Growth in pores

Xue, L. A., Thermodynamic benefit of abnormal grain growth in pore elimination during sintering, J. Am. Ceram. Soc., 72, 1536 37, 1989. 

V. Tikare, E. A. Holm. Simulation of grain growth and pore migration in a thermal gradient. J Am Ceram Soc 8PAS0, 1998. 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

The pores at the surface are smaller than those in the bulk of PS as, for example, shown in Figure 11, 8,16,24 Such an increase in pore diameter from the surface to bulk is due to the transition from pore initiation to steady growth. Also, two-layer PS, a micro PS layer on top of a macro PS can form for on illuminated n-Si or on lowly doped p-Si. For the micro PS layer formed on front illuminated n-Si, pore diameter is less than 2 nm and thickness of PS changes with illumination intensity and the amount of charge passed. Also, the diameter of macro pores on front illuminated n-Si changes with the amount of charge Passed.20 Pore size and depth variation of PS on n-Si are very different for front and back illuminated n-Si samples. 

When the surface is completely covered by an oxide film, dissolution becomes independent of the geometric factors such as surface curvature and orientation, which are responsible for the formation and directional growth of pores. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus have intrinsically identical structure in all orientations. Also, on the oxide covered surface the rate determining step is no longer electrochemical but the chemical dissolution of the oxide.1... 

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

The experimentally observed parabolic increase in pore depth and linear decrease in concentration shown in Fig. 9.18 c indicate that Eq. (9.6) is valid [Le9]. The macropore growth rate decreases linearly with l according to Eq. (9.6). If, therefore, a constant pore diameter is desired for a macropore array, a decrease in etching current or illumination intensity, respectively, with time is required. 

In some instances, to improve solderability, tin is deposited on nickel surfaces. In a short time, however, interdiffusion of the two metals results in the growth of an inter-metallic NiSn3 compound that is much less amenable to soldering. For tin over elec-trolessly deposited nickel surfaces, the interdiffusion results in pores in both films. Pores are to be avoided, of course, if conductivity and/or contact resistance is an issue. 

Nienow (1983a) observed a delay in the start of particle growth when binder was added to a bed of porous particles and stable fluidization under conditions which produced quenching with non-porous particles. Nitrogen adsorption measurements showed that the pore surface area of alumina decreased as spraying proceeded, indicafing that an effective reduction in pore volume was taking place. 

Fig. 8. Normalized tissue growth in amorphous PLLA devices of different pore sizes ( 500 pm ( ) 179 pm (O) and 91 pm (A)) as a function of implantation time [45], The error bars designate means + range of two sections...

Wake, M.C., Patrick, C.W and Mikos, A.G., Pore morphology effects on the fibrovas-cular tissue growth in porous polymer substrates. Cell Transplant. 3, 339,1994. Gion, T, Shimada, M., Shirada M., et al. Evaluation of a hybrid artificial liver using a polyurethane foam packed-bed culture system in dogs. Journal of Surgical Research 82, 132-136 1999. 

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