AG Calculations and Reaction Feasibility

Chemical thermodynamics is concerned with the interrelation of various forms of energy and the transfer of energy from one chemical system to another in accordance with the first and second laws of thermodynamics. In the case of CVD, this transfer occurs when the gaseous compounds, introduced in the deposition chamber, react to form the carbon deposit (and by-products gases). 

It should be noted that the negative free-energy change is a Veilid criterion for the feasibility of a reaction only if the reaction as written contains the major species that exist at equilibrium. 

The free energy of formation is not a fixed value but varies as a function of several parameters which include the type of reactants, the molar ratio of these reactants, the process temperature, and the process pressure. This relationship is represented by the following equation  

Zj = stoichiometric coefficient of species i in the CVD reaction (negative for reactants, positive for products) 

By definition, the free energy change for a reaction at equilibrium is zero, hence  

It is the equilibrium conditions of composition and activities (partial pressure for gases) that are calculated to assess the yield of a desired reaction. 

A demonstration of the feasibility of a reaction is illustrated in the following example regarding the formation of titanium diboride using either diborane or boron trichloride as a boron source, as shown in the following reactions  

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