Preparation And Characterization Of Doped Tetra-needle-like ZnO Whiskers

Aluminum doping modification was adopted to improve the dielectric constants and enhance the electric loss of tetra-needle-like ZnO whiskers(T-ZnOw).In this method,T-ZnOw was solid-state reacted with aluminum nitrate decomposed Al₂O₃ at high temperature.So T-ZnOw was doped by Al³⁺ and turned into n-type semi-conductor.When H₂ was used as reaction atmosphere,the volume resistivity of Al doped T-ZnOw decreased from 10⁹～10¹⁰Ω·cm to 10³ï½ž10⁶Ω·cm.With the increasing of Al doping content,the carrier density of treated T-ZnOw increased first and decreased afterward;it reached the peak value when the doping content was 4.0 mol%,and at this time the volume resistivity of Al doped T-ZnOw reached the lowest,10³Î©Â·cm.When the doping content was more than 4.0 mol%,the second phase,ZnAl₂O₄,was generated and made its volume resistivity rise.The tangent of electric loss of unmodified T-ZnOw in 2～18 GHz was between 0.31～0.52, and its extreme point was at 16.7 GHz.After treated by 4.0 mol%of Al doping,it became between 0.44～1.14 and increased strictly along with the raise of frequency.We used the self-made Matlab program and inputed the vector network analyzer tested data to simulate the wave-absorption effect of 4.0 mol%and 8.0 mol%of Al doped T-ZnOw at 2～18 GHz and found that,the absorption band of 1 mm thick sample was wide at high frequency(15～18 GHz), and moved forward to 5 GHz around for 2 mm or 3 mm thick samples.To enhance the absorption effect of T-ZnOw at low frequency band(2～18 GHz),we used ferric nitrate to instead aluminum nitrate as dopant in further study to bring T-ZnOw magnetic loss.The saturated Fe doping content was about 2.0 mol%.When it's more than 2.0 mol%,the excessive Fe reacted with ZnO to product ZnFe₂O₄,and this reaction consumed part of active molecules on needle points of T-ZnOw and destroied the micro-structure of T-ZnOw.When H₂ was used as the reaction atmosphere and Fe was excessive,part of excessive Fe was reduced to be simple substance.The real part of complex permeability of T-ZnOw maintained 1.0 and was not modified very much by doping Fe,but the imaginary part was obviously improved.The best doping content was 2.0 mol%,at this content the optimized imaginary part of complex permeability of T-ZnOw was 0.33 at 8 GHz,and the tangent of its magnetic loss was 0.32.Matlab program was used to simulate the reflectivity of Fe doped T-ZnOw and it was found that when the doping content was set as constant,the absorption peak moved to low frequency along with the increasing of thickness of simulated samples.When the thickness was assumed as constant,the absorption peak moved to low frequency along with the increasing of Fe doping content.When the thickness was assumed as 2 mm,the Fe doping content increased from 0.5 mlo%to 4.0 mol%,its simulated wave absorption peak position decreased from 10.5 GHz to 8.0 GHz.