Decomposition of precursors

The mechanisms, rates, and reactivity of the products markedly affect the success or failure of a synthesis. The calcination step is usually at atmospheric pressure in a suitably oxidizing, inert, or reducing atmosphere. Even under these straightforward conditions, various effects and reaction paths are still possible. 

The atmosphere initially surrounding the sample will change during the course of the decomposition because of the buildup of product gases or the depletion of the reactive gas. The equilibrium decomposition temperature for reversible reactions is determined by the partial pressure of the product. The temperature at the reacting interface, therefore, must increase during the decomposition as the outward 

The ill-defined nature of the temperature at the reaction interface is exacerbated by the enthalpy of the specific reaction. This energy, which is evolved or absorbed by the reaction, is generally large compared with the heat capacity of the reactants and products. Hence, the actual temperature at the reaction interface may differ substantially from that of the nearby temperature sensor. Consider the oxidation of carbon monoxide that occurs during the decomposition of oxalates in air. The thermocouple near the sample may indicate 300 °C while the actual specimen is glowing a bright red (indicating at least 600 °C). Conversely, for the more typical endothermic decomposition it is necessary to supply heat to maintain the reaction. 

The decomposition of calcium carbonate. Equation 8.4, is a good illustration of these effects  

The surface area of the product will be at a maximum immediately after each stage in the decomposition, as would be expected. The particles of the product powder grow and the porosity tends to close as the temperature is raised further. These changes are measured by a decrease in surface area and can also be confirmed by X-ray diffraction. Often, one assumes spherical particles and a particular density in order to calculate an effective average particle size from the surface area according to 

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It should be emphasized that citric acid is not the only possible acid employed in Pechini-type syntheses. Other polybasic carboxylic hydroxy acids (malic, tartaric, hydroxyglutaric, etc.) and polybasic carboxylic acids (e.g., succinic) have been probed in Refs. [4, 13-16], As far as amino acids are concerned, glycine seems to remain the only representative [13, 14]. However, the choice of each particular organic acid has never been justified, and no comparative studies are performed in order to find possible dependencies of the process (ability to form a sol, a gel, or a resin, easiness of thermal decomposition of precursors, etc.) on the steric factors, specifically, on the number of hydroxy and carboxylic groups in the molecule of an acid, as well as on the length of its carbon skeleton. 

Unfortunately, these reactions and those between tin atoms and various small molecules that have been reported (see Takahashi et al. [50] and references therein) are not expected to play a significant role in typical CVD processes. One reaction that very likely is important to the decomposition of precursors of the form RSnCls such as MBTC is... 

It was concluded that the catalyst lifetime is a function of the catalyst loading and does not relate to its way of synthesis [321]. With larger loadings, catalysts are active for a long time before they need reactivation. With regard to lifetime and activity, the four catalysts were ranked as follows wet impregnation incipient wetness > UV decomposition of precursors > sputtering. In case of loss of performance, two... 

During the decomposition of precursors as mentioned above, additional reactions may occur. With hydrazinium compounds reduction may well take place [40] ... 

Gao et al. tried two methods to s)mthesize EuS NPs based on thermal decomposition of Eu(Phen)(Ddtc)3 precursors. The direct decomposition of precursors in a nifrogen flowing furnace at 430 °C produced NPs in the range of 50-120 nm, while the decomposition in oleylamine at 200 °C... 

A rather novel route to metal nanoparticles of increasing interest and relevance is the soft displacement of ligands from organometalUc complexes with zero-valent metal atoms. An elegant way to do this is the decomposition of precursor molecules by CO and A prominent example is... 

StepU Transfer of Precursor to the substrate , (Step 2 and 3) Adsorption and Decomposition of Precursor,followed by... 

In the RTA process, large amount of gases are produced by the decomposition of precursor and fuel (i.e. urea and nitric acid), which leads to a foam structure. Figure 1 illustrates the photograph of so-produced samples. The ZrO foam is observed to brim over the container. 

These indicate that nitric acid plays important roles in the formation of the foam. Obviously, the foam structure and high BET surface area should be attributed to the synergetic effect of the decomposition of precursor and the combustion of fuel[4]. 

So it is not possible to determine whether the Murchison values are consistent with an abiotic source as far as N isotopes are concerned. However, the N sources of a-aminoisobutyric acid and sarcosine are clearly distinct. Glycine is a common decomposition product of other amino acids, so its 15N-depletion may reflect a kinetic isotope effect during decomposition of precursor amino acids. Alternatively, gylcine, which is a relatively simple compound, may have formed in interstellar space (Snyder 1997) from isotopically distinct precursor(s) prior to the synthesis of more complex amino acids on the Murchison parent body. At present there are insufficient data to conclude whether the stable isotopic composition of amino acids in the Murchison meteorite reflects the range associated with biosynthetic processes on Earth. 

The life-time of the four types of catalysts depends on the catalyst loading, which is related to the preparation route [41]. The larger the loading, the longer the catalysts could be used before reactivation. The four catalysts had the following sequence of life-time and activity wet-impregnation > incipient wetness > UV-decomposition of precursors > sputtering. 

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