There have been a number of review papers on layered silicate and carbon nanotube reinforced polymer nanocomposites, in which the fillers have high aspect ratios. Particulate–polymer nanocomposites containing fillers with small aspect ratios are also an important class of polymer composites. However, they have been apparently overlooked. Thus, in this paper, detailed discussions on the effects of particle size, particle/matrix interface adhesion and particle loading on the stiffness, strength and toughness of such particulate–polymer composites are reviewed. To develop high performance particulate composites, it is necessary to have some basic understanding of the stiffening, strengthening and toughening mechanisms of these composites. A critical evaluation of published experimental results in comparison with theoretical models is given.

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Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites

Mechanisms controlling the durability of thermal barrier coatings

During the past five years, the authors have developed in their laboratory a new type of solar cell that is based on a photoelectrochemical process. The light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor (i.e., titanium oxide (TiO{sub 2})). When excited by a photon, the dye has the ability to transfer an electron to the semiconductor. The electric field that is inside the material allows extraction of the electron, and the positive charge is transferred from the dye to a redox mediator that is present in solution. A respectable photovoltaic efficiency (i.e., 10%) is obtained by the use of mesoporous, nanostructured films of anatase particles. The authors show how the TiO{sub 2} electrode microstructure influences the photovoltaic response of the cell. More specifically, they focus on how processing parameters such as precursor chemistry, temperature for hydrothermal growth, binder addition, and sintering conditions influence the film porosity, pore-size distribution, light scattering, and electron percolation and consequently affect the solar-cell efficiency.

Nanocrystalline titanium oxide electrodes for photovoltaic applications

The notion of mimicking natural structures in the synthesis of new structural materials has generated enormous interest but has yielded few practical advances. Natural composites achieve strength and toughness through complex hierarchical designs extremely difficult to replicate synthetically. Here we emulate Nature's toughening mechanisms through the combination of two ordinary compounds, aluminum oxide and polymethylmethacrylate, into ice-templated structures whose toughness can be over 300 times (in energy terms) that of their constituents. The final product is a bulk hybrid ceramic material whose high yield strength and fracture toughness ({approx}200 MPa and {approx}30 MPa{radical}m) provide specific properties comparable to aluminum alloys. These model materials can be used to identify the key microstructural features that should guide the synthesis of bio-inspired ceramic-based composites with unique strength and toughness.

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Tough, bio-inspired hybrid materials.

Tough, bio-inspired hybrid materials E. Munch, 1 M. E. Launey, 1 D. H. Alsem, 1,2 E. Saiz, 1 A.P. Tomsia, 1 R. O. Ritchie 1,3∗ The notion of mimicking natural structures in the synthesis of new structural materials has generated enormous interest but has yielded few practical advances. Natural composites achieve strength and toughness through complex hierarchical designs extremely difficult to replicate synthetically. Here we emulate Nature’s toughening mechanisms through the combination of two ordinary compounds, aluminum oxide and polymethylmethacrylate, into ice-templated structures whose toughness can be over 300 times (in energy terms) that of their constituents. The final product is a bulk hybrid ceramic material whose high yield strength and fracture toughness (~200 MPa and ~30 MPa√m) provide specific properties comparable to aluminum alloys. These model materials can be used to identify the key microstructural features that should guide the synthesis of bio-inspired ceramic-based composites with unique strength and toughness. With the quest for more efficient energy-related technologies, there is an imperative to develop lightweight, high-performance structural materials that possess both exceptional strength and toughness. Unfortunately, these two properties tend to be mutually exclusive and the attainment of optimal mechanical performance is invariably a compromise often achieved through the empirical design of microstructures. Nature has long developed the ability to combine brittle minerals and organic molecules into hybrid composites with exceptional fracture resistance and structural capabilities (1-3); indeed, many natural materials like bone, wood and nacre (abalone shell) have highly sophisticated structures with complex hierarchical designs whose properties are far in excess what could be expected from a simple mixture of their components (2,4). Biological mineralized composites, in particular bone, dentin and nacre (5-7), can generate fracture toughness (i.e., resistance to the initiation and growth of a crack) primarily by extrinsic toughening mechanisms (8) that “shield” any crack from the applied loads. These mechanisms, which are quite different to those that toughen metals for Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Materials Science and Engineering, University of California, Berkeley, California, 94720, USA To whom correspondence should be addressed. E-mail: roritchie@lbl.gov

Tough, bio-inspired hybrid materials

The toughening behavior of whisker-reinforced ceramics is analyzed in terms of a whisker bridging zone immediately behind the crack tip. This approach is consistent with microscopy observations which reveal that intact bridging whiskers exist behind the crack tip as a result of debonding of the whisker-matrix interface. The theoretical results based on both the stress intensity and the energy change introduced by bridging whiskers reveal the dependence of toughening upon the composition and matrix, interface, and whisker properties. Furthermore, the analytical models of whisker bridging accompanied by very limited pullout accurately describe experimental observations of the toughening behavior in several SiC-whisker-reinforced ceramics. Such analytical descriptions also indicate that increases in whisker size and strength and modification of interface properties will result in further increases in toughness by whisker reinforcement.

Toughening Behavior in Whisker-Reinforced Ceramic Matrix Composites

The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPa{center_dot}m{sup 1/2} and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPa{center_dot}m{sup 1/2} coupled with a reduced fracture strength.

Microstructural Design of Silicon Nitride with Improved Fracture Toughness: I, Effects of Grain Shape and Size

A general closed-form solution for elastic deformation of multilayers due to residual stresses and external bending is derived. Based on the general solution, simplified solutions for residual stress distributions in multiple layers of thin films on a thick substrate are obtained. These simplified solutions can be expressed as functions of either mismatch strains between film layers and substrate or the curvature of the system. The simplified solution for the special case of one film layer on a substrate is also presented, and its accuracy is discussed.

Modeling of Elastic Deformation of Multilayers Due to Residual Stresses and External Bending

Stresses required to debond the fiber-matrix interface and to pull out a fiber are analyzed as a function of the embedded fiber length for a fiber-reinforced composite. The role of residual clamping stresses at the interface is considered. During fiber pull-out, Poisson contraction of the fiber in the radial direction reduces the resultant compressive stress at the interface and, hence, the interfacial frictional stress. This leads to a non-linear dependence of both the stress required for complete interfacial debonding and the stress for fiber pull-out on the embedded fiber length. Excellent agreement between present theoretical analyses and existing experimental results is obtained. Compared with the stress required for fiber pushout, where the fiber is subjected to compression in its axial direction, the stress required for fiber pullout is lower owing to Poisson's effect.

Interfacial debonding and fiber pull-out stresses of fiber-reinforced composites

Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated s-Si3N4 seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R-curve and extensive interfacial debonding between the elongated s-Si3N4 grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the s´-SiAlON growth layer formed on the elongated s-Si3N4 grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the s´-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct s´-SiAlON growth layer forms. It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed.

Chun-Hway Hsueh

Papers

Toughening Behavior in Whisker-Reinforced Ceramic Matrix Composites

Microstructural Design of Silicon Nitride with Improved Fracture Toughness: I, Effects of Grain Shape and Size

Modeling of Elastic Deformation of Multilayers Due to Residual Stresses and External Bending

Interfacial debonding and fiber pull-out stresses of fiber-reinforced composites

Microstructural design of silicon nitride with improved fracture toughness: II. Effects of yttria and alumina additives