Studies On The Synthesis Of Non-equilibrium Phases By Mechanical Alloying And Their Characterization

Since it was invented,mechanical alloying has been made great progress and became a fundamental material synthesizing method,especially in the synthesis of non-equilibrium materials.Due to its special thermodynamic and kinetic conditions, mechanical alloying has been widely used to prepare materials which can not be synthesized by conventional methods,although it may bring contamination.The synthesis of non-equilibrium phase by mechanical alloying becomes more and more important in the field of material science.In this dissertation,a series of non-equilibrium phases was synthesized by mechanical alloying,including Al(B,N) and W₅Sn solid solution,turbostratic boron nitride,high-temperature stabled molybdenum carbide and three unreported phases in the Ni-Sn-P system.At the same time,their structural evolution and reaction mechanism were studied.The microstructure and properties were also characterized.The major conclusions are summarized as follows:1.It was found that no new phase was formed during MA for Al/BN mixture,while the BN diffraction peaks are similarly absent like other systems after mechanically alloyed for a certain period of time.AlN and AlB₂ have formed after the milled samples were annealed at 773K for 2 hours.BN has gradually decomposed into boron and nitrogen atoms and the decomposed atoms then diffused into aluminum lattice to form solid solution as MA proceeded.The solid solution decomposed at around 500℃.The XRD patterns of h-BN with different crystalline sizes were simulated by general Debye equation.The simulation revealed that the refinement of BN crystallite was very obvious because of its cleavage.The XRD peaks of BN broadened and the intensity decreased much more obvious than those of metallic component.2.There have been wide arguments about the structural evolution of hexagonal BN during mechanical alloying.As to eliminate the difference of atomic scattering factors and absorption coefficient between BN and metallic component, hexagonal and cubic BN mixture was selected a starting material.It was found that the grain size of h-BN decreased remarkably with increasing milling time but grew up during annealing.Hexagonal BN was refined significantly during ball milling and a turbostratic structure rather than amorphous phase could be induced by ball milling.Based on general Debye equation,the XRD patterns of t-BN with different parameters were simulated,including rotation,translation,fluctuation of interlayer spacing,curvature of hexagonal layer and deviation of atom position in the hexagonal layer.The impact of above parameters on the XRD patterns of t-BN was also discussed.3.The difference of melting point between tungsten and tin is as high as 3175℃ and the melting of tungsten is even higher than the boiling point of tin(2603℃). So far no compound between tungsten and tin has been reported.Mechanical alloying of 5W-Sn mixture with a positive enthalpy of formation was performed for different hours.Single BCC phase was found after 40 hours' milling by means of XRD.The obtained experimental data and XRD pattern simulation showed that a W-Sn solid solution rather than a coherent composite or a nanosized mechanocomposite was synthesized by MA.The as-alloyed solid solution decomposed at 900℃.4.The molybdenum carbides reported so far are thermodynamically equilibrium phase.The high-temperature stabledη-Mo₃C₂ was retained at room temperature during the mechanical alloying of MoO₃-2Al-xC(x=0.5,1.0,1.25,1.5,1.75,2.0) mitures.β-Mo₂C and high temperature phaseη-Mo₃C₂ were mechanosynthesized by ball milling mixtures(MoO₃+2Al+xC)with different graphite contents(x=0.5, 1.0,1.25,1.5,1.75,2.0)at room temperature.The phase constitution of the resultants was quite different depending on the graphite content.The mechanosynthesis reactions were mechanically induced self-propagating reactions.The dependence of resulting phase constitution upon the graphite content is probably because that the increase of graphite depressed the adiabatic temperature of the reacting system and excess graphite tended to prevent aluminum from contacting with MoO₃ and prolonged the milling process.The oxidation of high temperature phaseη-Mo₃C₂ in air starts at about 360℃and the end product is MoO₃.Theρ-Mo₃C₂ phase is stable at 400℃in argon and it transforms intoβ-Mo₂C when heated to 700℃.5.There have been only orthorhombic Ni₂SnP and hexagonal Ni₃SnP phases prepared by conventional melting method.In this dissertation,three unreported phases in the Ni-Sn-P system were synthesized by mechanical alloying.Their lattice parameters were refined by the Rietveld method and the microstructure and properties were also characterized.The electrochemical properties of the tetragonal phase prepared from Ni₂-Sn-P were tested.The initial capacity of the tetragonal phase as anode material for lithium battery reaches 500.4mAh/g and it decreases to 181.8mAh/g after 25 cycles.The relatively low capacity is attributed to the low content of active tin element and the poor cycle life could be due to the amorphous phase in the sample and/or the metastable state of the new phase.Its initial irreversible capacity is only 102mAh/g,which makes the new phase a promising anode material after improvement.