Abstract: 　　Information functional ceramics is a class of ceramic materials which have unique electrical, magnetic, acoustic, optical, thermal, mechanical, chemical and biological properties which convert one to another, generate coupling effects. The ceramic materials used primarily for their electrical properties are generally called electroceramics or functional ceramics. They have become of increasing importance in many key technologies including computers, wireless communication, automotive and industrial control systems. The developments in the various subclasses of electroceramics have paralleled the growth of new technologies. Examples include ferroelectrics-MLCC, FeRAM; ferrites-filters, resonators; solid electrolytes-energy storage and conversion; piezoelectrics-sonar, transducers, actuators; semiconducting oxides PTC & NTC-environmental monitoring. The trend of electroceramics are thin films, low-dimensional materials, multiphase, multi-functional, textured, single crystal and high uniformity, low-cost, LTCC, environmental friendly. The new components and devices are to be miniaturized, low power comsuption, microwave, high-power, intelligent and high reliability under extreme conditions.
　　Recently, as the increasing awareness of environmental protection, the rapid growth in demand for energy and its impact on air-pollution have become increasingly prominent. Therefore, the studies on lead-free piezoceramics, high-performance dielectric for energy storage capacitors, as well as giant electrocaloric effect become three hot topics on the research of functional ceramics.
1  KNN based lead-free piezoceramics with d₃₃ > 400 pC/N
　　There are three main lead-free piezoelectric materials, e.g. BaTiO₃, Na_0.5Bi_0.5TiO₃ and K_0.5Na_0.5NbO₃, among which the KNN is the most promising candidate for substitute of PZT-based ceramics^[1-2].
　　It is reported that there is a phase boundary between two orthorhombic phases near the composition K/Na = 47/53 which resulting in superior piezoelectric properties^[3]. The pure KNN undergoes a series of phase transitions at 690 K(TC-T), 480 K(TT-O) and 158 K(TO-R). Ion substitution can change the phase transition temperature thus may improve the ferroelectric and piezoelectric properties. The A-site is usually substituted with Li⁺, Ag⁺ ions, and Ta⁵⁺, Sb⁵⁺ ions are in the B-site. While in unbalanced situation, the A-site is doped with Ca²⁺, Sr²⁺, Ba²⁺, even co-doped with(Bi_0.5Li_0.5)²⁺, (Bi_0.5Na_0.5)²⁺, (Bi_0.5K_0.5)²⁺, and Ti⁴⁺, Zr⁴⁺ in the B-site.
　　Li⁺ is the only dopant ion that can increase the TC of KNN. It was studied that the valence mismatch could cause the TC to rapidly decrease[4]. T-O and O-R phase transitions are strongly dominated by B-site ions. Both TT-O and TO?R decrease by substituting of Ti⁴⁺ substation in B-site and almost independent of A-site ions^[5]. Zr⁴⁺ ion also decreases TT-O but increases TO-R, 8% Zr⁴⁺ addition will stabilize the rhombohedral phase to room temperature. Sb5+ can also increase the TO-R, and 9% content Sb⁵⁺ substitution increases TO?R to room temperature. Ta⁵⁺ substitution decreases the TC-T and TT-O at the same time. It is noted that 40% Ta5+ addition decreases the TT-O to room temperature^[4,6]. It could concluded that the TO-R may not merely affected by the chemical pressure introduced by different B-site ionic radii but also the chemical bond strength. In short, through the optimization of MPB, in 0.90(Na_0.5K_0.5)NbO₃-xBaZrO₃-(0.10-x)(Bi_0.5Li_0.5)TiO₃ system, when x = 0.7-0.8, d₃₃ = 230-265 pC / N, k_p = 40.6%-41.9% could be obtained. For the ceramic with a composition of 0.92(Na_0.5K_0.5)NbO₃-0.06BaZrO₃- 0.02(Bi_0.5K_0.5)TiO₃-0.25wt% MnO₂, d₃₃ = 420 pC/N, k_p = 56%, T_c = 243℃ were achieved^[7-9]. These piezoelectric properties at room temperature are very close to that of PZT-4 ceramics. However, the temperature stability and long-term reliability remains questionable and to be tested further. The recent study on the "vertical MPB" has become an important issue^[10].
　　The phase transitions (MPB, PPT) of KNN-based ceramics with different ions substitution still need to be explored. KNN-based ceramics seems to be the most promising substitution of PZT for devices applications.
2  High-performance ceramic dielectrics for energy storage
　　Dielectric capacitors posess advantages of high energy density, fast charge and discharge, anti-aging cycle under extreme environmental conditions. It is highly demanded for electric vehicles, power electronics, pulse power systems, high energy density weapons, renewable energy and smart grid systems^[11] .
　　EEStor Inc. of US filed a number of patents on dielctric ultracapacitors, e.g. “Electrical-energy-storage unit (EESU) utilizing ceramic and integrated-circuit technologies for replacement of electrochemical batteries” ^[12]. EEStor reports a large relative permittivity (19818) at an unusually high electric field strength of 350 MV/m, giving 10⁴ J/cm³ in the dielectric. The capacitor can store more energy than lithium-ion batteries used in portable devices for cheaper than the low-cost lead-acid batteries used in gasoline powered cars. Its claims, if true, would radically change the electric car industry. However, the technology still has not publicly demonstrated.
　　Zhang, et al^[13] reported dielectric polymers with high dipole density have the potential to achieve a very high energy density (> 17 J/cm³, >575 MV/m) with fast discharge speed and low loss, by using defect modified poly(vinylidene fluoride) polymers. Recently, Liu, et al^[14] reported high-performance colossal permittivity material of (Nb/In) co-doped rutile TiO₂ ceramics, due to the formation of electron-pinned defect-dipoles (defect-dipole clusters). The new high-performance dielectric materials, exhibiting temperature- and frequency-stable, with a giant dielectric constant ( > 10⁴) and a small dielectric loss (< 0.05) .
　　Currently, the research work on the glass coated grains, glass with nano-polar region, as et al. well as BaTiO₃ coated with Al(OH)_x ionic layer, which may form electrochemical pseudo-capacitance. Randall, et al^[15] invented a new type of capacitor that combines electrochemical pseudo-capacitor with ceramic capacitor energy storage. The ionically conducting dielectric materials may have heterogeneously distributed conductivity that may generate barrier layer effects. The Ionic-Based-Barrier-Layer (IBBL) effect may generate an effective barrier layer capacitance. Meanwhile, a dielectric layer is also introduced the electric double layer capacitor to improve its working voltage. New concepts and mechanism of dielectric capacitors will greatly stimulate the studies and applications of capacitor energy storage technologies.
3  Giant electrocaloric effect
　　The electrocaloric effect (ECE) is a change in temperature (ΔT) in a polarable material by virtue of the change in entropy (ΔS) upon the application of or withdrawing of an electric field under adiabatic conditions. The ECE occurs in both ferroelectric and paraelectric phases, and the effect is larger in the paraelectric phase just above the F-P transition.
　　In 2006, Mischenko, et al^[16] reported that “giant” ECEs were found in ferroelectric ceramic Pb(Zr_0.95Ti_0.05)O₃ thin films by Sol-Gel preparation at temperatures near and above the F-P transition. The film with a thickness of 350 nm, working at 48 MV/m, showed an adiabatic temperature change (ΔT) of 12℃, the entropy change ΔS = 8 J/(kg?K). Shortly thereafter, Qiming Zhang, et al[17] reported that in the ferroelectric copolymer (P(VDF-TrFE), 68/32 mol%) large ferroelectric-paraelectric phase transition heat of 2.1×10⁴ J/kg, the entropy change ΔS = 56.0 J/(kg?K) were ontained, which draw geat attention to use the solid-state electrocaloric effects to developed new model of cooling system.
　　Lu, et al^[18] revealed that giant ECEs can be obtained in the high energy electron irradiated poly (vinylidene fluoride-trifluoroethylene) relaxor copolymer and in the La-doped Pb(ZrTi)O₃ relaxor ceramic thin films, which are much larger than that from the normal ferroelectric counterparts. The adiabatic temperature change of 40 K and isothermal entropy change of 50 J/(kg?K) were achieved . Recently, Huiqing Fan, et al.[19] found that a giant electrocaloric effect (ΔT = 45.3 K and ΔS = 46.9 J/(kg?K) at 598 kV/cm) was obtained in nanoscale antiferroelectric and ferroelectric phases coexisting in a relaxor Pb_0.8Ba_0.2ZrO₃ thin films at room temperature.
　　The above exited results of these studies indicate a new refrigeration technology to achieve the possibility of change, the use of solid-state electronic cards to replace the traditional principle of Carnot refrigeration compressor refrigeration ultra-small size, energy efficient, environmentally friendly and without refrigerants. A new generation of refrigerators, air conditioners become a new functional ceramics research focus .
　　The ECE may provide an efficient means to realize solid-state cooling technology. New refrigeration based on the ECE approach is more environmentally friendly and hence may also provide an alternative to the existing vapor-compression approach.