Nickel Powders from the Carbonyl Process

Metal powder is the base materials for the production of metallic component through the conventional powder metallurgy route or the emerging field of additive manufacturing. In any of these process routes, the properties of the finished product depends on the character of the base powder from which it is produced which is equally dependent on the process of production of the base powder. Therefore, there are different methods for producing metal powders with each method offering different particle morphology and purity. These methods include crushing (for brittle material), machining, mechanical pulverization, slotting, electrolysis, atomization of liquid metal using water, nitrogen, argon, or a combination of these, and reduction of metal oxides in hydrogen or using carbon. These metal oxides could be materials such as iron ore or iron oxide generated from pickling plants, in steel strip mills. Other methods include reduction of metal oxide with higher carbon containing, metal powder, chemical decomposition of metal carbonyls, and electrolytic processing of cathodic deposition from molten metal salts; and in some instances, recycling (Sharma, 2011). Each of these methods provides different particle morphology and characteristics. An illustration of typical powder shapes produced from some of these processes is shown in Figures 1 and 2.

New materials that can be tailored for individual applications are in constant demand. As the range of uses for powder metallurgy, hard metals and electronic materials expands, customer requirements are causing materials companies to come up with new products that have the necessary properties. Nickel can bring a number of benefits to these and other industries. It can improve the mechanical and fatigue properties of alloy steels, enhance conductivity and magnetic properties of electronic materials, act as a binder for holding together particulate materials and be used in filtration components in the form of high porosity products. These applications rely on high purity fine nickel powders and other special nickel forms being adapted to meet specific materials needs, for which a versatile production and processing technology is needed. The nickel carbonyl gas process fulfils these needs.

The Nickel Carbonyl Process

Nickel powder can be made by a number of different processes, including atomisation from melts or precipitation from solutions. However, these techniques tend to give relatively large particles and can be difficult to control economically at fine particle sizes. The nickel carbonyl gas process on the other hand tends to produce much finer particles, and with sufficient production know-how plus the latest computerised process controls, the particles produced can be precisely controlled to very accurate shapes and tolerances.

The nickel carbonyl gas process is used as a way of refining impure nickel. Nickel reacts with carbon monoxide to form nickel carbonyl gas (Ni(CO)4), which can be decomposed back to nickel metal at moderate temperatures with the recovery of carbon monoxide. Using thermal shock decomposition, fine or extra fine nickel powders can be made. Refineries in North America and Britain can each process up to 50,000 tonnes per year of nickel in his way, producing a wide range of different products. The use of such large volumes of carbonyl gas in the refineries allows the economic production of a range of nickel powders. New products can also be made by using the gas stream essentially as a coating medium. These new products include nickel coated graphite particulates, nickel coated carbon fibres and the large scale commercial production of high porosity nickel foam. Another benefit is that the process has no real waste products, with used gas is recycled back into the main refinery process.

Nickel Powders for Powder Metallurgy

The nickel powders produced for powder metallurgy applications have been developing step by step over recent decades as customer property specifications have become ever more stringent. Today, there are no ‘standard’ products, only certain families of powders that are based on different morphologies and subsequently fashioned for individual customer applications. Nickel powder production can now be controlled to give the powders the right particle size, density and especially particle shape to enhance the properties of low alloy steel powder metallurgy parts. Additions of nickel to the alloy typically range from 1.75-5%. Nickel-enhanced alloys are increasingly being used for making pressed and sintered parts, particularly in the automotive field.

Powders

Copper powder is produced by many processes including chemical precipitation, electrolytic deposition, oxide reduction, water atomization, gas atomization, and jet milling. Accordingly, Cu powders are commercially available in a wide range of particle shapes and sizes. Electrolytic and chemical powders exhibit poor packing and poor rheology in molding, so they have been largely unsuccessful for MIM (Wada, Kankawa, & Kaneko, 1997). Characteristics of examples of the other types are summarized in Table 20.2. Representative scanning electron micrographs are given in Fig. 20.4. These powders all have similar particle sizes but different morphologies. Typical purities reported by the manufacturers are about 99.85 wt%; however, oxygen contents can range up to 0.76 wt%. Powders are usually shipped containing desiccant and proper powder storage is essential to avoid oxidation between purchase and use.

It is important to try this experiment before doing it as a demonstration, as different samples of aluminium powder can react differently. The induction period for some samples can be quite long. However, this is an impressive and spectacular demonstration, proving that water can be a catalyst. It also shows that aluminium is a very reactive metal, and that its usual unreactive nature is due to the surface oxide layer.

The chemical properties of iodine are very similar to those of bromine and chlorine. However, its reactions are far less vigorous. It can also act as an oxidant for a number of elements such as phosphorus, aluminium, zinc and iron, although increased temperatures are generally required. Oxidation of finely dispersed aluminium with iodine can be initiated using drops of water. The reaction is strongly exothermic, and the excess iodine vaporises, forming a deep violet vapour.