Carbonized PAN fibers

Figure 5.60 The nitrogen content of carbonized PAN fibers up to 2500 C. Source Coiiated from information suppiied by RK Carbon Fibres and Courlauids.

FIGURE 8.7 SEM analysis of as spun, stabilized and carbonized PAN fibers (magnification of 5000x). 

Figure 7.12 Electrospun and carbonized PAN fibers decorated with Pt (a bright spots attributed to Pt particles) and an overview of the final freestanding porous electrode structure (b).

Mg. 8. Schematic representation of a carbonized PAN fiber. Adapted from Ref. 43. 

Recent research in Poland is helping PAN-based carbon fibers find new applications as biological implants. Low temperature carbonized fibers have found use in ligament and tendon prostheses and in surgical sutures (44,45). These fibers are heat treated to temperatures below 1300°C and are far less crystalline than traditional carbonized PAN fibers (46,47). Despite the promise of these studies, low temperature carbonized PAN fibers do not share the commercial success of their high performance cousins. 

Thus, the 2nd fiber category encompasses carbon and carbonized polymers. The foundation materials of this group are the heat convertible, fiber forming polymers, the most common of which today is PAN (polyacrylonitrile). Others are rayon, PBI, and pitch tar. One of the earliest to report results of PAN pyrolysis was Goodhow, et. al.(5i) in 1975. Later, in 1979, Fischbach and Komaki(5 and Brehmer, et. al.(di) in 1980 reported on the electrical properties of carbon fibers made fi om various polymers and described a dependency of resistivity upon the heat treat temperature (HTT) employed to carbonize the fiber which is now well known. The studies by Swift, et. al(52) in 1985 and more recently reported herein were undertaken to expanded upon this base of knowledge and to initiate studies of the stability of the electri properties of fibers made on commercially viable platforms. These studies have led to a launch point for what is believed to have been the first, relatively large scale, commercial application for partially carbonized PAN fibers(50), as resistive carbon fiber based static eliminator brushes. 

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

The use of a wet-spinning process with inorganic solvents has also been attempted. Although the details of this process are proprietary, it is clear that these inorganic wet-spun PAN fibers make higher quality carbon fiber precursors than those produeed with traditional organic solvents [5]. 

The as-spun acrylic fibers must be thermally stabilized in order to preserve the molecular structure generated as the fibers are drawn. This is typically performed in air at temperatures between 200 and 400°C [8]. Control of the heating rate is essential, since the stabilization reactions are highly exothermic. Therefore, the time required to adequately stabilize PAN fibers can be several hours, but will depend on the size of the fibers, as well as on the composition of the oxidizing atmosphere. Their are numerous reactions that occur during this stabilization process, including oxidation, nitrile cyclization, and saturated carbon bond dehydration [7]. A summary of several fimctional groups which appear in stabilized PAN fiber can be seen in Fig. 3. 

Carbon fibers have been used as filaments for lamps for nearly a century, since Edison first used them. In the early 1960s, Shindo developed the first modern carbon fiber when he pyrolyzed polyacrylonitrile (PAN) fibers [5]. 

Although there are many variations on how carbon fibers are made, the typical process starts with the formation of PAN fibers from a conventional suspension or solution polymerization process between a mixture of acrylonitrile plastic powder with another plastic, such as methyl acrylate or methyl methacrylate, and a catalyst. The product is then spun into fibers, with the use of different methods, in order to be able to achieve the internal atomic structure of the fiber. After this, the fibers are washed and stretched to the desired fiber diameter. This step is sometimes called "spinning" and is also vital in order to align the molecules inside the fiber and thus provide a good basis for the formation of firmly bonded carbon crystals after carbonization [7]. 

Schematic of the typical steps in carbon fiber manufacturing process from PAN fibers. Dotted borders indicate the steps that depend on the desired final product. (Modified from T. H. Ko.

For more information and details regarding the fabrication processes of PAN fibers and carbon fiber papers, please refer to Kinoshita [6], Decrecente, Layden, and Pike [8], and Mathias et al. [9]. 

Another style of DLs using PAN fibers was presented by Glora et al. [20]. They developed carbon aerogel sheets, which were then used as diffusion layers in PEMFCs. In order to fabricate these DLs, resorcinol-formaldehyde (RF) aerogels... 

Along with CFPs, carbon cloths have also been widely used materials for diffusion layers in fuel cells. Figure 4.6 shows SEM pictures of typical carbon cloth materials used in fuel cells. The majority of these fabrics are made from PAN fibers that are twisted together in rolls. For details regarding how normal PAN fibers and carbon fibers are fabricated, please refer to Section 4.2.I.I. In this section, we will briefly discuss the fabrication process of carbon cloths. 

T. H. Ko. The influence of pyrolysis on physical properties and microstmcture of modified PAN fibers during carbonization. Journal of Applied Polymer Science 43 (1991) 589-600. 

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