Influence of the powder preparation on the sintering of Yb-doped Sc2O3 transparent ceramics

Optical Materials 31 (2009) 734–739

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Influence of the powder preparation on the sintering of Yb-doped Sc2O3 transparent ceramics A.C. Bravo a, L. Longuet a,*, D. Autissier a, J.F. Baumard b, P. Vissie a, J.L. Longuet a a b

Commissariat à l’Energie Atomique, BP 16, 37260 Monts, France Laboratoire Science des Procédés Céramiques et de Traitements de Surface, SPCTS, UMR CNRS 6638, ENSCI, 47-73 Avenue Albert Thomas, 87065 Limoges Cedex, France

a r t i c l e

i n f o

Article history: Received 19 December 2007 Received in revised form 15 May 2008 Accepted 16 May 2008 Available online 14 July 2008 Keywords: Transparent ceramics Yb:Sc2O3 Solid state reaction Powder-chemical preparation Sintering

a b s t r a c t We report the preparation and the sintering of high quality Yb:Sc2O3 powders with the ultimate intent to make transparent Yb:Sc2O3 ceramics for laser applications. The aim of the study is to compare two routes of powder preparation and observe the influence of the powder processing on the sintering of Yb-doped Sc2O3 ceramics. The powders were prepared (i) either from commercial oxides by a classical ceramic way (solid state reaction process) (ii) or by a wet chemical route. In that case, a carbonate precursor was synthesized by a coprecipitation method from a mixed solution of scandium and ytterbium nitrates using aqueous ammonium hydrogen carbonate as a precipitating agent. The Yb:Sc2O3 powder was obtained after heat treatment of the carbonate precursor at 700 °C. The powders prepared by these two methods were characterized by BET measurements, XRD and SEM and their sintering behaviour was investigated by dilatometry. The effects of a grinding step and of the addition of a sintering aid addition (TEOS) were investigated. The microstructures of the vacuum sintered materials were observed by scanning electron microscopy. Electron probe microanalysis allowed to characterize the repartition of the dopant Yb3+ in the Sc2O3 matrix. Translucent ceramics with densities close to 99% were obtained in the best conditions. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Pore free ultra pure ceramics are of great interest in the field of the solid state laser sources. Indeed, they present several advantages such as higher thermal conductivities than glasses, lower costs than single crystals, and they also reveal the possibility of making large pieces. Thus, transparent ceramics appear as promising alternatives for laser applications. Rare earth sesquioxides have been already shown as encouraging hosts for high power solid state lasers [1] and in particular scandium sesquioxide which presents very interesting laser properties when doped moderately with Yb3+ [2–3]. Consequently we are looking here for the elaboration of Yb0.02Sc1.98O3 materials which can be noted Yb:Sc2O3 (1%). To elaborate high performance Yb:Sc2O3 transparent ceramics, the choice of the raw materials and the preparation of high quality starting powders are key points of the process. Indeed, the morphological characteristics and the purity degree of the starting powders exert a deep influence on the properties of the sintered materials. The aim of this study is to compare two routes of powder preparation and observe their influence on the sintering of Yb-doped Sc2O3 ceramics. The first part of the paper presents the two methods employed for the preparation of the Yb:Sc2O3 powders. The first method is a classical ceramic route, consisting in processing * Corresponding author. Tel.: +33 (0)2 47 34 48 04; fax: +33 (0)2 47 34 51 83. E-mail address: [email protected] (L. Longuet). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.05.004

the commercial oxide powders of Sc2O3 and Yb2O3. The oxides were weighed, mixed together with zirconia balls and finally dried in order to obtain homogeneous mixtures. The second method employed is a chemical route. Among the numerous techniques developed for the fabrication of nanocrystalline powders (combustion synthesis [4–5], sol gel processing [6], hydrothermal method [7], emulsion synthesis), the coprecipitation [8–11] in aqueous medium has been devised as an easy, reliable and inexpensive way to prepare reactive, pure and homogeneous powders. A carbonate precursor was synthesized by coprecipitation and calcined at 700 °C to obtain Yb:Sc2O3 powder. The chemical, structural and morphological characteristics of the different powders prepared by the two methods will be compared. The second part of the paper deals with the sintering behaviour of the powders prepared by the two routes. The effects of a grinding step and of a sintering aid on the properties of the powders (reactivity. . .) and on the characteristics of the sintered materials (final density, microstructure, homogeneity. . .) will be presented. Translucent ceramics were obtained with relative densities close to 99%. 2. Experimental 2.1. Powder processing by the solid state reaction route High purity powders of Sc2O3 (99.99%, Neyco) and Yb2O3 (99.99%, Rhodia) were used as starting materials to prepare three

A.C. Bravo et al. / Optical Materials 31 (2009) 734–739

itation, a molar ratio of 4 (R = 4), a synthesis temperature of 40 °C and a calcination temperature of 700 °C. This powder will be further noted P4.

Aqueous mixing of commercial powders: Sc2O3 – Yb2O3 Attrition grinding (AG)

Attrition grinding (AG) Sintering aid (TEOS) Drying (60˚C) Desagglomeration

Powder 2 P2

Powder 1 P1

735

Powder 3 P3

Fig. 1. Flow chart of the powders prepared by the classical ceramic route.

powders of Yb0.02Sc1.98O3 (Fig. 1). A first powder P1 was obtained by simply mixing the commercial powders in aqueous medium. Then, in order to investigate the influence of a grinding step, a second powder P2 was prepared with /8mm zirconia balls. Finally, 0.5 wt% of tetra-ethyl orthosilicate (TEOS) was added to a third powder P3 to check the influence of a SiO2 sintering aid. All the powders were dried at 60 °C before being desagglomerated. 2.2. Powder preparation by the coprecipitation technique An Yb:Sc2O3 precursor was prepared by the coprecipitation technique using Sc(NO3)3  5H2O (99.99%, Metall Rare Earth Limited), Yb(NO3)3  5H2O (99.9%, Strem Chemicals) and ammonium hydrogen carbonate further noted ‘‘AHC” (NH4HCO3, 99.9%, Rectapur VWR Prolabo) as starting materials. The salt solution was obtained by dissolving simultaneously Sc(NO3)3  5H2O and Yb(NO3)3  5H2O in deionized water to get a concentration of Sc3+ and Yb3+ of 0.1 M and 0.001 M, respectively. The aqueous solution of 1.0 M AHC was also prepared by dissolution in deionized water. The precipitating agent was introduced into the mixed nitrates solution under stirring and with a dropping rate of 3 ml/min. This precipitation way is commonly called the direct striking method whereas the reverse striking way consists in adding the nitrate solution to the precipitant. After completing the reaction, the precipitate slurry was aged for 3 h under continuous stirring. The precipitate was then separated by a centrifugation and washed several times with deionized water to completely remove NHþ 4,  by-products. After washing, the cake was dried at NO 3 and OH 60 °C for one night and then calcined in air at 700 °C for 2 h with a heating rate of 150 °C/h so as to obtain the oxide phase. In several previous works focused onto the precipitation synthesis of garnets [12–13] and sesquioxide nanocrystalline powders [14–18], it has been reported that the characteristics of the precursors (size, morphology, agglomeration state of the particles . . .) could vary according to different parameters of the precipitation process. A preliminary study [19] consisted in the observation of the effects of three parameters on the characteristics of the powders: (i) the precipitation technique (direct or reverse striking method), (ii) the value of the molar ratio R (R = AHC/Sc3+) and (iii) the synthesis temperature. The target was to highlight the best precipitation conditions and calcination temperature in order to get a reactive, pure and homogeneous Yb-doped scandia powder. The best conditions were identified as the normal striking precip-

2.3. Shaping and sintering of the as-prepared powders All the powders were shaped into cylindrical samples by uniaxial pressing under 150 MPa and the materials were sintered under vacuum in the 1600–1850 °C range for 2 or 5 h. 2.4. Characterizations The Brunauer–Emmett–Teller (BET) surface areas of the powders were measured with a specific surface area analyzer (Micromeritics ASAP 2010). Structural and microstructural investigations were performed by X-ray powder diffraction (XRD) with a D5000 X-ray diffractometer (CoKa1,2 radiation). Powders were observed in a scanning electron microscope equipped with a field emission gun (FEG-SEM) (Zeiss DSM 982 Gemini). The chemical composition of the precipitate prepared by coprecipitation and the purity of the powders and sintered materials were determined by chemical analysis (ICP-AES: Horiba Jobin Yvon Model Activa, ICP-MS: Thermoelectron Serie X7). The thermal behaviour of the precursor prepared by coprecipitation was investigated in air with a TG–DTA thermal analyzer (Setaram TAG 24) using a heating rate of 10 °C/min. The sintering behaviour of the whole powders was studied in air up to 1600 °C by dilatometry (Setaram TMA 92-16). The powder compacts were heated and cooled down at 5 °C/min and a dwell time of 2 h was applied at 1600 °C. The density of the sintered materials was determined by the Archimede method using deionized water with an accuracy of 1%. Their microstructure was observed after thermal etching on polished surfaces by scanning electron microscopy (LEO 435 VPi). The homogeneity of the doping element repartition was checked by electron probe microanalysis (CAMECA SX50). 3. Results and discussion 3.1. Powders prepared by the classical ceramic route (P1, P2, P3) Scanning electron micrographs of the oxides used as starting materials showed that the Sc2O3 powder is composed of nanoparticles assembled in micrometric platelets whereas the Yb2O3 powder is composed of elongated nanoparticles. The specific surface

Fig. 2. Scanning electron micrographs of the powder P3 attrited for 75 min.

A.C. Bravo et al. / Optical Materials 31 (2009) 734–739

Table 1 Morphology, particle size and BET area of the starting powders Powder

Solid state reaction

Sc2O3 Yb2O3 P3

Coprecipitation technique

Precipitate P4

Morphology and particles size Micrometric platelets composed of nanoparticles Elongated nanoparticles Nanoparticles assembled in little agglomerates Micrometric flakes composed of nanoparticles Micrometric spherical objects composed of fairly discrete and spherical crystallites of 50 nm

19 3 38

ATG

-10

BET area (m2/g)

20

-20

TG (%)

Powder preparation

40

0

0 ATD

-30

-2 -40 -40

-50

-60

-60

600

-70

90

Heatflow (mW)

736

0

200

400

600

800

1000

-80 1200

Temperature (°C) Fig. 4. TG and DTA curves of the precursor prepared by coprecipitation.

areas of the Sc2O3 and Yb2O3 powders measured by the BET method are, respectively, 19 m2/g and 3 m2/g. Fig. 2 shows the scanning electron micrographs of the powder P3 attrited for 75 min. This powder is composed of nanoparticles assembled in little agglomerates. The particle size was reduced during the grinding step and the surface specific area is about 38 m2/g. Table 1 summarizes the morphology, the size and the BET area of the starting powders. The XRD patterns of the powders P1, P2 and P3 can be indexed with the cubic Sc2O3 and Yb2O3 structures and no other phase is detected. The presence of the elements Sc, Yb and Si (in the powder P3) was checked by the chemical analysis. The ICP analysis also revealed the presence of about 500 ppm of zirconium in P2 and P3 resulting from the attrition grinding with zirconia balls. 3.2. Powder prepared by the coprecipitation method (P4) The precursor prepared by coprecipitation is a carbonate of chemical composition Yb0.01Sc(CO3)1.49(NH4)0.56(OH)0.61  1.33H2O. This composition was determined by chemical analysis such as ICP. XRD analysis shows that the precipitate is well crystallized (Fig. 3). No JCPDS Card is consistent with its XRD pattern. Nevertheless, it is similar to those presented by Li et al. [16] for carbonates with similar compositions.

Scanning electron micrographs indicate that the crystallized carbonate is flakes-like. In fact, it is composed of spherical nanoparticles as confirmed by the very high specific surface area of about 600 m2/g measured by the BET method (Table 1). The thermal behaviour of the precursor was studied by TG–DTA analysis coupled with mass spectrometry. Decomposition occurs in two distinct steps (Fig. 4), an endothermal peak associated to a fast and important weight loss around 200 °C followed by an exothermal peak around 475 °C. The investigations by mass spectrometry and X-ray diffraction allowed to attribute the first step to the removal of absorbed water and synthesis residues and the second to the crystallization into the Sc2O3 phase. Structural investigations were performed by XRD on precursor powders calcined in air at various temperatures in the 400– 1100 °C range. Fig. 3 shows that cubic Sc2O3 crystalline phase is formed above 400 °C. This temperature is lower than the one reported by the DTA analysis (about 475 °C). However, this shift can be easily explained by a kinetics difference between the two experiments. As the Sc2O3 phase is formed and no more weight loss is observed above 700 °C, the carbonate precursor was calcined at 700 °C for 2 h to produce reactive Yb-doped scandia powders (P4). Scanning electron micrographs indicate that powder P4 is composed of fairly discrete and spherical crystallites (Fig. 5b). Their average diameter is lower than 50 nm. This result is consis-

Sc2O3 JCPDS 00-042-1463

30000

Intensity (au)

1100°C 20000

900°C

700°C 10000

400°C Precipitate 0 20

30

40

50

60

70

2θ Fig. 3. XRD patterns of precursor powders calcined at various temperatures.

80

737

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Fig. 5. Scanning electron micrographs of powder P4.

2

100

S4

98,5

97,5

96,7

95,9

97,2

S1

-6 -10 -14

S2

S3

88,3

80

70 65,0

-18

61,4 S1

S4 -22 0

S2

87,2

60

S3

400

800

1200

1600

Temperature (°C)

Fig. 6. Shrinkage curves of the compacts S1, S2, S3 and S4.

tent with the specific surface area of 90 m2/g. The crystallites form almost spherical objects of about 5 lm in size (Fig. 5a). 3.3. Characteristics of the sintered materials Hereafter are presented the characteristics of the sintered samples S1, S2, S3 and S4, respectively, prepared from the powders P1, P2, P3 and P4. Fig. 6 shows the shrinkage curves of samples S1, S2, S3 and S4. It can be noticed that for all the samples, the sintering is not yet finished at 1600 °C. The shrinkage of sample S4 begins at 850 °C whereas for samples S1, S2 and S3, it starts at 1150 °C. The shrinkage rate is also more important (20%) for sample S4. These results

98,0 96,5

93,4

90 Relative density (%)

Shrinkage (%)

-2

98,7

50 1550

55,4 1600

1650

58,0

58,6

1700

1750

1800

1850

1900

Sintering temperature (°C)

Fig. 7. Evolution of the relative density with the sintering temperature. The compacts S1, S2, S3 and S4 were sintered for 2 h under vacuum.

emphasize the better reactivity of powder P4 prepared by coprecipitation. Besides, considering the respective shrinkage rates of S2 (16%) and S3 (18%) in comparison with S1 (6%), the grinding step and the addition of TEOS seem to be favourable to a better sintering in the classical ceramic route. In Fig. 7 are presented the density results for samples S1, S2, S3 and S4 fired under vacuum from 1600 to 1850 °C for 2 h. Whatever the sample, the relative density increases with the firing temperature. The sintering of sample S1 in the 1600–1850 °C range does not lead to satisfactory results. After a vacuum sintering at 1600 °C, the relative density (about 55%) is hardly higher than the green one (51%) and at 1850 °C, the density is not better than 65% of the theoretical density. Better results are obtained with

Fig. 8. Scanning electron micrographs of compacts S3 (a) and S4 (b) sintered at 1800 °C for 2 h under vacuum.

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Table 2 Relative density and grain size of compacts S3 and S4 sintered in the 1700–1800 °C range for 2 or 5 h under vacuum Sample

Sinteringtemperature (°C)

Dwell time (h)

Relative density (%)

Grains size (lm)

S3

1700 1800

2 2

95.9 97.2

1+9 12

S4

1750 1750 1800

2 5 2

98.5 98.0 98.7

13 48 60

Fig. 10. Translucent ceramics obtained with powders issued from the traditional solid state reaction process and the coprecipitation with relative densities close to 99%.

Fig. 9. Electron probe microanalysis of compacts S3 sintered at 1800 °C for 2 h under vacuum: (Yb, Si)-rich ScxOy (bright phase) and Sc2O3 (light grey phase).

samples S2 and S3 confirming (i) the necessity of a grinding step to increase the reactivity of the starting powders and (ii) the efficiency of an additive such as TEOS. These observations are consistent with the dilatometric analysis. Thus, the combined effects of a grinding step and a sintering aid allowed to reach 98% of theoretical density in the best case after a traditional solid state reaction process (sintering at 1850 °C of sample S3). However, it can be noticed that the densification rate of the sample elaborated with the coprecipitation powder P4 is higher than with the powders prepared by the classical ceramic way P1, P2 and P3. Fig. 8 shows SEM micrographs of the ceramics S3 and S4 sintered at 1800 °C for 2 h under vacuum. Table 2 shows the relative density and the grain size of ceramics S3 and S4 sintered from 1700 to 1800 °C for 2 or 5 h under vacuum. For both S3 and S4, there is an increase of the average grain size with the sintering temperature and the dwell time. Sample S4 elaborated by the coprecipitation technique has a higher grain size than sample S3 obtained by the classical ceramic way. Electron probe microanalysis on the ceramics S3 and S4 sintered at 1800 °C for 2 h under vacuum suggest that these two materials are quite comparable. Qualitative and quantitative analysis showed a similar composition of the main phase (Yb:Sc2O3#0.93% for S3 and Yb:Sc2O3#0.95% for S4) and the presence of few (Yb, Si)-rich ScxOy phases (Fig. 9). 4. Conclusion In order to elaborate Yb:Sc2O3 transparent laser ceramics, nanocrystalline Yb0.02Sc1.98O3 powders were prepared (i) by the traditional solid state reaction process and (ii) by coprecipitation, before being sintered under vacuum in the 1600–1850 °C range

for 2 or 5 h. The aim of the study was to compare the two ways of powder preparation and observe their influence on the sintering of Yb-doped Sc2O3 ceramics. Translucent ceramics were elaborated with the powders issued from the two techniques with near by 99% in relative density (Fig. 10). The sintered materials present similar compositions. The main difference between the two techniques is that the reactivity of the powder and the grain size of the sintered materials are higher in the case of the coprecipitation technique. Thus it is necessary to increase the reactivity of the powders and to use additives to improve the densification of the ceramics elaborated by the traditional solid state reaction process. The outlook of this work will be to increase the relative densities of the sintered materials issued from the traditional solid state reaction process and the coprecipitation technique. Different ways will be investigated: – the addition of a sintering aid in the powders synthesized by coprecipitation; – a shaping by slip casting or cold isostatic pressing (CIP) to improve the green densities of the samples; – an optimization of the sintering stage (heating rate, dwell time, etc.); – the use of the hot isostatic pressing (HIP) as a post treatment for the vacuum sintering in order to see if it’s possible to achieve the full transparency.

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