Formation Of Dust Particles Research Articles

Properties of the particles deposited from a low-temperature plasma arc discharge

The possible mechanisms for the formation of fractal particles of the plasma arc have been studies. The existence of dust structures of two types strongly and weakly magnetic was found. Paramagnetism of particles is the result of the synthesis of titanium carbonitride with embedded (encapsulated) magnetic substrate elements. The magnetization of particles indicates the presence in the test nanostructure particles. inhomogeneous distribution of electric charges in motion conducting bodies (drops) in a magnetized plasma is the reason for the formation of dust particles.

Modelling dust production in AGB stars

AbstractAsymptotic giant branch (AGB) stars are among the most important gas and dust polluters of the Universe. The latest AGB evolutionary models take into account dust production in the circumstellar envelope of these stars, starting from a detailed computation of the main physical processes and chemical surface variations occurring in this evolutionary phase. Following the formation and growth of dust particles, they provide the unique possibility of interpreting the AGB population observed in resolved galaxies. The first application was for the Spitzer observations of dusty AGBs in the Magellanic Clouds, characterising carbon-rich and oxygen-rich stars in terms of initial mass, epoch of star formation, evolutionary time on the AGB and dust contribution. The same set of models are able to interpret the CNO surface abundances observed for the PNe of the same galaxies.

The development and the tests of the electrostatic probe for dust particle collection in thermonuclear reactors

Formation of dust particles in thermonuclear reactors can greatly affect the plasma parameters and lead to accumulation of tritium. The rates of formation and deposition of dust need to be measured, and the parameters of formation of dust particles and clusters need to be studied. A model of a device for collection of fine conductive particles capable of removing them from the reactor chamber for future research is proposed in this paper. The dust collector's operation is based on a principle of applied electrostatic field. The model was tested in different operating conditions: in vacuum, at the atmospheric pressure in the atmosphere of air and dry nitrogen. The experiments were conducted with a stationary system and with the dust collector in motion relative to the dusty surface. It is shown that, during the probe moving relative to the surface, it can remove up to 95% of fine tungsten particles with sizes ranging from 1 to 10 μm.

Characterization of magnetically confined low-pressure plasmas produced by an electromagnetic field in argon-acetylene mixtures

Dust particles formation was investigated in magnetically confined low-pressure plasma produced in argon-acetylene mixtures. The plasma characteristics were measured in order to identify the species involved in the dust particles formation. Their dependence on the operating conditions including magnetic field intensity, acetylene fraction in the gas mixture and operating pressure was examined. In contrast with noble gases, in the presence of acetylene, the electron temperature increases with the magnetic field intensity, indicating additional charged particles losses in the plasma. Indeed, in these conditions, larger hydrocarbon ions are produced leading to the formation of dust particles in the plasma volume. The observed dependence of positive ion mass distribution and density and relative negative ion density on the operating parameters suggests that the dust particles are formed through different pathways, where negative and positive ions are both involved in the nucleation.

A solid-phase mechanism of shock-wave formation of dust particles of heavy metals

The possibility of formation of dust particles in solid as a result of shock-wave destruction of the initial crystalline material structure and subsequent coalescence of atomic clusters (nanoparticles), which leads to the aggregation of mesocrystalline particles (grains) in the shocked layer, is discussed.

Fast and interrupted expansion in cyclic void growth in dusty plasma

Low-pressure acetylene plasmas are able to spontaneously form dust particles. This will result in a dense cloud of solid particles that is levitated in the plasma. The formed particles can grow up to micrometers. We observed a spontaneous interruption in the expansion of the so-called dust void. A dust void is a macroscopic region in the plasma that is free of nanoparticles. The phenomenon is periodical and reproducible. We refer to the expansion interruption as ‘hiccup’. The expanding void is an environment in which a new cycle of dust particle formation can start. At a certain moment in time, this cycle reaches the (sudden) coagulation phase and as a result the void will temporarily shrink. To substantiate this reasoning, the electron density is determined non-intrusively using microwave cavity resonance spectroscopy. Moreover, video imaging of laser light scattering of the dust particles provides their spatial distribution. The emission intensity of a single argon transition is measured similarly. Our results support the aforementioned hypothesis for what happens during the void hiccup. The void dynamics preceding the hiccup are modeled using a simple analytical model for the two dominant forces (ion drag and electric) working on a nanoparticle in a plasma. The model results qualitatively reproduce the measurements.

Dissecting the Spitzer colour–magnitude diagrams of extreme Large Magellanic Cloud asymptotic giant branch stars

Abstract We trace the full evolution of low- and intermediate-mass stars (1 ≤ M ≤ 8 M⊙) during the asymptotic giant branch (AGB) phase in the Spitzer two-colour and colour–magnitude diagrams. We follow the formation and growth of dust particles in the circumstellar envelope with an isotropically expanding wind, in which gas molecules impinge upon pre-existing seed nuclei, favour their growth. These models are the first able to identify the main regions in the Spitzer data occupied by AGB stars in the Large Magellanic Cloud (LMC). The main diagonal sequence traced by LMC extreme stars in the [3.6] − [4.5] versus [5.8] − [8.0] and [3.6] − [8.0] versus [8.0] planes is nicely fit by carbon stars models; it results to be an evolutionary sequence with the reddest objects being at the final stages of their AGB evolution. The most extreme stars, with [3.6] − [4.5] > 1.5 and [3.6] − [8.0] > 3, are 2.5–3 M⊙ stars surrounded by solid carbon grains. In higher mass (>3 M⊙) models dust formation is driven by the extent of hot bottom burning (HBB) – most of the dust formed is in the form of silicates and the maximum obscuration phase by dust particles occurs when the HBB experienced is strongest, before the mass of the envelope is considerably reduced.

On the signature of positively charged dust particles on plasma irregularities in the mesosphere

Recent rocket payloads have studied the properties of aerosol particles within the ambient plasma environment in the polar mesopause region and measured the signature of the positively charged particles with number densities of (2000cm−3) for particles of 0.5–1nm in radius. The measurement of significant numbers of positively charged aerosol particles is unexpected from the standard theory of aerosol charging in plasma. Nucleation on the cluster ions is one of the most probable hypotheses for the positive charge on the smallest particles. This work attempts to study the correlation and anti-correlation of fluctuations in the electron and ion densities in the background plasma by adopting the proposed hypothesis of positive dust particle formation. The utility being that it may provide a test for determining the presence of positive dust particles. The results of the model described show good agreement with observed rocket data. As an application, the model is also applied to investigate the electron irregularity behavior during radiowave heating assuming the presence of positive dust particles. It is shown that the positive dust produces important changes in the behavior during Polar Mesospheric Summer Echo PMSE heating experiments that can be described by the fluctuation correlation and anti-correlation properties.

Anion dynamics in the first 10 milliseconds of an argon–acetylene radio-frequency plasma

The time evolution of the smallest anions (C2H− and H2CC−), just after plasma ignition, is studied by means of microwave cavity resonance spectroscopy (MCRS) in concert with laser-induced photodetachment under varying gas pressure and temperature in an argon–acetylene radio-frequency (13.56 MHz) plasma. These anions act as an initiator for spontaneous dust particle formation in these plasmas. With an intense 355 nm Nd : YAG laser pulse directed through the discharge, electrons are detached only from these anions present in the laser path. This results in a sudden increase in the electron density in the plasma, which can accurately and with sub-microsecond time resolution be measured with MCRS. By adjusting the time after plasma ignition at which the laser is fired through the discharge, the time evolution of the anion density can be studied. We have operated in the linear regime: the photodetachment signal is proportional to the laser intensity. This allowed us to study the trends of the photodetachment signal as a function of the operational parameters of the plasma. The density of the smallest anions steadily increases in the first few milliseconds after plasma ignition, after which it reaches a steady state. While keeping the gas density constant, increasing the gas temperature in the range 30–120 °C limits the number of smallest anions and saturates at a temperature of about 90 °C. A reaction pathway is proposed to explain the observed trends.

Dust particle formation due to interaction between graphite and helicon deuterium plasmas

The collection of dust particles using divertor simulation helicon plasmas has been carried out to examine dust formation due to the interaction between a graphite target and deuterium plasmas, which are planned to operate in the large helical device (LHD) at the Japanese National Institute for Fusion Science (NIFS). The collected dust particles are classified into three types: (i) small spherical particles below 400nm in size, (ii) agglomerates whose primary particles have a size of about 10nm, and (iii) large flakes above 1μm in size. These features are quite similar to those obtained through hydrogen plasma operation, indicating that the dust formation mechanisms due to the interaction between a carbon wall and a plasma of deuterium, which is the isotope of hydrogen, is probably similar to those of hydrogen.

Monitoring of hydrocarbon concentrations in dust-producing RF plasmas

In Ar and He radio-frequency (RF) plasmas with admixtures of C2H2 and CH4 the hydrocarbon chemistry has been studied in relation to dust particle formation by means of infrared tunable diode laser absorption spectroscopy (TDLAS) combined with Fourier transform infrared (FTIR) spectroscopy. The experiments were performed in a RF capacitively coupled parallel plate reactor at a frequency of f = 13.56 MHz, a pressure of p = 0.1 mbar and a flow rate of Φ = 8 sccm of Ar or He with admixtures of 0.5 sccm C2H2 or 1 sccm CH4. The power was P = 15 W. Using TDLAS, the temporal evolution of the concentrations of the methyl radical and of four stable molecules, C2H2, CH4, C2H4 and CO, was monitored in the plasma. Simultaneously, the growth process of the dust particles was analysed by FTIR spectroscopy. The degree of dissociation of the acetylene precursor was found to be nearly constant in the range of 96% under stabilized conditions for both the Ar and He plasmas. In contrast, the degree of dissociation of the methane precursor varied between 45% and 90% depending (i) on the appearance of dust particles in the reactor volume and (ii) on the Ar or He plasma conditions. The methyl radical concentration was found to be in the range of 1011 molecules cm−3. The concentrations of all hydrocarbon species were strongly correlated with the dynamic of the dust formation. Fragmentation efficiencies of acetylene (RF (C2 H2) = 3.2 × 1016 molecules J−1) and of methane (RF (CH4) = (0.16–2.5) × 1016 molecules J−1) and conversion efficiencies to the produced hydrocarbons (RC = (0.23–8.5) × 1014 molecules J−1) could be estimated in dependence on the discharge conditions in the RF plasma.

An optical analysis tool for avoiding dust formation in very-high frequency hydrogen diluted silane plasmas at low substrate temperatures

Control of the formation of dust particles in a silane deposition plasma is very important for avoiding electrical shunts in devices, such as thin film silicon solar cells. In this work we present a noninvasive in situ method for identification of the plasma regime, based on optical emission spectroscopy (OES), which can be applied to silane/hydrogen plasmas at low substrate temperatures. By monitoring the OES spectra as a function of the position perpendicular to the plasma electrodes we developed a method to identify the transition of a plasma from the dust free to a dusty regime, which was confirmed by TEM images of layers deposited in both regimes. Using this technique we mapped this transition as a function of applied forward very-high frequency (VHF) power and hydrogen dilution at different substrate temperatures. The advantage of this technique is that the experiment is insensitive to optical transmission loss at the viewport due to deposition of silicon films. As the transition from the dust free to the dusty regime is substrate temperature dependent and the transition from amorphous to nanocrystalline growth mainly depends on hydrogen dilution, a limited parameter window has been defined in which dust-free amorphous silicon can be deposited at low substrate temperatures. A single simple OES technique can be used for in situ monitoring of amorphous to nanocrystalline transition as well as the onset of the dusty regime in a thin film silicon cell fabrication process.

Formation of Small Dust Particles by Brittle Destruction

The size distribution of dust particles in nuclear fusion devices is close to the power function. In the paper it is shown that function of this kind can be the result of brittle destruction. From the similarity assumption it follows that the size distribution obeys the power law with the exponent between –4 and –1. The model of destruction has much in common with the fractal theory. The power exponent can be expressed in terms of the fractal dimension. An additional assumption about the structure of fragmentation offers that the exponent is close to –3. The exponent for the case of the biggest ball removing equals –3.4.

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.

Simulation of dust particles in dual-frequency capacitively coupled silane discharges

The behavior of nanoparticles in dual-frequency capacitively coupled silane discharges is investigated by employing a one-dimensional self-consistent fluid model. The numerical simulation tries to trace the formation, charging, growth, and transport of dust particles during the discharge, under the influences of the high- and low-frequency electric sources, as well as the gas pressure. The effects of the presence of the nanoparticles and larger anions on the plasma properties are also discussed, especially, for the bulk potential, electron temperature, and densities of various particles. The calculation results show that the nanoparticle density and charge distribution are mainly influenced by the voltage and frequency of the high-frequency source, while the voltage of the low-frequency source can also exert an effect on the nanoparticle formation, compared with the frequency. As the discharge lasts, the electric potential and electron density keep decreasing, while the electron temperature gets increasing after a sudden drop.

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 1015 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.

Controlled dust formation in pulsed rf plasmas

This paper deals with the formation of nanoparticles in a pulsed discharge. Experiments are performed in a capacitively coupled discharge operated in a mixture of argon and acetylene. The paper focuses especially on the influence of the pulse frequency on the dust formation. The experiments reveal the existence of a rather narrow frequency band that separates a frequency region with no dust formation from a frequency region where dust formation occurs. The decisive point in the observations is that a small change in the pulse frequency (from 700 to 725 Hz) is enough to induce or, respectively, suppress the formation of dust particles. The experimental results are discussed by means of a simple model that allows one to calculate the density of negative ions (C2H−, C4H−, etc.) as a function of the pulse frequency.

One-Dimensional Fluid Model for Dust Particles in Dual-Frequency Capacitively Coupled Silane Discharges

A self-consistent fluid model, which incorporates density and flux balances of electrons, ions, neutrals and nanoparticles, electron energy balance, and Poisson's equation, is employed to investigate the capacitively coupled silane discharge modulated by dual-frequency electric sources. In this discharge process, nanoparticles are formed by a successive chemical reactions of anion with silane. The density distributions of the precursors in the dust particle formation are put forward, and the charging, transport and growth of nanoparticles are simulated. In this work, we focus our main attention on the influences of the high-frequency and low-frequency voltage on nanoparticle densities, nanoparticle charge distributions in both the bulk plasma and sheath region.

Circumstellar dust shells around long-period variables

Context. Miras and long-period variables (LPVs) are radially pulsating, highly evolved stars on the Asymptotic Giant Branch. Because of peculiar conditions of this objects, their cool, extended atmospheres are ideal sites for the formation of dust particles. Carbon-rich circumstellar dust shells (CDSs) surrounding stars with high stellar luminosity, tend to become dynamically unstable. They develop a self-maintaining oscillatory pattern caused entirely by dust formation even without the additional input of mechanical momentum from the star (exterior κ-mechanism). Since this system obviously has an eigenmode, it is interesting to consider the interaction with an exterior mechanical force, i.e. the radial pulsation at the inner boundary. Aims. We investigate in great detail the complex dynamical behaviour of carbon-rich CDSs in a more systematic way. This is done by established methods of nonlinear dynamics. Methods. We consider CDSs as multioscillatory systems that can be analysed with tools of nonlinear dynamics. We also use a discrete Fourier transform to examine the eigenmodes and their behaviour and apply this to a typical model CDS as a representative example. Results. In the absence of external excitation, the dynamics of the shell are dominated by its eigenmode, which is determined by the characteristic timescale of the coupled system of dust formation, hydrodynamics, and thermodynamics. The input mechanical energy and momentum of an underlying stellar pulsation introduces a new timescale to the system. Depending on the ratio of these two timescales, the dynamical behaviour of the system is either dominated by the eigenmode of the shell, by the excitation force, or can be irregular. In the latter case, the strength of the excitation becomes especially important. However, even for a small excitation period when the system is dominated by the shell’s eigenmode, the oscillation of the shell tends to synchronise with the excitation force.

Some Aspects of Reactive Complex Plasmas

AbstractReactive plasmas are nowadays widely used for technological applications. The spontaneous formation and growth of dust is a phenomenon frequently observed in such plasmas. The formation of dust particles has been observed in a great variety of different discharge types and in different kind of gases or gas mixtures. Due to the large variety of different phenomena that can be observed in reactive complex plasmas this article will address some selected (general) problems and examples that are specific for the physics and chemistry of such systems. These examples concern the formation and growth of dust particles in reactive plasmas, the mechanisms responsible for that growth processes, the spatial distribution of the dust particles within the discharge, the response of the plasma to the formation and growth of dust particles and some technological aspects (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)