Condensation Of Metal Vapors Research Articles

Numerical analysis of the influence of particle charging on the fume formation process in arc welding

In order to clarify the influence of electrostatic forces caused by charging of particles on the coagulation process in fume formation in arc welding, a previously developed fume formation model is modified to consider the influence of charging, for both local thermodynamic equilibrium (LTE) and non-LTE conditions. The model takes into account formation of the particles from metal vapour by nucleation, growth of the particles by condensation of metal vapour and coagulation of the particles by collisions to form secondary particles. Results are obtained for both ballistic and Brownian motion of the particles. It is found that the growth of secondary particles is suppressed when the average particle charge becomes significant, because charging of the particle hinders collisions among secondary particles through the strong repulsive electrostatic force. Furthermore, deviations from LTE strongly affect the coagulation process, because the increased electron density at a given gas temperature increases the charging of particles. Brownian motion leads to larger secondary particles, since the average particle speed is increased. The influence of Brownian motion and particle charging cancel each other to a large extent, particularly when deviations from LTE are considered.

Some peculiarities in the magnetic behavior of aerosol generated NiO nanoparticles

Nickel oxide nanoparticles (NPs) 6–24nm size have been prepared by a levitation-jet method based on metal vapor condensation in a mixture of gaseous streams of helium and air (or oxygen). Particles were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) N2 adsorption and SQUID magnetometry. It is revealed that at room-temperature (RT) the nanoparticles show a small magnetic moment and the magnetic field dependence of their susceptibility a minimum lying below the high-field values. After the appropriate subtraction of both ferromagnetic and paramagnetic components from the initial magnetization curves, the appearance of a diamagnetic contribution is observed. The possible link between the latter contribution and the occurrence of some features related to RT superconductivity in NiO NPs is discussed.

Kinetics and energy states of nanoclusters in the initial stage of homogeneous condensation at high supersaturation degrees

The condensation of metal vapor in an inert gas is studied by the molecular dynamics method. Two condensation regimes are investigated: with maintenance of partial pressure of the metal vapor and with a fixed number of metal atoms in the system. The main focus is the study of the cluster energy distribution over the degrees of freedom and mechanisms of the establishment of thermal equilibrium. It is shown that the internal temperature of a cluster considerably exceeds the buffer gas temperature and the thermal balance is established for a time considerably exceeding the nucleation time. It is found that, when the metal vapor concentration exceeds 0.1 of the argon concentration, the growth of clusters with the highest possible internal energy occurs, the condensation rate being determined only by the rate of heat removal from clusters.

Electric field-assisted levitation-jet aerosol synthesis of Ni/NiO nanoparticles

Ni/NiO powdered nanoparticles with average sizes 10–30 nm were prepared by a levitation-jet method involving the condensation of Ni metal vapour in a mixture of helium with various amounts of air or oxygen. The process was undertaken with the application of a DC electric field up to 6.5 kV cm−1. The particles were characterized by X-Ray diffraction, transmission electron microscopy, BET adsorption and vibrating sample magnetometry. It was found that the intensity of the applied electric field and partial oxygen pressure correlated with the main structural and magnetic parameters of the nanoparticles, such as average particle size, residual ratio of nickel, coercivity and maximum magnetisation. The specific surface area of the particles correlated with the magnitude of the external electric field. Room-temperature hysteresis loops of weakly oxidized nanoparticles show ferromagnetic-like behaviour, whereas the strongly oxidized ones exhibit a low-field ferromagnetic feature superimposed to a paramagnetic signal, regardless of the particle size. Magnetic measurements allowed for the estimation of the residual metal Ni content in the powdered nanoparticles, which can be as low as 0.04 at.% depending on oxygen partial pressure and external electric field strength.

Mathematical simulation of the formation of metallic nanoparticles during the condensation of molten metal vapors

The efficiency of the vapor phase condensation of metallic nanoclusters and nanopowders is determined by setting optimum process parameters, the possibilities of experimental estimation of which is limited. In this work, mathematical and physical models are developed to perform computer analysis of the vapor phase condensation to describe the macroscopic characteristics of the process (temperature regime, gas mixture dynamics, diffusion and convective transport of clusters) with allowance for the properties of the components on a microscopic level.

Effect of HCl, SO2 and H2O on the condensation of heavy metal vapors in flue gas cooling section

Incineration of Pb, Cd, Zn and Cu‐loaded model waste with single mode and mixture mode has been carried out in a lab‐scale rotary kiln reactor to clarify the condensation distribution of their vapor species in a cooling zone. The influences of the gaseous compounds including HCl, SO2 and H2O have been investigated. A thermodynamic pseudo‐equilibrium model was developed to evaluate the condensation mechanisms of the metal vapor species. The results indicate that, Pb, Cd and Zn condensed as their chlorides whereas Cu deposited as a mixture of CuO and CuCl2 at the temperature range of 853–473K in the presence of HCl in the N2/O2/CO2 mixture. The presence of SO2 and/or steam in flue gas facilitated the conversion of chlorides into sulfates, thereby improving the deposition propensity formed at the temperatures above 853K. Coexistence of the four metal vapors in flue gas caused the increase of Cd, Zn and Cu sulfates fraction as a consequence of the heterogeneous nucleation of metal vapors on the pre‐existing PbSO4 nuclei. Model prediction implies that super‐cooling phenomenon reduced the temperature for the deposition of metal vapor species which was however compensated by the heterogeneous coagulation when SO2 and/or steam was introduced in flue gas.

Preparation of nickel nanoparticles for catalytic applications

Spherical oxidized nickel particles 15 to 200 nm in average size have been produced by a crucibleless aerosol method involving metal vapor condensation in an inert gas flow and oxidation processes. The particles have been characterized by scanning electron microscopy, X-ray microanalysis, X-ray diffraction, BET surface area measurements, and vibrating-sample magnetometry. The process parameters have been optimized for the preparation of particles with tailored size, specific surface area, and saturation magnetization. A dc electric field applied to the condensation zone during the oxidation process reduces the size and increases the extent of oxidation of the particles. We have studied low-temperature oxidation of carbon monoxide and propane on nickel nanopowders differing in particle size and extent of oxidation. The nanoparticles with optimized characteristics have been shown to have a marked catalytic effect on these processes.

The effects of metal vapour in arc welding

Metal vapour is formed in arc welding processes by the evaporation of molten metal in the weld pool, and in the case of gas–metal arc welding, in the wire electrode and droplets. The presence of metal vapour can have a major influence on the properties of the arc and the size and shape of the weld pool. Previous experimental and computational works on the production and transport of metal vapour in welding arcs, in particular those relevant to gas–metal arc welding and gas–tungsten arc welding, are reviewed. The influence of metal vapour on the thermodynamic, transport and radiative properties of plasmas is discussed. The effect of metal vapour on the distributions of temperature, current density and heat flux in arcs is examined in terms of these thermophysical properties. Different approaches to treating diffusion of metal vapour in plasmas, and the production of vapour from molten metal, are compared. The production of welding fume by the nucleation and subsequent condensation of metal vapour is considered. Recommendations are presented about subjects requiring further investigation, and the requirements for accurate computational modelling of welding arcs.

Effects of HCl, SO 2 and H 2O in flue gas on the condensation behavior of Pb and Cd vapors in the cooling section of municipal solid waste incineration

This study aims to clarify the temperature-dependence of the condensation behavior of heavy metal vapors upon flue gas cooling during municipal solid waste (MSW) incineration. Two heavy metals, lead (Pb) and cadmium (Cd), were investigated in a lab-scale rotary kiln reactor coupled with a multi-stage cooling zone. Three gas components, HCl, SO 2 and H 2O, were doped into flue gas to clarify their effects on the deposition behavior of inorganic vapors in cooling zone. The results indicate that, once vaporized, the gaseous PbCl 2 and CdCl 2 commenced to condense with flue gas temperature dropping to 853 K. A rapid quenching of flue gas, i.e. supercooling, decreased the high temperature (853–623 K) deposition fractions of the gaseous species, which were far lower than that predicted by thermodynamic equilibrium modeling. The presence of SO 2 and H 2O in flue gas facilitated the conversion of gaseous chlorides into sulfates, which in turn enhanced the deposition of Pd and Cd vapors at the temperatures above 853 K as the dew point of sulfate is higher than the corresponding chloride. Co-existence of Pb and Cd vapors in flue gas also influenced the deposition behavior of single metals. It has been confirmed that, the deposition of CdSO 4 was remarkably shifted to higher temperature range due to its heterogeneous nucleation with pre-existing PbSO 4 nuclei which were preferentially condensed as a result of its high partial vapor pressure at higher temperatures. The chemical reaction for inorganic vapors and their heterogeneous coagulation are influential in terms of the condensation of heavy metal vapors on flue gas cooling, which greatly compensated the negative effect of supercooling, and thus, shifted the composition of metallic deposits from toxic chlorides to less harmful sulfates which preferentially condense into solid particulates in flue gas.

Reactivity, Stored Energy, and Dislocations in Solid-Solid Reacting Systems

ABSTRACT Strong neutron bombardment of graphite can displace many atoms from their lattice positions; many of the dislocated atoms do not anneal during the irradiation process and the net result is an increase of internal energy of the graphite. This increase is usually referred to as stored energy, or in the older literature this phenomenon is sometimes called the total Wigner energy. In addition to irradiation, mechanical milling, decomposition, precipitation, reduction of chemical compounds, and condensation of metal vapors can also produce stored energy in powders. The stored energy associated with defects in solids is an important aspect of the nature of the defects and the processes that produce them. It has been shown experimentally that stored energy results in a large increase of reactivity of the solid materials, and the spontaneous release of the energy in an inert atmosphere can lead to a large temperature rise. This short communication reports on several typical features of stored energy and illustrates the problem with a few experimental examples. Since stored energy substantially increases the reaction rate we can carry out many reactions at much lower temperatures. Consequently, in technical applications for specific reactions high-temperature furnaces can be replaced by high-performance mills operating at room temperatures or by cheap low-temperature furnaces. Non-catalytic reactions, in spite of their importance, have not received as much attention as catalysis in the past. Major developments and working plants erected at the end of the last century without participation of the chemical engineering community, as, for example, cement manufacturing, steel production, and ore processing, did not encourage chemical engineers to study the governing phenomena in a systematic way. Consequently, modeling and experimental studies have been relatively few, and applied physics or metallurgy propelled development in the field. Reactions, that take place in gaseous or liquid systems can usually be well reproduced if the external conditions of the experiment are identical and the experiment is carried out with precursors of identical concentration. The situation is rather different in systems where solid precursors participate in the reaction scheme. In this short note we will report on some phenomena, that are not well known in the realm of chemical reaction engineering and that might play a very important role in experimental investigation and optimization of a particular solid reaction system.

Nucleation rates for the condensation of monovalent metals

We show that consideration of both cluster growth and magic numbers are necessary to accurately calculate nucleation rates for the condensation of alkali and coinage metal vapors. The effects are not additive. Rates calculated using the modified theory differ up to several orders of magnitude from typical classical calculations. Calculated rates compare favorably with experimental nucleation onset and rate data for lithium, sodium, cesium, and silver. Verifiable predictions are made for the other alkali and coinage metals.

Synthesis and Study of Binary Actinide and Lanthanide Compounds: XXIV. Curium-Cobalt Alloys

Two microsamples of 244Cm-Co alloys prepared by high-temperature condensation of curium metal vapor onto flat cobalt supports were studied. Both samples contained two intermetallic compounds with hexagonal lattices: Co17Cm2 and Co5Cm. The existence of Co2Cm or Co2(Cm,Th) intermetallic compound with a cubic lattice was assumed. It was found that Cm is insoluble in α- and β-Co at room temperature and at heating.

Numerical Modelling of an Anodic Metal Bath Heated with an Argon Transferred Arc

A new 3-D model has been developed to describe the interaction between a transferred electric arc and a liquid metal bath, and has been used to simulate a pilot axisymmetrical transferred arc furnace operating in the EDF Research and Development laboratory. This model enables calculations of the flow patterns, temperature distribution and electromagnetic fields in both the arc and the bath. The Navier-Stokes equation coupled with the electromagnetic relations are solved in each domain using a finite volume method. The source term in the radiative energy equation is modeled using the radiative transfer method in order to take into account the strong temperature variations in the electric arc. The transport and condensation of metal vapour in the arc domain are considered by solving a conservation equation for the vapour mass fraction. The arc flow calculation at the bath surface uses a one-dimensional sheath model taking account of the metal vapour, in order to ensure the coupling between the plasma and the bath by evaluating the boundary conditions at the arc/bath interface. The calculations were performed for an arc length of 0.25 m. Realistic predictions are obtained for the electrical, dynamic and thermal behaviour of the plasma and liquid metal, and also for the arc voltage. The results indicate that the effects of the arc impact and Lorentz forces are not sufficient to induce effective mixing throughout the metal bath, leading to a marked thermal stratification in the liquid metal. In the bath, the liquid/solid interface has been determined by calculations. Keywords

Condensed metal vapor on alumina ceramic in vacuum interrupters

Investigations have been carried out on the dielectric performance of the ceramic (high-purity alumina, Al/sub 2/O/sub 3/) surface in vacuum interrupters after switching. In order to examine the influence of the shielding on the protection of the ceramic surface against metal vapor condensation different types of vacuum interrupters (VIs) have been tested: VIs with and without shielding. Additionally, two contact materials CuCr: 75:25 wt% and WCAg: 56:4:40 wt% have been investigated to compare the adhesion of different metal vapors to alumina ceramic surfaces. After having performed a HV conditioning of the VIs, dc arcs with arbitrary arcing times were triggered between the contacts simulating the generation of metal vapor during high current interruption and load break switching. Between the arcing tests the insulation levels of all VIs have been tested by means of HV ac source. Afterwards the VIs were opened and the microstructure of the metallic condensate on the inner ceramic surface was analyzed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The integral chemical composition of the metallic film was investigated by inductive coupled plasma (ICP).

A semi-empirical model of the fume formation from gas metal arcwelding

The control of exposure to welding fume is necessary to meet healthand safety obligations. The work reported here examines the fundamentals ofwelding-fume formation. A physical chemistry model of the metal vapourmechanism for fume formation has been developed for non short-circuitingtransfer gas metal arc welding (GMAW). The model includes the importantcontribution made by direct condensation of metal vapour onto the weldpool andworkpiece, in removing a substantial fraction of the fume. The model showsthat droplet size and wire feed speed control the fine fume formation rate.The understanding developed so far, indicates that the smaller the detacheddroplet size, the lower the total fume formation rate. The physics behind thisis explained. The model gives an insight into how process modification mightbe used to control fume at source. Control at source is believed to be themost cost-effective and energy-efficient technique for dealing with weldingfume. It is anticipated that the understanding gained from this project willbe applied to determine the practical limits for the control of welding fumeat its source.

A Model of Post Combustor Condensation of Heavy Metal Vapors onto Suspended Sorbents

The feasibility of condensation for gas-to-particle conversion of heavy metal vapors from combustion sources as a control strategy was investigated by a model study using lead oxide (PbO) as the heavy metal species. Gas-to-particle conversion and particle growth by condensation were simulated using a computer program, CONDCAPT. The maximum saturation ratio of lead oxide was found for different concentrations of a monodisperse population of 1 μm diameter spherical sorbent particles. The maximum PbO vapor saturation ratio (Smax) attained was dependent on the aerosol concentration and the rate of decrease of flue gas temperature. Log-normal particle size distributions with one as well as two size modes were considered. The CONDCAPT simulation results were compared with results from another simulation (MAEROS2) that included coagulation as well as condensation as the particle growth mechanisms. The significance of the Kelvin effect was evaluated by comparing the CONDCAPT results with and without the Kelvin effect. An example case of a typical 100 Tons Per Day (TPD) capacity municipal waste incinerator was considered to assess the practical implications of the model results.

Synthesis and structural analysis of aluminum nanocrystalline powders

The morphology and structure of aluminum nanocrystalline powders are studied using X-ray diffraction and high resolution electron microscopy. The powders are produced by a cryomelting process, based on spontaneous metal vapour condensation within a cryogenic medium. The as-produced powders are formed of free aluminum particles with 60% of them having a size less that 70 nm. Particles are covered by a 3 nm thick alumina layer whose structure depends on the aluminum surface orientation. The particles which interact during the synthesis undergo high plastic yielding with the formation of deformation twins, grain boundaries and the flattening of particles surfaces.

THERMODIFFUSION AND VAPORIZATION OF METAL FROM LEVITATED DROPLET. II. VAPORIZATION RATE AT CONVECTIVE DIFFUSION

In the present paper the problem of convective thermodiffusion from a levitated droplet is solved respecting the temperature dependence of the diffusion coefficient and including the metal vapour condensation in the proximity of the droplet surface and the thermodiffusion. The relation for the mean density of the convective diffusion flow from the surface was derived. This relation describes the rate of metal evaporation in which the case of diffusion without convections is not included. The derivation was performed in the first approximation.

THERMODIFFUSION AND VAPORIZATION OF METAL FROM LEVITATED DROPLET. III. VAPORIZATION RATE AT MOLECULAR DIFFUSION AND TOTAL VAPORIZATION RATE

In the present paper the problem of the molecular diffusion in the proximity of the levitated droplet surface respecting the metal vapour condensation influence and temperature dependences of basic physical quantities was solved. The molecular diffusion flow density was derived and the final relation for the mean total thermodiffusion flow density from the droplet surface, i.e. the vaporization rate, was presented. The results are completed by some numerical values and the accuracy of some calculations performed in the linear approximations was analyzed.

Thermodiffusion and vaporization of metal from levitated droplet I. Calculation of temperature field

In the present paper the initial partial differential equations resulting from the thermodynamics of irreversible processes and Onsager theory are presented and transformed in a way describing convective thermodiffusion in the proximity of a levitated droplet in forced laminar flow. To solve the temperature field the procedure used by Levich for describing diffusion to the falling particle was applied. This procedure was generalized for the temperature dependence of the temperature conductivity coefficient. The results of this solution were compared with the case when the mean constant value of this coefficient is used and an acceptable agreement of the results was stated. The derived theory will be used as the basis for calculation of the vaporization rate of metal from the levitated droplet taking into account metal vapour condensation in the temperature and diffusion boundary layer.