|
The hexagonal BaRExFe,2.xO,,(RE denote La^ Nd> Sm and Gd) ultrafine powders with M-typestructure were firstly synthesized by a novel technique ------ sol-gel combustion synthesis methodwhich combines sol-gel method with low temperature combustion synthesis method.Based on theoretical analysis and experimental study, the effects of the composition of the gel, the different kinds of intermediate, strength of solution, the amount of complexing agent, the different kinds of anions, the adding of dispersant, auto - propagating combustion and the condition in heat treatment of the gel on crystal phase, particle size, morphology and magnetic properties of BaRE^Fe^O,, ultrafine powders were firstly investigated systematically to clarify the optimum forming conditions.The formation mechanism of BaRExFe12_xOl9in heat treatment was firstly deeply discussed, and the action mechanism of complexing agent (citrate) and dispersant (glycol) during the formation of the gel was also firstly studied.The effects of the different kinds of rare earths and the doping amount of them on magnetic properties of BaFe,2O19 ultrafine powders (including the specific saturation magnetization, the specific remanent magnetization and coercive force) were firstly discussed systematically.The composition of sample was identified by means of X-ray diffractometer(XRD). Magnetic properties were measured by vibrating sample magnetometer (VSM). The combustion behavior and crystallization process of the gel were studied by means of differential thermal analysis-thermogravimetric analysis (DTA-TG). Scan electron microscope (SEM) and transmission electron microscope (TEM) were used to examine the morphology. Some important results are concluded as follows:1) This investigation presents the formation mechanism of BaREiFe^Oi,, that is,organic precursor (gel) - Y -Fe2O3+Ba2+-BaFe204BaFe2O4+5 y -Fe2O3-BaFe,2O19Therefore, only when the formation of a -Fe2O3 intermediate is prevented, can the pure BaRExFe,2.xOI9 particles be obtained at relatively lower calcining temperature. This conclusion can also be applied in other synthesis methods such as chemical co-precipitation, aerosol synthesis, cryochemical method and hydrothermal synthesis.2) The results show that controlling the pH value of starting precursor solution at 7.0 or so and preprocessing the gel (that is, igniting the gel) into y -Fe^ before calcining are two key steps which can prevent the formation of a -Fe2O3 intermediate, thus the pure BaRE^Fe^O,, particles can be obtained at lower calcining temperature (850 "C).3) The BaRExFel2.x019 ultrafine powders with single phase, small size (about 30~70nm), narrow size distribution and high magnetic properties with coercive force (437kA/m) and specificmagnetization (65.54A ?m2/kg) that are similar to the theoretically predicted values and far more than those of barium ferrite produced by others without doping.4) The rule about the influence of rare earths (La, Nd, Sm, Gd) doping on magnetic properties of barium ferrite is concluded. With the increasing of the doping amount of these four rare earths, both the specific saturation magnetization and the specific remanent magnetization decrease gradually to the same extent. When the doping amount of these four rare earths is lower, coercive force of La and Sm-doped barium ferrite powders shows a similar regularity, that is, there exists a minimum, whereas there exists a maximum in Nd and Gd-doped barium ferrite powders. When the doping amount is higher, coercive force increases greatly with the increasing of the doping amount
|