WO2025254011A1 - Hydrogen-permeable membrane composed of pdcu-based alloy - Google Patents

Abstract

A hydrogen-permeable membrane according to the present invention is composed of a PdCu-based alloy. The PdCu-based alloy is composed of 47.0-49.0 atom% of Pd, 0.001-0.15 atom% of an interstitial element E_I, and 0.01-1.0 atom% of a substitutional element E_S, where the balance is Cu and unavoidable impurities. The PdCu-based alloy contains, as the interstitial element E_I, at least one element from among B, C, and N, and contains, as the substitutional element E_S, a metal element necessarily including at least one of Ag, Au, and Al. The hydrogen-permeable membrane according to the present invention has improved hydrogen permeability in a low-temperature region of 200℃ or lower. Specifically, in the hydrogen-permeable membrane, the ratio (φ¹⁰⁰/φ³⁰⁰) between the hydrogen permeation coefficient φ¹⁰⁰ at 100°C and the hydrogen permeation coefficient φ³⁰⁰ at 300°C is 0.4 or greater.

Description

Hydrogen-permeable membrane made of PdCu alloy

The present invention relates to a hydrogen-permeable membrane that selectively allows hydrogen to permeate from a hydrogen-containing gas. In particular, it relates to a hydrogen-permeable membrane with improved hydrogen permeability at low temperatures.

Hydrogen is widely used in a variety of fields, including as a hydrogen source and reducing agent in the synthesis of various compounds. In recent years, hydrogen has been attracting attention as a renewable energy source, and it is expected to be used as fuel gas in fuel cells that power automobiles and heavy machinery, and in hydrogen engines. Hydrogen is also attracting attention in the field of cutting-edge medical care; for example, the effectiveness of hydrogen inhalation therapy for post-cardiac arrest syndrome has been reported.

Hydrogen-permeable membranes are materials used in devices for handling hydrogen, which is utilized in the various fields mentioned above. For example, the use of hydrogen in the fuel field requires high-purity hydrogen gas, and hydrogen purification devices using hydrogen-permeable membranes have been developed. Hydrogen sensors are also needed to measure the hydrogen concentration inside the power source and in the exhaust gas of fuel cell vehicles, etc. Hydrogen sensors are also necessary in the medical field for accurate and precise measurement of the hydrogen concentration of therapeutic gases. Hydrogen-permeable membranes, which selectively allow only hydrogen to pass through from the gas being measured, can be ideally used as hydrogen sensors in these applications.

Hydrogen-permeable membranes are made of metal alloys that selectively absorb and release hydrogen while diffusing it internally. Known examples of such metal alloy membranes include Pd alloy membranes (such as PdAg alloys and PdCu alloys) that utilize the selective hydrogen permeability of Pd (palladium). Hydrogen-permeable membranes made of PdCu alloys in particular are becoming increasingly popular and are being mass-produced due to their reduced risk of hydrogen embrittlement and corrosion resistance (Patent Documents 1 and 2, Non-Patent Document 1).

Japanese Patent Application Laid-Open No. 2001-262252 JP 2008-12495 A

JamesRaphael Warren, “The Effect of Hydrogen on Palladium-Copper Based Membranes for Hydrogen Purification”, THE UNIVERSITY OF BIRMINGHAM, P22-29, 37-80.

The phenomenon of hydrogen permeation through hydrogen-permeable membranes made of PdCu alloy membranes is due to atomic diffusion, and the hydrogen permeability coefficient at this time is temperature dependent and is said to follow the so-called Arrhenius equation (Arrhenius plot). In an Arrhenius plot, the logarithm of the hydrogen permeability coefficient decreases linearly and inversely proportional to the reciprocal of temperature (1/T). Therefore, if the hydrogen permeability coefficient is measured in the high temperature range, it is possible to predict the hydrogen permeability coefficient in the low temperature range.

However, according to the inventors' investigations, it is difficult to predict the hydrogen permeability coefficient based on the Arrhenius plot described above for conventional hydrogen-permeable membranes made of PdCu alloys. Specifically, even if an Arrhenius plot is created based on the hydrogen permeability coefficient measured in the high-temperature range, the actual measured hydrogen permeability coefficient in the low-temperature range will be a low value that deviates from the Arrhenius plot. This means that hydrogen-permeable membranes made of PdCu alloys have lower-than-expected hydrogen permeability in the low-temperature range. While it is unavoidable that the hydrogen permeability coefficient has temperature dependency, it is undesirable for the hydrogen permeability coefficient to be lower than expected in the low-temperature range.

Previous studies into hydrogen-permeable membranes have mainly aimed to increase the hydrogen permeability coefficient, with not many studies aimed at improving the temperature dependency of hydrogen-permeable membranes. This is because it is recognized that the hydrogen permeability coefficient is the clearest indicator of a hydrogen-permeable membrane's functionality, and that its level is important. Another factor behind this is that hydrogen-permeable membranes have traditionally been used in many applications, such as hydrogen purification equipment, which is permitted to operate at high temperatures.

However, some devices that use hydrogen-permeable membranes must be operated at low temperatures. For example, hydrogen sensors for fuel cell vehicles and hydrogen sensors for medical applications are designed for use at room temperature. These applications of hydrogen-permeable membranes have only recently attracted attention, but hydrogen-permeable membranes used in these new fields are required to be able to exhibit higher hydrogen permeability at low temperatures.

The present invention was made against the background described above, and provides a hydrogen-permeable membrane made of a PdCu-based alloy that has improved hydrogen permeability in the low-temperature range. In this invention, the low-temperature range refers to the temperature range from room temperature (25°C) to 200°C.

In order to solve the above problem, the inventors investigated the causes of the decrease in hydrogen permeability in PdCu alloy membranes at low temperatures and countermeasures for this. The hydrogen permeability of PdCu alloys is exhibited in the beta phase, which has a B2 structure based on a body-centered cubic (bcc) lattice. Hydrogen exhibits hydrogen permeability by dissolving in the crystal lattice gaps of the PdCu alloy membrane in the beta phase and then moving and diffusing. Therefore, the hydrogen permeability of a PdCu alloy can be expressed as the product of the solid solubility of hydrogen and its mobility (ease of movement).

The present inventors have considered that the cause of the decrease in hydrogen permeability of the PdCu alloy membrane is the generation of vacancies due to structural defects (lattice defects) in the PdCu alloy crystal. Based on computational chemistry and experimental/empirical findings, the present inventors have concluded the following about the relationship between structural defects in the PdCu alloy crystal and the decrease in hydrogen permeability.
(i) Possible structural defects in PdCu alloy crystals include Pd atom vacancies at Pd sites, Cu atom vacancies at Cu sites, and Cu vacancy-like double defects caused by the diffusion of Cu atoms into vacancies formed at Pd sites. Although the occurrence frequency of these varies, in either case, there are sites near the defects that trap hydrogen atoms, and binding energy that fixes hydrogen is generated at these sites. This binding energy affects the hydrogen mobility.
(ii) For hydrogen permeation, the trapped hydrogen must be able to move, and for this to occur, energy greater than the binding energy must be applied.
(iii) The reason why the hydrogen permeability of PdCu alloys decreases at low temperatures is thought to be due to the binding energy near the defects mentioned above. When the PdCu alloy is in a high temperature range, the binding energy is relaxed by thermal energy, allowing hydrogen to migrate. However, this is difficult at low temperatures, preventing hydrogen from migrating, resulting in a decrease in hydrogen permeability.

As described above, the inventors have concluded that the decrease in hydrogen permeability at low temperatures is due to a decrease in hydrogen mobility caused by structural defects. The inventors have therefore conceived of adding an interstitial element such as B to the PdCu alloy as a means of alleviating binding energy and allowing hydrogen to migrate even at low temperatures. In this regard, the inventors have subsequently conducted computational chemistry studies that have confirmed that the interstitial element introduced into the PdCu alloy enters the lattice near the structural defects, thereby alleviating the aforementioned binding energy. Therefore, this method can be said to contribute to improving hydrogen mobility.

However, according to the inventors' research, further investigation is needed into the above measures. Light elements such as B and N, known as interstitial elements, experience low binding energy within the crystal lattice of PdCu alloys and are therefore easily desorbed. Converting a PdCu-based alloy into a hydrogen-permeable membrane requires several processes, including a melting process to produce an alloy ingot, annealing for processing, and heat treatment to convert the crystal structure to the β phase. There is a possibility that interstitial elements will desorb during this processing. If desorption of interstitial elements occurs, the aforementioned effect of relaxing the hydrogen binding energy will be lost.

The inventors therefore decided to try adding substitutional elements as additional additives in order to stabilize the interstitial elements introduced into the PdCu alloy. The substitutional elements are placed in the vacancies, which are the structural defects mentioned above. In this case, the substitutional elements generate repulsive or attractive forces due to interactions such as interatomic attractions with the surrounding Pd and Cu atoms, creating distortion in the crystal lattice. According to calculations by the inventors, it is believed that this lattice distortion can restrain and stabilize the interstitial elements between the lattice.

Based on the results of the above-mentioned multi-stage investigation, the inventors came up with the idea of simultaneously adding interstitial and substitutional elements to a PdCu alloy as a means of improving the hydrogen permeability of a PdCu alloy hydrogen-permeable membrane at low temperatures. They then conducted detailed research into the range of elements suitable for substitutional and interstitial elements, as well as the conditions for obtaining the B2 structure (β phase) required for the membrane to function as a hydrogen-permeable membrane, and came up with the present invention.

That is, the present invention, which solves the above-mentioned problems, provides a hydrogen-permeable film made of a PdCu-based alloy, characterized in that the PdCu-based alloy contains 47.0 atomic % to 49.0 atomic % of Pd, 0.001 atomic % to 0.15 atomic % of an interstitial element E _I , 0.01 atomic % to 1.0 atomic % of a substitutional element E _S , and the balance being Cu and unavoidable impurities, wherein the interstitial element E _I is at least one of B, C, and N, and the substitutional element E _S is a metal element that essentially contains at least one of Ag, Au, and Al.

The following describes the structure and hydrogen permeability of the hydrogen-permeable membrane according to the present invention, as well as its manufacturing method and application. Note that in this specification, ternary or higher alloys consisting of Pd, Cu, and other elements are referred to as PdCu-based alloys, and binary alloys consisting of Pd and Cu are referred to as PdCu alloys.

(A) Structure of the hydrogen-permeable membrane according to the present invention (A-1) Alloy composition The hydrogen-permeable membrane according to the present invention is made of a PdCu-based alloy (PdCuE _IES alloy) whose essential constituent elements are Pd, Cu, an interstitial element ( _EI ), and a substitutional element ( _ES ) _, excluding unavoidable impurities as described below. The functions and composition ranges of each of these constituent elements are as follows:

(A-1-1) Pd and Cu
Since the hydrogen-permeable membrane according to the present invention is composed of a PdCu-based alloy, Pd and Cu are essential and major constituent elements of the present invention. Pd is an essential metal for ensuring the hydrogen permeability of the hydrogen-permeable membrane made of a PdCu-based alloy. Furthermore, the hydrogen permeability of a PdCu-based alloy is exhibited when its crystal system is in the β-phase, which is a B2 structure. Cu is an essential additive metal for promoting the phase transformation from the α-phase (face-centered cubic lattice (fcc)) to the β-phase in the PdCu-based alloy and maintaining the phase structure necessary for exhibiting hydrogen permeability. Furthermore, Cu also has the effect of suppressing the reduction in strength of the PdCu-based alloy membrane due to hydrogen embrittlement.

In the PdCu-based alloy constituting the hydrogen-permeable membrane according to the present invention, the Pd concentration is set to 47.0 atomic % or more and 49.0 atomic % or less. If the Pd concentration exceeds 49.0 atomic %, it becomes difficult to develop the β phase, and it becomes difficult to obtain a sufficient amount of β phase even after heat treatment during the manufacturing process. On the other hand, since Pd is an essential element for exhibiting hydrogen permeability, a decrease in the Pd concentration leads to a decrease in the hydrogen permeability of the hydrogen-permeable membrane. Furthermore, if the Pd concentration is less than 47.0 atomic %, it becomes difficult to obtain sufficient hydrogen permeability even at high temperatures. The Pd concentration is preferably set to 47.25 atomic % or more and 48.8 atomic % or less, and particularly preferably set to 47.75 atomic % or more and 48.5 atomic % or less. The Cu concentration is the remainder of the Pd concentration, the _EI concentration and _ES concentration described below, and the concentration of inevitable impurities.

(A-1-2) Interstitial element (E _I )
As described above, in the present invention, when lattice defects occur in a PdCu alloy, an interstitial element E _I is used as an additive element to relax the binding energy of hydrogen trapped near the defects. This interstitial element E _I is at least one of B (boron), C (carbon), and N (nitrogen).

The concentration of the interstitial element _EI in the PdCu-based alloy constituting the hydrogen-permeable membrane of the present invention is set to 0.001 atomic % or more and 0.15 atomic % or less. If the concentration is less than 0.001 atomic %, the above effect is difficult to achieve, and the hydrogen permeability of the PdCu-based alloy at low temperatures is not improved. Furthermore, interstitial elements such as B significantly inhibit the formation of a β-phase in the PdCu-based alloy. Therefore, if an excessive amount of B is added, exceeding 0.15 atomic %, it becomes difficult to form a β-phase in the PdCu-based alloy. In this case, the resulting hydrogen-permeable membrane has poor hydrogen permeability not only at low temperatures but also at high temperatures. The concentration of the interstitial element _EI is more preferably set to 0.01 atomic % or more and 0.15 atomic % or less. Furthermore, the PdCu-based alloy of the present invention contains at least one of B, C, and N as the interstitial element _EI . One or more of these elements may be added.

(A-1-3) Substitutional element (E _S )
The substitutional element _ES is an additive element that improves the stability of an interstitial element ( _EI ) that is added to relax the binding energy of hydrogen trapped near the defect. Specifically, the substitutional element _ES substitutes for a Pd atom or Cu atom in the PdCu alloy and generates lattice distortion through interaction with the surrounding Pd and Cu atoms. This lattice distortion constrains the interstitial element and promotes solid solution of the interstitial element. This improves the stability of the interstitial element around the vacancy, maintaining the function of relaxing the binding energy of hydrogen.

According to the investigations of the present inventors, among the metal elements that can be substitutional elements, Ag (silver), Au (gold), and Al (aluminum) are capable of exerting the above-mentioned effects. Therefore, the substitutional element _ES in the present invention is a metal element that essentially contains at least one of Ag, Au, and Al.

The concentration of the substitutional element _ES in the PdCu-based alloy constituting the hydrogen-permeable membrane according to the present invention is set to 0.01 atomic % or more and 1.0 atomic % or less. At a concentration of less than 0.01 atomic %, the above-mentioned effects are difficult to achieve. On the other hand, the function of the substitutional element _ES is to stabilize the interstitial element _EI near the vacancies, and it does not directly improve hydrogen permeability. Rather, the addition of an excessive substitutional element leads to a decrease in the Cu or Pd concentration in the alloy, which may hinder β-phase formation and reduce hydrogen permeability across the entire temperature range, including high temperatures. Therefore, the upper limit of the concentration of the substitutional element _ES is set to 1.0 atomic %. It is more preferable that the concentration of the substitutional element _ES be set to 0.1 atomic % or more and 0.5 atomic % or less.

As described above, the substitutional element E _S may contain at least one of Ag, Au, and Al, and only one element may be added, or two or more elements may be added.

Furthermore, the substitutional element _ES is a metal element containing at least one of Ag, Au, and Al. In other words, the substitutional element _ES may contain metal elements other than Ag, Au, and Al. Other metal elements that can be substitutional elements are elements other than nonmetallic elements (hydrogen, halogen elements, and Group 18 elements), semimetallic elements (Si, Ge, As, Sb, Te, Se, Po, and At), and essential metal elements (Ag, Au, and Al). Specific examples of other metal elements include Mn, Cr, Fe, Ti, V, Co, Ni, Pt, Rh, Ru, Ir, Pt, Nb, Ta, Y, Ho, Hf, and Gd. While these other metal elements do not have the effects of Ag, Au, and Al, they have little effect on the hydrogen properties of the hydrogen-permeable membrane. Furthermore, Mn can be a useful additive element because it promotes β-phase formation. However, even if the other metal elements are contained as the substitutional elements _ES , the upper limit of the concentration of the substitutional elements _ES as a whole is 1.0 atomic %. As described above, if the concentration of the substitutional elements _ES becomes excessively high, the Pd concentration or Cu concentration will decrease, which may result in a decrease in hydrogen permeability. Furthermore, even when other metal elements are contained, the substitutional elements _ES of Ag, Au, and Al must be contained in an amount of 0.01 atomic % or more.

(A-1-4) Inevitable Impurities The PdCu-based alloy film of the present invention is composed of Pd, Cu, interstitial elements ( _EI ), and substitutional elements ( _ES ). However, the inclusion of unavoidable impurities is permitted. Examples of the unavoidable impurities include the above-mentioned metalloid elements. It is preferable that the total amount of these unavoidable impurities is 500 ppm or less.

(A-2) Crystal structure of PdCu-based alloy membrane Considering that hydrogen permeability in PdCu-based alloys is exhibited in the β-phase state, which is the B2 structure, and that the expected function of a hydrogen-permeable membrane is to allow hydrogen to permeate its cross section, it is preferable that the hydrogen-permeable membrane of the present invention has a high proportion of β-phase in its cross section. Specifically, it is preferable that the hydrogen-permeable membrane of the present invention has an area ratio of β-phase of 95% or more in any cross section.

"Arbitrary cross section" means that the above conditions are met in any arbitrarily selected cross section of the PdCu-based alloy film, regardless of direction. The area ratio should be calculated by observing the cross section in an area where both sides of the PdCu-based alloy film (both front and back ends) are visible, and calculating the area of the β phase relative to the total area of the observation area. The observation area should preferably be set to include both front and back ends of the PdCu-based alloy film, and include a width that is at least 10 times the length of the film thickness. It is more preferable that the area ratio of the β phase in this arbitrary cross section be 98% or more, and the upper limit of the area ratio of the β phase is preferably 100%.

An effective method for detecting the beta phase in any cross-section of a PdCu-based alloy film is analysis using electron backscattered diffraction (EBSD). EBSD makes it possible to obtain information on each crystal grain in the cross-section of the alloy film, allowing the distribution and area ratio of the beta phase in the cross-section of the alloy film to be measured and calculated.

Furthermore, the thickness of the PdCu-based alloy membrane that constitutes the hydrogen-permeable membrane of the present invention is preferably 1 μm or more and 250 μm or less. If it is less than 1 μm, the mechanical strength is insufficient and handling is difficult. Furthermore, if the membrane thickness exceeds 250 μm, the amount of hydrogen permeated will be reduced, resulting in a decrease in purification efficiency. Furthermore, there are no particular restrictions on the shape of the PdCu-based membrane of the present invention.

(A-3) Hydrogen Permeability of PdCu-Based Alloy Membrane According to the Present Invention The hydrogen-permeable membrane made of the PdCu-based alloy according to the present invention has excellent hydrogen permeability, particularly in the low-temperature range of 200°C or less. As described above, the hydrogen permeability coefficient of the PdCu-based alloy membrane deviates from the Arrhenius plot based on the high-temperature range in the low-temperature range, and the measured value is lower than the predicted value. In the present invention, such deviation of the hydrogen permeability coefficient in the low-temperature range is reduced.

A preferred specific embodiment of the hydrogen-permeable membrane according to the present invention is one in which the ratio of the hydrogen permeability coefficient ^φ100 at 100°C to the hydrogen permeability coefficient ^φ300 at 300°C is 0.4 or more. Since a decrease in the hydrogen permeability coefficient due to a decrease in temperature is inevitable, the ratio ^φ100 / ^φ300 is less than 1. The hydrogen-permeable membrane according to the present invention can achieve a ^φ100 / ^φ300 ratio of 0.4 or more by suppressing the decrease in the hydrogen permeability coefficient in the low temperature range. The hydrogen permeability coefficients at 100°C and 300°C are used as the performance evaluation criteria for hydrogen-permeable membranes because the decrease in hydrogen permeability coefficient tends to be significant around 100°C. The hydrogen permeability coefficient of a PdCu-based alloy membrane is greatest in the range of 300°C to 400°C, and it is convenient to apply the measured value at 300°C. The hydrogen permeability coefficient φ (mol/m s Pa ^1/2
) is calculated using the following formula:

The measurement range for measuring the hydrogen permeability ratio ^φ100 / ^φ300 is not particularly limited as long as it is a range that includes 100°C and 300°C. The measurement range is preferably 25°C or higher and 400°C or lower. This is because the hydrogen permeability coefficient rarely reaches its maximum at temperatures above 400°C, and a decrease in the hydrogen permeability coefficient due to decomposition of the β phase is observed at temperatures higher than this. Furthermore, measuring the hydrogen permeability coefficient at 25°C or lower (room temperature or lower) requires a measuring device equipped with a cooling means, making the measurement large-scale. However, there is no prohibition on measuring the hydrogen permeability coefficient over a wider temperature range than 25°C or higher and 400°C or lower.

The hydrogen permeability of a PdCu-based alloy membrane is a function of the combined effects of the Pd concentration, Cu concentration, and the area fraction of the β phase in the cross section, as described above. Even if a hydrogen-permeable membrane has a high hydrogen permeability coefficient at high temperatures due to an optimized alloy composition, it may be impossible to avoid a decrease in hydrogen permeability at low temperatures. The addition of the interstitial element _EI and the substitutional element _ES to the PdCu alloy of the present invention, in cooperation with the optimization of the alloy composition and the area fraction of the β phase, optimizes hydrogen permeability over a temperature range, including low temperatures.

(B) Method for manufacturing hydrogen-permeable membrane according to the present invention Next, a preferred method for manufacturing a hydrogen-permeable membrane according to the present invention will be described. The hydrogen-permeable membrane according to the present invention can be manufactured by preparing a PdCu-based alloy of the above-mentioned composition and thinning it through plastic working such as rolling. A preferred embodiment includes heat treatment to optimize the area ratio of the β phase in the cross section of the thin film. A preferred method for manufacturing a hydrogen-permeable membrane according to the present invention will be described below.

(B-1) Manufacturing Process of PdCu-Based Alloy Film The manufacturing method of the PdCu-based alloy film is not particularly limited and can be appropriately selected depending on the film thickness, dimensions, etc. The PdCu-based alloy film can be formed by various thin film formation processes such as sputtering, vacuum deposition, chemical vapor deposition, plating, etc. Furthermore, a plate-shaped or foil-shaped PdCu-based alloy film can be manufactured by rolling an alloy ingot, etc.

PdCu-based alloy films can be manufactured by the rolling method by producing a PdCu-based alloy ingot of the above composition using a melt casting method, and then processing it into an alloy film of the desired thickness using an appropriate combination of hot forging, hot rolling, cold rolling, etc. There are no particular restrictions on the processing steps from ingot to alloy film. However, since the introduction of processing strain in PdCu-based alloys can promote the phase transformation to the β phase, it is preferable to perform cold processing at a processing rate of 65% to 85% as the final processing step.

(B-2) Heat treatment process of PdCu-based alloy film (promoting phase transformation to β phase)
Furthermore, PdCu-based alloy films having the above composition manufactured by various manufacturing methods undergo a phase transformation to the β phase when heat-treated within a predetermined temperature range. The heat treatment temperature is 275°C or higher and 400°C or lower. The phase transformation temperature (α phase → β phase) of the PdCu-based alloy film of the present invention varies depending on the composition even within the above composition range, but is estimated to be within the range of approximately 300°C to 400°C. Heat treatment at temperatures below 275°C either does not cause a phase transformation to the β phase or makes it difficult to achieve a β phase area ratio of 95% or higher in the film cross section. On the other hand, the β phase of PdCu-based alloys is known to decompose to the α phase at high temperatures, and β phase decomposition tends to occur at temperatures above 400°C. Therefore, the heat treatment temperature range is preferably 275°C or higher and 400°C or lower.

The heat treatment atmosphere for phase transformation to the β phase is preferably a pressurized hydrogen-containing atmosphere. More preferably, the atmosphere should have a hydrogen partial pressure of 0.05 MPaG or more and 1.0 MPaG or less.

The heat treatment time is adjusted according to the film thickness of the PdCu-based alloy film. The generation of β phase by heat treatment progresses from both surfaces of the PdCu-based alloy film, and the phase transformation inside the film progresses as the treatment time increases. In the present invention, it is necessary to increase the area ratio of β phase in the cross section of the PdCu-based alloy film, so a sufficient heat treatment time is ensured to allow phase transformation to occur all the way to the inside, taking the film thickness into consideration. For PdCu-based alloy films with a film thickness within the preferred range described above, a treatment time of 5 hours or more is preferable. Furthermore, since decomposition of the β phase is unlikely to occur if the heat treatment is performed within the above temperature range, there is no problem with extending the treatment time.

(C) Uses of the Hydrogen-Permeable Membrane of the Present Invention The hydrogen-permeable membrane of the present invention has suitable hydrogen permeability over a wide range of temperatures, from high to low. This allows the present invention to be used in a variety of applications, such as hydrogen sensors, in addition to hydrogen purification devices (hydrogen purification processes).

(C-1) Hydrogen Purification Process and Hydrogen Purification Apparatus The hydrogen-permeable membrane according to the present invention is capable of purifying hydrogen by selectively allowing hydrogen to permeate from a hydrogen-containing gas (feed).

In this hydrogen purification process, it is preferable to set the treatment temperature (operating temperature) using the hydrogen-permeable membrane appropriately. The hydrogen purification method using a PdCu-based alloy membrane in this invention has a suitable treatment temperature of 25°C or higher and 400°C or lower. With regard to this treatment temperature range, the PdCu-based alloy membrane of this invention can exhibit suitable hydrogen permeability that is distinct from conventional technologies, particularly in the low temperature range. However, at temperatures above 400°C, even with the PdCu-based alloy membrane of this invention, decomposition of the β phase may occur, resulting in a decrease in the hydrogen permeability coefficient. Therefore, 400°C is set as the upper limit of the suitable treatment temperature.

The processing temperature here refers to the temperature in the region where the hydrogen-containing gas to be purified comes into contact with and permeates the hydrogen-permeable membrane. The processing temperature is adjusted by keeping at least one of the following temperatures within the above temperature range: the temperature of the hydrogen-containing gas, the temperature of the hydrogen-permeable membrane, and the ambient temperature inside the hydrogen production (purification) device.

When purifying gases containing hydrogen, the gas to be treated is supplied to one side (primary side) of a hydrogen-permeable membrane. At this time, the pressure on the primary side is increased relative to the other side (secondary side) of the hydrogen-permeable membrane, allowing the purified hydrogen that has permeated the hydrogen-permeable membrane to be extracted. There are no particular restrictions on the pressure difference at this time.

In addition, in the hydrogen purification process using a PdCu-based alloy membrane according to the present invention, a PdCu-based alloy membrane that has been heat-treated to form the β phase as described above may be used, but heat treatment may also be performed immediately before the hydrogen purification process. That is, an untreated PdCu-based alloy membrane may be prepared and heat-treated in a hydrogen atmosphere at a temperature of 275°C to 400°C to form a hydrogen-permeable membrane, and the treatment temperature may then be increased to 25°C to 400°C to allow the gas to be treated to permeate through the hydrogen-permeable membrane.

The above hydrogen purification method is carried out using a hydrogen purification device that uses the hydrogen-permeable membrane of the present invention. The main components of this hydrogen purification device, other than the hydrogen-permeable membrane, can be the same as known hydrogen purification devices. When installing the hydrogen-permeable membrane in the hydrogen-permeable device, a gas-permeable support may be combined with the hydrogen-permeable membrane to supplement its mechanical strength. Supports that can be used include metal mesh and porous sintered materials. However, a support is not essential, as mechanical strength may be ensured by the thickness of the hydrogen-permeable membrane.

(C-2) Hydrogen Sensor As mentioned at the beginning, highly sensitive hydrogen sensors are required to accommodate new applications of hydrogen, such as fuel cells and medical technology. The hydrogen-permeable membrane according to the present invention can also be suitably used as a hydrogen sensor.

Hydrogen sensors that use hydrogen-permeable membranes include gas sensors that use rare earth metals such as Y and La or semiconductor metal oxides such as _{Ga2O3 and SrTiO3} as the hydrogen detection element. In hydrogen sensors, hydrogen-permeable membranes are used as protective membranes that selectively allow hydrogen to permeate and supply hydrogen to the hydrogen detection element. In recent years, the development of concentration-cell-type hydrogen sensors has been reported. In concentration-cell-type hydrogen sensors, hydrogen-permeable membranes are used as the reference electrode and sample electrode. The hydrogen-permeable membrane imparts selective hydrogen permeability to both electrodes and supplies the permeated hydrogen to the electrolyte. The hydrogen-permeable membrane of the present invention exhibits excellent hydrogen selectivity over a wide temperature range and constitutes the sensitive portion of various hydrogen sensors.

As explained above, the hydrogen-permeable membrane made of the PdCu-based alloy of the present invention has excellent hydrogen permeability at low temperatures. The hydrogen permeability coefficient of a hydrogen-permeable membrane is expected to tend to have temperature dependence in line with the Arrhenius plot, and the present invention has hydrogen permeability that follows this tendency even at low temperatures.

FIG. 2 is a diagram showing the configuration of a hydrogen permeability coefficient measuring device used in the first and second embodiments. 1 is an Arrhenius plot of the hydrogen permeability coefficients of the PdCu-based alloy membranes of Example 1, Reference Examples 1 and 2, and Conventional Example 1, which are manufactured in the first embodiment. 10 is a diagram showing the results of EBSD analysis of the cross sections of PdCu-based alloy films of Example 2 (B concentration: 0.05 atomic %) and Comparative Example 1 (B concentration: 0.2 atomic %) produced in the second embodiment. FIG.

First Embodiment : Hereinafter, an embodiment of the present invention will be described. In this embodiment, a PdCu-based alloy membrane was manufactured to _which B was added as an interstitial element EI and Ag was added as a substitutional element _ES , and the hydrogen permeability coefficient was measured from a high temperature range to a low temperature range.

In this embodiment, the PdCu alloy to which each additive element is added has a basic composition of 47.25 atomic % Pd - 52.75 atomic % Cu (referred to as Conventional Example 1). To this, a PdCu-based alloy film (Example 1) was manufactured by adding both Ag and B, and a PdCu-based alloy film (Reference Examples 1 and 2) was manufactured by adding either Ag or B. In other words, the Pd concentration of the basic composition (Conventional Example 1) was left unchanged, but the Cu concentration was adjusted to add Ag and B. The PdCu-based alloy composition was set in this way because, since the hydrogen permeability of PdCu-based alloy membranes is largely dependent on Pd, it was considered preferable to make the Pd concentration uniform when comparing each sample. The PdCu-based alloy membrane was manufactured as follows.

[Production of PdCu-based alloy film]
A PdCu-based alloy ingot having a target composition was produced by melt casting, and the ingot surface was chamfered and cleaned. The PdCu-based alloy ingot was then subjected to repeated cold rolling processes to produce a thin film. The rolling process was repeated multiple times with intermediate annealing at 600 to 900°C, and the final rolling was performed at a processing rate of 70%. In this embodiment, a PdCu-based alloy film having a thickness of 30 μm was produced.

Next, the PdCu-based alloy film was heat-treated to promote the β-phase transformation. This heat treatment was carried out in hydrogen at 0.3 MPaG at a temperature of 400°C for a treatment time of 24 to 100 hours. The heat treatment time was adjusted by adding or not adding B. The composition of the PdCu-based alloy film produced in this embodiment is summarized in Table 1.

[Cross-sectional analysis of PdCu-based alloy film]
For various PdCu-based alloy films manufactured in this embodiment, cross sections were subjected to EBSD analysis, and the area ratio of the β phase in the cross section of the observation area was measured. As a pretreatment for EBSD analysis, the sample cross section was polished to a finish using 0.25 μm diamond paste, and then the surface was milled using an ion milling device (IM4000 manufactured by Hitachi High-Tech Corporation). The ion milling conditions were: stage control F2, acceleration 0.1 kV, discharge 1.5 kV, ion beam irradiation angle 70 degrees, eccentricity 4 mm, and argon gas flow rate 0.07 cm ³ /min, and the surface was milled for 20 minutes.

The EBSD analysis was performed using an ultra-high-resolution analytical scanning electron microscope (SU-70 manufactured by Hitachi High-Tech Corporation, NORDLYS-MAX3 manufactured by Oxford Instruments Ltd.). The analysis conditions were: pitch 0.2 μm, pinning mode 4x4, gain 0, exposure time auto, EBSD solver setting, number of bands 12, and Hough resolution 60. The analysis was performed using reflector 44 for the fcc phase (lattice constant 3.7653 Å) and reflector 43 for the B2 phase (lattice constant 2.9662 Å). The area fraction of the β phase (B2 phase) was measured using image analysis software installed in the analysis device. The EBSD analysis results showed that the area fraction of the β phase in the cross section was 100% for all of the PdCu-based alloy films manufactured in this embodiment, Example 1, Reference Examples 1 and 2, and Conventional Example 1.

[Measurement of water permeability coefficient of PdCu-based alloy membrane]
Next, the hydrogen permeability coefficients of the PdCu-based alloy membranes of Example 1, Reference Examples 1 and 2, and Conventional Example 1 were measured. The manufactured hydrogen-permeable membranes were cut into circles with a diameter of 21.3 mm. A sample (effective area: 2.08 ^cm2 ) was prepared by sandwiching this hydrogen-permeable membrane together with a stainless steel wire mesh (diameter: 18.4 mm) between an ICF34 flange gasket. This sample was set in a sample holder. The sample holder is a vacuum chamber that has a space on the primary side (gas supply side) and a space on the secondary side (permeation gas side) relative to the sample (hydrogen-permeable membrane), and is equipped with nozzles for gas supply and gas discharge.

Figure 1 shows an outline of the hydrogen permeability coefficient measurement device. The sample holder constructed as described above was set in an electric furnace and connected to the piping of a vacuum pump and various gas flow meters. Before measurement, the primary and secondary sides of the sample holder were evacuated and then replaced with hydrogen. Next, the furnace was heated to a predetermined measurement temperature, and hydrogen at a predetermined pressure was introduced into the primary side of the hydrogen-permeable membrane. The flow rate of hydrogen that had permeated to the secondary side was then measured. The permeability coefficient was calculated from the measured flow rate of the permeated gas (hydrogen), the supply-side pressure, the permeation-side pressure, and the thickness of the hydrogen-permeable membrane. The measurement conditions in this embodiment were as follows:
・Measurement temperature: 20℃ (293K) to 400℃ (673K)
Supply gas: hydrogen (hydrogen concentration 99.99%)
・Primary pressure: 0.3 MPa・G
Secondary pressure: 0 MPa G
・Exam time: 2.5 hours

[Evaluation results]
An Arrhenius plot showing the temperature dependence of the hydrogen permeability coefficient of the PdCu-based alloy membrane produced in this embodiment is shown in Figure 2. Furthermore, Table 2 summarizes the hydrogen permeability coefficient ^φ100 at 100°C, the hydrogen permeability coefficient ^φ300 at 300°C, and the ratio ( ^φ100 / ^φ300 ) of these for the PdCu-based alloy membrane in this embodiment.

2 and Table 2, the hydrogen permeability coefficients of the PdCu alloy membranes of Example 1, Reference Examples 1 and 2, and Conventional Example 1 show almost no difference in the hydrogen permeability coefficient at high temperatures (300°C). However, the PdCu alloy membrane of Conventional Example 1 (47.25 atomic % Pd-52.75 atomic % Cu) shows a drop in the hydrogen permeability coefficient from around 200°C (1/T = 0.0021), and shows the lowest values at 100°C and 25°C compared to the other PdCu alloy membranes. In contrast, the hydrogen permeability coefficient of the PdCu alloy membrane of Example 1 (47.25 atomic % Pd-52.1 atomic % Cu-0.5 atomic % Ag-0.15 atomic % B) to which B (interstitial element E _I ) and Ag (substitutional element E _S ) are added shows little drop in the low temperature range as in Conventional Example 1, and shows a change that is close to a linear decreasing trend at high temperatures. The improvement in the hydrogen permeability coefficient at low temperatures in Example 1 becomes clear when considering the ratio of hydrogen permeability coefficients ^φ100 / ^φ300 . That is, while ^φ100 / ^φ300 in Conventional Example 1 is 0.28, ^φ100 / ^φ300 in Example 1 is 0.5, and it can be said that the PdCu-based alloy membrane of Example 1 effectively maintains a hydrogen permeability coefficient in the low temperature range.

Furthermore, in Reference Example 1 (47.25 atomic % Pd-52.6 atomic % Cu-0.15 atomic % _B ) and Reference Example 2 (47.25 atomic % Pd-52.25 atomic % Cu-0.5 atomic % _Ag ), which are PdCu-based alloy films to which only either B (interstitial element E I ) or Ag (substitutional element E ^S ) is added, the ratio of hydrogen permeability coefficients φ ¹⁰⁰ /φ ³⁰⁰ is larger than that of Conventional Example 1. However, these ^ratios are also below 0.4. In other words, it is confirmed that the addition of both B (interstitial element E _I ) and Ag (substitutional element E _S ) is necessary to improve the hydrogen permeability coefficient in the low temperature range.

Second embodiment : In this embodiment, similar to the first embodiment, B was used as the interstitial element E _I and Ag was used as the substitutional element E _S , and the concentration of Ag added as the substitutional element E _S to the PdCu-based alloy of the basic composition (47.25 atomic % Pd-52.75 atomic % Cu) was changed to produce PdCu-based alloy membranes, and the hydrogen permeability was evaluated.

The manufacturing process for the PdCu-based alloy film was the same as in the first embodiment, with various PdCu-based alloy ingots produced by melt casting, and then subjected to intermediate annealing and repeated cold rolling processes to form thin films. Heat treatment (400°C, 24 to 100 hours) was then performed to transform the film into the β phase. The composition of the PdCu-based alloy film produced in this embodiment is summarized in Table 3.

The cross sections of various PdCu-based alloy films produced in this embodiment were subjected to EBSD analysis to measure the area ratio of the β phase in the cross section. As a result, it was confirmed that the PdCu-based alloy films of Examples 2 to 6 and Comparative Example 2 (with a B content of 0.15 atomic % or less) all had a β phase area ratio of 95% or more (100%) in the cross section. However, the β phase area ratio in the cross section of the PdCu-based alloy film of Comparative Example 1, in which the B content exceeded 0.15 atomic %, was 86%, less than 95%. The results of the EBSD analysis of the cross sections of the PdCu-based alloy films of Example 2 and Comparative Example 1 are shown in Figure 3. As shown in Figure 3, the PdCu-based alloy film of Example 2 was converted to β phase throughout the entire cross section, but the PdCu-based alloy film of Comparative Example 1 had a region (α phase) that was not converted to β phase in the center of the cross section. This is presumably because B, added as an interstitial element, has the effect of inhibiting the formation of the β phase, and the formation of the β phase was incomplete in Comparative Example 1, in which B was added at a concentration exceeding 0.15 atomic percent.

Next, the hydrogen permeability coefficients of the PdCu-based alloy membranes of Examples 2 to 6 and Comparative Example 2 were measured in the same manner as in the first embodiment. The measurements in this embodiment were performed using the same equipment and conditions as in the first embodiment. As for the PdCu-based alloy membrane of Comparative Example 1, as mentioned above, the central portion of the membrane was not converted to the β phase, and it was therefore determined that a sufficient hydrogen permeability coefficient could not be obtained even in the high temperature range, and so it was excluded from measurement.

Table 4 shows the hydrogen permeability coefficient φ ¹⁰⁰ at 100° C., the hydrogen permeability coefficient φ ³⁰⁰ at 300° C., and the ratio (φ ¹⁰⁰ /φ ³⁰⁰ ) of these for the PdCu-based alloy membrane produced in this embodiment.

From Table 4, it can be seen that the PdCu-based alloy membranes (containing 1.0 atomic % or less Ag) of Examples 2 to 6 all had a ratio of ^φ100 to ^φ300 , ^φ100 / ^φ300 , of 0.4 or more. Compared with the value of ^φ100 / ^φ300 (0.20) of the PdCu alloy membrane of Conventional Example 1 of the first embodiment, it was confirmed that the PdCu-based alloy membranes of Examples 2 to 6 also had significantly improved hydrogen permeability in the low temperature range.

On the other hand, in the PdCu-based alloy membrane of Comparative Example 2, in which the amount of Ag added exceeded 1.0 atomic %, the value of ^φ100 / ^φ300 significantly decreased to less than 0.4. This is presumably due to the excessive addition of the substitutional element E _s . Therefore, from the results of the study in this embodiment, it was confirmed that in order to function as a hydrogen-permeable membrane while ensuring hydrogen permeability in the low temperature range, it is necessary to set appropriate concentrations of the substitutional element E _s (Ag) and the interstitial element E _I (B).

Third Embodiment : In this embodiment, a PdCu-based alloy membrane using C as the interstitial element E _I and Ag as the substitutional element E _S , and a PdCu-based alloy membrane using B as the interstitial element E _I and Au and Al as the substitutional elements E _S were manufactured, and their hydrogen permeability was evaluated. In this embodiment, as a comparative conventional PdCu alloy, PdCu-based alloy membranes using the above-mentioned basic composition (47.25 atomic % Pd-52.75 atomic % Cu) and a basic composition with a higher Pd concentration (48.3 atomic % Pd-51.7 atomic % Cu: Conventional Example 2) were also manufactured. Furthermore, one PdCu-based alloy membrane with a composition (Pd concentration 47.8 atomic %) with an intermediate Pd concentration was manufactured.

The manufacturing process for the PdCu-based alloy film was the same as in the first and second embodiments. The composition of the PdCu-based alloy film manufactured in this embodiment is summarized in Table 5.

The cross sections of the various PdCu-based alloy films produced were subjected to EBSD analysis to measure the area ratio of the β phase in the cross section. As a result, it was confirmed that the area ratio of the β phase in the cross section of all PdCu-based alloy films produced in this embodiment was 95% or more.

The hydrogen permeability coefficients of the PdCu-based alloy membranes of Examples 7 to 13, Reference Examples 3 to 7, and Conventional Example 2 were measured in the same manner as in the first and second embodiments. Table 6 shows the hydrogen permeability coefficients ^φ100 at 100°C, ^φ300 at 300°C, and their ratios ( ^φ100 / ^φ300 ) for the PdCu-based alloy membranes produced in this embodiment.

First, for the PdCu-based alloy membranes (Examples 7 and 8) in which Al or Au was used as the substitutional element _ES , the ratio ^φ100 / ^φ300 ( ^φ100 / ^φ300) was 0.4 or greater. Therefore, it can be said that these metal elements are also effective in suppressing the decrease in hydrogen permeability coefficient at low temperatures. Meanwhile, in this embodiment, the effects of metal elements other than Ag, Au, and Al, such as Cr, Fe, and Mn, as substitutional elements _ES, were also investigated (Reference Examples 4 to 6). The PdCu-based alloy membranes in these Reference Examples had a ^φ100 / ^φ300 value of less than 0.4, indicating a decrease in hydrogen permeability coefficient at low temperatures. However, even for PdCu-based alloy membranes to which Cr, Fe, or Mn was added, the ^φ100 / ^φ300 value was 0.4 or greater if the alloy membrane was simultaneously added with the essential metal element Ag (Examples 11 to 13). These results confirmed that the effect of adding substitutional elements _ES on suppressing the decrease in hydrogen permeability coefficient is limited to Ag, Au, and Al. However, it was also confirmed that the effect is maintained even if other metal elements are added, as long as the above-mentioned essential metal elements are included.

In this embodiment, PdCu-based alloy films using C as the interstitial element _E1 were also investigated (Examples 9 and 10). These PdCu-based alloy films also had a ^φ100 / ^φ300 ratio of 0.4 or more. This result confirmed the effectiveness of C as the interstitial element _E1 .

In addition, Reference Examples 3 and 7 are PdCu-based alloy membranes to which only one of the interstitial element E _I or the substitutional element E _S was added. As in the evaluation results of the first embodiment, the PdCu-based alloy membranes of Reference Examples 3 and 7 did not show any improvement in hydrogen permeability in the low temperature range. It was also confirmed here that both the interstitial element E _I and the substitutional element E _S are necessary to improve hydrogen permeability in the low temperature range.

The hydrogen-permeable membrane of the present invention is made of a PdCu-based alloy thin film in which an interstitial element _EI and a substitutional element _ES are added to a PdCu alloy. Due to the action of these added elements, the present invention suppresses the decrease in hydrogen permeability coefficient at low temperatures observed in conventional hydrogen-permeable membranes made of PdCu alloy membranes. The present invention is useful for the operation at low temperatures of various devices and equipment to which hydrogen-permeable membranes are applied. The hydrogen-permeable membrane of the present invention is expected to be applicable not only to hydrogen purification devices but also to hydrogen sensors where operation at low temperatures is desirable.

Claims (4)

In a hydrogen-permeable membrane made of a PdCu-based alloy,
The PdCu-based alloy is
47.0 atomic % or more and 49.0 atomic % or less of Pd;
an interstitial element _EI of 0.001 atomic % or more and 0.15 atomic % or less;
A substitutional element E _S of 0.01 atomic % or more and 1.0 atomic % or less;
The balance is Cu and unavoidable impurities,
The interstitial element _EI is at least one element selected from the group consisting of B, C, and N,
A hydrogen-permeable film characterized in that the substitutional element _ES is a metal element that essentially contains at least one of Ag, Au, and Al. The hydrogen-permeable membrane of claim 1, wherein the area ratio of the β phase in any cross section is 95% or more. 3. The hydrogen-permeable film according to claim 1, wherein the ratio (φ ¹⁰⁰ /φ ³⁰⁰ ) of the hydrogen permeability coefficient φ ¹⁰⁰ at 100° C. to the hydrogen permeability coefficient φ ³⁰⁰ at 300° C. is 0.4 or more. 3. The hydrogen-permeable membrane according to claim 1, having a thickness of 1 μm or more and 250 μm or less.