# Microstructural insights on lithium aluminum silicate (LAS) glass ceramics

Maxence Vigier, Philippe Deniard, Eric Gautron, Nicolas Gautier, Cécile Genevois, Sandra Ory, Mathieu Allix, Ilyes Ben Kacem, Stéphane Jobic

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Maxence Vigier, Philippe Deniard, Eric Gautron, Nicolas Gautier, Cécile Genevois, et al., Microstructural insights on lithium aluminum silicate (LAS) glass ceramics. *Ceramics International*, 2024, 50 (16), pp. 29011-29015. 10.1016/j.ceramint.2024.05.135. hal-04636060v2

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Short communication

# Microstructural insights on lithium aluminum silicate (LAS) glass ceramics

<figure><img src="image_2.png" /></figure>

Maxence Vigier<sup>a,b</sup>, Philippe Deniard<sup>b,**</sup>, Eric Gautron<sup>b</sup>, Nicolas Gautier<sup>b</sup>, Cécile Genevois<sup>c</sup>, Sandra Ory<sup>c</sup>, Mathieu Allix<sup>c</sup>, Ilyes Ben Kacem<sup>a</sup>, Stéphane Jobic<sup>b,*</sup>

## ARTICLE INFO

Handling Editor: Dr P. Vincenzini

**Keywords:**

Glass-ceramics

LAS

Electron microscopy

Crystal orientation mapping

Hard sphere model

## ABSTRACT

LAS glass-ceramics stimulate considerable attention in academic and industrial fields due to exceptional properties, such as low thermal expansion coefficient, transparency or superior mechanical strength. We report here an experimental investigation of LAS structure and microstructure using conventional techniques as XRD or S/TEM, but also an innovative technique based on electron diffraction mapping. The later gives a topography of the sample and a clear picture of the distribution between the glass and the spodumene particles. All the data converge towards a model of hard spheres where 75 % of the volume is composed of spheroid particles and 25 % of the remaining volume is composed of glass, which is present in the inter-particle interstices. These findings provide a new knowledge about the LAS system and may offer useful guidance for other researchers in ceramic community.

## 1. Introduction

Glass-ceramics (GC) are composite materials that combine in once a crystalline and a glassy phase [1,2]. The great advantage of such materials compared to glasses or ceramics taken separately is their ability to combine the desirable properties of both components. Hence, the amorphous part of GC allows for the ease of fabrication and processing, while the synergy between the glass and crystalline components may confer to the composite high mechanical strength (high fracture strength and toughness), thermal stability, and chemical durability in comparison to glass. Glass-ceramics can also exhibit transparency or translucency, as well as opacity.

During the last five decades, GC materials have been the subject of intense researches due to their unique properties and applications in various domains [3–5]. In optics (e.g., telescope mirror substrate blanks [6]), cooking (e.g., cook top panel, cookware [7]), electronic packaging [8], dentistry [9] and transparent heat protection (e.g., oven window [10]), Li₂O–Al₂O₃–SiO₂ glass-ceramics (hereafter labelled LAS) specifically received much attention [11].

Namely, the LAS GC system is considered so far as one of the most important GC materials due to several outstanding properties, such as very low thermal expansion coefficient (near 10⁻⁷ K⁻¹ in the 0–300 °C

range), high transparency when desired, excellent chemical resistance, high mechanical strength, versatility in composition to access specific characteristics, and the ability to be polished [2,5,6,11–18]. The main reported crystalline phases observed in LAS GC vs. synthesis conditions are γ-LiAlSi₂O₆ and β-LiAlSi₂O₆ spodumene, β-LiAlSiO₄ eucryptite, SiO₂ keatite (an isomorphous form of β-spodumene), and a series of species belonging to the so called β-quartz solid solution (also named stuffed high-quartz solid solution) that correspond to Al-substituted β-SiO₂ compounds isomorphous with β-LiAlSiO₄ (let us notice that the labeling may be different for identical phases in the literature) [11].

As the demand for tailored materials with precise microstructural control continues to rise, it becomes increasingly imperative to delve deeper the LAS GC (micro)structure using Scanning/Transmission Electron Microscopy (S/TEM) that combines imaging, diffraction and spectroscopies at high spatial resolution. Hence, LAS glass-ceramics were examined for different thermal treatments or chemical compositions rate of major (Si, Al, Li) or minor (Zn, Na, Ca) common or exotic (B, La, Er, Y, La) constituents with ZrO₂, TiO₂ or ZrTiO₄ as nucleating agents [19–34].

In most of the work carried out on LAS, the structure is dominated by the presence of nanosized, more or less spherical, particles of spodumene [11,26,33]. Depending on the heat treatment used or the Li

<footnote><sup>a</sup> Arc France, 104 rue du Général de Gaulle, Arques, 62510, France</footnote>

<footnote><sup>b</sup> Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000, Nantes, France</footnote>

<footnote><sup>c</sup> Conditions Extrêmes et Matériaux: Haute Température et Irradiation (CEMHTI), UPR 3079 CNRS, 45100, Orléans, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: philippe.deniard@cnrs-imn.fr (P. Deniard), stephane.jobic@cnrs-imn.fr (S. Jobic).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Differential scanning calorimetry curve for a LAS glass heated at 10 °C/min (a) and STEM-ADF of the LAS GC (b) with a zoom-in of the red square zone at two different camera lengths (L = 1.150 (c) and 0.145 m (d)). For low L, the contrast observed is proportional to Z², i.e. the square of the atomic number. Conversely, for larger L, the contrast is linked to diffraction conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

concentration, these particles are well separated from each other or in contact. To date, only S/TEM images have been proposed to picture the structure of crystalline phases of LAS GC. Nevertheless, it is very difficult to observe directly the distribution of glass and crystals only with im-aging. This article looks at LAS glass-ceramic structures using a state-of-the-art microscopy technique rarely employed so far in the GC community, i.e. the electron diffraction phase mapping of the crystals in the GC with a resolution down to 2 nm.

## 2. Material and method

Glass raw materials were prepared with a 66SiO₂-22Al₂O₃-4Li₂O-1ZrO₂-3TiO₂-1.5BaO-1.4ZnO nominal composition (major elements expressed as weight percent oxides) from the respective binary oxides (except for Li) and lithium carbonate. Precursors were mixed and heated at 1500 °C for 48 h in air. The so-formed glass was then annealed at 550 °C for 4 h to relax internal stresses. To form GC, LAS parent glasses were heated up to 900 °C for 15 min with a 3 °C/min rate and quenched in air.

The thermal analysis curves of the glass-ceramics were measured at a constant heating rate of 10 °C/min between 500 and 1000 °C (ATG/DSC 3+, Mettler Toledo). Crystalline phases (structure and microstructure) and purity of the prepared samples were determined by X-ray powder diffraction (XRD). Patterns were collected at room temperature on a Bruker D8 Advance instrument (monochromatic Cu K-L₃ (λ = 1.540598 Å) X-ray radiation and LynxEye detector) in the 15°–85° 2θ range with a 0.0078° step and 0.97s/step. The quantification of amorphous part was carried out using MgO as standard [35,36].

Scanning/Transmission Electron Microscopy (S/TEM) characterization was performed on a Cs-probe corrected Themis Z G3 (Thermo Fisher Scientific). To limit sample degradation under electron irradiation (see S-I.1), first images were acquired at 80 kV and low temperature

(~178 °C) with a low probe current (~15 pA) and a small convergence angle (4.1 mrad). Second, in order to get image contrasts relative either to atomic number (i.e. proportional to Z²) or to diffraction patterns (i.e. linked to the orientation of crystals), S/TEM Annular Dark Field (ADF) images of the same area were collected at two different camera lengths (CL) to have different collection angles (5–32 mrad at 1.15 m CL, and 41–200 mrad at 0.145 m CL, respectively). The sample was crushed in ethanol in an agate mortar to obtain ca. 100 nm thick grains. A drop of this solution was then deposited on a gold grid covered with a thin amorphous lacey carbon film. Phase mapping was performed using the ASTAR tool (Automated Crystal Orientation Mapping technique, ACOM-TEM Nanomegas), at low temperature (103 K) with a 2 nm step size.

## 3. Results

Fig. 1 displays the differential scanning calorimetry (DSC) curve for a LAS glass in the 500–1000 °C domain and STEM-ADF images of the LAS GC. The glass transition temperature (Tg) collected on a monolithic sample occurs at 676 °C (Fig. 1a). Two exothermic peaks appear at ca. 814 °C and 855 °C. They are associated respectively with the nucleation and crystallization of a Li aluminosilicate [37], that turns to be spodumene (vide infra). After a monitored heat treatment up to 900 °C, the resulting glass-ceramic is brown-yellowish and transparent (see inset in Fig. 1a). Let us notice here that prior to this germination/crystallization step, on the basis of *in situ* TEM micrographs collected between 25 °C and 600 °C (i.e., before Tg in ambient atmosphere and before degradation of the sample under the electron beam in high vacuum conditions), the LAS glassy phase seems to be formed of two distinguishable amorphous components (no diffraction spot), a bright and a dark one, the contrast and the reticulation between the two interpenetrated subnetworks being strengthened with temperature (see S-I.2).

STEM micrographs of LAS-GC (Fig. 1b-d) show two different
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** a) Rietveld refinement of a LAS-GC X-ray pattern (Cu K-L₃ - λ = 1.540598 Å). b) Sketch-up of the LAS-GC with ca. 74 vol% of γ-LiAlSi₂O₆, ca. 25 vol% of glass and ca. 1 vol% of ZrTiO₄.</figcaption></figure>

structural features. First, 30–50 nm diameter crystallized particles with a roughly round shape are detected (crystals appear in grey or white depending on their orientation relative to the electron beam, Fig. 1 c.). They are assigned to a γ-spodumene phase (vide infra). Second, 2–5 nm large dots homogeneously distributed are observed (Fig. 1 d.). These particles are allocated by EDX-STEM and EELS mapping to a “ZrTiO₄” phase with a Ti:Zr ratio not defined with accuracy (see S.I.3, S.I.4). No amorphous phase was highlighted at this stage between the grains. Although conventional microscopy techniques provide a wealth of information, further analyses are needed to find out more about the structure of the GC. In that context, a Rietveld refinement on the collected X-ray pattern of LAS GC was carried out (Fig. 2a).

The diffraction pattern indicates undoubtedly the presence of γ-spodumene phase, i.e. γ-LiAlSi₂O₆, as the major phase. This is the thermodynamically metastable polymorph of the mineral. The observed refined cell parameters are lower than those expected in the literature (i. e., a = 5.184 (1) Å and c = 5.440 (2) Å vs. a = 5.217 (1) Å and c = 5.451 (2) Å [38]). This observation was already reported in the literature [39] and the aforementioned discrepancy was explained either by a Li⁺/M²⁺ (e.g. M = Mg or Zn) substitution process taking place along with the presence of Li vacancies or by a Si⁴⁺/Al³⁺ substitution balanced by Li interstitials. Based on a Rietveld refinement with a fundamental parameter approach, the calculated mean size of the spodumene crystallites is estimated around 38 ± 2 nm (in full agreement with STEM analyses). No size anisotropy was detected. The low intensity very broad peak at 30.5° in 2θ is only present when both TiO₂ and ZrO₂ nucleating

agents are used [28]. This unique peak is commonly associated to the (111) Bragg reflection of ZrTiO₄ [40,41]. Unfortunately, due to the sole presence of one low intense peak, no additional information could be obtained. Quantification of crystalline phases (γ-spodumene and “ZrTiO₄”) and glass lead to 72.7 and 27.3 ± 3 wt%, respectively, i.e. ca. 75.2 and 24.8 vol% after conversion taken into account the densities of crystalline phases. Although these values are tainted by errors due the inaccuracy of the exact “ZrTiO₄” chemical composition (vide supra), they strongly remind us the hard-sphere close-packing model (see Fig. 2b) in hexagonal compact structures with the ideal 74/26 vol% of sphere/void ratio.

As well known, XRD gives an averaged representation of the real structural morphology of GC. However, it is difficult at this stage to gain more information on the amorphous part of the glass ceramic. Indeed, in contrast, ACOM-TEM phase mapping can locally probe the glass-crystallized distribution in GC. Formally, electron diffraction is the technique of choice for the study of glass-ceramics, as its beam size of around 10 p.m. (vs. ~150 p.m. for X-rays) enables to probe the material very precisely and locally. This is particularly useful for investigating heterogeneities or interfaces within glass-ceramic samples. What's more, by coupling this technique with common microscopy techniques, the overall structure of GC, i.e. crystallized and amorphous parts, can be imaged at the local scale. A correlation coefficient map can be deduced of this set of diffraction patterns (DP). The intensities of the DP of each pixels are compared to the neighbour diffraction patterns. This technique highlights the grain boundaries, permitting to visualize the

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** a) TEM micrograph of GC sample at 1 µm scale. Correlation coefficient map of the highlighted area at 500 nm (b)), and 50 nm (c)) scale.</figcaption></figure>
crystallites and thus the amorphous zone of the sample [42,43]. The TEM and the associated correlation coefficient map are presented in Fig. 3.

The correlation coefficient map shows numerous spodumene crystallites of the order of 40 nm. Clearly, this technique makes possible to distinguish crystallites from particles, i.e. agglomerates of single crystals. Moreover, based on the examination of Fig. 3, γ-LiAlSi₂O₆ crystals turn to be in contact with each other with a spheroid shape. Thanks to these results, it is possible to describe the distribution of different objects in space with a high degree of accuracy combining the convergent results of XRD and ACOM-STEM techniques. There are many “voids” between crystallites that show no diffraction phase, i.e. an amorphous phase, in interstices defined by crystalline spheres.

## 4. Conclusion

The present investigation reports on the microstructure of the Li₂O–Al₂O₃–SiO₂ glass-ceramic nucleated with “ZrTiO₄”. Analyses carried out using XRD and STEM, and more specifically, phase mapping based on an automated crystal orientation technique, clearly validate a hard sphere close-packing model for the LAS GC. Indeed, this latter is made up of ca. 75% vol crystals composed mainly of nanosized γ-spodumene with “ZrTiO₄” as trace, and ca. 25 % of glass located in the interstices of the crystalline spheres.

In this way, we have been able to highlight a new characterization technique that is still rarely used, if at all, in the field of glass ceramics. Thanks to electron diffraction, “topographic” map of glass-ceramic samples could be achieved. Thus, from these results, crystalline parts can be identified and information obtained on their size and arrangement.

## CRediT authorship contribution statement

**Maxence Vigier:** Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Resources, Writing – original draft, Writing – review & editing, Visualization.
**Philippe Deniard:** Conceptualization, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.
**Eric Gautron:** Data curation, Formal analysis, Methodology, Writing – original draft, Writing – review & editing.
**Nicolas Gautier:** Conceptualization, Data curation, Investigation, Writing – original draft, Writing – review & editing.
**Cécile Genevois:** Data curation, Investigation, Methodology, Writing – original draft, Writing – review & editing.
**Sandra Ory:** Data curation, Investigation, Methodology.
**Mathieu Allix:** Data curation, Investigation, Methodology, Writing – original draft, Writing – review & editing.
**Ilyes Ben Kacem:** Conceptualization, Resources, Supervision, Writing – original draft, Writing – review & editing.
**Stéphane Jobic:** Conceptualization, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Maxence Vigier reports financial support was provided by Arc France. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors are indebted to Mr. Martin Payen and Ms. Catherine Blondel-Dacquin (ARC France, Arques, France) for synthesis of the studied materials. TEM measurements in Nantes were performed using the IMN’s characterization platform, PLASSMAT, and specifically the

“CIMEN” TEM microscope funded by the French Contrat Plan Etat-Région (CPER), the European Regional Development Fund of Pays de la Loire (FEDER), the Région Pays de la Loire, Nantes Métropole and CNRS. In Orléans, the project benefitted from the microscopy facilities of the Platform MACLE-CVL co-funded by the European Union and Centre-Val de Loire Region (FEDER).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ceramint.2024.05.135.

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