Dispersion Of Zirconia Nanoparticles And Preparation Of Cross-linked Zirconia Nanoparticle Films

Zirconia (ZrO2) material possessing the advantages of chemical inertness, excellent thermal stability, high refractive index and high hardness, is an ideal candidate to construct functional coatings with great potential applications as optical coatings, scratch resistant coatings, barrier coatings for polymer substrates, and etc. Inorganic ZrO2 coatings could be fabricated by filtered cathodic vacuum arc, ion beam induced chemical vapor deposition and sputtering. The resulted ZrO2 films generally have high quality with refractive index as high as 2.1-2.3. But all the processes involved have to be conducted under extremely high vacuum using special and expensive apparatus, which limits their large scale application and causes high cost. Sol-gel process using zirconium alkoxide is alternative choice for preparation of inorganic ZrO2 thin coatings. However, to dense and crystallize the ZrO2 phase, annealing at high temperature is usually employed, limiting their application on plastic substrate. Polymer/ZrO2 organic-inorganic nanocomposite coatings fabricated via a sol-gel process or blending with ZrO2 nanoparticles have been developed quickly in recent years. This approach is mild and the obtained organic-inorganic hybrid film can be applied large-scalely on various substrates. The shortcoming of nanocomposite coatings lies in its difficulty to achieve high ZrO2 level because phase separation of ZrO2 nanoparticles easily takes place. To work out this problem, it had better to avoid using polymer. Recently, we have successfully fabricated homogeneous ZrO2 nanoparticles dispersions deagglomerated at primary particle size level using nonaqueous synthesized ZrO2 nanoparticles with the help of ligands. These dispersions make it possible to construct transparent ZrO2 nanoparticle films with high ZrO2 content. In this article, the dispersion behaviors of ZrO2 nanoparticles in various organic solvents or water were investigated. Highly transparent ZrO2 nanoparticles coatings with high refractive index and prominent mechanical properties were fabricated from the transparent solvent-based or waterborne ZrO2 nanoparticles dispersions. The detailed experiments and results are described as follows.ZrO2 nanocrystals (4 nm), synthesized from zirconium-(IV) isopropoxide isopropanol complex and benzyl alcohol, were dispersed and functionalized in organic solvents using three kinds of bi-functional silane coupling agent (SCA), 3-glycidoxypropyltrimethoxysilane (GPTMS),3-aminopropyltriethoxysilane (APTES) and 3-isocyanatopropyltriethoxysilane (IPTES). Completely transparent ZrO2 dispersions were achieved in tetrahydrofuran (THF) with all three SCAs, in pyridine and toluene with APTES and IPTES, and in N, N-dimethylformamide with IPTES. Dynamic laser scattering (DLS) measurements and high resolution transmission electron microscopical (HRTEM) observation indicated that the ZrO2 nanocrystals are dispersed on primary particle size level. Fourier transform infrared spectroscopy, solid-state 13C-NMR and 29Si-NMR spectroscopy and thermogravimetric analysis demonstrated that all three SCAs are chemically attached to the surface of the ZrO2 nanoparticles, however, in different bonding modes. Except for GPTMS/ZrO2/THF dispersion and IPTES/ZrO2/pyridine dispersion, all other transparent dispersions have poor long-term stability. The increasing polarity, due to high amount of APTES attached and high hydrolysis and condensation degree of the bonded APTES, and the aggregation, due to inter-particle coupling via the bonded triethoxysilyl group, are the causes of the poor long-term stability for the ZrO2 dispersions with APTES and IPTES, respectively. Nevertheless, the APTES-functionalized ZrO2 precipitates can be de-agglomerated in water to get a stable and transparent aqueous ZrO2 dispersion via addition of a little hydrochloric acid or sodium hydroxide.UV-curable ZrO2 nanoparticle coatings, prepared by dispersing highly-crystalline ZrO2 nanoparticles in tetrahydrofuran with the aid of 3-glycidoxypropyl-trimethoxysilane (GPTMS) and following addition of a cationic photoinitiator, were cast on silicon wafers (or glass substrates) by dip-coating or spin-coating and photopolymerized to get transparent ZrO2 nanoparticle films. Ellipsometrical characterization indicates that the refractive index of the film changes from 1.63 to 1.77 at wavelength of 632 nm when the molar ratio of GPTMS-to-ZrO2 decreases from 0.30 to 0.15. Nano-indentation tests show that the films exhibit robust mechanical performance though they are not heat-treated.Highly-crystalline ZrO2 nanoparticles were functionalized with 3-(N-amino-ethyl)aminopropyltrimethoxysilane (AAPTMS) and dispersed in water at primary particle size level under basic condition (pH value of 13-14). The aqueous ZrO2 nanoparticles dispersion was cast on polycarbonate substrate with 1,4-butanediol digylcidyl ether as the cross-linker and then heated at 120℃for 1h to obtain ZrO2 nanoparticles films with as high as 81 wt% of ZrO2. The ZrO2 nanoparticle films are highly transparent and have refractive index changing from 1.70 to 1.77 at wavelength of 632 nm as the amount of AAPTMS attached to ZrO2 nanoparticles decreases. Nanoindentation tests show that the hardness of the film reaches 1.7 GPa, while punched tape abrasion and nanoscratch tests reveal that the films exhibit prominent scratch resistant performance.