Alumina-filled epoxy composites

Fracture Toughness and Rheology of Alumina-Filled Epoxy Composites... 

Fracto-emission (FE) is the emission of particles (electrons, positive ions, and neutral species) and photons, when a material is stressed to failure. In this paper, we examine various FE signals accompanying the deformation and fracture of fiber-reinforced and alumina-filled epoxy, and relate them to the locus and mode of fracture. The intensities are orders of magnitude greater than those observed from the fracture of neat fibers and resins. This difference is attributed to the intense charge separation that accompanies the separation of dissimilar materials (interfacial failure) when a composite fractures. 

Incorporate the neodymium magnet into the body of the paste of graphite-epoxy composite, 2 mm under the surface of the electrode [1] and continue placing the paste until filling all the cavity. Cure the conducting composite at 40°C during 1 week. Once the resin is hardened, polish the surface first with abrasive paper and then with alumina paper. 

The effects of ceramic particles and filler content on the thermal shock behavior of toughened epoxy resins have been studied. Resins filled with stiff and strong particles, such as silicon nitride and silicon carbide, show high thermal shock resistance, and the effect of filler content is remarkable. At higher volume fractions (Vf > 40%), the thermal shock resistance of these composites reaches 140 K, whereas that of neat resin is about 90 K. The highest thermal shock resistance is obtained with silicon nitride. The thermal shock resistance of silica-filled composites also increases with increasing filler content, but above 30% of volume fraction it comes close to a certain value. On the contrary, in alumina-filled resin, the thermal shock resistance shows a decrease with increasing filler content. 

Another composite structure we have investigated recently is a particulate-filled epoxy. The epoxy is EPON 828 (Z-hardener) filled with irregularly shaped alumina particles with an average diameter of approximately 10/xm. This material is quite strong and brittle so we fractured most of the samples in a three-point flexure mode. The cross section of the sample was 2 mm x 6 mm. A typical EE curve plotted on a log scale is shown in Fig. 9, where t = 0 corresponds to the instant of failure. The material for this emission curve is filled at an Al203/epoxy ratio a of... 

A comparison of critical temperature differences of resins filled with several ceramic particulates is shown in Figure 4. The volume fraction of all these composites is 34.2%. The critical temperature difference of epoxy filled with hard particulates was classified into three groups on the basis of thermal shock resistance. Composites filled with a strong particulate, such as silicon nitride or silicon carbide, showed high thermal shock resistance. Some improvement in thermal shock resistance was recognized for silica-filled composites. Composites filled with alumina or aluminum nitride showed almost comparable or lower resistance compared with the neat resin. 

Figure 1 Particle size distributions of some ATH filler grades. Re-drawn and modified from Wainwright, R.W. (1991) Mechanical properties of epoxy/alumina trihydrate filled compositions, Ph.D. thesis, Kingston Polytechnic, U.K.

Organic-ceramic composites may use an epoxy as the matrix and glass or ceramic powder as the filler. A common example is the fiberglass-reinforced epoxy used as a printed circuit laminate. An epoxy substrate filled with alumina and carbon black has also been developed. By weight, the composition is 10.8 percent epoxy resin, 89 percent alumina, and 0.2 percent carbon black. This material has a thermal conductivity of 3.0 to 4.0 W/(m K), compared to both glass-epoxy printed circuit material [0.2 W/(m K)] and glass-alumina low temperature cofired substrates [2.5 W/(m K)]. The TCE (17 ppm/°C) is substantially below that... 

Perforated PZT-polymer composites have been fabricated [4] by drilling holes in sintered PZT blocks and filling with epoxy. In some cases the perforation was left empty but capped with alumina plates and the whole structure encapsulated in epoxy these are referred to as perforated 3-2-0 or 3-1-0 composites. The maximum hydrostatic piezoelectric coefficients attained for these composites are generally significantly greater than those associated with solid PZT, and values of have been reported that are greater than 200 times the figure of merit for solid PZT. 

K would cause a AT of almost 100 °C. For at least two decades, alumina and crystaUine silica have been used to boost the thermal conductivity of epoxy resins. When highly conductive fillers such as boron nitride, aluminum niliide, and diamond powders become commercially available, these materials have been incorporated in adhesive compositions. The expected target was the attainment of kg values of at least 10 W m K if not better. Such high values have been claimed for diamond-filled adhesives but they remain currently questionable with regard to the experimental results. summarized in the graph of Fig. 12.18. 

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