Browsing by Author "Vu, TH"

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    Assessing carbonation in one-part fly ash/slag geopolymer mortar: Change in pore characteristics using the state-of-the-art technique neutron tomography
    (Elsevier, 2020-11-01) Vu, TH; Gowripalan, N; De Silva, P; Paradowska, AM; Garbe, U; Kidd, P; Sirivivatnanon, V
    Carbonation has long been recognised as a durability issue attributed to corrosion of steel reinforcement in geopolymer materials. The currently available information, however, is not sufficient to gain a deep understanding of this issue, particularly the facet of the carbonation impact on the pore structure of such materials. This paper, thus, assessed the influence of carbonation on porosity and pore size characteristics of one-part fly ash/slag geopolymer mortar, by using neutron tomography. The cutting-edge thermal neutron tomography used in this study provided the prowess of non-destructive 3D analysis of exploring different regions within geopolymers. The results obtained showed that carbonation in the investigated geopolymer mortars drew their porosity down approximately 30% and shifted pore size regions to smaller pore areas. Other evaluations such as changing pH, carbonation front depth and elemental mapping by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS) were also performed in this study, in order to supplement the findings of neutron tomography. © 2020 Elsevier Ltd.
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    Exploration of organic minerals on Saturn's moon Titan
    (International Union of Crystallography, 2021-08-14) Maynard-Casely, HE; Hodyss, R; Vu, TH; Malaska, MJ; Choukroun, M; Cable, ML; Runčevski, t
    Titan, the largest moon of Saturn, has been revealed by the Cassini-Huygens mission to be a fascinating and quite Earth-like world. Among the parallels to Earth, which includes the lakes, seas, fluvial and pluvial features on its surface, is an inventory of organic minerals [1]. However, where on Earth these organic minerals are only found in niche environments, on Titan they are likely to be the dominant surface-shaping materials. Titan’s organic minerals are formed primarily from photochemistry induced by UV radiation and charged particles from Saturn’s magnetosphere, which cause molecular nitrogen and methane (the primary components of the upper atmosphere) to generate into various CHN-containing species that deposit onto the surface [2]. Despite the ubiquity of these organic minerals upon the surface, it is difficult to understand their influence on the landscape and as, in some cases, even their crystal structure is unknown let alone wider physical properties[3]. Hence we have undertaken an experimental program to address this, and are currently focusing on the missing crystal structure and physical property understanding of a number of molecular solids and co-crystals that are likely to be organic minerals upon Titan. Using a combination of neutron diffraction, Xray diffraction and Raman scattering we have studied molecular solids including ethane, acrylonitrile, acetonitrile, butadiene and propyne, and explored what co-crystal form from the inventory of Titan’s molecules. This contribution will report highlights from these investigations. © The Authors
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    Hydrogen cyanide and butadiene as cryominerals on the surface of Titan
    (American Geophysical Union (AGU), 2021-12-17) Hodyss, RP; Vu, TH; Maynard-Casely, HE; Cable, ML; Malaska, MJ; Choukroun, M
    The Cassini-Huygens mission has revealed a wide variety of Earth-like landforms on Titan’s surface: plains, mountains, plateaux, dunes, lakes, seas and rivers. Titan’s surface appears to be constructed from organic materials and ice, rather than rocks and minerals that make up Earth’s surface. At a surface temperature of ~92 K, non-covalent interactions such as hydrogen bonding and van der Waals forces are sufficiently strong to enable stable interactions among these organic molecules, which form an entirely new class of cryogenic organic minerals (naturally occurring compounds with a specific composition). Photochemical models, partially validated by Huygens surface measurements and Cassini spacecraft measurements as well as Earth-based observations, allow us to make an initial guess for the composition of Titan’s surface. Simple organic molecules like acetylene, hydrogen cyanide, acetonitrile, etc. in their solid form are expected to be important constituents of the surface. However, many of their crystal structures and properties in solid state, at Titan relevant temperatures, are ambiguous. It is highly likely that crystalline polymorphs of some of these molecules are yet to be discovered. The crystal structure of a solid material is one of its most fundamental properties, and is necessary for understanding of intermolecular interactions and for prediction of mechanical and chemical properties – such as the ability to support deep valleys, high canyon walls, and resistance to erosion. We will present new data on the crystal structure and physical properties of two molecules thought to be present in significant quantities on Titan’s surface: hydrogen cyanide and butadiene. We have used Raman spectroscopy and cryogenic powder X-ray diffraction to better understand the phase behavior and structure of these materials under Titan conditions. While hydrogen cyanide is known to undergo a phase transition at ~170 K, the behavior of butadiene at low temperature has not been explored in detail. Our data indicates a new monoclinic structure for butadiene, and a possible new structure for HCN at low temperature. We will also present the implications of these results for Titan’s geology and evolution.
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    Prospects for organic minerals on Saturn’s moon Titan
    (Society of Crystallographers in Australia and New Zealand, 2017-12-03) Maynard-Casely, HE; Cable, ML; Malaska, MJ; Vu, TH; Choukroun, M; Hodyss, RP
    Titan, the largest moon of Saturn, contains a vast inventory of organic molecules and is considered a prebiotic chemical laboratory on a planetary scale. Active photochemistry in the atmosphere via solar radiation and energy from Saturn’s magnetosphere causes N2 and CH4 to dissociate and recombine, generating organics ranging from simple (ethane, acetylene, HCN) to complex (>10,000 Da) molecules. These molecules continue to react as they move through Titan’s atmosphere, forming aerosol haze layers and eventually depositing on the surface [1]. Additionally, the Cassini spacecraft revealed that Titan has standing bodies of liquid on its surface, in the form of lakes and seas. This is a remarkable discovery, as it makes Titan only the second planetary body known to have such features (after our own Earth). These lakes, which are evidenced to contain mainly methane and ethane, could dissolve many of the molecules that were generated in Titan’s atmosphere. These could subsequently form precipitates and create evaporite deposits similar to those observed by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) and Synthetic Aperture Radar (SAR) around some of the northern lakes [2]. Previous work has demonstrated [3] that two common organic molecules on Titan, ethane and benzene, form a unique and stable co-crystalline structure at Titan surface temperatures, which could comprise these evaporite deposits. Influenced by the discovery of a new solid phase for Titan, a survey has been undertaken outlining the current structural understanding of molecular solids under Titan conditions. Using the Cambridge Structural Database (CSD) a number of possible minerals ‘types’ that would be expected on the surface of Titan have been identified. The subsequent classification of possible Titan minerals is done on the basis of intermolecular interactions, with the materials organised into ‘Molecular solids’, ‘Molecular co-crystals’ and ‘Hydrates’ grouping. This classification is designed to aid future work in determining how a number of the features on Titan may have formed.
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    Titan cryomineralogy: what discoveries in the laboratory can tell us about Titan’s surface
    (American Geophysical Union (AGU), 2021-12-17) Cable, ML; Vu, TH; Malaska, MJ; Choukroun, M; Maynard-Casely, HE; Runcevski, T; Hodyss, RP
    Titan hosts a large, diverse menu of organic molecules and is considered to be a prebiotic chemical laboratory on a planetary scale. Photochemistry in the atmosphere initiates a chemical cascade, starting with the dissociation of N2 and CH4 and generating a wide variety of molecules, ranging from small molecules (acetonitrile, acetylene) to incredibly large macromolecular species (>10,000 Da). We and others have demonstrated previously that some organic molecules readily form co-crystals in Titan-relevant conditions. These molecular minerals, or cryominerals, represent an exciting new class of compounds for Titan’s surface. Here we report on the latest experimental and theoretical characterization of Titan cryominerals, and discuss implications for physical and chemical properties of various terrains on Titan’s surface that could be studied in situ by missions like Dragonfly. Co-crystals may influence Titan surface material characteristics such structural hardness and resistance to erosion. Enhanced thermal expansion and decreased crystal size, for example, may lead to fracturing and/or more rapid erosion of co-crystal-based deposits. Density changes upon co-crystal formation compared to pure compounds may also play a role in organic diagenesis and metamorphism on Titan. Some cryominerals with stability only under certain conditions could preserve evidence of surface evolution and exchange, such as cryovolcanic activity, ethane fluvial/pluvial exposure, or outgassing of CO2 from the moon’s interior. On a hydrocarbon world like Titan where organic chemistry dominates the atmospheric chemistry and surface geology, chemists have the opportunity play a significant role in planetary science discoveries, and likewise, discoveries motivated by planetary science may help inform fundamental organic and physical chemistry research. Plain-language Summary Titan is the largest moon of Saturn, and the only moon in our solar system with a thick atmosphere. Titan has similar weather processes to Earth, including clouds, rain, rivers, and lakes. However, it is too cold for this weather cycle to be driven by water. Instead, methane and ethane, the main components of natural gas, make up the liquids on Titan’s surface. Titan’s surface materials also appear to be made of different ‘stuff’ compared to Earth. Instead of rocks with silica, iron, and other heavy atoms, Titan’s terrain seems to be made up of organic molecules containing carbon, nitrogen, and hydrogen. We have discovered that some of these molecules form crystals that behave like minerals do here on Earth; we call them ‘molecular minerals’ or cryominerals. So far, we and others have reported seven cryominerals, with one more predicted by theoretical modeling. We will give an overview of the different properties of each of these cryominerals, and how that might give us clues as to how Titan’s surface features formed, or might be observed with landed spacecraft like Dragonfly.