Molecular orientation and mechanical property size effects in electrospun polyacrylonitrile nanofibers

Electrospinning of polymeric solutions entails high jet velocities which could orient the polymer molecules along the jet direction. Polarized Fourier Transform IR spectroscopy (FTIR), Wide angle X-ray diffraction (WAXD) and Microelectromechanical System (MEMS)-based single nanofiber mechanical property experiments were employed to investigate the molecular orientation and crystallinity in electrospun polyacrylonitrile (PAN) nanofibers produced under different electrospinning conditions with diameters mainly varying between 100 and 300 nm. FTIR measurements with nanofibers fabricated at three different electrospinning distances, but under the same electric field intensity, revealed an enhanced molecular orientation only for the longest electrospinning distance. At long electrospinning distances the fiber solvent content is substantially reduced resulting in high viscosity, and, therefore, sustained shear stresses, which, in turn, allows for permanent molecular orientation. The orientation factors from polarized FTIR were in good agreement with the mechanical property trends obtained from individual nanofibers, where high elastic moduli and yield strengths were recorded from nanofibers with diameters smaller than 300 nm, which were fabricated at the longest electrospinning distance. WAXD studies on bundles of aligned PAN nanofibers showed small crystallinity which did not follow the trends in the mechanical properties and varied rather non-monotonically from 7%, for fibers spun at the shortest distance, to 17% for fibers spun at the longest distance used in this study.     

Effects of grain size and temperature on mechanical and failure properties of ultrananocrystalline diamond

Molecular dynamics simulations of ultrananocrystalline diamond (UNCD), with random distribution of grain sizes and grain boundaries (GBs), are performed to investigate the effect of grain size and temperature on the mechanical properties and failure mechanisms under tensile loading. Results show that when the grain size of UNCDs decreases from 4.1 nm to 2.26 nm, the Young's modulus decreases from 891 GPa to 840 GPa, while the obtained intrinsic fracture strength, 113 GPa, is insensitive to the grain size. Elastic softening is attributed to the increased volume fraction of amorphous-like atoms. Our analysis reveals that at room temperature, UNCD fails via sliding along a grain boundary with a large shear stress. Such sliding triggers crack initiation at an adjacent triple junction and subsequent propagation along an adjacent grain boundary with a large normal stress. With increasing temperature, a crossover from grain sliding to a direct intergranular fracture is observed. The crossover is caused by a different dependence of GB shear and tensile strength on temperature. The present work provides information that may be useful to the design and optimization of the mechanical properties and failure behavior of UNCDs.     

Sintering,microstructure, and mechanical properties of vitrified bond diamond composite

The demand for high-performance grinding wheels is gradually increasing due to rapid industrial development. Vitrified bond diamond composite is a versatile material for grinding wheels used in the backside grinding step of Si wafer production. However, the properties of the vitrified bond diamond composite are controlled by the characteristics of the diamond particles, the vitrified bond, and pores and are very complicated. The main objective of this study was to investigate the effects of SiO₂–Na₂O–B₂O₃–Al₂O₃–Li₂O–K₂O–CaO–MgO–ZrO₂–TiO₂–Bi₂O₃ glass powder on the sintering, microstructure, and mechanical properties of the vitrified bond diamond composite. The elemental distributions of the composite were analyzed using electron probe micro-analysis (EPMA) to clarify the diffusion behaviors of various elements during sintering.The results showed that the relative density and transverse rupture strength of the composite sintered at 620 °C were 91.7% and 126 MPa, respectively. After sintering at 680 °C, the glass powder used in this study exhibited a superior forming ability without an additional pore foaming agent. The relative density and transverse rupture strength of the composite decreased to 48.2% and 49 MPa, respectively. Moreover, the low sintering temperature of this glass powder protected the diamond particles from graphitization during sintering, as determined by X-ray diffraction and Raman spectrum. Furthermore, the EPMA results indicate that Na diffused and segregated at the interface between the diamond particles and vitrified bond, contributing to the improved bonding. The diamond particles can remain effectively bonded by the vitrified bond even after fracture.     

The effects of the size of nanocrystalline materials on their thermodynamic and mechanical properties

This work has considered the intrinsic influence of bond energy on the macroscopic, thermodynamic, and mechanical properties of crystalline materials. A general criterion is proposed to evaluate the properties of nanocrystalline materials. The interrelation between the thermodynamic and mechanical properties of nanomaterials is presented and the relationship between the variation of these properties and the size of the nanomaterials is explained. The results of our work agree well with thermodynamics, molecular dynamics simulations, and experimental results. This method is of significance in investigating the size effects of nanomaterials and provides a new approach for studying their thermodynamic and mechanical properties.     

Computational study of thermal and mechanical properties of nylons and bio-based furan polyamides

We have investigated thermal and mechanical properties of bio-based furan polyamides and petroleum-based nylons with atomistic simulations. Glass transition temperatures estimated from a series of simulations at different temperatures were in good agreement with experimental measurements. Stress–strain relationships under uniaxial deformation conditions were also obtained and analyzed. Overall, polymers with smaller repeat units exhibited slightly higher glass transition temperatures and elastic moduli, which were attributed to higher cohesive energy densities. Furan polyamides displayed higher van der Waals cohesive energy densities and maintained more rigid planar structures near furan rings compared to nylons. As a result, bio-based furan polyamides showed higher glass transition temperatures and comparable mechanical properties despite having overall weaker hydrogen bonding than nylons.     

Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.     

Molecular dynamics study on mechanical cleavage mechanisms of GaAs and experimental verification

Laser bars are fast becoming a key device in a range of specific industrial applications. Mechanical cleavage technology is a new and efficient method for fabricating laser bars. However, there is little detailed analysis of the relationship between scratching step and cleavage step. To close this research gap, a molecular dynamics study on cleavage mechanisms of GaAs are reported investigating surface and subsurface damage, and molecular dynamics method could accurately describe the nano-scale processes in semiconductors at atomic scale. Simulation results show that the scratching depth has a significant effect on the scratch quality as compared with other parameters during the scratching process. Then, a series of cleavage experiments were carried out to verify the simulation results and further explore the influence of key scratching parameters on cleavage plane morphology. Experimental results correlate well with the simulations. Consequently, the achieved optimal combination of parameters have been found to be a scratching load of 10 g, scratching speed of 20 mm/s and scratching length of 0.6 mm, which provides direct guidance for the cleavage processing of GaAs materials. Under optimal conditions, the length and surface roughness of the undamaged area can reach 11.77 mm and 0.43 nm respectively.     

Effect of particle size and grafting density on the mechanical properties of polymer nanocomposites

End-grafting polymer chains to nanoparticles in polymer nanocomposite is a widely used method to disperse inorganic particles in a polymeric matrix in order to improve the material properties. While many fundamental studies have investigated how various factors influence the dispersion or aggregation of the nanoparticles, the effect of grafting on the resulting material properties has received considerably less attention. In particular, the effect of nanoparticle curvature and grafting density on the mechanical properties in polymer nanocomposites remains elusive. In this study, we develop a coarse-grained model of a polymer glass containing grafted nanoparticles and examine the resulting effects on the mechanical properties. By carefully designing the parameters of our polymer nanocomposites model, we can maintain dispersion of the nanoparticles whether they are grafted with polymer chains or not, which allows us to isolate the effect of end-grafting on the resulting mechanical properties. We examine how the nanoparticle size and grafting density affect the elastic constants, strain hardening modulus, as well as the mobility of the polymer segments during deformation. We find that the elastic constants and yield properties are enhanced nearly uniformly for all of our nanocomposite systems, while the strain hardening modulus depends weakly on the grafting density and the nanoparticle size.     

Phase coexistence and grain size effects on the functional properties of BaTiO3 ceramics

The functional properties of a series of BaTiO₃ ceramics, having grain sizes ranging from 75 nm to 2.25 μm, with polymorph coexistence around room temperature are presented. Large temperature ranges of phase coexistence were detected through structural analyses, with no apparent size effect on the extension of co-existence domain, while the transition temperatures vary with grain size. Permittivity values are among the highest reported and the typical maximum around 1 μm grain size is confirmed by low field measurements and sub-switching Rayleigh analysis. An interesting feature not reported elsewhere is the persistence of permittivity maximum above Curie temperature and under high dc field at saturation, where domain walls contribution is minimal, raising questions about the largely accepted interpretation concerning the domain walls role on permittivity maximum. The planar defects in the starting powders give rise to extended strained defects in the ceramics, impacting their functional properties and multi-phase character.     

Effect of SiC nanowires on the mechanical properties and thermal conductivity of 3D-SiCf/SiC composites prepared via precursor infiltration pyrolysis

In this study, SiC nanowires (SiC_NWS) were grown in situ on the surface of PyC interface through chemical vapor infiltration (CVI) to improve the mechanical characteristics and thermal conductivity of three-dimensional SiC_f/SiC composites fabricated via precursor infiltration pyrolysis (PIP). The effect of SiC_NWS on the properties of the obtained composites was investigated by comparing them with conventional SiC_f/PyC/SiC composites. After the deposition of SiC_NWS, the flexural strength of the SiC_f/SiC composites was found to increase by 46 %, and the thermal conductivity showed an obvious increase at 25?1000 °C. The energy release of the composites in the damage evolution process was analysed by acoustic emission. The results indicated that the damage evolution process was delayed owing to the decrease in porosity, the crack deflection and bridging of the SiC_NWS. Furthermore, the excellent thermal conductivity was attributed to the thermally conductive pathways formed by the SiC_NWS in the dense structure.     

Online monitoring of laser remelting of plasma sprayed coatings to study the effect of cooling rate on residual stress and mechanical properties

This investigation deals with laser remelting of plasma sprayed alumina and chromia coatings. The time-temperature history of the laser remelted zone was recorded using an infrared pyrometer during the remelting operation. Cooling rates, under varying scanning speed, were determined from the time temperature curve. Surface morphology, microstructure, and phases of the laser treated and as-sprayed coatings were characterized using scanning electron microscopy, optical microscopy, X-ray diffraction, respectively. X-ray diffraction was also employed to measure the surface residual stress of the coatings. Inherent features of plasma sprayed coatings like porosity and inter-lamellar boundary were obliterated upon laser remelting. A columnar grain growth perpendicular to the laser scanning direction was observed. The range of roughness of the as-sprayed coatings reduced from 6 to 8?µm to 1–2?µm in the remelted layers. For both coatings, more than 90% reduction in porosity was found upon laser remelting. Surface residual stress of the as-sprayed alumina and chromia coatings was found to be tensile and compressive, respectively. Within the limits of the testing condition the tensile residual stress of the remelted layers increased by up to around 500% in the alumina coatings. In the chromia coating a decrease of compressive stress by up to around 80% was recorded. In the remelted layer the tensile nature of the stress showed a tendency to increase with an increase in the cooling rate. However, the state of stress of the as-sprayed layer, i.e., tensile or compressive, was retained in the remelted layer. The residual stress was found to decrease in the remelted layer with an increase in the degree of overlap of the remelted tracks.     

ANPyO在不同温度下晶体感度和力学性能的分子动力学模拟

NPT系综下,用COMPASS力场对ANPyO超晶胞及沿其（4,0,-2）晶面切割的两种模型分别进行不同温度（195、245、295、345、395 K）下的分子动力学模拟。结果表明,随着温度的升高,ANPyO引发键最大键长递增,引发键双原子作用能和内聚能递减,这与炸药感度随温度升高而增大的事实相一致,一定条件下它们可作为炸药感度判定的理论依据。获得了5个温度下ANPyO和ANPyO（4,0,-2）的力学性能,从理论上揭示了其力学性能随温度递变的规律。     

Cleaner preparation of poly(vinylidene fluoride)/barium titanate composites: Thermal,mechanical, and dielectric properties and relaxation dynamics

Traditional polymer composite preparation techniques often employ organic solvents, which can damage the environment, to disperse inorganic fillers. In this article, classic nanocomposites with poly(vinylidene fluoride) (PVDF) polymer matrices and BaTiO₃ nanoparticle (BTP) fillers were created by a clean method combining planetary ball milling with an ultracentrifugal mill and then hot pressed into thin films. The microstructures, properties and relaxation dynamics of the thin films were characterized and analyzed. Scanning electron microscopy results demonstrated that BTP was homogeneously dispersed in the PVDF matrix. The thermal, mechanical, and dielectric properties were comparable to those of composite films prepared by solution mixing. Dielectric analysis revealed that the dielectric constant of the thin films reached 14 (10⁴ Hz) when the volume fraction of BTP was 30%; however, the dielectric loss was 0.1 (10⁴ Hz). Additionally, the dielectric loss spectra fitted with the Havriliak−Negami (H−N) and Vogel Fulcher equations were employed to analyze the relaxation dynamics of the nanocomposites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47254.     

Molecular dynamics study on thermal and mechanical properties of AOO modified DGEBA-anhydride insulating materials for high voltage GIS

Increases in rated voltage and power density of high voltage gas insulated switchgears (GIS) impose stringent requirements on the diglycidyl ether of bisphenol A (DGEBA)-anhydride thermoset, which is widely used as insulation materials for high GIS, for higher mechanical and heat-resistant properties. In this paper, 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (AOO) was copolymerized with DGEBA -anhydride system to increase the mechanical and thermal properties of the thermosets. Based on the molecular dynamics simulation method, the coefficient of thermal expansion, glass transition temperature (T_g), and modulus of AOO copolymer-modified DGEBA-anhydride thermosets were calculated. The AOO copolymer-modified DGEBA-anhydride thermosets specimens were further experimentally prepared and subjected to differential scanning calorimeter test, TGA test, tensile bending test, and dielectric test. The results of simulation and experiment simultaneously indicated that the T_g and modulus of the DGEBA-anhydride thermoset are enhanced after AOO modification. This is mainly due to the decrease in free volume percentage and mean square displacement of the epoxy cross-linked network structure after the introduction of AOO molecules, which makes the structure more compact and the cross-link density increased. Additionally, the dielectric performances of the thermoset was enhanced by AOO. The excellent thermal, mechanical and dielectric properties of AOO modified DGEBA-anhydride thermoset make it a promising application in the field of high voltage GIS.     

Elasticity,mechanical and thermal properties of submicron h-AlN: in-situ high pressure ultrasonic study

Compressional (V_P) and shear (V_S) wave velocities of high pressure (P) and temperature (T) sintered submicron hexagonal aluminum nitride (h-AlN) have been measured up to 15 GPa using ultrasonic interferometry. Using finite strain equation-of-state approaches, yielding K_S0 = 293.1 ± 2.9 GPa, G₀ = 141.9 ± 1.4 GPa and K′_S = 3.72 ± 0.04, G′ = 0.52 ± 0.02. It is found that the Vickers hardness of submicron h-AlN is as high as 11.7 GPa, and its fracture toughness is up to ～4.25 MPa m^1/2. Meanwhile, based on the elastic wave velocities and density data, the elastic-related properties of h-AlN are derived as Debye temperature Θ₀ = 1006 K, Grüneisen parameter γ = 1.72 and acoustic thermal conductivity λ_acoustic = 196 W/mK. These results may provide a comprehensive elasticity and elastic-related properties of submicron h-AlN under high pressure, which may be of great importance for its uses at extreme conditions.     

Effect of sintering temperature on the morphology,crystallinity and mechanical properties of carbonated hydroxyapatite (CHA)

Effect of sintering temperature on the physical and mechanical properties of synthesized B-type carbonated hydroxyapatite (CHA) over a range of temperature in CO₂ atmosphere has been investigated. The B-type CHA in nano size was synthesized at room temperature by using a direct pouring wet chemical precipitation method. The synthesized CHA powders were subsequently consolidated by sintering treatment from 800 to 1100 °C. The sintered CHA samples were evaluated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry, X-ray fluorescence (XRF), carbon-hydrogen-nitrogen-sulfur-oxygen (CHNS/O) elemental analyzer, Field emission scanning electron microscopy (FESEM), and Vicker's indentation technique. The results obtained from XRD and FESEM indicated that the synthesized B-type CHA powders were nanometer in size. The crystallinity and crystallite size of the sintered CHA samples were increased due to increasing sintering temperature. The heat treatment between 800 °C and 1000 °C has resulted in coarsening and increased hardness of the sintered CHA samples. However, these properties began to deteriorate when sintering beyond 1100 °C due the formation of calcium oxide.     

First-principles study on predicting the crystal structures,mechanical properties and electronic structures of HfCxN1-x

The crystal structures, mechanical properties and electronic structures of HfC_xN_1-x have been predicted by using evolutionary structure search followed by the first-principles calculations in this study. The crystal structures predicted indicate that there are 10 thermodynamic stable phases for HfC_xN_1-x, of which 8 are newly discovered crystal structures and 2 are already known. We investigated the mechanical properties, including the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and Vickers hardness, of all 10 stable phases, and further established the relationship between such properties and the ratio of nitrogen to carbon content. Besides, the Fermi energy level and electronic density of states of these 10 stable phases are calculated as well, and the results reveal the fundamental reason why the mechanical properties change with the ratio of nitrogen to carbon. The predictions of this study agree well with both the experimental data and the previous theoretical evaluations.     

The effects of fine WC contents and temperature on the microstructure and mechanical properties of inhomogeneous WC-(fine WC-Co) cemented carbides

Inhomogeneous WC-(fine WC-Co) cemented carbides with improved hardness and toughness were successfully prepared through the addition of fine WC using planetary ball milling combined with sinter isostatic hot pressing (SHIP) technology. The inhomogeneous microstructure of the alloys consisted of coarsened WC grains and WC-Co consisting of fine WC dispersoids and Co binder phase. The increase of temperature and the addition of fine WC enhanced the sintering process. The morphologies of the coarsened WC and of the fine WC consisted of triangular and near-hexangular prisms, respectively. Due to crack path deflection and crack bridging, the prism-like coarsened WC crystals efficiently hindered cracks propagation. Intergranular fracture became predominant when adding fine WC. However, the excessively coarsened WC and some pores in alloys with 20 wt% fine WC could decrease the mechanical properties. The inhomogeneous WC-(fine WC-Co) cemented carbides with 10 wt% fine WC, sintered at 1430 °C for 40 min, could provide a combination of superior hardness and toughness.     

Correlation between IR peak position and bond parameter of silica glass: Molecular dynamics study on fictive temperature (cooling rate) effect

The fictive temperature of glass is a consequence of its thermal history (cooling rate, primarily) and has a direct effect on physical and chemical properties of the glass. But, it is not easy to measure. The ability to nondestructively and spectroscopically measure it at room temperature would be of great benefit. Although empirical correlations have been established between fictive temperature and selected absorption peaks in the infrared spectra of silica glass, the fundamental understanding for this correlation has not been reported. Here, we use molecular dynamics simulations to show that the blue shift in the Si–O–Si asymmetric stretching peak of pure silica glass, which is known to correlate with a decrease in fictive temperature, can be attributed to a decrease in the average length of the Si–O bond in the silica network, not changes in the density or the Si–O–Si bond angle. The decrease in density at higher fictive temperatures of silica is associated with a decreased population of 5‐ and 6‐membered rings and broadening of the ring‐size distribution, and an increase in the average Si–O–Si bond angle.     

Nano‐crystalline cellulose,chemical blowing agent,and mold temperature effect on morphological,physical/mechanical properties of polypropylene

Polypropylene (PP)/nano‐crystalline cellulose (NCC) composites and foams were produced through extrusion compounding combined with injection molding. From the samples produced, a complete morphological, physical, and mechanical analysis was performed to study the effect of NCC concentration (0–5wt %), foaming agent content (0 to 2wt %) and mold temperature (30°C and 80°C). NCC was very effective to reduce cell size (42–71%) and increase cell density (5–37 times) of the foams, while slightly increasing the overall density (2–7%). The results showed that NCC addition increased the specific tensile modulus (15–22%), specific tensile strength (1–14%) and specific flexural modulus (13–26%) of PP, but decreased specific impact strength (10–20%) and specific elongation at break (50–96%). © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42845.