Patent Description:
In order to realize an energy-saving society as a measure against global warming, research and development that effectively utilizes waste heat at <NUM> to <NUM> discharged from automobiles and factories has increased. Thermoelectric conversion is a technique for performing a direct conversion from thermal energy to electrical energy using the Seebeck effect, or a direct conversion from electrical energy to thermal energy using the Peltier effect. For example, when a temperature difference is applied to a thermoelectric element, a thermoelectromotive force due to the Seebeck effect can be generated. Therefore, thermoelectric conversion is receiving attention as a technique that enables power generation from waste heat. As a thermoelectric material used in a medium temperature range, Mg<NUM>SiSn, which does not contain rare metals or toxic elements, is receiving attention, and thermoelectric conversion devices using Mg<NUM>SiSn have been developed.

Aluminum-based materials are used to join the thermoelectric materials that make up a thermoelectric conversion device and the electrodes. An advantage of using an aluminum-based material is that a junction temperature is lower than that of other junction materials (Ag-based materials, Ti-based materials, and Ni-based materials). Patent Literature <NUM> discloses a technique of forming a Ti layer between an aluminum brazing metal as a junction material and an electrode, and junction a thermoelectric material and the electrode. Patent Literatures <NUM> to <NUM> disclose a technique for forming a compound made of Al and Ni, or a compound layer of an element forming a thermoelectric element and Al on a junction interface between an Mg<NUM>Si-based, Si-Ge-based or MnSi-based silicide thermoelectric material with a composition including Si and the electrode, and junction the thermoelectric element to the electrode. Patent Literature <NUM> discloses a technique for forming a layer including Al<NUM>Ni<NUM> and Al<NUM>Ni at the junction interface between a Ni layer as a junction material and a Ni electrode. Other thermoelectric materials are disclosed in documents <CIT>, <CIT> and <CIT>.

Junction of Mg<NUM>SiSn and an electrode is an important technique for manufacturing high-power devices using Mg<NUM>SiSn. An upper limit temperature for using Mg<NUM>SiSn is <NUM>, and a joint capable of withstanding this temperature is required. Brazing in a furnace is considered to be the most suitable junction method, but the brazing temperature is generally a high temperature of around <NUM>. Active silver wax is often used for electrode junction of Mg<NUM>SiSn, but in this case there may be a problem in that cracks and voids are generated at the junction interface due to a high junction temperature and the diffusion of silver. Due to such circumstances, it is difficult to put into practical use and commercialize a thermoelectric conversion device that uses a medium temperature range of <NUM> to <NUM> as a heat source.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thermoelectric conversion element capable of obtaining a high output with high reliability, a method for manufacturing the same, and a thermoelectric conversion device.

In order to solve the above problems, the present invention is directed to a thermoelectric conversion element and to the corresponding manufacturing method, as set out in the appended claims.

According to the present invention, it is possible to provide a thermoelectric conversion element, a method for manufacturing the same, and a thermoelectric conversion device capable of obtaining a high output with high reliability.

Hereinafter, the present invention will be described in detail with reference to the drawings as appropriate. In the drawings used in the following description, the characteristic parts may be shown to be enlarged for convenience to make the features of the present invention easy to understand, and the dimensional ratios of each constituent element may differ from the actual ones. Materials, dimensions, and the like exemplified in the following description are merely examples and can be appropriately modified and carried out within the scope of the appended claims.

<FIG> is a plan view schematically showing the configuration of a π-type thermoelectric conversion element <NUM> according to an embodiment of the present invention. The thermoelectric conversion element <NUM> is mainly equipped with an n-type semiconductor <NUM>, two n-side junction layers <NUM>, a p-type semiconductor <NUM>, two p-side junction layers <NUM>, a first electrode <NUM>, and two second electrodes <NUM>. The n-type semiconductor <NUM> is joined to the first electrode <NUM> and the second electrode <NUM> via the n-side junction layer <NUM>, and the p-type semiconductor <NUM> is joined to the first electrode <NUM> and the second electrode <NUM> via the p-side junction layer <NUM>. (Hereinafter, the n-type semiconductor <NUM>, the n-side junction layer <NUM>, the p-type semiconductor <NUM>, the p-side junction layer <NUM>, the first electrode <NUM>, and the second electrode <NUM> may be referred to as a semiconductor <NUM>, a junction layer <NUM>, a semiconductor <NUM>, a junction layer <NUM>, an electrode <NUM>, and an electrode <NUM>, respectively.

The n-type semiconductor <NUM> has a composition represented by the following formula (<NUM>).

provided that <NUM>≤a<<NUM>, and A includes at least one of Sb, Bi, and Fe.

The p-type semiconductor <NUM> has a composition represented by the following formula (<NUM>):.

provided that <NUM>≤m≤<NUM>, <NUM><x≤<NUM>, <NUM>≤y≤<NUM>, z≤<NUM>, x+y+z=<NUM>, and -<NUM>. 00x+<NUM>≤z≤-<NUM>. 00x+<NUM> (<NUM><x≤<NUM>), -<NUM>. 00y+<NUM>≥z≥-<NUM>. 00y+<NUM> (<NUM>≥y≥<NUM>), -<NUM>. 00y+<NUM>≥z≥-<NUM>. 00y+<NUM> (<NUM><y≤<NUM>), and B includes at least one of Group 1A alkali metals, Au, Ag, Cu, Zn, Ca, and Ga.

The n-side junction layer <NUM> is formed on one main surface (a first surface) 101a and another main surface a (second surface) 101b of the surfaces of the n-type semiconductor <NUM>. The p-side junction layer <NUM> is formed on one main surface (a first surface) 103a and another main surface (a second surface) 103b of the surfaces of the p-type semiconductor <NUM>. Thicknesses of the n-side junction layer <NUM> and the p-side junction layer <NUM> are preferably <NUM> or more and <NUM> or less, and more preferably about <NUM>.

As the materials of the n-side junction layer <NUM> and the p-side junction layer <NUM>, for example, Al (aluminum), Ag (silver), Ti (titanium), Ni (nickel), alloys including 80wt% or more thereof (Al wax, silver wax, titanium wax, and nickel wax) and the like are adopted. Among these, an Al wax having a low junction temperature is particularly preferable. Examples of composition materials of Al waxes other than Al include Si, Fe, Mg, Cu, Mn, Cr, Zn, Ti, Bi and the like. When the Al wax is formed of Al, Si, and Fe, the compositional proportion of the Al wax can be, for example, Al: <NUM>. 2wt%, Si: 12wt%, and Fe :<NUM>.

A heating temperature (a junction temperature) for junction the n-type semiconductor <NUM> to the first electrode <NUM> and the second electrode <NUM> is proportional to a Si content in the n-side junction layer <NUM>. Similarly, the heating temperature for junction the p-type semiconductor <NUM> to the first electrode <NUM> and the second electrode <NUM> is proportional to the Si content in the p-side junction layer <NUM>. Therefore, a suitable junction temperature can be realized by adjusting the Si content. Specifically, if the Si content ratio is <NUM>1wt% or more and 13wt% or less, because the junction temperature can be set to about <NUM> to <NUM>, high reliability is obtained, and it is possible to realize a joined state in which a high output can be obtained when a device operates.

The n-side junction layer <NUM> may have a first n-side alloy layer (not shown) including at least one of the constituent materials of the first electrode <NUM> and Al in the vicinity of an interface on the first electrode <NUM> side. Similarly, the n-side junction layer <NUM> may have a second n-side alloy layer (not shown) including at least one of the constituent material of the second electrode and Al in the vicinity of the interface on the second electrode side (see examples to be described below).

When there are a first n-side alloy layer and a second n-side alloy layer, because a change in composition in a stacking direction is gradual, it is thought that the effect of stress relaxation can be obtained, the problem of peeling of the first electrode <NUM> and the second electrode <NUM> can be prevented, and reliability can be enhanced.

The p-side junction layer <NUM> may have a first p-side alloy layer (not shown) including at least one of the constituent material of the first electrode <NUM> and Al in the vicinity of the interface on the first electrode <NUM> side. Similarly, the p-side junction layer <NUM> may have a second p-side alloy layer (not shown) including at least one of the constituent material of the second electrode <NUM> and Al in the vicinity of the interface on the second electrode <NUM> side (see examples to be described below). When there are a first p-side alloy layer and a second p-side alloy layer, because the change in composition in the stacking direction becomes gradual, it is considered that the effect of stress relaxation can be obtained, the problem of peeling of the first electrode <NUM> and the second electrode <NUM> can be prevented, and reliability can be improved.

As an example, when the first electrode <NUM> and the second electrode <NUM> contain Ni as a main component, Ni is distributed to exude near the interface between the n-side junction layer <NUM> and the p-side junction layer <NUM> which are in contact with them. Then, an alloy layer of Ni and Al is formed. In this case, the n-side junction layer <NUM> has an AlNi layer and an Al<NUM>Ni<NUM> layer in this order from the first electrode <NUM> side or the second electrode <NUM> side (junction interface). Further, the p-side junction layer <NUM> has a Ni<NUM>Sn<NUM> layer, an AlNi layer, and an Al<NUM>Ni<NUM> layer in this order from the first electrode <NUM> side or the second electrode <NUM> side (junction interface).

It is preferable that Sn (tin) be included in at least one of the n-side junction layer <NUM> and the p-side junction layer <NUM>. When Sn is included, occurrence of cracks and the like is suppressed, and the joined state between the semiconductors <NUM> and <NUM> and the electrodes <NUM> and <NUM> can be improved. In the state in which Sn is included, this may be achieved, for example, by some of the Sn constituting the semiconductors <NUM> and <NUM> being activated and freed in the heating process, penetrating the junction layers <NUM> and <NUM>, and eventually segregating into the alloy layer near the interface of the electrodes <NUM> and <NUM>. (The method for segregating Sn may not follow this process. ) Actually, because a melting point of the p-type semiconductor <NUM> is lower than that of the n-type semiconductor <NUM>, and Sn tends to be free, Sn is segregated with respect to the p-side junction layer <NUM> with a higher frequency than that of the n-side junction layer <NUM>.

The shapes of the first electrode <NUM> and the two second electrodes <NUM> (106A, 106B) are not limited, but are preferably flat. In the first electrode <NUM>, one side 105a (here, a left side) is joined to the first surface 101a of the n-type semiconductor via the n-side junction layer <NUM>, and another side 105b (here, a right side) is joined to the first surface 103a of the p-type semiconductor via the p-side junction layer <NUM>. One side (here, the right side) of one second electrode 106A is joined to the second surface 101b of the n-type semiconductor via the n-side junction layer <NUM>, and one side (here, the left side) of another second electrode 106B is joined to the second surface 103b of the p-type semiconductor via the p-side junction layer <NUM>.

Examples of the materials of the first electrode <NUM> and the second electrode <NUM> include Ni, Cu, Ti, Fe, Au, Ag, Al and the like. However, in the manufacturing process of the thermoelectric conversion element, because there is a need for heating at a high temperature of <NUM> or more, Ni, which has high heat resistance, is preferable.

<FIG> is a perspective view of a thermoelectric conversion device <NUM> in which a plurality of thermoelectric conversion elements <NUM> are connected. Two adjacent thermoelectric conversion elements <NUM> share a second electrode <NUM>. Among the two adjacent thermoelectric conversion elements <NUM>, the n-side junction layer <NUM> constituting one thermoelectric conversion element and the p-side junction layer <NUM> constituting the other thermoelectric conversion element are connected with the shared second electrode <NUM> interposed between them.

The thermoelectric conversion element <NUM> of this embodiment can be manufactured mainly via the following procedure.

First, the n-type semiconductor <NUM>, the n-side junction layer <NUM>, the p-type semiconductor <NUM>, the p-side junction layer <NUM>, the first electrode <NUM>, and the second electrode <NUM> are stacked (assembled) to be aligned with the stacking order of the thermoelectric conversion element <NUM> in the completed state to form a stacked body (stacked body-forming process).

Specifically, a first n-side junction layer 102A and a second n-side junction layer 102B are disposed on the first surface 101a and the second surface 101b of the n-type semiconductor <NUM>, respectively. Similarly, a first p-side junction layer 104A and a second p-side junction layer 104B are disposed on the first surface 103a and the second surface 103b of the p-type semiconductor <NUM>, respectively.

In addition, the first electrode <NUM> is disposed to straddle both the junction layers with respect to the first n-side junction layer 102A and the first p-side junction layer 104A. That is, the first electrode <NUM> is disposed so that one side is in contact with the first n-side junction layer 102A and the other side is in contact with the first p-side junction layer 104A.

Further, one (here, the left side) second electrode 106A and the other (here, the right side) second electrode 106B among the two second electrodes <NUM> are disposed with respect to the second n-side junction layer 102B and the second p-side junction layer 104B, respectively. That is, one second electrode 106A is disposed so that one side is in contact with the second n-side junction layer 102B, and the other second electrode 106B is disposed so that one side is in contact with the second p-side junction layer 104B.

Next, in order to fix the arrangement of each layer with respect to the formed stacked body, pressure is applied (pressed) from both sides in the stacking direction L (pressing process). The pressure applied to the stacked body is preferably <NUM>/cm<NUM> or more and <NUM>/cm<NUM> or less.

Finally, the thermoelectric conversion element <NUM> of the present embodiment can be obtained by accommodating and heating the pressed stacked body in a vacuum furnace (heating process). The heating temperature (junction temperature) is <NUM> or more and <NUM> or less, and the heating time (junction time) is <NUM> minute or longer and <NUM> minutes or shorter. A temperature rise rate of the stacked body before the heating process is preferably <NUM>/min or more and <NUM>/min or less, and a temperature drop rate of the stacked body after the heating process is <NUM>/min or more and <NUM>/min or less.

When forming the thermoelectric conversion device <NUM>, in the stacked body-forming process, the pressing process and the heating process are performed after assembling so that each layer is disposed similarly to that in the thermoelectric conversion device <NUM>.

As described above, in the thermoelectric conversion element according to the present embodiment, both the n-type semiconductor and the p-type semiconductor are formed of Mg<NUM>SiSn-based thermoelectric materials, and both semiconductors have substantially the same coefficient of linear expansion. Therefore, the difference in the coefficient of linear expansion in a high-temperature state in the manufacturing process can be suppressed to a small level, damage to the junction portion with the electrode can be prevented, and reliability can be improved.

Further, in the thermoelectric conversion element according to the present embodiment, an aluminum-based material having a low junction temperature is used as the junction material. Therefore, it is possible to manufacture a thermoelectric conversion element at a temperature corresponding to the melting point (<NUM>) and the usage temperature (<NUM>) of Mg<NUM>SiSn constituting the n-type semiconductor and the p-type semiconductor, and it is possible to realize an optimum joined state in which damage such as cracks due to heat is suppressed. As a result, a high output can be obtained when operated as a device.

Hereinafter, the effects of the present invention will be made clearer by the examples. The present invention is not limited to the following examples, and can be appropriately modified and implemented without changing the gist thereof.

The thermoelectric conversion element was manufactured according to the above-mentioned procedure. The materials and compositions of each semiconductor and each layer were as follows.

Further, the dimensions of each constituent element were as follows.

These were stacked, pressed in the stacking direction, and then heated. The heating temperature (junction temperature) was <NUM>, the heating time (junction time) was <NUM> minutes, the temperature rise rate was <NUM>/min, and the temperature drop rate was <NUM>.

<FIG> is an SEM image of the junction portion of the n-type semiconductor <NUM>, the n-side junction layer <NUM>, and the second electrode <NUM> in the thermoelectric conversion element obtained after the temperature drop. <FIG> is an enlarged view of the vicinity of the junction portion between the n-side junction layer <NUM> and the second electrode <NUM> in the SEM image of <FIG>. It is known from these SEM images that the alloy layer <NUM> is formed between the n-side junction layer <NUM> and the second electrode <NUM>. The alloy layer <NUM> is made of two layers, and as a result of elemental analysis, it is known that the first layer 107A from the n-side junction layer <NUM> side is formed of Al<NUM>Ni<NUM> and the second layer 107B is formed of AlNi. The thickness of the first layer 107A of the alloy layer is <NUM> to <NUM>, and the thickness of the second layer 107B is <NUM> to <NUM>. A segregated portion <NUM> of Sn can be seen at a part of the interface between the alloy layer <NUM> and the n-side junction layer <NUM>.

<FIG> is an SEM image of the junction portion of the p-type semiconductor <NUM>, the p-side junction layer <NUM>, and the second electrode <NUM> in the thermoelectric conversion element obtained after the temperature drop. <FIG> is an enlarged view of the vicinity of the junction portion between the p-side junction layer <NUM> and the second electrode <NUM> in the SEM image of <FIG>. It is known from these SEM images that the alloy layer <NUM> is formed between the p-side junction layer <NUM> and the second electrode <NUM>. The alloy layer <NUM> is formed of three layers, and as a result of elemental analysis, the first layer 109A from the p-type semiconductor <NUM> side is formed of Al<NUM>Ni<NUM>, the second layer 109B is formed of AlNi, and the third layer 109C is formed of Ni<NUM>Sn<NUM>. The thickness of the first layer 109A of the alloy layer is <NUM> to <NUM>, the thickness of the second layer 109B is <NUM> to <NUM>, and the thickness of the third layer 109C is <NUM>.

The output characteristics of the obtained thermoelectric conversion element were measured. <FIG> is a graph showing the result.

A horizontal axis of the graph shows a temperature difference ΔT (°C) between the first electrode <NUM> side and the second electrode <NUM> side, and a vertical axis of the graph shows an output P (W) generated from the thermoelectric conversion element. The output when the temperature difference is <NUM> is <NUM>. 91µW, and the output when the temperature difference is <NUM> is <NUM> W. Since the plots of all the measurement results are on almost the same curve, it is known that stable output characteristics are obtained.

In the heating process, thermoelectric conversion elements were manufactured as Example <NUM> and Comparative Examples <NUM> and <NUM> when the heating time was <NUM> minutes and the heating temperatures were <NUM>, <NUM>, and <NUM>, respectively. The conditions other than the heating time and the heating temperature were the same as in Example <NUM>.

<FIG> are photographs of the junction portions of n-type semiconductors in the thermoelectric conversion elements of Examples <NUM> and Comparative Examples <NUM> and <NUM> of the present invention, respectively. When the heating temperatures were <NUM> and <NUM>, cracks and voids were generated, and there was a state which was not suitable for the thermoelectric conversion element. However, when the heating temperature was <NUM>, a good junction interface was obtained. Although not disclosed here, when the heating temperature was <NUM>, the reaction at the interface was insufficient, and the n-type semiconductor and the electrode could not be sufficiently joined.

In the heating process, when the heating temperature was <NUM> and the heating time was <NUM> minutes, <NUM> minutes, and <NUM> minutes, the thermoelectric conversion element was manufactured as Example <NUM> and Comparative Examples <NUM> and <NUM>, respectively. The conditions other than the heating time and the heating temperature were the same as in Example <NUM>.

<FIG> are photographs of the junction portion of the n-type semiconductors in the thermoelectric conversion elements of Examples <NUM> and Comparative Examples <NUM> and <NUM> of the present invention, respectively. When the heating time was <NUM> minutes, many small voids were generated, but when the heating time was <NUM> minutes and <NUM> minutes, such voids were not generated. Further, when the heating time was <NUM> minutes and <NUM> minutes, the junction interface between the n-type semiconductor and the electrode had an uneven structure. However, when the heating time was <NUM> minutes, the junction interface was flat.

When the heating temperature in the heating process was <NUM> and the temperature drop rate after heating in the heating process was <NUM>/min and <NUM>/min, thermoelectric conversion elements were manufactured as Example <NUM> and Comparative Example <NUM>, respectively. The conditions other than the heating temperature and the temperature drop rate were the same as in Example <NUM>.

<FIG> are photographs of the junction portion of the n-type semiconductors in the thermoelectric conversion elements of Example <NUM> and Comparative Example <NUM> of the present invention, respectively. When the temperature drop rate was <NUM>/min, the Al wax after junction had melted out, the thickness of the alloy layer was not uniform, and voids occupied <NUM>% of the alloy layer. In contrast, when the temperature drop rate was <NUM>/min, the alloy layer was normally formed, and a good junction was obtained.

In the heating process, thermoelectric conversion elements were manufactured as Example <NUM> and Comparative Example <NUM> when the heating temperatures were <NUM>, <NUM>, and <NUM>, respectively. The conditions other than the heating temperature were the same as in Example <NUM>.

Claim 1:
A thermoelectric conversion element, comprising:
an n-type semiconductor (<NUM>);
a p-type semiconductor (<NUM>);
an n-side junction layer (<NUM>) formed on each of a first surface (101a) and a second surface (101b) of the n-type semiconductor (<NUM>);
a p-side junction layer (<NUM>) formed on each of a first surface (103a) and a second surface (103b) of the p-type semiconductor (<NUM>);
a first electrode (<NUM>) which has one side joined to the first surface (101a) of the n-type semiconductor (<NUM>) via the n-side junction layer (<NUM>), and the other side joined to the first surface (103a) of the p-type semiconductor (<NUM>) via the p-side junction layer (<NUM>); and
two second electrodes (<NUM>), one of which is joined to the second surface (101b) of the n-type semiconductor (<NUM>) via the n-side junction layer (<NUM>) and the other of which is joined to the second surface (103b) of the p-type semiconductor (<NUM>) via the p-side junction layer (<NUM>),
characterized in that:
the n-type semiconductor (<NUM>) has a composition represented by the following formula (<NUM>),
the p-type semiconductor (<NUM>) has a composition represented by the following formula (<NUM>),
the first electrode (<NUM>) and the second electrode (<NUM>) include Ni as a main component,
the n-side junction layer (<NUM>) has an AlNi layer and an Al<NUM>Ni<NUM> layer in this order from the first electrode side or the second electrode side, and
the p-side junction layer (<NUM>) has a Ni<NUM>Sn<NUM> layer, an AlNi layer, and an Al<NUM>Ni<NUM> layer in this order from the first electrode side or the second electrode side, and
the n-side junction layer (<NUM>) and the p-side junction layer (<NUM>) include Al:

        Mg<NUM>SiaSn<NUM>-a+A     (<NUM>)

provided that, <NUM>≤a≤<NUM>, and A includes at least one of Sb, Bi, and Fe

        MgmSixSnyGez+B     (<NUM>)

provided that, <NUM>≤m≤<NUM>, <NUM>≤x≤<NUM>, <NUM>≤y≤<NUM>, z≥<NUM>, x+y+z=<NUM>, and -<NUM>.00x+<NUM>≥z≥-<NUM>.00x+<NUM>; if x is <NUM><x<<NUM>, -<NUM>.00y+<NUM>≥z≥-<NUM>.00y+<NUM>; if y is <NUM>≥y≥<NUM>, -<NUM>.00y+<NUM>≥z≥-<NUM>.00y+<NUM>; if y is <NUM><y≤<NUM>, and B includes at least one of Group 1A alkali metals, Au, Ag, Cu, Zn, Ca, and Ga.