Patent Description:
For example, a ceramic-carbon composite which has a plurality of carbon particles and a ceramics portion is known as a bonding material for bonding a carbon material and another member (<CIT>). Specifically, aluminum nitride or silicon carbide is used in the ceramics portion of the ceramic-carbon composite. The ceramic-carbon composite can be bonded to a metal material by being baked, for example, through heating at <NUM> to <NUM> and pressing. In addition, a bonded body which is configured from graphite and is to be bonded to a copper plate or the like by using an insert material which is configured from silver, copper, and titanium is also known (<CIT>). The bonded body disclosed in <CIT> is a bonded body which has a plate shape and is obtained by stacking graphene sheets. The bonded body can be bonded to a member which is to be bonded, by performing pressing in a state where an insert material is interposed between the member to be bonded and the bonded body. <CIT> and <CIT> describe alloys for soldering conductors to carbon and graphite.

According to the invention, a manufacturing method of a bonded body is provided as defined in claim <NUM>.

According to the invention, a bonded body is provided as defined in claim <NUM>.

Ahead of describing exemplary embodiments, problems in the related art will be briefly described.

It is necessary that carbon bonded by the bonding material disclosed in <CIT> is included in the bonding material in a particulate form. Thus, it is difficult to bond, for example, a carbon material formed to have a plate shape, to another member by the bonding material disclosed in <CIT>. Further, a metal material and carbon particles are bonded to each other by baking the carbon particles and a ceramics portion at a high temperature. Thus, there is a problem in that a ceramic-carbon composite after baking (that is, after bonding) has poor flexibility.

In addition, it is difficult to deform a bonded structure which uses the bonded body disclosed in <CIT> because of the high Young modulus (silver: <NUM> GPa, copper: <NUM> GPa) of an insert material which is configured from silver, copper, and titanium. Thus, high stress may be applied to a junction by heat shrinkage at a time of cooling in a manufacturing process, and thus cracks may occur.

To solve the above problems, an object of the disclosure is to provide a bonding material which can fix members having any shape to each other, particularly can be bonded to a carbon material, and can form a junction having flexibility.

According to the disclosure, a bonding material includes at least <NUM> wt% to at most <NUM> wt% of at least one element (compound-formable element) which may form a compound along with tin and carbon, and Sn as the main component of a remainder.

"At least one element (compound-formable element) which may form a compound along with tin and carbon" in the disclosure means any element for forming a compound along with tin and carbon. When two kinds or more of compound-formable elements are provided, the content percentage (wt%) of the compound-formable element in the bonding material indicates a percentage of the sum of the weights of the two kinds or more of compound-formable elements included in the bonding material, to the total weight of the bonding material.

In the disclosure, "the main component" means an element having highest abundance percentage among elements in the bonding material.

In a bonding material according to an exemplary embodiment, a compound-formable element includes at least one of titanium, zirconium, and vanadium.

In a manufacturing method of a bonded body according to the exemplary embodiment, a first member and a second member are prepared, and the first member and the second member are bonded to each other by using the bonding material. Thus, a bonded body is provided.

In the disclosure, "the first member" and "the second member" mean bonded members which are fixed to each other by the bonding material.

In the manufacturing method of a bonded body in the exemplary embodiment, at least one of the first member and the second member is a carbon material. When the first member and the second member are bonded to each other, a compound layer configured from a compound of the compound-formable element, tin, and carbon is formed at an interface between the bonding material and the carbon material.

In the disclosure, "the carbon material" means a bonded member which is fixed by the bonding material, has any shape, and is made of carbon.

In the manufacturing method of a bonded body in the exemplary embodiment, all of the first member and the second member are carbon materials. When the first member and the second member are bonded to each other, the compound layer configured from a compound of the compound-formable element, tin, and carbon is formed at the interface between the bonding material and the carbon material.

A bonded body according to the exemplary embodiment is a bonded body which includes a first member, a second member, and a junction provided between the first member and the second member. Each of the first member and the second member is a carbon material, and a compound layer including the compound-formable element is formed at an interface between the carbon material and the junction.

The bonded body according to the exemplary embodiment is a bonded body which includes a first member, a second member, and a junction provided between the first member and the second member. In the bonded body, all of the first member and the second member are carbon materials, and the compound layer including the compound-formable element is provided at the interface between the carbon material and the junction.

In the bonded body in the exemplary embodiment, the compound-formable element includes at least one of titanium, zirconium, and vanadium. Hereinafter, the bonding material as the exemplary embodiment of the disclosure will be described with reference to the drawings.

In the disclosure, the bonding material is an alloy which contains at least <NUM> wt% to at most <NUM> wt% of a compound-formable element and tin as the main component of the remainder.

The compound-formable element is not particularly limited so long as the element is an element which can form a compound along with both of tin and carbon. For example, titanium, zirconium, and vanadium may be used.

The content of the compound-formable element in the bonding material is equal to or greater than <NUM> wt%. Thus, in a case where the bonded member is a carbon material, the sufficient amount of a compound is formed at an interface between a junction and the bonded member (first member and second member). Accordingly, favorable bonding with high tensile strength can be performed at the interface between the junction and the bonded member. The content of the compound-formable element in the bonding material is equal to or smaller than <NUM> wt%. Thus, it is possible to prevent deterioration of heat conductivity at the junction which is formed by using the bonding material, and to prevent an occurrence of cracks in a compound layer formed between the junction and the bonded member in a case where the bonded member is a carbon material.

The remainder of the bonding material is configured from only tin. At this time, if one kind of compound-formable element is included in the bonding material, the bonding material is a two-element alloys configured from the one kind of compound-formable element and tin as the main component. The remainder of the bonding material may be configured from plural kinds of elements which includes tin as the main component. At this time, the bonding material is a multi-element alloy configured from the compound-formable element and the plural kinds of elements which includes tin as the main component.

A bonded body and a manufacturing method thereof according to the exemplary embodiment of the disclosure will be described below with reference to the drawings.

Firstly, as illustrated in <FIG>, first member <NUM>, second member <NUM>, and bonding material <NUM> are prepared.

First member <NUM> and second member <NUM> are carbon materials which are fixed to each other by the bonding material. In the exemplary embodiment illustrated in <FIG>, first member <NUM> and second member <NUM> are carbon materials. However, a member fixed by the bonding material may be a member formed by copper, nickel, aluminum, or the like. The reason that the various types of bonded members as described above may be obtained is because tin as the main component of bonding material <NUM> may cause an interface reaction with various types of metal. First member <NUM> and second member <NUM> may be any carbon materials. For example, a highly-aligned graphite sheet, an expanded graphite sheet, and an isotropic graphite may be used. However, it is not limited thereto.

It is preferable that first member <NUM> and second member <NUM> have heat conductivity of <NUM> W/m·K or greater. Since first member <NUM> and second member <NUM> have such heat conductivity, it is possible to use a bonded body to be formed, as a heat spreader.

Regarding easiness in manufacturing, first member <NUM> and second member <NUM> preferably have a vertical length of <NUM> or smaller, a horizontal length of <NUM> or smaller, and a thickness of <NUM> or smaller. However, it is not limited thereto, and first member <NUM> and second member <NUM> may have various kinds of dimensions.

The shape of bonding material <NUM> is a film shape. The thickness of bonding material <NUM> is preferably equal to or greater than <NUM>. Since the bonding material has such a thickness, first member <NUM> and second member <NUM> can be bonded to each other without a gap.

Then, while pressure is applied to first member <NUM>, second member <NUM>, and bonding material <NUM> illustrated in <FIG> so as to cause a junction which is to be obtained to have a desired thickness, hot pressing is performed on first member <NUM>, second member <NUM>, and bonding material <NUM> under an atmosphere of nitrogen over some time. Then, cooling is performed. Thus, as illustrated in <FIG>, bonded body <NUM> which includes first member <NUM>, second member <NUM>, and a junction is formed. The junction is configured by bonding material layer <NUM> and compound layer <NUM>.

A temperature for performing the hot pressing is variously selected in accordance with the type and the percentage of an element contained in the bonding material. However, this temperature may be equal to or higher than a melting point of bonding material <NUM>, and, is <NUM> to <NUM>. A time for performing the hot pressing is variously selected in accordance with the type and the percentage of an element contained in the bonding material. However, for example, <NUM> minutes may be provided.

The junction is formed by performing hot pressing on bonding material <NUM>, and is configured from bonding material layer <NUM> and compound layer <NUM>.

Compound layer <NUM> is a portion of the junction, and is provided at an interface between the first member <NUM> and the junction and between the second member <NUM> and the junction. Compound layer <NUM> is configured from a compound of the compound-formable element, tin, and carbon. Since such compound layer <NUM> is provided at the junction, first member <NUM> and second member <NUM> are significantly firmly bonded to each other at an atomic level by the junction, and thus it is possible to improve strength of bonded body <NUM>.

Concentration of the compound-formable element in compound layer <NUM> is higher than concentration of the compound-formable element in bonding material <NUM>. This is because the compound-formable element contained in bonding material <NUM> is thickened toward compound layer <NUM> when bonded body <NUM> is formed. A composition of compound layer <NUM> is variously selected in accordance with the type and the percentage of an element contained in bonding material <NUM>. However, it is preferable that compound layer <NUM> includes about <NUM> wt% to <NUM> wt% of the compound-formable element, <NUM> wt% to <NUM> wt% of tin, and <NUM> wt% to <NUM> wt% of carbon.

The thickness of compound layer <NUM> is <NUM> to <NUM>, and preferably <NUM> to <NUM>. Since the thickness of compound layer <NUM> is equal to or greater than <NUM>, the bonded member and the junction are bonded to each other by the sufficient amount of the compound which includes the compound-formable element, tin, and carbon. Thus, bonded body <NUM> has favorable bonding properties with high tensile strength. Since the thickness of compound layer <NUM> is equal to or smaller than <NUM>, compound layer <NUM> follows deformation of the bonded body <NUM>. Thus, it is difficult to damage bonded body <NUM>.

Bonding material layer <NUM> is a portion of the junction. At this portion, a situation in which the element included in first member <NUM> and second member <NUM> is diffused toward bonding material <NUM> when bonded body <NUM> is formed, does not occur. Compound-formable element contained in bonding material <NUM> is thickened toward compound layer <NUM> when bonded body <NUM> is formed. Therefore, the concentration of tin in bonding material layer <NUM> is further higher than the concentration of tin in bonding material <NUM> which includes tin as the main component. As described above, since bonding material layer <NUM> includes tin which is metal having the Young modulus of <NUM> GPa, as the main component, bonding material layer <NUM> is much softer than general ceramics. Since such bonding material layer <NUM> is provided at the junction, first member <NUM> and second member <NUM> are bonded to each other by the flexible junction, and thus it is possible to effectively reduce the degree of damage such as cracks in bonded body <NUM>.

The bonded body in the disclosure was manufactured as will be described in the following Examples <NUM> to <NUM>.

Highly-aligned graphite which had a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM> was used in first member <NUM> and second member <NUM>. A film obtained by performing processing with an alloy which contains <NUM> wt% of titanium as the compound-formable element and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

Firstly, bonding material <NUM> was disposed between first member <NUM> and second member <NUM>. Then, while pressing pressure was controlled in a heating furnace so as to cause the thickness of junction <NUM> to be <NUM>, hot pressing was performed at <NUM> for <NUM> minutes. Finally, natural cooling was performed, and thereby bonded body <NUM> was manufactured.

Bonded body <NUM> was manufactured under conditions similar to those in Example <NUM> except that a film obtained by performing processing with an alloy which contains <NUM> wt% of titanium as the compound-formable element and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

The same as those in Example <NUM> were used as first member <NUM> and second member <NUM>. A film obtained by performing processing with an alloy which contains <NUM> wt% of titanium as the compound-formable element and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

Firstly, bonding material <NUM> was disposed between first member <NUM> and second member <NUM>. Then, while pressing pressure was controlled in a heating furnace so as to cause the thickness of the junction to be <NUM>, hot pressing was performed at <NUM> for <NUM> minutes. Finally, natural cooling was performed, and thereby bonded body <NUM> was manufactured.

Bonded body <NUM> was manufactured under conditions similar to those in Example <NUM> except that highly-aligned graphite which had a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM> was used in first member <NUM> and second member <NUM>.

Bonded body <NUM> was manufactured under conditions similar to those in Example <NUM> except that a film obtained by performing processing with an alloy which contains <NUM> wt% of zirconium as the compound-formable element and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

The same as those in Example <NUM> were used as first member <NUM> and second member <NUM>. A film obtained by performing processing with an alloy which contains <NUM> wt% of vanadium as the compound-formable element and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

Bonded body <NUM> was manufactured under conditions similar to those in Example <NUM> except that a film obtained by performing processing with an alloy which contains <NUM> wt% of titanium and <NUM> wt% of zirconium as the compound-formable element, and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

Bonded body <NUM> was manufactured under conditions similar to those in Example <NUM> except that a film obtained by performing processing with an alloy which contains <NUM> wt% of titanium and <NUM> wt% of vanadium as the compound-formable element, and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

Bonded body <NUM> was manufactured under conditions similar to those in Example <NUM> except that a film obtained by performing processing with an alloy which contains <NUM> wt% of zirconium and <NUM> wt% of vanadium as the compound-formable element, and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

Bonded body <NUM> was manufactured under conditions similar to those in Example <NUM> except that a film obtained by performing processing with an alloy which contains <NUM> wt% of titanium, <NUM> wt% of zirconium, and <NUM> wt% of vanadium as the compound-formable element, and tin as the remainder was used as bonding material <NUM>. The film was processed to have a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM>.

Highly-aligned graphite having a vertical length of <NUM>, a horizontal length of <NUM>, and a thickness of <NUM> was prepared as a comparative example.

Bonded bodies <NUM> created in Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> were evaluated in a manner that a section was observed by an electron microscope. <FIG> illustrates an electron microscope image of a section of the bonded body manufactured in Example <NUM> in the disclosure. It is understood that first member <NUM> and bonding material layer <NUM> are reliably bonded to each other, because one compound layer <NUM> is substantially uniformly formed at the interface between first member <NUM> and bonding material layer <NUM>.

In addition, element analysis was performed on a portion of the section observed by the electron microscope, and a change of concentration of each element with a change of a position in a direction perpendicular to a direction in which each layer was expanded was examined. <FIG> is a diagram obtained by plotting an element analysis result for a portion corresponding to a broken line indicated by L in <FIG>, on the electron microscope image. It was detected that the percentage of tin (Sn) was highest in bonding material layer <NUM>. Compound layer <NUM> of bonded body <NUM> had a thickness of <NUM>. A composition ratio of compound layer <NUM> satisfied titanium/tin/carbon = <NUM> wt%/<NUM> wt%/<NUM> wt%. Thus, it was confirmed that a compound including three elements was formed in compound layer <NUM>.

Then, heat conductivity and flexibility of bonded bodies <NUM> created in Example <NUM> to <NUM> and Comparative Example <NUM> to <NUM> were evaluated, and heat conductivity of highly-aligned graphite prepared in Comparative Example <NUM> to <NUM> was evaluated. The evaluation was performed by using a tester illustrated in <FIG>. Firstly, a sample manufactured in each of the examples and the comparative examples was cut out to have a rectangular shape of <NUM> in length and <NUM> in width. This is used as replica obtained by replicating a shape when being disposed as a heat spreader, on a board. Evaluation was performed in a state where bonded body sample <NUM> fixed to flat plate <NUM> by fixing jig <NUM> was pushed onto flat plate <NUM> by holding jig <NUM>. Heating element <NUM> is provided in upper fixing jig <NUM>, and an input temperature is measured by thermocouple <NUM> for measuring an input temperature at an interface between heating element <NUM> and bonded body sample <NUM>. In this evaluation, temperature control of the heating element was performed such that the input temperature of the thermocouple <NUM> for measuring an input temperature was set to be <NUM>. The temperature of bonded body sample <NUM>, which was increased by transfer of heat from heating element <NUM> was measured by thermocouple <NUM> for measuring a transfer temperature, and the measured temperature was used as a transfer temperature. Heat conductivity was evaluated by calculating a difference between the transfer temperature and the input temperature. A test was performed while holding jig <NUM> was cooled at a flow rate of <NUM> liter/min by using water of <NUM>.

An object having a width of <NUM>, a depth of <NUM>, and a height of <NUM> was used as fixing jig <NUM>. An object having a width of <NUM>, a depth of <NUM>, and a height of <NUM> was used as holding jig <NUM>. A distance L between an end portion of fixing jig <NUM> and an end portion of holding jig <NUM> was <NUM>. An object having a width of <NUM>, a depth of <NUM>, and a height of <NUM> was used as heating element <NUM>. Heating element <NUM> was installed at the center of upper fixing jig <NUM> on the lower surface. Thermocouple <NUM> for measuring an input temperature was installed on the surface at the center of heating element <NUM> on the lower surface. Thermocouple <NUM> for measuring a transfer temperature was installed so as to interpose bonded body sample <NUM> between the center of holding jig <NUM> on the lower surface and thermocouple <NUM> for measuring a transfer temperature.

Table <NUM> shows the thickness of the compound layer in each of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>, the composition ratio of the compound layer, and test results and evaluation results of heat conductivity and flexibility, which were obtained by the above test and the above observation.

The heat conductivity was determined based on a decrease rate of the heat conductivity. The decrease rate was calculated from a temperature difference which was measured by performing the above test on highly-aligned graphite in Comparative Examples <NUM> and <NUM> in which the bonding material was not used, and a temperature difference as the test result in each of the examples. If described by using Example <NUM> as an example, the decrease rate of the heat conductivity refers to a percentage of a difference between the temperature difference in Example <NUM> and the temperature difference in Comparative Example <NUM>, to the temperature difference of highly-aligned graphite in Comparative Example <NUM>. The decrease rate of the heat conductivity is calculated to be (<NUM>-<NUM>)/<NUM>×<NUM>=<NUM>%. The highly-aligned graphite in Comparative Example <NUM> has the same thickness as the sum of the thickness of first member <NUM> and second member <NUM> in Example <NUM>.

A case where the decrease rate was equal to or smaller than <NUM>% was set to be "A". A case where the decrease rate was greater than <NUM>% and smaller than <NUM>% was set to be "B". A case where the decrease rate was equal to or greater than <NUM>% was set to be "C". In a case where the determination is "A", the bonded body has sufficient heat dissipation properties and does not cause degradation of performance of a CPU even when being used as a heat dissipation member, in a product. In a case where the determination is "B", degradation of performance of a CPU does not occur, but the temperature is increased. In a case where the determination is "C", a CPU is not operated because it is not possible to dissipate generated heat.

Regarding determination criteria of flexibility, a section was observed after the heat conductivity was evaluated. A case where cracks did not occur in the bonding material layer or the compound layer was set to be "α". A case where cracks occurred in the bonding material layer or the compound layer was set to be "B".

In the bonded body in each of Examples <NUM> to <NUM>, the thickness of the bonded member was <NUM>, and the titanium content of the bonding material was at least <NUM> wt% to at most <NUM> wt%. The temperature difference of such a bonded body was <NUM> to <NUM> and the decrease rate was <NUM>% to <NUM>%. This was a result which was substantially equivalent to the temperature difference of <NUM> in Comparative Example <NUM> in which the bonding material was not used and only a carbon material was used. Thus, favorable heat transferability was obtained. Even in a case of the result obtained by observing the section, cracks were not viewed in the bonding material layer or the compound layer.

In the bonded body in each of Comparative Examples <NUM> and <NUM>, the thickness of the bonded member was <NUM>, and the titanium content of the bonding material was <NUM> wt% in Comparative Example <NUM> and <NUM> wt% in Comparative Example <NUM>. The temperature difference of the bonded body was <NUM> in Comparative Example <NUM> and <NUM> in Comparative Example <NUM>. The decrease rate was <NUM>% in Comparative Example <NUM> and <NUM>% in Comparative Example <NUM>. As the result obtained by observing the section, in Comparative Example <NUM>, the bonding material had multiple voids without being wet by the carbon material. Thus, it is considered that the heat conductivity in Comparative Example <NUM> is largely decreased in comparison to that in Comparative Example <NUM>. In Comparative Example <NUM>, as the result obtained by observing the section, cracks were not viewed. When the observation is performed by an electron microscope, compound layer <NUM> is formed to exceed <NUM>. Thus, it is considered that the thickness of the bonding material layer including much Sn is small, flexibility of the junction is lost, and cracks occur. It is considered that the heat conductivity is largely reduced because cracks occur.

In the bonded body in Example <NUM>, <NUM> wt% of zirconium as the compound-formable element was used, and the thickness of the bonded member was <NUM>. The temperature difference of such a bonded body was <NUM> and the decrease rate was <NUM>%. This was a result which was substantially equivalent to the temperature difference of <NUM> in Comparative Example <NUM> in which the bonding material was not used and only a carbon material was used. Thus, favorable heat transferability was obtained. Cracks were not viewed in the bonding material or a bonding layer even from the result obtained by observing the section.

In the bonded body in Example <NUM>, <NUM> wt% of vanadium as the compound-formable element was used, and the thickness of the bonded member was <NUM>. The temperature difference of such a bonded body was <NUM> and the decrease rate was <NUM>%. This was a result which was substantially equivalent to the temperature difference of <NUM> in Comparative Example <NUM> in which the bonding material was not used and only a carbon material was used. Thus, favorable heat transferability was obtained. Cracks were not viewed in the bonding material or a bonding layer even from the result obtained by observing the section.

In the bonded body in each of Examples <NUM> to <NUM>, plural kinds of compound-formable elements for forming a compound along with tin and carbon were used so as to cause the total content thereof to be at least <NUM> wt% to at most <NUM> wt%, and the thickness of the bonded member was <NUM>. The temperature difference of such a bonded body was <NUM> to <NUM> and the decrease rate was <NUM>% to <NUM>%. Thus, favorable heat transferability as much as the performance of a CPU was not degraded even when the bonded body was used as a heat dissipation member, in a product. Even in a case of the result obtained by observing the section, cracks were not viewed in the bonding material layer or the compound layer.

Claim 1:
A manufacturing method of a bonded body (<NUM>), the method comprising:
a step of preparing a first member (<NUM>), a second member (<NUM>), and a film-shaped bonding material (<NUM>),
wherein each of the first member (<NUM>) and the second member (<NUM>) is a carbon material and wherein the film-shaped bonding material (<NUM>) includes at least <NUM> wt% to at most <NUM> wt% of at least one element which can form a compound along with tin and carbon,
wherein elements which can form a compound along with tin and carbon are selected from the group consisting of Ti, V and Zr, and
wherein the total amount of elements which can form a compound along with tin and carbon is at least <NUM> wt% to at most <NUM> wt%, to the total weight of the film-shaped bonding material (<NUM>); and Sn as single remainder;
a step of bonding the first member (<NUM>) and the second member (<NUM>) to each other, the bonding step including:
disposing the film-shaped bonding material (<NUM>) between the first member (<NUM>) and the second member (<NUM>);
hot pressing on first member (<NUM>), second member (<NUM>), and bonding material (<NUM>) under a nitrogen atmosphere at <NUM> or higher and <NUM> or lower for a suitable time;
and then cooling;
thereby forming a junction (<NUM>) configured by a bonding material layer (<NUM>) and compound layer (<NUM>),
wherein the compound layer (<NUM>) is configured from a compound of the at least one element, tin, and carbon, at an interface between the first member (<NUM>) and the bonding material layer (<NUM>) and an interface between the second member (<NUM>) and the bonding material layer (<NUM>).