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 powders 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.
Nickel Powder Morphologies
Although the versatile nickel carbonyl gas process can produce fine nickel powders with spherical, cubic and filamentary shapes, one structure dominates the market. The spiky dendritic structure shown in figure 1 is the PM industry standard particle as it gives all the necessary properties. The powder is produced to a tolerance of within 1µm in terms of particle size range. This allows the powder to meet industry demands for consistency and so limit any dimensional changes in PM components.
The needle-like dendrites at the surface of the particles help in the mixing with other powders and in binding to, for example, large iron powder particles. The distribution and shape of the particles is important to the diffusion that occurs during sintering. This ensures that the correct metallurgical structure is achieved in the finished part to give it, for example, the desired fatigue properties. The only other particle shape to find use in the PM industry is a larger, filamentary structure, which offers some specific benefits owing to its larger surface area.
Figure 1. Inco Type 123 nickel powder.
All nickel powders made by the carbonyl gas route have consistent batch by batch properties that can be measured by the powder producer to allow complete traceability and responsibility for consistency. In other words, all manufacturers using the route can achieve ISO 9002, which is demanded by the automotive industry.
Use in Hard Metals
Hard metal uses are another application area in which powder technology methods are used. Nickel powders of various types, especially spiky dendritic and fine filamentary types, can be used to bind the ‘hard’ components such as tungsten and titanium carbides, replacing expensive fine cobalt powder grades. The nickel powders also offer some corrosion resistance benefits.
Nickel's range of conducting and magnetic properties can be exploited in a variety of powder forms, including particulate nickel and nickel oxide, and in products such as nickel coated graphite and nickel coated carbon fibres. Nickel powders can be used to produce smooth coatings and nickel flakes can be incorporated in metal paints. Nickel alloy flakes can also be made to give specific conductivity or magnetic properties to a component.
Nickel coated graphite particles, are employed for electromagnetic shielding in seals and gaskets where they compete with expensive silver or silver coated powders. Electromagnetic shielding is also important in consumer electronic products such as laptop computers and portable phones, and this has led to the use of resin pellets incorporating nickel coated carbon fibres. These pellets are mixed with standard plastic pellets in the injection moulding operation, and form a conducting network in the plastic part, giving excellent electromagnetic shielding. Black nickel oxides of very high purity are also used in applications such as EMI suppression devices in the electronic industry, as part of ferrites.
Filamentary Fine Nickel Powders
Filamentary fine nickel powders with high surface area can be sintered together to make controlled porous structures that have applications as metal filters or in batteries and fuel cells. For example, sintered nickel powder electrodes can be made that have very fine pores, which allow high rates of charge and discharge in batteries. The powders can also be used to make controlled porosity filters, or mixed with other filter materials. The properties of the powder used vary for each particular use.
Nickel Coated Products
Other nickel-coated products can be made thanks to new technology that uses nickel carbonyl feed gas to coat nickel directly onto other substrates. This technique can be used to make high porosity pure nickel foams, figure 2, which have open structures featuring regular arrays of pores, as well as nickel fibremats and papers. These find applications as filters and in battery or fuel cell electrodes. A large commercial production plant for these products was started by Inco SPP in 1998.
Figure 2. Micrograph showing the structure of IncoFoam.
Fine Nickel Powders
Full commercial production has started recently of another new family of extremely fine nickel powders. Powders with particle sizes of just 1µm, in both filamentary, and non-filamentary forms can now be made. Applications for the filamentary particles include binders or conductivity additives, while the non-filamentary forms can be used for applications in which a paste or smooth, fine coating is needed, for example, in multi layer capacitors.
The very latest material on the market is a filamentary powder with a tiny particle size of only 0.3µm. This has an exceptionally high surface area and so has many potential uses as an additive or in catalytic roles.
Health and Safety Issues
Despite their wide range of successful applications, there is concern associated with the use of nickel and nickel powders. The European Union has classified nickel metal as a Category 3 suspect carcinogen, on the basis of animal tests. (it can be argued that these experiments do not represent employee exposure risks.) This means that information about the material has given cause for concern but that there is insufficient evidence to make a satisfactory assessment.
However, more positive news for the nickel industry comes from the most recent study on the subject by the American Congress of Government Hygienists. This led, earlier this year, to nickel metal being classified as ‘not suspected as a human carcinogen’ in the US.
Nickel is one of the most abundant elements on Earth. People are constantly exposed to it, including through the food chain. Industrially, more than 900,000 tonnes of nickel metal are produced and used each year worldwide in products such as stainless steel, automotive parts and rechargeable batteries. Many thousands of tonnes of fine nickel powders are also made and used every year. Studies show that there has been no evidence of any increased rate of cancer among employees handling such metallic nickel.
Of course, nickel dust, like all dusts, should be handled safely, using appropriate dust control methods to prevent direct and prolonged exposure. But in conclusion, nickel has an excellent track record in terms of both its safe use and its ability to be produced in different special forms to the benefit of new technologies. The use of fine nickel powders and the carbonyl gas process seems certain to continue evolving and growing.