Physical Properties Of CaCu₃Ti₄O₁₂-based High-dielectric Ceramics And BaTiO₃ Piezoelectric Ceramics

Dielectric and piezoelectric materials are two classes of important electronic materials.Dielectric ones are widely used to form capacitive devices such as capacitance,resonators and filters.Those dielectric materials with high dielectric permittivity have attracted considerable interest in recent years since they might offer the potential to enhance the performance or shrink the dimensional sizes of the microelectronic devices.Piezoelectric ones are the class of functional materials that realize the exchange between mechanical energy and electrical energy and thus are popularly utilized to fabricate sensors,actuators,transducers and other electronic devices.Currently,Pb(Zr,Ti)O₃(PZT)-based piezoelectric ceramics take the major market part of practical piezoelectric materials because of their excellent electrical properties.However,due to the toxicity of lead oxide that is largely used in the production,there is an increasing demand to replace PZT with the environment-benign lead-free alternatives.Under the circumstances,this thesis work concentrates mainly on the studies of material preparations,physical properties and the related mechanisms for CaCu₃Ti₄O₁₂(CCTO)-based high-dielectric ceramics and BaTiO₃ piezoelectric ceramics.CCTO is the oxide that has a cubic perovskite-related crystal structure and exhibits an enormously large dielectric permittivity(ε') at low frequencies in both forms of single crystals and ceramics.The dielectric permittivity is almost a constant in the low frequency range below 100 kHz at room temperature and is nearly independent of temperature over the wide temperature region.So far,several models have been proposed to explain the dielectric behavior but are quite controversial, including both the intrinsic and extrinsic mechanism explanations from the viewpoints such as crystal structure,internal barrier layer capacitance(IBLC) effect and contact-electrode depletion effect.On the other hand,despite having many advantageous properties,CCTO ceramics also show a serious shortcoming of relatively high dielectric loss(tanδ),which may result in the large signal decay or the instable operation of electronic circuit in practical applications.Therefore,it is essential to find a way that could somehow reduce the dielectric loss to a sufficiently low level and maintain simultaneously the desirable characteristic of high dielectric permittivity for CCTO ceramics.BaTiO₃ is historically the first polycrystalline piezoelectric material and had been once widely used as a piezoelectric material before the discovery of PZT.Nowadays, however,its main technical applications are no longer as a piezoelectric but as a dielectric material,largely because of the poor piezoelectric properties(usually,d₃₃≤190 pC/N) in comparison with PZT.Nevertheless,surprisingly high d₃₃ values (350,460 and 788 pC/N,respectively) were reported recently for those BaTiO₃ ceramics that were prepared from hydrothermally synthesized fine BaTiO₃ powders by some special fabrication techniques like microwave sintering,two-step sintering and templated grain growth(TGG).These results indicate that BaTiO₃-based ceramics possess a high possibility to become a good lead-free piezoelectric material.This thesis treats the CCTO-based high-dielectric ceramics and BaTiO₃ piezoelectric ceramics prepared by the conventional solid-state reaction as the study objects.For CCTO-based high-dielectric ceramics,the effects of sintering condition, Cu-content and La-doping on microstructure,crystalline structure,dielectric and electric properties are investigated,and the related high-dielectric mechanism is discussed.For BaTiO₃ piezoelectric ceramics,ceramic preparation conditions and its influence on the piezoelectric properties,microstructure and domain pattern are investigated,and the related high-piezoelectric mechanism is discussed.1.A series of CCTO ceramics sintered at different temperatures are investigated from the aspects of microstructures,dielectric properties and complex impedances. It has been revealed that their microstructures can be categorized into three different types:Type-A(with the small but uniform grain sizes),Type-B(with the bimodal distribution of grain sizes) and Type-C(with the large and uniform grain sizes), respectively.The largeness of low-frequencyε' at room temperature is closely related to the microstructure and particularly to the grain size.The real parts of dielectric spectra show drastic decreases and the imaginary parts of dielectric spectra exhibit correspondingly peaks in the frequency range above 100 kHz.At high temperatures,different types of CCTO ceramics show the diverse temperature-dependent behaviors of electrical properties according to their microstructures.However,there also exist some common characteristics among the various temperature-dependent behaviors.For all of the ceramics,the high-temperature dielectric dispersion displays a large low-frequency response and two Debye-type relaxations.Furthermore,they all show three semicircles in the complex impedance plane.An equivalent electrical circuit model,which contains three RC(R_gC_g,R_gbC_gb and R_xC_x) elements in series connection,is adopted to explain the experimentally observed dielectric and electric properties.These complex impedance semicircles are considered to individually represent different electrical mechanisms.The one in the low frequency range arises most probably from the contribution of the domain boundaries,and the other two are ascribed to the contributions from the domains and the grain boundaries,respectively.2.The influence of Cu content on microstructure,crystalline structure,dielectric and electric properties of CaCu_3+yTi₄O₁₂ ceramics sintered at 1050℃for 10 h are studied.The ceramics with Cu-deficiency(y < 0) exhibit the microstructures of uniform grain size distribution,whereas those ones with Cu-stoichiometry(y = 0) and Cu-excess(y > 0) show the microstructures of bimodal grain size distribution. The largeness of low-frequencyε' at room temperature is very sensitive to the Cu-content.Ceramics with Cu-stoichiometry show the highest low-frequencyε' and the lowest domain resistance.Off-stoichiometry(y≠0) results in the decrease of low-frequencyε' and the increase of domain resistance.The abovementioned equivalent electrical circuit model is also adopted here to interpret the observed dielectric and electrical properties.From analysis,it is suggested that the CuO-segregation layer plays the important role in affecting the low-frequencyε' and domain resistance.3.An important progress is achieved in solving the large dielectric-loss problem of CCTO ceramics by the invention of partially substituting Cu with La.The ceramics with the nominal chemical compositions of CaCu_3-xLa_2x/3Ti₄O₁₂(CCLTO) possess simultaneously the rather high dielectric permittivity values and sufficiently low dielectric losses.The existence of CaTiO₃ secondary phase in CCLTO ceramics is confirmed by X-ray diffraction.It is proposed that the segregation of CaTiO₃ secondary phase at grain boundaries is caused by La-doping and Cu-deficiency and contributes significantly to the obtained excellent dielectric properties.Using the currently invented method,the ceramics with a nominal composition of CaCu_2.90La_0.20/3Ti₄O₁₂(which was sintered at 1050℃for 20 h) show itsε' of 7500 and tanδlower than 0.05 in the frequency range from 120 Hz to 200 kHz and its dielectric temperature coefficient(TCK) of about±15 ppm/℃between -80 and 125℃.The dielectric properties of this ceramics meet the Z5U requirements of EIA standard(Electronic Industry Association standard) and is much better than the X7A requirement of TCK～±55 ppm/℃.This is the best result that has been ever obtained in the word-wide study of CCTO high-dielectric materials.4.BaTiO₃ ceramics with high piezoelectric coefficient(d₃₃) have been successfully obtained through the conventional solid-state reaction route starting from ordinary BaCO₃ and TiO₂ powders.They are fabricated through synthesizing the fine BaTiO₃ powder of the average particle size about 0.5μm and the tetragonal crystallographic structure and sintering at comparatively low temperature conditions. As a result,the BaTiO₃ ceramics sintered at 1210℃is found to have the excellent piezoelectric properties of d₃₃ = 419 pC/N,d₃₁ = -163 pC/N,k_p = 0.453,k_t = 0.354 andε'= 3181 with tanδ= 1.36%at 1 kHz.This d₃₃ value is two times larger than the long-known 190 pC/N and higher than 374 pC/N of the famous soft piezoelectric material PZT-5A.The result has an important meaning in that it is a breakthrough for realizing BaTiO₃-based ceramics as the low-cost and popular lead-free materials to at least partially replace PZT in the near future.Furthermore,besides the advantage of low production costs over the ones that were adopted by Japanese researchers using hydrothermally-synthesized BaTiO₃ powder and some unusual sintering ways,our conventional ceramic fabrrication technique has also the flexibility merit of further modifying the practically desired properties such as the temperature stability by additions.Exprimentally,it has also been disclosed that d₃₃ increases and the average domain width in poled BaTiO₃ ceramics remains approximately constant of about 240 nm with the decrease of average grain size from 9.5 to 2.0μm.Based on the collective information analysis of crystalline structure,orthogonal-tetragonal phase transition,density,microstructure and domain configuration,the largeness of 90°-domain wall in the BaTiO₃ ceramics is considered as an important factor which significantly influences the d₃₃ value.