Morphology control of α-Al2O3 platelets by molten salt synthesis

It is of great importance to control the morphology of α-Al₂O₃ plate-like powders since α-Al₂O₃ platelets with different shapes are needed in various applications. This paper was focused on how to control the morphology of α-Al₂O₃ platelets by molten salt synthesis. Results show that the morphology of α-Al₂O₃ platelets is affected by the heating temperature, heating time, the molten salts species, the weight ratio of salt to powders, additives and the addition of nano-sized seeds. Especially, it is very effective to control the morphology of α-Al₂O₃ platelets by adjusting the addition of additives such as Na₃PO₄·12H₂O and TiOSO₄. α-Al₂O₃ flakes with irregular shape are obtained by the addition of Na₃PO₄·12H₂O, while thick α-Al₂O₃ particles with hexagonal shape are obtained by the addition of TiOSO₄. The combination addition of Na₃PO₄·12H₂O and TiOSO₄ makes it possible to obtain thin α-Al₂O₃ platelets with discal shape. A small amount of nano-sized seeds addition also has a strong effect on the size of α-Al₂O₃ platelets. However, if the seeds are added too much, the overlapping and abnormal crystal growth of α-Al₂O₃ platelets occur, and the size distribution becomes nonuniform. The effect mechanism of additives and seeds on the morphology of α-Al₂O₃ platelets was also discussed in this paper.     

Preparation of micrometer-sized α-Al2O3 platelets by thermal decomposition of AACH

Micrometer-sized α-Al₂O₃ platelets with hexagonal shape were prepared by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) using AlF₃ as an additive. The precursor and the calcined product were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. The α-Al₂O₃ platelets with the size of 2-3 μm were obtained by calcining AACH at 1200 °C with 5 wt.% AlF₃. The morphology modification is attributed to the various growth rates along different crystal orientations due to the adsorption and uneven distribution of AlF₃. A step growth mode is responsible for the formation of the platelets.     

Solvent-assisted molten salt process: A new route to synthesise α-Fe2O3/C nanocomposite and its electrochemical performance in lithium-ion batteries

Nanostructured α-Fe₂O₃ was synthesised by a simple molten salt process using FeCl₂·4H₂O as a starting material and LiNO₃-LiOH·H₂O-H₂O₂ as a eutectic mixture at 300 °C. To synthesise α-Fe₂O₃/C composite, both α-Fe₂O₃ and malic acid were dispersed together in toluene, where malic acid was used as a carbon source. The morphology and microstructure of both compounds were confirmed by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. Electrochemical testing, including constant current charge-discharge and cyclic voltammetry (CV), was carried out. The α-Fe₂O₃/C composite anode exhibited much better electrochemical performance than the bare α-Fe₂O₃. The discharge capacities of the composite were measured to be 2112 mAh g⁻¹ at C/2 after 100 cycles and 584 mAh g⁻¹ at 20 C after 10 cycles. The superior electrochemical performance of α-Fe₂O₃/C composite can be mainly attributed to the combined effects of the nanostructure, the carbon layering on the α-Fe₂O₃ nanoparticles, and the porous ultra-fine carbon matrix, where the three factors would contribute to provide high electronic conductivity, reduce the traverse time of electrons and lithium-ions, and could also prevent high volume expansion of the anode film during cycling. Our results indicate that the prepared α-Fe₂O₃/C nanocomposite is a very promising anode material for Li-ion power batteries.     

Synthesis, characterization and photocatalytic activity of β-Ga2O3 nanostructures

Nanostructured β-Ga₂O₃ samples were prepared by a combination of the solvothermal processes and subsequent heat treatments. β-Ga₂O₃ samples with various morphologies were obtained by using different kinds of solvent, including water, isopropanol and ethylene glycol. One-dimensional β-Ga₂O₃ nanorods were obtained in water medium, while β-Ga₂O₃ spheres were prepared in alcohol. The possible mechanism related to the phase formation and morphology of the as-prepared materials was discussed. Photocatalytic performance of the synthesized β-Ga₂O₃ samples was also examined. Results revealed that β-Ga₂O₃ sample prepared with ethylene glycol showed the highest photocatalytic activity for the degradation of salicylic acid. This could be ascribed to the high surface area, abundant hydroxyl groups, and wide band gap of β-Ga₂O₃ sample synthesized in ethylene glycol.     

Synthesis and characterization of LZS/α-Al2O3 glass-ceramic composites for applications in the LTCC technology

In this work, α-Al₂O₃ (1–40 vol%) particles (350 nm) were added to a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (5 µm) glass-ceramic matrix to prepare composites with the purpose of studying the influence of α-Al₂O₃ on their mechanical, thermal and electrical properties in order to obtain materials for LTCCs applications. The composites, sintered between 800 and 950 °C for 30 min, with relative densities between 85% and 93%, showed zircon and β-spodumene as main crystalline phases. The maximum bending strength (290 MPa) was achieved for composites with 1% α-Al₂O₃. For composites containing between 1% and 10% α-Al₂O₃, sintered at 900 °C/30 min, the electrical and thermal conductivities and CTE varied between 3.35 and 1.21×10⁻¹⁰ S/cm, 4.65 and 2.98 W/m K, 9.54 and 3.36×10⁻⁶ °C⁻¹, respectively.     

α-Fe2O3 nano-platelets prepared by mechanochemical/thermal processing

A new method to prepare plate-shaped Fe₂O₃ nanoparticles was investigated and their magnetic properties were characterised. First, Fe₂O₃ nanoparticles of ~ 6 nm in diameter were synthesised by mechanochemical processing, involving the solid-state exchange reaction Fe₂(SO₄)₃ + 3 Na₂CO₃ → Fe₂(CO₃)₃ + 3 Na₂SO₄ → Fe₂O₃ + 3 Na₂SO₄ +  3 CO₂ (g) effected by high-energy ball milling and subsequent heat treatment at 400 °C. Next, nano-platelets were grown from the nanoparticles by further heat treatment in a salt matrix at 700 °C. Removal of the salt and by-product phases after heat treatment led to well-dispersed platy haematite particles of 20-200 nm in diameter with aspect ratio of 4-10. The hematite nano-platelets were weakly ferromagnetic with coercivity of ~ 160 Oe and remanent magnetisation of 0.2 emu/g.     

An electrochemical study of H2O2 decomposition on single-phase γ-FeOOH films

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2-12.6 and the H₂O₂ concentration range of 10⁻⁴ to 10⁻² mol dm⁻³. The Tafel slopes for H₂O₂ reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec⁻¹, respectively, independent of pH and the concentration of H₂O₂. Both the reduction and oxidation of H₂O₂ on γ-FeOOH have a first-order dependence on the concentration of molecular H₂O₂. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH⁻. For both cases the electroactive species is the molecular H₂O₂, not its base form, HO₂⁻. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HOOH⁻ and HO for the reduction, and HO₂H⁺, HO₂, and O₂⁻ for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH⁻ and H⁺ in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H₂O₂ to the corresponding intermediates, a reaction step which involves the use of Fe^III/Fe^II sites in the γ-FeOOH surface as an electron donor-acceptor relay. The rate constant for the H₂O₂ decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H₂O₂ decomposition.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

Co catalysts supported on SiO2 and γ-Al2O3 applied to ethanol steam reforming: Effect of the solvent used in the catalyst preparation method

Cobalt catalysts were prepared on supports of SiO₂ and γ-Al₂O₃ by the impregnation method, using a solution of Co precursor in methanol. The samples were characterized by XRD, TPR, and Raman spectroscopy and tested in ethanol steam reforming. According to the XRD results, impregnation with the methanolic solution led to smaller metal crystallites than with aqueous solution, on the SiO₂ support. On γ-Al₂O₃, all the samples exhibited small crystallites, with either solvent, due to a higher Co-support interaction that inhibits the reduction of Co species. The TPR results were consistent with XRD results and the samples supported on γ-Al₂O₃ showed a lower degree of reduction. In the steam reforming of ethanol, catalysts supported on SiO₂ and prepared with the methanolic solution showed the best H₂, CO₂ and CO selectivity. Those supported on γ-Al₂O₃ showed lower H₂ selectivity.     

Spinelisation and properties of Al2O3–MgAl2O4–C refractory: Effect of MgO and Al2O3 reactants

The effect of particle size of MgO and Al₂O₃ on the spinel formation associated with permanent linear change on reheating (PLCR) and microstructure of Al₂O₃–MgAl₂O₄–C refractory is investigated as a function of heating cycle at 1600 °C with 2 h holding at each cycle. It was found that rate of spinel formation and associated volume expansion is very much dependent on the reactivity and particle size of the reactant. When the reactants are very fine and reactive there is considerable amount of spinel formation, whereas coarser reactants with lower reactivity show negligible formation of spinel phase and associated expansion. Magnesia and alumina with moderate reactivity develops optimum PLCR of the refractory. It continuously increases with the number of heating cycles. The SEM photomicrographs show that in Al₂O₃–MgAl₂O₄–C refractory the spinel phase is formed in between the calcined bauxite grain and the EDX analysis indicates that the spinel phase formed is stoichiometric in nature.     

Synthesis of graphene/α-Fe2O3 nanospindles by hydrothermal assembly and their lithium storage performance

Nanocomposite of graphene/α-Fe₂O₃ (G/α-Fe₂O₃) nanospindles was synthesized by a simple hydrothermal assembly method followed by thermal treatment. The α-Fe₂O₃ nanospindles were evenly enwrapped by the graphene nanosheets. When evaluated as an anode for Li-ion batteries (LIBs), the G/α-Fe₂O₃ nanocomposite showed enhanced cycling performance and rate capability compared to pure α-Fe₂O₃. G/α-Fe₂O₃ retained a reversible capacity of 607 mAh g^?1 after 100 cycles at 100 mA g^?1-, which is far higher than that (160 mAh g^?1) of α-Fe₂O₃. The superior electrochemical performance of G/α-Fe₂O₃ can be attributed to the protection effect of graphene nanosheets to accommodate the volume change of α-Fe₂O₃ during lithiation/delithiation.     

Influences of La and Er on structure and properties of Bi2O3–B2O3 glass

Bi₂O₃·2B₂O₃ glasses doped with La₂O₃ and Er₂O₃ were prepared by the melting-quenching method with AR-grade oxides. IR analysis was used to investigate the glass network structure. The characteristic temperatures including the glass transition temperature (T_g), crystallization temperature (T_p), and melting temperature (T_m) were estimated by DSC. The coefficient of thermal expansion (α), mass density (D), and Vickers hardness (H_v) were also measured. The results show that the basic network structure of Bi₂O₃·2B₂O₃ glasses doped with rare-earth oxides consists of chains composed of [BO₃], [BO₄], and [BiO₆] units. La₂O₃ and Er₂O₃ act as network modifiers. As the doping concentrations of the rare-earth oxides were increased, T_g increased and α decreased, indicating that a more rigid glass was obtained. Er₂O₃ reduces the melting temperature and prevents glass crystallization. La₂O₃ contributes to the improvement of the microhardness of Bi₂O₃·2B₂O₃ glass.     

基于Fe2O3的片状α-Al2O3包覆研究

首先以片状α-Al₂O₃为基质,以Fe Cl₃为铁源,采用液相沉积法制备了Fe₂O₃/α-Al₂O₃珠光颜料,并借助SEM、EDS、XRD等手段对制备的珠光颜料进行性能表征,考察了反应p H值、反应温度、铁盐浓度、添加剂用量等因素对片状α-Al₂O₃包覆率的影响,得到了制备铁系包覆片状α-Al₂O₃的最佳制备工艺参数,同时了解了各参数影响包覆率的主次顺序,并在此基础上着重探讨了六偏磷酸钠对包覆率产生影响的机理。结果表明：影响包覆率的主次顺序依次为反应p H值、反应温度、添加剂用量、铁盐浓度;在最优的工艺参数下制备出来Fe₂O₃/α-Al₂O₃的包覆率为20.93%;样品的表面形貌光滑平整且包覆完整。     

Fabrication of rigid media tube filter bound with aluminum borate using Al2O3–B2O3–MgO frit for filtration of molten aluminum

We performed a study of a tube filter binding agent for molten aluminum (rigid media tube filter, RMF). In this study, we examined the formation mechanism of aluminum borate (9Al₂O₃·2B₂O₃; 9A2B) from Al₂O₃–B₂O₃–MgO frit composition, which used MgO instead of CaO in the Al₂O₃–B₂O₃–CaO frit in our previous report. The results indicated that crystallization of 9A2B is easier in Al₂O₃–B₂O₃–MgO compared to Al₂O₃–B₂O₃–CaO, in that crystals are formed with only a heating process, and the amount of crystallization increases depending on the calcination temperature and the heat load over the retention time. A tube filter made of Al₂O₃–B₂O₃–MgO frit contained a larger amount of 9A2B crystals and therefore its strength did not deteriorate until a very high temperature of 1200 °C was reached, and its coefficient of thermal expansion was low. This is an advantageous property for RMF used under constraints and high temperatures. In addition, when the amount of 9A2B crystal formation increases, the wettability of molten aluminum decreases, and it becomes more difficult for molten aluminum to impregnate the filter material, but we found that corrosion resistance was high.     

Effect of Fe2O3 on properties and densification of Ce0.8Gd0.2O2−δ by PCAS

Dense Ce_0.8Gd_0.2O_2−δ was sintered by pulsed current activated sintering (PCAS) within 6 min from Ce_0.8Gd_0.2O_2−δ nanopowder prepared by co-precipitation method. Sintering was accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense Ce_0.8Gd_0.2O_2−δ with relative density of up to 96.3% was produced under simultaneous application of an 80-MPa pressure and the pulsed current. The effects of Fe₂O₃ additions on the sintering behavior, ionic conductivities, and mechanical properties of the Ce_0.8Gd_0.2O_2−δ were investigated.     

Synthesis of γ-Ga2O3-Al2O3 solid solutions by spray pyrolysis method

Synthesis of the γ-Ga₂O₃-Al₂O₃ solid solutions by spray pyrolysis was examined. Spherical particles were obtained using an aqueous solution of Al(NO₃)₃ and Ga(NO₃)₃ with HNO₃. For Ga-rich composition, γ-phase solid solutions were directly crystallized by the spray pyrolysis. For Al-rich composition, spray pyrolysis gave amorphous products unless a sufficient thermal energy was supplied during the spray pyrolysis. Subsequent calcination of the amorphous products gave γ-Ga₂O₃-Al₂O₃ solid solutions. However, physical properties of the solid solutions were affected by the spray pyrolysis conditions.     

Densification and characterization of SiO2B2O3CaOMgO glass/Al2O3 composites for LTCC application

A glass/ceramic composite using lead-free low melting glass (SiO₂B₂O₃CaOMgO glass) with Al₂O₃ fillers was investigated. X-ray diffraction analysis revealed that the anorthite and cordierite phase appeared in the sintered composites. The dilatometric analysis showed that the onset of shrinkage took place at ∼624 °C for all the samples and the onset temperature was independent on the content of glass. The low melting glass significantly promoted densification of the composites and lowered the sintering temperature to ∼875 °C. The addition of 50 wt% glass sintered at 875 °C showed ε_r of 7.3, tan δ of 1.15×10⁻³, TEC of 5.41 ppm/°C, thermal conductivity of 3.56 W/m °C, and flexural strength of 184 MPa. The results showed that the SiO₂B₂O₃CaOMgO glass/Al₂O₃ composites were strong potential candidates for low temperature cofired ceramic substrate applications.     

Synthesis of α-Al2O3 at mild temperatures by controlling aluminum precursor,pH, and ethylenediamine chelating additive

This study demonstrates the synthesis of α-Al₂O₃ by sol–gel method according to various reaction parameters. Various Al₂O₃ phases were synthesized by a simple sol–gel method using three different aluminum precursors (aluminum isoporoxide (AIP), Al(OH)₃, and AlO(OH)) and pHs (3, 7, and 9). Thermally treating of the synthesized powders at 1200 °C produced rhombohedral structure α-Al₂O₃. When AIP was used as an aluminum precursor, α-Al₂O₃ was synthesized at all pH levels by calcination at 1200 °C. The structure was easily changed to α-Al₂O₃ by the addition of ethylenediamine as a chelating additive at the lower temperature of 1000 °C. In contrast, no α-Al₂O₃ structure was obtained by using Al(OH)₃ or AlO(OH) precursors at higher pH in spite of thermal treatment at 1200 °C. The specific surface areas were larger in α-Al₂O₃ synthesized using AIP precursor compared with that using Al(OH)₃ and AlO(OH) precursors. Electrophoretic light scattering (ELS) measurement in aqueous solution at pH=7 revealed positive surface charges in the α-Al₂O₃ synthesized using AIP precursor, but negative charges in that synthesized using Al(OH)₃ and AlO(OH) precursors. Most significantly, the α-Al₂O₃ synthesized with the ethylenediamine chelating additive had a negative charge, despite the use of AIP precursor, with a higher mobility and larger aggregated particle diameter.