A perspective on morphology controlled synthesis of powder by tuning chemical diffusion and reaction

This paper is a review of the progress and perspective of an approach for morphology controlled synthesis of particles by tuning chemical diffusion and reaction. Rational synthesis of materials with designed structures is a long term dream of scientists and engineers. The challenges of this dream lie in the poor understanding on the formation mechanism of diverse structures and the insufficient ability to program structure evolutions. From a view of chemical engineering, a shape-controlled synthesis of particles by regulating chemical diffusion and reaction was developed, which was experimentally confirmed by synthesizing diverse morphologies of silver particles at different diffusion and reaction and synthesizing similar dendritic structures of various materials at diffusion limitation. Diffusion and reaction determines the chemical distribution in the growth front of particles, which drives the anisotropic growth of particles, forming diverse morphologies. Ongoing study focuses on the dynamic stability of structure evolution, to establish a general model for structure design and rational synthesis by considering both the local environment and the nature of materials under the concept of mesoscience.     

Combined effects of diffusion and reaction on the growth or dissolution of spherical particles

An established implicit finite difference method was extended to predict the behaviour of particles when both diffusion and interface reaction effect the kinetics of growth or dissolution.A quasi-steadystate model was also obtained. Mixed behaviour must be expected for a relatively large range of conditions, 0.01<a ₀ k/D<10, wherek is a kinetic constant,a _o is the initial radius, andD is diffusivity. Evolution from initial mixed control to strictly diffusion controlled growth is possible for moderatea ₀ k/D, but only when the particle size becomes much larger than the initial size.     

A study of the temperature dependence of adsorption and silicidation kinetics at the Mg/Si(111) interface

The adsorption studies of magnesium on Si(111) substrate have been performed using AES, LEED and EELS at various substrate temperatures. It is observed that the sticking coefficient of magnesium on the silicide surface is close to zero for temperatures greater than 100 °C. It has been shown that the magnesium silicide grows as continuous films on Si substrate at temperature 100-140 °C, while for temperatures higher than 170 °C the magnesium silicide grows in the form of islands.     

Degradation and silicidation of Ta- and W-filaments for different filament temperatures

Using typical conditions for hot wire chemical vapour deposition (HWCVD) of high quality thin silicon films in a UHV deposition chamber, we studied the silicidation of different filaments mainly varying the filament temperatures between 1700 °C and 2130 °C. The experiments were done with constant current, running the filament for 5 to 8 h and even longer. The changes of filament resistance and filament temperature with time will be shown and discussed. We investigated the material changes over the whole filament by Scanning Electron microscopy (SEM), especially the thickness of the formed silicide layers. The change of filament resistance depending on the filament temperature was also monitored, pointing out the different behaviour of tungsten and tantalum filaments. As a result, optimum temperature regimes for tantalum and tungsten filaments could be derived with respect to the filament degradation reducing the filament lifetime. Using a specially developed protection for the cold ends, a tungsten filament could be run for more than 139 h under silane with a filament temperature of T_fil ≈ 2000 °C.     

Link between alginate reaction front propagation and general reaction diffusion theory

We provide a common theoretical framework reuniting specific models for the Ca(2+)-alginate system and general reaction diffusion theory along with experimental validation on a microfluidic chip. As a starting point, we use a set of nonlinear, partial differential equations that are traditionally solved numerically: the Mikkelsen-Elgsaeter model. Applying the traveling-wave hypothesis as a major simplification, we obtain an analytical solution. The solution indicates that the fundamental properties of the alginate reaction front are governed by a single dimensionless parameter λ. For small λ values, a large depletion zone accompanies the reaction front. For large λ values, the alginate reacts before having the time to diffuse significantly. We show that the λ parameter is of general importance beyond the alginate model system, as it can be used to classify known solutions for second-order reaction diffusion schemes, along with the novel solution presented here. For experimental validation, we develop a microchip model system, in which the alginate gel formation can be carried out in a highly controlled, essentially 1D environment. The use of a filter barrier enables us to rapidly renew the CaCl(2) solution, while maintaining flow speeds lower than 1 μm/s for the alginate compartment. This allows one to impose an exactly known bulk CaCl(2) concentration and diffusion resistance. This experimental model system, taken together with the theoretical development, enables the determination of the entire set of physicochemical parameters governing the alginate reaction front in a single experiment.     

Ion-beam-induced silicidation and its use in very-large-scale integration processing

The ion-beam-induced reaction of metal films on a silicon substrates is studied with emphasis on applications in very-large-scale integration processing. Using the Ti/Si system as an example it is shown that, in combination with subsequent rapid thermal annealing, silicides with excellent properties are formed in a uniform and reproducible way. When using dopant elements for the mixing, shallow junctions with a depth of the order of 0.1 μm are demonstrated, which show excellent electrical properties. The advantages of the ion-beam-induced metal-silicon reaction in a self-aligned silicide process are discussed.     

Microwave enhanced reaction kinetics in ceramics

 Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating or processing of a variety of ceramic, glass, and polymer materials. These empirical observations of microwave enhancements have been broadly called the ”microwave effect”. In the past, these claims have been the source of significant controversy, due in part to the lack of a credible and verifiable theoretical explanation. Moreover, certain notable microwave heating experiments have failed to observe any resolvable reaction or transport rate enhancements. This paper describes a series of recent experimental and numerical investigations that have established the fact that strong microwave electric fields induce a (previously unknown) nonlinear driving force for (ionic) mass transport near surfaces and structural interfaces (e.g., grain boundaries) in ceramic materials. This driving force can influence reaction kinetics by enhancing mass transport rates in heterogeneous solid-state reactions. Most of the previously reported observations regarding ”microwave effects” (both for and against) are consistent with the characteristics of this newly identified microwave-induced driving force. Received: 8 April 1997 / Accepted: 14 April 1997     

Self-triggering reaction kinetics between nitrates and aluminium powder

During the night between the 19 and 20 September 2003, a loud explosion occurred at about 3km from the town of Carignano that was clearly heard at a distance of some tens of kilometres. The explosion almost completely destroyed most of the laboratories of the Panzera Company that were used for the production of fireworks. The results of the research activities that were carried out using a differential scanning calorimeter (DSC) on the same raw materials that made up the pyrotechnical mixture that exploded are reported in this paper. This activity was carried out to identify the dynamics of the accident. It proved possible to verify how the event was produced because of a slow exothermic reaction which, after about 8h, caused the self-triggering of 120 kg of finished product. The detonation can therefore be put down to a runaway reaction in the solid phase, whose primogenial causes can be attributed to a still craftsman type production system, not conformed to the rigorous controls and inspections as those required by a safety management system for major risk plants, as the Panzera Company was.     

Asymptotic stability,Onsager fluxes and reaction kinetics

This paper constitutes a first step in the derivation of thermodynamics directly from the dynamics of physical systems. The existence of an asymptotically stable equilibrium point is used to construct a family of admissible entropy functions. These functions have nonnegative entropy production and assume an absolute maximum at the equilibrium point. A nonlinear generalization of the Onsager theory is then used to obtain a one-to-one correspondence between entropy production functions and the governing system of autonomous rate equations. The theory is applied to well stirred chemically reacting systems with constant temperature and pressure. This allows the derivation of chemical potentials, Gibbs' potentials and enthalpy for such systems. The rate equations for reaction kinetics and for classical thermodynamic reaction theory are obtained. Classic thermodynamic reaction theory is shown to give maximum entropy for constant enthalpy at the equilibrium point, while reaction kinetics gives this result only to within quadratic terms in the departure from equilibrium.     

Phase plane analysis of one-dimensional reaction diffusion waves with degenerate reaction terms

This paper demonstrates the existence of a minimum wave speed for reaction diffusion systems whose permanent form travelling wave solutions satisfy the nonlinear ordinary differential equation u' + cu' + f ( u , u' ; c ) = 0 when there are two equilibrium points, one of which is not hyperbolic, under certain restrictions on the behaviour of f . It also shows how to construct a trapping region in the phase plane, which leads to an upper bound on the value of the minimum wave speed. This class of systems includes autocatalytic reactions of order greater than one and combustion waves in premixed gaseous and solid fuels.     

The kinetics of oxidation of diffusion coatings in superheated steam

Abstract

The kinetics of oxidation of Cr-rich, Si–Cr-rich and Al-Cr-rich diffusion layers have been studied in superheated steam within the temperature range 650 and 750°C. The diffusion layers were generated in test-pieces of alloy type P-92 using the pack cementation approach. The thermogravimetric analytical technique was used to determine the kinetic in superheated steam over a period of 100 h. The results were benchmarked against the oxidation kinetic of the bare P92 alloy at 650°C. The results of these short-term experiments indicated a significant improvement in oxidation resistance where the diffusion layers did not experience any apparent attack. The oxidation kinetics of a test-piece of alloy type S304H were also determined at 750°C. The latter type of samples experienced oxidation with the formation of a 1 μm thick Fe–Cr spinel type oxide with discrete patches of break-away oxidation. By comparison the diffusion layers showed no apparent attack. The diffusion layer chemistry remained stable even at 750°C for all coated composition. The characteristics of the diffusion layers will be presented.     

An analysis of reaction diffusion in metal-non-metal systems

The formation of a single M _x N _y intermediate phase in a metal-non-metal system is governed by two parabolic growth regions if the metal side of the couple is finite in extent and initially unsaturated with non-metal. The first parabolic region is described by semi-infinite conditions on the metal side of the couple, while the second is according to saturated conditions. The intermediate phase reaction layer grows non-parabolically when conditions are such that the concentration profile cannot be described by either semi-infinite or saturated conditions on the metal side of the couple. Expressions have been obtained relating the growth constants to diffusion coefficients and information from an equilibrium phase diagram. The mathematical formulation of the problem was tested by application to the titanium-carbon system. The diffusion coefficient of carbon in titanium carbide was found to be in close agreement with values reported elsewhere in the literature.     

Object-oriented approach to simulation of chemical reaction kinetics

The structure and main elements of realization of a library of classes for simulation of the kinetics of chemical reactions are considered. Due to a modern approach used in the development of the library, it is characterized by high universality and adaptability and provides a high rate of calculations.__________Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 1, pp. 153–158, January–February, 2005.     

The kinetics of hydrogen embrittlement: A quantitative diffusion model

Stress assisted diffusion of hydrogen towards the tip of a notch or crack is described by adding a stress-dependent term to Fick's first and second law of diffusion. It is postulated that a critical combination of hydrogen concentration and stress will develop at the edge of the plastic enclave, and that this will produce an incremental crack growing from the edge of the plastic zone towards the notch or crack tip. Crack propagation is treated as a succession of incremental crack bursts. Equations are developed from which the primary and secondary incubation times can be calculated. Approximate solutions are presented for the case of a crack in an infinitely wide flat plate.     

Polarization controlled kinetics and composition of trivalent chromium coatings on aluminum

Combined in situ spectroscopic ellipsometry and electrochemistry have been employed to monitor, in real-time, the formation of trivalent Cr conversion coatings on polished Al substrates at applied sample potentials. It is found that the formation kinetics and chemical composition of the film can be controlled by adjusting the anodic and cathodic reactions. The growth kinetics are accelerated at more positive anodic potentials or more negative cathodic potentials. At more negative potentials, the percentage of chromium in the coating is found to increase, while the zirconium percentage decreases.