Several problems in the preparation of PZT piezoelectric ceramics

PZT piezoelectric ceramics have the characteristics of high curie temperature, strong piezoelectricity, easy doping modification and good stability. Since the 1960s, it has been a hot spot of attention and research, and it has dominated the field of piezoelectric ceramics. In terms of the preparation process of PZT piezoelectric ceramics, Pzt powder synthesis and densification sintering have the greatest influence on the quality of PzT products. PzT ultrafine powder has the advantages of fine particle size, large specific surface area and high reactivity, which can reduce sintering temperature, reduce lead volatilization, ensure accurate stoichiometry, and improve the performance of PZT products. Therefore, the preparation of ultrafine PZT powder has become underwater piezo tube. The focus of electrical ceramic research. In recent years, many new methods have been developed for the study of ultrafine PZT powder preparation. In addition to the traditional phase-phase method, the solid phase method includes the microwave radiation method , mechanical chemical method port, and the reaction sintering method. The liquid phase method has the advantages of low synthesis temperature, simple equipment, easy operation, low cost, etc., and which has been used for the preparation of PZT powder, such as sol-gel method, hydrothermal method , precipitation method". However, there are still many shortcomings in the preparation and performance of PZT piezoelectric ceramics, including powder agglomeration, stoichiometry and easy aging of product properties. the reasons for the above problems and the research progress of solving these problems are discussed as follows, and some suggestions for the development of Pzt piezoelectric ceramics are put forward.

1 powder agglomeration

Powder agglomeration generally includes soft agglomeration and hard agglomeration. Soft agglomeration is formed by the enhancement and interaction of van der Waals force, electrostatic attraction and capillary force between particles as the size of the powder particles decreases; hard agglomeration is due to the chemically bonded bond between the groups. The action forms a bridging oxygen bond, and the bridging oxygen chain between the particles interacts to form. The agglomeration problem is the most important factor affecting the excellent performance of ultrafine PzT powder, and it also greatly affects the quality of PZT products. This is because PZT materials are functional materials. The basic requirements for synthetic powders are high purity, ultrafine, uniform particle size distribution, good dispersion, stoichiometric accuracy, and uniform doping. In addition. The occurrence of agglomeration in PZT piezo ceramic pipe bismuth synthesis will lead to a decrease in bulk density and uneven morphology, and will introduce a large number of pores resulting in the uneven microstructure, seriously affecting the density of the sintered body, and may also lead to the second sintering process. Secondary recrystallization, thereby reducing the piezoelectric and thermoelectric properties of the PZT article. Therefore, reducing or avoiding agglomeration in the synthesis of ultrafine PzT powder is a prerequisite for the preparation of high performance PZT piezoelectric ceramics. The agglomeration in the preparation of PZT powder includes two forms of soft and hard agglomeration. The agglomeration mechanism is also different for different powder preparation methods. The traditional solid phase method for synthesizing PZT powder is characterized by repeated ball milling and high calcination temperature. Repeated ball milling not only introduces impurities, but also over-grinding leads to the formation of agglomeration. In particular, the mechanochemical method developed mainly utilizes mechanical energy completes the PZT phase synthesis, and the ball time is as long as the sky. It is easier to introduce impurities and cause powder agglomeration. Excessive calcination temperature may also cause powder agglomeration. The liquid phase method is used to synthesize PZT powder. Because solid phase particles are formed in the liquid phase, they generally undergo nucleation, growth, coalescence and agglomeration. Therefore, the agglomerated structure may be formed when it is a solid phase particle formed in the liquid phase. Because of the brownian motion, the particles are close to each other. When the kinetic energy between the particles is greater than the barrier forming the agglomerate, between the tender particles under the action of the movement, they reunite each other. Second, in the process of solid-liquid separation, with the elimination of the last part of the liquid phase, the solid phase particles are brought close to each other due to the surface tension. Especially the synthesis of PZT powder with water as solvent. The traces of water that eventually remain between the particles will bind the particles and particles tightly together by hydrogen. In addition, the prepared PZT powder precursor is generally calcined at 500 to 700°C, which may also cause powder agglomeration, and may also cause the formed agglomerates to aggravate the formation of agglomerated structures due to local sintering. It can be seen that PZT powder is prepared by coprecipitation method. Each stage of coprecipitation, grain growth to rinsing, drying, and calcination of the precipitate may result in particle growth and formation of agglomerates. According to the formation of agglomeration, reducing or avoiding particle growth and agglomeration in the preparation of ultrafine PZT powder can be considered from the following aspects: one is the separation of nucleation and growth process, promote nucleation, control growth; ensure large nucleation rate ten growth rates, that is, to ensure that the PZT powder precursor is generated under a large degree of supercooling or high supersaturation. The second is the prevention of PZT powder agglomeration. This includes how to inhibit the formation of agglomeration during powder preparation and how to eliminate agglomeration after agglomeration. The method for inhibiting agglomeration during powder preparation includes: (1) selecting reasonable reaction conditions (such as PH value, reaction concentration and temperature, ); (2) special treatment during powder synthesis or drying; including powder synthesis process surfactant is added and filtered with an organic solvent, and the surface tension of the surfactant is low. Therefore, a powder precursor with a lighter degree of agglomeration can be obtained; a special drying process is adopted in the drying process, mainly freeze drying, supercritical drying, and far infrared drying, etc. The basic principle is to eliminate the gas-liquid with a large surface tension. The interface, or the particles are fixed and cannot be close to each other. For example, lyophilization is to use a low temperature and a negative pressure to sublime the raw liquid medium frozen to a solid phase under a negative pressure, since the solid phase particles are frozen in the raw liquid medium. And there is no gas-liquid interface with a large surface tension in the capillary between the particles, thereby avoiding the problem of severe agglomeration caused by the liquid bridge.

(3) Select the best calcination conditions or use special processes, such as the use of microwave heating without heat transfer, high energy efficiency and other characteristics to replace the traditional high-temperature electric furnace, at 600. c .A single component PZT powder was obtained. Methods for eliminating agglomeration after formation of agglomerates are deposition or sedimentation, grinding and ultrasonic treatment, addition of a dispersant, and the like. For example, Wang Xicheng “uses metal alkoxide and nitrate as raw materials, strictly controls coprecipitation generation, washing, and the selection conditions of the dispersion, adopts freeze-drying technology, and then undergoes reasonable calcination process to form a uniform composition without hard agglomeration. Body, single perovskite phase, high sintering activity of micron-sized PzT (52/48) micropowder are formed by cold isostatic pressing (cIP) technology, densification sintering can be achieved at 800 °c, and its relative density is achieved.

The influence of stoichiometric stoichiometry on the quality of PZT products mainly includes z aspects: First, the lead volatilization during the sintering process of PZT piezoelectric ceramics causes the components to deviate from the accurate stoichiometry and the performance of the products is reduced; on the other hand, the zr/ in the components. The fluctuation of Ti affects the stability of PzT product performance. Lead volatilization is generally considered to be due to the high sintering temperature of PZT piezoelectric ceramics. Lead oxide has a relatively high saturated vapor pressure in high temperature environment, which leads to lead volatilization. The higher the saturated vapor pressure, the more easily lead is volatilized, and zr with the increasing of /Ti, the sintering temperature of PZT piezoelectric ceramics increases, and the saturated vapor pressure of lead oxide increases gradually. Lead loss will become more serious, so high zr/Ti PzT piezoelectric ceramics are more difficult to sinter, and too low oxygen partial pressure during sintering will lead to lead volatilization. The PZT material prepared by the conventional solid phase method has a sintering temperature of generally 1,200 due to the low activity of the synthetic powder. It is easy to cause a large amount of lead volatilization, product performance is not high, so the solid phase method of PZT products difficult to meet the high performance requirements of the field of application. At present, the main measures taken by material researchers at home and abroad for lead volatilization are as follows: First, excessive lead is added to the powder synthesis. Excess lead is added. In the initial stage of sintering, due to the formation of liquid phase, the contacting area of the reactants can be increased, the diffusion rate of zirconium, titanium and dopants can be accelerated, and the uniformity of the product can be improved; the liquid phase formed can also accelerate the diffusion movement of dissolution and precipitation, Conducive to the rearrangement and close packing of the particles, accelerate the densification of the product. However, when the amount of lead added is too much, the excess lead will deposit on the crystal on one hand, which will reduce the performance of the product. On the other hand, it will easily cause the local concentration of titanium in the PzT component to be too high, and the solubility in the liquid phase lead oxide is large dry zrOz. Solubility, thus resulting in a high local titanium content of the PZT product after sintering, especially at the grain boundary. Thereby affecting the uniformity of the microstructure of the product and reducing the performance of the product. The addition of excess lead affects the mechanical properties of PZT piezoelectric ceramic components: when the product is excessively excessive in lead, a fracture mode is a transgranular fracture. Second, in the sintering process of the product, according to the volatilization mechanism of lead, a reasonable sintering system and special measures are adopted. It is widely used to add a sintering atmosphere sheet and a double-layer bismuth technique to sinter, and control the sintering atmosphere to be an oxidizing atmosphere. This will reduce lead volatilization and prevent blackening of the product. Under a reducing atmosphere, Ti is easily reduced to T1 ̈ to darken the product.

The third is to add an appropriate amount of dopants. Doping reduces lead volatilization and on the other hand improves PzT product performance. The fourth is to further study the synthesis of high-activity PZT powder, so that PZT piezoelectric ceramics can be densified and sintered at a temperature lower than that of lead. In addition, the mechanism of volatilization of lead remains to be further studied. The volatilization temperature of lead oxide in the hydrothermal synthesis of PzT powder is 924.71. c, and the reaction temperature between the particles ̈. The reaction temperature between the solid phase synthesis PZT powder particles at 26 ° C was 47. c. The oxidation temperature of lead oxide is 29 ° C; the lowest eutectic temperature of the PZT system is 838. c. It can be seen that reasonable process measures must be taken according to the volatilization mechanism of lead and the preparation method of PZT powder to reduce lead volatilization and improve the performance of PZT products.

5 Doping

Studies have shown that PZT product performance is closely related. At present, for the study of PZT materials, zr/Ti is mainly concentrated in the range of 53/47 and 95/5. However, relying solely on different zr/T- to improve the performance of PZT products can not meet the requirements of PZT products in different fields. It is also necessary to improve the products by selecting some appropriate amount of dopants on the basis of zr/Tl. while reducing the effect of fluctuations in zr/Ti on the performance stability of pzt articles. The additive can be mutually soluble in the main crystal lattice and can be precipitated in the grain boundary in the form of the second phase.when it is mutually soluble, the performance or structure of the main crystalline phase can be changed according to the amount of addition; when it is not mutually soluble. As the second phase, the grain boundary is affected, which affects the bonding force or grain boundary property between the crystal grains. The main effects of doping include formation of vacancies, inhibition of grain growth, formation of bovine liquid phase, and expansion of the sintering temperature range. At the same time, according to the role of the dopant in the PZT piezoelectric ceramic, it can be divided into three types: donor doping, acceptor doping and variable ionic compound doping. In the doping modification study of PZT piezoelectric ceramics, La, Mn2- and Nb have been studied in large quantities. The effect of doped Nbzos on PzT65/35 material prepared by solid phase method was studied. It was found that the addition of niobium significantly promoted densification and sintering, and inhibited the growth of niobium with the increasing of niobium content due to Nb0. The strengthening of the octahedral electrostatic action causes the strain of the rhomboid unit cell of PzT to be strained. The unit cell tends to be stretched on the surface, which makes the product easy to polarize, and also contributes to the improvement of its anti-aging property. The "solubility" limit in the PZT material is 7 mol%. When the amount is less than 7. the aluminum and the perovskite crystal lattice are completely mutually soluble. At this point the product is completely in the perovskite phase. It exhibits donor impurity characteristics; when added in an amount greater than 7 mol, excess ruthenium will react with lead or titanium, resulting in the production of a second phase, such as a fluorite phase, which reduces the dielectric and piezoelectric properties of the article. The effect on the microstructure and piezoelectric properties of PZT materials, and the valence state of manganese in PZT materials was determined by EsR. The results show that the solubility of manganese in Pz-butyl materials is mainly in the form of Mn and Mn5+. 1.5nlol", when manganese content <o. At 5m01%, Mn will preferentially enter the Pb position in the form of Mn2_ and Mn", which improve the piezoelectricity of the material, and exhibit donor impurity characteristics; when the manganese concentration is 0.5 to 1.5 mol%, Mn or Mn In the mode of entry, the manganese doping material exhibits the piezoelectric properties of the "soft" and "hard" materials at the same time; when the manganese content is >15 mol, the excess Mn will accumulate at the grain boundary, reducing the piezoelectric activity of the product. In summary, for different types of dopants, the research focus is on completely dissolving the PZT main crystalline phase by seeking appropriate addition amount, improving the properties of the product, avoiding excessive doping and decomposing the grain boundary or forming the second phase. To reduce the performance of the product. In addition, the current doping modification research, the doping object mainly concentrates on the solid phase method to prepare PZT material, and is doped with oxide solid phase, so it is difficult to ensure the uniformity of components and accurate stoichiometry. How to achieve homogeneous doping will be one of the future research directions of PZT piezoelectric ceramics.

4. Aging

Aging refers to the characteristic that the electrical performance parameters of PZT articles change with time. We know that PZT piezoelectric ceramics belong to ABO. The perovskite structure comprises three crystal phases of cubic, tetragonal and rhombohedral. According to zr/T1, different crystal phases can be produced, and the activated cation is type A ion. Below the curie temperature, the A-type or B-type ions enter a certain position, producing spontaneous polarization and forming domains, but the angle between adjacent domains can only be 90 or 180. This is due to either The spontaneous polarization of the ferroelectric domain structure allows the orientation to be equivalent to the axis of the ferroelectric polarization axis in the prototype structure of the ferroelectric body. At the same time, the ferroelectric domain structure is also restricted by the spontaneous strain of the crystal. The phase must ensure that the spontaneous strain generated by the adjacent domains in the domain walls can be compatible, so the angle of the adjacent domains of F can only be 90 or 180. At the same time, the sintered piezo ceramic products must be in strong direct current. Under the action of the field, the spontaneous polarization of each domain is forced to be aligned, thereby generating the residual polarization, that is, exhibiting a piezoelectric effect. The polarized PZT piezoelectric ceramic disc is forced to chase the intra-cell disorder due to the polarization electric field. The arrangement of 90 slack and 180. domain steering, orderly arrangement, and the spontaneous polarization axis in the unit cell is a little longer than the non-spontaneous polarization axis, and the strain well generates a large internal stress when is turning. After removing the external electric field, this internal stress causes the internal imbalance of the unit cell. and store more internal energy, which leads to the 90 that has been turned. The new domain is restored to the disordered arrangement before polarization to gradually release the internal stress. Therefore, the residual polarization is related to these . Disordering and gradually decreasing, dielectric and piezoelectric parameters will change accordingly. It can be seen that the disorder of new domains is the root of the aging mechanism. The aging laws generally include linear, non-linear and gradual .There are three types of near-linear. The factors affecting the aging of PZT products include external and internal factors. External factors are related to the use environment. Internal factors such as temperature and humidity are closely related to domain structure and domain motion, which is due to high electric field. The polarization inversion process is completed by the movement of domain walls and the generation and movement of new domain walls. The domain structure is closely related to the microstructure after sintering such as grain size, grain boundary, etc. When the grain size is small .When the size of the domain wall is similar, the development of the ferroelectric domain structure and the movement of the domain wall will be hindered. Specifically, as the grain size increases, the grain boundary volume fraction decreases, and the coupling between the grain boundary and the domain wall decreases. Domain the orientation is more difficult, and the domain wall motion is severely suppressed, so the remnant polarization and dielectric constant increase with the increasing of the grain size, and the coercive field strength decreases. The thickness and properties of the grain boundary also have an effect on the domain structure. The grain boundary is favorable for the development of the domain across the grain boundary and improves the piezoelectricity of the product.