No abstract

ABSTRACT:Heat is transported by phonons through dielectric solids such as ceramics and organic insulating resins. Especially in organic insulating resins, phonons scatter intensely mainly by their amorphous structure, which affects their thermal conductivities, usually 1 to 3 orders lower than those of ceramics and metals. Here, we show that by the thermosetting resin system with a crystal-like structure that is microscopic anisotropy, the thermal conductivities of the resin themselves can be improved while keeping their macroscopic isotropy. We studied four kinds of diepoxy monomers with a biphenyl group or two phenyl benzoate groups as mesogens, and cured them thermally with an aromatic diamine curing agent. These thermal conductivities were maximally 5 times higher than that of conventional epoxy resins because mesogens were highly ordered to form crystal-like structures to suppress phonon scattering. We also succeeded in the direct confirmation of the existence of crystal-like structures in the epoxy resins by TEM observation. These results suggest a novel strategy to improve thermal conductivities of insulating resins themselves by controlling the high-order structures.

No abstract

A new interfacial-gel copolymerization technique is proposed to prepare steric gradient-index (GRIN) optical waveguides and lens arrays. Since the GRIN profile is obtained from the thin-gel phase formed on the wall of a polymer substrate, many geometrical GRIN materials, such as axial-, radial-, and spherical-GRIN, can be monolithically fabricated. In this paper the principle of GRIN formation is clarified, and a few steric radial -GRIN circuit components are demonstrated.

The molecular orientation effect of a liquid crystalline (LC) epoxy resin (LCER) on thermal conductivity was investigated, with the thermal conductivity depending on the surface free energy of amorphous soda–lime–silica glass substrate surfaces modified using physical surface treatments. The LC epoxy monomer was revealed to form a smectic A (SmA) phase with homeotropic alignments on the surfaces of substrates that possess high surface free energies of 71.3 and 72.7 mN m –1 , but forming a planar alignment on the surface of a substrate that possesses a relatively low surface free energy of 46.3 mN m –1 . The optical microscopy observations and the X-ray analyses revealed that the LC epoxy monomer also induced a homeotropically aligned SmA structure due to cross-linking with a curing agent on the high-free-energy surface. The orientational order parameter of the resulting homeotropic SmA structure was calculated from the grazing incidence small-angle X-ray scattering patterns to be 0.73–0.75. The thermal conductivity of the cross-linked LCER forming a homeotropically aligned SmA structure was also estimated to be 2.0 and 5.8 W m –1 K –1 for the average and maximum in the direction of the Sm layer normal. The value of the thermal conductivity was remarkable among the thermosetting polymers and ceramic glass, and the LCER could be applied for high-thermal-conductive adhesives and packaging materials in electrical and electronic devices.

No abstract

A highly thermoconductive resin with branched filler is reported. The filler was fabricated by calcination of aluminium isopropoxide adsorbed onto paper filter (cellulose fibers). Scanning electron microscopy and x-ray diffraction measurement demonstrated that the filler consists of α-alumina nanofiber with a branched structure. The thermal conductivity of the filler-resin nanocomposite was two to five times larger than that predicted by the well-known Bruggeman equation, which postulates the composite with spherical filler in the matrix. The branched shape of the α-alumina nanofiber increased the probability of formation of phonon paths with lower thermal resistance, leading to the high thermal conductivity of the nanocomposite.

No abstract