Additive-Coated Si 3 N 4 Powder Prepared By The Sol-Gel Method
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ABSTRACT The Y20 3-La20 3 additive-coated Si3N4 powders of about 1.5 pm in size were prepared by the sol-gel method. X-ray diffraction, X-ray photoelectron spectroscopy, differential thermal analysis, thermogravimetric analysis, electron microscopies, and particle size analysis were used to study the coating on the Si3N4 particles. The results show that properties of the bulk Si 3N4 powder are not affected by using the sol-gel coating and the powders are homogeneously coated by a thin layer of the Y20 3-La20 3 additives. The structure of the Y2 0 3 -La 20 3 coating layer is amorphous or microcrystalline with a submicron thickness. The coated powders are then more sinterable, and the mechanical properties of the ceramics prepared from such powders are improved. INTRODUCTION Sol-gel method has been widely used to prepare films on large areas especially when a homogeneous composition is required [1,2]. Recent studies indicated that the characteristics of the powders can be greatly modified by additive coating [3]. ZrO 2 powders coated with
ZnO 2 or A120 3 can be sintered to nearly full density whereas the powder of the same composition mixed by mechanical methods was not as sinterable [4]. Some similar results were obtained in other systems [5]. Additives coated evenly on the powders will greatly help sintering and improve the properties of the ceramics [4]. Si 3N4 ceramics are regarded as candidates of the future engineering materials. The sinterability of Si3N4 powders is poor because of its low self-diffusion coefficient [6]. Therefore, it is necessary to add some additives in order to improve its sinterability. Conventional methods generally introduce impurities and cause inhomogeneity [7]. In order to reduce impurities and inhomogeneity, in this study a sol-gel method has been used to introduce additives on the Si 3N4 powders other than using the conventional method. Experimental results in this study have showed that the Si3N4 powders are homogeneously coated by the additives as amorphous or microcrystalline films. The sintering properties of the additive-coated-Si3 N4 powders and the mechanical properties of the Si 3N4 ceramics prepared from these coated powders are effectively improved. The sol-gel coating process can be used to prepare high-quality Si 3N4 ceramics, composites, and oxynitride glasses.
667 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL Fabrication Processes The materials used in this study were Si3N4 powders with a number-average size of -0.55 pm, Y(OR) 3, and La(OR) 3. The solvent was pure alcohol. The composition of the Si3N4 powders is shown in Table 1 provided by the manufacturer. The self-prepared rareearth-metal alkoxides are not soluble. Therefore, an appropriate pre-treatment is necessary [8] to obtain a soluble precursor. The Si3N4 powders were dispersed into the Y20 3-La20 3 sol to form a suspension. Appropriate organic additives such as diethylenetriamine and acetylacetone were then added into the suspension to modify the pH value to 7.8. The s
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