| Space environmental engineering plays a vital role in exploring the universe,as well as utilizing and exploiting the space resources.Since the launch of the first man-made earth satellite,thousands upon thousands of various spacecrafts have been sent into the space,taking the tasks such as observation of cosmic space,information transmission,meteorological service and so on,and thus,greatly promoting the progress in human social civilization.Low earth orbit(LEO)spacecrafts are the space facilities,such as communication satellites and space stations,running in the environment of near-earth space within 1000 km to the earth surface,thus are important for each country to utilize space resources.With the improvement in the national strength,China started its plane to build its space station step by step in order to carry out large-scale experimental research related to space science in future.Based on this background,the projects of device physics related to LEO spacecrafts were proposed by this group associated with Key Laboratory of Beijing Satellite Environment Engineering,Institute of Reliability and Environmental Technology.In this dissertation,the main focus is the relevant problems in fabrication of Os-film resistance-type atomic oxygen(AO)sensor for detection of oxygen atoms in the atmosphere of LEO space environment.In addition,a novel low-pressure tubular plasma system invented in this group was studied for applications in AO generation,ion propulsion,spacecraft potential control,and materials modification.The main results and achievements are summarized below:(1)Preparation of metallic Os films and fabrication of AO sensor:high density of atomic oxygen in LEO environment exhibits a strong ability of corrosion,and then is one of the main reasons leading to invalidation of spacecraft materials.Because of its unique chemical properties,metallic Os is considered to be an ideal material for making AO sensors.However,the large elastic modulus of metallic Os leads to difficulty in preparation of Os films with sufficient thickness for fabrication of the AO sensor.In addition,the Os films for making the AO sensor are as thick as?3?m,beyond the ability of photolithography to make AO sensors with precision and quality.Therefore,many problems needs for solution in preparation of the Os films and fabrication of the AO sensors.In this dissertation,the growth of Os films was investigated as a function of growth conditions using a magnetron sputtering method.Using the pulsed bias technology and Ti-buffered layer,more than 3?m Os films were successfully grown on the quartz substrates,without the problems such as cracking and peeling.And then,laser precision machining technology was investigated and applied to fabrication of a batch of practical Os-film AO sensors for the basic research of AO detection in space environment.(2)Study on physical properties of Os films:mechanical performance and electrical resistivity of the Os films is crucial for the application of AO sensors.For this purpose,the aspects related to these issues were further studied in this dissertation.Measurement showed that the film hardness is 40%higher than that of bulk material,in which the thermal stress contributes to 13%due to the increase of elastic modulus and the other 24%is ascribed to the grain refinement.The film resistivity(?)of Os films could be fitted by ?=13.2+1.65/h(??cm),in which h is film thickness in ?m,indicating that the size effects are significant for the resistivity of Os films.Furthermore,Mayadas-Shatzkes electrical-resistivity model(MS model)was found to deviate from the experimental data of resistivity as a function of temperature.Using the code CASTEP,the electronic structure and Fermi surface for Os metal with hexagonal-close-packed structure was calculated,showing that the Fermi surface is far from the assumption in the MS model.And then,an analytical equation was suggested to correct the MS model for the contributions of electron scattering by grain boundaries,and successfully applied to fit the experimental data of resistivity for the Os films.As the resistivity depends on both of temperature and film thickness,mathematical models for operation of AO sensors in the modes of constant-and testing-temperature were further discussed.(3)Study of low-pressure tubular discharge:high-density plasma beam has potential to be widely applied to space environment engineering,such as spacecraft electrical-potential control and attitude determination.In this group,a specially designed low-frequency driving discharge tube,which is capable of generating high density of plasma beams at low pressure,has been developed with a platform for plasma diagnostics.In this dissertation,the discharges in N2+H2,O2,and Ar were studied for generation of high-density plasma beams applied to space environment engineering.In the discharge of N2+H2,hydrogen was found to have an ability to enhance ionization of the plasmas,thus leading to an increase in the ion density.Therefore,this discharge may be useful for spacecraft potential control.In O plasmas,high density of O atoms was detected and the plasma beams were demonstrated to etch polymer materials with extremely-high rate of reactive ion etching.It is evident that this tube discharge could be developed as a novel technique for generation of O atom beam that will be useful for testing AO sensors and other experiments related to AO oxidation.In comparison with the discharge of N2,the Ar discharge was found to create plasmas with large electrical current at relatively low input power,thus being promising in electric thruster and/or spacecraft potential control.(4)Study of low-temperature nitriding of pure iron:Ion nitriding is an important technology for surface modification of metals and alloys,and has been used in many fields including industrial equipments,defense armaments,and facilities in space environment engineering,etc.Low-temperature nitriding not only aims at energy conservation and cost production,but more importantly also at inurement of the workpiece precision and material performance during processing,thus is one of important goals in the research and also a key technology for space environmental engineering.In this dissertation,N2+H2 mixture was used as the working gas to generate the high-density plasma beam for study of ion nitriding of pure iron using the low-pressure discharge tube.The high-density plasma beam exhibited high efficiency of nitriding at relatively low temperature,as low as-250?.In addition,the tubular plasma source was demonstrated to have advantages of using relatively low content of hydrogen for ion nitriding with high efficiency.As the pulsed bias was found playing a crucial role in the nitriding,N2+ and N+ were further proved to be the dominant species responsible for ion nitriding of pure iron.With varying N2 in the mixed gas from 92.5%to 32.5%,estimation by plasma optical emission spectra revealed that the density of N2+ in the plasma plume is as high as 7.2± 0.6×1011 cm-3,thus leading to formation of a layer of compounds as thick as?5?m after nitriding at 450? for 120 min.If nitriding at 330?370?for 120 min,the plasma beam is capable of producing a layer of compounds with thickness larger than 4?m and a millimeter-scale diffusion zone.By fitting the optical emission spectra,the gas temperature of neutral species in the plasma plume was determined to be?300?,which is low enough to meet the requirements for low-temperature nitriding. |
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