Effects of polyethyleneimine adsorption on rheology of bentonite suspensions

The influence of the cationic polymer, polyethyleneimine polymer (PEI) on the flow behaviour of bentonite suspensions (2%, w/w), was studied. XRD, zeta potential and adsorption studies were done together with rheological measurements. The addition of PEI at concentration ranges of 10^-5-4.5 g/l and their rheological properties and stability of bentonite suspensions were studied. The adsorption rates for the bentonite suspensions are very fast. The XRD results showed that the PEG molecules did not intercalate into the layers of the clay.     

Effect of sodium dodecyl sulfate on flow and electrokinetic properties of Na-activated bentonite dispersions

The present study reports the effect of anionic surfactant sodium dodecyl sulfate (SDS, C₁₂H₂₅ OSO₃Na) upon the electrokinetic (electrophoretic mobility, zeta potential) and rheological (viscosity, yield value) properties of the Ca-bentonitic clay found in Turkey and its Na- activated form. The SDS dispersant was added in different concentrations in the range of 1 × 10⁻⁵−5 × 10⁻² mol/l. The results show that the viscosity and zeta potential values of bentonite dispersion are affected by the addition of anionic surfactant. The obtained data are analysed by considering the kind of exchangeable cations. Thixotropic property effect was observed in bentonite dispersions.     

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.     

温度对高压烧结氮化铝陶瓷热导率的影响

以直接氮化法合成的AlN微米粉为原料,添加3%(质量分数)的CaC2为烧结助剂,在5GPa的压力下烧结30min,考察不同烧结温度对AlN陶瓷热导率的影响。用阿基米德排水法、XRD、SEM等技术手段对AlN烧结体进行性能检测。研究表明,在1500～1800℃范围内,温度的升高能促使AlN陶瓷内部晶粒长大,晶型饱满,尺寸均一,晶界相减少,实现烧结致密化,利于热导率的提高。     

电场处理对碳化硅/聚合物复合材料电导特性的影响

通过在碳化硅(SiC)/低密度聚乙烯(LDPE)的热压成型和SiC/硅橡胶的硫化过程中施加不同形式的直流电场,研究了电场处理对SiC/聚合物复合非线性绝缘材料电导特性的影响.研究结果表明,在SiC/LDPE的热成型过程中待到模具中的物料流动结束后施加均匀电场仅使复合材料的电导率发生微弱变化,而在SiC/LDPE的热压流动过程中施加均匀电场导致复合材料的电导率明显增加;SiC/硅橡胶共混物的粘度较SiC/LDPE的粘度低,故均匀电场处理导致前者电导率增加的趋势较后者明显;非均匀电场处理导致复合材料的电导率明显增大,电导非线性特性明显得到改善,其作用效果明显好于均匀电场.     

The effect of spatial inhomogeneity in thermal conductivity on the formation of hot-spots

The steady-state microwave heating of a unit slab consisting of three layers of materials with different thermal conductivities is examined. The governing equations are a damped wave equation derived from Maxwell's equations and a heat-force equation for the temperature. As the primary concern is to investigate the dependence of the steady-state on the thermal-conductivity parameter, a simplifying assumption is made, namely that the electrical conductivity is temperature-independent. Under this assumption, the damped wave equation governing the electric field may be solved separately. An eigenfunction expansion for the problem based on the Galerkin method is described and a fundamental-mode approximation is presented. If this approximation is applied to a unit slab composed of three layers with different thermal conductivities, the hot-spot formation can be addressed and a global steady-state solution is found for the whole domain. Numerical results for some different cases of the three-layer combinations are interpreted to gain some insight in parameter dependence and the position of the low-thermal-conductivity inner layer related to hot-spot formation.     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

氮掺杂和空位对石墨烯纳米带热导率影响的分子动力学模拟

石墨烯是近年纳米材料研究领域的一个热点,其独特的热学性质受到了广泛关注,为了实现对石墨烯传热特性的预期与可控,利用氮掺杂和空位缺陷对石墨烯进行改性.采用非平衡态分子动力学方法研究了扶手形石墨烯纳米带中氮掺杂浓度、位置及空位缺陷对热导率影响并从理论上分析了热导率变化原因.研究表明氮掺杂后石墨烯纳米带热导率急剧下降,氮浓度达到30%时,热导率下降了75.8%;氮掺杂位置从冷浴向热浴移动过程中,热导率先近似的呈线性下降后上升;同时发现单原子三角形氮掺杂结构比多原子平行氮掺杂结构对热传递抑制作用强;空位缺陷的存在降低了石墨烯纳米带热导率,空位缺陷位置从冷浴向热浴移动过程中,热导率先下降后上升,空位缺陷距离冷浴边缘长度相对于整个石墨烯纳米带长度的3/10时,热导率达到最小.石墨烯纳米带热导率降低的原因主要源于结构中声子平均自由程和声子移动速度随着氮掺杂浓度、位置及空位缺陷位置的改变发生了明显变化.这些结果有利于纳米尺度下对石墨烯传热过程进行调控及为新材料的合成应用提供了理论支持.     

热处理温度对二维高导热炭/炭复合材料结构及热导率的影响（英文）

以高导热沥青基炭纤维布为增强体,中间相沥青为黏结剂,采用热模压成型及液相浸渍裂解工艺增密,并经高温石墨化处理制备二维高导热炭/炭复合材料。利用X射线衍射仪和透射电子显微镜对经不同温度处理后的沥青基炭纤维及二维高导热炭/炭复合材料的结构和形貌变化进行表征,并考察石墨化处理温度对复合材料热导率的影响。结果表明,随着热处理温度的升高,纤维及复合材料内部石墨微晶尺寸增大、取向度变好,纤维与基体间界面结合紧密、裂纹减少,而基体碳层间裂纹则呈扩大趋势。此外,二维高导热炭/炭复合材料的热导率随热处理温度的升高而线性增加,经3 000℃处理后,材料热导率高达443 W/m·K。     

超声振动对石墨烯微片/聚丙烯复合材料导电导热性能的影响机制

在挤出过程加入超声振动作用,研究超声振动对高石墨烯微片(GNP)含量的聚丙烯基(GNP/PP)复合材料微观形态、结晶、导电性和导热性的影响。结果表明:由于超声振动提供强烈的冲击波与微射流,挤出过程中加入超声振动可有效地减薄GNP片层厚度,减少GNP团聚,增强GNP在PP中的分散均匀性,有利于构建导电导热网络,从而提高GNP/PP纳米复合材料的导电导热性能。相较于无超声振动,加入100 W超声振动后,GNP含量越高,GNP/PP电导率和热导率提升幅度越大,在GNP含量为15wt%时,电导率升幅为85%,热导率升幅为9.7%。而在GNP含量同为12wt%时,随着超声振动功率的增加,电导率和热导率呈现先增大后减少的规律。当超声功率为200 W时,电导率升幅为214%,热导率升幅为17.2%。而超声功率达到300 W时,较高功率的超声振动使部分石墨烯微片的片径减少,导致片层间更难以搭建完整的导电导热网络,使GNP/PP性能均略有下降。     

钡含量对新型石榴石结构固态电解质导电性能的影响

用传统固相烧结法制备试样并研究钡含量对电导率的影响。综合运用XRD、SEM、EIS和密度测试对试样进行表征。XRD衍射分析表明,合成物质基本为石榴石结构,当钡含量过多时,出现二次相。立方晶格常数随着钡含量的增大而增大,当X>1时,变化无规律。测试了试样在20~250℃范围内的交流阻抗,实验结果表明,电导率随着X的增大而增大,并在X=1时达到最大值(8.77×10-6S/cm,20℃),继续增大X电导率反而降低。活化能随着X的增大而减小,并在X=1时达到最小值(0.41eV),X继续增大活化能反而增大。Li6BaLa2Ta2O12的SEM照片中烧结粉末颗粒不均匀,有团聚现象,相对密度为80.7%。     

Influence of graphene oxide with different degrees of oxidation on the conductivity of graphene/poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) composites

A modified Hummers method was used to synthesize graphene oxide (GO) with different degrees of oxidation. Single-factor and orthogonal experiments were designed to analyze the influence of the oxidant dose on the properties of the GO. The GO series were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, elemental analysis, and zeta potential measurements. We synthesized RGO/PEDOT:PSS composites by an in situ polymerization with the GO. Through analysis of the specific conductance of the composites, we determined the optimum conditions for forming RGO/PEDOT:PSS which can improve the specific conductance to 114.06 S/cm. Thus, by changing the oxidation degree of GO we were able to improve the conductivity of RGO/PEDOT:PSS composites.     

The effect of magnetic field on the thermal conductivity and electrical resistivity of different materials

In impregnated superconducting coils it is assumed that copper heat sinks extract the heat produced by ac losses.For this type of cooling we study the effect of magnetic field on the Wiedemann—Franz law for copper of different purities. We also look at the effect of magnetic field on the thermal conductivity tensor of resin with 2% silica gel and of a composite interlayer of an impregnated coil.     

Thermal conductivity of high-Tc crystals: Dependence on magnitude and orientation of magnetic field, and effect of Zn doping

We present precise measurements of in-plane thermal conductivity for superconducting single crystals of YBa₂Cu₃O_7–x (YBCO) withT _c =92 and 60 K, Bi₂Sr₂CaCu₂O₈ (BSCCO), and of Zn-doped YBCO. Magnetization and thermal conductivity data obtained with the same 90-K YBCO crystal demonstrate a close relationship between the magnetic thermal resistivity and the internal magnetic fieldB in a superconductor in the mixed state. For all superconductors studied here, the magnetic thermal resistivity is a sublinear function of magnetic field. The origins of the nonlinearity are discussed.Angular dependences of the magnetic thermal resistivity have been shown to depart from the anisotropic 3D superconductor model and are in quantitative agreement with a quasi-2D model. Implications for spatial modulation of the order parameter are made.     

超声与静电场协同作用对水电导率的影响

超声和静电场的协同作用对水的电导率产生明显的影响。首先通过单独的超声和静电场作用,研究了其对水电导率的影响,并通过改变超声和静电场的强度,发现在一定范围内,随着超声功率的增大,水电导率随时间逐渐提高,而单独的静电场对水的电导率没有明显影响。最后,通过超声和电场的协同作用,发现水的电导率相对其单独作用有了明显的提高,并且,随着超声电功率和静电场强度的增大,水的电导率提高明显。     

Effect of carbon nanofibers on the infiltration and thermal conductivity of carbon/carbon composites

Preforms containing 0, 5, 10, 15 and 20 wt.% carbon nanofibers (CNFs) were fabricated by spreading layers of carbon cloth, and infiltrated using the electrified preform heating chemical vapor infiltration method (ECVI) under atmospheric pressure. Initial thermal gradients were determined. Resistivity and density evolutions with infiltration time have been recorded. Scanning electron microscopy, polarized light micrograph and X-ray diffraction technique were used to analyze the experiment results. The results showed that the infiltration rate increased with the rising of CNF content, and after 120 h of infiltration, the density was the highest when the CNF content was 5 wt.%, but the composite could not be densified efficiently as the CNF content ranged from 10 wt.% to 20 wt.%. CNF-reinforced C/C composites have enhanced thermal conductivity, the values at 5 wt.% were increased by nearly 5.5-24.1% in the X-Y direction and 153.8-251.3% in the Z direction compared to those with no CNFs. When the additive content was increased to 20 wt.%, due to the holes and cavities in the CNF web and between carbon cloth and matrix, the thermal conductivities in the X-Y and Z directions decreased from their maximum values at 5 wt.%.     

碳纳米管的表面修饰及其对碳纳米管/氟橡胶复合材料导电性能的影响

分别采用混酸和四氟化碳(CF4 ) 等离子体处理技术对碳纳米管(MWCNTs) 进行了表面修饰, 将处理前后的碳纳米管进行了XPS 和SEM 测试, 获得了处理后前的表面形貌和结构, 并采用溶液浇注的方式制备了MWCNTs/氟橡胶(FE) 复合材料, 探讨了不同碳纳米管状态(未处理、混酸处理、CF4等离子体处理) 的导电性能, 结果表明两种表面处理方式可以使MWCNTs 表面接上极性官能团。而且在相同的碳纳米管添加量下(质量分数分别为0. 1 %、0. 5 %、1. 0 %、2. 0 %) , 酸处理MWCNTs/ FE 的渗流阈值最小, 达0. 5 %。      

石墨表面TiC涂层对高定向石墨/Cu复合材料热导率和抗弯强度的影响

以高温盐浴法对天然鳞片石墨粉体(GF)进行表面TiC镀层处理,然后采用真空热压烧结法制备TiCGF/Cu复合材料,研究了粉体表面涂层和GF体积分数对复合材料微观结构、热导率及抗弯强度的影响。系列测试结果表明:随着GF体积分数的降低以及粉体表面TiC镀层的形成,TiC-GF/Cu复合材料平行于GF片层方向的热导率有所降低,抗弯强度有所提升。其中在GF的体积分数占TiC-GF/Cu复合材料70%时,这种变化最为明显,平行于GF片层方向的TiC-GF/Cu复合材料热导率下降幅度最大,从676W/(m·K)下降到526 W/(m·K)。同时,TiC-GF/Cu复合材料的微观结构进一步说明,GF表面的TiC涂层对GF/Cu复合材料的断裂模型起着重要的作用。     

Particle shape effect on the viscosity and thermal conductivity of ZnO nanofluids

The viscosity and thermal conductivity of ZnO nanofluids with nanoparticle shapes of nearly rectangular and of sphere, were experimentally investigated under various volume concentrations of the nanoparticles, ranging from 0.05 to 5.0 vol.%. The viscosity of the nanofluids increased with increases in the volume concentration by up to 69%. In addition, the enhancement of the viscosity of the nearly rectangular shape nanoparticles was found to be greater by 7.7%, than that of the spherical nanoparticles. The thermal conductivity of the ZnO nanofluids increased by up to 12% and 18% at 5.0 vol.% for the spherical and the nearly rectangular shape nanoparticles, respectively, compared to that of the base fluid (water). The shape of the particles is found to have a significant effect on the viscosity and thermal conductivity enhancements.     

The effect of anodic oxide coatings on the mechanical behavior of niobium and tantalum single crystals

The effect of anodic oxide films on the mechanical behavior of single crystals of niobium and tantalum was investigated. Near room temperature, oxide films cause an increase in strength, in the manner generally observed for face-centered cubic crystals. At lower temperatures, oxide films reduce the yield stress and its temperature dependence and cause serrated flow over an appreciable range of strains. When the oxide films are removed from specimens deformed at lower temperatures, the serrations disappear during subsequent deformation, and the yield stress increases rapidly to that of the uncoated material. A model involving generation and motion of edge dislocations from the oxide-metal interface is used to explain the results.