Monique Comte’s research while affiliated with Corning Incorporated and other places

Structural, chemical, and microstructural changes in lithium aluminum silicate glass during ceramming are tracked by in‐situ impedance spectroscopy, X‐ray diffraction and transmission electron microscopy. The glass’ electric and dielectric properties are found to evolve during the ceram cycle. Scaling of frequency‐dependent electrical conductivity and dielectric properties provides temperature‐independent information of bulk phases and interfaces in presence. The initial glass is an ion conductor with Li‐ion mobility following simple Arrhenius‐type behavior. At the nucleation onset, a badly defined, broad feature appears in the impedance spectra, that represents early‐stage nuclei with their interfaces and evolves over time into separate contributions from internal interfaces and ion mobility in crystalline phase. Impedance tracking suggests that nuclei form by de‐mixing and phase separation of the glass, first producing interfaces in a very defective structure before the well‐defined structure and chemistry of crystalline β‐quartz crystal develop in a second time. Findings are supported by XRD and TEM.

Glass‐ceramics are materials produced by a twofold process whereby a glass is first formed and then partially crystallized by an appropriate subsequent thermal treatment. With respect to glass materials, glass‐ceramics acquire a complex microstructure during the crystallization step that gives them specific properties and makes a wide range of applications possible. The most widely used systems are lithium aluminosilicates, magnesium and zinc aluminosilicates, lithium silicates, and phosphates for which mechanical properties, resistance to thermal shock, chemical durability, and/or transparency have been optimized. Although some glass‐ceramics can be prepared through homogeneous crystallization, heterogeneous nucleation is generally required to precipitate the intended crystalline phases. With the exception of the surface crystallization used to produce some materials, heterogeneous crystallization generally requires the use of nucleating agents to ensure a uniform distribution of crystals in the bulk. The main nucleating agents are oxides (TiO2 and ZrO2), fluorides, or colloids of noble metals such as Ag, Pt, and Au. Either they precipitate themselves and create nucleating sites for the main crystalline phase or they first induce a phase separation such that one of the new amorphous phases is more prone to crystallization than the starting glass.

Li2O‐Al2O3‐SiO2 (LAS) glass‐ceramics have important industrial applications and bulk nucleation is usually achieved by using nucleating agents. In particular, P2O5 is an efficient agent in glasses containing a low level of Al2O3 but its role in the first stages of nucleation is not well established. In this study, we combine structural investigations from local to mesoscales to describe the structural evolution during crystallization of LAS glass‐ceramics. Local environment is probed using ²⁹Si and ³¹P MAS‐NMR, indicating organization of P in poorly crystallized Li3PO4 species prior to any crystallization. To better understand the detailed nanoscale changes of the glass structure, ³¹P‐³¹P DQ‐DRENAR homonuclear correlation experiments have been carried out, revealing the gradual segregation of P atoms associated with the formation of disordered Li3PO4. Small‐angle neutron scattering data also show the apparition of nanoscale heterogeneities associated with Li3PO4 species upon heating treatments and allow the determination of their average sizes. These new structural information enhance our understanding of the role of P in nucleation mechanisms. Nucleation is initiated by gradual change in P environment implying P segregation upon heating treatments, forming disordered Li3PO4 heterogeneities. The segregation of P atoms enables the precipitation of meta‐ and disilicate phases.

The influence of zirconium as a nucleating agent on the congruent crystallization and relevant physical properties of a supercooled calcium aluminosilicate melt of a composition close to CaAl2SiO6 has been investigated up to 6 mol% ZrO2. Zirconium marginally affects rheological and structural properties, decreasing the viscosity of the Zr-free melt by no more than 0.25 log unit and, as observed by Raman spectroscopy, not changing significantly the polymerization state of the material. Whereas the Zr-free melt crystallizes congruently and heterogeneously from the sample surface to yield yoshiokaite, a stuffed derivative of the nepheline structure, addition of zirconia promotes instead bulk crystallization of tetragonal ZrO2 and then of yoshiokaite. The latter process takes place in two stages: dissolved Zr first promotes homogeneous precipitation of zirconia before yoshiokaite crystallizes congruently from a Zr-depleted volume of melt around zirconia precipitates. This process makes zirconium, and probably other poorly soluble oxides, valuable to control congruent crystallization in silicate glass-ceramics. From the recorded thermograms, an enthalpy of crystallization of 40 and 46 kJ/mol has been determined at 1060 and 1140 K, respectively, for CaAl2SiO6 yoshiokaite, a very low value that is likely due to the extensive atomic disorder of crystals precipitating at high degrees of supercooling.

We studied the nucleation and growth of nano-sized crystals on two glass-ceramic systems: a conventional lithium-aluminosili-cate (LAS) and a (Mg,Zn) spinel. We combined several techniques: in situ Small Angle Neutron Scattering (SANS), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and laboratory X-ray diffraction (XRD). We observed by SANS, and confirmed by DSC, that during a temperature ramp, transient phenomena occur between the regions of nucleation and growth in the LAS, which do not follow classic kinetic theories. In contrast, the spinel material shows a smooth transition during the temperature ramp between the nucleation and the growth stages, and follows a more conventional growth pattern. In the spinel system the initial phase separation plays a very important role in determining the crystalline phase distribution in the glassy matrix, as crys-tallites are confined only in one phase.

The crystallization kinetics of a commercial lithium-aluminum silicate (LAS) glass were characterized by differential scanning calorimetry (DSC) under non-isothermal conditions, by in situ X-ray diffraction, and by three point beam bending viscosimeter (BBV). Non-isothermal DSC experiments were conducted at different heating rates. Results show that the crystal growth is controlled by a thermally activated process of the Arrhenius type. The activation energies obtained from isoconversional analysis are close to that extracted using the Johnson–Mehl–Avrami equation. While X-ray diffraction volume fraction data confirm the DSC analysis, it also shows that the crystallite size changes only at the end of the heat treatment protocol, during a hold at temperatures as high as 1000°C. In this latter case, the crystal growth follows the Ostwald ripening mechanism. Finally, the viscosity measured in the crystallization region by BBV provides the activation energy for viscous flow, and it is slightly higher than the values obtained by DSC.

Nucleation is the first step of the transition between the amorphous and crystalline states and thus plays a key role in Earth and Materials sciences whenever crystallization takes place. In spite of its considerable importance in igneous petrology and industrial applications (ceramics, glass-ceramics, etc.), nucleation remains known poorly because of the difficulties of investigating the structural rearrangements that take place at a nm scale when an ordered atomic packing begins to develop in a melt. In addition, the structure of amorphous phases is not only difficult to determine, but the wealth of information available for glasses is not necessarily applicable to nucleation because of the existence of temperature-induced structural changes in melts. In view of the basic geological and industrial importance of the SiO2-Al2O3-CaO system, we have investigated a calcium aluminosilicate whose crystallization has already been studied. And because elements such as Ti or Zr can promote rapid nucleation, information can be gained about the structural changes they induce by probing specifically their own environment. In this work we have thus performed a high-temperature study of the very first steps of crystallization in a calcium aluminosilicate with 7 mol percent ZrO2 by X-ray absorption measurements at the Zr K-edge et 1873 K on the homogenous melt and 1173 K on a nucleating supercooled liquid. To complement these results with information on medium range order (MRO) X-Ray diffraction experiments have also been performed under the same conditions. As a reference, the glass has been investigated by both techniques at room temperature.