A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

Introduction.- Phases and Equilibria in the W - C and W - Co - C Systems.- Ordering of Tungsten Carbides.- Nanocrystalline Tungsten Carbide.- Hardmetals WC - Co Based on Nanocrystalline Powders of Tungsten Carbide WC.

Tungsten Carbides: Structure, Properties and Application in Hardmetals

An understanding of the precise correlation between the presence of secondary phases and material damping has eluded investigators, partly as a result of the fact that often there are various mechanisms involved. As a step towards the clarification of damping phenomena in metals and alloys, this paper provides a systematic review of the studies that have been completed on the damping mechanisms present in metals and alloys, with particular emphasis on precipitation. The damping mechanisms associated with secondary phases in metals and alloys have been subdivided into four categories, namely interface damping theory, thermal mismatch-induced dislocation damping theory, interaction damping theory and the rule of mixtures damping theory. A number of alloy systems are discussed to demonstrate the applicability of the four types of theory and the level of understanding of these complex mechanisms. As an extension of precipitation damping theory, the damping behaviour and mechanisms in particle-reinforced metal matrix composites are extensively discussed.

Effects of secondary phases on the damping behaviour of metals, alloys and metal matrix composites

Studies of light alloys by positron annihilation techniques

Nanocomposite materials comprising a metal oxide bonded to at least one graphene material. The nanocomposite materials exhibit a specific capacity of at least twice that of the metal oxide material without the graphene at a charge/discharge rate greater than about 10C.

Nanocomposite of graphene and metal oxide materials

The positron‐annihilation spectrum for C8K obtained from the Doppler broadening of the 511 keV annihilation radiation is composed of two Gaussian bands: one due to annihilation with electrons in the interlayer region, the other mainly due to annihilation with electrons in the graphitic layers. Upon introduction of hydrogen to C8K, both bands sharpened, which indicates a conversion of hydrogen atoms to hybride ions through charge transfer from the graphitic layers and the formation of positron–hydride ion complexes in the interlayer region. (AIP)

Chemisorption of hydrogen into a graphite–potassium intercalation compound C8K studied by means of positron annihilation

Three lifetime components, one of which is extremely long (25±2 ns), have been observed in experimental studies of positron annihilation in porous silicon, made by anodization in hydrofluoric acid. The Doppler‐broadened spectrum of the porous silicon is sharp compared with that of crystal silicon and becomes even narrower in an applied magnetic field. The positronium yield in the porous silicon therefore is concluded from the long lifetime, narrow Doppler spectrum and its narrowing in a magnetic field. The porous structure is the cause of positronium formation.

Positron annihilation in porous silicon

Positron annihilation spectroscopy was carried out on nanocrystalline silver. Both lifetime and Doppler broadening were measured. High-resolution electron microscopy observations were also performed. Diffuse vacancy clusters, the sizes of which correspond to two to four vacancies, and voids of 1–5 nm diameter, were found to reduce the average atomic density of the grain boundary. Some grains grew to a diameter larger than about 70 nm during annealing the specimen from 50 to 100°C. A large fraction of the diffuse vacancy clusters and voids in the boundaries remained after annealing at 400°C. They were stabilized during annealing probably by gaseous atoms.

Structure and thermal stability of nanocrystalline silver studied by transmission electron microscopy and positron annihilation spectroscopy

The positron lifetime of undoped Liquid-Encapsulated Czochralski (LEC)-GaAs and Si-doped (1.3×1018 cm−3) LEC-GaAs was measured before and after irradiation with protons (dose 1×1015/cm2, 15 MeV). In Si-doped GaAs, the decrease of positron lifetime at temperatures between 10 and 300 K are due to the decrease of the positron-diffusion length and the increase of the effective shallow traps such as antisite GaAs. The annealing stage of the proton-irradiation-induced defects which show the different behavior from that of electron-irradiation-induced defects suggests that proton irradiation creates more complicated defect complexes, containing vacancies rather than isolated vacancy-type defects or simple complexes which have been observed during electron-irradiation processes. Above 700 K, proton-irradiation-induced defects such as vacancy-type defects and simple vacancy complexes are almost annealed out, while Si-induced defects such as SiGa-VGa complexes cannot be annealed out above 973 K.

Defect study of proton-irradiated liquid-encapsulated Czochralski GaAs using the positron-annihilation technique

The nanocrystallization mechanism of an amorphous alloy is discussed based on the kinetics of open nanospaces in Fe78B13Si9. There already exists a high concentration of Fe-enriched fluctuated sites with open nanospaces in the amorphous matrix. The structural and compositional fluctuation helps transient short-range Fe diffusion in the metastable amorphous matrix with an increase of temperature, triggering highly concentrated α-Fe nucleation. Along with the growth of α-Fe nucleus, Fe atoms are transferred from the intergranular amorphous phase to Fe-based nanocrystallites. The nanocrystallization of α-Fe is achieved through nucleation by short-range Fe diffusion and its growth by nanovoid-mediated long-range Fe diffusion.

Nanocrystallization mechanism of amorphous Fe78B13Si9

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