Increasing Preparation Temperature Research Articles

An EELS-based study of the effects of pyrolysis on natural carbonaceous materials used for activated charcoal preparation.

Electron energy-loss spectroscopy (EELS) has been used to characterize the electronic structure of charcoal phases at the nanoscale, thus demonstrating that the technique can be applied to environmental science. Activated charcoal is extensively used to remove pollutants from liquid and gaseous sewage. It is mainly obtained by activation of coke or charcoal produced from ligneous precursors. The present study concerns the use of by-products of local Caribbean agriculture, such as sugar cane bagasse, fruit stones and seeds, for use as activated charcoal precursors. Charcoal phases are prepared by high-temperature pyrolysis of lignocellulosic raw materials under a nitrogen gas flow. With the aim of optimizing the pyrolysis temperature and duration and oxygen content, the concentration of carbon sp2 hybridized chemical bonds and structural ordering have been followed by EELS for different treatment temperatures. To quantify the carbon sp2 content, near edge structure (NES) at the carbon K edge has been measured to determine the strength of pi --> pi* and 1s --> pi* transitions. Three precursors of plant origin, shells of Terminalia catappa and Acrocomia karukerana and seeds of Psidium guajava, with the pyrolysis temperatures between 600 and 900 degrees C, were investigated. The fraction of carbon sp2 bonding is found to increase when the temperature rises from 600 degrees C to the range 700-750 degrees C and becomes stable at higher temperatures. For temperatures in excess of 700 degrees C, structural ordering probably occurs and well-defined 1s --> sigma* NES is present, whose intensity increases with increasing preparation temperature. For the highest temperature of around 900 degrees C, the structure of the final product is less well organized than graphitized carbon but a few per cent of a highly ordered phase is found.

The effect of preparation temperature on the swelling behavior of poly (N-isopropylacrylamide) gels

The role of the preparation temperature of poly(N-isopropylacrylamide) (PNIPA) gels on their swelling behavior in water and in aqueous solutions of sodium dodecylbenzenesulfonate was investigated. PNIPA gels were prepared by free-radical crosslinking copolymerization of N-isopropylacrylamide and N,N'-methylene(bis) acrylamide in water at fixed monomer and crosslinker concentrations. The equilibrium swelling ratio of the gels increases first slightly up to about 20°C then rapidly with increasing gel preparation temperature. Magnitude of the collapse transition in water at 34°C becomes larger as the gel preparation temperature increases. Calculations indicate a decrease in the effective crosslink density of PNIPA gels with increasing preparation temperature. The gels prepared at temperatures higher then 20°C were heterogeneous consisting of highly crosslinked regions interconnected by the PNIPA chains.

Obtention and surface characterisation of several ash-free chars

Four ash-free cellulose chars were obtained at temperatures of 400, 500, 600 and 700 °C, respectively, at which the thermogravimetric plots approach a final constant mass, avoiding further high-temperature vapour activations to prevent micropore formation. Stable thermal behaviour was obtained, however, under further heat treatment at temperatures equal to or lower than those of the respective preparations. The surface structures were characterized using thermogravimetry, BET surface area, porosimetry, FTIR, ammonia adsorption and surface Brönsted acidity measurements. FTIR shows two main organic functional groups, i.e. aromatic lactones and carboxylic acid functions. Ammonia adsorption-desorption and cation exchange experiments show ammonia physisorption into the micropore system that is about three orders of magnitude higher than the adsorption by the Brönsted carboxylic acid functionality itself. The former increases and the latter decreases with increasing preparation temperature of the chars. These chars exhibit apparent BET surface areas between 133–337 m 2g −1 and relatively low micropore contents, especially those obtained at temperatures lower than 500 °C.

Improvement on the linearity of blue phosphor: Evaluation of hexagonal ZnS:Ag,Al

Abstract— ZnS:Ag,Al phosphors were prepared in an H2S flow without using flux at various temperatures. A cubic structure (a zinc‐blende structure) was obtained for the sample prepared at 900°C, and a hexagonal structure (a wurzite structure) was obtained at 1050–1200°C. The life was improved simply with increasing preparation temperature. The luminous efficacy and the current coefficient had their optimum temperature at 1050°C. They showed similar tendencies related to the preparation temperature. The diffuse reflectance also showed the similar tendency of the preparation temperature dependence. The thermoluminescence measurement showed that the traps at a depths of 1.3 eV (observed below 1050°C) and 1.15 eV (observed above 1050°C) disappeared at a preparation temperature of 1050°C.

Possibility of porosity control in nylon-6 membranes

The possibility of porosity control in nylon-6 membranes was investigated. By adding water onto the surface of a concentrated nylon-6 solution in calcium chloride-methanol mixture in a cylindrical glass bottle, a three-phase structure of solid, gel and solution (in order from the top) appeared. The thickness of the gel phase was 2.5 – 4 mm and became larger as the time elapsed after adding water. The interfaces between solid and gel, and between gel and solution, shifted downwards at very slow rates, 0.224 and 0.263 mm h −1 at 20°C, respectively. In the gel, the behaviour of pore growth was observed in real time. The membranes have spherical pores with a pore size distribution, and the pore size increases with increasing preparation temperature. On the basis of these observations, a schematic process of membrane formation was proposed as a fundamental of porosity control.

Influence of carbonization conditions on the gasification of acacia and eucalyptus wood chars by carbon dioxide

Gasification rates of cubic shaped acacia and eucalyptus wood chars were measured thermogravimetrically in a carbon dioxide atmosphere at temperatures in the range 810–960 °C. The effects of wood species and carbonization conditions, such as temperature, heating rate and soaking time, were determined. Both reactivity and the activation energy for the gasification of wood chars were found to be strongly influenced by the carbonization conditions employed during their preparation and wood type. The reactivities of both the acacia and eucalyptus wood chars decreased with increasing preparation temperature; while the activation energy for their gasification increased. Slow carbonization (heating rate: 4 °C min −1) led to the production of wood chars having lower reactivities and higher activation energies than those of the wood chars prepared under rapid carbonization (heating rate: 30 °C min −1) at the same temperature. With increasing soaking time, at carbonization temperatures of 800 and 1000 °C, the reactivity of resulting wood chars was reduced. The results also show that the reactivities of acacia wood chars are higher than those of similarly prepared eucalyptus wood chars.

An Intrinsic Relationship between Molecular Structure in Self-Assembled n-Alkylsiloxane Monolayers and Deposition Temperature

We have studied the effect of preparation temperature, in the range 5-65 OC, on the structures of n-octadecylsiloxane monolayers prepared by self-assembly from dilute solution of n-octadecyltrichlorosilane onto the surface of freshly hydrated, oxidized silicon substrates. Structural features of the films were characterized using a combination of liquid drop contact angle measurements, null ellipsometry, and infrared transmission spectroscopy. The contact angle data confirm a previously reported observation of a critical temperature, Tc - 28 f 5 OC, below which the surface energy is constant at a near-limiting value of a pure CH3 surface and above which the surface energy monotonically increases with increasing temperature. Coverages and chain organization, as measured by ellipsometry and vibrational spectroscopic features (peak positions and integrated intensities of methylene C-H stretching modes), respectively, show changes in the same temperature region as the wetting behavior. We conclude that when prepared below T,, the films exhibit a heterogeneous structure with closely spaced islands of densely packed, nearly all-trans alkyl chains arranged nearly vertical to the surface. In contrast, when prepared above T,, the films exhibit monotonically diminishing coverage with increasing preparation temperature and the alkyl chains increasingly assume higher contents of conformational disorder. Further, the infrared data indicate that these higher temperature films are heterogeneous with coexisting domains of high and low chain conformational ordering. All the data, taken together, are in good conformity with a film formation mechanism which involves, prior to siloxy group cross-linking, the intervention of intermediate structural phases of mobile alkylsiloxy species adsorbed on a water layer adjacent to the solid substrate surface. In support of this mechanism, a strong parallel is apparent between Tc and the triple point temperature at which gas (G), liquid-expanded (LE), and liquid-condensed (LC) phases coexist for chain Langmuir monolayers at the air/bulk water interface. Below Tc the self-assembled film structure is similar to that of the nearly pure LC Langmuir phase while above Tc the film structure is similar to that of coexisting LE and LC phases. Deviations of the self-assembled film structures for the analogous equilibrium Langmuir phase structures occur at higher preparation temperatures and are rationalized in terms of both the known occurrence of nonequilibrium phases in Langmuir films above the triple point temperature and the relative acceleration of the Si-O-Si cross-linking reaction in the self-assembled film to form structures with frozen-in defects.

The Structure of Aggregated NiO Particles in NiO/SiO<sub>2</sub> Particles Prepared by CVD Method

The structure of NiO particles in the NiO/SiO2 agglomerates prepared by CVD was investigated by XRD, TEM, XPS and TPR. TEM observation and XRD analysis indicated that the NiO particles obtained were in the aggregated state, and that the size of them became large, while that of primary particles became small, with increasing preparation temperature. TPR and XPS analyses indicated that a trace amount of nickel silicate coexisted with the NiO/SiO2 particles, most likely at the outer region of the NiO aggregates near the interface with the SiO2 deposited on them. From these results and the hydrogen reduction experiments, we proposed the following structure model of the NiO/SiO2 agglomeration; at high preparation temperature small primary NiO particles grow into large secondary particles, followed by coating with thin SiO2 layers, while at low temperature large primary NiO particles grow and are surrounded by SiO2 matrix.

The influence of the preparation temperature of colloidal ruthenium dioxide on the photosensitized reduction of H + ions

Samples of colloidal RuO 2were prepared by thermal decomposition of RuCl 3 at 300, 400 and 500 °C. X-ray diffraction showed that they all consisted of stoichiometric and crystalline RuO 2. The specific surface area decreases strongly with increasing preparation temperature. Catalytic properties for the photochemical production of hydrogen were tested in an aqueous system containing a photosensitizer (ruthenium trisbipyridyl), an electron relay (methylviologen) and an electron donor (EDTA). In this solution the RuO 2 particles form aggregates which have approximately the same size for all preparation temperatures. The rate of hydrogen gas production was measured as a function of weight concentration of RuO 2. For RuO 2 samples prepared at 300°C and at 400°C identical curves were obtained; for the 500 °C sample the production rate is slightly lower only for low RuO 2 concentrations. The aclivation energies for hydrogen evolution are the same for all RuO 2 samples, and do not change upon addition of anthracene carboxylate, which changes the mechanism of the homogeneous reactions. It is concluded that the small variation in hydrogen production rate with preparation temperature of the catalyst is due to changes in mass transfer of methylviologen radicals to the catalytic surface. Differences in catalytic properties of the RuO 2 samples are negligible.

Bulk defects in Co 3O 4, pure and slightly doped with lithium, revealed by EPR of the tetrahedral Co 2+ ions

The influence of the preparation temperature of Co 3O 4, pure and slightly doped with lithium (up to 2 mol%), on the EPR signal of tetrahedral Co 2+ ions was studied. The decrease in the preparation temperature of pure Co 3O 4 has a similar effect on the corresponding Lorentzian singlet as the increase in the lithium content. This effect is explained in terms of a Gaussian distribution of the Co 2+ coordination tetrahedra with respect to the tetrahedral angle arising out of bulk defects such as cationic vacancies or doped lithium ions and Co 4+ octahedral ions compensating the additional negative charges. The cationic vacancies are due to small deviations from stoichiometry decreasing with increasing preparation temperature of the pure Co 3O 4. The zero-field splitting energy of the Co 2+ ion groundstate, mostly determining the linewidth, is rather high even at small deviations of the local symmetry from the purely cubic one, due to the relatively high value of the single electron spin-orbit coupling constant of Co 2+ ions and the small cubic ligand field parameter.

Characterization of plasma polymer films of phenylsilane by X-ray photoelectron spectroscopy

Carbon 1 s and silicon 2 p X-ray photoelectron spectra of phenylsilane plasma polymer films prepared at substrate temperatures, T s, between 50 and 450°C were recorded. The binding energies, lineshapes, and shake-up satellite intensities are in accordance with a structure consisting of a silicon network with pendant phenyl groups, and the minor dependence on T s is consistent with the main effect of increasing preparation temperature being the loss of hydrogen and some pendant phenyl, with a concurrent increase in the interconnectivity of the network. The observed C 1 s binding energy and linewidth specifically rule out the presence of any significant amount of carbon in silicon carbide form. A simultaneous shift of about 0.6 eV in the binding energies of both the C 1 s and Si 2 p lines is tentatively interpreted as a shift in the Fermi level with respect to the valence band edge.

Tactiticy and stereochemistry in the ring-opening polymerization of 5,5-dimethylbicyclo[2.2.1]hept-2-ene initiated by metathesis catalysts

Ring-opened polymers of optically active 5,5-dimethylbicyclo[2.2.1]-hept-2-ene (5,5-dimethylnorbornene DMNBE), having cis double bond contents of 0 – 100%, were prepared using various olefin metathesis catalysts based on Mo, W, Re, OS, Ru and Ir compounds. The ring dyad tacticities in these polymers, with respect to both cis and trans double bonds, were determined from 13C NMR spectra. Cis double bonds are always asociated with 50 – 100% r dyads (syndiotacticity (σ r ) c = 0.5 – 1.0), while trans double bonds are always associated with 50 – 100% m dyads (isotacticity (σ m ) t = 0.5 – 1.0). The polymers may be divided into four groups: I, those of high tacticity, with (σ r ) c ) ∼ (σ m ) t ∼ 1.0; II, those of intermedia tacticity, with (σ r ) c ∼ (σ m ) t ∼ 0.6 – 0.9; III, those with (σ r ) c > (σ m ) t ; IV, those of low tacticity, with (σ r ) c ∼ (σ m ) t ∼ 0.5. These is no correlation between tacticity and cis content, but all- cis polymers are generally highly syndiotactic and all- trans polymers slightly isotactic when prepared from 3 M monomer at 20 °C. The tacticity falls with increasing preparation temperature but not always with decreasing monomer concentration. The results are interpreted in terms of sequential formation of a metallacarbene-monomer complex, 4, a metallacyclobutane, 5, a metallacarbene with the newly-formed double bond first coordinated to, 6, and achiral ‘symmetrical’ form of 6; there are corresponding mirror-image and symmetrical forms of 3. The first type of rupture accounts for cases of high tacticity or tacticity which falls with monomer dilution; the second explains how the tacticity can be independent of monomer dilution. Propagation is assumed to occur either by addition of monomer to 3 to form a cis or trans double bond, or by displacement of coordinated cis double bond from 6 by monomer to form a new cis doubled bond. With these assumptions a complete mechanism is developed (Scheme 6) to account not only for the results presented here but also for some more general features of olefin metathesis, especially stereospecificity and selectivity in cross-methathesis.

Paramagnetic moments and localization in1T-TaS2

The low-temperature magnetic susceptibility of $1T$-Ta${\mathrm{S}}_{2}$ is shown to have a contribution from localized paramagnetic moments that are not due to impurities. The density of paramagnetic moments increases as the preparation temperature is increased. This suggests that the moments are due to interstitital defects (intercalated Ta atoms). In some samples a weak remanent magnetization is observed below 4 K as the magnetic field is reduced toward zero, suggesting spin-glass-like ordering of the moments. Since the low-temperature electrical resistivity is dominated by Anderson localization, the magnitude of the resistivity at low temperatures also increases with increasing preparation temperature due to a higher defect density. These results are compared to recently proposed models.