Electronic Structure and Transport Coefficients of the Thermoelectric Materials Bi_2Te_3 from First-principles Calculations

The electronic structures of bulk Bi_2Te_3 crystals were investigated by the first-principles calculations.The transport coefficients including Seeback coefficient and power factor were then calculated by the Boltzmann theory,and further evaluated as a function of chemical potential assuming a rigid band picture.The results suggest that p-type doping in the Bi_2Te_3 compound may be more favorable than n-type doping.From this analysis results,doping effects on a material will exhibit high ZT.Furthermore,we can also find the right doping concentration to produce more efficient materials,and present the ＂advantage filling element map＂ in detail.

Calculation of railway transport capacity in an emergency based on Markov process

A method is proposed to calculate the railway transport capacity in an emergency to support the dispatching work of control centers. The effect of an emergency on section transport capacity is analyzed and the basic method to calculate the transport capacity is presented. The results show that the situation-changing process in emergency is actually a Markov process. The calculation rule is presented based on division of the time segment during which the emergency lasts. The algorithm is designed to calculate the transport capacity of each time segment. The pessimistic strategy and the fuzzy strategy are proposed to determine the computing value of the transport capacity of each time segment, to satisfy the calculating requirements in different occasions. Our study shows that the method is reasonable and practical and the method can be embedded in the train dispatching system to support the operation work in an emergency.

Numerical study of scattering Legendre moments and effect of anisotropic scattering on S_N shielding calculation

In neutron and photon transport problems,anisotropic scattering is of great importance for the particle flux,especially when the angular flux has a strong forward peak in shielding analyses.The conventional Legendre expansion is widely used in discrete ordinates transport codes because of algebraic simplifications with spherical harmonics for the scattering source.However,negative cross sections caused by the finitely truncated expansion may give rise to a negative source and flux.A simple method is adopted,based on integrating functions of scattering moments,to evaluate anisotropy and convergence of expanded functions.A series of problems were designed with angular fluxes of different anisotropy,and numerical simulations were performed using the ARES transport code to study different treatments and algorithms for scattering.Results show that the diagonal transport approximation is more stable and obtains a similar accuracy with the extended approximation.A conservative fixup for the negative source could ensure particle balance and improve computational accuracy significantly for photon transport.The effect of anisotropic scattering is problem-dependent,and no distinct differences among various methods are observed for volume source problems with a continuous energy source.For beam source problems,flux results are sensitive to negative scattering functions,and strictly nonnegative cross sections need to be implemented.

First principles calculations on the thermoelectric properties of bulk Au2S with ultra-low lattice thermal conductivity

Sulfide nanocrystals and their composites have shown great potential in the thermoelectric(TE)field due to their extremely low thermal conductivity.Recently a solid and hollow metastable Au2S nanocrystalline has been successfully synthesized.Herein,we study the TE properties of this bulk Au2S by first-principles calculations and semiclassical Boltzmann transport theory,which provides the basis for its further experimental studies.Our results indicate that the highly twofold degeneracy of the bands appears at theΓpoint in the Brillouin zone,resulting in a high Seebeck coefficient.Besides,Au2S exhibits an ultra-low lattice thermal conductivity(~0.88 W·m^-1·K^-1 at 700 K).At 700 K,the thermoelectric figure of merit of the optimal p-type doping is close to 1.76,which is higher than 0.8 of ZrSb at 700 K and 1.4 of PtTe at 750 K.Our work clearly demonstrates the advantages of Au2S as a TE material and would greatly inspire further experimental studies and verifications.

Electronic transport evolution across the successive structural transitions in Ni_(50-x)Fe_xTi_(50) shape memory alloys

TiNi-based shape memory alloys have been extensively investigated due to their significant applications,but a comprehensive understanding of the evolution of electronic structure and electrical transport in a system with martensitic transformations(MT) is still lacking.In this work,we focused on the electronic transport behavior of three phases in Ni_(50-x)Fe_xTi_(50)across the MT.A phase diagram of Ni_(50-x)Fe_xTi_(50) was established based on x-ray diffraction,calorimetric,magnetic,and electrical measurements.To reveal the driving force of MT,phonon softening was revealed using first-principles calculations.Notably,the transverse and longitudinal transport behavior changed significantly across the phase transition,which can be attributed to the reconstruction of electronic structures.This work promotes the understanding of phase transitions and demonstrates the sensitivity of electron transport to phase transition.

Effects of Rattling Behavior of K and Cd Atoms along Different Directions in Anisotropic KCdAs on Lattice Thermal Transport and Thermoelectric Properties

We employ advanced first principles methodology,merging self-consistent phonon theory and the Boltzmann transport equation,to comprehensively explore the thermal transport and thermoelectric properties of KCdAs.Notably,the study accounts for the impact of quartic anharmonicity on phonon group velocities in the pursuit of lattice thermal conductivity and investigates 3ph and 4ph scattering processes on phonon lifetimes.Through various methodologies,including examining atomic vibrational modes and analyzing 3ph and 4ph scattering processes,the article unveils microphysical mechanisms contributing to the lowκL within KCdAs.Key features include significant anisotropy in Cd atoms,pronounced anharmonicity in K atoms,and relative vibrations in non-equivalent As atomic layers.Cd atoms,situated between As layers,exhibit rattling modes and strong lattice anharmonicity,contributing to the observed lowκL.Remarkably flat bands near the valence band maximum translate into high PF,aligning with ultralowκL for exceptional thermoelectric performance.Under optimal temperature and carrier concentration doping,outstanding ZT values are achieved:4.25(a(b)-axis,p-type,3×10^(19)cm^(−3),500 K),0.90(c-axis,p-type,5×10^(20)cm^(−3),700 K),1.61(a(b)-axis,n-type,2×10^(18)cm^(−3),700 K),and 3.06(c-axis,n-type,9×10^(17)cm^(−3),700 K).

β-Ga_(2)O_(3)热电性能的第一性原理计算

通过使用第一性原理计算方法和玻尔兹曼输运理论,计算了β-Ga_(2)O_(3)的电学、热学和热电性质,并得出了热电性能因子随载流子浓度和温度的变化规律.此外,还考虑了声子散射和极性光学声子散射对平均电子弛豫时间的影响,并详细描述了不同载流子浓度和温度下β-Ga_(2)O_(3)的Seebeck系数、电导率、电热导率和功率因数的计算结果.最后,给出了β-Ga_(2)O_(3)的热电性能因子随载流子浓度和温度的变化趋势.结果表明,在高载流子浓度和高温下,β-Ga_(2)O_(3)具有较高的热电性能因子.

Charge Transport Properties of Tetrabenz[a,c,h,j]-anthracene Derivatives

Charge transport properties of F, OH, OCH3, SH and SCH3-substituted tetra- benz[a,c,h,j]- anthracene derivative molecules have been investigated theoretically at the B3LYP/6-31G＊＊ level using Marcus theory. The results showed that at 300 K, the hole or electron transport capability of F or SH-substituted molecules was better obviously than that of OH or OCH3-substituted molecules, The electron transport capability of SCH3-substituted and F or SH-substituted molecules was superior to their hole transport capability, respectively. F, SH or SCH3-substituted tetrabenz[a,c,h,j]-anthracene derivative molecules can be used as electron transport materials.

A comparison of the energy consumption and carbon emissions for different modes of transportation in open-cut coal mines

Transportation accounts for 80% of open-cut coal mine carbon emissions. With regard to the energy con- sumption and carbon emissions of transportation within an open-cut mine, this paper systematically compared the work and energy consumption of a truck and belt conveyor on a theoretical basis, and con- structed a model to calculate the energy consumption of open-cut mine transportation. Life cycle carbon emission factors and power consumption calculation model were established through a Process Analysis- Life Cycle Analysis （PA-LCA）. The following results were obtained： （1） the energy consumption of truck transportation was four to twelve times higher than that of the belt conveyor; （2） the C02 emissions from truck transportation were three to ten times higher than those of the belt conveyor; （3） with the increase in the slope angle for transportation, the ratio of truck to belt conveyor for both energy consumption and carbon emissions gradually decreased; （4） based on 2013 prices in China, the energy cost of transportation using a belt conveyor in open-cut coal mines could save 0.6-2.4 Yuan/（t kin） compared to truck transportation.

A methodology for calculating the passenger comfort benefits of railway travel

A comfortable environment in railway passenger coaches can be regarded as a resource for social consumption during the transport process. Railway passenger comfort benefits （RPCBs） can be regarded as a special generalized cost. In this paper, we select a series of objective and subjective indexes to formulate a quantitative method of calculating the RPCB with considering ticket fares. This method includes three steps： make the initial data dimensionless, calculate the weight of each index, and finally calculate the RPCBs. The proposed method was validated with the data collected from two types of trains： G13 from Beijing South to Shanghai Hongqiao and T109 from Beijing to Shanghai. Also, questionnaire survey was conducted in both trains. After data processing, the results show that there is a linear relationship between the RPCB and ticket fare with a correlation coefficient of 0.9616.

Single crystal growth, structural and transport properties of bad metal RhSb2

We have successfully grown an arsenopyrite marcasite type RhSb2 single crystal, and systematically investigated its crystal structure, electrical transport, magnetic susceptibility, heat capacity, and thermodynamic properties. We found that the temperature-dependent resistivity exhibits a bad metal behavior with a board peak around 200 K. The magnetic susceptibility of RhSb2 shows diamagnetism from 300 K to 2 K. The low-temperature specific heat shows a metallic behavior with a quite small electronic specific-heat coefficient. No phase transition is observed in both specific heat and magnetic susceptibility data. The Hall resistivity measurements show that the conduction carriers are dominated by electrons with ne = 8.62 × 10^(18) cm^(-3) at 2 K, and the electron carrier density increases rapidly above 200 K without change sign. Combining with ab-initio band structure calculations, we showed that the unusual peak around 200 K in resistivity is related to the distinct electronic structure of RhSb_2. In addition, a large thermopower S(T) about -140 μV/K is observed around 200 K, which might be useful for future thermoelectric applications.

First-principles Study of Electron Transport Through Oligoacenes

The electronic transport properties of oligoacenes sandwiched between two Au（111） surfaces with serial and parrallel configurations were investigeted by using a fully self-consistent nonequilibrium Green＇s function method combined with density functional calculations. This theoretical results show that the conductivity of oligoacenes with both sandwiched configurations at low bias voltage is mainly determined by the tail of the transmission peak from the perturbed highest occupied molecular orbital. When the molecular length increases, the zero-bias voltage conductance G（0） of oligoacenes with serial configuration neither follows Magoga＇s exponential law nor displays the even-odd oscillation effect, while the G（O） of the oligoacenes sandwiched with parallel configuration monotonically increases. The reduction of energy gaps, the alignment of the Fermi level, and the spatial distribution of the perturbed molecular orbitals are used to self-consistently explore the transport mechanism through oligoacenes.

Effect of boron/nitrogen co-doping on transport properties of C60 molecular devices

By using nonequilibrium Green＇s function method and first-principles calculations, the electronic transport properties of doped C60 molecular devices were investigated. It is revealed that the C60 molecular devices show the metal behavior due to the interaction between the C60 molecule and the metal electrode. The current-voltage curve displays a linear behavior at low bias, and the currents have the relation of MI〉M3〉M4〉M2 when the bias voltage is lower than 0.6 V. Electronic transport properties are affected greatly by the doped atoms. Negative differential resistance is found in a certain bias range for C60 and C58BN molecular devices, but cannot be observed in C59B and C59N molecular devices. These unconventional effects can be used to design novel nanoelectronic devices.

Steady-state and transient electronic transport properties of β-(Al_(x)Ga_(1-x))_(2)O_(3)/Ga_(2)O_(3)heterostructures:An ensemble Monte Carlo simulation

The steady-state and transient electron transport properties ofβ-(Al_(x)Ga_(1-x))_(2)O_(3)/Ga_(2)O_(3)heterostructures were investigated by Monte Carlo simulation with the classic three-valley model.In particular,the electronic band structures were acquired by first-principles calculations,which could provide precise parameters for calculating the transport properties of the two-dimensional electron gas(2DEG),and the quantization effect was considered in theΓvalley with the five lowest subbands.Wave functions and energy eigenvalues were obtained by iteration of the Schrödinger–Poisson equations to calculate the 2DEG scattering rates with five main scattering mechanisms considered.The simulated low-field electron mobilities agree well with the experimental results,thus confirming the effectiveness of our models.The results show that the room temperature electron mobility of theβ-(Al_(0.188)Ga_(0.812))_(2)O_(3)/Ga_(2)O_(3)heterostructure at 10 k V·cm^(-1)is approximately153.669 cm^(2)·V^(-1)·s^(-1),and polar optical phonon scattering would have a significant impact on the mobility properties at this time.The region of negative differential mobility,overshoot of the transient electron velocity and negative diffusion coefficients are also observed when the electric field increases to the corresponding threshold value or even exceeds it.This work offers significant parameters for theβ-(Al_(x)Ga_(1-x))_(2)O_(3)/Ga_(2)O_(3)heterostructure that may benefit the design of high-performanceβ-(Al_(x)Ga_(1-x))_(2)O_(3)/Ga_(2)O_(3)heterostructure-based devices.

高速铁路信号系统碳排放边界划分及分析

准确量化高速铁路信号系统的碳排放量,对高速铁路碳排放核算及低碳化发展有重要意义。本文提出在计算其碳排放时需要综合考虑信号系统相关关联活动的思路,并划分信号系统的碳排放边界;提出基于能源消耗的信号系统碳排放计算方法,进一步利用碳流理论分析计算信号系统内各设备的相关碳流指标;最后以甘肃省兰州西高铁站为例进行分析,验证本文所提方法的可行性和有效性。结果显示,控制及监测系统的碳排放主要与电力碳排放因子有关,维护保养电码化设备所产生的碳排放平均仅有0.12 kgCO_(2)·d^(-1);信号系统各设备间的平均支路碳流密度为0.0076 kgCO_(2)·kWh^(-1),各设备的节点碳势与支路碳流密度相等。结论表明,各子系统的碳排放量主要和其能耗类型有关,电能消耗引起的间接碳排放是信号系统碳排放的主要组成部分,信号系统相关活动引起的直接碳排放与一次能源消耗成正比。

畜禽车厢体内温度和流场数值CFD模拟及验证

为研究畜禽运输车厢体内环境的分布情况以及对温度场进行评估,利用计算流体力学(CFD)的方法,模拟出在空载及满载条件下内气流流场和温度场的分布情况,同时根据空载模拟结果提出优化方案并模拟。测量与分析厢体内关键位置的温度和风速,并与模拟值比较。结果表明:模拟空载时模拟值与测量值最大绝对误差1.7℃,模拟得到厢体的温度分布变化情况和测量值所得结果一致,风速值相对误差范围在3%以下的有22个点。优化后厢体内温度均匀度改善明显,温度变化幅度小;满载时,温度分布与空载相似,但整体温度有些许上升。验证CFD模型的正确性,为优化提供可靠基础。

不同沿岸输沙率计算公式对比分析

经历近100 a的发展,沿岸输沙率计算公式研究领域已形成许多代表性成果,系统研判各计算公式的预测精度对于快速准确判断研究区域沿岸输沙能力具有重要意义。依据使用率高、公式结构简洁、考虑参数相对全面等标准,选择了国内外7个沿岸输沙率计算公式,采用现场原型沙、室内原型沙和室内轻质沙等多种实测资料,对各公式的计算准确程度和适用范围进行了定量检验。结果表明,CERC公式和Kamphuis公式适用于现场原型沙预测,但对室内轻质沙预测的偏离程度较大;MH-Kamphuis公式和van Rijn公式对各种实测资料的预测均总体偏小;赵今声公式和Bayram公式的预测精度最低,预测偏小程度较大。经比较,孙林云公式对各项实测资料的符合程度总体较高,预测精度提高了36%~73%,具有更广泛的适用范围,推荐用于推导沙质海岸沿岸输沙模型相似律比尺关系。

基于匝道控制及尾气计算模型的高速公路绿色交通管理研究

随着高速公路车辆的增多,大量汽车排放的尾气导致高速公路附近的环境受到极大的威胁。匝道是在立交处连接立交上、下道而设置的单车道单方向的转弯道路。在高速公路的道路上通常都设有一个控制与检测点。为了提高高速公路附近的自然环境质量,研究提出了一种基于匝道控制和尾气计算模型的策略,该策略通过对匝道控制进行优化,减少车辆在道路上的停留时间,从而减少尾气排放。对研究提出策略的实际应用效果进行分析,结果说明,研究提出的交通策略可有效减少汽车尾气,从而提升高速公路附近的环境水平,促进绿色交通的发展。

超声速民机机动飞行的聚焦声爆全场预测方法研究

超声速民机在加速、俯冲等机动飞行阶段会产生强烈的聚焦声爆,其全场预测方法是当前国际上研究的热点和难点之一。由于目前无法承受全域非定常计算带来的成本,故而将聚焦声爆全场预测分为焦散计算和波形预测两部分,并在两部分之间迭代求解得到地面聚焦位置和声爆强度。首先,通过耦合求解机动飞行航迹和声射线传播路径,发展了迭代求交点和推进求包络线两类焦散计算方法。其次,在焦散计算和近场计算加中/远场传播之间迭代求得入射波形,作为求解广义Tricomi方程的输入,发展了一套聚焦声爆全场预测方法,并自主开发了预测程序FC-Boom。最后,采用AIAA第三届声爆预测研讨会标模算例和美国SCAMP项目飞行试验算例,验证了全场预测方法和程序的正确性。该工作可作为聚焦声爆抑制和低聚焦声爆设计的基础。

基于空间核反应堆事故工况下的仿真研究

随着我国航天事业的发展,空间核反应堆电源因其功率密度大、寿命长、环境适应性强、工作性能稳定等特点,已成为未来空间探索的理想动力能源。为了研究空间核反应堆在正常运行工况和掉落事故工况下的安全性,基于中子输运计算程序Open MC对空间核反应堆开展临界仿真研究,得到正常工况下堆芯反应性、控制鼓价值以及燃耗计算等一系列重要参数。同时模拟分析了空间核反应堆在水淹和沙埋2种事故工况下的堆芯反应性和中子能谱。结果表明,空间核反应堆在保持控制鼓功能正常的掉落事故工况下仍然有3000pcm左右的负反应性以防止反应堆达到临界状态,保证了空间核反应堆在此事故工况下的安全性,表明此堆芯的设计是安全可靠的。空间核反应堆仿真研究对我国未来空间核反应堆实际应用具有一定的指导意义。计算结果为空间核反应堆的安全分析和事故分析提供了重要的参考数据。