| With the development and utilization of electronic and electrical equipment toward to miniaturization,integration,high frequency and intelligence,the issues related to thermal and tribological management are becoming more crucial.It is difficult for traditional nanofluids to improve heat transfer enhancement and reduce tribological behavior of high-precision devices that require strict control of operating temperature and surface wear,which provide opportunities and challenges for the research of nanofluids.Therefore,the development of new multifunctional nanofluids for thermal management of microelectronic devices is a better solution.Therefore,the development of multifunctional nanofluids for thermal management and lubrication is very essential for the advancement of many energy efficient modern industries.Recently,hexagonal boron nitride (h-BN) has attracted increased attention due to its high thermal stability,excellent corrosion resistance and excellent electrical insulation.Especially,high thermal conductivity,lubricity and wear resistance at high temperature make BN materials reliable candidates for thermal lubricant additives of nanofluids.In this paper,the aim is to improve the friction behavior and thermal conductivity of nanofluids,as well as the heat collection and storage during friction process.Nanofluids and composite thermal storage phase change materials containing BN were prepared,and their tribological performance and thermal properties were investigated.The main research contents of this are as follows:1.Preparation of boron nitride nanospheres and their tribological properties in nanofluids:Solid lubricating additives can be used to improve the tribological properties of conventional liquid lubricants.Unfortunately,their low dispersion and poor stability have restricted their broader application.Herein,BN nanospheres were prepared by spray drying-assisted pyrolysis method,and the reaction conditions were optimized in order to obtain uniform BN nanospheres.The experimental results show that when the molar ratio of boric acid to urea is 1:2 and the pyrolysis temperature is1000°C,the obtained BN nanospheres have high uniformity and small size(~20 nm).BN/paraffin nanofluid-based lubricants were prepared by adding BN nanospheres into liquid paraffin,which can remain homogeneous stable state for more than three weeks.The tribological behaviors of BN/liquid paraffin using the four-ball friction and wear tester show that the friction coefficient and wear scar diameter of 1.5 wt%BN/liquid paraffin are reduced by 50%and 40%,respectively,when compared to pure liquid paraffin.These results demonstrate that the BN nanospheres have excellent friction reduction and wear resistance which are suitable for tribological management.2.Preparation of hollow boron nitride nanospheres and their heat transfer properties in nanofluids:Friction process will generate a large amount of heat,which will not only affect the stability of liquid lubricants,but also reduce the operating efficiency and working life of equipment.Herein,hollow BN nanospheres were prepared using hydrothermal carbon spheres as templates.Effect of the amount of template on the morphology of precursors and final products were investigated.The experimental results show that when the molar ratio of carbon sphere,boric acid and melamine is 9:3:1,hollow BN nanospheres with uniform morphology and diameter of~500 nm are synthesized.During the pyrolysis process,the carbon sphere template was removed in the form of a carbon-containing gas,thereby obtaining a hollow BN sphere similar to the template.The surface of BN hollow nanospheres has a large number of-OH groups,which were confirmed by FTIR spectrum and XPS spectra.Zeta potential measurements show that hollow BN nanospheres with high hydroxylation degrees exhibit good dispersion stability in water.When the volume fraction of BN nanospheres was 0.10 vol%,the thermal conductivity of BN/water nanofluids increased from 0.589 to 0.917 W/mK,thermal conductivity enhanced by 56%.These results suggest that the present water-based BN nanofluids are promising for thermal management systems.3.Preparation of boron nitride microrods and their heat transfer and tribological properties in nanofluids:The above experiments reveal that BN nanofluids have potential applications in tribological management and thermal management systems.However,investigations on the use of BN as a multifunctional additive to improve the tribological properties and heat transfer performance of nanofluids have not been rarely reported.Herein,BN microrods were prepared by spray drying-assisted pyrolysis method,and the effect of preparation parameters on the morphology,structure and chemical composition of the final products were investigated.Compared with drying-assisted pyrolysis,BN microrods prepared by spray drying-assisted pyrolysis exhibit fish-scale-like structure,large specific surface area (616 m~2/g),wide pore size distribution (4-60 nm) and a high oxygen content (12.7at%).A large number of hydrophilic groups are distributed on the surface of BN microrods,which can make them maintain homogeneous dispersion and long-term stability in water.The tribological and thermal properties of BN in water-based nanofluids show that the friction coefficient and wear scar diameter were reduced by79%and 41%,respectively,when a small amount of BN microrods (0.10 vol%) was added to water,the friction coefficient and wear scar diameter were reduced by 79%and 41%,respectively.In addition,the thermal conductivity of the BN/water nanofluids reached 1.117 W/mK,which is about 90%higher than that of pure water.This multifunctional BN nanofluids provides more options for various energy management fields in future.4.Preparation of boron nitride foams and their application in phase change thermal storage materials:Recovery and reuse of waste heat from the friction process is an effective way to conserve energy.In thermal energy storage systems,latent heat storage using inorganic phase change materials as one of the most promising heat storage methods.However,inorganic phase change materials have some the shortcomings,including low thermal conductivity,high supercooling degree,liquid leakage,poor shape stability and slow response rate,which have restricted their practical implementations.Herein,three-dimensionally interconnected porous BN foam was prepared using melamine sponge as template.The porous structure and hydrophilic surface help to confine the molten eutectic hydrate (EHS) phase change material within BN foam.When the content of BN foam is 4 wt%,the thermal conductivity of the EHS/BN foam composite reaches 1.359 W/mK,which is 10.4 times and 2.3 times than that of the pure EHS and EHS/5 wt%polymer composites.Interconnected BN networks with high quality could offer a‘‘high-speed rail''for heat transfer,which could effectively reduce the phonon-phonon and phonon-boundary scatterings.In addition,the properties of BN foams including large specific surface area and abundant oxygen-containing groups provide a large number of nucleation sites for EHS,which can effectively reduce supercooling degree.The introduction of BN foam can also greatly enhance the shape stability of EHS.After 300melting/solidification cycles,the DSC curve and FTIR spectrum of the EHS/BN composite phase change materials show negligible change,indicating that the as-prepared EHS/BN composite materials have excellent thermal reliability and outstanding cyclic stability.These results indicate that BN foam can be used as a promising thermally conductive packaging material for phase change energy storage,and it is desirable to collect,store and reuse waste heat generated by friction. |
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