Study On Preparation And Magnetic Properties Of Silicon Carbide Obtained Via Microwave Heating Method

As a typical kind of 3^rd generation wide band-gap semiconductor,silicon carbide?SiC?has wide band gap,high electronic mobility,strong critical breakdown field and tunable optical properties.Meanwhile,SiC has various fundamental properties including high mechanical strength,high hardness,large heat conductivity and low thermal expansivity.Because of these excellent electrical,optical,mechanical and thermal properties above,SiC may be utilized in the high-temperature and high-frequency devices.Compared with the properties of bulk SiC,nano-sized SiC materials have better comprehensive applying properties,thus SiC nano-materials draw considerable attentions in the field of functional materials and engineering&technology.Synthesis and obtaining of materials are the premise to realize the excavation of functional properties and the applications in various fields.Thus,exploitation of low-cost,easy-to-control,environment-friendly and high-yield methods is desirable for the practical applications of SiC.In the recent two decades,the synthesis of SiC with special morphologies and nano-or micro-size have drawn considerable attentions.Compared with the methods employing traditional heating,microwave heating method is good for the efficient synthesis of SiC because several features including high heating rate,lower reaction barrier,energy conservation and environment protection and so on.Improvements of the optical and electrical properties are conducive to the applications of SiC in microelectronic,and optical functional devices.Up to now,many researches have been carried out on the functional properties of SiC,but the researches on the magnetic properties and applications in the magnetic devices are less than that on the electrical and optical properties.The study on the magnetic performance of SiC,the origin of ferromagnetism and the regulation of the magnetic parameters can widen the application of SiC in the magnetic sensors,targeted drugs and electromagnetic shielding,and have important significance to the extensive use of SiC.In this work,we studied on the synthesis of SiC with different morphologies and micro-structures,especially on the synthesis of the one-dimensional SiC materials,under high-energy microwave irradiation by using different carbon and silicon sources.By employing comprehensive characterization methods including X-ray diffraction?XRD?,scanning electron microscope?SEM?and transmission electron microscope?TEM?,the morphologies and micro-structures of SiC were investigated and the growth mechanism of the SiC products were discussed.After the successful preparation of SiC products,the optical and magnetic properties of SiC materials were measured,and the origins of these functional properties were analyzed and evaluated.For the intensive study on the magnetic properties of SiC,based on the high-energy microwave irradiation technology,the magnetic properties of SiC were improved or regulated by tuning the carbon sources,adding metals in raw materials and employing ball-milling pre-treatment and so on.The main research contents are shown as follows:?1?SiC products with various morphologies and microstructures were prepared via microwave heating method with microwave power of 4⁵ kW by using graphite powders or graphite blocks as carbon sources and microwave absorbers,using silicon and silica powders as silicon sources.The graphite blocks were made into graphite crucible as reaction vessel,and the carbon atoms at the inner wall of the graphite crucible can react with silicon sources.The final products obtained by using graphite blocks as carbon sources were mainly composed of SiC nanowires with diameters of 50¹00nm and length up to dozens of micrometers.The SiC products with different morphologies covering one-dimensional,tapered,platy and granulate SiC were obtained in the different locations of the crucible by using graphite powders as carbon source.High-density stacking faults can be detected in the SiC products.Meanwhile,double crystal and forky SiC can be obtained in the SiC products.Most of the SiC nanowires were covered by amorphous SiO_x layers,and the layers could deform under high-energy electron irradiation.The growth mechanisms of SiC products were discussed,and we proposed that the formation of wire-like SiC were mainly relative to the difference on the surface energies of crystal planes and the concentration of vapor SiO and other intermediate products,and the difference on the temperature and the concentration of intermediate products result in the formation of the SiC products with different morphologies.?2?Following the successful synthesis of SiC,one-dimensional and micro-sized granulate SiC were selected as research objects for investigating the optical properties of them.Both of the two kinds of products can emit ultra-violet light with wavelength centered at 390 nm under excitation wavelength of 240 nm.Compared with the wavelength corresponding to the band gap?Eg?of bulk3C-SiC,the wavelength of emission peaks from the products showed the blue-shift phenomenon.By comparing the various photoluminescence spectra of the SiC products with different morphologies,the optical properties were mainly attributed to the point defects such as vacancies or surface defects.Furthermore,the emission peaks can be tuned by changing the excitation wavelength,and the tunable emission peaks may be attributed to the size distribution of the SiC products.Meanwhile,the diffusion-reflection spectra of the products were obtained and modified according to the Kubelka-Munk theory.According to the results of the modified diffusion-reflection spectra,the SiC products obtained via microwave heating were not totally transformed into direct-gap semiconductor,but the band-gap type of SiC products with different morphologies are different with each other.?3?In this study,we focused on magnetic properties of the SiC products,and this work can be divided into three parts:1)Measure of M-H curves of SiC products with different morphologies,and analyze the mechanism of the ferromagnetism exhibited by the products;2)exploration of the origins of the magnetic properties by comparing the M-H curves of SiC products obtained under different conditions;3)enhance or tune the magnetic parameters including the saturated magnetization?Ms?and coervivity?Hc?of SiC products.At first,the magnetic moment versus magnetic field?M-H?curves of one-dimensional and granulate SiC were measured,and the results indicate that both of them exhibit ferromagnetism around room temperature,and the latter have large saturated magnetization?Ms?.After that,magnetic metals?Fe and Ni?were added in the raw materials for improving the magnetic properties of the obtained SiC products,and the M-H curves showed the enhancement of Ms.The origins of magnetic properties the SiC products obtained by adding Ni were analyzed via thermal analysis and heat treatment at high temperature,and the co-exsitence of ferromagnetic and paramagnetic phases were attributed to the formation of Ni-Si alloys and the isolated Ni ions in the products.Furthermore,the changement of carbon source and ball-milling pre-treatment were employed to prepare the SiC products with different magnetic properties.It is found that the SiC products obtained by using flake graphite and employing ball-milling pre-treatment have Ms of around 0.4 and 0.9 emu/g,respectively,without adding any metals.The origins of the ferromagnetism or paramagnetism were analyzed by comparing the magnetic properties of different SiC products.Taking the ferromagnetic micro-sized granulate SiC as an example,the mechanisms of the ferromagnetism from SiC obtained via microwave heating method were analyzed through the corrosion by HF solution and the deposition of SiO_x on SiC under microwave irradiation.Through the comparative study,the magnetic properties of SiC obtained without adding any metal additives may mainly be attributed to the combined effect of vacancies and charge transfer around the interface between SiC and SiO_x.?4?At last,two ways including the addition of Mn or Cr and the composite of SiC and Ni for tuning the magnetic properties of SiC products were introduced.The coexistence of ferromagnetism and paramagnetism can be detected in the obtained SiC by adding Mn,and the SiC product by adding Cr exhibit typical paramagnetism but no ferromagnetic signal can be detected.SiC can be coated by Ni via electroless plating,while NiO was formed in the SiC/Ni composite obtained via microwave heating method.The SiC/Ni composites obtained via electroless plating and microwave heating both have positive magnetic susceptibility,and the Ms of the product obtained via electroless plating is up to 1 emu/g.Furthermore,the magnetic properties of the SiC/Ni obtained via electroless plating can be improved after two-hour heat treatment under high temperature of 600°C,the Ms and coercivity?Hc?were enhanced to 3 emu/g and 150 Oe,respectively.According to the above results,the electroless plating may enhance the magnetic hysteresis loss of the SiC products,and improve the electromagnetic shielding property.