Florian Bouville

Eric Maire

Sylvain Meille

Bertrand Van De Moortèle

Adam J Stevenson

Sylvain Deville

Strong, Tough, and Stiff Bioinspired Ceramics from Brittle Constituents

High strength and high toughness are usually mutually exclusive in engineering materials. Improving the toughness of strong but brittle materials like ceramics thus relies on the introduction of a metallic or polymeric ductile phase to dissipate energy, which conversely decreases the strength, stiffness, and the ability to operate at high temperature. In many natural materials, toughness is achieved through a combination of multiple mechanisms operating at different length scales but such structures have been extremely difficult to replicate. Building upon such biological structures, we demonstrate a simple approach that yields bulk ceramics characterized by a unique combination of high strength (470 MPa), high toughness (22 MPa.m 1/2 ), and high stiffness (290 GPa) without the assistance of a ductile phase. Because only mineral constituents were used, this material retains its mechanical properties at high temperature (600°C). The bioinspired, material-independent design presented here is a specific but relevant example of a strong, tough, and stiff material, in great need for structural, transportations, and energy-related applications.

Ceramics exhibit among the highest stiffness and strength of all known material classes 1 .

Because of the strong and directional bonding between constitutive atoms, they present a high

fusion temperature and thus a high thermal stability. Being composed of mostly light elements, they are also lightweight compared to most metals. This property combination predestined them to be the best material choice for high stress and high temperature operative conditions. However, the iono-covalent bonds limit most common plasticity mechanisms and therefore prevent any ductile behavior. This lack of plasticity is often the main limitation for the use of ceramic materials, resulting frequently in catastrophic and unpredictable failure [START_REF] Ritchie | The conflicts between strength and toughness[END_REF] which greatly limits their range of applications. Damage-resistant ceramics are thus in great demand.

However, countless damage-resistant biological materials comprise ceramics and are used for structural purposes [START_REF] Koester | The true toughness of human cortical bone measured with realistically short cracks[END_REF][START_REF] Peterlik | From brittle to ductile fracture of bone[END_REF] in nature. The most famous example is probably the nacreous part of seashells where the brick and mortar structure comprising 95 vol.% of platelets of polycrystalline aragonite (CaCO 3 ) and 5 vol.% of a protein is extremely effective at restricting crack propagation [START_REF] Barthelat | Toughness amplification in natural composites[END_REF] and presents a toughness orders of magnitude greater than either of its constituents [START_REF] Barthelat | On the mechanics of mother-of-pearl: A key feature in the material hierarchical structure[END_REF][START_REF] Okumura | Why is nacre strong ? Elastic theory and fracture mechanics for biocomposites with stratified structures[END_REF] . Nacre toughness is achieved through numerous extrinsic mechanisms that include viscoelastic deformation of the protein layer [START_REF] Song | Structural and mechanical properties of the organic matrix layers of nacre[END_REF] , mineral bridges rupture, and inelastic shearing and frictional sliding during platelets pull out [START_REF] Barthelat | Toughness amplification in natural composites[END_REF] .

Tough ceramic materials could be obtained by translating the specific microstructural and interfacial structures present in natural systems to engineering materials [START_REF] Meyers | Structural Biological Materials: Critical Mechanics-Materials Connections[END_REF] . This could be achieved by building "brick and mortar" organized structures using anisotropic particles as building blocks combined with a ductile phase. The alignment of anisotropic particles can be achieved by sequential deposition methods [START_REF] Bonderer | Bioinspired design and assembly of platelet reinforced polymer films[END_REF] for thin films but not for bulk materials.

Ceramic/polymer composites that replicate nacre features have been fabricated by various techniques, such as ice templating, magnetic particles alignment [START_REF] Meyers | Structural Biological Materials: Critical Mechanics-Materials Connections[END_REF] , or simply by gluing ceramic layers [START_REF] Clegg | A simple way to make tough ceramics[END_REF] and the toughness levels achieved are rather impressive [START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF] . Process using magnetic fields lead to structures with well-aligned platelets embedded in a polyurethane matrix [START_REF] Erb | Composites reinforced in three dimensions by using low magnetic fields[END_REF] . Such approaches nevertheless suffer from severe limitations. Because the design is based on the presence of a ductile phase, the materials exhibiting the highest toughness all comprise at least 20 vol.% of a polymeric phase, restricting de facto their useful temperature range to mild temperatures (200°C or less). In nacre and nacre-inspired materials, the intrinsic toughening is primarily a result of crack deflection at the platelets interfaces. Thus, as a first order approximation, increasing the local density of interfaces should increase the toughness of the material. Such increase can be achieved by the use of small constituents.

Finally, the processing route still includes too many steps to make it viable beyond the lab scale.

Here we demonstrate the use of a relatively simple processing route based on widespread ceramic processing techniques to obtain fully ceramic, layered bulk materials with submicrometer layer spacing that exhibit not only an unprecedented toughness for a ceramic material but also, because it comprises only mineral constituents, a very high strength and stiffness even at high temperature.

Processing strategy

Inspired by structural guidelines derived from the study of nacre, we fabricated dense ceramic materials defined by five structural features, spanning several length scales (Fig. 1A): (i) long range structural order, (ii) closely packed ceramic platelets of dimensions identical to that of nacre, (iii) ceramic bonds (bridges) linking the platelets, (iv) nano-asperities at the surface of platelets, and (v) a secondary phase with lower stiffness ensuring load redistribution, crack deflection, and delamination.

The process we used here is based on ice-templating. However, we did not rely on ice crystals to form the elementary bricks, but instead took advantage of their growth as a driving force for the local self-assembly of anisotropic particles (platelets). Those particles have the same dimensions as nacre platelets (500 nm thickness, 7 µm diameter) and constitute the elementary building blocks of the structure. When a suspension is directionally frozen, the metastable growth of the ice crystals [START_REF] Deville | Metastable and unstable cellular solidification of colloidal suspensions[END_REF] repels and concentrates the particles present in the suspension. The concentration of the particles occurs at a length-scale where self-assembly of platelets can occur [START_REF] Hunger | Platelets self-assemble into porous nacre during freeze casting[END_REF] (Fig. 1A). Alumina nanoparticles (100 nm) incorporated in the initial suspension serve as a source of both inorganic bridges between the platelets and nanoasperities at the surface of platelets, similar to that observed in nacre. Finally, smaller nanoparticles (20 nm) of liquid phase precursors (silica and calcia) are added to aid filling the remaining gaps during the sintering stage. The material is thus composed of 98.5 vol.% alumina, 1.3 vol.% of silica, and 0.2 vol.% of calcia. This simple strategy, where all the constituents are incorporated in the initial suspension and self-organized in one step, allows precise and easy tailoring of the final material composition. Long-range order of the ice crystals is obtained through a freezing under flow method (see Methods). Once the porous samples are obtained, they are simply pressed and sintered by field assisted sintering (FAST).

The materials are 86% porous after the freeze drying step. The pressing step (100MPa) priori to sintering is critical to remove almost all of the macroporosity. This is easily achieved thanks to the long-range order of the macropores induced by the controlled freezing step.

Due to their size and high aspect ratio, the platelets themselves are unlikely to densify by sintering. Instead, the liquid phase formation rearrange the particles through capillary forces, facilitating platelet packing under the applied load by lubricating the contact points, and by filling the pore space between them (Fig. 1B). At the same time, the nanometric alumina particles form strong bridges by sintering to the platelets. The high surface area and surface curvature of nanoparticles allow them to sinter at lower temperatures than the platelets [START_REF] Herring | Effect of Change of Scale on Sintering Phenomena[END_REF] (Fig. 1C). One important consideration is ensuring that the alumina nanoparticles are not completely dissolved in the liquid phase by either limiting the total amount of liquid phase, or by using short processing times like those employed here (enabled by the rapid, pressure assisted densification method). The sample with liquid phase starts shrinking at a higher temperature (Fig. 1C) than the sample without the liquid phase. We selected this glass composition (calcia and silica) for its very high melting temperature (pure silica start melting at around 1600°C). We believe that the precursor particles of the glass phase prevent most of the contacts between alumina platelets and the nanoparticles, and thus delay the densification of the alumina nanoparticles. The glass has to melt first before densification can proceed. In addition, because the liquid phase precursors are well dispersed in the system and in low concentration compared to the alumina platelets and nanoparticles, the local composition throughout the green body is not likely to be the liquid phase composition predicted from the phase diagram -there must be some diffusion and densification before the liquid phase formation. Along with the physical restraint described above, this may contribute to the slower densification kinetics observed.

The entire process thus consists of just three steps: (i) preparation of an aqueous colloidal suspension, containing all the required building blocks and processing additives, (ii) ice templating this suspension, and (iii) pressure assisted sintering step at 1500°C. The materials obtained are referred to as nacre-like alumina.

Nacre vs. nacre-like alumina microstructure

The final microstructure of the nacre-like alumina (Fig. 2) is characterized by a dense packing of platelets presenting long range order. The use of a fast sintering method limits grain growth so that the initial dimensions of the platelets are retained. A bulk material with submicronic layer spacing is thus obtained, a feature of nacre that had proved impossible to replicate so far in bulk materials. Compared to the microstructure of nacre (Fig. 2, A andB), the packing of platelets presents short range order (Fig. 2E), but the long range order is not as perfect. The waviness of the stacking (Fig. 2D) in our nacre-like ceramic, estimated at ± 15° around the main orientation, comes from the organization of the ice crystals. The long range order is nevertheless sufficient to obtain nearly fully dense samples (e.g., >98% of relative density). The relative density calculated by Archimedean method are very similar: platelets and nanoparticles: 98.9%, platelets and liquid phase: 98.4%, and platelets, nanoparticles and liquid phase: 98.8%.

Observations of the interfaces between platelets (Fig. 2, E andF) reveal the presence of a secondary phase mimicking the organic layer in nacre (Fig. 2B). It also reveals some alumina bridges between adjacent platelets and finally nanoasperities analogous to those in nacre (Fig. 2C). Based on the experimental conditions and the ceramic phases in presence, this secondary phase is most likely amorphous. A small fraction of the alumina platelets and nanoparticles is dissolved into this glass phase at the interface, so that the actual final fraction of this phase (estimated to be around 2 to 3 vol.%) is slightly higher than initially incorporated into the suspension (1.5 vol.%). We estimated the fraction of the glass phase based on the amount of glass precursors (initially 1.5 vol.%) and nano particles incorporated (initially 3 vol.%), and the corresponding ternary phase diagram (alumina-silica-calcia). Based on this phase diagram [START_REF] Zaitsev | Thermodynamic properties and phase equilibria in the system CaO-SiO2-Al2O3[END_REF] , we can estimate the composition of the glass phase at 10 wt.% SiO 2 , 65 wt.% CaO, and 25 wt.% Al 2 O 3 , although it is hard to tell whether equilibrium is achieved during the rapid densification.

The microstructure of the synthetic material is thus strikingly similar to that of natural nacre at several length scales, validating the possibility to fabricate bulk, centimeter-sized samples with only a few simple processing steps.

Ductile ceramics?

Single-edge notched beam (SENB) tests were carried out to measure the toughness and the behavior of the materials tested was compared with a reference polycrystalline alumina sample. The resulting stress-strain curves are plotted in Fig. 3A for three model compositions to illustrate the synergistic effects between the structural features introduced: platelets with nano-asperities, platelets with a liquid phase, and platelets with both (the nacre-like alumina structure). As expected, the samples containing the liquid phase only at the platelet interface exhibit a purely linear elastic response until a catastrophic failure occurs, with unstable crack propagation characteristic of brittle materials. The corresponding fracture surface, showing no crack deflection at all, is shown in Fig. 3D. The fracture toughness K Ic (6.1 MPa.m 1/2 ) is nevertheless already significantly higher than a reference alumina (3.5 MPa.m 1/2 ) since platelets are significantly harder to separate than isotropic particles due to their orientations [START_REF] Faber | Crack deflection processes-I[END_REF] .

Platelets with nano-asperities exhibit a very significant toughening stage. Progressive failure occurs by stable crack propagation, a very unusual behavior in pure ceramic materials.

When both reinforcements are combined, leading to the nacre-like alumina structure, we obtain stable crack growth combined with toughening, similar to that of nacre (reported for comparison in Fig. 3B). The nacre-like alumina, free of any ductile component, is clearly stiffer, with flexural modulus of 290 GPa (compared to 40 GPa for nacre). The mechanical response, where more than 50% of the strain to failure is inelastic, is strikingly reminiscent of that observed in ductile metallic or organic materials, a remarkable and unexpected behavior for a material exclusively composed of ceramic constituents. The reinforcement mechanisms (discussed in the next section) are extrinsic, which means that of course no true ductility (plastic deformation without crack propagation) is observed.

The fracture toughness K Ic describes only the resistance to a crack initiation and is significantly higher in this nacre-like structure (6.2 MPa.m 1/2 ) than in usual polycrystalline alumina (3.5 MPa.m 1/2 ). It is nevertheless unable to describe the multiple extrinsic toughening mechanisms that take place during crack propagation and greatly contributes to the dissipation of energy. The crack is deflected by the composite low stiffness interface [START_REF] Kolednik | Bioinspired Design Criteria for Damage-Resistant Materials with Periodically Varying Microstructure[END_REF] (Fig. 3E) and slowed down by various extrinsic toughening mechanisms which result in stable crack growth, similar to natural nacre [START_REF] Barthelat | Toughness amplification in natural composites[END_REF] . This toughening leads to an increase of the fracture resistance as the crack propagates. This behavior, known as an R-curve effect, is characteristic of many natural materials such as nacre [START_REF] Barthelat | Toughness amplification in natural composites[END_REF] , bone [START_REF] Koester | The true toughness of human cortical bone measured with realistically short cracks[END_REF][START_REF] Peterlik | From brittle to ductile fracture of bone[END_REF] , and tooth [START_REF] Koester | The effect of aging on crack-growth resistance and toughening mechanisms in human dentin[END_REF] . In order to measure the R-curve, the indirect crack length is measured by a compliance method (see Methods). Crack extension reported here satisfied a small-scale bridging conditions (see Methods). Analogous approaches have been used previously in similar structures [START_REF] Ritchie | The conflicts between strength and toughness[END_REF][START_REF] Koester | The true toughness of human cortical bone measured with realistically short cracks[END_REF][START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF] where multiple extrinsic toughening mechanisms needed to be described. The R-curves at room temperature and high temperature (600°C) are plotted in figure 3C. By taking into account the local deflection as well as the other dissipation mechanisms with J-integral and using the equivalence in stress intensity factor, the maximum increase of toughness is extremely high, around 22 MPa.m 1/2 . This corresponds to a 350% increase compared to the K Ic toughness (600% increase with respect to the reference alumina). This far exceeds that of nacre and is equivalent to the best brick and mortar polymer/ceramic composites developed previously [START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF] The second stage of crack growth, when the crack length exceeds 400 µm, is unstable.

Nevertheless, different energy dissipating mechanisms are present, eventually leading to the arrest of cracks before catastrophic failure can occur. Notch-free samples are not fully broken after being exposed to their maximum stresses, another evidence of the efficiency of the reinforcement in these materials.

The properties are retained at relatively high temperature (600°C, Fig. 3B), with a stress intensity factor for crack initiation (K Ic ) of 4.7 MPa.m 1/2 and a K JC maximum toughness of 21 MPa.m 1/2 (Fig. 3J).

Toughening mechanisms

We combine with these materials a number of toughening mechanisms, operating at different length scales. By restricting our choice of constituents to ceramic materials to ensure their use at high temperature and maximize the stiffness, we are also eliminating most of the usual toughening mechanisms seen in ductile materials such as dislocation plasticity or shear bands. The extrinsic reinforcement developed here is derived from the hierarchical architecture of the material and the load redistribution resulting from the brick and mortar architecture. Crack deflection (Fig. 3E), multiple cracking, crack bridging, and crack branching (Fig. 3F), and delamination (Fig. 3H) operate at the crack tip, effectively relieving the locally high stresses. The alumina nanoparticles provide both bridges between the platelets and nano-asperities. This leads to efficient energy dissipation by frictional sliding during platelets pull-out and breaking of the bridges. Pores located between adjacent platelets, as seen in Fig. 2F, may also contribute by acting as local crack arresters by a Cook-Gordon mechanism [START_REF] Cook | A Mechanism for the Control of Crack Propagation in All-Brittle Systems[END_REF] . The possibility to achieve toughness through a proper combination of brittle constituents has been recently hypothesized [START_REF] Dimas | Tough and stiff composites with simple building blocks[END_REF] . With the proper topological arrangement and stiffness ratio between two brittle phases, the stress concentration around the crack tip can be reduced thanks to more efficient load redistribution. The deformation is delocalized and strain redistributes throughout the entire structure. These mechanisms are likely to contribute to the level of toughness achieved in these structures. Further work will be necessary to quantify the relative contributions of all the toughening mechanisms identified here.

A high strength, high toughness material

The flexural strengths of our materials are reported in Fig. 3I. Platelets combined with either the liquid phase or the nano-asperities alone have, as expected, a flexural strength equivalent to or lower than the reference alumina. The nacre-like alumina exhibits a flexural strength of 470 MPa at room temperature and 420 MPa at 600°C, a value essentially similar to that of the reference alumina. The presence of pores (defects) in the platelets and nanoparticles material may explain its lower strength. The synergistic combination of both the liquid phase and the nano-asperities is thus strikingly demonstrated by these measurements. The strength is similar in the transverse direction (490 MPa perpendicular to the basal plane of the platelets). This material does not present strength anisotropy and thus any weakness in these two solicitation directions. Because of sample size limitations (maximum thickness around 5mm), the strength in the delamination direction cannot be assessed.

Strength and toughness are generally considered to be mutually exclusive 2 . Because intrinsic toughening mechanisms are linked to plasticity and thus strength, a compromise is always reached in structural materials where either one of the properties is set aside. In similar work [START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF] for instance, the introduction of 20 vol.% of a polymeric phase to increase the toughness resulted in a notable decrease of the flexural strength. Further toughness increases can be achieved through the addition of up to 60 vol.% Al-Si alloy [START_REF] Launey | A novel biomimetic approach to the design of high-performance ceramic-metal composites[END_REF] , but no appreciable gains in strength are obtained. The introduction of a polymer or a metallic phase is thus always done at the expense of strength, stiffness, and temperature range of use. The toughness of the nacre-like alumina is greater than other reported values for alumina [START_REF] Munro | Evaluated Material Properties for a Sintered alpha-Alumina[END_REF] , textured alumina [START_REF] Pavlacka | Fracture Behavior of Layered Alumina Microstructural Composites with Highly Textured Layers[END_REF] , or even for alumina composites [START_REF] Zhan | Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites[END_REF] , composed of either an organic phase [START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF] , a metallic phase [START_REF] Tuan | The preparation of AI2O3/Ni composites by a powder coating technique[END_REF][START_REF] Travitzky | Mechanical properties of composites fabricated by pressureless infiltration technique[END_REF][START_REF] Prielipp | Strength and fracture toughness of aluminum/alumina composites with interpenetrating networks[END_REF][START_REF] Aghajanian | Properties and microstructures of Lanxide Al2O3-Al ceramic composite materials[END_REF][START_REF] Chen | Toughening alumina with both nickel and zirconia inclusions[END_REF] , or carbon reinforcements such as carbon nanotubes [START_REF] Wei | The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite[END_REF][START_REF] Peigney | Carbon nanotubes in novel ceramic matrix nanocomposites[END_REF][START_REF] Yamamoto | A novel structure for carbon nanotube reinforced alumina composites with improved mechanical properties[END_REF] or graphene [START_REF] Liu | Mechanical properties of graphene platelet-reinforced alumina ceramic composites[END_REF] (Fig. 4A). As in nacre, the organization of the microstructure induces the emergence of improved properties without compromise, even at elevated temperature.

When it comes to materials choice, in particular for structural applications, the density is a major consideration. Metallic glasses are some of the toughest engineered materials produced so far but their high density limits their use. Thickness is also a major problem because the necessary cooling rates cannot always be achieved on bulk samples. The unique combination of specific strength (σ f /ρ) and specific toughness (K c /ρ) of our bioinspired ceramic material actually matches that of engineering aluminum and magnesium alloys (Fig. 4B) while exhibiting a higher hardness (16 GPa), stiffness, and operating temperature. Because only ceramic constituents are used, the stiffness, strength and operating temperature of our material are at the usual level of engineering, technical ceramics, highlighting their remarkable potential in structural and energy-related applications.

For now, the dimensions of the samples are limited to a few centimetres in diameter and a few millimetres in thickness. Two steps of the process induce size constraints: freezing and sintering. Although ice-templating has been restricted to the laboratory scale so far, there are no fundamental reasons why the process could not be scaled up. Large scale, low temperature processes are widespread in the industry (frozen food being the most notable example). The sintering process is actually very similar to Hot Pressing, except that an electric field is used to heat rapidly the graphite die. This kind of pressure-assisted equipment, assisted by an electric field, is now routinely used and widespread, pieces up to 80 cm diameter are routinely sintered for a wide range of applications. With a few exceptions such as refractory materials for furnaces, which can have a thickness of half a meter or so, technical ceramics pieces are almost always within this range of dimensions or even smaller. The majority of technical ceramics pieces have dimensions in the millimetre range. We therefore believe that this approach should fulfil the requirements for most structural applications of ceramics, such as molten metal processing and forming or high performances engines.

On a longer term, we believe that the design principles shown here are not restricted to ice templating, but could be adapted to standard colloidal processing routes for specific shapes, sizes, or directionality. We are currently investigating such ideas. The main issue we can expect, if standard colloidal processing routes are to be used, is how to control the orientation of anisotropic particles.

Concluding remarks

The bioinspired, material-independent design presented here is a specific but relevant example of a strong, tough, and stiff material, in great need for structural, transportations, and energy-related applications where catastrophic failure is not an option. The flexibility in the choice of materials permits an independent assessment (and thus tailoring) of the role of each structural feature on the mechanical properties, which is not possible in natural materials.

Beyond structural materials, we expect to see many other benefits of using ice crystals as a driving force for the bulk self-organization of elementary building blocks. We are now focused on extending the concepts demonstrated here to other materials combinations.

Methods summary

Suspensions preparation. A system composed of alumina platelets (7 µm diameter, 500 nm thickness), alumina nanoparticles (diameter around 100 nm), and a silica-calcia liquid phase (diameter around 20 nm) have been used. All the constituents where added in distilled water and then ball-milled for 24 hours, except the alumina platelets that have been added 3 hours before the end of the cycle to avoid any excessive breakage by the milling media.

Freezing under flow. To obtain dense structures by pressing the porous sample obtained after ice templating, further control of the crystal growth is needed to obtain parallel crystals over large dimension. We use a method in which we let the slurry flow on the cooling plate where freezing occurs, leading to a second perpendicular temperature gradient during the freezing. A 1°C/min cooling rate is used for all suspensions.

Field Assisted Sintering (FAST). The equipment used for sintering is a HPD25 device from FCT System GmbH. The linear shrinkage rates were obtained from the movement of the die during the sintering. A constant pressure of 100 MPa was applied during the heat treatment.

An electrical field is applied and mainly used to heat the graphite die by Joule effect. Due to the intensity of the input current, this method provides a high heating rate (around 100°C/min). Whether or not the current goes trough the sample is not completely clarified yet.

Microstructural characterization. SEM pictures were taken on uncoated samples by a

Supra 55 microscope (Zeiss) and a ZEISS NVision40.

Preparation of SENB and bending samples. Beam shaped specimens where cut from the sintered disks and mirror polished. SENB test specimen were first notched with a diamond saw of 200 µm thickness and then the bottom of each notch was sharpened by repeatedly passing a razor blade with diamond paste.

Hardness Measurement:

We used a MicroMet 6030 microdurometer to measure the hardness on mirror polished samples. The load applied was 0.3 kgf.

Determination of crack length. We used an equivalence between compliance and crack length on an SENB test. The crack length measured is thus the projection of the real crack length at the position of the notch. Samples deflection was measured by a linear variable differential transformer (LVDT) with micron accuracy. According to the ASTM criterion [START_REF]Metals-Mechanical testing; Elevated and low-temperature tests[END_REF] , the maximum crack extension is given by Δa max =0.25b (b being the uncracked ligament width), which corresponds in our results to Δa= 0.4 mm. However, toughness measurements can be considered valid until the data becomes geometry dependent due to large scale bridging [START_REF] Launey | Designing highly toughened hybrid composites through natureinspired hierarchical complexity[END_REF] , which here corresponds to Δa= 0.8 mm. The values reported here were thus always obtained within a valid range of crack extension.

Details on J-integral calculation.

To assess the different mechanisms that occurred during the stable crack propagation, a J-integral versus crack extension has been calculated as the sum of elastic and plastic contribution, a method already used to measure the properties of bone [START_REF] Koester | The true toughness of human cortical bone measured with realistically short cracks[END_REF][START_REF] Nalla | Fracture in human cortical bone: local fracture criteria and toughening mechanisms[END_REF] and similar structures [START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF][START_REF] Launey | Designing highly toughened hybrid composites through natureinspired hierarchical complexity[END_REF] .

Full Methods are available in the online version of the paper. showing an interaction between nanoparticles and liquid phase precursors. Improvements in toughness are obtained by the introduction of a ductile phase, such as a metallic phase [START_REF] Launey | A novel biomimetic approach to the design of high-performance ceramic-metal composites[END_REF][START_REF] Tuan | The preparation of AI2O3/Ni composites by a powder coating technique[END_REF][START_REF] Travitzky | Mechanical properties of composites fabricated by pressureless infiltration technique[END_REF][START_REF] Prielipp | Strength and fracture toughness of aluminum/alumina composites with interpenetrating networks[END_REF][START_REF] Aghajanian | Properties and microstructures of Lanxide Al2O3-Al ceramic composite materials[END_REF][START_REF] Chen | Toughening alumina with both nickel and zirconia inclusions[END_REF] , a polymer [START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF][START_REF] Libanori | Mechanics of platelet-reinforced composites assembled using mechanical and magnetic stimuli[END_REF] , or carbon reinforcements such as graphene [START_REF] Liu | Mechanical properties of graphene platelet-reinforced alumina ceramic composites[END_REF] , carbon nanotubes [START_REF] Wei | The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite[END_REF][START_REF] Peigney | Carbon nanotubes in novel ceramic matrix nanocomposites[END_REF][START_REF] Yamamoto | A novel structure for carbon nanotube reinforced alumina composites with improved mechanical properties[END_REF] , or whiskers [START_REF] Becher | Microstructural Design of Toughened Ceramics[END_REF] . Improvements in strength are achieved through a texturation of the grains [START_REF] Pavlacka | Fracture Behavior of Layered Alumina Microstructural Composites with Highly Textured Layers[END_REF] , hot isostatic pressing [START_REF] Rahaman | Ceramics for Prosthetic Hip and Knee Joint Replacement[END_REF] or the addition of strong reinforcements. (B) Ashby diagram of specific strength versus specific toughness for a range of engineering and natural materials [START_REF] Wegst | The mechanical efficiency of natural materials[END_REF] . The nacre-like alumina's have specific strength/toughness properties similar to that of titanium or magnesium metallic alloys. diameter). The heating of the sample is obtained by injecting electrical current through the punches. This leads to a very high heating rate and the presence of the electrical current accelerates the sintering. The FAST device is known to allow sintering ceramics and metals at lower temperature than conventional sintering resulting in finer microstructures due to the applied pressure. As described, a system composed of alumina platelets (7 µm diameter, 500 nm thickness), alumina nanoparticles (diameter around 100 nm), and a silica-calcia liquid phase (diameter around 20 nm) have been used. The molar composition of 75:25 (SiO 2 :CaO) have been chosen because of its high wettability of alumina surface and a relatively high melting point [START_REF] Pavlacka | Fracture Behavior of Layered Alumina Microstructural Composites with Highly Textured Layers[END_REF][START_REF] Bae | Determination of Critical Concentrations of Silica and/or Calcia for Abnormal Grain Growth in Alumina[END_REF][START_REF] Seabaugh | Texture Development by Templated Grain Growth in Liquid Phase Sintered alpha-Alumina[END_REF] , between 1300°C and 1500°C. The densification behavior of different compositions is described in figure 1B. The linear shrinkage rates were obtained from the movement of the die during the sintering. All the relative density of the sample prepared here are above 98% after sintering at 1500°C with 100°C/min heating rate and a constant applied pressure of 100 MPa.

Microstructural characterization

SEM pictures were taken on uncoated samples by a Supra 55 microscope (Zeiss) and a ZEISS NVision40. The pictures were taken at low acceleration voltage (typically 1 kV) to avoid any charging effects.

Preparation of SENB and bending samples

Disk shaped samples with 20 mm diameter were obtained after sintering. Beam shaped specimens with dimensions around 14x2x2 mm 3 where then cut from the sintered disks. The beams for bend testing where mirror polished and beveled to avoid any crack departure from the sides. SENB test specimen were first notched with a diamond saw of 200 µm thickness and then the bottom of each notch was sharpened by repeatedly passing a razor blade with diamond paste (1µm). Using this method, the final notch radiuses were always below 40 µm.

At least 5 specimens were tested for each composition and setup. Mechanical testing were carried out on an INSTRON machine. Values were determined by monotonically loading the specimens to failure at a constant displacement rate of 1 µm.s -1 . The beams deflections were measured by a linear variable differential transformer (LVDT).

Determination of crack length

The indirect method usually used to determine crack length is based on complaisance evolution during cyclic loading. However in our case this method has proven to be unusable here because the repeated cycles applied induce small crack propagation, even at low stress.

We instead used a simple equivalence between complaisance and crack length on an SENB test. The complaisance were calculated thanks to the relation C=u/f, where u and f are the displacement and force at each point after departure of a crack, respectively. Then the crack length was recursively calculated with (1).

𝑎 ! = 𝑎 !!! + !!! !!! ! ! ! !! !!! ! ! (1)
where W is the thickness of the specimen, a and C respectively the crack length and complaisance calculated at the n and n-1 step.

Details on J-integral calculation

To assess the different mechanisms that occurred during the stable crack propagation (crack length< 400 µm), a J-integral versus crack extension has been calculated as the sum of elastic and plastic contribution, a method already used to measure the properties of bone [START_REF] Koester | The true toughness of human cortical bone measured with realistically short cracks[END_REF][START_REF] Nalla | Fracture in human cortical bone: local fracture criteria and toughening mechanisms[END_REF] and similar structures [START_REF] Munch | Tough, bio-inspired hybrid materials[END_REF][START_REF] Launey | Designing highly toughened hybrid composites through natureinspired hierarchical complexity[END_REF] . The elastic contribution J el is based on linear-elastic fracture mechanics (J el =K IC 2 /E'),

The plastic component J pl is calculated with the relation (2):

𝐽 !" = !.!! !" !" (2)
where A pl represents the plastic area under the load-displacement curve, B the specimen lateral dimension and b the uncracked ligament.

A geometric mean of the local stress intensity factor leads to an equivalent stress intensity factor (3).

𝐾 !" = 𝐽 !" + 𝐽 !" 𝐸 (3) with K JC the back calculated stress intensity factor, J el and J pl elastic and plastic contribution of the J-integral and E the young modulus of alumina.

Fig. 1 .

 1 Fig. 1. Design strategy describing the control at multiple scale of structural self-

Fig. 2 .

 2 Fig. 2. Comparison of the microstructure of nacre (A-C) and nacre-like ceramic (D-F).

Fig. 3 .

 3 Fig. 3. Mechanical properties and toughness measurement of nacre-like ceramic and

Fig. 4 :

 4 Fig. 4: Comparison of the relative materials performance. (A) The conflict between

Acknowledgements:

We acknowledge the financial support of the ANRT (Association Nationale Recherche Technologie) and Saint-Gobain through a CIFRE fellowship, convention #808/2010. We are indebted to the Centre Lyonnais de Microscopie (CLYM) for access to the FIB microscope. Acknowledgements are due to Guillaume Bonnefont from MATEIS for his assistance on the sintering equipment, and Catherine Barentin from the ILM for tipping us on the Carbopol to obtain a yield stress suspension.

Supplementary Information is available in the online version of the paper.

Authors Contributions: SD and EM designed the research, FB processed the sample and performed the mechanical testing, FB and SM performed the high temperature mechanical testing, FB and BVdM investigated the structure, all authors analyzed and discussed the results, FB, AS, and SD wrote the paper.

Author Information: Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.

Correspondence and requests for materials should be addressed to S.D.

(sylvain.deville@saint-gobain.com).

Supplementary Materials

Methods

Suspensions preparation

All the constituents where added in distilled water and then ball-milled for 24 hours, except the alumina platelets that have been added 3 hours before the end of the cycle to avoid any excessive breakage by the milling media. A rheology modifier (Carbopol ETD 2691) was added to form a soft gel (yield stress suspension) in order to avoid any settling of the large particles during the freezing and thus ensuring the homogeneity of the sample throughout the specimen.

Freezing under flow

To obtain dense structures by pressing the porous sample obtained after ice templating, further control of the crystal growth is needed to obtain parallel crystals over large dimension. Indeed, the crystals are vertically aligned in the direction of the temperature gradient, but because nucleation occurs at different spots, different domains of lateral orientation are present [START_REF] Deville | In Situ X-Ray Radiography and Tomography Observations of the Solidification of Aqueous Alumina Particles Suspensions. Part II: Steady State[END_REF] . To induce long range order of the crystals, different strategies have been developed [START_REF] Münch | Architectural Control of Freeze-Cast Ceramics Through Additives and Templating[END_REF] . Here we use a method in which we let the slurry flow on the cooling plate where freezing occurs, leading to a second perpendicular temperature gradient during the freezing. The crystals thus guided in two directions are all parallel, and we repeatedly obtain sample of relatively large dimensions (a few centimeters) with aligned crystals. A 1°C/min cooling rate is used for all suspensions.

Field Assisted Sintering (FAST)

The equipment used for sintering is a HPD25 device from FCT System GmbH. The sample is compressed between two graphite punches inside a cylindrical graphite die (20 mm


Christian Guizard

Jérôme Leloup

Sylvain Deville
email: sylvain.deville@saint-gobain.com

Crystal-Templating With Mutually Miscible Solvents: A Simple Path To Hierarchical Porosity

Ice templating, a route where ice crystals are used to template macroporosity, has been used to process a variety of materials with one level of macropores. We demonstrate here a variant of ice templating based on the solidification of mutually miscible solvents. The solidification of different phases, each defined by their own size and morphology, provides a simple one-step processing route for materials with hierarchical porosity defined by up to three levels of macroporosity. These concepts are demonstrated with both ceramic (yttria-stabilized zirconia) and polymer (polyvinyl alcohol) materials.

Introduction

Macroporous materials comprising pores at multiple scales are required for a number of applications involving mass transfer (filtration), heat transfer, or surface reactions (catalysis). Many approaches have been developed to obtain ceramic materials with a hierarchical porosity 1 , such as extrusion [START_REF] Yang | Fine ceramic lattices prepared by extrusion freeforming[END_REF] or ice templating [START_REF] Mukai | Formation of monolithic silica gel microhoneycombs (SMHs) using pseudosteady state growth of microstructural ice crystals[END_REF] . They typically involve the presence of a macro-or mesopore former that is removed by a heat treatment. Such structures are usually defined by one level of macroporosity and mesoor microporosity. Ice templating is a well-established materials processing route suitable for making macroporous materials with a directional porosity [START_REF] Deville | Ice-templating, freeze casting: Beyond materials processing[END_REF] . When a solvent other than water is used, the more generic term "crystal-templating" might be used. The formation of the porosity is based on the templating effects of the solvent crystals, which thus define macroporosity on a size scale defined by the crystal size. The addition of organic pore formers [START_REF] Kim | Honeycomb Monolith-Structured Silica with Highly Ordered, Three-Dimensionally Interconnected Macroporous Walls[END_REF][START_REF] Zuo | Pore-forming agent induced microstructure evolution of freeze casted hydroxyapatite[END_REF] can add an additional level of macroporosity, but a careful heat treatment step is still required to eliminate the large quantity of pore formers.

Here we demonstrate a variant of crystal-templating based on the solidification of mutually miscible solvents that is able to create macropores at several scales without the assistance of organic pore formers. The solidification of different phases, each of them defined by their own size and morphology, provide a simple one-step processing route for materials with hier-archical porosity. We demonstrate the approach in the tert-butyl-alcohol (TBA)/water system using either a ceramic (8mol.% yttria-stabilized zirconia) or a polymer (polyvinyl alcohol). TBA is a solvent extensively investigated and used in the pharmaceuticals industry [START_REF] Vessot | A Review on Freeze Drying of Drugs with tert -Butanol (TBA) + Water Systems: Characteristics, Advantages, Drawbacks[END_REF] and its phase diagram with water (figure 1) is therefore well-known. In addition, its phase equilibria as a function of temperature and pressure are compatible with the usual ice templating conditions and equipment. The use of TBA in ice templating has already been largely documented [START_REF] Chen | Ceramics with Special Porous Structures Fabricated by Freeze-Gelcasting: Using tert-Butyl Alcohol as a Template[END_REF][START_REF] Hu | Control of pore channel size during freeze casting of porous YSZ ceramics with unidirectionally aligned channels using different freezing temperatures[END_REF][START_REF] Yang | Porous mullite composite with controlled pore structure processed using a freeze casting of TBA-based coal fly ash slurries[END_REF] , although always in its pure form.

Materials and Methods

The following products were used: distilled water, TBA (Sigma-Aldrich), 8mol.% yttria-stabilized zirconia (TZ8Y, Tosoh, Japan), dispersants (Prox B03, a polyacrylate ammonium salt from Synthron, Levallois-Paris, France), and a binder (polyethylene glycol PEG6M sold by Merck or polyvinyl-butyral (PVB) from Sigma-Aldrich) that is required to hold particles together during the freeze-drying step and avoid collapse of the porous structure. The macroporous PVA samples were obtained using a PVA (PVA LL6036) from Wacker (Burghausen, Germany). The ceramic suspensions are prepared by mixing the ceramic powder, the dispersant, the binder and the mix of solvents in predefined ratios. For TBA content up to 40wt%, ProxB03 was used as a dispersant and PEG6M as a binder. For TBA content above 70wt%, no dispersant was used and PVB was used as a binder, since it easily dissolves in concentrated TBA suspensions. The suspension was then ball-milled for 10 hours. The PVA solution was obtained by dissolving the PVA in water under magnetic stirring for 24 hrs. The suspension or solution were poured into a PTFE mold and cooled from the bottom, using a liquid-nitrogen cooled copper rod. The cooling rates were adjusted through a thermocouple and a heater. Details of the experimental setup can be found in previous papers 11 .

This setup provides a fine control of the ice growth velocity, along the temperature gradient direction. Once freezing is completed, the samples were freeze-dried for at least 48 hrs. in a commercial freeze-dryer (FREE Zone 2.5Plus, Labconco, Kansas City, Missouri, USA) to ensure a complete removal of the ice and TBA (or water/TBA hydrates) crystals. Ceramic samples were densified by a high temperature sintering treatment, with a temperature rise at a rate of 180°C/h up to 500°C, holding a 500°C for 1 hr, up to 1350°C at 300°C/hr, steady stage of 3 hours at 1350°C, temperature decrease at a rate of 350°C/h to room temperature. Samples were cut, perpendicular to the direction of the temperature gradient, using a low-speed diamond saw, and cleaned in an ultrasound bath to remove the debris. SEM observations were performed using either a TM1000 from Hitachi. Pore diameters and volume were measured by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics) with applied pressures up to 200 MPa.

Results and discussion

Structures with a hierarchical porosity are obtained when the solidification path goes through several solidification events at different temperatures. The resulting porous structures are then defined at multiple scales (figure 2). The temperatures of the solidification events are all different, crystallization of the various phases will thus occur at different moments of the process, ensuring a progressive organization towards the final structure. The basic requirement for the solvents is that their solubility limit for particles is very low (which is the case for ice [START_REF] Elliott | Particle Trapping and Banding in Rapid Colloidal Solidification[END_REF] ), so that pure phases are grown and effectively reject all the particles in the inter-crystal space. The pores characteristics were assessed by mercury porosimetry (figure 2C). Three distinct peaks, related to the three types of pores, are clearly observed in the pore size distribution. The solidification being directional, the tortuosity of the macropores is minimal. The resulting structures have thus a hierarchical porosity with three types of macropores of different morphologies and dimensions. Finer macropores (0.5 µm) are formed by the arrangement of concentrated particles at the end of freezing. Since breakthrough of the interface in the interparticle space occurs before maximum particle packing [START_REF] Deville | Influence of surface tension, osmotic pressure and pores morphology on the densification of ice-templated ceramics[END_REF] , we can expect the resulting pores to be larger than expected from the maximum packing.

The porosimetry results indicate good connectivity between the various pore size scales.

The structures obtained can be rationalized by considering the phase diagram (Figure 1) and the expected sequence of solidification events, and assuming that we are constantly close to the equilibrium during solidification. If the conditions are suitable (path b in Figure 1), ice crystals form first (large pores observed in Fig. 2A andB). As solidification proceeds, the TBA content in the liquid solution increases, until it reaches the eutectic point. A eutectic structure then forms, in the intercrystal space (smaller macropores observed in Fig. 2 A andB). The expected proportion of the relative crystals can be estimated using the lever rule. The lamellar spacing in eutectic structures is typically 1/10 th of that of dendrites grown under the same conditions [START_REF] Kurz | Fundamentals of Solidification[END_REF] , which seems consistent with the current observations. An equivalent behavior can be obtained in the TBA-rich side of the phase diagram, for instance with a 5wt% water/95wt% TBA zirconia suspension. A hierarchical structure with two populations of macropores is obtained (Figure 2D). The largest macropores present a facetted morphology typical of TBA-templated materials [START_REF] Li | Preparation of porous alumina ceramic with ultra-high porosity and long straight pores by freeze casting[END_REF] . The eutectic A in the water/TBA system (trajectory c in Figure 1) yields lamellar pores with a short spacing (1-3 µm, figure 3A), a range that has been difficult to achieve thus far in the other ice templating systems [START_REF] Waschkies | Investigation of structure formation during freeze-casting from very slow to very fast solidification velocities[END_REF] .

The pore size and morphology are thus consistent with that observed in Figure 2. If the TBA concentration is too low (4wt% of TBA, trajectory a in Figure 1), the fraction of particle in the liquid phase rejected in the intercrystal space is too high for any templating effect to occur. The concentration of particles is so high that they cannot rearrange when the eutectic is forming. The resulting structure comprises thus just two populations of macropores (Figure 3B), created respectively by the large ice crystals and the packing of particles. The final materials comprise polycrystalline platelets of large dimensions (10-20 µm in thickness, and hundreds of microns in length and width).

A variety of polymers have successfully been ice templated previously, including PVA 17 , cellulose 18 , hydrogels [START_REF] Zhu | Anisotropic tough poly(2-hydroxyethyl methacrylate) hydrogels fabricated by directional freezing redox polymerization[END_REF] , or composites 20 . The concepts demonstrated here are generic, and we successfully prepared a macroporous polyvinyl alcohol (PVA) material exhibiting two levels of macroporosity (Figure 4). Large macropores (100-200 µm wide, hundreds of microns long) are separated by macroporous walls defined comprising smaller macropores (5-30 µm wide, 20-100 µm long). Both populations of macropores are continuous along the freezing direction.

Conclusions

Crystal-templating based on mutually miscible solvents is thus a versatile approach to obtain porous ceramic or polymer materials with a variety of pore morphologies, including hierarchical porosity with up to three levels of macroporosity. The principles demonstrated here are generic and can easily be extended to other systems with appropriate phase diagrams. The temperature and phase separation kinetics must nevertheless be compatible with the usual ice templating temperature and pressure conditions. H. Zhang, I. Hussain, M. Brust, M.F. Butler, S.P. Rannard, and A.I. Cooper, "Aligned two-and three-dimensional structures by directional freezing of polymers and nanoparticles.," Nat. Mater., 4 [10] 787-93 (2005).

FigureFigure 1 .

 1 Figure

Figure 2 .

 2 Figure 2. TZ8Y porous structure with hierarchical porosity obtained from either 84wt% water/16wt% TBA (A-B) or 5wt% water/95wt% TBA (D). SEM micrographs parallel (A) or perpendicular (B, D) to the freezing direction, and pore size distribution (C) from mercury porosimetry intrusion for the 84wt% water/16wt% TBA sample. Cooling rates: 2°C/min. The crosssections shown in A and D were obtained after infiltration with an epoxy resin and polishing. Scale bars: A, B: 500μm, C: 150μm.

Figure 3 .

 3 Figure 3. SEM micrographs of the different types of porous, non-hierarchical TZ8Y structures obtained. Cross-section perpendicular to the freezing direction. (A) 80wt% water/20wt% TBA (close to eutectic composition) (B) 95wt% water/5wt% TBA . Scale bars: A: 30μm, B: 500μm.

Figure 4 .

 4 Figure 4. PVA porous structure with two levels of porosity (84wt% water/16wt% TBA). SEM micrographs perpendicular to the freezing direction. (B) Close-up view showing the finer macropores. Cooling rate: 2°C/min. Scale bars: A: 1mm, B:

H.E. Romeo, C.E. Hoppe, M.A. López-Quintela, R.J.J. Williams, Y. Minaberry, and M. Jobbágy, "Directional freezing of liquid crystalline systems: from silver nanowire/PVA aqueous dispersions to highly ordered and electrically conductive macroporous scaffolds," J. Mater. Chem., 22 [18] 9195 (2012).


Sylvain Deville
email: sylvain.deville@saint-gobain.com

Adam J Stevenson

Mapping ceramics research and its evolution

We show here how a simple data mining of bibliographic records can be used to follow and help understand the evolution of a research domain, at a level that cannot be captured by reading individual papers in a field of this size. We illustrate the approach by investigating 43 years of research on ceramic materials, covered by 253k bibliographic records. The patterns of keywords used reveal the trends and the evolution of research ideas and priorities within the field. Simple, interactive tools based on co-word network analysis help us better appreciate the organization and relationships of ideas or individuals, and hopefully allow identification of unexplored concepts, connections, or approaches on a given topic.

Introduction

We have many tools that can be used to try to understand and follow both the history of and developments in our field. To date, scientists have relied primarily on the advice and guidance of mentors, textbooks, targeted searches of bibliographic databases and literature, attending conferences and workshops, and following a few targeted periodicals. However, with the ever-increasing array of sub-fields, conferences, published articles [START_REF] Larsen | The rate of growth in scientific publication and the decline in coverage provided by Science Citation Index[END_REF] , and journals, it is increasingly difficult to keep tabs on even a small area of current research [START_REF] Rödel | Development of a roadmap for advanced ceramics: 2010-2025[END_REF] , let alone the historical developments that shaped the current environment. To fill this gap, a variety of new technologies, many social in nature, have been developed which merge the traditional discovery methods listed above with the social web: peer to peer interactions on the internet that aid discovery of new research and curation of information sources [START_REF] Bollen | Clickstream data yields high-resolution maps of science[END_REF] .

Along with the new social web technologies, other tools have emerged for obtaining, parsing, and understanding large text based data sets [START_REF] Michel | Quantitative analysis of culture using millions of digitized books[END_REF] , and these techniques have been recently applied to understanding -on a coarse grained level -the development of scientific literature within various fields [START_REF] Ding | Bibliometric cartography of information retrieval research by using co-word analysis[END_REF][START_REF] Boyack | Domain visualization using VxInsight for science and technology management[END_REF][START_REF] Mane | Mapping topics and topic bursts in PNAS[END_REF][START_REF] Skupin | The world of geography: visualizing a knowledge domain with cartographic means[END_REF][START_REF] Reid | Mapping the contemporary terrorism research domain[END_REF] .

Here we use simple data mining to analyze the field of ceramics research, in a way that cannot be captured by reading individual papers, considering the large size of the field. We analyzed 253k bibliometric records spanning 43 years of ceramic research. This exploration allows us to identify the publishing habits within the field of ceramic research, the rise and fall of specific ceramic materials, topics, and techniques, and explore the relationships between academic research and technological changes. At a lower level, we illustrate how co-word and co-author network analysis can be a powerful tool for individual researchers to understand trends within their fields, identify potential collaborators, and help target their research.

Rewinding the history of ceramic science

With bibliometric analysis, we can track changes to the entire field of ceramic research b . Here we illustrate several aspects of research: snapshots that capture the current research interest within a domain, and the evolution of specific ceramic materials, properties, characterization techniques, and applications revealed through the patterns of keywords usage.

Snapshots of current interests

A snapshot of the current interests of an entire field of research can be captured, for instance, using word clouds of keywords. In this representation, the size of each word is related to its statistical occurrence (in titles and abstracts) during the considered period. Although such analysis is very coarse, it can be used to reveal the dominant ideas, concepts, methods, or materials of interest at a given moment. In figure 1, examples are given for three different years: 1989 (6.1k records), 2000 (7k records), and 2011 (13.7k records). In 1989, the world of ceramic research was focused on exploring the superconductivity of ceramic materials. Three years before, Bednorz and Mueller revealed 10 that certain ABO 3 ceramic perovskite exhibit an unusually high critical temperature, and were later awarded b We must be careful as the meaning of words may change and be plural. Words can fall in three categories: theoretical, methodological, and observational (Leydesdorff1997). This is potentially problematic if we are to look at co-words analysis (i.e., which words are commonly used together). Bearing in mind these limitations, we believe we can still use the bibliometric record to explore the evolution of the field and its characteristics. Here, we simply investigate the occurrences of words. In a technical field like ceramic research, this simpler analysis is less susceptible to words having several meanings. with the Nobel prize for this discovery c . This discovery had potentially spectacular consequences, since having superconducting materials that operate at liquid nitrogen temperature is much more cost-effective than liquid helium temperature. The dominating keywords (film, phase, surface, magnetic, high-tc, layer, oxygen) are all related to the work on superconductivity. The impact of the work on superconductivity over the entire field of ceramic research can be observed at additional levels. The attention shifted for a while from the usual interests (as described below), but several other indicators reveal a change of pattern: the length of papers decreased (figure S2), as most papers were short communications reflecting a highly competitive and fast moving topic (the average length of superconductivity papers in 1988 is 3.5 pages, versus 5 pages for the average length of all papers), the average number of authors on a paper increased (figure S3) (as ceramists had to team up with physicists), and a spike in the number of papers (figure S1) and journals (figure S4) in which papers were published is observed. New journals were created to report specifically on this topic, such as Superconductor Science and Technology (est. 1988) or the Journal of Superconductivity and Magnetism (est. 1988) In 2000, the situation was more balanced, but ceramic composites attract most of the attention. Finally, the 2011 word cloud is fairly homogeneous, reflecting the variety and richness of interests in ceramic research. Some concepts associated with ceramic materials still dominate (sinter, phase, properties, microstructure, composite), but a wider variety of keywords is observed: properties (dielectric, electric, mechanical), materials (oxide, alumina, zirconia, carbon), techniques (analysis, x-ray, thermal, diffraction), applications (bone, coating, implant), etc. This increase of diversity can also be seen as a consequence of the ever-increasing number of records every year (see fig S1) and the increase of multidisciplinary studies.

Specific Interest for ceramic materials

The corpus of records was obtained by searching for records with the keyword "ceramic". The field of ceramic research is, at least for the past 40 years, dominated by work on technical (as opposed to traditional) ceramics: the process, structure, properties, and applications of such materials. A statistical analysis (figure 2) of the patterns of keywords usage reveals the specific interest for the various ceramic materials, and captures their rise and fall.

Plots shown in figures 2 to 6 comprise two parts. The upper part shows the evolution of the relative occurrences of a few selected keywords. The occurrences have been normalized to account for the increasing number of records every year (see Methods section). The lower part reveals the relative evolution of occurrences for an individual keyword, normalized by the maximum number of occurrences for this specific keyword. c The paper has now almost 13 000 citations as of today, and is probably one of the most cited papers in ceramic science.

Figure 2: Evolution the relative interest for various ceramic materials. Different fates can be observed: a gradual decrease of interest (quartz), a constant interest (alumina), a sudden burst (YBaCuO) which quickly disappears, a rising interest (titania), or the apparition of a new material (KNN).

Ceramists will not be surprised that alumina is the ceramic material attracting most of the attention as it is both an industrially important structural material and a model material for fundamental studies. We can see how the past forty years have seen the attention shifting from structural and traditional to technical ceramics. Titania, for instance, which was investigated 40 years ago, has slowly and steadily become one of the most investigated ceramic materials. Quartz, on the other hand, was the second most investigated ceramic material in 1970 and is now the object of much less attention.

The appearance of novel materials can also be tracked, such as Y 2 Ba 3 CuO 7-d , KNN, or BiFeO 3 , each of them appearing for different reasons, and having different fates. Y 2 Ba 3 CuO 7- d was suddenly of interest when its superconductivity properties at liquid nitrogen temperatures were discovered [START_REF] Wu | Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure[END_REF] . The promises offered by high temperature superconducting ceramics, considered as a disruptive technology for electricity transmission, triggered a frenzy that lasted a few years, but faded quite rapidly when other physical limitations (critical current densities, internal heating, etc.) were shown to be as important to application as the critical temperature itself. The rise of KNN, a lead-free ceramic material, can be related to the international regulations on lead-free materials (in particular in Europe), for health and environmental reasons, which triggered work to find alternatives to lead [START_REF] Saito | Lead-free piezoceramics[END_REF] , in particular in piezoelectric materials.

Properties

The evolution of specific properties can also be tracked (figure 3). Some historical properties of ceramics, such as mechanical strength, transparency, or piezoelectric properties have attracted a constant interest during the investigated period. In some cases, novel materials or concepts renewed previous interest (transparency). The properties related to nuclear applications have seen a spike of interest in the 80s (possibly as a consequence of the Three Mile Island nuclear accident in 1979) before falling to their lowest level today. The rising interest for dielectric applications can be linked to the rise of computers and more generally electronics and consumer electronics, where ceramic materials play a critical role. Superconductivity, as discussed previously, shows a unique pattern: an extremely rapid rise, until it became, by a large margin, the most investigated property for a few years, followed by a very rapid fall. Its influence is reflected in the sudden decrease observed, for instance, in mechanical and piezoelectric papers.

The evolution of techniques and tools

Characterization techniques have always been and still are a key component of materials science and of ceramic science in particular. The statistical analysis of the corresponding keywords reveals the predominance of imaging techniques, such as SEM, TEM, or microscopy (figure 4). Since microstructural characterization is crucial to understanding both processing and properties, it is no surprise to see the predominance of these techniques. Their evolution also indicates different fates, depending on whether a useful technique becomes affordable, widely applicable, and easy enough for non-specialists to perform and analyze. Scanning electron microscopy (SEM) for instance is now routinely used in every materials science lab, and the price of a basic SEM is constantly decreasing. The use of SEM is therefore steadily increasing. Transmission electron microscopy is much less accessible, both from a technical (sample preparation, operation) and financial point of view. After its initial development where its use rapidly increased, its use has been fairly constant over the past 20 years. Alternatively, techniques that were previously standard are slowly becoming less and less relevant, such as optical microscopy, as other techniques are developed and become more accessible. Finally, the birth of techniques can also be tracked, such as the introduction of atomic force microscopy, focused ion beam microscopy, or confocal microscopy, although such a bibliometric analysis is not required to do it.

Industry and applications: driving forces for research?

Application-related keywords (figure 5), can also reveal the driving forces for the work on ceramic materials. Historical applications of ceramic materials (bearing, resistor, turbine) have undergone a steady decline while other applications such as lasers, membranes, or solid oxide fuel cells increase in importance. Interestingly, ceramic materials were investigated for batteries applications before the interest faded. The last 15 years or so have seen a new rise of interest for this topic, due to both progress in ceramic materials and increasing energy considerations.

Industrialization may also have an impact on the number of publications in a specific area. For example, the number of records concerning turbine blades has decreased sharply since 1980, corresponding to the development (prior to 1980) and industrialization (post-1980) of thermal barrier coatings for turbine blades. This could be attributed to companies internalizing research in this area as it is industrialized, and these companies are less likely to publish their data than academic researchers that contributed heavily to the early development. Interestingly, we see some signs of a similar, but more recent, trend in solid oxide fuel cells. Beginning in 2000, there is a sharp increase in activity which stabilizes as the technology begins to mature from 2005-2010. In 2010, the publication activity begins to decrease. A possible explanation is that the technology has begun to be industrialized and is rapidly becoming commercially available. Thus, much of the research and development activity is moving inside of companies that are less likely to publish their results, following the pattern displayed by turbine blades. Observing the trend for superconductivity in Figure 3, we also see a sharp rise and subsequent fall in activity, but this trend cannot be related to commercial success. Instead, the sharp decline in activity surrounding high temperature superconductivity is most likely related to the fact that researchers quickly discovered several major and insurmountable challenges that prevented commercial adoption of the technology.

These three examples: turbine blades, solid oxide fuel cells (SOFC), and high temperature superconductivity are important for understanding how to use this historical literature data to target our research. By placing the research trends in context, we can better understand how or whether to enter certain fields. For example, to begin work with turbine blades, a field that has seen a substantial decrease in activity but has yielded many commercial products, a researcher should target industrial partners that have internalized much of the research and progress in this area over the last 30 years. For superconductivity, which follows a similar pattern but without commercial success, any new entrants into this field must clearly identify and have solutions to the fundamental problems that halted research in this area in the early 90s. With these two examples in mind, a researcher looking at these trends and with a desire to work with SOFCs must carefully consider whether the recent decline in SOFC publishing activity is a statistical oddity, a result of increasing commercialization that will necessitate industrial partnerships to develop research programs in this area, or whether it is a result of fundamental physical limits that must be solved for the technology to be developed further.

Figure 5: Evolution the relative interest for application-related keywords

Writing style and the positive bias

It is an unfortunate habit to report only positive results d , which are supposedly more interesting than failures. Scientific writing is also supposed to be objective and unbiased. This intuitively strong bias towards positive results [START_REF] Fanelli | Positive" results increase down the Hierarchy of the Sciences[END_REF] is revealed here by the statistical analysis of the words used in the titles and abstracts of the papers (figure 6). Keywords with a positive connotation, such as enhance, improve, innovative, remarkable, significant, successful, or promising have been increasingly used over the past forty years. Trying to attract attention, funding, and fame, the use of hype keywords is apparently ever increasing (although most journal editors do not encourage it), with no sign of stopping or slowing down anytime soon, although, in a interesting twist, new (new results, new materials, new mechanisms, new processes) show a relatively steady decrease.

Networks: linking concepts, understanding the organization of research

Another interesting use of data mining bibliographic records is the co-word network analysis. Two examples are described here: co-word (keywords) and co-authors (individuals) analysis, to illustrate how we can take advantage of such a tool.

The individuals' network was built from the author list on each record. Relationships (links) are established between co-authors of the same paper. No preferences were given to the first and to the corresponding author in this analysis, so that each author has the same weight on the network. The network obtained (figure 7) shows clusters of individuals. The size of each bubble is given by its statistical importance: individuals appearing more often in records appear as a bubble of a greater size. Bubbles of the same color belong to the same network. Such a network is a very convenient way to identify the key players in a specific domain, and reveal different organizational patterns depending on the domain. A corpus of records was obtained by running a search with two different keywords: transparent ceramic (4.4k records), and solar cells (98k records).

Several observations can be made on the characteristics and topology of the network. First, individual groups can easily be observed on the network, with two types of organizations: collaborative groups, where all the bubbles of a given group have the same size, and groups with a clear leader, having a much larger size than its co-authors. Although not shown on the figure, the name of each person is attached to each bubble, so that the most active individuals in a given domain can be rapidly identified.

These networks also reveal different habits depending on the domain of interest. For transparent ceramics, very few links exist between the different groups. Work within this field can be divided up into materials fabrication, optical properties measurements, and other (mechanical, etc.) properties measurements. Since many research organizations have groups that can fulfill these tasks, many small collaborative groups appear. Once a functional group is established, this group can work on many different materials and applications with little need to move outside the network. The network on solar cells shows a very different topology, with a much greater number of links between the various groups. This is possibly due to the complexity required to process, characterize, and analyze the performances of solar cells on multiple different levels from materials, to cells, to modules. A single group or even a single research center may be less likely to have all the required expertise. In particular, standard efficiency testing for solar cells can be performed in relatively few locations, and these sites act as links between various groups doing materials, cell, and module development. Such analysis is limited by the difficulty to address author's ambiguity and therefore giving correct credits to individuals. This problem is particularly prominent in countries such as China or Korea, where the same family name is shared by a very large number of individuals. Initiatives like ORCID should hopefully provide a solution for this, once (if?) they become widely adopted.

A similar network can be built if we analyze the keywords attached to the records. Links are established between keywords listed in the same paper. We then performed a class analysis: keywords sharing the same relationships belong to the same class, and are represented with the same color. This clustering allows identifying the organization of concepts and ideas within a given field. Similar to the individuals' network discussed previously, the bubble size of each keyword is related to its statistical occurrence. For the sake of clarity, only the keywords with the highest occurrence are shown (figure 8).

Although we can obtain the keyword network for the entire corpus of records investigated here, a printed figure is not the ideal representation for such a complex and detailed network. To illustrate the interest of such a tool, we will discuss the keyword network obtained for records related to transparent ceramics.

The main groups of concepts can be identified by their color: properties (light blue in the lower part: optical, electric, dielectric), materials (green on the right hand side: compound, carbide, sodium, aluminum, yttrium), processing methods (light green in upper part: method, chemical, deposition, sol, gel), densification methods (brown in upper part: sinter, plasma, hot pressing), microstructure (violet in lower part: structure, particle, growth, grain, pore, crack), etc. Such representation can be particularly useful, for instance, to identify the main materials or techniques investigated in a given domain. They may also be used to identify missing links and make new connections.

Conclusions

We illustrated here how a simple data mining can be a powerful tool to map a field of research and explore the relationships between academic research and technological changes. Such analysis can be done in real time, and eventually at a finer scale that shown here. Networks revealing the relationships between concepts can be built to understand the organization of research and possibly identify new, unexplored connections.

We exposed here a simple set of tools: bibliographic records, which are now widely available in machine-readable format, a script, and open-source software to build, represent, and interact with the network. Extensive computing resources are not required: the analysis was ran on a standard laptop. We can make the analysis a lot more complex, in particular if data mining of the full-text of records is available. We believe that such approach is complementary to the usual toolbox used in research. By offering a different look at the records, it may help discovering hidden connections and unexplored approaches. papers, review, book, and letters. Parsing and analysis of the records were made with a python script. We used a stop list to remove common words (the, and, with) and numbers, pronouns, conjugations of common words (have, will, can). Similar words have been merged (singular/plural, comparative and superlative, conjugations of verbs). Specialized nouns and adjectives (e.g. pore, porous and porosity) were equated. We then performed a co-word (words that frequently appear together) and co-authors (author that frequently appear together in a record) analysis.

With 619 records in 1970 and 13.5k records in 2013, there has been a constant increase in the number of bibliographic records over the past 40 years. At the same time, the average length of the records title also increased (figure S2 in SOM). Normalization was therefore performed by the relative volume of data (raw length of records list for the year). A second normalization was performed for individual keyword (subpanels in figures 2 to 6) by the maximum count of occurrences of the individual keyword, to explore their relative evolution and identify rapidly emerging or declining concepts.

The word clouds were obtained using a freely available online tool (http://wordle.net). The networks were built with Gephi (https://gephi.org/), an open source software.

Figure 1 :

 1 Figure 1: Word cloud of most commonly used keywords in 1989, 2000, and 2011. The size of each word is related to its statistical occurrence in titles and abstracts during the given period.

Figure 3 :

 3 Figure 3: Evolution of the relative interest for ceramic materials properties: mechanical, nuclear, permeability, photocatalysis, photoluminescence, piezoelectric, superconductivity, and transparency.

Figure 4 :

 4 Figure 4: Evolution the relative interest for characterization techniques

Figure 6 :

 6 Figure 6: Evolution of the use of keywords with a positive bias.

Figure 7 :

 7 Figure 7: Network of individuals with records on transparent ceramics (left) and solar cells (right).

Figure 8 :

 8 Figure 8: Network of keywords (co-word analysis) related to transparent ceramics records. A wider link indicates a stronger occurrence of the two words in the same record (e.g. grain/size, thin/film, solid/state, rare/earth).

Figure S4 :

 S4 Figure S4: Average number of journals in which records were found every year, since 1970. The 1, 2, 5 and 10 figures correspond to the number of journals which published at least 1, 2, 5 and 10 papers per year on the topic of ceramic research.

d with few exceptions such as the journal of negative results http://www.jnr-eeg.org , in ecology and evolutionary biology.

Acknowledgements

We acknowledge Stuart Cantrill (@stuartcantrill), Alexis Verger (@Alexis_Verger), and Benjamin Abecassis (@b_abk6) for the pre-publication peer-review of the paper through twitter.

Supplementary Materials