The metastable HCl center dot 6H(2)O phase - IR spectroscopy, phase transitions and kinetic/thermodynamic properties in the range 170-205 K

Simone Chiesa, Michel Rossi
Copernicus Gesellschaft Mbh, 2013
Article

Résumé

In this laboratory study a multidiagnostic experimental approach including Fourier transform infrared (FTIR) absorption of 1 to 2 mu m thick polycrystalline ice films, residual gas mass spectrometry (MS) and total pressure measurement were employed. Both amorphous HCl-H2O and crystalline HCl hexahydrate (HCl center dot 6H(2)O) have been investigated. After controlled doping with HCl and evaporation of excess H2O from the ice film, transmission FTIR of pure HCl center dot 6H(2)O films and use of calibrated mass spectrometry enabled the measurement of differential (peak) IR cross sections at several mid-IR frequencies, for example sigma = (6.5 +/- 1.9) x 10(-19) cm(2) molec(-1) at 1635 cm(-1). Two types of kinetic experiments on pure HCl center dot 6H(2)O have been performed under SFR conditions: (a) evaporation of pure HCl center dot 6H(2)O over a narrow T range after evaporation of excess H2O, and (b) observation of the phase transition from crystalline HCl center dot 6H(2)O to amorphous HCl-H2O under H2O-rich conditions at increasing T. The temperature dependence of the zero-order evaporation flux of HCl in pure HCl center dot 6H(2)O led to logJ(ev) molec cm(-2) s(-1) = (36.34 +/- 3.20) - (80 810 +/- 5800)/2.303 RT with R = 8.314 JK(-1) mol(-1), which turned out to be rate-limiting for evaporation. HCl center dot 6H(2)O has a significant intrinsic kinetic barrier to HCl evaporation of 15.1 kJ mol(-1) in excess of the HCl sublimation enthalpy of 65.8 kJ mol(-1) at 200 K but is kinetically unstable (metastable) at T >= 173 K. The atmospheric importance of HCl center dot 6H(2)O is questioned in view of its large nucleation barrier and its dependence on T and P(HCl) compared to the amorphous HCl-H2O phase at upper tropospheric-lower stratospheric (UT/LS) conditions.

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https://infoscience.epfl.ch/record/196083?ln=fr

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Simone Chiesa, Michel Rossi
Copernicus Gesellschaft Mbh, 2013
Article

Résumé

Official source

https://infoscience.epfl.ch/record/196083?ln=fr

À propos de ce résultat

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Concepts associés

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Design, characterisation and modelling of a high current DC arc plasma source for silicon and silicon carbide processing at low pressure

Lukas Derendinger

In the frame of this thesis, two similar high current DC arc (HCDCA) plasma sources were investigated in a low gas pressure regime (10-3-10-2 mbar). One of them was initially designed for the epitaxial growth of silicon and silicon germanium (LEP), the other for the industrial deposition of diamond (BAI). The LEP source was analysed using pure argon plasmas. Measurements of the ion saturation current were performed with a custom-built multi-Langmuir probe to analyse plasma density homogeneity. Plasma instabilities at 50 Hz were observed and studied by different means. One source of the instability is the AC current used for the filament heating, whereas lower frequency instabilities are due to the use of a ring shaped anode. A sensitive Hall sensor was used to measure the magnetic field induced by the discharge current. It was found that depending on the plasma parameters gas pressure and external magnetic field the current tends to attach at different points on the anode, leading to a complete loss of reactor symmetry. The installation of an additional cusp field around the reactor chamber lead to an increase of the overall homogeneity of the plasma density, but it could not resolve the problems of current attachment. In the following the ring anode was replaced by a point anode and the maximum external magnetic field strength was increased by a factor of ten. With a novel multi-Hall probe the current density of the now columnar shaped plasma was measured, showing a strongly peaked current density profile, when an external magnetic field is applied. Together with ion saturation current measurements made with a Langmuir probe, the electron temperature inside the plasma column was estimated to be about 4 eV. In earlier works made on the BAI reactor the high dissociation efficiency of the HCDCA plasma source has already been shown. Optical emission spectra were compared between RF plasmas, low pressure (1.5mbar) HCDCA plasmas and our very low pressure (10-3-10-2 mbar) HCDCA plasmas. The dominating species found in RF plasma spectra are molecular, the spectra of low pressure HCDCA plasmas are dominated by atomic species and the spectra of very low pressure HCDCA plasmas are dominated by ions, emphasising the high dissociation efficiency of this system. Silane and Methane was used as precursor gases in the BAI reactor for the deposition of silicon carbide films. Promising high deposition rates up to 9 nm/s were found, but FTIR spectroscopy showed high hydrogen and oxygen concentrations in the porous films, making the not optimised deposited material useless for the application of wear-resistant coatings. Microcrystalline hydrogenated silicon (µc-Si:H) is viewed as a cost- and energy-effective alternative to crystalline silicon for the production of solar cells. A detailed analysis of the deposition rate and the Raman crystallinity of µc-Si:H films deposited in the BAI reactor showed deposition rates up to 6.5 nm/s and a wide range of crystallinity from 0-80%. Film thickness inhomogeneity in silicon solar cells has to be less than 5%. To meet this standard a first substantial improvement was achieved with the installation of a linear gas injection along the plasma column. Tests on large surface glasses (47 × 37 cm2) revealed strong diffusive effects, which could be reproduced with a simple gas diffusion model. The model showed the necessity to reduce the dead volume around the plasma and to set the substrates as close as possible to the plasma column in order to minimise film thickness inhomogeneity due to diffusion. Deposition rate measurements made in these conditions assured the results of the model. The development of a method to estimate the dissociation efficiency of the plasma by simple pressure measurements showed also an important increase of the silane dissociation from 75-92% when the plasma is confined in a smaller volume. Therefore, compared to the reactor with a large dead volume, only a third of the initial silane is lost to the pumps.

EPFL2008

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Design, characterisation and modelling of a high current DC arc plasma source for silicon and silicon carbide processing at low pressure

Lukas Derendinger

EPFL2008