Progress in the research on the scale effect of nuclear silicon carbide connection in Ningbo

Silicon carbide and its composite material (SiC and SiC based composite) is a new generation with its low neutron absorption cross section, good radiation resistance, high temperature stability and excellent corrosion and oxidation resistance. One of the candidates for accident-tolerant nuclear fuel cladding materials.
In practical applications, due to the intrinsic brittleness and non-deformability of ceramic materials, it is very difficult to manufacture silicon carbide components with complex shapes. The use of smaller-sized components to join large-sized complex shapes is to solve silicon carbide and its composite materials. One of the methods of processing difficult problems. At present, the connection methods of silicon carbide and its composite materials include brazing, diffusion bonding, glass phase connection, and transient eutectic phase connection. Electric current field assisted sintering technology (FAST) is an effective method for sintering high-density fine ceramics at low temperatures, and has been widely used in the field of ultra-high temperature ceramic sintering.
The Nuclear Energy Materials team of the Institute of Materials Technology and Engineering of the Chinese Academy of Sciences collaborated with the surface technology team to heat the samples through the FAST transient generated plasma bodies to achieve the connection of the silicon carbide blocks. It is found that under the condition of high current or high field strength, the connection of composite materials 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 material. Local heating results in a high temperature gradient near the interface, which promotes elemental diffusion and ultimately equilibrium, so the sample can be joined in a short period of time. This technology can effectively avoid high temperature damage in non-connected areas, and has important reference significance for nuclear SiC-based composite cladding.
Titanium silicon carbon (Ti3SiC2) has excellent high temperature and corrosion resistance, quasi-plasticity at high temperature, and its lattice parameter is very compatible with silicon carbide (6H-SiC: a=3.079, c=15.12; Ti3SiC2:a =3.068, c = 17.669), is one of the candidate materials for the silicon carbide and its composite solder layer. Recently, the nuclear energy team of Ningbo Materials Institute successfully used FAST technology to make Ti3SiC2 cast film as the intermediate layer to realize the connection of SiC ceramics and carbon fiber reinforced carbon composites (Journal of Nuclear Materials, 466 (2015): 322-327; Carbon, 102 (2016): 106-115). In addition to using Ti3SiC2 as the intermediate layer directly, it is also possible to use Ti as the intermediate layer to form the Ti3SiC2 phase in situ on the interface to achieve the connection. It has been reported in the literature that Ti foil is used as an intermediate layer to connect silicon carbide and its composite materials, but Ti-Si brittle phase is 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 coefficients of Ti5Si3 in the a-axis and c-axis directions are ac=5.98×10−6K, respectively. −1, cc=16.64×10−6 K−1, the ratio of the two can reach αc/αa is equal to 2.7), which will seriously weaken the mechanical properties of the joint. From the data in the literature, it was found that the thicker the intermediate layer Ti foil used, the easier it is to form a Ti-Si brittle phase at the interface, and the Ti foil used in the research has been mostly on the order of micrometers.
The Ningbo Materials Institute used physical vapor deposition (PVD) technology to control the thickness of Ti film on the surface of silicon carbide (100nm, 500nm, 1μm, 6μm), and connected the 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 joint interface. The results show that when a 1μm Ti film is used as the intermediate layer, the connection of silicon carbide can be achieved at a low temperature of 600 °C for 20 minutes, and the 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 matrix silicon carbide in the intermediate layer, because at the same connection temperature, the energy is constant, and the difference in concentration will be shaped. The effects of nuclear and grain growth kinetics. At the beginning of the reaction, the silicon carbide at the interface is decomposed into Si atoms and C atoms, and diffused into the intermediate layer Ti. Since the C atom radius is relatively small and the diffusion speed is fast, it will preferentially diffuse to the intermediate layer, and a TiC is formed on the interface. The influence of the thickness of the intermediate layer can be specifically divided into the following two cases:
(I) The nano-scale Ti film is relatively thin in the intermediate layer. The concentration of C atoms in the intermediate layer is relatively high, and the nucleation is dominant. Therefore, the TiC formed at the interface is dense, and this dense TiC acts as a diffusion. Barriers prevent Si atoms from continuing to diffuse from the SiC matrix side to the unreacted Ti film, thereby preventing the formation of such a brittle phase of Ti5Si3. As the connection temperature increases, the newly formed TiC, a small amount of Si atoms diffused before the formation of the dense TiC layer, and unreacted Ti atoms react to form the ternary compound Ti3SiC2, and the reaction equation is 2TiC+Ti+Si=Ti3SiC2. Therefore, when the nano-scale Ti film is used as the intermediate layer, the evolution order of the upper layer on the intermediate layer is Ti→TiC→Ti3SiC2.
(II) Micro- or sub-micron Ti film is relatively thick in the intermediate layer, the concentration of C atoms in the intermediate layer is reduced, and the nucleation is relatively small, so the TiC layer formed at the interface is not dense and decomposed from silicon carbide. The Si atoms coming out pass through this layer of unimpressed TiC and enter the intermediate layer to form Ti5Si3. As the connection temperature increases, the TiC and Ti5Si3 reacted and the diffused Si atoms react to form the ternary compound Ti3SiC2, and the reaction equation is 10TiC+Ti5Si3+2Si=5Ti3SiC2. Therefore, when the micro- or sub-micron Ti film is used as the intermediate layer, the evolution order of the phase on the intermediate layer is Ti→TiC+Ti5Si3→TiC+Ti3SiC2.
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, while Si atoms decomposed by Ti3SiC2 will diffuse to the interface and capture C atoms at the interface. A SiC phase or a Si-rich Si1+xC amorphous phase is formed thereon to achieve partial seamless connection.
The above research results show that in the traditional silicon carbide connection, by controlling the thickness of the original intermediate layer, the interfacial reaction between the intermediate layer and SiC and the interfacial phase composition can be controlled, thereby realizing the low-temperature fast and effective connection of SiC. The corrosion resistance, radiation resistance and high temperature resistance of the joint layer under irradiation conditions depend on the phase composition and distribution of the intermediate connection layer. Therefore, this study has important reference for the nuclear silicon carbide connection technology and is highly evaluated by the reviewer. Evaluation. The work has been published online in the international journal Journal of the European Ceramic Society.
The research was supported by the National Natural Science Foundation of China (NO.91226202, NO.91426304 and NO.51502310) and the Chinese Academy of Sciences Strategic Pilot Project (NO.XDA03010305).

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