| LaB6 nanopowder has aroused great attention of academic and industry field due to its excellent absorbing and scattering effect to NIR and UV reported by Australian scientists in 2003.The exploration on synthesis and optical application of LaB6 nano-material has become research focus.The synthesis of LaB6 nanopowder by chemical synthesis method was studied in this paper,and the optimal process was obtained by adjusting main process parameters,finally LaB6 nanopowder with high purity was gained.Meanwhile,the microstructure of LaB6 nanopowder was characterized using FESEM and HRTEM,etc.Its high temperature stability,magnetic and optical properties were also investigated systematically,eventually,this paper made new approaching on LaB6 nanopowder/PI,PMMA composite material.The results showed that LaCl3-NaBH4 reaction system is better for high purity LaB6 nanopowder synthesis than La2O3-H3BO3 and La2O3-NaBH4 reaction systems.The main process parameters include reaction temperature,holding time,raw material proportion and additive(Mg).TG-DTA results showed that reactions of LaCl3 and NaBH4 occurred at 513℃,the product with low crystallinity was observed at 900℃, perfect crystals were obtained when temperature exceeded to 1200℃;the grain size coarsed as the holding time prolonged from 1h to 3h;with the increasing of B content (the ratio of La and B increasing from 1:6 to 1:12),a transitional product LaBO3 reduced;in addition,additive Mg could remove LaBO3,however,impurities doped with Mg such as MgO were formed which were difficult to remove.Finally,high purity LaB6 nanopowder was successfully achieved by optimizing main process parameters.The synthesized LaB6 nanopowder is well crystallized,whose crystals are in cubic shape with the size of 20～70nm.Composed of perfect crystallized LaB6 nano-single crystals covered with a thin(1.5～2.5nm) amorphous layer,and the lattice parameter is a0=0.4104nm,the preferential growing directions are[100]and[110],and the atomic ratio of La and B(≈1:4.05)is larger than theoretical value,that may be induced by the amorphous layers surrounding LaB6 single crystals.The results indicated that LaB6 nanopowder is of good high temperature stability,but the stability was far weaker than that of LaB6 micron powder.LaB6 nanopowder began to be oxidized dramatically at 640℃,when temperature exceeded to 1200℃, almost all of the powder was oxidized to be La(BO2)3,the oxides were poorly crystallized,and the grain sizes were far beyond nanoscale.LaB6 nanopowder exhibited diamagnetic,whose magnetic properties were weaker than the weak paramagnetic LaB6 micron powder.It is difficult to be magnetized and to maintain the state of magnetization.Outermost electrons of the central atom in B6 structure between LaB6 and CaB6 were different,that might result in that magnetic properties of CaB6 were much better than that of LaB6,and when powder fined into nanoscale,LaB6 became diamagnetic,whereas CaB6 turned ferromagnetic.There were five major absorption bands in IR spectra of LaB6 nanopowder,of which absorption bands at 1130 cm-1 and 3438 cm-1 corresponded to the two infrared active phonon T1u of B6 structure.In Raman spectra,there were four peaks at 212 cm-1,680 cm-1,1078 cm-1 and 1217 cm-1,respectively,corresponding to four phonons T1u,T2g, Eg and A1g of LaB6,which confirmed the formation of LaB6.LaB6 nanopowder could absorb most of the ultraviolet,a little visible light,and the near infrared in the range of 780～2020nm significantly.Compared with the micron powder,obvious Einstein shift and blue shift,band broadening and narrowing occurred simultaneously in IR and UV-VIS-NIR spectra of LaB6 nanopowder,even new absorption bands appeared in IR spectra,which should be related to stress-strain generated by preparation and nano-effect.Peak broadening observed in the Raman spectra might be associated with lattice distortion or internal stress.LaB6 nanopowder/PI,PMMA composite films were successfully fabricated by in-situ polymerization.Optical properties also were studied.LaB6 nanopowder dispersed directly in the prepolymer or monomer,the prepolymer(PAA) imidized(PI) or monomer(MMA) polymerized(PMMA) to generate nanopowder/polymer composite material.As coupling agent,Silane made LaB6 nanopowder well-distributed in the polymer.Detected IR spectra confirmed the successful synthesis of composite films, and transmittance was lower than 1.05%,especially in the region of 400-1800cm-1, where the transmittance could be as low as 0.LaB6 nanopowder/PI composite film could absorb all the ultraviolet and some visible light,and films with LaB6 content over 0.5%will have obvious absorption to the near-infrared light,and reduce the transmittance of the visible light at the same time.LaB6 nanopowder/PMMA composite films could substantially absorb the UV in region of 200-230nm and small amount of NIR light,meanwhile,let over 80%visible light transmit without affecting much on transparency of the films.As the content of LaB6 increasing,the absorption in composite films spectra enhanced.
|
PDF Full Text Request