Jack D Evans

François-Xavier Coudert
email: fx.coudert@chimie-paristech.fr

Microscopic Mechanism of Chiral Induction in a Metal-Organic Framework

The recently reported process of chiral induction in a metal-organic framework (MOF) by nonchiral guest adsorption, demonstrated on the prototypical MOF-5, may revolutionize the production of MOFs for enantioselective separation and catalysis. Herein, we describe an investigation employing multi-scale molecular simulation to discover the microscopic mechanism of chiral induction and investigate the stability of the resulting framework. Our results explain how the molecular size and chemical nature of N-methyl-2pyrrolidone (NMP) give rise to the chiral transformation in MOF-5, whereas it cannot occur for other guest molecules, such as N,N-dimethylformamide (DMF). Moreover, we show that the guest-free CMOF-5 structure is energetically unstable, with either the achiral conventional structure or a closed pore structure preferred, demonstrating that chirality will not be retained upon activation of CMOF-5. While this limits the usability of chiral induction in MOFs, our study opens new avenues for the use of other guest molecules and provides microscopic insight into this unexpected outcome of guestframework interactions in a soft porous crystal.

Metal-organic frameworks (MOFs) are a class of nanoporous materials constructed in a modular approach by the combination of inorganic nodes and organic links. [START_REF] Furukawa | The Chemistry and Applications of Metal-Organic Frameworks[END_REF] There is fervent research in the production of chiral MOF materials for particular applications in enantioselective separation and catalysis. [START_REF] Hu | Chiral Porous Hybrid Solids for Practical Heterogeneous Asymmetric Hydrogenation of Aromatic Ketones[END_REF][START_REF] Ma | Enantioselective catalysis with homochiral metal-organic frameworks[END_REF] Traditionally, chiral MOFs are produced by using chiral starting materials 4 or as a result of specific framework topologies. [START_REF] Kepert | A Versatile Family of Interconvertible Microporous Chiral Molecular Frameworks: The First Example of Ligand Control of Network Chirality[END_REF] Often, however, these materials are observed as racemic conglomerates. [START_REF] Morris | Induction of chiral porous solids containing only achiral building blocks[END_REF] Recently, Zaworotko and coworkers [START_REF] Zhang | Synthesis of a Chiral Crystal Form of MOF-5, CMOF-5, by Chiral Induction[END_REF] reported the exciting preparation of a chiral polymorph of the prototypical metal-organic framework MOF-5 (also known as IRMOF-1). 8 This was achieved by conducting conventional solvothermal synthesis in the presence of enantiopure proline or, alternatively, post-synthetic modification by immersion in N-methyl-2-pyrrolidone (NMP), an achiral solvent. The framework CMOF-5 crystallizes in the cubic chiral space group P2 1 3 and appears as a distorted structure of the familiar MOF-5, as demonstrated in Figure 1. This report of CMOF-5 suggested that the production and stability of a chirally induced MOF is strongly correlated to the adsorption of guest molecules: although induction of chirality by chiral guest adsorption has been reported before, 9,10 its occurrence upon adsorption of achiral molecules is novel and entirely unexpected. NMP was observed to be crucial for chiral formation; upon subsequent solvent exchange with a variety of organic solvents a crystal-to-crystal transformation to achiral MOF-5 occurs. It is unclear so far how the action of NMP leads to chiral induction and whether an activated sample of CMOF-5 retains a chiral structure after NMP evacuation.

In this investigation, we examine the intrinsic stability of CMOF-5 using quantum chemistry calculations at the density functional theory (DFT) level, which have been thoroughly used to probe the mechanical stability of crystalline materials. [START_REF] Ortiz | Anisotropic Elastic Properties of Flexible Metal-Organic Frameworks: How Soft are Soft Porous Crystals?[END_REF][START_REF] Ortiz | Prediction of flexibility of metal-organic frameworks CAU-13 and NOTT-300 by first principles molecular simulations[END_REF] DFT simulations suggest that chiral induction produces a metastable guest-loaded state which is unlikely to be stable upon guest removal. Furthermore, by combining DFT calculations with Monte Carlo (MC) simulations we probe the mechanism for this guest-induced polymorphism.

We began this study by looking at the behavior of the chiral CMOF-5 structure in the absence of guest molecules. Starting from the ∆-CMOF-5 and Λ-CMOF-5 crystal structures, we removed the NMP solvent molecules and produced an ordered model. We then performed DFT energy minimization, optimizing both atomic coordinates and unit cell parameters. Calculations were performed using the CRYSTAL14 software [START_REF] Dovesi | RYSTAL14 : A program for the ab initio investigation of crystalline solids[END_REF][START_REF] Peintinger | Consistent Gaussian basis sets of triple-zeta valence with polarization quality for solid-state calculations[END_REF] with localized TZVP basis sets and the solid-adapted hybrid exchange-correlation functional PBESOL0 [START_REF] Perdew | Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces[END_REF][START_REF] Adamo | Toward reliable density functional methods without adjustable parameters: The PBE0 model[END_REF] (see Supporting for full details and input files). We observe that although some differences are present in the crystallographic structures, the ∆ and Λ enantiomers of CMOF-5 behave identically. Upon energy minimization, in the absence of NMP, both structures relax back to the achiral MOF-5 structure. This indicates that CMOF-5 is not stable upon guest evacuation but spontaneously reverts back to the parent MOF-5 framework. We measure the energy difference between the two to be 9.33 -11.1 kJ/mol per Zn 4 O unit. This is likely to severely limit the use of chiral CMOF-5 to applications that do not require evacuation of the framework.

In an effort to capture the role of guests in chiral induction, we then placed 24 molecules of N-methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF) in the ∆-CMOF-5 unit cell. Guests were positioned in the asymmetric unit to create an ordered structure that retains the P2 1 3 symmetry of CMOF-5 while avoiding atom-atom overlap. Optimization of both lattice parameters and atomic positions resulted in the converged structures illustrated in Figure 2, with sharp contrast between the two guests. Like evacuated CMOF-5, the structure loaded with DMF relaxes to an achiral MOF-5 structure, Figure 2a,b with cell parameters and geometry similar to guest-free MOF-5 (no twisting of the linker). Conversely, the structure with NMP retained the chirality and geometric characteristics attributed to CMOF-5, Figure 2c,d; namely, a twist in the phenyl plane dihedral angle and adjacent Zn 4 O cluster angle, in good agreement to the reported crystal structure.

The contrasting interaction of DMF and NMP with the framework can be qualitatively understood by observation of the local clustering of guests in the converged structure, Figure 2b,d. DMF is found in close distances to the Zn 4 O cluster, and with a strong adsorption enthalpy of 98.1 kJ/mol (see Table 1). This is in agreement with a recent study which reports this strong binding is key to the formation of cobalt substituted MOF-5 analogues. [START_REF] Brozek | Dynamic DMF Binding in MOF-5 Enables the Formation of Metastable Cobalt-Substituted MOF-5 Analogues[END_REF] NMP, in contrast to DMF, is observed to converge closer to the phenyl ligand since it is unable to achieve close contact to the Zn 4 O cluster because of its size of NMP. Furthermore, NMP is found in close contact (≈ 4 Å) to adjacent NMP molecules, with guest-guest interactions stronger than DMF. This NMP-mediated "bridging" of neighboring aromatic rings creates the internal shearing stress that induces chirality in CMOF-5 by twisting the linkers. Notably, the interaction between NMP and the framework is dispersive in nature. Additionally, we find this "bridging" mechanism to be loading dependent revealed by further optimizations, with 12 NMP molecules per unit cell, converged to give the achiral MOF-5 structure.

While the DFT simulations detailed above give insight into NMP and DMF adsorption in MOF-5, and the reasons behind NMP inducing chirality, they form by their nature a static picture. To obtain a thermodynamic description of guest adsorption at a higher scale we employed classical Monte Carlo (MC) simulation in the NVT ensemble to describe DMF and NMP adsorption. We employed the RASPA classical simulation package, [START_REF] Dubbeldam | RASPA: molecular simulation software for adsorption and diffusion in flexible nanoporous materials[END_REF] and a detailed description of the methodology can be found in the Supporting Information.

In good agreement with the DFT simulations, MC simulations show that DMF and NMP adsorb in different sites in the framework's pore space. The distribution density of the guests averaged over the MC simulation is displayed in Figure 3. DMF adsorbs to the Zn 4 O clusters with a second site at the centre of the cage, in a similar appearance to that described of benzene. [START_REF] Amirjalayer | Molecular Dynamics Simulation of Benzene Diffusion in MOF-5: Importance of Lattice Dynamics[END_REF][START_REF] Amirjalayer | Mechanism of benzene diffusion in MOF-5: A molecular dynamics investigation[END_REF] As with the configuration observed by DFT optimization, we find NMP adjacent to the ligands in well-defined positions. Unlike DMF, the cloud of density probability for NMP shows the guest-mediated benzene-benzene bridging that is responsible at the microscopic level for chiral induction. This visualization is confirmed by calculation of radial distribution functions (RDFs) between the framework and oxygen atom of the guest, plotted in Figure 3c. This shows that average distances between NMP and the metal cluster is ≈ 1 Å larger than for DMF.

This detailed picture of guest adsorption, obtained by multi-scale molecular simulation, shows that it is the specific position of adsorbed NMP guests which is crucial to the formation of CMOF-5. Unsurprisingly, we find DFT optimizations of guest-free CMOF-5 to converge to give the achiral structure of MOF-5. To explore the energy landscape of guest-free CMOF-5 we conducted constrained cell DFT optimizations, beginning from the CMOF-5 structure, and compared these to the symmetry of conventional MOF-5, as plotted in Figure 4. The energy landscape of P2 1 3 (CMOF-5) crystal symmetry is in stark contrast to that of the achiral Fm 3m (MOF-5). Upon fitting third-order Birch-Murnaghan equations of state to the resulting energies [START_REF] Birch | Finite Elastic Strain of Cubic Crystals[END_REF] (see Supporting Information for full details) we find the Fm 3m crystal symmetry gives a bulk modulus of 13.5 GPa, equivalent to that reported for MOF-5 by alternative methods. [START_REF] Ortiz | Anisotropic Elastic Properties of Flexible Metal-Organic Frameworks: How Soft are Soft Porous Crystals?[END_REF][START_REF] Graham | The Effect of High Pressure on MOF-5: Guest-Induced Modification of Pore Size and Content at High Pressure[END_REF] How- ever, the P2 1 3 structure is softer with non Birch-Murnaghan behavior as identified by the poor fit and negative B 0 , the first derivative of bulk modulus with respect to changes of volume. A number of materials, including ZIF-8 [START_REF] Chapman | Pressure-Induced Amorphization and Porosity Modification in a Metal-Organic Framework[END_REF] and MIL-53(Cr) [START_REF] Ma | Guest dependent pressure behavior of the flexible MIL-53(Cr): A computational exploration[END_REF] show similar behavior, attributed to structural transitions with the application of hydrostatic pressure. Examining P2 1 3 framework structures at large deformations by DFT optimizations we discovered a local minimum energy structure corresponding to a previously unrealized closed pore analogue of MOF-5. This structure has -51.6% cell volume, compared to the conventional MOF-5 structure, and is found to exhibit no geometrically accessible surface area or pore volume. Though this previously undiscovered structure is stable, the transition requires NMP adsorption, the application of hydrostatic pressure and subsequent NMP desorption. Notwithstanding, similar features of this transition have been observed for MOF-5 during water adsorption and negative thermal expansion. [START_REF] Greathouse | The Interaction of Water with MOF-5 Simulated by Molecular Dynamics[END_REF][START_REF] Zhou | Origin of the exceptional negative thermal expansion in metal-organic framework-5 Zn 4 O ( 1 , 4 -benzenedicarboxylate ) 3[END_REF] Resulting from the simulations presented here, the mechanism for chiral induction in MOF-5 requires a very specific deformation of cell volume. In the case for CMOF-5, this is achieved by the occluded NMP guest, which owing to its size can not adsorb closely to Zn 4 O cluster. Notably, there is precedence for guest-induced volume shrinkage in MOFs. [START_REF] Coudert | Comment on "Volume shrinkage of a metal-organic framework host induced by the dispersive attraction of guest gas molecules[END_REF][START_REF] Joo | Volume shrinkage of a metal-organic framework host induced by the dispersive attraction of guest gas molecules[END_REF] However, NMP adsorption is a unique case where the framework deformation results in a chiral structure accomplished by more favorable torsional and angle deformation to the Zn 4 O cluster than compression of bond lengths. It is the specific adsorption pattern of NMP, creating symmetrylowering shear stress by bridging neighboring aromatic rings in the organic linker.

In summary, we have explored the recently reported chiral induction of MOF-5 by a multi-scale molecular simulation approach. Though it may have been foreseeable to be general in nature this effect is discovered to result from very specific interactions between NMP and MOF-5. Furthermore, DFT simulations have illustrated the instability of a guest-free CMOF-5 framework and the discovery of a closed pore analogue of MOF-5. To produce a stable guestfree framework by chiral induction, for applications including gas adsorption, an additional modification step is required to stabilize (or tie) the structure in the unstable chiral form. Similar methods have been recently exploited to stabilize a flexible porous organic cage. [START_REF] Liu | Acid-and Base-Stable Porous Organic Cages: Shape Persistence and pH Stability via Post-synthetic "Tying" of a Flexible Amine Cage[END_REF] Nonetheless, the details of chiral induction uncovered here give excellent insight for future experimental studies to discover new examples of this approach to chiral MOF synthesis.

Figure 1 .

 1 Figure 1. Structures of (a) achiral MOF-5 and (b) the ∆ enantiomer of chiral MOF-5 (CMOF-5). Hydrogen atoms have been omitted for clarity. (Zn 4 O, blue; O, red; C, grey)

Figure 2 .

 2 Figure 2. Local minima identified by DFT optimizations for DMF (a, b) and NMP (c, d) loaded structures of CMOF-5. 2 × 2 × 2 unit cells (a, c) illustrate the extent of chiral induction and a closer view (b, d) depicts the distinct local ordering of guest molecules. Additionally, close contacts (d, dotted lines) are depicted and the change in framework, simplified in two-dimensions. Hydrogen atoms have been omitted for clarity.

Figure 3 .

 3 Figure 3. Average density distributions of DMF and NMP in MOF-5 at 298 K, a and b respectively. The surface is shown here for a density value of 0.5 Å -3 . In addition, radial distribution functions (c) of DMF and NMP oxygen atoms and the framework present the different interactions observed.

Figure 4 .

 4 Figure 4. Energy landscape with respect to cell volume, computed by DFT simulation, for different symmetries of MOF-5. Moreover, the relative volume of CMOF-5 reported by single crystal X-ray crystallography (dotted vertical line) and third-order Birch-Murnaghan equations of state are depicted (solid lines).

Table 1 .

 1 Interaction energies of DMF and NMP in CMOF-5 as calculated from DFT optimizations with Grimme's DFT-D2 correction,[START_REF] Grimme | Semiempirical GGA-type density functional constructed with a long-range dispersion correction[END_REF] in units kJ.mol -1 per guest molecule. Energies are calculated for both guest-MOF and guest-guest interactions.

		∆E guest-MOF ∆E guest-guest
	24×DMF	-98.1	-10.1
	12×DMF	-173	7.56
	24×NMP	-48.9	-18.5
	12×NMP	-136	-15.7

Acknowledgement We thank Lydéric Bocquet for stimulating discussions. We acknowledge PSL Research University for funding (project DEFORM, grant ANR-10-IDEX-0001-02), HPC platforms provided by a GENCI grant (x2016087069) and the Chemical Structure Association Trust Grant.

Supporting Information Available

Details of computational methods, comparison of chiral enantiomers, tabulated structural features, interaction energies, Birch-Murnaghan coefficients and additional figures.


Serge Garlatti
email: serge.garlatti@enst-bretagne.fr

Sébastien Iksal
email: sebastien.iksal@enst-bretagne.fr

A Semantic Web Approach for Adaptive Hypermedia

Keywords: Adaptive navigation and presentation, Virtual Document, Composition Engine, Semantic Web

Adaptation/personalization is one of the main issues for web services. Adaptive web applications have the ability to deal with different users' needs for enhancing usability and comprehension and for dealing with large repositories. Indeed, adaptive web services -also often called Adaptive Hypermedia Systems -can provide different kinds of information, different layouts and different navigation tools according to users' needs.

We propose an open-ended adaptive hypermedia environment which is based on the virtual document and semantic web approaches and which is able to manage adaptive techniques at knowledge level. The aim is to simplify the creation and the management of adaptive web services by using ontologies and semantic properties for adaptation. Indeed, they are declarative parameters for computing on the fly services. Indeed, the specification of the adaptive mechanisms is defined by semantic properties associated to a hypermedia document by an author. These properties have the following roles: define how to evaluate the links/content for grouping them together in different classes according to a user model, determine how to manage these classes for each adaptive technique and assign user stereotypes to adaptive techniques. Then, an author can determine the relevant adaptive techniques for a given user group.

Introduction

Nowadays, numerous services are available on the Web, for instance, portals, e-learning, digital libraries, on-line information systems, virtual museums, e-business and digital newspapers are current services. Adaptation/personalization is one of the main issues for web services. Adaptive web services have the ability to deal with different users' needs for enhancing usability and comprehension and for dealing with large repositories. Indeed, adaptive web services -also often called Adaptive Hypermedia Systems -can provide different kinds of resources, different navigation tools and different layouts according to users' needs [14]. The creation and/or maintenance of adaptive web services from these repositories require the following features: i) methods to facilitate web service creation and management and ii) reuse, sharing and exchange of resources through the internet/intranet iii) selection of the relevant resources and their organization according to user's needs.

The new age of Internet is the Internet of meanings and provides some of these features. The Semantic Web is a vision for making the contents of the web understandable to the machines. It would be a basis for creating intelligent web services in the future. Then, this third web generation has the ability to enhance information retrieval, the reuse, sharing and exchange of resources through the internet/intranet and to deal with automatic or semiautomatic services. Indeed, it is well known that keyword-based information access presents severe limitations concerning precision and recall. On the contrary, intelligent search engines, relying on semantic web initiative [14] and semantic metadata, overcome these limitations [14]. Nevertheless, information space is so huge that it is not sufficient to have a precise search engine. It is necessary to take into account user interests and to have an accurate metadata schema to be sure to focus on relevant pieces of information. Adaptive Hypermedia systems can be viewed as automatic or semiautomatic services dealing with different users' needs and distributed resources. This kind of services can generate dynamically a service adapted to the users.

It is not sufficient to rely on the semantic web initiative. Indeed, flexible hypermedia and more particularly that of virtual documents can lead to methods facilitating web service design and maintenance. According to Watters, "A virtual document is a document for which no persistent state exists and for which some or all each instance is generated automatically at run time" [14]. They have the ability to select the relevant resources and their organization according to user's needs. A virtual document, as a service, is composed of an information space -resources -and mechanisms -at least a composition engine -to compute on the fly numerous different real documents from a specification. The service maintenance may be done by changing the specification. Moreover, a composition engine may also be used for creating new services as soon as these services are compatible with the principles underlying the composition engine. These principles are implemented in the composition rules for selecting resources according to a metadata schema and organizing them. For instance, if you have a composition engine able to compute a concert summary of ten minutes, you could reuse this composition engine for a TV show summary. It is possible whether the video segmentation, selection and organization principles are suitable for these shows.

We have designed an open-ended adaptive hypermedia environment which is based on the virtual document and semantic web approaches and which is able to manage selection, organization and adaptation at knowledge level. Virtual document and adaptive hypermedia are closely related -they can be viewed as the two faces of the same coin. At present, we have focused our study on five adaptive navigation techniques (direct guidance, annotation, hiding, sorting and partial hiding) from which adaptive content can be deduced. The specification of the adaptation mechanisms is defined by semantic properties associated to an adaptive document by an author. These properties have the following roles: define how to evaluate the links/content for grouping them together in different classes according to a user model, determine how to manage these classes for each adaptive technique and assign user stereotypes to adaptive techniques. Then, an author can determine the relevant adaptive techniques for a given user group. Indeed, some experiments have shown that it is necessary to provide the relevant adaptive techniques to the current user [14]. For instance, annotation technique is advised for expert and hiding technique for novice.

Adaptive documents rely on some principles which are firstly presented. Secondly, we define the different views of digital document and the corresponding architecture for our adaptive hypermedia environment. Thirdly, the adaptation will be analyzed via our adaptive semantic composition engine. Fourthly, the composition of the delivered document is presented. Finally, we conclude by some perspectives.

Design Principles

In our framework, we consider an adaptive hypermedia as an adaptive virtual document. We define it as follows: an adaptive virtual document consists of a set of information fragments, their corresponding metadata, different ontologies and a composition engine. The latter selects and/or filters the relevant information fragments1 -resources -, organizes and assembles them according to an author/designer specification. Composition and specification are the two stages of an adaptive virtual document. The composition consists of three functions which define the composition rules: the selection which retrieves the relevant set of fragments, the organization which provides an overall document structure and the assembly which defines the layout of the document. The organization determines how to access the relevant set of fragments and can be computed on the fly or specified by an author/designer. We are interested in author-oriented adaptive virtual documents. An author-oriented document has the following characteristics: authors have know-how which enables them to give coherence to a document. This coherence depends on the content and its organization according to user's needs. In such a framework, an author has to specify the content by means of the metadata and one or more organizations for this potential content. He also has to specify the different "rules" for adaptation. We call the outcome of this specification a generic document from which several adapted documents will be generated on the fly.

In a digital document, three different views may coexist: semantic, logical and layout [14]. These views are closely related to the semantic web architecture: i) semantic: logic, ontologies, RDFS/RDF, ii) logical: XML, iii) layout: XSL/XSLT. First of all the three views are presented. Secondly, our adaptive composition engine architecture based on these three views is analyzed.

Different views of a document

The three views have a specific structure organizing them. The semantic structure of a document conveys the organization of the meaning of the document content. As resources are distributed through Internet, there is no content at all in the semantic structure, but only a specification of the potential content. The semantic structure -as an overall document structure -plays the role of a site map in a static hypermedia document or the role of the "knowledge tree" in the approach of P. Brusilovsky [14].

The logical structure reflects the syntactic organization of a document. A document (for example books and magazines) can be broken down into components (chapters and articles). The logical view fits the syntactic level of the semantic web architecture. A logical structure is encoded in XML and represents a web page [14]. The layout view describes how the documents appear on a device and a layout structure describes it, (e.g. size and color of headings, texts, etc). An adaptive composition engine relying on these three views has been designed. In this paper, we focus on the semantic view and the corresponding adaptation mechanisms.

Adaptive Composition Engine

The adaptive composition engine architecture is based on two different studies: ICCARS2 Project and CANDLE [START_REF] Specht | Interaction of domain expertise and interface design in adaptive educational hypermedia in Second Workshop on Adaptive Systems and User Modeling on the World Wide Web[END_REF] Project which is a European project. Our composition engine is divided into three engines: semantic composition, logical composition and layout composition (cf. figure 1). They are sequential processes according to the three views described previously. In a virtual document framework, the three functions are distributed in these processes as follows: selection and organization are achieved in the semantic composition; assembly is divided into logical and layout compositions. The aim of the semantic composition engine is to compute an adapted document from an information space, a user model and a generic document. The latter is defined by an author and is a narrative structure for presenting a particular viewpoint on a set of articles or course elements. It is a directed graph in which nodes have a content specification according to a metadata schema and edges are semantic relationships. These relationships belong to those analyzed by Rhetorical Structure Theory (RST)4 [14]. The adapted document is an instance of the generic document in which content specification is replaced by fragments matching it. In our framework, the information space is reduced to a small subset of fragments to ensure the document coherence. This subset is defined by an author and is associated to the generic document. Nevertheless, it is possible to release this constraint and to provide access to internet/intranet on demand.

Fig. 1. The Composition Engine Architecture

The logical composition engine browses the adapted document according to user interactions and computes for each node an XML web page with content and navigation tools. An XML template and a user model are used to get this XML web page. In navigation tools, links have properties for managing adaptation [14]. The layout composition engine generates an HTML page from the XML web page in applying the layout rules given by the layout template and using an XSLT processor [14]. Adaptation processes can take place in the three engines. But we focus on the first one: the semantic composition engine.

For generating documents, the composition engine requires different types of models: the document model, the user model, the domain model and the metadata model. All these models are represented at a knowledge level by means of ontologies which are always organizedrelated -in the same way. They are separated to simplify their maintenance. As these ontologies are loosely coupled, they can be modified without effort. Consequently, it is easier to maintain adaptive documents and to design new adaptive documents.

The document model formalizes the specification of the generic document and its properties. It represents the author's competences and know-how for creating adaptive document. It defines all the different categories of fragments which may be specified in a generic document (for instance, news in-brief, chronicle, press review, editorial, record, research, poll, news items, etc.), their relationships and the RST relationships. The generic document can be considered as an instance of the document model. It also defines the different adaptive techniques and their properties. The domain model defines all the concepts and their relationships in a particular domain (for instance, for special reports about fishing, the different categories of fish, vessels and their equipments and tools, people, etc. are defined). The user model is composed of personal data (identity, age, town, professional activity, role (reader, author, etc.), history, session,preferences information about interests and preferred adaptive techniques, knowledge level about the domain concepts -it is an overlay model. The metadata schema is composed of six parts : General: identification of the resource : title, authors, etc. ; Lifecycle: information about the management of versioning, number, date, authors, etc.; Meta Metadata: Information about the metadata, creator, validator, language, date, etc.; Rights: Information about use, cost, license, access restrictions, etc.; Technical: Information about format, size, software version, etc.; Classification: Description of the content : domain concepts, necessary knowledge level, etc.. The metadata schema is related to the document model for specifying the type of fragments, the domain model for defining the main concepts of a fragment. For designing a generic document an author as to use a subset of the metadata entries for specifying the potential content. In the ICCARS project, an author chooses the relevant fragments for each node of the generic document and the authoring tool will deduce the corresponding specification in a limited information space to ensure semantic coherence. The metadata used to specify the content are as follows: MD. 6 Potentially, a subset of fragments -a small one -can match the specification. These fragments are called variants of fragments. Some of their other metadata entries differ. They will be used by the adaptation process for evaluating them. Then, another subset of the metadata entries will be reserved for variants of fragments. These two subsets of metadata entries are mutually exclusive and depend on the application.

The semantic and logical composition engines rely on OntoBroker. It is used to browse the different models -ontologies -and to retrieve fragments which match the content specifications by means of the fragment metadata included in [14]. OntoBroker contains four ontologies [14] -one per model -and metadata closely related to them. These ontologies are: a domain ontology for representing contents; a metadata ontology at the information level which describes the indexing structure of fragments; a user ontology which may define different stereotypes and individual features; a document ontology which represents the author's competences and know-how for creating adaptive document [14]. Ontobroker also contains resources' metadata for information retrieval. These ontologies are parameters for the composition engine and their organization and relationships have to be stable across services for ensuring the composition engine reusability.

Adaptation Specification

The main goal of adaptation in a hypermedia document is to provide the relevant navigation tools, information units and organization. The specification of the adaptation mechanisms is defined by semantic properties associated with a generic document by an author. Then, it can be used in various contexts which require different adaptation mechanisms.

Principles

Five adaptive navigation techniques are managed by our system: annotation, direct guidance, hiding, partial hiding and sorting [14]. Adaptive content is deduced from them. The properties specifying the adaptation mechanism have the following roles: define how to evaluate the links/content for grouping them together in different classes, determine how to use these classes in the different adaptive techniques and assign user stereotypes to adaptive techniques. Then, an author can determine the relevant techniques for a given user group. First of all the evaluation method is presented. Secondly, the use of the evaluation method for adaptive navigation techniques is analyzed. Thirdly, the association of stereotypes to adaptive techniques is presented.

Evaluation method

For adaptive navigation techniques, one can evaluate the relevance of the links or the relevance of the destination of links -fragments. In our framework, we propose to evaluate the fragments by means of a uniform evaluation method for all adaptive techniques. Indeed, we propose to define up to five disjoint and totally ordered classes of fragments -at least two classes. They are classified by an evaluation of the destination fragments of the links, that is to say the variants of fragments matching the content specification of each generic document node. These classes are called: very good, good, average, bad, very bad. An author defines the necessary and sufficient conditions for class membership. We have chosen to have up to five classes for user comprehension. Indeed, it could be difficult for a user to deal with too many classes. For instance, several studies have proposed up to five different classes for managing annotation [14], but not more.

The rules for class membership are defined by Boolean expressions composed of unary and binary operators (not, and, or) and comparators

(= | < | > | <= | >= | <> | in) between objects.
These objects can take into account a user stereotype -user model features coming from the user ontology -, a subset of metadata dedicated to variants of fragments -metadata ontologyand a knowledge level about domain concepts -computed from the metadata and the user model -percentage of known concepts or percentage of concepts having a sufficient level of knowledge. A rule is a condition which has to be satisfied by the fragment and the user model to be a member of the corresponding class. Four examples and an explanation:

• Very Good = "(JOB = Fish wholesaler) AND (PCTAGEKNOWN >= 100)"

• Good = "((JOB = Fisherman) AND (PCTAGEKNOWN >= 75)"

• The fragment is a member of the class "Good" if the user is a fisherman and he knows at least 75% of the domain concepts which defined the semantic content of the fragment.

• Average = "(JOB = Fisherman) AND (PCTAGEKNOWN < 50)"

• Bad = "(NOT (JOB = Sea Job))"

In this example, four different classes have been defined which are mutually exclusive and there are always rank in the same way (Very Good > Good > Average > Bad). The first type of conditions -(JOB = Fish wholesaler) -are compared to the current user model and the second type of conditions -(PCTAGEKNOWN < 50) -are used to compare the knowledge property -overlay model -of the current user model and the domain concepts representing the fragments in metadata. Each fragment matches the content specification of the current node of the generic document is classified in one of these classes according to the current user model.

For the knowledge level, two cases are distinguished: the known concept and the concept having a required knowledge level. Indeed, an author may specify a minimal knowledge level to each relevant concept describing the content. The computed elements are:

• PCTAGEKNOWN: percentage of known concepts.

• PCTAGEGOODLEVEL: percentage of concepts having a sufficient level of knowledge.

Evaluation management for adaptive navigation techniques

For managing the different adaptive navigation techniques, it is necessary to define how to manage the different classes of fragments. It is sufficient to decide which classes are kept or suppressed for a given technique. For instance, direct guidance, annotation and sorting are managed as follows: i) direct guidance: the best class is kept and the others are suppressed, ii) annotation: all classes are kept and links will be annotated according to the class relevance, iii) sorting: all classes are kept and links will be ordered according to the class order. For hiding and partial hiding, an author has to specify which classes are kept.

Stereotypes associated to adaptive navigation techniques

A user may have an adaptive navigation technique whether its stereotype matches the user model. All user model features can be used to define a stereotype. A stereotype is defined by Boolean expressions composed of unary and binary operators (not, and, or) and comparators

(= | < | > | <= | >= | <> | in)
between user model features. An example and its meaning:

• ANNOTATION = "((Age > 18) OR (JOB = Student)) AND (Location In Brittany)"

• Annotation is for user being 18 years old or being students in Brittany

The adaptive presentation is deduced from the applied adaptive navigation technique. Indeed, they are applied on links which are evaluated by the destination content, that is to say fragments. According to the current adaptive navigation technique, each evaluated fragment is presented to the user or not. When there is link filtering -hiding, partial hiding and direct guidance -some fragments are not presented because the corresponding links are suppressed. Then, there is a content adaptation -content adaptation -and a modification of the semantic structure of the document. For each generic document, the following semantic features are defined for adaptation management: the number of classes, membership rules for each class and the management of classes for hiding and partial hiding techniques and for each technique, a user stereotype.

Adaptive Composition

The main goal of the semantic composition engine is to compute on the fly an adapted document which is an instanciation of the generic document by selecting content and modifying the semantic structure. The semantic composition engine consists of three processes: the first one selects the relevant fragments for the current node, the second one evaluates the fragments and classifies them in the different classes specified by an author and the third one determines the allowed adaptive navigation techniques and applies them -that is to say kept or not some or all fragment variants.

1. First of all, the content specification is used to query the search engine -Ontobrokerand selects the relevant fragments from the information space associated to the generic document. The outcome of this process is a set of fragment variants. These fragments differ in a subset of the metadata schema, for instance, the knowledge level for some domain concepts, the technical properties (format, size, etc.), etc.

2. Secondly, all fragment variants are evaluated, that is to say each fragment metadata subset dedicated to variants is compared to the different fragment classes. Then, each fragment variant belongs to one class and it has a semantic property called "class member" whose value is "very good" or "good" or "average" or "bad" or "very bad". This property will be used to manage adaptive techniques. 3. Thirdly, the user model is compared to all stereotypes associated with the different adaptive techniques (annotation, hiding, direct guidance, etc.). Those which fit the user model are kept. For instance, whether the current user model only matches the stereotype of the hiding and direct guidance techniques. they are available to the user. According to the enabled adaptive navigation technique, a fragment variant is kept or not in the adapted document. For instance, if a user is allowed to have annotation, hiding and direct guidance techniques, all fragment variants are kept whatever their class. Indeed, it is necessary to keep all fragments to be able to annotate them. On the contrary, if a user is only allowed to have hiding and direct guidance, the fragment variants belonging to the best class are only kept -class "Very Good". The others are deleted because the user is not allowed to use them. Consequently, this kind of deletion leads to content adaptation. Some adaptive navigation techniques, like hiding, partial hiding or direct guidance, allow the removal of the irrelevant fragment variants whether they are the only techniques available. They have direct consequences on the document content and structure and then on content adaptation. Whether some fragment variants are deleted, the document content and structure may be modified.

The logical composition aims at computing the current web page structure -XML -with a content and navigation tools for accessing the rest of the adapted document. The navigation tools are the local and global guides for navigation [14] resulting from the available adaptive navigation techniques. Among the enabled adaptive navigation techniques, the user has to choose by means of his preferences one of them otherwise the default one is chosen. For the adaptive navigation techniques, author constraints have priority over user preferences. An XML web page is generated from an XML template. A template describes the logical structure of a web page but without any content or navigation tools. It contains queries for computing navigation tools and for loading the content. The content is given by the selected fragment in the current node of the adapted document. The navigation tools depend on the selected adaptive navigation technique. For defining the links in navigation tools, the logical composition engine has to browse the adapted document and then "translates" semantic relationships into hyperlinks. Let A, B be nodes and R1 be a relationship from A to B. As soon as B has several variants, the relationship R1 is considered as several relationships of the same type, one per fragment from the source A to each destination fragment in B. It has also to use the "class member" property of fragment variants to manage the current adaptive navigation technique, that is to say it has to decide which links are kept or not.

Conclusion and Perspectives

In this paper, we have presented an open-ended adaptive hypermedia environment which is able to manage selection, organization and adaptation at knowledge level. It is based on the virtual document and semantic web approaches. The composition engine uses four ontologies: document ontology, user ontology, domain ontology and metadata ontology. The first one defines the author know-how -how to select and to organize a document at knowledge leveland the available adaptation techniques and their properties. They are always organizedrelated -in the same way and are separated to simplify their maintenance. As these ontologies are loosely coupled, they can be modified without effort. Consequently, it is easier to maintain adaptive documents and to design new adaptive documents. These ontologies are parameters for the composition engine and their organization and relationships have to be stable across services for ensuring the composition engine reusability. They can also be reused, shared and exchanged through internet. The composition engine has been implemented with the adaptive mechanisms. At present, the authoring tool is only able to deal with metadata tagging, searching of fragments according to the metadata schema and non adaptive generic document. It is possible to define adaptive generic document but without the authoring tool.

Moreover, the specification of the adaptive techniques is defined by semantic properties associated to a generic document by an author. The semantic properties specifying the adaptation mechanism have the following roles: define how to evaluate the fragments for grouping them together in different classes, determine how to use these classes in the different adaptive techniques and assign user stereotypes to adaptive techniques. Then, an author can determine the relevant techniques for a given user group. The management of adaptive techniques is made at an abstract level for the entire document. Consequently, it is easier to generate new adapted documents because it is sufficient to modify these semantic properties.

Then, an author can deal with new adaptive documents and new user behaviors without changing the generic document a lot. Nevertheless, the management of adaptation at document level does not enable an author to change the adaptation mechanism at a very fine level of granularity. For instance, it is not possible to specify for each link or content how to manage adaptation like in AHA.

Our evaluation can be used for adaptive information retrieval. Indeed, a user can specify the adaptation criteria for adaptive information retrieval on the basis of combination of the following features: his interests and knowledge according to two different adaptive techniques. For example, a user can ask the search engine to sort the outcome by interests -up to five subsets -and to annotate each subset by knowledge. This possibility has been applied in the ICCARS project.

Table 1 .

 1 .1.1and MD.6.2.1 in table 1. Some metadata specifying the content

	MD.6	Classification	Domain value
	MD.6.1	Domain	List
	MD.6.1.1	Concept	Unique value Domain ontology
	MD.6.1.2	Level	Unique value User Ontology : knowledge level
	MD.6.2	Application	Unique value
	MD.6.2.1	Type of fragment Unique value Document Ontology: types of
			fragments

Fragments are reusable units. Then, they have their own metadata. Fragments can be atomic or abstract. Atomic fragments are information units and cannot be decomposed. Abstract fragments are composed of atomic fragments and/or abstract fragments and one or more semantic structures.

ICCARS : Integrated and CollaborativeComputer Assisted ReportingSystem (http://iccars.enst-bretagne.fr)

CANDLE : CollaborativeAndNetwork Distributed Learning Environment (http://www.candle.eu.org)

The main goal of the Rhetorical Structure Theory is to give coherence to a document;


est destinée au dépôt

Do non-steroidal anti-inflammatory drugs improve sport performance ? A meta-analysis of randomized trials Introduction

Catherine Cornu, Clémence Grange, Amanda Regalin, Justine Munier, Sonia Objective :

To perform a systematic review and meta-analysis of studies which evaluated the effect of NSAIDs on the athletic performances, i.e. maximum performance, time to exhaustion, and pain.

The intervention included NSAIDs (including aspirin), against any control. The primary outcome was the maximal performance (as defined in each study). The secondary outcomes were the time until self-reported exhaustion and the self-reported pain.

Do non-steroidal anti-inflammatory drugs improve sport performance ? A meta-analysis of randomized trials

Catherine Cornu 1,2 , Clémence Grange 2 , Amanda Regalin 2 , Justine Munier 2 , Sonia Ounissi 2 , Natane Reynaud 2 , Behrouz Kassai-Koupai 1,2 , Pierre Sallet 8 parallel group studies and 5 crossover studies were found, totaling 178 & 75 subjects respectively. 5 parallel group studies and 2 crossover studies tested an acute dosage, and 4 parallel group studies and 3 crossover studies tested a longer period of treatment. The dosages and drugs were very disparate. There was neither significant difference in the maximum performance between groups, nor in the time until exhaustion. Self-perceived pain was significantly lower in the NSAIDs group than in the control group.

Material and methods:

Databases were searched for randomized trials involving healthy males and females, untrained or athletes, from any setting.

Maximal performance: parallel group studies

Discussion / Conclusion:

Our meta-analysis does not allow concluding on the existence or not of an ergogenic effect of NSAIDs on sport performance, since the quality of the studies is low, the doses tested, and the exercises performed are very heterogeneous and far from those observed in real-life practices. However, the health risks for athletes is real and we recommend the conduct of new trials, adequately sized, methodologically sound, using adequate dosage in a real-life setting.

Risk of bias of studies: poor quality studies

  Ounissi, Natane Reynaud, Behrouz Kassai-Koupai, Pierre Sallet, Patrice Nony To cite this version: Catherine Cornu, Clémence Grange, Amanda Regalin, Justine Munier, Sonia Ounissi, et al.. Do non-steroidal anti-inflammatory drugs improve sport performance ? A meta-analysis of randomized trials Introduction. 23e Congrès de la Société Française de Pharmacologie et de Thérapeutique (SFPT 2019), Jun 2019, Lyon, France. hal-02143014

  Drugs (NSAIDs) are commonly used in sports medicine. NSAIDs are over-the-counter drugs and NSAIDs are frequently taken at high doses by athletes with unrestricted access. Severe adverse effects were reported such as acute renal failure, and gastro intestinal disorders. NSAIDs are not considered as performance enhancing drugs and are not included in the World Anti-Doping Agency (WADA) Prohibited List


A Maître

A Vande Put

J P Laval

S Valette

G Trolliard

Role of boron on the Spark Plasma Sintering of an ␣-SiC powder

Keywords: SiC, SPS, Sintering, Grain growth, Boron, Carbon

This study deals with the role of non-oxide sintering aids such as boron carbide (B 4 C) or -free boron (B) plus free carbon (C) -on the Spark Plasma Sintering treatment of silicon carbide. The results so obtained clearly show that free boron plus free carbon additions lead to the higher densification rates. This favourable behaviour with regards to the densification kinetics is accompanied by the absence of any abnormal grain growth. At the opposite, boron carbide additions do not significantly raise the densification kinetic after SPS treatment of SiC in comparison to pure silicon carbide. In this case, TEM investigations point out the formation of a borosilicate vitreous phase due to the dissolution process of B 4 C in contact with a native superficial silica layer surrounding the SiC grains. The resulting liquid phase leads to an abnormal grain growth coupled with undensifying process.

Introduction

Silicon carbide (SiC) is a very interesting ceramic due to its promising properties like high hardness, low bulk density, high oxidation resistance which makes it useful for a wide range of industrial applications (nuclear fuels coating for future reactors, thermal barrier for aeronautic or aerospace applications). In addition to the interest in its properties for structural applications, silicon carbide has also been the focus of investigations for its chemical resistance in cryolithic melts. [START_REF] Skybakmoen | Chemical resistance of sidelining materials based on SiC and carbon in cryolithic melts-a laboratory study[END_REF] It has more particularly received considerable attention as sidelining material in aluminum electrolysis cells. [START_REF] Skybakmoen | Evaluation of chemical resistance-oxidation of Si 3 N 4 -SiC sidelining materials used in Al electrolysis cells[END_REF] Unfortunately, like so many other covalent refractory ceramics, the silicon carbide is very difficult to sinter to a high density without using additives and/or pressure. For example, without any additives, the theoretical density could only be achieved at 2500 • C by hot-pressing under a pressure of 50 MPa. [START_REF] Nadeau | Very high pressure hot pressing of silicon carbide[END_REF] In previous studies, it is shown that using sintering aids could lower the sintering temperature. Several additives like YAG (i.e. the Y 3 Al 5 O 12 compound) and alumina were tested as sintering aids for silicon carbide powder both to enhance the densification rate and to slow down grain growth kinetic. [START_REF] Sakai | Effects of carbon on phase transformation of ␤-SiC with Al 2 O 3[END_REF][START_REF] Omori | Preparation of pressureless-sintered ␤-SiC with Al 2 O 3[END_REF] The sintering mechanism was determined for each of these additives considered separately and in the case of alumina, supplement adding of carbon or yttria were also performed. In these conditions, it is worth to notice that alumina together with carbon promoted silicon carbide sintering via solid-state sintering at temperature over 2000 • C [START_REF] Sakai | Effects of carbon on phase transformation of ␤-SiC with Al 2 O 3[END_REF] while alumina and yttria led to high density sintered sample via liquid phase sintering below 2000 • C. [START_REF] Omori | Preparation of pressureless-sintered ␤-SiC with Al 2 O 3[END_REF] In these approaches where additives are mainly oxides, the consolidation steps principally take place through liquid phase sintering, [START_REF] Omori | Preparation of pressureless-sintered ␤-SiC with Al 2 O 3[END_REF] giving rise to the formation of an interphase at the grain boundaries. In contact with melted salts, the so-formed secondary phases show lower corrosion resistance than the SiC matrix. [START_REF] Skybakmoen | Chemical resistance of sidelining materials based on SiC and carbon in cryolithic melts-a laboratory study[END_REF][START_REF] Skybakmoen | Evaluation of chemical resistance-oxidation of Si 3 N 4 -SiC sidelining materials used in Al electrolysis cells[END_REF] To achieve high densities through solid-state sintering and to avoid the appearance of low corrosion resistance intergranular phases, consolidation of silicon carbide with boron and carbon as sintering additives seems to be the most efficient way. [START_REF] Antonova | Preparation and some properties of materials based on silicon with boron and aluminium additions[END_REF][START_REF] Prochazka | The role of boron and carbon in the sintering of silicon carbide[END_REF][START_REF] Bind | Hot-pressing of silicon carbide with 1% boron carbide addition[END_REF][START_REF] Magnani | Pressureless sintering and properties of ␣-SiC-B 4 C composite[END_REF] Therefore, boron carbide was found to enhance the hotpressing behaviour of silicon carbide that could be so obtained in a fully dense state. [START_REF] Stobierski | Sintering of silicon carbide. Effect of boron[END_REF][START_REF] Stobierski | Sintering of silicon carbide. Effect of carbon[END_REF][START_REF] Ermer | Influence of sintering activators on structure of silicon carbide[END_REF] These additives permit to reach high density slightly over 2000 • C by means of the reduction of the surface energy of the grains (boron effect) [START_REF] Stobierski | Sintering of silicon carbide. Effect of boron[END_REF] and reaction with residual silica (carbon effect). Otherwise, several contributions [START_REF] Yamamoto | Consolidation of nanostructured ␤-SiC by Spark Plasma Sintering[END_REF][START_REF] Gao | Mechanical and microstructure of nano-SiC-Al 2 O 3 composites densified by Spark Plasma Sintering[END_REF] were devoted to the consolidation of silicon carbide powders to produce dense bodies by using Spark Plasma (or pulse electric current) Sintering (SPS). In all these studies, the main goal is to avoid or to minimize the grain growth of the starting SiC nanoparticles. Indeed, it is usually well admitted that grain growth can be suppressed during the SPS treatment of ceramics because of the very short sintering time. [START_REF] Takeuchi | Preparation of dense BaTiO 3 ceramics with submicrometer grains by spark plasma sintering[END_REF][START_REF] Gao | Superfast densification of oxide/oxide ceramic composites[END_REF][START_REF] Liu | Improved densification of Al 2 O 3 and SiC based ceramic composites by spark plasma sintering, advances and matrix composites VI[END_REF] However, Zhou et al. [START_REF] Zhou | Very rapid densification of nanometer silicon carbide powder by pulse electric current sintering[END_REF] revealed that the rapid densification of a nanometer sized ␤-SiC powder doped with 2.04 wt% Al 4 C 3 and 0.4 wt% B 4 C and conducted by SPS technique, was accompanied by significant microstructural and structural evolutions. Indeed, the densified SiC specimens underwent anisotropic grain growth and polytype transformation. According to these authors, [START_REF] Shen | Spark Plasma Sintering of alumina[END_REF] the ␤ → ␣ transformation during SPS would contribute both to the anisotropic grain growth of SiC (i.e. the formation of elongated grains) and to the modifications of the amount of the different polytypes. In the same manner, the SPS conditions (e.g. the heating rate) could strongly influence the ␤ → ␣ transformation. Unfortunately, the role of the sintering additives, especially boron carbide, was not considered to explain this significant microstructural evolution of SiC-based samples.

In the present study, the SPS method was used to produce fully dense SiC-based materials with non-oxide secondary phases in order to provide high corrosion resistance sidelining materials for Al electrolysis cells. The second objective was to focus on the role of boron during the Spark Plasma Sintering of a commercial ␣-silicon carbide powder. The structural and microstructural evolutions due to different boron additions (free boron plus free carbon or boron carbide (B 4 C)) were characterized by transmission electron microscopy (TEM) and Fourrier transform infra-red (FTIR) spectrometry techniques.

Experimental procedure

Starting powders

The main physical-chemistry properties of raw materials (specific area, average grain size, purity) were summarized in Table 1.

␣-Silicon carbide powder supplied by H.C. Starck TM (grade UF 25) was used as starting powder. From its X-ray diffraction pattern (Fig. 1), it can be noticed that this silicon carbide powder mainly contains 6H and 4H polytypes (JCPDS files 72-0018 and 29-1127, respectively).

Sintering additives provided by H.C. Starck TM , Prolabo TM and Cerac TM suppliers (Table 1) were added under several forms. When amorphous boron and carbon were introduced in the mixture as free elements, their respective amount was 3.1 wt% and 0.86 wt%, i.e. the same weight ratio as for B 4 C. Different symbols have been used in the present paper to make the sample description easier:

• "SBC" indicates samples in which both free boron and free carbon have been added to the starting silicon carbide powder; • "SB4C" stands for samples in which boron carbide additions are introduced in the same SiC powder.

Scanning electron microscopy (SEM) observations of silicon carbide powder are reported in Fig. 2. The powder consists in agglomerates of a few micrometers (Fig. 2a) which are formed of elementary particles of few hundred nanometres in size (Fig. 2b). Fig. 3 represents the microstructures of the sintering agent powders. Both free carbon and boron (Fig. 3a andb, respectively) are mainly composed of agglomerated equiaxed particles while B 4 C displays micron size angular platelet microstructure (Fig. 3c).

Sample preparation and sintering procedure

␣-SiC and sintering aids were mixed in petroleum ether (Riedel de Häen, "extra pure" quality) according to the chosen proportions and then dispersed by ultrasonic pulses. The mix was dried under vacuum at 80 • C in order to remove the solvent.

For SPS treatments, the powders were then put in a 15 mm diameter graphite die in which some thin papyex TM graphite foils were fetched on the inner surface. The sintering of SiC powder with or without sintering additives was conducted under vacuum (6 < P(Pa) < 14). The SPS apparatus (Model 2080 Sum- itomo Coal and Mining Co.) is a uniaxial 100 kN press combined with a 8000 A dc power supply to provide simultaneously pulsed current and pressure to the sample. The standard 12:2 pulse sequence [START_REF] Shen | Spark Plasma Sintering of alumina[END_REF] which corresponds to 12 dc current pulses and two pulses off was chosen. The SPS thermal treatment was as following:

• A degassing treatment performed at 600 • C during 5 min: 3 min under limited applied load (20 MPa) and 2 min under increasing load up to 40 MPa or 100 MPa.

• A heating up to 1850 • C or 1950 • C at 100 • C min -1 under
optimal applied load (40 MPa or 100 MPa), a soaking time of 5 min at maximum temperature then cooling down to room temperature.

The experimental parameters such as temperature, applied load, voltage and sample displacement were measured continuously during the sintering treatment. In particular, the temperature was measured thanks to a digital pyrometer focused on the graphite die close to the centre of the sample.

Characterization of sintered samples

The phase identification was performed using a X-ray Siemens D5000 diffractometer (λ Cu = 0.1541 nm) equipped with a back monochromator. The scanned angles (in 2θ scale) were ranging between 15 • and 120 • with a step of 0.02 • and a 13 s exposure time. The X-ray diffractograms were indexed using the DIFFRAC+ software (Socabim) which contained JCPDS files data base. Accurate measurements of the relative abundance of the various polytypes present in silicon carbide based samples were carried out using the Rietveld's method implemented into the TOPAS 3 program. [START_REF]TOPAS version 3[END_REF] The X-ray patterns were acquired from sintered samples previously ball milled in an agate mortar under cryogenic conditions.

The infrared spectroscopic measurements (FTIR) were carried out using a PerkinElmer Spectrum 1000 apparatus. The spectra were registered in the 400-4000 cm -1 range with a 2 cm -1 resolution. FTIR measurements were performed from starting powders and sintered samples which were previously ball milled.

Density of sintered samples was determined using Archimedes method. The microstructural analyses were carried out using SEM Philips XL30 apparatus. The identification of the intergranular phases required TEM investigations which were performed with a 2010 JEOL apparatus equipped with EDX facilities (Energy Dispersive X-ray analytical system) and operating at 200 kV. As such, specimens were prepared by cutting samples into thin disks with a 3 mm diameter. These disks were ground, dimpled and finally thinned to perforation by argon beam milling in a Gatan Precision Ion Polishing System (PIPS 692) at 4 kV.

Results

Densification kinetics

Fig. 4a shows the variation of the linear shrinkage for SiC-based samples doped with different sintering aids under a 100 MPa applied load as a function of temperature during the SPS treatment. From this figure, it can be noticed that the densification of SiC samples starts at lower temperatures when boron-based sintering additives are initially introduced. The temperature of the shrinkage beginning (T S ) is reported in Fig. 4b. T S values were determined using a polynomial interpolation in order to fit the curves showing the linear shrinkage (Fig. 4a) and then to determine the continuous variation of d( L)/dT as a function of temperature. For a given sample, the T S value is thus defined as the temperature where d( L)/dT deviates from the straight line in the low temperature range. The data so obtained were reported in Fig. 4b. From the latter, the temperature of the shrinkage beginning seems to be shifted towards lower temperatures by using sintering aids for the same applied load. For example, in the case of SBC, the T S value (i.e. ≈1200 • C) was 220 • C lower than that recorded for pure silicon carbide specimens under an 40 MPa applied load (see Fig. 4b).

Moreover, the higher the applied load during SPS treatment, the higher the densification kinetics whatever the sintering aids. In particular, the lowest temperature of the shrinkage beginning was reached (i.e. 1190 • C) when a 100 MPa load was applied during the treatment of SBC samples.

Table 2 provides the relative density for specimens treated by SPS at 1850 • C or 1950 • C for 5 min under a 40 MPa or 100 MPa load. The treatments at 1950 • C under 100 MPa gave relative density values higher than 97.5% regardless the sintering additive. This fact is confirmed by the observation of the sintering curves under isothermal conditions which denotes that fully dense materials are obtained at 1950 • C for a soak time fixed to 5 min (see Fig. 4c). Therefore, the theoretical density of SiC-based specimens was achieved when the maximum load was applied and the SBC composition was retained (Table 2). In the following and due to their high densification rates, the materials sintered for 5 min at 1950 • C under a 100 MPa load will be characterized by TEM, XRD and FTIR techniques.

Characterization of the different silicon carbide polytypes

X-ray diffraction patterns obtained from pure SiC, SBC and SB4C samples treated at 1950 • C for 5 min under 100 MPa have been reported in Fig. 5. Compared to the SiC starting powder (Fig. 1), all sintered samples are characterized by the occurrence of the 15R rhombohedral form (identified from JCPDS file 39-1196 reported in Fig. 5b) in addition to the 6H and 4H polytypes. To quantify the respective amount of the three polytypes, a simultaneous refinement of the three structures was performed, using the TOPAS 3 Rietveld program. [START_REF]TOPAS version 3[END_REF] Indeed, previous works demonstrated that the Rietveld's method yielded accurate phase composition measurements within a maximum 1 wt%. [START_REF] Ortiz | Quantitative polytype-composition analyses using X-ray diffraction: a critical comparison between the polymorphic and the Rietveld Methods[END_REF] The reference structures were taken from the ICSD database. [START_REF]ICSD (Inorganic Crystal Structure Database)[END_REF] Then, the structures were partly refined (scale factors and isotropic thermal vibration factors) keeping the atomic positions unchanged. After convergence to reasonable R factor values, the quantitative fraction of the three polytypes was determined with a good accuracy and the corresponding data is shown in Table 2. It is shown that the 15R polytype occurs in a significant amount even if the 6H and 4H polytypes remain predominant. Moreover, the 4H polytype amount in sintered samples seems to be particularly great for SB4C samples in comparison with that registered for "pure" SiC or SBC specimens (see Table 2) after SPS.

Microstructural study of the sintered samples 3.3.1. Pure silicon carbide

TEM observations of pure SiC samples revealed the presence of an equiaxed microstructure (Fig. 6a). No preferential crystalline orientation was observed. The grain size averages 500 nm and remains quite the same as that of the starting powder (see Fig. 2b). The electron diffraction study confirmed that the 6H polytype is the main phase presents within the sample (see inset in Fig. 6a).

SBC samples

A change in microstructure occurred when free boron and free carbon were added to the starting SiC powder (Fig. 6b). A heterogeneous grain size is observed contrasting with the microstructure of SiC samples presented above. Some of the grains have preferentially grown, showing elongated shape while most of them kept the equiaxed shape. No intergranular thin film of secondary phase linked to free boron and free carbon addings was detected either by conventional contrast imaging or by high resolution investigation of grain boundaries. These results however do not definitely preclude the possibility of atomic segregation of species at the grain boundaries as will be further discussed.

SB4C samples

Observations of SB4C sintered samples (Fig. 7) showed up microstructural changes compared to pure SiC and SBC samples presented above. The microstructure was characterized by abnormal grain growth with lath-type texture linked to the anisotropic character of the SiC polytypes. This grain growth was accompanied by a slight undensifying effect as shown by the results reported in Table 2.

The sample typically presents numerous spherical intragranular inclusions (Fig. 7: see arrows) as well as intergranular phase (Fig. 7: encircled areas) trapped between lath shaped SiC crystals and B 4 C crystals (Fig. 8a).

In fact, the intergranular phases are present in reactional zones involving B 4 C crystals. Indeed, the latter are highly corroded as shown by the presence of numerous corrosion gulfs all around the crystals (Fig. 8a: see arrows). In most cases, such crystals of parent phase are entirely dissoluted and relicts of the boron carbide crystals are only seldom seen. For that reason, this residual phase is not observed in XRD patterns presented in Fig. 5. When exist, the intergranular phase around the B 4 C crystals (Fig. 8a, c andd) consists of nanocrystals of about 50 nm embedded within an amorphous phase. An average chemical analysis performed with a defocused electron beam reveals that this phase is made of carbon, silicon and oxygen (Fig. 8b). A fine investigation of this area (Fig. 9) shows that the amorphous phase is mainly composed of a silicon oxide (Fig. 9b) while nanocrystals are composed of carbon (Fig. 9a) that could be present under graphite or diamond forms. The investigation of their reciprocal lattice by electron diffraction attests of a cubic structure and, consequently, unambiguously permits to conclude that diamond or more probably carbon-like diamond resulted from the SPS treatment.

The spherical intragranular inclusions (Fig. 10) are also formed by the same reactional product, i.e. carbon-like diamond crystals (Fig. 10b) embedded within the silicate phase (Fig. 10c). In this case, relicts of B 4 C crystals are never observed.

Finally, the comparison of FTIR patterns for both the starting silicon carbide powder and the sintered SB4C sample (Fig. 11) reveals the presence of additional Si-O (≈1100 cm -1 ) and O-Si-O (≈470 cm -1 ) bands [START_REF][END_REF] in the case of SPS treated samples. These results suggest that the oxidation of SiC grains occurs during the heat treatment.

Discussion

SPS treatment of pure SiC

This study has shown that Spark Plasma Sintering had a great effect on the kinetic of the densification of "pure" ␣silicon carbide. Indeed, to reach fully dense SiC-based materials (without sintering additives) by using conventional pressurized techniques such as hot-pressing or hot isostatic pressing, it is well known that severe experimental conditions are required: [START_REF] Bind | Hot-pressing of silicon carbide with 1% boron carbide addition[END_REF][START_REF] Dutta | Densification and properties of ␣-silicon carbide[END_REF] high sintering temperature (T ≥ 2100 • C) and high soaking time (t ≥ 30 min).

These drastic treatments usually lead to the thermal decomposition of the SiC compound and may involve the coalescence of grains and pores. [START_REF] Stobierski | Sintering of silicon carbide. Effect of carbon[END_REF] Conversely, the SPS treatment by reducing the holding time to 5 min and by lowering the sintering temperature to 1950 • C allows minimizing grain growth (Fig. 6a) and, consequently, provides equiaxed microstructure. These SPS conditions permit also to achieve high relative densities for SiC specimens (i.e. 97.5%) without sintering activators.

These results should be due to the characteristics of the SPS heating treatment such as self-heat generation by microscopic electric discharge between particles, this activation leading to high speed mass and heat transfer. [START_REF] Chen | Fundamental investigations on the spark plasma sintering/synthesis process: I. Effect of dc pulsing on reactivity[END_REF][START_REF] Anselmi-Tamburini | Fundamental investigations on the spark plasma sintering/synthesis process: II. Modeling of current and temperature distributions[END_REF] More particularly, the rapid densification could be correlated to mechanisms with faster kinetics, such as surface diffusion, diffusion through the melt, or time-independent process such as plastic deformation. [START_REF] Chaim | Densification mechanisms in spark plasma sintering of nanocrystalline ceramics[END_REF][START_REF] Tamari | Effect of Spark Plasma Sintering on densification and mechanical properties of silicon carbide[END_REF][START_REF] Gao | Spark Plasma Sintering technology[END_REF] So, the melting could occur only on the particle surface thanks to electron discharge which easily generates localized heating on the grains.

Effect of boron additions on the SPS behaviour of SiC-based specimens

The form under which the same amount of carbon and boron was added to the starting SiC powder plays a key role in the sintering process involved during the treatment. From all the results Fig. 10. TEM observation of an intragranular inclusion (see also arrows in Fig. 7) (a). The SAED pattern (b) and the EDS analysis (c) obtained on the overall area of the composite interphase show the presence of diamond nanocrystals embedded within a silica rich phase.

of sintering attempts and microstructural characterizations, the role of boron additions could be elucidated.

In SB4C samples, the B 4 C crystals are shown to be dissoluted at the contact of a silicate phase, giving rise to a liquid phase sintering process. FTIR investigations and EDXS analyses revealed that this phase would result from the reaction between the silica-based phase due to the native oxidation of silicon carbide grains and the boron carbide crystals introduced as sintering aids. The native oxidation of SiC is a well-known phenomenon which results from the sorption of water molecules at room temperature. [START_REF] Médout-Marère | Surface heterogeneity of passively oxidized silicon carbide particles: hydrophobic-hydrophilic partition[END_REF] The SPS treatment of SB4C samples would favour the reaction between the silica film surrounding the SiC grains and the B 4 C compound. The presence of thin jagged reaction fronts around boron carbide grains attest of the dissolution mechanism of B 4 C into the liquid silica-based phase (Fig. 8), finally providing a borosilicate glass after cooling. Therefore, the results obtained from the microstructural characterization of SB4C sintered samples permit to explain the role of a secondary fusible phase on the microstructure evolution during SPS. At the forming temperature, the borosilicate phase was partly a viscous liquid, which presumably controlled grain boundary migration. When the temperature increases, its viscosity drastically decreases, promoting the mobility of grain boundaries by favouring atomic diffusion within the liquid phase. It is reasonable to assume that the grain growth kinetic by dissolution/reprecipitation mechanism should be enhanced for some crystallographic orientations which probably correspond to the surfaces of lowest energy for the SiC phase. Thanks to the high motion of the grain boundary, some pockets of liquid phase were entrapped within the grains leading to spherical intragranular inclusions.

For the SBC samples, TEM investigations revealed that the formation of secondary phase is precluded, as no continuous thin film of interphase or glassy pockets at triple junctions were evidenced. This result is in fair agreement with a previous study [START_REF] Gu | Detection of boron segregation to grain boundaries in silicon carbide by spatially resolved by electron energy-loss spectrometry[END_REF] which showed that no distinct amorphous film can be found by HREM imaging of such interfaces. However, the interface seemed to be highly perturbed and thanks to spatially resolved electron energy-loss spectroscopy, these authors have shown that one monolayer of boron segregates at the SiC boundary. According to these authors, [START_REF] Shinoda | Effect of amount of boron doping on compression deformation of fine-grained silicon carbide at elevated[END_REF] the main effect of boron doping on the densification of silicon carbide should rely on the enhancement of the grain boundary diffusion. In fact, they indicated that most of the added boron would segregate at the grain boundary because of its low solid solubility at the sintering temperature (e.g. ≤ 0.1 wt% at T = 2200 • C). [START_REF] Tajima | Solid Solubility of aluminum and boron in silicon carbide[END_REF][START_REF] Linnarsson | Solubility limits of dopants in 4H-SiC[END_REF] Conversely, Elder and Krstic [START_REF] Elder | Effect of surface area on densification of ␤-SiC powders in the presence of B and c[END_REF] suggested that the additive boron is dissolved in the ␤-SiC matrix, forming a solid solution that enhanced lattice diffusion through the increase of carbon vacancies ratio in SiC. This latter phenomenon could explain the enhancement of the mass transfer during sintering. The same interpretation was retained by Datta et al. [START_REF] Datta | Sintering of nanocrystalline ␣-silicon carbide by doping with boron carbide[END_REF] for a ␣-SiC powder.

In the present study, despite the fact that the sintering additives are present in much higher amount (3.1 wt%), i.e. ten times higher than those reported in ref. [START_REF] Shinoda | Effect of amount of boron doping on compression deformation of fine-grained silicon carbide at elevated[END_REF] , no interphase was detected. This result would suggest that the sintering aids effect could rather rely on formation of solid solutions with the SiC compound, even if their location could be restricted to the interface vicinity. In this hypothesis, the role of free boron addition on the densification activation (Fig. 4) could be explained by the creation of further silicon and carbon vacancies in the lattice of the SiC compound following Eqs. ( 1) and ( 2):

B 4 C SiC -→4B Si + C × + 3V •••• C (1) SiB 4 SiC -→4B ••• C + Si × + 3V Si (2)
Finally, in these SBC samples, the free carbon addings present in well-distributed manner would reduce the native silica film on the surface of SiC grains during the sintering 37 by the overall reaction:

SiO 2(s) + 3C (s) → SiC (s) + 2CO (g) (3)

Conclusion and perspectives

This study confirms the favourable role of boron additions on the sintering of SiC by Spark Plasma Sintering processes.

The most efficient SPS treatment to reach fully dense SiCbased materials consists in both introducing free carbon and free boron and maintaining a 100 MPa pressure at 1950 • C for 5 min. Free carbon should favour the reduction of silica covering the surfaces of SiC grains whereas free boron additions could improve the densification kinetic by enhancing the formation of further point defects in the ␣-SiC lattice. It is thus possible to conclude that the volume diffusion certainly intervains as one of the mechanism that largely operates during the sintering of SiC. This does not preclude the possible influence of competitive process such as grain boundary diffusion.

In the case of boron carbide additions, the occurrence of a borosilicate liquid phase during the SPS treatment results from the reaction of dissolution of B 4 C in contact with the silica based phase coming from the native oxidation of the starting SiC powder. This secondary liquid phase promotes an abnormal grain growth leading to a slight non-densifying effect. The next studies should refine the mechanism of boron carbide dissolution and the main properties of the glassy phase obtained during the sintering treatment.

Fig. 1 .

 1 Fig. 1. X-ray diffraction pattern of the starting silicon carbide powder.

Fig. 2 .

 2 Fig. 2. SEM observations of the starting silicon carbide powder at different magnifications.

Fig. 3 .

 3 Fig. 3. SEM observations of the starting powders: carbon lampblack (a), boron (b) and boron carbide: B 4 C (c).

Fig. 4 .

 4 Fig. 4. Sintering curves as a function of temperature under a 100 MPa applied load for pure SiC, SBC and SB4C samples (heating rate = 100 • C min -1 ) (a). Temperature of shrinkage beginning (T S ) determined for the different SiC-based samples obtained by Spark Plasma Sintering (SPS) treatments under different applied loads (b). Densification curves as a function of soaking time under isothermal conditions (at 1950 • C) and a 100 MPa applied load for pure SiC, SBC and SB4C samples (c).

Fig. 5 .

 5 Fig. 5. X-ray diffraction patterns obtained on the samples sintered by SPS (1950 • C for 5 min under a 100 MPa pressure): pure SiC samples (SiC), SiC samples plus free boron and carbon addings (SBC), SiC samples plus boron carbide addings (SB4C) (a). Enlargement of the framed area in (a) showing the diffraction peaks of the different SiC polytypes (b).

Fig. 6 .

 6 Fig. 6. TEM micrographs obtained on the samples sintered by SPS (1950 • C for 5 min under a 100 MPa pressure): pure SiC samples (a), SiC samples with free boron and carbon as sintering aids (SBC) (b).

Fig. 7 .

 7 Fig. 7. TEM micrograph of a SiC samples after SPS at 1950 • C for 5 min under 100 MPa with boron carbide addings (SB4C). The lath-type microstructure accounts for abnormal grain growth. The presence of an intergranular phase at triple junctions (encircled areas) and of intragranular inclusions (see arrows) is noticeable.

Fig. 8 .

 8 Fig. 8. TEM micrograph of a reaction site (see also encircled areas in Fig. 7). Overview showing the B 4 C crystal surrounded by a composite glassy phase (a). It shows corrosion gulfs which attest that a dissolution process takes place. Energy dispersive spectrometry (EDS) analysis of the outer composite interphase (b). EDS analysis of the B 4 C grain (c). [0 0 01] SAED pattern of the B 4 C compound (d).

Fig. 9 .

 9 Fig. 9. TEM observations of the composite interphase shown in Fig. 8. Conventional imaging of a diamond nanocrystal (a) and its associated SAED pattern and EDXS analysis. The diamond nanocrystals are embedded within an amorphous silica rich phase (b).

Fig. 11 .

 11 Fig. 11. Infrared spectra of pure silicon carbide before SPS treatment (continuous line: SiC powder) and of SB4C samples (SiC powder plus boron carbide) sintered by SPS at 1950 • C for 5 min under 100 MPa (discontinuous line).

Table 1

 1

	Main physical-chemistry of the starting powders
	Powder (supplier)	Variety	Specific area (m 2 /g)	Mean diameter a (m)	Purity (%)
	Silicon carbide (H.C. Starck)	␣-SiC (6H, 4H)	26.70	0.07	98
	Boron carbide (H.C. Starck)	B 4 C	17.77	0.13	97
	Boron (Cerac)	Amorphous	16.05	0.16	99.9
	Carbon (Prolabo)	Lampblack	25.70	0.25	Ashes < 0.75

a The mean equivalent diameter Ø is calculated on the basis of the density ρ and of the measured specific area S by the classical relation: Ø = 6/(ρ × S).

Table 2

 2 Sintering conditions, relative density and polytype content of the SPS-sintered SiC ceramics

	Silicon carbide	Sintering	Heating rate	Holding	Load	Relative	Weight percentage of
		temperature ( • C)	( • C min -1 )	time (min)	(MPa)	density (%)	silicon carbide polytypes
							6H	4H	15 R
	Pure SiC	1950	100	5	100	97.5	88.35	4.95	6.70
		1850	40	5	40	92.8	-	-	-
	SBC sample (SiC + free	1950	100	5	100	98.8	88.60	5.66	5.84
	boron + free carbon)	1850	40	5	40	93.7	-	-	-
	SB4C sample	1950	100	5	100	97.1	84.59	11.29	4.12
	(SiC + boron carbide)	1850	40	5	40	91.9	-	-	-

Acknowledgements

This work was financially supported by ALCAN Company and especially by the Research Centre of Voreppe (CRV). The authors should like to express their appreciation for the technical assistance of Dr. Claude Estournes who is responsible for the French SPS platform (called PNF 2 and financially supported by CNRS). The authors are also grateful to Mr. Etienne Laborde, Mr. Bernard Soulestin and Dr. Daniel Tétard for their technical assistance.