A comparative study of promising filter materials for neutron imaging facilities

The effect of sapphire and bismuth single-crystal filters and their combinations on the quality of neutron radiographic images and neutron tomography data has been studied. The parameters of the contrast of the neutron image were analyzed depending on the monocrystalline filters. Neutron transmission spectra were obtained for sapphire and bismuth single crystals. Additionally, the effect of filters on the overall intensity of the thermal neutron beam and the background of gamma-rays was investigated. Based on the obtained data, we assume that a single-crystal sapphire filter can be most effectively used for radiographic and tomographic installations using thermal neutrons.

Fracture Behavior of Single-Crystal Sapphire in Different Crystal Orientations

In order to study the anisotropy of fracture toughness and fracture mechanism of single-crystal sapphire, the three-point bending tests and the single-edge V-notch beam (SEVNB) were used to test the fracture toughness of A-plane, C-plane, and M-plane sapphire, which are widely used in the semiconductor, aerospace, and other high-tech fields. Fracture morphology was investigated by a scanning electron microscope and three-dimensional video microscopy. The fracture toughness and fracture morphology of different crystal planes of sapphire showed obvious anisotropy and were related to the loading surfaces. C-plane sapphire showed the maximal fracture toughness of 4.24 MPa·m1/2, and fracture toughness decreases in the order of C-plane, M-plane, and A-plane. The surface roughness is related to the dissipation of fracture energy. The surface roughness of the fracture surface is in the same order as C-plane > M-plane > A-plane. The fracture behavior and morphology of experiments were consistent with the theoretical analysis. C-plane sapphire cleavages along the R-plane with an angle of 57.6 degrees and the rhombohedral twin were activated. M-plane and A-plane sapphire cleavages along their cross-section.

Experimental characterization of conformal hydrodynamic nanopolishing of a machined single crystal sapphire cavity

Sapphire is an important ceramic material which finds applications in fields such as temperature sensing, optics, electronics, and ceramic bearings. Polishing of sapphire has always been a difficult task for industries and research communities. Hydrodynamic polishing (HDP) is one of the prominent methods used for polishing of hard and profiled surfaces, whereas rigid tool-based methods such as diamond turning, grinding, and honing have many limitations. The HDP process involves deterministic flow of abrasive particles in the slurry between the workpiece surface and a rotating soft tool to obtain the desired surface finish. A novel experimental setup has been fabricated to realize the conformal hydrodynamic nanopolishing on single crystal sapphire cavity. In this study, the experiments were conducted to understand the effect of abrasive particle size, basicity of slurry, and change in temperature of slurry on the polishing of machined sapphire cavity. The effect of the initial surface roughness of the machined cavity on conformal hydrodynamic nanopolishing has also been investigated. A microcrack/pit-free surface has been found after the final polishing of the sapphire cavity. An improvement of 21% is found in surface finish after the final polishing using abrasive particle size of 0.06 µm. Abrasive slurry with higher basicity (pH 13) does not improve the surface finish. By heating the abrasive slurry to a temperature of 70°C–75°C, surface finish improves by ∼26% as compared to improvement of ∼ 21% at room temperature polishing.

Understanding the effect of thickness on the thermoelectric properties of Ca3Co4O9 thin films

We are reporting the effect of thickness on the Seebeck coefficient, electrical conductivity and power factor of Ca3Co4O9 thin films grown on single-crystal Sapphire (0001) substrate. Pulsed laser deposition (PLD) technique was employed to deposit Ca3Co4O9 films with precisely controlled thickness values ranging from 15 to 75 nm. Structural characterization performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the growth of Ca3Co4O9 on Sapphire (0001) follows the island growth-mode. It was observed that in-plane grain sizes decrease from 126 to 31 nm as the thickness of the films decreases from 75 to 15 nm. The thermoelectric power measurements showed an overall increase in the value of the Seebeck coefficient as the films’ thickness decreased. The above increase in the Seebeck coefficient was accompanied with a simultaneous decrease in the electrical conductivity of the thinner films due to enhanced scattering of the charge carriers at the grain boundaries. Because of the competing mechanisms of the thickness dependence of Seebeck coefficient and electrical conductivity, the power factor of the films showed a non-monotonous functional dependence on thickness. The films with the intermediate thickness (60 nm) showed the highest power factor (~ 0.27 mW/m-K2 at 720 K).