Nao.5Bi0.5TiO3 based lead-free piezoelectric ceramics
The past 20 years, the non-lead-based piezoelectric ceramics has been great development, some good system performance in low-end and midrange products have been practical. Currently there are lead-free piezoelectric ceramics system BaTiO3 (referred to as BT) based lead-free piezoelectric ceramics, alkali metal niobate-based lead-free piezoelectric ceramics, tungsten bronze structure lead-free piezoelectric ceramics, lead-free piezoelectric bismuth layer structure ceramics and BNT-based lead-free piezoelectric ceramics and other types. Which BNT strong due to the ferroelectric piezoelectric coefficient, good acoustic performance advantages and favored.
Research and Application of BT based lead-free piezoelectric ceramics to is already quite mature, the system material is one of the important material of the capacitor, but piezoelectric properties of BT-based ceramics are generally low Curie temperature (Tc = 120 ℃ ), narrow working temperature near 0 ℃ presence of a phase change, time stability, temperature stability are poor, more substantial by simply doping is difficult to improve its performance. While some of the properties of the ceramic part is satisfactory, in which Ti and Zr substituted Metal oxide formed BaxTi-yZryO3 (referred BZT) system, with small grain, the relative density of 90%, a piezoelectric constant d33 of up 340pC / N, electromechanical coupling coefficient k33 up to 65%, good performance. But through long-term modification of the BT-based piezoelectric ceramics, the current increase this performance of the piezoelectric ceramics have greater difficulty in the past 20 years time, BT-based ceramics patent only a dozen pieces.
Alkali niobate-based lead-free piezoelectric ceramics that ANbO3 type compounds (where A can of Li +, Na +, K +, etc.), the class of ceramic first synthesized by American scholars, such perovskite ferroelectric single crystal structure of the compound strong, then as the electro-optical crystal materials attention. Since Li, Na, K, alkali metal volatile at high temperatures, so a large number of traditional ceramic sintering process produces a great experience difficulties, it is better to obtain a dense test piezoelectric ceramic, coupled with its poor machining performance, the coercive high field, experience in the polarization and other shortcomings, and thus more sleepy practical application experience.
Lead-free piezoelectric ceramics tungsten bronze structure, is due to the four corners of such a ceramic crystal structure similar to tungsten bronze structure, named after its basic structure is characterized by the presence of [BO6] oxygen octahedra, where B is NB5 +, Ta5 + ions, can be divided into non-fill type (eg PbNb2O6), fill type (eg Ba4Na2Nb10O30) completely fill type (eg K6Li4Nb10O30) three types. Under normal circumstances, a compound having a tungsten bronze structure a large spontaneous polarization, high Curie temperature, a low dielectric constant, a large optical nonlinearity characteristics. Tungsten bronze structure of the compound over the years as an electro-optical crystal materials have been extensively studied in recent years, especially in the modification of the system of ceramic doped or substituted, has made great progress.
Lead-free piezoelectric ceramics bismuth layer-structured perovskite layer from the two-dimensional (Am-1BmO3m + 1) ^ 2 + and (Bi2O2) ^ 2 + layers alternately arranged regularly from its chemical formula is (Bi2) ^ 2 + (Am-1BmO3m + 1) ^ 2-, where a, B represent a coordination number of 6, 12 and the positive ions, generally as a + 1, + 2, + 3, + 4 ion or complex thereof ions, such as Bi ^ 3 +, Pb ^ 2 +, Ba ^ 2 +, Sr ^ 2 +, Ca ^ 2 +, Na ^ 2 +, K ^ +, U ^ 4 +, Th ^ 4 +, etc.; B is suitable octahedral ions or complex ions thereof, such as Fe ^ 3 +, Cr ^ 3 +, Ga ^ 3 +, Ti ^ 4 +, Zr ^ 4 +, Nb ^ 5 +, Ta ^ 5 +, Wo ^ 6 +, etc., m represents the number of the original research on cell indicates that the layer thickness direction, m is generally an integer between 1-5, m, the higher the activity of the corresponding piezoelectric, the lower Curie temperature Te. The Lead-free piezoelectric ceramics have low dielectric constant, strong spontaneous polarization, high Curie temperature, a high mechanical quality factor Qm, high resistivity, the aging rate, easy sintering, low-loss dielectric, piezoelectric and dielectric are anisotropic electrical properties, good resonance frequency time and temperature stability. Suitable for the production of filters, high temperature and high frequency devices in the field of energy conversion, application prospects more widely.
A. Nao.5Bi0.5TiO3 based lead-free piezoelectric ceramics
BNT is 1960 by Smolenskii etc. A bit complex synthesis of substituted perovskite ferroelectrics. As shown in Figure 1.
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When BNT ceramics at room temperature for tripartite ferroelectric phase, about 230 ℃ experiencing diffuse phase transition above 320 ℃ into paraelectric phase, cubic phase above 520 ℃, as shown in Figure 2.
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Because of its high Curie temperature, the ferroelectric strong (Pr = 38μC / cm²), the piezoelectric coefficient (kt, k33 is about 40-50%), low dielectric constant (of 240-340), the acoustic good performance (radial material constants Np = 3200HZ.m) and other characteristics, but also has characteristics of ferroelectrics relaxivity, sintering temperature and sintering temperature genus, easy to obtain better ceramic sintered body, is considered to be the most promising Lead-free piezoelectric material system.
