Description Of Bulk Research Articles

A physically based method to represent the thermo-mechanical behaviour of elastomers

A geometric nonlinear and thermodynamical consistent constitutive theory is proposed, which allows the representation of the thermomechanical behaviour of carbon black filled rubber. In a recent paper [1] it was shown that the mechanical behaviour of this material is mainly influenced by nonlinear elasticity coupled with some inelastic effects, in particular the Mullins-effect, nonlinear rate dependence and a weak equilibrium hysteresis. In the present paper, the Mullins-effect is not taken into consideration. At first we discuss a uniaxial approach, based on a simple spring dashpot system of viscoplasticity. The essential feature of this model is a decomposition of the total stress into a rate independent equilibrium stress and a nonlinear rate dependent overstress. The equilibrium stress is decomposed into a sum of two terms as well: The first term, the elastic part of the equilibrium stress, is a nonlinear function of the total strain, and the second term, the so-called hysteretic part, depends in a rate independent manner on the strain history. Both the overstress and the hysteretic part of the equilibrium stress are determined by nonlinear elasticity relations which depend on internal variables. These internal variables are inelastic strains, and the corresponding evolution equations are developed in consideration of the second law of thermodynamics. Accordingly, we demonstrate that the principle of non-negative dissipation is satisfied for arbitrary deformation processes. In a further step, we transfer the structure of this model to the three-dimensional and geometric nonlinear case. In a certain sense similar to finite deformation elasto-plasticity, we introduce two multiplicative decompositions of the deformation gradient into elastic and inelastic parts. The first decomposition is defined with respect to the overstress and the second one with respect to the hysteretic part of the equilibrium stress. Consequently, two intermediate configurations are induced, which lead two different decompositions of the Green's strain tensor into elastic and inelastic parts. The latter are the internal variables of the model. For physical reasons, we define the corresponding stress tensors and derivatives in the sense of the concept of dual variables [7], [39]. Theconstitutive equations for the overstress and for the hysteretic part of the equilibrium stress are specified by nonlinear elasticity relations, formulated with respect to the different intermediate configurations. In order to facilitate a separate description of inelastic bulk and distortional effects, we introduce kinematic decompositions of the deformation gradient into volumetric and distortional parts. Numerical simulations demonstrate that the developed theory represents the mechanical behaviour of a tread compound at room temperature very well. Thermomechanical heating effects, which are caused by inelastic deformations are also described by the theory. The method proposed in this paper can be utilised to generalise uniaxial rheological models to three-dimensional finite strain theories, which are admissible in the sense of the second law of thermodynamics.

Charge-consistency modelling of [formula omitted] chemisorption system

The bonding of CO to the NiO(100) surface has been investigated by means of the DV-Xα method. Emphasis is put on how to get a realistic description of the NiO bulk as well as the bonding mechanism in chemisorption. The calculation results demonstrate that it is very important to maintain the charge consistency so as to reproduce the experimental results of bulk NiO with an embedded cluster model. When compared with the bonding mechanism of CO in a surface electric field, the embedded cluster calculations with BSSE correction show that the interaction between CO and NiO(100) is mainly electrostatic.

Microscopic Description of Electron-Solid Interactions at a Surface

A microscopic quantum theory of the scattering of an electron at metal-vacuum interfaces is constructed. The interactions of the electron with the short-range electron-ion core potential, the bulk-density fluctuations (e.g., plasmons), and the induced surface charge all are incorporated into the theory in a systematic fashion. Models of the surface- and bulk-charge-density fluctuations are catalogued as appropriate limiting cases of theory. The structure and predictions of the various models are compared, and the suitability of the models as the basis of a theory of electron-solid scattering is examined. The approximations required to produce from the general theory the usual semiclassical description of bulk- and surface-plasmon emission by keV electron transmission are displayed. Similarly, the theory is applied to "derive" the model Hamiltonian recently used to construct a semiphenomenological quantum-field theory of inelastic low-energy ($10\ensuremath{\lesssim}E\ensuremath{\lesssim}{10}^{3}$ eV) electron diffraction. A distorted-wave-scattering theory of elastic low-energy electron diffraction is proposed in which bulk- and surface-plasmon inelastic processes appear as loss terms in a nonlocal complex optical potential whose nature is examined in detail. In appropriate limits, the optical potential is shown to lead both to the image force and to the empirical local-potential models which have been used in calculating elastic low-energy electron diffraction. The pronounced differences between the empirical models and the predictions of the microscopic theory for the optical potential are explored. Some of the consequences of these differences for the analysis of experimental elastic---low-energy---electron-diffraction data are derived.

Properties of polymer crystal aggregates. (1) Comparison of polyethylene crystal aggregates with bulk crystallized polyethylene

Although certain fundamental aspects of polymer single crystal physics are still under investigation and discussion, there is much to be gained by a direct approach to the problem of relating existing knowledge of single crystal behaviour to the description of bulk crystallized polymer. In this series of papers the experimental principle is quite simple: single crystals of polymer obtained from dilute solution are aggregated together to form macroscopic samples large enough for mechanical and physical testing. These aggregates or mats can be modified in various ways and compared with geometrically similar samples of bulk crystallized polymer. In this the first paper of the series, experiments are described in which mats of polyethylene crystals are compared directly with bulk crystallized polyethylene.