Surface Properties and Morphology of Boron Carbide Nanopowders Obtained by Lyophilization of Saccharide Precursors

materials Article Surface Properties and Morphology of Boron Carbide Nanopowders Obtained by Lyophilization of Saccharide Precursors Dawid Kozie ń 1,* , Piotr Jele ń 1 , Joanna St˛epień 2 , Zbigniew Olejniczak 3 , Maciej Sitarz 1 and Zbigniew P˛edzich 1,* 1 Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30 Mickiewicz Av., 30-059 Kraków, Poland; [email protected] (P.J.); [email protected] (M.S.) 2 Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, 30 Mickiewicz Av., 30-059 Kraków, Poland; [email protected] 3 Institute of Nuclear Physics, 152 Radzikowskiego St., 31-342 Kraków, Poland; [email protected] * Correspondence: [email protected] (D.K.); [email protected] (Z.P.) Abstract: The powders of boron carbide are usually synthesized by the carbothermal reduction of boron oxide. As an alternative to high-temperature reactions, the development of the carbothermal reduction of organic precursors to produce B4C is receiving considerable interest. The aim of this work was to compare two methods of preparing different saccharide precursors mixed with boric acid with a molar ratio of boron to carbon of 1:9 for the synthesis of B4C. In the first method, aqueous ◦ ◦ solutions of saccharides and boric acid were dried overnight at 90 C and pyrolyzed at 850 C for 1 h under argon flow. In the second method, aqueous solutions of different saccharides and boric acid Citation: Kozień,D.; Jeleń,P.; were freeze-dried and prepared in the same way as in the first method. Precursors from both methods ◦ St˛epień,J.; Olejniczak, Z.; Sitarz, M.; were heat-treated at temperatures of 1300 to 1700 C. The amount of boron carbide in the powders P˛edzich,Z. Surface Properties and depends on the saccharides, the temperature of synthesis, and the method of precursor preparation. Morphology of Boron Carbide Nanopowders Obtained by Keywords: boron carbide; saccharides; lyophilization; freeze drying; NMR; XAS Lyophilization of Saccharide Precursors. Materials 2021, 14, 3419. https://doi.org/10.3390/ma14123419 1. Introduction Academic Editors: Oleg A. Shlyakhtin Boron carbide (B4C), due to its specific properties (high hardness, low density, high and Alexander A. Lebedev melting point, high elastic modulus, etc.) [1,2], has been widely used in many applications such as in polishing as an abrasive, in ball mills as a neutron absorber and as a neutron Received: 18 April 2021 shield, and in boron neutron capture therapy (BNCT). In BNCT research, nuclear reactors Accepted: 8 June 2021 Published: 20 June 2021 or accelerators generate thermal neutrons, which are captured by a variety of nuclei, but the probability of capture by an isotope of boron (10B) is much higher than that of the Publisher’s Note: MDPI stays neutral capture of another isotope. The first studies of synthesized boron carbide (B4C) started with regard to jurisdictional claims in when Henri Moissan obtained boron carbide from the reduction of diboron trioxide (B2O3) published maps and institutional affil- by magnesium (Mg) in the presence of carbon (C) [1]. Since 1899, boron carbide has been iations. synthesized using various methods. The synthesis methods determine the properties of the products, morphology, and purity of obtained B4C[3,4]. The methods of boron carbide synthesis can be classified as carbothermic reduction [3–7], magnesiothermic reduction [8], synthesis from elements [9], vapor phase reaction [10], synthesis from polymer precursors [11], liquid phase reaction [12], ion beam synthesis [13], and vapor-liquid-solid (VLS) Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. growth [14]. Carbothermal reduction is commonly used in industry to obtain B4C as a This article is an open access article source of boron (boric acid (H3BO3) or boron oxide (B2O3), less commonly) and as a source distributed under the terms and of carbon fine crystalline graphite or petroleum coke. The reaction is carried out at a ◦ conditions of the Creative Commons temperature between 1700 and 2000 C under a protective atmosphere (Ar) [15]. Powders Attribution (CC BY) license (https:// obtained from carbothermal reduction are strongly agglomerated and aggregated, so they creativecommons.org/licenses/by/ require intensive crushing and grinding so that they become suitable for further use. The 4.0/). main problem is to reduce the cost of synthesis; many researchers have attempted to lower Materials 2021, 14, 3419. https://doi.org/10.3390/ma14123419 https://www.mdpi.com/journal/materials Materials 2021, 14, 3419 2 of 16 the synthesis temperature using various organic carbon precursors such as phenolic resin, citric acid, polyvinyl alcohol (PVA), and carbohydrates (cellulose, glucose, and sucrose) with boric acid (H3BO3)[16]. When the boron carbide is synthesized from an organic precursor, the main problem during synthesis is the removal of water from precursors, which avoids the aggregation of particles in precursors, thereby significantly affecting the size and morphology of the boron carbide after synthesis. Freeze-drying, also known as lyophilization or cryodesiccation, is a method of removing water by the sublimation of ice crystals from frozen material. Lyophilization is an effective method of drying materials without harming them and is commonly used mainly in the food and pharmaceutical industries [16–18]. Freeze-drying is also an effective method of removing water from the precursor obtained by the mixed solvent of boric acid and saccharides. Previous work by our research group [16] only focused on synthesized boron carbide from different types of saccharide precursors as the carbon precursor in carbothermal reduction and studied the influence of the used saccharide precursor with a molar ratio of carbon to boron of 9:1 on the morphology of the obtained boron carbide powders (B4C). In the present work, we aimed to verify the idea that the freeze-drying of mixed precursors (boric acid and different saccharides) has an important influence on the morphology and obtained size of boron carbide after synthesis. During the freeze-drying of saccharide precursors mixed with boric acid, water can be removed from the precursor without heat treatment. The second idea presented in the article focuses on how the type of precursor used and the molar ratio of boron to carbon in precursor influence the obtained morphology and aggregation of the powders. 2. Materials and Methods Powders of boron carbide were obtained from boric acid (H3BO3), and saccharides, including glucose, fructose, dextrin, and hydroxyethyl starch (HES), were all obtained from Sigma Aldrich (St. Louis, MO, USA) (99% pure). In the first method, boric acid (H3BO3) and mono- or polysaccharides were dissolved in distilled water in a molar ratio providing a boron to carbon ratio in the final powder of 1:9. After mixing the precursor in distilled water, the solutions were prepared as described in our earlier article [16], and then, they were dried overnight at 90 ◦C in a vacuum oven to obtain them in solid states. In the second method, the powders of boron carbide were prepared using a similar procedure as in the first method, but the only difference was that the solutions of boric acid mixed with different saccharides were freeze-dried to remove water from the precursor’s powders. All obtained precursors from both methods were pyrolyzed at 850 ◦C for 1 h under argon flow. The pyrolyzed powders were placed in graphite crucibles and heat-treated at a temperature ranging from 1300 to 1700 ◦C for 1 h under the argon flow. Changes in saccharide precursors were identified by infrared spectroscopy analysis. The FTIR measurements of both precursors and boron carbide materials were carried out using a Bruker Vertex 70 v spectrometer (Billerica, MA, USA). The standard KBr pellet method was employed, and 128 scans in the range of 4000 to 400 cm−1 were accumulated with a resolution of 4 cm−1. Raman spectroscopy measurements were recorded to determine the presence of carbon and carbon hybridization in the precursor. A WITec Alpha 300 M+ spectrometer (Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany) equipped with a 488 nm diode laser along with a 600 grating and a Zeiss 100× objective lens (Carl Zeiss AG, Oberkochen, Germany) was used. Such combination provides a laser spot of 661 nm in diameter. The power of the excitation source was set to prevent sample degradation. Each sample was measured in five spots with an accumulation of 2 scans, 120 s each, in each spot. Obtained sets of spectra were then averaged. To analyze the local structure in the near-surface region of the obtained precursors, both pyrolyzed and synthesized at 1700 ◦C, X-ray absorption spectroscopy (XAS) in total electron yield (TEY) detection mode was used. The measurements were performed using the PEEM/XAS beamline of the Solaris National Synchrotron Radiation Centre, Cracow, Poland [19]. PEEM/XAS Materials 2021, 14, 3419 3 of 16 beamline is a bending magnet-based beamline, providing the soft X-ray energy range (150 to 2000 eV) and equipped with plane grating monochromator of resolving power, E/DE > 4000. Samples in the form of powder on carbon tape were measured at boron K edge at room temperature. The carbon tape contribution was subtracted from the collected spectra, and they were normalized to a unit step by subtracting the constant value fitted in the preedge region and then dividing by the constant value fitted in the postedge region. The energy scale was calibrated so that the positions of characteristic spectral features of reference B4C were at the same energy values as in [20]. The beginning of absorption starts at 189 eV. On the spectra obtained from the boron edge, we can distinguish four bands from the π* excitation state: (A) 190.9 eV, (B) 191.7 eV, (C) 192.3 eV, and (D) 193.7 eV and three bands from the σ* excitation states: (E) 196 eV, (F) 200 eV, and (G) 204 eV.

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