Carbon Nanotube Assisted Electrical Treeing for Vascular Network Synthesis
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Carbon Nanotube Assisted Electrical Treeing for Vascular Network Synthesis Kristopher D. Behler and Eric D. Wetzel U. S. Army Research Laboratory (USARL), Composite and Hybrid Materials Branch, RDRLWMM-A, Aberdeen Proving Grounds, MD 21005-5069, U.S.A. ABSTRACT Electrical treeing (ET) is a stepwise dielectric breakdown process that generates a branched, hollow network of tubules in the dielectric between the electrode and ground. In this study, the controlled growth of electrical trees (ETs) in epoxies is demonstrated as a technique for fabricating synthetic vascular systems in engineering materials. A number of experimental conditions are explored, including AC versus DC voltage and geometric arrangement of the electrode and ground. AC growth tends to induce highly branched, "bush-like" trees while DC growth tends to produce lower-order branched structures. In addition, treating electrode surfaces with multi-walled carbon nanotubes (MWCNTs) is shown to promote ET initiation, most likely due to enhancements in the local electric field intensity. The utility of these structures for vascular applications is demonstrated by filling the channels with dyed liquids. INTRODUCTION When dielectrics are exposed to a global electric field that is greater than the dielectric strength of the material, a sudden, catastrophic dielectric breakdown process occurs [1]. This breakdown process usually leads to massive polymer degradation, resulting in substantial material loss and discoloration. A special type of dielectric breakdown, called electrical treeing (ET), results when the global field applied to the material is less than its dielectric strength, but inhomogeneities in the dielectric or electrode intensify the electric field and induce local dielectric breakdown [2]. If the localized breakdown event produces a new feature which promotes further breakdown, a continuous process results which takes the form of a branching figure that extends between electrodes. When the electrodes are bridged by the electrical tree, catastrophic dielectric breakdown follows. ET growth appears to be well-suited for use as a technique for growing synthetic vasculature in engineering materials. Their highly branched, hierarchical geometry is very similar to natural vascular structures found in living organisms. The mass loss associated with dielectric breakdown causes the ETs to be hollow and interconnected [3]. Finally, ETs can be grown in engineering materials such as polymers and ceramics [4, 5]. A few techniques are under development for the creation of synthetic vascular systems. Embedding hollow fibers into a polymer matrix [6, 7] is a simple and scalable approach, but does not allow for the degree of branching and multiple length scales associated with natural vasculature. Direct write techniques [8-10] allow for the creation of complex and detailed vascular networks, but may be challenging to scale to highly pervasive or fine-scaled networks. In this manuscript we explore the use of ETs as engineering vascular systems in polyme
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