Diffusion Of Silica Research Articles

Environmental Barrier Coatings for Silicon Nitride

Ytterbium silicate layers were deposited on Si3N4 ceramics as environmental barrier coatings (EBCs) by a dip coating-sintering method. Coated samples were tested in an atmosphere simulating the practical conditions of a gas turbine to investigate water vapor corrosion and recession mechanisms of ytterbium silicate coatings. Prior and after tests, phase compositions and morphologies of the coatings varied as the consequence of the formation of silica at the coating/substrate interface. Due to the evaporation and diffusion of silica into the upper layer, a porous interface was finally found, which led to the spallation of coating.

Laser-supported solid-state absorption fronts in silica

We develop a model based on simulation and experiment that explains the behavior of solid-state laser-supported absorption fronts generated in fused silica during high intensity (up to 5GW/cm{sup 2}) laser exposure. We find that the absorption front velocity is constant in time and is nearly linear in laser intensity. Further, this model can explain the dependence of laser damage site size on these parameters. This behavior is driven principally by the temperature-activated deep sub band-gap optical absorptivity, free electron transport and thermal diffusion in defect-free silica for temperatures up to 15,000K and pressures < 15GPa. The regime of parameter space critical to this problem spans and extends that measured by other means. It serves as a platform for understanding general laser-matter interactions in dielectrics under a variety of conditions.

Discrete free-boundary reaction-diffusion model of diatom pore occlusions

Background - Diatoms are unicellular algae, prolific in nearly all aqueous environments on earth. They are encased between two siliceous valves that each feature a variety of intricately patterned species-specific siliceous structures. How diatoms use biological and physical processes to form these tiny detailed structures is largely unknown. This work is concerned with the smallest regular structures in diatom valves, the pore occlusions, and the processes involved in their formation.Theory and method - We developed a discrete, free-boundary, reaction-diffusion computer model to assess a new physically motivated hypothesis: pore occlusion patterns in the genus Achnanthes are simply expressions of silica diffusion and deposition within a pore covered by a membrane (silicalemma), whose deformation interacts with the growth of the pore boundary to control the silica influx.Preliminary results and discussion - Simulations generate some promising pore features such as bifurcating and curved protrusions that grow towards one another, as seen in diatom pore occlusions. But they tend to be irregular and, to date, taking into account of smoothing and regularizing effects only partially symmetrized formations. Potential future work on this point is outlined.

Some pitfalls in the use of the Knudsen equation in modelling diffusion in nanoporous materials

The Knudsen model of diffusion in small pores, originally verified in macropores, is widely applied at the mesopore scale with adsorption effects neglected, largely based on linearity of the T/M correlation. Here, we show that this approach is misleading, and that the correlation masks inconsistencies arising from neglect of van der Waals forces in the Knudsen model. We examine the tortuosity for diffusion of light gases in nanoporous carbons using the Oscillator model of low pressure transport developed in the first author’s laboratory, which incorporates van der Waals interactions. Pore network effects are considered through a hybrid correlated random walk-effective medium theory approach. It is shown that in the presence of a pore size distribution the apparent tortuosity is not a porous medium property alone, but depends also on the temperature and on the diffusing molecule, because of the temperature and the gas-dependent short circuiting effects associated with pores that have high conductance. This short circuiting effect leads to a complex and rich variety of behaviour with respect to pore size, temperature and diffusing gas, which is consistent with experimental evidence, but is absent when the Knudsen model is used with adsorption effects neglected. It is shown that when effects of adsorption on equilibrium and transport are overlooked the commonly used correlation of diffusivity with T/M is deceptive, as the product of the adsorption equilibrium constant and diffusivity (or pore conductance) also approximately scales linearly with the Knudsen diffusivity (i.e. with T/M). Such behaviour is found for diffusion in mesoporous carbons and silica as well as in silicon. Consequently, claims of validity of the Knudsen model based on such a correlation may be misconceived.

Chloride ion diffusivity of fly ash and silica fume concretes exposed to freeze–thaw cycles

This research focuses on investigating the durability of concretes containing fly ash and silica fume exposed to combined mode of deterioration. For this purpose, the chloride ion diffusivity of concrete was evaluated before and after 300 freeze–thaw (F–T) cycles. It was found that the coefficient of chloride ion diffusivity (CCID) increased as water to cementitious material ratio (w/cm) and air content increased. Test results clearly showed that CCID for all concretes increased after F–T cycles. In addition, concrete incorporating silica fume showed the lowest CCID and highest durability factor (DF), regardless of curing regime, air content, and w/cm. However, fly ash concrete showed good resistance to chloride ion diffusivity before and after F–T cycles when low w/cm as well as a proper curing and air content were provided.

Oxygen and Silicon Diffusion in Silica under Varying Ambient Conditions

Diffusion of oxygen (O) in amorphous silicon dioxide (SiO2) was investigated by means of Si3N4/natSinatO2/28Si18O2/28Si isotope heterostructures grown by thermal oxidation and plasma enhanced chemical vapour deposition. Diffusion experiments with and without a silicon nitride (Si3N4) cap, which serves as diffusion barrier for the gasses in the ambient, were performed. In particular, we determined the impact of the ambient gas, of the thickness of the isotopically enriched SiO2 layer, and of the annealing time and temperature on diffusion. Our results are compared with data given in the literature on oxygen and silicon diffusion in silica and are discussed in the framework of the experimental conditions established at the sample surface and at the buried 28Si18O2/28Si interface. Taking into account a point defect model that is predicted by recent atomistic simulations all experimental results can be explained consistently.

Structure and diffusion of boron in amorphous silica: Role of oxygen vacancy related defects

Based on first-principles density-functional calculations, we present the structure and diffusion of boron in amorphous silica, as well as in crystalline silica for comparison purpose. We find that incorporation of a boron atom into the amorphous silica matrix results in various minimum-energy configurations with and without oxygen deficient centers, and also the B-related defects can undergo interconversions at elevated temperatures. While B atoms preferentially remain in the trigonal ${\text{BO}}_{3}$ form in amorphous silica, our work shows that B diffusion may require transformation of the immobile ${\text{BO}}_{3}$ state to a mobile B state by capturing oxygen vacancy related defects equivalent to an S center, which is a combination of an oxygen vacancy and a trivalent Si defect with an unpaired electron. Considering an energy cost for S center creation, our predicted activation energy of B diffusion is in good agreement with experiments. In addition, the defect-mediated diffusion model can explain the observed correlation of B diffusion and Si self-diffusion in amorphous silica.

Performance of perovskite-related oxide cathodes in contact with lanthanum silicate electrolyte

The cathodic performance of selected mixed-conducting electrodes, including perovskite-type SrMn 0.6Nb 0.4O 3 − δ , Sr 0.7Ce 0.3Mn 0.9Cr 0.1O 3 − δ and Gd 0.6Ca 0.4Mn 0.9Ni 0.1O 3 − δ , and Ruddlesden–Popper La 2Ni 0.5Cu 0.5O 4 + δ , LaSr 2Mn 1.6Ni 0.4O 7 − δ , La 4Ni 3 − x Cu x O 10 − δ ( x = 0–0.1) and La 3.95Sr 0.05Ni 2CoO 10 − δ , was evaluated in contact with apatite-type La 10Si 5AlO 26.5 solid electrolyte at 873–1073 K and atmospheric oxygen pressure. The electrochemical activity of porous nickelate-based layers was found to correlate with the concentration of mobile ionic charge carriers and bulk oxygen transport, thus lowering in the series La 4Ni 2.9Cu 0.1O 10 − δ > La 4Ni 3O 10 − δ > La 3.95Sr 0.05Ni 2CoO 10 − δ and decreasing on copper doping in K 2NiF 4-type La 2Ni 1 − x Cu x O 4 − δ . The relatively high overpotentials of nickelate-based cathodes, varying in the range − 240 to − 370 mV at 1073 K and current density of − 200 mA/cm 2, are primarily associated with surface diffusion of silica from La 10Si 5AlO 26.5, which partially blocks the electrochemical reaction zone. As compared to the intergrowth nickelate materials, the manganite-based electrodes exhibit substantially worse electrochemical properties, in correlation with the level of oxygen-ionic and electronic conduction in Mn-containing phases. The effects of cation interdiffusion between the cell components as a performance-deteriorating factor are briefly discussed.

The diffusion coefficient of dissolved silica revisited

The diffusion coefficient of dissolved silica was determined for two different salinities, 36 and 0, at temperatures ranging from 2 °C to 30 °C and at an average pH value of 8.1. Our results show limited influence of salinity and a variation by a factor of 2 to 3 of the silica diffusion coefficient within the temperature range considered in this study. The values obtained at 25 °C are in agreement with previous work carried out at room temperature for seawater and freshwater. The dependency on temperature and viscosity of the diffusion coefficient agrees well with the Einstein–Stokes equation. The composition of the solvent appears to be an important factor because it modifies the viscosity and allows for the complexation of the dissolved silica with less mobile ions, while its pH controls the dissolved silica speciation. In seawater, the higher viscosity and the presence of dissociated and polymeric species result in a decrease of the diffusion coefficient compared to freshwater systems.

An investigation of thick PZT films for sensor applications: A case study with different electrode materials

Lead zirconate titanate (PZT) is a piezoelectric material that can sense or respond to mechanical deformations and can be used in ceramic micro-electro-mechanical systems (C-MEMS). A thick-film paste was prepared from a pre-reacted PZT powder (PbZr0.53Ti0.47O3) and thick-film technology (screen-printing and firing) was used to deposit the PZT layers on LTCC tapes and on alumina substrates. The microstructural, electrical and piezoelectric characteristics of the thick PZT films on relatively inert alumina substrates and on LTCC tapes were studied. Preliminary experiments indicated that due to the interaction between the printed PZT layers and the LTCC substrates during firing the electrical characteristics deteriorate significantly. To minimise the influence of substrate-film interactions different electrode materials and the use of additional intermediate layers as a barrier were evaluated. The dielectric permittivities, dielectric losses, and piezoelectric coefficients (d 33) were measured. The dielectric permittivities of the thick films fired on LTCC substrates were lower (210 with gold electrodes and 430 with silver electrodes) than those measured on alumina substrates (500). The piezoelectric coefficients d33 were measured with a Berlincourt piezometer. The d 33 values measured on the LTCC substrates were relatively low (60–80 pC/N) compared with the values obtained for the alumina substrates (around 140 pC/N). The lower dielectric constants and piezoelectric coefficients d 33 of the films on LTCC substrates are attributed to the formation of phases with a lower permittivity. This was a result of the diffusion of SiO2 from the LTCC into the active PZT layer. The diffusion of silica was confirmed by the SEM and EDS analyses.

On the origin of nitrogen-induced retardation of boron diffusion in amorphous silica

The effect of N incorporation on B diffusion in amorphous SiO2 is presented based on spin-polarized density functional theory calculations. Our results show that N incorporation leads to the decrease of O vacancy concentration, which is largely responsible for the retarded B diffusion by reducing diffusion mediators such as E′ and S centers. We also determine the ground state structure of the BN complex, along with possible formation routes. The direct BN bonding interaction appears to only slightly increase the activation energy of B diffusion.

Comparison of Cleanroom Samplers for Inorganic Airborne Molecular Contaminants

The performances of inorganic airborne molecular contaminants (AMCs) via silica gel tubes, impingers, and diffusion denuder sampler (DDS) were compared in a cleanroom. The results showed silica gel tubes were not applicable for cleanroom sampling due to high blanks. While with optimal sampling conditions both impingers and DDS have much better performances, of which DDS has the lowest detection limits of the method for HF, HCl, HNO2, HNO3, SO2, and NH3 gases to be 0.15, 0.11, 0.13, 0.03, 0.07, and 0.42 µg/m3, respectively. Results indicated no significant difference for the HF and SO4 2− concentrations made by the DDS and impingers.

Polysulfone/silica nanoparticle mixed-matrix membranes for gas separation

Mixed-matrix membranes (MMMs) are based on polymeric membranes filled with inorganic particles as a means to improve their gas separation performance. In this study, MMMs were prepared from polysulfone (PSf) containing embedded nonporous fumed silica nanoparticles and the gas permeation properties of the resulting membranes were investigated. Physical properties such as film density, thermal degradation and glass transition temperature of PSf/silica MMMs were characterized. The distribution of the silica nanoparticles in PSf was observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Furthermore, the interface between the polymer and silica agglomerates was studied in relation with the gas transport properties. The gas permeabilities of hydrogen, helium, oxygen, nitrogen, methane, and carbon dioxide were measured as a function of silica volume fraction and diffusion and solubility coefficients were determined by the time-lag method. The effect of silica nanoparticles in PSf membranes on gas permeability is in contrast with predictions based on the Maxwell model. The O 2 permeability is approximately four times higher and CH 4 permeability is over five times greater than in a pure PSf membrane. However, the performance comprising permeability versus selectivity of PSf/silica MMMs for O 2/N 2 and CO 2/CH 4 follows a similar slope to that of the trade-off upper bound with increasing silica content.

Diffusion and structure in silica liquid: a molecular dynamics simulation

Diffusion and structure in liquid silica under pressure have been investigated by amolecular dynamics model of 999 atoms with the inter-atomic potentials of van Beest,Kramer and van Santen. The simulation reveals that silica liquid is composed of the speciesSiO4,SiO5 and SiO6 with a fraction dependent on pressure. The density as well as volume of voids can be expressedas a linear function of the fraction of those species. Low-density liquid is mainly constructed ofSiO4 and has a large number of O- and Si-voids and a large void tube. This tube contains mostO-voids and is spread over the whole system. The anomalous diffusion behavior is observedand discussed.

PZT thick films on different ceramic substrates; piezoelectric measurements

The piezoelectric, microstructural and electrical characteristics of thick PZT films on relatively inert alumina substrates and on two LTCC tapes, i.e., Du Pont 951 and Electro Science Labs 41020 were studied. A thick-film paste was prepared from the pre-reacted PZT powder (PbZr0.53Ti0.47O3) and printed and fired on LTCC tapes and on alumina substrates, respectively. Dielectric permittivities, dielectric losses, remnant polarizations and coercive fields were measured. The dielectric constants (100–150) of thick films fired on LTCC substrates are low. The piezoelectric coefficients d33 were measured by different methods, i.e. Berlincourt piezometer, interferometry and piezoresponse force microscope (PFM). The d33 values on LTCC substrates are low (30–70 pm/V) as compared with values obtained on alumina substrates (around 120 pm/V). Lower dielectric constants and piezoelectric coefficients d33 of films on LTCC substrates are attributed to the formation of phases with a low permittivity due to the diffusion of silica from LTCC substrates into PZT films. The d33 constants of samples with different thicknesses of PZT layers (from 20 to 160 μm) at first increase with the increasing thickness of PZT layers and then decrease for thicker films. As the cracks in the structure were not observed the reason for the decreasing d33 values for thicker films is still unclear.

Water diffusion in silica film during silicon wet oxidation

Water diffusion in silica glass is explained in terms of the migration of molecular water and its reaction with the silica network to form immobile hydroxyl water. It is assumed that the reaction is fast at high temperatures, and a local reaction equilibrium is maintained during water diffusion. While this mechanism appears to explain water diffusion in silica glasses determined from hydroxyl profiles reasonably well, we found that water diffusion-promoted phenomena, such as the oxidation of silicon, require a small quantity of molecular water diffusing into silica glasses ahead of the hydroxyl diffusion front, unrestricted by the local equilibrium. This was demonstrated by studying oxidation kinetics of silicon with a thick (∼15μm) oxide layer and measuring the delay time before the steady-state oxidation.

Thermodynamically Stable SixOyCz Polymer‐Like Amorphous Ceramics

Carbon can be used to create unusual nanostructures of Si–C–O by controlled pyrolysis of silsesquioxane organics. Unlike silica, these ceramics resist crystallization at ultrahigh temperatures. Their structure has been compared with that of polymers, where crosslinked chains of polymers in organics are replaced by crosslinked networks of graphene in the ceramics. The network sequesters nanoscale domains of SiO4 tetrahedra. The resistance to crystallization of these nanodomain networks has been attributed to kinetic factors, namely obstruction of long‐range diffusion of silica. In this work, we identify a thermodynamic hindrance to crystallization. Calorimetric measurements of heats of dissolution in a molten oxide solvent show that these ceramics possess a negative enthalpy relative to their crystalline constituents (silicon carbide, cristobalite, and graphite). The thermodynamic stability of the nanodomain structure is explained by a low free energy of the graphene–silica interfaces, perhaps related to the presence of mixed bonds of silicon bonded to both carbon and oxygen.

Physical property changes of crystalline and non-crystalline SiO 2 due to neutron irradiation and recovery by subsequent annealing

Physical property changes of quartz and vitreous silica due to neutron irradiation and recovery by subsequent annealing were examined. Specimens were fast-neutron-irradiated up to a fluence of 3.0 × 10 23 n/m 2 at 140 °C, and 6.9 × 10 23 n/m 2 at 300 °C. Vitreous silica shrunk ∼0.8% in length after both irradiations. On annealing, the length started to increase above 200 and 500 °C for the lower and higher fluence specimens, respectively, and gradually increased with increasing annealing temperature. Quartz expanded in an anisotropic manner, with larger expansion along the a-axis as compared to the c-axis, due to the irradiation. Length and lattice parameter of quartz started to reduce for annealing over 300–500 °C, and were almost fully recovered at 800 °C in the lower fluence case, and 1000 °C in the higher fluence case. Thermal diffusivity reduced ∼10% in vitreous silica and 60–70% in quartz by the irradiation. Recovery of thermal diffusivity of vitreous silica and quartz by annealing resembled the corresponding length change.

Electrochemical behavior of mixed-conducting oxide cathodes in contact with apatite-type La 10Si 5AlO 26.5 electrolyte

The electrochemical activity of porous La 2Ni 0.8Cu 0.2O 4+ δ , La 2Ni 0.8Cu 0.2O 4+ δ –Ag, La 0.8Sr 0.2Fe 0.8Co 0.2O 3− δ –Ce 0.8Gd 0.2O 2− δ and La 0.7Sr 0.3MnO 3− δ –Ce 0.8Gd 0.2O 2− δ electrodes in contact with apatite-type La 10Si 5AlO 26.5 solid electrolyte has been appraised at 873–1073 K in air. The polarization resistance of nickelate-based cathodes is substantially higher compared to similar layers applied onto (La 0.9Sr 0.1) 0.98Ga 0.8Mg 0.2O 3− δ , whilst the corresponding activation energies, 69–74 kJ/mol, are close to the E a values for ionic conduction in these electrolytes. The relatively low performance is primarily associated with the surface diffusion of silica from La 10Si 5AlO 26.5, which partially blocks the electrochemical reaction zone without formation of secondary phases detectable by X-ray diffraction. The oxygen reduction kinetics is also strongly influenced by the transport properties of solid electrolyte and by the exchange-related processes at the electrode surface. The role of the latter factor becomes evident on increasing current density, and in the cases when ionic conductivity of the electrode materials is low. As for other solid oxide electrolyte cells, the performance of mixed-conducting cathodes applied onto La 10Si 5AlO 26.5 can be improved by incorporating electrocatalytically-active components, such as Ag and PrO x , and by reducing electrode fabrication temperature.

Systematics of radon at the Wairakei geothermal region, New Zealand

222Rn and 220Rn in geothermal steam at Wairakei, NZ, range from 11 to 19 500 Bq kg −1, and 25 to 16 700 Bq kg −1, respectively, but do not cause toxic concentrations in air. The wide ranges are mainly due to differences in different physical conditions underground (e.g. thin silica diffusion barriers), not geochemical differences. Groundwater Rn from outside the area probably plays only a minor role. 210Po was found present in non-toxic levels in the steam. Historical records showed little change in Rn concentration over several decades, therefore potentially hazardous concentrations might be predicted from early exploration. 220Rn concentrations at Wairakei should decrease as the field becomes steam-dominated. Rock surfaces were variably leached or enriched with U, Th, Ra and 210Pb, providing a possible model for deposition in cooler regions near the field. Estimates of 222Rn permeability ranged from 2 to 77% of the maximum possible, with a median of 13%.