A sorption balance study of water vapour sorption on anhydrous cement minerals and cement constituents

The phenomenon of water vapour sorption by powdered cement constituents exposed to different relative humidities and temperatures was studied. The individual clinker phases C₃S, C₂S, C₃A, C₄AF, calcium sulfates and CaO were tested. Using a water sorption balance, the amount of chemically and physically sorbed water per unit of surface area of the powders and the relative humidity at which water sorption starts to occur on the phases were determined. Various cement clinker phases prehydrate very differently. CaO and C₃A were found to be most reactive towards water vapour whereas the silicates react less. CaO starts to sorb water at very low RHs and binds it chemically. Beginning at 55% RH, orthorhombic C₃A also sorbs significant amounts of water and binds it chemically and physically. Water sorption of C₃S and C₂S only begins at 74% RH, and the amount of water sorbed is minor. Calcium sulfates sorb water predominantly physically.     

Hard and tough boron suboxide based composites

Boron suboxide (B₆O) powder was synthesized at temperatures of about 1400 °C from the reaction of amorphous boron powder with boric acid. The synthesized B₆O powders were hot pressed at temperatures up to 1900 °C and at pressures of 50 MPa. Additionally to pure B₆O materials, composites with aluminium were prepared. The microstructure and properties of the sintered compacts were investigated. The addition of aluminium in the composites results in the formation of an additional aluminium borate phase. The composites showed a similar hardness (∼30 GPa) as the pure B₆O samples but increased fracture toughness (∼3.5 MPa m^1/2).     

Sorption-enhanced water gas shift reaction by sodium-promoted calcium oxides

The water gas shift reaction was evaluated in the presence of novel carbon dioxide (CO₂) capture sorbents, both alone and with catalyst, at moderate reaction conditions (i.e., 300-600 °C and 1-11.2 atm). Experimental results showed significant improvements to carbon monoxide (CO) conversions and production of hydrogen (H₂) when CO₂ sorbents are incorporated into the water gas shift reaction. Results suggested that the performance of the sorbent is linked to the presence of a Ca(OH)₂ phase within the sorbent. Promoting calcium oxide (CaO) sorbents with sodium hydroxide (NaOH) as well as pre-treating the CaO sorbent with steam appeared to lead to formation of Ca(OH)₂, which improved CO₂ sorption capacity and WGS performance. Results suggest that an optimum amount of NaOH exists as too much leads to a lower capture capacity of the resultant sorbent. During capture, the NaOH-promoted sorbents displayed a high capture efficiency (nearly 100%) at temperatures of 300-600 °C. Results also suggest that the CaO sorbents possess catalytic properties which may augment the WGS reactivity even post-breakthrough. Furthermore, promotion of CaO by NaOH significantly reduces the regeneration temperature of the former.     

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.     

Chemical preparation and characterization of nitrogen-rich carbon nitride powders

Creamy white powders were obtained by the chemical vapor reaction of carbon tetrachloride and ammonia at 1000 °C, followed by washing with boiling water. The composition of the material was C₃N_5.5O_0.5H_5.4, having a nitrogen content greater than that of the hypothetical hard material C₃N₄. The powders were harder than quartz whose Mohs hardness is 7, but were not as hard as sapphire (Mohs hardness: 9). The hardness could be explained by the C-N single bond observed in the ESCA C1s spectrum. The material washed with boiling water showed photoluminescence, which was mostly bright and white-blue in color to the naked eye.     

Synthesis of spherical shape borate-based bioactive glass powders prepared by ultrasonic spray pyrolysis

Spherical shape borate-based bioactive glass powders with fine size were directly prepared by high temperature spray pyrolysis. The powders prepared at temperatures between 1200 and 1400 °C had mixed phase with small amounts of fine crystal and an amorphous rich phase. Glass powders with amorphous phase were prepared at a temperature of 1500 °C. The mean size of the glass powders prepared by spray pyrolysis was 0.76 μm. The glass powders prepared at a temperature of 1200 °C had two distinct exothermic peaks (T_c1 and T_c2) at temperatures of 588 and 695 °C indicating crystallization. The glass transition temperature (T_g) of the powders prepared at a temperature of 1200 °C was 480 °C. Phase-separated crystalline phases with spherical shape were observed from the surface of the pellet sintered at a temperature of 550 °C. Crystallization of the pellet was completely occurred at temperatures of 750 and 800 °C. The pellets sintered at temperatures below 700 °C had single crystalline phase of CaNa₃B₅O₁₀. The pellet sintered at a temperature of 800 °C had two crystalline phases of CaNa₃B₅O₁₀ and CaB₂O₄.     

Fabrication and properties of CaZr0.9In0.1O3−δ prepared by an auto-ignition combustion process

One of the major challenges in developing proton conducting CaZr_0.9In_0.1O_3−δ is to achieve a high densification at low sintering temperature. In this work, an auto-ignition combustion process was first used to synthesize CaZr_0.9In_0.1O_3−δ powders aiming to improve its sinterability. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and dilatometry measurement. The results indicate that a calcination temperature of 1000 °C is sufficient to form the CaZr_0.9In_0.1O_3−δ phase. The as-obtained powders are fine, homogeneous and well crystallized, which strongly improves the sintering properties. Dense CaZr_0.9In_0.1O_3−δ ceramics with uniform grain size were obtained by sintering at 1350 °C, which is much lower than that required for the conventional solid state reaction method. In addition, the electrical properties of CaZr_0.9In_0.1O_3−δ ceramics were studied by electrochemical impedance spectroscopy.     

Synthesis and characterization of multiferroic BiFeO3 powders fabricated by hydrothermal method

The influence of processing parameters on phase formation and particle size of hydrothermally synthesized BiFeO₃ powders was investigated. BiFeO₃ powder was synthesized by dissolving bismuth nitrate and iron nitrate in KOH solution at temperatures ranging from 150 to 225 °C. X-ray diffraction patterns and scanning electron microscopy observation indicated that rod-like α-Bi₂O₃ phase was formed at initial stage of reaction and dissolved into ions to form thermodynamically stable BiFeO₃ phase. Single-phase perovskite BiFeO₃ has been formed using a KOH concentration of 8 M at a temperature of ≥175 °C in a 6 h reaction period. BiFeO₃ particle growth was promoted by lowering the KOH concentration, or increasing the duration time or reaction temperature. The effects of processing conditions on the formation of crystalline BiFeO₃ powders were discussed in terms of a dissolution–precipitation mechanism. The magnetization of the BiFeO₃ powders at room temperature showed a weak a ferromagnetic nature.     

Interactions of LiBr with calcite and calcium oxide powders

The interaction of LiBr with calcite and calcium oxide powders have been studied with and without the calcite decomposition reaction, using DTA and TG analysis plus SEM and RX observations. Interactions in LiBr  CaCO₃ system show two possible phase transformations at the temperature of 748 K and 786 K. The second one is due to the formation of an eutectic liquid phase in the LiBr-rich region. The rate of decomposition of calicite powders was measured in dry nitrogen and in a high partial pressure of CO₂ with and without LiBr. The addition of LiBr causes the decomposition reaction occur at a lower temperature. The LiBr changes the mechanism of the reaction. The decomposition process occurs through a liquid-phase path given by the eutectic of LiBrCaCO₃. The total rate of reaction as well as the temperature dependence are related to the liquid phase. The CaCO₃ powders obtained have a very dense structure with a high degree of crystallization. Probably the LiBrCaO eutectic provides a solution path for CaO recrystallization. The effectiveness of salt addition in lowering the decomposition temperature of carbonates seems promising in saving energy, as well as in promoting changes in morphology of important commercial oxide powders.     

Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst

In this study, transesterification of soybean oil to biodiesel using CaO as a solid base catalyst was studied. The reaction mechanism was proposed and the separate effects of the molar ratio of methanol to oil, reaction temperature, mass ratio of catalyst to oil and water content were investigated. The experimental results showed that a 12:1 molar ratio of methanol to oil, addition of 8% CaO catalyst, 65 °C reaction temperature and 2.03% water content in methanol gave the best results, and the biodiesel yield exceeded 95% at 3 h. The catalyst lifetime was longer than that of calcined K₂CO₃/γ-Al₂O₃ and KF/γ-Al₂O₃ catalysts. CaO maintained sustained activity even after being repeatedly used for 20 cycles and the biodiesel yield at 1.5 h was not affected much in the repeated experiments.     

An in situ high pressure-high temperature powder diffraction study of the formation of a precursor phase of bismuth manganite

The synthesis route for single phase BiMnO₃ from Bi₂O₃ and Mn₂O₃ has been undertaken, in situ, in a diamond anvil cell. The starting powders were mixed in a stoichiometric ratio and loaded into a diamond anvil cell with a gasket aperture of 150 μm. A ruby chip was used to measure the pressure and a circular resistive heater surrounded the central core of the cell. The reaction was studied as a function of pressure and temperature to 6.7 GPa and 480 °C by X-ray diffraction. There was evidence of the onset of a reaction between the starting oxides at 314 °C and 6.7 GPa but there was no evidence of transformation of the monoclinic Bi₂O₃ to the cubic form before the reaction began. A monoclinic phase related to the structure of multiferroic BiMnO₃ was observed to form with space group Cc, a = 10.21(1), b = 5.356(6), c = 10.38(1) Å and β = 116.87(6)°. A good fit for this structure was obtained by modelling a distorted BiMnO₂ unit cell with disordered Bi and Mn sites. We did not go sufficiently high in temperature to observe the formation of single phase BiMnO₃ but the precursor phase was observed to persist when returned to room temperature and pressure.     

Mechanical properties and micro-structures of calcium aluminate based ultra high strength cement

This paper describes the mechanical properties and microstructure of calcium aluminate based ultra high strength cement at early age. By using silica fume, polycarboxylate based superplasticizers and a hybrid defoaming mixer, which is anon-contact mixer, cement paste with water to powder ratio of 0.1 can be cast in a mold. When the water to powder ratio is 0.1, the bending strength of hardened samples can be obtained over 30 MPa. Samples were cured at 40 or 60 °C for 7 days. At 60 °C, C₃AH₆ is mainly produced, whereas C₃AH₆ and C₂AH₈ are produced at 40 °C. The mechanical properties of hardened samples with low water to powder ratio are related to the pore volume and pore size distribution.     

Catalytic effect of alkaline earth oxides on carbothermic formation of hexagonal boron nitride

Effect of MgO, CaO and BaO on carbothermic formation of hexagonal boron nitride (h-BN) was investigated. B₂O₃–C mixtures containing alkaline earth oxide additives were reacted at 1500 °C for 30–120 min in nitrogen atmosphere. Formed phases in the reaction products were determined by powder-XRD analyses, and amounts of the constituents were determined by chemical analyses. Particle size and morphology of the formed h-BN powders were examined by FESEM and particle size distributions were determined by particle size analyzer. Addition of alkaline earth oxides was found to increase the amount and grain size of h-BN significantly and to decrease the amount of B₄C formed in the system. Investigated alkaline earth oxides presented similar catalytic effects according to chemical analyses and FESEM observations. While the average particle size of h-BN powder obtained from plain mixture was 149 nm, those obtained from MgO, CaO and BaO containing mixtures were 297, 367 and 429 nm, respectively.     

Microwave-assisted preparation of submicron-sized FeTiO3 powders

FeTiO₃ powders were prepared from (Fe+Ti) mixed carbonate precursor by the microwave-assisted calcination method. The thermal analysis of the precursor was conducted by TG/DSC. It is demonstrated that the calcination process can be divided into three different stages: the removal of water, the decomposition of precursor and the formation of FeTiO₃. A variety of techniques, such as XRD, FT-IR, SEM and EDS were utilized to characterize the samples. The results indicated that FeTiO₃ powders can be prepared by microwave-assisted calcination for 20 min at 450 °C, while those cannot be obtained by conventional one until reaction time exceeds 120 min at 600 °C. The FeTiO₃ powders prepared by microwave-assisted calcination are nearly spherical with limited aggregation and have a mean particle size of 400–500 nm.     

Structural and electrical properties of La0.7Sr0.3Co0.5Fe0.5O3 powders synthesized by solid state reaction

This work investigates the effect of synthesis parameters (calcination temperature, milling conditions and sintering temperature) on the structural, morphological and electrical properties of La_0.7Sr_0.3Co_0.5Fe_0.5O₃ (LSCF) powders prepared by the solid state reaction. The thermogravimetric profile showed that the minimum temperature needed for the carbonate decomposition and formation of perovskite phase is 800 °C. SEM analysis revealed the loose and porous structure of the powder materials. The XRD patterns demonstrate that milling parameters such as grinding balls:sample ratio, rotational speed, and milling time influence the structural properties. The results revealed that powders synthesized with grinding balls:sample ratio of 8:1, 500 rpm and 4 h of milling present pure LSCF phase. Porosity of the pellets decreased with increasing sintering temperature from 950 to 1100 °C. Electrical conductivity was measured at 400–1000 °C and correlated with sintering temperature.     

Combustion synthesis of fine aluminum nitride powders under micropositive N2 pressure with water additive

Fine aluminum nitride (AlN) powders were prepared by a facile and efficient way of combustion synthesis under micropositive N₂ pressure of 0.15 MPa and with 3 wt% water as additive. By this approach, the maximum combustion temperature was well regulated to a low value. The influence of water on the reaction rate, the phase composition, and crystal growth of the products was systematically investigated. The addition of water was crucial to the complete nitridation of Al. Furthermore, H₂O vapor played a two-sided role in the reaction. It could accelerate the reaction by promoting the diffusion of Al vapor and N₂ and restrain the nitridation rate by absorbing heat.     

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.     

Modeling and simulation for the methane steam reforming enhanced by in situ CO2 removal utilizing the CaO carbonation for H2 production

The transient behavior of catalytic methane steam reforming (MSR) coupled with simultaneous carbon dioxide removal by carbonation of CaO pellets in a packed bed reactor for hydrogen production has been analyzed through a mathematical model with reaction experiments for model verification. A dynamic model has been developed to describe both the MSR reaction and the CaO carbonation-enhanced MSR reaction at non-isothermal, non-adiabatic, and non-isobaric operating conditions assuming that the rate of the CaO carbonation in a local zone of the packed bed is governed by kinetic limitation or by mass transfer limitation of the reactant CO₂. Apparent carbonation kinetics of the CaO pellet prepared has been determined using the TGA carbonation experiments at various temperatures, and incorporated into the model. The resulting model is shown to successfully depict the transient behavior of the in situ CaO carbonation-enhanced MSR reaction. The effects of major operating parameters on the transient behavior of the CaO carbonation-enhanced MSR have been investigated using the model. The bed temperature is the most important parameter for determining the amount of CO₂ removed by carbonation of CaO, and at temperatures of 600°C, 650°C, 700°C and 750°C, the CO₂ uptake is 1.43, 2.29, 3.5 and -CO₂/kg-CaO, respectively. Simultaneously with the increase in CO₂ uptake with increasing temperature, the corresponding amounts of hydrogen produced are 1.56, 2.54, 3.91 and -H₂/kg-CaO, at the same temperatures as above. Operation at high pressure, high steam to methane feed ratio, and the decreased feed rate at a given temperature are favorable for increasing the degree of the overall utilization of CaO pellets in the reactor bed, and for lowering the CO concentration in the product.     

Catalytic reduction of nitric oxide by methane over CaO catalyst

The selective catalytic reduction of nitric oxide by methane was studied over CaO catalyst in a bubbling fluidized bed in the temperature range of 800–900 °C, in which NO cannot be reduced by CH₄ without CaO catalyst. The nitric oxide conversion was found to depend on oxygen and CH₄ feed concentration, and also on temperature. In addition, the presence of water vapors in the flue gas enhanced the NO reduction admirably well in the absence of O₂. But water vapor has an inhibiting effect on the reaction while O₂ is present in the flue gas. The addition of CO₂ poisoned the CaO catalyst and exhibited a detrimental effect on NO conversion at the working temperature range, 800–900 °C. However, with a temperature rise to 900 °C the CO₂ poisoning effect on NO reduction was weakened. The mechanism was studied and discussed according to the references in the paper. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Treatment of chlorophenols-bearing wastewaters through hydrodechlorination using Pd/activated carbon catalysts

The catalytic hydrodechlorination of 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol in aqueous solution over Pd/activated carbon catalysts (0.5% w/w Pd) was studied in a fixed bed reactor. The reactor was fed with a 100 mg/l solution of chlorophenol and a H₂/N₂ (1:1) gas stream. The ranges studied for temperature, pressure and space-time were 25-100 °C, 1.8-6.0 bar and 14-55 kg h/mol, respectively. A commercial and some home-made catalysts were tested. The carbon supports were subjected to oxidation with nitric acid and sodium persulfate. Chlorophenols conversion was found to peak for pressure values ≈2.4 bar. In these conditions, an increase of reaction temperature increases conversion. In the runs carried out at high space-time both the reactants conversion and the selectivity towards end chain reaction products was enhanced. The oxidation of the carbon support with nitric acid prior to impregnation improves conversion. At the optimum conditions (2.4 bar, 75 °C and nitric acid oxidation of carbon support) conversion values over 95% were reached for all chlorophenols. As a result of this treatment the toxicity of the initial solutions was reduced by more than 90%.