Preparation And Properties Of Single Crystal Materials For Non-pb, Fe, Voltage

Ferroelectric and piezoelectric ceramic materials have wide applications to the fields of military, medicine and communications. The properties of ferroelectric single crystals, such as ferroelectric ,piezoelectric, dielectric, electromechanical coupling coefficients, are superior to the ceramic materials, which make them more promising on applications. At the same time, the ferroelectric single crystals are playing a key role in the basic study of material physics due to their simple structure of single crystal compared to the counterpart of the ceramic materials. At present, most of the ferroelectric materials in used are lead-zirconate-titanate (PZT). Represented by Pb(Zr,Ti)O₃, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMNT), and Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃ (PZNT), relaxor ferroelectric crystals have attracted much attention due to their high piezoelectric and electromechanical coupling coefficients. However, because of some disadvantages of the lead materials,such as the toxic pollution to the environment, the instability of the performances caused by the volatilization of the lead oxide and the aging of the devices. It is significant to explore the lead-free ferroelectrics and piezoelectric materials with high stability performance.The research of this work was focused on the lead-free ferroelectric and piezoelectric materials, which is the hot topic in this field to nowadays, alkaline-bismuth-titanate ( Na_0.5Bi_0.5TiO₃, abbreviated as NBT) is of superior properties of ferroelectric among numerous lead-free ferroelectric systems. It could form solid solutions at the present of the cations of Ba²⁺, Sr²⁺, Ca²⁺, K⁺ , which had the morphotropic phase boundary (MPB) . Nowadays, the study of Na_0.5Bi_0.5TiO₃-based ferroelectric materials were mostly focused on ceramic materials, and few reports on the Na_0.5Bi_0.5TiO₃-based ferroelectric crystals and even no report on the Bi_0.5(Na_1-xK_x)_0.5TiO₃system. Since NBT-based ferroelectric crystal was of multi-domain, its piezoelectricity could be improved by domain engineering.So in this thesis the alkaline bismuth titanate system and tungsten bronze ferroelectric ( Bai.xSrx) 2NaNb50i5 which had the MPB. It was chosen as study target.In this work, the growths of Bio.5(Nai-xKx)o.5Ti03 crystals with perovskite structure and BSNN crystals with tungsten bronze structure were studied. The main research works were: crystal growth, including growth of Bio.5(Nai.xKx)o.5Ti03 crystal and BSNN crystal, structure , thermoproperties and defects of BNTK crystals, dielectric , piezoelectric and ferroelectric properties of BNTK crystals,the structure, dielectric, piezoelectric and ferroelectric properties of the BSNN crystal.1. Growth of Bio.5(Nai.xKx)o.5Ti03-based lead-free ferroelectric crystalsBi2O3+Na2CO3 + K2CC>3 were used as self-flux. The original furnace was reconstructed with cooling O2 (at the room temperature) blowing onto the seed crystal by the tube with a hole. That is called the Modified Top Seed Solution Growth (M-TS SG). Selecting proper temperature and rotating-pulling rate, the BNTK single crystal with the size of 20X20X10mmJ can be obtained from melt for the first time. The XRD result showed that BNTK crystal was of pure perovskite structure. With the increasing of dopant K+, the structure of the crystal changed from rhombohedral structure to tetragonal structure, namly the morphotropic phase boundary (MPB) in the BNTK system existed near x = 0.15-0.20o DSC study showed that BNTK crystal had two phase transitions at 342.33°C and 574.67°C. This was consistent with the pure NBT crystal, and it also demonstrated the rhombohedral structure of BNTK15. The melt point of BNTK crystal is 1279.7°C . It was of good thermoproperties stability at above 600°C .2. Defects of the Bi0.5(Nai.xKx)o.5Ti03 single crystalThe defects of BNTK crystal included inclusions, cracks and inhomogeneity of the components in the crystals. The results of the study indicated that the crystal defects were formed due to the conditions of the matter, distributing and variation of temperature, rate of growth and segregation and the super-cooling of components. The defects could be decreased or eliminated by improving the conditions (such as growth equipment, seed defect, and so on) , or controlling the furnace temperature and the rotating-pulling rate during crystal growing , which would keep the flat interface and suitable temperature gradient. Perfect crystals would be gained by optimizing the matching between rate of growth and temperature gradient of the solide-solution interface, which could efficiently control the super-cooling. 3 Ferroelectric properties of Bio 5(Nai.xKx)o sTiCh single crystalThe measurement of the dielectric constant dependence with temperature and frequency showed that the dielectric behavior of BNTK15 and BNTK25 is similar. We found the dielectric anomaly at low temperature(250 °C ).The characteristric of relaxor ferroelectrics was present below the low temperature. Another dielectric anomaly was present at high temperature (350°C ) . At this temperature, the dispersive of the frequency occurred, which was different from the relaxor-ferroelectrics. The relative dielectric constant decreased with the increasing concentration of K+. From the behavior of dielectric, It was discovered that, due to the particularity of crystal structure, the interplay of its add-inside field and outside field made the crystal dielectric constant much larger than that of ceramic. The strong ion conductance of the crystal at high temperature greatly affected the dielectric behavior of the crystal. The loss of the dielectric in the low frequency increased greatly with the increasing of temperature.When the concentration of dopant K+ increased, the relaxor of the high temperature was indused by the ion conduction, namely conduction relaxor. But, the P-E hysteresis loops were not perfect due to the extreme high the leak current in the crystal. Bio.5(Nai.xKx)o.5Ti03 crystal had excellent ferroelectric properties and its piezoelectric surpassed far from that of the Bi0.5(Nai-xKx)o.5Ti03 ceramic. So it would have promising applications in the high power piezoelectric actuator. It was concluded that its MPB should exist near x = 0.20 from the variety curve between piezoelectric constant and component. During the measurement of d33, large d33 appeared in the direction of <100> when the polarizing field was applied in the direction of <001>. This abnormal phenomenon perhaps is owing to the existence of 90° domain in crystal. Under the effect of polizaring field, the rotation of 90 ° domains resulted in the sharply increasing of d33 in the direction of <100>. Therefore the lead-free crystal materials with high piezoelectric properties would be obtained by controlling the rotation of domains. 4. Study of the growth and properties of BSNN crystalWe have successfully grown BSSN crystals with high quality in atmosphere by using Pt crucible and the HF heating method. Selecting suitable temperature and rotating-pulling rate, the crystals with high quality can be obtained when the pulling rate was in 1.0-3.0mm/h.Study of the XRD showed that BSNN crystal had MPB when x = 0.6, i.e. from orthorhombic to tetragonal. Its melting point was identified as 1436 °C by DSC. It was also confirmed the existence of segregation in these crystals based on study of XFM, The molar ratio of Sr2+/ Ba2+ increased when the concentration of Sr2+ increasing.The BSNN crystal melts congruently. So the component inhomogeneity was present in the growing direction during the process of growth. And the inhomogeneity of chemical composition causedthe crystal lattice distort. So the ultra strain was induced inner the crystal which would resulted in cracksoThe BSNN crystal showed absorption edge at 353nm. It was almost transparent at the wavelength range from 353nm to lOOOnm ( >70%) . Its refractive index was reduced to 2.004 at 632.8nm. The frequency dependence of dielectric for a-plate> b-plate and c-plate of BSNN crystal showed obvious anisotropy. Little shift of its Curie temperature was observed when the frequency varied. However, the dielectric peak of a-plate and b-plate was not sharp near the Curie point, exhibiting certain relaxority. Therefore it is relaxor-ferroelectrics. The dielectric of c-plate near Curie point was about 70 times larger than that of a-plate and b-plate, and its Curie temperature was obviously higher than that of SBN crystal along with the addition of Na+. The measured results show that the piezoelectric and dielectric properties of the tungsten bronze ferroelectric crystals are better and more stable than those of other known lead-free perovskite ferroelectrics. As a result of the inhibition of the existed 90° domain wall movement, the process of polarization affected the dielectric constant greatly. Also, strong enhancement ofpiezoelectric and dielectric properties was observed for BSNN system close to the MPB composition. With some improvement using doping and making composition close to the MPB, it is expected that the BSNN system can be a very good lead-free candidate for piezoelectric applications, such as ultrasonic transducers and piezoelectric actuators.