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 terms describe the same set of techniques which has become known in the popular press as “3D printing” whether or not a printing process is involved! There are only a few strategies for doing this and they are classified as point, line or planar deposition methods: some deliver material to the building platform point-by-point (eg. ink-jet printing), some line-by-line (eg. extrusion freeforming) and some layer-by-layer (eg laminated object manufacture).
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. The same method is being used to make microwave 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
The popular versions of “3D printing” are monochrome but we have found a way to do what could be described as “3D printing in colour”. It means that we can place different materials in different positions in each plane as well as being able to deposit 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 [Mat. Sci. Eng. A.271, 344-352, 1999] but 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).
The dry powder delivery system is refined by the use of ultrasonic actuation which allows very fine doses of powder to be dispensed. See Dr Shoufeng Yang’s web page for videos of these processes.
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. This was 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.
LUSI has played 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 [Phys. Rev. E 73, Art. No. 021501, 2006].
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. We designate this “spontaneous manufacturing” because the shape is formed without equipment such as jiggers as would be required in the table-ware industry to form cups and bowls. Metal powders formed by spraying or atomization are likewise examples of spontaneous manufacturing but the form is limited to sphericity by surface energy demands. In this case, modification of the colloidal suspension can produce a range of shapes.
Well plates made by drying a droplet of well-dispersed suspension of ceramic powder.
Shown below is a 8 x 16 well plate made entirely of alumina and thus capable of holding samples for high temperature combinatorial work. [Adv. Appl. Ceram. 109, 51-55, 2010].
The drying of droplets of ceramic suspensions is a simple but vastly complicated process. The droplets “know” about their environment. Have a look at the four droplets here. They develop arches as they dry because they are influenced by the local relative humidity which has been modified by their neighbours [Langmuir, 25, 11299-11301, 2009].
The next image demonstrates
how extensively these arches can develop in droplets as they dry.
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.
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 continued 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 medicines, are hidden
in the galleries.
Ordered polymer clay nanocomposites: the quest to simulate the nacre microstructure
In the mollusc shell, nature uses polymorphs of calcium carbonate to confer higher elastic modulus than is possible from living tissue. Nobody in their right mind would use calcium carbonate in a structure that was critical for life-support because it has a fracture toughness of only about 0.2 MPa m1/2. Two factors influenced its selection: availability and solubility. But nature developed a composite with about 5 vol. % protein that presents a strength of 140 MPa and a fracture toughness of 8 MPa m1/2 in the abalone shell. The huge increase in mechanical properties is the result of the nano- and micro-structure of nacre. The message is simple: get the microstructure right and you don’t have to use expensive high performance materials.
The job of the mollusc shell is to protect soft living tissue from the rocks that get hurled around in strong waves. It’s pretty close to the job done by car body shells when cars get thrown around in motorway pile-ups. This is where the main application for biomimetic composites might turn out to be. There are nearly a billion cars in the world mostly with steel bodies weighing about 750 kg each. Fuel is consumed accelerating them. We need high strength to weight ratio and high stiffness to weight ratio materials to replace steel.
Volkswagen is pioneering a 250 mpg car and other manufacturers are on the same path. They are prototyping with carbon fibre composites. But this is a very expensive material, better suited to airframes (the Boeing 787 Dreamliner uses it), to F1 vehicles and top range cars. The whole purpose of fuel-saving vehicles is for them to penetrate the market fast. To do that, they must be cheap to buy.
That is why several groups around the world are trying to make ordered, layered nanocomposites with reinforcement platelets of smectite clays, layered double hydroxides, or graphene/graphene oxide [Applied Surface Science 258, 2098-2102, 2012].
In early work [Bioinspiration and Biomimetics, 3, 016005, 2008] we found it was quite easy to get ordered structures of montmorillonite clays by filtration, slip casting or sedimentation. They have a tendency, at the right pH, to self assemble with face-to-face interactions predominating over edge-to-face.
Ordered assemblies of aluminosilicate layers; the quest to simulate the structure of abalone.
Later we found that ordered structures can be made by drying the platelets from dilute suspension [Bioinsp. & Biomim. 7, No. 046004, 2012]. This idea has been picked in recent work from China [Nanoscale 5, 6356-6362, 2013] which shows that the assembly is unperturbed if a soluble polymer is included.
Layered double hydroxides can be processed in similar ways [Colloids and Surfaces A: Physicochemical and Engineering Aspects, 408, 71-78, 2012] and it may even be possible to use the ink-jet printer to arrange the microstructure [J.Coll. Interf. Sci 395, 11-17, 2013].
Using artificial neural networks for property-property correlations.
Materials science is predicated upon the
principle that structure (at all levels) determines properties but it can be
quite difficult to work out the connections in such a way that properties can
be predicted from structure. Since a given material has its own portfolio of
properties that each emerge from its structure, there ought to be some sort of
relationship between those properties. And there is. Refractive index, n, for
example is related to dielectric constant, κ,
by n = k 1/2 . Electrical and thermal conductivity are related because
electrons participate in both. But
are their more obscure relationships that could only be found by searching huge
amounts of data for correlations? Perhaps four or five properties taken
together can give a reasonable prediction of a sixth? Even if the prediction is
not perfect it would be a way to guide our searches and narrow down the number
of experiments that would have to be done in, for example, combinatorial work.
There are socio-economic influences on science and one of them is that science is structured so that each person is expert only in one area of property measurement. Someone making alloys for turbine blades wouldn’t dream of trying them out as catalysts for organic reactions. Someone making materials for hydrogen storage would be unlikely to test them for magnetic properties. Yet often correlations are found. The region of a ternary precious metal alloy phase diagram where the best catalysts occur might turn out to have very high reflectivity compared to the rest of the system. How can we ever find such correlations when science is so fragmented? The ANN, viewed as a simple but most capacious brain might be able to do such searches.
We began this work by looking at properties of the
elements, thinking somewhat naively, that there would be plenty of reliable
property data that we could use. We quickly found…I mean…the ANN quickly found…
that there are many errors in the data books and so we realised that the ANN is
a very good policeman for finding mistakes in datasets [Phil. Mag. 90,
4453-4474, 2010, J.Chem.
& Eng. Data, 56, 328-337, 2011]. In order for this type of venture to work,
it is necessary to inspect the data for accuracy very carefully before beginning
the search for correlations.
Polymers derived from biomass, particularly food waste
Despite fluctuations, the background rise in
oil price provides a market stimulus for biomass sourced fuels such as
bioethanol and biodiesel and for chemical and polymer production, each
associated with its own learning curve. This offers a prospect of
sustainability in materials. Thus the free market, responding to a combination
of oil price rises combined with regulatory measures to address climate change
may encourage the materials industries to use the biomass resource. This idea
is not new. In 1940, Henry Ford patented a car body shell made from a
soybean-based polymer supported on a tubular steel frame. It saved one third of
the weight of a steel body and Ford believed it was safer. Ford had a complex
relationship with his farming roots and saw the car as a way to combine
industrial and agricultural enterprises. This aspect of Ford’s vision is by no
means irrelevant today. The sectorization of economies, particularly between
industry and agriculture is reflected in the relative wealth of nations with
agriculturally-based economies tending to be poorer and the prospect for
biomass-sourced polymers represents a ‘synthesis’ in more ways than one and a
paradigm for the emerging idea of ‘integrated’ or ‘balanced’ sector
economics’. With these thoughts in
mind we began
to address the synthesis of
hydrophobic polymers soon realising that this would create the same
confrontation over land-use that has plagued the biofuel market. Our work has
therefore addressed routes to the synthesis of hydrophobic polymers from food
wastes of which there is 1.3 Pg annually. This approach extinguishes land-use
conflict. We have recently published an assessment of the available food waste
resource and a review of the routes to polymerisation [S.A.Sanchez-Vazquez, H.C.Hailes, J.R.G.Evans, Hydrophobic polymers from
food waste: Resources and synthesis, Polymer Reviews 53, 627-694, 2013].
Foams and their deployment for enhancement of oceanic albedo
It looks increasingly likely that the international community, faced with an impossible confrontation between climate change and the freedom of the market will ‘take it to the wire’ over the combustion of fossil fuels. For this reason the Royal Society has begun to accept that there may become a need for “Geoengineering” better denoted “Climate restoration”. [Geoengineering the Climate: Science Governance and Uncertainty, Royal Society Policy Document, 10/09, 2009]. Strategies include the injection of large quantities of sulphur into the stratosphere and the construction of space parasols but there are also less drastic proposals such as increasing the droplet number concentration in clouds by using seasalt as cloud condensation nuclei and the deliberate production of oceanic foams as proposed by British and US scientists [Evans JRG, Stride EPJ, Edirisinghe MJ, Andrews DJ, Simons R. (2010) Can oceanic foams limit global warming? Climate research 42:155–160Seitz R (2011) Bright water: hydrosols, water conservation and climate change. Climatic Change 105:365–381]. We are therefore exploring ways to prolong the lifetime of oceanic foams while preparing materials that are likely to be accepted as biodegradable agents.
- S.A.Sanchez-Vazquez, H.C.Hailes, J.R.G.Evans, Hydrophobic polymers from food waste: Resources and synthesis, Polymer Reviews 53, 627-694, 2013.
- Y.Zhang, J.R.G.Evans, Morphologies developed by the drying of droplets containing dispersed and aggregated layered double hydroxides, J.Colloid Interf. Sci. 395, 11-17, 2013.
- P.Walley, Y.Zhang, J.R.G.Evans, Self-assembly of montmorillonite platelets during drying, Bioinsp. & Biomim. 7, No. 046004, 2012.
- Y.Zhang, J.R.G.Evans Approaches to the manufacture of layered nanocomposites, Applied Surface Science 258, 2098-2102, 2012.
- X.S.Lu, J.R.G.Evans, S.N.Heavens, Ceramic domes fabricated by a combination of tape casting and vacuum forming, J.Euro. Ceram. Soc. 32, 681-687, 2012.
- X.S.Lu, L.Chen, N.Amini, S.Yang, J.R.G.Evans, Z.X.Guo, Novel methods to fabricate macroporous 3D carbon scaffolds and ordered surface mesopores on carbon filaments, J. Porous Mater. 19,529-536, 2012.
- X.Lu, J.R.G.Evans, S.N.Heavens, A comparison of the tape casting of alpha and beta alumina, J. Euro. Ceram. Soc. 32, 4219-4228, 2012.
- Y.Zhang, J.R.G. Evans, Alignment of layered double hydroxide platelets, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 408, 71-78, 2012.
- F.Gao, S.Yang, P.Hao, J.R.G.Evans, Suspension stability and fractal patterns, A comparison using hydroxyapatite, J.Amer. Ceram Soc. 94, 704-712, 2011.
- B.Q.Chen, J.R.G.Evans, Mechanical properties of polymer-blend nanocomposites with organoclays: polystyrene/ABS and high impact polystyrene/ABS, J.Polym. Sci. B-Polym Phys. 49 443-454, 2011.
- Y.M.Zhang, J.R.G.Evans, S.F.Yang, Corrected values for boiling points and enthalpies of vaporization of elements in handbooks, J.Chem. & Eng. Data, 56, 328-337, 2011.
- Y.Chen, J.R.G.Evans, S.Yang, A rapid doping method for high-throughput discovery applied to thick film PTCR materials, J.Amer. Ceram. Soc. 94, 3748-3756, 2011.
- A.Peart and J.R.G. Evans, A study of sea salt particles launched by bubble burst, Bubble Science, Engineering and Technology 3, 64-72, 2011
- L.F.Chen, J.R.G.Evans, Spontaneous manufacturing: ceramic 128-well plates made by droplet drying, Adv. Appl. Ceram. 109, 51-55, 2010.
- J.R.G.Evans, E.P.J.Stride, M.J.Edisiringhe, D.J.Andrews, R.R.Simons, Can oceanic foams limit global warming, Climate Res. 42 , 155-160, 2010.
- Y.M.Zhang, J.R.G.Evans, S.Yang, Detection of material property errors in handbooks and databases using artificial neural networks with hidden correlations, Phil. Mag. 90, 4453-4474, 2010.