Thermodynamics And Kinetics Of The Reaction Process Between Iron And Boron Nitride And Physical Properties Of The Products

Nanocomposites are usually prepared by dopping nano-particles with excellent physics properties in ceramic or metal matrix. They not only keep physics properties of matrix and nano-particles phases, but also reveal some physical and chemical properties better than those of matrix and nano-particles because of the interaction between matrix and nano-particles. Nanocomposites are usually classed as reinforced and function nanocomposites. The reinforced nanocomposites are usually prepared by dopping particles or fibers with nanometer scale in matrix and then sintering under vacuum or by precipitating nanophase from matrix. In the past years, many metal nitrides were usually produced by gas-solids reaction between metal and gases containing nitrogen atoms, such as, N2 and NH3, etc., but little by solid-solid reaction between metals and solids containing nitrogen atoms. It is well known that hexagonal boron nitride (h-BN) is a kind of non-magnetic material with high heat conductivity and is an ideal matrix material of nanocomposites. It is expected to be a new method to produce metal nitride by solid state reaction between metal and BN on use of mechanical milling and high pressure. Especially, new metal nitrides are expected to be prepared by solid state reaction between metal and BN under high pressure. In the present experiment, metal Fe and h-BN are chosen as precursor. Fe-N alloys were prepared by means of mechanical milling and high pressure technologies. Their structures are examined by X-ray diffractory (XRD). Fe/BN nanocomposites were prepared by mechanical milling, vacuum heat treatment ,the thermodynamic and kinetic mechanisms of formation ,phase transition , magnetism and electricity properties were studied by XRD, vibrating sample magnetometer (VSM) etc. An amorphous Fe–N alloy and a single phase ε?FexNalloy were prepared by milling a mixture of Fe and h-BN for 25 and 60 h, respectively. The Fe–N amorphous alloy was formed by reaction of Fe and a-BN with the volume ratio of 1:5.4, and ε?FexN alloy originated from a mechanically driven crystallization of the Fe–N amorphous alloy and reaction of Fe and a-BN. When annealing the 30 h-milled Fe/BN mixture for 1 h under atmospheric pressure at 770 and 1170 K, respectively, a small amount of γ?Fe4N was obtained at 770 K, and a small amount of γ?Fe(N) was produced at 1170 K. γ?Fe4N originates mainly from the crystallization of the Fe–N amorphous alloy and the reaction of a small amount of Fe and a-BN, while γ?Fe(N) is due to the phase transition from γ?Fe4N to γ?Fe(N). When the 30 h-milled mixture was annealed under 4GPa in a temperature ranging from 690 to 800 K, all the Fe reacted partially with the N atoms from the a-BN to form a single phase of ε?FexN, which is the same as the reaction product obtained by ball milling but different from that prepared by annealing at normal pressure. High pressure not only promotes the reaction of Fe and a-BN, but also affects the crystal structure of the reaction products very much. However, no Fe–B alloy was observed to form in the present experiment.The Gibbs free energies of formation for the Fe–N and Fe–B alloys were calculated by using the Miedema model and the entropy of formation reported in previous experiments, indicating that the Gibbs energy of the Fe–B alloy is much more negative than that of the Fe–N alloy, which contradicts the present result as regards the thermodynamics. Therefore, the formation of the Fe–N alloy in the reaction of Fe and a-BN is attributed to the fact that the Fe–N alloy has a smaller reaction potential barrier than the Fe–B alloy. This is due to the fact that N atoms have smaller atomic radii than the B atoms. The former can be incorporated into α?Fe to form an α?Fe(N) solid solution with limited solubility while the B atom cannot. The Fe/BN nanocomposites were prepared by milling Fe and h-BN with the volume ratio of 1:12.8. The changing of the saturation magnetization and the coercive force of Fe/BN with components and milling time show that: the formation of a-Fe-N shell on the α?Fe surface can reduce the saturation magnetization and increase the coercive force. The saturation magnetization of Fe/BN nanocomposites has a tendency to reduce with increasing the milling time. During the first 20-30h, there is no reaction and phase transition, the decrease of the saturation magnetization is originated from the transformation of α?Fe to superparamagnetic Fe. On the other hand, N doped into the Fe lattice interstitially testified from the increase of the lattice constant make the powder consist of paramagnetic a-Fe-N which decrease the saturation magnetization. The coercive force increases with the milling time. Many experiments show that, the internal stress during the milling process is the...