Synthesis and thermal behavior of Cu/Y2W3O12 composite

Cu metal matrix composite with Y₂W₃O₁₂ as a thermal expansion compensator was fabricated by high energy ball milling followed by compaction and sintering, and its thermal properties were explored for the potential applications as heat sinks in electronic industries, high precision optics, and space structures. The volume fraction of reinforcement was varied from 40% to 70% in order to tailor the composite for the simultaneous accomplishment of low thermal expansion and high thermal conductivity. The synthesis technique was optimized by varying the parameters like milling time from 1 to 20 h and sintering temperature from 600 to 1000 °C in order to achieve densified composites. The relative density of the composites is found to be around 90% for the 10 h milled powders followed by compaction at a pressure of 700 MPa and sintering at a temperature of 1000 °C. The thermal expansion of the composites exhibits linear behavior in the temperature range 200 to 800 °C and the low coefficient of thermal expansion (CTE) is found to be for Cu–70%Y₂W₃O₁₂ composite whose value, 4.32±0.75×10⁻⁶/°C, matches with that of Si substrate. The thermal conductivities are found to increase with a decrease in the volume fraction of the reinforcement and decrease with an increase in the temperature for all the samples. The experimentally determined CTE and thermal conductivity values are found to be comparable to those predicted by the thermal expansion based Kerner and Turner model and the thermal conductivity based Maxwell model, respectively.     

Preparation and characterization of negative thermal expansion Sc2W3O12/Cu core-shell composite

A composite of Sc₂W₃O₁₂/Cu where Sc₂W₃O₁₂, the core, is coated by the Cu shell was synthesized using simple electroless plating method. As-prepared Sc₂W₃O₁₂/Cu composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and thermomechanical analyzer (TMA) techniques. The study results show that the Pd-Sn activator was successfully formed on the surface of Sc₂W₃O₁₂ after the sensitization and activation. In the electroless plating process, Cu nanocrystals formed firstly, and then grew together to form a continuous coating. Sc₂W₃O₁₂/Cu core-shell composites exhibit a negative linear coefficient of thermal expansion CTE = −4.47 × 10⁻⁶ °C⁻¹ from room temperature to 200 °C.     

Phase transition and negative thermal expansion properties of Sc2−xCrxMo3O12

The crystal structure, phase transition and thermal expansion behaviors of solid solutions Sc_2−xCr_xMo₃O₁₂ (0≤x≤2) were investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). At room temperature, samples with x≤0.7 and x≥0.8 crystallize in orthorhombic and monoclinic structures, respectively. DSC result indicates that the phase transition of Sc_0.5Cr_1.5Mo₃O₁₂ from monoclinic to orthorhombic structure occurs at 203.66 °C. The linear thermal expansion coefficient of orthorhombic phases varies from −2.334×10⁻⁶ °C⁻¹ to 0.993×10⁻⁶ °C⁻¹ when x increases from 0.0 to 1.5. The near-zero linear thermal expansion coefficients of −0.512×10⁻⁶ °C⁻¹ and −0.466×10⁻⁶ °C⁻¹ are observed for compounds with x=0.5 and 0.7, respectively.     

Hydrothermal synthesis of LiNixCo1−xO2 cathode materials

Ultrafine powders of LiCoO₂, nonstoichiometric LiNiO₂ and LiNi_0.9Co_0.1O₂ were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/Co, Li/Ni and Li/(Ni + Co) was studied. The final products were investigated by XRD, TEM and EDS. To synthesize a stoichiometric LiNiO₂ under mild hydrothermal conditions was found to be a big challenge. Transmission electron microscopies (TEM) revealed the formation of well-crystallized LiCoO₂ and LiNi_0.9Co_0.1O₂ with average size of 100 nm and 10 nm, respectively.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Synthesis and negative thermal expansion property of Y2−xLaxW3O12 (0≤x≤2)

Y_2−xLa_xW₃O₁₂ solid solutions were successfully synthesized by the solid state reaction method. The microstructure, hygroscopicity and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and thermal mechanical analysis (TMA). Results indicate that the structural phase transition of the Y_2−xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phase can form for 0≤x≤0.4 with orthorhombic structure and for 1.5≤x≤2 with monoclinic one. High lanthanum content leads to a low relative density of Y_2−xLa_xW₃O₁₂ ceramic. Thermal expansion coefficients of the Y_2−xLa_xW₃O₁₂ (0≤x≤2) ceramics also vary from −9.59×10⁻⁶ K⁻¹ to 2.06×10⁻⁶ K⁻¹ with increasing substituted content of lanthanum. The obtained Y_0.25La_1.75W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.66×10⁻⁶ K⁻¹ from 103 °C to 700 °C.     

Amorphous LiCoO2 thin films on Li1+x+yAlxTi2−xSiyP3−yO12 prepared by radio frequency magnetron sputtering for all-solid-state Li-ion batteries

Amorphous LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 180 °C. The as-deposited LiCoO₂ thin films were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscope. All-solid-state Li/PEO₁₈-Li (CF₃SO₂)₂N/LATSP/LiCoO₂/Au cells were fabricated using the amorphous film. The electrochemical performance of the cells was investigated by galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. It was found that the amorphous LiCoO₂ thin film shows a promising electrochemical performance, making it a potential application in microbatteries for microelectronic devices.     

Combustion synthesis of (Ti1−xNbx)2AlC solid solutions from elemental and Nb2O5/Al4C3-containing powder compacts

Preparation of the (Ti_1−xNb_x)₂AlC solid solution (formed from the M_n+1AX_n or MAX carbides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element, and X is C) with x = 0.2-0.8 was investigated by self-propagating high-temperature synthesis (SHS). Nearly single-phase (Ti,Nb)₂AlC was produced through direct combustion of constituent elements. Due to the decrease of reaction exothermicity, the combustion temperature and reaction front velocity decreased with increasing Nb content of (Ti_1−xNb_x)₂AlC formed from the elemental powder compacts. In addition, the samples composed of Ti, Al, Nb₂O₅, and Al₄C₃ were adopted for the in situ formation of Al₂O₃-added (Ti,Nb)₂AlC. The SHS process of the Nb₂O₅/Al₄C₃-containing sample involved aluminothermic reduction of Nb₂O₅, which not only enhanced the reaction exothermicity but also facilitated the evolution of (Ti,Nb)₂AlC. Based upon the XRD analysis, two intermediates, TiC and Nb₂Al, were detected in the (Ti,Nb)₂AlC/Al₂O₃ composite and their amounts were reduced by increasing the extent of thermite reduction involved in the SHS process. The laminated microstructure characteristic of the MAX carbide was observed for both monolithic and Al₂O₃-added (Ti,Nb)₂AlC solid solutions synthesized in this study.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Fabrication and characterization of Li1+x−yNb1−x−3yTix+4yO3 substrates using aqueous tape casting process

An aqueous system for tape casting Li_1+x−yNb_1−x−3yTi_x+4yO₃ (LNT) ceramics was developed using poly(vinyl alcohol) (PVA) binder, ethylene alcohol (EG) plasticizer and ammonium salt of polycarboxylate (PCA-NH₄) dispersant. The zeta potential measurement showed that the isoelectric point of the LNT particles moved slightly toward more acid region after the dispersant absorbed on the particles, while the zeta potential increased significantly. The rheological test indicated that the ceramic slurry exhibited a typical pseudoplastic behavior without thixotropy. The effect of solid loading on the properties of the green tapes was investigated. The increase in the solid loading increased the tensile strength and the green density of the tapes. TGA analysis indicated that the organic additives in the green tapes can be completely removed by heat treatment at 600 °C. SEM micrographs showed that the microstructure of the green and sintered tapes was homogeneous.     

Study on the rapid synthesis of LiNi1−xCoxO2 cathode material for lithium secondary battery

LiNi_1−xCo_xO₂ (x = 0, 0.1, 0.2) cathode materials were successfully synthesized by a rheological phase reaction method with calcination time of 0.5 h at 800 °C. All obtained powders are pure phase with α-NaFeO₂ structure (R-3m space group). The samples deliver an initial discharge capacity of 182, 199 and 189 mAh g⁻¹ (25 mA g⁻¹, 4.35-3.0 V), respectively. The reaction mechanism was also discussed, which consists of a series of defect reactions. As a result of these defect reactions, the reaction of forming LiNi_1−xCo_xO₂ takes place in high speed.     

Effects of B-site ion (M) substitution on the ionic conductivity of (Li3xLa2/3−x)1+y/2(MyTi1−y)O3 (M=Al, Cr)

Effect on the ionic conductivity of B-site ion (M) substitution in (Li_3xLa_2/3−x)_1+y/2M_yTi_1−yO₃ (M=Al, Cr) has been investigated. It has been found that partial substitution of smaller Al³⁺ for Ti⁴⁺ is effective to enhance the ionic conductivity of Li_3xLa_2/3−xTiO₃. At 300 K, the maximum bulk conductivity of (1.58±0.01)×10⁻³ S cm⁻¹ is observed from the composition of (Li_0.39La_0.54)_1−y/2Al_yTi_1−yO₃ with y=0.02 (x=0.13), that is the highest yet reported for known perovskite solutions at room temperature. The conductivity enhancement is interpreted as being due to the substitution-induced bond-strength change rather than due to bottleneck size change for Li migration, TiO₆-octahedron tilting or A-site cation ordering.     

Synthesis and characterization of ZnxMg1 − xGa2O4:Co fabricated by low-temperature burning method

A series of Zn_xMg_1 − xGa₂O₄:Co²⁺ spinels (x = 0, 0.25, 0.5, 0.75, and 1.0) was successfully produced through low-temperature burning method by using Mg(NO₃)₂·4H₂O, Zn(NO₃)₂·6H₂O, Ga(NO₃)₃·6H₂O, CO(NH₂)₂, NH₄NO₃, and Co(NO₃)₂·6H₂O as raw materials. The product was characterized by X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The product was not merely a simple mixture of MgGa₂O₄ and ZnGa₂O₄; rather, it formed a solid solution. The lattice constant of Zn_xMg_1 − xGa₂O₄:Co²⁺ (0 ≤ x ≤ 1.0) crystals has a good linear relationship with the doping density, x. The synthesized products have high crystallinities with neat arrays. Based on an analysis of the form and position of the emission spectrum, the strong emission peak around the visible region (670 nm) can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴A₂(⁴F)] of Co²⁺ in the tetrahedron. The weak emission peak in the near-infrared region can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴T₂(⁴F)] of Co²⁺ in the tetrahedron.     

Studies on electrical properties of La0.8Sr0.2Ga0.8Mg0.2O2.80 (LSGM) and LSGM-SrSn1−xFexO3 (x = 0.8; 0.9) composites and their chemical reactivity

We report the electrical conductivity properties of solid-state synthesized perovskite-like La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.80 (LSGM) and LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites. LSGM exhibits both bulk and grain-boundary contribution in the ac impedance plots. The grain-boundary conductivity (σ_gb) is slightly (≤half-order of magnitude) higher than that of the bulk oxide ion conductivity (σ_bulk). Powder XRD study reveals that no chemical reaction occurs between LSGM and SrSn_1−xFe_xO₃ (1:1 wt.%) at 1000 °C (48 h) and forms a single-phase perovskite-like compound at 1300 °C (48 h) in air, while in hydrogen atmosphere, at 800 °C for 48 h, a growth of LaSrGaO₄ and LaSrGa₃O₇ impurity phases and formation of metallic Fe was observed. The LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites show a single or part of semicircle in air at low-temperature regime. The electrical conductivity of the composites were found to be much higher compared to pure LSGM and lower about an order of magnitude than those of pure Sn-doped SrFeO₃ perovskite.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Synthesis and thermal expansion properties of Y2−xLaxMo3O12 (x=0, 0.5, 2)

Y_2−xLa_xMo₃O₁₂ (x=0, 0.5, 2) ceramics were successfully synthesized by the solid state reaction method. The microstructure, composition and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and dilatometry. Results indicate that the Y_1.5La_0.5Mo₃O₁₂ crystallizes in monoclinic Tb₂Mo₃O₁₂-type structure and it is non-hygroscopic. The Y_1.5La_0.5Mo₃O₁₂ ceramic is denser than the Y₂Mo₃O₁₂ and La₂Mo₃O₁₂ ceramics, and its relative density can reach 94.12% of the theoretical value. Most importantly, it shows almost zero thermal expansion and its thermal coefficient is 0.87×10⁻⁶ K⁻¹ from 178 °C to 600 °C. Y₂Mo₃O₁₂ ceramic shows negative thermal expansion whereas La₂Mo₃O₁₂ ceramic shows positive thermal expansion, their thermal expansion coefficients being−12.06×10⁻⁶ K⁻¹ and 8.88×10⁻⁶×10⁻⁶ K⁻¹, respectively.     

Preparation and characterization of novel spinel Li4Ti5O12−xBrx anode materials

Br-doped Li₄Ti₅O₁₂ in the form of Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) compounds were successfully synthesized via solid state reaction. The structure and electrochemical properties of the spinel Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) materials were investigated. The Li₄Ti₅O_12−xBr_x (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li₄Ti₅O_12−xBr_x (x = 0.2) sample presented the excellent discharge capacity of 172 mAh g⁻¹, which was very close to its theoretical capacity (175 mAh g⁻¹), while that of the pristine Li₄Ti₅O₁₂ was 123.2 mAh g⁻¹ only.