The spark plasma sintering (SPS) technique was used to produce mid-infrared (IR) transparent alumina with the desired transmittance. An excellent transmittance of 85% has been obtained in a sample sintered at 13001C for 5 min. The heating rate, sintering time, and annealing have a significant influence on IR transmittance. The improvement in transmission may be attributed to the progressive elimination of residual porosity when applying a slower heating rate, longer sintering time during SPS, and postsinter annealing. It is suggested that localized residual strain/stress at grain boundaries and oxygen vacancy concentration are other factors influencing the optical properties of the SPS-sintered alumina.

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Spark plasma sintering (SPS) is a remarkable method for synthesizingand consolidating a large variety of both novel and traditional materials.The process typically uses moderate uni-axial pressures (< 100 MPa) in conjunction with a pulsing on-off DC current during operation. There are a number of mechanisms proposed to account for the enhanced sintering abilities of the SPS process. Of these mechanisms, the one most commonly put forth and the one that draws the most controversy involves the presence of momentary plasma generated between particles. This study employees three separate experimental methods in an attempt to determine the presence or absence of plasma during SPS. The methods employed include: in-situ atomic emission spectroscopy, direct visual observation and ultra-fast in-situ voltage measurements. It was foundSubmitted to the Journal of Applied Physics (2008) 1 using these experimental techniques that no plasma is present during the SPS process. This result was confirmed using several different powders across a wide spectrum of SPS conditions.

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