Influence of temperature on the spark plasma sintering of calcium fluoride ceramics
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e commercially abundant low purity calcium fluoride powder was directly loaded for spark plasma sintering (SPS). In a vacuum atmosphere with a constant pressure held at 70 MPa the sintering temperature was systematically varied in the range of 500–850 °C. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) techniques were used to characterize the raw powder; and for studying the microstructural properties and in-line transmittance of the finalized ceramics, SEM and Fourier transform infrared spectroscopy (FTIR) were used. Digital images of the 700 °C sintered translucent CaF2 ceramic were taken along with transmittance recordings. The grain growth mechanisms and activation energy values were determined; and the influences of temperature on the relative density, grain size, and optical transmittance were demonstrated. Furthermore, for the first time, a plausible predominant mechanism was proposed for describing the different sintering stages of calcium fluoride ceramics.
I. INTRODUCTION
In the pursuit of fabricating high-quality optical ceramics which are notably better than their glass and single-crystal counterparts, calcium fluoride ceramics have been gaining tremendous research attention.1–5 Due to advancements in nanotechnology the viability of the raw materials for producing superior optical ceramics has vastly increased. With their superior optical and chemical properties, calcium fluoride ceramics are highly suitable for a wide range of industrial and scientific applications such as laser technology,1,4,6 scintillators,7 photolithographic processing,5 and specialized window production.8 Hot pressing single-crystal deformation-fabricated yttrium-doped CaF2 ceramics have been demonstrated to have higher slope efficiency than that of single-crystal materials.4 A decade ago, europium-doped CaF2 ceramics were examined and determined to possess high light output and energy resolution with alpha particle excitation, making them useful for low transmittance applications.9 Furthermore, substantial mechanical strength enhancements have been recently achieved, inexorably improving the suitability of these specialized ceramics for more industrious infrared window applications.10 Calcium fluoride ceramic sintering research has been conducted for more than half a century. In the mid-1960s, Kodak developed the first fluoride ceramic laser a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.222 J. Mater. Res., 2014
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(CaF2 : Dy 31) by hot pressing coarse pulverized powder with a particle size distribution around 150 lm.11 To remove any potential contaminants, hazardous dry hydrogen fluoride was repeatedly used throughout the fabrication procedure. With the sintering temperatures ranging from 800 to 1000 °C, no significant grain growths were observed because the starting grain domains were already very big. A little earlier than the work from Kodak Company, using different sintering temperatures,
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