Modification of Graphite Surface by Intense Pulsed Ion-Beam Irradiation

Modification of Graphite Surface by Intense Pulsed Ion-Beam Irradiation

K. Kashine^a, H. Suematsu^b, W. Jiang^b, K. Yatsui^b

^aKagoshima National College of Technology, Hayato-cho, Kagoshima 899-5193, Japan ^bExtremely Energy Density Research Institute, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan

Abstract. The highly oriented pyrolytic graphite (HOPG) targets were irradiated with 70–120 J/cm² of pulsed ion beams. The deposited energy by the ablation plasma onto the target was 0.8 J with 100 J/cm² of the ion beam irradiation. The fast heating and fast quenching effects occurred on the target surfaces, and these effects can be enhanced at the higher energy density of beam irradiation. In the SEM analysis, sphere particles and whiskers which approximately 1 mm in diameter were observed on the irradiated surface. These whiskers may have been grown under the hydraulic pressure by the ablation plasma radiation.

INTRODUCTION

When irradiates a pulsed ion beam to the target material, due to strong interaction with matter, ions can travel a very short distance in solid material. This distance is called range. Large amount of the ion beam energy is deposited in a very thin layer, whose thickness is comparable to the range. For instance, the range is only ~13 mm in carbon for the proton with the energy of 1 MeV. With this fast energy deposition, the heated surface layer immediately turns into high-density ablation plasma. In our previous works, we have studied the kinetics of the ablation plasma by the ion beam irradiated aluminum targets experiment [1, 2]. By the pressure and the temperature caused by the ablation plasma, we can expect the surface modification of the target materials. Furthermore, from these works, we have found that the pressure and the temperature are enhanced at the high energy density. Therefore, we can control pressure and temperature in the targets by changing the ion beam energy density on the target surface.

Carbon remains the major thermodynamically stable such as graphite, diamond, liquid and vapor phases. For example, in the phase diagram of carbon, triple point of graphite / diamond / liquid phase exists at ~12 GPa and ~5000 K. Not only synthesis of bulk samples, but also modification of bulk surface of carbon have been performed by shock synthesis [3], ion implantation [4] and pulsed laser irradiation [5, 6]. As mentioned before, irradiation of target by pulsed ion beam also increases temperature and pressure in the vicinity of target surface. It is expected that surface modification of carbon targets by pulsed ion beam irradiation may be possible. In this paper, we attempted the surface modification of graphite targets by the pulsed ion beam irradiation. The highly oriented pyrolytic graphite (HOPG) targets were irradiated by the ion beams with energy density of 70–120 J/cm². The morphology of the irradiated surface was observed by a scanning electron microscopy (SEM).

EXPERIMENTAL SET-UP

FIGURE 1. Experimental setup using a magnetically insulated ion-beam diode

=160 mm) through a vane-structure cathode. The peak voltage of the diode was 1.1 MV. The pulse width was approximately 60 ns. The diode and the target chambers were evacuated to 2´10^–4 Torr of pressure during the experiments.

The ion-beam energy density was measured by a calorimeter at various distance between the anode and the target material (d_AT). The maximum average energy density of ~120 J/cm² was obtained at d_AT=150 mm, which is 10 mm closer than the geometric focal point due to a self magnetic field. The energy density is decreased at d_AT>150 mm.

The HOPG (Advanced Ceramics Corpora-tion, Grade ZYH) plates with thickness of 1 mm was used as the target. The physical properties of the HOPG are shown in Table.