| Author: | Tai, Cheuk-wai |
| Title: | Investigation of high dielectric constant ceramics and films with improved temperature stability of doped and undoped (x)Pb(In1/2Nb1/2)O3:(1-x)Pb(Mg1/3Nb2/3)O3 |
| Degree: | Ph.D. |
| Year: | 2004 |
| Subject: | Hong Kong Polytechnic University -- Dissertations Ferroelectric crystals Ferroelectric thin films Electronic ceramics Dielectrics |
| Department: | Department of Applied Physics |
| Pages: | ix, 294 leaves : ill. (some col.) ; 30 cm |
| Language: | English |
| Abstract: | Complex perovskite-structured relaxor ferroelectric ceramics of (x)Pb(In1/2Nb1/2)O3: (l-x)Pb(Mg1/3Nb2/3)O3 with x = 0.1 to 0.9 were studied extensively during the project. The ceramics were fabricated by conventional mixed oxide route of the two-step method. Measurements of their dielectric properties and ferroelectric hysteresis were performed to explore their potential for capacitor applications. The results show many features common to the relaxor behavior, including slim ferroelectric hysteresis loop and frequency dispersions in plots of relative permittivity. In addition, the ceramics with x = 0.3 to 0.7 show relative permittivity that is highly stable over the temperature range -30 C to 125 C. In order to explore structural alterations and their subsequent influence on dielectric properties, a variety of dopants were added to (0.3)Pb(In1/2Nb1/2)O3:(0.7)Pb(Mg1/3Nb2/3)O3 ceramics. The additives were Ba2+, Sr2+, La3+, Na+, Ti4+ and Yb4+ obtained from different raw materials of oxides or carbonates. The modified ceramics were also fabricated by the two-step method. Fourteen ceramics samples doped with 2 or 5 mole % of the above elements, 5 mole % Na + 2 mole % Ti and 5 mole % Na + 5 mole % Ti doped were characterized in total. The measured dielectric properties of these ceramics were different to those of the parent ceramics and some of these meet the EIA-standard for industrial ceramic capacitor applications. Since the 1:1 order-disorder behavior in most of the complex perovskite materials plays a central role in influencing the physical properties, transmission electron microscopy (TEM) studies were carried out to investigate the various structural long-range ordering, especially the 1:1 ordering. The undoped ceramics with x = 0.1, 0.3, 0.4 and 0.6 and all the modified ceramics were studied using their dielectric properties as a guide for selection. In the as-sintered undoped ceramics, the degree of the 1:1 structural ordering and the size of the ordered domains were dependent on the compositions. Besides the Bragg and the 1/2 1/2 1/2 superstructure reflections, the 1/2 1/2 0 forbidden reflection, diffuse streaking along < 110 > and diffuse scattering were also observed and imaged in dark-field for the first time. In addition, the temperature-dependence of the structural properties of the (0.6)Pb(In1/2Nb1/2)O3: (0.4)Pb(Mg1/3Nb2/3)O3 ceramic was studied starting from about liquid-nitrogen temperature to about 300 C. The 1:1 structural orderings in the modified ceramics were strongly influenced by the concentration and types of dopants. The local structures of the ceramics were investigated by high-resolution transmission electron microscopy (HRTEM) that showed nano-scale regions of varying contrast. A new model of mixed ordering was proposed to interpret the TEM and HRTEM results relating to the structural significance of mixed atomic arrangements constituting these nano-regions. Simulations of the [001] zone axis electron diffraction patterns and HRTEM images along [001] were performed by the EMS program. The model was extended to other complex perovskite materials. A well-known order-disorder ferroelectric material, Pb(Sc1/2Ta1/2)O3 (PST), was chosen for detailed studies for comparison with the results of both the undoped and doped (x)Pb(In1/2Nb1/2)O3:(1-x)Pb(Mg1/3Nb2/3)O3 ceramics. In addition to the 1/2 1/2 1/2 superstructure and the 1/2 1/2 0 forbidden reflections, the 1/2 0 0 forbidden reflection was found and the corresponding dark-field images were recorded for the first time for the vanishingly weak 1/2 1/2 0 and 1/2 0 0. To simulate the electron diffraction patterns, three models were proposed to interpret the 1/2 1/2 1/2, 1/2 1/2 0 and 1/2 0 0 and their possible structural origins. Simulated HRTEM images of PST, which were based on the mixed ordering model, confirmed the observed results. The models chosen successfully confirmed the results of the mixed ordering in PST. For the (x)Pb(In1/2Nb1/2)O3:(1-x)Pb(Mg1/3Nb2/3)O3 ceramics, the mixed ordering was further improved to interpret the diffuse streaking observed in electron diffraction patterns. It was found that small Pb-displacement can produce the streaking and also induce further contrast and feature differences in HRTEM images. The simulations of the HRTEM images of (x)Pb(In1/2Nb1/2)O3:(1-x)Pb(Mg1/3Nb2/3)O3 ceramics showed excellent agreement with the experimental results. An exploratory fabrication and study of thin films of the (0.4)Pb(In1/2Nb1/2)O3:(0.6)Pb(Mg1/3Nb2/3)O3 and two doped (0.3)Pb(In1/2Nb1/2)O3:(0.7)Pb(Mg1/3Nb2/3)O3 compositions were carried out to demonstrate their potential for MEMS or other micro- or nano-scale systems. The epitaxial films were grown successfully by pulsed laser deposition (PLD). Prior to deposition of the films, La0.7Sr0.3MnO3 (LSMO) bottom electrode was first grown on LaAlO3 substrate. The orientation relationship between film, electrode and substrate was characterized by x-ray diffraction. The dielectric properties and the ferroelectric hysteresis loops of the films were measured. |
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