18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS LASER REMELTED AL 2 O 3 /ER 3 AL 5 O 12 EUTECTIC IN SITU COMPOSTE CERAMICS FOR HIGH TEMPERATURE THERMAL EMISSION APPLICATIONS State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P. R. China H.J. Su,* J. Zhang, Y.F. Deng, W. Guo, L. Liu, H.Z. Fu *Corresponding author (shjnpu@yahoo.com.cn) Keywords : Eutectic ceramics; In situ composite, Laser remelting, Solidification characteristic; High temperature emission; Thermophotovoltaic aimed only to structural materials for high 1 Introduction temperature application [10-12]. Because the The strong demand for high temperature directionally solidified eutectics generally structural materials is the major motivation for present refined microstructure aligned along the the continuous development of new materials growth direction, they also possess great based on muti-compoment compounds, since potential to the functional applications. In the some of these compounds exhibit an interesting application field for these materials, a specific combination of high-temperature strength and feature of interest with the rare-earth oxides, for good oxidation resistance. In situ composite example, Er 2 O 3 , Yb 2 O 3 and Ho 2 O 3 have strong material systems combine two or more phases band emission, which ranges from the visible to homogeneously grown from melt to well the nearinfrared wavelength region. The bands optimize the overall material properties and permit strong thermal excitation at high performance, and consequently, attracting much temperatures, allowing their use as selective more attention in the past few years [1-2]. emitters applied in thermophotovoltaic (TPV) Directional solidification and single crystal generation systems [13]. As compared with growth technique can obtain directional and conventional generation systems, TPV single crystal solidification microstructure, generation systems have advantageous of no extremely improving the properties of materials. moving parts, high power density, wide-ranging Thus, many efforts have been devoted to heat source and low costs, which have been controlling and optimizing the microstructure by studied as one of the new generation systems of directional solidification methods [3-4]. electricity. Therefore, development of high In the field of composite ceramics, recent temperature thermal emission materials for developmental directionally solidified Al 2 O 3 - thermophoto-voltaic generation appilications is based eutectic in situ composite presents of current interest for a number of technology superior creep resistance, oxidation resistance sectors. and attractive high temperature strength In the present study, we focus on the retention [5,6], considering as the promising directionally solidified Al 2 O 3 /Er 3 Al 5 O 12 (EAG) candidate for high-temperature applications eutectic in situ composite ceramic to investigate above 165 0℃ . As a result, various preparation it as potential selective emitter for TPV systems. methods of oxide ceramics based on directional The Al 2 O 3 /EAG eutectic ceramic is rapidly solidification have been developed [7-9]. prepared by laser zone remelting technique However, most of previous researches were which allows no need of crucible, very high
melting temperature, large thermal temperature V gradient (>10 4 K/cm) and high growth rates [14]. Laser The processing method and the main appearance of this eutectic ceramic are x investigated. The effects of rapid solidification on the microstructure characteristic, the R solidification behavior and the emission properties are analyzed. 2 Experimental 2.1 Materials and Specimen Preparation z Starting materials are commercially available Fig. 1 Schematic diagram of the laser zone Al 2 O 3 (99.99%) and Er 2 O 3 (99.99%) powders remelting for growing ternary eutectic with diameter of 1-2 μ m. The powders with composite. R: solidification direction, V: eutectic composition (81mol% Al 2 O 3 and 19 scanning direction, x, z: worktable moving mol% Er 2 O 3 ) [15] are uniformly mixed by wet direction. ball milling with an aqueous solution of 3 Results and Discussion polyvinyl alcohol. The mixed powder is dried at 473K and then pressed into plate (40mm × 5mm 3.1 Microstructure Characteristic × 5mm) by uniaxial die pressing for 10 minutes Fig. 2(a) shows the photograph of the as- at 25 MPa. The precursor rods are sintered at solidified Al 2 O 3 /EAG eutectic ceramic plate. 150 0℃ for 2 hours to increase the density and The as-solidified eutectic composite presents handle strength. The laser zone remelting smooth surfaces with pink color, and contains method and a5 kW ROFIN-SINAR850 CO 2 no pores, cracks and grain boundaries, as shown laser were used to directionally solidify the in Fig. 2(b). There are no bubbles observed in eutectic rods in a vacuum chamber, as shown in the solidified samples by filling Ar gas during Fig.1. The sample is moved by the numerically- remelting. The thickness of the solidified layer controlled worktable with 5-axis and 4-direction is about 2-4 mm and the density is almost near coupled motion to realize the laser beam to the theoretical value (5.4 g/cm 3 ) of the scanning along the sample axis. The laser power composite. This indicates that laser zone is 200 W and the stage traveling speed (traverse remelting can achieve full density of the rate) is established at 800 μ m/s. ceramic composite, which is obviously superior than conventional sintering method [16]. The 2.2 Characterization solidified layer thickness is dependent on the The laser remelted samples are cut from the laser scanning rate and laser power density. transverse and longitudinal cross-section, and Fig. 3(a) shows the typical microstructure of prepared by diamond polishing in order to be the transverse cross-section for the composite. observed in a field emission scanning electron The eutectic microstructure displays a microscope (FSEM, Supra 55). The phase and continuous and homogeneous structure with fine composition of the composite are determined by entangled eutectic phases in an alternating energy disperse spectroscopy (EDS, Oxford) interpenetrating network of Al 2 O 3 (vol. 54%) and X-ray diffraction (XRD, Rigakumsg-158) and Er 3 Al 5 O 12 (vol. 46%) of sizes in the techniques. The emission property is tested by submicron range. The dark phase is Al 2 O 3 , and the UV-Vis spectrometer (UV-3150). the grey phase is EAG. No Al 2 O 3 solubility in EAG is 2
(a) (a) Al 2 O 3 Er 3 Al 5 O 12 1um (b) Fig. 3 SEM micrograph showing the near- Fig. 2 The macrograph of Al 2 O 3 /EAG eutectic surface zone of the transverse cross-section of plate: (a) surface; (b) longitudinal section. the eutectic Al 2 O 3 /Er 3 Al 5 O 12 grown at laser scanning rate of 800 μ m/s (a) and the interface found in the composite determined by XRD and EDS. The eutectic spacing is about 0.1 μ m, between the solidified layer and sintered precursor layer (b). which much smaller than the results of Nakagawa et al. [13] by Bridgman method (20~30 μ m) and consequently, improving the branched and refined toward to the solidified layer. The morphology of EAG phases properties of the composite. The great transforms from coarse block to fine lamellae. It refinement of microstructure is mainly means that the growth and evolution of EAG contributed to the high temperature gradient and phases basically control the microstructure rapid cooling rate. The growth of eutectic development of Al 2 O 3 /EAG eutectic ceramic. interfaces of rapid solidification shows a typical Furthermore, the eutectic spacing gradually faceted/faceted characteristic. The phase-size is decreases from the bottom to surface of the strongly dependent on the laser scanning rate, solidified layer (Fig.3a and b), which is decreasing at the nanometer range for the contributed to rapid increase of solidification samples grown at the highest rate. rate [14]. Moreover, for comparison, the interface of the solidified layer and unprocessed zone is 3.2 Emission Properties shown in Fig. 3(b). It can be seen that the un- processed ceramic by laser remelting presents The emission property is characterized by the polycrystalline ceramic structure with obvious absorbance spectrum, as shown in Fig. 4. The pores. At the interface between the sintered result shows that the laser remelted Al 2 O 3 /EAG layer and solidified layer, the coarse primary eutectic presents strong selective emission bands at wavelength 1.5 μ m. In comparison, the EAG phases are assembled and gradually are 3
Recommend
More recommend
