氯化物熔盐单质及其混合物比热容的分子动力学模拟及理论计算

由于具有良好的综合性能，氯化物熔融盐已被用于聚光太阳能的高温储热材料和传热介质。为进一步评估氯化物熔盐的储热能力，该文采用分子动力学模拟方法在高温范围对三种常用的氯化物熔盐单质(LiCl、KCl和NaCl)、二元混合氯化物熔盐(LiCl-KCl和LiCl-NaCl)的比热容进行了系统研究。结果表明三种单质的比热容模拟值与实验值相比平均误差均在5.2%以内，表现出较好的一致性。为了验证通过理论方法计算氯化物熔盐混合物比热容的可行性，采用混合法则和DP法对二元混合氯化物熔盐(LiCl-KCl和LiCl-NaCl)的比热容进行理论计算。与模拟值相比，除通过混合法则计算LiCl-KCl的误差为5%外，其余误差均在2%以内，表明通过理论计算方法对多元混合氯化物熔融盐比热容进行计算有一定的可行性。为了更好的从微观结构上理解氯化物熔盐的热物性，通过计算径向分布函数对熔融盐体系及局部结构进行了分析。该研究的结果及方法对混合氯化物熔盐的热物性预测具有参考和指导意义。     

NaCl-MgCl2-CaCl2相图计算方法研究

为满足规模化太阳能热发电中对传热蓄热的要求,针对储能材料开展探究。以熔融盐热物性及经济性作为筛选条件,选定来源广泛、价格低廉、工作温度范围宽、黏度低、相变潜热大的NaCl、MgCl₂和CaCl₂三元熔盐体系开展深入研究。应用修正的准化学溶液模型,在子二元系基础上推导计算三元混合熔盐NaCl-MgCl₂-CaCl₂相图。结果表明,该熔盐混合物共晶点温度为412.45 ℃,NaCl、MgCl₂和CaCl₂的摩尔分数分别为50.99%、22.78%和26.23%,与已有文献数据相比,误差在3%以内,验证了方法的准确性,为构建熔盐氯化物相图数据库奠定了部分基础。     

Preparation and thermal stability of ternary sulfate molten salt

以硫酸钠、硫酸钾和硫酸镁为原料,采用在硫酸钠-硫酸钾二元共晶盐中加入硫酸镁的方法制备三元硫酸熔盐。应用TG-DSC联用分析仪、热常数分析仪、X射线衍射仪以及热循环法对复合熔盐的熔点、相变潜热、热导率、比热容、分解点以及热稳定性进行表征。结果表明：所制备的三元硫酸熔盐熔点分布在667.5～669.7 ℃之间,较二元熔盐熔点降低了160 ℃左右,硫酸镁含量为30%（质量分数）的三元硫酸熔盐相变潜热值最大为94.3 J/g,比热容最大为1.13 J/(g·K)（720℃≤T≤800℃）,导热系数为0.41 W/(m·K),分解温度为1070 ℃,经50次热循环后,相变潜热值降低约4.34%,熔点和物相保持基本恒定,具有良好的热稳定性。该研究为硫酸盐作为高温传热蓄热介质提供了依据。     

Thermal properties of a low-melting-point nitrate molten salt system

利用硝酸盐熔盐熔点低、比热容高、热分解温度高的特性,制备一种新型硝酸盐熔盐[在Ca（NO₃）₂：KNO₃为0.47：0.53的熔盐体系中添加0.1%~15%的新型添加剂],并对该熔盐的热力学特性及使用成本进行了测试分析。结果表明,新型熔盐的熔点为120.1℃,熔化潜热为76.37 J/g,分解温度为588℃,平均比热容为1.598 J/（g·K）,相比较于传统的Solar salt和Hitec熔盐热力学性能具有较大提升。采用测量范围内比热容的积分平均值来代替整个温度范围内熔盐的比热容,通过计算得出该体系熔盐的显热蓄热成本为108元/（kW·h）。此外,对新型熔盐的热扩散系数以及导热系数进行的测试分析,进一步证明了该熔盐在太阳能光热发电中作为蓄热传热介质具有巨大的潜在应用价值。     

GaAs纳米线晶格热导率温度和尺度效应及声子非弹性散射

为了深入理解砷化镓(GaAs)纳米线的微观热输运机理,采用平衡分子动力学的方法模拟其晶格热导率。基于声子态密度的频域特性和热流自相关函数的时域特性,分别研究温度、尺寸对纳米线晶格热导率的影响规律。结果表明,纳米线的热导率有明显的温度与尺度效应。随温度升高,其晶格热导率先逐渐增大后减小,300 K左右达到最大值1.6 W/(m·K);晶格热导率随长度、直径的增大均先增加后趋于稳定。砷化镓纳米线晶格热导率的温度和尺度效应均可用声子非弹性散射解释。模拟的热导率比体材料的值小一个数量级,模拟结果可为改善半导体材料的热电性能提供指导。     

新型相同钠离子混合熔盐相图预测及物性测量

为了提高第三代聚光式太阳能热发电技术的效率,降低系统的运行成本,需要开发出具有更高使用温度的新型高温储热/蓄热材料.本文基于相同阳离子的原则,选取3种相同钠离子熔盐NaNO3、NaCl、Na2CO3作为基盐,借助相图原理指导混合熔盐的配制,通过FactSage软件对NaNO3-NaCl-Na2CO3三元体系进行相图热力学计算,利用差示扫描量热仪对预测最低共熔点附近多组共晶盐进行热物性分析.结果发现,当三元体系熔盐NaNO3:NaCl:Na2CO3物质的量比例为0.92:0.064:0.016时熔点最低,测得最低共熔点为279.9℃,相变潜热值为194.1 J/g,与相图预测结果(291℃)基本一致,验证了相图计算的准确性.对该体系其他热物性进行测量,得到其分解温度为595℃(质量损失率为3％),比热容为1.60 J/(g·K)(500℃),平均比热容为1.62 J/(g·K),相比于Solar Salt和Hitec平均比热容分别增加了0.12 J/(g·K)和0.28 J/(g·K).该新型三元体系熔盐具有潜热、比热容大和使用温度范围较宽的优点,在制备复合材料潜热储热方面有很大潜力,为开发新型太阳能热发电储热/蓄热材料提供了借鉴,也为借助相图理论指导混合熔盐的开发提供了参考.     

Li2CO3/Na2CO3/K2CO3及其混合熔融盐储热材料热物性分子动力学研究

对Li₂CO₃/Na₂CO₃/K₂CO₃及其二元和三元混合熔融盐的密度、比热容、黏度、热导率进行分子动力学模拟(MD),对比得出模拟结果与现有的实验数据和模拟值相近。结果表明：随着温度的升高,密度逐渐减小,离子之间的距离增加,导致对剪切应力的抵抗力变小,这说明单组分、二元和三元熔融盐黏度的负温度依赖性。对于熔融盐的热导率,单组分和二元熔融盐也呈现出负温度依赖性,而三元熔融盐趋势是随温度的升高呈上升状态。     

利用随机近似热探针方法的生物油热物性研究

提出了基于蒙特卡洛随机近似的热探针方法,该方法以热探针微分方程的精确解为基础,可同时进行热导率、比热容等热物性的测量.对水、甘油、硅油、酒精的测量结果显示,该方法热导率测量的最大相对误差仅为0.7%,比热容相对误差绝对值的平均值为2.6%,最大相对误差为3.8%.利用该方法对快速裂解所生产的生物油的热导率和比热容进行了测量,给出了温度、有机助剂对生物油热导率和比热容的影响,并对结果进行了分析.     

三元混合碳酸熔盐热物性实验研究

为满足超临界二氧化碳太阳能光热发电系统高温传热储热的要求,对3种不同比例的高温混合碳酸熔盐的热物性进行研究.实验结果表明,1号、2号、3号混合碳酸熔盐的熔点分别为388.7、337.9、391.3℃;初晶点分别为454.9、639.7、535.4℃;1号、3号混合碳酸熔盐的分解温度分别为916.2、920.4℃.通过分析发现2号混合碳酸熔盐的初晶点较高和熔化潜热较小,因此选取1号和3号混合碳酸熔盐进行比热容、导热系数和黏度的测定.通过分析发现,在实验温度范围内1号和3号混合碳酸熔盐的比热容随着温度的升高先降低再升高;1号混合碳酸熔盐的平均导热系数为1.024 W/(m··K),是3号混合碳酸熔盐平均导热系数的2.29倍;1号和3号混合碳酸熔盐的黏度都随着温度升高逐渐降低,1号和3号混合碳酸熔盐的最小黏度值分别为6.62 mPa··s和5.28 mPa··s.     

Molecular dynamics simulation of thermodynamic properties of Mg-Cu alloys

采用原子嵌入势对Mg-Cu合金块体进行了分子动力学模拟。研究了不同含量的Cu元素对Mg-Cu合金的热物性和微观结构的影响。基于能量-温度曲线与比热容-温度曲线对熔化温度以及熔化焓进行了研究，结果表明，随着Cu含量的增加，Mg-Cu合金的熔点先减小后增加，其熔化焓先增加后减小；所有合金的密度和比热容均随Cu含量的增加而降低，热导率随Cu含量的增加而增加。表征了Mg-Cu合金在常温下的微观结构，结果表明，Mg-Cu合金中不同原子之间的相互作用更强，有利于α+Mg₂Cu共晶组织的形成，其模拟结果证实了共晶体是影响合金熔化焓以及热导率的一个主要因素。     

Molten salt strategy to synthesize alkali metal-doped Co9S8 nanoparticles embedded,N, S co-doped mesoporous carbon as hydrogen evolution electrocatalyst

A molten salt strategy was proposed to prepare a series of electrocatalysts for hydrogen evolution reaction (HER) with salts as templates, which consisted of Co₉S₈ nanoparticles (NPs) and N, S co-doped mesoporous carbons. The porosities, heteroatoms contents, and crystalline structures of the final electrocatalysts were determined by the types of salts and the calcining temperatures. When KCl/NaCl, KCl/LiCl, and NaCl/CaCl₂ were adopted as the molten salts, Co₉S₈ NPs embedded, N, S co-doped carbons were obtained. However, when CaCl₂ and ZnCl₂ were used as the molten salts, Co₉S₈ could not be synthesized. The characterization results exhibited that alkali metal atoms could be introduced in the lattices of Co₉S₈. The combination of experimental and theoretical results revealed that Na and K atoms doping improved the electrocatalytic performance for HER. GTCo900-KCl/NaCl possessed the best HER activity, delivering a current density of 10 mA cm⁻² at 54 mV in acidic media, 142 mV in neutral media, and 103 mV in alkaline media.     

Thermodynamic calculation and reference electrode calibration for high temperature molten salts

Abstract

Molten salts are important reaction media for chemical and electrochemical processing and have recently attracted attention for their potential in reprocessing and partitioning spent nuclear fuels. Electrochemical measurements are a convenient tool for exploring thermodynamic and kinetic properties of molten salts, but inconsistency in acquired data may arise from the use of inaccurate reference electrodes and differences in thermodynamic calculations. A thermodynamic approach to the calculation of half cell potentials for reactions in molten salts is proposed. As examples, chlorine/chloride and lithium ion/lithium half cell potentials in LiCl–KCl eutectic are thermodynamically analysed. The Ag/AgCl reference electrode is discussed as an example of a high temperature reference electrode. A technique involving in situ transient reduction of constitutive metal ions for the calibration of high temperature reference electrodes is developed which may enable the consistency of acquired data using different reference electrodes in a variety of molten salts. The thermodynamic approach and calibration technique may be extended to ionic liquid and other media at high and low temperatures.     

Analytical prediction of heat capacity of molten AgCl and CuCl with application to thermochemical Cu-Cl cycle for hydrogen production

To sustain our power-dependent world, there is a need for technological innovation in all aspects of science and engineering. Many times, thermophysical and material properties are not well defined for the specific application, which leads to implementing assumptions and approximations from the published data. In the thermochemical copper-chlorine (Cu-Cl) cycle for hydrogen (H₂) production, heat is recovered from cuprous chloride (CuCl) molten salt and it is then reacted with hydrochloric acid (HCl) in stoichiometric proportions to produce the anolyte for the H₂ production step of the cycle. However, the lack of precise thermophysical properties on CuCl heavily hinders the detailed investigations of heat recovery from the molten salt as it cools from 450 °C to 90 °C. In this paper a new method is developed to determine the thermophysical property of CuCl and silver chloride (AgCl) as the molten salts are changing phases to solid. This is achieved by correlating electrochemistry data with thermal data. A model that predicts the specific heat capacity during phase change process is developed based on the existing electromotive force (EMF) and thermal data from literature. Developed model shows the EMF derived specific heat capacity values of AgCl and CuCl are similar with a slight offset since they have similar EMF's at higher temperatures.     

多元混合熔融盐的制备及其性能研究

采用静态高温混合熔融法制备以硝酸钾、亚硝酸钠、硝酸钠为三元基元和添加剂构成多元混合熔盐.通过热稳定性、放热实验以及X-射线衍射(XRD)相分析和DSC热分析等表征技术,确定在三元熔盐中加入5%添加剂additive A时,多元混合熔盐比三元熔盐的高温热稳定性好.在不提高熔盐固体析出温度的前提下,可以使其最佳操作温度由原来400～500℃提高到约550℃.从多元混合熔盐中NO_2~-含量变化和相变潜热损失方面可以发现添加剂adttive A的加入可以降低多元熔盐劣化程度,提高混合熔盐的蓄热效率.     

秸秆燃烧过程中KCl分压蒸发模型的建立与应用

为了研究秸秆燃烧中钾（K）类无机物的逸出特性,计算其逸出速率,利用X射线荧光能谱仪（XRF）和X射线衍射仪（XRD）分析玉米秸及其低温灰中K的化学形态,利用同步热分析仪（STA）研究秸秆及其灰中主要K类无机物在高温过程的逸出特性,建立KCl在多孔介质中逸出和在熔融相中分压的蒸发模型,并应用模型计算秸秆灰中KCl的蒸发速率。结果表明：秸秆低温灰中的K主要以KCl、K₂SO₄形态存在,两者约占灰总量的13%;常用燃烧温度下,KCl主要以蒸发形式逸出;KCl与K₂SO₄的共熔,可降低KCl在熔融相中的蒸气压;秸秆灰中KCl蒸发速率的计算值与实验值吻合良好。     

Natural circulation with DOWTHERM RP and its MARS code implementation for molten salt‐cooled reactors

Recently, molten salt has received attention as a promising coolant for advanced nuclear reactors, especially for fluoride salt‐cooled high‐temperature reactor. The heat transfer characteristics of molten salt provide great advantages for application as a primary coolant, because of its superior performance in terms of sustainability, economics, safety, and reliability compared with gas coolant. However, understanding the thermal‐hydraulic characteristics of molten salts by experimental method is difficult because of its high‐temperature corrosion and toxicity issue. Therefore, oil fluids were introduced as simulants for studying the heat transfer phenomena of high Pr (Prandtl number) molten salts. In this study, a scaled‐down experiment using simulant oil was conducted, and scaling laws were applied to investigate a single‐phase natural circulation, which is important in nuclear reactors as a part of their passive safety. DOWTHERM RP (Diaryl Alkyl) was considered as a heat transfer simulant in this study because it matches the relevant dimensionless numbers (Prandtl number, Ra, Grashof number, Reynolds number, etc.) with those of molten salt. Prior to the experiment, the thermophysical properties of both the liquid and vapor phases of DOWTHERM oils were implemented into thermal‐hydraulic system analysis code or multi‐dimensional analysis of reactor safety code, to enable simulation and further study of the molten salts. Then, natural circulation experiments were conducted with the scaled rectangular loop, to establish similarity and experimental feasibility. For the validation, two different codes (multi‐dimensional analysis of reactor safety and computational fluid dynamics (CFD) were used to simulate the same natural circulation loop. From the experimental data, new heat transfer correlation for a single‐phase natural convection was developed, and the existing heat transfer correlations were compared. Copyright © 2016 John Wiley & Sons, Ltd.     

熔盐与过冷沸腾水传热特性实验研究

为研究熔盐蒸汽发生器的传热特性,以Solar Salt二元硝酸盐为工质,对熔盐与过冷沸腾水在蒸汽发生器中的传热规律进行实验研究。实验结果表明:熔盐传热系数随着水侧与熔盐的质量流速的增大而增大,随着水侧压力的升高而减小;当水侧在过冷水状态时,熔盐传热系数随着熔盐入口温度的升高而增大;当水侧在过冷沸腾状态时,熔盐传热系数随着熔盐入口温度的升高而减小。以实验数据为基础,首次提出熔盐与过冷沸腾水的传热系数的实验关联式,其拟合误差为±5%。     

二元混合硝酸盐的配制及性能

按照不同配比、不同配制方法得到系列二元混合硝酸盐样品,并得到不同混合熔盐的差式扫描量热曲线,同时对配制的熔盐进行多次反复熔化和加热,研究其稳定性。结果表明:当硝酸钠占混合物总质量的40%～60%,或硝酸钠和硝酸钾的质量比为9∶1时,形成的混合物具备共晶性质,其他比例的混合熔盐无法达到共晶状态;两种配制方法均可达到共晶状态,其熔点十分接近;而且,配制的混合熔盐经过反复加热后,其熔点和分解点变化不大,证明对于二元混合熔盐可仅通过物理搅拌即可达到共晶状态。     

棕榈酸甲酯及棕榈酸乙酯高温热物理性质的研究

为了获取两种脂肪酸酯类燃料棕榈酸甲酯和棕榈酸乙酯的热物理性质,并为将其作为燃料和燃料添加剂提供理论和试验数据支持,利用表面光散射法、振动管法和全反射法在较宽温区下分别研究了两种棕榈酸酯类的表面张力、黏度、密度和折射率。密度和折射率数据分别关联成了温度的多项式和线性函数,棕榈酸甲酯和棕榈酸乙酯的密度试验值与方程计算值平均绝对偏差分别为0.033%和0.013%,折射率试验值与方程计算平均偏差均为0.004%;黏度数据关联成了温度倒数的多项式,棕榈酸甲酯和棕榈酸乙酯的试验值与方程计算值平均绝对偏差分别为1.76%和2.30%;表面张力数据利用了van der Waals方程进行关联,棕榈酸甲酯和棕榈酸乙酯的试验值与方程计算值平均绝对偏差分别为0.86%和1.70%。