Where high temperatures, aggressive chemical environments, and demanding mechanical conditions come together, ceramics can provide superior performance to metals. In such environments, ceramic-based composites and ceramic coatings are used to reduce corrosion and extend lifetimes of components. In applications such as aero engines, parts in direct contact with igniting fuel mixtures, and other demanding environments, are covered with ceramic heat-barrier coatings to give those parts a longer service life. It is important to know more about the performance of these materials under such demanding conditions, and so my research group in the University of Wales Aberystwyth (UWA) Physics Department has recently started a project using nuclear magnetic resonance (NMR) spectroscopy to study the structural evolution of ceramics subject to such extreme temperatures.
Structure and Properties of Ceramic Materials
Ceramics are effective in high-temperature applications because their atomic structure and their microstructure are well balanced. While their thermal insulation properties are mainly due to micro-scale porosity, the atomic structure governs the mechanical strength of the material. Similarly, the bonding of the ceramic or ceramic coating to the substrate depends on the chemistry of the interface between them. Thermal expansion coefficients are also determined by the atomic structure, since in most crystalline materials expansion is dependent on the crystallographic orientation. In a granular composite such as a ceramic, it is important that the thermal expansion and contraction of different components are balanced in a way that minimises stress at the contact points between grains. An amorphous interface component that is able to flow and whose thermal expansion is isotropic can reduce the thermally induced stresses. Thermal, mechanical, and chemical properties are interlinked, and it is not straightforward to predict the resilience of a ceramic without experimental input linking structure on various length scales and high-temperature behaviour.
Capabilities of Nuclear Magnetic Resonance Spectroscopy
Using NMR spectroscopy it is possible to probe the structural environment of all atoms of one particular isotope (e.g. 27A1, 29Si or 23Na) in the material under study. In a heterogeneous structure such as a ceramic, in which multiple components are present, the atomic environment of the components can be distinguished. For example, the spectral lines of aluminium atoms in tetrahedral and octahedral environments can easily be separated. But even more subtle differences in the site geometry can be analysed by modelling the line shape and comparing the lines to those of well-known reference structures.
The Affects of Crystallinity on Nuclear Magnetic Resonance Spectroscopy
Unlike diffraction, NMR allows the study of glassy, as well as crystalline phases and the ability to distinguish between them both. In materials with a large surface-to-volume ratio – fine-grained down to nanostructured ceramics - the distorted structure of the interface component can be separated from that of the bulk material.
Investigating High Temperature Behaviour
Using a special NMR probe and with samples heated by a 125W CO, (infrared) laser, we are studying the effect of high temperature (up to 2200°C) and large temperature gradients on materials in conditions very similar to those that they’re likely to encouter in ‘real-life’. Aerodynamic levitation of spherical or conical samples is used to provide containerless heating conditions, so minimising cooling by conduction and chemical reactions between the sample and the container.
Thermal Expansion and High Temperature Reactions
Different expansion coefficients of various components in the same material cause the individual spectral lines to shift differently. Reactive sintering and chemical reactions with the adjustable atmosphere cause existing lines to disappear and new ones to grow.
Such reactions are a major cause of corrosion in furnaces used in the glass-making and metal-smelting industries. These furnaces are lined with ceramic bricks, which are susceptible to corrosion owing to the dissolution of the container material in the melt and penetration of the melt into the channels and pores of the ceramic. The combined attack of thermal, chemical and mechanical influences leads to increased wear and tear at the flux line - the triple junction between melt, ceramic and furnace atmosphere. Novel, more efficient firing techniques, such as the use of oxy-fuel mixtures, increase both the temperature and the chemical reactivity, while the viscosity of the melt is reduced. This increases the need for optimised ceramic linings, and an understanding of the role of the atomic structure in corrosion will help in the design of better ones.
Laser Absorption Radiation Thermometry
Reaction kinetics are usually characterised by a very steep increase in the reaction rate around a critical temperature. This makes it necessary for us to monitor the temperature very accurately during any in-situ experiments. This is done by following a dual approach - by exploiting a novel emission-free thermometry technique and by developing a feedback control system for our laser. Contactless thermometry over a large range of temperatures is quite challenging because pyrometry is based on the assumption that the object under study is a grey body, i.e. that absorption and emission do not depend on the frequency of the radiation. Together with Dr Andrew Levick from NPL, we are implementing a novel technique developed by NPL to bypass the emissivity problem. In Laser Absorption Radiation Thermometry (LART), two low-power infrared lasers with different wavelengths are used to induce small temperature modulations in the sample. When probing the infrared emission from the sample at the two wavelengths of these lasers while knowing the period of the modulation, the emissivity cancels out and temperatures can be measured with an absolute accuracy of 15K.
The combination of a fast analytical technique for the characterisation of atomic environments - 27AI NMR - containerless infrared laser heating, and a contactless thermometry technique with previously unreached accuracy, enables us to carry out experiments on high-temperature ceramics under conditions similar to those encountered in real applications, and also to obtain time-resolved data tracing reasonably fast reaction kinetics or heat conduction pathways. Depending on the isotope analysed and the concentration of this isotope in the sample, time slices down to about ten seconds are achievable.