Glass welding method for nuclear silicon carbide ceramic

Abstract

The invention relates to the technical field of nuclear energy materials and ceramic connection, and particularly discloses a glass welding method for nuclear silicon carbide ceramic. The glass welding method for the nuclear silicon carbide ceramic comprises the following steps: performing pretreatment and pre-oxidation treatment on the SiC ceramic to form a pre-oxidized silicon dioxide layer on a to-be-welded surface; the treated LYAS glass brazing filler metal is placed between the pre-oxidized silicon dioxide layers of the to-be-welded face, and a to-be-welded piece is obtained; and calcining the to-be-welded part, and then quickly cooling to obtain the brazed joint on which the nano split-phase glass is uniformly distributed. A uniform and stable nano split-phase glass structure is constructed by utilizing a modification effect of lithium oxide on a glass network, a silicon dioxide layer prepared by a pre-oxidation process and a rapid cooling process, so that reinforcement and toughening collaborative optimization of a brazed joint is realized; the technical problems that in an existing glass brazing technology, irradiation amorphization is likely to happen to a micro-crystallized weld joint, and the mechanical property and the service reliability of a brazed joint are insufficient are solved.

Description

Technical Field

[0001] This application relates to the fields of nuclear energy materials and ceramic joining technology, and in particular to a glass welding method for nuclear silicon carbide ceramics. Background Technology

[0002] Silicon carbide (SiC) ceramics and their composites, with their excellent comprehensive properties of high temperature resistance, radiation resistance, and corrosion resistance, have become key candidate structural materials for advanced nuclear energy systems such as fourth-generation nuclear reactors and fusion reactors. Their engineering applications are an important direction for the research and development of advanced nuclear energy equipment. The large-scale fabrication of silicon carbide components relies heavily on reliable ceramic joining technology; therefore, the development of high-performance silicon carbide joining processes has become a key research focus and technological challenge in this field.

[0003] Currently, various methods for joining silicon carbide ceramics have been developed. Among them, glass brazing technology has become one of the mainstream technologies for joining silicon carbide ceramics for nuclear applications because it can achieve effective joining of silicon carbide in a pressureless, air-atmospheric environment, has strong process adaptability, and possesses irreplaceable technical advantages in the preparation of large and complex silicon carbide structural components. However, the weld formed by glass brazing has the problem of high intrinsic brittleness, which significantly restricts the mechanical properties and service reliability of the brazed joint. To solve this problem, existing technologies generally use microcrystallization treatment to modify the glass brazing filler metal, thereby achieving a synergistic improvement in the strength and toughness of the brazed joint by precipitating microcrystalline phases in the glass matrix.

[0004] However, under the irradiation service environment of nuclear energy systems, glass welds treated with microcrystallization exhibit significant application drawbacks: the crystalline phases in microcrystallized glass are prone to irradiation-induced amorphization transformation. This transformation causes weld volume expansion, leading to internal microcracks and even macroscopic cracking. This results in a sharp deterioration in the mechanical properties of the brazed joint, severely reducing its service stability and lifespan in a nuclear environment. This has become a major technical bottleneck for the large-scale application of glass brazing technology in the fabrication of nuclear silicon carbide components. Therefore, developing a silicon carbide ceramic joining technology that can effectively solve the problem of irradiation-induced amorphization failure of glass welds while simultaneously improving the overall mechanical properties and service reliability of brazed joints in a nuclear environment is of great significance for promoting the engineering application of silicon carbide ceramics and their composites in advanced nuclear energy systems. Summary of the Invention

[0005] In view of this, the purpose of this application is to provide a glass welding method for nuclear silicon carbide ceramics, which solves the technical problems of microcrystalline welds being prone to irradiation amorphization and insufficient mechanical properties and service reliability of brazed joints in existing glass brazing technology.

[0006] To achieve the above technical objectives, this application provides a glass welding method for nuclear silicon carbide ceramics, comprising the following steps:

[0007] Step S1: The SiC ceramic surface to be soldered is pretreated and pre-oxidized sequentially to form a pre-oxidized silicon dioxide layer on the surface to be soldered;

[0008] Step S2: Place the shaped LYAS glass solder between the pre-oxidized silicon dioxide layers on the SiC ceramic surface to be soldered to obtain the workpiece to be soldered;

[0009] Step S3: The workpiece to be welded is calcined, then rapidly cooled to 1000°C at a cooling rate of more than 10°C / min, and then slowly cooled to room temperature to obtain a brazed joint with uniformly distributed nano-phase glass.

[0010] Furthermore, LYAS glass solder is prepared by mixing basic glass powder and Li2O; wherein, the mass fraction of silicon dioxide in the basic glass powder is 46~48%; and the amount of Li2O added is 1%~3% of the mass of the basic glass powder.

[0011] Furthermore, the basic glass powder is composed of SiO2, Al2O3 and Y2O3, with a mass ratio of 45~50:15~20:30~35.

[0012] Furthermore, the thickness of the pre-oxidized silica layer is 0.5 μm to 3 μm.

[0013] Further, the pretreatment involves grinding the SiC ceramic surface to be welded using a 1000# diamond grinding wheel, followed by cleaning and drying of the SiC ceramic.

[0014] Furthermore, the pre-oxidation treatment is: sintering at 1300℃~1400℃ for 0.5~2h.

[0015] Furthermore, the process parameters for calcination are as follows: heating to 1430℃~1480℃ at a heating rate of 10℃ / min, and holding at that temperature for 5~30min.

[0016] Furthermore, the rapid cooling process involves rapidly cooling to 1000°C at a cooling rate of 20-30°C / min, then cooling to 500°C at a cooling rate of 5°C / min, and finally cooling to room temperature with the furnace.

[0017] Furthermore, the preparation method of LYAS glass brazing filler metal is as follows: SiO2, Al2O3 and Y2O3 are mixed evenly to obtain basic glass powder; Li2O is added to the basic glass powder and mixed evenly to obtain LYAS glass powder; the LYAS glass powder is ball-milled, melted and cooled in sequence to obtain glass blocks; the glass blocks are crushed and then ball-milled and dried to obtain LYAS glass brazing filler metal.

[0018] Further, the shaping process involves pressing the LYAS glass solder into sheets or using an adhesive to prepare it into a paste.

[0019] In summary, this application provides a glass welding method for nuclear silicon carbide ceramics, comprising the following steps: pre-treating the SiC ceramic surface to be welded, followed by pre-oxidation treatment to form a pre-oxidized silica layer on the surface to be welded; placing the shaped LYAS glass brazing filler metal between the pre-oxidized silica layers on the SiC ceramic surface to be welded, thereby obtaining a workpiece to be welded; calcining the workpiece to be welded, followed by rapid cooling to 1000°C at a cooling rate greater than 10°C / min, and then slow cooling to room temperature, thereby obtaining a brazed joint with uniformly distributed nano-phase glass. This application utilizes the modification effect of lithium oxide on the glass network to induce the spontaneous precipitation of nano-spherical quartz glass phase from the substrate phase above the softening temperature of LYAS glass brazing filler metal. Furthermore, by pre-oxidizing the SiC ceramic surface to prepare a silicon dioxide layer of a specific thickness and combining it with a rapid cooling process, a uniform and stable nano-phase glass structure was successfully constructed in the brazed joint. The shear strength of the brazed joint can reach 129.8±16.4MPa, realizing the synergistic optimization of reinforcement and toughening of the brazed joint, and significantly improving the service reliability and service life of the brazed joint in a nuclear environment.

[0020] Compared with existing technologies, the brazed joints prepared by this method have a strength increase of up to 44%, and their resistance to radiation swelling and radiation stability are significantly improved, effectively avoiding the failure problem of brazed joints caused by radiation amorphization. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 The images show the microstructure of the brazed joint prepared in Example 1 of this application; where a) is the overall microstructure of the brazed joint; b) is the cross-sectional morphology of the pre-oxidized silica layer; and c) is the interface microstructure of the brazed joint.

[0023] Figure 2 This is a microstructure diagram of the phase-separated glass in the brazed joint prepared in Example 1 of this application;

[0024] Figure 3 This is a microstructure diagram of the brazed joint prepared in Comparative Example 1 of this application;

[0025] Figure 4This is a microstructure diagram of the brazed joint prepared in Comparative Example 2 of this application;

[0026] Figure 5 This is a microstructure diagram of the brazed joint prepared in Comparative Example 3 of this application;

[0027] Figure 6 The images shown are backscattered electron images of the brazed joint prepared in Comparative Example 4 of this application; wherein, a) is a microstructure image of the brazed joint; b) is a microstructure image of the interface containing nanoparticles; and c) is a microstructure image of the interface containing silica crystals. Detailed Implementation

[0028] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments in this application specification, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection claimed in this application.

[0029] In the description of this application, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," indicating orientation or positional relationships, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0030] Unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0031] The raw materials used in this invention are not particularly restricted in their source; they can be purchased on the market or prepared using conventional methods known to those skilled in the art.

[0032] This application provides a glass welding method for nuclear silicon carbide ceramics, comprising the following steps:

[0033] Step S1: The SiC ceramic surface to be soldered is pretreated and pre-oxidized sequentially to form a pre-oxidized silicon dioxide layer on the surface to be soldered;

[0034] Step S2: Place the shaped LYAS glass solder between the pre-oxidized silicon dioxide layers on the SiC ceramic surface to be soldered to obtain the workpiece to be soldered;

[0035] Step S3: The workpiece to be welded is calcined, then rapidly cooled to 1000°C at a cooling rate of more than 10°C / min, and then slowly cooled to room temperature to obtain a brazed joint with uniformly distributed nano-phase glass.

[0036] It should be noted that, under conditions above the softening temperature, lithium oxide-modified yttrium aluminum silicon glass, due to the modification of the glass network by lithium ions, spontaneously precipitates glassy silica nanoparticles from the substrate glass. In this phase-separated glass, the silica nanoparticles are hard and brittle, while the lithium-rich glass matrix with a migratory network structure possesses a certain degree of plastic flow capability. This "rigid-plastic" composite structure significantly improves the indentation fracture toughness of the phase-separated glass. By pre-oxidizing the silicon carbide surface to prepare a silica layer of appropriate thickness, and combining it with a composite process of high-temperature rapid cooling and medium-low-temperature slow cooling, the crystallization behavior of the glass during brazing was successfully suppressed, while achieving sufficient phase separation of the weld glass, thus preparing a fully nano-phase-separated glass-type silicon carbide brazed joint.

[0037] The addition of lithium oxide plays a crucial regulatory role in achieving this phase separation process because the network structure of the basic glass powder is mainly composed of silicon-oxygen-silicon and silicon-oxygen-aluminum bonding units. The introduced lithium ions, possessing a high ionic potential (ionic charge / ionic radius ratio ≈ 1.28), participate in glass structure reconstruction as a glass network modifier; they competitively attract oxygen ions, triggering depolymerization reactions in the glass network. This process introduces structural inhomogeneity into the glass system: the chemical bonding modes and network connectivity differ significantly between lithium-modified and unmodified regions, leading to an increase in the overall Gibbs free energy of the system. When the lithium ion content exceeds a certain threshold, the system gains sufficient driving force for phase separation, causing the glass to spontaneously separate into a lithium-rich glass phase and interconnected SiO4 tetrahedral clusters forming a silicate glass phase.

[0038] The spherical nano-quartz precipitated during phase separation has an amorphous SiO2 chemical composition and possesses excellent mechanical and thermal properties, including high hardness, high elastic modulus, high strength, and extremely low coefficient of thermal expansion. When it is uniformly dispersed in the glass matrix, it can achieve a synergistic improvement in weld strength and toughness through dispersion strengthening, effectively improving the inherent brittleness of traditional glass welds. In addition, compared with the crystalline SiO2 contained in the microcrystalline modified welds in the prior art, the nano-quartz formed during the phase separation process of this application has a glassy structure. Its resistance to radiation swelling and radiation structural stability are significantly better than crystalline SiO2, which can effectively avoid failure problems such as weld volume swelling and cracking caused by radiation-induced amorphization, thereby greatly improving the long-term reliability and service life of brazed joints in nuclear irradiation service environments.

[0039] Pre-oxidation treatment of the SiC ceramic surface to be soldered serves a dual purpose: 1) Promoting the wetting and spreading of glass brazing filler metal on the SiC surface. This soldering process is conducted entirely in an air atmosphere. Without pre-oxidation, although a thin oxide layer will naturally form on the SiC ceramic surface, this oxide layer dissolves in the glass brazing filler metal, causing direct contact and reaction between the SiC ceramic and the filler metal. This reaction generates bubbles, leading to increased weld porosity defects and hindering the spreading and wetting of the glass brazing filler metal on the SiC ceramic surface, thus affecting the quality of the brazed joint; 2) Regulating the Si content of the glass system and promoting glass phase separation. Glass phase separation mainly occurs in high-Si content regions. The slow dissolution of the pre-oxidized layer during soldering gradually increases the Si content of the glass brazing filler metal, thereby providing a sufficient compositional basis for spontaneous phase separation of the glass. It should be emphasized that if the phase separation is promoted by directly increasing the Si content of the glass brazing filler metal, the technical effect of this application cannot be achieved. The core reason is that an excessively high Si content will significantly increase the viscosity and softening temperature of the glass brazing filler metal, causing the glass brazing filler metal to fail to achieve good wetting and spreading on the SiC ceramic surface after melting, ultimately affecting the connection performance of the brazed joint.

[0040] The rapid cooling process following the calcination and holding stage serves to suppress the precipitation and growth of crystalline phases in the weld. If the glass brazing filler metal remains in the weld in a molten state for too long, a large amount of Si from the SiC ceramic will dissolve into the molten glass. Simultaneously, the SiC ceramic interface will provide favorable nucleation sites for the SiO2 crystalline phase, leading to its nucleation and growth. During this growth, the Si from the surrounding area will be consumed, causing the local glass composition to deviate from the high-Si phase separation region, ultimately hindering the smooth progress of glass phase separation. It is worth noting that the rapid cooling process used in this application, as verified by experiments, does not increase the residual stress in the brazed joint, thus balancing phase separation effectiveness with the structural stability of the brazed joint.

[0041] In some embodiments, the LYAS glass solder is prepared by mixing a base glass powder and Li2O; wherein, the mass fraction of silicon dioxide in the base glass powder is 46-48%; and the amount of Li2O added is 1%-3% of the mass of the base glass powder.

[0042] It should be noted that the base glass powder of the LYAS glass used in this method mainly includes four elements: Si, Al, Y, and O. These elements have a small neutron absorption cross section, giving the glass excellent resistance to neutron irradiation. In this embodiment, the amount of Li2O added is small, and its main function is as an outer layer of the glass network to reduce the softening temperature and viscosity of the glass, thereby promoting phase separation. However, if the Li2O content is too low, a large number of crystalline phases will easily be generated during the welding process, and a nano-phase glass structure cannot be formed at the weld. If the Li2O content is too high, the softening temperature of the glass will be significantly reduced, resulting in the mechanical properties of the prepared brazed joint failing to meet the service requirements in a nuclear environment. It is worth noting that Li-7 has an extremely low neutron absorption cross section (only 0.033 target) and a high abundance in nature (approximately 92.5%), therefore, this LYAS glass brazing filler metal is suitable for use in nuclear service environments.

[0043] In some embodiments, the base glass powder is composed of SiO2, Al2O3, and Y2O3, with a mass ratio of 45~50:15~20:30~35. In some specific embodiments, the mass ratio of SiO2, Al2O3, and Y2O3 is 47.83:18.97:33.20 or 48:19:33.

[0044] In some embodiments, the thickness of the pre-oxidized silicon dioxide layer is 0.5 μm to 3 μm.

[0045] In some embodiments, the pretreatment is as follows: the SiC ceramic surface to be welded is polished with a 1000# diamond grinding wheel, and then the SiC ceramic is cleaned and dried in sequence.

[0046] In some embodiments, the pre-oxidation treatment is: sintering at 1300℃~1400℃ for 0.5~2h.

[0047] It should be noted that the core purpose of SiC ceramic pre-oxidation is to generate a dense SiO2 glass film on its surface in situ. This SiO2 glass film can significantly improve the wetting compatibility between the glass solder and the SiC ceramic surface, promoting uniform solder spreading. On the other hand, this film can slowly dissolve in the molten glass solder during welding, continuously increasing the Si content in the weld area, providing stable compositional support for glass phase separation, and thus ensuring the uniform formation of the nano-phase glass structure. SiC ceramic pre-oxidation is currently the simplest, most efficient, and cost-effective technique to achieve the above objectives. Other techniques cannot achieve the same results: for example, SiC ceramics are almost non-conductive at room temperature, making it impossible to prepare oxide films using arc oxidation technology; although magnetron sputtering and other coating technologies can deposit oxide films on the surface of SiC ceramics, the adhesion between the prepared film and the SiC substrate is weak, and it is very easy to fall off in the high-temperature environment of subsequent air atmosphere welding. Moreover, such coating technologies have the disadvantages of low efficiency and high equipment costs, making them unsuitable for large-scale production needs.

[0048] In some embodiments, the process parameters for calcination are: heating to 1430℃~1480℃ at a heating rate of 10℃ / min, and holding at that temperature for 5~30min.

[0049] In some embodiments, the rapid cooling step is as follows: rapidly cooling to 1000°C at a cooling rate of 20~30°C / min, then cooling to 500°C at a cooling rate of 5°C / min, and finally cooling to room temperature with the furnace.

[0050] It should be noted that the core function of rapid cooling is to suppress the precipitation and growth of crystalline phases in the weld. If the glass brazing filler metal remains in the weld in a molten state for too long, a large amount of Si in the SiC ceramic will dissolve into the molten glass. At the same time, the SiC ceramic interface will provide favorable sites for the nucleation of SiO2 crystalline phases, thereby initiating the nucleation and growth of SiO2 crystalline phases. During the growth of crystalline phases, the Si elements in the surrounding area will be consumed, causing the local glass composition to deviate from the high-Si phase separation region, ultimately hindering the smooth progress of glass phase separation. It is worth noting that the rapid cooling process adopted in this application has been experimentally verified not to increase the residual stress of the brazed joint, and can balance the phase separation effect with the structural stability of the brazed joint.

[0051] In some embodiments, the preparation method of LYAS glass brazing filler metal is as follows: SiO2, Al2O3 and Y2O3 are mixed evenly to obtain basic glass powder; Li2O is added to the basic glass powder and mixed evenly to obtain LYAS glass powder; the LYAS glass powder is ball-milled, melted and cooled in sequence to obtain glass blocks; the glass blocks are crushed and then ball-milled and dried to obtain LYAS glass brazing filler metal.

[0052] In some embodiments, the shaping process involves pressing the LYAS glass solder into sheets or using an adhesive to prepare it into a paste.

[0053] The applicant further provides the following specific embodiments to describe the present invention. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0054] Example 1

[0055] This embodiment provides a glass welding method for nuclear silicon carbide ceramics, including the following steps:

[0056] Step S1, prepare SiO2-Al2O3-Y2O3-Li2O (hereinafter referred to as LYAS) glass solder:

[0057] SiO2, Al2O3, and Y2O3 were mixed in a mass ratio of 48:19:33 to prepare basic glass powder. 1% Li2O was added to the basic glass powder to obtain LYAS glass powder. The LYAS glass powder was ball-milled for 2 hours. The ball-milled LYAS glass powder was placed in an alumina crucible and then placed in a high-temperature muffle furnace and melted at 1650℃ for 2 hours to obtain molten glass. The molten glass was quickly poured into water to obtain glass blocks. The glass blocks were crushed and then ball-milled. The ball mill speed was set to 200 rpm and the ball milling time was 1 hour. After ball milling, the glass was dried at 70℃ to obtain LYAS glass brazing filler metal with a median particle size of 34.82 μm.

[0058] Step S2, glass brazing:

[0059] The SiC ceramic surface to be soldered was polished with a 1000# diamond grinding wheel, and then ultrasonically cleaned with anhydrous ethanol for 5 min. The cleaned SiC ceramic was placed in a muffle furnace and sintered at 1400℃ for 1 hour to form a 2μm thick oxide layer on the surface of the SiC ceramic. This oxide layer is pre-oxidized silicon dioxide. The LYAS glass brazing filler metal was pressed into sheets to obtain LYAS sheets with a thickness of 100μm. The LYAS sheets were then placed between the pre-oxidized SiC ceramics to form the solder joint. The solder joint was placed in a muffle furnace and heated to 1460℃ at 10℃ / min, held for 10 min, then cooled to 1000℃ at 20℃ / min, and then cooled to 500℃ at 5℃ / min. The solder joint was then cooled with the furnace to obtain a brazed joint with uniformly distributed nano-phase glass.

[0060] In this embodiment, the brazed joint forms a uniformly distributed nanophase glass with a fracture toughness of 1.07 MPa·m. 1 / 2 The shear strength of the brazed joint reaches 121 MPa.

[0061] Figure 1 The images show the microstructure of the brazed joint prepared in Example 1; where a) is the overall microstructure of the brazed joint, with the weld completely transformed into a nano-phase glass; b) is a cross-sectional morphology of the pre-oxidized silica layer, where a 2 μm thick silica layer is formed on the surface of the silicon carbide substrate after pre-oxidation treatment; c) is a magnified view of the interface microstructure of the brazed joint, where silica nanoparticles with a particle size of 100–300 nm are uniformly distributed from the silicon carbide interface to the center of the weld.

[0062] Figure 2 Microstructure of the mid-phase glass of the brazed joint prepared in Example 1; wherein, a) is a bright-field image of the glass, and the inset is a selected area electron diffraction pattern of the marked area; b) is a scanning transmission electron image and the corresponding elemental plane distribution spectrum; to determine the phase characteristics of the precipitated nanoparticles in the glass phase, the micro-region was sampled using a focused ion beam and characterized by transmission electron microscopy; bright-field image ( Figure 2 As shown in a), a clear and tightly bonded interface is formed between the nanoparticles and the glass matrix. When the nanoparticles are individually distributed, they are nearly spherical; when they are close together, they deform into ellipsoids. This distribution characteristic indicates that the interfacial energy between the nanoparticles and the matrix is ​​extremely low, making them difficult to aggregate and grow. The elemental distribution results ( Figure 2 b) shows that the nanoparticles are rich in silicon and have low contents of yttrium and aluminum. Selected area electron diffraction analysis did not detect clear diffraction spots, indicating that both the nanoparticles and the glass matrix are glass phases.

[0063] Example 2

[0064] This embodiment provides a glass welding method for nuclear silicon carbide ceramics. The difference from Embodiment 1 is that the amount of Li2O added to the LYAS glass brazing filler metal is 3% of the mass of the base glass powder.

[0065] Comparative Example 1

[0066] This comparative example provides a glass welding method for nuclear silicon carbide ceramics. The difference from Example 1 is that the composition ratio of the base glass powder is different, with the mass ratio of SiO2, Al2O3 and Y2O3 being 51:18:31. Under this mass ratio, the silicon content in the base glass powder is increased by 6.25% compared to Example 1.

[0067] The fracture toughness of the brazed joint obtained in this comparative example is 0.75 MPa·m. 1 / 2 The shear strength was 72.5 MPa. The weld morphology of the brazed joint prepared in this comparative example was characterized, and the results are shown in [reference needed]. Figure 3 .Depend on Figure 3It is known that if the glass phase separation is promoted by directly increasing the Si content in the glass brazing filler metal, the technical effect of this application cannot be achieved. The reason is that an excessively high Si content will significantly increase the viscosity and softening temperature of the glass brazing filler metal, resulting in poor fluidity of the glass brazing filler metal in the molten state. It will be unable to achieve good wetting and spreading on the SiC ceramic surface to be welded, ultimately resulting in poor weld formation quality and no uniform nano-phase structure formation.

[0068] Comparative Example 2

[0069] This comparative example provides a glass welding method for nuclear silicon carbide ceramics, which differs from Example 1 in that the amount of Li2O added to the LYAS glass solder is 0% of the mass of the base glass powder.

[0070] The fracture toughness of the brazed joint obtained in this comparative example is 0.8 MPa·m. 1 / 2 The shear strength was 64.6 MPa. The weld morphology of the brazed joint prepared in this comparative example was characterized; the results are shown in [reference needed]. Figure 4 .Depend on Figure 4 It is known that if the Li2O content is too low, the glass brazing filler metal is prone to generating a large number of crystalline phases during the welding process, and the weld seam cannot form a nano-phase glass structure. This results in insufficient mechanical stability of the prepared SiC brazed joint under high-temperature service environment, which cannot meet the service requirements of nuclear environment.

[0071] Comparative Example 3

[0072] This comparative example provides a glass welding method for nuclear silicon carbide ceramics. The difference from Example 1 is that the SiC ceramic surface to be welded is not sintered, and there is no pre-oxidized silicon dioxide with a thickness of 0.5μm to 3μm on its surface.

[0073] The fracture toughness of the brazed joint obtained in this comparative example is 0.89 MPa·m. 1 / 2 The shear strength was 82.3 MPa. The morphology of the brazed joint prepared in this comparative example was characterized; the results are shown in [reference needed]. Figure 5 .Depend on Figure 5 It can be seen that the interior of the weld is mainly composed of a homogeneous glassy phase, with crystalline SiO2 forming in local areas. No nanophase separation of the glassy phase was observed. This experimental result further confirms that pre-oxidation treatment of the SiC ceramic surface to be welded is a necessary condition for achieving nanophase separation in the weld. Brazed joints with nanophase separation cannot be obtained after welding SiC ceramics that have not been pre-oxidized.

[0074] Comparative Example 4

[0075] This comparative example provides a glass welding method for nuclear silicon carbide ceramics. The difference from Example 1 is that the cooling process of the workpiece to be welded is different. Specifically, the workpiece is cooled to 500°C at a cooling rate of 5°C / min, and then cooled with the furnace.

[0076] The fracture toughness of the brazed joint obtained in this comparative example is 0.85 MPa·m. 1 / 2 The shear strength was 80.5 MPa. Backscattered electron imaging was used to characterize the brazed joint prepared in this comparative example. Figure 6 In the brazed joint, the white phase is β-yttrium disilicide, a high-temperature stable phase formed by the phase transformation of γ-yttrium disilicide at around 1200℃, and the dark phase is silicon dioxide. A large amount of silicon dioxide crystalline phase is distributed at the silicon carbide interface in the brazed joint. From c), it can be observed that under residual stress, cracks propagate along the silicon dioxide crystalline phase. The matrix of the brazed joint is a glassy phase. Detailed observation revealed a large number of dark-colored spherical nanoparticles (such as...) within the glassy matrix of the weld. Figure 6 As shown in a) and b) above, the particle size is approximately 100–400 nm, and the distribution is uneven, with no nanoparticles present in the region near the silica crystalline phase. Therefore, from Figure 6 It is known that the core function of rapid cooling is to suppress the precipitation and growth of crystalline phases in the weld: if the glass brazing filler metal remains in the weld in a molten state for too long, a large amount of Si element in the SiC ceramic will dissolve in the molten glass. At the same time, the SiC ceramic interface will provide favorable sites for the nucleation of SiO2 crystalline phase, which will eventually lead to the nucleation and growth of SiO2 crystalline phase. During the growth of crystalline phase, the Si element in the surrounding area will be consumed, causing the local glass composition to deviate from the high Si phase separation region, which will ultimately hinder the smooth progress of glass phase separation.

[0077] The above are merely preferred embodiments of this application and are not intended to limit the present invention. Although this application has been described in detail with reference to examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. However, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for glass welding of nuclear silicon carbide ceramics, comprising the following steps: Step S1: The SiC ceramic surface to be soldered is pretreated and pre-oxidized sequentially to form a pre-oxidized silicon dioxide layer on the surface to be soldered; Step S2: Place the shaped LYAS glass solder between the pre-oxidized silicon dioxide layers on the SiC ceramic surface to be soldered to obtain the workpiece to be soldered; Step S3: The workpiece to be welded is calcined, then rapidly cooled to 1000°C at a cooling rate of more than 15°C / min, and then slowly cooled to room temperature to obtain a brazed joint with uniformly distributed nano-phase glass.

2. The glass welding method for nuclear silicon carbide ceramics according to claim 1, characterized in that, The LYAS glass solder is prepared by mixing basic glass powder and Li2O; The base glass powder contains 46-48% silica by mass and 1%-3% Li2O by mass of the base glass powder.

3. The glass welding method for nuclear silicon carbide ceramics according to claim 2, characterized in that, The basic glass powder is composed of SiO2, Al2O3 and Y2O3, and the mass ratio of SiO2, Al2O3 and Y2O3 is 45~50:15~20:30~35.

4. The glass welding method according to claim 1, characterized in that, The thickness of the pre-oxidized silicon dioxide layer is 0.5 μm to 3 μm.

5. The glass welding method for nuclear silicon carbide ceramics according to claim 1, characterized in that, The pretreatment involves grinding the SiC ceramic surface to be welded using a 1000# diamond grinding wheel, followed by cleaning and drying of the SiC ceramic.

6. The glass welding method for nuclear silicon carbide ceramics according to claim 1, characterized in that, The pre-oxidation treatment is: sintering at 1300℃~1400℃ for 0.5~2h.

7. The glass welding method for nuclear silicon carbide ceramics according to claim 1, characterized in that, The process parameters for the calcination treatment are as follows: heating to 1430℃~1480℃ at a heating rate of 10℃ / min, and holding at that temperature for 5~30min.

8. The glass welding method for nuclear silicon carbide ceramics according to claim 1, characterized in that, The rapid cooling process is as follows: rapidly cool to 1000°C at a cooling rate of 20~30°C / min, then cool to 500°C at a cooling rate of 5°C / min, and finally cool to room temperature with the furnace.

9. The glass welding method for nuclear silicon carbide ceramics according to claim 1, characterized in that, The preparation method of the LYAS glass brazing filler metal is as follows: SiO2, Al2O3 and Y2O3 are mixed evenly to obtain basic glass powder; Li2O is added to the basic glass powder and mixed evenly to obtain LYAS glass powder; the LYAS glass powder is ball-milled, melted and cooled in sequence to obtain glass blocks; the glass blocks are crushed and then ball-milled and dried to obtain LYAS glass brazing filler metal.

10. The glass welding method for nuclear silicon carbide ceramics according to claim 9, characterized in that, The shaping process involves pressing the LYAS glass solder into sheets or using an adhesive to prepare it into a paste.

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