Extended Arc Thermal Plasma Research Articles

Co-plasma processing of banana peduncle with phosphogypsum waste for production of lesser toxic potassium–sulfur rich biochar

Production of macro-nutrient rich biochar is important to broaden its use as soil fertilizer. In this work, we report production of potassium–sulfur rich biochar through co-plasma processing of banana peduncle biomass with phosphogypsum waste. Biochars were produced using indigenous low-power (15 kW) extended arc thermal plasma reactor in 7 min under three different plasmagen gases i.e., argon, oxygen, and ammonia. Plasmagen gases showed differential and significant effect on potassium, sulfur and toxic element contents of biochar. Biochars showed relatively higher potassium (4.2–12.7%) and sulfur (13.3–17.8%) contents than phosphogypsum (potassium − 0.02% and sulfur − 12.5%). In addition, leachable fraction of fluoride and heavy metals decreases in biochars. Among plasmagen gases, retention of potassium and sulfur content was relatively higher in argon, whereas fluoride and heavy-metal leaching reduced maximum in ammonia. X-ray diffraction analysis showed the presence of potassium and sulfur as K2SO4 and CaS minerals in biochars. These findings highlights about application of co-plasma processing of nutrient-rich biomass with phosphogypsum waste for production of lesser toxic nutrient-rich biochar.

Synthesis of TiB2 by extended arc thermal plasma

Abstract Synthesis of titanium diboride powder has been systematically investigated by reacting a mixture of TiO 2 , B 2 O 3 and activated charcoal powder in an extended arc thermal plasma reactor using graphite electrodes in flowing Ar atmosphere. The influence of carbon content on the formation of TiB 2 was studied. The product obtained after reaction was subjected for compositional as well as microstructural analysis using XRD, FESEM, Raman and FTIR. From the XRD results, it was observed that TiB 2 formation was observed along with TiC indicating notably influence of carbon content and reaction time. The results also confirm the synthesis of TiB 2 is based on a TiC formation mechanism. It was also observed that increasing carbon content and reaction time had an obvious effect on the disappearance of unwanted phases. With increase in carbon content to 10 wt% excess and the reaction time for 20 min, the unwanted phases were completely removed and consequently a single-phase titanium diboride powder with crystallite size of about 78 nm was formed.

Studies on Carbothermal Reduction Nitridation of Alumina in Extended Arc Thermal Plasma

In this paper, carbothermal reduction and nitridation of alumina in an extended arc thermal plasma reactor operating in transferred arc mode is reported. The formation of hexagonal AlN (average particle size ∼5 to 7 µm) has been observed in 5 min of processing time. The reaction is completed around 25 min of processing time. The reaction mechanism at high temperature has been predicted based on the chemical equilibrium diagram. The phase analysis, morphology, and optical studies of plasma synthesized AlN powder are carried out by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and Raman spectroscopy, respectively. The compositional analysis is carried out by electron probe micro-analyzer (EPMA) and energy dispersive X-ray analysis (EDAX).

Production of pig iron from red mud waste fines using thermal plasma technology

Red mud, an insoluble residue produced during alkali leaching of bauxite, is considered as a low-grade iron ore containing 30% to 50% iron. The present paper deals with the use of thermal plasma technology for producing pig iron from red mud waste fines. The smelting reduction of red mud was carried out in a 35 kW DC extended arc thermal plasma reactor. Red mud was properly mixed with fluxes and graphite (fixed carbon, 99%) as a reductant as per stoichiometric requirement. The effect of various process parameters like a reductant, fluxes and smelting time on iron recovery was studied and optimized. An optimum condition for the maximum recovery of iron was obtained. A new thermal plasma process applicable to direct iron making from red mud waste fines that would achieve significant utilization of red mud was proposed.

Correlation between properties of zirconia mullite and Y2O3 stabilised zirconia mullite derived by various processing techniques

Zirconia mullite (MUZ) and Y2O3–MUZ composites were synthesised by plasma melting, plasma sintering and conventional sintering techniques. Mixtures of tetragonal and cubic phases have been observed in all the processed Y2O3–MUZ composites. This observation infers that though the available temperature in plasma sintered and conventionally sintered MUZ specimens is not sufficient for the transformation of monoclinic to cubic zirconia, the addition of Y2O3 favours this transformation and also stabilises the cubic zirconia phase at room temperature. The optical absorption studies completely reflect the improvement in optical band gap of Y2O3–MUZ composites to prove it as a suitable candidate for refractory applications. The bulk density, hardness and higher crystalline nature of plasma sintered composites indicate that better quality of products can be synthesised by direct current extended arc thermal plasma heating technique within a short duration of time.

Structural and Microstructural Properties of Fused ZrO2-Mullites

Silimanite and zircon rich sea beach sand is taken with alumina powder for the synthesis of fused zirconia mullite (MUZ) composites by extended arc thermal plasma reactor. The bulk density value of fused MUZ (∼3.79 g/cm3) is well comparable to the density value of commercial available MUZ (∼3.6 gm/cm3). The electron probe microstructure analysis predicts that alumina is rich in mullite grains, whereas the grain boundary region is rich with zirconia. A small percentage of zirconia is also present in the intragrain region, which reveals that mullite compound is surrounded by well-structured and compacted zirconia. The X-ray photoelectron spectrum reveals the presence of mullite phase along with crystalline Si in the composites.

Advanced WC–Co cermet composites with reinforcement of TiCN prepared by extended thermal plasma route

The synthesis of titanium carbonitride (TiCN) powders by thermal plasma using extended arc thermal plasma reactor and the effect of TiCN reinforcement for the development of advanced WC–Co cermets has been studied with respect to hardness and fracture toughness. These classes of materials are being investigated for future application in wear-resistant seals, cutting tools, etc. Metallurgical reactions and microstructural developments during sintering of cermets and functionally graded cemented carbonitrides are being investigated by analytical methods such as differential thermal analysis/thermo-gravimetric analysis, X-ray diffraction and analytical Scanning electron microscopy with energy dispersive X-ray spectroscopy. By an in-depth understanding of the complex phase reactions and the mechanisms that govern the sintering process and metallurgical reactions, new cermets and different types of functionally graded cemented carbonitrides with desired microstructures and properties have been attempted to develop. The significant improvement of micro-hardness was observed with optimal concentration of TiCN reinforcement addition in WC–Co system without sacrificing much fracture toughness value of the composite cermets.

Micro Raman Spectroscopy and Electron Probe Microanalysis of Graphite Spherulites and Flakes in Cast Iron

In this investigation, the evolution and formation of graphitized microstructure of cast iron has been studied by using micro Raman spectroscopy and electron probe microanalysis (EPMA). The samples were prepared by carbothermic reduction of iron ore powder by plasma smelting in an extended arc thermal plasma reactor. Magnesium was added in the ladle to spherodize the graphite. Elemental mapping of the sample across the spherulites and flakes was performed by EPMA. Raman scattering data were collected at different positions across graphite spherulites and flakes. Micro Raman analysis shows graphite peaks at 1350 cm−1 (D), 1580 cm−1 (G), and higher order graphite peaks for both spherulites and flakes. Additional peaks between 200 and 800 cm−1 are found to be present only in the case of spherulites. These extra peaks originate from cementite (Fe3C) present in and around the spherulites. It is inferred from the available experimental data that graphite spherulites are formed in areas richer in cementite.

Extended arc thermal plasma assisted sintering of Al-Zr oxide composites

An inexpensive and simple sintering technique using an extended arc thermal plasma assisted heating (EATPAH) source has been designed and fabricated for rapid sintering of Al x-Zr100- x ( X = 0, 10, 20,…, 100) oxide composites. High heat and mass transfer kinetics of thermal plasma assisted heating allows quick densification within a few minutes when compared with conventional heating source that takes tens of hours for identical densification. Sintering parameters such as sintering time, plasma power and plasmagen gas flowrate, and initial green density of composites are critically optimized. The green powder compacts sintered at a higher heating rate produced a sample with higher density and fine grain microstructure, whereas abnormal grain growth and lower density result at lower heating rate. This reveals that rapid sintering by EATPAH is effective for promoting the physical densification of the sintered end product. Hardness of the sintered products is studied to understand the density and shrinkage behaviour during plasma sintering. It is observed that highly dense sintered product with greater hardness and homogeneous phase distribution can be prepared within a few minutes by low cost EATPAH technique.

Studies on sintering behaviour of Al2O3–ZrO2 oxide composites processed by extended arc thermal plasma and conventional heating

Al2O3–ZrO2 composites were sintered using low cost extended arc thermal plasma reactor and conventional heating. Composites prepared in a wide range of composition were studied in terms of their density, shrinkage, hardness, structure, microstructure and dielectric response. Experimental parameter such as sintering time, sintering temperature and plasma power were optimized to achieve higher sintered end product. Highly dense sintered products were obtained by plasma heating route within short sintering time compared with conventional sintered method. Interesting development pertaining to structure and phase evolution, structure and dielectric response are analyzed. It is found that compositional variation in this composite produces structural phase separation at different sintering conditions, which is more in plasma heating product than conventional heated product. Plasma sintered product always shows less dielectric constant as compared to conventional sintered sample.

Low K dielectrics in sintered Al–Zr oxide composites processed by thermal plasma heating

A simple and low cost heating technique based on extended arc thermal plasma heating (EATPH) source has been developed to sinter high temperature Al x –Zr 100− x ( x=0, 10, 20, …, 100) oxides. Structure and morphology of sintered Al–Zr oxide samples were studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). Interesting result pertaining to the reduction of dielectric constant ( K) and dielectric loss (tan δ) were noted in thermal plasma sintered specimen as compared to the conventional sintered sample using resistive heating. It is observed that values of K are reduced up to 40% (maximum) w.r.t. conventional sintering. This low K behavior observed in thermal plasma sintered sample may be attributed to the incorporation of dynamic inclusion in different defect form during thermal plasma heating within a short sintering time. It is also further observed from the frequency and temperature variation of K and tan δ that surface charge polarization along with dipole screening play an important role at high frequencies and temperatures which can be explained by dipole pinning and depinning mechanisms.

Studies on dielectric properties of Al–Zr oxide composites sintered by thermal plasma

High-density sintered product of Al x –Zr 100− x ( x=0, 10, 20,…100) composites were produced using low-cost extended arc thermal Plasma heating (EATPH) source. Dielectric properties of these samples were studied as a function of different frequencies and at different temperatures. Particle sizes and density of the plasma-sintered products were controlled by controlling the plasma power, sintering time and plasmagen gas flow rate during sintering. Morphology and structure of sintered Al–Zr samples were examined by SEM and X-ray diffraction (XRD) to understand the plasma-sintered induced dielectric properties. It is noted that frequency- and temperature-dependent dielectric constant and loss factors are the consequence of polarization of dipole moment, which varied with the variation of plasma-sintered particle size. It is also observed that surface charge polarization along with dipole screening play crucial role at higher frequencies and at higher temperature. Dipole pinning and depinning mechanism are proposed to explain this complicated dielectric behavior.

Effect of thermal plasma heating on the dielectric properties of Al-Zr composites

A low-cost extended arc thermal plasma heating (EATPH) source has been used for the sintering of Al-Zr high-temperature ceramic oxides, and their dielectric properties have been studied. Pellets of Al-Zr composites were sintered using optimum sintering parameters such as sintering time, plasma power and plasmagen gas flow rate in an EATPH reactor. Samples of similar composites were also sintered using a conventional resistive heating furnace at 1500°C for 20 h, and their dielectric properties were studied and compared with the plasma sintered sample. Sintered pellets were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and dielectric measurement, carried out as a function of frequency and temperature. Highly dense sintered products were obtained by the plasma heating route within a very short sintering time compared with the conventional sintering schedule. Dielectric measurements (both dielectric constant and loss factor) showed a significant reduction in the dielectric constant, K, at room temperature under different test frequencies and also as a function of temperature. However, the change in dielectric loss factor, tan Δ, was not as pronounced as the change in K. The behaviour of the K and tan Δ curves changes drastically compared with the conventional sintered material. The reduction in K and the crossover behaviour of tan Δ in the plasma sintered specimen may be due to the action of non-reactive Ar plasmagen gas during plasma sintering, which favours mobile oxygen for the polarization network under the influence of frequency and temperature.

Sintering of Al–Zr based oxide ceramics using thermal plasma

Abstract A novel and low cost extended arc thermal plasma heating (EATPH) reactor [Roul et al., JMSP 6 (1) (1998)] has been used to sinter Al–Zr oxide ceramics within a few minutes in Ar atmosphere instead of a few tens of hours by conventional furnace heating. Critical experimental parameters such as plasma power (kW), plasmagen gas flow rate, sintering time and electrode spacing distance are optimized to achieve high density sintered materials. It is noted that sintering time is approximately reciprocal to plasma power. Higher plasma power with less sintering time can generate high-density homogeneous material without significant grain growth. XRD and SEM studies were carried out to characterize and evaluate the sintered materials. Surface morphology revealed uniform particle size distribution in long-range order, with no runway grain growth during this EATPH sintering process. This simple EATPH method provides an alternative and quick technique for tailoring high temperature ceramic materials over conventional sintering method due to fast heat and mass transfer kinetics inside thermal plasma reactor.

Production of High Aspect Ratio Silicon Carbide Fibres by Thermal Plasma Method

An attempt has been made to prepare β-SiC fibres by the reaction of SiO vapour with ‘C’ fibres at a temperature > 1600°C in an extended arc thermal plasma reactor. The ‘C’ fibres prepared by the thermal pyrolysis of cotton fibres are made to react with SiO vapour generated during the plasma heating of (SiO2 + Si) charge at a temperature > 1600°C. X-ray diffraction pattern confirms the formation of β-SiC fibres. The SEM studies reveal that the fibres thus synthesised have high aspect ratio with an average diameter of 10–12 μm and an average length of 250 μm.

Sintering of Calcia-Stabilized Zirconia an Extended Arc Thermal Plasma Reactor

Commercial-grade zirconia powder (average, 10-μm particle size) compacts with 5 wt% of calcia have been sintered (batchwise) in a short time using an indigenously made low-cost extended arc thermal plasma heating (EATPH) system with varying plasma reactor operating parameters such as plasma power, plasmagen gas flow rate, and sintering time. XRD, SEM, laser particle size analyzer, and density studies were carried out to evaluate the structural, morphological, and physical properties of the sintered samples. It is observed from the above studies that the sintered products are fully monophasic and stabilized to the cubic system with 92% of the theoretical density in 15 min. A uniform grain size with no runaway grain growth occurred during sintering by our EATPH technique. The pore-grain structure mechanism in relation to the grain growth and density of the materials is discussed.