Marjorie Cavarroc

Capacitively coupled plasma used to simulate Titan's atmospheric chemistry

A complex chemistry in Titan’s atmosphere leads to the formation of organic solid aerosols. We use a radio-frequency (RF) capacitively coupled plasma discharge produced in different N2–CH4 mixtures (from 0% to 10% of CH4) to simulate this chemistry. The work presented here was devoted to the study of the plasma discharge. In our experiment, the electron density is measured by the resonant cavity method and is about 10 15 m −3 in pure N2 plasma at 30 W excitation RF power. It decreases by a factor of 2 as soon as CH4 is present in the discharge, even for a proportion as small as 2% of CH4. An optical emission spectroscopy diagnostic is installed on the experiment to study the evolution of the N2 bands and to perform actinometry measurements using Ar lines. This diagnostic allowed us to measure variations in the electron temperature and to show that a decrease in the density of the electrons can be compensated by an increase in their energy. We have also used an experimental setup where the plasma is tuned in a pulsed mode, in order to study the formation of dust particles. We observed variations in the self-bias voltage, the RF injected power and the intensities of the nitrogen bands, which indicated that dust particles were formed. The characteristic dust formation time varied, depending on the experimental conditions, from 4 to 110 s. It was faster for higher pressures and for smaller proportions of CH4 in the gas mixture. (Some figures in this article are in colour only in the electronic version)

Single-crystal silicon nanoparticles: An instability to check their synthesis

An instability occuring in electrical signals of the discharge is used as a mark to detect the end of the single-crystal silicon nanoparticle formation in Ar∕SiH4 rf plasmas. Scanning electron microscopy and atomic force microscopy studies of depositions show that the exact beginning of the coalescence phase corresponds to the onset of the instability. At the end of the instability, no single-crystal nanoparticles are remaining in the gas phase. These results based on a nonperturbative method allow to control depositions of single-crystal silicon nanoparticles of a well-defined size distribution with the highest density available during dust particle growth.

Self-excited instability occurring during the nanoparticle formation in an Ar-SiH4 low pressure radio frequency plasma

An experimental investigation of an instability occurring during dust nanoparticle formation is presented in this paper. The present study has been performed in radio frequency low pressure plasma in an argon-silane mixture. The formation and growth of nanoparticles is followed, thanks to the analysis of the amplitude of the third harmonics (40.68MHz) of the discharge current and the self-bias voltage (Vdc). In some cases, at the end of the accumulation phase of the nanocrystallites an instability occurs. It seems to be an attachment induced ionization instability as observed in electronegative plasmas. A detailed study of the influence of different operating conditions (injected power, gas temperature, and silane flow rate) on this instability behavior and frequencies is presented. The paper concludes by examining a very particular case of the instability.

Dust particles in low-pressure plasmas: Formation and induced phenomena*

Formation of dust particles is a common mechanism in low-pressure plasmas. These big particles (in comparison with other plasma species) are sometimes the desired final products of the process, but they may also constitute a severe drawback in certain contexts. In either situation, it is necessary to understand growth mechanisms well, in order to control or avoid dust particle formation. One of the problems that has to be overcome is that dust particle growth is usually a continuous mechanism: once started, it can enter into a cyclic regime where new generations of dust particles are succeeding one after the other. This cyclic phenomenon often induces a side effect consisting of instabilities of a few tens of Hz. This paper discusses the main characteristics of dust successive generations, and particularly the importance of dust-free spaces (void) involved in this process. Finally, some aspects related to deposition when several generations coexist will be presented.

Plasma Emission Modifications and Instabilities Induced by the Presence of Growing Dust Particles

Formation of dust particles in a plasma can strongly change its properties due to electron attachment on dust surface. An easy way to detect dust formation is to analyze modifications of the plasma emission. In this paper, changes in the plasma emission are related to the growth of dust particles. We particularly show that dust formation induces low-frequency plasma instabilities. Another interesting induced effect is the formation of an enhanced emission region which is dust free and usually named ldquovoidrdquo.

Nanostructured Silicon Thin Films Deposited Under Dusty Plasma Conditions

Silane-based dusty plasmas are widely used in plasma-enhanced chemical vapor deposition processes to synthesize silicon nanoparticles and/or nanostructured thin films. Under certain conditions, it is possible to access to the inner structure of the thin film by scanning electron microscopy, using the ldquoholerdquo due to dust particles that moved when the sample is brought back to atmospheric pressure.

High Performance Plasma Sputtered Fuel Cell Electrodes with Ultra Low catalytic metal Loadings

Ultra-low Pt content PEMFC electrodes have been manufactured using magnetron co-sputtering of carbon and platinum on a commercial E-Tek_ uncatalyzed gas diffusion layer in plasma fuel cell deposition devices. Pt loadings lower than 0.01 mg cm-2 have been realized. The Pt catalyst is dispersed as small clusters with size less than 2 nm over a depth of 500 nm. PEMFC test with symmetric electrodes loaded with 0.01 mg cm-2 led to maximum reproducible power densities as high as 0.4 Wcm-2 with Nafion 212. Replacement of platinum was also realized by preparing MEAs composed of 0.01 mgPd cm-2 for the anode and 0.01 mgPd cm-2 + 0.001 mgPt cm-2 for the cathode and Nafion 212 membrane. Power density as high as 250 mW cm-2 were achieved at 80°C, which corresponds to a specific power density of 250 kW gPt-1

TCP Plasma Sputtering of Nanostructured Fuel Cell Electrodes

A transformer-coupled plasma sputtering reactor is used for depositing porous carbon-platinum proton exchange membrane fuel cell electrodes. Carbon nanocolumns decorated by platinum nanoclusters are thus obtained.

Original Polystyrene Nanoballs Grown by Plasma Polymerization

Plasma polymerization of styrene is widely used to synthesize polymer membranes for various applications. Under certain specific conditions, it is possible to grow polystyrene nanoballs. In this paper, we highlight the potential of plasma polymerization to synthesize polystyrene balls.