HEAT CONDUCTION MEMBER

Abstract

A heat conduction member includes a housing including a space therein, a wick structure located in the space, and a working fluid enclosed in the space. The housing includes one or more metal plates and a joint structure that connects the one or more metal plates. The joint structure includes two stacked metal plate layers and a boundary portion between the two metal plate layers. The boundary portion includes a first region including crystal grains straddling the two metal plate layers.

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. § 119 to Japanese Application No. 2019-203145 filed on Nov. 8, 2019 the entire contents of which are hereby incorporated herein by reference.

Field of the Invention

The present disclosure relates to a heat conduction member.

Background

Conventionally, a vapor chamber and a heat pipe, for example, have been known as heat conduction members using a working fluid. A conventional vapor chamber includes a container in which a cavity is formed by one plate-shaped body and another plate-shaped body facing the one plate-shaped body, a working fluid enclosed in the cavity, and a wick structure including glass fiber housed in the cavity.

A conventional heat pipe includes a long container in which a working fluid is enclosed, a first wick that is in contact with an inner wall of the container and faces a vapor flow path, and a second wick that includes therein a space extending in the longitudinal direction of the container.

The conventional vapor chamber and heat pipe include a housing in which a closed space is formed. Such a housing is usually formed by connecting one or more metal plates (e.g., copper plates) by diffusion joining or brazing. Here, diffusion joining or brazing requires special equipment and requires treatment at high temperature and high pressure for a long period of time, which may cause an increase in manufacturing cost. For this reason, the manufacturing cost of a heat conduction member including a housing formed by diffusion joining or brazing may increase.

SUMMARY

A heat conduction member according to an example embodiment of the present disclosure includes a housing with a space therein, a wick structure located in the space, and a working fluid enclosed in the space. The housing includes one or more metal plates and a joint structure that connects the one or more metal plates. The joint structure includes two stacked metal plate layers and a boundary portion between the two metal plate layers. The boundary portion includes a first region including crystal grains straddling the two metal plate layers.

The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of the example embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an example of a heat conduction member according to an example embodiment of the present disclosure.

FIG. 2 is a sectional view taken along section line II-II of FIG. 1.

FIG. 3 is a schematic diagram showing two metal plate layers that are not joined together.

FIG. 4 is a schematic diagram showing a joint structure formed by brazing.

FIG. 5 is a schematic diagram showing a joint structure formed by diffusion joining.

FIG. 6 is a schematic diagram showing a planar structure of a boundary portion of FIG. 2.

FIG. 7 is a schematic diagram of a modification of a heat conduction member according to an example embodiment of the present disclosure.

FIG. 8 is a schematic diagram showing a test performed in an example.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the present disclosure will be described while referring to the drawings as appropriate. Note that in the drawings, the same or corresponding elements or features will be denoted by the same reference symbols and description thereof will not be repeated. The size relationships among the dimensions, shapes, and elements in the drawings are not necessarily the same as the size relationships among the actual dimensions, shapes, and elements. In particular, the thickness and curvature of a housing and wick structure in a drawing may be greatly different from the thickness and curvature of an actual housing and wick structure.

A heat conduction member according to an example embodiment of the present disclosure includes a housing having a space formed therein, a wick structure arranged in the space, and a working fluid enclosed in the space. The housing has one or more metal plates and a joint structure that connects the metal plates. The joint structure has two stacked metal plate layers and a boundary portion between the two metal plate layers. The boundary portion has a first region configured of crystal grains straddling the two metal plate layers.

The heat conduction member according to the present example embodiment conducts heat according to the following principle. First, when a part of the housing of the heat conduction member according to the present example embodiment is heated, the working fluid evaporates at the heated portion (heating portion). At this time, the heating portion is cooled by absorbing the latent heat of vaporization. Next, the vapor generated by the evaporation of the working fluid moves at high speed in the housing and aggregates at a relatively low temperature portion (low temperature portion). At this time, the low temperature portion is heated by releasing the latent heat of vaporization. Next, the aggregated working fluid is adsorbed by the wick structure having a capillary structure. Next, the working fluid adsorbed on the wick structure is returned to the heating portion by capillarity. Then, the working fluid evaporates again in the heating portion. By repeating the above cycle, the heat conduction member according to the present example embodiment conducts heat from the heating portion to a cooling portion.

The heat conduction member according to the present example embodiment can conduct heat more efficiently than an ordinary metal plate or metal wire. Additionally, the heat conduction member according to the present example embodiment has a high degree of freedom in shape. For this reason, the heat conduction member according to the present example embodiment can be used as a heat radiating member of an electronic device (particularly, small electronic device represented by smartphone and tablet terminal), for example.

The heat conduction member according to the present example embodiment does not require brazing or diffusion joining in forming the housing. Specifically, the housing of the heat conduction member according to the present example embodiment can be formed by heating and pressurizing under a mild condition described later. For this reason, the heat conduction member according to the present example embodiment can be manufactured at low cost.

It is preferable that the boundary portion further have a second region configured of an interface between two metal plate layers. The boundary portion having the second region can be formed by heating and pressurizing under a milder condition. For this reason, by forming the housing under the condition for forming the second region in the boundary portion, the heat conduction member according to the present example embodiment can be manufactured at a lower cost.

Hereinafter, details of the heat conduction member according to the present example embodiment will be described with reference to the drawings. FIG. 1 is a schematic diagram of a heat conduction member 1 which is an example of the heat conduction member according to the present example embodiment. The heat conduction member 1 includes a housing 2 having a closed space 2a formed therein, a wick structure 3 arranged in the closed space 2a, and a working fluid (not shown) enclosed in the closed space 2a. The housing 2 is configured of two metal plates 4 facing each other. The housing 2 has the two metal plates 4 and a joint structure 5 that connects the metal plates 4. The heat conduction member 1 is suitably used as a vapor chamber.

The planar shape of the heat conduction member 1 is not particularly limited, and a planar shape (e.g., strip shape and square shape) according to the application can be adopted. The thickness of the heat conduction member 1 is not particularly limited, and may be 100 μm or more and 1000 μm or less, for example. The width of the heat conduction member 1 is not particularly limited, and may be 5 mm or more and 500 mm or less, for example.

FIG. 2 is a sectional view taken along section line II-II of FIG. 1. As shown in FIG. 2, the joint structure 5 has two stacked metal plate layers 5a and a boundary portion 5b between the two metal plate layers 5a. The two metal plate layers 5a are layers corresponding to the two metal plates 4, respectively. The boundary portion 5b has a first region A1 configured of crystal grains CP straddling the two metal plate layers 5a. In FIG. 2, at least 10 first regions A1 exist. The boundary portion 5b further has a second region A2 configured of an interface S (surface where displacement of metallographic structure is confirmed) between the two metal plate layers 5a. In FIG. 2, there are 11 second regions A2.

The difference between the joint structure 5 shown in FIG. 2 and a joint structure of a housing of a known heat conduction member will be described. The housing of a known heat conduction member has a joint structure formed by brazing or diffusion joining, for example. FIG. 3 is a schematic diagram showing two metal plate layers C1 and C2 that are not joined together. FIG. 4 shows a joint structure formed by brazing. In the joint structure shown in FIG. 4, a brazing material layer B made of a brazing material is formed between the metal plate layers C1 and C2. Therefore, in the joint structure shown in FIG. 4, the two metal plate layers C1 and C2 are not directly stacked on top of one another. Additionally, FIG. 5 shows a joint structure formed by diffusion joining. In the joint structure shown in FIG. 5, one metal plate layer C is formed by completely integrating the metal plate layers C1 and C2. For this reason, in the joint structure shown in FIG. 5, no clear boundary portion is confirmed. As described above, the joint structure 5 shown in FIG. 2 is different from the joint structure of the housing of the known heat conduction member in that the two metal plate layers 5a are directly stacked on top of one another, and that the boundary portion 5b exists between the two metal plate layers 5a.

The joint structure 5 is formed by the following method. First, heat and pressure treatment is performed with the two metal plates 4 that are materials of the housing 2 stacked on top of one another. As a result, the metallographic structure is gradually reconstructed at the contact point between the two metal plate layers 5a. If all of the temperature, pressure, and processing time are set to a certain value or more in the heat and pressure treatment, the two metal plate layers 5a are completely integrated. In this case, the joint structure shown in FIG. 5 (joint structure formed by diffusion joining) is formed. However, in the formation of the joint structure 5, at least one of the temperature, pressure and processing time is intentionally adjusted to a certain level or less in the heat and pressure treatment, so that the two metal plate layers 5a are not completely integrated. As a result, in the formation of the joint structure 5, the two metal plate layers 5a are partially integrated in the heat and pressure treatment. As a result, the boundary portion 5b is formed between the two metal plate layers 5a, the boundary portion 5b having the first region A1 which is a region where the metallographic structure is reconfigured and the crystal grains CP are newly formed, and the second region A2 which is a region where the metallographic structure is not reconfigured and the interface S remains. As described above, the joint structure 5 is formed by the heat and pressure treatment under milder conditions than diffusion joining.

The present inventors have discovered that the housing 2 having the joint structure 5 has a high hermeticity even though the two metal plate layers 5a are not completely integrated at the boundary portion 5b. This is presumed to be due to the following reasons. First, in the first region A1 of the boundary portion 5b, since the two metal plate layers 5a are completely integrated (two metal plate layers 5a are joined by strong metallic bond), naturally, permeation of fluid (e.g., working fluid and vapor of working fluid) is curbed to a great extent. Additionally, the boundary portion 5b is formed by the heat and pressure treatment that is performed with enough intensity to form the first region A1. For this reason, in the second region A2 of the boundary portion 5b, although the interface S exists between the two metal plate layers 5a, the two metal plate layers 5a are not completely separated. Specifically, in the second region A2 of the boundary portion 5b, the two metal plate layers 5a are joined together by a weak metallic bond. Hence, the second region A2 of the boundary portion 5b curbs permeation of fluid to some extent. Specifically, in the second region A2 of the boundary portion 5b, the joint between the two metal plate layers 5a is close to a metallic bond, which has an effect of sufficiently curbing permeation of water and high-temperature steam. As described above, in the boundary portion 5b, fluid permeation is curbed to a great extent in the first region A1, and fluid permeation is also curbed to some extent in the second region A2. For this reason, the housing 2 has a high hermeticity as a whole.

When the cross section of a 1 mm range of the boundary portion 5b is observed at 10 locations randomly selected from the joint structure 5, it is preferable that the first region A1 and the second region A2 exist alternately as shown in FIG. 2 in all the boundary portions 5b. Since the first region A1 and the second region A2 are thus mixed at the micro level in the joint structure 5, the hermeticity of the housing 2 is further improved.

The boundary portion 5b preferably has a sea-island structure including the first region A1 existing as a discontinuous phase and the second region A2 existing as a continuous phase in plan view. Hereinafter, such a sea-island structure will be described with reference to FIG. 6. FIG. 6 shows a planar structure of the boundary portion 5b. In FIG. 6, the right side shows the closed space 2a side of the housing 2, and the left side shows the outside of the housing 2. In FIG. 6, in the boundary portion 5b, multiple first regions A1 are irregularly scattered in the second region A2. In FIG. 6, reference symbol X represents a fluid (e.g., working fluid and vapor of working fluid). The arrow indicates the moving direction of the fluid X. In FIG. 6, the fluid X can pass through the boundary portion 5b at the shortest distance when it goes straight to the left. Hereinafter, the shortest movement direction for the fluid X to move from the closed space 2a of the housing 2 to the outside of the housing 2 may be referred to as a first direction. The fluid X tries to move as straight as possible in the first direction in the second region A2. However, as described above, the first region A1 curbs permeation of the fluid X to a great extent. That is, the fluid X is substantially prevented from passing through the first region A1. For this reason, when the fluid X tries to pass through the boundary portion 5b, the fluid X needs to pass through the second region A2 while avoiding the first region A1. Specifically, when the fluid X moving in the first direction comes to the first region A1, the fluid X is forced to moves in a second direction (vertical direction in FIG. 6) orthogonal to the first direction in order to avoid the first region A1. Then, the fluid X avoids the first region A1 by moving in the second direction. Then, the fluid X moves in the first direction again. Repeating the above, the fluid X does not go straight in the boundary portion 5b but moves in a bending manner. As a result, when the fluid X passes through the boundary portion 5b, the fluid X is forced to move for a long distance as if moving in a maze. Hence, the fluid X is hindered from passing through the boundary portion 5b. As described above, since the boundary portion 5b has the above-described sea-island structure, the housing 2 can exhibit high hermeticity even if the ratio of the first region A1 in the boundary portion 5b is relatively low.

When the cross section of a 1 mm range of the boundary portion 5b is observed at 10 locations randomly selected from the joint structure 5, it is preferable that the mean value of the number of first regions A1 included in the boundary portion 5b be 10.0 or more and 20.0 or less. When the mean value described above is 10.0 or more and 20.0 or less, the hermeticity of the housing 2 is further improved.

When the cross section of a 1 mm range of the boundary portion 5b is observed at 10 locations randomly selected from the joint structure 5, it is preferable that the mean value of the total length of the first regions A1 included in the boundary portion 5b be 0.05 mm or more and 0.95 mm or less. The mean value described above indicates the ratio of the first region A1 in the boundary portion 5b of the joint structure 5. As the mean value described above approaches 1.00 mm, the structure resembles the joint structure formed by diffusion joining. When the average value described above is 0.05 mm or more, the hermeticity of the housing 2 is further improved. The joint structure 5 having the above average value of 0.95 mm or less is easy to manufacture.

The housing 2 has the two metal plates 4 and the joint structure 5 that connects the two metal plates 4. One of the two metal plates 4 has a flat structure. The other metal plate 4 has a central portion recessed in a direction separating from the one metal plate 4. The joint structure 5 joins together the outer edge portions of the two metal plates 4. The housing 2 has the closed space 2a formed by disposing such two metal plates 4 facing each other. The closed space 2a is surrounded by an inner surface of the metal plate 4 and the joint structure 5.

The closed space 2a is preferably in a depressurized state (state in which pressure is lower than atmospheric pressure). Such a depressurized state of the closed space 2a promotes evaporation of the working fluid.

The metal plate 4 is a plate-shaped member whose main component is metal. Examples of the metal contained in the metal plate 4 include copper, iron, aluminum, zinc, silver, gold, magnesium, manganese, titanium, and alloys containing these metals (e.g., brass, stainless steel, and duralumin). The thickness of the metal plate 4 is 10 μm or more and 1000 μm or less, for example. When the metal plate 4 contains copper (i.e., when metal plate 4 is copper plate), the content ratio of copper in the metal plate 4 is preferably 60 mass % or more, more preferably 90 mass % or more, even more preferably 99 mass % or more. By setting the content ratio of copper in the metal plate 4 to 60 mass % or more, the joint structure 5 can be formed easily.

The housing 2 may have a columnar structure (not shown) that supports the closed space 2a from the inside so that the closed space 2a is not crushed. The columnar structure of the housing 2 increases the strength of the housing 2. The columnar structure may be unevenness formed on the metal plate 4. Alternatively, the columnar structure may be a member separate from the metal plate 4 and the wick structure 3. Additionally, the housing 2 may have a partition wall (not shown) that separates the working fluid and the vapor of the working fluid in the closed space 2a.

The working fluid is enclosed in the closed space 2a of the housing 2. The working fluid is not particularly limited as long as it is a liquid that evaporates and aggregates in the use environment of the heat conduction member 1. Examples of the working fluid include water, alcohol compounds (e.g., methanol and ethanol), CFC substitutes, hydrocarbon compounds, fluorinated hydrocarbon compounds, and glycol compounds (e.g., ethylene glycol). Water is preferably used as the working fluid.

The wick structure 3 is arranged in the closed space 2a of the housing 2. The wick structure 3 is not particularly limited as long as it is a member having a capillary structure. Here, a capillary structure refers to a structure capable of moving the working fluid by capillary pressure. Examples of a capillary structure include a porous structure, a fiber structure, a groove structure, and a mesh structure.

Examples of the wick structure 3 include a wire, a mesh, a nonwoven fabric, and a porous body (e.g., a sintered body). Examples of the material of the wick structure 3 include copper, aluminum, nickel, iron, titanium, and alloys of these materials (e.g., copper alloy, aluminum alloy, nickel alloy, stainless steel and titanium alloy), carbon fiber, and ceramics. Copper is preferably used as the material of the wick structure 3. Additionally, in FIG. 1, the wick structure 3 and the metal plate are depicted as separate members. However, in the heat conduction member according to the present example embodiment, the wick structure and the metal plate may be integrated. For example, in the present example embodiment, the wick structure may be a groove or an uneven structure formed on the metal plate.

The thickness of the wick structure 3 is not particularly limited, and can be 5 μm or more and 200 μm or less, for example. In plan view, the wick structure 3 is preferably arranged in the entire area of the closed space 2a. Note, however, that in plan view, the wick structure 3 may be arranged only in a part of the closed space 2a.

Next, a heat conduction member 11 according to a modification of the heat conduction member 1 will be described with reference to FIG. 7. The heat conduction member 11 includes a housing 12 having a closed space 12a formed therein, two wick structures 13 arranged in the closed space 12a, and a working fluid (not shown) enclosed in the closed space 12a. The housing 12 has a tubular shape. The housing 12 has two metal plates 14 and a joint structure 15 that connects the metal plates 14. The heat conduction member 11 is preferably used as a heat pipe. The joint structure 15 is the same as the joint structure 5 shown in FIG. 2.

The heat conduction member according to the present example embodiment has been described above with reference to the drawings. However, the heat conduction member according to the present example embodiment is not limited to the heat conduction member 1 of FIG. 1 and the heat conduction member 11 of FIG. 7.

For example, the number of metal plates forming the housing may be one, or three or more. Additionally, the shape of the housing of the heat conduction member according to the present example embodiment is not limited to the sheet shape like the heat conduction member 1 shown in FIG. 1 and the cylindrical shape like the heat conduction member 11 shown in FIG. 7, and other shapes (e.g., rectangular tube shape and semi-cylindrical shape) may be used. Moreover, the heat conduction member according to the present example embodiment may further include a member other than the housing, the wick structure, and the working fluid (e.g., radiation fin).

Hereinafter, a method of manufacturing a heat conduction member according to the present example embodiment will be exemplified. In the method of manufacturing a heat conduction member, first, one or more metal plates are formed in a predetermined shape and arranged to form a temporary housing having a space formed therein. The temporary housing is different from the housing included in the heat conduction member according to the present example embodiment in that it does not have a joint structure. Next, the metal plate of the temporary housing is subjected to heat and pressure treatment to form a joint structure (first heat and pressure treatment). Note, however, that in the first heat and pressure treatment, an opening is left in at least one part of the temporary housing. Next, a wick structure is arranged in the space described above through the opening described above, and the working fluid is poured into the space described above. Next, the joint structure is formed by performing heat and pressure treatment on the above-mentioned opening (second heat and pressure treatment). The heat conduction member according to the present example embodiment is obtained in the manner described above. Note that in the method of manufacturing the heat conduction member, the wick structure may be arranged in advance in the space inside the temporary housing before the first heat and pressure treatment.

In the method of manufacturing the heat conduction member, during the second heat and pressure treatment, the working fluid may be partially evaporated by heating. As a result, air in the space inside the temporary housing can be expelled by the vapor of the working fluid. Consequently, the closed space formed in the housing of the heat conduction member can depressurized.

The heating temperature in the first heat and pressure treatment and the second heat and pressure treatment is preferably 500° C. or higher and 1000° C. or lower, and more preferably 500° C. or higher and 800° C. or lower. By setting the heating temperature described above to 500° C. or higher, it becomes easy to form a joint structure that satisfies the characteristics described above. By setting the temperature described above to 1000° C. or lower, the manufacturing cost can be further reduced.

The rate of temperature increase during heating in the first heat and pressure treatment and the second heat and pressure treatment is preferably 5° C./sec or more and 50° C./sec or less, and more preferably 10° C./sec or more and 30° C./sec or less. By setting the rate of temperature increase to 5° C./sec or more and 50° C./sec or less, it becomes easy to form a joint structure that satisfies the characteristics described above.

The pressure in the first heat and pressure treatment and the second heat and pressure treatment is preferably 30.0 MPa or more and 300.0 MPa or less, and more preferably 45.0 MPa or more and 230.0 MPa or less. By setting the pressure described above to 30.0 MPa or more, it becomes easy to form a joint structure satisfying the characteristics described above. By setting the pressure described above to 300.0 MPa or less, the manufacturing cost can be further reduced.

In this example, conditions for forming the joint structure described in the example embodiment when the metal plate of the housing is a copper plate were examined.

As shown in FIG. 8, two cylindrical copper plates P having a diameter of 8 mm and a height of 4 mm were prepared. One of the two copper plates P was arranged below, and the other was arranged above. Additionally, a thermocouple T was set between the two copper plates P. Next, the two copper plates P were subjected to heat and pressure treatment using a hot working device. Specifically, a downward load L was applied to the copper plate P arranged above while heating at a predetermined rate of temperature increase (10° C./sec, 20° C./sec, or 30° C./sec). During the heat and pressure treatment, the temperature of the two copper plates P was measured by the thermocouple T. The heating was performed until the temperature measured by the thermocouple T reached a predetermined temperature (500° C. or 600° C.). As a result, the copper plate P arranged above was pressed against the copper plate P arranged below. The two copper plates P were integrated by the heat and pressure treatment described above. The heat and pressure treatment was performed until the total height of the two copper plates P contracted by 3.2 mm (strain: 2%) or 6.4 mm (strain: 4%). For samples with a strain of 2%, the pressure was recorded when the strain reached 2%. Additionally, for the sample with a strain of 4%, the pressure was recorded when the strain reached 4%.

Next, an attempt was made to separate the integrated two copper plates P with pliers. When the two integrated copper plates P could be peeled off with pliers, it was determined that the two copper plates P were not joined together sufficiently. In this case, it is determined that the joint structure of the two copper plates P cannot exhibit sufficient hermeticity. On the other hand, when the two integrated copper plates P could not be peeled off with pliers, it was determined that the two copper plates P were joined together sufficiently. In this case, it is determined that the joint structure of the two copper plates P can exhibit sufficient hermeticity.

Note that if two copper plates P are to be diffusion-joined, it is necessary to perform heat and pressure treatment at 600° C. to 800° C. for several hours to several tens of hours. Hence, it is determined that diffusion joining is not performed under the conditions of the test described above.

In Table 1 below, reference symbol “A” indicates that the two integrated copper plates P could not be peeled off with pliers. Reference symbol “B” indicates that the two integrated copper plates P could be peeled off with pliers. The pressure in the parenthesis indicates the pressure recorded during the heat and pressure treatment. Reference symbol “-” indicates that the test was not performed under the corresponding conditions.

TABLE 1
STRAIN
RATE OF TEMPERATURE	2%	4%
INCREASE	10° C./SEC	20° C./SEC	30° C./SEC	20° C./SEC
HEATING	500° C.	—	B (66.6 MPa)	—	A (197.8 MPa)
TEMPERATURE	600° C.	A (44.2 MPa)	A (30.6 MPa)	A (73.4 MPa)	A (139.5 MPa)

As is clear from Table 1, it is determined that the two copper plates P can be joined together sufficiently when the heating temperature is 600° C. or higher, the rate of temperature increase is 10° C./sec or more, and the load is 45 MPa or more.

Cross sections of the samples of Table 1 were observed with an electron microscope. The joint structure of the sample whose evaluation was A in Table 1 had two stacked copper plate layers and a boundary portion between the two copper plate layers. The boundary portion had the first region and the second region described in the example embodiment. Specifically, in the sample whose evaluation was A, when the cross section of a 1 mm range of the boundary portion was observed at 10 locations randomly selected from the joint structure, the first region and the second region existed in all the boundary portions. On the other hand, in the joint structure of the sample whose evaluation was B, no region corresponding to the second region was observed.

The present disclosure is suitably used as a heat conduction member for heat radiation of electronic components and the like, for example.

Features of the above-described preferred example embodiments and the modifications thereof may be combined appropriately as long as no conflict arises.

While example embodiments of the present disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The scope of the present disclosure, therefore, is to be determined solely by the following claims.

Claims

1. A heat conduction member comprising:

a housing including a space therein;

a wick structure located in the space; and

a working fluid enclosed in the space; wherein

the housing includes one or more metal plates and a joint structure that connects the one or more metal plates;

the joint structure includes two stacked metal plate layers and a boundary portion between the two metal plate layers; and

the boundary portion includes a first region includes crystal grains straddling the two metal plate layers.

2. The heat conduction member according to claim 1, wherein the boundary portion includes a second region including an interface between the two metal plate layers.

3. The heat conduction member according to claim 2, wherein when a cross section of an approximately 1 mm range of the boundary portion is observed at 10 locations randomly selected from the joint structure, the first region and the second region exist alternately in all the boundary portions.

4. The heat conduction member according to claim 2, wherein the boundary portion has a sea-island structure including the first region that is a discontinuous phase and the second region that is a continuous phase in plan view.

5. The heat conduction member according to claim 1, wherein when a cross section of an approximately 1 mm range of the boundary portion is observed at 10 locations randomly selected from the joint structure, a mean value of a number of the first regions included in the boundary portion is about 10.0 or more and about 20.0 or less.

6. The heat conduction member according to claim 1, wherein when a cross section of an approximately 1 mm range of the boundary portion is observed at 10 locations randomly selected from the joint structure, a mean value of a total length of the first regions included in the boundary portion is about 0.05 mm or more and about 0.95 mm or less.

7. The heat conduction member according to claim 1, wherein

the housing has a tubular shape; and

the heat conduction member defines a heat pipe.

8. The heat conduction member according to claim 1, wherein

the housing includes the two metal plates facing each other; and

the heat conduction member defines a vapor chamber.