Metal alkoxides volatility

Solution Deposition of Thin Films. Chemical methods of preparation may also be used for the fabrication of ceramic thin films (qv). MetaHo-organic precursors, notably metal alkoxides (see Alkoxides, metal) and metal carboxylates, are most frequently used for film preparation by sol-gel or metallo-organic decomposition (MOD) solution deposition processes (see Sol-GEL technology). These methods involve dissolution of the precursors in a mutual solvent control of solution characteristics such as viscosity and concentration, film deposition by spin-casting or dip-coating, and heat treatment to remove volatile organic species and induce crystaHhation of the as-deposited amorphous film into the desired stmcture. 

Mixed liquor volatile suspended solids (MLVSS), in biological waste treatment, 25 829-830 Mixed-metal alkoxides, titanium-vanadium, 25 100... 

Herrmann WA, Huher NW, et al (1995) Volatile Metal Alkoxides According to the Concept of Donor Fimctionalization. Angewandte Chemie-International Edition in English 34(20), 2187-2206... 

Perhaps the best studied group of titanium(IV) complexes is the alkoxides. The metal alkoxides generally have received a great deal of attention because of their ease of hydrolysis and reactivity with hydroxylic molecules, and their tendency to increase the coordination number of the metal which is opposed by the steric effect of the alkyl group. These properties result in materials, the characteristics of which range from polymeric solids to volatile liquids. The definitive review of this area is that by Bradley. ... 

On the other hand, to accelerate alcoholysis of NaH and KH in solutions of benzene or THF [22.1] - [2.22] - ctyptands are used (which bind alkaline metal into a rather stable chelate complex) [1004], Quite stable, volatile per-fluorotert-butoxides were first obtained in reactions of LiH or NaH with (CF3)3COH they distill at atmospheric pressure at 218 and 232°C, respectively [467] (the application of metals would presumably lead in this case to condensation of Wurtz type). Li and Na hydrides are used as cheaper than metal raw materials for production of the corresponding metal alkoxides. In particular it has been suggested that the equipment used in production of MH could be cleaned from its residue by the mixture ofEtOH and the aromatic hydrocarbon (40 to 60% by volume). After hydrogen evolution is completed the solvent is eliminated under vacuum at < 90°C the residue is MOEt with the content of the main product > 98% [342],... 

It may thus be concluded that application of metal alkoxides for preparation of PZT-based thin films in many aspects may be nowadays considered to be a routine technique widely used in many laboratories. The research work is presently mostly focused on electrical and microstructural aspects of these films rather than on chemical routes of their preparation. Irrespective of the technique for the precursor preparation, when 10% excess of Pb is introduced (to prevent loss of lead due to the PbO volatility) the ferroelectric perovskite PZT films are obtained after annealing at about 700°C, demonstrating nonlinear properties of the level acceptable for most of the desired applications. Their typical microstructure is presented in Fig. 10.3. 

The properties of Ge(OR)4 allow them to be considered more likely to be the esters of an inorganic acid than metal alkoxides these are colorless volatile liquids, containing monomeric tetrahedral molecules. The solid crystalline form is known only for R = Bu, OC6Hnc, and also 2,6-substituted phenoxides. All the members of the Ge(OR)4 homologous series are characterized by thoroughly determined physical characteristics — density, refraction index, surface tension, viscosity (and calculated parachor values), dipole moments in different solvents [222, 857, 1537] (Table 12.9). The results of the investigation of vapor pressure, density, viscosity polytherms, and so on. permitted rectification for the preparation of samples of high purity for sol-gel and MOCVD applications [682, 884]. 

Chemical vapor transport is used to synthesize thin films of materials on a substrate. The film can be the same composition as the substrate or different. In order to proceed with chemical vapor transport, the constituent elements of the compound to be deposited as a thin film must be brought into the vapor phase. Given that many of the thin films of commercial importance involve elements with little or no practical vapor pressure, a lot of attention has been focused on preparing volatile compounds that contain the elements needed in thin-film preparations. Most chemical supply companies carry these compounds as stock items. The major classes of compounds include metal alkyl, metal carbonyl, metal alkoxide, metal 3-diketonates, and organometallics. Examples of each are given in Table 3.1. 

Group 2 metal alkoxide compounds are potentially suitable as CVD precursors, especially because hydrolysis and subsequent thermally induced dehydration are likely to lead to complete removal of the supporting ligands. There are examples of Group 2 metal alkoxide compounds that are claimed to have suitable volatility for CVD, but to our knowledge these have not been used successfully. However, Group 2 metal alkoxide compounds have found widespread application in solution routes to metal oxide materials, an area that has been reviewed.1... 

In this class of compounds the nature of the alkyl group appears to determine the polarizability of the alkoxy group. The volatility of the compounds depends on the alkyl group. Groups such as neopentyl, f-butyl and triphenyl carbinol cause covalency in the metal alkoxides and hence to their volatility. 

The Nextel series of fibers produced by the 3M Company consists of a variety of aluminosilicate fibers. These contain mainly AljOj+SiOj and some B2O3. The compositions and properties of Nextel 312, Nextel 440, Nextel 550, Nextel 610, and Nextel 720 fibers are given in Table 6.3. Nextel 610 is, of course, the a-alumina fiber described above. The sol-gel manufacturing process used by 3M Co. has metal alkoxides as the starting materials. Metal alkoxides are M(OR)j -type compounds where M is the metal and n is the metal valence, and R is an organic compound. Selection of an appropriate organic group is very important. It should provide sufficient stability and volatility to the alkoxide so M-OR... 

The general properties of the significantly covalent behavior of metal alkoxides (both homo- and heterometallic), in spite of the polar character of the M5+—O5- bond, were already dealt with in some detail (6). The effect of steric and inductive factors on the extent of polarization of the M6+—O5- bond as well as the consequent degree of association and volatility can be exemplified by the boiling points (under 1-mm pressure) and the observed degrees of their association (given in parentheses) by the three isomeric butoxides of zirconium Zr(0-n-Bu)4 ( 250°C 3.5) Zr(0-sec-Bu)4 ( 150°C 2.0 Zr(0-/-Bu)4 ( 50°C 1.0). However, the similarities in the molecular association of the neopentyloxides of Ti, Zr, and A1 to the secondary rather than primaiy amy-loxides have been adduced to indicate the higher predominance of steric rather than inductive factors in the above directions. Similarly, the insolubility and... 

Volatility, solubility, and stereochemistry of metal alkoxides, [M(OR)J, tend to be limited both by their oligomeric nature and the steric demand of the alkoxide moiety, as a consequence of the formation of /t2 OR//x3-OR bridges... 

Although metal alkoxides proved to be excellent precursors for the preparation of ceramics by the sol-gel process, their applications in the MOCVD technique are not as widespread, in spite of the attractive features of their volatility and tendency to decompose to the metal oxides. Both of these desirable prerequisites appear to be enhanced, as discussed earlier, by choosing more ramified alkoxide groups. Despite these attractive features, the MOCVD applications of metal alkoxides appear to be limited by (a) their commercial nonavailability, (b) handling difficulties arising from their ready hydrolyzability, and (c) the lack of a clear understanding of their decomposition pathways, which could lead to improvement(s) in the purity of the deposited material. 

The syntheses, physical properties, and molecular structures of alkoxides and aryloxides have been discussed in CCC (1987).161 The alkoxides of scandium and yttrium were reviewed in CCC (1987).1 There have been more recent developments in this area and the impetus for this chemistry has been the developments in materials research. Metal alkoxides and /3-diketonates can be used as precursors for oxide and nonoxide thin films.162 The stable M—O bond and the volatility of the metal alkoxides are important features of this area of chemistry. This has lead to more research in this area particularly in synthesis, NMR, and X-ray crystallography. 

Several rare earth element oxides are components of high 7 superconducting materials. Their preparation is discussed elsewhere in this book (see Chapter 2). In general, rare earth metal oxides can be obtained by the chemical vapor deposition of the appropriate metal /S-diketonates or carboxylates [96]. Volatile metal alkoxides also should be potentially useful precursors for the preparation of rare earth oxides by CVD. Although several volatile rare earth element alkoxides have been reported in recent years [97-101], detailed information concerning their decomposition behavior is not available at this time. 

---