Open-cell Nickel Foams Research Articles

Strength and ductility of as-plated and sintered CVD nickel foams

Low density, open cell nickel foams produced by chemical vapor deposition (CVD) are made for electrochemical applications. One process involves nickel carbonyl decomposition within a plater whereby nickel is deposited onto a reticulated polyurethane (PU) foam substrate. The as-plated nickel foams are then sintered in a partially reducing atmosphere at high temperature in order to enhance ductility. The high strength and brittleness of the as-plated foams presents a material handling challenge in manufacturing. As-plated foams are much stronger than sintered foams of the same density. Our analysis shows that the strengthening in the as-plated nickel foams is due to a combination of grain size, solid solution strengthening, dislocations and nano-porosity. Of these, the effects due to interstitial carbon are dominant. This is lost upon sintering. Foam ductility does not show the usual inverse trend with strength. Instead, foam ductility and bendability show little variation with carbon or sulfur level or with the level of nano-porosity, which are among the strongest strengthening sources in the foams.

Production of titanium alloys by metal injection moulding

A titanium-copper-nickel braze powder is combined with various carrier polymers at a range of loadings and used to braze open cell nickel foams to Ti–6Al–4V plate. The microstructure of the interfaces and the chemical composition are examined, and these are correlated with the mechanical behaviour of the joints, as determined by shear and tensile tests, with the aim being to determine the influence the polymer binder and application method have on the final bond microstructure and properties. The investigations show that, as expected, the binder system used can have an important effect on the amount of oxygen, carbon and nitrogen contained in the braze material, and the mechanical properties obtained. However, the effect is complex, better properties in shear being obtained with an apparently lower quality bond, an observation that is explained by considering the constrained boundary layer formed by the bond. Further Weibull tests show that, although the density variations in the commercially available nickel based foam affect the properties, there is additional variation due to the high degree of process control required to achieve a good bond.

Mechanical properties and non-homogeneous deformation of open-cell nickel foams: application of the mechanics of cellular solids and of porous materials

The mechanical properties of open-cell nickel foams are investigated for the range of densities used in industrial applications for energy storage. The obtained Young’s modulus, compression yield stress and tensile fracture stress are compared to the predictions of models based on periodic, Penrose and Voronoi beam networks. It is found that Gibson and Ashby’s model [L.J. Gibson, M.F. Ashby, Cellular Solids, Cambridge University Press, Cambridge, 1998] provides the proper scaling laws with respect to relative density for almost all investigated properties. The strong anisotropy of the observed overall responses can also be accounted for. The two-dimensional strain field during the tension of a nickel foam strip has been measured using a photomechanical technique. Non-homogeneous deformation patterns are shown to arise. The same technique is used to obtain the strain field around a circular hole in a nickel foam strip. The observed deformation fields are compared to the results of a finite element analysis using anisotropic compressible continuum plasticity.