A Lucas

R Lebourgeois

F Mazaleyrat

E Labouré

Temperature Dependence Of Core Loss In Cobalt Substituted Ni-Zn-Cu Ferrites

Keywords: NiZnCu ferrites, cobalt substitution, core loss versus temperature, permeability

The temperature dependence of core loss in cobalt substituted Ni-Zn-Cu ferrites was investigated. Co 2+ ions are known to lead to a compensation of the magnetocrystalline anisotropy in Ni-Zn ferrites, at a temperature depending on the cobalt content and the Ni / Zn ratio. We observed similar behaviour in Ni-Zn-Cu and it was found that the core loss goes through a minimum around this magneto-crystalline anisotropy compensation. Moreover, the anisotropy induced by the cobalt allowed a strong decrease of core loss, a ferrite having a core loss of 350 mW/cm 3 at 80°C was then developed (measured at 1.5 MHz and 25 mT). This result represents an improvement of a factor 4 compared to the state of art Ni-Zn ferrites.

Introduction

Nickel-zinc-copper ferrites are essential materials because of their high permeability in MHz range. Moreover, their low sintering temperature makes them suitable for the realization of integrated components in power electronics. As for nickel-zinc ferrites, cobalt substitution is an efficient technique to decrease the permeability [START_REF] Byun | Factors affecting initial permeability of Co-substituted Ni-Zn-Cu ferrites[END_REF] and the magnetic losses of nickel-zinc-copper ferrites [START_REF] Lebourgeois | low losses NiZnCu ferrites (ICF8)[END_REF]. The effect of cobalt is to allow the pinning of the domain wall by inducing a magnetic anisotropy [START_REF] Néel | [END_REF]. The consequence is a decrease of tanδ µ at high frequency [4] and also an improvement of core loss [START_REF] Lebourgeois | low losses NiZnCu ferrites (ICF8)[END_REF]. The aim of this paper is to study the effect of cobalt substitution on core losses versus temperature. A lot of papers investigated core losses of spinel ferrites versus temperature but it was mostly for Mn-Zn power ferrites [START_REF] Zaspalis | The effect of Nb 2 O 5 dopant on the structural and magnetic properties of MnZn ferrites[END_REF] and not at high frequency. There are very few papers concerning core losses versus temperature of Ni-Zn or Ni-Zn-Cu ferrites [START_REF] Su | Effects of composition and sintering temperature on properties of NiZn and NiCuZn ferrites[END_REF] [7], moreover, they only investigated losses of a single composition measured at low frequency (50 kHz). In order to understand how the ferrite composition influences the core loss, ferrites with three Ni / Zn ratios were studied (Ni / Zn = 0.43, 1 and 3) with, for each ratio, cobalt substitutions up to 0.035 mol per formula.

2 Experimental procedure

Sample preparation

Ferrites were synthesized using the conventional ceramic route. The raw materials (Fe 2 O 3 , NiO, ZnO, CuO) were ball milled for 24 hours in water. Co 3 O 4 was then added before the calcination around 800°C in air for 2 hours. The calcined ferrite powder was then milled by attrition for 30 min. The resulting powder was compacted using axial pressing. The sintering was performed at 935°C for 2 hours in air. Magnetic characterizations were done on ring shaped samples with the following dimensions : outer diameter = 6.8 mm; inner diameter = 3.15 mm; height = 4 mm.

Sample measurements

Bulk density was deduced from weight and dimensions. Saturation magnetization was measured on a magnetic balance. Initial complex permeability was measured versus frequency between 1 MHz and 1 GHz using an HP 4291impedance-meter. Static initial permeability (µ s ) was defined as µ' at 1 MHz because for these ferrites µ' is constant from very low frequencies to the megahertz range. For the permeability versus temperature measurements, the rings were wound with a copper wire and placed in an oven going from -70°C to 150°C. µ s was deduced from the inductance measured at 100 kHz by an 4194A impedance-meter Agilent. Core losses were measured at 1.5 MHz and 25 mT using a Clark-Hess 258 wattmeter and a 100 W Kalmus HF amplifier.

Results and discussion

Physicochemical characterizations

Three different series of ferrites were studied: Four formulations were done for each series, with cobalt rate of 0, 0.014, 0.028 and 0.035 mol per formula. The copper rate was 0.20 mol to allow the densification below 950°C. Three Ni / Zn ratios were studied in order to investigate the influence of cobalt substitution on ferrite with different magneto-crystalline anisotropy. Indeed, K 1 of the host crystal increases with nickel content [START_REF] Smit | Ferrites, Philips technical library[END_REF].

The samples were sintered at 935°C for 2 hours in air. Figure 1 shows the X-ray diffraction pattern of a Ni 0.40 Zn 0.40 Cu 0.20 Fe 2 O 4 ferrite sintered at 935°C. Only the spinel structure can be observed, none of the precursor oxides are present in the sintered materials.

The bulk densities of the sintered ferrites are shown in figure 2. All the ferrites have a low porosity (ρ > 92 % of the theoretical density). The cobalt substitution does not seem to have a significant effect on densification, a slight increase of the densification can however be noticed. This phenomenon is probably a consequence of the slight increase in the iron default of the ferrite.

Microstructure of the ferrites were also observed, SEM pictures are shown in figure 3. The grain size is not affected by the Ni / Zn ratio. Even if the nickel is known to lead to a more difficult densification [START_REF] Ageron | [END_REF], it is not observed here. The cobalt rate does not change the microstructure. All the ferrites exhibit the same kind of microstructure, with a low porosity and an average grain size between 1.5 and 2 µm. The grain size repartition is relatively homogenous, although one can see a few larger grains with defaults inside.

Magnetic characterisations

Saturation magnetization (M s ) at room temperature was measured (see table 1). This parameter mainly depends on the Ni / Zn ratio. M s increases up to a Ni / Zn ratio close to 1, then, it decreases with the zinc content. The behaviour of the developed ferrites is coherent with previous studies concerning Ni-Zn [START_REF] Smit | Ferrites, Philips technical library[END_REF] and Ni-Zn-Cu [START_REF] Ageron | [END_REF] ferrites. The maximum of the saturation magnetization is obtained for the Ni 0.40 Zn 0.40 Cu 0.20 Co ε Fe 1.98 O 4-γ' ferrites. Copper slightly lowers M s compared to the corresponding Ni-Zn ferrite [START_REF] Joshi | Magnetisation, curie temperature and Y-K angle studies of Cu-substituted and non substituted Ni-Zn mixed ferrites[END_REF].

Cobalt substitution does not have a significant influence on the saturation magnetization. Cobalt introduction must lead to an increase of the saturation magnetization, due to Co 2+ ions magnetic moment, which is higher than the other divalent ions present in octahedral sites (Ni 2+ and Cu 2+ ) [START_REF] Rezlescu | Effect of Substitution of Divalent Ions on the Electrical and Magnetic Properties of Ni-Zn-Me Ferrites[END_REF]. The measurement device used for this study is probably not precise enough to discern a variation in M s with such a small amount of Co 2+ .

For each ferrite, the static initial permeability and the core loss were measured versus temperature. The results are shown in figure 4.

The initial static permeability versus temperature of the cobalt substituted ferrites is not monotonous ; A local maximum can be observed even if for high µ s samples, it is flattened. This is due to the contribution of the cobalt ions to the magneto-crystalline anisotropy (K 1 ). Several studies were carried out on this phenomenon, which can be useful to increase the permeability around a particular temperature [START_REF] Pascard | Basic concepts for high permeability in soft ferrites[END_REF]. The permeability is due to two contributions : the domain wall displacements and the magnetization rotation :

D K M s s . . 16 3 1 1 2 = - µ for the domain wall motion contribution [13] 1 2 . 3 2 1 K M s s ∝ - µ
for the spin rotation contribution [14] With D the average grain size and K 1 the magneto-crystalline anisotropy of the ferrite. In this study, we consider that the grain size is similar whatever the composition, so this parameter does not affect the permeability. The variation of permeability versus temperature is then only depending on the variations of M s and K 1 versus temperature.

As the saturation magnetization of these ferrites is monotonically decreasing versus temperature, such a change in permeability is therefore due to a change in magnetic anisotropy. K 1 of the Ni-Zn-Cu spinels is weak and negative (between -2 and -6,5.10 3 J/m 3 , for NiFe 2 O 4 , ZnFe 2 O 4 and CuFe 2 O 4 [START_REF] Smit | Ferrites, Philips technical library[END_REF]). The cobalt ferrite has a high and positive K 1 (close to 300.10 3 J/m 3 at room temperature [START_REF] Smit | Ferrites, Philips technical library[END_REF]). Addition of a small amount of cobalt will thus lead to a compensation of K 1 . Looking at the previous relations, if there is a magneto-crystalline anisotropy compensation, the permeability will go through a maximum. This explains the apparition of a local maximum in µ s (T) curves of cobalt substituted ferrites at a compensation temperature (T 0 ) increasing with the cobalt content and K 1 of the Ni-Zn-Cu host crystal.

The compensation temperature of the three series of ferrites are shown in table 2, these results are established from µ s (T) curves (Figures 4.A1, B1 and C1). For a given Ni /Zn ratio, T 0 increases with the cobalt content. For the same cobalt rate, T 0 increases with the nickel content, i.e. with magneto-crystalline anisotropy of the Ni-Zn-Cu host crystal. At low temperature, the magnetocrystalline anisotropy is positive due to Co 2+ ions contribution. At high temperature, Ni-Zn-Cuhost crystal contribution becomes preponderant and K 1 is negative. The measurements are thus coherent with the single-ion model theory.

The cobalt substitutions are also known to pin the domain walls and to lower the domain wall displacements contribution to the permeability. This results in a decrease of µ s with the increase of the cobalt content, as can be seen in figures 4.A1 and 4.B1. For the third series (Ni / Zn = 3, figure 4.C1), the magneto-crystalline compensation is particularly marked. The permeability of cobalt substituted ferrites can then become higher than that of the cobalt-free ferrite (curve B, figure 4.C1). It seems that for these ferrites, a slight cobalt substitution leads to a smaller K 1 than that of the cobalt-free ferrite.

Temperature dependence of core loss

Core loss variations mainly depend on saturation magnetization and effective anisotropy (since we consider that the microstructures are similar for all the materials). Indeed, it is known that larger grain size can increase core loss [START_REF] Lebourgeois | Influence of V 2 O 5 on the magnetic properties of nickel-zinc-copper ferrites[END_REF]. For the cobalt-free ferrites these two parameters decrease versus temperature.

The Ni 0.24 Zn 0.56 Cu 0.20 Fe 1.98 O 4 ferrite (curve A of the figure 4.A2) has core loss decreasing until 20°C and rapidly increasing above 120°C. To understand this behaviour, we have to look at the temperature dependence of the permeability. Until 80°C, the anisotropy decreases faster than the saturation magnetization, which results in an increase of the permeability and a decrease of the core loss. Above 80°C, the initial static permeability reaches 1000, shifting the µ" resonance frequency towards 3 MHz. As the measurements are done at 1.5 MHz, relaxation losses become therefore predominant and lead to a fast increase of the core loss at high temperature.

When the nickel content increases, the Curie temperature rises. For the Ni 0.40 Zn 0.40 Cu 0.20 Fe 1.98 O 4-γ' ferrite (curve A, figure 4.B2), T c is around 340°C. The core loss measurements between -50 and 150°C are continuously decreasing. This is again related to the variation of permeability which constantly increases while maintaining a resonance frequency far enough from 1.5 MHz.

For the Ni 0.60 Zn 0.20 Cu 0.20 Fe 1.98 O 4 ferrites (curve A, figure 4.C2), one can note that the permeability is almost constant in the studied temperature range. The core losses are thus nearly stable between -50 and 150°C, a slight decrease can be observed at high temperature. Concerning the core loss values of the Ni-Zn-Cu ferrites, the higher the magneto-crystalline anisotropy, the higher the core loss. The power losses at room temperature increase from 600 mW/cm 3 to 2800 mW/cm 3 when nickel content goes from 0.24 mol to 0.60 mol per formula.

Cobalt substituted ferrites have a completely different behaviour. The core loss goes through a minimum around T 0 (magneto-crystalline compensation temperature) and their values are also highly reduced near this compensation compared to the cobalt-free ferrites. This phenomenon appears to be similar to the K 1 compensation observed in Mn-Zn spinels [START_REF] Pascard | Basic concepts for high permeability in soft ferrites[END_REF]. This compensation, due to Fe 2+ ions, also leads to a minimum in the core loss. If this is well known for Mn-Zn ferrites, it is the first time that it is observed in Ni-Zn based ferrites.

Such low core losses around T 0 are not only due to the K 1 compensation but are also the result of the cobalt induced anisotropy (K u ). Small amounts of Co 2+ ions in Ni-Zn spinels are known to pin the domain walls, however, the temperature dependence of this induced anisotropy is not well known. In order to study this parameter, Core loss versus magnetic induction of the Ni 0.40 Zn 0.40 Cu 0.20 Co 0.028 Fe 1.98 O 4+γ ferrite was measured at different temperatures (figure 5). The cobalt substituted ferrites have not a classical behaviour of core loss versus induction. When induction increases, the core loss increases almost linearly until a threshold induction corresponding to the required energy to unpin the domain walls. Above this threshold induction (which depends on K 1 and K u ), the anisotropy induced by the cobalt is no longer efficient so the core loss increases fast. This phenomenon appears clearly on figure 5 for the measurement at 20°C, the core loss increases slightly up to 35 mT and then rises rapidly. When the temperature increases, the threshold induction decreases and, for the measurement done at 100°C, the core loss nearly recovers a classical behaviour with an evolution proportional to the square of the magnetic induction. This ferrite has a T 0 close to 10°C, therefore the measurements done on figure 5 tend to prove that the cobalt induced anisotropy is more efficient around the magneto-crystalline anisotropy compensation.

Consequently, around T 0 the magnetic configuration is doubly favourable in order to have low power loss with a weak effective anisotropy and an induced anisotropy that improve the linearity of the ferrite. This can then lead to very low core loss of respectively 150 and 200 mW/cm 3 (at 1.5 MHz, 25 mT and 20°C) for Ni / Zn = 0.43 and Ni / Zn = 1 ferrites with cobalt content of 0.028 mol. The third series of ferrites have higher core loss with a minimum of 400 mW/cm 3 . It seems that K 1 of the Ni-Zn-Cu host crystal (which increases due to high nickel content) is too high to have very low core loss.

Conclusion

The study of the temperature dependence of core loss of cobalt-substituted Ni-Zn-Cu ferrites highlighted that the core losses were minimum around the magneto-crystalline anisotropy compensation. This is extremely interesting for adapting the ferrite to the operating range temperature for power applications. Today, power electronics need materials that can work at high frequency and high temperature (> 80°C). The ferrites studied in this paper can perfectly answer to these issues. Indeed, for the Ni 0.40 Zn 0.40 Cu 0.20 Co ε Fe 1.98 O 4+γ ferrites (figure 4.B2), core loss at 80°C can be divided by a factor 4 thanks to cobalt substitution. These materials represent also a real improvement compared to the state of art Ni-Zn ferrite for radiofrequency, with core loss 4 times lower than a commercial ferrite with µ s =120 (figure 6).

Ferrites

Figure 1 :

 1 Figure 1 : X-ray diffraction pattern of Ni 0.40 Zn 0.40 Cu 0.2 Fe 1.98 O 4 ferrite (black curve), comparison with JCPDF 00-052-0277 (red peaks).

Figure 2 :Figure 3 :Figure 4 :Figure 5 :

 2345 Figure 2 : Density of the three series of ferrites sintered at 935°C for 2 hours in air.

Figure 6 :

 6 Figure 6 : Core loss versus temperature measured at 1.5 MHz and 25mT. Comparison between a commercial Ni-Zn ferrite (µ s =120) and the Ni 0.40 Zn 0.40 Cu 0.20 Co 0.028 Fe 1.98 O 4+γ ferrite (µ s =130)

-

  Ni 0.24 Zn 0.56 Cu 0.20 Co ε Fe 1.98 O 4-γ -Ni 0.40 Zn 0.40 Cu 0.20 Co ε Fe 1.98 O 4-γ' -Ni 0.60 Zn 0.20 Cu 0.20 Co ε Fe 1.98 O 4-γ''

Table 1 :

 1 Saturation magnetization at room temperature of materials sintered at 935°C for 2 hours in air.

	Ms
	(emu/g)

Table 2 :

 2 Magneto-crystalline anisotropy compensation of the three series of ferrites, determined from µ s (T) curves.


Keywords: argumentation, biotechnology education, ethics, qualitative research, science education socio-scientific issue, expertise, contextualization debate, socioscientific issue, argumentation, contextualization, expertise, decisionmaking, human embryonic stem cells

The present study analyses decision-making and argumentation by high school students in a debate situation on a socioscientific issue, the use of embryonic stem cells in research and therapy. We tested the influence on the debates of two different contexts. Adolescent students at the high school level in the same grade (mean age 16.4 years) from rural and urban zones of Provence, France participated in three debate sessions. During the first session students listed the background questions they want to ask the expert(s). They were also required to identify one or two major issues that would serve as an outline for the future debate. They then discussed these with the expert(s) during the second session and took note of the answers. During this session, control groups met with a neuroscientist whereas the experimental "contextualized" group met with the same neuroscientist together with a representative of an association of patients suffering from a neurodegenerative disease. Analysis of the students' arguments and decision-making revealed that contextualization introduced dynamism in the students' exchanges: they paid more attention to their peers' arguments and were more motivated to argue their own opinion. However, this type of contextualization may contribute to reinforcing ideology in scientific progress.

Introduction

Meetings between researchers and the public are a significant modality of contemporary science communication. By doing so, researchers express their will to meet lay people, and particularly students, to discuss science and society connections. The debates we study here are an example of such meetings. In formal and informal science education contexts, debate protocols are considered effective strategies to engage people in socioscientific issues. The training of participants in argumentation skills is one of the major challenges of scientific citizenship education. Argumentation is the way individuals make and justify claims and conclusions. In this study, we focused on the consequences of debate contextualization on students' argumentations when they are making a decision on a socioscientific dilemma: the use of human embryonic stem cells.

Socioscientific issues' research: rationale and objectives

This research uses the theoretical framework of socioscientific issue (SSI) didactics that arose from the Sciences -Technologies -Society movement. SSI relative to health, environment and techno-scientific innovations are defined as social dilemmas linked to science about which citizens have to make decisions. Many authors have proposed introduction of SSI in science curricula in order to promote development of responsible scientific citizenship [START_REF] Zeidler | Moral issues and social policy in science education: Closing the literacy gap[END_REF][START_REF] Aikenhead | The content of STS education[END_REF][START_REF] Driver | Establishing the norms of scientific argumentation in classrooms[END_REF][START_REF] Kolstø | Scientific literacy for citizenship : tools for dealing with the science dimension of controversial socioscientific issues[END_REF]. The initial purpose of the movement was to engage students in meaningful learning of "context knowledge" in order to help them identify the interdependence between science and society [START_REF] Sadler | Informal reasoning regarding socioscientific issues: A critical review of research[END_REF]. This mission is in line with the goal of enhancement of scientific literacy promoted by the American Association for the Advancement of Sciences since the 1990's. According to Simonneaux (2003), who participated in introducing SSI to French science educators1 , the issue for educationalists is how "to enable pupils, informed in scientific research methods, its practical applications and its possible implications, to propose and argue decisions in an uncertain context and to participate in debates as citizens". [START_REF] Sadler | Informal reasoning regarding socioscientific issues: A critical review of research[END_REF] pointed out that SSI research focuses on four main directions: relationships between the nature of science conceptualizations and socioscientific decision-making, ways of evaluating information regarding SSI, influence of conceptual understanding on reasoning regarding SSI, and socioscientific argumentation (Jimenez-Aleixandre & al., 2000;Simonneaux, 2001;[START_REF] Osborne | Enhancing the quality of argument in school science[END_REF][START_REF] Zohar | Fostering students' knowledge and argumentation skills through dilemmas in human genetics[END_REF]. In this study, we focus on the links between the nature of science conceptualizations and socioscientific decision-making, and on socioscientific argumentation, ie, how individuals make and justify claims and conclusions about a SSI. Debate between peers appears to be a pedagogical strategy for helping students to think about open ended and complex issues and to develop argumentative skills. The present study analyses students' argumentation during a debate situation on a SSI where they were asked to express their personal opinions.

Socioscientific issues and the nature of science

Most of the studies focused on the nature of science conceptualization assume that students' knowledge about science influences their interpretation of knowledge in science [START_REF] Kolstø | Consensus Projects : teaching science for citizenship[END_REF] [START_REF] Ratcliffe | Science education for citizenship. Teaching socio-scientific issues[END_REF]. This research concerns how students put into practice their conceptions about values and ways of thinking in science. According to [START_REF] Kolstø | Scientific literacy for citizenship : tools for dealing with the science dimension of controversial socioscientific issues[END_REF], we investigate "content transcending knowledge", that is to say "knowledge or skills and attitudes that do not have their focus on the products of science community. […] The focus is shifted from knowledge in science toward knowledge about science". For [START_REF] Kolstø | Assessing the science dimension of environmental issues through environmental education[END_REF], essential knowledge about science appears necessary to understand an SSI. The first is that science is not the only aspect to consider when studying an SSI. Financial, ethical, legal, political and social aspects also have to be considered. Because of the interrelatedness of science and society, science cannot be considered neutral. This is an important point to make if we want students to be able to evaluate scientific expertise. Second, science is process based, relying on argumentation and peer criticism. Third, an understanding of the temporary nature of scientific results is necessary in order to debate controversial issues. In order to escape from an absolute relativism, we can distinguish between « core science » and « frontier science », that is between consolidated knowledge and knowledge under construction.

Socioscientific issues and socioscientific argumentation

A majority of studies concerning SSIs focus on students' argumentation skills. Research designs often include debates between peers and individual decision-making about an SSI. Most of the studies reveal the weakness of students' skills in argumentation whatever their age (Khun, 1993). Many studies underline the lack of correlation between science conceptualizations and argumentation skills [START_REF] Perkins | Everyday reasoning and the roots of intelligence[END_REF]Sadler & Donnely, 2006). Several debate situations have been proposed to encourage students' argumentation. For example Simonneaux (2001) compared role-play to debate using an issue in animal transgenesis. She concluded that debate is more efficient than role-play to encourage students' discourses in science. Other studies suggested encouraging students to compare contradictory discourses. Students can integrate and internalize information from media reports and the diversity of the viewpoints expressed [START_REF] Ratcliffe | Science education for citizenship. Teaching socio-scientific issues[END_REF]Simonneaux, 2004;Federico-Agraso & Jiménez-Aleixandre, 2006). Zohar & Nemet's study (2002) also highlights the interest for students to get involved in a time consuming and metacognitive work about argumentation.

We are particularly interested in proposals taking into account the consequences of the debate context on students' argumentation and decision-making. Several authors [START_REF] Patronis | Students' argumentation in decision making on a socioscientific issue: implications for teaching[END_REF][START_REF] Kolstø | Consensus Projects : teaching science for citizenship[END_REF][START_REF] Jiménez-Aleixandre | Doing the lesson" or 13 "doing science": Argument in high school genetics[END_REF] dealing with the inclusions of SSI in science classrooms have pointed out the importance of context for improvement of argumentative skills. The main hypothesis is that personal connections to local, but not global, SSI will improve students' argumentative skills. The majority of these studies dealt with environmental SSI. For example Spanish students debated the Prestige oil spill, which hit the Galician coast in 2002 [START_REF] Jiménez Aleixandre | Scientific Authority and Empirical Data in Argument Warrants about the Prestige Oil Spill[END_REF]. However, few studies have addressed contextualization involving interactions with experts in the field, particularly researchers. These studies revealed that this setting does not allow students' opinions about science and scientific knowledge to be transformed (Trabelsi Chalgoumi, 2006;[START_REF] Simonneaux | La perception des sciences par des lycéens est-elle modifiée par la présentation par des chercheurs de leurs travaux ? Biannual meeting of Association pour la recherche en didactique des sciences et des techniques (ARDIST)[END_REF].

In this study, we chose to analyze a controversial health-related SSI and we assessed the effects of contextualization on students' argumentation, by encouraging direct interaction with specialists. The use of human embryonic stem cells (hESC) for research or treatment of neurodegenerative diseases is considered an 'acute' socioscientific issue for multiple reasons. In the 1990s neuroscientists proposed the use of embryonic stem cells to generate healthy neurons to replace brain neurons damaged by trauma or compromised by disease, like Parkinson's disease or Huntington chorea. Controversies on how to obtain and store hESC (issues of proliferation and differentiation) and on their therapeutic use (feasibility, therapeutic efficiency, risks for the patient to develop tumors…) exist within the scientific community. The use of the human embryo as a source of totipotent cells is influenced by social representations and value systems due to the fact that isolation of hESC requires embryo destruction and raises the question of the embryo's status. It is therefore highly controversial in society. The benefit-to-risk ratio for patients treated by cell therapy are debated in society and at the time of this study, the ethical framework for therapeutic use of stem cells in France was provided by restrictive legislation on hESC use that was in place at the time (Biomedical Agency) 2 . The last controversial point concerns therapeutic cloning, by which scientists create human embryos through nuclear transfer and extract stem cells for cell therapy, thus avoiding immuno-compatibility problems. Obviously, the large media coverage of the social dilemmas related to hESC use contributes to making this issue 'acute' in the classroom because students are not entirely naïve on the subject. However, teachers often do not believe students will be able to deal with it 3 .

Use of human embryonic stem cells as a socioscientific issue

Questions of research

Our work concerns a comparative assessment of debate situations within two frameworks. We both evaluated the effect of the location of the debates (a cultural science center within a scientific institute) and the effects of the contextualization of the debates (meeting with experts) concerning students' argumentation and decision-making. In the students' argumentation regarding hESC uses, we paid particular attention to knowledge in science and about science. The test consisted in evaluating the content and quality of students' argumentation in two situations. In the contextualization framework, we tested the effects of the presence of a representative of an association of patients suffering from a neurodegenerative disease, together with a researcher specialized in the topic (contextualized group). In the control framework, students benefited from discussion with only one expert, the researcher (control group).

Methods

Participants

Seven science classes of high school students (107 girls and 89 boys, mean age: 16.4 yrs) from the same grade-level, from French high schools of the Provence Region including urban and 2 In 2004, during the bioethics law revision, the French government declared a moratorium concerning hESC research. Starting in 2006, research was allowed, for a five-year period, on stem cells of supernumerary embryos from in vitro fertilization or on imported hESC lines. (See http://assemblee-nationale.fr/12/dossiers/bioethique.asp 3 Without developing media coverage of hESC uses, let us specify that national generalist newspapers (Le Monde, Le Figaro, Libération…) and specialised press (La Recherche, Pour la science) dealt with this issue about one time a month since 1990s'. According to political or legislative agenda, media coverage can increase (bioethics laws revision, popular consultation as 2005 Italian referendum…). Scientific papers are also relayed by the media as the well known example of first reported nuclear transfer in humans by south Korean researcher Hwang W.S. [START_REF] Hwang | Evidence of a pluripotent human embryonic stem cell line derived from a cloned blastocyst[END_REF][START_REF] Hwang | Patient-specific embryonic stem cells derived from human SCNT blastocysts[END_REF]. See also [START_REF] Jiménez Aleixandre | Justification and Persuasion about Cloning: Arguments in Hwang's Paper and Journalistic Reported Versions Research[END_REF].

Protocol of the structured debates

We assigned the theme of the debate to students 4-6 weeks in advance: Embryonic stem cells and human brain repair. They were required in a pre-test to give their definition of stem cells and to note their immediate questions. Then they had to seek background information and documentation during the 4-6 weeks preceding the debate.

The debate protocol included three successive sessions, each of one hour duration. The first day, students were provided with the entire 3-day protocol, objectives of a debate (to improve argumentative skills and to build a position regarding a SSI) and the elementary rules of a debate. During this first session (Day 1), students listed the background questions they wanted to ask the expert(s). They were also required to identify one or two major issues (questions) that would serve as an outline for the future debate. Then they discussed these questions with the expert(s) during the second session (Day 2) and took note of their answers. The debates occurred during the third session (Day 3) on the question identified in session 1.

We divided each class in two homogenous groups, control and contextualized 4 . The first and third sessions were identical for the two groups. In contrast, the second session differed. Control groups met a neuroscientist whereas the contextualized group met the same neuroscientist together with a representative of an association of patients suffering from a neurodegenerative disease (Parkinson's disease, Multiple sclerosis, Huntington chorea). After the last session, in a post-test, high school students were required once more to give their definition of embryonic stem cells (that may or may not have evolved) and their arguments for or against the use of embryonic stem cells in (1) scientific research and (2) treatment of neurodegenerative diseases. All participants agreed to be video and audio taped. One of us (G. M.) was the moderator and adopted the position of committed impartiality, that is to say he provided his own points of view while encouraging analysis of competing points of view on the controversial issues [START_REF] Kelly | Discussing controversial issues: Four perspectives on the teacher's role[END_REF] 5 .

Analysis of the debate content

We first performed a general analysis of pre-and post-tests in order to quantify the decision making regarding hESC uses, the eventual presence of arguments or not in the votes (claims with or without justifications or restrictions) and in particular of scientific knowledge based arguments to explain the votes [START_REF] Zohar | Fostering students' knowledge and argumentation skills through dilemmas in human genetics[END_REF]. These were counted, categorized and compared between control and contextualized groups. The general analysis allowed us to identify a classroom (one control group and one contextualized group) in which the group's argued 4 The science teachers of the class decided the group assignments on the bases of students' facilities to express themselves in task group. 5 Kelly postulated four positions which teachers might adopt: exclusive neutrality (positivistic approach surrounding sciences product free values truth), exclusive partiality (characterized by the deliberate intention to bring students to adopt a specific point of view), neutral impartiality (characterized by the unrealistic point of view sustaining that moderator should remain neutral and not reveal his own beliefs) and committed impartiality (the moderator, if asked to do so, tells his position on the subject of debate). 6 decision-making distribution was very close to the distribution when all of the students were quantified. We then performed a detailed qualitative analysis of the discourses of these two selected groups.

These were divided in sequences or episodes and underwent three levels of analysis (Simonneaux, 2003). In turn at speaking system, sequence is defined as dialogs with high semantic and / or pragmatic consistency [START_REF] Kerbrat-Orecchioni | Les interactions verbales. Approche interactionnelle et structure des conversations[END_REF]. For the first we analyzed the macrostructure of the debates: we identified the themes discussed, their recurrence and those common to both groups. Inductive analysis method was applied, that is we did not use any predefined set of categories for the analysis [START_REF] Kolstø | Assessing the science dimension of environmental issues through environmental education[END_REF]. We also quantified the total duration of the debates, the number of turns each student had at speaking, and the number of the moderator's speech acts. This analysis allowed us to characterize the dynamics of the debates and to select specific episodes concerning the human embryo's status for a more accurate comparative analysis. For the second level we performed a microscopic analysis of the quality of the arguments: we counted claims without justification, simple arguments relying on only one justification, and multiple level strategies involving several justifications that were linearly linked or interrelated, i.e. with counterclaims or rebuttals [START_REF] Toulmin | The uses of argument[END_REF][START_REF] Adam | Les textes: types et prototypes[END_REF] and finally the type of arguments used [START_REF] Breton | L'argumentation dans la communication[END_REF]. For the third level, we examined modalisation. Modalisation consists in the semantic way used by speakers to reveal their level of adhesion to their arguments. We classified the types of modalisation used into four categories, following [START_REF] Bronckart | Activité langagière, textes et discours. Pour un interactionisme sociodiscursif[END_REF]: logic, deontic, appreciative and pragmatic modalisation6 . The grid of analysis was therefore a combination of different grids (Simonneaux, 2003;[START_REF] Federico-Agraso | Therapeutic cloning? Discourse genres, ethical issues and students' perceptions[END_REF] for a more robust analysis.

Results

First we present the macroscopic analysis of the students' (n = 196) decision-makings and written arguments in the post tests, and then the microscopic analysis of the two selected groups' argumentation comparing the control and contextualized sessions of the oral debate.

Macroscopic analysis of the students' decision-making and arguments in their post-tests

More than 75% of the students from the control and contextualized groups voted in their post tests in favor of the use of human ESC in research only as early as 2006. In contrast, 80% of the students from control groups voted in favor of the therapeutic use of hESC, compared to only 60% of the contextualized group. This latter group, compared to the control group, showed an increase in the response "no" and in the "may be" response (figure 1).

Please insert Figure 1 about here On the whole, weakness of students' skills in argumentation was obvious when they expressed their decision-making during the post-tests. Some decision-making lacked justifications or restrictions, but the arguments given were rarely simple, often relying on many linearly linked Concerning arguments in favor of hESC uses, they were in order of occurrence:

-The hope "to cure", "to save lives", was the main argument used by students to justify their vote in favor of the use of hESC for research and therapy (63 occurrences). That category of arguments was twice as much in the contextualized groups (42 occurrences) than in the control ones (21 occurrences). Moreover, students of the contextualized groups expressed more often restrictions that took into account the potential risks of cellular therapy in their argumentation in favor of the use of hESC (14 occurrences vs 11 for students of the control groups). The most prevalent justification was: "To save lives, we must accept the sacrifice of other lives"

-The progress and freedom of scientific research was also used as justification. The written arguments were for example: "This can make the world better", "hooray for science and progress". We found 42 occurrences of that kind of arguments in favor of the use of hESC for scientific research, without significant differences between the control and contextualized groups.

Concerning students in favor of the therapeutic use of hESC, arguments for the progress of research were twice as much in the contextualized groups (8 occurrences) than in the control ones (4 occurrences) -The embryo is not a human being was an argument used 13 times: "the embryo is a group of cells, [a piece of meat], thus it does not think ". That kind of arguments was equally used in both groups.

In addition, there were 23 students who did not justify their vote in favor of hESC uses in the control groups compared to 11 in the contextualized ones. Even if students' skills in argumentation appeared globally weak, the contextualization participated in enhancing students' ability to justify their written opinions and to precise their limits.

Concerning the few students that were against the use of hESC for research or therapy (only 10% of the total), their arguments were more unsophisticated, often with only one justification:

-The embryo is a future human being and its legal status is not clear (12 occurrences). The words "crime", "to kill", "to sacrifice" were used, without significant difference between the control and contextualized groups. Theological or ideological arguments were presented: "Today society has gotten too far away from the principles of life", "I am a neoconservative", "We must leave natural selection to make decisions about life and death".

-The hESC therapy is risky and further research is needed "before being able to cure, we need research". That category of arguments was often associated with the one concerning the possibility of alternate therapies, such as those using adult stem cells (16 occurrences of which 10 were from the contextualized groups); -This could lead to oocyte trading and the commercialization of life (9 occurrences); -Therapeutic cloning can lead to reproductive cloning (3 occurrences).

In conclusion the majority of students were in favor of the use of hESC for research and in the clinic as early as 2006, although they were fully aware of the potential risks in such treatment of neurodegenerative diseases, as researchers explained them. Concerning nature of sciences, students expressed a strong ideology in favor of the progress of science but they did not clearly understand the differences between fundamental, clinical research and therapeutic To the extreme, a few students were against research on stem cells but in favor of their therapeutic use.

The meeting with both a researcher and a representative of a patient association (contextualized groups) dampened the above arguments, as the students better understood the risks of the therapeutic use of hESC previously explained by the researcher. This is a paradox as the patients generally lobby for the use of hESC in cellular therapy. Students that met both types of experts (a patient and a researcher) had the opportunity to hear two different viewpoints. This may also explain why their arguments are more complex, and include restrictions and refutations. For the other 50% of contextualized groups, the ideology of the progress of science was probably reinforced by the emotions brought by the meeting with a patient suffering from a neurodegenerative disease. The contextualization radicalized the decision-making.

Microanalysis of the argumentation in control and contextualized sessions

We chose to precisely analyze the control and contextualized groups of a single classroom because the decisions and arguments in post tests were representative of the mean (Figure 1). In the contextualized group, the duration of the discussions between students and the number of turns at speaking were higher that in the control group, where the majority of the feelings expressed did not lead to discussions. Conversely, the number of interventions of the moderator was, proportionately to the debate duration, a little bit lower. This indicates a better involvement of the students in the discussion (Table 1).

Please insert Table 1 about here Moreover, dividing debates in sequences allowed us to identify common themes to each group (human embryo's status, cellular therapy risks, therapeutic and reproductive cloning) and some specific to one or the other (Table 2). The latter were twice as numerous for the contextualized group. Therefore contextualization introduced dynamism in the exchanges between students, with more attention paid to what the other students are saying and motivation to promote their own opinions.

Please insert Table 2 about here

Comparative analysis of the debates showed that students of the contextualized group were twice more engaged in episodes of argumentation than in simple assertions. This comparative analysis corroborates the findings concerning argued decision-making analysis of the students in the post tests. In the contextualized groups, students were more involved in the debates. Emotion generated by the meeting with a patient suffering from Parkinson disease positioned debates in the complexity of the reality and led students to pay attention to their responsibility when they are debating about hESC uses. Contextualization helped students to develop argumentation skills (reinforced agreements, specifications or restrictions). Furthermore, contextualization drove many students to diversify their conceptions regarding the nature of science. Faced by the risks of cellular therapy for the patients, many students in the contextualized group considered alternative therapies such as deep brain stimulation or alternative research such as adult stem cell research. In addition, legal (diversity and changes in national legislations) or fiscal (funding systems of research) aspects of science were addressed only during contextualized group debates. We performed a detailed analysis of the microstructure of two episodes (one from a control group and one from a contextualized group) on embryo's status, which is the major issue of the debates. We identified the different strategies of argumentation, the speech acts, the rhetorical moves and the modalisation indices in the verbal interactions. We could not draw robust conclusions about the influence of the contextualization on the quality of the oral debates at this analysis scale. Microanalysis of the quality of arguments was inconclusive because it did not detect significant differences between the two groups, as we may be reaching the limits of such a comparative analysis. Indeed, at the other scales of analysis taking into account the whole debates, and not only a single episode, the nature of students' argumentation appeared better in the contextualized groups than in the control ones. Communication between high school students appeared to be more influenced by psychological and sociological factors (whether they were used to speaking in front of a large audience, whether they were militant for a cause, or familiar with acting as a leader, girl or boy…). This meant that some students (3 out of 15 students of the group) did not participate in the discussion. This does not mean that they did not pay attention to the discussion. In fact, they developed a panel of written arguments in the context of their decision-making, at the end of the third session.

Microanalysis of arguments during debates

Students had difficulties developing an argumentation while they were speaking. The moderator had to reopen the debate and to regulate it quite often in order to let someone speak, insist that students pay attention to the person speaking, or ask for explanations on what has been said. Often, declarations were imposed without arguments or with very simple ones. The large majority of arguments revealed a low level of adhesion from the speakers. Arguments relying on social or logical values, i.e. with deontic modalisation, were the least present. The analysis of the episode of debate on embryo status (Table 3) sheds light on the relationships between scientific knowledge and opinions expressed by students (control group). To elaborate their position on embryo status they used scientific arguments to prepare the audience to receive their opinion in favor of the use of embryonic stem cells. That kind of argument, called a framing argument (argument de cadrage in french) according to [START_REF] Breton | L'argumentation, entre information et manipulation[END_REF], is often used by students.

Please insert Table 3 about here Hassen and Corentin (at time 50,56,58 and 55,respectively), reduced the embryo to its biological definition ("a group of cells" time 50 and time 55), skipping any human dimension (time 56, 58). For Elsa, the embryo is not even alive (time 53). The scientific arguments concerning the absence of nervous system and therefore cognitive functions ("they do not yet have neurons" time 50, "it does not have a nervous system" 52) were aimed at confirming the non human character of the embryo. They refer to a calendar of development of the central nervous system that they never explain. These definitions of the embryo from a biological point of view only have the ability to make the audience accept its destruction or the use of its cells. And when Hassen introduces the researcher in the discussion (time 50, 'she' explained) it is as an authoritative argument. Rewording processes reducing the human embryo to his / its cell dimension occur in these kinds of sequences. The implicit purpose is to present the embryo as an object more than a subject and so to legitimize its / his use for research or therapy. Rewording processes concerning the embryo's status appears as a framing argument to make the others accept the human embryo as an object and not as a subject.

Discussion

In the contextualized situation, high school students participated much more in the debate on the possible use of hESC for research and therapy. Meeting a patient suffering from a neurodegenerative disease seemed to have generated emotions, motivation and a sense of responsibility that helped students to better understand the consequences of their votes. According to previous research, the type of contextualization tested in this study (meeting with experts and in particular a representative of an association of patients suffering from a disease) contributed to improve situated cognition by giving a meaning to the debates [START_REF] Jiménez-Aleixandre | Doing the lesson" or 13 "doing science": Argument in high school genetics[END_REF], 2004;[START_REF] Kolstø | Students' argumentations: Knowledge, values and decisions[END_REF][START_REF] Patronis | Students' argumentation in decision making on a socioscientific issue: implications for teaching[END_REF]. The diversity of viewpoints, scientific doubts, and uncertainty of the results and risks for patients, became more obvious when debates were in a real-life context. Also, students better recalled their scientific knowledge (ESC proliferation and differentiation, brain plasticity) as if having met with a patient added a new dimension to what they had learned before. Science then became a social activity with legal (the bioethics law and its evolution) and economic (how research is financed) aspects.

The above conclusions should not occlude the fact that concomitant consultation of a scientific researcher and a patient reinforced positivism and the idea that beneficial clinical applications always emerge from basic research. In a recent study Simonneaux and Simonneaux (2008) studied students' reasoning about three controversial SSI characterized by a local contextualization (the reintroduction of wolves in Mercantour and that of bears in Pyrenees in France) and a global one (global warming) and pointed out "the limits of a local contextualization that involves students too much". They observed a lower level of critical analysis and of knowledge appropriation in the case of local issue compared to a global one.

In addition, for most of the students, the place where the debate was organized (a scientific institute), the choice of scientific experts (scientific researchers who were all in favor of the use of ESC in research) probably had an impact on their decision-making. We think that the location reinforced the ideology of progress due to scientific activity that was formulated by students when they declared themselves favorable to cellular therapy, in spite of the risks raised by the researchers. This did not motivate them to challenge science authority and legitimacy as revealed by Federico-Agraso and Jiménez-Aleixandre (2008) concerning students' perception of scientific papers. Moreover, [START_REF] Jiménez-Aleixandre | Doing the lesson" or 13 "doing science": Argument in high school genetics[END_REF] had previously pointed out that often students give the arguments that teachers were waiting for, in particular for their first debate as in the present study. Science museums or centers for scientific culture may be placed more adapted to debates on SSI (appropriate public spaces for public use of reason according to [START_REF] Rasse | Les musées à la lumière de l'espace public[END_REF]. High schools also would be more adapted if the classroom is a democratic place where students can freely express their opinions without any religious pressure.

Weakness of students' skills in argumentation was the other prominent characteristic of the debates. Even if we detected a difference between students' argumentations in contextualized and control groups, both had difficulties in constructing arguments. Students used a small panel of arguments (they rarely mentioned ethical, legal, economic or social aspects of the question) and gave arguments without justification, with authority, or accompanied by simple analogies. Students deeply need to learn the difference between arguments and emotions, to gain ability to construct argumentation to defend their position i.e. to advance a reason for or against, a proposition or course of action. This confirms previous studies pointing out the difficulty of [START_REF] Jiménez-Aleixandre | Doing the lesson" or 13 "doing science": Argument in high school genetics[END_REF][START_REF] Kuhn | The skills of argument[END_REF][START_REF] Sadler | Informal reasoning regarding socioscientific issues: A critical review of research[END_REF]Simonneaux, 2003…). Only long-term efforts seem to give positive results, as proposed by [START_REF] Zohar | Fostering students' knowledge and argumentation skills through dilemmas in human genetics[END_REF] with a protocol including eleven sequences of work and a specific training to argumentation. Practices such as assessing alternatives, weighing evidence, interpreting texts, and evaluating the potential viability of scientific claims are all seen as essential components in constructing scientific arguments [START_REF] Latour | La Vie de laboratoire. La production des faits scientifiques. Paris : La découverte[END_REF]. So too are scientific controversies and the collective analysis of the different ways a scientific result has been communicated (press release, audio-visual supports, numerical, exhibitions….) (Federico-Agraso & Jiménez-Aleixandre, 2006;[START_REF] Sadler | Informal reasoning regarding socioscientific issues: A critical review of research[END_REF]. This is too often neglected in science curricula.

Science is usually "taught as a nearly unmitigated rhetoric of conclusions in which the current and temporary constructions of scientific knowledge are conveyed as empirical, literal and irrevocable truths" (Schwab, 1962, p. 24). As pointed out by [START_REF] Simonneaux | La perception des sciences par des lycéens est-elle modifiée par la présentation par des chercheurs de leurs travaux ? Biannual meeting of Association pour la recherche en didactique des sciences et des techniques (ARDIST)[END_REF], frontal transmission of scientific results via conferences given by a researcher to students without any upstream preparation or downstream discussion reinforces the image of the power of science and ideology of progress through science. On the contrary, students deeply need to understand the instability of scientific results as they rarely study epistemology in their curriculum. Also they did not understand how scientific data are produced by researchers, and that they are continuously debated within the scientific community and result from a collective work. The majority of students ignored the difference between basic and applied research and confused basic science with its technological applications. As students are usually keen and enthusiastic to meet researchers, if such meetings are well prepared, it gives them the opportunity to learn more about a subject and understand the instability of scientific results. When debates are thus inserted in a global strategy, students also learn how science is 'produced' in laboratories.

Placed in a socioconstructivist context, science teaching through debates implies trusting students' ability to generate knowledge and to develop scientific expertise that they will use outside classrooms. It implies that teachers used to being the one 'who knows' become facilitators of the collective construction of knowledge. Debates on SSI allow teaching of science within a social context. Learning how to debate appears therefore essential for the future citizens that will have to make scientific and technical choices during their life. To achieve such a goal, each classroom has to be seen as a community that will produce knowledge instead of one that will consume knowledge [START_REF] Jiménez-Aleixandre | Knowledge producers or knowledge consumers? Argumentation and decision making about environmental management[END_REF]. In this perspective, students would choose the question that they want to debate within the framework of the general topics proposed by the teacher. He also proposes that the aim of the debate sessions is that students' argumentation progressively evolves as they integrate new scientific and technical knowledge with societal aspects. Meeting with a diverse group of experts should help them to achieve this goal. This also helps students to identify their own position in an argument.

Are French science teachers trained in a way that would enable them to debate SSI with students? This would require knowledge regarding how science is produced, and an ability to place scientific within historical and social contexts. But initiation to scientific research, and studies of the historical aspects of science and epistemology are absent from their coursework. Lack of opportunities for the practice of argument within science classrooms, and lack of teacher's pedagogical skills in organizing argumentative discourse within the classroom are significant impediments to progress in the field. Structured classroom debates help advance two goals many teachers struggle to achieve with their students: classroom participation beyond the [START_REF] Oros | Let's Debate: Active learning encourages student participation and critical thinking[END_REF] and critical thinking on the scientific claims generated by the plethora of SSI that confront them in their everyday lives.

Conclusion

Meeting with researchers appears to be a positive experience for students debating a socioscientific issue, which increases their knowledge of science and the nature of science. Concerning health-related issues, their meeting with patient association representatives gives more meaning to their knowledge, providing a connection between science and their everyday lives. To make the most of these meetings, students must be prepared to put actors and discourse into perspective, and to evaluate the plurality of viewpoints with the necessary critical distance.

  High school students debate the use of embryonic stem cells: the influence of context on decision making

  engaged in debate following the protocol described below. They met with seven different scientists and (or not) four representatives of associations. Debates took place in2005 and 2006, that is to say, during the time in which restrictive legislation on hESC use was in effect.

  suspects'' present in every classroom

Figure 1 :

 1 Figure 1: Diagram showing the votes of high school students in response to

  cloning and its links to reproductive cloning Themes specific to the control group Science must go on The opportunity of the debate, the social legitimacy of adolescents compared to legislators The ethic committees and their roles Themes specific to the contextualized group Alternate therapies (deep brain stimulation) The different laws about the use of ESC for research in different countries Animal models (Parkinson) The neuroscientist and his work The patient suffering from Parkinson's disease, his symptoms, his everyday personal and social life Problems linked to a referendum (example of abortion)The cost of research The possible use of adult stem cells instead of embryonic ones

  ,

	2004;
	F o
	r
	P
	e e r
	R e v i e w
	O n
	l y
	3

Table 1 :

 1 Quantification of the exchanges in the two groups

	Control	Contextualized

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uk International Journal of Science Education

	F o
	r
	P
	e e r
	R e v i e w
	O n
	l y

Table 2 :

 2 The different themes discussed in the control and/or contextualized group

	Themes

Table 3 :

 3 Arguments given during a debate episode on the embryo status As to ethical problems, she explained that when they get the embryos they are just a group of cells, they do not yet have neurons, they are not human beings. religion, this a religious and ethical question / it has nothing to do with science/ it is a human embryo that could become a human being if we left it to grow / it's not something totally alive / it's alive but Objection based on scientific knowledge of the definition of the embryo status 55 Corentin The embryo has no shape since it is just a group of cells / it does not speak, it does not eat, it is unconscious of what it is

			Page 18 of 18
	Speaker	Oral sentences	Speech acts
	00 :10 :20		Argumentation
	50 Hassen	[…] Confused knowledge about the
			embryo status
			Simple argument (with authority)
	51 Moderator	Do you all agree?	Reopening of the debate
	52 Denis	it does not have a nervous system (he thinks), it is not	Agreement with addition of
		a human being	knowledge
	53 Elsa	We cannot say that it is alive, it becomes alive only	Non justified agreement
		later?
	54 Victor	It depends on Simple argument (de cadrage)
	56 Hassen	We can't say that it is human just because it is made	Same argument as 6 minutes
		of cells	before
	57 Victor	Scientifically speaking, it is not more developed than	Agreement reinforced
		a bacteria, or just a little more
	58 Hassen	than an animal.	Agreement reinforced

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In French context, whereas the « éducations à » (education to health, human reproduction, environment) existed in sciences curricula since 1970's, science and society interlinks were introduced per se during the last reform

2000- 2001. In this context, the field of research dealing with SSI is called 'didactique des questions socialement vives' and analyses the teaching of 'socially 'acute' questions'[START_REF] Legardez | La construction des objets d'enseignements scolaires sur des questions socialement vives: problématisation, stratégies didactiques et circulations des savoirs[END_REF] Simonneaux & Simonneaux, 2004).

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Logical modalisations consist in truth value judgment of formulated opinions (considered as certain, possible, uncertain, probable…). Deontic modalisations consist in formulated opinion appreciation considering social values (socially allowable, unsuitable, necessary, advisable…). Appreciative modalisations are the expression of a more subjective judgment, facts are considered as felicitous, unfortunate, strange… Lastly, pragmatic modalisations refer to someone's responsibility (will, objective, abilities, reasons…).

Acknowledgment:

G.M. thanks Claude Caussidier for invaluable help.

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International Journal of Science Education


Thevenet Jean

Siroux Monica
email: monica.siroux@univ-valenciennes.fr

Desmet Bernard

Keywords: two-color pyrometer, brake disc, temperature, emissivity

A fiber optic two-color pyrometer was developed for brake disc surface temperature and emissivity measurements. The two-color pyrometer consists of a fluoride glass optical fiber, two HgCdTe detectors equipped with bandwidth filters and a data conditioning and acquisition device. The two-color pyrometer measures the brake disc temperature in the 200-800°C range with a time resolution of 8 μs. The calibration formula for the signals obtained using a blackbody of known temperature is used to compute the true temperature. The uncertainty estimation for temperature and emissivity was obtained from the calibration results. Tests were carried out on known temperature target and a good correlation was found between results obtained with our two-color pyrometer and those obtained with a commercial two-color pyrometer. Hold braking and deceleration braking tests performed on a braking test bench enabled us to reach the brake disc surface temperature and emissivity during braking. Experimental results show a significant variation of emissivity during braking. Direct measurement of emissivity was carried out on the brake disc after braking and shows the emissivity dependence with the surface quality.

Measurements of brake disc surface temperature and emissivity by two-color pyrometry

Heat generated by the pad-disc contact during braking is the main cause of surface degradations such as oxidation, wear and thermo-mechanical failure, in particular, cracking [START_REF] Cristol-Bulthé | Mechanical and chemical investigation of the temperature influence on the tribological mechanisms occurring in OMC/cast iron friction contact[END_REF]. In order to predict the consequences of thermal damage, several techniques have been developed to measure the temperature of sliding mechanical components:

thermocouples techniques such as embedded thermocouples and dynamic thermocouples [START_REF] Kennedy | Thin film thermocouple arrays for sliding surface temperature measurement[END_REF][START_REF] Dufrenoy | Etude du comportement thermomécanique des disques de frein vis à vis des risques de défaillance[END_REF][START_REF] Denape | Aspect thermique du frottement : mise en évidence expérimentale et éléments de modélisation[END_REF] and infrared techniques: pyrometry [START_REF] Siroux | Thermal analysis of periodic sliding contact on a braking tribometer[END_REF] and thermography [START_REF] Majcherczak | Tribological, thermal and mechanical coupling aspects of the dry sliding contact[END_REF].

Concerning optical temperature measurements during braking, some difficulties must be overcame: reduced size of the hot spots (about some millimeters during a high speed train emergency braking), fast phenomena [START_REF] Benillouche | Thermographie infrarouge d'objets tournant à grande vitesse[END_REF] (some hot spots can have an observation time under 1 ms), difficulty of access. On the other hand, radiometric techniques need the knowledge of the brake disc emissivity [START_REF] Matteï | Traité Génie Énergétique Rayonnement thermique des matériaux opaques[END_REF]. This parameter depends on various factors, often time varying and depending of parameters such as the object temperature and its surface quality [START_REF] Hervé | Mesure de l'émissivité thermique[END_REF][START_REF] Hladik | Métrologie des propriétés thermophysiques des matériaux[END_REF][START_REF] Sacadura | Measurement Techniques for Thermal Radiation Properties[END_REF][START_REF] Cabannes | Pyrométrie optique, Traité : Mesures et contrôle[END_REF]. The bispectral or multispectral pyrometry can be a possible solution to overcome these difficulties: the radiometric measurements are simultaneously performed for two or more spectral ranges, allowing at the same time both the emissivity and temperature determination. Many authors have developed twocolor or multi-color pyrometers with or without optical fiber [START_REF] Cabannes | Température de surface : Mesure radiative[END_REF][START_REF] Ferdinand | Thermomètres à fibre optique sans contact[END_REF][START_REF] Komanduri | A review of the experimental techniques for the measurement of heat and temperatures generated in some manufacturing process and tribology[END_REF][START_REF] Ueda | Thermal Behaviour of Cutting Grain in Grinding[END_REF][START_REF] Ueda | Temperature of Work materials irradiated with CO 2 laser[END_REF][START_REF] Zhao | Optical Diagnostics for soot and temperature measurement in diesel engines[END_REF][START_REF] Müller | Time resolved temperature measurements in manufacturing[END_REF][START_REF] Tago | Spectoscopic Measurements of High Emissivity Materials Using Two-Dimensional Two-Color Thermometry[END_REF]. Others authors have used infrared cameras to determine thermal mapping by monochromatic or multichromatic methods [START_REF] Inagaki | Proposal of quantitative temperature measurement using twocolor technique combined with several infrared radiometers having different detection wavelength bands[END_REF][START_REF] Legrand | Thermographie multispectrale haute et basse température[END_REF][START_REF] Viellard | Mesure simultanée de la température et de l'émissivité sur un four industriel de recuit en continu[END_REF][START_REF] Inagaki | On the proposal of quantitative temperature measurement by using three-color technique combined with several infrared sensors having different detection wavelength bands[END_REF][START_REF] Tago | Spectroscopic Measurements of surface temperature and emissivity by two-dimensional four-color thermometry with narrow band[END_REF]. The bibliographical study shows that increasing the number of wavelengths does not necessarily imply a better accuracy on the temperature and need an adequate emissivity model to avoid a possible divergence of the temperature evaluation [START_REF] Hladik | Métrologie des propriétés thermophysiques des matériaux[END_REF][START_REF] Cabannes | Pyrométrie optique, Traité : Mesures et contrôle[END_REF][START_REF] Ferdinand | Thermomètres à fibre optique sans contact[END_REF]. Moreover, in using a multispectral method, one has to employ several detectors or several optical filters, which complicates the design and increases the cost of the apparatus.

The problematic is more complex in the case of braking given that the disc-rubbing surface evolves constantly during the test [START_REF] Pajani | Thermographie infrarouge[END_REF][START_REF] Panier | An experimental investigation of hot spots in railway disc brakes[END_REF][START_REF] Cartigny | Cartographies d'émissivité de la surface d'un disque de frein[END_REF][START_REF] Desmet | Métrologie thermique du contact en freinage[END_REF]. On the basis of this context, of the specific problems involved in the brake disc surface temperature measurement and while taking account the cost of the apparatus, a fiber optic two-color pyrometer was developed, able to make temperature and emissivity measurement of a brake disc. The pyrometer is composed of two HgCdTe detectors equipped with two bandwidth filters, a fluoride glass optical fiber and data conditioning and acquisition device. The fiber optic two-color pyrometer allows the brake disc surface temperature and emissivity measurements in the 200-800 °C range with a time response of 8 μs and a small measurement spot (2.4 mm).

Principle of the method

The theory of the two-color pyrometry is given in several references [START_REF] Hladik | Métrologie des propriétés thermophysiques des matériaux[END_REF][START_REF] Cabannes | Pyrométrie optique, Traité : Mesures et contrôle[END_REF][START_REF] Ferdinand | Thermomètres à fibre optique sans contact[END_REF].

This method uses the Wien approximation of Planck's law [START_REF] Cabannes | Pyrométrie optique, Traité : Mesures et contrôle[END_REF][START_REF] Cabannes | Température de surface : Mesure radiative[END_REF][START_REF] Ferdinand | Thermomètres à fibre optique sans contact[END_REF][START_REF] Borca-Tasciuc | Temperature measurement of fine wires by photothermal radiometry[END_REF][START_REF] Sacadura | Initiations aux transferts thermiques, Technique et Documentation[END_REF] to determine the monochromatic luminance:

  5 2 1 , exp C L T C T                (1)
with C 1 = 1.1910 -16 W.m 2 .sr -1 and C 2 = 1.4410 -2 K.m.

The two-color pyrometry method measures the infrared luminance at two different wavelengths λ 1 and λ 2 . In practice, the measurement signal is a voltage directly proportional to the luminance observed by the detectors:

1 1 1 1 1 2 2 2 2 2 5 2 11 1 5 2 12 2 exp exp C S A L A C T C S A L A C T                                   (2)

S S

:

                          5 2 1 1 2 2 1 2 2 1 ln 1 1         A A S S C T (3)
Consequently, the surface emissivity is expressed by substitution of T in

1  S or 2  S : 5 2 1 exp i i i i S A C C T               i=1 or 2 (4)
The temperature uncertainty is given by the total differential method [START_REF] Lefèvre | Métrologie thermique adaptée à un dispositif de freinage ferroviaire[END_REF]:

1 2 1 2 SS T K T S S                 (5)
where

i i S S   
is the relative uncertainty on the wavelengths outputs and K an uncertainty coefficient which depends on λ 1 , λ 2 , T and C 2 [START_REF] Lefèvre | Métrologie thermique adaptée à un dispositif de freinage ferroviaire[END_REF]:

12 2 2 1 ( ) T K C     (6)
Increasing the separation of the wavelengths ∆λ=λ 1 -λ 2 reduces the temperature measurement uncertainty but the assumption of the grey body behavior can be less valid for large value ∆λ. Therefore the choice of the two work wavelengths is a compromise between the grey body assumption, the temperature measurement uncertainty and the choice of equipment material [START_REF] Cabannes | Pyrométrie optique, Traité : Mesures et contrôle[END_REF][START_REF] Müller | Time resolved temperature measurements in manufacturing[END_REF][START_REF] Lefèvre | Métrologie thermique adaptée à un dispositif de freinage ferroviaire[END_REF]. For T = 1000 °C (which represents hot spots temperature on a TGV brake disc during an emergency braking), the curves of constant value of K are drawn in the space λ 1 , λ 2 (Figure 1). Taking into account the temperature measurement uncertainty, the grey body assumption and the filters availability, the wavelengths chosen in this study are λ 1 = 2.55 µm and λ 2 = 3.9 µm. These values are satisfactory to insure a sufficiently small wavelength separation and a reasonable effect on measurement uncertainty (K = 1.31).

To estimate the error corresponding to the grey body behavior assumption, an error coefficient K CR can be defined from the difference between the measured temperature T 1 and the real surface temperature T 2 [START_REF] Lefèvre | Métrologie thermique adaptée à un dispositif de freinage ferroviaire[END_REF]:

CR K C T T T T                     2 1 2 2 1 1 2 1 1 ln 2 1       (7)
The coefficient K CR is a factor representative of the measurement error due to the grey body behavior assumption. The value of K CR estimated for the two wavelengths previously chosen and taking into account the variations of the brake disc materials (steel, cast iron) emissivities given [START_REF] Bauer | Spectral emissivity of bright and oxidized metals at high temperatures, Fifteen Symposium on thermophysical Properties[END_REF] is within 5%.

Experimental device

The two-color pyrometer

The portable two-color pyrometer shown in Figure 2, consists of a flexible fluoride glass optical fiber, two HgCdTe detectors equipped with bandwidth filters (Table 1) and a data conditioning and acquisition device. The optical fiber is mounted inside a flexible black tube in order to reduce reflections. The fluoride glass fiber has a high transmittivity, above 0.85 for the spectral range 0.5 -4 µm [START_REF]Document technique[END_REF]. The optical fiber attenuation is 0.3 dB/m for 4 µm. The core and clad diameter are 450 and 500 µm, respectively and the numerical aperture of the fiber is 0.2. The two detectors are mounted on a three-stage Peltier cooling element, to reduce the detector noise. The time response of the HgCdTe detector is shorter than 2 µs. However, the detector signal is amplified by HgCdTe preamplifier. The preamplifier consists of two stages. The first stage is a transresistance amplifier equipped with a bias and offset supply. The second stage is an inverting gain amplifier. Therefore the preamplifier is inverting and supplies a decreasing signal when the detector is exposed to radiation. The bandwidth of the preamplifier is 0-150 kHz and consequently the time response of the detection system is about 8 µs. The detectors equipped with bandwidth filters and the data conditioning and acquisition device are mounted in an enclosure to isolate the detectors from the environment. This enclosure is ventilated to avoid the heating of the different components.

The measured spot diameter, noted d, is a function of the distance between the fiber and the target, noted h. An experiment was carried out to verify this theoretical value. A source of visible light was used to visualize the spot of the optical fiber, and a graph paper gave the measurement of the spot diameter. The experimental and theoretical data are very close. The use of the optical fiber gives small measurement spots. For example, if the distance between the fiber extremity and the brake disc surface is 5 mm the spot diameter is about 2.4 mm.

Calibration procedure

The calibration procedure, which is very important for the temperature evaluation, consisted in heating a blackbody and obtaining the calibration curve of each detector.

The two-color pyrometer was calibrated using an extended blackbody HGH (type ECN 100 H6, emissivity  = 0.98 ± 0.02, temperature range 50 -550 °C, absolute error temperature ∆T = ± 0.5 °C). The temperature of the blackbody was varied from 200 to 550°C by steps of 50°C. For each temperature, 10 acquisitions were made with the twocolor pyrometer. The target is alternatively the extended blackbody (blackbody signal) and the environment at ambient temperature (ambient signal).

From the calibration, it appears that for a given ambient temperature there is a drift of the signal. A recording of the ambient signal and blackbody signal was made and the evolution of the ambient signal was confirmed. However, the signal drift during braking test (about 150 s) is too weak to influence the measurement. It was also observed that the difference between the blackbody signal and the reference signal remained constant, so a relative value was used:

i i i S S réf       , (8)
For each point of calibration, Some tests were carried out for temperature lower than 200°C to verify the sensor thresholds. For these temperatures, the signals from the detectors did not vary. During the calibration, the temperature of the ambient source is lower than the sensor threshold so the ambient signal can be used as the reference signal.

The detector noise is evaluated using a spectrum analysis. Both blackbody and environment are used as source and gives two similar spectrums. The noise of the detector has a wide band spectrum. Because the signal to noise ratio is about 200 at 400°C, the detector noise contribution is not significant.

Figure 3 shows the proportionality between the luminance and the output signal. The radiances are calculated for the spectral bands of the detectors. The two amplification constants are determined using calibration curves:

 1  A 2.39×10 -9 V.W -1 .m 3 .sr;  2  A
3.16×10 -10 V.W -1 .m 3 .sr (9)

Influence of the surrounding radiation on measurements

The influence of the surrounding radiation [START_REF] Cabannes | Température de surface : Mesure radiative[END_REF][START_REF] Duvaut | Multiwavelength infrared pyrometry: optimization and computer simulations[END_REF][START_REF] N'guessan | Contribution à l'étude des propriétés radiatives des matériaux opaques ou semi-transparents, à température ambiante, par une méthode radiométrique en régime modulé[END_REF] on measurement results has been studied. The two-color pyrometer received both emitted and reflected flux. The total luminance incoming from the sample is:

  55 22 11 exp 1 exp env CC L C C TT                     ( 10
)
where T is the surface temperature of the object and env T the surrounding temperature.

During the braking tests the disc surface temperature is higher than 400°C, the environment temperature is about 20°C, the optical fiber is placed close to the pad-disc contact using a black mounting bracket held near room temperature. At temperature higher than 300°C the ratio between reflected and emitted flux estimated using equation ( 10) is about 0.004, so the reflected flux can be neglected.

Uncertainty analysis

The temperature and emissivity uncertainties can be obtained using the Law of Propagation of Uncertainty [START_REF] Coppa | Normal emissivity of samples surrounded by surfaces at diverse temperatures[END_REF][START_REF] Afnor | Guide pour l'expression de l'incertitude de mesure[END_REF][START_REF] Priel | Incertitudes de mesure et tolérances[END_REF][START_REF] Taylor | Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results[END_REF][START_REF] Siroux | [END_REF]: 11) and ( 12) the relative uncertainties of the temperature and the emissivity can be determined:

              1 2 1 2 1 2 1 2 12 12 2 2 2 2 2 2 2 2 2 22 22 12 12 2 cov , T T T T T A A AA T T T T                                                                                               (11)         22 2 2 22 2 2 2 ii ii i i AT AT                                                  (12)
  12 1 2 1 2 1 2 1 2 1 2 2 2 2 2 2 2 2 2 2 1 2 2 1 1 2 2 cov , 2 55 AA AA T K T CC TT                                                                                              (13) 22 2 2 2 2 22 5 ii ii i i i i A CC T T T A                                                        (14)
In equations ( 13) and ( 14) the estimated uncertainty of the central wavelength

i i  
 is 1 % according to the manufacturer specification.

The curve

 

T f T  is presented in the Figure 4, for temperatures higher than 300°C, the temperature uncertainty is lower than 18 °C.

The curve

) (T f  
is presented in the Figure 4, the emissivity uncertainty remains lower than 0.21 and 0.15 for temperatures higher than 300°C and 350°C respectively.

Experimental validation

To validate the two-color pyrometer, tests were carried out on a disc covered with a known emissivity black paint (ε = 0.93±0.02) [START_REF] Siroux | Experimental study using infrared thermograhy on the convective heat transfer of a TGV brake disc in the actual environment[END_REF]. The surface temperature was measured simultaneously by our new two-color pyrometer (T λ1/λ2 ) and a commercially available Impac TM two-color pyrometer (T bichro ) (pyrometer reference IGAR 12 LO, temperature range 300-1000°C, measurement wavelengths: 1.52 μm and 1.64 μm). The temperatures obtained with the two pyrometers are almost identical. The average deviation between the temperatures recorded by the two pyrometers is about 2 °C and remains lower than the two-color pyrometer uncertainty who is about 14 °C. These tests enabled us to validate the method of measurement.

Braking test bench experimental results

Experiments were carried out on a SCHENCK braking test bench at the C3T (Ground Transportation Technological Centre) (Figure 5). This test bench is composed of four parts: the test cell, the drive system, flywheels and the base. The test cell is an enclosure ensuring a control of the room temperature and allowing a correct ventilation of the disc. The enclosure is equipped with two port-holes for the radiometric measurements. One end of the driving shaft equipped with a rotary transmitter is in the test cell. The drive system is a DC machine. The flywheels and the drive system are used to simulate the brake inertia.

The brake system is composed of a 259 mm diameter ventilated disc equipped with 2 K-type thermocouples placed at the friction radius 6 mm under the disc surface and two pads (pad surface: 30 cm 2 ) equipped with 2 K-type thermocouples placed 5 mm under the contact surface (Figure 6). The optical fiber of the two-color pyrometer is placed close to the pad-disc contact using a black mounting bracket which was held near room temperature in order to avoid reflections on the brake disc surface. The distance between the fiber and the surface disc is 5 mm. According to the characteristics of the optical fiber, the spot diameter is 2.4 mm. To control the measurement of the two-color pyrometer, the disc surface temperature is also measured by a monochromatic Impac TM pyrometer (pyrometer reference IP 140, temperature range: 75 -550°C, spectral range : 2 -2.8 μm).

Two kinds of test were undertaken on the C3T test bench (Table 2):

 Deceleration braking: decreasing revolving speed  Hold braking: braking with constant torque and constant rotational speed.

The sampling frequency is 100 Hz, in order to satisfy the sampling condition according to Shannon's theorem.  ε is the emissivity of the surface measured by the two-color pyrometer

Deceleration braking

The temperature increases quickly at the beginning of the braking, the maximum temperature is reached after 20 s, and then it decreases. The monochromatic temperatures T λ1 , T λ2 are similar to the monochromatic pyrometer IMPAC TM temperature T pyro . The monochromatic temperatures was measured by the monochromatic pyrometer IMPAC TM (IP 140) and by each detector of our two-color pyrometer assuming a blackbody behavior for the disc surface (ε = 1). The monochromatic temperatures (T λ1 , T λ2 , T pyro ) are so underestimated [START_REF] Cabannes | Pyrométrie optique, Traité : Mesures et contrôle[END_REF][START_REF] Cabannes | Température de surface : Mesure radiative[END_REF]. A difference of around 150 °C between the monochromatic temperatures (assuming a blackbody behaviour for the disc surface) and the two-color temperature is observed.

This difference can be explained by the brake disc emissivity evolution during the test.

The brake disc emissivity increases with the temperature reduction (Figure 7). The emissivity value is 0.45 at the beginning of the braking, 0.4 for the maximum temperature and 0.7 at the end of the test. This increase of the emissivity with the reduction of the temperature is coherent with the bibliography [START_REF] Tago | Spectroscopic Measurements of surface temperature and emissivity by two-dimensional four-color thermometry with narrow band[END_REF][START_REF] Pajani | Thermographie infrarouge[END_REF][START_REF] Cartigny | Cartographies d'émissivité de la surface d'un disque de frein[END_REF][START_REF] Desmet | Métrologie thermique du contact en freinage[END_REF]. The variation of the emissivity is the consequence of the evolution of the properties of friction surface during braking.

Hold braking

Figure 8 shows the brake disc temperature and emissivity evolution during hold braking test. As it can be observed in Figure 8, the frequency of the temperature oscillations corresponds to the frequency of the disc rotation. It can be seen that the monochromatic temperatures T λ1 and T λ2 obtained from the two detectors are close to the monochromatic temperature measured by the IMPAC TM monochromatic pyrometer T pyro . However, a significant difference between the two-color temperature T λ1/λ2 and the monochromatic temperatures T λ1 , T λ2 and T pyro is observed. Once again, this difference can be explained by the brake disc emissivity evolution during braking.

The emissivity is around 0.5 at the beginning of braking, decreases to around 0.35 for t = 20 s and increases to around 0.4 at the end of the braking sequence. The evolution of the emissivity over time can be explained by the temperature variation and the modification of the brake disc surface during braking. While the two-color temperature value is about 500°C, the estimated temperature uncertainty is lower than 14 °C (Figure 4) and the estimated emissivity uncertainty is lower than 0.09 (Figure 4).

Direct measurements of emissivity were carried out on the brake disc after braking.

The disc was heated at 550 °C and a part of the heated disc was covered with a known emissivity black paint (ε = 0.93±0.02). The surface temperature was measured with an infrared camera (CEDIP, λ = 3-5 μm) and the two-color pyrometer. The disc emissivity is determined using the ratio of the disc and the black paint luminance. Figure 9 shows brake disc emissivity values obtained during braking using the two-color pyrometer and brake disc emissivity obtained after braking using the infrared camera and the two-color pyrometer. As it can be seen in Figure 9, a good correlation was found between pyrometer and infrared camera direct emissivity measurements. However, emissivity values obtained after braking using the direct method are higher (about 0.8) than emissivity values obtained during braking (about 0.4). This difference can be explained by the oxidation of the disc surface observed after braking. This corresponds to the bibliographical study [START_REF] Bauer | Spectral emissivity of bright and oxidized metals at high temperatures, Fifteen Symposium on thermophysical Properties[END_REF] which shows that the emissivity at T = 600 °C and for λ between 2.5 and 5 μm, varies between 0.35 and 0.2 for bright metals and between 0.7 and 0.9 for oxidized metals.

Conclusion

A fiber optic two-color pyrometer with a short time response (8 μs) and a small measurement spot (2.

where 12 , 1  S and 2 S

 1212 AA  amplification constant of each detector.The measurement signals are functions of the temperature and the emissivity of the surface. λ 1 and λ 2 (grey body behavior) the temperature and the emissivity are obtained from the wavelength outputs . The target temperature is obtained from the signal ratio



  of the ambient signal and i S  is the average of the blackbody signal. So for each temperature and each luminance, 10 values of i can be obtained and consequently the calibration curves σ = f(L) are easily obtained.

Figure 7

 7 Figure7shows the brake disc temperature and emissivity evolution during

Figure. 5 .

 5 Figure.5. Representation of the test bench

Figure.1. "Iso-K" curves in the space (λ 1 ,λ 2 )

Figure.2. Experimental device

Figure.3. Calibration curves σ = f(L)

Figure.4. Temperature and emissivity uncertainties

Figure.6. Brake disc and pin : Temperature measurements

Figure.7. Brake disc temperature and emissivity evolution during deceleration braking

Figure.8. Brake disc temperature and emissivity evolution during hold braking

Figure.9. Brake disc emissivity

Acknowledgments

The authors would like to thank the European Community and the Région Nord -Pas de Calais for the financial support given to the present study.

good correlation was found between results obtained with our two-color pyrometer and those obtained with a commercial two-color pyrometer. Some experiments carried out on a braking test bench enabled us to reach the transient brake disc surface temperature during braking and proved the importance of the emissivity on the measurement of the disc surface temperature. A significant variation of the emissivity has been observed for both hold braking and deceleration braking. Direct measurement of emissivity carried out on the brake disc after braking shows the emissivity dependence with the surface oxidation. Further, the two-color pyrometer can be used to investigate the temperature effect on the tribological behaviour of the brake disc during braking.

Optical Fiber

Fluoride Glass Spectral range: 0.