Synthesis of Boron Carbide Powder via Rapid Carbothermal Reduction Using Boric Acid and Carbonizing Binder

Abstract

Raw material is one of the most decisive factors for the quality of sintered boron carbide (B₄C) products, in the past, there were relatively successful efforts for the synthesis of B₄C powders via carbothermal reduction approaches. To prepare high-quality powder, a deeper understanding of the relationship between technological manufacturing parameters and resulting powder properties is required. In this paper, pure B₄C powders were synthesized by rapid carbothermal reduction (RCR) under B₂O₃ excess conditions using boric acid and a carbonizing binder as B₂O₃ and carbon source, respectively. The molar ratio of B₂O₃/C of starting mixtures was varied from 0.75:1 to 4:1. The effects of heat-treating temperature and starting composition on phase constitution, morphology as well as stoichiometry of the prepared powders were investigated. The studies show that the starting composition has no effect on the stoichiometry of the powders, all boron carbides synthesized at 1900 °C have a stoichiometric composition of B₄C. With increasing heating temperature and B₂O₃ content in the starting composition, the particle size of B₄C was reduced. Uniform B₄C powders with an average grain size of 300 nm were synthesized at 1900 °C from a starting powder mixture with a molar ratio of B₂O₃/C = 4. A formation mechanism is proposed under large B₂O₃ excess conditions. For the starting powder mixtures with a molar ratio of B₂O₃/C < 2, the formation of boron carbide occurs through both liquid–solid reaction and gas–solid reaction. Accordingly, the synthesized powders exhibit a morphology with mixed elongated platelets and small polyhedral particles. For the starting powder mixtures with a molar ratio of B₂O₃/C ≥ 2, fine-sized B₄C particles were formed by a liquid–solid reaction.

1. Introduction

Boron carbide is an advanced non-oxide ceramic. It possesses a set of outstanding properties such as a high melting point, high thermal stability, low density, high mechanical strength, extreme hardness and a high neutron absorption cross section [1,2,3]. These excellent properties make boron carbide a promising material for various industrial applications, including refractory devices, ballistic protection, wear resistance enhancement, material polishing, composite reinforcement and reactor control in nuclear technology [1,2,3,4,5]. In addition, boron carbide is a promising candidate material for high-temperature sensors and thermoelectric energy conversion due to its large Seebeck coefficients and material stability [6,7]. The outstanding properties of boron carbide are related to its unique crystal structure [1,2,3]. Boron carbide varies in a wide homogeneity range from 8.8 at% C (B_10.4C) to 20 at% C (B₄C) and has rhombohedral elementary cells composed of 12-atom icosahedra and 3-atom chains [1,2,3].

The properties of bulk sintered compacts are strongly influenced by the characteristics of the used raw materials. Therefore, preparation approaches for high-quality boron carbide have attracted great attention. Boron carbide can be synthesized by a variety of methods, such as vapor phase reaction of boron halides and hydrocarbon gases [8,9], carbothermal reduction of boron–oxygen compounds [10,11,12,13,14,15,16], magnesiothermic reduction of boron trioxide [17], self-propagating high-temperature synthesis (SHS) [18,19] and direct reaction of elemental boron and carbon [20,21]. Among these methods, carbothermal reduction has been widely used for the preparation of boron carbide owing to its cost-effective starting materials and simple synthesis process. The general reaction of boron carbide formation by carbothermal reduction can be described as:

2 B₂O₃ + 7 C → B₄C + 6 CO

(1)

Based on this reaction, great efforts have been devoted to the synthesis of boron carbide using different boron and carbon sources. Sinha et al. [10] reported on the synthesis of B₄C from boric acid and citric acid gel precursor, Alizadeh et al. [11] formed B₄C particles by reacting B₂O₃ with petroleum and carbon active, and Yanase et al. [12] prepared boron carbide powder from polyvinyl borate obtained by condensation of polyvinyl alcohol and boric acid. Nevertheless, free carbon residue and inhomogeneous particle size distribution of final powders are the common issues in these studies. For many industrial applications, high purity boron carbide powders with a fine particle size in the nanometer range are always desirable. For example, Moshtaghioun et al. [22] investigated the grain size dependence of hardness in sintered boron carbide ceramics. They showed that the harness increased from 31 MPa to 40 MPa if the average B₄C grain size was reduced from 17 µm to 120 nm. Hence, formation and morphology control of B₄C particles during the synthesis process are critical for property modification of sintered B₄C products with the desired grain size.

Weimer et al. [13,14] developed a graphite vertical tubular reactor and first reported a process for producing submicron B₄C particles using rapid carbothermal reduction (RCR) from an intimate mixture of B₂O₃ and cornstarch. It was found that the intrinsic kinetics of Equation (1) were inherently fast and heat transfer limitations played a dominant role in the overall reaction process. Rapid heating could improve heat conduction of the reduction and resulted in a small crystallite size of boron carbide. Several research works have also reported on the synthesis of ultrafine boron carbide powders using the same method [23,24,25]. Miller et al. [23] produced B₄C particles with a size ranging from 205 to 350 nm by the rapid carbothermal reaction of boric acid and carbon black. Toksoy et al. [24] obtained submicron B₄C powder (450 nm) using boric acid and Vulcan carbon black with a large surface area (~235 m²/g) through rapid carbothermal reduction. The RCR powders showed improved sinterability compared with commercial B₄C powders; consequently, the mechanical properties of the sintered B₄C ceramics could be improved significantly [23,24,25]. Furthermore, it is possible to use the RCR B₄C powders as starting materials for coating, spraying and producing functional composite ceramics.

Generally, preparation of B₄C by the carbothermal route by Equation (1) is associated with loss of boron in the form of boron trioxide and/or boron suboxide [10]. Therefore, an excessive amount of B₂O₃ in the starting composition (molar ratio B₂O₃/C > 2:7) is needed to compensate for the boron loss [23,26]. In addition, the morphology control of boron carbide particles in the rapid carbothermal reduction under B₂O₃-rich conditions is strongly dependent on the used boron and carbon sources.

In the present work, boron carbide powders were synthesized by rapid heating of B₂O₃ excess (carbon deficient) powder mixtures of boric acid and carbonizing binder. The effects of heat treatment temperature and starting composition on phase constitution, morphology as well as stoichiometry of prepared powders were studied.

2. Materials and Methods

Commercial boric acid powder (99.99% purity, ABCR, Karlsruhe, Germany) and carbonizing binder OPTAPIX CS 76 (20.0 wt% carbon yield after pyrolysis, Zschimmer & Schwarz, Lahnstein, Germany) were used as starting materials for B₂O₃ and carbon sources, respectively. The compositions and detailed processing conditions of the starting powders are given in

Table 1. Composition of starting powder mixtures, processing conditions, phase constitution and lattice parameters of synthesized boron carbide powders.

Samples	H3BO3/Carbonizing Binder Weight Ratio	B2O3/C * Molar Ratio	Final Temperature (°C)	Dwell Time (min)	Phases after Heat Treatment	Hexagonal Lattice Parameters of Boron Carbide #
						a (Å)	c (Å)
S1	1.54:1	0.75:1	1300	20	B2O3, amorphous C	-	-
S2	1.54:1	0.75:1	1400	20	B2O3, graphite	-	-
S3	1.54:1	0.75:1	1500	20	B2O3, graphite	-	-
S4	1.54:1	0.75:1	1600	20	B4C, B2O3, graphite	-	-
S5	1.54:1	0.75:1	1700	20	B4C, few graphite	-	-
S6	1.54:1	0.75:1	1800	20	B4C	5.6063 ± 0.0004	12.0877 ± 0.0013
S7	1.54:1	0.75:1	1900	20	B4C	5.6068 ± 0.0002	12.0898 ± 0.0008
S8	2.06:1	1:1	1900	20	B4C	5.5985 ± 0.0002	12.0819 ± 0.0007
S9	3.09:1	1.5:1	1900	20	B4C	5.6016 ± 0.0002	12.0823 ± 0.0006
S10	4.12:1	2:1	1900	20	B4C	5.5978 ± 0.0002	12.0693 ± 0.0006
S11	5.15:1	2.5:1	1900	20	B4C	5.6019 ± 0.0014	12.0825 ± 0.0030
S12	6.19:1	3:1	1900	20	B4C	5.6024 ± 0.0002	12.0841 ± 0.0005
S13	7.21:1	3.5:1	1900	20	B4C	5.6015 ± 0.0002	12.0796 ± 0.0006
S14	8.25:1	4:1	1900	20	B4C	5.6016 ± 0.0002	12.0783 ± 0.0006

* calculated B₂O₃/C molar ratio of the starting powders, the carbon yield of the carbonizing binder after pyrolysis is 20.0 wt%. It is assumed that there is no B₂O₃ and C loss during pyrolysis. ^# obtained by Rietveld analysis.

. The molar ratio of B₂O₃ to carbon (both pyrolysis products) was varied between 0.75:1 and 4:1. In the first step, different aqueous solutions were prepared by adding boric acid and carbonizing binder to deionized water under vigorous stirring at 80 °C. Subsequently, the solution mixtures were dried at 90 °C in an oven for 48 h to remove the water. The dried mixtures were pyrolyzed at 400 °C with a heating rate of 1 K/min for 1 h under an argon atmosphere. Spongy black masses were formed after pyrolysis. The obtained precursors, composed of B₂O₃ and carbon, were crushed and screened to powder with a maximum particle size of 315 µm.

The powder mixtures were filled into a specially designed high-density graphite cylinder of 30 mm inner diameter and 110 mm length, equipped with lids on both sides (Figure 1a). Two holes of 1 mm in diameter were drilled at the top of the graphite cylinder for degassing during reaction. The synthesis of boron carbide was performed in a high temperature furnace (Thermal Technology LLC, Santa Rosa, CA, USA, model No. 1000-45180-FP60) under flowing argon by a rapid heating up procedure (heating rate > 500 K/s, Figure 1b). The graphite cylinder with loaded precursor powders was first placed in a cold sample chamber of the furnace and then quickly lifted into the heating chamber, after the furnace was heated to the processing temperature of between 1300 °C and 1900 °C. The pressure inside the furnace was kept at 1 atm and the argon flow rate was 0.2 L/min. After a dwell time of 20 min at processing temperature, the graphite reactor was pulled from the heating chamber in the sample chamber and naturally cooled down to room temperature.

To study decomposition and phase reaction of the precursor mixtures during heat treatment, differential thermal analysis (DTA), thermogravimetry (TG) and derivative thermogravimetry (DTG) were carried out on an unpyrolyzed powder mixture with a molar ratio of B₂O₃/C = 2:1 using a Netzsch STA 449 F1 thermal analyzer at a temperature up to 1600 °C in argon. The heating rate of this experiment was 10 K/min. The phase composition of the synthesized boron carbide powders in Table 1 was determined by X-ray diffraction (XRD, Bruker D8) with Cu Kα (λ = 1.54178 Å) radiation, and the morphology was observed using field-emission scanning electron microscopy (FESEM, ULTRA 55; CARL ZEISS, Jena, Germany) equipped with an energy-dispersive X-ray (EDX) detector.

3. Results and Discussion

3.1. Thermal Analysis

The thermal analysis results of the unpyrolyzed powder mixture with a molar ratio of B₂O₃/C = 2:1 (sample S10) are shown in Figure 2. It shows that there are two stages of mass loss. In the range of 60–380 °C, the TG curve exhibited a sharp mass loss of about 45%, corresponding to a broad endothermic peak in the DTA curve, which could be ascribed to the thermal dehydration of boric acid and carbonizing binder. According to Pankajavalli et al. [27], H₃BO₃ decomposes through consecutive reactions, it firstly converts to metaboric acid HBO₂ between 100 °C and 130 °C, and further converts to boron trioxide above 160 °C.

$${\mathrm{H}}_3{\mathrm{BO}}_3\stackrel{100–130 °\mathrm{C}}{\to } {\mathrm{HBO}}_2\stackrel{ >160 °\mathrm{C}}{\to } {\mathrm{B}}_2{\mathrm{O}}_3$$

(2)

The overall reaction of the decomposition can be expressed as:

2 H₃BO₃ → B₂O₃ + 3 H₂O

(3)

In the temperature interval between 380 °C and 1300 °C, no thermal effects and no notable weight loss were observed. Therefore, we chose 400 °C as the pyrolysis temperature for the starting powders to obtain a fully converted mixture of B₂O₃ and carbon. The second stage of the mass loss started at around 1320 °C. The weight loss of this stage reached about 45% at 1600 °C, such a drastic mass loss is due to large amounts of gas generation during the carbothermal reduction. The evaporation of boron trioxide and boron suboxides formed in the heating process at high temperatures could be an additional contributor to the total mass loss [14,28].

3.2. Effect of Heat Treatment on Synthesis of Boron Carbide Powder

Figure 3 shows the XRD patterns of the starting powder mixture with a molar ratio of B₂O₃/C = 0.75:1 and products (samples S1–S7) obtained by heat treatment at different temperatures of 1300–1900 °C for 20 min. As can be seen from Figure 3, the starting powders consist of only B₂O₃ and amorphous carbon. After heat treating at 1300 °C, 1400 °C and 1500 °C, broad carbon peaks were found, indicating that nano-crystalline carbon was formed. The intensity of the B₂O₃ peaks decreased gradually, indicating B₂O₃ loss during heat treatment. After heat treatment at 1600 °C for 20 min, peaks of boron carbide were detected; however, there were still unreacted reactants in the product. By increasing the heat-treating temperature up to 1700 °C, the peaks corresponding to B₂O₃ and carbon decrease. For the powder products obtained at 1800 °C and 1900 °C for 20 min, only peaks assigned to B₄C were observed.

3.3. Effects of Heat Temperature on Morphology of Synthesized Boron Carbide Powder

SEM micrographs of the boron carbide powders (samples S5–S7) synthesized from a molar ratio of B₂O₃/C of 0.75:1 at 1700 °C, 1800 °C and 1900 °C for 20 min are illustrated in Figure 4a–d, respectively. The powders are characterized by the coexistence of fine particles and elongated platelets. When the heating temperature increased from 1700 °C to 1900 °C, more small polyhedral products with a particle size of 1–3 µm were formed. The reduction in particle size is probably attributed to the increasing reaction temperature and heating rate, which will be discussed further below. The carbothermal reduction for the formation of B₄C is considered an interface-controlled process containing nucleation and growth of nuclei [28]. The nucleation rate in the early reaction stage can be written as [28,29]:

$${\mathrm{I}=\mathrm{I}}_0{\mathrm{N}}_{\mathrm{s}}\exp (-\frac{\Delta \mathrm{G}}{\mathrm{kT}})$$

(4)

where I₀ is the frequency of adding one more atom to a nucleus, Ns is the available nucleation site density and ΔG is the nucleation barrier. Based on the classical nucleation theory, Foroughi et al. [28] studied the change of the nucleation barrier ΔG for the B₄C nucleation in dependence on reaction temperature. It is found that the driving force for the carbothermal reduction becomes larger and the nucleation barrier ΔG becomes smaller by increasing the process temperature. Accordingly, the nucleation rate from Equation (4) increases with increasing carbothermal reduction temperature. On the other hand, an increasing temperature gradient between the heating chamber and the cold chamber (Figure 1b) was created by increasing the temperature from 1700 °C to 1900 °C, which increased the heating rate for the reaction. At higher heating rates and higher reaction temperatures, enhanced heat transfer and increased nucleation occur, leading to the formation of many small crystallites [14].

During the heating process, liquid B₂O₃ is first formed (T_melting = 450 °C) and partially evaporated in the form of B₂O₃/ boron suboxides at high temperatures, as indicated by the thermal analysis and XRD analysis. The carbothermal reaction could occur through both the liquid and gaseous boron oxide with solid carbon. As a result, a morphology with mixed elongated platelets and small particles, like our work in Figure 4, is commonly reported in the B₄C products [16,28]. Jung et al. [26] attributed the formation of the elongated platelets with layered structure and smooth surfaces to a gas–solid interface reaction and the formation of fine-sized particles with truncated polyhedron morphology to a liquid–solid interface reaction.

3.4. Effect of Starting Composition on Stoichiometry of Synthesized Boron Carbide Powder

Theoretically, the molar ratio of B₂O₃/C for the synthesis of stoichiometric B₄C according to reaction (1) is about 0.286 (2:7). Due to the boron loss during the heating process, the application of excess B₂O₃ is a common approach to obtain carbon-free products. Furthermore, the results of the study by Weimer et al. [13] suggest that a large excess of B₂O₃ can yield boron-enriched boron carbide. To investigate the effects of starting composition on the stoichiometry of synthesized powders, the molar ratio of B₂O₃/C of the starting mixtures was varied from 0.75:1 to 4:1.

Figure 5 presents the XRD patterns of the prepared products (samples S7–S14), obtained at 1900 °C for 20 min. It reveals that the carbothermal reduction was completed for all powders and only peaks of the boron carbide phase were detected. Based on the XRD data, the hexagonal lattice parameters of the powders were determined using Rietveld refinement and are listed in Table 1. It is well known that the lattice parameters of boron carbides are correlated with the carbon content of the B-C lattice (Figure 6 insets). The lattice parameter a experiences a steady increase toward more boron-rich stoichiometries, whereas a change in the slope at about 13 at% C is observed for the compositional dependence of the lattice parameter c [3]. Comparison of the lattice parameters in our study shows that large excess B₂O₃ has hardly any influence on the stoichiometry of the resulting boron carbides (Figure 6). All prepared powders can be assigned to a stoichiometric boron carbide phase B₄C. The values of the lattice parameters are comparable with those of the standard B₄C (PDF 00-035-0798, a = 5.6003 Å and c = 12.086 Å). Weimer et al. [13] studied the effect of processing temperature and initial boron to carbon molar ratio on product composition for the B₂O₃-C system using thermodynamic equilibrium calculations. The results showed that B₄C is the stable reaction product for a stoichiometric feed composition (molar ratio B₂O₃/C = 0.276) up to its melting point (2743 K). In the presence of excess B₂O₃, boron-enriched boron carbides can be synthesized at temperatures above 2300 K. The formation of the stoichiometric B₄C phase in our study may be attributed to the lower heat-treating temperature.

3.5. Effect of Starting Composition on Morphology of Synthesized Boron Carbide Powder

Figure 7 shows SEM micrographs of boron carbide powders synthesized at 1900 °C for 20 min using a starting mixture with different B₂O₃/C molar ratios. It is seen that the amount of the elongated boron carbide platelets decreases with increasing B₂O₃/C ratios. For the powder products obtained from a B₂O₃/C ratio of 2 or greater, only fine-sized B₄C particles were formed. In addition, the particle sizes decreased from micron- to submicronsized range as the B₂O₃/C ratios increased from 2 to 4. The B₄C powders produced from a starting mixture with a molar ratio of B₂O₃/C = 4 exhibit a narrow particle size distribution and the average particle size is approximately 300 nm. The change of the morphology in Figure 7 indicates that the formation of B₄C for mixtures with a molar ratio of B₂O₃/C ≥ 2 may take place completely through the direct reaction of liquid boron oxide with carbon.

3.6. Formation Mechanism

Based on the study of process kinetics, Weimer et al. [14] reported that the formation of boron carbide by carbothermal reduction is highly dependent on the phase changes of reactant boron oxide (solid B₂O₃ → liquid B₂O₃ → gaseous B₂O₃/boron suboxides), the heating rate and the ultimate temperature. The formation process of boron carbide may include the following reactions:

B₂O₃(s) → B₂O₃(l) → B₂O₃(g)

(5)

B₂O₃(l,g) + C(s) → B₂O₂(g) + CO(g)

(6)

2B₂O₃(l) + 7C(s) → B₄C(s) + 6CO(g)

(7)

2B₂O₃(g) + 7C(s) → B₄C(s) + 6CO(g)

(8)

2B₂O₂(g) + 5C(s) → B₄C(s) + 4CO(g)

(9)

The gaseous boron suboxide (B₂O₂) is generated at about 1277 °C according to Equation (6), and the formation is more favorable at high temperatures [33]. Weimer et al. [14] observed a change in the mechanism from liquid–solid reaction (Equation (7)) to gas–solid reaction (Equation (9)) at about 1700 °C. However, in the case of a large B₂O₃ excess, the influence of starting composition on the reaction mechanism should be considered. Based on the results of our study, we propose the formation mechanism for boron carbide with respect to a large B₂O₃-rich environment, which is presented in Figure 8. For the starting mixtures with a molar ratio of B₂O₃/C < 2, part of the carbon surface was covered by liquid B₂O₃ during heat treatment. Both liquid–solid reaction and gas–solid reaction could occur simultaneously, resulting in fine-sized and elongated platelet B₄C powders, respectively. When the molar ratio of B₂O₃/C ≥ 2, all the carbon particles were covered by B₂O₃ liquid and there was no free carbon surface available for a gas–solid reaction. As a result, the formation of B₄C takes place only by liquid–solid reaction. The gaseous B₂O₃/B₂O₂ has no contribution to the synthesis of boron carbide, the resulting B₄C powders are uniform and very fine.

4. Conclusions

In this study, the synthesis of boron carbide powders using rapid carbothermal reduction from boric acid and carbonizing binder was investigated. Boron carbide powders without free carbon were successfully prepared from starting mixtures with a molar ratio of B₂O₃/C from 0.75:1 to 4:1. The starting composition has no effect on the stoichiometry of the products, all synthesized boron carbides have a stoichiometric composition of B₄C, which is attributed to the low processing temperatures. The particle size of the B₄C powders was strongly affected by the heat-treating temperatures and B₂O₃/C molar ratios. The nucleation rate of primary boron carbide was increased with increasing temperature and heating rate, resulting in the particle size reduction of B₄C.

In addition, it is found that for the starting mixtures with a molar ratio of B₂O₃/C < 2 the formation of boron carbide occurs through both liquid–solid reaction and gas–solid reaction. Accordingly, the synthesized powders exhibit platelet-shaped and fine-sized morphology. For the starting mixtures with B₂O₃/C ≥ 2, uniform and fine-sized B₄C particles were formed only by liquid–solid reaction. Our study demonstrates that the rapid carbothermal reduction approach provides a simple way to achieve high purity submicro B₄C powders. Further work should be done to identify the characteristics of the produced powders, such as purity, particle size distribution and bulk density. The sinterability and mechanical properties of sintered RCR powders should also be tested.