Lead-free piezoceramics - Where to move on?

Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensively searched for more than a decade worldwide. Some noteworthy outcomes such as the advent of commercial products for certain applications have been reported, but the goal, i.e., the invention of a lead-free piezocermic, the performance of which is equivalent or even superior to that of PZT-based piezoceramics, does not seem to be fulfilled yet. Nevertheless, the academic effort already seems to be culminated, waiting for a guideline to a future research direction. We believe that a driving force for a restoration of this research field needs to be found elsewhere, for example, intimate collaborations with related industries. For this to be effectively realized, it would be helpful for academic side to understand the interests and demands of the industry side as well as to provide the industry with new scientific insights that would eventually lead to new applications. Therefore, this review covers some of the issues that are to be studied further and deeper, so-to-speak, lessons from the history of piezoceramics, and some technical issues that could be useful in better understanding the industry demands. As well, the efforts made in the industry side will be briefly introduced for the academic people to catch up with the recent trends and to be guided for setting up their future research direction effectively.ope

Fabrication of relaxor-PT single crystals

The ultrahigh properties of relaxor‒lead-titanate (relaxor-PbTiO3) crystals have been proved to offer dramatic enhancements to electromechanical devices. The developmental stage of relaxor- PbTiO3 (PT) ferroelectrical crystals comprises three generations. The class of binary Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals represents the first generation, which exhibits giant electromechanical properties and piezoelectric coefficients. The ternary system Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) represents the second generation, which demonstrates a greater coercive field (EC), a higher rhombohedral-to-tetragonal phase transition temperature (Tr-t) and a higher Curie temperature (TC) than those of the first generation. The third generation is Mn-modified Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (Mn:PIN-PMN-PT), which possesses a higher mechanical quality factor (Qm) than those of the first and second generations, while it maintains comparable piezoelectric responses to those of generations I and II. There are several techniques for the growth of relaxor-PT single crystals. Flux growth is a simple method, but its small output is inconvenient for mass production. The solid-state conversion growth (SSCG) method offers large quantities of single crystals. Furthermore, its operation is simple and costeffective. The quality of the single crystal is low, however, due to porosity and defects. The modified Bridgman method is a straightforward way of synthesizing large quantities of relaxor-PT crystals. Single crystals are directly grown from a molten ingot passing through a temperature gradient and the solidus line of the solid solution phase diagram. Compositional segregation is an unavoidable disadvantage of this method, although several modifications have introduced to overcome this issue. Rare-earth doping and a continuous feeding approach have been confirmed to produce single crystals with only low segregation. In this work, PMN-PT, Sm-modified PMN-PT, and Mn-modified PIN-PMN-PT single crystals are grown using the modified Bridgman method. A new vertical Bridgman furnace was assembled in the Institute for Superconducting and Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong. Ceramic materials were prepared using the two-step precursor method. Optimal conditions for ceramic synthesis have been studied. The dielectric and piezoelectric properties of ceramic materials were investigated and confirmed to be good when compared them to reference values. PMN-PT single crystal was grown first. The physical appearance of the as-grown crystal was cloudy, and several grains were developed, as seen from cross-sectional view. The bottom part of the as-grown single crystal was also un-melted. The reason for these issues was that the charge was lifted to a position where the raw ceramic was not able to melt completely. Therefore, only the part in tapered section of the Pt crucible was melted, from which the crystallization was started with different grains. Learning from the first growth experiment, the next crystal growths were carefully carried out with operational condition’s changes. The growth procedure was carried out with a higher charge position and higher temperature in the upper zone to generate a greater temperature gradient. Consequently, pure and high quality Sm-modified PMN-PT and Mn-modified PIN-PMN-PT single crystals were grown. The recent development of relaxor-PT ferroelectric single crystals is also reviewed in this work. This review includes the growth methods, property improvement strategies, and application prospects based on the recent progress

Functional piezocrystal characterisation under varying conditions

Piezocrystals, especially the relaxor-based ferroelectric crystals, have been subject to intense investigation and development within the past three decades, motivated by the performance advantages offered by their ultrahigh piezoelectric coefficients and higher electromechanical coupling coefficients than piezoceramics. Structural anisotropy of piezocrystals also provides opportunities for devices to operate in novel vibration modes, such as the d36 face shear mode, with domain engineering and special crystal cuts. These piezocrystal characteristics contribute to their potential usage in a wide range of low- and high-power ultrasound applications. In such applications, conventional piezoelectric materials are presently subject to varying mechanical stress/pressure, temperature and electric field conditions. However, as observed previously, piezocrystal properties are significantly affected by a single such condition or a combination of conditions. Laboratory characterisation of the piezocrystal properties under these conditions is therefore essential to fully understand these materials and to allow electroacoustic transducer design in realistic scenarios. This will help to establish the extent to which these high performance piezocrystals can replace conventional piezoceramics in demanding applications. However, such characterisation requires specific experimental arrangements, examples of which are reported here, along with relevant results. The measurements include high frequency-resolution impedance spectroscopy with the piezocrystal material under mechanical stress 0–60 MPa, temperature 20–200 °C, high electric AC drive and DC bias. A laser Doppler vibrometer and infrared thermal camera are also integrated into the measurement system for vibration mode shape scanning and thermal conditioning with high AC drive. Three generations of piezocrystal have been tested: (I) binary, PMN-PT; (II) ternary, PIN-PMN-PT; and (III) doped ternary, Mn:PIN-PMN-PT. Utilising resonant mode analysis, variations in elastic, dielectric and piezoelectric constants and coupling coefficients have been analysed, and tests with thermal conditioning have been carried out to assess the stability of the piezocrystals under high power conditions

Single crystal PMN-0.33PT/epoxy 1-3 composites for ultrasonic transducer applications

Author name used in this publication: Hasou LuoAuthor name used in this publication: Kei C. Cheng2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

High density pixel array

A pixel array device is fabricated by a laser micro-milling method under strict process control conditions. The device has an array of pixels bonded together with an adhesive filling the grooves between adjacent pixels. The array is fabricated by moving a substrate relative to a laser beam of predetermined intensity at a controlled, constant velocity along a predetermined path defining a set of grooves between adjacent pixels so that a predetermined laser flux per unit area is applied to the material, and repeating the movement for a plurality of passes of the laser beam until the grooves are ablated to a desired depth. The substrate is of an ultrasonic transducer material in one example for fabrication of a 2D ultrasonic phase array transducer. A substrate of phosphor material is used to fabricate an X-ray focal plane array detector

High density pixel array and laser micro-milling method for fabricating array

A pixel array device is fabricated by a laser micro-milling method under strict process control conditions. The device has an array of pixels bonded together with an adhesive filling the grooves between adjacent pixels. The array is fabricated by moving a substrate relative to a laser beam of predetermined intensity at a controlled, constant velocity along a predetermined path defining a set of grooves between adjacent pixels so that a predetermined laser flux per unit area is applied to the material, and repeating the movement for a plurality of passes of the laser beam until the grooves are ablated to a desired depth. The substrate is of an ultrasonic transducer material in one example for fabrication of a 2D ultrasonic phase array transducer. A substrate of phosphor material is used to fabricate an X-ray focal plane array detector

Improved Performance of d₃₁-Mode Needle-actuating Transducer with PMN-PT Piezocrystal

Prototypes of a PZT-based ultrasound needle-actuating device have shown the ability to reduce needle penetration force and enhance needle visibility with color Doppler imaging during needle insertion for tissue biopsy and regional anesthesia. However, the demand for smaller, lighter devices and the need for high performance transducers have motivated investigation of a different configuration of needle-actuation transducer, utilizing the d 31 -mode of PZT4 piezoceramic, and exploration of further improvement in its performance using relaxor-type piezocrystal. This paper outlines the development of the d 31 -mode needle actuation transducer design from simulation to fabrication and demonstration. Full characterization was performed on transducers for performance comparison. The performance of the proposed smaller, lighter d 31 -mode transducer is comparable with that of previous d 33 -mode transducers. Furthermore, it has been found to be much more efficient when using PMN-PT piezocrystal rather than piezoceramic

INFLUENCE OF PIEZOELECTRIC ANISOTROPY ON ELECTROMECHANICAL PERFORMANCE OF ULTRASOUND NDT PROBES AT ELEVATED TEMPERATURES

When designing an ultrasound non-destructive (NDT) probe for high temperature applications, one has to take into account a multitude of temperature induced effects, such as the temperature dependence of key physical properties of probe materials. In this paper, the significant influence of temperature change on piezoelectric properties and piezoelectric anisotropy, and thus on the ultrasound properties of the whole probe, is discussed theoretically. This is demonstrated within the thermodynamic Landau-Ginzburg-Devonshire (LGD) framework and the KLM model of equivalent circuits for two different piezoelectric, acoustically active probe materials from the same family - barium titanate and lead titanate. These two materials are model materials for the whole family of piezoelectric perovskites, including the commercially widely used PZT and relaxor ferroelectrics PMN-PT and PZN-PT

Self-poling effect on Mn-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals synthesized by solid-state single crystal growth method

Department of Materials Science and EngineeringPiezoelectric single crystal such as PZN-PT [Pb(Zn1/3Nb2/3)O3-PbTiO3] and PMN-PT [Pb(Mg1/3-Nb2/3)O3-PbTiO3] which have magnificent piezoelectric properties, have been used for various applications, e.g., sensors, transducers and actuators. For piezoelectric applications, ferroelectric materials are usually employed because of their high performance after poling process. Poling process that forces ferroelectric domains to alignotherwise, randomly oriented, is essential in making a ferroelectric into a piezoelectric. It is typically performed at an elevated temperature by applying a certain amount of a unipolar electric field for some time since domain alignment is a time-dependent thermally activated process. However, induced piezoelectric properties generally disappear when ferroelectric material is heated up to Curie temperature (Tc) where aligned dipoles scatter. Because the synthesis of common ferroelectric materials is processed at high temperature, ferroelectric materials must be poled for piezoelectric application. In this paper, ferroelectric PMN-PT single crystals with doping Mn for inducing self-poling effect will be discussed. The Mn-doped PMN-PT exhibits a high piezoelectric response without any poling process. Moreover, high piezoelectric properties are re-induced after heating above TC with self-poling on cooling process. The defect-dipoles which is caused by Mn ions generate internal bias fields (Ei) which give forces aligning dipoles of PMN-PT to have spontaneous polarization. The Mn-PMN-PT crystal which is able to be self-poled has its own preferred poling direction. So opposite DC-poling can enhance piezoelectric and dielectric performances like AC-poling which is highly interested by ferroelectric single crystal society. The mechanism presented in this paper can offers a new perspective for enhancing the dielectric and piezoelectric properties of doped ferroelectric single crystals.clos