Fabrication of particulate reinforced magnesium composites by applying a spontaneous infiltration phenomenon

A new fabrication method of particulate reinforced magnesium matrix composites using a spontaneous infiltration phenomenon was developed. By mixing infiltration agent powder with reinforcement powder, the magnesium melt spontaneously infiltrated into the powder mixture. In this study, pure magnesium, SiO2 and SiC particle (SiCp, 1.2,2,3,4 and 8 m) powders were used as the matrix metal, infiltration agent and reinforcements, respectively. Experiments to clarify the conditions in which the spontaneous infiltration occurred were carried out under pure argon atmosphere at 973 K. Furthermore, the infiltration velocity was measured. Spontaneous infiltration occurred in the 2,3,4 and 8 m SiCp systems. A minimum SiO2 content necessary for spontaneous infiltration existed and it depended on SiCp diameter. Microstructural observation of the composites revealed that SiCp dispersed homogeneously, even though the diameter of SiCp was fine, and MgO and Mg2Si reaction products were observed. During infiltration, a temperature rise was observed, caused by Mg–SiO2 thermit reaction. Consequently, spontaneous infiltration was caused by improvement of wettability between the magnesium melt and SiCp, which resulted from Mg–SiO2 thermit reaction at the infiltration front.     

碳化硅轻量反射镜的超精密加工

运用在线电解磨削技术(ELID)加工大型碳化硅球面镜(口径ф360 mm),设计了一套专用的静压组合夹具用以抑制加工过程中因磨削力引起的工件变形.结合在线面形测量和基于误差补偿的补正加工方法,可以获得面形精度为P-V值0.8 μm、表面粗糙度为Ra7.8 nm的高质量碳化硅球面镜.     

A subsystem design approach to continuous-time performance of decentralized multirate sampled-data systems

A decentralized controller design is presented for multirate sampled-data systems, where stability and input-output performance are considered in terms of continuous-time signals. The design procedure consists of analysis for the overall system and controller synthesis for individual subsystems. For the subsystem designs, a unique technique of removing lifting operation which is used in the analysis is proposed. Robust performance and robust stability in terms of continuous-time signals can be achieved by using the proposed design. A numerical example shows that the design method results in satisfactory controllers which guarantee closed-loop stability and performance in continuous time.     

The relationship between microstructure and Young’s modulus of thermally sprayed ceramic coatings

An idealized model for the microstructure of thermally sprayed ceramic coatings, consisting of the stacking of lamellae a few micrometres thick, has been used to estimate Young’s modulus of the coating perpendicular to the coating plane. A theoretical relationship between Young’s modulus and the microstructural parameters has been established. There are two components of elastic strain of the coating under tensile stress, one arising from localized elastic strain at the regions of real-bonded area between lamellae, and the other arising from elastic bending of the lamellae between bonded regions. The bending component only becomes significant for a percentage bonding ratio between lamellae of less than 40%. The bending strain contribution depends strongly upon geometrical parameters of the coating microstructure. The estimated Young’s modulus for a typical alumina coating, based on quantitative microstructural data, was about 24% of that for the fully dense material. Taking into account the variable proportion of α-Al2O3 and γ-Al2O3 forms in an alumina coating, the comparison of the estimated Young’s modulus with published data gives reasonable agreement for the coating prepared over a wide range of processes and experimental conditions. This revised version was published online in November 2006 with corrections to the Cover Date.     

结合ELID磨削与磁流变光整加工的单晶硅反射镜超精密制造技术

分析了在线电解修整（ELID）磨削和磁流变光整加工（MRF）的加工原理与特点，充分结合这2种技术的优点对单晶硅反射镜进行纳米级精度的组合加工．首先进行ELID高效率磨削，在线检测工件表面误差后进行补偿磨削，使反射镜面加工成形，并获得较好的形状精度和表面质量．然后，利用磁流变技术进行确定性的光整加工，以减少反射镜的亚表面损伤，使加工表面的形状精度与表面粗糙度得到很大提高与改善．利用该组合工艺，对硅反射镜进行了系列的加工实验，高效率地得到了低于1nmRMS的表面粗糙度和69nmp-V形状精度的工件表面．     

EXTRACTION OF NIOBIUM AND TANTALUM WITH BIS-2-ETHYLHEXYL ACETAMIDE

ABSTRACT The extraction of Nb and Ta from acid solutions with bis-2-ethylhexyl acetamide and the stripping of these metals with sulphuric acid solutions were investigated. The organic phase was a binary solution of bis-2-ethylhexyl acetamide and xylene, while the aqueous phase was composed of hydrofluoric acid solution or hydrofluoric-sulphuric acid solution containing 3.5-13 Kg/m³ Nb and 5-10 Kg/m³ Ta. Sulphuric acid, hydrofluoric acid and nitric acid were used as salting out agents to understand the effect on the extraction.

Niobium and tantalum were not sufficiently extracted from hydrofluoric acid solutions, whereas the extraction of both metals remarkably increased with an addition of sulphuric acid to the aqueous phase. Both metals were completely co-extracted under the aqueous condition of 6N hydrofluoric acid and 8N sulphuric acid. The stripping occurred for both metals with high efficiency when water or dilute sulphuric acid was used as a stripping agent. The increase in sulphuric acid concentrations caused less stripping of Ta, while the stripping of Nb was maintained at 80% up to 7N sulphuric acid.     

Si nanowire FET and its modeling

Because of its ability to effectively suppress off-leakage current with its gate-around conffguration, the Si nanowire FET is considered to be the ultimate structure for ultra-small CMOS devices to the extent that the devices would be approaching their downsized limits. Recently, several experimental studies of Si nanowire FETs with on-currents much larger than those of planar MOSFETs have been published. Consequently, Si nanowire FETs are now gaining significant attention as the most promising candidate fo...     

Application of Monte Carlo method to optimal control for linear systems under measurement noise with Markov dependent statistical property

This paper presents an optimal control algorithm for linear systems with measurement noise which has a Markov dependent statistical property. Ordinarily, the optimal control for this problem involves a very large number of sequences, and the usual calculation method becomes impractical. In the algorithm proposed here, the optimal control is calculated with a relatively small number of sequences, sampled at random from the set of all the sequences.

Evidently, the algorithm for a control problem should be obtained directly from the performance criterion. Unlike the state estimation problem, the problem considered here has a difficulty that there exists an interaction between the algorithm and the state of the system. Because of this, a special consideration is required for the design of the algorithm. In this paper control-free measurement data are introduced to establish the convergence of the algorithm and to find a desirable way of sampling the sequences. Then, certain approximations are made to design a practical and efficient algorithm. A few digital simulation results appear to show the effectiveness of the proposed method.