Luminescence of Mn ions in Tb3Al5O12 garnet

The paper is dedicated to investigation of the Mn²⁺ luminescence in Tb₃Al₅O₁₂ (TbAG) garnet, as well as the processes of excitation energy transfer between host cations (Tb³⁺ ions) and activators (Mn²⁺ and Mn²⁺-Ce³⁺ pair ions) in single crystalline films of TbAG:Mn and TbAG:Mn,Ce garnets which can be considered as promising luminescent materials for conversion of LED's radiation. Due to the effective energy transfer between TbAG host and activator, Mn²⁺ ions in TbAG possess the bright orange luminescence in the bands peaked at 595 nm with a lifetime of 0.64 ms which are caused by the ⁴T₁→⁶A₁ radiative transitions. The simultaneous process of energy transfer is realized in TbAG:Mn,Ce: (i) from Tb³⁺ to Mn²⁺ ions; (ii) from Tb³⁺ cations to Ce³⁺ ions and then partly to Mn²⁺ ions through Tb³⁺ ion sublattice and Ce-Mn dipole-dipole interaction.     

掺杂水平对ZnO基变阻器电学性能的影响

根据相图规则设计、制备了三个系列不同Bi₂O₃与Sb₂O₃掺杂水平的ZnO基复合变阻器材料,研究了掺杂对氧化锌复合陶瓷电学性能的影响.研究发现,当Sb元素掺杂水平较低时,随着Sb₂O₃掺杂量的增加,所得氧化锌基变阻器材料漏电流的变化也很小,非线性系数(非线性系数α_L和击穿非线性系数α_B)将减小,而场强(场 关键词： 氧化锌 掺杂 复合陶瓷变阻器 电学性质     

Doping of polyaniline films with organic sulfonic acids in aqueous media and the effect of water on these doped films

Polyaniline (PANI) films in the form of emeraldine salt (ES) doped with aqueous organic sulfonic acids such as camphorsulfonic acid (CSA), p-toluenesulfonic acid (p-TSA) and dodecylbenzenesulfonic acid (DBSA) were studied. The ES films were obtained by treating the PANI in the form of emeraldine base (EB) with the aqueous solution of the acids. The dopant weight fraction (w), which is related to the mass gain during the redoping of EB, was in situ determined using a quartz crystal microbalance (QCM). The behaviour of PANI doping with different acids indicates that the uptake shows a slow diffusion process. The kinetics of the doping reaction is dominated by Fickian diffusion kinetics. The diffusion coefficients (D) of the dopant ions into the PANI chains were determined and were found to vary within the range of (1.6-18) × 10⁻¹⁵ cm² s⁻¹. Moreover, the effect of water on these doped ES films was studied. The starting point is the fact that PANI-coated the electrode of QCM shows significant frequency shifts on exposure to water. The changes in the frequency as a function of treatment time in water were quantitatively measured. The response of the device suggests that the mass decrease under water exposure is due to dopant ions release. The latter films were dedoped by exposure to ammonia solution to obtain the EB film form. A further decrease in the mass of the films was observed. The percentage of the mass loss due to water exposure is found to be less than w determined during the dedoping process.     

Nitrogen-doped carbon nanotube-encapsulated nickel nanoparticles assembled on graphene for efficient CO_2 electroreduction

Exploring 3 D hybrid nanocarbons encapsulated with metal nanoparticles(NPs) are recently considered as emerging catalysts for boosting CO_2 electroreduction reaction(CRR) under practical and economic limits.Herein,we report a one-step pyrolysis strategy for fabricating N-doped carbon nanotube(CNT)-encapsulated Ni NPs assembled on the surface of graphene(N/NiNPs@CNT/G) to efficiently convert CO_2 into CO.In such 3 D hybrid,the particle size of Ni NPs that coated by five graphitic carbon layers is less than 100 nm,and the amount of N dopants introduced into graphene with countable CNTs is determined to 7.27 at%.Thanks to unique CNT-encapsulated Ni NPs structure and N dopants,the achieved N/NiNPs@CNT/G hybrid displays an exceptional CRR activity with a high Faradaic efficiency of 97.7% and large CO partial current density of 7.9 mA/cm~2 at-0.7 V,which outperforms those reported metallic NPs loaded carbon based CRR electrocatalysts.Further,a low Tafel slope of 134 mV/dec,a turnover frequency of 387.3 CO/h at-0.9 V,and tiny performance losses during long-term CRR operation are observed on N/NiNPs@CNT/G.Experimental observations illustrate that the Ni NPs encapsulated by carbon layers along with N dopants are of great importance in the conversion of CO_2 into CO with high current density.     

Control of the Induced Handedness of Helical Nanofilaments Employing Cholesteric Liquid Crystal Fields

In this paper, a simple and powerful method to control the induced handedness of helical nanofilaments (HNFs) is presented. The nanofilaments are formed by achiral bent-core liquid crystal molecules employing a cholesteric liquid crystal field obtained by doping a rod-like nematogen with a chiral dopant. Homochiral helical nanofilaments are formed in the nanophase-separated helical nanofilament/cholesteric phase from a mixture with a cholesteric phase. This cholesteric phase forms at a temperature higher than the temperature at which the helical nanofilament in a bent-core molecule appears. Under such conditions, the cholesteric liquid crystal field acts as a driving force in the nucleation of HNFs, realizing a perfectly homochiral domain consisting of identical helical nanofilament handedness.     

Controlled Handedness of Twisted Lamellae in Banded Spherulites of Isotactic Poly(2‐vinylpyridine) as Induced by Chiral Dopants

Herein, we suggest a unique approach to control the handedness of twisted lamellae in banded spherulites of a stereoregular polymer, isotactic poly(2‐vinylpyridine) (iP2VP). When (R)‐ or (S)‐hexahydromandelic acid (HMA), which can associate with iP2VP, was introduced as a chiral dopant, mirror‐image CD spectra in the complex systems showed induced circular dichroism (ICD) of the iP2VP by chiral HMA. Banded spherulites resulting from lamellar twisting due to the imbalanced stresses at the opposite folding surfaces could be formed by crystallization of the iP2VP/HMA complexes, which had a crystalline structure similar to that of neat iP2VP. A preferential sense of the twisted crystalline lamellae was found in the iP2VP/HMA complex, thus suggesting homochiral evolution from conformational to hierarchical chirality.     

Assessing the Effect of Dopants on the C−H Activation Activity of γ-Al2O3 using First-Principles Calculations

In recent years, the high availability of methane in the shale gas reserves has raised significant interest in its conversion to high-value chemicals but this process is still not commercially viable. Metal oxides, due to their surface heterogeneity and the presence of Lewis acidic and basic site pairs are known to facilitate the activation of C−H bonds of methane. In this work, we investigate the C−H bond activation of methane on pristine and doped γ-Al₂O₃ clusters using density functional theory (DFT) calculations. Our results demonstrate that the polar pathway is energetically preferred over the radical pathway on these systems. We found that the metal dopants (boron and gallium) not only alter the catalytic activity of dopant sites but this effect is more pronounced on some of the adjacent sites (non-local). Among the selected dopants, gallium greatly improves the catalytic activity on most of the site pairs (including most active and least active) of pristine γ-Al₂O₃. Additionally, we identified a correlation between H₂ binding energies and the C−H activation free energies on Ga-doped γ-Al₂O₃.     

Helical twisting power and compatibility in twisted nematic phase of new chiral liquid crystalline dopants with various liquid crystalline matrices

ABSTRACT

Four chiral dopants exhibiting smectic LC phases themselves were prepared and their helical twisting power (HTP) and thermal phase behaviour in mixtures with four various LC hosts were studied. The influence of host liquid crystal on HTP was evaluated and generally higher values were found for hosts with high birefringence. Unexpectedly, high enhancement was found for an LC-chiral dopant pair, both having a similar aromatic core – biphenyl ring substituted with polar group. All studied chiral dopants exhibited limited compatibility with the LC hosts in twisted nematic phase at room temperature. For one of the studied mixtures, it was able to obtain single twisted nematic phase with selective light reflection band with maximum at wavelength about 1.0 µm. Carboxylic acid-type dopants exhibited total compatibility with the studied host in single twisted nematic phase at elevated temperatures, allowing preparation of mixtures with reflection band in the visible range. In case of the carboxylic acid dopants, blue phases for optimised compositions were observed. Intermolecular hydrogen bonding between carboxylic acid proton and pyridine nitrogen of chiral dopants was found. Doping the LC host with these dopants led to slight enhancement of HTP value and higher solubility in the LC host.