Patent Publication Number: US-9903768-B2

Title: Wireless thermionic sensor package and methods of using

Description:
TECHNICAL FIELD 
     The present disclosure relates to wireless thermionic sensor packages and methods of using the same. 
     BACKGROUND 
     Extreme environments pose a challenge to sensors configured to monitor a parameter in its surrounding environment. For instance, high temperature environments, high pressure environments, and/or environments containing corrosive gases all pose challenges for designing a sensor capable of sustained operation in such extreme environments. In view of these challenges, use of conventional sensors is either not viable in such environments, or requires modification such as a large and cumbersome size, e.g. due to one or more surrounding layers to protect the sensor from the surrounding extreme environment. For example, conventional sensors for use at high temperature are often designed to mitigate the effects of high temperature upon the sensors, such as by minimizing the flow of heat from the surrounding high temperature environment to the sensor. 
     Thermionic emission is a phenomenon in which heat induces the generation and flow of electrons from a metal surface. The flow of electrons occurs when the thermal energy of an electron is greater than the binding force of the electron to the metal (i.e., the thermal energy exceeds the work function of the metal). Thermionic emission, as a phenomenon, has been utilized in conventional devices, such as in vacuum tubes, where released electrons are collected on a positively charged anode. For example, some conventional devices include a heating coil to heat a metal surface to a sufficient temperature for thermionic emission to occur. For instance, the cathode is heated up with an external power source, but the anode is not. The enclosure remains in normal low temperature ambient conditions. 
     It would thus be useful to provide a sensor that can take advantage of the surrounding environment to facilitate its function, which would be capable of sensing a wide range of relevant process conditions, such as, for example, temperature, pressure, strain, flux, or flow rate. By providing a sensor for sustained use in an extreme environment, enhanced monitoring and control may be provided for systems subject to the extreme environment. 
     SUMMARY 
     According to various exemplary embodiments of the disclosure, a thermionic sensor comprising a sensor housing at least partially defining an emission chamber in which vacuum conditions are maintained; a cathode disposed in the emission chamber; an anode disposed in the emission chamber and spaced apart from the cathode; and an electrically conductive layer disposed in the emission chamber facing the anode and cathode is provided. The thermionic sensor is configured to output a detection signal when the anode and cathode are at substantially the same temperature. 
     According to further exemplary embodiments, a sensor package comprising a substrate; a package housing disposed on the substrate and at least partially defining a package chamber in which vacuum conditions are maintained; a thermionic sensor disposed in the package chamber; and a first wireless transmission device disposed on the substrate and configured to wirelessly transmit the sensor signal to an external device, is also provided. 
     According to still further exemplary embodiments, a method of using a thermionic sensor package, the method comprising exposing the sensor package to a high-temperature environment, such that an anode and a cathode of a sensor of the sensor package are both heated to at least about 600° C.; generating a sensor signal using thermionic emission between the cathode and the anode; and transmitting the sensor signal wirelessly to an external device, is also provided. 
     As described, sensors according to various exemplary embodiments described herein use heat from a high temperature environment to naturally enable thermionic emission, which is in turn used to measure one or more parameters of the surrounding environment. The sensor may be used to sense a wide range of relevant process conditions, such as, for example, temperature, pressure, strain, flux, or flow rate. By providing a sensor for sustained use in an extreme environment, enhanced monitoring and control is provided for systems subject to the extreme environment 
     Additional objects, features, and/or advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present disclosure and/or claims. At least some of these objects and advantages may be realized and attained by the elements and combinations particularly pointed out in the appended claims, although such is not necessary to be within the scope of the disclosure and claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the claims; rather the claims should be entitled to their full breadth of scope, including equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure can be understood from the following detailed description, either alone or together with the accompanying drawings. The drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more exemplary embodiments of the present disclosure and together with the description serve to explain certain principles and operation. 
         FIG. 1  shows a side cross-sectional view of a thermionic element, according to various embodiments of the present disclosure. 
         FIG. 2  shows a side cross-sectional view of a thermionic sensor package, according to various embodiments of the present disclosure. 
         FIG. 3  shows a plot of thermionic performance of a cathode and anode pair, according to various embodiments of the present disclosure. 
         FIG. 4  shows a schematic perspective view of a thermionic temperature sensor, with internal components of the sensor revealed, according to various embodiments of the present disclosure. 
         FIG. 5  shows a bottom view of the top plate and equipotential surface of the thermionic sensor of  FIG. 4 . 
         FIG. 6  shows a side view of the thermionic sensor of  FIG. 4 , taken along line  6 - 6  of  FIG. 4 . 
         FIG. 7  shows a top view of the bottom plate, anode, and cathode of the thermionic sensor of  FIG. 4 . 
         FIG. 8  shows a plot of temperature sensor performance, according to various embodiments of the present disclosure. 
         FIG. 9  shows a schematic perspective view of a thermionic pressure sensor, with internal components of the sensor revealed, according to various embodiments of the present disclosure. 
         FIG. 10  is a bottom view of a top plate of a pressure sensor, according to various embodiments of the present disclosure. 
         FIG. 11  is a top view of a substrate of a pressure sensor, according various embodiments of the present disclosure. 
         FIG. 12  is a side cross-sectional view taken along line  12 - 12  of  FIG. 9 . 
         FIG. 13  shows the pressure sensor of  FIG. 12  when an external pressure is applied to the sensor. 
         FIG. 14  shows a plot of pressure sensor performance, according to various embodiments of the present disclosure. 
         FIG. 15  shows a schematic perspective view of an amplifier, with internal components of the amplifier revealed, according to various embodiments of the present disclosure. 
         FIG. 16  is a side cross-sectional view taken along line  16 - 16  of  FIG. 15 . 
         FIG. 17  is a top view of a substrate of the pressure sensor of  FIG. 15 . 
         FIG. 18  is a schematic view of a wireless transmission device according to various embodiments of the present disclosure. 
         FIG. 19  is a side cross-sectional view of a power device, according to various embodiments of the present disclosure. 
         FIG. 20  is a side cross-sectional view of a thermionic sensor, according to various embodiments of the present disclosure. 
         FIG. 21  is a block diagram illustrating a method of using a sensor package, according to various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments described herein relate to a wireless thermionic sensor capable of sustained use in an extreme environment. For instance, sensors according to the exemplary embodiments described herein may be used at high temperature, such as, for example, up to about 1800° C., such as about 750° C. to about 1600° C. or about 1000° C. to about 1500° C.; high pressure, such as pressures up to about 1250 psi, for example up to about 1000 psi or up to about 750 psi; and/or gaseous environments including, such as, for example, hydrocarbon, oxygen, water vapor, carbon dioxide, carbon monoxide, sulfur oxide, and/or nitrous oxide gases, wherein the gaseous environment may provide a flow rate of up to about 1750 mph, such as up to about 1500 mph or up to about 1250 mph. 
     According to one embodiment, an exemplary thermionic sensor includes at least one thermionic element in a package capable of measuring at least one environmental parameter. The sensor may be a temperature sensor, pressure sensor, a strain sensor, a flux sensor, a flow rate sensor, and/or a sensor to measure one or more other parameters. According to one exemplary embodiment, the sensor may optionally include one or more amplifiers to enhance an output signal of the sensor. The sensor may optionally be wireless, according to at least one exemplary embodiment, although in at least one embodiment the sensor is not wireless. The sensor may include a thermionic power generator, according to one exemplary embodiment. The sensor may include a getter material to substantially maintain a vacuum or partial vacuum within the sensor, according to at least one exemplary embodiment. 
       FIG. 1  is a schematic diagram of a thermionic element  100  according to an exemplary embodiment of the present disclosure. Referring to  FIG. 1 , the thermionic element  100  includes a cathode  110  and an anode  120 . Herein, a cathode may also be referred to as an “emission electrode” or “electron emitter”, and an anode may be referred to as a “collection electrode” or “electron collector”. When thermionic element  100  is heated to a sufficient temperature to induce thermionic emission, electrons  140  are emitted from the cathode  110  to the anode  120 . A voltage source  130  may be provided to induce a small bias voltage, such as, for example, about 5 V, between the cathode  110  and anode  120 , to assist the flow of electrons from the cathode  110  to the anode  120 . The magnitude of the current between the cathode  110  and the anode  120  is directly related to the temperatures of the cathode  110  and anode  120 , and the geometry of the device. Due to this relationship, a relatively simple but effective temperature sensor may be provided by including the thermionic element  100  and measuring the current between the cathode  110  and anode  120 . Other conditions may be measured using the thermionic element  100 , such as by modulating the flow of current between the cathode  110  and anode  120  with a process condition to be measured, as will be discussed in additional exemplary embodiments described below. Further, a thermionic element  100  may be used in other components, which will be discussed in further detail below. 
       FIG. 2  illustrates a sensor package  200 , according to an exemplary embodiment of the disclosure. Referring to  FIG. 2 , sensor package  200  includes a substrate  214 , a housing  210  disposed on substrate  214 , and an encasement  212  that surrounds housing  210 . Herein, housing  210  may be referred to as a “package housing”. Within housing  210 , sensor package  200  includes a sensor  220 ; a sensor  230 ; amplifiers  222 ,  232 ; wireless transmission devices  240 ,  242 ; a scavenging material  260 ; and a power device  250  (power source). However, according to at least certain exemplary embodiments, one or more of these elements may optionally be omitted, as discussed below. 
     Sensor  220  may be configured to measure the ambient temperature of the sensor package  200 , and may be referred to as a temperature sensor. Sensor  230  may be configured to measure the ambient pressure of sensor package  200 , and may be referred to as a pressure sensor. However, according to various embodiments, sensors  220  and  230  may be other types of sensors. Temperature sensor  220  and pressure sensor  230  may be based upon the schematic thermionic element  100  of  FIG. 1 , for example. 
     In contrast to other sensor devices, sensor package  200  may optionally be wireless. For instance, sensor package  200  may lack a power connection to an external power source or transmitter, and may lack a vacuum connection to an external vacuum source to maintain a vacuum or partial vacuum within the interior of sensor package  200 . According to at least one exemplary embodiment, an outer surface of sensor package  200  may be continuous. For instance, the outer surface may be hermetically sealed without any apertures, connections, or passages extending through the housing of sensor package  200 , from between an exterior of sensor package  200  and an interior of sensor package  200 , and to any devices or components not directly attached to sensor package  200 . According to some embodiments, power device  250  may be attached to the outer surface of the sensor package  200 . However, sensor package  200  may lack a connection to any external power source, such that sensor package  200  is self-contained. 
     Wireless transmission devices  240 ,  242  respectively transmit signals output by sensors  220 ,  230  to one or more external devices. For example, a signal may be transmitted to an external control system that controls a parameter corresponding to the output signal. Amplifiers  222 ,  232  respectively amplify the signals output by sensors  220 ,  230 . However, according to some embodiments, amplifiers  222 ,  232  may be omitted, and sensors  220 ,  230  may be directly connected to wireless transmission devices  240 ,  242 . 
     Power device  250  provides power to components of sensor package  200 . Details of power device  250  will be discussed in exemplary embodiments below. Although power device  250  is schematically shown within the interior of sensor package  200  in  FIG. 2 , power device  250  may instead be disposed on the outer surface of sensor package  200 . Further, power device  250  may be connected to sensors  220 ,  230 . Power device  250  may also be connected to amplifiers  222 ,  232 . Power device  250  may supply a bias voltage to thermionic elements of sensors  220 ,  230 , similar to voltage source  130  of the exemplary embodiment of  FIG. 1 . 
     To facilitate the thermionic emission utilized by components of sensor package  200 , such as temperature sensor  220  and/or pressure sensor  230 , housing  210  may be maintained under vacuum conditions, i.e., at a complete or partial vacuum. According to various embodiments, the interior of sensor package  200  may be maintained at a pressure of, for example, about 100 micro Torr or less, for a period of, for example, about 3000 hours of operation or more. Because sensor package  200  is self-contained, the vacuum conditions may be maintained without a vacuum connection between the interior of sensor package  200  and a vacuum source exterior to sensor package  200 . Thus, sensor package  200  may operate without an external vacuum source. 
     As shown in  FIG. 2 , encasement  212  surrounds housing  210 . Encasement  212  may be, for example, a hermetic package that facilitates sealing of housing  210  to substantially maintain vacuum conditions in the interior of sensor package  200 . Sensor package  200  including housing  210  and encasement  212  may have a leak rate of less than about 10 −8  cm 3  atm/sec, according to at least certain exemplary embodiments. 
     Encasement  212  may comprise, for example, a refractory material such as a refractory metal. The refractory metal may include, for example, titanium, platinum, palladium, tungsten, tantalum, alloys thereof, or other suitable refractory metals. According to some embodiments, the encasement  212  may include tungsten. Encasement  212  may be in the form of a single layer that surrounds housing  210 . However, according to some embodiments, encasement  212  may include a plurality of layers. For example, encasement  212  may comprise a first layer, such as a layer of a refractory metal, and at least a second layer. The second layer of encasement  212  may comprise a coating upon the first layer, such as, for example, a metallic layer or a ceramic layer. Thus, an outermost layer of encasement  212  may form the outer surface of sensor package  200 . When the second layer of encasement  212  is metallic, the second layer may include, for example, titanium, platinum, or alloys thereof. When the second layer of encasement  212  is metallic, the second layer may include, for example, zirconium oxide. Encasement  212  may include a plurality of the first and second layers, which are alternately stacked on one another. 
     Scavenging material  260  operates as a getter material, to sequester gaseous molecules that infiltrate the interior of housing  210 . As such, scavenging material  260  helps maintain a vacuum or partial vacuum in housing  210 . Although housing  210  may be substantially sealed, some diffusion or leaking of gaseous molecules into housing  210  may occur. Accordingly, scavenging material  260  may act as a getter material that reacts with gaseous molecules the infiltrate the interior of sensor package  200 . Scavenging material  260  may be a material with a low vapor pressure and a high affinity for oxygen and/or other gaseous molecules at high temperature, such as, for example, zirconium. 
     In order to provide a thermoelectric element capable of sustained operation in an extreme environment, materials of the thermoelectric element should be carefully selected. For instance, materials may be subjected to extreme environments having, for instance, a high temperature, such as, for example, about 750° C. to about 1600° C., a high pressure, such as, for example, pressures up to about 1000 psi, and/or a gaseous environment including, such as, hydrocarbons, oxygen, water vapor, carbon dioxide, carbon monoxide, sulfur oxide, and/or nitrous oxide gases. The gaseous environment may have a flow rate of up to, for example, about 1500 mph. 
     A material may be selected for housing  210  of sensor package  200  that is capable of withstanding a surrounding environment under the extreme conditions noted above, for substantial periods of time. According to various embodiments, the housing  210  comprises a ceramic, such as, for example, alumina (Al 2 O 3 ). Typical ceramics, such as alumina, contain a small amount of porosity, and thus, have a high temperature oxygen conductivity that is unsuitable for use as a vacuum barrier at high temperatures. To address this, housing  210  may comprise a ceramic, such as alumina, sintered from nano-sized particles, so that the resulting ceramic has substantially zero porosity. According to exemplary embodiments, the alumina with substantially zero porosity comprises substantially pure alumina, such as, for example, alumina with a purity of about 99.9% or higher. Further, alumina is very strong. For instance, alumina having a flexural strength of over 150 MPa at 1600° C. can withstand over 1600 psi of pressure for a 1 cm 2  roof structure of a housing having a thickness of 1.5 mm, which is well in excess of a desirable target, which may be, for example, about 1000 psi. In addition, alumina is advantageously resistant to many chemicals and gases, including, for example, oxygen, steam, hydrocarbons, carbon dioxide, carbon monoxide, SO x , and NO x . 
     Substantially non-porous alumina is available as Dynallox HP, 99.9% Al 2 O 3  from Dynamic Ceramic of Cheshire, UK, and HIP Vitox, 99.9% Al 2 O 3  from Morgan Technical Ceramics of Fairfield, N.J. Dynallox HP, 99.9% Al 2 O 3  has a porosity of about 0%, an elasticity of approximately 350 GPa at room temperature and approximately 300 GPa at 1600° C., a maximum operating temperature of about 1800° C., a coefficient of thermal expansion of about 8.5 ppm/K, a flexural strength of approximately 350 MPa at room temperature and approximately 150 MPa at 1600° C., and is chemically compatible with carbon dioxide, oxygen, hydrocarbons, carbon monoxide, steam, NO x , and SO x  at 1600° C. HIP Vitox, 99.9% Al 2 O 3  has a porosity of about 0%, an elasticity of approximately 407 GPa at room temperature and approximately 350 GPa at 1600° C., a maximum operating temperature of about 1750° C., a coefficient of thermal expansion of about 6.8 ppm/K, a flexural strength of approximately 550 MPa at room temperature and approximately 210 MPa at 1600° C., and is chemically compatible with carbon dioxide, oxygen, hydrocarbons, carbon monoxide, steam, NO x , and SO x  at 1600° C. 
     According to at least one exemplary embodiment, housing  210  may comprise a plurality of pieces joined together. The pieces may be joined together via, for example, a paste of a compatible ceramic material, according to one exemplary embodiment. For instance, a paste in green form may be applied between pieces to be joined and then fired, such as in a high temperature co-fired ceramics (HTCC) process. The paste should be compatible with the materials that housing  210  is made of and should be capable of operating within an extreme environment for a substantial period of time without substantially compromising the vacuum capabilities of housing  210 . 
     According to exemplary embodiments, the paste comprises alumina (i.e., is an alumina-based paste). For example, the paste may be Ceramabond 569 alumina base paste from Aremco of Valley Cottage, N.Y., which has a maximum operating temperature of 1650° C., a coefficient of thermal expansion of 7.6 ppm/K, and is compatible with at least alumina, tungsten, and molybdenum. In other exemplary embodiments, the paste may be Resbond® 903HP alumina base paste from Cotronics of Brooklyn, N.Y., which has a maximum operating temperature of 1780° C., a coefficient of thermal expansion of 4.0 ppm/K, and is compatible with at least alumina, tungsten, and molybdenum. 
     Materials for the cathode and anode of a thermionic element, such as the thermionic element  100 , may be selected to withstand the conditions of an extreme environment. Further, the materials for the cathode and anode should be selected so the materials do not degrade at a high rate under the vacuum or partial vacuum within a sensor. The materials may also be selected to be substantially compatible with the material of the housing of a sensor. For instance, materials may be selected to have a similar coefficient of thermal expansion to the material of a housing, such as the ceramic and/or paste, at high temperature so the materials of the cathode and anode have minimal buckling and so popping off of the cathode or anode material from the housing material at high temperature is minimized or eliminated. Generally, the cathode material is electrically conductive and may be tied to ground or another voltage so that electrons can replenish those that have left the cathode. 
     A further consideration for materials of the cathode and anode is the work function of the materials. In various embodiments described herein, the cathode and anode are substantially at the same temperature. As a result, the cathode and anode materials should be selected so their work functions over a target operating range are sufficiently different to generate an adequate current density. For instance, thermionic emission is governed by Richardson&#39;s equation:
 
 J=AT   2   e   −Phi/kT  
 
where J=thermionic current density (A/m 2 ), A=thermionic constant dependent upon material properties, T=electrode temperature (K), phi=work function of the material, and k=Boltzmann constant.
 
     According to various exemplary embodiments, a cathode may comprise tungsten and another material. Tungsten may be used as a base material for a cathode because tungsten can withstand the high temperatures a sensor is subjected to, does not degrade at a substantial rate over a substantial period of time in vacuum conditions, and has a coefficient of thermal expansion that is well matched with the materials of a sensor housing. 
     In general, the work function of the cathode material can be the same, lower, or higher than that of the anode material, depending on a thermionic behavior desired for the sensor. 
     According to various exemplary embodiments, a cathode may comprise a low-work-function material in addition to a metal material, such as tungsten. The metal of the cathode may be coated, mixed and/or impregnated with one or more low-work-function materials, for example chosen from thorium (Th), thorium oxide (ThO 2 ), barium (Ba), barium oxide (BaO), strontium (Sr), strontium oxide (SrO), calcium oxide (CaO), lanthanum (La), lanthanum oxide (La 2 O 3 ), yttrium (Y), yttrium oxide (Y 2 O 3 ), cesium (Ce), cesium oxide (CeO 2 ), and combinations thereof. Suitable coating/impregnating low-work-function materials include, for example, BaO, SrO, CaO, La, La 2 O 3 , and/or combinations thereof. In certain embodiments, the low-work-function material may not be electrically conductive, and RF sputtering may be employed. 
     A cathode may comprise tungsten including a rare earth element, such as, for example, lanthanum. The rare earth element may be present in an amount of about, for example, 0.2 weight percent or more, based on the total weight of the cathode. The rare earth element may form an alloy with tungsten, which may include one or more phases, or may be present in an oxide form, such as oxide particles dispersed in the tungsten. According to exemplary embodiments, a cathode may comprise lanthanated tungsten. An example of lanthanated tungsten is tungsten that includes lanthanum oxide (i.e., La 2 O 3 ) in an amount of, for example, about 0.2 weight percent to about 5 weight percent, based on the total weight of the lanthanated tungsten. According to other exemplary embodiments, lanthanated tungsten includes lanthanum oxide in an amount of, for example, about 1 weight percent, based on the total weight of the lanthanated tungsten. Lanthanated tungsten may have a Richardson thermionic constant (A) of 6×10E 5  A/m 2 , a work function (phi) of about 4.5 eV, a coefficient of thermal expansion of about 4.7 ppm/K at room temperature and about 5.8 ppm/K at 1600° C., an elastic modulus of about 200 GPa at room temperature and about 400 GPa at 1600° C., and a tensile strength of about 300 MPa at room temperature and about 1000 MPa at 1600° C. 
     According to various exemplary embodiments, an anode may comprise substantially pure tungsten. Substantially pure tungsten may have a purity of, for example, approximately 99.99%. In other words, an anode may be formed of high purity tungsten, such as approximately 99.99% pure tungsten, so as to avoid impurities and elements that would alter the properties of the pure tungsten, such as, for example, the work function and/or the coefficient of thermal expansion of tungsten. Substantially pure tungsten may have a Richardson thermionic constant (A) of 6×10E 5  A/m 2 , a work function (phi) of about 2.5 to about 2.8 eV, a coefficient of thermal expansion of about 4.5 ppm/K at room temperature and about 5.5 ppm/K at 1600° C., an elastic modulus of about 200 GPa at room temperature and about 400 GPa at 1600° C., and a tensile strength of about 300 MPa at room temperature and about 1000 MPa at 1600° C. According to other exemplary embodiments, an anode may comprise platinated tungsten (i.e., tungsten including platinum and/or platinum oxide). For example, the anode may comprise from about 0.01 to about 10 wt % platinum, based on the total weight of the anode. According to another embodiment, an anode may be made of a different suitable material having a lower work function than a cathode material. 
     Cathode and anode materials may be paired by selecting materials from any of the previous exemplary embodiments described above. According to exemplary embodiments, a cathode may comprise lanthanated tungsten and an anode may comprise substantially pure tungsten according to the exemplary embodiments described above.  FIG. 3  shows a plot of thermionic performance for a lanthanated tungsten cathode and a substantially pure tungsten anode, with thermionic current plotted over temperature. Due to their large difference in work functions, the lanthanated tungsten cathode has more than five orders of magnitude of activity than the substantially pure tungsten anode, as shown in  FIG. 3 , resulting in current densities well in excess of those required for a functional thermionic device. However, cathode and anode material pairings are not limited to lanthanated tungsten and substantially pure tungsten but the respective materials for a cathode and anode may be selected from any of the exemplary embodiments described above. 
     The materials of other components or parts of a sensor package may also be selected to withstand the conditions of an extreme environment. According to exemplary embodiments, bond pads for making an electrical connection to cathodes and/or anodes within a sensor package may comprise the same material as the cathode or anode. According to other exemplary embodiments, bond pads may be made of tungsten, whether or not a cathode or anode is made of tungsten. 
       FIG. 4  illustrates a thermionic temperature sensor  300 , according to an exemplary embodiment of the present disclosure. Referring to  FIG. 4 , temperature sensor  300  includes a housing  310 , an anode  320 , and a cathode  330 . Housing  310 , anode  320 , and cathode  330  may be made of any of the materials discussed in the exemplary embodiments above. Herein, housing  310  may be referred to as “a sensor housing.” Housing  310  includes a top plate  316  and a bottom plate  318  that cooperate to form an emission chamber  312  in which anode  320  and cathode  330  are located. As discussed above with regard to the exemplary embodiment of  FIG. 2 , the emission chamber  312  of housing  310  may be maintained under vacuum conditions, such as at a pressure of, for example, about 100 micro Torr or less, to facilitate substantially unimpeded emission of electrons between cathode  330  and anode  320 . 
     Temperature sensor  300  is shown in  FIG. 4  as being a standalone temperature sensor. However, temperature sensor  300  may be included in a sensor package. For example, temperature sensor  300  may be utilized as temperature sensor  220  of sensor package  200  of  FIG. 2 , according to some embodiments. 
     Housing  310  may include other components, such as a spacer (not shown) between top plate  316  and bottom plate  318  to form chamber  312  in conjunction with top plate  316  and bottom plate  318 , as will be discussed below. However, housing  310  is not limited to this configuration and may have other arrangements. For example, a recess may be formed in a surface of top plate  316 , bottom plate  318 , or in both top plate  316  and bottom plate  318 , to form chamber  312 . Housing  310  may further include a substrate  314  upon which bottom plate  318  and top plate  316  are disposed. However, according to some embodiments, substrate  314  may be omitted. When included in sensor package  200 , substrate  214  may be used as substrate  314 . 
     Anode  320  may, in exemplary embodiments, be “C” shaped. Cathode  330  may, in exemplary embodiments, be circular and may be disposed inside anode  320 . In particular, opposing edges of cathode  330  and anode  320  may optionally be spaced apart by a substantially consistent minimum distance. In other words, a substantially consistent gap may be formed between anode  320  and cathode  330 . Anode  320  and cathode  330  may be formed, for example, by depositing the materials of anode  320  and cathode  330  upon bottom plate  318 , according to an exemplary embodiment. Temperature sensor  300  may further include leads  322  and  332  that respective extend from anode  320  and cathode  330  to the outside of housing  310 . Lead  332  extends from cathode  330  though an opening in anode  320 . A minimum distance between anode  320  and lead  332  may be substantially the same as the minimum distance between anode  320  and cathode  330 . 
       FIG. 5  is a bottom view of top plate  316 .  FIG. 6  is a side view of temperature sensor  300 , taken along line  6 - 6  in  FIG. 4 , where substrate  314  is omitted.  FIG. 7  is a top view of bottom plate  318 . Referring to  FIGS. 4-7 , temperature sensor  300  may further include an electrically conductive layer  340  disposed on top plate  316 , within chamber  312 . Layer  340  may be, for example, a layer of tungsten deposited on top plate  316 . Layer  340  may be disposed on a bottom side of top plate  316 , so that layer  340  faces anode  320  and cathode  330  disposed on bottom plate  318 . According to exemplary embodiments, layer  340  may serve as a semi-passive ceiling (i.e., equipotential surface). For instance, a small bias voltage (e.g., about 5 V) may be provided between anode  320  and cathode  330 , while the voltage of layer  340  is floated. 
     Because the rate of thermionic emission between cathode  330  and anode  320  is a direct function of the temperatures of cathode  330  and anode  320 , a current signal obtained from anode  320  and cathode  330 , such as via leads  322 ,  332 , can be directly correlated to temperature. The inventors prepared a simulation of temperature sensor  300  to estimate its performance. With a cathode area of 10 mm 2  and an anode area of 20 mm 2 , a cathode to anode bias voltage of 5 V, and a spacing between layer  340  and anode  320  and cathode  330  of 5 mm, a calculated response of temperature sensor  300  was determined, which is shown in  FIG. 8 . The response follows a power law, which may be used to calibrate temperature sensor  300 . It is expected that temperature sensor  300  has a resolution of about 10° C. (about 1%) in an environment at 1000° C. and a resolution of less than about 1° C. (about 0.06%) in an environment at 1600° C. 
       FIG. 9  illustrates a thermionic pressure sensor  400 , according to various embodiments of the present disclosure. Referring to  FIG. 9 , pressure sensor  400  include a housing  410 , which may include a top plate  412  and a substrate  414  that cooperate to form an emission chamber  416  in which a cathode  420 , an inner anode  430 , and an outer anode  434  are disposed. Herein, housing  410  may be referred to as “a sensor housing.” Cathode  420 , inner anode  430 , and outer anode  434  may be made with any of the materials of the exemplary embodiments discussed above, and may be formed by, for example, depositing such materials upon substrate  414 . 
     Cathode  420  may, for example, be substantially circular. Inner and outer anodes  430  and  434  may, for example, be “C” shaped. Outer anode  434  may optionally be wider than inner anode  430 . Opposing edges of cathode  420  and inner anode  430  may optionally be spaced apart by a substantially consistent first minimum distance. Opposing edges of inner anode  430  and outer anode  434  may optionally be spaced apart by a substantially consistent second minimum distance. The first and second minimum distances may be substantially the same. 
     As discussed above with regard to the exemplary embodiment of  FIG. 2 , the emission chamber  416  of housing  410  may be maintained under vacuum conditions, such as at a pressure of, for example, about 100 micro Torr or less, to facilitate substantially unimpeded emission of electrons. 
     Pressure sensor  400  may be incorporated into a sensor package, such as sensor package  200 . In order to determine the pressure outside of sensor package  200 , housing  410  may be part of housing  210  of  FIG. 2 , such that housing  410  and housing  210  form different portions of single housing. Other configurations are possible, so long as at least a portion of housing  410  is exposed to ambient environmental pressure. In addition, substrate  414  may be substituted with substrate  214 . 
     Housing  410  is shown schematically in  FIG. 9  and may include other components, such as a spacer (not shown) disposed between top plate  412  and substrate  414  and at least partially forming chamber  416 . However, housing  410  is not limited to this configuration and may have other arrangements, such as by providing a recess in top plate  412 , substrate  414 , or in each of top plate  412  and substrate  414 , to form chamber  416 . Further, although substrate  414  may be larger than top plate  412 , as indicated in the exemplary embodiment of  FIG. 9 , substrate  414  may be substantially the same size as top plate  412 . 
     Pressure sensor  400  may further include an electrically conductive layer  440  disposed on top plate  412 , within chamber  416 . Layer  340  may be, for example, a layer of tungsten deposited on top plate  412 , as shown in  FIG. 10 , which is a bottom view of top plate  412  with layer  440 .  FIG. 11  shows a top view of substrate  414 , which may have cathode  420 , inner anode  430 , and outer anode  434  disposed thereon. As shown in  FIG. 12 , which is a side view of temperature sensor  400  taken along line  12 - 12  of  FIG. 9 , layer  440  may be located on a bottom side of top plate  412 , so that layer  440  faces cathode  420 , inner anode  430 , and outer anode  434 . 
     Layer  440  may serve as a semi-passive ceiling (i.e., equipotential surface). For instance, a small separate bias voltage (e.g., about 5 V) may be provided between cathode  420  and inner anode  430  and between cathode  420  and outer anode  434 , while the voltage of layer  440  is floated. A split anode configuration (i.e., inner anode  430  and outer anode  434 ) may be provided, for instance, so that the separate anodes receive equal current at any operating temperature. 
     According to various exemplary embodiments, at least one surface of housing  410  is configured to flex when subjected to an external pressure from the ambient environment. A portion of housing  410  may be configured, for example, to flex under a pressure of up to about 1600 psi, while maintaining a vacuum or partial vacuum within chamber  416 . According to various non-limiting and exemplary embodiments, only one surface of housing  410  may be configured to flex when subjected to an external pressure while the remaining surfaces of housing  410  do not flex. In other words, housing  410  may include a flexible portion and an inflexible portion. A flexible portion of housing  410  may be, for example, a portion that layer  440  is attached to, such as top plate  412 , according to one exemplary embodiment. This is shown in the exemplary embodiment of  FIG. 13 , which shows the view of  FIG. 12 , but with an external pressure  450  applied to top plate  412 , which is configured to flex under pressure  450 , resulting in top plate  412  flexing and bending towards substrate  414 . When top plate  412  flexes in this way, the relative distances between top plate  412  and anodes  430 ,  434 , and cathode  420  are changed. As the relative distances change, the current detected to each anode  430 ,  434 , such as via respective leads  432 ,  436 , also changes. This change in current can be correlated to the external pressure  450  that causes flexion of top plate  412 . 
     A simulation of pressure sensor  400  was prepared in order to estimate its performance. With a cathode area of 10 mm 2 , an inner anode area of 20 mm 2 , an outer anode area of 100 mm 2 , a cathode to anode bias voltage of 5 V, and a spacing between layer  440  and cathode,  420 , inner anode  430 , and outer anode  434  of 5 mm, and a top plate  412  thickness of 0.2 mm, a calculated performance of the pressure sensor was plotted, which is shown in the exemplary embodiment of  FIG. 14 . The estimated pressure resolution of the pressure sensor is about 20 to about 70 psi (about 2% to about 7% of its full scale). 
     Because the overall rate of thermionic emission between the cathode and anodes is a function of temperature, the pressure sensor response is also dependent upon temperature. According to exemplary embodiments, pressure sensor  400  may be used in conjunction with temperature sensor  300 , to measure the change in temperature and correlate the temperature to a true ambient pressure. For example, pressure sensor  400  may be provided as a pressure sensor  230  along with a temperature sensor  220 , as shown in the exemplary embodiment of  FIG. 2 . According to exemplary embodiments, additional components, such as an amplifier  232 , and/or an amplifier  222  for temperature sensor  220  may be included as well. 
     Other types of thermionic sensors may be provided by using other configurations of thermionic elements. According to exemplary embodiments, an array of temperature sensors, which may be each configured according to temperature sensor  300 , may be arranged at known locations to detect a heat flux. The array may be arranged at known locations to measure heat flux in various coordinate arrangements, such as, for example, spherical coordinates, cylindrical coordinates, or other coordinate systems in which the sensors may be arranged. According to various exemplary embodiments, two or more pressure sensors  400  may be arranged in conjunction with temperature sensors  300 , so as to measure a flow rate. 
       FIG. 15  illustrates an amplifier  500 , according to various embodiments of the present disclosure. Referring to  FIG. 15 , amplifier  500  may be used with the temperature sensor of the exemplary embodiments of  FIGS. 4-8  and/or the pressure sensor of the exemplary embodiments of  FIGS. 9-14 . For instance, amplifier  500  may be provided in sensor package  200  as amplifier  222  or amplifier  232 , to enhance the output signal of a temperature sensor  220  and/or a pressure sensor  230 . 
     Amplifier  500  includes a housing  510  disposed on a substrate  514 . The housing  510  at least partially defines an interior chamber  516  in which a cathode  520  and an anode  530  are disposed. As discussed above with regard to the exemplary embodiment of  FIG. 2 , the interior chamber  516  may be maintained under a vacuum or partial vacuum, such as at a pressure of, for example, about 100 micro Torr or less. Substrate  514  may be substrate  214 , when amplifier  500  is included in sensor package  200 . Leads  522 ,  532  may respectively be connected to cathode  520  and anode  530 , to serve as input/output conduits. 
       FIG. 16  is a sectional view taken along line  16 - 16  of  FIG. 15 . Referring to  FIGS. 15 and 16 , housing  510  may include a top plate  512  upon which an electrically conductive layer  540  is disposed within chamber  516 . However, according to some embodiments, chamber  516  may be formed entirely within housing  510 , and substrate  514  may be omitted. For example, housing  510  may include a top plate and a bottom plate as shown in  FIG. 4 . Layer  540  may be, for example, a layer of tungsten deposited on top plate  512 , according to exemplary embodiments.  FIG. 17  shows a top view of substrate  514 , on which cathode  520  and anode  530  are disposed. As shown in  FIG. 16 , layer  540  may be disposed on a bottom side of top plate  512 , so that layer  540  faces cathode  520  and anode  530 . 
     In contrast to the sensor embodiments of  FIGS. 4-14 , layer  540  of amplifier  500  is electrically active in the exemplary embodiment of  FIG. 15 . A control voltage may be applied to layer  540 , which in turn changes the potential of layer  540  and changes the flow of current between cathode  520  and anode  530 . To supply the control voltage to layer  540 , an electrical connection  550  may be provided. As shown in  FIGS. 15 and 16 , electrical connection  550  may extend from substrate  514  to top plate  512 . A contact  542  may be provided on substrate  514 , to supply the voltage to electrical connection  550 , as shown in  FIGS. 16 and 17 . According to exemplary embodiments, electrical connection  550  may, for example, be a spring, as shown in  FIG. 16 , to accommodate any relative movement between top plate  512  and substrate  514 . 
     As discussed above with regard to the exemplary embodiment of  FIG. 2 , a wireless transmission device may be provided to wirelessly transmit data to a device external to a sensor according to various embodiments of the disclosure. For example,  FIG. 18  is a schematic diagram of a wireless transmission device  600 , according to various embodiments. Referring to  FIG. 18 , wireless transmission device  600  operates as a thermionic oscillator, such as a resistor-inductor (RL) relaxation oscillator. 
     As shown in the exemplary embodiment of  FIG. 18 , wireless transmission device  600  may be connected to a sensor  610 , such as the temperature sensor of the exemplary embodiment of  FIG. 4  and/or the pressure sensor of the exemplary embodiment of  FIG. 9 , a resistor, which may be fabricated as a film of tungsten or platinum on the same substrate as the electrodes, first and second amplifiers  620 ,  622 , and an antenna  630 . Amplifiers  620 ,  622  may be configured according to the exemplary embodiment of  FIGS. 15-17 . Thus, wireless transmission device  600  may include a thermionic element. 
     The frequency of the wireless transmission device  600  may be driven by the magnitude of the signal from sensor  610  (i.e., a voltage to frequency conversion), with the signal being wirelessly transmitted wireless transmission device  600  via antenna  630 . The signal may in turn be converted by a device external to a sensor that receives the signal. Such an external receiving device may also be external to the extreme environment so that the external device need not be designed to withstand the extreme environment. 
       FIG. 19  illustrates a section view of a thermionic power device  700 , according to various embodiments of the present disclosure. Referring to  FIG. 19 , power device  700  is a specific example of power device  250  that is included in the sensor package  200  of  FIG. 2 . Power device  700  is a thermionic power generator that includes a first substrate  710  and an opposing second substrate  720 . Thermionic power device  700  may be at least partially disposed within a sensor package, or may be disposed on an external surface of a sensor package. When power device  700  is disposed inside a sensor package, first substrate  710  may be at least a portion of a housing of the sensor package that is exposed to an extreme environment, such as a high temperature environment. When power device  700  is disposed on an external surface of a sensor package, second substrate  720  may be, for example, a wall that serves as a boundary to the extreme environment, such as a wall of a room or vessel that contains the extreme environment and to which a sensor package including power device  700  is attached. 
     Accordingly, first substrate  710  may be disposed closer to a harsh temperature environment than the second substrate  720 . As such, first substrate  710  may have a higher temperature than second substrate  720 . For example, when first substrate  710  is at a temperature of, for example, about 1600° C., second substrate  720  may be at a temperature of, for example, about 300° C. Such a temperature difference provides a thermal gradient and facilitates power generation. According to various embodiments, second substrate  720  may by a heat sink including fins  721  to facilitate heat transfer and thereby facilitate the thermal gradient. 
     Power device  700  includes a cathode  712  and an anode  722 , so that thermionic emission of electrons  730  may occur between cathode  712  and anode  722 . Cathode  712  may comprise, for example, lanthanated tungsten or other exemplary cathode materials discussed above. Anode  722  may comprise, for example, barium oxide or other anode materials discussed above. Power device  700  may include multiple cathode  712  and anode  722  pairs. Each cathode  712  and anode  722  pair may be capable of generating approximately 1.5 V, with a potential for up to 5 A/cm 2  of current. To achieve a higher voltage and power output, cathodes  712  and anodes  722  pairs may be arranged in series, with a conductor  732  providing a connection between cathodes  712  and anodes  722 . Thermionic emission of electrons  730  provides another connection between cathode  712  and anode  722  pairs. Conductor  732  may be a spring to accommodate any change in position between first substrate  710  and second substrate  720 , such as due to thermal expansion. According to exemplary embodiments, power device  700  may have an efficiency of about 15%, or more, and power density of about 10 W/cm 2 . 
       FIG. 20  illustrates sectional view of a thermionic sensor  800 , according to various embodiments of the present disclosure. Referring to  FIG. 20 , sensor  800  includes a substrate  862 , a cathode  820  and an anode  830  disposed on substrate  862 , a top plate  860  facing substrate  862 , a conductive layer  806  disposed on top plate  860 , and a spacer  810  disposed between substrate  862  and top plate  860 . Spacer  810  may be connected to substrate  862  and top plate  860  using a sealing paste  850 . Spacer  810  may be circular, but the present disclosure is not limited to any particular shape. 
     Substrate  862 , top plate  860 , and spacer  810  may be made of any of the materials discussed above for a sensor housing, such as, for example, alumina having substantially zero porosity. Sealing paste  850  may be a compatible paste and used in an HTCC process, as discussed above with regard to the exemplary embodiment of  FIG. 2 . A chamber formed between substrate  862 , top plate  860 , and spacer  810  may be at least partially evacuated and sealed. An encasement (not shown) may be applied to the exterior of top plate  860 , spacer  810 , and bottom plate  862  to facilitate the sealing. 
     An additional advantage of the exemplary embodiments of the thermionic sensors described herein is their compact size, although it should be noted that compact size is not required according to embodiments of the disclosure. Conventional sensors used in extreme environments are typically bulky, often because they are design to protect the sensor and mitigate the effects of the surrounding environment upon sensor components. However, the thermionic sensors according to at least certain exemplary embodiments described herein use the surrounding environment and therefore may have a more compact size. The surface area on an upper surface of a top plate of a sensor housing (i.e., top plate  316  of the exemplary embodiment of  FIG. 4  or top plate  412  of the exemplary embodiment of  FIG. 9 ) may be, for example, about 0.5 cm 2  to about 1.5 cm 2 . For example, such sensor housings may have lengths and widths ranging from about 0.7 cm to about 1.25 cm. 
       FIG. 21  is a block diagram illustrating a method of using a sensor package, according to various embodiments of the present disclosure. Referring to  FIG. 21 , in a first operation  10 , a sensor package is exposed to a high-temperature environment. In particular, at least a portion of the sensor package is exposed to temperatures of at least 500° C., for example, temperatures of at least 600° C. According to some embodiments the sensor package may be exposed to temperatures ranging from about 750° C. to about 1600° C. Further, the sensor package may be exposed to a high pressure, such as pressures up to about 1000 psi. As a result, the cathode and the anode a sensor of the sensor package are both heated to a temperature sufficient for thermionic emission to occur. 
     In addition, operation  10  may also include heating a cathode of a power generator of the sensor package to a relatively high temperature, while an anode of the power generator is kept at a relatively low temperature. For example, the cathode could be heated to a temperature that is from about 700° C. to 1200° C. higher than the temperature of anode. However, the present disclosure is not limited to any particular temperature variation. As a result, the power generator would generate sufficient current to power the sensor package. Therefore, the sensor package can be completely self-contained. 
     In operation  20 , once the sensor package is sufficiently heated, the sensor generates a sensor signal via thermionic emission. In operation  30 , the sensor signal may optionally be transmitted to an amplifier to amplify the sensor signal. However, according to some embodiments operation  30  may be omitted. 
     In operation  40 , the sensor signal is transmitted to a wireless transmission device, which transmits the signal to an external device. For example, the sensor signal could be converted into a radio frequency (RF) signal, and then broadcast using an antenna of the sensor package to an external device configured to receive the RF signal. 
     In operation  50 , the broadcast signal is received by an external device. The external device may extract sensor data from the received signal and store the same in a memory device. The sensor data may be stored with time and date information. The sensor date may include temperature data, pressure data, or any other data detected by a sensor of the sensor package. Operations  20 - 50  may be repeated, such that the conditions of the ambient environment of the sensor package may be tracked over time. For example, operations  20 - 50  may be repeated constantly, or may be repeated at a selected time interval. 
     In addition, data from multiple sensors may be transmitted concurrently. For example, data from multiple pressure and temperature sensors could be broadcast to an external device, such that a flow rate of an ambient environment of the sensor package could be determined. 
     For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages, or proportions, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term “about,” to the extent they are not already so modified. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. 
     It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the,” and any singular use of any word, include plural referents unless expressly and unequivocally limited to one referent. As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items. 
     Further, this description&#39;s terminology is not intended to limit the invention. For example, spatially relative terms—such as “beneath”, “below”, “lower”, “above”, “upper”, “proximal”, “distal”, and the like—may be used to describe one element&#39;s or feature&#39;s relationship to another element or feature as illustrated in the figures. These spatially relative terms are intended to encompass different positions (i.e., locations) and orientations (i.e., rotational placements) of a device in use or operation in addition to the position and orientation shown in the figures. For example, if a device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be “above” or “over” the other elements or features. Thus, the exemplary term “below” can encompass both positions and orientations of above and below. A device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     This description and the accompanying drawings that illustrate exemplary embodiments should not be taken as limiting. Various mechanical, compositional, structural, electrical, and operational changes may be made without departing from the scope of this description and the claims, including equivalents. In some instances, well-known structures and techniques have not been shown or described in detail so as not to obscure the disclosure. Effort has been made to ensure that like numbers in two or more figures represent the same or similar elements. Furthermore, elements and their associated features that are described in detail with reference to one embodiment may, whenever practical, be included in other embodiments in which they are not specifically shown or described. For example, if an element is described in detail with reference to one embodiment and is not described with reference to a second embodiment, the element may nevertheless be claimed as included in the second embodiment. 
     Further modifications and alternative embodiments will be apparent to those of ordinary skill in the art in view of the disclosure herein. For example, the systems and the methods may include additional components or steps that were omitted from the diagrams and description for clarity of operation. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the present teachings. It is to be understood that the various embodiments shown and described herein are to be taken as exemplary. Elements and materials, and arrangements of those elements and materials, may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the present teachings may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of the description herein. Changes may be made in the elements described herein without departing from the spirit and scope of the present teachings and following claims. 
     It is to be understood that the particular examples and embodiments set forth herein are non-limiting, and modifications to structure, dimensions, materials, and methodologies may be made without departing from the scope of the present teachings. 
     Other embodiments in accordance with the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit being indicated by the following claims.