Vibration energy harvesters based on piezoelectric resonators are promising for powering Wireless Sensors Nodes (WSNs). Yet, any mismatch between the resonant frequency of traditional harvesters and the vibration frequency can drastically decrease the scavenged power and make them ineffective. Electrical techniques able to tune the resonant frequency of piezoelectric harvesters has been proposed as a solution and opens up new perspectives. To be fully competitive, this approach requires energy harvesters with very strong global electromechanical coupling coefficients k² (>10%), whose design remains a challenge today. This work reports on a method to design strongly coupled piezoelectric cantilevers thanks to an analytical approach based on the Rayleigh-Ritz method and a two degrees-of-freedom model, which considers the proof mass inertia effects. Through an expression of the coupling coefficient, we provide design guidelines, which are experimentally validated. We show that a long proof mass is a very effective configuration to maximize the global electromechanical coupling coefficient and consequently the frequency bandwidth of the system. Three proposed prototypes exhibit some of the strongest squared global electromechanical coupling coefficients k² of the state-of-the-art of piezoelectric harvesters (16.6% for the PMN-PT cantilever, 11.3% and 16.4% for the narrow and wide PZT-5A cantilevers respectively) and demonstrate a wide bandwidth behavior (10.1%, 7.8% and 11.3% of the central frequency respectively). Using a strongly coupled prototype based on PZT-5A leveraged by a dedicated integrated circuit, we experimentally show that it can harvest enough power (more than 100µW) to supply a WSN over a frequency bandwidth as large as 21%.
The mechanical response of amorphous silica (or silica glass) under hydrostatic compression for very high pressures up to 25 GPa is modelled via an elastic-plastic constitutive equation (continuum mechanics framework). The material parameters appearing in the theory have been estimated from the ex situ experimental data from Rouxel et al. [Rouxel T, Ji H, Guin JP, Augereau F, Rufflé B J Appl Phys 2010;107(9):094903]. The model is shown to capture the major features of the pressure-volume changes response from the in situ experimental work of Sato and Funamori [Sato T, Funamori N Phys Rev Lett 2008;101:255502]. In particular, the onset and saturation of densification, the increase in elasticity parameters (bulk, shear and Young's moduli) andPoisson's ratio are found to be key parameters of the model.
International audienceThe determination of the contact area is a key step to derive mechanical properties such as hardness or an elastic modulus by instrumented indentation testing. Two families of procedures are dedicated to extracting this area: on the one hand, post mortem measurements that require residual imprint imaging, and on the other hand, direct methods that only rely on the load vs. the penetration depth curve. With the development of built-in scanning probe microscopy imaging capabilities such as atomic force microscopy and indentation tip scanning probe microscopy, last generation indentation devices have made systematic residual imprint imaging much faster and more reliable. In this paper, a new post mortem method is introduced and further compared to three existing classical direct methods by means of a numerical and experimental benchmark covering a large range of materials. It is shown that the new method systematically leads to lower error levels regardless of the type of material. Pros and cons of the new method vs. direct methods are also discussed, demonstrating its efficiency in easily extracting mechanical properties with an enhanced confidence
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