Sintering activity

Sintering, activated sintering and reaction sintering are used to produce maximum density in sintered bodies and to make special parts with controlled porosity. 

These phenomena have been given considerable attention, because they are typical of a number of raw materials, in particular oxides obtained in fine form by decomposition of hydroxides or carbonates. The relations between these properties and technological behaviour (shrinkage, sintering activity, reactivity) will be discussed in the chapters devoted to the ceramic materials. 

Pressure sintering is especially significant for covalent substances exhibiting a low sintering activity under normal pressure (e.g, SiC, Si3N4), With some substances of this kind, hot pressing is the only way of manufacturing a dense compact ceramic product. 

A completely new way of solving the problem of sintering activity is (he sol-gel method. Its principle is based on preparing a chemically homogeneous gei of the required composition by precipitation from a solution. This is then dehydrated at elevated temperature. The material thus prepared shows a high sintering activity so that a very dense product can be prepared at comparatively low temperatures. Vitreous (non-crystalline) products can be obtained by the sintering of silica gel or gel-type mixtures (Roy, 1969 and 1971 Dislich, 1971). 

It is technically easier to manufacture a molded component from SiC-powder by slip casting or dry pressing, working it mechanically and then sintering pressureless at 1950 to 2000°C. Due to the low sintering activity of silicon carbides, such processes have only been recently successfully carried out with the advent of fine particulate SiC-powders (specific surface area > 5 m /g) with low oxygen-contents (< 0.2%). Boron or aluminum and free carbon or boron carbide are added as sintering aids. 

Phase transformations are accompanied by an increase of the sintering activity and give rise to an enhanced pore growth [13] and so should usually be avoided. This goal can be achieved by suppressing the phase transformation or by shifting the phase transformation temperature to higher temperatures by... 

Specific surface area. The specific surface area is an important criterion for the sintering activity (solid state sintering), dissolution processes (liquid phase sintering), and reaction with gaseous or solid substances during carburization. Commonly, it is in the range of 0.01 m /g (coarse powders) up to 12m /g (very fine powders). 

Groza JR, Zavaliangos A (2000) Sintering activation by external electrical field. Mater Sci Eng A-Struct Mater Prop Microstruct Process 287 171-177... 

The perovskite lanthanum chromite (LaCr03) is one of the exceptional ceramic compounds that are chemically very resistant to both oxidizing and reducing ambients. Moreover, being an electronic conductor, it is eminently suitable as a bipolar connector in solid oxide fuel cells. It is evident that the thermal expansion coefficients of the different components in a fuel cell (electrolyte, electrodes, bipolar connector) must be closely matched. Doping the chromite with strontium or magnesium ions is necessary to increase its electronic conductivity as well as its sinter activity. 

However, it is quite difficult to prepare such materials as high density pellets. To improve the sintering activity, Babilo and Haile (2005) and Tao and Irvine (2006) introduced Zn into Y- and Zr- doped BaCeOj. Results show that it is an effective way to obtain a stable proton-conducting electrolyte BaCeo sZrojYo igZno MOj (BCZYZ) by co-doping of Zr and Zn, which could be sintered densely at lower temperature (150°C less) than the materials without Zn. Moreover, BCZYZ can be easily fabricated as a 20 pm thin membrane for electrolyte application (Ding et aL, 2010). 

Dasgupta, N., Krishnamoorthy, R., Jacob, T. Glycol-nitrate combustion synthesis of fine sinter-active yttria. Int. J. Inorg. Mater. 3,143-149 (2001)... 

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