Microwave-assisted Carbothermal Reduction Research Articles

Low-temperature synthesis and sinterability of high-purity submicron TiB2 powder via microwave-assisted carbothermal reduction

Microwave-assisted carbothermal reduction has been innovatively developed for the low-temperature synthesis of high-purity submicron TiB2 powders. The thermodynamic research of TiO2-B4C-C system and relevant reactions showed that suitable excess addition of B4C was required to compensate for the boron source evaporation loss. The effect of excess addition of B4C (0 −15 wt%) and synthesis temperature (1350 −1550 °C) on the phase composition, purity, and morphology of the products have been investigated. Submicron TiB2 powders (0.75 µm) with high-purity (0.36% residual C and 0.53% O) were successfully synthesized at 1450 °C for 20 min under microwave assistance with TiO2, excess 5 wt% B4C, and carbon black. Additionally, the as-synthesized TiB2 powders after spark plasma sintering at 2000 °C for 15 min showed a relatively higher sinterability, namely increased at 7.7% relative density of sintered ceramic when compared with the commercially available powders owing to the lower O content and submicron-scale size.

Research of Bayan Obo tailings characteristics and recovery methods

This paper presents a new method for the recovery of iron and rare earth elements (REEs) from Bayan Obo tailings, which are the waste materials from the extraction of iron and REEs from the Bayan Obo ore deposit in Inner Mongolia, China. The method is based on the combination of microwave-assisted carbothermal reduction and acid leaching. The microwave-assisted carbothermal reduction can effectively reduce the iron oxides and REE oxides in the tailings to metallic iron and REE metals, respectively, and separate them from the gangue minerals. The acid leaching can then dissolve the REE metals from the reduced product and obtain a high-purity REE solution. The effects of various parameters on the performance of the method were investigated and optimized, and the mechanisms and kinetics of the reactions were analyzed and modeled. The results showed that the method can achieve a high recovery rate of iron and REEs, a high selectivity of REEs, a low energy consumption, and a simple process. The economic and environmental benefits of the method are also significant, as it can utilize the Bayan Obo tailings and recover the valuable elements from them, and reduce the waste generation and greenhouse gas emission. The method provides a new and efficient technology for the utilization of Bayan Obo tailings and the recovery of iron and REEs, and contributes to the fundamental understanding of the microwave-assisted carbothermal reduction and acid leaching processes.

Study on the rapid synthesis of high-entropy carbonitride powder and its remarkable hardening effects on cemented carbide tool materials

As a novel high-entropy ceramic material with a multi-cation and multi-anion sublattice structure, high-entropy carbonitride ceramics (HECNs) have exhibited outstanding comprehensive properties and attracted increasing attention. In this study, a new WC-based cemented carbide material reinforced by high-entropy carbonitride (Ti0.2W0.2Mo0.2V0.2Nb0.2)(C0.7N0.3) (marked as HECN) was successfully fabricated by vacuum hot-pressed sintering. The mutual constraints between hardness and fracture toughness in extant WC-Co cemented carbide tool materials and their deficient high-temperature mechanical properties were effectively addressed. Moreover, for the low efficiency of the traditional carbothermal reduction nitridation, a novel method for rapidly preparing HECN powder was firstly proposed, namely the microwave-assisted carbothermal reduction nitridation, which could accelerate the heating rate and shorten the holding time. Single-phase HECN powder was successfully synthesized by this method, and the impacts of varying HECN content on the room and high-temperature mechanical properties of WC-based cemented carbide were investigated combined with the microstructure analysis by XRD\\SEM. The results revealed that HECN effectively inhibited the grain growth of WC in cemented carbide, and the refined WC grains provided a high-volume fraction of grain boundaries, which prevented dislocation movement and increased the hardness. With the optimized HECN content of 10 wt%, the Vickers hardness and fracture toughness of WC-HECN-Co composite reached 21.58 ± 0.36 GPa and 12.27 ± 1.17 MPa∙m1/2 respectively. Furthermore, WC-10HECN-10Co composites possessed outstanding high-temperature hardness and maintained a high hardness value of 13.16 ± 0.30 GPa at 800 °C, 37% higher than that of WC-10Co. The findings of this research hold significant practical implications for applying cemented carbide cutting tools in high-speed dry cutting.

Microwave-assisted carbothermal reduction of rice hull ash to biogenic silicon

Rice hull is an agricultural waste disposed of globally in large quantities. However, it is an attractive biomass resource, containing at least 15–20 wt% amorphous silica. Herein, biogenic Si production was attempted by direct carbothermal reduction of rice hull ash using a microwave heating source. Si was prepared from rice hull ash with a C-to-SiO2 ratio (in mole ratio) of 1.0 at 1564 °C after 15 min of 1500 W microwave irradiation. Although it is still difficult to evaluate the yield, the possibility of producing metallic Si at lower temperatures than the traditional method (>2000 °C) has been demonstrated, and it is hoped that this method will be applied in the future. We expect that these results can contribute to creating a new process that enables low-cost, energy-saving, and high-efficiency production of Si for solar cells and semiconductors.

Boron carbide amorphous solid with tunable band gap

Boron carbide BxC (x=1/6−10) powders were synthesized through a microwave-assisted carbothermic reduction reaction as a potential clean energy material. Their crystallographic structures and optical properties were characterized. X-ray diffraction and electron diffraction indicated that the synthesized BxC powders were amorphous. Electron energy-loss spectroscopy demonstrated that the composition of boron and carbon was in amorphous materials, and their chemical bonding were disclosed from Raman scattering spectroscopy. UV–vis absorption spectroscopy indicated that the bandgap of the bulks varied from 2.30eV to 3.90eV, tuned by the boron/carbon element ratio. The synthesized powders were potential photovoltaic materials. A short-range ordering model was established to explain the optical properties.

Microwave-Assisted Carbothermic Reduction of Discarded Rare Earth Magnets for Recovery of Neodymium and Iron Values

Neodymium-iron-boron (NdFeB) waste magnets consist of approximately 28% rare earth values in the form of the Nd2Fe14B phase. According to the results, carbothermic reduction through microwave irradiation of crushed magnets was observed to be promising for the separation of metallic iron and rare earth values. The effect of microwave exposure time and charcoal addition on the formation of different phases was evaluated. Carbothermic reduction (10% C) for 1.2 min raised the temperature of the sample to 990°C and resulted in the formation of iron balls with more than 85% purity. Higher exposure time favors the formation of the NdFeO3 phase, thereby encapsulating the rare earths, resulting in limited dissociation. Rare earth oxides with purity of more than 98% were recovered via a leaching and precipitation route. The microwave route was found to be superior to the conventional muffle furnace heating route with low energy consumption and metallic iron formation.

Efficient carbothermal reduction of diatomaceous earth to silicon using microwave heating

The microwave-assisted carbothermal reduction of silica (SiO2) to Si was studied using diatomaceous earth (DE) as SiO2 source instead of quartz to economically produce high-purity Si for solar cell use. Under microwave irradiation, polycrystalline Si was produced at 1580–1680 °C within 10–20 min, The temperature and reaction time were much lower and shorter, respectively than those required over 2000 °C and some decades of minutes for the conventional arc-furnace process. When the C/SiO2 mole ratios were 2.0 and 1.0–1.5, the main products were silicon carbide and Si, respectively. When the C/SiO2 mole ratio was lower than 0.8, the temperature was not high enough for the formation of Si. These temperature reductions and reaction rate acceleration effects were attributed to the reaction enhancement by the amorphous nature of DE, the catalytic action of impurities of DE such as iron, and the selective fast heating effect of microwaves.

Influence of Coal Petrography on Microwave-assisted Carbothermic Reduction Roasting of Banded Hematite Jasper Ore

ABSTRACT The study focuses on the use of microwave energy in the reduction roasting followed by magnetic separation studies of an Indian Banded Hematite Jasper (BHJ) ore assaying about 48% Fe. Two coals having different origins have been used as the reductants, and the effects of their petrographical characteristics on the microwave-assisted carbothermic reduction have been discussed. The impacts of various process variables such as microwave power, time, ore and coal size have been studied and subsequently optimized using the Taguchi based statistical design. Microwave power and time are found to be the most influential factors, while the particle sizes of the ore and coal play less significant roles. The coal having higher vitrinite reflectance and lower mineral matter is found to be a better reductant in the microwave roasting as it has given rise to a magnetic concentrate of ~64% Fe with a recovery of 68.4% at process conditions such as a power of 6 kW, time of 16 min and C/Fe ratio of 0.15. In contrast, even at the same C/Fe ratio, the coal having lower vitrinite reflectance and higher mineral matter could generate a concentrate with an iron recovery of 34.59%. The X-Ray Diffraction, optical micrographic, Scanning Electron Microscopic (SEM) and Vibrating Saturation Magnetometry (VSM) studies have been used to characterize the microwave roasted products with an intention to understand the phase changes.

Microwave-Assisted Carbothermal Reduction of Banded Hematite Jasper Ore

This study investigates the microwave carbothermal reduction of low-grade banded hematite jasper iron ore (Fe ~ 37%). The susceptibility of iron phases to microwave exposure and their selective absorptions help in the liberation of iron values from impurities. It was found that a small fraction of the microwave-irradiated ore-charcoal mixture was rapidly melted to produce pure ferrite balls with Fe ~ 90%. The ferrite balls were observed within 6-10 min of reduction at 8% and above charcoal at microwave power of 900 W. The iron grade deteriorated over prolonged exposure because of the formation of the fayalite phase. The optical micrograph of the ferrite ball reveals retained austenite and martensitic iron phases with an average hardness value ~ 302 HV and a saturation magnetization of ~ 190.4 emu/g.

Recovery of Iron Values from Discarded Iron Ore Slimes

The ever-increasing demand for steel and the subsequent depletion of high-grade ores requires the massive utilization of low-grade iron ores. In the present study, an iron ore slime sample containing 56.1% Fe was investigated using physical beneficiation, carbothermal reduction, and a microwave-assisted process; however, physical beneficiation and microwave exposure were found to be inefficient as they yielded lower iron recovery. Using coconut shell as an alternative reductant was found to be promising and does not lead to the formation of higher iron oxides, such as fayalite and wustite phases. The microwave-assisted carbothermal reduction was found at par with conventional reduction, saving time and heating. The optimum magnetic concentrate obtained in both carbothermal reduction routes (microwave and muffle furnace: Fe grade, Fe recovery, and yield of 64.3, 97, and 84%, and 67, 99, and 83%, respectively) are suitable for alternative iron-making or blast furnace units.

Microwave synthesis of ultra-high temperature ceramic ZrC nanopowders

Ultra-high temperature ceramic zirconium carbide (ZrC) nanopowders were synthesized for the first time via a microwave-assisted carbothermal reduction reaction using zirconia (ZrO2) and carbon black (C) as the starting materials. The effect of the reaction time on the formation of ZrC nanoparticles was examined. The as-synthesized samples were characterized using X-ray diffraction (XRD), helium pycnometry, Brunauer-Emmett-Teller (BET) N2 adsorption, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The lattice constant and interplanar distance (d111) were calculated, and the crystallite size was obtained using the Scherrer formula. Results from the experiment show highly crystalline ZrC, with crystallites less than 50 nm and nearly-spherical morphology, using 3 kW of power and only 50 min of reaction time, with a maximum reaction temperature of 1400 °C, lower than those involved using the conventional heating carbothermal reduction. The reaction time was ∼2 h shorter than that required by using a combined sol-gel and microwave-assisted carbothermal reduction method, used to prepare submicron ZrC powders (200–300 nm). The high heating rate achieved on the synthesis was favored by the excellent microwave absorption of carbon black, and the uniform heating at molecular level induced by the microwaves on the samples.

Design of polydopamine-encapsulation multiporous MnO cross-linked with polyacrylic acid binder for superior lithium ion battery anode

Investigating independent active materials is inadequate to settle the technological challenges involved in the booming lithium ion battery industry. The further development of energy-storage properties with both high energy density and superior cycling lifespan are highly dependent on the overall system optimization, which highlights the study of the interface between different components in a system. Herein, a system-level strategy of engineering MnO-based anode build on the plentiful electrolytic manganese dioxide are developed via a microwave-assisted carbothermic reduction, combined with a succeeding in-situ aggregation of polydopamine (PD) onto MnO along with chemical binding with polyacrylic acid (PAA) binder. Fourier transform IR spectra, X-ray photoelectron spectroscopy spectra validate the formation of cross-linking reaction between the PD buffer and PAA binder. Benefited from the prominent structure stability which endowed by well-connected three-dimensional porous structure, as well as two steady interfaces between MnO and PD buffer and between PD buffer and PAA binder, the cross-linked MnO anode exhibit a reversible capacity of 687 mAh g−1 at 300 mA g−1 and high Coulombic efficiency of 99% over 500 cycles and impressive rate capability of 320 mAh g−1 at 3000 mA g−1.

Microwave assisted synthesis, characterization and electrochemical performance of LiVPO4F cathode material

High-voltage cathode material, lithium vanadium fluorophosphate LiVPO4F, has been for the first time synthesized by a rapid two-step method based on microwave-assisted carbothermal reduction (CTR) and characterized with the use of scanning electron microscopy (SEM), X-ray diffraction and DTA/TG thermal analysis. Its electrochemical performance has been examined by galvanostatic charge-discharge cycling. Calculated lattice parameters of the obtained LiVPO4F compound were in accordance with literature. Its thermal stability up to 550 °C was confirmed. Charge-discharge cycling of LiVPO4F in the voltage range between 3.0 V and 4.4 V at a current rate of 7.75 mA⋅g−1 (C/20) after 5 cycles showed a charge capacity of 90 mA⋅h⋅g−1 and a discharge capacity of 58 mA⋅h⋅g−1. For a current rate of 15.5 mA⋅g−1 (C/10) after 10 cycles charge capacity achieved the value of 60 mA⋅h⋅g−1 whereas the discharge capacity was of 52 mA⋅h⋅g−1. The discharge plateau was at 4.18 V. The presented results suggest that LiVPO4F cathode material can be synthesized by the simplified two-step microwave-assisted reduction method. Although it needs to be further optimized, the method is simpler, faster and cheaper as compared to other preparation routes.

Preparation and characterisation of TiN by microwave-assisted carbothermic reduction–nitridation in air atmosphere

ABSTRACTTitanium nitride (TiN) is prepared from ilmenite (FeTiO3) powders by microwave-assisted carbothermic reduction–nitridation in air atmosphere followed by acid leaching treatment. The thermal analysis of the reduction–nitridation of FeTiO3 powders is conducted by thermogravimetry/differential scanning calorimeter. The phase transition sequence of microwave carbothermal synthesis of TiN–Fe composite is: FeTiO3 → Fe + TiO2 → Fe + TiN. There is no any other oxygen-deficient titanium oxides detected. Particularly, FeTiO3 can be transformed into TiN–Fe composite completely at 900°C for 60 min by microwave heating. The increase in both the reaction temperature and dwelling time is in favour of the transformation of FeTiO3. The product morphologies are spherical about 2–5 μm in size. Then TiN is obtained with the removal of Fe and their oxides from TiN–Fe composite by acid leaching treatment. This method not only reduces the reaction temperature significantly through microwave heating but also can simplify the operation process effectively.

Fast synthesis of porous chromium carbide by microwave-assisted carbothermal reduction

This work reports the preliminary synthesis of porous chromium carbide by microwave-assisted carbothermal reduction of simple reactants, Cr2O3 and carbon black, in only 20 min of reaction time. XRD revealed well-crystalized chromium carbides, depending the reaction parameters as forward power and time. The particles morphology is a hollow sphere-like morphology, with porous walls, confirmed by SEM, with potential application as microfiltration and thermal barrier coating. The neck size of walls is a time-temperature dependent.

Preparation and artificial neural networks analysis of ultrafine β-Sialon powders by microwave-assisted carbothermal reduction nitridation of sol–gel derived powder precursors

β-Sialon powders were synthesized by microwave-assisted carbothermal reduction of powder materials resultant from a sol–gel process using Si(OC2H5)4, Al(NO3)3·9H2O and sucrose (C12H22O11) as the main starting materials and artificial neural networks (ANNs) was applied to model and predict relative contents of β-Sialon in the final product samples. β-Sialon was formed at as low as 1250°C by using the technique developed with the present work. Furthermore, addition of Fe2O3 promoted the β-Sialon formation. As-prepared β-Sialon ultrafine powders were granular with primary size of about 69nm. A back propagation (BP) ANNs was used to establish a model to predict the reaction extents (relative contents of β-Sialon) under various processing conditions. The results indicated that the BP ANNs could be effectively used to establish the nonlinear relationships between the relative contents of β-Sialon and the processing conditions.

Synthesis of ultrafine (Ti,W,Nb)C solid solution powders by microwave-assisted carbothermal reduction

(Ti,W,Nb)C solid solution was obtained via microwave-assisted carbothermal reduction (MWCR) which was more cost-effective and timesaving than conventional carbothermal reduction (CR). Oxide–graphite mixtures were used as starting materials in the procedures. The phase evolution during MWCR has been discussed and compared with conventional CR in terms of the X-ray diffractometry. Microwave irradiation accelerated the formation of individual carbides (WC and NbC) as well as the formation of the final complete solid solution. The monolithic (Ti,W,Nb)C solid solution was synthesized at 1250°C for 30min. The agglomerates sizes of the final product were in the range of 100–350nm.

Synthesis of TiN from FeTiO3 by microwave-assisted carbothermic reduction–nitridation

Abstract Titanium nitride (TiN) was synthesized from ilmenite (FeTiO 3 ) through the microwave-assisted carbothermic reduction–nitridation followed by acid leaching treatment. The predominance area diagrams of Ti–Fe–N–C–O system at different temperatures were established by Outokumpu HSC Chemistry to analyze the stability of the species concerned. It is demonstrated that titanium and iron oxides can be transformed into TiN–Fe composite by controlling suitable CO and N 2 partial pressures at high temperatures. The TiN–Fe composite can be prepared from FeTiO 3 with microwave heating at 1000 °C for 40 min. TiN was obtained with the removal of iron from TiN–Fe composite by acid leaching at room temperature. The reaction mechanism for the carbothermic reduction–nitridation process was proposed according to experimental results.

Synthesis and electrochemical performance of La-doped Li3V2−x La x (PO4)3 cathode materials for lithium batteries

La-doped Li3V2−xLax(PO4)3 (x = 0.01, 0.02, and 0.03) cathode materials for lithium ion batteries were synthesized by the microwave-assisted carbothermal reduction method (MW-CTR). The structures and properties of the prepared samples were investigated by X-ray diffraction (XRD) and electrochemical measurements. The results showed that all the three Li3V2−xLax(PO4)3 samples had the same monoclinic structures and sharper diffraction peaks of the crystal plane compared with those of the undoped Li3V2(PO4)3. The initial charge/discharge specific capacity, coulomb efficiency, and discharge decay rate of all the three Li3V2−xLax(PO4)3 samples were superior to those of the undoped Li3V2(PO4)3 sample, and the Li3V1.98La0.02(PO4)3 sample exhibited the best features among the three La-doped Li3V2−xLax(PO4)3 samples. Electrochemical impedance spectroscopy (EIS) demonstrated that the Li3V1.98La0.02(PO4)3 sample had a lower charge transfer resistance and a higher Li ion diffusion coefficient compared with the undoped Li3V2 (PO4)3 sample.

Síntese de Al2O3/SiC em forno de microondas: estudo de parâmetros do processo

Estudos demonstram a eficiência do método de moagem prévia do aluminossilicato precursor para a síntese da fase Al2O3/SiC por meio da reação de redução carbotérmica em forno de microondas. No presente trabalho, além da moagem do precursor, outros parâmetros de reação foram estudados, como tempo de reação, potência da radiação emitida e fluxo de gás. As reações foram realizadas em forno de microondas semi-industrial, com adaptação para inserção de gás inerte. Dois tipos de reatores foram avaliados: um reator cilíndrico, termicamente isolado, e um reator tubular de leito fixo, nos quais foram colocados os precursores peletizados. Existe uma relação direta entre a saturação da atmosfera de reação com a cinética de redução carbotérmica do aluminossilicato. Esse comportamento, aliado a elevadas potências de emissão, favorecem a formação da fase Al2O3/SiC em períodos de tempo reduzidos.