Thermal conductivity gas sensor with amplification material

In one example, a thermal conductivity gas sensor is disclosed. The sensor includes a sensing element and an amplification material coupled to the sensing element. The amplification material has a target gas dependent thermal diffusivity. The sensing element measures the thermal diffusivity of the amplification material to determine a target gas concentration.

CROSS-REFERENCE TO RELATED OR CO-PENDING APPLICATIONS

This application may relate to and incorporates by reference co-pending U.S. patent application Ser. No. 13/557,042 entitled “Gas Sensor”, filed on Jul. 24, 2012, by Humbert et al., and Published as US20130042669 A1 on Feb. 21, 2013. This potentially related application is assigned to NXP B.V. of Eindhoven, Netherlands.

BACKGROUND

Brief Background Introduction

This specification relates generally to systems and methods for sensing a gas, and in one example to sensing a concentration of a target gas, such as carbon dioxide.

SUMMARY

A gas sensor, comprising: a sensing element; and an amplification material coupled to the sensing element, and having a target gas dependent thermal diffusivity; wherein the sensing element measures the thermal diffusivity of the amplification material.

The above summary is not intended to represent every example embodiment within the scope of the current or future Claim sets. Additional example embodiments are discussed within the Figures and Detailed Description below.

DETAILED DESCRIPTION

A thermal conductivity gas sensor, also known as hot wire detector, operates on the principle that gases differ in their ability to conduct heat. This property is used for measuring gas concentration in mixtures where component gases have different thermal conductivity. Such gas sensors can be used in a number of different applications to sense the composition and/or concentration of various gases. One example application is in the field of supply chain monitoring, in which a level of carbon dioxide (CO2) present in the air surrounding consumables such as food or beverages is monitored to determine suitability for consumption. The monitoring may typically be carried out at various stages in the distribution chain. Other applications include air quality monitoring, use in heating, ventilation and air conditioning (HVAC) system in buildings or automobiles, or CO2 monitoring in a greenhouse.

The gas sensor configuration in one example consists of a heating element and a sensing element although the heater and the sensing element can be the same element. One example sensor consists of a filament of platinum or tungsten wire exposed to a gas and heated by an electrical current. In other common configurations, the sensor die consists of two thin-film resistors, the first one responds to target gas concentration variation, and the second is used for compensation of other external ambient changes (e.g. external temperature).

Calibration of a thermal conductivity gas sensor may begin by first heating the sensor until thermal equilibrium in the presence of a reference gas. After calibration the sensor is exposed to one or more target gases to be measured. The thermal equilibrium temperature of the gas sensor then increases or decreases depending on the target gases' thermal conductivity relative to that of the reference gas. The temperature change in one example results in a resistance change in the measurement element that is measured with a circuit to produce a digital reading proportional to the gas concentration.

Direct contact between the gas sensor's sensing element and the target gases can provide a percentage level accuracy; however, small ppm variations (˜50 ppm) for smart building applications for example, require greater accuracy. Also, since the thermal conductivity of some target gases can be very similar to the thermal conductivity of the total gas mixture received measuring small changes in target gas concentration can be challenging. For example the thermal conductivity of CO2 is similar to the thermal conductivity of the main component of air (e.g. nitrogen). Cross sensitivities with relative humidity, temperature, pressure and gas flow rate can also add noise to the target gas concentration measurement.

Now described is an example gas sensor structure, which combines a thermal diffusivity measurement and an amplification material sensitive to a target gas, thereby enhancing a performance of the sensor in terms of sensitivity and cross sensitivity management. Using the amplification material, the sensing element in the gas sensor can be better tuned to a particular target gas, instead of being uniformly exposed to all gases in a received gas flow. Such tuning increases the gas sensor's sensitivity (e.g. amplifies sensitivity to the target gas) enabling better ppm detection especially for target gases such as CO2 which have a thermal conductivity only slightly different than air.

The amplification material may interact with the target gas in a variety of ways, including: a reversible chemical reaction of the target gas with the material, or by an irreversible absorption of the target gas in the material.

Use of the amplification material around all or a portion of the sensing element also helps isolate the sensing element from cross sensitivities due to relative humidity, temperature, gas pressure, and gas flow rate. Such amplification material, properly positioned can also create a more stable mechanical structure which can be useful in particularly turbulent gas flows (e.g. no free-hanging wires over a cavity).

Thus the thermal conductivity structure will not sense the target gas (e.g. CO2) concentration directly from the received gas flow, but will sense it via an indirect mechanism involving an interaction between the target gas and the amplification material. As the amplification material reacts with the target gas, the amplification material will induce a stronger effect than just probing for the target gas in the received volume of gas. In other example embodiments, the amplification material can also be selected to be more reactive to the target gas and less reactive to relative humidity, thereby reducing cross sensitivities to relative humidity or other gases present in the received gas flow.

FIG. 1is a first example of a gas sensor100. The gas sensor100can be a thermal conductivity gas sensor, including a sensing element104and an amplification material108located within a cavity106on a semiconductor substrate102, and fabricated using CMOS processes. In one example, the amplification material108chemically or molecularly attracts a target gas110(e.g. concentrates the target gas) more than the sensing element104.

The amplification material108is shown encapsulating (e.g. covering) the sensing element104(e.g. a sensing wire). In an alternate example the amplification material108does not encapsulate the whole sensing element104, but rather is just in contact with one or more portions of the sensing element104. The amplification material108has a thermal diffusivity which is dependent upon a target gas110. In one example the target gas110is carbon dioxide (CO2).

The sensing element104is used to measure the thermal diffusivity of the amplification material108. The thermal diffusivity measurement is then translated into a target gas concentration. Thermal diffusivity α is defined as α=k/(ρ*Cp), where k is heat conductivity, ρ is density and Cp is heat capacity. Cp, k and ρ can individually increase or decrease by absorbing or reacting with the target gas110.

The sensing element104is electrically connected to a module (not shown) which calculates a target gas concentration based on the thermal diffusivity measurements from the sensing element104. The module can be either a dedicated circuit, a firmware or software program, or some combination of the two. In one example the gas sensor100is heated with by passing a current through either the sensing element104or a separate heating element (not shown). Once the gas sensor100has reached a thermal equilibrium sensing element104measurements are taken.

Depending on how the amplification material108reacts with the target gas, at least three modes of gas sensor100operation are possible.

In a First Mode the target gas reversibly modifies the thermal diffusivity of the amplification material108. Reversible modification is herein defined as a property which permits the amplification material108to both absorb and/or react and expel the target gas110without requiring any additional energy input.

In a Second Mode the target gas irreversibly modifies the thermal diffusivity of the amplification material108, after which the thermal diffusivity of the amplification material108needs to be reset to an initial thermal diffusivity by heating the amplification material108either with a separate heating element or with the sensing element104.

In one example, quickly heating the sensing element104to a relatively high temperature (higher than the normal operating temperature where thermal diffusivity is determined) will release the captured target gas110(e.g. CO2) from the saturated amplification material108. In this mode, both the thermal diffusivity change and the absolute value of the thermal diffusivity are monitored. Once the amplification material108is saturated with the target gas110, regeneration by heat or another technique will reset the material108.

In a Third Mode the target gas irreversibly modifies the thermal diffusivity of the amplification material108, and the above mentioned module calculates a target gas concentration based on an amount of heat energy required to reset the thermal diffusivity of the amplification material108to an initial thermal diffusivity. This technique monitors the total heat consumption (calorimetry) and the heat flow during the regeneration step. The amount of energy required for reversing an irreversible reaction (=regeneration) is directly proportional to the amount of target gas110that has reacted with the amplification material108between two consecutive regeneration steps. This will provide a second measure of the gas concentration.

While the First mode in one example may use less energy, the Second and Third modes can operate with a much wider range of amplification materials.

In one example the amplification material108is at least one from a group consisting of: a single amine solvent, a blended amine solvent, and polymer or metal-organic frameworks having an increase in target gas molar density compared to a molar density of the target gas in air.

Some example amplification materials108which interact with carbon dioxide are now listed. These materials either reversibly react with CO2 or irreversibly capture CO2. The irreversible capture is mostly with chemical bonds and can be reversible when the amplification material108are heated.

Alternatively, blended amine solvents, such as a mixture of 2 or 3 amines with different features can be used.

Also liquid solvents, or polymers and metal-organic frameworks that show an increase in CO2molar density compared to the CO2molar density in air can be used, such as:Polymer absorbing CO2Polymer chemically reacting to CO2ZeolitesMOF

In one example the thermal diffusivity of the amplification material108selected to vary more when exposed to the target gas, than does a thermal diffusivity of a received gas containing the target gas, thereby increasing the sensitivity of the gas sensor100to the target gas110.

A more specific example of a gas sensor100where the sensing element104is a resistive transducer is now discussed. In this example, the sensing element104has a first resistance when the amplification material108is exposed to a reference gas and a second resistance when the amplification material108is exposed to a target gas. The circuit or software module (discussed earlier) then converts a difference between the first resistance and the second resistance into a target gas concentration. The module may additionally include a timing device (not shown) which provides for a concentration or integration time for the target gas110to react with the amplification material108.

In this specific example, the measurement technique is a constant temperature (e.g. constant resistance) method. In these methods, the drive current (I) or voltage (V) is adapted as the target gas concentration varies. Then I or V as a function of target gas concentration is monitored. A constant power method could also be used where an applied input power is kept constant as the target gas concentration varies and ΔR/R is then monitored. Alternatively, the heater temperature can be swept by increasing the power and simultaneously tracking its resistance. Using this technique, the heat dissipated to the environment as a function of temperature will be known and a DSC measurement (Differential Scanning calorimetry) can be used to then determine the actual target gas concentration.

In other examples the sensing element104is either a capacitive transducer, an impedance transducer, or an optical transducer, each of which take measurements that the module translates into target gas concentrations.

FIG. 2is a second example of a gas sensor200. In this example, the sensing element104, amplification material108, cavity106and semiconductor substrate102are as described inFIG. 1. One difference fromFIG. 1is that in one example the amplification material108does not fully encapsulate (e.g. cover) the sensing element104(e.g. a sensing wire). Another difference fromFIG. 1is that in another example the amplification material108is deposited on top of the sensing element104(e.g. thermal conductivity electrodes) and seals the top of the cavity106.

FIG. 3is a third example of a gas sensor300. In this example, a silicon substrate102is first covered by an amplification material302. A sensing element304is then fabricated on a side of the amplification material302opposite to the substrate102. Thus the sensitive material (i.e. the amplification material302) is deposited below the thermal conductivity electrodes (i.e. the sensing element304).

FIG. 4is a fourth example400of a gas sensor402paired with a matched gas sensor404. The matched gas sensor404includes a sensor element406formed within a cavity408in the substrate102. The gas sensor402is similar to the gas sensor100discussed inFIG. 1, however, the matched gas sensor404functions as a duplicate reference thermal conductivity structure which is not exposed to the target gas110(i.e. separated from the target gas110by a barrier410). The matched gas sensor404is used to compensate for other types of gas sensor402sensitivity (e.g. relative humidity, gas pressure, gas flow and so on).

FIG. 5is a fifth example of a gas sensor500. In this example, the sensing element104, cavity106and semiconductor substrate102are as described inFIG. 1. One difference fromFIG. 1is that in this example gas sensor500a liquid amplification material502encapsulates all or a portion of the sensing element104. A cap material504porous to the target gas110keeps the liquid amplification material502in partial or total contact with the sensing wire104.