**1. Introduction**

The manner and mechanisms involved on the sintering process are essential investigation to achieve the required microstructure and final properties in solids. During the conventional sintering of a compacted powder, densification and grain growth occur simultaneously through atomic diffusion mechanisms. Many researchers have been working on reducing the grain size below 1 μm aiming to improve some properties, such as strength, toughness and wear resistance in ceramics (Greer, 1998; Inoue & Masumoto, 1993; Morris, 1998). In order to obtain ultra-fine ceramic microstructures, nanocrystalline powders can be used. Although the sinterability of nanoparticles is superior to that of fine particles due to the higher sintering stress, densification of these powders is often accompanied by grain growth (Suryanarayana, 1995).

Hot pressing sintering (He & Ma, 2000; Porat et al., 1996), spark plasma sintering (Gao et al., 2000; Chakravarty et al., 2008) or pulse electric current sintering (Zhou et al., 2004) are typical techniques employed to produce nanostructured ceramics. However, many of these techniques are not economically viable depending on the use of the final product. Thus, conventional pressureless sintering is still a more attractive sintering method to produce ceramic products, mainly due to its simplicity and cost compared to other methods. In the conventional pressureless sintering, a controlled grain size with high densification could be achieved by adequate control procedures of the heating curve — herein defined as the maximization of the final density with minimum grain growth.

One hypothesis to the heating curve control can be achieved by improving the narrowing of grain size distribution in a pre-densification sintering stage followed by a final densification stage namely at a maximum densification rate temperature (Chu et al., 1991; Lin & DeJonghe, 1997a, 1997b). In a thermodynamics point-of-view, another hypothesis is regarded to control the heating schedule at temperature ranging the active grain boundary diffusion. Note, however, that the grain boundary migration is sufficiently sluggish and the

Mechanisms of Microstructure Control in Conventional Sintering 403

several processing difficulties. Main positive aspects include: increased reactivity between reagents and solid particles and between particles and the gas phase, which are important processes in synthesis; increased sintering rate and particularly lowering of sintering temperature, which can reduced by half the material's melting point (Hahn, 1993; Mayo, 1996). On the other hand, also due to the large surface area and large excess of free energy nanometric powder systems, there are many detrimental aspects to the processing and obtaining the refined and homogeneous microstructures. Some of these aspects are: a very strong tendency to agglomeration of primary particles of nanometric powders; difficulties of mixing and homogenization of compression due to the strong attraction between particles; demand for greater sintering atmosphere control, not only due to the higher reactivity, but also the possibility of formation of thermodynamically unstable phases and appearance of strong effect of adsorbed gases on the surface (Allen et al., 1996; Averback et al., 1992).

Many studies (Chen & Chen, 1996, 1997) on nanometric size particles have shown reduction of sintering temperature. Hahn et al. (Hanh, 1990), studying the sintering of nanometric TiO2 (12nm), Y2O3 (4 nm) and ZrO2 (8nm), found lower sintering temperatures than those conventional. The sintering of TiO2 occurred at 1000ºC while conventional TiO2 sintering requires temperatures above 1400ºC. The same pattern of reduced sintering temperature was observed for Y2O3 and ZrO2. In spite of the proven decrease in sintering temperature of nanometric powders, its densification is often accompanied by a large grain growth, causing

Production of polycrystalline ceramics with high density and small grain size have been studied for several processing routes. Among these routes may be cited: colloidal processes of powder with controlled particle size distribution (Sigmund & Bergström, 2000; Lim et al., 1997), sintering under pressure (He & Ma, 2000; Weibel et al.,1997), use of additives incorporated into a second phase or in solid solution (Novkov, 2006; Erkalfa et al., 1996), spark plasma sintering (Gao et al., 2000; Chakravarty et al., 2008, Bernard-Granger & Guizard, 2007), pulse electric current sintering (Zhou et al., 2004), etc. Usually these methods have several limitations on usage, in addition to requiring more complex and expensive equipment. Thus, sintering without pressure is even a more desirable sintering method to produce ceramic products, mainly due to its simplicity and cost when compared to other

In pressureless sintering, beyond the control of powders' characteristics, control of the sintering process has a major effect on final material's density and microstructure. This method is often unable to prepare dense ceramics with ultrafine grain size, once way the final sintering stage, both densification and grain growth occur by the same diffusion

Heating curve control to manipulate the microstructure during sintering is a route that has been studied and offers advantages such as simplicity and economy. The rate-controlled sintering (Brook, 1982, German, 1996) is one of the ways in which the relationship between densification rate and grain growth rate is determined to identify the sintering temperature at which densification rate is maximized (Chu et al., 1991). Ragulya and Skoroklod (Ragulya & Skoroklod, 1995) studied the rate-controlled sintering of ultra fine nickel powders

lost of their nanocrystalline ceramics characteristics.

**2.1 Effect of heating curve in the sintering** 

mechanisms (Mazahery et al., 2009).

methods.

densification could occur without grain growth. The aforementioned hypothesis was proposed by Chen and Wang (Chen & Wang, 2000) and has been successfully applied to different types of materials. A second phase can be added to preserve fine grains. In this case, grain boundary inhibition can be due to the pinning effect, which is associated with particles locations at grain boundaries or triple junctions (Chaim e al. 1998; Trombini et al., 2007). This drag pinning effect associated with heating curve control can be more effective to suppress the grain growth.
