Hasil untuk "Physical and theoretical chemistry"

Jana Fridrichová, Peter Bačík, Olena Rybnikova et al.

Abstract Topaz is frequently subjected to heat treatment and irradiation to enhance colour, particularly to produce the market’s most preferred salmon pink and sky blue varieties. However, an insufficient description of these processes can lead to fraudulent practices. This experimental and forensic mineralogical and gemmological study investigates eighteen heat-treated topaz samples from Ouro Preto (OP) and Caraí (CA), Brazil, using electron microanalysis, LA-ICP-MS, Raman, and optical absorption spectroscopy before and after heat treatment at various temperatures. The most significant optical changes were observed at 300 °C when the CA sample lost its colour from sky blue to colourless, while OP samples retained their imperial orange colour up to 500 °C before transitioning to pink at 700 °C. Chemically, the CA samples are rich in F (> 1.8 apfu) with low trace element concentration (Fe ≤ 125 ppm, Ge ≤ 153 ppm), falling to the pegmatite and greisen field of topaz origin. The OP samples contain less F (1.4–1.5 apfu) but higher trace element contents (Cr up to 204 ppm, Ti up to 115 ppm, Fe, Mn, Ge < 64 ppm), consistent with a hydrothermal origin. Raman spectra show no significant inter-sample variation, but their luminescence spectra feature strong differences: Mn acts as the luminophore in CA samples, while Cr3+ centers dominate in OP samples. The optical absorption spectra reveal distinct thermal responses. The OP samples heated to temperatures ≥ 500 °C developed new absorption bands at 530–532 nm, consequently resulting in a visible pink colour. On the other hand, the CA spectra exhibit strong absorption in the NIR region; the unheated sample has a broad absorption band at 634 nm, responsible for the sky-blue colour of topaz. Heating ≥ 300 °C eliminates the transmission window in the blue to cyan regions, removing blue colouration. These thermal-optical signatures serve as indicators of heat treatment in topaz declared from these two localities. Moreover, the combination of spectroscopic methods, which we successfully applied in recognizing heat treatment on the studied samples, provides a systematic approach for identifying treatment in topaz and potentially other gemstones.

Efrosyni Frousiou, Efstathios Tonis, Georgios Rotas et al.

The surface modification of fabrics composed of Kevlar^®, Nomex^®, or VAR was extensively investigated. Kevlar^® and Nomex^® are widely-utilized aramid materials, whereas VAR is a technical fabric comprising 64% viscose, 24% para-aramid (Kevlar^®), 10% polyamide, and 2% antistatic fibers. Both aramid materials and cellulose/viscose exhibit exceptional mechanical properties that render them valuable in a wide range of applications. For the herein studied modification of Kevlar^®, Nomex^®, and VAR, we used small organic molecules 3-allyl-5,5-dimethylhydantoin (ADMH) and 3-(acrylamidopropyl)trimethylammonium chloride (APTAC), which were anchored onto the materials under study via graft polymerization. By doing so, excellent antibacterial properties were induced in the three studied fabrics. Their water repellency was improved in most cases as well. Extensive characterization studies were conducted to probe the properties of the modified materials, employing Raman and FTIR spectroscopies, Scanning Electron Microscopy (SEM), and thermogravimetric analysis (TGA).

Xu Yaoqun, Wang Juan, Zhang Peng et al.

A study was conducted through quantitative calculations on the correlation between the micron-scale pores and the strength of nano-SiO2 (NS) sol reinforced cement mortar. The strength, pore structure, and microstructure of NS sol modified mortar were investigated, and the mortars were made equivalent to a two-phase material comprised of pores and mortar matrix; the model was applied to conduct a quantitative analysis of the correlation between pores and the strength. According to the research results, the modification made to the mortar using the NS sol led to significantly increased early strength and the level of porosity was also increased. Furthermore, the addition of NS caused a change to the C–S–H gel morphology of cement hydration products. As revealed by the results of quantitative analysis, the addition of 1.5 and 3% NS improved the mortar matrix strength by 29.3 and 56.6%, respectively. Moreover, the ratio between the mortar strength (f c) and matrix strength index (K) exhibited a nonlinear correlation with the porosity negatively. It was thus inferred that the increase in mortar porosity inhibited the improvement of mortar strength under the influence of NS sol.

Jiaxin Pan, Jia Nie, Bing Zeng et al.

The polyacrylonitrile/sulfur (PAN/S)composite was prepared by heating the mixture of polyacrylonitrile (PAN) and sulfur under sealed conditions. The effects of the ratio of raw materials, reaction temperature, reaction time and reaction container volume on the properties of the composite product were studied to optimize the preparation conditions of the PAN/S composite. The results showed that the PAN/S composite with the mass ratio of 1:1.5 had the best properties, which was prepared in a large container in a high temperature reaction kettle at 300°C-400°C for 12 hours. Based on the total mass of the composite, the initial discharge capacity of the composite is 856 mAh g-1 and the reversible specific capacity is 678 mAh g-1. After 100 cycles, the specific capacity of the composite is stable at 584 mAh g-1, and the capacity retention rate is 86.1 %, showing a good application prospect.

Creţescu, Igor, Lutic, Doina

Two commercial products containing titanium dioxide, P-25 and Kronos 7000, were used for the detailed investigation of the advanced degradation of crystal violet (CV) dye by photocatalysis. The parametric study concerning the influence of the initial dye concentration, photocatalyst dose, pH of the medium and hydrogen peroxide addition in the reaction batch was performed. The net superiority of P-25 in the decolorization was further highlighted by measuring the mineralization degree by TOC. The potential application of the results is confirmed by reaching 86–87% mineralization degrees after 180 min in the presence of $1~\mathrm{g}{\cdot }\mathrm{L}^{-1}$ P-25 or in 120 min using additionally 100 ppm H$_{2}$O$_{2}$.

Jing Zhang, Xiangyu Lu, Shengli Chen et al.

Ahmed A. El-Sherif, Elbastweesy R. Elbastweesy, Gaber M. Abu El-Reash et al.

The ligand (2E,2'E)-2,2'-(carbonylbis(hydrazine-2-yl-1-ylidene))dipropionic acid (CBHP) had been synthesized and characterized using different tools of analysis. Protonation constants of the novel (2E,2'E)-2,2'-(carbonylbis(hydrazine-2-yl-1-ylidene))dipropionic acid (CBHP) ligand and its corresponding metal-CBHP stability constants with Cu(II), Ni(II), Co(II) and Zn(II) ions were measured in aqueous solution at ionic strength (I) of 0.1 mol.dm-3 NaNO3. The order of stability for the formed complexes with reference to the metal ions complies with this order Cu(II) > Ni(II) > Co(II) > Zn(II) in harmony with the Irving-Williams stability order. The speciation of different species in solution has been assessed as a function of pH. The thermodynamic parameters were calculated and discussed. The effect of solvent on the protonation equilibria of the ligand was also discussed.

A. J. McCaffery

M. Mejía-López, L. Verea, A. Verde et al.

In this work a process to modify the surface of carbon electrode was studied with the goal of improving the adherence of the bacteria on it. This study was performed through an experimental design to determine the effect of the parameters tested for the biofilm formation. The biofilms were analyzed with cyclic voltammetry technique and the kinetic parameters of alpha and kapp were analyzed with a statistical tool called the surface response. The parameters varied in the treatment were: concentration of the substrate, temperature, potential applied and time of the treatment. The results showed differences on the biofilm formed mainly with the concentration of the substrate and the potential applied in the electrode treatment. An alpha of 0.5 obtained suggests an electron transport due to the confined redox compounds within the biofilm and the kapp varied from 0.07 s-1 to 0.4 s-1. Finally, the biofilms formed were used in a MEC to probe their capability as bio-anodes for hydrogen production and was obtained a production of 0.21 m3 H2/m3 d.

Dechuan Li, Yongxing Zhang, Guangping Zhu et al.

Low thermal expansion Ba0.5Sr0.5Co0.7Fe0.2Mn0.1O3-δ(BSCFM-0.1) material was prepared to improve the thermal compatibility between cathode and electrolyte for intermediate temperature solid oxide fuel cells. The pure perovskite BSCFM-0.1 can be obtained by a combined citrate and EDTA complexing method. The results show that Mn was introduced to replace Co in B-site of Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF) to depress thermal expansion. Thermal expansion coefficient of BSCFM-0.1 is 14.24×10-6 K-1 at 600oC, and 14.97×10-6 K-1 at 700oC, respectively. It is a smaller value comparing with BSCF for 15.62×10-6 K-1 at 600oC and 16.76×10-6 K-1 at 700oC. The small difference of thermal expansion is helpful for cathode adhering to the ceria-base electrolyte. As to the single cell, the maximum power density is 110 mW cm-2 at 600oC, which is slightly higher than that of BSCF cathode (102 mW cm-2 at 600oC). The results imply that the rate of lattice oxygen release has been slowed down to control the concentration of oxygen vacancies in the lattice by Mn partial substitution without reducing electrical properties. The gradual increase of thermal expansion is beneficial to improve the thermal compatibility between cathode and electrolyte for SOFCs.

Zhigang Xu, Zhen Geng, Guanghai Yi et al.

Porous activated carbon derived from corncob without further element-doping was employed as a coating interlayer on polypropylene (PP) separator in lithium sulphur (Li/S) battery. The highly electrically conductive activated carbon served as a pseudo current collector dramatically reducing the interface resistance and thus improved the reaction kinetics of the cell system. Furthermore, the mirco-/mesoporous structure and large pore volume of activated carbon provided an excellent environment to trap soluble polysulfide intermediates resulting in shuttle reaction and buffered the volume change of sulphur during cycling, prominently alleviating capacity fading. The cell with activated carbon coating layer maintained a high reversible specific capacity of 839.8 mA h g-1 after 100 cycles at a current rate of 0.3 C (1 C = 1672 mA g-1), comparing to 496.7 mAh g-1 of the cell with only pristine PP separator. Extended cycle life test exhibited a discharge capacity of 648 mAh g-1 after 350 cycles under 0.5 C. Moreover, thickness influence of activated carbon coating layer indicated that a thin layer of 29 μm coating was sufficient enough to improve the electrochemical performance of Li/S battery. Thicker coating was only able to slightly improve the performance of battery.

Lian Liu, Lan Yang, Ping Wang et al.

The development of anode materials originating from renewable resources with high performance and low cost has become an important research direction in the development of lithium ion batteries (LIBs). Herein, peanut shell, a common biomass, was used as a raw material to synthesize nitrogen-doped carbon applied in the anode of LIBs. The effects of calcination temperature and acid treatment on the electrochemical performance of the peanut shell-derived carbon material were first studied; it was found that a higher calcination temperature will improve the performance of the carbon material. The carbon prepared at 700°C presented a capacity at 180 mA h g-1 (0.1 C), much higher than for samples prepared at 300 and 500°C. The acid treatment can further improve the capacity to 320 mA h g-1. On this basis, nitrogen doping was introduced into the carbon material with melamine as the nitrogen source. It was found that the doping method will affect the final properties of the carbon; the nitrogen-doped carbon prepared by a one-pot method (doping and carbonization simultaneously) exhibited a capacity at 570 mA h g-1 with quite stable cycling performance, larger than that prepared by a successive method (carbonization followed by doping). This work demonstrates a promising pathway for the utilization of biomass to prepare active anode material for LIBs.

Nataliya Shcherban, V. Ilyin

N. Shcherban

Yue. Li, Ru-Quan Ren, Xiao-Juan Jin

Sulfur-and nitrogen-co-doped activated carbons were obtained from sulfur modified nitrogen-doped activated carbons prepared from waste medium density fiberboard (MDF) prepared by KOH at 850 C for 60 minutes at a weight ratio of 3:1. And the sulfur was adsorbed by nitrogen-doped activated carbon with different temperatures at 300°C, 400°C, 500°C, 600°C (AC300, AC400, AC500, AC600). The structural properties and chemical surface composition of the activated carbons were evaluated by Elemental analysis, Scanning electrical microscopy, Raman spectra, X-ray diffraction and Adsorption of nitrogen. The results showed that the content of sulfur ranging from 0.03 wt. %~1.64 wt. % in which the content of sulfur in AC400 (1.64 wt. %) is highest comparing with other activated carbons. The BET specific surface area is varying from 1805 m2g-1 to 2081 m2g-1. The activated carbons as the electrode material which was impregnated with 7 M KOH electrolytes were characterized and tested by galvanostatic charge-discharge curves, cyclic voltammetry and alternating current impedance. The specific capacitance was improved from 225 to 264 F/g and AC400 exhibited the best electrochemical behavior with a supreme specific capacitance (264 F/g) and rectangular cyclic voltammetry curves.

Boudieb Naima, M. Bounoughaz, A. Bouklachi

The main goal of this research is to improve the performance of positive plate of lead-acid battery using surfactants in 0.5 M H2SO4. The use of surfactants aims to increase the ability and the cycle life of the positive active material. The electrochemical behavior of pure lead and PbO2 electrodes interface was investigated at room temperature by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and potential-dynamic polarization curves. Scanning electron microscopy (SEM) was used to explore the film layer morphology deposited on the surface of working electrodes. Cyclic voltammograms of pure lead (Pb) electrode show that in the presence of PS the current peaks of the transition of PbO2 to PbSO4 and PbO2 to PbSO4 dropped at small concentrations of surfactants. We observed a remarkable improvement of the discharge capacity of the PAM for an amount of PS ranging between 10 and 50 ppm. EIS results indicate the improvement of mass transport by the decrease of resistance.In the other hand, the PbSO4 surface layers and the crystal size of PbSO4 changes with impact effect of surfactants. As a result of the morphological changes which occurred, the PbO2 films formed are hardly more reducible with PbSO4 and delays the appearance of resistive PbSO4 layers at the material interface. As a conclusion, we can say that the electrical performance of the battery increase by extending the life cycle.

K. Morgenstern, D. Marx, M. Havenith et al.

Markéta Ilčíková, Jaroslav Filip, Miroslav Mrlík et al.

The novel composites of gold nanoparticles and polypyrrole nanotubes (Aux@PNT) were prepared and used as a platform for fabrication of bioelectrode interfaces. Changing the conditions of composite preparation caused variations in a gold architecture, electrical conductivity and a biocompatibility. These features could be easily adjusted by setting up a proper fabrication protocol. The Aux@PNT-chitosan matrix was utilized for fabrication of bioelectrode interfaces with physisorbed oxidoreductases. Biocatalytic activity of such physisorbed fructose dehydrogenase (FDH), laccase (Lac) and bilirubin oxidase (BOD) was investigated with biocatalytic current density up to j = 2.45 mA cm-2 obtained for a bioanode based on direct electron transfer of FDH. Performance of biocathodes with immobilized Lac and BOD showed current density up to 232 μA cm-2 in presence of a redox mediator.

Jing Feng, B. Xiao

Meng Kang, Jianjun Fang, Sufang Li et al.

This study is focused on the effect of catalytic electrode morphology on the electrochemical performance of one of the most prominent solid state electrode systems, Pt(O2)/YSZ system. Two types of sintered Pt(O2)/YSZ electrodes (metal Pt electrode and metal-ceramic electrode) prepared by screen printing of different Pt pastes are investigated in this work. The morphology of the catalytic electrodes is characterized by scanning electron microscopy (SEM) and their electrochemical behaviors are investigated by CV techniques and chronoamperometry. Cyclic voltammetry on the Pt/YSZ catalytic electrodes evidences the characteristics of the oxygen charge exchange reaction. The corresponding redox reactions occur nearby the tripe phase boundary (tpb) and manifest themselves by one of more cathodic peaks and an anodic wave in the voltammograms. Series investigations demonstrate that the metal-ceramic electrodes show the high electrochemical activity of oxygen charge exchange reactions and can expect significant usage in the field of oxygen sensors and solid oxide fuel cells.