Relaxor ferroelectric compositions for field induced ultrasonic transducers

A relaxor ferroelectric composition that has the components lead magnesium niobate, lead titanate, and lead magnesium tungstate. The components are preferably present in relative molar amounts of (1-x-y) lead magnesium niobate, (x) lead titanate, and (y) lead magnesium tungstate, where 0.11.ltoreq.x.ltoreq.0.13 and 0.01.ltoreq.y.ltoreq.0.03. Also disclosed is a tunable ultrasonic transducer made of a relaxor ferroelectric composition that has the components lead magnesium niobate, lead titanate, and lead magnesium tunstate. A method of making a relaxor ferroelectric material comprising the step of adding an effective amount of lead magnesium tungstate to a lead magnesium niobate-lead titanate composition is also disclosed.

FIELD OF THE INVENTION 
The present invention relates to relaxor ferroelectric compositions and, 
more particularly, to a new relaxor ferroelectric composition particularly 
useful in ultrasonic transducer applications. 
BACKGROUND OF THE INVENTION 
Ultrasonic transducers are commonly used to analyze the interior of an 
object non-destructively. Imaging internal organs of the human body, such 
as the heart or the kidneys, for diagnostic purposes is a typical example. 
Transducers are typically formed of piezoelectric materials capable of 
generating ultrasonic waves. The piezoelectric materials convert 
electrical energy into mechanical energy to generate acoustic waves. The 
waves are sent into the body being imaged and reflect off objects within 
the body. The piezoelectric material then receives the reflected acoustic 
signals and converts them into electrical signals which may be sent to an 
imaging device. 
A known exemplary piezoelectric transducer material is lead zirconate 
titanate (PZT) ceramic. This material is formed from a PZT-based starting 
composition that is sintered, along with various dopants, into a dense 
polycrystalline ceramic. 
To induce piezoelectric properties in a PZT ceramic, and materials similar 
to it, the ceramic is polarized by applying d.c. voltage. In its polarized 
state, the ceramic exhibits the piezoelectric properties. The acoustic 
waves that are generated by the ceramic transducer are within a frequency 
range that is dependent upon the properties of the specific material used 
for the transducer. 
A material that exhibits piezoelectric properties is in what is known as 
the ferroelectric phase. When a material does not exhibit those 
properties, it is in the paraelectric phase. 
Upon removal of an applied d.c. voltage, "normal" piezoelectric materials, 
such as the PZT ceramic mentioned above, exhibit a remnant polarization at 
temperatures below T.sub.d, the temperature at which polarization (and 
hence piezoelectric effect) disappears. This means that the material 
remains polarized at these temperatures even after the d.c. bias voltage 
is removed. The material thus cannot be "tuned" on and off by applying and 
removing a d.c. bias voltage. The piezoelectric activity, and consequently 
the sensitivity, of these materials is relatively constant. 
Another class of materials, known as "relaxor" ferroelectric materials, are 
actively pursued for transducer applications. In these materials, above 
temperature T.sub.d, the piezoelectric phenomenon is present only when a 
d.c. bias is being applied. That is, there is little or no remnant 
polarization above T.sub.d, so that when the d.c. bias is removed, 
piezoelectric behavior stops instantly. Most importantly, only a modest 
voltage need be applied to induce the piezoelectric behavior. 
Thus, it is possible to construct transducers of relaxor ferroelectric 
materials that have variable sensitivity in piezoelectric properties 
responsive to the applied d.c. bias. Such transducers are referred to as 
"tunable" because they can be turned on and off by applying and removing a 
modest d.c. bias voltage. 
An important property of a transducer material is its dielectric constant 
K. In general, the highest possible dielectric constant K is desirable. 
Dielectric constant K is temperature dependent. Maximum dielectric 
constant K.sub.max occurs at a temperature T.sub.max. For relaxor 
ferroelectric materials, T.sub.max is considerably above T.sub.d. In the 
temperature range (T.sub.max -T.sub.d), relaxor ferroelectric materials 
not only are tunable using only a modest electric field, but also show a 
high dielectric constant K in that range. 
Another important property of a transducer material is thickness 
electromechanical coupling coefficient k.sub.t. Thickness 
electro-mechanical coupling coefficient k.sub.t is calculated from the 
electro-mechanical resonance and anti-resonance frequencies of the 
transducer, which may be measured by resonance techniques. The coupling 
coefficient k.sub.t represents the conversion efficiency of electrical 
energy to mechanical energy. Thus, for materials having a k.sub.t of 
0.45-0.47, only 45% to 47% of the energy is effective for useful work, and 
the rest is dissipated as heat. To avoid excessive heating and property 
degradation, k.sub.t should be as high as possible. 
Another property of a transducer material is the remnant thickness 
electromechanical coupling coefficient k.sub.t rem, which is the 
electro-mechanical coupling coefficient remaining after a d.c. bias is 
removed from a material. For practical applications, k.sub.t rem should be 
less than 0.15 for effective tunability. 
In some ultrasound transducer applications, particularly in the medical 
field, the transducer material is designed to operate at room temperature. 
For relaxor ferroelectric materials, this means that the tunable range 
(T.sub.max -T.sub.d) of the material should encompass room temperature. 
Because room temperature may vary depending on the geographical location, 
air conditioning, or localized heating of a transducer itself, (T.sub.max 
-T.sub.d) should be as broad as possible, preferably covering the range 
from 10.degree. C. to 40.degree. C. The properties of a transducer 
material should be optimized for this temperature range. In particular, a 
high dielectric constant and a high electro-mechanical coupling 
coefficient induced by moderate d.c. fields are most desirable, along with 
low dielectric loss (tan.delta.&lt;5%), high density, and small grain size. 
Specific relaxor ferroelectric materials considered for medical ultrasound 
transducers are lead magnesium niobate-lead titanate compositions 
(PMN-PT); lead zinc niobate-lead titanate-barium titanate compositions; 
and lead lanthanum zirconate titanate compositions. U.S. Pat. No. 
5,345,139, issued to Gururaja et al., discloses a PMN-PT composition doped 
with lanthanum, which produces a wider operating range (T.sub.max 
-T.sub.d). 
It is desirable to improve the properties of transducers operating in a 
wide temperature range around room temperature beyond those shown by the 
known materials, however. In particular, raising the electro-mechanical 
coupling coefficient k.sub.t at given applied d.c. biases is desirable. 
Raising k.sub.t improves the efficiency of the transducer and avoids 
excess heating. These properties are also desirable in other applications 
in which relaxor ferroelectric materials are used. 
SUMMARY OF THE INVENTION 
The present invention is a relaxor ferroelectric composition that has the 
components lead magnesium niobate, lead titanate, and lead magnesium 
tungstate. The components are preferably present in relative molar amounts 
of (1-x-y) lead magnesium niobate, (x) lead titanate, and (y) lead 
magnesium tungstate, where 0.11&lt;x&lt;0.13 and 0.01&lt;y&lt;0.03. 
The invention also provides a tunable ultrasonic transducer made of a 
relaxor ferroelectric composition that has the components lead magnesium 
niobate, lead titanate, and lead magnesium tungstate. 
In another aspect, the invention involves a method of making a relaxor 
ferroelectric material comprising the step of adding an effective amount 
of lead magnesium tungstate to a lead magnesium niobate-lead titanate 
composition.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is a new relaxor ferroelectric composition useful as, 
for example, an ultrasonic transducer. The composition includes lead 
magnesium niobate, lead titanate, and lead magnesium tungstate (PMW). Such 
a composition yields desirable properties over a wide temperature range 
around room temperature. 
Various exemplary compositions according to the present invention were 
prepared using a coulumbite precursor method as follows. 
First, magnesium niobate (MgNb.sub.2 O.sub.6) and magnesium tungstate 
(MgWO.sub.4) precursors were prepared. To form the magnesium niobate 
precursor appropriate stoichiometric amounts of magnesium oxide (MgO) and 
niobium oxide (Nb.sub.2 O.sub.5) were mixed in a ball mill for 24 hours 
with ethyl alcohol and 1/2" ZrO.sub.2 milling media. After drying, the 
mixture was calcined in a covered alumina crucible at 1000.degree. C. for 
10 hours. The calcined product was homogenized and recalcined at 
1100.degree. C. for 10 hours to improve the phase purity. After the second 
calcination, the powder was ball milled and dried. 
The magnesium tungstate precursor was prepared by calcining an appropriate 
stoichiometric mixture of MgO and tungstic oxide (WO.sub.3) at 
1000.degree. C. for 10 hours, and the product was ball milled and dried 
before further use. 
Appropriate stoichiometric amounts of litharge (PbO), MgNb.sub.2 O.sub.6, 
MgWO.sub.4, and titanium dioxide (TiO.sub.2) were then mixed in a ball 
mill for 24 hours. After drying, the mixture was calcined at 
950.degree.-960.degree. C. for 2-5 hours in a closed alumina crucible. The 
calcined powder was ground in a ball mill for 24 hours. After drying, it 
was mixed with 5 weight % of DuPont.RTM.5200 organic binder and was passed 
through a 100 mesh sieve. Several 1/2" pellets were prepared in a steel 
die by applying a uniaxial pressure of 24,000-26,000 psi. The binder was 
evaporated from the pellets by heating them to 300.degree. C. for 2 hours 
and 500.degree. C. for 5 hours. After the binder removal, the green 
pellets were sintered at 1,175.degree.-1,250.degree. C. for 1-4 hours over 
a platinum sheet in a closed alumina crucible. A mixture of PbO-ZrO.sub.2 
powder was used as a source to saturate the crucible atmosphere with PbO 
vapor. After sintering, the pellets were annealed at 
900.degree.-950.degree. C. for 1-5 hours. The sintered pellets were 
polished to appropriate thicknesses and given electrical characterization 
using electrodes. 
Twelve different compositions were prepared using this method. These 
compositions are labeled as compositions a-l in Table 1, which gives the 
composition in mole percent of the components. In general, the 
compositions have a formula (1-x-y)PMN-(x)PT-(y)PMW, where x=0.07-0.13 and 
y=0.00-0.03. 
For each composition, T.sub.max and K were measured at a frequency of one 
kilohertz with no d.c. bias applied. K and k.sub.t were specifically 
measured at 20.degree. C., an approximation of room temperature, with 
eight kilovolts per centimeter of d.c. voltage applied during the 
measuring of k.sub.t. The results are presented in Table 1. 
Table 2 presents various k.sub.t measurements for two of the compositions 
taken at 10.degree. C., 20.degree. C., and 50.degree. C., with different 
applied d.c. voltages of three, five, and eight kilovolts per centimeter. 
The remnant electro-mechanical coupling coefficient k.sub.t rem was also 
measured and is presented in Table 2. 
The data presented in Table 1 show that compositions b-j, those with PMW 
included, had many desirable properties. For these compositions, the 
tunable operating range (T.sub.max -T.sub.d) is expanded to as much as 
41.degree. C., as shown for composition j. Compositions f-i show an 
operating range between 34.degree. C. and 38.degree. C. This means that 
temperature variations of 34.degree. C. will not adversely affect the 
performance of transducers, low frequency actuators, or other devices made 
from these compounds. 
The dielectric constant K for all of the compositions including PMW, 
compositions b-j, are above 12,000 at 20.degree. C. Desirable K values at 
20.degree. C. are above 10,000. The K value for the exemplary compositions 
is as high as 18,950, as shown for composition j. 
Most significantly, k.sub.t for the compositions b-j that include PMW 
appears to be generally elevated in comparison to the compositions a, k, 
and l that do not include PMW. For the compositions without PMW, k.sub.t 
is 0.46-0.48. For the exemplary compositions with PMW, k.sub.t ranges from 
0.48-0.52. 
Any elevation in k.sub.t is a direct benefit in the use of relaxor 
ferroelectric compositions. Because k.sub.t represents the conversion 
efficiency for converting electrical energy to mechanical energy, 
compositions with higher k.sub.t values are more efficient and hence less 
costly to operate. In applications for relaxor ferroelectric materials 
such as ultrasound transducers, this also results in reduced heating of 
the device and reduced property degradation. 
Considering all of desirable factors and properties for relaxor 
ferroelectric materials, the data presented in Table 1 indicates that 
compositions f and i show the best overall performance. The k.sub.t 
values, along with the k.sub.t rem values, for these two compositions at 
three different temperatures and various applied voltages are shown in 
Table 2. At 20.degree. C., k.sub.t rem for both of these compositions is 
zero, and k.sub.t is 0.51 at eight kilovolts per centimeter. The k.sub.t 
values for these compositions vary only 10% from the maximum value over 
the entire temperature span at the various voltages. 
The tabulated properties for the exemplary compositions incorporating PMW 
illustrate that all of the compositions, and particularly compositions f 
and i, are quite suitable for use in applications, such as ultrasonic 
transducers and actuators, where such properties are desired. The 
compositions incorporating PMW provide a desired dielectric constant K 
over a broad temperature range with an elevated electro-mechanical 
coupling coefficient k.sub.t and low remnant coupling coefficient k.sub.t 
rem. 
Although described in connection with specific examples, the present 
invention is not intended to be limited thereto. Rather, the appended 
claims should be construed to encompass the present invention in its true 
spirit and full scope, including all such variants as may be made by those 
skilled in the art without departing therefrom.