Nanoscale calcium aluminate coated graphite for improved performance of alumina based monolithic refractory composite

https://doi.org/10.1016/j.materresbull.2013.03.006Get rights and content

Highlights

  • Sol–gel Ca-doped γ-Al2O3 accomplished graphite retention.

  • Nanocoating considerably improved matrix-aggregate bonding.

  • Less porous simulated matrix upgraded slag resistance.

Abstract

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.

Introduction

Several efforts are being taken to accommodate graphite/carbon in refractory castables for construction of furnaces employed in iron and steel industries. The reason is its desirable characteristics, especially high thermal shock resistance and low wettability towards molten metal and slag [1], [2], [3]. However its application is restricted due to poor oxidation resistance and low tendency to be mixed with water. A thin hydrophilic coating of oxide ceramics on graphite surface by sol–gel method can be an elegant approach to incorporate it in monolithic refractory composites [4], [5], [6], [7], [8], [9], [10], [11]. In our last communications, controlled sol–gel preparation of cost-effective calcium aluminate coatings on graphite at 600 °C had been reported [12]. It built up a nanodimensional selective coating that improved the oxidation resistance and hydrophilicity of the graphite underneath. A detail study on sol–gel calcium aluminate synthesis and the typical characteristics of coated graphite were also explained in those two papers. Some preliminary investigations on castable formulation and its physical properties had been touched upon too. However such surface-treated graphites can only be admissible to refractory researchers and users if an in depth refractory characteristics are studied on such castables. Such kind of literature is practically very little in number, although alumina–graphite and magnesia–graphite refractories are largely utilized in various important sectors of iron and steel plants.

In our previous work [13] the influence of mullite and spinel coated graphites on refractory composites had been discussed. However calcium aluminate finds a more widespread and versatile applications in several kinds of unshaped refractories categorized as conventional, low cement, ultra low cement and self flow castables. As such the industrial relevance of calcium aluminate coatings on graphite must be more significant and appreciated by the refractory researchers as well as consumers and producers. We report in this conclusive part the refractory fabrication in detail with its selective properties, e.g. slag corrosion resistance of alumina based castables containing these graphites. An extended and systematic research on Raman spectroscopy and microstructural analysis of coated graphites and fired castables had also been undertaken to ratify its application. In this regard, the interaction of simulated matrix of castable with aggressive slags is also documented. The materials chemistry concerned with the coating and significant high temperature performance of the refractory composites have been equally addressed in this paper.

Section snippets

Experimental

The wet chemical processing of oxide ceramic materials, especially by sol–gel method is very much practiced due to several advantages [14]. The sol–gel route adopted here included a hybrid set of precursors, namely aluminium-sec-butoxide and hydrous calcium nitrate. Propanol, acetic and nitric acids with acetylacetone were the other auxiliary chemicals for sol casting that involved less expensive instruments [12], [15]. Supplied graphite (97% carbon) was thoroughly mixed with the sol, dried,

Results and discussion

Raman spectroscopy has always been accepted as an important tool for characterizing carbon products, particularly graphite and graphene materials. Raman scattering of uncoated and coated graphites has been compared [Fig. 1(a and b)], to observe the changes in G and D bands, i.e. E2g and A1g modes respectively [19], [20], [21]. The higher the relative intensity ratio value (R = ID/IG ratio), the more disordered is the carbon [22]. Here this value was calculated to be 0.144 and 0.091 respectively

Conclusions

This work is the most important part of our laboratory scale investigation on graphite coated by nanosized calcium aluminates, from which the following inferences may be drawn:

  • (i)

    The Raman spectroscopy indicated the occasional exfoliation and graphite oxide formation and FESEM assured the stability of that coated graphite in the harsh environments of refractory production and application. Coating microstructure at nanoscale with immense surface area helped to considerably upgrade the

Acknowledgements

I thankfully acknowledge my students S. Dutta, S. Ghosh, A. Chakraborty and C. Mondal for their help rendered during the course of this work.

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