聚合物基功能梯度复合材料的研究进展

介绍了聚合物基功能梯度材料的概念和分类,综述了其制备研究进展(包括非平衡溶胀法,扩散共聚法,电场诱导法,离心法,纤维排列法)及其在航空,生物医学,光学工程领域的应用,并对其发展趋势作出了展望。     

离心法制备环氧树脂／碳纤维梯度功能材料——结构与耐磨性

梯度功能材料（functionally gradient materials,简称FGM）,又常称倾斜功能材料,严格地说应称为”梯度功能复合材料”,本文扼要介绍了用离心法制得的环氧树脂／短碳纤维梯度功能材料的结构及耐磨性能。     

Graphite–epoxy graded material by centrifugation

A graded distribution of graphite particles in epoxy resin matrix was obtained using a centrifugation technique. By varying the centrifugation time the graded profile could be effectively controlled. Scanning electron microscopy and optical microscopy revealed the graded dispersion of graphite particles in the epoxy matrix, which is sensitive to centrifugation time. Electrical or wear properties can accurately estimate the property profile of graded material. The abrasive wear test also provided a quick estimation of the extent of gradient formed in the sample. The increased centrifugation time increased the compaction of graphite particles in the graphite‐rich phase of graded material that could be correlated with the increased capacitance of the sample. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 550–556, 2005     

Experimental additive manufacturing of green body of SiC/Graphite functionally graded materials by stereolithography

Ceramic stereolithography (CSL)-additive manufacturing (AM) technology is used to create a functionally graded ceramic (FGC) green body made of silicon carbide (SiC) and graphite. For the SiC/graphite FGC, the mixing parameters of ceramics powders and ultraviolet (UV) curing resin are improved, and correlations of the resultant slurry curing depth with integrated light intensity are discovered. Therefore, the SiC/graphite FGC-produced green body has no flaws, pores, or cracks on its surfaces. According to the association between cure depth and integrated light density for each slurry's composition, several interfacial collapses discovered in a cracked cross-section might be decreased.     

Electrochemical processing of porosity gradients for the production of functionally graded materials

The present paper lays the theoretical foundations of a new production process for functionally graded materials (FGMs). The process is based on the evolution of porosity gradients in porous electrodes which undergo electrochemical dissolution or deposition. The electrodes with graded porosity serve as preforms for the production of graded composites by infiltration processing. A one-dimensional macroscopic model of the porous electrode has been used for the prediction of the porosity gradients. A numerical approach allows utilization of experimentally determined current–potential curves for nonporous electrodes, with the incorporation of changes of the pore structure during the course of the electrode reaction, to predict the porosity gradients. For porous copper cathodes and anodes the results of this model are compared with experimentally observed polarization behavior and porosity distributions for different current densities and electrolyte conductivities.     

Study on mixed‐mode fracture characterization in functionally graded materials

The fracture characterizations on mixed‐mode crack of functionally graded materials (FGMs) are investigated using digital speckle correlation method (DSCM). The stress intensity factors at mixed‐mode crack tip are obtained from digital speckle displacements fields. In combination with finite elements simulation results, the influences of gradient coefficients on fracture behavior of mixed‐mode cracks are analyzed. All the results show that the influence of gradient coefficients on fracture modes is not noticeable, and the stress intensity factor at the crack tip in graded materials are clearly influenced by the gradient coefficients, i.e., the stress intensity factors decrease with the increasing of gradient coefficients. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Estimation of Concentration of Particles in Polymerizing Fluid during Centrifugal Casting of Functionally Graded Polymer Composites

The concentration of uniformly distributed particles in a fluid changes with time in the direction of gravitational or centrifugal force to form a concentration gradient. The change in the concentration is an outcome of velocity variation of particles in a fluid. A modified equation for terminal velocity, v _m of particles in polymerizing-fluid under centrifugal force is proposed to estimate the changes in the volume fraction of particles in the graded composites. The proposed equation introduces the effect of cure kinetics of polymer and its effect on particle movement in the model that was based on the modified Stoke’s law, considering the parameters related to particle hindrance, centrifugal force, particle dimensions, viscosity variation etc. The model predictions of concentration changes at the different locations of samples were compared with calcium carbonate filled polysulphide-modified-epoxy graded composites prepared by centrifugal casting.. The effect of particle size, delayed curing rate of matrix were explored. The simulated results are in good agreement with those of experiments.     

Thermal residual stresses in adhesively bonded in-plane functionally graded clamped circular hollow plates

This study investigates the effects of in-plane compositional gradient exponent and direction on the thermal residual stress and deformations in adhesively bonded functionally graded clamped circular plates. The material composition was assumed to vary with a power law along an in-plane direction not through the plate thickness direction. The transient heat conduction and Navier equations in polar coordinates describing the two-dimensional thermo-elastic problem were discretized using finite-difference method, and the set of linear equations were solved using the pseudo-singular-value method. The material composition direction is designed as Ceramic-Metal (CM)–CM, CM–Metal-Ceramic (MC), MC–CM, and MC–MC for the inner and outer plates. The temperature decreased radially along the plates, but exhibited a sharp decrease across the adhesive layer. The compositional gradient exponent and direction affected evidently temperature levels and heat transfer period. The compressive radial and shear strains are more effective on the deformation in the adhesive layer and the plate regions near the plate–adhesive interfaces. The adhesive layer is subjected to considerable shear deformations. The equivalent strain and stresses are very low in a large region of the plates but exhibit sharp peaks on the plate regions near the plate–adhesive interfaces, and decrease towards the adhesive interfaces. These stress and strain peaks in the plates and adhesive layer are affected by the compositional gradient and direction. For an outer edge flux, the largest equivalent strain and stresses are observed in the CM–MC joint but the lowest levels occur in the MC–CM or secondly CM–CM joint. In addition, an inner edge flux results in the lowest and highest peak strains and stresses in the MC–CM and CM–MC joints, respectively. The MC–MC and CM–CM joints result in lower temperature, stress and strain levels around the adhesive layer and along the adhesive interfaces for outer and inner edge fluxes, respectively.     

Free vibration analysis of an adhesively bonded functionally graded double containment cantilever joint

In this study, Genetic Algorithms (GAs) combined with the proposed neural networks were implemented to the free vibration analysis of an adhesively bonded double containment cantilever joint with a functionally graded plate. The proposed neural networks were trained and tested based on a limited number of data including the natural frequencies and modal strain energies calculated using the finite element method. GA evaluates a value generated iteratively by an objective function and this value is calculated by the finite element method. The iteration process restricts us apparently to use directly the finite element method in our multi-objective optimisation problem in which the natural frequency is maximised and the corresponding modal strain energy is minimised. The proposed neural networks were used accurately to predict the natural frequencies and modal strain energies instead of calculating directly them by using the finite element method. Consequently, the computation time and efforts were reduced considerably. The adhesive joint was observed to tend vertical bending modes and torsional modes. Therefore, the multi-objective optimisation problem was limited to only the first mode which appeared as a bending mode. The effects of the geometrical dimensions and the material composition variation through the plate thickness were investigated. As the material composition of the horizontal plate becomes ceramic rich, both natural frequency and modal strain energy of the adhesive joint increased regularly. The plate length and plate thickness were more effective geometrical design parameters whereas the support length and thickness were less effective. However, the adhesive thickness had a small effect on the optimal design of the adhesive joint as far as the natural frequencies and modal strain energies are concerned. The distributions of optimal solutions were also presented for the adhesive joints with fundamental joint lengths and material compositions in reference to their natural frequencies and corresponding modal strain energies.     

Stress wave propagation in a functionally graded adhesive layer between two identical cylinders

This study investigates the stress wave propagation in circular aluminum cylinders bonded with a functionally graded adhesive layer subjected to an axial impulsive load. The adhesive joint consists of two identical (aluminum) cylinders and a functionally graded adhesive layer. The volume fractions of the two constituents: aluminum and epoxy in the adhesive layer were functionally tailored through the adhesive thickness by obeying a power-law. Therefore, the effective material properties at any point in the adhesive layer were predicted by the Mori-Tanaka homogenization scheme. The governing equations of the wave propagation in the joint were discretized by means of the finite difference method. The influence of the compositional gradient exponent on the displacement and stress distributions of the joint was examined. It was observed that changing the material composition of the adhesive layer had an evident effect on the displacement and stress levels, especially in the lower cylinder. On the contrary, the influence of the compositional gradient exponent was found to be minor on the displacement and stress distributions. The displacement and stress distributions were also investigated along the upper and lower cylinder-adhesive interfaces. Accordingly, with increasing the ductility of the adhesive layer the waves transmitted to the lower cylinder caused lower displacement levels. The normal stresses become peak at the bottom corners of the upper and lower cylinder-adhesive interfaces whereas the shear stresses concentrate in the middle region of the interfaces. In addition, the temporal variations of the displacement and stress components were evaluated at some critical points of the adhesive and lower cylinder. The compositional gradient exponent played an important role on the displacement and stress levels as well as the wave speeds in the adhesive and lower cylinder rather than in the upper cylinder. The stresses in the joints were observed to be alleviated by employing a functionally graded adhesive layer.     

Preparation of functionally gradient materials via frontal polymerization

Ascending frontal polymerization in a body with a moving boundary was accomplished experimentally. This process was shown to be a steady‐state process within a certain range of the parameters. Temperature profiles of the front were recorded. This new method gives an excellent opportunity to prepare functionally gradient materials because composition of a monomer feedstream can be varied in a programmable manner. Polymer samples with hyperbolic gradients of optical dye concentration were manufactured at ambient pressure and temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2398–2404, 2000     

Understanding centrifugal casting in the manufacture of functionally graded materials

Centrifugal casting is a shaping technique intended for ceramic tubular structure manufacture, where the particle size and density are exploited to produce asymmetrical membranes and functionally graded materials. However, some well-established insights about particle segregation are debatable and remain unclear. For instance, small particles do not necessarily stay in the inner region, and the bigger particles do not accumulate in the outer. Herein several manufacturing parameters were studied through Taguchi’s robust design, using discrete element method-based simulations to generate the data. Alumina powder was used as model material and water as the liquid phase, to assess the green cast formation time, the cast thickness, the roundness, the changes in relative density, and particle size distribution along the cast’s cross-section. The mean particle diameter and the rotation speed are the most influential parameters for the casting time. The volume fraction of solids on the precursor slurry is decisive regarding the cast structural properties, and particle segregation is negligible, except for size differences above one order of magnitude. When a fraction of denser nickel powder was added, density segregation was also observed, but the size differences can overshadow its effect. In addition, alumina and nickel particles were cast in a lab-scale centrifuge and experimentally validated the segregation of particles. The centrifugal casting method was successfully applied for producing the Al₂O₃-Ni composites with a gradient distribution of the Ni phase.     

Cobalt doped glass for the fabrication of percolated glass–alumina functionally graded materials

The present research was focused on the development of a new glass to produce glass–alumina FGMs. The glass formulation, belonging to the CaO–ZrO₂–SiO₂ system, was doped with cobalt, by adding a small molar percentage (about 0.1 mol%) of CoO, in order to obtain a blue glass, which could be useful to appreciate the final compositional gradient. The glass was accurately characterized, evaluating its thermal behaviour, its mechanical properties, and its attitude to crystallize during a thermal treatment. Subsequently, the glass was used to produce glass–alumina FGMs via percolation and the so obtained specimens were analysed in order to evaluate the effect of the glass infiltration. The possible development of new crystal phases, in particular, was tested via micro X-ray diffraction and the elastic properties gradient associated with the compositional gradient was measured via depth-sensing Vickers microindentation.     

One-step Sinter-HIP method for preparation of functionally graded cemented carbide with ultrafine grains

Ultrafine crystalline functionally graded cemented carbides (FGCCs) with a surface zone enriched in binder phase were prepared by a one-step Sinter-HIP method. The influence of sintering pressure and cubic carbide composition on the formation of gradient layer was examined. The results show that the ultrafine FGCC with surface zone enriched in binder phase can be formed by the one-step Sinter-HIP method. The process of the gradient layer formation is accelerated under higher sintering pressure; the gradient layer thickness increases with the sintering pressure increasing. The gradient layer thickness is controlled by diffusion distance of cubic carbide formers, such as Ti, Ta and Nb. The addition of (Ta,Nb)C leads to decrease the thickness of gradient layer.     

Fabrication process of smooth functionally graded materials through a real-time inline control of the component ratio

Functionally graded materials (FGMs) play an essential role in tissue engineering because of their satisfactory histocompatibility and excellent mechanical performance. While traditional manufacturing methods allow production of simple FGMs, precise control of composition and customized property at transition between the dissimilar materials is still a challenge. Here, an extrusion-based functionally graded additive manufacturing (FGAM) platform was developed to generate smooth graded parts by thrusting out monolithic cylindrical filaments with high viscosity. Furthermore, the rheological properties, hydrodynamic behavior, and mixed homogeneity of the non-Newtonian fluids were studied. Therefore, the appropriate solid contents, alternative energy-efficient mixers, and optimized printing parameters were proved to be beneficial for an outstanding deposition effect of the suspension. Ultimately, an object with smooth gradient was successfully manufactured. The validity of this strategy was verified via optical microscopy combined with an image processing method to gauge homogeneity and a scanning electron microscope to investigate graded composition and microstructure.     

电子塑封用脂环族环氧树脂研究进展

结合电子封装的现状、电子塑封材料的发展以及电子封装对塑封材料提出的高性能要求,介绍了新型脂环族环氧树脂及其作为塑封材料的应用前景,其包括耐热型液体、含磷三官能团型、有机硅多官能团脂环族环氧树脂。同时简略介绍了脂环族环氧树脂增韧改性研究动向。     

旋风分离器内颗粒质量浓度分布数值模拟

采用颗粒随机轨道模型和单元内颗粒源法,对旋风分离器内不同粒径颗粒质量浓度分布进行了数值模拟。结果表明,粒径较小的颗粒(dp≤4μm)大部分在旋风分离器分离空间锥段进行分离,而较大颗粒(dp>4μm)大部分在环形空间与分离空间筒段即被分离。随着颗粒粒径增加,分离器外壁的颗粒质量浓度逐渐呈螺旋灰带分布,内旋流夹带减小,环形空间顶板下方出现顶灰环。升气管入口0.25D(筒体直径)附近的短路流对小颗粒的影响较大。在分离空间下部排尘口附近0.5D有明显的颗粒返混,返混量随着颗粒粒径增大而减少。     

Preparation of epoxy resin elastic particles and their application in oil well cement

To solve the problem of high brittleness and easy cracking of cement stone, epoxy resin powder elastic particles (HEEP) were developed to improve the elasticity of cement stone. HEEP was prepared from epoxy resin and trimeric anhydride by solution polymerization. The surface wettability and particle size of HEEP were analyzed by contact angle test and laser particle size analysis respectively. Mechanical performance tests, scanning electron microscope (SEM) analysis and x-ray diffraction analysis were conducted on the cement stone containing HEEP. The results showed that HEEP was the target product, with a hydrophilic surface and a water contact angle of 75.2° on the HEEP surface. The particle size distribution of HEEP was uniform, and the median particle size was 36.5 μm. The initial decomposition temperature of HEEP was 300°C, indicating a good thermal stability. The stress–strain curve showed that when the HEEP dosage was 3%, 6%, and 9%, the elastic modulus of the cement stone was 6.382, 4.017, and 3.148 GPa, respectively. Compared with the blank cement without HEEP, the elastic modulus was reduced by 28.3%, 54.9%, and 64.6%, respectively. This showed that HEEP could effectively improve the elasticity of cement stone. In addition, the compressive strength and flexural strength of cement stone containing HEEP can meet the requirements of on-site applications. SEM analysis of cement stone showed that HEEP was well filled in cement stone. When the cement stone was subjected to complex stress, HEEP could effectively buffer and disperse the stress, thereby enhancing the resistance of cement stone to external force impact.     

CFD modelling of a mixing chamber for the realisation of functionally graded scaffolds

Biological tissues are characterised by spatially distributed gradients, intricately linked with functions. It is widely accepted that ideal tissue engineered scaffolds should exhibit similar functional gradients to promote successful tissue regeneration. Focusing on bone, in previous work we proposed simple methods to obtain osteochondral functionally graded scaffolds (FGSs), starting from homogeneous suspensions of hydroxyapatite (HA) particles in gelatin solutions. With the main aim of developing an automated device to fabricate FGSs, this work is focused on designing a stirred tank to obtain homogeneous HA–gelatin suspensions. The HA particles transport within the gelatin solution was investigated through computational fluid dynamics (CFD) modelling. First, the steady-state flow field was solved for the continuous phase only. Then, it was used as a starting point for solving the multi-phase transient simulation. CFD results showed that the proposed tank geometry and setup allow for obtaining a homogeneous suspension of HA micro-particles within the gelatin solution.     

In-vitro electrochemical and bioactivity evaluation of SS316L reinforced hydroxyapatite functionally graded materials fabricated for biomedical implants

Three sets of FGMs of stainless steel 316L (SS) reinforced with micro-, nano- and mixed (1:1 mass ratio) hydroxyapatite (HA) were fabricated by powder metallurgy route. The concentration of the HA was varied from 0 to 20 wt% with the increment of 5 wt% in all sets to strengthen the discrete layers of FGMs. The sintered densities along the discrete layers of FGMs continually decreased as a function of HA content which enhanced the bioactivity of the FGMs towards the end with high content of HA. All FGMs experienced an excellent corrosion response in the 0.9% NaCl solution with high passivity. Heavy diffusion of chromium (Cr) form SS to HA made the matrix Cr deficient. The chromium depletion regions around the interface of SS and HA caused an active corrosion behavior of FGMs in 0.1 M HCl solution. FGMs with micro-sized HA demonstrated the better corrosion properties than that of other FGMs. After being immersed in the simulated body fluid (SBF) solution for 7 days, the apatite layer formed on the surface of FGMs with micro-sized HA had a mature spherical morphology and leaf like shape for the other two FGMs. The apatite morphology and gained weight results proved the highest bioactivity for micro-sized HA reinforced FGMs.