Precursors types

Besides pH, other preparative variables that can affect the microstructure of a gel, and consequendy, the properties of the dried and heat-treated product iaclude water content, solvent, precursor type and concentration, and temperature (9). Of these, water content has been studied most extensively because of its large effect on gelation and its relative ease of use as a preparative variable. In general, too Httie water (less than one mole per mole of metal alkoxide) prevents gelation and too much (more than the stoichiometric amount) leads to precipitation (3,9). Other than the amount of water used, the rate at which it is added offers another level of control over gel characteristics. 

Chemokines and their receptors control the mobilization of naive DC precursors into the circulation in inflammation. Both DC precursor types express a common set of chemokine receptors such as CCR1, CCR5, and CXCR4,... 

Even though all three reactors share the same precursor delivery system, each tool offers specific advantages. For example, a cold-wall reactor (reactor B) helps prevent decomposition of the precursor before it reaches the substrate. A pulsed aerosol injection system at low pressure (reactor C) allows the film to grow under better-defined conditions than in a continuous process (reactor A) because of the minimization of undesirable transient effects caused by the high volatility of the solvents used.46 A more detailed description of each of the conditions for film growth, including reactor type, precursor type, delivery method, deposition temperature, growth time, and other parameters are summarized in Table 6.2. Depositions were done on bare and Mo-coated... 

For chalcogenide thin films it is possible to use elemental S, Se, Te as precursors provided that the other source is a volatile and reactive metal. ZnS deposition using elemental zinc and sulphur was the first ALD process developed [4]. Therefore for precursors other than metals, the reactivity of elemental chalcogens is not sufficient. For other precursor types, including halides, 6-diketonates and organometalHcs, simple hydrides, such as H2S, H2Se and H2Te, have typically been used as a second precursor. 

One other Si—N—B—X precursor type, where X = Ti, has been made by several groups85,90,91. The approach used has been to react Ti(NMe2)4 with a polysilazane as illustrated in equation 32. Note that the initial reaction with Ti(NMe2)4 can be exothermic and even violent if Si—H bonds are present. However, processable polymers are... 

To date, two precursor types have been identified that transform to nearly phase pure SiC. These are polymethylsilane (PMS, —Cl I3IISi—) and polysilaethylene (equation 33), which are related by the fact that heating PMS >300 °C transforms it via the Kumada rearrangement7 to polysilaethylene. 

The potential of the method is best demonstrated when the ring opening is preceded by chemo- and stereoselective alkylation reactions. As Fig. 12 shows, two main types of alkylation events can be carried out on the designed (3-lactam precursor. Type 1 alkylation takes place at the C3 position of the (3-lactam ring, with entrance of the electrophile R group from the side opposite to the Ar group at C4. The second one (type 2) takes place at the exocyclic carbon directly bonded to the... 

Catalyst preparation and activation conditions are included as parameters of importance in the optimisation algorithm. The preparation and activation procedures are very relevant aspects since minor variations in such conditions would cause major changes to the final phase of the solid and, consequently, to its catalytic properties. Typical preparation variables are promoter precursors, type of impregnation, calcination atmosphere, time and temperature, time and temperature for metal reduction and so forth. 

Further evidence of an indirect channel at low Ex came from the dependence of S on hydrogen coverage 0H [164], and this result is summarised in Fig. 32 which shows S(0H) for Ex = 63 meV ( ) (Ts = 300 K) and E = 13meV ( ) (Ts = 150 K). These correspond to conditions where the direct and indirect channel will dominate dissociation. Indeed precursor type behaviour is observed under the latter conditions, and a linear decrease (characteristic of direct dissociation of hydrogen on many metals) is evident under the former conditions. An observation which was more difficult to reconcile with the suggestion of an indirect channel at low ) on W(10 0) came from the observed absence of any significant surface temperature dependence in Sq. Figure 33 shows ( ) the dependence Sq(T ) on the... 

Further evidence that it is a step mediated channel responsible for the indirect dissociation on the W(1 0 0) surfaces comes from a comparison with the results for the step mediated dissociation on Pt(5 3 3) (Section 3.2) which exhibit very similar dynamical characteristics. A similar mechanism is likely to be responsible for the indirect dissociation channel on Ni(9 9 7) [89] (Fig. 25)., Y0 decays with Ei over the range 0 < Tq(meV) < 150 on all of the metal surfaces studied where steps are suggested to be responsible for indirect dissociation. S0 is also rather insensitive to 7 s in all cases, and, S (0n) exhibits precursor type dependencies. 

The pore volume and specific surface area of carbosils decrease with Cc values independent of pyrocarbon origin (Table 2, Figure 2).16 However, the precursor type as well as the initial pore structure of silica gels, e.g. Si-40 (Table 1) and Si-60 (Table 2), affect the pore characteristics of carbosils (Figures 1 and 2). The narrower the pores of the pristine silica gel, the larger the specific surface area reduction under the same pyrolysis conditions. 

Solution chemistry to form a gel type of precursor type of solvent pH (acid/base content) water content precursor concentration temperature... 

An intermediate position can be seen for pitches that are carbonized under pressure or by addition of elemental sulfur. Because of the high cost of precursor materials of the first group, the selection of precursor type and processing must be optimized carefully (20,32). 

The oxidation of chars prepared from nitrogen-containing precursors has been investigated. Chars produced from the nitrogen-containing compounds acridine and phenanthridine were oxidized at atmospheric pressure at temperatures of 773-873 K. The relative rates of nitrogen and carbon release and the formation of NO have been determined in relation to char nitrogen content and precursor type. 

However, high temperatures are not essential to produce unstable, reactive species in the gas phase under matrix conditions. This is exemplified by investigations of the lower sulfur and selenium fluorides. These can be prepared near room temperature and at low pressure by fast reactions between a solid and a precursor (type a) or between a gas and a precursor (type b) ... 

The porous structure and specific surface of activated carbons are determined by precursor type [13] and pyrolysis parameters, i.e. temperature [14] and heating rate jl5.16]. M y papers are dedicated to the synthesis of active carbons based on lignin-cellulose materials of various types [17]. There are empirical dependencies of texture on thermal treatment parameters for caibon materials from various precursors of plant nature [13,14,18]. Models for cellulose fibers pyrolysis are suggested [15). 

Very recently, Kim and co-workers have reported a new useful radical acylation approach which uses a series of sulfonyl oxime ethers, some of which function as a viable radical Cl synthon. Unlike carbon monoxide and isonitriles, which operate as a radical acceptor/radical precursor type Cl synthon, sulfonyl oxime ethers... 

Type I collagen accounts for approximately 90% of the organic matrix of bone. Type I collagen, a triple helix of two identical a-1(1) chains and a shghtly different a-2(I) chain, is synthesized as a precursor, type I procollagen, containing both N- and C-terminal extensions. ... 

Coking Results Temperature, precursor type, precursor partial pressure, and run duration all significantly affected the amount of coke for Incoloy 800 and aluminized Incoloy 800 coupons and Vycor glass. The... 

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