Go Yamamoto

By Go Yamamoto and Toshiyuki Hashida

Part of the book: Composites and Their Properties

By Keiichi Shirasu, Go Yamamoto and Toshiyuki Hashida

Electrothermal bimorph actuators have been widely researched, comprising two layers with asymmetric expansion that generate a bending displacement. Actuation performance greatly relies upon the difference of the coefficients of thermal expansion (CTE) between the two material layers. Since traditionally used bimorph materials have positive CTE values, the generated displacements are restricted because of their relatively low CTE difference. Currently, the synthesis and characterization of carbon nanotube (CNT)/polymer composite actuators are topics of intense research activity. CNTs have been attracting much interest because of their superior electrical, thermal and mechanical properties. In addition, the negative CTE value of CNTs in the axial direction has been investigated analytically, leading one to expect that the CTE of the composites in a direction parallel to the CNT alignment will drastically decrease by containing the aligned CNTs into polymer materials. In this chapter, an experimental method for determining the CTE of a CNT in the axial direction is discussed. Based on this result, we demonstrate an electrothermal bimorph actuator having a large bending displacement and high force output using an aligned CNT-reinforced epoxy composite and thin aluminum foil. Performance characteristics including power and work output per unit volume versus frequency are also reviewed.

Part of the book: Carbon Nanotubes

By Keiichi Shirasu, Go Yamamoto, Daniel Nelias and Toshiyuki Hashida

Carbon nanotubes (CNTs) have attracted much interest because of their superior electrical, thermal, and mechanical properties. These unique properties of CNTs have come to the attention of many scientists and engineers worldwide, eager to incorporate these novel materials into composites and electronic devices. However, before the utilization of these materials becomes mainstream, it is necessary to develop protocols for tailoring the material properties, so that composites and devices can be engineered to given specifications. In this chapter, we review our recent studies, in which we investigate the nominal tensile strength and strength distribution of multi-walled CNTs (MWCNTs) synthesized by the catalytic chemical vapor deposition (CVD) method, followed by a series of high-temperature annealing steps that culminate with annealing at 2900°C. The structural-mechanical relationships of such MWCNTs are investigated through tensile-loading experiments with individual MWCNTs, Weibull-Poisson statistics, transmission electron microscope (TEM) observation, and Raman spectroscopy analysis.

Part of the book: Carbon Nanotubes

By Go Yamamoto, Keita Koizumi and Tomonaga Okabe

The tensile strengths of unidirectionally aligned carbon fiber-reinforced plastic (UD-CFRP) were predicted by implementing a spring element model (SEM) that takes into account a stress concentration acting on an intact fiber surface originated from a fracture site in an neighboring fiber. The surface stress concentration was experimentally investigated by implementing multi-fiber fragmentation testing in combination with the SEM simulation. Four types of epoxy materials were selected to explore the effects of matrix polymer properties on the surface stress concentration. The size scaling results, coupled with the results of the SEM simulation, designed to take into account the surface stress concentration, were reasonably consistent with the experimentally obtained data on the tensile strengths of the UD-CFRP composites, irrespective of the differences in the matrix mechanical characteristics. The possible mechanisms by which additional stress concentration is generated on an intact fiber surface were analyzed numerically using the finite element method.

Part of the book: Strength of Materials