Enhanced thermal–mechanical properties of polymer composites with hybrid boron nitride nanofillers

The present work focuses on the investigation of the thermal–mechanical properties of the epoxy composites with hybrid boron nitride nanotubes (BNNTs) and boron nitride nanosheets (BNNSs). The stable dispersions of BNNTs–BNNSs were achieved by a noncovalent functionalization with pyrene carboxylic acid. The resulting epoxy/BNNTs–BNNSs composites exhibited homogeneously dispersed BNNTs–BNNSs and a strong filler–matrix interface interaction. The composites showed a 95 % increase in thermal conductivity and a 57 % improvement in Young’s modulus by addition of only 1 vol. % BNNTs–BNNSs. Meanwhile, the composites also retained a high electrical resistance of pure epoxy. Our study thus shows the potential for hybrid BNNTs–BNNSs to be successfully used as the nanofillers of polymer composites for applications in electrically insulating thermal interface materials.     

Electrical conductivity and equations of states of β-rhombohedral boron in the megabar dynamic pressure range

The pressure dependence of the conductivity of boron under conditions of a stepwise shock compression of megabaric range is studied. With this purpose, the following problems have been solved. The conductivity of boron has been measured in the range of dynamic pressures, where boron has different high-pressure phases. The equations of state of β-rhombohedral and amorphous boron have been constructed in a megabaric pressure range. The thermodynamic states of boron in the conditions of these experiments are calculated, which, in combination with the measurement data, made it possible to determine the change in the boron conductivity in the conditions of strong stepwise shock compression at dynamic pressures to 110 GPa. The increase in the conductivity of polycrystalline boron at megabar pressures is interpreted as a result of a nonmetal–metal transition.

     

Production of nanosized aluminum powders and their application as fillers for composite materials based on ultrahigh molecular weight polyethylene (UHMWPE)

A modified levitation-in-flow Gen-Miller technique is developed to produce nanodisperse aluminum particles 40–200 nm in size with a barrier coating of controlled thickness made of aluminum oxides and (oxy)nitrides for the use as a dielectric filler in composite materials. The polymerization filling technique (in situ polymerization) is used to synthesize new highly filled nanocomposite materials based on UHMWPE and nanodisperse aluminum with the barrier coating combining dielectric, heat-conducting, and plastic properties, which can replace brittle ceramics in a number of fields. The samples of such nanocomposites with a content of nanoaluminum ranging from 20 to 80 wt % were synthesized and characterized by electron microscopy and X-ray diffraction analysis.     

Relation between the Hurst Exponent and the Efficiency of Self-organization of a Deformable System

We have established the degree of self-organization of a system under plastic deformation at different scale levels. Using fractal analysis, we have determined the Hurst exponent and correlation lengths in the region of formation of a corrugated (wrinkled) structure in [111] nickel single crystals under compression. This has made it possible to single out two (micro-and meso-) levels of self-organization in the deformable system. A qualitative relation between the values of the Hurst exponent and the stages of the stress–strain curve has been established.     

Mössbauer study of the polymorphism in iron and iron-nickel alloys under deformation and high pressure

Structural and phase transitions in iron and binary iron-nickel alloys containing 6–32 at % nickel have been investigated by Mössbauer spectroscopy in situ under quasi-hydrostatic compression and compression shear. It is shown that the deformation decreases the onset pressure of the α-phase transition to the high-pressure ∈ and γ phases. Shear changes the transformation mechanism due to the formation of a large number of structural defects; it is an independent parameter of the phase transition kinetics. Compression shear leads to an irregular change in the hyperfine parameters of the initial and newly formed phases, which is due to the stress relaxation as in the trip effect.     

Ab initio study on the anisotropy of mechanical behavior and deformation mechanism for boron carbide

The mechanical properties and deformation mechanisms of boron carbide under a-axis and c-axis uniaxial compression are investigated by ab initio calculations based on the density functional theory.Strong anisotropy is observed.Under a-axis and c-axis compression,the maximum stresses are 89.0 GPa and 172.2 GPa respectively.Under a-axis compression,the destruction of icosahedra results in the unrecoverable deformation,while under c-axis compression,the main deformation mechanism is the formation of new bonds between the boron atoms in the three-atom chains and the equatorial boron atoms in the neighboring icosahedra.     

Effect of carbonization temperature on the microplasticity of wood-derived biocarbon

The uniaxial compression strength under stepped loading and the 325-nm-stepped deformation rate of biocarbon samples obtained by carbonization of beech wood at different temperatures in the 600–1600°C range have been measured using high-precision interferometry. It has been shown that the strength depends on the content of nanocrystalline phase in biocarbon. The magnitude of deformation jumps at micro- and nanometer levels and their variation with a change in the structure of the material and loading time have been determined. For micro- and nanometer-scale jumps, standard deviations of the differences between the experimentally measured deformation rate at loading steps and its magnitude at the smoothed fitting curve have been calculated, and the correlation of the error with the deformation prior to destruction has been shown. The results obtained have been compared with the previously published data on measurements of the elastic properties and internal friction of these materials.     

B掺入Cu∑5晶界间隙位性质的第一性原理研究

本文采用第一性原理方法对清洁Cu∑5晶界与有B掺杂 到间隙位的Cu∑5晶界进 行了拉伸和压缩的模拟研究. 结果分析表明, Cu∑ 5晶界结合因B的掺入得到加强. 清洁Cu∑5晶界处因有较大空隙而存在电子密度低的区域, 晶界结合相对较弱, 在拉伸过程中晶界从其界面处开始断裂. 有B掺杂在间隙位的Cu∑5晶界电子由Cu向Cu-B间积聚, 晶界结合相对较强, 拉伸时晶界从其近邻原子层开始断裂. 在形变小于20%的压缩过程中, B的掺入未对晶界产生明显影响. 关键词： 第一性原理 Cu晶界 B掺杂 拉伸压缩     

Crystallographic texture and microstructure evolution during hot compression of Ti–6Al–4V–0.1B alloy in the (α + β)-regime

Microstructure and texture are known to undergo drastic modifications due to trace hypoeutectic boron addition (~0.1 wt.%) for various titanium alloys e.g. Ti–6Al–4V. The deformation behaviour of such an alloy Ti–6Al–4V–0.1B is investigated in the (α?+?β) phase field and compared against that of the base alloy Ti–6Al–4V studied under selfsame conditions. The deformation microstructures for the two alloys display bending and kinking of α lamellae in near α and softening via globularization of α lamella in near β phase regimes, respectively. The transition temperature at which pure slip based deformation changes to softening is lower for the boron added alloy. The presence of TiB particles is largely held attributable for the early softening of Ti–6Al–4V–0.1B alloy. The compression texture of both the alloys carry signature of pure α phase defamation at lower temperature and α→β→α phase transformation near the β transus temperature. Texture is influenced by a complex interplay of the deformation and transformation processes in the intermediate temperature range. The contribution from phase transformation is prominent for Ti–6Al–4V–0.1B alloy at comparatively lower temperature.     

Amorphization and a Polymorphic Transformation of Boron Stimulated by High Dynamic Pressures

Structural transformations of polycrystalline boron have been studied under megabar multiple-shock compression. Shock-wave experiments on the compression and subsequent recovery of polycrystalline samples of β-rhombohedral boron β-B₁₀₆ have been performed. Thermodynamic states of boron under the conditions of the performed experiments have been calculated. According to these calculations, the maximum pressure in the samples is 115 GPa. X-ray diffraction analysis of the stored boron samples has been performed. It has been shown that the dynamic compression of polycrystalline β-B₁₀₆ to 115 GPa results in its partial amorphization and transformation to the tetragonal modification of boron T-B₁₉₂.     

Formation of three-dimensional arrays of magnetic clusters NiO,Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> by the matrix method

A method has been proposed for the formation of three-dimensional arrays of isolated magnetic clusters NiO, Co₃O₄, and NiCo₂O₄ in the sublattice of pores in the matrix of bulk synthetic opals through a single impregnation of the pores with melts of nickel and cobalt nitrate crystal hydrates and their thermal degradation. The method makes it possible to controllably vary the degree of filling of pores in the matrix with oxides within 10–70 vol %. The composition and structure of the synthesized materials, as well as the dependences of their static magnetic susceptibility on the magnetic field strength, have been investigated.     

Characterization of the tensile behaviour of a co-woven-knitted composite in the continuous and discrete frequency domain

Laplace-transform and Z-transform theories have been applied to analyze the tensile stress–strain curves of a co-woven-knitted (CWK) composite under quasi-static (0.001/s) and high strain rates (up to 2586/s) tension. The transform results were extended to characterize the tension failure and dynamic responses of the CWK composite in the frequency domain. Specifically, the Laplace-transform theory was employed to analyze the stress–strain curves of the CWK composite along 0°, 45° and 90° directions when the composite is assumed to be a continuous system, while the Z-transform theory was used for the discrete system for the composite. From the transformed results, it was found that the stress–strain curves of the CWK composite specimen under different strain rates tension have similar stability behaviours for the Laplace- and Z-transform. For the continuous system, few pole plots are distributed on the left side of the imaginary axis, which means the system is unstable. Nevertheless, the pole-plot distribution is stable before the post-critical deformation of the CWK composite. For the discrete system, most of the poles are located inside the unit circle before post-critical deformation, indicating the system is stable. From the stiffness–time history and fracture morphology, the stability of the pole-plot distribution corresponds to the stiffness stability and fracture uniformity. From continuous and discrete system analyses, it is found that the stress–time and strain-time histories of the CWK composite can be regarded as a digital signal system. Digital signal processing (DSP) methods can be extended to the investigation of the mechanical behaviour of composites.     

On the thermal expansion of CNT/Cu composites for electronic packaging applications

Carbon nanotubes (CNTs) exhibit both excellent high thermal conductivity and low coefficient of thermal expansion (CTE), which are an ideal reinforcement in composite materials for high performance electronic packaging applications. In the present study, CNT/Cu composites containing CNTs varying from 0 vol.% to 15 vol.% are prepared, and their CTE behavior is studied in detail. The results indicate that the CTE of 0–10 vol.% CNT/Cu composites is significantly decreased with increasing CNT content. However, as the CNT content increases to 15 vol.%, the decrease in CTE of the composites is pronouncedly reduced. Possible mechanisms are discussed in combination with CTE model predictions.     

Bacterial cellulose: the ultimate nano-scalar cellulose morphology for the production of high-strength composites

High-strength composites were produced using bacterial cellulose (BC) sheets impregnated with phenolic resin and compressed at 100 MPa. By utilizing this unique material synthesized by bacteria, it was possible to improve the mechanical properties over the previously reported high-strength composites based on fibrillated kraft pulp of plant origin. BC-based composites were stronger, and in particular the Youngs modulus was significantly higher, attaining 28 GPa versus 19 GPa of fibrillated pulp composites. The superior modulus value was attributed to the uniform, continuous, and straight nano-scalar network of cellulosic elements oriented in-plane via the compression of BC pellicles. PACS 81.05.Lg; 81.05.Qk     

Simulation of particulate-filled composite deformation diagrams on the basis of a constitutive model of large plastic deformation for a polymer matrix

Deformation diagrams of particulate-filled polymers have been calculated on the basis of specific constitutive equations [1] for large plastic deformation of the polymer. Composite structure is represented by the Hashin polydisperse model [2]. Original finite-element (FE) code with triangular elements has been elaborated and used for the numerical solution of boundary value problems. Local achievement of a critical value by the elastic main strain was used as a fracture criterion. Engineering (force-elongation) diagrams were found to exhibit maxima for arbitrary filler fraction if the interfacial bond was perfect and for low loading at zero adhesion. Stress-strain diagrams with a yield maximum and draw minimum provide macroscopic neck-type localization. Further, the loading in the case of facilitated deboading results in the diminution of the difference between maximum and minimum drawing forces and then in the disappearance of the latter, which in turn provides the transition from localized to macrouni-form deformation. Young's modulus and the yield stress increase with filling in the case of absolute adhesion and decrease in the opposite case. Ultimate elongation sharply drops with an increase in filler fraction, and embrittlement occurs at a small fraction of inorganic particles if a perfect interfacial bond is present. Contrary, a decrease in ultimate elongation is much more gradual, and composites conserve ductile properties of the matrix up to a high portion of inclusions. The laws found qualitatively coincide with what is observed for real materials.     

Validation of the rotational vector Preisach model with measurements and simulations of vectorial minor loops

Carbon nanotube reinforced Cu–Ti alloy (CNT/Cu–Ti) composites are fabricated by a powder metallurgical method. The interfacial bonding of CNT/Cu–Ti composites is evidently improved, which is attributed to the formation of a thin layer of TiC at the interface. The thermal conductivity of the composites increases by 7.5 % and 15.1 % compared to that of Cu–Ti matrix at CNT loadings of 5 vol.% and 10 vol.%, respectively. The matrix-alloying is therefore an effective way to enhance the thermal conductivity of CNT/Cu composites.     

Performance of glass woven fabric composites with admicellar-coated thin elastomeric interphase

Adequate stress transfer between the inorganic reinforcement and surrounding polymeric matrix is essential for achieving enhanced structural integrity and extended lifetime performance of fiber-reinforced composites. The insertion of an elastomeric interlayer helps increase the stress-transfer capabilities across the fiber/matrix interface and considerably reduces crack initiation phenomena at the fiber ends. In this study, admicellar polymerization is used to modify the fiber/matrix interface in glass woven fabric composites by forming thickness-controlled poly(styrene-co-isoprene) coatings. These admicellar interphases have distinct characteristics (e.g. topology and surface coverage) depending on the surfactant/monomer ratios used during the polymerization reaction. Overall, the admicellar coatings have a positive effect on the mechanical response of resin transfer molded, E-glass/epoxy parts. For instance, ultimate tensile strength of composites with admicellar sizings improved 50–55% over the control-desized samples. Interlaminar shear strength also showed increases ranging from 18 to 38% over the same control group. Interestingly, the flexural properties of these composites proved sensitive to the type of interphase formed for various admicellar polymerization conditions. Higher surface coverage and film connectedness in admicellar polymeric sizings are observed to enhance stress transfer at the interfacial region.     

The friction and wear properties of divinylbenzene-grafted UHMWPE fiber-filled serpentine/PTFE composite

Divinylbenzene-grafted Ultra-high-molecular-weight polyethylene (UHMWPE) fibers were used to reinforce the Polytetrafluoroethylene (PTFE) composite and the friction and wear behaviors of UHMWPE/PTFE composite were studied on the ring-block machine under vacuum condition. The worn surfaces of specimens were investigated using scanning electron microscopy and energy dispersive spectroscopy (EDS). The results showed that the friction coefficient and temperature of UHMWPE/PTFE composites with surface-treated UHMWPE fiber were apparently lower than that with untreated one. In conclusion, the surface treatment favored the improvement of the higher interface strength and so had good effect on improving the tribological properties of the composites. The dominant wear mechanisms were adhesion wear, plastic deformation, brittle facture, and spalling. The EDS analysis of the worn surface indicated the trend of the tribochemical reaction of the Fe related to the transfer of the PTFE.     

基于流场模拟的涡旋压缩机涡齿应力变形分析

为了更准确地计算涡旋压缩机涡旋齿的应力和变形,提出一种基于流场模拟的应力变形计算方法。通过对涡旋压缩机工作过程进行气体流动的数值模拟,得到其压力场和温度场分布。将流场分布作为载荷边界条件进行涡旋齿的受力变形计算,得到涡旋齿在气体压力、热载荷及其同时作用下的应力分布和变形规律。分析了涡旋齿的径向变形和轴向变形,讨论了不同主轴转角下的涡旋齿应力和变形,比较了动涡旋齿和静涡旋齿的变形。结果表明:在压缩结束时刻动涡旋齿的应力和变形最大,最大应力位于齿头根部,最大变形位于齿头顶部。     

Micromechanical analysis of transverse damage of fibre-reinforced composites

The mechanical behaviour of fibre-reinforced composites under transverse tension, compression and shear is studied using computational micromechanics. The representative volume element is constructed for fibre’s random distribution. The Drucker–Prager model and cohesive zone model are used to simulate the matrix damage and interfacial debonding, respectively. The stress distribution along the interface is studied using the model with only one fibre embedded in the matrix. It is found that the interface tensile failure at the equators of fibre firstly occurs under transverse tension; the interface shear failure firstly occurs under transverse compression; both the interface tensile failure and shear failure occur under transverse shear. The direction of fracture plane is perpendicular to the loading direction under transverse tension, 52.5° with the perpendicular direction under compression and 7.5° with the perpendicular or vertical direction under shear, respectively.