4470
J. Phys. Chem. B 2005, 109, 4470-4474
Surface Behavior of Nickel Powders in Aqueous Suspensions Nicola´ s Herna´ ndez,† Rodrigo Moreno,† A. Javier Sa´ nchez-Herencia,*,† and Jose´ L. G. Fierro‡ Instituto de Cera´ mica y Vidrio, CSIC, Cantoblanco, E-28049 Madrid, Spain, and Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Cantoblanco, E-28049 Madrid, Spain ReceiVed: NoVember 8, 2004; In Final Form: January 13, 2005
The colloidal behavior of nickel aqueous suspensions is studied and compared to that of NiO suspensions. Under acidic conditions, nickel readily dissolves, but no dissolution takes place at basic pH. ζ potential is studied as a function of pH, showing that the isoelectric point (IEP) occurs at pH 3.5-4. Above the IEP there is a ζ potential plateau, which is associated to the predominance of NiO(OH) species. At pH 9 a new decrease in ζ potential is associated to NiO predominance. XPS studies support that suspensions prepared at pH >9 lead to NiO-enriched species, while suspensions prepared at lower pH form NiO(OH) species.
Introduction Colloidal processing has proved to be a useful way for manufacturing ceramic bodies as well as complex structures.1,2 Ceramic powders dissolve to some extent when immersed in water. Generally speaking, ceramic oxides have a negligible dissolution rate, while other powders (most non-oxides) readily dissolve. There is a broad body of work dealing with the colloidal stability of oxidic powders in water, although the colloidal stability of non-oxides has received increased attention in the last years.3-5 The large effort devoted by ceramists to improve the properties and reliability of ceramics through a colloidal approach has not found a parallelism in the field of powder metallurgy, where some researchers have discouraged to study the dispersion of metal powders in water because of two major problems: (i) the complex colloidal behavior in which not only oxidation and hydrolysis but also dissolution occurs, depending on pH and temperature, and (ii) the large density of most metals that promotes a fast tendency to settle. In this sense, some papers describe the use of differential sedimentation of metals and ceramics to obtain functionally graded materials.6,7 Nickel is one of the most interesting metals for structural and functional applications in metal-ceramic composites, because of its excellent corrosion resistance, refractory properties and chemical stability. The colloidal approach has been recently used to obtain complex-shaped bodies of metal-ceramic composites7,8 with tailored microstructures9,10 A variety of slurry-based forming techniques has been considered for manufacturing complex devices such as fuel cells and thermal barrier coatings, including tape casting,11 low-pressure injection moulding,12 spray coating, screening, inkjet printing,13 etc. The use of water as the dispersion media is now in focus due to environmental considerations. When metallic nickel is exposed to air at room temperature a thin (3-5 monolayers thick) surface layer of nickel oxide develops. At room temperature, the oxygen uptake stops once this limit is reached.14 When the nickel particles are suspended * Corresponding author. Addres: Instituto de Cera´mica y Vidrio (CSIC), Camino de Valdelatas s/n, 28049 Madrid, Spain. Telephone: +34 -917 355 840. Fax: +34-917355843. E-mail:
[email protected]. † Instituto de Cera ´ mica y Vidrio. ‡ Instituto de Cata ´ lisis y Petroleoquı´mica
Figure 1. Log [C] vs pH diagram for a concentration of nickel of 10-3 M in water.
in water, the passive outer layers become hydroxylated and evolve to Ni(OH)2.15 The acid-base equilibrium diagrams describe the predominance of each species. Figure 1 plots the concentration of different species versus pH for a solution concentration of 10-3 M. Alkaline pH conditions are needed to avoid the dissolution of the passive nickel hydroxide layer. At low pH the evolution of the passive layer depends on the nature of the acid and its concentration; for example, diluted HNO3 dissolves metallic nickel, while concentrated HNO3 forms a passive layer and no dissolution occurs. Other strong acids, like HCl or HF, do not promote passivation of metal particles. The dissolution of Ni(OH)2 in water leads to a basic pH. From the red-ox diagram it can be observed that for pH
