Thermal densification

Weder s experiments were carried out with opposing body forces, and large current oscillations were found as long as the negative thermal densification was smaller than the diffusional densification. [Note that the Grashof numbers in Eq. (41) are based on absolute magnitudes of the density differences.] Local mass-transfer rates oscillated by 50%, and total currents by 4%. When the thermal densification dominated, the stagnation point moved to the other side of the cylinder, while the boundary layer, which separates in purely diffusional free convection, remained attached. 

Treatment of plasma-sprayed PSZ with a slurry of 0.1mm zirconia, and subsequent thermal densification. 

In this chapter, particular attention will be devoted to the study of the structure of sol-gel derived silica and modified silicates, at the molecular level, by IR absorption and reflection spectroscopies, as well as to microstructural aspects such as the elimination of residual porosity during thermal densification, which usually occurs together with a simultaneous elimination of residual OH species. Relevant results for other non-siUcate and non-oxide materials will also be reviewed. 

Campostrini R., Carturan G., Ferrari M., Montagna M., Pilla 0. Luminescence of Eu + ions durino thermal densification of Si02 gel. J. Mater. Res. 1992 7 745-753 Carlos L.D., Sd Ferreira R.A., De Zea Bermudez V., Ribeiro S.J.L. Full-color phosphors from amine-functionalized crosslinked hybrids lacking metal activator ions. Adv. Funct. Mater. 2001 11 111-115... 

Mustarelli P, Quartarone E, Benevelli F (1997) A B and i MAS-NMR study of sol-gel lithium Mborate glass subjected to thermal densification. Mat Res Bull 32 679-687... 

The small but definite densification enhancement resulting from isostatic pressing is consistent with material transport by dislocation motion. The pressing enhancement could result as follows. Dislocations are put into the structure of the partially sintered powder compacts by isostatic pressing at room temperature. On subsequent heating the dislocations move and affect the material transport. In oxides in which the dislocations move easily at room temperature, e.g., MgO and CaO, the effect on subsequent thermal densification appeared to be reduced or absent. Addition of particles presumed to act as pinning points resulted in densification enhanced by isostatic pressing. 

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

Chemical stabilization involves removing the concentration of surface hydroxyls and surface defects, such as metastable three-membered rings, below a critical level so that the surface is not stressed by rehydroxylation in use. Thermal stabilization involves reducing the surface area sufficiently to enable the material to be used at a given temperature without reversible stmctural changes. The mechanisms of thermal and chemical stabilization are interrelated because of the extreme effects that surface hydroxyls and chemisorbed water have on stmctural changes. Full densification of gels, such as the... 

Fig. 7. Thermal conductivity of composites obtained using various densification processes.

Thermal-Gradient Infiltration. The principle of thermal-gradient infiltration is illustrated in Fig. 5.15b. The porous structure is heated on one side only. The gaseous reactants diffuse from the cold side and deposition occurs only in the hot zone. Infiltration then proceeds from the hot surface toward the cold surface. There is no need to machine any skin and densification can be almost complete. Although the process is slow since diffusion is the controlling factor, it has been used extensively for the fabrication of carbon-carbon composites, including large reentry nose cones. 

Precursor Structure Effects. The precursor structure can impact a broad range of properties, including crystallization temperature, the formation of intermediate phases during thermal treatment and film density, among other properties. Table 2.4 reports some of the key precursor properties that may affect densification and crystallization behavior, as well as the final film microstructure. 

The majority of work done on VGCF reinforced composites has been carbon/carbon (CC) composites [20-26], These composites were made by densifying VGCF preforms using chemical vapor infiltration techniques and/or pitch infiltration techniques. Preforms were typically prepared using furfuryl alcohol as the binder. Composites thus made have either uni-directional (ID) fiber reinforcement or two-directional, orthogonal (0/90) fiber reinforcement (2D). Composite specimens were heated at a temperature near 3000 °C before characterization. Effects of fiber volume fraction, composite density, and densification method on composite thermal conductivity were addressed. The results of these investigations are summarized below. 

The results of combustion reactions are normally products in which the final density is somewhat lower than the theoretical value, normally about 70%. Increased densification can be achieved if the hot product, which is still relatively plastic, is compressed. This procedure combines the thermal... 

When the powder is isostatically compacted at elevated temperatnres, the process is called hot isostatic pressing (HIP). In this case, the flexible dies are often made of thin metals, and high-pressnre gases snch as argon are nsed to heat the part rapidly and rednce thermal losses. Pressnre np to 100 MPa and temperatnres in excess of 2000°C are possible nsing HIP, and parts up to 600 kg can be fabricated. A schematic diagram of a typical HIP apparatus is shown in Figure 7.18. Metals that are processed commercially by HIP include various specialty steels, superalloys, hard metals, refractory alloys, and beryllium. We will see in Section 7.2 that HIP is also particularly useful for the densification of ceramic components. 

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