HALOGENATED GAS SENSOR

Abstract

A halogenated gas sensor for detecting a halogenated gas, comprising at least a first metallic electrode and a second metallic electrode, which are connected with a sensing material, comprising at least one of NaAlSiO4, KAlSiO4, RbAlSiO4, CsAlSiO4.

Description

Materials containing at least one component with the chemical formula NaAlSiO₄, K₁SiO₄, RbAlSiO₄, or CsAlSiO₄ were used to fabricate a bead-type sensor for highly sensitive and selective detection of halogenated gases. The sensor comprises a central electrode, a coil, and a sensing material. The coil is heated by the current passing through it. The sensing material is porous. The conductance between the central electrode and the heated coil changes with the concentration of the halogenated gas. This invention relates to the improvement of a bead-type sensor for the highly sensitive detection of halogenated gases, especially refrigerant gases such as hydrofluoroolefin (HFO) and hydrofluorocarbons (HFC), which exhibit a much lower global warming potential compared to hydrochlorofluorocarbons (HCFC). Several technologies exist for the detection of halogenated gases. Tin-oxide-based metal oxide semiconductor (MOS) sensors have been used for halogenated gas detection; however, these sensors exhibit cross-sensitivity to many hydrocarbons and humidity. Non-dispersive infrared (NDIR) optical sensors Ref. 349309 English) are also used for the detection of halogenated gases; however, these sensors show limited sensitivity and their production is expensive. Solid-state bead-type sensors for detecting halogenated gases are relatively inexpensive to produce. These sensors are described by Loh in U.S. Patent No. 3751968, by Lee in U.S. Patent No. 5104513, by Stetter in U.S. Patent No. 5226309, and by Yannopoulos in U.S. Patent No. 5932176. Loh described a sensing element comprising a glass-ceramic containing a mixture of lanthanum oxide, lanthanum fluoride, and sodium silicate. Lee described a ceramic sensing element comprising a mixture of potassium silicate and a compound selected from the silicon dioxide and aluminum oxide group. Stetter described a sensing material comprising lanthanum sodium fluoride silicate, having the chemical formula NaLa(SiO4)3F. Yannopoulos described a sensing element comprising sodium titanate. The operating temperature of the sensor described in U.S. Patent No. 5226309 is 500°C to 600°C, which is too low to detect HFOs and HFCs with acceptable sensitivity. Lee described a ceramic sensing element comprising a mixture of potassium silicate and aluminum oxide in a ratio of approximately 11 The ratio of potassium silicate to aluminum oxide by weight is 0.25–4.0 parts per kilogram. This wide ratio and the poorly defined phase of the sensing material make it difficult to achieve reproducible sensor performance. The objective of this invention is to find well-defined materials for the selective and sensitive detection of HFOs and HFCs. Another objective is to find sensing materials with high melting points, enabling the sensor to operate with high sensitivity at temperatures ranging from 800°C to 1000°C. The halogenated gas sensor of the invention is defined in independent claim 1. Accordingly, the gas sensor comprises at least a first metallic electrode and a second metallic electrode, which are connected with a sensing material, comprising at least one of NaAlSiO4, KA1SiO4, RbAlSiO4, CsAlSiO4. The sensor material can be in the form of a bead, in which the two electrodes are at least partially embedded. One of the two electrodes can be a coil that surrounds the other of the two electrodes as a central electrode. The first electrode may be made of or comprise platinum and / or where the second electrode may be made of or comprise platinum. A voltage source can be connected at least to the first electrode to heat the electrode by means of a current or voltage applied to the electrode to a temperature in the range of 400°C-1200°C, 4000°C-1000°C, 600°C-1200°C or 600°C-1000°C. The first sensor may be in the form of a coil having a first end and an opposite second end, the two ends being connected to a voltage source (1), the second electrode being a central electrode in the form of a longitudinal straight element or bar extending through the center of the coil (4) along the longitudinal axis of the coil, the coil and the central electrode being surrounded by, and embedded in the sensor material. The invention also provides a method for detecting a halogenated gas with a sensor as described above, wherein the sensor material is heated to a temperature in the range of 400°C-1000°C, 400°-1200°C, 600°C-1000°C or 600°C-1200°C by applying a current or voltage to the first sensor or coil. The current through the coil after exposing the sensor to the gas to be detected can be divided by the current through the coil before exposing the sensor to the gas to be detected. Furthermore, the invention provides a method for manufacturing a halogenated gas sensor as described above, wherein the sensor material comprises at least a first component (A) made of a molecular sieve (3A), iviA / a / zuzo / ui uz 11 which is heated to a first temperature of several hundred °C, held at the first temperature for several hours, and preferably 3 hours, being heated thereafter to a second temperature, which is higher than the first temperature, and held at the second temperature for a second time, which preferably corresponds to the first time. The first component can then be ground to obtain fine particles with an average size of less than 5 pm, preferably approximately 3 pm. The first component may contain NaAlSiO₄ and KA₁SiO₄, preferably in a 1:1 ratio. The sensor material may comprise at least a second component (B) that is prepared by an ion exchange performed with a molecular sieve (4Ά) and CsNCn, the molecular sieve and the CsNO₂ that are preferably mixed in deionized water. The mixed suspension of molecular sieve, CsNOs and deionized water can be stirred for several hours, and preferably 24 hours, after which the suspension is preferably centrifuged, then heated preferably to a first temperature of several hundred °C and preferably about 900 °C for at least one hour and preferably for two hours, and then heated to a second temperature higher than the first temperature, preferably 1100 °C, for several additional hours, and preferably for about 3 hours. The second component (B) can be ground after heat treatment to obtain fine particles with an average size of a few pm and preferably about 4 pm. The first component (A) and / or the second component (B) can be mixed with a vehicle to obtain a suspension, the vehicle being preferably in the range of 5%-10% by weight of hydroxypropyl cellulose dissolved in water, the weight ratio of the mixture of components (A) and / or (B) with the vehicle being approximately 2:1. Brief Description of the Figures In the following exemplary embodiments of the invention, they are described with reference to the figures, in which: Figure 1 is a schematic view of a sensor according to one embodiment of the invention, Figure 2 shows the steps of a typical process for manufacturing the sensor with exemplary sensor materials, Figure 3 shows scanning electron microscopy images with 30 and 1600 magnifications, Figure 4 shows a typical response of the sensor to 100 ppm of R134a and 100 ppm of R1234yf, Figure 5 shows a typical response of the sensor to different concentrations of R134a, and Figure 6 shows a typical response of the sensor to different gases / vapors. Detailed Description of the Invention Sensor components The schematic diagram in Figure 1 shows that the invented sensor comprises a central electrode 7, preferably a platinum wire, surrounded by a metal coil 4, preferably a platinum coil. Both are embedded in a bead 5, which is composed of at least one of NaAlSiO4, KAlSiO4, RbAlSiO4, or CsAlSiO4. The coil is heated by applying voltage 1 to a temperature from 400 °C to 1000 °C. Current 6 is generated by applying voltage 3 between the central electrode 7 and the coil 4. The nominal resistance of the sensor is the ratio of voltage 2 to current 6, which varies with the concentration of the halogenated gas surrounding the sensor. Material A for bead 5 is synthesized from molecular sieve 3A, with the linear formula KnNal2-n[(Al1O2)12(SiO2)12] ·χH2O (n is approximately 6) from Alfa Aesar. The process for manufacturing the sensor is shown in Figure 2. For example, 50 g of molecular sieve 3A were placed in an oven and heated to 900 °C at a rate of 5 °C / min and held at 900 °C for 3 hours. Then, it was heated to 1100 °C at a rate of 5 °C / min and held at the same temperature for 3 hours. After the material cooled to room temperature, it was ground in a planetary ball mill (Retsch PM100) to obtain fine particles with an average size of approximately 3 µm. XRD diffraction spectra of the ground material were collected using the PANalytical X'Pert PRO XRD system. It was confirmed that it contains only NaAlSiO4 and KA1SiO4.Elemental analysis using energy-dispersive X-ray spectroscopy (EDS) confirmed that the ratio of NaAlSiO₄ to KA₁SiO₄ is 1:1. Material B was prepared by ion exchange using molecular sieve 4A (Na₂[(Al₁O₂)₁₂(SiO₂)₁₂]·nH₂O from Alfa Aesar) and CsNO₃. Typically, 5 g of molecular sieve 4A and 16.5 g of CsNO₃ are mixed in 85 mL of deionized water. The pH was adjusted to 8 with NaOH solution. The suspension was stirred for 24 hours and then centrifuged. The remaining material was ion-exchanged with 16.5 g of CsNO3 and 85 mL of the aqueous solution twice more. The final remaining material was washed with deionized water three times and dried at 80 °C with overnight air exposure. Further air-exposition of the material was then performed, including heating at 900 °C for 2 hours and an additional 3 hours at 1100 °C.After heat treatment, the material was ground using a pestle and mortar. Fine particles with an average size of approximately 4 µm were obtained. Elemental analysis by energy-dispersive X-ray spectroscopy (EDS) indicated that the ratio of NaAlSiO4 to CsAlSiO4 is 1:1. NaAlSiO4 and CsAlSiO4 have melting points of 1526 °C and 1750 °C, respectively. Therefore, they are expected to exhibit a long service life with an operating temperature of up to 1000 °C. Sensor manufacturing Material A or (and) material B were mixed with the vehicle to form a slurry. The vehicle is 5% to 10% (by weight) hydroxypropyl cellulose dissolved in water. The weight ratio of the material to the vehicle is approximately 2:1. The slurry is first applied to the center electrode and allowed to dry. The coated center electrode is then inserted into the heating coil. Additional slurry is added around the coil to form a complete bead covering the heating coil. The finished sensor is then heated to 850 °C for 0.5 to 2 hours at a rate of 1 to 5 °C / min. Once the sensors cool to room temperature, they are ready for testing. The sensor prepared with material A / B is referred to as sensor A / B. Figure 3 shows SEM images of a typical sensor A at 30x and 1600x magnification. The high-magnification image shows a highly porous morphology. Sensor performance Sensor A is heated to approximately 800 °C by passing a current through the coil during operation. The current (6) in Figure 1 changes with the refrigerant concentration. The current before exposure to the gas is denoted as 10, and the current after exposure is denoted as Ig. The ratio Ig / 10 is defined as the sensitivity. Figure 4 shows that sensor A responds very quickly to 100 ppm of R134a (CH2FCF3) and 100 ppm of R1234yf (CF3CF=CH2). Figure 5 shows the sensitivity of sensor A to different concentrations of R134a. The leak detector with a typical sensor A can detect a leak rate of 0.6 g / year of R134a intermittently for at least 3 months. Figure 6 shows the sensitivity of sensor A to 100 ppm of a different gas / vapor.The sensor shows no response to 100 ppm of isobutene (R600a), hydrogen, isopropanol, and methane (CH4), but it does show a good response to 100 ppm of R134a and 100 ppm of R1234yf. Sensors with material B show similar performance to sensor A; in addition, sensor B can be operated at a higher temperature of 800 °C to 1200 °C to achieve high sensitivity. In the preceding description, the invention was described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited to them. Different features and aspects of the above description may be used individually or in combination. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. A halogenated gas sensor for detecting a halogenated gas, characterized in that it comprises at least a first metallic electrode and a second metallic electrode, which are connected with a sensing material, comprising at least one of NaAlSiO4, KA1SiO4, RbAlSiO4, CsAlSiO4.

2. The sensor according to claim 1, characterized in that the sensor material has the shape of a bead, in which the two electrodes are at least partially embedded.

3. The sensor according to claim 1 or 2, characterized in that one of the two electrodes is a coil surrounding the other of the two electrodes as a central electrode.

4. The sensor according to one of claims 1 to 3, characterized in that the first electrode is made of, or comprises, platinum and / or where the second electrode is made of, or comprises, platinum.

5. The sensor according to any one of claims 1 to 4, characterized in that a voltage source is connected at least to the first electrode to heat the electrode by means of a current or voltage applied to the electrode to a temperature in the range of 400°C-1200°C, 400°C-1000°C, 600°C-1200°C or 600°C-1000°C.

6. The sensor according to any one of claims 1 to 5, characterized in that the first sensor has the form of a coil having a first end and an opposite second end, the two ends being connected to a voltage source, the second electrode being a central electrode in the form of a longitudinal straight element or bar extending through the center of the coil along the longitudinal axis of the coil, the coil and central electrode being surrounded and embedded in the sensor material.

7. A method for detecting a halogenated gas with a sensor according to one of claims 1-6, characterized in that the sensor material is heated to a temperature in the range of 400°C-1000°C, 400°C-1200°C, 600°C-1000°C or 600°C-1200°C by applying a current or voltage to the first sensor or coil.

8. The method for detecting halogenated gases according to claim 7, characterized in that the current through the coil after exposing the sensor to the gas to be detected is divided by the current through the coil before the sensor is exposed to the gas to be detected.

9. A method of manufacturing a halogenated gas sensor according to one of claims 1-6, characterized in that the sensor material comprises at least a first component (A) manufactured from a molecular sieve (3A), which is heated to a first temperature of several hundred °C, held at the first temperature for several hours, and preferably 3 hours, then heated to a second temperature, which is higher than the first temperature, and held at the second temperature for a second time, which preferably corresponds to the first time.

10. The method according to claim 9, characterized in that the first component is subsequently ground to obtain fine particles with an average size of less than 5 pm, preferably approximately 3 pm.

11. The method according to claim 9 or 10, characterized in that the first component contains NaAlSiO4 and KA1SiO4, preferably in a 1:1 ratio.

12. The method according to one of claims 1-11, characterized in that the sensor material comprises at least a second component (B) that is prepared by an ion exchange performed with a molecular sieve (4A) and CsNOs, the molecular sieve and the CsNCu being preferably mixed in deionized water.

13. The method according to claim 12, characterized in that the mixed suspension of the molecular sieve, the CsNCu and the deionized water is stirred for several hours, preferably 24 hours, after which the suspension is preferably centrifuged, then preferably heated to a first temperature of several hundred °C and preferably about 900 °C for at least one hour and preferably for two hours, and then heated to a second temperature higher than the first temperature, preferably 1100 °C, for several additional hours, and preferably for about 3 hours.

14. The method according to claim 13, characterized in that the second component (B) is ground after heat treatment to obtain fine particles with an average size of a few pm and preferably about 4 pm.

15. The method according to any one of claims 9-14, characterized in that the first component (A) and / or the second component (B) are mixed with a vehicle to obtain a suspension, the vehicle preferably being in the range of 5%-10% by weight of hydroxypropyl cellulose dissolved in water, the weight ratio of the mixture of components (A) and / or (B) with respect to the vehicle being approximately 2:1.

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