Fiber-directed preforms

Amoco has developed a family of ultra high modulus continuous graphite fibers and preforms with axial thermal conductivity to llOOW/mK. The extremely high thermal conductivity is a direct result of an extremely high degree of crystallinity during carbonization of the mesophase pitch precursor fiber. Table... 

SiC whiskers. The particulates or whiskers were uniformly distributed among the carbon fibers and preforms prepared from the treated fibers were directly infiltrated by molten A1 under applied stress. It was found that the longitudinal tensile strengths of hybrid composites were greatly improved, although their fiber volume fractions were quite low compared to those of conventional composites. With this hybridization method, it is also practical to tailor the fiber volume fraction of composites from 60 to 25 vol%, which is not possible in direct infiltration of fiber preforms by pressure easting. The results obtained lead to the conclusion that particulate or whisker additions do not act directly as reinforcements, but as promoters to improve the infiltration performances of fiber preforms and consequently, increase the strength transfer efficiency of carbon fibers. The addition of particulates or whiskers can also improve other properties of the composites, such as hardness and wear resistance. 

RIM molders n. Molders for reaction injection molding (RIM). These types of molders are lightly constructed machines and consists of preformer, clamp, metering unit and mixhead. The preformer types include thermoforming, fiber-directed, knitting, braiding, or a combination of two or more. Herman F Mark (2003) Encyclopedia of Polymer Science and Technology, John Wiley and Sons, New York. 

There are many methods to measure the RIP of the preform and the fiber. However, the RIP might be changed by the heat-drawing process, but some fabrication methods, such as coextrusion, can produce optical fibers directly from raw materials, not via preforms (see Chapter 5). In addition, we are interested in the final RIP of the fiber and not in the RIP of the preform during fabrication. Thus, this section focuses on measurements of the RIP of the fiber. 

In integrated photoelasticity it is impossible to achieve a complete reconstruction of stresses in samples by only illuminating a system of parallel planes and using equilibrium equations of the elasticity theory. Theory of the fictitious temperature field allows one to formulate a boundary-value problem which permits to determine all components of the stress tensor field in some cases. If the stress gradient in the axial direction is smooth enough, then perturbation method can be used for the solution of the inverse problem. As an example, distribution of stresses in a bow tie type fiber preforms is shown in Fig. 2 [2]. 

For nosetip materials 3-directional-reinforced (3D) carbon preforms are formed using small cell sizes for uniform ablation and small pore size. Figure 5 shows typical unit cell dimensions for two of the most common 3D nosetip materials. Carbon-carbon woven preforms have been made with a variety of cell dimensions for different appHcations (27—33). Fibers common to these composites include rayon, polyacrylonitrile, and pitch precursor carbon fibers. Strength of these fibers ranges from 1 to 5 GPa (145,000—725,000 psi) and modulus ranges from 300 to 800 GPa. 

Organoclay materials with higher-order organization can also be prepared by template-directed methods involving self-assembled supramolecular structures. In this approach, preformed organic architectures in the form of tubes, fibers, hollow shells, gyroids, helicoids, and so on are transferred into hybrid materials exhibiting structural hierarchy, complex form and ordered mesoporosity [47-55]. For example,... 

The majority of work done on VGCF reinforced composites has been carbon/carbon (CC) composites [20-26], These composites were made by densifying VGCF preforms using chemical vapor infiltration techniques and/or pitch infiltration techniques. Preforms were typically prepared using furfuryl alcohol as the binder. Composites thus made have either uni-directional (ID) fiber reinforcement or two-directional, orthogonal (0/90) fiber reinforcement (2D). Composite specimens were heated at a temperature near 3000 °C before characterization. Effects of fiber volume fraction, composite density, and densification method on composite thermal conductivity were addressed. The results of these investigations are summarized below. 

As noted earlier, CVl is nsed primarily to form ceramic-fiber-reinforced ceramic matrix composites. The most common of these combinations is SiC fiber/SiC matrix composites. One commercially available product has a two-dimensional 0/90 layup of plain weave fabric and fiber volume fraction of about 40%. This same composite can be fabricated with unidirectional fibers and with 45° architectures. The most commonly used SiC fiber for the preforms is Nicalon , the mechanical properties for which were provided earlier in Section 5.4.2.7. A number of other carbide and nitride fibers are also available, including Si3N4, BN, and TiC. Preform geometries can be tailored to the application in order to maximize strength and toughness in the direction of maximnm stresses. The reactions used to form the matrix are similar to those used in CVD processes (cf. Section 7.2.4) and those described previously in Eq. (3.105). 

Although the reinforcing fibers may be present in the liquid precursor prior to dispensing, better properties are typically obtained when the fibers are initially present in the mold as a preform. The liquid is then dispensed into the mold such that the final matrix fills the mold and surrounds the fibers. Preforms may be arranged as mats or meshes. The fibers within the preform may be randomly oriented or may be oriented in one or more directions. 

Electrospinning is a method allowing creation of polymer fibers with diameters in the range between a few tens of nanometers to a few micrometers, starting from a solution of preformed polymer. MIP nanoparticles have been included into nanofibers by electrospinning [126, 127], In another case, the nanofibers were directly produced by electrospinning and polymerizing an MIP-precursor solution [128]. Such MIP fibers can then be used, for example, for the preparation of affinity separation materials [129] or as affinity layers in biosensors [127, 130]. 

With the directed fiber system strands are blown onto a rotating preform screen from a flexible hose. Roving is directed into a chopper where air flow moves it to a preform screen. Use can be made of a vertical or horizontal rotating turntable. This process requires a rather high degree of skill on the part of the operator however, automated robots are used to provide a controlled system producing quality preforms. 

In this work, all carbon-carbon composites were fabricated by using pitch-based carbon fibers and pitch-derived carbon matrices. Two types of preforms, were mainly used. One is unidirectional (UD) preforms, and the other is 3-dimensional (3D) fabrics. 3D fabrics normally contain 55 vol.% of fibers that are introduced in three directions. Each volume fraction of fibers is 40 vol.% in the x-direction, 10 vol.% in... 

The examined C/C composite was fabricated via. preformed yam method[6]. The reinforcing fiber, fiber volume fraction, stacking sequence, and dimensions of specimens of it are Toray M40, 50%, 0790°, and 30mm x 30mm x 3mm, respectively. In the laminated C/C composite, periodical cracks along the fiber axis direction, transverse cracks (TCs), frequently appear as shown in Fig. 1. The surface layers of the TCs especially affect characteristics of the coating on C/C composites. 

---