Vitreous carbon properties

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. 

The redox properties of a series of heterometal clusters were assessed by electrochemical and FPR measurements. The redox potentials of derivatives formed in D. gigas Fdll were measured by direct square wave voltammetry promoted by Mg(II) at a vitreous carbon electrode, and the following values were determined 495, 420,... 

Properties of thin layers of lead electrodeposited on vitreous carbon have been found identical with that of metallic lead [304]. Therefore Pb and Pb02 coated reticulated vitreous carbon (RVC) electrodes [185] can be applied as electrodes in lead-acid batteries, as reviewed in [305]. The deposition of lead on carbon is through the diffusion-controlled process with instantaneous or progressive nucleation, for high and low Pb + concentration, respectively, and three-dimensional growth mechanism. The number of nucleation sites increases with deposition overpotential, as shown for vitreous [306] and glassy carbon [307] electrodes. The concentration dependence of the nucleation... 

Also known as vitreous carbon, glassy carbon has been the subject of intense electroanalytical research in the past 10 years. It is impermeable to liquids and gases, and thus porosity is not an issue, as it is with polycrystalline graphite. It is easily mounted, polishable, and compatible with all common solvents. These properties have led to widespread use in mechanistic electrochemistry, LC detection, and voltammetric analysis. 

Electropolymerized films have been used to prevent interferences and fouling in biosensors constructed from reticulated vitreous carbon and platinum disk electrodes (14,15). A biosensor constructed using electropolymerized films can have significantly improved diffusional properties due to the thinness of the film. By engineering the components and properties of a biosensor on a microscopic scale, rather than using "bulk-technology" and physically assembling discrete macroscopic components, as is the conventional practice, an all-chemical method of construction can be achieved. All-chemical methods of construction would... 

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. 

Most commonly, the battery will be configured with a stack of bipolar cells (10 -100 cells per stack) to give a useful output voltage and with parallel flows for the electrolytes to each of the cells in the stack. Hence, the electrodes will be bipolar with a solid core from carbon, graphite, or a carbon/polymer composite and the three-dimensional elements bonded or pressed onto either side of the solid core. The composites are a blend of a chemically stable polymer and a micron-scaled carbon powder, most commonly an activated carbon Radford et al. [127] have considered the influence of the source of the carbon and the chemical and thermal treatments on the properties of such activated carbons, especially the pore size and distribution [126]. Even though reticulated vitreous carbon has been used for the three-dimensional elements [117], the predominant materials are graphite cloths or felts with a thickness of up to 5 mm, and it is clear that such layers are essential to scale the current density and thereby achieve an acceptable power density. Details of electrode performance in the more developed flow batteries are not available but, for example, Skyllas-Kazacos et al. [124] have tabulated an overview of the development of the all vanadium redox flow battery that includes the electrode materials and the chemical and thermal treatments used to enhance activity and stability. 

Mercury electrodeposition is a model system for experimental studies of electrochemical phase formation. On the one hand, the product obtained is a liquid drop, corresponding very well with the liquid drop model of classical nucleation theory. Besides, electron transfer is fast [61] and therefore the growth of nuclei is controlled by mass transport to the electrode surface [44]. On the other hand, the properties of the mercuryjaqueous solution interface have been the object of study for over a century and hence are fairly well understood. The high overpotential for proton reduction onto both mercury and vitreous carbon favor the study of the process over a wide range of overpotentials. In spite of the complications introduced by the equilibrium between the Hg +, Hg2 " ", and Hg species, this system offers an excellent opportunity to verily the fundamental postulates of the electrochemical nucleation theory. In fact, the dependence of the nucleation rate on the oxidation state of the electrodepositing species is fiiUy consistent with theory critical nuclei appear with similar sizes and onto similar number densities of active sites... 

The materials reviewed in this chapter form another distinctive group of carbon materials the vitreous carbons. Like molded graphite, vitreous (glassy)carbon is processed by the carbonization (pyrolysis) of an organic precursor. Unlike most molded graphites, it does not graphitize readily and has characteristics and properties that are essentially isotropic. The difference between these two classes of materials stems from different precursor materials. 

Vitreous carbon is a relatively new material which was developed in the 1960 s. It has some remarkable properties, such as high strength, high resistance to chemical attack, and extremely low helium permeability. 

Because of its random structure, vitreous carbon has properties that are essentially isotropic. It has low density and a uniform structure which is generally free of defects. Its hardness, specific strength, and modulus are high. Its properties (as carbonized and after heat-treatment to 3000°C) are summarized in Table 6.2.1 i The table includes the properties of a typical molded graphite and of pyrolytic graphite for comparison (see Chs. 5 and 7). The mechanical properties of vitreous carbon are generally higher and the thermal conductivity lower than those of other forms of carbon. 

Table 6.2. Physical and Mechanical Properties of Vitreous Carbon and Other Carbon Materials at 25°C...

Figure 6.6. Variations in the properties of vitreous carbon as a function of temperature,...

In most cases, the chemical properties of vitreous carbon are similar to those of the graphite crystal, reviewed in Ch. 3, Sec. 7. Since the material has low permeability, is essentially non-porous and free of surface defects, and can be made with very low impurities, its resistance to chemical attack is generally excellent and is one of its outstanding characteristics. In many instances, it is far more chemically resistant than other forms of carbon, such as molded or pyrolytic graphites. 

Vitreous carbon loam is produced in several pore sizes, usually described as number of pores per inch (ppi). Commercially available foams are respectively 60,100, and 200 ppi (24,39 and 78 pores per cm). These foams have low density, with relatively even pore distribution. Their properties are listed in Table 6.4.i i... 

Catalytic Support. Vitreous carbon spheres are being considered as catalyst supports for iron and other metals. The material may offer some important advantages over other forms of carbon, such as lower inorganic impurities (which may poison the catalyst) and a more uniform pore structure. The activation mechanism and the properties and characteristics of catalytic materials are reviewed in greater detail in Ch. 10, Sec. 4.0. 

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