Stoichiometry control

Initially, the main disadvantage of using single-source precursors was their lack of volatility. However, the advent of ultralow-pressure and vacuum CVD techniques, along with the use of alternative delivery methods which do not rely primarily on precursor volatility, means that many compounds that are essentially nonvolatile can now be considered as potential CVD precursors. A major problem that remains, however, is that of stoichiometry control a molecular precursor containing two (or more) elements in a specific ratio, as desired within the thin film to be grown, will not necessarily deposit those elements in the same proportions. 

Timoshkin AY, Schaefer HE (2002) From parasitic association reactions toward the stoichiometry controlled gas phase synthesis of nanoparticles, A theoretically driven challenge for experimentalists. Chemical Record 2(5), 319-338... 

In experiments carried ont by ns to repeat these two methods, we have been able to deposit CdTe, although stoichiometry control was difficnlt, with Te oc-... 

The feed rate is manually set as required for either pH or reactant stoichiometry control. Soda ash is added to the fourth reactor as sodium makeup. Reactor effluent slurry flows by gravity to the thickener centerwell. Clarified liquor overflows from the thickener to the forward feed hold tank from which it is pumped to the tray tower. A horizontal belt filter is used for further dewatering of the thickener underflow solids. 

The plantwide control deals, mainly, with the mass balance of the species involved in the process. The species inventory can be maintained based on two different principles, namely self-regulation and feedback control. Control structures based on self-regulation set the flow rates of fresh reactants at values determined by the production rate and stoichiometry. Control of inventory by feedback consists of fixing one flow rate in each recycle loop, evaluating the inventory by means of concentration or level measurements, and reducing the deviations from the setpoint by change of the feed rate of fresh reactants. 

An explanation for this extrinsic phase separation is thus to imagine each crystalhte having its unit-cell parameters and stoichiometry controlled by its Mn + Mn + ratio, which ultimately determines whether the HTO phase transforms into to orbitally ordered LTO phase or the charge-ordered LTM one, or not at all (see Figure 5). 

Figure 11.16, is key to the selectivity of the desired product R. Here the reaction stoichiometry controls the yield of R, If the reactant >4 is consumed before B (molar feed ratio of B o A Ygf, greater than 1), the only reaction for the remaining B is the undesirable reaction according to Eq. (11-4) which further converts R, The end result is a reduced yield of R. If, on the other hand, is less than 1, B runs out before A and the conversion of A is limited to low values. 

The role of the solid phase was first developed from electronic theory (10) and then from the concept of valence and stoichiometry control (18, 19). Recent experimental results have shown that one thermodynamic phase, and not a simple gathering or sea" of ions at the surface of the active solid, can be responsible for catalytic activity and selectivity. 

Edmundson, J.T., Jenkins, D.P. and Mortimer,J. Multiple Burner Stoichiometry Control in Innovation in Process Energy Utilisation, IChemE, pp 47-60, 1987. 

Thin films of mixed metal oxides are usually obtained from a mixture of two different kinds of alkoxide precursors. However, this method suffers from problems with stoichiometry control since extensive efforts must be made to control the vapor phase concentration of two precursors with often dissimilar vapor pressures. Also of importance here is the near impossible task of matching rates of hydrolysis/oxidation to give pure , non-phase segregated films, i.e., those having a homogeneous composition and structure. In an effort to solve these problems, research effort has been aimed at single-source precursors, i.e., those containing both aluminum and silicon. 

To achieve proper stoichiometry control, the chamber above the container is closed, evacuated with a mechanical pump, and then filled with C02 which is allowed to flow through the system the boule has a sufficiently high conductivity that it can be remelted without a susceptor by reapplication of radio frequency power. The boule is kept under circulating C02/C0 for 90 min to achieve complete equilibration. The skull crucible is finally lowered out of the stationary power coil at a rate of IS mm/hr. 

Ternary systems, containing three different chemical elements, demand suitable precursors with element ratios corresponding to the material of choice. Further, the stoichiometry control present in the molecular carrier should be preserved during the processing steps, which is not trivial given the ambiguity associated with the structure and composition of intermediate species. Nevertheless, various single precursors tested in the material synthesis provide the proof of principle for the application of molecules-to-materials approach in multi-component ceramics. 

Recently, efforts have been devoted to the fabrication and characterization of PbZri- Ti c03 family thin films for their potential applications in nonvolatile memory devices (See Ref. 17, for example). Partly because of the convenient stoichiometry control during processing, it was found that chemical methods, such as sol-gel and metal organic decomposition (MOD), are superior to physical means in many aspects. To appreciate better the science and technology of ferroelectric thin-film fabrication, it is important to give a brief account of the past efforts and the present status and, it is hoped, shed some light on the future. 

In addition to a lattice-matched substrate, sol-gel epitaxy also requires excellent stoichiometry control of the solution. Most of the demonstrated sol-gel epitaxies were performed at relatively low temperatures. Therefore, molecular-level homogeneity is essential. To appreciate fully the delicate features of sol-gel epitaxy, it is now appropriate to compare sol-gel processing with other thin-film deposition techniques. 

Another potential source of processlble precursors Is the citric acid/ethylene glycol system which has been esiployed previously in the preparation of highly dispersed perovskite, spinel and related complex oxides. This method provides soluble, metal-organic, polymer precursors which have been used for the fabrication of oxide thin films as well as for the production of oxide powders with excellent homogeneity, good stoichiometry control and uniform sizes at relatively low temperatures(13,14). 

It is important to note that, in most cases, the DP remained low and did not complement the previous studies on the SPG protocol referred to above [6]. A logical explanation for this discrepancy is as follows. Dendronized macromonomers for all the discussed polymers are relatively complex, requiring multistep synthetic procedures, and this limits both the amounts and the level of purity of the prepared macromonomers. As monomer purity and the scale of the reaction are two key factors (to ensure sufficient stoichiometry control) in SPG [5], it is reasonable to assume that those polymers with higher molar masses would be accessible if additional efforts had been undertaken to satisfy these stringent requirements. On the other hand, an effective conjugation length for such polymers is reached already at short chain lengths hence, it is understandable that further efforts towards molar mass improvement were not pursued. 

Chemical vapor deposition High deposition rate, good stoichiometry control, low point defect concentration, high conformity and scalability Low precursor stabiUty, expensive... 

Stoichiometry control cannot be better than the accuracy of the determination of nonstoichiometry. In the hterature the nonstoichiometry of 123-Oj has been determined... 

Qv.ifuel and Qv i,ox are the stoichiometry controlled volumetric flow rates of fuel and oxidant, respectively, unless they are smaller than their minimum flow rates, Qv,fuel,min and Qv,ox,min-... 

The difficulty with stoichiometry control of multicomponent materials is possible... 

Excellent conformal coating and thickness seating Precise stoichiometry control... 

Centi G., Fornasari G. and Trifirh F. (1984). On the mechanism of n-butane oxidation to maleic anhydride Oxidation in oxygen-stoichiometry-controlled conditions, J. Catal., 89, pp. 44-51. 

---