Study On Electrical Performance Regulation And Dielectric Relaxation Behavior Of Copper - Titanate Yttrium - Based Dielectric Materials

The search for new high performance dielectric materials, exhibiting temperature-and frequency-stable, colossal permittivity （ε’> 1000） as well as sufficiently low dielectric loss, continues to arouse considerable interest motivated by their myriad device miniaturization and high-energy-density storage applications. Subramanian et al. firstly reported that CCTO ceramics possessed giant dielectric response in 2000. CCTO has a body-centered cubic perovskite-related structure and shows impressive dielectric properties （104～105） below 1 MHz. Moreover, the permittivity is practically independent of temperature between 100 and 600 K. Since CCTO was found to have a giant dielectric constant, it has attracted particular attention. During the last decade, owing to the high dielectric loss, the researches on CCTO were mainly focused on lowering the dielectric loss to realize practical application through doping, introducing binary phase, using new preparation methods and so on. Although, to some extent, the dielectric loss has decreased obviously, it could not meet the requirements of commercial application. It is noted that Y_2/3Cu₃Ti₄O₁₂ （YCTO） as one of ACu₃Ti₄O₁₂ （ACTO） materials exhibits the lowest dielectric loss about 0.049 at 100 kHz, however, the dielectric constant is relatively low about 1743 which is much lower than that of CCTO ceramics. Why do YCTO ceramics with a similar phase structure and composition as CCTO not show giant permittivity? Whether can we obtain giant dielectric properties in YCTO ceramics through refining the preparing conditions? How do the A site ions influence the dielelctric properties of ACTO materials?In this thesis, firstly, YCTO ceramics with giant permittivity, relatively low dielectric loss, good temperature- and frequency-independent dielectric properties were obtained by tuning the solid-state reaction conditions. The physical origins of giant dielectric constant were also revealed in detail. In addition, the dielectric properties of YCTO ceramics were further improved by A-site ions-doping, especially for decreasing the low-frequency dielectric loss. Finally, the high-temperature dielectric relaxation behavior of ACu₃Ti₄O₁₂ ceramics with different A-site ions were discussed and clarified systematically. The main conclusions obtained from experimental results and analyses were as follows:1. YCTO ceramics with giant permittivity, low dielectric loss, good temperature-and frequency-independent dielectric properties were prepared by the solid-state reaction method. Furthermore, the physical origins of giant dielectric constant were explored in detail. Effects of sintering conditions on microstructure, phase structure and the dielectric properties of Y_2/3Cu₃Ti₄O₁₂ ceramics were investigated in detail. Y_2/3Cu₃Ti₄O₁₂ ceramics sintered at 1010 ℃ for 25 h exhibited a giant dielectric constant （1.10×104） and a relatively low dielectric loss （0.033） around room temperature. The samples showed good temperature stability （ACT/C25℃=-6.7% to 9.5%） in the temperature range from -60 to 125 ℃ at 10 kHz. The complex impedance spectroscopy analysis suggested that Y_2/3Cu₃Ti₄O₁₂ ceramics were electrically heterogeneous, and they consisted of semiconducting grains and insulating grain boundaries, which could be modeled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The effects of dc voltage on the low frequency dielectric properties indicate that the electrode effect nearly does not contribute to the giant permittivity of YCTO ceramics. The surface layer slightly affects the low frequency giant dielectric properties, but, it is not the main reason of colossal permittivity. Totally, the giant permittivity phenomenon could be explained by internal barrier layer capacitance （IBLC） effect. According to the XPS results, the semiconductivity of grains should be resulted from the electron hopping between Ti_<sup>3+ and Ti_<sup>4+ aliovalences. And the insulating grain boundary is associated with Cu-rich phase at grain boundary.2. In order to further clarify the origin of semiconducting grain and insulating grain boundary of YCTO ceramics, the composition and electric response of grain boundary and grain were carefully modified by varying the Cu and Ti stoichiometry, respectively. The microstructure, electric and dielectric properties of Y_2/3Cu_<sup>3+xTi₄O₁₂ （-0.10= x= 0.10） ceramics as a function of Cu concentration were investigated in detail. Appropriate excess in Cu amount would improve the grain growth and give rise to a significant decrease in the low-frequency dielectric loss. The dielectric relaxation behaviors correlated with grain boundary response was obviously intensified by increasing the Cu content. When x≥ 0.10, the activation energies for the conduction in grain boundaries were substantially enhanced. The nonlinear I-V behavior further evidenced that the grain boundary resistance could be successfully modified by tuning the Cu amount. The enhanced grain boundary resistance should be responsible for the lowered low-frequency dielectric loss, the increased activation energies for the conduction and the improved nonlinear I-V behavior. Furthermore, the enhanced grain boundary resistance might be attributed to the segregation of Cu-rich phase at grain boundary which had been evidenced by EDX results. The variation of Ti stoichiometry （below or above 4） will give rise to a gradual decrease in grain size. Furthermore, the low-frequency and intrinsic dielectric constant were significantly enhanced by the variation of Ti stoichiometry. For the Ti-deficient samples, the intrinsic dielectric constant was substantially enhanced from 80 to 120. Complex impedance results suggest that the resistance of grain boundary increase and the resistance of grain decreases when the Ti stoichiometry is below 4. The decrease of grain resistance should be attributed to the increase of Ti³⁺/Ti⁴⁺. When the Ti stoichiometry is above 4, the resistance of grain boundary decrease and the resistance of grain nearly keep unchanged. The activation energies of relaxation behavior of grain increase with the variation of Ti’ stoichiometry, which is closely correlated with the increase of Ti³⁺/Ti⁴⁺.3. The A-site vacancy and low frequency dielectric loss were obviously depressed by K and Na doping. After ions doping, the dielectric constant of YCTO ceramics also keep at a relatively high value. The physical mechanisms of the variation of dielectric properties after ion doping were clarified in detail. K and Na doping is of great benefit to the growth of the grain size. Proper amount of K and Na substitution in Y_2/3Cu₃Ti₄O₁₂ ceramics makes the dielectric loss significantly decreased. For K doping damples, very large εr of～1.1×1O4 and relatively low tan S of ～2.6% are simultaneously observed for the KxY（2-x）/3Cu₃Ti₄O₁₂ samples with x= 0.020 and 0.035 when measured at～10 kHz. The lowered dielectric loss is closely associated with the enhanced localized behavior of conduction process at grain boundary. The dielectric relaxation behaviors of grain boundary become much more difficult after K doping. For Na doping samples, Na0.050Y0.650Cu₃Ti₄O₁₂ ceramics exhibit the lowest dielectric loss （about 0.022 at 1 kHz） and a relatively high dielectric constant （about 7500 at 1kHz）. The lowered dielectric loss is closely associated with the enhanced resistance of grain boundary. The conduction and dielectric processes of grain boundary become much more difficult after Na doping. Complex impedance analysis suggests that the same entities （doubly-ionized oxygen vacancy） are responsible for the conduction and dielectric relaxation behaviors of grain boundary of KxY（2-x）/3Cu₃Ti₄O₁₂ and NaxY（2-x）/3Cu₃Ti₄O₁₂ ceramics. Scaling behaviors indicate that the physical nature of their dielectric relaxation and conduction behavior are independent of the measurement temperature.4. The grain size of YCTO ceramics has been refined by rare-earth ions doping, which would enhance the volume and the resistance of grain boundary, finally, the low-frequency dielectric loss was depreseed successfully. The correlations between low-frequency dielectric loss and grain boundary resistance were revealed detailed. The effects of La or Nd content on the phase structure, microstructure, dielectric relaxation behavior, and grain boundary response of YCTO ceramics were studied in detail. For La-doped Y_2/3-LaxCu₃Ti₄O₁₂ （0.00≦x≦0.20） samples, proper amount of La substitution in Y2/3-xLaxCu₃Ti₄O₁₂ ceramics made the dielectric loss decreased. When x=0.10, Y_2/3-0.10La_0.10Cu₃Ti₄O₁₂ ceramics exhibited the highest grain boundary resistance （0.893 MΩ） and the lowest dielectric loss （about 0.025 at 1 kHz）, meanwhile the samples exhibited a relatively high dielectric constant above 6000 over a wide frequency range from 40 Hz to 1 MHz. With the increase of La concentration, the dielectric relaxation behaviors correlated to the grain boundary effects were significantly enhanced. By La doping, the activation energies for the conduction in grain boundaries were slightly depressed. For Nd-doped Y2/3-xNdxCu₃Ti₄O₁₂（0.00≦x≦0.12） samples, When x= 0.06 and 0.09, the samples exhibited the lowest dielectric loss （about 0.028 at 10 kHz） and a relatively high dielectric constant （above 10000）. Nd doping weakens the dielectric relaxation behavior, therefore, the temperature stability of dielectric constant was improved. Unlike La doped samples, Nd doping results in an increase of the activation energies for the conduction in grain boundaries. Furthermore, the activation energies for the relaxation process in grain boundaries were slightly changed by La or Nd doping. Electric modulus results that the temperature stability of grain boundary capacitance was improved by La doping, however, it is nearly unchanged by Nd doping. Finally, the decreased dielectric loss in La and Nd doped samples was attributed to the enhanced grain boundary resistance. The same defect charges are responsible for the conduction and dielectric relaxation behaviors of grain boundary in these samles.5. Na_0.5Y_0.5Cu₃Ti₄O₁₂ ceramics with giant permittivity have been prepared by the conventional solid-state reaction method. The physical origins of giant dielectric constant were also revealed in detail. The best performance of NYCTO ceramics were obtained when sintered at 1060 ℃ for 25 h. Two electrical responses were observed in the combined modulus and impedance plots, indicating the presence of Maxwell-Wagner relaxation. The contributions of semiconducting grains and insulating grain boundaries （corresponding to high-frequency and low-frequency electrical response, respectively） played important roles in the dielectric properties of Nao.5Yo.5Cu₃Ti4012 ceramics. The correlations between grain boundaries resistance and low frequency dielectric loss, grains resistance and the position of dielectric loss peak were addressed. Mixed-valent structures of Cu2⁺/Cu³⁺ and Ti³⁺/Ti⁴⁺ had been determined using X-ray photoelectron spectroscopy. Electron hopping between Cu2⁺ and Cu³⁺ and electron transport in Ti³⁺O--Ti⁴⁺ paths were proposed as the origin of the semiconducting nature of Na_0.50Y_0.50Cu₃Ti₄O₁₂ ceramics.6. The dielectric properties of NYCTO ceramics were successfully improved by tuning the Na/Y. The effects of Na/Y on the phase structure, microstructure, dielectric relaxation behavior and electric response of NYCTO ceramics were studied. The increase of Na/Y facilitates the grain growth. Compared with YCTO ceramics, the giant permittivity plateau of Na_0.20Y_0.60Cu₃Ti₄O₁₂, Na_0.35Y_0.55Cu₃Ti₄O₁₂ and Na_0.50Y₀.50)Cu₃Ti₄O₁₂ ceramics broadens to the high frequency. Compared with NYCTO, the frequency stability of low-frequency dielectric constant was substantially improved by decreasing the Na/Y, meanwhile, the dielectric loss was decreased. The decreased dielectric loss was linked with the enhanced resistance of grain boundary. Na_0.35Y_0.55Cu₃Ti₄O₁₂ ceramics exhibited relatively good dielectric properties: ε’=8000, tan δ=0.029 at 10 kHz. All the samples showed two dielectric relaxation behaviors in ε’（T） plots. The low temperature one could be attributed to the grain boundary response. With the increase of Na/Y, domain boundary responses start to contribute to the electric response of samples. Furthermore, the activation energies of conduction process of grain boundary were significantly enhanced by increasing the Na/Y.7. The high-temperature dielectric anomaly of ACu₃Ti₄O₁₂ (A=Y2/3, Na_0.5Y_0.5, Na_0.5Bi_0.5) were discussed and explored. For YCTO ceramics, a step-like increase of dielectric constant was observed 250 ℃, which should be associated with oscillations of space charge caused by accumulation of free charges at the two electrode interfaces. For NYCTO ceramics, a dielectric peak exhibiting relaxor-like behavior was observed around 250 ℃, which was linked with the formation of oxygen vacancies. Based on the electric modulus results, this behavior was explored in the light of the defect formation and explained in terms of a competition process depending on the combinational contribution to polarization between n- and p-type carriers rather than a relaxation process. In addition, the broad dielectric peaks were observed around 250 ℃ in Na_1/2Bi_1/2Cu₃Ti₄O₁₂ ceramics and could be fine fitted by the modified Curie-Weiss law and the Vogel-Fulcher relationship. Based on the universal dielectric response law and Maxwell-Wagner relaxation, the occurrence of the polarization mechanism transition from the grain boundary response to the electrode one with temperature is clearly evidenced in the low frequency range. It is likely to propose that the apparent relaxor-like behavior is related to an extrinsic effect instead of ferroelectricity.