Carbon threshold concentrations

In pressing, the threshold concentration of the filler amounts to about 0.5% of volume. The resulting distribution of the filler corresponds, apparently, to the model of mixing of spherical particles of the polymer (with radius Rp) and filler (with radius Rm) for Rp > Rm as the size of carbon black particles is usually about 1000 A [19]. During this mixing, the filler, because of electrostatical interaction, is distributed mainly on the surface of polymer particles which facilitates the forming of conducting chains and entails low values of the percolation threshold. 

For the second method the threshold concentration of the filler in a composite material amounts to about 5 volume %, i.e. below the percolation threshold for statistical mixtures. It is bound up with the fact that carbon black particles are capable (in terms of energy) of being used to form conducting chain structures, because of the availability of functional groups on their surfaces. This relatively sparing method of composite material manufacture like film moulding by solvent evaporation facilitates the forming of chain structures. 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Investigations of pure cultures of bacteria clearly show the existence of a threshold concentration for the carbon source below which replication does not occur. This value is about 18 pg/1 for Escherichia coli and Pseudomonas sp. growing on glucose, 180 pg/1 for Aeromonas hydrophila growing on starch,... 

Clear, colorless, heavy, watery liquid with a strong, sweetish, distinctive odor resembling ether. Odor threshold concentration in air ranged from 21.4 ppm (in a sampled derived from the chlorination of carbon disulfide) to 100.0 ppmv (dervived from the chlorination of methane) (Leonardos et ah, 1969). An odor threshold of 4.68 ppmv was determined by Leonardos et al. (1969). A detection odor threshold concentration of 3,700 mg/m (584 ppmv) was experimentally determined by Dravnieks (1974). The average least detectable odor threshold concentrations in water at 60 °C and in air at 40 °C were 1.08 and 3.6 mg/L, respectively (Alexander et al, 1982). 

Carbon dioxide has no essential effect on the stability of iron hydroxide colloids toward the action of electrolytes. The coagulation thresholds (concentrations of MgS04) prove to be the same for colloidal solutions of iron saturated with carbon dioxide (- co, bar) as for solutions in the air. 

A third important filler parameter is related to its shape. Figure 15.17 shows that the aspect ratio of carbon fiber affects conductivity. If the fiber is milled to almost spherical particles, its percolation threshold concentration is substantially increased. 

When aluminum borate whiskers were incorporated in SAN/PA-6 blends, the whisker, due to acid/based interaction had a better affinity to PA-6 than to SAN and for this reason they ended up in residence in the polyamide phase. A similar principle was used to obtain a conductive blend by compounding PA, PP and carbon black. Carbon black has a better affinity to polyamide and thus it prefers to reside in the polyamide phase or in the interphase formed between two immiscible polymers. Even when carbon black is added initially to polypropylene, it still transfers to the polyamide phase. This migration and preferential location of carbon black is not only an interesting scientific principle but it also has important practical implications. For the purpose of conductivity, a certain threshold concentration of carbon black is required to enable percolation and this threshold concentration is substantially lower if the carbon black settles in only one phase. This phase be-... 

All standard test metho ds in liquid conditions are based on respiratory tests with a diversity of methodologies measurement of oxygen consumption is easily achieved (i. e., closed bottles with DBO meter), whereas determination of the carbon dioxide concentration requires costly equipment. Moreover, inoculum shows huge variability (several sources, treatments and quantities), causing variability in the results. The test sets that combine various types of controls (blank, sterile, toxicity) are not the same from one standard test to another and are sometimes open to criticism in their present form. Finally, as seen in Table 8, there is not common agreement between the standardization committees about the threshold for... 

Threshold Chloride Concentration The average threshold concentration in carbon steel is 0.83 kg/m If the chloride concentration rebars increases beyond this value, the passive film is destroyed. 

Figure 3.16 shows that electrical conductivity can be dramatically improved by addition of carbon nanotubes.The electrical conductivity of composites abruptly increases by many orders of magnitude when the filler content exceeds the threshold concentration. ... 

Raising the temperature, the adsorption equilibrium between hydrogen and carbon monoxide, jointly adsorbing on platinum, shifts in favor of hydrogen adsorption. This raises the highest admissible threshold concentration of carbon monoxide. The effect could be seen in fuel cells with phosphoric acid electrolyte, which work at temperatures of about 180-200°C and admit carbon monoxide concentrations in hydrogen as high as 100 ppm, despite the fact that platinum catalysts are used. 

Table 18.3 Threshold concentrations for XE-2, a highly conductive carbon black, in various polymers.

---