NAVROTSKY INTERNATIONAL SYMPOSIUM (INTL. SYMP. ON GEOCHEMISTRY FOR SUSTAINABLE DEVELOPMENT)

Deeply appreciative of the symposium in my honor. I take this opportunity to present my science in the context of other scientific and societal events of the past five decades. In geochemistry I have utilized high-temperature oxide melt solution calorimetry to provide thermodynamic data and, more importantly, scientific insights into the microscopic sources of stability and metastability in spinels, high-pressure minerals, silicate glasses and melts, hydrous minerals and nanomaterials. In materials science I have studied the thermodynamics of ceramic, nuclear, and energy-related materials. Often the geochemical and materials projects are overlapping and synergistic. In this talk I highlight examples of my findings, roughly one per decade, that proved long-lived and transformational. However, my most important “product” may be the many students, postdocs and colleagues who have influenced me and been influenced by my ideas. I have seen the societal reasons and patterns for doing science evolve. Geochemistry is now more exciting than ever, especially in the broader planetary context. while materials science is rapidly discovering new materials which require fundamental understanding. Improvements in experimental and computational approaches promise future advances on both fundamental and applied fronts; thermodynamics remaining crucial to both science and technology.

Spent nuclear fuels will be eventually disposed of. They will be located in the canister, then the canister will be placed in the repository for disposal, surrounded by the buffer. Bentonite, a natural absorbent aluminum phyllosilicate consisting mostly of montmorillonite, is considered as buffer materials in South Korea. The spent nuclear fuels generate decay heat for a very long time. Thus the bentonite will be exposed to the elevated temperature condition, whereas groundwater will wet and saturate the bentonite with time. The bentonite will be exposed to dry, then to wet condition. For the safety of the disposal repository, the bentonite must maintain its required properties to delay groundwater reaching the surface of a canister [1]. Major concern to this include illitization, the transformation of smectite to illite [2,3].
In connection with our research program on Korean bentonite and its performance [4], we discuss geochemical characteristics (e.g., mineralogical changes, dehydration, volume changes, etc.) for Korean bentonite under the elevated temperature (i.e., higher than 100 ℃). The Korean bentonite loses interlayered waters at lower temperature, however holds them better at higher temperature as compared with the well-known MX-80 bentonite.

Our universe is wondrous, diverse and extreme, with temperatures scaling from near absolute zero in interstellar space to 10⁵ Kelvin at stellar surfaces, with pressures from near zero in interstellar space to gigabars in gas giant planet interiors, very large magnetic fields, and states of matter ranging from rarefied gases to organic, metallic and ceramic solids to dense hot plasmas. High levels of radiation arise from fusion factories producing chemical elements in stars, from natural radioactivity in planets, and from the solar wind. As atoms move closer together in the solid state because of low temperature and/or high pressure, the increased cooperative interactions of their electrons lead to quantum phenomena - ordering of electronic and magnetic states, superconductivity, and quantum confinement. Large magnetic fields can also alter the fundamental properties of chemical bonds and generate new emerging states. What appears as rare behavior on Earth may be common on other, very different, planets. Understanding and harnessing both the macroscopic (thermodynamic and kinetic) and the microscopic (atomistic and quantum) behavior of materials will open up new vistas for planetary science and space exploration.
The diversity and immense number of exoplanets poses an unprecedented inverse problem in materials science. Instead of asking “what are the properties of a planet with known physical and chemical parameters?” we must ask “what physical and chemical parameters are consistent with a few astronomical observations of a planet?” Addressing such inverse problems, which requires the collaboration of scientists from many fields ranging from astrophysics to geochemistry to physics, chemistry and engineering, is the focus of the newly established Navrotsky Eyring Center for Materials of the Universe (MotU) at Arizona State University. MotU also seeks to explore extreme conditions for the synthesis of new materials potentially applicable to aerospace and planetary science.
From the point of view of sustainability and the future of our own planet, both terrestrial geology through deep time and the study of other planets lends a long term and cautionary view. For example, were Mars and Venus once friendlier environments for life, and what factors (for example runaway greenhouse effects or loss of magnetic field) contributed to changing their conditions? Closer to home, are there environmental “tipping points” (anthropogenic or otherwise) leading to irreversible changes in surface and ocean conditions, atmosphere and climate? What can other planets, observable at present in different stages of their evolution) teach us about our past, present and future?

Interest in materials exhibiting oxygen ion and/or proton conduction has increased during the last years owing to their great importance for energy and environmental applications.
Ceria-based oxides are regarded as key oxide materials because rare earth-doped ceria shows a high oxygen ion conductivity even at intermediate temperatures. Using density-functional theory (DFT), we have investigated defect interaction and oxygen migration energies as well. By means of Kinetic Monte Carlo (KMC) simulations we then investigated the oxygen ion conductivity. We show that all interactions between the defects, namely vacancy-dopant attraction, dopant-dopant repulsion and vacancy-vacancy repulsion as well contribute to the so-called conductivity maximum of the ionic conductivity [1].
BaZrO₃-based oxides are proto-type proton conductors. Using density-functional theory (DFT), we have investigated defect interaction and proton migration energies in Y-doped BaZrO₃. The macroscopic proton conductivity was then investigated by means of KMC simulations. We discuss the resulting proton conductivities concerning special percolation pathways for protons [2].
Finally, we compare our theoretical results with experimental ones and discuss similarities and differences between oxygen ion and proton conductors.

In order to achieve a sustainable human presence on the Moon, Mars, and beyond, humanity must develop the capability to provide as close to 100% supply of resources in-situ as possible. Achieving this goal will require development of technologies to locate, extract, process, generate, and utilize said resources. To date, contemporary in-situ resource utilization-based space exploration architectures typically focus on the production of resources that have the most value for initial use in space such as O₂, H₂, and H₂O for the production of propellant and life support consumables [1]. However, critical metals indispensable to the terrestrial global economy such as Ni, Cu, Co, and the platinum-group elements will also likely be required to support the endeavor of becoming a multi-planetary species [2,3], and on this topic Mars becomes the focus. Based off compositional and petrographic similarities between terrestrial mantle-derived mafic/ultramafic magmas, meteorites known to come from Mars, and the physicochemical characteristics of the Martian surface, it is likely that massive and disseminated sulfide ores, which host these precious resources, were deposited at or near the surface [4,5]. In order to validate this belief, a more thorough exploration campaign is required to properly assess whether Mars is an ore-rich planet. Thus, this paper will provide an overview on the current state of knowledge and technologies available for prospecting for magmatic sulfide ores on Mars, with a particular focus on the capacity and necessity of integrating sustainable practices in upcoming space missions focused on in-situ resource utilization. Additionally, potential use cases of metals derived from magmatic sulfide ores in the space industry are considered.

The field of Solid State Ionics is concerned with the understanding and tailoring of defects, diffusion and reactions in solids. It has nowadays wide technological applications in energy conversion and storage, data storage, sensors etc. Thus, Solid State Ionics and its technological implications are inevitable for a future sustainable development of our world.
In this contribution I will, however, focus on some fundamental questions and still unresolved problems in the Science of Solid State Ionics. For this purpose, I will start with a brief history of Solid State Ionics showing the foundations of the field. Then I will focus on two major topics: The first is concerned with the role of defect interactions. This topic is of particular importance in materials with high defect concentrations where defect interactions are unavoidable. Interestingly, nearly all materials with technological importance belong to this class of materials. In contrast, the theoretical treatment of interactions is mostly limited to diluted systems. I will show a possible route to solve this problem by combining ab initio calculations with Monte Carlo simulations [1]. In this way, not only the problem of defect interactions can be solved, but also the link between the microscopic energetics and dynamics and the macroscopic thermodynamics and kinetics can be made. As examples, I will discuss our results for oxygen ion conductors and proton conductors [2,3].
The second topic is concerned with the number of components in a material. Nowadays, most materials in Solid State Ionics are multicomponent materials containing, e.g., three or more chemical elements. Thermodynamically, this is a challenge as the phase diagrams become rather complicated and are mostly unknown. On the other hand, there is another subtle problem which is concerned with the number of mobile species. Historically, in solid state kinetics only two mobile species were considered, e.g., two mobile cations during interdiffusion or one mobile anion and electrons in mixed conductors. The situation becomes, however, more complicated if there are three mobile species, e.g., two ionic defects and one electronic defect. I will discuss corresponding examples and the thermodynamic and kinetic implications [4,5].

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