1,721,064 research outputs found
Structural and elastic behaviour of aragonite at high-pressure: A contribution from first-principle simulations
Full text linkAragonite (CaCO3, space group Pmcn) is an important mineral for both geological and biological reasons, being one of the phases that recycles carbon in deep Earth conditions and the product of biomineralization of several terrestrial and marine organisms, respectively. Because of its ubiquity, aragonite has been the subject of several investigations to understand its elastic behaviour and stability at different P-T conditions, but the results reported in literature are still very scattered. Aiming at providing further details on this topic, in the present work we determined the structural and elastic properties of aragonite at absolute zero (0 K) within the Density Functional Theory framework, using a posteriori correction to include the weak long-range interactions. The equation of state parameters for this mineral phase, calculated between 0 GPa – 25 GPa, were K0 = 80.2(7) GPa, K’ = 4.37(10) and V0 = 223.00(6) Å3, in good agreement with the bulk modulus calculated from the elastic moduli (KR = 78.49 GPa). The results were compared to previous experimental and theoretical data, finding them in line with some specific studies, and show that some structural features (e.g., the carbonate ion aplanarity) could be related to the mechanism of phase transition to the post-aragonite phase at high pressure. The present work highlights the importance of including van der Waals interactions in the physical treatment of the structural and elastic properties of aragonite, and further extends the knowledge of the behaviour of this mineral as a function of pressure
QUANTAS: a Python software for the analysis of thermodynamics and elastic behavior of solids from ab initio quantum mechanical simulations and experimental data
Full text linkMineralogy, petrology and materials science are fundamental disciplines not only for the basic knowledge and classification of solid phases but also for their technological applications, which are becoming increasingly demanding and challenging. Characterization and design of materials are of utmost importance and usually need knowledge of the thermodynamics and mechanical stability of solids. Alongside well known experimental approaches, in recent years the advances in both quantum mechanical methods and computational power have placed theoretical investigations as a complementary useful and powerful tool in this kind of study. In order to aid both theoreticians and experimentalists, an open-source Python-based software, QUANTAS, has been developed. QUANTAS provides a fast, flexible, easy-to-use and extensible platform for calculating the thermodynamics and elastic behavior of crystalline solid phases, starting from both experimental and ab initio data
Equation of state and second-order elastic constants of portlandite Ca(OH)2 and brucite Mg(OH)2
No full textHydroxide minerals brucite Mg(OH) 2 and portlandite Ca(OH) 2 (space group P3 ̄ m1) are very important phases for several geological and industrial applications which often require the knowledge of the mechanical properties. In the present work, the equation of state (EoS) and the second-order elastic constants of the two minerals were calculated by ab initio quantum mechanical methods. The aims are extending the knowledge of their important applicative mechanical properties and providing a consistent relative dataset. In addition, the simple crystal-chemical composition and structure of Ca(OH) 2 and Mg(OH) 2 is ideal to simulate and characterize the effect of the proton disorder on the elastic properties, which could be useful for the comprehension of more complex hydrous minerals and synthetic phases. The third-order Birch–Murnaghan EoS parameters obtained in the present study were V 0 = 39.59(1) Å 3 , K 0 = 48.0(9) GPa and K′ = 9.1(3), and V 0 = 54.0(7) Å 3 , K 0 = 30.1(9) GPa and K′ = 8.5(4) for Mg(OH) 2 and Ca(OH) 2 , respectively. Axial compressibilities were found to be in ratio β(a):β(c) = 1.000:3.600 for brucite, and 1.000:3.777 for portlandite. The theoretical results agree with the general trend experimentally observed in the available literature, and further extend the knowledge of the mechanical properties of the two phases. The results could be very helpful for petro-geological investigations and for the synthesis and use of concrete nanocomposites and layered double hydroxides with tailored mechanical properties
Thermodynamic and thermoelastic data of georesources raw minerals: Zinc sulphide and apatite
No full textThis article reports a dataset on the thermodynamic and elastic properties of two important raw minerals exploited in georesources and ore mining. The presented data refers to two zinc sulphide polymorphs, namely zinc-blende (low-pressure polymorph, space group F4−3m) and rock-salt (high-pressure polymorph, space group Fm3−m) [1], and of type-A carbonated apatite, [CAp, Ca10(PO4)6CO3, space group P1] [2]. The data here reported were calculated from ab initio quantum mechanical simulations at the DFT/B3LYP level, all-electron Gaussian-type orbitals basis sets and from the analysis of the phonon properties of the zinc sulphide polymorphs and of type-A CAp by means of the quasi-harmonic approximation. In addition, a correction to take into account the effects of dispersive forces was considered to obtain the dataset of type-A carbonated apatite. This dataset, which was validated against experimental thermodynamic data reported in literature, has been employed to construct the phase diagram between the two zinc sulphide polymorphs and discuss their stability over the temperature and pressure range 0–800 K and 0–25 GPa. The thermodynamic and thermoelastic data of CAp were obtained between 0 and 600 K and 0–3 GPa, below the temperature of thermal decomposition of the mineral. The reported data can be of use in several application fields, for instance fundamental georesource exploration and exploitation, and also in applied mineralogy, geology, material science, and as a reference to assess the quality of other theoretical approaches. Furthermore, the data of type-A carbonated apatite could be useful for designing and processing new biomaterials with tailored properties
Study of the contrast in electric force microscopy images of RuO2-based thick-film resistors
No full textThe contrast mechanism of electric force microscopy (EFM) operating in static and dynamic modes have been investigated and applied to the clarification of the electrical conduction properties of RuO2-based thick-film resistors. Both the magnetic and the electrical contributions to the overall EFM signal and the corresponding contrast have been analysed and compared by using different types of atomic force microscopy tip (with a magnetic coating and with a Pt/Ir coating). It has been found that the EFM contrast changes on inverting the voltage polarity of the samples. The regions surrounding the RuO2 grains present an EFM signal which is lower for a negative bias than for a positive bias at low values of the applied voltage; this signal difference tends to disappear on increasing the absolute bias value. This behaviour, typical of semiconductors, ascribes to the above regions semiconducting properties
3D meso-nanostructures in cleaved and nanolithographed Mg-Al-hydroxysilicate (clinochlore): Topology, crystal-chemistry, and surface properties
No full textThe peculiar physico-chemical properties that a solid surface can present, unlike the bulk, are the key for a huge amount of important and widespread processes, including for instance, contaminant and biomolecules adsorption, solid-state and ion exchange reactions, soil aggregation, adhesion in micro and nanodevices. In this regard, the development of new materials and three-dimensional nanofabrication technologies becomes a fundamental challenge. Here, the authors present both natural and synthetic three-dimensional meso-nanostructures of a particular Mg-Al-hydroxysilicate (mineral clinochlore) produced by scanning probe microscopy related methods. Topology, crystal-chemistry, and surface properties were addressed by experimental and theoretical methodologies on nanocleaved and nanolithographed clinochlore. Scanning probe microscopy revealed a meso-nanostructured heterogeneous surface in terms of morphology, hydrophilic/phobic character and surface potential. The possibility to arbitrarily tailor and fabricate surface nanopatterns by SPM-based nanolithography is reported. Quantum mechanical simulations of the crystal-chemical structure and material properties supported and corroborated the experimental data. These findings suggest that the heterogeneous three-dimensional meso-nanostructured surface of clinochlore can represent an optimal effective substrate for exploring specific and innovative catalytic, electrochemical and biological processes at the nanoscale
Amino acids-clay interaction at the nano-atomic scale: The L-alanine-chlorite system
No full textFor technological purposes there is an urge towards the fundamental knowledge of biomolecules-clay interaction at a single molecular level. To this aim, in this work we investigated in detail the specific interaction between the basic amino acid L-alanine and the (001) clinochlore surface. The adsorption process of alanine was investigated by cross-correlated atomic force microscopy (AFM) and quantum mechanics simulations (QM) at the DFT/B3LYP level of theory. As similarly deduced for glycine, clinochlore surface was able to condense, organize, and agglomerate alanine molecules onto specific regions, the brucite-like ones. The amino acid was stably adsorbed onto the brucite-like surface of clinochlore, organized in dot-like structures, agglomerates and filament-like structures up to about 150 nm long. Single molecules of alanine were discriminated by AFM, with sizes in agreement with QM calculations (mean height of about 5 ± 0.5 Å and a mean width of about 11 ± 1 nm). Simulations provided also the lowest energy conformation of the molecule on the surface, the intermolecular bonding scheme and the amino acid/surface interaction energy, which could be as high as −250 kJ/mol. To further extend the knowledge of the specificity of the brucite-like sheet to interact with biomolecules, dedicated quantum mechanical investigations were also conducted to quantify the Brønsted−Lowry basic strength related to Al 3+ /Mg 2+ substitutions in the brucite-like sheet, typically occurring in this kind of clay mineral, simulating the interaction with probe molecules with increasing acidic strength (H 2 O, H 2 CO 3 , HNO 3 and HCl). The reported findings could be useful not only for biotechnological and environmental purposes, but also for prebiotic chemistry research, because clinochlore is one of the 420 mineral species likely to have been present in Earth's near-surface environment at the time of life's origins (Hazen, 2013)
SEM-EDS MICROANALYSIS OF ULTRATHIN GLASS AND METAL FRAGMENTS: MEASUREMENT STRATEGY BY MONTE CARLO SIMULATION IN CULTURAL HERITAGE AND ARCHAEOLOGY
Full text linkScanning electron microscopy (SEM) combined with energy dispersive X-ray spectrometry (EDS) has a very wide range of applications in cultural heritage and archaeology, because of the capability to provide morphological analysis with high spatial resolution, combined with chemical information at the microscale. However, when the size of the materials analyzed approaches the micro- and submicrometre scale, as often found in cultural heritage and archaeology investigations, several effects related to electron and X-ray generation and transport had to be considered to avoid quantification errors. In this work, Monte Carlo simulations are presented for the study of the effects of thickness and shape on quantitative microanalysis by SEM-EDS of ultrathin glass and metal alloys fragments, as usually found in cultural heritage and archaeology. Glass fragments with different chemical composition, elongated shapes, square section and thicknesses from 0.1 to 10 micrometers, and micro/nanoscale gold alloy fragments were simulated in realistic experimental conditions. The simulations showed an important contribution from the fragments thickness and shape on the X-ray intensity measured by EDS, which in turn affect the quantitification procedure. The results of this study are of general meaning and application, and can be used to develop the most appropriate specific measurement strategy and avoid analytical errors and misinterpretations
Correction factors for the effect of shape and thickness of SEM-EDS microanalysis of asbestos bundles and fibres by Monte Carlo simulation
No full textSEM-EDS quantitative microanalysis of asbestos mineral fibres still represents a complex analytical issue because of the variable fibre shape and small thickness (<5 μm) compared with the penetration of the incident electron beam. Size and shape of micro- and sub-micrometric particles may cause large errors in chemical quantification due to particle effects on the generation and measurement of X-rays intensity from the sample. These effects are related to the elastic scattering of electrons in the finite size (mass) of the fibre, with the scattering being strongly influenced by the average atomic number. For a given mean atomic number, the thickness of the particle is the main factor affecting X-rays intensity, with a component related to the particle shape that biases the contributions of absorption and fluorescence to the correction routine. To overcome these issues empirical methods were developed, however they are cumbersome and need characterized standards for thickness, geometry and composition. Here we present the results of Monte Carlo investigation for the thickness and shape effects on SEM-EDS and microprobe analysis of asbestos bundles. Crocidolite, amosite, tremolite-asbestos, chrysotile, anthophyllite-asbestos and actinolite-asbestos were simulated and correction factors for X-ray microanalysis were proposed. Monte Carlo simulation was used to investigate electron transport, X-ray generation and detection in asbestos bundles of variable thickness lying on a pure carbon holder. We report the results obtained on 100 μm long bundles of fibres of square section and thicknesses from to 0.1 μm to 10 μm. Realistic experimental conditions, such as sample geometry, SEM set-up and detector physics were taken into account. An electron probe of 40 nm in diameter was simulated, focussed in parallel illumination onto the surface of the bundle of fibres, in a mid position with respect to the edges. The modelled EDS detector has a resolution of 130 eV measured at Mn Kα, an elevation angle of 40°, and an azimuthal angle of 0°. The influence of thickness and shape on the simulated spectrum was investigated for electron beam energies of 5, 15 and 25 keV. A strong influence of the asbestos bundles thickness was observed. In general, the X-ray intensities as a function of bundle thickness showed a considerable reduction below about 0.5 μm at 5 keV, 2 μm at 15 keV, and 5 μm at 25 keV, with a non-linear dependence. Specific correction parameters, k-ratio, for the asbestos bundle thickness effect are here presented
First principle investigation of the thermomechanical properties of type A carbonated apatite
No full textCalcium apatites with the general chemical formula (Ca, X)10(PO4, Y)6Z2 represent a mineral family of utmost importance in several fields, for example, bone biology, biomaterials, and mineralogy. However, few works have focused on the mechanical properties of these phases, and in particular, no data are available on the thermomechanical and thermodynamic properties of carbonate-bearing (hydroxyl)apatites. In the present work, the equation of state of type A carbonated apatite (CAp, Ca10(PO4)6CO3, space group P1) was calculated by ab initio quantum mechanical methods within the density functional theory (DFT) framework. Starting from athermal results (at 0 K), the combined effect of temperature and pressure was investigated through the quasiharmonic approximation (QHA). In athermal conditions, the equation of state of the CAp unit cell volume can be described by a third-order Birch-Murnaghan formulation, with parameters V0 = 538.14(5) Å3, K0 = 106.2(7) GPa, and K′ = 4.6(4). The QHA well described the temperature and pressure dependence of the thermodynamics and mechanical properties of the mineral. For instance, the bulk modulus at 0 GPa and ambient temperature (300 K) is KT0 = 102.95 GPa, which is lower than that of stoichiometric apatite by about 6%. The unit cell thermal expansion coefficient between 0 and 600 K was also calculated and reported. The results are in line with the few available experimental data reported in literature on type AB carbonated hydroxylapatite. The reported findings further extend the knowledge of the mechanical and thermal behaviors of this important mineral found in biological environments, results that are useful for biotechnological and other applications of the (C)OHAp phases
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