Pyrolysis amorphous materials

The ceramic products of these pyrolyses were completely amorphous. Pyrolysis carried out at 1500 °C or lower yields only amorphous products. Amorphous materials can be converted to crystalline form by heating above the transition temperature. Pyrolysis must be carried out at 1550 °C or higher to obtain crystalline products. Thus, pyrolysis of the transamination product of Tris with ammonia at 1550 °C or higher gave high-purity, a-phase silicon nitride (33), as analyzed by powder X-ray difiraction (Figure 1 and Table I). 

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Thermal treatment of the polymeric precursor (3P) and molecular precursors (4) result in an amorphous material (Scheme 2). After the precursor-to-ceramic conversion, a black, dense ceramic was obtained. The density of the material was determined to be 1.6 — 1.7 g/cm. One of our main interests was to reduce the accompanying formation of gaseous by-products during pyrolysis. As shown in Fig. 2, TG measurements up to 1500 °C reveal a weight loss of ca. 37% in the case of 3P... 

Carbon molecular sieves, or carbogoric sieves are amorphous materials made by pyrolyz-ing coal, coconut shells, pitch, phenol-formaldehyde resin, or other polymers. EKslocations of aromatic microdomains in a glassy matrix give their porosity. Pores are slit-shaped. Pore structure is controlled by the temperature of the pyrolysis. Pore widths range from 3 A to 10 A. Acarbogenic sieve made from polyfurfuryl alcohol and combined with silica-alumina was selective for monomethylamine production from methanol and ammonia [54]. 

Moreover, polymers 6 and 8 were pyrolyzed in bulk. These pyrolysis experiments were performed in a slow stream of nitrogen and the samples were heated to 1000°C at a rate of 10°C min", remaining at this temperature for 30 minutes Both of the ceramic products were black powders and in X-ray powder diffraction studies they showed only broad peaks of low intensity, indicating the presence of mainly amorphous material To obtain crystalline materials, the ceramic products were heated slowly to 1400°C where they were held for 5 hours. The X-ray powder diffraction showed exclusively sharp peaks, characteristic of P-SiC, respectively. The increased ceramic yield obtained by pyrolysis of the metal modified carbosilane 8, as compared with the polycarbosilane 6, can be explained by an increased concentration of carbon as impurity, which was additionally evidenced by elemental analysis. 

Coordinately saturated polymers of composition (H2NBH2) are identified but not characterized. They are amorphous materials and their polymerization degree is believed to be quite low ( 15.2.5.3). A polymer of the same formula unit is prepared from pyrolysis of NH3BH3 under a low pressure of nitrogen. This polymer lacks sufficient solubility for molecular weight determination but is crystalline, in contrast to the materials of the same composition reported earlier, and its x-ray diffraction pattern differs from that of any known cyclic boranamine. 

Since the mid 1990s, many research groups have concentrated on the development of fibers via the pyrolysis of appropriate pre-ceramic polymers [ 112]. In particular, the system Si-B-N-C has been of major interest due to the excellent high-temperature and oxidation stability of the resultant amorphous material [113]. In this case, the onset of crystallization may be as high as 1800 °C, while decomposition starts at 2000 °C in protective atmospheres. 

A high aromatics selectivity, however, requires proper catalyst selection. Zhang et al. studied the fast pyrolysis of corncob in absence and presence of a catalyst (ie, ZSM-5) [287]. The presence of the catalyst increased the yields of noncondensable gas, water, and coke, while decreasing the liquid and char yields. The catalyst induced a decrease of the oxygen content of the liquid fraction by more than 25%. These studies indicate the importance of a catalyst during biomass pyrolysis. The most important catalytic parameters affecting the product distribution are pore structure and acid site type. This was demonstrated by testing siUcalite, a material with the same pore structure as ZSM-5 but with intrinsic different acid sites, and siUca-alumina, an amorphous material with Brpnsted acid sites, in the catalytic pyrolysis of... 

These carbonaceous catalysts can be obtained by the sulfonation of incompletely carbonized organic compounds [42]. Note that starch and cellulose can be used as carbon precursor [43, 44]. After the incomplete pyrolysis of the carbon precursor, the SO3H groups have been introduced by sulfonation with sulfuric acid (Scheme 3). After this treatment, the presence of phenolic hydroxyl, carboxylic acid, and sulfonic groups at the surface of these amorphous carbonaceous materials has been demonstrated. 

ZnO displays similar redox and alloying chemistry to the tin oxides on Li insertion [353]. Therefore, it may be an interesting network modifier for tin oxides. Also, ZnSnOs was proposed as a new anode material for lithium-ion batteries [354]. It was prepared as the amorphous product by pyrolysis of ZnSn(OH)6. The reversible capacity of the ZnSn03 electrode was found to be more than 0.8 Ah/g. Zhao and Cao [356] studied antimony-zinc alloy as a potential material for such batteries. Also, zinc-graphite composite was investigated [357] as a candidate for an electrode in lithium-ion batteries. Zinc parhcles were deposited mainly onto graphite surfaces. Also, zinc-polyaniline batteries were developed [358]. The authors examined the parameters that affect the life cycle of such batteries. They found that Zn passivahon is the main factor of the life cycle of zinc-polyaniline batteries. In recent times [359], zinc-poly(anihne-co-o-aminophenol) rechargeable battery was also studied. Other types of batteries based on zinc were of some interest [360]. 

In some instances, subtle changes in the precursor architecture can change the composition and microstructure of the final pyrolysis product. For example, pyrolysis of —[MeHSiNH] — leads to amorphous, silicon carbide nitride (SiCN) solid solutions at >1000°C (see SiCN section). At ca 1500 °C, these material transform to SisN SiC nanocomposites, of interest because they undergo superplastic deformation20. In contrast, chemically identical but isostructural — [F SiNMe] — transforms to Si3N4/carbon nanocomposites on heating, as discussed in more detail below21. 

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