Patent Description:
As a hydrogen purification method, there has been known a membrane separation method using a metal film. <CIT> discloses a porous filter in which defects opened on one surface of a porous ceramic membrane is closed by a metal, and a hydrogen separation membrane in which a palladium thin membrane or a palladium alloy thin film is formed on the one surface of the porous filter.

<CIT> discloses a hydrogen separation membrane on a stainless steal support with e.g. six layers of Pd, five layers of Ag alternated Pd-Ag. -Pd, the thickness of the layers is <NUM>-<NUM> micron, or in another embodiment <NUM> and <NUM> layers. In general layer thickness of <NUM> micron for Pd is disclosed and <NUM> micron for Ag layer, with a total thickness of those layers of <NUM> micron. <CIT> discloses a hydrogen separation membrane, prepared by layer deposition of Pd-Ag-Pd on a porous support, followed by annealing to obtain an alloy. The final layer thickness can be <NUM> (i.e. <NUM> micron). <CIT> discloses a Pd-Ag alloy for hydrogen separation on a porous support. Only two layers are disclosed.

Examples of metal that allows selective permeation of hydrogen include palladium (Pd), vanadium (V), tantalum (Ta), titanium (Ti), and niobium (Nb). These metals easily embrittle in a hydrogen atmosphere. In order to reduce or prevent the embrittlement of these metals, the hydrogen separation membrane is generally used at a high temperature (for example, approximately <NUM>). However, hydrogen purification under a high temperature requires a large amount of energy.

The present disclosure provides a hydrogen separation filter that allows hydrogen purification at a lower temperature than a conventional one.

The hydrogen separation filter of the present disclosure allows hydrogen purification at a lower temperature than a conventional one.

<FIG> is a schematic cross-sectional view of a hydrogen separation filter according to an embodiment.

The following describes embodiments with reference to the drawings as appropriate. In the drawings referred in the following description, the same reference numerals may be used for the same members or the members having similar functions, and their repeated explanations may be omitted in some cases. For convenience of explanation, dimensional ratios and shapes of respective units in the drawings are exaggerated, and different from actual dimensional ratios and shapes in some cases. A numerical range expressed herein using the term "to" includes respective values described before and after the term "to" as the lower limit value and the upper limit value. The upper limit values and lower limit values of numerical ranges disclosed herein can be used alone or in any combination to specify an appropriate range.

The term "include" and the term "contain" herein mean that an additional component may be included or contained, and encompass the term "consisting essentially of" and the term "consisting of. " The term "consisting essentially of" means that an additional component having substantially no adverse effect may be included. While the term "consisting of" means including only the described materials, but does not exclude inclusion of inevitable impurities.

The term "perpendicular" herein not only means to be accurately perpendicular, but encompasses being approximately perpendicular, and the term "parallel" not only means to be accurately parallel, but encompasses being approximately parallel. The term "on" herein encompasses both of "directly on" and "indirectly on" insofar as it is not especially specified in the context.

A hydrogen separation filter <NUM> according to an embodiment illustrated in <FIG> includes a porous substrate <NUM> and a super lattice layer <NUM> formed on the porous substrate <NUM>. The super lattice layer <NUM> includes at least one lattice expansion layer <NUM> and at least two hydrogen dissociation and permeation layers <NUM>. The lattice expansion layer <NUM> and the hydrogen dissociation and permeation layers <NUM> are alternately stacked. In this embodiment, the super lattice layer <NUM> may be formed directly on the porous substrate <NUM>.

The porous substrate <NUM> may be made of, for example, a metal, a metal oxide, or a resin, and may be made of a metal oxide because of its high durability. Examples of metal oxide include aluminum oxide, zirconium oxide, and zeolite. Especially, aluminum oxide is used in some embodiments because it is inexpensive. The porous substrate <NUM> may have any shape such as a flat plate shape and a cylindrical shape.

The porous substrate <NUM> is provided with pores through which hydrogen is allowed to pass. The pores are closed by the super lattice layer <NUM>. The porous substrate <NUM> may have an average pore diameter in a range of, for example, from <NUM> to <NUM>, or <NUM> to <NUM> in some embodiments. The average pore diameter in the above-described range allows the porous substrate <NUM> to have a sufficient hydrogen permeability while allowing the pores to be easily closed by the super lattice layer <NUM> without requiring the super lattice layer <NUM> to have an excessively large thickness. The average pore diameter may be less than <NUM>/<NUM> of the thickness of the super lattice layer <NUM>. Here, the average pore diameter of the porous substrate <NUM> is determined based on a pore diameter distribution obtained by mercury porosimetry according to JIS R <NUM>:<NUM>. The mercury porosimetry is a method in which mercury is infiltrated into open pores by applying a pressure, a relation between a volume of the mercury infiltrated into the open pores and a pressure value applied at the time is obtained, and the obtained relation is used for determining the diameters of the open pores with Washburn's equation assuming that the open pores have columnar shapes.

The porous substrate <NUM> may have a porosity in a range of <NUM>% to <NUM>%. This allows the porous substrate <NUM> to have the sufficient hydrogen permeability while having a sufficient mechanical strength.

The super lattice layer <NUM> includes three or more layers of the lattice expansion layer <NUM> and the hydrogen dissociation and permeation layer <NUM> in total. From the perspective of manufacturability and the hydrogen separation performance, the total number of the lattice expansion layer <NUM> and the hydrogen dissociation and permeation layer <NUM> is <NUM> to <NUM> layers in some embodiments, <NUM> to <NUM> layers in some embodiments, and <NUM> to <NUM> layers in some embodiments. In this embodiment, the uppermost layer <NUM> (that is, the layer farthest from the porous substrate <NUM>) of the super lattice layer <NUM> is the hydrogen dissociation and permeation layer <NUM>, and the lowermost layer <NUM> (that is, the layer closest to the porous substrate <NUM>) of the super lattice layer <NUM> is also the hydrogen dissociation and permeation layer <NUM>.

The function of the super lattice layer <NUM> will be described. Hydrogen molecules dissociatively adsorb on a surface <NUM> of the uppermost layer <NUM> of the super lattice layer <NUM> and thereby hydrogen atoms are generated. The hydrogen atoms diffuse inside the super lattice layer <NUM>, recombines in an interface <NUM> between the super lattice layer <NUM> and the porous substrate <NUM> to form hydrogen molecules, and leave the super lattice layer <NUM>. Then, the hydrogen molecules pass through the porous substrate <NUM>, and leave the hydrogen separation filter <NUM>. This is how the hydrogen separation filter <NUM> selectively permeates hydrogen.

The super lattice layer <NUM> may have a thickness exceeding seven times the average pore diameter of the porous substrate <NUM>. This allows the pores to be surely closed by the super lattice layer <NUM>, which leads to a satisfactory hydrogen separation performance of the hydrogen separation filter <NUM>. From the perspective of reducing raw material cost and production time of the hydrogen separation filter <NUM>, the total thickness of the super lattice layer <NUM> may be <NUM> or less.

The lattice expansion layer <NUM> contains a first material. The hydrogen dissociation and permeation layer <NUM> contains a second material.

The second material is Pd. Especially, Pd is appropriate as the second material because Pd has a high hydrogen dissociation and permeation capacity even at a low temperature of <NUM> or less.

The first material in the lattice expansion layer <NUM> has the same crystalline structure as the second material in the hydrogen dissociation and permeation layer <NUM>. The first material in the lattice expansion layer <NUM> and the second material in the hydrogen dissociation and permeation layer <NUM> may have the same crystal orientation.

A first bulk material having the same composition and the same crystalline structure as the first material has a lattice constant a<NUM>, bulk, a second bulk material having the same composition and the same crystalline structure as the second material has a lattice constant a<NUM>, bulk, and the lattice constant a<NUM>, bulk and the lattice constant a<NUM>, bulk satisfy Formula (<NUM>): <MAT> When the first material and the second material have the crystalline structure other than cubic system, the lattice constants along the same crystallographic axes of the first bulk material and the second bulk material satisfy the formula (<NUM>). Here, the bulk material means a completely relaxed material which is free-standing (i.e., not supported by another member). The first material and the second material having the same crystalline structure and the compositions satisfying the formula (<NUM>) can result in an average lattice constant a<NUM> of the second material in the hydrogen dissociation and permeation layer <NUM> larger than the lattice constant a<NUM>, bulk of the second bulk material.

The first material is Ag having the fcc structure, because Ag is relatively inexpensive and less likely to be oxidized. Table <NUM> illustrates the lattice constants of the bulk materials of these metals.

The average lattice constant a<NUM> of the second material in the hydrogen dissociation and permeation layer <NUM> satisfies Formula (<NUM>): <MAT> In some embodiments, the average lattice constant a<NUM> of the second material may satisfy Formula (<NUM>): <MAT> In some embodiments, the average lattice constant a<NUM> of the second material may satisfy Formula (<NUM>): <MAT>.

Here, the average lattice constant a<NUM> of the second material can be determined from a plane spacing between crystal planes perpendicular to an interface <NUM> between the lattice expansion layer <NUM> and the hydrogen dissociation and permeation layer <NUM>. In detail, electron diffraction patterns of the second material are obtained in three positions, two of which are positions in or in the vicinity of both surfaces (i.e., upper surface and lower surface) of each of the hydrogen dissociation and permeation layers <NUM> and the rest of which is an intermediate position between the upper surface and the lower surface of each of the hydrogen dissociation and permeation layers <NUM> by using a transmission electron microscope (TEM). Based on each of the electron diffraction patterns, the lattice constants are determined from the plane spacing between the crystal planes perpendicular to the interface <NUM> between the lattice expansion layer <NUM> and the hydrogen dissociation and permeation layer <NUM>. The determined lattice constants are averaged to produce the average lattice constant a<NUM> of the second material. Formula (<NUM>) described above indicates that a crystal lattice of the second material in the hydrogen dissociation and permeation layer <NUM> is expanded at least in a direction parallel to the interface <NUM> compared with a fully relaxed state of the crystal lattice. The crystal lattice of the second material in the hydrogen dissociation and permeation layer <NUM> may be expanded also in a direction perpendicular to the interface <NUM>. In a conventional hydrogen separation filter such as the one described in <CIT>, diffusion of hydrogen in the hydrogen dissociation and permeation layer at a low temperature causes the crystal lattice of the hydrogen dissociation and permeation layer to be repeatedly expanded and contracted, which results in embrittlement. However, in the hydrogen separation filter <NUM> according to the embodiment, the expanded crystal lattice of the hydrogen dissociation and permeation layer <NUM> reduces the expansion and contraction of the crystal lattice due to the hydrogen diffusion, and thereby preventing or reducing the embrittlement of the hydrogen dissociation and permeation layer <NUM> at a low temperature. Thus, the hydrogen separation filter <NUM> according to the embodiment allows hydrogen purification at a lower temperature than a conventional one.

Each of the lattice expansion layers <NUM> and the hydrogen dissociation and permeation layers <NUM> have a thickness in a range of <NUM> to <NUM>, or <NUM> to <NUM> in some embodiments, or <NUM> to <NUM> in some embodiments. These thicknesses prevent or reduce alloy formation of the first material and the second material and allow the crystal lattice of the hydrogen dissociation and permeation layer <NUM> to be sufficiently expanded.

An exemplifying method for manufacturing the hydrogen separation filter <NUM> according to this embodiment will be described. The second material and the first material are alternately deposited on the porous substrate <NUM> by a sputtering method. Accordingly, the super lattice layer <NUM> constituted of the hydrogen dissociation and permeation layers <NUM> and the lattice expansion layers <NUM> alternately stacked is formed on the porous substrate <NUM>. Thus, the hydrogen separation filter <NUM> according to this embodiment is obtained.

The present disclosure is not limited to the above-described embodiment, and various kinds of changes of design are allowed within a range not departing from the spirits of the present disclosure described in the claims.

While the following specifically describes the present disclosure by examples, the present disclosure is not limited to these examples.

A ceramic membrane filter ("Cefilt" manufactured by NGK INSULATORS, LTD. , ultrafiltration membrane, cutoff molecular weight <NUM>, average pore diameter <NUM>, hereinafter, simply referred to as a "substrate") was placed in a deposition chamber of a sputtering apparatus provided with a pure Ag target and a pure Pd target. After the surface of the substrate was cleaned by Ar-ion etching, a Pd layer having a <NUM> thickness was formed on the substrate, and subsequently, an Ag layer having a <NUM> thickness was formed by the sputtering method. Similarly, the forming of the Pd layer and the forming of the Ag layer were alternately repeated, and thus a super lattice layer constituted of four Pd layers and three Ag layers was formed on the substrate. The thickness of the super lattice layer was approximately <NUM> in total. Thus, the hydrogen separation filter (hereinafter, simply referred to as the "filter") was produced.

After the surface of the substrate was cleaned by Ar-ion etching similarly to Example <NUM>, a Pd layer having a <NUM> thickness was formed on the substrate by the sputtering method. Thus, the hydrogen separation filter (hereinafter, simply referred to as the "filter") was produced.

A TEM was used for obtaining electron diffraction patterns of the Pd layer in three positions, two of which were positions in or in the vicinity of both surfaces of each of the Pd layers of the filter of Example <NUM> and the rest of which was an intermediate position between the both surfaces. Based on each of the twelve electron diffraction patterns obtained, the Pd lattice constants were determined from plane spacings between crystal planes perpendicular to the interface between the Pd layer and the Ag layer, and an average value apd thereof was calculated. The average lattice constant apd was approximately <NUM> times as large as the bulk Pd lattice constant apd, bulk (<NUM>).

Similarly, electron diffraction patterns of the Pd layer were obtained in three places, two of which were positions in or in the vicinity of both surfaces of the Pd layer of the filter of Comparative Example <NUM> and the rest of which was an intermediate position between the both surfaces. Based on the obtained electron diffraction patterns, the Pd lattice constants were determined from plane spacings between crystal planes perpendicular to the surface of the Pd layer, and an average value apd thereof was obtained. The average lattice constant apd was approximately <NUM> times as large as the bulk Pd lattice constant apd, bulk (<NUM>).

Claim 1:
A hydrogen separation filter (<NUM>) comprising:
a porous substrate (<NUM>); and
a super lattice layer (<NUM>) on the porous substrate (<NUM>),
wherein the super lattice layer (<NUM>) comprises:
at least one Ag layer (<NUM>); and
at least two Pd layers (<NUM>),
wherein the at least one Ag layer (<NUM>) and the at least two Pd layers (<NUM>) are alternately stacked,
wherein each of the at least one Ag layer (<NUM>) and the at least two Pd layers (<NUM>) has a thickness in a range of <NUM> to <NUM>, and
wherein the Pd layer (<NUM>) has an average lattice constant aPd satisfying Formula (<NUM>): <MAT>
aPd,bulk being <NUM>, and
the average lattice constant aPd being determined by averaging Pd lattice constants calculated from a plane spacing between crystal planes perpendicular to an interface between the Ag layer (<NUM>) and the Pd layer (<NUM>) based on the electron diffraction patterns obtained by using a transmission electron microscope in three positions, two of which are positions in or in the vicinity of an upper surface and a lower surface of the Pd layer (<NUM>) and the rest of which is an intermediate position between the upper surface and the lower surface of the Pd layer (<NUM>).