Investigation on the Microstructure and Electrical Properties of the Compositionally Modified Pzt Ceramics Prepared by Mixed-oxide Method

The structural and electrical properties of Pb[Zr x Ti 0.95−x (Mo 1/3 In 2/3) 0.05 ]O 3 piezoelectric ceramics system with the composition near the morphotropic phase boundary were investigated as a function of the Zr/Ti ratio. Studies were performed on the samples prepared by the conventional method of thermal synthesis of mixed oxides. The materials structure was investigated by X-ray diffractometry to demonstrate the coexistence of the tetragonal and rhombohedral phases. In the present system, the MPB, in which the tetragonal and rhombohedral phases coexist, is in a composition range of 0.47 ≤ x ≤ 0.50. The lattice constants of the a and c axes for the samples were calculated from the XRD patterns. Microstructure of the sintered ceramics was observed by scanning electron microscopy (SEM) of free surfaces specimens. The relative permittivity, dielectric dissipation, piezoelectric coefficient and electromechanical coupling factor reach at maximum value x = 0.49 (ε r = 7300.345 (at the Curie temperature), tanδ = 0.002050, d 31 = 94.965 PC/N and k p = 0.513 and a Curie temperature of 430˚C). These properties are very promising for applications in ultrasonic motors.


Introduction
Lead zirconate titanate Pb(Zr,Ti)O 3 , a solid solution of perovskite ferroelectric PbTiO 3 and anti-ferroelectric PbZrO 3 with different Zr/Ti ratio, is an important material that is widely used in electronic sensors, actuators, resonators and filters [1][2][3].The materials with a perovskite structure of general formula ABO 3 (where A = mono or divalent ions, B = tri, tetra or pentavalent ions) have been found to be very useful and interesting for different solid-state devices [4][5][6][7][8].By making suitable substitution at A and/or B-site of ABO 3 structure, a large number of charge neutral or deficient compounds have been prepared [9][10][11] which have been found to be very suitable and useful for many industrial applications.
The influence of technological factors on the width of the co-existence region was investigated on the ternary system Pb[Zr x, Ti (0,95−x) (Mo 1/3, In 2/3 ) 0.05 ]O 3 by X-ray diffraction by varying the ratio Zr/Ti.The purpose of this work is to study the influence of sintering temperature on density, porosity and grain size on the ceramic in order to determine the width of co-existence of tetragonal and rhombohedral phases and the exact composition of the MPB rather than to determine the dielectric and the piezoelectric properties of these ceramics near the MPB in detail.

Experimental
All of the specimens were prepared by the conventional ceramics technologie.The compositions of the Pb[Zr x, Ti (0.95−x) (Mo 1/3, In 2/3 ) 0.05 ]O 3 system werewith 46 ≤ x ≤ 55.The commercially available PbO, ZrO 2 , TiO 2 , MoO 3 and In 2 O 3 , were used as the raw materials.Mixed oxides, after milling, were calcined at 800˚C for 2 h at heating and cooling rates of 2˚C/min.After calcinations, the ground and milled powders were pressed into disks 13 mm in diameter and about 1 mm in thickness.Pressed disks of 3PbO + 2ZrO 2 placed in a capped crucible to prevent PbO evaporation during sintering.Four sintering conditions were selected to be used with both methods ranging 1000˚C, 1100˚C, 1150˚C, 1180˚C and 1200˚C for 2 h.
The crystal structure of the samples was analyzed using X-ray diffractometry (XRD; Siemens D500).The voltage and currents ratings used were 40 kV, 30 mA respectively, and CuKα radiation was used.The diffraction data were collected with X-ray scan speed of 0.1˚•min −1 .The bulk density was measured using the Archimedes method.
To investigate the electrical properties, the electrodes were made by applying a silver paste on the two major faces of the sintered disks followed by heat treatment at 750˚C for thirty minutes.The dielectric constant ε was calculated from the capacitance at a frequency of one KHz.It was measured at temperature ranging from 25˚C to 400˚C with a heating rate of one ˚C/minute by using an impedance analyzer (HP4192A, Heweltt-Packard).The piezoelectric samples firstly were being poled in a silicone oil bath at 120˚C by applying a d.c.field of thirty KV/cm for thirty minutes; and, then, were being cold under the same electric field.
They were aged for twenty-four hours before testing.The electromechanical coupling factor K p was determined by the resonance and anti-resonance technique from the "Equation ( 1)" [19].

Phase Structure
The sintered powders were examined by X-ray diffractometry to ensure phase purity and to identify the phases.The phases of the samples were detected using XRD (at room temperature) for several compositions given in Table 1.
Triplet peaks indicate that the sample consists of a   mixture of tetragonal and rhombohedral phases.The multiple peak separation method was used to estimate the relative fraction of coexisting phases by which the relative phase fraction M R and M T were then calculated using the following Equations ( 2) and (3) [21]: phases coexist.The co-existence region is therefore quite narrow (Δx ≈ 0.05) and extends between x T and x R .The width Δx = x T − x R of the co-existence region from our work is close to that obtained by others [22,23].The coexistence of tetragonal and rhombohedral phases has, therefore, to be attributed to the first order nature of the MPB, this is marked contrast to the proposition of Kakegawa and al. [24,25] that the coexistence is invariably due to compositional fluctuations.The study of density is necessary to optimize the optimum sintering temperature.The quality of the material increases with the increase of density and it increases with the increase of the sintering temperature [26].The optimum temperature for sintering is determined from the pattern density as a function of sintering temperature d = f(T).So, the maximum density is the product of better quality electrical (low dielectric loss).Figure 3 gathers the curves of the density of all samples depending on the sintering temperature.A similar shape for all curves is that: the density is minimal for a sintering temperature T = 1000˚C, it begins to grow until it reaches a maximum value at a sintering temperature T = 1180˚C.Then, it decreases at the sintering temperature T = 1200˚C which means that the optimum temperature for sintering is 1180˚C.The increased density means fewer and pore size, so the volume of the cell decreases and consequently the structure becomes more compact.
The optimum sintering temperature depends on several factors such as the addition of impurities, the rate of heating, time of thermal treatment and protecting atmosphere.
Changes in the density of different samples of Pb[Zr x Ti 0,95−x (Mo 1/3 In 2/3 ) 0.05 ]O 3 sintered at 1180˚C depending on the rate of Zr is shown in Figure 4.
The shape of the curve shows that the density increases with the increase of Zr concentration until a maximum  value of 7.61g/cm 3 (94.18% of theoretical density) at Zr = 49% (sample No. 3) and then it decreases [20].
Figure 5 shows the variation of the porosity for different samples depending on the concentration of Zr% and the sintering temperature of 1180˚C, noting that the porosity decreases until itreaches the minimum value p = 5.81% for Zr = 0.49 (sample No. 3) and then it increases.
The parameters of the lattice were determined from the triplets (200) by using a nonlinear least squares method [27].The a R -parameter of the rhombohedral phase and the a T -parameter, c T -parameter, and the tetragonality (c T /a T ) of the tetragonal phase of Pb[Zr x Ti 0,95−x (Mo 1/3 In 2/3 ) 0.05 ]O 3 ceramics are plotted as a function of the ratio of Zr/Ti in Figure 6.The results showed that the parameters of the lattice of the tetragonalphase changed when the ratio of Zr/Ti was modified.While the value of the a T parameter increased, the one of c T parameter decreased and the a R parameter of  the rhombohedral phase increased.The resulting values of the parameters of the lattice of the tetragonal phase showed that the c T /a T axial ratio is decreased as a T it is increased and c T is decreased.The values of the parameters of the lattice were revealed to be practically the same as those which previously studied [28,29].
Figure 7 gives the SEM micrographs of free surfaces of the specimens sintered at 1000˚C -1180˚C for 2 h.From these images, small crystallites with large pores for specimens sintered at 1000˚C Figure 7(a) were noticed.As the sintering temperature increased, the grain grew and the average grain size increased slightly.Whereas, the number and the size of the pores is decreased.The grain size exponentially increased with the increase of temperature and this can be well explained by the phenomenological kinetic grain grow equation [30].The average grain size is 1.15 µm at temperature 1000˚C which increases to 1.56 µm at temperature 1180˚C [20].

Dielectric and Piezoelectric Properties
The dielectric properties at room temperature are plotted as a function of the Zr/Ti ratio in Thereafter, all the samples reveal dispersion due to Maxwell-Wagner [31,32] type interfacial polarization in agreement with Koop's phenomenological theory [33].The high value of dielectric constant observed at lower frequencies can be explained on the basis of space charge polarization due to heterogeneous in structure like impurities, porosity and grain structure [34].

Figure 4 .
Figure 4. Evolution of density as a function of the concentration of Zr% [20].

Figure 8 .
The ε increase with the increase of Zr/Ti ratio.It achieves a maximum value of 1331.955only when x = 0.49 and then it decreases significantly as the Zr/Ti ratio increases further.The tanδ shows an inverse trend and reaches the minimum value of 0.00205 when x = 0.49.The temperature dependence of the dielectric constant for Pb[Zr x Ti 0.95−x (Mo 1/3 In 2/3 ) 0.05 ]O 3 ternary ceramics at 1 KHz is shown in Figure 9.It is observed in all the compositions as the temperature increase, the value of dielectric constant increases and passes through a maximum (at T C ) and then decreases.The Curie temperature increased from 340˚C to 440˚C in our choosing composition.The maximum dielectric constant at MPB (x = 0.49) is about 7300.345.The variation in dielectric constant with frequency for Pb[Zr x Ti 0.95−x (Mo 1/3 In 2/3 ) 0.05 ]O 3 ternary ceramics is shown in Figure 10.The plots show that the dielectric constant decreases with an increase in frequency, showing dispersion in lower frequency rang.It attains a constant value and remains independent of frequency.

Figure 11 .
Figure 11.Variation of K p and d 31 for Pb[Zr x Ti 0.95−x (Mo 1/3 In 2/3 ) 0.05 ]O 3 ceramics sintered at 1180˚C.corresponds to the MPB and the maximum piezoelectric properties observed for this polycrystalline composition are reasonable.When the PZT content was varied away from the MPB, both the d 31 and k p value decreased asymmetrically.In the rhombohedral side, both the d 31 and k p value were decreased much more rapidly.