【作者】 李丽娟；

【导师】 杨凤霞；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2009， 硕士

【摘要】 自掺杂非化学计量比的硒化银和碲化银材料(简称硫族银化物材料)在较宽的温度范围(1K-300K)和超宽的磁场范围(1mT~60T)具有大的正线性磁电阻效应(LMR),且脉冲磁场直到60T,其LMR仍未达到饱和,这种奇异的线性磁电阻效应因其巨大的应用价值已引起了研究者的广泛关注。本文首次采用两步合成法成功地制备出完全化学计量比及非化学计量比的硒化银( Ag 2+δSe(δ≥0))晶体,分析了硒化银晶体的物相、微结构及形貌,探究了两步合成法制备硒化银晶体的最佳工艺条件,并研究了晶体的物理性能。本文通过改变化学反应方程式中Se的配比,利用室温合成法得到含不同过量银的硒化银纳米颗粒,然后通过固相合成法制备出晶体。XRD和SEM分析结果表明,室温合成法中,反应时间为10h时,可得到高纯的正交α相硒化银纳米颗粒;固相合成法中,温度为500℃时,可烧结出结晶完全的硒化银晶体,流通的氩气氛烧结及粉体预热处理工艺均可有效提高晶体的致密度。采用XRD和SEM对制备出的硒化银晶体进行微观性能的研究,结果表明,随着Se配比的降低(≥75%), Ag 2+δSe晶体中过量银的含量逐渐增多,其存在形式也由小微粒逐渐转变为纳米级的团聚体,同时,硒化银晶体的晶粒尺寸也逐渐减小。此外, Ag 2+δSe晶体中的过量银呈现出略显规则的链条式排列镶嵌在Ag 2S e母体中,链条的方向也基本一致。最后,采用多种测试手段,对硒化银晶体的物理性能进行了研究。结果表明,硒化银晶体为n型半导体,其禁带宽度约1.56~1.97eV,为窄禁带半导体,其中,非化学计量比硒化银的禁带宽度值略高;晶体的电阻率随温度的升高(105K~293K),先增大后减小,与杂质半导体的阻温特性一致;晶体具有正线性磁电阻效应,其中室温及1.73T下的磁电阻值可达10%,并且Se配比越低(≥75%), Ag 2+δSe晶体的磁电阻效应越明显,其临界磁场值越小。更多还原

【Abstract】 Self-doped nonstoichiometric silver chalcogenides have attracted lots of attention due to their unusually large, positive and linear magnetoresistance effect (LMR), which exhibits a linear dependence on magnetic field in a wide temperature range (1K-300K) and wide range of magnetic field (1mT ~ 60T). Especially, LMR has no saturation at the high magnetic field up to 60 T.In this thesis, both stoichiometric and nonstoichiometric silver selenide ( Ag 2+δSe(δ≥0)) crystals have been successfully synthesized by the two-step method for the first time. The structure and morphology of the samples were analyzed, the optimal preparation conditiones were investigated, and their physical properties were studied.Firstly, selenium proportion in chemical equation was changed and silver selenide nanoparticles with different silver excess ( Ag 2+δSe(δ≥0))were synthesized by room temperature method, and then were sintered into crystals by solid state method. The results of XRD and SEM indicated that 10h is the optimal reaction time with the result of high-purityα? Ag 2 +δSe nanoparticles and 500℃is the best sintering temperature with the result of completed- crystallized Ag 2 +δSe crystals. Moreover, the dense of crystals were improved when sintered in flowing argon of crystals as well as treated to the nanoparticles before sintered.Secondly, X-ray diffraction and scanning electron microscopy were used to characterize the microstructure of Ag 2 +δSecrystals. The results showed that, with the reducing Se in mixture ratio, the size of crystal reduced and there were more excess silver which exists as atoms or clusters and arranged chainlike in the silver selenide matrix. Moreover, the physical properties of Ag 2 +δSewere analyzed by using various test methods. The results indicated that Ag 2 +δSe crystals were n-type small-gap semiconductors with bandgap energies of 1.56~1.97eV, among samples, the bandgap of nonstoichiometric Ag 2 +δSe was slightly higher than those of Ag2Se; with increased temperature, the electric conductivity of crystals increased at first and arrived at a maximum, then reduced rapidly, which is coordinated with the conductivity- temperature proper of doped semiconductor; a positive and linear MR effect was discovered surprisingly with a large MR effect up to 10% at room temperature and magnetic fields of 1.73T. Also, the MR effect of Ag 2 +δSe crystals was improved and the transition field from OMR to LMR was reduced with reduced proportion of Se in mixture ratio(≥75%).更多还原