Temperature conventional sintering

Conventional Sintering. Ceramic sintering is usually accompHshed by heating a powder compact to ca two-thirds of its melting temperature at ambient pressure and hoi ding for a given time. Densification can occur by soHd-state, Hquid-phase, or viscous sintering mechanisms. 

Hot pressing (or pressure sintering) is the simultaneous application of pressure and heat to a powder mass enclosed in a die. This technique allows the use of lower temperatures and pressures and shorter processing times than those for conventional sintering and thus permits the production of bodies with finer grain size, lower porosity and higher purity. 

In conventional sintering, the dominant term in the free energy reduction is that due to surface area reduction. Recall from Eq. (2.61) that the surface energy is the increment of the Gibbs free energy per unit change in area at constant temperature, pressure, and total number of moles... 

Figure 11.8 graphically demonstrates this relationship, and shows that for conventional sintering temperatures (1400-1600°C), a reasonable partial pressure of oxygen (10 8-10 6 atm) is required to prevent the reaction... 

In addition to conventional sintering, reactive powder processing, also called combustion synthesis or self-propagating high-temperature synthesis (SHS), can be used if the target compounds can be synthezised from the starting powder mixture (Stangle and Miyamoto, 1995). This process comprises a rapid and exothermic chemical reaction to simultaneously synthesize some or all of the constituent phases in the FGM and density the component. 

Whether in minerals or man-made materials, the chemical bonding in CBCs is at room or warm temperatures, and this aspect distinguishes them from conventional sintered ceramics. Most of the CBCs are formed in the presence of water, though Wilson and Nicholson [8] have discussed several nonaqueous cements. In many of the aqueous CBCs, water is bonded chemically within their stmcture, but in some cases water may be expelled during the reaction. In aU cases, their formation is based on dissolution of individual components into an aqueous phase to form cations and appropriate anions. These ions react with each other to form neutral precipitates. If the rate of this reaction is controlled, then the reaction products will form network of connected particles and produce either well-ordered crystals or disordered structures. These CBCs comprise a composite of the crystallized and partly disordered structures. 

The industrial ceramics produced by conventional sintering of 8-phase Si3N4 powders synthesized by SHS (Petrovskii et al, 1981) can be used as high-temperature articles with attractive dielectric properties (tan6=4.4 10 at/= 10 Hz, and dielectric strength, Ea=9.2 kV/mm). Also, silicon nitride powders with a relatively high a-phase content ( 80%) have been used for the production of advanced structural ceramics with good mechanical properties ([Pg.109]

Coated spherical Th02- or U02-particles are increasingly utilized in the fuel of gas-cooled high temperature reactors. Their 50 to 1500 pm core of uranium(IV) oxide is manufactured using conventional sintering techniques. This is then pyrolytically coated with many layers of carbon and silicon carbide (see Section 5.7.5.1). 

Sol-gel process is proven to be an attractive fabrication method of multi-component oxide ceramics. In addition to the achieved homogeneity and purity of the products, the sol-gel method also enables a lower phase-formation and sintering temperature in comparison to the conventional sintering of powder With good size scaling possibility, colloidal sol-gel materials are suitable for depositing layers on macroporous substrates to serve as support of polymeric sol-gel derived layers preventing infiltration of the sol. 

The chemical methods have been used for the synthesis of bismuth nanoparticles by vapor flow condensation [6] and microemulsion process [7]. It is generally accepted that the microwave synthesis and sintering (MS) processes [8-10] can concentrate ceramic materials at a very rapid rate and at a substantially lower temperature than the conventional sintering (CS) process. 

Aonoand coworkers employed TG-DTA to follow the thermal decomposition of a heteronuclear La-Mn complex that at relatively low temperatures (600-700 °C) yields single-phase nanoparticles of hexagonal LaMnOs, which could not be obtained by conventional sintering even at 1200... 

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