Quenching, chemical

Collisional de-excitation is called quenching because it competes with spontaneous emission, and if significant the fluorescent signal will be reduced, or quenched. Chemical decay can also be important in some circumstances. 

The most common cure conditions for the networks under consideration are Tcure < T and Tcure = const. Under these conditions, the most important feature of the thermosetting process is the vitrification of the reacting mixture which practically quenches chemical crosslinking (see Sect. 2) and changes the physical and mechanical properties of the reacting mixture. In this section, we shall mainly consider the properties of epoxy-aromatic amine glasses prepared at Tcure = 40-110 °C. 

Four Star. [McGean-Rohco] Quenching chemicals. 

Counting efficiency for LS is reduced or degraded by several facfors, nofably chemical and color quenching. Chemical quenching occurs when an impurity in the... 

Hacl] Induction melting, annealing, rapid water quenching, chemical analysis, etching (nital), optical microscopy, TEM, selected-area diffraction (SAD), field-emission gun-transmission electron microscopy (FEG-TEM), energy-dispersive X-ray spectroscopy (EDS)... 

A broad residence time distribution and the associated varying reaction times in the solution can also have negative effects on the product distribution. This is especially tme for fast multi-step reactions, which need a very definite reaction time for the first step. This second step can also be a precipitation or a quench (chemical or thermal). 

Block copolymers are molecules composed of two or more distinct monomers chemically bonded in the same chain. We consider the simplest case where there are two types of elementary units A and B. These units are arranged into bonded linear sequences, or blocks, of variable length that are then repeated a variable number of times. To date, only one-component fluids composed of periodic block copolymers where the A and B block lengths are unique have been studied based on PRISM theory. However, random or statistical copolymers where there is quenched chemical or sequence disorder associated with the polymerization process are also of great interest. ... 

From adequate proportions of BijSj, CU2S and PbS in evacuated ampoules at 400 °C, quenched. Chemical transport reactions starting with the bulk material with iodine as carrier. 

Cu1.03Cr0.9jS2 c/a=5.36 the binary sulfides heated up to 850 °C and quenched, chemical transport with iodine at a temperature gradient of 870---820°C. Platelets. Stable above 400 °C. ... 

This is a rapidly developing field. Analytical procedures can be established by several methods specie fluorescence fluorescence quenching chemically induced fluorescence (e.g., chelation of non-fluorescent metal ions with fluorescent ligands) and enzymic reactions that produce fluorescent products.Sample concentrations and identities can be determined in solution, on powders, or on glasses. 

When the pyrolysis gases are quenched with a molar excess of iodine vapor, a yield of greater than 50% -xylylene diiodide is recovered. The observation of this effect offered the first direct chemical support for the idea that DPX pyrolysis results in PX (1) (3). 

In order for a soHd to bum it must be volatilized, because combustion is almost exclusively a gas-phase phenomenon. In the case of a polymer, this means that decomposition must occur. The decomposition begins in the soHd phase and may continue in the Hquid (melt) and gas phases. Decomposition produces low molecular weight chemical compounds that eventually enter the gas phase. Heat from combustion causes further decomposition and volatilization and, therefore, further combustion. Thus the burning of a soHd is like a chain reaction. For a compound to function as a flame retardant it must intermpt this cycle in some way. There are several mechanistic descriptions by which flame retardants modify flammabiUty. Each flame retardant actually functions by a combination of mechanisms. For example, metal hydroxides such as Al(OH)2 decompose endothermically (thermal quenching) to give water (inert gas dilution). In addition, in cases where up to 60 wt % of Al(OH)2 may be used, such as in polyolefins, the physical dilution effect cannot be ignored. 

The precursor glass powders may be produced by various methods, the simplest being the milling of quenched glass to an average particle size of 3—15 p.m. Sol gel processes, in which highly uniform, ultrafine amorphous particles are grown in a chemical solution, may be preferable for certain apphcations. 

Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

Because it was not possible to explain the differences in the effectiveness of hydrogen as compared to other gases on the basis of differences in their physical properties, ie, thermal conductivity, diffusivity, or heat capacity differences, their chemical properties were explored. To differentiate between the hydrogen atoms in the C2H2 molecules and those injected as the quench, deuterium gas was used as the quench. The data showed that although 90% of the acetylene was recovered, over 99% of the acetylene molecules had exchanged atoms with the deuterium quench to form C2HD and... 

Stoichiometric Air. If the stoichiometric amount of air is added to the combustion zone, the temperature ia that zoae is coatroUed by removing heat from the system. No eaergy leaves the system as chemical eaergy. The heat is removed by an external means such as generating steam or gas quenching. 

Although o2one can be formed in certain chemical reactions, eg, F2 + H2O and P + O2, and by rapid quenching of plasma-heated oxygen (>3000° C) with Hquid oxygen, these methods have no commercial importance. 

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