The influence of pore size distribution on thermal conductivity,permeability,and phase change behavior of hierarchical porous materials

Porous foams have been widely employed to improve heat storage rate and prevent leakage of phase change materials (PCMs).Actual porous foams have non-uniform or...     

多孔陶瓷孔径及其分布测定方法研究进展

根据发泡陶瓷形成的过程和结构特点,确定以扫描电子显微镜（scanning electron microscope,SEM）分析结构细节选取X射线断层扫描（X-ray computed tomography,X-CT）的体素,以泡孔形成及特征选取灰度阈值的方法.通过断层扫描图像、X-CT三维重构及可视化分析得到：发泡陶瓷由大孔和处于孔壁的小孔构成;发泡陶瓷泡孔基本处于连通状态,连通是由处于孔壁的小孔引起的;最大连通孔孔径不超过60 μm,最小连通孔径在14 μm左右;发泡陶瓷大孔的孔径处于0.5~1.0 mm,最大泡孔孔径值不超过2 mm,孔隙率为85.97%;孔壁厚度集中在20~60 μm.研究表明,X-CT在研究发泡陶瓷泡孔空间结构关系（即连通性）和泡沫材料大尺寸结构统计信息（孔径分布、壁厚分布等）方面具有明显优势.

     

Porosity and pore size distribution measurement of cement/carbon nanofiber composites by 1H low field nuclear magnetic resonance

The dispersion effect of carbon nanofibers （CNFs） in aqueous solution and the mechanical properties, porosity, pore size distribution and microstructure of CNFs reinforced cement-based composites were investigated in this paper. To achieve effective dispersion of CNFs, a method utilizing ultrasonic processing and a commercially surfactant were employed. CNFs were incorporated to cementitious materials with the addition of 0.1 wt% and 0.2 wt% of cement with a water/cement ratio of 0.35. The mechanical properties of CNFs/ cement composites were analyzed, the porosity and pore size distribution were characterized by ^1H low field nuclear magnetic resonance （NMR）, and the microstructure was observed by scanning electron microscopy （SEM）. The results indicate that the optimum concentration ratio of MC to CNFs is 2：1 for dispersing in aqueous solution. Moreover, in the field of mechanical properties, CNFs can improve the flexural strength and compressive strength. The increased mechanical properties and the decreased porosity of the matrices correspond to the increasing CNFs content and CNFs act as bridges and networks across cracks and voids.     

Filtration of fine particles in atmospheric aerosol with electrospinning nanofibers and its size distribution

In this study,the size distribution of atmospheric aerosol in Beijing was monitored by the scanning mobility particle sizer spectrometer and the optical particle sizer.The size of particles in atmospheric aerosol was primarily distributed in the range of less than 1 m.It showed different changes with the mass concentrations of particulate matters with an aerodynamic diameter of 2.5 m(PM2.5)for different sizes of fine particles.The amount of ultrafine particles(less than about 60 nm)decreased while the larger ones(60 nm)increased along with the mass concentration of PM2.5 in atmospheric aerosol.This was because of the formation of the secondary atmospheric aerosol.The polylactic acid(PLA)nanofibers were prepared for filtering the aerosol by electrospinning.PLA nanofiber mats were used as the middle layer to design the composite filter membranes.Atmospheric aerosol was used as dust source in the filtration test.The results showed that the filtration efficiency of the composite filter media increased along with the thickness of nanofiber mats,which was controlled by the collection time during electrospinning.Filtration efficiency can be improved obviously by compositing with a thin layer of nanofibers.     

砖的孔型及孔洞分布对砌体通缝抗剪强度的影响

对５种不同孔型及孔洞分布但孔洞率大体相同的烧结页岩多孔砖分成５组,采用统一强度等级的砂浆砌筑３０件砌体进行通缝抗剪试验,结果显示：烧结页岩多孔砖砌体通缝抗剪破坏具有脆性特征,其破坏形式多样;圆孔烧结页岩多孔砖砌体通缝抗剪强度高于矩形孔烧结页岩多孔砖砌体;由于砂浆键在砌体抗剪过程中发挥作用,使得实测平均值远大于规范平均值,也大于建议计算公式的平均值。试验表明该５种类型的烧结页岩多孔砖砌体抗剪强度满足安全要求,也反映出砖孔型及孔洞分布对砌体通缝抗剪强度有着重要的影响。     

Compressive Strength, Pore Size Distribution and Chloride-ion Penetration of Recycled Aggregate Concrete Incorporating Class-F Fly Ash

The effects of fly ash on the compressive strength, pore size distribution and chloride-ion penetration of recycled aggregate concrete were investigated. Two series of concrete mb：tures were prepared. The concrete mixtures in series I had a water-to-binder ratio and a cement content of 0.55 and 410 kg/ m^3 , respectively. The concrete rnixtures in series II had a water-to-binder ratio and a cement content of 0.45 and 400 kg/ ml respectively. Recycled aggregate was ased as 20% , 50% , and 100% replacements of natural coarse aggregate in the concrete mixtures in both series. In cutdition, fly ash was used as 0% , 25% and 35% by weight replacements of cement. The results show that the compressive strengths of the concrete decreased as the recycled aggregate and the fly ash contents increased. The total porosity and average porosity diameter of the concrete increased us the recycled aggregate content increased. Furtherrruore , an increase in the recycled aggregate content decreased the resistance to chloride ion penetration. Nevertheless, the replacement of cement by 25% fly ush improved the resistance to chloride ion penetration and pore diameters and reduced the total porosity of the recycled aggregate concrete.     

海相软土孔隙分布的分形特征及应用

利用压汞试验对琼州海峡浅海区软土的孔隙尺寸分布分形特征进行分析,发现原状土具有2个分形的无标度区,根据这一特点确定了团粒内孔隙和团粒间孔隙之间的分界尺寸为0.2μm.通过对固结过程孔隙分布的变化规律的研究,发现土体结构性的损伤与一定的团粒间孔径尺寸相联系,通过分形理论可以确定每级压力下的影响孔隙尺寸,得出琼州海峡软土宏观结构强度丧失时对应的孔径特征值为4μm.     

An effective thermal conductivity model for unsaturated compacted bentonites with consideration of bimodal shape of pore size distribution

An effective thermal conductivity model was proposed for unsaturated compacted bentonites with consideration of the bimodal shape of pore size distribution curves.The pores of soils were grouped into two dominant pore size modes corresponding to the intra-and inter-particle pores,and were simulated with randomly distributed spheroidal inclusions of different aspect ratios.With the assumption of preferential invasion of the wetting fluid(water)into pores of smaller sizes and by virtue of the analytical solution to the inhomogeneous inclusion problem in heat conduction,the model was developed using the Mori-Tanaka(MT),Ponte Casta?eda-Willis(PCW)and self-consistent(SC)homogenization approaches for different considerations of the interactions between pores and the solid phase.The proposed model is functions of the thermal conductivities of the solid,liquid and gas phases,porosity,the degree of saturation,the aspect ratios of pores and/or soil particles,and the fraction of the smaller group of pores.The proposed model was validated against five sets of laboratory measurement data on the MX-80,FEBEX,Kunigel-V1 and GMZ01 bentonites,showing a good agreement between the model predictions and the laboratory measurements.The responses of the model with respect to the geometries of pores and solid particles were examined.Compared to series-parallel structural models,the proposed model may overall exhibit better performance if proper homogenization schemes are adopted,but as an advantage,the model has clearer physical mechanisms and a smaller number of parameters.     

Effect of pore size and porosity distribution on radiation absorption and thermal performance of porous solar energy absorber

In this paper, experimental and numerical study were both conducted to investigate the effect of pore size and porosity distribution on radiation absorption and thermal performance of porous solar energy absorber. Ultraviolet-visible-near infrared(UV-Vis-NIR) spectrophotometer was used to measure the transmittance of porous media to reflect its radiation absorption capabilities. Numerical model was established based on the assumption of thermal nonequilibrium condition as well as using P1 model to consider the radiation heat transfer. The UV-Vis-NIR spectrophotometer measurement showed that:(1) With smaller pore size, the spectral transmittance of the porous media would be lower and the solar radiation absorption would be better;(2)Among the materials with different pore size distributions, pore-size-decreased combo and pore-size-increased combo have almost equal absorption coefficient which are higher than that of uniform structure. Numerical simulation demonstrated that:(3)For materials with different pore size distributions, pore-size-decreased structure has the best radiation absorption due to its ability of maximizing the volumetric absorption effect, which is agreed with the UV-Vis-NIR spectrophotometer experimental results;(4) For materials with different porosity distributions, porosity-gradually-increased structure has the highest mean fluid/solid temperatures because it can utilize the enhanced convective/conductive heat transfer to improve the overall thermal performance of porous receiver;(5) Porous structure with pore-size-decreased distribution and porosity-gradually-increased distribution has the best thermal performance of which the mean temperatures of fluid/solid phases are the highest among all the studied cases.     

制备条件对氧化铝前驱体形貌及孔径分布的影响

以硝酸铝溶液为原料,NH4HCO3为沉淀剂,采用共沉淀-溶胶工艺,制备了一系列前驱物Al(OH)3,通过正交实验考察前驱物生成条件对氧化铝孔径分布的影响.利用比表面孔径测试仪和TEM分别对氧化铝孔径分布和前驱物的形貌及聚集状态进行表征.结果表明,制备高分散度的前驱体是制备孔径分布集中的氧化铝的前提;前驱体生成条件对氧化铝孔径分布的影响从大到小依次为:陈化时间,过量系数(pH值),反应温度,加料方式,Al3+浓度.通过正交实验考察成型压力、黏结剂浓度和煅烧温度对氧化铝孔径分布的影响.高压下成型使Al2O3颗粒形成缺陷孔,孔径分布出现双峰结构,说明高压不利于形成孔径集中分布的Al2O3颗粒.成型条件对孔径分布的影响顺序为:成型压力,其次是煅烧温度和黏结剂.Al2O3孔径分布集中在5～10 nm.     

早龄期加载对粉煤灰混凝土变形及强度的影响

研究了粉煤灰掺量、加载龄期和加载应力对粉煤灰混凝土早期变形及加载后强度变化的影响。研究结果表明：随着粉煤灰掺量增加,混凝土的变形量逐渐降低,当掺量为30%时,变形量减少了33.6%;随着加载龄期提前或加载应力增大,粉煤灰混凝土的早期变形量增大,其中,加载应力的影响尤其明显,60%加载应力（60%的标准养护条件下7 d轴心抗压强度）比20%加载应力下混凝土最终变形量增加了277.2%;混凝土初始加载时间提前或加载应力增大会导致加载后粉煤灰混凝土强度下降,加载应力比加载龄期对加载后粉煤灰混凝土强度的影响更明显。     

高性能混凝土板式构件的早期收缩特性及预测模型

以桥面板等高性能混凝土板式构件为研究对象,通过实验测试了板式构件不同截面厚度位置处早期收缩分布情况,探讨混凝土早期收缩变形与温度、湿度发展过程。结果表明,混凝土板式构件的内部厚度方向不同位置处的相对湿度随龄期都呈现逐渐减小趋势,并呈现两阶段特征,同时,混凝土板式构件早期收缩变形厚度方向不同步,导致板式构件表面产生拉应力,当拉应力大于当时混凝土的抗拉强度时,混凝土表面就会出现浅层裂缝,这与实际工程中发现的板式构件表面开裂情况相吻合。混凝土板式构件早期收缩变形与相同配合比混凝土的早期自由收缩变形进行比较,引入相对约束度并建立了混凝土板式构件早期收缩变形的预测模型,并验证了该模型的准确性,其特点是能预测高性能混凝土板式构件不同截面厚度位置的早期收缩变形,对控制混凝土桥梁板式结构的早期收缩裂缝具有实际意义。     

水泥浆等效龄期与早期热变形相关性试验研究

水泥净浆早期热膨胀系数是变化的且具有规律性,而变化规律与其龄期存在着内在联系.通过10组水泥净浆热变形试验和凝结时间试验,对早期热膨胀系数的变化规律、等效龄期与其变化规律的相关性进行了研究和探讨.结果表明:凝结时间与水灰比有线性关系,等效龄期(终凝)是水泥净浆热膨胀系数出现最小值的时间位置.     

装配式结构新老混凝土竖缝界面区孔隙率分布模型

装配式混凝土结构中存在大量新老混凝土连接界面,模板效应导致界面区水泥砂浆孔隙率相对增大,从而引起其力学及耐久性能的弱化。为定量描述界面区水泥砂浆孔隙率分布特征,制作了不同水灰比搭配的新老混凝土竖缝平整界面区试件。利用扫描电镜获取各试件距离界面不同位置处的灰度图像,并采用数字图像处理工具进行图像信息增强处理和二值化处理,从而获得孔隙像素占总像素的比值(即名义孔隙率),据此分析了新老混凝土竖缝平整界面区名义孔隙率的分布特征;基于名义孔隙率与真实孔隙率之间的稳定关系,建立了新老混凝土竖缝平整界面区孔隙率的分布模型;考虑凿毛区新老混凝土含量连续变化关系,建立了凿毛界面区孔隙率的分布模型。结果表明：名义孔隙率在界面处最大,向混凝土内部逐渐减小,最后趋于稳定,总体变化趋势具有高斯函数特征;随水灰比增大,从界面到内部各部位的名义孔隙率均呈现相对增大趋势,但不同水灰比混凝土各自界面区与内部稳定区的相对名义孔隙率却接近一致。     

改性环氧PBA／PMMA核壳乳胶粒子的制备与应用

采用半连续种子乳液聚合工艺制备核壳结构的聚丙烯酸丁酯,（聚甲基丙烯酸甲酯-衣康酸-双酚A环氧树脂）（PBA／P（MMA—ITA—DGEBA））乳胶粒子。并通过红外光谱仪（IR）对乳胶粒子进行了表征,证明乳胶粒子具有壮粒子结构。将壳层带有环氧基的核壳乳胶粒子对环氧树脂进行增韧,通过扫描电子显微镜（SEM）对固化物冲击断面进行观测,改性核壳乳胶粒子对环氧树脂起到明显增韧效果,并提升了与环氧树脂的相容性。     

负载纳米TiO2、Ag复合微胶囊光催化性能的研究

以纳米Ag粒子掺杂光催化剂TiO2,采用原位聚合法成功制备负载纳米TiO2/Ag的复合微胶囊;利用扫描电子显微镜、紫外-可见吸收光谱等测试手段对其形貌和光谱性质进行了表征,探讨不同纳米TiO2、Ag加入量的微胶囊在不同光催化时间下对亚甲基蓝光催化效果的影响.结果表明:纳米Ag(1.94%)掺杂TiO(22.78%)的复合微胶囊对亚甲基蓝具有最大脱色率为10.82%,单独添加纳米TiO2和单独添加纳米Ag粒子的微胶囊样品与常规微胶囊比较没有显著差异.     

非等密度颗粒气固流化床的微观尺度模拟与分析

采用计算流体力学与离散单元法相耦合的CFD-DEM方法对两种表观气速下三维非等密度颗粒流化床内的气固运动进行了数值模拟研究,对比了两种气速下流化床内颗粒的分层和混合现象,发现在非等密度颗粒流化床内,有不同程度的颗粒分层现象存在。当表观气速较低,处于最小密度颗粒的临界流化速度和最大密度颗粒的临界流化速度之间时,颗粒体系出现了较为明显的分层现象,整体上为重颗粒在下、轻颗粒在上的分层结构;当表观气速较高,大于最大密度颗粒的临界流化速度时,分层现象不再明显。采用Lacey混合指数分析了流化床内颗粒之间的混合状况,发现颗粒密度差越小,混合指数越大,越难分离;颗粒密度差越大,则混合指数越小,分离越完全.     

Effect of coal gangue with different kaolin contents on compressive strength and pore size of blended cement paste

The effects of activated coal gangue on compressive strength, porosity and pore size distribution of hardened cement pastes were investigated. Activated coal gangue with two different kaolin contents, one higher and one lower, were used to partially replace Portland cement at 0%, 10%, and 30% by weight. The water to binder ratio（w/b） of 0.5 was used for all the blended cement paste mixes. Experimental results indicate that the blended cement of activated coal gangue mortar with higher kaolin mineral content has a higher compressive strength than that with lower kaolin mineral content. The porosity and pore size of blended cement mortar were significantly affected by the replacement of activated coal gangue.     

宽筛分颗粒高压热态最小流化速度的实验研究

在内径80 mm的加压热态流化床实验台上,以4种不同平均粒径的宽筛分颗粒为实验物料,研究了在压力(0.1～4.5 MPa)和温度(20～800℃)范围内实验物料的最小流化速度。实验结果表明:在相同的温度下,最小流化速度随压力的增加而减小。而在相同的压力下,温度对最小流化速度的影响随床料的种类的不同而有明显的差别。基于Ergun方程和床层受力分析,得到了最小流化速度的计算式,给出了高温高压宽筛分颗粒流化床最小流化速度合理计算步骤,并与其他研究者的关联式和实验结果进行了对比,结果表明本文提出的预测方法与实验结果吻合较好,相对误差在10%以内,为增压流化床反应器的设计和运行提供较可靠的参考依据。