Evolution of Carbon Self-Assembly in Colloidal Phase Diagram
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Evolution of Carbon Self-Assembly in Colloidal Phase Diagram V. Bouda Department of Mechanics and Materials, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, 166 27 Praha 6, Czech Republic ABSTRACT The growth of the self-assembled structure of carbon colloidal particles has been studied [1]. The system of carbon particles was processed in electrical field in polymer melt with controlled ionic concentration. The interpretation of the complex evolution of the selfassembled structure of carbon particles was given in terms of phase transitions of colloidal systems of carbon particles. Interactions between doublets of carbon black (CB) particles are interpreted in terms of DLVO approximation of interaction energy as multiples of average thermal fluctuation kT. Plots of the sum of energy of electrostatic repulsion and energy of van der Waals attraction versus separation between the doublets show the energy barriers to coagulation of high B and the energy wells with the secondary minima of depth W. The colloidal phase transitions appear at critical conjuncture of the concentration of ions in the medium and surface potential on the colloids. Six transition lines determine five phases of the assembly of carbon colloids in the proposed colloidal phase diagram: lateral vapor + axial vapor (vapor), lateral liquid + axial vapor (columnar liquid crystal), lateral liquid + axial liquid (smectic LC), lateral liquid + axial solid (nematic LC), and lateral solid + axial solid (solid). The diagram provides a tool to control the evolution of carbon self-assembly. The eventual morphology depends on the route of the steps of the processing. During the time elapsed in the LC state, the structure can reorganize and the eventual coagulation produces various crystals. On the contrary, the route outside the LC state can produce glass.
INTRODUCTION In a stable, dilute, dispersed system, all particles are in permanent Brownian motion and randomly distributed due to thermal energy. At moment the particles interact, the randomness is disturbed, and structures are built in the dispersion. The onset of structure formation manifests itself often by optical, electrical, or mechanical phenomena as turbidity, change in conductivity or viscosity, etc. Especially the electrical conductivity of structured carbon colloidal dispersion is of great practical relevance. Carbon and carbon-based composite materials may create their own revolution in the development of entirely new applications in electronics and micro- or nano-electromechanical systems. Carbon can form various agglomerates with a fine, solid or reversible structure using colloidal carbon-black (CB) particles, specific processing routes and control systems. The mechanics of such structures growth and metamorphosis is not well understood. The growth and metamorphosis of the selfassembled structure of CB colloidal aggregates in polymer melt has been studied recently both by theoretical modeling and experiments [1, 2].
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