Ammonia is one of the most important chemicals for modern civilization as well as a potentially invaluable intermediary component of a future sustainable H2 economy, yet its current production is decidedly unsustainable. Accordingly, researchers are attempting to devise new paradigms for ammonia production, one of which would involve the cyclical reaction of H2 with a nitride compound and the renitridation of that compound with N2 - a thermochemical loop that would allow for ammonia production with renewable inputs and at relatively low pressures. In this paper, researchers identified several ternary and quaternary metal nitrides with the potential to exhibit relatively favorable thermodynamics for both the reduction and nitridation steps of that reaction cycle. These compounds were synthesized via co-precipitation and Pechini synthesis and several were tested under gas flows of 75% H2/Ar at 100-700 C and 75% H2/N2 at 700 C to determine their behavior under these conditions. As suggested by the available literature, Co3Mo3N was found to be a far better candidate for thermochemical looping than Fe3Mo3N or Ni2Mo3N - with higher mass loss and mass regain. Interestingly, quaternary nitrides containing Fe and Co in addition to Mo also demonstrated remarkable reduction and nitridation capability under ambient pressures. Ultimately, this paper demonstrates the feasibility of synthesizing a variety of single phase ternary and quaternary nitrides and the potential that several of these nitrides hold for producing ammonia sustainably via cyclic thermochemistry.

MAX phases are layered hexagonal early transition metal carbides, sometimes nitrides, where M is an early transition metal, A is an A group element, most prominently groups 13 or 14, and X is either carbon or nitrogen.1 They are gaining a lot of attention because of their unusual properties. Particularly, their hardness, chemical stability at room temperature, and high melting points. These properties provide a material that is viable for a wide range of demanding applications.2,3 MAX phases display a combination of both ceramic and metallic characteristics. Furthermore, they also serve as a precursor for two-dimensional MXenes.4,5<br/>Generally, bulk synthesis of MAX phases is done through traditional solid state synthesis techniques. For example, three solid state synthesis techniques include solid state method, hot pressing and arc melting and annealing. During solid state method, the powder precursors are preheated between 350 and 400°C, allowing for decomposition of starting reagents and removal of volatile products leaving only the oxides. At this point the germination phase has completed, and the crystal growth phase begins. Under the effect of a concentration gradient and very high temperatures, cations migrate, forming well-ordered layers. Slow cooling rates are used in order to ensure crystallinity of the product.6 The second method, hot pressing, involves the mixing of powder precursors thoroughly and then cold pressed into a green body – a ceramic body powder pre-sintering. They are then heated under vacuum and often high pressure in order to form the product. Two variants of hot-pressing exits: reactive hot pressing, where the pressure during the reaction will vary throughout the reaction, and isostatic hot pressing, where the pressure is held constant throughout the entire reaction.7 Another solid-state technique is arc melting and annealing. During arc melting, alternating current is applied to the electrode inside an inert reactor, which is arranged as to generate an arc discharge. The heat produced by arcing causes rapid melting of the samples.8 While these methods are most common, they are not always viable due to the specialized equipment required in order to achieve the high temperature and pressure conditions. Furthermore, these specific techniques don’t allow for high control over particle size and morphology. <br/>Because of this, alternative, non-conventional synthesis techniques have been developed involving more readily available tube furnaces and microwaves, which lack the extreme pressures instead opting for ambient conditions.9 Sol-gel techniques have been developed by the group of Christina Birkel, and have successfully produced MAX phases through calcination of homogeneous citric acid-based gel-precursors. Some advantages of using these gel-precursors include shorter diffusion paths, and faster mass transport, thus, resulting in lower reaction temperatures and shorter reaction times. Ultimately, this allows for control over particle morphology and size.10<br/>The focus of this work is to discover optimal synthesis conditions to create spherical Cr2GaC. Spherical MAX phases have been briefly explored in existing literature using polymer-based hollow microsphere templates.10 These polymer microspheres have been used to synthesize spherical metal oxides. This is achieved by heating the metal oxide precursors which adhere to the spheres, then by thermal treatment, the template is then removed.11 <br/>Two different microsphere templates will be explored to study the advantages and disadvantages of different size distributions and surface conditions of the spheres. Furthermore, reaction temperature, reaction time, citric acid equivalents, and gel to microsphere ratio will be altered to determine optimal synthesis parameters for depositing Cr2GaC onto spherical templates. Cr2GaC serves as a model compound because it has been successfully synthesized through sol-gel chemistry in the past.10 This phase will be prepared through non-conventional sol-gel chemistry, with various heating profiles, both furnace and microwave, and will be characterized through X-ray diffraction (XRD), and Rietveld refinement. Further, the morphology and atomic composition will be analyzed through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).

MAX phases are ternary carbides or nitrides that possess unique material characteristics, often simplified as a mix of metallic and ceramic properties. Many aspects of MAX phases are still being researched, but they have exciting potential applications in high-temperature structural systems, the next generation of nuclear power plants, and concentrated solar power. This project aims to benefit further research into these applications by validating a rapid unconventional synthesis method: microwave-assisted sol-gel synthesis. Three MAX phases (Cr2GaC, Cr2GeC, and V2GeC) were successfully synthesized via this route, which should open the door for more rapid prototyping and ultimately more efficient research.