Host: The Mineralogical Society of Japan
Most of the solid materials in our solar system are believed to have originated in small grains that condensed during the outflow from a star. Grains are produced by a change from gas phase to solid phase. One of the most advanced methods of forming solid grains from gas phase in the laboratory is the evaporation method in inert gas or reactive gas. Since stars consist of plasma and plasmas exist in interstellar space, it is thought that dust evolves as it passes through the space plasma. Therefore, we tried passing carbonaceous grains into a plasma field. We found that the carbonaceous grains markedly evolved from the structure of original soot. The selective growths of fullerene grains, carbon tubes and carbon onions were found by the use of helium gas methane gases.
Fullerenes were recently detected in the Allende meteorite. The origin of the fullerenes in the Allende meteorite is an intriguing question. The observation of fullerenes together with polycyclic aromatic hydrocarbons (PAHs) suggests that PAHs may have been involved in fullerene synthesis, perhaps with circumstellar envelopes or other formation sites, since such environments generally contain a high abundance of atomic and/or molecular hydrogen.
The electron microscope images and the corresponding electron diffraction patterns of the carbon particles that were produced without passage through plasma and by passage through the plasma region in helium gas atmosphere. When carbon grains were produced without passage through the plasma, spherical carbon grains about 50 nm in size were predominantly produced. The ED pattern showed halo rings, and indicating that the carbon grains were amorphous. The C60 molecules generally comprised about 10 % of the soot produced in helium gas. If the growth conditions, such as gas pressure, source size and source temperature were selected, C60 soot grains with the FCC structure would be seen in the collected soot in smoke. In contrast, when carbon grains were produced by being passed into the plasma region, C60 crystals about 300 nm in size were produced. The growth of the C60 crystal was easily identified by EM imaging. The C60 crystal grown by passage through the plasma region was a plate crystal with a (111) surface plane of the FCC structure. Therefore, the coalescence growth of small crystalline fullerenes was accelerated by passage through the plasma region. Since crystals grow markedly 10 times larger crystal than the size of well-controlled condition in helium gas pressure, the amorphous carbon soot may also coagulate as the C60 crystal with passage through in plasma.
