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PLZT (X/70/30) ceramics with different La contents (X = 7.45, 7.51,7.57, 7.63) were fabricated to study the effect of anisotropy on ferroelectric, optical, and electro-optic properties in parallel and normal to the uniaxial hot-press sintering direction (abbreviated as parallel and normal), respectively. All samples show the square hysteresis loops but the tetragonality with ferroelectricity is more evident in a parallel direction than that in normal direction. The high transmittances (≥60%, 0.35mm, visible to near infrared light) are obtained. However, the transmittances and the light scattering contrast ratios are higher in the normal direction. The PLZT (7.63/70/30) ceramic exhibits the more stable electrically controlled light scattering behavior in parallel direction and its contrast ratio reaches to the maximum 1.37 in a normal direction in this system.
© 2016 Optical Society of America
1. Introduction
Lanthanum-modified lead zirconate titanate Pb1-xLax(Zr1-yTiy)1-x/4 (abbreviated as PLZT(x/(1-y)y) ceramics have been received extensive attention since early 1970s [1, 2] for their excellent performances, such as high transmittance, fast response speed, and large electro-optical effect, et al. Especially, the electrically controlled light scattering performances can be applied to attenuate the incident light intensity without using the polarizer, which means an important application in optical modulators, optical shutters, image devices, et al. [3–6].
Precious works have been mainly carried out on the PLZT (X/65/35, X = 7, 8.19) ceramics to meet the needs of optical photograph memory and display devices [7, 8], because they occupy high transmittance and relative high ratio. On the other hand, the PLZT(X/70/30, X = 7.3~8.0) ceramics are also found to present an excellent electrically controlled light scattering properties [4, 9, 10], which shows an promising application in optical modulators. Generally, the PLZT transparent ceramics are prepared by the hot-press sintering process to eliminate the pores completely in the material. But the anisotropy caused by the uniaxial hot-press in the sintering process in the PLZT ceramics is seldom to be reported. It is well known that the anisotropy of the material is very important to the optical devices, so it is necessary to figure out the relationship between the spatial orientations and the physical properties of the PLZT transparent ceramics prepared by hot-press sintering process.
In this study, PLZT(X/70/30) transparent ceramics with different La contents (X = 7.45, 7.51, 7.57, 7.63 mole %) were prepared by the uniaxial hot-press sintering techniques. The hysteresis loops, transmittances and electrically controlled light scattering properties of PLZT ceramics in different spatial orientations were examined to analyze the effect of anisotropy on the ferroelectric, optical and electro-optical properties of PLZT ceramics.
2. Experimental procedure
The raw materials PbO (99.7%), La2O3 (99.8%), ZrO2 (99.8%), TiO2 (99.99%) were selected to fabricate the PLZT(X/70/30) (X = 7.45, 7.51, 7.57, 7.63 mole %) ceramics by the uniaxial hot-press sintering techniques. 10wt% excess of PbO is required to compensate for the lead evaporation in the high temperature sintering process [11, 12]. The oxide mixtures were ball milled in the ethanol medium for 4 hours, then dried, and sieved. Subsequently the powders were cold pressed into cylinders with the thickness of 20 cm and the diameter of 35 cm. Then, the cylinders were sintered at 1240°C for 16 hours in an O2 atmosphere with the same axial pressure of ~160kg/cm2. To investigate the physical properties of different spatial orientations, all cylindrical samples were cut into pieces along parallel and normal to the uniaxial hot-press direction, respectively, see Fig. 1.
Fig. 1 Illustration showing how measured samples were taken from sintered cylinders; surfaces of parallel and normal cut samples were oriented normal and parallel, respectively, to hot-press direction. Shown are directions for XRD & SEM observation, polarization and incident light when properties were measured.
The phase structures of sintered samples were examined by X-ray diffraction (XRD, D/MAX-2550 V, Rigaku, Tokyo, Japan). The scanning electron micrographs of the fracture surfaces of the sintered samples were obtained using a Hitachi S-4800 (Tokyo, Japan), and the mean grain sizes were calculated by the line intercept method [13, 14]. The gold/chromium electrodes were evaporated on both sides of the major surfaces. The P-E hysteresis loops were examined on the WorkStation equipment (Radiant Technologies, USA) at room temperature. The transmittances of the polished samples with a thickness of 0.35 mm were measured by a U2800 spectrophotometer (Hitachi, Tokyo, Japan) over a wavelength range from visible to near IR. The tin doped indium oxide (ITO) transparent electrodes were deposited on both sides of the polished planes by a magnetron sputtering coater with ITO target (MSP-3200, Beijing, China) to investigate the electro-optical behaviors. The electrically induced light scattering performances were studied by examining the changes of samples’ transmittances at 632.8 nm under an applied electric field.
3. Result and discussion
3.1 Phase
Figure 2 shows the phase diagram of PLZT system at room temperature [1]. In this study, the compositions of the PLZT (X/70/30, X = 7.45, 7.51, 7.57, 7.63 mole %) ceramics are all located in the ferroelectric rhombohedral (FERh) phase edge region.
The XRD patterns of the PLZT ceramic samples are shown in Fig. 3(a)-3(b). The results indicate that all samples present a pure perovskite phase structure without the detectable second phase. The dependence of lattice parameter c/a on the La doping level in parallel and normal direction is shown in Fig. 3(c). It depicts that the values of c/a are found to approach to 1 as the La content increasing, but they deviate more from rhombohedral phase (c/a = 1) for all samples in parallel direction than those in normal direction which demonstrates the spatial anisotropy of the internal structure of these materials. The possible reason may be that when the material particles were rearranged and grew at high temperature under the enormous pressure, the greater induced internal stress was introduced into the unit cells in parallel direction and, therefore, more serious structure distortion occurred in parallel direction [15].
Fig. 3 The XRD patterns of PLZT ceramics: (a) in parallel direction, (b) in normal direction; and (c) the values of c/a for PLZT ceramics in parallel and normal direction.
3.2 Microstructure
The SEM images of fracture surfaces for PLZT (X/70/30) ceramic samples and the calculated mean grain sizes in parallel and normal direction are shown in Fig. 4. All samples exhibit fully dense microstructures with well-developed grains. The mean grain sizes for PLZT ceramic samples with the same La-doped content are slightly smaller in parallel direction than those in normal direction. It is deduced that when the great pressure at high temperature made the green body more compact, it restricted the grains to grow along parallel direction which led to the smaller grain size.
Fig. 4 The SEM images of fracture surfaces for PLZT ceramics: (a) 7.45/70/30, (b) 7.51/70/30, (c) 7.57/70/30, (d) 7.63/70/30 in parallel direction; (a1) 7.45/70/30, (b1) 7.51/70/30, (c1) 7.57/70/30, (d1) 7.63/70/30 in normal direction; and (e) the mean grain sizes for PLZT ceramics in parallel and normal direction.
3.3 Ferroelectric properties
The P-E hysteresis loops of PLZT ceramics and their remnant polarizations (Pr) & the coercive electric fields (Ec) are presented in Fig. 5. All samples show the square hysteresis loops with ferroelectric phase. But the slightly smaller remnant polarizations and the larger coercive electric fields appear in parallel direction for all ceramic samples, which agrees with the mean grain sizes (i.e., lower Ec means larger grain size and higher Ec means smaller grain size [16, 17]), see, Fig. 5(e). And the degree of loop squareness which represents the tetragonal ferroelectricity [16], is higher in parallel direction than that in normal direction (Fig. 5(a)-5(d)), which also fits with the XRD analyses results.
Fig. 5 The P-E hysteresis loops of PLZT ceramics with the same La content in parallel and normal direction at room temperature: (a) 7.45/70/30, (b) 7.51/70/30, (c) 7.57/70/30, (d) 7.63/70/30; and (e) the remnant polarizations & coercive electric fields of PLZT ceramics in parallel and normal direction.
3.4 Optical and electrically controlled light scattering properties
Figure 6 shows the transmittance spectra of PLZT ceramics as a function of wavelength with a thickness of 0.35 mm. Considering a large reflection loss R (R≈31%) [18], all PLZT ceramic samples exhibit high transmittances, above 60% over a wide wavelength range from visible to near IR. However, the transmittances of the samples in normal direction are overall higher than those in parallel direction. Generally, the transmittance of material is mainly related to the material lattice symmetry and the defects like grain boundaries in materials. The XRD analyses have revealed the more serious anisotropy in parallel direction. In the meanwhile, all these samples present the smaller mean grain sizes which means much more grain boundaries in parallel direction, and so the much more grain boundaries result in the higher light scattering [19], see Fig. 6(c). This is thought to be the main reason that the samples present the lower transmittances in parallel direction. The transmittance increases with the increasing of La content both in parallel and normal direction at 632.8 nm, see Fig. 6(d). And the PLZT (7.63/70/30) ceramic sample presents the highest transmittance of ≥65% in normal direction.
Fig. 6 The transmittance spectra versus wavelength cures of PLZT ceramics with a thickness of 0.35 mm: (a) in parallel direction, (b) in normal direction; (c) the transmittance versus mean grain sizes and (d) the transmittance of PLZT ceramics in parallel and normal direction at 632.8 nm.
The reversible curves of transmittance versus electric field for PLZT ceramics at λ = 632.8nm are shown in Fig. 7. With the increasing of La content, the curves of transmittance versus electric field gradually shift from peaks and valleys to only peaks near the coercive electric field (Ec) where the polarization direction changes reversely, whatever it is in parallel or normal direction. The domain walls as scattering centers, are proposed as refractive index discontinuities [20, 21]. When a proper electric field is applied, the inner domain and domain walls will grow and switch, leading to the light scattering and, consequently, the decreasing of transmittance. Especially, the compositions of the PLZT(X/70/30) ceramics are located near the mixed phase region, see Fig. 2, so the domains switch very sensitively near Ec. Thus, the curves show peaks and valleys there with low La contents while show only peaks with relatively high La content. As a result, the PLZT (7.63/70/30) ceramic exhibits the more stable electrically controlled light scattering behavior in parallel direction.
Fig. 7 The reversible curves of transmittance versus electric field for PLZT ceramics (λ = 632.8nm): (a) 7.45/70/30, (b) 7.51/70/30, (c) 7.57/70/30, (d) 7.63/70/30 in parallel direction; and (a1) 7.45/70/30, (b1) 7.51/70/30, (c1) 7.57/70/30, (d1) 7.63/70/30 in normal direction.
Here, the ratio of the maximum value near Ec and the value at E = 0 of the transmittance is defined as the contrast ratio to estimate the electrically controlled light scattering property, see Fig. 8. The results show that the contrast ratios in normal direction are higher for all samples and the maximum reaches 1.37 for PLZT (7.63/70/30) ceramic in normal direction.
Summary
PLZT (X/70/30, X = 7.45, 7.51, 7.57, 7.63) ceramics with pure perovskite phase structure and fully dense, uniform microstructure were prepared by the uniaxial hot-press sintering techniques. All samples exhibit the square hysteresis loops and the high transmittances (≥60%, 0.35mm, visible to near infrared light). However, due to the anisotropy of the internal structure for the uniaxial hot-press sintering PLZT (X/70/30) ceramics, some important differences in parallel and normal direction are observed. The ferroelectric tetragonality is more evident in parallel direction than that in normal direction, accompanied by the smaller mean grain sizes. The transmittances and the contrast ratios of the samples in parallel direction are lower than those in normal direction. In this study, the PLZT (7.63/70/30) ceramic exhibits the more stable electrically controlled light scattering behavior in parallel direction and its contrast ratio reaches to the maximum 1.37 in normal direction in this system.
Funding
Shanghai Army-Civilian Integration Specific Program (2015).
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