Fe 2 O 3 is widely used in the field of magnetism, and its magnetic properties are affected by particle size and crystalline form. Therefore, it is of great importance to study the effects of particle size on the structural transitions and magnetic properties. In this paper, accurate relations were derived to describe size dependence of the integral enthalpy and entropy of structural transitions of nanocrystals, and the influence mechanism of particle size on the thermodynamic properties of structural transitions is discussed. Different sizes of nano-Fe 2 O 3 were prepared, and differential scanning calorimetry (DSC) was used to obtain the temperature, enthalpy, and entropy of the structural transition from maghemite (γ-Fe 2 O 3 ) to hematite (α-Fe 2 O 3 ). The magnetic properties of nano-γ-Fe 2 O 3 were determined on the Magnetic Property Measurement System. We found that the thermodynamic properties of the structural transition decrease as particle size decreases and vary linearly with the reciprocal of particle size within the range of experimental particle size. In addition, it is found that the magnetic properties of nano-Fe 2 O 3 also show an obvious size dependency. As particle size increases, the saturated magnetization increases sharply and remains basically unchanged when reaches 60 nm; at the minimum particle size (19.3 nm), both the coercivity and the remanence approach zero and increase sharply with increasing particle size and reach the maximums at about 60 nm and then decrease. These findings improve our understanding of the effects of particle size on the thermodynamic properties of structural transitions and magnetic properties and provide a scientific guidance for the design, preparation, study, and application of Fe 2 O 3 and related materials.
To solve the problems of high temperature and non-uniformity of coloring on stainless steel, a new chemical coloring process, applying ultrasonic irradiation to the traditional chemical coloring process, was developed in this paper. The effects of ultrasonic frequency and power density (sound intensity) on chemical coloring on stainless steel were studied. The uniformity of morphology and colors was observed with the help of polarizing microscope and scanning electron microscopy (SEM), and the surface compositions were characterized by X-ray photoelectric spectroscopy (XPS), meanwhile, the wear resistance and the corrosion resistance were investigated, and the effect mechanism of ultrasonic irradiation on chemical coloring was discussed. These results show that in the process of chemical coloring on stainless steel by ultrasonic irradiation, the film composition is the same as the traditional chemical coloring, and this method can significantly enhance the uniformity, the wear and corrosion resistances of the color film and accelerate the coloring rate which makes the coloring temperature reduced to 40°C. The effects of ultrasonic irradiation on the chemical coloring can be attributed to the coloring rate accelerated and the coloring temperature reduced by thermal-effect, the uniformity of coloring film improved by dispersion-effect, and the wear and corrosion resistances of coloring film enhanced by cavitation-effect. Ultrasonic irradiation not only has an extensive application prospect for chemical coloring on stainless steel but also provides an valuable reference for other chemical coloring.
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