Mamoru Ishikiriyama

Tunable LCST behavior of poly(N-isopropylacrylamide/ionic liquid) copolymers

Poly(N-isopropylacrylamide), PNIPAM, is a thermoresponsive polymer widely known for its lower critical solution temperature (LCST) phenomenon at 32 °C in aqueous solutions. Precise tuning of the LCST of PNIPAM to a broader temperature range offers a larger window of applications especially in the field of biotechnology and nanotechnology. A series of free radical random copolymerizations between N-isopropylacrylamide (NIPAM) and various imidazolium based ionic liquids (ILs) were conducted. IL structures were varied in terms of their alkyl chain length at the N-3 (or N-1) position of the imidazolium ring and counter anion. The LCST behavior of the aqueous solutions of the copolymers was investigated through UV-VIS transmission measurements. The results confirm that introduction of IL into the PNIPAM offers a wide range of LCST behaviors with a synergism between the hydrophobic part of the ionic liquid and the basic strength of the counter anion, probably by varying the hydrogen bonding abilities of the copolymer.

Improvement in Durability of Piezoelectric Ceramics for Actuator

High-durability lead zirconate titanate (PZT) ceramics have been developed in order to prevent time-dependent degradation in displacement or failure under operation. The coercive field (E c) and mechanical strength of PZT have been improved by doping various additives such as In, La, and Pr, in this study. Neither decrease of displacement nor formation of cracks took place in this new PZT after over 108 cycles of dynamic durability tests. The results indicated that both E c and mechanical strength were important factors in improving the durability of PZT ceramics.

Durability of Piezoelectric Ceramics for an Actuator

Change of piezoelectric properties with the number of loading cycles was measured under various driving conditions, i.e., temperature, preset pressure and driving voltage. Although e33T/e0 changed very little, Kp decreased markedly. Kp decrease is affected by driving condition factors, i.e., minus voltage, preset pressure and temperature, in that order. By optimal arrangement of these factors, Kp decrease could be reduced considerably. Kp decrease is mainly due to 90° switching of the domain. Improvement of mechanical strength for the piezoelectric material is effective both for the suppression of microcrack occurrence and for prevention of Kp decrease. We found that microcrack occurrence caused domain switching. We suggest a deterioration mechanism for piezoelectric ceramics.

Modification of thermal conductivity and thermal boundary resistance of amorphous Si thin films by Al doping

We investigate the effects of Al doping on the thermal conductivity and thermal boundary resistance of a-Si thin films. Au/Al-doped a-Si/Si structures were prepared by depositing Al-doped amorphous Si films of different Al doping concentrations and thickness on Si substrates by magnetron sputtering. The thermal resistances of the structures were measured to calculate the thermal conductivities of the films. The thermal conductivities of the 150 nm-thick films were higher than those of 100 nm-thick films, and a sharp increase in thermal conductivity with increasing Al doping concentration was observed in the 150 nm-thick films but not in the 100 nm-thick films. Furthermore, the thermal boundary resistances at the two interfaces in the structures also increased with increasing Al doping concentration. Our findings could be used to tailor the thermal resistance of materials for thermal management in semiconductor devices as well as for development of thermal barrier coatings and thermoelectric materials with good performance.

Thermoelectric conversion efficiency in IV-VI semiconductors with reduced thermal conductivity

Mid-temperature thermoelectric conversion efficiencies of the IV-VI materials were calculated under the Boltzmann transport theory of carriers, taking the Seebeck, Peltier, and Thomson effects into account. The conversion efficiency was discussed with respect to the lattice thermal conductivity, keeping other parameters such as Seebeck coefficient and electrical conductivity to the same values. If room temperature lattice thermal conductivity is decreased up to 0.5W/mK, the conversion efficiency of a PbS based material becomes as high as 15% with the temperature difference of 500K between 800K and 300K.

Amorphous/epitaxial superlattice for thermoelectric application

An amorphous/epitaxial superlattice system is proposed for application to thermoelectric devices, and the superlattice based on a PbGeTeS system was prepared by the alternate deposition of PbS and GeTe using a hot wall epitaxy technique. The structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray analysis, and it was found that the superlattice consists of an epitaxial PbTe-based layer and a GeS-based amorphous layer by the reconstruction of the constituents. A reduction in thermal conductivity due to the amorphous/epitaxial system was confirmed by a 2ω method. Electrical and thermoelectric properties were measured for the samples.

Ultra-low thermal conductivity of high-interface density Si/Ge amorphous multilayers

Owing to their phonon scattering and interfacial thermal resistance (ITR) characteristics, inorganic multilayers (MLs) have attracted considerable attention for thermal barrier applications. In this study, a-Si/a-Ge MLs with layer thicknesses ranging from 0.3 to 5 nm and different interfacial elemental mixture states were fabricated using a combinatorial sputter-coating system, and their thermal conductivities were measured via a frequency-domain thermo-reflectance method. An ultra-low thermal conductivity of κ = 0.29 ± 0.01 W K−1 m−1 was achieved for a layer thickness of 0.8 nm. The ITR was found to decrease from 8.5 × 10−9 to 3.6 × 10−9 m2 K W−1 when the interfacial density increases from 0.15 to 0.77 nm−1.

Equivalence of the EMD- and NEMD-based decomposition of thermal conductivity into microscopic building blocks

Thermal conductivity of a material can be comprehended as being composed of microscopic building blocks relevant to the energy transfer due to a specific microscopic process or structure. The building block is called the partial thermal conductivity (PTC). The concept of PTC is essential to evaluate the contributions of various molecular mechanisms to heat conduction and has been providing detailed knowledge of the contribution. The PTC can be evaluated by equilibrium molecular dynamics (EMD) and non-equilibrium molecular dynamics (NEMD) in different manners: the EMD evaluation utilizes the autocorrelation of spontaneous heat fluxes in an equilibrium state whereas the NEMD one is based on stationary heat fluxes in a non-equilibrium state. However, it has not been fully discussed whether the two methods give the same PTC or not. In the present study, we formulate a Green-Kubo relation, which is necessary for EMD to calculate the PTCs equivalent to those by NEMD. Unlike the existing theories, our formulation is based on the local equilibrium hypothesis to describe a clear connection between EMD and NEMD simulations. The equivalence of the two derivations of PTCs is confirmed by the numerical results for liquid methane and butane. The present establishment of the EMD-NEMD correspondence makes the MD analysis of PTCs a robust way to clarify the microscopic origins of thermal conductivity.