J. Ceram. Sci. Tech., 03 [02] 81-88 (2012) DOI: 10.4416/JCST2012-00003 available online at: http://www.ceramic-science.com © 2012 Göller Verlag Robocasting of Alumina Lattice Truss Structures T. Schlordt * , F. Keppner, N. Travitzky, P. Greil Department of Materials Science, Glass and Ceramics, Friedrich-Alexander- University Erlangen-Nuernberg, Martensstr. 5, 91058 Erlangen, Germany received January 20, 2012; received in revised form February 8, 2012; accepted February 25, 2012 Abstract Robocasting of aqueous colloidal a-Al 2 O 3 gels for manufacturing cellular ceramics with periodical lattice truss structures was investigated. Coagulation of gels loaded with 48 vol% a-Al 2 O 3 was induced by adding CH 3 COONH 4 . The gels exhibit shear-thinning behavior, shear elastic moduli ranging from 6.7 to 390 kPa and yield-stresses from 25 to 570 Pa. Continuous filaments with a diameter of 0.5 mm were extruded with a deposition speed of up to 35 mm/s on a high-precision six-axis robotic system equipped with a single-screw micro-extruder. The lattice structures consist of alternating layers formed by a linear array of circular rods aligned parallel with a distance of 1 mm and an angle of 90° between alternating layers. After being dried for 12 h, the robocast grids were sintered in air at 1650 °C for 2 h resulting in a fractional strut density > 0.95, a mean filament diameter of 400 lm, a volume filling fraction of 0.49 (sealed walls) and 0.35 (meshed walls), and macro-cells in the deposition plane of quadratic shape with a mean area of 0.136 mm 2 ± 0.017 mm 2 Based on gravitation-driven viscous flow, model conditions for attaining free spanning ligaments were discussed. Keywords: Robocasting, alumina gel, lattice truss structures I. Introduction Moldless robocasting of ceramics is a solid freeform tech- nique working on the principle of continuous writing of a filament and layer-by-layer build-up of a three-dimen- sional filamentary preform 1, 2 . Aqueous colloidal gels and colloid-loaded thermoplastic polymers were applied to demonstrate the capability of robocasting technology to manufacture three-dimensional preforms with excellent shape variability including high aspect walls as well as un- supported spanning structures 3 . While robocasting in air was successfully performed applying nozzles with diam- eters exceeding 500 lm, decoupling the deposition kinet- ics from the drying process by extrusion into a non-wet- ting oil bath allowed the generation of filament diameters smaller than 100 lm 4 . The dried extruded preform can be sintered without a separate debinding process as only a low organic content (< 3 wt%) is required. Applications include manufacturing of grid and lattice structures for composites, bone restoration and meshes for filters. Ce- ramic gels based on silica 1 , alumina 5–7 , mullite 8 , lead- zirconate-titanate 9 , tricalciumphosphate 10 , hydroxyap- atite 11 , lead-magnesium-niobate (PMN) 12 , porcelain 13 and barium-titanate 14 were applied to robocasting. Gela- tion was induced by lowering the pH-value, increasing the ionic strength or adding a polymeric flocculant. In order to prevent sedimentation and syneresis, cellulose derivatives were added which may give rise to apprecia- ble yield-stress of the particle suspension 15 . Depending on the extrusion speed and rheological behavior, flow of a * Corresponding author: tobias.schlordt@ww.uni-erlangen.de colloidal gel through the extrusion nozzle may cause a pro- nounced shear-rate gradient over the filament diameter. Plug-flow with an unyielded core and a surface region de- pleted of particles (slip-plane) was reported 4 . The oppor- tunities and challenges of robocasting e.g. direct filament writing were surveyed in an excellent review by Lewis et al. 1 . In this work, a novel six-axis robot system was coupled to a single-screw micro-extruder to provide high geomet- rical precision and line control flexibility for continuous filament writing. Regular grids of alumina were manufac- tured with an aqueous-based alumina gel feedstock. The rheological behavior of the colloidal alumina gel was sys- tematically varied by adding CH 3 COONH 4 (NH 4 Ac). Yield stress, shear elastic modulus and relaxation kinet- ics of the gel filaments were analyzed to select optimum parameters for control of shape and dimensional stability. Three-dimensional lattice truss structures of alumina were manufactured, and the potential of robot-assisted contin- uous filament deposition was demonstrated. II Experimental Procedure (1) Preparation of the colloidal alumina gel An aqueous slurry containing 52 vol% (81 wt%) of a submicron a-Al 2 O 3 powder (CT3000 SG, Almatis GmbH, Ludwigshafen, Germany, d 50 = 0.35 lm, S V = 8.3 m²/g) was prepared in a tumbling mixer (Turbula T2F, Willi A. Bachofen AG, Muttenz, Switzerland) using ZrO 2 milling balls. The slurry contained 1.6 wt% NH 4 - polymethacrylate dispersant (Darvan C-N, R.T. Van-