The Control of Zn for ZST Microwave Ceramics With Low Sintering Temperature
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roperties characterization The sintered densities of the samples were measured by the Archimedes method. Dielectric properties in the microwave frequency range were measured, using Hakki- colemann method. A network analyzer (HP 8720C, Hewlett-Packard Co.) was used for the microwave measurement system. X-ray powder diffraction (M18XHF-SRA, McScience, Japan) was used to determine the crystalline phases, using Cu Ka radiation. The ZST ceramics was were processed by polishing, dimpling, and ion milling (Gattan, Model DuoMill 600) to observe by transmission electron microscope (CM 20, Philips Electronic Instruments, Inc., Amsterdam, Netherlands) and energy dispersive x-ray spectrometer for microstructure and composition, respectively. RESULTS Fig. 1. shows XRD patterns of ZST ceramics as-sintered at 13501C for 2h and annealed at 900 - 1100 'C for 5 h in the oxygen atmosphere. In the samples as-sintered as well as annealed at the all temperature, second phases and lattice constant as well as distortion were not observed.
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20 CuKa Fig. 1. XRD patterns of ZST ceramics (a) as-sintered, post-annealed at (b) 900°C, (c) 1000'C and (d) 1100'C for 2hr in the oxygen atmosphere, respectively. Dielectric properties of post-annealed ZST ceramics at 900 - I100°C are shown in Fig. 2. Although samples are annealed, relative density, dielectric constant, and temperature coefficient of the resonant frequency remained virtually unchanged at as-sintered values; 97.5 %, 38 and -3ppm/*C, respectively. On the other hand, it was observed that among specimens, Q x f value of ZST post-annealed at 9001C increased to 46000 from the assintered value of 40000. Therefore, it could be assumed that Q x f value in this case was aftected by certain factors other than the existence of second phase and/or change of lattice parameter and distortion.
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Fig. 2. Dielectric properties of ZST ceramics sintered at (a) 13501C and post-annealed at (b) 900 1C, (c) 1000 IC,(d) 1100 'C for 5 h in the oxygen atmosphere, respectively. Fig. 3. shows TEM image and X-ray microanalysis of grain boundary phases and internal grain on the ZST ceramics prepared by process A. Comparing sample as-sintered at 13501C to annealed at 900 'C for 5h in the oxygen atmosphere, Zn content at certain depth from grain boundary remarkably decreased after post-annealing. That is, annealing treatment of assintered ZST ceramics can causes Zn to out-diffuse. Therefore, it is inferred that the incorporation of Zn into the grains are solely responsible for Qx f value of ZST in the absence of other factors mentioned above.
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Depth of detection from grain boundary (to) Fig. 3. TEM image and X-ray microanalysis of grain boundary area of ZST prepared by process A;(a) sintered at 1350'Cfor 2h, (b) post-annealed at 900 'C for 5h in the oxygen
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