Fused Silica Particles Research Articles

Influence of β‐Si3N4 whiskers on sintering and crystallization of fused silica ceramics

AbstractFused silica ceramics are widely applied for radome materials, crucibles, and vanes, but the mechanical properties were deteriorated due to the cristobalite crystallization. The fused silica ceramics added with by β‐Si3N4 whiskers were prepared by a slip‐casting method to retard the cristobalite crystallization. The influences of the sintering environments and the β‐Si3N4 whiskers on the microstructure and phase structure were investigated. The silanol (Si‐(OH)n) and oxygen vacancies (VO) in the fused silica in formed in different conditions were studied by Fourier Transform Infra‐Red (FT‐IR) and X‐ray photoelectron spectroscopy (XPS), and the results indicated that the ball‐milled produced a large amount of the silanol groups onto the surface of the fused silica particles. The fused silica heated in the vacuum created the maximum oxygen vacancies (24.2%) on the surfaces. Silanol groups reacted with the β‐Si3N4 whiskers, and the O atoms in the silanol groups were fixed into the bulk materials. And the crystallization kinetics and the activation energy of Si3N4w/SiO2 ceramics at the temperature ranging from 1200 to 1400°C were calculated based on the JMA(Johnson‐Mehl‐Avrami) model. The activation energy of the fused silica ceramics with the addition of the β‐Si3N4 is 506.2 kJ/mol, increased by 23.6% than that of the pure fused silica ceramic.

Effects of micro‐sized mullite on cristobalite crystallization and properties of silica‐based ceramic cores

AbstractSilica‐based ceramic cores are extensively used in investment casting process, during which they must exhibit sufficient flexural strength and deformation resistance. In this study, micro‐sized mullite was used as an additive to silica‐based ceramic cores to optimize their high temperature properties. To investigate the effects of micro‐sized mullite on cristobalite crystallization, mechanical and thermal properties of silica‐based ceramic cores, ceramic cores with different amounts of micro‐sized mullite were fabricated. The XRD results showed that additional micro‐sized mullite diminished the crystallization of cristobalite at high temperatures, primarily caused by the mullite related compressive stresses on the surface regions of fused silica particles. Three‐point bending tests and SEM results showed that micro‐sized mullite had a more significant effect on the flexural strength of ceramic cores compared with conventional additives. Particularly, the fracture mechanism of silica‐based ceramic cores had been changed from intergranular fracture into a mixed fracture consisting of both intergranular and transgranular fracture. The mechanical and thermal properties of ceramic cores were all reduced slightly as the mullite content exceed 4.6 wt%. Hence, to optimize the properties of silica‐based ceramic cores, the micro‐sized mullite content should not exceed 4.6 wt%.

Investigation of mechanical, thermal and electrical properties of hybrid composites reinforced with multi-walled carbon nanotubes and fused silica particles

In this study, epoxy composites were reinforced with multi-walled carbon nanotubes and fused silica particles, dispersing the fillers within the epoxy resin based on a simple physical method using only shear mixing and ultrasonication. The hybrid composite specimens with 0.6 wt% of carbon nanotubes and 50 wt% of silica particles showed improved mechanical properties, with increase in tensile strength and Young’s modulus up to 12 and 37%, respectively, with respect to those of the baseline specimens. The experimental results showed that the low thermal expansion of the silica particles improved the thermal stability of the composites compared with that of the baseline specimen, whereas the thermal expansion slightly increased, due to the increased heat transfer from the exterior to the interior of specimens by the carbon nanotube filler. The coefficient of thermal expansion of the hybrid composite specimen reinforced with 0.6 wt% of carbon nanotubes and 50 wt% of silica particles was decreased by 25%, and the thermal conductivity was increased by about 84%, compared with those of the baseline specimen.

Fumed silica induces co-continuity across a wide composition range in immiscible polymer blends

We examine ternary blends composed of two immiscible polymers, polyisobutylene (PIB) and polyethylene oxide (PEO), and fumed silica particles which have strong affinity for the PEO. Samples with up to 10 vol% fumed silica and a wide range of PIB:PEO ratios were melt-blended and their morphology characterized. The unusual structure of fumed silica – which comprises 5–50 nm primary particles permanently bonded together into highly porous, fractal-like aggregates – strongly influences the morphology at all compositions. At relatively low PEO loadings, the PEO is absorbed into the pores within the fumed silica aggregates. The morphology consists of a network of interlocked particle aggregates bonded together by PEO. At higher PEO loadings, the PEO and the particles form a combined phase that is solid-like even at low fumed silica loadings. The morphology somewhat resembles a conventional co-continuous morphology where one phase is PIB and the other is a fumed silica-filled PEO. However, the microstructure is very irregular and the interface between the PIB and the PEO phase appears extremely rough on the scale of the primary particles. Morphologies with two percolating phases appear across almost the entire composition range examined. Compared to spherical fused silica particles examined previously, fumed silica affects the morphology of polymer blends at far lower particle loadings, and greatly widens the range of co-continuity.

High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings

A nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction. The effects of the coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and flexural strength of silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the specimens, and the results indicated that the formed nanosized alumina coatings could retard cristobalite formation by inducing compressive stress on the fused silica particle surface. A mullite phase was also found due to the reaction of the nanosized alumina coating and the surface of the fused silica when the sintering temperature was increased to 1300 °C. Analysis using scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) suggested that alumina nanoparticles in the coated layer dispersed into a liquid phase and formed a barrier layer to impede the movement of the liquid phase, preventing the pore-filling process and increasing the open porosity of the ceramic specimens. Flexural strengths at room temperature were tested, indicating that increases in the sintering temperature of the specimens without coated fused silica powders had little effect on flexural strength. However, the flexural strength of the specimens with coated fused silica powders increased with increases in sintering temperature. The improvement in flexural strength was related to the reinforcement by sintering necks between particles and the improvement in the strength of the coated fused silica powder.

Preparation of silica ceramic cores by the preceramic pyrolysis technology using silicone resin as precursor and binder

Abstract A facile and effective preceramic pyrolysis technology was used to prepare the fused silica ceramic cores, among which fused silica powders were used as the raw materials and the silicone resin with -Si-O-Si- as the main chains was used as the precursor. Effects of the silicone resin content and sintering temperature on the properties of fused silica ceramic cores were researched. Results showed that the silicone resin powders had the good cohesiveness, which was beneficial for directly shaping the silica ceramic cores. The organic to inorganic transformation of silicone resin occurred with the increase of temperature, forming the amorphous SiO2 at the temperature less than 1250 °C and the cristobalite by the crystallization process of amorphous SiO2 at the temperature more than 1250 °C. Based on TG analysis, it was found that the ceramic yield (SiO2) of silicone resin had 83.55wt%. In green bodies, the silicone resin was mainly coated on the surface of fused silica particles via the function of pressing force. After the pyrolysis process was completed, the silicone resin formed some small amorphous silica particles, which was coated on the surface of raw fused silica and filled into the gaps between fused silica particles. With the increase of silicone resin content, the apparent porosity gradually increased, and bending strength showed a decreasing trend. The increasing temperature from 1200 °C to 1300 °C promoted the densification of silica ceramic cores when the silicone resin content was the same. However, the highest bending strength was obtained at 1250 °C. Further increasing temperature was detrimental to the bending strength of silica ceramic cores owing to the microcracks caused by the surface crystallization of fused silica to cristobalite.

Optical tweezers for trapping in a microfluidic environment.

Optical tweezers use the force from a light beam to implement a precise gripping tool. Based purely on an optical principle, it works without any bodily contact with the object. In this paper we describe an optical tweezers that targets an application within the framework of nuclear magnetic resonance (NMR) spectroscopy of small objects, which are embedded inside a microfluidic channel that will be integrated in a micro-NMR detector. In the project's final stages, the whole system will be installed within the wide bore of a superconducting magnet. The aim is to precisely maintain the position of the object to be measured, without the use of susceptibility disturbing materials or geometries. In this contribution we focus on the design and construction of the tweezers. For the optical force simulation of the system we used a geometrical optics approach, which we combined with a ray fan description of the output beam of an optical system. By embedding both techniques within an iterative design process, we were able to design efficient optical tweezers that met the numerous constraints. Based on details of the constraints and requirements given by the application, different system concepts were derived and studied. Next, a highly adapted and efficient optical trapping system was designed and manufactured. After the components were characterized using vertical scanning interferometry, the system was assembled to achieve a monolithic optical component. The proper function of the optical tweezers was successfully tested by optical trapping of fused silica particles.

Reduction of [11 C]CO2 to [11 C]CO using solid supported zinc.

A new method for the reduction of no-carrier-added [11 C]carbon dioxide into [11 C]carbon monoxide ([11 C]CO) is described, in which the reductant (zinc) is supported on fused silica particles. Using this setup, which allows for a reduction temperature (485°C) well above the melting point for zinc (420°C), radiochemical yields of up to 96% (decay-corrected) were obtained. A slight decrease in radiochemical yield was observed upon repeated [11 C]CO productions (93±3%, n=20). The methodology is convenient and efficient and provides a straightforward path to no-carrier-added production of [11 C]CO.

Three-dimensional deformations in transparent soil using fluorescent markers

This paper presents details of a new, simple and low-cost technique that allows three-dimensional (3D) movements of individual particles, and particle aggregations, to be observed in geotechnical physical model experiments. This has been achieved by seeding a transparent granular mass formed from sand-sized fused silica particles and a pore fluid with matching refractive index with crushed uranium glass particles, which do not significantly impact transparency. When illuminated by low-power lasers, the particles fluoresce and appear bright green, which greatly increases the sample's texture, and can be easily detected by photography. These tracer particles have been used to obtain two-dimensional soil deformations using both digital image correlation and particle tracking; and by using stereo-photogrammetry, 3D positions and displacements can also be analysed. Displacements were measured through 85 mm of the transparent model, and the technique should allow a single plane to be illuminated and measured at much greater depths. Fluorescent uranium glass tracer particles enable both individual glass particles and particle aggregations to be tracked. The stereo-photogrammetric system developed is shown to be a cost-effective and viable method of quantifying 3D deformations within a transparent soil model.

Influence of fused Silica and chills incorporation on Corrosion, Thermal and Chemical composition of ASTM A 494 M Grade Nickel alloy

A review of a host of relevant literature on the composites leads to some important observations on the gap that prevails for developing the composite with increased strength to weight ratio, improved thermal properties and reduced corrosion rate with the addition of fused SiO2 dispersoid for the nickel based alloy. In the arena of engineering, metallurgists look for techniques to improve the thermal, corrosion and chemical properties of the materials.In this connection an investigation has been carried out to fabricate and evaluate the corrosion, chemical and thermal properties of chilled composites consisting of nickel matrix with fused silica particles (size 40-150 μm) in the matrix. The main objective of the present research is to obtain fine grain Ni/SiO2 chilled sound composite having very good properties. The dispersoid added ranged from 3 to 12 wt. % in steps of 3%. The subsequent composites cast in molds containing metallic and non-metallic chill blocks (MS, SiC & Cu) were tested for their microstructure, chemical, thermal properties and corrosion behavior.

Effect of dispersant on the rheological properties of gelcast fused silica ceramics

Fused silica ceramics with high flexural strength, low porosity, low dielectric constant and loss tangent were fabricated by gelcasting, a near-net shape fabrication technique. Fused silica suspensions with solid loading as high as 73 vol.% with low viscosity has been prepared using various dispersants in acidic and alkaline regions/medium. Commercially available Darvan 821A, Darvan C-N, Dolapix A88 and Dolapix CE64 were used as dispersants. Investigations were carried out to determine the suitable dispersant and effects of dispersant percentage, pH value, zeta potential, and solid loading on the rheological properties of the suspension. Darvan 821A showed better results in the suspension of fused silica particles in aqueous gelcast system. At 1250°C the flexural strength of fused silica bodies is as high as 52.3 MPa, and the dielectric constant and loss tangent (1 MHz) were as low as 3.25 and 1 X L52M0-3 for solid loading of 70 vol.% respectively. Such properties are highly desirable for ceramic radomes used in lower range missiles.

Preparation of functional lupine protein fractions by dry separation

Lupine protein concentrate is a promising ingredient that can be obtained by a combination of milling and air classification, generally called dry fractionation. This is a more sustainable route than conventional wet extraction and delivers a protein concentrate with native functional properties. Critical is the detachment of the protein bodies from other seed components during milling. Ideally, the protein bodies are released during milling, whereas the other components remain in larger particles (D0.5 > 40 μm) to facilitate effective air classification. Coarse milling (down to 100 μm) followed by air classification gave concentrates with protein contents between 54 and 59 g protein/100 g dry solids and yields up to 13%. The application of flowability aids (fused silica particles) during air classification doubled the yield of the protein-rich fraction. The air classified protein concentrate could provide a 2.3 times extended half-life of the foam compared to an intensively heated protein concentrate. In addition, the viscosity of the native concentrate was lower, while after (in vitro) digestion the amount of proteins smaller than 3 kDa was higher in native and mildly heated concentrates compared to intensively heated concentrate. These results suggest promising development of liquid-like formulations from air classified lupine protein concentrates.

Characterization of TC-3 Glass/Fused Silica Composites for LTCC Applications

A glass powder of the SiO2-Al2O3-K2O-Na2O system was added to fused silica particles to form composite green compacts by tape casting. This procedure was able to lower the sintering temperature of the fused silica particles making the composites suitable for being co-fired with the silver-palladium (Ag-Pd) paste commonly used as a conductive circuit in several microwave applications. The resulting new ceramic composite with the composition of 50 wt% fused silica and 50 wt% glass (brand name: TC-3) had a low dielectric constant and a low loss tangent of 2.7 and 2.5-3.7 x10-3 , respectively, and was able to be co-fired with the Ag-Pd conductive paste at the temperature of 895 °C, resulting in a potential low temperature co-fired ceramic (LTCC) system for microwave applications.

Sintering Properties of Fused Silica/Nano-Zirconia Composite Ceramic

Zirconyl chloride was used as zirconium source and fused silica particles were used as main raw material. First of all, the composite powders were prepared by wet chemical synthesis using ammonia as the precipitator and polyethylene glycol as the dispersant. Then, fused silica nanozirconia composite ceramic containing nanometer particle zirconia with different contents (5%, 15%, 25% and 35%) were fabricated in reduction atmosphere at 1300°C, 1350°C and 1400°C for 1 h. The bulk density and bending strength were measured, microstructure was observed by SEM. The result indicated bulk density and bending strength of composite ceramic increase and microstructure becomes denser with content of zirconia increasing. Bulk density of composite ceramic increases and bending strength which reaches maximum at 1350°C firstly increases then decreases with the increase of sintering temperature. Both high sintering temperature and nanozirconia possessing high energy interface can improve the composite ceramic sintering.

Effect of zircon content on chemical and mechanical behavior of silica-based ceramic cores

Abstract In this work, the effect of zircon content on the mechanical and chemical behaviors of injection molded silica-based ceramic cores has been investigated. In order to simulate a casting process condition, the sintered samples at 1220 °C were consequently heated up to 1430 °C. Three point bending tests were carried out on all the prepared samples. The chemical resistance of the prepared cores was evaluated by leaching of samples in 43% KOH solution at its boiling point. Phase evolution and microstructure were investigated by XRD and SEM, respectively. Results showed that increasing zircon content led to an increase in MOR and decrease in leachability owing to the decrease in content of fused silica with lower strength and chemical stability. The leaching rate of sintered ceramic cores was decreased after the simulated casting heat treatment at all zircon contents due to crystallization of cristobalite, with a higher chemical stability, on the surface of fused silica particles.

High Volume Fraction Aluminum /Alumina-Fused Silica Hybrid Particulate Metal Matrix Composite

High volume fraction Aluminum/alumina-fused silica hybrid metal matrix composites containing alumina with 0, 10, 30 and 50 wt% fused silica were produced by melt squeezing casting method. Microstructure of hybrid composite was investigated by optical microscope and scanning electron microscopy (SEM). The SEM images showed uniform distribution of fused silica particles in composite microstructure. Also compressive strength of the composites changed (310-110 MPa) with amount of fused silica.

A Study on Crystallization of Fused Silica/Zirconia Ceramic Composites

Fused silica particles and zirconyl chloride were used as main raw material. Meanwhile, ammonia was used as precipitator and polyethylene glycol as dispersant. Firstly, the composite powders were prepared by wet chemical synthesis. Then, fused silica/zirconia ceramic composites containing zirconia with different contents (5%, 15%, 25%, 35% and 45%) were fabricated in reduction atmosphere at 1300°C, 1350°C and 1400°C for 1 h. The thermal expansion ratios and XRD of samples were examined. The analysis of XRD indicated that the cristobalite peaks intensity of sample with more zirconia is lower at the same sintering temperature, and the intensity of cristobalite was higher while sintered at higher temperature. Zircons were found in all samples’ XRD patterns. The results of thermal expansion ratios showed the ratios of samples with more zirconia were lower, especially at higher sintering temperature. So, existence of zirconia can inhibit crystallization of fused silica/zirconia ceramic composites effectively.

Characterization of ejected fused silica particles following surface breakdown with nanosecond pulses

The light emission produced near the surface of fused silica following laser-induced breakdown on the exit surface was spatially and spectrally resolved. This signal is in part generated by ejected particles while traveling outside the hot ionized region. The thermal emission produced by the particles can be separated from the plasma emission near the surface and its spectral characteristics provide information on the temperature of the particles after ejection from the surface. Assuming the emission is thermal in origin, data suggest an initial average temperature on the order of at least 0.5 eV.

Abrasive and slurry wear behavior of chilled aluminum alloy (A356) reinforced with fused silica (SiO2p) metal matrix composites

This paper describes the fabrication and testing of aluminum alloy MMCs reinforced with fused silica particles cast in sand moulds containing metallic and nonmetallic chills. Fused silica particles of size 50–100μm were dispersed (3–12wt.%) into the matrix. The resulting composites cast were tested for their microstructure, strength, hardness, and wear behavior. Microstructural studies indicate good bonding along with uniform distribution of the dispersoid. Strength, hardness and wear resistance increase up to 9wt.% additions of dispersoid and copper chill was found to be the best because of its high volumetric heat capacity.

Rational design in suspension rheology of cellular silica

Slurry based processing route involving the use of partially hydrophobized fused silica particles, binders, long chain surfactants and drying control additives have been optimized for the fabrication of cellular silica. The major accomplishments of this study are the optimization of the fused silica powder based suspension composition, foaming duration and foam stabilization to prepare cellular silica-based components. Rheological studies have been carried out on the suspensions to examine the effect of solids loading, and volume fractions of individual additives on the viscosity, with the objective of optimizing the slurry composition and viscosity for maximum air entrainment and foam stability. The rheological studies on silica-based suspensions have exhibited that the final properties of the foams depend on the slurry composition. The suspensions prepared in the study have been found to be non-Newtonian, and have shown shear thinning behavior at higher shear rates. The role of viscosity and surface tension has shown a significant effect on the flow behavior, foaming characteristics and stability of the air bubbles in a suspension, which in turn affects the overall pore volume fraction and porous architecture of the finished foams.