Polycrystalline fibers alumina

An important polycrystalline a-alumina fiber has been developed by 3M Co. This a-alumina fiber, trade name Nextel 610, is made via a sol-gel route. The sol-gel process of making fibers involves the following steps common to all sol-gel processing ... 

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. 

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. 

Other Ceramic Fibers. Alumina polycrystalline filaments (AI2O3) are of interest mainly for metal matrix composites. They have a high resistance to oxidation, tensile strength and modulus of up to 2 and 380 GPa, respectively, and an excellent strength retention up to 1370°C. Their density is 3.95 g/cm. DuPont s FP polycrystalline alumina yam has a 100% a-aliunina composition, and a remarkably high compressive strength of about 6.9 GPa. 

Thorne K., Ting S.J., Chu C.J., Mackenzie J.D. Synthesis ofTiC via polymeric titanates the preparation of fibres and films. J. Mater. Sci. 1992 27 4406-4414 Towata A., Hwang H.J., Yasuoka M., Sando M., Niihara K. Fabrication of fine Y AG-particulate-dispersed alumina fiber. J. Am. Ceram. Soc. 1998 81 2469-2472 Towata A., Hwang H.J., Yasuoka M., Sando M., Niihara K. Preparation of polycrystalline YAG/alumina composite fibers and YAG fibers by the sol-gel method. Composites, Part A 2001 32 1127-1131... 

The 3M Company manufactures a continuous polycrystalline alumina—silica—boria fiber (Nextel) by a sol process (17). Aluminum acetate is dissolved in water and mixed with an aqueous dispersion of colloidal silica and dimethylformamide. This mixture is concentrated in a Rotavapor flask and centrifuged. The viscous mixture is then extruded through spinnerettes at 100 kPa (1 atm) the filaments are collected on a conveyor and heat-treated at 870°C to convert them to metallic oxides. Further heating at 1000°C produces the 10-pm diameter aluminum borosilicate fibers, which are suitable for... 

Several fiber types have been mentioned so far, and several other types have been neglected that have been worked on over the past few years. Some of those not discussed may become important fibers for reinforcement in the years ahead. To date though, they have not been available in sufficient quantity for thorough evaluation in composite specimens. Included in this group are boron carbide, spinel, polycrystalline alumina and silica, titanium diboride, and miscellaneous silicides and intermetallics. Ten years from now as we look back on the 70s we no doubt will have an entirely different view of some of these materials. 

D. J. Pysher and R. E. Tressler, Tensile Creep Rupture Behavior of Alumina-Based Polycrystalline Oxide Fibers, Cer. Eng. Sci. Proc., 13[7-8], 218-226 (1992). 

Figure 6.14 A comparison of various polycrystalline alumina and aluminosilicate fibers in the Nextel series. Note the superior creep resistance of the Nextel 720 fiber. 3M data.

Adsorption of aniline on minerals, such as kaolinite, montmorillonite and vermiculite as well as on a-alumina and iron powders, has been studied in order to better understand the interaction of the adsorbate with natural adsorbents154-157. However, the results, similarly to those obtained on a smooth polycrystalline platinum electrode158, are not useful for the removal of aniline from waste water. The commonly used activated charcoal is also deficient owing to slow kinetics of the removal of dissolved pollutants. In this situation, activated carbon fibers, woven as a C-cloth, seem to be the ideal adsorbents for removal of aniline from waste streams. The reports of Niu and Conway159,160 explained the adsorptive and electrosorptive behavior of aniline on C-cloth electrodes and suggested a methodology for clean-up of industrial waste waters. 

Use the Halpin-Tsai equations to determine the five elastic constants of a unidirectional fiber composite in which alumina fibers are dispersed in a glass matrix. The Young s modulus and Poisson s ratio of polycrystalline AljOj are 400 GPa and 0.23 and for the glass, 70 GPa and 0.20. 

FIGURE 3-19 Typical creep curves for polycrystalline alumina-based oxide fibers at 1,090°C (1,994 F) in air. 

FIGURE 3-21 One-hour bend stress relaxation ratio for directionally solidified eutectic YAG/alumina fibers grown by edge-defined film growth (EFG), for polycrystalline alumina-based fibers, and for c-axis sapphire fibers. Source Morscher et al., 1995. 

Additional investigation of the layered perovskites (KCa2Nb30io and BaNd2Ti30io) should be limited because of the low probability that these compounds will be stable with available creep-resistant fibers. Layered perovskites may be stable with a creep-enhanced polycrystalline alumina fiber although it has been reported that these coatings are stable with alumina only at moderate temperatures ( 1,250°C [2,282°F]). The simpler chemistry of layered P-aluminas and magnetoplumbites are more likely to be used as coatings for available oxide fibers. 

Pysher, D.J., and R. E. Tressler. 1992b. Tensile creep rupture behavior of alumina-based polycrystalline oxide fibers. Ceramic Engineering and Science Proceedings 13(7-8) 218-226... 

Sabol, S.M., and R.E. Tressler. 1990. Microstructural tailoring of polycrystalline alumina fibers. HiTemp Review 15 1-11. ... 

Yun, H.M., J.C. Goldsby, and J.A. DiCarlo, 1993. Stress Rupture Behavior of Small Diameter Polycrystalline Alumina Fibers. NASA Technical Memoranda 106256. 

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