The rhombohedral structure of boron carbide (B4C) is an important engineering material, its hardness is second only to diamond and cubic boron nitride, with high hardness (>30GPa), high modulus, good wear resistance, low density (2.52) ), oxidation resistance, acid and alkali resistance, and good neutron absorption properties. In addition, due to the high neutron capture cross section of boron, the absorption energy spectrum is wide, the thermal neutron cross section containing isotopes is as high as 4000 Barn, and the radiant energy of secondary emission is low, so the boron carbide structural material can be applied to radiation protection, atom Nuclear safety areas such as reactor control and shielding. However, boron carbide materials have low fracture toughness and high sintering temperature, which poses high challenges for the formation and sintering of their components. In the field of nuclear energy, it is very important to study the low-temperature sintering mechanical toughening of such materials.
Conventional sintering of boron carbide is often obtained by powder metallurgy, that is, the raw materials are sufficiently uniformly mixed by ball milling, and then boron carbide and the second phase are generated in situ by a pulse current sintering device (such as PECS/SPS), and a fine microscopic texture is formed. . In the early cooperation with the team of Prof. Song Songlin from Anhui University of Technology, the Ningbo Materials Institute improved the surface activation energy of the reactive powder by using high-energy ball milling, and achieved good results in low-temperature sintering (such as titanium carbide and no The shaped boron (B) powder was ball-milled for 12 h to obtain a highly active mixed powder; then sintered in situ at 1900 ° C / 50 MPa using SPS equipment to obtain B4C (41 vol%) with an average grain size of less than 1 μm. - TiB2 composite material. The composite material has a relative density of 97.9%, a three-point bending strength of 891 MPa, a Vickers hardness of 28 GPa, and a fracture toughness of 4.4). Related work has been published in Scripta Materialia, Journal of European Ceramic Society and Ceramics International (Scripta Materialia 135 (2017) 15-18; Journal of European Ceramic Society 35 (2015) 1107-1112 and Ceramics International 40 (2014) 15341-15344 ).
Recently, the researchers of the Nuclear Energy Materials Engineering Laboratory of Ningbo Materials Institute further adopted the technology of high-temperature molten salt to break through the problem of low efficiency of traditional ceramic ball milling process, and successfully prepared a two-phase composite powder with uniform distribution of submicron. The synthesis sintering aid uniformly coats the core-shell structure of boron carbide: and. Boron carbide composites were successfully prepared by subsequent pulse current sintering equipment under relatively low temperature (1700 ° C) and 45 MPa pressure conditions. The mechanical properties and microstructure of the material were characterized by Vickers hardness tester, nanoindenter and scanning electron microscope. It was found that the second phase was uniformly distributed at the grain boundary of boron carbide. Among them, the fracture crack caused by the thermal expansion mismatch at the grain boundary is deflected at the grain boundary, and the fracture mode is changed from a single transgranular fracture to a transgranular-intergranular mixed fracture mode, which greatly improves the boron carbide composite. Mechanical properties. Among them, the B4C (18wt.%)-Al3BC composite has a relative density of 100% and excellent mechanical properties (elastic modulus 495GPa, Vickers hardness 37GPa and fracture toughness 6.32). The B4C (29.8 vol%)-TiB2 composite has a relative density of 98.2%, a Vickers hardness of 32.1 GPa and a fracture toughness of 4.38. Moreover, the material has excellent electrical and thermal properties in addition to excellent mechanical properties. (Conductivity, thermal conductivity 33W/mK), fully meet the requirements of the later EDM cutting process. Related work has been published in the journals of the ceramics, Journal of European Ceramic Society (2017, 4242-4531) and Journal of American Ceramic Society (2018, doi.org/10.1111/jace. 15541).
The project was funded by the National Natural Science Foundation's major research and development program, “Advanced Nuclear Fission Energy Fuel Proliferation and Transmutation†(91426304).
Figure 1 Scanning electron microscope shows (a) original boron carbide powder, (b) and (c) sintering aid evenly coated with boron carbide powder, (d)-(f) elemental surface scanning analysis
Fig. 2 Grain boundary dispersion-enhanced boron carbide composite prepared by SPS sintering
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