Infiltration Kinetics

In this chapter, we describe the synthesis and characterisation of the microstructure and properties of layered-graded alumina-matrix composites through liquid infiltration. This approach is relatively simple and offers excellent control over the depth of the graded layer. The presence of a graded dispersion of reinforced particles in the alumina matrix has a profound influence on the physical and mechanical properties of the composites. An overview of the infiltration kinetics and the use of the infiltration process as a new philosophy for tailoring novel graded ceramic systems are also presented. 

There are a number of formulae which are relevant for modelling the infiltration kinetics of a liquid into preforms. The first equation to calculate the height of infiltration against time was formulated by Washburn (1921) ... 

In order to successfully model the infiltration kinetics in terms of the effects of presintering temperature, type of infiltrant, infiltration environment, and multiple infiltrations, the pore radius of alumina preform (presintered at 1000°C) was measured using water as infiltrant, since the viscosity and... 

The pore radius determined from infiltration kinetics can be reconciled to the experimentally determined radii from SEM and porosimetry, by assuming a two-pore-size model (pore neck and pore bulge), instead of a single capillary pore-size [Dullien etal., 1977 Einset, 1996]. The schematic diagram for this two-pore-size model is shown in Fig. 5.2(b). 

Huang, J., Zhang, R. Y., Li, C. M. (2005). Infiltration kinetics analysis of molten salt infiltrating porous ceramic. Journal of Guangdong University of Technology, 22(2), 1-5. 

CVI is a special CVD process in which the gaseous reactants penetrate (or infiltrate) a porous structure which acts as a substrate and which can be an inorganic open foam or a fibrous mat or weave. The deposition occurs on the fiber (or the foam) and the structure isgradually densified to form a composite.The chemistry and thermodynamics of CVT are essentially the same as CVD but the kinetics is different, since the reactants have to diffuse inward through the porous structure and the by-products have to diffuse out.f l Thus, maximum penetration and degree of densification are attained in the kinetically limited low-temperature regime. 

Phosphoenolpyruvate carboxykinase (PEPCK) deficiency is distinctly rare and even more devastating clinically than deficiencies of glucose-6-phosphatase or fructose-1,6-bisphosphatase. PEPCK activity is almost equally distributed between a cytosolic form and a mitochondrial form. These two forms have similar molecular weights but differ by their kinetic and immunochemical properties. The cytosolic activity is responsive to fasting and various hormonal stimuli. Hypoglycemia is severe and intractable in the absence of PEPCK [12]. A young child with cytosolic PEPCK deficiency had severe cerebral atrophy, optic atrophy and fatty infiltration of liver and kidney. 

Due to the fact that industrial composites are made up of combinations of metals, polymers, and ceramics, the kinetic processes involved in the formation, transformation, and degradation of composites are often the same as those of the individual components. Most of the processes we have described to this point have involved condensed phases—liquids or solids—but there are two gas-phase processes, widely utilized for composite formation, that require some individualized attention. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) involve the reaction of gas phase species with a solid substrate to form a heterogeneous, solid-phase composite. Because this discussion must necessarily involve some of the concepts of transport phenomena, namely diffusion, you may wish to refresh your memory from your transport course, or refer to the specific topics in Chapter 4 as they come up in the course of this description. 

In Section 3.4.2, we introdnced the concept of chemical vapor infiltration, CVI, in which a chemical vapor deposition process is carried out in a porous preform to create a reinforced matrix material. In that section we also described the relative competition between the kinetic and transport processes in this processing technique. In this section we elaborate npon some of the common materials used in CVI processing, and we briefly describe two related processing techniques sol infiltration and polymer infiltration. 

Kinetics Trans- formations, Corrosion Devitrification, Nucleation, Growth Polymerization, Degradation Deposition, Infiltration Receptors, Ligand binding... 

NACs in a laboratory column system containing aquifer material from the banks of a river-groundwater infiltration site (Fig. 14.11). The columns were run under ferrogenic conditions. Note that zero-order kinetics suggests that the reactive sites were always saturated such as encountered in enzyme kinetics at saturation (Box 12.2). In this system, all model compounds as well as other NACs including again TNT, ADNTs, and DANTs (data not shown, see Hofstetter et al., 1999) reacted at virtually the same rate. However, when present in mixtures, the compounds showed competition for the reactive sites. A competition quotient, Qc (competition with the reference compound 4-C1-NB present at about equal concentrations) was determined for all model compounds ... 

Liquid infiltration into dry porous materials occurs due to capillary action. The mechanism of infiltrating liquids into porous bodies has been studied by many researches in the fields of soil physics, chemistry, powder technology and powder metallurgy [Carman, 1956 Semlak Rhines, 1958]. However, the processes and kinetics of liquid infiltration into a powdered preform are rather complex and have not been completely understood. Based on Darcy s fundamental principle and the Kozeny-Carman equation, Semlak Rhines (1958) and Yokota et al. (1980) have developed infiltration rate equations for porous glass and metal bodies. These rate equations can be used to describe the kinetics of liquid infiltration in porous ceramics preforms, but... 

To study the kinetics of infiltration, a bar of the alloy under study was placed in the centre of the cell, and a diamond powder being studied was placed between the bar and the tube heater [2]. The upper disc is made of a conducting while the bottom one of nonconducting material. 

The kinetics of diamond powder infiltration with cobalt of VK15 sintered carbide and Co-Mo and Co-Ti melts was studied experimentally at 8 GPa (Fig, 1). Confidence intervals for T and k values, the reliability being a= 0,95, do not exceed 8 %. According to [3], the limit of WC solubility in Co attains 10 mass % or 3.2 at. %. The additive contents of Co-Mo and Co-Ti alloys was 10 mass % (accordingly, the atomic portions were 0.12 Ti and 0,064 % Mo). Samples of alloys were sintered from mixtures of cobalt-molibdenum and cobalt-titanium hydride powders in a vacuum furnace at 1000 °C. 

In both the cases, the activation energy of the infiltration process (Fig. 1) is practically the same ( 40 kJ/mol) and close to that of the cobalt viscous flow (37.3 kJ/mol) [8], i.e. the observed variations in kinetics of the infiltration are caused by the difference between viscosities of cobalt and its alloys. According to [8] y= Aq exp(Ea/RT), where y is the kinematic viscosity, Aq is the constant, Ea is the activation energy of viscous flow, R is the universal gas constant, T is the absolute temperature. Correlating this formula with Eq. (2) and considering that q = py, where p is the density of the liquid, we have for k ... 

Calculated Eq.(4) and experimental values of the k/kq are given in the Table. It is seen that kinetics of a diamond powder infiltration with the Co-WC and Co-Ti alloys is described by Eq.(4), while for the Co-Mo alloy, k/Kq experimental and theoretical values do not agree and though the k value decrease as compared with that of pure cobalt, but not to the extent that would be expected from Eq.(4). 

B.W. Sheldon and T.M. Besmaim, Reaction and Diffusion Kinetics During the Iititial Stages of Isothermal Chemical Vapor Infiltration, Journal of American Ceramic Society, Vol.74, 1991, p.3046. 

Borovinskaya, I. P, Merzhanov, A. G., Mukasyan, A. S Rogachev, A. S., and Khusid, B. M Macro-kinetics of structure formation during infiltration combustion of titanium in nitrogen. Dokl. Akad. Nauk SSSR, 322,912 (1992a). 

Nakamura M. and Watson E. B. (2001) Experimental study of aqueous fluid infiltration into quartzite implications for the kinetics of fluid redistribution and grain growth driven by interfacial energy reduction. Geofluids 1, 73—89. 

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