V. Saltas

Use of engineering geophysics to investigate a site for a building foundation

The combination of geophysical data and geotechnical measurements may greatly improve the quality of buildings under construction in civil engineering. A case study is presented here at a vacant building site. Initially, boreholes indicated a complex geology. A dipole–dipole configuration was selected for electrical resistivity tomography (ERT) implementation and the data were processed and interpreted by applying 2D and 3D inversions. An electromagnetic survey was also carried out at a different time periods and successfully used to verify the results of the resistivity measurements. It is demonstrated that engineering geophysics is able to provide solutions for determining subsurface properties and that different prospection techniques are necessary for developing a reasonable model of the subsurface structure.

A thermodynamic approach of self- and hetero-diffusion in GaAs: Connecting point defect parameters with bulk properties

The self- and hetero-diffusion in GaAs is investigated in terms of the cBΩ thermodynamic model, which connects point defect parameters with the bulk elastic and expansion properties. Point defect thermodynamic properties, such as activation enthalpy, activation volume, activation Gibbs free energy, activation entropy and isobaric specific heat of activation, are calculated as a function of temperature for Ga, H and various n- and p-type dopants (Si, Be, Cr, Fe and Zn) diffused in GaAs. The present calculations are in good agreement with the reported experimental results. The pressure dependence of Ga self-diffusion is also investigated and the diffusivities and activation volumes are predicted at different temperatures from ambient pressure up to 10 GPa, above which GaAs is transformed into the orthorhombic structure. The activation volumes of dopants are also estimated at high temperature (1124 K), as a function of pressure.

Composition and temperature dependence of self-diffusion in Si 1−x Ge x alloys

The knowledge of diffusion processes in semiconducting alloys is very important both technologically and from a theoretical point of view. Here we show that, self-diffusion in Si1−xGex alloys as a function of temperature and Ge concentration can be described by the cBΩ thermodynamic model. This model connects the activation Gibbs free energy of point defects formation and migration with the elastic and expansion properties of the bulk material. The approach allows the systematic investigation of point defect thermodynamic parameters such as activation enthalpy, activation entropy and activation volume, based on the thermo-elastic properties (bulk modulus and its derivatives, mean atomic volume and thermal expansion coefficient) of the two end-members of the Si1−xGex alloy. Considerable deviations from Vegard’s law are observed, due to the diversification of the bulk properties of Si and Ge, in complete agreement with the available experimental data.

Modelling solid solutions with cluster expansion, special quasirandom structures, and thermodynamic approaches

Modelling solid solutions is fundamental in understanding the properties of numerous materials which are important for a range of applications in various fields including nanoelectronics and energy materials such as fuel cells, nuclear materials, and batteries, as the systematic understanding throughout the composition range of solid solutions for a range of conditions can be challenging from an experimental viewpoint. The main motivation of this review is to contribute to the discussion in the community of the applicability of methods that constitute the investigation of solid solutions computationally tractable. This is important as computational modelling is required to calculate numerous defect properties and to act synergistically with experiment to understand these materials. This review will examine in detail two examples: silicon germanium alloys and MAX phase solid solutions. Silicon germanium alloys are technologically important in nanoelectronic devices and are also relevant considering the rec...

Investigation of oxygen self-diffusion in PuO2 by combining molecular dynamics with thermodynamic calculations

In the present work, the defect properties of oxygen self-diffusion in PuO2 are investigated over a wide temperature (300–1900 K) and pressure (0–10 GPa) range, by combining molecular dynamics simulations and thermodynamic calculations. Based on the well-established cBΩ thermodynamic model which connects the activation Gibbs free energy of diffusion with the bulk elastic and expansion properties, various point defect parameters such as activation enthalpy, activation entropy, and activation volume were calculated as a function of T and P. Molecular dynamics calculations provided the necessary bulk properties for the proper implementation of the thermodynamic model, in the lack of any relevant experimental data. The estimated compressibility and the thermal expansion coefficient of activation volume are found to be more than one order of magnitude greater than the corresponding values of the bulk plutonia. The diffusion mechanism is discussed in the context of the temperature and pressure dependence of the activation volume.

Tin diffusion in germanium: a thermodynamic approach

Point defect properties including diffusion properties are technologically important in semiconductor materials particularly as the characteristic dimension of devices are a few nanometers. Tin is an isovalent dopant in germanium that is presently considered to form radiation tolerant devices and in defect engineering strategies aiming to contain the high n-type dopant diffusion. In the present investigation we show that the cBΩ thermodynamic model linking the defect Gibbs energy to the isothermal bulk modulus and the mean volume per atom, can describe tin diffusion in germanium. The model is used to calculate point defect thermodynamic parameters such as the activation entropy, activation enthalpy and activation volume of tin diffusion in germanium as a function of temperature.

Using acoustic emissions to enhance fracture toughness calculations for CCNBD marble specimens

Rock fracture mechanics has been widely applied to blasting, hydraulic fracturing, mechanical fragmentation, rock slope analysis, geophysics, earthquake mechanics and many other science and technology fields. Development of failure in brittle materials is associated with microcracks, which release energy in the form of elastic waves called acoustic emissions. In the present study, acoustic emission (AE) measurements were carried out during cracked chevron notched Brazilian disc (CCNBD) tests on Nestos marble specimens. The fracture toughness of different modes of loading (mode-I and –II) is calculated and the results are discussed in conjunction with the AE parameters.

Complexity in Laboratory Seismology: From Electrical and Acoustic Emissions to Fracture

Abstract The use of electrical and acoustic signals emission as fracture precursors when brittle materials like rocks are subjected to mechanical stress has been evaluated and found to be applicable by several researchers. A detailed review and description within the framework of the complexity approach will be performed on laboratory experiments that are conducted and show up the generation and behaviour of electrical and acoustic signal emissions mainly when geomaterials are subjected to mechanical stress. The experimental results will be examined in the framework of their capacity to provide information regarding the initial stages of microcrack generation, propagation and coalescence, aiming to be used as fracture precursors. Specifically, the temporal variation of the electrical and acoustic emissions (AEs) in combination with the adopted mechanical stress application protocol and the type of stress test (compression, bending, etc.) is discussed. Furthermore, modelling that involves statistical physics and algorithmic statistics that are currently used on the analysis of the electrical and AEs are described and the latest findings reported. The similarities with the observations associated with fracture would be viewed in relation to the electrical and acoustic signal laboratory results.