2 Homometallic Alkoxides

674 Alkoxo and Aryloxo Derivatives of Metals
been obtained by hydrolysis of zirconium alkoxides with hydrogen peroxide and
nitric acid. 26
3 CERAMICS AND GLASSES
In recent years there has been considerable interest in using metal alkoxides as molecular
precursors for preparing ceramic oxide materials and speciality glasses. In particular
the sol–gel process has been applied because it provides a relatively low temperature
mechanism for producing solid oxide materials in contrast to the traditional “grind and
bake” method.
A comprehensive account of the sol–gel technique has recently been published by
Brinker and Scherer 27 who discuss the advantages and disadvantages of the technique
together with references to a wide range of applications. Essentially the sol–gel process
involves the controlled hydrolysis of the metal alkoxide in a suitable organic solvent
(water miscible) to form a gel from which solvent is removed leaving finely divided
oxide particles which can be compacted and heated to form a ceramic or glass. Most
metal alkoxides undergo extremely rapid hydrolysis and chemical additives are often
used to control the rate of hydrolysis. These additives are usually polyols, carboxylic
acids or ˇ-diketones. By contrast the hydrolysis of tetra-alkoxysilanes Si⊲OR⊳4 is so
slow that acid or base catalysts are required to accelerate the process.
The sol–gel process is particularly useful for nonvolatile metal alkoxides which
cannot be used in the MOCVD process, and for forming heterometal oxides, where
control of stoichiometry is important. Since 1980, numerous excellent reviews have
been published on the sol–gel chemistry of metal alkoxides 1–3,28–38 to which readers
are referred for detailed bibliography. Numerous publications have appeared detailing
the preparation of binary metal oxides and heterometal oxides using the sol–gel technique
and a few representative examples only are given here.
The binary metal oxides Al2O3, 39 Y2O3, 40 TiO2, 41 TiO2, 42 V2O5, 43 Nb2O5 44 and
Ta2O5 45 have attracted considerable interest covering a range of applications. Although
tetra-alkoxysilanes are not metal alkoxides they cannot be ignored because they are
precursors for the preparation of various forms of silica and as components for many
glasses and ceramics. The acid catalysed hydrolysis of tetra-alkoxysilanes is an essential
stage in the sol–gel synthesis of quartz and various glasses, and a host of publications 46
have appeared covering a variety of applications.
The requirement for inorganic electronic materials prepared at low temperatures has
stimulated enormous activity in the heterometal oxide field. The sol–gel technique
using metal alkoxides has been especially effective in producing ferroelectric, piezoelectric,
and pyroelectric materials, e.g. BaTiO3; 47 LiNbO3; 48 LiTaO3; 49 PbTiO3; 50
Pb⊲Zr,Ti⊳O3⊲PZT⊳; 51 (Pb,La) ⊲Zr,Ti⊳O3⊲PLZT⊳; 52 Pb⊲Fe,Nb⊳O3; 53 and Pb⊲Mg,Nb⊳O3. 54
Considerable attention has been devoted to the preparation of high Tc superconductors
based on the heterometal copper oxide systems. Most reports have been concerned
with the deposition of films of the “1,2,3” superconductors YBa2Cu3O7 x on a variety
of different substrates. 55
Other superconducting materials deposited by the sol–gel process using metal
alkoxide precursors are the “1,2,4” compound YBa2Cu4O8, 56 bismuth strontium
calcium copper oxides, 57 bismuth lead strontium calcium copper oxide, 58 and thallium