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General characteristics of SiC-based woven fabrics The typical XRD patterns of the as-received and heat-treated SiC-based ceramic woven fabrics are shown in Fig. 2. The peaks show some broadening due to the small grain size of the fibers. The existence of three peaks in the 2θvalues of 35.9, 60.2, and 71.8°showed the presence of β-SiC, detected in both the as-received and heat-treated samples. However, after the heat treatment an additional phase, SiO2(α-cristobalite), was identified at around the 2θvalue of 22.0°, indicating oxidation of the sample. This passive oxidation leads to the formation of a protective silica film, protecting the fibers from further oxidation [22]. Under equilibrium conditions, quartz is the stable phase at room temperature; however, in this case cooling from 1573 K (heat-treatment temperature) was not slow enough for reconstructive phase transitions from cristobalite to quartz, which resulted in a displacive transition, causing the distortion of cristobalite and the existence of α-cristobalite at room temperature [23]. Figure 3a shows the SEM micrographs of the as-received PN-type fibers. A continuous oxide layer can be observed on the surface of the fibers on the SEM micrograph of the heat-treated PN (PN-H) fabric (Fig. 3b). In some areas the continuity of the oxide layer was lost,most probably during handling of the bundles in the course of the observation of the fracture surface. At a higher magnification, the thickness of the oxide layer around the fiber was determined to be ~1 μm (Fig. 3c). The morphology and structure of the S8-type fibers showed similar characteristics to the PN-type in both as-received and heat-treated states. Micrographs of the as-received and heat-treated ZE8(ZE8-H)-type fibers are shown in Fig. 4a and b, respectively. In the case of the ZE8-H, detailed observations revealed that there was no clear continuous oxide layer around the fibers, but rather only localized oxidation of the fiber, indicated by arrows in Fig. 4b. The ZE8 fabric is described as heat resistant by the manufacturer, a claim that is verified by the results of our experiments. |
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RXMCDM: 金币+1 2014-06-12 20:47:34
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基于SiC的编织纤维（这个词我查的百度）的基本表征： 购买的和热处理过的基于SiC的陶瓷编织纤维的XRD谱图如图2所示。由于纤维的晶粒尺寸比较小，所以谱图中的峰宽比较宽。在购买的和热处理过的样品中，均出现了2θ=35.9, 60.2, 和 71.8°的衍射峰，说明这两个样品中都有β-SiC存在。在经过热处理后，因为有2θ=22.0°的衍射峰出现，所以可以确定样品中存在SiO2(α-cristobalite)。这也说明了在热处理过程中样品被氧化了。这种氧化导致了一层保护性二氧化硅膜在样品表面形成，从而避免了纤维的进一步氧化。在平衡态下，石英在室温下是一种稳定相。然而，从1573K冷却的时候，由于冷却速度没有足够慢，因此，晶相从方晶石相向石英相的转化没有彻底完成，从而导致了位移性转化（这个词不确定），引起方晶石相的扭曲，因而在室温下也存在方晶石相。 图3a为购买的PN-type纤维的扫描电镜照片。从照片中，可以看到在热处理的PN纤维表面上出现了一个连续的氧化层（图3b）。在有些区域，这种连续的氧化层也会出现断裂，这可能是因为在观察断裂表面的过程中对样品处理时造成的。在更高的放大倍数下，氧化层的厚度被测定为1微米（图3C）。S8纤维的形貌和结构特征与购买的和经过热处理的PN纤维类似。 购买的和经过热处理的ZE8(ZE8-H)型纤维的形貌如图4a和4b所示。对于ZE8-H来说，通过仔细观察发现在它的表面没有明显的连续氧化层，然而这种纤维的局部出现了氧化现象，这些区域在图4b中已经被箭头标注出来。生产商认为ZE8纤维是一种抗热型材料，我们的实验结果验证了这一观点。 楼主。别忘了悬赏的35个金币啊！ |
2楼2014-06-12 20:09:10
