Carbon-Free Precursors

Another possibility is also to use carbon-free precursors, as follows. 

It appears that if homogeneous gels or coprecipitates can be obtained using carbon-based chemicals, they do not yield much better ceramics than conventional solid state synthesis. This is primarily attributed to the resulting carbonate-containing materials whose 

Nitrate solutions are also suitable for aerosol flow production of fine powders (50)(51). This technique yields small spherical particles that should be suitable for good sintering. However, because of the resulting small size of the superconducting grains, ceramics may have very poor flux expulsion properties if they are not thoroughly sintered (50). 

Hydroxides Precipitation of hydroxides may be done through addition of a base to an aqueous solution of salts. Yttrium and copper hydroxides have a similar precipitation range but for the less polarizing Ba cation, very high pH is necessary to start the precipitation. However, two processes have been described that increase the pH of a solution of nitrates in water by addition of either NaOH (52) or tetramethyl ammonium hydroxide (53). It appears that at 60°C and 

There have been many solution techniques used to synthesize the superconducting perovskite-related phases, but no experiment has really achieved a substantial improvment of the synthesis, compared to conventional solid state reaction. 

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By using carbon-free precursors, the synthesis temperature could be significantly lowered but this did not result in any improvement of the properties since the particle size remained too small. 

The carbon-free precursor [In(NH3)3]n is useful in this regard. In a two-step procedure it is quantitatively decomposed to InN ... 

For the hydroformylation, (PPli j) Rli( H)(CO) with host 11 was used as the catalyst. An excess of PPhj (stemming from the catalyst precursor) was needed to avoid isomerization, as was found when phosphine-free precursors were used (at the concentrations used even bidentates should be added in excess to prevent substantial exchange with carbon monoxide). Linear to branched ratios of 2 1 were obtained and no isomerized alkene could be detected. These results are similar to those obtained by Kalck and coworkers [41]. As expected, catalysis for 11 is slower than that for (PPh3)3Rh(H)(CO) as the host is a bidentate phosphine catalysis with (PPh3)3Rh (H)(CO) strongly depends on the concentrations of rhodium and PPh3 and comparison of the rates of the two systems does not make sense. 

CVD is an established method for the synthesis of CNTs [59, 60]. Supported transition metals that catalyze the growth of CNTs, such as iron, nickel, or cobalt, are situated in a tubular reactor, and CNTs are grown at elevated temperature on the surface of the catalyst particles by decomposition of a carbon-containing precursor. The catalyst particles have to be removed by chemical treatment /washing in order to obtain a metal-free final product. 

Sen, R. Govindaraj, A. Rao, C. N. R. Metal-filled and hollow carbon nanotubes obtained by the decomposition of metal containing free precursor molecules. Chem. Mater. 1997 9. 207B. 

There are several ways in which to adjust the composition of the solution. For one, different elemental precursor chemicals can be used. The choice of precursors depends on their solubility in a given solvent or solvent system. Similarly, the choice of solvent is primarily dictated by the requirement to fully dissolve the precursor. Furthermore, carbon-free solvents, such as water, and low carbon residue (soot) solvents, such as methanol, may be used to control the amount of elemental carbon in the flame. The elimination of impurities can also be extremely significant in certain applications. For example, sulfur and sodium can be detrimental to many coatings in such cases solvents and precursors must be free of these and other common impurities. 

Samoilenkov grew carbon-free high-quality films of /3-PbO on SrTiOs and a-PbO on MgO, below 400 °C substrate temperatures, using Pb(thd)2 (23c) as precursor in the presence of water vapor and oxygen. Addition of water resulted in a fivefold increase in growth rate at 300 °C, similarly to other metal enolates mentioned above... 

Van Cappellen and Berner (1991) smdied the growth kinetics of FAP in seeded precipitation experiments using carbonate-free solutions. The growth of FAP was inhibited by Mg " " at concentrations typical of marine pore waters and enhanced by H" " ions in the pH range of 7-8.5. At moderate degrees of supersaturation, a precursor phase similar in composition to octacalcium phosphate formed on the FAP... 

The tri-azide [Ga(N3)3]oo is an ideal carbon-free (no N—C or Ga—C bonds) precursor to GaN . In its native form it can explode under the appropriate conditions. However, it is remarkably stable when complexed to Lewis bases. Although these compounds do not sublime, they can be used to form GaN in condensed phases ... 

In general, for the deposition of carbon-free films, it has proven prudent to focus growth on precursors which are comprised of saturated pendant groups on the central element. The prevailing concept is that only sp carbon atoms should reside in the substituents for achievement of the objective of carbon-free depositions. This correlation originates in the feeling that sp and sp carbon atoms are more closely related in their... 

Bu 3Ga-Sb(Pr )3 as single-molecule precursors for the synthesis of carbon-free GaSb nanocrystals and whiskers. ... 

E. R. Camargo, M. Popa, J. Frantti, and M. Kakihana. Wet-chemical route for the preparation of lead zirconate An amorphous carbon- and halide-free precursor synthesized by the hydrogen peroxide based route. Chem. Mater, 13 3943-3948, 2001 E. R. Camargo, J. Erantti, and M. Kakihana. Low-temperature chemical synthesis of lead zirconate titanate (PZT) powders free from halides and organics. J. Mater. Chem., 11 1875-1879,2001... 

Ozen M, Mertens M, Luyten J, Snijkers F, D Hondt H, Cool P (2012) Hydrothermal S3mthesis of carbonate-free submicron-sized barium titanate from an amtnphous precursor S3mthesis and characterization. Ceram Int 38 619-625... 

Formation of gaseous carbon black precursors at high temperature. This involves dehydrogenation of primary hydrocarbon molecular species to atomic carbon or primary free radical and ions which condense to semisolid carbon precursors (or polynuclear aromatic sheet) and/or formation of large hydrocarbon molecules by polymerization, which then is dehydrogenated to particle precursors. 

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