A new lead-free transparent electro-optic ceramic for optoelectronic application

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A new lead-free transparent electro-optic ceramic for optoelectronic application

 

Author: Li, Faliang
Title: A new lead-free transparent electro-optic ceramic for optoelectronic application
Degree: Ph.D.
Year: 2013
Subject: Electrooptics -- Materials.
Ceramics.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xxi, 135 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2652772
URI: http://theses.lib.polyu.edu.hk/handle/200/7244
Abstract: The most widely used electro-optic (EO) materials in the integrated optics are single crystals and ceramics. As compared to the EO single crystals, the EO ceramics have several advantages, such as strong EO responses, ceramic ruggedness, easy fabrication, low fabrication cost and large available size. However, most of the EO ceramics available in the marked contain a large amount of lead (e.g. PLZT). For environmental protection reason, there is a need of replacing the lead-based materials by environmental-friendly lead-free materials. In this work, new lead-free transparent ceramics with good EO properties have been successfully developed. Via the co-modifications with Li and Bi, the optical transparency of potassium sodium niobate (K₀.₅Na₀.₅)NbO₃ ceramics has been improved. The (K₀.₅Na₀.₅)1-xLixNb1-xBixO₃ (KNNLB-100x) ceramics with x ≥ 0.05 become optically transparent. If proper antireflection coatings are applied on the surfaces to eliminate the reflection loss, the transmittance of the ceramics can reach as high as 95% in the near-IR region. The ceramics also exhibit a strong linear EO effect, giving a high effective linear EO coefficient (120-200 pm/V), which is about 10 times greater than that of LiNbO₃ crystal (a widely used EO material in industries). The ceramics are also as good as the PMN-PT single crystals with rhombohedral structure and better than those with tetragonal structure. Hot-press sintering is a mature technique and is widely used in industries to produce pore-free ceramics. In this work, it is used to fabricate the KNNLB ceramics so as to improve their densification and hence optical quality to crystal-like transparency. The effects of sintering pressure on the microstructure, optical, dielectric and electro-optic properties of the KNNLB-5-2 (i.e. KNNLB-5 ceramic added with 2 mol% excess Bi₂O₃) are investigated. It is found that the sintering pressure is effective in enhancing the optical transmittance of the KNNLB-5-2 ceramics. A low sintering pressure (e.g. 2 MPa) is enough to enhance the optical transmittance, and the transmittance becomes saturated at around 10 MPa. Unlike the lead-based EO ceramics, the sintering pressure does not have significant effect on the grain growth. It is effective in improving the densification of the KNNLB-5-2 ceramics. The enhancement in the optical transmittance should be partly due to the reduction of defects such as pores. Due to the better densification, the dielectric properties are also improved. However, the EO responses of the ceramics are not dependent on the sintering pressure.
The effects of the co-modification with Li and Bi are investigated. It is found that the grain growth of the ceramics is considerably suppressed by the co-modification with Li and Bi. The ceramics generally possess a dense and fine-grained structure, and the light scattering by the grains (< 0.5 um) become negligible. Our results also reveal that the co-modification induces a transformation of crystal structure from orthorhombic to nearly cubic. As a result, the optical anisotropy and hence the scattering of light at the grain boundaries are greatly reduced. The co-modification also induces a diffuse phase transition, making the ceramics become more relaxor-like and contain more polar nano-regions. The amount of domain walls and then the resulting light scattering are further eliminated. Accordingly, the KNNLB-100x ceramics with x ≥ 0.05 become optically transparent, giving a high optical transmittance (~60%) in the near-IR region. Owing to the large distortion of BO6 octahedron and increase in εr arisen from the substitution of Li+ in the KNN lattices, the Li/Bi co-modified ceramics exhibit strong EO response, giving a large effective linear EO coefficient (120-200 pm/V). The effects of excess Bi₂O₃ on the microstructure, optical, dielectric and electro-optic properties of KNNLB-5 (as examples) are investigated. It is found that the excess Bi₂O₃ does not have significant effect on the grain growth of the ceramics. Although the crystal structure of the ceramics added with excess Bi₂O₃ becomes more cubic and the ferroelectric behavior becomes more relaxor-like, their optical and EO properties remain unchanged, giving a high optical transmittance (~60%) in the near IR-region and a large effective linear EO coefficient (~120 pm/V). The feasibility of preparing the transparent EO KNNLB ceramics by pressureless sintering is investigated. Due to the effective suppression of the grain growth by the Bi-doping, the KNNLB ceramics prepared by pressureless sintering also possess a fine-grained structure. However, our results reveal that the excess Bi₂O₃ is effective in promoting uniform densification under pressureless sintering. Because of the reduction of defects such as pore, the KNNLB-10 ceramics added with 4 and 6 mol% excess Bi₂O₃ possess a dense structure and become optical transparent, exhibiting a high optical transmittance (60-70%) in the near-IR region. The good optical transparency may also be resulted from the changes to the more cubic-like crystal structure and more relaxor-like characteristics arisen from the diffusion of the excess Bi³⁺ into the A-sites of the lattices. Nevertheless, the EO responses of the ceramics become weakened, showing a low effective linear EO coefficient of 40 pm/V and 29 pm/v, respectively.

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