Precipitation Of Atoms Research Articles

Precipitation of boron atoms implanted in silicon as detected by channeling analysis

The precipitation of boron atoms in a silicon crystal which had been heavily implanted with boron ions at room temperature and then postannealed at high temperatures was studied by channeling analysis of the α-particles induced by the 11B(p,α)8 Be nuclear reaction. It was found that boron atoms which exceed the solubility limit precipitate on 〈110〉 strings in the region of the highest boron concentration (around the boron ion range) with a lattice distance somewhat different from that of the host crystal after annealing at 900 °C.

Dislocation Etch Hillock Formation in White Tin Single Crystals

The mechanism for producing pyramidal etch hillocks at the sites of dislocation on the {001} surface of white tin is studied. The hillock formation results from the etch resistant materials existing at the dislocation sites on the crystal surface. These materials come from (1) the segregation of impurities contained in the crystal itself along the dislocation lines, and (2) the precipitation of copper or silver atoms contained in the etching solution at the clean dislocation sites. The role of impurities to produce hillocks is demonstrated and the mechanism for the development of hillocks is derived from the nature of dissolution of the {101} surface enclosing hillocks. Application of this mechanism to another crystal is suggested.

焼き戻したFe-1%C-0.5%As合金における異常析出

When Fe-1.0%C-0.5%As alloy containing fine spheroidized cementite is quench-aged in the range of the alpha phase, anomalous precipitation is found in the neighborhood of 500°C. In order to clarify the origin of this phenomenon, measurements of electrical resistivity and internal friction, and direct observation with electron microscope were carried out.The experimental results obtained are as follows:(1) The anomalous precipitation seems to be the process for the solute carbon atoms to precipitate onto fine spheroidized cementites which have been formed previously.(2) Free carbon atoms exist after quenching even in the alloy in which the anomalous precipitation occurs. These atoms precipitate with a normal two-stage precipitation process at 150°∼300°C. The above experimental results may be most plausibly explained by the following model:(3) The As-C complex exists in the alpha solid solution of Fe-C-As alloy. The energy for the formation or decomposition of this complex is larger than the energy for the migration of carbon atoms in alpha iron. When the complex decomposes at about 500°C to bring supersaturation of carbon, it precipitates onto fine spheroidized cementites which have been formed previously. This process seems to correspond to the anomalous precipitation.

Autoradiographic Observations on the Behavior of Gas Atoms in Neutron Irradiated Aluminum-Lithium Alloys

The behavior of radiation-produced gas atoms in Al-Li alloys was observed by means of 3H autoradiography. The technique is extremely useful for studying the behavior of gas atoms in specimens containing 3H in concentrations of the order of 10−5 in atom fraction. Cold-worked microstructures of the specimen are also revealed in the autoradiographs. However, the effect of β-phase on the behavior cannot be observed by autoradiography. Precipitation of the gas atoms could not be discerned within the grain in specimens irradiated in JRR-2 at ambient-water temperature. Upon heating to 200°C, 3H was seen to precipitate along the dislocation within the grain and denuded zones free of the precipitate appeared along the grain boundaries. The amount of 3H remaining in the specimen decreases with rising heating temperature. In the case when the concentration of radiation-produced gas atoms contained is the order of 10−5 in atom fraction, gas bubbles are observed in auto-radiographs of specimens heated at a temperature...

Behavior of excess vacancies during the nucleation of precipitates in aluminum-silicon alloys

Precipitation processes of vacancies and of silicon atoms were investigated by means of resistometric measurement and transmission electron microscopy, and the mechanism of vacancy enhanced nucleation of silicon precipitates was discussed. When a specimen quenched from 580°C and preaged to enhance the nucleation of silicon precipitates is aged at near 200°C, the resistivity decreases in four stages. In the first stage, dislocation loops are formed by condensation of vacancies. The loops disappear and silicon atom clusters become visible in the second stage where the resistivity decreases rather slowly. The third and fourth stages are due to precipitation of silicon atoms in a stable form. Nucleation of silicon atom clusters is considered to take place during the first stage. Growth and shrinkage of dislocation loops due to condensation and evaporation of vacancies during the initial stage of aging are considered important in the nucleation of silicon clusters. The possibility for silicon atoms to flow to dislocation loops together with excess vacancies is also discussed.

Heterogeneous Precipitation at Dislocations

Precipitation phenomena in concentrated aluminum-base silver alloys have been studied with a variety of techniques including electron microscopy. The purpose of the present work was to investigate the dislocation reactions that occur as silver atoms precipitate (or segregate) under a relatively low supersaturation. Specimens (0.1 mm thick) of Al-1 at. % Ag were quenched from ~500°C into an oil bath at room temperature and aged 30 min. at 265°C. The initial configurations available as sites for heterogeneous precipitation will therefore include perfect prismatic dislocation loops, Frank sessile loops and random segments of grown-in dislocations.

Expected spatial distribution of low-energy protons precipitated in the auroral zones

A Monte Carlo method was used to compute the latitudinal dispersion in the auroral zone of 5- to 20-kev protons injected on a magnetic field line with equatorial pitch angles less than 3°. Neutral hydrogen atoms resulting from charge exchange at altitudes of 300–500 km were found to travel large distances across field lines. An isotropic injection pitch angle distribution results in precipitation of protons and hydrogen atoms in the atmosphere over an area as wide as 600 km. The associated auroral Balmer α emission is also expected to originate in a region several hundred kilometers wide. The mechanism described here seems adequate to account for the diffuseness of observed auroral hydrogen emission.

The sputtering of metals with noble gas ions; an experiment to determine the fate of the injected gas and its influence on the sputtering process

Micrographs of thin copper foils irradiated at 20, 350, and 500 deg C with 10/sup 17/ A/sup +/ ions are shown and the results are discussed. It is concluded that the majority of the rare gas atoms precipitate into bubbles, leaving the surrounding metal free from dissolved impurities. Collision sequences travel directly to the surface and are not influenced by the injection gas. (L.B.S.)

Contribution of specific heat measurements below 4° K to the observation of helium diffusion after annealing in irradiated beryllium oxide

A l'aide de plusieurs méthodes expérimentales nous avons montré qu'il est possible d'établir à chaque instant le bilan d'hélium dans BeO, irradié et recuit. La chaleur spécifique à très basses températures (près du point −λ de l'hélium) détermine la masse de l'hélium précipité; la Chromatographie donne la quantité d'hélium désorbée pendant le recuit et la microscopie électronique détermine le rayon et la distance moyenne des bulles d'hélium dans BeO. On peut suivre ainsi le processus de diffusion et de précipitation de l'hélium dans BeO avec précision. La précipitation de l'hélium a lieu déjà pendant l'irradiation; la nucléation des bulles commence à partir de 600 °C; elle est maximale vers 1200 °C.Le coefficient de diffusion de l'hélium dans BeO satisfait à la relation: D = 2,5×10−3 exp (−A/kT) avec une énergie d'activation A = 3,45 eV entre 1080 et 1400 °C.With the help of several experimental methods we have shown that it is possible to determine from moment to moment the balance of helium in irradiated and annealed BeO. The specific heat at very low temperatures (near the λ-point of helium) establishes the mass of helium which has been precipitated; chromatography gives the amount of helium desorbed during the anneal and electron microscopy determines the radius and mean separation of helium bubbles in BeO. In this way the process of diffusion and precipitation of helium in BeO can be followed accurately.Precipitation of helium begins during irradiation; bubble nucléation begins from 600 °C and peaks at 1200 °C. The diffusion coefficient of helium in BeO satisfies the relation: D = 2.5 × 10−3exp (−A/kT), where A = 3.45 eV between 1080 and 1400 °C.Die Benutzung verschiedener experimenteller Methoden erlaubt es, in jeder Phase die Bilanz des in neutronenbestrahltem und getempertem BeO enthaltenen Heliums aufzustellen: — Messung der spezifischen Wärme bei tiefen Temperaturen (in der Nähe des λ-Punkts, 2,2 °K) ergibt die Masse des in Blasen agglomerierten Heliums; — gaschromatographisch wird die Heliummenge gemessen, die während des Temperns die Probe verlässt; — elektronenmikroskopisch werden Durchmesser und Anzahl der Blasen bestimmt. Es ist damit möglich, den Diffusions- und Agglomerationsprozess von He in BeO genau zu verfolgen.Die Agglomeration (Blasenbildung) des Helixims setzt bei etwa 600 °C ein; sie hat ein Maximum nahe 1200 °C. Der Diffusionskoeffizient für He in BeO ergibt sich zu D = 2,5 × 10−3exp (−A/kT) mit einer Aktivierungsenergie von A = 3,45 eV im Temperaturbereich von 1080–1400 °C.

Dislocations in Silicon Single Crystals

Dislocations and layer structures in silicon single crystals pulled by the Czockralski method are investigated by means of copper decoration as well as of etching with Dash's etchant. In the seeding process, a dislocation network is formed in the neighborhood of the interface between the seed and the grown crystal. Some of the dislocations propagate from the network into the grown crystal, their density being reduced under a favorable circumstance of crystal growth. Two types of layer structures are observed, one in dislocation free crystals and the other in crystals containing dislocation networks. These structures are ascertained to be caused by stratified accumulation of oxygen atoms in the form of clusters for dislocation free crystals and by stratified precipitation of oxygen atoms on the nodes of dislocation network for crystals containing many dislocations. Some interesting phenomena concerning the slips of the dislocations are discussed.

Radiation-enhanced helium precipitation in copper

Abstract The effect of neutron irradiation on the precipitation of helium atoms in copper has been studied at temperatures up to 425°c. At 160°c end 300°c no effect was observed. At 425°c the irradiated sample exhibited considerable precipitation around grain boundaries as compared to an unirradiated sample held at the same temperature. It is concluded that the additional vacancies introduced during the irradiation enhance the precipitation process. The resultant voids are equal in size to those obtained at higher temperatures in the absences of radiation.