Alumina fibers single crystal

Note The principal reinforcement, with respect to quantity, is glass fibers, but many other types are used (cotton, rayon, polyester/TP, nylon, aluminum, etc.). Of very limited use because of their cost and processing difficulty are whishers (single crystals of alumina, silicon carbide, copper, or others), which have superior mechanical properties. 

Whiskers are short fibers, usually a single crystal with an aspect ratio of 10/1 or greater. They have high strength but are difficult to process. Their applications so far has been limited SiC whiskers are reported as random reinforcement for ceramics and in alumina cutting tools. CVD is a common production method.O Beside SiC, the following whiskers have recently been produced by CVD ... 

Figure 6.11 Schematic of the edge-defined fihn-fed growth (EFG) method of making a single crystal alumina fiber.

In Figure 3-21, BSR data (one hour) for several oxide fibers are compared, including the single-crystal alumina monofilament (Morscher et al., 1995)... 

A comparison of creep rates of single crystal a-alumina fibers and directionally solidified YAG-alumina fibers, at 1400°C, reveals that the former shows no creep at a rate... 

This includes single crystal silicon [15], germanium [22] and alumina [10] fibers. Polycrystalline fibers can grow either by a VLS or a VS phase transformation when the incident laser power (focal temperature) is intermediate, and supports the growth of a fiber with a semisolid tip. This includes polycrystalline silicon [15], boron [5] and silicon carbide fibers [23]. Amorphous fibers are obtained by a VS phase transformation when the incident laser (focal temperature) is low, and supports the growth of a fiber with a hot but solid tip. This includes amorphous silicon [15], boron [12], carbon [13] [16], silicon carbide [23], and silicon nitride [17] fibers. 

The highest modulus of a given substrate is obtained with a single crystal structure. Single crystal CVD-SiC whiskers (578 GPa) have a stiffen more highly ordered, structure than polycrystalline CVD-SiC fibers (190-400 GPa), and sapphire whiskers and fibers (415 GPa) are stiffer than slurry spun polycrystalline alumina fibers such as Fiber FP (380 GPa). Superimposed upon this relationship is a compositional factor. Fiber modulus and structural order generally also decrease with increasing compositional complexity, e.g., silicon carbide is intrinsically stiffer than silicon oxycarbide such as Nicalon, and slurry spun alumina fibers are stiffer than sol-gel or melt spun aluminate fibers. 

GPa for single crystal alumina fibers. The overall trend reflects the weight percent increase in alumina. Differences in each category reflect super-imposed effects of other oxides (not shown) or process-induced differences. 

In the laser heated float zone (LHFZ) or pedestal (LHPG) growth process, a circumferential laser is placed around a preform rod (e.g. polycrystalline alumina) to zone refine a segment of the material while at the same time updrawing a single crystal fiber (e.g., sapphire). 

The main interest of YAG fibers is their creep resistance. The BSR behavior of polycrystalline fibers prepared from diphasic gel [107] is intermediate between those of commercially available alumina based polycrystalline fibers (except coaindum/mullite Nextel 720) and that of monocrystalline sapphire (c-axis) fiber. Hence, the creep resistance of polycrystalline YAG fibers (m = 0.5 at 1300°C for a 1 h BSR test) represents a significant improvement over that of the other alumina based fibers (m = 0.5 at 950-1100°C). However, it is much lower than the creep resistance of the monocrystalline fibers. Polycrystalline YAG creeps four orders of magnitude faster than single crystal alumina and five orders of magnitude faster than single crystal YAG. 

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