Sintering surface tension

Ultimately, the surface energy is used to produce a cohesive body during sintering. As such, surface energy, which is also referred to as surface tension, y, is obviously very important in ceramic powder processing. Surface tension causes liquids to fonn spherical drops, and allows solids to preferentially adsorb atoms to lower tire free energy of tire system. Also, surface tension creates pressure differences and chemical potential differences across curved surfaces tlrat cause matter to move. 

During pressure sintering, interiDarticle compressive stress, approximated by the externally applied stress and nonnalized by the relative density of the compact p, supplements the surface tension driving force for pore shrinkage ... 

Herring C 1949 Surface tension as a motivation for sintering The Physics of Powder Metallurgy ed W E Kingston (New York McGraw-Hiii) pp 143-79... 

In the same year as Kuezynski s research was published, Shaler (1949), who had done excellent work on measuring surface energies and surface tensions on solid metals, argued that surface tension must play a major part in fostering shrinkage of powder compacts during sintering his paper (Shaler 1949) led to a lively discussion, a feature of published papers in those more spacious days. 

In electrostatic atomization, an electrical potential is applied between a liquid to be atomized and an electrode placed in the spray at a certain distance from liquid discharge nozzle. As a result of the mutual repulsion of like charges accumulated on the liquid surface, the surface becomes unstable and disrupts when the pressure due to the electrostatic forces exceeds the surface tension forces of the liquid. Droplets will be generated continuously if the electrical potential is maintained above a critical value consistent with liquid flow rate. Both DC and AC systems have been employed to provide high electrical potentials for generating fine droplets. Many configurations of electrode have been developed, such as hypodermic needles, sintered bronze filters, and cones. 

The studies reported in the literature have suggested that the surface tension of Cu depends on its surrounding environment it is higher in vacuum and varies as vacuum > H2 > CO. Well-rounded particles are likely to form when the surface tension is low. In CO, the surface tension is lowered to the extent that the Cu prefers to spread out as sheets rather than as three-dimensional spherical particles. Experiments carried out on real (practical) powder catalysts are consistent with the data from the model systems. As in the model systems, sintering by Cu particles is dominant, the particles growing to several tens of nanometres. The type and extent of sintering depend on the exact composition of the bimetallic catalyst. For Cu > Ru, ETEM studies show the sintering of Cu to be primarily by particle coalescence. 

Although more expensive than melt fiberization, the sol processes offer advantages in fiber chemistry selection. In melt fiberization, viscosity and surface tension are gready influenced by additions of small quantities metallic oxides. In the sol process, where viscosity can be controlled independently, any number of metal salts may be added without adverse effects. These salts can serve as grain growth inhibitors, sintering aids, phase stabilizers, or catalysts. 

The dominant mechanism and transport path—or combinations thereof—depend upon material properties such as the diffusivity spectrum, surface tension, temperature, chemistry, and atmosphere. The dominant mechanism may also change as the microstructure evolves from one sintering stage to another. Sintering maps that indicate dominant kinetic mechanisms for different microstructural scales and environmental conditions are discussed in Section 16.3.5. 

When solid particles come in contact with each other at elevated temperatures, they tend to coalesce, thereby decreasing the total surface area. This process is called sintering (18). It is usually accompanied by a decrease in the total volume of the particulate bed. A decrease in surface area brings about a decrease in (surface) free energy hence, the surface tension is the driving force for the coalescence process. 

Sintering of PS Pearls Calculate the rate of coalescence of PS pearls made from suspension polymerization, which are 0.2 cm in diameter. The temperature of the sintering process is 180°C. Use the Power Uaw constants of the unmodified PS in Appendix A. The surface tension of the melt can be taken to be 32.4 dyne/cm.6... 

Porous plates produced by sinteration of glass powders are widely used, especially in laboratories. However, both the size of pores and their cross-section along the plate height vary in a wide range. Hence, the number of active pores depends on gas pressure and surface tension of the solution. Increase in pressure activates all smaller pores. 

Fig. 2.4 presents a measuring cell with a porous plate made of sintered glass (similar to variant C, Fig. 2.2). Porous plates of various pore radii can be used (usually the smallest radius is about 0.5 p.m) [23]. In this case the meniscus penetrates into the pores and their radius determines the radius of curvature, i.e. the small pore size allows to increase the capillary pressure until the gas phase can enter in them. The radius of the hole in which the film is formed is usually 0.025 - 0.2 cm. To provide a horizontal position of the film the whole plate is made very thin. In the porous plate measuring cell (Fig. 2.4) the capillary pressure can be varied to more than 10s Pa, depending on the pores size and the surface tension of the solution. When the maximum pore radius is 0.5 (tm, the capillary pressure is 3- 10s Pa at a - 70 mN/m.

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