Al 2 O 3 -CeO 2 ceramics were found to be transparent in the IR range and high-traslucent in the visible range. The surface of nanometric α-alumina particles was modified by deposition-precipitation of small fractions of ceria nanoparticles. The powders were sintered using Spark Plasma Sintering. Values of Real In-line Transmittance up to 70% in the IR-Vis range have been measured. Transparency enhancement has been attributed to Cerium oxide nanoparticles located at the grain boundaries and triple points. These particles are hindering alumina grain growth during SPS at a temperature as high as 1430°C. This effect is found to be effective under SPS low vacuum conditions and short dwell times. The optimum ceria content was found to be 0.7 wt.%. Diffusion in Al 2 O 3 -*Manuscript 2 CeO 2 as a function of the atmosphere has been studied in diffusion couples. The results obtained by the proposed route are discussed considering the data reported in the literature for SPS ed transparent alumina.

It is well known that obtaining transparent alumina is of special interest for applications that require the combination of optical and mechanical properties. Sapphire and polycrystalline alumina are being used in industrial and military applications, such as armour parts, discharge lamps, airborne infrared sensors and lasers. [1] In addition, the high cost of the production of transparent sapphire crystals and the difficulties of carrying out the synthesis process for large-sized alumina monocrystals has focused attention on the development of polycrystalline transparent alumina materials (PTAMs). [2] Obtaining PTAMs requires that two main challenges be dealt with: i) the porosity in the final material must be lower than 0.05 vol.-% to avoid scattering effects and so to achieve transparency, [3] and, ii) the birefringence character of alumina grains, which is inherent to the material and seems to be more critical for larger grains, must be balanced. In this context, the key factor to achieve PTAMs seems to be the control of the grain growth in order to decrease the flaw size (< 100 nm) and, therefore, the volume of porosity, as well as to minimize the influence of birefringence in the optical properties of alumina.Several authors have reported that the presence of second phases, such as MgO, Y 2 O 3 , SiO 2 TiO 2, CaO, etc., modifies the grain growth rate of alumina when sintering. [4][5][6] Nevertheless, this is not enough to reach transparency due to the anisotropic character of alumina. Moreover, the effect of these elements on decreasing porosity is not well understood currently.The influence of oxides (such as MgO, TiO 2 , CaO, etc.) on the transparency of polycrystalline alumina compacts are widely studied in the literature. In this work, a completely different approach is developed, consisting of precipitating 0.5 wt.-% CeO 2 nanoparticles (< 5 nm) on the surface of the starting alumina nanopowder (d 50 approximately 170 nm) using cerium(III) acetate as precursor. It is shown that the ceria nanoparticles strongly enhance the transparency of the spark plasma sintered compacts due to: i) the ceria nanoparticles acting as powder lubricant, increasing by around 15% the initial density of the powder in the SPS die, and, ii) the CeO 2 nanoparticles, having a very low solid solubility in the alumina grains, locating at grain boundaries, hindering alumina grain growth by pinning during SPS sintering at 1 430 8C, 80 MPa for 2 min. This effect is found to be effective only under SPS vacuum conditions. In order to explain the light scattering behavior in the near-infrared and visible range, a light scattering model under the Rayleigh-Gans-Debye approximation for polycrystalline alumina is used. This model offers an additional and simple tool for a completed bulk evaluation of the SPS compacts microstructure. 1154 wileyonlinelibrary.com ß