Prof Julian Evans
Ceramic and powder processing
Since most ceramics cannot be melted and cast or forged into shape, we often resort to making them from powders which, when heated to allow atom mobility, sinter to near full density. Two problems emerge; the first is how to get complicated shapes from powder and the second is that the fine powders needed for sintering are ‘sticky’ and don’t pack together well (the van der Waals attractive force between adjacent particles over-rides the force on an individual particle in the gravitational field).
One way around these difficulties is to use injection moulding, which is widely applied for thermoplastics. The powder is incorporated into a wax or polymer vehicle, moulded and the vehicle is removed, usually by heating and sometimes assisted by capillary action.
There are many other processes used for shaping thermoplastics and so the question arises; can they also be used for making ceramics. The answer is that they all can, with different degrees of success.
There are also many ways for making ceramic and metal foams that rely on thermosetting and thermoplastic polymers as vehicles:
Solid freeforming and rapid prototyping
These two unnecessarily complicated terms describe the same set of techniques. On your pc, you can press ‘print’ and get a two dimensional image of what was on the screen. In solid freeforming, you press print and a get a three dimensional replica of the CAD picture on the screen. There are only a few methods of doing this. Some deliver material to the building platform point-by-point, some line-by-line and some layer-by-layer. Perhaps the most well known is stereolithography in which a UV laser scans across a thin layer of photo-sensitised monomer, converting it to polymer in the forming areas and leaving it as monomer in the non-forming areas. Then a new layer of monomer is spread across and the next layer is scanned. In this way a 3D object is created and if ceramic powder is included in the monomer, a ceramic object can be made by removing the polymer and firing the assembly of particles. Another well-known method is selective laser sintering. It’s a similar idea but this time the laser is more powerful and scans a layer of powder (polymer, metal or ceramic) in order to melt it. The non-forming areas are left as powder to support subsequent layers.
Direct ceramic inkjet printing
We have developed direct ceramic inkjet printing as a solid freeforming method in which the ceramic powder is made into an ink and deposited through a nozzle exactly where it is wanted. The picture below shows a model of the maze at Hampton Court Palace made from zirconia with an inkjet printer and fired at 1450°C. Such structures could perhaps be used in high temperature microreactors.
The structure below is also made by inkjet printing and it has a number of possible applications including microfluidic mixing and as a bandgap metamaterial functioning in the microwave region
The same technique can be used to pattern clay tiles individually with raised motifs to assist visually impaired persons in their homes or workplaces.
We are using the building platform shown below to create ceramic or metal latticework for hard tissue scaffolds particularly for use in maxillofacial surgery where we work with surgeons at Guy’s and St Thomas’ Hospital. The same method is being used to makemicrowave metamaterials where we work with colleagues in the Department of Electronic Engineering at Queen Mary, University of London. The platform consists of a three-axis table (X and Y are linear motors) with two extrusion heads using micro-steppers. The extrudate is a ceramic paste. The image shows a hydroxyapatite/tricalcium phosphate lattice with three hierarchical levels of porosity. The larger pores provide pathways for vascularisation, the smaller cavities provide for cell development. Finally the sintering of the ceramic itself controls sub-micron porosity which influences dissolution rate. The overall shape is also computer controlled. This equipment is used to make microwave metamaterials with bandgap structures for antennae and for biomedical applications.
Lattices with a central hole can also be assembled for example, to fill with a growth promoter and we are building concentric dies to produce duplex structures. The equipment can also be used to make miniature helices with a central hole and we have recently build devices suitable for high temperature microfluidic applications.
Lattices based on spider web structures provide ‘combinatorial’ samples for testing the effect of pore sizes. A CAD pattern and part of the assembled ceramic lattice are shown.
Solid freeforming of 3D functional gradients
We can liken solid freeforming to printing in three dimensions rather than two and, pursuing the analogy, we can divide it into “monochrome” and “colour” printing. Colour printing in 3D means you could place any material at any point as well as depositing mixtures of materials of predetermined composition. This means we could build three dimensional functional gradients into an object and the computer file would not just control the external shape and form of the object but also the composition at every point. Direct ceramic inkjet printing already allows this. We have also found a method of dry powder deposition which can be used with selective laser sintering or similar solid freeforming processes that use coarser powders, such as metals.
The dry powder dispensing system has similarities to an ancient artform of the Navajo Indians known as sand-painting. Acoustic waves generated from the design file are sent to a glass capillary hopper via a connecting rod from a transducer (using 100-300 Hz). This provides both on/off switching and flow rate control. High amplitude produces low flow rates. An explicit model with no disposable constants, explains the physical basis of flow rate control [Phil. Mag. 85 (2005) 1089-1109]. This process was exhibited at the Royal Society Summer Science Exhibition 2007
The image shows a 2D picture of the Olymic games logo printed with five colours. An orchestra of six valves is positioned over a three axis building platform. Since selective laser sintering involves metallurgical damage and residual stress, we also devised a method of making complex shapes with 3D functional gradients in which mould and part powders are layered simulataneously. The assembly can either be compacted and sintered or loose sintered. The image shows sections of step wedges built from stainless steel powder and infiltrated with tin-bronze.
The Chinese characters (title of the Tao Teh Ching) demonstrate the start-stop capability of dry powder direct writing (copper powder on SiC paper).
Combinatorial and high throughput methods for materials discovery
My team has built two ink-jet printers for combinatorial searches of ceramic compositions. The first mixes ceramic inks behind the nozzle using pulsed electromagnetic valves fed from pressurised reservoirs to create multiple discrete compositions. Thiswas subjected to rigorous calibration for flow rate as a function of viscosity, and for mixture assays of both inorganic liquids and ceramics. This is a simple inexpensive device and paved the way for the larger gantry robot known as LUSI, the London University Search Instrument funded by EPSRC in response to a grant application by myself and Peter Coveney.
Combinatorial ink-jet printer based on electromagnetic valves and capable of anterior ink mixing: physical arrangement and pipework diagram.
On the basis of these preliminaries, LUSI was built from an EPSRC equipment grant and is now playing a role in an EPSRC collaborative grant with IC, UCL, LSBU and QMUL to explore new functional oxide compositions. One of the problems (now solved) of this approach is the segregation of particles of different oxides during droplet drying.
The LUSI printer showing the robot pick and place arm preparing to collect a library slide.
Complicated flow paths are found during the drying of droplets of a powder suspension. They allow us to make ceramic well plates from well-dispersed suspensions. The photograph shows dried and fired TiO 2 wells; the shape, which is reproducible, results solely from the drying of the droplets.
Well plates made by drying a droplet of well-dispersed suspension of ceramic powder.
We started this work in 2001 at a stage when there was little UK activity on the effects of smectite clays on polymers with an EPSRC grant with Peter Coveney and Durham University. As well as pioneering computer modelling from Peter Coveney’s group, fundamental studies emerged on preferential intercalation of high molecular weight polymer fractions [J. Phys. Chem. 108 (2004) 14986-14990], driving force for intercalation [Phil. Mag. 85 (2005 1519-1538], calculation of effective volume fractions [Macromolecules 39 (2006) 1790-1796] and modulus of clay platelets [Scripta Mat. 54 (2006) 1581-1585]. This work continues with an emphasis on using smectite clays to improve the properties of polymer blends, especially for recycled materials. Other aspects include the preparation of foams by making use of blowing agents concealed in the galleries [Nanotechnology 16 (2005) 2334-2337] which opens up the idea of a ‘Trojan Horse’ approach to materials in which additives, perhaps medicaments, are hidden in the galleries. In the most recent developments, we are making ‘bricks and mortar’ structures that simulate the nacre of abalone using groups of clay platelets.
Ordered assemblies of aluminosilicate layers; the quest to simulate the structure of abalone.
- X.Lu, Y.Lee, S.Yang, Y.Hao, J.R.G.Evans, C.G.Parini, Solvent-based paste extrusion solid freeforming, J.Euro. Ceram. Soc. 30, 1-10, 2010
- F.Akthar, J.R.G.Evans, High porosity (>90%) cementitious foams, Cement Conc. Res. 40, 352-358, 2010.
- L.F.Chen, J.R.G.Evans, Spontaneous manufacturing: ceramic 128-well plates made by droplet drying, Adv. Appl. Ceram. 109, 51-55, 2010.
- H.Y.Yang, X.P.Chi, S.Yang, J.R.G.Evans, Mechanical strength of extrusion freeformed calcium phosphate filaments, J. Mat. Sci. Mater. in Medicine 21 , 1503 2010
- R.C.Pullar, Y.Zhang, L.Chen, S.Yang, J.R.G.Evans, A.N. Salak, D.A.Kiselev, A.L.Kholkin, V.M.Ferreira, N.McN.Alford, Dielectric measurements on a novel Ba1-xCaxTiO3 (BCT) bulk ceramic combinatorial library, J. Electroceram. 22, 245-251, 2009.
- X.Lu, Y.J.Lee S.Yang, Y.Hao, J.R.G.Evans, C.G.Parini, Fine lattice structures fabricated by extrusion freeforming: process variables, J. Mater. Process. Technol. 209, 4654-4661, 2009.
- X.Lu, Y.Lee, S.Yang, Y.Hao, R.Ubic, J.R.G.Evans, C.G.Parini, Fabrication of millimetre wave electromagnetic bandgap crystals using microwave dielectric powders, J.Amer. Ceram. Soc. 92, 371-378, 2009.
- X.Lu, Y.Lee, S.Yang, H.Yang, J.R.G.Evans, C.G.Parini, Rapid prototyping of microwave electromagnetic crystals, Rapid Prototyping J. 15, 42-51, 2009.
- Y.J.Lee. Y.Hao, C.G.Parini, X.Lu, S.Yang, J.R.G.Evans, Cylindrical EBG antenna for short range gigabit wireless communications at millimetre-wave bands, Electronics Letters, 45, 136-138, 2009.
- S.Yang, X.Lu, Y.Lee, Y.Hao, J.R.G.Evans, C.G.Parini, Metamaterials fabrication by solid freeforming fabrication (invited lecture), Int. Conf. Advances in Functional Materials, 8-12 June 2009, Jiuzhaigou, China.
- X.Lu, S.Yang, J.R.G.Evans, Microfeeding with different ultrasonic nozzle designs, Ultrasonics, 49, 514-521, 2009.
- B.Chen, J.R.G.Evans, Exploration of Biodegradable Polymer-clay Nanocomposites, 1st Int. Conf. on Multifunctional, Hybrid and Nanocomposites, Tours, France, Mar. 2009.
- X.Lu, Y.Lee, S.Yang, Y.Hao, J.R.G.Evans, C.G.Parini, Fabrication and evaluation of solid freeformed electromagnetic bandgap structures, J.Phys. D- Appl. Phys. 42 (14) 145107, 2009.
- Z.Zhang, J.B.M.Goodhall, D.J.Morgan, S.Brown, R.J.H.Clark, J.C.Knowles, N.J.Morden, J.R.G.Evans, A.F.Carley, M.Boker, J.A.Darr, Photocatalytic activities of N-doped nano-titanias and titanium nitride, J.Euro. Ceram. Soc. 29, 2343-2353, 2009.
- Z.C.Zhang, S.Brown, J.B.M.Goodall, X.L.Weng, K.Thompson, K.N.Gong, S.Kellici, R.J.H.Clark, J.R.G.Evans, J.A.Darr, Direct continuous hydrothermal synthesis of high surface area nanosized titania, J.Alloys & Compounds, 476, 451-456, 2009.
- B.Chen, J.R.G.Evans, Impact strength of polymer-clay nanocomposites, Soft Matter, 5, 3572-3584, 2009.
- L.F.Chen, J.R.G.Evans, Arched structures created by colloidal droplets as they dry, Langmuir, 25, 11299-11301, 2009.
- Y.Lee, X.Lu, Y.Hao, S.Yang, J.R.G.Evans, and C.G.Parini, Low-Profile Directive Millimeter-wave Antennas Using Free-formed Three-Dimensional (3D) Electromagnetic Bandgap Structures, IEEE Transactions on Antennas and Propagation 57, (Special Issue Pt.i), 2893-2903, 2009.
- X.Weng, J.K.Cockcroft, G.Hyett, M.Vickers, P.Boldrin, C.C.Tang, S.P.Thompson, J.E.Parker, J.C.Knowles, I.Rehman, I.Parkin, J.R.G.Evans, J.A.Darr, High throughput continuous hydrothermal synthesis of an entrire nanoceramic phase diagram, J.Combinatorial Chem. 11, 829-834, 2009.
- D. J. Scott, S. Manos, P. V. Coveney, J. C. H. Rossiny, S. Fearn, J. A. Kilner, R. C. Pullar, N. Mc N. Alford, A-K Axelsson, Y. Zhang, L. Chen, S. Yang, J. R. G. Evans and M. T. Sebastian, Functional Ceramics Materials Database: An online resource for materials research. J. Chem. Information and Modelling, 48, 449-455, 2008.
- S.Yang, M.M.Mohebi, J.R.G.Evans, A Novel Solid Freeforming Method using Simultaneous Part and Mould Construction, Rapid Prototyping Journal, 14, 35-43, 2008.
- Y.M.Zhang, S.Yang, J.R.G.Evans, Revisiting Hume Rothery's Rules with an Artificial Neural Network, Acta Materialia 56, 1094-1105, 2008.
- B.Chen, J.R.G.Evans, H.C.Greenwell, P.Boulet, P.V.Coveney, A.A.Bowden, A.Whiting, A Critical Appraisal of Polymer-clay Nanocomposites, Chemical Society Reviews, 37, 568-594, 2008.
- H.Y.Yang, S.F.Yang, X.P.Chi, J.R.G Evans, I.Thompson, R.J.Cook, P.Robinson, Sintering Behaviour of Calcium Phosphate Filaments for Use as Hard Tissue Scaffolds, J.Euro. Ceram. Soc. 28, 159-167, 2008.
- J.R.G.Evans, Seventy Ways to Make Ceramics, J.Euro. Ceram. Soc. Special Issue for Professor Sir Richard Brook's 70th Birthday, 28, 1421-1432, 2008.
- T.Liu, B.Chen, J.R.G.Evans, Ordered Assemblies of Clay Nano-platelets, Bioinspiration and Biomimetics, 3, 016005, 2008.
- X.Lu, S.Yang, J.R.G.Evans, R.Ubic, Y.Lee, Y.Hao, C.G.Parini, Fabrication of Electromagnetic Crystals by Extrusion Freeforming, Metamaterials, 2, 36-44, 2008.
- J.R.G.Evans, M.J.Edirisinghe, I.Ford, Adjustment of Aerosol Seasalt Concentration: A Preliminary Look at Technical Strategies, Annual Aerosol Conference April 2008, The Aerosol Society, Somerset UK pp.4-5.
- S. Yang, X.P.Chi, H.Y.Yang, J.R.G.Evans, Rapid Prototyping of Ceramic Lattices for Hard Tissue Scaffolds, Materials and Design. 29, 1802-1809, 2008.
- B.Q.Chen, S.Liu, J.R.G.Evans, Polymeric Thermal Actuation using Laminates Based on Polymer-clay Nanocomposites, J.Appl. Polym. Sci. 109, 1480-1483, 2008.
- X.S.Lu, S.F.Yang, J.R.G.Evans, Ultrasound-assisted Microfeeding of Fine Powders, UK-China Particle Technology Forum, 1-2 April 2007 Leeds UK: Particuology, 6, 2-8, 2008.
- Y.Zhang, S.Yang, L.Chen, J.R.G.Evans, Shape Changes on Drying of Droplets of Suspensions, Langmuir, 24, 3752-3758, 2008.
- H.Y.Yang, I.Thompson, S.F.Yang, X.P.Chi, J.R.G.Evans, R.J.Cook, P.Robinson, Dissolution Characteristics of Extrusion Freeformed Hydroxyapatite-tricalcium Phosphate Scaffolds, J.Mater. Sci. Mater. In Medicine, 19, 3345-3353, 2008.
- J.R.G.Evans, S.Yang, Solid Freeforming and Combinatorial Research at University of London, Int. conf. Rapid Prototyping and Biomaterials, Nov. 2008 Beijing. Proc. ICRPM-BM 08 Tsinghua Univ. Nov. 2008
- B.Chen, J.R.G.Evans, Impact and Tensile Energies of Fracture in Polymer-clay Nanocomposites, Polymer, 49, 5113-5118, 2008.
- Y.Lee, X.S.Lu, Y.Hao, C.G.Parini, J.R.G.Evans, Directive Millimetrewave Antennas using Freeformed Ceramic Metamaterials in Planar and Cylindrical Forms, IEEE Antennas and Propagation Soc. Int. Symp. Vols 1-9, pp.2242-2245, 2008.
- J.C.H.Rossiny, J.Julis, S.Fearn, J.A.Kilner, J.R.G.Evans, Combinatorial Characterisation of Mixed Conducting Perovskites, 16th Int. Conf. Solid State Ionics, Shanghai, 2007: Sol. State Ionics, 179, 1085-1089, 2008.