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_0.5Mg_0.5Fe_0.5Al_1.5O₄) 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 Co₃O₄, MgO, Fe₂O₃, and Al₂O₃ were also measured. The NIR spectra of Pigments A show two major broad bands. One arises from a ⁴A₂→ ⁴ T₁ (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 Fe₂O₃. The Co(II) band contains three component bands that are characteristic of a spinel structure. A shoulder arising from (A_1-xB _x )^Th(A _x B_2-x)^OhO₄ (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-xB _x )^Th(A _x B_2-x)^OhO₄ (A≡Co, Mg, B≡Fe, Al).