Characterization of di-phasic nanoscale composites derived from xerogels

Di-phasic xerogel-derived composites, such as SiO₂AgCl, SiO₂-AIPO₄, SiO₂-CePO₄, SiO₂ -Nd₂O₃, SiO₂-CdS, SiO₂CrPO₄, SiO₂-BaSO₄ and SiO₂-PbCrO₄ have been characterized in detail by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) techniques. The SiO₂-AgCl photochromic composites with small amounts of AgCl did not show any crystallinity either by XRD or by SAED. Thin edges of these SiO₂-AgCl composites did not reveal discrete AgCl particles because these are too small to be resolved even by TEM and are expected to be in the range 1.5 to 2.5 nm in size based on the pore size of silica gel. A few large AgCI-Ag particles precipitated on the outside of silica gel were, however, detected by TEM-SAED in silica gels with higher concentrations of AgCl. The SiO₂-AIPO₄ and SiO₂-Nd₂O₃ composites are noncrystalline and did not show any periodic structure by TEM and SAED. Heat treatments to 400 or 600° C did not crystallize the AIPO₄ or Nd₂O₃ phases. On the other hand, SiO₂-CePO₄ and SiO₂-CdS composites showed lath-like particles of CePO₄ and irregular particles of presumably CdS on the surfaces of silica gels. The SiO₂-BaSO₄ and SiO₂-PbCrO₄ composites showed crystals of BaSO₄ and PbCrO₄ which are too large to be incorporated in the silica gel pores. These results show that the size and crystallinity of a second phase within silica gels can be controlled by the appropriate manipulation of the different parameters, and to do so is an important advantage for this new class of diphasic nanoscale composite xerogel materials.     

Preparation and interface modification of Si_3N_(4f)/SiO_2 composites

In order to modify the interface, SiON coating was introduced on the surface of silicon nitride fiber by perhydropolysilazane conversion method. Si₃N_4f/SiO₂ and Si₃N_4f/SiON_c/SiO₂ composites were prepared by sol-gel method to explore the influence of SiON coating on the mechanical properties of composites. The results show that with the protection of SiON coating, Si₃N₄ fiber enjoys a strength increase of up to 24.1% and Si₃N_4f/SiON_c/SiO₂ composites have a tensile strength of 170.5 MPa and a modulus of 26.9 GPa, respectively. After 1000 °C annealing in air for 1 h, Si₃N_4f/SiON_c/SiO₂ composites retain 65.0% of their original strength and show a better toughness than Si₃N_4f/SiO₂ composites. The improvement of mechanical properties is attributing to the healing effect of SiON coating as well as its intermediate coefficient of thermal expansion between Si₃N₄ fiber and SiO₂ matrix.     

Large-scale synthesis of ear-like Si3N4 dendrites from SiO2/Fe composites and Si powders

Large-scale ear-like Si₃N₄ dendrites were prepared by the reaction of SiO₂/Fe composites and Si powders in N₂ atmosphere. The product was characterized by field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results reveal that the product mainly consists of ear-like Si₃N₄ dendrites with crystal structures, which have a length of several microns and a diameter of 100-200 nm. Nanosized ladder-like Si₃N₄ was also obtained when changing the Fe content in the SiO₂/Fe composites. The Si₃N₄ nanoladders have a length of hundreds nanometers to several microns and a width of 100-300 nm. The ear-like Si₃N₄ dendrites are formed from a two-step growth process, the formation of inner stem structures followed by the epitaxial growth of secondary branches.     

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant

Silica (SiO₂) bonded porous silicon nitride (Si₃N₄) ceramics were fabricated from α-Si₃N₄ powder in air at 1200–1500 °C by the oxidation bonding process. Si₃N₄ particles are bonded by the oxidation-derive SiO₂ and the pores derived from the stack of Si₃N₄ particles and the release of N₂ and SiO gas during sintering. The influence of the sintering temperature and holding time on the Si₃N₄ oxidation degree, porosity, flexural strength and dielectric properties of porous Si₃N₄ ceramics was investigated. A high flexural strength of 136.9 MPa was obtained by avoiding the crystallization of silica and forming the well-developed necks between Si₃N₄ particles. Due to the high porosity and Si₃N₄ oxidation degree, the dielectric constant (at 1 GHz) reaches as low as 3.1.     

Preparation,characterization, and photocatalytic properties of composite materials of copper(II) porphyrin/TiO2

Three new CuPp–TiO₂ composite materials were prepared by impregnating copper(II) porphyrin with different peripheral substituent (–OH, –COOC₂H₅, and –COOH) onto the surface of polycrystalline TiO₂ at room temperature and characterized by SEM, energy-dispersive X-ray spectrometry, X-ray diffraction, FT-IR, UV–Vis DRS, and photoluminescence. The effects of metalloporphyrins on the surface of TiO₂ have been detected by the photodegradation of 4-nitrophenol (4-NP) and rhodamine B (RhB). The loading of metalloporphyrins onto TiO₂ results in strong visible light absorption by the composite and, more importantly, a 1.47–2.47 times increase in visible light photocatalytic activity in the degradation of RhB. The metalloporphyrins dispersed on the TiO₂ surface can act as a small-band-gap semiconductor to absorb visible light, giving rise to electron–hole separation. What’s more, these CuPp–TiO₂ with different peripheral substituent (–OH, –COOC₂H₅, and –COOH) in meso-sites of porphyrin ring displayed different catalytic activities in the degradation of 4-NP and RhB, the CuPp containing –OH and –COOH showed better catalytic activity due to their strong interaction with TiO₂. A possible mechanism of these higher photocatalytic efficiencies was proposed based on the relative experiments.     

Cement-based composites endowed with novel functions through controlling interface microstructure from Fe3O4@SiO2 nanoparticles

In this paper, Fe₃O₄@SiO₂ nanoparticles (NPs) were introduced in the surface layer of cement-based materials derived by magnetic field to create a wave adsorbing layer. The cement-based materials treated with Fe₃O₄@SiO₂ NPs revealed superior microwave-absorption property comparing with the samples treated with pure Fe₃O₄ NPs. Because of a SiO₂ coating on Fe₃O₄ NPs, water absorption rates of cement mortars treated with Fe₃O₄@SiO₂ NPs have reduced by 45.3%. In addition, the SiO₂ coating on Fe₃O₄ NPs bonded wave absorbing materials on the surface of cement-based composites by forming a mass of SiO₂ and calcium silicate hydrate (C-S-H) gels. The Fe₃O₄@SiO₂ NPs can be considered as an ideal wave absorption surface-treatment agent for cement-based composites.     

Development of light composite materials with low coefficients of thermal expansion

Abstract

Composite materials based on aluminium are used in different fields where weight, thermal expansion, and thermal stability are key requirements. The aim of the present study was to develop a universal method and scientific approach for evaluating the design of lightweight, Al matrix composites with low coefficients of thermal expansion (CTE) and high dimensional stability, and to produce such composites using the vacuum plasma spray (VPS)process. The methodology is general and could be applied to other composite systems. The VPS-produced Al and Al alloy 6061 based composites were reinforced with a variety of ceramic particles including Si₃N₄, B₄C, TiB₂, and 3Al₂O₃.2SiO₂. These composites have low CTE values ((12–13)×10^-6 K^-1), similar to that of steel, and high dimensional stability (capable of keeping dimensions stable with changes in temperature). They have low porosity (98–99%dense) and a uniform distribution of the strengthening particles. Hot rolling of the VPS-formed composites, followed by heat treatment, resulted in a significant improvement in the mechanical properties. Deformed and heat treated 6061 based composites, containing 20 wt-%TiB₂ and 40 wt-%3Al₂O₃?2SiO₂, showed excellent mechanical properties (ultimate tensile strength 210–250 MPa, elongation >4%).     

Effect of silicon on the formation of silk fibroin/calcium phosphate composite

The silk fibroin/calcium phosphate composites were prepared by adding the different amount of Na₂SiO₃ to assess the effect of silicon on the HA (hydroxyapatite) formation in the composites. FTIR and XRD results suggested that the inorganic phase was constituted mainly by the amorphous DCPD (dicalcium phosphate dehydrate), a precursor of HA in the bone mineral, when the composites were prepared at the final Na₂SiO₃ concentration lower than 0.008%. Otherwise, HA was formed as the predominant one in the as-prepared composite, accompanied with a conformational transition in the organic phase of silk fibroin protein from silk I (α-helix and/or polyglycine II (3₁–helix) conformations) to silk II (antiparallel β-sheet conformation). SEM images showed the different morphologies with the samples, i.e., sheet-like crystals in the composites prepared at a low Na₂SiO₃ concentration and rod-like bundles in other composites. The rod-like bundles were connected together to form the porous network, due to the fact that the HA crystals grew with the aggregation of silk fibroin, and further accreted onto the silk fibroin fibrils. TG curves indicated that the composites prepared with a certain amount of additional SiO₃²⁻ had the higher thermal stability because of its high molecular orientation and crystallinity, and high water-holding capacity due to the porous microstructure.     

Syntheses and proton conductivity of mesoporous Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> and NdOCl–SiO<Subscript>2</Subscript> composites

Two mesoporous oxide composites of Nd₂O₃–SiO₂ and NdOCl–SiO₂ were synthesized using SBA-15 as a template and neodymium nitrate or neodymium chloride as a precursor. The porous Nd₂O₃–SiO₂ with a SBA-15-like structure has amorphous walls and the porous NdOCl–SiO₂ with a replicated structure of SBA-15 has crystalline walls. These porous materials were characterized by X-ray diffraction, transmission electron microscopy and nitrogen adsorption/desorption. They exhibited significant proton conductivities in the presence of moisture at low temperatures and the highest conductivity observed was 4.55 × 10⁻⁴ S/cm at 47 °C in wet air (RH = 28.6%).     

Properties of porous Si3N4 ceramic electromagnetic wave transparent materials prepared by technique combining freeze drying and oxidation sintering

A simple and low-cost technique combining freeze drying and oxidation sintering is explored to prepare Si₃N₄ ceramics with high porosity and complex shape. The effects of sintering temperature and time on the phase composition, microstructure, porosity, pore size and dielectric constant of the porous Si₃N₄ ceramics are studied. Due to the variations of phase composition and microstructure, the porous Si₃N₄ ceramics sintered at different temperature possess characteristic in flexural strength. The porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h have the highest flexural strength of 71–74 MPa. The changes of porosity and composition have much effect on the dielectric constant of porous Si₃N₄ ceramics. Because of the high porosity and SiO₂ volume fraction, the porous Si₃N₄ ceramics sintered at 1,300 °C for 2–3 h possess low dielectric constant of 3.4–3.6 and small pore size of 0.9 μm. The porous Si₃N₄ ceramics are good structural/functional and promising electromagnetic wave transparent material.     

Effect of the SiO2/TiO2 ratio on the solid acidity of SiO2-TiO2/montmorillonite composites

SiO₂-TiO₂/montmorillonite composites with varying SiO₂/TiO₂ molar ratios were synthesized and the effect of the SiO₂/TiO₂ ratio on the solid acidity of the resulting composites was investigated. Four composites with SiO₂/TiO₂ molar ratios of 0, 0.1, 1 and 10 were synthesized by the reaction of colloidal SiO₂-TiO₂ particles prepared from alkoxides with sodium-montmorillonite at room temperature. The composites showed slight expansion and broadening of the XRD basal reflection, corresponding to the intercalation of fine colloidal SiO₂-TiO₂ particles into the montmorillonite sheets and incomplete intercalation to form disordered stacking of exfoliated montmorillonite and colloidal SiO₂-TiO₂ particles. The colloidal particles crystallized to anatase in the low SiO₂/TiO₂ composites but remained amorphous in the high SiO₂/TiO₂ composites. The specific surface areas (S_BET) of the composites measured by N₂ adsorption ranged from 250 to 370 m²/g, considerably greater than in montmorillonite (6 m²/g). The pore size increased with decreasing SiO₂/TiO₂ molar ratio of the composites. The NH₃-TPD spectra of the composites consisted of overlapping peaks, corresponded to temperatures of about 190 and 290 °C. The amounts of solid acid obtained from NH₃-TPD were 186-338 μmol/g in the composites; these values are higher than in the commercial catalyst K10 (85 μmol/g), which is synthesized by acid-treatment of montmorillonite. The present sample with SiO₂/TiO₂ = 0.1 showed the highest amount of acid, about four times higher than K10.     

A comparison of various dielectric/metal sidewall diffusion barriers for Cu/porous ultra-low-K interconnect technology in terms of leakage current and breakdown voltage

A very thin dielectric flash layer (DFL) was used in addition to Ta for Cu/ultra-low-K (porous SiLK™) back-end technology. Atomic force microscopy showed that SiC DFL is superior to Si₃N₄ or SiO₂ DFL in terms of smaller surface roughness. It appears to the authors that the key point to get a very smooth DFL is to avoid the use of nitrogen-containing gases like NH₃, N₂ or N₂O during the DFL deposition process. An alternative explanation is that trimethylsilane is the better Si source compared to silane. Electrical testing showed that SiC DFL is superior to Si₃N₄ or SiO₂ DFL in terms of higher breakdown voltage.     

Dielectric properties in GHz range of porous Si3N4–BN–SiO2 ceramics with considerable flexural strength prepared by low temperature sintering in air

Abstract

Porous Si₃N₄–BN–SiO₂ ceramics with ultimate apparent porosities between 0·140 and 0·799 were fabricated in air at 1100°C by partial sintering using core starch as both consolidator and pore former in the green bodies. The pores were derived from burning off the starch, the partial oxidation of silicon nitride and the stack of particles of the start materials. Effect of retaining time on the microstructure of sintering bodies was analysed by SEM analysis. Reference intensity ratio (RIR) technique based on the X-ray diffractometry results demonstrated the phase components content of sintered bodies. Influence of porosity on the flexural strength of porous Si₃N₄–BN–SiO₂ ceramics was investigated. The ceramic with a porosity of 0·140 attained a maximal flexural strength of 60±4·11 MPa. In addition, the dielectric constants and loss tangents were presented for porous Si₃N₄–BN–SiO₂ triphase ceramics in the frequency range of 18–40 GHz, and the real part of dielectric constant of the materials reached as low as 2·67 at the porosity of 0·732 at a frequency of 20 GHz.     

Preparation of granular X-type zeolite/activated carbon composite from elutrilithe by adding pitch and solid SiO2

The preparation of granular X-type zeolite/activated carbon composites from a locally available elutrilithe by adding pitch powder and solid SiO₂ was studied, and the variations in the synthesis process of zeolite X were investigated. The preparation steps of the composite involved (1) calcination of pre-shaped mixture (2) activation of the carbonaceous material from elutrilithe and pitch to prepare activated carbon and (3) hydrothermal conversion (zeolitisation) of aluminosilicate in elutrilithe and additional SiO₂ to zeolite X in alkaline medium. The adding of additional SiO₂ in the reaction system to adjust SiO₂/Al₂O₃ ratio of the reaction mixture was necessary for the formation of zeolite X. The characterization of XRD, SEM and N₂ adsorption of the resulting composites had a hierarchical pore structure, which shows that pure X-type zeolite phase with high crystallinity could be obtained regardless of the content of carbon in the composites.     

Development of preparation method to control silica sol–gel synthesis with rheological and morphological measurements

In this paper, we report the effects of pH and salt on the gelling properties of silica (SiO₂) sols and gels which were measured both by dynamic viscoelasticity measurement and by creep test equipment. The network structures built by SiO₂ particles formed under different conditions were observed by a scanning probe microscope (SPM). N₂ adsorption measurements were also conducted in order to analyze the characteristics of dried gel powders including specific surface area, pore volume, and pore size distribution. The objective of this study is to characterize the SiO₂ sol, gels, and powders obtained by a Y-shaped reactor under different pH and salt conditions.     

Porphyrin intercalation into a layered niobate derived from K4Nb6O17

The incorporation of guest species into two-dimensional inorganic structures can lead to materials with interesting chemical, catalytic, electronic, optical or mechanical properties. Concerning porphyrins and metalloporphyrins intercalation compounds, nanostructured materials have been obtained and evaluated in studies about photoprocess and catalytic reactions in confined media. The intercalation of bulky species such porphyrins into layered niobates is not easy to perform due to their high layer charge densities when compared to other layered materials. In this work we describe a method for TMPyP [5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H, 23H-porphyrin] intercalation into a layered niobate derived from K₄Nb₆O₁₇. The potassium precursor was converted into the acidic-exchanged form and then intercalated with n-butylamine to produce an expanded material that was later used in the production of a dispersion containing exfoliated niobate sheets. The niobate dispersion was dropped into a porphyrin solution originating an organic-inorganic hybrid composite of formula TMPyP_0.35H_0.6K₂Nb₆O₁₇·3H₂O. XRD data suggest a tilted arrangement of the TMPyP ring with respect to the layers. Spectroscopic data (uv-visible absorption, fluorescence and resonance Raman) showed that TMPyP is intercalated in a non protonated form and that the interaction with the niobate layers surface is weak, corroborating with the proposed tilted orientation in the interlayer region.     

Thickness dependence of high-k materials on the characteristics of MAHONOS structured charge trap flash memory

The electrical characteristics of tunnel barrier engineered charge trap flash (TBE-CTF) memory of MAHONOS (Metal/Al₂O₃/HfO₂/SiO₂/Si₃N₄/SiO₂/Si) structure were investigated. The stack of SiO₂/Si₃N₄/SiO₂ films were used as engineered tunnel barrier, HfO₂ and Al₂O₃ films were used as charge trap layer and blocking oxide layer, respectively. For comparison, the electrical characteristics of MONOS (Metal/SiO₂/Si₃N₄/SiO₂/Si), MONONOS (Metal/SiO₂/Si₃N₄/SiO₂/Si₃N₄/SiO₂/Si), and MAHOS (Metal/Al₂O₃/HfO₂/SiO₂/Si) were also evaluated. The energy band diagram was designed by using the quantum-mechanical tunnel model (QM) and then the CTF memory devices were fabricated. As a result, the optimized thickness combination of MAHONOS structure was confirmed. The tunnel barrier engineered MAHONOS CTF memory showed a considerable enhancement of program/erase (P/E) speeds, retention time and endurance characteristics.     

The Degradation Behavior of SiC<Subscript>f</Subscript>/SiO<Subscript>2</Subscript> Composites in High-Temperature Environment

SiC_f/SiO₂ composites had been fabricated efficiently by Sol-Gel method. The oxidation behavior, thermal shock property and ablation behavior of SiC_f/SiO₂ composites was investigated. SiC_f/SiO₂ composites showed higher oxidation resistance in oxidation atmosphere, the flexural strength retention ratio was larger than 90.00%. After 1300 °C thermal shock, the mass retention ratio was 97.00%, and the flexural strength retention ratio was 92.60%, while after 1500 °C thermal shock, the mass retention ratio was 95.37%, and the flexural strength retention ratio was 83.34%. After 15 s ablation, the mass loss rate was 0.049 g/s and recession loss rate was 0.067 mm/s. The SiO₂ matrix was melted in priority and becomes loosen and porous. With the ablation going on, the oxides were washed away by the shearing action of the oxyacetylene flame. The evaporation of SiO₂ took away large amount of heat, which is also beneficial to the protection for SiC_f/SiO₂ composites.     

Room temperature synthesis of Si-MCM-41 using polymeric version of ethyl silicate as a source of silica

Synthesis of mesoporous MCM-41 materials at room temperature using less expensive polymeric version of ethyl silicate (40 wt% SiO₂) as a source of silica was established. The influence of crucial synthesis parameters such as molar ratios of H₂O/NH₄OH, NH₄OH/SiO₂ and CTMABr/SiO₂ in gel on the quality of the phase formed was investigated. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and low temperature N₂ adsorption-desorption isotherms have been employed to characterize the products. The magnitude of orderness, textural properties and thermal stability of the Si-MCM-41 samples prepared under identical judiciously pre-controlled synthesis conditions using ethyl silicate and conventional tetraethyl orthosilicate (TEOS) were assessed. Even though, ethyl silicate has proved to be suitable source for the preparation of MCM-41 at room temperature, there exists an optimum value of H₂O/NH₄OH for different NH₄OH/SiO₂ molar ratios in the gel. Changes in the morphology were observed when NH₄OH/SiO₂, H₂O/NH₄OH molar ratios in the gels were changed.