Carbothermal Reduction And Nitridation Research Articles

Synthesis of Si3N4 powder with a controllable alpha/beta ratio by sol-gel assisted carbothermal reduction and nitridation

The controllable α/β ratio of Si3N4 powder was synthesized using tetraethyl orthosilicate (TEOS), glucose, MgCl2 and YCl3·6H2O by sol-gel assisted carbothermal reduction and nitridation (CRN). This study aimed to investigate the impact of various parameters, including a H2O/TEOS mol ratio of the sol, a C/Si mol ratio, sintering aids, and the temperature of CRN on the phase transformation of α-Si3N4. The results demonstrated that the H2O/TEOS ratio significantly influenced the relative content of α and β phase in Si3N4 by changing the morphological characteristics of SiO2. When the H2O/TEOS ratio was 100, β-Si3N4 powder with an approximate β content over 88 wt% was synthesized at 1500 °C. This remarkable phase transformation was likely facilitated by the eutectic liquid phase consisting of MgSiO3/Mg2SiO4. The SEM image revealed rod-like whiskers with a length around 1.4 μm for the β-Si3N4 grains. Due to the presence of liquid phase, the synthesis of spherical-like α-Si3N4 nano-powder with an α-phase content of 91 wt% was achieved at a lower temperature of 1450 °C, with an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The size of the particles was about 70 nm due to the reduced reaction temperature. Furthermore, without the addition of any sintering aids or Si3N4 seeds, high purity α-Si3N4 with an α phase content approaching 96 wt% could be synthesized at 1500 °C, using an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The resulting α-Si3N4 powder exhibited an extremely regular hexagonal prism shape with a length of approximately 2 μm.

Effect of varied amount of photocatalytically active TiO2 loaded on AlON carrier-type photocatalyst

An AlON carrier with light-transmitting performance and through-hole structure was prepared by carbothermal reduction and nitridation (CRN). The TiO2/AlON photocatalyst was prepared by hydrothermal method. Under vertical irradiation of the light source, the degradation rate of MB by TiO2(15%)/AlON photocatalyst was 97.3% after 180 min. And 88.8% MB was mineralized by determining the total organic carbon in the solution. An increase in TiO2 loading can accelerate the efficiency of TiO2/AlON degradation of MB within 0-90 min. At the same time, it will also exacerbate the blockage of AlON carrier and increase the amount of free TiO2. These can all exacerbate light loss, thereby shortening the penetration distance of light in the mixed liquid. Within the range of 90-180 min, the photocatalytic performance of TiO2(15%)/AlON powders was better than that of TiO2(25%)/AlON (97.3% vs 82.5%). In addition, TiO2/AlON photocatalyst was easy to cycle and has stable photocatalytic performance.

Study on one-step fast CRN mechanism for preparing fine AlON powder using carbon coated γ-Al2O3/C precursor

By pre-milling in glucose aqueous solution, γ-Al2O3 particles were uniformly mixed with carbon via hydrothermal treatment technique. Then, directly heating the γ-Al2O3/C precursor at 50 °C/min to 1775 °C for 15–30 min in N2, single phase AlON powder was fast obtained through one-step carbothermal reduction and nitridation (CRN). By approaching the evolution of phase assemblages and morphology during heating and dwelling, the one-step fast CRN mechanism for preparing fine AlON powder was revealed. The carbon around γ-Al2O3 particles delayed the phase transformation of Al2O3 from γ phase to α one. The undesired particle aggregation and growth before formation of AlON was also auxiliarily inhibited by swiftly achieving AlON formation temperature via high heating speed. Further elevating synthesis temperature enabled high reaction kinetics to boost the speed of AlON formation. By these ways, CRN reactions were quickly completed to fast obtain pure AlON powder with small primary particles due to limited particle aggregation and growth. The as-prepared fine AlON powder was easy to be ball-milled to prepare highly transparent AlON ceramics by pressureless sintering.

Preparation of transparent AlON ceramics with controlled shapes by a novel spontaneous coagulation casting and pressureless sintering method

Aluminum oxynitride (AlON) exhibits excellent mechanical properties and good optical transparency in the ultraviolet to near-infrared range and can be used in military and civilian fields. However, the troublesome molding of AlON, especially for complex shapes, limits its promotion and application. Therefore, transparent AlON ceramics with complex morphologies were prepared for the first time by a novel spontaneous coagulation casting (SCC) and pressureless sintering method. In this study, we report a simple carbothermal reduction and nitridation (CRN) method for synthesizing high-purity AlON powder, which is especially suitable for wet casting without anti-hydration treatment. The dispersion and gelation behavior of AlON slurries by isobutene maleic anhydride copolymer (Isobam104) and tetramethylammonium hydroxide (TMAH) were extensively investigated. With the addition of 0.9 wt% Isobam104 and 15 mL/L TMAH, the AlON slurries can achieve a solids loading of up to 45 vol% with the lowest viscosity (460.4 mPa·s-10 r/min) and the AlON green body is free of cracks and deformation during drying and demolding processes. Finally, the AlON green bodies with square and polygonal irregular morphologies were fabricated via a simple SCC method. High transparent AlON ceramics of a maximum transmittance up to 77.3% at 2000 nm and 72.2% at 632 nm were obtained by pressureless sintered at 2000 °C for 6 h. The SCC method has great potential in preparing ceramics with controllable shapes.

Microstructure control of porous Si3N4 whisker scaffolds by in-situ carbothermal reduction and nitridation reaction

Si3N4 whisker scaffolds have great potentials owing to the superior properties of Si3N4 and their unique porous structures. Obviously, the properties of the scaffolds are closely related to the morphology and distribution of phases, and the bonding between whiskers. This study proves that the above-mentioned microstructural features can be tailored to a great extent by the in-situ growth of secondary Si3N4 products via carbothermal reduction and nitridation (CRN) of SiO2 inside a β-Si3N4 whisker network. Specifically, the yield and morphology (grain size, aspect ratio, etc.) of the CRN product are found to be strongly dependent on the composition and amount of the reactants (SiO2 and C/SiO2 molar ratio) and reaction temperature. When reacted at 1300 ℃, a distinctive microstructure is formed in which nano sized and CRN-derived Si3N4 whiskers are uniformly distributed in the original micron sized whisker network. The whisker scaffolds reveal improved flexural strength if compared with that without CRN-derived Si3N4. This finding indicates the formation of a strong bonding between the β-Si3N4 whiskers by the growth of the CRN product. The measured low dielectric constant (2.6 ∼ 3.1) and low dielectric loss (<1.1 ×10−2) of the scaffolds makes them suitable for wave transmission application.

Light-transmitting AlON carrier with enhanced photocatalytic degradation performance

A skeleton structure AlON (aluminum oxynitride) carrier with light transmission performance was prepared by carbothermal reduction and nitridation (CRN). The AlON carrier can disperse nano active components and enhance its photocatalytic performance. The microstructure of the TiO2/AlON composite photocatalyst shows that the dispersion of the agglomerated TiO2 is achieved by the physical adsorption mechanism, the embedding mechanism and the single particle dispersion mechanism. The morphology of the BiVO4/AlON and g-C3N4/AlON composite photocatalysts further proves the universality of the AlON carrier in the dispersion of nano photocatalyst. The photocatalytic performance of the nano active component was enhanced by combining with AlON carrier. After 240 min of visible light irradiation, the degradation rate of MB increased by 7.4% (BiVO4/AlON) and 8.1% (g-C3N4/AlON), respectively. High light transmittance was the main reason for AlON carrier to improve the photocatalytic performance of loaded active components. In addition, AlON carrier type composite photocatalyst can be reused for many times because it has physicochemical stability and can be recovered easily.

Preparation of single phase nano-sized AlN powder by carbothermal reduction and nitridation of hydrothermal synthesized carbon coated γ-Al2O3 from glucose via fast heating

By twice pre-milling γ-Al2O3 in absolute ethyl alcohol plus in glucose aqueous solution, γ-Al2O3 was first coated by carbon via hydrothermal treatment technique. Then, at a fast heating speed of 50 °C/min, nano-sized pure AlN powders were synthesized through carbothermal reduction and nitridation (CRN) at 1550–1600 °C for 60–120 min in N2. The pre-milling in absolute ethyl alcohol effectively broke the initial aggregation of γ-Al2O3, and the subsequent milling in glucose aqueous solution led to well contact between γ-Al2O3 and glucose, which contributed to the formation of intact carbon coating on the surface of γ-Al2O3 during hydrothermal treatment. The coated carbon served as one reactant and inhibitor of particle aggregation for CRN, which also led to retention of γ-Al2O3 and its morphology during heating with the help of high heating speed. As a result, fine AlN of ∼100 nm was directly formed by nitridation of γ-Al2O3. The synthesis mechanism of nano-sized AlN powder by the carbon coated Al2O3/C precursor was discussed.

Regulation of crystal defect concentration in AlON

AlON powders were synthesized by carbothermal reduction and nitridation (CRN) with α-Al2O3 and C as raw materials. The crystal defect concentration of AlON can be regulated by changing the C content in the α-Al2O3/C mixture. The results show that the oxygen vacancy defect of AlON is caused by N replacing O, and the N content increases with the increase of C content, resulting in the lattice expansion of AlON. However, the change in defect concentration has no obvious effect on the morphology of AlON powders.

Carbon‐containing droplet combustion–carbothermal synthesis of well‐distributed AlN nanometer powders

AbstractA novel three‐step technique was employed to synthesize the well‐distributed AlN nanopowders. First, the hollow spherical precursor particles with an average diameter of 2–5 μm, consisting of an amorphous structure mixture of Al2O3 and C, was prepared by carbon‐containing droplet combustion method by using glucose, urea, and aluminum nitrate as starting materials. The carbothermal reduction and nitridation (CRN) was carried out at 1500°C under N2 flow for 2 h and subsequently the CRN product was calcined at 700°C in air for 1 h to remove residual carbon and transform the CRN product to high‐purity AlN powders consisting of nanostructured hollow spheres. The formation mechanism of precursor and AlN hollow spheres was discussed in detail. The AlN powder exhibited well‐distributed spherical particles with a size of 30–50 nm and good sinterability. After additive‐free and pressureless sintering at 1800°C for 2 h, the relative density of the sintered AlN sample was measured to be 99.02%.

Effect of carbon and α-Al2O3 on controlled synthesis of AlON powder

The α-Al2O3/C mixtures were prepared by ball milling, and then AlON powders were synthesized by carbothermal reduction and nitridation (CRN). The effects of α-Al2O3 particle size, carbon powders morphology and particle size on the morphology and particle size of the synthesized AlON powders were studied. The results showed that as the particle size of α-Al2O3 increases, the particle size of the synthesized AlON powders will also increase, but the surface morphology will not be affected. The increase of the carbon particle size will increase the particle size of the synthesized AlON powders. The pore size and number of pores of the synthetic AlON powders were very similar to the morphological characteristics of the carbon powders. In addition, the mechanism of controlling the synthesis of AlON powder with α-Al2O3 and carbon as raw materials was also discussed.

Fast one‐step carbothermal reduction and nitridation method using nano‐sized Al2O3 and carbon black to prepare aluminium oxynitride powder for highly transparent ceramics

AbstractBy fast heating the nano‐sized Al2O3 and carbon black mixtures at 50°C/min to 1750°C for 30–120 min, single‐phase AlON powders were successfully obtained by a fast one‐step carbothermal reduction and nitridation (CRN) method. The AlON ceramics pressureless sintered at 1880°C for 150 min by these powders show high transmittances up to 83%–84%, which indicates that the proposed fast one‐step CRN method is an effective and efficient way with strong robustness to synthesize single‐phase AlON powder for highly transparent AlON ceramics. It was found that α‐Al2O3 particles do not have enough time to aggregate and coalesce during heating due to the tremendously shortened heating span, which significantly inhibited particle coarsening until the formation of AlON starts. The fast‐formed AlON further inhibits the coarsening of α‐Al2O3 during dwelling. Consequently, single‐phase AlON powder of small primary particles can be obtained after 30 min dwelling at 1750°C.

Preparation of transparent AlON from powders synthesized by novel CRN method

Aluminum oxynitride (AlON) powders were synthesized by a novel carbothermal reduction and nitridation (CRN) method. Homogenous and fluffy AlOOH/C core-shell nanoparticle precursor was hydrothermally synthesized with aluminum nitrate hydrate, sucrose and urea as starting materials. Then single-phase AlON powders were synthesized by CRN method at 1700 °C for 2 h. The phase transition and growth of Al2O3 particles was effectively retarded by the amorphous carbon nano-layers on the surface of precursor, resulting in significantly lower reaction temperature and further smaller particle size. Based on above fine AlON raw material, transparent AlON ceramic was prepared by pressureless sintering at 1880 ℃ with the in-line transmittance above 80 %.

One-step carbothermal reduction and nitridation (CRN) for fast fabrication of fine and single phase AlON powder by using hydrothermal synthesized α-Al2O3/C mixture

Glucose and ball-milled α-Al2O3 mixed powder was hydrothermal treated to synthesize Al2O3/C mixture, which was directly heated to 1700–1750 °C for 10 min holding in N2. Fine and single phase AlON powders were successfully fast prepared by one-step carbothermal reduction and nitridation (CRN). With aggregation of as-purchased α-Al2O3 being effectively disconnected by ball milling, α-Al2O3 particles were perfectly coated with carbon by hydrothermal treatment, which inhibited coalescence of Al2O3 during heating and enabled fast formation of AlON to further shorten holding duration. As a result, one-step fast fabricated pure AlON powder has fine primary particles with weak interconnections. At 170 rpm, powders were ball-milled to D50 ≤ 1.00 μm for 4 h, and D50 = 0.33 μm for 24 h. Moreover, primary particle size is controllable by adjusting holding time, synthesis temperature and heating speed. One-step CRN method provides a practical way to prepare pure AlON powder with the required primary particles.

Highly transparent AlON fabricated via SPS synthesized by Al2O3/C core–shell starting powder and CRN method

AbstractTwo types of starting powders were used to synthesis AlON powder via the carbothermal reduction and nitridation (CRN) route, the Al2O3 and C mixture, and the Al2O3/C core–shell. Al2O3/C core–shell resulted in lower carbon content and lower synthesis temperature for CRN method. AlON powder was synthesized using Al2O3 and C then was sintered via SPS at 1500, 1600, and 1700°C for 10‐30 min in a vacuum atmosphere. A sample with the highest transmittance was annealed at 1250°C for 2, 4, and 8 h in air. The characteristics of AlON powder and bulk samples were investigated by XRD, DLS, FT‐IR, elemental analysis of carbon and sulfur, SEM, EDS, and UV–Vis spectroscopy analyses. The annealing process for 2 h improved the transmittance from 31% to 58% at the wavelength of 600 nm. AlON powder synthesized using Al2O3/C core–shell, was SPSed at 1700°C for 15 min and annealed at 1250°C for 2 h was used to make a sample of 1.7 mm thickness whose transmittance was found to be 81%.

Towards a sustainable technology for production of extra-pure Ti metal: Electrolysis of sulfurized Ti(C,N) in molten CaCl2

A new concept for producing highly pure Ti metal powder from ilmenite (FeTiO3) is proposed in this article. Titanium nitride (TiN) or titanium oxycarbonitride (TiOxCyNz) could be synthesized in the first step via the simultaneous carbothermal reduction and nitridation (CTRN) of FeTiO3 to remove oxygen roughly. To separate oxygen completely, high-quality TiS2 samples were then synthesized from TiN and TiC using S2 gas, and the clean sulfides were finally reduced to α-Ti powders with spherical morphology using electrolysis in molten CaCl2. X-ray diffraction (XRD), scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDS), and elemental LECO analysis were used to study the phases and microstructures of the sulfides and the electrochemically reduced powders. The Ti powder showed no carbon contamination and consisted of high-purity foil-like Ti sheets with very low oxygen, carbon, and nitrogen contents of less than 0.15wt% O, 0.02wt% C, and 0.003wt% N, respectively. The quality of the Ti powder was much higher than that of the powder obtained using the conventional OS process (proposed by K. Ono and R.O. Suzuki) starting directly from the oxides.

Preparation of AlN powder of low oxygen content via carbothermal reduction and nitridation by active gas exchange technique

By generating a periodic impulse-like pressure (2.0–4.4 kPa, 84 s) to actively exchange the gas in synthesis furnace, pure AlN powder of low oxygen content was synthesized via additive free carbothermal reduction and nitridation (CRN) of Al2O3 powder. Compared with the conventional CRN method, the proposed extra gas exhaust process can more effectively remove the side-produced CO from the reaction sites to accelerate nitridation process and decrease the residual oxygen content in the obtained AlN powder. For example, with 39 wt% activated carbon loaded in the raw material at 1650 °C for 4 h, the prepared AlN powder by the proposed synthesis scheme has only 0.68 wt% residual oxygen. The effects of carbon content, synthesis temperature and holding time on the residual oxygen content in AlN powder by the proposed synthesis scheme were also studied. The ball-milled as-prepared AlN powder was pressureless sintered at 1880 °C for 2.5 h to obtain a translucent AlN ceramics (37.6% at ~5700 nm), which demonstrates the excellent sinterability of the as-prepared AlN powder.

A contrast of carbothermal reduction synthesis of MgAlON and AlON powders for transparent ceramics

MgAlON and AlON powders have been synthesized by carbothermal reduction and nitridation (CRN) from C–Al2O3–7.8 wt% MgO and C–Al2O3 systems, respectively. The phase and morphology of the systems were investigated under 1100–1750 °C via XRD, DTA, SEM and NMR. The results showed that, MgO could significantly effected the stabilization region and morphology of the (Mg)AlON products. The formation of spinel MgAl2O4s intermediate phase was key to obtain MgAlON at lower temperature with smaller particles than that of AlON. Under the optimized compositions, single-phase MgAlON and AlON were obtained at 1600 °C and 1720 °C, respectively. When sintered at 1850 °C for 24 h, the MgAlON sample was clear with highly transparency (82.5% at 3.7 μm), while AlON was opaque with lower transmittance (∼8.0% at 4.7 μm) due to lower sintering activity of the powder.

Direct production of Si3N4 foams by carbothermal reduction and nitridation of SiO2

In this study, Si3N4 foams are successfully produced by a combination of replica and carbothermal reduction and nitridation (CRN) methods. Slurries prepared from C, SiO2 (molar ratio of C:SiO2 was kept to 2.2), bentonite and three types of oxide additives (Y2O3, CeO2 and Yb2O3) are impregnated into polyether sponges. With the addition of bentonite, sufficient green strength is provided and durable foams are produced after the sponge burn-out process. Bentonite also enhances the CRN process by increasing the amount of eutectic melt and, hence, a higher amount of β-Si3N4 is obtained. In addition to bentonite, the yield of β-Si3N4 is highly dependent on the type of oxide additive. The highest yield is achieved when Y2O3is used as a result of the formation of a lower viscosity eutectic melt. Vapor-liquid-solid mechanism is favored by the lower viscosity. Foams have β-Si3N4 as the major phase with minor amounts of α-Si3N4 and secondary phases originate from the eutectic melt. The formation and growth mechanisms of β-Si3N4 are directly affected by the eutectic melt. SiO is deposited in the eutectic melt and super-saturation of β-Si3N4 grains occur. During the growth of these grains, they contact each other, hindering the formation of typical elongated β-grains. The final microstructures of the foams consist of hexagonal β-grains with a low aspect ratio.

Highly photocatalytic activity of porous skeleton structure AlON powder synthesized by CRN under ultraviolet-light irradiation

A large-sized (∼20 μm) pure aluminum oxynitride (AlON) powder characterized by its porous skeleton structure was synthesized by carbothermal reduction and nitridation (CRN) method using α-Al2O3. The photocatalytic performance of the obtained AlON powder was studied under ultraviolet-light in methylene blue (MB) solution. In 40 ml MB solution doped with 30 mg as-synthesized AlON powder, a photogradation efficiency as high as 97% was measured after irradiating 120 min, and 80% MB was mineralized by determining the total organic carbon (TOC) in the solution, i.e., mineralization is the main degradation mode of MB in the solution due to the added AlON powder. Further radical scavenger’s experiments reveal that both h+ and OH were the dominant reactive species in the photogradation process of MB by the AlON powder. It should be pointed out that the specific surface area of the obtained AlON powder is smaller than that of solid small-sized powder (∼800 nm) due to its porous skeleton structure. However, the measured photogradation efficiency of the obtained AlON powder is slightly higher than the reported value, which is mainly attributed to light reflection and scattering within its skeleton. The as-synthesized AlON powder has excellent dispersibility for its large particle size. Moreover, the obtained AlON powder also exhibits high reusability. Therefore, the obtained large-sized porous skeleton structure pure AlON powder is an efficient and promising photocatalyst.

Kinetics of microwave synthesis of AlN by carbothermal‐reduction‐nitridation at low temperature

AbstractAluminum nitride (AlN) was synthesized at 1000‐1400°C from a mixture of alumina and carbon powders using 2.45 GHz microwaves in a N2 atmosphere. High nitridation ratios (&gt;0.90) were obtained in the temperature range 1200‐1400°C. The apparent activation energy of the carbothermal reduction and nitridation (CRN) reaction using 2.45 GHz microwave irradiation was calculated from the nitridation ratio. The value obtained, 79.9 kJ/mol, is 11% of the energy reported for conventional synthesis using α‐Al2O3 as the raw material. This result indicates that 2.45 GHz microwave irradiation could promote the kinetics of the CRN reaction, and AlN could be effectively synthesized at low temperature.