| Transparent ceramic is an important direction for the development of solid laser medium because of its advantages such as lower cost, higher doping level comparing with single crystal. To date, the materials used as laser transparent ceramics are mostly oxides and fluorides both belonging to cubic system. Among the fluorides, calcium fluoride (CaF2) has accepted extensive attentions because of its broad range of transmittance, low refraction index and phonon energy and high threshold of damage. The rare earth (RE) ion Er3+ has emission channel for laser with wavelength of 1.5μm and 2.7-3μm which are safe for human eyes; furthermore, it is rich in the up-conversion energy level. So the Er:CaF2 laser ceramics have potential application in the fields such as optical communication, medical treatment, laser detection, ranging and up-conversion luminescence. In this dissertation, CaF2 and Er:CaF2 nanopowders were synthesized by wet chemical method firstly. Then taking these obtained powders as raw materials, CaF2 transparent ceramics were prepared by hot-pressing (HP) method/hot isostatic pressing (HIP) as post-treatment, and Er:CaF2 transparent ceramics were fabricated by HP method. At last, the research was carried out on the fabrication of Er:CaF2 by spark plasma sintering (SPS).Firstly, CaF2 nanopowders were synthesis by wet chemical method, of which two kinds of methods were studied, i.e. reverse micelle method and directly chemical precipitation method. The obtained powders were characterized by XRD, FE-SEM and TEM. For the reverse micelle method, the effects on the obtained powders of three issues, i.e., concentration of agents, molar ratio of surfactant to cosurfactant, and molar ratio of water to surfactant, were studied. The results indicated that grain size increased with that of concentration of agents, relative amounts of cosurfactant and water, this phenomenon could be attributed to the damaged "micro-reactor". The grain size obtained by reverse micelle method ranged from 20 to 40 nm, with spherical morphology and better dispersion. However, this method needs large amounts of surfactant and organic solvents, with high cost, complicated operation and small output.The effects of supersaturation ratio (SR) on the obtained powders were studied for the directly chemical precipitation method, e.g., the effects of SR on grain size, morphology and aggregation by TEM and laser particle size analyzer. When the SR was smaller than a3, grains were mostly cubes; grain size increased with that of SR, and was about 1μm when SR was a3. When the SR was larger than a3, grains transformed from cubic morphology to approximately hexagonal, spherical or irregular morphology; grain size decreased with the increase in SR. the particle size distribution analysis showed better dispersion when SR was smaller than a7.Aging time also had an important effect on grain size and morphology. The grain size of powders synthesized with SR of a7 with aging time of 0 day, 1day, 12days and 30 days were 46.55 nm,87.37 nm,98.65 nm and 110.53 nm, respectively. The growth rate of grains was largest within 24 hours after synthesis. With the increase in aging time, grains transformed from spherical morphology to approximately hexagonal morphology.The study of effect of Er doping level on CaF2 nanoparticles indicated that, with doping level of 10%, grain size increased with that of SR when SR was smaller than a4 and grains showed cubic morphology. When the SR was bigger than a4, grain size decreased with increase in SR and grains transformed from cubic morphology to spherical or hexagonal morphology. With the same SR, the aggregation of doped powders was more serious than that of undoped powders. For the doped CaF2 nanoparticles, grain size decreased, lattice constants increased and fluorescence intensity was weakened with increase in doping concentration of Er3+ ions.The preparation of CaF2 transparent ceramics indicated that:CaF2 ceramics by HP method cracked and were opaque; carbon penetration happened when sintered at higher temperature. Although the CaF2 ceramics with x% LiF as sintering addictive still cracked, the transmittance increased from W1℃to W3℃, however, the samples sintered at W4℃and W5℃were opaque because of carbon penetration. With the combination of HP method and HIP post-treatment, as well as x% NaF and x% LiF as sintering addictive, respectively, the transmittance of ceramics at 1100 nm were 26% and 39%, with grain size of 50-100μm and 200-300μm. NaF and LiF were incorporated in the matrix of CaF2 in the form of solid solution, resulting in the increase in lattice constants. For both HP method and HIP post-treatment, there were some amounts of "micro-pores" in obtained ceramics, resulting in the poor transmittance.Er:CaF2 transparent ceramics were fabricated by HP method. The results of 5%Er:CaF2 transparent ceramics sintered at different temperatures showed that T4℃was preferred temperature. The doping level had an important effect on the property of transmittance, lower transmittance for low doping level (e.g.1%) and higher transmittance for higher doping level (e.g.5%,7% and 10%). The fluorescence spectra showed that green light was dominant for low doping level, however, green light and red light was dominant at high doping level, and the intensity of blue light increased. The attenuation coefficient about 1.6545 cm-1 was deduced according to the dependence of transmittance on thickness. This higher attenuation coefficient could be attributed to the penetration of carbon cement. x% LiF could improve the transmittance of Er:CaF2 ceramic, taking 5%Er:CaF2 as examples, the transmittance of 5%Er:CaF2-x%LiF transparent ceramic at 1100 run was 90.45%, close to the theoretical value of 93.98%.The SPS sintering of Er:CaF2 transparent ceramics was researched preliminarily. The samples sintered by SPS tend to rupture but with fine grains; the sample with x% LiF as addictive was opaque because of the second phase existed at the boundaries. Slow heating rate was in favor of completeness of samples; appropriate annealing temperature (b℃) improve the transmittance and decrease the absorption coefficient. |
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