Tomonori Iizuka

Bio: Tomonori Iizuka is an academic researcher from Waseda University. The author has contributed to research in topics: Epoxy & Materials science. The author has an hindex of 13, co-authored 38 publications receiving 847 citations.

Role of interface on the thermal conductivity of highly filled dielectric epoxy/AlN composites

Abstract: The interface between filler and matrix has long been a critical problem that affects the thermal conductivity of polymer composites. The effects of the interface on the thermal conductivity of the composite with low filler loading are well documented, whereas the role of the interface in highly filled polymer composites is not clear. Here we report on a systematic study of the effects of interface on the thermal conductivity of highly filled epoxy composites. Six kinds of surface treated and as received AlN particles are used as fillers. Three kinds of treated AlN are functionalized by silanes, i.e., amino, epoxy, and mercapto group terminated silanes. Others are functionalized by three kinds of materials, i.e., polyhedral oligomeric silsesquioxane (POSS), hyperbranched polymer, and graphene oxide (GO). An intensive study was made to clarify how the variation of the modifier would affect the microstructure, density, interfacial adhesion, and thus the final thermal conductivity of the composites. It was f...

Development of epoxy/BN composites with high thermal conductivity and sufficient dielectric breakdown strength partI - sample preparations and thermal conductivity

Abstract: The aim of this research is to find a way to achieve the epoxy composites with both high thermal conductivity and acceptable dielectric breakdown (BD) strength. As high thermal conductivity, low permittivity and low thermal expansion coefficient of filler can endow composite with higher thermal conductivity, higher BD strength and lower thermal expansion coefficient respectively, BN (boron nitride) with high thermal conductivity, low permittivity and low thermal expansion coefficient was adopted as main filler in the research. Thermal conductivity was investigated in this part. The BD strength of samples will be discussed in Part II. Neat epoxy and other 25 kinds of epoxy/BN composites were prepared by a hot press method. Most of BN fillers were surface modified with silane coupling agent through ethanol/water reflux method to improve thermal conductivity. The values of 2.91 W/m·K, 3.95 W/m·K and 10.1 W/m·K as thermal conductivity were obtained for the composites that was single-loaded with h-BN(hexagonal boron nitride), c-BN (cubic boron nitride) or conglomerated h-BN, respectively. They were further improved to 5.26 W/m·K, 5.94 W/m·K and 12.3 W/m·K, respectively, by adding extra smaller A1N (aluminum nitride) to fill the voids in sample. Thermal conductivity of samples changes with the ratio of c-BN and h-BN when c-BN and h-BN were co-loaded. A value of 5.74 W/m·K as maximum was obtained at their ratio of 1 to 1 when total filler content is 80 wt%. A much higher value of 7.69 W/m·K was obtained by adding extra AIN. From the experiment data, it is concluded that the filler orientation in vertical direction of sample surface and the decrease of voids in sample are very important to obtain high thermal conductivity, and that the filler surface modification is also necessary to improve thermal conductivity especially for epoxy/c-BN composites, and addition of nano silica in small amount can also increase thermal conductivity if sample is prepared appropriately.

Development of epoxy/BN composites with high thermal conductivity and sufficient dielectric breakdown strength part II-breakdown strength

Abstract: The aim of this research is to find a way to achieve the epoxy composites with high thermal conductivity and acceptable dielectric breakdown (BD) strength. A value 12.3 W/m·K is the highest thermal conductivity obtained for epoxy composite in Part I. Dielectric breakdown performances such as short-time dielectric breakdown strength (BD strength), partial discharge (PD) resistance and BD time for composites were investigated in the Part II. In general, micro filler inclusion will increase thermal conductivity and decrease dielectric breakdown performance. Influencing factors are considered to be the orientation of filler, the content of void space, the content ratio in the case of co-mixing, the addition of nano filler, and filler surface modification. Twenty six kinds of composites were prepared in consideration of the above influencing factors. There are two options for most appropriate ones among the composites evaluated in the research. One is an epoxy/ conglomerated h-BN composite with co-loaded nano SiO2 and micro AIN filler. It has 12.3 W/m·K in thermal conductivity, 75.1 kVpeak/mm in BD strength and 260 % of BD time for neat epoxy. It is most suitable when low BD strength and high thermal conductivity is needed. The other one is an epoxy/ h-BN composite with co-loaded nano silica and AIN filler for requirement of very high BD strength but lower thermal conductivity. Optimum thermal conductivity is obtained if flaky h-BN filler is oriented in parallel to heat flow. Since it is difficult to realize full orientation, the use of conglomerated h-BN filler is a suitable option. Optimum BD performance is obtained if void space is reduced by certain methods such as co-dispersion of different size fillers and addition of nano filler.

Electrical properties of epoxy/POSS composites with homogeneous nanostructure

Abstract: The knowledge of the structure-property relationship at nanoscale level is important to develop advanced dielectric polymer composites. Herein dielectric epoxy/polyhedral oligomeirc silsesquioxanes (POSS) composites with homogeneous nanostructure were prepared. Unlike the conventional inorganic nanoparticles (e.g., silica) used for polymer nanocomposite preparation, the POSS molecules used in this work have three advantages: a comparable size with the segments of polymer chains, being capable of reacting with the base polymer, good solubility in many solvents. These three advantages make the POSS be dispersed in polymers at a molecular level and thus their nano-effect could be fully utilized. Microstructure analysis by transmission electron microscopy, atomic force microscopy and X-ray diffraction confirmed the molecular-level dispersion of POSS in the epoxy composites. On this base, the partial discharge erosion resistance, frequency/temperature dependence of dielectric response, space charge distribution and breakdown strength of the epoxy/POSS composites were investigated. Moreover, the correlation between the nanostructure and properties of epoxy/POSS composites was documented.

Characteristics of initial trees of 30 to 60 μm length in epoxy/silica nanocomposite

Abstract: Many studies were carried out on breakdown (BD) V-tb characteristics to bring about 1 to 3 mm long trees in the past. Instead, tree initiation V-ti characteristics are intensively investigated in this paper in order to investigate interactions of trees with nano-filler in their early growth period. Initial trees are defined as tiny trees of 30 to 60 μm length that can be detected by a recently developed detection method. It was shown that tree initiation V-ti characteristics show the same trends with voltage and treated and untreated nano-filler as the BD V-tb characteristics though they are much shorter in time. From V-ti characteristics the following four results are obtained. (1) Tree initiation time is prolonged when epoxy is filled with 5 wt% nano silica of any type in general. (2) It is longer for 40 nm silica than for 100 nm silica. (3) Surface treatment of filler by a coupling agent helps contribute to the further increase in initiation time. (4) Fumed silica gives longer time for tree initiation than fused silica. From SEM observation it is indicated that filler agglomeration is larger in epoxy with surface-treated fused silica than in epoxy with original silica, and that it is larger in epoxy with untreated fumed silica than in epoxy with untreated fused silica. Adhesion of nano-filler with epoxy must be more effective to increase tree initiation time than filler agglomeration. One of the most important findings is a positional relation between initial trees and nano-filler particles. SEM images support that most of initial trees are untouched with nano-filler particles, while some of initial trees are touched with them. Some discussion is made on how initial trees will interact with nano-filler in the path of their growth.

Recent Progress on Ferroelectric Polymer-Based Nanocomposites for High Energy Density Capacitors: Synthesis, Dielectric Properties, and Future Aspects.

Abstract: Dielectric polymer nanocomposites are rapidly emerging as novel materials for a number of advanced engineering applications. In this Review, we present a comprehensive review of the use of ferroelectric polymers, especially PVDF and PVDF-based copolymers/blends as potential components in dielectric nanocomposite materials for high energy density capacitor applications. Various parameters like dielectric constant, dielectric loss, breakdown strength, energy density, and flexibility of the polymer nanocomposites have been thoroughly investigated. Fillers with different shapes have been found to cause significant variation in the physical and electrical properties. Generally, one-dimensional and two-dimensional nanofillers with large aspect ratios provide enhanced flexibility versus zero-dimensional fillers. Surface modification of nanomaterials as well as polymers adds flavor to the dielectric properties of the resulting nanocomposites. Nowadays, three-phase nanocomposites with either combination of fillers...

Polyhedral Oligosilsesquioxane‐Modified Boron Nitride Nanotube Based Epoxy Nanocomposites: An Ideal Dielectric Material with High Thermal Conductivity

Abstract: Dielectric polymer composites with high thermal conductivity are very promising for microelectronic packaging and thermal management application in new energy systems such as solar cells and light emitting diodes (LEDs). However, a well-known paradox is that conventional composites with high thermal conductivity usually suffer from the high dielectric constant and high dielectric loss, while on the other hand, composite materials with excellent dielectric properties usually possess low thermal conductivity. In this work, an ideal dielectric thermally conductive epoxy nanocomposite is successfully fabricated using polyhedral oligosilsesquioxane (POSS) functionalized boron nitride nanotubes (BNNTs) as fillers. The nanocomposites with 30 wt% fraction of POSS modified BNNTs exhibit much lower dielectric constant, dielectric loss tangent, and coefficient of thermal expansion in comparison with the pure epoxy resin. As an example, below 100 Hz, the dielectric loss of the nanocomposites with 20 and 30 wt% BNNTs is reduced by one order of magnitude in comparison with the pure epoxy resin. Moreover, the nanocomposites show a dramatic thermal conductivity enhancement of 1360% in comparison with the pristine epoxy resin at a BNNT loading fraction of 30 wt%. The merits of the designed composites are suggested to originate from the excellent intrinsic properties of embedded BNNTs, effective surface modification by POSS molecules, and carefully developed composite preparation methods.