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In recent years, with the revolutionary development of piezoelectric material, single crystal materials with better piezoelectric properties than the existing ceramics have already been widely used in many fields. These new materials will potentially provide advanced performance in ultrasonic applications such as sonar and biomedical diagnosis and imaging.
There are three stages of producing piezoelectric materials used in medical devices. First, Ferroelectric single crystals ,such as quartz and barium titanate, were the first materials to be used in ultrasonic transducers, Next , Piezoelectric ceramics, like the PZT family, were developed after ferroelectric single crystals, Finally ,through major efforts in the crystal growth community, single crystal piezoelectric materials, Including PMN-PT, have recently been developed. So we will have comparison between transducers based on the single crystal lead magnesium niobate-lead titanate(PMN-PT) and the ceramic lead zirconate titanate(PZT-5H), PbTiO3 and the piezoelectric polymer polyvinylidene (PVDF).
Furthermore, Pb(Mg1/3Nb2/3)O3-/PbTiO3 (PMN-PT) single crystal has widely used for applications as electro-mechanic transducers due to its excellent piezoelectric properties. It is also a potential nonlinear optical material because of its large electro-optical coefficients. And also we know that much inhomogeneity and uncertainty of the structure of this solid solution system combined by the rhombohedral PMN and the tetragonal PT can be produced by the relaxor nature of PMN and the coexistence of possible structures.
It is believed that research on some aspects of PMN-PT should be useful in deepening the understanding of the microstructure and properties of PMN-PT system and promoting its practical applications. And in my opinion, the dependence of their dielectric and piezoelectric performance on the composition, crystal orientation and poling field are involved.
2. Background and history
Before World War II the only crystals that were at all widely used were quartz, Rochelle salt and tourmaline. Quartz was used in all oscillator and filer applications because of their excellent mechanical properties. Rochelle salt in most low-frequency transducer applications and tourmaline was used solely for measuring hydrostatic pressures. Stimulated by the need for a piezoelectric transducer for underwater sound applications that was more stable and less temperature sensitive than Rochelle salt, considerable work was done during World War II in searching for new piezoelectric crystals. This resulted in the discovery and application of ammonium dihydrogen phosphate (ADP) to underwater sound transducers.
As the first generation piezoelectric material, quartz received wide application because of their excellent mechanical properties. Stable quartz crystal grew out of the original work of Professor W.G.Cady of Wesleyan University. These have been applied to controlling the frequency of broadcasting stations and radio transmitters in general. But the Ferroelectric single crystals are now almost restricted to used for which relatively weak properties 
3. Materials Properties
After learn about the background and introduction of piezoelectric material's development. We focus on the different types of piezoelectric materials and their specific parameters. There are four typical examples of piezoelectric materials. We make a table to compare the characteristic between the four typical materials. 
Table 1 Material properties for comparison between major piezoelectric materials for ultrasonic transducers design
Piezoelectric strain constant
Piezoelectric voltage constant
Relative permittivity at constant stress
Electromechanical coupling coefficient
Piezoelectric figure of merit(FOMp)
Thickness mode velocity
Specific acoustic impedance
The sheet shows the main piezoelectric parameters for the four materials. Piezoelectric polymers such as polyvinylidene (PVDF) and its copolymer with trifluoroethylene (TrFE) have also been found to be useful for producing transducers because of PVDF possess a high degree of flexibility, low density, and low acoustic impedance (4MRayl)especially advantages media like water and biological tissues with impedance matching . In general, transducers made with this material are typically very wideband. However, PVDF is not an ideal transmitting material because of its low electromechanical coupling coefficient; it actually does have a low dielectric constant and a fairly high receiving constant. Alternatively, polycrystalline ferroelectric ceramic materials such as lead zirconate titanate, Pb(Zr,Ti)O3 or PZT, have very strong piezoelectric properties ( kt of 0.51 and d33 value about 600 pC/N), a wide range of relative permittivity and low electrical and mechanical losses. Certain Piezoelectric properties of PZT can be enhanced by doping. Because of the outstanding properties of lead-based ceramics, especially the PZT, it has been used as the material of choice for electromechanical devices, such as actuators, sensors and transducers for several decades. However, the increasing success of PZT releases more and more Pb into the environment.As a result, many types of PZT-based piezoceramics are easy to be made so available on the market.
And also It is noticed that d33 piezoelectric coefficient for PMN-PT are significantly larger than the other three types including PVDF, PbTiO3, PZT-5H. As a result, it has much larger piezoelectric figure of merit (FOMP). This suggests that single crystal will have better sensitivity both in transmission and reception modes. In comparison with polycrystalline ceramics, single crystals exhibit dielectric constants ranging from 1000 to 5000 with low dielectric loss <1%, and high frequency (30-60 MHz) transducers fabricated from PMN-PT and PZN-PT single crystals with two-way bandwidths of 40-60% and insertion loss as high as 8 dB have been reported. However, the high values of the relative permittivity of PMN-PT in comparison with PZT-5H could lead to poorer electrical matching in some circumstances.
According to the thickness mode electromechanical coupling coefficient kt, which corresponds to the fundamental thickness resonance of a bulk plate poled through its thickness, is higher for PMN-PT than PZT-5H. Selected mechanical coefficients; the lower mechanical quality factor of PMN-PT in comparison with PZT can be translated into a broader frequency bandwidth. The only data where PZT-5H clearly outperforms PMN-PT in the Curie temperature, Tc, indicate that the single crystal will limit their thermal stability during device fabrication and operation in medical imaging devices.
Thus, exploring materials with similar piezoelectric properties but higher Curie temperature is people's new goal to achieve. It has been reported that single crystal Pb(Yb1/2Nb1/2)O3-PbTiO3 (PYbN-PT) can separatively achieve a Curie temperature and d33 as high as 360 â„ƒ and 2500 pC/N. To sum up,The figures predict that the single crystal material will have enhanced performance over the ceramic and other Piezoelectric polymers both in bulk form and as 1-3 composite.
4. New single crystal material------PMN-PT
In recent years, scientists spend many years on synthesizing a new class of single-crystal piezoelectric materials. As the most common one, lead magnesium doped with lead titanate (PMN-PT) has already used by people and also we found it can further enhance the electro-mechanicalcoupling factor and piezoelectric constant compared to PZT.
Within the past few years, PMN-PT single crystals have been tried in the field of medical diagnosis and imaging, and another advantage of the material is the ability of enhancing the sensitivity. Comparing with ceramic composite arrays, the array transducer with PMN-PT single crystal array was fabricated have outstanding performance. The PMN-PT single crystal ultrasonic transducer was studied recently in order to develop an ultrasonic transducer for highly attenuative material, we find a major disadvantage in the manufacturing of PMN-PT single crystal transducer is the high cost but it can be solved with the perfection of technology. 
The most important property compared with lead zirconate titanate (PZT) ceramics is their extremely high dielectric and piezoelectric constants.
Unlike PZT materials, PMN-PT and PZN-PT single crystals can be grown with relative ease from either high temperature solution and/or congruent melt. As early as in the late 1950s, Myl'nikova and Bokov have successfully synthesized PMN single crystals from a PbO flux. Since then, Smolemskii , Afanas'ev et al. , Setter and Cross , and Petrovskii et al. have also reported the growth of PMN single crystals using the same flux. In 1990, Shrout et al. successfully synthesized single crystals of PMN-PT with PbO-B2O3 fluxes. 
4.1 Morphology of PMN-PT
In both fabrication processes, single crystals grew in the platinum crucible, and most of the single crystals were found on the inside wall and bottom of the platinum crucible. Crystals from different fabrication processes showed different morphology.
In the fabrication a process, there are two kinds of colour crystals, most of the produced single crystals were yellow, and some others were white. The shape of yellow single crystals was octahedral with sizes from 2 to 7 mm, and the shape of the white crystals was cubic with sizes less than 1 mm. Most of the single crystals grown in the fabrication process were white, and cubic or octahedral. The dimension of the crystals ranges from 1 to 4 mm. Fig. 1 shows the typical morphology of the single crystals PMN-PT. 
Fig. 1 Morphology of PMN-PT single crystals.
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4.2. X-ray diffraction analysis of single crystals
XRD technique was used to identify the crystal structures of the produced single crystals. View Within Article The XRD pattern of the white single crystals matches completely with the standard PMN XRD pattern. It is concluded that the structure of the obtained single crystals is, indeed, the structure of PMN crystals. EDS of the white single crystals is shown in Fig. 3. It indicates that the single crystals are composed of elements of Pb, Mg, Nb, and Ti. The combination of XRD and EDS results strongly support that the white single crystals are PMN-PT solid solution. The standard XRD of PMN is shown in Fig. 2.
Fig. 2. XRD pattern of the crystals produced in the B process (upper pattern corresponds to single crystal; lower pattern corresponds to PMN standard).
Fig. 3. EDS pattern of PMN-PT single crystals.
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4.3. Dielectric Properties
Fig. 4 shows the dielectric constant and dielectric loss of PMN-PT single crystals as a function of frequency. The dielectric constant of PMN-PT crystals decreases with the increasing frequency. Fig. 5 shows the dielectric constant and dielectric loss of PMN-PT single crystals obtained from B process as a function of temperature at different frequency. The maximum dielectric constant at room temperature is 13,281 at frequency of 100 Hz. The transition temperatures depend on the frequencies of the applied electric field. The transition temperature is 38°C at 100 Hz, and moves to higher temperature with increasing frequency: 42°C at 10 kHz and 50°C at 1 MHz. The dielectric constant increases with increasing temperature up to the transition temperature, then rapidly decreases as the temperature further increases. The maximum dielectric constant of 15,200 is observed at 38°C at 100 Hz. The value of dielectric loss is as small as 0.078 at the room temperature, and slightly increases with increasing temperature up to about 45°C.
Fig. 5. Temperature dependence of dielectric constant and dielectric loss for PMN-PT single crystal.
Measurement also indicated the frequency dependence and temperature dependence of the dielectric constant and dielectric loss of poled PMN-PT single crystals. The behaviour is similar to that of unpoled samples. It can be seen that the polarization has no effect on the transition temperature. The ferroelectric transition temperature of poled samples is the same as that of unpoled samples.
4.4. Hysteresis loop
Fig. 6 shows the electrical polarization hysteresis loop of PMN-PT single crystals. When the electric field is turned off, the polarization does not go to zero but remains at a finite value called the remnant polarization (Pr). It results from the oriented domains being unable to return to their original random state without an addition energy input by an oppositely directed field. It implies that energy is needed for a change in domain orientation. The strength of the electric field that is required to return the polarization to zero is known as the coercive field Ec.
Fig. 6. Hysteresis loop of PMN-PT single crystals at room temperature.
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Prior to transducer simulation, PVDF, PbTiO3, PZT-5H and PMN-PT have been compared in terms of their piezoelectric properties including typical characteristic. We know that the new class of single-crystal piezoelectric materials such as lead magnesium doped with lead titanate (PMN-PT) has been synthesized and was found to further enhance the many factors and piezoelectric constant compared to PZT and other piezoelectric polymers.
After that, we focus on the research on PMN-PT, from analyzing some aspects like Morphology, x ray diffraction, Dielectric Properties and Hysteresis loop; we can find the nature and processing of the new single crystal material PMN-PT. As the revolutionary development of piezoelectric material, single crystal materials with better piezoelectric properties than the existing ceramics have already been widely used in many fields.