Info

Figure 5. XRD pattern of MgAl2O4/MgO nanocomposite obtained from [Mg2Al3(OPr')13] precursor.

building units are intermediates between the extended network and the atoms forming it. Attention is paid to the connectivity between the elements that should be present in the final material after all of the processing steps have been performed. In other words, the molecular precursor for MDN provides intermediate control of the process of formation of large structures.

Some of the salient features and promises of the single molecular source approach are (i) the possibility of tuning the metal and organic contents in a molecule according to the requirements of the material; (ii) low decomposition/ crystallization temperatures, since the metals are already mixed on a molecular scale and no diffusion of ions is necessary; and (iii) ultrahigh purity of the resulting ceramics or composites because of the neat decomposition processes that can be tuned by the appropriate ligand elimination mechanism. In addition, the use of a single molecular species avoids phenomena like preferential hydrolysis or crystallization during solution processing and selected pyrolysis of one of the constituents in thermolysis reactions, which are typical features of a multisource synthesis. A large number of chemical compounds belonging to different organometallic groups have been exploited to obtain solid materials of different compositions. The main classes of precursors used in (nano)material synthesis are presented in Table 5.

One of the major advantages of using metal-organics in processing new materials is the flexibility of varying both the organic and metallic parts of the molecular source. In the case of metal alkoxides, a common problem is their propensity to expand the coordination number at the metal center, either by intermolecular association (oligomerization) or by reaction with adventitious water or oxygen [45, 49-51, 322-326]. This adversely affects their properties, such as volatility, which is an important feature for their easy transport in the gas phase. A simple approach to preventing this is to modify the alkoxide precursor with chelating or donor functionalized ligands, that produces species with coordi-natively saturated metal centers. Jones et al. have shown that dimeric Nb and Ta alkoxides can be modified by alkoxide groups containing one donor function, such as dimethyl

Hydrides/halides Nitrates

Oxalates, acetates

Alkyls

Carbonyls

Cyclopentadienylides

Amides Silanes Silazanes ß-diketonates

Alkoxides Siloxanes

Thiolates (selenolates, tellurates) Thiocarbamates

M(NOj)J yH2O M(HOOCCOO)x, M(CH3COO)x MRx, MRxRy

M(CO)x, (CO)xMLy, L = donor ligand M(C5H5)x, (C5H5)M(CO)x Ry,

MR,(NH2)y, M(NR2)x, (N3)xM(RN)y RxSiLy, L = OR, halide

M(j3-dik)x, ß-dik = acac, dpm, tmhd, od, hfac, fod

M(OR)x, LM(OR)x, L = R, O, OH, halide m(oSíR)x, R3Si-O-MRx, M(R)x(OSiR3)y m(eR)x , E = S, Se, Te

Me, Et, 'Pr, 'Bu, n Bu, 'Bu, Ph, and substituted alkyl or aminoethoxide (OCH2CH2NMe2), to obtain monomeric species (Scheme 5) with a 10-fold increase in the vapor pressure (Ta2(OEt)10, 0.075 torr/108 °C; Ta(OEt)4(OCH2CHr NMe2), 0.75 torr/108 °C) [327]. Furthermore, multidentate ligands can be used to modify the reactivity of alkoxide precursors, for example, by decreasing the number of hydrolyz-able (terminal) ligands. The modification of ligand sets not only alters the physical and chemical properties of the precursor systems, but also seems to have a pronounced influence on the properties of the resulting materials. Boyle et al. have investigated the densification behavior of TiO2 films obtained with the use of various carboxylic acid-modified titanium alkoxides in a sol-gel process. The structure of the Ti-O framework depends on the steric bulk of the lig-ands and the Ti(OR)4/carboxylic acid ratio [328]. The results indicate that small nuclearity clusters favored fully dense materials, which illustrates the relationship between precursor structure and final film properties. If the modification of precursor is achieved with polymerizable organic ligands such as methacrylic acid or methyl methacrylate, clusters capable of undergoing polymerization or cross-linking reactions are obtained. Such organically modified clusters entail a synergetic combination of the properties typical of each of the constituents, giving rise to the class of inorganic-organic hybrid materials [329].

OEt OEt

OEt Me Me

Scheme 5.

0 0

Post a comment