The influence of Na2O on the structure and properties of 3CaO.Al2O3

Four crystal forms of solid solutions of Na₂O in 3CaO.Al₂O₃ have been synthesized. Chemical, crystal-optical, X-ray diffraction and IR-spectroscopic investigations have been made.The hydration activity at the early and later stages has been studied. It is shown that the hydration process can be divided into three stages. The analysis of the kinetic curves permitted the conclusion that the retardation of the hydration process is controlled by diffusion of the liquid phase through the product layer.     

Effect of surface-modified Al2O3 on the thermomechanical properties of polybutylene succinate/Al2O3 composites

This study investigates the effect of the incorporation of alumina particles on the thermomechanical properties of polybutylene succinate (PBS)/Al₂O₃ composites. The alumina surface was modified with the carboxylic groups of maleic acid through simple acid-base and in situ polymerization reactions. Scanning electron microscope (SEM) results revealed the introduction of maleic acid treated alumina significantly affect the morphology of the PBS/Al₂O₃ composites as compared to the neat PBS. The thermal conductivity of the composite (0.411?W?m^?1 K^?1) was more than twice that of neat PBS. The composite containing polymerization-modified alumina showed a 50% increase in storage modulus compared with that of neat PBS. In addition, universal testing machine (UTM) and differential scanning calorimetry (DSC) measurements indicated an increase in the tensile strength and degree of crystallinity after the incorporation of modified alumina in the PBS/Al₂O₃ composite.     

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.     

Fe2O3/Al2O3 catalysts for the N2O decomposition in the nitric acid industry

Fe₂O₃ catalysts supported on Al₂O₃ were used to remove nitrous oxide from the nitric acid plant simulated process stream (containing O₂, NO and H₂O). Catalysts were prepared by the coprecipitation method and were characterized for their physico-chemical properties by BET, XRD, AFM and TPR analysis. A strong influence of the post-preparation heating conditions on the structural and catalytic properties of the catalysts has been evidenced. Laboratory tests revealed 95% conversion of N₂O at temperature 750 °C and a slight decrease in activity in the presence of H₂O and NO. The catalysts were inert towards decomposition of NO. The pilot-plant reactor and real plant studies (up to 3300 h time-on-stream) confirmed high activity and very good mechanical stability of the catalysts as well as no decomposition of nitric oxide.     

Effect of Al2O3 on mechanical properties of Ti3SiC2/Al2O3 composite

The effect of Al₂O₃ on mechanical properties of Ti₃SiC₂/Al₂O₃ composite fabricated by SPS was studied systematically. The results show that the hardness of the Ti₃SiC₂/Al₂O₃ composite can reach 10.28 GPa, 50% higher than that of pure Ti₃SiC₂. However, slight decrease in the other mechanical properties was observed with Al₂O₃ addition higher than 5–10 vol.%, which is believed to be due to the agglomeration of Al₂O₃ in the composite.     

复合固体超强酸SO42-/Fe2O3/Al2O3/ZnO的制备及其催化性能的研究

利用共沉淀法由Fe(NO3)3、Al(NO3)3、Zn(Ac)2和H2SO4制备了SO42-/Fe2O3/Al2O3/ZnO催化剂,并用于催化合成苯甲醛丙二醇缩醛,研究了醛醇摩尔比、催化剂用量、反应时间等因素对产品收率的影响。结果表明,在n(苯甲醛)∶n(丙二醇)=1∶1.2,催化剂用量为反应物总质量的1%,反应时间50 min的最佳条件下,苯甲醛丙二醇缩醛的收率可达97.6%。     

Conditions and mechanism of interconvertibility of compounds 12CaO.7Al2O3 and 5CaO.3Al2O3

Based on a critical analysis of the data reported in the literature and our own experimental studies relating to the melt and glass formation from compositions corresponding to 12CaO.7Al₂O₃ under various atmospheric conditions, the liquid structure of the melt phase and conditions for stabilization of 12CaO.7Al₂O₃ and 5CaO.3Al₂O₃ in crystalline state have been highlighted. The role of “stabilizers” like O^2?, F^?, Cl^?, S^2? for the crystalline structure of 12CaO.7Al₂O₃, effect of temperature and atmosphere on melt structure and properties, and conditions favouring the appearance of 12CaO.7Al₂O₃ and 5CaO.Al₂O₃ crystalline phases from the given melt have been specifically dealt with.     

Thermodynamic investigation of the CaO---Al2O3---CaSO4---H2O system at 50°C and 85°C

The CaO---Al₂O₃---CaSO₄---H₂O system has been investigated at 50°C and 85°C, by calculations of the equilibrium solubility surfaces of AH₃, C₃AH₆, CH, ettringite, monosulphoaluminate and gypsum. The results indicate that monosulphoaluminate becomes a stable phase at elevated temperature. Since ettringite also remains stable at 85°C, new invariant points are created, for which calculated aqueous phase compositions are given.     

Activity and characterization of Rh/Al2O3 and Rh-Na/Al2O3 catalysts for the SCR of NO with CO in the presence of SO2 and HCl

SO₂ and HCl are major pollutants emitted from waste incineration processes. Both pollutants are difficult to remove completely and can enter the catalytic reactor. In this work, the effects of SO₂ and HCl on the performance of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts for NO removal were investigated in simulated waste incineration conditions. The characterizations of the catalysts were analyzed by BET, SEM/EDS, XRD, and ESCA. Experimental results indicated the 1%Rh/Al₂O₃ catalyst was significantly deactivated for NO and CO conversions when SO₂ and HCl coexisted in the flue gas. The addition of between 2 and 10 wt.% Na promoted the activity of the 1%Rh/Al₂O₃ catalyst for NO removal, but decreased the CO oxidation and BET surface area. The catalytic activity for NO removal was inhibited by HCl as a result of the formation of RhCl₃. Adding Na to the Rh/Al₂O₃ catalyst decreased the inhibition of SO₂ because of the formation of Na₂SO₄, which was observed in the XRD and ESCA analyses. SEM mapping/EDS showed that more S was residual on the surface of the Rh-Na/Al₂O₃ catalyst than Cl.     

Characterization and tribological investigation of sol-gel Al2O3 and doped Al2O3 films

Thin films of Al₂O₃ and doped Al₂O₃ were prepared on a glass substrate by dip coating process from specially formulated ethanol sols. The morphologies of the unworn and worn surfaces of the films were observed with atomic force microscope (AFM) and scanning electron microscope (SEM). The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of obtained thin films sliding against Si₃N₄ ball were evaluated and compared with glass slide on a one-way reciprocating friction tester. XPS results confirm that the target films were obtained successfully. The doped elements distribute in the film evenly and exist in different kinds of forms, such as oxide and silicate. AFM results show that the addition of the doped elements changes the structure of the Al₂O₃ films, i.e., a rougher and smoother surface is obtained. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. As the results, the doped films exhibit better tribological properties due to the improved toughness. Sever brittle fracture is avoided in the doped films. The wear of glass is characteristic of brittle fracture and severe abrasion. The wear of Al₂O₃ is characteristic of brittle fracture and delamination. And the wear of doped Al₂O₃ is characteristic of micro-fracture, deformation and slight abrasive wear. The introduction of ZnO is recommended to improve the tribological property of Al₂O₃ film.     

Mechanical properties of Al2O3/ZrO2 composites

In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al₂O₃/(t-ZrO₂+m-ZrO₂) composites were prepared by sintering pressurelessly at 1600 °C. The microstructure of the composites are characterized, the elastic modulus, strength and toughness determined. Because the ZrO₂ inclusions are close to each other in the Al₂O₃ matrix, the yttrium ion originally in t-ZrO₂ particles can diffuse to nearby m-ZrO₂ particles during sintering, and the m-phase zirconia is thus stabilized after sintering. The strength of the Al₂O₃/(t-ZrO₂+m-ZrO₂) composites after surface grinding can reach values as high as 940 MPa, which is roughly three times that of Al₂O₃ alone. The strengthening effect is contributed by microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al₂O₃ alone. The toughening effect is attributed mainly to the zirconia t–m phase transformation.     

The mechanism of SO2 effect on NO reduction with propene over In2O3/Al2O3 catalyst

An In₂O₃/Al₂O₃ catalyst shows high activity for the selective catalytic reduction of NO with propene in the presence of oxygen. The presence of SO₂ in feed gas suppressed the catalytic activity dramatically at high temperatures; however it was enhanced in the low temperature range of 473–573 K. In TPD and FT-IR studies, the formation of sulfate species on the surface of the catalyst caused an inhibition of NO_X adsorption sites, and the absorbance ability of NO was suppressed by the presence of SO₂, and the amount of ad-NO₃⁻ species decreased obviously. This leads to a decrease of catalytic activity at higher temperatures. However, addition of SO₂ enhanced the formation of carboxylate and formate species, which can explain the promotional effect of SO₂ at low temperature, because active C₃H₆ (partially oxidized C₃H₆) is crucial at low temperature.     

Crystal structure and properties of Al2O3-Cr2O3 solid solutions with different Cr2O3 contents

To enable the comprehensive application of Al₂O₃-Cr₂O₃ solid solutions, the crystal structures and properties of Al₂O₃-Cr₂O₃ solid solutions with different Cr₂O₃ contents were studied. It was observed that Al₂O₃ and Cr₂O₃ form a complete substitutional solid solution over the entire composition range at 1650 °C, with no compounds being formed. Lattice parameters “a” and “c” both increase linearly with an increase in the Cr₂O₃ content. The doping of the Cr³⁺ ions causes a more severe lattice strain in the c-axis direction. The diffraction angles of the diffraction peaks decrease in a linear manner with the increase in the Cr₂O₃ content. The relationship between the theoretical density of the solid solution and the Cr₂O₃ content could be fitted using a second-order polynomial. It was also observed that the linear expansion coefficient of the solid solutions decreases with an increase in the Cr₂O₃ content.     

Synthesis and properties of lithium disilicate glass-ceramics in the system SiO2–Al2O3–K2O–Li2O

The purpose of this study was the synthesis of lithium disilicate glass-ceramics in the system SiO₂–Al₂O₃–K₂O–Li₂O. A total of 8 compositions from three series were prepared. The starting glass compositions 1 and 2 were selected in the leucite–lithium disilicate system with leucite/lithium disilicate weight ratio of 50/50 and 25/75, respectively. Then, production of lithium disilicate glass-ceramics was attempted via solid-state reaction between Li₂SiO₃ (which was the main crystalline phase in compositions 1 and 2) and SiO₂. In the second series of compositions, silica was added to fine glass powders of the compositions 1 and 2 (in weight ratio of 20/100 and 30/100) resulting in the modified compositions 1–20, 1–30, 2–20, and 2–30. In the third series of compositions, excess of silica, in the amount of 30 wt.% and 20 wt.% with respect to the parent compositions 1 and 2, was introduced directly into the glass batch. Specimens, sintered at 800 °C, 850 °C and 900 °C, were tested for density (Archimedes’ method), Vickers hardness (H_V), flexural strength (3-point bending tests), and chemical durability. Field emission scanning electron microscopy and X-ray diffraction were employed for crystalline phase analysis of the glass-ceramics. Lithium disilicate precipitated as dominant crystalline phase in the crystallized modified compositions containing colloidal silica as well as in the glass-ceramics 3 and 4 after sintering at 850 °C and 900 °C. Self-glazed effect was observed in the glass-ceramics with compositions 3 and 4, whose 3-point bending strength and microhardness values were 165.3 (25.6) MPa and 201.4 (14.0) MPa, 5.27 (0.48) GPa and 5.34 (0.40) GPa, respectively.     

Study of the system CaO---SiO2---CO2---H2O in relation to scawtite under hydrothermal conditions

Hydrothermal studies showed the location of scawtite in the narrow internal tetrahedronfoshagite-xonotlite-CO₂---H₂O of the system CaO---SiO₂---CO₂---H₂O and its compatibility with the calcium silicate hydrates in the 4-phase assemlages. The phase assemblages in this region involve scawtite, foshagite, xonotlite, tobermorite, hillebrandite, calcite and silica. The carbonation series of the 4-phase assemblages were determined in the sections C₄S₃---CO₂---H₂O (C₄S₃ = 4CaO · 3SiO₂), C₇S₆---CO₂---H₂O and C₆S₆---CO₂---H₂O at 473 K and saturated steam pressure in dependence on the CO₂ content. The carbonating influence of CO₂ takes place in the volume of the CaO---SiO₂---CO₂---H₂O system up to the CO₂ concentration limited by the calcite-silica-H₂O triangle. Above this CO₂ content, carbon dioxide appears in a gaseous form together with gaseous H₂O in the 4-phase assemblage calcite-silica-CO₂H₂O. The instability of scawtite under the CO₂ excess (CO₂/C₇S₆ > 1) is influenced by its structural arrangement. The present channels enable penetration of CO₂ molecules into the structure and formation of carbonated phases.     

Rapid solidification in the Al2O3ZrO2 system

Eutectic compositions of Al₂O₃ and ZrO₂ have been melted and rapidly solidified by shock-wave quenching, flame-pressure atomization and high-pressure water atomization. Quenching rates > 10⁴ K/s resulted in amorphous particles which, on annealing, crystallized at 1310°C. Microcrystalline particles with tetragonal ZrO₂ distributed in an ?-Al₂O₃ matrix formed at lower quenching rates. ?-Al₂O₃ transformed into α-Al₂O₃ on annealing at 953°C.     

Physicochemical surface and catalytic properties of CuO–ZnO/Al2O3 system

A series of CuO–ZnO/Al₂O₃ solids were prepared by wet impregnation using Al(OH)₃ solid and zinc and copper nitrate solutions. The amounts of copper and zinc oxides were varied between 10.3 and 16.0 wt% CuO and between 0.83 and 7.71 wt% ZnO. The prepared solids were subjected to thermal treatment at 400–1000°C. The solid–solid interactions between the different constituents of the prepared solids were studied using XRD analysis of different calcined solids. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at −196°C and their catalytic activities were determined using CO-oxidation by O₂ at temperatures ranged between 125°C and 200°C.

The results showed that CuO interacts with Al₂O₃ to produce copper aluminate at ≥600°C and the completion of this reaction requires heating at 1000°C. ZnO hinders the formation of CuAl₂O₄ at 600°C while stimulates its production at 800°C. The treatment of CuO/Al₂O₃ solids with different amounts of ZnO increases their specific surface area and total pore volume and hinders their sintering (the activation energy of sintering increases from 30 to 58 kJ mol⁻¹ in presence of 7.71 wt% ZnO). This treatment resulted in a progressive decrease in the catalytic activities of the investigated solids but increased their catalytic durability. Zinc and copper oxides present did not modify the mechanism of the catalyzed reaction but changed the concentration of catalytically active constituents (surface CuO crystallites) without changing their energetic nature.     

The solubility of selenate-AFt (3CaO·Al2O3·3CaSeO4·37.5H2O) and selenate-AFm (3CaO·Al2O3·CaSeO4·xH2O)

The Se(VI)-analogues of ettringite and monosulfate, selenate-AFt (3CaO·Al₂O₃·3CaSeO₄·37.5H₂O), and selenate-AFm (3CaO·Al₂O₃·CaSeO₄·xH₂O) were synthesised and characterised by bulk chemical analysis and X-ray diffraction. Their solubility products were determined from a series of batch and resuspension experiments conducted at 25 °C. For selenate-AFt suspensions, the pH varied between 11.37 and 11.61, and a solubility product, log K_so=61.29±0.60 (I=0 M), was determined for the reaction 3CaO·Al₂O₃·3CaSeO₄·37.5H₂O+12 H⁺⇔6Ca²⁺+2Al³⁺+3SeO₄²⁻+43.5H₂O. Selenate-AFm synthesis resulted in the uptake of Na, which was leached during equilibration and resuspension. For the pH range of 11.75 to 11.90, a solubility product, log K_so=73.40±0.22 (I=0 M), was determined for the reaction 3CaO·Al₂O₃·CaSeO₄·xH₂O+12 H⁺⇔4Ca²⁺+2Al³⁺+SeO₄²⁻+(x+6)H₂O. Thermodynamic modelling suggested that both selenate-AFt and selenate-AFm are stable in the cementitious matrix; and that in a cement limited in sulfate, selenate concentration may be limited by selenate-AFm to below the millimolar range above pH 12.     

High temperature neutron diffraction study of the Al2O3–Y2O3 system

The phase diagram of the Al₂O₃–Y₂O₃ system has been investigated with five different compositions by XRD and in situ high temperature neutron diffractometry. High purity YAG, YAP and YAM compounds have been produced successfully through a melt extraction technique. High temperature neutron diffraction has made it possible to follow, in real time, the reactions involved in this system, especially to determine the temperature range of each reaction, which would have been impossible to determine by other means. A good agreement between the experimental results and the phase diagram of the Al₂O₃–Y₂O₃ system has been observed.     

HF/HCl acid resistance mechanisms of alumina ceramics in the Al2O3-MgO-CaO-SiO2-Y2O3 system

Silicon compounds in raw materials are the main reason for the low HF/HCl acid resistance of alumina ceramics. Y₂O₃ can improve the acid resistance of alumina ceramics. This work aimed to reveal the mechanisms of the effects of Y₂O₃ on the form of Si and the durability of the ceramic. An experiment on a high-temperature reaction between Y₃Al₅O₁₂ and a polycrystalline alumina ceramic was designed. The effect of corrosion time on the acid solubility of the alumina ceramic was investigated. The results show that Si can dissolve in Y₃Al₅O₁₂ to generate solid solutions, impeding the generation of Si-containing compounds with bad acid resistance, and decreasing the content of amorphous Si. The acid solubility of the ceramic was only 0.95%, even when the corrosion time was extended to 60 times the industry standard. This revelation of the acid resistance mechanisms can provide a new idea for designing corrosion-resistant ceramics.