Configurational constraints on glass formation in the liquid calcium aluminate system

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Structure of levitated Si-Ge melts studied by high-energy x-ray diffraction in combination with reverse Monte Carlo simulations

The short-range order in liquid Si, Ge and binary Six-Ge1-x alloys (x = 0.25, 0.50, 0.75) was studied by x-ray diffraction and reverse Monte Carlo simulations. Experiments were performed in the normal and supercooled liquid states by using the containerless technique of aerodynamic levitation with CO2 laser heating, enabling deeper supercooling of liquid Si and Si-Ge alloys than previously reported. The local atomic structure of liquid Si and Ge resembles the beta-tin structure. The first coordination numbers of about 6 for all compositions are found to be independent of temperature indicating the supercooled liquids studied retain this high-density liquid (HDL) structure. However, there is evidence of developing local tetrahedral ordering, as manifested by a shoulder on the right side of the first peak in S(Q) which becomes more prominent with increasing supercooling. This result is potentially indicative of a continuous transition from the stable HDL beta-tin (high pressure) phase, towards a metastable low-density liquid phase, reminiscent of the diamond (ambient pressure) structure

Structure from glass to melt: a case study along the MgSiO3_{{3}}–CaSiO3_{{3}} join using neutron and X-ray diffraction

Glass and melt structures are inherently complex and disordered with significant changes expected to occur with temperature. In the present paper, a comparison of the structure of glasses and liquids along the MgSiO3–CaSiO3 join is carried out using neutron and X-ray diffraction (XRD). Empirical Potential Structure Refinement (EPSR) simulations were used to fit the experimental data. The average coordination number (CN) and site distribution is obtained for Mg and Ca showing distinct sites between the two cations and higher coordinated sites in the liquids. The major glass to melt modifications is observed at the scale of intermediate range order (IRO) by rearrangement of the (Ca,Mg)–Si and (Ca,Mg)–(Ca,Mg) connections. The structural evolution with temperature, especially concerning the cationic environments, illustrates the differences between glass and melt organization. These changes highlight the important contribution of cations to thermodynamical properties, diffusion and glass forming ability

Structure of liquid tricalcium aluminate

International audienceThe atomic-scale structure of aerodynamically levitated and laser-heated liquid tricalcium aluminate (Ca 3 Al 2 O 6) was measured at 2073(30) K by using the method of neutron diffraction with Ca isotope substitution (NDIS). The results enable the detailed resolution of the local coordination environment around calcium and aluminum atoms, including the direct determination of the liquid partial structure factor, S CaCa (Q), and partial pair distribution function, g CaCa (r). Molecular dynamics (MD) simulation and reverse Monte Carlo (RMC) refinement methods were employed to obtain a detailed atomistic model of the liquid structure. The composition Ca 3 Al 2 O 6 lies at the CaO-rich limit of the CaO:Al 2 O 3 glass-forming system. Our results show that, although significantly depolymerized, liquid Ca 3 Al 2 O 6 is largely composed of AlO 4 tetrahedra forming an infinite network with a slightly higher fraction of bridging oxygen atoms than expected for the composition. Calcium-centered polyhedra exhibit a wide distribution of four-to sevenfold coordinated sites, with higher coordinated calcium preferentially bonding to bridging oxygens. Analysis of the MD configuration reveals the presence of ∼10 % unconnected AlO 4 monomers and Al 2 O 7 dimers in the liquid. As the CaO concentration increases, the number of these isolated units increases, such that the upper value for the glass-forming composition of CaO:Al 2 O 3 liquids could be described in terms of a percolation threshold at which the glass can no longer support the formation of an infinitely connected AlO 4 network

Neutron diffraction of calcium aluminosilicate glasses and melts

International audienceThe combination of neutron diffraction with aerodynamic levitation and laser heating, pioneered by Neville Greaves and co-workers about 15 years ago, is an important tool for studying the structure of liquid melts. Since the first work on liquid Al2O3 published in 2001, the technique has been largely improved and experiments are now routinely performed at neutron sources, providing interesting structural information on various materials.In this paper, the structure of glass-forming compounds in the system CaO-Al2O3-SiO2 was measured by applying neutron diffraction with aerodynamic levitation. Results obtained in the liquid state above the melting point and from the glass at room temperatures are presented. Various compositions were studied by increasing the silica content and by changing the ratio CaO/Al2O3. As observed using other methods, the main structural changes relate to modification of the Al-O short range order

Structure from glass to melt: a case study along the MgSiO3_{{3}}–CaSiO3_{{3}} join using neutron and X-ray diffraction

Glass and melt structures are inherently complex and disordered with significant changes expected to occur with temperature. In the present paper, a comparison of the structure of glasses and liquids along the MgSiO3–CaSiO3 join is carried out using neutron and X-ray diffraction (XRD). Empirical Potential Structure Refinement (EPSR) simulations were used to fit the experimental data. The average coordination number (CN) and site distribution is obtained for Mg and Ca showing distinct sites between the two cations and higher coordinated sites in the liquids. The major glass to melt modifications is observed at the scale of intermediate range order (IRO) by rearrangement of the (Ca,Mg)–Si and (Ca,Mg)–(Ca,Mg) connections. The structural evolution with temperature, especially concerning the cationic environments, illustrates the differences between glass and melt organization. These changes highlight the important contribution of cations to thermodynamical properties, diffusion and glass forming ability

From Molten Calcium Aluminates through Phase Transitions to Cement Phases

Crystalline calcium aluminates are a critical setting agent in cement. To date, few have explored the microscopic and dynamic mechanism of the transitions from molten aluminate liquids, through the supercooled state to glassy and crystalline phases, during cement clinker production. Herein, the first in situ measurements of viscosity and density are reported across all the principal molten phases, relevant to their eventual crystalline structures. Bulk atomistic computer simulations confirm that thermophysical properties scale with the evolution of network substructures interpenetrating melts on the nanoscale. It is demonstrated that the glass transition temperature (T-g) follows the eutectic profile of the liquidus temperature (T-m), coinciding with the melting zone in cement production. The viscosity has been uniquely charted over 14 decades for each calcium-aluminate phase, projecting and justifying the different temperature zones used in cement manufacture. The fragile-strong phase transitions are revealed across all supercooled phases coinciding with heterogeneous nucleation close to 1.2T(g), where sintering and quenching occur in industrial-scale cement processing

Structural transformations on vitrification in the fragile glass-forming system CaAl₂O₄

International audienceThe structure of the fragile glass-forming material CaAl 2 O 4 was measured by applying the method of neutron diffraction with Ca isotope substitution to the laser-heated aerodynamically levitated liquid at 1973(30) K and to the glass at 300(1) K. The results, interpreted with the aid of molecular dynamics simulations, reveal key structural modifications on multiple length scales. Specifically, there is a reorganization on quenching that leads to an almost complete breakdown of the AlO 5 polyhedra and threefold coordinated oxygen atoms present in the liquid, and to their replacement by a predominantly corner-sharing network of AlO 4 tetrahedra in the glass. This process is accompanied by the formation of branched chains of edge and face-sharing Ca-centered polyhedra that give cationic ordering on an intermediate length scale, where the measured coordination number for O around Ca is 6.0(2) for the liquid and 6.4(2) for the glass. Calcium aluminates ðCaOÞ x ðAl 2 O 3 Þ 1Àx (0 x 1) have been extensively studied on account of their geological , technological, and scientific importance [1-20]. For example, they are a significant component of the Earth's mantle so that the liquid structure is of interest for understanding magma-related processes [21], they are an integral component of aluminous cement [22], the glasses have a favorable infrared transmission window that extends up to a wavelength $6 m [23] giving them optical applications [24,25], and the rare-earth-metal-doped materials exhibit persistent luminescence [26]. From a glass physics perspective, calcium aluminates are very fragile glass for-mers [1,4] and, in contrast to strong network glass formers such as SiO 2 , large structural alterations should accompany the rapid change in viscosity and other dynamical properties as the glass transition temperature T g is approached [27]. Experimental information on the extent of structural transformation is therefore essential to understanding the processes occurring around T g and the material properties to which they are linked. An experimental exploration of liquid aluminates is, however, challenging because of the high temperatures involved. The containerless method of aerodynamic levitation offers a way forward, and by minimizing heterogeneous nucleation, it extends the narrow glass-forming region centered at x ¼ 0:65 in the calcium aluminate system to include the equimolar composition CaAl 2 O 4 [16] which has a fragility index of m ¼ 116 [1,28]. At this composition , the O:Al ratio is 2:1 such that it is just feasible to form an ideal network of fully connected corner-sharing AlO 4 tetrahedra where the oxygen atoms are twofold coordinated, as in the crystalline phase which has a tridymite-like structure where the tetrahedra form a fully polymerized network of six-membered rings [29]. This has motivated a range of experimental and computer simulation studies on the liquid and glass structure [2-20]. It has, however, proved difficult to measure unambiguously the Al and Ca coordination environments. For example, in the liquid state 27 Al nuclear magnetic resonance (NMR) experiments observe the fast exchange limit such that individual Al coordination environments cannot be identified [2-5], and in diffraction experiments , the nearest-neighbor Ca-O and other pair correlations are strongly overlapped [16-19]. The powerful method of neutron diffraction with isotope substitution has been used to probe directly the coordination environment of Ca in ðCaOÞ 48 ðSiO 2 Þ 49 ðAl 2 O 3 Þ 3 glass [30,31], but the method is usually limited to large samples [32]. In this Letter we show, however, that the neutron diffraction with isotope substitution method can be used to measure the detailed atomic structure of a single aerodynamically levitated laser-heated drop of liquid CaAl 2 O 4 at 1973(30) K. The structure of the glass at 300(1) K is also investigated. The results, interpreted with the aid of molecular dynamics (MD) simulations, characterize the nature of the structural transformations that occur on vitrification on both the local and intermediate atomic length scales. The total structure factor measured by neutron diffraction is given by FðQÞ ¼ P P c c b b ½S ðQÞ À 1

Joint diffraction and modeling approach to the structure of liquid alumina

The structure of liquid alumina at a temperature ∼2400 K near its melting point was measured using neutron and high-energy x-ray diffraction by employing containerless aerodynamic–levitation and laser-heating techniques. The measured diffraction patterns were compared to those calculated from molecular dynamics simulations using a variety of pair potentials, and the model found to be in best agreement with experiments was refined using the reverse Monte Carlo method. The resultant model shows that the melt is composed predominantly of AlO4 and AlO5 units, in the approximate ratio of 2:1, with only minor fractions of AlO3 and AlO6 units. The majority of Al-O-Al connections involve corner-sharing polyhedra (83%), although a significant minority involve edge-sharing polyhedra (16%), predominantly between AlO5 and either AlO5 or AlO4 units. Most of the oxygen atoms (81%) are shared among three or more polyhedra, and the majority of these oxygen atoms are triply shared among one or two AlO4 units and two or one AlO5 units, consistent with the abundance of these polyhedra in the melt and their fairly uniform spatial distribution