Views:2 Author:Site Editor Publish Time: 2019-10-23 Origin:Site
Piezoelectric crystal transducer are widely used in the fields of electronics, light, heat, and acoustics, and have become important functional materials in the defense industry, civil industry, and daily life. They are a major research direction of current functional materials. At present, the most used piezoelectric ceramic is still lead zirconate titanate (PZT) and its ternary or quaternary ceramics. The polarization process is a key process in the fabrication of piezoelectric ceramic devices. The polarization process is the process of the movement and development of domain structures in piezoelectric ceramics. Piezoelectric ceramics are isotropic bodies before artificial polarization, and do not exhibit piezoelectric effect externally; after polarization, they become anisotropic bodies due to remanent polarization, thus having a piezoelectric effect. The dielectric, and elastic properties of the polarized piezoelectric ceramic are related to the degree of polarization. In order to make the piezoelectric ceramics have a high degree of polarization and give full play to their potential piezoelectric properties, it is necessary to adopt the optimum polarization conditions, that is, to select the appropriate polarization electric field strength (E) and polarization temperature (T). And polarization time (t). The three conditions of the polarization process are interrelated. If the polarization electric field is weak, it can be compensated by increasing the temperature and prolonging the polarization time; if the electric field is strong and the temperature is high, the polarization time can be shortened. However, the three conditions of polarization are closely related to the composition of piezoelectric ceramics. For PZT piezoelectric ceramic materials, the coercive electric field is reduced. The traditional method is to adjust the zirconium to titanium ratio. The larger the zirconium to titanium ratio, the smaller the coercive electric field, so that the polarization electric field is smaller. It is increasing the zirconium to titanium ratio does not significantly improve the polarization process conditions.
In the production and scientific research, certain oxides and compounds are often used as trace additives to improve the performance of piezoelectric ceramic materials. These trace additives replace the positions of some titanium ions and zirconium ions in PZT, which makes the domain in the grains move easily, which leads to a significant reduction of the coercive electric field and also reduces the three conditions of polarization. Easy to polarize. After a long period of repeated experiments, it is determined that the 6. 5 MHz piezoelectric ceramic filter is made of modified PZT and its composition is Pb0. 90 Sr0. 05Mg0. 03Ba0. 02 (Zr0. 53 Ti0. 47 ) O3 +CeO2 + After the piezoelectric ceramic raw material is pre-fired, formed, fired and polished, a round piezo ceramic disc of 24 mm × 0.35 mm is formed, and after being silver on both sides of the round piezo ceramic piece, it is placed in an oven at 100 ° C. Bake for more than 10 min, and remove the tiles from the silver layer. Then, the silver piezo plate is placed in a box furnace, and the temperature is raised to 100 ° C at a constant temperature of 15 ° C / 6 min, and the temperature is raised at 0.5 ° C. The temperature is raised to a constant temperature of 15 ° C / 6 min. At 400 °C, the temperature was raised to 700 °C at a constant temperature of 20 °C / 6 min. After a constant temperature of 20 min, the temperature was slowly lowered to below 100 °C. The silver-plated porcelain pieces were placed at room temperature for 12 h, placed in a silicone oil bath, and subjected to polarization treatment under the different polarization conditions. The piezoelectric properties of tangential piezoelectric tube were measured after standing for 24 h.
Effect of Polarized Electric Field on Piezoelectric Properties
In the polarization process, the polarization electric field is the external driving force for steering the domain. In the case of not exceeding the saturation field strength of the material, the larger E, the greater the effect of the orientation of the domain alignment, and the degree of polarization The more complete, the better the piezoelectric performance. Electrons that are difficult to deflect or reorient at the low pressure are more susceptible to deflection or reorientation under high pressure, which is making polarization more complete. For a 180° inversion domain, the inversion of the domain does not steer the reverse domain through the lateral movement of its domain wall, but rather grows a lot of polarization near the electrode along the edge of the sample inside the inversion domain. A new, sharp-like domain with a direction consistent with the direction of the electric field. After nucleation of the new domain, it advances under the action of an electric field and penetrates the entire sample. When the electric field is enhanced, new domains appear continuously, and the forward development propagates to the entire reverse domain. Finally, the reverse domain becomes the same as the direction of the external electric field, and combines with adjacent isotropic domains to form a larger volume. For a 90° domain, the domain wall can move laterally, and the critical electric field required for lateral movement of the 90° domain is smaller than the critical electric field required for the sharp-shaped new domain core, but the 90° domain steering and the external electric field direction are required. Consistent requires a larger electric field, and the development of its new domain mainly relies on the external electric field to push the lateral movement of the 90° domain wall. Under the condition of t = 15 min and T = 130 °C, the polarization of the piezoelectric ceramic piece was changed by E, and the piezoelectric constant d33 changed with E. It can be seen that when E < 1. 5 kV/mm, d33 increases slowly with the increase of E; when E > 1. 5 kV/mm, d33 increases rapidly with the increase of E, but when E > 2. 5 kV/ mm , d33 suddenly drops rapidly. This is because when E < 1. 5 kV/mm, the polarization can only make the material easily turn to 180° domain orientation in the direction of the external electric field, so the d33 value is lower and the increase is slower; when E > 1. 5 kV ,the external electric field is larger than the coercive electric field of the material, so that the 90° domain that is difficult to turn the material.which tends to the direction of the external electric field, so the d33 increases rapidly; continue to increase the external electric field strength ,when E > 2. 0 kV/ At mm, the piezoelectric domain turn in the material is almost complete, so the increasing of d33 tends to be slow. But when E reaches a certain value (E > 2. 5 kV/mm), the free electrons in the piezo ceramic get more energy in the electric field than the lost energy. According to the ionization collision theory, the free electrons can be after each collision. Accumulating energy causes the temperature of the ceramic sheet to rise continuously, the piezoelectric performance is continuously degraded, and finally thermal breakdown occurs. Moreover, when the applied electric field is sufficiently high, due to the tunneling effect of quantum mechanics, the forbidden band electrons may enter the conduction band, and under the action of the strong field, the free electrons are accelerated, causing the electrons to collide and ionize. At this time, due to the increasing of the current, the local temperature of the piezo crystal rises, causing the piezo crystal to partially melt and destroy its structure, so that the properties of the piezo ceramic are degraded, and finally breakdown occurs.
Effect of polarization temperature on piezoelectric properties
Under the condition of E = 2. 0 kV/mm and t = 15 min, T is changed to polarize the piezoelectric ceramic. The variation of d33 and d33 starts to increase faster. After the temperature reached 130 °C, the value of d33 remained basically unchanged. This is because at lower temperatures, as the temperature increases, the piezo crystal axis ratio becomes smaller, the domain activity increases, and the internal stress caused by the 90° steering of the domains becomes smaller, that is, the domain steering is affected. The resistance is small, and the domains are easily oriented, so polarization is easier to perform. When T reaches 130 °C, most of the piezoelectric domains are turned and the steering is saturated, so the value of d33 does not change.
Polarization conditions have a great influence on the performance of piezoelectric ceramics, and the polarization electric field is the main factor in the polarization conditions. Theoretically, when the applied electric field exceeds the coercive field strength, most of the domains should be turned and polarized rearranged and fully polarized, but under such an electric field, even if it is polarized for a long time, it cannot be obtained. Better piezoelectric properties. In order to make the piezoelectric properties of the material fully exerted, the electric field must be added to the saturation field strength, which is 3 to 4 times of coercive field strength. Therefore, the coercive electric field is the lower limit of the electric field selected during polarization, and the saturation field strength It can be considered that the upper limit of the field strength is selected at the time of polarization, and if the saturation field strength is exceeded, the breakdown is easily found. After comprehensive consideration, the optimal polarization process parameters of the 6. 5 MHz piezoelectric ceramic filter are determined: the polarization electric field strength is 2. 2 kV/mm, and the polarization temperature is 130 °C. Based on this, the pole is determined. The time is 15 min. The experimental results show that when the polarization time exceeds 15 min, the effect on the piezoelectric performance is not obvious. It was also found in the experiment that the use of low-temperature conductive paste silver paste instead of the commonly used high-temperature silver paste in the silver-sintering process can improve the piezoelectric and mechanical properties of the ceramic sheet to a certain extent, but the bonding strength is lower and the cost is higher. High and unsuitable for industrial production. In the experiment of the firing process, it was found that the piezo ceramic sheet is having a temperature of more than 1 250 ° C and a holding time of more than 2 h was prone to breakdown during polarization, resulting in an increasing in cracks. This is because the higher the firing temperature and the longer the holding time, the more severe crystallization occurs, so that smaller grains become large grains, which usually leads to an increasing of ceramic porosity and a decrease in ceramic density. It reduces the mechanical strength and dielectric constant, and at the same time reduces the mechanical quality factor of the piezo ceramic.
When the handover arrives, the handover is authenticated. If it is successful, the resources allocated in the preparation phase are handed over to the protocol access module and a call is initiated to the control. At this time, the call control considers the call to be a normal terminal call. When the protocol access module reports to the HOM that the terminal has actually accessed the message, the switch can be considered to be in a stable state. If the terminal that is switched in requires other handovers, such as internal handover or subsequent handover, it can be completed according to the HO function description. It should be pointed out that the switched OT ends have nothing to do with the OT ends of the call. The T end and the O end of the call can be the switched O end or the switched T end. After research on GSM and UMTS handover, if there are GSM and UMTS handovers in the mobile softswitch, there are many similarities between the signaling process and the media control, especially the handover messages of BSSAP and RANAP. In the design process of the state machine, it is considered to implement the fusion of the two protocols, and finally adopt the scheme of separation implementation. The signaling process of a single protocol in handover is not complicated, and the complexity of handover mainly comes from the cooperation of protocols on multiple different interfaces. The fusion of the handover-related messages in the two protocols of the Base Station Subsystem Application Part (BSSAP) and the Radio Access Network Application Part (RANAP) cannot be greatly simplified for the design of the handover state machine, and will also be for BSSAP. The design adapted to the RANAP protocol adds complexity. In addition, the fusion will result in redundancy of messages and parameters or loss of functionality.