Synthesis and Dielectric Properties of ( 0 . 80 − x ) Pb ( Cr 1 / 5 , Ni 1 / 5 , Sb 3 / 5 ) O 3-xPbTiO 3-0 . 20 PbZrO 3 Ferroelectric Ceramics

Perovskite PZT variants were synthesized from stoichiometric oxide ratios of Pb, Zr, Ti, Cr, Ni and Sb. The oxide powders were mixed mechanically and calcinated, and then sintered to form the desired perovskite phase. The detailed structural and ferroelectric properties were carried out for sintered specimens. The results of X-ray diffraction showed that all the ceramics specimens have a perovskite phase. The multi-component ceramic system consists of the (0.80 − x)Pb(Cr1/5,Ni1/5,Sb3/5)O3-xPbTiO3-0.20PbZrO3 (PZT-CNS), with 0.30 ≤ x ≤ 0.42, and the ternary system near the rhombohedral/tetragonal morphotropic phase boundary(MPB) was investigated by X-ray diffraction and dielectric properties. In the present system, the MPB that coexists with the tetragonal and rhombohedral phases is a narrow composition region of x = 0.38 0.42. The scanning Electron Microscopy (SEM) showed an increase of the mean grain size when the sintering temperature was increased. A sintered density of 92.93% of the theoretical density was obtained for Ti = 42% after sintering at 1180 ̊C. Ceramics sintered at 1180 ̊C with Ti = 42% achieve excellent dielectric properties, which are as follows εr = 4262.48, and Tc = 340 ̊C.


Introduction
Lead zirconate titanate (PZT) with the perovskite structure is the most popular ferroelectric material, which plays a remarkable role in modern electroceramic industry [1].Moreover, PZT has high dielectric constant, high electromechanical coupling and high piezoelectric coefficient and has been employed as sensors, actuators and transducers [2][3][4][5][6].The PZT are often modified by the introduction of the doping agents into the sites A or/and in the sites B of perovskite ABO 3 structure.The principal role of the doping agents is generally the improvement of the physical and mechanical properties of these materials.Substitutions in the crystal lattice called doping are often led with the aim to improve the specific properties of the PZT or sometimes to adapt them to specific applications.These properties are generally improved by the additions of one or more cations which will replace Pb 2+ in site A and/or couple (Zr 4+ /Ti 4+ ) in site B of perovskite structure (ABO 3 ) [7].The selection of dopants or substitutions to tailor some physical properties of PZT was based on many factors which are the following: 1) charge neutrality, 2) tolerance factors, 3) ionic radius, and 4) solubility/miscibility.However, the sintering of PZT at high temperatures gives rise to a lead loss, which drastically degrades the device performance.Generally, a lead loss at high temperatures can be prevented by atmosphere controlled sintering of PZT.However, such composition requires sintering at a high temperature (>1250˚C) in a controlled atmosphere to contain lead volatilization so as to avoid a shift in composition.To get around the problem, different sintering aids have been tried by various workers [8][9][10].However, for practical applications, such sintering aids need proper selection so that the electrical and piezoelectric properties of the ceramics do not degrade.The width and the properties of the coexistence region are associated with the occurrence of the compositional fluctuation of Ti 4+ and Zr 4+ ions in the PZT materials [11].The compositional fluctuation, which is due to a non-uniform distribution of Titanium and Zirconium ions, leads to a broad variation in the dielectric constant accompanied with a Titanium concentration in the MPB region [12].The width of this coexistence region and the structure of the PZT ceramics were greatly affected by the firing time and temperature [13].
In this study, (0.80 − x)Pb(Cr 1/5 ,Ni 1/5 ,Sb 3/5 )O 3 -xPbTiO 3 -0.20PbZrO 3 piezoelectric ceramics were investigated near the MPB by varying the ratio of Zr/Ti.The purpose of this work was to study the phase structure, the dielectric, and the piezoelectric properties of these ceramics near the MPB in detail.
Raw materials were mixed in acetone medium by using a magnetic stirrer during two hours.The obtained paste is being dried at 80˚C in a drying oven for two hours, and then crushed in a mortar out of glass during six hours.After crushing, the obtained powder is compacted in a form of pastilles with a pressure of 300 kg/cm 2 .Then, a preliminary calcination with 800˚C is carried out during two hours with a heating rate of 2˚C/mn.The calcined mixture is crushed for a second time during four hours, and then was quickly crushed in a form of pellets with a pressure of 1000 kg/cm 2 .These pellets are agglomerated at various temperatures of sintering (1100˚C, 1150˚C, 1180˚C, and 1200˚C) during two hours.It is important to note that a lead loss is possible by evaporation of PbO which is very volatile in T ≥ 900˚C.To limit this effect; an atmosphere rich in PbO was maintained with the powder of PbZrO 3 to the minimum to reduce this loss during sintering.The pastilles are metalized by using a thin layer of silver paste on the two faces.
X-ray diffraction (XRD, Siemens D500) was used to determine the crystalline phases present in the powder.The compositions of the PZT phases were identified by the analysis of the peaks [(002)T, (200)R, (200)T] in the 2θ range 43˚ -46˚.The tetragonal (T), rhombohedral (R) and tetragonal-rhombohedral phases were characterized and their lattice parameters were calculated.The rhombohedral lattice parameter was calculated on the assumption that the rhombohedral distortion was constant (unit cell angle α R = 89.9˚)[14,15].In order to ensure an ac-curate determination of the lattice parameters, the X-ray peaks were recorded gradually with 0.01˚ steps.
Electronic micrographs scanning (SEM) were taken from fractured as well as chemically etched surfaces.A section of the sintered sample was etched in a 5% HCl solution for 3 minutes.The fractured surfaces were used for grain size and morphology determination.The size distribution of the grains was measured and the results compared with each other.The size distribution of the pores and the total value of porosity were determined on a polished cross-section of the samples with an image analyzer.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 a 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 temperatures ranging from 25˚C to 450˚C with a heating rate of one ˚C/minute by using an impedance analyzer (HP 4192A, Hewlett-Packard).

Phase Analysis and Microstructure
Sintered powders were examined by X-ray diffractometry to ensure phase purity, and to identify the crystal structure.The coexistence of tetragonal and rhombohedral phases near the morphotropic phase boundary implies the existence of compositional fluctuations which can be determined from the width of the X-ray diffraction peaks.However, determination of the compositional fluctuation for samples near the morphotropic phase boundary is difficult.XRD patterns of PZT powders were analyzed for detecting the characteristic rhombohedral and tetragonal splittings.The (2 0 0) reflections form a doublet in the tetragonal phase while (1 1 1) is a singlet.For the rhombohedral phase, (1 1 1) is a doublet while the (2 0 0) is a singlet.The powder X-ray patterns of (0.80 − x)Pb(Cr 1/5 ,Ni 1/5 ,Sb 3/5 )O 3 -xPbTiO 3 -0.20PbZrO 3 ceramics with different x values are shown in Figure 1.For x in the range of 0.39 -0.42, the diagram indicates that a mixture of phases should be present, which is illustrated by the (0 0 2) and (2 0 0) tetragonal doublet enclosing the (2 0 0) rhombohedral line.For x in the range of 0.30 -0.36 there is a little evidence of the (2 0 0) R peak, indicating a virtually single-phase tetragonal structure.It is evident from "Figure 1" that as the Ti content increases, the morphotropic phase becomes more prominent whereas the tetragonal decreases.
Figure 2(a) shows the variation of density with sintering temperature.The density increases in the initial period with sintering temperature and saturates beyond 1180˚C.From these results, the optimum firing temperature for the maximum density, ρ, of the ceramic is be-  tween 1150 and 1200˚C.At 1180˚C and Ti = 42%, 92.93% of the theoretical value was achieved.Sintering at 1200˚C caused the density to decrease.The optimum value of the sintering temperature was affected by the additions of impurities and other processing parameters, such as the rate of heating, time of thermal treatment, and composition of the protecting atmosphere.The optimum sintering temperature was taken as the point when the PbO vapor pressure evaporation-recondensation equilib-rium for the reaction: PbO-PbO (vapor)-Pb(vapor) + 1/2 O 2 was established [16].Increase of the porosity for temperatures higher than the optimum can, therefore, be attributed to a greater rate of evaporation of PbO compared to that recondensed.Additions of different oxides to PZT-type ceramics influence the densification and the grain size.The process involves a decrease in the number and size of the pores together with an increase in the grain sizes.The porosity, determined by means of the image analyzer as a function of sintering temperature, is given in Figure 2(b).
Figure 3 shows the lattice constant at room temperature as function of x.It can be seen that the tetragonal lattice parameter a T increases linearly with increasing x, while the c T parameter decreases linearly to a smaller extent.In all the composition range where the tetragonal phase is present, c T and a T are closing to gather a T when Ti content increases, particularly inside the co-existence region, meaning that the structure is approaching the cubic geometry.The rhombohedral lattice parameter aR appears to oscillate between 4.023 and 4.024 Å.According to these results, we find that there is a region where the two phases tetragonal (T) and rhombohedral (R) coexist.This region is detected for compositions: Ti = 39%, Ti = 42%.As against the compositions correspond to Ti ≤ 36%, show that the material obtained is of tetragonal structure.The influence of the substitution of Zr/Ti ratio on the structure of the parameters can be explained by the difference between the ionic rays of Zr and Ti (0.68 and 0.79 Å, respectively).This cannot provide a total homogeneity in the solid solutions containing both tetragonal and rhombohedral phases.
Figures 4(a)-(c) shows the SEM images of PZTCNS (20/36), PZTCNS (20/39) and PZTCNS (20/42) ceramics sintered at 1180˚C.All the sintered ceramics appear to be very dense and of a homogeneous granular structure.At first sight, the three compositions seem homogeneous and there do not seem to be grains of the pyrochlore

Dielectric Properties
Figure 5 shows the variation of the dielectric constant as a function: of composition and of temperature at sintering temperatures 1100˚C, 1150˚C and 1180˚C.For the three temperatures of sintering 1100˚C, 1150˚C and 1180˚C, observed that the permittivity increases gradually with the increase in the composition of x and takes a maximum of 290,15 for the sample with Ti = 42% included in the morphotropic phase boundary (MPB) at the temperature 1180˚C.This maximum of dielectric activity can be explained by the presence of several directions of spontaneous polarization relating to the existence of the two structures rhombohedral and tetragonal.Sample No. 4 (20/39/41) at the sintering temperature of 1180˚C has an exception in the evolution of ε r (T).The dielectric constant increases continuously as a function of temperature, so this sample does not have a Curie temperature to a temperature between (0, 450˚C) [18,19].

Conclusion
The compounds of the solution solid zirconate-titanate lead, noted PZT, general formula (0.80 − x)Pb (Cr 1/5 ,Ni 1/5 ,Sb 3/5 )O 3 -xPbTiO 3 -0.20PbZrO 3 as x varies from 0.30 to 0.42 by setup of 0.03, and it has been prepared from a mixture of oxides by the method ceramics.The effect of sintering temperature on density and porosity was studied to achieve the optimum sintering temperature corresponding to the maximum density and minimum value of porosity, because this temperature (1180˚C) corresponds to a better quality product.The parameters of the lattice: a T and c T of the tetragonal structure and a R of the rhombohedral structure were found to change when composition is modified.The 0.20PbZrO 3 -0.42PbTiO 3 -0.38Pb(Cr1/5 ,Ni 1/5 ,Sb 3/5 )O 3 ceramics sintered at 1180˚C exhibit good Dielectric properties at the new MPB ε r = 4262.48,and Tc = 340˚C.

Figure 3 .
Figure 3. Variation of the unit cell dimensions as a function of composition (Ti%).

Figure 4 .
Figure 4. Microstructure of (0.80 − x)Pb(Cr 1/5 ,Ni 1/5 ,Sb 3/5 )O 3 -xPbTiO 3 -0.20PbZrO 3 ceramics sintered at 1180˚C for 2 h, (a) Ti = 36; (b) Ti = 39; (c) Ti = 42.phase which are identifiable by their pyramidal form.The ruptures with the grain boundaries are synonymous with a good sintering.It is noticed that the average diameter of the grains increases significantly with increasing TiO 2 .The intermediate size of the grains is 1.842 μm for the sample "Figure 4(a)" with Ti = 36%.For cons, the intermediate size of sample "Figure 4(b)" of the grains is larger (2.283 μm).In the case of ceramics "Figure 4(c)" with Ti = 42%, the intermediate size of the grains is larger than that of "Figure 4(a)" and "Figure 4(b)" (of the order 2.521 μm); and the broader the granulo-metric distribution "Figure 4(c)", the more the size of the grains gets bigger [17].

Figure 5 .
Figure 5. Dielectric constant (ε) according to the variation of x (Ti) in the composition and temperature.