Amorphous Silica Powder Research Articles

Structural evolutions of hydrothermally prepared mesostructured MCM-48 silica using differently manufactured amorphous silica powders

The hydrothermal synthesis of Si-MCM-48 mesoporous molecular sieves was carried out using a ternary SiO 2:CTAOH:H 2O system wherein differently manufactured amorphous silica powders such as fumed silica (FMDS), spray dried precipitated silica (SDPS) and flash dried precipitated silica (FDPS) were used as silica source materials. The changes in structural/textural properties were evaluated using powder XRD, N 2 adsorption–desorption and scanning electron microscopy techniques. Studies on the progressive development of MCM-48 mesophases revealed that, the reactivity of the silica source follow the trend: FMDS > SDPS > FDPS. MCM-48 synthesized using low cost FDPS has exhibited thicker pore walls but poorer orderness, while MCM-48 prepared from relatively expensive FMDS has thinner pore walls and more ordered structure. Moreover, the extent of contraction caused by calcinations, agglomerate size and structural stability were found to depend on the reactivity of the silica source used.

ELECTROSPRAYING A NANOPARTICULATE SUSPENSION

This paper reports an investigation into electrospraying a nanosuspension containing nanoparticles sized at 5 nm and suspended in a polymer grade ethylene glycol. Hence, the processing of this nanosuspension in the stable cone-jet mode having an amorphous silica powder loading of 30 wt.% is elucidated. A comparison is made with established literature on jet break-up to identify the break-up mechanism in this investigation. The ensuing electrospray characteristics for both the polymer grade ethylene glycol and the nanosuspension are also presented. Using the data of collected droplet relics together with a volume equivalence method, the generated droplet sizes are estimated for both media and are compared with the well-known theoretical expression for calculating the droplet sizes generated for that respective jet break-up regime and medium. Finally, transmission electron microscopy of the collected deposits concludes the discussion presented in this investigation.

A study of silicon carbide synthesis from waste serpentine

There are 60 000 tons of serpentine wastes produced in year 2004 in Taiwan. This is due to the well-developed joints in the serpentine ore body as well as the stringent requirements of the particle size and chemical composition of serpentine by iron making company. The waste also creates considerable environmental problems. The purpose of this study is reutilization of waste serpentine to produce a high value silica powder after acid leaching. These siliceous microstructure products obtained from serpentine would be responsible for high reactivity and characteristic molecular sieving effect. In this study, the amorphous silica powder was then synthesized to silicon carbide with the C/SiO 2 molar ratio of 3. The experiment results show that silicon carbide can be synthesized in 1550 °C. The formed silicon carbide was whisker β type SiC which can be used as raw materials for industry.

Crystallization of zeolite L from Na 2O–K 2O–Al 2O 3–SiO 2–H 2O system

Zeolite L powder was prepared from the substrate mixture of Na 2O–K 2O–Al 2O 3–SiO 2–H 2O system at temperatures of 373–443 K. In order to investigate the factors which influence the synthesis outcome, a reference system which yields zeolite L in a reproducible manner was chosen and subjected to controlled changes in synthesis parameters. The crystalline zeolite L samples obtained were characterized by elemental chemical analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM). It was established that phase purity, morphology, and the size of crystals of crystalline product were affected by molar ratios of the substrate, such as SiO 2/Al 2O 3, (K 2O+Na 2O)/SiO 2, Na 2O/(K 2O+Na 2O), and H 2O/(K 2O+Na 2O). Amorphous silica powder (Zeosil) was the preferred silica source, and the crystallization rate was promoted by introducing gel aging, seeding, and rapid heating rate.

Analysis of the High-Temperature Cohesion Behavior of Ash Particles Using Pure Silica Powders Coated with Alkali Metals

This paper analyzes the increasing mechanism of cohesive behavior of ash powders at elevated and high-temperature conditions. Model ash powders were prepared from fine pure amorphous silica powder coated and heat-treated on the surface with 0.5 wt % sodium and/or potassium. The cohesive behavior and deformation properties of model ash powder beds were determined by using a new split-type tensile strength tester of powder beds. The adhesion behavior of fly ash collected in a pulverized coal combustion (PC) and a pressurized fluidized bed combustion (PFBC) system was compared with the results of model ash powder samples. In the high-temperature range above 800 °C, a rapid increase of tensile strength and plastic rupture deformation occurred in both the natural ash samples and the model ash powder samples prepared from alkali metal and pure silica powder. It is suggested that the coating alkali metal reacted with the amorphous silica phase and formed a small amount of low-melting-point eutectic materials, such as K2O·4SiO2. Since these eutectic materials formed a small amount of liquid phase at interparticles at about 800 °C, the tensile strength and plastic rupture deformation rapidly increased at high temperatures above 800 °C.

Preparation of high purity, low water content fused silica glass

High purity, low water content silica glasses were prepared by melting crystallized, sol–gel-derived powders. Sol–gel-derived amorphous powders have been used to produce high purity silica glass, but a small quantity of water (hydroxyl) tends to remain in the resulting silica glass. High purity crystalline silica powders were prepared by heat-treating the sol–gel-derived amorphous powders in a silica crucible. Both amorphous silica powders and crystallized silica powders were melted in vacuum at 1800 °C for 1 h. The water content of the glass from amorphous silica powders was 44 ppm while that of the glass from the crystallized silica powder was less than 0.1 ppm.

Rice hull-derived silica: applications in Portland cement and mullite whiskers

Silica was extracted from rice hulls by three processing routes, starting with acid treatment followed by burning and milling. The amorphous white silica powder showed a surface area of 260 to 480 m²/g, purity above 99% and average particle size of 2.0 to 0.6 mm. This silica is suitable for the preparation of mullite whiskers employing the rare earth aluminosilicate glass technique. Due to its highly pozzolanic reaction, this prepared silica is used as an additive in high performance concrete.

Synthesis of silicon nitride fibers from ferrosilicon

Silicon nitride occurs in two crystalline forms: α-Si3N4 and β-Si3N4 and as amorphous material. There is a current interest in making silicon nitride fibers with a high tensile strength for use in composite materials especially, at high temperature (≈1400 ◦C) and in various atmospheres. Chollon et al. [1, 2] have recently produced amorphous silicon nitride (oxide) fibers from a starting mixture of amorphous silica and titanium powders spread over a silicon carbide substrate. The resulting fibers had a diameter of 1–6 μm and an average strength of 2.38 GPa. However, they contained about 11 (at.%) oxygen which is present not as free silica but as an integral component in the silicon oxide nitride phase, hence this is a ternary phase material. Osendi and Miranzo [3] have prepared β-Si3N4 monocrystalline fibers by a self-propagating-high-temperature method (SHS). The fibers showed ribbon like aspects, but no oxygen analysis was given. A simple high temperature method for producing colorless and predominantly amorphous silicon nitride fibers with a very low oxygen content from relatively inexpensive ferrosilicon (FeSi) alloy has been discovered and is reported here. 3 g of ferrosilicon (FeSi) powder (−45 μm) was placed in an alumina crucible and introduced into the working tube of an electrically heated tube furnace. The tube was sealed with silicon bungs and connected to associated gas flow apparatus to allow ammonia to flow through the tube with a flow rate of 80 ml/min. The furnace was heated at a rate of 5 ◦C/min and the sample held at the independently measured temperature of 1370 ◦C for 24 h. This temperature was selected since it is just 40 ◦C below the melting point of FeSi. After cooling to room temperature (5 ◦C/min) the furnace was flushed with argon and the sample examined. A similar experiment was performed in which the sample was heated to the much lower temperature of 400 ◦C. Upon opening the end of the furnace working tube a highly expanded mat of very fine colorless fibers was found in and above the crucible. This mat had a pronounced rubbery texture such that the fibers were held intimately together with very little loose material. In the bottom of the crucible lay the remains of a grey powder with some evidence of partial fusion. By comparison, no reaction was evident from the experiment run at 400 ◦C. The fibers were removed, ground and analyzed by powder X-ray diffractometry using a Philips diffractometer PW1710 with Cu Kα1 radiation (λ= 1.54050 A). The diffraction pattern (Fig. 1) showed two very broad regions between approximately 20–40 and 60–80 ◦2θ which is characteristic of amorphous material and when integrated indicated that this is the major phase. The sharp diffraction peaks in the pattern all correspond to α-Si3N4 (ICDD card 41-360) which is present as the minor phase. No other material was detected. The fibers were analyzed by infra-red spectroscopy and comparison made with a silica glass reference material to see if they contained any silicon-oxygen bonds. Fig. 2 shows clearly the two large infra-red absorbance peaks for the silica glass (≈470 and≈1100 cm−1). The coincidence of the 1100 cm−1 peak with the Si3N4 sample does not exclude the presence of Si-O bonds in the silicon nitride sample, but the absence of the

Various Types of Bubble-Containing Silica Glasses Fabricated by Sintering Powder Mixtures of Silica with Silicon Nitride.

Bubble-containing silica glasses were fabricated by sintering amorphous silica powders with small amount of silicon nitride powders. Sintering was performed at 1800°C in N2 gas atmosphere. It was found that the addition of Si3N4 is effective for forming bubbles. By controlling the Si3N4 content, various types of bubble-containing silica glasses, such as the so-called the opaque silica glass and the foamed silica glass, could be obtained. Addition of 0.004mol% Si3N4 produced highly opaque silica glasses containing a large number of small bubbles, while 0.431mol% Si3N4 resulted in low-density foamed silica glasses containing large 1mm-sized bubbles. Wide variations of bubble size and density were observed on these glasses. Thermal conductivities were estimated and compared with several literature values. The relation between thermal conductivity and bubble density was discussed based on a model simulation.

Correlation between microstructure and electrical properties of tungsten-silica nanocomposites

The electrical properties and the microstructure of nanocomposite materials composed of nano-sized amorphous silica powder and micron-sized tungsten powder were studied as a function of composition and heat treatment temperature. The dielectric constant and the electrical conductivity of the nanocomposites are higher than those of pure silica powder at all compositions. The percolation threshold of this system is about 20% vol. W. Above this composition a sharp increase in the dielectric constant and in the electrical conductivity occurs. This increase becomes even sharper after the nanocomposites are heat treated at 950°C for 1 hour. No changes in the microstructure are observed up to this temperature. It is suggested that the electrical properties of the nanocomposites are dominated mainly by dangling bonds at the W/SiO 2 interfaces. Further research should be undertaken to investigate the factors controlling the dielectric constant and the electrical conductivity in order to obtain high capacitance materials with high dielectric constant and low electrical conductivity.

Vitrification of radioactive waste by reaction sintering under pressure

Silicate nuclear waste glasses were synthesized by reaction sintering of powdered precursors under pressure. The glass samples contained a glass matrix phase with embedded zirconia (baddeleyite) particles. A waste composition with 38 wt% of ZrO 2 was prepared with a waste loading of 30–50 wt% at 800°C and 28 MPa, by hot isostatic pressing. The glass former was commercial amorphous silica powder to which simulated waste was added as calcined oxides. Phase compositions and microstructure of the sintered glass samples were characterized using scanning and analytical electron microscopy. The results show that extensive sintering took place and that a continuous glass phase was formed, particularly at higher waste loading. Waste components such as Na 2O, CaO, MnO 2, La 2O 3, Fe 2O 3, Cr 2O 3, and P 2O 5 dissolved completely in the glass phase. ZrO 2 was also dissolved but recrystallized from the glass as aggregates of baddeleyite crystallites surrounding the original silica particles. MCC-1 type chemical durability tests showed that the glasses are durable with dissolution rates similar to or lower than that of the highly durable French R7T7 borosilicate glass. This glass contains 13 wt% high-level radioactive waste from light water reactor fuel reprocessing and has a melting temperature of 1150°C. The long-term chemical durability of our sintered glasses is expected to be as high as that of rhyolitic glasses, based on hydration energies of 3.7 and 3.3 kJ/mole, respectively. Rhyolitic glasses show little alteration over geological periods of time with a typical corrosion rate of 1 μm/1000 yr.

Dissolution study of potassium feldspars using hydrothermally treated sanidine as an example

The surfaces of sanidine grains (KAlSi3O8 ), leached in acid solutions (pH 1) in the presence of amorphous silica powder, have been examined by SIMS, XPS, Fourier transform infrared spectroscopy (FTIR), SEM, transmission electron microscopy (TEM) and electron probe microanalysis (EPMA). Results reveal a layer at the mineral's surface that is depleted in Na, K and Al in comparison to the bulk material. The purpose of this study was to perform a detailed characterization of the altered surface layer and to decide whether this layer was formed by actual leaching of the mineral's surface or/and by precipitation of silica on the surface. The thickness of the residual hydrated layer has been observed to increase with elevated reaction times and temperatures. The amount of dissolved silicon plays an important role in the lifetime of this layer. Diffusion coefficients calculated from the K/Si SIMS depth profiles yielded realistic numbers for the out-diffusion of K at the fresh sanidine/altered layer boundary. Fourier transform infrared spectroscopy analyses reveal that a large part of the H signal observed in the SIMS profiles is adsorbed water, although some hydrogen is preferentially bound to AlO. The amount of adsorbed H2O, together with the increasing number of etch pits, supports the hypothesis that penetration of molecular water is a key step in the dissolution process of feldspar minerals. Transmission electron microscopy allows imaging of the contact region between the crystalline sanidine and the amorphous surface layer: the observations strongly support the hypothesis of an Na, K, Al-depleted layer. Secondary ion mass spectrometry measurements were also carried out on sanidine samples leached in the presence of isotopically enriched, amorphous silica powder. This allowed confirmation that the Na, K, Al-depleted layer was at least partly formed by leaching but also showed an exchange of silicon between solution and mineral. © 1999 John Wiley & Sons, Ltd.

Enhanced sintering of iron by addition of amorphous silica powder

Enhanced sintering of iron by addition of amorphous silica powder

Vitrification of High-Level Radioactive Waste by Sintering Under Pressure

ABSTRACTSintered silicate glass waste forms with a continuous glass phase and embedded but not completely dissolved waste particles were prepared by powder technology. We used a typical defense waste compositions, containing 38 wt.% of ZrO2 and prepared sinter glass samples with 30, 35, and 40 wt.% waste loading at 830°C and 27.6 MPa, by hot isostatic pressing. The glass former was amorphous silica powder. Simulated waste was added as calcine. Samples were characterized using scanning and transmission electron microscopy. The results show that extensive sintering took place, particularly with the 40 wt.% samples. Waste components, such as Na2O, CaO, MnO2, La203, Fe2O3, NiO, Cr2O3, and P2O5, dissolved completely in the continuous glass phase. ZrO2 was partly dissolved and occurred as aggregates of tiny baddeleyite crystals. The porosity of the samples was 2-3%, indicating that the optimum sintering conditions have not been met to reach porosities <1%.Chemical durability tests were carried out with sintered glass with 35 wt.% waste loading in deionized water at 90°C under static conditions, showing that the glass is at least as chemically durable as a borosilicate glass with 31 wt.% melted at 1350°C.

Microwave sintering of amorphous silica powders

The objective of this work was to further study the microwave (MW) processing of noncrystalline materials by examining the densification of amorphous silica powder compacts heated by exposure to MW fields.

Contributions to the structural and chemical composition of plasma-chemically synthesized silica powders

Ultrapure amorphous silica particles of size < 4 nm were generated in a capacitively coupled radiofrequency argon plasma (CCP) by reaction of silicon tetrachloride with oxygen. The particles were analyzed mainly by means of transmission electron microscopy, thermogravimetry and infrared spectroscopy. They were deposited within the reactor at high quenching rates of the plasma-activated species and formed agglomerates with rugged, reactive surface areas as well as with a great number of internal structure bonded hydroxyl groups. These properties of the particles point to a higher solubility of amorphous plasma silica powders which is advantageous to their availability for the synthesis of molecular sieves.

Dispersion of alumina and silica powders in non-aqueous media: Mixed-solvent effects

Colloidal amorphous silica and α-alumina powders were dispersed in organic media (non-aqueous solvents and dispersants) and stabilized by electrostatic, and steric, or electrosteric repulsive forces. The effects of variables such as the physicochemical properties of both solvent media and dispersants, the proportions of mixed solvent media, etc., were studied. The electrostatic contribution to the stabilization was shown to be much greater than expected. The electrostatic, steric, and electrostatic interactions between particles could be tailored by changing the organic-liquid media.

Synthesis of nitrogen ceramic povvders by carbothermal reduction and nitridation Part 3 Aluminium nitride

AbstractSilicon nitride powders of high α content were prepared from mixtures of very fine amorphous silica and carbon powders in flowing nitrogen at 1400–1500°C. The higher the reaction temperature within this range the lower the yield of silicon nitride in spite of a higher rate of SiO generation. The overall reaction rate increased with increasing nitrogen flowrate. An increase of carbon content of the starting mixtures increased the reaction rate by increasing the rate of SiO formation as well as the rate of Si3N4 nucleation. It is thought that the effect of the carbon particle size on the reaction rate is more significant than that of the specific surface area. The amount of silicon carbide formed and the α/β ratio in the reaction products were strongly affected by a change of the source of carbon used in the starting mixtures.MST/1346a

Synthesis of nitrogen ceramic povvders by carbothermal reduction and nitridation Part 1 Silicon nitride

Silicon nitride powders of high α content were prepared from mixtures of very fine amorphous silica and carbon powders in flowing nitrogen at 1400–1500°C. The higher the reaction temperature within this range the lower the yield of silicon nitride in spite of a higher rate of SiO generation. The overall reaction rate increased with increasing nitrogen flowrate. An increase of carbon content of the starting mixtures increased the reaction rate by increasing the rate of SiO formation as well as the rate of Si3N4 nucleation. It is thought that the effect of the carbon particle size on the reaction rate is more significant than that of the specific surface area. The amount of silicon carbide formed and the α/β ratio in the reaction products were strongly affected by a change of the source of carbon used in the starting mixtures.MST/1346a

Muonium reaction with ethylene on powder surfaces

The reaction rates of muonium with ethylene adsorbed on the surface of amorphous silica powder grains were measured between 6 K and 300 K. kf values characterizing the two-dimensional surface reactions and the 2 1/2 dimensional reactions with muonium in the gas phase were obtained.