Ind. Eng. Chem. Res. 2008, 47, 9913–9918
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Solid-State NMR Characterization of Silicon Nitride Bonded Silicon Carbide Refractories Zoran D. Zujovic,* Ronny Etzion, and James B. Metson Light Metals Research Centre, Department of Chemistry, UniVersity of Auckland, PriVate Bag 92019, Auckland, New Zealand
Solid-state 29Si NMR spectroscopy and X-ray diffraction have been used to analyze commercial samples of silicon nitride bonded silicon carbide (SNBSC) refractories. Spectra of samples before and after exposure to aluminum electrolysis conditions from the peripheral and core parts of as manufactured SNBSC bricks from two different refractory producers are presented. The quantitative analysis of SiC/Si3N4 and R-Si3N4/β-Si3N4 using NMR is complicated due to the fact that SNBSC can contain paramagnetic impurities. Thus, from the NMR data, these ratios can only be tentatively estimated. It is shown that in order to achieve a comprehensive understanding of the microstructure of SNBSC materials, a combination of NMR and X-ray diffraction methods should be applied. Introduction Aluminum Industry. In modern high amperage aluminum reduction cells, the service life of the cell and the energy efficiency of operation are strongly influenced by the performance of the sidewall refractory lining. The use of Si3N4 bonded SiC (SNBSC) refractories has become the state of the art for sidewalls, largely replacing the traditional carbon materials. The primary task of the sidewall is the containment of the molten salt cryolite (Na3AlF6) based electrolyte, and liquid aluminum. During cell operation the transmission of heat through the sidewall maintains a layer of frozen molten salt electrolyte (ledge), protecting the sidewall from exposure to high temperature (960-1000 °C), corrosive liquid, and vapor attack, which can lead to chemical degradation and corrosion. Several operational circumstances can lead to cell instabilities and temperature raising causing the frozen ledge to melt and exposing the sidewall lining to the molten electrolyte. In addition the refractory located above the level of the molten electrolyte is subjected to attack by highly corrosive vapors, such as HF released from the cell. Thus there is considerable interest in the resistance of these materials to such corrosive attack.1-3 Silicon nitride bonded silicon carbide refractories are formed by mixing SiC grains with silicon powder and an organic binder. This mixture is sintered and shaped into bricks using vibration and pressing. The green bricks are fired in a furnace in an air atmosphere at low temperature (up to 300 °C) and then in a nitrogen atmosphere at temperatures up to and sometimes in excess of 1400 °C. During this stage the nitridation process converts the Si to Si3N4 which becomes the binder phase, encapsulating the SiC grains. The final brick typically consists of 72-80 wt % SiC and 20-28 wt % Si3N4.1 High temperature is needed to initiate the nitridation reaction but the exothermic nature of the reaction can lead to thermal “run-away” within the brick, with temperatures above 1400 °C resulting in formation of β-Si3N4 crystals.4 Thus the refractories are typically zoned with outer and inter regions which differ in the distribution of porosity, the R to β-Si3N4 ratio and in the observed reactivity.1-4 * To whom correspondence should be addressed. Tel: +64 9 373 7599 ext. 85876. Fax: +64 9 3737422. E-mail: z.zujovic@ auckland.ac.nz.
Temperature control plays a crucial role in the exothermic nitridation in terms of reactivity of Si and selection of the dominant Si3N4 phase, with the formation of β-Si3N4 favored by higher temperatures. Temperatures above 1400 °C can also lead to formation of liquid Si during the fabrication of the block. This silicon liquid phase is then difficult to convert to the Si3N4 bond phase due to low surface area and thus the slow kinetics of this reaction. Thus the manufacture of these materials presents challenges, and the performance of the refractories can be somewhat unpredictable.2,3 Therefore it is highly desirable to have better methods to determine the composition and chemistry of these materials in order to better understand performance. Anecdotal evidence from post mortem cell autopsies shows strong oxidation and degradation of the refractory in the sidewall above electrolyte level.3 Refractories initially show good cryolite resistance at the hot face and in the gas zone above the electrolyte, but then show accelerated failure once the exterior zone is breached. A further observation from the literature throws some light on the importance of the SiC/binder phase interface in the degradation process. Selective removal or alteration of the bond phase was observed-particularly the formation of silicates, at the interface between the matrix and the SiC grains.3 This selective attack of the bond phase, especially along the SiC/matrix boundary, leads to grain release and is likely responsible for the “crumbly” nature of the used refractory. A degradation mechanism suggested in the literature involves formation of a protective oxide layer that is subsequently attacked by NaAlF4, HF, and CO2 in the gas zone,3 or a mechanism involving oxidation of the SiC grain surface by moisture to produce silicate products, that were further attacked by HF to produce Si-fluoride vapors.1 Silicon Nitride. The two crystalline forms of silicon nitride are classified as R and β. The R-phase is converted to β during liquid-phase sintering. The common characteristic of both structures is that the silicon atom is tetrahedrally bounded to four nitrogen atoms. Both structures consist of interleaved sheets of 8- and 12-membered rings of silicon and nitrogen. In β-Si3N4, the SiN4 tetrahedra are oriented in such a way that the edges are in a straight line parallel to the unit cell c axis. There is only one unique Si site and there is only one peak in the 29Si NMR spectrum of β-Si3N4 centered at about -48.7 ppm.5 In R-Si3N4 there are two stacking sequences which are distorted
10.1021/ie800759c CCC: $40.75 2008 American Chemical Society Published on Web 11/13/2008
9914 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
with respect to one another by about 60-70° around the c axis. Consequently there are two nonequivalent Si sites and the 29Si spectrum of R-Si3N4 consists of two peaks centered at around -46.8 and -48.9 ppm.5 Silicon Carbide. Silicon carbide (SiC) exists in many crystalline forms. These forms (polytypes) are based on a cubic structure classified as β, or a variety of hexagonal structures collectively called R. Typically, a single resonance is observed in the NMR spectrum of β-SiC, while up to three closely spaced resonances characterize the R-material.6-8 Silicon carbide impurities are quite varied, and some have been extensively studied. On the basis of the fact that oxygen is a common contaminant of industrial silicon carbide, tentative peak assignments of the impurities can be made.8 Oxide contamination, sometimes formulated as SixCyOz indicates the close association of oxygen with the SiC lattice and is a well-known problem in the compaction of silicon carbide powders. Oxidation to SiO2 can occur creating a film on silicon carbide surfaces. Amorphous silicon carbide can contain considerable amounts of SiO2 and carbon as well as its principal component, microcrystalline β-SiC.8 Effect of Material Properties on Chemical Resistance. Oxide Content. SNBSC samples with additives of oxide phases, like alumina and/or silica, show more corrosion than materials without such additives. Although the density improves, corrosion resistance declines. Tests with SiAlON bonded SiC have shown accelerated corrosion in fluoride electrolytes compared to Si3N4 bonded SiC.3 Content of Free Si. A high level of silicon can influence the strength and generally indicates insufficient temperature control in the nitration process.3 In this case a premature local melting of the silicon powder can led to the formation of glass as an amorphous phase which could reduce the corrosion resistance of these materials. Elemental silicon is readily detected in the NMR spectrum as it absorbs at -80.6 ppm, while SiO2 absorbs at -108.1 to -119.4 ppm.9,10 Amount of Binder Phase, Si3N4. Thermodynamically, silicon nitride is less stable than the carbide phase, so it is anticipated that the binder will be attacked more easily than SiC. Literature reports samples with 26.6% binder showing more corrosion than samples with 22.5% binder.3 Amount of Si2ON2. Relatively low contents of Si2ON2 (
