Polycrystalline fibers

Next, the alkali-resistance of SA fiber is described in detail compared with other type of SiC polycrystalline fiber using boron instead of aluminum as the... 

Figure 4. X-ray diffraction pattern from a well oriented and polycrystalline fiber of potassium gellan using CuKa radiation. (Reproduced with permission from Ref. 13. Copyright 1988 Elsevier.)...

Pinnow DA, Gentile AL, Standlee AG, Timper AJ, Hobrock LM (1978) Polycrystalline fiber optical waveguides for infrared transmission. Appl Phys Lett 33 28-29... 

Step 3. Firing. The gel fibers are fired to produce a ceramic. The zirconia is stabilized in a cubic fluorite structure by the presence of yttrium in the structure. The polycrystalline fibers are typically 5-10 pm in diameter. The grain size depends on the sintering temperature. At temperatures <1000 C the grain size is <0.1 pm. If the sintering temperature is 1500 0 the grains are -1 pm in diameter. 

In the last decade, many new oxide fibers with improved high-temperature performance have been commercialized. The keys to these improvements has been (1) the design of fiber microstructures to reduce the volume of amorphous phases and (2) the development of multiphase polycrystalline fibers. Eliminating amorphous phases prevents rapid, viscous deformation under load at high temperatures. Multiphase polycrystalline microstructures appear to inhibit creep, particularly at elevated temperatures. Examples of developmental fibers with improved high-temperature properties include polycrystalline AI2O3, YAG, and mullite filjers. 

Segregant-weakened interfaces are an attractive concept based on the possibility of eliminating the coating phase from the composite fabrication process. This concept has only been demonstrated, however, on model composite systems. A major technical issue is whether the matrix or fiber, or both, would have to be doped to control the fiber-matrix interfacial energy. If polycrystalline fibers require doping, then the effects of dopants on creep performance must be considered. Therefore, additional basic research of this interface concept, utilizing available polycrystalline fibers and candidate oxide matrices, is recommended. 

Other manufacturers have modified the production technique to reduce the diameter of the alpha-alumina fibers that they have produced. This reduction of diameter has an immediate advantage of increasing the flexibility and hence the weaveability of the fibers. Mitsui Mining and 3M Corporation have introduced polycrystalline fibers, the Almax and the Nextel 610 fibers with diameters of 10 p.m, that is half the diameter of Fiber FP. 

Metal particle catalyzed chemical vapor deposition is the most versatile VLS process (Table II) yielding a wide range of single crystal whiskers and nanowhiskers [1-2] [5-6], short amorphous or polycrystalline fibers [1] [7], and nanotubes [8]. Laser ablation of selected metal alloys is a recent VLS process used for the synthesis of semiconductor nanowire [74]. Metal particle catalyzed carbothermal reduction, another VLS process, yields single crystal whiskers [9-10]. Metal catalyzed arc discharge [11], metal particle catalyzed laser ablation [12], and metal particle catalyzed plasma arc discharge [13] yield nanotubes by a VLS mechanism. 

The highest internal order, that of a single crystal, is obtained when the tip temperature is above the liquidus of the substrate, yields a liquid tip, and proceeds by a vapor-liquid-solid phase transformation. The lowest internal order, that of an amorphous structure, is obtained when the tip temperature is below the glass transition temperature of the substrate. A solid tip yields a vapor solid phase transformation. Between the liquidus and the glass transition temperature of a substrate, intermediate internal order is that of a polycrystalline fiber. In this case, whisker growth is either governed by a VLS and/or by a VS phase transformation. 

Amorphous and polycrystalline fibers and near-single crystal whiskers were obtained having diameters of 10-150 nanometers and lengths of several micrometers. Growth of whiskers and short fibers by this method is believed to proceed by a solution-liquid-solid (SLS) phase transformation, suggesting that similar synthesis routes may now also become available for other covalent short fibers and perhaps whiskers. 

Amorphous and polycrystalline boron fibers (Equation 1) are obtained by chemical vapor deposition from boron trichloride [1] [5] [7], At the lowest temperatures where deposition occurs, the rate is controlled by chemical kinetics, and amorphous fibers are obtained with a growth rate of 2 jm/s. At higher temperatures, the deposition process becomes limited by gas phase transport, and polycrystalline fibers with a p-rhombohedral crystal structure grow with a growth rate 5 pm/s. The 2.5x increase in growth rate occurs around the glass transition temperature for boron, which is -1500 K. 

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 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. 

Sol-gel methods may also be used to prepare continuous, refractory, polycrystalline fibers that exhibit high strength and stiffness in addition... 

Man-made fibers ean be amorphous, polycrystalline, or ciystalline. The amorphous fibers inelude the man-made vitreous fibers typified by the insulation wools, which form the bulk of man-made inorganic fibers. Polycrystalline fibers include continuous carbon fibers used in composite material and specialty fibers such as Saffil. Crystalline man-made fibers include ceramic whiskers, such as silicon carbide and silicon nitride, which are used in reinforce metals and other composite materials. 

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