Fabrication processes powder pressing

Kim Y. S., 1976, Effects of powder characteristics, in F. F. Y. Wang (Ed.), Treatise of Materials Science and Technology, Vol. 9 Ceramic Fabrication Processes, Academic Press, New York. 

As shown in Figures 1.36(a) and 1.36(b), the glycine nitrate process-derived powder is highly porous. The pore size ranges from tens of nanometers to several micrometers. The powder is thus named as foam powder, which shows extremely low fill densities of less than 1/100 of the theoretical density value. The low fill density makes it possible to prepare thin films of doped ceria by dry pressing, in which the films are processed by means of punches in a hardened metal die. Figures 1.36(c) and 1.36(d) show a cross-sectional view of an 8 /tm-thick GDC film fabricated by dry pressing. 

Once the structural support layers have been fabricated by extrusion or EPD for tubular cells or by tape casting or powder pressing for planar cells, the subsequent cell layers must be deposited to complete the cell. A wide variety of fabrication methods have been utilized for this purpose, with the choice of method or methods depending on the cell geometry (tubular or planar, and overall size) materials to be deposited and support layer material, both in terms of compatibility of the process with the layer to be deposited and with the previously deposited layers, and desired microstructure of the layer being deposited. In general, the methods can be classified into two very broad categories wet-ceramic techniques and direct-deposition techniques. 

Powder pressing is a very flexible process in that materials that are difficult to melt or deform may be produced in a variety of shapes and sizes. Adding to the flexibility of the process may be the fact that the starting powder does not need to be uniform. Different size powders, or more commonly, different types of powders may be used to produce composite microstructures. These different powders may be distributed uniformly throughout the fabricated component, or isolated to form a functionally graded material. Powder pressing is commonly used to... 

Molding under pressure is called compression molding. In this process, powders or mold materials are usually preheated, and then simultaneously pressed and hardened (Figure 36-1). Generally, only thermosets containing a great deal of filler are used as mold materials, i.e., phenolic, urea, melamine, and unsaturated polyester resins. Inlays such as mats and fabrics are also frequently used. 

The fuel fabrication process consists of conversion of enriched UFg into UO2 powder that is then pressed into pellets that typically are about 1 cm in diameter and 1 cm in length. These pellets are inserted into fuel rods that are combined in fuel assemblies and placed in the reactor core as shown in Figure 1.16. 

Powder pressing generally forms boron nitride substrates. Various sihca and/or calcium compounds may be added to lower the processing temperature and improve machinabihty. Diamond substrates are typically formed by chemical vapor deposition (CVD). Composite substrates, such as AlSiC, are fabricated by creating a spongy structure of SiC, and forcing molten aluminum into the crevices. 

Crystal size is usually less sensitive and is independent of environment. Heat treatment does, of course, cause an increase in crystal size, and crystal growth may be followed all the way through a complete ceramic fabrication process. Thus while a particular oxide or metal powder produced by a chemical process may have a crystallite size of 10-100 A, subsequent heat treatment (calcination) by which the powder is prepared for slip-casting, dry pressing, etc., may increase the crystal size to about 0.01-0.1 //, and from the final sintering process a crystal size of about 100 // may result. 

Disc/flat-sheet-shaped membranes are mostly applied in dense ceramic membrane reactors due to the ease of the fabrication process the ceramic material powder is pressed into discs in a stainless steel mould under an isostatic or hydraulic pressure, followed by sintering at a high temperature. Such disc-shaped membranes usually have a thickness of about 1 mm so as... 

There are two further processes. Silicon-based ceramics can be fabricated by sintering or by hot-pressing. But a new route, reaetion bonding (Fig. 19.6), is cheaper and gives good precision. If pure silicon powder is heated in nitrogen gas, or a mixture of silicon and carbon powders is sintered together, then the reactions... 

All AB, alloys are very brittle and are pulverized to fine particles in the hydrid-ing-dehydriding process (see Sec. 7.2.1). Thus electrodes must be designed to accommodate fine powders as the active material. There are several methods of electrode fabrication Sakai et al [35] pulverize the alloy by subjecting it to several hydrogen absorption-desorption cycles, before coating the resulting particles with Ni by chemical plating. The powder is mixed with a Teflon dispersion to obtain a paste which is finally roller-pressed to a sheet and then hot-pressed to an expanded nickel mesh. The fabrication of a simple paste electrode suitable for laboratory studies is reported by Petrov et al. [37],... 

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. 

Enriched UF6 is processed into U02 powder at fuel fabrication facilities using one of several methods. In one process uranium hexafluoride is vaporized and then absorbed by water to produce uranyl fluoride, U02F2, solution. Ammonium hydroxide is added to this solution and ammonium diuranate is precipitated. Ammonium diuranate is dried, reduced, and milled to make uranium dioxide powder. The powder is pressed into fuel pellets for nuclear reactors. 

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