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Synthesis of Nanocrystalline Materials

#A-947


Mechanically Activated Synthesis of Micro and Nanocrystalline Materials

Tech Area / Field

  • MAT-SYN/Materials Synthesis and Processing/Materials
  • CHE-IND/Industrial Chemistry and Chemical Process Engineering/Chemistry

Status
3 Approved without Funding

Registration date
21.10.2002

Leading Institute
A.I. Alikhanyan National Science Laboratory, Armenia, Yerevan

Collaborators

  • Korea Institute of Science and Technology / Nano-Materials Research Centre, Korea, Seoul\nUniversity of Oxford / Department of Engineering Science, UK, Oxford\nUniversity of Maryland, Baltimore County, USA, MD, Baltimore\nGoucher College, USA, MD, Baltimore

Project summary

Mechanically induced chemical reactions have been of increasing interest lately driven by both scientific curiosity and potential applications. Many inorganic compounds having amorphous, micro and nanocrystalline structure, such as silicides, carbides, magnetic and super-conductive materials etc. have been successfully prepared using different apparatus of mechanical processing. As the basic equipment for mechanical activation, vibration and planetary ball mills as well as attritors of various types having rather simple and serviceable construction are frequently used. By these facts are conditioned several evident merits of this technology such as simplicity of practical realization, environmental friendly manner, and possibility to scale up to tonnage quantities. In spite of practical attainment and simplicity of the method, the theoretical description of mechanically induced chemical reactions is a difficult problem due to the complex combination of interrelated processes on several length and time scales. There is no general theory that could explain the available data and orient future investigations and the development of applications. Further progress in this area requires better fundamental knowledge of the mechanisms of the relevant chemical interaction and the role of mechanical activation in these processes. The operation of the milling device and the way its energy is converted into the mechanical activation of the charge have to be understood. From these points of view one of the basic goals of the presented project is the detailed investigation of the mechanisms of the definite groups of chemical reactions under different conditions of mechanical processing. In particular, we intend to investigate reduction reactions under various conditions of ball milling and under influence of high pressure and shear deformation, to determine the basic principles of their behavior, which should allow to predict scientifically the main characteristics of the final reaction products.

Along with the progress of practical mechanochemistry, there are new ideas using mechanical processing in connection with other steps in a complex technology. Particularly, the reactive milling experiments in combination with electrical discharge conditions were found to result in completely different reaction paths for the same reacting compounds. A new species of metastable and nanocrystalline materials were synthesized recently under the above conditions of joint influence. So, another important goal of the project is investigation of the similar reaction systems both under pure ball milling and electrical discharge assisted ball-milling conditions in order to understand the role of electrical discharge on the behavior of mechanochemical reactions and for synthesis of new materials.

Thus, the main goals of the proposed project can be formulated as follows:

A) to investigate mechanically induced reduction reactions in order to clarify their mechanisms and provide their well-controlled progress and reproducibility;


B) to develop practical applications for synthesis of micro and nanocristalline materials on the basis of these reactions;
C) to combine mechanical alloying with the electrical discharge method in order to synthesize new species of materials.

We have chosen reduction reactions for the detailed investigation because of the following important facts: 1. Many of these systems have shown a definite critical state of activation, reached after a reproducible interval of milling. It can be detected easily via the abrupt temperature increase of the milling vial. 2. The relatively simple composition of these systems allows rather comprehensive analysis of the current processes and the obtained results. 3. Many interesting practical applications are based on these systems, namely synthesis of super-hard materials, nanostructural metals and oxides, etc. Although we propose to work with some selected systems, the results will contribute to research of other reactions as well, since it is known that the mechanochemical reduction of many sulphides, chlorides, and other salts occurs by similar mechanisms.

Regarding the apparatus for mechanical processing, we will mostly use the vibration ball mill both for reactive and spark discharge assisted ball milling. Hydraulic and screw presses with Bridgemen’s anvils and specific accessories will be used during high-pressure and shear deformation investigations. Reactive and electrical discharge assisted ball milling investigations will be very important for practical applications, while the investigations under the influence of high pressure and shear deformation will help us to understand the mechanisms of current complex processes during the operation of milling devices. The standard techniques of X-ray diffraction, DTA, electron and optical microscopy, electrical conductivity etc. available in our Institution will be used for various investigations of the activated systems and the obtained products during all the time of experimentation.

A) Mechanically induced reduction reactions. Within the framework of the Project we plan to investigate two groups of reduction reactions induced by mechanical activation. The first group is metal oxide - metal reaction (i.e. (МеnОm - Me1) systems, where МеnОm are reducible oxides and Me1 is the reducing agent) that take place as a self-sustaining processes after a definite activation time. The second one is the reaction of metal oxides with organic solvents, basically alcohol derivatives.

The behaviour of 14 reactions system (described in detail below) from each reaction group will be investigated depending on several complex factors. Particularly, the influence of the amplitude and frequency of the mechanical processing, chemical and structural properties of the starting compounds, inert additives, temperature as well as the values of pressure, shifting angle and electrical power supply will be considered.

B) Practical applications based on the investigated reactions. The ultimate goal of the project is to use the investigated mechanochemical reactions for synthesis of various kinds of micro and nanocrystalline materials. More exactly the following technologies will be considered.

1. Synthesis of superhard materials. As mentioned, many of (МеnОm - Me1) mixtures have shown a definite critical state of activation, reached after a reproducible interval of milling. Moreover, after reaching this state, some of them can interact with detonation. So, by applying the explosive kinetics of such reactions, when the bulk of exothermic energy is liberated within 10-3-10-5 sec, we intend to carry out the syntheses of various superhard, micro and nanocomposite materials. The principal possibility to use the mechanically activated systems for synthesis of such kind of materials has already been shown in our earlier works. (Diamond micropowder was synthesized by means of mechanically activated (CuO-Mg-carbon) system). Thus, using the specific feature of definite (МеnОm - Me1) systems to interact with detonation, we will develop a simple technology for syntheses of various kinds of superhard micro and nanocomposites. Particularly, unique carbide and oxide containing diamond composites will be synthesized.

2. Synthesis of amorphous, micro and nanocrystalline composites. For mechanically activated (МеnОm - Me1) systems the low temperature synthesis is also possible. For example, if the milling process of (МеnОm - Me1) system is interrupted close to the critical state of activation (i.e. before beginning of self-propagation or explosion) and the activated mixture is subjected to the thermal treatment in an inert gas atmosphere a little below of the ignition temperature, the reaction will go on. As was shown in our several works, at that course of reaction the obtained products are basically amorphous or nanocrystalline substances. Particularly, X-ray amorphous metallic Mo-MgO and Fe-SiOx nanocomposites were obtained from activated (MoO3-Mg) and (Fe2O3-Si) systems. Thus, we can use these specific features of activated (МеnОm - Me1) systems for synthesis of (Me-Me1nOm) or (Me1nMekOm) type amorphous or nanocrystalline composites.

3. Synthesis of nanocrystalline metals and organometallic complexes. An interesting practical application can find also the metal oxide - organic solvent reactions, (i.e. МеnОm – alcohol derivatives). As was shown in some of our previous works, some metal oxides can be reduced under ball milling conditions in organic solvent media. Particularly, a multistage reduction of Fe3O4 up to pure metallic Fe has been investigated and reported. Similar reactions were also observed for a range of other metal oxides including TiO2, WO3, PbO etc., and in many cases the reduced metals were in nanocrystalline state. So we can use this method for synthesis of nanocrystalline metallic powders. For other group of oxides such as CuO, SiO2 etc., the mechanical processing in organic solvent media can result in the formation of organometallic complexes like metal alchoholites Me (OR). These reaction systems will be used for syntheses of amorphous and nanostructured metal oxides.

C) Electrical discharge assisted ball milling. In this part of the Project we will investigate the effect of high voltage electrical pulses on the process of mechanochemical reduction of the above-mentioned reaction systems. The reactions and their products obtained by both conventional milling techniques and electrical discharge assisted milling will be examined and compared using standard techniques of X-ray diffraction, DTA, electron and optical microscopy. We plan also to use spark discharge conditions for reactive milling in order to promote direct formation of new phases, rapid solid-solid and solid-liquid reduction reactions. The direct formation of carbides from elemental metallic W, Mo and organic solvents under the combined influence of mechanical energy and electrical discharge will be also considered.

It should be noted that all the necessary basic equipment for starting investigations already exists in our Institutions. It is important also that we as well as our foreign collaborators have sufficient experience in the area of mechanochemical synthesis. Particularly, our previously published works in the field are good indications of it.


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