High rate performance of Li[Ni1/3Co1/3Mn1/3]O2 synthesized via co-precipitation method by different precipitators

In the present study, we investigated the effect of three different precipitators (NaOH, Na₂CO₃ and (NH₄)₂CO₃) on the synthesized layered Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials via co-precipitation method. The obtained compounds were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and galvanostatic charge–discharge measurements. The XRD patterns analysis showed that all the resulted Li[Ni_1/3Co_1/3Mn_1/3]O₂ materials possess a layered hexagonal structure. It was found that at high discharge rate (2C), the prepared Li[Ni_1/3Co_1/3Mn_1/3]O₂ system using Na₂CO₃ as the precipitator exhibits better cycling performance in the charge–discharge tests compared to others, indicating that Na₂CO₃ is an optimum precipitator. After 100 cycles at 2C discharge rate in the voltage range from 2.8 to 4.5 vs. Li/Li⁺, the Li[Ni_1/3Co_1/3Mn_1/3]O₂ system using Na₂CO₃ as the precipitator retains 97% of its initial discharge capacity.     

The vibration spectra of sodium chloride doped with Na2CO3 and NaOH

The paper investigates absorption and reflection spectra, in the infra-red region from 2 to 25, of single crystals of sodium chloride (NaCl) doped with 0·1 to 1% mol. of Na₂CO₃ and NaOH. A correlation between the absorption bands and the corresponding groups CO ₃ ^2– and OH^– was found. It was also shown that in crystals pulled in air by the Kyropoulus method from a melt doped with NaOH the latter is practically completely transformed into Na₂CO₃.The work was carried out at the suggestion of Dr. A. Bohun, whom the author thanks for valuable discussions.     

X-ray diffraction and magnetic susceptibility of sodium fullerides NaxC60

The X-ray diffraction (XRD), magnetic susceptibility and electron spin resonance (ESR) measurements have been carried out for Na_xC₆₀. The XRD profiles with x<4 can be assigned to a face-centered cubic (fcc) lattice, while those with 4≦x to a hexagonal one. The temperature dependence of magnetic susceptibility χ for Na_xC₆₀ using SQUID was fitted to the Curie law, and estimated temperature-independent component χ₀. The composition x dependence of the χ₀ for Na_xC₆₀ shows two maxima at around x=3 and x=10, and minimum at x=6. The absence of Pauli contribution at x=6 was confirmed using ESR. A trace of superconducting transition at 14 K has been found for some Na_xC₆₀ specimens with 8<x<9.     

Enhancement of the superconducting transition temperature near a phase instability in NaxWO3

The superconducting transition temperature of tetragonal I sodium tungsten bronze (Na_xWO₃) has been found to increase rapidly as the x-value is decreased to the metal-semiconductor phase transition. It is suggested that a soft mode instability is responsible for the increased electron-phonon interaction near the phase transition.     

Formation mechanism of Si(1 0 0) surface morphology in alkaline fluoride solutions

Formation mechanism of Si(1 0 0) surface morphology in alkaline fluoride solutions was investigated both theoretically and experimentally. By analysis of Raman spectra of silicon wafer surfaces and three kinds of etching solutions (NaOH, NaOH/NH₄F, and NaOH/NH₄F/Na₂CO₃) with and without addition of Na₂SiO₃·9H₂O, no Si-F bond is formed, F⁻ and CO₃²⁻ ions accelerate the condensation of Si-OH groups. Based on experimental results, it is proposed that bare silicon and silicon oxide coexist at the wafer surface during etching process and silicon oxide of different structure, size, and site at the surface manufacture different surface morphology in alkaline fluoride solution.     

Formation of nickel magnetic nanoparticles and modification of nickel phthalocyanine matrix by sodium doping

Data for the vapor-phase doping (300°C) of nickel phthalocyanine (NiPc) by sodium taken in different concentrations (x), as well as structural analysis data for Na _{x = 0.2}NiPc, Na _{x = 1}NiPc, and Na _{x = 3}NiPc samples, have been reported. The structure of the samples and their atomic configuration versus the doping level have been studied by transmission electron microscopy, Raman scattering, X-ray diffraction, X-ray absorption spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The structural parameters of Ni–N, Ni–C, and Ni–Ni bonds have been determined, and it has been found that, at a low level of doping by sodium, local structural distortions are observed in some molecules of the NiPc matrix near nickel atoms. The fraction of these molecules grows as the doping level rises from x = 0.2 to x = 1.0. It has been shown that doping changes the oscillation mode of light atoms, which indicates a rise in the electron concentration on five- and six-membered rings. At a high level of sodium doping (x = 3.0), nickel nanoparticles with a mean size of 20 nm and molecule decomposition products have been observed in the NiPc matrix. It has been found that the fraction of nickel atoms in the Na _{x = 3}NiPc nanoparticles as estimated from EXAFS data is sufficient for the room-temperature magnetic properties of the samples to persist for a long time.     

Effects of impurity Na ions on the structural and magnetic properties of Ni-Zn-Cu ferrite powders: An improvement for chemical coprecipitation method

Ni-Zn-Cu ferrite powders with nominal composition Ni_0.4−xZn_0.6Cu_xFe₂O₄ (x=0.00-0.20) were prepared via chemical coprecipitation method. X-ray diffractometer, vibrating sample magnetometer, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry and energy dispersive spectrum were used to study the effects of impurity Na⁺ ions on the structural and magnetic properties. As a result, it was found that the impurity Na⁺ ions affect the crystalline structures and magnetic properties greatly. Moreover, the heterogeneous distribution of impurity Na⁺ ions and the formation of Na compounds retard the phase formation and the grain growth of specimens. Our study also reveals that for the chemical coprecipitation method, a second washing process introduced after drying can eliminate the impurity Na⁺ ions effectually and thus helps in the formation of single-phase structure and the growth of grains, which is very important for the improvement of magnetic properties and the preparation of ferrites via chemical coprecipitation method.     

Temperature impedance spectroscopy of (1 − x)Na1/2Bi1/2TiO3-xLaMg1/2Ti1/2O3 solid solutions

The dielectric properties of (1 ? x)Na_1/2Bi_1/2TiO₃-xLaMg_1/2Ti_1/2O₃ solid solutions (0 ≤ x ≤ 0.4) were studied over the temperature range 650–1030 K by measuring the impedance spectra over the frequency range 25–10⁶ Hz. The Curie temperature T _C was determined as a function of the composition of the solid solutions. It is shown that T _C decreases linearly with increasing x. The temperature dependences of the dc component (σ_dc) and the polarization component (σ′_ac) of the real part of the conductivity are found. The activation energy for the conductivity σ_dc is shown to increase abruptly as the temperature increases above approximately 770 K for all solid solution compositions. Over the temperature range corresponding approximately to the region of existence of the tetragonal phase of Na_1/2Bi_1/2TiO₃, the polarization component σ′_ac of the solid solutions, as well as that of the pure compound, is anomalously high at low frequencies, which can be due to structural inhomogeneities.     

Effects of initial water content on steam reforming of aliphatic hydrocarbons with nonthermal plasma

The effects of initial water content on steam reforming of aliphatic hydrocarbons such as methane, propane, and neopentane with nonthermal plasma were analyzed in terms of substrate conversion, carbon recovery, and product selectivity. It was found that water addition increased CO₂ yield despite a decrease in substrate conversion. The number of carbon atoms in the substrate hydrocarbon affected the additive effect due to the insufficient supply of oxygen atoms from water. Plausible reaction pathways for the conversion of substrate to CO_x are proposed to explain the relative concentrations of CO_x species; these pathways involved different precursors for CO and CO₂.     

Effects of excessive Zn2+ ions on intrinsic magnetic and structural properties of Ni0.2Zn0.6Cu0.2Fe2O4 powder prepared by chemical coprecipitation method

Zn(OH)₂ is a kind of amphoteric compound. Therefore, for chemical coprecipitation method, the precipitation of Zn²⁺ ions may be incomplete if using NaOH as precipitator. In this study, single-phase powder specimens with a nominal composition Ni_0.2Zn_0.6Cu_0.2Fe₂O₄ were prepared with chemical coprecipitation method, and the effects of excessive Zn²⁺ content (x, x = 3%, 5%, 7%, 9%) in working solution on intrinsic magnetic and structural properties were studied by vibrating sample magnetometer and X-ray diffractometer, respectively. It was found that the magnetization when H_m = 398 kA/m (5000 Oe) reached a maximum when x = 5%, and then decreased with the increase of x, which was attributed to the effect of different amount of Zn²⁺ in A sites on the A–B and B–B exchange interaction. Moreover, it was found that the lattice parameter was affected by the Zn²⁺ and Fe³⁺ ions due to their different ion radius to a certain extent.     

The system Na2CO3–MgCO3 at 3 GPa

It was suggested that Na–Mg carbonates might play a substantial role in mantle metasomatic processes through lowering melting temperatures of mantle peridotites. Taking into account that natrite, Na₂CO₃, eitelite, Na₂Mg(CO₃)₂, and magnesite, MgCO₃, have been recently reported from xenoliths of shallow mantle (110–115?km) origin, we performed experiments on phase relations in the system Na₂CO₃–MgCO₃ at 3?GPa and 800–1250°C. We found that the subsolidus assemblages comprise the stability fields of Na-carbonate?+?eitelite and eitelite?+?magnesite with the transition boundary at 50?mol% Na₂CO₃. The Na-carbonate–eitelite eutectic was established at 900°C and 69?mol% Na₂CO₃. Eitelite melts incongruently to magnesite and a liquid containing about 55?mol% Na₂CO₃ at 925?±?25 °C. At 1050 °C, the liquid, coexisting with Na-carbonate, contains 86–88?mol% Na₂CO₃. Melting point of Na₂CO₃ was established at 1175?±?25 °C. The Na₂CO₃ content in the liquid coexisting with magnesite decreases to 31?mol% as temperature increases to 1250°C. According to our data, the Na- and Mg-rich carbonate melt, which is more alkaline than eitelite, can be stable at the P–T conditions of the shallow lithospheric mantle with thermal gradient of 45?mW/m² corresponding to temperature of 900 °C at 3?GPa.     

Study of the etching characteristics of -mixed NaOH solution

In the present paper the effect of the presence of Na₂CO₃ on the etching characteristics of NaOH has been presented quantitatively. Six CR-39 detectors were etched at 50, 60 and in 6 M NaOH solutions containing 0%, 1%, 2%, 3%, 4% and 5% concentration of Na₂CO₃. Etching was performed in 22 steps of 5–10 min starting from 15 min up to 210 min. These detectors were previously exposed to ²⁵²Cf source. Lengths of 35 randomly selected fission fragment tracks were measured after each etching time interval. Similarly, diameters of 25 randomly selected fission fragments having 90 incidence angle were also measured as mentioned above. Track etch rate, bulk etch rate, etching efficiency and activation energies of both track as well as bulk etching have been determined.     

Ferroelectric-relaxor behavior of (Na0.5K0.5)NbO3-based ceramics

We report that ferroelectric-relaxor behavior is induced by doping of SrO and TiO₂, or BaO and TiO₂ into classic ferroelectric (Na_0.5K_0.5)NbO₃. It is found that [(Na_0.5K_0.5)_0.9Sr_0.1](Nb_0.9Ti_0.1)O₃ ceramics exhibit a pronounced ferroelectric-relaxor behavior, comparable to that of [(Na_0.5K_0.5)_0.9Ba_0.1](Nb_0.9Ti_0.1)O₃ ceramics. Our results indicate that the relaxor behavior is closely related to the appearance of micropolar regions in these systems. The relaxor behavior should arise from the dynamic response of micropolar clusters. Raman spectra of [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics measured in the wavenumber range from 100 to 1200 cm⁻¹ confirm that the first order scattering is dominant in phonon bands should result from both short-range ordered region (micropolar regions) and disordered matrix. The frequency dependence of dielectric permittivity measurements show that the relaxor behavior of SrO and TiO₂, or BaO and TiO₂ doped (Na_0.5K_0.5)NbO₃ ceramics is not a Debye type in the radio frequency range.     

Dielectric and piezoelectric properties of lead-free (Na0.5K0.5)NbO3-SrTiO3 ceramics

In this article, we report successful preparation of dense [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ (x=0.005-0.100) ceramics by ordinary sintering in air. The dependence of phase structure on doping content of SrO and TiO₂ has been determined by the X-ray diffraction technique. It was found that the crystal structure changed from orthorhombic to tetragonal at x≈0.040. Dielectric study revealed that the dielectric relaxor behavior was induced by doping of SrO and TiO₂ into (Na_0.5K_0.5)NbO₃. The samples in the composition range from x=0.005 to 0.020 exhibited excellent electrical properties, piezoelectric constant of electromechanical planar and thickness coupling coefficients of k_p=26.6-32.5% and k_t=39.8-43.8%. The results show that the [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics are one of the promising lead-free materials for electromechanical transducer applications.     

Ion-exchange between Na2Ti3O7 and H2Ti3O7 nanosheets at different pH levels: An experimental and first-principles study

The models of Na_2−xH_xTi₃O₇ (x=0, 1, and 2) nanosheets were proposed to investigate the formation energies of ion-exchange using first-principles calculations. The calculated results demonstrated that sodium titanate nanosheet is energetically favorable for ion-exchange in a wide pH range, from acidic solution to even highly concentrated alkaline aqueous solution due to the negative formation energies. Therefore, the composition of sodium titanate nanosheet in alkaline solution should be Na_2−xH_xTi₃O₇ (0<x≤2) rather than Na₂Ti₃O₇. The formation energies of ion-exchange decrease with the pH decreasing. As a result, the thermodynamic driving force of ion-exchange is enhanced at low pH level. To further verify the calculated results, the ion-exchange properties of a series of titanate nanosheets in aqueous solutions at different pH levels were investigated. The experimental results are in good agreement with the theoretical deduction.     

Density functional theory study into the adsorption of CO2, H and CHx (x = 0-3) as well as C2H4 on α-Mo2C(0 0 0 1)

The structures and energetics of the chemisorbed CO₂, CH_x species and H as well as C₂H₄ on the α-Mo₂C(0 0 0 1) surface have been computed at the GGA-RPBE level of density functional theory. It is found that CO₂ adsorbs dissociately into CO and O, in agreement with the experimental finding. The adsorbed O, CH_x and H species prefer the site of three surface molybdenum atoms over a second layer carbon atom (V_C site). On the basis of the calculated adsorption energies of CH_x and H, the sequential dehydrogenation of CH₄ and the C/C coupling reaction of CH_x have been discussed.     

Effects of Li2CO3 on the sintering behavior and piezoelectric properties of Bi2O3-excess (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics doped with Li₂CO₃ and Bi₂O₃ as sintering aids were manufactured, and their micro structural, dielectric and piezoelectric properties were investigated. All specimens could be well sintered at a low-temperature of 1080 °C. The bulk density of the specimens doped with a small amount of Li₂CO₃ was enhanced. The dielectric and piezoelectric properties of ceramics were investigated with different amounts of Li₂CO₃ substitutions. High electrical properties of d₃₃ = 167 pC/N, k_p = 0.34, P_r = 40 μC/cm² and E_c = 38 kV/cm were obtained from the specimen containing 0.1 mol% of Li₂CO₃ sintered at 1080 °C.     

Potentiometric sensor for sulfur oxides using NASICON as a solid electrolyte

The ac conductivity of NASICON is higher by two orders of magnitude than that of Na₂SO₄ at 1000 K. The dc polarization measurement reveals that NASICON shows sodium ion conduction even at the temperature of about 1200 K, and that the electronic transference number is of the order of 10^?5. The SO₂-O₂-SO₃ concentration cell using NASICON electrolyte gives essentially the same electromotive force as in the cell using Na₂SO₄ electrolyte because a thin layer of Na₂SO₄ if formed on NASICON at the electrodes. The high sinterability of NASICON offers a dense electrolyte without permeation of gases. The SO_x sensor using NASICON electrolyte exhibits good response and excellent selectivity against CO₂ and NO₂.     

Fundamental and practical aspects of CO2 sensors based of nasicon electrolytes

Potentiometric CO₂ sensors based on the Na⁺ conducting solid electrolyte Nasicon have been investigated. The sensor arrangement may be described as chemical sensor of type III, $$( - )Pt(or Au), Na_{0.9} CoO_2 |Nasicon|Na_2 CO_3 , Pt(or Au) ( + ),$$ , with Na_0.9CoO₂ as reference electrode and Na₂CO₃ as auxiliary or sensing electrode. It is shown that major problems are related to the magnitude of the voltage and long-term stability of this type of sensor. The measured EMFs are generally reduced compared to the values calculated from literature data of standard Gibbs energies of formation. This observation is ascribed to reactions at the interfaces and competing surface reactions. The interfacial processes may occur at both electrolyte / sensing and electrolyte / reference electrode, and represent charged capacitors. It has also been observed that the slope of the measured EMF as a function of the CO₂ partial pressure is reversed in some cases. This phenomenon may be related to the possible reduction of Na₂O which is dissolved in Nasicon during the formation of Na₂CO₃ upon an increase of the CO₂ partial pressure.     

The effects of power ultrasound (24 kHz) on the electrochemical reduction of CO2 on polycrystalline copper electrodes

The electrochemical CO₂ reduction reaction (CO2RR) on polycrystalline copper (Cu) electrode was performed in a CO₂-saturated 0.10 M Na₂CO₃ aqueous solution at 278 K in the absence and presence of low-frequency high-power ultrasound (f = 24 kHz, P_T ~ 1.23 kW/dm³) in a specially and well-characterized sonoelectrochemical reactor. It was found that in the presence of ultrasound, the cathodic current (I_c) for CO₂ reduction increased significantly when compared to that in the absence of ultrasound (silent conditions). It was observed that ultrasound increased the faradaic efficiency of carbon monoxide (CO), methane (CH₄) and ethylene (C₂H₄) formation and decreased the faradaic efficiency of molecular hydrogen (H₂). Under ultrasonication, a ca. 40% increase in faradaic efficiency was obtained for methane formation through the CO2RR. In addition, and interestingly, water-soluble CO₂ reduction products such as formic acid and ethanol were found under ultrasonic conditions whereas under silent conditions, these expected electrochemical CO2RR products were absent. It was also found that power ultrasound increases the formation of smaller hydrocarbons through the CO2RR and may initiate new chemical reaction pathways through the sonolytic di-hydrogen splitting yielding other products, and simultaneously reducing the overall molecular hydrogen gas formation.