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Sodium and barium fluorosilicates precipitated and thermally decomposed into silicon tetrafluoride. Barium fluorosilicate thermally decomposes at lower temperatures in shorter times, to give same percentage yield of silicon tetrafluoride gas, than sodium fluorosilicate. Salt byproducts of decompositions recycled to precipitate fluorosilicates from hexafluorosilicic acid, the primary reactant.
In order to study the access of solar wind ions to the magnetosphere, together with the processes that transport and accelerate magnetospheric particles, the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission will release and monitor lithium and barium tracer ions in both the solar wind and the magnetosphere. A single, massive release of barium in the dawn magnetosheath will in addition create a visible artificial comet in the flowing solar wind plasma, within which studies of a range of different plasma effects will be undertaken. The AMPTE will obtain comprehensive measurements of natural magnetospheric particle populations' elemental composition and dynamics. AMPTE comprises three spacecraft: the Ion Release Module, the Charge Composition Explorer, and the United Kingdom Subsatellite.
The hydrated surfaces of a zirconium barium fluoride glass, which has potential for application in optical fibers and other optical elements, were observed by scanning electron microscopy. Crystalline zirconium fluoride was identified by analysis of X-ray diffraction patterns of the surface crystals and found to be the main constituent of the surface material. It was also found that hydrated zirconium fluorides form only in highly acidic fluoride solutions. It is possible that the zirconium fluoride crystals form directly on the glass surface as a result of its depletion of other ions. The solubility of zirconium fluoride is suggested to be probably much lower than that of barium fluoride (0.16 g/100 cu cm at 18 C). Dissolution was determined to be the predominant process in the initial stages of the reaction of the glass with water. Penetration of water into the glass has little effect.
Numerous plasma wave effects were detected by the AMPTE/IRM spacecraft during the artificial comet experiment on December 27, 1984. As the barium ion cloud produced by the explosion expanded over the spacecraft, emissions at the electron plasma frequency and ion plasma frequency provided a determination of the local electron density. The electron density in the diamagnetic cavity produced by the ion cloud reached a peak of more than 5 x 10 to the 5th per cu cm, then decayed smoothly as the cloud expanded, varying approximately as t exp-2. As the cloud began to move due to interactions with the solar wind, a region of compressed plasma was encountered on the upstream side of the diamagnetic cavity. The peak electron density in the compression region was about 1.5 x 10 to the 4th per cu cm. Later, a very intense (140 mVolt/m) broadband burst of electrostatic noise was encountered on the sunward side of the compression region. This noise has characteristics very similar to noise observed in the earth's bow shock, and is believed to be a shocklike interaction produced by an ion beam-plasma instability between the nearly stationary barium ions and the streaming solar wind protons.
NASA Goddard Space Flight Center, Wallops Flight Facility has developed, flown, and recovered a unique plasma physics payload. This sounding rocket payload was developed to measure varying aspects of Alfven's critical velocity effect in a space plasma by using conical-shaped barium explosives. These measurements could possibly duplicate conditions that existed in the early solar system. This paper provides details of the payload and subpayload development, with specific emphasis on the extensive dynamic analysis of the barium release modules. Other key elements which are expanded on in the paper are: (1) design, development, and testing acceptance for the science/inertia booms using a viscous damping system for high spin rate deployment; (2) vehicle dynamic analysis; (3) apogee and impact dispersion analysis to satisfy the science and NASA safety requirements; (4) a comparison of predicted versus actual flight events.
The Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM spacecraft performed a series of eight chemical releases in essentially comparable pairs. There were two lithium releases in the solar wind, two lithium and two barium releases in the near-Earth magnetotail, and two barium releases to simulate artificial comets, one in the solar wind and the other in the magnetosheath. A variety of plasma waves were observed in conjunction with each of the releases. Although there were unique features associated with some of the individual releases, the comparable releases generally produced similar wave emissions. This paper discusses the similarities and differences among the releases.
Thin films of YBa2Cu3O(7-beta) have been grown by sequential evaporation of Cu, Y, and BaF2 on SrTiO3 and MgO substrates. The onset temperatures were as high as 93 K while T sub c was 85 K. The Ba/Y ratio was varied from 1.9 to 4.0. The Cu/Y ratio was varied from 2.8 to 3.4. The films were then annealed at various times and temperatures. The times ranged from 15 min to 3 hr, while the annealing temperatures used ranged from 850 C to 900 C. A good correlation was found between transition temperature (T sub c) and the annealing conditions; the films annealed at 900 C on SrTiO3 had the best T sub c's. There was a weaker correlation between composition and T sub c. Barium poor films exhibitied semiconducting normal state resistance behavior while barium rich films were metallic. The films were analyzed by resistance versus temperature measurements and scanning electron microscopy. The analysis of the films and the correlations are reported.
Barium aluminosilicate glasses are being investigated as matrix materials in high-temperature ceramic composites for structural applications. Kinetics of crystallization of two refractory glass compositions in the barium aluminosilicate system were studied by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). From variable heating rate DTA, the crystallization activation energies for glass compositions (wt percent) 10BaO-38Al2O3-51SiO2-1MoO3 (glass A) and 39BaO-25Al2O3-35SiO2-1MoO3 (glass B) were determined to be 553 and 558 kJ/mol, respectively. On thermal treatment, the crystalline phases in glasses A and B were identified as mullite (3Al2O3-2SiO2) and hexacelsian (BaO-Al2O3-2SiO2), respectively. Hexacelsian is a high-temperature polymorph which is metastable below 1590 C. It undergoes structural transformation into the orthorhombic form at approximately 300 C accompanied by a large volume change which is undesirable for structural applications. A process needs to be developed where stable monoclinic celsian, rather than hexacelsian, precipitates out as the crystal phase in glass B.
Barium aluminosilicate glasses are being investigated as matrix materials in high-temperature ceramic composites for structural applications. Kinetics of crystallization of two refractory glass compositions in the barium aluminosilicate system were studied by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). From variable heating rate DTA, the crystallization activation energies for glass compositions (wt percent) 10BaO-38Al2O3-51SiO2-1MoO3 (glass A) and 39BaO-25Al2O3-35SiO2-1MoO3 (glass B) were determined to be 553 and 558 kJ/mol, respectively. On thermal treatment, the crystalline phases in glasses A and B were identified as mullite (3Al2O3-2SiO2) and hexacelsian (BaO-Al2O3-2SiO2), respectively. Hexacelsian is a high-temperature polymorph which is metastable below 1590 C. It undergoes structural transformation into the orthorhombic form at approximately 300 C accompanied by a large volume change which is undesirable for structural applications. A process needs to be developed where stable monoclinic celsian, rather than hexacelsian, precipitates out as the crystal phase in glass B.
A rocket borne Critical Ionization Velocity (CIV0 experiment was carried out from Wallops Island at dusk on May 4, 1989. Two barium shaped charges were released below the solar terminator (to prevent photoionization) at altitudes near 400 km. The ambient ionospheric electron density was 50,000/cu cm. The neutral barium jet was directed upward and at an angle of nominally 45 degrees to B which gives approximately 3 x 10 to the 23rd neutrals with super critical velocity. Ions created by a CIV process in the region of the neutral jet would travel up along B into sunlight where they can be detected optically. Well defined ion clouds (max. brightness 750 R) were observed in both releases. An ionization rate of 0.8 percent/sec (125 sec ionization time constant) can account for the observed ion cloud near the release field line, but the ionization rate falls off with increasing distance from the release. It is concluded that a CIV process was present in the neutral jet out to about 50 km from the release, which is significantly further than allowed by current theories.
Twenty pyroelectric type infrared detectors were flown onboard the Long Duration Exposure Facility (LDEF). The detector chips were of three different pyroelectric materials: lithium-tantalate, strontium-barium-niobate, and triglycine-sulfide. The experiment was passive; no measurements were taken during the flight. Performance of the detectors was measured before and after flight. Postflight measurements revealed that detectors made of lithium-tantalate and strontium-barium-niobate suffered no measureable loss in performance. Detectors made of triglycine-sulfide suffered complete loss of performance, but so did the control samples of the same material. Repoling of the triglycine-sulfide failed to revive the detectors.
Four hollow cathode electron sources were analyzed via boroscopy, scanning electron microscopy, energy dispersive x ray analysis, and x ray diffraction analysis. These techniques were used to develop a preliminary understanding of the chemistry of the devices that arise from contamination due to inadequate feed-system integrity and improper insert activation. Two hollow cathodes were operated in an ion thruster simulator at an emission current of 23.0 A for approximately 500 hrs. The two tests differed in propellant-feed systems, discharge power supplies, and activation procedures. Tungsten deposition and barium tungstate formation on the internal cathode surfaces occurred during the first test, which were believed to result from oxygen contamination of the propellant feed-system. Consequently, the test facility was upgraded to reduce contamination, and the test was repeated. The second hollow cathode was found to have experienced significantly less tungsten deposition. A second pair of cathodes examined were the discharge and the neutralizer hollow cathodes used in a life-test of a 30-cm ring-cusp ion thruster at a 5.5 kW power level. The cathodes' test history was documented and the post-test microanalyses are described. The most significant change resulting from the life-test was substantial tungsten deposition on the internal cathode surfaces, as well as removal of material from the insert surface. In addition, barium tungstate and molybdate were found on insert surfaces. As a result of the cathode examinations, procedures and approaches were proposed for improved discharge ignition and cathode longevity.
In both the ionospheric barium injection experiments CRIT 1 and CRIT 2, a long duration oscillation was seen with a frequency close to the gyro frequency of barium and a time duration of about one second. A model for the phenomena which was proposed for the CRIT 1 experiment is compared to the results from CRIT 2 which made a much more complete set of measurements. The model follows the motion of a low Beta ion cloud through a larger ambient plasma. The internal field of the model is close to antiparallel to the injection direction v sub i but slightly tilted towards the self polarization direction E sub p = -V sub i by B. As the ions move across the magnetic field, the space charge is continuously neutralized by magnetic field aligned electron currents from the ambient ionosphere, drawn by the divergence in the perpendicular electric field. These currents give a perturbation of the magnetic field related to the electric field perturbation by Delta E/Delta B approximately equal to V sub A. The model predictions agree quite well with the observed vector directions, field strengths, and decay times of the electric and magnetic fields in CRIT 2. The possibility to extend the model to the active region, where the ions are produces in this type of self-ionizing injection experiments, is discussed.
Measurements from rocket-borne sensors inside a high-velocity neutral barium beam show a-factor-of-six increase in plasma density in a moving ionizing front. This region was colocated with intense fluctuating electric fields at frequencies well under the lower hybrid frequency for a barium plasma. Large quasi-dc electric and magnetic field fluctuations were also detected with a large component of the current and the electric field parallel to B(0). An Alfven wave with a finite electric field component parallel to the geomagnetic field was observed to propagate along B(0), where it was detected by an instrumented subpayload.
Auger, ESCA, electron emission, evaporation, and desorption measurements were made on three different types of scandate surfaces. They are: (1) an impregnated top layer scandate cathode, (2) an unimpregnated top layer scandate cathode with a deposited barium or barium oxide adsorbate surface layer, and (3) a synthesized scandate surface, which replicates a scandate cathode surface. The purpose of these experiments was to determine the role that Sc2O3 plays in making the scandate cathode a more copious electron emitter than the conventional impregnated-type cathode. The synthesized scandate surface experiments consisted of depositing multilayer scandium on a tungsten surface, oxidizing the scandium, and then depositing either Ba or BaO on the scandium oxide surface. The results of these measurements showed that the low work function portions of the thin-film scandate cathode are where the Sc2O3 is the substrate and BaO is the adsorbate.
Vector electric field measurements were made inside two ionizing, high-velocity streams of barium atoms in the earth's ionosphere, and a variety of electrical phenomena across the frequency spectrum are reported. A very large quasi-dc electric field was detected antiparallel to the beam velocity at a roughly 45 deg angle with the magnetic field B0. A very large component of E is found parallel to B0. The fluctuating electric fields are also quite large, of the same order of magnitude as the quasi-dc pulse. The wave energy maximizes at frequencies below the barium lower hybrid frequency and includes strong signatures of the oxygen cyclotron frequency. Measurements made on a subpayload separated across B0 by several hundred meters and along B0 by several km do not show the large pulse. Very large amplitude magnetic field fluctuations were observed in both bursts.
The quasi-dc electric fields measured in the CRIT I ionospheric release experiment are studied. In the experiment, two identical barium shaped charges were fired toward a main payload, and three-dimensional measurements of the electric field inside the streams were made. The relevance of proposed mechanisms for electron heating in the critical ionization velocity (CIV) mechanism is addressed. It is concluded that both the 'homogeneous' and the 'ionizing front' models probably are valid, but in different parts of the streams. It is also possible that electrons are directly accelerated by a magnetic field-aligned component of the electric field. The coupling between the ambient ionosphere and the ionized barium stream is more complicated that is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes.
Four hollow cathodes have been examined using boroscopy, scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffraction analysis to characterize the life-limiting phenomena that resulted from their extended operation. The first pair of cathodes was operated for approximately 500 h in an ion thruster simulator at a 5.0 kW-equivalent power level using different cathode activation and propellant feed-system procedures. The second pair of cathodes was the main discharge cathode and neutralizer cathode from an ion thruster which have completed an 89-hour life-test at a 5.5 kW power level. The greatest change to the cathode condition was found to be removal of refractory metals, primarily tungsten, and their subsequent deposition on other cathode surfaces. Barium tungstate and barium molybdate were also found in all four cathodes. Criteria for improved cathode longevity and discharge operation are proposed for use in hollow cathode and full thruster testing facilities.