Synthesis,Characterization,Tribological Properties Of 1,4-Diazabicyclo[2.2.2]octane Derived Dicationic Ionic Liquids And Their Hybrids With Carbon Quantum Dots

Ionic liquids have many advantages,such as low volatility,high thermal stability,non-flammability,low melting point and excellent solubility,which meet the performance requirements of ideal lubricant additives.In recent years,ionic liquids have attracted wide attention as a new type of high-performance lubricant additive.The unique designability of ionic liquids makes it possible to obtain countless kinds of ionic liquids through the combination of anions and cations,while their solubility,thermal,chemical stability can be tuned by changing the type of anions and cations.In this thesis,a novel dicationic ionic liquid with symmetrical structure was designed based on 1,4-diazabicyclo[2.2.2]octane cation(DABCO).Furthermore,the functionalized ionic liquids based on 1,4-diazabicyclo[2.2.2]octane were designed and synthesized to achieve the preparation of hybrids based on the 1,4-diazabicyclo[2.2.2]octane-based ionic liquids and carbon quantum dots.Their physicochemical properties and tribological properties in polyethylene glycol(PEG)synthetic base oils were thoroughly investigated.(1)[C4DABCOC4][Br]2 dicationic ionic liquid was synthesized based on 1-bromobutane and 1,4-diazabicyclo[2.2.2]octane.Three dicationic ionic liquids with different anions were synthesized via ion exchange reaction.The structure of the ionic liquids was confirmed by fourier transform infrared(FTIR)spectroscopy,nuclear magnetic(1H,13C,11B,31P and 19F)and mass spectrometry.Simultaneously,the effects of different anion types on the thermal properties,wettability,kinematic viscosity and friction properties of the ionic liquids were investigated.Among them,[C4DABCOC4][NTf2]2 showed superior anti-wear and friction-reduction properties as a lubricant additive than others.PEG with 2wt%ionic liquids achieved remarkably improved anti-wear property(81.28%)and friction-reduction property(19.27%)at loads of 392 N.A similar effect was also observed when PEG with ionic liquids were tested under 588N load,which remarkably improved anti-wear property(85.82%)and friction-reduction property(17.70%),respectively.The anti-wear and friction-reduction mechanism of the ionic liquids were explored based on the multiple surface analyses techniques:the ionic liquids formed a physical adsorption layer on the surface of the friction pair via electrostatic adsorption.Along with the rubbing time,adsorpted ionic liquids started to partially decomposed under the action of high pressure,friction heat and mechanical energy generated by friction.Then the active elements combining with the friction pair material formed a chemical reaction film containing various compounds.(2)Hydroxyl functionalized ionic liquid[C3BHDABCOC3][C1]2 was synthesized based on 3-chloro-1-propanol and 1,4-diazabicyclo[2.2.2]octane,followed by the preparation of ionic liquid hybrid carbon quantum dot(CDs-Cl)via in-situ co-carbonization of the[C3BHDABCOC3][Cl]2 and carbon dot precursor.The hybrid structure was analyzed by fourier transform infrared(FTIR)spectroscopy,nuclear magnetic(1H,13C),mass spectrometry,X-ray photoelectron spectroscopy(XPS)and transmission electron microscopy(TEM).The results indicated that the CDs-Cl have a narrow size distribution with an average diameter of 2.6-3 nm and exhibited excellent dispersion stability in the PEG base oil.The addition of CDs-Cl could effectively improve the anti-wear and friction-reduction properties of PEG.The wear volume of PEG was improved by 39.62%,and the friction coefficient was reduced by 36.36%at loads of 392N.Moreover,CDs-Cl can significantly improve the running-in process,reducing the coefficient of friction in a shorter period of time.(3)Three hydroxyl functionalized ionic liquids[C3BHDABCOC3][BScB]2,[C3BHDABCOC3][PF6]2 and[C3BHDABCOC3][NTf2]2 were synthesized via anion exchange reactions between[C3BHDABCOC3][C1]2 and various anions.Furthermore,the ionic liquid hybrid carbon quantum dots(CDs-IL)were prepared via in-situ co-carbonization,which showed luminescence dependence and uniform size morphology The results showed that CDs-IL with different morphologies,compositions and physicochemical properties can be obtained by adjusting the anion structure of functionalized ionic liquids.These three kinds of CDs-IL as lubricant additives can significantly improve the anti-wear and friction-reduction properties of PEG,of while the CDs-PF6 has the best performance in PEG.Compared with pure PEG,the wear spot volume can be reduced by up to 96.49%,and the friction coefficient can be reduced by up to 72.72%.In addition,the friction coefficient of PEG with lwt%CDs-IL reaches the minimum and remains stable in a very short period of time after the friction starts(less than 500s).It is found that CDs-IL transfers continuously from the oil to the friction surface under the action of friction to achieve a "rolling effect",thus reducing the friction coefficient.On the other hand,CDs-IL can form a carbon-rich lubrication film on the friction surface,thus improving the wear resistance.