Study On The So-gel Autocombustion Synthesis And Magnetic Properties Of Sevral Kinds Of Magnetic Materials

Sol-gel is a commonly used method in the synthesis of various kinds of materials.This method shows many advantages in the synthesis of materials,such as simple preparation process,low-cost apparatus,nontoxic raw materials and products,differen elements in the products having good homogeneity at atomic level.Autocombustion is also a commonly used method in the synthesis of materials.Its best advantage is that the combustion is a self-sustaining process caused by the heat that generated from its exothermic reaction.Except the ignition process,no additional energy supply is required.This advantage will save energy in the mass production.By combining these two methods,a novel method called "sol-gel autocombustion" has been proposed in the synthesis of materials.After that,sol-gel autocombustion is widely used to synthesize metal oxides.Recently,our group extend this method succesfully to the synthesis of metals and metal alloys.Based on these studies,we now extend this method to the synthesis of magnetic materials.In this dissertation,we have prepared sevral kinds of magnetic materials containing porous magnetite foams,large saturation magnetization Fe-Co alloy foams,large coercivity Fe-Pt alloy nanoparticles,superparamagnetic magnetite nanotubes,nickel nanotubes and nickel nanowires.The main contents of this dissertation are as follows:1.Porous Fe3O4 foams are synthesized successfully by the sol-gel autocombustion method by using citric acid,iron nitrate,and ammonia as the starting materials.By controlling the reductive atmosphere produced by the autocombustion of the gels,one third Fe atoms are reduced from the Fe3+ ions.So the Fe3O4 products are obtained.In this study,the molar ratios ?of citric acid to iron nitrate are adjusted from 0.1:1 to 1.5:1.The phase structure characterization indicate that magnetite can be synthesized within a large range of O.Porous morphologies of the Fe3O4 foams with micron-size pores are observed.The highest saturation magnetization of the Fe3O4 samples synthesized in argon atmosphere is as high as 91 emu/g,which is about 99%of its bulk value(92 emu/g).This indicates the purity of the as-prepared Fe3O4 foams is very high.Without the protection of inert gas,porous Fe3O4 foams are also synthesized in air by controlling the reductive atmosphere produced by the autocombustion of the gels.By further annealing the Fe3O4 foams at 250 ? for 1 hour in air,porous y-Fe2O3 foams can be obtained.2.Fe1-xCox alloys are synthesized successfully by the sol-gel autocombustion method using citric acid,iron nitrate,cobalt nitrate,and ammonia as starting materials.The atomic ratios of Fe:Co in the products are controlled by adjusting the relative concentration of Fe3+ and Co2+in the preparation of sols.Morphology characterization reveal the porous structure with micron-size pores.Synthetic mechanism is studied based on thermogravimetry and differential scanning calorimetry characterization.Magnetic properties of the alloys are investigated at room temperature.From the results,Fe0.7Co0.3 alloy possesses the highest saturation magnetization Ms,which is found to be about 231 emu/g,a value that is much larger than that of the corresponding alloys synthesized through other chemical methods.Compared with the maximum value 211 emu/g obtained by other chemical methods,the Ms is increased 9.5%.3.Ll0 FePt nanoparticles are synthesized successfully by the sol-gel autocombustion method using nontoxic glycine,iron nitrate,and hexachloroplatinic acid as the starting materials.Different from the commonly used methods,the high temperature annealing process is not needed for the formation of L10 FePt phase.The entire ignition and combustion process last no more than one minute.By adjusting the glycine to metal ions molar ratio ? from 0.5 to 6.0,its effects on the phase transformation and magnetic properties of the final products are investigated.When ?=1.5,pure phase L10 FePt is obtained.The L10 FePt nanoparticles are monodispersed with an average particle size of about 20 nm.Magnetic measurements showed that the L10 FePt nanoparticles have very large coercivity of 15.8 kOe at 300 K,and 23.2 kOe at 5 K.4.Magnetite nanotubes are synthesized successfully by anodic aluminum oxide template-assisted sol-gel autocombustion method.Iron nitrate,citric acid and ammonia are used as the starting materials.The average diameter of the magnetite nanotubes is about 140 nm,which is consistent with the pore diameter of the anodic aluminum oxide templates.The magnetite nanotubes are longitudinally continuous and have very high aspect ratios.Magnetic measurements indicate that the nanotubes have superparamagnetic characteristic at room temperature,with a blocking temperature of 95 K.5.Ni nanotubes and Ni nanowires with an average diameter of about 140 nm are synthesized successfully by anodic aluminum oxide template-assisted sol-gel autocombustion method.Nickel nitrate,citric acid and ammonia are used as the starting materials.The products having tubular or wire-like morphologies can be controlled by adjusting the experimental parameters and experimental conditions.The morphology characteristics of the Ni nanotubes and Ni nanowires are found to be very different.The Ni nanotubes are very friable and can be easily broken into pieces of short nanotubes,but the Ni nanowires are compact and have very high aspect ratios.Magnetic measurements indicate that both the Ni nanowires and the Ni nanotubes possess superparamagnetism at room temperature,with a blocking temperature of 250 K.