Pyrolytic graphite particles

Pyrolytic graphite was first produced in the late 1800s for lamp filaments. Today, it is produced in massive shapes, used for missile components, rocket nozzles, and aircraft brakes for advanced high performance aircraft. Pyrolytic graphite coated on surfaces or infiltrated into porous materials is also used in other appHcations, such as nuclear fuel particles, prosthetic devices, and high temperature thermal insulators. 

The deposition of pyrolytic graphite in a fluidized bed is used in the production of biomedical components such as heart valves, ] and in the coating of uranium- and thorium-carbides nuclear-fuel particles for high temperature gas-cooled reactors, for the purpose of containing the products of nuclear fission.fl" The carbon is obtained from the decomposition of propane (CgHg) or propylene (CgHg) at 1350°C, or of methane (CH4) at 1800°C. Its structure is usually isotropic (see Ch. 4). 

FIG. 16 Current versus bias voltage for a CdS nanoparticle on the end of an STM tip. The CdS particles were formed by exposing a bilayer of cadmium arachidate on the STM tip to H2S gas. The other conducting surface is a highly oriented pyrolytic graphite electrode. The inset is a plot of differential conductance versus the bias voltage. (Reproduced with permission from Ref. 202. Copyright 1996 National Academy of Sciences, U. S. A.)... 

As is well known, when hydrocarbons transported in an inert gas such as helium are heated, carbon or pyrolytic graphite will deposit on the walls. In the HTCVD, the ethylene helps make the particles stable and, in doing so, carbon is transported into the chamber together with the silicon. It is a sort of symbiosis in the transport between the silicon and carbon. 

An STM probe has been used to isolate individual MS (M = Cd, Pb) particles and to measure electronic phenomena (55,56,81). The MS films were prepared either by exposure of metal ion/fatty acid films to H2S (55,56) or by transfer of a compressed DDAB-complexed CdS monolayer (81). All the films were transferred onto highly oriented pyrolytic graphite (HOPG) for the STM measurements. A junction was created at an individual CdS particle with the STM tip as one electrode and the graphite as the other, and the current/voltage characteristics of the panicles were measured. For the particle prepared in the fatty acid films the I/V curves exhibit step-like features characteristic of monoelectron phenomena. In the case of the DDAB-coated CdS particles the I/V measurements demonstrated n-type semiconductor behavior. The absence of steps in this system is probably a reflection of the larger size of the particles in the DDAB films (8 nm by AFM) compared to the 2-nm particle size typically found for MS particles formed in fatty acid films. 

New methods for improving the sustainability of the methane thermocatalytic decomposition process have been developed. Studies indicate that the presence of small amounts of moisture and H2S (<3 v.%) in the hydrocarbon feedstock is not detrimental for the catalyst activity and process efficiency. This implies that commercial hydrocarbon fuels could potentially be employed as feedstocks for the process. A bench-scale thermocatalytic reactor was designed, fabricated and operated using methane and propane as feedstocks. The TCR produced hydrogen-rich gas free of CO/CO2 impurities the gas was directly fed to PEM fuel cell. Material characterization studies indicated that depending on operational conditions, carbon could be produced in several valuable forms including turbostratic carbon, pyrolytic graphite, spherical carbon particles, or filamentous carbon. 

The manufacture of heterogeneous catalysts from pre-prepared nanometal colloids as precursors via the so-called precursor concept ll has attracted industrial inter-est.l l An obvious advantage of the new mode of preparation compared with the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors can be tailored for special applications independently of the support. In addition, the metal particle surface can be modified by lipophilic or hydrophilic protective shells, and covered with intermediate layers, e.g. of oxide. The addition of dopants to the precursor is also possible. The second step of the manufacture of the catalyst consists in the simple adsorption of the pre-prepared particles by dipping the supports into organic or aqueous precursor solutions at ambient temperature. This has been demonstrated, e.g., for charcoal, various oxidic support materials, even low-surface materials such as quartz, sapphire, and highly oriented pyrolytic graphite. A subsequent calcination step is not required (see Fig. 1). 

B.J. Hwang, R. Santhanam, and Y.L. Lin, Electrochemical deposition of gold metal particles into 3-dimensional polypyrrole film deposited on a highly oriented pyrolytic graphite. Electroanalysis, 15,1667-1676 (2003). 

Characterisation of these fullerene tubes was done by electro-deposition [developed by Smalley and cowoikers, see above references] which drives the snspended tnbes onto the surface of highly oriented pyrolytic graphite surface and are scanned by AFM. The molecular nature of the tubes was demonstrated by converting their CO2H groups to COCl, followed by reaction with H2N(CH2)nSH and exposure to lOnm gold particles. Gold (which reacts with SH) was only fonnd on the carboxylated fullerene tubes. 

An electrochemically heterogeneous electrode is one where the electrochemical activity varies over the surface of the electrode. This broad classification encompasses a variety of electrode types [1, 2] including microelectrode arrays, partially blocked electrodes, electrodes made of composite materials, porous electrodes and electrodes modified with distributions of micro- and nanoscale electroactive particles. In this chapter, we extend the mathematical models developed in the previous chapter, in order to accurately simulate microelectrode arrays. Fbrther, we explore the applications of a number of niche experimental systems, including partially blocked electrodes, highly ordered pyrolytic graphite, etc., and develop simulation models for them. 

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