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The influence of the content and nature of impurities on the rotational temperature of three probe molecules (OH, NH and ) in a helium plasma created in a microwave resonant cavity is investigated by the synthetic spectra method and by mass spectrometry. OH is found to be the only reliable probe to obtain a satisfactory estimation of the gas temperature. In our experimental conditions, depending on the impurity, the gas temperature varies strongly and at 500 ppm, for instance, a difference of 700 K is possible (∼30% of the mean temperature).

A collisional–radiative model is proposed to describe the behaviour of a helium plasma sustained in a resonant cavity at atmospheric pressure. A set of rate constants is proposed at 2450 K. Indeed, most of the available data in the literature are reliable below 500 K. It is shown that the time post-discharge is mainly controlled by ambipolar diffusion during the first ten µs, next by electron-assisted recombination of helium ions and finally by chemionization from the excited state. This post-discharge lasts for hundreds of µs. It is mainly due to the slow recombination of electrons together with chemionization. Then, at steady state in CW mode, the electron temperature is found to be lower than 1 eV since low reduced fields are needed to sustain the discharge.

The ever-increasing number of connected objects requires novel ways to power them and make them fully autonomous. In this context, photovoltaic, piezoelectric or thermoelectric energy-harvesting technologies show great promises as...

Thermoelectric generators (TEGs) offer a versatile solution to convert low‐grade heat into useful electrical power. While reducing the length of the active thermoelectric legs provides an efficient strategy to increase the maximum output power density pmax, both the high electrical contact resistances and thermomechanical stresses are two central issues that have so far prevented a strong reduction in the volume of thermoelectric materials integrated. Here, it is demonstrated that these barriers can be lifted by using a nonconventional architecture of the legs which involves inserting thick metallic layers. Using skutterudites as a proof‐of‐principle, several single‐couple and multi‐couple TEGs with skutterudite layers of only 1 mm are fabricated, yielding record pmax ranging from 3.4 up to 7.6 W cm−2 under temperature differences varying between 450 and 630 K. The highest pmax achieved corresponds to a 60‐fold increase per unit volume of skutterudites compared to 1 cm long legs. This work establishes thick metallic layers as a robust strategy through which high power density TEGs may be developed.

International audienceModes of energy dissipation in impacts made on various materials (Al, Cu, Fe, and Si) by discharges in heptane are investigated for micro-gap conditions. Bulk metals and thin films of 300 nm in thickness deposited on silicon wafers are used as samples. Positive high voltage pulses with nanosecond rise times make it possible to isolate a single discharge and to study the way the charge delivered by the power supply is transferred to the larger electrode (the sample) in a pin-to-plate configuration. The diameter of the impacts created by the plasma varies linearly versus the charge raised at a power close to 0.5. However, the exact value of the power depends on the material. We also show how the impact morphologies change with the applied charge. At high charges, the diameters of impacts on thin films behave as those made on silicon. At low charges, they behave as the bulk material. Finally, we show that the energy dissipated in impacts is below a few percent. [http://dx.doi.org/10.1063/1.4780786

No abstract

Filamentation in an argon plasma is studied using a microwave cavity at atmospheric pressure. We show that the size and gas temperature of the filaments increase with the power absorbed by the plasma. The appearance of an additional filament occurs at specific values of the absorbed power. Each new filament appears with a smaller diameter than that of its parent filament but the sum of the diameters of all filaments evolves linearly with the absorbed power. A secondary filament emerges from a set of microfilaments created by a perturbation of the electric field (a slight increase in the incident power above a threshold value). This perturbation occurs over a larger radius than that of the parent filament. By resorting to modeling, we found that the filamentation process involves either a decrease in the effective frequency for momentum-transfer collisions, i.e., a lower electron temperature, or an increase in the electron density. We could show that a small change in the relative positions occupied by two filaments in the microwave cavity requires a strong variation in the electron temperature.