Solid-state reaction of Cr2O3 and SnO2

Various mixtures of SnO₂---Cr₂O₃ and SnO₂---K₂Cr₂O₇, were subjected to different thermal treatments. The crystalline phases in the calcination products have been identified by X-ray diffraction. The non-formation of definite compounds was also proved by diffuse reflectance: The structural environment of the cation Cr(III) was also studied.     

Visible light induced photocatalytic activity of Nb2O5/carbon cluster/Cr2O3 composite materials

Nano-sized Nb₂O₅/carbon cluster/Cr₂O₃ composite material was prepared by the calcination of NbCl₅/chromium acetylacetonate/epoxy resin complex under an argon atmosphere. The Pt-loaded Nb₂O₅/carbon cluster/Cr₂O₃ composite material shows the photocatalytic activity under visible light irradiation. The composite material successfully decomposed the water into H₂ and O₂ in the [H₂]/[O₂] ratio of 2. Electron spin resonance spectral examination suggests a two-step electron transfer in the process of Nb₂O₅ → carbon cluster → Cr₂O₃ → Pt.     

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.     

Study of color change and microstructure development of Al2O3–Cr2O3/Cr3C2 nanocomposites prepared by spark plasma sintering

The densification behaviors of Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposites prepared by a Spark Plasma Sintering (SPS) were investigated in this work. The initial powders used for sintering were Al₂O₃–Cr₂O₃, which were prepared by metal organic chemical vapor deposition (MOCVD) in a spout bed. Different colors of the compacts such as green, purple and black were observed after densification process at different SPS temperatures from 1200 °C to 1350 °C. These changes of color were relevant to the existence of secondary phase of green Cr₂O₃, pink solid solution of Cr₂O₃–Al₂O₃ and black Cr₃C₂, which were formed under the different SPS temperature. The secondary phase of Cr₂O₃ retarded the processing of densification for spark plasma sintering at 1200 °C. The Cr₂O₃ reacted with Al₂O₃ to form solid solution of Cr₂O₃–Al₂O₃ and with carbon to form Cr₃C₂ as sintering temperature increased to 1350 °C. The characteristics of high heating rate, shorter sintering time for SPS and the formation of secondary phase of Cr₃C₂ effectively reduced the substrate's grain growth, making Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposites with small grain size.     

Preparation and characterization of Cr2O3-TiO2-Al2O3-V2O5 green pigment

Green pigments with high near infrared reflectance based on a Cr₂O₃-TiO₂-Al₂O₃-V₂O₅ composition have been synthesized. Cr₂O₃ was used as the host component and mixtures of TiO₂, Al₂O₃ and V₂O₅ were used as the guest components. TiO₂, Al₂O₃, and V₂O₅ were mixed into 39 different compositions. The spectral reflectance and the distribution of pigment powder were determined using a spectrophotometer and a scanning electron microscope, respectively. It was found that a pigment powder sample S9 with a Cr₂O₃-TiO₂-Al₂O₃-V₂O₅ composition of 80, 4, 14 and 2 wt%, respectively, gives a maximum near infrared solar reflectance of 82.8% compared with 49.0% for pure Cr₂O₃. The dispersion of pigment powders in a ceramic glaze was also studied. The results show that the pigment powder sample S9 is suitable for use as a coating material for ceramic-based roofs.     

Mechanical properties of Al2O3-Cr2O3/Cr3C2 nanocomposite fabricated by spark plasma sintering

The fine grains of Al₂O₃-Cr₂O₃/Cr-carbide nanocomposites were prepared by employing recently developed spark plasma sintering (SPS) technique. The initial materials were fabricated by a metal organic chemical vapor deposition (MOCVD) process, in which Cr(CO)₆ was used as a precursor and Al₂O₃ powders as matrix in a spouted chamber. The basic mechanical properties like hardness, fracture strength and toughness, and the nanoindentation characterization of nanocomposites such as Elastics modulus (E), elastic work (W_e) and plastic work (W_p) were analyzed. The microstructure of dislocation, transgranular and step-wise fracture surface were observed in the nanocomposites. The nanocomposites show fracture toughness of (4.8 MPa m^1/2) and facture strength (780 MPa), which is higher than monolithic alumina. The strengthening mechanism from the secondary phase and solid solution are also discussed in the present work. Nanoindentation characterization further illustrates the strengthening of nanocomposites.     

Effects of PTFE activation and carbon sources on combustion synthesis of Cr2AlC/Al2O3 composites

Formation of Cr₂AlC/Al₂O₃ in situ composites was investigated by self-propagating high-temperature synthesis (SHS) involving both PTFE activation and aluminothermic reduction. In addition to Al and Cr₂O₃ as the starting materials, carbon black, graphite, and Al₄C₃ were used as the carbon sources. PTFE was employed not only as a reaction promoter, but also as a carburizing agent. Depending on different sources of carbon, the threshold amounts of PTFE for inducing self-sustaining combustion were 1.5, 4.0, and 3.0 wt% for the samples adopting carbon black, graphite, and Al₄C₃, respectively. The combustion front velocity and temperature increased with increasing PTFE content. Moreover, the sample using carbon black was the most exothermic, while the Al₄C₃-based sample was the least. For the powder compacts adopting carbon black or graphite, Cr₂AlC/Al₂O₃ composites were produced with no impurities. Due to relatively weak reaction exothermicity, however, the synthesized composites containing small amounts of Cr₇C₃ and Al₄C₃ were obtained from the Al₄C₃-based reaction scheme.     

Thermodynamic Evaluation of Cr2O3-Al2O3 Refractory Corrosion by Smelting Reduction Ironmaking Slag

The interfacial reactions of Cr2O3- Al2O3 refractory materials with smelting reduction ironmaking slag were investigated by thermodynamic modeling. The calculation results indicated that different spinel phases were produced after the interactions between the slag and three types of refractory materials( 10Cr2O3- 90Al2O3,50Cr2O3- 50Al2O3 and 90Cr2O3- 10Al2O3) at 1 500℃. Increased contents of Al2O3 and Cr2O3in the remained liquid slag after interaction implied the dissolution of refractory. Thermodynamic modeling predicted that the process of dissolution might lead to much deeper penetration into the refractory materials by the present compositional slag. Accordingly,corrosion testing was also conducted to validate the thermodynamic prediction.     

Cr2O3 doping in ZnO–0.5 mol% V2O5 varistor ceramics

The effects of the amount of Cr₂O₃ (0.5–4 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO–0.5 mol% V₂O₅ system. The microstructure of the samples consists mainly of ZnO grains with ZnCr₂O₄ and α-Zn₃(VO₄)₂ as the minority secondary phases. The addition of Cr₂O₃ is found to be effective in controlling the abnormal ZnO grain growth often found in V₂O₅-doped ZnO ceramic system, and a more uniform microstructure can be obtained. The varistor performance is also improved as observed from the increase in the non-linear coefficient α of the Cr₂O₃-doped ZnO–V₂O₅ samples. The α value is found to increase with the amount of Cr₂O₃ for up to 3 mol% Cr₂O₃ content. Further increase in Cr₂O₃ is found to cause a decrease in the α value. The highest α value of 28.9 is obtained for the ZnO–0.5 mol% V₂O₅–3 mol% Cr₂O₃ sample.     

Surface modification of Cr2O3 nanoparticles with 3-amino propyl trimethoxy silane (APTMS). Part 1: Studying the mechanical properties of polyurethane/Cr2O3 nanocomposites

The surface of Cr₂O₃ nanoparticles was modified with various amounts of 3-amino propyl trimethoxy silane (APTMS). Thermal gravimetric analysis (TGA), turbidimeter and Fourier transform infrared (FTIR) spectroscopy were utilized in order to investigate APTMS grafting on the nanoparticles. Then, polyurethane nanocomposites were prepared using various loadings of silane modified Cr₂O₃ nanoparticles. The nanoparticles dispersion in the coating matrix was studied by a field emission scanning electron microscopy (FESEM). Dynamic mechanical thermal analysis (DMTA) and tensile test were utilized in order to investigate the mechanical properties of the nanocomposites. Results obtained from FTIR, TGA and turbidimeter measurements revealed that the organic functional groups of the silane were successfully grafted on the surface of the nanoparticles. The mechanical properties of the polyurethane were significantly enhanced using 2 wt% Cr₂O₃ nanoparticles modified with 0.43 g silane/5 g pigment compared with other samples.     

Preparation and characterization of LaCrO3 and Cr2O3 methane combustion catalysts supported on LaAl11O18- and Al2O3-coated monoliths

A series of LaAl₁₁O₁₈- and Al₂O₃-supported LaCrO₃ and Cr₂O₃ combustion catalysts was prepared. Different active phase–support combinations were prepared and applied to cordierite monoliths. The washcoat materials were aged in flowing humid air at temperatures between 1100°C and 1400°C, after which they were characterized by BET, XRD, TPR, and EDS. The monolith catalysts were evaluated in methane combustion. The presence of an active phase retarded sintering of the Al₂O₃ support, whereas the active phase slightly decreased the thermal stability of LaAl₁₁O₁₈. X-ray measurements revealed extensive interaction between support and active phase in the washcoat materials. A substituted perovskite, LaCr_1−xAl_xO₃, is proposed to be formed in nearly all samples containing both lanthanum and chromium. The accessibility of chromium decreased rapidly after aging. The activities of the Al₂O₃-supported catalysts were higher than of those supported on LaAl₁₁O₁₈, which was related to the higher surface area of the former.     

Promoting effects of CO2 on dehydrogenation of propane over a SiO2-supported Cr2O3 catalyst

The effects of carbon dioxide on the dehydrogenation of C₃H₈ to produce C₃H₆ were investigated over several Cr₂O₃ catalysts supported on Al₂O₃, active carbon and SiO₂. Carbon dioxide exerted promoting effects only on SiO₂-supported Cr₂O₃ catalysts. The promoting effects of carbon dioxide over a Cr₂O₃/SiO₂ catalyst were to enhance the yield of C₃H₆ and to suppress the catalyst deactivation.     

Transmission electron microscopy characterization of nanostructured carbon derived from Cr3C2 and Cr(C5H7O2)3

The preparation, characterization and comparison of nanostructured carbons derived by direct chlorination of Cr₃C₂ and Cr(C₅H₇O₂)₃ are reported in this work. Cr₃C₂ precursor was treated at 400 and 900 °C with a reaction time of 1 h. The nanostructure of the products has been characterized in some detail by means of transmission electron microscopy and associated techniques, such as electron energy-loss and X-ray energy dispersive spectroscopies and high-angle annular dark field imaging. Remains of Cr₃C₂ encapsulated in an amorphous carbon shell were observed at 400 °C, whereas carbon with higher ordering degree was produced at 900 °C. In the latter case, the sample can be described as a continuous variation from poorly-stacked graphene-like carbon to graphitic agglomerates. Remains of the reaction by-product, CrCl₃, are detected in the carbon particles, forming monolayers intercalated inside the graphitic agglomerates and amorphous nanoparticles. As a comparison, carbon samples derived from Cr(C₅H₇O₂)₃ were prepared at 300 and 900 °C. They mainly consist of highly disordered carbon, with local graphite-like stacking in the sample prepared at 900 °C.     

Effects of the CaO/SiO2 ratio and Cr2O3 on the hydrothermal synthesis of xonotlite

Formation of xonotlite was attempted by hydrothermal reactions of Ca(OH)₂, silica gel and coprecipitated SiO₂Cr₂O₃ gel at C/S molar ratios of 0.65, 0.83, 1.0 and 1.2 at a saturated water vapor pressure with addition of 0–10% of Cr₂O₃. Cr₂O₃ enters 10% or more into CS(I) to form a solid solution, interferes with the formation of tobermorite and expands the range of formation of CSH(I) towards the higher temperature side. The temperature of formation of xonotlite rises above that in the absence of Cr₂O₃: from 210 to 450°C at C/S = 0.83 from 160 to 280°C at C/S = 1.0 and from 250 to 270°C at C/S=1.2.     

The effect of Al2O3 doped multi-walled carbon nanotubes on the thermal conductivity of Al2O3/epoxy terminated poly(dimethylsiloxane) composites

Al₂O₃ doped multi-walled carbon nanotubes (MWCNT) were synthesized as a conducting additive in alumina–epoxy terminated poly(dimethylsiloxane) (ETDS). The addition of Al₂O₃ doped MWCNT improved the thermal conductivity of the composites, which was a function of the Al₂O₃ loading. The mechanisms underlying this enhanced conductivity were examined in the context of the Hashin–Shtrikman (HS) boundaries and interconnectivity. The measured thermal conductivity revealed more enhanced thermal conductivity than expected by analytical predictions at a fixed micro Al₂O₃ concentration. Further analytical investigations showed that the addition of Al₂O₃ doped MWCNT affected the interconnectivity between the conducting particles because of their high aspect ratios. Overall, Al₂O₃ doped MWCNT may be useful for establishing three-dimensional heat conducting percolating networks in a matrix that affect the thermal conductivity of a composite.     

Effect of TiO2 supporting layer on Fe2O3 photoanode for efficient water splitting

For an electrochemical water splitting system, titanate nanotubular particles with a thickness of ∼700 nm produced by a hydrothermal process were repetitively coated on fluorine-doped tin oxide (FTO) glass via layer-by-layer self-assembly method. The obtained titanate/FTO films were dipped in aqueous Fe solution, followed by heat treatment for crystallization at 500 °C for 10 min in air. The UV–vis absorbance of the Fe-oxide/titanate/FTO film showed a red-shifted spectrum compared with the TiO₂/FTO coated film; this red shift was achieved by the formation of thin hematite-Fe₂O₃ and anatase-TiO₂ phases verified using X-ray diffraction and Raman results. The cyclic voltammetry results of the Fe₂O₃/TiO₂/FTO films showed distinct reversible cycle characteristics with large oxidation–reduction peaks with low onset voltage of I–V characteristics under UV–vis light illumination. The prepared Fe₂O₃/TiO₂/FTO film showed much higher photocurrent densities for more efficient water splitting under UV–vis light illumination than did the Fe₂O₃/FTO film. Its maximum photocurrent was almost 3.5 times higher than that obtained with Fe₂O₃/FTO film because of the easy electron collection in the current collector. The large current collection was due to the existence of a TiO₂ base layer beneath the Fe₂O₃ layer.     

Sintering behavior and mechanical properties of nano-sized Cr3C2/Al2O3 composites prepared by MOCVI process

A process using metal-organic chemical vapor infiltration (MOCVI) conducted in fluidized bed was employed for the preparation of nano-sized ceramic composites. The Cr-species was infiltrated into Al₂O₃ granules by the pyrolysis of chromium carbonyl (Cr(CO)₆) at 300–450 °C. The granulated powder was pressureless sintered or hot-pressed to achieve high density. The results showed that the dominant factors influencing the Cr-carbide phases formation, either Cr₃C₂ or Cr₇C₃, in the composite powders during the sintering process were the temperature and oxygen partial pressure in the furnace. The coated Cr-phase either in agglomerated or dispersive condition was controlled by the use of colloidal dispersion. The microstructures showed that fine (20 –600 nm) Cr_xC_y grains (≤8 vol.%) located at Al₂O₃ grain boundaries hardly retarded the densification of Al₂O₃ matrix in sintering process. The tests on hardness, strength and toughness appeared that the composites with the inclusions (Cr₃C₂) had gained the advantages over those by the rule of mixture. Even 8 vol.% ultrafine inclusions have greatly improved the mechanical properties. The strengthening and toughening mechanisms of the composites were due to grain-size reduction, homogenous dispersion of hard inclusions, and crack deflection.     

Atomic layer deposition of Al2O3 thin films on diamond

Atomic layer deposition (ALD) of aluminum oxide thin films on diamond was demonstrated for the first time, and the film properties as a gate insulator for diamond field effect transistor (FET) were examined. The interface between the aluminum oxide and the diamond was abrupt, and the ratio of aluminum to oxygen in the film was confirmed to be stoichiometric by Rutherford back scattering. Even a bumpy surface of polycrystalline diamond film was conformally covered by the Al₂O₃ films. To evaluate the feasibility of the film for FET gate insulator, the electrical characteristics of the Al₂O₃ films deposited by ALD on diamond were measured using metal–insulator–semiconductor structure. It was found that the Al₂O₃ films deposited by ALD were better than those deposited by conventional methods, which indicates that the ALD-Al₂O₃ films are feasible for gate insulators of diamond FETs.