SAMPE Journal - May/June 2011 - 7

Figure 2. Optical micrograph of low volume fraction dispersion of nickel nanostrands, identifying nanostrand animals. to help solve the electrical conductivity problem in composites. Specifically, newly available conductive nanoparticles have shown great advances over previously available conductive particles for increasing the conductivity of polymer composites. These new nanomaterials demonstrate some fundamental differences from previously available conductive particles, primarily in geometry. There are many important factors in adding conductive particles to non-conductive matrixes. Particle geometry is of primary importance. While previous conductive additives (such as milled powders, coated spheres, platelets, etc.) have aspect ratios on the order of 1 to 10, newer nanoparticles (such as carbon nanofibers) have aspect ratios in the thousands. This fact, in combination with the nanoscale diameter of these particles, means that less material is required, both in terms of volume percent and weight percent, to achieve the same conductivity levels1-4. Nickel Nanostrands in Nanocomposites A new class of conductive metal nanomaterial has shown highly improved geometric and material advantages. Nickel Nanostrands5 are a relatively new material. They are a sub-micron diameter, high aspect ratio nano-structure. Nanostrands feature a (patented) three dimensionally interconnected structure that creates interconnecting loops and branches.

Nanostrands exhibit an interesting and unique dispersion in composites systems. They are manufactured as a continuously interconnected “cake” of nanostrand structures (Figure 1). This “cake” can be used as a pre-form, or reduced to a nanostructured aggregate powder form. The latter practice is most typical. The cake is subjected to a process which results in discrete nanostrand clusters, which likewise are composed of individual strands. These clusters will contain long nanostrands that interconnect and branch. Some of the branches will also interconnect, and some are open ended. An analogy is to imagine a pure nickel nanoscale “tumbleweed” with three dimensional interconnections. These dispersed nanostructures are the key to the performance of nanostrand polymer composites. These structures are seen in bulk in the optical micrograph shown in Figure 2. Additional images of the dispersed nanostructure, including in cured polymers, are shown in Figure 3. There are unique properties of nanostrands that are well suited to conductivity in polymers, relative to other metal fillers or nanoparticles. The high aspect ratio of individual strands requires fewer particles for effective volumetric percolative properties. The looped and branched nature of the nanostrand structure allows for a high number of three dimensional interconnects, and therefore higher conductivity. The branches of a nanostrand establish radial interconnects and provide three-dimensional connection opportunities (for example, two parallel nanostrands do not need to intersect along their major axis, as they can connect with branches extending transverse from the major axis of each). The branches can also serve as a multiplicity of antennas. Another key feature found in nanostrand geometry is that the three dimensional particle can be viewed as a “skeleton” rather than a “body.” The void space of the nanostrand structure is thus filled with the matrix material, facilitating better bonding and preservation of material properties while providing a conductive skeleton structure. The effective diameter required for inter-cluster connection displaces much less than volume than would be required with solid fillers. Nanostrand mixtures percolate to higher conductivity levels than have been demonstrated with carbon nanomaterials1,2,6-8. A comparison percolation curve for nickel nanostrands and carbon nanofibers is shown in Figure 4.

(a) (b) (c) Figure 3. Nickel nanostrands (a) as manufactured, 2500 X (10 mm scale bar); (b) 10 vol% in thermoplastic polyurethane, 2500 X (10 mm scale bar); (c) 5 vol% in epoxy, 500 X (50 mm scale bar).
SAMPE Journal, Volume 47, No. 3, May/June 2011 7

SAMPE Journal - May/June 2011

Table of Contents for the Digital Edition of SAMPE Journal - May/June 2011