Effect of Europium Concentration on Densification of Transparent Eu:Y 2 O 3 Scintillator Ceramics Using Hot Pressing Stephen R. Podowitz, w Romain Gaume´ , and Robert S. Feigelson Department of Materials Science and Engineering, Stanford University, Stanford, California 94305 Europium (Eu) was found to act as a solid-state sintering aid in Y 2 O 3 optical ceramics by controlling ionic diffusivity, which in turn leads to enhanced optical transparency. Transparent ce- ramic samples of Eu-doped Y 2 O 3 , with no additional additives, were sintered by uniaxial vacuum hot pressing under 40 MPa and maximum temperature of 15801C. Optical attenuation was found to decrease with increasing Eu concentrations between 0 and 5 at% for ceramics processed under the same sintering con- ditions. In order to study the effect of Eu concentration on ce- ramic densification, the strain rate and grain size during sintering at constant temperature and varied pressure were mea- sured. A diffusional flow densification model was used to derive instantaneous effective diffusion constants for the densification process. Diffusion constants were found to increase with in- creasing Eu concentration according to a log–linear relation- ship. Eu 21 was detected in samples after hot pressing through fluorescence spectroscopy, and the extrinsic defect chemistry was found to be dominated by the reduced Eu in solid solution with Y 2 O 3 . A sintering model with diffusion rate limited by yt- trium interstitial transport and controlled by the incorporation of Eu 21 onto the cation sublattice was found to be in good agreement with experimental diffusivity data. I. Introduction T HE yttrium oxide (Y 2 O 3 ) is a material of interest for infrared windows, high-power lasers, and radiation detection appli- cations. Transparent Y 2 O 3 ceramics offer potential advantages over single crystals, including reduced processing temperatures, near-net shape fabrication, and enhanced mechanical properties. Many applications for Y 2 O 3 ceramics require doping with lu- minescent ions, particularly rare earths. For radiological detec- tion, europium (Eu)-doped Y 2 O 3 is a scintillator material of interest and shares the bixbyite cubic structure with Eu:Lu 2 O 3 ,a higher radiation stopping power material. Sintering aids are widely used in ceramic fabrication process- ing to enhance densification. Control of the relative velocities of grain boundaries and pores during sintering is an important de- terminant leading to the production of a pore-free material. Control of these parameters may be achieved through the use of additives, which generally modify the mass transport kinetics at the grain boundaries through either changes in ionic diffusivity or the addition of a drag force. It is important that these addi- tives do not form permanent secondary phases, which may alter the optical properties of the material by introducing scattering centers. One successful approach to preparing transparent ce- ramics of Y 2 O 3 is through the use of sintering aids that form transient solid and liquid secondary phases. Additives that have been used for this method of sintering include La 2 O 3 , Al 2 O 3 , LiF, MgO, and MgAl 2 O 4 . 1–5 Another approach, based on the introduction of dopants to alter ionic diffusivity through modification of the host defect chemistry, has been reported in ThO 2 -, HfO 2 -, TiO 2 -, BeO-, and SrO-doped Y 2 O 3 . 6–10 Ionic de- fect concentration may also be controlled by the sintering at- mosphere, which has been shown to have a pronounced effect on densification. 11,12 Although sintering aids serve to enhance transparency in ce- ramics, their addition may have deleterious effects on ceramic performance. In transparent Eu 31 -doped yttria-based ceramics, Greskovich et al. 13 reported a substantial decrease in light yield with the addition of ThO 2 and other transparency promoting additives. The addition of Gd 2 O 3 to Y 2 O 3 has been shown to diminish light yield even at low concentrations. With this in mind, the production of Y 2 O 3 -based scintillators in the absence of a traditional sintering aid is of interest. Pressureless sintering of fully dense ceramics often relies on the use of sintering aids, because the driving potential for dens- ification without applied pressure is minimal. However, more recent work has focused on the sintering of fine particles as an alternative route to obtain highly transparent ceramics without sintering aids. 14,15 Conversely, the application of pressure during sintering has been effective in producing transparent ceramics because of the driving force associated with pressure-enhanced mass transport. In some instances, sintering aids have been uti- lized to enhance transparency in conjunction with pressure sinte- ring. 3,4,16 Song et al. 17 introduced an analytical model where densifi- cation is controlled by a vacancy flux driven by a concentration gradient between pores and the grain-boundary regions in be- tween. An increased vacancy concentration will be present near the pores, because of the decrease in free energy of vacancy for- mation due to curvature at the pore surface. This difference in vacancy concentration during intermediate-stage sintering is given by DC v ¼ C v0 g sv O=kTr (1) where C v0 is the vacancy concentration at the center of the boundary, g sv is the surface energy, O is the vacancy volume, and r is the radius of the pore. With an applied pressure, a pressure-dependent term is incorporated into the equation de- scribing the vacancy concentration difference DC v ¼ C v0 O=kT ðg sv =r þ jPÞ (2) where P and j represent the applied pressure and the stress in- tensification factor, respectively. For ceramics with micrometer- size grains sintered under typical hot pressing conditions, the pressure-dependent term will dominate over the curvature term, and any alteration to the surface energy or average pore size from an additive should have a negligible effect on the concen- tration gradient. Therefore, the vacancy flux under applied pres- sure will be affected in the same proportion by a change in the vacancy concentration at the grain boundary as it is by an equal change under pressureless sintering. Consequently, even under J. Hellmann—contributing editor Supported by the Defense Threat Reduction Agency under grant no. HDTRA1-06- CWMDBR. w Author to whom correspondence should be addressed. e-mail: podowitz@stanford.edu Manuscript No. 26142. Received April 13, 2009; approved July 29, 2009. J ournal J. Am. Ceram. Soc., 93 [1] 82–88 (2010) DOI: 10.1111/j.1551-2916.2009.03350.x r 2009 The American Ceramic Society 82