Effects of heat-treatment temperature and starting composition on morphology of boron carbide particles synthesized by carbothermal reduction

Carbothermal reduction using B₂O₃ and carbon black was applied for synthesis of B₄C powder and the effects of heat-treatment temperature and starting composition of raw mixture on morphology of B₄C particles were investigated. Morphology of B₄C particles synthesized at 1450 °C was mainly spherical shapes. The B₄C powder synthesized at 1550 °C was large and changed in morphology from polyhedral to skeletal shape, and particle size of B₄C increased with an increase in the amount of B₂O₃ in the starting mixtures. The B₄C powder synthesized beyond 1650 °C consisted from dendrite-like particles aggregated by small primary particles. Morphology of the primary B₄C particles synthesized at 1750 °C changed from polyhedral to rounded shape with increasing the amount of B₂O₃ in the starting mixtures. It is clarified that heat-treatment temperature and the starting compositions of raw mixtures mainly affected B₄C nuclei number along with primary particle size and morphology of primary B₄C particles, respectively.     

Molten-salt-assisted combustion synthesis of B4C powders: Synthesis mechanism and dielectric and electromagnetic wave absorbing properties

A novel electromagnetic wave (EMW) absorber was prepared by combustion synthesis. Boron carbide (B₄C) powders with different grain sizes using a molten-salt-assisted combustion technique with B₂O₃, CB (carbon black), and Mg powders as starting materials, and NaCl as an additives. The effects of the NaCl content on the phase compositions and the microstructure of the products were characterized. A combustion front quenching method was used to elucidate the mechanism for the B₄C powders synthesis. The dielectric, and EMW absorbing properties in the X-band were also investigated. The results showed that the addition of NaCl significantly reduced the grain size of B₄C powders. Nanoscale B₄C powders with cubic polyhedral structures were synthesized using 6 wt% NaCl (labeled as N-6). According to the quenching test results can be obtained that the first step in the combustion synthesis was melting B₂O₃ into a glassy substance. At the same time, Mg melted and formed a liquid pool into which the NaCl dissolved, followed reduction of the B₂O₃ to B. The formed B eventually reacted with CB to form B₄C, and the B₄C particles precipitated from the matrices. The N-6 sample exhibits optimal dielectric and EMW absorbing properties, because of a high specific surface area that enhances interfacial and space charge polarization.     

Understanding the morphological variation in the formation of B4C via carbothermal reduction reaction

Despite the relatively successful effort for the synthesis of fine boron carbide (B₄C) powders, very little is known about the underlying interrelationships between processing, compositions and the resulting product morphology for B₄C formation via the carbothermal reduction reaction (CTR) process. In this paper, B₄C, including uniform submicron powders, were synthesized using CTR of finely mixed boron trioxide and carbon obtained from low-cost water-soluble precursors. To understand the aforementioned interrelationships and to better control product morphology, the effects of factors such as CTR thermal profile, CTR atmosphere and precursors on product phase purity and morphology were systematically studied. Among all the factors, it was found that CTR temperature, along with total reaction time, plays a dominant role in determining the morphology of B₄C products. Such observation was understood by analysis of CTR reaction kinetics and comparing it with microstructural evolution: significant non-uniformity in B₄C product morphology arises from the competition between nucleation and growth at low to intermediate CTR temperatures (e.g., ~1150–1450 °C) while highly uniform submicron or nano B₄C products result from very fast nucleation at high CTR temperatures (e.g., ≥~1750 °C) within a few minutes or even seconds.     

Thermodynamic investigation and reaction mechanism of B4C synthesis based on carbothermal reduction

The reaction mechanism of B₄C synthesis as yet remains unclear due to the lack of sufficient thermodynamic investigation, which limits the large-scale controllable production of B₄C ceramic material. In this paper, thermodynamic investigation of B₄C synthesized by carbothermal reduction was calculated at atmospheric pressure over a wide temperature range (1500-2500 K) coupled with the effects of pressure on B₄C synthesis. Besides, predominance diagrams for B–C–O system at different temperatures and pressures were further established. Upon thermodynamic results, reaction mechanism was proposed and then verified by the synthesis experiment of B₄C at desired temperatures. The thermodynamic investigation indicates that B₄C may be synthesized by the reactions of C with liquid B₂O₃ (LS mechanism) or gaseous B₂O₂ (VS mechanism). At low temperature (~1823 K), B₄C is polyhedral shape via LS mechanism, while two-dimensional flake-like B₄C is synthesized by VS mechanism at high temperature (2123 K~). The obtained two types of B₄C verify the accuracy of thermodynamic results. This study is instructive for the optimization of temperature, pressure for large-scale controllable production of B₄C ceramic material.     

Preparation of Al2O3–SiC composite powder from kyanite tailings via carbothermal reduction process

ABSTRACT

Al₂O₃–SiC composite powders were prepared from kyanite tailings mixed with 20% excess carbon coke via carbothermal reduction (CR) reaction. The optimised synthesis condition for synthesising Al₂O₃–SiC composite powders was at 1600°C for 6?h. The equilibrium relationship curves of the condensed phases were presented and the temperature dependence of the phase composition was also studied. The results show that irregular Al₂O₃ and SiC grains first formed at 1500°C, and the elements C, O, Al, and Si randomly distributed in the each crystal particles. The amount of Al₂O₃–SiC composites increased with the increasing synthesis temperature and reaction time. Finally, Al₂O₃–SiC composite bulk materials were further prepared by pressureless sintering using the synthesised Al₂O₃–SiC composite powders as raw materials, and their mechanical properties were investigated in detail. All these results indicate that the CR method can offer a niche application for the development of kyanite tailings.     

Effects of boron oxide composition,structure, and morphology on B4C formation via the SHS process in the B2O3–Mg – C ternary system

This study investigates the formation of B₄C in the B₂O₃–Mg–C ternary system via a magnesiothermic reduction process using two kinds of boron oxide (B₂O₃) ‒ the commercial B₂O₃ and that synthesized from boric acid via the coupled chemical-thermal process. In addition to the raw materials, the products were subjected to XRD, FTIR, SEM, and FESEM analyses to determine the effects of microstructural and morphological properties of boron oxide as an important raw material, on B₄C formation in the combustion synthesis process. For this purpose, powder mixtures of B₂O₃:Mg:C were prepared at a stoichiometric molar ratio of 2:6:1 and compacted into pellets using a uniaxial hydraulic press, which were subsequently subjected to the combustion synthesis process based on the self-propagating high-temperature synthesis (SHS). Finally, the samples thus obtained were leached in an aqueous hydrochloric acid solution. Analysis of the commercial B₂O₃ revealed the presence of large amounts of such by-products as magnesium borate (Mg₃B₂O₆) and magnesium oxide (MgO) along with relatively small amounts of boron carbide after the leaching process, while those obtained for the chemically-thermally synthesized B₂O₃ showed a relatively large amount of B₄C (from micron-sized particles to nanoparticles) together with a remaining carbon phase and very small amounts of magnesium borate as by-products. It can be, therefore, concluded that the changes in chemical composition and introduction of a hydrous HBO₂ phase in the boron oxide in the B₂O₃–Mg–C mixture as well as its varied microstructure, morphology, and particle size have significant effects on the efficiency of B₄C production through the SHS process.     

Effect of processing parameters on the formation process of nano-sized ZrB2 powders by the high energy ball milling

ABSTRACT

Nano-sized ZrB₂ powders were synthesised using the high energy ball milling with ZrO₂ and B₂O₃ as raw materials and Mg as the reducing agent. The resulting powders were characterised by X-ray diffraction, scanning electron microscopy, laser particle size analysis, transmission electron microscopy, energy dispersive spectrometry, and X-ray photoelectron spectroscopy. The influence of the synthesis parameters, including the ratios of ZrO₂ to B₂O₃, milling medium, and reaction time, on the synthetic course of the ZrB₂ nanopowders were studied systematically. The mechanisms by which these parameters influence the synthetic course of and the resulting product quality are determined. Ultimately, the diameter of the resulting particles is about 200–400?nm, which are an agglomeration composed of many individual small particles with an average diameter of ～50?nm. In addition, the oxidation of ZrB₂ powders has also been studied.     

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

Abstract

Abstract

This paper reports on synthesis of MgAl₂O₄ spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45?vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5?wt‐%Al₂O₃ into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1?h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.     

Combustion synthesis of B4C/Al2O3/C composite powders and their effects on properties of low carbon MgO-C refractories

To improve the dispersity and oxidation resistance of nano carbon black (CB) in low carbon MgO-C refractories, B₄C/Al₂O₃/C composite powders were prepared by a combustion synthesis method using B₂O₃, CB and Al powders as the raw materials. The phase compositions and microstructures of the synthesized products were characterized by X-ray diffraction (XRD), Raman spectroscopy, and a scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The results show that an 80 wt% excess of CB is the maximum amount of CB that can be added under the condition of a self-propagating combustion wave, and the phase compositions of the products are B₄C, α-Al₂O₃ and CB. B₄C particles with uniform sizes and cubic polyhedral structures are embedded in the Al₂O₃ matrix. The combustion-synthesized B₄C/Al₂O₃/C powders and mechanically mixed B₄C/Al₂O₃/C powders were added to the low carbon MgO-C refractories, and their corresponding properties were compared. The apparent porosity (AP) of the refractories with the synthesized powders (labelled as M3) is lower than those of the refractories with mechanically mixed powders (labelled as M2) and without composite powders (labelled as M1). The oxidation ratio and slag erosion depth of M3 were lower than those of M2 and M1. The thickness of the decarburized layer of M3 was 10.2% and 22.4% less than that of M2 and M1, respectively. The penetration depth of M3 was 12.0% and 27.9% less than that of M2 and M1, respectively. The thermal shock resistance of M3 was better than that of M2 and M1. The residual strength ratio of M3 was 15.8% and 17.2% more than that of M2 and M1, respectively. These results suggest that the combustion-synthesized B₄C/Al₂O₃/C composite powders can be used as new and promising additives for low carbon MgO-C refractories.     

Ceramizable phenolic adhesive modified by different inorganic particles: A comparative study of thermal stability,bonding strength and bonding mechanism

In this work, two ceramizable phenolic adhesives were prepared using ZrSi₂ particles or ZrSi₂/B₄C mix particles as the inorganic fillers. The thermal stability, bonding strength, microstructure and phase composition of the adhesives were investigated by TGA, shear strength of Al₂O₃ joints, SEM, EDS, XRD and XPS. The results show that these two adhesives have different bonding performances and ceramicization evolutions above 600 °C in air due to the addition of the second phase particles B₄C. The bonding strength of ZrSi₂/B₄C modified phenolic adhesive after treatment at 1200 ℃ can be as high as 36.6 MPa, while the bonding strength of ZrSi₂ modified phenolic adhesive under the same conditions is only 17.5 MPa. B₄C undergoes oxidation reaction before ZrSi₂, and the oxidation product B₂O₃ liquid phase not only reacts with ZrSi₂ to form oxidation-resistant ZrB₂, but also can dissolve the high temperatures defects of the adhesive and chemically bond with the Al₂O₃ substrates at the interface.     

Growth behavior of aluminum borate whiskers on zirconia toughened alumina (ZTA) particle surface

In this work, B₄C-covered zirconia-toughened alumina (ZTA) particles are prepared and oxidised at 1050 °C for different times (0, 2, 4, and 8 h) in air. The X-ray diffraction and electron probe micro-analysis results show that the covering layer is mainly composed of oxide B₂O₃ intermetallics, residual B₄C particles, and Al₁₈B₄O₃₃ whiskers. The scanning electron microscopy results show that the growth of Al₁₈B₄O₃₃ whiskers on the ZTA particles enhances with increasing heat preservation time; the optimum holding time is determined to be 8 h Al₂O₃ in the ZTA particles diffuse into the covering layer and combine with B₂O₃ to form Al₁₈B₄O₃₃ whiskers; the Al₁₈B₄O₃₃ whiskers grow via the liquid-solid mechanism.     

Synthesis and characterisation of dental composite materials reinforced with fluoroapatite–mullite glass–ceramic particles

Abstract

Glass–ceramic filler particles containing various amounts of fluoroapatite–mullite crystalline phases were synthesised using SiO₂–Al₂O₃–P₂O₅–CaF₂–CaO system as the base glass formulation. Different additives were used to promote crystallisation in this system. Composite samples were prepared by incorporating the silane treated glass–ceramic particles into the Bis-GMA/TEGDMA (60∶40 mass ratio) epoxy matrix. Structural and microstructural characterisations were conducted using Fourier transform infrared spectroscopy, X-ray diffractometry and scanning electron microscopy (SEM). The mechanical properties of the light cured samples were examined by measuring flexural and diametral tensile strength, as well as conducting Vickers microhardness test. Results obtained in this study showed strong dependence of the flexural strength on the composition of the filler particles. Diametral tensile strength and microhardness values demonstrated lesser sensitivity to the filler composition. Microstructural examination of the samples by SEM revealed particle pull-out, debonding and crack deflection as the major energy consuming mechanisms in the fracture process.     

Glycine–nitrate synthesis of Sr doped La2Zr2O7 pyrochlore powder

Abstract

Abstract

Materials with A₂B₂O₇ (pyrochlore) structure have received significant attention for their applications as new protonic conductors and materials used in electronic devices. One of the unique synthesis routes for La₂Zr₂O₇ (pyrochlore) powders is the glycine–nitrate combustion method, which shows superior properties of the synthesised powder using glycine as a complexing agent. The Sr doped La₂Zr₂O₇ powders in pure pyrochlore structure were produced using this approach. Selected characteristics of the synthesised powders, such as crystal structure, lattice parameters, crystallite size, the vibrational properties, the morphology of the particles, along with the specific surface area and particle size, have been investigated. The dependence of some properties on annealing temperatures of the powders has been studied.     

Low‐Temperature Synthesis of Calcium Hexaboride Powder via Transient Boron Carbide Formation

Calcium hexaboride (CaB₆) powder was synthesized by carbothermal reduction using a low‐temperature synthesis method for boron carbide (B₄C) powder. A B₄C precursor consisting of boron oxide (B₂O₃) and carbon components was prepared from a condensed boric acid (H₃BO₃)‐poly(vinyl alcohol) (PVA) product by thermal decomposition in air, which was then mixed with calcium carbonate (CaCO₃) powder. CaB₆ was formed via the transient formation of calcium borate (Ca₃B₂O₆) and B₄C, which were in close contact owing to the finely dispersed B₂O₃/carbon structure of the B₄C precursor. CaB₆ powder with fine particles was synthesized by heat treatment at 1400°C for 10 h in an Ar flow.     

Improving sintering behavior of MWCNT/BaTiO3 ceramic nanocomposite with Bi2O3-B2O3 addition

The present research systematically investigated the novel low-temperature fabrication of a multi-walled carbon nanotube (MWCNT)/barium titanate nanocomposite using a two-step mixing technique. The synthesis was conducted using different amounts of MWCNT (0.25%, 0.5%, 1%, 2%, 4%, and 8% wt) with different compositions of (Bi₂O₃ + B₂O₃) as a sintering aid. Scanning and transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, three-point bending strength, Vickers hardness indentation and Archimedean technique were used to characterize the as-synthesized specimens. It was found that the appropriate content of sintering aid (Bi₂O₃+B₂O₃) strongly decreased the sintering temperature from 1200 °C to 950 °C. The results also revealed that any sintering aid with the optimum composition that included 30% (mol) Bi₂O₃ was optimal for a sintering aid content of 6% (wt). Consequently, the highest values of the flexural strength and fracture toughness were achieved by applying the optimal amounts of MWCNT (1% wt) and sintering aid (6% wt).     

Thermochemical modeling and experimental studies on the formation of TiB2 through carbothermic synthesis from TiO2 and B2O3 or B4C

In the present study, a combined modeling and experimental approach were chosen to compare the carbothermal reaction mechanisms of TiO₂ with B₂O₃ or B₄C and to obtain a high yield of TiB₂ as a final product. A thermochemical modeling software, FactSage, was used to estimate the possible products and their quantities by means of Gibbs Energy Minimization approach. In the experimental step, different reaction temperature (1400–1700 °C) and time (0–60 min) combinations on TiB₂ synthesis were investigated at fixed initial molar ratios of TiO₂:B₂O₃:C (1:1:5) and TiO₂:B₄C:C (1:0.5:1.5). The experimental products were characterized by using X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and BET analysis methods. It was found that calculated modeling results obtained through FactSage agree well with the experimental results. The Magneli phases formed as intermediate products only when B₂O₃ was used as starting material. Full conversion to TiB₂ was obtained at 60 min and 1600 °C for both initial sources of boron.     

B4C‒TiB2 composite with modified microstructure and enhanced properties from optimal size coupling of raw powders

B₄C‒15 vol% TiB₂ composites were fabricated by in situ reactive spark plasma sintering with B₄C, TiC, and amorphous B powders as the raw materials. The size coupling of initial B₄C and TiC particles was optimized based on the reaction mechanism to derive B₄C‒TiB₂ composites with enhanced microstructure and properties. During the reactive sintering, fine B₄C–TiB₂ particles were firstly formed by an in situ reaction between TiC and B. Then, large B₄C particles tended to grow at the cost of small B₄C particles. The in situ TiB₂ grains gradually grew up and interconnect, distributing around the large B₄C grains to form an intergranular TiB₂ network. The results showed that the B₄C‒15 vol% TiB₂ composite prepared from 3.12 μm B₄C powder and 0.80 μm TiC powder had the optimal comprehensive properties, with a relative density of 99.50%, a Vickers hardness of 31.84 GPa, a flexural strength of 780 MPa, a fracture toughness of 5.77 MPa·m^1/2, as well as an electrical resistivity of 3.01 × 10⁻² Ω·cm.     

Effects of reactants proportions on features of in-situ magnesiothermic self-propagating high temperature synthesized boron carbide powder

The effects of reactant proportions were investigated on features (phase composition, micromorphology and crystal development) of B₄C (boron carbide) powder synthesized by in-situ magnesiothermic SHS (self-propagating high temperature synthesis) method, which was based on the perspective of thermodynamic design. The results showed that the reactant proportions were the fundamental reasons affecting the phase composition of the products during the reaction process. Mg addition could decrease the Mg₃B₂O₆/MgO mass ratio of the SHS products. Moreover, C addition would increase the C/B mass ratio of the leached products. When the C molar ratio was 0.2, free boron appeared in the leached products, and the composition of boron carbide was B₁₃C₂. When the C molar ratio ≥0.6, the composition of boron carbide changed into B₄C, while free carbon began to appear. The lattice parameters a and c of boron carbide crystal dropped with the increase of the C/B mass ratio from 0.1 to 0.3. Meanwhile, carbon atoms gradually entered the boron carbide unit cell. When the C/B mass ratio exceeds 0.3, the lattice parameters tended to be constants of a = 5.608 Å, c = 12.039 Å, while free carbon began to appear in the leached products.     

Fabrication of B4C from Na2B4O7 + Mg + C by SHS method

This paper deals with the formation of boron carbide (B₄C) powders from Na₂B₄O₇ + Mg + C system by self-propagating high-temperature synthesis (SHS) method. B₄C without impurities could be obtained after the acid enrichment and distilled water washing. The reaction mechanism of SHS of B₄C was proposed: the synthesis of B₄C is a process involving the decomposition of Na₂B₄O₇ into the intermediate phase B₂O₃, which reacts with Mg and carbon to form B₄C.     

Structure and mechanical properties of hot-pressed B4C-NdB6 composites

High-performance B₄C-NdB₆ composites were fabricated by hot-pressing sintering at the temperature of 2050 °C for 20 min holding time and 20 MPa pressure with Nd₂O₃ (1～4 wt%) as the aiditive. The effects of Nd₂O₃ on the sintering process of the B₄C were studied. The reaction mechanisms of B₄C and Nd₂O₃ at different temperature were investigated. Based on the results of TG-DSC and thermodynamic calculation,. NdB₆ was formed via Nd₂O₃ react with B₄C in the sintering process, which greatly enhanced the densification of B₄C and promoted the sintering process. The flexural strength, fracture toughness and hardness of the B₄C-NdB₆ composites rose to 366.42 MPa, 5.27 MPa m^1/2 and 38 GPa by adding 3 wt% Nd₂O₃, respectively. The coexistence of transgranular and intergranular fracture is the major fracture mode. The phenomenon of pull-out contributed to improvement of the fracture toughness.