Abstract:
A gas sensor is provided for detecting one or more gases in a gas sample. The gas sensor includes a substrate, a solid electrolyte layer including lanthanum oxide for sensing carbon dioxide, a heating element thermally coupled to the solid electrolyte layer, and a controller coupled to the heating element and the solid electrolyte layer. The controller heats the heating element so that the solid electrolyte layer reaches an operating. Methods of sensing carbon dioxide and humidity are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to gas sensors, and more specifically, to gas sensors for detecting one or more gases in a sample of an environment or flow stream. 
     There is growing interest in monitoring and controlling air quality in both indoor and outdoor environments, including carbon dioxide concentration. There are several types of gas sensors that can monitor carbon dioxide (CO 2 ) or other gases. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a gas sensor, and more specifically, a gas sensor for detecting carbon dioxide and other gases in a gas sample, and in some cases humidity. The gas sensor includes a sensor for sensing a desired gas and a heater for heating the sensor. During operation, a controller provides power to the heater to heat the sensor to an operating temperature, which is above ambient temperature. In some embodiments, the sensor and heater are thermally isolated from some or all of the remainder of the sensor, such as the sensor substrate. This may help reduce the amount of power that is required to heat the heater and sensor to the operating temperature. This may make it more energy efficient to heat the sensor to an operating temperature at spaced time intervals. The gas sensor of the present invention may be ideally suited for battery powered and/or wireless applications. 
     Methods of sensing a gas are also disclosed. One illustrative method includes the steps of providing a solid electrolyte layer including lanthanum oxide, contacting the solid electrolyte layer with a gas sample, heating the solid electrolyte layer from 100 degrees Celsius to an operating temperature with a first amount of energy, and determining a concentration of carbon dioxide in the gas sample based on first amount of energy. 
     Another illustrative method includes the steps of providing a solid electrolyte layer including lanthanum oxide, contacting the solid electrolyte layer with a gas sample, heating the solid electrolyte layer to about 100 degrees Celsius with a water desorbing amount of energy, heating the solid electrolyte layer from about 100 degrees Celsius to a carbon dioxide desorbing temperature with a carbon dioxide desorbing amount of energy, and then determining a humidity level in the gas sample based on the water desorbing amount of energy and determining a concentration of carbon dioxide in the gas sample based on the carbon dioxide amount of energy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of an illustrative gas sensor in accordance with the present invention; 
         FIG. 2  is a schematic top view of the illustrative gas sensor of  FIG. 1 ; 
         FIG. 3  is a cross-sectional side view of another illustrative gas sensor in accordance with the present invention; 
         FIG. 4  is a time verses temperature graph showing differential thermal analysis for determining a concentration of carbon dioxide in a gas sample; 
         FIG. 5  is a time verses temperature graph showing differential thermal analysis for determining a concentration of water and carbon dioxide in a gas sample; and 
         FIG. 6  is a schematic side view of an illustrative gas sensor assembly in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure is directed toward a gas sensor, and more specifically, a gas sensor for detecting carbon dioxide and other gases, and in some cases, humidity, in a gas sample.  FIG. 1  is a cross-sectional side view of one illustrative gas sensor.  FIG. 2  is a schematic top view of the illustrative gas sensor of  FIG. 1 . The illustrative gas sensor is generally shown at  10 , and includes a sensor  12  formed on or above a substrate  14 . The illustrative sensor  12  includes a heater layer  16 , a buffer layer  18 , a lower electrode layer  20 , a solid electrolyte layer  22 , and an upper electrode layer  24 , as best shown in  FIG. 1 . It should be understood that the specific layers shown, as well as their relative positions, may be changed and still be within the scope of the present invention. All that is important is that the heater layer  16  is thermally coupled to the solid electrolyte layer  22 , and contacts are provided from the solid electrolyte layer  22 . 
     In the illustrative embodiment, the heater layer  16  is made from a resistive material that generates heat when a current is passed therethrough. To increase the heat the can be delivered to the sensor  12 , as well as the uniformity of the heat, the heater layer  16  may be configured to meander back and fourth along the area of the sensor  12 , as better shown in  FIG. 2 . 
     In the illustrative embodiment, the solid electrolyte layer  22  may be made from a suitable solid electrolyte material. For example, if the gas to be detected is CO 2  and/or humidity, the solid electrolyte may be lanthanum oxide, La 2 O 3  (CAS No.: 1312-81-8) available from Sigma Aldrich Chemical Company, Milwaukee Wis. The solid electrolyte layer  22  can be a layer of La 2 O 3  or a layer of material (such as silica, for example) doped with La 2 O 3 , as desired. 
     Lanthanum oxide is a useful solid electrolyte since it absorbs water and carbon dioxide at ambient temperature and desorbs water as it is heated to 100 degrees Celsius and then desorbs carbon dioxide as it is heated above 100 degrees Celsius. As such, a single heating cycle of the gas sensor can provide accurate concentration measurements of both water (e.g., humidity) and carbon dioxide in a gas sample. With the physical properties (i.e., specific absorption capacity, volume, and mass) of lanthanum oxide and the amount of heat applied to lanthanum oxide, a concentration of both water and carbon dioxide can be determined based on the change in thermal mass with differential thermal analysis. One illustrative differential thermal analysis sensor is described in U.S. Pat. No. 6,238,085, and is incorporated by reference herein. In one illustrative embodiment, the gas sensor has an ultimate carbon dioxide sensitivity of 5 ppm. 
     Control electronics  28  may be provided on or in the substrate  14 , or elsewhere, as desired. Control electronics  28  can be coupled to the heater layer  16  via traces  30   a  and  32   b , and the lower electrode layer  20  and the upper electrode layer  24  via traces  32   a  and  32   b , as best shown in  FIG. 2 . During operation, control electronics  28  can provide power to the heater layer  16  to heat the sensor  12  to an operating temperature, which is above an ambient temperature. The application of heat to the sensor  12 , and more specifically, to the solid electrolyte layer  22 , causes the absorbed gases to desorb. 
     Referring to  FIG. 4 , and in one illustrative embodiment, the control electronics  28  may provide power to the heater layer  16  to heat the sensor to the operating temperature during a first period of time with a first amount of energy.  FIG. 4  shows an illustrative time verses temperature graph of a control sensor and a CO 2  sensor according to the present disclosure. At time zero, the sensors are at ambient temperature. Energy (i.e., current) is applied to both sensors at a constant rate and at a constant voltage, thus providing a constant power to the heater. Both sensors reach 100 degrees Celsius at about the same time, with about the same amount of energy (which can be calculated by integrating the area under each curve.) At 100 degrees Celsius, carbon dioxide begins to desorb from the CO 2  sensor, causing the CO 2  sensor to heat up at a slower rate than the control sensor. At time equal to T C , the control sensor reaches a temperature of 500 degrees Celsius. At time equal to T CO2 , the CO 2  sensor reaches a temperature of 500 degrees Celsius. At a temperature of about 500 degrees Celsius substantially all of the carbon dioxide has desorbed from the sensor. The difference in the areas under each curve (A CO2 ) corresponds to the amount of energy required to desorb the carbon dioxide from the sensor. Knowing the physical properties of the solid electrolyte and carbon dioxide, a total amount of carbon dioxide desorbed from the sensor can be determined. A concentration of carbon dioxide in the gas sample can then be determined based on known equilibrium constants of carbon dioxide and the solid electrolyte at ambient absorption temperature and pressure. 
       FIG. 5  shows an illustrative time verses temperature graph of a control sensor and a relative humidity (RH) and CO 2  sensor according to the present disclosure. At time zero, the sensors are at ambient temperature. Power (i.e., current/voltage) is applied to both sensors at a constant rate. Here, the RH/CO 2  sensor reaches and begins to rise above 100 degrees Celsius at time equal to T H2O , and the control sensor reaches 100 degrees Celsius at time equal to T C1 . The difference in the total amount of energy required for each sensor to reach and just exceed 100 degrees Celsius is related to the amount of water that desorbed from the RH/CO 2  sensor. Knowing the physical properties of the solid electrolyte and water, a total amount of water desorbed from the sensor can be determined. A concentration of water in the gas sample can then be determined based on known equilibrium constants of water and the solid electrolyte at ambient absorption temperatures and pressures. Relative humidity can then be determined using known techniques. 
     Above 100 degrees Celsius, carbon dioxide begins to desorb from the RH/CO 2  sensor, causing the CO 2  sensor to heat up at a slower rate than the control sensor. At time equal to T C2 , the control sensor reaches a temperature of 500 degrees Celsius. At time equal to T CO2 , the CO 2  sensor reaches a temperature of 500 degrees Celsius. At a temperature of about 500 degrees Celsius substantially all of the carbon dioxide has desorbed from the sensor. The difference in the areas under each curve (above 100 degrees Celsius) is the amount of energy required to desorb the carbon dioxide from the sensor. Knowing the physical properties of the solid electrolyte and carbon dioxide, a total amount of carbon dioxide desorbed from the sensor can be determined. A concentration of carbon dioxide in the gas sample can then be determined based on known equilibrium constants of carbon dioxide and the solid electrolyte at ambient absorption temperatures and pressures. 
     In some embodiments, a control sensor may, or may not, be provided. The control sensor can be identical in construction to the gas sensor without the lanthanum oxide. The control sensor can be coupled to the controller and provide a control heating profile for the gas sensor. Thus, a differential heating profile, or differential energy amount can be determined and used to determine desorbed carbon dioxide and/or humidity from the gas sensor. When a control sensor is not provided, the desorbed carbon dioxide and/or humidity can be determined from calculated sensor characteristic data previously known or determined and may be stored in a memory within the controller. 
     In some embodiments, the sensor  12  may be thermally isolated from some or all of the remainder of the gas sensor  10 . In the embodiment shown in  FIG. 1 , a pit  52  may be etched into the substrate below the sensor  12  leaving a gap or space between the sensor  12  and the substrate  14 . The gap may be an air gap, or filled with a material with a low coefficient of thermal conductivity. Supporting legs  50   a - d  may be left in tact to support the sensor  12  above the pit  52 . In this configuration, the sensor  12 , which includes the heater  16  and the solid electrolyte layer  22 , is suspended above the substrate, which helps thermally isolate the sensor  12  from the remainder of the gas sensor  10 . This may help reduce the amount of power and time that is required to heat the sensor  12  to the operating temperature. 
     Because the amount of power required to heat the sensor  12  to the operating temperature is reduced, and/or because the sensor  12  is only heated when a reading is desired, the gas sensor  10  may be suited for battery powered and/or wireless applications. For example, the control electronics  28  may be powered by a battery  56 , and/or the control electronics  28  may wirelessly transmit an output signal from the gas sensor  10  via an antenna  58 . 
       FIG. 3  is a cross-sectional side view of another illustrative gas sensor in accordance with the present invention. The illustrative gas sensor is generally shown at  80 , and includes a substrate  82 , a support structure  84 , a sensor  86  and control electronics  88 . This embodiment is similar to that shown and described above with respect to  FIGS. 1-2 . However, rather than suspending the sensor above an etched pit  52  in the substrate, as shown in  FIG. 1 , a support structure is provided on the substrate that suspends the sensor  86  above the substrate  82 . A gap  90  or the like may be provided below the support structure  84  to help provide thermal isolation. Alternatively, or in addition, the support structure  84  may be formed from a material that has a low coefficient of thermal conductivity. Regardless of which approach is used, the sensor  10  and sensor  86  may be relatively thermally isolated from the substrate. 
       FIG. 6  is a schematic side view of an illustrative gas sensor assembly in accordance with the present invention. The gas sensor assembly is generally shown at  100 , and includes a housing  102 , a gas sensor  104 , and an absorber  106 . The gas sensor may be similar to those shown and described above with respect to  FIGS. 1-5 . 
     A gas sample  110  from an environment may be provided to the gas sensor  104  through the absorber  106 . The absorber may include an absorbent material that absorbs unwanted constituents or gases from the sample  110  before the sample  110  reaches the gas sensor  104 . For example, the absorber may absorb one or more interference gases. In some cases, interference gases can reduce the reliability or accuracy of the measurements made by the gas sensor  104 . 
     The gas sensor assembly  100  may further include a number of leads  108 . The leads  108  may provide a mechanical and/or electrical connection between the gas sensor assembly  100  and an external board or the like, when desired. 
     Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.