Novel Preparation Processes And Structure-Property For Alumina/Nickel Composites

Alumina has been and will continue to be widely used in structural,electronic, optical and biomedical applications due to its outstanding properties of low cost,high rigidity,high electrical resistance and chemical stability.However,the brittleness is a major limitation of alumina in many applications.It is well known that the brittleness of alumina is resulted from the chemical bonding characteristics among components and the microstructure,so that optimizing the microstructure and state of chemical bonds among components are keys to reduce the brittleness of alumina.Moreover,with the functionality has been a general tendency for materials technology,combining the functional and structural characteristics of ceramics and developing novel alumina-based composites have been a hotspot.Based on the structural design of microspheres,nano-ferromagnetic metals (Fe,Co,Ni and alloys)-coated alumina microspheres were prepared by heterogeneous precipitation,followed thermal-reduction process.Then,alumina-based Ni composite cermets were prepared by hot-press sintering alumina/Ni composite microspheres.The microstructure and interfacial bonding states for alumina/Ni composite cermets were tailored via adopting different sintering raw materials and hot-press sintering processes. Furthermore,alumina-based Ni and zirconia composites were prepared by coating Ni and yttrium-stablized zirconia successively on alumina microspheres.The morphologies, microstructure and composition or crystalline phases of the composite microspherical precursors,thermal-reduction products and resultant hot-press sintering cermets were characterized by scanning electron microscope(SEM),energy dispersive spectroscope (EDS) and X-ray diffraction(XRD),respectively.The microstructure and interfacial chemistry of alumina/Ni cermets were analyzed by transmission electron microscope(TEM) and electron energy loss spectroscopy(EELS).The Archimedes' method was used to measure the density of alumina/Ni cermets.The flexural strength and fracture toughness were estimated via the three-point bending and single-edge-notched beam methods respectively.And by means of the LCR automative meter and vibration sample magnetmeter(VSM) the electro-and magnetic properties of alumina/metal composites were investigated.The main results and innovations obtained in this study include:(1) A novel preparative route for core-shell structural nanometal-coated alumina microspheres has been developed and a series of nano-metals(Fe,Co,Ni and alloys)-coated alumina powders were successfully prepared by this novel heterogeneous precipitation coating-thermalreduction process.These spherical powders were characterized with controllable properties,such as metal content and constitutes,coating thickness,metal distributing state and size of metal particles,and excellent magnetic performance.The aqueous heterogeneous precipitation process were mainly influenced by the concentration of coated particles,adding raw materials rate,reaction time and surface active reagent,et al.At the optimized precipitation parameters,i.e.,10～15g/L of the concentration of alumina micropowders,4～5mL/min of the rate of adding reactants, 5mL/L of the surface active reagent,and 1～2 hr of the reaction time,the mechanical stirring speed of 1000r/min,the designed precursors of Ni-coated alumina microspheres were fabricated.Then Ni-coated alumina powders were prepared after the hydrogen reduction of the precursors.(2)Based on the microstructural design concept,three kinds of composite cermets with intergranular structure,intragranular structure or mixture of these two named according to the Ni particle location in the alumina matrix were prepared by hot-press sintering Ni-coated alumina microspheres,alumina-coated Ni microspheres and alumina/Ni coprecipitates respectively.These processes could adjust the distribution manners of Ni particles,and made the microstructural design for the alumina/Ni composites come true.(3)By adopting metal-coated powders,the structural uniformity of composite cermets were improved,and the interfacial bonding state between Ni and alumina was also amended by the formed strong bonds of -Ni-Ni-Al-O-Al at some hot-press sintering conditions.The introduction of Ni particles was also favorable to fine grain sizes of alumina.The toughening alumina was enhanced by strengthening matrix grain boundaries and changing fracture mode of alumina and a maximum mean values of about 600MPa for fracture strength and 6～8MPa·m^1/2 for fracture toughness were obtained.And these cermets exhibited high saturation magnetism,coercive force and relative high dielectric constants.(4)Comparing with monolithic alumina,the inclusions of Ni and ZrO₂ could increase the fracture strength of alumina by 28%and fracture toughness by 141%. However,the toughness enhancement of Al₂O₃/(ZrO₂+Ni) composites was not higher than the sum of the toughness enhancement of Al₂O₃/ZrO₂ and of Al₂O₃/Ni composites, because the toughness by phase transformation of ZrO₂ was absorbed by large Ni particles neighboring to ZrO₂ phase.And higher dielectric constants of composites were due to the intensification of space charge polarization by means of the inclusion of Ni and ZrO₂.This heterogeneous precipitation coating—thermal-reduction process for preparation of nanometal-coated alumina microspheres is featured with simple processing,rich raw materials resource,and easy scale-up production.These composite microspheres with excellent fluidity,high filling density,high specific surface areas and surface activeness,combined the properties of nanometals with alumina matrix and possessed the cooperative features of metal shell and alumina core,are promising excellent electromagnetic wave absorbers and advanced catalysts in hydrogen-production industry.Furthermore,by adopting these kinds of microspheres as sintering raw materials,many technical issues such as weak interfacial combinations between metal and alumina matrix,poor uniformity in chemical components etc.,will be resolved,and these materials have great potential uses in the fields of ceramic tool materials,large inert anode materials for aluminium electrolysis,and ceramic-metal seals,etc.