Elodie Drouelle

Véronique Brunet

Jonathan Cormier

Patrick Villechaise

Pierre Sallot

Foad Naimi

Frederic Bernard

Sylvain Dubois

Oxidation resistance of

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I -Introduction

MAX phases are a family of nanolaminated ternary nitrides and carbides, with the general formula Mn+1AXn (n=1 to 3), where M is a transition metal, A is an element from group A, and X is either carbon or nitrogen 1 . Depending on the n value, they are now often named 413 (n=3), 312 (n=2) or 211 (n=1) MAX phase. These compounds were first discovered in the 1960s 2-4 , however, they were revived in 1995 when Barsoum et al. discovered that they exhibit unique physical and chemical properties 5 . The three compounds crystallize in a hexagonal P63/mmc lattice characterized by M6X octahedra separated by A atomic layers. Their unique structure, combining both strong covalent M-X bonds and weaker M-A bonds, confers to MAX phases properties of both classes of materials. Like ceramics, they are stiff, lightweight, chemically stable and oxidation resistant. Like metals, they are relatively soft, machinable, resistant to thermal shock and they exhibit good electric and thermal conductivity as well as good damage tolerance [5][6][7] .

Among the 312 phases, Ti3AlC2 and its solid solution have attractive properties. Ti3AlC2 possesses quite remarkable properties such as low density (4.25g/cm 3 ), high Young's modulus (297 GPa), high strength at high temperature, self-lubrication, good machinability, high thermal and electrical conductivity etc [8][9][10] . Moreover, Alumina-forming MAX phases are well-known for their excellent oxidation resistance 11-19b , rivaling many metallic NiAl, NiCrAl, and FeCrAl counterparts PhD thesis Elodie and with upper temperature capability possible to ~1400 °C. Hence, these remarkable properties make Ti3AlC2 a promising material for various applications.

However, before they can be used it is imperative to be able to predict the oxide thicknesses that would form after long times at elevated temperatures. This, in turn implies that the oxidation kinetics be well understood and documented. Pietzka and Schuster first synthesized bulk polycrystalline Ti3AlC2 by sintering cold-compacted powder mixtures of Ti, TiAl, Al4C3 and C at 1573 K under pure hydrogen for 20h [START_REF] Pietzka | Summary of constitutional data on the Aluminum-Carbon-Titanium system[END_REF] . Tzenov and Barsoum prepared bulk polycrystalline Ti3AlC2 (4 vol.% Al2O3 as impurity in the sample) by reactive Hot Isostatic Pressing (HIP) at 70MPa and 1400 o C for 16h using a mixture of Ti, C and Al4C3 powders and investigated some properties of the as-synthesized samples 8 . Wang and Zhou produced fully dense Ti3AlC2 using a solid-liquid reaction synthesis and simultaneous in situ hot pressing process using a mixture of Ti, Al, and graphite as starting powders 21 . Zhou et al. fabricated Ti3AlC2 by Self-propagating High-temperature Synthesis (SHS) from elemental Ti, Al and C, followed by a densification step using the Spark-Plasma Sintering (SPS) method [START_REF] Zhou | Synthesis and mechanical properties of Ti3AlC2 by spark Plasma Sintering[END_REF] . However, the purity of the Ti3AlC2 was not easy to control, because of the thermal explosion that occurred among the elemental raw materials. Zhu et al. synthesized Ti3AlC2 by hot pressing TiC, Ti, and Al powders at 1573-1673 K for 2 h under a pressure of 30 MPa 23 . Finally, a variety of methods are implemented to prepare Ti3AlC2 bulk samples or Ti3AlC2 powder samples. Nevertheless, it is difficult to obtain a single-phase Ti3AlC2 material due to the narrow phase domain in the Ti-Al-C phase diagram. Impurities such as TiC, TixAly intermetallics, Al2O3 or the 211 MAX phase (Ti2AlC) are commonly detected in the end-products 10,[24][25][26][27][28][29][30] . Especially, the presence of TiC invariably deteriorates the high electrical conductivity 7 , the oxidation resistance 18 and some other properties of Ti3AlC2. Thus, several sintering additives such as Si 31 , Sn 32,33 and B2O3 34 were introduced into Ti-Al-C system to avoid impurity formation and to get the Ti3AlC2 single phase. The former research work by Ai et al. [START_REF] Ai | Synthesis of Ti3AlC2 powders using Sn as an additive[END_REF] and Li et al. [START_REF] Li | Formation of a singlephase Ti3AlC2 from a mixture of Ti, Al and TiC powders with Sn as an additive[END_REF] has proved that Sn is an efficient additive to inhibit the TiC impurity formation. A nearly pure Ti3AlC2 was thus obtained in a relative wide temperature range by pressureless sintering 3Ti+Al+1.8C with 0.2 mol of added Sn. Ai et al. [START_REF] Ai | Synthesis of Ti3AlC2 powders using Sn as an additive[END_REF] also detected some Sn in the final products by Energy Dispersive X-ray Spectrometry (EDXS) and reported more Sn distributed in the grain boundary rather than in the Ti3AlC2 structure. Bei et al. have studied, in details, the synthesis of Ti3Al1-xSnxC2 solid solution by Hot Isostatic Pressing 33b,33c .

The first paper describing the oxidation behaviour of Tin+1AlCn has been published in 2001 14 . Since that date, the high-temperature oxidation behavior of Ti3AlC2 has been extensively studied 11-19c ; a review paper can also be found 15 . However, to our knowledge, the oxidation behaviour of Ti3Al(1-x)SnxC2 solid solution has not been studied. Barsoum studied the oxidation behavior of Ti3AlC2 at 1073-1373 K in air for exposure times up to 100 h 8,14 . The oxide layers mainly consist in a rutile-based solid solution, (Ti1-yAly)O2-y/2 with y< 0.05, and some Al2O3. Barsoum's specimens formed thick and highly striated oxide layers during hightemperature oxidation. Barsoum further explained that the subjection of the (Ti1-yAly)O1-y/2 layer to an oxygen chemical potential gradient resulted in its demixing, with the dissolution of the Al 3+ into the rutile at a lower oxygen partial pressure and its precipitation as Al2O3 at a higher partial pressure. When the demixing occurred extensively, the resulting microstructure became highly striated, consisting of three layers, an Al2O3-rich layer, an (Ti1-yAly)O1-y/2-rich layer, and a porous layer, which repeated numerous times 8 . The C atoms were presumed to diffuse through the reaction layers and oxidize. The rate-limiting step was thought to be the diffusion of oxygen and/or Ti 8 . Wang and Zhou studied Ti3AlC2 oxidation behavior at 1273-1673 K in air for exposure times up to 20h 11 . The results indicated that Ti3AlC2 oxidized following parabolic kinetics, and its excellent oxidation resistance was attributed to the formation of an adherent, continuous inner Al2O3 layer, which formed below an outer TiO2 layer. It is proposed that the inner Al2O3 layer might has been formed by the inward diffusion of oxygen through the outer TiO2 layer, and the outer TiO2 layer by the outward diffusion of Ti. In 2006, Lee and Park 19b reported that the oxidation kinetics can lead either to the predominant formation of TiO2 layers or to the predominant formation of -Al2O3 layers.

Needless to add, the samples that formed an Al2O3 layer were quite oxidation resistant. In such a case, an inner, thick Al2O3 barrier layer formed below the outer, thin TiO2 layer 11,15,17,19b . However, when the oxide layers formed were TiO2-based, the resulting oxidation resistance was poor. In such a case, either alternating (thick TiO2 layer)/(thin Al2O3 layers) or a thin, intermediate Al2O3 barrier layer plus a thick, (TiO2+Al2O3)-mixed layer formed 14,15,19b . In all cases, the outermost scale is consistently composed of TiO2 14,15,19b . It has been demonstrated that the thin Al2O3 layer forms first due to the fast diffusion of Al in the basal plane of the MAX phase 19b-19e . The TiO2 layers that forms on top of the Al2O3 layer results from an outward diffusion of Ti atoms through the grain boundary of preformed Al2O3 19b,19d . Finally, the (TiO2+Al2O3)-mixed layer results from an inward diffusion of oxygen 19b .

In this context, the present study takes the advantage of the comparison of Ti3AlC2 and Ti3Al0.8Sn0.2C2 solid solution synthesized by SPS in order to better understand the role of Sn substitution on A sites on oxidation properties. Hence, oxidation products, kinetics and resistance of the Ti3AlC2 and Ti3Al0.8Sn0.2C2 compounds are compared to finally conclude on the influence of the Al substitution by Sn on the oxidation resistance of 312 MAX phases.

II-Experimental procedures

Ti3AlC2 samples were synthesized by powder metallurgy. Titanium, aluminum and titanium carbide powders were blended in quasi stoichiometric proportions (i.e. 1.9TiC-1.05Al-Ti) for 20 min in a Turbula® mixer. The reactant powder mixture is encapsulated in a glass container in vacuum and thus, sintered in argon atmosphere for 2h at 1450°C to produce porous Ti3AlC2 bulk material which is subsequently crushed into powder. After the Ti3AlC2 synthesis, XRD analysis was systematically performed; the corresponding X-Ray diffractograms (not shown) do not show any impurity at the X-Ray scale. The same is true for the Ti3Al0.8Sn0.2C2 commercial solid solution provided by Jinghe Technology Co., Ltd. Beijing. For comparison, this in-house Ti3AlC2 powder and a commercial Ti3Al0.8Sn0.2C2 batch were densified by Spark Plasma Sintering (SPS, model HPD 125 FCT system installed in a "Flash Sintering platform" within the University of Burgundy) in vacuum at 1315°C for 2 min at a pressure of 75 MPa with a cooling rate adjusted to 70°C/min. The sample diameters were 60 mm. X-Ray Diffraction (XRD) analyses were performed using a Bruker D501 diffractometer with Cu-Kα radiation in order to identify the impurities contained in bulk materials and also to identify the oxidation products. XRD data were refined using the MAUD software in order to extract the composition of the different Ti3AlC2 and Ti3Al0.8Sn0.2C2 samples [START_REF] Lutterotti | MAUD: A Friendly Java Program for Material Analysis using Diffraction[END_REF] . After an appropriate mechanical polishing of the SPSed samples, Polarized Light Microscopy method (PLM, Zeiss Imager Vario Z2) was used to investigate the grain size distribution and to quantify the amount of the different impurities. The porosity content was measured using the Archimedes method.

The microstructure of the Ti3AlC2 and Ti3Al0.8Sn0.2C2 bulk samples (grain size, distribution of porosity, location and composition of secondary phases) and the one of the oxidation products (morphology and composition of the oxide scale) were examined by Scanning Electron Microscopy using a high resolution field-emission gun scanning electron microscope (FESEM, Jeol 7001F-TTLS) coupled with energy-dispersive X-ray spectrometer (EDXS, Oxford Energy) for chemical analyses.

Cylindrical (diameter: 6-10mm, thickness: 0.5-2mm) and parallelepiped (2.5X2.5X3 mm 3 ) specimens were polished with SiC paper up to 2400 grade prior to oxidation tests.

Oxidation tests were performed under isothermal conditions over the temperature range 800-1000°C. The oxidation kinetics of Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX phase compounds were measured using isothermal thermogravimetric tests (Setaram Setsys TG16 and Setaram Setsys Evolution) under synthetic dry air for 30 to 100h. Heating and cooling were performed in argon atmosphere.

III -Results and discussion

III-1-Ti3AlC2 and Ti3Al0.8Sn0.2C2 microstructural characterization

Figure 1 shows the XRD patterns obtained on the Ti3AlC2 and Ti3Al0.8Sn0.2C2 bulk samples. Rietveld refinement of the XRD data underlines the presence of about 0.1-0.2 wt. % of titanium carbide in the sintered samples. As shown in figure 1, the X-ray peaks of the solid solution are displaced to smaller angles reflecting an increase of the cell parameters. Rietveld refinement allows determining the cell parameters (a=3.0747(6) Å and c=18.5720(5) Å) and (a=3.0841(6) Å and c=18.5865(5) Å) of the Ti3AlC2 and Ti3Al0.8Sn0.2C2 compounds respectively. The cell parameters of Ti3AlC2 and of Ti3Al0.8Sn0.2C2 are in very good agreement with the ones published previously 20, 33b, 33d, 33e .

Figure 2 shows back-scaterred SEM micrographs obtained on the Ti3AlC2 and Ti3Al0.8Sn0.2C2 SPSed samples. Figure 2 and EDXS analyses allow demonstrating the presence of small amount of TixAly intermetallics (variable x and y contents) and Al2O3 grains randomly distributed at Ti3AlC2 and Ti3Al0.8Sn0.2C2 grain boundaries. EDXS analyses enable to confirm the presence of TiC. Al2O3 and TixAly impurities are either formed during the powder synthesis or during the densification by SPS. However, TiC is very likely formed during the SPS densification as its presence was not detected by XRD after the powder synthesis. As Al2O3 displays rather weak XRD lines and as some of the alumina may be amorphous, this alumina phase is not detected using XRD although the amount of Al2O3 is in the same range as the one of TiC. In the same vein, TixAly may not be detected as a consequence of the large composition range of the formed TixAly and also as a result of the superimposition of the main TixAly peaks with the ones of the MAX phase. Using the light microscopy protocol, one can notice that:

-The amount of pores is about the same for both materials (in the range 1-2 vol.% in Ti3AlC2 and 1-3 vol.% in Ti3Al0.8Sn0.2C2) -The amount of Al2O3 is slightly larger in Ti3AlC2 (in the range 1.2-1.4 vol.%) compared to Ti3Al0.8Sn0.2C2 (in the range 0-1.3 vol.%).

-The amount of TixAly intermetallics is slightly larger in Ti3Al0.8Sn0.2C2 (in the range 1-5 vol.%) compared to Ti3AlC2 (0.2-1 vol.%).

-The grain size, as determined from the PLM method and from Electron Backscattered Diffraction, is 4.4 m for the Ti3Al0.8Sn0.2C2 grains and 3.9 m for the Ti3AlC2 ones (mean equivalent diameter).

III-2-Oxidation behaviors

Figure 3 shows the weight gain per surface area recorded as a function of the oxidation time and at different temperatures on the Ti3AlC2 and on the Ti3Al0.8Sn0.2C2 solid solution. One can notice that the weight gain is very high for the Ti3Al0.8Sn0.2C2 solid solution compared to the one obtained for Ti3AlC2. As an example, it is about 3 g.m -2 for Ti3AlC2 after 30h at 1000°C and around 300 g.m -2 for the solid solution.

After a transient stage, the oxidation kinetics, m/A=f(t), can be described either by a parabolic or by a cubic law as shown for the Ti3AlC2 SPSed samples in figure 4. It has to be noticed that M. W. Barsoum et al. and J. L. Smialek have shown that a cubic law better fits the experimental results than a parabolic one 15,16 . It thus makes more sense to assume the kinetics to be cubic.

Nevertheless, in order to compare our results with other published results on more conventional alloys, a parabolic law was chosen to evaluate corresponding parabolic rate constants (kp). The obtained values of the parabolic rate constant are given at different temperatures for Ti3AlC2 and Ti3Al0.8Sn0.2C2 in table 1. A comparison of the oxidation kinetics of Ti3AlC2 and Ti3Al0.8Sn0.2C2 shows that the parabolic rate constants of Ti3Al0.8Sn0.2C2 are, at the same temperatures, 2-4 orders of magnitude greater than those of Ti3AlC2. Such a result demonstrates that Ti3AlC2 has better oxidation resistance than Ti3Al0.8Sn0.2C2. Figure 5 shows the Arrhenius plot of kp versus the inverse of temperature. For both materials, a linear fit is obtained. The slope of the straight line allows calculating the apparent activation energies which are 69 and 336 kJ.mol -1 for Ti3AlC2 and Ti3Al0.8Sn0.2C2 respectively. Such a slope can be referred to a true activation energy only in cases where the rate-controlling step is well defined, unique and controlled by diff usion through the oxide scale. It may be the case for the fine-grained Ti3AlC2 where Al2O3 is the main formed oxide (see figure 8b and9) however, it is unlikely the case for the solid solution. Thus, the slope of the straight line refers to an apparent activation energy.

The apparent activation energy determined for the fine-grained Ti3AlC2 sample is smaller than the ones that have been published previously (in the range 136-258 kJ.mol -1 ) 11,17,18 . Thus, the apparent activation energy that is obtained for our fine-grained Ti3AlC2 specimens (69 kJ.mol - 1 ) implies that the first stages of oxidation, which allows alumina formation, is easier than the ones implied for fine-grained Ti3Al0.8Sn0.2C2. One has also to notice that the parabolic rate constants are less temperature dependent for the former specimens.

Figure 6 shows a cross-section view of the oxide layer observed by SEM on the Ti3AlC2 SPSed specimen oxidized during 1000h at 1000°C. Ti, Al, C and O corresponding EDXS maps of the oxide layer are once more presented. After 1000h of oxidation, the thickness of the oxide layer, mainly composed of Al2O3, is about 3 m. Some nodules of TiO2 are observed on top of the Al2O3 layer. Such a microstructure of the oxide scale has been observed in a lot of works 11,12,15,19c,19d,19e and is characteristic of a Ti3AlC2 with a good oxidation resistance 15 . The Al2O3 layer effectively inhibited the oxidation, the scale being dense and adherent; carbon is not detected in the oxide layer. As demonstrated by Rao et al. 19e One can also notice that carbon is very likeley present inside the oxide scale. Such a microstructure of the oxide scale has been observed and discussed by Lee and Park 19b . The authors have shown, using Pt marker on the Ti3AlC2 surface, that Al2O3 thin layer is primarily formed and that TiO2 nodules are formed on top of the Al2O3 thin layer by outward diffusion of Ti 4+ . Song et al. 19d , noticing that the amount of TiO2 after 0.5 and 80 h oxidation remains the same, suggest that the TiO2 nodule formation is terminated once a dense and continuous Al2O3 layer is formed. Lee and park 19b demonstrated that the mixed (Al2O3+TiO2) layer results from the internal oxidation of Ti3AlC2 down to the thin Al2O3 layer ; it is formed due to inward diffusion of O 2-when the alumina scale is not dense enough.

Figure 8 shows a cross-section of the oxide layer observed by SEM on the Ti3Al0.8Sn0.2C2 SPSed specimen oxidized for 30h at 850°C, simultaneously with Ti, Al, C, O and Sn corresponding EDXS maps of the oxide layer. The oxide thickness is about 30 m. It consists in six layers: the first one, adjacent to the MAX phase, is a mixture of TiO2 and Al2O3, the second one consits in a tin oxide whose stoechiometry has not been determined due to the proximity of the other oxides (it is nevertheless quite reasonable to assume that the tin oxide consists in SnO2), the third layer is also a mixture of TiO2 and Al2O3, the fourth one is a tin oxide, the fifth one is Al2O3 and finally the last one consists in TiO2. Some pores are observed inside the oxide scale, mainly at the interface between the external TiO2 layer and the Al2O3 adjacent layer. It is now well known that, during the oxidation of Ti3AlC2 and Ti2AlC MAX phases, a protective -Al2O3 layer forms first followed by a discontinuous TiO2 layer formed on top after long time of oxidation 19b,19d,19e . The lower bond-breaking energy of Al-Ti bonds in the MAX phase is the driving force of this preferential oxidation process. During the oxidation of the Ti3Al0.8Sn0.2C2 solid solution, it is possible that a SnO2 layer forms first. Indeed, ab initio calculations of the Al-Sn substitution predict a cohesive energy reduction of 21.7% which is supposed to contribute to a reduction in the thermal stability and to an easier mobility of Sn compared to Al in the A planes of the MAX [START_REF] Liu | Theoretical Investigation of A-Element Atom Diffusion in Ti2AC (A=Sn, Ga, Cd, In, and Pb)[END_REF] . Once a first SnO2 layer is formed, a continuous -Al2O3 layer forms followed by the formation of the discontinuous TiO2 upper layer. Due to the inward diffusion of oxygen through the grain boundary, the internal oxidation of the solid solution can continue and lead to the multilayered microstructure shown in figure 8.

After oxidation at 950°C for 30h, the same oxides are formed as shown in figure 9. The Al2O3 and TiO2 external layers are mixed at this temperature to form a single layer consisting in a crystallite's mixture of the two oxides. Moreover, the adhesion of the oxide layer seems to be poorer than the one formed at 850°C; the external TiO2, Al2O3 and SnO2 layers are scaled (see red arrow in figure 9) whereas microcracks and debonding are observed in the oxide layer and also at the interface between the 312 phase and the oxide layer (see blue arrow in figure 9).

Oxidation tests performed on Ti3Al0.8Sn0.2C2 solid solution demonstrate the harmful effect of Al substitution by Sn atoms. Although the tin oxide formation is thermodynamically less favourable than the Al2O3 or TiO2 ones, the atomic position of tin atoms in the hexagonal structure of the MAX phase (in the A planes) and the weak M-A bonds lead to an easy outward diffusion of Sn (and Al) atoms to the interfaces (oxide/air, MAX/oxide). Moreover, a quite large amount of non protective rutile TiO2 is formed on the surface of the Ti3Al0.8Sn0.2C2 solid solution. Such a deleterious TiO2 formation likely results from the Al/Ti atom's ratio, about 0.27 in this case, which is far from 0.32. Indeed, Li et al. have shown that, for Al/Ti atom's ratio smaller than 0.32, aluminum atom supply at the MAX phase/oxide interface is insufficient to inhibit TiO2 growth 19 .

IV-Summary

Highly pure (secondary phases in the range 2-5 weight %) and dense (porosity smaller than 2 vol.%) Ti3AlC2 and Ti3Al0.8Sn0.2C2 bulk samples have been synthesized by coupling natural sintering and SPS densification. The PLM method demonstrates that grain size is small (mean equivalent diameters are smaller than 5 m). Finally, oxidation tests were performed on the Ti3AlC2 compound and on the Ti3Al0.8Sn0.2C2 solid solution. It is demonstrated that the substitution of Al atoms by Sn atoms in the A plane of the MAX phase has a harmful effect on the oxidation resistance. Indeed, the oxidation kinetics is very high for the solid solution compared to the one of the Ti3AlC2 compound (parabolic rate constant is 3 order of magnitude smaller in the case of Ti3AlC2). The presence of Sn atoms in the A planes of the solid solution leads to an easy diffusion of Sn atoms out of the MAX phase. As a result, tin oxide, a non-protective oxide, is formed. Moreover, Al/Ti atom's ratio being small, non-protective rutile-TiO2 is also formed. The oxide layer, on top of the Ti3Al0.8Sn0.2C2 solid solution thus consists in TiO2, SnO2 and Al2O3. On the opposite, the oxide layer formed on top of the Ti3AlC2 phase mainly consists in a protective Al2O3 layer; a few TiO2 grains being observed on top of this oxide layer.

Figures captions

Figure 1: XRD patterns obtained on Ti3AlC2 and Ti3Al0.8Sn0.2C2 SPSed samples.

  , -Al2O3 nuclei nucleate on the Ti3AlC2 matrix and grow in the initial stage of oxidation, forming the inner Al2O3 oxide scale. Once the depletion of Al atoms occurs in the surface layer of Ti2AlC matrix, Ti atoms start to diffuse out and form the outer TiO2 layer.

Figure

  Figure 7a and 7b show a cross-section view of the oxide layer observed by SEM on the

Figure 2 :

 2 Figure 2: Back-scattered SEM micrographs of (a) Ti3AlC2 and (b) Ti3Al0.8Sn0.2C2 SPSed

Figure 3 :

 3 Figure 3: Weight gain per surface area unit (m/A) measured as a function of time at different

Figure 4 :

 4 Figure 4: (a) (m/A) 2 and (b) (m/A) 3 versus time. Experimental data have been recorded on

Figure 5 :

 5 Figure 5: Arrhenius plot of the parabolic rate constant as a function of the inverse of temperature

Figure 6 :

 6 Figure 6: Cross-section view of the oxide layer observed by SEM on the Ti3AlC2 SPSed

Figure 7 :

 7 Figure 7: Cross-section view of the oxide layer observed by SEM on the Ti3AlC2 SPSed

Figure 8 :

 8 Figure 8: Cross-section view of the oxide layer observed by SEM on the Ti3Al0.8Sn0.2C2 SPSed

Figure 9 :

 9 Figure 9: Cross-section view of the oxide layer observed by SEM on the Ti3Al0.8Sn0.2C2 SPSed

Figure 1 :

 1 Figure 1: XRD patterns recorded on Ti3AlC2 and Ti3Al0.8Sn0.2C2 SPSed samples.

Figure 2 :Figure 3 :

 23 Figure 2: Back-scattered SEM image of (a) Ti3AlC2 and (b) Ti3Al0.8Sn0.2C2 SPSed samples.

Figure 4 :

 4 Figure 4: (a) (m/A) 2 and (b) (m/A) 3 versus time. Experimental data have been recorded on

Figure 5 :

 5 Figure 5: Arrhenius plot of the parabolic rate constant as a function of the inverse of temperature

Figure 6 :

 6 Figure 6 : Cross-section view of the oxide layer observed by SEM on the Ti3AlC2 SPSed

Figure 7 :

 7 Figure 7 : Cross-section view of the oxide layer observed by SEM on the Ti3AlC2 SPSed specimen oxidized during 24h (a) and 500h (b) at 800°C. Ti, Al, C and O corresponding EDXS maps of the oxide scale. A schematic representation of the different oxide's layer is also given.

Figure 8 :Figure 9 :

 89 Figure 8: Cross-section view of the oxide layer observed by SEM on the Ti3Al0.8Sn0.2C2 SPSed

Table 1 :

 1 Kinetic rate constant determined at different temperatures on the Ti3Al0.8Sn0.2C2 and Ti3AlC2 MAX phase.

Acknowledgements:

The authors acknowledge SAFRAN for the financial support and for E. Drouelle PhD grant.


This paper draws on the first results of an ongoing research (2017)(2018)(2019)(2020)(2021), on the implementation of a Youth Popular University (YPU) in a French metropolitan area (Northern France) [START_REF] Azizi | L'UPJ, une promesse émancipatrice : Les stratégies rhétoriques d'un appel à projet polyphonique[END_REF]. Our objective is to study the actions developed under this project, and specifically, during the first phase, how young people (18-30 years old) are mobilizing, in particular when involving in speakers' corner experiences.

Few studies exist on YPUs, but are numerous on popular education. Many of them highlight a youth participation "through action", and a commitment "by doing"; they refer to a political vision tending to "the promotion of activation and empowerment of users", at the opposite of a consumerist approach [START_REF] Allemand | Education populaire pour et par les jeunes : pratiques numériques, lieux innovants et medias des jeunes[END_REF]Loncle, 2008). In this perspective, "speakers' corners" (SC) are seen as opportunities, in non formal environments, to live rich and complex experiences. Recent studies insist on their role as "meeting places", where actors can interact, practicing free expression, debating, verbalizing their experiences...; and as "potential spaces", promoting problematization of knowledge from experiences [START_REF] Gehl | City to Waterfront: Public Spaces and Public Life[END_REF], along with the development of social, relational and informational skills. Seen through the lens of knowledge and learning-training, it is part of "participatory culture" [START_REF] Fortin | The appropriation of a digital speakers' corner: lessons learned from the In-the-wild deployment of Megaphone[END_REF][START_REF] Jenkins | Youth Voice, Media, and Political Engagement. Introducing the Core Concepts[END_REF], and closely related to the ability of "critical consciousness", essential to the building of a coherent and reasoned vision of reality [START_REF] Elmborg | Critical information literacy: Implications for instructional practice[END_REF]. However, studies point out that participation is not equally educative, and it does not necessarily create opportunities for deeper learning and broader thinking, able to activate connections.

What can we say about the speakers' corner experience and its role in citizen commitment and political empowerment? What do the actors' practices reveal about its potential? To what extent does it contribute -or not -to the development of participatory and critical (information) culture(s)? In terms of methodology, we use a qualitative and comprehensive approach, collecting field data in immersion, close to the actors (young people, facilitators), to better understand their experiences: ethnographic observations with "thick descriptions", informal conversations, semi-structured interviews, photographs, collection of documents... The results are based on the data collected throughout the SC implementation process: from the reflection on a previous SC experience (strengths, limitations), to the mobilization of young people during a training phase, until the key moment of its effective implementation. In an anthropological perspective, we consider the SC as an evolutive process, as it is experienced by the actors (Turner & Bruner, 1986), paying attention, beyond their narratives, to their "ways of being" in the world, between individual and collective, sensible and intelligible.

As designed in the project, as potential space, the "speakers' corners" (SC) offer opportunities to live complex experiences. "Young people have critical faculties, critical sense, naturally developed, but not always well controlled, when expressed through violence or anger. How to channel criticism, with method, to make it constructive, sending a clear message? " asks the project manager.

From enclosed space to public space (plaza in front of a station), the speakers' corners" are a platform for free speech, for expression of critical consciousness on transverse issues (City as a playground; Youth and environment "Free your voice") : more theatrical on esplanade (outdoors), they attract passersby, arousing curiosity ; placemaking reinforces connections between people and spaces they share: reflection on how take ownership of the city, thinking everyday spaces and their potential (making common spaces, the heart of community).

From the microphone to the megaphone, the voice amplification brings a performative dimension. The power of words and images is reinforced by the amplification of sound (increased presence, in other physical spaces; happening/appropriation of public space). The live interventions on the plaza find an xtension via the Facebook of the People's Youth University (YPU): this virtual presence is at the origin of interactions (likes, comments, questions), it contributes to make SC and YPU known.

Towards a speakers' corner literacy? Young people develop communication and expression skills, both oral and written (only a small digital part) : how to communicate and promote ideas (points of view, experiences, information), preparing for participation. The project manager insists on three key words: thinking a message; attracting and engaging the audience; conversing, reinforcing dialogue.

In terms of limitations and challenges, these experiences allow for commitment through action, "by doing", but these are two events that "happen" without deployment over time: there are difficulties in attracting and recruiting young people: events in different cities and places, with local recruitment; difficulties in broadening recruitment: young people are already informed and/or engaged in actions, in existing networks; difficulties in retaining them: those who intervene tend not to return, and to be part of YPU on a regular basis (consumption, post-it commitment). At this stage of the project, we can speak of commitment (vs. embodied participation): attention is focused on some objects (vs. civic agency), related to practical, do-able activities [START_REF] Dahlgren | Media and Political Engagement: Citizens, Communication, and Democracy[END_REF]. Moving from a minimalist to a maximalist approach is a challenge for the future.


domain: spi chim sdv

text:
V A Oleshchenko

A Yu Kharin

A F Alykova

O V Karpukhina

N V Karpov

A A Popov

V V Bezotosnyi

S M Klimentov

I N Zavestovskaya

A V Kabashin
email: kabashin@lp3.univ-mrs.fr

V Yu Timoshenko

Localized infrared radiation-induced hyperthermia sensitized by laserablated silicon nanoparticles for phototherapy applications

Keywords: Pulsed laser ablation in liquids, Nanoparticles, Silicon, Laser diode, Photoexcitation, Hyperthermia, Paramecium Caudatum

Highlights:

 Laser-ablated silicon nanoparticles act as sensitizers for spatially and temporally localized photohyperthermia under irradiation in the tissue transparency window.  Irradiation with millisecond pulses of laser diode promotes hyperthermia effect due to non-stationary temperature distribution nearby silicon nanoparticles.  In vitro experiments confirm the hyperthermia sensitized by silicon nanoparticles.

Introduction

Silicon (Si) nanoparticles (NPs) exhibit a series of properties, which make them extremely promising for biomedical applications [START_REF] Canham | Handbook of Porous Silicon[END_REF][START_REF] Ischenko | Nanosilicon: Properties, synthesis, applications, methods of analysis and control[END_REF][START_REF] Santos | Porous silicon for biomedical applications[END_REF]. Si nanocrystals with sizes below 10 nm possesses photoluminescence in the window of relative tissue transparency (650-850 nm), which can be used for their detection in biological cells [START_REF] Erogbogbo | Biocompatible luminescent silicon quantum dots for imaging of cancer cells[END_REF][START_REF] Gongalsky | Laser-synthesized oxide-passivated bright Si quantum dots for bioimaging[END_REF] and tissues [START_REF] Gu | In vivo time-gated fluorescence imaging with biodegradable luminescent porous silicon nanoparticles[END_REF]. On the other hand, Si-NPs can offer sensitizing properties to initiate therapies under external stimuli, including photodynamic therapy (PDT) [START_REF] Yu | Silicon nanocrystals as photosensitizers of active oxygen for biomedical applications[END_REF], ultrasound [START_REF] Sviridov | Porous silicon nanoparticles as sensitizers for ultrasonic hyperthermia[END_REF], radiofrequency (RF) radiation- [START_REF] Tamarov | Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy[END_REF] and infrared (IR) radiation-induced [START_REF] Lee | Porous silicon as an agent for cancer thermotherapy based on near-infrared light irradiation[END_REF] hyperthermia. However, biomedical prospects of nanosilicon structures are complicated by the difficulty of their synthesizing in pure, uncontaminated and waterdispersible form. Indeed, conventional wet chemical routes for the fabrication of Si-NPs cannot normally avoid the use of toxic substances and by-products, such as non-saturated organic compounds [START_REF] English | Size tunable visible luminescence from individual organic monolayer stabilized silicon nanocrystal quantum dots[END_REF] or solvents [START_REF] Neiner | Low-temperature solution route to macroscopic amounts of hydrogen terminated silicon nanoparticles[END_REF], while Si-NPs synthesized by "dry" routes are typically not water dispersible [START_REF] Gupta | Luminescent colloidal dispersion of silicon quantum dots from microwave plasma synthesis: Exploring the photoluminescence behavior across the visible spectrum[END_REF][START_REF] Mangolini | High-yield plasma synthesis of luminescent silicon nanocrystals[END_REF] and require similar wet chemical steps to enable this option. Porous NPs prepared by mechanical milling of electrochemically fabricated porous Si seem to be well suitable for biomedical applications [START_REF] Canham | Handbook of Porous Silicon[END_REF][START_REF] Osminkina | Porous Silicon as a Sensitizer for Biomedical Applications[END_REF], but such NPs can be contaminated by hydrofluoric acid derivatives [START_REF] Koynov | Purification of nano-porous silicon for biomedical applications[END_REF] and present a widely size-and shape-dispersed population [START_REF] Osminkina | Porous Silicon as a Sensitizer for Biomedical Applications[END_REF] complicating their transport in vivo.

Si-NPs prepared by methods of laser ablation in gaseous [START_REF] Kabashin | Photoluminescence characterization of Sibased nanostructured films produced by pulsed laser ablation[END_REF][START_REF] Kabashin | Laser-induced treatment of silicon in air and formation of Si/SiO x photoluminescent nanostructured layers[END_REF] or aqueous [START_REF] Rioux | Silicon nanoparticles produced by femtosecond laser ablation in water as novel contaminationfree photosensitizers[END_REF][START_REF] Blandin | Femtosecond laser fragmentation from waterdispersed microcolloids: Toward fast controllable growth of ultrapure Si-based nanomaterials for biological applications[END_REF][START_REF] Intartaglia | Bioconjugated silicon quantum dots from onestep green synthesis[END_REF] ambient present a novel object, which promises the solution of the above-stated toxicity and other problems. When produced in ultrapure, well-controlled ambient (residual helium gas, deionized water), laser-synthesized nanostructures are free of any toxic impurities and water-dissolvable with the dissolution rate controllable by conditions of laser-ablative synthesis [START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF]. As follows from results of our recent studies, a systemic administration of Si-NPs in a small animal model does not provoke any toxicity effects, while the NPs rapidly biodegrade and excrete from the body within several days [START_REF] Baati | Ultrapure laser-synthesized Si-based nanomaterials for biomedical applications: In vivo assessment of safety and biodistribution[END_REF]. More importantly, laser-synthesized Si-NPs can initiate a series of bioimaging and therapy functionalities based on their unique physico-chemical properties [START_REF] Kabashin | What theranostic applications could ultrapure laser-synthesized Si nanoparticles have in cancer?[END_REF][START_REF] Kabashin | Laser Processed Nanosilicon: A Multifunctional Nanomaterial for Energy and Healthcare[END_REF]. As an example, small (2-4 nm) Si nanocrystals can enable imaging functionality based on the photoluminescence (PL) via quantum confined excitonic states [START_REF] Gu | In vivo time-gated fluorescence imaging with biodegradable luminescent porous silicon nanoparticles[END_REF], while larger NPs (10-100 nm) can offer dual non-linear bioimaging modality based on the combination of Second Harmonic Generation (SHG) and Two-Photon Excited Luminescence (TPEL) channels [START_REF] Yu | Bi-Modal Nonlinear Optical Contrast from Si Nanoparticles for Cancer Theranostics[END_REF].

On the other hand, laser-synthesized Si NPs can be efficient sensitizers of therapeutic modalities such as PDT [START_REF] Yu | Silicon nanocrystals as photosensitizers of active oxygen for biomedical applications[END_REF] and RF radiation-induced hyperthermia [START_REF] Tamarov | Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy[END_REF], as well as serve as carriers of therapeutic radionuclides [START_REF] Petriev | Nuclear nanomedicine using Si nanoparticles as safe and effective carriers of Re-188 radionuclide for cancer therapy[END_REF]. Such a combination of different modalities in one Si-based nanoformulation looks as an effective strategy to develop novel theranostic (therapy + diagnostics) agents for biomedical applications. In this sense, additional diagnostic and therapeutic functionalities based on laser-synthesized NPs can be considered as new important assets.

In this paper, we show that laser-synthesized Si-NPs can serve as efficient sensitizers of infrared radiation-induced hyperthermia similarly to how it was earlier reported for electrochemically produced porous silicon-based NPs [START_REF] Lee | Porous silicon as an agent for cancer thermotherapy based on near-infrared light irradiation[END_REF][START_REF] Hong | Porous silicon nanoparticles for cancer photothermotherapy[END_REF]. We suppose that such a modality can be used in parallel with other approaches to improve the efficiency of antibacterial and cancer treatments.

Materials and methods

Crystalline silicon (c-Si) wafers having resistivity of 10-20 Ohmcm and crystallographic surface orientation (100) were used as targets for laser ablation experiments. The wafers were treated with an aqueous solution of HF (48%) for 1-2 s to remove a natural silicon oxide from the surface. In laser ablation experiments, we used a linearly polarized beam from a diode pump Teta 10 system (Avesta, Russia), operating at wavelength of 1030 nm, pulse length 270 fs, pulse energy 100 µJ, repetition rate 10 kHz. A laser beam (3 mm in diameter) was focused onto target immersed in 10 ml of deionized water at normal incidence. The experiment was carried out for 1 h at room temperature. Aqueous suspensions of laser-synthesized Si-NPs having initial concentration of 0.1 mg/mL were centrifuged (12000 g, 20 min) until the concentration reached 4 mg/mL. Diluted suspensions of Si-NPs were investigated by transmission electron microscopy (TEM) using a LEO912 AB OMEGA system. Dissolution kinetics for diluted Si-NPs solutions in media having different pH were investigated by optical extinction spectroscopy using Cary 60 UV-Vis spectrometer. Size and charge characteristics of Si-NPs in aqueous suspension were studied by dynamic light scattering (DLS) using a Malvern Zetasizer ZS analyzer.

To carry out photothermal and biological tests, we used a home-made laser diode providing 808 nm radiation at the average power of 3 W in continuous wave (CW) or quasi-continuous wave (QCW) regimes [START_REF] Bezotosnyi | Laser diodes emitting up to 25 W at 808 nm[END_REF]. The QCW mode provided periodic 0.3 s pulses at power of 10 W followed by 0.7 s offset mode. The laser irradiation was applied from the bottom side of a transparent plastic cuvette having a cross-sectional size of 10x10 mm 2 and the length of 40 mm (inset in Figure 1b). Laser beam cross-section area at the cuvette bottom was about 0.02 mm 2 .

A 1-mm in diameter temperature sensor was located at 15 mm from the bottom of the cell, in one of its corners, to avoid direct laser irradiation.

To analyze toxicity of Si NPs and assess effects of continuous and pulsed laser illumination, we used suspensions of living cells of Paramecium Caudatum. Despite the simplicity of organization, this type of cells combines features of both a single cell and the whole organism [START_REF] Plattner | My favorite cell -Paramecium[END_REF]. These cells are interesting from analytical point of view as they can be considered as simple receptor systems that have the ability to respond to thermal effects of a whole set of biological, physiological and biochemical stimuli [START_REF] Fujishima | Paramecium caudatum acquires heat-shock resistance in ciliary movement by infection with the endonuclear symbiotic bacterium Holospora obtusa[END_REF][START_REF] Krenek | Thermal performance curves of Paramecium caudatum: A model selection approach[END_REF]. As an example, the movement velocity of Paramecium Caudatum cells drastically increases when temperature is increased from 25 to 30 o C, while a further increase of temperature above 40 o C typically leads to the cell death [START_REF] Krenek | Thermal performance curves of Paramecium caudatum: A model selection approach[END_REF][START_REF] Pace | The effect of temperature on respiration in Paramecium aurelia and Paramecium caudatum[END_REF]. The latter temperate is close to the well-known hyperthermia threshold for cancer cells (see for example review in Ref. [START_REF] Baronzio | A brief overview of hyperthermia in cancer treatment[END_REF]). Prior laser irradiation, NPs aqueous suspension (1 mL with concentration 1.2 mg/mL ) was mixed with a cell suspension (2 mL) in a plastic cuvette, and the mixture was incubated for 30 min. The cell death was visually estimated by the absence of cell movement, often associated with the deformation and lysis of the cell wall. As a control test, we monitored the number of cells in 10 ml of the medium containing the intact culture of infusoria. The evolution of temperature of aqueous suspensions under laser irradiation was monitored by using a digital medical thermometer TMTSP-1 (MIREA, Russia) having a temperature sensor tip from the Exacon Scientific (Denmark). The range of thermometer measurements was between +5 and +50°C, measurement accuracy and sampling rate were 0.01°C and 15 Hz, respectively. In order to avoid direct heating of the temperature sensor under laser irradiation, the sensor was located at the periphery of the cuvette outside the irradiation region.

Calculations

In order to access temperature in the center of cuvette filled with NPs solutions, a threedimensional finite-element model was developed with using COMSOL Multiphysics software.

Temperature distribution was modeled via a numerical solution of the thermal conductivity equation:

𝜕𝑇(𝑥,𝑦,𝑧,𝑡) 𝜕𝑡 = 𝐷 • ∆𝑇(𝑥, 𝑦, 𝑧, 𝑡) + 𝐻(𝑥, 𝑦, 𝑧, 𝑡) , ( 1
)
where D is the thermal diffusion coefficient (𝐷 = 1.43 • 10 -7 m 2 /s for water at 25 o C) and 𝐻(𝑥, 𝑦, 𝑧, 𝑡) is the heat generation source function. The boundary conditions consider constant temperature of cuvette walls (corresponding to ambient temperature), and presence of air above the liquid in the cuvette. Geometry of the cuvette and laser beam divergence were set in the form of an inverted truncated pyramid with height, base, and area of the initial cross-section corresponding to experimental conditions. While the laser beam propagates along the zcoordinate, the heat source can be approximated as follows:

𝐻(𝑥, 𝑦, 𝑧, 𝑡) ≈ 𝑃(𝑡)𝐾(𝑥, 𝑦, 𝑧)

𝑘•𝑒𝑥𝑝(-𝑘•𝑧) (𝑥 0 +𝑧•𝑡𝑔𝛼)(𝑦 0 +𝑧•𝑡𝑔𝛼) , ( 2
)
where 𝑃(𝑡)is the excitation power at the cuvette entrance, K(x,y,z) is the dimensionless factor, which describes the heat source geometry, in which K=1 inside and K=0 outside of the excitation region. x0 =0.1 mm and y0=0.2 mm are dimensions of the cross section of the laser beam at the cuvette entrance, and 𝛼 ≈ 18 𝑜 is the laser beam divergency angle, which was determined experimentally. The numerator shows the absorbed light intensity in accordance to Beer-Lambert law, where k is a linear absorbance coefficient for current concentration of nanoparticles at wavelength 808 nm. The denominator shows the area of rectangular beam cross-section at height z.

Results and discussion

A typical TEM image of dried Si-NPs and a corresponding electron diffraction pattern are shown in Figure 1a. One can see that the formed NPs had an ideal spherical shape, while their mean size was about 20-30 nm. As follows from electron diffraction data (inset of Fig. 1a), Si-NPs had a polycrystalline structure. As shown from DLS spectrum from as-prepared suspension of Si-NPs (Figure 1b), mean hydrodynamical size (diameter) and zeta-potential of the NPs were about 100 nm and -15 mV, respectively. A larger value of the hydrodynamic size compared to the TEM data can be explained by the appearance of an ionic shell around Si-NPs in aqueous media. The negative zeta-potential of the prepared Si-NPs was obviously related to the presence of hydroxyl groups on the Si-NPs surface [START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF][START_REF] Baati | Ultrapure laser-synthesized Si-based nanomaterials for biomedical applications: In vivo assessment of safety and biodistribution[END_REF]. Structural and dissolution properties of laserablated Si-NPs were studied in our earlier works [START_REF] Tamarov | Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy[END_REF][START_REF] Rioux | Silicon nanoparticles produced by femtosecond laser ablation in water as novel contaminationfree photosensitizers[END_REF][START_REF] Blandin | Femtosecond laser fragmentation from waterdispersed microcolloids: Toward fast controllable growth of ultrapure Si-based nanomaterials for biological applications[END_REF][START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF]. According to Fourier transform infrared spectroscopy (FTIR) [START_REF] Tamarov | Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy[END_REF] and X-ray photoelectron spectroscopy (XPS) [START_REF] Blandin | Femtosecond laser fragmentation from waterdispersed microcolloids: Toward fast controllable growth of ultrapure Si-based nanomaterials for biological applications[END_REF][START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF] data, the upper layer of laser-ablated Si-NPs is oxidized, with a certain amount of hydrogen still remaining on the NPs surface. Such a silicon oxide-related coating of Si-NPs is supposed to condition hydrophilic properties of their surface. It was also found from Raman spectroscopy and TEM data [START_REF] Tamarov | Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy[END_REF][START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF] that laser-ablated Si-NPs could completely dissolve in physiological environment. In this case, the dissolution rate depended on whether laser ablation was performed in normal oxygen-saturated water (oxygen-rich conditions) or in water deoxygenated by pumping with noble gases (Ag or He) before and during the experiment (oxygen-free conditions) [START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF]. Here, more oxidized Si-NPs prepared in oxygen-rich conditions demonstrated much faster dissolution kinetics (4-7 days compared to 3-4 weeks), which was explained by the relative friability of the oxygen defect-rich nanoparticle core [START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF].

To access photohyperthermia applications of laser-ablated Si-NPs, it is important to check the stability of these nanostructures in projected biological conditions. Therefore, we first explored dissolution properties of Si-NPs in aqueous solutions having different acidity, i.e. pHlevel. Figure 2a shows extinction spectra of Si-NPs (as-prepared and aged in water buffer at pH7). As shown in the Figure, the extinction peak dropped 10-fold in intensity after its aging in pH7 buffer for 10 days, evidencing the dissolution process of Si-NPs. Based on extinction spectra data, we then measured kinetics of NPs dissolution in pH5, pH7, pH8 buffers (Figure 2b). As shown in the figure, the degradation rate depended on pH, with higher rates for solutions having higher pH, while characteristic degradation times for all samples were in the range of several days. It should be noted that the degradation time was much longer than projected laser treatment times (tens of minutes) and longer than typical times of NPs accumulation in vivo [START_REF] Baati | Ultrapure laser-synthesized Si-based nanomaterials for biomedical applications: In vivo assessment of safety and biodistribution[END_REF]. Such a degradation kinetics looks favorable for applications of laser-synthesized Si-NPs in bioimaging and therapies. The simulated effect of local heating of aqueous NPs suspensions under their irradiation at 808 nm can be explained by particular properties of laser-synthesized Si-NPs. Since the measured extinction coefficient of about 0.5 cm -1 (for 1 g/L concentration of NPs) can hardly be related to phonon-assisted electron transitions in indirect band gap silicon, we can attribute such enhanced optical absorption to local field effects at the nanoscale [START_REF] Yu | Bi-Modal Nonlinear Optical Contrast from Si Nanoparticles for Cancer Theranostics[END_REF] or/and electron defect states in Si-NPs. The involvement of the first effect was confirmed by the appearance of non-monotonic features in the extinction spectra (Fig. 2a), evidencing Mie resonances in laserablated Si-NPs. Possible contribution of defect states follows from defect-rich structure of lasersynthesized Si-NPs as a result of their formation under strongly non-equilibrium aqueous conditions [START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF]. Indeed, such Si-NPs presented a polycrystalline structure comprising numerous intrinsic defects such as twins and grain boundaries (Fig. 1a). Finally, we carried out a series of cell viability measurements in order to assess the efficiency of laser-induced treatment using Si-NPs as sensitizers of local photo-hyperthermia. Such tests revealed a significant difference in the cell survival rates after applying CW and QCW irradiation modes. Indeed, the application of QCW mode led to a complete destruction of Paramecium Caudatum cells with incubated Si-NPs after 60 min irradiation, while 50% of cells were still alive after similar application of the CW mode (Figure 5). Before laser irradiation, Paramecium Caudatum cells presented unicellular organisms, which freely moved in the liquid medium (Fig. 6a, this image was taken by a fast snapshot of moving object), while after laser irradiation in the QCW mode (0.3 s pulses, peak power density 100 W/cm 2 ) they were dead stationary objects (Fig. 6 b,c). Just after the first pulse, we observed a change in the nature of cell movement, while after the third pulse we could detect only slow motions over a circular trajectory. Then, after 6-7 pulses the movement nearly stopped. Comparing images in panels (a) and (b) of Figure 6, one can reveal changes in the shape of microorganisms and their internal structure. Before laser irradiation, the shape was elongated; the internal structure had a clear outline. After the irradiation, the elongation became shapeless, while the internal structure was broken (intracellular organelle contrast was still high due to NPs accumulation inside the cells).

The cell destruction is illustrated in Fig. 6 b,c by snapshots taken in 60 sec and 180-sec intervals after the exposition. Within 3 minutes of observation after the exposition digestive vacuoles inside the cells increased in size, while the cell volume decreased probably due to a partial destruction of the membrane as the contrast of the cell membrane became significantly reduced.

The thermolysis of the cell membrane and its interior was the most probable reason of cell death under hyperthermia conditions. In our tests, the threshold power density for cells death was below 100 W/cm 2 even for relatively low NPs concentrations (< 0.08 g/L). For comparison, we carried out cell irradiation tests in the absence of Si-NPs.

Step-by-step increasing laser power density we managed to evaluate the "threshold" laser intensity, which can provide cell death in water without the NPs. This threshold intensity was about 150 W/cm 2 , which was much higher than in the case of NPs. After the treatment by a single laser pulse at this intensity, microorganisms were alive exhibiting a very slow movement for 30-50 minutes, while the application of many pulses led to a gradual decrease of the movement ability and a subsequent cell death. Our analysis showed that in the absence of Si-NPs laser-induced treatment damages the most absorbing, contrasting cell organelles such as macronucleus, micronucleus, and digestive vacuoles. In general, the sensitizing effect of Si NPs was pronounced making possible the destruction of cells under much lower radiation intensities.

Thus, laser-synthesized Si-NPs can be efficient sensitizers of local overheating under nearinfrared laser excitation. As follows from our tests in Paramecium Caudatum cell model, the QCW mode appeared to be more efficient in the elimination of cells with the cell death reaching 100% under 60 min irradiation at the mean power of 3W. A better efficiency of QCW mode can be explained by a very high peak power at the maxima of laser pulses (10 W), which caused a major damage to the cells. We believe that the proposed photo-induced hyperthermia sensitized by laser-ablated Si-NPs can be used solely or in combination with other approaches profiting from the unique physical and chemical properties of Si-based nanostructures. On the one hand, because the mean size of laser-ablated Si is usually above the quantum confinement regime, they are hardly applicable in the conventional fluorescent bioimaging [START_REF] Canham | Handbook of Porous Silicon[END_REF][START_REF] Ischenko | Nanosilicon: Properties, synthesis, applications, methods of analysis and control[END_REF][START_REF] Santos | Porous silicon for biomedical applications[END_REF][START_REF] Erogbogbo | Biocompatible luminescent silicon quantum dots for imaging of cancer cells[END_REF]. On the other hand, Si-NPs with sizes above 10 nm can be visualized by means of the nonlinear optical bioimaging in the TPEL and SHG modes [START_REF] Yu | Bi-Modal Nonlinear Optical Contrast from Si Nanoparticles for Cancer Theranostics[END_REF]. Moreover, the NP-sensitized hyperthermia is well combined with earlier demonstrated alternative therapy methods, including the RF-induced hyperthermia [START_REF] Tamarov | Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy[END_REF] and PDT [START_REF] Yu | Silicon nanocrystals as photosensitizers of active oxygen for biomedical applications[END_REF], which could be enabled in parallel profiting from the sensitizing properties of Si-NPs.

We believe that the use of laser-synthesized Si-NPs as sensitizers of photo hyperthermia can offer several advantages compared to porous silicon and other nanosilicon-based counterparts:

(i) In contrast to porous silicon nanostructures, laser-synthesized Si-NPs have ideal spherical shape and are low size-dispersed, which promises a much improved transport in biological systems in vivo [START_REF] Petriev | Nuclear nanomedicine using Si nanoparticles as safe and effective carriers of Re-188 radionuclide for cancer therapy[END_REF]; (ii) Laser-synthesized Si-NPs can have a defect-rich polycrystalline structure, which conditions a much faster dissolution and excretion of these nanoparticles from the body in vivo (4-7 days [START_REF] Al-Kattan | Ultrapure laser-synthesized Si nanoparticles with variable oxidation states for biomedical applications[END_REF][START_REF] Baati | Ultrapure laser-synthesized Si-based nanomaterials for biomedical applications: In vivo assessment of safety and biodistribution[END_REF] compared to more than 6 weeks in the case of porous silicon [START_REF] Park | Biodegradable luminescent porous silicon nanoparticles for in vivo applications[END_REF]); (iii) Laser-synthesized Si-NPs are produced in ultrapure deionized water, which excludes any contamination of nanoparticles by toxic by-products of synthesis, as it takes place in many alternative methods [START_REF] English | Size tunable visible luminescence from individual organic monolayer stabilized silicon nanocrystal quantum dots[END_REF][START_REF] Neiner | Low-temperature solution route to macroscopic amounts of hydrogen terminated silicon nanoparticles[END_REF][START_REF] Koynov | Purification of nano-porous silicon for biomedical applications[END_REF]; (iv) Laser-synthesized Si-NPs can offer a series of imaging [START_REF] Gongalsky | Laser-synthesized oxide-passivated bright Si quantum dots for bioimaging[END_REF][START_REF] Yu | Bi-Modal Nonlinear Optical Contrast from Si Nanoparticles for Cancer Theranostics[END_REF] and therapy (Refs. [START_REF] Tamarov | Radio frequency radiation-induced hyperthermia using Si nanoparticle-based sensitizers for mild cancer therapy[END_REF][START_REF] Rioux | Silicon nanoparticles produced by femtosecond laser ablation in water as novel contaminationfree photosensitizers[END_REF][START_REF] Petriev | Nuclear nanomedicine using Si nanoparticles as safe and effective carriers of Re-188 radionuclide for cancer therapy[END_REF]) modalities, which can be enabled in parallel with the considered photothermal modality.

Conclusions

We showed that laser-synthesized Si NPs can be efficient sensitizers of local hyperthermia under their illumination by laser radiation in the region of relative tissue transparency (808 nm).

As follows from both the calculations and experimental results, the millisecond pulses QCW regime appeared to be advantageous compared to the CW-based one to reach higher temperatures under a high spatial localization of the heating release. The efficiency of hyperthermia-based phototherapy therapy was illustrated for the model of Paramecium Caudatum cells. Here, our experiments confirmed that the QCW regime was indeed more efficient to initiate the hyperthermia leading to 100% cell death even for relatively low NPs concentrations.

The presented simplified approach can be the basic step for the development of practical hyperthermia methods, which should take into account specific effects accompanying laser light propagation in living tissues including light scattering, blood flow, etc. It is implied that such methods can enable the hyperthermia therapy at a given depth in the tumor area in the absence of traumatic effects on the surrounding healthy tissues. We believe that long-pulse QCW regime looks especially promising taking into account its efficiency and the availability of compact and inexpensive laser diode modules emitting radiation in the tissue transparency window.

Figure 3

 3 Figure3shows simulated kinetics of temperature growth during laser heating of Si-NP colloids under CW (a) and QCW (b) irradiation regimes. Here, the extinction coefficient k = 0.21 cm -1 for 808 nm wavelength under concentration of 0.4 g/L was found experimentally, while the extinction coefficient of pure water was below 0.05 cm -1 under the same laser irradiation conditions. Red and blue curves in panels (a) and (b) show temperature at different distances from the cuvette bottom. Two modes, CW and QCW had the same average power of 3 W, but the QCW mode had a higher instant power (10 W) as it comprised a series of rectangular pulses having the peak power of 10 W, duration of 0.3 s and frequency of 1 Hz.One can see that the irradiation in QCW mode led to temperature peaks that exceeded the temperature level in CW mode by 3-5 o C. One can also mention that temperature peaks became smoothened at peripheral parts of the cuvette due to a low thermal diffusivity of water (CW and QCW modes have the same effect). Color insets in panels (a) and (b) of Figure3show simulated images of isothermal surfaces inside the cuvette after the irradiation for 5 min in CW and QCW modes. Here, a red isothermal surface corresponded to heating by more than 30 °C relative to the initial temperature. It is visible that in QCW mode this temperature was recordable even at points located 10 mm from the bottom (b), while in CW mode the relevant temperature could be reached only for close points (< 3 mm). In general, our calculations showed that QCW mode looked much more efficient for the implementation of local hyperthermia under the same values of the average power (3 W).

Figure 4

 4 Figure 4 shows time dependencies for temperature of aqueous suspensions of Paramecium Caudatum cells alone and the cells with incubated Si-NPs under CW (a) and QCW (b) laser irradiation. As shown in the figure, the temperature increase of the cell suspension incubatedwith Si-NPs was much more pronounced compared to the cells without NPs, while the mean temperature growth was similar for the CW and QCW irradiation regimes. This fact indicates that Si-NPs acted as efficient absorbers of the applied near-infrared laser radiation. Here, a direct measurement of the absorbed power revealed the extinction coefficient about k = 0.21 cm -1 for the suspension of 0.4 mg/ml Si-NPs. It is also important that maximal temperature achieved at the peak of laser pulse in the QCW regime significantly exceeded the mean temperature in this regime (inset of Fig.4b). Note that the absolute value of the temperature growth during the action of laser pulse was underestimated because of a limited time resolution and peripheral location of the thermometer used.

Figure 1 .

 1 Figure 1. (a) A TEM image of laser-synthesized Si-NPs. An inset shows a corresponding electron diffraction pattern; (b) Size distribution of Si NPs in aqueous suspension measured by DLS. An inset shows the digital image of a plastic cuvette with aqueous suspension of Si-NPs.

Figure 2 .

 2 Figure 2. (a) Extinction spectra of as-prepared aqueous suspension of Si-NPs with concentration of 0.1 g/L (black curve) and from the same sample after 10 days of storage in aqueous pH7 buffer (red). (b) Dependences of the Si-NPs extinction at 300 nm versus time

Figure 3 .

 3 Figure 3. Calculated temperature growth at two points having the coordinate of z=5 mm (red color) and 10 mm (blue color) from the bottom of a cuvette filled with an aqueous suspension of Si-NPs under irradiation by a laser diode at 808 nm in CW (a) and QCW (b) regimes. Inserts in panels (a) and (b) show 3D-isothermal surfaces in the cuvette after

Figure 4 .

 4 Figure 4. (a) Temperature transients for aqueous suspensions of Paramecium Caudatum cells (black curve) and those with incubated Si-NPs (blue curve) under CW laser irradiation. (b) Temperature transients for the same cell suspensions (black curve) and those with incubated Si-NPs (red curve) under QCW laser irradiation (pulse duration =0.3 s, period T=1 s). An inset in panel (b) shows parts of these transients expanded into the time interval of 0.2-0.24 s.

Figure 5 .

 5 Figure 5. Relative number of living cells of Paramecium Caudatum versus time after injection of Si-NPs (black squares), followed with the irradiation in CW (red circles) and QCW (blue triangles) regimes (808 nm, mean power 3 W).

Figure. 6 .

 6 Figure.6. Digital images of unicellular organisms Paramecium Caudatum in aqueous suspension of Si-NPs (a) before exposition to laser beam, (b) 1 min after laser irradiation, (c) 3 min after laser irradiation. Arrows in panel (b) indicate bubbles near the perforated membrane. The laser irradiation was done at 808 nm in QCW regime for 15 s. The initial temperature was 22.5 o C.

Acknowledgements

This work was partially supported by the Ministry of Science and Higher Education of the Russian Federation (project 16.2969.2017/4.6). The authors also acknowledge contribution of the Russian Science Foundation (Project 19-72-30012) for fabrication of Si-NPs samples and the support from MEPhI Academic Excellence Project (Agreement with the Ministry of Education and Science of the Russian Federation of 27.08.2013, project #02.a03.21.0005) for providing the research equipment and the related scientific infrastructure.