Chitosan nanocomposites based on distinct inorganic fillers for biomedical applications

Chitosan (CHI), a biocompatible and biodegradable polysaccharide with the ability to provide a non-protein matrix for tissue growth, is considered to be an ideal material in the biomedical field. However, the lack of good mechanical properties limits its applications. In order to overcome this drawback, CHI has been combined with different polymers and fillers, leading to a variety of chitosan-based nanocomposites. The extensive research on CHI nanocomposites as well as their main biomedical applications are reviewed in this paper. An overview of the different fillers and assembly techniques available to produce CHI nanocomposites is presented. Finally, the properties of such nanocomposites are discussed with particular focus on bone regeneration, drug delivery, wound healing and biosensing applications.     

Intercalation of natural flake graphites

Natural flake graphites are characterized in terms of their ability to form intercalation compounds. Factors such as amounts and distribution of mineral matter, extents of intercalation by bisulphate and bromine, flake thickness and fissuring which control extents of intercalation are examined. Differences in stabilities of intercalates in the natural flake graphites are assessed. Techniques used to examine the flake graphites include image analysis to assess flake thickness and the degree of fissuring as well as bromine intercalation and desorption as an indication of crystal perfection in flake graphites. Amounts of mineral matter in the flakes and the amount and distribution of crystalline sulphur (not previously reported) in the intercalated flake graphites are studied using EDAX. The intercalated graphites were exfoliated at 1200° C and examined by SEM to assess their extent of expansion and the structure and extent of pinning within the graphite flakes. Results indicate that flake thickness is a primary factor influencing extents of intercalation in these materials, the optimum thickness for bromine intercalation being 25m. Bromine uptake could indicate the perfection of stacking within a graphite. Mineral impurity in the graphite flake is of importance as it influences flake thickness and cleavage properties.     

Brittleness and thermal stability of graphites

The correlation between calculated values of thermal-stability criteria and experimentally measured values of the failing temperature gradient for 10 graphites is investigated. It is demonstrated that use of the brittleness measure makes it possible to divide the materials into two groups: relatively brittle (<0.85) and transitional (>0.85). A linear correlation between the criterion R and the failing temperature gradient T is established for materials of the transitional group, whereas a correlation between the criterion T_c and T is indicated for the relatively brittle materials.Translated from Problemy Prochnosti, No. 9, pp. 61–63, September, 1990.     

旅游场景下的基于深度学习的文本方面级细粒度情感分类

方面级细粒度情感分类是指针对文本数据,分析其在指定方面的情感极性.由于获取到的评论样本往往涉及不同的方面,导致各个方面的情感极性不平衡.为了减少不平衡数据对模型训练的影响,本文提出了一种新的数据平衡方法——批处理平衡方法(BB),用来平衡多标签多类别数据.同时,由于评论文本蕴含多个方面,传统模型结构往往每次只能预测一个...     

Contact thermal resistance of some graphites

Experimental data on the contact thermal resistance of VPP, ARV, and MPG-6 graphites in relation to the compression and finish of the contact surfaces are given.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 3, pp. 455–459, September, 1977.     

Effect of surface nature of carbonaceous nanosized fillers on properties of polyurethane based nanocomposites

Polyurethane (PU) composites containing uniformly dispersed nanosized and macrosized carbonaceous fillers (fullerene C60, fullerene soot, detonation nanodiamonds, and nanodiamonds soot) with loadings varying from 0.1 to 0.5?wt.% were prepared by in situ polymerization. The kinetics of PU synthesis catalyzed by dibutyltin dilaurate was studied as a function of amount of nanosized filler. It was found that the reaction is zero-order. The kinetics of the hard domain growth of the PU matrix was studied and described successfully using the two-parameter Avrami model for isothermal crystallization. It was found that the growth of polymers induces athermal primary nucleation for hard domain growth, and that nanosized fillers do not change the nucleation mechanism. Our results show that the mechanical strength of PU composites increases in the presence of all types of nanoparticles under study. This can be related to the antiplasticization effect of the nanoparticles on the polymer matrix. The dielectric spectroscopy demonstrated that T_g values of the composites increase in comparison with neat PU. However, the effects of hydrophilic/hydrophobic properties of nanoparticles on the secondary relaxations are not obvious Microphase separation in these nanocomposites was studied using FTIR spectroscopy, but no effect of surface properties of the nanoadditives on this phenomenon has been observed.     

基于离散元法的双组份复合涂料搅拌螺杆参数优化

为了提高双组份复合涂料的混合均匀度,对搅拌螺杆的参数进行优化。以混合销钉式搅拌螺杆为对象,建立其三维模型后导入EDEM软件,然后基于离散元法对双组份复合涂料在该搅拌螺杆中的搅拌过程进行仿真分析,研究了螺杆的转速、螺距和长径比对混合均匀度的影响。最后通过单轴压缩实验对螺杆参数优化后的搅拌效果进行了对比验证。结果表明：螺杆转速、螺距和长径比分别通过改变颗粒抛撒程度、螺旋叶片间隙和螺杆长度来影响复合涂料的混合均匀度;稳定出料后,螺杆转速为200 r/min、螺距为80 mm、长径比为5.57∶1时的搅拌效果最佳;实验验证了利用优化后搅拌螺杆搅拌时复合涂料的力学性能优于手工搅拌时的。研究结果可为自动化搅拌螺杆的结构设计和参数优化提供理论依据。     

Electric heating films based on m-aramid nanocomposites containing hybrid fillers of graphene and carbon nanotube

m-Aramid nanocomposite films containing 1.0 wt% hybrid fillers of different compositions of graphene and multi-walled carbon nanotube (MWCNT) are prepared by an efficient solution-casting method, and their electric heating behavior is investigated as a function of the composition of hybrid fillers. Electron microscope images and X-ray diffraction patterns reveal that the hybrid fillers are well dispersed in the m-aramid matrix by forming interconnected networks among graphene sheets and MWCNTs. The electrical resistivity of the nanocomposite films is decreased gradually from ~10⁵ to 10¹ Ω cm with increasing the MWCNT content in the hybrid fillers. Accordingly, maximum temperature attained at a given applied voltage for the nanocomposite films can be finely controlled by the graphene/MWCNT composition of 1.0 wt% hybrid fillers. The m-aramid/hybrid filler nanocomposite films also exhibit excellent electric heating performance in aspects of rapid temperature response and high electric power efficiency at applied voltages of 1–100 V.     

Complementary percolation characteristics of carbon fillers based electrically percolative thermoplastic elastomer composites

Electrically percolative composites of thermoplastic elastomers (TPE) filled with different concentrations of carbon nanotubes (CNT), carbon black (CB) and (CNT–CB) hybrid fillers were fabricated by melt blending. The effects of filler type and composition on the electrical properties of the percolative TPE composites were studied. Percolation threshold for CB-, CNT- and (CNT–CB)-based composites was found to be 0.06, 0.07 and 0.07 volume fraction respectively. Compared to CB-based composites and earlier reported results, CNT- and (CNT–CB)-based ones revealed an unexpectedly high percolation threshold, which otherwise considered an unwelcome phenomenon, lead to distinct and rare percolation characteristics of CNT filled percolative composites like per-percolation conductivity and a relatively steep percolation curves. CB-based composites showed a comparatively sharp insulator–conductor transition curve complementing the percolation characteristics CNT- and (CNT–CB)-based composites. Percolation threshold conductivity of the fillers was in the order of CB > CNT > (CNT–CB), while maximum attained conductivities followed the order of CNT > (CNT–CB) > CB. Conductivity order of fillers not only denied much reported synergic effect in (CNT–CB) filler but also highlighted the effect of percolation characteristics on the outcome of conductivity values. Results obtained were of theoretical as well as practical importance and were explained in the context of filler morphology and different dispersion characteristics of the carbon based fillers.     

球磨时间对硼掺杂石墨材料抗氧化行为的影响

通过球磨分散方法制备了掺杂细颗粒碳化硼石墨材料。采用热分析仪考察了经不同球磨时间制备的复合材料在室温至1400℃温度范围内的抗氧化行为;利用扫描电镜分析了样品在800℃和1000℃恒温氧化后的表面形貌。研究结果表明:经球磨50h制备的细颗粒碳化硼掺杂石墨材料在1000℃氧化时,有优良的自愈合抗氧化性能。其抗氧化机理为:均匀分散的细颗粒B4C可以在氧化时迅速形成连续的、均匀的保护膜覆盖在基体表面,从而有效地抑制氧气进入和基体反应。     

Thermal conductivity improvement of composite carbon foams based on tannin-based disordered carbon matrix and graphite fillers

Carbonaceous porous matrices were prepared from a tannin-based resin by physical foaming, having improved thermal properties by addition of various kinds and various amounts of graphite fillers. The resultant composite carbon foams presented much higher thermal conductivity, making them suitable for hosting phase-change materials with the aim of using them in seasonal storage applications. These materials were investigated in terms of porous structure, thermal and mechanical properties. It was shown that, unlike what was a priori expected, smaller particles were far more suitable for getting conductive, strong and porous matrices. The smaller were the particles, the better were the results. These findings were explained and justified, making such biomass-based composite carbon foams interesting and cheap candidates for thermal storage applications.