The synthesis and properties of high alumina castable containing nanostructured calcium aluminate coated graphite were studied in terms of slag resistance and overall physical characteristics. Raman spectroscopy, BET surface area and field emission scanning electron microscopy (FESEM) were performed to exclusively understand the coating characteristics and its compatibility in refractory composite. The coating not only secured graphite in castable for prolonged period but also noticeably improved matrix to aggregate contact. The microstructural aspects of castables were investigated, with special emphasis on a representative matrix prepared and infiltrated with slag at elevated temperature. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of fired composite containing surface-treated graphite was quite prospective. It circumvented the problems of incorporating as-received graphite in castables and should be in the attention of refractory researchers and producers. © 2013 Elsevier Ltd. All rights reserved.

SUNANDA MUKHOPADHYAY

Chemical Technology

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

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It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

Materials Research Bulletin

The present contribution focused mainly on the fundamental understanding of the influence of surface-modified graphite on rheology of the matrix of a castable slurry in comparison to the same containing the uncoated one. A special observation on the rheology of a graphite-free matrix of similar high alumina castable has been described. The change in rheological behavior due to the presence of other microfine constituents has also been noticed with a particular attention to the refractory cement bond. The variation in viscosity and shear stresses with shear rate of castable matrices containing as-received and surface-modified graphites has been compared. The ‘ball-in-hand’ consistency test of respective castables had also been differentiated. The effect of coated and uncoated graphites on the loss and storage moduli of the respective suspensions has been explored. Zeta potential values of individual constituents as well as the slurries have also been measured and correlated with the flow characteristics of castable matrices. The X-ray diffraction (XRD), SEM (scanning electron microscope) and EDS (energy dispersive spectroscopy) studies of the coated graphite have been observed to substantiate the flow pattern of the slurry. The physical properties of refractory castables at different temperatures have been estimated to supplement the rheological pattern. © 2019 Elsevier Ltd and Techna Group S.r.l.

Ceramics International

The influence of surface-modified graphites on the rheological pattern of alumina-carbon cementitious refractory castable matrix

This investigation entails our continued research on calcium aluminate coated graphite for application in alumina-carbon monolithic refractories. Surface treatment of graphite flakes by a scalable sol-gel route has been utilized to develop stable bonds between ceramic phases with the functionalized graphite sheets. This study has been primarily conducted to differentiate between the coating evolution pattern at green and calcined conditions. In this regard, C-H-N analysis, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), x-ray diffraction (XRD) with Rietveld analysis, Fourier transformed infrared spectroscopy (FTIR) and zeta potential studies of three kinds of graphites had been compared and critically estimated. These results were supplemented with thermogravimetry (TG), pore size distribution, oxidation resistance and scanning electron microscopy (SEM) of selected samples. A schematic outline of the work has also been proposed. An appreciable improvement of bulk density, apparent porosity and crushing strength of the respective castable has been correlated with the compatibility of the coating to the reactive matrix constituents of the refractory in wide temperature region (110-1500°C). © 2017 The American Ceramic Society

International Journal of Applied Ceramic Technology

Extended studies on surface-treated graphite vis-Ã -vis its application in high alumina refractory castable

The performance of alumina- and magnesia-based carbon-containing refractory castables, suitable for different sectors of steel plants, has been compared. Natural flaky graphites have been modified by their surface treatment via sol-gel route. The sustainability of graphite in oxidizing atmosphere was noticeably increased by this method with a synergistic possibility of pollution abatement. The hydrophilic nanocoatings of ceramic oxides derived by inorganic precursors reduced the water demand of the composite batch and simultaneously enhanced its oxidation resistance. The oxide coating compositions together with the precursor chemistry have been varied to substantiate their common beneficial features. Scanning electron microscopy (SEM), Raman spectroscopy, Differential thermal analysis (DTA), Thermogravimetry (TG), X-ray diffraction, thermal conductivity, and zeta-potential studies have been carried out to compare the influence of heat treatment on as-received and surface-modified graphites. The properties of refractory castables, namely, bulk density, apparent porosity, and cold crushing strength have been measured within wide temperature region (110–1500°C), to confirm the positive influence of nanoengineering on graphite surface. The sequence of important phase reactions during sintering of both the composite materials has been discussed. Finally the slag resistance behavior at aggressively fired condition has been investigated to corroborate the improved matrix characteristics due to coated graphites. © 2017 The American Ceramic Society

Effect of surface-treated graphites on the properties of carbon-containing monolithic refractory materialsâ€”A comprehensive comparison

The thermomechanical analysis (TMA) and thermal characteristics of carbon-containing refractory castable matrix with 20.0 wt% graphite have been compared with graphite-free high alumina based similar castable matrix. Graphite has been added both in uncoated and coated forms, the latter having a thin sol-gel calcium aluminate coating on as-received graphite flakes. The influence of systematic variation of microfine constituents e.g. reactive alumina, high alumina cement etc in the matrix has been investigated in terms of the heat flow studies obtained by differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). The densification behavior of graphite-free and graphite-containing castable matrices has been critically estimated by the dimensional changes in dynamic heating regime up to 1500 °C. The role of coated graphite on improved densification behavior of refractory was explored by microstructure and phase assemblage studies of the respective fired castable. It was further corroborated by the transmission electron microscope (TEM) studies of surface-modified graphites. © 2016 Elsevier Ltd and Techna Group S.r.l.

Thermal and thermomechanical characteristics of monolithic refractory composite matrix containing surface-modified graphite

Some newer insights had been reported in this paper on synthesis and application of calcium aluminate coated graphite in alumina–carbon refractory castables. The course of evolution of AlO4 (tetrahedral) and AlO6 (octahedral) units in the nanocoating was investigated by XRD (X-ray diffraction), Raman spectroscopy and FTIR (Fourier transformed infrared spectroscopy) studies. An extended research on the resemblance and disparity between sol–gel and conventional refractory cements were also explored. FESEM (Field Emission Scanning Electron Microscopy), XRD of coated and uncoated graphites had been revisited with a special study on ESEM (environmental scanning electron microscopy) of those two. The hydrophilic thin film containing Ca-doped γ-Al2O3 phases rendered a decisive influence on graphite retention in castable matrix. A comparison between pore size distribution and thermogravimetric analysis (TGA) of two fired castable refractories advocated the superiority of the one containing coated graphite. Determination of corrosion and penetration indices by the static slag corrosion test, supplemented with microstructural studies, confirmed that the less porous castable containing coated graphites excelled again in the harsh environment. © 2015 Elsevier Ltd and Techna Group S.r.l.

In depth studies on cementitious nanocoatings on graphite for its contribution in corrosion resistance of alumina based refractory composite

This paper entails an extended investigation on sol-gel thin film of calcium aluminate (CaAl 2O 4) over graphite flakes that improved their oxidation resistance and water wettability. The commercial preparation of calcium aluminate has been compared with the sol-gel synthesis by differential thermal analysis (DTA) and X-ray diffraction (XRD) to assess the feasibility of the latter for coating preparation. Poorly crystalline nanostructured Ca-doped γ-Al 2O 3 is considered to be an important intermediate for this preparation. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of the calcined gel have been carried out to ascertain its composition. Quantitative chemical analysis of sol gel derived calcium aluminate was also estimated. Atomic force microscopy (AFM) has been conducted to ensure the evolution of hydrophilic nanosized cementitious phases on graphite. Zeta potential values of coated and uncoated graphites with increasing pH have also been determined to distinguish between their compatibility in a refractory castable matrix. Improved physical properties of that high alumina castable containing coated graphite, e.g. apparent porosity (AP), bulk density (BD), cold crushing strength (CCS) have been measured to evaluate the refractory quality. The reasons for its better performance are explored by taking further insight on the microstructural analyses of the fired castable (1500°C) soaked for an extended period. © 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Comparison of solid state and sol-gel derived calcium aluminate coated graphite and characterization of prepared refractory composite

The performance of high alumina castables containing mullite and spinel coated graphites was investigated to explore their commercial application in carbon containing refractory composites. It was compared in terms of thermal shock resistance, refractoriness under load, static slag resistance test, pore size distribution, and physical properties, e.g., apparent porosity and cold crushing strength. The microstructures of these two castables, along with the other containing uncoated graphites, were also studied. Differential thermal analysis and X-ray diffraction patterns of heat-treated carbon-containing castable composites have also been noted. Castables containing mullite-coated graphite were found to be more resistant to repeated thermal shock cycles, whereas those containing spinel-coated graphites were more encouraging when slag resistance and overall physical properties are concerned. © 2012 Copyright Taylor and Francis Group, LLC.

Materials and Manufacturing Processes

Characteristics of refractory castables containing mullite and spinel coated graphites

Mullite and spinel forming sols were prepared from hybrid precursors having both organic and inorganic origins. Refractory grade graphite flakes were coated by these sols and their performances were compared in terms of oxidation resistance and water-wettability. Particle size distribution, structural evolution and related characteristics of both mullite and spinel gels have been investigated by dynamic light scattering (DLS), differential thermal analysis (DTA), X-ray diffraction (XRD) and infrared spectra (IR) studies. Coated graphites have also been studied by IR and XRD tests along with scanning electron microscopy (SEM) with energy dispersive spectral (EDS) analysis. The better performance of spinel coated graphite was confirmed. The mechanism of spinel formation on graphite was suggested to take place via intermediate gamma alumina phase formation from boehmite sol. It was clarified by DLS, XRD and microstructural analysis of dried and calcined gels. © 2010 Elsevier Ltd and Techna Group S.r.l.

Development of mullite and spinel coatings on graphite for improved water-wettability and oxidation resistance

An intimate mixture of common hydrated magnesium salt and cheaper boehmite sol had been used as a cost-effective precursor of reactive nanometric spinel (1:1) powder. It was gelled at controlled pH, temperature and time, and calcined at significantly lower temperature after soft mechanochemical treatment. X-ray diffraction (XRD), infrared spectra, differential thermal analysis (with thermogravimetry), proton-nuclear magnetic resonance (NMR) and transmission electron microscope studies were carried out to investigate the incipient formation of low crystalline nanosized domains in the still-hydrated spinel. Spinel nanoparticles are protected by a thin chemisorbed layer of (OH) groups held around the active surface of the aggregates. Selected properties of conventional Al2O3-spinel and Al2O3{single bond}MgO castables were compared with the same formulated by the chemical-route spinel fines. Cold and hot strengths of the refractory castable containing commercial spinel with excess alumina powder were the best, while the same bonded by reactive magnesia and coprecipitated spinel fines were not satisfactory. Performance of sol-gel spinel is comparable to the preformed spinel with 78% alumina. © 2008 Elsevier Ltd and Techna Group S.r.l.

Structural analysis on spinel (MgAl 2 O 4 ) for application in spinel-bonded castables

Oxidation resistance and water wettability of graphite flakes have been improved by a thin sol-gel film of magnesium aluminate spinel (MgAl 2O4) over its surface. The hydrosol has been synthesized by less expensive precursors and the spinel formation has been studied by scanning electron microscopy (SEM), supplemented with energy dispersive spectral analysis. After an easy-to-use mixing procedure, drying (110°C), and subsequent calcination (550°C), coated graphites were sieved to below 75 μm. The coating over the powder contained 1.5 wt% MgAl2O 4, which enormously increased the oxidation resistance (performed at 600°, 900°, and 1200°C) and water wettability, as revealed by hydrophilic functional groups from infrared spectra. Defective, intermediate spinel structure of fine, lamellar Mg-doped γ-Al2O3 has been considered to be significant for this improvement. An approximate (1:2) stoichiometry of (Mg:Al) in the coting composition was confirmed by an X-ray photoelectron spectroscopy test. Castables prepared by this graphite remarkably improved their bulk density and apparent porosity compared with those prepared by the as-received graphite. Casting water was reduced along with the amount of antioxidants. This also enhanced the resistance toward the basic slag by retaining the graphite in the refractory. © 2009 The American Ceramic Society.

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ISSN	0025-5408
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