Resonant degenerate four-wave mixing in I(2): effect of buffer gas pressure

The influence of molecular collisions on the production of the degenerate four-wave mixing signal in I(2) is presented. Measurements were performed on gaseous molecular iodine, I(2), contained in a glass cell in which pressure, temperature, and species concentration are easily and independently varied. Frequencydoubled outputs from a seeded Nd:YAG laser and an excimer-pumped dye laser were used as excitation sources. We have studied the dependence of signal strength versus buffer gas pressure, with pump intensity as a third parameter. It is evident from our results that, for pump intensities of less than 1 MW/cm(2), the pressure dependence of the signal follows that given by a simple two-level model in the homogeneously broadened regime. In this regime collisional deexcitation becomes significant, leading to changes in saturation intensity. This is evidenced by a reduction in the signal with an increase in buffer gas pressure. This behavior is similar to that seen in laser-induced fluorescence. At higher pump intensities, the signal is seen to increase with pressure; this behavior cannot be described by the simple two-level model.     

Buffer-gas-induced narrowing of electromagnetically induced transparent spectra for an open system

We have investigated the spectra of the electromagnetically induced transparency (EIT) when a cell is filled with a buffer gas. Our theoretical results show that the buffer gas can induce a narrower spectra line and steeper dispersion than those of the usual EIT case in a homogeneous and Doppler broadened system. The linewidth decreases with the increase of the buffer gas pressure. This narrow spectra may be applied to quantum information processing, nonlinear optics and atomic frequency standard.     

Investigation on a Stirling refrigerator with thermal buffer tube driven by an oil-lubricated compressor

In this article, a special configuration of Stirling refrigerator for domestic refrigeration purpose is introduced. A thermal buffer tube is installed between the refrigerator cold-end and the expansion piston to improve the system reliability by moving the expansion piston from low temperature to ambient temperature. Furthermore, a commercial oil-lubricated dual-piston compressor is modified to drive the refrigerator, inside which an elastic membrane is used to transfer acoustic work and separate the working gas of the refrigerator from that of the compressor. Experimental investigations on the refrigerator are performed using helium as the working fluid and a cooling power of 200 W at −78 °C is achieved at 15 Hz working frequency and 2.5 MPa mean pressure. Meanwhile, a rough estimation of the refrigerator COP in terms of cooling power divided by input acoustic power gives the value of 0.64. It gives the possibility of building a low-cost, high efficiency domestic refrigerator.     

Network model approach for calculating oscillating frequency of thermoacoustic prime mover

The network model is given and used to calculate the oscillating frequency of thermoacoustic prime mover. Theoretically calculated frequency by the model is plural, whose real part denotes actual frequency of the system, and imaginary part means attenuation coefficient of acoustic pressure. Numerical calculation is made under different conditions including different acoustic cavities, spacing between each parallel plate, gas pressures and system lengths. Theoretically calculated frequency and experimentally measured frequency are in quite good agreement.     

LNG燃料汽车储罐自增压及供气特性

研究了车用液化天然气储罐自增压过程以及稳压供气特性,研究表明液化天然气储罐在自增压过程中储罐压力以及气腔气体温度稳步上升,而液腔主体液体温度基本保持不变。在稳压供气过程中,主要分析影响储罐压力波动的因素,因为压力波动频率太快不利于压力调节系统的准确控制和使用寿命。分析结果表明工作压力范围太窄以及汽车行驶震动越剧烈都将引起储罐压力波动频率的加快。     

基于Unisim动态模拟的高压缓冲罐内气体状态的分析

根据高压缓冲罐的实际应用情况,结合Unisim的动态模拟功能,从充压、泄压和备用三个工况对缓冲罐内的气体状态进行了研究,得出了各个工况下的温度压力随时间变化的特点,辨析出各种工况下存在的潜在风险,并提出了预防措施,以便在日常工作中能多一个考量维度,设计出更加完善的后备系统.     

Oxygen Potentials and Oxygen-potential Diagrams for Buffer Gas System at Normal Pressure

The principles of chemical equilibrium were used to derive a new set of formulae representing the oxygenpotentials for five buffer gas mixtures at normal pressure.Various sorts of classical formulae for them are on-ly a particular representation under ignoring the effects of oxygen,thereby being unavailable for accurate cal-culation of oxygen potentials.In this paper,the oxygen potentials set by CO_2-H_2 gas mixtures and the mis-takes and errors of the classical expressions for them were discussed emphatically.The deviation in partialpressures of oxygen in some of previous experiments under the oxygen potentials controlled by CO_2-H_2 gasmixtures was explained quantitatively.The oxygen-potential diagrams predicting the equilibrated gas com-positions from the initial conditions have been also given.     

The effect of pressure on ultrasonic testing sensitivity of hot steel blooms by pressure contact method

The effect of exerted pressure of the buffer rod on the sensitivity of ultrasonic testing of hot steel blooms by the pressure contact method is discussed. The formula describing their linear relation is introduced. Results show that, at a given temperature, sensitivity increases linearly with pressure until a certain pressure is reached where sensitivity is rather insensitive to pressure variations. However, this pressure region is not always used. The proper choice of exerted pressure for practical use is analyzed.     

Electric conductivity of molecular hydrogen plasma with alkali metal additive

The electric conductivity of a gas mixture as a possible working medium for a magnetohydrodynamic generator (a minor amount of cesium or potassium in argon and molecular hydrogen) is calculated in the 1000–5000 K temperature range at a constant pressure of 1 atm. The advantages of molecular hydrogen as a possible buffer gas for magnetohydrodynamic generators are discussed.     

高温高固气质量比条件下旋风预热器换热管压降的研究

为了掌握旋风预热器换热管压降随温度、风速和固气质量比的变化规律,在各种不同条件下,对旋风预热器换热管压降进行测试。结果表明:在气流速度固定不变的情况下,当固气质量比z≤1时,随着温度的升高,压降先增大后减小,在200℃附近出现高点;当固气质量比z≥1.5时,随着温度的升高,压降逐渐减小,在600℃附近趋于平缓,之后随着温度的升高略有增大。压降总是随着风速的提高而增大,随着固气质量比的增加而增大。     

Extensional bundle waveguide techniques for measuring flow of hot fluids

A bundle of acoustically slender metal rods, each thin compared to wavelength, tightly packed within a sheath, and welded closed at each end, provides a dispersion-free waveguide assembly that acts as a thermal buffer between a transducer and the hot fluid medium the flow of which is to be measured. Gas and steam flow applications have ranged up to 600 degrees C. Liquid applications have ranged from cryogenic (-160 degrees C) to 500 degrees C and include intermittent two-phase flows. The individual rods comprising the bundle usually are approximately one millimeter in diameter. The sheath, made of a pipe or tube, typically has an outside diameter of 12.7 to about 33 mm and usually is about 300 mm long. Materials for the sheath and bundle are selected to satisfy requirements of compatibility with the fluid as well as for acoustic properties. Corrosion-resistant alloys such as 316SS and titanium are commonly used. The buffers are used with transducers that are metal-encapsulated and certified for use in hazardous areas. They operate at a frequency in the range of 0.1 to 1 MHz. The radiating end of the buffer is usually flat and perpendicular to the buffer's main axis. In some cases the end of the buffer is stepped or angled. Angling the radiating faces at approximately 2 degrees to overcome beam drift at Mach 0.1 recently contributed to solving a high-temperature high-velocity flow measurement problem. The temperature in this situation was 300 degrees C, and the gas molecular weight was about 95, with pressure 0.9 to 1.1 bar.     

射频溅射ZnO作为GaN厚膜生长缓冲层的工艺研究

采用射频磁控溅射法制备出了适用于HVPE-GaN厚膜生长的ZnO缓冲层,利用X射线衍射(XRD)和原子力显微镜(AFM)和光致发光(PL)等分析方法表征了ZnO缓冲层以及HVPE-GaN厚膜的晶体性能。实验结果表明,采用溅射功率为60W、氩气压强为2.0Pa、蓝宝石衬底为室温条件下的溅射工艺获得了(0002)单一取向、晶界清晰、晶粒尺寸均一的ZnO薄膜,以它为缓冲层获得的GaN厚膜XRD的(0002)衍射峰半高宽(FWHM)为265secarc,室温PL谱未见明显黄光发射带。     

高频微型脉冲管制冷机的数值研究

利用数值方法模拟了高频微型同轴脉冲管制冷机内部气流交变流动和换热过程。给出了小孔型，双向进气型和多路旁通型脉冲管制冷机内部气流动态参数的瞬态变化。分析了各动态参数变化对制冷机整机性能的影响。     

Influence of Variations of the Composition of Natural Gas from Different Fields on the Results of Mass and Volumetric Flowrate Measurement under Standard Conditions in Gas Distribution Organizations

Formulas are given for the conditionally-constant values of density under standard conditions as well as for the dynamic viscosity and the adiabatic exponent. The dynamic viscosity and adiabatic exponent depend only on the natural gas temperature and pressure that we recommend be used for mass and volumetric flowrate measurements under standard conditions in gas distribution organizations.The paper considers the feasibility of correcting meter readings not by entering the daily conditionally-constant composition of natural gas into a calculator, but by correcting the readings of all the instruments participating in commercial operations during the accounting period. This would be done by using correction factors that depend on the density under standard conditions and fixed consumption patterns (absolute pressure and temperature of the gas).     

Caracterization of AIN buffer layers on (0001)-sapphire substrates

It is now established that low temperature-grown buffer layers are needed to improve the structural and electronic properties of GaN layers grown on sapphire. Using X-ray diffraction, we have studied the dependency on temperature and annealing time of the recrystallization of AlN buffer layers grown by low pressure MOVPE. The Warren–Averbach method applied to the broadening and shape analysis of the (0002) and (0004) X-ray diffraction peaks has allowed us to separate grain size distribution from micro-strain effects. The obtained evolution of the relative frequency distribution of the grain sizes with annealing conditions is correlated with atomic force microscopy experiments (dynamic mode). The angular distribution of the c-axis of the grains is determined from X-ray rocking-curves experiments. X-ray reflectometry experiments and a simulation procedure have given us access to the roughness and the chemical composition of the sapphire-buffer layer interface and to the thickness and the roughness of the AlN grown.     

Modeling resonator frequency fluctuations induced by adsorbing and desorbing surface molecules

Resonator frequency fluctuations due to adsorption and desorption of molecules on plate electrodes are studied using the principle of mass-loading effects of adsorbed molecules. The study is based on a 525 MHz, AT-cut quartz resonator enclosed in a small crystal holder. Equations relating the surface adsorption rates of the crystal holder to pressure were derived and found to be quadratic polynomial functions of the adsorption rates. Calculations based on these equations show that a contaminant gas with a higher desorption energy creates larger changes in pressure when the temperature is varied. The function describing the frequency fluctuations due to any one contaminant site is a continuous-time Markov chain. Kolmogorov equations and an autocorrelation function for the Markov chain are derived. The autocorrelation and spectral density function of resonator frequency fluctuations are derived. The spectral density of frequency fluctuations at 1 Hz is studied as a function of pressure, temperature, and desorption energy of molecules. The noise levels for a contaminant gas with one type of molecules are found to be lower for lower desorption energies, and higher at lower pressures.     

A Proposed Dynamic Pressure and Temperature Primary Standard

Diatomic gas molecules have a fundamental vibrational motion whose frequency is affected by pressure in a simple way. In addition, these molecules have well defined rotational energy levels whose populations provide a reliable measure of the thermodynamic temperature. Since information concerning the frequency of vibration and the relative populations can be determined by laser spectroscopy, the gas molecules themselves can serve as sensors of pressure and temperature. Through measurements under static conditions, the pressure and temperature dependence of the spectra of selected molecules is now understood. As the time required for the spectroscopic measurement can be reduced to nanoseconds, the diatomic gas molecule is an excellent candidate for a dynamic pressure/temperature primary standard. The temporal response in this case will be limited by the equilibration time for the molecules to respond to changes in local thermodynamic variables. Preliminary feasibility studies suggest that by using coherent anti-Stokes Raman spectroscopy we will be able to measure dynamic pressure up to 10⁸ Pa and dynamic temperature up to 1500 K with an uncertainty of 5%.     

室温磁制冷机的实验性能研究

介绍了利用1.5 T永磁体提供磁场、选取氦气作为换热流体、采用主动式磁回热器的室温磁制冷机的实验性能研究情况.应用现有实验装置,通过调节气体压力与循环频率,最大达到了30.93 K的高低温端温差,在高低温端温差为10.97 K达到了25.20 W制冷量.研究结果表明室温磁制冷机的制冷能力显著,并且为室温磁制冷机的设计以及进一步的发展提供了有益的参考.     

Effect of different working gases on the performance of a small thermoacoustic Stirling engine

The performance of a small thermoacoustic Stirling heat engine (TASHE) was investigated with three kinds of working gases experimentally and numerically. The examined performances focused on the operating frequency, onset temperature, pressure amplitude and some temperature characteristics after onset. The working frequency with nitrogen, argon and helium as the working gas was 45 Hz, 42 Hz and 130 Hz, respectively. The engine worked with helium in a much wider range of mean pressure than with nitrogen and argon. There was an optimal mean pressure for the minimum onset temperature for each working media. Using nitrogen and argon as working gas rather than helium, another optimal mean pressure for the highest pressure ratio was obtained in the experiment. The loop dimension was indispensable in determining the frequency and the highest pressure ratio was observed in the resonator cavity.     

On the mechanism of encapsulated particle formation during pulsed laser deposition of WSe x thin-film coatings

We have studied factors influencing the formation of particles with the structure of a spherical metal W core inside a WSe₂ shell during pulsed laser deposition (PLD) of thin films of tungsten diselenide under variable conditions (buffer gas (Ar) pressure, substrate temperature). It is established that the metal core is formed at the stage of laser ablation of a synthesized WSe₂ target, while the shell grows as a result of condensation, migration, and redistribution of atoms during deposition of a laser-initiated atomic flow on the surface of a growing film. Retardation of the atomic flow by a buffer gas at pressures within 2–10 Pa does not ensure activation of the shell condensation process on the metal core in the gas phase. Increasing the substrate temperature from room temperature up to 250°C leads to transformation of the shell structure from amorphous into laminar.