Synthesis, Characterization, and Densification of Samaria Doped Ceria Ultra-Fine Powders

Synthesis, characterization and densification of Al2-xScx(WO4)3 ceramics for low-expansion infrared-transparent windows

Niladri Dasgupta*, Bruce Butler*, Erinn Sörge*, Tzu-Chien Wen** and Dinesh K. Shetty**

* Materials and Systems Research, Inc. Salt Lake City, UT 84104

** Department of Materials Science & Engineering, University Of Utah, Salt Lake City, UT 84112

ABSTRACT

Materials and Systems Research, Inc. is developing a material with a low coefficient of thermal expansion (CTE) that could be used in an infrared-transparent window. The material is derived from a solid solution of Al2(WO4)3, which has positive thermal expansion, and Sc2(WO4)3 with a negative thermal expansion. An optimum composition of Al0.5Sc1.5(WO4)3 was identified by synthesizing solid solutions, Al2-xScx(WO4)3, by a solid-state route with compositions ranging from x = 0 to 2.0. A single orthorhombic phase was obtained at all compositions. A composition corresponding to x = 1.5 had a low CTE value of -0.15 x 10-6/oC in the temperature range, 25-700ºC. A low temperature solution combustion process was developed for this optimum composition resulting in a single phase powder with a surface area of ~ 14 m2/g and average particle size (as determined from surface area) of 91 nm. Preliminary densification experiments via dry uniaxial pressing and pressureless sintering at 1100°C for 2 hours resulted in a sintered compact 97.5% in density and submicron grain size. Keywords: Tungstates, Low-thermal-expansion, IR transparency, Thermal-shock resistance

1. INTRODUCTION

Transparent optical ceramics have widespread applications such as optical switches, laser amplifiers and lenses, infrared (IR) windows and domes, IR night vision devices and optical window materials for gas lasers. Many of these applications require high optical transmittance in the 3-5 and 8-12 µm wavelength ranges. Two materials currently in wide use for IR windows and domes in the 3-5 µm range are hot-pressed, polycrystalline magnesium fluoride (MgF2) and melt-grown sapphire (single crystal Al2O3)1. A number of polycrystalline oxide ceramics are under various stages of development as more durable and less expensive counterparts to magnesium fluoride and sapphire, respectively1,2,3. These include spinel (MgAl2O4), aluminum oxynitride (ALON) and yttria (Y2O3)1,2,3. Although the polycrystalline oxides have optical properties comparable to sapphire and offer significant savings in cost of fabrication of domes and windows, their durability under thermal-shock conditions is poor. One way to improve the thermal-shock durability of optical windows fabricated from polycrystalline materials is to make them out of materials that have low CTE. Two classes of oxide ceramics that are known to exhibit low CTE values are tungstate solid solutions having the Sc2(WO4)3 type structure4,5 and sodium or potassium zirconium phosphate [NaZr2P3O12 (NZP) and KZr2P3O12 (KZP)]6,7. Of these two families, the phosphates are expected to exhibit lower absorption edge wavelengths in the 3-5 µm midwave IR windows due to the weaker P-O bonds. Accordingly, tungstates are more desirable from the optical-absorption point of view. Suzuki and Omote5,6 reported a zero thermal expansion, single-phase material, Al2x(HfMg)1-x(WO4)3, obtained by forming a solid solution of two isostructural materials with positive Al2(WO4)3 and negative HfMg(WO4)3 thermal expansions. The thermal expansion coefficient increased from a value of -2.0 ppm/°C for HfMg(WO4)3 to a value of 2.0 ppm/°C for the solid solution, Al1.4(HfMg)0.3(WO4)3, as the atomic fraction of Al3+ increased from x = 0 to x = 1.4. Their

Window and Dome Technologies and Materials XII, edited by Randal W. Tustison, Proc. of SPIE Vol. 8016, 80160D · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.884035

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data suggested that a composition corresponding to Al0.3(HfMg)0.85(WO4)3 should yield nearly zero thermal expansion coefficient. An attractive feature of this material was that the near-zero thermal expansion was achieved with a single-phase solid solution by fine tuning the ionic bonds, and not through two different phases with phase boundaries. It was subsequently demonstrated at MSRI that a solid-state synthesis route could produce a single orthorhombic phase powder of the solid solution Al0.3(HfMg)0.85(WO4)3. However, experimental work at MSRI demonstrated that although this material exhibited a low CTE (0.3 ppm/oC) in the temperature range 170-800oC, a significantly higher thermal expansion was observed in the temperature range, 25-170oC. The material was also found to be unstable under typical hot isostatic pressing conditions of high pressure, temperature and relatively low oxygen partial pressures. Mary and Sleight8 reported near zero thermal expansion for the solid solution AlSc(WO4)3. However systematic structural and thermal expansion data were not reported for the complete range of solid solutions formed between the end members Al2(WO4) and Sc2(WO4)3. More recently, Sugimoto et al.9 systematically measured the variation in thermal expansion of Al2(WO4)3 as ‘Al’ was progressively replaced by ‘Sc’, ie. for Al2-xScx(WO4)3 where 0 ≤ x≤ 2. They reported that a near zero coefficient of thermal expansion was obtained for the composition) Al0.5Sc1.5(WO4)3 and the phase transition in Al2(WO4)3 which occurred at -21.9oC was not seen for this composition even at -140oC. However the thermal expansion traces were shown for the temperature range -125oC to 150oC and no data were reported for the cooling cycle. They also reported XRD data for solid solutions over a range of compositions between the two end members. In this paper we report the synthesis of a series of solid solutions Al2-xScx(WO4)3 by the solid state route where 0 ≤ x≤ 2. An analysis of the variation of lattice parameter with composition is reported and results for thermal expansion in the temperature range 25oC to 800oC for both heating and cooling stages are presented. The eventual fabrication of transparent components from these orthorhombic materials would require extremely fine grain sizes and high purity which may only be achieved if the powder is made by a solution route. The preparation of the solid solution Al0.5Sc1.5(WO4)3 by a low temperature combustion route using ethylene glycol as a fuel is also described.

2. EXPERIMENTAL PROCEDURE

Compositions corresponding to x = 0, 0.1, 0.4, 0.8, 1.2, 1.5, 1.7 and 2.0 for the family of solid solutions Al2-

xScx(WO4)3 were selected for synthesis by the solid state route. The compounds were synthesized from constituent oxides WO3, Sc2O3 and Al2O3 procured from Alfa Aesar. The constituent oxides were weighed in the required stoichiometric proportions and milled for 24 hrs. in a ball mill using zirconia grinding media and ethanol. The obtained slurry was dried and the dry powder was pressed into a pellet and calcined on a coarse bed of powder of similar composition. The pellet was covered with an alumina crucible to prevent the loss of tungsten. The calcination was carried out at 1100oC for 4 hours. The calcined powders were milled in a Fritsch planetary mill at 300 RPM in ethanol for 3 hours using zirconia grinding media and were recalcined at 1100oC for 4 hours. X-ray diffraction (XRD) patterns of the samples were obtained by using Phillips X’Pert PW3040 Diffractometer with Cu Kα radiation. The lattice parameters of the solid solutions were calculated from the powder XRD data by using the ‘unit cell’ software. The recalcined powders were planetary milled again for 3 hours, dried and uniaxially pressed into bars. The bars were sintered in a high temperature furnace at 1150oC for 4 hours. The sintered bars were characterized for density and it was ensured that they had a density higher than 98% of the theoretical density of the composition. The bars were then ground to approximate dimensions of (25.4 x 5 x 5) mm. The thermal expansion was measured between 25 and 800oC by an Orton dilatometer with a silica sensing rod, during heating as well as the cooling cycle. The solid solutions Al0.8Sc1.2(WO4)3, Al0.5Sc1.5(WO4)3 and Al0.3Sc1.7(WO4)3 were characterized by high temperature XRD measurements. High temperature XRD measurements were carried out at the University of California at Santa Barbara.



Measurements were made using a Bruker D8 2 theta XRD equipment with an Anton Parr HTK – 16 high temperature platinum stage and a LynxEye dectector. Each powder sample was mixed with a little DI water to make a slurry. The XRD measurements were made for each sample at 30, 300 and 600oC from 10 to 80 degrees 2-theta range at 0.01 degrees per step and each step was measured for 0.2 seconds. The generator was set at 40 kV and 30 mA. The heating stage was equipped with a platinum strip heating element which also supported the powder bed. A thermocouple was also present below the powder bed for measuring the exact temperature of the powder sample. Since the powder was placed on the heating strip, the XRD pattern for the powder also included diffraction peaks for platinum. The platinum peaks served as an internal standard for the powder sample and were used to compensate for the effect of stage movement at elevated temperatures on the sample peak positions. The lattice parameters were accurately determined for each solid solution at the three temperatures by Rietveld Analysis.

The solid solution Al0.5Sc1.5(WO4)3 was synthesized by a combustion process using ethylene glycol as a fuel based on work reported on the synthesis of nanoparticles of yttrium oxide10 and a family of solid solutions of the general formula Nd1-xSrxFe1-yCoyO3-δ

11,12 by one of the authors. In this process, ethylene glycol (EG) is used as a fuel for combustion. A solution is made (usually in water) of the respective salts of the components in stoichiometric amounts and an appropriate quantity of EG is dissolved in the same solution. The solution is heated on a hot plate with constant stirring, and as the water evaporates the EG forms a gel that enmeshes the various cations within its network structure thus preserving their homogeneity and maintaining mutual proximity. On further heating, a mild exothermic auto combustion reaction occurs (very often without a visible flame) with the emission of NOx fumes and the reaction mix converts into a dry foamy mass. The foamy mass is partially crystalline and controlled calcinations may be carried out to obtain the crystalline phase and eliminate all volatile residues. The reaction occurs in a matter of minutes and therefore unlike coprecipitation, the process is quick. In this particular case, a solution was made in water with the starting materials scandium nitrate, aluminum nitrate, ammonium metatungstate and ethylene glycol. On completion of the reaction, the powder was calcined at 750oC for two hours. The powder obtained after combustion and the calcined powder were characterized for phase composition by XRD. The powder was subsequently planetary milled at 300 RPM for 3 hours to break down any agglomerates formed during synthesis and calcination. The specific surface area for the milled powder was determined by BET measurements using TriStar 3000 (Micromeritics Instrument Corporation, Norcross, GA) equipment. The milled powder was dispersed in ethanol and a drop of the dispersion was placed on a glass slide and observed under the SEM. The milled powder was uniaxially pressed into a disc (25.4 mm in diameter and 3 mm thick) and sintered at 1100oC for 2 hours in a high temperature furnace. The density of the sintered disc was measured by the Archimedes’ liquid displacement technique.

3. RESULTS AND DISCUSSION

Aluminum and scandium tungstates are isostructural and have the orthorhombic space group Pnca. Their lattice parameters differ by only 6% and hence upon substitution of Al by Sc at the cation site, one expects the solid solution to remain a single phase. The lattice parameters of the solid solutions typically vary linearly between the values of the end members (Vegard’s Law). Figure 1 shows the powder X-ray diffraction patterns of Al2-xScx(WO4)3 solid solutions as ‘x’ varies from 0 to 2. It is apparent that the diffraction peaks shift to lower 2θ values for increasing scandia contents, indicating an increase in the d-spacings. This is expected as the ionic radius of Sc3+ (0.745 Å) for octahedral coordination is larger than that of Al3+ (0.535 Å) resulting in an increase in the unit cell parameters with substitution of ‘Al’ with ‘Sc’. All the peaks could be indexed and attributed to the Pnca orthorhombic space group and no impurity peaks were detected.



Lattice Parameters for the A]2 Sr (IWO4) System

a (A) b (A) (A)

12591 9050 9020

02 054 9000 9000

04 12722 9020 9224

08 12111 9244 9220

12 12975 9205 9405

IS 02007 9402 9497

17 12115 9400 9502

20 02200 9575 9059

The lattice parameters of the Al2-xScx(WO4)3 solid solutions were calculated as functions of ‘x’ from the powder XRD data by using the ‘unit cell’ software. Table I lists these values. As expected, the lattice parameters increased monotonically with ‘x’. Similar results have been reported by Imanaka et al.13 and Sivasubramanian et al.14. Figures 2 a, b and c compare the lattice parameters measured in this study with those reported by Imanaka et al.13 The results in this study are in reasonable agreement with those reported by Imanaka et al.13 It may be concluded that Vegard’s law is followed in the entire composition range. Lattice parameter values have also been reported for this system of solid solutions by Sugimoto et al.9. However, they reported three regions where different linear relationships were observed. The boundaries to these regions were located at x = 0.2~0.3 and 0.8~0.9. The data obtained at MSRI did not show any such regions.

Figure 3 is a graph showing the thermal expansion traces of Al2-xScx(WO4)3 (for various ‘x’ values ranging from 0.1 to 1.7) bar specimens as a function of temperature. The direction of the heating cycle is indicated by an arrow. It is apparent that the coefficient of thermal expansion varies from positive to negative values with increase in the scandium content. This is to be expected as it had been determined by us earlier that Al2(WO4)3 has a positive value of thermal expansion while Sc2(WO4)3 has a negative value of thermal expansion. The degree of hysteresis during the heating and cooling cycles also varies with the composition and it is generally lower for high scandium compositions. The hysteresis in thermal expansion seen during thermal cycling of a sample is usually attributed to micro-cracking due to the presence of grains that exceed

Figure 1: Powder X-ray diffraction patterns of

Al2-xScx(WO4)3 solid solutions

Table 1: Lattice parameters for the Al2-xScx(WO4)3 system



Figure 2: Comparison of the variation in lattice parameters with composition for) Al2-xScx(WO4)3 solid solutions as determined

at MSRI with values reported by Imanaka et al.

a specific threshold size limit in materials with anisotropic thermal expansion14. Since the melting point of Al2(WO4)3 is lower than that of Sc2(WO4)3, relatively dense samples of alumina rich solid solutions are expected to have larger grain-sizes than scandia rich samples when sintered at the same temperature. This could offer a possible explanation for the higher hysteresis observed in alumina rich samples although the specific case of very large hysteresis seen for the thermal expansion curve for x = 0.4 seems to be an anomaly. Table-2 below illustrates the coefficient of thermal expansion values extracted from the heating cycle of the graph for the various compositions. The temperature range has been selected based on the linear region available for measurement. The value corresponding to x = 1.5 is the smallest at -0.15 ppm/oC in the 25 to 700oC temperature range. The CTE values reported for the same composition by Sugimoto et al.9 are ≈ -0.1 ppm/oC in the -50 to 100oC temperature range and -0.8 ppm/oC in the 100 to 450oC range. The exact value of ‘x’ corresponding to a CTE of zero, extracted from the CTE values of Table-2 by intrapolation is 1.45. However, this value does not have much meaning since the experimental variables that determine the final composition and grain-size may vary enough to off-set such minor changes in the targeted composition. At grain-sizes that are higher than a threshold size (unique to a specific composition) the formation of micro-



Composition Temperature Range Coefficient of Thermal Expansion (CTE)

X = 0.1 25 -550©C 2.171 ppm

X = 0.4 25 -550©C 1.779 ppm/©C

X = 0.0 25 775©C 1.546 ppm/©C

X = 1.2 25 775©C 0.010 ppm/©C

X 1.5 25-700©C700 000©C

- 0.lSppint©C- 1.7 ppint©C

X17 25-000©C -1 lppint©C

Figure 3: Thermal expansion plots for Al2-xScx(WO4)3 solid solutions.

Table 2: Coefficients of thermal expansion for the Al2-xScx(WO4)3 system



cracks may affect the value of the CTE. To eliminate the effect of microcracks on thermal expansion, one must either ensure that the grain-size is below this threshold value or determine the thermal expansion from change in lattice parameters with temperature, by high temperature X-ray diffraction. Accordingly high temperature XRD measurements were done for select compositions belonging to the Al2-xScx(WO4)3 system.

The lattice parameters obtained from XRD data obtained at elevated temperatures are shown as functions of temperature in Figures – 4 A, B and C. The compositions measured are Al0.8Sc1.2(WO4)3, Al0.5Sc1.5(WO4)3 and Al0.3Sc1.7(WO4)3. Lattice parameter ‘a’ increases with temperature for all three compositions. Lattice parameter ‘c’ decreases with temperature for all three compositions. Lattice parameter ‘b’ decreases with

Figure 4: Variations of A) lattice parameter ‘a’ B) lattice parameter ‘b’ C) lattice parameter ‘c’ and D) unit cell

volume ‘V’ with temperature for compositions x = 1.2, 1.5 and 1.7.



CompositionlTemp. (ppm/°C) u (ppnil°C) a0 (ppnil°C) a0j50 (ppntl°C) a.(p p m/° C)

X1.2 5.14 -0.017 -1.5 1.21 0.010

X1.5 3.45 -2.1 -2.3 -0.32 -0.32

X=L7 341 -26 -36 -0941 -11

temperature for the Al0.8Sc1.2(WO4)3 and Al0.5Sc1.5(WO4)3 while it is almost constant for Al0.3Sc1.7(WO4)3. Figure–4D shows the change in unit cell volume (a.b.c) with temperature. The volume takes into account the variations in all three lattice parameters with temperature. It is apparent from Figure-3D that the volume increases with temperature for Al0.8Sc1.2(WO4)3 , decreases with temperature for Al0.3Sc1.7(WO4)3 and remains almost constant for Al0.5Sc1.5(WO4)3.

The average CTE values obtained from high temperature XRD in the temperature range, 30-600oC along the three axes (αa, αb and αc) and the average linear thermal expansion (αabc) [where αabc = average volumetric thermal expansion / 3] which corresponds to the value measured by the dilatometer are given in Table-3. The last column shows the corresponding CTE values obtained previously by dilatometry. It is apparent that Al0.5Sc1.5(WO4)3 still shows the lowest CTE value among the three compositions studied. It is also the composition closest to the point at which the CTE changes sign. The data also suggest that a small addition of aluminum oxide to Al0.5Sc1.5(WO4)3 will result in a near zero CTE. It may also be reasonably concluded that micro-cracking does not occur in the test samples to any appreciable degree. Table 3: Comparison of the average CTE values obtained from high temperature XRD and dilatometry

The solid solution Al0.5Sc1.5(WO4)3 was synthesized by a low temperature combustion synthesis process using ethylene glycol as a fuel. Our previous experience has shown that by using an appropriate glycol/nitrate molar ratio, it is possible to restrict the reaction temperature to 400 to 500oC.10 The lower combustion temperature prevents hard agglomerates in the powder as compared to powders obtained by using conventional fuels such as urea and glycine. The initial powder formed after the reaction is X-ray amorphous (Figure – 5 A). A subsequent calcination step is required to ensure that the desired pure phase is formed and that the excess nitrates or fuel components such as carbon are eliminated. All such components are normally eliminated from the powder by about 750oC for nitrate systems in general using ethylene glycol as the fuel10. Accordingly, 750oC was selected as the calcination temperature for our initial trials. The XRD pattern for the powder calcined at 750oC is shown in Figure – 5 B. The presence of all peaks corresponding to Al0.5Sc1.5(WO4)3 indicates that the crystalline phase has formed at this temperature. The crystallite size for this powder was calculated to be 37 nm from Scherrer’s formula for X-ray line broadening. The powder was subsequently planetary milled at 300 RPM for 3 hours to break down the loose agglomerates that are usually present in combustion synthesized powders especially when they are made in a flat bed. While these agglomerates do not affect the surface area to any appreciable extent, they do not allow the particles to pack together efficiently, thereby leading to lower green densities. The specific surface area of the milled powder was 13.9 m2/g. Assuming the particles to be spherical in shape, the average particle diameter may be calculated from the specific surface area using the formula

D = 6/Sρ (1)



where ρ is the density of Al0.5Sc1.5(WO4)3 and S is the specific surface area. The average particle diameter was determined to be 90.4 nm. Figure 6A and B shows SEM micrographs of the milled powder at two different

Figure 5: XRD plots of combustion synthesized Al0.5Sc1.5(WO4)3 powder A) as synthesized, B) calcined at 750oC

for 2 hours.

magnifications. Most of the particles appear to be sub-micron in size with many that are smaller than 0.5 µm. The magnified micrograph shows the degree of agglomeration with a clustering of 100 to 200 nm size

Figure 6: SEM micrographs of Al0.5Sc1.5(WO4)3 powder synthesized at MSRI by the glycol-nitrate combustion

route.

500 nm

B



particles to form 500 nm particles. Figure 7 shows the SEM micrograph of a uniaxially pressed disc of the milled powder sintered at 1100oC for 2 hours. The sample was polished and thermally etched to reveal the grain structure. Although the grain boundaries are not well demarcated, it is evident that a majority of the grains are submicron in size. The bulk density was measured to be 97.5 % of the theoretical density of Al0.5Sc1.5(WO4)3. The results obtained are for powders that were prepared in a flat glass container on a hot plate, uniaxially die-pressed in the dry condition and pressureless sintered.

Figure 7: SEM micrograph of Al0.5Sc1.5(WO4)3 uniaxially pressed, sintered at 1100oC for 2 hours and polished and

etched.

4. CONCLUSIONS

A systematic investigation was conducted to determine the structural variations and thermal expansion behavior of a series of solid solutions corresponding to the general formula Al2-xScx(WO4)3. The compositions were synthesized by a solid state route and a single orthorhombic phase was obtained in each case. The variation of lattice parameters with composition followed Vegard’s law. The coefficient of thermal expansion changed from positive to negative values as the ‘Al’ in the composition was gradually replaced with ‘Sc’. The smallest value of CTE obtained was -0.15 ppm/oC in the temperature range 25 to 700oC for the solid solution Al0.5Sc1.5(WO4)3. Single phase Al0.5Sc1.5(WO4)3 was synthesized by a low temperature combustion route . Submicron grain sizes were obtained for uniaxially pressed specimens sintered at 1100oC for 2 hours.



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Synthesis, Characterization, and Densification of Samaria Doped Ceria Ultra-Fine Powders

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