Preparation And Properties Of Multi-walled Carbon Nanotubes/Alumina Composites By Sol-Spray Drying

Aluminum oxide（Al₂O₃）ceramics possess notable attributes including high mechanical strength,chemical stability,and temperature resistance.However,their widespread application potential is severely constrained by drawbacks such as brittleness,low conductivity,and limited thermal conductivity.Thus,the enhancement of alumina ceramics to improve toughness,conductivity,and thermal conductivity holds great research significance and practical value.Multi-walled carbon nanotubes（MWCNTs）,renowned for their exceptional mechanical,electrical,and thermal properties,are regarded as an ideal reinforcement for composite materials aiming to address the limitations outlined above.Unfortunately,MWCNTs exhibit structural characteristics such as their substantial aspect ratio,extensive specific surface area,and poor surface wettability that render them prone to tangling or aggregation.Therefore,achieving uniform dispersion within alumina ceramic substrates and establishing robust interfacial bonding pose considerable challenges.To address these concerns,this study leveraged the composite design concept and employed a powder metallurgy process to synthesize MWCNTs/Al₂O₃ composite powders with uniform dispersion via a sol-gel spray drying.Subsequently,MWCNTs/Al₂O₃ composites were prepared using spark plasma sintering（SPS）.Furthermore,the investigation explored the impact of Zr O₂ and Si O₂ nanoparticles,present on the surface of MWCNTs,on the interface structure and properties of the composite material.The primary research content and conclusions of this work are as follows:The SPS exhibits a commendable physical purification effect on MWCNTs.Subjecting them to a vacuum at 1600℃for 10 minutes in vacuum not only effectively reduces the content of free carbon and metal impurities,such as Ni and Al,but also efficiently graphitizes the carbon nanotubes.Through the sol-spray drying method,MWCNTs/Al₂O₃ composite powders are prepared,showcasing excellent dispersion performance.Notably,the mild acidic and alkaline environment during the preparation process does not cause noticeable damage to the surface structure of MWCNTs.Moreover,rapid dehydration process effectively inhibits the entanglement and reaggregation of MWCNTs.Mechanical tests reveal that the 2wt%MWCNTs/Al₂O₃ composite materials significantly enhances their bending strength and fracture toughness,resulting in a 55.3%and 77.9%increase,respectively,compared to pure alumina ceramics.Specifically,when the length of MWCNTs is approximately 1μm,the composite material exhibits the highest bending strength and fracture toughness,experiencing a remarkable increase of 62.6%and 87.4%,respectively.Furthermore,when the outer diameter is around 50nm,the bending strength and fracture toughness of the composite material increase by58.3%and 83.0%,respectively.Additional testing of the 2wt%MWCNTs@5wt%Zr O₂/Al₂O₃ composite materials exhibit favorable mechanical properties compared to the Zr O₂-free sample,the bending strength and fracture toughness are increased by 20.7%and 8.4%respectively.In contrast,the Si O₂-loaded sample exhibits a modest improvement in bending strength and fracture toughness,but showcases a remarkable Vickers hardness of 2601.3 HV.The strengthening mechanism primarily involves the synergistic effect of matrix grain refinement,MWCNTs extraction and tearing,as well as crack deflection and bridging.Electrical conductivity tests demonstrate that when the addition of MWCNTs is0.5 wt%,the composite materials exhibit a conductivity increase an extraordinary 12orders of magnitude over pure Al₂O₃ ceramics.As the length and outer diameter of MWCNTs increase,the conductivity of the composite material shows a gradual increase,albeit with a relatively small impact.The electrical conduction of 0.5wt%MWCNTs@5wt%Zr O₂/Al₂O₃ are increased by 41.3%compared to the Zr O₂-free sample.However,the 0.5wt%MWCNTs@5wt%Si O₂/Al₂O₃ sample exhibits a significant deterioration in electrical with decreased 75.5%compared to the Si O₂-free sample.The conductive mechanism in these composite materials can be explained by the percolation theory in conductor/insulator composites.The percolation threshold of composite material is f_c=0.32±0.02 wt%with fitting the percolation concentration formula.The size of the percolation threshold can be used as an indicator to evaluate the dispersion of MWCNTs in the composite materials.Thermal diffusivity tests show that when the addition of MWCNTs is 0.5 wt%,the composite material exhibits the highest thermal diffusion coefficient,with an average improvement of 26.2%compared to pure Al₂O₃ ceramics.Notably,when the length of MWCNTs is 5μm and the out diameter is 50nm,the thermal diffusion coefficient is higher.The thermal diffusion coefficient of 0.5wt%MWCNTs@5wt%Zr O₂/Al₂O₃ are increased by 41.9%compared to the Zr O₂-free sample.However,the thermal diffusion coefficient of 0.5wt%MWCNTs@5wt%Zr O₂/Al₂O₃ are hardly improved.The thermal conductivity enhancement mechanism of composite materials is that MWCNTs form an effective thermal conductivity network in the matrix,forming a synergistic thermal conductivity mechanism of phonons and electrons.In the MWCNTs/Al₂O₃ composite material,there is an optimal critical value for the mass fraction and morphology size of MWCNTs.At this time,the composite material can achieve good bending strength and fracture toughness,and the conductivity and thermal diffusion coefficient are also improved the best.This provides theoretical support and data reference for the preparation of multifunctional composite ceramic materials.Surface modification of MWCNTs with Zr O₂ prior to inclusion in the composite further enhanced performance.Zr O₂ improves the dispersion of MWCNTs and the interface bonding with the matrix.After loading Si O₂,the mechanical properties only slightly improved,while the conductivity and thermal diffusion coefficient both decreased.This deterioration can be attributed to Si O₂ exacerbating MWCNT aggregation,increasing porosity and interfacial density.finally increases the interfacial thermal resistance effect.