Carbothermal Reduction Nitridation Method Research Articles

Investigation on the synthesis mechanism and size control of (Ti,W,Mo,Cr)(C,N) solid solution powder

Complex interfaces in cermets limit its application as cutting or heat-work die materials. Interface modification of hard particles by composition optimization is proved to be an effective strategy for high-performance Ti(C,N)-based cermets. Herein, we prepared (Ti,W,Mo,Cr)(C,N) solid solution powder through carbothermal reduction nitridation method. The phase and microstructure were characterized to discover the synthesis mechanism. Oxides of Mo, W, Ti, and Cr participated in the reaction successively, MoO2 transformed to M2X (X is C and/or N, M is W, Mo and/or Cr) directly, both W2C and W formed simultaneously as the reduction products of WO3, TiO2 followed TiO2 → TinO2n-1 → (Ti, M)(C,N) (M is W, Mo and/or Cr), and Cr diffused into (Ti, M)(C,N) and M2X phases directly from its oxide. Finally, the solid solution powder with composition (Ti0.83W0.07Mo0.09Cr0.01)(C0.62N0.38) in the particle size of 0.73 μm with ∼0.14 wt% oxygen and ∼ 0.43 wt% free carbon was obtained at 1750 °C for 2 h. Our findings provide insight into developing advanced cermets with better comprehensive properties.

Synthesis of high‐quality vanadium nitride particles by one‐step vacuum carbothermal reduction–nitridation method: Theoretical and experimental study

AbstractOwing to its high hardness, favorable stability, and good electrical performance, vanadium nitride (VN) has been extensively used as an alloy additive, an electrode material, a ceramic material, and a catalyst. However, traditional VN synthesis methods require high temperatures and long soaking times. In this study, high‐quality VN particles were prepared from V2O3 powder at a lower temperature and shorter soaking time by a one‐step vacuum carbothermal reduction–nitridation (VCRN) method. Thermodynamic and kinetic analyses elucidated the mechanism of this process. Thermodynamic analysis showed that vacuum was conducive to the carbothermal reduction of V2O3, reducing the initial reaction temperature, and the main reaction path was V2O3→VO→VxCy(VC/ V2C/ V8C7/ V6C5)→VN. Kinetic results indicated that the one‐step VCRN method was a gas–solid reaction. Within the temperature range of 1373–1473 K, the rate‐determining step was controlled by the carbothermal reduction–nitridation reaction, combination control of the reduction–nitridation reaction and the interlayer diffusion, and interlayer diffusion control of N2 during the different reaction processes. High‐quality VN particles obtained at a temperature of 1473 K, pressure of 300 Pa, and soaking time of 3 h were characterized by X‐ray diffraction, transmission electron microscopy, energy dispersive X‐ray spectroscopy, and particle size distributions. The O and N contents, measured by an O/N analyzer, were 0.85% and 19%, respectively, and the C content, measured by a C/S analyzer, was 0.92%. These analyses indicate that high‐quality VN can be obtained by a one‐step VCRN method.

Preparation of high-entropy (Ti,Nb,Ta,Mo,W)(C,N) ceramics via carbothermal reduction nitridation using different carbon source

The (W,Mo,Ta,Nb,Ti)(C,N) high-entropy carbonitride powders were synthesized by the carbothermal reduction nitridation method, utilizing equimolar proportions of either graphite or carbon black. The subsequent utilization of these pre-synthesized powders yielded carbonitride ceramics after spark plasma sintering. The choice of carbon source played an important role in the microstructure, phase compositions and mechanical properties. By employing carbon black with its fine particle size and high reactivity, the synthesized powder maintained finer size, narrow powder size distribution (350 ± 12 nm) and have less free-carbon (∼0.032 wt%), which contributed to the high hardness (26.8 ± 1.4 GPa) of the sintered ceramic. Conversely, utilizing graphite with its larger particle size and lower reactivity would increase the fraction of free-carbon, leading to the deterioration of the ceramic's performance.

Preparation of AlN with low agglomeration using polyethylene glycol and emulsifier to disperse the ultrafine raw powders

In this study, aluminium nitride (AlN) powder was prepared by the carbothermal reduction–nitridation method using ultrafine alumina and carbon black as the raw materials. A polyethylene glycol (PEG) and emulsifier (OP-10) disperse system was used to distribute the raw powders during ball milling. The mixing slurries and as-received AlN products were investigated using a viscometer, X-ray diffraction, field emission scanning electron microscopy, oxygen nitrogen hydrogen analyser, TG analysis and a laser particle size analyser. The results showed that the OP-10/PEG composite system reduced the viscosity of the slurry containing alumina and carbon during ball milling and significantly improved the wettability of water on the carbon black powder. The viscosity decrease improved the distribution of alumina and carbon under the same ball-milling conditions. At the same time, a dispersed structure was formed in which the OP-10 micelles separated alumina particles with carbon particles. Furthermore, the micelles with carbon were separated by the long PEG chains. This increased the number of primary particles below 2 μm and decreased the number of agglomerates above 5 μm in the final CRN AlN products. Based on the experimental results, herein the dispersion mechanism of the OP-10/PEG composite system on fine alumina and carbon black is proposed. Moreover, the rationality of the composite system is discussed.

Mechanism and experimental study on preparation of high-quality vanadium nitride by one-step vacuum carbothermal reduction nitridation method

With high hardness, high electrical conductivity, and good catalytic activity, VN is widely used in alloy materials, catalysis and electrode materials, and other fields. However, the conventional carbothermal reduction nitridation (CRN) methods are often limited by high temperatures and long reaction times. In this paper, the reaction process of vanadium trioxide one-step vacuum carbothermal reduction nitridation (VCRN) synthesis of vanadium nitride was studied by combining theoretical calculation and experimental research. The experimental results show that reducing the pressure is favorable for the V2O3-CRN reaction and high-quality VN was successfully synthesized under the conditions of 1473 K (300 Pa, 3 h) and the carbon mass ratio (mC/mV2O3) is 0.25. In addition, the reaction mechanism of V2O3-VCRN was elucidated by ab initio molecular dynamics (AIMD) simulations. The results show that carbon plays important role in the VCRN reaction process of vanadium trioxide. First, Carbon catalyzes the dissociation of nitrogen molecules on the surface of V2O3–C, which promotes the formation of the V–N bond, second is to react with O in the V2O3 lattice and reduces V2O3.

Low temperature synthesis and densification of (Ti,V,Nb,Ta,Mo)(C,N) high-entropy carbonitride ceramics

The single-phase (Ti0.2V0.2Nb0.2Ta0.2Mo0.2)(C0.9N0.1) high-entropy carbonitride powder with an average particle size of 203 nm and a low oxygen content of 0.2915 wt% was synthesized by the carbothermal reduction-nitridation method at 1600 °C for 4 h. Subsequently, (Ti,V,Nb,Ta,Mo)(C,N) high-entropy carbonitride ceramics were prepared by hot pressing at temperatures of 1600 °C to 1900 °C. Increasing the sintering temperature is conducive to the uniform distribution of metallic elements, however, the hardness of high-entropy carbonitride ceramics is also reduced. (Ti,V,Nb,Ta,Mo)(C,N) ceramic hot-pressed at 1600 °C for 1 h has a relative density of 99.8%, an average grain size of 0.37 ± 0.08 µm, nanoindentation hardness of 32.4 ± 0.9 GPa, Vickers’ hardness of 24.0 ± 0.7 GPa at 9.8 N and fracture toughness of 4.87 ± 0.25 MPa·m1/2, which is comparable to those of high-entropy carbides and nitrides. In this study, a novel and convenient method for the preparation of (Ti,V,Nb,Ta,Mo)(C,N) high-entropy carbonitride ceramics was established.

Synthesis and luminescence properties of red‐emitting AlN:Mn2+ spherical phosphors

AbstractIn this study, we successfully prepared red‐emitting AlN:Mn2+ spherical phosphors by using nitrogen pressure–assisted carbothermal reduction–nitridation method. The optimum preparation parameters were explored by changing nitrogen pressure, CaF2 content, reaction temperature, and the amount of MnCO3. It was confirmed that uniform spherical shape, large particle size, less impurity content, and suitable Mn2+ concentrations could effectively enhance the luminous intensity of AlN:Mn2+ phosphors. The structural compositions of the AlN:Mn2+ phosphors were characterized by X‐ray diffraction and X‐ray photoelectron spectroscopy, indicating that Mn2+ ions were successfully doped into AlN lattice by substitution of Al3+ ions, thus providing the red‐emitting luminescence centers. Based on the experimental results, we further proposed the underlying growth mechanism of spherical AlN phosphors. It was suggested that an adequate amount of liquid Ca‐aluminates was essential to achieve uniform spherical morphology and high fluorescence efficiency.

Thermodynamic and kinetic analyses of vacuum synthesis of AlN by the alumina carbothermal reduction nitridation method

AbstractUltrafine single‐phase aluminium nitride (AlN) particles were successfully synthesized by the vacuum carbothermal reduction nitridation (VCRN) method at shorter times and lower temperatures. The mechanism of the process was investigated. By analyzing the X‐ray diffraction (XRD) patterns and thermodynamic calculations, the main reaction path was determined to be Al2O3→Al2CO→Al4C3→Al5C3N→AlN. By investigating the kinetics mechanism, the process was a gas–solid reaction model, which was controlled by the crystalline chemical reaction from 0.5 to 1.5 h, combination control from 1.5 to 2 h, and gas diffusion control from 2 to 2.5 h. Moreover, the apparent activation energy (Ea) was calculated from the nitridation ratio, the value obtained, 237.71 kJ mol–1, is 31% of the energy reported for the conventional synthesis method. These results show that ultrafine AlN could be effectively synthesized at 1823–1873 K and 300 Pa N2 pressure for 2.5 h.

Carbothermal synthesis of high-aspect-ratio AlN whiskers using graphite felt as carbon source

In this study, aluminum nitride (AlN) whiskers were successfully synthesized via carbothermal reduction nitridation (CRN) method. Graphite felt was used as carbon source and growth substrate, while no any catalyst was introduced. It is found from TEM and SEM analysis that the AlN whiskers grew along [001] direction, presenting uniform morphology and smooth surface with a length of up to microns. The high purity and high crystallinity of the whiskers were confirmed by XRD, XPS and Raman scattering spectrum. In addition, the influence of reaction temperature was systematically investigated, and it was indicated that excessively low or high temperature could both lead to the reduced whisker yield. On basis of the working results, the underlying gas-solid (VS) growth mechanism of AlN whiskers was significantly proposed.

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si3N4 powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si3N4 seeds content on the products were studied. It was found that high-purity α-Si3N4 (>99.0 wt%) was synthesized from C/SiO2 = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO2 and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si3N4 increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si3N4. Furthermore, higher nitriding temperature and Si3N4 seeds content both decreased the grain size and increased β-Si3N4 content. The forming mechanism of non-agglomerated and submicron-sized α-Si3N4 was clarified.

Fabrication of in-situ Ti(C,N) phase toughened Al2O3 based ceramics from natural bauxite

Al2O3–Ti(C,N) ceramics were fabricated via carbothermal reduction nitridation method with high-titania special-grade bauxite as the raw material. The formation mechanism of in-situ Ti(C,N) phase and its effect on the properties of materials are discussed. After nitrided at 1700 °C, Ti(C,N) phase could be formed in-situ with appropriate C/TiO2 molar ratio. Due to the residual stress field formed by Ti(C,N) particles, the path of crack propagation is changed, leading to the crack deflection and pinning. Therefore, the mechanical properties of the materials are improved by forming in-situ Ti(C,N) phase. With a C/TiO2 molar ratio of 2.2 and nitridation temperature of 1700 °C, Al2O3–Ti(C,N) ceramic with a hardness of 13.9 GPa, a fracture toughness of 8.28 MPa m1/2 and a flexural strength of 387 MPa could be fabricated.

Thermal insulation TiN aerogels prepared by a combined freeze-casting and carbothermal reduction-nitridation technique

With the rapid development of energy sources and aerospace engineering, high-performance thermal insulation materials with multiple functions are attracting more and more attentions. Non-oxide ceramic aerogels have application potential in thermal insulation under harsh conditions owing to their good thermal insulation performance and chemical stability. In this work, TiN aerogels were prepared by a combined freeze-casting and carbothermal reduction-nitridation method utilizing TiO2 and chitosan as raw materials, at 1473 K for 3 h, with a C/TiO2 molar ratio of 3/1. They showed a specific surface area of 167 m2/g and ultralow density of 0.07 g/cm3 (the relative density of 1.3 %). Their thermal conductivity value was only 0.166 ± 0.0054 W/(m K at 673 K (TiO2 solid content of 5 wt%). Moreover, the prepared aerogels possessed reasonably high mechanical strength and good high-temperature service performance, suggesting that they can be used under high-temperature conditions as an insulation material.

Ultrafine AlN synthesis by alumina carbothermal reduction under vacuum: Mechanism and experimental study

Ultrafine AlN powders were synthesized by carbothermal reduction nitridation method under vacuum (VCRN) in a double temperature zones vacuum furnace with alumina and carbon black as raw materials. The mechanism of the process was investigated. The results show that when the soaking time was 1.5 h at 1650 °C, 300 Pa N2 pressure, the purity ultrafine AlN powders were obtained. By analyzing the results of density function theory (DFT) calculation and XRD, the mechanism of VCRN can be identified as Al2O3 → Al2OC/Al4C3 → Al6C3N2/AlN → AlN. The results of theoretical study further proved that the compound of Al6C3N2 can be nitrogenized quickly through the reaction of Al6C3N2 + 2 N2(g) → 6AlN + 3C.

Synthesis of high-quality ferrovanadium nitride by carbothermal reduction nitridation method

High-quality ferrovanadium nitride (FeV45N, FeV55N and Fe65N) was fabricated using the raw materials of Fe3O4, V2O5 and graphite via carbothermal reduction nitridation method. Compared with the traditional methods, it shortens the production process of ferrovanadium nitride by avoiding the preparation of ferrovanadium. The effects of C/O molar ratio and reaction temperature on phase transition, density, carbon, oxygen and nitrogen contents and microstructure were investigated. The appropriate C/O molar ratio is crucial to obtain the products with high nitrogen content. It is also found that a higher temperature is beneficial for the densification, and the density of the products obtained at 1550 °C is much higher than that at 1500 °C. Moreover, a higher temperature contributes to the increase in nitrogen content owing to the higher reaction kinetics. The carbothermal reduction nitridation method is proved to be a facile route to fabricate cost-effective ferrovanadium nitride and is possible to be applied for industrial production.

Effects of AlN content on mechanical and optical properties of AlON transparent ceramics

In this study, AlON powders with different AlN contents were synthesized by the carbothermal reduction nitridation method and used to prepare transparent AlON ceramics. Lattice parameters of AlON powders were calculated from XRD patterns to evaluate the effects on microstructure. The influences of the AlN content on the properties of the resulting AlON ceramics was investigated in aspect of density, mechanical properties and optical properties. With an increase in the AlN content, the density of the AlON ceramics increased, the grain size decreased, and the hardness and fracture toughness increased. Transparent AlON ceramics were obtained at the AlN content of 21.6–29.7 mol%. The refractive index of the AlON ceramics increased from 1.688 to 1.702 at 4000 nm with an increase of AlN content. The correlation between the composition and properties of the AlON ceramics obtained in this study will provide both theoretical and experimental reference for industrial AlON ceramic manufacturing.

Synthesis of blue‐emitting AlN:Eu2+ spherical phosphors by carbothermal reduction nitridation method

AbstractIn this study, blue‐emitting AlN:Eu2+ spherical phosphors were successfully synthesized for the first time by the carbothermal reduction nitridation (CRN) method, assisted with high nitrogen pressure, appropriate synthesis temperature, and the addition of CaF2. The influence of typical experimental parameters, such as N2 pressure, heating temperature, CaF2 content and Eu2+ concentration on the morphologies and luminescence properties of AlN phosphors were comprehensively investigated. The formation mechanism of spherical morphology were significantly proffered, indicating that sufficient liquid Ca‐aluminates during the AlN growth stage were essential for the spheroidization process under the action of surface tension. The synthesized AlN:Eu2+ spherical phosphors presented an intense blue emission band centered in the range of 427‐ 476 nm relative to the reaction temperature. The lifetime of AlN:Eu2+ phosphor was calculated to be around 1.89 μs. The temperature‐dependent PL spectra suggested that the emission band did not shift until 225°C. In addition, the spectral analysis strongly suggested that the luminescence property of AlN:Eu2+ phosphors was significantly enhanced by the large particle size, spherical morphology, reduced impurity content, and appropriate Eu2+ concentration.

Preparation of AlN under vacuum by the alumina carbothermal reduction nitridation method

With good electrical, thermal and mechanical properties, aluminium nitride is widely used in microelectronics and other fields. However, high temperatures are still needed for the method of alumina carbothermal reduction nitridation (CRN) for AlN preparation. Herein, thermodynamics calculation and experimental research were adopted to investigate the preparation of AlN under vacuum by the alumina CRN reaction in order to lower reaction temperature. The results demonstrate that lower pressure benefits the alumina CRN reaction. A single phase of AlN was successfully obtained at 1823 K for 2.5 h with a N2 gas pressure of 300 Pa. Moreover, an underlying growth mechanism of AlN during the alumina CRN process under vacuum was proposed, which is different from that under atmosphere or elevated pressure. The rate limiting step of alumina CRN process was determined. The results also indicate that Al4C3 and Al2OC play important roles in the process of alumina CRN under vacuum.

Preparation and Performance of Si3N4 Hollow Microspheres by the Template Method and Carbothermal Reduction Nitridation.

Silicon nitride (Si3N4) hollow microspheres with smaller particle size and narrower distribution can be used to prepare closed-cell ceramics as the pore-forming agent to improve heat insulation performance and wave-transparent performance of porous Si3N4 ceramics. In this work, Si3N4 hollow microspheres with a diameter of about 1 μm and a wall thickness of approximately 150 nm were prepared by using the template method combined with the carbothermal reduction nitridation method. The optimum preparation temperature of the Si3N4 hollow microspheres is 1450 °C. The morphology, microstructure, and phase composition of the prepared Si3N4 hollow microspheres were characterized. The formation mechanism of the Si3N4 hollow microspheres was discussed. The dielectric properties of Si3N4 hollow microspheres were measured using the waveguide method at 8.2-12.4 GHz. The results show that the wave-transparent performance of the Si3N4 hollow microspheres is similar to those of α-Si3N4 particles. It can be used as the pore-forming agent or matrix for preparing lightweight, heat-insulating, wave-transparent, and high-strength porous Si3N4 ceramics.

Preparation of (Cr1-x, Mox)2(C, N) infinite solid solution powders

(Cr1-x,Mox)2(C,N) (x = 0–1) infinite solid solution powders were successfully prepared through carbothermal reduction nitridation method, for the first time. The phase and morphology evolutions, with the various synthesis temperature, isothermal time and Mo content, were systematically discussed by using TG-DSC, XRD, SEM and chemical composition analysis. The results show that phase evolution follows the sequences of (Cr2O3, MoO3) → (Cr2O3, MoO2) → (Cr2O3, Mo2C) → (Cr2O3, Cr3C2, (Cr1-x,Mox)2(C,N)) → (Cr2O3, (Cr1-x,Mox)2(C,N)) → single-phase (Cr1-x,Mox)2(C,N). Properly elevating synthesis temperature and prolonging isothermal time benefit the reduction of oxygen and nitridation of (Cr1-x,Mox)2(C,N). The optimal preparation temperature and isothermal time should be 1200 °C and 1 h, respectively. Increasing Mo content enlarges the lattice parameters of (Cr1-x,Mox)2(C,N) by obeying the Vegard's law, meanwhile, refines the particles from ~5 μm (x = 0) to ~100 nm (x = 1).

Effects of reducing environment and fusible components on carbothermal reduction–nitridation reaction of coal gangue at high temperature under N2 atmosphere

The synthesis of SiAlON materials by the carbothermal reduction–nitridation (CRN) method paves the way for utilization of coal gangue with significant economic benefits. Excessive carbon (C), which provides a high residual C content in the product, is added to the raw materials to ensure a high yield of SiAlON. The industrial solid waste of coal-coke powder is used as a reductant. In this study, the effects of the reducing environment and C content on the CRN reaction of coal gangue using corundum and graphite crucibles are investigated and a method to reduce the amount of C added to the raw materials is proposed. The process is carried out under a N2 flow in the range of 1300–1500 °C with a holding time of 3 h. The effects of fusible components including CaO, Fe2O3, and MgO on the CRN reaction and micromorphology are discussed. The graphite crucible considerably promotes the conversion of mullite to β-SiAlON and is effective for the reduction in residual C content in the product. When the graphite crucible is used, the mullite wholly transforms to β-SiAlON and corundum, and β-SiAlON and AlN at 1400 °C when the C contents in the raw materials are 11% and 18%, respectively. The content of residual C in the product is approximately 1% (or lower). The β-SiAlON content decreases with the additions of CaO and MgO in the raw materials owing to sintering and melting. The addition of Fe2O3 promotes the formation of β-SiAlON whiskers according to the vapor–liquid–solid mechanism. These results would be useful in parameter optimization for a cleaner production of β-SiAlON powder using the CRN method.