点缺陷石墨烯的电导

Graphene is one of the most promising materials in nanotechnology and has attracted worldwide attention and research interest owing to its high electrical conductivity, good thermal stability, and excellent mechanical strength. Perfect graphene samples exhibit outstanding electrical and mechanical properties. However, point defects are commonly observed during fabrication which deteriorate the performance of graphene based-devices. The transport properties of graphene with point defects essentially depend on the imperfection of the hexagonal carbon atom network and the scattering of carriers by localized states. Furthermore, an in-depth understanding of the effect of specific point defects on the electronic and transport properties of graphene is crucial for specific applications. In this work, we employed density functional theory calculations and the non-equilibrium Green's function method to systematically elucidate the effects of various point defects on the electrical transport properties of graphene, including Stone-Waals and inverse Stone-Waals defects; and single and double vacancies. The electrical conductance highly depends on the type and concentration of point defects in graphene. Low concentrations of Stone-Waals, inverse Stone-Waals, and single-vacancy defects do not noticeably degrade electron transport. In comparison, DV585 induces a moderate reduction of 25%–34%, and DV55577 and DV5555-6-7777 induce significant suppression of 51%–62% in graphene. As the defect concentration increases, the electrical conductance reduces by a factor of 2–3 compared to the case of graphene monolayers with a low concentration of point defects. These distinct electrical transport behaviors are attributed to the variation of the graphene band structure; the point defects induce localized states near the Fermi level and result in energy splitting at the Dirac point due to the breaking of the intrinsic symmetry of the graphene honeycomb lattice. Double vacancies with larger defect concentrations exhibit more flat bands near the Fermi energy and more localized states in the defective region, resulting in the presence of resonant peaks close to the Fermi energy in the local density of states. This may cause resonant scattering of the carriers and a corresponding reduction of the conductance of graphene. Moreover, the partial charge densities for double vacancies and point defects with larger concentrations exhibit enhanced localization in the defective region that hinder the charge carriers. The electrical conductance shows an exponential decay as the defect concentration and energy splitting increase. These theoretical results provide important insights into the electrical transport properties of realistic graphene monolayers and will assist in the fabrication of high-performance graphene-based devices.     

参杂缺陷石墨烯的高分子复合材料导热特性分子动力学模拟

Polymers are widely used advanced materials composed of macromolecular chains, which can be found in materials used in our daily life. Polymer materials have been employed in many energy and electronic applications such as energy harvesting devices, energy storage devices, light emitting and sensing devices, and flexible energy and electronic devices. The microscopic morphologies and electrical properties of the polymer materials can be tuned by molecular engineering, which could improve the device performances in terms of both the energy conversion efficiency and stability. Traditional polymers are usually considered to be thermal insulators owing to their amorphous molecular chains. Graphene-based polymeric materials have garnered significant attention due to the excellent thermal conductivity of graphene. Advanced polymeric composites with high thermal conductivity exhibit great potential in many applications. Therefore, research on the thermal transport behaviors in graphene-based nanocomposites becomes critical. Vacancy defects in graphene are commonly observed during its fabrication. In this work, the effects of vacancy defects in graphene on thermal transport properties of the graphene-polyethylene nanocomposite are comprehensively investigated using molecular dynamics (MD) simulation. Based on the non-equilibrium molecular dynamics (NEMD) method, the interfacial thermal conductance and the overall thermal conductance of the nanocomposite are taken into consideration simultaneously. It is found that vacancy defects in graphene facilitate the interfacial thermal conductance between graphene and polyethylene. By removing various proportions of carbon atoms in pristine graphene, the density of vacancy defects varies from 0% to 20% and the interfacial thermal conductance increases from 75.6 MW·m⁻²·K⁻¹ to 85.9 MW·m⁻²·K⁻¹. The distinct enhancement in the interfacial thermal transport is attributed to the enhanced thermal coupling between graphene and polyethylene. A higher number of broken sp² bonds in the defective graphene lead to a decrease in the structure rigidity with more low-frequency (< 15 THz) phonons. The improved overlap of vibrational density states between graphene and polyethylene at a low frequency results in better interfacial thermal conductance. Moreover, the increase in the interfacial thermal conductance induced by vacancy defects have a significant effect on the overall thermal conductance (from 40.8 MW·m⁻²·K⁻¹ to 45.6 MW·m⁻²·K⁻¹). In addition, when filled with the graphene layer, the local density of polyethylene increases on both sides of the graphene. The concentrated layers provide more aligned molecular arrangement, which result in better thermal conductance in polyethylene. Further, the higher local density of the polymer near the interface provides more atoms for interaction with the graphene, which leads to stronger effective interactions. The relative concentration is insensitive to the density of vacancy defects. The reported results on the thermal transport behavior of graphene-polyethylene composites provide reasonable guidance for using graphene as fillers to tune the thermal conduction of polymeric composites.     

基于二维光子晶体点缺陷可调谐光功率分配器

在二维矩形阵列结构硅光子晶体中去除中间一排硅柱形成线波导,在线波导右侧引入点缺陷。利用时域有限差分法(FDTD)模拟仿真以及数值分析研究线波导中光耦合特性,计算出两个通道的分光比,发现改变光子晶体的温度可以明显改变这两个通道的光功率比。基于此结构,提出了一种新的光功率分配器,可以获得大范围的光功率比值,从1∶1～90∶1都可以通过改变光子晶体的温度来实现,并且当温度从0℃～200℃就可以实现这一功能,最后设计了一款三通道可调谐光功率分配器,通过调节两个点缺陷区域内温度来实现光功率的分配。     

点缺陷对表面生长动力学标度行为的影响

基于含噪声Kuramoto-Sivashinsky方程, 采用动力学重正化群技术, 研究生长表面存在点缺陷或杂质对表面生长动力学标度行为的影响, 得到了相应的粗糙度指数α 和动力学标度指数z. 所得结果表明, 点缺陷的存在使生长表面粗化, 并缩短达到稳定生长的弛豫时间.     

铁电马氏体中的电致形状记忆效应

与温度驱动形状记忆的缓慢响应及磁场驱动形状记忆的巨大体积相比较，电场驱动形状记忆具有响应快和体积小的优点。但是，基于传统反压电效应的电场驱动形状记忆由于变形量较小而收到限制。本研究表明，通过运用基于点缺陷短程序对称性遵循的普适性原理的可逆畴翻转机制，可以在铁电马氏体中获得巨大的电致形状记忆效应；其产生的变形量理论上是反压电效应所产生变形量的几十倍。采用原位畴观察实验给出了可逆畴翻转的直接证据。并且，在铁电多晶陶瓷中也获得了这种大的电致形状记忆效应。这种效应在很宽的频率范围内都很稳定，在疲劳测试中也显示了很好的可靠性。运用这一新的电致形状效应有望制备出新一代非线性驱动器材料。     

铁电马氏体中的电致形状记忆效应

与温度驱动形状记忆的缓慢响应及磁场驱动形状记忆的巨大体积相比较,电场驱动形状记忆具有响应快和体积小的优点.但是,基于传统反压电效应的电场驱动形状记忆由于变形量较小而收到限制.本研究表明,通过运用基于点缺陷短程序对称性遵循的普适性原理的可逆畴翻转机制,可以在铁电马氏体中获得巨大的电致形状记忆效应;其产生的变形量理论上是反压电效应所产生变形量的几十倍.采用原位畴观察实验给出了可逆畴翻转的直接证据.并且,在铁电多晶陶瓷中也获得了这种大的电致形状记忆效应.这种效应在很宽的频率范围内都很稳定,在疲劳测试中也显示了很好的可靠性.运用这一新的电致形状效应有望制备出新一代非线性驱动器材料.     

从铝合金的应变时效内耗考察位错与点缺陷的交互作用

研究了合金含量、合金元素和形变条件对铝合金应变时效内耗行为的影响．由此描述了点缺陷的分布状态和位错组态；指出了控制应变时效回复过程的因素是溶质原子的动性和溶质原子与位错的作用能。提出了能描述应变时效过程中位错─—点缺陷交互作用的位错弦脱钉和位错气团拖曳共同作用的复合模型，相应的理论处理合理地解释了实验现象。 关键词：     

Cu3Au与Au3Cu中点缺陷的分子动力学研究——兼论L12型合金点缺陷的性质

讨论了Ｌ１２型合金中点缺陷的各种可能构型，然后运用分子动力学的方法，采用由Ａｃｋ-ｌａｎｄ等提出的Ｃｕ-Ａｕ体系的多体势函数，计算了具有Ｌ１₂结构的Ｃｕ₃Ａｕ与Ａｕ₃Ｃｕ中点缺陷的构型、形成能及缺陷体积，并进一步讨论了目前研究较为深入的Ｌ１₂型金属间化合物中点缺陷的性质 关键词：     

复合声子结构的声场模拟及噪声-压电转化设计

城市噪声作为一种污染源,给人们的生活带来不便;同时噪声中蕴含的能量没有被合理利用,造成浪费。为了解决此问题,该文设计引入了点缺陷的声子晶体、纤维层、压电材料复合结构,对入射噪声进行有效吸收并在点缺陷处对入射声压有较明显的声压加强。采用有限元和边界元的方法,对该复合结构的降噪发电效果进行数值模拟。结果显示在输入声压为2 Pa时,声子晶体结构带隙1.1 kHz处,吸声系数达到0.6,压电片有最高的输出电压0.5 V,输出功率密度达到308.49μW/cm~3。与传统降噪结构相比能更好地实现对声能的吸收与利用。     

漏斗型完全光子带隙光波导单向传输

光波单向传输器件在全光计算和信息处理方面具有重要应用.本文提出一种具有完全光子带隙的硅基空气孔光子晶体漏斗型光波导结构,在光通信波段可实现单向传输特性.漏斗型光波导可打破光波对称传输,引入点缺陷通过模式转换与失配进一步抑制反向透射,并研究了不同的点缺陷类型与位置对反向透射的影响.采用时域有限差分法进行数值计算,优化选取了最佳的点缺陷模式.结果显示,单柱型点缺陷在向左移动5a(a为光子晶体晶格常数,a=470 nm)时,横电(TE)偏振光在工作波长1550 nm处正向透射率为0.716,透射对比度为0.929,工作带宽为111 nm.另外,本文提出的光波单向传输器件结构简单、工艺要求低,有望为集成光路中单向传输器件设计提供新的解决方案.