Vacuum Sintering

The first commercial use of tantalum was as filaments ia iacandescent lamps but it was soon displaced by tungsten. Tantalum is used ia chemical iadustry equipment for reaction vessels and heat exchangers ia corrosive environments. It is usually the metal of choice for heating elements and shields ia high temperature vacuum sintering furnaces. In 1994, over 72% of the tantalum produced ia the world went iato the manufacturiag of over 10 x 10 soHd tantalum capacitors for use ia the most demanding electronic appHcations. 

Typical equipment made from tantalum includes heat exchangers, reaction vessels liners, thermowells, and heating elements or heat shields for high temperature vacuum sintering furnaces. Tantalum fabricated parts are found in the manufacturing of pharmaceuticals, explosives, insecticides, dyes, acidic baskets for silver cyanide barrel platers, and in hydrochloric and hydrobromic acid condensers. 

Residual traces of zinc are released during vacuum sintering of cemented carbides made with recovered powders. This can be troublesome when a buildup of zinc occurs in the furnace. Teledyne Advanced Materials further developed this process on a commercial basis by achieving zinc levels in the low ppm range (<30 ppm). The fact that the materials were vacuum-sintered in their original form where certain impurities are removed leads to lower impurity levels in the recovered powders. There is a slight oxidation or loss of carbon that must be compensated, otherwise the recycled powder is not in any way inferior to the original. 

Cold-isostatic-pressing foUowed by vacuum sintering or HIP is also used to manufacture smaller intricate shapes. In this instance beryUium powder is loaded into shaped mbber bags and pressed isostaticaUy in a pressure chamber up to 410 MPa (60,000 psi). After the pressing operation the mbber bag is stripped from the part which is then vacuum sintered to about 99% of theoretical density at about 1200°C. If full theoretical density is required, the sintered part may be simply given a HIP cycle because there is no open porosity after vacuum sintering. In a similar manner, conventional axial cold-pressing... 

The high melting point and reactivity of tantalum with the permanent gases at high temperatures prevents conventional consolidation by melting and casting in air. The metal is in fact consolidated by vacuum sinter-... 

Rolling and swaging Vacuum-sintered bar can be cold rolled, and reductions up to 90% between anneals are possible. Arc-cast and electron-beam-melted material is generally forged at room temperature prior to rolling and swaging. 

In order to interpret correctly the results of electrophysical measurements conducted on vacuum - sintered ZnO semiconductor films one should answer the question concerning the origin of contacts between specific crystallites controlling the electric conductivity of the material. This was accomplished in paper [37] using the method of prerelaxation VAC to run a comparative analysis of mechanisms of charge transfer in thin sintered (in vacuum) films and pressed polycrystalline ZnO samples. 

Thus, the whole complex of existing experimental data indicates that the major part of polycrystalline contacts in vacuum sintered polycrystalline oxides are provided by bridges of open type. Moreover, the vacuum sintering at moderate temperatures 300 - 350°C leads to formation of a unified pattern (see Fig. 2.4, b) which cannot be disjoint into specific microcrystals and connecting bridges [37, 40]. The structure of adsorbents obtained presents a complex intertwining of branches of various thickness. 

The mixture was filtered through a pad of Celite (5 g) using a vacuum sinter funnel and... 

Using the same experimental procedure, it was found that the vacuum-sintered samples produced curves similar to that shown in Fig. 3. 

Such a temperature-time-conductance curve for a vacuum-sintered sample is shown in Fig. 5. A sample, pretreated at 100°C as described above, was heated to 125°C at time zero. The conductance changes as a function of time were followed for 160 minutes (point B), and then the temperature was suddenly raised to 150°C. The time-dependent conductance changes proved to be quite reversible, contrary to the irreversible decrease of conductance with increasing temperature for samples kept in an oxygen atmosphere. 

Pressure sintering. Sometimes, hardmetals exhibit residual porosity (A and B type), which can occur for several reasons. Since the 1970s pressure sintering (HIP-ing) has been applied to remove this porosity. For that purpose, the sintered specimens are loaded in a HIP-ing device and heated again under Ar or He pressure of 50-150 MPa (temperatures and commonly 25-50 °C below the vacuum sintering temperature). Hence any residual porosity can be removed. Exceptions are gas porosities, which either can occur due to incomplete outgassing of the carbon monoxide, or due to the presence of impurities. 

In recent years, this rather expensive two-stage treatment (vacuum sintering and subsequent HIPping) was substituted by the sinteiiiip process, which was developed in the... 

FIGURE 9.25. Three-chamber vacuum sintering furnace for hardmetal production (Ipsen). By courtesy of WIDIA GmbH., Germany. 

Ducheyne, P., Radin, S., Heughebaert, M., and Heughebaert, J.C. (1990) Calcium phosphate coatings on metallic porous surfaces. The effect of structure and composition on the electrophoretic deposition, vacuum sintering, and in vitro dissolution behavior. Biomaterials, 11, 244- 254. 

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