PMN ceramics

The ferroelectric Pb(Mgy3Nb2/3)03 (PMN) ceramic has been the snbject of extensive investigations due to its high dielectric coefficient and high electrostrictive coefficient, which renders it suitable for use in capacitors and electrostrictive actuators. However, the successful exploitation of this material is limited by the difficulty of producing a single-phase material with the perovskite structnre. Conventional solid state synthesis techniques invariably resnlt in the formation of one or more pyrochlore phases, which exhibit poor dielectric properties. 

For comparison, the same dependencies for bulk PMN ceramics are presented in Fig. 2.12. It is seen that maximal values of t (f,T) and tg >(f,T) decrease with thickness decreasing, being several times less than those in the bulk samples. The... 

Fig. 2.12 The temperature dependence of relative dielectric permittivity and losses for bulk PMN ceramics of 0.49 mm thickness for different frequencies, shown in the legends [28]...

There is also growing iaterest ia thin-film dielectric capacitors. For example, through the use of processiag techniques such as sol—gel solution deposition, thin (--- 0.25 fim) ceramic layers having dielectric constants ranging from 500 to 2000 ia the PZT, Pb(Zr,Ti)03, and PMN—PT, Pb(Mn2 3Nb2 3)03-PbTi03, compositional families respectively, have been prepared (3). 

These lead-based materials (PZT, PLZT, PMN) form a class of ceramics with either important dielectric, relaxor, pie2oelectric, or electrooptic properties, and are thus used for appHcations ia actuator and sensor devices. Resistive properties of these materials ia film form mirror the conduction processes ia the bulk material. Common problems associated with their use are low dielectric breakdown, iacreased aging, and electrode iajection, decreasiag the resistivity and degrading the properties. 

Electrostrictive materials offer important advantages over piezoelectric ceramics in actuator applications. They do not contain domains (of the usual ferroelectric type), and so return to their original dimensions immediately a field is reduced to zero, and they do not age. Figure 6.24(a) shows the strain-electric field characteristic for a PLZT (7/62/38) piezoelectric and Fig. 6.24(b) the absence of significant hysteresis in a PMN (0.9Pb(Mg1/3Nb2/303-0.1 PbTi03) electrostrictive ceramic. 

Fig. 7.9 shows the temperature dependence of the dielectric constant and dielectric loss at 1 kHz for the PMN-PT ceramics obtained by sintering the calcined powders from a soft-mechanochemical route at 1200°C for 2 h. A diffuse phase transition, being typical for a relaxor, is observed for each ceramics. As x increases from 0 to 0.2, the maximum dielectric constant, K, , increases from 13000 to 27000. The temperature correspondent to K ,... 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

Lam et al. [2005] also reported the evolution of the pyroelectric coefficient (pe) with the volume fraction of PMN-PT. The pyroelectric coefficients of ceramic and copolymer have the same sign, but not their 33 coefficients. The maximum increase was obtained for a parallel polarization procedure. In both cases, the increase was quasilinear as a function of filler content from 5 to 40% of PMN-PT to 40%, the pyroelectric coefficient, pe, increased by a factor of 3. A linear increase in the piezoelectric coefficients of composites has also been shown in a PA-11/BT system [Capsal et al., 2007]. It was found that BT particles increase the piezoelectricity of the composite up to 6pC/N for piezoelectric activity with decreasing filler size, due to the decrease in tetragonality (ferroelectric phase). 

Ceramic PLZT has a number of structures, depending upon composition, and can show both the Pockels (linear) electro-optic effect in the ferroelectric rhombohedral and tetragonal phases and the Kerr (quadratic) effect in the cubic paraelectric state. Because of the ceramic nature of the material, the non-cubic phases show no birefringence in the as-prepared state and must be poled to become useful electro-optically (Section 6.4.1). PMN-PT and PZN-PT are relaxor ferroelectrics. These have an isotropic structure in the absence of an electric field, but this is easily altered in an applied electric field to give a birefringent electro-optic material. All of these phases, with optimised compositions, have much higher electro-optic coefficients than LiNb03 and are actively studied for device application. 

The typical representatives of this family include Pbi La (ZryTii y)i /403 (PLZT), Pb(Mgi/3Nb2/3)03-PbTi03 (PMN-PT), and Pb(Zny3Nb2/3)03-PbTi03 (PZN-PT). The PLZT formula assumes that La substitutes for Pb " in the A-site and the B-site vacancies are created for electrical balance. To achieve highest transparency and electro-optic coefficient, some elements, such as Ba and/or La, are usually introduced into the solid solutions of PMN-PT and PZN-PT. Material synthesis and properties of transparent electro-optic ceramics, including PLZT and PMN-PT, have well been summarized and documented [223]. 

Detailed investigation on the optical characteristics, including the electro-optic phase modulation, electric hysteresis property, and thermo-optic coefficient, of transparent PMN-PT electro-optic ceramics have been conducted [229]. A polarization independent PMNT electro-optic switch by using s -shifted fiber Sagnac interferometer stmcture was constracted and analyzed experimentally. Some switch performances, including thermal characteristic and different switching frequency response, were also realized. 

Preparation and characterization of transparent ceramics of La-doped PMN-0.25PT by a two-stage sintering method from conventional raw materials were reported in Ref. [236]. Optical characteristics of Er -doped PMN-PT transparent... 

Fig. 2.21 Photographs of the Er -doped PMN-0.25PT transparent ceramics with a thickness of 1.5 mm. The numbers in the photographs are the Er doping levels in mol%. Reproduced with permission from [237]. Copyright 2012, Elsevier...

Kong LB, Ma J, Zhu W, Tan OK (2002) Translucent PMN and PMN-PT ceramics from high-energy ball milling derived powders. Mater Res Bull 37 23-32... 

Fig. 7.13 Transmission curves of the PMN-PT 3/75/25 ceramics with a thickness of 0.5 mm (a) and SEM images of the samples sintered with different schedules b sample A, c sample B, and d sample C. Reproduced with permission from [59]. Copyright 2010, John Wiley Sons...

Figure 10.13 shows experimental setup for the optical characteristic measurement of PMNT ceramics [133]. The size of PMN-PT ceramic sample was 5 mm X 2 mm x 1 mm for length x width x thickness. Ti/Pt/Au layers were sputtered on both surfaces of the ceramics as electrodes. Two collimators were used to collimate the incident beam and receive the transmission beam. The output beam was detected by using an optical spectrometer and phase demodulation. Because the PMN-PT electro-optic ceramics have a large refractive index, i.e., n = 2.465, the ceramic samples could be considered as a Fabry-Perot (FP) resonator, which can be used to measure the electric hysteresis and thermo-optic coefficient. The applied voltage generated a transverse electro-optic effect for the transmission light beam. 

Figure 10.14 shows the variation of phase shift of incident beam of the PMN-PT ceramic samples, as a function of the applied voltage [133]. A quadratic curve... 

Figure 11.15a and b show the crystal morphology of individual Pb(Mgi/3Nb2/3) 03-35mol% PbTiOs (PMN-35PT) ceramics constantly in contact with vapor and with liquid during heat treatment, respectively. No noticeable trace of the development of any curved surfaces is detectable in (b), while curved surfaces - which are the... 

Figure 11.18 (a) Optical micrograph of an abnormally grown twinned PMN-35PT ceramic with its 3-D shape (b) Electron-beam back-scattered diffraction analysis shows the crystallographic relationship between two domains in the twinned PMN-35PT ceramic. After Refs [77, 78]. 

Fig. 4.59 X-ray diffraction patterns of random and textured PMN-32.5PT ceramics [33]. With kind permission of John Wiley and Sons...

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