Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-03-08
2002-10-15
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06465937
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improved cut orientations and dimensions for lead based single crystal compositions including Pb(Mg
1/3
Nb
2/3
)O
3
—PbTiO
3
(“PMN—PT”), Pb(Zn
1/3
Nb
2/3
)O
3
—PbTiO
3
(“PZN—PT”), and Pb(Sc
1/3
Nb
2/3
)O
3
—PbTiO
3
(“PSN—PT”), type relaxor single crystal compositions. Specifically, improved electromechanical properties of single crystals were obtained by orienting the crystal's thickness and width along certain directions. In addition, improved properties were also obtained by preparing single crystals of specific dimensions. These combinations of cut orientations and dimensions give rise to improved crystal processibility and excellent electromechanical properties.
BACKGROUND OF THE INVENTION
A transducer is a device that converts one form of energy to another. For example, ultrasonic transducers convert electrical energy to mechanical energy and vice versa. An ultrasonic transducer includes an ultrasonic transmitting/receiving element(s) typically consisting of piezoelectric element(s) connected to electrodes. The electrical energy supplied to the electrodes electrically excites the ultrasonic probe element(s) causing them to vibrate at a given frequency. The vibrations then give rise to acoustic waves (in this case, ultrasonic waves) which, upon impinging on an interface representing a junction between two media, are either reflected or transmitted. The reflected waves can be detected by the same piezoelectric probe. This reflection and transmission of acoustic waves at the interface between two media is the basis of ultrasonic imaging. An ultrasonic imaging apparatus incorporating this ultrasonic probe has been used to examine the interior of a human body or to detect flaws in a metal welded portion.
B-mode imaging, color flow mapping (CFM), and Doppler are the common ultrasonic diagnostic imaging methods used on human bodies. CFM is capable of two-dimensionally displaying in color the blood flow velocity in organs such as the heart, liver, kidney, spleen or carotid artery by using a Doppler shift of ultrasonic waves caused by the bloodstream, as well as displaying tomographic images (the so-called B-mode images where the echo signals are represented as intensity-modulated lines in a display) of human bodies. Diagnostic capability has been dramatically improved by these medical diagnostic methods.
A commonly used ultrasonic probe configuration comprises an array of a few tens to about 300 ultrasonic transmitting/receiving elements each of which is made of a strip of piezoelectric material. With this configuration, it is difficult to obtain matching with a transmitting/receiving circuit because the impedance of each piezoelectric element increases as the number of ultrasonic transmitting/receiving elements increases. Also, for certain applications, the surface of a phased-array probe has to be kept as small as possible if, for example, the probe used to image the heart through rib spacing or used internally on a live subject.
Ultrasound as an imaging method has tradeoffs. When high frequencies are used, the resolution is improved but the penetration is reduced. Thus, in many cases, more than one transducer is needed to perform a diagnosis because of the necessary penetration depth and resolution. But good penetration and resolution cannot be obtained at the same time. The human tissue has strong non-linear characteristics. When it is imaged by an ultrasonic signals, it generates harmonic signals, such as first, second, and third harmonic signals. Recently, with the advent of tissue harmonic imaging, it is possible to increase penetration by transmitting at a lower fundamental frequency (f
0
) and at the same time, to increase resolution by detecting the second harmonic signal (2f
0
) arising from the nonlinear response of the subject.
Another ultrasound application utilizing harmonics is the contrast harmonic imaging. In this type of imaging, the contrast agents used are typically gas-filled microspheres (bubbles) which resonate at certain ultrasonic frequencies. When the contrast agents are insonified at one frequency, they generate large harmonic signals due to the contrast agents' nonlinear response. The use of contrast agent significantly improves the detection of blood-filled structures and blood flow velocity in the arterial systems.
For harmonic imaging, broadband transducers must be used to both transmit and received a bandwidth wide enough to encompass the fundamental and harmonic frequencies. Because current PZT-type transducers do not fulfill this bandwidth requirement, their performance in harmonic imaging is lower than transducers that meet the requisite bandwidth.
Nonetheless, piezoelectric materials such as PZT based ceramics are widely used for medical ultrasound transducers. Two of the important criteria for choosing a piezoelectric material for ultrasound transducer applications are high values of the longitudinal coupling constant (k
33
) and dielectric constant (K). A high coupling constant is desirable because it represents the efficiency of conversion of electrical energy to mechanical energy and vice versa. A high dielectric constant leads to better electrical impedance matching with the system electronics especially for small element phased array transducers. PZT ceramics have a typical k
33
value of 0.70, but even higher coupling constants are preferred because they would increase not only the transmit and receive efficiency but also the bandwidth of the transducer. The recent discovery of high coupling in lead-based single crystal materials have generated a lot of interest in this regard.
Lead-based ferroelectric single crystals with the general formula Pb(B′B″)O
3
where B′=Mg
2+
, Zn
2+
, Sc
3+
. . . and B″=Nb
5+
, Ta
5+
. . . , and the solid solution of these compounds with PbTiO
3
have been shown to exhibit excellent electromechanical properties near the morphotropic phase boundary (MPB), the boundary separating the rhombohedral phase (spontaneous polarization along <111>) and the tetragonal phase (polarization along <001>). Some of the important compounds include Pb(Mg
1/3
Nb
1/2
l
3
)O
3
—PbTiO
3
(“PMN—PT”), Pb(Zn
1/3
Nb
2/3
)O
3
—PbTiO
3
(“PZN—PT”), and Pb(Sc
1/3
Nb
2/3
)O
3
—PbTiO
3
(“PSN—PT”).
FIG. 1
shows the phase diagram of PZN—PT and PMN—PT.
The electromechanical properties of PZN-9%PT (ratio of PZN to PT is ~10 to 1) single crystals were first reported by Yonezawa et al. in 1969 (
J. Jpn. Soc. Powder Metallurgy,
16, 253-258 (1969)), then by Kuwata et al. in 1982 (
Ferroelectrics,
37, 579-582 (1981);
Jpn. J. Appl. Phys.
21, 1298-1302(1982)). The high coupling constants of these single crystals make them attractive for transducer and actuator applications. These crystals with a composition on the rhombohedral side of the MPB and cut with the thickness along the [001] direction showed very high coupling (k
33
>0.92) and piezoelectric constants (d
33
>1500 pC/N). U.S. Pat. Nos. 5,295,487, 5,402,791, and 5,998,910 describe PZN—PT and PMN—PT systems at various compositions for ultrasonic transducer applications, the contents of which are hereby incorporated in their entirety. Coupling constants for slivers (k
33
′=0.82) and for bars (k
33
=0.92) have been reported for these systems.
Previous work has focused primarily on the <001> longitudinal orientation, and the properties, especially the dielectric properties were found to be unstable near the MPB compositions because the compositions undergo a phase transition from rhombohedral to tetragonal phase. While the <111> orientation has also been investigated for bar-shaped elements, the electromechanical properties along this orientation have been shown to be inferior to those along the <001> orientation. For example, U.S. Pat. No. 5,998,910 and Kuwata et al. in
Jap. J. Appl. Phys.
21 1298-1302 (1982) report low k
33
values of only about 0.35-0.68 along the <111> direction. Thus, in
Beck Heather
Chen Jie
Gururaja Turuvekere R
Panda Rajesh Kumar
Dougherty Thomas M.
Medley Peter
Vodopia John
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