Carbothermal Reduction Nitridation Process Research Articles

Carbothermal Reduction Synthesis of Aluminum Nitride from Al(OH)3/C/PVB Slurries Prepared by Three-Roll Mixing.

Polyvinyl butyral (PVB) was used in the Al(OH)3/carbon black/ethanol slurries by the three-roll mixing to prepare AlN powder using the carbothermal reduction–nitridation (CRN) process in the experiments. The effects of PVB addition on the synthesis of AlN powder were studied by viscosity, tap density, XRD, SEM and TG measurements. The results showed that the PVB layer covering on the surface of Al(OH)3 particles reduced the viscosity of Al(OH)3/carbon/ethanol slurry and increased the dispersion homogeneity of Al(OH)3/carbon raw powder. The tap densities of the Al(OH)3/carbon mixtures after three-roll milling could be increased with the increase in PVB addition. In the CRN process, most of the PVB covering Al(OH)3 particles evaporated and supplied the passage for nitrogen removal to the particles. Based on the experimental data, the role of PVB on the mixing and CRN process was discussed.

Directly growing nanowire-assembled nanofibrous ceramic foams with multi-lamellar structure via freeze-casting process

Herein, a novel and simple approach for directly growing nanowire-built nanofibrous ceramic foams with tunable hierarchical pores has been put forward, where silicon particles, Ludox® and carbon black are selected as the initial materials. Firstly, cellular green bodies were constructed by freeze casting method, and their oriented pores provided sufficient but designated space for the generation of nanowires during carbothermal reduction nitridation process, meanwhile ensured the adequate nitrogen supply. The prepared multi-laminate ceramic foams assembled by nanowires with low density of 0.40 g/cm3 exhibit excellent heat insulation performance with low thermal conductivity of W/(m·K) and outstanding dielectric properties with low dielectric constant of 2.35 at frequency of 10.0 GHz, which could be attributed to the hierarchical porous structure built by nanowires level by level, and they would have promising applications in the thermal insulators, absorptions, catalyst supports, filters for gas or liquid as well as wave-transparent materials.

Synthesis of pure 15R–SiAlON polytype phase & its crystal structure under carbothermal-reduction-nitridation

Synthesis of pure 15R–SiAlON polytype phase, unexplored so far, through carbothermal-reduction-nitridation (CRN) of a mixture of alumina with stoichiometrically proportionate silica has been successfully attempted. The probable reaction sequences of formation are described in terms of the obtained phases. It is argued that the role of CRN process is limited to the formation of intermediate phases, AlN and β-SiAlON, which finally react to form the polytype phase. FTIR studies provide supportive evidence for the different chemical reactions taking place. Formed particles are mostly agglomerated which are actually consisted of fine crystallites (100–300 nm). The powder is well sinterable without external densification aid under spark plasma sintering process producing plate-like elongated grains. The refinement of X-Ray data revealed lattice parameters, lattice volume, crystallite size, microstrain as well as atom coordinates and the corresponding Wycoff positions of the obtained 15R–SiAlON. Six independent atom sites in the unit cell, one (Si/Al) site at 3a, two (Si/Al) sites at 6c and three (O/N) sites at 6c are found. The crystal structure as revealed is silicon rich, ordered but distorted with respect to both bond length and direction for the presence of N4− electronic state.

β-Si3N4 Microcrystals Prepared by Carbothermal Reduction-Nitridation of Quartz

Single phase β-Si3N4 with microcrystals was synthesized via carbothermal reduction-nitridation (CRN) of quartz and carbon coke powder as starting materials. The effects of reaction parameters, i.e., heating temperature, holding time, C/SiO2 ratio, Fe2O3 additive and β-Si3N4 seeds on the phase transformation and morphology of products were investigated and discussed. Rather than receiving a mixture of both α- and β- phases of Si3N4 in the products, we synthesized powders of β-Si3N4 single polymorph in this work. The mechanism for the CRN synthesis of β-Si3N4 from quartz and the formation mechanism of Fe3Si droplets were discussed. We also firstly reported the formation of Fe3Si Archimedean solids from a CRN process where Fe2O3 was introduced as additive. Comparing to the gear-like short columnar morphology observed in samples without β-Si3N4 seeding, the addition of β-Si3N4 seeds led to an elongated morphology of final products and much finer widths. In addition, the β-Si3N4 microcrystals exhibited a violet‒blue spectral emission range, which could be highly valuable for their future potential optoelectronic applications.

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.

Synthesis Ca-α-SiAlON powder by carbothermal reduction nitridation of colloidal sol-gel derived powder precursor

Abstract Ca-α-SiAlON powder was prepared by carbothermal reduction nitridation process at nitrogen atmosphere without using high pressure and long holding time. The precursor was prepared by a novel colloidal sol-gel process consisting of silica, alumina, calcium oxide and carbon. The obtained powders were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The XRD and SEM analysis results revealed that by heating at 1200 °C for 2 h, spherical particles consisting of amorphous calcium aluminosilicate liquid phase were formed. With increasing temperature to 1360 °C and held for 2 h, the spherical particles with the dominant phase of O–SiAlON were observed. Finally, as the temperature climbed to 1500 °C and held for 2 h, spherical particles disappeared and Ca-α-SiAlON was formed. The XRD analysis of the products showed that with increasing temperature from 1360 °C to 1500 °C, O–SiAlON, β-SiAlON and Ca-α-SiAlON were formed, respectively, and colloidal sol-gel method, due to the high homogeneity of the reactants, had a significant role in increasing the purity of Ca-α-SiAlON.

Effect of seeding and carbon content on the formation and microstructure of Ca-α-SiAlON

Ca-α-SiAlON powder was synthesized by carbothermal reduction nitridation process at a low temperature (i.e., 1400 °C, 2 h). The precursor was prepared by a novel sol-gel process. The effect of carbon content and seeding on the microstructure and purity of the Ca-α-SiAlON powder was investigated by X-ray diffraction and scanning electron microscopy. It was found that carbon, in addition to reduction the raw materials, preventing agglomeration of the final product via its steric hindrance. There was an optimal amount of carbon in the precursor and an excessive increase in that caused impurities such as AlN would increase again in the final product, which could be due to an increase in the diffusion distance of the reagents. The results showed that in some cases Ca-α-SiAlON with a distribution of X-values is formed by different reaction path. Seeding has an effective role in the purity and morphology of the products, and single phase Ca-α-SiAlON powder was prepared with the addition of 2 wt% Ca-α-SiAlON seed to the precursor at 1530 °C for 2 h.

Hot-pressed sintered Ca-α-Sialon ceramics with grains from short prismatic to elongated morphology synthesized via carbothermal reduction and nitridation

Ca-α-Sialon powders was initially synthesized by carbothermal reduction-nitridation (CRN) of SiO2, AlN, CaF2 and carbon black at 1700 °C, subsequently was used as the raw material to prepare Ca-α-Sialon ceramic by hot-pressed (HP) sintering at 1900 °C. The main intermediate nitridated products were Si2N2O and O-Sialon formed at 1400 °C–1600 °C. The resultant Ca-α-Sialon just had hexagonally short prismatic structure at 1700 °C. In order to obtain its elongated crystals, α-Sialon seeding was introduced during CRN process. The results revealed that the seeding facilitated crystal refinement of Ca-α-Sialon and the increase in its aspect ratios from 2 to 5.45. The resultant HP-sintered Ca-α-Sialon ceramic with seeding at 1900 °C had compact structure with fine and elongated grains, forming a highly interlocked microstructure. They had a Vickers indentation hardness of ∼21 GPa independent of the microstructure. The flexural strength and fracture toughness of the Ca-α-Sialon ceramics drastically increased with increasing the amounts of seeds, which was in accordance with that in grains aspect ratio of α-Sialon crystals synthesized by CRN as function of amounts of seeds. The improved flexural strength and fracture toughness should be ascribed to the development of elongated α-Sialon grains, which could enhance crack deflection and grain pullout.

Carbothermal synthesis of spherical AlN particles using sucrose as carbon source

In this work, spherical AlN particles were innovatively synthesized by the carbothermal reduction nitridation (CRN) route, using Al2O3 and sucrose as raw materials. The influence of Y2O3 additive, N2 gas pressure and carbon source on the nitridation rate and particle morphology were studied. The results indicated that the elevated N2 gas pressure hampered the reaction rate but were beneficial to increase the AlN particle size. The additive of Y2O3 could react with Al2O3 to form Y-aluminates, which accelerated the CRN process and promoted the formation of spherical morphology. In addition, it was proposed that higher nitridation rate and smaller particle size were achieved when sucrose was used as carbon source rather than carbon black.

Phase, microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition

Aluminum nitride powders were synthesized by carbothermal reduction-nitridation method using Al(OH)3, carbon black and Y2O3 as raw materials. The change of phase, microstructure and densification during the AlN synthesis and sintering process were investigated and the effects of Y2O3 was discussed. The results showed that Y2O3 reacted with Al2O3 to form yttrium aluminates of YAlO3 (orthorhombic and hexagonal phases), Y4Al2O9 and Y3Al5O12 at the low temperature of 1350°C. YAlO3 could firstly be transformed into Y2O3 and then completely into YN when the firing temperature and holding time increased. However, YN could be oxidized into Y2O3 again after the carbon removal at 700°C in the air atmosphere. There were two ways generating AlN when adding Y2O3 and the possible mechanism was proposed. Y2O3 from YN oxidation favored the densification of AlN ceramics because the liquid had better flowability and distribution in the sintering process at 1800°C.

Synthesis of well‐dispersed β‐Si3N4 with equiaxed structures using carbothermal reduction–nitridation method

AbstractWell‐dispersed β‐Si3N4 powders with a novel equiaxed structure and eminent crystal integrity were prepared by carbothermal reduction–nitridation (CRN) strategy with the assistance of CaF2 additive. The growth mechanism of Si3N4 particles in the CRN process was elucidated. It is proposed that the liquid phase formed by SiO2 and CaF2 additive is crucial to the formation of equiaxed β‐Si3N4, and with an appropriate content of CaF2, Si3N4 powders with pure β phase, superior dispersity and crystal integrity can be obtained.

Mechanism and kinetics of combustion-carbothermal synthesis of AlN nanopowders

Abstract In this work, the carbothermal reduction-nitridation process of low-temperature combustion synthesis (LCS) (Al 2 O 3 +C) precursor was investigated in detail. Compared with conventional precursor, the LCS precursor possesses many advantages such as amorphous structure, nanosized particles, homogeneous mixing at molecular level. The experimental results indicate that the methods for preparing precursor exert great influence on phase transformation of Al 2 O 3 , onset temperature of nitridation and reaction activity. During the calcination, the phase transformation of Al 2 O 3 is hindered by a large amount of surrounding C particles rendering Al 2 O 3 maintains high reactivity. Accordingly, the nitridation reaction initiates at 1300 °C and completes at 1500 °C for 2 h. Furthermore, the reaction mechanism was also discussed on the basis of experiments. More significantly, it is established that the activation energy of carbothermal reduction-nitridation reaction using LCS precursor is Eα=336 KJ/mol.

In-situ synthesis of AlON reinforcing phases in resin bonded Al 2 O 3 composite materials

Abstract AlON reinforced Al2O3 composite materials were prepared from fused corundum, tabular alumina, active Al2O3 powders and phenolic resin through a carbothermal reduction nitridation process. The as-synthesized composite materials are characterized by X-ray diffraction, Scanning electronic microscope and Energy-dispersive X-ray spectroscopy. The results show that the major phase is Al2O3 in the composite materials synthesized at various sintering temperatures. And the in-situ formed AlON reinforcing phases show an evolution, which changes from 21R polytype (Al7O3N5), to γ-AlON (Al5O6N) and finally to φ′-AlON (Al11O15N), with the elevation in sintering temperature. Due to the fact that different AlON crystals are connecting neighboring Al2O3 matrix particles, the physical properties of the as-synthesized composite materials are improved. Based on the conducted characterizations and calculations, AlON phases are formed from the reaction between small quantities of residual carbon from phenolic resin and active Al2O3 powders.

Preparation of high quality ferrovanadium nitride by carbothermal reduction nitridation process

High quality ferrovanadium nitride was prepared successfully by carbothermal reduction nitridation process by using V2O5 and Fe as the raw materials. The effects of reaction temperature and C/O molar ratio (content of O is defined as the oxygen in V2O5) on the quality of ferrovanadium nitride were investigated in detail. It was demonstrated that C/O molar ratio had a great influence on the residue carbon, as well as the nitrogen content in the final products. It was found that the nitrogen content of the final product achieved 11.52% when the C/O molar ratio was 90% of the stoichiometric value. However, the nitrogen contents of the final products decreased with the further increase of C/O molar ratio. For the temperature higher than 1773 K, Fe became liquid phase and surrounded the solid V(C, N, O) particles tightly. The formation of liquid Fe was beneficial for the further removal of oxygen atoms left in the V(C, N, O) lattice, since it provided a liquid tunnel for carbon diffusion which greatly enhanced the reaction kinetics. From the results of X-ray diffraction (XRD) and thermodynamic calculations, phase evolution sequence of V2O5 can be obtained as: V2O5?VO2?V2O3?V(C, N, O). Moreover, the kinetic process from V2O3 to VN was studied and the results shown that the reaction rate can be described by Jander diffusion model with the extracted activation energy of 188.173 kJ/mol.

Effects of additives on the synthesis of spherical aluminum nitride granules by carbothermal reduction-nitridation process

In this work, the effects of various additives on the AlN conversion fraction, weight loss, phase compositions and surface morphology during the carbothermal synthesis of spherical AlN granules were investigated in detail. The results inferred that additives played an important role in the improvement of nitridation rate, the formation of spherical morphology and the growth of particle size. The simultaneous addition of CaF2 and YF3 resulted in the highest nitridation rate in all samples, while excessive CaF2 appeared to retard the nitridation process at a particular temperature. However, Ca-aluminates forming in the synthesis process tended to be reduced and further volatilized at high temperature. Furthermore, by analyzing the weight loss during the carbothermal reduction-nitridation process, the reaction mechanisms in the systems using different additives were also attentively disclosed.

Phase and morphological transformation stages during carbothermal reduction nitridation process: From coal gasification slag wastes to Ca-α-SiAlON powders

Effective utilization of industrial solid wastes is of great benefit for environmental and economic needs. In order to clarify the theoretical issues and pave the way to synthesize high quality Ca-α-SiAlON powder from coal gasification slag by carbothermal reduction nitridation (CRN), in this study, the theoretical issues including the network structure of the slag as well as the phase and morphological transformation stages of raw slag during CRN process were comprehensively studied at different temperatures (1350, 1400, 1450 and 1500°C). It was found that the CRN process was dominated by nitridation of the Ca-Si-Al-O glass component in slag. With the increase of processing temperature, spherical Ca-Si-Al-O particles were firstly nitrided into porous Ca-Si-Al-O-N spheres, plate-like O-SiAlON grains, and then nitrogen-rich SiAlONs (β-SiAlON, 15R polytypoid and α-SiAlON). At the end of CRN, Ca-α-SiAlON was found to be the only SiAlON phase presenting elongated prismatic morphology. In addition, high concentration Ca-α-SiAlON powder was obtained through a two-step purification process with a conversion rate as high as 45wt%. The obtained results shed light on the preparation of Ca-α-SiAlON products from coal gasification slag and related wastes.

Aluminum Nitride Formation From Aluminum Oxide/Phenol Resin Solid-Gel Mixture By Carbothermal Reduction Nitridation Method

Abstract Hexagonal and cubic crystalline aluminum nitride (AlN) particles were successfully synthesized using phenol resin and alpha aluminum oxide (α-Al2O3) as precursors through new solid-gel mixture and carbothermal reduction nitridaton (CRN) process with molar ratio of C/Al2O3 = 3. The effect of reaction temperature on the decomposition of phenol resin and synthesis of hexagonal and cubic AlN were investigated and the reaction mechanism was also discussed. The results showed that α-Al2O3 powder in homogeneous solid-gel precursor was easily nitrided to yield AlN powder during the carbothermal reduction nitridation process. The reaction temperature needed for a complete conversion for the precursor was about 1700°C, which much lower than that when using α-Al2O3 and carbon black as starting materials. To our knowledge, phenol resin is the first time to be used for synthesizing AlN powder via carbothermal reduction and nitridation method, which would be an efficient, economical, cheap assistant reagent for large scale synthesis of AlN powder.

Synthesis of Sialon-SiC Composites from Kyanite Tailings by Carbothermal Reduction Nitridation

Sialon-SiC composites were synthesized from kyanite tailings via the carbothermal reduction nitridation (CRN) technique. The phase composition, morphology, and microstructure of the CRN samples were investigated by x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, and elected-area electron diffraction. A brief discussion on the synthesis mechanism of CRN process was investigated based on the thermodynamic analysis of the SiO2-Al2O3-C-N2 system. The results show the phase compositions depended on the synthesis temperature. With the increasing heating temperature, SiO2 was first changed into Si2N2O and then transformed into Si3Al3O3N5 through reacting with N2, Al2O3, and C. At the same time, the excess SiO2 changed into SiC by a carbothermal reduction reaction. The optimized synthesis temperature for the CRN reaction was found to be at 1550°C for 4 h. The Si3Al3O3N5 and SiC phases were proved coexisting in the same CRN particle. This transformation provided a feasible utilization route of kyanite tailings for high-temperature application.

Fabrication and Analysis of β-Sialon Whiskers from Fly Ash by Carbon Thermal Reduction-Nitridation

β-Sialonwhiskers which the molecular structuralformula of β-Sialonis Si3Al3O3N5(z = 3) were synthesized from fly ash and graphite under appropriate technological conditions by carbothermal reduction–nitridation process. The effects of carbon content, reaction temperature and reaction time on synthesis ofβ-Sialonwere analysed by XRD, SEM techniques. The results proved that, the condition of the carbon content over 80% is the best parameter to promote theβ-Sialon powder production. Compared to other kinds of temperature, 1430 °C is the optimal temperature to promoteβ-Sialon powder generation. Compared to 3h, holding time of 6h is promoting theβ-Sialon powder generation.The main morphology of β-Sialon was rod-like whisker.

Phase transformation of high aluminium fly ash in carbothermal reduction–nitridation at high temperatures

Sialons (Si5AlON7, Si4Al2O2N6, Si3Al3O3N5 and SiAl4O2N4) were synthesised from high aluminium fly ash by using a carbothermal reduction–nitridation process. The effects of temperature and the addition of carbon black on the phase compositions and morphologies have been investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Thermodynamic analysis has been used to study phase transformations. For the sample with the stoichiometric carbon black content, the product formed at 1500°C was single phase short prismatic Si3Al3O3N5. The products at 1600°C were Ca–α-Sialon accompanied by AlN, whereas at 1700°C, Si4Al2O2N6 and SiAl4O2N4 formed. Increasing the carbon black at 1500°C, Si3Al3O3N5, β-SiC, 15R and FexSi were produced as reaction products.