An apparatus for first determining the thermal conductivity and / or the specific heat capacity of a gas mixture and second determining the density and / or the viscosity of the gas mixture

Abstract

The invention relates to a device comprising a chip (10; 110), which is prepared in particular from an SOI wafer and which contains a first oscillatable cantilever (20; 120) with a first piezoelectric transducer (30; 130) for exciting a cantilever oscillation, wherein the first cantilever (20) projects into a recess (11; 111) in the chip, at least one first temperature sensor element (42) arranged on the cantilever side in an end portion of the first cantilever for detecting the temperature of a gas mixture, at least one second temperature sensor element (50) spaced apart from the first cantilever (20; 120) for detecting the temperature of the gas mixture, at least one heating element (44) in a free end portion of the cantilever (20; 120), which heating element is separated from the second temperature sensor element by at least one recess portion, and an evaluation unit (60) for determining the density and the viscosity and also the thermal conductivity or the specific heat capacity of the gas mixture on the basis of the oscillation behavior of the at least one cantilever (20; 120) or on the basis of the measured temperature values of the first temperature sensor element (42) and the second temperature sensor element (50; 150, 240) arranged on the cantilever side and the power consumption of the heating element (44).

Description

Technical Field

[0001] This invention relates to an apparatus for first determining the thermal conductivity and / or specific heat capacity of a gas mixture and secondly determining the density and / or viscosity of the gas mixture. Based on density and viscosity, the energy content or Wobbe index of a combustible gas can be determined via related methods, such as those described, for example, in International Publication WO 2017 063 795A1. For example, a microsystem apparatus for determining the viscosity and density of a gas, as well as trace gas concentrations or moisture, is disclosed in patent application DE 10 2019 123 874.9, which has not yet been published. Background Technology

[0002] However, relying solely on the parameters, density, and viscosity mentioned above is insufficient to accurately account for the influence of hydrogen. This is not a problem, as long as the hydrogen content in the gas mixture under investigation is negligible. This condition is not frequently met in cases where the importance of hydrogen is foreseeably increasing. However, the hydrogen fraction can be correctly determined when the thermal conductivity of the gas mixture is additionally determined, as described in international publication WO 2018 082 875A1. Publication WO 2017 131670A1 discloses a system for determining the density of a gas mixture using a vibration sensor and for determining other parameters of the gas mixture, particularly the thermal conductivity, using an external sensor. However, the system described there has large space requirements and is too expensive for large-scale applications. Therefore, the object of the present invention is to provide a remedy in this situation. Summary of the Invention

[0003] This objective is achieved by the apparatus as defined in independent claim 1.

[0004] The apparatus of the present invention for first determining the thermal conductivity and / or specific heat capacity of a gas mixture and secondly determining the density and / or viscosity of the gas mixture comprises: a chip fabricated from a semiconductor wafer, particularly an SOI wafer, and including at least a first oscillating cantilever having a first piezoelectric transducer for exciting oscillation of the cantilever at a first frequency, wherein the chip has a cavity and the first cantilever extends into the cavity with an oscillating free end; at least a first cantilever-side temperature sensor element, the first cantilever-side temperature sensor element being specifically implemented in the free end of the first cantilever and having a temperature-dependent resistance value for recording the temperature of the gas mixture; at least a second temperature... The system includes a sensor element, the second temperature sensor element being spaced apart from the first cantilever and having a temperature-dependent resistance value for recording the temperature of the gas mixture; at least a first heating element, the first heating element being specifically implemented in the free end section of the cantilever, wherein at least one segment of the cavity extends between the first heating element and the second temperature sensor element; and an evaluation and / or computer unit adapted to determine the density and viscosity of the gas mixture based on the oscillation characteristics of the cantilever, and to determine the thermal conductivity and / or specific heat capacity of the gas mixture based on the measured temperatures of the first cantilever-side temperature sensor element and the second temperature sensor element and the power consumption of the heating element.

[0005] Determining the two thermophysical variables, thermal conductivity and specific heat capacity, provides redundancy in determining calorific value and thus increases reliability. Fundamentally, compared to the method described in WO 2018 082 875A1, this provides more information about the gases involved and their concentrations when determining the composition of binary, ternary, or quaternary gas mixtures, thereby enabling a more accurate determination of the gas composition.

[0006] In a further development of the invention, the chip includes a second oscillating cantilever with a second piezoelectric transducer for exciting oscillation at a second frequency, wherein the second cantilever extends into a cavity with an oscillating free end and can be exposed to a gas mixture.

[0007] In a further development of the invention, a second heating element is arranged in the free end section of the second cantilever.

[0008] The cavity into which the cantilever extends can comprise, for example, a rectangle with a side length not longer than 2 mm, particularly not longer than 1 mm, wherein the cantilever extends into the cavity with a length of several 100 μm, for example, 300 μm to 500 μm, wherein the longitudinal axes of the cantilever are separated from each other by several 10 s to, for example, 20 10 s or 200 μm. The cantilever can extend into the cavity, particularly from opposite sides. The heating elements and temperature sensor elements of the different cantilevers can be segmented, for example, by a shared transverse plane that extends perpendicular to the longitudinal axis of the cantilever. This means that the associated thermal coupling between the heating elements and temperature sensor elements of the different cantilevers is via the gas mixture. Due to the small separation, the timescale of heat transfer between the heating element and the temperature sensor element is correspondingly short, which in turn enables the measurement of the device's rapid response time to changes in the composition of the gas mixture.

[0009] In a further development of the invention, a second cantilever-side temperature sensor element for recording the temperature of a gas mixture using temperature-dependent resistance values ​​is arranged in the free end section of the second cantilever.

[0010] In a further development of the invention, the chip includes an edge region surrounding the cavity, wherein a reference temperature sensor element is disposed in the edge region, and wherein at least one segment of the cavity extends between the reference temperature sensor element and a free end segment.

[0011] According to another development of the invention, the piezoelectric transducer of the cantilever is arranged between segments in the edge region in each case, wherein the cantilever is connected to the chip and the cantilever-side temperature sensor element or heating element.

[0012] In a further development of the invention, the evaluation and / or computer unit is adapted to use the temperature measurement value of the reference temperature sensor element as the temperature measurement value of the second temperature sensor element.

[0013] In a further development of the invention, the evaluation and / or computer unit is adapted to use the temperature measurement value of the second cantilever side temperature sensor element as the temperature measurement value of the second temperature sensor element.

[0014] In a further development of the invention, the evaluation and / or computer unit is adapted to determine the thermal conductivity of the gas mixture in a steady state or by a transient method.

[0015] For example, the product information for Xensor's XEN-SP3880 describes an example of determining thermal conductivity under steady-state conditions.

[0016] For example, the determination of thermal conductivity in the transient method is described in Woodfield et al., Int. J. Thermophysics (2008) 29: 1299-1320, and in Gustafsson et al., 1979, Transient hot-strip method for simultaneously measuring thermal conductivity and thermal diffusivity of solids and fluids. Journal of Physics D: Applied.

[0017] Thermal conductivity measurements generally assume that the relative movement between the medium and the measuring device is negligible. However, in current measuring devices, at least one heating element involved in the measurement and one temperature sensor element involved in the measurement are arranged on an oscillating cantilever. As the cantilever oscillates, heat transfer must travel via shear waves. The degree of this measurement interference corresponds to the penetration depth of the shear waves into the gas mixture. This can be estimated using the thickness δ of the viscous boundary layer according to the following formula:

[0018]

[0019] Where η and ρ are the dynamic viscosity and density of the medium, respectively, and f is the oscillation frequency of the cantilever. With frequencies between 10 kHz and 100 kHz and pressures between 0.1 MPa and 1 MPa, penetration depths ranging from a few μm to, for example, 100 μm are generated, for example, by gases from a list including Ar, N2, CH4, CO2, He, and H2. Depending on the separation between the heating sensor and the temperature sensor element in the thermal conductivity measurement, the penetration depth can be neglected or not. When the penetration depth, for example, corresponds to less than 1% of the separation, sufficiently accurate results are possible, allowing thermal conductivity measurements to be performed with an oscillating cantilever. In contrast, when the penetration depth corresponds to 10% or more of the separation, depending on accuracy requirements, performing thermal conductivity measurements when the oscillation is paused by a stationary cantilever can be appropriate. Furthermore, the effect of oscillation in the case of thermal conductivity measurement can be determined by using an oscillating cantilever and then a stationary cantilever to perform the thermal conductivity measurement, wherein correction values ​​are determined from the deviation between the measurement results, and these correction values ​​can then be used to correct the preliminary results of the thermal conductivity measurement with an oscillating cantilever. It is possible to determine correction values, for example, in a factory calibration setting, and, given conditions, to describe the correction values ​​using a corresponding model. It is possible to follow equivalent procedures, either periodically or in a controlled manner, during ongoing measurement operations, for example, after establishing changes in gas composition based on measurements of density, viscosity, and thermal conductivity.

[0020] In a further development of the invention, the evaluation and / or computer unit is adapted to determine the specific heat capacity of a gas mixture in a method using variable heating power in a first heating element, wherein the temperature of a second temperature sensor element is evaluated over time in response to the temperature of the first heating element to determine the product of the specific heat capacity and density of the gas mixture. Methods for determining specific heat capacity are generally known and are described, for example, in the articles by Woodfield et al. and Gustafsson et al. mentioned above.

[0021] In a first embodiment of this additional development of the invention, method steps are provided for evaluating the step function response of a second temperature sensor element to a heating power step function of a first heating element.

[0022] In a second embodiment of this additional development of the invention, the temperature of the second temperature sensor element is evaluated over time in terms of phase and amplitude in response to harmonic excitation of the first heating element.

[0023] In a further development of the invention, the evaluation and / or computer unit is adapted to determine the specific heat capacity by dividing the product by a density determined based on the oscillation frequency of one of the cantilever arms.

[0024] In a further development of the device of the present invention, at least one cantilever includes a functionalized surface for selective adsorption of components of a gas mixture surrounding the device, said components being in particular water, wherein the evaluation and / or computer unit is further adapted to influence the amount of adsorbed components by means of a heating element of the cantilever and to determine the amount of adsorbed components based on a measurement frequency of the oscillating cantilever compared with at least one reference frequency.

[0025] In a further development of the invention, at least one reference frequency includes the frequency of the cantilever under conditions of complete desorption of the component and / or the frequency of the nonfunctional cantilever.

[0026] In a further development of the invention, the chip has a functionally related structured layer sequence fabricated on an SOI substrate having an oxide layer, an adhesion-promoting layer, a first electrode layer, a piezoelectric layer, a second electrode layer, and a passivation layer. The piezoelectric transducer or piezoelectric transducer region comprises two electrode layers and a piezoelectric layer. A temperature sensor element is formed in each case by constructing regions of the electrode layers, particularly the first electrode layer. A first heating element or heating element is formed in each case by constructing regions of one of the electrode layers, particularly the second electrode layer. The adhesion-promoting layer and piezoelectric layer particularly comprise aluminum nitride. The first and second electrode layers particularly comprise platinum. The passivation layer particularly comprises aluminum oxide. Method steps for fabricating and constructing this layer sequence are described in the unpublished patent application DE 102019 123874.9. The passivation layer, not mentioned therein, can be fabricated, for example, by means of ALD (atomic layer deposition). Attached Figure Description

[0027] The invention will now be described in more detail based on examples of embodiments shown in the accompanying drawings. The figures are shown below:

[0028] Figure 1 A schematic longitudinal section of a cantilever, representing an embodiment of the device of the present invention;

[0029] Figure 2 A plan view of a chip in a first example embodiment of the device of the present invention;

[0030] Figure 3 A plan view of the chip in a second example embodiment of the device of the present invention; and

[0031] Figure 4 A schematic general view of an example embodiment of the device of the present invention. Detailed Implementation

[0032] Figure 1The layer structure of a single cantilever 20 of the chip of the device of the present invention is shown in a longitudinal section, wherein the layer structure is preferably prepared and constructed on an unseparated wafer before the wafer is separated into individual chips. For example, an SOI wafer (SOI stands for silicon on insulator) can be used as the basis for producing the cantilever 20. Such an SOI wafer includes a thin silicon layer 10c with a thickness of, for example, 3 μm to 5 μm, which is separated from the silicon substrate 10a by an insulating layer 10b (e.g., SiO2). An insulating layer 10d is applied on the thin silicon layer 10c. The insulating layer 10d can be, for example, a silicon oxide layer, which is preferably formed on the thin silicon layer 10c via a wet oxidation step. Optionally applied on the insulating layer 10d in a next manufacturing step is an adhesion promoting layer 10e. The adhesion promoting layer 10e can include, for example, aluminum nitride (AlN), which is preferably applied with a layer thickness of less than 50 nm.

[0033] A piezoelectric transducer 30 is applied to the adhesion promoting layer 10e. The piezoelectric transducer 30 includes regions of a layer sequence comprising: a first conductive electrode layer 10f applied to the adhesion promoting layer 10e; a piezoelectric layer 10g applied to the first electrode layer 10f with a thickness of several 100 nm (e.g., 500 nm); and a second electrode layer 10h applied to the piezoelectric layer 10g. For example, electrode layers 10f and 10h can comprise platinum, wherein the first electrode layer 10f has a thickness of, for example, 20 nm to 30 nm, while the second electrode layer 10h has a thickness of, for example, 150 nm to 250 nm. This layer structure is typical for the cantilever of the chip in the device of the present invention.

[0034] Temperature sensor element 42 and heating element 44 are still fabricated on the free ends of the cantilever element 20 in the two electrode layers 10f and 10h. This can be achieved, for example, by a peeling process. Preferably, the electrode layers in this case are configured such that they have a meandering structure serving as the temperature sensor element and heating element, wherein the heating element has a resistance of several hundred ohms, while the temperature sensor element has a resistance equivalent to 1 to 2 kiloohms. The length of the cantilever is, for example, 300 μm to 500 μm, resulting in an oscillation frequency on the order of 20 kHz to 80 kHz.

[0035] The temperature sensor element 42 and the heating element 44 are collectively referred to as the thermal functional block 40, which is separated from the piezoelectric transducer by an isolation groove with a width of several μm.

[0036] The entire surface of the chip is coated with a passivation layer of aluminum oxide with a thickness of several 10 nm, for example, in the ALD process.

[0037] Optionally, a selective adsorption layer 10j, such as a hygroscopic layer, like a hydrogel, zeolite, or silica gel, can still be deposited on the thermal functional block above the passivation layer. By influencing the temperature using the heating element 44, the adsorption or desorption of the components can be controlled, wherein the amount of adsorption can be determined via the oscillation frequency of the cantilever.

[0038] To improve sensitivity, an adsorption layer 10j can also be deposited on the piezoelectric transducer 30, wherein the region of the second electrode layer 10h that forms the electrode of the piezoelectric transducer 30 is preferably also constructed as a heating element.

[0039] Figure 2 A plan view of chip 10 is shown, in which the cantilever 20 of chip 10 is shown. Figure 1 In the middle, the cantilever 20 is freed by creating a cavity 11 using DRIE. The temperature sensor element of the thermal functional block 40 is contacted via a first conductive trace pair 12 prepared in the first electrode layer. The heating element of the thermal functional block 40 is contacted via a second conductive trace pair 14 prepared in the second electrode layer. The electrode of the piezoelectric transducer 30 is contacted via a third conductive trace pair 16, which in each case has one conductive trace in an electrode layer. The reference temperature sensor element 50 in the edge region 22 of the chip 10 and the fourth conductive trace pair 52 contacting it are prepared in the first electrode layer.

[0040] Figure 3 A plan view of chip 110 is shown, in which cantilevers 120, 220 are freed by means of cavities 111 fabricated using DRIE. Each cantilever has thermal functional blocks 140, 240 and piezoelectric transducers 130, 230. For the first cantilever 120, the following contact pattern applies: the first temperature sensor element of the first thermal functional block 140 is contacted via a first conductive trace pair 112 fabricated in the first electrode layer. The heating element of the thermal functional block 140 is contacted via a second conductive trace pair 114 fabricated in the second electrode layer. The electrode of the first piezoelectric transducer 130 is contacted via a third conductive trace pair 116, which in each case has one conductive trace in one electrode layer. For the second cantilever 220, the following contact pattern applies: the second temperature sensor element of the second thermal functional block 240 is contacted via a first conductive trace pair 212 fabricated in the first electrode layer. The electrodes of the first piezoelectric transducer 230 are contacted via a third conductive trace pair 216, which in each case has one conductive trace in an electrode layer. The reference temperature sensor element 150 in the edge region 122 of the chip 110 and the fourth conductive trace pair 152 contacting it are fabricated from the first electrode layer.

[0041] Figure 4The illustration shows an example of the typical construction of an embodiment of the device 1 of the present invention, which is connected to a pipeline 70 for analyzing a gas mixture transported in the pipeline. Device 1 includes the aforementioned chip 10, which is disposed in a sample chamber 62 communicating with the pipeline 70 via connecting lines 64, 66. A diaphragm 72 is disposed in the pipeline between the connection points of the connecting lines and the pipeline 70. This diaphragm 72 influences the pressure gradient under volumetric flow through the pipeline, which in turn influences the volumetric flow through the sample chamber 62, ensuring that the chip is always supplied with a currently representative sample of the gas mixture. Device 1 further includes an evaluation and / or computer unit 60, to which the chip 10 is connected, and is adapted to determine the density and viscosity of the gas mixture based on the oscillating characteristics of at least one cantilever, and to determine the thermal conductivity and / or specific heat capacity of the gas mixture based on temperature measurements from a first cantilever-side temperature sensor element and a second temperature sensor element, as well as the power consumption of a heating element. When the cantilever includes a functionalized surface, the concentration of the component may also be specifically determined.

[0042] A particular advantage of this invention lies in the fact that the thermal functional blocks, having temperature sensors and heating elements, are positioned relative to each other at the free oscillating end of an oscillating cantilever element, making reliable measurement of the thermophysical parameters of the gas mixture surrounding the cantilever, such as thermal conductivity and heat capacity, possible with high accuracy. Since the mechanical properties of the gas mixture, such as density and viscosity, are measured via the oscillation frequency and damping of the same cantilever element, they are actually measured at the same location, so various measurement data come from identical samples in virtually the same thermodynamic state. This achieves highly accurate measurements with minimal effort. Furthermore, the method according to WO 2018 082875A1 can be implemented more accurately and cost-effectively. Similarly, the methods for measuring thermal conductivity and specific heat capacity according to Woodfield et al. or Gustafsson et al. can be performed with significantly reduced effort. For the procedure for measuring moisture and determining concentration by adsorption of components, refer to the unpublished patent application DE 10 2019 123 874.9.

Claims

1. An apparatus for first determining the thermal conductivity and / or specific heat capacity of a gas mixture and secondly determining the density and / or viscosity of the gas mixture, comprising: A chip (10; 110), said chip (10; 110) is fabricated from a semiconductor wafer and includes a first oscillating cantilever (20; 120) having at least a first piezoelectric transducer (30; 130) for exciting the oscillation of the first oscillating cantilever (20; 120) at a first frequency. The chip (10; 110) has a cavity (11; 111), and the first oscillating cantilever (20) extends into the cavity (11; 111) with an oscillating free end section; At least a first cantilever-side temperature sensor element (42) is specifically implemented in the free end section of the first oscillating cantilever and has a temperature-dependent resistance value for recording the temperature of the gas mixture; At least a second temperature sensor element (50) is arranged at a distance from the first oscillating cantilever (20; 120) and has a temperature-dependent resistance value for recording the temperature of the gas mixture; At least a first heating element (44), specifically implemented in the free end segment of the first oscillating cantilever (20; 120), wherein at least a portion of the cavity (11; 111) extends between the first heating element (44) and the second temperature sensor element (50; 150, 240); and An evaluation and / or computer unit (60) is adapted to determine the density and viscosity of the gas mixture based on the oscillation characteristics of the first oscillating cantilever (20; 120), and to determine the thermal conductivity and / or specific heat capacity of the gas mixture based on the measured temperatures of the first cantilever-side temperature sensor element (42) and the second temperature sensor element (50; 150, 240) and the power consumption of the first heating element (44).

2. The device (1) according to claim 1, wherein, The semiconductor wafer is an SOI wafer.

3. The device (1) according to claim 1, wherein, The chip (110) includes a second oscillating cantilever (220) having a second piezoelectric transducer (230) for excitation oscillation at a second frequency, wherein the second oscillating cantilever (120) extends into the cavity with an oscillating free end and can be exposed to the gas mixture.

4. The device (1) according to claim 3, wherein, A second cantilever-side temperature sensor element (240) for recording the temperature of the gas mixture with a temperature-dependent resistance value is specifically implemented in the free end section of the second oscillating cantilever (220).

5. The device (1) according to claim 3 or 4, wherein, The second heating element is arranged in the free end section of the second oscillating cantilever (220).

6. The device (1) according to any one of claims 1 to 4, wherein, The chip includes an edge region surrounding the cavity, wherein a reference temperature sensor element is disposed in the edge region, and wherein at least a portion of the cavity extends between the reference temperature sensor element and the free end segment.

7. The device (1) according to claim 6. in, The evaluation and / or computer unit is adapted to use the temperature measurement value of the reference temperature sensor element as the temperature measurement value of the second temperature sensor element.

8. The device (1) according to claim 4, wherein, The evaluation and / or computer unit is adapted to use the temperature measurement value of the second cantilever side temperature sensor element as the temperature measurement value of the second temperature sensor element.

9. The device (1) according to any one of claims 1 to 4, wherein, The evaluation and / or computer unit is adapted to determine the thermal conductivity of the gas mixture under steady-state conditions or in a transient manner.

10. The device (1) according to any one of claims 1 to 4, wherein, The evaluation and / or computer unit is adapted to determine the specific heat capacity of the gas mixture using a method with variable temperature of the first heating element, wherein the temperature of the second temperature sensor element is evaluated over time in response to the temperature of the first heating element in order to determine the product of the specific heat capacity and density of the gas mixture.

11. The device (1) according to claim 10, wherein, The method evaluates the step function response of the second temperature sensor element to the temperature step function of the first heating element.

12. The device (1) according to claim 10, wherein, In response to the harmonic excitation of the first heating element, the temperature of the second temperature sensor element is evaluated over time in terms of phase and amplitude.

13. The device (1) according to claim 10, wherein, The evaluation and / or computer unit is adapted to determine the specific heat capacity by dividing the product by the density determined based on the oscillation frequency of one of the cantilever arms.

14. The device (1) according to any one of claims 1 to 4, wherein, At least one cantilever includes a functionalized surface (10j) for selective adsorption of components of a gas mixture surrounding the device (1), wherein the evaluation and / or computer unit is further adapted to influence the amount of adsorbed components by means of the heating element of the at least one cantilever, and to determine the amount of adsorbed components based on the measurement frequency of the at least one cantilever compared with at least one reference frequency.

15. The device (1) according to claim 14, wherein, The component is water.

16. The device (1) according to claim 14, wherein, The at least one reference frequency includes the frequency of the at least one cantilever under conditions of complete desorption of the component and / or the frequency of the nonfunctional cantilever.

17. The device (1) according to any one of claims 1 to 4. in, The chip (10) has a functionally related structured layer sequence, which is fabricated on an SOI substrate (10a, 10b, 10c) having an oxide layer (10d), an adhesion-promoting layer (10e), a first electrode layer (10f), a piezoelectric layer (10g), a second electrode layer (10h), and a passivation layer (10i). The first piezoelectric transducer and the second piezoelectric transducer include the two electrode layers (10f, 10h) and the piezoelectric layer (10g). The first cantilever-side temperature sensor element and the second temperature sensor element are formed by constructing a region of one of the electrode layers (10f, 10h). The first heating element is formed by constructing regions of electrode layers (10f, 10h).

18. The device (1) according to claim 17. in, The adhesion-promoting layer (10e) and the piezoelectric layer (10g) comprise aluminum nitride.

19. The device (1) according to claim 17. in, The first electrode layer (10f) and the second electrode layer (10h) comprise platinum.

20. The device (1) according to claim 17. in, The passivation layer (10j) comprises aluminum oxide.

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