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Silicon carbide and its composite material (SiC and SiC based composite) has become a new generation with its low neutron absorption cross section, good radiation resistance, high temperature stability, and excellent resistance to corrosion and oxidation. One of the candidates for accident-tolerant nuclear fuel cladding materials.
In practical applications, because of the inherent brittleness and non-deformability of ceramic materials, it is very difficult to manufacture silicon carbide components with complicated shapes. The use of smaller-sized components to connect large-sized and complex-shaped devices is to solve silicon carbide and its composite materials. One of the ways to deal with difficult problems. At present, the connection methods of silicon carbide and its composite materials include brazing, diffusion bonding, glass bonding, and transient eutectic bonding. Electric field assisted sintering technology (FAST) is an effective method for sintering high-density dense ceramics at low temperatures, and has been widely used in ultra-high temperature ceramic sintering and other fields.
The Nuclear Materials Group of the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, teamed up with the Surface Technology team to heat the sample through a plasma sample created by FAST transients to achieve the connection of silicon carbide blocks. It is found that under high current or high field strength conditions, the connection of composites has a local sintering mechanism, and the temperature distribution is limited to the vicinity of the interface, which can effectively control the heat-affected zone and protect the body material. Local heating results in a very high temperature gradient near the interface, which promotes element diffusion and eventually reaches equilibrium, so that the sample can be connected in a very short time. The technology can effectively avoid the high-temperature damage in the non-connected region, and has important reference significance for the nuclear silicon carbide-based composite cladding.
Titanium-silicon-carbon (Ti3SiC2) has excellent high temperature and corrosion resistance, is quasi-plastic at high temperatures, and its lattice parameters are very compatible with silicon carbide (6H-SiC:a=3.079, c=15.12; Ti3SiC2:a =3.068,c=17.669), which is one of the candidate materials for the welding layer of silicon carbide and its composite materials. Recently, Ningbo Nuclear Material Group successfully used FAST technology to make Ti3SiC2 cast film as an intermediate layer to achieve the connection of silicon carbide ceramics and its carbon fiber reinforced carbon composites (Journal of Nuclear Materials, 466 (2015): 322-327; Carbon, 102(2016):106-115). In addition to using Ti3SiC2 directly as an intermediate layer, it is also possible to use Ti as an intermediate layer to generate the Ti3SiC2 phase in-situ at the interface and achieve the connection. It has been reported in the literature that Ti foil is used as an interlayer to connect silicon carbide and its composite materials, but Ti-Si brittle phases are formed in the reaction zone. The Ti-Si brittle phase is easily amorphized under neutron irradiation conditions, and its thermal expansion coefficient anisotropy is very obvious (for example, the thermal expansion coefficient of Ti5Si3 in the a-axis and c-axis directions is ac =5.98×10−6, respectively. K−1, cc =16.64×10−6 K−1, the ratio between the two can reach αc/αa≅2.7), which can seriously weaken the mechanical properties of the joint. By studying the data in the literature, it is found that the thicker the Ti foil used in the intermediate layer, the easier to form the Ti-Si brittle phase at the interface, and the Ti foils used in the existing studies are mostly on the micrometer scale.
The Ningbo Material Institute team used Physical Vapor Deposition (PVD) technology to control the thickness of Ti film on the surface of silicon carbide (100nm, 500nm, 1μm, 6μm) and realized the connection of silicon carbide with FAST technology. Studies have shown that the thickness of the intermediate layer has an important influence on the phase composition and mechanical properties of the interface. The results show that when a 1 μm Ti film is used as the intermediate layer, silicon carbide can be connected at a low temperature of 600° C. for 20 minutes, and its four-point bending strength can reach 169.7 (±37.5) MPa. In-depth mechanistic studies have shown that the thickness of the intermediate layer determines the concentration of Si atoms and C atoms diffused from the silicon carbide substrate in the intermediate layer, because at the same connection temperature, the energy is constant, and the difference in concentration will shape the Nuclear and grain growth kinetics have an effect. In the initial stage of the reaction, the silicon carbide on the interface decomposes into Si atoms and C atoms, and diffuses into the intermediate layer Ti. Due to the relatively small radius of C atoms and fast diffusion, it preferentially diffuses into the intermediate layer and forms a layer of TiC at the interface. The influence of the thickness of the intermediate layer can be specifically divided into the following two cases:
(I) Nanoscale Ti film as interlayer
The middle layer is relatively thin, the concentration of C atoms in the middle layer is relatively high, nucleation dominates, so the TiC formed at the interface is dense, and this dense TiC as a diffusion barrier will prevent Si atoms from continuing from the side of the SiC matrix. Diffusion to the unreacted Ti film prevents the formation of a brittle phase such as Ti5Si3. As the connection temperature increases, newly formed TiC, a small amount of Si atoms diffused in before the formation of a dense TiC layer, and unreacted Ti atoms react to form a ternary compound Ti3SiC2 with a reaction equation of 2TiC+Ti+Si=Ti3SiC2. Therefore, when the nano-scale Ti film is used as an intermediate layer with the increase of the connection temperature, the order of the phases in the intermediate layer is: Ti→TiC→Ti3SiC2.
(II) Microlayer or submicron Ti film as intermediate layer
The intermediate layer is thicker, the concentration of C atoms in the intermediate layer decreases, and the nucleation is relatively small, so the TiC layer formed at the interface is not dense, and the Si atoms decomposed from the silicon carbide enter through the layer of TiC that is not dense. Ti5Si3 is formed in the intermediate layer. With the increase of connection temperature, TiC reacts with Ti5Si3 and the diffused Si atoms to form ternary compound Ti3SiC2. The reaction equation is 10TiC+Ti5Si3+2Si=5Ti3SiC2. Therefore, when the micron or sub-micrometer Ti film is used as an intermediate layer, the order of the phases in the intermediate layer is: Ti→TiC+Ti5Si3→TiC+Ti3SiC2 as the connection temperature increases.
When the temperature is increased to 1500°C or higher, a small amount of Ti3SiC2 will decompose to form TiC:Ti3SiC2→Si(g)↑+3TiC0.67. Simultaneously, Si atoms decomposed by Ti3SiC2 will diffuse toward the interface and trap C atoms at the interface. Formed on the SiC phase or Si-rich Si1 + xC amorphous phase, to achieve partial seamless connection.
The above research results show that by controlling the thickness of the original intermediate layer in the connection of the traditional silicon carbide, the interfacial reaction between the intermediate layer and SiC and the interface phase composition can be controlled, thereby realizing the fast and effective connection of SiC at a low temperature. The corrosion resistance, radiation resistance, and high temperature resistance of the connection layer under irradiation conditions depend on the phase composition and distribution of the intermediate connection layer. Therefore, this research has important reference functions for nuclear silicon carbide connection technology and is highly recommended by reviewers. Evaluation. This work has been published online in the international journal "Journal of the European Ceramic Society".
The study was supported by the National Natural Science Foundation of China (NO.91226202, NO.91426304 and NO.51502310) and the Chinese Academy of Sciences' Strategic Initiative (NO.XDA03010305).
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