| Summary: | 碩士 === 國防大學理工學院 === 應用化學碩士班 === 98 === In the study, advanced zinc oxide products were synthesized by using hydrothermal, sol-gel, combustion synthesis, i.e. Then, the effects of the growth time, the possible formation mechanism, the luminescence properties and the microwave absorption behaviors for the as-prepared ZnO products were also discussed. The study content included four parts. Firstly, ZnO particles of ~25 nm in crystalline size were synthesized using an aqueous solution composed of zinc acetate dihydrate and urea. The relevant experimental parameters for ZnO synthesis via a hydrothermal route, i.e., the hydrothermal time, the calcination temperature and the calcination time, were analyzed using the Taguchi method. It was found that the crystalline size and the luminescence properties of the ZnO nanoparticles were dependent on the calcination time and calcination temperature. By using the appropriate parameters derived from the Taguchi method, it was verified that the confirmed experiment provided a yield of ~70 % and that the synthesized ZnO particles possessed the characteristics of small crystalline size and improved luminescence properties. Secondly, a two-step wet chemical process with economic and practical advantages was developed to prepare arrayed ZnO nanorods on glass substrates by using zinc acetate dihydrate [Zn(CH3COO)2．2H2O, ZnAc2] and monoethanolamine (NH2CH2CH2OH, MEA) as raw materials. The proposed method includes the pre-deposition of a thin ZnO seed layer using the sol-gel technique and the subsequent hydrothermal growth of ZnO nanorods. Each ZnO nanorod was confirmed to be a single crystal with a wurtzite structure and its growth direction was nearly perpendicular to the substrate. Photoluminescence (PL) measurements confirmed that the ZnO nanorods exhibited near-UV emission at ~385 nm together with a green emission that was centered at ~500 nm. We note that the PL properties may be affected by the hydrothermal time. Thirdly, a self-propagating high-temperature synthesis (SHS) process has been developed for synthesis of multipod-shaped ZnO whiskers (mZnOw) in static oxygen atmosphere. Commercial powdered Zn (purity: 99.9 %, particle size: 4-6 μm) was pressed into a compact, which was placed in a closed chamber. The compact was firstly heated at ~380 ℃ under air to form a refractory ZnO shell on the Zn particle so as to prevent the compact form collapsing during the following-up combustion reaction. Then, the SHS reaction was ignited by rapid heating the compact, and high reaction conversions (~95 %) could be achieved under O2 gauge pressures (PO2) of 0.05-0.25 MPa. It was confirmed that the as-obtained products consisted mainly of multipod whiskers. Additionally, the Zn/O composition ratio in the product and the branch size as well as the aspect ratio of the mZnOw were affected by the PO2. Photoluminescence (PL) measurements showed that the products possessed a near-UV emission at ~375-380 nm together with a defect-related green emission that was centered at ~500 nm. Fourthly, the composite materials were prepared by mixing equal-weighted ZnO particles/whiskers with epoxy. The microwave-absorbing efficiency of the mZnOw composites was much better than that of the composite containing nanosized ZnO particles.It was found that the 1.5-mm-thickness 50 wt% mZnOw-epoxy resin composites exhibited good microwave-absorbing efficiency with low reflection losses of ≦ -35 dB in the frequency range of 12-16 GHz.
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