NanoConsulting

Stutensee, Germany

NanoConsulting

Stutensee, Germany

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Fischer F.D.,University of Leoben | Leindl M.,University of Leoben | Dirschmid H.,Vienna University of Technology | Vollath D.,NanoConsulting | Clemens H.,University of Leoben
ZAMM Zeitschrift fur Angewandte Mathematik und Mechanik | Year: 2010

Heat can be released or consumed in particles in a nearly adiabatic way due to phase transformation, thermal fluctuation, etc. In this case a temperature field, strongly varying in space and time, develops in the particle in addition to the global temperature field. Analytical as well as numerical solutions for this additional temperature field are presented for identical and different thermal properties of both the particle and its environment. The case of heat conduction and of heat transfer is dealt with. Consequences for nanoparticles are discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Vollath D.,NanoConsulting | Fischer F.D.,University of Leoben
Progress in Materials Science | Year: 2011

Based on the definition of fluctuation in connection to phase transformations, structural fluctuations are spontaneous transitions from an equilibrium phase to a non-equilibrium phase, followed by a back-transformation into the equilibrium phase, criteria for the fluctuation of isolated single particles and ensembles of nanoparticles are developed. It is important to realize that, in case of ensembles, the probability for fluctuation depends on the number of transformed particles. Especially the latter criteria are deduced from a statistical model describing fluctuation processes. Furthermore, this statistical model leads to the conclusion that the equilibrium state of fluctuation processes is characterized by a minimum of the free enthalpy. Interestingly, this equilibrium state is independent of the character of the nanoparticles either being conventional particles or ones characterized by indistinguishability. A detailed thermodynamic analysis, studying isothermal and adiabatic processes, of the behavior of a single isolated particle and an ensemble under isothermal and adiabatic conditions allows formulating a set of seven theorems. In the adiabatic case, the calculations indicate the existence of bistability or hysteresis in the temperature range of transformation. Experimentally, these phenomena are well documented, however, in most cases, attributed to activation phenomena. As a result of this study, at least connected to nanoparticles, the interpretation of these experiments needs thorough examination. Furthermore, a complete or partial adiabatic enclosure of the specimen, which is in experimental reality unavoidable, causes a shift of the transformation temperatures. This result enforces a new view on phase diagrams, especially on those for nanoparticles. © 2011 Elsevier Ltd. All rights reserved.


Vollath D.,NanoConsulting | Fischer F.D.,University of Leoben
Journal of Materials Chemistry | Year: 2011

A statistical model delivering average equilibrium concentrations and indications for the time evolution of ensembles of phase transforming nanoparticles, showing fluctuations, gives clues for a new approach. It is shown that the temporal evolution of transformation of the ensemble depends on the distinguishability of the particles. However, the equilibrium configuration of fluctuating ensembles does not depend on the distinguishability of the particles. The outcome of the presuppositionless statistical fluctuation model is the proof that any fluctuations connected to phase transformations occur in the minimum of the free enthalpy. Additionally, the analysis allows drafting a new fluctuation condition for ensembles. © 2011 The Royal Society of Chemistry.


Vollath D.,NanoConsulting
International Journal of Materials Research | Year: 2012

Phase transformations of freestanding nanoparticles are well analyzed. The situation is different for particles embedded in a second matrix. The special case of melting and crystallizing of nanoparticles in a rigid matrix is the topic of this study based on the energy balance, resulting in a size dependency of these phase transformations primarily on variation of the materials properties with the particle size. Therefore, an important outcome of this study is the result that thermodynamic data of bulk materials are insufficient to describe phenomena connected to nanoparticles. The differences may be significant. © 2012 Carl Hanser Verlag, Munich, Germany.


Vollath D.,NanoConsulting | Fischer F.D.,University of Leoben
Journal of Nanoparticle Research | Year: 2010

A thermodynamic analysis of phase transformations of nanoparticles under hydrostatic pressure has revealed important differences between phase transformations under isothermal or adiabatic conditions. This presuppositionless analysis fully explains a hysteresis with respect to the phase fraction and the pressure observed experimentally. It is important to mention that the results of this analysis may be transferred to the role of any external volumetric field acting on phase transforming nanoparticles. Typical examples are phase transformations of ferromagnetic intermetallics subjected to the influence of magnetic fields.


Vollath D.,NanoConsulting
Advanced Materials | Year: 2010

Many applications require nanoparticles that exhibit high magnetic moment and luminescence. Compounds exhibiting this combination of properties do not exist. However, this combination of properties may be obtained by nanocomposites. There are two possible configurations for these composites: the core-shell design, leading to the smallest composite particles, and agglomerates containing separated particles with the properties in question. The magnetic core is, in most cases, maghemite or magnetite, whereas the luminescence carrier is either an organic molecule or an inorganic quantum dot. One of the basic problems in designing such composites, to be overcome by the appropriate layout choice, is the potential incompatibility between the magnetic core and the lumophore. Experimentally realized solutions of these problems are presented. Many high-value-added applications require nanoparticles that exhibit significant magnetic moment and luminescence. This combination of properties is obtained by using nanocomposites, either of the core-shell type or the type that consists of agglomerates containing particles of both properties in question. The figure displays the properties of core-shell-type composites using a maghemite core and an anthracene shell as lumophore. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Vollath D.,NanoConsulting
Journal of Nanoparticle Research | Year: 2011

Nanoparticles intended for high value added applications often require special size distributions. Based on model calculations, this article compares the particle size distributions obtained with conventional and plasma processes. The model is based on an estimation of the probability for collisions; either for neutral or equally charged particles, whereas the growth of the particles is calculated using a model derived from Markov chains. The results of these calculations confirm the empirical knowledge that, under the special conditions of particles carrying electric charges of equal sign, plasma processes deliver products with the narrowest particle size distribution. Synthesis of extremely small particles with conventional processes leads to a significant residue of unreacted precursor. This finding is important in cases of expensive educts. The results of these model calculations are in perfect agreement with experimental findings. © Springer Science+Business Media B.V. 2011.

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