Monitoring of a calcination reaction of high reflective green-black (HRGB) pigments by using near-infrared electronic spectroscopy: calcination temperature-dependent crystal structural changes of their components and calibration of the extent of the reaction

This paper demonstrates the potential of near-infrared (NIR) electronic spectroscopy in nondestructive monitoring of a chemical reaction of inorganic functional material. For this purpose NIR spectra in the 12,000-4000 cm(-1) region were measured for high reflective green-black (HRGB) pigments (Co(0.5)Mg(0.5)Fe(0.5)Al(1.5)O(4)) calcined at 1000, 1100, and 1200 °C and pigments with the same components as HRGB but calcined at different temperatures (500-900 °C) (hereafter, called "Pigments A") . NIR spectra of their components such as Co(3)O(4), MgO, Fe(2)O(3), and Al(2)O(3) were also measured. The NIR spectra of Pigments A show two major broad bands. One arises from a (4)A(2)→(4)T(1) (T(h)) d-d transition of Co(II) in the 9000-6000 cm(-1) region. The other band in the 12,000-9000 cm(-1) region is assigned to a foot of the charge-transfer (CT) band of Fe(2)O(3). The Co(II) band contains three component bands that are characteristic of a spinel structure. A shoulder arising from (A(1-x)B(x))(Th)(A(x)B(2-x))(Oh)O(4) (A≡Co, Mg, B≡Fe, Al; inverse spinel structure) emerges near 5900 cm(-1) in the spectra of Pigments A calcined in the temperature range of 700-900 °C, indicating that the Pigments A calcined in this temperature range assume an inverse spinel structure. When the calcination temperature is above 1000 °C, the final product, HRGB, is produced. This is confirmed from the fact that HRGB shows peaks characteristic of a spinel structure that have different wavenumbers from those of the corresponding peaks of Pigments A. Wide-angle X-ray diffraction (WAXD) patterns were also measured for HRGB, Pigments A, and their components. Based on the NIR and WAXD data we investigated calcination-temperature-dependent crystal structural changes of the components. We also developed partial least squares (PLS) calibration models for the 9000-6000 cm(-1) region of the NIR spectra of HRGB and Pigments A. The score plot of latent variable (LV) 2 of the calibration model for calcination temperature demonstrates clearly the existence of an intermediate of the calcination reaction, which may be (A(1-x)B(x))(Th)(A(x)B(2-x))(Oh)O(4) (A≡Co, Mg, B≡Fe, Al).