Boron-doped mullite derived from single-phase gels

Mullite with low dielectric constant and high transparency in infrared and microwave range has potential applications in communication industry. To improve the above properties of mullite, boron-doped mullite single-phase gels with a constant molar ratio of Al/Si = 3/1 and various B/Al ratios (B/Al = 0–0.4/3) were prepared in this study by slow hydrolysis of aluminum nitrate, boric acid and tetraethoxysilane. It was found that boron reduces the mullite formation temperature and suppresses spinel formation. The cell unit lattice parameters and cell volume in boron-doped mullite generally decrease with the increase of boron amount. The SEM observation shows that a small amount of boron reduces the grain sizes of mullite sintered bodies while a large amount of boron facilitates the formation of elongated grains and the amorphous glass phase. Boron decreases the transmittance of mullite ceramic and produces additional intensive absorption bond at 3.9 μm and also reduces the dielectric constants in the frequent range of 1 M–1 GHz.     

Thermal evolution and structural study of 2:1 mullite from monophasic gels

Single phase mullite gels with composition 2Al₂O₃·SiO₂ (2:1) were prepared by the slow hydrolysis method using aluminium nitrate nonahydrate and tetraethylorthosilicate as reagents. The evolution to mullite from gels was studied by infrared (IR) spectroscopy and X-ray diffraction (XRD). Gels thermally treated under fast schedules showed mullite formation below 900 °C. Compositional and microstructural changes in 2:1 mullites through the range of temperature from 900 to 1600 °C were determined by the measurement of lattice parameters and field emission scanning electron microscopy. The alumina-rich mullites formed at low temperatures become almost the nominal 2:1 at 1600 °C. This result is consistent with available thermodynamic data for mullite formation from alumina and silica. Microstructural examination indicated an almost constant grain size for mullite from 900 to 1600 °C.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Solubility and microstructural development of TiO2-containing 3Al2O3·2SiO2 and 2Al2O3·SiO2 mullites obtained from single-phase gels

The interdependence of the titanium oxide amount and the anisotropic growth of mullites prepared from single-phase gels were investigated. Gels with stoichiometries 3(Al_2−xTi_xO₃)·2(SiO₂) and 2(Al_2−xTi_xO₃)·(SiO₂), with 0 ≤ x ≤ 0.15 were prepared by the semialkoxide method. Gels and specimens heated at temperatures between 1200 and 1600 °C were characterized by using infrared spectroscopy (IR), X-ray diffraction (XRD) and transmission and field emission scanning electron microscopies (TEM and FESEM). Al₂TiO₅ as minor impurity was detected in both series of mullites for gel precursor compositions x = 0.10 and x = 0.15, obtained at temperatures between 1200 and 1600 °C. Variations of lattice parameters of mullite, processed at temperatures from the range between 1400 and 1600 °C, with the starting nominal amount of titanium oxide indicated that the solubility limit of titanium oxide was in ranges 3.8–4.1 and 4.1–4.4 wt% TiO₂ for 3:2 and 2:1 mullites series, respectively. The anisotropic growth of titanium-doped mullite crystalline grains was significant only when the nominal amount of titanium oxide exceeded the limit of solubility into the mullite structure (for both mullite series). Stronger anisotropy occurred for the 3:2 series specimens, i.e. for the SiO₂-richer mullites. In both series of mullites, the anisotropic grain growth was observed for the process temperatures higher than 1400 °C; the crystalline grains of mullites processed at lower temperatures were equiaxials and of almost the same size.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.     

Origin of extrinsic dielectric loss in 1:2 ordered,single-phase BaMg1/3Ta2/3O3

Among the family of temperature-compensated microwave dielectric ceramics, BaMg_1/3Ta_2/3O₃ shows the lowest dielectric loss and remains a material of choice for state-of-the-art airborne and land-based communication systems. We report on the compositional stability range, microwave dielectric properties, and the degree of atomic order of the title compound within the BaO–MgO–Ta₂O₅ ternary diagram. In most cases an atomic order is robust to the deviation from stoichiometry with an exception of Ba-rich and/or Ta-deficient samples which favor (partial) disorder. We further demonstrate that the dense, atomically ordered BaMg_1/3Ta_2/3O₃ ceramic shows large variation of dielectric loss within a single phase composition region – a clear message that the dielectric loss in practical ceramics is dominated by extrinsic sources and that the cation order alone is insufficient to achieve a minimum dielectric loss in BaMg_1/3Ta_2/3O₃. The low-temperature dielectric relaxation studies suggest that the extrinsic dielectric loss in the title compound is due to the ‘rattling’ of the off-centered Mg²⁺ ions misplaced at the Ba sites. Controlled deviation from the BaMg_1/3Ta_2/3O₃ stoichiometry toward the Mg-deficient region leads to suppression of the extrinsic dielectric loss as a result of the reduced chemical activity of Mg ion.     

Industrial waste silica-alumina gel recycling: Low-temperature synthesis of mullite whiskers for mass production

The increasing demand for mullite whisker-reinforced, toughened ceramic materials and mullite raw materials that meet industrial requirements has prompted the search for new and alternative sources, as well as effective technologies to obtain the target products. In this work, mullite whiskers of high purity were synthesized by a vapor-liquid–solid (V-L-S) process using industrial waste silica-alumina gel and Al₂(SO₄)₃·18H₂O as raw materials, with AlF₃·3H₂O and Na₂SO₄ as additives. The effects of sintering temperatures on the mullitization reactions and mullite morphology were investigated by XRD, TG-DTA, SEM and so forth. The results suggest that the introduction of AlF₃·3H₂O and Na₂SO₄ alters the mullitization reaction path, which leads to an initial mullitization reaction temperature of 720 °C. The SEM results demonstrate that mullite whiskers transformed from secondary growth to anisotropic growth when the sintering temperature was increased from 720 °C to 825 °C. By analyzing the experimental results, the mechanism of AlF₃·3H₂O-assisted growth of mullite whiskers with Na₂SO₄ as the liquid phase template is proposed based on the “dissolution-precipitation” process. Herein, a novel and feasible solution for the recycling of silica-rich industrial waste is proposed, which offers new and simple insights into the high value-added recycling of industrial waste, which provides new ideas for the actual mass production of mullite whiskers.     

Porous mullite ceramics through diphasic gels of large boehmite and small silica particles

Mullite formation process has been studied in stoichiometric mullite (3Al₂O₃·2SiO₂) diphasic gel containing large boehmite (1 m) and small silica (10 nm) particles. It has been found that initial mullitization did not take place inside the silica phase (cristobalite), but took place in the defect -alumina phase. -alumina was stabilized by silica when the temperature was below 1350°C. At temperatures above 1350°C, mullite crystallized directly. It was suggested that silica diffused into the pores (<1.8 nm) of -alumina and prevented the collapse of -alumina pore structure. On the other hand, when silica was not present, the pore structure of -alumina collapsed and -alumina crystallized at 1100°C. Porous mullite ceramics were obtained by using special diphasic gels containing large boehmite and small silica particles.     

Lightweight and thermal insulation mullite fibers/whiskers hierarchical framework materials: Low-temperature in-situ growth method and mechanism

Mullite fibers/whiskers hierarchical structure materials (MFWs) were prepared via the three-stage method, i.e. seeds breeding, precursors introducing and whiskers growth. The mechanism of low-temperature in-situ synthesis of mullite whiskers during gas-phase reaction process has been discussed in detail. The seeds bred on mullite fibers (MFs) are the growth points and can effectively reduce the subsequent growth temperature of mullite whiskers (MWs). The precursors composed of aluminum source, silicon source and catalyst provided raw materials for whiskers growth. Under the heat treatment temperature of 800 °C, mullite seed grains were guided to in-situ transform into MWs. Moreover, MFWs fabricated via low-temperature in-situ growth mechanism on the MFs present low density (0.103–0.147 g/cm³) and ultralow thermal conductivity (0.0426–0.0514 W·m^?1·K^?1). Due to the lower whiskers growth temperature in this work than the ones in the most recent literatures, the three-stage method can be regarded as a viable strategy for low-temperature in-situ growth whiskers.     

莫来石结合Al2O3-SiC浇注料的显微结构特征

借助SEM和EDAX研究了莫来石结合Al2O3 -SiC浇注料的显微结构特征。结果表明 :在莫来石结合Al2 O3 -SiC浇注料中 ,莫来石相与填充在其间隙的玻璃相形成连续基质 ;刚玉颗粒与基质中SiO2 反应生成的二次莫来石与基质中的原生莫来石交错存在 ,构成网状结构 ;SiC颗粒表面高温氧化生成的SiO2 膜 ,改善了SiC颗粒与基质的润湿性 ,是其与基质直接结合的媒介。     

Obtaining shrink-free mullite lightweight ceramics in the AlF3-Al2O3-SiO2 system

Conclusions Using the mullite-forming mechanism in the system AlF₃-Al₂O₃-SiO₂ it is possible to synthesize mullite during firing of the goods without preheating the starting components. The solid phase synthesis of crystals of mullite leads to self-reinforcing of the ceramic material. The experimental batches of goods showed that it is possible to obtain mullite articles with an apparent density of 0.65–2.00 g/cm³.Translated from Ogneupory, No. 2, pp. 44–47, February, 1987.     

铝在路易斯酸性1-烯丙基-3甲基米唑氯铝酸离子液体中的低温电沉积(英文)

Lewis acidic 1-allyl-3-methylimidazolium chloroaluminate ionic liquids were used as promising elec-trolytes in the low-temperature electrodeposition of aluminium.Systematic studies on deposition process have been performed by cyclic voltammetry and chronoamperometry.The surface morphology and X-ray diffraction(XRD) patterns of deposits prepared at different experimental conditions were also investigated.It was shown that the nu-cleation density and growth rate of crystallites had a great effect on the structure of aluminium deposited.The crys-tallographic orientation of deposits was mainly influenced by temperature and current density.Smooth,dense and well adherent aluminium coatings were obtained on copper substrates at 10-25 mA?cm?2 and 313.2-353.2 K.More-over,the current efficiency of deposition and purity of aluminium have been significantly improved,demonstrating that the ionic liquids tested have a prospectful potential in electroplating and electrorefining of aluminium.     

莫来石结合刚玉-碳化硅质浇注料的组成与性能

研究了莫来石结合刚玉-碳化硅质浇注料的组成和性能的关系。借助SEM分析了浇注料的显微结构。掺入适当比例的氧化铝微粉和氧化硅微粉,有利于系统中二次莫来石的反应生成,可以改善浇注料的性能。     

White light emission from novel host-sensitized single-phase Y2WO6: Ln3+ (Ln3+=Eu3+, Dy3+) phosphors

A series of Eu³⁺- or Dy³⁺-doped and Eu³⁺/Dy³⁺ co-doped Y₂WO₆ in pure phase was synthesized via high-temperature solid-state reaction. X-ray diffraction, diffuse reflection spectra, photoluminescence excitation and emission spectra, the CIE chromaticity coordinates and temperature-dependent emission spectra were exploited to investigate the phosphors. Upon UV excitation at 310 nm, efficient energy transfer from the host Y₂WO₆ to dopant ions in Eu³⁺ or Dy³⁺ single-doped samples was demonstrated and those phosphors were suitable for the UV LED excitation. The intense red emission was observed in Y₂WO₆: Eu³⁺, and blue and yellow ones were observed in Y₂WO₆: Dy³⁺. Concentration quenching in Y₂WO₆: Dy³⁺ phosphors could be attributed to the electric dipole-dipole interaction. In Eu³⁺/Dy³⁺ co-doped Y₂WO₆ phosphors energy transfer process only took place from the host to Eu³⁺/Dy³⁺ ions and warm white-light emission can be obtained by adjusting the dopant concentrations. The temperature-dependent luminescence indicated Eu³⁺/Dy³⁺ co-doped Y₂WO₆ was thermally stable. Our overall results suggested that Y₂WO₆: Ln³⁺ (Ln³⁺=Eu³⁺, Dy³⁺) as warm white-light emitting host-sensitized phosphor might be potentially applied in WLEDs.     

Multi-phase glass-ceramics containing CaF2: Er3+ and ZnAl2O4:Cr3+ nanocrystals for optical temperature sensing

Oxyfluoride transparent glass-ceramics (GC) containing CaF₂ and ZnAl₂O₄ nanocrystals have been fabricated with melt-quenching method. By carrying out the heat treatment of the precursor glass (PG), Er³⁺ and Cr³⁺ were selectively partitioned into CaF₂ and ZnAl₂O₄ nanocrystals, respectively. The obtained multi-phase GC exhibited strong upconversion (UC) fluorescence of Er³⁺ as well as intense down-conversion (DC) fluorescence of Cr³⁺. Under 980 nm excitation, the green UC fluorescence of Er³⁺ due to ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition and the red DC fluorescence lifetime of Cr³⁺ due to ²E, ⁴T₂ → ⁴A₂ transition were found to be highly dependent on the temperature and makes them possibly suitable for Optical Thermometry. With least-square fitting methods, the FIR of Er³⁺ from thermally coupled energy states (²H_11/2 and ⁴S_3/2) produced maximum temperature sensing sensitivity values of 0.33% K⁻¹ at 437 K and 0.36% K⁻¹ at 267 K, respectively. Similarly, fluorescence lifetime of Cr³⁺ attributed to the parity forbidden (²E → ⁴A₂) and spin allowed (⁴T₂ → ⁴A₂) produced the maximum temperature sensor sensitivity value equal to 0.67% K⁻¹ at 535 K.     

Effects of Al2O3 crystal types on morphologies,formation mechanisms of mullite and properties of porous mullite ceramics based on kyanite

Porous mullite ceramics with different crystal shapes of mullite are fabricated by in-situ reaction with middle-grade kyanite as raw material, Al(OH)₃, γ-Al₂O₃, ρ-Al₂O₃ and α-Al₂O₃ as alumina sources. Effects of Al₂O₃ crystal types on morphology evolution and formation mechanisms of mullite, and properties of porous ceramics are investigated. Results show that mullite in the sample with Al(OH)₃ mainly shows acicular morphology, because its (001) plane has the minimum interplanar crystal spacing and maximal attachment energy, it grows fast along [001] direction by screw dislocation mechanism. With a successive slowdown in reactivities of Al(OH)₃, γ-Al₂O₃, ρ-Al₂O₃ and α-Al₂O₃, the amount and aspect ratio of mullite reduce, its growth mechanism gradually transforms into two-dimensional nucleation. Acicular mullite not only reinforces samples, but makes effective pore sizes smaller, which enable the sample with Al(OH)₃ to present low bulk density, high apparent porosity and linear changes, small average pore size and good mechanical strength.     

Growth of whisker nanocrystals in the (1 ? x)In2O3 · xSeO2 system

PbSe and In films with surface areas of 3 × 3 mm² and thicknesses of 1?C1.5 ??m were consecutively deposited on substrates made of glass of S-29 grade using the vacuum deposition method. The films were oxidized in the dry air atmosphere at a temperature of 550°C. The possibility of forming whisker nanocrystals (1 ? x)In₂O₃ · xSeO₂ on glass substrates was demonstrated. The results of X-ray microanalysis demonstrated the presence of not only lead and indium selenide, but also (1 ? x)In₂O₃ · xSeO₂, PbSeO₃, PbO and SeO₂ on the surfaces of the films under study.     

Halogen-content-dependent photoluminescence of Mn2+-doped CsPbCl3 nanocrystals

The thermodynamically viable halogen vacancy defects in CsPbCl₃ perovskite nanocrystals (NCs) are considered the main factor in weakening their optical quality. However, their separate impact on the dopant emission in Mn²⁺-doped CsPbCl₃ perovskite NCs is still under exploration owing to the absence of comparable samples. Herein, a series of Mn²⁺-doped CsPbCl₃ samples were synthesized with different oleylamine-Cl (OAm-Cl) contents based on a halogen-hot-injection strategy. It is found that the Mn²⁺ concentration of as-prepared samples fixed at 0.65%, and the Mn²⁺ photoluminescence (PL) also exhibited an OAm-Cl content-independent lifetime of ∼ 1.78 ms, implying the efficient and identical luminescence of isolated Mn²⁺ ion in all samples. In contrast, the excitonic and Mn²⁺ PL intensities of the NCs are strong related to the amount of OAm-Cl, which are attributed to the coordination effect of the suppressed excitonic nonradiative recombination owing to the passivation of chloride vacancies and the enhanced energy transfer from the host exciton to Mn²⁺ ions in the samples with sufficient OAm-Cl contents. The temperature-dependent of Mn²⁺ PL disclosed an initial increase and was followed by a decrease with increasing temperature for the samples with relatively higher OAm-Cl contents, while it monotonously decreases for the sample with lower OAm-Cl content of 0.4 mL. The different PL intensity change trend results from the effective passivation of chloride vacancies by extra OAm-Cl. Above results present here will help clarify the underlying influence mechanism of halogen vacancy on the PL properties of Mn²⁺-doped perovskite NCs, which is essential for targeted modulation of their optical properties.