In Situ Hyperspectral Raman Imaging: A New Method to Investigate Sintering Processes of Ceramic Material at High-temperature

Abstract

In the last decades, Raman spectroscopy has become an important tool to identify and investigate minerals, gases, glasses, and organic material at room temperature. In combination with high-temperature and high-pressure devices, however, the in situ investigation of mineral transformation reactions and their kinetics is nowadays also possible. Here, we present a novel approach to in situ studies for the sintering process of silicate ceramics by hyperspectral Raman imaging. This imaging technique allows studying high-temperature solid-solid and/or solid-melt reactions spatially and temporally resolved, and opens up new avenues to study and visualize high-temperature sintering processes in multi-component systems. After describing in detail the methodology, the results of three application examples are presented and discussed. These experiments demonstrate the power of hyperspectral Raman imaging for in situ studies of the mechanism(s) of solid-solid or solid-melt reactions at high-temperature with a micrometer-scale resolution as well as to gain kinetic information from the temperature- and time-dependent growth and breakdown of minerals during isothermal or isochronal sintering.

In Situ Hyperspectral Raman Imaging of Ternesite Formation and Decomposition at High Temperatures

“…The best achievable the lateral and axial (depth) resolution of the Raman measurements with the focus at the sample surface is usually estimated from the theoretical diffraction limit given by d l ≈ 1.22λ/NA and d a ≈ 4λ/NA 2 , yielding 1.3 and 8.5 µm in our case, respectively. However, the real lateral and axial resolution may rather be in the order of d l ≈ 2 µm and d a ≈ 15 µm, respectively [16,17]. The confocal hole was set to 300 µm, which has to be empirically proven to provide the best compromise between depth resolution and the signal-to-noise ratio for our instrument [16].…”

“…However, the real lateral and axial resolution may rather be in the order of d l ≈ 2 µm and d a ≈ 15 µm, respectively [16,17]. The confocal hole was set to 300 µm, which has to be empirically proven to provide the best compromise between depth resolution and the signal-to-noise ratio for our instrument [16]. It is important to note that the axial resolution at the surface also depends on the absorption properties of the material investigated.…”

“…In particular, ternesite can serve as a temperature indicator once its thermal stability field is known, which can help kiln operators to optimize the combustion temperatures. In the present study, samples of calcium oxide, quartz, and anhydrite were fired to about 1100 • C and the reactions were monitored in situ by confocal hyperspectral Raman imaging, which allowed monitoring of the HT solid-solid reactions at the micrometer scale without the need to quench the sample to room temperature (RT) before analyzing it [15][16][17]. However, before performing the multiphase sinter experiments, we studied the thermal behavior of a natural ternesite crystal between RT and 1230 • C in 10 • C-steps by Raman spectroscopy.…”

Hyperspectral Imaging Techniques for the Study, Conservation and Management of Rock Art

“…Another advantage is its high spectral resolution with multiple bands and narrow bandwidth, i.e., it allows a continuous spectral response to be obtained for each pixel of the image. Because of these unique features, hyperspectral images have been used in conservation [8] and the study of artefacts [9] and to reveal hidden details in ancient manuscripts and paintings, although its use in rock art [10], and in cultural and industrial heritage objects [11], has been scarce and relatively recent.The main objective of this study is to propose a method of generating the cartography of pigments and improving the visualization of aspects of rock art not visible to the naked eye due to the causes previously mentioned, as well as to understand its process of creation or the sequence of execution of different motifs.…”

“…Another advantage is its high spectral resolution with multiple bands and narrow bandwidth, i.e., it allows a continuous spectral response to be obtained for each pixel of the image. Because of these unique features, hyperspectral images have been used in conservation [8] and the study of artefacts [9] and to reveal hidden details in ancient manuscripts and paintings, although its use in rock art [10], and in cultural and industrial heritage objects [11], has been scarce and relatively recent.…”