Developing transparent polycrystalline zinc selenide (ZnSe) pellets for laser applications

Transparency in particular regions of the electromagnetic spectrum is well recognized and is utilized in many applications, including solid-state lasers, optical windows in optical spectroscopy instruments, and transparent armors. Improving the quality of transparency, mechanical endurance, thermal stability, and corrosion resistance in this material class has attracted academic and industrial interest. Nowadays, numerous transparent ceramics are available in various compositions employed in different applications due to their physical, chemical, and optical characteristics. Among many functional transparent ceramics, ZnSe, in particular, exhibits a number of unique properties: (i) it does not oxidize at temperatures up to 300 °C in the ambient atmosphere, (ii) its refractive index doesn't vary to a large extent with temperature, (iii) it exhibits homogeneity in refractive index at different wavelengths, (iv) it is non-toxic, (v) it has low phonon energy, (vi) it exhibits isotropic properties because of its cubic crystal structure, and (vii) it exhibits transparency in a broad range of the electromagnetic spectrum (0.6–21μm). ZnSe is widely chosen as a host material for lasers that operate in a broad spectrum, particularly in the mid-infrared range. Until now, the polycrystalline ZnSe materials having the highest light transmittance have been synthesized using chemical vapor deposition. However, this approach needs the use of very hazardous gases such as H2Se and has a relatively slow crystal growth rate and is an expensive process. Therefore, new synthesis methods should be developed to produce transparent ZnSe materials. In this thesis, polycrystalline ZnSe materials have been prepared via solid-state synthesis, co-precipitation, and mechanical alloying (high-energy ball milling) methods. Furthermore, single-crystalline ZnSe was obtained via the chemical vapor transport method. The polycrystalline ZnSe materials synthesized by one of the aforementioned methods have been consolidated using the spark plasma sintering (SPS) process with a variety of sintering parameters. Chemical, structural, and optical analyses have been performed on all as-obtained ZnSe pellets. According to the characterization results, the formation of a trace amount of impurity in the powder material to be sintered, a broad particle size distribution, and small crystallite sizes considerably lower the optical transmittance of the ZnSe pellets obtained by SPS. Additionally, carbon diffusion to the sintered material has been detected from the graphite die after SPS process, evidenced by the material's black/gray color. By optimizing the SPS parameters, it has been possible to hinder carbon contamination of the material to be sintered. Besides, by applying a purification step (heat treatment either under Argon or vacuum atmosphere) for the powder to be sintered and by tuning the SPS parameters, pellets with large grain size and less porous structure have been obtained. In this way, the transparency of the pellets has been largely improved. The best sintered ZnSe sample has been obtained by using the ZnSe powder prepared by the solid-state method for 150 minutes at 1200 °C. This sample provided a light transmittance of around 50% in the far-infrared range. To the best of our knowledge, to synthesize single-crystalline ZnSe materials, ZnCl2 was used as a transport agent for the first time in this thesis. Single-crystalline ZnSe synthesis up to 2 mm in length has been accomplished using this method in trials conducted under various heat gradients and with varying concentrations of transport agents.