High temperature microscopy is the use of microscopy techniques on samples at high temperatures. Often, this means the microscopy instrumentation needs to be specifically designed to cope with the effects of high temperatures, including many optical components undergoing temperature-dependent changes in their optical properties.
Image Credit: Maniock/Shutterstock.com
One of the key applications of high temperature microscopy is to examine material behavior under high temperature conditions.1 For example, high temperature microscopy can be used to understand the ferric grain formation sites in steel alloys and some of the material behavior associated with the phase transitions seen when the temperature is changed.
As well for applications on metal alloys, high temperature microscopy can be used to study phenomena such as high temperature crystallization processes and the thermal stability of different material types, including glass-ceramics.2 Such studies can be performed over longer thermal aging timescales to ensure there is no degradation of the material properties or performance after extended periods of heating.
High-temperature microscopy is sometimes otherwise known hot-stage or temperature-controlled microscopy, where a heated stage is used to increase the sample temperature and a number of different measurements and sample analyses are performed. Hot-stage microscopy can involve a full analysis of the sample of interest using methods such as differential scanning calorimetry (DSC) and thermomechanical analysis (TMA).3
Hot-stage microscopy can be used in the analysis of pharmaceuticals, where using microscopy to observe the crystallization behavior of the active pharmaceutical ingredient under temperature ramping conditions can be used to determine the thermal stability of the active compound.3 Access to methods such as DSC and TMA combined with optical imaging information can also be used to study polymorphism in detail, as different polymorphs of a pharmaceutical compound may differ in their physicochemical properties.
Microscopy Instrumentation
High temperature microscopy is compatible with a number of different imaging approaches, such as confocal microscopy, phase-contrast microscopy and widefield microscopy. There are now a number of commercial solutions available for hot-stage or high-temperature microscopy from companies such as Mettler Toledo and Thermo Fischer Scientific.
A typical hot-stage instrument will consist of the microscopy apparatus, a computer-controllable hot stage, any necessary filters and a camera to record the image information. Often translation stages will be used to control the sample position or allow for rastering over the sample area. Some instrumentation also comes with cooling, as well as heating capabilities for recording a wider range of temperature conditions.
Most high temperature microscopes have a heating stage with a crucible that is capable of providing intense temperatures in the local sample environment. Depending on the type of heating process to be studied, the heating process may be done with an electrical heater or, in some cases, a laser can be used to initiate the heating.4
The advantage of using a microscope alongside traditional thermal analysis methods like TMA or DSC is that, as well as the latter being used to determine the phase transition points, the imaging information can be used to analyze any physical changes in the structure of the sample. Sometimes, when the thermal changes associated with a phase transition are very small, it is easier to identify the phase transition temperatures by visual inspection of the sample than through the results of methods like DSC.
Many microscopes make use of polarizers and filters to improve image contrast and quality. Looking at the temperature-dependence of the optical properties of materials, such as the change in birefringence via the microscope is also a powerful way of visualizing regions of stress and strain in the sample.
Recent Work
Materials engineering, the pharmaceutical industry and, more recently, even the food industry are some of the key application areas for hot-stage microscopy. Recent work looking at engineered edible colloids has made use of hot-stage microscopy to perform a thermal analysis of how formulations collapse over time when samples are heated above 65 °C.5
Functional and engineered colloids are possible routes for complex drug delivery and researchers have been recently exploring how colloids could be exploited to tune food-body interactions. The material properties of foodstuffs are very important for determining consumer perception of the goods and determining how they will interact with the body and foams, which are one form of food that colloids could help stabilize.
Hot-stage microscopy can also be used to look at food formation processes, such as the gelatinization of starch, in real-time.6 Gelatinzation is an important step in industrial food processing and food use and not all starch compounds produce gels with the same kinds of properties. Hot-stage microscopy provides a route to seeing how the gelatinization process occurs and how certain steps can be optimized to produce foodstuffs with the right final properties.
Overall, hot-stage microscopy is a highly versatile technique that offers a great deal of insight into material properties. While often the spatial resolution is not as good as other standard microscopy methods, the ability to capture temperature-dependent behavior is very important for any material that will experience a range of temperature conditions in its application.
References and Further Reading
Mu, W., Hedstrom, P., Sibata, H., Jonsson, P. G., & Nakajima, K. (2018). High-Temperature Confocal Laser Scanning Microscopy Studies of Ferrite Formation in Inclusion-Engineered Steels : A Review. Advanced Real Time Optical Imaging, 70(10), pp. 2283–2295. https://doi.org/10.1007/s11837-018-2921-1
Gödeke, D., & Dahlmann, U. (2011). Study on the crystallization behaviour and thermal stability of glass-ceramics used as solid oxide fuel cell-sealing materials. Journal of Power Sources, 196, pp. 9046–9050. https://doi.org/10.1016/j.jpowsour.2010.12.054
Molinaro, C., Béné, M., Gorlas, A., Cunha, V. Da, Robert, H. M. L., Catchpole, R., Gallais, L., Forterre, P., & Baffou, G. (2022). Life at high temperature observed in vitro upon laser heating of gold nanoparticles. Nature Communications, 13, p. 5342. https://doi.org/10.1038/s41467-022-33074-6
Kumar, A., & Singh, P. (2020). Hot stage microscopy and its applications in pharmaceutical characterization. Applied Microscopy, 50, p. 12. https://doi.org/10.1186/s42649-020-00032-9
Patel, A. R. (2020). Functional and Engineered Colloids from Edible Materials for Emerging Applications in Designing the Food of the Future. Advanced Functional Materials, 30, p. 1806809. https://doi.org/10.1002/adfm.201806809
Cai, C., Cai, J., Zhao, L., & Wei, C. (2014). In situ Gelatinization of Starch using Hot Stage Microscopy. Food Sci Biotechnol, 23(1), pp. 15–22. https://doi.org/10.1007/s10068-014-0003-x
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.