Different amounts of boron compounds (i.e. boric acid and borax) are mixed with natural rubber to form a thermal neutron radiation shielding block. With proper treatment on natural rubber, uniform distribution of a boron compound can be achieved, which results in an enhancement of thermal neutron shielding capability. It is found that the neutron shield capability of those shielding blocks depends sensitively on both borax and boric acid's concentration, and the thickness of the shielding block. Even though the boron content in a natural rubber shielding block mixed with boric acid is higher than that with borax at the same amount, the shielding blocks with borax can shield thermal neutron better due to the loss of boric acid by a bubble formation during the process. An inclusion of lead oxide (PbO 2 ) in the shielding block can slightly increase its thermal neutron shielding capability and results in good gamma ray shielding capability, in which a minimum thickness of shielding blocks is required to be effective.
In this study, a prototype of commercialized-style radiation shielding blocks based on natural rubber mixed with radiation shielding substance (i.e. lead and tungsten compound) was developed. The gamma shielding test was carried out using 137 Cs (662 keV) source and 60Co (1173, 1332 keV) source. The results reveal that based on the gamma attenuation property and forming capability, the optimum formula was a shielding block with 60% lead oxide, which was able to completely shield gamma radiation (> 99%). In comparison with a radiation shielding block with lead oxide, a radiation shielding block with tungsten oxide showed similar characteristics. Due to the higher price of tungsten oxide, natural rubber block with lead oxide is preferred. However, with an environmental concern, natural rubber block with tungsten oxide is selected. Moreover, it was found that the mechanical properties of these radiation shielding blocks (including hardness, tensile strength and elongation at break) were reduced with increasing of the amount of radiation shielding substance mixed in the radiation shielding blocks. However, the reduction of these mechanical properties does not affect the actual utilization since these radiation shielding blocks are normally not subjected to any strong force or pressure. In addition, the SEM images showed the uniform dispersion of radiation shielding substances in the rubber block texture.
Subcritical debonding is of particular concern for microelectronic packaging, coating, and adhesive applications. Time-dependent subcritical debonding at polymer=substrate interfaces occurs at lower mechanical loads and strain energy release rates compared with those required for catastrophic interface fracture. In this work, the role of organosilane adhesion promoters, 3-aminopropyltriethoxysilane and glycidoxypropyltrimethoxysilane, in subcritical debonding of epoxy=glass interfaces under hygrothermal condition is investigated. The epoxy systems studied included a thermally cured bisphenol F-based epoxy resin and a bisphenol F-based epoxy resin cured with a UV active curing agent. Subcritical debonding results revealed that there are two regions, the threshold strain energy release rate (G TH ) and the power law region, observed in subcritical debonding curves. Hygrothermal aging not only lowers the critical debonding driving energy required for debond extension (G C ) but also lowers G TH , below which interfacial crack growth does not occur. Interestingly, applying silane adhesion promoters on glass surfaces increased G TH values and decreased debonding growth rate. Therefore, the subcritical debonding growth rate mechanism was found to be sensitive to interface chemistry. An attempt to correlate the results of critical and subcritical debonding was undertaken. It was found that as aging time increased, the role of subcritical debonding became less important compared with the critical debonding component. However, the presence of subcritical debonding at applied driving energies significantly below G C has important implications for the long-term reliability of interfaces.
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