Z. X. Tang

Preparation of manganese ferrite fine particles from aqueous solution

Fine manganese ferrite particles have been prepared by a coprecipitation method and subsequent digestion process (below 100°C). Manganous salts mixed with either ferric or ferrous salts were coprecipitated with sodium hydroxide. Particles produced from ferric salts were MnFe2O4 with relatively smaller sizes (5 to 25 nm) while ferrous salts created MnxFe3−xO4 (0.2 < × < 0.7) with bigger sizes up to 180 nm. In either case, the particle size appeared to be a unique function of the ratio of metal ion concentration to hydroxide ion concentration when the digestion conditions were fixed. For the system with ferric salts, the undigested samples were polycrystals with crystallite sizes of about 2 nm. Digestion, which could be described as an Ostwald ripening process, did not change the crystalline structure but increased both the crystallite size and the particle size. A basic solution was essential for an effective digestion process in this system. The system with ferrous salts, on the contrary, needed an acidic solution to create a single ferrite phase. Digestion changed both the crystalline structure and the particle size of the precipitated precursors. This process involved a dissolution and renucleation/growth mechanism. Cation and anion effects on the particle size and the evolution during digestion were also studied.

Size‐dependent magnetic properties of manganese ferrite fine particles

We have developed synthetic techniques involving coprecipitation of iron(III) and manganese(II) ions from aqueous solutions using NaOH to create single‐crystal, stoichiometric MnFe2O4 particles. The particle diameter could be varied between 5 and 25 nm by varying the metallic‐ion to hydroxide‐ion concentration ratio. Magnetic measurements revealed that the saturation magnetization decreased, the ferrimagnetic to paramagnetic transition broadened, and the apparent transition temperature increased, all with decreasing particle size.

Magnetic properties of aerosol synthesized barium ferrite particles

Barium ferrite fine particles have been synthesized from ferric nitrate and barium nitrate aqueous solution by an aerosol technique. The as-received particles showed a spin-glass behavior with a history-dependent low-field magnetization below 180 K. An exothermic peak with an initial crystallization temperature of 687 degrees C was found by DSC (differential scanning calorimetry) analysis. Heat treatments above this temperature greatly changed the magnetic properties and morphologies without changing the Ba/Fe atomic ratio, which was 1/12. Heat-treated samples had coercivities and saturation magnetizations as high as 5360 Oe and 70.6 emu/g, respectively. >

Magnetic properties of aerosol synthesized iron oxide particles

Abstract Fine iron oxide particles have been prepared by an aerosol technique. The particles are spherical with a mean size of 0.1 μm. Heat treatment in air or nitrogen at various temperatures for various times leads to a variety of phase mixtures of α-Fe 2 O 3 , γ-Fe 2 O 3 and Fe 3 O 4 . The magnetic properties of these various samples are presented.

Magnetic studies of fine iron and iron‐oxide particles

The magnetic and structural properties of fine iron and iron‐oxide particles have been studied. The samples of iron oxide were prepared by the aerosol method and those of pure iron were prepared by the vapor deposition technique. Electron micrographs showed the size of aerosol particles to be 200 nm and that of vapor deposition particles to be 10–20 nm. The maximum coercivity at 10 K obtained for aerosol particles was approximately 700 Oe, and for vapor deposited particles it was 1540 Oe.

Magnetization reversal in ferrite magnets

Abstract The effect of particle size on the magnetization reversal in ferrite magnets has been examined by correlating the hard magnetic properties with the microstructure. Initial magnetization curves, field and temperature dependence of coercivity and the remanence curves were obtained and compared with the predictions of theoretical models. The coercivity of bulk samples can be explained by the nucleation model. In ultrafine particles the coercivity is very close to the value predicted for single domain particles while the larger particles show a behavior which is a mixture of both the bulk magnets and ultrafine particles behavior.

Aerosol Spray Pyrolysis Synthesis Techniques

Aerosol techniques provide a convenient method for generation of small particles of high purity and a wide range of chemical formulations. Here we will emphasis the method of aerosol spray pyrolysis wherein solutions of precursor salts are nebulized and then dried and pyrolyzed in a furnace reactor. This method can be used to create a wide variety of complex, multicomponent particles with typical sizes in the 50 nm to 500 nm range. Problems involving nondense morphology (often hollow and/or porous particles) and chemical phase segregation must be addressed, and schemes to defeat these problems are given. Examples from our work to create complex metal oxides (ferrites and garnets) will be used to show phase and morphology evolution during synthesis. The magnetic properties of these particles are similar to the bulk materials but with variations due to particle size and preparation scheme.

Preparation of Fine Particles

Abstract The techniques of vapor deposition, aerosol spray pyrolysis and chemical reduction have been used to prepare fine magnetic particles. Fe particles were prepared with an average size in the range of 50-300 A by varying the gas pressure during evaporation (1–30 torr). Fe-B particles have been produced by chemical reduction with NaBH 4 . The size (100–500 A) and the boron concentration of the particles were varied by changing the reduction conditions. Amorphous and crystallized barium ferrite particles with a size of about 1000 A have been synthesized from metal salts by an aerosol technique. The magnetic and structural properties of all these particles were studied with SQUID magnetometry and transmission electron microscopy, respectively. The magnetic properties of the ultrafine metallic particles can be explained by a particle morphology consisting of an oxide shell around a metallic core.