Design,Fabrication And Thermal Conductivity Investigation For Diamond/W Composites With Doped Diamond

Tungsten(W)is regarded as the most promising Plasma Facing Materials(PFM)for future nuclear fusion devices and power plants.However,due to the relatively low thermal conductivity of W,its heat transfer capacity is insufficient in the environment of high thermal load for divertor to remove heat quickly and efficiently,resulting in recrystallization,corrosion or cracking on the surface of PFM.Therefore,it is urgent to develop tungsten-based materials with high thermal conductivity.Because diamond(Dia)has a high thermal conductivity,this thesis designed and fabricated W matrix composites with doped Dia to efficiently improve the thermal conductivity of W materials..The Dia surfaces were coated with Si,SiC,SiC-Si and Ni coatings by interface design respectively,and the composites were prepared via the Spark Plasma Sintering(SPS)technique.The effects of the coatings on the interface bonding,interface structure and thermal conductivity of the composites were studied,and the relationship between the interface structure and the interface thermal resistance of the composites was determined.The following are the key research findings and conclusions:(1)Raw Dia was used to prepare Dia/W composites.When the volume fraction of raw Dia was 40%,the composite possessed the highest density and thermal conductivity of 94.75%and 138 W·m-1·K-1 respectively,which was slightly higher than that of SPS pure W(i.e.121 W· m-1·K-1).The WC and W2C with low thermal conductivity were generated during the sintering process,and the discontinuous carbide interface layers were formed at the interfaces.The total interface thermal resistance of composite was increased by two different types of interface structures(i.e.Dia-WC/W2C-W and DiaW),leading to a relatively low thermal conductivity of the composite.The interfacial carbides nucleated inhomogeneous,and its growth was controlled by the diffusion of C atoms and Ostwald aging process.(2)Si coating was designed to prepare Dia(Si)/W composite.Under the combined action of reaction diffusion between W-C and Si-C atoms,and mutual diffusion between W-Si atoms,Si coating can enhance the interface bonding of the composite and the effects of diffusion and reaction became more noticeable with an increase in volume fraction.However,Si layer was unable to entirely prevent the formation of WC/W2C,leading to two interface structures,i.e.Dia-Si-W and Dia-WC/W2C-W.Compared with raw Dia/W composite,the interface thermal resistance was reduced,and 60vol.%Dia(Si)/W composite had the highest density and thermal conductivity of 96.18%and 193 W·m-1·K-1,respectively.(3)SiC coating was designed to prepare Dia(SiC)/W composite.SiC was prone to convert to WC through the SiC-W reaction,and the composite had a low density(94.81%),forming two interface structures of Dia-WC-W and Dia-SiC-WC-W,which increased the thermal resistance and decreased the thermal conductivity of the composite(i.e.152 W·m-1·K-1).Based on this,Dia(Si)was vacuum heated to form a SiC transition layer,resulting in the Dia-SiC-Si interface structure.SiC-Si layer effectively decreased the generation of WC in the composite,formed the Dia-SiC-SiW interface structure and enhanced interface bonding.Additionally,there was a parallel phase relationship between the surfaces of(111)Dia and(111)β-SiC,resulting in a semiconformal interface,which further reduced the interface energy and made the thermal conductivity of the composite reach to 205 W·m-1·K-1.(4)Dia(Ni)/W composite was designed.Through diffusion solution,Ni formed amorphous NiW or crystalline NiW with W,and induced C atoms on the Dia surface to form amorphous C layer,which effectively improved the interface bonding between Dia and W matrix,avoided the generation of WC and W2C,and significantly decreased the interface thermal resistance of composite.Combined with the low thermal resistance interface structure of Dia(Ni)/W composite,the effects of the particle size and volume fraction of Dia(Ni)on the thermal conductivity of the composite were further studied.When the size and the volume fraction of Dia(Ni)were 400 μm and 60%respectively,the maximum density 98.63%and thermal conductivity composites 298 W·m-1·K-1 were acquired.Its thermal conductivity was 71.3%higher than that of W.(5)The results of the finite element simulation analysis showed that,when compared to pure W module,the Wf/W composite-Dia/W composite module prepared with 60vol.%Dia(Ni)/W composite had a lower temperature difference(819℃)and equivalent plastic strain(0.001369),which effectively improved the low-cycle thermal fatigue life of composited module.