Flux-driven cellular precipitation in open system to form porous Cu3Sn

Recently, a new morphology of porous Cu₃Sn with lamellar structure is observed. Several possible explanations of the formation are proposed and compared. The most reasonable one seems to be the one based on a theory of flux-driven cellular precipitation in open system. Outflux of Sn from Cu₆Sn₅ generates simultaneously supersaturation with vacancies and with copper leading to the eutectoid-like transformation β → α + γ (where γ is void). The transformation is complete due to a complete outflux of Sn from the Cu₆Sn₅ phase. Simple formulae for prediction of the lamellar structure parameters and the propagation velocity are obtained and compared reasonably with experimental data. The suggested model can be interpreted as one more case of the flux-driven phase transformations in open systems.     

On the dielectric glass relaxation process of polymers: Ball-like labels in polystyrene

Ball-like molecules with strong dipoles (labels) were mixed with technical polystyrene (PS168N) in low concentrations (<0.5% wt) and measured dielectrically in the frequency range 10^–2–10⁷ Hz, and the temperature range 100°–135°C (glass relaxation region). The measurements showed that these ball-like molecules relax cooperatively with the polymeric segments with relaxation times lying at the high-frequency tail of the glass process. The activation energy of the main label process is found to be very similar to that of the glass process of the polystyrene segments and also has the same temperature dependence. This finding implies the existence of an additional mode of relaxation in the dielectric spectrum of the glass process of polystyrene (compared to polyisoprene). Considering the different behavior of the ball-like molecules in polystyrene and polyisoprene and the temperature dependence of the half-width of dielectric loss peak in different polymers, we suggest that the polymers could be classified into three classes according to the available dielectric relaxation modes in the glass process. In addition, the label molecules showed a high-frequency local relaxation process. The relaxation strength ratio of the local process (X _local) to the total relaxation strength of the label was found to be dependent on the volume of the label. This phenomenon could supply a new method for the determination of the mean size of the holes (voids) representing the free volume of the host matrix.     

Impossibility of localization in linear thermoelasticity with voids

In this note we prove the impossibility of the localization in time of the solutions of the linear thermoelasticity with voids. This means that the only solution for this problem that vanishes after a finite time is the null solution. From a thermomechanical point of view, this result says that the combination of the thermal and porous dissipation in the linear theory is not sufficiently strong to guarantee that the thermomechanical deformations will vanish after a finite time. The main idea to prove this result is to show the uniqueness of solutions for the backward in time problem.     

Small‐angle X‐ray scattering investigation of carbon nanotube‐reinforced polyacrylonitrile fibers during deformation

Structural changes during deformation in solution‐ and gel‐spun polyacrylonitrile (PAN) fibers with multi‐ and single‐wall carbon nanotubes (CNTs), and vapor‐grown carbon nanofibers were investigated using synchrotron X‐ray scattering. Previously published wide‐angle X‐ray scattering (WAXS) results showed that CNTs deform under load, alter the response of the PAN matrix to stress, and thus enhance the performance of the composite. In this article, we find that the elongated scattering entities that give rise to the small‐angle X‐ray scattering (SAXS) in solution‐spun fibers are the diffuse matrix‐void interfaces that follow the Porod's law, and in gel‐spun fibers these are similar to fractals. The observed smaller fraction of voids in the gel‐spun fibers accounts for the significant increase in the strength of this fiber. The degree of orientation of the surfaces of the voids is in complete agreement with those of the crystalline domains observed in WAXS, and increases reversibly upon stretching in the same way as those of the crystalline domains indicating that the voids are integral parts of the polymer matrix and are surrounded by the crystalline domains in the fibrils. The solution‐spun composite fibers have a larger fraction of the smaller (<10 nm) voids than the corresponding control PAN fibers. Furthermore, the size distribution of the voids during elongation changes greatly in the solution spun PAN fiber, but not so in its composites. The scattered intensity, and therefore the volume fraction of the voids, decreases considerably above the glass transition temperature (T_g) of polymer. Implications of these observations on the interactions between the nanotubes and the polymer are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2394–2409, 2009     

Thermal Stresses in Plane Strain of Porous Elastic Solids

This paper is concerned with the linear theory of thermoelastic materials with voids. We present a method to reduce the thermoelastic problem to an isothermal one with zero body loads and with certain known boundary data. The results are used to study the thermal stresses in a tube and the thermoelastic deformation of a cylinder subjected to a uniform temperature gradient.     

Detecting shrinkage voids in plastic gears using magnetic levitation

Shrinkage voids have a large influence on the quality of plastic gears, and it is still a problem to detect the voids inside gears accurately and conveniently. This paper presents a novel method for detecting shrinkage voids via magnetic levitation. The porosity levels of plastic gears can be calculated using magnetic levitation because the density of plastic gears is influenced by the shrinkage voids. The distribution of shrinkage voids is quantified by the moment of volume, hence a theoretical model for the distributions of shrinkage voids and levitating posture can be established. Computer tomography (CT) detections were also carried out to verify the accuracy of magnetic levitation for detecting the shrinkage voids. Experimental results show that the average relative error of calculated porosity level is less than 7%, and the theoretical model for distribution of shrinkage voids agrees well with the results from CT detections, with the correlation coefficient being up to 99.8%. The proposed method has great potential for mass detection of plastic gears.     

Selective CO2 Sorption Using Compartmentalized Coordination Polymers with Discrete Voids**

Carbon capture and storage with porous materials is one of the most promising technologies to minimize CO₂ release into the atmosphere. Here, we report a family of compartmentalized coordination polymers (CCPs) capable of capturing gas molecules in a selective manner based on two novel tetrazole-based ligands. Crystal structures have been modelled theoretically under the Density Functional Theory (DFT) revealing the presence of discrete voids of 380 ų. Single gas adsorption isotherms of N₂, CH₄ and CO₂ have been measured, obtaining a loading capacity of 0.6, 1.7 and 2.2 molecules/void at 10 bar and at 298 K for the best performing material. Moreover, they present excellent selectivity and regenerability for CO₂ in mixtures with CH₄ and N₂ in comparison with other reported materials, as evidenced by dynamic breakthrough gas experiments. These frameworks are therefore great candidates for separation of gas mixtures in the chemical engineering industry.     

Dilatational viscoplasticity of polycrystalline solids with intergranular cavities

We propose constitutive models for polycrystalline aggregates with intergranular cavities and test them against full-field numerical simulations. Such conditions are prevalent in many engineering applications and failure of metallic components (e.g. HIPing and other forming processes, spallation under dynamic loading conditions, etc.), where the dilatational effects associated with the presence of cavities must be accounted for, and standard polycrystalline models for incompressible plasticity are not appropriate. On the other hand, it is not clear that the use of porous plasticity models with isotropic matrix behavior is relevant, particularly, when large deformations can lead to significant texture evolution and therefore to strong matrix anisotropy. Of course, finite strains can also lead to significant changes in the porosity and pore shape, resulting in additional anisotropy development. In this work, we make use of ‘variational linear-comparison’ homogenization methods to develop constitutive models simultaneously accounting for texture of the matrix, porosity and average pore shape and orientation. The predictions of the models are compared with full-field numerical simulations based on fast Fourier transforms to study the influence of different microstructural features (e.g. overall porosity, texture of the matrix phase, single-crystal anisotropy, etc.) and type of loading (triaxiality) on the dilatational viscoplastic behavior of voided polycrystals. The results are also compared with the predictions of isotropic-matrix porous plasticity models to assess the effect of the possible matrix anisotropy in textured samples.     

Analytical criterion for porous solids containing cylindrical voids in an incompressible matrix exhibiting tension–compression asymmetry

A new analytic plastic potential is developed using a rigorous limit analysis approach. Conditions of homogeneous boundary strain rate are imposed on every cylinder concentric with the cavity. It is shown that, due to the tension–compression asymmetry of the incompressible matrix, the third invariant of the stress deviator has a strong influence on the yielding of the porous solid. New and intriguing results are obtained; namely, for axisymmetric loadings and plane strain conditions, the stress state at yielding is not hydrostatic. In the case when the matrix has the same yield in tension as in compression, the new criterion reduces to Gurson’s criterion for cylindrical voids.     

Size effects due to secondary voids during ductile crack propagation

In the present study the size-effect due to a secondary void population during ductile fracture is investigated. Discrete primary voids are resolved in the process zone at the crack tip. A non-local GTN model is employed to describe the evolution of the secondary voids in the intervoid ligaments. The non-local GTN model contains an intrinsic length scale related to the size of the secondary voids. Hence, the ratio of the size of the primary and that of the secondary voids can be varied. The results show that small secondary voids can toughen the material. Such a behavior is in contrast to the prediction of cell model simulations. A theoretical reasoning of this effect and conclusions are given.