| Giant dielectric materials have become increasingly important due to the strong technological needs for the further reduction of dimensional size and the enhancement of performance in capacitance-based components like capacitors. In2000, the CaCu3Ti4O12(CCTO) with perovskite-related structure has been extensively investigated because of its giant dielectric constant and good temperature stability. As a potential material for microelectronic devices, CCTO has a relatively high dielectric loss, which is disadvantageous for the application. In recent years, in order to obtain giant dielectric material of excellent performance and good practicability, the researches mainly focus on in two aspects:One is the ACu3Ti4O12ceramics with similar structures to CCTO, which was obtained by replacing Ca2+in CCTO with different ions, so the giant dielectric properties of CCTO was improved. Second, ACu3Ti4O12giant dielectric materials with excellent performance were prepared by adopting new technology, optimizing parameters. La2/3Cu3Ti4O12(LCTO) is a member of the ACu3Ti4O12family. But so far, there are limited literatures reporting the La2/3Cu3Ti4O12(LCTO) ceramics, and the microstructure and electrical properties of the LCTO ceramics prepared by the sol-gel method have not yet been reported. Interestingly, the LCTO ceramics prepared by the sol-gel and solid-state methods show entirely difference dielectric behaviors under the effect of temperature and frequency according to our previous research, which attracts our research groups’ attention, while the differences have not yet been researched and analyzed in ACu3Ti4O12ceramics. In this paper, obtaining the dielectric materials with high dielectric constants, low dielectric loss and good temperature stability is the objective of the study. The ACu3Ti4O12(A=La2/3, NaxLay) ceramics prepared by sol-gel method is the main object of the study. The effects of sol and sintering conditions on the ceramic microstructure and electric properties were detailed researched. The dielectric materials with superior properties were prepared and choosed by optimizing the preparation technology. The differences in microstructure and electrical properties of ACu3Ti4O12(A=La2/3,A=La2/3,Na0.5La0.5) ceramics prepared by solid-state and sol-gel methods were analyzed. The effects of different "A" site ionic (La3+, Li+, Na+, K+. NaxLay) and different content of "A" site ionic (NaxLay) on microstructure and electrical properties of ACu3Ti4O12ceramics were also investigated. Through the above research work, the main conclusions obtained from experimental results and analyses were as follows:1. La2/3Cu3Ti4O12(LCTO) ceramics were prepared by sol-gel method (SG) and solid-state method (SS), which were indicated as LCTO-SG and LCTO-SS, respectively. Effect of Ti4+concentration, water content (molar ratio of the water and titanium) and pH value of the sol on microstructure and dielectric properties of LCTO-SG ceramics were investigated in detail, respectively. The optimum sol conditions were determined as follows:[Ti4+]=1.00mol/L, H2O/[Ti]=5.6, pH=1.0. Through optimizing the sintering process of LCTO-SG and LCTO-SS ceramics, it was found that when sintered at1105℃for15h, LCTO-SG and LCTO-SS ceramics exhibited the best microstructure and dielectric properties. In addition, the microstructure and electrical properties of LCTO precursor powders and ceramics prepared by sol-gel method and solid-state method were compared and analyzed. The results showed the formation temperature of crystal phase structure for LCTO-SG ceramics was lower. It was suggested that sol-gel method has advantage on reducing the reaction temperature. Compared with LCTO-SS ceramics, the LCTO-SG ceramics showed larger grain size, higher density and especially higher dielectric constant up to about0.9-1.6x104at102-105Hz. The higher dielectric constant of the LCTO-SG ceramics might be due to the stronger internal barrier layer capacitor (IBLC) effect. From the dielectric temperature spectrum, impedance spectrum, electric modulus and conduction as well as related activation energies, the LCTO-SG ceramics showed third kind of dielectric anomalies peak, which might be attributed to the distribution state transitions from static state to dynamic state of oxygen vacancies, two values of conduction activation energy and conduction activation energy decrease with increasing temperature, compared with LCTO-SS ceramics. The activation energies for the conduction and relaxation process in LCTO-SG ceramics showed great difference below about210℃, suggesting that the mechanism of electrical conduction and dielectric relaxation seem to be different in LCTO-SG ceramics. The different dielectric behaviors might be attributed to different defects (oxygen vacancy concentration and form) besides grain size. It suggested that the different preparation methods had different effects on defect structure of ceramics, which affected the ceramics electrical behavior.2. Na0.5La0.5Cu3Ti4O12(NLCTO) ceramics, Na+substituting for La3+in LCTO ceramics, were prepared by solid-state method and sol-gel method, which were signified as NLCTO-SG and NLCTO-SS, respectively. Effect of Ti4+concentration, water content (molar ratio of the water and titanium) and pH value of the sol on dielectric properties of NLCTO-SG ceramics were investigated in detail, respectively. It was found that the optimum concentration, the molar ratio of the water and titanium, and pH value of the sol were1.00mol/L,11.0and0.3, respectively. Through optimizing the sintering process of NLCTO-SG and NLCTO-SS ceramics, it was found that NLCTO ceramics sintered at1080℃for10h exhibited higher dielectric constant (1.1-1.8×104,102-105Hz), lower dielectric loss (0.050-0.064,1-10kHz), better frequency stability and temperature stability, compared with LCTO ceramics. From the results of research, it concluded that when using0.5mol Na+substituting for La+in LCTO ceramics, the dielectric properties of the LCTO ceramics effectively were improved. Analyzing microstructure and electrical properties of NLCTO precursor powders and ceramics prepared by sol-gel method and solid-state method, it concluded the formation temperature of crystal phase structure for NLCTO-SG ceramics were lower and sol-gel process had advantage on reducing the reaction temperature. Comparing with NLCTO-SS ceramics, NLCTO-SG exhibited homogeneous microstructure, higher dielectric constant and lower dielectric loss. The higher dielectric constant of the NLCTO-SG ceramics might be due to the stronger internal barrier layer capacitor (IBLC) effect. From the dielectric temperature spectrum, impedance spectrum, electric modulus and electric conduction as well as related activation energies, it was clearly found that NLCTO-SG ceramics showed the third dielectric abnormal peak and two conductance activation energy values which decreased with increasing temperature comparing with NLCTO-SS ceramics, which similar to LCTO-SG ceramics. At the same time, two dielectric relaxation peaks were observed in the NLCTO-SG electric modulus figure, which might be due to the response of grain boundary and domain wall, respectively, appearing two types of Maxwell-Wagner relaxation. The NLCTO ceramics prepared by sol-gel method and solid-state method exhibited different dielectric behaviors, which might also be attributed to different defects (Oxygen vacancy concentration and form) besides grain size. It further suggested that the different preparation methods had different effects on defect structure of ceramics, which affected the ceramics electrical behavior. Based on results of chapter3and chapter4, it was concluded that the sol-gel method not only can optimize process parameters and improve microstructure and dielectric constant of ceramics, but also seem to change electrical behavior of the ceramics, comparing with the solid-state method.3. Li1/2La1/2Cu3Ti4O12(LLCTO) and K1/2La1/2Cu3Ti4O12(KLCTO) ceramics were prepared by sol-gel method, respectively. Effect of sintering condition on dielectric properties of Li1/2La1/2Cu3Ti4O12(LLCTO) and K1/2La1/2Cu3Ti4O12(KLCTO) ceramics were investigated in detail, respectively. The excellent dielectric properties of the LLCTO ceramics (ε=6.03×l04, tanδ=0.072,1kHz) which were pre-calcined at650℃and sintered at1080℃for5h and KLCTO (ε=6.30×104, tanδ=0.494,1kHz) which were pre-calcined at800℃and sintered at1080℃for5h were obtained. Microstructure and electrical properties of LLCTO, NLCTO, KLCTO ceramics by different ion (Li+, Na+, K+) substituting for parts La3+in LCTO ceramics were compared and analyzed. All ceramics obtained pure cubic perovskite structure, only KLCTO ceramic appeared secondary phases. With the increase of the substituting ionic (Li+, Na+, K+) radius, the cell parameters of corresponding ceramics increased, and microstructure showed obvious difference. When using Li+substituting for parts La3+, the corresponding ceramics showed better frequency and temperature stability, but its dielectric constant was lower. Meanwhile, it was found that the third dielectric abnormal peak was not observed in the dielectric temperature spectrum. When using the Na+substituting for parts La3+, the corresponding ceramic showed the higher dielectric constant, better frequency and temperature stability, and showed the third dielectric abnormal peak and two conductance activation energy values, appeared another Maxwell-Wagner relaxation that might be due to the response of domain wall. When using K+substituting for parts La3+, corresponding ceramics performance was deteriorated. From the above results, although Li+, Na+, K+had similar structure, there were greatly different effects on the microstructure and electrical properties of the ceramics. This might be due to the different degree of discrepancy between substituting ionic radius and La3+radius, which leaded to different degree of Li+, Na+, K+substituting for La3+. This corresponding ceramics presented different grain size and defect structure (concentration and existing form of’A’site vacancies and oxygen vacancies etc.), so the ceramics showed different electrical behavior.4. Na0.5-xLa0.5+xCu3Ti4O12(x=0.0,0.1,0.2,0.3) ceramics were designed, prepared and researched on the premise of unchanging the mole ratio of atoms. With decreasing the content of Na+and increasing the content of La3+, all ceramics obtained pure cubic perovskite structure, and the cell parameters of corresponding ceramics increased, but the grain size hardly changed. The dielectric constant decreased after the first increased When x=0.1, the dielectric constant reached largest (17510,1kHz), but the dielectric loss gradually increased. At1kHz, the dielectric constants were15000,17510,15044,13950respectively. The giant dielectric properties might be due to IBLC effect. It was found that with decreasing the content of Na+and increasing the content of La3+, grain boundary resistance (Rgb) and grain resistance (Rg) gradually decreased. This might be attributed to the charge compensation effect enhancements. The charge compensation effect enhancements, gathering charge on both sides of the grain boundary increases, according to the IBLC effect, grain boundary capacitance increased accordingly. But the contribution of charge compensation effect to dielectric constant seems to have an extreme value, so the dielectric constants showed decrease after increase first. In addition, it was discovered in dielectric temperature spectrum that with decreasing the content of Na+and increasing the content of La3+, temperature stability of ceramics gradually worsened. But all the ceramics showed similar dielectric abnormal peak, one of which was the typical thermal activation Debye relaxation, which might be attributed to the distribution state transitions from static state to dynamic state of oxygen vacancies. From electric modulus, Imaginary part of Impedance vs frequency and corresponding Arrhenius plots as well as related activation energies, it was seen that with decreasing the content of Na+and increasing the content of La3+, the ceramics showed similar relaxation characteristic, two types of Maxwell-Wagner relaxation caused by grain boundary and domain boundary, respectively. Meanwhile it was found that when x=0.1, the ceramics only had a conduction activation energy. This indicated that on the premise of unchanging the mole ratio of atoms, with the content change of Na+and La+, appeared charge compensation effects, which had obvious effects on conductivity characteristics of ceramics, but had a smaller effects on dielectric relaxation characteristics of ceramics.5. NaxLa(2-x)/3Cu3Ti4O12(x=0.50,0.35,0.20,0.05) ceramics were designed, prepared and researched on the premise of charge balance. With decreasing the content of Na+and increasing the content of La3+, all ceramics obtained pure cubic perovskite structure, and the cell parameters of corresponding ceramics increased, the grain size decreased, dielectric constant decrease after increase firstd. When x=0.35, the dielectric constant reached largest (17300,1kHz). At1kHz, the dielectric constants were15000,17300, 7330,5892respectively, but the dielectric loss gradually increased. The giant dielectric properties might be attribute to the IBLC effect. It was found that when x=0.35, corresponding ceramics Rg and Rgb slightly decreased. When x=0.20,0.05, corresponding ceramics Rg and Rgb significantly increased. The increase of the grain boundary resistance and grain resistance, could be interpreted as association effects between a large amount of "A" site vacancy (equivalent to negative charged) and oxygen vacancy (equivalent to positively charged), hindering move of conductance carriers, and so the grain resistance and grain boundary resistance increased. According to effect of the IBLC model, change of ceramics boundary resistance (Rgb) and grain resistance (Rg), resulted in the change of ceramics dielectric properties. In addition, it was discovered from dielectric temperature spectrum that when x=0.20, only showed the typical thermal activation Debye relaxation. Based on the analytic results from electric modulus, Imaginary part of Impedance vs frequency and corresponding Arrhenius plots as well as related activation energies, it was found that when x=0.50,0.35, the ceramics showed two types of Maxwell-Wagner relaxation caused by grain boundary and domain boundary, respectively. When x=0.20,0.05, the Maxwell-Wagner relaxation caused by domain disappeared. But with decreasing the content of Na+and increasing the content of La3+, all ceramics showed a linear area, a conductance activation energy values except x=0.50. From the above results, on the premise of charge balance, with the content change of Na+and La3+, content of "A" site vacancy increased, which had obvious effects dielectric relaxation characteristics of ceramics, but had smaller effects on conductivity characteristics of ceramics.6. From the results of chapter6and chapter7, it was found that when x=0.1and x=0.35, dielectric constants of corresponding ceramics showed abnormal increase. Corresponding ceramics appeared two types of Maxwell-Wagner relaxation caused by grain boundary and domain boundary, and two activation energies for relaxation process were very close, and corresponds closely with activation energies for conduction process. It suggested that ceramics appeared multiple internal barrier layer effect, and Maxwell-Wagner relaxation mechanism caused by the barrier layer was same as the conduction mechanism of the ceramics. The ceramics might exhibit a higher dielectric constant. |
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