Formation Of Oxide Nanocrystals Research Articles

Group 13 precursor structures and their effect on oxide nanocrystal formation

Group 13 precursor structures and their effect on oxide nanocrystal formation

Fracture and fatigue behaviour of a laser additive manufactured Zr-based bulk metallic glass

Laser additive manufacturing of bulk metallic glass (BMG) provides an effective bypassing of the critical casting thickness constraints that limit the size of components that can be produced; however, open questions remain regarding the resulting mechanical properties. In this work, a Zr-based BMG known as AMZ4 with composition Zr59.3Cu28.8Nb1.5Al10.4 was printed using a laser powder bed fusion (LPBF) technique. Micro X-ray computed tomography results together with electron microscopy imaging revealed porous processing defects in LPBF produced AMZ4 that led to a loss in tensile strength. Fatigue crack growth studies revealed a fatigue threshold, ΔKth., of ∼1.33 MPa√m and a Paris law exponent of m = 1.14, which are relatively low values for metallic materials. A KIC fracture toughness of 24−29 MPa√m was found for the LPBF BMG samples, which is much lower than the KQ of 97−138 MPa√m and KJIC of 158−253 MPa√m measured for the cast alloy with the same composition. The lower fracture toughness of the laser processed AMZ4 was attributed to ∼7.5× higher dissolved oxygen in the structure when compared to the cast AMZ4. Despite the higher level of oxygen, the formation of oxide nanocrystals was not observed by transmission electron microscopy. Oxygen induced toughness loss was confirmed by dissolving elevated concentrations of oxygen into cast AMZ4 rods, which led to a reduction in bending ductility and changes in the short-range order of the glass structure, as revealed by synchrotron X-ray diffraction.

Chemistry of Shape-Controlled Iron Oxide Nanocrystal Formation.

Herein, we demonstrate that meticulous and in-depth analysis of the reaction mechanisms of nanoparticle formation is rewarded by full control of the size, shape, and crystal structure of superparamagnetic iron oxide nanocrystals during synthesis. Starting from two iron sources, iron(II) and iron(III) carbonate, a strict separation of oleate formation from the generation of reactive pyrolysis products and concomitant nucleation of iron oxide nanoparticles was achieved. This protocol enabled us to analyze each step of nanoparticle formation independently in depth. The progress of the entire reaction was monitored via matrix-assisted laser desorption ionization time-of-flight mass spectrometry and gas chromatography, thus providing insight into the formation of various iron oleate species prior to nucleation. Interestingly, due to the intrinsic strongly reductive pyrolysis conditions of the oleate intermediates and redox process in early stages of the synthesis, pristine iron oxide nuclei were composed exclusively from wüstite irrespective of the oxidation state of the iron source. Controlling the reaction conditions provided a very broad range of size- and shape-defined monodispersed iron oxide nanoparticles. Curiously, after nucleation, star-shaped nanocrystals were obtained that underwent metamorphism toward cubic-shaped particles. Electron energy loss spectroscopy tomography revealed ex post oxidation of the primary wustite nanocrystal, providing a full 3D image of Fe2+ and Fe3+ distribution within. Overall, we developed a highly flexible synthesis, yielding multi-gram amounts of well-defined iron oxide nanocrystals of different sizes and morphologies.

In Situ Study of Spinel Ferrite Nanocrystal Growth Using Liquid Cell Transmission Electron Microscopy

We report transition metal oxide nanocrystal formation in a liquid cell using transmission electron microscopy (TEM). The growth of M–Fe–oxide (M = Ni, Mn, Co, or Zn) nanoparticles from a growth solution of metal acetylacetonates dissolved in oleylamine, oleic acid, and benzyl ether was studied. Nickel iron oxide nanocrystals with spinel structure were obtained under electron beam irradiation of the Ni–Fe growth solution, whereas iron oxide nanocrystals were achieved with Mn remaining in the Mn–Fe growth solution. Similarly, we achieved cobalt iron oxide nanocrystals in the Co–Fe precursor solution, while iron oxide nanoparticles were obtained in the Zn–Fe solution. By tracking nanoparticle size evolution as a function of time along the Ni–Fe–oxide nanoparticle growth trajectories, we found the growth kinetics follow a Lifshitz–Slyozov–Wagner (LSW) model suggesting surface reaction-limited growth. Ex situ characterization shows elemental distribution and structural and valence state of the different nanop...

Towards Shape Control of Metal Oxide Nanocrystals in Confined Molten Media

AbstractSpray synthesis is a robust route to compositionally complex materials as fine powders; however, the solid‐state synthetic conditions in most cases limit access to discrete nanoparticles with defined structural features. Yet, by maintaining a dynamic droplet phase through integration of molten salt chemistry, product formation can proceed in manners similar to lower temperature solution routes to nanomaterials. This Focus Review connects concepts from classical nucleation theory and molten salt chemistry to provide a framework for nanocrystal formation via aerosol‐assisted molten salt synthesis (AMSS). Examples of metal oxide nanocrystal formation by AMSS are surveyed and highlight how the spatial and temporal confinement of aerosol droplets limits crystal growth in molten media. Ultimately, we strive to provide practical guidelines, not limited to AMSS, towards the synthesis of compositionally complex materials as shape‐defined nanocrystals.

Kinetics of Monodisperse Iron Oxide Nanocrystal Formation by “Heating-Up” Process

We studied the kinetics of the formation of iron oxide nanocrystals obtained from the solution-phase thermal decomposition of iron-oleate complex via the "heating-up" process. To obtain detailed information on the thermal decomposition process and the formation of iron oxide nanocrystals in the solution, we performed a thermogravimetric-mass spectrometric analysis (TG-MS) and in-situ magnetic measurements using SQUID. The TG-MS results showed that iron-oleate complex was decomposed at around 320 degrees C. The in-situ SQUID data revealed that the thermal decomposition of iron-oleate complex generates intermediate species, which seem to act as monomers for the iron oxide nanocrystals. Extensive studies on the nucleation and growth process using size exclusion chromatography, the crystallization yield data, and TEM showed that the sudden increase in the number concentration of the nanocrystals (burst of nucleation) is followed by the rapid narrowing of the size distribution (size focusing). We constructed a theoretical model to describe the "heating-up" process and performed a numerical simulation. The simulation results matched well with the experimental data, and furthermore they are well fitted to the well-known LaMer model that is characterized by the burst of nucleation and the separation of nucleation and growth under continuous monomer supply condition. Through this theoretical work, we showed that the "heating-up" and "hot injection" processes could be understood within the same theoretical framework in which they share the characteristics of nucleation and growth stages.