Low-temperature synthetic route for boron carbide

Boron carbide is one of the hardest materials with diamond-like mechanical properties, and is already used for a variety of applications including armor plating, blasting nozzles, and mechanical seal faces, as well as for grinding and cutting tools. It is produced on an industrial scale by classical carbothermal reduction of boric oxides at high temperatures, but the formation of pure boron carbide in processed forms, such as films and fibers, is difficult. As an alternative to high-temperature powder techniques, there is recently great interest in the development of polymer precursors to produce ceramic materials. The aim of the present work is to develop a cost effective and low-temperature manufacturing process to synthesize boron carbide from cheap and easily available raw materials. The initial objective of our research is the design and synthesis of a new type of boron–carbon polymer, which would serve as precursor for boron carbide. The polymeric precursor is synthesized by the reaction of boric acid and polyvinyl alcohol that after pyrolysis at 400 °C and 800 °C gives boron carbide. The polymeric precursor and its pyrolyzed products are characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). X-ray diffraction shows that boron carbide (B₄C) obtained from this method has an orthorhombic crystal structure. It is a unique low-temperature (∼400 °C) synthetic route for boron carbide.     

Synthesis of nanocrystalline boron carbide by sucrose precursor method-optimization of process conditions

Nanocrystalline boron carbide powder was synthesized by a precursor method using B₂O₃ as the source of boron and sucrose as the source of carbon. Precursor was prepared at different temperatures ranging from 300 to 800 °C. The optimum temperature for the precursor preparation was found to be 600 °C. All the precursors were heat treated at different temperatures from 1000 to 1600 °C for different duration of heating, ranging from 5 to 240 min under vacuum. The products thus obtained after heat treatment were characterized using X-ray diffraction. The boron carbide obtained was nanocrystalline and the average X-ray crystallite size was found to be ~ 33 nm. Boron, total carbon and free carbon contents also were determined. The free carbon content was found to be less than 3% for samples heated at 1600 °C for 10 min. Effect of heat treatment temperature on the morphology of the synthesized product was studied using scanning electron microscope.     

Synthesis of ultra-fine hafnium carbide powders combining the methods of liquid precursor conversion and plasma activated sintering

Ultra-fine hafnium carbide (HfC) powders were synthesized using a novel method combining liquid precursor conversion and plasma activated sintering (PAS). Solution-based processing was used to achieve a fine-scale mixing of the reactants, and further treatment by PAS allowed fast formation of HfC. We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders, where mixtures were prepared using salicylic acid, citric acid, or a combination of these. The results show that pure HfC powders (with an average particle sizes of 350 nm) were obtained at a relatively low temperature (1550 °C) using a HfOCl₂·8H₂O precursor with the mixed acids. The oxygen content of the synthesized powders was only 0.97 wt%. The type of acid had a significant effect on the synthesis product. When using only citric acid, the temperature required to produce pure hafnium carbide increased to 1700 °C. In the case of a salicylic acid precursor, pure HfC was not obtained, even at a synthesis temperature of 1700 °C.     

Carbothermal reduction synthesis of TiB2 ultrafine powders

Submicrometric TiB₂ powders were synthesized by carbothermal reduction process using titanium dioxide, boron carbide and carbon black as the starting materials. The influence of different amount of boron carbide (22.0–26.8 wt%), calcination temperature (1400–1900 °C) and holding time (15–90 min) on the composition and microstructure of the product was investigated. The resultant powders were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Results showed that hexagonal impurity-free TiB₂ crystalline powders with the grain size below 1.0 μm could be successfully prepared at 1600 °C for 30 min in Ar atmosphere when the amount of boron carbide was 25.3 wt%. The increase in temperature contributed to reaction completion and grain growth, but the abnormal grain growth and oversintering took place above 1800 °C.     

Synthesis,characterization and use of synthesized fine zirconium diboride as an additive for densification of commercial zirconium diboride powder

Zirconium diboride (ZrB₂) was synthesized by a solution-based technique using zirconyl chloride (ZrOCl₂·8H₂O, ZOO), boric acid (H₃BO₃, BA) and gum karaya (GK) as the sources of zirconium, boron and carbon, respectively. The initial formation temperature of ZrB₂ was 1200 °C and complete conversion was achieved by 1400 °C. Preceramic precursors and as-synthesized ZrB₂ powders were characterized by XRD, TG-DTA, SEM, TEM, EDX and compared with commercial ZrB₂ powder made by carbothermic reduction. FT-IR of as-synthesized dried preceramic precursor revealed the formation of Zr–O–C and Zr–O–B whereas SEM showed agglomerated spherical particles with mean diameter of <1 µm. Commercial ZrB₂ and as-synthesized fine ZrB₂ powder were spark plasma sintered (SPS) at 1900 °C for 10 min. Addition of 10 wt% of synthesized fine powder improved the fired density from 87% to 93% of theoretical. A significant cost benefit arises for the utilization of cheap synthesized fine powder as an additive for the densification of the more expensive commercial powder.     

A novel route to obtain B4C nano powder via sol–gel method

Boron carbide nanopowders and nanowhiskers were synthesized using phenolic resin and boron alkoxide as precursors through sol–gel process in water–solvent–catalyst–dispersant system. The effects of soaking time on free carbon content and synthesized B₄C particles morphology were evaluated at 1270 °C. The synthesis process of B₄C nanopowders was completed at 1270 °C after 1 h while B₄C nanowhiskers were heterogeneously nucleated and grown from obtained nanopowders after 3 h.     

Effect of particle size on the mechanical properties of reaction bonded boron carbide ceramics

In the present communication, effect of boron carbide particle size on the mechanical properties such as hardness, fracture toughness and flexural strength of reaction bonded boron carbide (RBBC) ceramics were investigated. RBBC composites were produced by the reactive infiltration of molten silicon into porous preform containing boron carbide and free carbon. Boron carbide powders with mean particle size of 18.65 μm, 33.70 μm and 63.35 μm were chosen for the RBBC composites. The experimental results show that hardness increases from 1261.70±64.74 kg/mm² to 1674.90±100.00 kg/mm² and fracture toughness drops from 5.76±0.26 MPa m^1/2 to 3.4±0.37 MPa m^1/2. However, flexural strength decreases from 403.41±5.70 MPa to 256.15±25.05 MPa with the increase in particle size. Indentation induced cracks in RBBC are mainly median type and number of cracks increase with the increase of starting particle size.     

Microstructure and deposition mechanism of CVD amorphous boron carbide coatings deposited on SiC substrates at low temperature

Amorphous boron carbide (α-B₄C) coatings were prepared on SiC substrates by chemical vapor deposition (CVD) from CH₄/BCl₃/H₂/Ar mixtures at low temperature (900–1050 °C) and reduced pressure (10 kPa). The deposited coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that two kinds of α-B₄C coatings were deposited with different microstructures and phase compositions, and the effect of deposition temperature was significant. When deposited at 1000 °C and 1050 °C, the coatings exhibited a nodular morphology and had a relatively low content of boron. The free carbon was distributed in them inhomogeneously; in contrast, when deposited at 900 °C and 950 °C, the coatings presented a comparatively flat morphology and had a uniform internal structure and high boron content. They did not contain free carbon. At the last of this paper, the pertinent mechanisms resulting in differences in microstructure and phase composition were discussed.     

Preparation of carbon foams with enhanced oxidation resistance by foaming molten sucrose using a boric acid blowing agent

Boric acid was used as a blowing agent as well as a boron precursor for the preparation of boron-doped carbon foams from molten sucrose. The H⁺ generated, due to the formation of a complex between sucrose and boric acid, catalyzes the –OH to –OH condensation reaction leading to the polymerization and the foaming of the molten sucrose. The char yield of the solid organic foams increased from 24 to 39 wt.% when the boric acid concentration increased from 0 to 8 wt.%, due to the formation of the B–O–C cross-links between sucrose polymer by B–OH to C–OH condensation. The inductively coupled plasma analysis showed the presence of 0.44–3.4 wt.% boron in the carbon foams. The density and compressive strength decreased and cell size increased with boric acid concentration. The room temperature thermal conductivity of the boron-doped carbon foams was in the range of 0.057–0.043 W m⁻¹ K⁻¹. The weight loss studies by dynamic and isothermal heating showed the increased oxidation resistance with boron concentration.     

Synthesis and pyrolysis evolution of glucose-derived hydrothermal precursor for nanosized zirconium carbide

Nanosized zirconium carbide (ZrC) was synthesized successfully by a novel hydrothermal precursor conversion method using chelation of polydentate glucose as the carbon source. During the pyrolysis, the core-matrix structure of intimate nanosized ZrO₂ and amorphous carbon mixture forms, resulting in short diffusion path and limit of grain growth. ZrC first appears at a much lower temperature of 1200 °C and completes conversion at 1400 °C in comparison with that of precursor without hydrothermal treatment. By raising the heating temperature to 1600 °C, oxygen content could be reduced (0.55 wt%) with a low residual carbon content (2.3 wt%), and the average size of the spherical crystallite increases from 100 nm to 200 nm. Based on above ZrC powders, the additive-free ceramic with 99.4% relative density by spark plasma sintering (SPS) at a low temperature of 1700 °C has been achieved.     

Gel-cast hierarchical porous B4C/C preform and its role in fabricating reaction bonded boron carbide composites

The resorcinol-formaldehyde (RF) gel-casting system is employed for the first time to fabricate a hierarchical porous B₄C/C preform, which was subsequently used for the fabrication of reaction bonded boron carbide (RBBC) composites via a liquid silicon infiltration process. The effect of the carbon content and carbon structures of this perform on the microstructures and mechanical properties of B₄C/C preform and the resultant RBBC composites is reported. The B₄C/C preform (16 wt% carbon) exhibit a strength of 34±1 MPa. The obtained RBBC composites shown uniform microstructure is consisted of SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase. The Vickers hardness, flexural strength and fracture toughness of the RBBC composites (16 wt% carbon) are 24 GPa, 452 MPa and 4.32 MPa m^1/2, respectively.     

Wear resistant boron carbide compacts produced by pressureless sintering

Boron carbide compacts were produced by pressureless sintering at 2200 °C/2 h and 2250 °C/2 h in Ar atmosphere, using a starting powder with a particle size smaller than 3 µm. Effects of carbon addition (3.5 wt%) and methanol washing of the starting powder were investigated on the densification, Vickers hardness, and micro-abrasive wear resistance of the samples. The removal of oxide phases by methanol washing allowed the production, with no sintering additive, of highly densified (93.6% of theoretical density), hard (25.4 GPa), and highly wear resistant (wear coefficient =2.9×10^–14 m³/N.m) boron carbide compacts sintered at 2250 °C. This optimized combination of properties was a consequence of a reduced grain growth without the deleterious effects associated to the carbon addition. Methanol washing of the starting powder is a simple and general approach to produce, without additives, high quality, wear resistant boron carbide compacts by pressureless sintering.     

Synthesis of sub-micro sized high purity zirconium diboride powder through carbothermal and borothermal reduction method

Sub-micro sized zirconium diboride (ZrB₂) powders were successfully prepared via the boro/carbothermal reduction method using zirconium oxide and boron carbide as the primary raw materials. The prepared mixtures were thermally reacted at 1250 °C for 1 h. The optimized composition range containing the lowest oxide and carbide impurity, which was 0.14% of oxygen and 0.3% of carbon contents, was determined using crystallographic and elemental analysis. The particle size was reduced from 5 µm to 245 nm by the addition of B₄C as a reductant within a composition range that maintained the highest purity. The morphology changed from faceted to angular hexagonal bar-like with a simultaneous growth in particle size. Changes in the particle structure were a result of the existing liquid B₂O₃ phase during the reaction. The 245-nm particles contained 12.1% oxygen content and 16.2% oxygen content for the 5-μm particle in the circumstance in which limited oxides could be produced.     

Bulk boron carbide nanostructured ceramics by reactive spark plasma sintering

Bulk boron carbide (B₄C) ceramics was fabricated from a boron and carbon mixture by use of one-step reactive spark plasma sintering (RSPS). It was also demonstrated that preliminary high-energy ball milling (HEBM) of the B+C powder mixture leads to the formation of B/C composite particles with enhanced reactivity. Using these reactive composites in RSPS permits tuning of synthesized B₄C ceramic microstructure. Optimization of HEBM + RSPS conditions allows rapid (less than 30 min of SPS) fabrication of B₄C ceramics with porosity less than 2%, hardness of ~35 GPa and fracture toughness of ~ 4.5 MPa m ^1/2     

Characterization of nanocrystalline porous SiC powder by electrospinning and carbothermal reduction

Nanocrystalline porous silicon carbide (SiC) powder was prepared from various ratios with SiO₂ as silicon source and carbon black as carbon source by electrospinning method and subsequent carbothermal reduction. Their microstructure, thermal stability and porosity of prepared porous silicon carbide (SiC) powder were evaluated as function of Si to C ratios in precursor. 3C/Si could not often maintain the spherical form against oxygen–carbon reaction during calcination owing to relatively low carbon content. On the other hand, the uniformed spherical forms were presented with deep wrinkles in the other prepared samples. They were believed that the wrinkled surfaces were formed by vaporization of carbonaceous gas made from carbon and oxygen. When the carbon ratio to silicon (C/Si) was 5, the β-SiC was observed without remained silica and silicon metal. Further increase in the carbon content in the precursor led to sharp and high intensity of SiC peak. Initial decomposition temperature of prepared sample was increased with decreasing C/Si ratio. We confirmed that whereas 3C/Si and 4C/Si had higher big macropore, 5C/Si and 6C/Si had much large pore volume in the range of mesopore diameter (2–50 nm) and small macropore (50–100 nm).     

Synthesis and pyrolysis behavior of a soluble polymer precursor for ultra-fine zirconium carbide powders

A soluble polymer precursor for ultra-fine zirconium carbide (ZrC) was successfully synthesized using phenol and zirconium tetrachloride as carbon and zirconium sources, respectively. The pyrolysis behavior and structural evolution of the precursor were studied by Fourier transform infrared spectra (FTIR), differential scanning calorimetry, and thermal gravimetric analysis (DSC–TG). The microstructure and composition of the pyrolysis products were characterized by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscope (SEM) and element analysis. The results indicate that the obtained precursor for the ultra-fine ZrC could be a Zr–O–C chain polymer with phenol and acetylacetone as ligands. The pyrolysis products of the precursor mainly consist of intimately mixed amorphous carbon and tetragonal ZrO₂ (t-ZrO₂) in the temperature range of 300–1200 °C. When the pyrolysis temperature rises up to 1300 °C, the precursor starts to transform gradually into ZrC, accompanied by the formation of monoclinic ZrO₂ (m-ZrO₂). The carbothermal reduction reaction between ZrO₂ and carbon has been substantially completed at a relatively low temperature (1500 °C). The obtained ultra-fine ZrC powders exhibit as well-distributed near-spherical grains with sizes ranging from 50 to 100 nm. The amount of oxygen in the ZrC powders could be further reduced by increasing the pyrolysis temperature from 1500 to 1600 °C but unfortunately the obvious agglomeration of the ZrC grains will be induced.     

Synthesis of fine dispersed titanium diboride from nanofibrous carbon

This paper presents the experimental data on the synthesis of titanium diboride (TiB₂) fine dispersed powder carried out in laboratory scale. TiB₂ powder was prepared by the reduction of titanium dioxide with boron carbide and nanofibrous carbon in an argon atmosphere. The powders of TiB₂ were characterized by X-ray diffraction (XRD), elemental analyses, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), low-temperature nitrogen adsorption, particle size analysis, simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). The resulting material contains a single phase – titanium diboride. The particles of the powder were predominantly aggregated. The average size of the particles and the aggregates were 7.4–8.0 µm with a wide size of distribution. The specific surface values of samples obtained were 2.4–5.8 m²/g. The oxidation of titanium diboride began from the temperature of 450 °C. In this work, the optimal synthesis conditions were estimated: the molar ratio was TiO₂:B₄C:C=2:1:3 (according to stoichiometry), the temperature was 1600 °C, the process duration was 20–30 min.     

Precursor synthesis and properties of nanodispersed tungsten carbide and nanocomposites WC:nC

Nanoscale tungsten carbide (WC) and WC:nC nanocomposites have been synthesized by the precursor method. The precursor, obtained in the form of a glassy mass by thermal treatment of a mixture of (NH₄)₁₀W₁₂O₄₁∙7H₂O and glycerol, was heated in inert gaseous atmosphere up to 1050–1100 °C. The concentration of chemically active carbon in the precursor and nanocomposites depends on the W/C ratio in the initial mixture. At W/C=1/3 pure tungsten carbide is formed; at W/C>1/3 composites of WC and free carbon (WC:nC) are formed. Heating of the precursor with W/C=1/6 up to 1100 °C in helium atmosphere results in the formation of carbon-encapsulated tungsten carbide nanoparticles. An increase in the precursor-heating rate leads to the formation of chain-like structures. Each chain consists of hexagonal WC grains with unit cell parameters a=2.93 Å and c=2.83 Å. Free carbon in WC:nC composites forms agglomerates of carbon “nano-onions” of spherical or multi-layered tubular shapes.     

The dispersion of boron carbide powder in aqueous media

In the present work, the dispersion of boron carbide powder in aqueous media was studied in terms of surface treatment, surface characterization, the particle stability as well as rheological behavior, etc. Initially, the influence of the impurity and boron oxide from commercial B₄C powder on the slurry dispersion was studied. The surface compositions, bonding and surface charge behavior of acid-treated B₄C powder were investigated systematically. The positive effect of TMAH (tetramethylammonium hydroxide) on the dispersion of as-treated B₄C powder was studied and the content was optimized based on rheological measurement. Results showed that well dispersed B₄C slurries could be developed with the solid loading of 55 vol.% using 0.18 wt% TMAH as the dispersant. The dispersion mechanism was also investigated before and after TMAH addition.     

Fabrication and characterization of a new-style structure capillary channel in reaction bonded silicon carbide composites

A new-style structure capillary channel was fabricated by using boron carbide powder mixtures with an appropriate multimodal particle size distribution to promote the liquid silicon infiltration in reaction bonded silicon carbide composites. Two types of core–rim structure were observed and the secondary SiC produced in the siliconisation reaction existed in two forms: nucleating on the original SiC and occupying the original positions of the residual silicon. The size of the secondary SiC in the latter form was in a range of tens to hundreds nanometers. These nano-sized SiC grains and the additive of fine boron carbide particles refined the crystalline structure and broke up the residual silicon phase into small pieces. Using this method, the microstructure was refined and the mechanical properties improved significantly. The lowest residual silicon volume fraction was 4.0% and the flexural strength and fracture toughness reached peak values of 526 ± 21 MPa and 6.2 ± 0.4MPa m^1/2, respectively.