Biomedical subjects

J de Urquijo

Publications and source records attributed to J de Urquijo.

7 recordsLinked to original sources

PubMed2005

Electron drift velocities in mixtures of helium and xenon and experimental verification of corrections to Blanc's law.

Measurements of electron drift velocities were performed in pure Xe and He and in a number of mixtures ranging up to 70% of Xe. The data were obtained by using a pulsed Townsend technique over the density-normalized electric field strength E/N between 1 and 100 Td . Even for pure gases there are no data in the entire range covered here, and these data represent an extension of accurate drift velocities to higher E/N. A selection of well-established cross sections for low energies, which was extended to higher energies, led to a reasonably good agreement of the calculated transport coefficients with the available data. At the same time we have applied the standard (common E/N) Blanc's law and two forms of common mean energy (CME, due to Chiflykian) procedures. Blanc's law fails for most mixtures at low and moderate E/N, while the CME procedure is capable of following the experimental data for the mixtures much more closely, and even predicting the negative differential conductivity region when such effect does not exist for pure gases. Thus the present paper also represents an experimental test of procedures to correct the standard Blanc's law. Finally, we have used the data for two mixtures to obtain results for the third mixture and in all cases this procedure gave excellent results even though only the standard Blanc's law was used in the process.

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PubMed2005

Measured and calculated SF-6 collision and swarm ion transport data in SF6 -Ar and SF6 -Xe mixtures.

The measurement of the mobility of SF-6 in the mixtures SF6 -Ar and SF6 -Xe is reported over the density-reduced electric field strength E/N 1-180 Td (1 Townsend = 10(-17) V cm(2)), from a time-resolved pulsed Townsend technique. Simultaneously, the mobility of SF-6 in the same binary mixtures has been calculated from a set of collision cross sections for SF-6 -Ar, SF-6 -Xe, and SF-6 - SF6 using a Monte Carlo simulation procedure for ion transport. The good agreement between measured and calculated mobilities in these gas mixtures has led us to conclude that the validation of our cross section sets is confirmed. The elastic collision cross section, a predominant process for ion energies lower than about 10 eV, was determined from a semiclassical JWKB approximation using a rigid core potential model for the ion-neutral systems under consideration. This elastic cross section was then added to several other inelastic collision cross sections found in the literature for ion conversion, electron detachment of SF-6 and charge transfer. Moreover, the calculations of the mobility and the ratios of the transverse and longitudinal diffusion coefficients to the mobility were extended into a much wider E/N range from 1 to 4000 Td. Additionally, we have also calculated the energy distribution functions and the reaction coefficients for ion conversion and electron detachment. Finally, we have shown that the range of validity for the calculation of the mobility in gas mixtures from Blanc's law is only valid for the low E/N region, where the interaction is dominated by elastic collisions and the ion distribution function remains essentially Maxwellian.

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PubMed2003

Ion mobilities in Xe/Ne and other rare-gas mixtures.

The ion mobility or drift velocity data important for modeling glow discharges in rare gas mixtures are not generally available, nor are the ion-neutral scattering cross sections needed to calculate these data. In this paper we propose a set of cross sections for Xe+ and Ne+ collisions with Xe and Ne atoms. Ion mobilities at 300 K calculated using this cross section set in a Monte Carlo simulation are reported for reduced field strengths, E/N, up to 1500 x 10(-21) V m(2), in pure gases and in Xe/Ne mixtures containing 5% and 20% Xe/Ne, which are mixtures of interest for plasma display panels (PDPs). The calculated Xe+ mobilities depend strongly on the mixture composition, but the Ne+ mobility varies only slightly with increasing Xe in the mixture over the range studied here. The mobilities in pure gases compare well with available experimental values, and mobilities in gas mixtures at low E/N compare well with our recent measurements which will be published separately. Results from these calculations of ion mobilities are used to evaluate the predictions of Blanc's law and of the mixture rule proposed by Mason and Hahn [Phys. Rev. A 5, 438 (1972)] for determining the ion mobilities in mixtures from a knowledge of the mobilities in each of the pure gases. The mixture rule of Mason and Hahn is accurate to better than 10% at high field strengths over a wide range of conditions of interest for modeling PDPs. We conclude that a good estimate of ion mobilities at high E/N in Xe/Ne and other binary rare gas mixtures can be obtained using this mixture rule combined with known values of mobilities in parent gases and with the Langevin form for mobility of rare gas ions ion in other gases. This conclusion is supported by results in Ar/Ne mixtures which are also presented here.

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PubMed2003

Negative ion motion in the mixtures of SF6 with CF4 and CH4-Ar.

This paper deals with the measurement of the mobility of negative ions in the mixtures of SF6 with CF4 and the CH4-Ar (50:50) binary mixture with SF6 contents up to 50%. The pulsed Townsend technique was used to observe the integrated ionic avalanches over a range of the density-reduced electric field E/N for which ionization is either negligible or absent, and attachment processes are significant, leading to the formation of mostly SF-6. The E/N range of measurement was from 1 to 70 Td (1 Td=10(-17) V cm(2)), over which the measured mobilities were found to be almost constant. The mobility of the negative ions was also measured for trace amounts of SF6 in CH4 and Ar and 1% CF4, thereby providing a good value of the mobility of SF-6 in these pure gases, in order to test the measured mobilities with Blanc's law. We have found good agreement, within quoted experimental uncertainties, between calculated and measured values.

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PubMed2001

Motion of SF+x (x=1-3,5) and ion conversion in SF6-N2 mixtures.

This paper reports on the mass-analyzed measurement of the drift velocity of the positive SF6 daughter ions, SF+x (x=1-3,5) in the mixture of SF6 with N2 for SF6 concentrations of 10% and 50%. A double mass spectrometer-drift tube was used for these measurements. The density-normalized electric field intensity E/N, was varied from 30 to 360 Td (1 Td=10(-17) V cm(2)). We have found an increasingly monotonic dependence of the drift velocity with E/N, and an inverse dependence of the above parameter with the decrease of the SF6 content in the mixture. A test of Blanc's law for the zero-field reduced mobilities of the above ions shows a fair qualitative agreement. No drift velocities of these ionic species could be measured in pure N2, since we found that these ions reacted strongly with the nitrogen molecules. The cases for SF+3 and SF+5 are discussed. A similar behavior was observed for the nitrogen ions drifting in pure SF6.

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PubMed2000

N+ charge transfer and N+2 dissociation in N2 at swarm energies.

This paper reports a drift-tube-mass-spectrometer measurement of the relative abundances of N+ and N+2 in pure nitrogen, over a ratio of electric field to gas density, E/N, from 800 to 7200 Td [1 townsend (Td)=10(-17) V cm(2)]. A proposed charge transfer dissociation scheme between the above two ions and N2 allowed us to obtain spatial rate coefficients for charge transfer and dissociation over the E/N range 800-2800 Td. Using previously measured cross sections for the above processes, and assuming a Maxwellian distribution of ion velocities, we calculated the reaction coefficients, which were found to be in good agreement with our measured values. In particular, the present results support the trend toward fairly high charge transfer cross section values for N+ energies above 10 eV. In the overlap range between 2.4 and 7.2 kTd, our concentration ratio [N(+)/N(+)(2)] is about five times smaller than that measured previously from a diffuse Townsend discharge in which electron impact is involved in addition to N+2 collisional dissociation with N2, but has the same trend. Thus it seems that, besides N+2 dissociation by electron impact, collisional dissociation becomes important at elevated values of E/N. In connection with previous discharge work in nitrogen, the present study may help explain the enhanced cathode yields observed.

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PubMed1999

Ionization, electron attachment, and drift in CHF3.

Using a pulsed Townsend technique, we have measured the effective ionization coefficient and the electron drift velocities in CHF(3). The density-normalized electric field intensity E/N ranged from 4 to 250 townsends (Td) (1 Td=10(-17) V cm(2)). The E/N value at which the effective ionization coefficient becomes zero was estimated to be 66 Td. For E/N<20 Td, the electron attachment coefficients are practically constant, and are compatible to within about +/-70% with previously measured values at thermal energies and above.

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