Functionally Graded Zircon–Molybdenum Materials without Residual Stresses

A layered, zircon–molybdenum functionally graded material was obtained by starting from commercial powders of molybdenum (median particle size ( d ₅₀) of ∼3 μm) and zircon ( d ₅₀∼ 0.8 μm). Conventional processing led to a material that was free of internal residual stress. The sintering behavior of green compacts with compositions that corresponded to the different layers was studied via dynamic sintering. The thermal expansion coefficients of each layer were measured using conventional dilatometry. The presence of residual stresses was determined using Vickers indentations. Both thermal expansion mismatch and differential shrinkage between the layers were negligible.     

高分子功能梯度材料的研究进展

通过不同聚合物材料以及聚合物与其它材料的巧妙梯度复合得到的高分子功能梯度材料,目前已成为材料领域的重要分支。本文从高分子功能梯度材料的类型、制备方法和应用等方面,综述了高分子功能梯度材料近年来的研究进展。     

Temperature and Stress Distributions in a Hollow Cylinder of Functionally Graded Material: The Case of Temperature-Independent Material Properties

We have presented a numerical technique for analyzing one-dimensional transient temperature distributions in a circular hollow cylinder that was composed of functionally graded ceramic–metal-based materials, without considering the temperature-dependent material properties. The functionally graded material (FGM) cylinder was assumed to be initially in a steady state of gradient temperature; the ceramic inner surface was exposed to high temperature, and the metallic outer surface, which was associated with its in-service performance, was exposed to low temperature. Then, the FGM cylinder was cooled rapidly on the ceramic surface of the cylinder, using a cold medium. The transient temperature and related thermal stresses in the FGM cylinder were analyzed numerically for a model of the mullite–molybdenum FGM system. The technique for analyzing the temperature distribution is quite simple and widely applicable for various boundary conditions of FGMs, in comparison with methods that have been proposed recently by other researchers.     

Fabrication and Properties of Tape-Cast Laminated and Functionally Gradient Alumina–Titanium Carbide Materials

A tape-casting process was used to prepare various Al₂O₃–TiC green tapes, from which laminated and functionally gradient Al₂O₃–TiC materials (FGM Al₂O₃–TiC) were produced by cutting, stacking, and laminating the material, removing the binder, and hot-pressing the green bodies. The bending strength of the FGM Al₂O₃–TiC composites was not much less than that of the laminated Al₂O₃–TiC composites. However, the fracture toughness was >50% higher; the fracture toughness of the FGM composite sintered at 1700°C was 9.43 MPa·m^1/2. This increased toughness was attributed to the stress distribution that was caused by variations in the composition of the FGM composite layers. The present results demonstrate that the FGM design is a useful method for modifying the mechanical properties of ceramic composites.     

Ti/Al2O3系梯度功能材料研究动态

介绍了目前用于Ti/Al2O3系梯度功能材料制备的几种主要工艺,提出了利用共沉降法制备Ti/Al2O3系梯度功能材料的设想,简述了Ti/Al2O3复合材料制备技术的基础理论及其研究现状,并指出了其不足和未来的发展方向.     

Calculation of the Composition Profile of a Functionally Graded Material Produced by Centrifugal Casting

Functionally graded materials have designed inhomogeneous distributions of different components on the scale of the material. They can be made by suspension processing, in which particles are stacked in a controlled manner. Segregation effects can be used to obtain the required gradient if the particles in suspension have different velocities. A model is derived for the composition profiles that develop as a result of these effects. The model can be used to determine the most suitable process conditions. The outcome of the model shows fair agreement with experiments found in the literature in which an Al₂O₃/ZrO₂ composite has been produced by uniaxial centrifugal casting.     

Ni／Ni3Al—TiC系梯度功能材料的热应力缓和特性设计

本文选择Ｎｉ／ＮｉＡｌ－ＴｉＣ体系的ＦＧＭ，对其在制备过程中的残余絷应力进行了计算机有限元模拟。在综合考虑热应力最小，应力强度比值最小以及纯ＴｉＣ侧应力状态等因素的基础上，完成了ＦＧＭ体系的热应力缓和特性设计，得到组成分布指数Ｐ＝１．６的最佳设计结果。     

Dense Layered Molybdenum Disilicide–Silicon Carbide Functionally Graded Composites Formed by Field-Activated Synthesis

Dense, layered, single- and graded-composition composites of MoSi₂ and SiC were formed from elemental powders in one step, using the field-activated pressure-assisted combustion method. Compositions ranging from 100% MoSi₂ to 100% SiC were synthesized, with relative densities ranging from 99% to 76%, respectively. X-ray diffractometry results indicated the formation of pure phases when the concentration of MoSi₂ was high and the appearance of a ternary phase, Mo_4.8Si₃C_0.6, when the concentration of SiC was high. Electron microprobe analysis results showed the formation of stoichiometric and uniformly distributed phases. A layer-to-layer variation in composition of 10 mol% was sufficient to prevent thermal cracking during formation of the layered functionally graded materials.     

Fabrication and Evaluation of Porous Piezoelectric Ceramics and Porosity–Graded Piezoelectric Actuators

Porous ceramics of lead zirconate titanate (PZT) were prepared by sintering powder compacts consisting of PZT and stearic acid powders in an air atmosphere; stearic acid was added as a pore-forming agent (PFA). The dielectric, elastic and piezoelectric properties of uniformly porous PZT ceramics were investigated as a function of the porosity volume fraction. Furthermore, a beam-shaped PZT actuator sample with a graded porosity content across its thickness was fabricated by sintering PFA-graded powder compacts. The electric-field-induced bending displacement characteristics of the actuator samples were measured by using strain gauges and were found to be in good agreement with the theoretical predication based on a classical lamination theory.     

功能梯度材料及其复合电镀制备现状与进展

综述了功能梯度材料的概念、性能、研究动态及最新进展。总结了功能梯度材料的各种制备方法。重点探讨了采用复合电镀技术制备功能梯度材料的方法。该方法成本低,易于操作,所得镀层孔隙率低,结合力好,耐磨,耐蚀性好,具有广阔的应用前景。     

聚合物基梯度功能材料研究进展

对正在兴起的聚合物基梯度功能材料的研究状况，特别是其制备技术的研究和开发进行了综述，并对其潜在的应用价值进行了展望。利用物质传输原理制备梯度功能材料的方法包括了基于质量传输的溶液扩散、互穿网络、相转变和溶胶一凝胶等方法；而基于能量传输的制备方法涉及到了利用电场能、磁场能、热能、重力势能、辐射能、光能以及化学能等多种方法。同时还对界面聚合、纤维排列、组分含量连续调控、复合共挤、逐层浇注、静电喷涂等基于构造原理所开发的制备技术作了简要介绍。     

Functionally Graded Polyurethane/Cellulose Nanocrystal Composites

The preparation and investigation of functionally graded polymer nanocomposites, which have a concentration gradient of cellulose nanocrystals (CNCs) along one direction, is reported here. As a test bed, a series of nanocomposites consisting of a thermoplastic polyurethane (PU) and 0–15% w/w CNCs is prepared via solvent casting and the mechanical properties of films of these materials are characterized by dynamic mechanical analyses and tensile tests. The formation of graded materials is accomplished by lamination of films with varying CNC content. The processing conditions are optimized to achieve intimate fusion of the individual layers. The elimination of internal interfaces is evidenced by an elongation at break of up to 500%. In order to explore potential applications of graded PU/CNC nanocomposites, structure‐dependent actuation in response to water is demonstrated in a bioinspired architecture. In addition, the damping behavior of cylindrical shaped composites is investigated by way of compression tests. The results show that functionally graded PU/CNC composites show good damping behavior over a much larger range of forces than the neat PU or the homogeneous nanocomposites.     

电沉积梯度功能镀层的研究进展

论述了功能梯度镀层的概念,性能,应用及制备方法,复合电沉积法是制备梯度功能材料的重要方法之一,具有控制简单,易于操作,投资少,可处理复杂工件等优点,本文综述了电沉积法用于制备梯度功能镀层的原理,特点,研究现状及应用前景。     

非自然环境中制备PMMA梯度材料的研究

探讨了梯度材料的合成与制备方法的研究进展,提出在非自然环境中制备PMMA梯度折射率材料的制备方法和模型研究。研究表明,增加反应物中两相物质分子或颗粒的作用力差是制备具有应用前景的梯度高聚物材料的关键。并就非自然环境中制备高聚物梯度材料的实验方法、数学模型及利用该方法制备出的高聚物梯度材料的性能表征进行了研究。     

Effect of Porosity on the Electrical Properties of Polycrystalline Sodium Niobate: II, Dielectric Behavior

The dielectric behavior of dense and porous NaNbO₃ ceramic samples, synthesized by a suitable chemical route, was investigated by impedance spectroscopy between room temperature and 800°C in dry air. The dielectric behavior and thermal stability of the samples were evaluated as a function of several thermal cycles. The dielectric constant was calculated from the relaxation frequency, and from an alternative approach based on the variation of the opposite of the imaginary part of impedance as a function of reciprocal angular frequency. The values obtained using both relations were in a good agreement. After the porosity was corrected, the porous and dense samples presented the same dielectric constant. All samples evaluated displayed a broad dielectric anomaly between 300°–400°C. Neither the orthorhombic-tetragonal- nor the tetragonal-cubic-phase transitions were detected by dielectric measurements. The Curie-Weiss law was found to be valid above the transition temperature, whereas the corresponding phase transition presented a diffuse nature. The origin of the related thermal hysteresis is discussed herein.     

微生物燃料电池中碳材料改性阳极和碳基复合材料阳极的最新研究进展

微生物燃料电池(MFC)是一种利用微生物的代谢作用将化学能转化为电能的新技术,近年来受到广泛关注。文章综述了MFC阳极碳材料改性和优化的最新研究成果,介绍了碳材料改性阳极和碳基复合材料阳极的种类、理化特性、产电性能及其在MFC中的应用。     

Innovative Electrospinning Method of Depositing Functionally Graded and Aligned Multidirectional Polymer Nanofibers

An innovative setup in which multiple electrodes, whose potentials can be switched on or off, was used for producing electrospun fibers. By controlling the activation time and sequence of switching between adjacent electrodes, the electrical field and thus the charge-carrying fibers can be directed to different locations. It has been demonstrated that a variety of functionally graded architectures having preferred alignments can be produced. The simulation results point to possible mechanisms for directional deposition of fibers. The electrospun fibers can be varied in a controlled manner to produce mats containing random, aligned, or multidirectional nanofibrous patterns.     

Drying of Ceramic Layers with a Graded Pore Structure

Thin self-supporting ceramic TiO2-layers with a graded pore structure were prepared by using centrifugal deposition of powders and sols with different particle size distributions from mixed, diluted suspensions. During the evaporation drying step the layers have a strong tendency to warping and crack-formation because of the resulting difference in the capillary pressure in the upper and the bottom side pores. Four drying methods were investigated concerning their suitability for diminishing or eliminating capillary forces and for the production of planar, crack-free dried specimens. The drying techniques used in the experiments are briefly introduced. It should be emphasized that the most successful drying method for the layers mentioned above is a combination of microwave drying and subsequent critical point drying.     

Ultralight Silicon Nitride Ceramic Foams from Foams Stabilized by Partially Hydrophobic Particles

Ultralight Si₃N₄ ceramic foams have been successfully prepared through particle‐stabilized foams method, which is based on the adsorption of in situ hydrophobized Si₃N₄ particles to the liquid/air interface of the foams. Here, we firstly used a long‐chain surfactant cetyltrimethylammonium chloride to render the Si₃N₄ particles partially hydrophobic. By tailoring the surfactant concentration and pH values of the suspensions, the wet foams were stabilized to avoid coarsening and coalescence. SEM results show that the Si₃N₄ ceramic foams possess single strut walls with elongated β‐Si₃N₄ grains interlocking with each other, and their pores are uniform with an average pore size of 95 μm. The obtained ceramic foams maintain compressive strength of 1.34 ± 0.13 MPa with porosity of 92.0%, when the suspension contains 3 mmol/L surfactant at the pH of 11.0.     

Preparation of Chamottes as a Raw Material for Low‐Cost Ceramic Membranes

Low‐cost ceramic membranes are usually prepared from a mixture of natural raw materials and some organic porogen agent such as starch. The fact that porogen must be completely eliminated during firing, leaving an interconnected porous structure, impose large firing times, increasing the final price. A study about the synthesis of porous chamottes as an alternative to organic pore formers was conducted to reduce firing costs. Chamottes were obtained from mixtures of clay and starch. Different starches were used and the influence of the composition and processing variables were studied. The viability of the porous chamottes was demonstrated.