Amorphous Alumina Nanoparticles: Structure, Surface Energy, and Thermodynamic Phase Stability

Abstract

To provide a complete picture of the energy landscape of Al 2 O 3 at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al 2 O 3 ). a-Al 2 O 3 nanoparticles were obtained by condensation from gas phase generated through laser evaporation of α-Al 2 O 3 targets in pure oxygen at25 Pa. As-deposited nanopowders were heat-treated at different temperatures up to 600 °C to provide powders with surface areas of 670−340 m 2 /g. The structure of the samples was characterized by powder X-ray diffraction, transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The results indicate that the microstructure consists of aggregated 3−5 nm nanoparticles that remain amorphous to temperatures as high as 600 °C. The structure consists of a network of AlO 4 , AlO 5 , and AlO 6 polyhedra, with AlO 5 being the most abundant species. The presence of water molecules on the surfaces was confirmed by mass spectrometry of the gases evolved on heating the samples under vacuum. A combination of BET surface-area measurements, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry was employed for thermodynamic analysis. By linear fit of the measured excess enthalpy of the nanoparticles as a function of surface area, the surface energy of a-Al 2 O 3 was determined to be 0.97 ± 0.04 J/m 2 . We conclude that the lower surface energy of a-Al 2 O 3 compared with crystalline polymorphs γand α-Al 2 O 3 makes this phase the most energetically stable phase at surface areas greater than 370 m 2 /g.

Thermodynamics of formation of coffinite, USiO 4

“…S6), and MS shows that the water is removed gradually up to 450°C, which suggests that at least part of the H 2 O in this sample is strongly bonded. Assuming water in coffinite is adsorbed on the surface with an integral adsorption enthalpy of −80 kJ/mol per mole of H 2 O, similar to the values observed for water adsorption on alumina and titania (44)(45)(46) and, through proper thermochemical cycles (SI Appendix, Table S4), a value of ΔH f,ox of coffinite (24.1 ± 3.5 kJ/mol) is obtained, in agreement with the results for coffinite-F ( Table 1). The tightly bound water could originate from metaschoepite (UO 3 ·2H 2 O), which could be formed from partial oxidation of U(IV) (47) that originally was not incorporated in the coffinite structure, but rather was embedded in the gelatinous layer of excess amorphous silica (33).…”

Strong enhancement in thermal conductivity of ethylene glycol-based nanofluids by amorphous and crystalline Al2O3 nanoparticles

“…10 Owing to the general availability and unique characteristics, such as good structural, chemical and thermal stability at elevated temperatures, high electrical resistivity, low gate leakage, and excellent behavior in optics and electronics, Al 2 O 3 in both amorphous and polymorphs (material with similar composition but different crystal structure) forms are registered among the technologically most crucial ceramic materials. [11][12][13][14][15][16][17]27,28 In this Letter, we evaluated the effects of ordered/disordered structures of Al 2 O 3 NPs, volume fraction, and temperatures on the TC enhancement in EG nanofluid for different volume concentrations. As described in a previous work, three different structures of NPs, i.e., amorphous and crystalline c-, and a-Al 2 O 3 , were synthesized by sol-gel method.…”

“…The long-range order in nano-crystalline may resist to the liquid-particle interface and resulted in the lower enhanced TC of the crystalline c-and a-Al 2 O 3 /EG nanofluids. Moreover, the amorphous structure of Al 2 O 3 has random distribution of Al 3þ and vacancies over tetrahedral (AlO 4 ), polyhedral (AlO 5 ), and octahedral (AlO 6 ) sites with the prominent tetrahedral and octahedral units; whereas the crystal structure of c-Al 2 O 3 is composed of a mixture of tetrahedral and 12,15,27,28 We infer that the predominant fractions of a-Al 2 O 3 NPs may highly interact with the base-fluid and enhance the TC of nanofluids. The lower TC enhancement achieved by the crystalline c-and a-Al 2 O 3 /EG nanofluids can be ascribed due to the presence of short-range interfacing between solid-particle and liquidphase as compared to the a-Al 2 O 3 /EG nanofluids.…”

Green Synthesis of Aluminium Oxide Nanoparticles by using Aerva Lanta and Terminalia Chebula Extracts

“…It is commonly called alumina. The oxides of aluminium materials are widely used in ceramics, refractories and abrasives due to their hardness, chemical inertness, high melting point, non-volatility and resistance to oxidation and corrosion [1][2][3][4][5]. Predominantly, aluminum oxide nanoparticles commonly known as alumina (Al 2 O 3 NPs) have trapped the awareness of many researchers due to its great catalytical activities.…”

“…Predominantly, aluminum oxide nanoparticles commonly known as alumina (Al 2 O 3 NPs) have trapped the awareness of many researchers due to its great catalytical activities. An alumina nanoparticles can be synthesized by using many techniques including ball milling, spray combustion, hydrothermal, sputtering, sol-gel, microwave and laser ablation [5][6][7][8][9][10]. Out of all the methods, hydrothermal method proved more helpful to obtain well shaped materials with designed texture and composition at low processing temperatures [11][12].…”