Tribology-related space mechanism anomalies and the newly constructed high-vacuum mechanism test facilities in NASDA

In the past many functional anomalies of mechanisms and mechanical components have been experienced on board spacecraft and their launch vehicles. Such anomalies sometimes lead to the total loss of the spacecraft. Among the major causes of mechanical anomalies, vacuum tribology-related malfunctions have received central attention in Japan due to the continued occurrence of spacecraft failures. Phenomena such as the increase of frictional forces between moving parts and the binding of metallic parts in high-vacuum environments are sometimes very difficult to examine in full scale on the ground, and only limited prototype verification is conducted in orbit. Responding to the strong need for the improvement of space mechanism and tribology research work, integrated multiple high-vacuum mechanism test facilities were constructed in NASDA Tsukuba Space Center in 1998. These facilities are intended to provide fundamental data on the performance of mechanical components and materials in high vacuum for validating space mechanism designs. One of the facilities also makes possible the examination of combined environmental factors, such as electron beams, ultraviolet radiation, atomic oxygen and temperature, for mechanisms and materials. This paper introduces typical features of the space-based mechanism functional anomalies and describes the new test facilities.     

Morphological Effects of Grinding on Bismuth-Based Cuprate Superconductors

Morphological changes of bismuth-based cuprate superconductors, such as the 2223 phase, via a small mechanical force were studied mainly by means of high-resolution transmission electron microscopy and atomic force microscopy. Even light mechanical grinding of the sintered 2223 samples led to the formation of superfine particles that had a width of ∼5 nm. Moreover, we found first that the superfine particles had a discrete thickness of odd numbers of a half unit length c /2 of the 2223 phase, e.g. 1.9 nm, 5.6 nm, and so on. The formation of the superfine particles was due to cleavage at Bi—O double layers that were bound weakly to each other in a unit cell.     

Carbon oxidation with platinum supported catalysts

The effect of the support oxide, Pt precursor and reactant gas composition on the catalysis of soot oxidation was investigated using carbon black as a model soot and simulated exhaust gases. The Pt precursors used were Pt(NH₃)₄(OH)₂, H₂PtCl₆·6H₂O, Pt(NH₃)₄(NO₃)₂, and Pt(NH₃)₄Cl₂. The support metal oxides used were SiO₂, Al₂O₃, and ZrO₂. Pt/SiO₂ prepared from Pt(NH₃)₄(OH)₂ showed the highest carbon oxidation activity. It had much higher activity in the condition of N₂+O₂+H₂O+NO+SO₂ than without NO and SO₂.     

Nitrate percolation and discharge in cropped Andosols and Gray lowland soils of Japan

Contamination of groundwater with nitrate (NO₃) derived from agricultural activity is serious problem in many countries worldwide. We investigated the annual (growing and non-growing seasons) behavior of NO₃–N in the soil pore water of cropped Andosols and Gray lowland soils under eight crop groups (Type A: paddy rice, Type B: winter crops, Type C: vegetables 1, Type D: vegetables 2, Type E: vegetables and forages, Type F: legume crops, Type G: orchard, and Type H: grass). In the vegetable group (Type C) and the orchard group (Type G), which required large amounts of fertilizer and frequent top-dressing, NO₃–N concentrations in the soil pore water were extremely high. In these agricultural lands, it was clear that the inorganic nitrogen produced by nitrification in surface soil was dominantly discharged from a depth of 90 cm in July to September. The descending order of the amount of discharge of NO₃–N (N-discharge) was Type C > G > D > E > F > B > H > A for the Andosol, and Type G > C > F > D > E > B > H > A for the Gray lowland soil. If fertilization of the vegetables and orchard was performed based on the standard application amount, the annual average NO₃–N concentration at a depth of 90 cm exceeds 10 mg L^?1. To reduce the risk of groundwater contamination by NO₃–N, we calculated the annual cumulative water flux density and annual cumulative NO₃–N flux density. We examined the calculated fertilizer amount and proposed reduced fertilizer application amounts so that the annual average concentration of NO₃–N in soil pore water would not exceed 10 mg L^?1. The standard application amount of nitrogen fertilizer for vegetables should be reduced by 65.8 and 30.8 kg ha^?1 in the Andosol and the Gray lowland soil, respectively. We also proposed that the standard application amount of nitrogen fertilizer be reduced by 59.9 and 40.7 kg ha^?1 in Andosol orchards and Gray lowland soil orchards, respectively.     

High-Voltage Screening of Unidirectionally Surface-Ground Titania Ceramics

The effects of high-voltage screening were examined for perpendicularly and parallelly surface-ground titania rectangular bars. A screening field at or below which 30% of titania samples break electrically was applied to each group samples. After high-voltage screening, the surviving samples were subjected to mechanical strength measurement and the resultant strength distribution was compared with the original distribution. After screening, Weibull plots of perpendicularly ground samples bent to become convex curves while plots in the high-strength region remained almost the same, indicating that low-strength samples were selectively eliminated by the high-voltage screening. On the other hand, the screening effect on parallelly surface-ground samples was very small.     

An Investigation of the Nanomechanical Properties of 0.5Ba(Ti0.8Zr0.2)O3–0.5(Ba0.7Ca0.3)TiO3 Thin Films

For practical application, the functional piezoelectric film in microelectromechanical systems should meet the requirement of physical properties, as well as the mechanical properties. In this article, 0.5Ba(Ti_0.8Zr_0.2)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (0.5BZT–0.5BCT) thin films with varied properties were prepared on (100) Si substrates via a sol–gel technique at different annealing temperatures. The effects of the annealing temperature on the morphology, piezoelectricity, hardness, and elastic modulus were studied. Particular attention was paid to the surface frictional behavior of films, and the changes in the friction force can be radically explained in terms of differences in the hardness/elastic modulus ratio and the residual stress of films. And, it reveals that the higher ratio of hardness to elastic modulus and tensile residual stress can contribute to a lower friction force for 0.5BZT–0.5BCT film during sling friction.     

Estimation of diffusivity of alkaline metals in graphite lattice from oxidation rate measurements

A reaction model has been developed for the estimation of diffusivity of the intercalated species in graphite on the basis of a material balance expressed in terms of cylindrical coordinates, where the species in the layered structural cylinder diffuse radially from the center to the surface and react with gas. The resultant kinetic equations are used to analyze the experimental data for the oxidation rate of potassium atoms intercalated in natural graphite powder of initially different sizes intercalated at the first stage.     

Effects of oxygen supply condition and specific biofilm interfacial area on phenol removal rate in a three‐phase fluidized bed bioreactor

The effects of oxygen supply conditions and specific biofilm interfacial area on the phenol removal rate in a three‐phase fluidized bed bioreactor were evaluated. The experimental data were well‐explained by the semi‐theoretical equation based on the assumption that the reaction rate follows first‐order reaction kinetics with respect to oxygen and zero‐order one with respect to phenol. Two cases, biological reaction as rate‐controlling step and oxygen absorption as rate‐controlling step, were both explicable by this semi‐theoretical equation. The maximum volumetric phenol removal rate was 27.4 kg·m^?3·d^?1.     

Characterization of simple and reproducible vascular stenosis model in hypercholesterolemic hamsters

The importance of low-density lipoprotein (LDL) in the etiology of atherosclerosis is well recognized. We have established a reproducible stenosis model in hypercholesterolemic hamsters, and the process of arterial stenosis by thrombus or neointima was studied and compared with that in normal hamsters. The level of plasma LDL was 4.6 times higher in hamsters fed a high-cholesterol diet than in hamsters fed normal food. Endothelial injury in right common carotid arteries was induced using a modified catheter. Arterial blood flow was monitored continuously using a Doppler flow probe. Arterial patency after the initiation of injury in high-cholesterol hamsters was significantly changed as compared with that of normal hamsters. Neointima was observed 2 wk after the vascular injury. The neointimal area of high-cholesterol hamsters was significantly larger than that of normal hamsters. To characterize the stenosis in hypercholesterolemic hamsters, we measured platelet aggregation, thrombin time, activated partial thromboplastin time, and proliferating smooth muscle cells (SMC) in vitro and in vivo. The half-maximal inhibitory concentration value for platelet aggregation induced by thrombin or collagen, the DNA synthesis stimulated by plateletderived growth factor (PDGF)-BB, and 5-bromo-2-deoxy-uridine labeling indices (proliferating index of SMC in vivo) in high-cholesterol hamsters were each significantly higher than the comparable value from normal hamsters. However, specific binding of PDGF-BB in SMC was not different between the two types of hamsters. Furthermore, we investigated the inhibitory effects of probucol or losartan on neointima formation using this model. Probucol, but not losartan, significantly reduced the neointimal area in hypercholesterolemic hamsters. These findings indicated that high levels of plasma LDL strongly contributed to the development of thrombus and neointima formation via both up-regulation of platelet aggregation and the enhancement of SMC proliferation. This stenosis model may be useful for the investigation of hypercholesterolemia-associated cardiovascular diseases.     

Molecular characteristics and gelling properties of carrageenan family 2. Tri-sulfated and tetra-sulfated carrageenans

Summary New series of tri-sulfated (τ-carrageenan) and tetra-sulfated (ρ-carrageenan) carrageenans were prepared by the sulfating reaction from ι-carrageenan. The chain conformations of new carrageenans in aqueous solutions were investigated in the presence of various metal salts by means of static/dynamic light scattering and small-angle X-ray scattering, and their conformational characteristics were discussed with respect to the gelation behaviour. Although the overall conformation of newly prepared carrageenans is specified as a random coil, the local chain retains a rigid-rod-like structure by forming a helical conformation. The sulfate groups on C2 of (1→3)-linked β-D-galactoses were found primarily responsible to prevent gelation. Received: 28 August 2001/Accepted: 17 September 2001