Sintering of metals

H. H. Hausner, "Compacting and Sintering of Metal Powder Without the Apphcation of Pressure," International Symposium Agglomeration, Apr. 12-14,1961. 

The above syntheses of metal nanowires are based on the thermal hydrogen reduction. However, we found that the reproducible synthesis is difficult by this method, because the hydrogen reduction needs careful control of the reaction conditions. For the reproducible and selective synthesis of wires and particles, we need to clarify the factors controlling the sintering of metals. The key factors are the concentration of residual solvent and the relative rate of reduction and migration of metal ions. The details are shown in the next section. 

Coalescence of drops is caused by capillary pressure. Analogously it may be expected that sintering of metals, which is an important industrial process, also would be governed or at least influenced by the surface energy of the metal particles. An instance of the theoretical treatment of this problem can be found in Ref.141. ... 

An important attribute of these materials comes from the two distinct cationic sites (Oh and Td), and the feasibility of cations migration among them due to the redox nature of metalions, while keeping the spinel structure intact. This particular aspect helps to avoid possible segregation/sintering of metal-ions and remain stable for longer period, compared to a mixed metal oxide catalyst. This unique property of spinels makes them an attractive candidate for number of catalytic reactions. 

There is much more to be said concerning sintering, especially with regard to the role that diffusion plays. Many of the models that describe the sintering processes are based on the diffusivity of solid species, which are addressed in Section 4.3. The practical aspects of the sintering of metals and ceramics are described in Sections 7.1 and 7.2, respectively. 

Gerdes, T., Willert-Porada, M., (1994), Microwave sintering of metal-ceramic and ceramic-ceramic composites , Mat. Res. Sco. Symp. Proc. 347, 531. 

Fischer, P., Blatter, A., Romano, V. and Weber, H.P. (2004) Highly precise pulsed selective laser sintering of metal powders. Laser Physics Letters, 620-628. 

A reliable simulation is important, for example, in planning coke burning from catalytic reformers, where excessively high temperatures can cause sintering of metal crystallites. 

H.H. Hausner, Compacting and sintering of metal powders without the application of pressure, in W.A. Knepper (Ed.), Agglomeration, Interscience, New York, 1962, pp. 55-91. 

An important property of a passivating solid coating is that it should be stable in spite of subsequent thermal treatments, such as the sintering of metal connections. The problem of thermal stability is especially worrisome in the case of a-Si H that is known to evolve hydrogen above about 350°C (Pankove and Carlson, 1977). Heating the sample for 30 min at 500°C caused the leakage current to increase by a factor of about three, as shown by the dashed line in Fig. 3. Note that the current for the heated sample is still about one order of magnitude lower than that of the oxide-passivated p-n junction, which is well known for its thermal stability. 

The theoretical treatment of these various mechanisms has been reviewed by Ruckenstein [35]. The essential features of sintering (of metal particles) can be qualitatively summarized on the basis of these simple processes. Consider a supported metal catalyst with a broad particle-size distribution ranging from single... 

Supports are in most cases used to prevent the sintering of metal particles. This is also the role of the otherwise inert promoters (e.g. AI2O3 in Fe). Small metal particles can be prepared as colloids (see e.g. ref. 31) or dispersed on a support. Only the second way and the fused metal technique have been applied to the Fischer-Tropsch synthesis catalysts. 

The role of tbe additives, which may be used in bi- or muitnnetaltic combinations (Re, Ir, Ru, Ag, Au, Ge etc.) is poorly umterstood. They help to enhance the properties of platinum hy keeping it in a suitable state of dispersion, and to modulate the acidic character of the support. They appear to oppose the sintering of metallic crystallites by the formation of alloys or polymetallic dusters. 

Metal fibers can be produced by metal-cutting processes, by foil cutting processes, powder metallurgically by the sintering of metal powders which can be extruded with the help of organic binders to fibers, by metallization of non-metalic fibers and also by the controlled chemical dissolution of wires to the required fiber thickness. Thin metal wires and thick metal fibers can in principle be produced by the same methods. 

Kang and co-workers [79,80,84] prepared Au/NaY and Au-Fe/NaY catalysts by ion exchange from a solution of HAuCR. When only gold was present, they obtained gold particles of 10 to 40 nm diameter, while in the presence of iron, the mean diameter decreased to 5-8 nm. These results suggest that the presence of iron retards the sintering of metallic gold. 

Since this book is focussed on chemorheological processes, we will focus on rapid prototyping and manufacturing processes that include reactive processing (mther than the rapid-prototyping techniques involving only physical transformations, i.e. sintering of metals, ceramics or thermoplastics). 

T. Gerdes, M. Willert-Porada, Microwave Sintering of Metal-Ceramic and Ceramic-Ceramic Composites", MRSProc. Vol. ML, S. 531-537 (1994)... 

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