WO2014012951A1

WO2014012951A1 - Mems device with improved filter

Info

Publication number: WO2014012951A1
Application number: PCT/EP2013/065040
Authority: WO
Inventors: Hervé DELPRAT
Original assignee: Sgx Sensortech Sa
Priority date: 2012-07-16
Filing date: 2013-07-16
Publication date: 2014-01-23

Abstract

A gas sensor comprising: at least one active element; and a filtering cap comprising a porous filtering element for counteracting the ingress of one or more atmospheric gases to the active element. There is a separation between the active element and the filtering element of the cap and the filtering element comprises a mechanically densified powder of filtering material.

Description

MEMS DEVICE WITH IMPROVED FILTER

Field of the invention

This invention relates to gas sensors. In particular, though not exclusively, this invention relates to filtering components for gas sensors, gas sensors comprising such filtering components, and associated methods.

Background to the invention

Gas sensors are known in the art for detecting gases based on a variety of techniques. Such sensors include at least one sensing (or active) element sensitive to the presence or concentration of one or more gases. One known class of gas sensors are catalytic gas sensors, or pellistors, which detect the presence of combustible gases. In pellistors, a detector element provides an electrical resistance measurement dependent on the presence of a combustible gas. In particular, the resistance in the detector element varies with changes in temperature produced by the catalytic oxidation of a combustible gas if present. To facilitate combustion of gas, the sensors are operated at an elevated temperature, such as greater than 300°C, for example in the range of from 450 °C to 750°C.

The catalytic sites in a pellistor detector element are prone to inhibition or poisoning. Inhibition of catalyst tends to be reversible following removal of the inhibitor, whereas poisoning species tend to cause long-term damage. Inhibiting species include sulphur species, such as H₂S and organosulfur compounds, which have a particular affinity with catalytic metals, such as Group 10 metals, including palladium and platinum, typically employed in detector elements of pellistors. Poisoning species include siloxanes, for example

hexamethyldisiloxane, which form a layer of silicon dioxide over active sites at the elevated operating temperature of the sensor. Inhibition or poisoning of active sites reduces the sensitivity of a sensor and ultimately leads to failure. This problem occurs not only in pellistors but also in other gas sensors sensitive to inhibiting and/or poisoning species, hereinafter collectively referred to as harmful species. EP1 151285 discloses a gas sensor comprising an active element surrounded by a porous insulating material. The insulating material helps to protect the active element. However, this solution is unsuitable for sensors comprising small active elements, such as MEMS structures, where thermal losses to a surrounding material would lead to inaccuracies and unacceptably high power consumption and where mechanical fragility may be an issue.

There remains a need for effective solutions to counteract the ingress of inhibiting and/or poisoning species to active sites in detector elements of gas sensors.

It is an object of the invention to overcome or mitigate at least one problem associated with the prior art.

Statements of the invention

According to a first aspect of the present invention there is provided a gas sensor comprising: at least one active element; and a filtering cap comprising a porous filtering element for counteracting the ingress of one or more atmospheric gases to the active element, wherein there is a separation between the active element and the filtering element of the cap and wherein the filtering element comprises a mechanically densified powder of filtering material.

The filtering cap may be made and sold as an independent component. Thus, from a second aspect, the invention embraces a filtering cap for a gas sensor, the cap comprising a porous filtering element comprising a mechanically densified powder of filtering material, the cap being shaped to receive an active element of a gas sensor with a separation between the active element and the filtering element. The term "densified powder" is used herein to describe powders having a densified or compacted bulk density (mass/volume ratio) which is greater than a Standard Bulk Density of the powder measured according to method I in United States Pharmacopeial Convention Chapter 616 "Bulk and Tapped Density", 1 December 2012. In preferred embodiments of the invention the densified powder has a compacted bulk density that is at least two times, at least three times, at least four times, at least five times or even at least ten times greater than the relevant Standard Bulk Density. In embodiments the densified powder has a compacted bulk density which is greater than a Standard Tapped Density of the powder measured according to method I in United States Pharmacopeial Convention Chapter 616", 1 December 2012. The powder is densified mechanically, i.e. by compaction through the application of a pressure. In embodiments of the invention the powder may be mechanically densified at low temperature, such as a temperature that substantially avoids sintering of the powder. For example, the powder may conveniently be densified at a

temperature below 100°C, such as below 50°C or at ambient temperature.

Advantageously, the separation may comprise a gap. In one embodiment the filtering cap comprises a formation, such as a cavity, for accommodating the active element. The filtering cap may comprise a structure for supporting the filtering element. In one embodiment the filtering element is itself cap-shaped.

A separation between the filtering element and the active (or sensing) element has been found advantageous particularly in the context of avoiding thermal losses from the active element. Furthermore, mechanical damage to the active element is more readily avoided. Thermal losses have been found to be of particular concern in the design of small-scale active elements, for example based on MEMS technology. Firstly, thermal losses lead to an increase in power consumption, whereas low power consumption is a key objective of miniaturisation. Secondly, on account of the low thermal mass of small-scale active elements, thermal losses rapidly lead to temperature variations which affect sensor accuracy. Also, mechanical damage to the active element tends to be a particular concern in small-scale active elements.

In preferred embodiments of the invention, the active element is a small-scale active element. Such an element may be defined as having a heated active volume of less than 0.1 mm³ or even of less than 0.01 mm³.

In one embodiment, the active element is a catalytic (or pellistor) element designed to catalyse an exothermic oxidation of combustible gases. For example, the active element may comprise or consist of a detector composition comprising a combustion catalyst. The detector composition may

advantageously be ceramic or based on one or more refractory oxides. An active catalytic element may be present in conjunction with a resistive element for heating the active element and determining the temperature of the active element to provide a signal representative of the presence or concentration of a combustible gas. A parallel compensator element may also be provided in such as sensor as is known in the art.

In another embodiment, the active element is a chemoresistive gas-sensitive element designed to change its resistivity in the presence of gas. Suitably a chemoresistive gas-sensitive element may comprise one or more pure or doped semiconducting metal oxides, as is known in the art. In an embodiment the gas sensor comprises a heating element for heating the active element. In an embodiment, the heating element may operate at an energy consumption of less than 300 mW or even less than 100 mW when in continuous operation to achieve an operating temperature, for example in the range of from 300 to 700°C, such as about 500 °C. In an embodiment, the heating element has a maximum diameter in plan of less than 1 mm. In an embodiment the heating element is generally circular in plan but other shapes may also be used. The heating element may, for example, be a resistive heating element. Micro- hotplates comprising such resistive heating elements may be formed using MEMS techniques, e.g. from vapour deposited layers of metal or

semiconductors. The heating element may be integral with the active element. In an embodiment the heating element is as described or defined in our parallel international application also claiming priority from PCT/EP2012/063931.

The filtering element may comprise one or more filtering materials consistent with counteracting the ingress of one or more inhibiting or poisoning species to the active element. In an embodiment, the filtering element has a thickness in the range of from 0.4 mm to 3 mm, such as in the range of from 0.5 to 2 mm.

On account of the separation between the filtering element and the active element a particularly wide range of materials may be used. The filtering material or filtering element may, for example, comprise or consist of metal or non-metal oxides, zeolites, ceramics, silica, alumina, micro/nano porous carbons, or other porous compounds such as nitrides and carbides which are

activated/impregnated or not, organic polymers, and mixtures thereof. In one embodiment, the filtering material comprises a transition metal oxide. In one embodiment the filtering material comprises one or more of ZnO, CuO and Ti0₂. Metallic dopants may be used to increase the activity of the filtering materials. Such metallic dopants may be, for example, one or more of Pd, Pt, Cu, Ag, etc.

Properties of the filtering material may be optimised in the context of particular active elements, sensor designs and operational environments. However, in all sensors a degree of porosity is required in the filtering material to permit the ingress of gas to be detected by the active element. A paradox therefore exists between maintaining sufficient porosity to provide a sensitive sensor whilst counteracting the ingress of harmful species.

In preferred embodiments of the invention, the filtering element, or the filtering cap as a whole, has a pore volume in the range of from 0.1 to 2.0 cm³/g, preferably in the range of from 0.15 to 1.5 cm³/g, more preferably in the range of from 0.2 to 1.3 cm³/g.

Surface properties of filtering materials play a crucial role in overcoming the paradox between sensitivity and ingress of harmful species. However, such surface properties may in turn be affected by processing to achieve a particular porosity. A complex picture therefore arises for optimising filtering materials, further accentuated in miniaturised sensors on account of limited space.

The present invention provides certain benefits independently of optimising filtering materials. Accordingly, whilst optimisation is preferred, the invention also embraces embodiments in which the filtering element is not optimised.

Mechanical densification of the powdered filtering material has been found to offer a solution to achieving suitable porosities coupled with excellent retention of surface properties. The filtering element may preferably be free from thermally sintered filtering material. Heating, and in particular sintering, may affect the surface properties negatively. Similarly, the filtering element need not include binders which could detrimentally affect the properties of the filtering material.

As aforesaid, mechanical densification of filtering material is facilitated in the present invention since there is separation between the active element and the filtering element. Without separation, in-situ densification of filtering material might more easily result in damage to the active element.

The particle size of a filtering material has an impact on its porosity and surface properties. In embodiments of the invention the filtering material comprises a powder having a (weight) average particle size of less than 500 nm, preferably less than 300 nm, more preferably less than 100 nm, such as in the range of from 5 to 30 nm. Finer powders having a (weight) average particle size of less than 50 nm have been found to perform well in mechanically densified filtering elements. In an embodiment, the filtering materials or the components which result from their shaping have a specific surface area in the range of from 5 m²/g to 6000 m²/g, e.g. 10 m²/g to 1000 m²/g. In some embodiments the specific surface area may be in the range of from 400 m²/g to 1000 m²/g, such as 500 to 800 m²/g. In other embodiments the specific surface area may be in the range of from 10 m²/g to 100 m²/g.

Of course the chemical nature of the filtering material may be chosen to target particular harmful species. In embodiments where the ingress of sulphur compounds, in particular organosulfur compounds and H₂S, is to be

counteracted, the filtering material may advantageously be selected from the group comprising: activated carbon, transition metal oxides such as copper oxide, zinc oxide, iron oxide, manganese dioxide, aluminium oxide and mixtures thereof.

In one embodiment, the filtering material comprises transition metal oxide, such as zinc oxide and/or copper oxide. Advantageously the oxide may have a weight average particle size of less than 250 nm and/or a specific surface area in the range of from 20 to 200 m²/g. In an embodiment the oxide has a Standard Bulk Density in the range of from 0.1 to 1.5 g/cm³ and/or a true density in the range of from 4 to 15 g/cm³. In an embodiment the mechanically densified oxide has a pore volume in the range of from 0.2 to 0.4 cm³/g and/or a compacted bulk density in the range of from 1 to 2.5 g/cm³

In embodiments where the ingress of organosilicon compounds is to be counteracted, the filtering material may advantageously comprise one or more of: silica, alumina, zeolite, and mixtures thereof. In one embodiment, the filtering material comprises a silica powder. In an embodiment, the particles of the silica powder may be substantially spherical. In preferred embodiments the silica powder comprises a particle size as defined hereinabove and/or a specific surface area as defined hereinabove. In an embodiment the silica powder has a Standard Bulk Density in the range of from 0.02 to 0.1 g/cm³, e.g. 0.04 to 0.08 g/cm³. As aforesaid, the compacted bulk density may preferably be at least two times, at least three times, at least four times, at least five times or even at least ten times greater than the Standard Bulk Density. The filtering element formed from silica powder may

advantageously have a pore volume as defined hereinabove.

Silica powder has been found to provide excellent filtering performance against organosilicon compounds (e.g. siloxanes). Advantageously, such powders may be mechanically densified according to the invention without thermal sintering to offer suitable porosities without excessive loss of surface properties. In embodiments of the invention the filtering cap may comprise a first filtering element designed to filter (i.e. counteract the ingress of) a first species, for example selected from the group comprising organosulfur compounds and H₂S, and a second filtering element designed to filter a second species, e.g.

organosilicon compounds. Of course other filtering elements may be readily formed from densified powders comprising activated carbon, or other transition metal oxides such as iron oxide, manganese dioxide, aluminium oxide and mixtures thereof, zeolites or the like.

The invention also embraces, from a third aspect, a method of making a gas sensor, or a filtering cap for a gas sensor, the method comprising mechanically densifying a powder of filtering material to form a filtering element and incorporating the filtering element into a filtering cap comprising a cavity for accommodating an active element of a gas sensor. Suitably the method may comprise operating at least one piston or pressurising member to apply a pressure to the powder. In one embodiment the powder is densified by applying a pressure of at least 10 kg/ cm² or at least 30 kg / cm² to a surface of the powder.

In an embodiment, the filtering element is formed in-situ within a body of the filtering cap. Mechanical densification allows the porosity of the filtering element to be adjusted with greater precision than is achievable, for example, by sintering. Furthermore, particularly where a filtering element is formed in-situ, mechanical densification tends to facilitate the formation of a better seal between the filtering element and other parts of the filtering cap compared to that formed by sintering.

From a fourth aspect, the invention resides in a method of protecting an active element of a gas sensor against one or more harmful gaseous species, the method comprising fitting a filtering cap according to the second aspect of the invention to intersect a flow path of gas to the active element. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and

"comprises", mean "including but not limited to", and do not exclude other moieties, additives, components, integers or steps. However, for the avoidance of doubt such words may, in embodiments, embrace "consist". Moreover the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Other features of the invention will become apparent from the following examples. Generally speaking the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings). Thus features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. Moreover unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose. Where upper and lower limits are quoted for a property, for example for the concentration of a fuel component, then a range of values defined by a combination of any of the upper limits with any of the lower limits may also be implied.

In this specification, references are - unless stated otherwise - to properties measured under ambient conditions, i.e. at atmospheric pressure and at a temperature of 25°C.

The present invention will now be further described with reference to the following examples and the accompanying illustrative drawings, of which:

Figure 1 is a schematic cross-sectional view of a gas sensor in accordance with a first embodiment of the invention;

Figure 2 is a schematic cross-sectional view of a gas sensor in accordance with a second embodiment of the invention;

Figure 3 shows a plot comparing the output of "MEMS pellistor" sensors according to embodiments of the invention with a commercially available gas sensor in the presence of CH₄ and H₂S; and

Figure 4 shows a plot comparing the output of a pellistor gas sensor including unsintered/non-pre-heated silicon dioxide filtering material with those of pellistor gas sensors including silicon dioxide filtering material that has been pre-heated, in the presence of hexamethyldisiloxane (HMDSO).

Detailed description of exemplary embodiments

With reference to Figure 1 , in a first embodiment, a gas sensor 2 comprises a filtering cap 4. The filtering cap 4 includes a filtering element 6 separated from an active element 8 of the gas sensor 2.

The filtering cap 4 comprises a support 10 formed of a suitably rigid material which defines a socket for the filtering element 6. Together the support 10 and filtering element 6 of the filtering cap 4 define a cavity or gap 12 in which the active element is disposed together with an associated hotplate and circuitry (not shown) and optionally a compensator arrangement (also not shown). The filtering cap and the active element are borne by a substrate 14 which is a silicon wafer.

Notably the active element 8 has no direct contact with the filtering element 6 provided in the cap 4. However, the filtering element intersects a flow path of gases to the active element 8. The filtering element 6 comprises a mechanically densified powder of filtering material. With reference to Figure 2, in a second embodiment a gas sensor is substantially identical to that of the first embodiment, with like reference numerals being used for like parts, save that the filtering cap 4 is formed entirely of the filtering element 6.

It will be appreciated from the forgoing disclosure that modifications can be made to these embodiments. For example, numerous variations of filtering materials may be employed in the first embodiment without departing from the scope of the invention. Similarly, a number of structural modifications may be made without departing from the scope of the invention. For example, multiple differing or identical filtering elements may be employed in a single gas sensor.

Examples

Example 1

A pellistor gas sensor was constructed in accordance with the structure of the first embodiment above.

The filtering material of the filter was chosen to counteract the ingress of organosulfur compounds or H₂S. The filtering material was mechanically densified zinc (II) oxide (ZnO) with the following characteristics: average particle size: 20 nm

- specific surface area: 50 m²/g

The filtering material was mechanically densified at 25 °C to a compacted bulk density of about 1.8 g/cm³. This density is roughly five times the Standard Bulk Density of the material. The thickness of the filter was about 0.8 mm.

The gas sensor was tested by injecting H₂S (100ppm) in presence of CH₄ (1 %). A sensor fitted with such a filter had a loss of sensitivity of only 0.1 % after 45 minutes of testing. Figure 3 shows a comparison with a commercially available traditional "coil+bead" sensor.

In a variation copper (II) oxide with a surface area of 36 m²/g was substituted for the zinc oxide. A sensor fitted with such a filter had a loss of sensitivity of only 0.5% after 45 minutes of testing under the same conditions. The results are also shown in Figure 3.

Example 2

The impact of compaction on preventing sensitivity losses was studied in a pellistor gas sensor constructed in accordance with the first embodiment above. The pellistor was fitted with differing filters comprising ZnO as in Example 1 , of 0.8 mm thickness but with differing levels of compaction.

After an initial period of injecting CH₄ (2.5%) the gas sensors were tested by injecting H₂S (100ppm) in presence of CH₄ (2.5%). The results are shown in Table 1.

Filter No Filter Mass Mass/vol Sensitivity Decrease of CH₄

Thickness ZnO ratio ZnO under CH₄ sensitivity after 45 (mm) (mg) (mg/mm³) (mV) min in H₂S

1 24.7 2.46 40.7 -3.4%

2 0.8 18.5 1.84 54.2 -3.8%

3 15.5 1.54 58.8 -5.4%

Table 1 - Comparison of ZnO filters with differing compaction

It is apparent from the results in table 1 that compaction (mechanical densification) of the filtering material increases the effectiveness of the filter. The associated decrease in absolute sensitivity still allows for the construction of suitably sensitive sensors.

Example 3

A pellistor gas sensor was constructed in accordance with the structure of the first embodiment above. The filtering material of the filter was chosen to counteract the ingress of organosilicon compounds. The filtering material was mechanically densified silicon dioxide with a weight average particle size of less than 100 nm.

The filtering material was mechanically densified at 25 °C to a compacted bulk density about ten times the Standard Bulk Density of the material. The gas sensor was tested by injecting HMDSO hexamethyldisiloxane) (20ppm) in presence of CH₄ (2.5%). A sensor fitted with such a filter had a loss of sensitivity of only 9% after 40 minutes of testing. A commercially available traditional "coil+bead" sensor, claimed as silicone poison resistant, had no sensitivity only a few minutes after injecting HMDSO.

To demonstrate the advantage of mechanical densification of the filtering material, the HDMSO filtering characteristics of the material were tested in unsintered/non-pre-heated form and compared to identical material pre-heated for 48 hours at 1000°C, by incorporation of the materials into respective identical filtering caps of gas sensors. The results, shown in Figure 4, illustrate that unsintered/non-pre-heated filtering material provides superior performance compared to powder that was pre-heated before incorporation. Accordingly, it can be inferred that filtering caps formed by mechanical densification, in which heating or sintering is not required, can offer superior performance.

Claims

A gas sensor comprising: at least one active element; and a filtering cap comprising a porous filtering element for counteracting the ingress of one or more atmospheric gases to the active element, wherein there is a separation between the active element and the filtering element of the cap and wherein the filtering element comprises a mechanically densified powder of filtering material.

The gas sensor of claim 1 , wherein the active element is a small-scale active element having a power consumption of less than 300 mW.

The gas sensor of claim 1 or claim 2 wherein the active element is a catalytic element or a chemoresistive gas-sensitive element.

The gas sensor of any preceding claim, wherein the filtering element has a compacted bulk density which is greater than a Standard Bulk Density of the powder measured according to method I in United States

Pharmacopeial Convention Chapter 616 "Bulk and Tapped Density", 1 December 2012.

The gas sensor of claim 4 wherein the compacted bulk density of the filtering element is at least four times greater than the relevant Standard Bulk Density of the filtering material.

The gas sensor of any preceding claim, wherein the filtering element is substantially free from thermally sintered filtering material and/or components other than the filtering material.

The gas sensor of any preceding claim, wherein the filtering material has a weight average particle size of less than 100 nm.

8. The gas sensor of any preceding claim, wherein the filtering material has a specific surface area in the range of from 10 m²/g to 6000 m²/g.

9. The gas sensor of any preceding claim, wherein the filtering element has a pore volume in the range of from 0.15 to 1.5 cm³/g.

10. The gas sensor of any preceding claim, wherein the filtering material is selected from the group comprising: transition metal oxides, zeolites, ceramics, silica, alumina, micro/nano porous carbons, activated carbon, nitrides, organic polymers, and mixtures thereof.

1 1. The gas sensor of claim 10, wherein the filtering material comprises zinc oxide and/or copper oxide.

12. The gas sensor of claim 10, wherein the filtering material comprises one or more of silica, alumina, zeolite, and mixtures thereof.

13. The gas sensor of claim 12, wherein the filtering material comprises a silica powder.

14. A filtering cap for a gas sensor, the cap comprising a porous filtering

element comprising a mechanically densified powder of filtering material and being shaped to receive an active element of a gas sensor with a separation between the active element and the filtering element.

15. A method of making a filtering cap for a gas sensor, the method comprising mechanically densifying a powder of filtering material to form a filtering element and incorporating the filtering element into a filtering cap comprising a cavity for accommodating an active element of a gas sensor.

16. The method of claim 15 wherein densifying the powder comprises

operating at least one piston or pressurising member to apply a pressure to the powder.

17. The method of claim 15 or claim 16 wherein the powder is densified at a temperature below 50 °C

18. The method of any one of claims 15 to 17 wherein the powder is densified such that the filtering element has a compacted bulk density which is greater than a Standard Bulk Density of the powder measured according to method I in United States Pharmacopeial Convention Chapter 616 "Bulk and Tapped Density", 1 December 2012.

19. The method of any one of claims 15 to 18 wherein the compacted bulk density is at least four times greater than the relevant Standard Bulk Density.

20. The method of any one of claims 15 to 19, wherein the filtering material has a weight average particle size of less than 100 nm.

21. The method of any one of claims 15 to 20, wherein the filtering material has a specific surface area in the range of from 10 m²/g to 6000 m²/g

22. The method of any one of claims 15 to 21 wherein the filtering material is selected from the group comprising: transition metal oxides, zeolites, ceramics, silica, alumina, micro/nano porous carbons, activated carbon, nitrides, organic polymers, and mixtures thereof.

23. A method of protecting an active element of a gas sensor against one or more harmful gaseous species, the method comprising fitting a filtering cap according to claim 14 to intersect a flow path of gas to the active element.