Preparation of ultrafine high density gamma ferric oxide using aerosol OT microemulsions and its characterization

Kinetic modeling for thermal dehydration of ferrous oxalate dihydrate polymorphs: a combined model for induction period-surface reaction-phase boundary reaction

In this study, ferrous oxalate dihydrate polymorph particles, α- and β-phases, with square bipyramidal and quadratic prismatic shapes, respectively, were synthesized. Thermal dehydration of the samples was subjected to kinetic study as a typical reaction that indicates a significant induction period and a sigmoidal mass-loss behavior. On the basis of the formal kinetic analysis of the mass-loss traces recorded under isothermal, nonisothermal, and constant transformation rate conditions and the morphological observations of the surface textures of the partially reacted sample particles, a combined kinetic model for the induction period-surface reaction-phase boundary reaction was developed. The sigmoidal mass-loss behavior after the significant induction period under isothermal conditions was satisfactorily simulated by the combined kinetic model. The kinetic parameters for the component processes of induction period, surface reaction, and phase boundary reaction were separately determined from the kinetic simulation. The differences in the kinetic behaviors of the induction period and the phase boundary reaction between α- and β-phase samples were well described by the kinetic parameters. The applicability of the combined kinetic model to practical systems was demonstrated through characterizing the physicogeometrical kinetics of the thermal dehydration of ferrous oxalate dihydrate polymorphs.

Controlled Growth of Gold Nanoparticles in AOT/C(12)E(4)/Isooctane Mixed Reverse Micelles

Stable anisotropic gold nanoparticles were prepared by the reduction of a relatively high concentration of tetrachloroauric acid with hydrazine in mixed reverse micelles formed with anionic surfactant AOT and nonionic surfactant tetraethylene glycol dodecyl ether (C(12)E(4)) in isooctane. It was found that the C(12)E(4) serves not only as a structure modifier but also as a stabilizer for Au particles, to prevent their further growth and precipitation. By the analyses of a high-resolution electron microscope, electron diffraction patterns, and energy-dispersive X-ray analysis (EDX), the resultant particles have been found to be pure gold of face-centered cubic structure. In the presence of C(12)E(4), the Au particle size is larger than that in the absence of C(12)E(4), while the particle size decreases with increases in the concentration of C(12)E(4). The molar ratio of hydrazine to HAuCl(4) was found to be an important parameter in the control of size and shape for the production of gold nanoparticles. A decrease in the molar ratio of hydrazine to HAuCl(4) resulted in larger Au particles with significantly more polydispersity. When the HAuCl(4) was injected directly into the mixed reversed micelles containing hydrazine, anisotropic gold nanoparticles, such as cylinders and trigons, were obtained at the molar ratio of hydrazine to HAuCl(4) of less than 0.5. Copyright 2001 Academic Press.

Synthesis of Strontium Ferrite Ultrafine Particles Using Microemulsion Processing

The strontium ferrite ultrafine particles have been prepared using the microemulsion processing. The mixed hydroxide precursor was obtained via the coprecipitation of Sr(2+) and Fe(3+) in a water-in-oil microemulsion of water/CTAB/n-butanol/isooctane. According to the investigation on the thermochemical properties by TGA/DTA and the phase analysis by XRD, it was shown that the precursor could yield pure strontium ferrite after calcination at 700 degrees C for 5 h while using an appropriate molar ratio of Sr/Fe in microemulsions. From TEM measurement, the diameters of the precursor and calcined particles were 3.8+/-0.7 and 50-100 nm, respectively. The magnetic properties characterized by a SQUID magnetometer showed that the saturation magnetization, remanent magnetization, coercivity, and squareness ratio were 55 emu/g, 28 emu/g, 492 Oe, and 0.51, respectively. The magnetization was also observed to increase with the decrease of temperature at 5-400 K. Compared with those reported earlier, the quite low coercivity implies the potential application of final product in the high-density perpendicular recording media. Copyright 2001 Academic Press.

Controlled Growth of Gold Nanoparticles in Aerosol-OT/Sorbitan Monooleate/Isooctane Mixed Reverse Micelles

Stable anisotropic gold nanoparticles were prepared by the reduction of tetrachloroauric acid with hydrazine in mixed reverse micelles formed with anionic surfactant Aerosol-OT and nonionic surfactant sorbitan monooleate (Span80) in isooctane. It was found that the Span80 serves not only as a structure modifier but also as a stabilizer for Au particles, to prevent their further growth and precipitation. The control of particle size, shape, and degree of dispersion was achieved by varying the process variables, such as molar ratio of reduction agent to metal salt, size of water droplets (omega(o)), concentration of metal salt, and sequence of addition of metal salt into the mixed reverse micelles. When the HAuCl(4) was injected directly into the mixed reversed micelles containing hydrazine, nonspherical gold nanoparticles, such as rods and cubes, were obtained at the molar ratio of hydrazine to HAuCl(4) of less than 1.0. The nonspherical Au particles were preferably formed at larger omega(o) value and lower gold salt loading. By the analyses of high-resolution electron microscope, electron diffraction pattern, and energy-dispersive X-ray analysis (EDX), the resultant particles have been found to be pure gold of face-centered cubic structure. Copyright 2000 Academic Press.

Synthesis of Platinum Ultrafine Particles in AOT Reverse Micelles

The synthesis of platinum ultrafine particles by the reduction of H2PtCl6 with hydrazine in AOT/isooctane reverse micellar solutions has been studied. By high-resolution electron microscope, electron diffraction pattern, and XRD analyses, the resultant particles have been found to be pure platinum of fcc structure. Their sizes were observed to increase with the increases in the molar ratio of water to AOT (omegaO) and in the concentration of H2PtCl6, while they decreased with the increase of hydrazine concentration. At a constant omegaO value, the size of platinum ultrafine particles was not affected significantly when the concentration ratio of hydrazine to H2PtCl6 was above 10, the AOT concentration increased from 0.1 to 0.6 M, and the temperature varied from 15 to 35 degrees C. Furthermore, the kinetic study of particle formation indicated that the nucleation time needed several minutes. The time for the growth of platinum ultrafine particles to their final size after nucleation was about one to several hours. It was observed that the formation rates increased with the increase of omegaO value and the concentrations of AOT and H2PtCl6, but they were not affected by hydrazine concentration when the concentration ratio of hydrazine to H2PtCl6 was above 10. Copyright 1999 Academic Press.

Preparation of Palladium Ultrafine Particles in Reverse Micelles

The preparation of palladium ultrafine particles by the reduction of Pd(NH3)4Cl2 with hydrazine in AOT/isooctane reverse micellar solutions has been studied. By the analyses of high-resolution electron microscope, electron diffraction pattern, and XRD, the resultant particles have been found to be pure fcc palladium. Their sizes were observed to increase first and then approach constant values with increases in the molar ratio of water to AOT (omega0) and in the concentration of Pd(NH3)4Cl2, but were not significantly affected when the concentration ratio of hydrazine to Pd(NH3)4Cl2 was above 10, the AOT concentration increased from 0.1 to 1.0 M, the temperature varied from 15 to 35 degreesC, and the pH of aqueous phase of Pd(NH3)4Cl2 was between 7 and 12. In addition, the kinetic study of particle formation indicated that the nucleation time needed several minutes. After nucleation, the particles grew to their final sizes within several to tens of minutes. The formation rates were found to be faster at larger omega0 values and higher AOT and Pd(NH3)4Cl2 concentrations. Copyright 1999 Academic Press.