Volume 72, Issue 11

Now we can use several temperature control modes, for example, isothermal run including stepwise heating and cooling, constant rate heating (or cooling), temperature control for sample thermal history, sample-controlled thermal analysis (SCTA or controlled-rate thermal analysis, CRTA), temperature jump, rate jump, temperature modulation, and repeated temperature scanning. Their advantages and drawbacks are reviewed with some illustrative examples, especially for application to kinetic analysis. The combined use of these varieties of temperature control mode is recommended by showing examples. Temperature modulation and repeated temperature scanning are discussed in comparison with temperature-modulated differential scanning calorimetry (DSC), and common and analogous points are elucidated. In relation to this, the possibility of the imaginary part of the overall reaction rate constant in complex reactions is postulated. Finally, these modes are classified and tabulated from two viewpoints, and other possible modes are shown.

The exact knowledge of hydrocarbon oxidation kinetics is very important due to the fact that this process is involved in many technological processes: combustion in engines and furnaces, flame synthesis of materials, partial oxidation processes in chemical technology, catalytic combustion, and exhaust gas treatment, etc. An overview is given on the present state of the art with respect to kinetic data on gas-phase and (shortly) surface oxidation of hydrocarbons. Furthermore, some applications are described in the areas mentioned above. Examples for the importance of the gas-phase oxidation of hydrocarbons are ignition and combustion in engines and furnaces and partial oxidation processes in industrial chemical reactors. In many applications, both gas-phase and surface chemistry are taking place. Examples here are flame generation of diamonds and syngas generation.

Existing thermodynamic and vaporization data for liquid refractories are based either on estimates or on data extrapolated from studies on the solids obtained at much lower temperatures. Previously, we have shown that pulsed laser heating, coupled with time-dependent mass spectrometry of the free-expansion vapor plume, can be used for semi-quantitative measurements of vaporization thermochemistry. The present work extends this approach with the development of (a) more direct, and more accurate, methods for determining the system temperature and pressure; (b) improved experimental and theoretical determinations of key parameters such as ionization cross sections; and (c) improved characterization of the gas dynamic expansion and thermal equilibration processes. Example material systems considered include C, SiC, Al 2 O 3 , ZrO 2 —7%Y 2 O 3 , and Y 2 O 3 at temperatures and total pressures typically in the range of 3000 to 5000 K and 0.01 to 10 bar, respectively (1 bar = 10 5 Nm -2 ).

Containerless conditions allow well-controlled investigation of liquids at high temperatures. Levitation methods used for this purpose are reviewed, and their application is illustrated by discussion of the properties and behavior of deeply undercooled yttrium-aluminum-oxide melts.

The kinetics of solid-solid interfaces controls in part the course of heterogeneous reactions in the solid state, in particular in miniaturized systems. In this paper, the essential situations of interface kinetics in solids are defined, and the basic formal considerations are summarized. In addition to the role interfaces play as resistances for transport across them, they offer high diffusivity paths laterally and thus represent two-dimensional reaction media. Experimental examples will illustrate the kinetic phenomena at static and moving boundaries, including problems such as exchange fluxes, boundary-controlled solid-state reactions, interface morphology, nonlinear phenomena connected with interfaces, and reactions in and at boundaries, among others.

Photoelectron spectra of negatively charged Ni n - , Cu n - , and Co n - clusters are presented. The spectra of the Ni n - clusters contain information about the degree of localization of the 3d valence orbitals, which is strongly related to the magnetic properties. In the clusters with less than 7 atoms, the 3d orbitals seem to be almost totally localized. For Co 13 - we find indications of a narrowing of the 3d-derived valence "bandwidth" corresponding to a presumed high symmetry and a high degree of degeneracy of the 3d electronic states.

An overview is given of transport reactions and corrosion phenomena in metal halide gas discharge lamps filled with a mixture of alkali halides with scandium or rare-earth iodides. The phenomena that are discussed are: (a) pressures of light-emitting species above the molten salt mixture, (b) interaction of the metal halides with the vessel wall, quartz glass as well as polycrystalline alumina, (c) transport processes along the vessel wall giving rise to wall corrosion, and (d) transport to and from the electrodes (i.e., electrode corrosion and wall blackening).

Chemical vapor deposition (CVD) processes have a large throwing power and can operate at atmospheric pressure. Therefore, they are economical for many applications. High-temperature CVD processes give a good control of the crystal structure (e.g., epitaxial layers or other special layer structures) and support diffusion processes. The are used, for example, for the following deposition processes: 1) perovskites, 2) yttrium-stabilized ZrO 2 layers as ion conductors or heat barrier coatings, 3) aluminide diffusion coatings in long tubes for corrosion protection, and 4) BN on fibers for fiber-reinforced materials. The special properties of the perovskite CVD are discussed in more detail. These processes are described for small- and large-scale applications. The CVD process is simulated by the computer code Fluent.

The influence of thermal and electrical conductivities and relative density on the dynamics of self-sustaining reactions activated by an electric field was investigated by modeling. Under a given field, the variation of the wave velocity and maximum temperature with normalized electrical and thermal conductivities and density exhibited a maximum at an optimum value of each of the three parameters. The results are discussed in terms of a Fourier relationship modified by the Joule heat contribution of the imposed field. A simulation was also made to investigate the configuration of the reaction front in systems that are ignited and sustained by a current. A transition from a volume (uniform) combustion reaction to a wavelike form was found to occur as the sample size is increased. These changes are also attributed to the interrelation between the thermophysical parameters.

Experiments on laser flash photolysis (308 nm) of Fe(OH) 2+ (aq) complex in aqueous solution with addition of nitrobenzene demonstrate the formation of hydroxyl radical in the primary photochemical process.

In performing risk assessments for plant protection products by applicants or regulators in relation to the registration of the products, an important aspect to take into account is the foliar interception and retention of the active substance of the product on the plant. An overview is given of the approaches to this item in several parts of the world. The relevant circumstances and influencing variables, such as growth phase, planting density, and some physicochemical characteristics (e.g., vapor pressure and Henry's coefficient) are dealt with. Finally, a proposal is presented for how to take into account the phenomenon of foliar interception and retention in the initial phase, first tier, of the risk assessment process.