The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

Pattern formation and transient thermocapillary flow in a rectangular side-heated open cavity

Marangoni convection in an open, side-heated container of a rectangular cross-section is investigated experimentally. The cavity’s length is much larger than the distance between the heated walls. Therefore, the flow is quasi-two-dimensional if the thermocapillary Reynolds number and the Rayleigh number are small. On an increase of the temperature difference between the side walls the flow becomes three-dimensional. We measure the onset of the three-dimensional flow and its structure. Morover, microgravity experiments have been performed using the Drop Tower facility of ZARM at the University of Bremen. In these latter experiments the dynamics of the flow upon a step change from 1g to μg was measured. The result of the experiments and corresponding numerical simulations show a significant change of the flow pattern within the first 1.5 seconds after the step change.     

Band gap reduction and redshift of lattice vibrational spectra in Nb and Fe co-doped PLZT

Nb and Fe co-doped PLZT (Pb_0.97La_0.02(Zr_0.52Ti_0.48)_1?2x (Nb_0.5Fe_0.5)_2x O₃ for x = 0.00, 0.02, 0.04, 0.06 and 0.08) samples have been prepared using sol–gel method. X-ray diffraction (XRD) and Raman spectroscopy studies confirmed that the samples are single phase even for the highest tested doping of 8 mol% of Nb and Fe. Incorporation of Nb and Fe atoms into PLZT lattice has been confirmed by XRD study where a systematic peak shift has been observed with increasing dopant concentration. The lattice parameters are found to decrease gradually with increase in Nb and Fe contents. From Raman spectroscopic investigation, redshift of several modes has been observed. Rietveld refinement has been performed to correlate XRD results with the fitting of Raman spectra. A total of 14 distinguished modes have been identified by de-convolution of Raman spectra, and they are in good agreement with the theoretically calculated modes for PbTiO₃ and also with those reported on PZT and PLZT previously. The Burstein–Moss shift of absorption edge has been observed by diffuse reflectance spectroscopy experiment, and the analysis shows change in band gap from 3.21 eV (for x = 0.00) to 2.59 eV (for x = 0.08). The underlying mechanisms and the observed electronic behavior have been confirmed and analyzed by photoluminescence study which revealed several transitions and supported the effect of Nb and Fe co-doping as observed from XRD and Raman spectroscopy.     

SnO-SnO2 modified two-dimensional MXene Ti3C2Tx for acetone gas sensor working at room temperature

Acetone,as widely used reagents in industry and laboratories,are extremely harmful to the human.So the detection of acetone gas concentrations and leaks in special environments at room temperature is essential.Herein,the nanocomposite combining SnO-SnO2 (p-n junction) and Ti3C2Tx MXene was successfully synthesized by a one-step hydrothermal method.Because of the existence of a small amount of oxygen during the hydrothermal conditions,part of the p-type SnO was oxidized to n-type SnO2,forming in-situ p-n junctions on the surface of SnO.The hamburger-like SnO-SnO2/Ti3C2Tx sensor exhibited improved acetone gas sensing response of 12.1 (Rg/Ra) at room temperature,which were nearly 11 and 4 times higher than those of pristine Ti3C2Tx and pristine SnO-SnO2,respectively.Moreover,it expressed a short recovery time (9 s) and outstanding reproducibility.Because of the different work functions,the Schottky barrier was formed between the SnO and the Ti3C2Tx nanosheets,acting as a hole accumulation layer (HALs) between Ti3C2Tx and tin oxides.Herein,the sensing mechanism based on the formation of hetero-junctions and high conductivity of the metallic phase of Ti3C2Tx MXene in SnO-SnO2/Ti3C2Tx sensors was discussed in detail.     

Prototyping and modeling of the centrifugal casting process for paraffin waxes

Centrifugal casting is a technology used for manufacturing hybrid rocket paraffin grains. This technology helps avoiding voids formation inside the solid paraffin as it cools. Voids are formed because of air bubbles being entrapped while pouring and because the liquid wax shrinks by 17–19% upon cooling. In this work, the centrifugal casting process for the manufacturing of paraffin cylinders was prototyped at two different scales considering critical casting issues. The effects of process parameters (rotational speed, melt temperature, and flow rate) on the tensile properties of the manufactured grains were analyzed. The results of the optimization conducted at the lower scale (2.5?kg) were up scaled to manufacture 25?kg grains. The resulting mechanical properties complied with the design specifications, and they were better than those characterized from the gravity cast wax. A numerical model of growth and dissolution of bubbles during the process was then developed to predict the quality of the castings. The numerical results showed how increasing the mold rotational speed up to 1800?rpm reduced the removal time. However, compared to grains solidification time, the predicted removal times were much shorter, proving the advantage of centrifugal casting in counteracting voids formation.     

Solid base catalysis of calcium oxide for a reaction to convert vegetable oil into biodiesel

For the purpose of investigating solid base catalysis of calcium oxide for transesterification of soybean oil with refluxing methanol, the catalyst collected after the reaction was characterized by several instrumental methods: X-ray diffraction, scanning electron microscopy, solid state ¹³C-NMR. The collected catalyst consisted of calcium glyceroxide, Ca[O(OH)₂C₃H₅]₂, due to the direct combination of calcium oxide with glycerol by-produced from soybean oil. Also, the collected catalyst was active in the soybean oil transesterification, and we found that the yield of fatty acid methyl esters reached 70% after 1 h. Although the transformation of the catalytically active phase brought about a slight decrease in the reaction efficiency, calcium glyceroxide was catalytically tolerant to air-exposure.     

Hydrothermal synthesis of TiO<Subscript>2</Subscript> nano-particles using novel water-soluble titanium complexes

Hydrothermal treatment of a series of water-soluble titanium complexes resulted in the formation of TiO₂. Rutile, anatase or a mixture of both can be synthesized by varying the ligands. The titania obtained was composed of nano-sized particles with large specific surface areas. These TiO₂ powders exhibited high photocatalytic activity for NO _x decomposition. In particular, they demonstrated higher activity than P25 (Degussa) under visible-light irradiation.     

Optical characterization of vanadium-titanium oxide films

Composite films of titanium and vanadium oxides (TVO) with various compositional ratios were prepared by sputtering deposition. The optical properties, crystalline structure and film morphology were investigated as a function of the composition. The results of thermochromism and X-ray diffraction suggest that the TVO films at any compositional ratios form substitutional solid solution of Ti and V, that is, Ti_xV_1−xO₂, where 0 ≤ x ≤ 1. Dielectric constants of the TVO films at any compositions were consistently determined at photon energies between 0.75 to 3 eV by employing the Lorentz-oscillator formula. With wide variation in x, the dielectric constants at visible and near-infrared wavelengths monotonically decrease down to the values of TiO₂, which suggests that dielectric constants of the TVO film can be precisely controlled by adjusting rf power in co-sputtering deposition.     

Extensions of polyacrylic acid ammonium salts in the adsorption layer to fluidize alumina slurries

An average shell volume occupied in the adsorption layer on alumina by a polyacrylic acid ammonium salts molecule (PAA) defined as the average area occupied by an adsorbed PAA on alumina multiplied by the average thickness of the water layer at the limit of thickening, has been calculated from the adsorbed amount of PAA and from the flow points of alumina in the presence of PAA of different molecular weights. A steric effect of the PAA dispersant on the dispersion of alumina resulted in a flow with no yield stress. This was due to the change of the extension of PAA in the adsorption shell from a sphere to an ellipsoid with a long axis which exceeded the effective distance that the van der Waals attraction force reaches at a molecular weight for the PAA of between 10 000–20 000.