Hydrogen sensor material

Abstract

The invention relates to a hydrogen sensor material comprising at least one rod-shaped single crystal having wurtzite structure composed of a semiconductor and a conversion layer formed by an MOF on the surface of the single crystal, where the single crystal has a length of 100 nm to 100 pm and a lateral extent of 500 nm to 10 pm and where the aspect ratio of the single crystal of length to lateral extent is greater than 3. The invention further relates to a production process for a hydrogen sensor material.

Description

[0001] HYDROGEN SENSOR MATERIAL

[0002] The invention concerns a new sensor material for a hydrogen sensor.

[0003] As the energy sector transforms from fossil fuels to renewable, carbon-free energy sources, hydrogen (H2) is also becoming increasingly important. The plan is to utilize existing infrastructure and inject up to 20% H2 into natural gas pipelines. In this context, the precise detection of even the smallest amounts of H2 is of great importance, among other things because leaks in the pipelines must be detected immediately.

[0004] Gas sensors based on zinc oxide nanostructures are seen as promising candidates. [VS Bhati, M. Hojamberdiev, M. Kumar, Enhanced sensing performance of ZnO nanostructures-based gas sensors: A review, Energy Rep. 6 (2020) 46-62]

[0005] A disadvantage of gas sensors made of semiconducting metal oxides (MOS) is their low selectivity toward reducing gases, meaning they can detect them but cannot be distinguished from one another. One approach to solving this problem is molecular filters made of metal-organic frameworks (MOFs), which are applied as a conversion layer at the gas-sensor interface to prevent signal contamination.

[0006] US 2021 / 0016245 describes MOFs coated on various substrates and the use of these coated substrates as sensor material.

[0007] CN107991350A discloses rod-shaped sensor material made of zinc oxide (ZnO) coated on the surface with the metal-organic framework compound ZIF-8 ([Zn(C4HsN2)2)]) (ZnO@ZIF-8).

[0008] Further zinc oxide structures coated with ZIF-8 (ZnO@ZIF-8) intended as sensor material for the detection of hydrogen are known from the following publications.

[0009] F. Cui, W. Chen, L. Jin, H. Zhang, Z. Jiang, Z. Song, Fabrication of ZIF-8 encapsulated ZnO microrods with enhanced sensing properties for H2 detection, J. Mater. Sci. Mater. Electron. 29 (2018) 19697–19709. https: / / doi.org / 10.1007 / s10854-018-0095-9 reveals that zinc oxide is a popular sensing material with limited sensitivity, selectivity, and stability for gas detection, especially H2 detection, while zeolite imidazolate framework-8 (ZIF-8), a type of metal-organic frameworks (MOFs), possesses tunable porosity, a large specific surface area, and good thermal stability. FREUND, Ralph et al.: Understanding the Chemistry of Metal Oxide to Metal-Organic Framework Reactions for Morphology Control. In: Chemistry of Materials, 2023, 35, 1891-1900. DOI: 10.1021 / acs.chemmater.2c02946 reveals that metal-organic frameworks (MOFs) are a class of porous materials whose three-dimensional (3D) morphological control could be of interest. Complex 3D ZIF-8 and ZnO@ZIF-8 composite structures composed of various ZnO precursors are conceivable.

[0010] M. Drobek, J.-H. Kim, M. Bechelany, C. Vallicari, A. Julbe, SS Kim, MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity, ACS Appl. Mater. Interfaces. 8 (2016) 8323-8328. https: / / doi.org / 10.1021 / acsami.5b12062

[0011] P. Ji, X. Hu, R. Tian, ​​H. Zheng, J. Sun, W. Zhang, J. Peng, Atom-economic synthesis of ZnO@ZIF-8 core-shell heterostructure by dry gel conversion (DGC) method for enhanced H2 sensing selectivity, J. Mater. Chem. C. 8 (2020) 2927-2936. ttps: / / doi.org / 10.1039 / C9TC06530J

[0012] X. Wu, S. Xiong, Z. Mao, S. Hu, X. Long, A Designed ZnO@ZIF-8 Core-Shell Nanorod Film as a Gas Sensor with Excellent Selectivity for H2 over CO, Chem. - Eur. J. 23 (2017) 7969-7975. https : / / doi . org / 10.1002 / chem .201700320

[0013] R. Lv, Q. Zhang, W. Wang, Y. Lin, S. Zhang, ZnO@ZIF-8 Core-Shell Structure Gas Sensors with Excellent Selectivity to H2, Sensors. 21 (2021) 4069. https: / / doi.org / 10.3390 / s21124069

[0014] A. I. Khudiar, A.K. Elttayef, M.K. Khalaf, A.M. Oufi, Fabrication of ZnO@ZIF-8 gas sensors for selective gas detection, Mater. Res. Express. 6 (2020) 126450. https: / / doi.org / 10.1088 / 2053- 1591 / ab69c2

[0015] MISHRA, Yogendra Kumar [et al.]: Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing. In: ACS Applied Materials and Interfaces 2015, 7, 14303-14316: DOI: 10.1021 / acsami.5b02816.

[0016] RASCH, Florian [et al.]: Highly selective and ultra-low power consumption metal oxide based hydrogen gas sensor employing graphene oxide as molecular sieve. In: Sensors & Actuators: B.

[0017] Chemical, 2020, 320, 128363. DOI: 10.1016 / j.snb.2020.128363.

[0018] MENG, Jiashen [et al.]: Advances in metal-organic framework coatings: versatile synthesis and broad applications. In: Chemical Society Review, 2020, 49, 3142. DOI: 10.1039 / c9cs00806c.

[0019] A disadvantage of the known sensor materials for hydrogen sensors is the operating temperature of the sensor, which in some cases is well above 120°C. Especially at high concentrations of hydrogen, high sensor temperatures pose a significant risk due to flammability. A further disadvantage is that heating to these high temperatures consumes more energy. Another disadvantage of the known sensor materials for hydrogen sensors is their high cross-sensitivity to other gases and thus low selectivity. Another disadvantage is that all known sensor materials for hydrogen sensors require oxygen to be present for detection. In particular, the sensitivity of the known sensor materials for hydrogen sensors needs to be improved.

[0020] Therefore, it is an object of the invention to provide a hydrogen sensor material which has an increased sensitivity compared to all known hydrogen sensor materials.

[0021] Furthermore, it is an object of the invention to provide a hydrogen sensor material that provides a high selectivity of the sensor.

[0022] Furthermore, it is an object of the invention to provide a hydrogen sensor material that can be used in an operating range of less than 120°C.

[0023] A further object of the invention is to provide a hydrogen sensor material that can also be used in the absence of oxygen.

[0024] The object of the invention is achieved by a hydrogen sensor material comprising at least one rod-shaped single crystal with a wurtzite structure made of a semiconductor and a conversion layer formed by a MOF on the surface of the single crystal, wherein the single crystal has a length of 100 nm to 100 pm and a lateral extent of 500 nm to 10 pm and wherein the aspect ratio of the single crystal of length to lateral extent is greater than 3.

[0025] In a preferred embodiment, the object of the invention is achieved by a hydrogen sensor material comprising at least one rod-shaped single crystal with wurtzite structure selected from the group ZnO, ZnTe, ZnSe.

[0026] In a particularly preferred embodiment, the object of the invention is achieved by a hydrogen sensor material comprising at least one rod-shaped single crystal with a wurtzite structure made of ZnO.

[0027] In a further preferred embodiment, the object of the invention is achieved by a hydrogen sensor material comprising at least one rod-shaped single crystal with a wurtzite structure made of zinc oxide (ZnO) and a conversion layer formed by the MOF ZIF-8 ([Zn(C4H5N2)2)]) on the surface of the single crystal.

[0028] In a particularly preferred embodiment, the object of the invention is achieved by a hydrogen sensor material comprising at least one rod-shaped single crystal with a wurtzite structure from the arm or parts thereof of a tetrapodal zinc oxide (t-ZnO) and a conversion layer formed by the MOF ZIF-8 ([Zn(C4H5N2)2)]) on the surface of the single crystal, wherein the single crystal has a length of 100 nm to 100 pm and a lateral extent of 500 nm to 10 pm and wherein the aspect ratio of the single crystal of length to lateral extent is greater than 3.

[0029] In a further embodiment of the invention, several single crystals according to the invention are connected to form a diffusion-open network.

[0030] The term semiconductor is understood by those skilled in the art to be solid-state materials with a medium band gap greater than 0 eV, which behave like an electrical insulator at a temperature of 0 K and like an electrical conductor at a finite temperature below the melting point of the material, preferably no more than 30 °C from the application temperature. The average conductivity at this temperature is typically between that of an insulator and an electrical conductor.

[0031] The wurtzite structure refers to the structure of compounds with the composition AB that crystallize in the space group PQzmc or space group number 186, i.e., in a hexagonal crystal system. Each type of atom is surrounded by the other types of atoms in a tetrahedral arrangement.

[0032] The electrical properties of semiconductor single crystals can be characterized by their defect density. The defect density can be determined by the decay time of photoluminescence. [Zhong et al. J. Phys. Chem. C. 2008, 112, 16286-16295] The rod-shaped single crystals with a wurtzite structure according to the invention have a decay constant of more than 1 nm, determined by photoluminescence.

[0033] Sensor sensitivity refers to the degree to which the sensor switches its electrical resistance with and without the presence of the analyte at a fixed concentration. The measure of sensitivity is the quotient of the conductivities and corresponds to the sensor response.

[0034] The selectivity of the sensor is understood as the comparison of the measured sensitivities for different gases. If a sensitivity of 1 is shown for a gas, this means that the resistance does not change after the addition of this gas; thus, the sensor shows no sensitivity to this gas. In this case, the cross-sensitivity is minimal and the selectivity is maximum.

[0035] Surprisingly, it was found that the hydrogen sensor material according to the invention has a sensitivity increased by a factor of 50 to 100 compared to known sensor materials for hydrogen sensors.

[0036] Also surprisingly, it was found that the hydrogen sensor material according to the invention exhibits excellent selectivity compared to known sensor materials for hydrogen sensors. The sensitivities for the gases methane, acetone, ethanol, 2-propanol, n-butanol, ammonia, and CO2 are 1. This indicates an almost infinite selectivity with respect to the gas hydrogen.

[0037] Surprisingly, it was found that the hydrogen sensor material according to the invention can be used for hydrogen sensors at operating temperatures of less than 120°C, compared to known sensor materials.

[0038] It was also surprisingly found that the hydrogen sensor material according to the invention can be used for measurement even in the absence of oxygen, compared to known sensor materials for hydrogen sensors.

[0039] In a further aspect, the object of the invention is achieved by a method for producing a hydrogen sensor material comprising the following steps.

[0040] I. Determination of the reaction temperature in the range between 60°C and 160°C, the reaction time and the molar ratio in relation to the desired layer thickness based on Table 1, which shows the minimum ratio of ZnO to HMelM, an increase in the proportion of HMelM is possible without changing the overall result and Table 2 and the RGT rule

[0041] Table 1 : ZnO to HMelM ratio

[0042] Table 2: Average film thickness of the ZIF-8 coating that can be achieved at a reaction temperature of 140 °C for a given reaction time.

[0043] II. Placing ZnO material in a sealable reactor

[0044] III. Addition of 2-methylimidazole (HMelM),

[0045] IV. Closing the reactor

[0046] V. Temperature control of the reactor

[0047] VI. Cooling the reactor

[0048] VII. Optional: Removal of excess HMelM by treating the crude product under reduced pressure and / or at elevated temperature, whereby steps II. and III. can be carried out in a variable order.

[0049] Figure 1 shows the SEM images of the single crystals according to the invention, here the tetrapods of zinc oxide t-ZnO, which exhibit a surface conversion layer formed by the MOF ZIF-8 ([Zn(C4HsN2)2)]). (t-ZnO@ZIF-8) after 4 hours (c), 8 hours (d), 20 hours (e), and 60 hours (f) of reaction time.

[0050] The images demonstrate the good homogeneity of the ZIF-8 coating. The thickness of the layers can be found in the table below.

[0051] Table 3: Thickness of the ZIF-8 layer on the ZnO tetrapods. The quantitative determination of the ZIF-8 content was performed using thermogravimetric analysis. The samples contain the ZIF-8 contents listed in Table 2. Table 4: Content of ZIF-8 in t-ZnO@ZIF-8 determined by TG analysis

[0052] The specific surface areas were determined using nitrogen sorption measurements and are listed in Table 5. Table 5: Specific surface area As, BET of the t-ZnO@ZIF-8 samples as a function of the reaction time Figure 2 shows a schematic representation of the process according to the invention for producing the sensor material also according to the invention (here t-ZnO@ZIF-8).

[0053] To measure hydrogen detection from the t-ZnO@ZIF-8(4h) sample, one arm of a single tetrapod was connected to gold contacts on a SiO2 / silicon wafer using the focused ion beam (FIB) method to measure the current flow in a single tetrapod when exposed to different sample gases. Figure 3 (a) shows the SEM images of a t-ZnO@ZIF-8(4h) microrod that was contacted with platinum via the FIB method to the gold contacts on a SiO2 / silicon wafer. Figure 3 (b) shows the cross-section of a t-ZnO@ZIF-8(4h) microrod.

[0054] The response behavior of the sensor according to the invention was tested for the gases hydrogen, methane, acetone, ethanol, 2-propanol, n-butanol, ammonia, and CO2 in the temperature range of 20–175 °C. The particularly high selectivity towards hydrogen is demonstrated in Figure 4. It can be seen that none of the other analytes tested reacted with the sensor (S=1). Therefore, the selectivity towards hydrogen is absolute. High sensitivity (= high sensor response) is already evident at 100 °C. The selectivity is maintained even at higher temperatures, up to 175 °C.

[0055] Figure 5 shows the short response times and regeneration times in the range of seconds of the sensor according to the invention.

[0056] The sensor according to the invention demonstrates sensitivity and selectivity towards H2 even in an oxygen-free atmosphere. In this case, measurements were taken in a CH4 atmosphere. The results are shown in Figure 6. This result is very surprising, since the state-of-the-art semiconductor sensors using the ZnO@ZIF-8 sensor material require the presence of oxygen in the atmosphere to measure H2. It shows that although the sensor response is reduced compared to the air atmosphere, it is still high enough for reliable detection in a methane atmosphere.

[0057] Zinc oxide structures coated with ZIF-8, which are intended as sensor material for the detection of hydrogen, are known from the following publications.

[0058] 1) M. Drobek, J.-H. Kim, M. Bechelany, C. Vallicari, A. Julbe, S.S. Kim, MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity, ACS Appl. Mater. Interfaces. 8 (2016) 8323-8328. https: / / doi.org / 10.1021 / acsami.5b12062

[0059] 2) P. Ji, X. Hu, R. Tian, H. Zheng, J. Sun, W. Zhang, J. Peng, Atom-economical synthesis of ZnO@ZIF-8 core-shell heterostructure by dry gel conversion (DGC) method for enhanced H2 sensing selectivity, J. Mater. Chem. C. 8 (2020) 2927-2936. https: / / doi.Org / 10.1039 / C9TC06530J 3) X. Wu, S. Xiong, Z. Mao, S. Hu, X. Long, A Designed ZnO@ZIF-8 Core-Shell Nanorod Film as a Gas Sensor with Excellent Selectivity for H2 over CO, Chem. - Eur. J. 23 (2017) 7969-7975. https: / / doi.org / 10.1002 / chem.201700320

[0060] 4) F. Cui, W. Chen, L. Jin, H. Zhang, Z. Jiang, Z. Song, Fabrication of ZIF-8 encapsulated ZnO microrods with enhanced sensing properties for H2 detection, J. Mater. Sei. Mater.

[0061] Electron. 29 (2018) 19697-19709. https: / / doi.org / 10.1007 / s10854-018-0095-9

[0062] 5) R. Lv, Q. Zhang, W. Wang, Y. Lin, S. Zhang, ZnO@ZIF-8 Core-Shell Structure Gas Sensors with Excellent Selectivity to H2, Sensors. 21 (2021) 4069. https: / / doi.org / 10.3390 / s21124069 6) AI Khudiar, AK Elttayef, MK Khalaf, AM Oufi, Fabrication of ZnO@ZIF-8 gas sensors for selective gas detection, Mater. Res. Express. 6 (2020) 126450. https: / / doi. org / 10.1088 / 2053-1591 / ab69c2

[0063] The following Table 6 provides an overview of the respective sensor material in comparison to the inventive material and to ZnO without conversion layer.

[0064] Table 6: Overview of known ZnO@ZIF-8 sensor materials and ZnO without conversion layer (ZnO) as a sensor material in comparison to the sensor material according to the invention. It can be clearly seen that with the hydrogen sensor material according to the invention, the sensor response at 100 ppm H2 and 100°C measuring temperature in the presence of oxygen is more than 100 (546), which is a factor of 50 to 100 higher than for the comparison materials. Furthermore, it can be seen that the selectivity of the sensor material according to the invention is extremely good. There is no sensor response for the other gases, in this case methane, acetone, ethanol, 2-propanol, n-butanol, ammonia, and CO2, and thus no cross-sensitivity, i.e. maximum selectivity.

[0065] The measurement of H2 in an oxygen-free atmosphere is also possible. Materials and Methods

[0066] Information on the chemicals and equipment used is summarized in Tables 7 and 8. Table 7: List of reagents used in the synthesis including manufacturers.

[0067] Table 8: Measurement methods and equipment used for the production of t-ZnO@ZIF-8

[0068] For the synthesis, approximately 200 mg of ZnO tetrapods (2.46 mmol) are placed into one half of a 30 mL Teflon reactor. 200–400 mg of 2-methylimidazole (HMelM) (2.44–4.87 mmol) are placed in a smaller Teflon vessel within the reactor. The reactor is sealed in a steel autoclave. The reaction is carried out for 2–60 hours at 140 °C. After the reaction, the reactor is quickly cooled and opened. The product is evaporated at 100 °C under reduced pressure (<10'). 2 kPa) to remove excess HMelM from the samples. The exact reaction conditions are summarized in Table 9, and the general synthesis setup is shown in Figure 2.

[0069] Table 9: Synthesis details for the solvent-free coating of ZnO tetrapods with ZIF-8. Analytics

[0070] The homogeneity of the coating was checked using SEM images (Figure 1), and the layer thickness of the ZIF-8 layers was determined for four samples and summarized in Table 1. The quantitative determination of the ZIF-8 content was carried out using thermogravimetric analysis. The samples contain the amounts of ZIF-8 listed in Table 2. The specific surface areas were determined using nitrogen sorption measurements and are listed in Table 3. For the measurement of hydrogen detection of the t-ZnO@ZIF-8(4h) sample, one arm of a single tetrapod was connected to a SiO2 / silicon wafer with gold contacts using the focused ion beam (FIB) method to measure the current flow in a single arm when exposed to different measurement gases. The response of the sensor was tested for the gases hydrogen, methane, acetone, ethanol, 2-propanol, n-butanol, ammonia and CO2 in the temperature range 20-175 °C.The particularly high selectivity towards hydrogen is demonstrated in Figure 4. High sensitivity (= high gas response) is already evident at 100°C. The selectivity is maintained even at higher temperatures, up to 175°C. Short response and regeneration times (Figure 5) in the range of seconds are also observed. The observed sensitivity and selectivity towards H2 in a CH4 atmosphere (Figure 6) is atypical for semiconductor sensors.

[0071] Sensor manufacturing and testing

[0072] The t-ZnO@ZIF-8 tetrapods or individual microrods were contacted with platinum via focused ion beam (FIB) on a SiO2 / silicon wafer with gold contacts using a method developed by Lupan et al. [O. Lupan, V. Cretu, M. Deng, D. Gedamu, I. Paulowicz, S. Kaps, Y.K. Mishra, O. Polonskyi, C. Zamponi, L. Kienle, V. Trofim, I. Tiginyanu, R. Adelung, Versatile Growth of Freestanding Orthorhombic a-Molybdenum Trioxide Nano- and Microstructures by Rapid Thermal Processing for Gas Nanosensors, J. Phys. Chem. C. 118 (2014) 15068-15078. https: / / doi.org / 10.1021 / jp5038415]

[0073] [O. Lupan, L. Chow, Th. Pauporte, LK Ono, B. Roldan Cuenya, G. Chai, Highly sensitive and selective hydrogen single-nanowire nanosensor, Sens. Actuators B Chem. 173 (2012) 772-780. https: / / doi.Org / 10.1016 / j.snb.2012.07.111.]

[0074] They were tested in air with a range of gases: acetone, n-butanol, methane, ethanol, hydrogen, ammonia, 2-propanol, and CO2 at various operating temperatures from 20 to 175 °C using 100 ppm of the previously reported analytes. The operating temperature was limited to a maximum of 175 °C, which is well below the decomposition temperature of the MOF.

[0075] The sensor response (S) was calculated using the ratio of the current during gas exposure (I-gas) and in air (I-air): S = I-gas / I-air. For sensor response experiments in air with F / CFL mixtures, the ^ concentration was increased stepwise from 5 to 10 to 20 ppm. After equilibration in each step, 10 ppm of CH4 was added for a short time. In addition, a sensor response experiment of H2 in pure CH4 as the carrier gas with 100 ppm H2 was performed. For these measurements, only samples formed after 4 h of reaction time (-t-ZnO@ZIF8(4h)) were used.

[0076] List of figures:

[0077] Fig. 1 SEM images of the sensor material t-ZnO@ZIF-8

[0078] Fig. 2 Schematic representation of the inventive method for producing the sensor material (t-ZnO@ZIF-8)

[0079] Fig. 3: SEM images of a t-ZnO@ZIF-8(4h) microrod

[0080] (a) with platinum brought into contact with the gold contacts on a SiO2 / silicon wafer via the FIB

[0081] (b) Cross-section of a t-ZnO@ZIF-8(4h) microrod

[0082] Fig. 4 Response behavior of the sensor according to the invention for different gases

[0083] Fig. 5 Response times and regeneration times of the sensor according to the invention

[0084] Fig. 6 Response behavior of the sensor according to the invention in an oxygen-free atmosphere

Claims

CLAIMS 1. Hydrogen sensor material comprising at least one rod-shaped single crystal with a wurtzite structure made of a semiconductor and a conversion layer formed by a MOF on the surface of the single crystal, wherein the single crystal has a length of 100 nm to 100 pm and a lateral extent of 500 nm to 10 pm and wherein the aspect ratio of the single crystal of length to lateral extent is greater than 3.

2. Hydrogen sensor material according to claim 1, characterized in that the rod-shaped single crystal with wurtzite structure is formed from zinc oxide (ZnO) or ZnTe or ZnSe.

3. Hydrogen sensor material according to one of the preceding claims, characterized in that the conversion layer is formed by the MOF ZIF-8 ([Zn(C4HsN2)2)]).

4. Hydrogen sensor material according to one of the preceding claims, characterized in that the rod-shaped single crystal with wurtzite structure is formed from tetrapodal zinc oxide (t-ZnO).

5. Hydrogen sensor material according to one of the preceding claims, characterized in that the sensor response at 100 ppm H2 and 100°C measuring temperature in the presence of oxygen is more than 100.

6. Hydrogen sensor material according to one of the preceding claims, characterized in that the sensor response to the gases methane, acetone, ethanol, 2-propanol, n-butanol, ammonia and CO2 at 100 ppm and 100°C measuring temperature in the presence of oxygen is 1.

7. Hydrogen sensor material structure with a hydrogen material according to one of the preceding claims, characterized in that several rod-shaped single crystals with wurtzite structure are connected to form a diffusion-open network.

8. A method for producing a hydrogen sensor material and / or structure according to any one of the preceding claims 1 to 7, in particular 3 to 7, comprising the following steps I. Determination of the reaction temperature in the range between 60°C and 160°C, the reaction time and the molar ratio in relation to the desired layer thickness using Tables 1 and 2 and the RGT rule Table 1 : ZnO to HMelM ratio Table 2: Average film thickness of the ZIF-8 coating that can be achieved at a reaction temperature of 140 °C for a given reaction time. II. Placing ZnO material into a sealable reactor; III. Addition of 2-methylimidazole (HMelM); IV. Closing the reactor; V. Tempering the reactor; VI. Cooling of the reactor, whereby steps II and III can be carried out in a variable order.

9. A process for producing a hydrogen sensor material according to the preceding claim, characterized in that excess HMelM is removed by treating the crude product under reduced pressure and / or at elevated temperature (step VII).

10. Use of a hydrogen sensor material and / or structure according to any one of claims 1 to 7 at a measuring temperature of less than 120°C.

11. Use of a hydrogen sensor material and / or structure according to any one of claims 1 to 7 in an oxygen-free atmosphere.

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