Compact QEPAS sensor for trace methane and ammonia detection in impure hydrogen

A compact two-gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was developed for trace methane and ammonia quantification in impure hydrogen. The sensor is equipped with a micro-resonator to confine the sound wave and enhance QEPAS signal. The normalized noise-equivalent absorption coefficients (1σ) of 2.45×10^?8 cm^?1?W/ $\sqrt{}$ Hz and 9.1×10^?9 cm^?1?W/ $\sqrt{}$ Hz for CH₄ detection at 200 Torr and NH₃ detection at 50 Torr were demonstrated with the QEPAS sensor configuration, respectively. The influence of water vapor on the CH₄ channel was also investigated.     

一种稳健的柔性阵波束形成方法

由于航向、航速的改变以及水流、风浪等因素的影响,柔性线列阵在实际应用过程中阵型畸变是不可避免的。一方面,这种畸变使得许多假设阵型无畸变的波束形成方法的性能受到严重的影响;另一方面,目前很多的阵型估计方法仍不是很成熟,很难准确估计阵元的实际位置。本文提出将柔性阵列分为一段阵元位置已知的子阵和若干段阵元未知的子阵,利用不同子阵接收信号的互相关即可获得所需要的波束形成加权系数,从而在不需要知道阵元确切位置信息的情况下实现目标方法的估计。     

Open-path trace gas detection of ammonia based on cavity-enhanced absorption spectroscopy

A compact open-path optical ammonia detector is developed. A tunable external-cavity diode laser operating at 1.5 μm is used to probe absorptions of ammonia via the cavity-enhanced absorption (CEA) technique. The detector is tested in a climate chamber. The sensitivity and linearity of this system are studied for ammonia and water at atmospheric pressure. A cluster of closely spaced rovibrational overtone and combination band transitions, observed as one broad absorption feature, is used for the detection of ammonia. On these molecular transitions a detection limit of 100 ppb (1 s) is determined. The ammonia measurements are calibrated independently with a chemiluminescence monitor. Compared to other optical open-path detection methods in the 1–2 μm region, the present result shows an improved sensitivity for contactless ammonia detection by over one order of magnitude. Using the same set-up, a detection limit of 100 ppm (1 s) is determined for the detection of water at atmospheric pressure. Received: 19 January 2000 / Revised version: 6 March 2000 / Published online: 7 June 2000     

Mild hydrothermal treatment to prepare highly dispersed multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) with improved dispersion property have been prepared by a mild and fast hydrothermal treatment. The hydrothermal process avoids using harsh oxidants and organic solvents, which is environmental friendly and greatly decreases the damage to intrinsic structure of MWCNTs. The modified MWCNTs were highly soluble in polar solvents such as water, ethanol and dimethylformamide. Morphological observation by TEM indicated that the diameter and inherent structure were well reserved in modified MWCNTs. X-ray photoelectron spectroscopy and Raman spectroscopy were used to quantify functional groups created on the MWCNT surface, and to determine rational parameters of hydrothermal process.     

One-pot synthesis of highly fluorescent amino-functionalized graphene quantum dots for effective detection of copper ions

In this work, a green and simple one-pot route was developed for the synthesis of highly fluorescent amino-functionalized graphene quantum dots (a-GQDs) via hydrothermal process without any further modification or surface passivation. We synthesized the a-GQDs using glucose as the carbon source and ammonium as a functionalizing agent without the use of a strong acid, oxidant, or other toxic chemical reagent. The as-obtained a-GQDs have a uniform size of 3–4 nm, high contents of amino groups, and show a bright green emission with high quantum yield of 32.8%. Furthermore, the a-GQDs show effective fluorescence quenching for Cu²⁺ ions which can serve as effective fluorescent probe for the detection of Cu²⁺. The fluorescent probe using the obtained a-GQDs exhibits high sensitivity and selectivity toward Cu²⁺ with the limit of detection as low as 5.6 nM. The mechanism of the Cu²⁺ induced fluorescence quenching of a-GQDs can be attributed to the electron transfer by the formation of metal complex between Cu²⁺ and the amino groups on the surface of a-GQDs. These results suggest great potential for the simple and green synthesis of functionalized GQDs and a practical sensing platform for Cu²⁺ detection in environmental and biological applications.     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

A simple way to prepare monolayer films of the magnetite nanocrystal by the Langmuir-Blodgett-like technique

A simple preparation of water-soluble magnetite nanoparticles protected by sodium oleate has been demonstrated. The magnetite particles are nanocrystal, which are confirmed with XRD. This kind of magnetite nanocrystal can be changed into amphiphilic particles and steadily exist at the interface of water/toluene. The amphiphilic nanocrystal can form monolayer film on hydrophilic substrate by the Langmuir-Blodgett-like technique. The results of SEM indicate that the magnetite nanocrystal film is a uniform compact monolayer film that is composed of nanosized magnetite particles. We also proposed a possible mechanism for the formation of the nanocrystal monolayer film.     

A magnetron sputtering technique to prepare a-C:H films: Effect of substrate bias

Amorphous hydrogenated carbon (a-C:H) films were deposited by magnetron sputtering with a mixture gas of Ar and CH₄. The a-C:H films deposited by this method have relatively low internal stress (<1 GPa) compared to some films deposited by conventional deposition process. The effects of substrate bias voltage on microstructure, surface morphology and mechanical properties of the films were investigated by various techniques. It has been found that the polymer-like structure is dominated at low bias voltage (−100 V), while the diamond-like structure with the highest hardness and internal stress is the main feature of the a-C:H films deposited under high bias voltage (−300 V). With increasing the bias voltage further, the feature of diamond-like structure decreases associating with the increase of graphitization. The frictional test shows that the friction coefficient and wear rate of the a-C:H films are depended strongly on structure and mechanical properties, which were ultimately influenced by the deposition method and bias voltage.     

A technique for almost Doppler-free spectroscopy of highly ionised atoms

A technique is described which uses “longitudinal” observation of a recoil-ion source and offers the promise of high spectral resolution for highly ionised atoms. Transitions in helium-like neon have been observed from which an estimate of the Doppler contribution to the linewidth has been made.     

结构微裂纹混频非线性超声检测方法研究

给出了一种基于混频效应的非线性超声微裂纹检测方法。首先,对结构损伤混频检测机理及信号特征提取方法进行了理论分析,之后,根据试件中差频分量及和频分量幅值分布随激励信号频率变化关系,优化确定出混频检测参数。最后,进行了异侧混频激励下无损检测试验研究,并分析了激励信号频率变化对混频检测效果的影响。结果表明,异侧激励混频检测模式不仅可以实现结构中疲劳微裂纹检测,而且可以实现缺陷的定位。且检测信号频率选择对混频检测信噪比有一定的影响。当检测信号中的混频分量幅值最大时,混频检测效果最佳。因此,在优化检测参数基础上,异侧混频激励检测模式可以很好实现结构微裂纹的检测与定位。     

A generalized modelling technique for linearized motions of mechanisms with flexible parts

Analysis and control of vibrations of agricultural machines to improve machine performance and vibration comfort of the operator is a major concern of manufacturers these days. In this paper, an analytical method to build the linearized equations of motion of an elastic tree structured mechanism, is presented. The method is based on the principle of virtual work resulting in a set of parameterized linear equations that are functions of the mechanical parameters and the geometry and the interconnection structure of different bodies in the mechanism. The rigid-body motions of the mechanical system are represented by Lagrangian generalized co-ordinates while elastic deformations are described by nodal co-ordinates from a finite element formulation. Explicit expressions for external distributed and concentrated forces and internal concentrated forces acting on the mechanism are given.     

A technique for designing highly inclined spacecraft orbits using gravity-assist maneuvers

A highly accurate algorithm for synthesizing chains of cranking gravity-assist maneuvers (GAMs) has been constructed using formalization of the search for ballistic scenarios followed by the adaptive involvement of tens of millions of variants. Its use results in a significant change in the orbital inclination of a research spacecraft without a significant expenditure of propellant with a reasonable mission time.     

A new technique for Raman spectroscopy in highly absorbing liquid systems

In this paper we describe a new technique for Raman spectroscopy in highly absorbing liquid samples. Such a technique, called Jet Flow Raman Spectroscopy (JFRS) is used in the case of saturated solutions of strong electrolytes such as CuCl₂ and CoCl₂. A comparison of spectra obtained with JFRS and a semicapillary cell is shown. The power of the JFRS technique is also proved with the evidence of a resonant Raman effect in saturated aqueous solutions of CuCl₂.     

Bayesian estimation for selective trace gas detection

We present a Bayesian estimation analysis for a selective trace gas detection technique with species separation provided by differential diffusion. The proposed method collects a sample containing multiple gas species into a common volume, and then allows the mixed cloud to diffuse across a linear array of optical absorption detectors using, for example, high-finesse Fabry–Perot cavities. The estimation procedure assumes that all gas parameters (e.g., diffusion constants, optical cross sections) are known except for the number population of each species, which are determined from the time-of-flight absorption profiles in each detector.     

Photoacoustic spectroscopy for fast and sensitive ammonia detection

A photoacoustic (PA) spectrometer with H-type first longitudinal resonant cells for ammonia detection is developed. A new PA cell structure is designed to accelerate the drift velocity of the sample gas near the cell surface, so that the short response time at the flow rate of 100 sccm (standard cubic centimeter per minute) is achieved. The response time of 5 min and detection limit of 0.86 ppbv is reached for ammonia concentration measurement with a Teflon polytetrafluoroethylene (PTFE) cell. Further improvement could be expected when using a brass cell with a high quality Teflon fluorinated ethylene propylene (FEP) coating.     

A fluorescent probe for zinc detection based on organically functionalized SBA-15

In this study, highly ordered mesoporous silica material (SBA-15) functionalized with N-(quinoline-8-yl)-2-(3-triethoxysilyl-propylamino)-acetamide (QTPA) as zinc probe has been reported. The anchoring to the surface of the SBA-15 was carried out by the reaction between the precursor and the hydroxyl groups available on the inner surface of the support. The primary ordered mesoporous structure of SBA-15 was well preserved after the grafting procedure. Fluorescence characterization showed that the obtained organic-inorganic hybrid composite displayed highly selective and sensitive to Zn²⁺ ion over other cations such as Cd²⁺, Pb²⁺, Ni²⁺ and Co²⁺. And the hybrid material has ideal chemical and spectroscopic properties for further biological and environmental applications.     

A guided wave technique for the characterization of highly attenuative viscoelastic materials

The measurement of the acoustic properties of highly attenuative materials such as bitumen is very difficult. One possibility is to use measurements of the extent to which filling a cylindrical waveguide with the material affects the dispersion relationship of the cylinder. Torsional modes have been excited using piezoelectric transducers placed at one end of the cylinder, while the phase velocity and attenuation spectra have been measured by means of laser scanning. At each frequency, under the hypothesis of linear viscoelasticity, the phase velocity and attenuation of the fundamental torsional mode have been calculated as a function of the bulk shear velocity and the bulk shear attenuation of the inner core at that frequency. The resulting phase velocity and guided wave attenuation contour plots have been employed for deriving the unknown shear properties from the measured velocity and attenuation of the guided wave. The monochromaticity of the approach has not required a particular frequency dependence of the material properties to be assumed. Results for bitumen are given.     

Facile and highly effective synthesis of nitrogen-doped graphene quantum dots as a fluorescent sensing probe for Cu2+ detection

Nitrogen-doped graphene quantum dots (N-GQDs) with high blue fluorescence efficiency were synthesized by the hydrothermal method from p-Phenylenediamine and p-Coumaric acid. The N-GQDs possess several superiorities, most significantly in excellent solubility and superior photostability. Besides, the as-prepared N-GQDs exhibit a uniform size distribution with a diameter of about 3.8 ± 0.5 nm. After dispersing the N-GQDs in water, the formed aqueous solution still presents a stable and homogeneous phase even after 2 months at room temperature. The N-GQD dispersion was further utilized as sensing probes for the selective detection of copper ions (Cu²⁺), which is realized by the photoluminescence (PL) quenching of N-GQDs after adding Cu²⁺. The detection limit for Cu²⁺ was found to be 57 nM L⁻¹, with superior selectivity in the presence of other commonly interfering metal ions. The presented results in this study provide a facile and high-efficiency method for synthesizing N-GQDs, with ultra-high detectivity and selectivity for Cu²⁺ detection, offering numerous opportunities for the development of biosensing, bioimaging, environment monitoring, and others.     

Label-free tungsten disulfide quantum dots as a fluorescent sensing platform for highly efficient detection of copper(Ⅱ) ions

A fluorescent probe for the sensitive and selective determination of copper ion(Cu~(2+)) is presented. It is based on the use of tungsten disulfide quantum dots(WS_2 QDs) which is independent of the p H of solution and emits strong blue fluorescence. Copper ions could cause aggregation of the WS_2 QDs and lead to fluorescence quenching of WS_2 QDs. The change of fluorescence intensity is proportional to the concentration of Cu~(2+), and the limit of detection is 0.4 μM. The fluorescent probe is highly selective for Cu~(2+) over some potentially interfering ions. These results indicate that WS_2 QDs,as a fluorescent sensing platform, can meet the selective requirements for biomedical and environmental application.     

New fluorescent labels for time-resolved detection of biomolecules

New dyes with characteristic fluorescence lifetimes have been developed for bioanalytical applications. Based upon the concept of multiplex dyes, we have designed rhodamine dyes with nearly identical absorption and emission spectral characteristics but different fluorescence lifetimes. Extending this principle to applications with laser diodes, new rhodamines with functional groups for covalent coupling of analytes have been developed. The new labels exhibit absortion and fluorescence beyond 600 nm and have a high quantum efficiency, even in aqueous buffer systems.