Elastic properties of perovskite-type hydride NaMgH3 for hydrogen storage

Mechanical properties of NaMgH₃ were investigated using the norm-conserving pseudopotentials and plane waves (PP–PW) within the general gradient approximation (GGA) in the frame of density functional theory (DFT). The elastic constants of NaMgH₃ were calculated for the first time. The NaMgH₃ compound is found to be mechanically stable at ambient pressure. The linear bulk modulus and the bulk modulus along crystallographic axes of single crystals have been derived using elastic constants. The calculated linear bulk moduli are found to be in good agreement with the theoretical value reported in the literature. Shear and Young's moduli as well as Poisson's ratio for ideal polycrystalline NaMgH₃ are also calculated. According to the obtained results, NaMgH₃ can be classified as brittle material. The shear anisotropic factors and the elastic anisotropy are also discussed. A Debye temperature of 648 K was also determined using theoretical elastic constants.     

Elastic modulus and internal friction of SOFC electrolytes at high temperatures under controlled atmospheres

Mechanical properties such as Young's modulus, shear modulus, Poisson's ratio and internal friction of conventional electrolyte materials for solid oxide fuel cells, Zr_0.85Y_0.15 O_1.93 (YSZ), Zr_0.82Sc_0.18O_1.91 (ScSZ), Zr_0.81Sc_0.18Ce_0.01O_2−δ (ScCeSZ), Ce_0.9Gd_0.1O_2−δ (GDC), La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O_3−δ (LSGMC), La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM), were evaluated by a resonance method at temperatures from room temperature to 1273 K in various oxygen partial pressures. The Young's modulus of GDC gradually decreased with increasing temperature in oxidizing conditions. The Young's moduli of the series of zirconia and lanthanum gallate based materials drastically decreased in an intermediate temperature range and increased slightly with increasing temperature at higher temperatures. The Young's modulus of GDC considerably decreased above 823 K in reducing atmospheres in response to the change of oxygen nonstoichiometry. However, temperature dependences of the Young's moduli of ScCeSZ and LSGMC in reducing atmospheres did not show any significant differences with those in oxidizing atmospheres.     

Axisymmetric and One-Dimensional Thermoelastic Fields of Radially Graded Bodies

In this paper, both Young's modulus and Poisson's ratio along with thermal expansion coefficient are allowed to vary across the radius in a solid ring and a curved beam. Effects of non-constant Poisson's ratio on the thermoelastic field in these graded axisymmetric and one-dimensional problems are studied. A governing differential equation in terms of stress function is obtained for general axisymmetric and one-dimensional problems. Two linearly independent solutions in terms of hypergeometric functions are then attained to calculate the stresses and the strains. Using Green's function method, a form of a solution for the stress functions in terms of integral equations for a curved beam and a solid ring are obtained. Specifically, closed form solutions for the stress functions, when Young's modulus and Poisson's ratio are expressed as power law functions across the radius, are calculated. The results show that the effect of varying Poisson's ratio upon the thermal stresses is considerable for the solid ring. In addition, a non-constant Poisson's ratio has significant influences on the thermal strain field in solid rings. The effect of varying Poisson's ratio upon the thermal stresses is negligible for the curved beam. However, non-constant Poisson's ratios have substantial effects on the thermal strain field in curved beams. Finally, the effects of varying Poisson's ratio on the thermal stresses in thick solid rings and curved beams are also investigated.     

The effect of hydrogen on the mechanical properties of FeTi for hydrogen storage applications

In this paper, the density functional theory (DFT) within the generalized gradient approximation (GGA) was used. The single crystal elastic constants for the intermetallic FeTi and its hydrides FeTiH and FeTiH₂ are successfully obtained from the stress–strain relationship calculations and the strain energy-strain curves calculations, respectively. The shear modulus, Young's modulus, Poisson's ratio and shear anisotropic factors are also calculated. The bulk moduli derived from the elastic constants calculations of the cubic FeTi, orthorhombic P222₁ FeTiH and Cmmm FeTiH₂ are calculated. For cubic FeTi compound, the bulk modulus is in a good agreement with both theoretical results and experimental data available in the literature. More importantly, it is found that, the insertion of hydrogen into the FeTi crystal structure causes an increase in the bulk modulus. From the analysis of shear-to-bulk modulus ratio, it is found that FeTi compound and its hydrides are ductile and that this ductibility, changes with changing the concentration of hydrogen.     

Thermal Stresses in Thin Auxetic Plates

This article investigates the effect of auxeticity on the thermal stresses of isotropic plates. The thermal stress is non-dimensionalized against the coefficient of thermal expansion, the change in temperature and at least one of the moduli so as to express the dimensionless thermal stresses solely in terms of Poisson's ratio of the plate material. Results show that increasing auxeticity leads to mild and significant drop in the thermal stresses under the conditions of constant Young's modulus and constant shear modulus, respectively. However, increasing auxeticity causes increase in the thermal stress under the condition of constant bulk modulus. It is also shown that increasing auxeticity under the condition of constant product of all the three moduli reduces the thermal stress if Poisson's ratio falls within a wide range of ?1 and 0.303. These results suggest that, under most circumstances, the replacement of conventional plate materials with auxetic solids is useful for reducing thermal stresses therein. The use of auxetic materials, therefore, provides an additional choice for the reduction of thermal stresses in plates other than selecting materials of lower modulus and low coefficient of thermal expansion.     

Temperature Effect on Young's Modulus of Boron Nitride Sheets

In this article, based on the oscillations of atoms due to the thermal effects (i.e., thermal phonons), Young's modulus of a hexagonal boron nitride sheet at different environment temperatures is investigated. To this end, the density functional theory (DFT) and quasi-harmonic approximation (QHA) are applied to calculate the energies of electrons and phonons, respectively, and then to obtain the total energy of the system. Unlike graphene, Young's modulus of boron nitride sheets tends to considerably increase with the increase of temperature to a specific value about 800 K. For the temperatures greater than 800 K, variation of Young's modulus with temperature is not considerable so that it can be neglected at high temperatures. It is also discerned that when temperature is high, the effect of phonon energy on Young's modulus is negligible.     

Investigation of structural,electronic and lattice dynamical properties of XNiH3 (X = Li,Na and K) perovskite type hydrides and their hydrogen storage applications

XNiH₃ (X = Li, Na, and K) perovskite type hydrides have been studied by using Density Functional Theory (DFT) and these materials are found to be stable and synthesizable. The X-ray diffraction patterns have been obtained and they indicate that all materials have the polycrystalline structure. The electronic properties have been investigated and it has been found that these structures show metallic character. The Bader partial charge analysis has also been performed. In addition, the elastic constants have been calculated and these materials are found to be mechanically stable. Using these elastic constants, the mechanical properties such as bulk modulus, shear modulus, Poisson's ratio have been obtained. Moreover, the Debye temperatures and thermal conductivities have been studied. The anisotropic elastic properties have been visualized in three dimensions (3D) for Young's modulus, linear compressibility, shear modulus and Poisson's ratio as well as with the calculation of the anisotropic factors. Additionally, the dynamical stability has been investigated and obtained phonon dispersion curves show that these materials are dynamically stable. Also, the thermal properties including free energy, enthalpy, entropy and heat capacity have been studied. The hydrogen storage properties have been examined and the gravimetric hydrogen storage capacities have been calculated as 4.40 wt%, 3.57 wt% and 3.30 wt% for LiNiH₃, NaNiH₃ and KNiH₃, respectively. Furthermore, the hydrogen desorption temperatures have been obtained as 446.3 K, 419.5 K and 367.5 K for LiNiH₃, NaNiH₃ and KNiH₃, respectively.     

A novel constitutive stress-strain law for compressive deformation of the gas diffusion layer

Substantial compressive deformation occurs in the gas diffusion layer (GDL) under the pressure applied during the fuel cell assembly. The GDL deformation has a direct impact on the efficiency and performance of the fuel cell since it leads to the alteration of the GDL microstructure and porosity. This makes the accurate characterization of the GDL compressive behavior crucial for analyzing the fuel cell performance and its optimal design. In this paper, analytical, experimental, and numerical methods have been employed to comprehensively study the constitutive law of the GDL under compression. Starting from the recently developed stress-density relations, the constitutive stress-strain equations are derived for the GDL and the relation between the stress-density and stress-strain laws are revealed. Experimental compression tests have been performed on GDL samples and the capability of the proposed constitutive law in capturing the real behavior of the material has been proved. It has been observed that the simplifying assumption of constant zero Poisson's ratio in the through-plane direction made in many previous studies cannot accurately represent the GDL material behavior and a modification is proposed. The developed constitutive law has been successfully implemented in a finite element model of the GDL-bipolar plate assembly in the fuel cell structure and the variations of the GDL porosity, density, and through-plane Young's modulus and Poisson's ratio have been investigated for different vertical displacements of the bipolar plate.     

Young's modulus of polycrystalline Li22Si5

In order for Li-Si alloys to be used in Li-ion batteries as anodes, knowledge of their mechanical properties, such as Young's moduli, is crucial. Young's modulus of polycrystalline Li₂₂Si₅ was determined from nanoindentation testing. The value of Young's modulus was 35.4 ± 4.3 GPa. This value is approximately one-half of the predicted value based on density functional theory calculations. This difference was not a result of the testing procedure or microstructural variables.     

Effect of Temperature on Young's Modulus of Graphene

The temperature dependence of the thermo-mechanical behavior of materials is of great importance in many engineering applications where the precise properties of materials over an extended temperature range are needed. The objective of this work is to present a density functional study to predict the temperature variation of Young's modulus of graphene. To this end, the energies of phonons as well as thermodynamic functions are calculated from phonon calculations via the quasi-harmonic approximation. It is observed that with the increase of strain the phonon energy decreases. Also, by increasing temperature up to a special value which is around 400 K, Young's modulus decreases appreciably. For the temperatures higher than 400 K, Young's modulus decreases with a lower rate and tends to be constant at high temperatures. The results obtained are in a good agreement with the experimental data previously reported in the literature.     

Compressive behaviour of thin catalyst layers. Part I - Experimental study

In this study, the effect of compression is investigated experimentally on deformation and porosity of catalyst layers (CLs). Compression tests are performed on five CL samples with various microstructures using a thermomechanical analyzer and a custom-made machine Tuc-Ruc (Thickness under compression-Resistivity under compression). The results indicate that CLs have a linear behaviour with no plastic deformation at pressures less than 2 MPa even after 12 cycles. However, CLs showed plastic deformation, work hardening, and elastic shakedown under cyclic compression up to 5 MPa. In this pressure range, the material becomes stiffer and Young's modulus has increased by 50–113% after 8 loading cycles. Moreover, the material “settles down” after 6 cycles showing no further significant plastic deformation at higher pressures (up to 5 MPa). This behaviour suggests that CLs enter elastic shakedown region since after several cycles, plastic strain diminished, and they behave elastically afterwards. The compression tests on five samples yield Young's modulus of 30–45 MPa for pressures up to 2 MPa and Young's modulus of 37–70 MPa for pressures up to 5 MPa. The reason for slight change in Young's modulus is that the microstructure of CL changed, and the porosity decreased at higher pressures.     

The applications of 2D elasto-plastic stochastic finite element method in the field of fracture mechanics

In this paper the stochastic finite element method (SFEM) is applied to the field of fracture mechanics and cracked structures are analysed by using an elasto-plastic methodology. The main random variables considered are Young's modulus, Poisson's ratio and the crack size. The formulae of the mean and variance value of the J-integral for elasto-plastic deformation are discussed and an effective method for the probabilistic safety assessment of cracked structures is given. Some elasto-plastic problems are solved by this method, and numerical examples demonstrate that this method is appropriate for engineering problems and that the related computer program developed in this paper has sufficient precision.     

First-principle study on the effects of hydrogen in combination with alloy solutes on local mechanical properties of steels

The first-principle calculation method has been utilized to study the effects of hydrogen in combination with alloy elements on the mechanical properties of iron-based super-cells, with the aim to bring new insight in the design of hydrogen-resistant steels. It is found that hydrogen prefers the second and third tetrahedral interstitial sites of the alloy solutes. Nb/Zr/Mo/W/Al alloyed super-cells dropped the most in the bulk modulus by hydrogen addition. Ni/Nb/Cu/Co/Zr dropped the most in the shear modulus/Young's modulus/hardness. The underlying influencing mechanisms on the elastic mechanical properties are discussed in perspective of atomic parameters/density of states/charge density/difference charge density/Bader charge analysis.     

Elastic,electronic properties and QTAIM of new H-enriched hydrogen storage material Mg(BH4)2⋅(NH3)2 (NH3BH3)

Mg(BH₄)₂⋅(NH₃)₂ (NH₃BH₃) is an important H-enriched hydrogen storage material capable of releasing high-purity hydrogen. This work investigated the elastic and electronic properties of Mg(BH₄)₂⋅(NH₃)₂ (NH₃BH₃) using first-principle calculations for the elastic constants, bulk modulus, Young's modulus, B/G. Results show this compound is mechanically stable and classified as a brittle material. Three-dimensional curves indicate significant anisotropy exists in the (010) and (100) planes for bulk modulus and the (001) plane for Young's modulus. The (001) plane elastic anisotropy is larger than those of the (010) and (0‾1 0) planes. An electronic properties analysis indicates this compound is a semiconductor with a 1.151 eV band gap. There exist the strong B-H, Mg-N interactions from the analysis of density of state, which are further confirmed by Bader's quantum theory of atoms in molecules (QTAIM).     

Numerical exploration of 1 + 2 type laminar natural convection in a differentially heated square cavity using Chebyshev spectral collocation method

In this paper, the Chebyshev spectral collocation method is applied to explore the unsteady two dimensional (1 + 2 type) laminar natural convection in a differentially heated square cavity at a Rayleigh number (Ra) of 10⁷. The method has embedded the traditional Chorin's algorithm so as to avoid the trouble of seeking the pressure field in the buoyancy driven wall-jet flow. The sensitivity of the δ− parameter has been numerically investigated. It is found that when the δ value is over 11.6173, numerical instability occurs. Comparing the maximum horizontal velocity component with the existing numerical data obtained by solving the Poisson's equation of pressure field reveals that the Chorin's algorithm should be inapplicable for the solution of the benchmark problem of natural convection at Ra = 10⁷ in thermal science.     

A study of safe CO2 storage capacity in saline aquifers: a numerical study

An effective way of reducing greenhouse gas content in the atmosphere is carbon dioxide (CO₂) geo‐sequestration in saline aquifers. The main objective of this study is to develop a 3‐D numerical model to identify the optimum CO₂ storage capacity in saline aquifers by studying the factors affecting it and the possibility of the injected CO₂ back‐migrating into the atmosphere. A 1000m×1000m×184 m saline aquifer, lying 800 m below the ground surface, was therefore considered to develop a model using the COMET 3 reservoir simulator. The effects of injecting CO₂ properties (injection pressure) and the aquifer's properties (depth, temperatures and salinity) on the CO₂ storage capacity were examined first. According to the results of the model, CO₂ storage capacity increases with increasing injection pressure and salinity and decreasing depth and temperature, and 100% variations in injection pressure, depth, temperature and salinity levels cause the CO₂ storage capacity to be changed by 54%, 36%, 18% and 1.8%, respectively. The next stage of the study involved the determination of cap rock failure due to CO₂ injection pressure and the identification of the factors influencing it. A detailed parametric study was conducted, with changes to the depth, temperature and salinity with respect to injection pressure, to detect the effects of these factors on the optimum CO₂ injection pressure. According to the results, optimum CO₂ injection pressure clearly depends on the aquifer depth and the effects of salinity and temperature are negligible. An increment of 0.8 to 1.4 km in aquifer depth causes the optimum injection pressure to be increased from 19.55 to 42 MPa, which is about 10⁵ and 10⁷ higher than the effects of temperature (20 to 110 °C increment) and salinity level (100,000 to 160,000 ppm increment), respectively. The model can be used effectively in field studies to safely enhance CO₂ storage capacity in saline aquifers. Copyright © 2012 John Wiley & Sons, Ltd.     

Thermal and mechanical properties of LaCoO3 and La0.8Ca0.2CoO3 perovskites

In this work, the thermal and mechanical properties such as coefficient of thermal expansion, strength, Young's modulus and fracture toughness of LaCoO₃ and La_0.8Ca_0.2CoO₃ perovskites have been studied, as well as slow crack growth of La_0.8Ca_0.2CoO₃. The mechanical performance of the two cobaltites have been evaluated in terms of their ferroelastic hysteresis properties such as non-symmetry in bending of both stress and strain distributions, non-linear deformation upon applied load from the arbitrary low stresses, and ferroelastic toughening.     

Shallow gravel aquifers and the urban ‘heat island’ effect: a source of low enthalpy geothermal energy

Northern European countries with no high temperature geothermal resources can utilise the urban ‘heat island’ effect to generate low enthalpy geothermal energy for space heating/cooling systems in buildings, provided a suitable aquifer underlies the urban area. Buried valleys, formed at the height of the Pleistocene glaciation 15,000 years ago, when sea level was 130 m lower than present, and infilled with gravels as sea level rose again at the end of the Pleistocene, underlie many European cities. These high yielding aquifers exist at only a few metres depth, and can provide a supply of groundwater at temperatures elevated 3–4 K above the average rural groundwater temperatures. This can produce a marked improvement both in the output and in the efficiency of a geothermal system making use of this source. When passed through a heat pump operating at a Coefficient of Performance (COP) of 4.5:1, a well yielding 20 l/s of groundwater at 13 °C can generate 865 kW heat, sufficient to supply space heating for buildings with a footprint in excess of 12,000 m² with a peak heating intensity of 70 W/m². The economics of this low enthalpy geothermal energy source are outlined. Although development costs are minimal, at current low natural gas fuel prices in Ireland, heating-only applications will be less attractive, and a real cost saving will only accrue if dual heating/cooling functions can be developed.     

Geochemical monitoring of the Krafla and Námafjall geothermal areas, N-Iceland

The Krafla and Námafjall high-temperature geothermal areas in N-Iceland have been exploited for steam production since the late and early 1970s, respectively. Power generation at Krafla was 30 MW until 1998, when it was increased to 60 MW. At Námafjall the steam has been utilized for operating a 3 MW back-pressure turbine unit, drying of diatomaceous earth and heating of fresh water for space heating. A total of 34 wells have been drilled at Krafla, of which 18 are producing at present. At Námafjall 12 wells have been drilled but only three are productive. The highest temperatures recorded downhole are 320 and 350 °C at Námafjall and Krafla, respectively. Geochemical monitoring in the two fields during the last 20–25 years has revealed decreases in the Cl concentrations in the water discharged from most of the wells that have been producing for more than 10 years. The cause is enhanced colder water recharge into the producing aquifers of these wells due to depressurization by fluid withdrawal from the geothermal reservoir. Such recharge is particularly pronounced in the central part of the Leirbotnar wellfield at Krafla but it is also extensive in the only producing well in the Hvíthólar wellfield. At Námafjall incursion of cold groundwater into the reservoir was particularly intense subsequent to the volcanic-rifting event in the area in 1977. Solute (quartz, Na/K, Na/K/Ca) geothermometry temperatures have decreased significantly in those wells where Cl concentrations have decreased but only to a limited extent in those wells which have remained constant in Cl. This indicates that the changes in the concentrations of the reactive components, on which these geothermometers are based, is largely the consequence of colder water recharge and not partial re-equilibration in the depressurization zone around wells where cooling of the fluid occurs in response to extensive boiling. Aqueous SO₄ concentrations increase as Cl concentrations decrease. Except for the hottest wells, which are low in SO₄, sulphate concentrations are controlled by anhydrite solubility. Increase in SO₄ concentrations is a reflection of cooling as anhydrite has retrograde solubility with respect to temperature. H₂S-temperatures are similar to the solute geothermometry temperatures for wells with a single feed. They are, on the other hand, higher, for wells with multiple feeds, if the feed zones have significantly different temperatures. H₂-temperatures are anomalously high for most wells due to the presence of equilibrium steam in the producing aquifers. The equilibrium steam fraction amounts to 0–2.2% by wt. of the aquifer fluid (0–47% by volume). CO₂ temperatures are anomalously high for some Krafla wells due to high flux of CO₂ from the magma intruded into the roots of the geothermal system during the 1975–1984 volcanic-rifting episode. During the early phase of this episode the Leirbotnar wells were the ones most affected. The new magma gas flux has migrated eastwards with time. Today some wells in the Sudurhlídar wellfield are the ones most affected whereas the Leirbotnar wells have recovered partly or fully. The depth level of producing aquifers in individual wells at Krafla and Námafjall has been evaluated by combining data on temperature and pressure logging and geothermometry results. The majority of wells at Krafla receive fluid from a single aquifer, or from 2–3 aquifers having similar temperature. The same applies to two of the three productive wells at Námafjall.     

Measurement and interpretation of terrestrial heat flow in Israel

Sixty-eight new determinations of terrestrial heat flow in Israel have a range of 0.17-11.07 μcal/cm²s. The average value of deep conductive heat flow in the undisturbed complex of the Arabo-Nubian Massif is 0·94 μcal/cm²s; it is least affected by circulation of groundwater. This value is only slightly higher than the heat flow of 0·88 μcal/cm²s in the Levantine Basin of the Mediterranean Sea. Several values that exceed 2·0 μcal/cm²s are due either to (probable) deep hydrothermal activity or to small domal structures of the basement.Within the sedimentary sequence which blankets the crystalline massif, terrestrial heat flow is often redistributed by circulating groundwater. Recharge regions, particularly Judean-Samarian Galilee, where cool meteoric waters percolate into the subsurface have anomalously low heat flow, ranging from 0·17 to about 1·0 μcal/cm²s. Part of the original deep thermal flux in those regions is intercepted at moderate depths by the recharge flow, and is carried into deeper aquifers of the Foothills, Coastal Plain, or the Jordan-Dead Sea Rift. Movement of groundwater occurs mainly along faults.Deep faults associated with the Jordan-Dead Sea Rift system act as conduits for hot waters ascending from deep confined aquifers. The most tangible surface expression of the convective hydrothermal system are the numerous warm to hot springs, emerging along the margins of the Rift. However, the waters emerging on the surface as the warm and hot springs are a minor fraction of the convective system. Most of the ascending thermal waters are absorbed by shallow aquifers with lower hydraulic potential. Such regions are characterized by anomalously high heat flow; several values exceed 2 and one value is 11 μcal/cm²s.