Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation

A commercial granular activated carbon (GAC) was subjected to thermal treatment with ammonia for obtaining an efficient carbon dioxide (CO₂) adsorbent. In general, CO₂ adsorption capacity of activated carbon can be increased by introduction of basic nitrogen functionalities onto the carbon surface. In this work, the effect of oxygen surface groups before introduction of basic nitrogen functionalities to the carbon surface on CO₂ adsorption capacity was investigated. For this purpose two different approaches of ammonia treatment without preliminary oxidation and amination of oxidized samples were studied. Modified carbons were characterized by elemental analysis and Fourier Transform Infrared spectroscopy (FT-IR) to study the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties. The texture of the samples was characterized by conducting N₂ adsorption/desorption at −196 °C. CO₂ capture performance of the samples was investigated using a thermogravimetric analysis (TGA). It was found that in both modification techniques, the presence of nitrogen functionalities on carbon surface generally increased the CO₂ adsorption capacity. The results indicated that oxidation followed by high temperature ammonia treatment (800 °C) considerably enhanced the CO₂ uptake at higher temperatures.     

Variable-temperature FT-IR studies on the thermodynamics of carbon dioxide adsorption on a faujasite-type H-Y zeolite

Adsorption of carbon dioxide on a faujasite-type H-Y zeolite (Si:Al = 2.6:1) was studied by variable-temperature (200-290 K range) infrared spectroscopy. Adsorbed CO₂ molecules interact with the Brønsted acid Si(OH)Al groups located inside the zeolite supercage, bringing about a characteristic bathochromic shift of the O-H stretching mode from 3645 cm⁻¹ (free OH group) to 3540 cm⁻¹ (hydrogen-bonded CO₂ adsorption complex). Simultaneously, the asymmetric (ν₃) mode of adsorbed CO₂ is observed at 2353 cm⁻¹. From the observed variation of the integrated intensity of the 3645 and 2353 cm⁻¹ IR absorption bands upon changing temperature, corresponding values of standard adsorption enthalpy and entropy were found to be ΔH° = −28.5(±1) kJ mol⁻¹ and ΔS° = −129(±10) J mol⁻¹ K⁻¹. Comparison with the reported values of ΔH° for CO₂ adsorption on other zeolites is briefly discussed.     

电场调谐InAs单量子点的发光光谱

采用稳态和时间分辨发光光谱,研究了生长在p-i-n二极管结构内的InAs单量子点发光光谱的电场调谐特性.随着电场强度的增加,观察到量子点中激子发光的Stark 效应.通过选择不同波长的激光线激发量子点样品,发现随着电场强度的增加导致量子点发光强度的减弱,这是由于量子点俘获载流子概率的减小所致,而激子寿命的增加源于电场导致激子的Stark效应. 关键词： InAs单量子点 Stark效应 电子-空穴分离     

Fabrication and electrical properties of a carbon nanotube quantum dot

Single-walled carbon nanotubes (SWNTs) were synthesized by pyrolyzing methane (CH₄) at a temperature of 900℃ on SiO₂ substrates pre-coated with iron nano-particles. Electrical contacts were fabricated onto one of the SWNTs by using an electron beam lithography process. Coulomb blockade and single-electron tunnelling characters were found at low temperatures, indicating that the SWNT in-between the electrodes forms a quantum dot. It is found that the Coulomb gap of the quantum dot is about 8.57 meV, and the factor \alpha , which converts the gate voltage to the true electrostatic potential shift, is around 200 for this device.     

一种二氧化碳-离子液体吸收式制冷系统性能的分析研究

吸收式制冷利用低品位热源为驱动,具有结构简单、运转安静、节能环保等特点,有很大的发展空间。适当的离子液体和CO2可以构成吸收式制冷的工质对,这类吸收制冷工质对可以工作在较高压力,有利于吸收制冷系统的小型化,具有潜在的应用前景。以1-丁基-3-甲基咪挫六氟硼酸盐[bmim][PF6]为例,计算分析了一种离子液体-CO2跨临界吸收式制冷循环的性能,发现该循环的热力性能还并不理想,然后从工质对溶解度和反应热方面分析了原因,给出了进一步研究的方向。     

Universal transport properties of a quantum dot system with a laterally-coupled Majorana zero mode

We present a comprehensive analysis about the transport properties of a quantum dot (QD) system with a side-coupled Majorana zero mode. Our calculation result shows that when the coupling manners between the two leads and QDs are identical, the local Andreev reflection and the interlead normal tunneling have the same magnitude at the zero-bias limit. Accordingly, the zero-bias conductance value is always equal to e²/2h, which is exactly one half of the resonant-tunneling conductance. This result is independent of the level number and the level distribution in the single-QD case, and in the coupled-QD case it is irrelevant to the geometry of the QD molecule. The universal transport property is a powerful evidence for the feasibility to detect the MBSs based on a QD circuit. This result also means that the QD condition is not a key factor to achieve the detection. On the other hand, if the decoupling phenomenon appears, the Majorana zero mode may play a trivial role in contributing to the conductance property.     

First-principles calculations of contact effect on quantum transport in carbon nanotubes

We report first-principles calculations of conductance of carbon nanotubes between metallic electrodes. The electronic states are calculated using a numerical atomic orbital basis set in the framework of the density functional theory, and the conductance is calculated using the Green's function method. We show transmission spectra of carbon nanotubes connected to electrodes and reveal the contact effect of electrodes on the transport properties of nanotubes.     

Carbon nanotubes supported Cu-Ni bimetallic catalysts and their properties for the direct synthesis of dimethyl carbonate from methanol and carbon dioxide

Multi-walled carbon nanotubes (MWCNTs) supported Cu-Ni bimetallic catalysts for the direct synthesis of dimethyl carbonate (DMC) from CH₃OH and CO₂ were synthesized and investigated. The supporting materials and the synthesized catalysts were fully characterized using FTIR, scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS) techniques. The catalytic activities were investigated by performing micro-reactions. The experimental results showed that the metal phase and Cu-Ni alloy phase in the catalyst were partially formed during the calcination and activation step. Active metal particles were dispersed homogeneously on the surface of the MWCNTs. Cu-Ni/MWCNTs catalysts were efficient for the direct synthesis of DMC. The highest conversion of CH₃OH was higher than 4.3% and the selectivity of DMC was higher than 85.0% under the optimal catalytic conditions of 120 °C and around 1.2 MPa. The high catalytic activity of Cu-Ni/MWCNTs in DMC synthesis can be attributed to the synergetic effects of metal Cu, Ni and Cu-Ni alloy in the activation of CH₃OH and CO₂, the unique structure of MWCNTs and the interaction between the metal particles and the supports.     

Monte Carlo simulation of carbon monoxide,carbon dioxide and methane adsorption on activated carbon

In this study, the adsorption capacity of pure and activated carbon with regard to carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄) gases at 298?K and pressure from 0.01 up to 2.0?MPa has been investigated computationally. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a graphite model with varying density of activation sites. The Grand Canonical Monte Carlo (GCMC) simulation technique was employed to obtain the uptake of each adsorbed gas by considering a graphite model of parallel sheets activated by carboxyl and hydroxyl groups, as observed experimentally. The simulation adsorption data for these gases within the examined carbon pore material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions between adsorbate species and adsorbent material. We found that the simulated adsorption uptake of the examined graphite model under these conditions with regard to the aforementioned fluids increases in the order CO?<?CH₄?<?CO₂.     

The plastic properties of carbon tetrabromide

The shear strength of polycrystalline carbon tetrabromide was measured over the temperature range from 275–375°K which includes the well-known polymorphic transition near 320°K. A large discontinuity in strength was observed at the transformation. The results are compared with earlier measurements on carbon tetrachloride and other soft solids.     

Environmentally benign nanomixing by sonication in high-pressure carbon dioxide

Due to the increased use of nanocomposites, mixing at nanoscale has become important. Current mixing techniques can be classified into: (a) dry mixing (mechanical mixing), (b) wet mixing, and (c) simultaneous production of mixed nanoparticles (when possible). Dry mixing is in general not effective in achieving desired mixing at nanoscale, whereas wet mixing suffers from different disadvantages like nanomaterial of interest should be insoluble, has to wet the liquid, and involves additional steps of filtration and drying. This paper examines the use of pressurized carbon dioxide having high density and low viscosity to replace the liquids (e.g., n-hexane, toluene). Ultrasound is applied to the suspension of nanopowders in gaseous and supercritical carbon dioxide where high impact collisions during sonication help mixing and the final mixture is obtained by simple depressurization. The method is tested for binary mixture of alumina/silica, silica/titania, MWNT (multiwalled carbon nanotubes)/silica, and MWNT/titania. The effects of sonication intensity and pressure on the degree of mixing are studied. Comparative study is also done with liquid n-hexane as a mixing media. Quantitative characterization (e.g., mean composition standard deviation, intensity of segregation) of mixing of alumina/silica and silica/titania is done with energy-dispersive X-ray spectroscopy, and that of MWNT/silica and MWNT/titania is done using field-emission scanning electron microscopy and day-light illumination spectrophotometry. Results show that mixing in carbon dioxide at higher ultrasound amplitudes is as good as in liquid n-hexane, and the final mixed product does not contain any residual media as in the case of liquid n-hexane.     

Near infrared spectroscopy of carbon dioxide I. OCO line positions

High-resolution near-infrared (4000-9000 cm⁻¹) spectra of carbon dioxide have been recorded using the McMath-Pierce Fourier transform spectrometer at the Kitt Peak National Solar Observatory. Some 2500 observed positions have been used to determine spectroscopic constants for 53 different vibrational states of the ¹⁶O¹²C¹⁶O isotopologue, including eight vibrational states for which laboratory spectra have not previously been reported. Calibration by simultaneous use of CO near 4200 cm⁻¹ and C₂H₂ near 6500 cm⁻¹ provides absolute line position accuracies of 6.0 × 10⁻⁵ cm⁻¹ (RMS) for strong, isolated transitions throughout the observed range. Fits with RMS errors <3.8 × 10⁻⁵ cm⁻¹ have been obtained for the 20013 ← 00001, 20012 ← 00001, and 20011 ← 00001 bands, RMS errors <6 × 10⁻⁵ cm⁻¹ have been obtained for the 30014 ← 00001, 30013 ← 00001, 30012 ← 00001, and 00031 ← 00001 bands, and RMS errors <5 × 10⁻⁴ cm⁻¹ for even the weakest fitted bands. This work reduces CO₂ near-infrared line position uncertainties by a factor of 10 or more compared to the 2000 HITRAN line list, which has not been modified since the comprehensive work of Rothman et al. [J. Quant. Spectrosc. Rad. Transfer 48 (1992) 537]. The new line list satisfies the line position accuracies required for the next generation of CO₂ remote sensing instruments, improves the capability of solar-viewing spectrometers to retrieve precise column CO₂ measurements, and provides a secondary frequency standard in the near-infrared.     

Tuning the cross-gap transition energy of a quantum dot by uniaxial stress

We show that a piezoelectric actuator can be used to apply uniaxial stress to a layer of self-assembled quantum dots. The applied stress leads to a change of the quantum dot's ground state exciton energy by up to a few hundred μeV. This approach allows the possibility of an in situ and continuous tuning of the stress at temperatures down to 4 K and offers an alternative to tuning by temperature and Stark effect. We measure the relative change in the charging energy to the n-doped back contact by capacitance and the change in the exciton energy by photoluminescence. By tuning the uniaxial stress we are able to perform reflection spectroscopy on a single dot.     

Influence of quantum well states on transport properties of double barrier junctions

A strong asymmetric behavior in the I–V characteristics and the tunnel magnetoresistance in asymmetric magnetic double-barrier junctions is predicted. This effect relates to formation of quantum well states in the middle metallic layer. The influence of the random fluctuations of the barrier and the middle metallic layer thickness on the statistics of resonant levels is investigated.     

Molecular dynamics simulations of pure methane and carbon dioxide hydrates: lattice constants and derivative properties

ABSTRACT

We report extensive molecular dynamics simulation results of pure methane and carbon dioxide hydrates at pressure and temperature conditions that are of interest to various practical applications. We focus on the calculation of the lattice constants of the two pure hydrates and their dependence on pressure and temperature. The calculated lattice constants are correlated using second order polynomials which are functions of either temperature or pressure. Finally, the obtained correlations are used in order to calculate two derivative properties, namely the isothermal compressibility and the isobaric thermal expansion coefficient. The current simulation results are also compared against reported experimental measurements and other simulation studies and good agreement is found for the case of isothermal compressibility. On the other hand, for the case of isobaric thermal expansion coefficient good agreement is found only with other simulation studies, while the simulation studies are in disagreement with experiments, particularly at low temperatures.     

Optimizing mechanical and thermal expansion properties of carbon/carbon composites by controlling textures

The present study explored the effect of medium texture (MT) content on flexural properties and thermal expansion coefficients (CTE_S) of carbon/carbon (C/C) composites with multilayered pyrolytic carbon. The specimen with 39% MT exhibited maximum flexural strength of 221.55 MPa, increasing by 52% compared with pure high texture. While the flexural strength decreased when the MT content exceeded 39%. The excellent strength can be attributed to crack deflection between multilayered texture and the strong interface bonding between fibers and matrix. Moreover, the four specimens expressed a similar trend of CTE_S in the direction of XY and Z. In the direction of XY, the specimen with 39% MT had the lowest CTE_S from 800 °C to 2100 °C. Therefore, the C/C composites with 39% MT have the best mechanical and thermal expansion properties, which means that the properties of C/C composites can be optimized by controlling the texture.     

固态二氧化碳电子结构及性能的理论研究

采用基于赝势平面波理论的第一性原理计算方法,研究了9种结构的固态二氧化碳的结构和性质.经过结构的几何优化,得到α石英稳定结构晶格常数与他人的计算值基本一致.通过对平衡态能量的分析,我们得出β方石英(cristobalite)结构是能量最低的结构.这与文献的研究结果一致.对弹性性质的计算结果表明,除了超石英(stishovite)和立方黄铁矿(cubic-pyrite)结构之外,其他的结构都是弹性稳定的.利用基于Mulliken轨道重叠布居数的共价固体本征硬度计算方法,预测了各个结构的本征硬度值.结果表明, 关键词： 第一性原理计算 固态二氧化碳 电子结构 硬度     

可调谐半导体激光吸收光谱法监测二氧化碳的通量

含碳温室气体浓度增加所加剧的温室效应是气候变化的重要原因,大面积范围内二氧化碳气体通量的测量对于评价各类陆地生态系统对大气中主要温室气体浓度的贡献具有重要的意义。可调谐半导体激光吸收(TDLAS)光谱技术具有高分辨率、高灵敏度以及快速响应等特点,是痕量气体高灵敏快速监测的新方法。文章以可调谐分布反馈半导体激光器作为光源,通过波长调制方法对1.573μm附近二氧化碳气体某一吸收线的二次谐波信号测量,结合激光分束技术,实现对不同高度层面700多米光程范围内二氧化碳气体浓度的快速在线检测。结合大口径闪烁仪测量出来的莫宁-奥布霍夫长度和特征速度,通过公式计算得到一天内二氧化碳气体的通量在-1.5～2.5mg·(m2·s)-1范围内的波动,突破了目前对近地面痕量气体通量的监测只能提供局地结果的状况,使大面积范围内痕量气体通量的测量成为可能。     

利用溶液理论计算饱和甲烷中二氧化碳溶解度

求取二氧化碳在饱和液态甲烷中的溶解度,对于在较高温度下实现液化天然气至关重要。文中在理想溶液基础上,采用正规溶液关系式和改进的Scatchard-H ildebrand关系式进行二氧化碳的溶解度计算,并且在临界点附近采用经验公式对其进行修正。将上述计算结果与Davis实验数据进行比较后表明,改进的正规溶液理论计算方法在低于140K温区时可推荐用于此项溶解度计算,经验公式可用于接近临界温度区域的溶解度计算。