We investigate the mechanical behavior of granular suspensions subjected to coupled vibrations and shear. At high shear stress, whatever the mechanical vibration energy and bead size, the system behaves like a homogeneous suspension of hard spheres. At low shear stress, in addition to a dependence on bead size, vibration energy drastically influences the viscosity of the material that can decrease by more than 2 orders of magnitude. All experiments can be rationalized by introducing a hydrodynamical Peclet number defined as the ratio between the lubrication stress induced by vibrations and granular pressure. The behavior of vibrated wet and dry granular materials can then be unified by assimilating the hookean stress in dry media to the lubrication stress in suspensions.

By means of a stress imposed rheometer coupled with a “vibrating cell,” generating a Brownian motion at a macroscopic scale into the samples, we have shown that dense-phase vibrated powders exhibit rheological behaviors archetypal of non-Newtonian viscoelastic fluids. These behaviors have been accurately described through a free volume structural model based on simple “stick-slip” granular interactions. As a result, the evolution of the steady-state viscosity has been accurately expressed as a function of the shear rate, the frictional stress, the granular pressure, the mass of the samples, the vibration frequency, the vibration energy, the intergranular contact network mean life, and the free volume distribution. The model is consistent with Hookean, Coulombian, and Newtonian limits and is not only descriptive but also explicative and predictive of the encountered phenomena. In particular, a “time-granular temperature superposition principle,” theoretically predicted by the model, has been experimentally verified, the “granular temperature” being controlled through the vibration energy and frequency. Moreover, this superposition principle has been precisely described by a “Vogel-Fulcher-Tammann” law, leading to very close analogies with molecular systems near their glass transition point.

BACKGROUND AND PURPOSE:The purpose of this work was to evaluate the possible use of low-dose multidetector CT (MDCT) in cervical clearance of patients with blunt trauma.

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In this paper, we present a model aimed at predicting the rheological response of a 3D dry granular system to nonstationary mechanical solicitations, subjected or not to vibrations. This model is based on a phenomenological two-state approach related to the inherent bimodal behavior of chain forces in granular packing. It is set up from a kinetic equation describing the dynamics of the contact network. To allow experimental assessment, the kinetic equation is transformed into a differential constitutive equation, relating stress to strain, from which rheological properties can be derived. Its integration allows predicting and describing several rheological behaviors, in stationary and nonstationary conditions, including viscous (Newtonian) and frictional (Coulombian) regimes, as well as elastic linear (Hookean and Maxwellian) and nonlinear behaviors. Despite its simplicity, since it involves only three independent parameters, the model is in very close agreement with experiments. Moreover, within experimental errors, the values of these parameters are independent of the type of test used to determine them, evidence of the self-consistency of the model.

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3D die stacking is a promising technique to allow miniaturization and performance enhancement of electronic systems. Key technologies for realizing 3D interconnect schemes are the realization of vertical connections, either through the Si die or through the multilayer interconnections. The complexity of these structures combined with reduced thermal spreading in the thinned dies complicate the thermal analysis of a stacked die structure. In this paper a methodology is presented to perform a detailed thermal analysis of stacked die packages including the complete back end of line structure (BEOL), interconnection between the dies and the complete electrical design layout of all the stacked dies. The calculations are performed by 3D numerical techniques and the approach allows importing the full electrical design of all the dies in the stack. The methodology is demonstrated on a 2 stacked die structure in a BGA package. For this case the influence of through-Si vias (TSVs) on the temperature distribution is studied. The modeling results are experimentally validated with a dedicated test vehicle. A thermal test chip with integrated heaters and diodes as thermals sensors is used to successfully validate the detailed temperature profile of the hot spots in the top die of the die stack.