Yuri I Kantor
email: kantor.yuri1956@gmail.com

Juan Horro

Emilio Rolán

Nicolas Puillandre

Paraclavatula (Gastropoda; Conoidea; Clavatulidae), a new genus with a distinctive radula type from West Africa

A unique radular configuration for Conoidea, consisting of five teeth in a transverse row (acuspate plate-like central and laterals and duplex marginal teeth), was found in three species previously described in the genus Clavatula: C. delphinae, C. pseudomystica, and C. christianae. Analysis of the COI gene demonstated that they belong to the family Clavatulidae.

The new genus Paraclavatula n. gen. is described. No similar radulae have been found previously among Conoidea and their morphology suggests that the presence of well-defined lateral teeth is more broadly distributed within Conoidea than previously anticipated. Based on radular morphology alone it would not be possible to attribute the genus to any presently recognized family of Conoidea.

INTRODUCTION

Clavatulidae Gray, 1853 as now understood is a relatively small family of Conoidea, that presently encompasses 13 Recent genera, two of them included tentatively [START_REF] Bouchet | A new operational classification of the Conoidea (Gastropoda)[END_REF]. The representatives of the family are distributed in the Atlantic, Indian and Pacific oceans from intertidal to bathyal depths, and are particularly diverse in West, South, and Southeast Africa. Overall, the shell and radular characters display much disparity within the family.

The radulae and soft body morphology of Clavatulidae remain rather poorly studied. Line drawings of radulae of several species from South Africa and Mozambique were provided by [START_REF] Kilburn | Turridae (Mollusca: Gastropoda) of southern Africa and Mozambique. Part 2. Subfamily Clavatulinae[END_REF], and scanning electron micrographs of a few additional species were published by [START_REF] Bouchet | A new operational classification of the Conoidea (Gastropoda)[END_REF].

The genus Clavatula Lamarck, 1801 (type species Clavatula coronata Lamarck, 1801 by monotypy) is particularly diverse in West Africa and includes over 40 species, several being described recently (eg. [START_REF] Nolf | Clavatula nathaliae (Mollusca: Gastropoda: Conoidea: Turridae) a new species from Gabon: an old mystery finally elucidated[END_REF][START_REF] Nolf | Two new turrid species from West Africa: Clavatula delphinae and Clavatula pseudomystica (Mollusca: Gastropoda: Clavatulidae)[END_REF][START_REF] Nolf | Clavatula christianae, a new turrid from West Africa (Mollusca: Gastropoda: Clavatulidae)[END_REF], using shell characters alone.

In the course of a systematic revision of the genus Clavatula from West Africa by Juan Horro, Emilio Rolán, and Peter Ryall (unpubl. data), an extremely unusual radula was found in two species. This unique radular morphology posed the question of the systematic position of the species. We were able to obtain one specimen with a similar radula, collected in the Republic of the Congo, and adequately preserved for DNA sequencing. A molecular phylogenetic analysis confirmed its position within the family Clavatulidae. Based on morphological and molecular evidence, we provide a detailed description of the radula and describe the new genus Paraclavatula gen. nov. for three species from West Africa previously attributed to Clavatula.

MATERIAL AND METHODS

Specimens examined in this study were mostly collected by Xico Fernandes in Angola during the 1990's. The shells were preserved in ethanol and stored in the collections of the second and third authors. One specimen (MNHN-IM-2009-26535) was obtained during fieldwork in the Republic of the Congo and is stored in the Muséum National d'Histoire Naturelle in Paris (MNHN). For the latter specimen a tissue clip of the foot was preserved on the spot in 95% ethanol. To test the taxonomic affinities of MNHN-IM-2009-26535, several specimens belonging to various genera of Clavatulidae, as well as samples belonging to other families of Conoidea were included in the dataset (Table 1). These samples were collected during several expeditions conducted by the MNHN and from other sources (see Table 1 for details). All of them are stored in the collections of the MNHN. Details of localities and photos of the specimens are available on the website of the MNHN at https://science.mnhn.fr/institution/mnhn/collection/im/item/ by adding the registration number of the specimen (without prefix IM-) after the last backslash.

DNA was extracted using the Epmotion 5075 robot (Eppendorf), following the manufacturers' recommendations. A fragment of the cytochrome oxidase subunit I (COI) was amplified using universal primers LCO1490/HCO2198 [START_REF] Folmer | DNA primers for amplification of mitochondrial cytochrome c oxidase subunit I from diverse metazoan invertebrates[END_REF]. The protocol for the PCR reactions followed [START_REF] Puillandre | A quest for the lost types of Lophiotoma (Gastropoda: Conoidea: Turridae): integrative taxonomy in a nomenclatural mess[END_REF]. PCR products were purified and sequenced by the Eurofins sequencing facility. Sequences were deposited in BOLD (Barcode of Life Datasystem) and GenBank (Table 1). The phylogenetic tree was reconstructed using MrBayes (Huelsebeck, Ronquist & Hall, 2001), as implemented on the Cipres Science Gateway (MrBayes 3.2.2 on XSEDE; http://www.phylo.org/ portal2). The three codon positions of the COI gene were treated as independent partitions, and the substitution model was set to GTR+I+G; the parameters of the substitution model were evaluated independently for each partition during the analyses. Each of the two runs consisted of 8 Markov chains and 10,000,000 generations, with 5 swaps at each generation, a sampling frequency of one tree each 1,000 generations and a chain temperature set at 0.02. Convergence of each analysis was evaluated using TRACER 1.4.1 (Rambaut & Drummond, 2014) to check that all effective sample size values exceeded 200. Consensus trees were calculated after omitting the first 25% trees as burn-in. Nodal support was assessed with posterior probability (PP) of each node.

Radulae were prepared using standard methods [START_REF] Kantor | Evolution of the radular apparatus in Conoidea (Gastropoda: Neogastropoda) as inferred from a molecular phylogeny[END_REF] and investigated with a JEOL JSM 840A scanning electron microscope at the MNHN.

RESULTS

The tree based on COI sequences is characterized by relatively low support of the nodes (Fig. 1). The Clavatulidae, albeit monophyletic, is not statistically supported (PP = 0.86). Two species of the genus Makiyamaya Kuroda, 1961 are sister to a highly supported clade (PP = 1) that contains all other clavatulid species (except Gemmuloborsonia, the inclusion of which in the family is doubtful; [START_REF] Bouchet | A new operational classification of the Conoidea (Gastropoda)[END_REF]. Mostly nodes within this clade are unsupported.

The genus Clavatula itself does not constitute a monophyletic group as the species attributed to it are scattered in three clades, two with high support (PP = 1; Clavatula taxea + Clavatula Description: Shells of small to moderate size, between 10 and 30 mm, general shape rhomboid to oval. Smooth protoconch of 2.5 to 3 whorls, teleoconch with 6 to 8 whorls. Body whorl occupies from 1/2 to 2/3 of shell length. Sculpture of usually oblique and flexuous axial ribs, 14 to 25 on last whorl and numerous spirals cords, from 15 to 20, some more distinct, and strong subsutural cord nodulose at intersections with axials. Siphonal canal short and open, sinus U-shaped, on subsutural ramp, parietal callus usually lacking or very weakly developed.

Colour from brownish to bluish or greyish, with variable darker brown, and yellowish to white dots, blotches and/or flammules. When fresh, periostracum brown.

Radula with five teeth in transverse row; marginal teeth duplex with sharp-edged major limb and more or less pronounced socket where accessory limb is inserted. Accessory limb from large, about 2/3 of major limb length, to very small, vestigial, about 1/3 of major limb length (P. delphinae). Lateral teeth irregularly shaped, plate-like, with elevated posterior outer corner forming obtuse large cusp. Central teeth with strongly to moderately arcuate anterior margin, and forming two or three elevated transverse ridges.

Species composition: Three species, all originally described in Clavatula, are assigned to the new genus based on their similar radular type: P. delphinae [START_REF] Nolf | Two new turrid species from West Africa: Clavatula delphinae and Clavatula pseudomystica (Mollusca: Gastropoda: Clavatulidae)[END_REF], the type species, P. christianae [START_REF] Nolf | Clavatula christianae, a new turrid from West Africa (Mollusca: Gastropoda: Clavatulidae)[END_REF] and P. pseudomystica [START_REF] Nolf | Two new turrid species from West Africa: Clavatula delphinae and Clavatula pseudomystica (Mollusca: Gastropoda: Clavatulidae)[END_REF].

Besides the three species which we attribute to this genus there were some others described recently as Clavatula that may belong to Paraclavatula, having similar shell characters and habitat, among them C. congoensis Nolf &Verstraetten, 2008, andC. hattenbergeri Nolf &Verstraetten, 2008. However, in The figured shell (Fig. 2 A-D, SL 11.3 mm) of the sequenced specimen from which a radula was extracted, came from the type locality, Pointe-Noire. It is a somewhat atypical specimen, due to the presence of the dark blotches; it is not fully adult (the species reaches SL 16.3 mm), however it agrees in most features. Frank Nolf kindly confirmed the identification.

Protoconch multispiral, first 2.75 whorls smooth, strongly convex, then there are narrow but distinct arcuate axial ribs, 9 in total, occupying approximately half a whorl. The border with the teleoconch is indistinct, but can be detected by slight changes in the character of the axial ribs, which become broader and less arcuate.

Anterior foregut: Proboscis medium long, conical. Proboscis retractors numerous, attached to rhynhodaeum in its posteriormost part. Buccal mass large, occupying posterior half of inverted proboscis and strongly protruding backwards. Oesophagus leaving the buccal mass forming a short loop before passing through massive nerve ring. Salivary glands paired, acinous, small. Venom gland large, strongly convoluted, passing through the nerve ring towards its opening posterior to the radular sac. Radular sac relatively very large, situated at the right side of the buccal mass.

Radula with duplex marginal teeth with sharp-edged major limb with flattened dorsolateral area, where vestigial minor limb (Fig. 2H, arrow) inserts; length of minor limb about 1/3 of major limb length. Lateral teeth irregularly shaped, narrowing towards rounded posterior margin. Posterior margin elevated in side view (Fig. 2G), producing blunt cusp. Central teeth in dorsal view heart-shaped, with notched anterior margin. Central part of the tooth elevated in side view (Fig. 2G), producing blunt but broad triangular cusp.

Paraclavatula pseudomystica [START_REF] Nolf | Two new turrid species from West Africa: Clavatula delphinae and Clavatula pseudomystica (Mollusca: Gastropoda: Clavatulidae)[END_REF] The figured shell (Fig. 4 A-C), from which a radula was extracted, has SL 18.3 mm. Similar specimens were collected in different locations from Ghana to south Angola. We believe it agrees with specific characters stated in the description of this species, particularly in that the, "…lower part of columella has a typical white blotch changing into dark brown on the parietal wall" (Nolf, 2008: p. 7).

Protoconch (Fig. 4D) partially eroded, multispiral, of about 3 whorls, upper ones smooth and strongly convex, posteriormost part before transition to teleoconch with few narrow arcuate axial ribs.

Radula with duplex marginal teeth with sharp-edged major limb with rather deep socket in which relatively large accessory limb inserts; length of accessory limb about 2/3 of major limb length. Lateral teeth irregularly trapeziform, with thickened anterior margin and shallowly notched posterior margin, which is elevated producing blunt outer cusp and narrower and more pointed inner cusp. Central teeth in dorsal view trapeziform, with shallowly notched anterior margin. Basal plate in side view (Fig. 4 H) forming two transverse elevated ridges, one at anterior margin, another close to posterior margin, which is weakly irregularly serrated.

Paraclavatula christianae [START_REF] Nolf | Clavatula christianae, a new turrid from West Africa (Mollusca: Gastropoda: Clavatulidae)[END_REF] Material examined: Cacuaco Bay, Angola, from very low tide to 1 m, 1 specimen, coll. J.

Horro; Sacomar, Angola, 2-5 m., coll. E. Rolán, 1 specimen.

The figured shell (Fig. 5 A-C, SL 14.6 mm) was taken from the type locality. Similar specimens from different locations from Ghana to Angola are also in author's (JH) posession.

Although the studied specimen differs from the holotype in having a shorter siphonal canal, it falls within the intraspecific variability of the species as defined by [START_REF] Nolf | Clavatula christianae, a new turrid from West Africa (Mollusca: Gastropoda: Clavatulidae)[END_REF] and its identification was confirmed by Frank Nolf.

Protoconch multispiral (Fig. 5 D), of about 3 whorls, upper ones smooth and weakly convex, posteriormost part before transition to teleoconch with few narrow arcuate axial ribs.

Radulae were examined in two specimens (Figs 5 E-H), and are very similar. Marginal teeth duplex, with sharp-edged major limb with pronounced socket in which relatively large accessory limb inserts; length of accessory limb about 2/3 of major limb length. Lateral teeth irregularly trapeziform, with thickened anterior margin and nearly straight oblique posterior margin, which is elevated producing blunt outer cusp and narrower and more pointed inner cusp. Central teeth in dorsal view crescent shaped, with arcuate notched anterior margin. In side view (Fig. 5G) it is seen that the basal plate of the central teeth forms three transverse parallel elevated arcuate ridges, on both anterior and posterior margins and in central part of the tooth. [START_REF] Powell | The molluscan families Speightiidae and Turridae[END_REF] considered the conoidean radula with five teeth in a transverse row as the "prototypic form" confined to Drilliidae (as Clavinae). In this family of Conoidea, well-defined lateral teeth are arcuate, comb-shaped, and multicuspid (Taylor et al., 1993;[START_REF] Bouchet | A new operational classification of the Conoidea (Gastropoda)[END_REF]. The viewpoint of radular morphology in Drilliidae as "least derived" was accepted also in Taylor et al. (1993). At the same time, it was suggested that in Turridae and Clavatulidae, usually with a broad central tooth, the "insubstantial, lateral 'wings' [of the central tooth] may represent vestiges of lateral teeth that have fused with the central tooth" (Taylor et al. (1993: 135). Later, Taylor (1994: 195, pl. 5) illustrated the radula of Turricula javana (Linnaeus, 1767) (Clavatulidae), and commented that the broad central tooth may be formed by fusion of the plate-like lateral teeth with the smaller true central tooth. Finally, [START_REF] Kantor | On the morphology and homology of the "central tooth" in the radulae of Turrinae (Conoidea: Turridae)[END_REF] illustrated a number of different radular morphologies in Turridae s.s. and concluded that, in this family, the "central tooth" is also formed by complete or incomplete fusion of central and plate-like lateral teeth. The term central formation was proposed for this composite structure to distinguish it from the true central tooth. Nevertheless, in addition to Drilliidae, the radula of Paraclavatula is the second case of having five well defined separate teeth in a transverse row among Conoidea.

Discussion

The radulae of the species assigned to Paraclavatula are unique for Clavatulidae and Conoidea in general.In Clavatulidae, the central formation is variously developed, sometimes appearing as a pronounced tooth (e.g., Turricula nelliae (E.A. Smith, 1877); Fig. 6C), sometimes it looks like three incompletely fused elements (Clionella sinuata (Born, 1778) -Taylor, Kantor & Sysoev, 1993; fig. 18A,B; Tomellana lineata (Lamarck, 1818) -Fig. 6B). In some clavatulids the central formation is very reduced and is probably represented by a central tooth alone (Clavatula xanteni Nolf & Verstraeten, 2006; Fig. 6D). Finally it may be nearly completely reduced (Pusionella compacta Strebel, 1914-Bouchet et al., 2011, fig. 18C). On the contrary, in Paraclavatula central and lateral teeth are completely separate, very well defined, rather broadly spaced and of a strange trapeziform shape without distinct cusps.

Somewhat similar lateral teeth were found only in some species of Turridrupa Hedley, 1922(Turridae -Kantor, 2006: fig. 4 C,D, as Turridrupa n. sp.), but they are less defined with an inner margin fused to the radular membrane, while the central teeth are narrowly cuspate. In Paraclavatula all sides of the lateral teeth are well defined and the central tooth has a very different morphology. These two families, Clavatulidae and Turridae are not closely related as demonstrated by molecular phylogeny [START_REF] Puillandre | The dragon tamed? A molecular[END_REF]. Thus the similarity between the shape of the lateral teeth in Paraclavatula and Turridrupa is superficial.

The central tooth of Paraclavatula is very different from other Conoidea. The central tooth when present is always narrow, monocuspid with a pointed cusp. In Paraclavatula, although being variable in shape, the central tooth is plate-like, broad, cuspless and with two or three characteristic elevated transversal ridges. So far it may be considered a synapomorphy of the genus.

Thus the radula of Paraclavatula is unique in the shape of the central tooth, since such well defined plate-like laterals with an elevated posterior margin have not been recorded neither in Clavatulidae, nor in Conoidea.

The discovery of one more group of Conoidea (in addition to Drilliidae) with five well defined teeth in a transverse radular row is yet more evidence that this condition is more broadly distributed within Conoidea than previously anticipated . The homology of the elements of the central portion of the radula in Conoidea is not always easy to establish. In Cochlespiridae there is no evidence that the central tooth is a composite structure (Bouchet et al., 2011, fig. 12). On the contrary, in Clavatulidae we observe different stages of fusion of three central elements and sometimes their reduction (e.g., reduction of lateral teeth in Clavatula xanteni with retention of the narrow central one).

The radular morphology is broadly utilized in the taxonomy of Conoidea (as well as other Neogastropoda), since in many demonstrated cases the shell characters were not sufficient for revealing the relationships and usually radula is a good morphological proxy for taxonomic decisions at least on family level. Nevertheless in case of a very unusual or unique radula its morphology is also not sufficient for attribution. For example the genus Strictispira McLean, 1971 was assigned to the family Strictispiridae based on an unusual radula and foregut anatomy (Taylor et al., 1993;[START_REF] Puillandre | The dragon tamed? A molecular[END_REF], but the molecular phylogenetic analysis finally placed the genus in Pseudomelatomidae (Puillandre, unpublished). Similarly, inclusion of Paraclavatula in Clavatulidae would be difficult on the basis of the radula alone, but was confirmed by molecular analysis. [START_REF] Nolf | Two new turrid species from West Africa: Clavatula delphinae and Clavatula pseudomystica (Mollusca: Gastropoda: Clavatulidae)[END_REF].

Abbreviations: asg, accessory salivary gland; bm, buccal mass; bt, buccal tube seen through proboscis wall; lsg, left salivary gland; mb, muscular bulb of the venom apparatus; nr, circumoesophageal nerve ring; oe, oesophagus; prr, proboscis retractors; rs, radular sac; rsg, right salivary gland; vg, venom gland.

  tripartita; Clavatula caerulea + C. pyramidata) and one with low support (PP = 0.90; C. regia + C. muricata). Since the tree based on the single gene obviously cannot resolve relationships within the family we refrain from any taxonomic decisions at the current stage. However the position of the new genus Paraclavatula within the Clavatulidae is supported. urn:lsid:zoobank.org:act:C851F468-8B91-4C14-82ED-BD0190EC5CCB Type species: Clavatula delphinae Nolf, 2008; here designated

  the absence of data on radular morphology we abstain from attributing those species to Paraclavatula. Habitat and range: The species of the genus extend along the West African coast, from Ghana (and probably from Senegal) to south Angola. No records are known from islands in the Gulf of Guinea. They live in shallow water, from low tide to 10 m, usually in sandy mud, or under rocks and rock crevices. Remarks: Paraclavatula differs from all other clavatulid genera in presence of welldeveloped, plate-like, and acuspate lateral teeth, as well as in the acuspate central teeth with elevated transverse ridges. Paraclavatula delphinae (Nolf, 2008) comb. nov. (Figs 2-3) Clavatula delphinae Nolf, 2008: 6-7, plate I, figs 1-8; plate II, figs 9-10; 16 (holotype, Royal Belgian Institute for Natural Sciences, Bruxelles (RBINS); Plage Koraf, Pointe-Noire, mouth of the Songolo River, the Republic of the Congo, 4 m) Material examined: MNHN-IM-2009-26535, shell length (SL) 11.4 mm, the Republic of the Congo, stn. CP505, 4°42.250' S; 11°47.000' E, 16-17 m, adult male

  comb. nov. (Fig. 4) Clavatula pseudomystica Nolf, 2008: 7-8, pl. III, figs 17-21, pl. IV, figs 22-25 (holotype, RBINS; Saco Mar, Namibe, South Angola, in rock crevices, 3 m) Material examined: Cacuaco Bay, Angola, 3 m, 2 specimens, coll. J. Horro.

  figs.13-18 (holotype, RBINS IG.31835; Cacuaco, Bengo Province, Angola, on intertidal rocks).

  phylogeny of the Conoidea (Mollusca, Gastropoda). Journal of Molluscan Studies, 77: 259-272.RAMBAUT, A., & DRUMMOND, A. J. 2014. Tracer v1.6. [http://beast.bio.ed.ac.uk/Tracer, 2014, accessed 14 March 2016]. TAYLOR, J.D. 1994. Foregut anatomy of the larger species of Turrinae, Clavatulinae and Crassispirinae (Gastropoda: Conoidea) from Hong Kong. In: The Malacofauna of Hong Kong and Southern China III (B. Morton, ed.), pp. 185-213. Hong Kong University Press, Hong Kong. TAYLOR, J.D., KANTOR, Yu.I. & SYSOEV, A.V. 1993. Foregut anatomy, feeding mechanisms, relationships and classification of Conoidea (=Toxoglossa) (Gastropoda).Bulletin of the natural History Museum, London (Zoology), 59: 125-169.

  Figure captions.

Figure 1 .

 1 Figure 1. Bayesian phylogenetic tree obtained with partial COI sequences. Only posterior probabilities > 0.80 are shown.

Figure 2 .

 2 Figure 2. Paraclavatula delphinae (Nolf, 2008). A-C. Shell, MNHN IM-2009-26535, SL 11.4 mm. D. Protoconch, lateral view, scale bar 0.5 mm. E-H. Radula of the same specimen. E. Dorsal view of the central part of radula. F. Enlarged central portion. G. Lateral view of central portion of radula. H. Enlarged marginal teeth, arrow marks accessory limb. Scale bars: E,G,H = 50 µm; F = 20 µm.

Figure 3 .

 3 Figure 3. Left and right views of foregut of Paraclavatula delphinae[START_REF] Nolf | Two new turrid species from West Africa: Clavatula delphinae and Clavatula pseudomystica (Mollusca: Gastropoda: Clavatulidae)[END_REF].

Figure 4 .

 4 Figure 4. Paraclavatula pseudomystica (Nolf, 2008). A-C. Specimen for which radula was examined, SL 18.3 mm. D. Protoconch, scale bar 0.5 mm. E-H. Radula. E. Dorsal view of the central part of radula. F. Enlarged central portion. G. Young forming part of radula. H.

Figure 5 .

 5 Figure 5. Paraclavatula christianae (Nolf, 2011). A-C. Specimen for which radula on figs E-F was examined, SL 14.6 mm. D. Protoconch, scale bar 0.5 mm. E-H. Radulae (E-G -Cacuaco Bay, Angola; H -Sacomar, Angola). E. Dorsal view of the radula on the bending plane. F. Enlarged central portion. G. Lateral view of central portion of the radula. H. Anterior part of the radula. Scale bars: E = 50 µm; F, G, H = 20 µm.

Figure 6 .

 6 Figure 6. Comparative morphology of the radulae of different Clavatulidae. In upper panel for each speciesdorsal view of entire transverse radular row, in lower panelenlarged central portion. A. Paraclavatula delphinae (Nolf, 2008). B. Tomellana lineata (Lamarck, 1818), MNHN uncataloged, Republic of the Congo, Bay of Pointe-Noire, 15-16 m (Port ZANAGA stn. CP510). C. Turricula nelliae (E.A. Smith, 1877), NHMUK MOEA 20100551, Danang, Vietnam. D. Clavatula xanteni Nolf & Verstraeten, 2006, MNHN IM-2007-17829, Angola, Loanda Island, 40-60 m. Abbreviations: cf, central formation; ct, central tooth; lt, lateral tooth; mt, marginal tooth.

ACKNOWLEDGEMENTS

We would like to thank Peter Ryall (Maria Rain, Austria) for sharing material and opinions about described species, and to Frank Nolf, for his help in identification.

The specimens used in the molecular analysis were collected during the following expeditions organized by the MNHN: Port ZANAGA, ATIMO VATAE, TAIWAN 2004, NANHAI 2014, EBISCO, PANGLAO 2004 and 2005. Special thanks to Bernard Thomassin, who invited Alexander Fedosov to participate in the Port ZANAGA field survey and to A.

Fedosov himself, who collected the sequenced material during this survey.

We thank, among others, Virginie Héros, Philippe Maestrati and Barbara Buge for their role in specimen processing during the expeditions and their help in curating the specimens.

We gratefully acknowledge two anonymous referees for their corrections and suggestions that improved the manuscript. This project was partly supported by the Service de Systématique Moléculaire (UMS 2700 CNRS-MNHN) and by the CONOTAX project funded by the French ANR (grant number ANR-13-JSV7-0013-01). Contribution of Yuri Kantor was funded by grant No. 16-14-10118 from the Russian Science Foundation (PI Yu. I. Kantor).

And it is always a pleasure to deal with the friendly and meticulous editors of the journal.


Loren Acher

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Davy Carole
email: davy.carole@univ-lyon1.fr

Thermochromic properties of pure NiTiO3 and its Cu-or Co-doped derivatives

Keywords: Thermochromism, C. Optical properties, C. Colour, D. BaTiO3 and titanates

Nickel titanate is a yellow-brown pigment which adopts the ilmenite crystal structure as corundum and hematite. Since Cr doped Al2O3 and Fe2O3 present a thermochromic behaviour, NiTiO3 was also studied for its potential ability to change colour with temperature. Powders were synthesized via a modified Pechini method followed by a calcination step at 700 °C. The thermochromic properties in the 20 °C -400 °C temperature range were collected via the determination of the colorimetric L*a*b* parameters. Namely, the hue continuously shifts from yellow-brown to red-brown, with L*a*b* parameters moving from 60.4/19.2/58.5 to 42.9/30.5/43.1. This color change is fully reversible even after several cycling.

Temperature dependent UV-vis spectroscopy showed that the transparency window at 550 -650 nm at room temperature shifts to 575 -650 nm at 400 °C. This fully explains the observed colour change. Finally, the effects of Cu or Co doping on NiTiO3 optical and thermochromic properties were also investigated.

Introduction

Thermochromism refers to the ability of matter to undergo a colour change vs. temperature.

Thermochromic materials are potentially used in a wide range of applications such as temperature memory sensors (cold chain break sensor, threshold temperature sensor for aeronautic application…), temperature sensors for safety (kitchen tools…), smart materials for specific application (smart windows or concrete for building energy efficiency…) or in day-to-day life as smart pigment (textile, ink, packaging…) [START_REF] White | Thermochromism in commercial products[END_REF][START_REF] Aburas | Thermochromic smart window technologies for building application: A review[END_REF][START_REF] Nguyen | Synthesis and Thermochromic properties of Cr-doped Al2O3 for a reversible thermochromic sensor[END_REF][START_REF] Ramlow | Smart textiles: An overview of recent progress on chromic textiles[END_REF]. Colour change with temperature variation can be reversible or irreversible, and continuous (gradual change in colour) or discontinuous (sudden change in colour). In the case of inorganic thermochromic compounds [START_REF] Day | Thermochromism of inorganic compounds[END_REF], the colour change can be due to a chemical composition alteration (e.g. dehydration, decarbonation, decomposition) [START_REF] Robertson | Structural transformation and thermochromic behavior of Co 2+ -doped Zn3(PO 4)2•4H2O hopeites[END_REF] leading then to an irreversible change. In contrast, reversibility may be obtained with phase transition [START_REF] Robertson | Investigation of the First-Order Phase Transition in the Co1-xMgxMoO4 Solid Solution and Discussion of the Associated Thermochromic Behavior[END_REF][START_REF] Gaudon | Unprecedented "One-Finger-Push"-induced phase transition with a drastic color change in an inorganic material[END_REF][START_REF] Gaudon | Adaptable thermochromism in the CuMo1-xWxO4 series (0≤x<0.1): a behavior related to a first-order phase transition with a transition temperature depending on x[END_REF][START_REF] Thiry | On the Cyclability of the Thermochromism in CuMoO4 and Its Tungsten Derivatives CuMo1-xW xO4 (x< 0.12)[END_REF], band gap shrinkage [START_REF] Dai | Temperature dependence of band gap in CdSe nanocrystals[END_REF] and/or change in the ligand field [START_REF] Serier-Brault | Thermochromism in yttrium iron garnet compounds[END_REF] since the integrity of the material is not affected at very first sight. In order to choose the best suited material for the desired thermochromic application, the main properties to consider are the initial colour, the transition temperature range, the colour contrast and the reversibility/cyclability if needed.

Among the reversible inorganic thermochromic material, hematite α-Fe2O3 is well-known. It has a reddish brown colour coming from two 2p(O 2-)3d(Fe 3+ ) charge transfers [START_REF] Pailhé | Correlation between structural features and vis-NIR spectra of α-Fe2O3 hematite and AFe2O4 spinel oxides (A= Mg, Zn)[END_REF]. Upon heating, the thermal expansion results in an increase of the metal-oxygen bonds distances [START_REF] Saito | The anomalous thermal expansion of hematite at a high temperature[END_REF][START_REF] Willis | Crystal structure and antiferromagnetism in haematite[END_REF]. The evolution of the local environment of Fe 3+ sites (size and degree of distortion) with increasing temperature leads to a change in the positioning of iron energy levels, which results in red shift of the optical absorption and so to thermochromic property. In the case of -Al2O3, which is isostructural to Fe2O3, Cr doping gives a red colour to the crystal at room temperature and it turns to green upon heating [START_REF] Mcclure | Optical spectra of transition-metal ions in corundum[END_REF]. This colour change is explained by a decrease of the crystal field strength with increasing temperature, which induces a downward shift in energy of the parity forbidden but spin allowed 4 A2  4 T2 and 4 A2  4 T1 electronic transitions.

NiTiO3 is a yellow-brown pigment reported as a semiconductor with an optical gap around 2.2 eV and Ni 2+ cations anti-ferromagnetically coupled. It adopts the ilmenite crystalline structure, i.e. the same as -Fe2O3 and -Al2O3 and is actively studied for photocatalytic and multiferroic applications [START_REF] Ruiz-Preciado | Nickel titanate (NiTiO3) thin films: RF-sputtering synthesis and investigation of related features for photocatalysis[END_REF][START_REF] Jing | Visible-light photocatalytic activity of novel NiTiO3 nanowires with rosary-like shape[END_REF][START_REF] Yuvaraj | Investigations on the temperature dependent electrical and magnetic properties of NiTiO3 by molten salt synthesis[END_REF] but not for thermochromic ones. Due to the above-mentionned structural similarities with known thermochromic compounds and the fact that NiTiO3 is already naturally coloured at room temperature, we suspect that NiTiO3 could also be thermochromic. This will be investigated in the present study. Besides, since metal doping of this compound is known to change its colour [START_REF] Tursun | Effect of doping on the structural, magnetic, and ferroelectric properties of Ni1-xAxTiO3 (A= Mn[END_REF], we also investigate the effect of partial Ni substitution by Cu and Co on optical properties, and specifically on the thermochromism colour rendering. These elements were chosen due to their effective ionic radius, close to the one of Ni: Cu 2+ (0.73 Å), Co 2+ (0.745 Å) and Ni 2+ (0.690 Å) (high spin configuration, coordination number: six) [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Such radius similarity explains why their insertion on Ni site is easy, as already observed by Guiet et al. for copper in the range 0 -20 at.% [START_REF] Guiet | Copper-Substituted NiTiO3 Ilmenite-Type Materials for Oxygen Evolution Reaction[END_REF]. In our case, the doping rate was limited to 2 at.% to prevent the formation of secondary phases that could trigger undesired darkening of the product.

Material and methods

Materials preparation

NiTiO3, Ni1-xCuxTiO3 and Ni1-xCoxTiO3 powders were synthesized via a modified Pechini method (protocol adapted from [START_REF] Lin | Synthesis and characterization of ilmenite NiTiO3 and CoTiO3 prepared by a modified Pechini method[END_REF]). Typically, for pure NiTiO3 sample, 3.88 g of nickel(II) nitrate (97 %, Fluka), 4.40 g of titanium(IV) n-butoxide (97 %, Sigma-aldrich) and 5 g of citric acid (99 %, Alfa Aesar) were first separately dissolved in 20 ml of ethanol. The citric acid solution was then added drop by drop to the Ti 4+ solution (the resulting solution remained translucent).

After 10 min of homogenization under stirring at room temperature (RT), the Ni 2+ solution was finally added in order to form the metal-citrate complex (the solution still remained translucent).

The overall solution was then heated at 90 °C in a water bath until the complete evaporation of the solvent and the formation of a green coloured gel. The as obtained precursor was first calcined at 300 °C during one hour, and second heated at 700 °C during three hours in air. After cooling to room temperature, a yellow-brown NiTiO3 powder was achieved. Ni0.98Cu0.02TiO3 and Ni0.98Co0.02TiO3 were prepared with copper(II) nitrate (99 %, Carl Roth) and cobalt(II) nitrate (98 %, Carl Roth) as precursors, respectively. The protocol was the same as for pure NiTiO3, the precursors being dissolved separately in ethanol and the amount of Ni precursor being adjusted according to the desired amount of dopant. Typically, for the same above mentioned proportion of Ti and citric acid in 20 ml of ethanol each, 3.71 g of Ni, 0.063 g of Cu or 0.075 g of Co precursors were dissolved in 20, 10 and 10 ml of ethanol respectively. The asprepared solutions containing dopant ions were then added in the Ni/Ti solution, just after the Ni 2+ addition.

Sample characterization

High-Temperature X-Ray Diffraction (HT-XRD) was performed using a Panalytical diffractometer (X'Pert Pro MPD) equipped with an external heating chamber provided by Anton Paar (HTK 1200N). A fast detector (PIXcel 1D), in the Bragg Brentano geometry, and a copper anode (wavelength (Cu Kα): 1.5419 Å) were used. All data were collected using the same conditions, in the angular range 12 < 2θ < 100°, with step intervals of 0.026° (in 2θ) and a scan set time of 21 minutes (scan rate of 4.2°/min). Measurements were done first at RT, then from 100 °C up to 400 °C with a step of 50 °C and finally after Back to Room Temperature (BRT).

Phase identification and structural analysis (Rietveld refinements [START_REF] Rietveld | A profile refinement method for nuclear and magnetic structures[END_REF]) were performed using the Match! software (version 3, Crystal Impact) and the Crystallography Open Database.

Morphologies characterizations were performed by Field-Emitter Scanning Electron

Microscopy (FE-SEM) in a Zeiss Merlin VP Compact apparatus using an accelerating voltage of 1 kV.

A homemade set-up was used for both qualitative and quantitative colour estimation in combination with heating and with the advantage of allowing a direct visualisation of the colour evolution in temperature (Fig.SI.1). It is composed of a heating stage (thermocouple-controlled) positioned inside a closed black box and uniformly illuminated from the top using commercial white light-emitting diodes which simulate the daylight (light temperature 5500 K). An external camera (Canon EOS 550D) equipped with a macro lens (EF-S 24mm f/2.8 STM), perpendicular to the observed sample, was used in order to take pictures from the top side after reaching the targeted temperature. Pictures were taken at RT, 50 °C and from 100 °C up to 400 °C every 25 °C.

In order to evaluate thermochromism reversibility, pictures were also taken after cooling BRT. the CIELAB parameters at room temperature. Let us mention that a color difference leading to 1< ΔE < 2 can only be detected by an experienced observer. For an unexperienced observer, ΔE has to be higher than 2. From ΔE above 3.5, a clear colour difference is noticed by anyone [START_REF] Mokrzycki | Color difference Delta E -A survey[END_REF].

Room-temperature UV-visible diffuse reflectance spectra were recorded on a Perkin Elmer UV-NIR 1050+ spectrophotometer equipped with an integrating sphere coated with Spectralon. For temperature-resolved measurements, a praying mantis accessory from Harrick equipped with a high temperature reaction chamber (UV-Vis-NIR) (HVC-VUV-5) was used. This setup was installed inside the 1050+ photospectrometer. The reflectance values were measured from 200 to 1200 nm at 1 nm intervals. The spectra were recorded every 25 °C up to 250 °C. The Kubelka-Munk equation (𝐾𝑀 = ( ) , with R the reflectance at a given wavelength) was used to transform diffuse reflection spectra into absorption ones.

Results and discussion

Pure NiTiO3

Nickel titanate adopts the ilmenite structure type (R-3c space group) [START_REF] Barth | The Crystal Structure of Ilmenite[END_REF][START_REF] Ohgaki | Electron-density distribution in ilmenite-type crystals III. Nickel (II) titanium (IV) trioxide, NiTiO3[END_REF][START_REF] Wilson | Structure and properties of ilmenite from first principles[END_REF]. The XRD pattern of undoped NiTiO3 sample recorded at room temperature was indexed with the COD (Crystallography Open Database) card no. 1544718 [START_REF] Ohgaki | Electron-density distribution in ilmenite-type crystals III. Nickel (II) titanium (IV) trioxide, NiTiO3[END_REF] and is in perfect accordance with the expected ilmenite structure (Fig. 1a, top). Considering the full width at half maximum (FWHM) of the diffraction peaks, an average crystallite size L of 55 nm was estimated using Scherrer's formula:

𝐿 = 𝐾 × 𝜆 𝐹𝑊𝐻𝑀 × 𝑐𝑜𝑠𝜃
where K is the Scherrer constant (= 0.94), λ the wavelength of the radiation and θ the diffraction angle of the peak. Here, crystallite means a coherent scattering domain, which is, of course, different from the grain observable by SEM. Indeed, SEM pictures show that NiTiO3 powder is composed of micrometric agglomerates made of 30 -120 nm large grains (Fig. 2) which decently correlates with Scherrer's formula results.

Considering now the other XRD patterns of Figure 1a, recorded with increasing temperature, no phase transformation can be detected in the range 25 -400 °C. The main effect of temperature is limited to a slight shift of all the peaks towards smaller 2θ angles (this is better shown in Figure 1b), corresponding to an increase of crystal cell volume with temperature, as expected from thermal expansion. When estimating the separate effects for a and c parameters, one can evidence a slight anisotropy in thermal expansion: c parameter increases more rapidly than a one (Figure 3). This leads to a linear increase of the c/a ratio with temperature, being found 0.07% higher at 400 °C as compared to RT value (calculated as follow: δ(c/a)/(c/a) (%)). These experimental data are in good agreement with the ones already reported by Boysen et al. who determined a variation of c/a ratio of 0.1 % between 27 and 427 °C [START_REF] Boysen | A neutron powder investigation of the hightemperature phase transition in NiTiO3[END_REF].

Coming now to the optical properties of pure NiTiO3, Kubelka-Munk transformed reflectance spectra collected from RT to 400 °C are depicted in Figure 4 in the 300-800 nm interval. The overall shape of the absorption spectra is very similar to those reported in the literature [START_REF] Guiet | Copper-Substituted NiTiO3 Ilmenite-Type Materials for Oxygen Evolution Reaction[END_REF][START_REF] Rossman | Origin of the yellow color of complex nickel oxides[END_REF][START_REF] Llusar | Synthesis and coloring performance of Ni-geikielite (Ni, Mg)TiO3 yellow pigments: Effect of temperature, Ni-doping and synthesis method[END_REF][START_REF] He | Synthesis and coloration of highly dispersed NiTiO3@TiO2 yellow pigments with core-shell structure[END_REF] namely with three observable domains.

A broad absorption takes place at 450 nm and at the highest wavelengths (>740nm) due to respectively Ni on site the 3 A2g(F)  3 T1g( 3 P) and 3 A2g  3 T1g( 3 F) spin-allowed transitions.

Absorption shoulders observed around 510 and 740 nm can be attributed respectively to spinforbidden transitions 3 A2g(F)  1A1g( 1 G) and 3 A2g(F)  1 T2g( 1 D) in one part and 3 A2g  1 Eg( 1 D) in the other part. At lowest wavelengths (320-400 nm), the O 2--Ti 4+ (250 -320 nm) and O 2--Ni 2+ (320 -390 nm) charge transfers can explain the absorption bands. In the 400-600 nm window, the 3 A2g(F)  3 T1g(3P), 3 A2g(F)  1A1g(1G) and 3 A2g(F)  1 T2g(1D) transitions are overlapped.

The former, spin allowed, is much more intense than the two latter that are spin forbidden. When temperature increases, a down shift is observed for the 3 A2g(F)  3 T1g( 3 P) that will progressively overlap the 3 A2g(F)  1A1g( 1 G) and 3 A2g(F)  1 T2g( 1 D) ones as predicted by the Tanabe-Sugano diagram associated for a d 8 configuration of Ni in octahedral site [START_REF] Sugano | Multiplets of transition-metal ions in crystals[END_REF]. At the end, the absorption at wavelengths lower than 400 nm is associated to charge transfer transitions. Namely Ni 2+ to Ti 4+ , O 2-to Ni 2+ and O 2-to Ti 4+ charge transfer may compete, the two first lying a priori at lower energy than the latter, Ni-d levels contributing both to the top and the bottom of the valence and conduction band built respectively on O-sp and Ti-d orbitals. As common, position of charge transfers is significantly impacted by temperature due to change in the orbital overlap. Let us notice that the assignment of the absorption bands may differ with authors. Hence, the band occurring in the blue region is assigned to a Ni -Ti charge transfer by Rossman [START_REF] Rossman | Origin of the yellow color of complex nickel oxides[END_REF], the observed shoulders being related to a splitting of the Ti-d block. Nevertheless, we may notice that the absorption at about 2.2 eV disappears for NiTiO3 prepared as thin films suggesting that this band is associated rather to parity forbidden d-d transition than to a charge transfer, the difference in the oscillator strength between the two transition type being of several orders of magnitude.

Whatever, the transparency window shifts roughly from 550 -650 nm to 575 -650 nm going from RT to 400 °C. This fully explains the colour change from yellow-brown to red-brown, two colours far from being saturated.

In order to get quantitative information about the effective thermochromism of NiTiO3 material, the L*a*b* parameters were determined as a function of temperature using our homemade setup (see Table 1). First of all, NiTiO3 is clearly a thermochromic material, changing from yellow-brown at room temperature to darker brown at 400 °C (Table 1). L* and b* parameters both decrease whereas a* increases with temperature. The colour change, i.e. the thermochromism, is continuous and totally reversible as L*a*b* parameters come back to their original values when BRT (see bottom of Table 1). This was repeated up to 10 times without significant change of the results. Colour contrast ΔE, calculated from these data, was used to further quantify the thermochromism of NiTiO3 (Fig. 5). The progressive increase of ΔE with temperature confirms that the phenomenon is continuous. The maximum value obtained at 400 °C is ~26, which is clearly detectable by human eyes. When comparing with ΔE values reported in the literature for other yellowish (at RT) thermochromic compounds, i.e. Y3Fe5O12 and CaZr4(P0.95V0.05O4)6, we can see that NiTiO3 is less thermochromic than Y3Fe5O12 (ΔE ~24 in the 25 -255 C range [START_REF] Serier-Brault | Thermochromism in yttrium iron garnet compounds[END_REF] while we obtained ΔE= 17.5 for NiTiO3 in the same range) but more thermochromic than CaZr4(P0.95V0.05O4)6 (ΔE ~4.1 in the 30 -170°C range [START_REF] Yanase | Substitution effect of V, Ca, and Ba on the thermochromic properties and thermal expansion of SrZr4(PO4) 6[END_REF] versus ΔE =12 for our NiTiO3 in the same range).

Note that the few values of L*a*b* parameters for pure NiTiO3 (at RT) which can be found in the literature are quite different from our results [START_REF] He | Synthesis and coloration of highly dispersed NiTiO3@TiO2 yellow pigments with core-shell structure[END_REF]. These parameters were found to vary, depending on calcination temperature (750 -950 °C), in the range 71.41 < L* < 74.6, 3.07 < a* < 8.16 and 38.48 < b* < 42.41. Note that our samples were calcined at lower temperature (700 °C) than in this study, which can explain these L*a*b* differences. Despite this, these reported CIELAB parameters leads to rather close yellow/brown hues as compared to the ones of our samples.

Having evidenced for the first time the thermochromism of undoped NiTiO3, we extended the study to the effect of dopant addition on this colour change.

Cu and Co-doped NiTiO3

The purity of the 2 % Cu and Co-doped samples was checked by XRD. No secondary phase containing copper or cobalt was detected while slight exces of NiO (bunsenite, COD card no. 9008693) and/or TiO2 (rutile, COD card no. 9009083) were found which depended on the type of sample. For Co-doping, 1.9 weight % of TiO2 and < 1 weight% of NiO was obtained. For Cu doping, only TiO2 in excess of 2.0 weight% was detected. Note that TiO2 and NiO traces are often formed during NiTiO3 synthesis, even in undoped sample [START_REF] Guiet | Copper-Substituted NiTiO3 Ilmenite-Type Materials for Oxygen Evolution Reaction[END_REF][START_REF] Ruiz-Preciado | Comparison of nickel titanate (NiTiO3) powders synthesized by sol-gel and solid state reaction[END_REF]. These two phases were also found randomly in some undoped powders but we did not notice any detectable impact of these phases on the colour of the synthesized powders. Indeed, the obtained L*a*b* parameters were found identical in the uncertainty range compared to an undoped "pure" sample.

The addition of 2 mol % Cu in NiTiO3 structure induces a colour change at room temperature from yellow-brown to brown (Table 2). This colour change can be correlated to the evolution of the UV-Vis reflectance spectrum which clearly shifts toward lower reflectance values on the entire visible range when doping with Cu (Fig. 6). Also, the reflection band centered around 580 nm for undoped NiTiO3 is shifted to lower energy (~600 nm) after Cu doping. This shift to lower energy was also observed by Guiet et al. but for higher doping level (≥ 5 %) [START_REF] Guiet | Copper-Substituted NiTiO3 Ilmenite-Type Materials for Oxygen Evolution Reaction[END_REF]. Both effects correlate well with the brownishing/darkening of the powder colour due to higher red contribution and overall higher absorption of visible light.

Interestingly, in the case of Co-doped NiTiO3, the change in colour is more pronounced since the powder becomes now greenish with 2 mol % Co doping (Table 2). Here, a clear absorption band appears around 600 nm, which could be attributed to the 4 T1g → 4 T1g (P) transition of Co 2+ in an octahedral site [START_REF] Brik | Titania-supported cobalt and cobaltphosphorus catalysts: characterization and performances in ethane oxidative dehydrogenation[END_REF]. It reduces the yellow contribution of the colour and thus shifts it towards the green (Fig. 6).

Finally, the colorimetric and thermochromic properties of these Cu and Co-doped samples were evaluated (see Fig. 7 and Tables 2 &3). Alike for undoped NiTiO3, all doped samples show a reversible thermochromism in the range 20 -400 °C. Despite generating different colors at RT, both Cu and Co doping lead to similar dark brownishing at 400 °C. Their ΔE evolution with temperature is also very similar but always lower than for undoped material. This means that doping with Cu or Co have negative impact on the thermochromic properties of NiTiO3, even though the case of Co doping is intrinsically interesting since it shifts the RT colour towards green.

Conclusion

Nickel titanate was synthesized using modified Pechini method using nickel nitrate and titanium(IV) n-butoxide. The undoped powder exhibits a thermochromic behavior with a ΔE regularly increasing up to 26 at 400 °C. When doping with Cu or Co elements, such thermochromism decreases in both cases. Co doping leads to greeninshing of the powder while Cu doping leads to brownishing.

For

  reproducibility reasons, samples were put on a dull white boron nitride crucible. It thus avoids both chemical reactivity with the metallic heating stage and metallic reflection of the light on the heating stage, which could induce colour estimation errors. A white reference material (barite BaSO4 pellet) was also placed outside of the heating stage but still in the photography-captured area for image treatment. After adjusting the white balance colour using BaSO4, the L*, a*, b* CIELAB colour parameters of the studied powders were extracted from the photographs using GIMP (Gnu Image Manipulation Program) software (2.10 version), considering an average area of 30x30 pixels taken on the powder. CIELAB is a three-dimensional colour space where L* is a measure of lightness (100 = white, 0 = black), and a* and b* are colour parameters (-a* = green, +a* = red; -b* = blue, + b* = yellow with a* and b* ranging from -120 to + 120). The colour difference, i.e. colour contrast, between room temperature and the measured temperature (ΔE) was determined using the following formula: 𝛥𝐸 = (𝐿 * -𝐿 * ) + (𝑎 * -𝑎 * ) + (𝑏 * -𝑏 * ) with L * T, a * T and b * T the CIELAB parameters at the measured temperature, and L * RT, a * RT and b * RT

Figure captions : Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .Fig. 7 .

 :1234567 Figure captions:

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .Fig. 7 .

 1234567 Fig. 1. Evolution of XRD patterns of undoped NiTiO3 from room temperature up to 400 °C: a) general patterns and b) zoom on the (113) peak of NiTiO3 position as a function of temperature

Table 1 .

 1 CIELAB parameters of NiTiO3 as a function of temperature, together with the corresponding pictures and their numerical images in order to highlight the colour evolution

	Temperature (°C)	L*	a*	b*	Real pictures	Numerical representation
	25	60.4	19.2	58.5
	50	59.4	20.3	57.8
	100	57.3	23.7	55.9
	125	56.2	25.3	54.9
	150	55.0	25.9	53.9
	175	54.0	26.9	52.7
	200	52.4	27.8	51.4
	225	51.3	28.8	50.6
	250	49.6	28.9	49.0
	275	48.6	29.1	47.7
	300	47.1	30.2	46.5
	325	46.5	30.7	46.3
	350	44.9	30.0	45.3
	375	43.9	31.2	44.3
	400	42.9	30.5	43.1
	Back to RT (30 °C)	60.5	19.3	58.8

Table 2 .

 2 Numerical

	Temperature (°C)	Cu-doped Undoped Co-doped
	20
	100
	200
	300
	400
	Back to RT (20 °C)

images of undoped, Cu-doped and Co-doped NiTiO3 samples showing the evolution of colour versus temperature

Table 3 .

 3 CIELAB parameters of Ni1-xCuxTiO3 and Ni1-xCoxTiO3 (x = 0.02) as a function of

			temperature
			Cu-doped			Co-doped
	Temperature (°C)	L*	a*	b*	L*	a*	b*
	20	46.9	17.9	40.9	44.2	0.0	31.4
	100	45.1	20.7	39.8	42.7	3.4	31.5
	200	41.7	21.5	37.2	40.3	8.3	30.6
	300	38.4	21.7	33.8	38.3 11.5 29.7
	400	35.1	21.1	30.0	35.7 13.9 28.0
	Back to RT (20 °C)	46.3	18.3	40.6	44.0	-0.1	32.4

Acknowledgments

The authors would like to thank the CTμ platform for SEM characterizations and the CMTC Grenoble-INP platform for XRD measurements. The authors are very grateful to S. Coindeau and T. Encinas for fruitful discussions.

Funding sources

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Highlights


Kaiyuan Li

Yanyan Zou

Serge Bourbigot

Jie Ji

Xianfeng Chen

Pressure effects on morphology of isotropic char layer, shrinkage, cracking and reduced heat transfer of wooden material

Keywords: Wood, Pyrolysis, Heat transfer, Pressure, Char

HAL is

Introduction

Char layer has been found as an important thermal barrier blocking the external heat flux from Please cite this article as: K. Li, Y. Zou, S. Bourbigot et al., Pressure effects on morphology of isotropic char layer, shrinkage, cracking and reduced heat transfer of wooden material, Proceedings of the Combustion Institute, https://doi. org/10.1016/j.proci.2020.07.072 pyrolyzing the raw material of burning wood and other charring materials during combustion [1] . The "charring behaviors" including shrinkage and cracking induced by combustion can significantly affect the material pyrolysis, for instance, the char fissures induced by cracking will become important paths for heat transferred into the material and flame flowing out. Li et al. [2 -5] investigated the charring behaviors of natural wood and wooden materials under regular pressure. However, these behaviors are still weakly implemented in the current modeling techniques [6 -8] as the related research is not sufficient. Hence, to enhance the accuracy of combustion modeling, in-depth study should be carried out to reveal the effects of charring behaviors on the combustion process of wood and wooden materials, which make it possible to consider the behaviors of char layer in combustion and fire safety engineering.

In the past few decades, researchers have identified the charring behaviors of wood in regular conditions and reported in several different works [9 -18] . Detail literature reviews have been conducted in the previous researches [2 , 3] therefore it is not repeated in this paper. As addressed previously, in the combustion process, the raw wood turns into porous media as part of the wood substance is gasified by pyrolysis reactions. The porous media usually has a lower inside pressure which causes the pressure difference between the internal space and surrounding environment, leading to char shrinkage. Moreover, unbalanced shrinkage led by different levels of pyrolysis across the sample depth results in char cracking and fissures. The theory of shrinkage cracking was first established from the drying processes in food and concrete industries [19 , 20] . The association between shrinkage and cracking has been identified [2 , 3] . Based on the hypothesis of shrinkage changing the ambient pressure might affect the char cracking behaviors. However, previous study [3] focused on the fracture mechanics of cracking under regular pressure without considering the effects of ambient pressure. In the past, most of the studies on the pressure effect have concentrated on the gas phase flame behaviors [21 , 22] in the combustion and fire community. There is a lack of research on the pressure effect on solid phase pyrolysis. Previous research has shown that the ambient pressure has ignorable effect on the chemical kinetics of pyrolysis [23] . Thus, the changes of thermo-physical properties and geometric deformations induced by pressure variations might strongly affect the charring behaviors. For instance, it has been found that as the ambient pressure increased the material tensile strength increased as the molecules become more compacted [24 , 25] . The change of tensile strength will significantly affect the char cracking. Besides, it has been pinpointed that the heat transfer is crucial to the pyrolysis of wood materials [26] . Therefore, the effects of charring behaviors on the Fig. 1. Experimental apparatus [2] .

heat transfer process should be addressed for the potential modification of numerical model.

To investigate the effects of ambient pressure on charring behavior and heat transfer, a number of MDF samples were pyrolyzed under different incident heat fluxes and ambient pressures. The MDF has an isotropic surface which simplifies the fire problem [27] . The mechanism of char cracking at different ambient pressures is revealed theoretically and moreover, the effects of charring behaviors on the heat transfer to the solid phase are discussed.

Experiments

Experiment facility

Figure 1 shows the configuration of the experiment facility [2] . A gas supply system with 2 pipes was used supplying both highly purified nitrogen and ambient air to a low-pressure compartment. A valve on each pipe controls the gas flowrate and thus the internal pressure and gas concentration in the compartment. The size of low-pressure compartment is 1.0 m (length) × 1.0 m (height) × 0.6 m (width). There is a 0.3 m square panel radiator inside the compartment pyrolyzing the samples with near uniform radiative heat fluxes. Digital camera was used to record the experiment processes. Apart from the regular experiments, a set of shrinkage experiments with two rulers as the holder were also conducted (refer to supplemental materials). In the shrinkage experiments, the real time vertical and horizontal shrinkages as well as the charring rate were measured. The experiments were controlled by a control system and more details can be found elsewhere [2 , 3] .

Materials

The MDF samples were made of pine from mid-China. The bulk density of MDF panel is 730 ±17 kg/m 3 the smoke in the compartment). It should be noted that the moisture was removed from the MDF samples before the experiments.

Experimental condition and procedure

The MDF samples were pyrolyzed under 3 ambient pressures (inside the compartment) of 95, 60, 30 kPa and 4 heat fluxes of 20, 30, 40, 50 kW/m 2 . The nonuniformity of incident (radiative) heat flux was found less than 2.7% of the nominal value at the sample center for the 100 mm square area.

To simplify the problem, an inert atmosphere with pure nitrogen was created inside the lowpressure compartment by replacing the air with proper ventilation process, to avoid flaming and also the effects of flame on the samples during pyrolysis. The ambient pressure inside the compartment was adjusted by varying the amount of nitrogen in the compartment using simultaneously the gas supply system and vacuum pump shown in Fig. 1 . Figure 2 presents the pressure records during the experiments. The experiment times are 2000s, 1300s, 1000s and 850 s for respective 20, 30, 40 and 50 kW/m 2 . Each experiment will be repeated for 3 times to ensure the repeatability. The experimental error is found less than 10% for all measured variables and more details regarding the experimental procedure can be referred to Ref. [3] .

Results and discussion

Shrinkages, char densities and fissures at different ambient pressures

The sample began to char and produced pyrolysis gas when the panel radiator was turned on. There was no flame due to the lack of oxygen. The sample surface started turning into black in a few seconds with noticeable deformation and shrinkage observed. After a certain period of time, the sample cracked generating a lot of smoky gas. The char fissures then took place owing to cracking and in the current set of experiments it was found that the number of char fissures remained mostly the same during the whole process.

Figure 3 shows the char residue volume, mass and densities under different experimental conditions. We define the shrinkage ratio as shrunk length over original length. As reported previously [3] , the shrinkage ratio at the vertical direction was measured as 30 ±2%, which is much higher than the one at the horizontal direction of 11 ±1%. In the literature, the longitudinal and tangential shrinkages of wood fibers are respective 3-8% and 12-15% [16] . As shown in the scanning electron microscope results in our previous work [3] , the MDF is in fact made of layers of wood fibers randomly orientated in various directions. Therefore, the average value of longitudinal and tangential shrinkages of 7.5-11.5% can be expected for the horizontal shrinkage of MDF while the current measurement falls into this range. The vertical Please cite this article as: K. Li, Y. Zou, S. Bourbigot et al., Pressure effects on morphology of isotropic char layer, shrinkage, cracking and reduced heat transfer of wooden material, Proceedings of the Combustion Institute, https://doi. org/10.1016/j.proci.2020.07.072 shrinkage of MDF is caused by the interaction between wood fiber layers and there is no data to compare in the literature. However, in the current research we did both horizontally and vertically (supplemental materials) orientated experiment to confirm the shrinkage ratio. From the experiment, it is difficult to identify the pressure effect based on the shrinkage ratio. Although Fig. 3 a shows that the char volume will drop with increasing heat flux and ambient pressure, it is seen that the variation is not able to fully overcome the uncertainty of measurement. Therefore, the char volume could be regarded as constant in the current study. Nevertheless, the differences in the measured mass of char residue under difference experimental conditions can be easily identified, as shown in Fig. 3 b. From Fig. 3 b, the char yield decreases with decreasing ambient pressure and increasing heat flux. Therefore, the same trend can be expected for the char density. It is understandable that when the heat flux increases more substance will be decomposed under higher temperature and heating rate [28 , 29] , resulting in less residue. This is in line with the findings in the literature [28 , 29] . Meanwhile, it was found that increasing pressure increases the yields of char due to the increases of secondary reactions for microscale samples [30 , 31] . Therefore, the current finding is consistent with the previous researches, however, more bench-scale experiments are demanded for engineering applications.

Figure 4 a presents the photos of char fissure. From Fig. 4 a, as the ambient pressure decreases and heat flux increases the number of fissures increases, indicating the enhanced extent of cracking. It should also be noted that there is boundary effect on cracking. As seen in Fig. 4 a, the char blisters close to the sample edges are generally larger than those at the sample center, especially for high ambient pressures. For low ambient pressures, the sizes of char blisters are more even, such as the case under 30 kPa and 50 kW/m 2 , indicating an early cracking time with weak shrinkage. The boundary effect is not involved in the current model. Figure 4 b shows the number and average area of char blisters. From Fig. 4 b, it is found that the number of char blisters increases while its average area decreases with decreasing ambient pressure and increasing heat flux. If the shape of the char blister is assumed as a square, the number of fissures at one direction on the surface can be approximated as

N 30 kPa -3 ≈ N 60 kPa -1 ≈ N 95 kPa ( 1
)
where N is the number of char fissures, which can be determined using the square root of number of char blisters minus 1. Equation ( 1) is an empirical correlation from the experimental results and in the following section we will prove its validity through theoretical analysis.

Theoretical analysis on the number of char blisters

In our previous research under regular pressure [3] , the mechanism of cracking initiation has been partly investigated, which could be caused by the thermal shock effect induced by pyrolysis shrinkage. In Ref. [3] , the surface tensile stress of each char blister induced by unbalance shrinkage is calculated. This tensile stress should not exceed the surface tensile strength according to fracture mechanics. Otherwise, new crack takes place reducing the size of char blister. The detail derivation of cracking initiation equations can be found in [3] . The current paper uses the same mechanism as the one in Ref. [3] . However, as both size of char blister and surface tensile strength change under Please cite this article as: K. Li, Y. Zou, S. Bourbigot et al., Pressure effects on morphology of isotropic char layer, shrinkage, cracking and reduced heat transfer of wooden material, Proceedings of the Combustion Institute, https://doi. org/10.1016/j.proci.2020.07.072 low pressures, the surface tensile stress should be re-calculated. For isotropic shrinkage at the sample surface, the shrinkages at the two directions on the surface plane are identical therefore the surface stress is

σ s = E ε s 1 -ν (2)
where σ s is the surface tensile stress, ε s denotes the surface strain, ν is the Poisson ratio, E is the Young's modulus. The surface strain is a function of the angle of relative shrinkage, α [3] . As shown in Fig. 5 , when α is near π /2, the surface strain is 0 (surface and lower materials have the same shrinkage) regardless of the deformation compared to raw material, whereas when it is close to 0, the surface strain is close to L s0 /L s0 . The surface strain can be expressed as

ε s = C L s 0 L s 0 = 2 D py L s 0 ctgα ( 3
)
where C is the constant related to relative shrinkage, α is the angle led by relative shrinkage of char layer, D py is the pyrolysis depth (m) which is defined as the distance from the sample surface to the location where raw material starts pyrolyzing, L s 0 is the length of shrinkage at the sample surface (m), L s 0 is the original length of sample or char blister (m). Therefore,

σ s ∝ ctgα (4)
Assuming the same shrinkage under different ambient pressures, as discussed above, the surface stress decreases when the number of char fissures increases, as the α increases. Then,

ctgα ∝ 1 N + 1 ∝ σ s ≤ σ critical (5)
where σ critical is the surface tensile strength for cracking. Detailed analysis on the angle of relative shrinkage can be referred to Ref. [3] . However, it should be noted that Eq. ( 5) cannot be used to compare the stresses at different incident heat fluxes as the pyrolysis processes are different. Very little research has focused on the mechanical properties Fig. 6. Peak surface tensile stresses at different pressures ( σ s,peak is determined using the measured data in Fig. 4 ).

of wooden materials under different pressures. Wu et al. [32] conducted a set of measurements under different pressures for corn straw bio-board which has a lower tensile strength than MDF. It was found that the tensile strength of engineering board has a linear correlation with the pressure, which is

σ critical ∝ P ( 6
)
where P is the ambient pressure, while the Young's modulus maintains nearly constant although at high pressures it might slightly decrease. One possible explanation for the trends of tensile strength and Young's modulus is that under high pressure the cross-linked structure of wood fibers is more compacted, enhancing its strength and stiffness.

Using the numerical results in Ref. [3] and Eq. ( 5) , the peak surface tensile stresses of char blister against the number of fissures under different heat fluxes are plotted in Fig. 6 . From Fig. 6 , it is seen that increasing the number of fissures reduces the surface tensile stress. According to the mechanism of shrinkage cracking, the number of fissures is mainly determined by the shrinkage during pyrolysis and the tensile strength of material. As the shrinkage under different pressures are similar, the tensile strength dominates the cracking and as it drops with decreasing pressure the peak surface tensile stress should also decrease. The decrease of stress can be seen in Fig. 6 , as a result of tensile strength variation. However, although the peak surface tensile stresses are similar with different heat fluxes, the one of 20 kW/m 2 is generally lower than the others, which is mainly caused by the longer cracking time [3] , leading to the failure of elastic assumption of MDF. Meanwhile, it is found that the calculated values, σ s,peak , are systematically higher than the theoretical tensile strengths, σ critical . This discrepancy could be a result of the uncertainties of Eq. ( 6) , boundary effect and 2D model of stress. Therefore, future works with more accurate numerical simulations are demanded.

Effects of char fissures on pyrolysis modeling 3.3.1. Mass loss rate

It has been long debated on the effect of fissure on the heat transfer during solid combustion in the community [33] . In theory, shrinkage and cracking will expose the raw material to the external heat fluxes in conduction, convection and radiation modes, therefore, enhance the heat transfer into the material. However, so far there is not any quantitative assessment on the effects of shrinkage cracking on heat transfer in the literature, perhaps due to the experimental difficulties. The current experiments lead to some new findings in this regard. Figure 7 a shows the measured mass loss rates under 50 kW/m 2 . As shown in Fig. 7 a, it has been found that overall the mass loss rates at different pres-sures are similar, however, the first peak of mass loss rate increases with increasing pressure. The difference of the first peaks at 30 and 95 kPa cases is approximately 20% which is much over the uncertainty of repeated experiments. Therefore, we can confirm that the first peaks of mass loss rate are different under different heat fluxes. Similar trends have been identified in other heat fluxes (supplemental materials). It should be noted that the major difference takes place at the early stage of mass loss rate curve while weak difference is identified in the curvatures and the second peaks. This implies that the heat transfer enhancement mainly affects the materials near the surface while the influences on the center and back zones are weak, as highlighted in Fig. 7 a. The measured surface temperatures (refer to the Supplemental material) are another evidence proving the effects of char fissures on heat transfer, which are higher under lower pressures. This is reasonable as the radiation dominates the current pyrolysis process therefore it can be expected that both the radiative heat flux and view factor decrease as goes deep into the fissure, which reduces the effects of radiation enhancement. It should also be noted that due to the higher average sample temperature and the reduction of secondary reactions [30 , 31] , the mass loss lasted longer at the late stage of experiment under low pressure (material pyrolyzes more completely), leading to low char residue mass as shown in Fig. 4 b. For the near surface zone, the mass loss rate is determined by the radiative heat flux ˙ Q R and the heat transfer process affected by the fissures. Under the same shrinkage, the amount of radiative heat flux penetrating the fissure is determined by the area of fissure which increases with increasing ctg α and decreasing N , which is

A f issure ∝ ctgα ∝ 1 N + 1 (7)
In the current paper, we proposed a simple way for correlating the first peak of mass loss rate with the radiative heat flux ˙ Q R and number of fissures N . Figure 7 b summarizes the measured first peaks and fitting result which is

˙ m ∝ 1 N + 1 0 . 5 ˙ Q R 1 . 5 (8)
where ˙ m is the first peak of mass loss rate. Equation ( 8) is empirical and it should be noted that both kinetics and heat transfer determine ˙ m , however, since MDF is identical the heat transfer dominates the change of ˙ m in this work. The first peak takes place when the char layer effect is weak therefore it is related to the depth of char layer [2 , 3] which increases with thermal conductivity and heating time using semi-infinite solid hypothesis as [2 , 34] ˙ m ∝ δ p ∝ k s t (9) Please cite this article as: K. where k s is the solid thermal conductivity and t is the time to first peak of mass loss rate which has a correlation with the radiative heat flux as [35] t ∝ 1 ˙ Q R 2 [START_REF] Spearpoint | Predicting the Ignition and Burning Rate of Wood in the Cone Calorimeter Using an Integral Mode[END_REF] Therefore, combining Eqs. ( 8) - [START_REF] Spearpoint | Predicting the Ignition and Burning Rate of Wood in the Cone Calorimeter Using an Integral Mode[END_REF] gives

k s ˙ Q R ∝ 1 N + 1 ˙ Q R 2 (11)
As k s is a material property, in Eq. ( 11) it only relates to the number of fissures, which gives

k s ∝ 1 N + 1 ( 12
)
From Eq. ( 12) , under the same incident heat flux, the thermal conductivity has a linear trend with 1/(N + 1 ) (refer to supplemental material for the limitation of Eq. ( 12) ). With this principle, the thermal conductivities of both raw material and char are simultaneously modified in the near surface zone and numerical simulations are carried out to view the effect of thermal conductivity modification.

Solid phase pyrolysis model

The current work uses a pyrolysis model that solves the pyrolysis process of material using a set of 1D mesh taking into account the chemical kinetics and physical heat transfer simultaneously. The transport process of pyrolysis gas within the solid was neglected. A general pyrolysis gas was generated at the same temperature as the local solid, and was assumed to leave the solid phase instantaneously.

Kinetics and thermo-physical properties

In the previous works, a set of thermogravimetric (TG) experiments were conducted under multiple heating rates, based on which a non-first-order ( n = 1) kinetic model with 4 parallel reactions of four components (resin, hemicellulose, cellulose, and lignin) was proposed for MDF [3] . The kinetics properties, initial mass fractions and char yields were determined using genetic algorithm previously [3] . In the pyrolysis model, the average thermal conductivity of solid phase was calculated in the volumetric basis as

k s = ρ N i=1 Z i ρ i k i ( 13
)
and it changes with the local density. The average specific heat capacity of solid phase was estimated based on the masses of components as

c s = N i=1 Z i c i (14)

Modeling results

Using Eq. ( 12) , the modified thermal conductivity was assigned to near surface zone and the typical modeling results are shown in Fig. 8 . It can be seen that with the modified thermal conductivities the first peak of mass loss rate noticeably drops as the ambient pressure decreases from 95 to 30 kPa, while the rest mass loss rates maintain nearly identical between different pressures. The modeled results match the experiments better at 30 kW/m 2 while the model over-predicts the mass loss peaks and under-predicts the curvature at 50 kW/m 2 . From the trial and error process, it is found that the depth of the modified thermal conductivity zone should be within 1 to 3 mm. It should also be noted that from the current set of experiments the thermal conductivity under 95 kPa could be over 2 times higher than its value under 30 kPa (20 kW/m 2 case).

Please cite this article as: K. Li, Y. Zou, S. Bourbigot et al., Pressure effects on morphology of isotropic char layer, shrinkage, cracking and reduced heat transfer of wooden material, Proceedings of the Combustion Institute, https://doi. org/10.1016/j.proci.2020.07.072

Conclusions

Char shrinkage and cracking and their effects on the heat transfer during pyrolysis are investigated using isotropic wooden surface under varied ambient pressures. It is found that the pressure has some effect on the char shrinkage however it is insignificant compared to the decrease of tensile strength with decreasing pressure. As a result, the char layer shrinkage creates more fissures under low pressures compared to regular pressure. It also indicates that the pressure difference between interior space of char layer and ambient environment may not be the main mechanism leading to shrinkage. The experimental result shows that as the number of char fissures increases the peak mass loss rate decreases. The increase of char fissures reduces the fissure width (area) induced by horizontal shrinkage, which could weaken the radiation heat transfer into the sample. By modifying the thermal conductivity of near surface zone, the numerical model can reproduce the different first peaks of mass loss rates at the early pyrolysis stage. However, the modeling results show that the zone affected by cracking is less than 3 mm in the current experiments and the thermal conductivity under near regular pressure could be 2 times higher than its value under low pressure.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 3 .

 3 Fig. 3. Measured char layer volume, char yield and density.

Fig. 4 .

 4 Fig. 4. Photo of char fissures and number and average area of char blisters.

Fig. 5 .

 5 Fig. 5. Surface strains when the angle of relative shrinkage equals to π /2 and 0.

Fig. 7 .

 7 Fig. 7. Typical mass loss rate curves and correlation of peak MLR against N and ˙ Q R .

Fig. 8 .

 8 Fig. 8. Modeling results with modified thermal conductivities in the near surface zone.

  and the panel thickness is 15 mm. For general charring experiments, the sample size was 100 mm square while for those shrinkage experiments the sample size was 50 mm square (to reduce Please cite this article as: K. Li, Y. Zou, S. Bourbigot et al., Pressure effects on morphology of isotropic char layer, shrinkage, cracking and reduced heat transfer of wooden material, Proceedings of the Combustion Institute, https://doi. org/10.1016/j.proci.2020.07.072 Fig. 2. Pressure records.

Please cite this article as: K. Li, Y. Zou, S. Bourbigot et al., Pressure effects on morphology of isotropic char layer, shrinkage, cracking and reduced heat transfer of wooden material, Proceedings of the Combustion Institute, https://doi. org/10.1016/j.proci.2020.07.072

Acknowledgment

This work was supported by National Natural Science Foundation of China (NSFC) under Grant Nos. 51876148 and 51774221 and the European Research Council (ERC) under the European Union's H2020 -the Framework program for Research and Innovation (2014-2020) / ERC Grant Advances Agreement no. 670747 -ERC 2014 AdG/FireBar-Concept.

Supplementary materials

1. MLR data under 20 and 40 kW/m 2 . 2. Detail derivation of thermal conductivity equation and an example in FDS model.

3. Shrinkage experiment of MDF samples with different orientations.

4. Measured surface and center temperature data under 30 and 50 kW/m 2 . Supplementary material associated with this article can be found, in the online version, at doi: 10. 1016/j.proci.2020.07.072 .