The properties and preparation of talc have long been investigated due to its diverse industrial applications, which have expanded recently. However, its comminution behavior is not yet fully understood. Therefore, having better control of the particle size and properties of talc during manufacturing is required. In this study, we investigate the effect of the ball size in a high-energy ball mill on the comminution rate and particle size reduction. High-energy ball milling at 2000 rpm produces ultrafine talc particles with a surface area of 419.1 m²/g and an estimated spherical diameter of 5.1 nm. Increasing the ball size from 0.1 mm to 2 mm increases the comminution rate and produces smaller talc particles. The delamination of (00l) layers is the main comminution behavior when using 1 mm and 2 mm balls, but both the delamination and rupture of (00l) layers occurs when using 0.1 mm balls. The aggregation behavior of ground talc is also affected by the ball size. Larger aggregations form in aqueous solution when ground with 0.1 mm balls than with 1 mm or 2 mm balls, which highlights the different hydro-phobicities of ground talc. The results indicate that optimizing the ball size facilitates the formation of talc particles of a suitable size, crystallinity, and aggregation properties.

Nanoparticles are known to have physical properties different from their bulk analogs, a characteristic that bears wide technological potential. Detailed knowledge of the atomic structures of diverse metastable/stable polymorphs in alumina nanoparticles with varying particle size is essential to understand their macroscopic properties and the behavior of their phase transitions. In this study, we report high-resolution solid-state ²⁷Al 2D triple-quantum (3Q) magic-angle spinning (MAS) and 1D MAS nuclear magnetic resonance (NMR) spectra for alumina nanoparticles with varying temperature and particle size with an aim to explore the nature of phase transitions in alumina nanoparticles. Although the ²⁷Al MAS NMR spectra of alumina nanoparticles cannot fully resolve all the crystallographically distinct Al sites for metastable aluminas such as γ-, δ-, and θ-Al₂O₃, the simulation of ²⁷Al MAS NMR spectra collected at different magnetic fields following the Czjzek model allows us to obtain the quantitative fractions of alumina polymorphs in nanoparticles and the NMR characteristics of their Al sites. The ²⁷Al 3QMAS NMR spectra resolved crystallographically distinct ^[6]Al and ^[4]Al sites in (γ,δ)- and θ-Al₂O₃ in the isotropic dimension for the first time. The fraction of θ-Al₂O₃ gradually increases up to ~1473 K at the expense of a decrease in (γ,δ)-Al₂O₃. The onset of formation of α-Al₂O₃ from metastable aluminas is observed above ~1493 K. Several phase transitions in alumina nanoparticles observed in the current study include, γ,δ → θ-Al₂O₃, γ,δ → α-Al₂O₃, and θ → α-Al₂O₃. Although the phase transition γ,δ → θ-Al₂O₃ occurs gradually with increasing annealing temperature from 873 to 1473 K, the phase transitions γ,δ → α-Al₂O₃ and θ → α-Al₂O₃ occur dramatically within a narrow temperature range between 1473 and 1573 K. The observed difference in temperature range (gradual vs. dramatic) for phase transition γ,δ → θ-Al₂O₃ and γ,δ,θ → α-Al₂O₃ originates from the different structural disorder in the metastable aluminas (i.e., γ-, δ-, θ-) and α-Al₂O₃. The effect of particle size on the phase transition (γ,δ) → θ-Al₂O₃ between 298 and ~1473 K is not observed significantly. On the other hand, the transition temperature for γ,δ,θ → α-Al₂O₃, where the 50% for alumina is α-Al₂O₃, apparently increases as the particle size increases (as evidenced by TEM observation), indicating a larger energy penalty for phase transitions into α-Al₂O₃ in larger alumina nanoparticles. This could be due to higher surface energy of θ-Al₂O₃ than that of α-Al₂O₃ and/or the fact that the transition from θ-Al₂O₃ to α-Al₂O₃ is kinetically favored for smaller nanoparticles. The mechanistic details of phase transitions among alumina polymorphs provided in the current study yield insights into the nature of the phase transition mechanisms for other oxide nanoparticles ubiquitous in the Earth’s surface environment.

Despite efforts to understand the amorphization mechanisms of zeolites upon heating and subsequent dehydration, little is known about the extent of Si-Al disorder and topological variations in both crystalline and amorphous phases during amorphization. In this study, we investigated the atomic structure and the extent of configurational disorder (e.g., Si-Al ordering) in Na-zeolite A and other dehydrated phases during their temperature-induced amorphization using multi-nuclear solid-state NMR. We also report the first multi-nuclear (¹⁷O, ²⁹Si, and ²⁷Al) NMR spectra of the intermediate amorphous phases. ²⁹Si MAS NMR results confirm the prevalence of amorphous phases up to ~1073 K and variation in Q-species for the crystalline phases. The ²⁷Al quadrupolar coupling constant of the ^[4]Al peak in Na-zeolite A and the intermediate amorphous phases increase with increasing temperature, which suggests an increase in the topological disorder associated with the structural distortion around ^[4]Al. 2D ¹⁷O 3QMAS NMR spectra resolve the crystallographically distinct Si-O-Al sites in Na-zeolite A and three types of oxygen linkages namely, Si-O-Al, Si-O-Si, and Al-O-Al in the intermediate amorphous phases, which provides an unambiguous experimental evidence for an increase in the Si-Al disorder during the amorphization of zeolite. The detailed structural changes in Na-zeolite A and other dehydrated phases at various temperatures provide insights into the structural changes of other aluminosilicates during amorphization, thereby highlighting the changes in Si-Al ordering.