Hydrogen-permeable membrane composed of pdcu-based alloy

Abstract

The present invention relates to a hydrogen-permeable membrane composed of a PdCu-based alloy. A PdCu-based alloy according to the present invention is characterized by comprising 47.0-49.0 atom% of Pd and 0.01-0.75 atom% of Ag, the balance being Cu and inevitable impurities. In a hydrogen-permeable membrane according to the present invention, 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, and any decrease in the hydrogen permeation coefficient in a low-temperature region is suppressed. The hydrogen-permeable membrane according to the present invention has improved hydrogen permeability in a low-temperature region of 150°C or lower.

Description

Hydrogen-permeable membrane made of PdCu alloy

[0001] The present invention relates to a hydrogen-permeable membrane that selectively allows hydrogen to permeate from a hydrogen-containing gas, and in particular to a hydrogen-permeable membrane that has improved hydrogen permeability at low temperatures compared to conventional techniques.

[0002] Hydrogen is widely used in various fields, such as as a hydrogen source or 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 for fuel cells and hydrogen engines that power automobiles and heavy machinery. Furthermore, hydrogen is also attracting attention in the field of advanced medical care, and the effectiveness of hydrogen inhalation therapy for post-cardiac arrest syndrome has been reported, for example.

[0003] Hydrogen-permeable membranes are materials used in devices for handling hydrogen, which are 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 and the like. 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 permeate from the gas to be measured, can be suitably used as hydrogen sensors in these applications.

[0004] The hydrogen-permeable membrane is made of a metal alloy that can selectively absorb and release hydrogen while diffusing it within. Among such metal alloy membranes, Pd alloy membranes (e.g., PdAg-based alloys and PdCu-based alloys) are well known, taking advantage of the selective hydrogen permeability of palladium (Pd). Hydrogen-permeable membranes made of PdCu-based alloys are particularly well-known for their reduced hydrogen embrittlement and corrosion resistance, and are therefore increasingly being put to practical use and mass-produced (Patent Documents 1 and 2, Non-Patent Document 1).

[0005] JP 2001-262252 A JP 2008-12495 A

[0006] JamesRaphael Warren, “The Effect of Hydrogen on Palladium-Copper Based Membranes for Hydrogen Purification”, THE UNIVERSITY OF BIRMINGHAM, P22-29, 37-80.

[0007] The hydrogen permeation phenomenon in a hydrogen-permeable membrane made of a PdCu-based alloy membrane 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 the Arrhenius plot, the logarithm of the hydrogen permeability coefficient is negatively proportional to the reciprocal of temperature (1 / T). Therefore, by measuring the hydrogen permeability coefficient in a suitable temperature range, it is possible to predict the hydrogen permeability coefficient in other temperature ranges.

[0008] However, according to the studies of the present inventors, the predictability of the hydrogen permeability coefficient based on the Arrhenius plot described above is not sufficient for hydrogen-permeable membranes made of PdCu-based alloys. Specifically, even if an Arrhenius plot is created based on hydrogen permeability coefficients measured at high temperatures, the actual measured hydrogen permeability coefficients at low temperatures deviate from the Arrhenius plot and are lower than the predicted values. In other words, hydrogen-permeable membranes made of PdCu-based alloys exhibit a greater-than-expected decrease in hydrogen permeability at low temperatures. While the temperature dependency of the hydrogen permeability coefficient itself cannot be avoided, such a greater-than-expected decrease in the hydrogen permeability coefficient is undesirable.

[0009] Previous studies on hydrogen-permeable membranes have focused mainly on increasing the hydrogen permeability coefficient, with not many studies on improving the temperature dependency of hydrogen-permeable membranes. This is because the hydrogen permeability coefficient is the clearest indicator of a hydrogen-permeable membrane's function. 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.

[0010] However, some devices that use hydrogen-permeable membranes must operate at low temperatures. For example, hydrogen sensors for fuel cell vehicles and medical applications are intended for use at room temperature. These applications of hydrogen-permeable membranes have only recently attracted attention, but the hydrogen-permeable membranes used in these applications require higher hydrogen permeability at low temperatures.

[0011] The present invention has been made in light of the above background, and provides a hydrogen-permeable membrane made of a PdCu-based alloy, which has improved hydrogen permeability in the low-temperature range, which in the present invention is defined as the temperature range from room temperature (25°C) to 150°C.

[0012] To solve the above problems, the inventors investigated the causes of the decrease in hydrogen permeability of PdCu-based alloy membranes at low temperatures and countermeasures for this. The hydrogen permeability of PdCu-based alloys is exhibited in the β-phase state of the B2 structure based on a bcc (body-centered cubic lattice). Hydrogen absorbed in a PdCu alloy membrane in this state exhibits hydrogen permeability by diffusing through the gaps in the PdCu body-centered cubic lattice.

[0013] According to the inventors' estimation, the cause of the decrease in hydrogen permeability of PdCu-based alloy membranes is thought to be the generation of vacancies due to lattice defects in the PdCu alloy crystal. Vacancies in PdCu alloys (body-centered cubic lattice) can occur at both Pd and Cu sites. When vacancies occur, hydrogen is trapped there and cannot be desorbed unless a certain amount of energy is applied. If the hydrogen-permeable membrane is at a high temperature, hydrogen can be desorbed by applying thermal energy. However, hydrogen desorption is difficult in low temperature regions where thermal energy is scarce, and this is thought to cause a greater-than-expected decrease in hydrogen permeability.

[0014] If the decrease in hydrogen permeability at low temperatures is due to hydrogen trapping in vacancies, one possible solution is to add another element to the alloy system. When vacancies occur due to lattice defects, the added element replaces the vacancies, thereby continuing hydrogen diffusion and maintaining hydrogen permeability. The inventors investigated additional elements that have such an effect and found that Ag is a particularly preferable additional element, leading to the invention.

[0015] The present invention, which solves the above problems, provides a hydrogen-permeable membrane made of a PdCu-based alloy, characterized in that the PdCu alloy contains 47.0 atomic % to 49.0 atomic % of Pd, 0.01 atomic % to 0.75 atomic % of Ag, and the balance being Cu and unavoidable impurities. The structure and hydrogen permeability of the hydrogen-permeable membrane according to the present invention are described below, along with its manufacturing method and applications.

[0016] (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 (PdCuAg alloy) whose constituent elements are Pd, Cu, and Ag, excluding unavoidable impurities as described below. The functions and composition ranges of these constituent elements are as follows:

[0017] (A-1-1) Pd and Cu Because the hydrogen-permeable membrane according to the present invention is a PdCu-based alloy containing Ag, Pd and Cu are essential and major constituent elements of the present invention. Pd is an essential metal for ensuring the hydrogen permeability of a 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 bcc structure. Cu is an essential additive metal for promoting the phase transformation from the α phase 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 decrease in strength of the PdCu-based alloy membrane due to hydrogen embrittlement.

[0018] 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 Ag concentration described below, and the concentration of inevitable impurities.

[0019] (A-1-2) Ag Ag is an additive element that preferentially substitutes for vacancies when lattice defects occur in a PdCu-based alloy, thereby suppressing hydrogen trapping and fixation and suppressing a decrease in hydrogen permeability at low temperatures. Such vacancy filling and suppression of a decrease in hydrogen permeability at low temperatures can also be achieved with metal elements other than Ag. However, according to the inventors' studies, Ag is particularly effective in improving hydrogen permeability at low temperatures. This is presumably due to Ag's high selectivity for vacancies. Therefore, in the present invention, Ag is specified as an additive element to the PdCu alloy membrane.

[0020] The Ag concentration in the PdCu-based alloy of the hydrogen-permeable film according to the present invention is set to 0.01 atomic % or more and 0.75 atomic % or less. If the Ag concentration is less than 0.01 atomic %, the addition of Ag is ineffective. On the other hand, if the Ag concentration exceeds 0.75 atomic %, the hydrogen permeability of the PdCu-based alloy film is reduced overall, resulting in deterioration of hydrogen permeability not only in the low temperature range but also in the high temperature range. The Ag concentration is preferably set to 0.1 atomic % or more and 0.5 atomic % or less, and particularly preferably to 0.15 atomic % or more and 0.38 atomic % or less.

[0021] (A-1-3) Inevitable Impurities The PdCu-based alloy film of the present invention is composed of Pd, Cu, and Ag, and does not contain any intentionally added elements other than these. However, the inclusion of unavoidable impurities is permitted. Examples of unavoidable impurities include Fe and Si. It is preferable that the total amount of these unavoidable impurities is 500 ppm or less.

[0022] (A-2) Crystal structure of PdCu-based alloy membrane Considering that hydrogen permeability in PdCu-based alloys is exhibited in the β-phase state 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.

[0023] The term "arbitrary cross section" means that the above conditions are satisfied in any cross section selected arbitrarily regardless of the direction of the PdCu alloy film. The area ratio should be calculated by observing the cross section of the PdCu alloy film in a region where both sides (both front and back ends) can be seen, and calculating the area ratio of the β phase relative to the total area of ​​the observation region. The observation region is preferably set to include both front and back ends of the PdCu alloy film and a width 10 times or more longer than the film thickness. The area ratio of the β phase in this arbitrary cross section is more preferably 98% or more, and the upper limit of the area ratio of the β phase is preferably 100%.

[0024] An effective method for detecting the β phase in an arbitrary cross section of a PdCu alloy film is analysis by electron backscattered diffraction (EBSD). EBSD can obtain information on each crystal grain in the cross section of the alloy film, and this allows the distribution and area ratio of the β phase in the cross section of the alloy film to be measured and calculated.

[0025] The thickness of the PdCu-based alloy film constituting the hydrogen-permeable film according to the present invention is preferably 1 μm or more and 250 μm or less. If the thickness is less than 1 μm, the mechanical strength is insufficient and handling is difficult. If the thickness exceeds 250 μm, the amount of hydrogen permeated is reduced, resulting in a decrease in purification efficiency. There are no particular restrictions on the shape of the PdCu-based film according to the present invention.

[0026] (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 150°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, this deviation of the hydrogen permeability coefficient in the low-temperature range is reduced.

[0027] A preferred specific embodiment of the hydrogen-permeable film according to the present invention is one having a hydrogen permeability coefficient φ at 100°C. １００ and hydrogen permeability coefficient φ at 300°C ３００ Since a decrease in the hydrogen permeability coefficient due to a decrease in temperature is inevitable, the ratio φ １００ / φ ３００ is less than 1. The hydrogen-permeable film according to the present invention suppresses the drop in hydrogen permeability coefficient in the low temperature range, thereby １００ / φ ３００ can be set to 0.4 or more. The reason why the hydrogen permeability coefficients at 100°C and 300°C are used as the characteristic evaluation standard for hydrogen-permeable films is that the hydrogen permeability coefficient tends to drop significantly around 100°C. The hydrogen permeability coefficient of PdCu-based alloy films is maximum 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:

[0028]

[0029] The ratio of the above hydrogen permeability coefficients φ １００ / φ ３００The measurement range for measuring the hydrogen permeability coefficient 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. Measurements below 25°C are not particularly meaningful. In addition, 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. However, there is nothing to prevent the measurement of the hydrogen permeability coefficient from being measured over a wider range than 100°C or higher and 400°C or lower.

[0030] 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 Ag to the PdCu alloy in 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.

[0031] (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.

[0032] (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.

[0033] In the production of a PdCu-based alloy film by the rolling method, a PdCu-based alloy ingot having the above composition is produced by a melting and casting method, and then processed by an appropriate combination of hot forging, hot rolling, cold rolling, etc. to form an alloy film of a predetermined thickness. There are no particular restrictions on the processing step from the ingot to the alloy film. However, since the introduction of processing strain in a PdCu-based alloy can promote 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.

[0034] (B-2) Heat Treatment Process of PdCu-Based Alloy Film (Promoting Phase Transformation to β Phase) The PdCu-based alloy film having the above composition, manufactured by various methods, undergoes 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 result in 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 into 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.

[0035] The atmosphere for the heat treatment for the phase transformation to the β phase is preferably a pressurized hydrogen-containing atmosphere, more preferably an atmosphere with a hydrogen partial pressure of 0.05 MPaG or more and 1.0 MPaG or less.

[0036] The heat treatment time is adjusted depending on the film thickness of the PdCu-based alloy film. The generation of β-phase by heat treatment proceeds from both surfaces of the PdCu-based alloy film, and the phase transformation inside the film progresses with the treatment time. In the present invention, since it is necessary to increase the area ratio of β-phase in the cross section of the PdCu-based alloy film, a sufficient heat treatment time is ensured so that the phase transformation occurs to the inside while taking the film thickness into consideration. For PdCu-based alloy films with a film thickness within the above-mentioned preferred range, the treatment time is preferably 5 hours or more. Note that, since decomposition of the β-phase is unlikely to occur if the heat treatment is performed within the above-mentioned temperature range, there is no problem with extending the treatment time.

[0037] (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).

[0038] (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).

[0039] In this hydrogen purification process, it is preferable to set the treatment temperature (usage temperature) using the hydrogen-permeable membrane appropriately. The hydrogen purification method using a PdCu-based alloy membrane of the present invention has a suitable treatment temperature of 25°C or higher and 400°C or lower. In this treatment temperature range, the PdCu-based alloy membrane of the present invention can exhibit suitable hydrogen permeability that is distinguishable from conventional techniques, particularly in the low-temperature range. However, at temperatures exceeding 400°C, even the PdCu-based alloy membrane of the present invention may experience decomposition of the β phase, resulting in a decrease in the hydrogen permeability coefficient. Therefore, the upper limit of the suitable treatment temperature is 400°C.

[0040] The treatment temperature here refers to the temperature at which the hydrogen-containing gas to be purified comes into contact with and permeates the hydrogen-permeable membrane. The treatment temperature is adjusted by adjusting at least one of the temperature of the hydrogen-containing gas, the temperature of the hydrogen-permeable membrane, and the ambient temperature within the hydrogen production (purification) device within the above temperature range.

[0041] In the purification of hydrogen-containing gases, 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 made higher than that on the other side (secondary side) of the hydrogen-permeable membrane, and purified hydrogen that has permeated the hydrogen-permeable membrane is extracted. There are no particular restrictions on the pressure difference at this time.

[0042] 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, or the heat treatment may 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 then the treatment temperature may be increased to 25°C to 400°C to allow the target gas to permeate through the hydrogen-permeable membrane.

[0043] The above hydrogen purification method is carried out by a hydrogen purification device that employs 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 those of 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. Examples of the support that can be used include metal mesh and porous sintered materials. However, a support is not essential, as the thickness of the hydrogen-permeable membrane may be sufficient to ensure mechanical strength.

[0044] (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.

[0045] 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.

[0046] The hydrogen permeability coefficient of a hydrogen-permeable membrane is expected to have a temperature dependency that conforms to an Arrhenius plot, and the present invention exhibits hydrogen permeability that conforms to this trend even at low temperatures. The hydrogen-permeable membrane made of a PdCu-based alloy according to the present invention exhibits hydrogen permeability that is better than that of conventional techniques, even at low temperatures of 150°C or less.

[0047] 1 is a diagram showing the configuration of a hydrogen permeability measurement device used in this embodiment. An Arrhenius plot showing the temperature dependence of the hydrogen permeability coefficient of a PdCu-based alloy membrane (basic composition: 48.3 atomic % Pd-51.7 atomic % Cu) of group A produced in this embodiment. An Arrhenius plot showing the temperature dependence of the hydrogen permeability coefficient of a PdCu-based alloy membrane (basic composition: 47.25 atomic % Pd-52.75 atomic % Cu) of group B produced in this embodiment. An Arrhenius plot showing the temperature dependence of the hydrogen permeability coefficient of a PdCu-based alloy membrane (basic composition: 48.5 atomic % Pd-51.5 atomic % Cu) of group C produced in this embodiment. An example of an Arrhenius plot showing the temperature dependence of the hydrogen permeability coefficient of a PdCu-based alloy membrane (PdCuAl alloy membrane) produced as a reference example. An example of an Arrhenius plot showing the temperature dependence of the hydrogen permeability coefficient of a PdCu-based alloy membrane (PdCuMn alloy membrane) produced as a reference example.

[0048] Hereinafter, an embodiment of the present invention will be described. In this embodiment, a hydrogen-permeable membrane made of a PdCu alloy and a hydrogen-permeable membrane made of a PdCuAg alloy in which Ag was added to a PdCu alloy were manufactured. Then, for each hydrogen-permeable membrane, the hydrogen permeability coefficient was measured over a range from high to low temperatures. In this embodiment, PdCuAg alloy membranes were manufactured by using the PdCu alloys of the following three groups A to C as the basic alloy compositions and adding 0.15 atomic % to 1.0 atomic % of Ag to these PdCu alloys. Group A: 48.3 atomic % Pd-51.7 atomic % Cu Group B: 47.25 atomic % Pd-52.75 atomic % Cu Group C: 48.5 atomic % Pd-51.5 atomic % Cu

[0049] In the composition adjustment of the PdCuAg alloys of the above three groups, the Pd concentration was not changed, but the Cu concentration was adjusted and Ag was added. This was done to replace the added Ag with Cu. The reason for setting the composition in this way was that the hydrogen permeability of the PdCu-based alloy membrane is largely dependent on Pd, and as mentioned above, Ag exerts its effect by substituting for vacancies in the PdCu-based alloy. The PdCu-based alloy membrane was manufactured as follows.

[0050] [Production of PdCu-based alloy film] A PdCu-based alloy ingot with the 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, resulting in a final rolling reduction rate of 70%. In this embodiment, a PdCu-based alloy film with a thickness of 30 μm to 100 μm was produced. Next, the PdCu-based alloy film was heat-treated to promote the β-phase transformation. The heat treatment was performed in hydrogen at 0.30 MPaG, at a heat treatment temperature of 300°C or 400°C, for 24 hours. Table 1 summarizes the composition, film thickness, and heat treatment temperature of the PdCu-based alloy film produced in this embodiment.

[0051]

[0052] [Cross-section analysis of PdCu alloy film] For various PdCu alloy films produced, the cross section was analyzed by EBSD, and the area ratio of the β phase in the cross section in the observation area was measured. As a pretreatment for EBSD analysis, the sample cross section was polished to a fineness of 0.25 μm using 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, argon gas flow rate 0.07 cm ３ The surface was milled for 20 minutes under the condition of 0.15 rpm / min.

[0053] The EBSD analysis was performed using an ultra-high resolution analytical scanning electron microscope (SU-70 manufactured by Hitachi High-tec 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. The area fraction of the β phase (lattice constant 2.9662 Å) was measured using image analysis software provided with the analysis device.

[0054] As a result of EBSD analysis, it was confirmed that the PdCu-based alloy films (A-1 to A-5, B-1 to B-3, C-1 to C-2) produced in this embodiment all had a β-phase area ratio of 95% or more in the cross section. In particular, the PdCu-based alloy films B-1 to B-3 had a β-phase area ratio of 100%. Furthermore, the β-phase area ratios of the PdCu-based alloy films A-1 to A-5 were in the range of 99.9% to 100%, and the β-phase area ratios of the PdCu-based alloy films C-1 to C-2 were 99.0% or more.

[0055] [Measurement of water permeability coefficient of PdCu-based alloy membrane] Next, the hydrogen permeability coefficient was measured for the hydrogen-permeable membrane (PdCu-based alloy membrane) produced by heat treatment at each temperature. The produced hydrogen-permeable membrane was cut into a circle with a diameter of 21.3 mm. This hydrogen-permeable membrane was sandwiched together with a stainless steel wire mesh (diameter 18.4 mm) between an ICF34 flange gasket to produce a sample (effective area 2.08 cm). ２ This sample was set in a sample holder. The sample holder is a vacuum vessel that has a primary space (gas supply side) and a secondary space (permeation gas side) for the sample (hydrogen-permeable membrane), and is equipped with nozzles for gas supply and gas discharge.

[0056] FIG. 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 vacuum pump and the piping of various gas flow meters. Prior to 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 permeable 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°C (293K) to 600°C (873K) Supply gas: hydrogen (hydrogen concentration 99.99%) Primary-side pressure: 0.3 MPa·G Secondary-side pressure: 0 MPa·G Test time: 2.5 hours

[0057] [Evaluation Results] Arrhenius plots showing the temperature dependence of the hydrogen permeability coefficient for the PdCu-based alloy membranes (PdCu alloy membrane and PdCuAg alloy membrane) of Groups A to C manufactured in this embodiment are shown in Figures 2 to 4. These figures show predicted lines of hydrogen permeability coefficients estimated by linear approximation from measured values ​​at high temperatures (250°C to 400°C) for PdCu alloy membranes without added Ag. However, for Figure 4 (Group C), the predicted line of hydrogen permeability coefficient for PdCuAg alloy membranes is also shown because there is a difference in measured values ​​between the presence and absence of added Ag. As can be seen from Figures 2 to 4, the measured hydrogen permeability coefficients of conventional PdCu alloy membranes without added Ag deviate from the predicted line near 150°C (1 / T = 0.0024), and the deviation becomes larger at lower temperatures. In contrast, for the Ag-added PdCuAg alloy membrane, although there is some deviation from the predicted line, the deviation is small, and it is clear that the decrease in hydrogen permeability coefficient at low temperatures is suppressed. Therefore, it was confirmed that the addition of Ag to the PdCu alloy membrane contributes to the improvement of the hydrogen permeability coefficient at low temperatures.

[0058] However, the benefit of adding Ag to a PdCuAg alloy film is limited to a small amount. The PdCuAg alloy film with an Ag concentration of 1.0 atomic % shown in Figures 2 and 3 does not significantly improve the decrease in hydrogen permeability coefficient at low temperatures. Furthermore, these PdCuAg alloy films have low hydrogen permeability coefficients across the entire temperature range. Adding a large amount of Ag does not contribute to improving the properties of the PdCu alloy film at all.

[0059] In this embodiment, in addition to the above sample, PdCuAg alloy membranes with Ag additions of 1.5 atomic %, 3 atomic %, and 6 atomic % were manufactured and the hydrogen permeability coefficients were measured. However, these membranes had extremely low hydrogen permeability coefficients across the entire temperature range, making them difficult to function as hydrogen-permeable membranes.

[0060] Regarding the hydrogen permeability of the PdCu alloy membrane and the PdCuAg alloy membrane in this embodiment, the hydrogen permeability coefficient φ at 100°C is １００ and hydrogen permeability coefficient φ at 300°C ３００ , and the ratio of these (φ １００ / φ ３００ ) are summarized in Table 2 below.

[0061]

[0062] From Table 2, in the PdCuAg alloy membrane with appropriate Ag addition, the ratio of hydrogen permeability coefficients φ １００ / φ ３００ It was confirmed that the hydrogen permeability coefficient was 0.4 or more, and that the hydrogen permeability coefficient was effectively maintained in the low temperature range.

[0063] Reference Example (Comparison with Other Metal Additions) Here, in order to confirm that Ag is suitable as a metal element to be added to a PdCu alloy film, the inventors produced PdCu-based alloy films to which other metals were added and evaluated in the same manner as above. The manufacturing process of the PdCu-based alloy film is the same as above. In this reference example, PdCu-based alloy films (PdCuAl alloy films, PdCuMn alloy films) were also produced and evaluated, using the PdCu alloys of Groups A to C as the basic composition and adding Al and Mn as additive metals. Note that Al and Mn were investigated as reference examples because these elements, like Ag, are presumed to have the effect of stabilizing the phase transformation from an fcc structure (α phase) to a bcc structure (β phase).

[0064] Regarding the Arrhenius plots, which are the evaluation results of the PdCuAl alloy film and the PdCuMn alloy film as reference examples, the results for the PdCu-based alloy film of the basic composition group A are shown in Figure 5 (PdCuAl alloy film) and Figure 6 (PdCuMn alloy film) as representative examples. When Al and Mn are applied as additive elements to the PdCu alloy film, the hydrogen permeability coefficient in the low temperature range is slightly improved when the additive concentration is about 0.5 atomic %. However, the improvement is extremely narrow, and the improvement effect is not as great as that of Ag. The ratio φ of the hydrogen permeability coefficient １００ / φ ３００ The hydrogen permeability coefficient of the PdCu-based alloy is also less than 0.4. This tendency was also observed for the PdCu-based alloy membranes of other basic compositions (Groups B and C). It was confirmed that Ag is the optimum metal to fill the vacancies that cause a decrease in the hydrogen permeability coefficient of the PdCu-based alloy.

[0065] The hydrogen-permeable membrane made of a PdCu-based alloy according to the present invention suppresses the decrease in hydrogen permeability coefficient at low temperatures observed in conventional hydrogen-permeable membranes made of a PdCu alloy by adding Ag. This makes the present invention useful for the operation at low temperatures of various devices and equipment to which the hydrogen-permeable membrane is applied. The hydrogen-permeable membrane according to 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

1. A hydrogen-permeable film made of a PdCu-based alloy, characterized in that the PdCu-based alloy is composed of 47.0 atomic % or more and 49.0 atomic % or less of Pd, 0.01 atomic % or more and 0.75 atomic % or less of Ag, and the remainder being Cu and unavoidable impurities.

2. The hydrogen-permeable membrane according to claim 1, wherein the area ratio of the β phase in any cross section is 95% or more.

3. Hydrogen permeability coefficient φ at 100℃ １００ and hydrogen permeability coefficient φ at 300°C ３００ Ratio to (φ １００ / φ ３００ 3. The hydrogen-permeable membrane according to claim 1, wherein the value of (a) is 0.4 or more.

4. The hydrogen-permeable membrane according to claim 1 or 2, having a thickness of 1 μm or more and 250 μm or less.

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