Buffer Gas Mixture Research Articles

Electron Impact Cross Sections and Transport Studies of C3F6O

Electron impact scattering from C3F6O is studied in this work. The R-matrix method was used for the calculations of elastic, momentum transfer, and excitation cross sections. The attachment cross section was obtained through a parametric estimator based on the R-matrix outputs. The Binary-Encounter-Bethe (BEB) method was used for computing the ionization cross section. The obtained cross section set was used for the transport studies using the BOLSIG+ code, which is a two-term Boltzmann equation solver. The present calculation was performed for steady-state Townsend experimental conditions for E/N, covering a range of 100–1000 Td. The critical dielectric strength of pure C3F6O was found to be 475 Td, which is much greater than that of SF6 (355 Td). The effect of the addition of different buffer gases, such as CO2, N2, and O2, was also examined. For the C3F6O–CO2, C3F6O–N2, and C3F6O–O2 mixtures with 65%, 55%, and 60% C3F6O, respectively, the critical dielectric strength was determined to be essentially the same as that of pure SF6. The presence of synergism was confirmed for these gas mixtures. We further derived the Paschen curve using a fitting method with the transport parameters as the basic inputs. The minimum breakdown voltage of C3F6O accounted for only 55% of that of SF6. The buffer gas mixture improved the condition; however, the performance of CO2 and O2 mixtures was not satisfactory. The addition of N2 as the buffer gas significantly improved the breakdown property of the gas. The mixture of ≥99% of N2 or ≤1% of C3F6O gave a better breakdown characteristic than SF6. Any proportion ≥90% of N2 or ≤10% of C3F6O was suitable in the higher pressure ranges. The present work demonstrates the potential of C3F6O as a substitute gas for SF6 with a negligible environmental threat.

Negative DC Dielectric Breakdown Characteristics and Synergistic Effect of HFO-1336mzz(E) Mixtures

HFO-1336mzz(E), a substitute for sulfur hexafluoride (SF6), has received much attention owing to its excellent insulation and environmental performance. To provide a reference for selecting suitable buffer gas type and mixing ratio in engineering applications, this article investigated the saturated vapor pressure characteristics, negative direct current (−dc) breakdown properties, and synergistic effect of HFO-1336mzz(E)/CO2 and HFO-1336mzz(E)/N2 mixtures. Experimental results show that although the allowable percentage of HFO-1336mzz(E) in the HFO-1336mzz(E)/CO2 mixture is lower than that of the HFO-1336mzz(E)/N2 at the same gas pressure and limiting temperatures, the former one has a higher breakdown voltage in the gas pressure range from 0.1 to 0.7 MPa. Compared with HFO-1336mzz(E)/N2, the shorter distance within the bimolecular composite structures of HFO-1336mzz(E)/CO2 leads to stronger interaction energy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {int}}$ </tex-math></inline-formula> ). As a result, HFO-1336mzz(E)/CO2 has a better synergistic effect in view of breakdown voltage. Based on the comprehensive analysis of saturated vapor pressure, liquefaction temperature, and insulation properties of HFO-1336mzz(E)/buffer gas mixtures, CO2 is superior as a buffer gas to N2 being mixed with HFO-1336mzz(E). The negative dc breakdown voltage of 0.7 MPa HFO-1336mzz(E)/CO2 occupies 99.17%, 93.03%, and 85.92% of that of 0.7 MPa 20%SF6/80%N2 mixture under −5 °C, −15 °C, and −25 °C limiting temperatures, guaranteeing its application as an insulation medium in electrical equipment under 220 kV.

Yields of HONO2 and HOONO Products from the Reaction of HO2 and NO Using Pulsed Laser Photolysis and Mid-Infrared Cavity-Ringdown Spectroscopy.

The reaction of HO2 with NO is one of the most important steps in radical cycling throughout the stratosphere and troposphere. Previous literature experimental work revealed a small yield of nitric acid (HONO2) directly from HO2 + NO. Atmospheric models previously treated HO2 + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO2 and ozone (O3), among others. Here, the yield of HONO2, φHONO2, from the reaction of HO2 + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO2, produced by pulsed laser photolysis of Cl2 in the presence of methanol, reacted with NO in a buffer gas mixture of N2 and CO between 300 and 700 Torr at 278 and 300 K. HONO2 and its weakly bound isomer HOONO were directly detected by their v1 absorption bands in the mid-IR region. CO was used to suppress HONO2 produced from OH + NO2 and exploit a chemical amplification scheme, converting OH back to HO2. Under the experimental conditions described here, no evidence for the formation of either HONO2 or HOONO was observed from HO2 + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO2 detected in the system could be accounted for by OH + NO2. At 700 ± 14 Torr and 300 ± 3 K, φHONO2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO2 was attributed to the HO2 + NO reaction, φHONO2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φHONO2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.

Spectroscopy of high-pressure rubidium–noble-gas mixtures

Spectroscopy of alkali-metal--buffer-gas mixtures at high pressures from single-digit to several hundred bars in the regime of substantial collisional broadening is relevant in a wide range of fields, ranging from collisional redistribution cooling to laboratory astrophysics. Here we report on spectroscopic measurements of dense rubidium--noble-gas mixtures recorded in a pressure cell equipped with soldered sapphire optical viewports, which allows for the controlled realization of extreme conditions of high temperature and high pressure in a table top laboratory experiment. In the gas cell, we have recorded absorption and emission spectra of rubidium subject to 188 bars of helium buffer-gas pressure at 548-K temperature. The spectra to good accuracy follow a Boltzmann-type Kennard-Stepanov frequency scaling of the ratio of absorption and emission spectral profiles. Further, the long optical path length in the cell allowed us to both record spectra of rubidium-argon mixtures at moderate temperatures and high pressures and observe redistributional laser cooling in this system.

Kr-Based Buffer Gas for Rb Vapor-Cell Clocks.

Optically pumped Rb vapor cell clocks are by far the most used devices for timekeeping in all ground and space applications. The compactness and the robustness of this technology make Rb clocks extremely well fit to a large number of applications, including GNSS, telecommunication, and network synchronization. Many efforts are devoted to improve the stability of Rb clocks and reduce their environmental sensitivity. In this article, we investigate the use of a novel mixture of buffer gas based on Kr and N2, capable of reducing by more than one order of magnitude the barometric and temperature sensitivities of the clock, with possible improvement of their long-term stability.

A physical method for investigating defect chemistry in solid metal oxides

The investigation of the defect chemistry of solid oxides is of central importance for the understanding of redox processes. This can be performed by measuring conductivity as a function of the oxygen partial pressure, which is conventionally established by using buffer gas mixtures or oxygen pumps based on zirconia. However, this approach has some limitations, such as difficulty in regulating oxygen partial pressure in some intermediate-pressure regions or the possibility of influencing the redox process by gases that can also be incorporated into the oxide or react with the surface via heterogeneous catalysis. Herein, we present an alternative physical method in which the oxygen partial pressure is controlled by dosing pure oxygen inside an ultra-high vacuum chamber. To monitor the conductivity of the oxide under investigation, we employ a dedicated four-probe measurement system that relies on the application of a very small AC voltage, in combination with lock-in data acquisition using highly sensitive electrometers, minimizing the electrochemical polarization or electro-reduction and degradation effects. By analyzing the model material SrTiO3, we demonstrate that its characteristic redox behavior can be reproduced in good agreement with the theory when performing simultaneous electrical conductivity relaxation and high-temperature equilibrium conductivity measurements. We show that the use of pure oxygen allows for a direct analysis of the characteristic oxygen dose, which opens up various perspectives for a detailed analysis of the surface chemistry of redox processes.

Laser refrigeration of gas filled hollow-core fibres

We evaluate prospects, performance and temperature limits of a new approach to macroscopic scale laser refrigeration. The considered refrigeration device is based on exciplex-mediated frequency up-conversion inside hollow-core fibers pressurized with a dopant - buffer gas mixture. Exciplexes are excited molecular states formed by two atoms (dopant and buffer) which do not form a molecule in the ground state but exhibit bound states for electronically excited states. The cooling cycle consists of absorption of laser photons during atomic collisions inducing light assisted exciplex formation followed by blue-shifted spontaneous emission on the atomic line of the bare dopant atoms after molecular separation. This process, closely related to reversing the gain mechanism in excimer lasers, allows for a large fraction of collision energy to be extracted in each cycle. The hollow-core fiber plays a crucial role as it allows for strong light-matter interactions over a long distance, which maximizes the cooling rate per unit volume and the cooling efficiency per injected photon while limiting re-absorption of spontaneously emitted photons channeled into unguided radiation modes. Using quantum optical rate equations and refined dynamical simulations we derive general conditions for efficient cooling of both the gas and subsequently of the surrounding solid state environment. Our analytical approach is applicable to any specific exciplex system considered and reveals the shape of the exciplex potential landscapes as well as the density of the dopant as crucial tuning knobs. The derived scaling laws allow for the identification of optimal exciplex characteristics that help to choose suitable gas mixtures that maximize the refrigeration efficiency for specific applications.

Surface tension of liquid Ti with adsorbed oxygen and its prediction

Surface tensions of electromagnetically levitated liquid Ti-samples were measured under the influence of oxygen. The partial pressure of oxygen is controlled by means of yttria-stabilized zirconia tubes as well as Ar/H2/H2O buffer gas mixtures. In addition, TiO samples were prepared by adding different amounts of TiO2 powder to pure Ti. No significant dependence of the surface tension on the applied oxygen partial pressure was found for the samples processed. However, the surface tension was found to strongly depend on the bulk oxygen mole fraction determined by chemical analysis. From some experiments a small time dependence of the surface tension became evident.The observed dependencies of the surface tension on oxygen mole fraction as well as on time could be described by the model developed in the present work. It is based on adsorption/desorption rate equations and the surface tension is calculated from the ideal Butler equation. Non-ideal interactions ΔH≠0 between oxygen and titanium are taken into account.It is generally observed that, due to the strong attractive interaction between Ti and oxygen, the surface tension of Ti is much less sensitive to the presence of oxygen than the surface tension of other metallic systems.

The emission spectroscopy of the [formula omitted] system of the CD radical

The emission spectrum of the 0-0,1-1 and 2-2 bands of the C2Σ+-X2Π system in the CD molecule have been obtained using a stainless steel hollow-cathode lamp with two anodes and filled with a mixture of He buffer gas and CD4. The emission from the discharge was observed with high accuracy dispersive optical spectroscopy using a plane-grating spectrograph and recorded by a photomultiplier tube. In total, 428 transition frequencies belonging to the three bands were precisely measured and rotationally analyzed. The current data were elaborated with the help of precise ro-vibrational and equilibrium ground state parameters reported by Zachwieja et al. (2012) from investigation of the A2Δ-X2Π transition. The spin-rotation interaction constants γv and γD,v of the C2Σ+,v=0,1,2 levels were obtained for the first time. Moreover, the main equilibrium parameters of the third excited state were obtained as follows: Te=31,825.944(12) cm−1, ωe=2078.803(27) cm−1, ωexe=59.558(12) cm−1, Be=7.86810(23) cm−1, αe=0.25315(58) cm−1 and γe=-0.01518(28) cm−1.

Hydrogen gas sensing performance of low partial oxygen-mediated nanostructured zinc oxide thin film

In this work, an innovative design was presented to fabricate ZnO nanostructured thin film-based sensor by thermal oxidation at low values of oxygen partial pressure using a buffer gas mixture of H2 and H2O at different operating temperatures. Microstructural observations of the as-fabricated films prepared at different values of oxygen partial pressure showed significantly improved surface roughness and variations in porosity with reference to those obtained by thermal oxidation of sputtered Zn films in air and ZnO prepared by DC reactive sputtering followed by heated in argon. In low operating temperatures of oxygen partial pressure, ZnO nanostructures possessed larger grain sizes and higher porosity. ZnO film prepared by oxidization of sputtered Zn in low oxygen partial pressure possessed more oxygen vacancies that lead to the formation of more active sites to target gas and to be efficient gas sensor. As-fabricated ZnO nanostructures were characterized to be a platform for H2 gas sensing. ZnO nanostructured films with substantially improved surface roughness and porosity were found to show better gas sensing performance toward low levels of H2 (75–1200ppm). This work paves a possibility for developing new porous metal oxides sensors with high computability and good repeatability toward different gases.

The emission spectroscopy of the [formula omitted] system of CD

The visible spectrum of CD has been investigated at high resolution between 24,500 and 27,500cm−1 using a high accuracy dispersive optical spectroscopy technique. The CD molecules were produced and excited in a stainless steel hollow-cathode lamp with two anodes and filled with a mixture of He buffer gas and CD4. The emission from the discharge was observed with a plane grating spectrograph and recorded by a photomultiplier tube. The 0–0, 1–0 and 1–1 bands of the B2Σ-–X2Π transition have been registered and measured, while 2–0 and 2–1 absorption bands (Herzberg and Johns, 1969) have been reanalyzed. The current data were elaborated with help of recent X2Π ground state parameters reported by Zachwieja et al. (2012) from investigation of the A2Δ–X2Π transition. This way, the improved spectroscopic constants for the B2Σ- state of CD have been provided as follows: νe=26,050.787(11)cm−1, ωe=1653.019(25)cm−1, ωexe=123.899(12)cm−1, Be=7.08296(32)cm−1, αe=0.30741(84)cm−1, and γe=-0.10727(42)cm−1.

Electromagnetically Induced Transparency in Potassium Vapors: Features and Restrictions

Features of electromagnetically induced transparency (EIT) in potassium vapors at the D1 line of the 39K isotope are studied. EIT resonances with a subnatural width of 3.5 MHz have been recorded upon excitation by two independent narrow-band diode lasers in a 1-cm-long cell filled with a natural mixture of potassium isotopes and buffer gas. The splitting of EIT resonances in potassium vapors in longitudinal and transverse magnetic fields has been studied for the first time. The splitted components also have a subnatural width. The smallness of the coupling factor of the hyperfine structure in 39K atoms leads to a transition to the Paschen—Back regime at relatively weaker magnetic fields than in the case of Cs, Rb, and Na atoms. Practical applications of the phenomena under study are noted. The theoretical model well explains the experiment.

Rotational analysis of the 0–0 and 1–1 bands of the [formula omitted] system of the AlD isotopologue

The emission spectrum of the A1Π–X1Σ+ system of AlD has been observed in the 22,400–23,700cm−1 spectral region at high resolution using a high accuracy dispersive optical spectroscopy. The AlD molecules were formed and excited in an aluminum hollow-cathode lamp with two anodes, filled with a mixture of Ne buffer gas and a trace amount of ND3. In total, 133 transitions belonging to the 0–0 and 1–1 bands were precisely measured and rotationally analyzed. The present data have been combined with available high resolution measurements of the ro-vibrational bands by White et al. (1993) to provide improved spectroscopic constants for the A1Π, v=0,1 state of AlD. With the help of previous results of Holst and Hulthén (1934) the main equilibrium constants of the A1Π state were obtained as follows: νe=23,652.8343(15)cm−1, ωe=1015.2849(14)cm-1, ωexe=[86.005]cm-1, Be=3.239806(45)cm-1, αe=0.127934(55), γe=[-0.052]cm-1, De=2.91922(76)×10-4cm-1, βe=3.6662(13)×10-4cm-1 and δe=[2.11×10-4]cm-1.

Cs vapor microcells with Ne-He buffer gas mixture for high operation-temperature miniature atomic clocks.

We report on the characterization of Cs vapor microfabricated cells filled with a Ne-He buffer gas mixture using coherent population trapping (CPT) spectroscopy. The temperature dependence of the Cs clock frequency is found to be canceled at the first order around a so-called inversion temperature higher than 80°C whose value depends on the buffer gas partial pressure ratio. This buffer gas mixture could be well-adapted for the development of miniature atomic clocks devoted to be used in specific applications such as defense and avionic systems with high operating temperature environment (typically higher than 85°C). This solution suggests an alternative to buffer gas mixtures generally used in optically-pumped vapor cell atomic clocks.

Nanostructured vanadium carbide thin films produced by RF magnetron sputtering.

In this paper, nanostructured vanadium carbide thin films were deposited on glass substrates and their optical and surface properties were analyzed. All produced samples were transparent in the optical region. Refractive index values were calculated using the Drude model. According to contact angle measurements of the coated surfaces, the samples show high wettability. The surface free energies of the samples were found to be very similar. The influence of the nitrogen content in the buffer gas mixture was determined; it was concluded that the microstructure, refractive index, surface morphology, surface free energy, and thickness of thin films can change in response to the nitrogen concentration of the radio frequency (RF) buffer gas.

Protective coatings with a mixed composition for low-pressure discharge amalgam lamps

The decrease of the ultraviolet radiation of high-power low-pressure discharge amalgam lamps with a protective coating with a mixed composition based on spinels and oxides has been experimentally studied. It has been shown that these coatings exhibit fairly high protective properties at low pressures of the buffer gas mixture and provide useful lifetime of the lamps comparable to that of the best oxide coatings.

A second-generation constrained reaction volume shock tube.

We have developed a shock tube that features a sliding gate valve in order to mechanically constrain the reactive test gas mixture to an area close to the shock tube endwall, separating it from a specially formulated non-reactive buffer gas mixture. This second-generation Constrained Reaction Volume (CRV) strategy enables near-constant-pressure shock tube test conditions for reactive experiments behind reflected shocks, thereby enabling improved modeling of the reactive flow field. Here we provide details of the design and operation of the new shock tube. In addition, we detail special buffer gas tailoring procedures, analyze the buffer/test gas interactions that occur on gate valve opening, and outline the size range of fuels that can be studied using the CRV technique in this facility. Finally, we present example low-temperature ignition delay time data to illustrate the CRV shock tube's performance.

Detection of atomic oxygen and nitrogen created in a radio-frequency-driven micro-scale atmospheric pressure plasma jet using mass spectrometry

In this paper we show mass spectrometry results for a radio-frequency-driven micro-atmospheric pressure plasma jet (μ-APPJ) discharge obtained using a mass analyzer with triple differential pumping allowing us to sample directly in ambient atmospheric pressure environment (Hiden HPR-60). The flow of the buffer gas (mixture of helium and 1% oxygen) was 2 slm and 3 slm and the excitation frequency was 13.56 MHz. We monitored production of atomic oxygen and nitrogen in the plasma for different flows and powers given by the RF power supply. These measurements were made for energies of electrons emitted from the ionization filament below the threshold for dissociation of O2 and N2. In addition to oxygen and nitrogen atoms, yields for O2, N2, NO and O3 are recorded for different powers and gas flows. It is shown that the μ-APPJ is symmetrical and operates in α-mode. The power transmitted to the discharge was below 5 W in all measurements.

The A2Δ–X2Π band system of the CD radical

The A2Δ–X2Π, Δv=0 sequence of the CD radical was observed in the 22800–24000cm−1 spectral region using a conventional spectroscopic technique. The CD molecules were formed and excited in a stainless steel hollow-cathode lamp with two anodes and filled with a mixture of He buffer gas and CD4. The emission from the discharge was observed with a plane-grating spectrograph and recorded using a photomultiplier tube. The observed lines were assigned to the 0–0, 1–1, 2–2, and 3–3 bands. In total, 1189 transition wavenumbers were precisely measured, with an estimated accuracy of 0.003cm−1, and rotationally analyzed. In the final global fit, the present data were combined with available high-resolution measurements of the vibration–rotation transitions [Morino et al., J. Mol. Spectrosc. 174 (1995) 123–131] and pure rotational transitions [Brown and Evenson, J. Mol. Spectrosc. 136 (1989) 68–85; Halfen et al., Astrophys. J. 687 (2008) 731–736]. This procedure enabled the extraction of molecular constants for the A2Δ and X2Π states of CD. The equilibrium parameters were compared with the calculations performed using the Born–Oppenheimer approximation, and a slight difference was observed in some cases due to the partial breakdown of the B–O approximation. The electronic isotopic shift, Δνe, for the A–X transition was determined to be 32.105cm-1, and the shift independent of the nuclear mass coefficient, ΔU=64.762(38)cm-1, was also calculated.

First-order cancellation of the Cs clock frequency temperature-dependence in Ne-Ar buffer gas mixture

Through the detection of Coherent Population Trapping (CPT) resonances, we demonstrate the temperature-dependence cancellation of the Cs clock frequency in microfabricated vapor cells filled with a mixture of Ne and Ar. The inversion temperature at which the Cs clock frequency temperature sensitivity is greatly reduced only depends on the partial pressure of buffer gases and is measured to be lower than 80°C as expected with simple theoretical calculations. These results are important for the development of state-of-the-art Cs vapor cell clocks with improved long-term frequency stability.