Production of metal oxide particles with nano-sized grains

Details Production of metal oxide particles with nano-sized grains Production of metal oxide particles with nano-sized grains

2000-11-21

2004-06-22

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

C423S593100, C423S263000

Reexamination Certificate

active

06752979

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to very fine-grained particulate material and to methods for producing such very fine-grained particulate material. In preferred aspects, the present invention relates to oxide materials of very fine-grained particulate material and to methods for producing such material. Most suitably, the particulate material has grain sizes in the nanometer scale.
BACKGROUND OF THE INVENTION
Metal oxides are used in a wide range of applications. For example, metal oxides can be used in:
solid oxide fuel cells (in the cathode, anode, electrolyte and interconnect);
catalytic materials (automobile exhausts, emission control, chemical synthesis, oil refinery, waste management);
magnetic materials;
superconducting ceramics;
optoelectric materials;
sensors (eg gas sensors, fuel control for engines);
structural ceramics (eg artificial joints).
Conventional metal oxides typically have grain sizes that fall within the micrometer range and often are supplied in the form of particles having particle sizes greater than the micrometer range. It is believed that metal oxides that are comprised of nanometer sized grains will have important advantages over conventional metal oxides. These advantages include lower sintering temperatures, potentially very high surface areas, and sometimes improved or unusual physical properties. However, the ability to economically produce useful metal oxide materials with nanometer-sized grains has proven to be a major challenge to materials science. It has proven to be difficult to make such fine-scale metal oxides, particularly multi-component metal oxides, with:
(a) the correct chemical composition;
(b) a uniform distribution of different atomic species;
(c) the correct crystal structure; and
(d) a low cost.
Many important metal oxides have not yet been produced with very fine grains, especially multi-component metal oxides. This is because as the number of different elements in an oxide increases, it becomes more difficult to uniformly disperse the different elements at the ultra-fine scales required for nanometer-sized grains. A literature search conducted by the present inventors has shown that very small grain sizes (less than 20 nm) have only been attained for a limited number of metal oxides. The reported processes used to achieve fine grain size are very expensive, have low yields and can be difficult to scale up. Many of the fine grained materials that have been produced do not display particularly high surface areas, indicating poor packing of grains.
At this stage, it will be realised that particles of material are typically agglomerated of a number of grains. Each grain may be thought of as a region of distinct crystallinity joined to other grains. The grains may have grain boundaries that are adjacent to other grain boundaries. Alternatively, some of the grains may be surrounded by and agglomerated with other grains by regions having a different composition (for example, a metal, alloy or amiorphous material) to the grains.
Methods described in the prior art for synthesising nano materials include gas phase synthesis, ball milling, co-precipitation, sol gel, and micro emulsion methods. The methods are typically applicable to different groups of materials, such as metals, alloys, intermetallics, oxides and non-oxides. A brief discussion of each will follow:
Gas-Phase Synthesis
Several methods exist for the synthesis of nano-particles in the gas phase. These include Gas Condensation Processing, Chemical Vapour Condensation, Microwave Plasma Processing and Combustion Flame Synthesis (H. Hahn, “Gas Phase Synthesis of Nanocrystalline Materials”, Nano Structured Materials, Vol 9, pp 3-12, 1997). In these methods the starting materials (suitable precursors to a metal, alloy or an inorganic material) are vaporised using energy sources such as Joule heated refractory crucibles, electron beam evaporation devices, sputtering sources, hot wall reactors, etc. Nano-sized clusters are then condensed from the vapour in the vicinity of the source by homogenous nucleation. The clusters are subsequently collected using a mechanical filter or a cold finger. These methods produce small amounts of non-agglomerated material, with a few tens of gram/hour quoted as a significant achievement in production rate.
Ball Milling
Mechanical attrition or ball milling is another method that can be used to produce nano-crystalline materials (C. C. Koch, “Synthesis of Nanostructured Materials by Mechanical Milling: Problems and Opportunities”, Nano Structured Materials, Vol 9, pp 13-22, 1997). Unlike the aforementioned methods, mechanical attrition produces the nano-materials not by cluster assembly but by the structural decomposition of coarser-grained materials as a result of severe plastic deformation. The quality of the final product is a function of the milling energy, time and temperature. To achieve grain sizes of a few nanometers in diameter requires relatively long processing times (several hours for small batches). Another main drawback of the method is that the milled material is prone to severe contamination from the milling media.
Co-Precipitation
In some special cases it is possible to produce nano-crystalline materials by precipitation or co-precipitation if reaction conditions and post-treatment conditions are carefully controlled (L. V. Interrante and M. J. Harnpden-Smith),
Chemistry of Advanced Materials—An Overview
, Wiley—VCH (1998)). Precipitation reactions are among the most common and efficient types of chemical reactions used to produce inorganic materials at industrial scale. In a precipitation reaction, typically, two homogenous solutions are mixed and an insoluble substance (a solid) is subsequently formed. Conventionally, one solution is injected into a tank of the second solution in order to induce precipitation, however, simultaneous injection of the two solutions is also possible. The solid that forms (called the precipitate) can be recovered by methods such as filtration.
The precursor material has subsequently to be calcined in order to obtain the final phase pure material. This requires, in particular, avoidance of phenomena that induce segregation of species during processing such as partial melting for example. Formation of stable intermediates also has to be avoided since the transformation to the final phase pure material might become nearly impossible in that case. Typical results for surface areas for single oxides can be of several tens of m
2
/g. However, for a multi-cation compound, values less than 10 m
2
/g become more common.
Sol-gel Synthesis
Sol-gel synthesis is also a precipitation-based method. Particles or gels are formed by ‘hydrolysis-condensation reactions’, which involve first hydrolysis of a precursor, followed by polymerisation of these hydrolysed percursors into particles or three-dimensional networks. By controlling the hydrolysis-condensation reactions, particles with very uniform size distributions can be precipitated. The disadvantages of sol-gel methods are that the precursors can be expensive, careful control of the hydrolysis-condensation reactions is required, and the reactions can be slow.
Microemulsion Methods
Microemulsion methods create nanometer-sized particles by confining inorganic reactions to nanometer-sized aqueous domains, that exist within an oil. These domains, called water-in-oil or inverse microemulsions, can be created using certain surfactant/water/oil combinations.
Nanometer-sized particles can be made by preparing two different inverse microemulsions (eg (a) and (b)). Each microemulsion has a specific reactant dissolved in the aqueous domains. The inverse microemulsions are mixed, and when the aqueous domains in (a) collide with those in (b), a reaction takes place that forms a particle. Since the reaction volumes are small, the resultant particles are also small. Some microemulsion techniques are reviewed in “Nanoparticle and Polymer Synthesis in Microemulsion”, J. Eastoe and B. Warne,
Current Opinion in Colloid and Interface Science
, vol. 1 (1996), p800

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