In situ cracking of silica beads in the SEM and TEM — Effect of particle size on structure–property correlations

• Published in: Powder technology 2015-01, Vol.270, p.337-347
• Main Authors: Paul, J., Romeis, S., Mačković, M., Marthala, V.R.R., Herre, P., Przybilla, T., Hartmann, M., Spiecker, E., Schmidt, J., Peukert, W.
• Format: Article
• Language: English
• Subjects: Brittle to ductile transition; Fracture; In situ SEM; In situ TEM; Mechanical properties; Stöber silica particles
• ISSN: 0032-5910; 1873-328X
• DOI: 10.1016/j.powtec.2014.10.026

Stöber Fink Bohn (SFB) silica is a well-known colloidal system: monodisperse spherical particles are readily obtained by this wet chemical synthesis route. Despite the frequent application of SFB silica, its mechanical properties are not comprehensively understood. Within this account, size dependent mechanical properties of single SFB particles are systematically investigated using in situ SEM and TEM techniques: particle properties are addressed in the size range of 1.5μm down to 200nm. Both, untreated particles and heat treated particles densified at 1000°C are investigated. Structural characterization by vibrational spectroscopy, by solid state nuclear magnetic resonance spectroscopy, by scanning electron microscopy and by focused ion beam machining allows correlating mechanical properties and the internal particle structure. For untreated particles comprised of a weakly cross-linked silica network a high degree of plasticity associated with low Young's moduli and hardness values is found. At large strains only cracking without full fracture of the particles occurs, whereby the crack path is clearly linked to the synthesis conditions: for larger particles obtained from a multistep growth process cracks propagate along weak intraparticle interfaces and are deflected around an inner core structure. After the thermal treatment cross-linking of the silica network is significantly enhanced and Young's moduli comparable to vitreous silica accompanied by increased hardness values are found. Only the smallest particles exhibit cracking; fracture into two or more pieces is distinguished for particles above 500nm. It is found by SEM imaging that the relative occurrence of cracking into several pieces is increasing with size — a behaviour associated with the brittle to ductile transition of silica. By testing a large quantity of particles, statistical significance is ensured. [Display omitted] •In situ SEM and TEM testing of statistical significant particle ensembles•Mechanical properties of Stöber particles in the size range 200nm to 1.5μm•Mechanical properties of untreated and thermally densified particles•Size dependent crack propagation reflects particle synthesis conditions.•Assignment of a brittle to ductile transition to thermally densified particles