Hot-Pressing Behavior of Si3N4

The densification behavior of Si₃N₄ containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si₃N₄. Evidence of a second-phase material between the Si₃N₄ grains was obtained through etching studies. Transformation of α- to β-Si₃N₄ during hot-pressing is not necessary for densification.     

ABA block copolymers without tapering by isokinetic living anionic polymerization in a series of stirred tank reactors

Equations for the molecular weight distribution and the average molecular weights of a living anionic polymerization with very rapid or slow initiation are presented for a series of stirred tank reactors. It is shown that polydispersity of the polymer in the last vessel of a cascade takes for nearly complete conversion of monomer then a minimum, when the reaction is performed isokinetically with concern to monomer conversion, i.e. when monomer conversion in each vessel of a cascade of n tanks is 1/n. Now, polydispersity of the polymer in the mth vessel of the cascade (1 ≤ m ≤ n) is D_m = 1 + 1/m. Values of all systems variables needed for isokinetic control of the polymerization may be precalculated from kinetic models. For vessels of equal size and with equal residence times reaction temperature is suggested as control variable increasing stepwise from vessel to vessel. The proposed procedure was checked for the systems styrene/n-butyllithium/toluene, styrene/s-butyllithium/benzene, and isoprene/s-butyllithium/n-heptane. A satisfying agreement between calculated and measured values of the objectives monomers conversion, number average molecular weight, and polydispersity was found. It is proposed to use the described procedure for the preparation of ABA block copolymers with negligible tapering between the blocks by living anionic polymerization in a series of stirred tank reactors.     

Dielectric relaxation of poly(chlorostyrenes) and their copolymers in the primary transition region

Dielectric relaxation measurements were carried out on a series of bulk poly(chlorostyrene) homopolymers and random copolymers over the frequency range from 50 to 100 kHz and at temperatures in the neighbourhood of the glass transitions of the polymers thus encompassing the α relaxation. Homopolymers examined were polystyrene (PS), poly(2-chlorostyrene) (P2CS), poly(3-chlorostyrene) (P3CS), and poly(4-chlorostyrene) (P4CS). Copolymers were poly(styrene-co-2-chlorostyrene) (PS2CS), poly(styrene-co-4-chlorostyrene) (PS4CS), and poly(2-chlorostyrene-co-4-chlorostyrene) (P2CS4CS). The dielectric data were analysed to yield dipole moments and Kirkwood—Fröhlich correlation parameters. The shapes of the dielectric loss curves were also taken into account. Glass transition temperatures were determined by differential scanning calorimetry (d.s.c.). It was concluded that the phenyl ring rotates freely in the α relaxation regions of PS, P4CS, and P3CS, but not in P2CS. The dipole moments of the copolymers are correlated with dyad distributions calculated from reactivity ratios.     

Heat transfer

Conclusions Numerous new heat transfer developments have been reported in the last few years. These developments are useful for a better understanding of the basic theory and for developing improved equipment. In many cases both heat and mass transfer occur simultaneously in commercial operations.     

Technical new feature

A continuous process for the commercial production of isopropenyl stearate (IPS) from triple pressed stearic acid and a stabilized form of propyne has been developed. Cost estimates, including capital costs, operating costs, and profitability, for commercial scale plant production which show the process to be economically feasible are presented. This potentially profitable process offers the advantages of reliable raw material sources, minimal external thermal requirements, and usable process waste streams. For a plant producing 5 million pounds of IPS per year, the selling price range is 80 to 107 cents/lb IPS, corresponding to a raw material cost range of 27 to 54 cents/lb of IPS. For a 20 million pound per year plant, the selling price range is 58 to 85 cents/lb IPS. The selling prices include a 20% annual return on fixed capital investment. Fixed capital requirement ranges from 2.7 to 10.9 million dollars (3rd quarter, 1975) for plants ranging in size from 5 to 50 million pounds of IPS per year, respectively.     

Stereochemistry and mechanism of the hydrogenation of dialkyl cyclohexenes

In order to investigate the steric effect of the substituents to determine the product distribution, disubstituted cyclohexenes were hydrogenated over several transition metal catalysts. Some cyclohexenes which have two large substituents at the vicinal carbon atoms, at least one of which is trigonal, were not hydrogenated over Raney Ni at all but were hydrogenated over Pt catalyst under our experimental conditions. Presumably, the stereoselectivity depends on the competitive operation of the torsional angle strain and the catalyst hindrance with substituents. In the hydrogenation over Pd catalyst, thermodynamically more stable products were dominant at the standard condition but at high substrate to catalyst ratio the less stable products were slightly preferred. No appreciable stereoselectivity was observed in the hydrogenation of 1,4-disubstituted cyclohexenes.     

Pile electrique a anode d'aluminium—I electrolyte acide

An acid electrolyte (pH ) 3) for an aluminium primary battery has been studied. It contains: Al(CIO₄)₃. 1 M: AlCl₃. $\text{1}\text{30}$ M; (NH₄)₂CrO₄, $\text{1}\text{50}$ M. The performance of three binary aluminium alloys AlZn 2%; AlZn 5%; AlGa 0·3% are compared. Capacity of Al primary battery made with Al 2% is twice that of a conventional Leclanché cell on continuous discharge test. Unfortunately CrO⁼₄ disappears in presence of MnO₂ which is prejudicial to the good conservation of the aluminium cell.     

Homogeneous Catalyzed Ozone Decomposition in the Presence of Co(II).

The homogeneous decomposition of ozone in the presence of a Co(II) catalyst has been investigated in aqueous solution. Under the conditions investigated (Co(II) concentration: 0.0 – 2.0?ppm, pH: 1.6- 8.4, O₃ concentration: 5 10^?5 – 2.0 10⁴?M) the process can be assumed to follow pseudo first order kinetics with respect to ozone. Cobalt concentration dependency also obeys first order kinetics although it may be considered to reach a steady state concentration. pH exerts a positive influence on the decomposition rate from 1.6 to 7.1, the process leveling off at pH 8.4. An Arrhenius analysis of the temperature effect gave a moderate activation energy of the global reaction (E=10917?cal mol^?1). A more detailed radical mechanism than a simple pseudo first order reaction can be postulated for simulation purposes. By numerical optimization of some unknown kinetic constants the influence of several operating variables can be adequately predicted.     

Modeling of fast pulse responses in the Multitrack: an advanced TAP reactor

A detailed transport model for the Multitrack setup, a TAP-like system, has been developed, which allows further analysis of adsorption, diffusion and catalysis phenomena. This includes the transport in the void part between the pulse valve and the reactor inlet. The effects of viscous flow and thermal transpiration, aspects that have not been studied in detail before for this type of setup, have been analyzed. A new expression for the modeling of the output signal is proposed depending on the positioning of the MS detector used in the study. The transport parameters of the model have been estimated by the analysis of the experimental pulse responses of the empty reactor system and the reactor charged with an inert packed bed. The proposed model reproduces the experimental pulse responses very well, and therefore can be extended to study systems with reacting or adsorbing beds by including the corresponding rate equations for the processes occurring in the bed of particles.     

Aqueous electrochemical intercalation of bromine into graphite fibers

For the first time, graphite fibers have been electrochemically intercalated with Br⁻ that have the same structure and properties as those intercalated from vapor phase Br₂. This was accomplished by intercalating pitch-based Thornel^® K-1100 graphite fibers at low temperature (near 0 °C) and high currents (2 A) for long times (6 h). The mechanism appears to be that Br⁻ is oxidized to aqueous Br₂ which, when sufficient local concentration builds up, intercalates the fiber. This was confirmed by intercalating K-1100 fiber in a saturated aqueous Br₂ solution without passing an electrical current. The applied voltage does apparently lower the activation energy of the reaction as evidenced by the observation that P-120 and P-100 fibers will not intercalate in aqueous Br₂ unless a voltage is applied.