# Rapid Laser Reactive Sintering Synthesis of Colossal Dielectric CCTO Ceramics
## 3 Results and Discussion
As shown in Figure 3a the sample sintered at 25 W showed a intergranular porous structure with grain size of 50-100 nm in diameter. This indicates that the atomic diffusion and grain growth were not sufficient to form compact ceramics with subordinate laser power. When laser power increased to 45 W, the intergranular distance became smaller with denser grains. For the sample prepared at 65 W, it contained both smaller and larger grains, and some smaller grains grew into larger grains. Meanwhile, the particles started to come into closer contact and grow into compact structures. The sample prepared at 85 W showed a fine-grained, dense microstructure with an average grain size of 30 nm. This is consistent with the previous calculated grain size result from [PERSON]'s formula according to X-ray diffraction patterns. Figure 2: X-ray diffraction (XRD) patterns of CCTO ceramics prepared by rapid laser reactive sintering at different laser power rates. Figure 4 shows the frequency dependence of the dielectric permittivity and \(\tan\delta\) of CCTO ceramics at room temperature in the frequency range of 20-10\({}^{7}\) Hz. All the ceramics showed high permittivity (almost more than 10\({}^{4}\)) in the low-frequency region (\(f<\) 10\({}^{2}\) Hz), and the dielectric constant increased with raised laser power over the whole \(f\) range as shown in Figure 4a. The corresponding \(\varepsilon_{r}\) changed from about 10\({}^{4}\) to 10\({}^{2}\) in the range of 20-10\({}^{7}\) Hz for the samples synthesized at laser power of 25-65 W, showing the sharp decrease in permittivity. However, the permittivity of CCTO at 85 W experienced a slow decline from 10\({}^{5}\) to 10\({}^{4}\). It could be observed that the \(\varepsilon_{r}\) maintained almost constant values in the range of 10\({}^{2}\)-10\({}^{7}\) Hz, demonstrating good frequency stability. The highest permittivity value for CCTO was 10\({}^{5}\) at 100 Hz, and 3 \(\times\) 10\({}^{3}\) for CCTO prepared at 25 W at 100 Hz. The reported common value of permittivity can reach 10\({}^{3}\) for CCTO thin films, 10\({}^{4}\) for poly-crystalline ceramics, and 10\({}^{5}\) for single-crystalline [9, 11, 12, 19]. As per the diffraction patterns and SEM images, the CCTO sintered by laser should belong to a poly-crystalline structure. The \(\varepsilon_{r}\) increased quickly with increasing laser power, which can be seen from the changes in microstructures in the SEM images shown in Figure 3. The denser microstructures had a higher dielectric permittivity than those of a looser structure. When exceeding 10\({}^{5}\) Hz, the relationship curves of permittivity vs. frequency for the samples under 65 W linearly declined, which could be ascribed to the well-known Debye-like relaxation [19, 24]. As a result, the frequency stability should be closely connected with the grain size, density of grain boundaries, and the compactness of ceramics. In addition, the laser power should be regulated for a suitable value to synthesize pure CCTO, and the arbitrary power increase will lead to the appearance of impurities such as oxygen vacancies in CCTO. Figure 4b shows the frequency dependence of dielectric loss (\(\tan\delta\)) at room temperature. It can be seen that the \(\tan\delta\) of CCTO ceramics declined from a low frequency to a platform in middle frequency and then gently increased at high frequency. In the low-frequency range of 20-10\({}^{2}\) Hz, \(\tan\delta\) decreased slowly for all the samples with increasing laser power, which can be attributed to the direct current (dc) conduction effect in the bulk [25]. [PERSON] et al. [26] studied the CCTO-based nano-composite by the conventional solid-state synthesis method and concluded that the higher \(\varepsilon_{r}\) and loss at the low \(f\) may be ascribed to the appearance of interfacial space charge polarization.