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Iridium(IV) oxide
Iridium(IV) oxide, IrO2, is the only well-characterised oxide of iridium. It is a blue-black solid. The compound adopts the TiO2 rutile structure, featuring six coordinate iridium and three coordinate oxygen.[1] It is used with other rare oxides in the coating of anode-electrodes for industrial electrolysis and in microelectrodes for electrophysiology research.[2] As described by its discoverers, it can be formed by treating the green form of iridium trichloride with oxygen at high temperatures:
A hydrated form is also known.[3] ApplicationIridium dioxide can be used as an anode electrode for industrial electrolysis and as a microelectrode for electrophysiological studies.[4] Iridium dioxide can be used to make coated electrodes.[5] Mechanical PropertiesOxide materials are typically hard and brittle, which means it can fracture under stress without significant prior deformation.[6] Iridium oxide is also a stiff material and do not easily deform under stress.[7] Since iridium oxide’s applications focus on electrode coating and catalytic materials for electrolysis, the discussion of mechanical properties is related to these applications. Young’s ModulusYoung’s modulus is a material property that measures the stiffness of the material. By experimentally measuring Young’s modulus, people could understand how much a material will deform under a specific load, which is essential in designing structures and preventing deformations.[8] For iridium oxide films, the young’s modulus data is crucial for accurate modeling and design of electromechanical devices where the mechanical properties of the electrode material significantly affect device performance. Therefore, researchers used the cantilever bending method to determine Young’s modulus of iridium oxide thin film.[7] First, iridium oxide was deposited onto a silicon wafer and fabricated to cantilever beams. Using an atomic force microscope (AFM), a fine tip is aligned to the free end of the beam and a tiny force is applied. The force exerted and the resulting deflection were precisely measured to calculate the stiffness and then the Young’s modulus of iridium oxide. The experimental measurement of the young’s modulus of Iridium oxide thin film is reported to be 300 ± 15 GPa.[7] Compared to metal Iridium, which has a young’s modulus of 517 GPa,[9] the oxidation of iridium lower the stiffness of the material. Fracture and Delamination of Iridium Oxide Film on SubstrateFracture and delamination are well-known problems when fabricating devices that incorporate iridium oxide film. The delamination is typically due to stresses that develop between the IrO2 layer and its substrate during manufacturing processes. One potential cause of delamination is lattice mismatch between iridium oxide and the substrate material. Iridium oxide has a tetragonal lattice with lattice parameters of 4.5Å and 3.15Å.[10] In contrast, popular substrates like gold and platinum have lattice constants of approximately 4.08 Å and 3.92 Å, respectively.[11] The difference in lattice parameter can lead to strain at the interface between the iridium oxide layer and the substrate, resulting in fracture and delamination of the iridium film. Iridium oxide sputtered on liquid crystal polymer could be a potential way to avoid delamination.[12] Another cause of delamination is the incorporation of high temperature processes during fabrication, such as annealing. Annealing involves heating iridium oxide to a high temperature but under melting point (around 750-900 °C) and then cooling it, relieving internal stresses and improving the iridium oxide’s crystallinity and mechanical properties.[13] However, if the lattice parameter of the iridium oxide layer changes significantly compared to the substrate following annealing, it can result in a greater lattice mismatch, which increases the surface tension and assist the formation of long cracks (similar to mechanically stressed cracks reported by Mailley et al.[14]). The cracks create a breakpoint where the surface strain is relieved, leading to delamination and other types of mechanical failure. Even if the iridium oxide film remains intact under equilibrium conditions, it may still delaminate during operation. Cogan et al. reported that sputtered iridium oxide films could delaminate after several cyclic voltammetry cycles, which suggests that the film could delaminate under operational loads.[15] The team then limits the maximum potential bias to 0.9V and no visible delamination occurs. References
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