Particulate-composite systems

Properties of particle-filled composites, such as strength, toughness, and impact resistance, depend on the material properties of both matrix and filler. Of importance are the size and shape of the filler particles and interactions between them, the total volume occupied by the filler, the presence of voids in the structure, and the degree of adhesion between filler and matrix. When the elastic modulus of the filler is higher than that of the matrix, the modulus of the composite usually increases in proportion to the volume fraction of filler. Opposite behavior is expected if the matrix is more rigid than the filler. 

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Physical properties of blends consisting of a continnons matrix and one or more dispersed (discrete) components can be predicted by nsing adapted models proposed for particulate composite systems (216-220). Most of these models do not consider effects of the particle size, but only of volnme fractions of components in the system. Thus, the increase in particle size dne to particle coalescence is not presumed to perceptibly affect mechanical properties, except for fractnre resistance, which is controlled by particle size and properties of dispersed rnbbers. As polymer blends with three-dimensional continuity of two or more components are isotropic materials, simple parallel or series models or models for orthotropic or quasi-isotropic materials are not applicable. Physical properties of blends with partially co-continuous constituents can be calculated by means of a predictive... 

Unlike fibre- or whisker-reinforced composites, particulate composites have the advantage of being compatible with conventional powder processing, and in many cases can be pressurelessly sintered. As with other ceramic microstructures, a myriad of other ingenious fabrication routes have also been reported, but these are too numerous and system-specific to describe here. This section merely outlines the main points of powder processing where the production of composites in chemically compatible systems (i.e. those in which the components do not react chemically with one another) differs from that of monolithic ceramics. 

Wasche, R. and Klaffke, D. Ceramic particulate composites in the system SiC-TiC-TiB2 sliding against SiC and A1203 under water , Tribology International 32 (1999) 197-206. 

Concentrated ambient particle system (CAPS) is a new technology that has been developed to allow ambient particles to be concentrated in real time by factors of 25 or more (Sioutas et al. 1995). CAPS chambers are typically smaller in size (3-5 m in volume) and mimic the ambient conditions in a particular geographical area. Variations in particulate composition between cities can affect the impact of the findings (Mills et al. 2008). Furthermore, exposure concentrations can vary from day to day depending on ambient levels of PM from which the air is taken, as well as the concentrating factor of the instrument used. Exposure concentrations in CAPS can range from relatively low levels to concentrations much higher (23-311 pg/m ) than those commonly found in polluted cities (USEPA 2004). 

Two sets of keys open the way to understanding solidification. One is the basics of phase equilibria - liquid-vapor, liquid-liquid, and liquid-solid - of reaction equilibria, polymer gelation and vitrification, and colloidal transitions. The other is heat and mass transport processes reaction, transformation, and shrinkage kinetics and stress phenomena in polymeric systems and polymer-particulate composites. Engineering approximations... 

For the N22 CMC system, remaining open porosity in the CVI SiC matrix was filled by room-temperature infiltration of SiC particulate or slurry casting, followed by the melt-infiltration (MI) of silicon metal near 1400°C. This yielded a final composite with 2% closed porosity within the fiber tows and 0% porosity between the tows. The final composite system (often referred to as a slurry-cast MI composite) typically displayed a thermal conductivity about double that of a full CVI SiC composite system in which the CVI matrix process was carried to completion. Also the composite did not require an oxidation-protective over-coating to seal open porosity. Decreasing the porosity of the hybrid matrix also increased the N22 CMC elastic modulus, which in turn contributed to a high proportional limit stress. However, since the filler contained some low-modulus silicon, the modulus increase was not as great as if the filler were completely dense SiC. 

For this testing, the composite system interrogated was a Melt Infiltrated In-Situ BN SiC/SiC composite (Ml SiC/SiC). The interface coating for this material is form a two step process it is initially heat treated to create a fine layer in in-situ BN and then it is followed with CVI deposited Si-doped BN. The MI SiC/SiC system has a stochiometric SiC (Sylramic ) fiber in a multiphase matrix of SiC deposited by chemical vapor deposition followed by slurry casting of SiC particulates with a final melt infiltration of Si metal. The specific MI SiC/SiC tested for this effort had 36% volume fraction fibers using a 5 HS weave at 20 EPI. The fibers are 10 pm diameter and there are 800 fibers per tow. This material system was developed by NASA-GRC and is sometimes referred to as the 01/01 material [11]. A cross section of this material is shown in Figure 1. 

Particle reinforced composite systems can be either large particle or dispersion strengthened. If a composite is reinforced by large particles (larger than 0.1 [xm and equiaxed, which are harder and stiffer than the matrix), mechanical properties are dependent on volume fractions of both components and are enhanced by increase of particulate content. Concrete is a common large particle strengthened composite where both matrix and particulate phases are ceramic materials. 

Large particle reinforced composite systems are utilised with all three types of materials (metals, ceramics and polymers). Concrete is a common large particle strengthened composite where both matrix and particulate phases are ceramic materials. 

The interlayer model was developed by Maurer et al. The model of the particulate-filled system is taken in which a representative volume element is assumed which contains a single particle with the interlayer surrounded by a shell of matrix material, which is itself surrounded by material with composite properties (almost the same as Kemer s model). The radii of the shell are chosen in accordance with the volume fraction of the fQler, interlayer, and matrix. Depending on the external field applied to the representative volmne element, the physical properties can be calculated on the basis of different boundary conditions. The equations for displacements and stresses in the system are derived for filler, interlayer, matrix, and composite, assuming the specific elastic constants for every phase. This theory enables one to calculate the elastic modulus of composite, depending on the properties of the matrix, interlayer, and filler. In... 

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