Formation Of Boron Carbide Research Articles

EXPERIMENTAL STUDY OF BORON CARBIDE FORMATION IN COMBUSTION OF BORON-CONTAINING SOLID PROPELLANTS

Using model aluminum dodecaboride (AlB<sub>12</sub>)/binder mixtures imitating the matrix of boron-containing solid propellants, the process of formation of boron carbide in combustion of such propellants has been experimentally studied. To characterize the samples under study, thermogravimetric analysis and differential thermal analysis were used, which made it possible to determine the initial stage of decomposition of samples with different contents of AlB<sub>12</sub>. Experiments on the rapid thermal decomposition of samples were carried out on the original URAN-1 installation, which makes it possible to create heat flows to the sample surface in the range of 70 to 300 W/cm<sup>2</sup>. It has been shown that the condensed residue formed during the pyrolysis of the binder consists mainly of carbon, which, upon interaction with boron, forms boron carbide. The dependence of the mass of the carbonaceous residue on the boron content in the propellant matrix (mixture of binder and powdered boron-containing fuel) has been determined. The mass rate of matrix pyrolysis has been studied as a function of the incident energy flux density and the content of powdered boron-containing fuel in the matrix. It is shown that during high-speed heating of the matrix, the amount of boron carbide formed strongly depends on the rate of energy supply. The dependence of the relative mass of the resulting boron carbide on the temperature in the pyrolysis zone was determined, which can be conditionally divided into four areas: (1) up to a temperature of 1400°C, boron carbide is practically not formed; (2) in the temperature range from 1400°C to about 1700°C, the amount of boron carbide formed increases sharply with increasing temperature from zero to about 10-15%; (3) in the temperature range from 1700°C to about 2500°C, the amount of boron carbide formed is slightly dependent on temperature; (4) for temperatures above 2500°C, the amount of boron carbide formed in the condensed products of the studied samples sharply increases with temperature, and at T ≈ 2600°C reaches 50% of the mass of the entire sample residue. A theoretical analysis of the investigated process has been carried out.

High‐temperature carbothermal synthesis and characterization of graphite/h‐BN bimaterials

AbstractHexagonal boron nitride (h‐BN) and graphite have similar crystal structures, comparable lattice parameters, and coefficients of thermal expansion, but vastly different electrical and thermal transport. Despite their key differences, it is possible to couple h‐BN and graphite in a bimaterial system allowing the unique properties of both materials to be utilized in a single component. Through a carbothermal reduction of B2O3 in nitrogen, the surface of graphite can be converted to h‐BN. This results in a layered system that is electrically insulating on the surface due to h‐BN, and more compliant as well as conductive within the substrate due to the graphite structural body. We discuss the high‐temperature synthesis and characterization of this layered material, focusing on the processing–microstructure relationship as well as the interface of graphite/h‐BN to assess the chemical and mechanical adhesion of the layers, and to establish how such properties are contingent on the reacting phase of B2O3. This is achieved by investigating the origin of h‐BN formation and the unwanted side reaction of boron carbide formation, through the evaluation of the thermochemistry and kinetics governing the carbothermic reactions. We establish that a reaction temperature and holding time of 1700°C for 18 h produced the thickest h‐BN layers which exhibited the highest fracture toughness over all lower temperature synthesis conditions.

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.

Production of Spherical Boron-Carbide Particles Encapsulated in a Graphite Shell

Laser ablation makes it possible to obtain boron-carbide nanoparticles suitable for subsequent use in boron neutron capture therapy. However, particles of boron and boric acid are formed in parallel. Boric acid is harmful to the human body, and boron can naturally form boric acid. A method is proposed for reducing the toxicity of the synthesis of boron-carbide nanoparticles by laser ablation by adding free carbon to the reaction and replacing the traditionally used buffer medium, ethyl acetate. Purified water is used as a dispersing medium. It is shown that the described technology has prospects for reducing the level of toxicity due to the choice of a suitable buffer medium. The formation of boron carbide is recorded by transmission electron microscopy and XRD analysis. The formed particles are predominantly spherical in shape and have a carbon shell. A mechanism is proposed for the formation of such a shell due to the diffusion of carbon from the bulk of the particle through the domain boundaries.

Gas‐phase kinetic of boron carbide chemical vapor deposition using BCl3+CH4+H2 mixture

AbstractA detailed chemical kinetic mechanism consisting of 472 reactions and 61 species for the gas‐phase thermal decomposition of BCl3+CH4+H2 mixtures is used to model the chemical vapor deposition (CVD) of boron carbide. The mechanism is constructed using a reaction mechanism generator (RMG), which is automatic and requires no or less human intervention. New functionality, considerable thermodynamic and kinetic data have been added to RMG to account for boron chemistry. The model considers all necessary reactions in the reaction pathways and reasonably predicts the experimental data available from the literature. The sensitivity analysis identifies crucial species responsible for boron carbide formation. Besides, a reduced mechanism (15 species and 26 reactions) is derived from a detailed mechanism, which could be used to expedite an effective CVD reactor design through numerical simulations. The performance of the reduced model is also evaluated concerning the complete mechanism, which shows that the reduced model can predict the reactor behavior reasonably well.

Synthesis of boron carbide from its elements up to 13 GPa

The synthesis of boron carbide from its elements (boron and carbon) has been studied under pressures up to 13 GPa and optimum parameters have been determined by varying the (P, T, reactants) conditions. Stoichiometric mixtures of amorphous boron and amorphous carbon have been subjected to a range of temperatures from 1673 K to 2273 K at the pressure values of 2 GPa and 5 GPa. The formation temperatures have been compared to those obtained from mixtures of β rhombohedral boron and graphite, and β rhombohedral boron and amorphous carbon at 2 GPa and 5 GPa. The formation temperature is thus shown to be affected by the pressure and the choice of the reactants. The carbon concentration of boron carbide is also shown to be affected by the pressure at which it is synthesised from elements, and we propose pressure as a means to control the carbon content. Temperature cycling has also been shown to reduce the formation temperature of boron carbide at 13 GPa. The formation of α rhombohedral boron as an intermediate phase is seen at 5 GPa before the formation of boron carbide.

Sintering of composite materials for tool appointment, based on impact diamonds, under high pressure and temperatures

The article presents the results of a study of composite materials based on diamond-lonsdaleite abrasive (DLA) and various binders (Fe–Ti mechanocomposite, silicon carbide SiC). A metal-matrix composite material with a multimodal nano- and microlevel structure, characterized by increased adhesion of diamond grains to the binder, is obtained on the basis of impact diamonds and a Fe–Ti nano-mechanical composite. It is shown that the use of impact diamonds in comparison with synthetic diamonds makes it possible to reduce the pressure of thermobaric treatment by 30–50 % at the same sintering temperatures. The use of Fe–Ti–DLA composites in the process of magnetic-abrasive polishing (MAP) makes it possible to increase the removal rate of material based on silicon by 1.5–2 times and reduce the processing time by 30 % compared to ferroabrasive powder (FAP) based on synthetic diamonds. The effect of adding of silicon carbide on the process of obtaining a superhard composite material impact diamond – SiC is investigated. It is found that adding of SiC helps to reduce the defectiveness of the material and increase the homogeneity of its structure in comparison with the material without adding of a binder. In this case, an increase in the content of SiC and Si also leads to an inversion of the structure type of the superhard composite from polycrystalline to matrix. It is found that the additional use of amorphous soot and boron affects the refinement of the matrix structure of the composite material due to the formation of boron carbide and secondary finely dispersed silicon carbide.

Synthesis of boron carbide from its elements at high pressures and high temperatures

The formation of boron carbide under high pressures and from elemental reactants has been studied. To this end, stoichiometric mixtures of commercial crystalline beta rhombohedral boron and amorphous glassy carbon have been subjected to temperatures ranging from 1473 K to 2473 K at pressures of 2 GPa and 5 GPa. Similar syntheses have been repeated for crystalline boron and graphite. As the synthesis pressure increases, the temperature of formation of boron carbide is found to increase as well. Moreover, the reaction temperature is shown to be affected by the choice of the reactants.

Morphological changes of calcium hexaboride powder synthesized from condensed boric acid-polyol product

We previously reported a novel low-temperature synthesis method for calcium hexaboride (CaB6) powder via the transient formation of boron carbide (B4C) using a condensed boric acid (H3BO3)-polyol product. In this study, the effect of the type of polyol on the morphology of the CaB6 powder was investigated. Three types of polyol, glycerin, mannitol, and poly(vinyl alcohol) (PVA), were used for the preparation of condensed H3BO3-polyol products since the B4C particles prepared from these polyols exhibit different morphologies. CaB6 powder was successfully synthesized at a low temperature of 1400 °C regardless of the polyol used. The size of the obtained CaB6 particles varied with the polyol used, which reflected the size of the B4C particles prepared using the corresponding polyol. These results indicate that the morphology of the obtained CaB6 powder is derived from that of the transiently formed B4C particles.

Low-temperature green synthesis of boron carbide using aloe vera

The unique structural and physical properties of boron carbide, which make it suitable for a wide range of applications, demands the development of low-cost and green synthesis method. In the present work, the commonly available leaves of aloe vera are hydrothermally treated to form the carbon precursor for the synthesis of boron carbide. The morphological characterization reveals the porous nature of the precursor turning into a tubular structure upon boron carbide formation. The structural characterization by x-ray diffraction and other spectroscopic techniques such as Fourier transform infrared, Raman, photoluminescence and uv-visible near-infrared spectroscopy confirm the formation of boron carbide. The thermogravimetric analysis of the sample is found to exhibit good thermal stability above 500 °C. When the sample is annealed to 600 °C, boron carbide with phase purity is obtained, which is confirmed through XRD and FTIR analyses. The optical emission properties of the sample are studied through CIE plot and power spectrum. Compared with other natural precursors for boron carbide, the aloe vera is found to give a good yield above 50%.

Low temperature synthesis of multilayered-hollow microspheres of boron carbide from castor oil for photonic applications

The development of boron carbide microspheres of various types is of greater interest because of their applications in thermal insulation, imaging, luminescent coating, and drug delivery. The present methods of synthesis involve costlier equipment, toxic chemicals, and high temperature. This work describes low temperature, green synthesis of multilayered-hollow microspheres of boron carbide using the natural carbon precursor—castor oil. Refluxion of boric acid with castor oil and successive cooling results in the formation of boron carbide and is subjected to thermal deposition at 300 °C for 1 h to get a thin film. The field emission scanning electron microscopic analysis shows the formation of multilayered hollow microspheres in the film whose surface morphology is studied using an atomic force microscope. The X-ray diffraction, X-ray mapping, and energy dispersive spectroscopic and Fourier transform infrared spectroscopic analyses confirm the formation of boron carbide which is thermally stable and gives a yield of 37% at 800 °C. The optical characterization of the sample is carried out with ultraviolet-visible and photoluminescence spectroscopic techniques. The chromaticity diagram and the power spectrum suggest the possible use of the material as a blue light emitter.

Natural cotton as precursor for the refractory boron carbide—a hydrothermal synthesis and characterization

Boron carbide (B4C) is an excellent covalent carbide that finds applications in industries and nuclear power plants. The present synthesis methods of boron carbide are expensive and involve the use of toxic chemicals that adversely affect environment. In the present work, we report for the first time the use of the hydrothermal method for converting the cellulose from cotton as the carbon precursor for B4C. The carbon precursor is converted into functionalized porous carbonaceous material by hydrothermal treatment followed by sodium borohydride. It is further treated with boric acid to make it a B4C precursor. The precursor is characterized by UV–visible diffuse reflectance, Raman, Fourier transform infrared, photoluminescent and energy dispersive spectroscopy. The morphology and structure analysis is carried out using field emission scanning electron microscopy and x-ray diffraction techniques. The results of structural and optical characterization of the sample synthesized are compared with the commercial B4C. The thermal stability of the sample is studied by thermogravimetric analysis. The sample annealed at 700 °C is found to be B4C devoid of amorphous carbon with a yield of 44.7%. The analysis reveals the formation of boron carbide from the sample.

High thermal conductivity of Cu-B/diamond composites prepared by gas pressure infiltration

A gas pressure infiltration method was used to prepare diamond particles reinforced Cu-B alloy matrix composites (Cu-xB/diamond, x = 0.3 and 1.0 wt%). The Raman spectroscopy and X-ray diffraction (XRD) analysis confirm the formation of boron carbide (B4C) at the Cu/diamond interface. The amount of interfacial boron carbide increases with increasing boron content in the Cu matrix. A high thermal conductivity of 868 W/mK and corresponding coefficient of thermal expansion of 5.3 × 10−6/K are obtained in the Cu-0.3 wt%B/diamond composite. When the boron content increases to 1.0 wt% B, the thermal conductivity decreases to 647 W/mK. It is found that boron carbide plays a critical role in improving the interfacial bonding and enhancing the thermal conductivity of the Cu-B/diamond composites; however, too much interfacial boron carbide will deteriorate the thermal conductivity of the composites.

Preparation of Boron Carbide from BF3 and BCl3 in Hydrogen Plasma of Arc RF Discharge

A comparative study was carried out of the process of plasma chemical deposition of boron carbide from hydrogen plasma containing the mixtures of BF3 + CH4 and BCl3 + CH4 sustained by RF arc (13.56 MHz) discharge. It was shown that in the case of synthesis of B4C from a mixture of BF3 + CH4, carbon and complex coordination compound [X3B]−H+ (R3B·FH) are formed as the by-products of condensed products. In the case of synthesis of B4C from the BCl3 + CH4 mixture, the only condensed product is carbon. Mechanisms for the formation of boron carbide on the surface of heated electrodes are proposed. The main feature of these mechanisms is the preliminary deposition of a graphite layer from CH4 and then the precipitation of boron with the participation of the radicals BF2, BF and BCl. B4C samples were obtained and the impurity composition, morphology and structure of bulk boron carbide samples obtained using both of its halides were studied. It was found that in both cases a carbon phase is present in boron carbide samples. The main impurities entering the B4C, in the case of using a mixture of BF3 + CH4, is silicon, and in the case of a mixture of BCl3 + CH4, is tungsten.

Synthesis of boron carbide nanoparticles via spray pyrolysis

Abstract

Modification of the surface properties of highly dispersed AlB12 by a high-power carbon ion beam

Modifications in the morphology, elemental composition, and microstructure of aluminum dodecaboride (AlB12) surface layers exposed to a high-power ion beam are studied using scanning electron microscopy, X-ray microanalysis, and optical metallography. It is demonstrated that the topography of the ion-modified aluminum dodecaboride surface is determined by the dose and power density of the carbon ion beam. It is established that the effect of the high-power pulsed carbon ion beam on the AlB12-based material fabricated by self-propagating high-temperature synthesis leads to the formation of boron carbide on the surface of the latter.

Boron Carbide B<sub>25</sub>C and B<sub>8</sub>C<sub>18</sub> Synthesized Using Boric Acid-Glucose and Boric Acid-Active Carbon at Low Temperature without Coreductor Materials

This study examines the formation of Boron carbide with a wide homogeneity range B8C18 and B25C, from boric acid-glucose and boric acid-active carbon as precursor materials. The samples was analysed by means of X-ray diffraction and Fourier Transform Infrared spectrometer. X-ray diffraction pattern was analysed by GSAS software. X ray diffraction pattern of B8C18 shows peaks at 27o, 39o, 45o, 65o and 79o, orthorhombic crystal system and lattice parameters a = 13.640 Ǻ, b = 7.8500 Ǻ and c = 12.910 Ǻ and space group P212121, whereas for B25C sample show peaks at 2θ angle 28o and 40o, tetragonal crystal system, and the lattice parameters a = b = 8.753 Ǻ and c = 5.093 Ǻ and space group P-4 2 m. FTIR results show that for B8C18 have absorption bands with B-C bond at 1196.5 cm-1, 0-H 3216.7 cm-1 and B-O at 1477.1 cm-1, whereas the formation B25C have absorption bands with B-C bond at 1195.1 cm-1, O-H 3216.7 cm-1 and B-O 1460.8 cm-1.

Formation of boron carbide under the action of low-voltage nonstationary electrical discharges on trialkyl orthoborates

Application of a low-voltage nonstationary electric arc to trialkyl orthoborates at low medium temperatures (£ 130 C) leads to boron carbide and carbon as the main products.

Effect of Carbon and Oxygen on the Densification and Microstructure of Hot Pressed Zirconium Diboride

The role of carbon additions and oxygen content on the densification of zirconium diboride (ZrB2) was studied. ZrB2 with up to 1 wt% added carbon was hot pressed at temperatures of 2000°C and 2100°C. Nominally pure ZrB2 hot pressed at 2100°C achieved relative densities >95.5%. Carbon and oxygen analysis indicate that oxygen removal was facilitated by the reduction of oxides with carbon or the removal of boria (B2O3) as a vapor. Therefore, by removing oxides from the particle surfaces, carbon additions of ≥0.5 wt% enabled densification to proceed to >96.5% of theoretical at 2000°C. Raman spectroscopy revealed the formation of boron carbide (B4.3C) in specimens with carbon additions of ≥0.75 wt%. The formation of B4.3C was eliminated via a 1 wt% addition of zirconium hydride (ZrH2), as a source of zirconium, resulting in the formation of carbon as the only residual second phase. Grain sizes were in the range of 7–10 μm (2000°C) and 12–16 μm (2100°C) and only appeared to be controlled by temperature, as no trends due to the evaluated carbon or oxygen contents were observed.

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.