12 research outputs found

    Scale-up of electrospray atomization using linear arrays of Taylor cones

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    Linear arrays of Taylor cones were established on capillary electrode tubes opposite a slotted flat plate counterelectrode to investigate the feasibility of increasing the liquid throughput rate in electrospray atomizers. It was found that individual Taylor cones could be established on each capillary over a wide range of the capillary radius to spacing ratio R/S. The onset potential Vs required to establish the cones varied directly with R/S, but the liquid flow rate per cone and current per cone were nearly independent of R/S for a given overpotential ratio P=V/Vs. Only six working capillaries were used, but the results per cone are applicable to larger arrays of cones since end effects were minimized

    A noncontact measurement technique for the specific heat and total hemispherical emissivity of undercooled refractory materials

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    A noncontact measurement technique for the constant pressure specific heat (c(pl)) and the total hemispherical emissivity (epsilon(T1)) of undercooled refractory materials is presented. In purely radiative cooling, a simple formula which relates the post-recalescence isotherm duration and the undercooling level to c(pl) is derived. This technique also allows us to measure epsilon(Tl) once C(pl) is known. The experiments were performed using the high-temperature high-vacuum electrostatic levitator at JPL in which 2-3 mm diameter metallic samples can be levitated, melted, and radiatively cooled in vacuum. The averaged specific heats and total hemispherical emissivities of Zr and Ni over the undercooled regions agree well with the results obtained by drop calorimetry: C(pl,av(Zr)=40.8+/-0.9 J/mol K, epsilon(Tl,av) (Zr)=0.28+/-0.01, c(pl,av)(Ni)=42.6+/-0.8 J/mol K, and epsilon(Tl,av)(Ni)=0.16+/-0.01

    Inward electrostatic precipitation of interplanetary particles

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    An inward precipitator collects particles initially dispersed in a gas throughout either a cylindrical or spherical chamber onto a small central planchet. The instrument is effective for particle diameters greater than about 1 µm. One use is the collection of interplanetary dust particles which are stopped in a noble gas (xenon) by drag and ablation after perforating the wall of a thin-walled spacecraft-mounted chamber. First, the particles are positively charged for several seconds by the corona production of positive xenon ions from inward facing needles placed on the chamber wall. Then an electric field causes the particles to migrate toward the center of the instrument and onto the planchet. The collection time (of the order of hours for a 1 m radius spherical chamber) is greatly reduced by the use of optimally located screens which reapportion the electric field. Some of the electric field lines terminate on the wires of the screens so a fraction of the total number of particles in the chamber is lost. The operation of the instrument is demonstrated by experiments which show the migration of carbon soot particles with radius of approximately 1 µm in a 5-cm-diam cylindrical chamber with a single field enhancing screen toward a 3.2 mm central collection rod

    Ablation of silicate particles in high-speed continuum and transition flow with application to the collection of interplanetary dust particles

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    A model for the ablation and deceleration of spheres in continuum and slip flow is presented. Experiments were conducted in which initially spherical 7.1 micron diameter soda-lime glass particles were launched from vacuum at ~4500 m s^(-1) through a 0.5 mil (13 micron) plastic film into a capture chamber containing xenon at 0.1 and 0.2 atm and 295 K. Samples of ablated particles were collected and inspected using scanning electron microscopy (SEM). It was found that the ratio of the ablated particle radius (R_f) to the initial radius (R_0) depends on the gas pressure such that at 0.1 atm, R_f/R_0 = 0.67 ± 0.08, and at 0.2 atm, R_f/R_0 = 0.88 ± 0.08. The model agrees with these results if the heat of ablation Q is set to 1.5 ± 0.2 MJ kg^(-1). This value of Q approximately corresponds to the energy needed to raise the particle temperature from 295 to 1300 K, the working point of soda-lime glass. This indicates that the mechanism of ablation is melting and blowing of material from the particle's surface

    Electrospray Atomization of Electrolytic Solutions

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    Results of experiments on droplet production by electrospray atomization of electrolytic solutions are described. The spray results from the formation of liquid droplets at the tip of a Taylor cone. These experiments using sodium iodide dissolved in n-propyl alcohol show that as the solution concentration increases, the volumetric flow rate decreases, the electrical current increases, and the aerosol size distribution of the solid residue particles shifts to smaller sizes, a counter-intuitive result which occurs because the atomized droplet size decreases with increasing specific electrical conductivity of the solution. There is no complete analytical description of the electrospray but some analytical insights will be discussed regarding the behavior of its three distinct yet interacting parts: the Taylor cone, the jet, and the charged droplet spray

    Synthesis of Yttria Powders by Electrospray Pyrolysis

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    Electrospray atomization of high-concentration (∼400 g/L) chemical precursor solutions was applied to the synthesis of yttria powders. Conditions were found which led to high-quality powders, composed of dense, spheroidal, submicrometer, and nanocrystalline oxide particles. The precursor solutions were hydrated yttrium nitrates dissolved in n-propyl alcohol at concentrations ranging from 44.1 to 455 g/L. Electrospray atomization produced submicrometer precursor droplets which were dispersed in air and carried through an electric furnace for thermal decomposition at 500°C for several seconds residence time. X-ray powder diffraction patterns indicated the expected cubic phase. Transmission electron micrographs showed that the particle structure varied with solution composition, ranging from hollow, inflated spheres for 6-hydrated nitrates to dense spheroids for 5-hydrated nitrates. The use of 6-hydrated nitrates in the solutions appeared to form particle surfaces which were impermeable to alcohol vapor evolved during thermal decomposition, leading to hollow, inflated spheres