Wet-chemistry processing of powdery raw material for high-tech ceramics
© Trusova et al; licensee Springer. 2012
Received: 19 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
The purpose of this study was to develop wet-chemistry approaches for the synthesis of ultradispersed and mesoporous metal oxide powders and powdery composites intended for usage in the production of ceramic materials with desired properties. The focus is on the development of template synthesis of mesoporous metal silicates as well as obtaining nano- and subnanopowders by a modified sol-gel technique and template methods. Families of mesoporous (2 to 300 nm) metal silicates and nano-oxides and subnanopowders (4 to 300 nm) were synthesized by the template method and modified sol-gel technique, respectively. Texture and morphology of the obtained objects have been studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller analysis, and N2 adsorption-desorption. It was found that morphological parameters of the metal oxide obtained by the modified sol-gel technique depend nonlinearly on the initial molar ratio value of the sol stabilizer and metal in the reaction medium as well as the nature of the stabilizer. It has been shown that the nature of structure-directing components determines the morphology of the silicate obtained by the template method: dispersion and shape of its particles. The developed laboratory technology corresponds to the conception of soft chemistry and may be adapted to the manufacture of ultradispersed materials for catalysis, solar cells, fuel cells, semiconductors, sensors, low-sized electronic devices of new generation, etc.
In the last two decades, the wet-chemistry methods became the most promising commercial approaches. A specific feature of wet chemistry is the use of liquid phases: aqueous and organic solutions as well as aqueous-organic mediums . Wet-chemistry approaches allow the control of the particle growth and pore structure parameters of materials up to several nanometers. To obtain materials with desired physicochemical properties, in the course of synthesis, it is necessary to carefully control the following process parameters: stirring rate, concentration of components and their quantitative ratio, electrical conductivity, density, pH, process temperature, viscosity, and other parameters which described the state of the liquid mediums. These techniques could be considered as soft chemistry with a good reason: because they do not need high temperature and pressure, they do not need to use a large number of expensive energy carrier for their technological realization and can be attributed to ecotechnology. In the technology marketplace, wet-chemistry methods have the ability to obtain nanoparticles with a narrow size distribution, to form the coatings with a controlled particle packing, and to design new-generation catalysts with high phase purity and chemical homogeneity at the atomic level. The purpose of this study was to develop wet-chemistry approaches for the synthesis of ultradispersed and mesoporous metal oxide powders and powdery composites intended for usage in the production of ceramic materials with desired properties. The focus is on the development of the template synthesis of mesoporous metal (Al, Ge, Fe, Ni, Ti, and Zr) silicates as well as obtaining the nano- and subnanopowders by a modified sol-gel technique and template methods. In these methods, it is possible to control the growth of particles in a colloid, their form, and size by changing the mentioned parameters. The dendrimer-assisted method is well known to obtain nanoparticles with a narrow size distribution [2, 3]. In our research, the concept of three-dimensional [3D] oligomeric template formation in situ was used and developed for wet-chemistry synthesis in aqueous-organic mediums.
Wet methods of producing nanostructured raw materials win the increasing confidence of material scientists and technologists. Their implementation is based on the use of aqueous and organic solutions and aqueous-organic fluids. Using the wet method provides control over the structure of the resulting materials at the nanoscale size. A simple adaptation of wet methods to the fabrication conditions makes them promising for a wide range of materials for spintronics, fuel cells, solar cells, implants, medical equipment, catalysts, and fine grain ceramics.
The salts of mineral acids (chlorides, nitrates, and sulfates) and organic derivatives (alcoholates or acetylacetonates) of metals were used as sources of metals. Silicic acid or tetraethoxysilane was used as a source of silicium. Hexamethylenetetramine [HMTA], N,N-dimethyloctylamine [DMOA], monoethanolamine [MEA], and tetraethylammonium hydroxide [TEAH] were used as templates or sol stabilizers [St]. The different St were used with different molar ratio values of St/metal which were varied in a wide range from 1 to 20. Syntheses were carried out in aqueous-organic mediums (deionized water, alcohols). The obtainment of mesoporous metal silicates by the template method was realized in an autoclave at 80°C to 150°C and at an autogenous pressure by stirring. Texture and morphology of the obtained powders have been studied by X-ray diffraction [XRD], scanning electron microscopy [SEM], transmission electron microscopy [TEM], Fourier transform infrared [FTIR] spectroscopy, Brunauer-Emmett-Teller [BET] analysis, and N2 adsorption-desorption.
Result and discussion
We have developed (1) a template method to obtain mesoporous (2 to 30 nm) metal silicates, in which a part of silicium ions were substituted isomorphically by metal ions in the SiO2 lattice and (2) a modified sol-gel synthesis of nanosized (≥ 4 nm) powdery oxides of group II-VIII metals with a BET surface area ranging from a few to 150 m2/g. In the case of the template method, a catalytic co-solvolysis of organic and inorganic derivatives of silicium and metal was carried out in aqueous-organic mediums. The formation of a 3D oligomeric gel intermediate occurs from low-molecular components of the reaction mixture: co-solvents, structure-directing agents, and ligands, previously included in the composition of the metal and silicium derivatives. OH- or H+ groups were catalysts of the formation of oligomeric organic-inorganic gel intermediates.
In the case of the modified sol-gel technique, syntheses were carried out by the use of N-containing structure-directing components, which promote sol stabilization and formation of phase interface boundaries. In both cases, a thermo-treatment schedule of the obtained gel intermediate is of great importance for structure formation.
A modified sol-gel method
In the development of a modified sol-gel technique, the choice of N-containing compounds (DMOA, HMTA, MEA, TEAH) was stipulated by several reasons: the use of small molecules in the synthesis of nanostructured objects by a 'bottom up' approach gives an ability to simulate the structure at the atomic and molecular levels; a mechanism of sol stabilization by DMOA is unknown; and there is no report on the use of DMOA for sol stabilization in the literature, but from an economic point of view, it is very attractive because of its low market value.
The choice of acetylacetone as a complexing agent was due to the fact that it forms into metal acetylacetonates which are insoluble in water. Consequently, the proposed method of obtainment can be used for the preparation of a large number of metal oxide nanopowders. Methanol and ethanol were used as co-solvents for hydrosol stabilization.
Obtained nanopowders, crystallite size (according to XRD data), and their perspective purposes
Crystallite size (nm)
Catalysts for petrochemistry (FTS, obtainment of alcohols, HDS), medicine materials (for endoprosthesis), laser equipment
Varistor ceramics, hybrid conductivity membranes
7 to 60
Fuel cells, environmental catalysis (CO oxidation, HC, and soot)
30 to 170
Catalysts for petrochemistry (FTS, obtainment of alcohols, HDS), varistor ceramics
30 to 80
30 to 40
Medicine materials (for endoprosthesis)
160 to 300
Catalysts for petrochemistry (FTS, obtainment of alcohols, HDS)
4 to 10
90 to 100
20 to 30
Laser equipment, varistor ceramics
8 to 18
Medicine materials (for endoprosthesis), hybrid conductivity membranes
7 to 10
CO, HC, and soot oxidation catalysts
A complex wet-chemistry-based approach is developed for obtaining nanostructured powdery materials for different assignments. The developed method allowed obtaining the mesoporous oxides and silicates of metals with high crystallinity and given morphological parameters. Morphological parameters of the metal oxide obtained by the modified sol-gel technique depend nonlinearly on the initial molar ratio value of the sol St and metal in the reaction medium as well as on the nature of the St. The nature of structure-directing components determines the morphology of the silicate obtained by the template method: dispersion and shape of its particles. The developed laboratory technology corresponds to the conception of soft chemistry and may be adapted to manufacture ultradispersed materials for catalysis, solar cells, fuel cells, semiconductors, sensors, low-sized electronic devices of new generation, etc.
This work was supported by RFBR grant no. 09-08-00917-a, Fundamental Research Program no. 22 of the Presidium of RAS, and Program no. 7 of the Department of Chemistry and Materials Science of RAS.
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