40 2 Carbon Fibers and Nanofillers matrix of a carbon-matrix composite.In general,the more crystalline the carbon, the higher the onset temperature for oxidation 2.3 Nanofillers Nanocomposites refer to composites with nanometer-scale structure.This struc- ture can relate to the grain size,the filler size,the pore size,etc.A small pore size helps to improve the mechanical strength of a porous material.A small grain size is attractive since it leads to a high yield strength in metals and a high toughness in ceramics.A small filler size provides a large filler-matrix interface area per unit volume.The large area can be advantageous or disadvantageous,depending on the type of property.For example,it may result in a composite of low strength (in spite of the possibly high strength within a single unit of the nanofiller,e.g., within a single nanofiber)due to the mechanical weakness of the interface;it may also result in a composite of high electrical resistivity (in spite of the possibly low electrical resistivity within a single unit of the nanofiller,e.g.,within a single nanofiber)due to the electrical resistance associated with the interface.On the other hand,the high interface area may be good for the performance of electro- chemical electrodes due to the fact that the electrochemical reaction occurs at the interface between the electrode and the electrolyte,as in a battery;it may also be good for shielding from electromagnetic interference(abbreviated to "EMI")that occurs in the near-surface region of a conductive filler unit,due to the skin effect (the phenomenon in which high-frequency electromagnetic radiation penetrates only the near-surface region of an electrical conductor).Thus,nanocomposites are not necessarily superior to conventional composites. A nanofiller refers to a filler in the nanometer size range(0.5-500nm,typically 1-100 nm)along at least one of its dimensions.For example,a nanoplatelet is in the nanometer size range in the direction perpendicular to the plane of the platelet,but 2m10000N Figure 2.3.A scanning electron microscope photograph of fumed alumina,which takes the form of porous agglomerates of nanopartides around 13 nm in size40 2 Carbon Fibers and Nanofillers matrix of a carbon-matrix composite. In general, the more crystalline the carbon, the higher the onset temperature for oxidation. 2.3 Nanofillers Nanocomposites refer to composites with nanometer-scale structure. This struc￾ture can relate to the grain size, the filler size, the pore size, etc. A small pore size helps to improve the mechanical strength of a porous material. A small grain size is attractive since it leads to a high yield strength in metals and a high toughness in ceramics. A small filler size provides a large filler–matrix interface area per unit volume. The large area can be advantageous or disadvantageous, depending on the type of property. For example, it may result in a composite of low strength (in spite of the possibly high strength within a single unit of the nanofiller, e.g., within a single nanofiber) due to the mechanical weakness of the interface; it may also result in a composite of high electrical resistivity (in spite of the possibly low electrical resistivity within a single unit of the nanofiller, e.g., within a single nanofiber) due to the electrical resistance associated with the interface. On the other hand, the high interface area may be good for the performance of electro￾chemical electrodes due to the fact that the electrochemical reaction occurs at the interface between the electrode and the electrolyte, as in a battery; it may also be good for shielding from electromagnetic interference (abbreviated to “EMI”) that occurs in the near-surface region of a conductive filler unit, due to the skin effect (the phenomenon in which high-frequency electromagnetic radiation penetrates only the near-surface region of an electrical conductor). Thus, nanocomposites are not necessarily superior to conventional composites. A nanofiller refers to a filler in the nanometer size range (0.5–500nm, typically 1–100nm) along at least one of its dimensions. For example, a nanoplatelet is in the nanometer size range in the direction perpendicular to the plane of the platelet, but Figure 2.3. A scanning electron microscope photograph of fumed alumina, which takes the form of porous agglomerates of nanoparticles around 13nm in size