CC BY
23SHS OF BN NANOPOWDER USING BORON-CONTAINING
COMPOUNDS AND SODIUM AZIDE
Yu. V. Titova*", A. P. Amosov", D. A. Maidan", D. R. Safaeva", M. V. Suslov",
and D. V. Ostroukhov"
aSamara State Technical University, Samara, 443100 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10175
Hexagonal boron nitride (BN) has a unique set of properties and a wide range of applications, which led to the creation of a large number of methods to obtain it [1]. However, the known methods for producing BN are based on long-term heating to high temperatures, are energy-intensive and expensive, and therefore energy-saving technologies based on the SHS process represent an effective alternative to traditional technologies for producing hexagonal BN [1]. The first SHS technologies were based on nitriding the pure boron powder or its chemical compounds by gaseous nitrogen during their combustion in the SHS reactor in a nitrogen atmosphere [1-3]. However, for nitriding the pure boron, it is necessary to create very high nitrogen pressures of 10-500 MPa, which greatly complicates the design of the SHS reactor and the technology for obtaining BN, or significantly dilute the initial boron powder with the final product BN. In the case of nitriding the chemical compounds (B2O3, KBF4), a metal-reducer (most often magnesium Mg) is added to the charge, which lowers the required nitrogen pressure in the reactor to an average of 5 MPa, but also leads to the formation of by-products along with the target product BN, which have to be separated by subsequent acid washing, which also complicates the SHS technology.
Then, methods of azide SHS with the use of solid nitriding reagent-sodium azide NaN3 instead of gaseous nitrogen were used to obtain hexagonal BN [4-7]. In the case of nitriding the boron oxide B2O3, a metal-reducer Mg or Ca was used [4]:
3B2O3 + 9Mg + 2NaN3 = 6BN + 9MgO + 2 Na|
The combustion process took place at atmospheric nitrogen pressure of 0.1 MPa with the formation of 90-95% BN and 5 to 10% of MgO or CaO impurities that are removed by subsequent acid washing. The disadvantage of the method was also the formation of fire-hazardous atomic sodium. Powders of pure boron and boron-containing halide salts KBF4 or NH4BF4 were used as initial reagents at Samara State Technical University [5-7]:
8B + 3NaN3 + KBF4 = 9BN + 3NaF + KF
The SHS process took place at a nitrogen pressure in the reactor of 5 MPa with the formation of 97-98.5% BN and 1.5-3% NaF and KF impurities easily removed by subsequent water washing. In this paper, an attempt is made to replace the KBF4 or NH4BF4 salt with cheaper and available boron-containing compounds: boric anhydride B2O3 and boric acid H3BO3.
The aim of this work is to study the possibility of obtaining and determining the conditions for the synthesis of boron nitride powder by SHS from the systems "boric acid-sodium azide", "boric anhydride-sodium azide" with the addition of amorphous boron. The process of obtaining boron nitride from these systems consisted in the following sequence of operations. The powders of the initial components, taken in the desired ratio, were mixed manually in a porcelain mortar for 10 minutes. The resulting mixture was poured into a tracing paper cup to
form a sample with a diameter of 30 mm and a height of 45 mm, and then placed in a constant pressure reactor. The samples were burned in a nitrogen atmosphere at a pressure of 4 MPa, and the temperatures and combustion rates of the studied systems were recorded. Table 1 presents the results of experimental studies of the temperature and combustion rate of mixtures intended for the synthesis of boron nitride.
Table 1. The results of experimental studies of the temperature and combustion rate of mixtures
Initial mixture Combustion temperature, °C Combustion rate, cm/s
8B + B2O3 + 6NaN3 2497 1.4
16B + B2O3 + 6N 2944 1.6
16B + 2H3BO3 + 6NaN3 2563 1.4
17B + H3BO3 + 6NaN3 2877 1.7
It can be seen from the presented results that with an increase in the boron content in the initial mixture, the temperature and combustion rate increase. The phase composition of the synthesized products was determined on the Dron-2 X-ray diffractometer. X-ray spectra were measured using Cu-radiation. The results of the X-ray phase analysis of the products are shown in Figs. 1, 2.
Fig. 1. XRD pattern of the products of combustion of the mixture 8B + B2O3 + 6NaN3.
Fig. 2. XRD pattern of the products of combustion of the mixture 16B + B2O3 + 6NaN3.
It can be seen from the presented XRD patterns that the combustion products of the mixtures 8B + B2O3 + 6NaN3 and 16B + B2O3 + 6NaN3 consist of two phases: target is boron nitride (BN) and side is sodium oxide (Na2O). With an increase in the content of boron in the initial mixture, the amount of sodium oxide is slightly reduced. The study of the surface topography and morphology of reaction product particles was carried out using the Jeol scanning electron microscope JSM-6390A with the prefix Jeol JED-2200. The results are presented in Figs. 3 and 4.
(a) (b)
Fig. 3. SEM image of particles of combustion products of mixtures 8B + B2O3 + 6NaN3 (a) and 16B + B2O3 + 6NaN3 (b).
(a) (b)
Fig. 4. SEM image of particles of combustion products of mixtures 16B + 2H3BO3 + 6NaN3 (a) and 17B + H3BO3 + 6NaN3 (b).
Comparing the photos presented in Fig. 3 and taking into account the results of XRD analysis presented below, we can conclude that the combustion of mixtures of 8B + B2O3 + 6NaN3 and 16B + B2O3 + 6NaN3 formed particles of lamellar and irregular shape of boron nitride and sodium oxide. The size of boron nitride particles varies in the range from 150 to 400 nm. The particle size of BN increases with the boron content rise.
Figure 5 shows the XRD pattern of combustion products of mixture 17B + H3BO3 + 6NaN3. It can be seen from the XRD pattern that the products of combustion of the mixture 17B + H3BO3 + 6NaN3 consists of two phases: the task is boron nitride (BN) and side is sodium oxide (Na2O). Moreover, the yield of the target product is slightly lower than in systems using boron oxide as a starting component.
Fig. 5. XRD pattern of the products of combustion of the mixture 17B + H3BO3 + 6NaN3.
Comparing the photos presented in Fig. 4 and taking into account the results of X-ray phase analysis presented below, it can be concluded that the combustion of mixtures of 16B + 2H3BO3 + 6NaN3 and 17B + H3BO3 + 6NaN3 formed particles of lamellar and irregular shape of boron nitride and sodium oxide. The size of boron nitride particles varies in the range from 150 to 450 nm. The particle size of BN synthesized from boric acid systems is larger than that of boron oxide systems.
Thus, the experiments conducted to obtain micro-and nanopowders of target boron nitride by SHS using boron-containing compounds and sodium azide led to the following results. It was found that the synthesized boron nitride has lamellar and irregular particles ranging in size from 150 to 450 nm. The final products contain sodium oxide along with the target boron nitride. The addition of boron to the initial charge increases the yield of the target product, but the particle size increases.
The authors express their gratitude to the company ETIMADEN-Etiproducts Ltd. for the donation of samples of boric acid H3BO3 ULSPOWDER and boron oxide B2O3 POROUS.
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