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The Forefront of Space Science

Research on Ultra-High Temperature Liquids Using the Electrostatic Levitation Technique
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Introduction

In our daily lives, we use containers to carry beverages. If the liquid spills out of the container, it spatters. Whether the temperature of a liquid is 100 deg. C or 500 deg. C, we still need containers to retain it. When a liquid’s temperature exceeds 1,000 deg. C, the selection of a container becomes difficult because, if the container’s melting point is low, the container itself melts. In some cases, the liquid and container cause a chemical reaction with each other. When the temperature rises to over 2,000 deg. C, there is no available material for a container.

When we examine the characteristics of a liquid, we need to retain the liquid in one place to measure it precisely. For this reason, there are many unsolved questions regarding the properties of liquids that cannot be held in containers due to their high melting points. For liquids with high melting points such as silicon (1,412 deg. C) and iron (1,535 deg. C), two important materials from an industrial viewpoint, we still do not fully understand their fundamental characteristics.

We are researching extremely high-temperature liquids with melting points of 1,000 deg. C and 2,000 deg. C. Our target is to measure the most basic material data Eatomic structure, electronic structure and density Eand to identify thermophysical properties, such as viscosity and surface tension, which are important from an industrial aspect. Eventually, we intend to utilize such information for material development.


Electrostatic levitation technique

If it were possible to levitate liquids that no container can hold, we could avoid the problem of a container. Merely levitating a liquid, however, allows it to move around and hamper accurate measurement. To conduct an experiment, it is important to keep a liquid stable and levitated, but this is not easy.

JAXA has been engaged in the research and development of the electrostatic levitation technique with the aim of performing levitation dissolution experiments in the International Space Station (ISS) (see the article on August 17, 2005, of the Forefront of Space Science). The electrostatic levitation technique lifts samples by utilizing Coulomb's force (gravitation is caused between plus and minus while repulsive force is caused between plus and plus or minus and minus). Coulomb’s force works between the charged sample and the electrodes surrounding it (Fig. 1). The basic technology of this technique was developed at the Jet Propulsion Laboratory (JPL) of NASA. The technique has several advantages, for example, any material that can be charged can be levitated. The levitated sample is dissolved by laser radiation that heats the sample. So far, we have successfully dissolved several ultra-high-temperature melting-point materials such as tungsten (melting point 3,410 deg. C) and rhenium (3,180 deg. C) for the first time in the world.

Our device for the electrostatic levitation dissolution test has been developed for experiments on the ISS. To this end, the system is designed to be compact and portable so that it can be launched by rocket and used for experiments in the limited space on the ISS. Accordingly, the device can be installed not just on the ISS or our research laboratory, but also in various external sites. We devised a plan to install the electrostatic levitation and dissolution test system in a site other than the ISS to study atomic structure and electronic structure of ultra-high-temperature liquids.


Figure 1
Figure 1. Interior of the electrostatic levitation and dissolution system
A 2mm-diameter metal ball is levitated between upper and lower copper electrodes (distance between two electrodes is 8mm). Voltage between electrodes is about 15kV. To prevent discharge, a vacuum (about 10-5Pa) is maintained within the chamber.


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