Porous Piezoelectric Ceramics

Materials, 2015

The use of porosity to modify the functional properties of piezoelectric ceramics is well known in the scientific literature as well as by the industry, and porous ceramic can be seen as a 2-phase composite. In the present work, examples are given of applications where controlled porosity is exploited in order to optimise the dielectric, piezoelectric and acoustic properties of the piezoceramics. For the optimisation efforts it is important to note that the thickness coupling coefficient k t will be maximised for some non-zero value of the porosity that could be above 20%. On the other hand, with a good approximation, the acoustic velocity decreases linearly with increasing porosity, which is obviously also the case for the density. Consequently, the acoustic impedance shows a rather strong decrease with porosity, and in practice a reduction of more than 50% may be obtained for an engineered porous ceramic. The significance of the acoustic impedance is associated with the transmission of acoustic signals through the interface between the piezoceramic and some medium of propagation, but when the porous ceramic is used as a substrate for a piezoceramic thick film, the attenuation may be equally important. In the case of open porosity it is possible to introduce a liquid into the pores, and examples of modifying the properties in this way are given.

Journal of Electroceramics, 2007

Porous ceramics are of interest for ultrasonic transducer applications. Porosity allows to decrease acoustical impedance, thus improving transfer of acoustical energy to water or biological tissues. For underwater applications, the d h g h figure of merit can also be improved as compared to dense materials. In the case of high frequency transducers, namely for high resolution medical imaging, thick film technology can be used. The active films are generally porous and this porosity must be controlled. An unpoled porous PZT substrate is also shown to be an interesting solution since it can be used in a screen-printing process and as a backing for the transducer. This paper describes the fabrication process to obtain such materials, presents microstructure analysis as well as functional properties of materials. Modelling is also performed and results are compared to measurements. Finally, transducer issues are addressed through modelling and design of several configurations. The key parameters are identified and their effect on transducer performance is discussed. A comparison with dense materials is performed and results are discussed to highlight in which cases porous piezoceramics can improve transducer performance, and improvements are quantified.

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2020

For transducer design, it is essential to know the acoustic properties of the materials in their operating conditions. At frequencies over 15 MHz, standard methods are not well adapted because layers are very thin and backings have very high attenuation. In this article, we report on an original method for measuring the acoustic properties in the 15-25 MHz frequency range, corresponding to typical skin-imaging applications, using a backing/piezoelectric multilayer structure. Onto a porous Pb(Zr 0.53 Ti 0.47)O 3 (PZT) substrate, a piezoelectric PZT-based layer with a thickness of ∼20 µm was deposited and directly used to excite an acoustic signal into water. Herein, the measured signal corresponds to the wave that is first reflected on a target in water, then propagates back to the multilayer structure, and is transmitted through the thick film and further to the rear face of the porous backing, where it is again reflected and returns to the piezoelectric thick film, thus avoiding overlap with the electrical excitation signal. Two types of PZT backings with similar porosity of ∼20% and spherical pores with size of 1.5 and 10 µm were processed. The ultrasound group velocities were measured at ∼3500 m/s for both samples. The acoustic attenuation of the backings with pore size of 1.5 and 10 µm were 12 and 33 dB/mm, respectively, measured at 19 MHz. This advanced measuring technique demonstrated potential for the simple measurements of acoustic properties of backing at high frequencies in operating conditions. Importantly, this method also enables rapid determination of the minimum required thickness of the backing to act as a semi-infinite medium, for high-frequency transducer applications. Index Terms-Acoustic characterization, backing, porous Pb(Zr 0.53 Ti 0.47)O 3 (PZT).

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2003

PZT-air (porous PZT) and PZT-polymer (polymer impregnated porous PZT) piezocomposites with varying porosity/polymer volume fractions have been manufactured. The composites were characterised in terms of hydrostatic charge ( d h ) and voltage (gh) coefficients, permittivity, hydrostatic figure of merit (dh.gh), and absolute sensitivity ( M ) . With decreasing PZT ceramic volume, gh increased, and dh.gh had a broad maximum around 80 to 90% porosity/polymer content. The absolute sensitivity was also increased. In each case, PZT-air piezocomposites performed better than PZT-polymer piesocomposites. Hydrophones constructed from piesocomposites showed slightly lower measured receiving sensitivities than sensitivity of thc hydrophone, is related to thc value of dh a,nd the pcrmittivit,y under constant stress ( E $ ) :

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000

Optimizing the dynamic resonant characteristics of ultrasonic therapeutic transducers depends most importantly on fine-grain piezoceramics with good resonant properties. In this paper, we prepare and compare modified Pb 0.99 Sr 0.01 [0.03(Mn 1/3 Nb 2/3 )-0.97(Zr 0.51 Ti 0.49 )]O 3 piezoceramics with 0.1 wt% CaCO 3 and 0.8 wt% PbO additives (PMZT3) synthesized by B-oxides precursor (BO) and conventional ceramic mixed-oxide methods (MO). Our experimental results show that the BO-type piezoceramics have better grain microstructure and better material properties [e.g., d 33 = 340 pc/N, k t = 0.52, Q m = 1250, temperature coefficient of change rate of resonant frequency (TCF) = 0.01%/°C, and temperature coefficient of change rate of clamped capacitance (TCC) = 0.18%/°C]. We construct 1-MHz transducers using our BO and MO types of piezoceramics and examine their dynamic resonant characteristics as the transducers are driven by a power driver with open-loop control.

SPIE Proceedings, 1998

Piezoelectric materials lie at the heart of ultrasonic transducers. For transducers used in medical imaging (3-7 MHz), PZT-5H ceramics offer high electromechanical coupling (k 33 ≥ 75%), resulting in good bandwidth and sensitivity. As transducer arrays become smaller with increasing frequency, the development of high permittivity ( 000 , 7 > T R ! vs. 3,400 for PZT-5H), piezoelectrics based on polycrystalline PMN-PT, provide improved electrical impedance matching. Advanced medical diagnostic techniques, including contrast and harmonic imaging, have taken advantage of the recent development in single crystal Relaxor-PTs that offer coupling k 33 's > 90% and subsequently, significant increases in bandwidth. For small animal, ophthalmology and cellular imaging, higher resolution is demanded, thus requiring transducers operational in the range of 20-100 MHz. Advancements in ceramic processing include pore-free and fine-grain (≤1 micron) piezoelectric ceramics of PT and PZT, being an "enabling" technology, allowing the fabrication of high frequency single element and annular arrays. Innovations in the fabrication of high frequency arrays (≥ 30 MHz) include tape casting and sol-gel molding techniques. Of particular significance, DRIE (deep reaction ion etching), has demonstrated the ability to mill out ultrafine features, allowing 1-3 crystalpolymer composites operational at frequencies > 60 MHz, far beyond that achieved by current state-of-the-art dicing.

Journal of the European Ceramic Society, 2010

In this work a functionally graded porous Nb-doped PZT material (PZTN) was produced by tape casting. Each step of the production process (slurry formulation, lamination and thermal treatments) was thoroughly investigated. Tapes with different carbon black (CB) amounts were produced. The conditions necessary to laminate 6 layers of different CB concentration were optimized by tailoring the binder to plasticizer volume ratio of each single green layer. Cracks and delaminations were eliminated by gradually increasing CB content and adjusting the binder burn-out procedure. The optimization process led to a well developed, crack-free porosity-graded multilayer, less than 400 m thick and with porosity along the thickness ranging from 10 to 30 vol.%. The application of a load during the heating treatments was absolutely required to obtain warpage-free planar multilayer specimens.

Journal of The European Ceramic Society, 2005

The goal of this work is to prepare doped PZT-type materials with improved radial piezoelectric properties. Ceramic materials were obtained by solid-state reaction at different sintering temperatures. SAED, bright field TEM and HRTEM methods were used for the microstructure studies. The influence of the composition and nanostructure on the dielectric and piezoelectric properties of the materials is discussed. One of the materials with high coupling constant was used in the construction of a miniature flexural ventilator. Designated to function at an emf of 220 V/50 Hz, this device is characterized by high efficiency, reliability and low energy consumption.

The Journal of the Acoustical Society of America, 1999

The construction and evaluation of a hydrophone based on porous piezoelectric ceramics with high d h g h figure of merit ͑FOM͒ is described. It has been shown that, in order to improve the hydrophone signal-to-noise ratio, a piezoelectric material with a high FOM should be employed. A porous piezoelectric material has been prepared by mixing calcined lead zirconate titanate ͑PZT͒ powder with fine particle starch powder. Square plate samples have been cold pressed from this material, which were then heated to eliminate the organic component, sintered, electroded, and poled in a high electric field. An optimum pore volume fraction of approximately 40% has been selected in order to obtain materials with high piezoelectric coefficients and reasonably good mechanical resistance. For this composition a hydrostatic figure of merit of approximately 10 Ϫ11 m 2 /N has been obtained that is a few orders of magnitude higher than traditional piezoceramics. Square plate elements were assembled in a planar hydrophone which was made watertight with polyurethane resin. The hydrophone was characterized by different measurements performed in a water tank, by using a pulsed sound technique. Results on acoustical sensitivity measurements, directivity, equivalent noise pressure level, and sensitivity variation with pressure are presented and discussed.

Acta Physica Polonica A, 2019

Lead free porous piezoelectric ceramics have been the motivation behind research by numerous scientists in perspective of environmental friendly, less dense, wide performance advantages over monolithic and polymer piezoelectric materials with enhanced figure of merit. In this review, a collective entity regarding the processing of porous piezoelectric compounds by various methods, lead free piezoelectric families like BaTiO3 (BT), Ba1−xCaxTi1−y ZryO3 (BCZT), (Bi0.5Na0.5)TiO3 (BNT), (K0.5Na0.5)NbO3 (KNN) and their solid solutions are reported. Porosity configuration, pore forming agents, and various applications in porous lead free piezoelectric ceramic have also been discussed. This review herein has discussed in terms of the tendency to work with lead free porous piezoelectric ceramics.