O Benamara

K Lebbou

Orientations and seed type effect on Al 2 O 3 -YAG-ZrO 2 eutectic microstructure solidified from the melt by the micro-pulling down technique

Keywords: Eutectic, Seed, Solidification, Microstructure, µ-PD

) YAG, and c(0001), A(1-210), M(10-10) sapphire seeds, the YAG and ZrO 2 phases are oriented along the <100> direction parallel to the growth direction. The zirconia (111) seeding X-ray diagram eutectic presents additional peaks and the monoclinic ZrO 2 phase appears at the solidification rate of 1mm.min-1. The rods grown through ZrO 2 seeding are more stressed than those solidified by using eutectic, YAG and sapphire seeds, respectively.

1-Introduction

Unidirectional solidified eutectic oxides ceramics have a number of properties that make them of interest as potential engineering materials [START_REF] Minford | Crystallography and Microstructure of Directionally Solidified Oxide Eutectics[END_REF][START_REF] Hulse | The Effect of Eutectic Microstructures on the Mechanical Properties of Ceramic Oxides[END_REF][START_REF] Stubican | Eutectic Solidification in Ceramic Systems[END_REF]. They can be considered as composite material elaborated from the melt with microstructures strongly affected by the solidification parameters [START_REF] Llorca | Directionally solidified eutectic ceramic oxides[END_REF]. An important advantage of unidirectional solidified eutectic for high temperature applications is that their microstructure is extremely stable, practically up to melting temperature [START_REF] Nakagawa | High temperature strength and thermal stability for melt growth composite[END_REF][START_REF] Waku | A ductile ceramic eutectic composite with high strength at 1,873 K[END_REF]. This stability results from the facts that microstructures and morphologies were elaborated from the liquid state under thermodynamic equilibrium conditions [START_REF] Bayles | The effect of elevated-temperature exposure on the microstructure and tensile strength of Al sub 3 Ni whisker-reinforced aluminum(Microstructure stability of aluminum reinforced with Al-Ni whiskers, noting tensile strength and mechanical property dependence on temperature)[END_REF]. If grain boundaries are present in the microstructure they are relatively, few in number and quite often parallel to the solidification direction [START_REF] Llorca | Directionally solidified eutectic ceramic oxides[END_REF]. Due to the physical and chemical limit of non-oxide materials at high temperature, eutectic ceramics with high melting point (>1700°C), lower density (4.5g/cm 3 ) and high thermal conductivity are an excellent alternative to these materials. In the recent years, important research have been conducted to improve the knowledge of the microstructure morphology aspect of the eutectic oxides solidified from the melt.

Among eutectic oxide ceramic, the Al 2 O 3 -YAG-ZrO 2 [START_REF] Cherif | Croissance de la céramique eutectique Al₂O₃-YAG-ZrO₂∶Y et étude de la microstructure Chinese Script[END_REF][START_REF] Nagira | Chain structure in the unidirectionally solidified Al2O3-YAG-ZrO2 eutectic composite[END_REF][START_REF] Oelgardt | Sintering, microstructure and mechanical properties of Al2O3-Y2O3-ZrO2 (AYZ) eutectic composition ceramic microcomposites[END_REF][START_REF] Carroz | Tuning the sapphire EFG process to the growth of Al2O3/YAG/ZrO2:Y eutectic[END_REF][START_REF] Londaitzbehere | Céramiques eutectiques oxydes à microstructure interconnectée préparées par solidification dirigée : élaboration, microstructure, mécanismes de déformation par fluage et réactivité en présence de vapeur d'eau, phdthesis[END_REF] ternary system is of great importance. Directional solidification of this system has been carried out previously and most of these studies have centered around Bridgman, Floating zone and micro-pulling down techniques [START_REF] Waku | Temperature dependence of flexural strength and microstructure of Al2O3/Y3Al5O12/ZrO2 ternary melt growth composites[END_REF][START_REF] Mazerolles | New microstructures in ceramic materials from the melt for high temperature applications[END_REF][START_REF] Peña | Microstructure of Y2O3 doped Al2O3-ZrO2 eutectics grown by the laser floating zone method[END_REF][START_REF] Sayir | The effect of the microstructure on mechanical properties of directionally solidified Al2O3/ZrO2(Y2O3) eutectic[END_REF][START_REF] Laidoune | Yttria stabilized Al2O3-ZrO2 eutectic crystal fibers grown by the laser heated pedestal growth (LHPG) method[END_REF][START_REF] Lee | Microstructure of Y2O3 doped Al2O3/ZrO2 eutectic fibers grown by the micro-pulling-down method[END_REF][START_REF] Benamara | Microstructure and crystallography of Al2O3-Y3Al5O12-ZrO2 ternary eutectic oxide grown by the micropulling down technique[END_REF][START_REF] Benamara | Shaped ceramic eutectic plates grown from the melt and their properties[END_REF]. The melting temperature is around 1750°C, this system is stable and presents high-performance chemical and mechanical properties [START_REF] Waku | Temperature dependence of flexural strength and microstructure of Al2O3/Y3Al5O12/ZrO2 ternary melt growth composites[END_REF][START_REF] Lee | Microstructure of Y2O3 doped Al2O3/ZrO2 eutectic fibers grown by the micro-pulling-down method[END_REF]. During the solidification from the eutectic point, growth of single-phase dendrites, colonies or cells are observed. Their presence are strongly connected to the solidification rate, axial thermal gradient along the growth direction and the chemical composition in Al 2 O 3 -Y 2 O 3 -ZrO 2 equilibrium diagram.

In the case of Al 2 O 3 -YAG-ZrO 2 eutectic crystallisation, all phases are highly textured along preferred orientation. During eutectic solidification by using the micro-pulling down technique, when the growth rate became higher than 0.75 mm.min -1 , the crystallographic orientations change completely [START_REF] Benamara | Microstructure and crystallography of Al2O3-Y3Al5O12-ZrO2 ternary eutectic oxide grown by the micropulling down technique[END_REF][START_REF] Benamara | Shaped ceramic eutectic plates grown from the melt and their properties[END_REF]. Increasing the solidification rate results in a smaller grains size. The grains are chemically homogeneous and have either limited or no grain boundary segregation of impurities. In view of the advantage and the importance of this solidified system from the melt, the examination and the understanding of the growth parameters appears appropriate to control the performance and the application of this material.

However, most of the previous research have been made to explore stability of the microstructure under different experimental processes. To our knowledge, there are not studies on the ternary eutectic microstructures and their stability as function of the seed type and its orientation. In this paper, the effect of the seed type on the microstructure formation in Al 2 O 3 -YAG-ZrO 2 eutectic solidified from the melt by using the micro pulling down technique was investigated.

2-Experimental

The samples were made from commercial high-purity (>99.99%) Al 2 O 3 , Y 2 O 3 and ZrO 2 powders of nano-spherical shape. The powders were mixed in agate mortar at eutectic proportion according to the ternary eutectic composition 65mol%Al 2 O 3 /16mol%Y 2 O 3 /19mol%ZrO 2 [START_REF] Lakiza | Stable and Metastable Phase Relations in the System Alumina-Zirconia-Yttria[END_REF] . The mixture was sintered at 1400°C for 10 hours in air.

Eutectic ceramics rods were solidified by the micro pulling down technique (µ-PD) and the experimental set-up was described in our previous works [START_REF] Benamara | Microstructure and crystallography of Al2O3-Y3Al5O12-ZrO2 ternary eutectic oxide grown by the micropulling down technique[END_REF][START_REF] Benamara | Shaped ceramic eutectic plates grown from the melt and their properties[END_REF]. The growth process is carried out under flowing argon atmosphere. A CCD camera allows following in situ the growth process during all the experiments. Table 1 summarizes the different oriented sapphire, YAG and zirconia seeds used for the growth process. The rods are pulled under two pulling rates (0.5 and 1 mm.min -1 ). The whole melt in the crucible is solidified, and the crucible is empty at the end of the solidification. To study the homogeneity and the microstructures, samples are characterized along the rods.

Longitudinally and transversally, cut pieces about 2 to 5 mm in length are embedded in organic resin and polished. The SEM observations of the rods are performed with a Phenom desktop Scanning Electron Microscope (SEM) at 5KV.

Room temperature X-ray diffraction characterization is used to define the purity and the crystallographic texture of the eutectic rods. The X-Rays diagrams are carried out using a Bruker D8 advanced diffractometer with Cu-K α1 and Cu-K α2 X-rays (λ=0.15406 and 0.15444 nm). The diffraction diagram is scanned over a 2θ range from 15° to 100° in steps of 0.02° with a counting time of 16 s/step and continuous rotation (120 rpm) for bulk measurement.

The 2 E 4 A 2 fluorescence lines of chromium (Cr 3+ ) ions as natural impurity in the Al 2 O 3 host phase of the eutectic composite are used to evaluate the residual stresses in the solidified rods [START_REF] He | Polarization Dependence of the Cr3+ R-Line Fluorescence from Sapphire and Its Application to Crystal Orientation and Piezospectroscopic Measurement[END_REF]. The displacements of the fluorescence lines of Cr 3+ probe ions are measured at room temperature using a Horiba Jobin-Yvon micro-Raman spectrometer equipped with a CCD detector under confocal condition. A 473 nm laser wavelength is used as excitation source and focused on the surface of the sample through a 10X Olympus MIR plan objective with 0.25 NA in order to cover a large sample area. For each measurement, the argon emission spectrum is recorded simultaneously for calibration and was used as external reference. For each sample, a 50 point of measurement are taken on different areas of the polished transversal section and small algorithm program was developed to calculate the position of R1, R2, argon peak and the errors measurement. Finally, all the measured spectra are fitted using a Gaussian profile in order to determine the exact peak positions of Cr 3+ fluorescence lines.

3-Results

Microstructure and morphology

Fig. 1 shows the as solidified eutectic rods as a function of the seeds orientation. Controlled crystallization was obtained using pulling rates of 0.5 and 1mm.min -1 . The rods had a white color, macroscopic defects free (except with zirconia seeds), stable diameter (=5mm) and length around 100mm depending on the starting charge in the crucible. Except solidification through eutectic seed, the growth behavior was quite difficult using YAG, zirconia and sapphire seeds. This was because of the crystallization interface, which spontaneously moves from the steady-state position, resulting in freezing of the rods at the crucible bottom, or disconnecting, especially when zirconia seeds are used. The microstructural evolution can only be analyzed after interrupting the seeding solidification process. The detailed mechanisms by which the eutectic microstructures are found to be dependent on seeding is shown in figure 2. The microstructure and the morphology near the interface, the kinetics of solidification, the competition for the nucleation of phases and the nature of the successive phases formed are strongly affected by the seed type.

Whatever the initial seed, the system is initially filled with a binary system in the melt state with a composition C 0 , at a given temperature T. Due to heat extraction from the seed, the average temperature of the system decreases and the solidification starts with a given undercooling. Three zones could be identified, the solid zone, mixture zone composed of liquid and solid phases and finally a fully liquid zone.

The crystallization of eutectic from the melt through seeding different than the eutectic seed causes a local change in the composition, which is due to the equilibrium condition between three phases (Al 2 O 3 , YAG, ZrO 2 ). This difference between the phases distribution between the liquid and solid phases is connected to the solute redistribution or segregation, which has an important impact on the final microstructure as well as on the phase's volume fraction.

For this ternary eutectic system, the microstructure can be Chinese script, geometrical pattern, regular or irregular growth as a function of the growth parameters and the entropy of fusion (Table 2) of the three solid phases growing together [START_REF] Su | Rapid solidification of Al2O3/Y3Al5O12/ZrO2 eutectic in situ composites by laser zone remelting[END_REF]. On one hand, in the Al 2 O 3 -YAG-ZrO 2 ternary system, the entropies of fusion of YAG and Al 2 O 3 are much higher than 2R (R=8.3144 J mol -1 K -1 ), so they present strongly faceted growth. On the other hand, the ZrO 2 entropy of fusion is close to 2R, so the growth is slightly faceted and form rods or lamellae morphology. An interesting observation to point out when considering all connection steps in figure 2 is a systematic correlation between zirconia distribution and seeding. At the beginning of the growth, the average concentration of zirconia is higher when connection is realized through sapphire seed and the solidification process is very close to (Al 2 O 3 -ZrO 2 ) binary system [START_REF] Levin | Phase diagrams for ceramists[END_REF].

As observed in Figure 2, the size of ZrO 2 phase in Al 2 O 3 matrix is larger than that corresponding size in the standard Al 2 O 3 -YAG-ZrO 2 ternary eutectic [START_REF] Benamara | Microstructure and crystallography of Al2O3-Y3Al5O12-ZrO2 ternary eutectic oxide grown by the micropulling down technique[END_REF]. There is a delay of YAG precipitation appearing after 30µm of growth for sapphire a-axis seed and around 50 µm for c and m-axis, respectively. The most part of YAG accumulated in the interface starts to reach saturation, the solidification front is destabilized and a strong projection of YAG into the melt is accompanied by random YAG phase precipitation. The coarseness of the YAG into the melt is due to its entropy of fusion higher than Al 2 O 3 and ZrO 2 phases. It is clearly seen that the variation of the zirconia size is narrower when c and m-axis sapphire seeds are used. After few time, nucleation and growth of the eutectic microstructure start. This could be explained by the important thermal gradient after the connection by sapphire seed. Another hand, in addition, using eutectic and YAG seeds, the priming of the solidification process is controlled through Al 2 O 3 -YAG binary system [START_REF] Viechnicki | Eutectic solidification in the system Al2O3/Y3Al5O12[END_REF]. The microstructure is mainly composed of YAG lamella, sapphire and few ZrO 2 particles imbedded on YAG interface. The same scale is used for all seed connection. The evolution of each grain can be individually followed during the solidification process. As a function of seeds, experimental observation tend to show how the different phases interact in the melting zone near the crystallization interface. Figure 3 shows the film, where some grains can be seen in the molten zone as well as their movements before and after incorporation in the rod. In the case of eutectic seeding, the liquid is homogeneous in the front of crystallization interface and we have not seen any particles movement in the melting zone.

The crystallization front is plane from the beginning (t=4'15"), the stationary stable growth regime is reached after few minutes (6'08") of pulling (Fig 3c) and the growth doesn't introduce any undercooling or meniscus length increasing (Fig. 3d). The situation is completely different when sapphire, YAG and ZrO 2 seeds are used. In Fig. 3e corresponding to sapphire M(10-10) seeding, after 4:40 minutes of growth (Fig. 3f) two elongated grains (white color) are observed near the center of the crystallization interface as well as their movement and interaction before incorporation in the rod (t=6:55m, Fig. 3g). It should be noted that we have observed irregular grain coalescence until complete penetration in the eutectic rod. The liquid is a bit electrically conductive [START_REF] Cherif | On the growth and structure of Al2O3-Y3Al5O12-ZrO2:Y solidified eutectic[END_REF], electrostatic effects are likely to play a role in the stabilization of the grains position close to the interface. The overall force acting on the grain is apparently large enough to prevent the grains from flowing upward. After connection (Fig. 3j) and 4:31m later (Fig. 3k), few grains appear in the melting zone and successive particles are formed. As a function of the time (t=4:34-4:39m), the grain number increases and the average size varied from few µm to hundred µm (Fig 3i,9m). The progression and the shape of the visible grains in the melting zone are found closely correlated to the pulling rate, also explaining the geometry of the crystallization front. As the entropy of fusion of YAG is the higher one, the observed grown grains (Fig. 3m) in the melting zone are expected to be YAG in good argument with SEM image of YAG seeding (Fig. 2). In addition, the coalescence along the solidification axis are expected to the formation of YAG lamella as it is observed at the crystallization interface. The examination of this region of the samples revealed directional but poorly oriented microstructures. Extensive nucleation of new grains occurred near the solidification front, but these grains were only approximately oriented. At least more than 100µm of solidification is required before the better-oriented grains began to predominate, but soon after, the grain orientation became random and eutectic microstructure appears. The thermal gradient at the interface is high and the crystallization interface is convex. However, the thermal gradient decrease as the amount of the solidification increases. The decreasing of the interface thermal gradient resulted from the increasingly effective thermal insulation presented by the solidified eutectic along the growth direction. After few minutes of solidification, the interface crystallization becomes flat and stationary stable growth is observed.

By using ZrO 2 seed (Fig. 3n), a lot of tentative of seeding has been done to prime the solidification process. It was quite difficult to obtain melting zone and mix ZrO 2 seed with eutectic melt because of the Zr 4+ charge higher than Al 3+ and Y 3+ . Until proven otherwise, may be liquid immiscibility control the beginning of the solidification. In Fig 3o , after the first period of connection (t=11s), spherical particles remained stable very close or contacting the solid-liquid interface. However, in a second step, after 23s (Fig. 3p) the particles in the center of the image start to oscillate in the molten zone until fragmentation in two particles followed by penetration in the rod. The color contrast of the liquid changes and new particle appears at the center of the interface. After 7min53s (Fig. 3q), a small particle connected to the interface appears. The change from small particle to lamella (t=9min23s) is shown primarily by the diffusion requirements in growth (Fig . 3r). Few second later (10min01s), a cooperative growth phenomena is observed and two lamellas grow with a common interface (Fig. 3s). The cooperative growth of the two lamellas with a common interface can be shown to be faster process than either phase individually. After 12min: 23s, the growth indicates instability of the edge of the lamella (Fig. 3t). Therefore, only one of the lamella of the unstable edge will grow and again become unstable until complete fragmentation after 12min36s (Fig. 3u). The quantity of the crystallized lamellas near the solidification front increase with time (Fig 3v,3w). After growth and cooling down, it was possible to find back those particles at the beginning of the grown rod. The phase's precipitates are mainly ZrO 2 : Y and sapphire Lamella's.

It is important to note that we use iridium crucible, so we have note reaction of the melt with crucible. Chemical reactions of oxide (Al 2 O 3 , Y 2 O 3 , ZrO 2 ) in the growth chamber, in order to produce CO gas, are not possible in our case where no graphite parts are used in the chamber.

The incorporation of the grains in solidifying rod as a function of seeding is a very important phenomenon, which we followed during the solidification process. Most likely various mechanism play a role in this phenomenon like instability of the liquid as a function of seeding, local thermal or chemical effects, capillarity, viscous drag, thermal conductivity of the particles, or chemical segregation. In comparison to the others seeds (Eutectic, Sapphire, YAG), the ZrO 2 seeding has a memory effect on the stability of the microstructure and the morphology. shows that the microstructures crystallized at pulling rate of 1mm.min-1 are smaller than those pulled at slower solidification rate (0.5mm.min -1 ), which is the expected behavior.

At solidification rate 0.5mm.min -1 , whatever the seed types and their orientation, homogeneous Chinese script microstructure and coupled eutectic growth are observed. Some instability involving the formation of colonies are observed in all the samples. This instability problem has been observed by many researcher [START_REF] Lee | Growth and characterization of Al2O3/Y3Al5O12/ZrO2 ternary eutectic fibers[END_REF][START_REF] Wang | Crystallography and interfacial structure in a directionally solidified Al2O3/Y3Al5O12/ZrO2 eutectic crystal[END_REF][START_REF] Peña | Processing, microstructure and mechanical properties of directionally-solidified Al2O3-Y3Al5O12-ZrO2 ternary eutectics[END_REF][START_REF] Calderon-Moreno | Al2O3-Y3Al5O12(YAG)-ZrO2 ternary composite rapidly solidified from the eutectic melt[END_REF]. It is based on the competitive growth kinetic in which leading phase is the one that is growing at higher interface temperature. The formation of colonies has been shown to occur due to the presence of nonplanar eutectic growth interface [START_REF] Ester | Processing and microstructural study of surface laser remelted Al2O3-YSZ-YAG eutectic plates[END_REF][START_REF] Fu | Microstructure refinement approaches of melt-grown Al2O3/YAG/ZrO2 eutectic bulk[END_REF][START_REF] Song | Longitudinal cross-section microstructure of growth striation in Al2O3/Y3Al5O12/ZrO2 directionally solidified eutectic ceramic prepared by laser floating zone[END_REF]. As shown in figure 4, the solidified eutectic using eutectic seed under the same conditions presents a slight magnification of the phases.

The zirconia phase appears as a fine homogeneous dispersion of precipitates at the YAG The microstructure crystallized at pulling rate of 1mm.min-1 is smaller than that solidified at slower solidification rate (0.5mm.min -1 ). In comparison with the typical eutectic microstructure and the samples grown by using eutectic, sapphire and YAG seeds, for both pulling rate poor microstructures are obtained and ZrO 2 particles are randomly dispersed in to correspond to monoclinic phase in good agreement with Ref [35]. These traces of monoclinic Y-ZrO particles are observed at the colonies periphery.

Eutectic spacing's () are measured as a function of the seed type, orientation and pulling rate (Figure 7). The variation with growth rate is found to show more significant deviation than the seed type and orientation. The eutectic spacing is found to decrease with increasing pulling rate in good agreement with references [START_REF] Benamara | Microstructure and crystallography of Al2O3-Y3Al5O12-ZrO2 ternary eutectic oxide grown by the micropulling down technique[END_REF][START_REF] Benamara | Shaped ceramic eutectic plates grown from the melt and their properties[END_REF]. The lamellar eutectic growth revealed is (100) which are in good agreement with that reported in references [START_REF] Mazerolles | New microstructures in ceramic materials from the melt for high temperature applications[END_REF][START_REF] Mazerolles | Microstructures, crystallography of interfaces, and creep behavior of melt-growth composites[END_REF]. For the pulling rate of 1mm.min -1 , as a function of the seeds (except with zirconia seeds), the crystallographic planes became (01-12) for Al 2 O 3 , (100) and additional YAG garnet peaks for YAG phase, and finally (110) (420) for zirconia phase. For both pulling rates, in case of zirconia seed, the X- observed in rods solidified at 1 mm.min -1 through eutectic, sapphire and YAG seeds are not observed in the rods grown at 0.5 mm.min -1 and zirconia seeding for both pulling rates.

The rod grown at pulling rate of 1mm.min -1 by zirconia seed contains two additional peaks (011) and (-102) belonging to the zirconia monoclinic phase. According to the ZrO 2 -Y 2 O 3 binary phase diagram [START_REF] Selsing | Internal Stresses in Ceramics[END_REF], only cubic ZrO 2 is stable for Y 2 O 3 greater than 8 mol%. For lower compositions tetragonal or monoclinic ZrO 2 are formed. In ZrO 2 compound, formation of the monoclinic structure is athermal, diffusionless and shear displacive. This transformation can be stress induced because of the strong thermal gradient regarding the high pulling rate (1 mm.min -1 ). The presence of the monoclinic ZrO 2 phase can be explained by two hypothesis:

1-The melting temperature of zirconia (2600°C) is higher than melting temperature of sapphire (2050°C) and YAG (1980°C). Approaching zirconia seed to melting zone will extract the heat (melt temperature decreases) causing larger thermal gradient and consequently faster growth rate can be attained in the melting zone. The liquid on the surface is sufficiently undercooled to destabilize the cubic zirconia and precipitate the monoclinic phase.

2-Precipitation of the monoclinic phase is a function of particles size and shape, thermal properties of the particles and its surrounding matrix. The monoclinic phase formation will occur if the surface energy and yttrium concentration favor its precipitations. In Figure 6a, the small ZrO 2 particles emerged in alumina matrix at the border of the cells could be the driving force for the monoclinic phase formation. The utilization of ZrO 2 seed to initiate the solidification from the melt at pulling rate 1mm.min -1 causes an off eutectic composition (zirconia enrichment) at the connection zone near the crystallization interface. This behavior affect the shape of the crystallization front and the composition of the melting zone favoring the destabilization of cubic zirconia and the precipitation of the monoclinic phase.

It should be noted that the monoclinic ZrO 2 in the Al 2 O 3 -Y 2 O 3 -ZrO 2 ternary eutectic composition was never reported before and its origin is not fully understood. Further experimental work is required to look if the zirconia destabilization occurs via the cubicmonoclinic transformation path. It will be the subject of another paper. It is clear that the seed type and pulling rates play an important role in determining not only the microstructure morphology, but also the crystallography.

3-3 Residual stresses

The ternary eutectic crystallized from the melt at high temperature and the different reaction occurred during the solidification process leads to the nucleation of three phases with strong interfacial bonding and different thermal expansion coefficient. The origin of the stress in the phases composing the eutectic ceramics is quite often explained by the thermal expansion mismatch between the Sapphire, YAG and ZrO 2 phases, which induce a thermal elastic residual stresses during the solidification process. The Thermal expansion coefficient of zirconia (12.6 x10-6 °C-1 ) is higher than alumina (9.2x10-6 °C-1 ) and YAG (8x10-6 °C-1 ) [START_REF] Llorca | Directionally solidified eutectic ceramic oxides[END_REF][START_REF] Sayir | The effect of the microstructure on mechanical properties of directionally solidified Al2O3/ZrO2(Y2O3) eutectic[END_REF]. Because of the low room temperature thermal expansion coefficient difference between alumina and YAG (∆α = α(Al 2 O 3 ) -α(YAG ) = 1.2 x 10-6 °C-1 ), the residual stress in sapphire measured by piezo spectroscopy is about -70 MPa in Al 2 O 3 /YAG binary eutectic [START_REF] Gouadec | Ruby micropiezospectroscopy in GdAlO3/Al2O3(/ZrO2), Er3Al5O12/Al2O3(/ZrO2) and Y3Al5O12/Al2O3(/ZrO2) binary and ternary directionally solidified eutectics[END_REF][START_REF] Dickey | Residual stresses in high-temperature ceramic eutectics[END_REF] and becomes -363MPa [START_REF] Pardo | Piezospectroscopic Study of Residual Stresses in Al2O3-ZrO2 Directionally Solidified Eutectics[END_REF] in ternary composite system containing zirconia phases.

As a function of seed and pulling rates,

Conclusion

In this work, using different oriented seeds Al 2 O 3 -YAG-ZrO 2 ternary eutectic ceramics was solidified from the melt by using the µ-PD technique. The impact of the type and the orientation of the seed effect on the morphology, the microstructure, the stress and the crystallographic orientations have been investigated. The most important finding is the evolution of the melting zone during the growth and the different phenomena involved near the crystallization front. Following the results, this eutectic system shown that over the range of the composition and seeding three phases growing cooperatively from the liquid.

Cooperative growth of the phases with a common interface can be shown to be faster than either phase individually. At pulling rate 0.5 mm.min -1 , whatever the seed type Chinese script microstructure was obtained. On the other hand, at pulling rate 1mm.min -1 , for (100) YAG and (0001) sapphire seeds, a localized geometrical pattern microstructure was observed.

Whatever the seed orientation for pulling rates of 0.5 and 1mm.min -1 a few colonies were observed. Through (111) zirconia seeding, due to the entropy of fusion of ZrO 2 and off eutectic initial composition, a complex poor irregular cellular microstructure was observed in the entire rods and monoclinic structure was registered at the solidification rate of 1mm.min -1 .

Compressive stress in the irregular cellular microstructure (zirconia seeding) is more important than Chinese script microstructure (eutectic, sapphire and YAG seeding).

Figure 1 .

 1 Figure 1. Eutectic rods as a function of the starting seed grown from the melt by the µ-PD technique (v=0.5mm.min -1 ).

Figure 2 .

 2 Figure 2. Impact of seeding and the first step of the growth as a function of the seed orientations: (a) M(10-10) sapphire, (b) c(0001) sapphire, (c) M(10-10) sapphire, (d) Eutectic ; (e) YAG [100], (f)YAG [111]. The black color corresponds to Al 2 O 3 phase, the gray is YAG and the white correspond to Y-ZrO 2 phase.

After 8 '

 8 41'' (Fig 3i), the coalesced grain was completely incorporated in the rod and stable growth with slightly convex interface is observed. Whatever sapphire seed orientation, the SEM images (Fig 2) showed zirconia particles are the most dominated phase at the beginning of the growth. Fig 3(jm) shows direct observation of the grains propagation and the phase's nucleation through YAG seeding. The main drawback is the difficulty to measure the thermal history of an individual grain during its apparition, coalescence and penetration in the rod.

Figure 3 .

 3 Figure 3. Video sequence of the melt behavior, particles precipitation and the different steps before their incorporation in the rods as a function of the seeds. Diameter of the capillary die at the bottom of the crucible is 5mm.Figure 4 and Figure 5 show the transversal and longitudinal SEM images of the eutectic

Figure 4 and

 4 Figure 3. Video sequence of the melt behavior, particles precipitation and the different steps before their incorporation in the rods as a function of the seeds. Diameter of the capillary die at the bottom of the crucible is 5mm.Figure 4 and Figure 5 show the transversal and longitudinal SEM images of the eutectic microstructures 4 cm from the seed connection. The eutectic microstructure is composed of three phases distinguished by their different geometries and colors. The black matrix

in figure 5 .

 5 interfaces. Part of the zirconia is distributed over the alumina matrix. Under solidification rate of 1mm.min -1 , a change in morphology at the center of the colonies in the samples solidified through YAG (100) and sapphire (0001) seeds. Fig.4h, 4k 5h and 5k show the Geometrical Pattern microstructure formed at the center of the colonies. Then approaching the edge of the colonies, the Chinese Script microstructure is formed. The phases have a very angular appearance. The zirconia is in the form of a fiber inside the Al 2 O 3 phase close to the Al 2 O 3 -ZrO 2: Y binary system[START_REF] Peña | Microstructure of Y2O3 doped Al2O3-ZrO2 eutectics grown by the laser floating zone method[END_REF][START_REF] Lee | Microstructure of Y2O3 doped Al2O3/ZrO2 eutectic fibers grown by the micro-pulling-down method[END_REF][START_REF] Gnesin | Textures of Phases in Al2O3 -ZrO2 (Y2O3) Eutectic Composites[END_REF]. At pulling rate of 1mm.min -1 , the colony microstructure merged into nearly homogeneous cell structure with lamellae. Except the rods pulled by the eutectic seed (Fig5a,5g) a strong elongation of the microstructure is observed along the direction of solidification as shown by the observation in longitudinal section presented No striation defect, which would divide the connection of the rods along the growth direction were observed in any sample.

Figure 4 .

 4 Figure 4. Transversal microstructure evolution as a function of the seeds types and pulling rates.

  (a,b,c,d, e,f) correspond to v=0.5mm/min, (g,h,i,j,k,l) correspond to v=1mm.min -1 (a,g): eutectic seed, (b,h): (100) YAG seed, (c,i): (111) YAG seed, (d,j): A(1-210) sapphire seed, (e,k): c(0001) sapphire seed, (f,l): ), M(10-10) sapphire seed.

Figure 5 .

 5 Figure 5. Longitudinal microstructure evolution as a function of the seed types and pulling rates. (a,b,c,d, e,f) correspond to v=0.5mm.min-1 , (g,h,i,j,k,l) correspond to v=1mm.min -1 (a,g): eutectic seed, (b,h): (100) YAG seed, (c,i): (111) YAG seed, (d,j): A(1-210) sapphire seed, (e,k): c(0001) sapphire seed, (f,l): ), M(10-10) sapphire seed.

Figure 6

 6 Figure6shows the transversal and longitudinal SEM images of the eutectic microstructures solidified through (111) ZrO 2 seed after pulling 40 mm of the rod. It is difficult to control the microstructure and the morphology of the growth seeded through zirconia seed. Eutectic solidified using zirconia seed show instabilities of the microstructures. The oscillating structures are predicted at off-eutectic composition. A large irregular cellular microstructure limited by ZrO 2 border are observed. The cell walls are relatively weak and contain higher concentration of ZrO 2 . The cells solidified at pulling rate of 0.5mm.min -1 are larger than those grown at 1mm.min -1 .

Al 2 O

 2 3 matrix. Few amount of ZrO 2 particles are imbedded on YAG interface different that seeding sapphire, YAG and eutectic seeds. Whatever the pulling rates, some ZrO 2 particles are crystalized in lamella shape of length up to 50µm. At pulling rate 0.5mm.min -1 some zirconia particles have a fibrous structure. The sapphire grows in a lamellar manner and ZrO 2 prefers to grow in the Al 2 O 3 phase. This is connected to the melt excess of ZrO 2 phase coming from the melted part of ZrO 2 seed at the beginning of connection. EDX measurements of the aluminum, yttrium and zirconium are performed on different regions of the samples solidified through eutectic polycrystalline seed and YAG, Sapphire and ZrO 2 single crystals seeds. Except ZrO 2 grains which exhibit an important concentration of yttrium atoms due to the substitution of Zr by Y in ZrO 2 phase, all the composition analysis of YAG (Y 3 Al 5 O 12 ) and Al 2 O 3 phases are very close to the theoretical stoichiometric composition. The presence of Zr in YAG and sapphire phases corresponds to the accuracy measurement (<1%). The average of yttrium oxide (Y 2 O 3 ) composition in YZT phase measured by EDX is about 19 mol % ± 3%. It is the necessary concentration to stabilize cubic zirconia (ZrO 2 ) phase. In the samples solidified using zirconia seed and pulling rate 1mm.min - 1 we registered some Y-doped ZrO 2 traces of low Y 2 O 3 concentration (<1.5mol%) susceptible

Figure 7 .- 2 CrystallographyFig. 8

 728 Figure 7. Eutectic spacing variation as a function of seeds and pulling rates

  ray diagrams are different than others seeds orientations. The samples present additional YAG, Al 2 O 3 and ZrO 2 peaks showing others crystallographic orientations. There is a distinct difference between the zirconia seeds X-ray diagram and the others seeds concerning the presence of peak of ZrO 2 (311). No apparent peak of ZrO 2 (311) have been seen in the rods solidified through eutectic, sapphire and YAG seeds. The Al 2 O 3 (01-12) and (02-24) peaks

Figure 8 .

 8 Figure 8. Room temperature bulk XRD patterns of the transversal sections of Al 2 O 3 /YAG/ZrO 2 eutectic grown by using different seeds. (a) v= 0.5 mm.min -1 , (b) v=0.5 mm.min -1

  Fig 9 shows the evolution of the average hydrostatic stress measured on the transversal section of the samples. Whatever the solidification rate and

Table 1 .

 1 Seeds used to initiate eutectic solidification

	Seed	Standard	Sapphire Sapphire Sapphire	YAG	YAG ZrO 2 :Y
		Eutectic
	Orientation Poly-crystal C(0001) A(1-210) M(10-10) (100)	(111)	(111)

Table 2 .

 2 Melting entropies of YAG, Al 2 O 3 and ZrO 2 oxides.

	S melting (J K -1 Mol -1)	YAG (Y 3 Al 5 O 12 ) 122	Al 2 O 3 48	ZrO 2 30

Acknowledgements.

The authors acknowledge the French MAT&PRO ANR program ''CiNATRA'' ref: ANR-12-RMNP-0008 for financial support. We also thanks M. Podgorski ODGORSKI from SAFRAN for his cooperation and assistance. The authors express their gratitude to Dr A.

Nehari and Dr G. Alombert-Goget for their great collaboration and exchange. Thanks to Dr.

H. Cabane and M. Dumortier from Cristalinnov for their technical assistance and polishing samples.

that the spacing eutectic is not very sharp and a band of stable interlamellar spacing is distributed under pulling rates 0.5 and 1 mm.min -1 . Figure 6. Microstructure evolution of eutectic solidified through (111)zircnia seeding.

-Transversal section: ((a) v=0.5 mm.min -1 ,(b) v=1 mm.min -1 )) -Longitudinal section: ((c) v=0.5 mm.min -1 ,(d) v=1 mm.min -1 ))

the seed type, the negative values of the hydrostatic stress indicate that the sapphire phase is under compression [START_REF] Sayir | The effect of the microstructure on mechanical properties of directionally solidified Al2O3/ZrO2(Y2O3) eutectic[END_REF]. The rod solidified through eutectic seed has a stress around -250

MPa and decrease to -400 MPa under zirconia seeding at pulling rate of 1 mm.min -1 .

Compared to seeding effect, we didn't observed an important impact of the pulling rate. The variation of the residual stresses, not only depends on cooling rate, but also on seed type and


Wen Lu

Jie Xu

Qingsong Song

Kaiting Bian

Jun Guo

Jian Liu

Dongzhen Li

Peng Liu

Chaojin Zhang

Xiaodong Xu

Jun Xu
email: xujun@mail.shcnc.ac.cn

Kheirreddine Lebbou
email: kheirreddine.lebbou@univ-lyon1.fr

Spectroscopic properties of Tm:Bi 4 Ge 3 O 12 crystals grown by the micro-pulling-down method

Keywords: Tm:Bi 4 Ge 3 O 12, Micro-pulling-down method, Spectroscopic property

Tm 3+ -doped bismuth germanate crystals (Tm:BGO) with various doping concentrations of 0.5, 0.75, 1.0, 1.5, and 2.0 at.% were grown by the micro-pulling-down method (µ-PD). The effect of thulium concentration on the growth, structure, absorption spectra, emission spectra and fluorescence lifetime of 3 H 4 and 3 F 4 state was studied. Under excitation of 792 nm, Tm:BGO crystals exhibit two fluorescence bands corresponding to the 3 H 4 → 3 F 4 and the 3 F 4 → 3 H 6 transitions. The fluorescence lifetime of 3 H 4 state decreased strongly as the thulium concentration increased, while the lifetime of 3 F 4 state are relatively constant for all thulium concentrations.

Introduction

Tm 3+ -doped laser materials have attracted much attention due to their potential applications in medical surgery, tele-communication, remote sensing, and pumping sources of optical parametric oscillators [START_REF] Honea | 115-W Tm:YAG diode-pumped solid-state laser[END_REF][START_REF] Zayhowski | Tm:YVO 4 microchip laser[END_REF][START_REF] Zhang | Enhanced 1.4 µm emissions of Tm 3+ via Tb 3+ deactivation in (Gd 0.5 Lu 0.5 ) 2 SiO 5 crystal[END_REF]. During the last decade, different Tm doped crystalline host materials have been investigated for their favourable properties to obtain an efficient laser emission. Tm 3+ ions can be efficiently excited by commercial AlGaAs laser diode around 800 nm (Tm 3+ , 3 H 6 → 3 H 4 transition) and emission spectral intervals are around 1.5 µm [START_REF] Ryba-Romanowski | Relaxation dynamics of excited states of Tm 3+ in SrGdGa 3 O 7 crystals activated with Tm 3+ and Tm 3+ +Tb 3+[END_REF], 1.9 µm [START_REF] Zheng | Crystal structure and optical study of Tm:Sc 2 SiO 5 single crystal[END_REF] and 2.3 µm [START_REF] Li | Crystal growth, spectroscopic characteristics, and Judd-Ofelt analysis of Tm:GdScO 3[END_REF]. Laser oscillation of Tm 3+ has been demonstration in numerous hosts such as LiYF 4 [START_REF] Huang | High power simultaneous dual-wavelength CW and passively-Q-switched laser operation of LD pumped Tm:YLF at 1.9 and 2.3 µm[END_REF][START_REF] Stoneman | Continuous-wave 1.50-µm thulium cascade laser[END_REF], BaY 2 F 8 [START_REF] Muti | Tunable continuous-wave laser operation of Tm 3+ :BaY 2 F 8 near 2.3 µm[END_REF], CaF 2 [START_REF] Thouroude | Widely tunable in-band-pumped Tm:CaF 2 laser[END_REF],

MgWO 4 [START_REF] Wang | Single-walled carbon-nanotube saturable absorber assisted Kerr-lens mode-locked Tm:MgWO 4 laser[END_REF], CaGdAlO 4 [START_REF] Lan | Passively Q-switched Tm:CaGdAlO 4 laser using a Cr 2+ :ZnSe saturable absorber[END_REF], YAlO 3 [START_REF] Guillemot | Continuous-wave Tm:YAlO 3 laser at ~2.3 µm[END_REF], LuScO 3 [START_REF] Xu | First laser oscillation of diode-pumped Tm 3+ -doped LuScO 3 mixed sesquioxide ceramic[END_REF], and Y 2 O 3 [START_REF] Huang | VBG-locked continuous-wave and passively Q-switched Tm:Y 2 O 3 ceramic laser at 2.1 µm[END_REF].

Bismuth germanate crystal Bi 4 Ge 3 O 12 (BGO) belongs to cubic system with the space group of I43d [START_REF] Farias | The local structure around the Nd impurity incorporated into the Bi 4 Ge 3 O 12 crystal matrix: An ab initio study[END_REF]. It has been studied as a host for Er 3+ , Eu 3+ , Dy 3+ , Pr 3+ and Nd 3+ ions, etc., with applications as optical temperature sensors, scintillators and laser gain media [START_REF] Huang | Effects of Er3+ concentration on upconversion luminescence and temperature sensing properties in Bi 4 Ge 3 O 12 crystal[END_REF][START_REF] Shim | Radios-, photo-and thermo-luminescence characterization in Eu 3+ -doped Bi 4 Ge 3 O 12 single crystal for scintillator application[END_REF][START_REF] Kuzmicheva | Local structural features and composition of the Bi 4 Ge 3 O 12 :Dy 3+ crystals: effect of doping concentration[END_REF][20][START_REF] Kaminskii | Spectroscopic properties and stimulated emission in the 4 F 3/2 → 4 I 11/2 and 4 F 3/2 → 4 I 13/2 transitions of Nd 3+ ions from cubic Bi 4 Ge 3 O 12 crystals[END_REF]. In addition, as a host material, BGO possesses many qualities desirable for laser applications such as excellent physical and chemical properties [START_REF] Williams | Optical, thermo-optic, electro-optic, and photoelastic properties of bismuth germanate (Bi 4 Ge 3 O 12 )[END_REF]. In 1979, A.A. Kaminskii et al. reported the absorption and emission properties of Tm 3+ -doped BGO crystal grown by Czochralski method [START_REF] Kaminskii | Growth, spectroscopy, and stimulated emission of cubic Bi 4 Ge 3 O 12 crystals doped with Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , or Yb 3+ ions[END_REF]. The micro-pulling-down (µ-PD) method was used to grow BGO crystals with high quality and transparency, where the crystals have low thermal strain compared to other growth methods [START_REF] Shim | Growth of Bi 4 Ge 3 O 12 single crystal by the micro-pulling-down method from bismuth rich composition[END_REF][START_REF] Chani | Evaporation induced diameter control in fiber crystal growth by micro-pulling-down technique: Bi4Ge3O12[END_REF][START_REF] Farhi | BGO fibers growth by μ-pulling down technique and study of light propagation[END_REF]. Eu:BGO and Nd:BGO crystals have also been grown by the µ-PD method [START_REF] Li | Spectroscopic properties of Eu:Bi 4 Ge 3 O 12 single crystal grown by the micro-pulling down method[END_REF][START_REF] Li | Optical properties of Nd:Bi 4 Ge 3 O 12 crystals grown by the micro-pulling down method[END_REF][START_REF] Shim | Eu 3+ doped Bi 4 Ge 3 O 12 fiber crystals grown by the by the micro-pulling down method[END_REF]. BGO melt congruently and relatively high rate of evaporation of Bi 2 O 3 occurs during melt overheating [30][START_REF] Chani | Evaporation Induced Diameter Control Growth Book, Advances in Materials Research, Shaped Crystal Growth by Micro-Pulling-Down Technique[END_REF].

In this work, Tm:BGO crystals with different doping concentration were successfully grown by the micro-pulling down method. Achievement of stationary conditions for the growth of uniformly shaped (constant diameter) fiber crystals was considered as final goal and positive outcome of this paper. Spectral properties of the crystals were studied at room temperature.

Experimental

Crystal growth

Tm:Bi 4 Ge 3 O 12 crystals were grown by the µ-PD method [32]. The Tm 2 O 3 (99.999%), Bi 2 O 3 (99.999%) and GeO 2 (99.999%) powders were used as starting materials and weighed according to the formula Tm 4x Bi 4(1-x) Ge 3 O 12 (x=0.005,0.0075,0.01,0.015,0.02). After thoroughly mixed in an agate mortar, they were pressed into bulks and sintered in the air at 750℃ for 24h. The polycrystalline Tm:BGO was melted in a platinum crucible, and then the melt was pulled down through a capillary channel at the bottom of the crucible. The Tm:BGO crystals were grown with constant pulling rate of 0.3 mm/min by pulling down a <210> oriented BGO seed in the air atmosphere. Whatever the starting composition prepared in this work (Tm≤2at%), we had not seen any evolution of growth Kinetic. As shown in Fig. 1(a)-(e), five crack-free Tm:BGO single crystals with dimensions of Φ2 × 44-68 mm 3 were obtained and present a light yellow color. The crystals produced from the crucibles were very uniform in shape according to the measurements performed directly from the the fibers with accuracy of about ± 20 μm. The crystals have a rough appearance on the outside, but the inner part of the crystals was transparent and free of visible defects. It is important to note that, once the melt becomes Bi-poor there is no way to establish constant properties (solidification temperature and following shape of the meniscus) of the melt Thus, for all Ge-rich systems evaporation of the bismuth oxide will only increase dissolution of the melt and its transformation from melt to flux system. 1.5 at.% and (e) 2.0 at.%.

X-ray powder diffraction

In order to investigate the structure of Tm:BGO crystals, small section of the samples were cut from the as-grown crystals and grounded into powder in an agate mortar. And then the structure was examined by X-ray diffraction method (XRD, Bruker-D2, Germany) at a scan width of 0.05° with a range 5°-90°.

Spectra measurements

For spectroscopic measurements, the Tm:BGO samples were cut from the as-grown crystals

(a) (b) (c) (d) (e)
and then optically polished to flat and parallel faces with the size of Φ2×2 mm 3 . The absorption spectra were measured from 400 to 2000 nm by a UV-VIS-NIR spectrophotometer (Lambda 1050, Perkins Elmer, U.S.A.) at room temperature. The fluorescence spectrum as well as the decay time were recorded by a fluorescence spectrometer (Edinburg Instrument, FLS1000, U.K.) under the excitation 792 nm. All the measurements were carried out at room temperature.

Results and discussion

Table 1. Lattice parameters of Tm:BGO crystals as a function of Tm concentration

x at.% Tm:BGO a (Å) 0 0.5 10.497 [34] 10.453 The room temperature absorption spectra of BGO crystals doped with 0.5, 0.75, 1.0, 1.5, and 2.0 at.% of Tm 3+ ions in the wavelength region of 400-2000 nm are displayed in Fig. 3. We note that all the samples have the same absorption spectra. Five absorption bands are located at approximately 474, 680, 792, 1211 and 1737 nm, which are attributed to the transitions from the 3 H 6 ground state to 1 G 4 , 3 F 2 + 3 F 3 , 3 H 4 , 3 H 5 and 3 F 4 excited states, respectively. Among five main peaks, the peak at 792 nm ( 3 H 6 → 3 H 4 ) matches well with the emission of the commercial AlGaAs LD pumping. As the doping concentration increases from 0.5 at.% to 2.0 at.%, the absorption coefficient at 792 nm increases from 0.17 cm -1 to 0.62 cm -1 . For 2.0 at.% Tm:BGO crystal, the full width at half-maximum (FWHM) of the absorption band at 792 nm is about 3.3 nm. The value is smaller than that of Tm:YAG (4 nm at 780 nm [START_REF] Fan | Spectroscopy and diode laser-pumped operation of Tm, Ho:YAG[END_REF]), but larger than that of Tm:KY 3 F 10 (2.8 nm at 778.6 nm [START_REF] Loiko | Emission properties of Tm 3+ -doped CaF 2 , KY 3 F 10 , LiYF 4[END_REF]), which is suitable for LD pumping. increases with the increase of thulium concentration. The upper level 3 F 4 can be populated by a cross-relaxation process ( 3 H 4 + 3 H 6 → 3 F 4 + 3 F 4 ) and the effectivity of this process increases with higher thulium concentrations. For 2.0 at.% Tm:BGO crystal, the FWHM of the emission band at 1452 nm was calculated to be 3.9 nm and the FWHM of the emission band at 1879 nm is about 44.6 nm. The FWHM of 3 F 4 → 3 H 6 transition is larger than the value of Tm:YAG (31.37 nm [START_REF] Fei | Spectroscopic properties and laser performance of Tm:YAG ceramics[END_REF]).

We did not register the emission around 2.3 µm. The energy gap between 3 H 4 and 3 H 5 energy level is about 4100 cm -1 and the Tm 3+ ions at the 3 H 4 level can relax nonradiatively to the 3 H 5 level due to the lager phonon energy (≈ 900 cm -1 ) of BGO crystal [START_REF] Kaminskii | Spectroscopic properties and stimulated emission in the 4 F 3/2 → 4 I 11/2 and 4 F 3/2 → 4 I 13/2 transitions of Nd 3+ ions from cubic Bi 4 Ge 3 O 12 crystals[END_REF]. 3.4 Fluorescence lifetime Fig. 5(a) presents the fluorescence decay curves of Tm:BGO crystals in correspondence with the 3 H 4 → 3 F 4 transition at 1452 nm at room temperature. The fluorescence lifetime was calculated to be 352 µs, 318 µs, 313 µs, 267 µs and 218 µs for the doping concentration of 0.5, 0.75, 1.0, 1.5 and 2.0 at.%, respectively, by a single exponential fitting. The lifetime of the 3 H 4 state decreased strongly as the thulium concentration increased, which is due to the concentration quenching effect caused by the energy transfer between thulium ions at higher thulium concentration [38].

The fluorescence lifetime at 1452 nm of 1 at.% Tm:BGO is much longer than that of 1% Tm:KGd(WO 4 ) 2 (175 µs [38]) and 1% Tm:YVO 4 (100 µs [START_REF] Ermeneux | Growth and fluorescence properties of Tm 3+ doped YVO 4 and Y 2 O 3 single crystals[END_REF]).

The fluorescence decay curves of Tm:BGO crystals corresponding to the 3 F 4 → 3 H 6 transition at 1879 nm are shown in Fig. 5 [38]. The fluorescence lifetime at 1879 nm of 1 at.% Tm:BGO is much longer than that of 1% Tm:KGd(WO 4 ) 2 (1.71 ms [38]) and 1% Tm:YVO 4 (1.03 ms [START_REF] Ermeneux | Growth and fluorescence properties of Tm 3+ doped YVO 4 and Y 2 O 3 single crystals[END_REF]). The results indicated that Tm:BGO single crystal may be a promising laser gain medium.

For 1.45 µm emission, the lifetime of the terminal 3 F 4 level is much longer than that of the initial 3 H 4 level, making the 3 H 4 → 3 F 4 transition self-terminating . The sensitive co-doping ions like Pr 3+ , Ho 3+ and Tb 3+ ions were proposed to depopulate 3 F 4 state efficiently [START_REF] Mazurak | Optical properties of the Tm 3+ and energy transfer between Tm 3+ →Pr 3+ ions in P 2 O 5 -CaO-SrO-BaO phosphate glass[END_REF][START_REF] Stoneman | Continuous-wave 1.50-µm thulium cascade laser[END_REF][START_REF] Li | Growth and spectroscopic properties of Tm 3+ and Tb 3+ co-doped GdScO 3 crystal[END_REF], which will be our next investigation in the near future.

Conclusion

Tm-doped BGO crystals with different doping concentration were successfully grown by the µ-PD method and uniformly single crystals fibers of constant diameter was reported. The room temperature XRD revealed that the cell parameter decreased with the increasing of doping concentration in good agreement with Vegard Low [43]. The absorption spectra, emission spectra and fluorescence lifetimes of Tm:BGO crystals were measured at room temperature. The absorption peak at 792 nm matches well with the emission of the commercial AlGaAs LD pumping. The intensity of emissions of the 3 F 4 → 3 H 6 transition increases with the increase of thulium concentration. The fluorescence lifetime of 3 H 4 state decreased strongly as the thulium concentration increased, while the lifetime of 3 F 4 state are relatively constant for all thulium concentrations. The results indicate that Tm:BGO crystal could be a promising laser gain medium.

Fig. 1 .

 1 Fig.1. Photographs of the as-grown Tm:BGO crystals, (a) 0.5 at.%, (b) 0.75 at.%, (c) 1.0 at.%, (d)

3. 1

 1 Crystal structure The room temperature XRD patterns of Tm:BGO crystals are shown in Fig. 2. The crystal structure of undoped and Tm-doped BGO belongs to the cubic space group I -43d . The diffraction peaks of Tm:BGO crystals are in agreement with JCPDS 84-0505 of Bi 4 Ge 3 O 12 . No additional impurity phases were found in the patterns. Due to the different doping concentration of Tm 3+ ions, the relative intensity of each diffraction peak is different and the diffraction peaks of the XRD patterns were slightly offset to the right compared to the JCPDS 84-0505 of BGO. The lattice constants of Tm:BGO crystals are listed in Table1. With the increasing of doping concentration x, the calculated lattice constant decreased, which is attributed to the smaller radius of Tm 3+ ion (0.869 Å) compared with that of Bi 3+ ion (0.96 Å)[33]. The results reveal that a small amount of Tm 3+ ions did not change the structure of BGO crystal without any composition evolution resulting in a second-phase formation.

Fig. 2 .

 2 Fig.2. Room temperature XRD patterns of Tm:BGO crystals with a standard reference of undoped BGO (JCPD 84-0505)

Fig. 3 . 4 → 3 H 6

 3436 Fig.3. Absorption spectra of Tm:BGO crystals as a function of Tm concentration.

Fig. 4 .

 4 Fig.4. Fluorescence spectra of Tm:BGO crystals with different Tm doping concentration in (a) 1350-1550 nm and (b) 1700-2150 nm.

Fig. 5 .

 5 Fig.5. Fluorescence decay curves of the 3 H 4 and 3 F 4 level of Tm:BGO crystals with different doping concentration.

  (b). The measured decay curves showed a single exponential decaying behavior, and the fluorescence lifetime was measured to be 5.43, 5.16, 5.36, 5.69 and 5.64 ms, respectively. In contrast to the lifetime of the 3 H 4 state, the fluorescence lifetimes of 3 F 4 state are relatively constant for all thulium concentrations. This can be explained by two different mechanisms: the reabsorption effect between 3 F 4 → 3 H 6 resonant transitions and repopulation of the 3 H 4 state via the 3 H 4 + 3 H 6 → 3 F 4 + 3 F 4 cross-relaxation process

Acknowledgements

This work is partially supported by National Natural Science Foundation of China (No. 61621001) and "Qinglan Project" of the Young and Middle-aged Academic Leader of Jiangsu College and University.