Glass behaves in the same way as hardened honey; its fluidity increases with heat. Cracks simply flow through.
Material researchers enjoy using nature as a source of inspiration. For example, life cycles play an important role in nature, and in materials as well. It is fascinating to watch how nature handles predicaments, such as injuries. It’s not uncommon for organisms or even inorganic materials to develop self-healing powers in nature, thus living for longer – wounds heal, and cracks close up by themselves without external help. How can technical materials also be spurred to heal themselves? For Dr. Ralf Müller and Carsten Blaeß from the BAM Glass Division, this is vital to know. Should they succeed in activating self-healing powers in materials such as glass and glass ceramics, this could prolong the life of complex technical systems.
It is often the case that when a single defect occurs in a complex system, the entire system no longer functions. An example of such a complex system is the solid oxide fuel cell (SOFC).
In a SOFC, individual cells are connected in series of so-called ‘stacks’, and are sealed with a material that must be a culmination of the most diverse properties: it must withstand the operating temperature of approx. 850°C for tens of thousands of hours, it must be dimensionally stable and gas-tight, ensure electrical insulation and inhibit the transport of hydrogen and harmful corrosion products. And above all else: it must endure numerous heating and cooling processes without cracking. This means the used sealing material will undergo extreme stress.
Dr. Müller knows that: “The glass ceramic sealants currently used are well-developed and already fulfil the complex requirement profile quite well. No problems arise during active operation and constant temperature.” However, after being switched on and off, vast fluctuations in temperature occur and the sealing materials expand and contract again. In the course of this, cracks must not arise in the sealant, and if they do after all, it would be beneficial if they could ‘heal’, thus avoiding functional loss.
But, what influence does the glass-ceramic microstructure of the sealant have on crack healing? Is there an optimal microstructure?
Glass without any crystalline inclusions seem to be particularly well-suited for healing cracks. This is because glass has an amorphous, i.e. disordered, structure. It starts to flow viscous at temperatures above the so-called glass transition. For window glass, this temperature is approx. 500°C. At this temperature, glass begins to behave like hardened honey, its fluidity increases with heat. Cracks simply fill up by viscous flow and the components in the glass newly arrange themselves, all without the chemical amorphous structure of the glass undergoing any change. The original properties are retained. However, the mobility comes at a price: The glass seal is not very dimensionally stable and damaging corrosive products can easily spread within the material. The glass reacts with the bordering materials, e.g. chrome containing steel.
The other extreme is fully crystallised glass, which more resemble ceramics. These are also very stiff in high temperatures above 1000°C, making them more mechanically resilient and inhibit the transport of corrosion products. However, they do not have the desired self-healing powers.
The search for the perfect microstructure
Dr. Müller and Carsten Blaeß are now facing the task of balancing the two extremes. Their aim was to develop a safety mechanism to ‘locally’ restore the damages in sealing glass materials. The mechanism is intended to work precisely where damages emerge. The glass's natural ability to heal cracks should be used, i.e. its ability to flow viscously at certain temperatures. At the same time, the ‘global’ transport of corrosive products in the overall system should be contained as far as possible.
From model systems to applications in practice
Carsten Blaeß reports: “In the beginning, we searched meticulously for a suitable model combination of glass and ceramics for the experiments. This was not easy to find.” Next, Carsten Blaeß manufactured test specimens with various mixing contents from glass and ceramics, which he cracked with the help of so-called Vickers-Indentation. He then stored the samples in the furnace at a constant temperature, examining them at certain time intervals with a laser scanning microscope (LSM) to document changes in crack length, width and depth. The decisive observation was that the cracks closed up depending on the time and viscosity of the respective glass. The data were recorded and evaluated. Both scientists presented these mechanisms optically in a master curve in which all healing curves for different temperatures were on the same line. In practical terms this means that: When certain factors such as temperature and glass viscosity are known, scientists can predict the duration of the crack healing.
Furthermore, the two scientists found a combination of glass and ceramic that is still free-flowing ‘locally’ but behaves adequately rigid and stiff ‘globally’. This made it possible to combine the advantages of the two sealing concepts glass and glass ceramic. This is promising in terms of increasing the life cycles of SOFCs.
Individual results of the investigations will be published in a dissertation. The first specialist article has already been released.
You will find more information on hydrogen and fuel cells in the Hydrogen Dossier