Enhanced Photoacoustic Beam Profiling of Pulsed Lasers

An improved version of a photoacoustic beam profiler of pulsed lasers is presented. The new model resorts to high-bandwidth condenser microphones to register higher-order, excited acoustic modes, thus enabling more accurate profiling. In addition, Xe was used as a buffer gas since its high atomic weight further reduces the eigenfrequencies. Furthermore, a new gas-handling system makes up for some deficiencies found in the first model. The system was calibrated using the Airy pattern generated with a pinhole illuminated by a frequency-doubled Nd:YAG laser that excited $\text{ NO }_{2}$ traces. Once calibrated, the beam profile of a TEA $\text{ CO }_{2}$ laser was obtained, using ethylene as the absorbing species. This profiler returns more accurate profiles than thermal paper.     

Thermal Diffusivity Mapping of Solids by Scanning Photoacoustic Piezoelectric Technique

Quantitative thermal diffusivity mapping of solid samples was achieved using the scanning photoacoustic piezoelectric (PAPE) technique. Based on the frequency-domain PAPE theoretical model, the methodology of the scanning PAPE thermal diffusivity mapping is introduced. An experimental setup capable of spatial and frequency scanning was established. Thermal diffusivity mapping of homogeneous and inhomogeneous samples was carried out. The obtained thermal diffusivity images are consistent with the optical images in image contrast and consistent with the reference values in thermal diffusivity. Results show that the scanning PAPE technique is able to determine the thermal diffusivity distribution of solids, hence providing an effective method for thermal diffusivity mapping.     

Pulsed Photoacoustic Spectroscopy of I2 and NO2

A novel approach based on the use of ?? = 532?nm pulsed radiation obtained by the second-harmonic generation from the Q-switched Nd: YAG laser and its application for excitation of acoustic modes in a specially designed multi-resonant high-Q factor photoacoustic (PA) cell is reported. The cell is employed for time-domain spectroscopic studies of gaseous NO₂ and I₂ molecules. The time-resolved spectra of NO₂ and I₂ were recorded up to 8000?Hz and 26?000?Hz, respectively. It has been demonstrated for the first time that excited cavity modes which correspond to different transition lines of the I₂ molecules show different behaviors under different pressure and input laser energy. The lowest limit of detection attained with the proposed PA system for NO₂ buffered in realistic air was 14.6?ppbV.     

A Modified Photoacoustic Piezoelectric Model for Thermal Diffusivity Determination of Solids

The conventional photoacoustic piezoelectric (PAPE) model did not take into account the influence of the piezoelectric transducer (PZT) on the vibrations of the sample, and this approximation brought certain error. In this article, a simple method has been proposed to investigate the vibrations of the sample–PZT combination, and the theory of the PAPE technique has been modified. By introducing an equivalent thickness parameter, the two-layered model has been simplified and an analytical expression for the phase of the PAPE output signal has been obtained. The experimental system has been set up, and the thermal diffusivities of several metal samples have been measured. The experimental results show that the modified model has a higher accuracy than the conventional model.     

Theoretical Equation of State for Highly Anharmonic Solids

Thermodynamic properties of Lennard–Jones (LJ) face-centered cubic crystals are calculated in a wide range of temperatures along sublimation and melting lines on the basis of statistical mechanical theory which accounts for anharmonicity, pair and triple correlations in the displacements of atoms. Results are compared with the most accurate Monte Carlo data available. Excellent agreement was found. Anharmonic and correlation contributions to entropy, internal energy, and the compressibility factor of LJ solids are computed and their relative importance analyzed. An analytical equation of state approximating the theoretical results is developed.     

Pulsed Laser Photoacoustic Detection of SO(2) Near 225.7 nm

A trace concentration of SO(2) near 225.7 nm has been detected with a master-oscillator power-oscillator laser system for the first time, to our knowledge. A photoacoustic absorption spectrum of SO(2) has been recorded on the (1)A(2)-(1)B(2) (pi-pi*) transition. Parametric dependence of the photoacoustic signal has been investigated. A detection limit (signal-to-noise ratio of 1) of 1.3 parts in 10(9) [1.3 ppbv (parts per billion by volume)] for SO(2) have been determined at 1 atmospheric pressure inside a photoacoustic cell.     

压痕尺寸效应的理论分析

针对微纳米尺度下材料硬度测试中出现的压痕尺寸效应(ISE),指出目前用于解释压痕尺寸效应较为常用理论方法;介绍了应变梯度塑性理论的发展过程,引入了几何必需位错密度的概念,详细地描述了Nix-Gao模型及其修正模型的产生,对于预测硬度随压痕深度的变化趋势提供了很好的预测;介绍了能量平衡方法的形成过程,通过描述Meyer方程、Hays-Kendall方法和比例试样阻力模型,推导出能量平衡方程,为分析试验结果提供了理论模型;通过理论公式推导,分析两种理论分析方法对于压痕尺寸效应分析的一致性.     

\text{ CO}_{2} Laser-Based Pulsed Photoacoustic Ammonia Detection

Detecting ammonia traces is relevant in health, manufacturing, and security areas, among others. As ammonia presents a strong absorption band (the $\nu _{2}$ mode) around 10  $\upmu $ m, some of the physical properties which may influence its detection by means of pulsed photoacoustic (PA) spectroscopy with a TEA $\text{ CO}_{2}$ laser have been studied. The characteristics of the ammonia molecule and the laser intensity may result in a nonlinear dependence of the PA signal amplitude on the laser fluence. Ammonia absorption can be described as a simple two-level system with power broadening. As $\text{ NH}_{3}$ is a polar molecule, it strongly undergoes adsorption phenomena in contact with different surfaces. Therefore, physical adsorption–desorption at the cell’s wall is studied. A theoretical model, based on Langmuir’s assumptions, fits well to the experimental results with stainless steel. Related to these studies, measurements led to the conclusion that, at the used fluenced values, dissociation by multiphotonic absorption at the 10P(32) laser line may be discarded. A calibration of the system was performed, and a detection limit around 190 ppb (at 224 $\text{ mJ}\cdot \text{ cm}^{-2}$ ) was achieved.     

Photoacoustic Analysis of Blue Corn Pigments in Nixtamalized Flours

Anthocyanins are natural pigments with antioxidant properties, and recently they have received more attention because these pigments are consumed in nutritional diets with therapeutic effects. In addition, anthocyanins are important in the treatment of diseases caused by oxidation of free radicals in live systems, which could be the cause of chronic diseases, like cancer. Anthocyanins, found in flowers and some fruits, are also present in blue Mexican corn (Zea mays L.). In this work, photoacoustic spectroscopic, chemical and spectrophotometric analyses of nixtamalized blue corn flours are presented. Different Ca(OH)₂ concentrations were used in the fractioned nixtamalization process, and the total anthocyanin concentrations of these flours were obtained by chemical extraction of these pigments and compared with relative intensity of optical absorption obtained by photoacoustic spectroscopy.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Photoacoustic Analysis of Pigments from Archeological Ceramics

Photoacoustic spectroscopy (PAS) is widely used for diverse applications in different areas. These include studies in material, environmental, and life sciences. In the present work the study of pigments from pottery surfaces and volumes of Mexican (Aztec) and Poblana cultures that were developed in central Mexico from 1325 to 1521 and 1521 to 1800, respectively, is reported. The optical absorption spectra from each archeological sample was obtained using PAS. The superficial spectra were also compared with standard color pigments and archeological registers. Complementary energy dispersive spectroscopy (EDS) analysis of these archeological potteries gave us their elemental composition which agreed with other studies about their composition and technology of the pottery manufacturing.     

Photoacoustic Spectroscopy Analysis of Traditional Chinese Medicine

Chinese medicine is a historic cultural legacy of China. It has made a significant contribution to medicine and healthcare for generations. The development of Chinese herbal medicine analysis is emphasized by the Chinese pharmaceutical industry. This study has carried out the experimental analysis of ten kinds of Chinese herbal powder including Fritillaria powder, etc., based on the photoacoustic spectroscopy (PAS) method. First, a photoacoustic spectroscopy system was designed and constructed, especially a highly sensitive solid photoacoustic cell was established. Second, the experimental setup was verified through the characteristic emission spectrum of the light source, obtained by using carbon as a sample in the photoacoustic cell. Finally, as the photoacoustic spectroscopy analysis of Fritillaria, etc., was completed, the specificity of the Chinese herb medicine analysis was verified. This study shows that the PAS can provide a valid, highly sensitive analytical method for the specificity of Chinese herb medicine without preparing and damaging samples.     

Determination of Tequila Quality by Photoacoustic Analysis

A pulsed laser photoacoustic (PLPA) technique is proposed to distinguish original from adulterated tequila. In fact, it brings a reliable cheaper and more sensible method in adulteration detection, in comparison with traditional techniques. The method proposed is comparative and non-destructive, and it is based on a correlation analysis of photoacoustic signals, obtained by exciting tequila samples with short laser pulses (7 ns), in the UV region (355 nm). Eleven samples of tequila were analyzed. From a reference sample, all other samples were classified.     

横向巨磁阻抗效应中磁导率张量的理论研究

建立了比较完善的二维铁磁体在横向巨磁阻抗 (GMI)效应中的磁化模型 ,采用将静态磁化过程和动态磁化过程分开计算的方法 ,同时考虑畴壁移动和磁畴转动两种磁化机制的影响 ,推导出比较完善的用于二维铁磁体横向巨磁阻抗效应理论研究的磁导率张量的表达式。     

A Theoretical Framework for Core Material Properties Identification in Cellular Solids using Novelty Detection

Abstract:  This paper presents a novelty detection-based technique to identify core material properties of honeycombs and cellular structures. A numerical model (finite element) representing full scale and/or reduced size of the cellular solid is used to generate transmissibilities between topological homologous points at cells in different locations. In order to make the procedure robust against noise, these transmissibilities are artificially corrupted. This step is representative of a series of experimentally obtained measurements which automatically return information relating to the size and shape of data noise along with the mean measurement. The next stage, both in this paper and in the proposal for the experimental approach, is to generate several further sets of transmissibilities from the finite element model; the only difference being that the core material properties are altered from the original values. A novelty detection framework is then adopted to find a similarity measure between each of these 'test' transmissibilities and the original set thereby identifying the material properties. Although this work is concerned with identifying only one material property, the methodology extends to identifying several properties.     

Photoacoustic Effect Generated from an Expanding Spherical Source

Although the photoacoustic effect is typically generated by amplitude-modulated continuous or pulsed radiation, the form of the wave equation for pressure that governs the generation of sound indicates that optical sources moving in an absorbing fluid can produce sound as well. Here, the characteristics of the acoustic wave produced by a radially symmetric Gaussian source expanding outwardly from the origin are found. The unique feature of the photoacoustic effect from the spherical source is a trailing compressive wave that arises from reflection of an inwardly propagating component of the wave. Similar to the one-dimensional geometry, an unbounded amplification effect is found for the Gaussian source expanding at the sound speed.     

Signal-to-Noise Analysis of the Differential Open Photoacoustic Helmholtz Cell

When open photoacoustic cells are considered, attenuation of external noise is a serious issue. This paper describes how mechanical dimensions of a differential open photoacoustic Helmholtz cell influence its signal-to-noise (S/N) ratio. The analysis was performed by means of computer simulations based on the loss-improved transmission line model. This research showed that the mechanical parameters noticeably affect the signal-to-noise ratio. According to the presented results, optimal selection of the dimensions is a factor which can substantially improve the signal-to-noise ratio of the examined photoacoustic cell. Due to the very good acoustic properties of the cell, it should be possible to obtain an S/N ratio of over 100 dB.     

Parametric Analysis of the Improved Open Photoacoustic Helmholtz Cell

Many photoacoustic cells use a Helmholtz structure for amplification of the photoacoustic signal, but very few of them are open cells. Open Helmholtz cells known from the literature exhibit very poor attenuation of the external acoustic noise, which severely limits their applications. Recent research showed that this problem can be overcome in a so-called improved open photoacoustic Helmholtz cell structure. The paper describes how mechanical dimensions of such a cell affect the frequency response of the cell and its penetration by the external acoustic noise. Properties of the cell were investigated by means of computer simulations based on the loss-improved transmission line model. The analysis showed that appropriate selection of the duct dimensions and buffer volumes should result in over 50 dB attenuation of the external acoustic noise, without noticeably affecting the Q-factor and resonance frequency of the cell.