Gas Temperature Gradient Research Articles

Synthesis of FeCo nanoparticles by pulsed laser deposition in a diffusion cloud chamber

Magnetic FeCo nanoparticles were successfully synthesized in a diffusion cloud chamber setup within pulsed laser deposition (PLD) equipment. The variation of morphology and size of FeCo nanoparticles with the number of laser pulses, ambient gas pressure and temperature gradient was studied. It was observed that the morphology of the nanoparticles changes from “cloud-like” fractal clusters to particle chains; average particle size increased at higher argon gas pressure. Increasing the temperature gradient considerably reduced the agglomeration of the nanoparticles. Nanoparticles deposited using the diffusion cloud chamber are found to be crystalline.

Circulation' dynamo in complex plasma

In ground based experiments (performed in Ar using a radio frequency (rf)-discharge), we observe the interaction of individual agglomerated particles with a monodisperse (bulk) complex plasma cloud containing (melamine-formaldehyde (MF)) microparticles of 7.17 μm± 3% diameter. The particles are levitated by thermophoresis. For this purpose, a gas temperature gradient of 2000 K m−1 is applied. The particle cloud has a complicated ‘sandwich-like’ vertical structure of two dense slabs (filled by particles), separated by a void, a central particle-free region. The void is impenetrable for small particles, but not for heavier and/or accelerated agglomerates, which may slide through the entire void and therefore can be used as natural test particles for determination of the force acting inside the void. The bulk particles remain in quasi-equilibrium for a long time and are dynamically active, e.g., intense edge rotations (vortices) and nonlinear vertical waves. We traced particle motions in detail and studied the correlation of particle vibrations inside the clouds and the motion of agglomerates and/or accelerated particles penetrating through the void. A possible physical explanation of the cloud's activity is based on the assumption that the phenomenon can be considered as a consequence of the non-Hamiltonian character of complex plasmas.

CO J = 6-5 Observations of TW Hydrae with the Submillimeter Array

We present the first images of the 691.473 GHz CO J = 6-5 line in a protoplanetary disk, obtained along with the 690 GHz dust continuum, toward the classical T Tauri star TW Hya, using the Submillimeter Array. Imaging in the CO J = 6-5 line reveals a rotating disk, consistent with previous observations of CO J = 3-2 and 2-1 lines. Using an irradiated accretion disk model and two-dimensional Monte Carlo radiative transfer, we find that additional surface heating is needed to simultaneously fit the absolute and relative intensities of the CO J = 6-5, 3-2, and 2-1 lines. In particular, the vertical gas temperature gradient in the disk must be steeper than that of the dust, mostly likely because the CO emission lines probe nearer to the surface of the disk. We have used an idealized X-ray heating model to fit the line profiles of CO J = 2-1 and 3-2 with χ2 analysis, and the prediction of this model yields CO J = 6-5 emission consistent with the observations.

Hot electrons blast wave generated by femtosecond laser pulses on thin Au(1 1 1) crystal, monitored by subpicosecond X-ray diffraction

Femtosecond time-resolved X-ray diffraction is used to probe reversible lattice changes of a 150-nm Au(1 1 1) crystal on mica heated with 100-femtosecond laser pulses with fluence below the damage and melting threshold. The lattice strain generated throughout the crystal is larger than estimated from the inhomogeneous lattice temperature distribution and acoustic wave produced by nonequilibrium heating. The broadening and shift of the rocking curve observed across the depth of the crystal support the presence of an additional ‘blast’ wave arising from hot electron gas temperature gradient, which is observed experimentally for the first time.

Nanoparticle manipulation in the near-substrate areas of low-temperature, high-density rf plasmas

Manipulation of a single nanoparticle in the near-substrate areas of high-density plasmas of low-temperature glow discharges is studied. It is shown that the nanoparticles can be efficiently manipulated by the thermophoretic force controlled by external heating of the substrate stage. Particle deposition onto or repulsion from nanostructured carbon surfaces critically depends on the values of the neutral gas temperature gradient in the near-substrate areas, which is directly measured in situ in different heating regimes by originally developed temperature gradient probe. The measured values of the near-surface temperature gradient are used in the numerical model of nanoparticle dynamics in a variable-length presheath. Specific conditions enabling the nanoparticle to overcome the repulsive potential and deposit on the substrate during the discharge operation are investigated. The results are relevant to fabrication of various nanostructured films employing structural incorporation of the plasma-grown nanoparticles, in particular, to nanoparticle deposition in the plasma-enhanced chemical-vapor deposition of carbon nanostructures in hydrocarbon-based plasmas.

Role of the thermophoretic force on the transport of nanoparticles in dusty silane plasmas

A comparison, based on numerical modeling, is made between the different forces experienced by nanoparticles present in a low pressure capacitively coupled parallel-plate silane (Si H4 ) discharge. We investigate in particular the influence of the thermophoretic force on the spatial distribution of the nanoparticle density profiles, due to a thermal gradient in gas temperature induced by heating or cooling of the electrodes. A series of simulations with a one-dimensional fluid model are performed with asymmetrical variation of the electrode temperatures. It appears that the resulting density profile of the nanoparticles experiences a significant shift towards the cooler electrode as soon as a temperature difference is applied. Thus thermophoresis is capable of influencing the force balance of suspended nanoparticles in the plasma, even at relatively small electrode temperature differences.

Predicting heat and mass transfer to a growing, rotating preform during soot deposition in the outside vapor deposition process

During the manufacture of optical fibers using the outside vapor deposition (OVD) process, porous soot preforms are made by depositing soot onto a rotating cylindrical target from a soot-laden flame traversing along the target axis. The dominant mechanism of soot deposition in the OVD process is thermophoresis, which is the tendency of particles to migrate down the local gas temperature gradient. Accurate methods to estimate the heat and mass transfer rates from the flame to the growing preform are critical, as these rates dictate important preform characteristics. The heat and mass transfer during the OVD process are coupled due to particle thermophoresis and growth of the preform. We here present methods to predict the growth rate of a rotating preform along with the evolution of temperature profile at different radial locations within the preform for specified process parameters of flame temperature, burner traverse speed and number of burners. Sensitivity of preform temperature profile and deposition rate to each of the process parameters is presented, along with the critical discussion of our principal results.

Verification studies of thermophoretic protection for extreme ultraviolet masks

A “thermophoretic pellicle” has been proposed as an alternative to the traditional organic pellicle as a means of protecting extreme ultraviolet (EUV) lithographic photomasks from particle contamination. The thermophoretic pellicle protects a mask from particles by exploiting the thermophoretic force, which is exerted on a particle by a surrounding gas in which a temperature gradient exists. Two critical requirements of the thermophoretic pellicle are: (1) the mask is kept warmer than its surroundings and (2) the surrounding gas pressure is kept sufficiently high to enable thermophoretic protection. Experiments are presented which verify the viability of thermophoretic protection for EUV masks. In these experiments, wafers are exposed to a monodisperse, polystyrene-latex-sphere aerosol under carefully controlled experimental conditions. Robust thermophoretic protection is observed over a wide range of argon gas pressures (50–1600mTorr or 6.66–213Pa), particle sizes (65–300nm), and temperature gradients (2–15K∕cm). Numerical simulations of the thermophoretic pellicle show good agreement with the data.

Prediction of retention times and efficiency in linear gradient programmed pressure analysis on capillary columns

A method for the prediction of the retention time and the resolution of chromatographic peaks in different experimental conditions by starting from few experimental data measured in isothermal and isobaric analyses was published previously. In this paper, the same mathematical model was implemented for calculating the retention times and the column efficiency in programmed pressure runs. Some models originated from the Golay equation and reported in the literature are compared, and a new modified equation for the calculation of the peak width at half height is proposed. The procedure for the prediction of the retention time and the peak width at half height at programmed pressure of the carrier gas and different column temperature and linear gradient by using retention data of different compounds obtained in few isobaric runs is described. The prediction of the retention time and the separation efficiency of compounds with different polarity gave good results for the programmed pressure runs with linear gradient. The effect of the variation of the initial parameters of the experimental analyses and of the mathematical model on the accuracy of the prediction has been evaluated.

Sound Attenuation by Glow Discharge Plasma

A unique method of blocking sound waves using a glow discharge plasma sheet was demonstrated experimentally. The main thrust of the investigation was the determination of the effectiveness of using glow discharge plasma as a sound barrier in aerospace applications. Experiments were conducted in an anechoic chamber where the attenuation of single-frequency (12.5-kHz) sound propagating through a plasma sheet formed between two electrodes was studied. The sound attenuation was measured at different locations inside the chamber with the discharge plane oriented normal to the direction of sound-wave propagation. Some measurements were also carried out with the plasma plane oriented 45 deg to the sound-wave propagation direction. The measurements clearly demonstrated that the plasma attenuates sound. The highest attenuation of about 23 dB was obtained when the plasma was oriented 45 deg to the sound propagation direction. In this configuration, sound reflection from the glow discharge region was also observed. The present results suggest that the dominant mechanism responsible for the sound attenuation is the change in the index of refraction caused by gas temperature gradient at the plasma boundaries.

Reproducing the entropy structure of clusters and groups

We describe a semi-analytic model for the X-ray emitting gas in clusters and groups in which the gas is preheated before halo formation. The model relies on physically sound prescriptions for the formation and evolution of halo structure. For a gas temperature gradient corresponding to a polytropic index of 1.2 and an energy injection of 0.6 keV per gas particle, this model successfully reproduces the observed correlations between gas luminosity, gas temperature and halo mass. We use the model to investigate the detailed entropy structure of groups and clusters of galaxies and compare the results with the observational data.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Temperature characterization in the collection tank of the NIST 26 m3 PVTt gas flow standard

Gas temperature gradients created during the filling stage of a pressure–volume–temperature–time (PVTt) calibration cycle, and those imposed by inhomogeneous room conditions, lead to uncertainties in the average gas temperature in the collection tank. Because these temperature uncertainties dominate the overall flow uncertainty, NIST upgraded the temperature-averaging scheme used in its 26 m3 PVTt system. Instead of arithmetically averaging 10 thermistors to obtain the mean gas temperature, we now calculate this value via a volume-weighted trapezoidal integration procedure using 35 thermistors. Applying the new temperature-averaging scheme, the mean gas temperature can be determined with a standard uncertainty of 89 mK after only 2700 s of fan mixing. As a result, the flow uncertainty in the NIST 26 m3 PVTt system has decreased from 0.22% to 0.13% (with a coverage factor of 2). This paper highlights the temperature improvements and presents a detailed analysis for estimating the lower temperature uncertainty.

Modeling of self-excited dust vortices in complex plasmas under microgravity.

A two-dimensional hydrodynamic model for a dusty argon plasma in which the plasma and dust parameters are solved self-consistently has been supplemented with a separate dust particle tracing module to study the behavior of dust vortices. These coherent vortices appear in plasma crystal experiments performed under microgravity conditions. The nonconservative total force exerted by the discharge on the dust particles is responsible for the generation of the vortices. The contribution of the thermophoretic force driven by the gas temperature gradient plays an insignificant role in the generation of the vortices, even when the gas heating via the dust particles is taken into account. The forces related to the electric field, including the ion drag force, are dominant.

Cold Fronts in Cold Dark Matter Clusters

Recently, high-resolution Chandra observations revealed the existence of very sharp features in the X-ray surface brightness and temperature maps of several clusters. These features, called cold fronts, are characterized by an increase in surface brightness by a factor 2 over 10-50 kpc accompanied by a drop in temperature of a similar magnitude. The existence of such sharp gradients can be used to put interesting constraints on the physics of the intracluster medium (ICM) if their mechanism and longevity are well understood. Here, we present results of a search for cold fronts in high-resolution simulations of galaxy clusters in cold dark matter models. We show that sharp gradients with properties similar to those of observed cold fronts naturally arise in cluster mergers when the shocks heat gas surrounding the merging subcluster, while its dense core remains relatively cold. The compression induced by supersonic motions and shock heating during the merger enhance the amplitude of gas density and temperature gradients across the front. Our results indicate that cold fronts are nonequilibrium transient phenomena and can be observed for a period of less than a billion years. We show that the velocity and density fields of gas surrounding the cold front can be very irregular, which would complicate analyses aiming to put constraints on the physical conditions of the ICM in the vicinity of the front.

Nucleation and subsequent growth of clusters in reactive plasmas

Nucleation and subsequent growth processes of clusters, i.e. particles below a few nm in size, have been studied in capacitively coupled high-frequency SiH4 discharges. Time evolution of size and density of clusters has clearly shown the existence of bottleneck in the size distribution at around Si4Hx's. After the nucleation, the clusters of the large size group, SinHx's (n>4), grow mainly due to influxes of those of the small size group, SinHx's (2⩽n⩽4) and other molecular species SiHx's (0⩽x⩽3). These results have led to the growth model of clusters, which can reasonably explain the results obtained until now. Furthermore, effects of gas flow, discharge modulation, gas temperature gradient and hydrogen dilution on growth and behaviour of clusters have been studied to control their growth. The reactor, which suppresses the growth of clusters both by thermophoretic force and by gas flow and evacuation without stagnation, has been newly developed to make a-Si:H films of high qualities. The qualities of films deposited using the reactor are much better than those of conventional high-quality films.

Modeling of transport processes and kinetics of silicon carbide bulk growth

The diameter of silicon carbide (SiC) single crystal grown by the physical vapor transport method has increased significantly in recent years. Process modeling has played an important role in designing and developing the large diameter SiC growth systems. The numerical algorithm incorporates the calculations of radio-frequency, time-harmonic magnetic field by induction heating, radiation and conduction heat transfer in the system, as well as the growth kinetics. The generated power density in the graphite susceptor is obtained by solving the magnetic vector potential equations, and radiative heat transfer is calculated from the integrated equations for radiation. Chemical reactions and transport of gaseous species, Si2C, SiC2, SiC and Si, are also considered. A growth kinetics model is proposed for the first time, which uses the Hertz–Knudsen equation to relate the growth rate to the supersaturation of a rate-determining vapor species, the driving force for the deposition. The theoretical predictions compare reasonably with the published experimental data. The growth rate curves are obtained as a function of growth temperature and system pressure. The growth kinetics is greatly influenced by the inert gas pressure, temperature and temperature gradient. Since the vapor pressure is an ascending function of the temperature, for low temperature growth, a larger temperature gradient is needed in order to achieve the desired level of supersaturation (or growth rate). A low temperature growth is usually associated with small diameter systems, which maintain larger temperature differences. At a high growth temperature, since the vapor pressure is high, only a small temperature difference is required to achieve the same level of supersaturation. Desirable growth temperature and growth rate profiles can be obtained across the seed surface by optimizing the furnace components, such as the graphite susceptor, induction coil, and insulation materials.

The effect of the gas temperature gradient on dust structures in a glow-discharge plasma

An experimental investigation is performed of the effect of the neutral gas temperature gradient on plasma-dust formations in the positive column of a glow discharge. It is demonstrated that the thermophoretic forces arising due to the temperature gradient are comparable with radial electric fields and define the condition of formation and different shapes of plasma-dust structures, in particular, the formation of rings in the vicinity of tube walls. A model description of this effect is given.

Detailed thermal boundary conditions in the 3D fluid-dynamic modelling of horizontal MOVPE reactors

A three-dimensional (3D) detailed heat transfer model describing the effective thermal behaviour and fluid dynamics of a horizontal metalorganic vapour-phase epitaxy (MOVPE) reactor chamber is reported. Both H 2 conduction and convection were considered, along with the presence of a cooling H 2 flow outside the main MOVPE chamber, the quartz heat conduction through the chamber walls and the convective heat transfer to the ambient air. Computational fluid-dynamic (CFD) simulations indicated that relatively large gas temperature gradients occur normal to the wafer surface both in the process and cooling gas. In the former a fully developed thermal profile is observed. Reduced temperature gradients occur instead in the lateral directions within the process gas flow, giving rise to a good lateral uniformity of the temperature field close to the susceptor. However, large gradients build up in the cooling gas as a result of heat transfer to the surrounding ambient by the free convection. Correspondingly, large buoyancy appears in the cooling flow. A laminar behaviour is instead obtained for the process flow, although the substrate rotation introduces an asymmetry of the gas pathlines above the wafer. Comparing simulated susceptor-ceiling temperature differences (Δ T) with experimental ones indicates that a free convection parameter α≈7.5 W/m 2 K occurs for our system. This is consistent with Ra∼10 5 for the ambient outside the MOVPE chamber, suggesting subcritical-free convection conditions. CFD simulations performed as a function of susceptor temperature show close agreement between simulated and experimental Δ T, indicating the usefulness of a 3D detailed heat transfer approach for the correct simulation of temperature fields in a horizontal MOVPE reactor.

Effects of Gas Temperature Gradient, Pulse Discharge Modulation, and Hydrogen Dilution on Particle Growth in Silane RF Discharges

The effects of gas temperature gradient, pulse discharge modulation, and hydrogen dilution on the growth of particles below about 10 nm in size in silane parallel-plate RF discharges are studied using a high-sensitivity photon-counting laser-light-scattering (PCLLS) method. Thermophoretic force due to the gas temperature gradient between the electrodes drives neutral particles above a few nm in size toward the cool RF electrode which is at room temperature. Pulse discharge modulation is much more effective in reducing the particle density when it is combined with the gas temperature gradient, and particles above a few nm in size cannot be detected by the PCLLS method even after 2 h. Hydrogen dilution of a high H2/SiH4 concentration ratio above about 5 is also useful in suppressing particle growth in the radical production region around the plasma/sheath boundary near the RF electrode.

Methods of Suppressing Cluster Growth in Silane RF Discharges

ABSTRACTThe effects of gas temperature gradient, pulse discharge modulation, hydrogen dilution, gas flow, and substrate materials on growth of clusters below about 10 nm in size in silane parallelplate RF discharges are studied using a high-sensitivity photon-counting laser-light-scattering (PCLLS) method. Thermophoretic force due to the gas temperature gradient between the electrodes drives neutral clusters above a few nm in size toward the cool RF electrode. Pulse discharge modulation is much more effective in reducing the cluster density when it is combined with the gas temperature gradient, and clusters above a few nm in size cannot be detected by the PCLLS method even for the discharge over a few hours. Hydrogen dilution and gas flow are also effective in suppressing growth of clusters, when the H2/SiH4 concentration ratio is above about 5 and the flow velocity is above about 6 cm/s, respectively. Cluster growth rate with a glass or Si substrate is found to be considerably higher than that without the substrate.