Synthesis of Rare-earth Hexaborides and Metal Vanadates One-dimentional Nanostructures and Their Properties

Author:Li Qi Dong

Supervisor:zhao yan ming


Degree Year:2018





Materials in nanoscale are deemed to be different from their bulk phase on the basis of the special functionality like quantum size effect derived from nanomaterials.In this thesis,we focused on the build and regulation of rare earth hexaborides(RB6,including La/Pr-doped NdB6,La B6/carbon cloth,LaB6,CeB6,PrB6,NdB6,SmB6,La-doped SmB6)as well as metal vanadates(including ZrV2O7 and Na2ZnV2O7)one-dimensional(1-D)nanomaterials.Furthermore,performances like field emission,optical absorption,thermal expansion,electronic conductivity and lithium storage are also explored on the grounds of the materials physical and chemical chracteristics.We attempt to use La/Pr to replace part of cations in NdB6 on account of the totally same structure of RB6 system,aiming at optimizing the morphology of as-synthesized 1-D nanostructures as well as their field emission properties.In fact,by utilizing the lower melting point of La-Nd and Pr-Nd alloy,we do fabricate better morphologies with much smoother surfaces and higher aspect ratio when using same synthesis method.When evaluated as field emission cathode materials,LaxNd1-xB6 presented a lower turn-on field of 4.12 V/μm,which indicate its potential in application of photoelectric display devices.This part demonstrates the doping strategy in optimizing and regulating the morphologies and performances of RB6 1-D nanosatructures,which is of reference value for further research.For cold field emission aspect,we designed LaB6/carbon cloth(LaB6/CC)by using flexible,high electronic and thermal conductivity carbon cloth as chemical vapor deposition(CVD)substrates.For the first time using low melting point and high reductibility NaBH4 as boron source,La metal powder as La source and Ni as catalyst,this method is proved to be high yield and repeatability.Finally,we tested the field emission properties of LaB6/CC long nanowires and the turn-on field,threshold filed are 2.51 V/μm and 4.28 V/μm,respectively.The fluctuation during 1 min of 4.2 V/μm applied external electric field is 5.09%,indicating the favorable stability.As the first report of flexible LaB6-based filed emitter,this work would be meaningful and helpful for next-episode development of LaB6 materials.RB6 were found to be sensitive to optical source.For the first time,we explored the optical property of 1D RB6 nanostructures.In the experiments,we using a low temperature solid state reaction to fabricate dual-RB6(LaB6,CeB6,PrB6,NdB6,SmB6)and ternary RB6(LaxSm1-xB6(x=0.2,0.4,0.6,0.8)).In combination of the reaction condition and change of morphologies,we concluded a growth mechanism which can be analogized to CVD approach.By measuring the optical properties of the samples in range of ultraviolet-visible light-near infrared region(UV-VL-NIR),transmittance in VL and absorption in NIR can be obtained.For LaxSm1-xB6nanowires,the absorption region would shift upon x value.The build of ZrV2O7 1-D nanostructure was based on a solution-based method.After certified by XRD and SEM,we found the as-synthesized ZrV2O7 1-D nanofibre was in its superstructure form.We for the first time studied the thermal expansion properties of ZrV2O7superstructure by using in-situ XRD technique.When evaluated as cathode material for Li-ion battery,a discharge plateau of 2.48 V and specific capacity of 282.6 mA h/g can be obtained.Furthermore,in-situ XRD was used to study the mechanism of Li insertion/extraction during first cycle.Results show that with Li inserted into ZrV2O7 superstructure,it experienced a solid solution reaction with subsequent conversion reaction till totally amorphous structure.On account of the high capacity and conductivity,this materials are promising candidate in thermal expansion as well as Li-ion battery areas.Na2ZnV2O7 1-D nanorods were synthesized by hydrothermal method.By adjusting the experimental parameters,we concluded the forming process of Na2ZnV2O7 nanorods.When using as an anode material for Li-ion battery,the first discharging capacity can reach 1102.3mA h/g.The acceptable rate and cycle capability indicated the promising usage.By using ex-situ XRD,EIS and TEM,we found Na2ZnV2O7 nanorods experienced an intercalated process and conversion reaction during Li insertion process.The formed amorphous state act as main phase in subsequent cycles.