Amorphous fibers

The axial elastic modulus of PET fibers E) depends, as with other kinds of fibers, on the value of the elastic modulus of crystalline material ) and amorphous fiber... 

Non-crystalline polymers or copolymers can also be used to generate fibers with relatively low softening temperatures. Such fibers can be blended with regular fibers, e.g. staples, and bonded together by applying sufficient heat to melt the low-temperature component. Such fibers need not be exotic. The use of undrawn, amorphous fibers suffices for many such purposes, for example, bonded nonwo-ven webs formed from a mix of drawn and undrawn PET staple fibers. Without crystalline structure, the undrawn fibers will soften and become tacky at relatively low temperatures, so providing bond sites. 

MMVF are synthetic fibers with glasslike structures. The term usually refers to silicate-based glass fibers, because these compositions form the largest volume of fibers produced. However, in addition to fiberglass and fused silica (Si02), there are other amorphous fibers used in commerce alumina (AI2O3) and silica combinations, rock and slag wool, as well as fibers with nonsilicate compositions such as carbon. Many of these amorphous fibers have proprietary names. 

One continuing and unresolved issue is whether or not a crystalline fiber is more desirable than an amorphous fiber. Thus, efforts to make amorphous, SiC-based ceramics are still an area of interest as suggested by the following section. 

Figure 5. Is2 vs. s diagram for a well-crystallized PET fiber (----) and an amorphous fiber sample... 

Shape of products Powder—spherical Film—amorphous Fiber—long fiber, short fiber, nonwoven cloth... 

The acrylics and modacrylics are among the most important of the amorphous fibers. They are based on the acrylonitrile unit —CH2CH(CN)— and are usually manufactured as copolymers. When the aaylonitrile content is 85% or higher, the polymer is an acrylic fiber but if this drops to between 35 and 85%, it is known as a modacrylic fiber. Vinyl chloride and vinylidene chloride are the most important comonomers, and the copolymers produce high-bulk yams, which can be subjected to a controlled shrinking process after fabrication. Once shmnk, the fibers are dimensionally stable. 

For amorphous fibers composed of C,ooH202 and with thicknesses of 5-8nm, the anisotropies of the individual chains, as measured by the principal moments of the radius of gyration tensor, are comparable with those expected for chains in the bulk. However, the radius of gyration tensors tend to be oriented within the fiber, with the nature of the orientation depending on the distance of the center of mass from the axis of the fiber. Chains with their centers of mass close to the fiber axis tend to have the largest component of their radius of gyration tensor ahgned with the fiber axis. 

Wide-angle X-ray scattering and DSC measurements of cold drawn PEN stretched from an amorphous fiber that is spun at low speeds indicate that the strain-induced crystallization can occur at a temperature be-... 

Fig. 4.3 Cartoon representation of the structures of orientated semicrystalline (a), and amorphous (/>) fibers.

Recommendations 2, 3, and 4 (listed in order of decreasing priority) are related to performance, which is also considered to be a high priority. Recommendation 2 is the most important in this category. The oxidation resistance of oxide fibers is attractive, but poor creep resistance is a significant limitation. Thus, Recommendation 3 addresses the need to improve this property. Recommendation 4 (regarding non-oxide fibers) is last in this category because the committee concluded that resources directed toward property improvement in fiber coatings and oxide fibers was more important. The committee is satisfied that the preliminary properties reported for Si-B-N-C amorphous fibers are sufficiently attractive to stimulate the research needed to verify them. 

The development of an amorphous fiber in the Si-N-C-B system has revealed two unexpected possibilities. First, this fiber appears to be stable in the amorphous state up to 1,600°C (2,912°F), even in air, and retains its tensile strength up to 1,600°C (2,912°F). Second, oxidation of the Si-N-C-B fiber forms silica at rates comparable to the rate that silica is formed upon oxidation of crystalline SiC, and also creates a thin ( 50 nm [0.002 mils]) buried interphase of hexagonal BN. It has been hypothesized that this in-situ BN would act as a crack deflection layer in a CMC. If so, the regenerative nature of the deflection interphase could avert composite degradation by stress oxidation. 

Silicon - amorphous fiber endless solid one exper. solid fiber 3.2.1... 

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. 

The fourth example shows that some amorphous HP-LCVD fibers having binary silicon-nitrogen compositions [17] were silicon rich, others were near stoichiometric Si-N compositions, and only a few were representative of stoichiometric silicon nitride. Thus, the process offers wide latitude in the design of fibers with amorphous or glassy structures. These design options facilitate the production of amorphous fibers from equilibrium and nonequilibrium melt compositions and of amorphous fibers having stoichiometric or non-stoichiometric, binary compositions. 

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. 

Single crystal whiskers Amorphous fibers Single crystal fibers Amorphous fibers... 

C. Two very different SiC fibers meet these requirements [35]. One of these fibers (see Chapter 10) is Nicalon, an amorphous fiber having a fiber diameter of 10-20 jm. It is commercially fabricated by thermal decomposition of a polycarbosilane precursor fiber. The other fiber, which is discussed here, is a polycrystalline 3-SiC fiber made by Textron. It has a sheath/core structure and a fiber diameter of 142 jm. The core consists of a 37 jm diameter graphite coated carbon fiber. 

Forming amorphous fibers from inviscid liquids... 

Rapid solidification and inviscid melt spinning suppress crystallization, which would otherwise occur with aluminate and YAG melts which are derived from highly crystalline materials. Fibers from liquefied gases, fibers from inviscid melt spun oxides and ribbons from rapidly solidified metal alloy melts are amorphous. Fibers spun from inviscid metal melts are predominantly amorphous but contain a minor crystalline phase. 

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