Hydrogen effusion from tritiated amorphous silicon

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a-Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300 °C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600 h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritium outgassing with a time constant of 15 h, seems to be related to surface desorption of tritiated water (HTO) with a free energy of desorption of 1.04 eV. The second process, slow tritium outgassing with a time constant of 200-300 h, appears to be limited by oxygen diffusivity in a growing oxide layer. This material of lowest H stability would lose half of the hydrogen after 60 years. © 2008 American Institute of Physics

Self-irradiation enhanced tritium solubility in hydrogenated amorphous and crystalline silicon

Experimental results on tritium effusion, along with the tritium depth profiles, from hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) tritiated in tritium (T2) gas at various temperatures and pressures are presented. The results indicate that tritium incorporation is a function of the material microstructure of the as-grown films, rather than the tritium exposure condition. The highest tritium concentration obtained is for a-Si:H deposited at a substrate temperature of 200°C. The tritium content is about 20 at. % on average with a penetration depth of about 50 nm. In contrast, tritium occluded in the c-Si is about 4 at. % with penetration depth of about 10 nm. The tritium concentration observed in a-Si:H and c-Si is much higher than the reported results for the post-hydrogenation process. β irradiation appears to catalyze the tritiation process and enhance tritium dissolution in the silicon matrix. The combination of tritium decay and β-induced ionizations results in formation of reactive species of tritium (tritium atoms, radicals, and ions) that readily adsorb on silicon. The electron bombardment of the silicon surface and subsurface renders it chemically active thereby promoting surface adsorption and subsurface diffusion of tritium, thus leading to tritium occlusion in the silicon matrix. Gaussian deconvolution of tritium effusion spectra yields two peaks for a-Si:H films tritiated at high temperature (250°C), one low temperature (LT) peak which is attributed to tritiated clusters and higher order tritides, and another high temperature peak which is attributed to monotritides. Activation energy of 2.6-4.0 eV for the LT peak was found. © 2011 American Institute of Physics

Static and dynamic fractomechanics parameters determination applying the discrete element method composed by bars

El cálculo de parámetros fractomecánicos utilizando métodos numéricos computacionales es siempre un área activa de investigación. Tradicionalmente los métodos más empleados son el Método de los Elementos Finitos (MEF) y el Método de los Elementos de Contorno (MEC). Por su parte el Método de los Elementos Discretos compuesto por barras (MED) es otra alternativa de uso no tan extendido en el área de mecánica de los sólidos. El éxito del MED para simular mecanismos de falla y la nucleación de defectos motiva su aplicación a problemas de mecánica de fractura. Con el objetivo de explorar sus potencialidades en este tipo de problemas se presenta en este trabajo el cálculo de parámetros fractomecánicos estáticos y dinámicos de una placa modelada con el MED. Para tal fin son empleadas las mismas metodologías numéricas que se utilizan con métodos numéricos tradicionales, como los son el MEF y el MEC. Los resultados obtenidos comparados con resultados analíticos y numéricos de otros autores, permiten realizar la validación del método para este tipo de problemas donde la mecánica de fractura tiene que ser considerada indefectiblemente en el proceso de ruptura.Peer Reviewe

Applications of lattice method in the simulation of crack path in heterogeneous materials

The simulation of critical and subcritical crack propagation in heterogeneous materials is not a simpleproblem in computational mechanics. These topics can be studied with different theoretical tools. In the crackpropagation problem it is necessary to lead on the interface between the continuum and the discontinuity, andthis region has different characteristics when we change the scale level point of view. In this context, this workapplies a version of the lattice discrete element method (LDEM) in the study of such matters. This approach letsus to discretize the continuum with a regular tridimensional truss where the elements have an equivalent stiffnessconsistent with the material one wishes to model. The masses are lumped in the nodes and an uni-axial bilinearrelation, inspired in the Hilleborg constitutive law, is assumed for the elements. The random characteristics of thematerial are introduced in the model considering the material toughness as a random field with defined statisticalproperties. It is important to highlight that the energy balance consistence is maintained during all the process.The spatial discretization lets us arrive to a motion equation that can be solved using an explicit scheme ofintegration on time. Two examples are shown in the present paper; one of them illustrates the possibilities of thismethod in simulating critical crack propagation in a solid mechanics problem: a simple geometry of grade material.In the second example, a simulation of subcritical crack growth is presented, when a pre-fissured quasi-brittlebody is submitted to cyclic loading. In this second example, a strategy to measure crack advance in the model isproposed. Finally, obtained results and the performance of the model are discussed

Tritium locked in silica using 248 nm KrF laser irradiation

In this Letter we report on selectively occluding tritium in a silica film on a silicon substrate using a combination of high-pressure tritium loading and 248 nm KrF laser irradiation. Sixty percent of tritium dissolved in the silica film was bonded by laser irradiation. The concentration of the bonded tritium was proportional to the total laser fluence. Tritium effusion experiments indicated that the laser-locked tritium existed stably in the glass matrix up to 400 °C. In this work we point a way to a safe and simple approach for the integration of on-chip radioisotope micropower sources for micromechanical and microelectronic applications. © 2006 American Institute of Physics

Tritiation of amorphous and crystalline silicon using T <inf>2</inf> gas

Incorporation of tritium in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) at 250 °C using tritium (T 2) gas at pressures of up to 120 atm is reported. The tritium is stored in a surface layer which is approximately 150 and 10 nm for a-Si:H and c-Si, respectively. The concentration of tritium occluded in planar and textured c-Si is linearly dependent on the total surface area. The tritium is stable and the dominant tritium evolution occurs at temperatures above 300 °C. The concentration of tritium locked in a-Si:H and c-Si was 20 and 4 at. %, respectively. Self-catalysis appears to be important in the tritiation process. © 2006 American Institute of Physics

Long-Range Correlations and Natural Time Series Analyses from Acoustic Emission Signals

This work focuses on analyzing acoustic emission (AE) signals as a means to predict failure in structures. There are two main approaches that are considered: (i) long-range correlation analysis using both the Hurst (H) and the detrended fluctuation analysis (DFA) exponents, and (ii) natural time domain (NT) analysis. These methodologies are applied to the data that were collected from two application examples: a glass fiber-reinforced polymeric plate and a spaghetti bridge model, where both structures were subjected to increasing loads until collapse. A traditional (AE) signal analysis was also performed to reference the study of the other methods. The results indicate that the proposed methods yield reliable indication of failure in the studied structures