Indirect selective laser sintering of epoxy resin-Al2O3 ceramic powders combined with cold isostatic pressing

To fabricate Al₂O₃ ceramic components with complex shape, selective laser sintering (SLS) combined with cold isostatic pressing (CIP) process was used to consolidate Al₂O₃ powder with additive of epoxy resin E06 (ER06) and polyvinyl alcohol (PVA). The starting material preparation combined spray drying with mechanical mixing to formulate compound powder consisting of PVA (1.5 wt%), ER06 (8 wt%) and Al₂O₃ and provide a good fluidity for SLS. Experimental investigations were carried the shrinkage, relative density, bending strength of Al₂O₃-ER06 SLS specimens in order to optimize the laser sintering parameters. It was found that Al₂O₃-ER06 SLS specimens represented acceptable shrinkage, high density and bending strength when laser power, scanning speed, scanning space and layer thickness were, respectively, 21 W, 1600 mm/s, 100 μm and 150 μm. Following that, the SLS specimens were processed through CIP to eliminate the pores in green ceramics. Finally, the optimized SLS/CIP Al₂O₃ specimens were debinded, sintered to produce crack-free Al₂O₃ bodies. The final Al₂O₃ components achieved a relative high density of more than 92% after furnace sintering. The study shows a novel and promising approach to fabricate complex ceramic matrix and ceramic components via indirect SLS and CIP process.     

Plastic shaping of NiFe2O4-based cermets using granular feedstock: microstructure and mechanical properties

Carbon-free aluminum electrolysis is crucial for achieving the low-carbon development and carbon neutrality in the future aluminum industry. NiFe₂O₄-based cermet is the potential candidate of inert anode, benefiting from the satisfactory corrosion resistance and high-temperature conductivity. Herein, complex-shaped 25(Cu–20Ni)/(NiFe₂O₄–10NiO) cermets were fabricated via a plastic shaping method using the polyformaldehyde (POM)-based granular feedstock. The feedstock showed uniform microstructure with evenly dispersed powders wrapped in polymers, which exhibited shear-thinning behavior during molding. Roles of the feedstock with different powder loadings in the morphology and bending strength of the cermets were investigated. High-precision gear parts exhibited no deformation, a high relative density over 98%. Investigation of different powder loadings revealed that parts prepared with a 56 vol% loading demonstrate the excellent performance, and possessed an impressive flexural strength of 178.4 MPa. This achievement provided a foundation for the future utilization of complex-shaped inert anode material components in industrial applications.     

Sintering kinetics and properties of NiFe2O4-based ceramics inert anodes doped with TiN nanoparticles

Sintering kinetics of NiFe₂O₄-based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe₂O₄-based ceramics possessed high-temperature semiconductor essential characteristics. The electrical conductivity of NiFe₂O₄-based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe₂O₄-based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.     

Effect of TiN content on properties of NiFe2O4-based ceramic inert anode for aluminum electrolysis

NiFe₂O₄-based ceramic inert anodes for aluminum electrolysis doped with various TiN nanoparticles were prepared by a two-step cold-pressing sintering process to investigate how TiN affected the sintering behavior and properties of the composites. The differential scanning calorimetry-thermogravimetry (DSC-TG), X-ray diffraction (XRD), and microstructure analysis results indicated that the Ti and N were evenly distributed in the NiFe₂O₄ matrix to form a solid solution. The maximum linear shrinkage and linear shrinkage rate were enhanced with the increase of TiN nanoparticles contents, and the sintering activation energy of initial stage was lowered from 382.63 to 279.58 kJ mol⁻¹ with the TiN nanoparticles additive range from 0 to 9 wt%. When the content of TiN nanoparticles was 7 wt%, the relative density, bending strength, and elastic modulus reached their maximum values of 97.24%, 73.88 MPa, and 3.77 GPa, respectively, whereas the minimum static corrosion rate of NiFe₂O₄-based ceramic of 0.00114 g cm⁻² h⁻¹ was obtained, mainly attributed to the relatively dense and stable microstructure. The electrical conductivity of NiFe₂O₄-based ceramics presented a clear ascending trend with increasing TiN nanoparticles content and elevated temperature, attributed to the increased concentration and migration rate of carrier.     

Microstructure and mechanical properties of Al2O3/Er3Al5O12 eutectic rods grown by the laser-heated floating zone method

Eutectic rods of Al₂O₃-Er₃Al₅O₁₂ were grown by directional solidification using the laser-heated floating zone method at rates in the range 25-1500 mm/h. Their microstructure and mechanical properties (hardness, toughness and strength) were investigated as a function of the growth rate. A homogeneous and interpenetrated microstructure was found in most cases, and interphase spacing decreased with growth rate following the Hunt-Jackson law. Hardness increased slightly as the interphase spacing decreased while toughness was low and independent of the microstructure. The rods presented very high bending strength as a result of the homogeneous microstructure, and their strength increased rapidly as the interphase spacing decreased, reaching a maximum of 2.7 GPa for the rods grown at 750 mm/h. The bending strength remained constant up to 1300 K and decreased above this temperature. The relationship between the microstructure and the mechanical properties was established from the analysis of the microstructure and of the fracture mechanisms.     

Electrochemical performances of poly(3,4-ethylenedioxythiophene)-NiFe2O4 nanocomposite as electrode for supercapacitor

Poly 3,4-ethylenedioxythiophene (PEDOT)-based NiFe₂O₄ conducting nanocomposites were synthesized and their electrochemical properties were studied in order to find out their suitability as electrode materials for supercapacitor. Nanocrystalline nickel ferrites (5-20 nm) have been synthesized by sol-gel method. Reverse microemulsion polymerization in n-hexane medium for PEDOT nanotube and aqueous miceller dispersion polymerization for bulk PEDOT formation using different surfactants have been adopted. Structural morphology and characterization were studied using XRD, SEM, TEM and IR spectroscopy. Electrochemical performances of these electrode materials were carried out using cyclic voltammetry at different scan rates (2-20 mV/s) and galvanostatic charge-discharge at different constant current densities (0.5-10 mA/cm²) in acetonitrile solvent containing 1 M LiClO₄ electrolyte. Nanocomposite electrode material shows high specific capacitance (251 F/g) in comparison to its constituents viz NiFe₂O₄ (127 F/g) and PEDOT (156 F/g) where morphology of the pore structure plays a significant role over the total surface area. Contribution of pseudocapacitance (C_FS) arising from the redox reactions over the electrical double layer capacitance (C_DL) in the composite materials have also been investigated through the measurement of AC impedance in the frequency range 10 kHz-10 mHz with a potential amplitude of 5 mV. The small attenuation (∼16%) in capacitance of PEDOT-NiFe₂O₄ composite over 500 continuous charging/discharging cycles suggests its excellent electrochemical stability.     

Synthesis of highly crystalline spinel LiMn2O4 by a soft chemical route and its electrochemical performance

Highly crystalline spinel LiMn₂O₄ was successfully synthesized by annealing lithiated MnO₂ at a relative low temperature of 600 °C, in which the lithiated MnO₂ was prepared by chemical lithiation of the electrolytic manganese dioxide (EMD) and LiI. The LiI/MnO₂ ratio and the annealing temperature were optimized to obtain the pure phase LiMn₂O₄. With the LiI/MnO₂ molar ratio of 0.75, and annealing temperature of 600 °C, the resulting compounds showed a high initial discharge capacity of 127 mAh g⁻¹ at a current rate of 40 mAh g⁻¹. Moreover, it exhibited excellent cycling and high rate capability, maintaining 90% of its initial capacity after 100 charge-discharge cycles, at a discharge rate of 5 C, it kept more than 85% of the reversible capacity compared with that of 0.1 C.     

Processing and physical properties of Al2O3/aluminum alloy composites

Porous aluminum oxide (Al₂O₃) preforms were formed by sintering in air at 1200 °C for 2 h. A356, 6061, and 1050 aluminum alloys were infiltrated into the preforms by squeeze casting in order to fabricate Al₂O₃/A356, Al₂O₃/6061, and Al₂O₃/1050 composites, respectively, with different volumes of aluminum alloy content. The content of aluminum alloy in the composites was 10–40% by volume. The resistivity of Al₂O₃/A356, Al₂O₃/6061, and Al₂O₃/1050 composites decreased dramatically from 6.41 × 10¹² to 9.77 × 10⁻⁴, 7.28 × 10⁻⁴, and 6.24 × 10⁻⁴ Ω m, respectively, the four-points bending strength increased from 397 to 443, 435.1, 407.2 MPa, respectively, and the deviations were smaller than 2%. From SEM microstructural analysis and TEM bright field images, the pore volume fraction and the relative density of the composites were the most important factors that affected the physical and mechanical properties. The ceramic phase and alloy phase in Al₂O₃/aluminum alloy composites were found to be homogenized and uniformly distributed using electrical and mechanical properties analysis, microstructure analysis, and image analysis.     

Molten salt synthesis of spherical LiNi0.5Mn1.5O4 cathode materials

This paper describes a novel simple redox process for synthesizing monodispersed MnO₂ powders and preparation of spherical LiNi_0.5Mn_1.5O₄ cathode materials by molten salt synthesis (MSS) method. Monodispersed MnO₂ powders have been synthesized by using potassium permanganate and manganese sulfate as the starting materials. By using this redox method, it was found that monodispersed MnO₂ powders with average particle size ∼5 μm can be easily obtained. Resultant MnO₂ and LiOH, Ni(OH)₂ was then used to synthesis LiNi_0.5Mn_1.5O₄ cathode materials with retention of spherical particle shape by MSS method. The discharge capacity was 129 mAh g⁻¹ in the first cycle and 127 mAh g⁻¹ after 50 cycles under an optimal synthesis condition for 12 h at 800 °C.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Gold nanoparticles mediate the assembly of manganese dioxide nanoparticles for H2O2 amperometric sensing

Aggregates of gold nanoparticles (AuNPs) that mediate the assembly of manganese dioxide nanoparticles (nano-MnO₂) for hydrogen peroxide (H₂O₂) amperometric sensing have been developed. The aggregates were prepared by directly mixing citric-capped AuNPs and poly(allylamine hydrochloride) (PAH)-capped nano-MnO₂ using an electrostatic self-assembly strategy. The prepared sensor exhibited excellent electrochemical behaviors and a wide linear range from 7.80 × 10⁻⁷ to 8.36 × 10⁻⁴ M with a detection limit of 4.68 × 10⁻⁸ M (S/N = 3) because of the synergistic influence of excellent catalytic ability of MnO₂ and good electrical conductivity of AuNPs. In addition, its applicability to practical samples for measuring H₂O₂ in toothpastes has obtained a satisfactory result. Due to the ease of preparation and excellent properties of the sensor, indicating the MnO₂-AuNP material may be a potential H₂O₂ sensor.     

Stable Ti/RuO2-Sb2O5-SnO2 electrodes for O2 evolution

RuO₂-based electrodes are generally known to be unstable for O₂ evolution. In this paper, a stable type of RuO₂-based electrode, Ti/RuO₂-Sb₂O₅-SnO₂, is demonstrated for O₂ evolution. In the ternary oxide coating, RuO₂ serves as the catalyst, SnO₂ as the dispersing agent, and Sb₂O₅ as the dopant. The accelerated life test showed that the Ti/RuO₂-Sb₂O₅-SnO₂ electrode containing 12.2 molar percent of RuO₂ nominally in the coating had a service life of 307 h in 3 M H₂SO₄ solution under a current density of 0.5 A cm⁻² at 25 °C, which is more than 15 times longer than other types of RuO₂-based electrodes. Instrumental analysis indicated that RuO₂-Sb₂O₅-SnO₂ was a solid solution with a compact structure, which contributed to the stable nature of the electrode.     

Effects of precursor treatment with reductant or oxidant on the structure and electrochemical properties of LiNi0.5Mn1.5O4

LiNi_0.5Mn_1.5O₄, a lithium-ion battery cathode material, is prepared using co-precipitation via a two-step drying method with Ni-Mn mixed hydroxide as the precursor. This study examines the effects of precursor pretreatment with hydrazine (a reductant) or with H₂O₂ (an oxidant) in solutions of NiSO₄ and MnSO₄. The results indicate substantial differences in the structure and electrochemical properties of LiNi_0.5Mn_1.5O₄ depending on whether the precursor is pretreated with reductant or oxidant. For the hydrazine-treated precursor, the synthesized LiNi_0.5Mn_1.5O₄ has a very pure spinel phase and an ordered, octahedral crystal morphology (ca. 100-300 nm). In contrast, the material synthesized using the H₂O₂-treated precursor shows numerous impurity phases (Na_0.7MnO_2.05) with a layer-by-layer crystal structure. The control sample (prepared without precursor pretreatment) maintains an octahedral structure but still retains a few impurity phases of Na_0.7MnO_2.05. The electrochemical results show that LiNi_0.5Mn_1.5O₄ synthesized using a hydrazine-treated precursor has a higher specific capacity (especially under high discharge current) and a higher cyclic life than the control sample, whereas the sample using H₂O₂-treated precursor shows almost no special capacity due to changes in crystal structure.     

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.     

Synthesis of spherical spinel LiMn2O4 with commercial manganese carbonate

Spherical spinel LiMn₂O₄ particles were successfully synthesized from a mixture of manganese compounds containing commercial manganese carbonate by sintering of the spray-dried precursor. Different preparation routes were investigated to improve the tap density and to enhance the electrochemical performance of LiMn₂O₄. The structure and morphology of the LiMn₂O₄ particles were confirmed by X-ray diffraction (XRD) and scanning electron microscopy. The results showed that hollow spherical LiMn₂O₄ particles could be obtained when only commercial MnCO₃ was used as the manganese source. These particles had a low tap density (ca.0.8 g/cm³). Perfect micron-sized spherical LiMn₂O₄ particles with good electrochemical performance were obtained by spray-drying a slurry composed of MnCO₃, Mn(CH₃CHOO)₂ and LiOH, followed by a dynamic sintering process and a stationary sintering process. The as-prepared spherical LiMn₂O₄ particles comprised hundreds of nanosize crystal grains and had a high tap density(ca. 1.4 g/cm³). The galvanostatic charge-discharge measurements indicated that the spherical LiMn₂O₄ particles had an initial capacity of 121 mAh/g between 3.0 and 4.2 V at 0.2 C rate and still delivered a reversible capacity of 112 mAh/g at 2 C rate. The retention of capacity after 50 cycles was still 96% of its initial capacity at 0.2 C. All the results showed that the as-prepared spherical LiMn₂O₄ particles had an excellent electrochemical performances. The methods we used for preparing spherical LiMn₂O₄ are energy-saving and suitable for industrial application.     

Fabrication of novel multilayer Al2O3-(m-ZrO2)/t-ZrO2 fibrous ceramics composites

A novel layered microstructure in the Al₂O₃/ZrO₂ composites system was fabricated by the multipass extrusion method. The microstructure consisted with very fine alternate lamina of Al₂O₃-(m-ZrO₂) and t-ZrO₂. The composites were designed in such a way that a small group of 7 cylindrical alternate layers of Al₂O₃-(m-ZrO₂) and t-ZrO₂ made a concentric microgroup around 40 μm in diameter, with a common boundary layer between the adjacent groups. The thickness of both layers was around 2-3 μm. The microstructure was unidirectionally aligned throughout the composites. The composite microstructure was fibrous due to the unidirectional orientation of these microgroups. Detailed microstructure of the fabricated composites was characterized by SEM. The effect of the concentric layered microstructure on mechanical behavior was discussed. Material properties such as density, bending strength, Vickers hardness and fracture toughness were measured and evaluated depending on different sintering temperatures.     

Microstructural, crystal structure and electrical characteristics of shock-consolidated Ga2O3 doped ZnO bulk

Ga₂O₃ (5 wt.%) doped zinc oxide (ZnO, 95 wt.%) bulk was fabricated by underwater shock compaction technique. The microstructural, crystal structure and electrical properties of shock-consolidated samples were investigated and compared to a commercially available sintered Ga₂O₃ (5 wt.%) doped ZnO (95 wt.%). The relative density of shock-consolidated sample was about 97% of the theoretical density, and no grain growth and lattice defects were confirmed. The grain boundary resistance was remarkably higher than that of commercial sintered Ga₂O₃ doped ZnO and nonlinear current-voltage (I-V) characteristics of shock-consolidated ZnO and Ga₂O₃ doped ZnO were very lower than that of commercial ZnO varistor.     

Pressureless sintering of B4C-TiB2 composites with Al additions

The effects of Al addition on pressureless-sintering of B₄C-TiB₂ composites were studied. Different amounts of Al from 0% to 5 wt.% were added to B₄C-TiB₂ mixtures (containing up to 30 wt.% TiB₂) and the samples were pressureless sintered at 2050 °C and 2150 °C under Ar atmosphere. Physical, microstructural and mechanical properties were analysed and correlated with TiB₂ and Al additions and sintering temperature. Addition of Al to B₄C-TiB₂ results in increased shrinkage upon sintering and final relative density and lower porosity, the effect is being more evident when both Al and TiB₂ are present. Fracture strength, elastic modulus and fracture toughness of 450 MPa, 500 GPa and 6.2 MPa.m^1/2, respectively were measured.     

Structural modification of Li[Li0.27Co0.20Mn0.53]O2 by lithium extraction and its electrochemical property as the positive electrode for Li-ion batteries

In our previous report, we have synthesized Li₂MnO₃-LiCoO₂ solid solutions and have investigated electrochemical properties [J.-M. Kim, T. Sho, N. Kumagai, Electrochem. Commun. 9 (2007) 103]. These materials have showed a long charge plateau at above 4.5 V during the first charge, which disappears with the subsequent cycles. This phenomenon is usually observed in Li₂MnO₃ and Li₂MnO₃-LiMeO₂ system (Me = Ni_1/2Mn_1/2 [Z. Lu, D.D. MacNeil, J.R. Dahn, Electrochem. Solid State Lett. 4 (2001) 191], Co [K. Numata, C. Sakaki, S. Yamanaka, Solid State Ionics 117 (1999) 257; Y.J. Park, Y.-S. Hong, X. Wu, M.K. Kim, K.S. Ryu, S.H. Chang, J. Electrochem. Soc. 151 (2004) A720], Fe [M. Tabuchi, A. Nakashima, H. Shigemura, K. Ado, H. Kobayashi, H. Sakaebe, H. Kageyama, T. Nakamura, M. Kohzaki, A. Hirano, R. Kanno, J. Electrochem. Soc. 149 (2002) A509], or Cr [B. Ammundsen, J. Paulsen, Adv. Mater. 13 (2001) 943]). In this study, we investigate the relationship between the first lithium extraction process and the electrochemical property of the synthesized Li[Li_0.27Co_0.2Mn_0.53]O₂ material. The crystal structure and electrochemical performance of the synthesized Li[Li_0.27Co_0.20Mn_0.53]O₂ are modified by the Li⁺ extraction.     

Effect of Al2O3 particle size on preparation and properties of ZTA ceramics formed by gelcasting

Zirconia-toughened alumina (ZTA) ceramics were prepared using three different kinds of Al₂O₃ powders (marked PW-A average particle size: 7.53 μm, marked PW-B average particle size: 1.76 μm, marked PW-C average particle size: 0.61 μm) by gelcasting. Effect of Al₂O₃ particle size on zeta potential, dispersant dosage and solid volume fractions of ZTA suspensions as well as the mechanical properties of ZTA green bodies and ceramics were investigated. The optimum dosages of dispersant for ZTA suspensions prepared by PW-A, PW-B and PW-C are 0.4 wt%, 0.5 wt% and 0.7 wt%, respectively. The highest solid volume fractions of ZTA suspensions can reach 62 vol% (SP-A), 60 vol% (SP-B) and 52 vol% (SP-C), respectively. The green bodies show a bending strength as high as 20 MPa, which can meet the requirement of machining. The Al₂O₃ powder with fine particle size is beneficial to the improvement of mechanical properties. The ZTA ceramics prepared by PW-B Al₂O₃ powder show the highest bending strength (680 MPa) and toughness (7.49 MPa m^1/2).