Graphite dies

Die compaction of simple shapes can be carried out at elevated temperature using carbide, superalloy, refractory metal, or graphite dies. An inert gas atmosphere or vacuum is often used to protect the die and/or the powder. For example, beryllium powder is compacted at about 1350°C in a graphite die under vacuum with pressures of 2-4 MPa. 

Hoyt and Chorne [230] have recently reported the preparation of several self-bonded dense borides. EuBe (90%) was made by hot vacuum pressing in graphite dies at 1800—2000° C. Temperature, pressure and time are important variables. Unsuccessful attempts have been made to prepare EUB4. 

Campbell and Van Wyk hot-pressed their iron-palladium and iron-platinum composites at 1090°C (2000°F) and 50MPa. They prepared a variety of composites containing nickel in a similar manner, but none of these was successful. They used graphite dies, and considered that this helped to create a reducing environment during the hot-pressing, which prevented oxidation of the molybdenum disulphide. Hubbell... 

Boron carbide pellets and structures can be produced by cold pressing and sintering (70-80% density) or by hot pressing. In the latter the B4C powder is first cold-pressed into pellet form and then hot-pressed in graphite dies at 2050-2300°C under 10.3 MPa (1500 psi). The density is controlled by varying the temperature and the pressure. 

In the EFG process, commercialised at Schott Solar (under development in the past by Mobil Tyco, ASE and RWE), the silicon ribbon is pulled to heights of up to 7 m from the top of a graphite die. The molten silicon is fed through the die by capillary action (shape of meniscus Ml). Extensive temperature control by radiation shields, cold shoes and afterheating realises maximum temperature gradient where plastic flow is possible, to allow for a maximum growth rate as described above [30,31] (Fig. 7.2). 

Figure 4 The stepped graphite die, used in this study, for the control of residual stress profile of FGM via pulse- electric discharge consolidation.

Figure 5 Temperature profile at the surface of the stepped graphite die in heating, taken by thermography.

Figure 7 shows densification of five layered compacts graded from amorphous TiAl to PSZ via temperature gradient controlled electric discharge resistance consolidation using the stepped graphite die as shown in Fig.4. The consolidation of FGMs having hf = 7.40 and 6.52 under two different temperature gradient show...

The materials were Cu (Nippon Atomized Powders Co. Ltd., 5 pm) and polyimide (Kaneka Co. Ltd.) powders. Polyimide and Cu powders in a graphite die were first consolidated independently using spark plasma system (Sumitomo Coal Mining Co. Ltd., SPS-1050) in a vacuum. Polyimide and Cu powders were consolidated by the spark plasma system under pressures of 9.8 and 39.8 MPa using an ordinary die. Then those powders were mixed using an agate mortar with a pestle for 20 min. 

By using a boron-nitride powder barrier (hot pressed or deposited onto graphite mold and punches), it is possible to prepare dense and pure boron and boron-rich phases, especially carbides (between B,o.4C and B4C) by hot pressing in graphite dies " . 

The ZrB2-SiCp (ZS) composite were fabricated by hot pressing in a graphite die at Materials and Machines, Inc, Tucson, AZ, and contained 20 v/o SiC particles. Cu-clad-Mo plates, obtained from H.C. Starck, Inc, Newton, MA, had Cu-to-Mo-to-Cu layer thickness ratio of 13%-74%-13%. The commercial brazes, Palco, Palni, Cusil-ABA and Ticusil were obtained in foil form (thickness 50 pm) from Moigan Advanced Ceramics. The compositions, liquidus and solidus temperatures, and selected properties of the braze alloys are given in Table 1. 

Why are graphite dies widely used for hot pressing Under what conditions would the use of graphite not be appropriate ... 

The brown powder was first ball milled with high purity zirconia balls for 12 h at a rotational speed of 200 rpm, in isopropyl alcohol with 3 wt% polyethylene glycol. The ball milled mixture was dispersed supersonically for 30 min and then dried by using a rotary evaporator. The dried powder was further calcined at different temperatures for 2 h. Rnally, the calcined powders were ball milled for second time with zirconia balls for 12 h in isopropyl alcohol. The milled powders were dried and then sintered by using SPS, with graphite dies with an inner diameter of 15 mm at 50 MPa uniaxial pressure. The temperature was increased to 600 °C within 10 min, from where and onwards the pressure was increased to 100 MPa. The samples were sintered at 1200-1350 °C for 5 min at a heating rate of 100 °C min. ... 

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