Excess Silica Research Articles

ChemInform Abstract: Coffinite, USiO4, Is Abundant in Nature: So Why Is It So Difficult to Synthesize?

AbstractConditions are established under which coffinite can be reproducibly synthesized.

Effect of Autoclave Curing on the Microstructure of Blended Cement Mixture Incorporating Ground Dune Sand and Ground Granulated Blast Furnace Slag

Investigating the microstructure of hardened cement mixtures with the aid of advanced technology will help the concrete industry to develop appropriate binders for durable building materials. In this paper, morphological, mineralogical and thermogravimetric analyses of autoclave-cured mixtures incorporating ground dune sand and ground granulated blast furnace slag as partial cementing materials were investigated. The microstructure analyses of hydrated products were conducted using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), differential thermal analysis (DTA), thermo-graphic analysis (TGA) and X-ray diffraction (XRD). The SEM and EDX results demonstrated the formation of thin plate-like calcium silicate hydrate plates and a compacted microstructure. The DTA and TGA analyses revealed that the calcium hydroxide generated from the hydration binder materials was consumed during the secondary pozzolanic reaction. Residual crystalline silica was observed from the XRD analysis of all of the blended mixtures, indicating the presence of excess silica. A good correlation was observed between the compressive strength of the blended mixtures and the CaO/SiO2 ratio of the binder materials.

Coffinite, USiO4, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Coffinite, USiO4, is the second most abundant U(4+) mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

Experimental evidence of bulk chemistry constraint on SiO2 solubility in clinopyroxene at high-pressure conditions

We have experimentally confirmed that the solubility of SiO2 in clinopyroxene at ultrahigh-pressure metamorphic conditions is buffered by coesite and kyanite. The present findings were derived from high-pressure experiments on metapelite glass, powdered andesite and eclogite glass under anhydrous conditions. The metapelite glass and powdered andesite were recrystallised in boron nitride capsules at 8GPa and 1100–1500°C. The eclogite glass was heated in an AuPd capsule, both ends of which were welded, at 3GPa and 1000°C.Clinopyroxene nucleated from metapelite glass, the bulk composition of which is saturated in both SiO2 and Al2SiO5 components plotting within the Jd (Na,K)(Al,Cr)(Si,Ti)2O6−Qtz(Si,Ti)O2−GrtM3(Al,Cr)2(Si,Ti)3O12−Als(Al,Cr)2(Si,Ti)O5 tetrahedron (M=Fe,Mn,Mg,Ni,Zn,Ca), coexists with garnet, coesite and kyanite. The average excess silica content of the clinopyroxene ranges from 23.4 to 35.4mol%. In contrast, an andesite experiment saturated in SiO2 but undersaturated in Al2SiO5 within the Jd–Qtz–Aug M(Si,Ti)O3–Grt tetrahedron produced clinopyroxene, garnet and coesite but no kyanite. The average excess silica in the clinopyroxene was 9.7–15.5mol%, which is comparable to previous experimental data. Experiment on the eclogite glass with similar composition to andesite yielded clinopyroxene, garnet and coesite. An average excess silica content in clinopyroxene counts 6.4mol%, which is much lower than that obtained from the andesite.The SiO2 content of clinopyroxene coexisting with garnet, coesite and kyanite is much higher than that of clinopyroxene coexisting with garnet and coesite without kyanite. Although the temperature dependence is unclear, the SiO2 solubility increases with pressure and Fe/(Fe+Mg). Clinopyroxene forms the solid solution series Jd–Es □0.5M0.5Al(Si,Ti)2O6 and Aug–Es, rather than Jd–Ts MAl2(Si,Ti)O6 and Es–Ts joins. Our experimental data suggest the probable existence of octahedral Si which may accompany the M2 vacancies in clinopyroxene.

A new UHP metamorphic complex in the 1.8 Ga Nagssugtoqidian Orogen of West Greenland

The Nagssugtoqidian Orogen is a ca. 1.8 Ga belt of east-west trending, highly deformed rocks that bisects central Greenland. Although a variety of data have suggested this belt marks the location of a continent-continent collision zone, evidence of subduction has been lacking. We report here mineralogical evidence from four samples within a well-defined lithologic unit of metabasic and metasedimentary ocean floor rocks of a previously unrecognized UHP metamorphic episode. The UHP episode is recorded by remnants of orthopyroxene exsolved from majoritic garnet, graphitized diamond, exsolution of rutile from garnet and pyroxenes, exsolution of magnetite from olivine, and complex exsolution textures in ortho- and clinopyroxenes (including omphacite). Associated with these mineralogical features is an unusual occurrence of quartz needles in Mn-rich fayalite. From textural characteristics, we infer that the quartz needles exsolved from the fayalite. To our knowledge, olivine with exsolved silica has not been reported. We note, however, that experimental studies have shown that b-spinel can incorporate excess silica. We therefore speculate these quartz needles may be silica that exsolved from Mn-rich ahrensite, the Fe analog of ringwoodite, upon decompression and inversion to fayalite. If correct, this occurrence would be the first reported sample of naturally occurring olivine (fayalite) that inverted from ahrensite. Corroborating an early UHP history are reaction relationships that delineate a path through high-pressure and high-temperature conditions during decompression. P-T conditions inferred for the UHP episode are ~7 GPa at ~975 °C. The unusually low T for this UHP system at ~1.8 Ga may reflect either very rapid subduction rates at that time, or unexpectedly cool mantle conditions. Preservation of the UHP assemblages probably is due, in large part, to the exceptionally low a H2O during decompression and cooling. These UHP rocks establish that the location of the subduction and suture zones that must have existed prior to and during the collision of continents was along what is now the northern edge of the Nordre Strømfjord shear zone.

Densification behavior and microstructure of mullite obtained from diphasic Al2O3–SiO2 gels

Stoichiometric and silica-rich diphasic Al2O3–SiO2 gels were prepared through a sol–gel process. Structural evolution of the gels with heat-treatment up to 1300°C was carried out by combination of differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. Results showed that the homogeneity level of the gels was the determining factor in the phase transformation sequence and the mullite formation temperature. For the gels with high homogeneity level, the crystallization to mullite occurred at low temperature. The presence of ammonium nitrate in the gels promoted the initial formation of alumina–silica spinal at about 950°C, which further reacted with free silica to form elongated orthorhombic mullite grains at about 1250°C.Linear shrinkage, density measurement and scanning electron microscopy (SEM) analyses were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 200°C. It was found that the homogeneity level and amount of silica both have profound effects on the densification behavior. For the gels with high homogeneity level and thus low mullite formation temperature, the densification behavior was worse, owing to the overlapping with the temperature for densification by viscous flow and reaction sintering. In addition, the inhibitory effect for the densification of elongated mullite grains was stronger than that of equaxial ones. Herein, the derived ceramics containing elongated mullite grains and an excess amount of silica sintered to higher densities, since the presence of excess silica promoted the densification by viscous phase sintering.

Phosphorus burial and diagenesis in the central Bering Sea (Bowers Ridge, IODP Site U1341): Perspectives on the marine P cycle

To reconstruct the cycling of reactive phosphorus (P) in the Bering Sea, a P speciation record covering the last ~4Ma was generated from sediments recovered during Integrated Ocean Drilling Program (IODP) Expedition 323 at Site U1341 (Bowers Ridge). A chemical extraction procedure distinguishing between different operationally defined P fractions provides new insight into reactive P input, burial and diagenetic transformations. Reactive P mass accumulation rates (MARs) are ~20–110μmol/cm2/ka, which is comparable to other open ocean locations but orders of magnitude lower than most upwelling settings. We find that authigenic carbonate fluorapatite (CFA) and opal-bound P are the dominant P fractions at Site U1341. An overall increasing contribution of CFA to total P with sediment depth is consistent with a gradual “sink switching” from more labile P fractions (fish remains, Fe oxides, organic matter) to stable authigenic CFA. However, the positive correlation of CFA with Al content implies that a significant portion of the supposedly reactive CFA is non-reactive “detrital contamination” by eolian and/or riverine CFA. In contrast to CFA, opal-bound P has rarely been studied in marine sediments. We find for the first time that opal-bound P directly correlates with excess silica contents. This P fraction was apparently available to biosiliceous phytoplankton at the time of sediment deposition and is a long-term sink for reactive P in the ocean, despite the likelihood for diagenetic re-mobilisation of this P at depth (indicated by increasing ratios of excess silica to opal-bound P). Average reactive P MARs at Site U1341 increase by ~25% if opal-bound P is accounted for, but decrease by ~25% if 50% of the extracted CFA fraction (based on the lowest CFA value at Site U1341) is assumed to be detrital. Combining our results with literature data, we present a qualitative perspective of terrestrial CFA and opal-bound P deposition in the modern ocean. Riverine CFA input has mostly been reported from continental shelves and margins draining P-rich lithologies, while eolian CFA input is found across wide ocean regions underlying the Northern Hemispheric “dust belt”. Opal-bound P burial is important in the Southern Ocean, North Pacific, and likely in upwelling areas. Shifts in detrital CFA and opal-bound P deposition across ocean basins likely occurred over time, responding to changing weathering patterns, sea level, and biogenic opal deposition.

Geology and diagenesis of a zeolitic Foça tuff unit deposited in a Miocene phreatomagmatic lacustrine environment (western Anatolia)

This study explains the relations between the depositional environment of a zeolitic tuff unit and its diagenesis. It gives new ideas of the juvenile phreatomagmatic origin of the zeolitic unit with its bentonitic cap rock, and finds that the diagenetic alteration of the volcanic ash deposition in a hot hydrologic system is penecontemporaneous and not epigenetic. A massive, fine-grained zeolitic unit has a sharp contact with the underlying shallow subaqueous rhyolitic dome intrusions and their surrounding volcanoclastic ejecta. Juvenile emanations from the basal subaqueous intrusions activated thorough zeolitic diagenesis within the overlying rhyolitic tuff deposits extending as far as the periphery of the underlying intrusions. The bentonitic cap rock suggests that the diagenesis diminished vertically with the weakened phreatomagmatic activity up to the overlying limestone. The lack of sedimentary evaporite minerals and scarcity of boron-bearing authigenic K-feldspar indicate a nonsaline-alkaline depositional and diagenetic environment during the zeolitic transformation. Geochemical data from the zeolitic tuff samples indicated that the main diagenetic factors were hydrolysis of the glassy tuff in an open hydrologic system, under high heat flow rates and one of several scales of ion transfer. Zeolitisation developed with a significant loss of alkaline elements and iron oxide, which were compensated for by an important gain in the alkaline earth elements and absorption of strontium. The rhyolitic glass was altered by hydrolysis to form smectite and clinoptilolite, resulting in the release of excess silica that was not removed from the system but was changed in crystal form to opal-CT.

Anti-citrullinated protein antibodies in rheumatoid arthritis: a bridge between genetic predisposition and autoimmunity

Anti-citrullinated protein antibodies in rheumatoid arthritis: a bridge between genetic predisposition and autoimmunity

Age-differentiated subduction regime: An explanation of regional scale upper mantle differences beneath the Alps and the Variscides of Central Europe

The upper mantle beneath the Alps and the Variscides of Central Europe has a varied seismic structure as a result of the accretionary and evolutionary processes that have shaped it. Natural earthquake data, the raypaths of which pass beneath these regions, have enabled the calculation of travel times out to 3000km. The source-receiver geometry ensured there was broadband station coverage at offsets between 900 and 3000km, enabling the detection of the appropriate upper mantle phases over an azimuthal span embracing the studied structures. The resulting data allowed us to model first order discontinuities to a depth of about 420km. We have used modelling based on ray-tracing in a cylindrical coordinate system reflecting the geometry of the wave propagation medium. The data allow determination of the thickness of the upper mantle low velocity zone (LVZ) beneath the Alpine orogen. It appears to be thicker than beneath surrounding areas, with its upper ‘lid’ at a depth of about 90km and its base, the ‘Lehmann discontinuity’, being depressed to 220km. Such thickening of the LVZ is typical below young orogens. A seismic discontinuity at a depth of around 300km, with the P-wave velocity increasing to 9km/s, has been observed for all the sub-Alpine raypaths. We consider this discontinuity to have originated as a result of Alpine orogen subduction. This is responsible not only for changes in the thermal field as a result of transporting colder material to depths probably exceeding 200km but also for delivering the necessary amounts of excess silica required for the coesite–stishovite phase transition. For rays passing beneath the Variscides, there is a scatter in travel times over the 8–15° distance range. However, the scattering does not indicate significant thickening of the LVZ as identified beneath the Alpine orogen, and the Lehmann discontinuity at the base of the LVZ does not exceed 200km depth. Also, in contrast to the Alps, no high velocity phases are observed for a discontinuity near 300km depth. This supports our contention that the 300-km discontinuity is a regional feature ascribed to the subduction regime beneath the younger orogen. For all azimuths from which structures beneath the Alps are illuminated, we clearly observe both the refraction as well as the reflection branch from the 410-km discontinuity.

Effects of Silica Sol on Structure and Properties of Core-Shell Silicon-Acrylic Materials

Silica sol prepared by sol-gel method was introduced into poly (butyl acrylate) (PBA)/poly (butyl acrylate-styrene-methacryloxypropyl trimethoxysilane) (PSBM) core-shell emulsions to prepare a series of paper surface sizing agents. The rheological measurement indicated that PSBM emulsions exhibited shear-thinning behavior, and the phenomena became more pronounced with increasing silica sol concentration. Dynamic mechanical analysis (DMA) demonstrated that the stronger interfacial interaction between silica sol and polymer matrix, but microphase separation took place with excess silica sol. Thereby the tensile strength and thermal stability of emulsion films were increased with desirable silica sol concentration, and when silica sol concentration was greater than 6 wt%, the tensile strength leveled off and the decomposition temperature decreased from 351.19℃ to 331.63℃. The degree of crystallinity increased from 5.12% to 10.98% with 4% silica sol addition, resulting in enhanced rigidity of films. Furthermore, the interaction between polymer and fiber was improved with certain amount of silica sol, resulting in improved sizing degree, ring crush strength, surface strength and folding strength. However, excessive crosslinking will be harmful for the properties of sized paper.

The role of polymethylhydrosiloxane in the sol-gel synthesis of high surface area porous silicon carbide

Abstract High surface area porous silicon carbide was synthesized via a modified sol-gel method. In the sol-gel method, furfuryl alcohol and tetraethoxysilane were used respectively as carbon and silicon precursors, polymethylhydrosiloxane (PMHS) was employed as a pore-adjusting agent for preparing a carbonaceous silica xerogel. SiC was obtained by the carbothermal reduction of the carbonaceous silica xerogel at 1 300 °C in argon flow and then purified by removing excess silica, carbon and other impurities. Depending on the amount of PMHS added in the sol-gel process, the SiC products are found to have high specific surface area ranging from 120 to 167 m2 g−1 and pore volume ranging from 0.49 to 1.01 cm3 g−1. It is therefore suggested that PMHS enhances the production of mesoporous pores in the SiC products.

Rare-Earth Disilicates As Oxidation-Resistant Fiber Coatings for Silicon Carbide Ceramic-Matrix Composites

Current SiC-based ceramic–matrix composites (SiC–SiC CMCs) rely on carbon or boron nitride fiber–matrix interphases for toughness and flaw tolerance. However, oxidation of these interphases can be performance limiting in many CMC applications. The γ-polymorph of the rare-earth disilicates (RE2Si2O7) is a potential oxidation-resistant alternative to carbon or BN. The formation of γ-Y2Si2O7 and γ-Ho2Si2O7 at different temperatures and processing environments was investigated. Silica–yttrium hydroxide and silica–holmium hydroxide dispersions were made and heat treated at 1200°–1400°C for 8 h in air and argon. LiNO3 was added to the dispersions to enhance the formation of γ-Y2Si2O7 and γ-Ho2Si2O7. The effects of excess silica and LiNO3 dopant on the formation of γ-Y2Si2O7 were investigated. Coatings of Y2Si2O7 and Ho2Si2O7 were made on α-SiC plate and SCS–0 SiC fiber using these dispersions. These were heat treated in argon and argon—500 ppm oxygen mixtures at 1400°C/8 h. For coatings heat treated in argon—500 ppm oxygen mixtures, X-ray diffraction showed the formation of single phase γ-Ho2Si2O7 and a mixture of γ and β-Y2Si2O7 at 1400°C. Scanning electron microscopic image analysis gave an estimate of 18 vol% of excess silica for γ-Y2Si2O7 formed with high Si:Y ratio and ∼5 vol% excess silica for material formed with lower Si:Y ratio. Transmission electron microscopy of samples directly beneath indentations showed both extensive dislocation slip and fracture.

Influence of silica content in sulfonated polysulfone/ phosphotungstic acid hybrid membranes on properties for fuel cell application

AbstractThe sulfonated polysulfone (SPSU)/phosphotungstic acid (PWA)/silica (SiO2) hybrid membranes were researched for proton exchange membranes. The influence of silica content on the properties of composite membranes was studied in detail. The study displays that inorganic silica particles have a great influence on membrane properties, such as membrane durability, PWA extraction, proton and methanol transport behavior, thermal stability and water uptake. Interestingly, the hybrid membrane with 2 wt.% silica content showed the best results: low PWA extraction, long membrane life time under oxidative condition, high proton conductivity, and low methanol permeability. However, excessive silica content caused a large aggregation of SiO2 particles, leading to the deterioration of membrane properties.

Synthesis of High Surface Area Porous Silicon Carbide by Employing Soft Template in the Sol-Gel Process

High surface area porous silicon carbide was synthesized by a modified sol-gel method. In the sol-gel method, furfuryl alcohol and tetraethoxysilane were used respectively as carbon and silicon precursors for preparing a carbonaceous silica xerogel. Polymethylhydrosiloxane (PMHS) was employed as pore-adjusting agent in the sol-gel process. SiC was obtained by the carbothermal reduction of the carbonaceous silica xerogel at 1300 oC in argon flow and then purified by removing excess silica, carbon and other impurities. XRD、FTIR、SEM、HRTEM and BET were used to characterize the SiC samples. The results show that the SiC products are found to have high specific surface area of 135 m2 /g. PMHS has important effect on the surface area, pore volume of the SiC products. It is therefore suggested that PMHS plays the role of structure-directing agent that enhances the production of mesoporous pores in the SiC products.

Effects of solid particle content on properties of o/w Pickering emulsions

The control of droplet size and stability of oil-in-water Pickering emulsions stabilized by hydrophobized fumed silica was investigated. Three regimes were observed according to the silica content: instability at low silica content, stable emulsions with droplet size controlled by the silica content, and emulsions of constant size set by the emulsification process at high silica concentrations. The oil-to-silica ratio was the relevant parameter of the size control in the medium concentration regime. Centrifugation experiments and particle size distribution measurements gave evidence of the presence of excess silica present as dispersion in the aqueous phase in the high silica content regime. Adsorption of silica to the droplet surface did not follow adsorption equilibrium; strong adsorption prevailed. Lastly, aggregation of silica particles appeared a crucial parameter. Oil adsorption and capillary condensation of oil within the silica aggregates provided a supplementary mechanism of silica aggregation that contributed to the stability of emulsions.

Fluid+melt intrusions in lechatelierite from the Popigai suevites: A product of dynamic interaction melts and fluids during the shock melting of the target gneiss

The paper presents data on lechatelierite form suevites of the Daldyn Formation in the Popigai astrobleme. Some of the lechatelierite samples show a complicated structure and contain block of diaplectic quartz glass and dynamic “intrusions” of glasses of types I, II, and III. The glasses of types I and II abound in fluid inclusions and display evidence of partial homogenization with lechatelierite. The glasses of type III are clearly separated from all other glasses but show evidence of dynamic interaction with them in the molten state. Fluid inclusions in the glasses of types I and II are syngenetic but have notably different densities from those of completely liquid or gaseous inclusions at 20°C. As is indicated by cryometric data, the liquid phase of the inclusions is aqueous solution of low salinity (5–8 wt % NaClequiv). The bulk petrochemistry of the glasses of type I characterizes them as highly silicic (96.04 wt % SiO2 on average), with elevated K and Na concentrations (Na2O + K2O = 0.72 wt % on average), with 0.73 wt % Al2O3 (on average) and analytical totals 1.97 wt % less than 100%. The glasses of type II are also rich in SiO2 (91.51 wt % SiO2 on average) but contain a broader spectrum of concentrations of major oxides (totaling 5.53 wt % on average) and deficient analytical totals (by 2.96 wt % on average). The glasses of type III are completely equal to impactites produced by melting gneisses of the Popigai astrobleme. The glasses of type I are interpreted to be the intrusion products of the “early” highly mobile and H2O-rich fluid+melt mixtures, whose protolithic material was K-Na feldspars of the target rocks. The derivation of these melts was associated with the capturing of much silica and water at a highly mobile behavior of K and Na and an inert behavior of Al. The glasses of type II were produced by the extensive mixing of silica and water at the limited involvement of apogneiss melts, and these glasses are sometimes deficient in Al. The glasses of type III are usual mixed apogneiss melts. Excess silica in the glasses of types I and II and their richness in water and deficiency in Al suggest impact anatexis and the selective separation of components during their derivation; the parental fluid-melt mixtures of these glasses were derived from such “hydrous” varieties of the target gneisses as diaphthorized and fractured rocks. The evolution and partial vitrification of lechatelierite and the glasses of types I and II proceeded under residual shock pressures, as follows from data on the dense (from ∼0.5 to 1 g/cm3) aqueous inclusions in these glasses, which suggest that the inclusions were captured in the glasses under pressures from ∼0.8 to 3.3 GPa. It follows that our lechatelierite samples have a complex multistage genesis, and their quenching facilitated the preservation of “intrusions” of various stages of shock melting, including the products of the “early” impact anatexis of the gneisses with the selective separation of components at the active participation of water.

Silica Gel as a Surrogate for Biogenic Silica in Batch Dissolution Experiments at pH 9.2: Further Testing of the Shrinking Object Model and a Novel Approach to the Dissolution of a Population of Particles

Recently, the increase in dissolved concentration in the batch dissolution of various salts or sucrose has been successfully modelled with three equations, one a cubic in time. However, from three separate earlier investigations with ocean sediments and phytoplankton frustules, there is residual suspicion that biogenic silica does not follow this behaviour. This paper shows that the Shrinking Object Model applies to the dissolution of sieved silica gel particles, as well as to a sample of unsieved, freeze-dried frustules of Odentella sp. Silica gel, being readily available in quantities that can be sieved, is a useful surrogate for biogenic silica in allowing problems of experimental design to be overcome. The dissolutions covered three possible analytic integrations that arise from the model: an exponential for approach to saturation with excess solid silica; the approach to near saturation with either a slight excess or deficiency of silica; dissolution at high under-saturation. Good agreement was found between experimental results and mathematical modelling. The paper provides template calculations by which future raw results can be parameterized. Nevertheless, the reasons for non-linear kinetics reported in earlier work have not been identified, and so controversy over non-linear dissolution kinetics is enhanced. Stirring regime was found to be important with silica gel dissolution, and so biogenic silica dissolution is therefore likely to be ‘transport limited’ at low stirring rate. Accordingly, all archived and future data should be scrutinized for stirring effects before being applied to the oceanic environment. A rigorous test for determining whether a substance’s dissolution deviates from the model is recommended as a preliminary to any future dissolutions, whether in batch or with the chemo-stat. A fixed amount of frustule sample is added to a series of buffered mixtures containing increasing background silicic acid concentrations. Absence of any problem is marked by a linear plot between the increase in silicic concentration accruing over a fixed reaction period and that of the background silicic acid. A novel mathematical proof is provided to justify the test’s use. The reasons for the earlier deviations from expected behaviour of, for example, oceanic sediments, are discussed. Lastly, the paper provides a novel approach to the dissolution of a population of particles of mixed sizes which will probably find ready future application in oceanography.

PDMS–silica composite membranes with silane coupling for propylene separation

Polydimethylsiloxane (PDMS) coated membranes on a polysulfone (PSf) support were fabricated for propylene recovery from off-gas stream. PDMS coated membrane, however, showed trade-off trends that permeability and selectivity are inversely proportional to each other. Addition of silica nanoparticles of 12–400 nm size into PDMS matrix enhanced both separation factor and propylene permeance from 5.2 to 7.3 from 31 to 36 GPU, respectively. Silica nanoparticles in composite membrane tended to agglomerate with each other. Coupling of silica nanoparticles with a silane coupling agent modified the silica surface to enhance the compatibility with PDMS. Several coupling agents for silica nanoparticles in the PDMS/organic solvent system were tested and mercapto silane showed the best characteristics as a coupling agent in terms of hydrophobicity and thermal stability. Successful coupling was confirmed by FT-IR, and better dispersion of coupled silica nanoparticles in PDMS matrix was attained. Composite membrane with coupled silica nanoparticles still enhanced separation factor up to 8.5 and propylene permeance up to 42 GPU. Effects of size and amount of the silica nanoparticles were examined by comparing fumed silica and sol–gel silica. Fumed silica enhanced the sorption of propylene into membrane matrix to increase the separation factor maintaining the propylene permeance. Sol–gel silica disrupted chain packing to increase the propylene permeance maintaining the selectivity. Increase of silica content enhanced the separation factor as well as propylene permeance. However, excessive silica loading in PDMS solution caused problems in coating process, and optimum silica nanoparticle content was 15 wt%. This work showed the novelty in preparation of organic–inorganic composite membranes with enhanced permeance and selectivity by properly coupling the silica particles before loading.

Aluminum Phosphate–Mullite Composites for High‐Temperature Radome Applications

An AlPO4–SiO2 powder with a composition of Al:P:Si=1.5:1:0.1 was synthesized by the sol–gel method using aluminum nitrate, phosphate acid, and tetraethoxysilane. The structural evolution of this material was characterized by thermal gravimetric analysis‐differential scanning calorimetry, Fourier transform infrared, and X‐ray diffraction. By adding silica in AlPO4, the sinterability of the AlPO4 was enhanced because of the reactions between excess alumina and silica to form mullite. The sintered composites have a high strength and good dielectric properties at 10 GHz. Because of the formation of mullite at high temperatures, the composites showed a hydrophobic property. These unique properties indicate that the sintered AlPO4–mullite composites are suitable for the radome application.