Fourier transform infrared analysis of ceramic powders: Quantitative determination of alpha, beta, and amorphous phases
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F. E. Brinckman and R. A. Faltyneka) National Bureau of Standards, Ceramics Division, Gaithersburg, Maryland 20899 (Received 11 July 1988; accepted 21 October 1988) Fourier transform infrared spectroscopy (FT-IR) forms the basis for determining the morphological composition of mixtures containing alpha, beta, and amorphous phases of silicon nitride. The analytical technique, involving multiple linear regression treatment of Kubelka-Munk absorbance values from diffuse reflectance measurements, yields specific percent composition data for the amorphous phase as well as the crystalline phases in ternary mixtures of 0 - 1 % by weight Si3N4 in potassium bromide.
I. INTRODUCTION The physical properties of objects and films composed of silicon nitride are highly dependent on morphological composition and impurity levels in the ceramic, both before and after densification. For example, Luongo's correlation of hydride contaminants in plasma-deposited amorphous Si3N4 films with property failure noted in earlier reports,1"3 and the observation by Chu et al. that crystalline phases of Si3N4 embedded in protective ceramic coatings lead to adhesion loss and cracking,4 underscore the necessity for chemical quality control in Si3N4 formulations. Traditional Si 3 N 4 processing, yielding either reaction-bonded or hot-pressed products, relies on the monitoring of phase transitions as the amorphous material is converted first to an alpha and then to a beta crystalline morphology, since maximum density and minimum porosity are achieved only in the beta form.5 Sialon technology, designed to add processibility to the hot-pressed densification protocol, is based on the introduction of aluminum oxides and nitrides into Si3N4: a procedure of purposefully including a desirable impurity in the ceramic.6'7 Identification and quantitation of phases and impurities thus become crucial components in Si3N4 processing, and rapid, nondestructive measurement methods are needed to gauge reaction progress in this system. Infrared spectroscopy is an attractive technique for silicon nitride analysis, since it is capable of identifying not only the likely chemical impurities, but also all morphologies, including the amorphous phase, present in a given Si3N4 sample. Indeed, qualitative aspects of infrared analysis of Si3N4 are well documented in the literature, and studies have appeared describing the relative amounts of alpha and beta phases in Si3N4 powders,8"10 the level of S i - 0 functionality in O-Si-N phases,1112 the surface properties of Si3N413 and Si2ON2,14 and a comparison of band a)
Address correspondence to this author. J. Mater. Res., Vol. 4, No. 2, Mar/Apr 1989
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positions in Si3N4 vs Ge3N4.15 The recent advent of rugged commercial Fourier transform infrared (FT-IR) spectrometers has made it possible to conduct in situ infrared analyses as powders are being synthesized and processed. We present in this report the details of a technique for measuring all three Si3N4 morphologies quantit
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