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32MULTIFUNCTIONAL BORON NANOPARTICLS: AN ECOLOGICAL METHOD OF PRODUCTION, PROPERTIES
P. A. Khaptakhanova*", S. A. Uspenskii"A, T. S. Kurkin", A. N. Zelenetskii", and S. Yu. TaskaevA
aEnikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, Moscow, 117393 Russia
bBudker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10059
In recent years, there has been considerable interest in using biofuels as alternative energy sources. Moreover, many types of biofuels have a low energy density compared to energy resources sold, such as gasoline or kerosene. Therefore, a lot of research is currently carried out in the field of changing energy parameters of biofuels by adding various additives with a higher energy density.
Nanoscale powders are attractive as potential fuel additives because of their high energy density combined with high specific surface area. Furthermore, nanodispersions have a lower flash point than micron-sized particles. The main requirements for nanofillers as an additive to the fuel are as follows: the average size of nanodispersions should be from 5 to 20 nm, there should be no oxide layer on the surface of particles, the concentration of nanoparticles should not exceed 10 vol % [1].
Materials based on boron nanoparticles have a wide range of application as semiconductors such as protective coatings for increasing mechanical wear resistance, drugs for treating oncological diseases, and are also used in various combinations, including combustible nanofluids, gel rocket fuels, solid rocket fuels, and also as high density fuel additives.
Elemental boron has the largest volumetric heat of reaction with oxygen (138 kJ/cm3), compared to other elements. However, its ignition is greatly hindered by the presence of a B2O3 oxide layer on the surface of the particles, the ignition temperature of which in an oxygen-containing medium is 1500-1950 K, regardless of the particle size. Moreover, the energy release from boron particles is significantly limited by the diffusion of elemental boron through the molten shell of the oxide layer, as well as during combustion in gases containing hydrogen, due to the formation of metastable HBO2 particles [2]. The removal of the oxide layer occurs by evaporation, which is a slow process, accounting for a significant proportion of the total particle burning time. All these difficulties limit the scope of boron. The presence of an oxide layer on the surface of the nanodispersion of particles is due to technological methods of their preparation.
In general, methods of producing elemental boron nanoparticles implemented in technological practice are the methods of chemical deposition from the gas phase or methods of CDGP. There are many ways to implement CDGP, that are fundamentally different among themselves in the ways of launching chemical reactions and the conditions of the process. For example, there are electron-beam, laser, plasma, arc evaporation methods, or combinations thereof [3].
The exclusive advantages of the CDGP methods are tonnage, high yield of the target product (close to 98%) and high purity of elemental boron (99.6-99.9%).
One of the disadvantages of the above methods of obtaining is the fact that such methods allow to obtain crystalline powders of elemental boron, in which micron-sized particles
ISHS 2019 Moscow, Russia
predominate in the powder as a result of the sintering process. Also on the surface of the particles obtained using the CDGP method, there is a layer of boron oxide, which makes it difficult to use them as an additive to biofuels.
Ultrasonic technology is a new approach to the improvement of the implemented methods of deposition from the gas phase, namely the dispersion of the sintered powder with micron particles to nanoscale. The proposed method is economical and technically easy to implement. By using this method it is possible to obtain a sufficient amount of boron nanoparticles within a short period of time. With ultrasonic dispersion, acoustic cavitation is the main factor of destruction, which is an effective energy concentration mechanism. The total energy of a collapsing bubble has a small energy value, however, the spherical convergence of the bubble leads to the formation of very large local energy densities, and therefore, high temperatures (5000-25000 K) and pressures (100 MPa) [4]. Under the influence of such loads, micron boron is crushed.
The line of dispersions of particles less than 100 nm was obtained with the use of prolonged exposure to ultrasound and cascade fractionation. When ultrasound was applied to the 80 nm fraction, particles smaller than 20 nm were obtained, plus there was a tendency to their decrease in size and their ovalization (Table 1). Data on the characteristics and parameters of the developed method of ultrasonic dispersion for boron nanoparticles, namely - data from dynamic light scattering devices and an electron microscope are presented in Table 1.
Table 1. Parameters and conditions for obtaining nanoparticles of elemental boron by method of ultrasonic dispersion.
№
Concentration
of boron nanoparticles, mg/ml H2O
Particle size BEFORE ultrasonic treatment, nm
Ultrasonic processing time, min
Particle shape AFTER ultrasonic treatment
The size diapason of the
obtained nanoparticles,
nm
Crystal elemental boron
Ultrasonic generator power - 0.6 ± 0.03 kW
1
2
3
4
5
6
30 mg/ml H2O
Diapason: 1000-2000
25-30 50-60 80-90 115-120 250-270 300-320
Irregularly shaped with sharp edges
Spherical
350±100 200±50 80±10 50±20 20±15 5±3
To assess the effect of ultrasound on the crystal structure of elemental boron, X-ray structural analysis was used. Under the influence of ultrasound, the monolithic crystal structures degrade over the crystallographic planes of their contact, i.e. the dispersion of the original micron boron powder increases to nanoscale size. The initial sample of boron on the diffractogram revealed a peak at 28°, which corresponds to crystalline boron oxide on the surface of microdispersion. Upon long-term treatment of micron boron with ultrasound, on the diffractogram of the nanosample the boron oxide reflex was not detected. Thus, the composition of the boron nanoparticles powder is itself a pure elemental boron.
Thus, ultrasonic dispersion allows obtaining of nanodispersed boron particles with a size less than 20 nm. During the process of ultrasonic treatment, their monodispersity increases and the oxide layer disappears from the surface, which leads to increase in the potential of their use as a fuel additive. At the same time, elemental boron nanoparticles obtained by the new technological method represent promising material for further scientific research in various fields of technology.
This work was supported by Russian Science Foundation (grant no. 19-72-30005). Electron Microscopy Studies were performed with the financial support from Ministry of Science and Higher Education of the Russian Federation. X-ray analysis and dynamic light scattering research were performed with the financial support from Ministry of Science and Higher Education of the Russian Federation using the equipment of Collaborative Access Center «Center for Polymer Research» of ISPM RAS.
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