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Astron. Astrophys. 330, 1080-1090 (1998)

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2. Experiments

The presolar diamonds were extracted using the method described by Tang & Anders (1988). A 10.8 g piece of the Allende meteorite was dissolved by alternating treatment with 10 N HF-1 N HCl and 6 N HCl (10 ml/g meteorite) to remove the silicates. The sulfur was removed with a CS2 treatment and the reactive kerogen was destroyed by oxidation with HNO3 and HClO4. To separate the diamonds the sample was dispersed by ultrasonification in 0.1 M NH4 OH, producing a diamond colloid, which was extracted after centrifugation.

The CVD diamonds were prepared as a hetero-epitaxial diamond film on a silicon (100) substrate which had been polished with 0.25 µg of diamond powder before depositing the CVD diamonds. The CVD diamonds were deposited in a hot filament reactor with 8% CH4 in the H2 source gas. The filament temperature was between 2200°C and 2400°C, the substrate temperatures were between 700°,C and 900°C and the pressure was 5 mbar. This gives a growth rate of about 1 µg/hour. The CVD diamonds were scraped off from the silicon substrate with a diamond needle.

Transmission electron microscopy (TEM) was carried out on both the presolar diamonds and the CVD diamonds, using a Philips EM 430 transmission electron microscope operated at 300 keV (Fig. 1). With this instrument we performed conventional imaging, electron diffraction and energy-dispersive X-ray spectroscopy (EDS) with a sensitivity down to Boron. We estimated that the sizes of the CVD diamonds are between 1.4 nm and 14 nm, from visual inspection of the TEM images. The CVD diamonds are hence, approximately three times bigger than the meteoritic diamonds.

[FIGURE] Fig. 1. A transmission electron microscope bright-field micrograph showing a cluster of presolar nm-sized diamond crystallite. Each cluster consists of about 1000 diamonds. The corresponding diffraction pattern is shown indicating the (111), (220), (311) and (400) spacings for diamond crystallite (d = 2.065 (2.06), 1.281 (1.26), 1.035 (1.0754) and 0.8892 (0.8916) Å, respectively, with the table values in parenthesis).

The spectral measurements were carried out in the infrared (400-4000 cm-1; 2.5-25 µm) and in the UV/VIS (12200-52600 cm-1; 190-820 nm) region.

The UV/VIS spectra of the presolar and CVD diamonds were taken on a very dilute solution of 350 µg diamonds in distilled water. The spectra were obtained with a HP 8452A Diode-Array Spectrophotometer, which is a single-beam microprocessor-controlled spectrophotometer with a deuterium lamp as the light source giving 190 nm - 820 nm wavelength range. The wavelength accuracy was [FORMULA] 2 nm. The sample cell was a small quartz container with dimensions 40 mm [FORMULA] 10 mm [FORMULA] 1 mm. During the measurements, a reference spectrum of distilled water was used. The difference of the signals obtained for the quartz container filled with distilled water and the one filled with diamonds in distilled water was attributed to the absorption of the diamonds.

The infrared spectra of the presolar and CVD diamonds were obtained with a FT-IR Perkin Elmer 1760 Infrared Spectrometer. The spectrometer operates in the region 400-5000 cm-1. The instrument operation is driven by a computer station which allowed us to record the spectra in digital form. All measurements were performed with 64 scans and a resolution of 1-2 cm-1. For sample preparation the KBr technique was used, where small quantities of the sample are mixed throughly with powdered KBr (300 mg). Because of the softness of KBr and its bulk transparency between 40 and 0.2 µm, the KBr and particle mixture can be pressed into a clear pellet (diameter = 13 mm) (Colthup et al. 1990). Around 300 µg of presolar diamonds was added to the KBr as a suspension in ethanol. The diamonds were suspended in order to separate them, so that they would not be stuck in big clumps, because the diamonds are likely to have formed individually, not in agglomeration in stellar atmospheres. By suspending them we therefore expect to obtain a spectrum which compare best with the possible stellar spectrum. Around 200 µg of CVD diamonds was added to the KBr in the same way and the KBr for the reference pellets were also suspended in ethanol. The ethanol was removed by pumping on the sample before pressing the tablet. Despite this, we still had traces of ethanol in our spectra, so it was necessary to compensate for this by the use of the reference spectra.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998