JPS59190277A - Ceramic pipe bonded body and bonding method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] The present invention relates to a ceramic tube bonded body and a bonding method, and more specifically to a ceramic tube bonded body and a ceramic tube bonded body for heat exchange tubes used in high-temperature equipment such as heat exchangers. Regarding the layer method.

Conventionally, high-temperature heat exchangers have used heat exchange tubes made of metal such as heat-resistant alloys, but in order to improve thermal efficiency and processing capacity, higher i! ! There is a need for a replacement with ceramic heat exchange tubes that can be used in However, heat exchange tubes often have a length of several meters, a U-shaped bend, i11≦, etc. Obtaining a real ceramic tube that satisfies thermal properties has been difficult in terms of molding power and firing ability. Therefore, it is desirable to join ceramic tubes of known or simple shapes to obtain a ceramic prefabricated assembly of the required shape and size.

Such a joined body of ceramic tubes is required to have the following characteristics. First, since such a bonded body is used at high temperatures of 1000° C. or higher, not only the ceramic tube but also the adhesive layer must have heat resistance. Next, such a joined body becomes long and has a considerable weight,
In particular, sufficient reinforcing strength is required to prevent the joints from sagging or bending when held horizontally. Dust with different properties flows inside and outside the ceramic tube; 11-
Since it is not desirable for the two to mix together, it is necessary that the joints be airtight as well. Furthermore, gases in an oxidizing atmosphere, such as high-temperature 1A gas and preheated air, are often flowed inside or outside the heat exchange tube, so the joints must have sufficient chemical stability in the high-temperature oxidizing atmosphere. be. - It goes without saying that in order to obtain a single bond, it is desirable to use an adhesive layer or a mold material before it, or an inorganic material that is easy to prepare by hand.

An object of the present invention is to provide a VC assembly and a joining method for ceramic tubes that satisfy these performances and can be used for applications such as heat exchange tubes.

The present invention provides a joined body of ceramic tubes made of a pair of ceramic tubes with a joint made of rhinoceramic along the joint, which contains 50 to 95% metal oxide and 5 to 50% S
A first adhesive layer containing iC and/or ij Si3N+ k and having an amorphous phase of more than 50% is interposed between the ceramic tubes, and a first adhesive layer containing more than 50% of a metal oxide and having a crystalline phase of more than 50%. A second adhesive layer having a coefficient of greater than 50 is interposed between the ceramic pipe and the joint, and the absolute value of the difference between the coefficient of expansion of the second adhesive layer and the coefficient of expansion of the joint is 1×
10-6/°C A joint of ceramic tubes with a temperature of 10-6/℃ and a joint of ceramic tubes with two joints made of ceramic force applied to the joint part of a pair of ceramic tubes. 0 sun, 50 ~ as an active ingredient
Contains gold 70,000 oxide with a ratio of 95% and S1c and/or Si3N4 with a ratio of 5 to 50%, and contains an amorphous phase or 5% as an active ingredient.
For more than 0% contact) n, 1 large 1 ↓ 1 piece is interposed between the two ceramic tubes, and the maximum temperature is 165Q.
After the first step of heat treatment at a first temperature of ~1650°C and the V first step, a metal oxide is added as an active ingredient at 51. ] A second bonding composition containing a large amount of silane and having a crystalline phase of more than 50 mol as an active ingredient is interposed between the ceramic tube and the joint, so that the maximum temperature force and the bull's temperature are lower. Part-d
The present invention proposes a method for joining ceramic tubes, which includes a second step of heat treatment at a temperature of 4+. The joined body shown in FIG. 1 has a first adhesive layer 4 having excellent airtightness interposed between the end faces of a pair of ceramic members 1.2 that are to be joined. Additionally, a tubular ceramic joint 6 having an inner diameter slightly larger than the outer diameter of the lamic pipes 1 and 2 is used.
On the outside of the ceramic tubes 1 and 2, the first adhesive layer 4 is placed at the longitudinal center of the joint (the device is shifted, and the first adhesive layer 4 is placed behind the ceramic tubes 1 and 2, and the first adhesive layer 4 is mainly reinforced between the ceramic tubes 1 and 2 and the joint 6. The second adhesive layer 5 has ligatures interposed therebetween, which contributes to strength.

The joined body shown in FIG. WJ2 is the same as that shown in FIG. 1, except that a tubular ceramic joint having an outer diameter slightly smaller than the inner diameter of the ceramic tubes 1 and 2 is disposed inside the ceramic tubes 1 and 2. The joined body shown in FIG. 6 has a plurality of curved plate-shaped 5-pieces (4 pieces in the 51st Kan) spaced apart, and four rings are placed on the end face of the ceramic tube 1, and four rings are placed on the end face of the ceramic tube 2. The structure is the same as that shown in FIG. 1 except that a convex groove is provided. Therefore, the arrangement of the constituent elements of the bonded body of the present invention, such as the ceramic plate, the joint, the adhesive layer, etc. (the arrangement shown in FIG. 1 is preferable from the viewpoint of ease of manufacture).

However, it is necessary to compensate for the contraction force of the joint i)・1i
Since it is not necessary for the development of 5'fr degrees, it can be placed inside the ceramic container as shown in Fig. 2. This is one of the features of the present invention. Reinforcement strength fI, maintenance 1-c-1-j, airtight forehead protection (・de・8
- In order for the adhesive layer 4 to act as a king, the joint is connected to the first adhesive layer 4
It is not necessary to cover the entire circumference of the platform, and it is possible to arrange it as shown in Figure 3. One of the advantages of the present invention is that it facilitates operations such as uniformly pouring the composition into narrow spaces.In addition, in Figure 3 (f), a joint of similar shape is connected to a ceramic tube. Inside ((
May be arranged. Furthermore, by forming convex points, concave points, etc. on the end surfaces of the ceramic tubes 1 and 2, the first adhesive layer (
・It is also possible to increase the size of the ceramic tube 1゜2, or to create appropriate irregularities on at least one of the joint surfaces of the ceramic tube 1゜2 and the joint surface. You may increase the connectivity by the Niyonuma layer.

The ceramic tube and joint used in the present invention are made of aluminum.
Various oxide-based high-temperature grooves such as 2O3, ZrO2, mullite, and cordierite are also used, but non-acid ceramics are used.

Among them, SiC quality, Si3N4 quality, SiC-
Si3 N4 composite material and non-oxide-oxidation v/J composite ceramic.

Among these, the aronous material is preferable because it has a high seed quality, is rich in atmosphere stability, is dense, has high strength, and has excellent thermal shock resistance.A stiff non-oxide type or non-oxide
Oxide composite ceramic (sintering aid)
A1201. Y2O3, MyO, B, BN.

C or the like may be blended, or some metals such as Si and C may remain due to sintering by the reaction sintering method.

As a bonded body, it is preferable from the viewpoint of dimensional stability during use that the ceramics of Toto 17 are sufficiently sintered, but the ceramics of Konera before bonding are not necessarily sufficiently sintered. You don't have to do it. :j=)
This is because they can be mixed and sintered during heat treatment. It is preferable that both the pair of ceramic tubes and the fittings are made of the same material (although it is preferable to use different materials depending on the desired physical properties and economical efficiency).

The first adhesive layer mainly provides good airtightness as well as reinforcing strength, and has a diameter of 50 to 95 mm, and is made of a metal oxide of 60 to 85 mm and a metal oxide of 5 to 50 mm. , especially s i C &(J' / or Si3N
4 included @'l-, -(inari-J:Jl'd:
:? The first adhesive layer 4 and 2 are amorphous! i iJ ; g, especially cl, which must be greater than 65 yen. Note that in this specification, the term "row" refers to weight percentage. If the metal oxide content is less than 50%, the high temperature oxidation resistance will be poor and it will not be suitable for SIC and/or
Alternatively, if the content of Si3N4 is less than 5 parts, this adhesive layer IfC cracking occurs, which is not appropriate. Also, the amorphous phase is 50
If it is less than %, the airtightness will deteriorate and it is not suitable. Here, it is thought that the majority of the metal oxides probably form an amorphous phase and participate in the formation of airtight 7-trisoxes, and SiC and/or Si3N4 are dispersed within these 7-trisoxes and form a matrix. It is thought that it is involved in the increase in the strength of i.

The metal oxide of the first adhesive layer is a mixture of multiple types of metal oxides or a composite oxide with a required content of amorphous + 1
” can be easily obtained, which is preferable. The combination of metal species constituting this metal oxide is not necessarily limited to A2, but may include Rta group and IIIa group elements, especially My, Ca, Y, La, etc.

and one or more selected from Ce and A1. A combination of three or more different metals J containing Si is preferred because it can easily form one required amorphous phase and has excellent adhesion to ceramic materials. In this case, the content in the first adhesive layer is A120313-43, 5i02.1
4 to 40 CI), 20 to 45% of one or more oxides selected from the lla group and IIIa group elements, and 415 to 40% of SiC and/or S3N, and has a special fc
(Hasosewaka 16-26%, 24-64 staff, 24-26%
It is preferable that it is 38% and a ratio of 15 to 65. In addition, in the specification, Al2O3 refers to 3 Al, 2 in Murai) (3A1203/2sio2), for example.
A, 12 when displaying oxides like 03 equivalent car length
It may represent a portion represented by 03 or 1~, and is not necessarily limited to Al2O3 in a free state. The same applies to 5i02 and other metal oxides.

In contrast to the crystalline phase in the first adhesive layer, the amorphous phase contributes to improving the fluidity of the adhesive composition during bonding and reducing air bubbles and voids in the adhesive layer. Most of the amorphous phase is a metal oxide, and most of the metal oxide is often an amorphous phase.Substantially all of the metal oxide may be an amorphous phaseQ second adhesive layer mainly contributes to reinforcement strength, and so
%, especially more than 55% metal oxide, and the crystalline phase in the second adhesive layer must be 50%,
In particular, there must be more than 55 sections. Furthermore, the absolute value of the difference between the expansion coefficient of the second adhesive layer and the expansion coefficient of the joint (hereinafter referred to as
(simply referred to as expansion coefficient difference) must be smaller than 1x10-〆°C. In this specification, the expansion coefficient is 2.
It refers to the average linear expansion coefficient at 0 to 1000°C. If the content of the metal oxide is less than 50%, the high temperature oxidation resistance will be poor and it will not be suitable. If the content of the crystal phase is less than 50%, the reinforcing strength will decrease and it is not suitable. Also, the expansion coefficient difference is 1
If it is larger than x 1o-6/°C, adhesion between the second adhesive layer and the joint will not be performed properly in the joining process,
Or, when the obtained joined body is used for one period at high temperature, defects such as cracks occur in the ceramic pipe or joint, making it unsuitable.

The metal oxide of the second adhesive layer is also preferably a mixture of multiple types of metal oxides or a composite oxide because the second adhesive layer having an expansion coefficient of rupture can be easily obtained. Further, the second adhesive layer may be substantially entirely of a crystalline phase, or may contain 5 to 20% of an amorphous phase in order to obtain appropriate fluidity during bonding. In order to satisfy the stiffness and also achieve the desired expansion coefficient difference, the second adhesive layer should be AlXSi, M! It is desirable that the composition contains more than 50% of a crystal phase composite oxide of two or more metals selected from , I, Ti, and Zr.

Preferred examples of such crystal phase composite oxides include mullite, cordierite, zircon, and alumina-titania.

In addition, preferred embodiments of the first adhesive layer and the second adhesive layer Vc fa described above, their purpose, and effects will be described later in the first adhesive layer: fi
The composition No. 11 and the active ingredient of the second adhesive composition can be applied in the same manner. In addition, the amorphous phase component and crystalline phase component in the adhesive layer are often supplied mainly by the amorphous phase component and crystalline phase component in the adhesive composition, and although it is desirable, the amorphous phase component and crystalline phase component in the adhesive layer are supplied mainly by the amorphous phase component and crystalline phase component in the adhesive composition, and although it is desirable, A part of the amorphous phase in the adhesive composition may become a crystalline phase in the adhesive layer, or vice versa.

When the ceramic pipe or joint is made of non-oxide ceramic or non-oxide-oxide composite ceramic, it is preferable that the second adhesive layer contains 15 to 65% of SIC and/or Si3N4. Alternatively, it is often preferable to improve the affinity between the joint and the second adhesive layer. For this purpose, it is preferable that the second adhesive composition contains 15 to 50% of one or more metals selected from metals s1, c, SiC, and 813N4 as immobile components.

- Furthermore, it is essential that the joining method of the present invention includes a first step and a second step after the first step. The first step is mainly a final step for forming the first adhesive layer, and the effective/min.
5 to 50'ly, especially 15 to 40'
% of SiC and/or 3 i3 N4, and has an amorphous phase as an active ingredient of 50%, especially 65%, by interposing the first adhesive composition between the ceramic tubes to achieve maximum The heat treatment is performed at a first temperature of 1350 to 1650°C. It should be noted that both the first adhesive composition and the second adhesive composition described below are prepared using a dispersion medium such as water or an organic solvent, or a dispersion of methylcellulose (hereinafter abbreviated as M○) or alginic acid (hereinafter abbreviated as SA). The active ingredient may be applied to a specified area in the form of a paste or suspension with the addition of other auxiliary agents, or may be applied as a powder to the specified area. It refers to components and their precursors, and does not include water or organic components. If the maximum temperature at about 1-8 is lower than 1,350°C, the active ingredient of the first adhesive composition will not flow sufficiently, and the airtightness of the first adhesive layer will not be ensured. As a result, the fluidity becomes high, and many of the components that should form the first adhesive layer flow out to areas outside the space between the ceramic tubes, which is not a good idea. In addition, in the first step, a pair of ceramic tubes are placed between the end faces.
Vertical concentricity (/rc maintained with an adhesive composition interposed,
By heat-treating the first adhesive composition while applying some or all of the gravity acting on the upper ceramic tube to the first adhesive composition, the active ingredient of the first adhesive composition that has become fluidized by the heat treatment. It is possible to form a more airtight first adhesive layer by gradually flowing while reducing air bubbles and voids. ! Furthermore, the heat treatment in the first step may be performed while applying an external force to press the other of a pair of ceramic tubes held horizontally and concentrically.

The second step is mainly a heat treatment step for forming the second adhesive layer, and one of the features of the present invention is that the maximum temperature of this step is lower than the maximum temperature of the first step. This is because the second adhesive layer can be formed by heat treatment at this temperature, and if this temperature is higher than the maximum temperature of the first step, the properly formed first adhesive layer will flow again. This is because the maximum temperature in this second step is 1,200 to 1,450°C, which allows the second adhesive layer to be easily formed and prevents the functional deterioration of the first adhesive layer. It's nice to be able to do it.

The active ingredient of the second adhesive composition for forming the second adhesive layer contains a metal oxide in an amount of 50 parts, especially 55 parts, and a crystalline phase of 50 parts, especially 55 parts. Contains a lot. Furthermore, regarding the adhesive layer formed from the second adhesive composition in the second step of the bonding method of the present invention, that is, the second adhesive layer, the difference in expansion coefficient is 1x 1o-t"
It is preferable that the temperature is smaller than /°C.

When the trench second adhesive composition contains metal Si or metal S1 and C, C becomes metal S1 through heat treatment.
It is important that metal S1 reacts with C or nitrogen in the atmosphere to form SiC or 5i31i4. Therefore, the second heat treatment step -4
It is preferable to carry out the reaction in a nitriding atmosphere, that is, an atmosphere that does not contain nitrogen in the molecular or compound form, and particularly does not substantially contain oxidizing substances such as oxygen.

The active ingredient of the trench second adhesive composition may be substantially only a crystal phase composite oxide, such as mullite, but in this case, the second adhesive layer formed must have sufficient reinforcing strength. Despite being resistant to high temperature oxidation, it is often brittle.

In such a case, as shown in FIG. 5, a second adhesive composition whose active ingredient is substantially only a crystalline phase composite oxide is interposed between the center portion of the joint and the ceramic tube. to form a second adhesive layer, and a third adhesive composition containing 10 to 50% of metal S1 and/or amorphous metal oxide as an active ingredient is applied between the end of the joint and the ceramic tube. By forming the third adhesive layer therebetween, the fragile portion is not exposed to the outside, which is preferable. At this time, the second adhesive layer and the third adhesive layer may be formed simultaneously in the second step, or after the second step, the third adhesive composition may be interposed in the required area, and then the third adhesive layer may be formed in the third step. A layer may also be formed.

Furthermore, we will discuss the features of the present invention.

The first adhesive layer is melt-softened, and the second adhesive layer is sinterable due to a reaction between solid phases or between a solid phase and a gas phase. If both adhesive layers are melt-softening, mixing and diffusion will occur at the contact interface of both adhesive layers, and both adhesive layers will become easily softened at that part. However, the present invention solves this problem. It doesn't happen.

It is not preferable to replace the intervening part of the first adhesive layer and the second adhesive layer. The circumferential surface to be bonded compared to the clearance,
This is because even if a considerable amount of the adhesive composition is applied to the clearance and then heat treated, it often tends to leave an airway communicating with the outside. On the other hand, the first point of the present invention is that an adhesive layer, which does not necessarily have to be airtight, is provided in this area.

It is also an advantage of the present invention that the second step is performed after the first step. In other words, if the second process is performed first, the position of the pair of ceramic tubes will be fixed by the second contact layer that provides reinforcing strength, and even if the first process is performed after that, the first adhesive layer will remain in the same layer. This is because airtightness cannot be achieved simply by expanding the volume occupied by air bubbles, etc.

The present invention will be explained below by way of examples.

Example 1 A pair of SIC
The tubes were joined. The outer diameter is 1.05n++n and the inner diameter is 90mm, obtained by sintering using the pressureless sintering method.

Two S'lC circular tubes with a length of + 500 + nrn, each with a paste-like first adhesive composition applied to the upper end surface for approximately 6 m.
It was coated to a thickness of m=f=J and plated. This 1-layer adhesive composition was made by mixing equal weights of powders A, ], 203.5102, and Y203, heat-treating the mixture at 1500°C to vitrify it, and then pulverizing it into glass powder (hereinafter abbreviated as 1 glass). , S1. It is a mixture of powder C and kneaded with an appropriate amount of 5% MC water bath liquid added as an auxiliary agent for making a paste.
It had a chemical composition of 0225%, Y2O3 25%, and SiC25, and the amorphous phase in the active ingredient was 75%. this! After properly drying the fl compound coated area, hold the two SiC circular tubes vertically with the coated areas facing each other, and transfer the weight of the upper SiC circular tube to the lower SiC circular tube. The maximum temperature of 1500℃ [3
Both tubes were joined for 0 minutes. The thickness of the obtained first adhesive layer was 0.1 to 0.4 mm, 68% of the thickness was in an amorphous phase, and 95% of the metal oxide was in a glass phase. Is this joint 34 KL at room temperature? /+ran2. 1000
The strength and leakage amount of 21.7Kg/mm2 at ℃ are
The airtightness was required to be less than X 100-8at-cc/see (detection limit value based on helium leak detector).

Then, on the outside of this joint, the outer diameter l3 obtained in the same manner as above.
5mm, inner diameter 113mm, length 100+ml.

A SiC circular straight joint with an expansion coefficient of 4.5 x 1o-6/'C is arranged, and as shown in Fig. A suitable amount of s % MC aqueous solution was added to mullite powder to form a paste, which was then filled, and both ends were filled with A'i, 203 26 width, 5i0254.
%, glass powder to cure the chemical composition of Y2O340 (hereinafter abbreviated as glass B) 20 parts, metal; 31 powder 20 parts,
5% of applied to powder mixture consisting of mullite powder 60c$
A paste made by adding an MC aqueous solution was filled. This bonded body was then maintained at a maximum temperature of 1650° C. for 1 hour in a N2 atmosphere to form a second adhesive layer and a second adhesive layer.

Obtain'i1. The second adhesive layer is made of metal oxide and 1
The crystalline phase accounted for 95%. Almost all of the metal Si was still nitrided, and the presence of metal S1 was not recognized in X-ray qualitative analysis of the third adhesive layer. The expansion coefficient of the Murai I plate obtained under the same composition and treatment conditions as the second adhesive layer of this example was 5.6 x 1o-6/°C.

The conjugated body thus obtained and the same shape as the kneaded body (''-
A pair of SiC circular tubes are bonded using the above method only using the first adhesive layer, and the joined body is not bonded using the second adhesive layer and the third adhesive layer, and the joined body is held horizontally and supported at both ends. 'l Durability was evaluated by maintaining it in an oxidizing atmosphere at 300°C for 1000 hours. Although there was no increase in the amount of leakage in either case, the former had sufficient reinforcing strength with no sinking in the center part, whereas sinking of approximately 2 mm was observed in the latter fc.

Examples 2 to 5 The method shown in FIG. 4 was used instead of FIG. Example 1 except that the heat treatment conditions in Section 2-8 were as shown in the Softness and Softness Table.
A zygote was obtained in the same manner as above. The physical properties of the first and second adhesive layers of the obtained joined body are also shown in the table. Even after the same durability test as in Example 1, the bonded body obtained in Example 1 was
It had the same airtightness and reinforcement strength.

Comparative Example The method shown in Fig. 4 was used instead of Fig. 5, and the active ingredients of the second adhesive composition were a powder mixture consisting of 32 parts of metal Si powder, 62 parts of SiC powder, and 6 parts of My F2 powder. A joined body was obtained in the same manner as in Example 1 except for the above. This conjugate and the conjugate obtained in Example 1 were maintained in an acidic and arsenic atmosphere at 1,000°C for 1,000 hours to evaluate chemical stability.
is. Whereas the former joint had cracks in several places,
No equally significant changes were observed in the latter junction.

Further, a rectangular parallelepiped compact obtained by hydraulically pressing the active ingredient powder mixture of the second adhesive composition used in both bonded bodies was maintained at 1350°C for one period in an odor atmosphere. It was exposed to air at 1000° C. for 1000 hours, and the relationship between the elapsed time and volume increase was investigated. The results are shown in FIG. It is clear that the 2h difference in the joint of the joined body obtained in this example is due to the increase in the volume of the second adhesive layer.

[Brief explanation of the drawing]

1 to 3 are partially cutaway sectional views of the joint portion of the joined body of the present invention. 4 and 51 are both detailed explanatory diagrams of the present invention. FIG. 6 is a behavior diagram showing changes in the volume of the obtained/ζ second contact layer depending on the composition and heat treatment conditions, similar to Example 1 and Comparative Example. 1.2: Ceramic tube 3: Joint 4: First adhesive layer, 5: Second adhesive layer 6: First adhesive composition 7: Second adhesive composition 8: Third adhesive composition 2) ￥I 3) 'A 41n
Otama,

Claims (1)

[Claims] A joint of ceramic tubes formed by placing a joint made of ceramic along the joint portion of a pair of ceramic tubes. 5-50% c/) Sj, C and/or f: I Si
, 3 containing N4 and having an amorphous phase of more than 50% is interposed between the ceramic tubes. 4) Please confirm that there is a valve between the Hiramic pipe and the joint, and check the expansion coefficient of the second adhesive layer and the expansion coefficient of the joint. The absolute value of the difference is 1×1o
Joined body of ceramic tubes 4 characterized by a temperature smaller than -6/℃. The ceramic tube layer () and joints of “2-G.” are all made of Si.
0%, Si3N4'fl, SiC-Si3N4 composite matrix, or zyalonium. 3 First adhesive layer: 13 to 43% of XAl・IO3,
5io2; 114-40%, Group IIa and III
One or more oxides selected from group a elements, total 20 to 45, and SiC and/or (Si3N4)
Q-containing special ¥ + claim, box 1 or (2-mentioned, combined,) 4 second tangent M layer bear, A1, sl, M7, T1, Zr
5. Joining of a pair of ceramic tubes with a high fit and one side according to claim 1. In the method of joining ceramic pipes by aligning a joint made of ceramic along part G,
50-95% metal oxide and 5-50% 311″: and θ・/′ or fret )・], 3 N4 as an active ingredient.
Contains (2, and amorphous phase% +
A first adhesive composition having a higher Kso ratio is interposed between the ceramic tubes, and the maximum temperature is 1350 to 1650.
Heat treatment at the first temperature of °C! pfru:'fy-,,['
and a second adhesive composition containing more than 50 parts of a metal oxide as an effective component and having more than 50 parts of a crystalline phase as an active ingredient. A maximum of 11 objects are inserted between the ceramic tube and the joint, j'T+j
A second step of heat treatment at a lower second temperature. 6 The absolute value of the number of expanded fibers of the adhesive layer formed from the second adhesive composition in the second step and the expansion coefficient of the joint is 4.
X 1 o-", smaller than l'C! The joining method described in 5.
Claim 5 or 6 containing 15 to 30 types of one or more selected from Cz SiC and S 13 N4
Joining method described. 8. The first step is the joining method described in Chapter 12, Volume 5, 6, or 7, of applying force to the ceramic tubes in a direction that brings them closer together and heat-treating them. 9. The joining method according to claim 5.6.7 or 8, wherein the second step is heat-treated in a nitriding atmosphere.