Vacuum decomposition process

The role of bulk diffusion in controlling reaction rates is expected to be significant during surface (catalytic-type) processes for which transportation of the bulk participant is slow (see reactions of sulphides below) or for which the boundary and desorption steps are fast. Diffusion may, for example, control the rate of Ni3C hydrogenation which is much more rapid than the vacuum decomposition of this solid. 

The vacuum decomposition [1106] of nickel maleate at 543—583 K was predominantly a deceleratory process. Following an initial surface reaction, data fitted the kinetic expression... 

Bruder and Brenn (1992) studied the spinodal decomposition in thin films of a blend of deuterated polystyrene (dPS) and poly(styrene-co-4-bromostyrene) (PBrxS) by TOF-ERDA. They examined the effect of different substrates on the decomposition process. In one series of experiments, a solution of the polymers in toluene was spread on a silicon wafer to form a film of thickness 550 nm which was then heated in vacuum at 180°C for various times. 

A number of special vacuum distillation processes have been developed, whereby thermal decomposition of the oil is kept to a minimum. One of these employs thin film vaporization, in which mercury vapor is employed as an indirect heating medium (89, 71). Another employs a carrier other than steam—for example, kerosene—in the vaporization of the heavy lubricating oil fractions (16). These processes are being operated on a commercial scale, but apparently because of their special nature they have not been widely adopted by the petroleum industry. 

Over the last ten years, the vacuum pyrolysis process has been developed at the bench and pilot scales at the Universite Laval and Pyrovac Institute Inc. in Quebec, Canada. This thermal decomposition process enables a large variety of solid and semi-Uquid wastes to be transfoimed into useful products. Vacuum pyrolysis is typically carried out at a temperature of 400-500°C and a total pressure of 2-20 kPa [1], These vacuum conditions allow the pyrolysis products to be rapidly withdrawn from the hot reaction chamber, thus preserving the primary fragments originating from the thermal decomposition reactions. 

While the model is successful in simulating the results for vacuum pyrolysis, care must be taken in extending the results to other thermal decomposition processes where the reactions may be more complicated. 

Rienaecker and Werner [36] suggested that Baj(Mn04)2 is the initial decomposition product, by analogy with results for KMn04- Isothermal ar-time curves for vacuum decomposition at 413 to 463 K were sigmoid [37] following an initial (or= 0.04) burst of gas. The acceleratory process was fitted by the Prout-Tompkins equation ( , = 151 kJ mol ) and the power law with = 2. Fragmentation into thin platelets was observed. The deceleratory period could be described by the first-order equation with , = 124 kJ mol. ... 

A comparative study of the thermal reactions in oxygen of the above three reactants [118], together with nickel formate, showed that the shapes of the ar-time curves were comparable with those for vacuum decompositions. The magnitudes of the Anhenius parameters (with the exception of the decomposition of the formate which does not give carbide product) were diminished. It was concluded that the initiation of reaction was unaffected by the presence of oxygen, so that the geometry of each rate process was unchanged. In the presence of Oj, interfacial reactions on the nickel oxide product (-+ COj + HjO) proceeded appreciably more rapidly by a common mechanism and all values of E, were close to 150 kJ mol. ... 

Earlier laboratory processes used to prepare metallic protactinium include vacuum decomposition of the oxide by 35-keV electrons [G6] and thermal decomposition of the pentahalides [G2]. More recently, protactinium has been prepared by reducing the tetrafluoride by barium vapor [CS, S3, Zl], by calcium at 1250°C [M2] and by zinc-magnesium. The purest protactinium has been prepared by reduction in a barium-fluoride crucible at 1300°C [L2]. 

The thermal decomposition of single-crystal and powder samples of KN3, in an ultra-high-vacuum environment, has been studied. Mass spectrographic detection was used to monitor the decomposition products, and, from the temperature dependence of the partial pressure of nitrogen, three decomposition regions were found. Azide radicals and potassium atoms were also detected. The application of an electric field across the sample during decomposition was investigated, and models for the decomposition process were discussed. ... 

The tensile strength of Si-C-0 fibers decreases after exposure to elevated temperatures. When Nicalon NL 200 fibers are exposed for 1 hour to 1300 C in argon (P = 100 kPa), their mean tensile strength and scale parameter, Co, decrease by 45% while their Weibull modulus remains unchanged [80-83]. Fibers exposed to more severe conditions (e.g., for 5 hours in a vacuum at 1500°C) are so weak that they cannot be tested. Finally, the fact that oxygen-free fibers maintain their tensile strength under similar conditions relates to the absence of silicon oxycarbide and its decomposition process. 

As will be seen below, we obtained similar and sometimes even better results with carbon layers. It is well known that so-called isotropic carbon is especially compatible with blood /7/. It is deposited as pyrolytic carbon in a fluidized bed of a hydrocarbon-noble gas mixture at relatively low pyrolysis temperatures between 1200 and 1500 C (LT = low temperature). Since this process can on ly be used with a heat-stable substrate material with low thermal expansion coefficients, we (and others /8/) used vacuum-coating processes for the deposition of similar coatings onto polymers. In our work we used two processes with one, the results of which will be reported in sect. 5, the carbon is separated out of a hydrocarbon-noble gas mixture in the set-up shown in Fig.2. However, not heat, as in pyrolysis, but rather a glow discharge is used for the decomposition. 

In thermal debinding, the binder is removed as a vapor by heating at ambient pressure in an oxidizing or nonoxidizing atmosphere or under a partial vacuum. The process is influenced by both chemical and physical factors. Chemically, the composition of the binder determines the decomposition temperature and the decomposition products. Physically, the removal of the binder is controlled by heat transfer into the body and mass transport of the decomposition products out of the body. In practice, binder systems consist of a mixture of at least two components that differ in volatility and chemical decomposition. The ceramic powder may alter the decomposition of the pure polymer. In view of the complexity of real systems, we first consider the basic features of thermal debinding for a simplified system consisting of a powder compact with a single binder, e.g., a thermoplastic polymer such as poly(methyl methacrylate) or polyethylene. Later,... 

Chances for successful identification and quantification are considerably enhanced when analytes are separated. For solutions, chromatography is the supreme tool, whereas for solids some form of thermal treatment may achieve fractionation of matter according to volatility. Vapour evolution from polymers may be controlled and studied by various means, such as sublimation, thermal distillation, vacuum TG-MS, thermal evolution analysis (TEA) including TVA, headspace techniques or thermal desorption. It is obviously desirable that evaporation of the additives takes place below the decomposition temperature of the polymer. In principle, this can also be realised in thermal-programmed pyrolysis (dry distillation in vacuum). Desorption processes are controlled by diffusion. 

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