ZĞĂĐƚŝǀĞĨĨĞĐƚŽĨDŝĐƌŽǁĂǀĞ,ǇĚƌŽŐĞŶWůĂƐŵĂǁŝƚŚ^ŝůŝĐĂƚĞWŽǁĚĞƌ^Ğŝũŝ<ŝŵƵƌĂĂŶĚ^ĞƚƐƵŬŽtĂĚĂdŚĞhŶŝǀĞƌƐŝƚǇŽĨůĞĐƚƌŽͲŽŵŵƵŶŝĐĂƚŝŽŶƐ͕ŚŽĨƵͲƐŚŝ͕dŽŬǇŽϭϴϮͲϴϱϴϱ͕:ĂƉĂŶ
Introduction
A component of crystalline silicates was detected in IR spectra of some evolvedstars or around some young stars. On the other hand, amorphous silicates aresuggested as the carrier of a broad 9.7 and 18µm absorption feature detected ininterstellar medium (ISM). The disappearance of crystalline silicates in the ISM issupposed to be due to their amorphization caused by a continuous processing suchas annealing, cosmic ray and shockwave in interstellar environments. Ion irradiation
experiments aimed at simulating this process were performed by several groups.
Demyk et al. performed the irradiation of olivine with 4-10keV He+ ions [1]. Jäger etal. performed the irradiation experiments on enstatite with He+ and Ar ions in the 50400keV energy range [2]. These experiments by energetic ion bombardment showed

Experimental procedure

TEM observation

1. Natural olivine (Mg1.8Fe0.2SiO4)
2. Natural enstatite (Mg1.8Fe0.2Si2O6)

silicate powders

that crystalline silicates are transformed into amorphous structures. Ion and solid
reactions with high energy ion irradiation lead to physical sputtering of atoms andformation of defects in their structure. During their life cycle, silicate dust are supposed
to suffer from a processing by an ionized gas. During their life cycle, silicate dusts aresupposed to suffer from a processing due to an ionized gas. In order to study thereaction between silicates and hydrogen plasma with low energies, we exposed thesilicate powders to hydrogen plasma generated by a microwave discharge at 2.45GHz.
We investigated the structural and chemical alteration of crystalline silicate powders
before and after hydrogen plasma treatment.

Olivinequartz boat
discharge tube
H2 gasVacuumMicrowave
2.45GHz

Before plasma treatment

Enstatite
Look of the plasma

Figure is a schematic drawing of
the apparatus used for exposing
the silicate powders to hydrogenplasma. We placed the silicate
powders on a quartz boat and thenput the boat into the discharge tube.
We exposed the silicate powders tohydrogen plasma generated by amicrowave discharge at 2.45GHz.
When the olivine powder wasexposed to the plasma, the color of
the powder changed from white tobrownish gray. On the other hand,the color of the enstatite changed
to brown. When the silicatepowders were exposed to Ar
plasma, the color didn’t change.

afterolivineafter

Plasma conditionsĞůĞĐƚƌŽŶĞŶĞƌŐǇ

͗ƐƉĞĐŝĞƐ
ƐĂŵƉůĞ͛ƐƚĞŵƉĞƌĂƚƵƌĞ
ŐĂƐĚĞŶƐŝƚǇ䌾ϭϬĞs,Ϯ͕,͕,н͕,Ϯн͕ĞͲ͗ ϲϬϬϴϬϬ<͗ ϭϭϱdŽƌƌ͗䌾䌾

Before plasma treatment

UV-VIS spectra
Olivine80

Reflectance (%)

Reflectance (%)

TEM images of untreated enstatite powder and enstatite powder afterhydrogen plasma treatment. Small grains with the size of about
10nm are formed on the surface of the particles after plasma
treatment. TEM image of different position observed in same sample isshown in right figure. A growth of comet-like particles indicates acondensation from vapor phase [3].

Enstatite

80

6040–:untreated–:H2 plasma 90min–:Ar plasma 90min

60

ESR spectra
Olivine
40(b)

20

1000
1500
2000
Wavelength (nm)

2500

500

1000
1500
2000
Wavelength (nm)2500

Reflectance spectra of olivine powder and enstatite powder before and after hydrogen plasma
treatment. The reflectance of olivine powder exposed to hydrogen plasma or Ar plasma decreases
to lower values in the range from ultraviolet to near-infrared wavelength. Although the reflectance ofenstatite powder after hydrogen plasma treatment decreases in the range of visible wavelength, thefeature doesn’t change in near-infrared region. When the enstatite powder was exposed to Ar
plasma, spectral change can be seen at near-infrared. The difference of the spectral changesuggests that olivine reacts with the plasma more easily than enstatite. The decrease ofreflectance in the visible region is due to a change of scattering effect resulted from the alteration ofthe surface or the formation of Fe particles.

Hydrogen plasma effectŵŝĐƌŽǁĂǀĞ ,ͲĐŽŶƚĂŝŶŝŶŐƐƉĞĐŝĞƐ hsƌĂǇƐ;,н͕,͕,ϮͿ䊶ƚŚĞƌŵĂůĞĨĨĞĐƚ;,ĂƚŽŵ͕ŵŝĐƌŽǁĂǀĞͿїŝŶĐƌĞĂƐĞŽĨƚĞŵƉĞƌĂƚƵƌĞ΀ϱ͕ϲ΁;ĐƌǇƐƚĂůůŝǌĂƚŝŽŶŽĨĂŵŽƌƉŚŽƵƐƐƚƌƵĐƚƵƌĞͿ䊶ŶŽŶͲƚŚĞƌŵĂůĞĨĨĞĐƚ;,ĂƚŽŵͿ

Enstatite

(a)

0200

(c)

(a) : 0min(b) : 90min–:untreated–:H2 plasma 90min–:Ar plasma 90min

20500

After plasma treatment

400

Magnetic Field (mT)

600(a) : 0min(b) : 270min(c) : 540min(b)

ESR Intensity

beforeEnstatite

ESR Intensity

beforeAfter plasma treatment

TEM images of untreated olivine powder and olivine powder after hydrogen plasma treatment. Although the surface of
the untreated particle is angular shape, the surface is changed into round shape after plasma treatment. As shownby arrows, black spots with the size of about 1nm are seen in high resolution TEM image. This result suggests that theparticles react with hydrogen plasma to cause the surface alteration.

(a)

0200

400600Magnetic Field (mT)ESR spectra of olivine powder and enstatite powder before and after hydrogen plasmatreatment. Strong ESR signal is detected in the plasma-treated samples. The g-value isestimated to be 2.07, which is very close to the value of Fe-bearing meteorites [4]. ESR
spectra suggest that an iron ion in silicates is reduced to give a ferromagnetic material byhydrogen plasma treatment. ESR signal were not detected in the samples of olivine andenstatite with Ar plasma treatment. The difference is caused by a highly reactive effect due tohydrogen atom in the plasma.

їĚĞĐƌĞĂƐĞŽĨĂĐƚŝǀĂƚŝŽŶĞŶĞƌŐǇ΀ϳ΁^ŝůŝĐĂƚĞƉĂƌƚŝĐůĞƐƉƌŽŵŽƚŝŽŶŽĨƚŚĞƌĞĂĐƚŝŽŶďǇĞŶŚĂŶĐŝŶŐĂƚŽŵŵŽďŝůŝƚǇĂŶĚĚŝĨĨƵƐŝŽŶ΀ϴ΁

SummaryThe hydrogen plasma promotes chemical and structural alterations of silicates effectively.

䋱䋮TEM images reveal that the shape of silicate particles become round after hydrogen plasma treatment.
dŚĞƐĞĞĨĨĞĐƚƐƉƌŽŵŽƚĞƚŚĞƌĞĂĐƚŝŽŶ͘ǀĂƉŽƌĂƚŝŽŶŽĨĂƚŽŵƐŽƌŵŽůĞĐƵůĞƐďǇĐŚĞŵŝĐĂůƌĞĂĐƚŝŽŶƐ

When the silicate powders are exposed to hydrogen plasma, microwave, UV rays, and Hcontaining species affect the particles. Chemical reaction of H-containing species is considered
to be an important process for the alteration of the silicate powders because the change was notfound in the powders with Ar plasma treatment. Even if the silicate powders are heated at 600K,
the change doesn’t occur. The hydrogen plasma promotes effectively chemical and structural
alterations of silicates at the low temperature of about 600K.

Many small grains with about 1nm in diameter are produced on the surface of olivine particles. In the case ofenstatite particles, many grains with about 10nm in diameter are produced.

䋲䋮The reflectance of silicates treated by hydrogen plasma decreases in the range of visible wavelength.
When silicates are exposed to Ar plasma, the reflectance of olivine decreases in the region from UV to nearinfrared wavelength. In the case of enstatite, the reflectance doesn’t change in the range from UV to visible
wavelength. The result suggests that the erosion rate of olivine is large than that of enstatite.

䋳䋮A strong ESR signal is detected in the plasma-treated samples of olivine and enstatite. The g-value isestimated to be 2.07, which is very close to the value of Fe-bearing meteorites. ESR spectra suggest that aniron ion in silicates is reduced to give a ferromagnetic material by hydrogen plasma treatment.
䋴䋮The change of particle shape is caused by the effect of highly reactive sputtering due to hydrogen plasma.

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