石墨烯力常数和弹性波波速随温度变化规律研究

在哈里森键联轨道法框架下,考虑到原子的短程相互作用和原子的非简谐振动,建立石墨烯弹性模型并求出原子振动的简谐系数和非简谐系数。在此基础上,确定了石墨烯的力常数和弹性波波速随温度和形变势参量的变化关系式,探讨了原子非简谐振动对它们的影响。结果表明：（1）石墨烯的力常数和弹性波波速均随温度升高而非线性减小,但有不同的变化特征：在C11、C12、C44这三个力常数中,以C12随温度的变化最大,以C44的变化最小;纵波波速随温度的变化要大于横波;（2）石墨烯的三个力常数随形变势参量的变化有不同的特征：C11随η和γ的增大而增大、而C12则随η的增大而减小,随γ的增大而增大;C44随η的增大而增大,而几乎不随γ而变;（3）若不考虑原子振动的非简谐项,则石墨烯的力常数和弹性波波速均为常数。考虑到非简谐项后,不仅它们均随温度的升高而减小,而且,非简谐情况的力常数Cij和弹性波波速vi与简谐近似的值Cij0、vi0的差|Cij－Cij0|、|vi－vi0|均随温度的升高而增大,即温度愈高,非简谐效应愈显著。     

石墨烯力常数和弹性波波速随温度变化规律研究

在哈里森键联轨道法框架下,考虑到原子的短程相互作用和原子的非简谐振动,建立石墨烯弹性模型并求出原子振动的简谐系数和非简谐系数。在此基础上,确定了石墨烯的力常数和弹性波波速随温度和形变势参量的变化关系式,探讨了原子非简谐振动对它们的影响。结果表明：（1）石墨烯的力常数和弹性波波速均随温度升高而非线性减小,但有不同的变化特征：在C11、C12、C44这三个力常数中,以C12随温度的变化最大,以C44的变化最小;纵波波速随温度的变化要大于横波;（2）石墨烯的三个力常数随形变势参量的变化有不同的特征：C11随η和γ的增大而增大、而C12则随η的增大而减小,随γ的增大而增大;C44随η的增大而增大,而几乎不随γ而变;（3）若不考虑原子振动的非简谐项,则石墨烯的力常数和弹性波波速均为常数。考虑到非简谐项后,不仅它们均随温度的升高而减小,而且,非简谐情况的力常数Cij和弹性波波速vi与简谐近似的值Cij0、vi0的差|Cij－Cij0|、|vi－vi0|均随温度的升高而增大,即温度愈高,非简谐效应愈显著。     

石墨烯力常数和弹性波波速随温度变化规律研究

在哈里森键联轨道法框架下,考虑到原子的短程相互作用和原子的非简谐振动,建立石墨烯弹性模型并求出原子振动的简谐系数和非简谐系数.在此基础上,确定了石墨烯的力常数随温度和形变势参量的变化关系式,以及弹性波波速随温度和形变势参量的变化关系式,并探讨了原子非简谐振动对它们的影响.结果表明:(1)石墨烯的力常数和弹性波波速均随温度升高而非线性减小,但有不同的变化特征:在C_(11)、C_(12)、C_(44)这三个力常数中,以C_(12)随温度的变化最大,以C_(44)的变化最小;纵波波速随温度的变化要大于横波;(2)石墨烯的三个力常数随形变势参量的变化有不同的特征:C_(11)随η和γ的增大而增大、而C_(12)则随η的增大而减小,随γ的增大而增大;C_(44)随η的增大而增大,而几乎不随γ而变;(3)若不考虑原子振动的非简谐项,则石墨烯的力常数和弹性波波速均为常数.考虑到非简谐项后,不仅它们均随温度的升高而减小,而且,非简谐情况的力常数C_(ij)和弹性波波速v_i与简谐近似的值C_(ij)~0、v_i~0的差|C_(ij)-C_(ij)~0|、|v_i-v_i~0|均随温度的升高而增大,即温度愈高,非简谐效应愈显著.     

石墨烯的声子热学性质研究

声子是石墨烯导热过程中的主要载体,而声子的弛豫时间又是其中最基本、最重要的物理量.本文采用简正模式分解法研究了石墨烯声子的弛豫时间,并且借此分析了不同声子在导热过程中的贡献.该方法通过平衡分子动力学模拟实现,首先通过模拟得到单个声子的能量自相关函数衰减曲线,并进一步采用拟合和积分两种方法得到单个声子的弛豫时间.然后,研究了弛豫时间与波矢、频率和温度的关系.结果发现,弛豫时间随波矢的变化与对应的色散关系相近,弛豫时间与频率和温度的关系符合理论模型:1/τ=νnTm,其中声学支的n为1.56,而光学支结果较为发散,指数m对于不同声子支结果略有不同.最后,还研究了不同频率声子对导热的贡献,发现低频声子在态密度上占有绝对优势,并且其弛豫时间整体高于高频声子,所以低频声子对导热的贡献占据主导地位.     

非简谐振动对SiC的热膨胀系数数和介电性能的影响

利用抛物型电子能谱模型,考虑到原子的非简谐振动,求出了SiC中原子振动的简谐系数与非简谐系数,用固体物理理论和方法,得到了SiC的热膨胀系数和格林乃森参量以及介电常数随温度变化的解析式,探讨了原子非简谐振动对的影响。结果表明：的格林乃森参量和热膨胀系数均随温度升高而非线性增大,而介电常数随温度升高而非线性减小;原子振动的非简谐项（特别是第二非简谐项）对的热膨胀等热学性质和介电性能有重要影响,温度愈高,非简谐振动项的影响愈大。     

界面尺寸对铜/石墨烯界面热导的影响

采用原子格林函数(AGF)方法研究了界面尺寸对铜/单层石墨烯(SLG)界面热导的影响.建立了有限和无限界面尺寸的AGF计算模型,计算得到的界面热导均在铜的迪拜温度(343 K)附近收敛,但两者计算得到的透射系数和界面热导存在明显差别:有限界面尺寸时计算得到的透射系数在1.5 THz附近达到峰值0.84,而无限界面尺寸时...     

基于石墨烯电极的蒽醌分子器件开关特性

研究了基于石墨烯电极的蒽醌分子器件的开关特性.分别选取了锯齿型和扶手椅型的石墨烯纳米带作为电极,考虑蒽醌基团在氧化还原反应下的两种构型,即氢醌(HQ)分子和蒽醌(AQ)分子,构建了双电极分子结,讨论了氧化还原反应和不同的电极结构对蒽醌分子器件开关特性的影响.研究发现,无论是锯齿型石墨烯电极还是扶手椅型石墨烯电极,HQ构...     

以石墨烯为电极的有机噻吩分子整流器的设计及电输运特性研究

传统硅基半导体器件受到了量子尺寸效应的限制,发展分子电子学器件有可能解决这一难题.本文提出了由石墨烯电极和有机噻吩分子相结合构造分子器件的思想,建构了"石墨烯-噻吩分子-石墨烯"结构的分子器件,并运用非平衡态格林函数结合密度泛函理论的方法研究了其电输运特性.系统地分析了电子给体"氨基"和电子受体"硝基"两种取代基的位置对有机噻吩分子电输运的影响.计算表明,有机噻吩二聚物被"氨基"和"硝基"取代后会产生明显的负微分电阻效应和整流效应.进一步对产生这些效应的物理机制进行分析,发现氨基的位置可以调整负微分电阻的强弱,硝基的位置可以改变整流的方向.     

聚酰亚胺电导率随温度和电场强度的变化规律

介质深层充电对航天器安全运行构成了重大威胁.以聚酰亚胺为代表的此类聚合物绝缘介质的电导率受温度影响显著,又因为充电过程中局部产生强电场(10~7V/m量级),因此,其电导率模型需要综合考虑温度和强电场的影响,这对介质深层充电的仿真评估意义重大.已有的两类模型,不是低温区间不适用,就是没有充分考虑强电场的影响.基于跳跃电导理论,本文分析对比了现有电导率模型,提出了适用于较宽温度范围且合理考虑强电场增强效应的电导率新模型,并采用某型聚酰亚胺电导率测试数据做出验证.此外,为了提高新模型在强电场下的低温适用范围,尝试对强电场因子中的温度做变换,取得了满意的效果.参数敏感度分析表明新模型在电导率拟合与外推方面具有参数少、适用性强的优势.     

基于石墨烯电极的齐聚苯乙炔分子器件的整流特性

以齐聚苯乙炔分子为研究对象,采用密度泛函理论与非平衡格林函数相结合的第一性原理方法,对基于石墨烯电极的齐聚苯乙炔分子器件整流特性进行了研究,系统地分析了官能团对分子器件整流特性的影响.通过研究发现,官能团对齐聚苯乙炔分子器件整流特性影响显著,当添加失电子官能团氨基(NH_2)时出现正向整流,添加得电子官能团硝基(NO_2)时出现反向整流,当同时添加氨基和硝基官能团时,会出现正反向整流交替现象,研究结果表明通过添加不同类型的官能团能有效控制分子整流器的整流特性.     

Negative thermal expansion in NbF_3 and NbOF_2:A comparative theoretical study

Thermal expansion control is always an obstructive factor and challenging in high precision engineering field. Here,the negative thermal expansion of Nb F_3 and Nb OF_2 was predicted by first-principles calculation with density functional theory and the quasi-harmonic approximation(QHA). We studied the total charge density, thermal vibration, and lattice dynamic to investigate the thermal expansion mechanism. We found that the presence of O induced the relatively strong covalent bond in Nb OF_2, thus weakening the transverse vibration of F and O in Nb OF_2, compared with the case of Nb F_3.In this study, we proposed a way to tailor negative thermal expansion of metal fluorides by introducing the oxygen atoms.The present work not only predicts two NTE compounds, but also provides an insight on thermal expansion control by designing chemical bond type.     

Generalized Grüneisen parameters and low temperature limit of lattice thermal expansion of cadmium and zirconium

The generalized Grüneisen parameters (γ′ _j ) and (γ″ _j ) for cadmium and zirconium were calculated from the second- and third-order elastic constants to determine the low temperature limit of the volume thermal expansion of these metals of hexagonal symmetry. The low temperature limit of cadmium and zirconium was calculated to be positive values indicating a positive volume expansion down to 0 K even though many Grüneisen gammas were found to be negative.     

Effect of optical pumping on the momentum relaxation time of graphene in the terahertz range

The momentum relaxation time of a photoexcited graphene in the THz frequency range has been studied by using terabertz time domain spectroscopy under optical pumping at room temperature. It is found that the momentum relaxation time of the graphene as a function of the optical pumping intensity exhibits a threshold behavior. The features of the momentum relaxation time as a function of the optical pumping intensity are also investigated. The results are useful for understanding the basic underlying physics of graphene scattering as well as finding the possible applications in carbon- based electronics.     

Coupled temperature dependences of volume and compressibility

We present a new method for understanding the changes with temperature of the volume and compressibility of solids. These changes depend on five parameters: V ₀, B ₀, Θ, γ _G and δ _T. V ₀ and B ₀ are the atomic volume and bulk modulus at T?=?0?K, Θ is the Debye temperature, γ _G is the Grüneisen parameter, and δ _T is the Anderson–Grüneisen parameter. Equations for the temperature-dependent volume, bulk modulus and thermal expansion are given, and examples of the use of these equations are provided, with particular emphasis on the light actinides. For the elements, we examine the relationship between experimental values of γ _G and δ _T, and find no clear correlation. In particular, Swenson's rule, which states that the bulk modulus does not change with temperature if the volume is held constant, is a poor approximation to the data. We reveal a new useful approximate relationship between dB/dP and γ _G. We find that the thermodynamic quantity q, which describes the variation in γ _G with volume, distributes around 2, not around 1, as often assumed. We show that the thermal- expansion behavior of Si and Ge (including negative thermal expansion at low temperatures) are well described with the use of a two-level invar model.     

X-ray determination of thermal expansion of cadmium fluoride and lead fluoride

The lattice parameters of CdF₂ andβ-PbF₂ have been determined over the temperature range 300–670 K. The coefficient of expansion at room temperature is 21·3 × 10⁻⁶ K⁻¹ and 25·4 × 10⁻⁶ K⁻¹ for CdF₂ and PbF₂ respectively and it increases linearly with temperature over the range of temperature covered. The Grüneisen parameter decreases with temperature in both the crystals.     

Thermodynamic properties of OsB under high temperature and high pressure

The energy-volume curves of OsB have been obtained using the first-principles plane-wave ultrasoft-pseudopotential density functional theory (DFT) within the generalized gradient approximation (GGA) and local density approximation (LDA). Using the quasi-harmonic Debye model we first analyze the specific heat, the coefficients of thermal expansion as well as the thermodynamic Grüneisen parameter of OsB in a wide temperature range at high pressure. At temperature 300 K, the coefficients of thermal expansion α_V by LDA and GGA calculations are 1.67×10⁻⁵ 1/K and 2.01×10⁻⁵ 1/K, respectively. The specific heat of OsB at constant pressure (volume) is also calculated. Meanwhile, we find that the Debye temperature of OsB increases monotonically with increasing pressure. The present study leads to a better understanding of how the OsB materials respond to pressure and temperature.     

Higher order elastic constants and generalized Gruneisen parameters of elastic waves and low temperature thermal expansion of gadolinium

Expressions for the higher order elastic constants are derived using the sublattice displacements to the second degree in strains. These expressions are used to obtain the higher order elastic constants and their pressure derivatives in gadolinium. The higher order elastic constants are used to find out the generalized Gruneisen parameters of the elastic waves propagating in different directions in gadolinium. The Brugger gammas are evaluated and the low temperature limit of the Gruneisen gamma is obtained. The results are compared with the available reported values.     

良导体导热系数实验中稳态时间、稳态温度与水流量的相关性

通过改变参数进行实验测定,分析了稳态法测量良导体导热系数实验中系统达到稳态的时间、水流量、稳定温度及测得导热系数结果的准确性等相关问题,提供了可供参考的经验数据,对学生实验过程有指导作用.