Bulk Decomposition Research Articles

On some energetic and chemical properties of the calcite surface

The mass spectrometry studies of the gas released by calcite powders when heated to between room temperature and 500 °C show that neither the surface of crystalline calcite, limited by rhombohedral faces, nor the surface of ground calcite dissociates throughout the whole temperature range. However, when treated by carbon dioxide, the ground calcite surface releases a quantity of this gas equivalent to some 30 % of the calcium carbonate monolayer and at a tempera ture much lower than that of bulk decomposition.

XPS studies of surfaces and thin films of small-band-gap inorganic azides

Since XPS samples only a few surface layers of the solid for most materials, the signal from these first few layers makes a dominant contribution to the total signal. For the inorganic azides available, cross sections indicate that in most cases the surface layers contain less than the stoichiometric amount of azide (nitrogen). Since most of the metals are reactive, the excess metal is predominantly in the form of compounds, e.g. oxides. Carbon and oxygen are always observed as surface contaminants and in many cases the amounts of C and O detected are of the same order of magnitude as the metals and azide. The lack of stoichiometry and C and O contamination may be important for electrical properties involving the surface, e.g., electric-field initiation to detonation of explosive azides. The surface films may tend to provide protective coatings, so that twenty-year-old lead azide does not show significant bulk decomposition. The decomposed surfaces can be reactivated for most azides by exposure to hydrazoic acid. Short exposure of copper metal to HN3 vapor produces a thin film of copper azide.

On some energetic and chemical properties of the calcite surface

The specific heat of immersion of calcite in water depends on the powders used. For ground crystals, the heat of immersion is very high because of the breaking of surface bonds during grinding, and, also, because of a partial surface decomposition of about 30 % during thermal treatment. Calcite crystals, limited only by singular faces, have a low heat of immersion in water because their surface is of high crystallinity and does not dissociate (below the temperature of bulk decomposition).

Pressureless sintering of Si3N4

An investigation of the pressureless sintering of Si3N4 powder with the addition of 5 wt % MgO revealed that shrinkage by a liquid phase mechanism and bulk decomposition are two countervailing processes. Within the temperature range studied, i.e. between 1500 and 1750° C, high densities can be achieved when sintering is performed either for long periods at low temperatures or short periods at higher temperatures. A model is presented showing that pore growth due to decomposition causes a decrease in the driving force for sintering and causes shrinkage to cease.

Catalytic electroreduction of oxygen at a dropping mercury electrode in the presence of colloidal gold

The electroreduction of oxygen at the DME in the presence of colloidal gold has been studied. The first wave of O2 reduction undergoes an increase of its amplitude, while the second wave is split and partially shifted toward less negative potentials. The first and second wave are both dependent on the colloidal gold concentration, the pH and the temperature. The catalytic exaltation of the first wave is attributed to the decomposition of the hydrogen peroxide produced at the electrode. Comparison between the kinetic constants obtained by employing the equations for the catalytic currents with partial regeneration of depolarizer and those experimentally measured from bulk decomposition of H2O2 show that a primary role must be played by the mercury itself. This assumption is supported by the variation of the visible spectrum of the gold colloid caused by the presence of mercury. Analysis of the polarographic measurements, as well as additional evidence from the pertinent literature, indicates that the most likely mechanisms is Au(Hg) + HO2− → Au(HgO) + HO− Au(HgO) + O2−H → Au(Hg) + HO− + O2

Catalytic electroreduction of oxygen at a dropping mercury electrode in the presence of colloidal gold

Summary The electroreduction of oxygen at the DME in the presence of colloidal gold has been studied. The first wave of O2 reduction undergoes an increase of its amplitude, while the second wave is split and partially shifted toward less negative potentials. The first and second wave are both dependent on the colloidal gold concentration, the pH and the temperature. The catalytic exaltation of the first wave is attributed to the decomposition of the hydrogen peroxide produced at the electrode. Comparison between the kinetic constants obtained by employing the equations for the catalytic currents with partial regeneration of depolarizer and those experimentally measured from bulk decomposition of H2O2 show that a primary role must be played by the mercury itself. This assumption is supported by the variation of the visible spectrum of the gold colloid caused by the presence of mercury. Analysis of the polarographic measurements, as well as additional evidence from the pertinent literature, indicates that the most likely mechanisms is Au(Hg) + HO2− → Au(HgO) + HO− Au(HgO) + O2−H → Au(Hg) + HO− + O2

The formation of silicone polymer films on metal surfaces at high temperatures and their boundary lubricating properties

In this paper we have studied the lubrication of heavily-loaded sliding copper contacts by a low viscosity (10 cSt) dimethylsilicone fluid containing a small quantity of a fatty acid surfactant (stearic acid). Effective lubrication is provided by an interfacial monomolecular film of coadsorbed dimethylsilicone and fatty acid molecules. This film continues to maintain low friction and wear with increasing temperature due to the reaction of the acid with the metal surfaces and the subsequent formation of the high-melting-point copper soap. At approx. 100°C, a further marked enhancement in lubrication is observed. Electrical resistance measurements show this to be due to the formation of a thin non-conducting interfacial film, which provides protection for the sliding surfaces up to temperatures in excess of 150°C. Infrared spectroscopic analysis has shown this film to consist of a high concentration of copper stearate embedded in a cross-linked polysiloxane matrix. The molecular structure of the metal soap is complex and possible interactions with the polysiloxane are considered. The results show that polysiloxane film formation first occurs at temperatures of the order of 95°C which is much lower than the bulk decomposition temperature of the dimethylsilicone fluid. Thermal decomposition occurs at these low temperatures due to the catalytic oxidative nature of metal oxide interfaces. A convenient method of analysing the observed variations in electrical resistance across the sliding contacts is described which provides an insight into the effects of increasing load, temperature, repeated traversals of the same wear groove, surface deformation and work hardening. Static electrical resistance measurements employing a delicately loaded gold wire probe show that the load-bearing properties of the film are dependent on the high concentration of copper stearate trapped at the metal interface. The polysiloxane network simply serves as a matrix and in the absence of the fatty acid is readily worn away by the sliding process. Experiments under controlled atmosphere confirm the role of oxygen and metal oxide in the formation of both the polysiloxane and the metal soap. The results emphasize the possible importance of such films in limiting the friction and wear of heavily-loaded metal surfaces at elevated temperatures.

Hot-Atom Chemistry and Thermal Decomposition of Sodium Azide Crystals Containing Nitrogen-13 Recoil Atoms

Summary The reactions of recoil 13N atoms produced by the (n, 2n) reaction in sodium azide have been studied: labelled azide and nitrogen (N2) are the principal products, with yields of about 30 and 65% respectively. The yield of azide shows little dependence on temperature. When samples of the irradiated salt are subjected to thermal decomposition, the specific activity of the nitrogen evolved first falls, then rises and shows a peak during the "induction period" for bulk decomposition. This behavior is interpreted in terms of displacement spike theory.