Acid-catalysed Al2O3 sol and its gel material: influence of HNO3 content and stability

Abstract

A series of Al₂O₃ sols were prepared by HNO₃ catalysis of aluminum isopropoxide (AIP), using different molar ratios of HNO₃/AIP (n_H). The changes in sol densities, viscosities, refractive indices, stability and particle sizes were reported. Derived properties such as Gibbs energies (ΔG*) and the reaction rate constant (k) have been calculated from the experimental density and viscosity data. The phase-chemical structures of Al₂O₃ materials were characterized using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) measurement. The results show that, when the molar ratio of HNO₃/AIP is 0.25, the prepared Al₂O₃ sol was transparent, uniform and stable, and the sol average particle size gets the minimum. The chemical structure of the non-calcined Al₂O₃ gel material is dominated by Al–O–H and Al–O bonds. After calcination at 350?°C, the Al–O–H bond will break and is transformed into Al–O bonds, and the phase structure is changed from γ-AlOOH to γ-Al₂O₃.     

Preparation and Performance of Li4Ti5O12 by Sol-Gel Method

Abstract

In This paper, butyl titanate and LiOH·H₂O was used as titanium source and lithium source respectively. Li₄Ti₅O₁₂ as battery negative material was prepared by sol-gel method. The effects of chelating agent (triethanolamine), raw material ratio, calcination temperature and calcination time on Li₄Ti₅O₁₂ products were studied by single factor method. The experimental results show that The optimum synthetic conditions are as follows: the spinel Li₄Ti₅O₁₂ with pure phase, uniform grain size, 100–200?nm particle size and no obvious agglomeration can be prepared under the conditions of molar ratio of lithium to titanium is 0.9–1.0, the calcination temperature can be 730–750?°C at 8?h. After assembled into CR2016 button cell, the first charge-discharge capacity of CR2016 button cell can reach 169 mAh/g at 0.1?C rate, which is very close to the theoretical value of 175 mAh/g.     

Sintering and dielectric properties of Al2O3 ceramics doped by TiO2 and CuO

The effects of CuO and TiO₂ additives on the microstructure and microwave dielectric properties of Al₂O₃ ceramics were investigated. Al₂O₃ ceramics with CuO and TiO₂ additions can be well sintered to achieve 93∼98% theoretical densities below 1,360 °C due to Ti₄Cu₂O liquid phase sintering effect. The Qf values decreased with increasing CuO and TiO₂ content, due to the formation of the second phase Ti₄Cu₂O. However, the varying behaviors of the dielectric constant (ɛ _r ) and temperature coefficients (τ _f ) were associated with phase constitutions, as a result of the change of CuO and TiO₂content. The τ _f can be shifted close to 0 ppm/°C by controlling the content of CuO and TiO₂. The specimens with 0.5 wt.% CuO and 7 wt.% TiO₂ sintered at 1,360 °C for 4 h showed ɛ _r of 11.8, Qf value of 30,000 GHz, and τ _f of −7 ppm/°C.     

Mixed Hydroxide Precursors for La2Ti2O7 and Nd2Ti2O7 by Homogeneous Precipitation

Feasibility of formation of stoichiometric precursors of either M₂ (TiO)₂(C₂O₄)₅ 4H₂O (M = La and Nd) or coprecipitated hydroxides of M(OH)₃+TiO(OH)₂ was investigated by two solution routes at different pH values. Composition of precipitates obtained at pH = 7.0 by coprecipitation method starting from La or Nd nitrates and potassium titanyl oxalate corresponded to a physical mixture of La or Nd(C₂O₄)₃ 9.5H₂O and TiO(OH)₂7·H₂O which on thermal decomposition did not yield phase pure M₂Ti₂O₇. However, precipitation from La or Nd nitrates and titanium tertrachloride by urea hydrolysis yielded homogeneous mixture of hydroxides of La or Nd and Ti, which on pyrolysis at 950°C yielded phase pure La₂Ti₂O₇ and Nd₂Ti₂O₇. Use of potassium titanyl oxalate as precursor for Ti, led to selective precipitation of La or Nd oxalate even at pH as low as 0.1 leading to sequential precipitation of La or Nd oxalate followed by Ti hydroxide at pH = 3.0. The resultant precipitate on pyrolysis underwent typical solid-state reaction.     

Low-temperature sintering and microwave dielectric properties of Mg4Nb2O9 ceramics

The effects of V₂O₅ and Li₂CO₃ on the sinterability and microwave dielectric properties of Mg₄Nb₂O₉ (MN) ceramics were investigated. With addition of 1.5wt% V₂O₅, the dielectric constant (?) and Q·? value of MN ceramics sintered at 1,000 °C are comparable to those of pure MN sintered at 1,400 °C. The good results are because of the enhancement of the density by liquid sintering at the lower temperatures. With the mixtures of V₂O₅ and Li₂CO₃, the sintering temperature of MN was further reduced to 925 °C at the expense of the quality factor (Q·?) value. Typically, ? of 13.7 and Q·? value of 78,000 GHz were obtained for the specimens with mixtures of 1.5wt% V₂O₅ and 1.5wt% Li₂CO₃ and sintered at 925 °C for 5 h.     

Enhanced electrochemical and thermal properties of Sm2O3 coated Li[Li1/6Mn1/2Ni1/6Co1/6]O2 for Li-ion batteries

For EV vehicles of the future, a lithium ion battery with highly enhanced electrochemical and thermal properties has been widely investigated. Here we introduce Sm₂O₃ as a new surface coating source for the cathode material of Li-ion batteries and explore the effects of the Sm₂O₃ coating using XRD, EIS, and DSC measurements. The cathode material used is Li[Li_1/6Mn_1/2Co_1/6Ni_1/6]O₂. The XRD and TEM results indicate that the Sm₂O₃ is coated on the surface of the cathode material without changing the layered structure of the core Li[Li_1/6Mn_1/2Co_1/6Ni_1/6]O₂. With the Sm₂O₃ coating, the cycling performance during 50 cycles using 6 C rate, as well as 1 C rate, is highly improved, five times better. The EIS results show that film resistance and the charge transfer resistance decrease in comparison with the pristine Li[Li_1/6Mn_1/2Co_1/6Ni_1/6]O₂. Furthermore, the DSC results show a highly enhanced thermal property. We highly recommend the Sm₂O₃ coating for Li-ion batteries for various applications.     

Ionic Doping Effects in SrBi2Nb2O9 Ferroelectric Ceramics

Ionic doping effects of various ions in Bi-layered ferroelectric SrBi₂Nb₂O₉ (SBN) ceramics were studied. Un-doped and doped SBN ceramics with Ba²⁺, Pb²⁺, Ca²⁺, Bi³⁺, La³⁺, Ti⁴⁺, Mo⁶⁺, and W⁶⁺ ions were made with solid state reactions. Temperature dependent dielectric constants were measured. Ferroelectric transition temperature (T_C) decreased with Ba²⁺ and Pb²⁺ ions but increased with Ca²⁺ ion which substitutes the 12-coordinated Sr²⁺ site. T_C increased with Ti⁴⁺, Mo⁶⁺, and W⁶⁺ ions which substitute the 6-coordinated Nb⁵⁺ sites. With trivalent Bi³⁺ and La³⁺ ions, T_C increased with Bi³⁺ ion but much decreased with La³⁺ ion. These results showed that the ion size plays an important role in ferroelectricity of SBN ceramics.     

High dielectric permittivity of SrBi2Nb2O9(SBN) added Bi2O3 and La2O3

In this paper, the structural and dielectric properties of SrBi₂Nb₂O₉ (SBN) as a function of Bi₂O₃ or La₂O₃ addition level in the radio (RF) and microwave frequencies were investigated. The SBN, were prepared by using a new procedure in the solid-state reaction method with the addition of 3; 5; 10 and 15 wt.% of Bi₂O₃ or La₂O_3. A single orthorhombic phase was formed after calcination at 900 °C for 2 h. The analysis by x-ray diffraction (XRD) using the Rietveld refinement confirmed the formation of single-phase compound with a crystal structure (a?=?5.5129 Å, b?=?5.5183 Å and c?=?25.0819 Å; α?=?β?=?γ?=?90°). Scanning Electron Microscope (SEM) micrograph of the material shows globular morphologies (nearly spherical) of grains throughout the surface of the samples. The Curie temperature found for the undoped sample was about 400 °C, with additions of Bi³⁺, the temperature decreases and with additions of La³⁺ the Curie temperature increased significantly above 450 °C. In the measurements of the dielectric properties of SBN at room temperature, one observe that at 10 MHz the highest values of permittivity was observed for SBN5LaP (5%La₂O₃) with values of 116,71 and the lower loss (0.0057) was obtained for SBN15LaP (15%La₂O₃). In the microwave frequency region, Bi₂O₃ added samples have shown higher dielectric permittivity than La₂O₃ added samples, we highlight the SBN15BiG (15 % Bi₂O₃) with the highest dielectric permittivity of 70.32 (3.4 GHz). The dielectric permittivity values are in the range of 28–71 and dielectric losses are of the order of 10^?2. The samples were investigated for possible applications in RF and microwave components.     

Defect CHEMISTRY and Charge TRANSPORT in SrBi2Nb2O9

Equilibrium dc conductivity and thermopower measurements at 650–800°C on undoped and 1% acceptor-doped SrBi₂Nb₂O₉, SBN, indicate that the n-type conductivity is similar to that of a simple transition metal oxide that contains 1–2% donor excess. The donor content is attributed to the presence of Bi⁺³ on Sr⁺² sites in the perovskite-like layers of the structure. These centers arise from cation place exchange between these ions in the alternating layers of the crystal. This exchange is apparently not completely self-compensating, and there is local charge compensation in each layer. While the equilibrium conductivity of SrBi₂Ta₂O₉, SBT, is dominated by ionic conduction in the Bi layers, in SBN conduction by electrons in the perovskite-like layers prevails. The difference in behavior is attributed to the expected smaller band gap of the niobate. The electron mobility in SBN is extremely small, of the order of 10^–5 cm²/v · sec at 750°C, and is highly activated with an activation energy of about 1.6 eV. The resulting low mobility at ambient temperatures is proposed as the basis for the observed resistance to ferroelectric fatigue. Reports of metallic Bi on the surface of SBT and SBN by XPS analysis are shown to result from the highly reducing atmosphere of the XPS apparatus.     

Comparison of ZrO2-SiO2 sols synthesized by two methods: Preparation,stability and physical-chemical structure

Abstract

In the present work, zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O) was used as ZrO₂ precursors. The ZrO₂-SiO₂ sols with the Zr/TEOS molar ratios (n_Zr) ranging from 0 to 2.0 were synthesized by two different methods, which were called ZrO₂-SiO₂-1 and ZrO₂-SiO₂-2 sols, respectively. The Gibbs energies of activation for viscous flow (ΔG*) and reaction rate constants (k) of the sol system were calculated. The effects of n_Zr, ΔG* and k on the sol stability were investigated. The phase, chemical and pore structures of the two ZrO₂-SiO₂ materials calcined at 400?°C were investigated. The results show that, with the increase of n_Zr, the ρ, η and ΔG* values of the sol system increase, and the sol stability deteriorates. The ZrO₂-SiO₂-1 sol has a better stability than ZrO₂-SiO₂-2 sol. The Zr-O-Si bond was found in the ZrO₂-SiO₂-1 and ZrO₂-SiO₂-2 materials. The ZrO₂-SiO₂-2 material fired at 400?°C clearly showed sharp peaks corresponding to a crystalline tetragonal structure of zirconia, while ZrO₂-SiO₂-1 material didn’t. The ZrO₂-SiO₂-2 material has a smaller pore size than ZrO₂-SiO₂-1 material. The result reveals that preparation method of sol plays an important role in the physicochemical properties of ZrO₂-SiO₂ sol.     

Effect of Sb2O3 addition on the varistor characteristics of ZnO-Bi2O3-ZrO2-MtrO (Mtr = Mn, Co)

Effect of Sb₂O₃ addition on the varistor characteristics of pyrochlore-free ZnO-Bi₂O₃-ZrO₂-M_trO (M_tr = Mn, Co) system previously proposed has been studied. With Sb₂O₃ up to 0.1 mol%, a gradual enhancement of densification and the grain growth inhibition were seen in the system sintered between 900 and 1200^∘C. In X-ray diffraction patterns, small amount of pyrochlore appeared in the specimens doped with Sb₂O₃ (>0.06 mol%), which is thought responsible for the sintering behavior. Enhanced values of non linear coefficient (α) were obtained in ZnO-Bi₂O₃-ZrO₂ (ZBZ) doped with 0.001 mol% Sb₂O₃, but was leveled off at higher concentrations. In ZBZ added with M_trO (M_tr = Mn, Co), significant increase of nonlinear coefficient (α > 30) along with low leakage current (I _L ≪ 100 μA/cm²) was attained. The α-enhancement effect of Sb₂O₃, however, was not observed in high-α ZBZ added with M_trO. As for degradation, addition of a trace amount (0.001 mol%) of Sb₂O₃ to ZBZM_tr was efficient, especially in I _L.     

Effect of Aluminum Doping on the Performance and Microstructure of Methyl-Modified SiO2 Sol

Abstract

With methyltriethoxysilane as hydrophobic precursors and aluminum isopropoxide as the aluminum source, the Al₂O₃/SiO₂ sols and their gel materials were prepared. The effects of aluminum content (n_Al) on the viscosity (η), density (ρ), reaction rate constant (k), Gibbs energies of activation for viscous flow (ΔG*), particle size of Al₂O₃/SiO₂ sols were studied. And the high temperature calcined materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). The results show that, with the increase of n_Al, the η, n and ΔG* values of the Al₂O₃/SiO₂ sols decrease while the k and ρ values and average particle sizes increase. There is strong intermolecular interaction between the Al₂O₃ and SiO₂ sol molecules. The Al-O-Si bond exists in the Al₂O₃/SiO₂ materials before and after calcination at 350?°C. Calcination at 350?°C in N₂ atmosphere can change the phase structure of Al₂O₃/SiO₂ sample greatly, which can make the γ-AlOOH in the Al₂O₃/SiO₂ gel material convert to γ-Al₂O₃ through dehydration.     

Enhancement of ferroelectric phase and dielectric properties of P(VDF-HFP) by NiCl2⋅6H2O nucleating agent

Abstract

This work focuses on an improvement of dielectric properties of poly(vinylidene fluoride-co-hexafluoropropylene) or P(VDF-HFP) via mixing with different nickel(II) chloride hexahydrate (NiCl₂?6H₂O) contents (0–2?wt%). The nanocomposite films have been prepared by a conventional solution technique. The results reveal that the surface roughness increases with increasing salt content. The NiCl₂?6H₂O salt has no significant effect on the crystallinity of films. Additional incorporation of NiCl₂?6H₂O in the polymer leads to the growing conductivity. However, the considerable enhancement in dielectric constant with the relatively low dielectric loss tangent (0.1–0.3) can be found because of the formation of β-phase nucleation of NiCl₂?6H₂O in the P(VDF-HFP) matrix. The maximum value of calculated β-phase fraction of the nanocomposite is achieved 86% for salt content 0.5?wt%. Moreover, the largest dielectric constant (29.2 at 10?Hz) of the film doped 2?wt% NiCl₂?6H₂O is about 7 times greater than that of the pure P(VDF-HFP). The findings in this research suggest that the NiCl₂?6H₂O/P(VDF-HFP) nanocomposite is a candidate dielectric material for the next generation of energy conversion components.     

Processing and Properties of Ferroelectric Pb(Zr,Ti)O3/Silicon Carbide Field-Effect Transistor

Metal-ferroelectric-(insulator)-semiconductor MF(I)S structures have been fabricated and the properties of pulsed laser-deposited PZT/Al₂O₃ gate stacks have been studied on n- and p-type 4H-SiC. Among several polytypes of SiC, 4H-SiC is considered as the most attractive one because of its wider bandgap (E _g ? 3.2 eV) as well as higher and more isotropic bulk mobility than other polytypes. Single PZT phase without a preferred orientation was confirmed by x-ray diffraction. The interface trap densities N _IT, fixed oxide charges Q _F, and trapped oxide charges Q _HY have been estimated by C-V curves with and without photo-illuminated measurements at room temperature. It is found that the charge injection from SiC is the dominant mechanism for C-V hysteresis. Importantly, with PZT/Al₂O₃ gate stacks, superior C-V characteristics with negligible sweep rate dependence and negligible time dependence under the applied bias were obtained compared to PZT directly deposited on SiC. The MFIS structures exhibited very stable capacitance-voltage C-V loops with low conductance (<0.1 mS/cm², tan δ ～ 0.0007 at 400 kHz) and memory window as wide as 10 V, when 5 nm-thick Al₂O₃ was used as a high bandgap (E _g ～ 9 eV) barrier buffer layer between PZT (E _g ～ 3.5 eV) and SiC (E _g ～ 3.2 eV). The structures have shown excellent electrical properties promising for the gate stacks as the SiC field-effect transistors (FETs). Depletion mode transistors were prepared by forming a Pb(Zr_0.52Ti_0.48)O₃/Al₂O₃ gate stack on 4H-SiC. Based on this structure, ferroelectric Pb(Zr,Ti)O₃ (PZT) thin films have been integrated on 4H-silicon carbide (SiC) in a SiC field-effect transistor process. Nonvolatile operation of ferroelectric-gate field-effect transistors in silicon carbide (SiC) is demonstrated.     

Effect of CaO and SiO2 addition on magnetic properties of anisotropic Ba-Zn system W-type hexagonal ferrite magnets

An experiment was carried out to investigate the effect of CaO and SiO₂ addition on the magnetic and physical properties of anisotropic Ba-Zn W-type hexagonal ferrite magnets. It was found that magnetic properties of BaO.2ZnO.8Fe₂O₃ compounds added with CaO and SiO₂ were improved. The optimum conditions of typical specimens are as follows: chemical analysis composition-Ba_0.857 Zn_1.643 Ca_0.150 Si_0.266 Fe²⁺_0.030 Fe³⁺_15.857 O₂₇; semisintering condition-1275°C × 1.0 h in air; sintering condition- 1250°C × in air, magnetic and physical properties are J_m = 0.436 T, J_r = 0.393 T. H_cj = 64.4 kA/m, H_cB = 64.1 kA/m, (BH)_max = 16.1 kJ/m³, T_c = 357°C, H_A = 971 kA/m, K_A = 2.03 × 10⁵ J/m³ and n_B = 33.2 μ_B.     

Preparation and characterization of LiNi0.80Co0.20– x Al x O2 as cathode materials for lithium ion batteries

LiNi_0.80Co_{0.20− x} Al _x O₂ samples (x = 0.025, 0.050 and 0.100) were prepared by a solid-state reaction at 725^∘C for 24 h from LiOH⋅H₂O, Ni₂O₃, Co₂O₃ and Al(OH)₃ under oxygen flow. LiNiO₂ simultaneously doped by Co-Al has been tried to improve the cathode performance. The results showed that substitution of optimum amount Al and Co at the Ni-site in LiNiO₂ improved cycling performance. As a consequence, LiNi_0.80Co_0.15Al_0.05O₂ has 178.2 mAh/g of the first discharge capacity and 174.0 mAh/g after 10 cycles. Differential capacity vs. voltage curves indicated that the Co-Al co-doped LiNiO₂ showed suppression of the phase transitions compared with pure LiNiO₂.     

Influence of B2O3/SiO2 Ratio on the Fabrication of Nd:YAG Ceramics

B₂O₃/SiO₂ are used as composite sintering aids to fabricate Nd:YAG ceramics by solid-state reaction and vacuum sintering method at 1750°C for 5h using Nano-Al₂O₃, Y₂O₃, Nd₂O₃ as starting materials. In this article, we focus on the influence of B₂O₃/SiO₂ ratio on grain size, porosity and relative density. Finally, with the increase of B₂O₃/SiO₂ ratio, the density and shrinkage rate of transparent ceramics increase, the grain size becomes uniform and the porosity reduces, for the reason that B³⁺ begins to vaporize at 1300°C and is reduced to trace levels by 1600°C. The best B₂O₃/SiO₂ ratio is 4: 1.     

Preparation of size-controlled nanocrystalline infrared-to-visible upconverting phosphors Gd2O3:Yb,Er by using a water-in-oil microemulsion system

Nanocrystalline infrared-to-visible upconverting phosphors Gd₂O₃:Yb,Er with narrow size distribution were prepared by using a microemulsion system (MES) of stability, which includes phases of Triton X-100 (surfactant phase (s)), n-hexanol (assistant surfactant phase (As)), cyclohexane (oil phase (O)) and solution (water phase (W)). From the microemulsion system we could get appearance-controllable nanocrystalline by adjusting the ratio of oil and surfactant. The X-ray diffraction results showed that the powders were cubic phase of Gd₂O₃ after being calcined in the air at 1273 K. The scanning electronic microscopy revealed that the particles were spherical. Red and green upconversion luminescences were obtained by 980 nm infrared excitation. We also investigated contributions of different ratio of surfactant to the appearance.     

Thermodynamic and transport properties of CO2, CO2–O2, and CO2–H2 mixtures at temperatures of 300 to 30,000 K and pressures of 0.1 to 10 MPa

This paper provides the theoretical calculation results of thermodynamic and transport properties of CO₂, CO₂–O₂ mixture, CO₂–H₂ mixture under thermal equilibrium condition at temperatures of 300 to 30,000 K and at pressures of 0.1 to 10 MPa. The gas CO₂ is one of the candidates for the environmentally benign arc‐quenching medium in a circuit breaker. Furthermore, the effect of additional gases O₂ and H₂ on the thermodynamic and transport properties of CO₂ was also investigated in this paper. The hydrogen atom included CO₂ is similar to the polymer ablated vapor in switching devices. First, equilibrium compositions of CO₂, CO₂–O₂ mixture, CO₂–H₂ mixture were calculated through the Gibbs free energy minimization method. Second, thermodynamic properties were computed using the calculated composition. Finally, transport properties were calculated by the first‐order approximation of Chapman– Enskog method using the collision integrals between species. Inclusion of H₂ increases the electrical conductivity of CO₂ in the range 3000 to 6000 K because CHO molecules produced in this temperature range emit more electrons due to the lower ionization potential of CHO. It also increases the thermal conductivity of CO₂ especially due to dissociation reactions of H₂ around 3900 K and ionization of H around 15,000 K. These properties provided here can be used for CO₂ thermal plasma simulation. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(4): 18–29, 2008; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/eej.20467     

Lithium ion conduction in Li5La3Ta2O12 and Li7La3Ta2O13 garnet-type materials

Electrical properties of the lithium garnets Li₅La₃Ta₂O₁₂ (L5LTO) and Li₇La₃Ta₂O₁₃ (L7LTO) are reported over a wide frequency range from 10 MHz to 0.1 Hz at different temperatures. The structural properties are characterized by powder X-ray diffraction, Scanning electron microscopy with energy dispersive X-ray spectroscopy and Fourier transformation Infrared spectroscopy. By means of the frame work of classical brick layer model (BLM) and of a finite element approach, the ion transport properties of grain and grain boundary for the lithium garnets were analyzed. The specific grain conductivity of 5.0?×?10^?6?S/cm at 40 °C is found for both lithium garnets. Specific grain conductivities and grain boundary conductivities are thermally activated, with activation energies found to be in the range of 0.55–0.61 eV. The total conductivity is found to be depending on the ion conduction of grain boundary. The information on the fraction of contact area α_contact between grains <0.25 is obtained by the finite element approach for Li₇La₃Ta₂O₁₃.