PRECURSORS AND PROCESSING

Vitreous carbon is obtained by the carbonization of organic polymers, commonly known as plastics. The types of polymers and the carbonization process and mechanism are reviewed in broad terms in Ch. 3, Sec. 2. 

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The ineffective observation element of the extrusion sub-process enables the reoccurring product purity problem, which had no effect on any safety barrier present. It appears that the focus of the analysis so far should be more on safety relevant precursors and processes, to avoid results that do not affect any safety barrier. This problem will also be discussed further in Section 5.2. 

As a result of these disadvantages, tremendous efforts over the past 20 years have focused on developing chemistries/processes to improve or replace the Yajima process. At this point, as discussed below, several precursors can be synthesized and/or processed to produce phase pure SiC shapes with controlled microstructures that offer the exceptional properties expected of SiC. Thus, this area of precursor chemistry can now be considered to be mature. Consequently, only engineering and/or cost considerations now dictate which precursor systems are useful for a given application. Furthermore, because the general area of SiC precursors has been reviewed in detail6,7, we focus here only on those precursors and processing methods that provide phase pure SiC. 

Markussen J, Fiil N, Hansen MT et al (1990) DNA-sequence encoding biosynthetic insulin precursors and process for preparing the insulin precursors and human insulin. US Patent 4,916,212... 

Figure 7.1D. Peptide precursors and processing products (taken from Ref. 100).

Isotopic Analysis. Isotopic study of humic substances is important for obtaining information on precursors and processes involving formation and diagenesis (Nissenbaum and Kaplan, 1972 Stuermer et al., 1978). However, little work has been carried out for humic substances in lake sediments. Table 6 lists isotopic data of lake sediment humic substances. 

The fabrication of the super alloys of today and those of the future requires improved molds, cores, and binders. Chemical processes and oil refining require catalysts with improved activity, selectivity, and mechanical properties. Classic ceramics and the new ceramics for electronic microcircuits (microelectronics) require purer and more reliable precursors and processes. Chromatographic processes require substrates with improved physical, mechanical, and surface properties. And there is an ever-increasing need of better absorbents, binders, and pigments. 

The target molecule is now entered and ready for synthetic analysis. The molecule may be saved in a disk file with the WRITE command so the work of input will not be lost in the event of telephone disconnection. It can be read back in by a READF command. To begin analysis with the standard default strategies (which will be described later) the user simply types RUN or pushes light button "PROCESS. This generates the first level of the retrosynthetic tree. Each precursor is shown to the user on his terminal. He is then free to VIEW a precursor and PROCESS it further. Thus the standard usage of SECS is quite simple for the user. 

Formed from spun polymeric fibers which are subsequently pyrolysed to yield structures of unusual strength and stiffness. The properties are a function of polymer precursor and processing history. More recently, carbon fibers have been grown from the vapor phase. 

Secondary problems that must be addressed include the need for facile, inexpensive synthetic routes to useful precursors and, processing methods that limit the introduction of deleterious elements (e.g. oxygen). 

The sol-gel process leads to clear sols which can be applied by conventional coating techniques such as dipping, spraying, flow-coating, roll-coating, screen printing, etc. The molecular precursors and processing conditions used in the sol-gel process... 

A number of factors have to be considered in designing an efficient process for the production of ceramic fibres by this route. These are considered below, first in a general sense and then with reference to specific precursors and processing routes which have been evaluated. 

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