低气压环境下的大空间温度均匀度模拟方法研究

高空环境模拟中一个重要的试验指标是低气压条件下的高低温温度均匀度模拟,为了达到中国国家军用标准相关要求,提出采用阶梯式全面孔板、混合风道和内置大面积冷板组合式气流组织方案,采用正升压式空气制冷技术,在大空间的高空模拟舱内进行了试验研究.试验结果表明该低气压环境下的大空间温度均匀度模拟技术能够使大空间内的常压温度均匀度达到2 ℃、低压低温温度均匀度达到1 ℃、低压高温温度均匀度达到1.2 ℃,优于中国国家军用标准要求,从而解决了大空间低气压下温度均匀性的技术难题.     

Oxygen mass transfer at liquid-metal-vapour interfaces under a low total pressure

The problem of oxygen exchange at the interface between a gas and a liquid metal is treated for systems under a “vacuum” (Knudsen regime, pressures lower than 1 Pa), where, due to the large mean free path of gas molecules in a vacuum, transport processes in the gas phase have no influence on the total interphase mass exchange, which is controlled by interface phenomena and by oxygen partition equilibrium inside the liquid. Owing to the double contribution of molecular O₂ and volatile oxides to the oxygen flux from the surface, non-equilibrium steady-state conditions can be established, in which no variations in the composition of the two phases occur with time, as the result of opposite oxygen exchanges. The total oxygen and metal evaporation rates are evaluated as a function of the overall thermodynamic driving forces, and an account of the transport kinetics is given by using appropriate coefficients. A steady-state saturation degree s _r, is defined which relates the oxygen activity in the liquid metal to the O₂ pressure imposed and to the vapour pressures of volatile oxides. When metals able to form volatile oxides are considered, pressures of molecular O₂ higher than those defined under equilibrium conditions have to be imposed in the experimental set-up in order to obtain a certain saturation degree, as a consequence of the condensation of the oxide vapours on the reactor walls. Effective oxidation parameters are determined, which define the conditions leading the liquid to a definite steady-state composition under a “vacuum” when it is out of equilibrium. The effective value of the oxygen pressure which corresponds to the complete oxygen saturation of the metal, $P_{O_{2,} s}^E $ , is evaluated at different temperatures for the systems Si-O and Al-O. The results are represented as curves of $P_{O_{2,} s}^E $ against T, which separate different oxidation regimes; these results agree well with the experimental data found in the literature.     

The growth of diamond in microwave plasma under low pressure

The synthesis of diamond particles in a low pressure plasma has been studied, with emphasis on the investigation of the substrate effect and the plasma conditions. It was found that a special pre-treatment of silicon substrate made it possible to form dense films, and a thickness of about 15m could be reached by 20 h discharge. Unfortunately, however, the prepared films had poor adhesion. Observations by scanning electron microscope (SEM) showed that the poor adhesion was due to the fact that the film consisted of large particles with a diameter of about 10m, and each particle had contacted to the substrate only by a point, not by a face. In addition, the plasma diagnostics of optical and ultraviolet emission spectroscopy (OES, 200–750 nm) revealed that CH and H radicals have come to be criteria for the formation of diamonds, and the ratio of radicals drastically affected the characteristics of the deposits. Nucleation and growth mechanism are also discussed.     

Probing multilayer stack reflectors by low coherence interferometry in extreme ultraviolet

We use low coherence interferometry to investigate the depth structure of a complex multilayer stack reflector. The probing instrument is an interferometer based on a Fresnel's bi-mirror illuminated by relatively wide-band synchrotron undulator light near 13.5 nm. Simulations clearly confirm that our test object generates two back propagated signals that behave as if reflected on two effective planes. First results in this spectral range may open the way to a new physical approach to extreme ultraviolet sample characterization in the form of line-scan optical coherence tomography.     

Improvement in oxidation resistance of the intermetallic compound titanium aluminide by heat treatment under a low partial pressure oxygen atmosphere

The improvement in oxidation resistance of an intermetallic compound TiAl was investigated by means of a new type of surface treatment: heat treatment under a low partial pressure oxygen atmosphere. The specimens treated by this method showed superior oxidation resistance compared with a nickel-base superalloy Inconel 713C during cyclic heating to a temperature of 1173 K in static air. The best conditions for the heat treatment under a low partial pressure oxygen atmosphere were found to be: pressure, 6.7 X 10-³ Pa; temperature, 1273 K; time, 7.2 ks (2 h) or more. It was presumed that the excellent oxidation resistance resulting from this method is due to preferential formation of a thin, strong Al₂0₃ surface layer.     

Synthesis of a phase-pure orthorhombic YBa2Cu3Ox under low oxygen pressure

Reaction of Y₂O₃, BaCo₃ and CuO for 4 h at 800 °C in flowing O₂ with a total pressure of about 2.7 × 10² Pa, followed by cooling in O₂ at ambient pressure, has produced phase-pure orthorhombic YBa₂Cu₃O_x. Keeping the ratio of O₂ to evolved CO₂ above 50 was necessary to ensure phase purity. The resultant powder yielded pressed and inserted pellets with improved superconducting properties.     

A cost‐effective approach for rapid manufacturing a low pressure wax injection mold with high surface finish and high dimensional accuracy

Investment casting process is considered as an economic method for mass production of metal parts. Improvement of mold surface quality and geometric accuracy in the fused deposition rapid tooling is a major concern. In this paper, a new technique is proposed for rapid manufacturing a low pressure wax injection mold with high surface finish and high dimensional accuracy. Wax patterns produced from this mold have not only better dimensional accuracy but also better surface finish. The average relative error of dimension of wax patterns can be reduced from 1.76% to 0.66%. Surface roughness improvement rate of wax patterns of up to 85.71% can be achieved. Advantages of this technique include low manufacturing cost, simple manufacturing process and flexible process capability.     

Destruction of porous bodies by a liquid under high pressure

On the basis of experimental data and theoretical investigations and with the aid of the obtained equation of filtering of viscous compressible liquids into porous bodies under high pressure (1000 MPa) the optimal conditions of destruction of these bodies are determined. The mechanism of destruction is established and the strength criterion substantiated.Translated from Problemy Prochnosti, No. 4, pp. 69–73, April, 1991.     

Multi-walled carbon nanotubes grow under low pressure hydrogen,air, and argon ambient by arc discharge plasma

Multi-walled carbon nanotubes (MWCNTs) were grown on cathode deposit by arc discharge plasma under H₂, Ar, and air ambient environment. The influence of ambient gas pressure on the structure and physical properties of carbon nanotube were compared. Herein, we highlight the influence of ambient environment and pressure to grow high quality carbon nanotubes. Field emission scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction were used for structural characterization and yield determination. The result revealed that background gas and pressure were crucial factor for growing highly crystalline and highly graphitic with I_D/I_G ratio 0.237 obtained for MWCNTs' synthesized in H₂ environment with extreme low defects.     

Ionization dynamics in the laser plasma in a low pressure gas target

In Xe-laser-plasma short-wave-radiation sources, the laser-energy-to-EUV conversion efficiency (CE) turns out to be substantially lower than theoretical expectations. An estimation made in the present work is evidence of what a long period of the primary ionization, lasting up to a moment when high-Z ions appear to emit short-wave photons, can be considered as a main cause for the low CE values. During that period the plasma remains low-ionized and absorbs weakly the laser energy. Data deduced from laser light absorption measurements confirm the estimation above. A preionization of the gas target with the UV excimer laser pulse is proposed as a method to accelerate the ionization process.     

Synthesis of high purity silicon nanoparticles in a low pressure microwave reactor

The formation of pure single crystalline silicon nanoparticles by microwave induced decomposition of silane in a low pressure flow reactor is reported. The morphology and crystal structure of the particles are characterized in situ by particle mass spectrometry (PMS) and ex situ by means of X-ray diffraction, high resolution transmission electron microscopy, electron energy loss spectroscopy, and infrared spectroscopy. The preparation method allows for the adjustment of the mean particle diameter in the range 6 nm < or = dPM < or = 11 nm by controlling the precursor concentration, gas pressure, and microwave power. Spectroscopic investigations reveal that the particles are single crystal silicon. The potential on n- or p-type doping is in progress.     

Heat transfer to a cylindrical surface immersed in a fluidized bed at low pressure

Results are given of an investigation of heat transfer in a fluidized bed to a cylindrical surface over the range of pressures from atmospheric to 133 N/m². Equations for determining _max are presented.     

Thermohydraulic processes in a heat pipe with a central circulation insert at low pressure

Results are given of an experimental investigation of thermohydraulic processes occurring in a compact natural-circulation loop (heat pipe with a central circulation insert) at a low pressure of steam-water mixture, including low vacuum p _top = 22–176 kPa. Consideration is given to axial heat transfer in nonboiling water under conditions of bubbling mode of loop operation; to superheating of water, which rises up the adiabatic region, relative to the saturation temperature; and to true void fraction and hydrodynamic stability of natural circulation.