Seyit O. Ural

Loss mechanisms and high power piezoelectrics

Heat generation is one of the significant problems in piezoelectrics for high power density applications. In this paper, we review the loss mechanisms in piezoelectrics first, followed by the heat generation processes for various drive conditions. Heat generation at off-resonance is caused mainly by dielectric loss tan δ′ (i.e., P-E hysteresis loss), not by mechanical loss, while the heat generation at resonance is mainly attributed to mechanical loss tan ′. Then, practical high power materials developed at Penn State is introduced, which exhibit the vibration velocity more than 1 m/s, leading to the power density capability 10 times of the commercially available “hard” PZTs. We propose a internal bias field model to explain the low loss and high power origin of these materials. Finally, using a low temperature sinterable “hard” PZT, we demonstrated a high power multilayer piezoelectric transformers

LOSS DETERMINATION METHODOLOGY FOR A PIEZOELECTRIC CERAMIC: NEW PHENOMENOLOGICAL THEORY AND EXPERIMENTAL PROPOSALS

The key factor to the miniaturization of piezoelectric devices is power density, which is limited by the heat generation or loss mechanisms. There are three loss components in general in piezoelectric vibrators/resonators, i.e., dielectric, elastic and piezoelectric losses. The mechanical quality factor, determined by these three factors, is the Figure Of Merit (FOM) in the sense of loss or heat generation. In this paper, we introduce a new loss phenomenology and innovative measuring methods based on the theory. First, quality factors at resonance and antiresonance for the k31, k33, kt and k15 vibration modes are derived theoretically, and the methodology for determining loss factors in various orientations (i.e., loss anisotropy) is provided. For simplicity, we focus on materials with ∞ mm (equivalent to 6 mm) crystal symmetry for deriving the loss factors of a polycrystalline ceramic, and 14 different loss factors among 20 in total can be obtained from the measurements. Second, we propose the experimental methods for measuring both mechanical quality factors QA and QB at the resonance and antiresonance modes: a continuous admittance/impedance spectrum measuring method (traditional with temperature rise) and a burst mode (to circumvent the temperature effect).

Development of a High Power Piezoelectric Characterization System and Its Application for Resonance/Antiresonance Mode Characterization

We developed a new high power piezoelectric characterization system, and report its application for characterizing the resonance and antiresonance vibration performance in this paper. Although the traditional constant voltage measurement was improved by using a constant current measurement method, the conventional technique was still limited to the vicinity of the resonance. In order to identify a full set of high power electromechanical coupling parameters and the loss factors of a piezoelectric, both resonance and antiresonance vibration performance should be precisely measured simultaneously. However, the high power characterization across antiresonance has not been addressed previously in the literature. Our new high power characterization system reported here is capable of measuring the impedance/admittance curves by keeping the following various conditions: (1) constant voltage, (2) constant current, (3) constant vibration velocity of a piezoelectric sample, and (4) constant input power. In addition, the system is equipped with an infrared image sensor to monitor the heat generation distributed in the test sample. We demonstrated the usefulness of the new system in a rectangular piezoelectric plate in the whole frequency range including the resonance and antiresonance frequencies. The results clearly concluded that compared to the resonance mode, the antiresonance mode exhibits a higher mechanical quality factor QM and the same vibration amplitude/velocity under a smaller input electrical power and lower heat generation. This may suggest a superiority of the antiresonance mode usage to the resonance mode from the high power application viewpoint (i.e., ultrasonic motors, transformers).

Multilayered Unipoled Piezoelectric Transformers

This study describes a multi-layer piezoelectric voltage and power transformer which has one direction poling, operates in a wide-frequency range and delivers both step-up and step-down voltages by inverting the electrical connections. In this design, the input and output electrodes are on the same side of the disk and are isolated from each other by a fixed gap. Investigations were performed on a disk of diameter 29.1 mm. The electrode pattern is a ring/dot structure, where a strip connects the dots. Various ratios of input to output area were studied and it was found that area ratio in the range of 2.8–3.3 or the output diameter in range of 13–15 mm yields high power and efficiency. The power density for the optimized single layer transformer was 40 W/cm3 while that for the 3-layer structure was 25 W/cm3. Though the power increased with multilayer structure, the effective power density decreased because of the interlayer constraints.

Derivation of Piezoelectric Losses from Admittance Spectra

High power density piezoelectrics are required to miniaturize devices such as ultrasonic motors, transformers, and sound projectors. The power density is limited by the heat generation in piezoelectrics, therefore, clarification of the loss mechanisms is necessary. This paper provides a methodology to determine the electromechanical losses, i.e., dielectric, elastic and piezoelectric loss factors in piezoelectrics by means of a detailed analysis of the admittance/impedance spectra. This method was applied to determine the piezoelectric losses for lead zirconate titanate ceramics and lead magnesium niobate-lead titanate single crystals. The analytical solution provides a new method for obtaining the piezoelectric loss factor, which is usually neglected in practice by transducer designers. Finite element simulation demonstrated the importance of piezoelectric losses to yield a more accurate fitting to the experimental data. A phenomenological model based on two phase-shifts and the Devonshire theory of a polarizable–deformable insulator is developed to interpret the experimentally observed magnitudes of the mechanical quality factor at resonance and anti-resonance.

Mn dopant on the “domain stabilization” effect of aged BaTiO3 and PbTiO3-based piezoelectrics

We report that an obvious difference in the “domain stabilization” effect between 1.0 mol. % Mn doped (Ba1−xSrx)TiO3 and (Pb1−xSrx)TiO3 piezoelectrics with a similar c/a ratio and aging treatment, though typically “increased” stabilization effect occurs with the increase of c/a in each system. The three-time larger microscopic defect dipole field Ei in lead-system from P-E measurements suggests the more aligned defect dipoles through kinetically easier hopping of oxygen vacancy originated from local structure rather than the average structure like c/a may be a primary cause of the strong domain stabilization effect. This may help on the hardening functionality improvement of lead-free systems.

Design of Translation Rotary Ultrasonic Motor with Slanted Piezoelectric Ceramics

We designed and manufactured a multi-degree-of-freedom (MDoF) ultrasonic motor (USM) by using a single actuator. Although there have been numerous MDoF USMs reported in the literature, only a few of them cover the combined rotary and translational motions in a cylindrical-joint type configuration. Combining these two motions in one joint and controlling this combined motion through a novel USM is the main aim of this work. Here, we use smooth impact drive method (SIDM) with two distinct modes obtained at two distinct frequencies by the help of slanted ceramics and uneven square shaped excitation signals. The translational-rotary dual-mode motor can be driven by single source. The prototype of the motor (5 mm diameter, 25 mm total length) has 5 mm/s translational and 3 rad/s rotary speed under 4 mN blocking force, when the input signal is 20 Vp–p square wave.

High Power (Na0.5K0.5)NbO3-Based Lead-Free Piezoelectric Transformer

Though there is a lot of research related with lead-free piezoelectric materials, high power characteristics of the candidate lead-free materials have not been looked into in the literature under equilibrium conditions. This paper reports high power characteristics of a sodium potassium niobate (NKN) based ceramic under equilibrium conditions, and its application as a candidate material for piezoelectric transformers compared with hard lead–zirconate–titanate (PZT). (Na0.5K0.5)(Nb0.97Sb0.03)O3 was prepared with 1.5 mol % CuO addition. Disk-shaped samples were sintered with conventional sintering methods. High power characteristics were investigated with our high power piezoelectric characterization system (HiPoCS). Distinctly different from PZTs, the NKN ceramics did not exhibit a decrease in mechanical quality factor (Qm) with increasing vibration velocity (up to 0.4 m/s). Ring-dot piezoelectric transformers made from a disk shaped NKN ceramic revealed power density as high as 25 W/cm3, almost comparable to that obtained for the conventional PZT transformers. In conclusion, NKN ceramics possess good high power characteristics, which are satisfactorily applicable to piezoelectric transformers.

Errata - High power universal piezoelectric transformer

This study describes a multilayer piezoelectric voltage and power transformer that has one direction poling, operates in a wide-frequency range and delivers both step-up and step-down voltages by inverting the electrical connections. In this design, the input and output electrodes are on the same side of the disk and are isolated from each other by a fixed isolation gap. The electrode pattern is a ring/dot structure such that it uses radial mode for both input and output part that are built-in on the same ceramic disk. A prototype transformer was fabricated of size 15 2 78 mm2 having mass of 3.8 gm. In the step-down configuration at the constant output power of 6 W, the transformer characteristics across a 100 Ω load were found to be efficiency = 92%, gain = 0.21 input voltage = 110 Vrms, and temperature rise = 20 C from the room temperature. In the step-up configuration at the constant output power of 5 W, the transformer characteristics across a 5 kΩ load were found to be efficiency = 97%, gain = 9.5, input voltage = 16 Vrms, and temperature rise = 8 C from the room temperature. A detailed equivalent circuit analysis of the transformer was done, and the results were found to be in excellent agreement with the experimental results.

Analysis on Loss Anisotropy of Piezoelectrics with ∞ mm Crystal Symmetry

The key factor for the miniaturization of piezoelectric devices is power density, which is limited by the heat generation or loss mechanisms. There are three loss components for piezoelectric vibrators, i.e., dielectric, elastic and piezoelectric losses. The mechanical quality factor, determined by these three factors, is the figure of merit in the sense of loss or heat generation. In this paper, quality factors of resonance and antiresonance for both k31 and k33 vibration modes are derived, and the method to determine loss factors in various directions is provided. For simplicity, we focus on materials with ∞ mm (equivalent to 6 mm) crystal symmetry for deriving the loss factors of polycrystalline ceramics, and ten different loss factors can be obtained from the measurements. Finite element simulations are made to prove the theory, and the analysis also demonstrates the significance of the piezoelectric loss factor which has usually been neglected by previous piezoelectric device designers.