Development of Wet Chemical Synthesis Strategies for the Class of MAX Phases

MAX phases are an intriguing class of materials with exotic combinations of properties, essentially turning them into metallic ceramics. Despite this unique feature, no commercialization has been accomplished yet. Looking at the state of the art within the MAX phase community, almost all published studies can be summarized using the term “traditional high temperature synthesis”. Contrasting the scientific interest that has been on the rise especially since the discovery of MXenes, the synthetic spectrum has been largely the same as it has been over the past decades.Herein, the newly-emerging sol-gel chemistry is being explored as an alternative non-conventional synthetic approach. Building on the successful sol-gel synthesis of Cr2GaC, this study focuses around the expansion of sol-gel chemistry for MAX phases. Starting with a thorough mechanistic investigation into the reaction pathway of sol-gel synthesized Cr2GaC, the chemical understanding of this system is drastically deepened. It is shown how the preliminary nano-structured metal-oxide species develop into bulk oxides, before the amorphous and disordered graphite partakes in the reaction and reduces the metals into the MAX phase. Furthermore, the technique is extended to the two Ge- based MAX phases V2GeC and Cr2GeC, a critical step needed to prove the viability and applicability of the newly developed technique. Additionally, by introducing Mn into the Cr-Ga-C system, a Mn-doping was achieved, and for the first time for (Cr1–xMnx)2GaC, a unit cell increase could be recorded. Based on magnetometry measurements, the currently widely accepted assumption of statistically distributed Mn in the M-layer is challenged. The versatility of wet chemistry is explored using the model system Cr2GaC. Firstly, the MAX phase can be obtained in a microwire shape leveraging the branched biopolymer dextran, eliminating the need for any post-synthesis machining. Via halide intercalation, the electrical transport properties could be purposefully engineered. Secondly, leveraging the unique and linear biopolymer chitosan, Cr2GaC was obtained as thick films and dense microspheres, drastically opening potential areas of application for MAX phases. Lastly, hollow microspheres with diameters of tens of μm were synthesized via carboxymethylated dextran. This shape once more opens the door to very specific applications requiring sophisticated structures.