Papers by Author: Kunihito Nagayama

Abstract: The relationship between the initial temperature dependence of the detonation velocity and the initial density is examined using a proposed unified form of the EOS for pentaerythritol tetranitrate (PETN). First, the derivatives of the variables at Chapman-Jouguet (C-J) points are calculated. These derivatives are used to investigate the initial state dependence of the detonation velocity using the relationship between the detonation velocity and the initial state variables. As a result, the contributions of the coefficient of thermal expansion and the increment of the initial internal energy to the detonation velocity are clarified. We found that the initial temperature derivative of the detonation velocity can be estimated from the initial density derivative of the detonation velocity.

Abstract: The purpose of this study is to reconstruct the equation of state (EOS) whose parameters can be applied for high energetic material of arbitrary initial density without any modification. The simulation for detonation propagation in arbitrary initial density was proposed as the new method for obtaining the information of the EOS for detonation products of arbitrary initial density. At the same time, to collect the experimental data which verify the applicability of the numerical simulation, the detonation velocity for the system consisting of the pellet explosives and air gaps were conducted. The thickness of the 20 mm diameter pellet explosive was 10 mm, and air gaps were varied 0.5 mm to 2.0 mm. The relationship between detonation velocity and experimental condition was clarified for composition A5. The proposed one dimensional simulation was also conducted. The relationships between the pressure and the specific volume for detonation products were extracted from the proposed simulation

Abstract: A compositionally graded thin film of Fe/Si was fabricated by a gravity-assisted pulsed laser ablation (GAPLA) system. By this method, a compositionally graded structure along gravity direction was successfully produced under a gravity field of 5,400 G. Systematic experiments were conducted by several parameters, including gravity, distance between target and substrate, and laser fluence in case of typical target material of iron disilicide (FeSi₂). We demonstrate that the atomic fraction of Fe, the heavier component of the thin film, showed increasing spatial distribution with the direction of gravity. Relatively high laser fluence as well as a very narrow space between the target and the substrate are found to be essential to the compositionally gradient of thin film.

Abstract: Growth of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films without initial nucleation was realized by an coaxial arc plasma gun at a substrate-temperature of 550 °C and hydrogen-atmosphere of 53.3 Pa. The pulsed arc discharge was triggered at a repetition rate of 1 Hz. The deposition rate was 80 nm/min. X-ray diffraction measurements with 12-keV X-rays from synchrotron radiation indicated extremely broad rings from diamond and none from graphite. The UNCD crystallite diameters were estimated to be approximately 1.3 nanometers by using Scherrer’s equation. The sp3/(sp2+sp3) was estimated to be approximately 57% from the X-ray photoemission spectroscopy. The coaxial arc plasma gun is a new powerful method that might enable us to realize the supersaturated condition with highly energetic ions for the growth of UNCD.

Abstract: We have previously reported that -AlN crystallites with diameters of 0.5–1 µm were occasionally grown on sapphire(0001) by pulsed laser deposition, which implied that the migration mobility of the species deposited on the substrate surface might be an insufficient for the film growth of -AlN. In the present study, in order to enhance the crystal growth of -AlN, sapphire(0001) substrates with an atomically smoothness (step-sapphire) were employed. The growth conditions of - and -AlN extended to higher nitrogen-pressures, as compared to those using normal surface sapphire(0001) substrates (normal-sapphire). This is due to the enhancement in the mobility of the deposited species on the substrate surface.

Abstract: The growth of ultrananocrystalline diamond (UNCD) by pulsed laser deposition necessitates hydrogen atmospheres during the deposition. Optical emission spectroscopy was used to study the roles of the hydrogen atmosphere on the UNCD growth. Time-resolved images of a plume that expanded from a laser-irradiation spot toward a substrate were taken using a high-speed ICCD camera equipped with narrow-bandpass filters. While the plume disappeared at the longest within 1 s in vacuum, the emission from C+ lasted above the substrate surface for approximately 7 microseconds at a hydrogen pressure of 53.3 Pa. Since emission lifetimes of species are approximately 10 nanoseconds, this implies that C+ ions collided actively for such a long time. The hydrogen atmosphere has a role of forming a high number density of C+ ions. In addition, we believe that atomic hydrogen that might be generated by the collisions with carbon species contributes to the UNCD crystallite formation by their terminating the dangling bonds of carbon clusters as theoretically predicted in previous reports.

Abstract: Polymer materials have widespread applications in various situations for structural materials by themselves as well as by combining with other materials such as carbon fiber. Some of them are also candidates for energetic materials in space applications.[1] Due to their general use, shock response of them has attracted attention for many researchers.[2-4] One of the striking characteristics of the dynamic response of them is that stress and/or particle velocity profile has a relaxation structure of s range.[5, 6]

Abstract: The behaviors of the high explosive near the critical conditions for shock initiation of detonation are investigated by high speed photography and pressure measurements in gap tests. The sample is RDX base explosive, and the inner diameter of donor and acceptor charges is 26 mm. Gap material is PMMA. Near the critical condition, the results under the following conditions have been discussed. 1) Shock to detonation transition (SDT) take place in acceptor, 2) The SDT does not occur, but the reaction wave affects the leading shock front in acceptor, and 3) The gap length in which the effect of the reaction wave to shock front almost disappears. These results are very useful to construct the initiation model for solid explosive.