Electrospray device

An electrospray device includes an electrospray emitter adapted to receive electrospray fluid; an extractor plate spaced from the electrospray emitter and having at least one aperature; and a power supply for applying a first voltage between the extractor plate and emitter for generating at least one Taylor cone emission through the aperature to create an electrospray plume from the electrospray fluid, the extractor plate as well as accelerator and shaping plates may include a porous, conductive medium for transporting and storing excess, accumulated electrospray fluid away from the aperature.

FIELD OF THE INVENTION

This invention relates to an improved electrospray device and more particularly to an electrospray device which avoids excess accumulation of electrospray fluid.

BACKGROUND OF THE INVENTION

Basic operation of electrosprays is well-known and characterized in a large number of publications and used in many applications. Most of these applications are distinguished by relatively short-duration operation, especially where the electrospray is emitted through one or more apertures in grids composed of some solid material. During electrospray operation, and particularly during startup or with certain applied voltages and/or liquid flowrates applied to the electrospray, the electrospray may deposit some amount of sprayed liquid upon the solid edges of the grids (overspray), especially proximate the aperture. Accumulated build-up of this deposited liquid adversely impacts performance of electrospray operation over time, particularly when low vapor pressure liquids are used. Because this accumulation is gradual and most applications are short-duration, performance impacts had been negligible and no solutions to liquid accumulation had been sought.

For extended electrospray operation e.g. for durations exceeding 100 hours, or for particular operating conditions, no effective means for transport/removal of accumulated liquid exists. Consequently, duration of electrospray operation is limited to the point where the accumulated liquid interferes with, and ultimately prevents, proper function. Proper function is restored by performing manual cleaning of accumulated liquid and/or replacement of affected components.

Frequent cleaning or replacement of components is costly, and for applications where cleaning or replacement is impossible, the duration of electrospray operation is limited to the time it takes for some critical buildup preventing proper operation.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an improved electrospray device.

It is a further object of this invention to provide such an improved electrospray device which prevents buildup of excess electrospray fluid.

It is a further object of this invention to provide such an improved electrospray device which prevents back spray and shorting due to electrospray fluid accumulation.

It is a further object of this invention to provide such an improved electrospray device which enables extended operation and enables use in environments where cleaning or replacement is not practical.

It is a further object of this invention to provide such an improved electrospray device which reduces time and cost for cleaning or replacement necessitated by electrospray accumulation.

It is a further object of this invention to provide such an improved electrospray device which provides improved operation by eliminating the effect of free surfaces in electric fields affecting electrospray.

It is a further object of this invention to provide such an improved electrospray device which enables smaller and more compact devices through denser arrays of electrospray emitters.

The invention results from the realization that an improved electrospray device which reduces excess electrospray accumulation is achieved by making the extractor plate and one or more accelerator plates and shaping plates from a porous, conductive medium for transporting excess, accumulated electrospray fluid away from the aperature(s) in the plates.

This invention features an electrospray device including an electrospray emitter adapted to receive electrospray fluid, an extractor plate spaced from the electrospray emitter and having at least one aperture, and a power supply for applying a first voltage between the extractor plate and emitter for generating at least one Taylor cone emission through the aperture to create an electrospray plume from the electrospray fluid, the extractor plate including a porous, conductive medium for transporting excess, accumulated electrospray fluid away from the aperture.

In a preferred embodiment the emitter may include a capillary tube for delivering the electrospray fluid. The emitter may include an externally wetted needle. The emitter may include a porous core with a circular emission rim. The emitter may include a porous core with a linear emitter. The electrospray fluid may include an ionic liquid. The electrospray fluid may include a mixture of ionic liquid and a solvent. The porous, conductive medium may include a metal. The porous, conductive medium may include a sintered metal. The aperture may include a hole. The aperture may include a gap distance from the extractor plate. There may be an accelerator plate spaced from the extractor plate on the other side from the emitter. The power supply may provide a second voltage between the extractor place and the accelerator plate. The second voltage may have the same polarity as the first voltage. The second voltage may have the opposite polarity from the first voltage. There may be shaping plate for shaping the electrospray plume. The shaping plate may be disposed between the emitter and the extractor plate. The power supply may apply a voltage to the shaping plate similar to that applied to the emitter. The shaping plate may include a porous, conductive medium. The accelerator plate may include a porous, conductive medium. At least one of the extractor, accelerators, and shaping plates may be connected to a liquid storage.

DETAILED DESCRIPTION OF THE INVENTION

As explained in the Background, electrospray devices such colloid thrusters employ extraction and acceleration grids to create and accelerate liquid propellant as charged droplets and ions. A small fraction of the emitted propellant, usually much less than 1%, is intercepted by the grids where it may accumulate. The majority of this interception occurs during start up transients. Because the liquid is conductive, sufficient accumulation can bridge across high voltage gaps, and short the thruster, terminating its useful life or detrimentally altering the electrostatic field between the grids.

There is shown inFIG. 1an electrospray device according to this invention including an emitter10, which may be a capillary tube, adapted for connection to an electrospray fluid source12to receive electrospray fluid13. There is an aperture plate14, which in this configuration would typically be an extractor plate, having an aperture16through which the electrospray18is formed. A power supply20shown simply as a battery is connected across plate14and emitter10to create a voltage potential difference which results in a Taylor cone22being created. Power supply20can be a battery21connected as shown with the positive electrode connected to plate14and the negative to emitter10or the polarity may be reversed depending upon the configuration and application. It is the Taylor cone which results in the electrospray18and the concomitant thrust which is generated. The electrospray fluid may be a variety of materials preferably ionic materials such as available from Covalent Associates, Inc. of Corvallis, Oreg., Alfa Aesar of Ward Hill, Mass., and Sigma Aldrich of St. Louis, Mo. One of the desirable attributes of ionic fluids is that they have very low vapor pressure and so do not evaporate quickly. This normally beneficial feature contributes to the problem explained in the Background because when the electrospray fluid accumulates on plate14especially in and around aperture16it does not quickly evaporate.

In accordance with this invention aperture plate14is made of a porous conductor. It is a conductor in order to contribute to the establishment of the electric field between emitter10and plate14. In accordance with this invention it also must be porous. Metals are a good choice for plate14, especially sintered metals. The porosity depends upon the viscosity and flowability of the electrospray fluid used. For example, using an electrospray fluid such as EMIIm available from Covalent Associates, Inc. plate14may be made of stainless steel and have a porosity of 0.5 μm. Using ionic fluid such as EMIBF4available from Alfa Aesar the plate14may be made of stainless steel and have a porosity of 0.5 μm. If the electrospray fluid is EMIPF6available from Sigma Aldrich then plate14may be made of stainless steel and have a porosity of 0.5 μm. By making plate14porous in accordance with this invention the excess electrospray fluid which would accumulate on plate14in and around hole16is instead wicked away and absorbed into the porous plate14. The enlarged detail view ofFIG. 2shows the Taylor cone22more clearly. It is the Taylor cone that gives birth to the electrospray which forms through aperture16. The invention is not limited to an emitter using only a capillary tube, such as shown inFIGS. 1 and 2.

InFIG. 3, Taylor cone22is created by the flow of the electrospray fluid13which flows around a needle emitter10ato create the same phenomena of a Taylor cone22and electrospray18through aperture16of porous aperture (extractor) plate14.

In another alternative,FIG. 4, porous plate14according to this invention may be used with a circular emitter10b, having an emission rim28around whose periphery a plurality of Taylor cones, not shown, are generated contributing to the electrospray18, also not shown, generated through aperture16. Emitter10bwith its emission rim28is supplied with electrospray fluid13from an electrospray fluid source via a circular emission source, center core30, which may also be a type of porous material to feed the fluid up in a controlled fashion to emission rim28.

While thus far the porous plate14according to this invention has been shown with an aperture that is basically a hole or a round opening this is not a necessary limitation of the invention for as shown inFIGS. 5 and 6, here referred to together, the fluid may be fed into a propellant inlet40from thence to an elongated manifold pipe42not visible inFIG. 5, which feeds the electrospray fluid to a porous core44that in turn delivers the electrospray fluid13to the emission area or split ridge46. Here the aperture plate14ahas an elongate slot or gap48for the aperture instead of a hole or a more rounded opening. Split ridge46is formed by a space between the two halves14aaand144aaaof plate14a.

The action promoted by the porous structure of the aperture plate according to this invention is shown inFIG. 7where aperture plate14bis shown having an electrospray buildup50concentrated in the area of aperture16. Due to the porosity of plate14bthis electrospray fluid50is wicked away as indicated by the random paths of lines52and54for example to distal portions of aperture plate14b. In a preferred embodiment of the invention aperture plate14bmay be connected with an added volume of porous material56which acts as a reservoir to receive even more of the electrospray fluid which is wicked away through the absorbing nature of plate14band proximate areas of porous reservoir56which may be made of the same material as plate14b.

Although thus far the invention has been applied to a single aperture plate, typically the extractor plate of an electrospray device, this is not a necessary limitation of the invention. For example, as shown inFIG. 8, plate14is an extractor plate accompanied by a second aperture plate, accelerator plate60, having a similar aperture62. Accelerator plate60may be a porous medium, too. Accelerator plate60is also connected to power supply20awhich now includes a second voltage source shown simply as a battery64. In this case battery64polarizes accelerator plate60with the same polarity as that of extractor plate14with respect to emitter10but this is not a necessary limitation for the accelerator to be at a negative voltage, in which case the polarity connection of battery64to plate60would be reversed.

The invention is applicable to all manner of arrangements and configurations of aperture plates, shaping electrodes and the like. Some examples of the variety of aperture plates including extractor plates, accelerator plates, shaping electrodes and lenses which are accommodated by this invention are shown inFIGS. 9A-E. For example inFIG. 9A, extractor plate14is accompanied by a shaping electrode70for shaping the electrospray. In this case the shaping electrode70would also be porous in accordance with this invention. InFIG. 9A, the shaping electrode70is on the opposite side of extractor plate14from emitter10. This is not a necessary limitation as the shaping electrode70a, FIG.9B, may be closer to emitter10while extractor plate14is farther from it. InFIG. 9Cextractor plate14is accompanied by a set of shaping electrodes80,82,84which together form an Einzel Lens for procuring a particular shape to electrospray18. InFIG. 9D, a shaping electrode70is positioned between extractor aperture plate14and accelerator aperture plate90. Any or all of the extractor plate14, accelerator90, and one or more shaping electrodes70may be connected to liquid resolve150which may be porous as shown inFIG. 7. InFIG. 9E, extractor plate14and accelerator plate80are positioned interstitial three shaping electrodes100,102, and104.