纳米流体导热系数实验研究进展

导热系数是反映介质换热能力的主要参数,具有重要的理论和应用意义。详细介绍国内外纳米流体制备及其导热系数的研究进展情况,对比和总结国内外研究者的研究结果。表明:纳米流体能显著提高基液的导热系数,但不同的研究者对于纳米流体导热系数的研究得出的结果存在一定差异,有待于进一步的深入研究。     

瞬态热丝法测量纳米流体的导热系数

推导了用双铂丝测量流体导热系数的瞬态热丝法的实验关联式,并据此设计实验装置,分析误差因素,给出了系统误差修正系数.用该装置测量了几种纳米流体的导热系数并作了相关分析.     

纳米导热材料的研究进展

纳米导热材料的发展为日常生活和工业生产的传热节能提供了大的帮助。文章重点介绍了纳米流体、纳米导热聚合物的种类、制备以及在工业领域的应用,详细阐述了国内外的最新研究成果,分析了纳米导热材料的问题,展望了纳米导热材料的未来。     

Cu-水纳米流体对流换热特性研究

建立了测量纳米流体流动与对流换热性能的实验系统,探讨了不同pH值、分散剂浓度和纳米粒子质量分数对Cu-水纳米流体对流换热性能的影响。结果表明:pH值对Cu-水纳米流体对流换热系数的影响较小,这个现象启发了我们将纳米流体应用到未来工业中,可以不考虑pH值对纳米流体对流换热性能的影响。分散剂加入量是影响Cu-水纳米流体对流换热系数的重要因素,从分散稳定、导热系数和对流换热系数提高三个方面来考虑,在0.1%Cu-H2O纳米流体中,0.07%十二烷基苯磺酸钠被选为最优化浓度。另外,Cu-水纳米流体的对流换热系数随纳米粒子质量分数的增大而增大,但其对流换热系数的增加明显低于导热系数的增加。     

影响纳米流体导热系数实验测试的因素

导热系数是衡量纳米流体强化换热的最重要的参数,但在不同学者的研究中,对于同一种纳米流体所测得的导热系数却有很大差别。本文针对影响纳米流体导热系数实验测量的因素进行研究,在相同的实验条件下,分别运用Hotdisk导热仪和闪光法导热仪对水基二氧化钛纳米流体的导热系数进行了测量。实验结果表明,用Hotdisk法的测量结果比用闪光法测得的高21%～34%。通过计算分析发现:自然对流是引起纳米流体导热系数测量结果多变的重要原因之一。     

石墨烯无溶剂纳米流体的制备及其在防腐导热涂料中的应用

通过在氧化石墨烯(GO)表面沉积Fe₃O₄纳米粒子制备得到GO@Fe₃O₄复合纳米粒子,再进一步通过表面接枝KH560-M2070,成功制备出石墨烯无溶剂纳米流体,从而改善石墨烯类材料在环氧树脂基体中的分散性和相容性。采用TEM、XRD分析方法对制备的石墨烯无溶剂纳米流体进行表征,同时对石墨烯流体/环氧树脂涂层的耐水、耐酸碱和导热性能进行了探究。     

纳米流体复合水泥固结导热及力学性能试验研究

地热井固井水泥材料的力学强度及导热特性是目前众多研究者的关注重点。本工作开展了纳米流体复合水泥材料研制、固结导热特性评价及机理分析,选用铁粉为基本导热填料,研究了纳米流体种类、颗粒质量分数、纳米流体用量对铁粉复合水泥石力学及导热性能的影响,最终得出优化配方：G级油井水泥+15%铁粉(以水泥用量外掺)+20%Al₂O₃-水纳米流体(以拌合用水用量内掺,纳米颗粒质量分数0.5%,水灰比0.44)。性能测试结果表明：添加Al₂O₃-水纳米流体能够降低水泥石孔隙率,提高中、后期抗压强度,提升导热性能近32%,而干混式加入Al₂O₃纳米颗粒对水泥石力学强度及传热性能几乎无提升。观察了水泥石的微观结构,分析了其物相成分,对纳米流体复合水泥的固结导热机理进行了研究。分析结果表明：纳米流体中均匀分散的Al₂O₃纳米颗粒起到促进水化、填充孔隙的作用。纳米流体的使用为地热井固井水泥材料的力学及导热性能优化提供了新思路。     

基于粒子吸附层的纳米流体有效导热系数模型

基于纳米粒子表面吸附层的分析,构建包含纳米粒子、分散剂层、类固相层和分散基液的导热单元,利用最小热阻法建立纳米流体的有效导热系数模型,推导出其表达式,并分别讨论纳米粒子粒径、分散剂层和类固相层对纳米流体的有效导热系数的影响。结果表明,考虑类固相层的影响得到的纳米流体的有效导热系数比不考虑其影响得到的数值大;添加分散剂后,纳米流体的有效导热系数随着分散剂导热系数的增大而增大,当纳米粒子较小时分散剂在纳米粒子表面的吸附层对纳米流体导热系数的影响更大。     

Cu-水纳米流体的稳定性及其粘度的实验研究

采用"两步法"制备了含不同质量分数的Cu-水纳米流体,分别对质量分数为0.10%、0.5%以及1%纳米流体的德定性及其粘度进行了实验研究,并与经典两相混合物粘度模型进行了比较.结果表明,添加分散剂SDBS能有效改善Cu-水纳米流体的稳定性,且当其浓度与Cu纳米颗粒的质量分数相当时,纳米流体的稳定最好;纳米流体的粘度随颗...     

Cu-水纳米流体的分散行为及导热性能研究

通过测定Cu-水纳米悬浮液的Zeta电位和吸光度,采用Hotdisk热物性分析仪测量了其导热系数,探讨了不同pH值和分散剂浓度对Cu-水纳米悬浮液分散稳定性和导热性能的影响.结果表明,pH值和分散剂加入量是影响Cu-水纳米悬浮液分散稳定和导热系数的重要因素.最优化的pH值和分散剂加入量能显著提高水溶液中Cu表面Zeta电位绝对值,增大了颗粒间静电排斥力,悬浮液分散稳定性较好,导热系数较高.从分散稳定和导热系数提高两个方面来考虑,pH=9.5左右被选为最优化值,在0.1%Cu-H2O纳米流体中,0.07%SDBS被选为最优化浓度.另外,Cu-水纳米流体的导热系数随纳米粒子质量分数的增大而增大,呈非线性关系,且比现有理论(Hamilton-Crosser模型)预测值大.     

Preparation and characterization of reduced graphene oxide using ascorbic acid and sodium citrate as binary reductant

In this article, we successfully prepared transparent and gauze-like reduced graphene oxide (RGO) film with the thickness of only 1.32 nm using modified Hummers method and binary reductant consisting of ascorbic acid and sodium citrate. SEM, TEM, AFM, XRD, UV-Vis, and FTIR results all convincingly demonstrated that the reduction of graphite oxide using binary reductant is a feasible method to prepare RGO. In addition, the results also revealed that extending the oxidation time and increasing the ultrasonic oscillation reasonably might improve the oxidation degree of the graphite oxide, the ratio of the D-band Raman intensity to that of the G band (I_D/I_G) reached up to 1.244 in the as-obtained graphite oxide. Moreover, sodium citrate not only increased the reducibility of binary reduction system but also improved the dispersion of graphite oxide. Furthermore, reasonable annealing treatment could reduce the defects of RGO. It is a potential route to prepare RGO films and increase practical applications in large-scale production of graphene.     

Study of space charge in gallium nitride by the thermal step method

In this paper, we report the electric investigation of thin nitride gallium films by the capacitance voltage technique and the thermal step method (TSM). The C–V analysis at 1 MHz of Au/GaN diode reveals MOS behaviour and shows strong capacitance hysteresis.

This may be due to the presence of trapped charge in this structure. The space charge dynamics is studied by thermal step method at different applied voltages. The TS currents are reverted from negative ones to positive ones above inversion threshold of +0.2 V. This change corresponds to charge modulation from accumulation to the inversion one, in good agreement with the C–V characteristics. The stored charge in this sample is related to the nature of gallium nitride and to the manufacturing processes. The results confirm the possibility to apply the TSM for the measurement of the space charge in the semiconductor materials.     

A finite element method for non-Fourier heat conduction in strong thermal shock environments

Non-Fourier effect is important in heat conduction in strong thermal environments. Currently, generally-purposed commercial finite element code for non-Fourier heat conduction is not available. In this paper, we develop a finite element code based on a hyperbolic heat conduction equation, which includes the non-Fourier effect in heat conduction. The finite element space discretization is used to obtain a system of differential equations for the time. The transient responses are obtained by solving the system of differential equations, based on the finite difference, mode superposition, or exact time integral. The code is validated by comparing the numerical results with exact solutions for some special cases. The stability analysis is conducted and it shows that the finite difference scheme is an ideal method for the transient solution of the temperature field. It is found that with mesh refining (decreasing mesh size) and/or high-order elements, the oscillation in the vicinity of sharp change vanishes, and can be essentially suppressed by the finite difference scheme. A relationship between the time step and the space length of the element was identified to ensure that numerical oscillation vanishes.     

柠檬酸镍制备金属镍纤维热分解过程研究

以柠檬酸和碱式碳酸镍为原料,采用溶胶-凝胶法制备柠檬酸镍前驱体纤维,在Ar气气氛下热分解得到金属镍纤维。通过FE-SEM/EDS、DSC-TG、XRD、FT-IR和XPS对金属镍纤维的形貌、前驱体的热分解过程及纤维的物相组成进行表征。结果表明柠檬酸镍前驱体纤维在300℃热处理时出现金属Ni,且热分解过程中没有氧化镍出现。故金属镍纤维是由前驱体直接热分解形成,不经过氧化镍热还原过程。     

Measurement of the thermal diffusivity by the thermal wave method using low-power heat sources

The errors in measuring the thermal diffusivity by the plane thermal wave method are considered as a function of the thermal flux power density. The minimum values of the thermal flux power density required for measurements with a specified error and the optimum parameters of the samples and of the heat source are determined. __________ Translated from Izmeritel’naya Tekhnika, No. 8, pp. 44–46, August, 2007.     

Analytical study for the estimation of thermal properties of processed meat based on hyperbolic heat conduction model

This study proposes an analytical method in conjunction with existing experimental temperature to estimate the unknown relaxation time and thermal diffusivity of processed meat based on the hyperbolic heat conduction model. This analytical method is a combination of the Laplace transform and least squares methods. The thermal contact resistance at the interface between adjacent samples at different temperatures is assumed to be negligible. The relaxation time is estimated from the temperature jump at a specific measurement location. The thermal diffusivity is determined from the definition of the dimensionless spatial coordinate and the resulting relaxation time. The results show that the relaxation time and thermal diffusivity obtained are in good agreement with the existing results. The obtained dimensionless temperature history at a specific measurement location is close to the experimental temperature data. This means that the Cattaneo–Vernottee (CV) model can be suitable for this study. The proposed analytical inverse method can be applied to determine a more accurate estimate of such problems. A comparison of the estimate obtained from CV and dual phase lag models is made.     

柠檬酸钠调控水热合成羟基磷灰石微球

本研究采用四水硝酸钙(Ca(NO₃)₂·4H₂O)和磷酸氢二铵((NH₄)₂HPO₄)分别作为钙源和磷源, 以丙酰胺为pH调节剂调控溶液的过饱和度, 以柠檬酸钠为钙源缓释剂调控羟基磷灰石(Hydroxyapatite, HA)的形貌, 经水热法处理成功制备出高结晶度、形貌均一和分散性良好的HA微球。采用X射线衍射(XRD)、傅里叶变换红外光谱(FT-IR)和扫描电镜(SEM)对反应产物进行了表征, 研究了初始pH值、柠檬酸钠加入量和水热反应温度对HA结晶度、组成及形貌的影响。研究结果表明, 当初始pH值为3, 柠檬酸钠与钙源的摩尔比为1︰1.5, 反应温度为180℃时, 有利于HA微球的形成。     

Use of the Laplace transformation for data reduction in the flash method of measuring thermal diffusivity

This paper presents a new way to reduce data in the laser flash method of measuring thermal diffusivity. Experimental temperature vs time data are first transformed by using the Laplace transformation, and then they are fitted with an appropriate theoretical formula. The data reduction procedure is more efficient and enables the use of more realistic models of heat conduction in the sample, because the theoretical formulae for transformed temperatures have a simpler form than those for nontransformed ones. Some examples of the theoretical formulae of transformed temperatures are included here for one- and two-dimensional heat transfer, respectively. The models described take into account a finite pulse time and heat losses from the sample. Two fitting algorithms are proposed. Experimentally, the data reduction procedure has been tested for a correction of the finite pulse time effect in the flash method. The results show that the accuracy of our procedure is comparable with other data reduction methods. Provided that the shape and duration of the pulse are known, this procedure allows elimination of the finite pulse time effect on calculation of the thermal diffusivity for any transformable heat pulse time function, even in cases where the other specialized data reduction procedures have failed.     

识别薄板中兰姆波和体波的阶梯板方法

斜探头在某些频率下激励出的兰姆波,其群速度与体波的传播速度相近,所以通过判断传播速度不易区分出兰姆波和体波。通过数值模拟和实验,分别研究了激励频率为2 MHz的纵波和S0模态兰姆波在阶梯板上的反射特性,发现:在阶梯板上入射S0模态兰姆波时,有反射回波;而入射纵波时,无反射回波。基于这种反射特性的差别,提出了一种利用阶梯板区别薄板中兰姆波和体波的方法,该方法可用于确认探头的激励特性。     

热针法瞬态测量热物性研究

该文从一维圆柱面传热模型的定解问题出发,分析热线法测量材料热导率的局限性,推证热针法瞬态测量材料热物性的基本原理、热扩散系数和热导率的计算公式,设计和开发针式热物性瞬态测量的软硬件,试制SHT-C针式热物性瞬态测量仪,实现对材料热物性的瞬态测量,并实测证明该测量仪有很好的重复性和实用价值.