A rough hard-sphere model for the thermal conductivity of molten salts

A new model based on the rough hard-sphere theory is proposed for the correlation and prediction of the thermal conductivity of molten salts. The model is capable of predicting the thermal conductivity of all the members of a family of molten salts characterized by a common anion if the behavior of any single member of the family of salts is known. Only the molar volumes of the molten salt and the solid salt at the melting temperature are required in the calculations. In addition, the model is easily extended to mixtures with a simple mixing rule.     

Corresponding states correlation for the thermal conductivity of molten alkali halides

The principle of corresponding states has been applied to the thermal-conductivity data for molten alkali halides which have been obtained by recent forced Rayleigh scattering measurements. The theory, which was developed by Harada et al. for the transport properties of uni-univalent molten salts, is based on the fluctuation-dissipation theorem with the pair interaction between ions composed of core repulsive and Coulombic potentials. Four characteristic parameters specific to each salt have been used to reduce the thermal conductivity and temperature. It has been found that the thermal conductivity of molten alkali halides is adequately correlated by the corresponding-states correlation ( ^* 1/T ^*) within experimental accuracy. By employing the correlation, the thermal conductivity of molten alkali fluorides, which could not be measured by the forced Rayleigh scattering method, is predicted.Paper dedicated to Professor Joseph Kestin.     

纳米材料改善硝酸熔盐传蓄热性能的研究进展EI北大核心CSCD

聚光式太阳能热发电是解决能源和环境矛盾的理想途径,传热蓄热技术是光热发电的重要环节,在此需要解决的关键问题是传热蓄热介质。熔盐作为储蓄热介质具有明显优势。国内外运行的光热电站中大多使用二元硝酸熔盐(Solar salt)与三元硝酸熔盐(Hitec),但二者传蓄热性能均欠佳,影响了太阳能的利用效率。纳米材料的独特空间结构,使其具有优异的导热性能、良好的稳定性等,将其作为添加剂引入到硝酸熔盐体系中,有望改善材料的传热蓄热等热物性能,进而提高太阳能光热利用的效率,降低发电成本。本文综述了纳米金属粒子、纳米金属氧化物、纳米碳材料和其他无机纳米材料作为添加剂掺杂到硝酸熔盐体系中的相关研究,论述了改性后熔盐热物性的变化并探讨了作用机理,以期为制备优异热性能的储能熔盐提供参考。未来的研究可重点关注热物性测试、传热机理、构效关系和工业化中试,将具有优异的传蓄热性能的硝酸熔盐应用在太阳能光热发电领域,在清洁能源开发利用方面发挥更重要的作用。     

A Novel Instrument for the Measurement of the Thermal Conductivity of Molten Metals. Part I: Instrument’s Description

The paper reports the design and construction of a new instrument for the measurement of the thermal conductivity of molten metals and salts. The apparatus is based on the transient hot-wire technique, and it is intended for operation over a wide range of temperatures, from ambient up to 1200 K. The present experimental technique overcomes problems of convection and thermal radiation, and it is demonstrated that it operates in accord with a theoretical model. The uncertainty of the thermal conductivity results is estimated to be ±2% which is superior to that achieved in most earlier work.     

Experimental determination of the thermal diffusivity of molten alkali halides by the forced Rayleigh scattering method. I. Molten LiCl,NaCl, KCl,RbCl, and CsCl

As a series of experimental determinations of the thermal diffusivity of molten alkali halides, this paper describes measurements on five molten alkali metal chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) in the temperature range up to 1440 K by the forced Rayleigh scattering method. K₂Cr₂O₇ is employed as a dye substance to color the transparent molten salts. The accuracy is estimated to be ± 4 to ±11 % depending on the measured salts. In comparison with the present results converted into thermal conductivity, most of the previous experimental data obtained by steady-state methods show larger values, up to about five times, which may be due to the systematic error caused by the presence of convection and radiation. It is found that the thermal conductivity of these series of molten alkali metal chlorides decreases with increasing molecular weight, and their temperature coefficients are weakly negative.     

熔盐法合成高导热磷化硼及其热管理性能研究

随着电力电子器件封装密度提高, 开发导热性能优异的热界面材料受到了广泛关注。绝大多数传统导热填料的热导率较低, 因此合成新型高导热填料是提高热界面材料导热性能的重要途径。本研究通过简单的熔盐法合成了高导热的磷化硼(BP)颗粒, 与氮化硼(h-BN)混合并通过搅拌和浇注的方法填充到环氧树脂(EP)基体中制备得到树脂基复合材料(BP-BN/EP)。实验结果表明：采用三盐法(NaCl : KCl : LiCl)合成的BP产率最高达到74%, 相对于单盐法(41%)和双盐法(39%)分别提高了33%和35%。对于BP-BN/EP复合材料, 复合材料的微结构显示BP和BN颗粒均匀分布在环氧树脂基体。当混合填料体积分数为30%时, 该复合材料的热导率达到1.81 W•m^-1•K^-1, 是纯树脂热导率(0.21 W•m^-1•K^-1)的8.6倍, 这与BP颗粒作为桥梁连接相邻BN颗粒形成导热网络有关。除此以外, 相较于不含BP的复合材料(SBN-BN/EP), BP-BN/EP复合材料展现出更加优异的热导率、热稳定性和较好的热力学性能。因此, 熔盐法合成的BP在热管理领域具有较大的应用前景。     

Experimental determination of the thermal diffusivity of molten alkali halides by the forced Rayleigh scattering method. II. Molten NaBr,KBr, RbBr,and CsBr

This is a companion to an earlier paper (on molten alkali metal chlorides) which gives experimental results for the thermal diffusivity of four molten alkali metal bromides: NaBr, KBr, RbBr, and CsBr. The measurements were performed with a forced Rayleigh scattering instrument at temperatures up to 1326 K. The overall uncertainty in the measured thermal diffusivity is estimated to be ±3 to ±11%, depending on the measured salts. The results converted to thermal conductivity show one of the smallest values among other earlier experimental data obtained mainly by the steady-state methods. It is also found that the temperature dependence of the thermal conductivity is weakly negative.     

Thermal conductivity of liquid mixtures of benzene and 2,2,4-trimethylpentane at pressures up to 350 MPa

This paper contains the results of new measurements of the thermal conductivity of mixtures of benzene and 2,2,4-trimethylpentane in the liquid phase within the temperature range 313 to 344 K at pressures up to 350 MPa. The measurements were carried out with a transient hot-wire instrument and have an estimated accuracy of ±0.3%. The study is the first conducted at high pressures on mixtures of components of greatly differing volatilities and therefore provides a further test of methods of representing the thermal conductivity of liquid mixtures based upon the hard-sphere theory of transport in liquids. It is shown that the procedure is capable of representing all of the present experimental data within ±5%. A more detailed examination of the results reveals small, but systematic, deviations from universality of the behavior of the thermal conductivity as a function of density implied by the hard-sphere theory, which merit further investigation.     

The thermal conductivity of the molten NaNO3-KNO3 eutectic between 525 and 590 K

Molten salts are one of the few remaining classes of fluids for which standardquality (±1% accuracy) data on thermal conductivity have not hitherto been available. We have therefore developed a new apparatus based on the transient hot-wire technique to obtain reference-quality measurements of the thermal conductivity of molten salts at high temperatures. Liquid metal-filled quartz capillaries served as insulated hot wires in our method, and in addition, a two-wire technique was used in order to obtain absolute values of the thermal conductivity. New data for the NaNO₃-KNO₃ eutectic between 525 and 590 K are reported in this paper and comparisons with other recent measurements are shown.     

Measurements of thermophysical properties by a stepwise heating method

An outline of the stepwise heating method for measuring thermal diffusivity and specific heat capacity of samples in both solid and liquid phases is described. The method is based on the measurement of temperature response at the surface of a solid sample when the other surface is heated in step-function. By making the best use of the characteristic points of this method, applications to samples in the liquid state, especially to high temperature melts such as molten salts, have been tried. As examples of measurement results, the thermal diffusivity, specific heat capacity, and thermal conductivity of zirconia brick and the thermal diffusivity of molten salts are shown in graphic form.Presented at the Japan-United States Joint Seminar on Thermophysical Properties, October 24–26, 1983, Tokyo, Japan.     

Simultaneous Measurements of the Thermal Conductivity and Thermal Diffusivity of Molten Salts with a Transient Short-Hot-Wire Method

A transient short-hot-wire technique has been successfully used to measure the thermal conductivity and thermal diffusivity of molten salts (NaNO₃, Li₂CO₃/K₂CO₃, and Li₂CO₃/Na₂CO₃) which are highly corrosive. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a short wire with the same length-to-diameter ratio and boundary conditions as those used in the actual experiments. In the present study, the wires are coated with a pure Al₂O₃ thin film by using a sputtering apparatus. The length and radius of the hot wire and the resistance ratio of the lead terminals and the entire probe are calibrated using water and toluene with known thermophysical properties. Using such a calibrated probe, the thermal conductivity and thermal diffusivity of molten nitrate are measured within errors of 3 and 20%, respectively. Also, the thermal conductivity of the molten carbonates can be measured within an error of 5%, although the thermal diffusivity can be measured within an error of 50%.     

不同煅烧介质中纳米氧化镁的合成技术研究

以化学沉淀法首先制备了氢氧化镁(Mg(OH)2)纳米材料,并且进一步在LiNO3和NaCl 2种盐介质中分别进行液相和固相辅助煅烧使其转变成纳米氧化镁.研究表明,液相LiNO3熔盐介质比固相NaCl熔盐介质更有利于纳米MgO的生成,并且纳米MgO产率也更高.简要分析了2种熔盐辅助煅烧的不同机理.     

面向太阳能光热发电的NaNO3-KNO3-Mg(NO3)2三元硝酸熔盐

Solar Salt(60%NaNO_3-40%KNO_3)熔盐作为传蓄热介质,在太阳能光热发电领域应用广泛。以Solar Salt熔盐为基础,采用熔融共混法,通过添加Mg(NO_3)2·6H_2O制备了低熔点NaNO_3-KNO_3-Mg(NO_3)_2三元熔盐(NKM熔盐),优化了制备工艺,获得了最佳工艺参数及条件,获得了三元熔盐热物性变化规律。通过热物性分析,确定NKM熔盐配比为49.5%NaNO_3-33%KNO_3-17.5%Mg(NO_3)_2。与Solar Salt和HTS(53%KNO_3-40%NaNO_2-7%NaNO_3)熔盐相比,NKM熔盐具有较低的熔点和良好的温度使用范围,黏度较小,储热性和导热性良好,适合作为中温储能材料应用于光热发电。     

Thermal conductivity of carbon tetrachloride in the temperature range 310 to 364 K at pressures up to 0.22 GPa

The paper reports new measurements of the thermal conductivity of carbon tetrachloride in the temperature range 310 to 364 K at pressures up to 0.22 GPa. The experimental data have an estimated uncertainty to ±0.3%. The hard-sphere theory of transport in dense fluids is employed to formulate a correlation scheme for the thermal conductivity as a function of density. A single equation represents the dependence of the thermal conductivity on density for all isotherms, the isotherms being distinguished by a characteristic value of the molar volume. It is shown that earlier measurements of the viscosity and self-diffusion coefficient of carbon tetrachloride may be represented in a similar fashion using consistent values of the characteristic volume.     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.     

The thermal conductivity of molten NaNO3 and KNO3

The thermal conductivity data for molten NaNO₃ and KNO₃ have been examined in order to propose recommended data sets for these two popular heat carriers and to establish the reference values above the temperature range covered by toluene and water. It is known that the measurement of the thermal conductivity of molten salts is very difficult, owing mainly to their corrosiveness and high melting temperatures, which introduce complications in apparatus design and significant systematic errors due to radiation and convection. However, some recent measurements seem to manifest more trustworthy values than obtained before. All available data have been collected and critically evaluated. The temperature range covered is 584 to 662 K for molten NaNO₃ and 662 to 712 K for molten KNO₃, with the confidence limits better than ± 5%.     

Measurement of thermophysical properties of molten salts: Mixtures of alkaline carbonate salts

The purpose of this study is to develop measuring methods for the thermal diffusivity, the specific heat capacity, and the density of molten salts, as well as to measure these properties of mixtures of alkaline carbonate salts. The thermal diffusivity is measured by the stepwise heating method. The sample salt is poured into a thin container, and as a result, a three-layered cell is formed. The thermal diffusivity is obtained from the ratio of temperature rises at different times measured at the rear surface of the cell when the front surface is heated by the stepwise energy from an iodine lamp. The specific heat capacity is measured using an adiabatic scanning calorimeter. The density is measured by Archimedes' principle. Thermal conductivity is determined from the above properties. Measured samples are Li₂CO₃-K₂CO₃ (42.7–57.3, 50.0-50.0, and 62.0-38.0 mol%).Invited paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Modeling Thermal Conductivity of Electrolyte Mixtures in Wide Temperature and Pressure Ranges: Seawater and Its Main Components

A model has been established for calculating the thermal conductivity of aqueous electrolyte solutions containing the Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, and Br⁻ ions. The model is based on a previously developed computational framework for the thermal conductivity of mixed-solvent electrolyte systems, which has been expanded by explicitly accounting for pressure effects in addition to temperature and electrolyte composition effects. The model consists of a contribution of the solvent, a contribution of individual species expressed using modified Riedel coefficients, and an ionic strength-dependent term that is due to interactions between species. The model accurately represents the thermal conductivity of solutions containing single and multiple salts at temperatures ranging from 273 K to 573 K, pressures up to at least 1400 bar, and concentrations up to the limit of solid saturation. Further, the model has been applied to seawater and used to elucidate the discrepancies between the experimental data for seawater and those for Na–K–Mg–Ca–Cl–SO₄ salt solutions. With parameters evaluated on the basis of data for binary and multicomponent salt solutions, the model provides reliable predictions of the thermal conductivity of seawater.     

Thermal conductivity of isopentane in the temperature range 307–355 K at pressures up to 0.4 GPa

This paper reports the results of new, absolute measurements of thermal conductivity of isopentane in the temperature range 307–335 K at pressures up to 0.4 GPa. The experimental data have an estimated uncertainty of ±0.3%. The density dependence of the thermal conductivity along the various isotherms has been represented with the aid of a single universal equation derived for a series of alkanes and based upon the hard-sphere model of dense fluids. An even more general prediction scheme for the thermal conductivity of liquids developed initially for normal alkanes is found to predict the present data within ±5%.     

A physical model for predicting the thermal conductivity of brine-wetted snow

Recent measurements by Crocker (1984) have shown that the snowcover overlying young sea ice contains large quantities of salt (concentrations as high as 100 ppt). The influence of the salt, which is incorporated into the snowpack in the form of brine, on the thermal conductivity of the snow is analysed using a modified form of the physical thermal conductivity model developed by Pitman and Zuckerman (1967). The new model indicates that at snow densities commonly encountered in young sea ice, the presence of the brine increases the thermal conductivity of the snow by up to 50%. When the thermal conductivities predicted by the model are input into an ice-growth model, better association between observed and predicted ice growth (on Resolute Passage, North West Territories, 1982) is obtained than with any of the empirical thermal conductivity formulae tested.