Sintering conventional

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. 

Conventional Sintering Equipment. Like drying furnaces, sintering furnaces (29,76,85) can be periodic or continuous in nature. Periodic kilns offer greater flexibiHty continuous tunnel kilns are more economical. Advanced ceramics are typically siatered in high purity, controUed atmosphere furnaces by electric resistance heating. Ceramic furnaces used to fire traditional ceramic ware are generally heated with inexpensive natural gas, oil, wood, or coal. 

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. 

Activating additives (mainly transition metals) ean be used as in conventional sintering allowing maximal densification at much lower T, thus preventing only chemical interaction between the sample and the die ". 

The electronic conductivity depends significantly on the preparation and crystalline structure of the LSM specimen. Li et al. [7] prepared LSM samples by conventional sintering and plasma spraying. Depending on the composition, the electronic conductivity of the sintered sample is between 40 and 485 Scm 1 at 1,000°C and that of plasma-sprayed coatings between 50 and 201 Scm . The electronic conductivity of the plasma-sprayed samples is 50% lower than that of the sintered sample. On an... 

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). 

Fig. 3 Profiles of microhardness on Co content graded cemented carbide-steel by Spark Plasma Sintering (1283K) compared with conventional sintering(1673K)...

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