基于热分析的非晶合金非等温晶化动力学参数计算

为了对非晶合金晶化动力学参数的各种计算方法的优缺点进行分析比较,为正确合理的选用数据处理方法提供依据,介绍了非等温晶化激活能及晶化机理函数的各种计算方法及其计算原理,并进行了数学归纳.分析表明,在计算激活能的各种方法中,无模式等转化率法是最可靠的一类方法,分为晶化速率-等晶化体积分数法、基于温度积分近似解析解的等晶化体积分数法、最大速率法3种类型.当热分析曲线基线难以精确确定时,用第二种类型计算出的激活能比第一种类型精度高.在确定机理函数的两种方法(Surinach曲线拟合法和Popesc曲线拟合法)中,Popesc法可以直接利用实验数据的积分而不需要提前知道速度常数和激活能,其结果的可靠性更高.     

A TEM study and non-isothermal crystallization kinetic of tellurite glass-ceramics

The glass transition temperature was studied via differential thermal analysis of glasses in the system (100 − x)TeO₂–5Bi₂O₃–xZnO and (100 − x)TeO₂–10Bi₂O₃–xZnO where x = 15, 20, 25 in mol%. The crystallization behavior and microstructure development of the 0.7TeO₂/0.1Bi₂O₃/0.2ZnO glass during annealing were investigated by non-isothermal differential thermal analysis (DTA), X-ray diffractometry, and transmission electron microscopy. The glass transition temperature, crystallization temperature, and the nature of crystalline phases formed were determined. From the heating rate dependence of the glass transition temperature, the glass transition activation energy was derived. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 305.8 and 197 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Moreover, synthesized crystalline Bi_3.2Te_0.8O_6.4, Bi₂Te₄O₁₁, and Zn₂Te₃O₈ were investigated. In addition, the change in particle size with increasing annealing time was observed by high-polarized optical microscope.     

Mechanical properties and morphological characterization of exfoliated graphite–polypropylene nanocomposites

This research explores the potential of using exfoliated graphite nanoplatelets, xGnP, (graphene sheets ∼10 nm thickness, ∼1 μm diameter), as reinforcement in polypropylene, PP. xGnP–PP nanocomposites were fabricated by melt mixing and injection molding. The feasibility of using xGnP–PP nanocomposites was investigated by evaluating the flexural strength, modulus and impact strength and studying the morphology of this system as a function of xGnP loading and aspect ratio and by comparing the xGnP–PP with composites made with commercial available reinforcements such as carbon fibers, carbon black and clays. It is concluded that the smaller aspect ratio xGnP has the strongest impact on the mechanical properties of PP, at loadings up to 5 vol.%, compared to the other reinforcements used, which reflects the compatibility between the exfoliated graphite nanoplatelets and the PP matrix and the exceptional mechanical properties of xGnP, similar to crystalline graphite.     

Dynamic mechanical analysis and non-isothermal kinetics of EVA/PPy carbon black nanocomposites

Pure ethylene-co-vinyl acetate (EVA) and its conductive blend-nanocomposite with polypyrrole (PPy)/carbon black (CB) were prepared using a melt mixing process. Dynamic mechanical analysis and non-isothermal thermal gravimetrical analysis were performed on the samples. The storage modulus, loss modulus, and damping factor of the EVA/PPy/CB nanocomposites were significantly affected by the incorporation of PPy/CB. Two thermal decomposition stages were detected for pure and blended nanocomposite samples. Peak analysis was used to deconvolve the first complex decomposition stage. The Coats-Redfern method was used to determine the kinetic parameters. Improving the thermal and mechanical properties of the EVA co-polymer will enable the three-phase blend/nanocomposites to be used in several industrial applications.     

High density polyethylene and silane treated silicon nitride nanocomposites using high-density polyethylene functionalized with maleate ester: Mechanical,tribological and thermal properties

In this study, attempt has been made to improve the overall performance of high-density polyethylene (HDPE) composites by blending with silane treated silicon nitride (SN) using HDPE grafted dibutyl maleate as compatibilizer. A small quantity of surface modified nanoclay has also been added in order to improve the mechanical properties. The mechanical properties show that both silane modification of silicon nitride accompanied by nanoclay addition has led to the substantial enhancement in mechanical properties. In addition, compatibilization has further improved the mechanical properties showing 10% (w/w of silicon nitride) as optimal compatibilizer content. The wear results showed that, as SN content increased, the slide wear loss decreased. Addition of SN along with compatibilizer also improved the thermal stability of the nanocomposites.     

Biomicrofibrilar composites of high density polyethylene reinforced with curauá fibers: Mechanical,interfacial and morphological properties

Vegetal fibers are used in polymer composites to improve mechanical properties, substituting inorganic reinforcing agents produced by non renewable resources, like fiberglass. The highest performance formulation in high density polyethylene, HDPE, composites reinforced with curauá fibers were studied, aiming to improve the interphase interaction and optimize the mechanical properties. The fiber content, the type and the concentration of coupling agent were tested. The composites and the pure materials were characterized by Fourier transform infrared spectroscopy and the fiber/matrix phase adhesion was evaluated by scanning electron microscopy. The mechanical properties and the micrographs showed that the best formulation is: 20 wt.% of milled curauá fibers and 2 wt.% poly(ethylene-g-maleic anhydride). The coupled composites are also less hygroscopic than the uncoupled composites. We conclude that the composites reinforced with curauá fibers have mechanical properties comparable to commercially produced composites of HDPE reinforced with fiberglass.     

Kinetics of non-isothermal crystallization process and activation energy for crystal growth in amorphous materials

An equation expressing the volume fraction,x, of crystals precipitating in a glass heated at a constant rate, α, was derived. When crystal particles grow m-dimensionally,x is expressed as In [- ln(1 -x)] = -n (nα - 1.052mE/RT + Constant whereE is the activation energy for crystal growth andn is a numerical factor depending on the nucleation process. When the nuclei formed during the heating at the constant rate,α, are dominant,n is equal tom + 1, and when the nuclei formed in the previous heat-treatment before thermal analysis run are dominant,n is equal tom. The validity and usefulness of this equation was ascertained by applying it to a Li₂O·2SiO₂ glass. A method for determining the values ofn andm from DSC curves was proposed and it was concluded that the modified Ozawa-type plot is very useful and convenient to obtain the activation energy for crystal growth.     

Synthesis and characterizations of modified expanded graphite/emulsion styrene butadiene rubber nanocomposites: Mechanical,dynamic mechanical and morphological properties

The present research work demonstrated the reinforcing effect of expanded graphite (EG) and modified EG (MEG) with and without carbon black (CB) on the physical, mechanical and thermo-mechanical properties of emulsion polymerized styrene butadiene rubber (SBR) vulcanizates. In separate batches, EG and MEG flakes with and without CB were incorporated into the SBR by melt blending. The microstructures of the nanocomposites were precisely characterized by wide angle X-ray diffraction (WAXD) analysis and high resolution transmission electron microscope (HR-TEM). EG and MEG filled SBR compounds showed improvement in the curing features, mechanical, thermal and dynamic mechanical properties than their respective controls.     

Mechanical reinforcement and crystallization behavior of poly(ethylene 2,6-naphthalate) nanocomposites induced by modified carbon nanotube

Poly(ethylene 2,6-naphthalate) (PEN) nanocomposites reinforced with a very small quantity of carbon nanotube (CNT) were prepared by melt compounding using a twin-screw extruder. Morphological observations revealed that the modified CNT was uniformly dispersed in the PEN matrix and increased interfacial adhesion between the nanotubes and the polymer matrix, as compared to the untreated CNT. Furthermore, a very small quantity of the modified CNT can improve the mechanical and thermal properties of the PEN nanocomposites. This study also demonstrates that the non-isothermal crystallization behaviors of the PEN nanocomposites are strongly dependent on the presence of the modified CNT and cooling rate. The variations of the nucleation activity and activation energy for crystallization reflected the enhancement of crystallization of the PEN nanocomposites induced by the modified CNT. Combined Avrami and Ozawa analysis was found to be effective in describing the non-isothermal crystallization of the PEN nanocomposites in the presence of the modified CNT.     

Structural analysis of graphene oxide/silver nanocomposites: optical properties,electrochemical sensing and photocatalytic activity

In the present study, graphene oxide/silver (GO/Ag) nanocomposites were synthesized via a facile simple one pot chemical reduction method using ethylene glycol/sodium borohydrate (EG/NaBH₄) as solvent and reducing agent. GO was selected as a substrate and stabilizer to prepare GO/Ag nanocomposites. The synthesized GO/Ag nanocomposites were characterized by a series of techniques. Highly monodispersed stable crystalline silver nanoparticles having a face-centered cubic (fcc) phase were confirmed by X-ray powder diffraction (XRD) on GO signature. Scanning electron microscopy images showed that Ag nanoparticles are deposited on the GO sheet with a narrow size distribution. Transmission electron microscopy observations revealed that large numbers of Ag nanoparticles were uniformly distributed on GO sheet and well separated with an average size of 18 nm. Ultraviolet–visible (UV–Vis) spectroscopic results showed the peak of GO and surface plasmon resonance (SPR) of Ag nanoparticles. The SPR property of GO/Ag nanocomposites showed that there was an interaction between Ag nanoparticles and GO sheet. The intensities of the Raman signal of GO/Ag nanocomposites are gradually increased with attachment of Ag nanoparticles i.e. there is surface-enhanced Raman scattering activity. Electrochemical investigations indicated that the nanocomposites possessed an excellent performance for detecting towards 4-nitrophenol. An application of the obtained GO/Ag nanocomposites as a catalyst in the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄ was demonstrated. The GO/Ag nanocomposites exhibited high activity and stability for the catalytic reduction of 4-nitrophenol. The prepared GO/Ag nanocomposites act as photo-catalysts.     

BEM approaches and simplified kinetic models for the analysis of damping in deformable MEMS

Microflows occurring in MEMS are addressed by applying a continuum BEM code for quasi-static Stokes flow accounting for slip boundary condition. Working conditions typical of transition and rarefied flows are treated in a simplified manner by employing a viscosity corrected according to semi-analytical solutions for the linearized BGK model of Couette and Poiseuille flows. Numerical results are compared with experiments showing excellent agreement     

Fabrication of high-quality silver nanowire conductive film and its application for transparent film heaters

Here,we report a facile method to produce pure silver nanowires (AgNWs) with high yield.A highly conductive dispersant was used to ensure uniform dispersion of the AgNWs.Without any posttreatment,the AgNW networks,deposited on flexible substrates,showed excellent optoelectrical performance owing to minimal junction resistance between the AgNWs.To explore their potential in flexible optoelectronic devices,a transparent film heater was constructed based on the present AgNW networks.The heater could achieve rapid response at low input voltage and reach a relatively high temperature in a short response time.Since this high-quality AgNW film exhibits relatively low production costs and fast production time,it may have value for future electronic industry applications.     

Bamboo fibers for composite applications: a mechanical and morphological investigation

Bamboo fibers are very promising reinforcements for polymer composites production due to its high aspect ratio and strong mechanical performances. In order to better understand their reinforcing potential, the mechanical properties of single bamboo fibers extracted from eleven commercial bamboo species in China were measured with a newly developed microtensile technique. For comparison, the mechanical properties of mature single Chinese Fir and Masson Pine wood fibers were measured. The results show that the average longitudinal tensile modulus of the eleven kinds of bamboo fibers ranges from 25.5 to 46.3 GPa with an average value of 36.7 GPa. For tensile strength, the value ranges from 1.20 to 1.93 GPa with an average value of 1.55 GPa. The tensile strength and modulus of bamboo fibers are nearly two times of that of single Chinese Fir and Masson Pine fibers, and significantly higher than most of the published data for other softwood fibers. The average elongation at break of bamboo fibers is about 4.84 %, only a little lower than the value 5.15 % of the tested mature softwood fibers. Additionally, bamboo fibers were found to have smaller diameters and larger aspect ratio than most documented wood fibers, which favored an improved reinforcing effect. These combined mechanical and morphological advantages highlight the potential of bamboo fibers as the reinforcing phase in polymer composites for structural purpose.     

Chemical,morphological, and mechanical analysis of rice husk/post-consumer polyethylene composites

Composites were obtained from post-consumer high-density polyethylene (PE) reinforced with different concentrations of rice husk. PE and rice husk were chemically modified to improve their compatibility in composite preparation. Rice husk was mercerized with a NaOH solution and acetylated. The chemically modified fibers were characterized by FTIR and ¹³C NMR spectroscopy. The composites were prepared by extrusion of modified and unmodified materials containing either 5 or 10 wt.% fibers. The morphology of the obtained materials was analyzed by SEM. The chemical modification of the fiber surface was found to improve its adhesion with matrix. Flexural and impact tests demonstrated that PE/rice husk composites present improved mechanical performance comparatively to the pure polymer matrix, on the contrary no benefit is observed in the tensile strength over the pure PE.     

Mechanical,rheological, morphological and water absorption properties of maleated polyethylene/hemp composites: Effect of ground tire rubber addition

Natural fibre composites were produced from maleated polyethylene (MAPE) and hemp fibres, while impact modification was performed via ground tire rubber (GTR) addition. Incorporation of hemp fibre increased significantly the tensile (strength and modulus) and flexural properties of the MAPE matrix. Impact strength however, decreased with increasing hemp content, but GTR addition led to a noticeable increase in impact strength (up to 50% at 10% GTR). Increase in hemp content produced also higher water uptake and longer saturation time. After ageing in water, the mechanical properties and thermal stability were unchanged for samples up to 30% hemp, but samples at higher concentrations showed some degradation.     

Non-isothermal crystallization of polyamide 6 matrix in all-polyamide composites: crystallization kinetic, melting behavior, and crystal morphology

The difference in the melting points of polyamide 66 (PA66) fiber and polyamide 6 (PA6) film permits the preparation of all-polyamide (all-PA) composites by film-packing. Good interface performance and integrated consolidation structure in this all-PA composite are contributed to the similar chemical composition between PA66 fiber and PA6 matrix. In this paper, the non-isothermal crystallization kinetics and melting behaviors of PA6 matrix in all-PA composite are studied by differential scanning calorimetry (DSC), in which the modified Avrami equation, Ozawa model, and Mo equation combining Avrami and Ozawa equation are employed. It is found that the Mo equation exhibits great advantages in treating the non-isothermal crystallization kinetics for both neat PA6 and PA6 matrix in all-PA composite. The crystal morphologies of single PA66 fiber–PA6 composite by polarizing microscope (POM) clearly show a transcrystallinity layer of PA6 around PA66 fiber that proves a remarkable nucleation effect of PA66 fiber surface on the crystallization of PA6 matrix.     

A new method for estimating the isothermal devitrification and crystallization of mold powder slags from non-isothermal DSC data

The rate of formation of crystalline phases from liquid and glassy mold powder slags is of foremost importance in the performance of molds used for continuous casting of steel. This study shows how the Induction Period (of Šimon and Kolman) and the Kissinger methods can be combined in a kinetic model to evaluate the isothermal rate of formation of crystalline phases from thermo-analytical data – onset temperature, T_i, peak maximum temperature, T_m, shape index, S, and conversion at peak maximum, x_m – collected at various linear heating and cooling rates. The diagram of the extent of isothermal transformation as a function of time calculated for a commercial mold powder, used for casting low carbon steels, shows good agreement with the degree of transformation observed in photomicrographs of glass disks devitrified isothermally, at several temperatures for different times. Additionally, qualitative and quantitative X-ray diffraction results obtained at room-temperature from glass powder samples treated isothermally and quenched also show good accord with the degree of transformation predicted with the kinetic model developed in this work.     

Multi-scale mechanics of nanocomposites including interface: Experimental and numerical investigation

The addition of minor amounts of fillers that have characteristic dimensions on the nanometer length scale to polymer matrix-based materials has attracted wide attention. A high aspect ratio of nanoelements and extraordinary mechanical properties (strength and flexibility) provide the ultimate reinforcement for the next generation of extremely lightweight but highly elastic and very strong advanced composite materials. However, the resultant physical behavior imparted by such an addition, has been, to date, difficult to quantify, especially for advanced thermoset composites. In an effort to better understand the phenomenological changes across multiple length- and time-scales, we first review currently reported methods of calculating nanoelement reinforced composite mechanical properties. Secondly, we describe recent experimental data along with a multi-scale modeling methodology for the calculation of elastic constants and local/interface properties for systems with statistically homogeneous distribution of embedded nanofillers (nanofibers, nanoparticles, nanoplates, or other heterogeneities, that are either aligned or randomly oriented).     

Gas bubbles generated by thermal aging in thin film bicrystals of silver I: Observations and quantitative analysis

Gas bubble generation in thin film bicrystals of silver under thermal and room temperature aging was studied by transmission electron microscopy. Bicrystals were prepared by superposing two epitaxial films obtained by evaporation in vacuum. After aging they were separated by a (001) twist grain boundary with an angle ranging from 0° to 45°. Gas bubbles only appeared in bicrystals in which welding had produced the boundary and they were absent from bicrystals that were badly welded and from single films which had undergone the same aging. A quantitative analysis of the gas bubbles showed that their density decreases if the aging temperature or the sample thickness are increased. A correlation with the mean size of the gas bubbles was also found, and a relation between the gas bubble density and the growth rate of the films was established.