Multiporoelasticity of Hierarchically Structured Materials: Micromechanical Foundations and Application to Bone

We here extend the theory of microporomechanics by Dormieux et al. to multiple pore spaces. As an application, we reveal, on the basis of a recently validated multiscale elastic model for bone tissues by Fritsch and Hellmich, the effects of multiple pore pressures in various, scale-separated pore spaces, on the overall behavior of the multiporous composite material. Thereby, our focus is on the lacunar pore space, and on its interplay with the pore spaces found further below: not only those between the mineral crystals (of some 10?nm characteristic pore size) but also those of the collagen molecules building up (micro-)fibrils (with a little more than 1?nm distance between these molecules). Our results clearly show that the interplay between pore pressure and skeleton deformation depends strongly on the loading direction and on the characteristic size of the pores—hence, we can conclude that the consideration of these strongly hierarchical and anisotropic effects in whole-organ simulations including fluid mass transport, would allow for valuable new insights into the ongoing discussion on poromechanobiology of bone.     

Chemoporoplasticity of Calcium Leaching in Concrete

This paper presents a macroscopic material model for calcium leaching in concrete, for the quantitative assessment, in time and space, of the aging kinetics and load bearing capacity of concrete structures subjected to severe chemical degradation (such as radioactive waste disposal applications). Set within the framework of chemically reactive porous continua, the model accounts explicitly for the leaching of calcium of portlandite crystals and C-S-H, and its cross-effects with the elastic deformation (chemical damage) and irreversible skeleton deformations (chemical softening) treated within the theory of chemoplasticity. In the first part of this paper the governing equations are derived focusing on the chemomechanical couplings between calcium dissolution, increase in porosity, and deformation and (micro-) cracking of concrete. Without any a priori assumption concerning local equilibrium between the solid calcium concentration s and the interstitial calcium concentration c the well-known calcium leaching state function s = s(c) is then derived using combined thermodynamic equilibrium and dimensional arguments relating to the structural dimension of containment structures. In the second part, this paper addresses the experimental determination of chemical damage and chemical softening of the calcium leaching. For chemical damage, a simple mixture rule involving different skeleton constituents suffices to capture the main chemoelastic features of leaching; in turn, microhardness measurements allow access to the chemical softening state function capturing chemoplastic cross-effects. The intrinsic nature of these functions, and of the proposed procedure, is validated by means of finite-element analysis of experimental compression tests of a degraded specimen with nonhomogeneous chemical degradation states.     

Alignment of Semiconductor Nanowires Using Ion Beams

Gallium arsenide nanowires are grown on 〈100〉 GaAs substrates, adopting the epitaxial relation and thus growing with an angle around 35° off the substrate surface. These straight nanowires are irradiated with different kinds of energetic ions. Depending on the ion species and energy, downwards or upwards bending of the nanowires is observed to increase with ion fluence. In the case of upwards bending, the nanowires can be aligned towards the ion beam direction at high fluences. Defect formation (vacancies and interstitials) within the implantation cascade is identified as the key mechanism for bending. Monte Carlo simulations of the implantation are presented to substantiate the results.     

Effect of Inclusions on Friction Coefficient of Highly Filled Composite Materials

Highly filled composite material systems exhibit, in triaxial compression, a composite strength that is greater than either the weaker particulate or matrix strength. This is due to an amplification of the local confinement in the matrix activating frictional mechanism. The paper quantitatively addresses this increase of the friction coefficient of a matrix reinforced by rigid inclusions using assorted means of nonlinear micromechanics. The approach is based on a nonlinear elastic representation of a Drucker–Prager type frictional strength behavior of the matrix at failure. The key to success of the homogenization procedure relies on the appropriate definition of effective strains in the matrix, to capture local confinement effects and shear effects in the connected matrix phase. It is shown that an effective strain concept based on linear volume averaging (i.e., classical secant method) leads to overestimate the inclusion effects; while an effective strain concept based on quadratic volume averaging (i.e., modified secant method) provides a more realistic representation of shear strains and local confinement effects that develop in triaxial compression in the matrix. Finally, a combination of these two methods leads to a mixed secant method, which gives a relative friction increase of (volume fraction fI). This estimate accurately predicts the experimentally observed frictional behavior of unleached and leached cement-based mortars, composed of a cement paste matrix and rigid sand inclusions.     

Residual design strength of cement-based materials for nuclear waste storage systems

Calcium leaching of concrete may be critical for the mechanical integrity of hazardous waste storage systems, in which cement-based materials are employed as construction material or grouting material. Results from a recent triaxial strength test campaign on calcium depleted cementitious materials highlight that these materials exhibit decohesion and frictional softening and become increasingly pressure sensitive. But the overall emerging picture is one of a material with a residual, finite accountable strength, which is well above either the particulate or the matrix strength. This allows us to propose a ‘safe’ lower bound for calcium depleted cement-based composites, which may serve for the durability performance design of concrete structures subjected to calcium leaching-one critical design scenario of e.g. nuclear waste storage systems.     

A new noise model of HFET with special emphasis on gate-leakage

A new temperature noise model, including the influence of a gate-leakage current on the noise performance of a microwave HFET, is presented. Based on an extended small-signal equivalent circuit of the HFET and three equivalent noise temperatures the noise model allows the exact prediction of the four noise parameters in a wide frequency range. The validity of the new model is demonstrated by noise measurements at room temperature. It is shown that the three equivalent noise temperatures are frequency independent and that one of them (T_p ) especially represents the noise contribution caused by the gate-current I_G. The advantages of the new model are clearly demonstrated in comparison with a well established temperature noise model     

Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm², which is important for potential applications in concentrator solar cells.