Measurement of the working parameters of an air-post vertical-cavity surface-emitting laser

We present a complete experimental evaluation of the effective parameters necessary to describe the dynamical behavior of an air-post vertical-cavity surface-emitting lasers, on the basis of theoretical equations which are also derived in this paper. The experimental investigation is composed of several steps, including power versus current measurement, noise spectrum analysis, linewidth evaluation. The complete set of parameters derived, in particular the linewidth-enhancement factor and the spontaneous emission factor, is particularly important for accurate comparisons of theoretical models on the laser dynamics with experiments.     

Coding approaches to fault tolerance in linear dynamic systems

This paper discusses fault tolerance in discrete-time dynamic systems, such as finite-state controllers or computer simulations, with focus on the use of coding techniques to efficiently provide fault tolerance to linear finite-state machines (LFSMs). Unlike traditional fault tolerance schemes, which rely heavily-particularly for dynamic systems operating over extended time horizons-on the assumption that the error-correcting mechanism is fault free, we are interested in the case when all components of the implementation are fault prone. The paper starts with a paradigmatic fault tolerance scheme that systematically adds redundancy into a discrete-time dynamic system in a way that achieves tolerance to transient faults in both the state transition and the error-correcting mechanisms. By combining this methodology with low-complexity error-correcting coding, we then obtain an efficient way of providing fault tolerance to k identical unreliable LFSMs that operate in parallel on distinct input sequences. The overall construction requires only a constant amount of redundant hardware per machine (but sufficiently large k) to achieve an arbitrarily small probability of overall failure for any prespecified (finite) time interval, leading in this way to a lower bound on the computational capacity of unreliable LFSMs.     

Processing techniques for particulate-reinforced metal aluminium matrix composites

The critical need for high strength, lightweight and high stiffness materials has, in recent years, resurrected much interest in discontinuously reinforced powder metallurgy metal matrix composites. These hybrid materials have combined both standard wrought alloys of aluminium and a wide variety of discontinuous reinforcements such as particulates and whiskers of ceramic materials. Renewed interest in these materials as attractive candidates for use in the aerospace and transportation industry has resulted from an attractive and unique combination of physical and mechanical properties, and an ability to offer near isotropic properties coupled with the low cost of these materials when compared with existing monolithic materials. In this paper, the primary processing categories for discontinuously-reinforced metal-matrix composites are highlighted and the salient features of the various techniques in each category are discussed. The variables involved in each processing technique are examined, and the influence of alloy chemistry highlighted. Novel processing techniques for these materials such as the variable co-deposition method is presented as a means to process these novel engineering materials in order to improve their overall mechanical performance.     

Effect of temperature and cyclic heating on the structure and strength of glass textolite

The article established the regularities of change of the physicomechanical characteristics and strength of glass textolites in single and repeated thermal loading. It shows that under multiple loading the intense loss of strength of the investigated materials is due to increased porosity on account of the large number of microcracks appearing in the structure. A correlation is obtained between the elastic and strength characteristics and porosity in dependence on the temperature of multiple heating which in the first approximation is described by a polynomial of second degree with respect to temperature, multiplied by the reciprocal of porosity.Translated from Problemy Prochnosti, No. 6, pp. 53–58, June, 1991.     

Comfort limits for asymmetric thermal radiation

Groups of 32 and 16 subjects of both sexes were exposed in an environmental chamber to radiant asymmetry caused by a cool wall, a warm wall, and a cool ceiling. Each subject was tested individually while seated and clothed at 0.6 clo. During each 3.5-hour experiment the subject was exposed to six radiant temperature asymmetries. He was asked whether and where he experienced any local cool or warm sensation, and whether it was felt to be uncomfortable. During the entire experiment he was kept thermally neutral by changing the air temperature according to his wishes.For cool walls, warm walls, and cool ceilings curves have been established showing the percentage of dissatisfied subjects as a function of the radiant asymmetry. Radiant asymmetry at a warm wall caused less discomfort than a cool wall. A cool ceiling caused less discomfort than a warm ceiling. Accepting that 5% of the subjects may feel uncomfortable. a radiant temperature asymmetry of 10°C is allowable at a cool wall, 23°C at a warm wall, and 14°C under a cool ceiling. A previous study showed that 4°C is allowable under a warm ceiling. Radiant asymmetry had no significant impact on the operative temperatures preferred by the subjects. No significant differences were observed between the responses of men and women exposed to radiant asymmetry.     

Relationship between structure parameters and chemical properties in some MPS3 layered phases

Cationic thermal motions in the MPS₃ have been related to substitution abilities in this layer family. They have also been correlated to the crystal field stabilization energy. These considerations explain why some MPS₃ materials may or may not undergo a low temperature substitution intercalation reaction. From interatomic distances comparisons, the bond between (PS₃) groups in the anionic (P₂S₆)^4? octahedra, is found to be easily stretched upon cation substitution. Cationic radii follows the expected variation according to their high spin configuration in octahedral sulfur environment.     

The preparation of high purity Mg2Ni

The intermetallic compound Mg₂Ni has been prepared by the vapour exchange/diffusion reaction of the elements sealed in a stainless steel container. The product has been characterised by powder X-ray diffractometry with Mo-K_α radiation, using step-scanning and profile refinement techniques. The hexagonal unit cell has constants a = 5.2117(4), $\text{c}\text{= 13.2538(9)}\text{A}\text{?}$.     

Transparent monolithic zirconia gels: effects of acetylacetone content on gelation

Transparent yttria-stabilized zirconia monolithic gels were synthesized under controlled conditions for hydrolysis and chemical polymerization. The influence of acetylacetone on the previous chemical reactions was explored, and a plausible explanation was proposed concerning the outstanding role of this additive during the gelation process. Small-angle X-ray scattering was used to estimate the size of the polymeric species and to investigate the influence of acetylacetone on the structure of the gels prepared.