Electrocaloric Performance of Multilayer Ceramic Chips: Effect of Geometric Structure Induced Internal Stress.

Driven by an ever-growing demand for environmentally benign cooling systems, the past decade has witnessed the booming development in the field of electrocaloric (EC) cooling technology, which is considered as a promising solid-state cooling approach. Multilayer ceramic chip capacitors (MLCCs) represent the optimum structure for EC cooling elements because of large breakdown strengths, low driving voltages, and high macroscopic volumes of active EC materials. However, fundamental relationships between the geometric parameters of MLCCs and the EC coefficient are less understood. In this study, 0.92Pb(Mg1/3Nb2/3)O3-0.08PbTiO3 (PMN-PT) MLCCs with controlled configurations, such as active/inactive layer thickness, number of layers, and active volume ratio, were fabricated, and their EC performance was evaluated. The electric properties of the MLCCs are confirmed to be closely related to the geometric structure, which influences not only the heat flow but also the internal stress, resulting in the variability of EC performance and reliability/breakdown strength. The internal stress arises due to the residual thermal stress originating from the densification-related shrinkage, thermal expansion mismatch during the sintering, and clamping stress arising from the inactive area due to the large strain from the active area under a high electric field. The geometric structure-based stress distribution and the magnitude of stress on the active layers in MLCCs were determined by finite element modeling (FEM) and correlated with the experimental EC coefficients. The results reveal that a low inactive volume percentage is beneficial toward increasing the breakdown field and enhancement of EC performance because of reduced clamping stress on active EC material.

Instabilities of an annulus flow between rotating cylinders in a helical magnetic field.

The stabilities of an annulus flow between the rotating inner and outer cylinders with an external helical magnetic field are studied by using the quasistatic approximation. It is shown numerically that for the spiral base flow with a zero axial pressure gradient, the helical magnetic field yields a helical traveling wave at a critical Reynolds number. This wave mode is revealed to be the most unstable mode by linear stability analysis. At higher Reynolds numbers, the first wave mode is superposed by a second antisymmetric helical wave mode, which travels with a higher phase velocity than the first mode. When the Reynolds number is increased further, the flow becomes turbulent, but the key features of the flow structure are still dominated by the first and the second wave modes. Furthermore, when a finite axial pressure gradient is applied to guarantee a zero axial flow rate, the annulus flow is found to be more unstable than the case with zero axial pressure gradient.