Raman-Spectroscopy – A key to ceramic investigation

Have you ever felt the impulse to observe your ceramic while sintering? Would you like to follow the interaction between refractory material and slag? You would like to watch the molecules at work as they polymerize?

Asking these questions and performing thought experiments is easy. In practice, however, with most ideas and concepts you will find yourself standing in front of closed oven doors – or at least that is what you might believe. Quoting Albert Einstein: “In the midst of difficulty lies possibility” you will find the possibility in Raman Spectroscopy. It opens the door to new and innovative research possibilities – because almost every molecule can “dance”.

What is meant by that? We would like to explain this to you in the following.
Perhaps you had this one chemistry teacher at school, too, who demonstrated the behavior of water molecules in various states of aggregation. In the frozen state there was no movement, the “warmer” it became, the more the person fidgeted with arms and legs. This fidgeting simulates vibrations along so-called covalent atomic bonds that are generated by the influence of energy, in this case heat.
Depending on their structure, molecules have several different types of vibration, known as vibrational modes.

The easiest to understand are bending and stretching vibrational movements. Figure 1 shows two simplified examples.

Figure 1: Vibrational modes of one molecule. Left: symmetric stretching, right: asymmetric stretching.

C. V. Raman and his co-worker K. S. Krishnan used this phenomenon of molecular vibration in 1928 for the analysis of organic substances. They illuminated molecules with light of a defined wavelength (laser beam) to incite vibration and detected shifts (differences) in the wavelengths of the backscattered light. These shifts were caused by the transfer of energy to the molecule. Since covalent atomic bonds in molecules of different kinds are different, too, the energies required to incite oscillation and the shifts in the wavelengths are characteristic. The analysis of phases using this method is called Raman spectroscopy.

Why Raman spectroscopy is an attractive tool is the fact that almost every material consists of molecules. This enables spatially resolved analysis of crystalline, amorphous and organic solids as well as fluids. Therefore, it is no wonder, Raman spectroscopy is highly used for synthetic substance or forensic investigations. Polymerisation and sol-gel-processes are frequently observed by means of Raman spectroscopy, too.

At WZR ceramic solutions we mainly deal with ceramic materials. We have been using scanning electron microscopy as an imaging, spatially resolved observation tool since the beginning. So why is Raman spectroscopy an enrichment for our ceramic analysis?
Scanning electron microscopy shows the status quo of the sample, important processes – sintering, phase transitions, slag attacks on refractory material – have not yet taken place or are already finished. But precisely these processes are the most important to understand to fully characterize the material.

Here the Raman spectroscopy gives the perfect solution for us: It enables real time measurements of material in place, meaning in-situ during these processes happen – an important tool to decipher ceramics. Because, what could be more of importance for analyses of technical ceramics than live-observing chemical reactions at 1000 °C during sintering or the refractory-slag interaction)
Figure 2 shows an example of how complex reactions at higher temperature can be and how they can be resolved with Raman spectroscopy.

Figure 2: Interaction along grain boundaries between quartz (red) and lime (yellow) at 929 °C and 973 °C resulting in the formation of dicalciumsilicate (Ca2SiO4). A replacement of quartz after Ca2SiO4 takes place. More information can be found at [1].

All over the world, institutions that work with Raman spectroscopy in the high temperature range are small in count. We are therefore pleased to call the leading working group in high temperature Raman spectroscopy on ceramics our new cooperation partner.

Do you have ideas that you could realize with the help of Raman spectroscopy? Contact us and start a new and innovative development of your ceramics.

[1] Hauke, K., Kehren, J., Böhme, N., Zimmer, S., & Geisler, T. (2019): In situ hyperspectral Raman imaging: A new method to investigate sintering processes of ceramic material at high-temperature. Applied Sciences, 9(7), 1310.

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