Synthesis Of Boron Carbide Powder Research Articles

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

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.

Synthesis of boron carbide powders from mono- and polysaccharides

Boron carbide powders are usually synthesized by carbothermal or magnesiothermic reduction of boron oxy-compounds, a carbon source is usually graphite or some organic phases. The aim of the work is using of mono- and polysaccharides for synthesis of boron carbide powders in which the final grains are separated from each other by an unreacted excess of carbon preventing both grain growth and formation of strong aggregates. Aqueous solutions of glucose, fructose, dextrin or hydroxyethyl starch and boric acid were dried, pyrolyzed at 850 °C for 1 h and then heat-treated at temperatures from 1300 to 1700 °C for 1 h. The amount of boron carbide in the powders depends both on the saccharide and on the temperature. The grains prepared from precursors containing monosaccharides had oval shapes whereas polysaccharides were applied they had rhombohedral shapes corresponding to the boron carbide structure. This effect can be attributed to presence of molten boron oxide.

Ethylene glycol assisted low-temperature synthesis of boron carbide powder from borate citrate precursors

B4C powders were synthesized by carbothermal reduction of ethylene glycol (EG) added borate citrate precursors, and effects of EG additions (0–50 mol% based on citric acid) on the morphologies and yields of synthesized B4C powders were investigated. The conditions most suitable for the preparation of precursor were optimized and optimum temperature for precursor formation was 650 °C. EG additions facilitated low-temperature synthesis of B4C at 1350 °C, which was around 100–300 °C lower temperature compared to that without EG additions. The lowering of synthesis temperature was ascribed to the enlargement of interfacial area caused by superior homogeneity and dispersibility of precursors enabling the diffusion of reacting species facile. The 20% EG addition was optimal with free residual carbon lowered to 4%. For smaller EG additions, the polyhedral and rod-like particles of synthesized product co-existed. With higher EG additions, the morphology of synthesized product was transformed into needle and blade-like structure.

Effect of addition of tartaric acid on synthesis of boron carbide powder from condensed boric acid–glycerin product

The effect of the addition of tartaric acid on the synthesis of boron carbide (B4C) powder from a condensed boric acid (H3BO3)–glycerin product was investigated in this study. The condensed product was prepared by dehydration condensation after directly mixing equimolar amounts of H3BO3 and glycerin with the addition of 0–50mol% tartaric acid (based on glycerin), which was followed by thermal decomposition in air to obtain a precursor powder from which excess carbon had been eliminated. The dispersion state of the boron oxide (B2O3) and carbon components in the precursor prepared from the condensed product with 25mol% tartaric acid added was finer than that without tartaric acid added, in which both precursors had a three-dimensional bicontinuous B2O3/carbon network structure. The complete formation of crystalline B4C powder was achieved at 1250°C within a shorter heat treatment time for the precursor with a fine dispersion state. The synthesized B4C powders became fine owing to the increased number of nucleation sites.

Low-Temperature Synthesis of Boron Carbide Powder by Boric Acid-Polyvinyl Alcohol Gel Precursor

Low-Temperature Synthesis of Boron Carbide Powder by Boric Acid-Polyvinyl Alcohol Gel Precursor

Microstructure in precursor formed by controlling composition of condensed boric acid-poly(vinyl alcohol) product for low-temperature synthesis of boron carbide powder

The dispersion state of boron oxide (B2O3) and carbon components in a precursor prepared from a condensed boric acid (H3BO3)-polyol product by thermal decomposition in air was found to be closely related to the formation of boron carbide (B4C) powder at a low synthesis temperature. The microstructure in the precursor was fabricated by controlling the composition of a condensed H3BO3-poly(vinyl alcohol) (PVA) product. The size of B2O3 particles dispersed in the carbon matrix of the microstructure decreased with increasing PVA content of the condensed product. Crystalline B4C powder with a little free carbon was synthesized from a precursor with a more finely and homogeneously dispersed structure consisting of B2O3 particles and the carbon matrix by heat treatment at 1200°C for 5 h in an Ar flow. Furthermore, the formation and subsequent grain growth of B4C particles were promoted even at a low synthesis temperature.

Effect of Molecular Structure of Polyols with Different Molecular Characteristics on Synthesis of Boron Carbide Powder

Crystalline boron carbide (B4C) powder was synthesized by the carbothermal reduction of condensed products formed from boric acid (H3BO3) and polyols with different molecular characteristics, i.e., glycerin, mannitol, and poly (vinyl alcohol) (PVA). The effect of the molecular structure of the polyol on the thermal decomposition conditions and the obtained morphology of the B4C powder was discussed in this study. The thermal decomposition in air of each condensed product was performed before the carbothermal reduction in order to eliminate the excess carbon, where the decomposition conditions varied with the type of polyol. Crystalline B4C powder with less residual free carbon was synthesized from the thermally decomposed products by heating at 1250 °C for 5 h in an Ar flow. The thermal decomposition conditions and the particle size of the obtained B4C powder reflected the molecular characteristics of the polyols.

Synthesis of boron carbide powder in relation to composition and structural homogeneity of precursor using condensed boric acid–polyol product

Crystalline boron carbide (B4C) powder was synthesized by the carbothermal reduction of boron oxide (B2O3) using a condensed boric acid (H3BO3)-mannitol product. The condensed product was prepared by the dehydration condensation of H3BO3 and mannitol at several molar ratios. The composition of the precursor was controlled by thermal decomposition in air before the carbothermal reduction of the condensed product, which had a homogeneous structure but an excessive carbon component compared with that required for the carbothermal reduction. B4C powder with less free carbon could be synthesized by heat treatment at 1250°C for 5h in an Ar flow of the thermally decomposed product prepared from the condensed product by thermal decomposition at 400°C for 3h in air. Furthermore, the structural homogeneity of the precursor affected the B4C formation behavior at the low synthesis temperature. These results demonstrated that the compatibility of the composition, the dispersibility, and the homogeneity of the B2O3 and carbon components in the precursor, which was achieved by thermal decomposition of the condensed product in air, leads to the low-temperature synthesis of crystalline B4C powder using a condensed H3BO3–polyol product.

Low-temperature synthesis of boron carbide powder from condensed boric acid–glycerin product

Crystalline boron carbide (B4C) powder was synthesized by the carbothermal reduction of a condensed product formed from boric acid (H3BO3) and glycerin (C3H8O3). The condensed product was prepared by dehydration after directly mixing equimolar amounts of H3BO3 and glycerin, which was followed by pyrolysis in air to obtain a precursor powder from which the excess carbon had been eliminated. The prepared precursor powder had a bicontinuous boron oxide (B2O3)/carbon network structure. Crystalline B4C powder without residual carbon was successfully synthesized from this precursor powder by heating at 1250°C for 5h in an Ar flow.

Effect of boron oxide/carbon arrangement of precursor derived from condensed polymer-boric acid product on low-temperature synthesis of boron carbide powder

The effect of the boron oxide (B2O3)/carbon arrangement of the precursor on the low-temperature synthesis of boron carbide (B4C) powder was investigated in this study. The precursor was prepared by low-temperature heat treatment without thermal decomposition and subsequent pyrolysis in air of the condensed product formed from poly(vinyl alcohol) (PVA) and boric acid. The dispersion state of B2O3 particles in the carbon matrix of the precursor significantly affected the acceleration of the synthesis reaction. Crystalline B4C powder with little free carbon was obtained by heat treatment at 1100°C, which was also the critical formation temperature, for 20 h in an Ar flow starting from a precursor with a B2O3/carbon structure homogeneously arranged at the nanometer scale.

Synthesis of boron carbide powder from hexagonal boron nitride

We report the synthesis of boron carbide powder via the reaction of hexagonal boron nitride with carbon black. The reaction between hexagonal boron nitride and carbon black completed at 1900 °C for 5 h in vacuum. The particle sizes of the synthesized boron carbide powder were about 100 nm from transmission electron microscopy. The possible reaction mechanism was that hexagonal boron nitride decomposed into elemental boron and nitrogen even when there was no carbon at a relatively low rate, and introduction of carbon into hexagonal boron nitride powder facilitated the decomposition process; the boron from the decomposition of boron nitride reacted with carbon to form boron carbide.

Synthesis of boron carbide powder from polyvinyl borate precursor

Polyvinyl borate (PVBO) was prepared by the condensation of poly(vinyl alcohol) (PVA) and boric acid, and used as a precursor for boron carbide. Boron carbide powder was synthesized by the pyrolysis of the PVBO precursor in air at 600 °C for 2 h, followed by heat treatment in Ar flow at 1300 °C for 5 h, which is a relatively low temperature compared with conventional carbothermal methods. Pyrolysis of the PVBO precursor resulted in submicron-size particles of B 2O 3 dispersed in a carbon matrix. In addition, the pyrolysis temperature governed the carbon content in the pyrolyzed product of the PVBO precursor, which led to the synthesis of crystalline boron carbide powder with little free carbon.

Production of boron carbide powder by carbothermic reduction from boron oxide and petroleum coke or carbon active

Carbothermic reduction process is recognised as a beneficial method to produce boron carbide powder. In the present article, this method was utilised to produce a boron carbide powder using commercial purity raw materials. Boron oxide being a source of boron and carbon active and petroleum coke as reducing agents were used. Mixtures of boron oxide and carbon holding material with a particle size of <44 μm were put in a graphite crucible and heated under a flow of argon atmosphere in a tube furnace to 1400–1550°C for 1–5 h, resulting in the formation of boron carbide powder with or without unreacted starting raw materials. It was found that the optimum weight ratio of boron oxide to carbon bearing material was 1·8 and 1·6 for petroleum coke and carbon active respectively. Heat treatment of these mixtures at 1470°C for 5 h resulted in the synthesis of boron carbide powders, which contained 0·75 and 0·65 wt-% free carbon respectively.

Synthesis of boron carbide powder by a carbothermic reduction method

Carbothermic reduction process is an economic method to produce boron carbide powder. In the present article this method was utilized to produce a boron carbide powder using commercial purity raw materials. Boric acid as a source of boron, and carbon active and petroleum coke as reducing agents were used. Mixtures of boric acid and carbon bearing material with a particle size of less than 44 μm were placed in a graphite crucible and heated under a flow of argon atmosphere in a tube furnace to 1400–1550 °C for 1–5 h. This resulted in the formation of boron carbide powder with or without un-reacted starting raw materials. It was found that the optimum weight ratio of boric acid to carbon bearing material was 3.5 and 3.3 for petroleum coke and carbon active, respectively. Heat treatment of these blends at 1470 °C for 5 h resulted in the synthesis of boron carbide powders, which contained 0.82 and 0.59 weight percent free carbon, respectively.