Nickel Sheets from Suspensions of the Metallic Powders in Water

Abstract

Materials designs evolving nickel sheets combined with ceramics has applications in either electrical or structural applications. Examples can be taken from the fabrication of high J c -HTS (high temperature superconductors) tapes [1,2] or the high temperature SOFCs where the metallic nickel has been used because of good oxidation resistance and good match with the ceramic components (YBCO [3] and YSZ). On the other hand the same properties of nickel can be employed for thermo-mechanical purposes. The joining of alternate metallic and ceramics tapes permits the fabrication of materials with graceful failure and high work of fracture. [4][5][6] Processing of those laminated materials is a critical point as the final reinforcement depends strongly on the distribution of the layers. Colloidal processing techniques have proved to be an accurate method to develop laminated and graded microstructures in both ceramic-ceramic [7,8] and metal-ceramic [9] materials by a diverse techniques such as slip casting, tape casting, electrophoretic deposition or screen-printing. For any of those processing techniques, the rheology of the slurries is a critical parameter to be controlled in order process suitable samples. [10][11][12] But the use of metals together to ceramics requires of processing methods to force the microstructural developments or to favor the co-sintering and joining between tapes. In this sense, tape casting has been extensively used to produce substrates and pieces in a wide range of ceramic materials and microstructures [7,[13][14][15] with precise control of the thickness. Usually the dispersion media has been an organic liquid due to their compatibility with the most common binders and plasticizers, [16] but environmental and economical reasons are promoting the employment of water as the dispersion media. [6,13,15] Aqueous slurries for tape casting implies an increment in the solid content of the slurries as a key issue to fabricate handily green tapes. [12,13] In this sense, previous works employed electrochemical [17] and rheological [11,18] investigations to optimize slurries of metallic nickel in water up to solid contents of 40 vol%. Dense and porous material has been later fabricated starting with those optimized slurries by colloidal techniques employed in ceramic processing. [17][18][19] In this paper the fabrication of nickel dense sheets has been faced by the employment of aqueous based tape casting. Previous studies demonstrated that better stability of Ni powders in water was reached when assuring a pH high enough to prevent dissolution (Ni 2+ ) and hydroxylation (through NiO(OH) and Ni(OH) 2 species), so that pH must be higher than 9. Furthermore, a maximum zeta potential of about -50 mV is reached when adding 1 wt% polyelectrolyte at a pH = 10. [17] Well-dispersed suspensions of nickel in water prepared under those conditions were rheologically characterized with different binder contents. The flow curve measured in the CR mode for suspensions prepared to solids loadings of 31 vol % and 40 v...

Colloidal processing and sintering of porous percolative Ni-YSZ layers

“…Shear thinning, or pseudoplasticity, is characterized by a decreasing viscosity as the shear rate increases, being specially important the viscoelastic transition zone at the lower shear rates [30]. This shear thinning has been previously observed in aqueous nickel and nickel-alumina slurries employed for colloidal processing [22,31].…”