Initial SiO Research Articles

Study on Mesoporous Al-SBA-15 with Enhanced Acidity and Hydrothermal Stability for Heavy Oil Hydrocracking Conversion

To increase heavy oil conversion by hydrocracking, the larger pore opening mesoporous materials with enhanced acidity and hydrothermal stability are required. A series of mesoporous Al-SBA-15 was studied in acid-free and different HCl concentration solutions with various initial SiO₂/Al₂O₃ molar ratio. The final product samples were characterized with N₂ adsorption, SAXS/XRD, NH₃-TPD, TEM, ²⁸Si NMR, and ²⁷Al NMR. The results concluded that the acid-free and weak acidic medium favor the formation of the Al-SBA-15 with better textual properties, higher acidity, and higher wall-thickness. A considerable amount of Al was present in the tetrahedral form.

Evolution of shape, size, and areal density of a single plane of Si nanocrystals embedded in SiO2 matrix studied by atom probe tomography

The dependence of the shape, size, and areal density of Si NCs on the thickness of the initial SiO layer was investigated using atom probe tomography and validated by energy filtered transmission electron microscopy.

Synthesis of photoluminescent Si/SiO x core/shell nanoparticles by thermal disproportionation of SiO: structural and spectral characterization

Core/shell Si/SiO x nanoparticles (Si/SiO x -NP) having bright red-infrared photoluminescence were obtained by a three-stage synthesis based on the thermal disproportionation of microdispersed SiO. Transformation patterns of structure and spectroscopic properties of the material during passage through all process stages (starting from initial SiO microparticles and up to the Si/SiO x -NP sols) have been revealed by using Raman, photoluminescence and ESR spectroscopy, XPS, XRD, and electron microscopy. Thermal annealing of SiO microparticles (stage I) results in formation of amorphous-crystalline Si nanophase in the matrix of SiO2, as well as generation of paramagnetic Pb centres with the concentration up to 4 × 1018 particles/g. At the annealing temperature, T an > 900 °C, a rapid growth of nanocrystal sizes takes place, and, simultaneously, a rapid growth of paramagnetic Pb centre concentration occurs. Elimination of SiO2 from the annealed sample by etching in HF (stage II) stimulates further crystallization of amorphous-crystalline core, caused by stress relaxation inside the Si core when removing SiO2 matrix. Functionalization of nanoparticle surface (stage III) allows obtaining core/shell Si/SiO x -NP with a bright red-infrared photoluminescence and their sols. Average size of the crystalline Si core increases from 4.7 to 11.1 nm when T an at the stage I rises from 350 to 1100 °C. At relatively low T an = 350 °C, the nanoparticles with monocrystalline Si cores are mainly formed, while at T an > 1100 °C, a large number of polycrystalline Si nanoparticles are also observed. Our TEM images have revealed the existence of monocrystalline Si nanoparticles having significantly different contrast even at comparable nanoparticle sizes. We attribute that to the formation of both bulk (with a high TEM contrast) and flat (2D) Si nanocrystals (with a low TEM contrast) in the course of SiO annealing.

Organic template-free synthesis of ZSM-5/ZSM-11 co-crystalline zeolite

A series of ZSM-5/ZSM-11 co-crystalline zeolites with various compositions and morphologies were successfully synthesized via an organic template-free hydrothermal route and characterized by XRD, XRF, SEM, NMR and N 2 adsorption/desorption technologies. The effects of raw materials and batch composition were investigated systematically. Various silicon sources can be employed in the organic template-free synthesis of ZSM-5/ZSM-11 co-crystalline zeolite, however only a few types of aluminum sources are available. This organic template-free system is favorable to the aluminum-rich zeolite. With the increase of initial SiO 2/Al 2O 3 ratio, the ZSM-5 percentage in the ZSM-5/ZSM-11 co-crystalline zeolite increases as well as the crystal size, and especially the morphology of ZSM-5/ZSM-11 co-crystalline zeolite prepared from the colloidal silica–NaAlO 2 solution system changes gradually from nano-rod aggregation, micro-spindle to single hexagon and then to twinned hexagon crystals. Moreover, Na + and OH − in the initial materials can promote the nucleation of the ZSM-5/ZSM-11 co-crystalline zeolite significantly and are beneficial to the formation of crystals with relatively low length/width ratio, while K + postpones the crystallization process seriously.

Crystallization and morphology of mordenite zeolite influenced by various parameters in organic-free synthesis

A series of mordenite zeolites with different morphologies were synthesized via a facile organic-free hydrothermal route, and characterized by X-ray diffraction, scanning electron microscopy, X-ray fluorescence spectrometer and N 2 adsorption–desorption techniques. Influences of synthetic parameters, including seed crystal, silicon precursor, SiO 2/Al 2O 3, Na 2O/SiO 2 and H 2O/SiO 2, on mordenite crystallization were investigated systematically. It was found that mordenite zeolites with various morphologies, such as fiber-like, rod-like, prism-like and needle-like ones could be synthesized in control. Especially, novel nano fiber-like MOR crystals with high c/ b aspect ratio were prepared from low silica concentration system, which was manipulated by using small initial SiO 2/Al 2O 3 ratio, large H 2O/SiO 2 and silicon source with slow dissolution rate. Moreover, mordenite samples with various morphologies exhibited different mesopore property and thermal stability.

Oxidation behaviour of arc-melted and uniaxial hot pressed MoSi 2 at 500 °C

Samples of MoSi 2 were synthesised by arc melting and uniaxial hot pressing from different starting materials in order to distinguish the effects of sample preparations on the occurrence of the pest phenomenon, i.e. the rapid fragmentation of the pieces. The arc-melted samples presented cracks and porosities after preparation whereas the hot pressed samples were well densified (>95%). The pest appeared after no more than a few hours of exposure in air at 500 °C for arc-melted samples, whereas the sintered MoSi 2 samples with our experimental conditions were not at all affected. The corrosion products consisted of stoichiometric MoO 3 and SiO 2 mixtures. Both isothermal and cyclic oxidation measurements over long durations (>2000 h) support these results. It appears that the oxidation kinetics of hot pressed samples are governed by surface or phase boundary reactions and consequently depends on the grain size and the distribution of the initial SiO 2 particles, i.e. on the MoSi 2 microstructure.

Removal of silicon from pulping whitewater using integrated treatment of chemical precipitation and evaporation

This article investigates and evaluates the technical applicability of integrated physico-chemical methods to treat thermo-mechanical pulping (TMP) whitewater collected from a local mill in Finland. Under optimized conditions, both chemical precipitation and evaporation were applied to remove silicon in the whitewater samples. The mechanism of silicon removal by the precipitant is also presented. Their treatment performances in removing silicon were statistically evaluated and compared to those of other reported studies. It is evident that both the treatments are applicable and effective to remove target pollutant from the samples. At the same SiO 2 concentration of 200 mg/L, 0.10 g/L of dose, pH 10.5 and 303 K, the integrated chemical precipitation using Fennofix type FF40 and evaporation could remove 96% of silicon. This finding is slightly higher than those of similar precipitation using CaO and electro-coagulation in other studies, which could accomplish 93% and 95% of silicon removal respectively at initial SiO 2 concentrations of 954 and 250 mg/L. Both chemical precipitation and evaporation could effectively treat a varying strength of TMP whitewater, as their treated effluents could meet the required silicon limit of less than 50 mg/L set by environmental legislation. Therefore, subsequent treatments are not required to complement the removal of silicon in the effluents prior to their discharge.

Surface and Interface Processes during Atomic Layer Deposition of Copper on Silicon Oxide

The initial surface chemistry and growth mechanisms of the atomic layer deposition (ALD) of metallic copper on SiO(2) surfaces are investigated using an amidinate precursor (copper(I) di-sec-butylacetamidinate, [Cu((s)Bu-amd)](2)) and molecular hydrogen. Using in situ Fourier transform infrared spectroscopy together with calculations based on density functional theory, we show that the initial surface reaction of [Cu((s)Bu-amd)](2) with hydroxylated SiO(2) takes place by displacement of one of the sec-butylacetamidinate ligands at a surface -OH site, thus forming a Si-O-Cu-((s)Bu-amd) surface species, evident by the stretching vibrations of Si-O-Cu and the chelating -NCN- bonds. Molecular hydrogen exposure during a subsequent pulse dissociates most of the sec-butylacetamidinate ligands bound to surface Cu, which releases free amidine vapor, leaving Cu atoms free to agglomerate on the surface and thus opening more reactive sites for the next [Cu((s)Bu-amd)](2) pulse. Copper agglomeration is evident in the IR absorbance spectra through the partial recovery of the intensity of SiO(2) optical phonon modes upon H(2) reduction, which was lost after the reaction of [Cu((s)Bu-amd)](2) with the initial SiO(2) surface. The thermally activated ligand rearrangement from a bridging to a monodentate structure occurs above 220 degrees C through hydrogenation of the ligand by surface hydroxyl groups after exposure to a [Cu((s)Bu-amd)](2) pulse. As Cu particles grow with further ALD cycles, the activation temperature is lowered to 185 degrees C, and hydrogenation of the ligand takes place after H(2) pulses, catalyzed by Cu particles on the surface. The surface ligand rearranged into a monodentate structure can be removed during subsequent Cu precursor or H(2) pulses. Finally, we postulate that the attachment of dissociated ligands to the SiO(2) surface during the [Cu((s)Bu-amd)](2) pulse can be responsible for carbon contamination at the surface during the initial cycles of growth, where the SiO(2) surface is not yet completely covered by copper metal.

Nano Si Superlattices for the Next Generation Solar Cells

We present a study on amorphous SiO/SiO2 superlattice formation on Si substrate held at room temperature and annealed in the temperature range 600-1100 degrees C. Grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity were used to study such samples. Amorphous SiO/SiO2 superlattices were prepared by high vacuum physical vapor deposition of 4 nm thin films of SiO and SiO2 (10 layers each) from corresponding targets on silicon substrate. Rotation of the Si substrate during evaporation ensured homogeneity of the films over the whole substrate. We observed that the inhomogeneities introduced into the SiO and SiO2 layers during the deposition (evaporation) give rise to small angle scattering at lower annealing temperatures. After an initial SiO layer thickness reduction for 600 degrees C annealing, these thicknesses remain virtually unchanged up to 1000 degrees C, where they start to decrease again which leads to particle formation. Nevertheless, this compacting at low temperatures may lead to the seed formation in SiO layers that will facilitate later Si nanoparticles growth.

Elaboration and characterization of barium silicate thin films

Room temperature depositions of barium on a thermal silicon oxide layer were performed in ultra high vacuum (UHV). In-situ X-ray photoelectron spectroscopy (XPS) analyses were carried out as well after exposure to air as after subsequent annealings. These analyses were ex-situ completed by secondary ion mass spectrometry (SIMS) profiles and transmission electron microscopy (TEM) cross-sectional images. The results showed that after air exposure, the barium went carbonated. Annealing at sufficient temperature permitted to decompose the carbonate to benefit of a barium silicate. The silicate layer was formed by interdiffusion of barium with the initial SiO 2 layer.

Medium-term phase stability of Na 2O–Al 2O 3–SiO 2–H 2O geopolymer systems

The phase evolution and microstructural development of a series of geopolymer mixtures comprising SiO 2, Al 2O 3, Na 2O and H 2O prepared by alkali reaction of metakaolin, have been studied. The study also included the effects of cure duration and its impact on physical properties such as compressive strength. The characteristic molar ratios of the geopolymer mixtures were of the range SiO 2/Al 2O 3 [2.50–5.01] and Al 2O 3/Na 2O [0.60–1.70], respectively. The formulations were subjected to continuous curing at 40 °C for 7 months, and were analyzed periodically by XRD and SEM techniques. Amorphous Na–Al–Si phase(s), observed at early ages, gradually transformed to crystalline phase(s) with prolonged curing. The initial SiO 2, Na 2O, and Al 2O 3 contents of mixtures appeared to be critical factors governing the observed amorphous → crystalline transformation. Well-developed crystalline zeolitic phases, including chabazite, faujasite, zeolite A and zeolite P, were identified in some of the mixtures investigated. In most cases, with prolonged curing, some correlation emerged relating compressive strength development with corresponding phase changes. In essence, the mixture formulations that developed crystalline phases after prolonged curing tended to produce low strengths. The relevance of these findings on the phase development of mild- to warm-temperature prolonged curing of geopolymer systems is discussed.

Interfacial properties of cetyltrimethylammonium-coated SiO 2 nanoparticles in aqueous media as studied by using different indicator dyes

In this paper, we compare the properties of SiO 2/water interface modified by cetyltrimethylammonium bromide (CTAB) with those of CTAB spherical micelles. The suspension of uniform silica nanoparticles coated with CTAB adlayer was investigated by using a set of acid–base indicators. The study of the colloidal system has been provided using electron microscopy and dynamic light scattering methods; the diameter of the initial SiO 2 particles in dried state was ca. 40 nm. The increase in the ζ-potential value of nanoparticles from −34 to +(37–54) mV on going from pure silica suspension to the CTAB-containing system points on the silica surface recharging and formation of surfactant bilayer (or multilayer) on the silica/water interface. To obtain further information about the interfacial surfactant adlayer, the behavior of different indicator dyes has been studied in CTAB-modified SiO 2 suspension. Comparison of the indices of apparent ionization constants, i.e., p K a a values of phenol red, bromothymol blue, and fluorescein with those determined in CTAB micellar solutions have confirmed the supposition about certain similarity between CTAB-covered silica nanoparticles and common spherical surfactant micelles. However, the experiments on kinetics of bromophenol blue fading, as well as the spectral properties of methyl orange and solvatochromic Reichardt's indicator and some other data revealed the specificity of surfactant-coated silica nanoparticles, presumably, originating from their surface morphology.

Influence of the thickness of the tunnel layer on the charging characteristics of Si nanocrystals embedded in an ultra-thin SiO 2 layer

In this paper, we have studied the effect of the thickness of the initial SiO 2 layer (5–7 nm) on the charge and discharge properties of a 2D array of Si nanoparticles embedded in these SiO 2 layers fabricated by ultra-low-energy ion implantation (ULE-II) and annealing. The structural characteristics of these nanocrystal-based memories (position of the nanocrystals with respect to the electrodes, size and surface density of the particles in the plane) were studied by transmission electron microscopy (TEM) and energy filtered TEM (EF-TEM). Electrical characterizations were performed at room temperature using a nano-MOS capacitor to be able to address only a few nanoparticles (nps). EFTEM gives the measurements of oxide thickness, injection, control and nps distances, size and density. I– V and I– t measurements exhibit current peaks and random telegraph signal fluctuations that can be interpreted as due to quantized charging of the nps and to some electrostatic interactions between the trapped charges and the tunnelling current. We have shown that these characteristics strongly vary with the initial oxide thickness, exhibiting several charging/discharging events for the 7-nm-thick layer while charging events prevail in the case of 5-nm-thick layer. These results indicate that the probability of discharging phenomena is reduced when the tunnel layer thickness decreases.

Modeling the kinetics of silica nanocolloid formation and precipitation in geologically relevant aqueous solutions

The kinetics of the formation and precipitation of nanocolloidal silica from geologically relevant aqueous solutions is investigated. Changes in monomeric (SiO 2(mono)), nanocolloidal (SiO 2(nano)) and precipitated silica (SiO 2(ppt)) concentrations in aqueous solutions from pH 3 to 7, ionic strengths (IS) of 0.01 and 0.24 molal, and initial SiO 2 concentrations of 20.8, 12.5 and 4.2 mmolal (reported in [Icopini, G.A., Brantley, S.L., Heaney, P.J., 2005. Kinetics of silica oligomerization and nanocolloid formation as a function of pH and ionic strength at 25 °C. Geochim. Cosmochim. Acta 69(2), 293–303.]) were fit using two kinetic models. The first model, termed the concentration model, is taken from Icopini et al. (2005) and assumes that the rate of change of SiO 2(mono) as a function of time has a fourth-order dependence on the concentration of SiO 2(mono) in solution. The second model, termed the supersaturation model, incorporates the equilibrium concentration of amorphous silica and predicts that polymerization will be a function of the degree of silica supersaturation in solution with respect to amorphous silica. While both models generally predicted similar rate constants for a given set of experimental conditions, the supersaturation model described the long-term equilibrium behavior of the SiO 2(mono) fraction more accurately, resulting in significantly better fits of the monomeric data. No difference was seen between the model fits of the nanocolloidal silica fraction. At lower pH values (3–4), a metastable equilibrium was observed between SiO 2(mono) and SiO 2(nano). This equilibrium SiO 2(mono) concentration was found to be 6 mmolal, or three times the reported solubility of bulk amorphous silica under the experimental conditions studied and corresponds to the predicted solubility of amorphous silica colloids approximately 3 nm in diameter. Atomic force microscopy was used to determine the average size of the primary nanocolloidal particles to be ∼3 nm, which is in direct agreement with the solubility calculations. Larger aggregates of the primary nanocolloids were also observed to range in size from 30 to 40 nm. This work provides the first kinetic models describing the formation and evolution of nanocolloidal silica in environmentally relevant aqueous solutions. Results indicate that nanocolloidal silica is an important species at low pH and neutral pH at low ionic strengths and may play a more important role in geochemical cycles in natural aqueous systems than previously considered.

Size and density control of Si-nanocrystals realized by SiO x/SiO 2 superlattice

A process which allows the preparation of size and density-controlled Si nanocrystals (Si-NCs) with a crystal diameter down to 2 nm and a nanocrystal density up to 10 19/cm 3 has been developed. The process is based on the growth of amorphous SiO x /SiO 2 superlattice, and produces Si-NCs due to the thermally induced phase separation of SiO x layers within the films and the crystallization of the resulting Si-nanoclusters. Amorphous SiO x films with adjustable oxygen content have been deposited on Si wafers by reactive evaporation of SiO powder in different O 2 atmospheres. The oxygen content of the deposited films can be controlled by changing the oxygen pressure during evaporation. Photoluminescence spectra and transmission electron microscopy images of the superlattice samples indicate that the size of Si-NCs is determined by the initial SiO x layer thickness and, the density of Si-NCs within the layer varies with the stoichiometry of the SiO x layer in the superlattice. In this way, the size and density of Si-NCs within a layer can be controlled separately.

Synthesis and characterization of SiC materials with hierarchical porosity obtained by replication techniques

Porous silicon carbide monoliths were obtained using the infiltration of preformed SiO(2) frameworks with appropriate carbon precursors such as mesophase pitch. The initial SiO(2) monoliths possessed a hierarchical pore system, composed of an interpenetrating bicontinuous macropore structure and 13 nm mesopores confined in the macropore walls. After carbonization, further heat treatment at ca. 1,400 degrees C resulted in the formation of a SiC-SiO(2) composite, which was converted into a porous SiC monolith by post-treatment with ammonium fluoride solution. The resulting porous SiC featured high crystallinity, high chemical purity and showed a surface area of 280 m(2) g(-1) and a pore volume of 0.8 ml g(-1).

Carbon deposits on the surface of CaO/SiO 2 as active catalysts for the oxidative dehydrogenation of ethylbenzene

The oxidative dehydrogenation (OD) of ethylbenzene (EB) has been investigated over different catalysts of xCaO/SiO 2 and yP/ xCaO/SiO 2 composition. It has been shown that the texture characteristics of initial SiO 2 exert an important influence on the activity of catalysts obtained on their basis. Higher conversion of ethylbenzene and selectivity on styrene are observed in the case of macroporous silica gels. The yield of styrene and quantity of carbon deposits (CD) in the initial reaction period are increased symbately. In the stationary reaction period achievable in 1–6 h after beginning of reaction, depending on texture characteristics of catalysts, maximal conversion of EB and minimal rate of accumulation of carbon deposits are observed over catalyst surface. In this period of process catalytically active system represents carbon deposits formed over catalyst surface and inclusive various carbonyl-containing groups. Catalysts are not passive carriers on which carbon deposits are formed; on the contrary, structure and catalytic activity of carbon deposits strongly depend on composition, texture characteristics and acid–base properties of used catalysts.

A preferred method for the determination of bulk compositions of coarse-grained refractory inclusions and some implications of the results

Analyses of coarse-grained refractory inclusions typically do not have the solar CaO/Al 2O 3 ratio, probably reflecting nonrepresentative sampling of them in the laboratory. Many previous studies, especially those done by instrumental neutron activation analysis (INAA), were based on very small amounts of material removed from those restricted portions of inclusions that happened to be exposed on surfaces of bulk meteorite samples. Here, we address the sampling problem by studying thin sections of large inclusions, and by analyzing much larger aliquots of powders of these inclusions by INAA than has typically been done in the past. These results do show convergence toward the solar CaO/Al 2O 3 ratio of 0.792. The bulk compositions of 15 coarse-grained inclusions determined by INAA of samples >2 mg have an average CaO/Al 2O 3 ratio of 0.80 ± 0.18. When bulk compositions are obtained by modal recombination based on analysis of thin sections with cross-sections of entire, large, unbroken inclusions, the average of 11 samples (0.79 ± 0.15) also matches the solar value. Among those analyzed by INAA and by modal recombination, there were no inclusions for which both techniques agreed on a CaO/Al 2O 3 ratio deviating by >∼15% from the solar value. These results suggest that: individual inclusions may have the solar CaO/Al 2O 3 ratio; departures from this value are due to sample heterogeneity and nonrepresentative sampling in the laboratory; and it is therefore valid to correct compositions to this value. We present a method for doing so by mathematical addition or subtraction of melilite, spinel, or pyroxene. This yields a set of multiple, usually slightly different, corrected compositions for each inclusion. The best estimate of the bulk composition of an inclusion is the average of these corrected compositions, which simultaneously accounts for errors in sampling of all major phases. Results show that Type B2 inclusions tend to be more SiO 2-rich and have higher normative Anorthite/Gehlenite component ratios than Type B1s. The inclusion bulk compositions lie in a field that can result from evaporation at 1700–2000K of CMAS liquids with solar CaO/Al 2O 3, but with a wide range of initial MgO (30–60 wt%) and SiO 2 (15–50 wt%) contents.

Silica scale inhibition by polyaminoamide STARBURST ® dendrimers

This paper reports the inhibition efficiency of a family of polyaminoamide (PAMAM) STARBURST ® dendrimers in colloidal SiO 2 growth. PAMAMs of various generations (0.5, 1.5, 2.5 with –COOH termini, and 1, and 2 with –NH 2 termini) were tested and compared to control samples. Superiority of the dendrimer inhibitors with –NH 2 termini (maintaining ∼380 ppm soluble SiO 2 after 12 h) versus those with –COOH termini (maintaining ∼180 ppm soluble SiO 2 after 12 h) was discovered in solutions containing 500 ppm initial SiO 2. These inhibitors present potential for water treatment applications.

H2O and SiO maser emission in Galactic center OH/IR stars

We have performed targeted surveys for 22 GHz H2O and 43 GHz SiO maser emission in Galactic center OH/IR stars using the Very Large Array. Some of the detections have been used in a previous paper to investigate the possibility of measuring milli-arcsecond accurate positions (to obtain stellar proper motions) in the Galactic center. Here we report on the detection of at least 25 H2O masers and 18 SiO masers associated with stars within 2 and 15 0 of Sgr A*, respectively. This survey has more than doubled the total number of proper motion candidates to at least about 50 stellar objects. The stellar SiO masers are best suited for proper motion studies, although the initial OH, H2O or SiO maser positions may not be well enough determined to use for a-priori VLBI positions. Serendipitously we also found 8 H2O masers in the molecular cloud M 0.13 0.08.