Showing papers by "Tanya Prozorov published in 1999"

Self-Assembled Monolayers of Alkanesulfonic and -phosphonic Acids on Amorphous Iron Oxide Nanoparticles

Abstract: We have functionalized amorphous Fe2O3 nanoparticles with alkanesulfonic and octadecanephosphonic acids. TEM reveals nanoparticles 5−10 nm in diameter. FTIR spectra suggest that while in all cases the alkyl chains are packed in a solid-like arrangement, packing disorder increased with decreasing chain length. TGA of the sulfonic acid-functionalized Fe2O3 nanoparticles shows that moieties started to decompose and desorb from the iron oxide surface at about 260 °C. In the case of the octadecanephosphonic acid (OPA)-functionalized Fe2O3, moieties started to decompose and desorb at 340 °C. It is suggested that free Fe−OH groups can serve as proton donors to assist in the sulfonic acid desorption process and that because of the diprotic nature of the phosphonic acid these free surface Fe−OH groups may no longer be available. Among all, the octadecanesulfonic acid coating displays the lowest magnetization, which may be explained by the high packing and ordering of the alkyl chains on the particle surface. The s...

Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

Abstract: Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ samples composed of regular amorphous, acicular amorphous, and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time-dependent barrier yields a natural explanation for the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define an energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data.

Sonochemical synthesis and characterization of iron oxide coated on submicrospherical alumina : a direct observation of interaction between iron oxide and alumina

Abstract: Coating of iron oxides on submicrospheres of alumina was carried out by the sonochemical method. Three kinds of alumina were used, namely, the as-prepared, 700 and 1000 °C heated-alumina. TEM results reveal various coating effects on different heat-treated substrates. The optimum coating is obtained on as-prepared amorphous alumina, in which most of the iron oxide particles are adhered to alumina spheres tightly, while in the sample coated on crystallized alumina heated at 1000 °C, most of the iron oxide particles remain separate from the alumina spheres. The strong interaction between iron oxides and alumina substrate was directly observed by XRD, TEM, and IR and magnetic measurements. Owing to the strong interaction between adhered iron particles and alumina substrate, the complete transformation of γ-Fe2O3 to α-Fe2O3 was retarded to higher than 700 °C; conversely, the presence of haematite can induce the formation of α-Al2O3 at the high temperatures. TEM images clearly show the changes of particle size...

Sonochemical Synthesis and Characterization of Iron Oxide Coated on Submicrospherical Alumina: A Direct Observation of Interaction Between Iron Oxide and Alumina.

Abstract: Coating of iron oxides on submicrospheres of alumina was carried out by the sonochemical method. Three kinds of alumina were used, namely, the as-prepared, 700 and 1000 °C heated-alumina. TEM results reveal various coating effects on different heat-treated substrates. The optimum coating is obtained on as-prepared amorphous alumina, in which most of the iron oxide particles are adhered to alumina spheres tightly, while in the sample coated on crystallized alumina heated at 1000 °C, most of the iron oxide particles remain separate from the alumina spheres. The strong interaction between iron oxides and alumina substrate was directly observed by XRD, TEM, and IR and magnetic measurements. Owing to the strong interaction between adhered iron particles and alumina substrate, the complete transformation of γ-Fe2O3 to α-Fe2O3 was retarded to higher than 700 °C; conversely, the presence of haematite can induce the formation of α-Al2O3 at the high temperatures. TEM images clearly show the changes of particle size...