瞬态热线法导热系数测量的数值模拟

利用有限元方法对瞬态热线法导热系数测量进行了数值模拟,对各种因素如加热功率、热线半径以及实验温度等对测量过程的影响进行了分析,并将模拟得到的温升曲线与实验测量得到的温升曲线进行了比较,结果表明:通过选择适当的参数值,模拟曲线可以与实测曲线吻合得很好,实验值与模拟值的偏差小于实验结果的不确定度.本结果的获得对进一步理解瞬态热线法导热系数测量过程,提高导热系数测量技术水平具有借鉴意义.     

静电悬浮无容器方法测量热膨胀系数的研究

传统接触测量的方法无法对高温范围金属材料的热膨胀系数进行测量,结合静电悬浮无容器技术和图像测量方法,通过半导体激光器加热悬浮样品,在加热的过程中利用CCD采集悬浮样品的图像,采用方向梯度算法提取图像中样品的边缘,并利用球谐函数拟合样品的体积,计算得到材料在不同温度下的热膨胀系数。采用纯锆的金属材料样品进行实验,在750~1 800℃的温度范围内,测量得到固态样品锆的热膨胀系数随温度变化的表达式,验证该测量方法的有效性和准确度。     

纳米铜/石蜡复合相变蓄热材料的导热性能研究

采用HotDisk热分析仪测试了Cu/石蜡体系在不同纳米颗粒质量分数、温度和热循环次数下的导热系数。研究表明,Cu/石蜡体系的固、液态导热系数随纳米Cu颗粒含量的增加呈非线性增加;温度变化对相变材料导热系数的影响并不明显,但当温度升高至相变温度区间时,相变材料的导热系数急剧增加;复合材料在经历100次热循环后,材料的导热系数值仍较稳定。     

基于非稳态传热过程的真空玻璃性能建模

由于真空玻璃的导热系数测量复杂,本文提出一种基于非稳态传热过程的软测量表征真空玻璃导热系数的模型。首先,在连续介质与半无限大物体的前提下,采用能量守恒定律与拉普拉斯变换法,对数学物理模型进行求解,建立非加热面的中心温度计算公式。然后,利用数值仿真软件FLUENT对物理模型进行温度场仿真。仿真结果表明:检测时间少于5分钟,达到节约时间的目的;为保证温度测量准确,取多个测量点的测量值;为减小测量误差,采用圆形加热片时,加热片的半径与真空玻璃厚度之比要大于3。这些为后续研究真空玻璃导热系数与温度变化之间的关系奠定一定的理论和应用基础,对实验有指导意义,为真空玻璃导热系数的在线检测和其过程的优化控制提供指导。     

基于复合相变材料的梯级组合蓄热特性研究

针对潜热蓄热装置内部相变材料(PCM)导热系数偏小,蓄热速率过低的问题,对基于复合相变材料的两级串联式梯级蓄热装置的相变过程进行了数值研究。通过对不同热物性PCM工况的对比与分析,得到了装置在不同工况下的蓄热特性。结果表明:PCM存在最佳的热扩散系数使固定熔点的PCM实现"均匀等速相变"。同时,增大PCM的热扩散系数可以有效降低加热面温度,但随着热扩散系数的增大,加热面温度降低幅度减小。通过分析Stefan数,得到了装置最佳的参数,使工况蓄热效果最佳。最后通过Stefan数为2. 88时的实验工况验证了相关规律的正确性。     

一种新型复合相变材料导热性能的实验研究

针对相变材料在实际应用过程中交替存在升温液化和降温固化的复杂传热过程,采用JW-Ⅲ建筑材料热流计式导热仪,分别对升温和降温过程中处于固态、混合态、液态的新型复合相变材料导热性能进行了测试和分析。研究结果表明,复合相变材料在加热和冷却过程中的导热系数随温度的变化存在明显的规律性差异,导热系数在混合态时差值达到20%;升温过程中,复合相变材料在混合态和液态时的导热系数值相差不大,但与固态时相比有明显减小;降温过程中,在液-固相变的过程中导热系数随温度减小而增大,有利于加速相变材料的固化。     

脂肪酸相变材料导热系数测试及相变传热过程的数值模拟

相变材料在相变过程中由于材料本身状态的变化,其相变传热过程属于具有移动边界的非线性过程,针对相变材料相变传热过程中非线性传热特征,对月桂酸-癸酸混合脂肪酸相变材料的热性能进行了研究,利用差示扫描量热法(Differential Scanning Cal-orimetry)对相变材料的相变潜热和相变温度进行了测试,利用DRE-2C导热系数测定仪测试了不同摩尔比例脂肪酸相变材料以及相变材料在不同测试温度下的导热系数,通过将无机多孔材料硅藻土和脂肪酸相变材料混合制备了一种无机复合相变材料,并对其导热系数和蓄放热性能进行了测试,利用有限元法对相变材料的相变过程进行了数值模拟。研究表明,脂肪酸相变材料的导热系数和其相变温度呈反比关系,相变材料的相变温度越高,其导热系数越低。对于同一相变材料来说,相变材料的导热系数随着材料温度的升高而升高,硅藻土的掺入明显增加了相变材料的导热系数,复合相变材料蓄放热速率加快,改善了相变材料的传热性能。有限元模拟分析法可以较好地描述相变材料的传热过程,相变材料的导热性能需要强化以提高其蓄放热的速率和频率。     

固化吸附剂导热系数测量参数选择

在开发高导热吸附剂过程中经常需要测量材料的导热系数,稳态平板法测量导热系数因方法简单直接而应用广泛.通过数值模拟和实验发现侧面散热对导热系数测量影响很大,采用大空间自然对流换热实验关联式对试验数据处理公式进行修正,可得到较满意的实验结果,并进一步研究了热源温度、样品厚度、试样导热系数对侧面散热的影响.为减少侧面散热以及热电偶波动对测量结果的影响,并考虑到试样加工难易程度,就所用实验装置而言,试样厚度在12mm较为适宜,热源温度选用55℃较为适宜.     

探针法测量低温下食品导热系数研究

在分析了探针法测量原理的基础上,用探针法对低温下食品材料的导热系数进行了测量研究。实验表明探针在用甘油和蒸馏水进行标定后,可以方便准确地测量不同水分的土豆在25℃-50℃温度范围内的导热系数。     

电流热退火效应对碳纤维导热性能的影响

利用瞬态电热技术(TET)和电流热退火对聚丙烯腈基碳纤维(PAN-CF)进行原位测量,基于数值模拟方法分析实验现象。研究了退火温度对PAN-CF导热性能的影响,分析了电流热退火条件下PAN-CF的石墨化与导热性能之间的关系。在PAN-CF的退火区域,导热系数随退火温度的升高而增大,最高平均导热系数为16.27 W/(m·K),是原始平均导热系数的3.45倍。电流退火存在沿纤维轴向温度分布不均匀的现象,样品在大约中点位置燃断。燃断点的温度和导热系数最大,分别为3 867 K和96.74 W/(m·K)。其中,断点导热系数比样品的平均导热系数高出6.51倍。研究表明,经电流退火后PAN-CF的石墨化程度升高,C原子的杂化效应促使材料结构缺陷密度降低,最终导致PAN-CF的导热性能提高。     

Thermal conductivity and heat capacity per unit volume of poly(methyl methacrylate) under high pressure

The thermal conductivity and heat capacity per unit volume of poly(methyl methacrylate) (25 and 350 kg · mol^– in molecular weight) have been measured in the temperature range 155–358 K at pressures up to 2 GPa using the transient hot-wire method. The bulk modulus has been measured up to 1.0 GPa at 294 K and yielded a constant valueg = 3.4 ± 0.3 for the Bridgman parameter. No dependence on molecular weight could be detected in the properties we measured.     

Micro- and Nano-scale Measurement of the Thermophysical Properties of Polymeric Materials Using Atomic Force Microscopy

To realize the benefits and optimize the performance of micro- and nano-structured materials and thin films, designers need to understand and thus be able to characterize their thermal, thermophysical, and thermomechanical properties on appropriate length scales. This paper describes the determination of glass-transition temperatures of polymers on the micro-scale, obtained from contact force–distance curves for poly(methyl methacrylate) and poly(vinyl acetate) measured using an atomic force microscope (AFM). Measurements were made using a standard AFM tip where thin films were heated using a temperature controlled hot stage and by using a scanning thermal microscopy (SThM) probe. The latter was used either with the hot stage or with the SThM probe providing a localized heating source via Joule heating. Differences in the glass-transition temperature measured using the hot stage and Joule heating were apparent and considered to be due to heat transfer effects between the probe, specimen, and surroundings. Gradients of force–distance curves, pull-off and snap-in forces, and adhesion energy were obtained. The results suggest that the onset of changes in the material’s mechanical properties at the glass transition was found to be dependent on the mechanical property measured, with pull-off force values changing at lower temperatures than the snap-in force and adhesion energy.     

原位聚合法制备炭黑/聚甲基丙烯酸酯导电复合材料

通过原位聚合法制备了炭黑/聚甲基丙烯酸甲酯(CB/PMMA)、炭黑/聚甲基丙烯酸丁酯(CB/PB-MA)和炭黑/聚甲基丙烯酸2-乙基己酯(CB/PEHMA)三种导电复合材料,研究了这三种聚合物基体对复合材料的导电性影响。结果表明:由于炭黑的阻聚作用,当复合材料的炭黑含量增加时,均聚物的数均分子量呈下降趋势,多分散性系数PDI变大。而且炭黑粒子表面发生了接枝聚合反应,这有利于炭黑粒子在聚合物基体中的分散。当聚合物基体的玻璃化温度Tg较高时,复合材料逾渗阈值较低。     

Modeling of thermo-physical properties for FRP composites under elevated and high temperature

A decomposition model for resin in glass fiber-reinforced polymer composites (GFRP) under elevated and high temperature was derived from chemical kinetics. Kinetic parameters were determined by four different methods using thermal gravimetric data at different heating rates or only one heating rate. Temperature-dependent mass transfer was obtained based on the decomposition model of resin. Considering that FRP composites are constituted by two phases – undecomposed and decomposed material – temperature-dependent thermal conductivity was obtained based on a series model and the specific heat capacity was obtained based on the Einstein model and mixture approach. The content of each phase was directly obtained from the decomposition model and mass transfer model. The effects of endothermic decomposition of the resin on the specific heat capacity and the shielding effect of evolving voids in the resin on thermal conductivity are dependent on the rate of decomposition. They were also described by the decomposition model; the effective specific heat capacity and thermal conductivity models were subsequently obtained. Each model was compared with experimental data or previous models, and good agreements were found.     

Conductivity studies on poly(methyl methacrylate)–Li2SO4 polymer electrolyte systems

Thin-film ion-conducting polymer electrolytes based on poly(methyl methacrylate) (PMMA) complexed with Li₂SO₄ have been prepared using the solution casting technique. The complexation of salt with PMMA was confirmed by structural and thermal studies. The results of the electrical conductivity measurements in these electrolytes are reported. A Vogel–Tammann–Fulcher-like temperature dependence of ionic conductivity implies coupling of the ion movement with the segmental motion of the polymer chains in the cross-linked environment. Predominant formation of contact ion pairs of Li₂SO₄ has been consistently observed through conductivity, X-ray diffraction, Fourier transformation and thermal analysis studies.     

Improved thermal properties of graphene oxide-incorporated poly(methyl methacrylate) microspheres

Graphene, a single layer of carbon atoms in a two-dimensional lattice, has attracted considerable attention owing to its unique physical, chemical and mechanical properties. In particular, because of its excellent thermal properties such as high thermal conductivity and good thermal stability, graphene has been regarded as a one of the promising candidates for the reinforcing fillers on the polymer composites field. In this study, we prepared the poly(methyl methacrylate) (PMMA)/graphene oxide (GO) nanocomposite by a simple solution mixing process, and examined the thermal reinforcing effects of GO on a PMMA matrix. Using thermogravimetric analysis, differential scanning calorimeter, and thermal conductivity meter, we investigated the effects of GO on the thermal properties of PMMA/GO nanocomposites. With 3 wt% of GO loading, the glass transition temperature (Tg) of the PMMA/GO nanocomposite were increased by more than 7 degrees C and the thermal conductivity of which also improved 1.8 times compared to pure PMMA.     

Real-time shape evolution of nanoimprinted polymer structures during thermal annealing

The real-time shape evolution of nanoimprinted polymer patterns is measured as a function of annealing time and temperature using critical dimension small-angle X-ray scattering (CD-SAXS). Periodicity, line width, line height, and sidewall angle are reported with nanometer resolution for parallel line/space patterns in poly(methyl methacrylate) (PMMA) both below and above the bulk glass transition temperature (T(G)). Heating these patterns below T(G) does not produce significant thermal expansion, at least to within the resolution of the measurement. However, above T(G) the fast rate of loss in pattern size at early times transitions to a reduced rate in longer time regimes. The time-dependent rate of polymer flow from the pattern into the underlying layer, termed pattern "melting", is consistent with a model of elastic recovery from stresses induced by the molding process.     

Effect of C<Subscript>60</Subscript> fullerene additives on the thermal conductivity of poly(methyl methacrylate) films

The thermal conductivity of a composite comprising C₆₀ fullerene particles dispersed in a poly(methyl methacrylate) (PMMA) matrix exhibits a significant decrease, even at small concentrations of the fullerene. It is concluded that the main factor that determines this behavior is a decrease in the mean free-path length of phonons, which is caused by increasing inhomogeneity of the material and growing number of the scattering centers.     

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.