Preparation, Structure And Characterization Of Metal (Metal Oxide)/Alumina Composites

Ceramic is one of the oldest members in materials fields.Ceramic has much excellent properties such as its structuralcharacterization for resisting high temperature, corruption and wearor friction;and its functional characterization for novel electricity,magnetism and piezoelectricity, but brittleness is its fatal defect.Generally say, metal and metal alloy has fine thermalstabilization and elasticity, combining metal with ceramic to formmetal-ceramic composites which possess high hardness, toughness,strengthen, corruption-resistance, friction-resistance and lowercoefficient of expansion and so on. While nano-meter-sized ceramiccompound doped with metals both improve fracture strengthen andcontain inherent electric and magnetic properties of metals.Alumina ceramic is the most stable material, which has highmechanical strengthen, hardness and fine corruption-resistance,friction-resistance and high insulation, so it is widely used to makefundamental materials for machine parts.Doped with second phase or strengthen with fiber or granular,some properties of alumina ceramic could be improved to somedegree, but at the same time ceramic's brittleness also increases. Sosuitable choice of metals to be injected into alumina matrix is veryimportant. Some metal such Fe, W, Mo, Ni, Cu can combineadvantages of both metal and ceramic so that make the compositesnot only has excellent structural properties but also contains goodmagnetism, conductivity and tenacity.We choose Al₂O₃ as matrix to prepare metal oxide (metal andalloy) -Al₂O₃ compound by means of sol-gel method throughuniform doping and surface coating. We investigated the effect ofingredients, concentration, doping method on the structure andcharacterization of compounds. The phase, structure, magnetism andhardness were studied through XRD, TG-DT, MS, TEM and VSM.Crystallization temperature of as prepared α-Al2O3 was reduced200℃ by uniform dope compared with traditional method. α-Al2O3/α -Fe2O3 nano-composites were obtained at 900℃ whenconcentration of Fe2O3 doping in alumina matrix was 40wt%.Tthepresence of iron oxide introduced preferred nucleation sites, thusreducing the activation barrier for nucleation and lowering thetemperature of transformation from γ-to α-Al2O3 phase. On theother hand, the crystallization temperature of pure α-Fe2O3 isusually lower than 773K, while in our samples, the nucleation andgrowth of α-Fe2O3 grains was restrained by the amorphous structureof alumina matrix. The restriction effect between the two phasescould control the grain size at a reasonable range, which is favorablefor the preparation and sinter process of ceramics.While surface coating method could drop the crystallizationtemperature ofα-Al2O3 more remarkably. Stableα-Al2O3 phasecould form at lower temperature by surface coating than uniformdoping with the same Fe2O3 doping concentration. This is becauseα-Fe2O3 crystal exit on the surface of alumina, providing the seed forthe crystallization ofα-Al2O3, for the reason of the identical structureof two phase. The grain size ofα-Fe2O3 increased rapidly because itwas on the surface of matrix so it grew more fast than alumina.Uniform doping follows the process first as â‘ Fe2O3+2Al2O3+H2→2FeAl2O4+ H2O and then â‘¡FeAl2O4+H2→Fe+Al2Ox+H2O soreduce course is more thorough, while the process of surface coatingis the competing course between â‘¢Fe2O3+3H2→2Fe+3H2O andâ‘£Fe2O3+Fe+Al2O3→3FeAl2O4.The products of coating process isthe coexistence of and Fe3+ ion, Fe2+ ion and Fe.The as prepared powder with the 40wt% Fe2O3 concentration isα-Al2O3/α-Fe2O3 composite when it was heat-treated at airatmosphere, while it is Fe /γ-Al2O3 composite after treated athydrogen atmosphere. The magnetic metal-ceramic nano-compositewith superfine grain size could be obtained if we hot-pressed the Fe/γ-Al2O3 composite at high temperature.Reducing temperature affected the structure and grain size ofsamples. For the uniform doping method, sample is amorphous whentemperature is 500℃, there is some Fe2+reduced;when temperatureincreased to 700 ℃, more were reduced to Fe2+ ions and Fe, but thereFe3+ions still existed. When reducing temperature is 900℃, all Fe3+ions and Fe2+ ions were reduced to Fe. There is an in-process productFeAl2O4 during the reduce course. Magnetic properties are dependentwith grain size of α-Fe in specimens. With the increasing of size, thecoercive force decreases and saturation magnetization increases.Stableα-Fe/α-Al2O3 chunk composite could be obtained at1080℃ by uniform doping. Compared with powders, there isFeAl2O4 appear in samples prepared by uniform doping method. Soreduction is easy for powders.For the chunk materials, with the increasing of concentration, thehardness and coercive force decreases while density and saturationmagnetization increases for the coating samples. When reducingtemperature increased, the hardness, density and Ms value of samplesincreased except the specimen reduced at 500 ℃.Fe-Ni alloy provided the magnetism. With the increase of Niconcentration, coercive force decreases, the Hc value of Fe-Ni alloyincreases while the Hc value of FeNi3 alloy decreases. For cupperdoped samples, Hc, Mr and Ms decrease with the increase of Cuconcentration.For Co-doped samples, the interaction of Fe-Co alloy and grainsize affected magnetic properties of samples. When magnetism arisefrom Fe-Co alloy only, Hc, Mr and Ms decrease with the increase ofCo concentration. When Co and Fe-Co alloy coexisted, Mr and Msincrease with the increase of Co concentration, the Hc is adverse.