Formation and escaping of positronium in porous SiO2 films at low temperature

Positronium formation and escaping has been studied in porous silica thin films at temperature ranging from 13 to 300 K by 2-3 gamma ratio of positronium (3γ-PAS) measurements. Nanoporous silica thin films were deposited by spin coating on p-type (1 0 0) Si substrates and thermal treated in air at temperatures of 600 °C. Two different molar ratios of porogen (polyvinylpyrrolidone) were used in the TEOS-ethanol mixture to obtain samples with close porosity and connected porosity with the surfaces. In both types of sample a reduction of the 2-3 gamma ratio of positronium was observed by decreasing the temperature. This finding, in disagreement with the theoretical expectation, is discussed on the basis of the possible quenching mechanisms.     

Evolution of pores in mesoporous silica films: Porogen loading effect

Mesoporous silica films were synthesized via a sol-gel process under an acidic condition. Various amounts of triblock copolymer F38 were loaded to precursor sols as the pore generator. The evolution of the pores generated by porogen decomposition was investigated as a function of F38 loading by positron annihilation gamma-ray energy spectroscopy and positron annihilation lifetime spectroscopy based on slow positron beams. The threshold of pore percolation is found to be around 10 wt% of F38 loading by positron annihilation gamma-ray energy spectroscopy. Positron annihilation lifetime spectroscopy in the films show that the pore size increases from 1 nm to 3 nm with increasing F38 loading from 5 wt% to 30 wt%.     

Orthopositronium annihilation and emission in mesostructured thin silica and silicalite-1 films

Mesoporous silica films and MFI-type pure silica zeolite films were investigated using slow positrons. Detection of the 3γ annihilation fraction was used as a quick test to estimate the emission of orthopositronium (o-Ps) into vacuum. Positronium time-of-flight (TOF) spectroscopy, combined with Monte-Carlo simulation of the detection system was used to determine the energy of o-Ps emitted from the films. Evidence for an efficient o-Ps emission was found in both the mesoporous and silicalite-1. A 3γ fraction in the range of 31-36 % was found in the films with the highest o-Ps yield in each type of porous material, indicating that 40-50 % of the implanted positrons form positronium in the pore systems with very different pore sizes. Time-of-flight measurements showed that the energy of the orthopositronium emitted into vacuum is below 100 meV in the film with 2-3 nm pores at 3 keV positron energy, indicating an efficient slowing down but no complete thermalization in the porous films of 300-400 nm thickness.     

Control on wetting properties of spin-deposited silica films by surface silylation method

Control on the wettability of solid materials by liquid is a classical and key issue in surface engineering. Optically transparent water-repellent silica films have been spin-deposited on glass substrates at room temperature (∼27 °C). The wetting behavior of silica films was controlled by surface silylation method using dimethylchlorosilane (DMCS) as a silylating reagent. A coating sol was prepared by keeping the molar ratio of methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent, water (H₂O) constant at 1:8.8:2.64 respectively, with 4 M NH₄OH as a catalyst throughout the experiments and the amount of DMCS in hexane was varied from 0 to 12 vol.%. It was found that with an increase in vol.% of DMCS, the water contact angle values of the films increased from 78° to 136°. At 12 vol.% of DMCS, the film shows static water contact angle as high as 136° and water sliding angle as low as 18°. The hydrophobic silica films retained their water repellency up to a temperature 295 °C and above this temperature the films show superhydrophilic behavior. These results are compared with our earlier research work done on silylation of silica surface using hexamethyldisilazane (HMDZ) and trimethylchlorosilane (TMCS). The hydrophobic silica films were characterized by taking into consideration the Fourier transform infrared (FT-IR) spectroscopy, thermo gravimetric-differential thermal (TG-DT) analyses, scanning electron microscopy (SEM), atomic force microscopy (AFM), % of optical transmission, thermal and chemical aging tests, humidity tests, static and dynamic water contact angle measurements.     

Transparent Superhydrophobic silica coatings on glass by sol-gel method

Wetting behavior of solid surfaces is a key concern in our daily life as well as in engineering and science. In the present study, we demonstrate a simple dip coating method for the preparation of Thermally stable, transparent superhydrophobic silica films on glass substrates at room temperature by sol-gel process. The coating alcosol was prepared by keeping the molar ratio of methyltriethoxysilane (MTES), trimethylmethoxysilane (TMMS), methanol (MeOH), water (H₂O) constant at 1:0.09:12.71:3.58, respectively with 13 M NH₄OH throughout the experiments and the films were prepared with different deposition time varied from 5 to 25 h. In order to improve the hydrophobicity of as deposited silica films, the films were derivatized with 10% trimethylchlorosilane (TMCS) as a silylating agent in hexane solvent for 24 h. Enhancement in wetting behavior was observed for surface derivatized silica films which showed a maximum static water contact angle (172°) and minimum sliding angle (2°) for 25 h of deposition time. The superhydrophobic silica films retained their superhydrophobicity up to a temperature of 550 °C. The silica films were characterized by field emission scanning electron microscopy (FE-SEM), surface profilometer, Fourier transform infrared (FT-IR) spectroscopy, thermo-gravimetric and differential thermal analysis (TG-DTA), percentage of optical transmission, water contact angle measurements. The imperviousness behavior of the films was tested with various acids.     

Mechanical stress in ALD-Al2O3 films

Mechanical stress in atomic-layer deposition (ALD)-Al₂O₃ films was investigated at room temperature and during thermal cycling up to 870 °C. The films were generally under tensile stress. Thicker films (25-60 nm) showed a sharp stress increase at about 780-790 °C. X-ray diffraction (XRD)-, X-ray reflectance (XRR)- and X-ray photoelectron spectroscopy (XPS)-measurements indicate an irreversible phase transition from amorphous AlO(OH) to a mixture of different crystalline Al₂O₃-phases. Annealing at higher temperatures leads to a stress reduction as a result of diffusion and recovery processes. The stress behaviour of thinner films (<20 nm) during thermal cycling is quite different. Tensile stress increases with increasing temperature and decreases to nearly the same value during cooling down. The process is continuous and reversible.     

Influence of temperature annealing on optical properties of SrTiO3/BaTiO3 multilayered films on indium tin oxide

We have prepared SrTiO₃/BaTiO₃ thin films with multilayered structures deposited on indium tin oxide (ITO) coated glass by a sol-gel deposition and heating at 300-650 °C. The optical properties were obtained by UV-vis spectroscopy. The films show a high transmittance (approximately 85%) in the visible region. The optical band gap of the films is tunable in the 3.64-4.19 eV range by varying the annealing temperature. An abrupt decrease towards the bulk band gap value is observed at annealing temperatures above 600 °C. The multilayered film annealed at 650 ° C exhibited the maximum refractive index of 2.09-1.91 in the 450-750 nm wavelength range. The XRD and AFM results indicate that the films annealed above 600 ° C are substantially more crystalline than the films prepared at lower temperatures which were used to change their optical band gap and complex refractive index to an extent that depended on the annealing temperature.     

The structure and optical properties of evaporated Samarium fluoride films

Samarium fluoride (SmF₃) films have been deposited on quartz, silicon and germanium substrates by vacuum evaporation method. The crystal structure of the films deposited on silicon substrate is examined by X-ray diffraction (XRD). The films deposited at 100 °C, 150 °C and 250 °C have the (1 1 1) preferred growth orientation, but the film deposited at 200 °C has (3 6 0) growth orientation. The surface morphology evolution of the films with different thickness is investigated with optical microscopy. It is shown that the microcrack density and orientation of thin film is different from that of thick film. The transmission spectrum of SmF₃ films is measured from 200 nm to 20 μm. It is found that this material has good transparency from deep violet to far infrared. The optical constants of SmF₃ films from 200 nm to 12 μm are calculated by fitting the transmission spectrum of the films using Lorentz oscillator model.     

XPS study of the phase transition in pure zirconium oxide nanocrystallites

Pure zirconium oxide nanocrystallites with diameters 6-140 nm are fabricated from ultrafine metallo-organic complexes by thermal hydrolysis at 120 °C and/or heat treatment at 125-1025 °C. X-ray photoelectron spectroscopy shows that effective ionic valence of Zr decreases with decreasing particle diameter. The size dependence of the ionic valences suggests that the phase transition from cubic to tetragonal occurs at an effective Zr valence of 2.0 near 3 nm in diameter and that the phase transition from tetragonal to monoclinic takes place at a critical size of 25 nm diameter with an effective Zr valence of about 2.6.     

Thermal stability of Ag films in air prepared by thermal evaporation

The thermal stability of silver films in air has been studied. Pure Ag films, 250 nm in thickness, were prepared on glass substrates by thermal evaporation process, and subsequently annealed in air for 1 h at temperatures between 200 and 400 °C. The structure and morphology of the samples were investigated by X-ray diffraction, Raman spectra and atomic force microscopy. It is found that the crystallization enhances for the annealed films, and film surface becomes oxidized when annealing temperature is higher than 350 °C. The electrical and optical properties of the films were studied by van der Pauw method and spectrophotometer, respectively. Reflectance drops sharply as Ag films are annealed at temperatures above 250 °C. Film annealed at 250 °C has the maximum surface roughness and the minimum reflectance at 600 nm optical wavelength. Film annealed at 200 °C has the minimum resistivity, and resistivity increases with the increasing of the annealing temperature when temperature is above 200 °C. The results show that both oxidization on film surface and agglomeration of silver film result in infinite of electrical resistivity as the annealing temperature is above 350 °C.     

Synthesis and characterization of nanoporous silica film via non-surfactant template sol–gel technique

Nanoporous silicon dioxide has received growing interests owing to their peculiar application potentials in conservation and storage energy. Therefore, the development of novel and simple techniques is required for raising these nanoporous materials to industrial level. In this research, we report novel strategy for the synthesis of nanoporous SiO₂ via non-surfactant template sol–gel technique for the first time. The morphology and structure of the as prepared and annealed nanoporous silica films were studied using X-ray diffraction, scanning electron microscope, Fourier transform infrared spectroscopy and nitrogen absorption/desorption technique. The results showed that highly order nanoporous silica film has been obtained at annealing temperature 600 °C with average diameter 5.1 nm and average pore volume 3.6 cm³ g⁻¹.     

Nanofibers and nanoplatelets of MoO3 via an electrospinning technique

Polyvinyl alcohol (PVA)/ammonium molybdate composite fibers were prepared by using sol-gel processing and electrospinning technique. After calcinations of the above precursor fibers at 500 °C, MoO₃ nanofibers with a diameter of 100-150 nm were successfully obtained. MoO₃ nanoplatelets and submicron platelets were prepared by further calcinations of the MoO₃ nanofibers at 600 and 700 °C. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). A possible growth mechanism for the MoO₃ nanofibers and nanoplatelets was suggested.     

Depth resolved Doppler broadening measurement of layered Al-Sn samples

The accumulation of positrons in a two-dimensional layer of tin embedded in aluminum is examined by Doppler broadening spectroscopy (DBS). For this purpose samples are grown out of high purity materials consisting of a step-shaped layer (0.1-200 nm) of tin on a substrate of aluminum and covered by an aluminum layer of constant thickness (200 nm).The positron implantation profile is varied by different positron acceleration energies of up to 15 keV. The pre-thermal implantation profile of the monoenergetic positron beam is examined since the effect of thermal positron diffusion is less significant at tin layers thicker than 50 nm. At thin layers (<50 nm), the positrons greatly accumulate either at the aluminum-tin interface or in the tin layer due to its higher positron affinity compared to aluminum. Thus a very high sensitivity of the measurement for low densities of tin is observed. Consequently from the experimental results, a sensitivity threshold for the detection of a low amount of tin in an aluminum matrix with DBS is determined. The DB results are compared to theory by an approximation for pre-thermal implantation in layered materials.     

Effect of H2 and O2 plasma etching treatment on the surface of diamond-like carbon thin film

In this study, we investigated the surface properties of diamond-like carbon (DLC) films for biomedical applications through plasma etching treatment using oxygen (O₂) and hydrogen (H₂) gas. The synthesis and post-plasma etching treatment of DLC films were carried out by 13.56 MHz RF plasma enhanced chemical vapor deposition (PECVD) system. In order to characterize the surface of DLC films, they were etched to a thickness of approximately 100 nm and were compared with an as-deposited DLC film. We obtained the optimum condition through power variation, at which the etching rate by H₂ and O₂ was 30 and 80 nm/min, respectively. The structural and chemical properties of these thin films after the plasma etching treatment were evaluated by Raman and Fourier transform infrared (FT-IR) spectroscopy. In the case of as-deposited and H₂ plasma etching-treated DLC film, the contact angle was 86.4° and 83.7°, respectively, whereas it was reduced to 35.5° in the etching-treated DLC film in O₂ plasma. The surface roughness of plasma etching-treated DLC with H₂ or O₂ was maintained smooth at 0.1 nm. These results indicated that the surface of the etching-treated DLC film in O₂ plasma was hydrophilic as well as smooth.     

Effect of post-annealing on the properties of copper oxide thin films obtained from the oxidation of evaporated metallic copper

Thin films of copper oxide were obtained through thermal oxidation (100-450 °C) of evaporated metallic copper (Cu) films on glass substrates. The X-ray diffraction (XRD) studies confirmed the cubic Cu phase of the as-deposited films. The films annealed at 100 °C showed mixed Cu-Cu₂O phase, whereas those annealed between 200 and 300 °C showed a single cubic Cu₂O phase. A single monoclinic CuO phase was obtained from the films annealed between 350 and 450 °C. The positive sign of the Hall coefficient confirmed the p-type conductivity in the films with Cu₂O phase. However, a relatively poor crystallinity of these films limited the p-type characteristics. The films with Cu and CuO phases show n-type conductivity. The surface of the as-deposited is smooth (RMS roughness of 1.47 nm) and comprised of uniformly distributed grains (AFM and SEM analysis). The post-annealing is found to be effective on the distribution of grains and their sizes. The poor transmittance of the as-deposited films (<1%) is increased to a maximum of ∼80% (800 nm) on annealing at 200 °C. The direct allowed band gap is varied between 2.03 and 3.02 eV.     

Freeze casting of aqueous coal fly ash/alumina slurries for preparation of porous ceramics

A porous mullite-matrix composite with a bimodal pore structure has been prepared by a freeze casting route using water/coal fly slurry system. The top and bottom parts of the sintered freeze cast body consisted of solid particles and micropores, which were irregularly distributed. However, the middle section was made up of small lamellar pores and porous ceramic walls, aligned along the solidification direction. The porosity of mullite composites was in the range 67-55% after sintering at 1300-1500 °C. The addition of 3Y-ZrO₂ reduced the porosity, especially material in sintered at 1500 °C due to relatively high densification. The compressive strength of the porous composite with 10 wt% 3Y-ZrO₂ addition, sintered at 1500 °C exhibited a maximum value of ∼41 MPa.     

The effect of heat treatment on the physical properties of sol-gel derived ZnO thin films

Zinc oxide (ZnO) thin films were deposited on microscope glass substrates by sol-gel spin coating method. Zinc acetate (ZnAc) dehydrate was used as the starting salt material source. A homogeneous and stable solution was prepared by dissolving ZnAc in the solution of monoethanolamine (MEA). ZnO thin films were obtained after preheating the spin coated thin films at 250 °C for 5 min after each coating. The films, after the deposition of the eighth layer, were annealed in air at temperatures of 300 °C, 400 °C and 500 °C for 1 h. The effect of thermal annealing in air on the physical properties of the sol-gel derived ZnO thin films are studied. The powder and its thin film were characterized by X-ray diffractometer (XRD) method. XRD analysis revealed that the annealed ZnO thin films consist of single phase ZnO with wurtzite structure (JCPDS 36-1451) and show the c-axis grain orientation. Increasing annealing temperature increased the c-axis orientation and the crystallite size of the film. The annealed films are highly transparent with average transmission exceeding 80% in the visible range (400-700 nm). The measured optical band gap values of the ZnO thin films were between 3.26 eV and 3.28 eV, which were in the range of band gap values of intrinsic ZnO (3.2-3.3 eV). SEM analysis of annealed thin films has shown a completely different surface morphology behavior.     

Effect of nanostructured AlN coatings on the oxidation-resistant properties of optical diamond films

Diamond film is an ultra-durable optical material with high thermal conductivity and good transmission in near-infrared and far-IR (8-14 μm) wavebands. CVD diamond is subjected to oxidation at temperature higher than 780 °C bared in air for 3 min, while it can be protected from oxidation for extended exposure in air at temperature up to 900 °C by a coating of aluminum nitride. Highly oriented AlN coatings were prepared for infrared windows on diamond films by reactive sputtering method and the average surface roughness (R_a) of the coatings was about 10 nm. The deposited films were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). XRD confirmed the preferential orientation nature and AFM showed nanostructures. Optical properties of diamond films coated AlN thin film was investigated using infrared spectrum (IR) compared with that for as-grown diamond films.     

Preparation of activated carbons from cherry stones by activation with potassium hydroxide

Using cherry stones, the preparation of activated carbon has been undertaken in the present study by chemical activation with potassium hydroxide. A series of KOH-activated products was prepared by varying the carbonisation temperature in the 400-900 °C range. Such products were characterised texturally by gas adsorption (N₂, −196 °C), mercury porosimetry, and helium and mercury density measurements. FT-IR spectroscopy was also applied. The carbons prepared as a rule are microporous and macroporous solids. The degree of development of surface area and porosity increases with increasing carbonisation temperature. For the carbon heated at 900 °C the specific surface area (BET) is 1624 m² g⁻¹, the micropore volume is 0.67 cm³ g⁻¹, the mesopore volume is 0.28 cm³ g⁻¹, and the macropore volume is 1.84 cm³ g⁻¹.     

Crystallization and grain growth characteristics of yttria-stabilized zirconia thin films grown by pulsed laser deposition

Knowledge about the crystallization and grain growth characteristics of metal oxide thin films is essential for effective microstructural engineering by thermal post-annealing and the integration to Si-based miniaturized electroceramic devices. Finite size and interface effects may cause fundamentally different behavior compared to three dimensional macroscopic systems. This work presents a comprehensive investigation of the crystallization kinetics and microstructural evolution upon thermal post-annealing of amorphous 200 nm and 1.2 μm thin films of 8 mol% yttria-stabilized zirconia grown by pulsed laser deposition (PLD) using ex- and in-situ X-ray diffraction, Raman spectroscopy, and electron microscopy techniques. The layers exhibit a remarkably low crystallization temperature of 200-250 °C while exposure to energetic electrons induces the formation of randomly dispersed ~ 20 nm sized crystallites already at ambient temperature. The isothermal amorphous to crystalline phase transformation kinetics can be described quantitatively by the Johnson-Mehl-Avrami-Kolmogorov model. They reveal characteristics of a three dimensional growth under cation bulk diffusion control with heterogeneous nucleation that changes from continuous to instantaneous initial seeding at temperatures above 300 °C. Large (> 100 nm) equiaxed grains are formed rapidly without a stabilization of transient nanocrystals during the thermally induced phase transformation. A stagnation of normal grain growth resulting in a logarithmic normal size distribution is observed once the average grain dimensions approach the film thickness. The results on the crystallization and grain growth of the PLD-grown YSZ films are evaluated with regards to the fabrication of YSZ solid electrolyte membranes for Si-supported micro solid oxide fuel cells and gas sensors.