Influence of Temperature on the Properties of Polycarbosilane

A polycarbosilane precursor of SiC fiber was synthesized at high temperature under high pressure from liquid polysilane (LPS), which was obtained by thermal decomposition of poly(dimethylsilane). The effect of reaction temperature on the Si–H bond content, degree of linearity, Si–Si bond content, molecular weight, molecular weight distribution, elemental composition, softening point, and yield of the polycarbosilane (PCS) was studied spectroscopically (FTIR, UV, ¹H-NMR, and ²⁹Si-NMR) and by gel permeation chromatography (GPC). The results showed that the molecular weight, yield and softening point of the PCS increased, the molecular weight distribution broadened, and the Si–Si bond content and degree of linearity decreased when the reaction temperature increased. The Si–H bond content increased when the thermolysis reaction temperature was less than 450°C and decreased when the temperature was over 460°C. Increasing of the reaction temperature affected the composition with a general decrease in the amount of carbon, hydrogen and oxygen in the product. A middle molecular weight region of PCS in the GPC appears when the reaction temperature approaches 450°C; and, the as-synthesized PCS was stable with low Si–Si bond content. Synthetically, the conversion process is initially the formation of PCS by thermal decomposition of LPS, which is followed by an increase in molecular weight via condensation of PCS molecules.     

Cationic substitution in tricalcium aluminate

Cubic and orthorhombic crystals of tricalcium aluminate doped with Na₂O, Fe₂O₃ and SiO₂ were prepared and examined using an electron probe microanalyzer. The cationic ratios based on six oxygen atoms were derived from the oxide compositions. These data, together with those in previous studies for clinker aluminates containing Mg²⁺ and K⁺, provided excellent correlations between Al+Fe and Si (Al+Fe=2.001−1.03Si) and Ca+Mg and Na+K+Si [Ca+Mg=3.006−0.51(Na+K+Si)]. The chemical variation that is constrained by these equations is well accounted for by the general formula (Na,K)_2x(Ca,Mg)_3−x−y[(Al,Fe)_1−ySi_y]₂O₆, where x is the amount of Ca substituted by Na and K (0≤x<0.158), and y is the amount of Al substituted by Si (0≤y<0.136). The crystal system changed from cubic to orthorhombic with increasing x value. The substitution of Si and Fe for Al extended the solid solution range of the orthorhombic phase to lower values of x, while its effect on the solid solution range of the cubic phase was reversed.     

Positive modification on the mechanical,tribological and oxidation properties of AlCrNbSiN coatings by regulating the Nb/Si-doping ratio

The precise control of Nb/Si-doping ratio is the critical factor to tailor AlCrNbSiN coatings with superior comprehensive properties. In this study, the effect of Nb/Si-doping ratio on the microstructure, mechanical, tribological and oxidation properties of AlCrNbSiN coatings was systematically researched. With the increase of Nb/Si-doping ratio, coatings’ microstructures changed from a featureless dense structure to a columnar and equiaxed mixed microstructure gradually. The main phase was transformed from the solid solution phase of h-Al(Cr)N for Nb-free coating (Nb/Si = 0:1) to c-Al(Cr)N solid solution for three Nb-containing coatings (Nb/Si = 1:2, 1:1 and 2:1). When Nb/Si ratio is 1:1, the formation of harmful h-NbN phase was found in the coating. The performance results indicated that, (1) The AlCrNbSiN coating with the Nb/Si ratio of 2:1 achieved optimal hardness (~34.9 GPa), toughness (CPRs ~569.3) and the minimum wear rate of 2.34 × 10⁻⁶ mm³/(N·m); (2) When the Nb/Si-doping ratio is 1:2, the coating exhibited the best oxidation resistance, attributing to the sufficient (Al, Si)O_x oxidation protective layer and only a small amount of AlNbO₄ and CrNbO₄ formed at 1200 °C.     

SU-57 – An aluminosilicogermanate with a DFT topology and variable composition

An aluminosilicogermanate with a DFT zeotype framework (denoted as SU-57) was synthesized for the first time by ethanol-assisted hydrothermal synthesis at 160 and 170 °C. The compound was characterized by single crystal and powder X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy (EDS), thermogravimetry (TG) and solid state nuclear magnetic resonance (NMR) spectroscopy. SU-57 crystallizes in space group P4₂/n with estimated a ≈ 10.38–10.46 Å, c ≈ 8.88–8.91 Å, V ≈ 957–975 Å³. It has variable Al–Si–Ge composition with an approximate formula |C₂H₁₀N₂| [Al(Si_xGe_1?x)O₄]₂ (x ≈ 0.3–0.9), which results in a super-structure originated from different cation occupancies of the two unique tetrahedral (T) sites. Single crystal X-ray structure refinements, together with results from X-ray powder diffraction (XRPD) and EDS analysis, showed that (i) the AlO₄ and (Si, Ge)O₄ tetrahedra are only partially ordered over the DFT framework and do not follow a strict alternating manner. (ii) Al resides predominantly on one T site and Si and Ge predominantly on the other. (iii) The Al cation concentration (Al/(Al + Si + Ge)) is nearly constant and slightly less than 50 at%, while the Si and Ge cation concentrations vary over a large range. (iv) Al and Ge occupy both T sites. The cation disorder was confirmed by ²⁷Al and ²⁹Si solid state NMR. TG analysis and in situ XRPD showed that SU-57 was stable up to 375 °C in N₂ atmosphere.     

Synthesis and optical properties of intense blue colors oxides based on Mn5+ in tetrahedral sites in Ba7Al2-xMnxO10+y

Mn-doped Ba₇Al_2-xMn_xO_10+y (0 ≤ x ≤ 0.7) samples with intense blue colors are successfully prepared by solid state reactions. With the increase of Mn content, the colors of compounds are tuned from cyan to ocean blue. FT-IR absorption spectrum and ²⁷Al NMR spectrum indicate that the structure contains a dominating portion of Al–O tetrahedral sites while a small number of Al–O octahedral sites may also exist. The oxidation state of Mn is proved to be +5. Based on all the analysis, the intense blue shades of the samples are proposed as the d-d transition of Mn⁵⁺ ions in [AlO₄] tetrahedral sites.     

Experimental study of Si-Al substitution in calcium-silicate-hydrate (C-S-H) prepared under equilibrium conditions

C-A-S-H of varying Al/Si and Ca/(Al + Si) ratios have been prepared introducing C-S-H (Ca/Si = 0.66 and 0.95) at different weight concentrations in a solution coming from the hydration of tricalcium aluminate (Ca₃Al₂O₆) in water. XRD and EDX (TEM) analyses show that using this typical synthesise procedure, pure C-A-S-H is obtained only for calcium hydroxide concentrations below 4.5 mmol L^− 1. Otherwise, calcium carboaluminate or strätlingite is also present beside C-A-S-H. The tobermorite-like structure is maintained for C-A-S-H. A kinetic study has shown that the formation of C-A-S-H is a fast reaction, typically less than a few hours. The Ca/(Al + Si) ratio of C-A-S-H matches the Ca/Si ratio of the initial C-S-H, in the ionic concentration range studied i.e., less than 4.5 and 3 mmol L^− 1 of calcium hydroxide and aluminium hydroxide respectively. The Al/Si ratio increases with the aluminium concentration in the solution and reaches a maximum value of 0.19.     

Aluminum Incorporation in the C–S–H Phase of White Portland Cement–Metakaolin Blends Studied by 27Al and 29Si MAS NMR Spectroscopy

The composition and structure of the calcium‐silicate‐hydrate (C–S–H) phases formed by hydration of white portland cement–metakaolin (MK) blends have been investigated using ²⁷Al and ²⁹Si MAS NMR. This includes blends with 0, 5, 10, 15, 20, 25, 30 wt% MK, following their hydration from 1 d to 1 yr. ²⁹Si MAS NMR reveals that the average Al/Si ratio for the C–S–H phases, formed by hydration of the portland cement–MK blends, increases almost linearly with the MK content but is invariant with the hydration time for a given MK content. Correspondingly, the average aluminosilicate chain lengths of the C–S–H increase with increasing MK content, reflecting the formation of a C–S–H with a lower Ca/Si ratio. The increase in Al/Si ratio with increasing MK content is supported by ²⁷Al MAS NMR which also allows detection of strätlingite and fivefold coordinated aluminum, assigned to AlO₅ sites in the interlayer of the C–S–H structure. Strätlingite is observed after prolonged hydration for MK substitution levels above 10 wt% MK. This is at a somewhat lower replacement level than expected from thermodynamic considerations which predict the formation of strätlingite for MK contents above 15 wt% after prolonged hydration for the actual portland cement–MK blends. The increase in fivefold coordinated Al with increasing MK content suggests that these sites may contribute to the charge balance of the charge deficit associated with the incorporation of Al³⁺ ions in the silicate chains of the C–S–H structure.     

Carbonation of C–S–H and C–A–S–H samples studied by 13C, 27Al and 29Si MAS NMR spectroscopy

Synthesized calcium silicate hydrate (C–S–H) samples with Ca/Si ratios of 0.66, 1.0, and 1.5 have been exposed to atmospheric CO₂ at room temperature and high relative humidity and studied after one to 12 weeks. ²⁹Si NMR reveals that the decomposition of C–S–H caused by carbonation involves two steps and that the decomposition rate decreases with increasing Ca/Si ratio. The first step is a gradual decalcification of the C–S–H where calcium is removed from the interlayer and defect sites in the silicate chains until Ca/Si = 0.67 is reached, ideally corresponding to infinite silicate chains. In the seconds step, calcium from the principal layers is consumed, resulting in the final decomposition of the C–S–H and the formation of an amorphous silica phase composed of Q³ and Q⁴ silicate tetrahedra. The amount of solid carbonates and of carbonate ions in a hydrous environment increases with increasing Ca/Si ratio for the C–S–H, as shown by ¹³C NMR. For C–A–S–H samples with Ca/Si = 1.0 and 1.5, ²⁷Al NMR demonstrates that all aluminium sites associated with the C–S–H are consumed during the carbonation reactions and incorporated mainly as tetrahedral Al(–OSi)₄ units in the amorphous silica phase. A small amount of penta-coordinated Al sites has also been identified in the silica phase.     

Synthesis and characterisation of gel-derived mullite precursors from rice husk silica

The sol–gel synthesis and characterization of mullite precursor derived from rice husk silica and aluminum nitrate hydrate [(Al(NO₃)₃·9H₂O] has been investigated. The samples were characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) coupled with Rietveld analysis, and scanning electron microscopy (SEM). FTIR results showed the presence of Si–O–Si, Al–O–Al, and Si–O–Al functional groups, which were associated with mullite, corundum, quartz, and cristobalite, as verified by XRD analysis. It is concluded that mullite formation started at 1150 °C, and its abundance increased rapidly with an increase in temperature from 1150 to 1350 °C, resulting in increased phase content from 30.9 to 67.7 wt%. Although mullite was formed at a low temperature, the complete reaction between corundum and silica to form mullite was not achieved. This finding demonstrated that rice husk silica is a potential alternative raw material for the production of mullite ceramic.     

Variation of crystal structure and defect luminescence in Ca3−xSrxAl2O6 phosphors

Ca_3−xSr_xAl₂O₆(x=0, 1.07, 1.88, 1.98, 3) phosphors were prepared through solid state reaction route under different atmospheres and two kinds of oxygen-defects were identified in Ca_3−xSr_xAl₂O₆ by means of X-ray photoelectron and electron spin-resonance spectra. Their defect luminescence behavior was discussed based on photoluminescence spectra as well as the calculation of bond volume polarizability of chemical bonds. Interstitials oxygen O_i is easily formed by the lattice oxygen (O1) with increasing Sr²⁺ content and air atmosphere may also provide O_i(O_i=1/2O₂). Besides, emission intensities of samples regularly changed by accommodating more O_i in air. By selecting proper excitation energy (276/355 nm), emission occurred from different O_i defect centers, with Spectral regions from 350 nm to 600 nm. Analysis suggested that the tunable emission characteristic at different excitation energies was linked with abnormal O_i, which was located in the interior of [AlO₄] six-member rings and the capacity of accommodating O_i is related to the size of six-member rings. Based on the theoretical and experimental results, a model was proposed to explain the mechanisms related to emission tunability accordingly. This research may help elucidate emission induced by an oxygen defects system.     

Effect of Ce-doping on microstructure and electrical properties of LaAlO3 ceramics

A series of novel negative temperature coefficient (NTC) thermistor materials based on La_1-xCe_xAlO₃ (0 ≤ x ≤ 0.2) ceramics were synthesized via the solid-state route. X-ray diffraction results confirmed the successful doping of Ce in the La_1-xCe_xAlO₃ crystal and the formation of a good solid solution. Scanning electron microscopy results indicated that Ce doping is beneficial for grain growth and reduces the porosity of the samples. With the increase in the Ce doping amount, the average grain size increased from 2.1793 to 10.7344 μm, and densities of the ceramics increased from 93.15% to 99.26%. The temperature vs resistance curve indicated that Ce doping reduces the resistivity of LaAlO₃ materials, while reducing the B_200/1400 value of the LaAlO₃ ceramic. For a doping amount of 0.2, the B_200/1400 value of the LaAlO₃ ceramic decreased from 18175.1 to 4897.7K, and the resistivity at 1000 °C decreased from 68971.87 to 1105.15 Ω cm. In addition, the La_1-xCe_xAlO₃ (0 ≤ x ≤ 0.2) series materials exhibited good linear NTC characteristics. X-ray photoelectron spectroscopy results revealed that the resistivity of the LaAlO₃ materials decreased after Ce doping owing to the transformation between the Ce⁴⁺ and Ce³⁺ valence states，and the concentration of Ce³⁺ increased with the increase in the Ce doping amount. Ce³⁺ increases the concentration of oxygen vacancies, decreasing the resistance. Impedance analysis findings suggested that the resistance of the La_1-xCe_xAlO₃ (0 ＜ x ≤ 0.2) material mainly originates from the grain. These results indicate that Ce doping is an effective method to reduce the resistivity of LaAlO₃. Consequently, La_1-xCe_xAlO₃ (0 ≤ x ≤ 0.2) is a promising material for NTC applications.     

Synthesis and photoluminescence properties of La1−xAlO3:xTb green phosphors for white LEDs

Tb³⁺-doped La_1−xAlO₃ phosphor powders are successfully synthesized by the solution combustion method, using citric acid as the combustion fuel. The crystal structure and photoluminescence properties of La_1−xAlO₃:xTb³⁺ phosphors are studied, depending on Tb³⁺ content. The strongest emission peak is found at 543 nm, which originates from the ⁵D₄→⁷F₅ transition of Tb³⁺ ions, indicating green emission. Among the fabricated phosphors, the La_0.9AlO₃:0.1Tb³⁺ phosphor emits the strongest green light. The excellent luminescent properties make it a possible candidate for white light-emitting diodes and various photonic applications.     

The Effect of Alkali Ions on the Incorporation of Aluminum in the Calcium Silicate Hydrate (C–S–H) Phase Resulting from Portland Cement Hydration Studied by 29Si MAS NMR

The incorporation of aluminum in the calcium–silicate–hydrate (C–S–H) phases formed by hydration of three different white Portland cements has been investigated by ²⁹Si MAS NMR. The principal difference between the three cements is their bulk Al₂O₃ contents and quantities of alkali (Na⁺ and K⁺) ions. ²⁹Si MAS NMR allows indirect detection of tetrahedral Al incorporated in the silicate chains of the C–S–H structure by the resonance from Q²(1Al) sites. Analysis of the relative ²⁹Si NMR intensities for this site, following the hydration for the three cements from 0.5 d to 30 weeks, clearly reveals that the alkali ions promote the incorporation of Al in the bridging sites of the dreierketten structure of SiO₄ tetrahedra in the C–S–H phase. The increased incorporation of Al in the C–S–H phase with increasing alkali content in the anhydrous cement is in accord with a proposed substitution mechanism where the charge deficit, obtained by the replacement of Si⁴⁺ by Al³⁺ ions in the bridging sites, is balanced by adsorption/binding of alkali ions in the interlayer region most likely in the near vicinity of the AlO₄ tetrahedra. This result is further supported by similar ²⁹Si MAS NMR experiments performed for the white Portland cements hydrated in 0.30M NaOH and NaAlO₂ solutions.     

Electronic and structural properties of reduced-charge montmorillonites

Solid state silicon (²⁹Si) and aluminum (²⁷Al) nuclear magnetic resonance (NMR) spectroscopies were applied to a series of reduced-charge montmorillonites (RCMs) to discern changes in electronic and structural properties that are induced by Li fixation. Room temperature ²⁹Si MAS NMR spectra revealed a consistent chemical shift to more negative values and increased line width of the main Q³(0Al) Si resonance with increasing levels of Li fixation in the RCM series. A decreased line width of the octahedral Al (Al_[6]) environment was observed and may be attributed to formation of a more uniform electronic environment surrounding Al_[6] as charge reduction occurs. No appreciable changes in the tetrahedral Al (Al_[4]) peak were observed for the series, except for line broadening. Correlations of ²⁹Si NMR chemical shifts with layer charge and infrared-active structural vibrations indicated that distortions in the Si–O–T bond angles (T=tetrahedral Si or Al) occurred, with the mean Si–O–T bond angle increasing, following charge reduction. These results are interpreted as evidence of a redistribution both of layer charge and an abatement of the fit between octahedral and tetrahedral sheets following Li fixation and charge reduction. Our results are consistent with the formation of pyrophyllite-like character in reduced-charge montmorillonite.     

Development of novel perovskite based oxide ion conductor

The phase evolution and electrical conductivity of undoped and calcium-doped GdAlO₃ samples have been investigated. The undoped and doped compositions of GdAlO₃ were prepared through citrate gel process. Analysis of the phases was carried out using X-ray diffraction (XRD). The microstructural evaluation of the samples was carried out by SEM/EDS. The electrical conductivity of Gd_1−xCa_xAlO_3−δ (x = 0-0.3) was measured using ac impedance spectroscopy as a function of temperature ranging from 300 to 1000 °C under air. Among the doped compositions, Gd_0.85Ca_0.15AlO_3−δ exhibited a conductivity of 0.057 S/cm at 1000 °C.     

The Sites of Molecular and Dissociative Hydrogen Adsorption in High-Silica Zeolites Modified with Zinc Ions. III DRIFT Study of H2 Adsorption by the Zeolites with Different Zinc Content and Si/Al Ratios in the Framework

study of dihydrogen adsorbed at 77K by the zinc-modified hydrogen forms of mordenite and ZSM-5 zeolites with the different Si/Al ratios in the framework indicated the existence of several adsorption sites connected with the modifying zinc ions. The fraction of the sites of the strongest perturbation of adsorbed molecular hydrogen with the H–H stretching frequency of ca. 3930–3950 cm^-1 is the highest at the highest Si/Al ratios of 41 or 80. These sites dissociatively adsorb hydrogen at moderately elevated temperatures. Adsorption of hydrogen with the higher H–H stretching frequency of about 3970–4000 cm^-1 predominates at the lower Si/Al ratios. It does not result in the dissociation of adsorbed molecules. It was concluded that the most active sites are most probably connected with the zinc ions, which compensate two distantly separated negatively charged [AlO₄]^- tetrahedra localized in the adjacent five- or six-membered rings on the walls of the large channels of the pentasil's framework. The sites of the weaker perturbation of adsorbed hydrogen are most probably connected with Zn⁺² ions at the conventional exchangeable cationic positions with two aluminum atoms in the same five- or six-membered ring.     

Optimum synthesis of Li2Fe1−xMnxSiO4/C cathode for lithium ion batteries

Li₂Fe_1−xMn_xSi0₄/C cathode materials were synthesized by mechanical activation-solid-state reaction. The effects of Mn-doping content, roasting temperature, soaking time and Li/Si molar ratio on the physical properties and electrochemical performance of the Li₂Fe_1−xMn_xSi0₄/C composites were investigated. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), charge-discharge tests and AC impedance measurements. SEM images suggest that the morphology of the Li₂Fe_1−xMn_xSi0₄/C composite is sensitive to the reaction temperature. Samples synthesized at different temperatures have different extent of agglomeration. Being charged-discharged at C/32 between 1.5 and 4.8 V, the Li₂Fe_0.9Mn_0.1Si0₄/C synthesized at the optimum conditions shows good electrochemical performances with an initial discharge capacity of 158.1 mAh g⁻¹ and a capacity retention ratio of 94.3% after 30 cycles. AC impendence investigation shows Li₂Fe_0.9Mn_0.1SiO₄/C have much lower resistance of electrode/electrolyte interface than Li₂FeSiO₄/C.     

Method for preparing polyaluminocarbosilane

Polyaluminocarbosilane (PACS) was synthesized directly by the one‐pot reaction of polydimethylsilane (PDMS) with aluminum acetylacetonate [Al(acac)₃] in an autoclave. In this closed system, all the aluminum in Al(acac)₃ was converted into PACS. Therefore, the content of aluminum could be readily controlled quantitatively. On the basis of Fourier transform infrared, ¹H‐NMR, ¹³C‐NMR, ²⁹Si‐NMR, and ²⁷Al magic‐angle spinning NMR analysis, the reaction mechanism was proposed as follows: PDMS dissociated during pyrolysis to generate silicon free radicals, and then they reacted with Al(acac)₃ to yield PACS containing (Si? O)_nAl groups (n = 4, 5, or 6). Meanwhile, these reactions resulted in the cleavage of O? C and/or O?C bonds in Al(acac)₃. Some of the free‐radical fragments generated by this cleavage continued to react with the silicon free radicals and were incorporated into the structural units of PACS; the rest of them may have been converted into small oxygen‐containing compounds, which were removed in the subsequent processing after the reactions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Facile synthesis of AlOx dielectrics via mist-CVD based on aqueous solutions

Aluminum oxide (AlO_x) thin films were synthesized by mist-chemical vapor deposition (mist-CVD) using aluminum acetylacetonate (Al(acac)₃) dissolved in an aqueous solvent mixture of acetone and water. Nitrogen gas was used to purge the precursor solution and growth rates between 7.5–13.3 nm/min were achieved at substrate temperatures of 250–350 °C. The AlO_x layers deposited at temperatures below 350 °C exhibit 3–5 at% residual carbon levels, however those grown at 350 °C exhibit only 1–2 at% carbon impurity. Reasonable dielectric properties were obtained in the latter, with a dielectric constant (κ) of ~ 7.0, breakdown field of ~ 9 MV/cm and relatively low leakage current density of ~ 8.3×10⁻¹⁰ A/cm².     

Si–Al thin film anode material with superior cycle performance and rate capability for lithium ion batteries

To improve the electrochemical performance of Si thin film, we have investigated the effect of the addition of Al in the film. The Si–Al thin film were prepared by co-deposition from Si target embedded with Al rods on Cu foil. The atomic ratio of Al in the film is 18.69% estimated by energy-dispersive spectroscopy. The XRD and TEM analysis revealed that the Si–Al thin film was a complete amorphous structure. The electrochemical performance of the Si–Al thin film as anode material for lithium ion battery was investigated by the cyclic voltammetry and charge/discharge tests. The Si–Al thin film delivered a high reversible capacity of 2257.8 mAh g⁻¹ and an initial Coulombic efficiency of 85.9% at 0.05C rates. Compared with pure Si thin film with the same thickness, Si–Al thin film showed superior rate capability and cycle performance. And the Li⁺ diffusion coefficient of Si–Al thin film is much higher than that of Si thin film.