Ageing studies on some titania gels

Hydrous titania gels, precipitated from titanous chloride and aged in different aqueous media have different textural, thermal and structural properties as demonstrated by nitrogen adsorption, differential thermal analysis, X-ray diffraction and scanning electron microscopy studies. Ageing in ammonia leads to the development of microporosity, whereas ageing in water causes an extension of the pore system in the primary particles with little change in the surface texture. Heat-treatment of the precipitated gels prior to ageing seems to afford a certain stability to the primary particle by allowing hydroxo and oxo bridges to be formed, thus leading to the formation of a more rigid structure. The nature of the ageing medium and its effect on the inner coordination sphere of the titanium ions are important parameters affecting the textural and thermal behaviours of the titania gels.     

The three-dimensional structure of Arabidopsis thaliana O-methyltransferase predicted by homology-based modelling

O-methylation of flavonoid compounds is an important enzymatic reaction since it not only reduces the chemical reactivity of their phenolic hydroxyl groups but also increases their lipophilicity and, hence, their intracellular compartmentation. Several genes encoding flavonoid O-methyltransferases (OMTs) have been isolated and characterized both at the molecular and biochemical levels. In contrast with mammalian enzymes, plant OMTs exhibit narrow substrate specificities as well as position-specific activities, so that the homology comparison, derived using programs such as BLAST can not provide sufficient information on the enzyme function or its substrate preference. In order to study these characteristics, therefore, another approach, homology-based modelling is being carried out. We report here the determination of the 3-D structure of Arabidopsis thaliana O-methyltransferase, AtOMT1 as well as its dynamics when complexed with its substrate. The predicted structure obtained by homology-based modelling is conserved during molecular dynamics simulations. AtOMT1 exhibits a structure similar to that of caffeic acid O-methyltransferase, COMT when the latter was used as a template. Whereas COMT includes 20 alpha-helices and nine beta-sheets, AtOMT1 has 16 and 9, respectively. Although the homology between both proteins is higher than 77% and all amino acids surrounding the active sites, except one residue, are similar in their primary sequences, the two proteins exhibit different substrate preferences. The differences in substrate specificity may be explained on the basis of the predicted structures of the protein and its complex with the substrate. In addition, docking the substrate into the active site of the protein allowed the study of the structural change of the active site on the dihedral angle distribution of the residues surrounding the active site.     

Parallel components of an equivalent circuit of two-dimensional island gold films

Two-dimensional island gold films [2D-I(Au)Fs] were prepared by the thermal evaporation technique where Corning 7059 glass slides, held at ambient temperature during deposition, were used as substrates. After exposing the films to air at a room temperature of 300 K, the d.c. resistance of the films was monitored until the films attained short-term stability, namely, invariance of resistance over time. The different island gold films were identified by the values of their stabilized surface resistivity ( _s ); these values in units of M/ are 23, 275, 4.4×10³, 23.2×10³ and 37.5×10³. The values of the parallel components (r _p , c _p ) of an equivalent circuit of the investigated films were obtained via a.c. measurements on the films performed by a lock-in amplifier at six temperatures (T) 100, 140, 180, 220, 260, and 300 K, where at each of them the frequency (f) of the a.c. voltage imposed on the film was varied over the range 0.1–80 kHz. It is found that: (i) regarding r _p , its value decreases with the increase in (T) and (f) while it increases as ( _s ) does. (ii) with respect to c _p , its magnitude decreases with the increase in (f) and ( _s ) but increases as (T) does. Our results are interpreted, qualitatively, by relating r _p and c _p to three frequency-independent parameters that reflect what happens physically in an island film when a steady voltage is applied across it. These parameters are R _b , R _g , and C _g ; where R _b is the resistance associated with the conduction of free electrons within the bulk (interior) of islands themselves, R _g is the resistance that accompanies the conduction across the gaps between the islands and C _g is the capacitance resulting from the presence of separated metallic islands at different potentials. To the best of our knowledge, the present results are the first to be reported for 2D-I(Au)Fs.     

Analysis and Optimization of Noise in Bipolar RF Harmonic Mixers

A noise analysis of bipolar harmonic mixers (BHM) used for direct-conversion receivers is presented in this paper. Analytical and simulated results for the transfer function of the mixer are presented. Simple analytical expressions describing noise contribution from all sources are derived. Estimation of flicker noise quite agrees with harmonic-balance simulation results. Based on the derived expressions, total time average noise power spectral density (PSD) at the output is compared with simulation results. For the recommended regions of operation, error is less than 20%. The overall BHM noise figure (NF) is calculated and optimized based on a simple extracted formula. Errors introduced by analysis remain within a 1.5-dB margin with respect to simulation results. The validity of analysis for high frequencies is justified. The effect of flicker noise coefficient on the overall mixer NF is compared for different available processes.     

Surface and bulk properties of ancient Egyptian mortars. Part I: X-ray diffraction studies

A series of Ancient Egyptian Mortars were extracted from the great Giza Pyramid, the second Giza Pyramid and the Sphinx and its adjoining temple. X-Ray Diffraction studies revealed the presence of gypsum, calcite and silica in most of the samples with a clearcut predominance of gypsum in the samples extracted from the Sphinx. Calcium silicates of different crystallographic modifications seem to be present in a large proportion of the samples. X-Ray studies carried out on heat-treated samples indicated the prevalence of the hemihydrate or γ-CaSO₄ modification until a temperature as high as 400°C.     

Characterization of hydrous titania from titanous chloride. III: Thermal studies (B) and surface texture

The present investigation deals with the surface properties of titania gels prepared from titanous chloride and ammonium carbonate under acidic and alkaline conditions of pH. The dehydration characteristics of such hydrous oxides are also investigated by means of simultaneous differential thermal analysis, thermogravimetric analysis and differential thermogravimetric analysis. Contrary to hydroxide-precipitated gels, even at a low pH of precipitation, very little ‘coordinated’ water remains in these gels. It is suggested that it is the concomitant presence of ammonium and carbonate ions in the precipitating agent, acting together, that provide the conditions favorable for the development of microporosity and high surface area in the gels.     

Modelling of thermal failure of metallic interconnects under electrostatic discharge transients

Temperature dependence of metal electrical resistivity is taken into account in modelling of thermal failure of metallic interconnects under electrostatic discharge (ESD) events. SPICE-based electro-thermal modelling is used to calculate the maximum temperature rise in the interconnect during stress. New ESD design guidelines for interconnects, based on the threshold temperature of latent failure, are proposed to optimise the interconnect width.     

Surface and related studies in sulphated oxides of zirconium

Zirconium oxide superacids with different sulphate ion contents were prepared from Zr(SO₄)₂ using urea as precipitating agent. These oxides were subjected to different heating temperatures. Characterization was carried out by means of infrared spectroscopy, X-ray diffraction and nitrogen adsorption studies. The surface acidity was determined by the method described by Boehm [1]. Infrared results confirmed the presence of the sulphate ions and suggested the gradual lowering through heat-treatment of the symmetry of the sulphate ions by complex formation. Surface acidity was favored when the sulphate ions were in a bidentate state of ligation. The latter ligands seemed in turn to be stabilized by water. The strongest acidity was displayed in systems where a high concentration of bidentated sulphate ions was present in conjunction with high surface areas and micropore volumes.     

Liquid crystal behavior of cellulose nanoparticles-ethyl cellulose composites: Preparation,characterization, and rheology

This work deals with assessing the approach for preparation of cellulose nanoparticles (CNPs) to be acted as synergistic component in liquid crystal (LC) ethyl cellulose composite (EC-CNPs). In this respect different structures of CNPs were prepared by acid and salt agents. These prepared CNPs were characterized by carboxyl content, IR, transmission electron microscope (TEM), and zeta potential, while their composites with EC were characterized by rheological measurements as a key factor for measuring the critical concentration of LC behavior. The results showed that, the crystallinity of CNPs obtained by ammonium persulfate exceeded that prepared by sulfuric acid hydrolysis. TEM images of stained CNPs showed both methods led to produce nanoparticles have rod like shape with aspect ratio (L/W) between 7.69 ± 3 and 31.3 ± 5. For the rheological measurements, it demonstrated the efficient of incorporating the CNPs to EC (EC-CNPs composites) to decrease the critical concentration of EC from 40 wt% to approximately 34 wt%.     

Hydrous oxides of titanium: cation exchange properties and kinetics of exchange

The ion exchange properties of hydrous titania gels of different particle sizes, precipitated from titanous chloride through the agency of ammonium carbonate and hydroxide have been studied. Such studies were carried out under acidic and alkaline conditions with respect to Cu²⁺, Ni²⁺, Co²⁺ and Cr³⁺ ions.In the case of gels precipitated by ammonium carbonate, oxygen gas was used as the oxidizing agent whereas with ammonium hydroxide as precipitant, oxidation was performed with hydrogen peroxide.Ion exchange capacities were determined by visible spectrophotometry. Increasing the pH of preparation lead to an increase in exchange capacities of the hydroxide precipitated gels that are characterized to be mesoporous. Such an increase is not observed in the case of carbonate precipitated microporous gels. It is shown that in the latter case the NH ₄ ⁺ ions generated by the initial interaction of (NH₄)₂CO₃ with the acidic titanous chloride lead to the formation of titania exchangers that are predominantly in the ammonium form. The textural characteristics of the exchanger resulting from different conditions of preparation is a significant contributing parameter to the resulting data.Ageing of the microporous titania samples markedly reduces the exchanger capacity of the smaller Ni²⁺ ions but increases that of the bulkier Cr³⁺ as a result of the presence of some wide pores that appear upon agglomeration. The presence of Cr³⁺ ions in the hydroxo form in solution seems to inhibit its exchange with the appropriate surface species.Studies on the kinetics of exchange with respect to the Ni²⁺ ions seem to indicate that a particle diffusion mechanism is partly or completely responsible for the rate of exchange.