Silica-doped alumina cryogels with high thermal stability

Alumina cryogels with a dopant of silica in the content range of 0–10 wt% were synthesized from aqueous boehmite sol through the sol-gel technique and subsequent freeze drying. The higher thermal stability was achieved by the addition of 10 wt% silica; a γ-Al₂O₃ phase still remained after heating at 1200 °C for 5 h, and the surface area and pore volume were 47 m² g⁻¹ and 105 mm³ g⁻¹, respectively. The marked stability was ascribed to the synergetic effect of the very low bulk density (0.05 g cm⁻³) and the dopant. The thermal stability was lower for the cryogels than for the corresponding aerogels; however, it was also suggested that cryogel was highly durable in water in contrast to aerogel.     

Effect of nanosilica type on protective properties of composite ceramic coatings deposited on steel 316L by sol–gel technique

Trilayer SiO₂ coatings were obtained from sol modified with nanosilica differing in its specific surface and hydrophilic and hydrophobic properties. Each of the successively deposited coating layers was sintered at ever lower temperatures: 300, 250 and 200 °C. Examinations under a JSM 5800LV Joel scanning electron microscope showed that each SiO₂ coating includes a smooth, uncracked, thin layer covering the entire steel 316L basis and an outer layer made up of grains forming clusters of different size and density depending on the nanosilica used. The corrosion resistance of the coatings in Ringer’s solution was evaluated on the basis of polarization studies. A comparison of the passive region widths, the passive region current intensities, the cathode current densities at potential E_SCE = ?750 mV, the anode region potentials corresponding to a current density of 2 μA/cm², the through-coating porosity and the polarization resistance values shows the coatings obtained from sol modified with hydrophilic nanopowder to be superior. As regards the tested hydrophobic powders, additive ^®R972 (S_BET = 110 m²/g) ensures good protective properties. If hydrophobic silica with larger specific surface (^®R974, ^®R812, S_BET with respectively 170 and 260 m²/g) is used, the deposited coatings only slightly improve the protecting properties of steel 316L. The deposited coatings are uncracked. Even at their highest compactness, the surface grains and grain clusters, do not form a tight outer shell as evidenced by, for example, the poor protective properties of the coatings produced from sol modified by hydrophobic silica ^®R812. The latter coatings’ protective properties are the poorest at the highest grain compactness. If hydrophobic nanosilica with small specific surface (^®130 and ^®150) is used the resulting layers have low grain compactness and good protective properties (probably smooth thicker layers directly coat the steel basis).     

Preparation of polymer/silica hybrid hard coatings with enhanced hydrophobicity on plastic substrates

In this study, an easy method to increase hydrophobicity of the polymer/silica hybrid coating was demonstrated. UV-curable nano-sized colloidal silica was synthesized and surface-modified both by a coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA), and a capping agent, trimethyethoxysiliane (TMES). The formed particles were introduced into the poly(2-hydroxyethyl methacrylate) (PHEMA) matrix to yield PHEMA/silica hybrid hard coatings on plastic substrates via a UV-curing process. Differential scanning calorimetric (DSC) analyses of the hybrids indicated increases of the glass transition temperature (T_g) with increasing silica content in the hybrids; in general, an increase of 23 °C could be achieved for hybrids doped with 15 wt.% silica. Thermal decomposition temperature (T_d), as measured by the thermal gravimetric analyzer (TGA), was found to depend on the silica content in a trend similar to that on T_g. Specifically, a large increase of 25 °C was observed when the sample contained 15 wt.% silica. The pencil hardness of the PHEMA/silica hybrids coated on poly(methyl methacrylate) (PMMA) substrates can reach 5H, in comparison with 2H for pure PHEMA coating. Abrasion resistance was enhanced when silica nanoparticles were incorporated. Furthermore, due to the incorporation of TMES, hydrophobicity of the hybrid coating increased considerably as the TMES content was increased. In the extreme case, a hard surface with a water contact angle (92°) has been obtained.     

Correlation between the method of preparation and the properties of the sol–gel HfO2 thin films

This work describes the preparation of HfO₂ thin films by the sol–gel method, starting with different precursors such as hafnium ethoxide, hafnium 2,4-pentadionate and hafnium chloride. From the solution prepared as mentioned above, thin films on silicon wafer substrates have been realized by ‘dip-coating’ with a pulling out speed of 5 cm min^?1. The films densification was achieved by thermal treatment for 10 min at 100 °C and 30 min at 450 °C or 600 °C, with a heating rate of 1 °C min^?1. The structural and optical properties of the films are determined employing spectroellipsometric (SE) measurements in the visible range (0.4–0.7 μm), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The main objective of this paper was to establish a correlation between the method of preparation (precursor, annealing temperature) and the properties of the obtained films. The samples prepared from pentadionate and ethoxide precursors are homogenous and uniform in thickness. The samples prepared starting from chloride precursor are thicker and proved to be less uniform in thickness. Higher non-uniformity develops in multi-deposition films or in crystallized films. A nano-porosity is present in the quasi-amorphous films as well in the crystallized one. For the samples deposited on silicon wafer, the thermal treatment induced the formation of a SiO₂ layer at the coating–substrate interface.     

Effect of acid,water and alcohol ratios on sol-gel preparation of antireflective amorphous SiO2 coatings

Varying amounts of nitric acid catalyst, water and ethyl alcohol were used in the preparation of SiO₂ sols by hydrolysis and condensation reactions of tetraethyl orthosilicate in a one step acid catalysis process. Hydrolysis of TEOS was followed by FT-IR analyses. Size of SiO₂ particles was seen to vary in 8–41 nm range with respect to changing HNO₃ and water amounts in the sols. Gelation occurred in some systems. Surfaces of films were examined by FESEM and AFM, after coating on glass substrates by dip coating. Thicknesses of the films were measured to be in the range of 80–120 nm. 5.6 ± 0.2% point increase in light transmittance was obtained when HNO₃/TEOS (mol/mol) ratio of 4.74 × 10^? 4 and H₂O/TEOS (mol/mol) ratio of 9.08 were utilized. Sols were found to be stable for months and coatings prepared after 45 days still provided 5.2 ± 0.2% point increase in light transmittance.     

Influence of Ar/C2H2 ratio on the structure of hydrogenated carbon films

The amorphous hydrogenated carbon films (a-C:H) were obtained on Si (1 1 1) wafers by plasma jet chemical vapor deposition (PJCVD). a-C:H coatings have been prepared at 1000 Pa in argon/acetylene mixture. The Ar/C₂H₂ gas volume ratio varied from 1:1 to 8:1. It was demonstrated that by varying the Ar/C₂H₂ ratio the composition, growth rate of the coatings, and consequently the structure of the film, can be controlled. The growth rate and surface porosity of coatings deposited at Ar/C₂H₂ = 8:1 ratio decrease slightly with an increase in the distance between the plasma torch nozzle and substrate from 0.04 to 0.095 m. The transmittance of the coatings in the IR region of 2.5–25 μm slightly increases, while the absorption peaks at 2850–2960 cm^?1 related with sp³ CH_2–3 modes remain unchanged with an increase in the distance. The Raman spectroscopy indicated that the a-C:H coating formed at the Ar/C₂H₂ = 8:1 and 0.06 m has the highest sp³ C–C fraction. The proposed PJCVD technique allows to achieve the growth rates up to 300 nm/s.     

Recombination luminescence of oxygen-deficient centers in silica

The luminescence of silica glass, prepared by plasma chemical vapor deposition (PCVD) and quartz glass of type IV (trade mark KS-4V) methods, were studied while irradiated with pulses of ArF laser (193 nm) light in the range of sample temperatures between 10 and 300 K. The samples contain less than 0.1 ppm metallic and hydroxyl impurities. The samples synthesized by PCVD were of two kinds. The first one (amorphous) was as-deposited from plasma at a substrate tube temperature of ～1200 °C. The second one (fused) was prepared from the first by the tube collapsing with an external burner. In this process, a section of the substrate tube with the deposited glass was installed in a lathe and processed at a temperature of ～2100 °C during ～20 min until the tube was transformed to a rod. After such processing, the rod was cooled down to room temperature in air at an average rate of about 400 °C per min. The only observed luminescence possesses two broad bands, with not well defined position, one at 2.6–2.9 eV (a blue band) and another in the range of 4.4 eV (an UV band). There is a correspondence in luminescence properties between KS-4V silica and fused PCVD silica. Those bands have been attributed to oxygen deficient centers (ODC). No luminescence is observed in amorphous PCVD silica under irradiation with 193 nm laser light. So, formation of the sample by melting at least stimulates formation of ODCs at 193 nm. The blue band decays obeys to power law ～t^?1 and is detected in the range of time 10 ns to 300 μs. The UV band possesses a fast, practically repeating excitation pulse, and a slow component (～30 μs). The obtained new kinetics data are compared with known in literature for lone twofold-coordinated silicon having exponential decay for the blue band equal to 10 ms and 4.5 ns for the UV band. That shows the blue band of new studied samples under ArF laser possesses decay component faster and the UV band slower than that of the twofold-coordinated silicon center. This corresponds to the recombination process of luminescence excitation by laser. We propose a model of the processes as charge separation under excitation with creation of a nearest self-trapped hole and electron trapped on the twofold-coordinated silicon, modified by its surrounding atoms or ions. This pair is recombining then with luminescence.     

Water diffusion in silica film during silicon wet oxidation

Water diffusion in silica glass is explained in terms of the migration of molecular water and its reaction with the silica network to form immobile hydroxyl water. It is assumed that the reaction is fast at high temperatures, and a local reaction equilibrium is maintained during water diffusion. While this mechanism appears to explain water diffusion in silica glasses determined from hydroxyl profiles reasonably well, we found that water diffusion-promoted phenomena, such as the oxidation of silicon, require a small quantity of molecular water diffusing into silica glasses ahead of the hydroxyl diffusion front, unrestricted by the local equilibrium. This was demonstrated by studying oxidation kinetics of silicon with a thick (∼15 μm) oxide layer and measuring the delay time before the steady-state oxidation.     

The study of removing hydroxyl from silica glass

The removal of hydroxyl from silica glass produced by melting quartz powder under an atmosphere containing hydrogen was investigated. After heat-treatment at the temperature range (700–1200 °C) in nitrogen atmosphere, the effective hydrogen diffusion coefficients were evaluated based on the law of nonsteady-state diffusion. The activation energy obtained is 254 kJ mol⁻¹ for the dehydroxylation process in the heat-treatment temperature range of 700–900 °C, and a different activation energy calculated is 32 kJ mol⁻¹ in the temperature range of 900–1200 °C. The activation energies for the dehydroxylation process at the temperature (700–900 °C) and the higher temperature (900–1200 °C) correspond to the binding energy of SiO–H bond and the activation energy for the diffusion of hydrogen in silica glass respectively, which indicate there is a change of mechanism for dehydroxylation with heat-treatment temperature.     

Strengthening of soda-lime-silica glass by surface treatment with sol–gel silica

In this study, the failure resistance of soda-lime-silica glass was increased by surface treatment with sol–gel silica. Samples annealed and ion-exchanged in KNO₃ for 24 h at 450 °C were considered. Sol–gel silica coating was carried out by dipping the glass samples into a sol suspension prepared by hydrolysis of Si(OEt)₄ in ethanol/water solution. The deposited layer was consolidated in air for 24 h and subjected to mild thermal treatment at 300 °C for 1 h. The surface treatment increased the fracture resistance of annealed glass of about 35 MPa; conversely, ion-exchanged specimens showed an average increase of about 90 MPa. The strengthening effect induced by the surface treatment was attributed to the reduction of the effective crack length generated by the silica coating. The different strength increase between annealed and ion-exchanged samples is discussed in terms of fracture toughness which, for ion-exchanged glass, is not constant, due to the presence of the surface residual stresses and thus the reduction of the crack length due to the silica coating determines a higher strength increase than for annealed glass.     

Improved method for preparation of anhydrous silica by microwave irradiation with spectroscopic characterization and toxicity assay

Amorphous anhydrous silica SiO_{2 (mw)} (99.99%) is successfully synthesized through microwave irradiation technique and time of reaction is reduced up to 1 h. The dehydration phase study of Si–water, Si–OH, Si–O–Si networking, elemental analysis and surface morphology was carried out by FTIR, FTNIR, SEM and EDAX spectroscopic techniques. The broad absorption stretching and bending of Si–OH and H₂O at 3695.38–2832.96 cm^? 1, 1638 cm^? 1 and 1191.20–1017.14 cm^? 1 completely disappeared and appearance of new bands at 946.93 and 797.63 cm^? 1 confirmed the amorphous anhydrous silica with Si–O–Si networking. The SEM images of SiO_{2 (mwc)} described the smooth and fine particle texture and confirmed 99.99% Si–O–Si networking of anhydrous silica. The 99.99% purity was verified by EDAX spectra which exhibited sharp signals only for oxygen and silicon. Toxicity against Monomorium minimum and Tribolium castaneum with 100% mortality and LT₅₀ 91 min and 7.5 h respectively is being reported. It can be used for long storage of grains in the future.     

Roughness of glass surfaces formed by sub-critical crack growth

This paper presents a study on the roughness of glass fracture surfaces formed as a consequence of sub-critical crack growth. Double-cantilever-beam specimens were used in these studies to form fracture surfaces with areas under well-defined crack velocities and stress intensity factors. Roughness depends on crack velocity: the slower the velocity, the rougher the surface. Ranging from approximately 1 × 10⁻¹⁰ m/s to approximately 10 m/s, the velocities were typical of those responsible for the formation of fracture mirrors in glass. Roughness measurements were made using atomic force microscopy on two glass compositions: silica glass and soda lime silica glass. For silica glass, the RMS roughness, R_q, decreased from about 0.5 nm at a velocity of 1 × 10⁻¹⁰ m/s to about 0.35 nm at a velocity of 10 m/s. For soda lime silica glass, the roughness decreased from about 2 nm to about 0.7 nm in a highly non-linear fashion over the same velocity range. We attributed the roughness and the change in roughness to microscopic stresses associated with nanometer scale compositional and structural variations within the glass microstructure. A theory developed to explain these results is in agreement with the data collected in the current paper. The RMS roughness of glass also depends on the area used to measure the roughness. As noted in other studies, fracture surfaces in glass exhibit a self-affine behavior. Over the velocities studied, the roughness exponent, ζ, was approximately 0.3 for silica glass and varied from 0.18 to 0.28 for soda lime silica glass. The area used for these measurements ranged from (0.5 μm)² to (5.0 μm)². These values of the roughness exponent are consistent with values obtained when the scale of the measurement tool exceeds a critical size, as reported earlier in the literature.     

Evaluation of structural and mechanical properties of aluminum oxide thin films deposited by a sol–gel process: Comparison of microwave to conventional anneal

Thin films of Al₂O₃ have been deposited on polished silica glass substrates at room temperature by sol–gel dip coating technique followed by two different exposure methods. One set was annealed at different temperatures ranging from 200 °C to 800 °C for 10 h and a second set was exposed to microwave (2.45 MHz) radiation at different powers for 10 min. The lower temperature and shorter time with microwave irradiation might be ascribed to the activating and facilitating effect of microwaves on solid phase diffusion. Unlike other preparation methods, microwave heating is generally quite faster, and energy efficient. X-ray diffraction (XRD) and scanning electron microscopy (SEM), energy dispersive X-ray analysis techniques have been employed to characterize structural, morphological and elemental compositions of the films. Adhesion strength failure measurements on films performed by scratch test in progressive loading sequence have shown critical loads up to 25 N (partial perforation) for both annealed films and films exposed to microwave irradiation. Nanohardness indentation tests of the films exposed (800 W) to microwave have shown hardness of 8.3 GPa with elastic modulus of 120 GPa compared to the conventional annealed film (800°) of 4.5 GPa with elastic modulus of 90 GPa.     

Cation diffusion in soda-lime-silicate glass melts

Cation diffusion was experimentally investigated in soda-lime-silicate glass melts (composition in mol%: 74SiO₂–16Na₂O–10CaO) at temperatures from 1000 to 1200 °C using the diffusion couple technique. One half of each diffusion couple was doped with 11 trace elements (500 ppm by weight of Rb, Cs, Sr, Zn, Cd, Nd, Eu, In, Sn, Ge and 1000 ppm by weight of Fe). Experiments were performed in an internally heated gas pressure vessel at a confining pressure of 100 MPa to avoid convective fluxes in the diffusion samples. The distribution of major elements was analyzed by electron microprobe. IR spectroscopy was used to quantify concentrations of dissolved water in the run products. Trace element diffusion profiles were measured simultaneously employing synchrotron X-ray fluorescence microanalysis. In all analyzed glasses the highest diffusion coefficients were observed for Rb whereas Nd was always the slowest element, e.g. at 1000 °C the diffusivity decreases from (1.51 ± 0.35) × 10⁻¹¹ m²/s for Rb to (1.29 ± 0.34) × 10⁻¹³ m²/s for Nd. The diffusivity of Nd is close to the chemical diffusivity of network former calculated from viscosity data using the Eyring relationship. Surprisingly, the rare earth elements Nd (3+) and Eu (mixed 2+, 3+) diffuse more slowly than the tetravalent Ge. Activation energies for diffusion increase from (132.1 ± 1.5) kJ/mol for Rb to (205 ± 16) kJ/mol for Eu. Based on the diffusion data for Eu, Sr and Nd we estimated that Eu²⁺/Eu_total ratios in soda-lime-silicate glass melts are below 0.04 both at reducing and oxidizing conditions.     

Electrochromic properties of nickel oxide and mixed Co/Ni oxide films prepared via sol–gel route

The Ni oxide and mixed Co/Ni oxide films were prepared by sol–gel dip coating method at optimum conditions. The XRD analysis reveals the pure and Co mixed nickel oxide films to be in amorphous state. The field emission SEM images reveal nanopore like structure for Ni oxide film and well defined grains with pores for Ni oxide films containing 5 wt.% of Co. Electrochromic properties have been studied using cyclic voltammetric (CV) and in situ spectro-electrochemical techniques. The pure and cobalt mixed (5 wt.%) Ni oxide films exhibit anodic/cathodic diffusion coefficient of 4.93 ± 0.14/3.74 ± 0.10 × 10^?10 cm²/s and 10.00 ± 0.24/7.60 ± 0.20 × 10^?10 cm²/s respectively after 300 cycles. The cobalt mixed (5 wt.%) Ni oxide films exhibit the bleached/coloured state transmission of 90.42/7.21% with a photopic constrast ratio of 12.54 and the colouration and bleaching time were 5.9 and 2.4 s respectively. The addition of cobalt beyond 5% leads to poor transparency and inhibited electrochromic switching character.     

Vacuum ultraviolet excitation of the 2.7 eV emission band in neutron irradiated silica

The temperature dependence of the photoluminescence induced at 2.7 eV by ultraviolet (UV) and vacuum ultraviolet (VUV) excitation of neutron irradiated (10²¹ n/m² and 10²² n/m²) KU1 and KS-4V high purity silica, with different OH content, have been studied. Commercial silica Infrasil 301 has also been studied for comparison. At the highest neutron fluence and at the same temperature, the three irradiated silica grades show similar excitation spectra. Two close UV excitation bands, which show opposite temperature dependence, are observed at 4.8 and 5.1 eV. The 4.8 eV band, related to the triplet–singlet transition in SiODCs(II), decreases on decreasing temperature from 300 to 10 K and the band at 5.1 eV, probably related to SiODCs(I), is observed only at very low temperatures (～10 K). An important VUV excitation structure, observed at low temperature, could also be related to SiODCs(I). A shift of the irradiated bands is detected at low temperature.     

Tracer diffusion of 22Na and 45Ca,ionic conduction and viscosity of two standard soda-lime glasses and their undercooled melts

The tracer diffusivities of ⁴⁵Ca in two different high purity standard soda-lime silica glasses have been measured by the radiotracer method below and above their calorimetric glass transition temperatures. Calorimetric glass transition temperatures (T_g) of 845 K and 867 K have been obtained for standard glasses I and II, respectively, using differential scanning calorimetry (DSC) at a heating rate of 20 K/min. In this paper, we focus on the results of ⁴⁵Ca diffusion and conductivity of the two standard soda-lime glasses and compare them with ²²Na diffusivities also obtained in our laboratory [E.M. Tanguep Njiokep, H. Mehrer, Solid State Ionics 177 (2006) 2839, E.M. Tanguep Njiokep, H. Mehrer, Defect Diffus Forum 237–240 (2005) 282]. The ⁴⁵Ca diffusion coefficients obtained are found to follow the Arrhenius law, both below (Tanguep Njiokep and Mehrer, 2006, 2005) and above T_g. In the Arrhenius diagram a change of slope of the ⁴⁵Ca diffusivities appears at 835 K for standard glass I and at 790 K for standard glass II. At the same time, the ionic conductivities display a change in slope at 790 K and 778 K for standard glasses I and II, respectively. These temperatures are somewhat smaller than the calorimetric glass transition temperatures obtained at a heating rate of 20 K/min. Rather, they appear to be close to values of T_g obtained by extrapolation to a vanishing heating rate (Tanguep Njiokep and Mehrer, 2006). The viscosity diffusion of standard glass I is considerably smaller than the conductivity diffusion coefficient and both tracer diffusivities. In both glasses the ionic conductivity is essentially due to the motion of Na ions. The contribution of Ca ions to the conductivity is negligible.     

Ion beam synthesis of C-based optically-active nanoclusters in silica

Carbon nanoclusters formed using ion implantation and thermal annealing are shown to photoluminescence in the visible range. Silica samples were implanted with a fluence of 2 × 10¹⁷ atoms/cm², 70 keV carbon ions and thermally annealed for 4 h at 1100 °C. Photoluminescence measurement made at select intervals during the anneal process show continued growth of the nanoclusters within the silica throughout the process. However, Rutherford backscattering showed a rapid loss of carbon during the initial 15 min of annealing indicating a competition between the growth of the second-phase nanoparticles and the formation of CO, a volatile form of carbon.     

Growth of pure ZnO thin films prepared by chemical spray pyrolysis on silicon

Structural, morphological, optical and electrical properties of ZnO thin films prepared by chemical spray pyrolysis from zinc acetate (Zn(CH₃COO)₂ 2H₂O) aqueous solutions, on polished Si(1 0 0), and fused silica substrates for optical characterization, have been studied in terms of deposition time and substrate temperature. The growth of the films present three regimes depending on the substrate temperature, with increasing, constant and decreasing growth rates at lower, middle, and higher-temperature ranges, respectively. Growth rate higher than 15 nm min⁻¹ can be achieved at T_s=543 K. ZnO film morphological and electrical properties have been related to these growth regimes. The films have been characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy.     

Silica nano-rings and nano-hollows: Preparation and UV photoluminescence emission

By annealing fused silica coated with ultra-thin Ag film, silica nano-rings and nano-hollows were prepared on the substrate. The Ag nano-particles attached on the wall of nano-hollows or embedded in silica were confirmed with energy dispersive spectroscopy and transmission electron microscopy. Besides the well-known characteristic stretching bands of silica, three novel stretching bands around 1579, 1320 and 270 cm^?1 were found in the annealed Ag-coated silica by Raman scattering spectroscopy, which have been attributed to the O₂ in ground state, O–O and metal–oxygen stretching bands, respectively. The formation mechanism of nano-rings and nano-hollows has been discussed based on the experimental results. An ultraviolet photoluminescence emission of 360–370 nm from annealed Ag-coated silica was found when the excitations were 230 nm and 280 nm or longer. The possible photoluminescence emission mechanism has been discussed, which suggests that oxygen excess defects are responsible for the photoluminescence emission, and photoexcitation occurs in the silica as well as in Ag⁺ ions.