Studies On Piezoelectric And Dielectric Properties Of Two Kinds Of Lead-free Ceramics

Piezoelectric effect was found in quartz crystal by the Curie brothers in 1880. 1940's, BaTiO₃ piezoelectric ceramics were produced. Herefrom piezoelectric materials had been widely applied. As conversion materials of mechanical energy and electric energy, piezoelectric materials play an important role in electric, magnetic, acoustic, optical, thermal, mechanical conversion devices. Lead zirconate titanate (PZT) based ceramics can be changed by adjusting ingredients in a wide range to meet different needs. PZT based ceramics have better piezoelectric properties due to the high piezoelectric activity near the morphotropic phase boundary(MPB). So PZT based ceramics have been widely applied. However, PZT contains a lot of lead, and because of the toxicity of lead it will give rise to environmental pollution, endangering people's health. The use of materials containing lead has been restricted. It is important to develop lead-free piezoelectric materials with better piezoelectric properties to replace lead-based piezoelectric ceramics.Alkali metal niobate-based piezoelectric ceramics are a kind of lead-free piezoelectric ceramics. Potassium sodium niobate (KNN) piezoelectric ceramics have been paid attention because of its good performance,and become a substitute to lead-based piezoelectric ceramic material. However, because of the volatility of potassium and sodium, the ceramics have low density which results in the deterioration of their piezoelectric properties. To enhance the KNN ceramics piezoelectric properties, it has been taken exceptional preparations. Hot pressed (K_0.5Na_0.5)NbO₃ ceramics have good piezoelectric propertities of d₃₃=160pC/N and k_p=45%. The doped KNN ceramics have been adopted to improve its piezoelectric properties. The morphotropic phase boundary has been found in many systems, and they have excellent piezoelectric properties.In the piezoelectric section of the paper (1-x)(K_0.5Na_0.5)NbO₃—x (Bi_0.5Na_0.5)TiO₃ ceramics system [abbreviated as (1-x) KNN-xBNT] was studied .Sodium bismuth titanate [(Bi_0.5Na_0.5)TiO₃, BNT] is relaxor ferroelectrics with perovskite structure and at room temperature with trigonal phase, and relatively large remanent polarization 38uC/cm² ,and a relatively high Curie temperature of 320°C,and in recent years has been extensively studied. In this paper, (1-x) KNN-xBNT ceramics were prepared by solid state reaction. The structural characteristics, dielectric and piezoelectric properties were studied. BNT inhibits the grain growth. BNT lowered the Curie temperature of KNN. The phase transition temperatures of x>0.03 samples extended. The x=0.03 samples have the maximum piezoelectric constant , electromechanical coupling factor and the maximum density. X-ray diffraction data also showed that the crystalline structure of the x=0.03 samples changed. It shows that the morphotropic phase boundary exists in the vicinity of this component. X=0.03, d33=190pC/N, kp=38.7%.In the dielectric section of this paper, (1-x) BaTiO_3-xCaTiO₃ [ abbreviated as (1-x) target BT-xCT] was studied. BaTiO₃ has three phase transitions at -90°C and 0°C and 120°C. From low to high temperatures, BaTiO₃ followed through trigonal, orthorbombic ,tetragonal and cubic structures. Due to the phase transitions, the temperature stability of BaTiO₃ dielectric constant was destroied. In this paper, (1-x)BT-xCT ceramics were prepared by solid state reaction. The microscopic structures ,dielectric spectrum and the temperature dependences were studied. When x ≤ 0.30, CaTiO₃ and BaTiO₃ can form solid solutions. The Curie temperature and ferroelectric to ferroelectric phase transition temperatures of BaTiO₃ are lowered by CaTiO₃, and the dielectric constant temperature stability of BaTiO₃ is improved.