The structure-tunable synthesis and magnetic properties of Fe3O4 nanocrystals

Magnetite nanocrystals with tunable crystalline structures (orthorhombic and cubic) were synthesized via a simple oxidation-coprecipitation approach basing on the reaction of FeSO₄·7H₂O with C₆H₁₂N₄. The average diameter of prepared orthorhombic Fe₃O₄ nanorods were 15 nm while their lengths were approximately 150~200 nm. As-prepared cubic counterparts were composed of 10 nm sized nanoparticles. XRD, FESEM, TEM, SAED and HRTEM were then used to characterize our samples. In addition, magnetic measurements showed the saturation magnetization of orthorhombic magnetite was lower than that of cubic ones. Mössbauer spectroscopy verified the samples possessed the nature of cubic magnetite. Finally, a possible growth mechanism for orthorhombic nanorods and cubic nanoparticles was also discussed.     

Comparative investigation of resistance and ability to trigger high voltage discharge for single and multiple femtosecond filaments in air

A comparative investigation of resistance and ability to trigger high voltage (HV) discharge for single filament (SF) and multiple filaments (MF) has been carried out. The experimental results show that the trend of the breakdown threshold of the SF exactly follows that of its resistance, but this is not the case for the MF. The MF's resistance is much smaller than SF's. However, the MF shows a bit higher HV breakdown threshold than the SF. The underlying physics is that the measured resistance of the MF is collectively contributed by every filament in the MF while the HV breakdown threshold is determined by only one single discharging path.     

碳纳米管锁模双包层光纤激光器的实验研究

本文采用双包层掺镱光纤作为增益介质,用单壁碳纳米管作为饱和吸收体,获得最高输出功率为336 mW的锁模脉冲激光.用飞秒激光诱导水击穿法直接在单模光纤上制备出D形区,通过在D形光纤上滴涂单壁碳纳米管溶液,成功制备出碳纳米管饱和吸收体,并对其饱和吸收特性进行测试,发现其调制深度为27％.利用该饱和吸收体作为锁模器件,制备出具有环形腔结构的锁模光纤激光器.当抽运功率为4W时,获得了脉宽为93.8 fs,中心波长为1083.8 nm,3 dB谱宽为8.6 nm,重复频率为5.59 MHz,平均功率为336 mW的飞秒脉冲激光输出.     

A Quantum Watermark Protocol

Quantum watermarking is the technique which embeds the invisible quantum signal such as the owner’s identification into quantum multimedia data (such as audio, video and image) for copyright protection. In this paper, a watermark strategy for quantum images is proposed based on flexible representation for quantum images (FRQI). Compared with the former strategy, which can only be used to verify the identity of the true owner of a carrier image, the proposed method can be used to find out who is the real owner, and our strategy’s capacity reaches the largest. It is impossible for anyone except the copyrighter to clear off or extract the watermark images.     

Hydrothermal synthesis of LiMnO2 microcubes for lithium ion battery application

Two kinds of LiMnO₂ microcubes were successfully synthesized by hydrothermal method using solid or hollow Mn₂O₃ microcubes as precursors. One was made up of nanoparticles varying in size and the other was made up of interlaced polygonal nanoplates with the thickness of 70 nm. Both kinds of LiMnO₂ microcubes were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Charge–discharge curves were carried out to investigate their electrochemical properties. LiMnO₂ microcubes with interlaced nanoplates showed much better capacities than the ones with nanoparticles indicating it is more suitable for application in the lithium ion batteries. The former material could deliver the capacities of 197 and 134 mAh/g at 0.1 and 1 C, respectively. And its capacity fading after 50 cycles did not exceed 7 %. The excellent electrochemical performance of the former material could be ascribed to the smaller size which could shorten the path length for lithium ion transport and increase the electrode and electrolyte contact.     

非局域暗孤子及其稳定性分析

由于其在通信领域潜在的应用前景, 非局域空间孤子一直是研究热点. 但空间非局域暗孤子由于其边界的特殊性, 对其特别是稳定性方面研究甚少. 提出了非局域暗孤子稳定性分析理论, 并对热非线性体介质中1+1维基态和二阶暗孤子的稳定性进行了数值分析和研究, 得到了稳定性图. 从稳定性分析图可知: 在热非线性体介质中, 1+1维基态暗孤子在其存在区域总是稳定的, 而1+1维二阶暗孤子是震荡不稳定的, 其不稳定区域的宽度与传播常数以及介质的非局域程度有关. 为了验证非局域暗孤子稳定性分析理论的正确性, 对加噪声初始输入的传输进行数值模拟得到了传输图, 传输图表明稳定性分析理论的正确性. 关键词： 非局域 稳定性 暗孤子     

光滑粒子流体动力学方法固壁处理的一种新型排斥力模型

与传统网格法相比, 光滑粒子流体动力学方法不能直接施加壁面边界条件, 这就限制了该方法在工程中的应用.为此, 本文基于Galerkin加权余量法并结合传统排斥力方法, 推导出一种新的排斥力公式来施加壁面边界条件.该方法不含未知参数, 能在不减小边界粒子尺寸的情形下有效地防止流体粒子穿透壁面, 同时可避免邻近边界的流体粒子的速度及压力振荡. 分别通过静止液柱算例、液柱坍塌算例、容器中液体静止算例及溃坝算 例来验证本文方法的有效性, 并与传统边界处理方法进行对比, 结果表明: 本文方法克服了传统方法存在的缺陷, 是一种有效的固壁边界处理方法. 关键词： 光滑粒子流体动力学法 固壁边界 排斥力 加权余量法     

晶体相场方法研究二维六角相向正方相结构转变

应用双模PFC模型,计算二维PFC相图,模拟二维六角晶格向正方晶格的结构转变过程, 观察新相(二维正方相)的形核、长大特点,以及相结构转变的动力学特征. 结果表明:六角结构相向正方结构相的转变,正方相最易在六角相晶界处, 尤其是在三晶粒的交汇处首先生成正方相的晶核,之后是正方相逐渐通过吞噬六角相的边缘, 向六角相内部推进,并不断长大.对于结构转变生成的正方相晶粒,其晶粒取向几乎是随机的, 与原先六角相晶粒取向角没有明显的关系.正方相转变的面积分数随时间变化的动力学曲线 呈现典型的"S"形.由Avrami曲线可将相变曲线看成由两阶段组成. 计算模拟得到的Avrami曲线的第二阶段直线斜率K的范围在2.0和3.0之间, 与JMAK理论的指数n相符合. 关键词： 结构转变 晶体相场 相图 晶粒取向     

Ni熔体凝固过程中临界晶核和亚临界晶核的分子动力学模拟

本文用分子动力学模拟研究了Ni熔体以不同冷速凝固后微观结构的演变规律, 并通过理论计算确定出了Ni熔体凝固后获得理想非晶的临界条件. 模拟结果发现冷速小于10¹¹ K/s时, Ni 熔体凝固后形成晶态组织; 冷速在10¹¹ K/s到10^14.5 K/s之间时, Ni熔体凝固后形成由晶态结构与非晶态结构组成的混合组织. 冷速小于10¹⁰ K/s, Ni 熔体凝固后形成的晶态组织具有fcc结构; 冷速在10¹⁰ K/s到10^14.5 K/s之间时, Ni熔体凝固后组织中的晶态由fcc和hcp结构层状镶嵌排列构成. 通过分析模拟结果和计算结果, 确定出了Ni熔体凝固后形成理想非晶的临界冷速为10^14.5 K/s. 并发现Ni熔体中临界晶核(冷速等于10^14.5 K/s)和亚临界晶核(冷速大于10^14.5 K/s) 均由fcc和hcp组成层状偏聚结构, 这表明Ni熔体中生长的晶体、临界晶核和晶胚的结构是相同的. 关键词： 分子动力学模拟 晶体团簇 临界冷速 结构