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32STRUCTURE AND PROPERTIES OF BORON CARBIDE PRODUCED BY SHS AND MECHANOCHEMICAL SYNTHESIS: A COMPARATIVE STUDY
Zh. V. Eremeeva", S. Vorotilo", V. S. Panov", L. V. Myakisheva", A. I. Lizunov", A. A. Nepapushev", D. A. Sidorenko", and D. Y. Mishunin"
aNational University of Science and Technology MISiS, Moscow, 119049 Russia
DOI: 10.24411/9999-0014A-2019-10036
High requirements are looked for the absorbing components of modern nuclear reactors. These requirements determine different properties of regulatory bodies, such as: high efficiency of neutron absorption, low burning rate of absorbing isotopes during the exploitation of the reactor, high resistance to radiation damages, volume stability at normal-operating temperatures and overheat. The usage of the boron carbide in absorbing components is determined by the special physical properties of the 10B isotope - the effective capture cross-section of thermal neutrons as well as high mechanical properties at high temperatures.
It has been established, that the application of nanostructured boron carbide ceramics allows one to receive improved physical-mechanical properties as compared with coarse-grained ceramics. In the case of materials which are used in the nuclear and space fields, the application of the boron carbide in the nanocrystalline state will reduce the negative effect of radiation on these materials i.e. reducing of radiation swelling and embrittlement.
The object of this research were boron carbide (B4C) powders produced received by the two methods: (a) mechanical synthesis (MCS) of a mixture of the soot PM-15 and grade A amorphous boron (splintered particles of 5-20 p,m), and (b) self-propagating high-temperature synthesis (SHS) method (flaky particles of 7-28 p,m).
Figure 1 shows microstructures of the boron carbide powder particles.
(b)
Fig. 1. SEM images of boron carbide powder prepared by (a) mechanosynthesis and (b) SHS.
Mechanochemical synthesis (MCS) was carried out in the planetary mill "Activator 2S" with planetary disk rotation speed 600 to 900 rpm and drum rotation speed 1000-1800 rpm. The "balls/charge" ratio is 30^45:1, argon pressure in drums is 3-5 atm, treatment duration is 5 to 120 min.
Hot pressing was carried out using special hydraulic press DSP-515 SA (Dr. Fritsch Sondermaschinen GmbH, Germany) which is equipped with the temperature control system.
■SHS 2019 Moscow, Russia
The determination of the specific area of initial powders and obtained boron carbide powders was carried out using the analyzer of the specific surface and porosity NOVA 1200e (USA) by the method of low-temperature nitrogen adsorption (BET-method). The universal laser device FRITSCH ANALYSETTE 22 MicroTec plus was used for determination of grain-size composition of boron carbide powder obtained by the SHS-method. The submicron particle analyzer Beckman COULTER No. 5 was used for determination of grain-size composition of amorphous nano-sized boron carbide powder obtained by mechanosynthesis according to GOST 19440-94.
X-ray analysis of compounds obtained was carried out using DRON-2.0 X-ray diffractometer (copper Ka irradiation, diffraction angle range 10° to 130°). The Raman spectra were measured on a Horiba Jobin Yvon T64000 Raman spectrometer.
Analytical electron microscope JEM-2100 was used to obtain electron microscopic images and electron diffraction patterns. The content of impurities was determined by the atomic absorption and the emission spectral analysis.
Based on grain-size composition and SEM-data it was established that boron carbide powders synthesized by mechanical synthesis are nanoscale particles of unequal shape with diameters of 50-500 nm, combined into agglomerates.
Table 1 shows some properties of boron carbide powders.
Table 1. Properties of boron carbide powders.
B4C Flow rate, s Bulk density, Specific Average size of Particle
g/cm3 area, m2/g agglomerates, ^m size, nm
mechanosynthesis SHS No No 1.195 1.25 16-24 18-32 5-10 5-25 100-300 200-500
The compactibility of boron carbide powders obtained was investigated. It was established that the SHS B4C-powder and MCS-B4C-powder are densified quite well in the pressure range 200 to 900 MPa, and have density of 2.85 and 2.9 g/cm3 at 600 MPa respectively, which is significantly higher than the one for standard boron carbide powder obtained by carbothermic method.
Hot pressing of these powders was carried out using the following regime: total duration 200 min, isothermal holding 60 min, the rate of temperature rises 10°C/min, the temperature of hot pressing 2000 to 2150°C.
Figure 2 shows the dependence of the sample relative density for boron carbide powders obtained by various methods on the hot pressing temperature.
2000 2050 2100 2150
Fig. 2.
Figure 3 shows the structure of hot-pressed samples for boron carbide powders obtained by various methods.
(a) (b)
Fig. 3. SEM images of hot-pressed samples of boron carbide powders prepared by (a) SHS and (b) MCS.
The experiments carried out showed the possibility of the obtaining of nanosized boron carbide powders by mechanochemical synthesis of soot and amorphous boron and obtaining the theoretical density of boron carbide after hot pressing.
The work was supported by RFBR, project no. 17-08-00204.
