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  1. 3 Acta Physica Polonica A

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  1. 2 2014

  2. 1 2012

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  1. 3 Takacs L.

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What Is Unique About Mechanochemical Reactions?

100%
Takacs L.
Acta Physica Polonica A
|
2014
|
vol. 126
|
issue 4
1040-1043
EN
Mechanochemical reactions can provide compounds, phases, and microstructures that are essentially different from the products of ordinary reactions. In this paper, the origin of this uniqueness is discussed in light of the recent advances of the field. It is claimed that the local availability of large batches of energy, well above kT, is the key feature of mechanochemical reactions. As a consequence, reactions that cannot occur thermally become possible, similarly to the reactions induced by the energy of photons in photochemistry. However, the situation is more complex, as macroscopic deformation affects many defect sites simultaneously. The direction of the mechanical load relative to the orientation of a molecule or the crystallographic axes of a solid can be important. Many mechanochemical reactions of organic compounds take place at low milling energy that is not sufficient to break primary bonds, but the gentle mechanical grinding can influence the relative position of macromolecules, leading to the formation of unique cocrystals and compounds. In inorganic systems, unusual products form due to forced mixing and the high defect density generated by intense milling.
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Mechanochemistry and the Other Branches of Chemistry: Similarities and Differences

100%
Takacs L.
Acta Physica Polonica A
|
2012
|
vol. 121
|
issue 3
711-714
EN
It is argued that mechanical action can induce a unique chemical reaction, if enough mechanical energy is concentrated in the bonds involved in the process to bypass the activation energy. This can happen at crack tips, at the core of dislocations, or at the asperities of colliding or sliding surfaces. A mechanical reaction is always complex, as the macroscopic work is distributed among many possible reaction sites. In comparison, an elementary photochemical reaction is induced by a single photon, while thermochemical reactions rely on the accidental concentration of energy by thermal fluctuations. The paper also compares mechanochemical synthesis in a ball mill with reactions under well-defined loading conditions and mechanochemical experiments carried out on the molecular scale. Closer interaction among those branches of mechanochemistry is urged.
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Gradual and Self-Sustaining Processes in the Sn-Zn-Se System

100%
Takacs L.
Acta Physica Polonica A
|
2014
|
vol. 126
|
issue 4
1032-1039
EN
Ball milling induces self-sustaining reaction in binary Sn-Se and Zn-Se powder mixtures. But if such mixtures are blended, the ignition time increases at either end of the concentration scale and the suppression of ignition can take place in an intermediate concentration interval. This phenomenon was investigated in (1-x)(Sn+Se)+x(Zn+Se) and (1-x)(Sn+2Se)+x(Zn+Se) mixtures, by measuring the ignition time as a function of both composition and milling conditions and investigating activated and reacted mixtures using X-ray diffraction and scanning electron microscopy. At the Sn-rich compositions of the first system, ignition happened as soon as the mill was started, in spite of the rather low adiabatic temperature of the reaction. Simultaneous local melting of Sn and Se is suggested as a possible explanation for immediate ignition. It can also explain the asymmetry of the properties of the binary reactions, namely that Sn+Se is less exothermic but easy to ignite, while Zn-Se is more exothermic but difficult to ignite. Similar asymmetry is considered as the reason for the increase of the ignition time and the loss of ignition in other mixed metal-chalcogen systems.
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