Correlating structure and mechanical properties for submicron amorphous silica spheres

Abstract

Spherical amorphous silica particles obtained from the well‐known Stöber‐Fink‐Bohn (SFB) synthesis are frequently used as standards for size or shape and as proxy for the behavior of amorphous silica [1]. So far, several studies have dealt with the peculiarities of the internal structure of these particles (see e.g. references in [2–3]): particles from the SFB synthesis exhibit a highly hydroxylated weakly condensed silica network with a certain microporosity at least for water and ions. Larger particles which are generally obtained from a reseeded multi‐step growth protocol show a ring like internal structure. Upon thermal treatment, the particles' silica network can be condensed ‐ the structure of vitreous silica is approached. Despite the abundant use of the particles, however, detailed investigations which correlate the mechanical properties and the internal structure of (heat‐treated) SFB silica are absent. Within this account, the size dependent mechanical and structural properties of SFB silica and thermally derived vitreous silica spheres are assessed for particle diameters of 200 nm to 5 µm. For the mechanical characterization a scanning electron microscope (SEM) supported custom‐made indenter [4] and the Hysitron PI95 TEM Picoindenter TM in a transmission electron microscope (TEM) are used to test a statically representative amount of particles (at least 50). Structural characterization is performed by nitrogen sorption, vibrational spectroscopy, colloid titration and solid‐state nuclear magnetic resonance spectroscopy. For SFB spheres with a mean diameter of 500 nm it is shown that hardness, yield strength, and Young's modulus of the SFB particles are significantly increased after thermal treatments at temperatures exceeding 400°C (Fig. 1). With an increasing treatment temperature the Young's modulus of bulk fused silica is approached. However, hardness, yield strength and the sustained plastic deformation till catastrophic failure of the spheres occurs clearly exhibit bulk values. The underlying changes of the internal structure are in accordance: a slight shrinkage (~28 vol.%) is accompanied by an overall homogenization, densification, increased cross‐linking and dehydroxylation of the particles (Fig. 2) [3]. The size‐dependent characterization of the failure modes of SFB particles and corresponding derived vitreous silica spheres provides information on the underlying deformation modes and allows a classification of the different cracking types (Fig. 3 and Fig. 4). Independent of size, the untreated SFB particles exhibit only the formation of (presumably) ductile cracks; full fragmentation does not occur. For the derived vitreous silica spheres, however, a clear brittle‐to‐ductile transition is observed in the size range of 500 – 800 nm: particles above this size are fragmented into two or more individual parts. The latter behavior is also indicative for bulk fused silica which is known for its brittleness on the mesoscopic scale [5]. In contrast to bulk fused silica, the silica spheres still sustain high plastic deformations (in the order of 40%). By ex situ Raman spectroscopy on the single particle level the observed plasticity can be attributed to local densification of the vitreous silica spheres directly below the contacts of the sphere with the diamond flat punch and the substrate [6]. It is noteworthy that fine details of the contact zones and the role of molecules on the surface might be addressed by non‐linear spectroscopy [7].