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The charge constant of piezoelectric material is one of the crucial constant values. At the present time, the measurement of this value is mostly realized by means of three techniques: the frequency method, the laser interferometer technique, and the quasi-static method. These techniques have been practically applied to piezoelectric ceramic samples. Our paper presents comparison of the individual methods with regard to their accuracy and the demands placed on the instrumentation and preparation of the piezoelectric material samples. A soft ceramic product marketed under the production code of NCE51 was used in the experiments. The methods are described in detail, including the process of sample preparation, description of the experiments, and procedure of alculating coefficients from the measured values. Nábojová konstanta piezoelektrického materiálu je jednou z nejdůležitějších. Pro její měření jsou dnes nejčastěji používány tři metody – frekvenční metoda, měření pomocí laserinterferometru a kvazistatická metoda. Náš příspěvek se věnuje porovnání jednotlivých metod z pohledu přesnosti metod, nároků na přístrojové vybavení a nároků na přípravu vzorků piezomateriálu. Jednotlivé metody jsou podrobně popsány, včetně postupu přípravy vzorků, popisu experimentů a způsobu výpočtu koeficientů z naměřených hodnot.

Depolarization at high temperatures around the Curie point constitutes an important yet difficultly measurable material property of piezoelectric (PZT) ceramics. The common vibrometric technique (d33 meters) is not suitable for the measurement of temperature dependences, and therefore we used the frequency method to perform the desired procedures. The indicator selected to show the depolarization state in the piezoelectric ceramics consisted in the piezoelectric charge coefficient, whose value can be effectively measured via the above-mentioned frequency technique. The accuracy of the method was verified via comparing a d33 meter constructed by the authors (as described in this paper) and also by means of differently sized cylinders of ceramics NCE51, which are designed for longitudinal length modes. Based on the obtained results, we established a measurement methodology to exactly determine the value of the Curie point that corresponded to the phase transition to a cubic crystallographic structure. The experiment also confirmed the applicability of progressively controlled depolarization of PZT ceramics by high temperature in the range of between 350 and 370 °C, and it defined the temperature limits at which there occur irreversible changes of the piezoelectric properties of PZT ceramics. In the measured NCE51 material, the limit for irreversible changes was equal to 95% of the Curie temperature (368 °C). Depolarizace zvýšenou teplotou v okolí Curieova bodu je významnou materiálovou vlastností PZT keramiky, která je však obtížně měřitelná. Běžně dostupné d33-metry měření teplotních závislostí neumožnují, proto byla použita pro stanovení teplotních závislostí frekvenční metoda. Jako indikátor zobrazující stav depolarizace PZT keramiky byl zvolen piezoelektrický nábojový koeficient, jehož hodnotu lze frekvenční metodou dobře měřit. Přesnost frekvenční metody byla ověřena porovnáním s d33-metrem vlastní konstrukce, který je v textu rovněž popsán, na různých rozměrech válečků z PZT keramiky NCE51, určených pro podélné typy kmitů. Výsledkem je metodika měření, která umožňuje dosáhnout přesné stanovení hodnoty teploty Curieova bodu. Experiment rovněž prokázal možnost postupné řízené depolarizace PZT keramiky pomocí vysoké teploty a současně i hranice teploty, při kterých již začíná docházet k nevratné změně piezoelektrických vlastností PZT keramiky. Pro měřenou keramiku NCE51 je to 95% Curieovy teploty (368 °C).

This article describes the experimental research of a ring of PZT ceramics used in knock sensors. The main aim of the paper is to investigate the temperature effects related to the parameters of piezoelectric materials and to characterize the influence and changes exerted by these parameters on the properties of knock sensors. The purpose of the research is to facilitate the fault diagnosis of an active piezoelectric element, thus enabling the self-diagnostics of a sensor in normal engine operation. Such self-diagnostics is necessary to prevent engine damage and can be performed via online health monitoring of knock sensors. The properties of the material forming the active element based on PZT ceramics are significantly affected by temperature changes. In the paper, the temperature deviations are compared with the impedance and phase characteristics in the working temperature range of the engine and also related to different types of sensor damage. Based on the material defects (cracks, fissures, fractures) and changes in the material coefficients connected with the frequency characteristics, we can assess the degree of the damage in the examined sensors and evaluate the quality of the inside piezoelement. The results of the realized experimental comparison are shown within the paper. A series of experiments was conducted on a knock sensor, in which we used piezoelectric elements with a differing number of cracks. The experiments demonstrate that the proposed methods can be used for the above-mentioned purpose and are applicable to eliminate any possible damage of an automotive engine caused by vibrations.

The behaviour of material coefficients of PZT ceramics at temperatures approaching the Curie point is a problem hitherto not examined in detail. However, this field offers several interesting perspectives, such as the possibility of controlled depolarization of the piezoceramic material in the volume of a sample. The measurement of temperature dependences in piezoelectric materials used for sensors is very significant with respect to determining the degree of stability that characterizes the sensitivity and accuracy of a sensor; moreover, such measurement enables us to establish the maximum temperature ranges for regular operation. In the experiment described within the paper, we applied two temperature ranges according to the parameters of the calibration furnaces. While the first range spanned between -20°C and 140 °C, the second one was defined by the boundary temperatures of 30°C and 330°C, where the latter value (330°C) corresponds to the Curie point of the applied NCE 51 piezoelectric ceramics. The samples were cycled at gradually increasing maximum temperatures, and changes of the material constants were observed. We applied the frequency method to measure samples for the radial, thickness, transverse longitudinal, longitudinal, and thickness shear oscillation types. In samples of PZT ceramics, the measurement of temperature dependences up to the Curie temperature is very demanding in view of the characteristics of the applied measuring devices. Thus, the authors have included in this article a detailed discussion of suitable apparatuses and measurement procedures. The major result acquired from the performed research activities consists in both the measured temperature dependences and the sum of recommendations concerning the correct approach to be adopted in the measurement of PZT ceramics at a wide range of temperatures and with minimum error.

This article describes in detail the measurement method and the calculation of the complete matrix of piezoelectric coefficients from one PZT ceramics disc. A set of piezoelectric samples is produced from a disc of appropriate dimensions for all types of vibrations that comply with the world and the European standards. The samples produced are a thin plate for traverse length mode, a thin disc for radial mode and thickness extension mode, a cylinder for longitudinal length mode and a thin plate for thickness shear mode. Soft PZT ceramics, type NCE51, was used for calculation of the matrix coefficient and for description of the measurement method. Frequency method was used for the measurements and for the calculation of all coefficients. Impedance analyzer Agilent 4294A with a range from 40 Hz to 110 MHz and measuring tweezers “Tweezers Contact Test Fixture 16334A” were used for the measurements. The experiments verified the possibility of determining the complete matrix of the coefficients on piezoelectric ceramics without major requirements regarding the set of samples needed for the measurement.

The paper compares the main methods used for measuring of the piezoelectric material constants. It outlines the principle of three measuring methods most commonly used today, i.e. the frequency method, the laser interferometry method and the quasi-static method. These methods have been practically applied to piezoelectric ceramic samples. The paper describes the production of the piezoelectric ceramic samples of defined sizes in accordance with the current regulations. An NCE51 production code soft ceramic was used in the experiments. A piezoelectric charge coefficient was measured. The final values of the piezoelectric charge coefficient obtained through all the methods were compared to the catalogue values of the piezoelectric ceramic. All three methods can be described as appropriate; compared with the frequency method, the laser interferometry and quasi-static methods are rather time-consuming and more demanding with respect to preparation of the measurement experiment. The frequency method provides results within a smaller value range.