Journal of the Ceramic Society of Japan 126 [8] 602-608 2018 -Japan DOI http://doi.org/10.2109/jcersj2.18093 JCS SPECIAL ARTICLE The 72th CerSJ Awards for Advancements in Ceramic Science and Technology: Review Low-temperature synthesis of boride powders by controlling microstructure in precursor using organic compounds Masaki KAKIAGE1,³ 1 Institute for Fiber Engineering, Shinshu University (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, 3–15–1 Tokida, Ueda, Nagano 386–8567, Japan The carbothermal reduction of boron oxide (B2O3) is an important process for the synthesis of boride powders. As a low-temperature synthesis method for boron carbide (B4C) powder by carbothermal reduction, we focused on an approach using a condensed product prepared from boric acid (H3BO3) and an organic compound with a number of hydroxyl groups (a polyol) such as glycerin, mannitol, or poly(vinyl alcohol). A borate ester bond was formed by the dehydration condensation of H3BO3 and a polyol, leading to the homogeneous dispersion of the boron and carbon sources at the molecular level. The thermal decomposition of a condensed H3BO3-polyol product in air was performed to control the amount of carbon to the stoichiometric C/B2O3 ratio required for carbothermal reduction. Within the thermally decomposed product consisting of B2O3 and carbon compo- nents (B4C precursor), a B2O3/carbon structure at the nanometer scale was formed. The improved dispersibility and homogeneity of the B2O3/carbon microstructure accelerated the B4C formation at a lower temperature. Consequently, crystalline B4C powder with little free carbon was synthesized by heat treatment at a low temperature of 1200°C in an Ar flow. This low-temperature synthesis approach was applied to the low- temperature synthesis of other boride powders, i.e., boron nitride and calcium hexaboride powders. ©2018 The Ceramic Society of Japan. All rights reserved. Key-words : Low-temperature synthesis, Carbothermal reduction, Precursor, Microstructure, Polyol, Boron carbide (B4C), Boride powders [Received April 28, 2018; Accepted May 28, 2018] hardness, and thus a large amount of energy is required for 1. Introduction the pulverization process. Therefore, the development of a Carbothermal reduction is an important industrial proc- low-temperature synthetic route has been strongly expect- ess for the synthesis of non-oxide ceramic powders such ed for avoiding the volatilization loss of boron components as carbides, borides, and nitrides. The carbothermal reduc- and reducing the manufacturing cost. tion of boron oxide (B2O3) is the most common indus- In order to reduce the synthesis temperature of B4C trial manufacturing method for boron carbide (B4C) pow- powder by carbothermal reduction, the B2O3 and carbon der.1)­3) The overall reaction of carbothermal reduction is components must be dispersed well to increase the contact given by area between the B2O3 and carbon components and to reduce the diffusion distance of the reacting species. Many 2B2O3 þ 7C ! B4C þ 6CO: ð1Þ studies reported to synthesize B4C powder at lower tem- This process is suitable for large-scale synthesis because peratures by using a condensed product which employed the starting materials, which include boric acid (H3BO3)or various organic compounds as a carbon source such as 4),5) 6)­8) 9),10) 11) B2O3 as a boron source and activated carbon or petroleum glycerin, citric acid, sugar, phenolic resin, coke as a carbon source, are inexpensive and nonhazar- and poly(vinyl alcohol) (PVA),12) and they could reduce dous. However, this process is conducted at a high tem- the synthesis temperature to 1500­1600°C. However, in the perature of approximately 2000°C. The volatilization loss case of heat treatment at lower temperatures of less than of boron components is significant at the high synthesis 1500°C, the product contained residual free carbon derived temperature. Furthermore, the obtained ingot must be from the organic compound used as the raw material. crushed, refined, and granulated to produce B4C powder We have focused on both a molecular approach and a suitable for practical use. B4C exhibits extremely high structural approach to further reduce the synthesis temper- ature of B4C powder without residual free carbon using a ³ 13)­19) Corresponding author: M. Kakiage; E-mail: kakiage@ condensed H3BO3-polyol product. The compatibility shinshu-u.ac.jp of the composition, the dispersibility, and the homogeneity 602 ©2018 The Ceramic Society of Japan Journal of the Ceramic Society of Japan 126 [8] 602-608 2018 JCS-Japan of the B2O3 and carbon components in a precursor was with excess H3BO3, which contained an isolated H3BO3 achieved by the combination of the bond-forming reaction component without a B­O­C bond.16) Therefore, the simul- between H3BO3 and a polyol and a thermal decomposition taneous pursuit of dispersibility and compositional control process in air (molecular approach). Furthermore, a finely of a condensed product contains a major contradiction. and homogeneously arranged B2O3/carbon structure in the In an attempt to resolve the above contradiction, we precursor leaded to a larger interface between the B2O3 performed the thermal decomposition in air before the and carbon components, enabling synthesis of B4C pow- carbothermal reduction of a condensed H3BO3-polyol der at a low temperature of 1200°C (structural approach). product prepared at the stoichiometric ratio for the dehy- – – dration condensation to control the amount of carbon to 2. Molecular approach: formation of B O C the stoichiometric C/B O ratio required for the carbo- bond and compositional control 2 3 thermal reduction given by Eq. (1) (C/B2O3 = 3.5). The by thermal decomposition in air thermal decomposition in air eliminates the excess carbon Carbothermal reduction using a condensed product as a component while maintaining the dispersibility. Figure 2 precursor that consists of H3BO3 and an organic compound shows X-ray diffraction (XRD) patterns of the products with a number of hydroxyl (­OH) groups (a polyol) is obtained by heat treatment at 1250°C for 5 h in an Ar flow attractive as a low-temperature synthetic method for B4C. of thermally decomposed products (precursor powders) A condensed H3BO3­polyol product forms a borate ester prepared from the condensed H3BO3-mannitol product by (B­O­C) bond by a dehydration condensation reaction thermal decomposition at (a) 300­500°C for 2 h and (b) 20) 16) between H3BO3 and the polyol [Fig. 1(a)]. The forma- 400°C for 1­4 h in air. The XRD patterns changed sys- tion of this bond leads to the homogeneous dispersion of tematically with the thermal decomposition temperature the boron source and carbon source at the molecular level, [Fig. 2(a)] and holding time [Fig. 2(b)]. A peak attributed and thus the synthesis temperature is reduced owing to the to amorphous carbon was observed at lower thermal increased surface-active area between the B2O3 and carbon decomposition temperatures or for shorter holding times, components with superior reactivity. We used glycerin indicating that the precursor had excess carbon, and peaks 14),19) 16) 13),15),18) (C3H8O3), mannitol (C6H14O6), or PVA as a attributed to B2O3 were observed at higher thermal decom- polyol that has a strong complexation ability and can easily position temperatures or for longer holding times, indicat- 5),20),21) form a B­O­C bond with H3BO3. Expected molec- ing that the precursor had excess B2O3. Note that the 5) ular structures of a condensed H3BO3-glycerin product, structural homogeneity of the condensed H3BO3-mannitol 20) a condensed H3BO3-mannitol product, and a condensed product dominated the B4C formation behavior at a low H3BO3­PVA product are shown in Fig. 1. However, the synthesis temperature (see Figs. 11 and 12 in Ref. 16). The obtained product contained a large amount of residual formation of B4C was induced simultaneously within a carbon derived from the polyol, which is a common dis- short time throughout the entire homogeneous precursor advantage of B4C synthesis using an organic compound, (the thermally decomposed product prepared from the since a condensed product has excessively large carbon condensed product with the stoichiometric ratio for the component compared with that required for carbothermal dehydration condensation) even at a low synthesis tem- 8),10) reduction. In previous research, an excessive amount perature. In contrast, widely spaced B2O3 and carbon com- of H3BO3 was used as a raw material to prevent the forma- ponents, which have less reactivity, existed in the hetero- tion of residual free carbon in the product. However, the geneous precursor (the condensed product prepared with homogeneity was low for the condensed product prepared excess H3BO3, which contained an isolated H3BO3 com- ponent without a B­O­C bond). The heterogeneity of the synthesis reaction, which reflects the structural hetero- geneity, resulted in a time lag in the complete formation 16) of B4C, particularly at a low synthesis temperature. Consequently, the low-temperature synthesis of crystal- line B4C powder with little free carbon was achieved by carbothermal reduction using a condensed H3BO3-polyol product with a highly and homogeneously dispersed struc- ture and a suitable C/B2O3 composition. The C/B2O3 composition of the precursor can be controlled by varying the thermal decomposition conditions in air. 3. Structural approach: morphological control of B2O3/carbon microstructure in precursor The formation reaction of B4C from a condensed Fig. 1. (a) Schematic interpretation of dehydration condensa- H3BO3-polyol product is carbothermal reduction, i.e., the tion reaction between H3BO3 and polyol and molecular structures reaction of B2O3 and carbon [Eq. (1)]. Hence, we propose of (b) condensed H3BO3-glycerin product, (c) condensed H3BO3- an approach to developing lower-temperature synthesis mannitol product, and (d) condensed H3BO3­PVA product. routes by clarifying in detail the relation between the 603 JCS-Japan Kakiage: Low-temperature synthesis of boride powders by controlling microstructure in precursor using organic compounds Fig.