METHOD AND APPARATUS FOR A POROUS METAL ELECTROSPRAY EMITTER

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of and priority to U.S. Provisional Patent

Application No. 61/050,847 filed on May 6, 2008, which is owned by the assignee of the instant application and the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The technology generally relates to devices and methods of generating ions.

More specifically, the invention relates to methods and devices for a porous metal electrospray emitter.

BACKGROUND OF THE INVENTION

[0003] Existing colloid thrusters utilize pressure fed capillary emitter geometry to transport liquid to the base of Taylor Cones. Figure 1 shows a schematic of Taylor Cone formation from a pressure fed capillary emitter. A voltage can be applied to a capillary emitter 10, relative to an electrode 20. The balance between surface tension and electric pressure forms a Taylor Cone 30 and generates emission of ions 40. Droplets can be emitted, due to instability, at apex of cone 50. Droplets can carry most of the ejected mass (i.e., since droplets are relatively heavy) while delivering little impulse (i.e., as droplets move relatively slowly). This can translate into inefficient operation. In ion beam etching, droplets can also contaminate the substrate.

[0004] Pressure fed capillary emitters, however, can require pressurization systems

(e.g., onboard the spacecraft using the emitters), that adds mass/weight and complexity to the system. The difficulties in fabricating small, uniform capillaries can pose problems in the miniaturization of needle arrays. One way to avoid the issues of pressure fed capillary emitters is to use externally wetted emitter geometries where liquid is drawn from a reservoir by capillary forces. Such passively fed systems can supply liquid at the rate established by the electrospray emission process. The use of externally fed emitters in vacuum, however, is possible with ionic liquids.

[0005] Ionic liquids (ILs) are molten salts at room temperature and exhibit extremely low vapor pressures. ILs are formed by positive and negative ions which can be directly extracted and accelerated to produce thrust when used in bipolar operation. ILs have been shown to emit a purely ionic current when exposed to a strong applied potential. ILs generate a substantially pure ionic emission and have a relatively low starting voltage (e.g., less than approximately 2 kV required to generate ions from the Taylor Cone). ILs allow for a scalable specific impulse of the electrospray emitter(s) from approximately 500 seconds to 5000+ seconds. Some ILs can display super-cooling tendencies in which they remain as liquids well below their nominal freezing points. Just as their inorganic cousins (simple salts like NaCl, KBr, etc.) at their melting points (typically > 850°C), ILs exhibit appreciable electrical conductivity at room temperature, making them suitable for electrostatic deformation and subsequent Taylor Cone formation. ILs are thermally stable over a wide range of temperatures (they do not boil, but decompose at temperatures ~250-500°C) and are apparently non-toxic being able to be used with applications with green standards, such as in the synthesis and catalysis of chemical reactions. ILs can be used in electrochemical systems, such as in high energy density super-capacitors. ILs' electrochemical window (i.e., the maximum potential difference sustainable by the liquid before electrochemical reactions are triggered) is higher than in conventional aqueous solutions. ILs have low vapor pressures at, or moderately above, their melting points. This allows for use in high vacuum equipment in open architectures such as externally wetted needles/emitters.

[0006] Ion sources using ILs can produce positive or negative ion beams with: (1) narrow energy distributions, (2) high brightness, (3) small source size, and (4) wide selection of liquids with diverse molecular compositions. IL ionic sources can be used as a simple and compact source of nearly-monoenergetic negative ions, which can reduce the charge build-up that limits the ability to focus non-neutralized positive ion beams onto dielectrics (insulators or some biological samples) or conductive, but electrically floating targets, and act as a chemically reactive etch agent for materials micro- and nanoprocessing applications.

SUMMARY OF THE INVENTION

[0007] Porous metal Electrospray emitters have been shown to emit more current than a comparably sized solid externally wetted emitter (e.g., needle), due to the increased capillary flow capacity (e.g., greater flow through volume) provided by the volumetric

porosity of the emitter substrate. Porous metal emitters also have the benefit of being a passive, self-regulating capillary supply that reduces complexity over pressure fed capillary systems coupled with the benefit of increased flow through volume that permits the porous metal emitters to emit greater current and provide greater thrust. A typical thrust of a single porous metal emitter operating in the ionic mode can be about 0.05 - 0.1 μN/μA. Passively fed porous metal Electrospray emitters can emit purely in the ionic regime, which allows for high specific impulse (high ISP) operation and high polydispersive efficiency. Multiple emitters can be grouped to produce a desired amount of current, for example, in space applications. Micro-fabrication techniques can be used to manufacture a single emitter or an array of emitters. Porous metal electrospray emitters can be manufactured using, for example, photolithography and electrochemical etching.

[0008] In one aspect, an ionic liquid ion source includes a microfabricated body including a base and a tip and formed of a porous metal compatible (e.g., does not react or result in electrochemical decaying or corrosion) with at least one of an ionic liquid, or room- temperature molten salt. The microfabricated body can have a pore size gradient that decreases from the base of the body to the tip of the body, such that the ionic liquid is capable of being transported through capillarity from the base to the tip.

[0009] In another aspect, an ionic liquid ion source includes a plurality of emitters microfabricated from a porous metal compatible with at least one of an ionic liquid, or room- temperature molten salt. Each emitter can have a pore size gradient that decreases from the base of the emitter to the tip of the emitter, such that the ionic liquid is capable of being transported through capillarity from the base to the tip of each emitter.

[00010] In yet another aspect, a system for producing ions includes a source of at least one of ionic liquid or room-temperature molten salt and an array of emitters microfabricated from a porous metal compatible with the at least one of ionic liquid or room-temperature molten salt, where each emitter can have a pore size gradient that decreases from the base of the emitter to the tip of the emitter such that the ionic liquid is capable of being transported through capillarity from the base to the tip of each emitter. The system can also include an electrode positioned downstream relative to the array of emitters and a power source for providing a voltage to the array of emitters with respect to the electrode.

[00011] In another aspect, a method for manufacturing an array of electrospray emitters can include applying polyimide to a first side of a sample comprising a porous metal compatible with an ionic liquid, applying photoresist to the first side of the sample and applying a transparency mask to the first side of the sample and exposing the sample to UV light to form an emitter geometry pattern. The method can also include removing the photoresist from the sample, curing the sample to harden the polyimide, electrochemically etching the sample to form an emitter geometry and removing the polyimide resulting in an array of electrospray emitters. The method can include the step of treating and/or processing a tip of each emitter to vary a pore size between each tip and each base of each emitter in the array.

[00012] In yet another aspect, a method for manufacturing an ion emitter can include forming a body from a porous metal compatible with at least one of an ionic liquid or room temperature molten salt, the body having a pore size gradient that decreases from a first end of the body to a second end of the body. The method can also include microfabricating the body to form a base relative to the first end of the body and a tip relative to the second end of the body, wherein the ionic liquid is capable of being transported through capillarity from the base to the tip.

[00013] In another aspect, a method for manufacturing an ion source can include forming an emitter geometry pattern on a unitary substrate comprising a porous metal compatible with at least one of an ionic liquid, or room-temperature molten salt. The method can also include electrochemically etching the unitary substrate to form a plurality of emitters, where each emitter comprises a base at the first end of the substrate and a tip at the second end of the substrate. A tip of each emitter can be processed/treated to form a pore size gradient that varies from the base to the tip of each emitter.

[00014] In other examples, any of the aspects above, or any apparatus or method described herein, can include one or more of the following features.

[00015] In some embodiments, ionic liquid is capable of being continuously transported through capillarity from the base of a microfabricated body to the tip of the microfabricated body. The body can be a cylindrical needle. In some embodiments, the body is a flat ribbon-like needle. The tip of the microfabricated body can be formed by

electrochemical etching. In some embodiments, a radius of curvature of the tip is about 1-20 μm.

[00016] In some embodiments, the porous metal is at least one of tungsten, nickel, magnesium, molybdenum or titanium.

[00017] In some embodiments, an ion source includes a plurality of emitters and ionic liquid is capable of being continuously transported through capillarity from the base to the tip of each emitter. The emitters can be formed by electrochemical etching. In some embodiments, a spacing between the emitters is less than about 1 mm. In some embodiments, a system for producing ions includes an array of emitters and ionic liquid is capable of being continuously transported through capillarity from the base to the tip of each emitter.

[00018] A method for manufacturing an array of emitters can include filling the porous metal with photoresist and exposing the porous metal with a UV light to block pores of the porous metal to form the sample. In some embodiments, the method includes blocking the porous metal surface by the uniform deposition of mono-layers of a compatible material (e.g., compatible with the ionic liquids and the porous metal substrate and does not react or result in electrochemical decaying or corrosion) using Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD). The step of applying polyimide to the first side of the sample can include prebaking the sample. The method can include developing the sample to transfer the emitter geometry pattern by removing positive photoresist and etching the polyimide.

[00019] In some embodiments, electrochemically etching the sample includes the step of removing excess porous metal to form the emitter geometry. The step of electrochemically etching the sample can include etching the sample to form a conical emitter geometry. In some embodiments, the porous metal is at least one of tungsten, magnesium, molybdenum, titanium or nickel.

[00020] The method for manufacturing an ion emitter can include microfabricating a body to form a base and a tip. In some embodiments, the ionic liquid is capable of being continuously transported through capillarity from the base to the tip. Microfabricating the body can include shaping the body into a flat ribbon-like needle.

[00021] A surface of the tip of an emitter (e.g., one or more emitters in an array) can be treated/processed by applying a layer of compatible metal (e.g., a porous layer of metal compatible with the ionic liquids and the porous metal substrate that does not react or result in electrochemical decaying or corrosion) or other readily condensable metal to the porous metal emitter (e.g., on the surface at or substantially near the tip of the emitter). For example, a layer of zinc can be applied to a porous tungsten emitter. A surface of the tip of an emitter (e.g., an emitter in an array) can also be treated/processed by attaching carbon nanotubes to a surface of each emitter at or substantially near the tip of the emitter(s) (e.g., so that a pore size at the tip of each emitter is smaller than a pore size at a base of each emitter).

[00022] Other aspects and advantages of the invention can become apparent from the following drawings and description, all of which illustrate the principles of the invention, by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

[00023] The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

[00024] Figure 1 is a schematic of Taylor Cone formation from a pressure fed capillary emitter.

[00025] Figure 2 is a schematic of an ion source, according to an illustrative embodiment of the invention.

[00026] Figure 3 shows a schematic for a method for manufacturing a porous metal electrospray emitter, according to an illustrative embodiment of the invention.

[00027] Figure 4 shows a schematic of a setup for electrochemical etching, according to an illustrative embodiment of the invention.

[00028] Figure 5 shows a schematic of a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00029] Figure 6 is a drawing of a porous electrospray emitter array assembly, according to an illustrative embodiment of the invention.

[00030] Figure 7A is a graph showing time of flight measurements for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00031] Figure 7B is another graph showing time of flight measurements for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00032] Figure 8 A is a graph showing thrust measurements for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00033] Figure 8B is another graph showing thrust measurements for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00034] Figure 9A is a graph showing current and voltage measurements for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00035] Figure 9B is another graph showing current and voltage measurements for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00036] Figure 1 OA is a graph showing the percentage of current for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00037] Figure 1OB is another graph showing the percentage of current for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00038] Figure 1 IA is a graph showing the specific impulse for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

[00039] Figure 1 IB is another graph showing the specific impulse for a porous metal electrospray emitter array, according to an illustrative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[00040] Figure 2 is a schematic of an ion source 100, according to an illustrative embodiment of the invention. The ion source 100 includes a body 105 (e.g., an emitter body)

that includes a base 110 and a tip 115. The body 105 can be made of a porous metal (e.g., a microfabricated emitter body formed from a porous metal substrate) compatible with an ionic liquid or a room temperature molten salt (e.g., does not react or result in electrochemical decaying or corrosion). The body 105 can be mounted relative to a source 120 of ionic liquid or a source of a room temperature molten salt. The body 105 includes a pore size gradient that decreases from the base 110 of the body 105 to the tip 115 of the body 105, such that ionic liquid can be transported through capillarity (e.g., through capillary forces) from the base 110 to the tip 115. The ionic liquid can be continuously transported through capillarity from the base 110 to the tip 115 so that the ion source 100 (e.g., emitter) avoids liquid starvation. An electrode 125 can be positioned downstream relative to the body 105. A power source 130 can apply a voltage to the body 105 relative to the electrode 125, thereby emitting a current (e.g., a beam of ions 135) from the tip 115 of the body 105. The application of a voltage can cause formation of a Taylor cone (e.g., as shown in Figure 1) at the tip 115 and cause the emission of ions 135 from the tip 115.

[00041] In some embodiments, the body 105 is an emitter that is a cylindrical needle or a flat ribbon-like needle. Emitter geometry (e.g., shape and/or configuration of the emitter body) can affect the current generated by the emitter. For instance, flat ribbon-like configurations yield more current than traditional cylindrical solid needles. A tungsten externally wetted emitter can generate about 0.2 μA per emitter. In contrast, a flat ribbon tungsten emitter can generate up to about 10 μA per emitter. In some embodiments, a radius of curvature of the tip 115 of the body 105 can be in the range of about 1 μm to about 20 μm in the horizontal direction (e.g., along the z axis) and a radius of curvature of about 2 μm to about 3 μm in the vertical direction (e.g., along the y axis).

[00042] The body 105 can be microfabricated from a porous metal substrate. Body

105 can be formed by electrochemical etching. In some embodiments, the body can be formed of a porous metal substrate (e.g., tungsten) but other materials may be present. The body 105 can be microfabricated from a porous metal compatible (e.g., does not react or result in electrochemical decaying or corrosion) with ionic liquids and/or room temperature molten salts. Examples of such porous metals include tungsten, nickel, magnesium, molybdenum, or titanium.

[00043] The pore size gradient of the body 105 can allow ionic liquid from the source

120 to be transported from the base 110 to the tip 115. In some embodiments, the size of the

pores in the base 110 are larger than the pores in the metal at the tip 115, which allows for the ionic liquid to be transported through capillarity (e.g., capillary forces) from the base 110 of the emitter to the tip 115. By transporting the ionic liquid through capillarity, the pore size gradient can act as a passive, self-regulating capillary supply that reduces mass and complexity over capillary systems (e.g., by substantially reducing the need for a pressurized system). The pore size gradient can continuously provides ionic liquid to the tip 115, reducing the chances that the ion source will suffer from liquid starvation. Flow throughout the body (e.g., increased volume flow from the pores) can allow for even more current than solid ribbon emitters.

[00044] Figure 2 depicts an ion source comprising an emitter body 105, however, a plurality of emitters (e.g., an array of emitters) can be used in a ID or 2D array. The array of emitters can also be microfabricated from a porous metal (e.g., a unitary porous metal substrate) compatible with the at least one of ionic liquid or room-temperature molten salt. Each emitter, as described above, can have a pore size gradient that decreases from the base of the emitter to the tip of the emitter so that the ionic liquid is transported through capillarity from the base to the tip of each emitter. An electrode (e.g., electrode 125) can be positioned downstream relative to the array of emitters and a power source (e.g., power source 130) can provide a voltage to the array of emitters with respect to the electrode.

[00045] Figure 3 shows a schematic for a method for manufacturing a porous metal electrospray emitter, according to an illustrative embodiment of the invention. Single emitters (e.g., emitter body 105 as shown in Figure 2) or arrays of emitters (e.g., ID or 2D arrays) can be manufactured from porous metallic substrates using micro fabrication techniques, such as photolithography and electrochemical etching. Porous metal emitter(s) can be microfabricated using electrochemical etching with a polyimide film as a masking layer. A method for manufacturing the emitters can include the following steps: (1) filling a porous substrate with positive photoresist (Step 300), (2) applying a layer of polyimide to the sample (e.g., the porous substrate with the photoresist) (Step 310), (3) applying a layer of positive photoresist on the polyimide and exposing the photoresist to transfer the intended geometry (Step 320), (4) developing the photoresist (e.g., to remove the exposed photoresist) and etching the polyimide (Step 330), (5) removing the photoresist (e.g., leaving only the polyimide mask defined by the intended emitter geometry) (Step 340), and (6) etching the sample to form the emitter/emitter arrays (Step 350). The method can also include

processing and/or treating a tip of each emitter to vary a pore size between the base and the tip of the emitter.

[00046] A method for manufacturing the emitters can include the step of providing a porous metal substrate 360. The emitter body can be formed from a porous metal compatible (e.g., does not react or result in electrochemical decaying or corrosion) with an ionic liquid or room temperature molten salt. For example, tungsten sheets (e.g., porous tungsten sheets with a 0.25 mm thickness and 2 micron porosity from American Elements, Los Angeles, CA) can be cut into 1 cm by 2.5 cm pieces using a diesaw (e.g., Disco Abrasive System Model DAD-2H/6T from DISCO, Tokyo, Japan) and cleaned in acetone followed by isopropanol. Other porous metals compatible with ionic liquids and room temperature molten salts can be used as well. For example, the porous metal can be nickel, magnesium, molybdenum, titanium, or any combination thereof. In some embodiments, a unitary substrate of a porous metal can be used to form more than one emitter (e.g., an emitter geometry pattern that can be used to form an emitter array). The use of porous metal results in the increased capillary flow capacity provided by the volumetric porosity of the emitter substrate. The emitters can be manufactured from one or more substrates to form one or more flat emitters (e.g., needles).

[00047] The porous metal substrate can be developed to form a sample that includes porous metal substrate 360 (e.g., porous tungsten substrate) with the pores blocked (Step 300), for example, with a photoresist 370. The porous metal substrate can be filled with photoresist 370 (e.g., Shipley 1827 positive photoresist) and the substrate exposed (e.g., both sides) with UV light to block pores of the porous metal to form the sample. In some embodiments, the substrate can be allowed to soak in the photoresist 370 (e.g., for 20 seconds). The sample (e.g., the porous metal substrate with the photoresist) can be spun for 60 seconds starting at 700 rpm and increased to 1700 rpm with an acceleration of 200 rpm/s. The sample can then be baked by heating on a hotplate for 20 seconds at 7O ⁰ C followed by 30 seconds in an oven at 90° and 30 seconds at 130°. In some embodiments, both sides of the sample can be exposed using a broadband aligner (e.g., a Karl Suss MJB3 from Suss- MicroTec, Waterbury Center, CT) for 150 seconds and immersed in a developer (e.g., a high pH solution developer such as MF-319 to "wash" away the material to be eliminated upon UV exposure) until both surfaces are cleared of photoresist. A broadband aligner can be used in microfabrication to transfer a pattern to a photoresist-coated substrate by shining UV light. If no pattern is to be applied, the UV light can be used to produce the decay on exposed

surfaces. The photoresist 370 can be left substantially filling the bulk of the porous media to prevent polyimide from entering the pores. The samples can be cleaned in deionized (DI) water and dried.

[00048] In some embodiments, the surface of the porous metal 360 can be blocked by the uniform deposition of mono-layers of a compatible metal using Chemical Vapor Deposition (CVD) (e.g., thermal evaporation technique involving boiling and depositing a material on to a relatively colder surface, such as, for example, depositing the compatible metal on to the porous substrate). Mono-layers of a compatible metal using CVD can be deposited instead of soaking the porous metal substrate in photoresist polymers. One benefit is that pores can be substantially clear of potential contamination that could hurt the etching process and electrospray operation. A "compatible metal" can be a metal that is compatible with ionic liquids and the porous substrate material (e.g., does not react or result in electrochemical decaying or corrosion). For example, if the porous metal substrate 360 is a porous tungsten substrate, the surface of the porous tungsten substrate can be blocked by the uniform deposition of mono-layers of tungsten using CVD.

[00049] The method can also include the step of adding a layer of polyimide 380 (e.g.,

PI2556 polyimide from HD Microsystems) to a first side of the sample (e.g., the porous metal substrate 360 with the pores blocked with the photoresist 370) (Step 310). The sample can be prebaked to drive off solvents. Polyimide 380 can be used as the masking material for its resistance to Sodium Hydroxide and ability to be precisely patterned using standard photolithography techniques. In some embodiments, a 1.5 μm layer of polyimide 380 is spun onto one surface of the sample. The sample can be prebaked by pooling the polyimide 380 on the surface for 10 seconds, spun at 500 rpm for 5 seconds and slowly ramped up to 1300 rpm and spun for 50 seconds. The polyimide 380 can be heated on a hotplate at 55° for 30 seconds and 70° for 30 seconds followed by oven bakes at 90° for 60 seconds and 130° for 60 seconds. The gradual heating protocol employed can limit the amount of holes in the polyimide 380 caused by gas trapped in the bulk of the porous media escaping during rapid heating.

[00050] The method can also include the step of applying photoresist 370 to the first side of the sample (e.g., porous metal substrate with the blocked pores and including a layer of polyimide) (Step 320). A layer of photoresist 370 can be applied on top of the polyimide 380 (e.g., the layer of polyimide 380 applied to the porous metal substrate 360 in Step 310

above). In some embodiments, a layer of photoresist 370 having a thickness of about 5 μm is spun onto the polyimide 380. The sample can be heated at 70° for 30 seconds on a hotplate and 130° for 90 seconds in an oven. A transparency mask (e.g., photolithography transparencies from Page Works, Cambridge, MA) can be applied to the first side of the sample and exposed with a UV light to form the emitter geometry pattern 390 and exposed parts of positive photoresist 400.

[00051] The sample can be developed to transfer the emitter geometry pattern 390 to the sample. The sample can be developed to remove the exposed positive photoresist 400 from the sample (Step 330). The exposed parts of the positive photoresist 400 can be removed to etch the underlying polyimide 380 (e.g., to remove the portion of the polyimide 380 covered by exposed parts of the photoresist and leave the portion of the polyimide covered by the unexposed photoresist), thereby transferring the desired emitter geometry pattern 390. In some embodiments, samples are exposed for 120 seconds and developed in MF-319 until the pattern is transferred to the polyimide.

[00052] The method can also include the step of cleaning photoresist off the sample

(e.g., cleaning off the layer of the unexposed photoresist from Step 320) and then curing the sample (Step 340). Curing the polyimide 380 (e.g., the remaining polyimide defined by the emitter geometry pattern 390) in an oven hardens the polyimide 380 against the electrochemical etch chemistry. The samples can be immersed in acetone for 1 hour in an ultrasonic cleaner to remove the photoresist 370 from the surface and the bulk (e.g., to remove the photoresist 370 that was unexposed from Step 320 and also the photoresist 370 that filled the pores of the porous metal substrate 360 in Step 300). The samples can be baked in an anneal furnace to partially cure the polyimide 380 using the following temperature profile: a slow ramp rate from room temperature to 150°C, hold at 150° for 10 minutes then ramp up to 200° and hold for 10 minutes in nitrogen, then a ramp up to 240° and hold for 1.5 hours in nitrogen followed by a slow cool down period.

[00053] The sample can be then electrochemically etched to form the emitter geometry

390 (Step 350). The sample (e.g., a unitary substrate of porous metal that has undergone the Steps 300-340 above) can be etched to remove excess porous metal 360 to form the desired emitter geometry 390 (e.g., one or more emitters where each emitter has a base at the first end of the substrate and a tip at the second end of the substrate). The sample can be etched, for example, in Sodium Hydroxide until the excess porous metal 360 (e.g., excess tungsten or

other metal) is removed to shape the emitter(s) according to a desired geometry. The emitter(s) can be microfabricated by etching the sample to remove excess porous metal to form, for example, one or more conical shaped emitters or a one or more flat ribbon-like needles/emitters. The remaining polyimide 380 can be then removed, thereby providing the porous metal emitter array with the desired emitter geometry pattern 390.

[00054] A tip of an emitter, or an individual emitter in an emitter array, can be processed and/or treated to vary a pore size between the base of the emitter body to a tip of the emitter body. In some embodiments, the smallest pores (e.g., relative to the other pores in the emitter) are near the emitter tips. The emitter body can be manufactured to have a pore size gradient that decreases from a first end of the body to a second end of the body (e.g., the pore size becomes smaller towards the second end/tip of the body, so that the sizes of the pores at the second end/tip are smaller than the pore size at the first end/base of the body). The pore size gradient allows the ionic liquid to be continuously transported through capillarity from the first end of the body to the second of the body (e.g., from the base of the emitter to the tip of the emitter).

[00055] A nano/meso porous layer of a compatible, electrically conductive material

(e.g., zinc on porous tungsten) can be applied to the surface substantially near/around the tip of each emitter to vary a pore size (e.g., to form smaller pores at the tip relative to the base). The size of the pores in the layer of the compatible material can be substantially smaller than the size of the pores in the porous substrate (e.g., the porous emitter). A "compatible metal" can be a metal that is compatible with ionic liquids and the porous substrate material (e.g., does not react or result in electrochemical decaying or corrosion). A compatible metal (e.g., zinc for porous tungsten) can be deposited through thermal evaporation. The compatible metal (e.g., zinc) can form aggregates over the porous metal emitter (e.g., porous tungsten). In some embodiments, a layer about 1-5 microns thick of compatible metal can be deposited. Carbon nanotubes can also be attached to the surface of the emitter substantially near/around tip of each emitter to form the pore size gradient (e.g., to form smaller pores at the tip relative to the base). Carbon nanotubes can be deposited on the surface (e.g., at or substantially near the tip of the emitter) forming a relatively well-organized porous "forest." In both cases, the introduction of dissimilar porosities for preferential flow (e.g., pore sizes smaller at the tip than the base of the porous metal emitter) facilitates liquid transport to the emission sites (e.g., the tip of the emitter).

[00056] Traditional ion sources using normal solvents (with non-zero vapor pressures) do not, in principle, use controlled pore variation since the liquid/vapor interface is in equilibrium (e.g., water with water vapor) and is convected outwards through evaporation, including inside the pores. Porous metal electrospray emitters, however, have no preferential direction for convection since there is no thermal evaporation from ionic liquids. There is only ion evaporation, but for it to occur, the liquid is transported to the tips through capillarity (e.g., capillary forces).

[00057] Figure 4 shows a schematic of a setup 401 for electrochemical etching, according to an illustrative embodiment of the invention. As noted above, emitters can be manufactured from electrochemically etched porous metal substrates (e.g., porous tungsten) with a polyimide layer acting as an etch mask. Isotropic etching (e.g., step 350 in Figure 3) can be performed to form emitter geometry. The masked sample 410 (e.g., the sample from Step 340) can be placed into a container filled with an etchant solution 420 (e.g., IN sodium hydroxide (NaOH)). An electric potential (e.g., DC electric potential) can be applied using a power source 429 between the sample 410 and a cathode 430 (e.g., a stainless steel cathode) to initiate the etching process. The etching can be performed, for example, in a glass beaker 440 with a circular cathode surrounding the piece. To aid in the formation of even tips and to enhance the etching rate, the porous metal sample 410 can be removed periodically and immersed in an ultrasonic cleaner to clear the surface of the residue and to remove bubbles that form on the surface. The etching can also be carried out in a uniform flow of etchant, which can reduce the effect of eddies and bubble formation on the etch. Following the completion of the etch, sodium hydroxide can be rinsed off the sample in DI water. The remaining polyimide mask (e.g., the remaining polyimide mask defining the emitter geometry as described in Steps 340 and 350 in Figure 3 above) can be removed in Piranha (e.g., 4:1 mixture of sulfuric acid and hydrogen peroxide). Following a rinse in DI water, the emitters can be blown dry with nitrogen.

[00058] Figure 5 shows a schematic of a porous metal electrospray emitter array 500, according to an illustrative embodiment of the invention. The emitters 510A-510E can be manufactured to have emitter spacing 520 of less than about 1 mm. Each emitter can have a pore size gradient that decreases from the base 530A-530E of the emitter to the tip 540A- 540E of the emitter 510A-510E. The ionic liquid can be continuously transported through capillarity (e.g., capillary forces) from the base 530A-530E to the tip 540A-540E of each

emitter 510A-510E. In some embodiments, any one of emitter(s) 510A-510E can have a radius of curvature of about 10 μm to about 20 μm in the horizontal direction (e.g., along the y-axis) and a radius of curvature of about 2 μm to about 3 μm in the vertical direction (e.g., along the x-axis). The emitter array 500 can include a plurality of emitters 510A-510E microfabricated from a porous metal. The porous metal can be compatible (e.g., does not react or result in electrochemical decaying or corrosion) with an ionic liquid or room- temperature molten salt. The emitters can be formed by electrochemical etching. In some embodiments, the porous metal is tungsten, nickel, magnesium, molybdenum, titanium, or any combination thereof.

[00059] Figure 6 is a drawing of a porous metal electrospray emitter array assembly

600, according to another illustrative embodiment of the invention. A plurality of emitters (e.g., two or more emitters) can be grouped together to form a thruster. The typical thrust of a single emitter/needle operating in the ionic mode is on the order of about 0.05 μN/μA to about 0.1 μN/μA, therefore emitters can be grouped to produce as much current as possible (e.g., for different space applications). A thruster assembly can include emitter sheets 620A- 620C (e.g., where each sheet includes one or more one emitters) together to create a 2D array of emitters. The emitters can be placed relative to an extractor 630, resulting in the generation of an ionic beam.

[00060] In this embodiment, the ion source 600 is a thruster that includes a set of 3 flat needle arrays 620A-620C, each containing up to 18 individual emitters giving a maximum of 54 emitters with a tip to tip separation of about 1 mm. The thruster 600 provided an emitter density of a little under 0.5 tips per mm ² . In this embodiment, individual emitter sheets 620A-620C are clamped in place between two bars 640 (50 x 7.9 x 7.9 mm stainless steel). Emitter sheet separation can be provided by plates 641 (e.g., 1.5 mm inch thick stainless steel plates 165 cut by a waterjet).

[00061] Extractor 630 can be made from stainless steel (e.g., a 0.635 mm thick stainless steel sheet). In this embodiment, the individual extractor slits 650 are 1 mm wide which gives clearance for a beam spreading half angle of 51 degrees when the emitter tips are just touching the extractor slit plane. The extractor 630 can be attached to the holder bars 670 using fastening mechanism 660 and 670 (e.g., two polycarbonate #6-32 screws with two polyethylene spacers). The combination of screws 660 (e.g., or other similar fastening device) and spacers 670 can provide electrical insulation between the extractor 630 and

emitters 620A-620C and can inhibit the liquid fuel from migrating to the extractor and causing a short.

[00062] The thruster assembly 600 can provide precise alignment between the emitter sheets 620A-620C and the extractor grid 630 to reduce beam impingement. The thruster assembly 600 can provide adequate insulation (e.g., electrical and fluidic insulation) between the extractor 630 and emitters 620A-620C to reduce the risk of electrical shorting. The assembly 600 includes materials that are compatible (e.g., does not react or result in electrochemical decaying or corrosion) with ionic liquids for long periods of time. The thruster assembly 600 is easy to assemble, to reduce the risk of breaking emitters.

[00063] Figures 7A-7B show graphs 700 and 710 show time of flight measurements for a porous metal electrospray emitter array. Plots 700 and 710 show the normalized intensity of a beam of ions generated by a porous metal Electrospray emitter array, as a function of time. The emitters tested used EMI-BF4 (3-ethyl-l-methylimidazolium tetrafluoroborate) as the Ionic Liquid. EMI-IM (l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) can also be used as the ionic liquid. The time-of-flight technique was used to determine the composition of the emitted beam on a single flat needle array containing 6 emitters. The time of flight curves for positive (1915.28 V) and negative (-1898.33 V) emission are shown in Graphs 700 and 710, respectively. Time-of-flight mass spectrometry shows that the emitted beam is purely ionic and includes two species of ions in both positive and negative modes of operation. The contribution of each ion species to the total current can be calculated by looking at the relative changes in measured current during the time-of-flight tests. The lack of an elongated tail following the steps indicates that there are no droplets contributing to the emitted current. Operating an electrospray source in a mixed droplet-ion regime could be very costly in terms of efficiency and specific impulse.

[00064] In Figures 7A-7B, the time it takes for particles to travel a known distance was measured and then a charge to mass ratio was calculated based on the velocity. From the drift time measured for the particles to travel between the given distance between the electrostatic gate and the detector (L = 751.57mm) the specific charge ratio and subsequently the mass of the species can be calculated using EQN 1.

where "L" is the drift distance in the time-of-flight spectrometer, "m" is the particle mass, "q" is the particle electric charge, "ti" is the time of flight over drift distance, and "φB" is the on-axis accelerating potential. As an approximation, the on-axis accelerating potential can be taken to be equal to the extraction potential. In reality, it can be up to 7 eV lower. The results are tabulated in table 1.

Table 1. Emitted Beam Composition

[00065] Figures 8A-8B are graphs 800 and 810 showing thrust measurements for the porous metal electrospray emitter array containing 6 emitters from Figures 7A-7B. Graphs 800 and 810 show the measured thrust as a function of extraction potential for a negative extraction voltage range and a positive extraction voltage range, respectively. Thrust measurements were conducted at the Busek Company, Natick, MA using a torsional balance capable of measuring sub micro newton forces. Porous metal electrospray emitters have been shown to support an increase in current of over an order of magnitude as compared to solid cylindrical emitters. The results show that the thruster produced from about 0.82 μN to about 2.33 μN in the -1282 V to -2088 V negative extraction voltage range 800 and from about 1.08 μN to about 5.67 μN in the 1391 V to 2437 V positive extraction voltage range 810. This corresponds to a thrust per emitter tip of about 0.048 μN at -2088 V and about 0.116 μN at 2437 V. The leveling off of thrust in the negative mode can be due to the thruster approaching the limit of its ability to transport liquid to the tip.

[00066] Figures 9 A-9B show current as a function of extraction voltage measurements for the porous metal electrospray emitter array containing 6 emitters from Figures 7A-7B. Graphs 900 and 910 show the measured current in a beam of ions generated by the porous metal Electrospray emitter array as a function of extraction voltage for a negative extraction voltage range and a positive extraction voltage range, respectively. Plot 940 is the current measured/lost in the extractor (e.g., electrode), plot 930 is the current measured in the beam of ions and plot 920 is the total current collected. Figures 10A-10B show the percentage of

current for the same emitter array. Graphs 1000 and 1010 show the percentage of total current lost to the extractor as a function of extractor voltage for a negative extraction voltage range and a positive extraction voltage range, respectively. A small fraction of the beam current (about 10% - 20%) was lost to the extractor which is due to beam impingement. The extractor geometry can be changed aligned to minimize the current lost to the extractor. Extractors can be, for example, positioned a distance about 1 emitter height away from the tip and can be aligned through the fabrication of alignment features on the substrates. The extractor thickness can also be reduced to minimize current lost to the extractor.

[00067] The beam current can be extracted using the following equation:

where F is the measured thrust and IB is the Electrospray beam current. The specific charge ratio can be calculated as described above in EQN. 1 for the time of flight measurements. In addition, there can exist some beam current unaccounted for in the extractor current and collected current, due to the effect of secondary electrons caused by the high energy ions hitting the extractor and the collector. The collector can be biased to trap the secondaries and reduce this effect.

[00068] Figures 1 IA-I IB show the specific impulse for the porous metal electrospray emitter array containing 6 emitters from Figures 7A-7B. Graphs 1100 and 1110 show the specific impulse measured as a function of extraction voltage for a positive extraction voltage range and a negative extraction voltage range, respectively. Plot 1120 charts the measured specific impulse and plot 1130 charts the maximum specific impulse. In the negative polarity regime 1110, the emitters produced up to about -57.17 μA in current and a thrust of up to about 2.33 μN, yielding a specific impulse (ISP) of about ISP of about 2000 to 3000 seconds. In the positive polarity regime 1100, the emitters produced up to about 69.84 μA in current and a thrust of up to about 5.67 μN, yielding an ISP of about 3000 to 5000 seconds.

[00069] While the invention has been particularly shown and described with reference to specific illustrative embodiments, it should be understood that various changes in form and detail may be made without departing from the spirit and scope of the invention.