Sol Gel-Based Synthesis of the Phosphorus-Containing MAX Phase V2PC.

More than 150 MAX phases are known to date. Their chemical diversity is the result of mixing-and-matching early-to-mid transition metals (M), main group elements (A), and carbon and/or nitrogen (X). The vast majority of the respective carbides and (carbo)nitrides contain group 13 and 14 as the A element, such as Al, Ga, and Si. V2PC is among the least studied members of this family of materials; as a matter of fact, it is only mentioned in two pieces of original literature. The solid-state synthesis is extremely vaguely described and working with elemental phosphorus poses additional synthetic challenges. Here, we confirm these experimental difficulties and present an alternative sol gel-based approach to prepare almost single-phase V2PC. The versatility of the sol gel chemistry is further demonstrated by variation of the gel-building agent moving beyond citric acid as the carbon source. DFT calculations support the experimentally obtained structural parameters and show V2PC is a metal.

Mechanistic Insights into the Nonconventional Sol-Gel Synthesis of MAX Phase M2GeC (M = V, Cr).

Despite their intriguing properties, MAX phases to date remain a class of materials overwhelmingly synthesized and studied with conventional approaches that date back to their discovery. With an ever-increasing demand for new and better materials and areas of application, developing new synthesis techniques must be at the forefront of our scientific efforts and cannot be overlooked. Sol-gel chemistry, while being a very traditional approach (especially for oxides), has so far hardly been leveraged within the MAX phase community. As a newly emerging technique to access nonoxide compounds, such as MAX phases, it offers a variety of advantages over classical solid-state chemistry, namely, milder reaction conditions and greater processibility (as previously shown for Cr2GaC). Here, the sol-gel synthesis of the two MAX phase members V2GeC and Cr2GeC, in combination with both conventional and nonconventional (microwave) heating techniques, is presented. In all instances, high yields were achieved, with only minor impurities remaining in the product. This expansion of the method to other members (apart from Cr2GaC) is a critical milestone in proving the technique's viability. Additionally, using simultaneous calorimetry and mass spectrometry, first insights into the underlying carbothermal reduction reaction are presented. Understanding the chemistry and formation mechanism will help broaden the sol-gel-based synthesis technique and increase its applicability.

Shape Control of MAX Phases by Biopolymer Sol-Gel Synthesis: Cr2GaC Thick Films, Microspheres, and Hollow Microspheres.

The class of MAX phases represents intriguing materials, as they combine ceramic and metallic properties quite exotically. Although many potential areas of application have been identified, a commercialization is still to be realized. This is particularly odd considering their existence of more than 60 years, however, less so considering the common synthesis techniques used. In fact, MAX phases are typically studied in either bulk or thin films, considerably hindering their integration into highly functional applications. Here, a facile and versatile sol-gel-based approach for the biopolymer-templated synthesis of MAX phase Cr2GaC is introduced, capable of preparing the layered ternary carbide in a variety of technological useful shapes. We demonstrate for the first time how our wet chemical synthesis strategy immensely increases the accessibility of specific shapes and morphologies via the targeted synthesis of thick films, microspheres, and hollow microspheres.

The synthesis and electrical transport properties of carbon/Cr2GaC MAX phase composite microwires.

While MAX phases offer an exotic combination of ceramic and metallic properties, rendering them a unique class of materials, their applications remain virtually hypothetical. To overcome this shortcoming, a sol-gel based route is introduced that allows access to microwires in the range of tens of micrometers. Thorough structural characterization through XRD, SEM, EDS, and AFM demonstrates a successful synthesis of carbonaceous Cr2GaC wires, and advanced low temperature electronic transport measurements revealed resistivity behavior dominated by amorphous carbon. The tunability of electronic behavior of the obtained microwires is shown by a halide post-synthesis treatment, allowing purposeful engineering of the microwires' electrical conductivity. Raman studies revealed the polyanionic nature of the intercalated halides and a slow decrease in halide concentration was concluded from time-dependent conductivity measurements. Based on these findings, the process is considered a viable candidate for fabricating chemiresistive halogen gas sensors.