Microstructure characterization of short-chain branching polyethylene with differential scanning calorimetry and successive selfnucleation/annealing thermal fractionation

A series of the copolymers of ethylene with 1-hexene(M1–M9) synthesized by metallocene catalyst Et[Ind]2ZrCl2/MAO was studied by differential scanning calorimetry and successive self-nucleation and annealing(SSA) thermal fractionation. The distribution of methylene sequence length(MSL) in the different copolymers was determined using the SSA method. The comonomer contents of samples M4 and M5 are 2.04 mol% and 2.78 mol%, respectively. Both M4 and M5 have low comonomer content and their MSL distribution profiles exhibit a monotonous increase trend with their MSL. The longest MSL of M5 is 167, and its corresponding molar percent is 43.95%, which is higher than that of M4. Moreover, the melting temperature(Tm) of M5 is also higher than that of M4. The comonomer contents of samples M7, M8, and M9 are 8.73 mol%, 14.18 mol% and 15.05 mol%, respectively. M7, M8, and M9 have high comonomer contents, and their MSL distribution profiles display unimodality. M7 has a lower peak value of 33 and a narrow MSL distribution, resulting in a Tm lower than that of M8 and M9. The MSL and its distribution are also key points that influence the melting behavior of copolymers. Sometimes, MSL and its distribution of copolymers have a greater impact on it than the total comonomer contents, which is different from traditional views.     

Performance of a parallel viscoelastic-viscoplastic model for a microcellular thermoplastic foam on wide temperature range

This paper is concerned with the experimental testing and the constitutive modelling of a thermoplastic microcellular polyethylene-terephthalate (MC-PET) foam on the temperature range of 21–210 °C in order to investigate the temperature-dependent performance of the applied parallel viscoelastic-viscoplastic material model. By means of carefully designed uniaxial mechanical tests in temperature chamber, the viscous, elastic and yielding behaviours of the investigated material are identified, which are then applied for selecting suitable viscoelastic-viscoplastic constitutive models. The material characterization process is conducted using finite-element-based fitting method, including also the analysis of the applied numerical optimization algorithm. The fitting results are used to analyse the parameter sensitivity and to propose closed-form analytical relations for the temperature dependency of the material parameters. Finally, the utilisation of the analytical temperature functions for speeding up the parameter-fitting process is also demonstrated.     

Nanocomposites induced by two-dimensional titanium carbide nanosheets for highly efficient energy storage

Surface group-rich titanium carbide nanosheets (TCNSs) were successfully fabricated by simply etching Ti₃AlC₂ powders and used as dielectric fillers to promote the dielectric and energy storage performances of poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP)-based composites. The PVDF-HFP/TCNS composites realize a high dielectric constant and low dielectric loss of 16.3 and 0.034 at 10² Hz, respectively. Importantly, a high energy storage density (U_e) of 0.367 J cm⁻³ at 900 kV cm⁻¹ and a high energy storage efficiency (η ≥ 78.9%) at a TCNS content of only 0.5 wt% are obtained, which indicates that incorporating TCNS is an efficient route in enhancing U_e while maintaining a high level η of the PVDF-HFP-based composites. According to detailed characterization results, a mechanism related to the reduction of lamellar crystals in the PVDF-HFP matrix is suggested. The above mechanism restricts the movement of polymer chains near the filler-matrix interface and is proposed to be responsible for the outstanding dielectric and energy storage performances. Consequently, this work provides a simple and effective method for fabricating highly efficient energy storage nanocomposites.     

Highly porous PEEK and PEEK/HA scaffolds with Escherichia coli-derived recombinant BMP-2 and erythropoietin for enhanced osteogenesis and angiogenesis

The present study reports the results of structural and mechanical analysis, as well as proteins release kinetics and osteointegration in mice craniotomy model of highly porous PEEK (PolyEther Ether Ketone) and PEEK/HA (PolyEther Ether Ketone/HydroxyApatite) biomimetic scaffolds loaded with Escherichia coli-derived recombinant Bone Morphogenetic Protein-2 (BMP-2) and ErythroPOietin (EPO). Porous scaffolds were obtained by thermopressing with NaCl as a pore-forming filler. Two fractions of pore-forming filler were used to imitate natural trabecular bone tissue by making a preferential porosity using large fraction and creating an extended surface and special microrelief using small fraction. Hydroxyapatite (HA) was added up to 20% to activate bioinert PEEK providing loading of recombinant growth factors and osteointegration as well as sufficient level of mechanical properties imitating human trabecular bone. Unexpectedly, the non-activated PEEK produced by our technology was also able to spontaneously bind both BMP-2 and EPO. Loading of both BMP-2 and EPO to both types of implants resulted in enhanced neoosteogenesis and angiogenesis in a critical-size cranial defect model in mice in 3–6 weeks. Considering good mechanical characteristics and excellent osteoinductive and angiogenic properties, both materials in combination with BMP-2 and EPO can find their application in regenerative medicine.     

Effect of working temperature on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present study, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) in the working temperature range from 23 °C to 90 °C was studied by experimental and constitutive methods. Monotonic and cycle loading-unloading single-lap-shear tests were employed to determine the interfacial properties of hybrid composites. The experimental results show that both interfacial shear strength and shear stiffness decrease with increasing working temperature. A regression function was adopted to evaluate the decaying degree of interfacial properties with increasing working temperature. The shear stress-displacement relationship under monotonic loading exhibits nonlinear behavior after an initial elastic region. The envelope lines of shear stress-displacement of hybrid composites under cyclic loading indicate that the nonlinearity in the curve is caused by the plastic deformation of polypropylene in the interphase region. A constitutive model was built to describe the nonlinear shear stress-displacement relation of hybrid composites at different working temperatures. A full suite of temperature-dependent plastic parameters in the model was obtained from cyclic loading-unloading tensile tests. The predicted shear stress–displacement curves agreed well with experimental results from different working temperatures. In addition, the failure mode of hybrid composites varied with working temperature.     

PEFC catalyst layers: Effect of support microstructure on both distributions of Pt and ionomer and cell performance and durability

In order to improve the performance and durability of polymer electrolyte fuel cells (PEFCs), various improvements in the microstructures of cathode catalyst layers (CLs) were initiated in the early 1990s. More recent advances in CL materials are highlighted, including carbon supports for improved accessibility of Pt nanoparticles (NPs), adsorption of ionomer on the Pt surface, high-oxygen-permeability ionomers, corrosion resistance of mesoporous and microporous carbons, and conductive ceramic supports with a fused-aggregate network structure. These approaches are summarized as stepwise improvements. The influences of the support structure on the distribution of Pt NPs and ionomer are reviewed, as well as their effects on performance and durability. These approaches for carbon supports are extended to conductive ceramic supports and the unique advantages are discussed.     

Nonlinear free vibrations of porous composite microplates incorporating various microstructural-dependent strain gradient tensors

The main objective of the present numerical analysis is to predict the nonlinear frequency ratios associated with the nonlinear free vibration response of porous composite plates at microscale in the presence of different microstructural gradient tensors. To achieve this end, by taking cubic-type elements into account, isogeometric models of porous composite microplates are obtained with and without a central cutout and relevant to various porosity patterns of distribution along the plate thickness. The established unconventional models have the capability to capture the effects of various unconventional gradient tensors continuity on the basis of a refined shear deformable plate formulation. For the simply supported microsized uniform porous functionally graded material (U-PFGM) plate having the oscillation amplitude equal to the plate thickness, it is revealed that the rotation gradient tensor causes to reduce the frequency ratio about 0.73%, the dilatation gradient tensor causes to reduce it about 1.93%, and the deviatoric stretch gradient tensor leads to a decrease of it about 5.19%. On the other hand, for the clamped microsized U-PFGM plate having the oscillation amplitude equal to the plate thickness, these percentages are equal to 0.62%, 1.64%, and 4.40%, respectively. Accordingly, it is found that by changing the boundary conditions from clamped to simply supported, the effect of microsize on the reduction of frequency ratio decreases a bit.

     

激光打孔对氮化铝绝缘性能的影响

氮化铝陶瓷具有优良的绝缘和导热性能,是传导冷却超导电力装置绝缘与导热连接件的首选,激光切割是对其进行加工的最有效方法,但激光切割可能对其绝缘性能产生影响,进而影响超导电力装置的安全运行。基于此,通过对激光打孔前后氮化铝基片孔周区域绝缘电阻的测量、金相结构和扫描电镜成分的比较,分析了激光打孔对氮化铝绝缘性能的影响,并总结了选用氮化铝基片作为超导电力装置电流引线的绝缘与导热垫片应注意的问题。     

X7R型MLCC流延瓷膜显微结构的AFM研究

片式电容是由电介质陶瓷薄膜和内电极相互重叠而成的多层独石结构,又称多层陶瓷电容器(mul-tilayer ceram ic capacitors,简称MLCC)。具有体积小、内部电感低、绝缘电阻高及漏电流小、介质损耗低、价廉等优点,被广泛应用于各种电子整机中的振荡、耦合、滤波和旁路电路,尤其是高     

Microstructure evolution of room-temperature-sputtered ITO films suitable for silicon heterojunction solar cells

Thickness influence on structural, optical and electrical properties of sputtered indium tin oxide (ITO) with thickness ranging from 60 up to 430 nm films has been studied. At the increase of the film thickness crystallinity degree and grain size increased, whereas tensile structural distortion as well as resistivity decreased. It was observed that a microstructure evolution takes place: the initial amorphous layer evolved in polycrystalline phase, with a grain–subgrain surface morphology. Carrier concentration increased at the increase of the film thickness and a general relationship between electrical characteristics and structural distortion has been found. In thinner films larger tensile distortion allowed to include larger amount of interstitial O and/or Sn atoms in the lattice. An appreciable impact of the thickness was also observed on electro-optical properties in terms of changes in energy gap, resistivity and optical absorption. Silicon heterojunction solar cells have been produced and J_sc as high as 33.0 mA/cm² has been obtained.