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

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

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

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

An electrostatic levitator for high-temperature containerless materials processing in 1-g

This article discusses recent developments in high-temperature electrostatic levitation technology for containerless processing of metals and alloys. Presented is the first demonstration of an electrostatic levitation technology which can levitate metals and alloys (2–4 mm diam spheres) in vacuum and of superheating-undercooling-recalescence cycles which can be repeated while maintaining good positioning stability. The electrostatic levitator (ESL) has several important advantages over the electromagnetic levitator. Most important is the wide range of sample temperature which can be achieved without affecting levitation. This article also describes the general architecture of the levitator, electrode design, position control hardware and software, sample heating, charging, and preparation methods, and operational procedures. Particular emphasis is given to sample charging by photoelectric and thermionic emission. While this ESL is more oriented toward ground-based operation, an extension to microgravity applications is also addressed briefly. The system performance was demonstrated by showing multiple superheating-undercooling-recalescence cycles in a zirconium sample (Tm=2128 K). This levitator, when fully matured, will be a valuable tool both in Earth-based and space-based laboratories for the study of thermophysical properties of undercooled liquids, nucleation kinetics, the creation of metastable phases, and access to a wide range of materials with novel properties

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

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

Part 1. Synthesis of ceramic powders by electrospray pyrolysis. Part 2. An approach to interplanetary particle sampling

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. In Part 1, the application of electrospray atomization for the production of ceramic powders is described. A model of the Taylor cone was developed which predicts the droplet size, volumetric flow rate, and electrical current for atomization of electrolytic solutions as functions of the liquid's specific electrical conductivity, surface tension, absolute viscosity, and density, as well as the applied electrical potential and atomizer geometry. Experimental verification was obtained for atomization of sodium iodide in n-propyl alcohol. The knowledge gleaned from the sodium iodide experiments was used to apply electrospray atomization to the production of submicron, spheroidal, and yttria particles by atomization of yttrium nitrate in n-propyl alcohol and thermal decomposition of the resulting aerosol droplets. This process is named "electrospray pyrolysis." A means of increasing the ceramic powder production rate for industrial and more extensive laboratory use via arrays of Taylor cones was experimentally tested. In Part 2, a method of sampling interplanetary dust particles (IDPs) is described. IDPs move at speeds [...]10 km/s relative to an interplanetary probe. A gas-filled balloon stops the particles by drag and ablation after they penetrate the balloon's skin. A model for ablation and deceleration of particles in the continuum and transition regimes was developed and experimentally tested using 7 [...] diameter glass spheres moving with an initial speed of 5 km/s through xenon gas at 0.1 and 0.2 atm. To analyze stopped IDPs by mass spectrometry, microscopy, etc., they must be deposited on a small sample substrate. To this end, a method of inward electrostatic precipitation was devised, modelled, and experimentally tested for [...] carbon soot particles in a cylindrical chamber. By this method, [...] IDPs can be intercepted by a [...] m noble gas-filled balloon, and deposited on a [...] mm centrally located sample substrate for subsequent analysis

Electrospray Atomization of Electrolytic Solutions

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

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