R. Grob

Photoconductivity studies in some nonpolar liquids

Measurements of the photoconductivity of liquid n‐pentane, n‐hexane, cyclopentane, cyclohexane, neopentane, neohexane, and tetramethylsilane induced by far UV radiation (7⩽hν⩽10 eV) have been performed. We have studied the influence of the incident photon energy, of the applied electric field strength, and of the concentration of two electron scavengers (carbon tetrachloride and perfluoromethylcyclohexane). Our results show that in going from the gas phase to the liquid phase, the ionization energy threshold values of these low dielectric constant liquids are lowered by about 1.6 eV. The introduction of an electron scavenger reduces drastically the solvent photocurrent. Our results are consistent with the quenching, by the scavenger, of a solvent expanded excited state, precursor of the geminate ion pair, and not with an epithermal electron scavenging as in high energy photon irradiation.

Extraction of organophosphorus pesticides from soil by off-line supercritical fluid extraction

Supercritical fluid extraction (SFE) conditions were optimized for the isolation of organophosphorus pesticides (OPPs) from soil. Results using pure carbon dioxide and carbon dioxide modified with acetone, ethyl acetate or methanol were compared in terms of recoveries, extraction rates and matrix effects. Despite the good solubility of the OPPs in pure CO2, the addition of methanol was necessary to achieve high recoveries (90%) when spiked soil was extracted. The extent to which matrix effects, which probably cause the decrease in recoveries, occur depends on both the analyte polarity and the spiking method used. Similar extraction efficiencies were achieved by adding microlitre amounts of the modifier directly to the soil in the extraction cartridge and by using a pre-mixed gas cylinder. SFE of OPPs in soil was compared with solvent extraction.

Spark decomposition of SF/sub 6//H/sub 2/O mixtures

The spark decomposition of mixtures of SF/sub 6/ (partial pressure from 100 to 500 kPa) and water (concentrations between 600 and 2000 VPM) has been studied. The sparks were generated between a point (stainless steel) and a plane (aluminum) either under 50-Hz AC voltage or by discharging a capacitor. Owing to the high water content the only stable gaseous oxyfluorides detected were, in order of decreasing importance, SOF/sub 2/ and SO/sub 2/F/sub 2/. The yields of these two byproducts have been measured as a function of the number of sparks, the SF/sub 6/ pressure, and the H/sub 2/O pressure. When the sparks additionally struck a solid insulator (polytetrafluoroethylene or biphenolic resin), CF/sub 4/ was detected. Its appearance was paralleled by a sharp increase in the yield of SOF/sub 2/, whereas that of SO/sub 2/F/sub 2/ remained unchanged. >

Study of the decomposition of SF/sub 6/ in the presence of water, subjected to gamma irradiation or corona discharges

The influence of water on the degradation of SF/sub 6/ was studied by subjecting mixtures of SF/sub 6/ and H/sub 2/O either to irradiation by /sup 60/Co gamma -rays or to corona discharges (point-plane electrodes; voltage: DC negative or positive polarity). The stable-reaction products formed were analyzed and assayed with various analytical techniques including gas-phase chromatography, gas chromatography-mass spectrometry, specific electrode, colorimetry, atomic absorption, and EDAX (energy-dispersive analysis by X-rays). The influence of the energy input, the water content, the pressure of SF/sub 6/ and, for the corona discharges, the value of the discharge current and the polarity of the applied voltage on the formation of SOF/sub 2/ and SO/sub 2/F/sub 2/ which were the only two stable gaseous compounds detected, were all studied. The results obtained were compared with data from the literature and with the reaction schemes proposed to date, to explain the decomposition of SF/sub 6/ under the effect of similar or different input energies. >

Photoconductivity of liquid 2, 2-dimethylbutane and liquid 2, 2, 4-trimethylpentane

Abstract The photoconductivity dependence on photon wavelength has been studied between 120 and 170 nm for liquid 2,2-dimethylbutane and for liquid 2,2,4-trimethylpentane. The values of the liquid phase photoionization energy threshold for these two alkanes are respectively: 8.49±0.05 eV and 8.38±0.05 eV .

Dependence of free ion yield on electrical field strength in a dielectric liquid irradiated by 60Co γ rays

A corrected curve of free ion yield versus electric field is given for n‐hexane as well as new curves for 2‐methylpentane, 2,2,4‐trimethylpentane, and carbon tetrachloride. Comparison with the Onsager theory of geminate ion recombination does not give reasonable agreement over the entire range of field strengths for any of the functions used.(AIP)

Influence of sample preparation and conditioning procedures on the electrical properties of silicone oil

Abstract In order to assess the influence of polar groups and manufacturing residues on the electrical behavior of silicone oil, we have studied different preparation and conditioning procedures. We find that the best filters were Teflon membranes and breakdown varies significantly with the number, nature and dimension of particles. Electrical breakdown, conductivity and dielectric losses are all varying with water content ; d.c. conductivity and tan δ show a saturation above a water concentration well below the oil saturation level. Elution through activated silica gel modifies irreversibly the current-voltage characteristics. These results demonstrate that electrical properties of silicone oil depend strongly of preparation and drying conditions.

Transient Currents in Silicone Oils Subjected to Voltage Steps with Polarity Reversal

We have studied the transient currents flowing in silicone oils subjected to voltage steps with polarity reversal. The experiments have been carried out, at room temperature, on two 50 cSt viscosity polydimethylsiloxane silicone oils (Rhône-Poulenc 604 V 50 and Dow Corning 561) in a parallel plate conductivity cell as a function of the applied electric field strength (0.4 ≦ E(kV.cm−1) ≦ 10), of the cell gap (0.05 ≦ d(cm) ≦ 0.55), of the oil viscosity (50 cSt to 300 cSt) and of the oil purity. From our measurements we can assert that: 1) the peaks appearing on the wave forms corresponding to the commercial oils are due to some impurities present in these oils; 2) these peaks cannot reflect the migration of a charged layer between the electrodes; 3) they are more certainly correlated to some dipole reorganization near the electrodes after each voltage polarity reversal.

Effect of some electron scavengers on the photoconductivity of several non polar liquids

Abstract The effect of some electron scavengers on the photoionization current of seve- ral non polar liquids irradiated by far U.V. photons has been studied as a func- tion of the scavenger concentration, c, (a1 -3 ⩽c(M) ⩽1.10 -1 , of the photon ener- gy (7⩽hv(eV) ⩽10) and of the applied electric field strength (3⩽E (kV.cm -1 ⩽ 15). We have noticed a decrease of the photocurrent as the scavenger concentra- tion increased. This scavenging effect is similar to that already observed in the same mixtures irradiated by high photons ( 60 Co γ-rays). The plots of I 0 /I c (I o and I c being respectively the solvent photocurernt without scavenger and with a scavenger concentration c) versus scavenger concentration give straight lines whose slopes are independent of the applied electric field strength, at least between 0 and 15 kV.cm -1 and the photon energy, at least between the solvent photoionization energy threshold value and 10 eV. The linear regressions that we have obtained for I c /I c versus c indicate that at the low excitation energies we have used, the scavengers react with metasta- ble, neutral, excited states of the solvents and not with the geminate ion-pairs.

Photoionization of a 50 cSt viscosity polydimethylsiloxane silicone oil

We have studied the influence of incident photon energy (6⩽hv⩽10 eV) and of applied electric field strength (0 < E ⩽ 15 kVcm-1) on the photoconductivity of Rhone-Poulenc 604 V 50 silicone fluid. Its photoionization energy threshold value is: 7.29±0.05 eV. Within the studied electric field range we observed a strong influence of the field on the shape of the I = f(hv) curves.