Patent Publication Number: US-10334714-B2

Title: Atom and ion sources and sinks, and methods of fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 62/403,970 titled “ATOM AND ION SOURCES AND SINKS, AND METHODS OF FABRICATING THE SAME,” filed on Oct. 4, 2016, which is herein incorporated by reference in its entirety for all purposes. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under grant No. HR0011-14-C-0115 awarded by DARPA. The U.S. government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     Various aspects and embodiments disclosed herein relate generally to solid state atom and ion sources and sinks and to methods of fabricating the same. 
     BACKGROUND 
     Some existing approaches to introducing atoms and ions (e.g., alkali atoms and ions) into an environment from a source of same involve: heated chemical reactions using, for example, alkali sources sold by SAES Getters S.p.A. of Milan, Italy (“SAES”) or Alvatec Alkali Vacuum Technologies GmbH of Althofen, Austria (“Alvatec”); using solid state atom sources; pulsed laser ablation of metal in a vacuum system; and using ovens filled with alkali metal. One example of a solid state atom source is described in U.S. Pat. No. 8,999,123. These exemplary atom and ion sources may be used to provide a controlled partial pressure of atoms in atomic sensor systems, such as atomic clocks, atomic magnetometers, and cold atom inertial systems (e.g., gyroscopes and accelerometers). Moreover, ion beams may be used to provide thrust for spacecraft, in ion beam etching, and to source ions to ion-traps for atomic sensors. 
     The exemplary approaches for producing atoms and ions described above suffer, however, from several drawbacks. For example, the products sold by SAES and Alvatec typically draw large amounts of current and power, are heated to a high temperature, and produce magnetic fields (from the high currents), which are all undesirable traits for most atomic sensors. In addition, undesirable gasses are produced as a by-product of use with some of these alkali sources. 
     Existing solid state atom sources often use shadow masked, evaporated electrodes that are inefficient due to large line-width and a low density of triple-phase boundaries (TPBs)—i.e., a low density of regions where a body of the solid state ion-conducting atom source, electrodes, and an environment into which the atoms are to be released or from which atoms are to be absorbed meet. Currently, shadow masking of electrodes can only produce wide metal fingers having widths of at least about 100 micrometers or more that trap most of the atoms to be produced in the body of the solid state atom source below the electrodes, resulting in low current efficiency. The resulting current conversion efficiency based on atoms produced per electron flowing through the system is generally less than 1%. Narrow slot shadow masks can be fabricated by, e.g., laser machining of sheet metal, or using microelectromechanical systems (MEMS) etching techniques. However, these techniques typically do not achieve interpenetration of the metal and ion-conducting phases. As such, adhesion of the metal electrodes is not optimized. 
       FIG. 1  shows an exemplary prior art system  10  including a solid state source  12  with metal finger electrodes  14  produced by shadow masking. The source  12  also includes a copper contact  16  that is connected to the metal finger electrodes  14  and can be connected to a voltage source. The width of the narrow metal electrode finger electrodes  14  is limited to about 130 micrometers (130 μm) by available shadow mask technology. The pictured source  12  has a diameter D of 12 millimeters (12 mm). 
       FIG. 2  is a schematic cross-sectional view of a system  10  as shown in  FIG. 1 .  FIG. 2  shows a metal finger electrode  14 , which serves as a cathode. The metal finger electrode has a width of w m . The metal finger electrode  14  extends over a solid state ionic conductor  18 , which extends over an anode  20 . In operation, mobile ions, such as rubidium (Rb) ions within the solid state ionic conductor  18 , migrate towards the metal finger electrode  14  under the influence of an electric field, and accumulate in a region  22  adjacent to the metal finger electrode  14  where they are neutralized. The region  22  extends beyond the edges of the metal finger electrode  14  by a distance of t sc  on each side of the metal finger electrode  14 . Atoms that arrive near the edge of the metal finger electrode  14  can evaporate into the surrounding environment, which is often a vacuum. These edge regions form TPBs  24  where the electron conductor (in this case, the metal finger electrode  14 ), ionic conductor  20 , and empty space, also referred to herein as voids or pores, meet. Most of the mobile atoms arrive under the wide metal finger electrode  14  and are trapped due to limits on the width of the metal finger electrode  14 . Furthermore, a continuous layer of the alkali element (e.g., Rb, Cs) under the metal finger electrode  14  can result in failure of the system  10 , particularly if it is exposed to air and/or moisture during or after operation. 
     An alternative to forming metal finger electrodes on the surface of an ionic conductor with a shadow mask to fabricate an ion/atom supply/sink system is to use photolithography to create narrower interconnected lines by liftoff and/or etching. However, these surface lines generally still suffer from poor adhesion to the ionic conductor. Furthermore, many fast ionic conductors are incompatible with photoresist and developer chemistries or with processes including photolithography for defining electrodes on surfaces of the ionic conductors, since the ionic conductors, such as ceramic ion conductors, are hygroscopic. 
     Pulsed laser ablation typically requires the addition of a high power pulsed laser to the system, and oven sources generally consume power and cannot be easily switched on and off. 
     Accordingly, there is a need for an improved electrode system for solid state atom and ion sources and sinks. 
     SUMMARY 
     According to one aspect of the present disclosure, a bi-directional device is provided for generating or absorbing atoms or ions. In some embodiments, the bi-directional device comprises a solid-phase ion-conducting material, the solid-phase ion-conducting material including an element selected from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal; a first electrode positioned on a first surface of the solid-phase ion-conducting material; a second electrode positioned on a second surface of the solid-phase ion-conducting material; a plurality of triple phase boundaries, each triple phase boundary located at an interface between the solid-phase ion-conducting material and the first electrode; and a density of the triple phase boundaries in the range of about 10 4  m/m 2  to about 2×10 7  m/m 2  on the first surface of the ion-conducting material. 
     In some embodiments, the first electrode covers less than 10% of the first surface. 
     In some embodiments, the first electrode covers less than 3% of the first surface. 
     In some embodiments, the first electrode includes a plurality of contiguous ion-conducting particles disposed on the first surface, and the plurality of contiguous ion-conducting particles leave contiguous interstitial spaces. 
     In some embodiments, a largest dimension of each interstitial space is between about 0.1 microns and about 10 microns. 
     In some embodiments, the first electrode is positioned in a plurality of grooves in the first surface of the solid-phase ion-conducting material. 
     In some embodiments, the second electrode comprises one of silver and copper. 
     In some embodiments, the solid-phase ion-conducting material is selected from a material capable of generating or absorbing an atom or an ion. 
     In some embodiments, the device further comprises a temperature control device operatively connected to the solid-phase ion-conducting material. 
     According to another aspect of the present disclosure, a method is provided for generating or absorbing atoms or ions. In some embodiments, the method includes connecting a bi-directional device to a voltage source, the bi-directional device being capable of generating or absorbing atoms or ions, the bi-directional device comprising a first electrode positioned on and covering less than 10% of a first surface thereof and a second electrode positioned on a second surface thereof; determining whether to generate or absorb atoms or ions; selectively applying a voltage of a correct polarity to the first electrode of the bi-directional device in response to the step of determining whether to generate or absorb ions. 
     In some embodiments, the bi-directional device has a conversion efficiency of between one atom and five atoms per 10 electrons flowing through the first electrode. 
     In some embodiments, the method further comprises determining a desired partial pressure of atoms in an atomic sensor system; sensing a partial pressure of atoms in the atomic sensor system; and controlling the voltage to release atoms into or to absorb atoms from the atomic sensor system based on the sensed partial pressure of atoms in the atomic sensor system to achieve the desired partial pressure. 
     In some embodiments, the method further comprises directing the atoms to provide thrust for a vehicle. 
     In some embodiments, the method further comprises ion beam etching a surface of a workpiece, wherein controlling the voltage causes the bi-directional device to release ions. 
     According to another aspect of the present disclosure, a method of fabricating a bi-directional device for generating or absorbing atoms or ions is provided. In some embodiments, the method comprises selecting a solid-phase ion-conducting material comprising a material from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal; positioning a first electrode on a first surface of the solid-phase ion-conducting material, the first electrode having a plurality of triple phase boundaries, each triple phase boundary located at an interface between the solid-phase ion-conducting material and the first electrode, and a density of the triple phase boundaries in the range of about 10 4  m/m 2  to about 2×10 7  m/m 2  on the first surface of the ion-conducting material; and positioning a second electrode on a second surface of the solid phase ion-conducting material. 
     In some embodiments, the first electrode covers less than 10% of the first surface. 
     In some embodiments, positioning the first electrode on the first surface of the solid-phase ion-conducting material comprises: creating grooves within the first surface; and positioning an electrically conductive material within the grooves. 
     In some embodiments, selecting the solid-phase ion-conducting material comprises selecting a ceramic material and the method further comprises firing the ceramic material. 
     In some embodiments, the method further comprises removing a first portion of the electrically conductive material extending above an upper surface of the solid-phase ion-conducting material after positioning the electrically conductive material within the grooves such that a second portion of the electrically conductive material remains in the grooves. 
     In some embodiments, positioning the first electrode on the first surface of the solid-phase ion-conducting material comprises positioning a mixture of an ion-conducting powder and an electron-conducting powder on the first surface. 
     In some embodiments, method further comprises sintering the mixture onto the ion-conducting material. 
     In some embodiments, creating grooves within the first surface comprises: molding grooves into the first surface; and firing the solid-phase ion-conducting material. 
     In some embodiments, selecting the solid-phase ion-conducting material further comprises: selecting a first ceramic material having a first grain size and a second ceramic material having a second grain size; and positioning a layer of the second ceramic material on a layer of the first ceramic material. 
     In some embodiments, selecting the first ceramic material comprises selecting β″ alumina. 
     In some embodiments, selecting the second ceramic material comprises selecting β″ alumina. 
     In some embodiments, the method further comprises firing the first ceramic material and the second ceramic material. 
     In some embodiments, positioning the first electrode further comprises disposing a metal layer on the second ceramic material after firing the first ceramic material and the second ceramic material. 
     In some embodiments, disposing the metal layer comprises disposing the metal layer in a grid pattern by one of shadow masking, screen printing, and aerosol jet printing. 
     In some embodiments, disposing the metal layer further comprises disposing the metal layer at an angle relative to a line normal to an upper surface of the solid-phase ion-conducting material. 
     In some embodiments, selecting the solid-phase ion-conducting material further comprises depositing a ceramic material via electrophoresis over a carbon mold. 
     In some embodiments, the method further comprises sintering the ceramic material by heating the ceramic material and the carbon mold in an oxidizing atmosphere; and removing the carbon mold. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
         FIG. 1  shows an exemplary prior art electrode system; 
         FIG. 2  shows a schematic cross-sectional view of the system of  FIG. 1 ; 
         FIG. 3  shows a solid state atom or ion source with a porous cathode; 
         FIG. 4  shows an ion source/sink with a porous layer on both an upper and lower side; 
         FIGS. 5A-5H  show steps of a method of forming fine featured metal conductive traces in a glass body; 
         FIG. 6  is a flowchart of a method of forming a solid state device; 
         FIG. 7  shows a cross-sectional view of an ion-conducting bi-directional device; 
         FIG. 8  shows a cross-sectional view of a portion of an ion-conducting ceramic with a porous surface layer; 
         FIG. 9A  shows a perspective view of a MEMS shadow mask; 
         FIG. 9B  shows a top view of a MEMS shadow mask; 
         FIG. 10  shows a grid electrode deposited on a porous surface layer device; 
         FIGS. 11A-11F  show steps of a method of electrophoretic deposition of ceramic on a carbon mold to form fine featured grooves in the ceramic; 
         FIG. 12  is a flowchart of a method of generating or absorbing ions; and 
         FIG. 13  is a flowchart of a method of fabricating a bi-directional device for generating or absorbing ions. 
     
    
    
     DETAILED DESCRIPTION 
     Various aspects and embodiments disclosed herein feature an electrode system for solid state atom or ion sources and sinks. In one embodiment, a porous electrode region that includes or consists of a mix of an electron conductor, an ion conductor, and connected porosity is employed. The porous region or porous layer may be formed on one side or two opposing sides of the ion conductor. In various embodiments, a device disclosed herein improves upon currently available solid state atom or ion sources and sinks, as it has improved current efficiency, and electrodes which exhibit improved adhesion to an ion conductor, and improved durability as compared to currently available solid state atom or ion sources and sinks. The deficiencies of currently available solid state atom or ion sources and sinks are overcome as a result of an increased density of TPBs and the inter-connectedness of the ion and electron conductors in the device. 
     Aspects and embodiments of the device disclosed herein feature several advantages over current technology, as it allows for the generation and absorption of alkali metals at low power, low current, and low temperature. For example, aspects and embodiments of the device disclosed herein may operate at a temperature below about 200° C. Additionally, aspects and embodiments of the device disclosed herein are compatible with high vacuum systems. In some embodiments, the device can quickly generate or absorb alkali atoms or ions in vacuum systems, presenting a speed advantage over atom or ion sources and sinks currently available on the market. 
     Aspects and embodiments disclosed herein improve upon currently available solid state atom or ion sources and sink systems, such as those described with reference to  FIGS. 1 and 2 , by employing micro-structured, inter-penetrating electron-conducting and ion-conducting phases that increase reliability and adhesion between the electron-conducting and ion-conducting phases. Simply decreasing the width of the metal fingers of the electrode illustrated in  FIGS. 1 and 2  by, for example, forming the metal finger electrode by photolithography or by utilizing a shadow mask with finer slots may increase the current efficiency of the device, but does not allow for the interlocking of the electron-conducting and ion-conductive phases, which is desirable for increasing adhesion between the electron-conducting and ion-conducting phases and durability of the device. 
     Aspects and embodiments disclosed herein may increase the current efficiency of the resulting device as compared to that of previously known devices through the use of a micro-structured porous surface layer that combines an ion conductor, an electron conductor, and voids. 
     According to one aspect of the present disclosure, a bi-directional device for generating and/or absorbing atoms or ions is provided. In some embodiments, the bi-directional device comprises a solid-phase ion-conducting material (ion-conducting material), a first electrode, such as a cathode, positioned on a first surface of the solid-phase ion-conducting material, and a second electrode, such as an anode, positioned on a second surface of the ion-conducting material. 
     Embodiments of the bi-directional device of the present disclosure can include any materials used for solid state electrochemical alkali sources and sinks. Embodiments of the bi-directional device of the present disclosure can be made of materials described in “Solid State Electrochemical Alkali Sources for Cold Atom Sensing” (J. Bernstein, A. Whale, J. Brown, C. Johnson, E. Cook, L. Calvez, X. Zhang and S. Martin, Technical Digest, Solid-State Sensors, Actuators and Microsystems Workshop, Hilton Head Island, Jun. 5-9, 2016). Materials described in U.S. Pat. No. 8,999,123 can also be used in embodiments of the bi-directional device of the present disclosure. 
     The solid-phase ion-conducting material may be any material that is useful for conducting an ion. The solid-phase ion-conducting material may be capable of generating or absorbing atoms or ions. The solid-phase ion-conducting material may be non-conductive to electrons. In some embodiments, the solid-phase ion-conducting material comprises an element selected from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal. In some embodiments, the solid-phase ion-conducting material is a doped glass. In some embodiments, the solid-phase ion-conducting material is useful for conducting Cesium (Cs) ions or Rubidium (Rb) ions, among others. In some embodiments, the solid-phase ion-conducting material is useful for conducting aluminum or mercury. 
     The solid-phase ion-conducting material may provide alkali atoms or ions, as well as atoms or ions of other elements such as alkaline earth elements (e.g., Strontium (Sr)) or rare earth elements. Alternatively or additionally, in some embodiments, the device is adapted for use as a solid state atom or ion sink. In some embodiments, the solid-phase ion-conducting material includes β″ alumina. β″ alumina is typically manufactured as a sodium compound. However, other alkali or alkaline earth or rare earth elements can be substituted for the sodium by an ion exchange process, thereby creating, e.g., Rb substituted β″ alumina or Cs substituted β″ alumina. 
     The solid-phase ion-conducting material may be between about 0.1 millimeters and about 10 millimeters thick in some embodiments, depending on how many atoms or ions a device including the solid-phase ion-conducting material is intended to release and/or absorb. The thickness of the solid-phase ion-conducting material can be selected based on the intended use of the bi-directional device. The greater the volume of the solid-phase ion-conducting material, the greater capacity it has to store ions. A bi-directional device that is intended to release and/or absorb a greater quantity of ions may include a larger volume of ion-conducting material than a bi-directional device that is intended to release and/or absorb a lesser quantity of ions. 
     Chalcogenide glasses based on sulfur, selenium, or tellurium doped with alkali-halides have high ionic mobility, and may be used in or as the solid-phase ion-conducting material. Chalcogenide glasses are also less hygroscopic than oxide glasses that have high alkali content. Sodium β″ alumina is a ceramic phase of Al 2 O 3  and sodium with high ionic mobility. Another alkali can be substituted in place of the sodium via high temperature diffusion. β″ alumina has a high ionic conductivity at a low temperature relative to other ceramics and relative to doped glasses. β″ alumina is also useful because of its durability and its availability. 
     When currents in the microamp to milliamp range are passed through chalcogenide doped with cesium, rubidium, and/or rubidium-β-alumina, the glass produces a high purity alkali by electrolysis. When the polarity of voltage supplied from a voltage source to the first and second electrodes is reversed, the doped glass can absorb alkali. The doped chalcogenide glass is useful for generating ions when a first electrode is a cathode and a second electrode is an anode. Alternatively, the doped glass is useful for absorbing ions when the first electrode is an anode and the second electrode is a cathode. 
     β″ alumina has a higher electrical conductivity than selenide glass doped with cesium or rubidium. Selenide glass doped with cesium or rubidium has an electrical conductivity that varies with the alkali concentration. The respective activation energies for the Rb and Cs doped selenide glass and β″ alumina are similar. The higher the electrical conductivity of an ion-conducting material is, the lower the voltage and material temperature that may be required to operate a device including the ion-conducting material to release or absorb a desired amount of ions or atoms and/or affect a rate of release or absorption of ions or atoms. Thus, high electrical conductivity ion-conducting materials are desirable for applications in which low voltage operation is desired. 
     The electrodes of the bi-directional device of the present disclosure may be formed of metal, carbon, or any other conductive material. The electrodes of the bi-directional device of the present disclosure may include electron-conducting material that is non-conductive to ionic species. 
     Although reference is made herein to an anode and a cathode, in some embodiments, the bi-directional device includes a first electrode and a second electrode, which each may be an anode or a cathode, depending on the polarity of the voltage applied to the first electrode and the second electrode. 
     The cathode may comprise or consist of a metal, for example, silver and/or copper or any metal or alloy that can be used in an electrode. In some embodiments, the cathode covers less than about 10% of a first surface of the solid-phase ion-conducting material though which ions and/or atoms are released and/or absorbed (a “first surface”) in a device as disclosed herein. In some embodiments, the cathode covers less than about 3% of the first surface of the solid-phase ion-conducting material. The cathode supplies electrons for neutralizing the alkali ions that migrate to the first surface of the ion-conducting material. 
     The anode may comprise or consist of a metal, for example, silver and/or copper, or any metal or alloy that can be used in an electrode. 
     In some embodiments, the anode is a second electrode that can act as a cathode when the polarity of the voltage at the first and second electrode is reversed. In some embodiments, the anode covers less than about 10% of a second surface of the solid-phase ion-conducting material in a device as disclosed herein. In some embodiments, the anode covers less than about 3% of the second surface of the solid-phase ion-conducting material. The second surface may be a surface opposite the first surface. 
     The anode may supply ions into or absorb ions from the solid-phase ion-conducting material, and may have a volume of about one quarter that of the solid-phase ion-conducting material. For example, if the solid-phase ion-conducting material has the same length and width as the anode, the anode may have a depth that is about one fourth the depth of the solid-phase ion-conducting material. In some embodiments, the solid-phase ion-conducting material may have a circular profile when viewed from above. In some embodiments, the circular profile may have a diameter in the range of from about 1 centimeter to about 10 centimeters, although embodiments disclosed herein are not limited to having these dimensions. 
     The anode may serve as an ion source for the solid-phase ion-conducting material. The size of the anode can be configured based on the needs of a particular implementation. In some embodiments, the anode can provide a mass of ions equal to about 25% of its total mass. The anode may be a solid that has a surface that is in direct engagement with the solid-phase ion-conducting material. 
     Each of the anode and the cathode may be positioned on the solid-phase ion-conducting material to allow for a durable connection to extend the life of the bi-directional device. As used in relation to positioning the anode or positioning the cathode, the term “positioning” may include any manufacturing method useful for adhering the anode and/or the cathode to the ion-conducting material. For example, positioning may include providing an anode or cathode having an electron conducting phase that interpenetrates an ion-conducting phase of the ion-conducting material to increase reliability and adhesion of the electron-conducting phase to the ion-conducting material. Positioning may include placing metal cathode or anode material in a groove in the ion-conducting material. Positioning may include depositing grains of electron-conducting material on the ion-conducting material. 
     To improve adhesion of an electrode to the solid solid-phase ion-conducting material, an adhesion layer including a thin layer, for example, a layer of less than a micrometer in thickness, of titanium or chromium can be applied between the solid-phase solid ion-conducting material and the respective electrode. In some embodiments, there is no adhesion layer between the solid-phase ion conducting material and the electrode that forms the anode. In some embodiments, there is no adhesion layer between the solid-phase ion conducting material and the electrode that forms the anode, and the anode is made of copper or silver. 
     A plurality of triple-phase boundaries is formed at an interface between the solid-phase ion-conducting material, the cathode, and the environment in a system in which the device is disposed. In some embodiments, triple-phase boundaries are located at edges of contiguous interstitial spaces defined by a plurality of contiguous conducting particles (contiguous grains) in the cathode. As the term is used herein “contiguous conducting particles” are conducting particles that are each electrically connected to one another. In some embodiments, the largest dimension (such as a length) of each interstitial space is between about 0.1 microns and about 10 microns. In some embodiments, the triple-phase boundaries are located at edges of grooves in the ion-conducting material. For example, in some embodiments, the cathode includes a plurality of indentations, such as grooves or channels, positioned on a surface of the ion-conducting material. In some embodiments, the cathode includes a metal disposed in each indentation (such as in each groove or in each channel). In such embodiments, triple-phase boundaries are formed at an interface between an outer edge of the ion-conducting material and an outer edge of the metal. 
     In some embodiments, the anode and/or cathode has a high density of triple-phase boundaries. The density of the triple-phase boundaries can be measured as a number or a total length of triple-phase boundaries per unit of surface area of the ion-conducting material. In some embodiments, the density of triple-phase boundaries can be measured as a number or total length of triple-phase boundaries per unit of length of a surface of the ion-conducting material. 
     The density of the TPBs for a square array of grooves each having width w surrounding square mesas of width a on the upper surface of the solid-phase ion-conducting material is 4a/(a+w) 2 . The TPB density for a spherical particle having a radius r, where one hemisphere of the particle is coated with a metal layer, is 2/r. 
     For example, in an embodiment of the device with a square array of grooves that are 1 μm to 5 μm wide and spaced apart by 1 μm to 10 μm, a density of triple-phase boundaries would be between 100,000 and 1,000,000 in the units of meters −1  (meters of triple phase boundaries/meter 2  of surface area of ion-conducting material). 
     In an embodiment of the device with a porous media with circular grains having a grain size between 0.1 μm and 10 μm, the ratio of perimeter to area of the grains, the density of the TPBs is between 2×10 5  and 2×10 7  meter/meter 2 . Smaller grains in a porous solid (e.g. 0.1 μm) yield a higher density of TPBs compared to the TPB density associated with micro-molding, assuming the micro-mold is limited to 1 μm dimensions or larger. In some embodiments, the density of TPBs is between 10 4  and 2×10 7  meters/meter 2 . Such TPB densities are not achievable with previously known shadow mask electrode formation technologies due to the limits on electrode width that may be formed via a shadow mask. Ion-conducting glasses useful in the devices disclosed herein are typically not compatible with methods of forming thin electrodes exhibiting acceptable adhesion to the ion-conducting glasses via photolithographic techniques as used in the semiconductor industry and thus TPB densities achievable with the devices and methods disclosed herein would not be achievable with previously known photolithographic electrode formation methods. 
     In some embodiments, the device includes a temperature control device. In some embodiments, the temperature control device senses a temperature of the ion-conducting material, the anode, and/or the cathode. In some embodiments, the temperature control device is operatively connected to the ion-conducting material. The temperature control device can adjust the temperature of the bi-directional device during operation of the bi-directional device. The temperature control device may include a resistive heater, or could be heated by a laser or other optical source. The temperature control device may include a ceramic heater and a thermocouple. Some embodiments of the bi-directional device of the present disclosure operate at temperatures below about 200° C. Some embodiments of the bi-directional device of the present disclosure operate at temperatures between about 50° C. and about 120° C. 
     Some embodiments of the bi-directional device may operate with a current between about 1 micro-amp and about 100 micro-amps. By controlling the voltage and/or current supplied to the electrodes, the output of atoms and/or absorption of atoms of the bi-directional device is controlled. 
     Another aspect of the present disclosure relates to a method of generating or absorbing ions. In some embodiments, a method of generating or absorbing ions includes connecting a bi-directional device to a voltage and/or current source. The bi-directional device is capable of releasing or absorbing atoms and ions. The bi-directional device can be any bi-directional device disclosed herein. In some embodiments, the method includes determining whether to generate or absorb atoms and selectively applying a voltage of the correct polarity to a first electrode and a second electrode of the bi-directional device in response to the step of determining whether to generate or absorb atoms. 
     Embodiments of the bi-directional device operate at low power compared to currently available solid state atom/ion supply/absorption devices. In some embodiments, the voltage source may provide a voltage of, for example, about 10 volts or less, between the anode and cathode of the bi-directional device. In some embodiments, the current through the anode and cathode may be about 7 microamps or less. 
     The bi-directional device of the present disclosure has a high conversion efficiency compared to currently available solid state atom/ion supply/absorption devices. In some embodiments, the bi-directional device has a conversion efficiency of generating or absorbing between one atom (or ion) and five atoms (or ions) per ten electrons flowing through the cathode. One atom (or ion) absorbed or released per ten electrons flowing through the cathode corresponds to a 10% conversion efficiency level. Five atoms (or ions) absorbed or released per ten electrons flowing through the cathode corresponds to a 50% conversion efficiency level. 
     Determining whether to generate or absorb atoms or ions may be based on a sensed partial pressure of atoms or ions in a contained system surrounding the bi-directional device. In some embodiments, the contained system may be a vacuum chamber that contains the bi-directional device therein. In some embodiments, the contained system may be an atomic sensor system. The method may include sensing a partial pressure of ions in an atomic sensor system, and controlling the current that is directed through the cathode to release ions into the atomic sensor system or to absorb ions from the atomic sensor system based on the partial pressure of ions in the atomic sensor system. Thus, the bi-directional device may be useful for achieving and maintaining a desired partial pressure of ions in a contained system. 
     The bi-directional device can be incorporated into various systems. In some embodiments, the method includes directing ions to provide thrust for a vehicle. The solid state device of the present disclosure is particularly useful for space vehicles, in which weight restrictions and dimensional constraints limit the weight and size of components that can be included in the vehicle. In some embodiments, the method includes controlling the current to cause the solid state ion source to release ions for ion beam etching a surface of a workpiece. 
     Another aspect of the present disclosure is related to a method of fabricating a bi-directional device for generating or absorbing ions. In some embodiments, the method includes selecting a solid-phase ion-conducting material comprising a material from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal; positioning a cathode on a first surface of the ion-conducting material, the cathode covering less than about 10% of the first surface; and positioning an anode on a second surface of the ion-conducting material. 
     In some embodiments, selecting a solid-phase ion-conducting material can include selecting a material, such as glass that is doped with an alkali metal, an alkaline earth metal, and/or a rare earth metal, as discussed above. In some embodiments, selecting a solid-phase ion-conducting material can include selecting an ion-conducting material that consists of an alkali metal, an alkaline earth metal, and/or a rare earth metal. 
     In some embodiments, selecting the solid-phase ion-conducting material includes selecting a ceramic material and firing the ceramic material. Selecting the solid-phase ion-conducting material may include selecting a ceramic material and depositing the ceramic material via electrophoresis over a carbon mold. Some methods may include heating the ceramic material and the carbon mold in an oxidizing atmosphere, thereby sintering the ceramic material, and then removing the carbon mold. 
     In some embodiments, positioning the cathode on a first surface of the solid-phase ion-conducting material includes positioning the cathode on the first surface such that the cathode covers less than about 3% of the first surface. In some embodiments, an anode may be positioned on a second surface of the ion-conducting material such that the anode covers less than 3% of the second surface. 
     Positioning the cathode on the first surface of the ion-conducting material can include one or more manufacturing method or fabrication method, as discussed above. In some embodiments, positioning the cathode on the first surface of the solid-phase ion-conducting material includes creating grooves within the first surface of the solid-phase ion-conducting material, and positioning a metal or other conductive material within the grooves. 
     The grooves may be pressed, stamped, machined, and/or manufactured using another technique. In some embodiments, creating grooves within the first surface of the solid-phase ion-conducting material includes molding the grooves into the first surface. In some embodiments, creating grooves within the first surface includes firing the ion-conducting material after molding grooves into the first surface of the ion-conducting material. In some embodiments, the grooves may be between about 1 μm and about 5 μm wide, and spaced apart by about 1 μm to about 10 μm. 
     In some embodiments, the method further includes removing a first portion of the metal after positioning the metal within the grooves of the ion-conducting material such that a second portion of the metal remains in the grooves. Removing the first portion of the metal may be performed by any appropriate method, such as polishing the metal, grinding the metal, using electrical discharge machining, or by performing another machining method. In some embodiments, the first portion of the metal is the metal that extends above an upper surface of the solid-phase ion-conducting material. In some embodiments, removing the first portion of the metal exposes the upper (first) surface of the ion-conducting material and no metal remains on the upper surface of the ion-conducting material above the grooves. TPBs remain at the edges of the grooves, where an edge of the metal extends adjacent an edge of the groove on an outer surface of the bi-directional device. 
     In some embodiments, an electrically conductive material other than or in addition to a metal can be disposed in the grooves. 
     In some embodiments, positioning the cathode on the first surface of the ion-conducting material includes positioning a mixture of an ion-conducting glass or ceramic powder and an electron-conducting powder on the first surface. In some embodiments, the method further includes sintering the mixture onto the ion-conducting material. 
     In some embodiments, positioning the cathode on the first surface of the solid-phase ion-conducting material may include selecting a first glass or ceramic material having a first grain size and a second glass or ceramic material having a second grain size, the second grain size being greater than the first grain size; positioning the first glass or ceramic material on the first surface of the ion-conducting material; and positioning the second glass or ceramic material on the first glass or ceramic material. In some embodiments of the device, the ion-conducting material comprises the first glass or ceramic material. In some embodiments, the ion-conducting material of the device comprises the first glass or ceramic material and the second glass or ceramic material. In some embodiments, the ion-conducting material of the device consists of the first glass or ceramic material and the second glass or ceramic material. The first grain size may be less than about 0.1 μm. The second grain size may be between about 0.1 μm and about 10 μm. Grain size may refer to an average diameter or maximum diameter or other characteristic dimension of the grains. The first ceramic material and the second ceramic material may each be β″ alumina. The first and second ceramic material may be provided as a green ceramic material that is then fired. In some embodiments, after firing the first ceramic material of the cathode and the second ceramic material of the cathode, a metal layer is positioned over the second ceramic material of the cathode. The metal layer may be a metal film or a metal powder. The metal layer may include any metal that is useful in a cathode. The metal layer may be positioned in a pattern of lines, in a grid pattern, or in another geometrical shape. In some embodiments, positioning the metal layer includes disposing the metal in a grid pattern by one of shadow masking, screen printing, and aerosol jet printing. TPBs are located at edges of the metal layer on the grains of the first ceramic material and edges of the metal layer on the grains of the second ceramic material. This structure yields a high density of TPBs on the bi-directional device. The grains and the metal layer are interpenetrating, so the ion-conducting phase and the electron-conducting phase are interpenetrating. This increases adhesion of the metal layer to the solid-phase ion-conducting material, and also increases the conversion efficiency of the bi-directional device because of the high density of TPBs. 
     In some embodiments, forming the solid-phase ion-conducting material may include selecting a first ceramic material having a first grain size and a second ceramic material having a second grain size, the second grain size being greater than the first grain size; forming an internal portion of a body of the solid-phase ion-conducting material with the first ceramic material; and positioning the second ceramic material on the first ceramic material. The first grain size may be less than about 0.1 μm. The second grain size may be between about 0.1 μm and about 10 μm. The first ceramic material and the second ceramic material may each be β″ alumina. The first and second ceramic material may be provided as a green ceramic material that is then fired. Firing of the first and second ceramic material may result in the first ceramic material being sintered into a substantially void-free body, while the second ceramic material forms a layer or coating including voids defined between sintered grains of the second ceramic material on the substantially void-free body. 
       FIG. 3  shows a cross section of a portion of an embodiment of a solid state bi-directional device  300  in which a cathode having a porous surface layer  302  is formed on a first side  304  of a solid state ion-conducting material  306 . The porous surface layer may include interpenetrating ion-conducting material and electron-conducting material. The electron-conducting material may be contiguous. An anode  308  is formed on a second side  310  of the solid state ion-conducting material  306 . 
     In some embodiments, the porous surface layer  302  may be present on a first surface  304  of the solid-phase ion-conducting material, as illustrated in  FIG. 3 . In other embodiments, the porous surface layer  302  may be present on both sides  304 ,  310  of an ion-conducting material. In some embodiments, a porous surface layer may be present on another ion-conducting surface in the device. 
     In one embodiment, the electron conductor of the porous surface layer covers less than about 75% of the device surface on which the porous surface layer is located. In other embodiments, the electron conductor of the porous surface layer covers less than 50%, less than 35%, less than 20%, less than 10%, less than 5%, or less than 3% of the device surface on which the porous surface layer is located. As the fraction of the surface covered by the electron conductor increases, it increasingly blocks the escape of atoms/ions (e.g., alkali) and reduces the faradaic efficiency for atom/ion (e.g., alkali) generation. Where the device is used as an alkali absorber, it is desirable to have the impinging alkali atoms land on the ion conductor. Any atom that lands on the electron conductor would first have to diffuse to a TPB to lose an electron and become ionized to enter the lattice of the solid-phase ion-conducting material. Decreasing the fraction of the surface covered by the electron conductor thereby improves the efficiency of the atom and ion sources and sinks described herein. Increasing the density of TPBs improves the efficiency of the atom and ion sources and sinks described herein. 
     By employing a porous, mixed phase electrode with small particle sizes, an improved electrode with a high density of TPB regions may be obtained. One embodiment is illustrated in  FIG. 3 . In this embodiment, the device is configured to function as either a solid state source or a solid state sink. In some embodiments, the pore size and particle size (for example, average diameter) in the porous electrode are between about 0.1 μm and about 10 μm. 
       FIG. 4  shows an embodiment of a solid state bi-directional device  400  with a first porous layer  402  and a second porous layer  404  each on a surface of an ion-conducting solid  406  that includes or consists of an ion-conducting material. In this embodiment, one surface serves as an alkali emitter, while the other surface serves as an alkali absorber. For example, the device illustrated in  FIG. 4  may serve as an electrically operated bi-directional pump for alkali ions. A voltage source  408  is connected to a lead  410  that is connected to an electron conductor embedded in the first porous layer  402 . The voltage source  408  is also connected to a lead  412  that is connected to an electron conductor embedded in the second porous layer  404 . The voltage source  408  can thus provide a current through the device  400 . 
     The leads  410 ,  412  may be made of a conductive material. Silver and copper are good conductors, and may be used in the leads  410 ,  412 . In some embodiments, the leads  410 ,  412  include inert materials that do not react with the alkali. For example, one or more lead may be made of aluminum. One or more leads may be made of gold or platinum. 
     A temperature controller  414  may be provided in thermal communication with the solid state bi-directional device  400 . The temperature controller  414  may include a source of heat, for example a resistive heater and a temperature monitor, for example, a thermocouple. A power source and circuitry for controlling the temperature controller  414  may be provided internal to the temperature controller, or remote from the source of heat and/or temperature monitor. 
     The exemplary devices depicted in  FIGS. 3 and 4  may be manufactured using one of several exemplary methods described below. These described methods of manufacture present several advantages over existing methods, as they allow for the creation of a porous cathode and/or anode with a high density of electron conductor, ion conductor, and voids (i.e., a high density of TPBs). None of the methods described below involve driving a chemical reaction by heating with very high current, power or temperature, or create unnecessary magnetic fields or impurity gasses, as do existing methods of manufacture. 
     As described below, the method of manufacturing a porous, mixed phase electrode region depends on the material of the solid state ionic conductor and its melting point. 
     Example Method 1 
     Because glasses have softening points (which crystalline ceramics do not have) generally at a low temperature, different techniques for porous layer formation are available, such as molding or hot pressing a porous metal or carbon electrode into the surface of a glass body. Porous metal electrodes can be made using well known powder metallurgy methods. 
     In one embodiment, a micro-mold is used to press an array of holes or a network of channels into a surface of an ion-conducting glass. Following a sputter or evaporation step, which deposits a metal layer on the surface of the glass, the top surface of the metal is polished off, leaving a continuous network of metal lines along the bottom and sides of the glass channels.  FIGS. 5A-5H  shows the steps involved in forming a mold, molding the glass, and forming the electrode. In  FIG. 5A , the starting mold substrate  500  is shown. In some embodiments, this is a metal plate with a polished surface (e.g., a nickel alloy). In  FIG. 5B , a layer of photoresist  502  is applied to the starting mold substrate  500 , exposed, and developed to open a pattern where the mold metal is to be electroplated. In  FIG. 5C , metal (e.g. Ni, Ni—Co or another metal or alloy)  504  is electroplated into the openings  506  in the photoresist  504 . The openings  506  are longitudinally extending openings in the end view  FIG. 5C . In  FIG. 5D , the photoresist  502  has been removed, leaving the metal  504  on the starting mold substrate  500 . The metal  504  is in the form of longitudinally extending protrusions extending upwardly from the substrate  500 . The resulting mold  508  comprises the metal  504  and the substrate  500 . In  FIG. 5E , the mold  508  is placed on a glass body  510  in a furnace for a time and temperature suitable to press the mold into the ion-conducting glass  510 . In  FIG. 5F , the mold  508  has been removed from the glass  510 , leaving longitudinally extending grooves  512  in the glass  510 . In  FIG. 5G , a metal conductive layer  514  has been deposited on the upper surface of the molded glass  510  so that the metal conductive layer  514  extends into the grooves  512 . In  FIG. 5H , a surface layer of metal conductive layer  514  that extends above the upper surface  516  of the glass  510  has been removed (e.g., by polishing) leaving a network of fine-featured metal lines  518  inside the glass grooves  512  produced by molding. This process produces a high density of fine metal lines  518  embedded in the glass, whose surface boundaries form TPBs  520  capable of emitting or absorbing alkali or other mobile metals. In this method, the longitudinally extending borders between the upper edges of the U-shaped grooves  512  and the metal lines  518  form the TPBs. 
     In some embodiments, an alternate technique for use with glass is to melt a mixture of a glass powder (frit) and an electron conducting powder to form a porous interpenetrating phase where both ionic and electronic phases are continuous while retaining porosity. The electron conducting powder may be a metal, an alloy, or carbon, among others. This mixed conductor may be formed on the surface of a solid glass disc or other shaped substrate. A flow chart describing the steps in forming a mixed phase electrode is shown in  FIG. 6 . In the method  600 , an ion-conducting body is prepared at  602 . At  604 , a powdered frit of ion-conducting glass is prepared. At  606 , the electron conducting powder is mixed with glass frit. At  608 , the mixture of frit and electron conducting powder is sintered on the glass body to form a porous electrode. At  610 , a metal anode is deposited on a side of the glass opposite the porous electrode. 
     An illustration of an exemplary device fabricated using the method  600  of  FIG. 6  is shown in  FIG. 7 . In this embodiment, the device  700  is fabricated with a non-blocking electrode  708 , typically copper (Cu) or silver (Ag). The device  700  includes a porous cathode  702  that includes a mixture of frit and electron conducting powder that has been sintered. The porous cathode  702  is positioned on a first side  704  of an ion-conducting glass  706 . An anode  708  is positioned on a second side  710  of the ion-conducting glass  706 . 
     Example Method 2 
     In some embodiments, ceramics such as β″ alumina may be used for an ion-conducting body of a device as disclosed herein. However, the melting point of β″ alumina (T≈2000° C.) is too high to conveniently hot press a metal electrode into the surface. In one embodiment, a porous surface layer is formed by appropriately choosing a particle size of the β″ alumina during formation of the green (unfired) ceramic. It is well known to ceramists that the time and temperature necessary to achieve full density from a green body (unfired ceramic) is a function of the particle size, with coarser powders taking longer to sinter to full density. 
     In this embodiment, the ion-conducting body, or substrate, of the green ceramic is made of a fine-grained powder, which sinters to near full density. A surface layer made of larger grain sizes is deposited on the fine grained layer, resulting in a porous surface layer after firing. 
     After the firing step, the electrode, or electron conductor, is created through a metal deposition step (e.g., evaporation), which deposits a continuous thin film of metal down onto the surface layer, providing a high density of micro-structured TPBs.  FIG. 8  shows an illustration of a solid, β″ alumina substrate  800  fabricated from fine-grained particles, a porous layer  802  formed of large-grained particles  804 , and an evaporated metal layer  806 . In some embodiments, the porous layer  802  is formed as thin as possible while providing for a desired electrical current to flow through the metal layer  806 . Providing for a thin porous layer  802  provides only a small volume in which water vapor or other moisture may accumulate that may need to be removed from the porous layer  802  when the device including the structure illustrated in  FIG. 8  is utilized in a vacuum chamber. A reduction in the moisture content of the porous layer  802  may reduce a pump-down time of the vacuum chamber. In some embodiments, the porous layer  802  may have an average or median thickness of between one and ten, between one and five, or between one and three particles  804 . The evaporated metal layer  806  is a contiguous layer that is formed in a pattern over the large-grained particles  804 . 
     In some embodiments, the metal is deposited in a grid pattern (e.g., in a grid of metal lines), allowing the coverage ratio, which is the ratio of the surface area coated with the metal to the entire (i.e., coated and uncoated) surface area, to be reduced to a low level, e.g., about 10% or less, while still maintaining a layer of metal thick enough to ensure electrical continuity. The metal grid may be deposited through a shadow mask, by screen printing, or written by aerosol jet printing. In the case of a shadow mask, the metal thickness may be between 0.1 μm and 5 μm. In the case of screen printing, the metal may be thicker—e.g., up to several hundred microns. In the case of aerosol jet direct writing (e.g., Optomec aerosol jet printing), the metal lines may be 1 μm to 20 μm thick and 10 μm to 50 μm wide. 
     In the embodiment of  FIG. 8 , the metal is deposited along a line  808  that is at an angle α to a normal line  810  extending from an upper surface  812  of the β″ alumina substrate  800 . By depositing the metal layer  806  along the line  808 , or another line that is at an angle to the normal line  810 , more of the upper surface of the porous layer  802  is left uncovered by the metal layer  806  than would be if the metal layer  806  were deposited along the normal line  810 . When the metal layer  806  coats a particle  804  that is in the form of a spherical grain along the normal line  810 , the entire upper hemisphere of that grain is coated with the metal layer  806 , which can lead to trapping of neutralized atoms below that metal layer  806 . When α is between 0° and 90°, some of the upper hemisphere of the spherical grain is left uncoated by the metal layer  806 , reducing the tendency of the metal layer  806  on that spherical grain to trap neutralized atoms beneath it. 
       FIG. 9A  shows a perspective view of a MEMS shadow mask  900  and  FIG. 9B  shows a top view of the MEMS shadow mask  900 . The MEMS shadow mask shown in  FIGS. 9A and 9B  may be fabricated by a process starting with a Silicon on Insulator (SOI) wafer. In one embodiment, the thin device layer is first etched to form the desired stripe pattern, and then a back-side etch is performed to open parallel openings  902  in the substrate  904 . Finally, the buried oxide layer is etched in hydrofluoric acid (HF). The advantage of this MEMS process is that much narrower openings  902  may be fabricated using MEMS processes (e.g., lines 2-5 μm wide) than by conventional sheet-metal etching (125 μm minimum slot width) or by laser cutting (50 μm minimum slot width). Alternatively, conventional etched or laser cut metal shadow masks may be utilized to form a coarser grid pattern. 
     In one embodiment, the porous side of the ion-conducting device is first metalized by evaporating or sputtering metal through the shadow mask  900  to form a first set of metal grid lines  1002 , then the mask  900  is rotated 90° and a second set of metal grid lines  1004  is deposited orthogonal to the first layer of metal grid lines  1002 , thereby forming a continuous square grid  1006 .  FIG. 10  shows one example of a grid electrode  1006  deposited on a porous ion-conducting surface layer  1008 . The porous ion-conducting surface layer  1008  is positioned on an ion-conducting material  1010 . On the opposite side of the ion-conducting material  1010 , a solid non-blocking back electrode  1012  is placed. 
     Example Method 3 
     It is well known that ion-conducting ceramics can be molded before firing, in the green state. In some embodiments, a fine featured mold can be used to form fine grooves in the unfired, ion-conducting ceramic. Such a mold may be made identically to the mold shown in  FIGS. 5A-5H . In this embodiment, fine features are formed on the mold by electroplating through a photoresist mask, forming protruding ribs which are molded into the green ceramic before firing. The process shown in  FIGS. 5A-5H  is applicable except that an additional firing step is added between step  5 F (removing the mold) and step  5 G (metallization). This firing step sinters the ceramic particles together forming a dense body, retaining the grooves that were formed by molding. 
     Example Method 4 
     In this embodiment, ceramic particles are suspended in a liquid and deposited on a conductive surface by the application of a voltage across the suspension. A fine featured conductive mold of carbon is formed that may be removed by oxidation during the sintering step. In greater detail,  FIG. 11A  shows an inert, high temperature substrate  1100 , such as alumina.  FIG. 11B  shows two layers of deposited photoresist, such as SU-8, on the substrate. The first layer of photoresist  1102  is a planar layer exposed and cross-linked everywhere. The second layer of photoresist  1104  forms a series of narrow ridges  1106  that form an interconnected network, typically a hexagonal, square, or triangular grid. The substrate  1100 , the first layer of photoresist  1102 , and the second layer of photoresist  1104  are heated in an inert, non-oxidizing atmosphere (e.g., N 2 ) at a temperature between 600° C. and 800° C. to convert the photoresist to pyrolytic carbon. As shown in  FIG. 11C , features such as posts or ridges  1106  shrink by a factor of 2-3 during the pyrolysis. 
       FIG. 11D  shows the masked off regions of the substrate with a third layer of photoresist  1108 . The masked regions will not attract ceramic deposit, whereas the open areas will grow a layer of ceramic during an electrophoretic deposition.  FIG. 11E  shows the ion-conducting ceramic  1110  deposited in the open areas, forming discs or other shapes. The ceramic  1110  grows over the fine carbon features  1106 . In  FIG. 11F , the ceramic  1110  and mold  1100  are heated to a temperature where the ceramic sinters to full density in an oxidizing atmosphere, removing the carbon layers, and leaving the ceramic bodies with fine featured grooves  1112 . These grooves can then be metalized as shown in  FIGS. 5G and 5H . 
       FIG. 12  is a flowchart  1200  of a method for generating or absorbing ions. At block  1202 , a bi-directional device is connected to a voltage source. Block  1204  includes determining whether to generate or absorb ions. Block  1206  includes selectively applying a voltage of a correct polarity to an electrode in response to the determination made at block  1204 . 
       FIG. 13  is a flowchart  1300  of a method of fabricating a bi-directional device for generating or absorbing ions. At block  1302 , the method  1300  includes selecting an ion-conducting material comprising a material from the group consisting of an alkali metal, an alkaline earth metal, and a rare earth metal. At block  1304 , a first electrode, such as a cathode, is disposed on a first surface of the ion-conducting material. A density of triple-phase boundaries formed by the first electrode and the upper surface of the ion-conducting material is in the range from of about 10 4  m −1  to about 2×10 7  m −1 . At block  1306 , a second electrode, such as an anode, is positioned on a second surface of the ion-conducting material. 
     Prophetic Example of a Bi-Directional Device 
     To test the alkali generation and absorption of a bi-directional device according to the present disclosure, the bi-directional device is placed in a vacuum chamber of a test system. The vacuum chamber may be a stainless steel cube. The vacuum chamber is pumped to a pressure of 10 −7  torr. The test system includes connections to a small ceramic heater, a thermocouple, and a voltage source to drive the bi-directional device. In some embodiments, the test system includes a connection to drive an alkali source in the chamber for demonstrating absorption of alkali from the vacuum. 
     To measure alkali absorption, a laser is tuned to an appropriate wavelength, such as a wavelength for detecting cesium or rubidium. Using a controller and a processor, absorption of the laser energy in the chamber can be analyzed. 
     A bi-directional device is placed on a ceramic heater within the chamber, and the bi-directional device is connected to a voltage source. The controller uses laser absorption to measure the partial pressure of the alkali vapor in the chamber. 
     When providing a voltage of 10 volts and a current of 7 microamps to a bi-directional device according to the present disclosure, the controller can measure an efficiency of 10% for some embodiments an efficiency of up to 50% for some embodiments. The measured alkali vapor concentration in the chamber can be reduced by 50% in a matter of minutes. In some embodiments, the measured alkali concentration in the chamber can be reduced by 90% in a matter of minutes or over a few seconds, depending on the size of the chamber. 
     A thermocouple may detect a temperature of less than 200° C. on the ion-conducting material in some embodiments. A thermocouple may detect a temperature of less than 170° C. on the ion-conducting material in some embodiments. 
     By applying a voltage of a first polarity to a first electrode and a second polarity to the second electrode, the bi-directional device can be used for absorbing ions. By applying a voltage of the second polarity to the first electrode and the first polarity to the second electrode, the bi-directional device can be used for generating ions. 
     The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. The features and functions of the various embodiments may be arranged in various combinations and permutations, and all are considered to be within the scope of the disclosed invention. Unless otherwise necessitated, recited steps in the various methods may be performed in any order and certain steps may be performed substantially simultaneously. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive. Furthermore, the configurations, materials, and dimensions described herein are intended as illustrative and in no way limiting. Similarly, although physical explanations have been provided for explanatory purposes, there is no intent to be bound by any particular theory or mechanism, or to limit the claims in accordance therewith.