Effects of CaSiO3 addition on sintering behavior and microwave dielectric properties of Al2O3 ceramics

The effects of CaSiO₃ addition on the sintering behavior and microwave dielectric properties of Al₂O₃ ceramics have been investigated. The addition of CaSiO₃ into Al₂O₃ ceramics resulted in the emergence of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which acting as liquid sintering aids can effectively lower the sintering temperature of Al₂O₃ ceramic. The Q × f value of Al₂O₃-CaSiO₃ ceramics decreased with the CaSiO₃ addition increasing because of the lower Q × f value of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈. Compared with the pure CaSiO₃ ceramic, the Al₂O₃-CaSiO₃ ceramic with 20 wt% CaSiO₃ addition possessed good dielectric properties of ?_r = 9.36 and Q × f = 13,678 GHz at the similar sintering temperature.     

Enhancement of oxygen storage capacity by reductive treatment of Al2O3 and CeO2–ZrO2 solid solution nanocomposite

Oxygen storage capacity (OSC) of CeO₂–ZrO₂ solid solution, Ce_xZr_(1−x)O₄, is one of the most contributing factors to control the performance of an automotive catalyst. To improve the OSC, heat treatments were employed on a nanoscaled composite of Al₂O₃ and CeZrO₄ (ACZ). Reductive treatments from 700 to 1000 °C significantly improved the complete oxygen storage capacity (OSC-c) of ACZ. In particular, the OSC-c measured at 300 °C reached the theoretical maximum with a sufficient specific surface area (SSA) (35 m²/g) after reductive treatment at 1000 °C. The introduced Al₂O₃ facilitated the regular rearrangement of Ce and Zr ions in CeZrO₄ as well as helped in maintaining the sufficient SSA. Reductive treatments also enhanced the oxygen release rate (OSC-r); however, the OSC-r variation against the evaluation temperature and the reduction temperature differed from that of OSC-c. OSC-r measured below 200 °C reached its maximum against the reduction temperature at 800 °C, while those evaluated at 300 °C increased with the reduction temperature in the same manner as OSC-c.     

Effect of ceria on the structure and catalytic activity of V2O5/TiO2–ZrO2 for oxidehydrogenation of ethylbenzene to styrene utilizing CO2 as soft oxidant

The present study was undertaken to investigate the influence of ceria on the physicochemical and catalytic properties of V₂O₅/TiO₂–ZrO₂ for oxidative dehydrogenation of ethylbenzene to styrene utilizing CO₂ as a soft oxidant. Monolayer equivalents of ceria, vanadia and ceria–vanadia combination over TiO₂–ZrO₂ (TZ) support were impregnated by a coprecipitation and wet impregnation methods. Synthesized catalysts were characterized by using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature programmed reduction, transmission electron microscopy and BET surface area methods. The XRD profiles of 550 °C calcined samples revealed amorphous nature of the materials. Upon increasing calcination temperature to 750 °C, in addition to ZrTiO₄ peaks, few other lines due to ZrV₂O₇ and CeVO₄ were observed. The XPS V 2p results revealed the existence of V⁴⁺ and V⁵⁺ species at 550 and 750 °C calcinations temperatures, respectively. TEM analysis suggested the presence of nanosized (<7 nm) particles with narrow range distribution. Raman measurements confirmed the formation ZrTiO₄ under high temperature treatments. TPR measurements suggested a facile reduction of CeO₂–V₂O₅/TZ sample. Among various samples evaluated, the CeO₂–V₂O₅/TZ sample exhibited highest conversion and nearly 100% product selectivity. In particular, the addition of ceria to V₂O₅/TZ suppressed the coke deposition and allowed a stable and high catalytic activity.     

Thermal shock resistance of in situ formed Si2N2O–Si3N4 composites by gelcasting

Thermal shock resistance of Si₂N₂O–Si₃N₄ composites was evaluated by water quenching and subsequent three-point bending tests of strength diminution. Si₂N₂O–Si₃N₄ composites which was prepared with in situ liquid pressureless sintering process using Yb₂O₃ and Al₂O₃ powders as sintering additives by gelcasting showed no macroscopic cracks and the critical temperature difference (ΔT_c) could be up to 1400 °C. A mass of pores existed in the sintered body and the irregular shaped fibers extended from the pores increased the thermal shock property.     

Effects of Al2O3 addition on the sintering behavior and microwave dielectric properties of CaSiO3 ceramics

The effects of Al₂O₃ addition on the densification, structure and microwave dielectric properties of CaSiO₃ ceramics have been investigated. The Al₂O₃ addition results in the presence of two distinct phases, e.g. Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which can restrict the growth of CaSiO₃ grains by surrounding their boundaries and also improve the bulk density of CaSiO₃-Al₂O₃ ceramics. However, excessive addition (≥2 wt%) of Al₂O₃ undermines the microwave dielectric properties of the title ceramics since the derived phases of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈ have poor quality factor. The optimum amount of Al₂O₃ addition is found to be 1 wt%, and the derived CaSiO₃-Al₂O₃ ceramic sintered at 1250 °C presents improved microwave dielectric properties of ?_r = 6.66 and Q × f = 24,626 GHz, which is much better than those of pure CaSiO₃ ceramic sintered at 1340 °C (Q × f = 13,109 GHz).     

Dielectric properties and microstructures of CaSiO3 ceramics with B2O3 addition

The effects of B₂O₃ additives on the sintering behavior, microstructure and dielectric properties of CaSiO₃ ceramics have been investigated. The B₂O₃ addition resulted in the emergence of CaO–B₂O₃–SiO₂ glass phase, which was advantageous to lower the synthesis temperature of CaSiO₃ crystal phase, and could effectively lower the densification temperature of CaSiO₃ ceramic to as low as 1100 °C. The 6 wt% B₂O₃-doped CaSiO₃ ceramic sintered at 1100 °C possessed good dielectric properties: _r = 6.84 and tan δ = 6.9 × 10⁻⁴ (1 MHz).     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.     

Low-temperature firing and microwave dielectric properties of 16CaO–9Li2O–12Sm2O3–63TiO2 ceramics with V2O5 addition

The sintering behaviors and microwave dielectric properties of the 16CaO–9Li₂O–12Sm₂O₃–63TiO₂ (abbreviated CLST) ceramics with different amounts of V₂O₅ addition had been investigated in this paper. The sintering temperature of the CLST ceramic had been efficiently decreased by nearly 100 °C. No secondary phase was observed in the CLST ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLST ceramics with small amounts of V₂O₅ addition could be well sintered at 1200 °C for 3 h without much degradation in the microwave dielectric properties. Especially, the 0.75 wt.% V₂O₅-doped ceramics sintered at 1200 °C for 3 h have optimum microwave dielectric properties of Kr = 100.4, Q × f = 5600 GHz, and TCF = 7 ppm/°C. Obviously, V₂O₅ could be a suitable sintering aid that improves densification and microwave dielectric properties of the CLST ceramics.     

Sintering behavior and microwave dielectric properties of a new low-permittivity ceramic system Ca(Mg1−xAlx)(Si1−x/2Alx/2)2O6

The diopside ceramics with a formula of Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆ (x=0.01–0.3) were synthesized via a traditional solid-state reaction method, and their solid solubility, sintering behavior and microwave dielectric properties were investigated. The results revealed that the solubility limit of Al₂O₃ in Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆, which is defined as x, was between 0.15 and 0.2, and a second phase of CaAl₂SiO₆ presented when the x value reached 0.2. Appropriate Al³⁺ substitution for Mg²⁺ and Si⁴⁺ could promote the sintering process and lower the densification temperature, and a broadened densification temperature range of 1250–1300 °C was obtained for the compositions of x=0.08–0.15. With the increase of the x value, the dielectric constant (ε_r) increased roughly linearly, and the temperature coefficient of frequency (τ_f) showed a rising trend. The Q×f values increased from 57,322 GHz to 59,772 GHz as the x value increased from 0.01 to 0.08, and then they were saturated in the range of x=0.08–0.2. Further increase of the x value (x≥0.25) deteriorated the microwave dielectric properties. Good microwave dielectric properties of ε_r=7.89, Q×f=59,772 GHz and τ_f=−42.12 ppm/°C were obtained for the ceramics with the composition of x=0.08 sintered at 1275 °C.     

Fabrication of SiCN-Sc2Si2O7 coatings on C/SiC composites at low temperatures

SiCN-Sc₂Si₂O₇ environmental barrier coatings were fabricated on the surface of C/SiC composites at low temperatures by adding Li₂CO₃ as sintering aids. With this addition, the fabrication temperature could be lowered about 100-200 °C. The shrinkage of the polysilazane-Sc₂Si₂O₇ bars with and without Li₂CO₃ was tested by dilatometer. The results indicate that the shrinkage speed of the polysilazane-Sc₂Si₂O₇ bar with Li₂CO₃ is faster than the one without Li₂CO₃, indicating that the Li₂CO₃ greatly promotes the sintering of polysilazane-Sc₂Si₂O₇. Water-vapor corrosion behavior of the SiCN-Sc₂Si₂O₇ coated C/SiC composites was carried out at 1250 °C. The results reveal that the SiCN-Sc₂Si₂O₇ coatings can effectively protect the C/SiC composites. The corrosion resistance of SiCN-Sc₂Si₂O₇ coatings is not degraded by adding Li₂CO₃.     

Luminescence properties of Eu activated Ba2Si5N8 red phosphors with various Eu contents

This study was carried out to characterize the crystal structure and luminescence properties of Eu²⁺ doped red-emitting Ba₂Si₅N₈ phosphor. In this research, Ba₂Si₅N₈ phosphors with various Eu compositions were prepared by normal pressure sintering (NPS). Ba₃N₂, Si₃N₄ and Eu₂O₃ were sintered at a high temperature in a mixture of N₂ and H₂. The crystal structure was analyzed by X-ray diffraction(XRD), and the photoluminescence(PL) properties of the Eu²⁺ - activated Ba₂Si₅N₈ phosphors were evaluated as a function of the Eu²⁺ activator concentration. The red-emitting Ba₂Si₅N₈ phosphors showed a broad excitation band range as well as high quantum output.     

The effect of SiO2 additions on barium aluminate cement formation and properties

Barium aluminate cements have been synthesized by barium carbonate, alumina, kaolin and colloidal silica as starting materials. The effects of the source of SiO₂ and of firing temperature on phase formation and physical properties of the fired cements have been studied. Cement samples were characterized using XRD, SEM, EDX. The setting time and heat of hydration of cements were also evaluated. The barium aluminate cements were mixed in castables. Cold crushing strengths evaluated, and values compared to those obtained using calcium aluminate cement (Secar 71). Mixtures of BaCO₃ and Al₂O₃ were targeted to produce BaAl₂O₄; which had fast set time, expansive behavior and lower strength compared to samples with SiO₂ additions. SiO₂ additions, regardless of source, resulted in BaAl₂Si₂O₈ (celsian) formation. The prepared samples had short setting times and higher mechanical properties in comparison with standard calcium aluminate cement.     

The influence of different CaO source in the production of anorthite ceramics

The effects of starting raw materials and firing conditions on anorthite (CaAl₂Si₂O₈) phase formation are investigated by differential thermal analysis (DTA)–thermogravimetry (TG) and X-ray powder diffraction (XRD). Four different sources of CaO were used for anorthite production such as Ca(OH)₂, CaCO₃, marble powder and gypsum mould waste. The mixture of raw materials was prepared in stoichiometric ratio of anorthite. Sintering of samples was carried out at various temperatures (1000–1300 °C). In all samples before the formation of anorthite phase, formation of layered alimunosilicate phase (LAS) and of gehlenite phase were observed at low temperatures (<1200 °C). All the samples showed similar crystallization behaviour at 1200 °C. The densification characteristic and the flexural strength of samples were affected by the nature of starting raw materials. The maximum density (～80%) was reached in sample ACH which was prepared from Ca(OH)₂.     

Characterisation of PEO-Al2O3 composite polymer electrolytes

The behaviour of PEO₈LiClO₄ with different quantities of α-Al₂O₃ or γ-Al₂O₃ was investigated using DSC, AC conductivity and ⁷Li NMR experiments. DSC results showed that the presence of the filler does not change the glass transition temperature of the electrolyte but, on the other hand, modifies the quantity of its crystalline phase. From the AC impedance measurements, it was observed that the sample with the highest conductivity at room temperature is PEO₈LiClO₄ 5.3 wt.% α-Al₂O₃. The change in the quantity of crystalline phase cannot alone explain the conductivity data, and it is suggested that the space charge contribution in the interphase of the filler particles and the polymeric chains influences the behaviour of the samples. The ⁷Li NMR results showed that line width narrowing begins at temperatures close to T_g. From the hydrogen decoupling experiments it was possible to estimate the LiH average distances as 2.7 Å. The LiLi distance was calculated as being between 2.6 and 3.5 Å depending on the number of near neighbours lithium nuclei used in the model.     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

Supported PdCl2CuCl2 catalysts for carbon monoxide oxidation II. XAFS characterization

A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCl₂CuCl₂ was active for the oxidation of CO near room temperature. The structure of the catalyst was studied by X-ray absorption fine structure (XAFS). The active phase of Pd was a Pd¹¹ species containing chlorine and, probably, carbonyl ligands. Direct interaction of PdPd or PdCu was not detected. The active phase of copper was found to be solid Cu₂Cl(OH)₃ particles in agreement with the X-ray diffraction (XRD) results. The presence of Cu was essential to keep the Pd in the Pd¹¹ state during the reaction. The rates of CO oxidation measured at temperatures of 30–70°C showed a minimum at 40°C, which was attributable partly to an unusual structure change of the active palladium species during the reaction at this temperature.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Novel promoting effect of SO2 on the selective catalytic reduction of NOx by ammonia over Co3O4 catalyst

Spinel nano-Co₃O₄ was prepared by solid-state reaction at room temperature and investigated for selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). Although suffering from pore filling and plugging, treatment of this catalyst by SO₂ showed novel promoting effect on NH₃-SCR above 250 °C. Bulk cobalt sulfate was observed over the sulfated Co₃O₄ with XRD, which would be an active component for NH₃-SCR. The sulphated Co₃O₄ catalyst exhibited good resistance to SO₂ (500 ppm, 100 ppm) and 10% H₂O at a space velocity of about 25 000 h⁻¹ at 300 °C, as tested for 12 h.     

Simultaneous desorption–crystallization of CO2–CaCO3 in multi-stage flash (MSF) distillers

In this work, the desorption–crystallization of CO₂–CaCO₃ in MSF distillers was simulated by coupling mass transfer with chemical reaction and correlating the CaCO₃ crystallization rates to the CO₂ release rates. The model was applied to two 20-stage reference MSF once-through and recycles distillers. The CO₂ release rates decreased exponentially from the first stage to the last stage. The CO₂ release rates increased with increasing top brine temperature (TBT) and so CaCO₃ deposition rates did. The CaCO₃ deposition rates increased by 76.9, 102.5 and 123.0 g per ton distillate at 90, 100 and 110 °C, respectively. This corresponded to fouling resistance of 0.64, 0.83 and 1.00 m²K/kW, respectively. The results were fully discussed and interpreted.     

Effect of in situ formation of Ti(C,N) on the properties of Al2O3–ZrO2–C slide plate material

Al₂O₃–ZrO₂–C slide plate was ordinarily used in sliding nozzle system because of its excellent mechanical properties and slag corrosion resistance. This study improved the slag corrosion resistance of the slide plate by the carbothermic reduction of TiO₂ and aluminothermic reduction of TiO₂ under high temperature in coke bed to generate Ti(C,N) phase in situ. The result revealed that TiC of the high melting point phase could enter into anorthite (CaAl₂Si₂O₈) of the low melting point phase, forming eutectic phase CaAl₂Si₂O₈–TiC, and improve the viscosity of the slag. Furthermore, TiC and CaAl₂Si₂O₈ captured FeO and MnO from the slag, resulting in the increase of the slag viscosity, inhibiting the penetration corrosion of the slag, and improving the slag corrosion resistance of the materials. In general, compared with the material without TiO₂ powder, the slag corrosion resistance of the material introduced 2 wt.% TiO₂ powder was increased by 24%. Meanwhile, the cold crushing strength of fired specimen at room temperature was increased by 35.6%.