time. Results of preliminary experimental tests showed that mean crystallite size changes systematically in accordance with excitation amplitude (see supplementary section “Optimization of electrical excitation parameters”).