聚噻吩单链量子热输运的第一性原理研究

聚噻吩块体通常被视为绝热材料,其热导率小于1W·m~(-1)·K~(-1).但近年发现对于室温下沿聚噻吩分子链方向排列的无定形聚噻吩纳米纤维,其热导率高于聚噻吩块体,可达4.4W·m~(-1)·K~(-1).为了相对准确地揭示纳米尺度聚噻吩单链热输运的微观特征,从量子力学出发,在密度泛函理论计算的基础上,应用中间插入延展方法结合非平衡格林函数方法,对长度为25.107nm、包含448个原子的聚噻吩单链的量子热输运及其同位素效应进行了研究,并与分子动力学方法模拟的结果进行了详细比较.结果表明:室温下32 nm长的纯聚噻吩单链热导率上限高达30.2 W·m~(-1)·K~(-1),与铅的热导率35 W·m~(-1).K~(-1)相近;相同掺杂比例(原子百分数)下C元素热导的同位素效应比S元素显著;室温下聚噻吩单链中~(12)C,~(13)C等比例随机掺杂时的同位素效应最为显著,此时聚噻吩单链的平均热导至少降低了30%;室温下纯聚噻吩单链的热导随C的相对原子质量增加近似呈反比例减小,随S的相对原子质量增加呈非线性单调增加.该研究对认识和调控聚噻吩这种新型功能材料的热输运特性具有积极的价值.     

四声子费米共振与反常巨大声子散射现象

近日,南方科技大学物理系教授何佳清团队在具有极低晶格热导率的AgCrSe2材料中的热输运机理研究方面取得重要进展.团队基于多体微扰理论,系统地研究了具有赝二维层状结构AgCrSe2在低温至室温下的非谐晶格动力学,发现该材料中由Ag原子的四阶非谐相互作用以及横波声学支的平坦能量色散关系所诱发的独特的四声子费米共振行为.进...     

二维材料XTe_2(X=Pd,Pt)热电性能的第一性原理计算

利用密度泛函理论结合玻尔兹曼输运方程,预测了二维层状热电材料XTe_2 (X=Pd, Pt)的热电性质.两种材料都具有较低的热导率,材料的晶格热导率随温度的升高而降低,且表现出各向异性.而电子热导率随温度的升高而升高.在较低温时,晶格热导率对总热导率的贡献占据主导地位.较高的载流子迁移率、电导率及塞贝克系数也对材料的热电转换效率产生极大的影响,展现出较为优异的电输运性能.对比分析PdTe_2和PtTe_2两种材料的ZT值,发现两种材料的热电性能以p型掺杂为主. PtTe_2单层的ZT值高于PdTe_2单层,并且PtTe_2单层在常温下的ZT峰值可达到2.75,是一种极具潜力的热电材料.     

类石墨烯氮化碳结构(C3N)热传导机理研究

类石墨烯氮化碳结构(C₃N)作为一种全新的碳基二维半导体材料,由于其优异的机械和电子性能引起了研究者们的广泛关注,不同结构C₃N的热输运和声子输运机制还待进一步研究.本文构造了4种不同结构的C₃N,采用非平衡分子动力学与晶格动力学方法对不同结构的C₃N的热传导机理进行了研究.研究结果表明:1)在4种结构中M3热导率最高,M1次之,M4热导率最低;2)不同结构的C₃N的热导率具有明显的尺寸效应和温度效应.当样本长度较短时,声子主要以弹道输运的方式进行传输;当样本长度增大,扩散输运占主导地位;随着温度的升高,Umklapp散射在热输运中占据主导地位,使得热导率与温度具有1/T的依赖性.3)与M3相比,M1和M4结构中都存在更大的声子带隙,色散曲线进一步软化,低频和高频声子同时出现了局域化的特征,对热导率产生了显著的抑制作用.本文为更好地设计热管理材料提供了思路.     

阶梯化对快离子导体LixTiS2的结构与能量性质的影响

根据密度波理论建立了stage-2有序LixTiS2(x=1/8,1/6,1/4,1/3,3/8,1/2)系统的晶胞模型.采用平面波赝势方法,对stage-2有序系统和stage-1有序LixTiS2(x=0,1/4,1/3,1/2,2/3,3/4,1)系统进行了几何结构优化和总能量计算,并进行了对比研究.发现stage-2晶胞的参量c(d)与stage-1晶胞的参量2c0(d)有相似的变化规律.但在stage-2有序系统中,有插层离子的夹层间距c1(d)相对于stage-1有序系统的夹层间距c0(d)额外膨胀,而与之对应的无插层离子的夹层间距c2(d)被压缩.这种额外膨胀和压缩有一定的互补关系.阶梯化对TiS2夹层中的原子结构也有显著影响.stage-2有序Li1/2TiS2系统的形成能明显高于stage-1有序Li1/2TiS2系统,表明后者更稳定.这与已报道的实验结果一致.stage-2有序Li1/4TiS2系统的形成能低于stage-1有序Li1/4TiS2系统,预言了一种新的有序结构.     

Ca_2Y_2Cu_5O_(10)的单晶生长与低温物性

本文针对S=1/2 J1-J2阻挫自旋链材料Ca_2Y_2Cu_5O_(10)的浮区法单晶生长条件进行了详细的探究,并研究了其磁性质、比热、热导率行为.采用德拜模型与爱因斯坦模型叠加的方式拟合1.8-300K的声子比热,进而得到磁比热以及磁熵.对单晶的热导率采用德拜模型进行拟合发现垂直于自旋链方向的热导率是声子的贡献,而沿自旋链方向的热导率出现磁激发导热的贡献,进一步的分析认为该磁热导是非弹道的输运方式.     

Co替代La2／3Ca1／3Mn0．9Co0．1O3输运特性和双峰效应的研究

系统研究了Mn位替代的La2/3Ca2/3Mn0.9Co0.1O3的电输运特性和磁诱导行为,这里替代Co离子的多价态对应有多重的自旋态变化.发现了存在于Co替代La2/3Ca1/3Mn0.9Co0.1O3电输运行为上的双峰效应,具体表现为除了居里温度Tc附近电阻-温度曲线的金属-绝缘(M-I)转变行为(TMIH约为182K)以外,在低温区域电输运行为出现二次性的类M-I转变现象(TMIL乱约为124K).通过对外加磁场下输运特性的研究表明,随外加磁场的增加,电阻-温度曲线整体呈现降低行为,即表现为负的磁电阻效应;同时,高温区峰值温度TMIH向高温区移动,这与传统CMR锰氧化物M-I转变特性完全一致;而低温峰值温度了TMIL则几乎不变,即TMIL表现出某种磁场无关的特征.结合样品磁特性测量结果,我们发现,除了场冷(FC)和零场冷(ZFC)曲线的分叉现象可能证明Co替代诱导Mn位的磁无序而导致体系出现不均匀的铁磁/反铁磁(FM/AFM)团簇外,在低温峰值温度TMIL乱附近没有观察到磁性异常变化.对照对Cu和Cr等相关体系的类似研究,证明这种低温二次M-I转变可能与Co替代导致体系的氧缺陷有关.     

全固态锂离子电池内部热输运研究前沿

本文简要阐述了全固态锂离子电池的特点及其内部热输运研究的意义.介绍并总结了国内外与正极材料、负极材料、固态电解质,以及电极与电解质界面热输运性质相关的实验和理论工作.针对脱嵌锂过程对电极材料热导率的影响机理尚不明确,非晶态转变对电极材料热输运研究的挑战,界面热输运模型与方法不足等问题,系统梳理了全固态锂离子电池内部热输运的重要前沿科学问题.     

准一维电荷有序半导体(NbSe4)3I低温物性研究

近期,准一维电荷密度波(CDW)材料(TaSe_4)_2I的低温相被报道为轴子绝缘体[Nature 575,315(2019)],引起了广泛的关注.电荷有序半导体(NbSe_4)_3I与(TaSe_4)_2I属于同一个体系,晶体结构也表现为准一维的链状结构,低温下的物态值得进一步研究.本工作利用拉曼光谱和电输运测量实验方法对化学气相输运法(CVT)制备的高质量type-II(NbSe_4)_3I单晶样品进行了低温下的电输运和振动性质研究.通过实验研究发现,(NbSe_4)_3I在低温下并不存在轴子绝缘态.此外,电输运实验表明(NbSe_4)_3I在T_(C1)=270.4K处发生了第一次结构相变,即P4/mnc (D~6_(4h))→P42_1c (D~4_(2d)).随温度进一步降低,拉曼光谱测试发现在T_(C2)=180K和T_(C3)=110K附近有新峰的出现,分别归因于反铁电(AFE)长程有序相的形成而导致的第二次结构相变以及对称性进一步降低导致的第三次结构相变.值得一提的是,与其他拉曼峰的演化不同,57.4cm~(-1)右侧出现的新峰在T_(C3)以下迅速向高波数移动,发生明显的蓝移,且伴随强度的急剧增强,有可能在低温温度传感器领域有着潜在的应用前景.     

Mo低掺杂La_(0.67)Sr_(0.33)MnO_3的磁电特性及室温磁电阻增强(英文)

采用固相反应法成功制备了Mo低掺杂La0.67Sr0.33Mn1-xMoxOc(x=0,0.02)单相多晶样品,系统研究了样品的磁性、电输运特性和磁电阻效应.发现Mo低掺杂对La0.67Sr0.33MnO3 的居里温度影响很小,但在La0.67Sr0.33MnO3铁磁基态背景下诱导出团簇自旋玻璃态行为.在低温区(0.1相似文献   

Electrical transport and optical properties of vanadyl phosphate—polyaniline nanocomposites

The nanocomposites of conducting polyaniline and layered vanadyl phosphate, VOPO₄·2H₂O are synthesized by redox intercalation method. Water content decreases with insertion of polyaniline molecules. In scanning electron micrographs plate like structures are observed for both VOPO₄·2H₂O and intercalated nanocomposites. Protonation of polyaniline and interaction with vanadyl phosphate are observed in infrared and UV absorption spectroscopy. Intercalation improves conductivity of pristine vanadyl phosphate. Thermally activated electrical dc conductivity at low temperature shows two distinct slopes around 210 K for both the nanocomposites. The optical band gap of vanadyl phosphate decreases from 4.0 to 3.7 eV due to insertion of polyaniline.     

Influence of copper intercalation on the resistive state of compounds in the Cu-HfSe<Subscript>2</Subscript> system

Polycrystalline samples of intercalated compounds Cu _x HfSe₂ have been synthesized for the first time and their electrical resistivity has been measured at both direct current and alternating current (with a frequency ranging from 200 Hz to 150 kHz) in the temperature range 80–300 K. It has been shown that the intercalation of copper atoms between three-layer Se-Hf-Se blocks leads to an increase in the electrical resistivity of the samples, as well as to a more pronounced activated character of the temperature dependence of the electrical resistivity. A time dependence of the electrical resistivity of the Cu _x HfSe₂ samples at room temperature, which is associated with the presence of copper ions in the sample, has been determined.     

Thermal transport in CuCl2 intercalated pitch-derived carbon fibers

The effect of intercalation with CuCl₂ on the temperature variation of the thermal conductivity, thermopower and electrical resistivity of pitch-derived carbon fibers in the temperature range 2<T<300 K is studied. At high temperature, the lattice thermal conductivity is smaller than that of the pristine fibers while at low temperature an enhancement of both the lattice and the electronic thermal conductivities is observed. The thermopower is positive and exhibits a temperature dependence very similar to that generally reported in acceptor-type graphite intercalation compounds.     

Features of electrical conductivity in an incommensurate phase of graphite intercalation compound C10HNO3

Electrical conductivity and conduction-electron spin resonance (CESR) have been studied in stage-2 acceptor α-graphite-nitric acid intercalation compound C₁₀HNO₃. It is found that the electrical conductivity σ_c along the c axis in the structurally incommensurate phase of this compound is temperature independent, whereas the electrical conductivity σ_a along carbon layers exhibits “metallic” temperature behavior. Analysis of the temperature dependences of σ_c, σ_a, and the CESR linewidth demonstrates that, in the incommensurate phase of the graphite intercalation compound, the electrical conductivity along the c axis is realized through a nonband mechanism—the transfer of free charge carriers along thin high-conductivity channels shunting the carbon layers adjacent to the intercalate.     

C-axis thermoelectric transport in low stage SbCl5 graphite intercalation compounds

c-axis thermal conductivity, electrical resistivity and thermopower measurements performed on stages-2, 3 and 4 SbCl₅-graphite intercalation compounds in the temperature range 3 < T < 300 K are reported. Contrary to the electrical resistivity and thermopower data, the temperature variation of the thermal conductivity is qualitatively different from that previously observed on other intercalation compounds.     

Enhanced thermoelectric properties of bismuth intercalated compounds BixTiS2

The thermoelectric properties of Bi intercalated compounds Bi_xTiS₂ have been investigated at the temperatures from 5 to 310 K. The results indicate that Bi intercalation into TiS₂ leads to substantial decrease of its electrical resistivity (one order low for x=0.05 and two orders low for x=0.15, 0.25 at 300 K) and lattice thermal conductivity (22, 115 and 158% low at 300 K for x=0.05, 0.15 and 0.25, respectively). Specially, the figure of merit, ZT, of lightly intercalated compound Bi_0.05TiS₂ has been improved at all temperatures investigated, and specifically reaches 0.03 at 300 K, which is about twice as large as that of TiS₂.     

常压烧结和高压合成Nd2-xCexCuO4的结构特征与导电性质

 本文在常压高温和高压高温条件下合成出了Nd_2-xCe_xCuO₄（x=0~0.20）系列样品，对比研究了两种不同条件下合成产物的结构特征与室温至液氮温区的导电性质。测试分析结果表明，高温高压（1.7 GPa，800 ℃，10 min）合成的样品与常压高温（1 000 ℃，10 h）烧结的样品具有相同的四方结构，但晶格常数随掺杂量变化有所不同，高压合成产物的c轴随掺杂量基本不变，而常压烧结样品c轴随掺杂量增加呈下降趋势。两种条件合成的样品在液氮温区均呈现出不同程度的半导体特征，经过一次高温淬火后处理后，高压样品的导电性质明显优于常压样品。实验结果表明，高压可以降低固相反应的合成温度，缩短反应时间，特别是高压的还原作用有利于产物导电性质的改善。     

CuCl2-FeCl3-H2SO4石墨插层化合物的结构特征

"采用水热法在150 ℃合成出了CuCl2-FeCl3-H2SO4的石墨插层化合物．通过X射线衍射、EDX、高分辨透射电子显微术分析了该石墨插层化合物的结构和成分．分析表明,CuCl2-FeCl3-H2SO4已经成功地插入到石墨的片层之间．测量了从5 K到300 K的磁化率随温度变化曲线,发现在低温时,该石墨插层化合物存在两个反铁磁转变,转变温度分别为50和100 K．"     

A new semiconducting organic-inorganic nanocomposite, 1,5-diaminonphthalene-saponite intercalation compound

We synthesized a new intercalation compound, 1,5-diaminonaphathalene(DAN)-saponite where intercalated DAN molecules were shown to have a formal charge of +0.67 The measurement of optical diffuse reflectance spectra revealed the formation of electronic bands with a gap of ca. 1 eV suggesting semiconducting behaviour of this system. From ESR measurements, the radical formation in DAN-saponite was confirmed and the spin concentration was determined to be 1 spin per 200 and 300 DAN-molecules at 290 and 7.9 K, respectively. This temperature dependence of the spin density also implies the semiconductive nature of DAN-saponite.     

高压低温下固体C60中的取向有序态及再取向的弛豫行为

通过构造C₆₀分子间的相互作用，研究了高压下及玻璃化转变温度附近固体C₆₀的取向状态占有概率分布及再取向的弛豫行为，所得结论均能较好地解释热导率实验中的异常现象. 关键词：