WO2023025716A1

WO2023025716A1 - Method and device for removing residual oxygen from inert gases by synthesizing metal nanoparticles

Info

Publication number: WO2023025716A1
Application number: PCT/EP2022/073308
Authority: WO
Inventors: Vinzent OLSZOK; Malte BIERWIRTH; Alfred P. Weber
Original assignee: Technische Universität Clausthal
Priority date: 2021-08-24
Filing date: 2022-08-22
Publication date: 2023-03-02
Also published as: EP4392164A1; DE102021121928A1

Abstract

The aim of the invention is to remove residual oxygen from an inert gas (8). This is achieved in that a voltage is applied between two electrodes (5, 6) adjoining the inert gas (8), said voltage producing a direct gas discharge (9) in the inert gas (8). As a result of the gas discharge (9), metal is removed from at least one of the electrodes (5, 6). The metal forms nanoparticles (12) in the inert gas (8), said nanoparticles spontaneously oxidizing, thereby using the residual oxygen.

Description

REHBERG HÜPPE + PARTNER - 1 - Version originally filed 21582PCT 08/22/2022

RHP Ref.: 21582PCT ZBA6

Patent application: file number not yet assigned

Priority: 08/24/2021 (DE 10 2021 121 928.0)

Title: Method and device for removing residual oxygen from

Inert gases by synthesis of metal nanoparticles

Applicant: Clausthal University of Technology

METHOD AND DEVICE FOR REMOVING RESIDUAL OXYGEN FROM INERT GASES BY SYNTHESIS OF METAL NANOPARTICLES

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method and a device for removing residual oxygen from an inert gas.

Many materials, such as most metals, form oxide layers on their surface within a short time at room temperature in air. This oxide formation is disruptive for various processes, for example in semiconductor production. Such oxide layers can also be a hindrance when processing metals.

STATE OF THE ART

It is known to carry out oxygen-sensitive processes in ultra-high vacuum (UHV) systems. By removing the atmosphere, the oxygen partial pressure in such a system is also reduced to such an extent that oxidation can no longer take place. However, UHV systems are expensive to purchase and operate. In addition, the implementation of processes in UHV systems is time-consuming because the formation of an ultra-high vacuum requires a long pumping time that increases with falling residual pressure. In addition, materials with a low vapor pressure, for example, cannot be processed in an ultra-high vacuum.

As an alternative to carrying out oxygen-sensitive processes in UHV systems, it is known to carry out such processes under an oxygen-free atmosphere, ie a so-called protective gas atmosphere. Inert gases such as argon or nitrogen are particularly suitable for forming the protective gas atmosphere. Even 99.999% pure inert gases (purity 5.0) still have so much residual oxygen that oxygen-sensitive processes can be significantly disrupted.

To selectively reduce the oxygen partial pressure, it is known to bind oxygen by oxidizing so-called oxygen scavengers. For example, titanium chips are heated in ultra-high vacuum systems in order to stimulate the formation of titanium oxide, as a result of which the oxygen partial pressure drops.

It is also known to use monosilane (SiF) as an oxygen scavenger, which reacts with residual oxygen in an inert gas to form silicon dioxide (SiO2) and hydrogen (H2). However, monosilane is itself a highly reactive compound and the reaction product, hydrogen, is also a highly reactive impurity in the inert gas from which the oxygen has been removed.

Another known way of removing residual oxygen from an inert gas is to pass the inert gas through a sodium-potassium alloy that is liquid at room temperature and reacts with the residual oxygen. However, this alloy of alkali metals is highly reactive and should be handled with great care from a safety point of view.

DE 44 26 081 B4 discloses a gas cleaning device for cleaning gases contaminated with pollutants, with a housing having an inlet opening and an exhaust gas opening and with a fan having at least one fan blade located between them and a device for generating a glow discharge between a housing inner wall and the fan blade . The device includes a magnetic layer on the outside of the housing and electrodes on the inside surface of the housing and at least on the fan blade tip. In this case, the inner wall of the housing and/or the fan blade are provided with a catalytically active metal layer which is selected from platinum, palladium, ruthenium and rhodium. Upstream of the gas purification device, there are provided a cathode and an anode opposed to each other, and a deoxidizing element formed of zinc or indium and interposed between the anode and the cathode on the cathode side. Exhaust gas is not directly introduced into the gas purification device but is first introduced into the deoxidizing element. The oxygen contained in the exhaust gas comes into contact with zinc or indium or a site from which oxygen has been removed by sputtering zinc oxide (or indium oxide), thereby selectively trapping or removing the oxygen. absorbed and removed from the exhaust gas. The oxygen-free exhaust gas is then fed to the gas cleaning device, where the gas is cleaned. When an electric voltage is applied across the anode and the cathode, a discharge phenomenon takes place between the anode and the cathode, and cations generated by the discharge are accelerated to the cathode side and cause a sputtering phenomenon that converts ZnO or InO into Zn or In or reduced. Thus, this sputtering phenomenon regenerates the oxidizing element with the trapped oxygen atoms.

OBJECT OF THE INVENTION

The invention is based on the object of demonstrating a method and a device for removing residual oxygen from an inert gas which are highly effective but do not require the use of highly reactive substances and also do not contaminate the inert gas with highly reactive substances.

SOLUTION

The object of the invention is achieved by a method having the features of independent patent claim 1 and a device for carrying out this method having the features of patent claim 7 . Preferred embodiments of the invention are defined in dependent claims 2-6 and 8-10.

DESCRIPTION OF THE INVENTION

In the method according to the invention for removing residual oxygen from an inert gas, a voltage is applied between two electrodes adjacent to the inert gas, which causes a direct gas discharge in the inert gas. The direct gas discharge removes metal from at least one of the electrodes, forming nanoparticles in the inert gas. The nanoparticles oxidize spontaneously, consuming the residual oxygen.

It is generally known that a direct gas discharge between two electrodes in a working gas can be used to synthesize nanoparticles from the material of the electrodes. The well-known method is also referred to as spark synthesis, see for example https://parteq.net/products/articlesynthesis/funkensynthesis/. The tendency of metallic nanoparticles to oxidize is also known in principle, see for iron nanoparticles W. Karim et al.: "Size-dependent redox behavior of iron observed by in-situ single nanoparticle spectro-microscopy on well-defined model systems", SCIENTIFIC REPORTS , 6: 18818, DOI: 10.1038/srep18818, published January 06, 2016 and for aluminum nanoparticles A. Rai et al.: "Understanding the mechanism of aluminum nanoparticle oxidation" Combustion Theory and Modelling, Vol. 5, October 2006, 843-859, http://dx.doi.org/10.1080/13647830600800686, wherein the aluminum nanoparticles are synthesized by arc discharge between an aluminum anode and a tungsten cathode.

Nevertheless, it is surprising that the formation of metal nanoparticles by spark synthesis can reduce the residual oxygen in an inert gas to extremely low residual oxygen contents of less than 1×10′ ⁹ ppm with overall comparatively very little effort. No substances are used for this that are highly reactive as such. The oxidation tendency of the nanoparticles, which are formed as a result of the gas discharge between the electrodes made of the metal of at least one of the electrodes, is sufficient.

It goes without saying that two electrically conductive electrodes are used in the method according to the invention for forming the direct gas discharge. Typically, both of these electrodes also consist of metal, which is removed as a result of the gas discharge and forms nanoparticles in the inert gas, which then oxidize using the residual oxygen. In principle, however, the other of the electrodes can be formed from a rather inert metal, which is removed to a lesser extent and also forms less reactive nanoparticles than the metal of the at least one of the electrodes.

Specifically, the at least one of the two electrodes can be formed at least predominantly from aluminum or iron. This can also apply to the other of the two electrodes. Alternatively, the other of the two electrodes can consist of a material, such as tungsten, which is not removed or is removed to a lesser extent as a result of the direct gas discharge. Tests have shown that the tendency of iron and aluminum nanoparticles to oxidize is completely sufficient to reduce the residual oxygen content in inert gases to well below ^10'9 ppm. It can be assumed that other metals such as copper, magnesium and titanium are also suitable for the formation of nanoparticles in the method according to the invention, which oxidize spontaneously while consuming the residual oxygen in the inert gas. In the case of copper, however, due to a comparatively low production rate of the nanoparticles a rather small effect. Magnesium has an outstanding effect that can be used in particular for argon and other noble gases as inert gases. However, magnesium has no long-term stability under nitrogen due to nitride formation on electrode surfaces.

In the method according to the invention, metal is to be removed from the at least one electrode in a significantly more than stoichiometric amount in order to form a sufficient number of nanoparticles with a sufficiently large reactive surface area to remove the residual oxygen from the inert gas to the desired extent. However, there is also no need for an extreme superstoichiometric ratio of the metal to the oxygen to be removed. Thus, 4 mol to 100 mol, preferably 8 mol to 40 mol, of the metal can be removed per 1 mol molecule of residual oxygen. In order to remove the residual oxygen from 1 liter of inert gas with a purity of 5.0 (the volume refers to normal pressure), metal in the order of 1 pg can be removed in order to form the nanoparticles from it.

Depending on the metal and the degree of oxidation or oxidation product, the oxidized nanoparticles are inert. Nevertheless, it is usually useful to filter the oxidized nanoparticles from the inert gas. This can easily be achieved with a series connection of particle filters with increasing degrees of separation. The series connection of two 99.5% particle filters and one 99.999% particle filter has proven itself to essentially completely remove the oxidized nanoparticles from the inert gas without the 99.999% particle filter having to be changed frequently because it becomes contaminated with the filtered nanoparticles added.

In this way, it has been possible with the method according to the invention to reduce the residual oxygen content in inert gases such as argon and nitrogen not only below 1 x 10' ⁹ ppm, but also below 1 x 10' ¹ ppm or below 1 x 10' ¹¹ ppm and up to to 1 x 10' ¹² ppm or down to 1 x 10' ¹³ ppm and even lower. At normal pressure, a residual oxygen content of 1×10' ¹³ ppm corresponds to less than 3 oxygen molecules in 1 cm ³ of inert gas. All details of the residual oxygen content in ppm here refer to the relative number of oxygen molecules still present.

If the method according to the invention is carried out repeatedly, ie the inert gas is brought into contact at least twice in succession with the metal nanoparticles being formed, the residual oxygen content can even be reduced to below 1×10′ ¹⁴ ppm. Concrete a residual oxygen content of 3.5× ^10′15 ppm was achieved when the process according to the invention was carried out twice.

A device according to the invention for removing residual oxygen from inert gas according to the method according to the invention has a reaction space between a gas inlet and a gas outlet, two electrodes adjoining the reaction space and a voltage source. The voltage source is designed to apply a voltage between the electrodes, which causes a direct gas discharge into the reaction space filled with inert gas, metal being removed from at least one of the electrodes by the direct gas discharge, forming nanoparticles in the inert gas. These nanoparticles oxidize spontaneously, consuming the residual oxygen.

As in the method according to the invention, the at least one of the electrodes can be formed at least predominantly from aluminum or iron or another of the metals copper, magnesium and titanium.

In the device according to the invention, an oxygen probe, for example a lambda probe, can be arranged in the reaction space. A controller of the device can then be designed to control the voltage source depending on the signal from the oxygen probe in such a way that so much material is removed that the residual oxygen content in the inert gas is reduced to 1 x 10' ¹⁰ ppm, preferably 1 x 10' ¹¹ ppm, more preferably to 1 x 10' ¹² ppm and most preferably to 1 x 10' ¹³ ppm. The controller can also control a device that determines the flow of the inert gas through the reaction space, for example a valve between a pressure vessel for the inert gas and the gas inlet into the reaction space.

In order to achieve particularly low residual oxygen contents, two or even more devices according to the invention can be connected in series, so that the inert gas flows through their reaction chambers one after the other.

A filter device, which is designed to filter the oxidized nanoparticles from the inert gas, is preferably arranged downstream of the reaction space. The filter device can have a series connection of particle filters with increasing degrees of separation, as has already been explained in connection with the method according to the invention. A Changing the particle filter is only seldom necessary during operation of the device according to the invention because the particle filters only clog very slowly if their degree of separation is designed appropriately due to the small absolute number of nanoparticles formed and correspondingly to be filtered off with the particle filters.

The oxidation of the nanoparticles by the residual oxygen in the inert gas also takes place in the filter, i. H. as long as the nanoparticles are in contact with the inert gas.

Advantageous developments of the invention result from the patent claims, the description and the drawings.

The advantages of features and combinations of several features mentioned in the description are merely exemplary and can have an effect alternatively or cumulatively without the advantages necessarily having to be achieved by embodiments according to the invention.

The following applies to the disclosure content - not the scope of protection - of the original application documents and the patent: Further features can be found in the drawings - in particular the illustrated geometries and the relative dimensions of several components to one another as well as their relative arrangement and operative connection. The combination of features of different embodiments of the invention or of features of different patent claims is also possible, deviating from the selected dependencies of the patent claims and is hereby suggested. This also applies to those features that are shown in separate drawings or are mentioned in their description. These features can also be combined with features of different patent claims. Likewise, features listed in the patent claims can be omitted for further embodiments of the invention, but this does not apply to the independent patent claims of the granted patent.

The features mentioned in the patent claims and the description are to be understood with regard to their number in such a way that exactly this number or a larger number than the number mentioned is present without the need for an explicit use of the adverb "at least". So if, for example, we are talking about an oxygen probe, this is the case understand that there is exactly one oxygen probe, two oxygen probes, or more oxygen probes. The features listed in the patent claims can be supplemented by further features or can be the only features that the subject matter of the respective patent claim has.

The reference signs contained in the claims do not limit the scope of the subject-matter protected by the claims. They only serve the purpose of making the claims easier to understand.

BRIEF DESCRIPTION OF THE FIGURES

The invention is further explained and described below with reference to preferred exemplary embodiments illustrated in the figures.

Fig. 1 is a schematic representation of a device according to the invention and

FIG. 2 is a plot of a residual oxygen content in an inert gas versus time after the device according to FIG. 1 has been switched on until after the device has been switched off again.

FIGURE DESCRIPTION

The device 1 shown schematically in FIG. 1 has a reaction chamber 2 between a gas inlet 3 and a gas outlet 4, which are indicated by arrows. Two metal electrodes 5 and 6 are arranged in the reaction chamber 2 and are connected to a voltage source 7 . In Fig. 1, the voltage source 7 is shown as a DC voltage source; alternatively, the voltage source 7 can be designed as an alternating voltage source and/or as a pulsed voltage source. The voltage source 7 applies a high voltage between the electrodes 5 and 6 which exceeds the breakdown voltage of an inert gas 8 arranged in the reaction space 2 . This results in a direct gas discharge 9 between the electrodes 5 and 6, which is visible as a spark gap 10 between the electrodes. More specifically, the current from the voltage source 7 charges a capacitor 11 connected across the gap between the electrodes 5 and 6 between the outputs of the voltage source 7 . This increases the voltage across the capacitor 11, which is applied to the electrodes 5 and 6 is present. When the breakdown voltage, which is linearly dependent on the distance between the electrodes 5 and 6, is reached, a rapid gas discharge 9 occurs between the electrodes 5 and 6. The electrical energy stored in the capacitor 11 is released in the form of a spark. Accordingly, the gas discharge 9 does not take place continuously, but in the form of individual sparks with a frequency of the order of 1 Hz to a few kHz, often from 10 to 100 Hz. This frequency increases with the strength of the current from the voltage source 7 charging the capacitor 11 . On the other hand, a larger capacitance of the capacitor 1, which is in a typical order of magnitude of a few to a few 10 nF, leads to higher energies of the individual sparks and thus to higher rates of removal of the metal from the electrodes 5 and 6.

The gas discharge 9 leads to a removal of metal from the electrodes 5 and 6, from which nanoparticles 12 form within the inert gas 8 in the reaction chamber 2. These nanoparticles

12 have a typical agglomerate size between 20 and 120 nm, and they can be present in a particle number concentration between 1×10 ⁴ and 1×10 ⁹ particles/cm ³ . The nanoparticles 12 oxidize spontaneously while consuming the residual oxygen in the inert gas 8 and can thus reduce the residual oxygen content to 1×10′ ¹³ ppm. A filter device 13 for filtering off the nanoparticles 12 from the inert gas 8 is arranged in front of the gas outlet 4 of the reaction chamber 2 . In concrete terms, this can involve a series connection of two 99.5% particle filters 14 and 15 and a 99.999% particle filter 16 . The residual oxygen in the inert gas can also react with the nanoparticles 12 in the filter device 13 . The residual oxygen in the inert gas exiting through the gas outlet 4 is detected using an oxygen probe 17 . A controller 19 controls the voltage source 7 as a function of the signal 18 from the oxygen probe 17 .

FIG. 2 documents the time course of the oxygen concentration in nitrogen in ppm measured with the oxygen probe 17 according to FIG. 1 after the device 1 according to FIG. 1 was switched on and later switched off again. First, the residual oxygen content drops quickly to 1× ^10′1 ppm after the gas discharge 9 has started. With increasing loading of the filter

13 with nanoparticles made of aluminum here, the residual oxygen content is further reduced to about 1×10 ^-11 ppm. When the device is switched off, the residual oxygen content rises again to about 1 ppm, which corresponds to the supplied nitrogen gas of purity 5.0. By increasing the voltage between the electrodes 5 and 6 of the device 1 and reducing the flow of the inert gas 8 through the reaction space 2, the residual oxygen content could be reduced to 1× ^10-13 ppm. With two devices 1 connected in series, a residual oxygen content of 3.5× ^10′15 ppm was even achieved.

REFERENCE LIST

contraption

reaction space

gas inlet

gas outlet

electrode

voltage source

Inert gas direct gas discharge

spark gap

capacitor

nanoparticles

filter device

particle filter

oxygen probe

Oxygen probe signal 17

steering

Valve

Claims

PATENT CLAIMS

1. A method for removing residual oxygen from an inert gas (8), characterized in that a voltage is applied between two electrodes (5, 6) adjacent to the inert gas (8), which causes a direct gas discharge (9) in the inert gas (8 ) causes, whereby the direct gas discharge (9) removes metal from at least one of the electrodes (5, 6), forming the nanoparticles (12) in the inert gas (8), the nanoparticles (12) oxidizing spontaneously, consuming the residual oxygen .

2. The method according to claim 1, characterized in that the at least one of the electrodes (5, 6) is formed at least predominantly from aluminum or iron.

3. The method according to any one of the preceding claims, characterized in that per 1 mol of residual oxygen 10 mol to 1000 mol, preferably 50 mol to 500 mol of the metal is removed.

4. The method according to any one of the preceding claims, characterized in that so much metal is removed that the residual oxygen content in the inert gas (8) to 1 x 10 ^'10 ppm, preferably to 1 x 10' ¹¹ ppm, more preferably to 1 x 10' ¹² ppm and most preferably to 1 x 10' ¹³ ppm.

5. The method according to any one of the preceding claims, characterized in that the oxidized nanoparticles (12) are filtered from the inert gas (8).

6. The method according to claim 5, characterized in that the oxidized nanoparticles (12) are filtered with a series connection of particle filters (14-16) with increasing degrees of separation.

7. Device (1) for removing residual oxygen from inert gas (8) according to one of the preceding claims, with a reaction chamber (2) between a gas inlet (3) and a gas outlet (4), with two electrodes adjoining the reaction chamber (2). (5, 6), and with a voltage source (7) which is designed to generate a voltage between the electrodes (5, 6), which causes a direct gas discharge (9) in the reaction chamber filled with inert gas (8), metal being removed from at least one of the electrodes (5, 6) by the direct gas discharge (9), the nanoparticle (12) in the inert gas (8), wherein the nanoparticles (12) oxidize spontaneously with consumption of the residual oxygen.

8. Device (1) according to claim 7, characterized in that the at least one of the electrodes (5, 6) is formed at least predominantly from aluminum or iron.

9. Device (1) according to one of the preceding claims, characterized in that an oxygen probe (17) is arranged in the reaction chamber (2) or downstream of the reaction chamber (2) and that a controller (19) is designed to control the voltage source ( 7) depending on a signal (18) from the oxygen probe (17) so that so much metal is removed that the residual oxygen content in the inert gas (8) falls to 1 x 10' ¹⁰ ppm, preferably to 1 x 10' ¹¹ ppm, more preferably to 1 x 10' ¹² ppm and most preferably to 1 x 10' ¹³ ppm.

10. Device (1) according to one of the preceding claims, characterized in that a filter device (13) is arranged in the gas outlet (4) of the reaction chamber (2) or downstream of the reaction chamber (2), which is designed to filter the oxidized nanoparticles ( 12) to filter out.

11. Device (1) according to claim 10, characterized in that the filter device (13) has a series connection of particle filters (14-16) with increasing degrees of separation.