Nanocrystalline yttrium aluminium garnet (YAG, Y3Al5O12) and Ce-doped YAG ceramics were synthesized by two ‘soft chemistry' sol–gel processes using (i) mixtures of 
inorganic salts or oxides and (ii) mixtures of alkoxides of the respective elements. In the first approach the metal ions, generated by dissolving metal oxides or nitrates in acetic acid and/or water, were complexed by ethylene glycol to obtain the precursors for pure and doped YAG samples. In the alkoxide route monolithic gels were obtained by hydrolysis and condensation of a compositional mixture of Al, Y and Ce alkoxides in a PriOH solution. The molecular level mixing and the tendency of partially hydrolysed alkoxide species to form extended networks of cross-linked metal centers facilitates the structure evolution thereby lowering the crystallization temperature, in the latter case. The X-ray diffraction (XRD) patterns of the ceramic sintered at 700 °C were identical with the stoichiometric YAG composition which is the lowest temperature reported for the synthesis of crystalline and single phase Y3Al5O12 while well developed YAG phases in the non-alkoxide synthesis were obtained only at 1000 °C. Cerium doped YAG powders (CeO2 + Y3Al5O12) were synthesized using [NH4]2[Ce(NO3)6] (8 mol%) and [Ce3(OBut)9(ButOH)2] (5 mol%), as dopants. A homogeneous distribution of cerium in the YAG lattice was achieved in both cases. The thermal behaviour, phase transformations, composition and microstructural features in the gels and polycrystalline samples were studied by TG/DTA, XRD, FT-IR, solid-state 27Al MAS NMR spectroscopy, SEM, TEM, energy dispersive X-ray analysis and high resolution electron microscopy studies. The quality of the resulting products (homogeneity, crystallization temperature, grain size, grain size distribution, etc.) and economical aspects (synthetic skill, cost of precursors, etc.) of the two approaches are discussed.

Low temperature synthesis of nanocrystalline Y3Al5O12 (YAG) and Ce-doped Y3Al5O12via different sol–gel methods

Sol-gel synthesis of nano-sized BaTiO3, BaZrO3 and BaTi05Zr05O3 ceramics using alkoxide and semi-alkoxide routes has been investigated and the pervoskites obtained have been compared with respect to crystallisation temperature, crystallite size and compositional purity Heterometal alkoxides containing two (for BaTiO3 and BaZrO3) and three (for BaTi05Zr05O3) different metals were used as single-source precursors in the alkoxide route while semi-alkoxide synthesis was performed by reacting barium hydroxide or acetate with Ti and/or Zr alkoxides Semi-alkoxide synthesis also produces stoichiometric and phase-pure oxides, however, at temperatures higher than 1000°C At temperatures below 1000°C, BaCO3 and small amounts of other undesired phases (eg, BaTi2O4) were present in the oxides derived from semi-alkoxide synthesis Thermal behaviour, studied by TGA/DTA measurements, shows that thermal decomposition occurs in three major steps and depends on the educt composition and the synthesis route Among alkoxide derived powders, crystalline BaTi05Zr05O3 phase is formed at 400°C while complete crystallisation of BaMO3 ceramics occurs around 600°C The cubic to tetragonal phase transition for BaTiO3 is clearly observed at relatively low-temperature of 800°C The stoichiometry and phase homogeneity of the obtained powders were demonstrated by energy dispersive X-ray analysis and powder diffractometry The averaged crystallite size of the obtained nano-ceramics was evaluated using the FormFit programme SEM and TEM observations revealed a high microstructural uniformity

Sol-Gel Synthesis of Nano-Scaled BaTiO3, BaZrO3 and BaTi0.5Zr0.5O3 Oxides via Single-Source Alkoxide Precursors and Semi-Alkoxide Routes

Nanocrystalline aluminate spinels with cobalt, nickel or copper as bivalent cations have been prepared in different particle sizes using single-source heterometal alkoxides of the type [M{Al(OR)
 4
}
 2
] (M
 II
=Co, Ni, Cu; R=Pr
 i
, Bu
 t
) in a microemulsion assisted sol-gel process. The compatibility of the metal stoichiometry in the precursor molecules with respect to the spinel requirement was established by elemental, spectroscopic and single crystal X-ray diffraction analyses. Synthesis of CoAl
 2
O
 4
 using both single- and multi-component routes gave different results: whereas the single-source precursor approach yielded monophasic high purity spinels, phase contamination was evident in the diffractograms of ceramics obtained using a mixture of individual components. The alkoxide precursors were hydrolysed in a homogeneous dispersion of colloidal water droplets in a non-aqueous phase. The diameter of water droplets in the microemulsion can be tuned, inter alia, by varying the hydrophilic chain length of the surfactant molecules which influences the size distribution of nanoparticles in the MAl
 2
O
 4
 ceramics. This parameter is used to obtain nano-spinels with particle sizes in the range 5-45 nm and reveals a qualitative correlation between the initial droplet size and the crystallite size of the resulting spinels. The TG-DTA measurements showed the thermal decomposition to be a three step process with crystallisation of the spinel phase occurring around 900 °C. 27
Al MAS NMR spectra reveal the inverse nature of the obtained spinels. XRD, TEM, SEM, EDX, FTIR and UV-VIS spectral studies were used for characterisation of the powders.

Microemulsion mediated sol-gel synthesis of nano-scaled MAl2O4 (M=Co, Ni, Cu) spinels from single-source heterobimetallic alkoxide precursors

The use of molecular Single Source Precursors (SSP) containing all elements necessary for the formation of the final solid material in CVD (Chemical Vapor Deposition) or sol–gel routes is classified with respect to the control of the reaction. Three different cases have been distinguished: In SSP-I the stoichiometric ratio of the elements in the precursor corresponds to the ratio required in the solid whereas the number and kind of side products of the reaction (ligands) are not controlled. In SSP-II, not only does the ratio of the elemental components of the precursor have the correct stoichiometry but also the ligands are chosen in such a way that a cascade of reactions is induced at moderate temperatures leading to few products and therefore low contamination of the solid material. In SSP-III, the precursor decomposes to form at least two solid phases with perfect stoichiometric and structural control. All three processes allow the obtention of nanocrystalline materials.

Molecular precursors for (nano) materials — a one step strategy

New perspectives in the tailoring of hetero (bi- and tri-) metallic alkoxide derivatives

While exploring new and effective routes to composites of ceramics and metals we have used the heterometallic alkoxide barium bis[tert-butoxistannate(II)], BaSn2(OtBu)6, as molecular precursor in metalorganic chemical vapour deposition (MOCVD) techniques. The solid material obtained is characterized by highly dispersed metal in a ceramic matrix as determined by electron microscopy, electron spectroscopy and X-ray diffraction and therefore has peculiar chemical and physical properties. Furthermore the material is exclusively built of globular particles. In the one-component CVD process at low pressure heating is achieved by a microwave system. To yield the new solid composite we have designed a CVD apparatus as shown in Fig. 1 which is especially useful for alkoxide precursors. The decomposition takes place in the temperature range 300-500 °C. This temperature is sufficient to decompose the precursor completely and is low enough to prevent carbide formation (the highest C-content found is 5% by photoelectron spectroscopy (XPS)). The precursor barium bis[tert-butoxi-stannate (II)], obtained as described [1] and recrystallized from toluene, is incorporated into the system and the substrate is heated by induction to the temperature of decomposition which is 350 °C for Ba Sn2(OtBu)6. The temperature of the outer furnace is successively raised until a continuous flow of precursor gas is reached which can be monitored by the unchanging mass spectrum of the reaction gases. If

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New metal-ceramic composites grown by metalorganic chemical vapour deposition

Metal alkoxides of subvalent Ge, Sn and Pb, M(OtBu) 2 , as well as their mixed metal alkoxides of the general formula M'M z (OtBu) 6 (M' = Ca, Sr, Ba; M = Ge, Sn, Pb; tBu = tert-Butyl) have been found to be excellent precursors for CVD processes. When heated to 350 - 450°C they decompose to yield iso-butene and tert-butanol as the gaseous products. The solid residue, which can be easily grown on any surface, consists of two different phases: a metallic phase and a metal-oxide phase (M . MO z or M MM'O 3 ). Electron microscopy, X-ray diffraction, elemental analysis, EDX and XPS (ESCA) spectroscopy have been used to characterize the composites. The reaction seems to proceed quantitatively and the driving force is the disproportionation of the low valent main group element following reaction: 2 M 2+ → M 0 + M 4+ (M = Ge, Sn, Pb). The two phases in the composites form particles of a few nanometer diameters.The core of the spheres consists of metallic elements (Ge, Sn, Pb) wrapped-up in a metal oxide matrix. Another precursor, [AlH 2 OtBu] 2 , developed by our group, on heating in a CVD process gives a nano-scaled composite of composition Al.Al 2 O 3 . Again the exact stochiometry of the composite as well the reproduction of the surface is excellent.

S. J. Kneip

Papers

New metal-ceramic composites grown by metalorganic chemical vapour deposition

Metal/metal oxide composites by single source precursor CVD