M C Seghini

F Touchard

F Sarasini

V Cech

L Chocinski-Arnault

D Mellier

J Tirillò

M P Bracciale

M Zvonek

TWENTY-SECOND INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS (ICCM22) INTERFACIAL ADHESION OF BASALT FIBRE REINFORCED POLYMER COMPOSITES

Keywords: Basalt fibres, Interface, Fibre/matrix adhesion, Fragmentation, Micro-CT analysis

The aim of the present work is to compare the effects of a commercial coupling agent, a thermal desizing treatment and a plasma polymerization process on the interfacial adhesion of basalt fibres with polymer matrix. The different basalt fibres were characterized in terms of surface morphology, by FE-SEM observations, and chemical composition, performing FT-IR analysis. Fragmentation tests allowed the determination of the interfacial shear strength (IFSS) at fibre/matrix interface, while the adhesion quality was assessed in terms of critical fragment length and debonding length. The plasma polymerization process was able to produce a homogeneous tetravinylsilane (pp-TVS) coating on the surface of basalt fibres, which resulted in a significant increase in the adhesion between basalt fibre and epoxy resin. The surface roughness of the untreated and treated basalt fibres has been measured by Atomic Force Microscopy (AFM) and the results demonstrate how surface topography can play a significant role in the domain of fiber/matrix interfacial adhesion. High-resolution microtomography (µ-CT) has been used to support the analysis of the damage mechanisms during fragmentation tests.

INTRODUCTION

In recent years, there has been substantial interest on the application of basalt fibres as reinforcement of polymer matrices. These can be proposed as a replacement for glass fibres in view of their advantages in terms of environmental cost relative to their chemical and physical properties. Basalt fibres are extruded at a temperature around 1500-1700°C from a melt of basalt rock. In particular, their similar chemical structure to glass, even though their density is slightly higher (2.8 g/cm 3 compared to 2.54 g/cm 3 of glass), makes easier its replacement. Also, the chemical stability of the basalt fibres is higher than that of glass fibres. High modulus, good strength and elastic behaviour make this kind of fibres a good alternative to the glass ones [START_REF] Fiore | A review on basalt fibre and its composites[END_REF]. Unfortunately, the mechanical performance of a fibrous composite is not only dictated by the properties of the single constituents (fibres and matrix), but also by fibre/matrix adhesion [START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF]. A crucial issue for developing basalt fibres reinforced polymer composites is to analyze and optimize their interface quality. Much academia and industry efforts have gone into increasing the fibre/matrix adhesion. Several strategies have been proposed over the years in order to enhance the interfacial adhesion of basalt fibres: (i) incorporation of nanofillers into sizing formulations [START_REF] Wei | Surface modification and characterization of basalt fibers with hybrid sizings[END_REF][4], (ii) bulk modification of the matrices by using coupling agents [START_REF] Greco | Mechanical properties of basalt fibers and their adhesion to polypropylene matrices[END_REF] [START_REF] Sarasini | Effect of basalt fibre hybridisation and sizing removal on mechanical and thermal properties of hemp fibre reinforced HDPE composites[END_REF] or by nanofillers dispersion [START_REF] Lee | Modification of surface functionality of multi-walled carbon nanotubes on fracture toughness of basalt fiber-reinforced composites[END_REF]. Another way to improve the performance of composites via interface engineering is the plasmachemical process, which is considered as one of the most effective methods to optimize both strength and toughness if materials for coating and process parameters are carefully selected [START_REF] Liu | Optimising the interfacial response of glass fibre composites with a functional nanoscale plasma polymer coating[END_REF] [START_REF] Kim | High strength, high fracture toughness fibre composites with interface control -A review[END_REF]. Unlike plasma treatment, which is used to increase the surface roughness and introduce functional groups into the surface layer of fibres with limited increase in interfacial shear strength [START_REF] Kim | Study on an oxygen plasma treatment of a basalt fiber and its effect on the interlaminar fracture property of basalt /epoxy woven composites[END_REF], the plasma polymerization process is able to produce homogeneous and pinhole free films on the fibres in a clean environment without use of solvents and at low temperature, thus minimizing the risks of mechanical properties degradation of the substrate. This process proved to be successful for glass fibres [START_REF] Marks | Plasma polymerised coatings for engineered interfaces for enhanced composite performance[END_REF]- [START_REF] Cech | The influence of surface modifications of glass on glass fiber/polyester interphase properties[END_REF] but no study, to the best of authors' knowledge, is available on basalt fibres. In this work, the surface, morphological and mechanical properties of plasma-polymerized basalt fibres with a tetravinylsilane film are investigated. Plasma-polymerized organosilicones represent a class of versatile materials with a sound scientific background and their use as a tailored interlayer to improve interfacial adhesion of surface modified basalt fibres in epoxy-based composites is tested. Comparisons with commercially sized basalt fibres and basalt fibres after sizing removal are made. The adhesion to epoxy matrix is studied by single fibre fragmentation tests (SFFT).

MATERIALS AND METHODS

Materials

A commercial continuous roving of basalt fibres (Kamenny Vek), characterized by a nominal diameter of 13 µm and by the presence of a commercial sizing optimized for epoxy resin, has been selected. Concerning the polymer resin, the epoxy Prime 27 infusion resin, supplied by Gurit, has been used for the manufacture of composite specimens.

Treatments

In order to remove the commercial sizing from the basalt surface, fibre bundles were heat treated asreceived in a tube furnace (Lenton Thermal Designs) at a temperature of 400°C in air for 1 h and then removed from the furnace and allowed to cool in room temperature air. After the thermal treatment, a plasma polymerized tetravinylsilane (pp-TVS) film has been deposited on the unsized basalt fibres by Plasma-Enhanced Chemical Vapor Deposition (PECVD) process. The polymer deposition treatment has been divided in two steps: a first non-polymerising gas plasma step, a pre-treatment with oxygen plasma (5.8 Pa, 100 W) for 30 min to remove contaminants, activate the fibre surface and improve film adhesion, and a second step of polymerising gas plasma, in which tetravinylsilane monomer Si(-CH=CH 2 ) 4 was fragmented and ionized in plasma producing excited species, free radicals and ions which were recombined forming a thin plasma polymer film onto the fibre surface. Tetravinylsilane plasma (3.8 Pa, 10 W) was employed for 15 min.

Tensile testing

Tensile properties of basalt fibres have been determined by single fibre tensile tests in accordance with ASTM C-1557. Tensile tests were carried out at room temperature using a Zwick/Roell Z010 machine equipped with a 100 N load cell. Each test has been performed in displacement control at a cross-head speed of 2 mm/min. Fibre diameter was evaluated through an optical microscope Nikon Eclipse 150L.

Single fibre fragmentation tests

According to previous works [START_REF] Seghini | Interfacial adhesion assessment in flax/epoxy and in flax/vinylester composites by single yarn fragmentation test : Correlation with micro-CT analysis[END_REF], the single fibre fragmentation test has been used to assess the interfacial properties of basalt fibres with epoxy matrix. Tests were performed with an Instron E1000 ElectroPuls test machine with a load cell of 2 kN, using a crosshead speed of 0.005 mm/min. The loading phase was stopped if the specimen failed, or when the fragmentation saturation level was achieved. This last case was defined when no new fibre breaks appeared during a subsequent strain increase by 0.5%. After each fragmentation test, all the fragment lengths were determined by using a ZEISS Axio Imager optical microscope. To study in a proper manner the interfacial debonding phenomena between the basalt fibre and the epoxy resin, a birefringence analysis has been performed for the untreated basalt fibre/epoxy, the thermally treated basalt fibre/epoxy and the tetravinylsilane plasma treated basalt fibre/epoxy systems. During optical microscopy observation, the fragmented single fibre composite samples were placed into a circular polariscope in order to observe the stress state near the fibre breaks and the interface between the single basalt fibre and the matrix.

Morphological and chemical characterization

Morphological investigations have been carried out through a field-emission gun scanning electron microscope (FE-SEM) Zeiss Auriga and a Philips XL40. The roughness of the untreated and treated basalt fibres has been measured using a Bruker Dimension Icon AFM. The chemical composition has been studied by Fourier-transform infrared (FTIR) analysis with Bruker Vertex 70 spectrometer (Bruker Optik GmbH).

RESULTS AND DISCUSSION

Basalt fibres characterization

At first, a morphological investigation of the lateral surfaces of the different basalt fibres have been carried out through FE-SEM. Figure 1 shows the micrographs found for the untreated and the heattreated basalt fibres. The comparison between Figure 1-a and Figure 1-b highlights the ability of the heat treatment in removing the protective sizing layer, exposing the superficial defects present on the as received fibres. Concerning the plasma deposition process, a continuous pp-TVS film has been homogeneously deposited on all over the lateral surface of basalt fibre after the PECVD treatment. The presence of the pp-TVS film has been demonstrated by intentionally producing some scratches on the fibre surface, Figure 2. The surface roughness of the untreated and treated basalt fibres has been measured by AFM [Seghini basalt 2019]. Differences in the surface topography of the different basalt fibres have been found. From results described in Table 1, it is possible to observe how a lower surface roughness has been found for the untreated basalt fibres, while an increase in surface roughness has been found for both thermally treated and plasma treated basalt fibres.

Average RMS roughness [nm]

As-received Basalt These results indicate that the heat treatment is able to remove, at least partially, the sizing layer present in the commercial fibres, thus exposing surface defects. The chemical composition of the treated and untreated basalt fibres was investigated employing the Fourier Transform Infrared Spectroscopy. The FT-IR spectra of as-received, thermally treated and plasma treated basalt fibres are shown in Fig. 2. The spectral analysis revealed that the characteristic bands of O=C-NH of imine at 1650 cm-1, the -NH bending at 1510 cm-1 and small peaks of -CH at 2852 and 2924 cm-1 were visible in the as-received sample (Fig. 2a) [START_REF] Kim | Formation, structure, and reactivity of aminoterminated organic films on silicon substrates[END_REF]. These bands should be due to the presence of the sizing in as-received basalt fibre and it is possible to observe how they disappeared after the thermal treatment (Fig. 2b). The presence of plasma-polymerized tetravinylsilane (pp-TVS) films deposited on thermally treated basalt fibres was confirmed by FT-IR analysis. At the wavenumber range 3700-3100 cm-1 a significant band of -OH occurs. Following, wide bands in the range 3100-2750 cm-1 are due to vibrations of -CH 2 and -CH 3 groups. Significant bands due to C=O vibration at 1699 cm-1, C=C stretching at 1651 cm-1, -CH 2 scissoring at 1455 cm-1 and =CH 2 deformation at 1418 cm-1 in vinyl group can be observed. Finally, an increase in the band belonging to Si-O-Si stretching at 980 cm-1 is clearly visible in plasma-coated sample [START_REF] Cech | Influence of oxygen on the chemical structure of plasma polymer films deposited from a mixture of tetravinylsilane and oxygen gas[END_REF] [START_REF] Davidson | The vibrational spectrum of tetravinylsilane[END_REF]. A mechanical characterization of untreated and treated basalt fibres has been performed in order to analyze the effect of the different treatments on the tensile behavior. A comparison between the tensile results found for the commercially sized basalt fibres, the basalt fibres after the thermal sizing removal process and the plasma treated basalt fibres, is presented in Table 2. Tensile test on single fibres highlights the importance of the surface defects on the mechanical behavior of basalt fibres. In particular, a reduction and an increase in tensile strength has been found for the thermally treated and plasma treated basalt fibres, respectively. This result is clearly linked to the protective effect of the sizing agents present on the surface of the commercial basalt fibre. As highlighted by the morphological analysis, the heat treatment is able to deteriorate the protective sizing layer of the fibre, producing an exposure of the superficial defects. Zinck [START_REF] Zinck | Mechanical characterisation of glass fibres as an indirect analysis of the effect of surface treatment[END_REF] have investigated the influence of the sizing layer on the mechacal properties of the fibres because it can fully or partially fill the surface cavities. The exposure to the plasma polymerization process after the heating treatment results in a partial recovery of the tensile properties of fibres. Comparing the results found for the thermally treated and plasma treated basalt fibres, an increase in the tensile strength has been detected after the pp-TVS film deposition. This recovery phenomenon may be ascribed to the healing effect of the pp-TVS deposited film on the surface defects of the thermally treated basalt fibres.

Gauge Length l 0 [mm] σ f [MPa] ε f [%]

As-received Basalt

Fibre -Matrix adhesion analysis

At first, the shape of the fibre breaks inside polymer resin was analysed. This is an indicative factor of the adhesion between basalt fibre and the epoxy matrix. As reported by Kim [START_REF] Kim | Observations of Fiber Fracture and Interfacial Debonding Phenomena Using the Fragmentation Test in Single Fiber Composites[END_REF], when a fibre fails and fractures earlier than the polymer matrix during the fragmentation test, the fibre ends may slip leaving empty space at the point of the fibre break. Figure 4 reports the optical micrographs found for the fragmented epoxy samples reinforced with the as-received (Fig. 4a), thermally treated (Fig. 4b) and plasma treated basalt fibres (Fig. 4c). The optical micrographs show that, for each sample, a black area corresponding with the fibre break zone has been detected. The micro-CT results confirmed that the black area found during the optical microscopy investigation corresponds to the empty zone inside the matrix produced during tensile loading. The untreated basalt fibre/epoxy samples exhibited a shape of breaks characteristic of a weak interface system, characterized by a wide breaking gap. On the contrary, both the epoxy samples reinforced with thermally treated and plasma treated basalt fibres were characterized by the presence of a reduced break gap between the fibre ends. As it is possible to see from the micrographs, Fig. 4 b-c, the epoxy matrix cracks forming a characteristic "bat-shape" break zone [START_REF] Schutte | The use of a single-fibre fragmentation test to study environmental durability of interfaces / interphases between DGEBA / mPDA epoxy and glass fibre : the effect of moisture[END_REF]. As reported by Feih [START_REF] Feih | Testing procedure for the single fiber fragmentation test[END_REF], this shape is distinctive of a strong interface system. The birefringence method has been used to observe the interfacial debonding phenomena between the different basalt fibres and the epoxy matrix. Table 3 reports all the debonding length values found for the different specimens. Figure 5 a-b and Figure 6 a-b show the birefringence patterns found for the as received basalt fibre/epoxy system and for the single filament epoxy specimens characterized by the presence of the thermally and plasma treated basalt fibres. From Fig. 5-b it is possible to note that the maximum of the flat colour birefringence zone coincides with the black line present at the interface between the fibre and the epoxy matrix. The length of this zone has been considered for the debonding length measurement. The as-received basalt fibre/epoxy samples have shown the highest debonding length values. A symmetrical birefringence pattern has been found, Fig. 5-a, which confirms that the saturation level occurred at the end of fragmentation test. After the thermal treatment and the polymer plasma deposition process, a decrease and a lack in the flat colour zone were detected, respectively Fig. 6-a and Fig. 6-b. After the pp-TVS deposition, basalt fibres showed an increase in compatibility with the epoxy matrix evidenced by a significant reduction in the debonding phenomenon. The measurement of the critical fibre fragment length has been performed through optical microscopy. As reported by Ohsawa [START_REF] Ohsawa | Temperature dependence of critical fiber length for glass fiber -reinforced thermosetting resins[END_REF], the critical fragment length has been evaluated using equation 1:

l c = • l (1)
where l is the average value of the fragment length, determined as the distance between the fibre ends. According to Kelly and Tyson [START_REF] Kelly | Tensile properties of fibre-reinforced metals: Copper/Tungsten and Copper/Molybdenum[END_REF], the interfacial shear strength value (IFSS) may be estimated using equation 2:

𝐼𝐹𝑆𝑆 • • ( 2
)
where d is the fibre diameter, l c is the critical fragment length and σ f (l c ) is the fibre strength at a length equal to the critical fibre length. In Table 3 are reported all the critical fragment length values, the σ f (l c ) values and the IFSS results obtained. The best results in terms of both critical fragment length and IFSS were found for the basalt fibres treated by the TVS plasma. From the results reported in Table 3 it may be observed that a marked increase in the values of IFSS was produced after the deposition of the polymeric film. Comparing the critical fragment lengths and the IFSS values found for the thermally treated and the plasma treated basalt fibres, it is possible to note that the thermal treatment seems to have a positive effect on the interfacial adhesion of the basalt fibre with the epoxy matrix.

l debonding [µm] l c [µm] σ f (l c ) [MPa] IFSS [MPa]

As-received Basalt 192. Despite the sizing degradation carried out by the thermal exposure, a decrease in the critical length and an increase in the IFSS values has been produced compared to the values found for the as-received fibres. This is an interesting result because the sizing formulations are studied and optimized in order to promote the chemical bonding across the fibre/matrix interface. A possible explanation of this result may be ascribed to the increase in the surface roughness after the fibre thermal exposure that may create static frictional stresses able to affect the IFSS value.

CONCLUSIONS

The present experimental work investigates the effects of a commercial coupling agent, a thermal de-sizing treatment and a plasma polymerization process on the fibre/matrix interfacial strength. The determination of the interfacial shear strength at fibre/matrix interface has been realized performing single fibre fragmentation tests. The study showed that the plasma deposition process is able to realize a polymeric coating able to promote the interfacial bond between the basalt fibers and the epoxy matrix. The adhesion qualities of the plasma-treated basalt fibres were found to be superior to those of commercial fibres. A reduction of the debonding and the critical fragment length as well as an increase in the value of IFSS have been obtained as a result of the pp-TVS. The study demonstrates how surface topography can play a significant role in the quality of the fiber/matrix interfacial adhesion. Indeed, an increase in the fibre surface roughness is able to promote the formation of mechanical interactions between the fibre and the polymer matrix.

Figure 1 :

 1 Figure 1: Scanning electron micrographs detailing the lateral surface of untreated (a) and thermally treated (b) basalt fibres.

Figure 2 :

 2 Figure 2: Scanning electron micrograph showing several uniformly plasma treated fibres and an intentionally scratched fibre.

Figure 3 :

 3 Figure 3: Infrared spectra of as-received (a), thermally treated (b) and plasma treated (c) basalt fibres.

Figure 4 :

 4 Figure 4: Optical micrographs of the fragmented basalt fibres: (a) as-received, (b) thermally treated and (c) plasma treated basalt fibres.

Figure 5 :

 5 Figure 5: Optical micrograph showing the interface patterns (a) and the debonding length measurement (b) for the as-received basalt fibre/epoxy samples.

Figure 6 :

 6 Figure 6: Optical micrograph showing the interface patterns for the thermally treated basalt fibre/epoxy (a) and the plasma treated basalt fibre/epoxy systems (b).

Table 1 :

 1 Average RMS roughness and IFSS values for different basalt fibres.

	3.72 ± 0.45

Table 2 :

 2 Summary of tensile properties of untreated and treated basalt fibres.

		20	2515 ± 277	2.41 ± 0.32
		30	2440 ± 164	2.6 ± 0.24
		40	2410 ± 277	2.5 ± 0.4
	Basalt 400 °C -1h	20 30	1197 ± 311 1231 ± 387	1.47 ± 0.43 1.42 ± 0.44

Table 3 :

 3 Debonding length, critical fragment length, and Interfacial Shear Strength of the untreated and treated basalt fibres.

		5 ± 80 675.2 ± 109	3591 ± 55	35.4 ± 6
	Basalt 400 °C -1h	48 ± 8	353 ± 13	3282 ± 27	60.5 ± 3
	Basalt 400 °C -1h + PECVD	23.5 ± 14	346 ± 48	3948 ± 101	75.4 ± 10

ACKNOWLEDGEMENTS

Plasma surface modification of basalt fibers was supported by the Czech Science Foundation, grant no. 16-09161S


Pierrick Roperch

Jovid Aminov

Guillaume Dupont-Nivet

Stéphane Guillot

France Lagroix

Large field impressed anisotropy of magnetic susceptibility (AMS) in metamorphic volcanoclastic rocks from the western Central Pamir with ilmeno-hematite as the main magnetic carrier

• A Field impressed magnetic fabric is the change in the anisotropy of magnetic susceptibility after a sample has been subjected to a strong DC or AF magnetic field.

• Field impressed AMS fabric, although it has been recognized for a very long time, has been the subject of very few publications in the paleomagnetic literature. This effect has been mainly described in samples with magnetite as a main magnetic carrier. This fabric is usually of low magnitude and observed mainly in nearly isotropic rock after application of static AF demagnetization or after acquisition of an isothermal remanent magnetization (IRM). The Bartang volcanoclastic sequence was deformed during metamorphism as described by Stearns et al. (2014;doi : 10.1002/2014TC003774). The temperature was not high enough to permit a TRM like remanent magnetization to be recorded except at one site (BG39). At all other sites the samples' NRMs are in a nearly demagnetized state where ChRMs are mainly controlled by the samples' magnetic fabric.

Field impressed AMS

• In the course of the paleomagnetic study, we discover that the AMS fabric was modified after imparting an IRM in the samples • Here we show results obtained on several samples. • In addition to samples that were not heated for thermal demagnetization, we also made experiments on samples heated in the lab up to 680°C.

•

  Forty four paleomagnetic sites have been sampled in a >2 km thick sequence of Cretaceous volcano-clastic rocks from the western Central Pamir mountain (Tadjikistan). • These rocks present a medium grade level of metamorphism characterized by fine grained recrystallisation of biotite and incipient schistosity. First part of the presentation : Paleomagnetic study of the Bartang volcanoclastic rocks Second part : description of the AMS fabric induced by IRM General geological map showing the location of the study area Forty four paleomagnetic sites have been sampled in a >2 km thick sequence of Cretaceous volcano-clastic rocks in the Bartang valley from the western Central Pamir mountain (Tadjikistan). Paleomagnetic sampling 44 sites were sampled along a ~2km section of the volcanoclastic sequence. 8 samples were drilled at each site in the most fine-grained siltstones. All samples except those from one site (BG39) have low intensity of Natural Remanent Magnetization (~0.01 Am-1) and low magnetic susceptibility (< 0.0004 SI). Acquisition of isothermal remanent magnetization indicates near saturation above 600mT. Thermal demagnetization of the IRM shows a main unblocking temperature between 610-650°C. Acquisition of thermoremanent magnetization (TRM) in the laboratory in a 50µT field give TRM values of ~0.5Am-1. Site BG39 is the only site with intensity of the natural remanent magnetization compatible with a TRM. All samples contain quartz, calcite, biotites and oxides. Large grains (~25 µm) of Hematite are the dominant oxides. Rutile inclusions are numerous in the hematite grains. EDS data show that all the hematite grains contain ~5% Titanium. The irregular shape indicates recrystallisation. The titanium content explains unblocking temperatures significantly lower than the hematite Curie point (~680°C). MPMS measurements (at IPGP) of samples from the Bartang volcanoclastic rocks show no evidence for the Morin transition or the Vervey transtion. Thus there is no trace of magnetite. The Morin transition is likely suppressed by 10 to 15% of ilmenite in a solid solution with hematite as suggested by the EDS/SEM data. The Bartang sequence records a w e l l d e f i n e d a n i s o t r o p y of magnetic susceptibility (AMS) fabric (a). The magnetic foliation is dipping to the E-SE with a steep dipping magnetic lineation. This fabric is consistent with the regional NNE-SSW deformation at 15Ma. The shape of the magnetic ellipsoids varies from oblate to p r o l a t e . I n A M S f a b r i c , paramagnetic minerals like biotite may contribute as much as hematite. In order to assess the magnetic anisotropy due to oxides, we have carried anisotropy of IRM in a field of 950mT and anisotropy of TRM acquired in low field by cooling below 670°C (b). The degree of magnetic anisotropy is stronger and up to 2.0. While the magnetic foliation is also dipping to the E-SE as the AMS foliation, lineations of anisotropy of TRM and IRM are distributed along a girdle. Samples were progressively thermally demagnetized. Exemples of demagnetization plots are shown in a, b and c (figure next slide). At several sites the characteristic magnetization is scattered. d) A mean ChRM was determined at 13 sites and the directions are distributed along a girdle which is not different from the one observed on the magnetic lineations of the ellipsoids of anisotropy of remanent magnetization. Site BG39 is the only site where strong magnetization and antipodal ChRMs of normal (a) and reverse polarity (b) is observed. e) A whole rock 40Ar/39Ar experiment (Gilles Ruffet, Univ. Rennes) on one sample from site BG39 provides a plateau age of 15.2±0.1 Ma that confirm metamorphism during dome emplacement in the Central Pamir.

Magnetic susceptibility along X after IRM acquisition along Z

The magnetic susceptibility is almost constant in the direction orthogonal to the IRM Comparison of the induced AMS anisotropy degree with increasing field value during IRM acquistion

The AMS fabric was measured after each step of IRM acquisition. We can see that the magnitude of the induced magnetic fabric increases with the imparted field and mirror the IRM acquisition curve.

The sample with no significative effect corresponds to one heated sample for which a large increase in magnetic susceptibility upon heating was detected. Thus the mineralogy of this sample was modified.

Samples that have been heated up to 680°C have slightly higher coercivities than non heated samples, likely due to oxidation of the titanohematite grains. This difference was also observed in the shape of the curve of the degree of the field-impressed magnetic fabric versus the magnetizing field value used for the IRM.

(see previous slide)

Can we detect the effect in hematite-rich red sandstone? Is there a fraction of metamorphic hematite in the red beds from the nearby Late Miocene sedimentary basin?

Five samples in red sandstones with hematite as the main magnetic carrier were processed. IRM acquisition shows that saturation is not reached by 2 Teslas. The field-impressed AMS is much lower and no greater than 1.03. Further work is needed to test the possibility that these sandstones contain a small amount of metamorphic hematite.

Samples from red sandstones

Conclusions

• Metamorphism in the upper greenschist facies may lead to the formation of metamorphic hematite • The hematite contains 10 to 15% of ilmenite in solid solution • Samples with this magnetic carrier present a strong fieldimpressed AMS fabric after IRM acquisition • The magnetic susceptibility decreases strongly in the direction of the IRM • AMS data may not reflect the initial magnetic fabric when they are measured after IRM acquisition


Patryk Filipiak
email: patryk.filipiak@inria.fr

Fabien Almairac

Théodore Papadopoulo

Denys Fontaine

Lydiane Mondot

Stéphane Chanalet

Rachid Deriche

Maureen Clerc

Demian Wassermann

Diffusion MRI-based Connectivity Enriched with Microstructure Information Predicts the Propagation of Cortico-Cortical Evoked Potentials

Propagation of Cortico-Cortical Evoked Potentials (CCEPs) varies depending on numerous structural features of brain tissue. In this work, we show that dMRI-based connectivity enriched with microstructure data has the potential to measure cortico-cortical communication as it predicts CCEP-based effective connectivity. Our multiple linear regression model incorporates q-space indices like Q-space Inverse Variance, Non-Gaussianity and Return to Plane Probability with minimum streamline lengths obtained from tractography to predict delays and amplitudes of the N1 peaks in CCEPs. In our experiment, we use presurgical dMRI and intrasurgical ECoG recordings of 9 patients operated on brain tumor in the awake condition.

Introduction

Propagation of Cortico-Cortical Evoked Potentials (CCEPs) varies depending on numerous structural features of the brain tissue [START_REF] Keller | Mapping human brain networks with cortico-cortical evoked potentials[END_REF][START_REF] Yamao | Intraoperative dorsal language network mapping by using single-pulse electrical stimulation[END_REF]. In this work, we show that dMRI-based connectivity enriched with microstructure data has the potential to measure cortico-cortical communication as it predicts CCEP-based effective connectivity. For this, we studied a group of 9 patients undergoing brain tumor resection in the wide awake condition.

Methods

For each of the 9 patients (5 female, aged 40 ± 13), we acquired presurgical multishell dMRI (b ∈ {400, 800, 1550, 3100} [s/mm 2 ] with {6, 13, 29, 51} directions, respectively) and intrasurgical ECoG recordings in the exposed perisylvian language area, as illustrated in Figure 1. Direct Electrical Stimulation (DES) of the cortex induced a series of repetitive CCEPs, which we quantified with delays and amplitudes of N1 peaks [START_REF] Keller | Mapping human brain networks with cortico-cortical evoked potentials[END_REF][START_REF] Vincent | Electrophysiological brain mapping: Basics of recording evoked potentials induced by electrical stimulation and its physiological spreading in the human brain[END_REF]. Then, we trained linear regression models to predict the above effective connectivity measures using variables describing structural links between DES sites and ECoG recording electrodes, i.e. (i) logtransformed streamline counts, (ii) minimum and (iii) median streamline lengths, (iv) distances measured along the surface of white matter (WM).

Taking into account that propagation of evoked potentials is related to tissue microstructure [START_REF] Goldman | Computation of impulse conduction in myelinated fibers; theoretical basis of the velocity-diameter relation[END_REF], we extended our set of variables obtained from dMRI with common tensor-based and q-space indices: Fractional Anisotropy (FA); Mean (MD), Axial (AD), and Radial Diffusivities (RD); Return to Origin (RTOP), Axis (RTAP), and Plane Probabilities (RTPP); Mean Squared Displacement (MSD), Q-space Inverse Variance (QIV), Non-Gaussianity (NG), and parallel (NG ) and perpendicular Non-Gaussianity (NG ⊥ ). Our approach was strictly data-driven. We applied stepwise regression on the full set of indices for a feature selection. Also, we arranged the streamlines in the ascending order with respect to their lengths and tested various subsets of streamlines restricted with low-Fig. 1: ECoG electrodes for recording CCEPs in Patient 1. The scheme illustrates the presurgical planning of the locations of electrodes (red circles) and stimulation sites (green crosses) using the tractography-based dissections of Arcuate Fasciculus and Superior Longitudinal Fasciculus III (both pictured jointly as blue streamlines).

pass and high-pass filters with cut-off values defined by percentiles of lengths p lower , p upper ∈ {0, 10, 20, ..., 100}.

Results

The linear regression models which used macrostructure information only (abbr. macrostructure only) produced comparable mean residuals for each of the four input measures, nonetheless a dispersion of residuals was relatively high (Table 1). Variances of the effective connectivity data were best explained by minimum streamline lengths (Figure 2), for which R 2 scores were the highest (Table 1).

The stepwise regression method gave consistent results regarding microstructure features selection (Table 1): QIV was chosen in all the four models aimed at predicting N1 delays, while FA, NG ⊥ , and RTPP appeared in the models based on the streamline lengths. Analogously, for predicting N1 amplitudes, the stepwise regression method selected FA and NG ⊥ .

The models based on a combination of macro-and microstructure data reached higher prediction accuracy than the ones using macrostructure only. However, their performance varied depending on the length of streamlines along which the microstructure indices were computed (Figure 3). While predicting the N1 delays with minimum streamline lengths and {QIV, FA, NG ⊥ , RTPP}, the R 2 scores were highest and least dispersed when all the streamlines were included. Meanwhile, for predicting the N1 amplitudes, it was better to remove a shorter half of the streamlines from the data set. Also note that the macrostructure only models visibly overestimated the low values of N1 delays and N1 amplitudes, yet understimated the high ones (Figure 3). An inclusion of the microstructure indices helped reduce this bias.

Table 1: Macro-and microstructure information allowed to predict the N1 delays and N1 amplitudes using the linear regression models. The combined model outperformed the macrostructure only variant in N1 delays (t-statistic: -1.72) and N1 amplitudes (t-statistic: -1.19). The included microstructure indices are provided in the last column. Abbreviations: QIV -Q-space Inverse Variance, FA -Fractional Anisotropy, NG ⊥ -perpendicular Non-Gaussianity, and RTPP -Return To Plane Probability.

Discussion

We hypothesized that the propagation of N1 peaks in CCEPs depends on the macro-and microstructural properties of the WM fibers connecting the pyramidal cells excited with DES and the distal recording areas. The observed increases of the R 2 scores and decreases of the mean squared residuals after including the microstructure-related indices in our linear regression models imply tangibly that these variables contained information about the propagation of CCEPs and thus can serve as a measure of cortico-cortical comunication.

The presence of FA among the features selected with the stepwise regression method is by far the most intuitive. This tensor-based index relates, although non-specifically, to numerous properties of WM microstructure, including fiber density, neurites dispersion, axonal diameter, and myelination level [START_REF] Basser | Microstructural and physiological features of tissues elucidated by quantitative-diffusion-tensor MRI[END_REF]. Also note that QIV appeared systematically in all the studied regression models for the N1 delays. As pointed out by Fick et al. [START_REF] Fick | A sensitivity analysis of q-space indices with respect to changes in axonal diameter, dispersion and tissue composition[END_REF], this index reflects changes in tissue composition and is probably more sensible in doing so than FA. Another notable example is RTPP which is typically attributed to the length of a pore [START_REF] Özarslan | Mean apparent propagator (MAP) MRI: A novel diffusion imaging method for mapping tissue microstructure[END_REF]. Additionally, it doesn't require an angular integral and doesn't suffer from the power law phenomena [START_REF] Veraart | Universal power-law scaling of water diffusion in human brain defines what we see with MRI[END_REF]. However, RTPP feature was selected only once, so its relevance in predicting the effective connectivity measures might be questionable. Finally, the role of NG ⊥ is most difficult to interpret. This index quantifies the non-Gaussian, hence restricted, portion of the diffusion signal measured perpendicularly to the principal fiber direction. However, one must keep in mind that NG ⊥ is computed under the assumption of an axial symmetry of a tissue, which is often not satisfied.

Conclusion

In this study, we showed that brain tissue microstructure features help explain the propagation of CCEPs. Particularly, the N1 delays and amplitudes measured instrasurgically in brain tumor patients were linearly related to FA and q-space indices quantifying axon dispersion and WM tissue composition. We believe that our findings extend the clinical significance of microstructure indices and contribute to the goal of understanding the propagation of CCEPs.

Fig. 2 :

 2 Fig. 2: Scatter plots showing the positive correlation between: (a) N1 delays and minimum streamline lengths, (b) N1 amplitudes and the inverse of minimum streamline lengths (both in Patient 7).

Fig. 3 :

 3 Fig. 3: The accuracy of prediction of N1 delays and N1 amplitudes varied depending on the lengths of streamlines used for computing microstructure information. The mean R 2 scores with standard deviations for various lower and upper cut-off percentiles are shown for (a) N1 delays and (d) N1 amplitudes. The four remaining figures are Bland-Altman plots illustrating the prediction accuracy obtained with the leave-one-patient-out cross-validation for the macrostructure only cases (b and e) or macro-and microstructure cases with best cut-off values.

Acknowledgements

This work has received funding from the ANR/NSF award NeuroRef; the MAX-IMS grant funded by ICM's The Big Brain Theory Program and ANR-10-IAIHU-06.