Whisker surface chemistry

In addition to the bulk chemistry, the SiC whisker surface chemistry is also very important in alumina matrix composites. This is because it directly influences the nature of the interface bond between the alumina matrix and the SiC whisker. The properties of the... 

Apart from manifold structures, carbons can have various shapes, forms, and textures, including powders with different particle size distributions, foams, whiskers, foils, felts, papers, fibers [76, 77], spherical particles [76] such as mesocarbon microbeads (MCMB s) [78], etc. Comprehensive overviews are given, for example in [67, 71, 72], Further information on the synthesis and structures of carbonaceous materials can be found in [67, 70, 72, 75, 79]. Details of the surface composition and surface chemistry of carbons are reviewed in Chapter II, Sec. 8, and in Chapter III, Sec. 6, of this handbook. Some aspects of surface chemistry of lithiated carbons will also be discussed in Sec. 5.2.2.3. 

The toughening of a ceramic by the addition of whiskers that are different from the matrix material is the most traditional and the best explored methodology. The advantage of this method is the broad spectrum of ceramics that can be prepared in whisker form. Consequently, there is the potential for a broad range of ceramic matrix-ceramic whisker combinations. Since the whiskers are manufactured in a separate process, it is possible to control their size, morphology, and surface chemistry without affecting the matrix material. Additionally, whiskers can be chemically or thermally treated to change their future interactions with the matrix. 

The bulk chemistries, surface chemistries, and morphologies of silicon carbide whiskers vary widely depending on the type of process used, the stage of the process development, and the postsynthesis treatments practiced by the producer. The synthesis of the whiskers as described earlier is... 

The bulk analysis of /3-SiC whiskers shows the least variation in chemistry. In some whiskers, the residual metals content can vary, most likely, as a result of additives that used as catalysts during synthesis. These include iron, cobalt, and chromium. Studies by Karasek et al. [56] have shown that the physical properties of silicon carbide whisker-reinforced composites do not correlate to the bulk properties of the whiskers significantly. This lack of significant correlation is mainly due to the fact that the important phase chemistry of the whisker-matrix interface is controlled by the matrix chemistry and the surface chemistry of the whiskers. There seems to be little impact of the diffusion of materials into or out of the bulk whisker material. 

As can be deduced from the discussion above, the characteristics of the whiskers, such as diameter and strength, have a direct effect on the toughening behavior and mechanical behavior, The surface chemistry of the whiskers influences the nature of the interface bond between the whiskers and the matrix, which is addressed in the y /y/ term in Eq. 2. It has been found that whiskers from various manufacturers have different characteristics, which affect their performance as reinforcements. 

The chemistry ofthe whiskers is important since it affects many areas in fabrication and final composite properties. The bulk chemistry influences the stability ofthe whiskers at high temperatures and in reactive environments. The surface chemistry can change the interface properties between the whisker and the matrix and thereby change the debonding behavior... 

Fig. 13.2 The lipid whisker model. Cell membranes of senescent or apoptotic cells contain oxidized phospholipids with sn-2 oxidized fatty acid acyl chains protruding into the extracellular space. This conformation renders them accessible to interact with scavenger receptors and other pattern recognition receptors on the surface of probing macrophages of the innate immune system (Greenberg et al., 2008 Permission for use of this figure has been requested from The Journal of Biological Chemistry.)...

SiC and SisNa powders have been studied extensively by XPS or x-ray-in-duced AES [42,43,48-57]. Powders, whiskers, and platelets were treated at elevated temperatures, and surface analysis was carried out by measuring Si 2p XPS or Si KLL AES intensities for Si02 and SiC (or Si3N4). Subsequently, surface oxide film growth rates were measured and surface oxidation activation energies were calculated [42,43,48-51]. In some cases, the effects of the presence of yttria, a sintering aid, and boron, an impurity, were studied [43,48,49]. The effects on surface oxidation chemistry caused by different powder processing routes were also studied [50]. 

Yang Z., Xia Y. and Mokaya R. (2004). High Surface Area Silicon Carbide Whiskers and Nanotubes Nanocast Using Mesoporous Silica. Chemistry of Materials, 16(20), 3877-3884. 

Fourier-transform infrared spectroscopy (FT-IR) is the most commonly used technique for evaluation of the chemistry of surface-modified cellulose microfibrils and nanocrystals (whiskers) [25,51,52,57-60]. However, FT-IRanalysis is not confined to the surface of the material, and is therefore best suited for qualitative evaluations of the chemistry of microfibrillar cellulose. 

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