SiO2 Doping Research Articles

Creep of polycrystalline yttrium aluminum garnet (YAG) at elevated temperature in air and in steam

Compressive creep of high-purity polycrystalline yttrium aluminum garnet (YAG, Y3Al5O12) was investigated at 1300°C and 50–200MPa in air and in steam. Compressive creep behavior of silica-doped polycrystalline YAG (Y3Al5O12–0.14wt% SiO2) was also studied. Creep specimens were microstructurally characterized by optical microscopy and TEM before and after creep. Steam slightly increased creep rates of material with grain size less than 1μm (the undoped YAG), but otherwise had little effect. The flow stress exponent was n≈1 for both SiO2-doped YAG and undoped YAG. Creep rates and microstructural observations are consistent with the Nabarro-Herring creep mechanism, with creep rate limited by lattice diffusion of yttrium cations (Y3+). Silica-doped YAG had a larger grain size of 2.41μm and lower creep rates than undoped YAG with 0.92μm grain size. However, creep rates normalized by grain size for Nabarro-Herring creep were higher in SiO2-doped YAG. Possible effects of SiO2 doping and steam on creep of YAG are discussed.

Preparation of nanocrystalline titanium dioxide fibers using sol–gel method and centrifugal spinning

This paper reports the study of the preparation and structural characterization of nanocrystalline titanium dioxide (TiO2) fibers, which were prepared using sol–gel method with titanium tetrachloride (TiCl4) as the titanium source and acetylacetone as chelating agent to synthesize the poly-acetylacetonatotitanium (PAT) spinning solution, followed by centrifugal spinning and steam atmosphere heat-treatment. The molecule structure of the PAT was analyzed. Using the fourier transform infrared and 1H nuclear magnetic resonance spectrum. The microstructure of the TiO2 fibers was observed by using scanning electron microscope, which showed that the fibers’ diameters are in the range of 5–15 μm. The effects of the SiO2 doping were investigated by using high-resolution transmission electron microscope and X-ray diffractometer, and the conclusion is that SiO2 doping can efficiently inhibit the grain growth of TiO2 fibers and defer the phase transformation of anatase-to-rutile. In fact, the grain size is 33.2 nm for the un-doped TiO2 fibers heat-treated at 700 °C, while the grain size is only 7.6 nm for the TiO2 fibers doped with 15 wt% SiO2, which shows the possibility to obtain nano-crystalline anatase TiO2 fibers at higher heat-treatment temperature.

Dielectric Properties of Barium Titanate Ceramics Modified by CuO in Different Methods

CuO has been doped solely and together with BaO and SiO2 into barium titanate by mixing and calcination. X-ray diffraction showed that the sample doped by CuO-BaO has stable second hexagonal phase while the samples doped by CuO, CuO-SiO2 were of a pure perovskite phase. For CuO-BaO doping, the Curie temperature was decreased accompanied with higher εmax; while for CuO doping and CuO-SiO2 codoping, the Curie temperature was increased with εmax decreased. The difference can be explained considering that Cu2+ ion could not only incorporate into the BaTiO3 lattice, but also assemble in the boundary phase when CuO was doped solely. Besides, the mechanisms of CuO doping and SiO2 doping could happen at one time.

Effect of Iron Oxide and Silica Doping on Microstructure, Bandgap and Photocatalytic Properties of Titania by Water-in-Oil Microemulsion Technique

The microemulsion method was successfully used to prepare a series of TiO2, Fe oxide and SiO2 doped TiO2 nanoparticles at Fe/Ti atomic ratio of 10% and Si/Ti atomic ratio of 15%. The molar ratio of water to surfactant (W0) was 2. The samples were calcinated at 350°C. The structural features of TiO2, Fe oxide and SiO2-TiO2 were investigated by XRD, UV-Visible spectroscopy, SEM and TEM. XRD data verified the formation of typical characteristic anatase form in all the prepared Fe and SiO2-doped TiO2 samples. In comparison with the pure TiO2, Fe oxide and SiO2-TiO2 samples were relatively large in particle size, indicating that doping with Fe oxide and SiO2 can help in increasing the particle size. The role of SiO2 and Fe oxide doping could be related to the reduction in bandgap of the titania particles; this, in turn, could increase the ability of the particles in absorbing photons with energy in the visible spectrum. The doping of highly stabilized SiO2, Fe oxide in the titania matrix lead to the significant red shift of optical response towards visible light owing to the reduced bandgap energy. Among all the pure Fe and SiO2 doped TiO2, Fe oxide-TiO2 sample showed the highest activity of the photocatalytic decomposition of methylene blue (k = 2.1 × 10–2 min–1).

Understanding in vivo response and mechanical property variation in MgO, SrO and SiO2 doped β-TCP

The aim of this work is to evaluate the influence of MgO, SrO and SiO₂ doping on mechanical and biological properties of β-tricalcium phosphate (β-TCP) to achieve controlled resorption kinetics of β-TCP system for maxillofacial and spinal fusion application. We prepared dense TCP compacts of four different compositions, i) pure β-TCP, ii) β-TCP with 1.0wt.% MgO+1.0wt.% SrO, iii) β-TCP with 1.0wt.% SrO+0.5wt.% SiO₂, and iv) β-TCP with 1.0wt.% MgO+1.0wt.% SrO+0.5wt.% SiO₂, by uniaxial pressing and sintering at 1250°C. β phase stability is observed at 1250°C sintering temperature due to MgO doping in β-TCP. In vitro mineralization in simulated body fluid (SBF) for 16 weeks shows excellent apatite growth on undoped and doped samples. Strength degradation of TCP samples in SBF is significantly influenced by both dopant chemistry and amount of dopant. Compressive strengths for all samples show degradation in SBF over the 16 week time period with varying degradation kinetics. MgO/SrO/SiO₂ doped sample shows no strength loss, while undoped TCP shows the maximum strength loss from 419 ± 28 MPa to 158 ± 28 MPa over the 16 week study. In case of MgO/SrO doped TCP, strength loss is slow and gradual. TCP doped with 1.0wt.% MgO and 1.0wt.% SrO shows excellent in vivo biocompatibility when tested in male Sprague-Dawley rats for 16 weeks. Histomorphology analysis reveals that MgO/SrO doped TCP promoted osteogenesis by excellent early stage bone remodeling as compared to undoped TCP.

Effect of SiO2 on Densification and Microstructure Development in Nd:YAG Transparent Ceramics

This paper examines the influence of SiO2 doping on densification and microstructure evolution in Nd3xY3−3xAl5O12 (Nd:YAG) ceramics. Nd:YAG powders were doped with 0.035–0.28 wt% SiO2 and vacuum sintered between 1484° and 1750°C. 29Si magic-angle spinning nuclear magnetic resonance showed that Si4+ substitutes onto tetrahedrally coordinated Al3+ sites. High-resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600°–1750°C. Coarsening was limited by a solute drag mechanism as suggested by cubic grain growth kinetics and transmission electron microscopy energy-dispersive X-ray spectroscopy observations of increased Nd3+ concentration near grain boundaries. Increasing SiO2 content increased both densification and grain growth rate and led to increasingly coarsening-dominated sintering trajectories. Fine-grained ( 82% real in-line transmission) ceramics were produced by sintering 0.035 wt% SiO2-doped ceramics at 1750°C for 8 h. Coarse-grained (18 μm), transparent samples were obtained with 0.28 wt% SiO2-doped Nd:YAG when sintered at 1600°C for 8 h.

Structural characterization for L1-ordered FePt films with (001) texture by x-ray diffraction

The nanostructure of L10-ordered FePt films with a [001] orientation prepared by both an epitaxial growth and nonepitaxial method was characterized by fitting X-ray diffraction profiles. Good fitting results for both types of films were obtained by considering the structural inter-and intragrain disorders in a calculation model. The distributions of the long range order and film thicknesses were considered to be part of the structural disorder among the grains. The fluctuation of the lattice spacing and the depth of x-ray interference along the surface normal were also taken into account to explain the structural disorder within the grain. The epitaxial FePt films grown on the MgO(100) underlayer were gradually transformed from a disordered A1 phase to an ordered L10 phase with increasing deposition temperature. A thinner film of 5 nm indicated a relatively low long range order with the fluctuation of lattice spacing. This is attributed to a large mismatch between the lattice of the initial growth layer and the MgO underlayer. On the other hand, for nonepitaxial FePt films prepared by SiO2 doping and rapid thermal post-annealing, high [001]-oriented L10 grains without the distribution of long range order were obtained at an initial transformation stage at a low annealing temperature, and the L10 grains grew along the film surface with increasing temperature. The fluctuation of lattice spacing was independent of the crystallite height in the nonepitaxial films, whereas the structural disorders such as the lattice fluctuation and chemical disorder in epitaxial films increased with a decrease in the crystallite height.

Sulfonated poly(ether sulfone)/silica composite membranes for direct methanol fuel cells

AbstractA series of sulfonated poly(ether sulfone) (SPES)/silica composite membranes were prepared by sol–gel method using tetraethylorthosilicate (TEOS) hydrolysis. Physico–chemical properties of the composite membranes were characterized by thermogravimetric analysis (TGA), X‐ray diffraction (XRD), scanning electron microscope–energy dispersive X‐ray (SEM–EDX), and water uptake. Compared to a pure SPES membrane, SiO2 doping in the membranes led to a higher thermal stability and water uptake. SEM–EDX indicated that SiO2 particles were uniformly embedded throughout the SPES matrix. Proper silica loadings (below 5 wt %) in the composite membranes helped to inhibit methanol permeation. The permeability coefficient of the composite membrane with 5 wt % SiO2 was 1.06 × 10−7 cm2/s, which was lower than that of the SPES and just one tenth of that of Nafion® 112. Although proton conductivity of the composite membranes decreased with increasing silica content, the selectivity (the ratio of proton conductivity and methanol permeability) of the composite membrane with 5 wt % silica loading was higher than that of the SPES and Nafion® 112 membrane. This excellent selectivity of SPES/SiO2 composite membranes could indicate a potential feasibility as a promising electrolyte for direct methanol fuel cell. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Influence of SiO2 Addition on the Dielectric Properties and Microstructure of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 Ceramics

In this study, the effect of SiO2 doping on the sintering behavior, microstructure, and dielectric properties of BaTiO3‐based ceramics was investigated. Silica was added to (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 (BCTZ) powder prepared using the solid‐state method. SiO2‐doped BCTZ ceramics with a high density and a uniform grain size were obtained and sintered at 1220°C in a reducing atmosphere. A second phase (BaTiSiO5) existed in samples when SiO2 was added in excess of 1%. The amount of the second phases was observed to increase as the number of SiO2 additives increased. It was found that BCTZ ceramics sintered with SiO2 are helpful in reducing the sintering temperature for a typical thick film and MLCC applications. However, there were disadvantageous effects on the dielectric properties with mere addition of SiO2 addition (3% and 5%) due to higher formation of BaTiSiO5. Doping with a small amount of silica can improve the sintering and dielectric properties of BCTZ ceramics. In addition, to understand the effect of the BaTiSiO5 phase on the dielectric properties of BCTZ ceramics, the BaTiSiO5 composition was synthesized from individual BaCO3, TiO2, and SiO2 powders using conventional solid‐state methods. X‐ray diffraction results show the presence of mainly the crystalline phase, BaTiSiO5, in the sintered ceramics.

Hot Isostatic Pressing of Transparent Nd:YAG Ceramics

This paper demonstrates that fine‐grained (2–3 μm), transparent Nd:YAG can be achieved at SiO2 doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry‐pressed samples, tape‐cast samples were free of large, agglomerate‐related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO2 levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid‐state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO2, especially at higher temperatures, because the SiO2 solubility limit is exceeded.

Synthesis and characterization of polyaniline/silicon dioxide composites and preparation of conductive films

AbstractThe focus of this study was to synthesize the inherently conductive polymer polyaniline using an optimized process to prepare polyaniline/silicon dioxide (PANI/SiO2) composites by in situ polymerization and ex situ solution mixing. PANI and PANI/SiO2 composite films were prepared by drop‐by‐drop and spin coating methods. The electrical conductivities of HCl doped PANI film and PANI/SiO2 composite films were measured according to the standard four‐point‐probe technique. The composite films exhibited an increase in electrical conductivity over neat PANI. PANI and PANI/SiO2 composites were also investigated by spectroscopic methods including UV‐Vis, FT‐IR, and Photoluminescence. UV‐Vis and FT‐IR studies showed that SiO2 particles affect the quinoid units along the polymer backbone and indicate strong interactions between the SiO2 particles and the quinoidal sites of PANI (doping effect). The photoluminescence properties of PANI and PANI/SiO2 composites were studied and the PANI/SiO2 composites showed increased intensity as compared to neat PANI. The increase of conductivity of PANI/SiO2 composite may be partially due to the doping or impurity effect of SiO2 where the silicon dioxides compete with chloride ions. The morphology of particles and films were examined by a scanning electron microscope (SEM). SEM measurements indicated that the SiO2 were well dispersed and isolated in composite films. Copyright © 2009 John Wiley & Sons, Ltd.

Doping effect on microwave reflection characteristics for W-type ferrite/epoxy composites

The reflection properties have been studied for BaCoZnFe16O27 composites doped with 0.5 wt% CaO, Bi2O3, SiO2, and 1 wt% Nb2O5 and V2O5. For composites undoped and doped with SiO2, Nb2O5 and V2O5 two matching thicknesses, tm1 and tm2, and two corresponding matching frequencies, fm1 and fm2, are observed. tm1 has its origin in the quarter wavelength matching condition and tm2 is related to the magnetic resonance. As compared with the original composite, doping of CaO, Bi2O3 and SiO2 deteriorates the reflection properties of composites due to a large increase in ϵ′, which leads to a mismatch between the material and free space. On the other hand, doping of Nb2O5 and V2O5 can significantly improve the reflectivity and bandwidth, due to enhanced μ′0 and μ″max, and almost unchanged ϵ′. The two composites doped with Nb2O5 and V2O5 are potential candidates for use as EM materials with low reflectivity and broad bandwidth at microwave frequency.

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al2O3 catalysts with 1wt% SiO2 or TiO2 doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NOx using propene under lean conditions. Ag/Al2O3 showed 44% NOx conversion at 623K, which was drastically reduced to 21% when exposed to 20ppm SO2. When Al2O3 support in Ag/Al2O3 was doped with 1wt% SiO2 or TiO2 the NOx conversion remained constant in presence of SO2 showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al2O3 catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NOx conversion to N2. In the case of Ag/Al2O3 doped with SiO2 or TiO2, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al2O3 catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.

Optimal Doping for Enhanced SnO2 Sensitivity and Thermal Stability

AbstractTin oxide nanocrystals (5–10 nm) doped with silica (0–15 wt %) were made by flame‐spray‐pyrolysis direct deposition onto the sensing electrodes and in situ stabilization by rapid flame annealing. Although increased SiO2‐doping reduced the SnO2 crystal and grain size, its sensing performance to ethanol vapor (0.1–50 ppm) exhibited an optimum with respect to SiO2 content. The thermal stability and morphology of SiO2‐doped SnO2 nanoparticles were evaluated by sintering at 200–900 °C for 4–24 h in air. At low SiO2 content, sintering of SnO2 was prevented only partially resulting in small sinter necks (bottlenecks) between SnO2 primary particles (smaller than twice the Debye length). This morphology drastically enhanced the sensitivity toward the analyte by maintaining a thermally stable high surface area and fully depleted connections at the primary particle necks. This enhancement is attributed mostly to the decreasing neck size of the SnO2SiO2 heterojunctions rather than the decreasing SnO2 crystallite and grain sizes with increasing SiO2 doping. At high SiO2 contents, SnO2 sintering was inhibited as its grains were separated effectively by dielectric SiO2; this resulted in isolated SnO2 nanocrystals with drastically reduced sensitivity, thereby effectively being insulators.

SiO2첨가가 AIN 세라믹스의 고온 비저항에 미치는 영향

The effects of SiO₂ impurity on the high temperature resistivities of AlN ceramics have been investigated. When SiO₂ was added into 1 wt% Y2O3-doped AlN, DC resistivities have decreased and electrode polarizations disappeared. Impedance spectroscopy showed two semi-circles at 600℃, which were attributed to grain and grain boundary, respectively. SiO₂ doping had more significant effects on the grain resistivity than grain boundary resistivity, implying that doped Si acted as a donor in AlN lattice. In addition, voltage dependency of DC resistivity was observed, which might be related to dependency of size of grain boundary semi-circle on the bias voltage in impedance spectroscopy.

Doping effect on complex permeability and permittivity for W-type barium ferrite composites

Doping effects on crystal structure, static, and dynamic magnetic properties have been investigated for BaCoZnFe16O27 doped with 0.5 wt % CaO, Bi2O3, SiO2, 0.5 and 1.0 wt % Nb2O5 and V2O5, as well as their ferrite/epoxy composites. The results show that the doping can greatly modify the high-frequency dynamic properties of the composites. As compared to the undoped composite, doping of CaO, Bi2O3, and SiO2 significantly increases complex permittivities ϵ′ and ϵ″. The increase has its origin in the formation of Fe2+ ions caused by the doping. Doping of CaO, Bi2O3, Nb2O5, and V2O5 leads to a considerable increase in static permeability μ0′. Further, the doping of V2O5 can significantly increase the maximum imaginary permeability μmax″. Based on the curve-fitted results to complex permeability spectra, the former is attributed to the enhanced wall permeability and the latter is related to the decrease in effective damping coefficient. However, the resonance is shifted to lower frequency for all composites doped with various oxides.

Effect of SiO2 Doping on Growth Retardation of WO3 Nanoplate

Tungsten oxide (WO3) is recently recognized as one of the most widely studied gas sensitive ceramics. To maintain good thermal stability at moderate operating temperature, the gas sensitive materials were typically heated at several hundreds degree Celsius. Such treatment caused grain growth, resulting to reduction of specific surface area which is considered to be one of the important properties determining gas sensor's performance. In this present work, grain growth of the WO3 nanoplate was retarded by incorporation of silica (SiO2) particle. To obtain homogeneous structure, the SiO2 was incorporated by mixing tetraethyl orthosilicate into the W precursor during precipitation of the tungsten oxide. It was found that the SiO2 dramatically retarded the particle growth, preserved the nanoplate structure, and resulted to more porous WO3 powder. The effect was very significant when the powders were calcined at high temperatures.

Solid‐State Reactive Sintering of Transparent Polycrystalline Nd:YAG Ceramics

Transparent polycrystalline Nd:YAG ceramics were fabricated by solid‐state reactive sintering a mixture of commercial Al2O3, Y2O3, and Nd2O3 powders. The powders were mixed in methanol and doped with 0.5 wt% tetraethoxysilane (TEOS), dried, and pressed. Pressed samples were sintered from 1700° to 1850°C in vacuum without calcination. Transparent fully dense samples with average grain sizes of ∼50 μm were obtained at 1800°C for all Nd2O3 levels studied (0, 1, 3, and 5 at.%). The sintering temperature was little affected by Nd concentration, but SiO2 doping lowered the sintering temperature by ∼100°C. Abnormal grain growth was frequently observed in samples sintered at 1850°C. The Nd concentration was determined by energy‐dispersive spectroscopy to be uniform throughout the samples. The in‐line transmittance was >80% in the 350–900 nm range regardless of the Nd concentration. The best 1 at.% Nd:YAG ceramics (2 mm thick) achieved 84% transmittance, which is equivalent to 0.9 at.% Nd:YAG single crystals grown by the Czochralski method.

Grain Growth Phenomena in the Interface Region of α-Alumina Bilayer Composites

Microstructural development in the interface region of α-Al2O3 bilayer composites has been systematically investigated in terms of the sintering additive CaO–SiO2, residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO–SiO2 doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO–SiO2-Al2O3 liquid at the interface region, thereby strongly influencing α-Al2O3 grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO–SiO2 dopant in the α-Al2O3 bilayer. This provides an important mechanism for controlling two-dimensional structures in coatings, films, and layered ceramic materials for various engineering applications.

Preparation and characterization of silica-doped aluminum nitride–aluminum nitride polytypoid composites

Fine-grained AlN–AlN polytypoid composites doped with SiO 2 were prepared in situ by pressureless sintering of AlN–Al 2O 3 mixtures (3.7 and 10.5 mol% Al 2O 3) at 1870 °C using Y 2O 3 as sintering aid. SiO 2-doping (1 wt.%) was used to stabilize the AlN polytypoids and reduce the average grain size of the polytypoids. The composites consisted of elongated SiO 2 containing AlN polytypoids grains (30 and 61 vol.%) embedded in equi-axed AlN matrix. Vickers hardness and bending strength increased with polytypoid composite formation. The Young's modulus showed a maximum at 3.7 mol% Al 2O 3 (30 vol.% polytypoids). The fracture toughness increased up to 4.0 ± 0.3 MPa m 1/2 for the composites. The toughening was caused by crack deflection due to residual strain both in the polytypoids and the AlN matrix.