Formation of gyrolite during hydrothermal synthesis in the mixtures of CaO and amorphous SiO2 or quartz

Parameters of gyrolite hydrothermal synthesis were determined when primary mixtures consisting CaO and amorphous SiO₂·nH₂O or quartz and a sequence of intermediary compound formation were examined and explained. The molar ratio (C/S) of the primary mixtures was 0.66 (C—CaO; S—SiO₂) and the water/solid ratio (W/S) of the unstirred suspension was 10. Hydrothermal synthesis was carried out in saturated steam at 150, 175, and 200 °C temperatures. The duration of isothermal curing was 4, 16, 24, 32, 48, 72, and 168 h.Gyrolite does not form even after a week in the mixtures of CaO and amorphous SiO₂ at 150 °C temperature in saturated steam. Increase in the temperature positively affects the synthesis of this compound—pure gyrolite at 175 °C was obtained after 72 h at 200 °C—after 32 h. It should be noted that while synthesizing gyrolite, intermediary compounds C-S-H (I) and Z-phase are always formed. The mechanism of hydrothermal reactions and the sequence of compounds to be formed in the mixtures of CaO and quartz are totally different. Due to low quartz solubility rate at temperature range from 150 to 200 °C, neither Z-phase nor gyrolite forms even during 72 h of hydrothermal curing. In the beginning of the synthesis, α-C₂S hydrate prevails, which gradually recrystallizes into 1.13 nm tobermorite and xonotlite. Almost half of the quartz reacts during the first 4 h at 150 °C temperature, and the further decrease of its quantity depends much on the duration of hydrothermal curing. However, about 10% of the quartz does not react at all, when the C/S in the products approach approximately 0.8, stable calcium silicate hydrates—1.13 nm tobermorite and xonotlite—are formed. They are relatively stable. Experimentally obtained data and preconditions were approved by thermodynamic calculations.     

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.     

Effect of Mg-and Fe-ions on formation mechanism of aluminium titanate

The presence of Mg²⁺- and Fe³⁺-ions has an effect on the formation of Al₂TiO₅. Crystalline phases produced under the influence of the heat treatment have been identified in a heated X-ray diffraction chamber in the temperature range of 20–1500 °C. In the presence of Mg²⁺- and Fe³⁺-ions transitional phases are formed in the temperature range of 1000–1350 °C during Al₂TiO₅ formation. The XRD technique was used to identify the crystalline phases formed. On addition of MgO, chemical composition of the transitional phase formed is Mg_0.3Al_1.4Ti_1.3O₅, whereas on addition of Fe₂O₃ we have calculated a Powder Diffraction File card data for the transitional phase. Determination of the lattice parameters of the Al₂TiO₅ ceramics produced enabled verification of incorporation of Mg²⁺- or Fe³⁺-ions into the crystal lattice of Al₂TiO₅, i.e. the formation of Mg²⁺- and Fe³⁺-containing solid solutions.     

Kinetics of the CaO  quartz  H2O reaction at 120° to 180°C in suspensions

Kinetics of hydrothermal reactions have been studied for mixtures of CaO and quartz (<10 μm 10–20 μm) with Ca/Si = 0.8 and 1.0 in stirred suspensions at 120 – 180°C. Reaction proceeds through the sequence: Ca(OH)₂ + SiO₂ → Ca-rich C-S-H + SiO₂ (at 120°C) → poorly crystalline tobermorite (at 140°C)→ highly crystalline tobermorite (at 180°C) → xonotlite at 180°C and Ca/Si = 1.0 and 180°C and Ca/Si = 0.8 if 10–20 μm quartz is used. Reaction is controlled by dissolution of the quartz. For both Ca/Si ratios the radius of the 10–20 μm quartz decreases at a constant rate, viz 0.85 μm/h at 180°C, 0.13 μm/h at 140°C, 0.04 μm/h at 120°C.     

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.     

Improving the efficiency of dye-sensitized Zn2SnO4 solar cells: The role of Al ions

The dye-sensitized Zn₂SnO₄ solar cells were treated with Al³⁺ ions to enhance the power conversion efficiency for the first time. Usually, the surface treatment on photoanodes with Al³⁺ ions generated an overlayer of Al₂O₃. For Zn₂SnO₄ photoanode, another reaction pathway was found. The treatment with Al³⁺ ions led to decreasing open circuit voltage, and a 22.6% enhancement of efficiency. Mott–Schottky measurements revealed that the flat band of Zn₂SnO₄ had a positive shift owing to the introduction of Al³⁺ ions. XPS confirmed that Al³⁺ ions were introduced into the lattice of Zn₂SnO₄ and occupied the position of Sn⁴⁺, resulting in decreased conduction band edge. TEM demonstrated the size of Zn₂SnO₄ nanoparticles became larger due to the reaction of Al³⁺ with Zn₂SnO₄. Although the adsorption amounts of dyes lowered by 21%, the driving force for electron injection was greatly enhanced as a result of decreased conduction band edge, resulting in significantly enhanced cell efficiency.     

Microstructure and mechanical behavior of alumina-zirconia-mullite refractory materials

The synthesis of Al₂O₃-ZrO₂-SiO₂ (AZS) refractory materials for use in furnace chambers for glass melting has been studied in this work. Several mixtures with different alumina-zirconia-silica ratios, corresponding to compositions selected within the ternary phase diagram Al₂O₃-ZrO₂-SiO₂, were processed by attrition milling followed by pressing and reactive sintering at different temperatures (1450, 1550 and 1650 °C). The density of the sintered samples varied significantly with respect to that of the green materials, with increases of up to 90% in this property after sintering at 1650 °C. The X-ray diffraction patterns revealed the formation of the predicted phases (ZrO₂, Al₆Si₂O₁₃ and Al₂O₃) according to the ternary phase diagram for all compositions studied, with varying amounts of each formed phase. SEM characterization showed that the sintering temperature employed made a significant difference regarding the final microstructure of the sintered materials. For the studied compositions, the maximum resistance to fracture (MRF) was 185 MPa, with the highest values of fracture toughness obtained at 1650 °C.     

Calcium oxides for CO2 capture obtained from the thermal decomposition of CaCO3 particles coprecipitated with Al ions

Calcium-carbonate powders were coprecipitated with Al³⁺ and then decomposed in air and/or under a CO₂ flux between 590 °C and 1150 °C. The data were analysed using a consecutive-decomposition-dilatometer method and the kinetic results were discussed according to the microstructure analysis done by N₂ adsorption isotherms (78 K), SEM and FT-IR measurements. Below 1000 °C, CaCO₃ particle thermal-decomposition was pseudomorphic, resulting in the formation of a CaO grain porous network. When the CaO grains were formed, the Al³⁺ diffused among them, producing AlO₄ groups that promoted the CaO grain coarsening and reduced O²⁻ surface sites available to CO₂ adsorbed molecules to form CO₃²⁻. In pure CaO, CO₃²⁻ diffused through the grain boundary, enhancing Ca²⁺ and O²⁻ mobility; AlO₄ groups reduced CO₃²⁻ penetration and CaO sintering rate. Above 1000 °C, the sintering rate of the doped samples exceeded that of the undoped, likely because of Al³⁺ diffusion in CaO and viscous flow.     

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).     

Synthesis of chemically and structurally modified dicalcium silicate

This paper describes the synthesis of cements, chemically and structurally related to Ca₂SiO₄. Silica was obtained from rice hull after heating at 600 °C. Calcium oxide and small amounts of barium chloride were mixed in order to obtain a final (Ca/Si) or (Ca+Ba)/Si ratio equal to 1.95, 1.90, and 1.80, which is lower than in the conventional cement. The solids were mixed and ultrasonically treated for 1 h with a water/solid ratio of about 20. After drying and grinding, the mixtures were heated up to 1100 °C. It was possible, in some cases, to obtain a cementitious material. These cements are structurally related to β-Ca₂SiO₄ and the lower (Ca+Ba)/Si ratio obtained was 1.95. The initial chemical compositions of these cements are: (Ca_1.83+Ba_0.12)SiO₄ and (Ca_1.79+Ba_0.16)SiO₄. A further lowering in the (Ca+Ba)/Si ratio changes the nature of the silicates.     

Microwave dielectric characteristics of Nb2O5-added 0.9Al2O3–0.1TiO2 ceramics

The sintering characteristics, phase composition, and microwave dielectric properties of Nb₂O₅-added 0.9Al₂O₃–0.1TiO₂ ceramics sintered at 1300–1500 °C have been investigated. Results show that Nb⁵⁺ and Al³⁺ can co-substitute for Ti⁴⁺ and form Ti_0.8Al_0.1Nb_0.1O₂, which can lower effectively the sintering temperature, and improve the quality factor of 0.9Al₂O₃–0.1TiO₂ ceramics.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

Low-temperature synthesis of cements from rice hull ash

Rice hull is an agricultural by-product containing about 20% of silica. Usually, this material is burned at the rice fields generating small silica particles, which may cause respiratory and environmental damage. This work describes the use of rice hull ash as a raw material to prepare Ca₂SiO₄-related cements, which is a component of commercial Portland cement. Rice hull was heated at 600 °C rendering silica with a surface area of 21 m² g⁻¹. This material was mixed with CaO and BaCl₂·2H₂O in several proportions, added stoichiometricaly in order to keep a ratio (Ca+Ba)/Si=2. The solids were mixed with water 1:20 (w/w) and sonicated for 60 min. The suspensions were dried and heated at several temperatures (from 500 to 1100 °C). The resulting solids were analyzed by FT-IR spectroscopy and X-ray diffraction. Cements with structure similar to that of β-Ca₂SiO₄ were obtained at temperatures as low as 700 °C, according to the composition.     

Copper active sites for the selective reduction of nitrogen monoxide by propane on Cu-MFI catalysts and non-zeolitic supported-copper solids

The selective reduction of NO by C₃H₈ is performed on copper-based catalysts: Cu/Al₂O₃, Cu/SiO₂, Cu/SiO₂–Al₂O₃ solids, fresh and hydrothermally-treated Cu-MFI with various Si/Al ratios. For all the Cu-MFI solids and for the non-zeolitic supported-copper solids with low copper loadings, O₂ promotes the reduction of NO. The C₃H₈–NO reaction rate correlates with the number of accessible and isolated Cuⁿ⁺ ions deduced from the infrared spectroscopy of adsorbed CO. When only one type of sites is detected, the FTIR spectroscopy of adsorbed CO allows an estimation of the copper dispersion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of grinding and firing conditions on CaAl2Si2O8 phase formation by solid-state reaction of kaolinite with CaCO3

The effects of grinding and firing conditions on CaAl₂Si₂O₈ phase formation by solid-state reaction of kaolinite with CaCO₃ were investigated by differential thermal analysis (DTA)–thermogravimetry (TG), X-ray powder diffraction (XRD) and ²⁹Si and ²⁷Al MAS NMR. Unground and ground samples showed similar crystallization behavior at about 850 °C, and the crystallizing temperature was relatively unaffected by grinding. On the other hand, the crystalline products were strongly influenced by the grinding. Gehlenite (Ca₂Al₂SiO₇) was the dominant phase in the unground samples but layer-structured CaAl₂Si₂O₈ was dominant in the ground samples, together with a small amount of anorthite, which is the stable phase. The amount of anorthite gradually increased with higher firing temperature, the sample fired at 1000 °C being almost completely anorthite. Grinding treatment before firing was effective in accelerating the decomposition of CaCO₃ and extending the temperature range for the formation of CaAl₂Si₂O₈, a phase with local structure similar to that of layered CaAl₂Si₂O₈.     

A direct electrochemical route from oxides to Ti-Si intermetallics

The titanium silicide intermetallics have been directly prepared from the mixture of titanium oxide (TiO₂) and silicon oxide (SiO₂) powder by using the solid-oxygen-ion-conducting membrane (SOM) electrolysis process. The electrochemical process was carried out in a molten flux CaCl₂ at 950 °C with a potential of 3.5-4.0 V. The effects of the stoichiometry of the initial mixture on the electrolysis characteristics and the final product compositions were investigated. It has been found that the molar ratio of TiO₂:SiO₂ dominates the composition of final products. A single-phase silicide Ti₅Si₃ intermetallic was obtained when the TiO₂:SiO₂ molar ratio is 5:3; the TiSi was identified as the dominant phase with a minor amount of TiSi₂ at TiO₂:SiO₂ molar ratio 1:1; three silicide phases, Ti₅Si₄, Ti₅Si₃ and TiSi, were found coexisting in the final product produced from TiO₂-SiO₂ mixture of molar ratio 5:4; the product of electrolysis consisted of the compound Ti₅Si₃ and the pure metal Ti as TiO₂:SiO₂ molar ratio equals to 3:1; and two silicide phases, TiSi and TiSi₂, are formed as TiO₂:SiO₂ molar ratio equals to 1:2. The preliminary experimental results suggest that the electro-deoxidization process is fast and the current efficiency reached 75%.     

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.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Effects of different sintering additives on the synthesis of γ-Y2Si2O7 powders

Pure γ-Y₂Si₂O₇ powders were synthesized by the solid-liquid reaction method using Y₂O₃ and SiO₂ powders with Li₂O, MgO or Al₂O₃ additives. The effects of the metallic ions Li⁺, Mg²⁺ and Al³⁺ on the synthesis process were systematically investigated by X-ray diffraction and differential scanning calorimetry. The chemical kinetics of the Y₂Si₂O₇ synthetic process was calculated to illuminate the influences of the different metallic ions on the formation of silicate. The results indicate that the additives could effectively reduce the synthesis temperature by 100–300 °C. The apparent activation energy of the synthetic reaction was reduced by 79.75%, 65.16%, or 56.77% when 6 mol.% of Li₂O, MgO, or Al₂O₃ was added, respectively, and the reaction rate was also significantly increased.     

Co-precipitated ZnAl2O4 spinel precursor as potential sintering aid for pure alumina system

Ultra-fine ZnAl₂O₄ spinel hydrogel precursor synthesized from mixed salt solutions of Zn²⁺ and Al³⁺ ions using ammonium hydroxide–hexamethylenetetramine as basic media for co-precipitation was used as bonding material and sintering aid for pure alumina system. The hydrogel powder exhibited some well-defined ZnAl₂O₄ spinel phases at 800 °C. Alumina compacts were fabricated by incorporating small proportions of the precursor in alumina powder and firing at different temperatures (1350–1500 °C). The degree of densification was studied by measurement of fired shrinkage, apparent porosity, bulk density and cold crushing strength. Phase compositions and microstructural features of sintered samples were evaluated by XRD and SEM respectively. Addition of 0.2% hydrogel powder to alumina exhibited remarkable influence on development of high mechanical strength. The in situ formed ZnAl₂O₄ spinel dopant acted as a grain growth inhibitor in the alumina system.