Adjustable thermal expansion in La(Fe,Si)_(13)-based conductive composites by high-pressure synthesis

Providing a suitable contact interface, where a high conductivity material with a desirable coefficient of thermal expansion(CTE) adjoins the target micro-electric devices, is very crucial to optimize the properties and service life of the relevant instruments. Regrettably, a high conductivity, low thermal expansion and relatively inexpensive material is very rare. Composites, fortunately, can offer a method to design materials with adjustable properties by mixing two or more diverse constituents. In this paper, high conductivity composites with adjustable thermal expansion were successfully prepared by a high-pressure synthesis. The composites are based on combining La(Fe,Si)_(13)-based compounds, the materials showing a giant, isotropic negative thermal expansion(NTE) properties, within Cu matrix. The La(Fe,Si)_(13)-based compounds were used to adjust the CTE of the composites, while the Cu phase is in charge of tuning the thermal conductivity properties. Thus, by changing the relative amount of the two components, the composites with high conductivities and adjustable CTE were achieved. Furthermore, the thermal expansion and magnetic properties of the composites were investigated by a physical property measurement system. The present results highlight the potential applications of the Cu-based high conductivity composites with room-temperature NTE properties in the thermal contacts to various semiconductor and microelectronic devices.     

纤维素纳米晶对胶凝复合材料耐酸性能的影响

Acid mine drainage presents an important threat to cementitious structures. This study is aimed at investigating the effect of cellulose nanocrystals (CNCs) on the acid resistance of cementitious composites. CNCs were added to mortar mixtures as additives at cement volume ratios of 0.2vol%, 0.4vol%, 1.0vol%, and 1.5vol%. After 28 d of standard curing, the samples were immersed in a sulfuric acid with a pH value of 2 for 75 d. The unconfined compressive strength (UCS) test, the density, water absorption, void volume test, and thermogravimetric analysis were carried out to investigate the properties of CNC mixtures before sulfuric acid immersion. It was found that the addition of CNC reduced the volume of permeable voids and increased the hydration degree and mechanical strength of the samples. Changes in mass and length were monitored during immersion to evaluate the acid resistance of mixtures. The mixture with 0.4vol% CNC showed a reduced mass change and length change indicating its improved acid resistance.     

氧化铁/氧化锌超疏水复合材料的制备及其光催化性能

A facile approach was developed to construct Fe₂O₃-modified ZnO micro/nanostructures with excellent superhydrophobicity and photocatalytic activities. The effects of stearic acid (SA) and Fe₂O₃ on the morphological characteristics, water contact angle (WCA), and photocatalytic degradation were investigated. Superhydrophobicity results showed that WCA increased from 144° ± 2° to 154° ± 2° when the weight of SA increased from 5 to 20 mg because of the formation of a hierarchical or rough structure. Furthermore, Fe₂O₃-modified ZnO micro/nanostructure surfaces before and after SA treatment (20 mg) were chosen to evaluate the photodegradation of methylene blue (MB) dye under the support of visible light. MB degraded after 80 min of irradiation, and its photodegradation efficiencies were 91.5% at the superhydrophobic state and 92% at the hydrophilic state. This improvement in photocatalytic activity at both states might be attributed to an increase in surface area and improvement in charge carrier separation.     

SiCf/Ti2AlNb复合材料的界面反应及热稳定性

界面反应层是影响SiC纤维(SiC_f)增强钛基复合材料力学性能的重要因素,本文研究了SiC_f/Ti₂AlNb复合材料在热等静压成型以及热暴露过程中的界面反应、界面元素分布规律和界面热稳定性.研究结果表明:SiC_f/Ti₂AlNb复合材料内部元素扩散形成的界面产物主要为TiC,在热暴露过程中出现了TiSi₂和NbSi₂相.SiC_f/Ti₂AlNb复合材料界面反应层的厚度长大符合Arrhenius定律,其界面反应层厚度长大速率随着热暴露温度的升高而增加.界面反应层长大激活能为24.27kJ/mol,界面层长大频率因子为2.80×10^-4 m/s^1/2.SiC_f/Ti₂AlNb复合材料界面在700℃及以下温度具备良好的热稳定性.     

工程水泥基复合材料高温损伤超声特性

为了研究工程水泥基复合材料(ECC)在高温作用后的损伤机理及超声特性,对不同温度(20,105,250,400,600和800℃)作用后的ECC试件进行超声波和抗压强度测试.结果表明:超声波通过高温作用后ECC的波形幅值、主频幅值、能量和ECC抗压强度的变化趋势相同;不同温度作用后40~50kHz频段的能量占比均最大,约为0~300kHz总能量的15%;在20~250℃作用后声速变化不大,温度高于400℃后声速随温度升高而降低.原因是高温后自由水汽化、水化产物分解和骨料物性改变,导致孔隙蒸汽压力升高和骨料界面损伤,ECC内部裂纹产生并扩展.扫描电镜测试结果表明,PVA纤维随温度升高发生软化、熔化和汽化,ECC基体中产生空隙和孔道并与裂纹连通形成网络,有利于释放孔隙蒸汽压力,减弱ECC高温损伤.研究表明超声特性可以有效反映ECC的高温损伤演化过程.     

二维碳/碳复合材料氧化机理研究

研究了两种二维碳／碳复合材料的氧化行为和氧化机理，通过对其恒温氧化动力学曲线的测定和分析，得到氧化速率与温度、时间、氧的体积流量等的对应关系。研究结果表明，两种碳／碳复合材料的氧化动力学过程基本相同，其氧化过程均分为两个阶段：氧化失重率小于约60％时为线性氧化阶段，大于60％时为对数型氧化阶段。不同阶段的氧化速率不同，活化能也有差别。线性氧化阶段的氧化机理从低温时以化学反应为控制步骤转变为高温时以化学反应及扩散共同为控制步骤。     

碳纳米管复合材料的有效热导率

考虑到碳纳米管的空间取向分布特征,建立了碳纳米管复合材料的有效热导率模型。根据Maxwell理论,推导出了计算碳纳米管复合材料有效热导率的简单公式。应用实例表明,该模型能够很好地解释在碳纳米管纳米流体中观察到的热导率异常增加现象,还可以用来分析纳米管复合材料的渗流性质,而且理论计算结果与实验结果吻合较好。     

复合材料桥联问题的近似解析分析

纤维增强复合材料发生损伤时出现的桥联问题，最终可归结于对一个积分方程的求解，且通常该积分方程只能采用数值解法．在分析数值解的基础上进行了解析分析，并给出了数个较有价值的近似解析结果．     

TGS—PVDF复合材料的试制与性能研究

试制了 TGS—PVDF 复合材料,发现该材料具有良好的热电性与柔软性,适于33℃以下使用。33℃以上材料的稳定性变坏。极化机构是 TGS、PVDF 及相界三部分的贡献。     

大掺量冶金渣制备高强度人工鱼礁混凝土的试验研究

通过正交试验研究了用作制备高强度人工鱼礁的钢渣--矿渣--熟料--石膏体系胶凝材料的强度.净浆正交试验表明:钢渣∶矿渣的复合比为5∶3,并与10%的水泥熟料和10%的脱硫石膏复合的胶凝材料具有较高的强度.以优化后的胶凝材料代替水泥,并以热闷法稳定化的钢渣颗粒为骨料,可以制备出抗压强度达到50 MPa以上的人工鱼礁混凝土.利用X射线衍射和扫描电镜分析净浆的水化过程,发现体系在早期水化主要生成AFt相和C--S--H凝胶,在后期钢渣和矿渣的火山灰活性反应对强度的增长起主要作用.