Gel casting of aqueous suspensions of BaTiO3 nanopowders

Abstract

Commercial nano BaTiO 3 powders have been formed into green bodies using colloidal forming routes. A study of the rheological behaviour of the suspensions as a function of dispersant concentration and homogenisation time was made in order to prepare stable concentrated suspensions of the nanopowders. Bulk components were then manufactured using aqueous slip and gel casting involving polysaccharides that gel on cooling, i.e. agar. The performance of theses consolidation techniques for obtaining dense green bodies from the BaTiO 3 nanopowders was studied. It was possible to prepare relatively big gel cast samples with a similar density and microstructure and in a shorter time compared to those obtained by slip casting. Another difficulty found in nanoceramics manufacturing is the shaping of homogeneous green bodies. In general, colloidal processing allows the production of complex-shaped parts with a decreased number and size of pores and hence higher reliability 6 . In colloidal systems the interfaces play a key role in the surface properties, such as adsorption, surface charge, electric double layer, etc [6,7]. In the case of 3 nanosized particles, their massive surface areas and hence substantial interaction forces between the particles make it more difficult to prepare stable nano-particle suspensions and study their colloidal properties.Over the last few years, significant effort has been devoted to developing new forming methods for manufacturing complex-shaped ceramic products with minimal number of detrimental defects [8][9][10] This work aims to study the preparation and rheological characterisation of concentrated aqueous suspensions of commercial nanosized powders of barium titanate (nanoBT) for shape forming by slip and gel casting, the latter using thermogelling polysaccharides. The performance of these consolidation techniques for obtaining dense green bodies from the BT nanopowder is discussed. ExperimentalThe starting material was a commercial nanoBT powder (HPB-1000, TPL Inc., was added as a deflocculant to the solution [20,27] prior to the addition of sufficient of the nanoBT powder to generate suspensions with solids loadings of 70, 75 and 80 wt% (28, 33 and 40 vol%, respectively). The suspensions were mixed using by mechanical agitation with helices and further homogenised using ultrasound (a 400 W sonication probe, UP400S, Hielscher Ultrasonics GmbH, Stuttgart, Germany) for up to 10 min.Suspensions were cooled in an ice-water bath during ultrasonication in order to avoid heating.A commercially available agar (Grand Agar, Hispanagar S.A., Burgos, Spain) was used as the gelling agent. Solutions of agar were prepared at concentrations of 6 wt% by heating in a pressure vessel [14,15] in order to achieve the 110ºC required for its full dissolution without the water boiling.The rheological behaviour of the suspensions was studied using a rheometer (Haake RS50, Karlsruhe, Germany) with a double-cone and plate measuring system, provided with a solvent trap to reduce evaporation. Flow curves...

Processing of bulk nanostructured ceramics

“…Although well formed green bodies were produced, they all lost strength very significantly on drying resulting in bodies that could not be handled without them falling apart. It should be noted, however, that related work performed on the gel casting of a commercial nano barium titanate powder and using a different gelation system was more successful in producing handleable green bodies 21 ; although in this case the quality of precursor nanopowder was much lower than the nano 3YSZ used in the present work.…”

Slip casting of nanozirconia/MWCNT composites using a heterocoagulation process

“…Both suspensions were thixotropic although the viscosity of the former decreased due to the better stabilization. 30,31,[38][39][40] For this reason, TMAH was added in the preparation of the nanozirconia suspensions. The suspensions with PAA and TMAH were homogenized by using an ultrasound probe for a few minutes.…”

“…30 As deflocculant, aqueous solutions of an ammonium salt of polyacrylic acid-based polyelectrolyte (PAA, Duramax D3005, Rohm & Haas, USA) were prepared in deionised water to concentrations of 1.0-3.0 wt%. The amount of deflocculant added is expressed in terms of the active matter present in the dispersant with respect to suspension solids loading.…”

“…Tetramethylammonium hydroxide, TMAH (Aldrich-Chemie, Germany), supplied as a 25 wt% aqueous solution was added to the suspensions in order to obtain the desired value of pH (8.0 ± 0.1) although it also contributes to a better stabilisation of the suspensions. 26,30,31 The suspensions were homogenised using mechanical agitation with helices for 30 min and dispersed using a 400 W ultrasound probe (UP400S, Hielscher Ultrasonics GmbH, Stuttgart, Germany). Suspensions were cooled in an ice-water bath during ultrasonication (US) in order to avoid excessive heating.…”

Thermal decomposition of monosaccharides derivatives applied in ceramic gelcasting process investigated by the coupled DTA/TG/MS analysis

“…There are many shaping techniques in ceramic technology but most of them have important disadvantages such as nonhomogeneity of powder compacts, size and geometry limit of produced element, use of high pressure, long binder burnout and high costs. In order to overcome these limitations some novel shaping technologies have been recently developed on the basis of colloidal processing of ceramic powders, for example gelcasting [1][2][3], monomer based sol-gel, electrophoretic deposition [4,5] or direct coagulation casting [6,7]. Gelcasting allows obtaining high-quality, complex-shaped ceramic elements by means of an in situ polymerization, through which a macromolecular network is created to hold ceramic particles together (Fig.…”