Volume resistivity encapsulants

Values of "dry" leakage currents for encapsulated samples which are presumed accurate and measure in the lOOpA-lnA range, should be easily verifiable by a volume resistivity measurement of the polymer, since the polymer s resistivity would have to be less than approximately lxlO1 ohms-cm. 

Addition of a filler such as AKO s an alternate method to reduce creep. As shown in Table 3 this filler also increases mechanical strength as well as lowering the coefficient of thermal expansion. The latter aspect can be an important consideration when trying to match material coefficients of expansion in an encapsulating situation. It should also be noted that volume resistivity measurements of unfilled material (see Table 3) indicate these formulations provide satisfactory electrical insulation for encapsulating purposes. 

Retaining a high electrical volume resistivity in the presence of moisture is one of the most important properties of moulding powder resin systems which are used for the encapsulation of electronic components. 

Ethlylene propylene liquid polymers are frequently used as electrical encapsulation compounds. The dielectric constant of the cured materials is 2.1-2.2, and the volume resistivity is 10 ohm-cm. They have low moisture permeability and low strength and are UV resistant. Applications include automobile and construction sealants, waterproofing membranes, and electrical encapsulants. From a safety standpoint, these materials have an extremely low level of dermal, oral, and inhalation toxicity and low eye irritation. 

To maintain insulation betweai pins, volume resistivity must be more than 10 ohm-cm. On the other hand, when devices are operated at higher temperature, leakage occurs depending upon the device and/or encapsulation material. This p4ienomenon is well known as parasitic MOS Two independent diffusion layers are ccninected... 

These siloxanes are easily hydrolyzed by ambient humidity and water bound to the substrates to generate transient silanols 77 and 78. Subsequent dehydration initially provides a precursor 79 of crosslinked polydimethylsiloxane network, which is formed by further condensation of the silanol groups. With many variants, this process constitutes the base of the RTV silicones that are mainly used as protection shields in the glob top encapsulation process. Low-stress conductive adhesives have been prepared by the condensation reaction of ethylphenylsila-nediol, l,4-phenylenebis(dimethylsilanol), and trimethylsilyl-terminated poly-diethylsiloxane. When loaded with carbon particles and cured at 180°C, this composition exhibits a volume resistivity of 1.2 X 10 flcm and a thermal conductivity of 2.1 W m K. ... 

The encapsulation process deals with the dried wastes. The dried waste is first chemically treated and then coated with a binder, usually polyethylene. The advantages of this process are that the encapsulated waste is very durable and resistant to water and deterioration and the final product need not be stored in containers. The disadvantage is that it is an expensive process and applicable only to small volumes of wastes. 

If empty zeolite LTA is exposed to cesium gas, and redox reaction with the exchangeable cations of the zeolite can occur, then the product Cs" " ions may seal the zeolite crystals. Everything within the zeolite may be encapsulated, including the product atoms, perhaps as neutral clusters. Well, nearly all cations, including Na" ", are reduced by Cs(g) only some of the other alkali-metal cations may resist reaction. Depending upon the mobility of the Cs" " ions at the reaction temperature, the surfaces of the crystals may be closed initially. If the reaction proceeds, the entire volume of the zeolite may be sealed. The ultimate outcome of the process will depend upon the mobilities of all the atoms and ions in the structure at the reaction temperature, and upon considerations of the stability and lability of any structural subunits which may form. 

Mix the lipid dispersions along with the material to be encapsulated, in a heat-resistant flask such as a pyrex beaker, and add glycerol to a final volume concentration of 3% (see Note 14). Alternatively a heat-resistant bottle with six baffles can be used for the process as explained and pictured in reference (49). 

Encapsulation refers to the confinement of a liquid solution within small capsules enclosed by a polymer or a surfactant. A potentially high interfacial area is thus created and the recovery of the catalyst is facilitated. The selective sorption through the membrane can further increase catalytic performances. Scaling-up is easy, but capsules should be as small as possible to prevent extra resistance to mass transfer in the non-agitated encapsulated volume. A problem associated with such capsules is the fact that there is no way to provide a fresh solution to the inner portion of the capsule or to continuously remove product from that phase. The capsules have to be either leached or broken at the end. 

As described above, the most effective composite catalysts demonstrate the specific volume activity close to that for bulk intermetallides. This is apparently caused by specificity of the composites structure and texture, though variation of the state of AC surface caused by encapsulation could be important as well. As for the texture effects, a significant decrease of the encapsulated AC particle size versus that for pure AC minimizes the effect of the internal diffusion on the total reaction rate. Similarly, while condensed hydrocarbons usually fill meso- or micropores of traditional FTS catalysts, they are nearly absent in macropores of cermets, thus minimizing the diffusion resistance of the irmer space of matrix... 

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