Vitreous carbon structure

A similar, but highly porous, vitreous carbon material—reticulated vitreous carbon (RVC)—has found widespread application for flow analysis and spectro-electrochemistry (25). As shown in Figure 4-10, RVC is an open-pore ( spongelike ) material such a network combines the electrochemical properties of glassy carbon with many structural and hydrodynamic advantages. These include a very high surface area ( 66 cm2 cm-3 for the 100-ppi grade), 90-97% void volume, and a low resistance to fluid flow. 

FIGURE 4-10 The open-pore structure of reticulated vitreous carbon. 

The feasibility of benzenic ring amines, benzenic ring structures and aliphatic acids oxidation by means of Fenton chemistiy was tested in synthetic, acidic wastewaters by Fenton s reagent electrogenerated at a reticulated vitreous carbon cathode using the flow-cell. The organic molecules considered were phenol (Ph), cresol (Cr), aniline (An) hydroquinone (HQ), catechol (Cat), parabenzoquinone (pBQ) and oxalic acid (OxAc). Their initial... 

Several reticulated vitreous carbon (RVC)-plant tissue composite electrodes have also been reported where the open-cell structure of RVC serves as a template for the biocomponent used. One such sensor was constructed by press-fitting 100 pore/in RVC (2-mm thick x 3 or 6 mm OD) cylinders into a 6-mm diameter cavity of a thin-layer cell. The inner side of the disc was pressed into an edge of a copper wire, which provided electrical contact. 

Figure 9.10 Assembly of sandwich-type optically transparent electrochemical cell for extended x-ray absorbance fine structure (EXAFS) spectroelectrochemistry. Cell body is of MACOR working electrode is reticulated vitreous carbon (RVC). [From Ref. 64, with permission.]...

In the case of vitreous carbon, the component Ogv of csv was measured by the technique used for graphite (Donnet et al. 1982) and found to be 32 mJ/m2. This is five times lower than that of the graphite basal plane and may be explained by the low density of vitreous carbon (1.5 x 103 kg/m3) compared to that of graphite (2.26 x 103 kg/m3) (see also Section 8.1). Because the atomic structure of polished surfaces of vitreous carbon is not known, it is not possible to evaluate the percentage of [Pg.170]

Molecular and large-scale structural parameters of vitreous carbon have been elucidated as a function of heat-treatment temperature, using X-ray diffraction and electron microscopic techniques. Short-range order in the carbon layers is characterized by an interatomic distance of 1.42 0.01 A (c/. graphite) but with a... 

An e.s.r. study has also been carried out to assess the influence of both heat-treatment and neutron irradiation on the structure of vitreous carbon. 

Recently, a cellular, structural biomaterial comprised of 15 to 25% tantalum (75 to 85% porous) has been developed. The average pore size is about 550 p,m, and the pores are fully interconnected. The porous tantalum is a bulk material (i.e., not a coating) and is fabricated via a proprietary chemical vapor infiltration process in which pure tantalum is uniformly precipitated onto a reticulated vitreous carbon skeleton. The porous tantalum possesses sufficient compressive strength for most physiological loads, and tantalum exhibits excellent biocompatibility [Black, 1994]. This porous tantalum can be mechanically attached or diffusion bonded to substrate materials such as Ti alloy. Current commercial applications included polyethylene-porous tantalum acetabular components for total hip joint replacement and repair of defects in the acetabulum. 

Tantalum, which is used for a number of applications, was recently made into a porous material that could be used for bone reconstruction. The porous structure is made by depositing the metal onto a vitreous carbon scaffold using chemical vapor deposition/infiltration techniques. Its low stmctural density means that its stiffness (2.5-4 GPa) is closer to that of natural bone than the solid metal, and the porosity means that bone can fully integrate into the stmcture. forming an excellent bond. " ... 

Vitreous carbon is a fine-grained polycrystalline material formed by slow heating of a polymer. On heating, the more volatile components diffuse from the structure, and only carbon remains (Hench and Ethridge, 1982). Since the process is diffusion-mediated and potentially volatile, heating must be slow, and the dimensions of the structure are therefore limited to approximately 7 mm (Bokros, 1978). Salient properties of all three forms of carbon are summarized in Table 13.1. 

A thicker cell is possible if reticulated vitreous carbon (RVC) is used. For example, transmission of 13—45% for 100 p.p.i. (pores per inch) is possible for a 1.2-0.5 mm thick RVC plate. LIGAs are Llthogra-phic-GAlvanic structures that present some advantages over indium tin oxide (ITO) and metal meshes, namely improved stability, faster response times, and a greater range of electrode materials that can be used in the same cell. The electrodes are sandwiched between quartz rods coupled to the spectrometer using fiber optics. 

Amorphous and vitreous carbon The first attempt of modelling the structure of a-C vacuum-deposited films using the continuous network approach was made by Kakinoki et al (I960). Their experimental technique (electron diffraction, Smax 35 A ) yielded a carefully measured/(s) curve (Figure 2.31) and a RDF of high resolution which showed two well defined but rather broad peaks at r = 1.50 and r2 = 2.54 k. Using Debye s Eq. (2.2) the authors tried to match without success the whole measured I(s) function by a calculated curve for a spherically symmetrical array of atoms in which only these two interatomic distances were present. 

Note The terms Glassy Carbon and Vitreous Carbon are trademarks and should not be used and, moreover, as implied, there is no similarity in the structures of silicate glasses, other than the pseudo-glassy appearance of the surface. 

In order to clarify the terminology, it is necessary to define what is meant by carbon and its polymorphs. When used by itself, the term "carbon should only mean the element. To describe a carbon material, the term is used with a qualifier such as carbon fiber, pyrolytic carbon, vitreous carbon, and others. These carbon materials have an sp atomic structure, and are essentially graphitic in nature. 

Vitreous carbon has a structure that is more closely related to that of a glassy material (i.e., non-crystalline), with high luster and glass-like fracture characteristics, hence the name vitreous (or glassy). Vitreous carbon is also frequently called polymeric carbon since it derives mostly from the carbonization of polymeric precursors. In the context of this book, the terms vitreous, glassy and polymeric carbons are synonymous. 

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