Book Description

Effective manufacture of piezoelectric ceramic resonator elements applied in acoustic transmission technology requires careful thermal process development and control to ensure stability of field-dependent electromechanical response. Variation in fundamental properties of the sintered ceramic elements strongly depends on processing conditions, and sensitivity to variation influenced by the sintering dynamics can be traced to the underlying ceramic matrix composition. The observations of the study herein evaluate variability as a function of key processing parameters and composition for cases of PZT ceramics applied in sonar technologies. The primary objective is to identify sources of variability and process-property sensitivity. Through this comprehensive sintering evaluation, guidance can be provided to ensure minimized internal losses of the transducer piezoelectric ceramic elements and ensure linear response with applied electrical energy. For this study, two chemically distinct morphotropic-phase PZT formulations were evaluated at variable sintering temperatures and atmospheric effects of oxygen partial pressure ratios (P/PO2). The powders were prepared conventionally through solid state reaction and mixed with organic binder components and underwent atomization for spray-dried granule formation. The spray-dried powders were uniaxial pressed into cylindrical compacts to achieve a green density that was approximately 5 g/cm3 or 63% theoretical density. All organic binder constituents were burned out of the green bodies tracking carefully the geometric dimensions and mass changes prior to sintering. The dried pellets were sintered in covered alumina crucibles embedded in the source powders of their own respective compositions. Careful control of atmospheric oxygen partial ratio and sintering dwell temperatures was made possible by sintering in a closed tube furnace with sealed edge flanges and gas-flow controlled by mass flow controllers with downstream sorbent vapor trap vented to an atmospheric exhaust. For each data point, structural or dielectric measurement, is the average of three values as each sintering series contained 3 samples for each composition for replication and verification of property values. The processing window of sintering temperature investigated effects in the range of 1200 to 1325 °C. Atmospheric effects of oxygen partial pressure was evaluated at these temperatures using air as a control (P/PO2 ~0.22) then increasing incrementally for P/PO2 of 0.50 and 1.00. The sintering soak period was constant for all experiments at 2 hours as an analog to manufacturing-type process. The evaluations of processing and compositional effects are enabled through complementary analysis of the sintered grain structures, and measurements of dielectric properties at low-signal and as a function of applied field, as well as piezoelectric and electromechanical properties. Further insight into the polarization and strain response of the materials as a function of applied field are assessed via unipolar and bipolar AC field hysteresis to document property trends as a function of composition and processing. The intrinsic and extrinsic contributions to piezoelectric response were assessed utilizing the Rayleigh method for poled ceramics in unipolar strain-field behavior below half of the expected value of coercive field. Additionally, impedance analysis is performed as a function of frequency to assess the resonance characteristics of radial and thickness-mode excitations for thin disk sample geometry machined from the sintered bodies. The resonance characteristics are utilized to document shifts in electromechanical coupling coefficients as well as dependence of elastic stiffness properties to provide insight on structural material characteristics impact on applications for the PZT materials targeted for high-power underwater transducers for acoustic signal transmission and communication. The results of the comprehensive data package suggest that properties of standard PZT compositions which have a deep base of processing knowledge and history of property assessments can be utilized as baselines to guide future processing development for new vendors upstarting processes to produce material with expected properties. The case of an application-specific composition such as PZT-C requires more careful process development and assessment to understand variability as a function of the base material chemistry, with particular attention paid to the compositional modifiers that can have dramatic effects on sintered properties and ceramic characteristics that show instability or unsuitable performance for the application. The optimization of properties must be carefully assessed as a function of the thermal process variables to a larger degree than what may be needed for standard catalog compositions but can be used to guide modifications to compositions or process conditions to tailor a specific composition to meet alternative target applications.