مجله مواد و فناوری های پیشرفته
سال هشتم شماره 1 (بهار 1398)

Nowdays, AAO (Anodic Aluminium Oxide) films obtained by anodizing, have vast  applications in different fields of nano. In this study, AAO films production by mild and hard anodizing of 1100 aluminium alloy was investigated in three different conditions, and the characteristic parameters of these films were studied. Results showed that AAO films produced in oxalic acid and sulfuric acid mixture as an anodizing electrolyte needs to lower voltage compared with films produced in oxalic acid electrolyte alone. Mixture of oxalic acid and sulfuric acid generated AAO films with smaller interpore distance and pore diamaeter without any reduction in oxide film quality. Thickness of the barrier layer which is an important parameter for the growth of materials, was also reduced from 155 nm to about 110 nm. Phase analysis results using XRD patterns showed amorphous-crystalline structure of AAO films. EDS results showed the presence of sulfur in these films with addition of sulfuric acid to the anodizing electrolyte, and the more sulfuric acid was added to the solution, the amount of sulfur was increased. This shows high acidic potential of sulfuric acid and its peneteration into the film during anodizing process.

The probable structures of format anion adsorbion on the 101 surface of anatase allotropy of titanium dioxide were investigated by quantum mechanical calculations in order to analyze mechanism of action of this advanced photocatalyst in this adsorption process. At the first, in order to get the optimized structures, the structures of adsorbent surface alone, adsorb species alone and adsorbent surface including adsorb species were exposure to the structural optimization calculations. These optimizations of structures were done using cluster and periodic computational methods. Results of both methods show that dissociative adsorption of formic acid on the 101 surface of anatase is more stable than associative one. Also, stability of the adsorbed formate ion is in the order of bidentate state on the surface with oxygen defency, bidentate state on the perfect surface and monodentate state on the perfect surface. The atom in molecule (AIM) and nuclear magnetic resonance (NMR) theorems were employed in order to certify the results of adsorption energy investigations. The nature of the oxygen-titanium bonds were identified using quantum theory of atoms in molecules and it was shown that they have electrostatic nature. Results of these theorems certified the former results of adsorpsion strength.

Titanium oxide coatings were prepared on commercially pure titanium substrates using plasma electrolytic oxidation process under current density of 0.15 A/cm2 in electrolyte containing 5-15 g/L potassium phosphate for period of 120-600 s. The effects of oxidation time and electrolyte concentration on surface morphology, thickness, roughness, relative porosity, pores size distribution, microstructure and corrosion behavior were evaluated. Investigation of voltage-time variations showed that increasing the electrolyte concentration up to 15 g/l caused an acceleration of microdischarge events and reduced the breakdown voltage from 593 V to 516 V. Morphological observations revealed that the pores were located at long distances and protuberances around the pores were more prominent. By increasing the oxidation time to 600 s, the thickness and roughness of the coatings increased, and due to the presence of 11% porosity of the coating surface, the total number of pores with diameters greater than 1 μm to 18% of the total surface pores reached. Investigation of the corrosion behavior revealed that the thickness of 8.2 ± 0.3 μm and low porosity, increased the corrosion resistance of the sample coated in the concentration of 15 g/L of potassium phosphate up to 1670 kΩ.cm2.

Copper-tin alloys with three tin contents of 0.18, 0.3, and 0.5wt.% was produced by continuous casting process at the temperature of 1150 and 1180 ℃ and speed of 3 and 5 m/min; then, microstructure, electrical, and mechanical properties of produced alloys were determined and compared with the pure copper. It was found that all casting samples have similar columnar grains, elongated radially from walls to the center. Also, the effect of Sn content on the columniation of grains is more sizeable than both other casting parameters. The results of scanning electron microscopy indicated that the produced alloys have a single-phase structure and Sn distribution is almost homogeneous in the copper. Additionally, electrical conductivity is slightly reduced by the increment of the alloying element whereas casting temperature and speed do not have a considerable effect.

Cathode material of Li-ion batteries is responsible for performances and behaviours of these high energy storage devices. In this study, olivine, LiFePO4 (LFP), with Pnma space group (orthorhombic) is evaluated by density functional theory (DFT) using Wien2k program. Calculations were performed by different methods, i. e. LSDA, PBE-GGA, LSDA+U, GGA+U, modified Becke-Johnson (mBJ) and PBE-Fock-a (a Hybrid Functionals method, named HF). Assessments showed structural stability after extraction of one Li per formula. The relaxed structure using LSDA (GGA) calculations was underestimated (overestimated) regarding experimental structure data.  According to the calculations, the most shrinkage after Li extraction is occurred in ab plane, which could cause uniform domino-cascade Li extraction in c direction. This phenomenon would lead to constant voltage in charging/discharging. Considering experimental reaction voltage value is 3.2 V, the closest theoretical calculated voltage values belonged to GGA+U, GGA and mBJ methods. According to calculated density of states (DOS) diagrams, lithiated structure (LiFePO4) is N-type semiconductor and delithiated structure (FePO4) is P-type semiconductor. Inversely-biased-diode phenomenon turns this material as low rate capable in comparison by the oxide cathode materials. However, based on the DOS configurations, its rate capability is better than many of polyanion cathodes. Extraction of Li (turning Fe ionic state from II to III) leads to conductivity enhancement. The empty 3d-Fe orbitals are responsible for this phenomenon.

In the present study, in order to increase the wear resistance and surface hardness of Ni-Br-Al alloy, coating process of electroless plating was used to make nickel-boron coating. Microstructural investigations by field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) showed that the nickel-boron coating was uniformly formed with completely amorphous structure, thickness of about 7 µm, and the cauliflower morphology on the substrate. The resulting coating with a hardness of 788 (Hv50) increased the hardness of the substrate by about 40%. To increase the hardness, the coated samples were heat treated at three temperatures of 310, 410, and 510°C for 70 minutes. After the heat treatment, the hardness of the samples increased to 1365 (Hv). By increasing the temperature of the heat treatment from 410°C, the hardness of the samples decreased. XRD results showed that at 310°C temperature, the coating structure was maintained amorphous and at temperatures of 410 and 510 °C, a completely crystalline structure was created. Also, XRD analysis showed the formation of Ni2B and Ni3B precipitates in samples after the heat treatment. According to results of the wear test on the disk, the highest wear resistance was related to the sample coated and heat treated at 410 °C temperature.

In this study, the effect of phosphorous concentration has been investigated on performance of porous silicon solar cells. For this purpose, silicon substrates were etched into aqueous etching solution containing hydrofluoric acid / hydrogen peroxide with molar ratios 0.82, 0.87 and 0.89 via metal-assisted chemical etching. Surface characteristics were studied by optical microscopy, field-emission electron microscopy and antireflection analysis. In the following, solar cells were fabricated by phosphorous diffusion into optimized porous silicon substrate which phosphorous concentrations had values of 3%, 9%, and 15%. The result presented that in 0.82 molar ratio, the etched silicon had uniform porosities whose sizes were nearly to Ag nano-particles. The reflectance of this substrate was decreased to 11% and was the best substrate for absorbing incident light. Survey of fabricated solar cell performance showed 98% increasing of short circuit current in the solar cell which was made by P2O5 3%. This concentration due to is near the solubility of phosphorous in silicon, caused the increasing efficiency to 10.6%.

Pure and cobalt doped zinc oxide nanopigments were synthesized by chemical sol-gel method. The structural properties of the prepared nanopigments were investigated be x-ray diffraction (XRD) and transition electron microscopy (TEM) and the optical properties by UV-vis spectroscopy. Both TEM micrographs and size strain plot (SSP) analysis of the XRD results showed that the crystalline size of the samples increased by the dopant increases. It was also indicated that the optical band gap of the prepared nanopigments were increased by the cobalt content increases that is related to the crystalline size increases in presence of cobalt. The colorimetric investigations showed that the color of the prepared white zinc oxide nanopigments were changed to green by adding cobalt to the matrix. As expected, the intensity of the green color was increased as the Co content increases.