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.     

Synthesis and characterization of manganese oxide-doped dicalcium silicates obtained from rice hull ash

This work describes the synthesis and hydration behavior of dicalcium silicates doped with manganese. The syntheses were performed using silica obtained from rice hull ash. The solids (SiO₂, CaO and MnO) were weighed in stoichiometric proportions to prepare silicates having a ratio (Ca + Mn)/Si = 2. Insertion of manganese varied from 1 to 10% (mol). Solids were grounded and water was added rendering aqueous suspensions. The suspensions were sonicated for 60 min in an ultrasonic bath. After drying, the resulting solids were grounded and burned at 800 °C. The preparation of calcium silicates containing up to 10% of manganese oxide was observed.Hydration degree of a dicalcium silicate and calcium silicate containing 5% of manganese was determined by thermal analysis. Both materials present similar behavior. Hydration degree reaches approximately 70% after 60 days.     

Effect of rice husk ash addition on CO2 capture behavior of calcium-based sorbent during calcium looping cycle

Rice husk ash/CaO was proposed as a CO₂ sorbent which was prepared by rice husk ash and CaO hydration together. The CO₂ capture behavior of rice husk ash/CaO sorbent was investigated in a twin fixed bed reactor system, and its apparent morphology, pore structure characteristics and phase variation during cyclic carbonation/calcination reactions were examined by SEM-EDX, N₂ adsorption and XRD, respectively. The optimum preparation conditions for rice husk ash/CaO sorbent are hydration temperature of 75 °C, hydration time of 8 h, and mole ratio of SiO₂ in rice husk ash to CaO of 1.0. The cyclic carbonation performances of rice husk ash/CaO at these preparation conditions were compared with those of hydrated CaO and original CaO. The temperature at 660 °C–710 °C is beneficial to CO₂ absorption of rice husk ash/CaO, and it exhibits higher carbonation conversions than hydrated CaO and original CaO during multiple cycles at the same reaction conditions. Rice husk ash/CaO possesses better anti-sintering behavior than the other sorbents. Rice husk ash exhibits better effect on improving cyclic carbonation conversion of CaO than pure SiO₂ and diatomite. Rice husk ash/CaO maintains higher surface area and more abundant pores after calcination during the multiple cycles; however, the other sorbents show a sharp decay at the same reaction conditions. Ca₂SiO₄ found by XRD detection after calcination of rice husk ash/CaO is possibly a key factor in determining the cyclic CO₂ capture behavior of rice husk ash/CaO.     

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.     

Nickel-niobia interaction induced by the reduction of NiNb2O6 supported on SiO2

Nickel niobate (NiNb₂O₆) supported on SiO₂ was prepared by a chemical mixing method using mixed Ni and Nb citrate solutions. X-ray diffraction study showed that the NiNb₂O₆ compound was reduced to Ni metal and NbO₂ after H₂ treatment at 600 ° C, and a strong Ni-niobia interaction was induced after the decomposition of the compound: the ethane hydrogenolysis activity was suppressed severely after high temperature reduction at 600 ° C, and recovered by O₂ treatment at 500 ° C followed by low-temperature reduction at 200 ° C. The selectivity of cyclohexane dehydrogenation was improved significantly by the Ni-niobia interaction, if compared with unpromoted Ni/SiO₂ catalyst, and the structural change and catalytic behaviors were compared with those of Rh double oxides such as RhNbO₄.     

Cs2.5H0.5PWO40/SiO2 as addition self-humidifying composite membrane for proton exchange membrane fuel cells

In this paper, we first reported a novel self-humidifying composite membrane for the proton exchange membrane fuel cell (PEMFC). Cs_2.5H_0.5PWO₄₀/SiO₂ catalyst particles were dispersed uniformly into the Nafion^® resin, and then Cs_2.5H_0.5PWO₄₀-SiO₂/Nafion composite membrane was prepared using solution-cast method. Compared with the H₃PWO₄₀ (PTA)_, the Cs_2.5H_0.5PWO₄₀/SiO₂ was steady due to the substitute of H⁺ with Cs⁺ and the interaction between the Cs_2.5H_0.5PWO₄₀ and SiO₂. And compared with the performance of the fuel cell with commercial Nafion^® NRE-212 membrane, the cell performance with the self-humidifying composite membrane was obviously improved under both humidified and dry conditions at 60 and 80 °C. The best performance under dry condition was obtained at 60 °C. The self-humidifying composite membrane could minimize membrane conductivity loss under dry conditions due to the presence of catalyst and hydrophilic Cs_2.5H_0.5PWO₄₀/SiO₂ particles.     

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.     

Improvement of oxidation resistance of unidirectional Cf/SiO2 composites by the addition of SiCp

Unidirectional carbon fiber reinforced fused silica composites (uni-C_f/SiO₂) with addition of different contents of SiC particle (SiC_p) were prepared by slurry infiltrating and hot-pressing. The model of oxygen infiltrating into the composite was supposed according to the characterization of fiber/matrix interface observed by transmission electronic microscope (TEM). The oxidation process of the composite was analyzed by thermo-gravimetry and differential scanning calorimeter (TG-DSC) method and the oxidation resistance was evaluated by the residual flexural strength and the fracture surface of the composite after heat treatment at elevated temperatures method. The results showed that the oxidation of carbon fiber started at 480 °C and ended at 800 °C and the oxidation of SiC_p started at above 1000 °C in the composite. The addition of 20 wt.% SiC_p had a better oxidation resistance. According to the characterization of fiber/matrix interface observed by TEM, gaps existed at the fiber/matrix interface which resulted from the CTE mismatch of carbon fiber and SiO₂ matrix. While the CTE mismatch between SiC_p and SiO₂ matrix could also result in the pre-existing gaps in the matrix. The oxygen penetrated along the gaps and simultaneously reacted with carbon fiber ends and SiC_p, which filled the gaps at the fiber/matrix interface and the pre-existing gaps in the matrix and subsequently prevented oxygen from infiltrating inward.     

Chemical stability and electrochemical properties of CaMoO3−δ for SOFC anode

In an effort to develop alternative anode materials based on mixed conducting ceramics capable of offering high mixed ionic-electronic conductivity, stability to redox cycles, and limited activity for carbon formation to Ni/YSZ cermets, CaMoO₃ ceramics for application as a solid oxide fuel cell (SOFC) anode material were synthesized as a function of temperature and oxygen partial pressure (pO₂). CaMoO₃ perovskite-dominant powders were obtained by reducing the CaMoO₄ showing a structure of orthorhombic unit cells with the following lattice parameters: a = 5.45 Å, b = 5.58 Å, and c = 7.78 Å. The equilibrium total conductivity of CaMoO₃, measured by DC 4-probe method in 5% H₂/balance N₂ condition (pO₂ ≈ 10⁻²² atm) at various temperatures, decreased with increasing temperature below 400 °C, indicating metallic properties with an activation energy of 0.028 eV. Between 400 °C and 600 °C, the equilibrium total conductivity slightly increased, and finally sharply decreased at 800 °C. The Mo metal precipitation during measurement was thermodynamically proved by the predominance diagram for CaMoO₃. Finally, a fuel cell with CaMoO₃ anode exhibited poor performance with a maximum power density of only 14 mW/cm² at 900 °C, suggesting that further research is needed to enhance the ionic conductivity and thus improve the catalytic properties.     

Effect of the metal oxide loading on the activity of silica supported MoO3 and V2O5 catalysts in the selective partial oxidation of methane

Precipitated silica catalysts loaded with either MoO₃ (0.2–4.0 wt%) or V₂O₅ (0.2–5.3 wt%) have been studied in the selective partial oxidation of methane to formaldehyde with molecular oxygen at 520 °C. The functionality of the SiO₂ surface towards the formation of HCHO is significantly promoted by V₂O₅, while it is depressed by the MoO₃.     

Low hydrothermal temperature synthesis of porous calcium silicate hydrate with enhanced reactivity SiO2

This study presents a novel approach for the synthesis of porous calcium silicate hydrate (CSH) at a low hydrothermal temperature of 110 °C based on enhanced reactivity SiO₂ (i.e. silica/polyethylene glycol (PEG₂₀₀₀) composites) as the source silica material. The as-prepared CSH materials exhibited a porous microstructure with a large number of small mesopores. The porosity formation mechanism of CSH was apparent that cavitation, resulting from sonication, enabled PEG₂₀₀₀ (via intercalation on silica) to break apart Si–O–Si structural units, thereby enhancing SiO₂ reactivity at a low hydrothermal temperature. In addition, the presence of PEG₂₀₀₀ effectively prevented the aggregation of particles during the formation process of the porous CSH solid. The low temperature synthesis proposed herein represents a viable and effective method for the further development of porous CSH as a functional ceramic material.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Structural investigation relating to the cementitious activity of bauxite residue — Red mud

The cementitious behavior of red mud derived from Bauxite-Calcination method was investigated in this research. Red mud were calcined in the interval 400–900 °C to enhance their pozzolanic activity and then characterized in depth through XRD, FTIR and ²⁹Si MAS-NMR techniques with the aim to correlate phase transitions and structural features with the cementitious activity. The cementitious activity of calcined red mud was evaluated through testing the compressive strength of blended cement mortars. The results indicate that red mud calcined at 600 °C has good cementitious activity due to the formation of poorly-crystallized Ca₂SiO₄. The poorly-crystallized Ca₂SiO₄ is a metastable phase which will transform into highly-crystallized Ca₂SiO₄ with the increase of calcination temperature from 700 °C moving to 900 °C. It is the metastable phase that mainly contributes to the good cementitious activity of red mud. This paper points out another promising direction for the proper utilization of red mud.     

Photochemical Degradation of 1- and 2-methylphenanthrenes in Acetonitrile and on Some Solids

In order to obtain basic information about photochemical degradability and degradation products for 1- and 2-methylphenanthrenes (MPs), photodegradation of these compounds in an organic solvent (acetonitrile) and on some solids (soil, silica gel [SiO₂], and titanium dioxide [TiO₂]) were carried out in a laboratory using solar simulators.

The results showed that the degradation rate (%) increases in the order of TiO₂ > SiO₂ > acetonitrile > soil. Products formed in acetonitrile included aromatic compounds with molecular ion [M⁺] peaks at m/z = 148, 162, 198, 206, 208, 210, 222, 224 and 238. In case of 1 MP, a compound with M⁺ 206, which was identified as 1-phenanthrene-carbaldehyde, was predominantly formed. However, 2MP showed no predominance for M⁺ 206 and also yielded M⁺ 210 and M⁺ 224 as major products. Degradation on soil and SiO₂ gave similar products to those detected in acetonitrile, while that on TiO₂ gave different products from the formers. Based on the results of product analysis, three degradation pathways which may occur in acetonitrile are proposed.     

Crystallization kinetics of amorphous alumina–zirconia–silica ceramics

Crystallization kinetics of amorphous alumina–zirconia–silica ceramics was studied by nonisothermal differential scanning calorimetry (DSC). Different amorphous materials were produced by plasma spraying of near-eutectic Al₂O₃–ZrO₂–SiO₂ mixtures. Phase composition and microstructure of the amorphous materials and nanocrystalline products were analyzed. All of the investigated materials show an exothermic peak between 940 and 990 °C in the DSC experiments. The activation energies calculated from DSC traces decrease with increasing SiO₂ concentration. Values of the Avrami coefficients together with results of the microstructural observations indicate that tetragonal zirconia crystallization from materials containing more than 10 wt.% SiO₂ proceeds by a diffusion-controlled mechanism with nucleation occurring predominantly at the beginning of the process. In contrast, material with almost no SiO₂ exhibited a value of the Avrami exponent consistent with the crystal growth governed by processes at the phase boundary.     

Polyacrylamide gel synthesis and sintering of Mg2SiO4:Eunanopowder

A Mg₂SiO₄:Eu³⁺ nanopowder was synthesized by a polyacrylamide gel method. In this route, the gelation of the solution is achieved by the formation of a polymer network which provides a structural framework for the growth of particles. The densification of the powders was also studied. An amorphous nanopowder was synthesized and crystallized to Mg₂SiO₄ after heat-treatment via a solid-state reaction at a relatively low temperature of about 700 °C. The powders prepared by the polyacrylamide gel method showed better sinterability than the powders synthesized by the conventional sol–gel method. The relative density of the sample was 97% at 1500 °C.     

Phosphorus supplying power of some thermally promoted reaction products of phosphate rocks

The P availability in soil and agronomic efficiency of the products of non-premium grade, unreactive Purulia phosphate rock (PPR) heated alone or with Na₂CO₃ or KCl at different temperatures were investigated in two P deficient soils. The heated products of PPR alone did not improve the P availability in soil or P utilisation by rice over the original PPR. The products of PPR-KCl mixtures heated at 300-900°C were not effective at all. Out of several products of PPR with Na₂CO₃, the product prepared from PPR and Na₂CO₃ mixture in the weight ratio 2:1 heated at 900°C was comparable to superphosphate (SP) with respect to P availability in soil, straw and grain yield and P uptake by rice. The effectiveness of the products of PPR-Na₂CO₃ mixtures heated at 700°C though inferior to SP were superior to that of the original PPR in the highly acidic P deficient soil from Choudwar. However, products of another phosphate rock (PR) from Jordan and NA₂CO₃ mixtures heated at 900°C were less effective in comparison to SP. The amount of inherent silica present in Jordan PR was inadequate to promote the apatite-NA₂CO₃-SiO₂ reaction towards completion thus leading to an inferior product. On the other hand, similar products of non-premium grade Kasipatnam and Mussoorie PRs which are not suitable for direct application were comparable to SP in their effectiveness when these PRs were fused with Na₂CO₃ in the weight ratio 2:1 at 900°C. X-ray diffraction studies indicated presence of water and citrate soluble phosphate phases viz., Na₃PO₄, NaCaPO₄ and possibly Ca₇ (PO₄)₂ (S10₄)₂ in these products of PR-Na₂CO₃ mixture heated at 900°C. The water and citrate soluble phases of these products could release adequate P for absorption by crop.     

Effects of Elevated CO2 and Temperature on Methane Production and Emission from Submerged Soil Microcosm

Incubation experiments were conducted under controlled laboratory conditions to study the interactive effects of elevated carbon dioxide (CO₂) and temperature on the production and emission of methane (CH₄) from a submerged rice soil microcosm. Soil samples (unamended soil; soil + straw; soil + straw + N fertilizer) were placed in four growth chambers specifically designed for a combination of two levels of temperature (25 °C or 35 °C) and two levels of CO₂ concentration (400 or 800 mol mol^–1) with light intensity of about 3000 Lx for 16 h d^–1. At 7, 15, 30, and 45 d after incubation, CH₄ flux, CH₄ dissolved in floodwater, subsurface soil-entrapped CH₄, and CH₄ production potential of the subsurface soil were determined. The results are summarized as follows: 1) The amendment with rice straw led to a severalfold increase in CH₄ emission rates, especially at 35 °C. However, the CH₄ flux tended to decrease considerably after 15 d of incubation under elevated CO₂. 2) The amount of entrapped CH₄ in subsurface soil and the CH₄ production potential of the subsurface soil were appreciably larger in the soil samples incubated under elevated CO₂ and temperature during the early incubation period. However, after 15 d, they were similar in the soil samples incubated under elevated or ambient CO₂ levels. These results clearly indicated that elevated CO₂ and temperature accelerated CH₄ formation by the addition of rice straw, while elevated CO₂ reduced CH₄ emission at both temperatures.     

Preparation of highly dispersed SiO2 and Pt particles in Nafion112 for self-humidifying electrolyte membranes in fuel cells

We have developed preparation protocol of practically large size self-humidifying polymer electrolyte membranes (PEMs) with highly dispersed nanometer-sized Pt and/or SiO₂ for fuel cells. The Pt particles were expected to catalyze the recombination of H₂ and O₂, leading to a suppression of the chemical short-circuit reaction at the electrodes, while the SiO₂ particles were expected to adsorb the water produced at the Pt particles together with that produced at the cathode reaction. Stable SiO₂ particles were formed in a commercial PEM (Nafion^®112) via in situ sol-gel reactions at 70 °C. It was found by SAXS that the hydrophilic cluster size increased by water adsorbed SiO₂, which may contribute to the increase in the proton conductivity once SiO₂ adsorbed water. Pt particles were uniformly dispersed in a Na⁺-form normal-PEM or SiO₂-PEM by an ion-exchange reaction with [Pt(NH₃)₄]Cl₂, followed by a reduction with 1-pentanol at 125 °C. The newly prepared Pt-SiO₂-PEM was found to perform a self-humidifying operation in a standard-size PEFC (25 cm² electrode area) with H₂ and O₂ humidified at 30 °C. The performance of the Pt-SiO₂-PEM cell operated with the low humidity reactant gases was as high as the normal-PEM cell fully humidified, because the ohmic resistance of the former cell was as low as the latter cell.     

Reductive Activation of Dioxygen in Catalytic Systems Including Platinum and Heteropoly Compounds: Oxidation of Cyclohexane

Thermal activation of clayzic catalyst has been thoroughly investigated by calcining anhydrous ZnCl₂ impregnated on Mont-K10 with different ZnCl₂ loadings (0.5-2.0 mmol g^-1) under static or flowing air at 270 °C and also under N₂ flow at different temperatures (150-400 °C). Depending upon the ZnCl₂ loading and calcination temperatures, an appreciable amount of HCl is evolved in the thermal activation, indicating an occurrence of reaction between the ZnCl₂ and the surface hydroxyl groups of the clay (- OH + ZnCl₂ - O - Zn - Cl + HCl), leading to formation of new Lewis acid sites (- O - Zn -Cl). The Cl/Zn ratio, surface area and catalytic activity (in the benzene benzylation by benzyl chloride at 80 °C) of the clayzic formed in the thermal activation are influenced markedly by the ZnCl₂ loading and calcination conditions (temperature and gas atmosphere). The maximum catalytic activity for the clayzic is observed at the optimum ZnCl₂ loading (about 1.0 mmol g^-1) and calcination temperature (about 270 °C). The thermal activation at 270 °C under flowing N₂ led to a most active clayzic catalyst for the benzene benzylation. A temperature-programmed evolution of HCl in the calcination of ZnCl₂/Mont-K10 with different ZnCl₂ loadings from 25 to 400 °C at a linear heating rate of 2 °C min^-1 has also been investigated.