Carl C. M. Wu

High authority piezoelectric torsional actuators

The results on the development of torsional actuators presented here are intended to demonstrate the basis for the utilization of novel, piezoelectric shear response, electromechanical actuators in smart materials and structures. The actuator design developed in this study is a segmented, piezoelectric tube, with the individual segments driven in a shear mode. The assembled tube can be driven to produce precise and significant angular displacement and high torque output directly from this solid state device. Techniques were developed for continuous poling, assembly and testing of actuators. Projected actuator outputs based on the electromechanical test results are discussed relative to the requirements of actual helicopter rotor blade applications, in terms of driving trailing edge flap and direct blade twist actuation.

The Effect of Interfacial Polarization on the Energy Density of Ferroelectric Glass-Ceramics

In the last two years, the US Naval Research Laboratory has been able to synthesize barium strontium titanate (BST)-based glass-ceramics with dielectric breakdown strength as high as 800 kV/cm and dielectric constant up to 1200. Unfortunately, the energy density of the candidate glass-ceramics was only ~1 joule/cc when measured using a discharge measurement circuit. Polarization-electric field measurements revealed wide open hysteresis loops, indicating that most of the electrical energy was not released during discharge. Subsequent experiments showed that the buildup of interfacial polarization was the likely cause in this composite dielectric system. Using the Maxwell-Wagner capacitor model, we were able to quantify the dielectric response of composites based on the permittivities and conductivities of the constituent phases. The response was used to plot polarization-electric field hysteresis for energy density predictions. The results indicated that the aluminosilicate glass phase is the major contributor to the interfacial polarization in this glass-ceramic system.

Microwave Joining of Ceramics

This paper presents a process developed at Quest Research Corporation for using microwave energy to join ceramics. It begins with a brief overview of the concept and a recent history of microwave ceramic processing. A mathematical treatment of the microwave heating process then is presented followed by a description of the joining technique, and a summary of the experimental results and joint characterizations. For one set of ceramic samples used, electron microscopy and fracture toughness and hardness measurements were made at the Naval Research Laboratory. Another set of sample material was provided by Coors and photomicrographic evaluations of joints produced by Quest using these samples were performed by Coors.

High authority, telescoping actuators

A family of high authority actuators was developed at the Naval Research Laboratory. These actuators are based on displacement amplification within a compact, solid state, monolithic piezoelectric actuator, by using a telescoping tube design. In this design, concentric tubes are mechanically connected in series. This gives an effective actuator length that is equal to the sum of the lengths of the individual concentric elements. The high displacement output of this actuator permits efficient coupling of the actuator output into a load of similar impedance, and thereby much greater effective actuator output. Initial prototypes were made of commercially available PZT tubes of three different diameters and wall thickness. These tubes were pulsed through the thickness of the walls and the change in their lengths were used for actuation. Their actuation is therefore making use of the d31 piezoelectric coefficient. Alternatively, electrodes can be applied to the ends of the individual concentric tubes and their lengthwise displacement will subsequently be proportional to the d33 parameter of the material. The tubes were bonded at their ends to alumina plates using epoxy-based adhesive. The displacement obtained from the assembly is close to the sum of those of the three individual tubes at the same applied field. Other parameters such as blocking force and energy densities are also reported. These actuators have applications where high force and simultaneously large displacement are required and space is limited. Potential uses include high end aerospace as well as low tech commercial applications.

High-authority telescoping actuators with single-crystal piezoelectric materials

The displacement output of the telescoping actuators, as developed in the NRL, depend on the material properties. Recently developed relaxor ferroelectric materials, with compositions near the morphotropic phase boundary and in single crystal forms, show high strain output capability and high coupling coefficients. These materials, when design into devices, would provide high performance, in terms of displacement output, and high transduction efficiency. We have utilized one of these single crystal materials, namely, lead zinc niobate-lead titanate, PZN-PT, for the construction of the NRL telescoping actuators. The evaluation of the piezoelectric performance of these single crystal-based telescoping actuators showed much higher displacement outputs than that for the ones previously made with polycrystalline PZT ceramics.

Crystallization Kinetics of Barium Strontium Titanate Glass-Ceramics

Barium strontium titanate has been targeted as one potential ferroelectric glass-ceramic for high energy density dielectric materials. Previous testing has shown that the dielectric constant of these materials was as high as 1000 and dielectric breakdown strength up to 800 kV/cm. This did not, however, result in exceptional energy density (~ 0.90 J/cm3). In order to increase overall energy density refining agents can be added to the melt, but the nucleation and growth of the ceramic particles can also play a role. Therefore, in this study the crystallization kinetics were observed to more fully understand how the barium strontium titanate (BST) phase forms so that the optimal energy density could be achieved. It was found that the activation energy was 400 -430 kJ/mol, while the average Avrami parameter was 2.2 -2.5 for BST 70/30 with various additives. The activation energy is close to the disassociation of the Si-O bonds, while crystallization most likely occurs in the bulk with the mechanism of growth being interface controlled.

Piezoelectric Rotary Actuator Driven Devices and Applications

Novel piezoelectric cylindrical ceramic actuators have been developed utilizing the d 15 piezoelectric shear coefficient. These solid state actuators are segmented piezoelectric tubes that generate high torque and angular displacement. This rotary actuation can be applied to the trailing edge flap of a helicopter rotor blade to reduce vibration and to provide real time active blade tuning. For long stroke applications, rotary inchworm/motor devices were designed and fabricated using roller clutches to accumulate the minute displacement of the actuator over many cycles of the applied ac field. Operating at the resonant frequency of the actuator, the amplitude of the rotary angle is amplified by the mechanical quality factor of the device.

Initial Evaluation of the Fracture Behavior of Piezoelectric Single Crystals Due to Artificial Surface Defects

This study is part of a new research program to develop fundamental understanding of the fracture and fatigue behavior of piezoelectric single crystals through the combination of computational and experimental approaches. In this work we present 1) experimental results on the creation of artificial surface defects in piezoelectric single crystals using a focused ion beam (FIB) system and 2) initial observations on the crystals fracture behavior under an electrical field. The major advantage of using a FIB is that one can control the size, shape, and orientation of artificial defects precisely, allowing realistic surface defects, e.g., half-penny-shaped, 100 mum long, <1 mum wide, and 50 mum deep. We have demonstrated that multiple artificial defects with varying inclination angles relative to the specimens crystallographic orientation can be machined in a few hours. In this paper, we report the experimental details of the FIB milling, typical defect shape, and initial results on the effects of high electric field on the fracture behavior of single crystals.

Strength and Toughness Measurements of Ceramic Fiber Composites

A recent modification of the applied moment double cantilever beam (AMDCB) fracture toughness test is described. This test, the L-arm AMDCB test; is used to measure the fracture toughness of UTRC Compglas unidirectional and 0/90 crossply ceramic fiber-ceramic matrix composites. It is shown that the test permits characterization of the fracture toughness and fracture behavior of such high toughness and highly anisotropic materials. The measured KIC values, for the high toughness directions, were found to be as high as 60 MPa·m1/2, with indications of a change in fracture mode with increase crack velocity.

Strength, Toughness And Fracture Initiation Of Fluoride Glasses

A ZBLAL fluoride glass produced at NRL was characterized mechanically both in bulk and fiber forms. Fracture toughness measurements gave Ki values of 0.38 ± 0.05 MPa.m1/2, about 1/2 that of Si02 glasses. Flexural strength of machined bars of the current ZBLAL glasses was about 60MPa, about 80% of similarly finished silicate glasses. While the fluoride glass has lower strength than Si02 glass, it had much less slow crack growth, which should give it a longer useful life over a range of stresses. Although the average tensile strength of fibers from initial fiber drawing trials was only about 170 MPa, fractographic examinations indicated that the useful strength of this fiber could be raised considerably with improved processing.