极低温制冷技术的发展与应用

近年来,空间探测和凝聚态物理等领域对极低温制冷技术(1K)的潜在需求越来越大。本文总结了绝热去磁制冷、稀释制冷与吸附制冷三种目前主要制冷方法的原理与优缺点,回顾了三类制冷技术的发展历程与应用现状,提出了绝热去磁制冷、稀释制冷与吸附制冷技术未来的发展趋势与研究重点。     

Photoacoustic detection of two photon absorption in formaldehyde using pulsed dye laser

Absorption spectra of formaldehyde molecule in the gas phase have been recorded using photoacoustic (PA) technique with pulsed dye laser at various power levels. The spectral profiles at higher power levels are found to be different from that obtained at lower laser powers. Two photon absorption (TPA) is found to be responsible for the photoacoustic signal at higher laser power while the absorption at lower laser power level is attributed to one photon absorption (OPA) process. Probable assignments for the different transitions are given in this paper.     

高浓度气体共振光声光谱信号饱和特性研究

采用恒流驱动耦合机械斩波技术在激光光声光谱装置上系统测量了5%—100%宽浓度范围甲烷气体的共振光声信号,发现在高浓度区共振光声信号呈现异常的饱和特征.基于气体吸收和光声光谱原理定量分析了光声信号饱和的主要原因及影响因素,研究表明,气体样本对入射光强吸收而导致的声源与本征共振模式的耦合系数改变是异常饱和的主要原因,并导出判定光声信号饱和深度的准则以用于判定高浓度气体饱和深度。     

Photoacoustic spectroscopy of formaldehyde with tunable laser radiation at the parts per billion level

Selective and sensitive detection of formaldehyde (CH₂O) was achieved using a grazing-incidence optical parametric oscillator (GIOPO), pumped at high repetition rate, as a light source for photoacoustic spectroscopy. The photoacoustic spectrum of formaldehyde was measured in the range from 2785 to 2840 cm^-1 and an absorption line at 2805.0 cm^-1 was selected for detection. Concentrations down to 20 ppbv (parts per billion by volume) formaldehyde in nitrogen were recorded. The detection limit determined by background fluctuations was 3 ppbv (S/N=1) for 3 s lock-in time constant and 3 min acquisition time. The 2805.0 cm^-1 absorption line of the ν₁ vibrational mode was chosen because of the absence of interference with water and carbon-dioxide bands. This allowed the direct detection of formaldehyde in laboratory air without filtering. PACS 42.62.Fi; 42.68.Ca; 42.65.Yj; 07.07.Df     

Quantum oscillation measurements in high magnetic field and ultra-low temperature

The physical properties of a solid are determined by the electrons near the Fermi energy and their low-lying excitations. Thus, it is crucially important to obtain the band structure near the Fermi energy of a material to understand many novel phenomena that occur, such as high-T_c superconductivity, density waves, and Dirac-type excitations. One important way to determine the Fermi surface topology of a material is from its quantum oscillations in an external magnetic field. In this article, we provide a brief introduction to the substation at the Synergetic Extreme Condition User Facility(SECUF),with a focus on quantum oscillation measurements, including our motivation, the structure of and the challenges in building the substation, and perspectives.     

工作于可见和紫外波段的脉冲激光光声装置

报道一种工作于可见和紫外波段的脉冲激光光声装置 .该装置以YAG泵浦的染料激光为光源 (脉宽为8ns ,工作频率为 10Hz) .用此装置记录了C2 H2 分子的泛频激发谱和A1Au←X1Σ+ g 电子跃迁谱 ,并测量了光声强度随气体压力及激光能量的变化关系 .将光声谱用于分子的电子跃迁 .由于脉冲激光光声装置和实验操作十分简便 ,因而除需超高分辨的场合外 ,它均可用来代替共振 (或非共振 )调制cw激光光声装置 .还讨论了脉冲方式和cw调制方式的不同特点     

Photoacoustic cavitation and heat transfer effects in the laser-induced temperature jump in water

The finite element method is employed to analyze photoacoustic cavitation and heat transfer occurring when modest temperature jumps (T-jumps) are induced by a laser in D₂O solution, which may contain a small concentration of a protein or peptide sample. Cavitation can be initiated through a photoacoustic mechanism at intensities well below optical breakdown thresholds. Cavitation probability is related to test medium properties, initial temperature, T-jump magnitude and test region geometry. Parameters affecting thermal conduction losses are also examined because such losses limit the useful duration of the T-jump induced in protein folding experiments. From this study, guidelines are offered for reducing the occurrence of cavitation and extending the useful duration of the T-jump. Received: 2 April 2001 / Revised version: 17 October 2001 / Published online: 29 November 2001     

Photoacoustic characteristics of dynamical adhesion of films on a metallic substrate

In this communication, we report a numerical model that predicts the mechanical deformations associated with the pulsed laser irradiation of a film surface, based on thermal diffusion theory. The model is consequently advanced to produce a method for evaluating film adhesion strength. The epicenter surface displacements within the irradiated fields have been measured using a heterodyne interferometer. The comparison of the experimental data and the displacements calculated by the model shows good agreement. By investigating the propagating acoustic modes under non-destructive and destructive modes, we reveal that, with or without interface delamination, the phase structure of the longitudinal waves will be altered due to the change of reflection mode at the interface. Applying shock dynamics theory, we evaluate the adhesion strength of the TiN/stainless interface. We also indicate the strain rate can be up to 10⁵∼10⁶ s^-1 during film interface delamination. Received: 5 April 2002 / Accepted: 24 June 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +86-511/879-1919, E-mail: mzhou@ujs.edu.cn     

温度和压力对Au反射特性影响及动高压下的测温应用分析

基于金属电子气模型,进行了温度、压力对Au反射率变化影响的研究与分析。利用DAC装置开展了压力对Au反射率变化测量实验,以及激光加热的动态温升条件下温度对Au反射率变化测量实验,获得了探测光束波长为488 nm条件下,温度(室温至350 ℃)和压力(11 GPa范围内)对Au反射特性影响的实验结果。结果表明：在11 GPa压力范围内,与温度因素相比,压力对Au的反射率变化影响可忽略;Au对488 nm波长激光的反射率变化趋势为单调递增,变化幅值达约10%,且具有反射率与温度的一一对应特性。通过动高压加载下材料温度瞬态测量要求分析,认为基于Au在488 nm波长下的反射变化特性,可建立一种适用于动高压加载下低温段(低于1000 K)的瞬态测温方法,用于解决材料动高压领域的瞬态测温技术难点。     

温度和压力对Au反射特性影响及动高压下的测温应用分析

基于金属电子气模型,进行了温度、压力对Au反射率变化影响的研究与分析。利用DAC装置开展了压力对Au反射率变化测量实验,以及激光加热的动态温升条件下温度对Au反射率变化测量实验,获得了探测光束波长为488 nm条件下,温度(室温至350 ℃)和压力(11 GPa范围内)对Au反射特性影响的实验结果。结果表明:在11 GPa压力范围内,与温度因素相比,压力对Au的反射率变化影响可忽略;Au对488 nm波长激光的反射率变化趋势为单调递增,变化幅值达约10%,且具有反射率与温度的一一对应特性。通过动高压加载下材料温度瞬态测量要求分析,认为基于Au在488 nm波长下的反射变化特性,可建立一种适用于动高压加载下低温段(低于1000 K)的瞬态测温方法,用于解决材料动高压领域的瞬态测温技术难点。     

Kinetics and mechanism of nickel ferrite formation under high temperature ultrasonic treatment

The effect of simultaneous ultrasonic and thermal treatment on kinetics and mechanism of nickel ferrite formation was studied. It was established that sonication leads to notable increase of the mean rate of this reaction and decrease of effective activation energy from 190+/-5 to 125+/-7 kJ/mol. XRD data show that ultrasonic treatment significantly affects the microstructure of both initial reagent (Fe(2)O(3)) and reaction product (NiFe(2)O(4)) thus promoting formation of well developed reaction zone. A general model of ferrite formation mechanism under high temperature ultrasonic treatment was proposed.     

Photosynthesis in eucalyptus studied by the Open Photoacoustic technique: Effects of irradiance and temperature

This work demonstrates the usefulness of the Open Photoacoustic Cell Technique to study the effects of irradiance and temperature on photosynthesis. In vivo and in situ photosynthetic induction measurements were performed in three different species of eucalyptus plants (E. grandis, E. urophylla, and E. urograndis) previously dark-adapted at different temperatures. Photosynthetic activity curves were built as a function of light intensity, indicating the occurrence of photosynthesis saturation. E. urograndis presented higher photosynthetic activity than the other species, especially at low temperature, indicating its tolerance to stress conditions. The incidence of background saturation light of various intensities allowed the in situ study of photoinhibition in eucalyptus plants through open photoacoustics.     

ZnSe/ZnS/L-Cys核壳结构量子点光声与表面光伏特性

ZnSe量子点光电子特性的研究对于其微观电子结构探测和应用领域的扩展具有重要的意义.本文结合表面光伏与光声技术以及激光Raman研究了不同回流温度下制备L-半胱氨酸(L-Cys)为配体核壳结构ZnSe量子点的微结构和光声与表面光伏特性.结果发现,具有n-型光伏特性的ZnSe量子点在近紫外到可见光范围内展示出优良的表面光伏性质.尤其在波长为350-550 nm范围内光子能量绝大部分用于产生表面光伏效应,而不是用于无辐射跃迁导致的晶格热振动,同时证实了光声与表面光伏效应之间的能量互补关系.实验指认ZnSe量子点在300-350 nm短波区域出现的光声信号和在1120,1340和1455 cm~(-1)高频区域出现的Raman峰与配体L-Cys的多声子振动模式密切相关.实验结果表明,随着回流温度的降低,ZnSe量子点的平均粒径有减小趋势,这在改善样品的表面效应和小尺寸效应的同时,有利于提高核壳结构ZnSe量子点的光伏转换效率.     

外电场下甲醛的分子光谱与解离特性研究

摘要：以CC-PVQZ为基组,采用筛选的B3LYP研究了不同外电场对CH2O分子基态结构、电荷分布、总能量、电偶极距、最高占据轨道(HOMO)能级、最低占据轨道(LUMO)能级、能隙、红外光谱、拉曼光谱及CH2O分子中1C-2H键解离的影响。研究表明：电场由-0.04 a.u.变化至0.04 a.u.时,CH2O的结构参数与电场有明显依赖关系;体系总能量呈下降趋势;偶极矩呈增加趋势;HOMO能级与LUMO能级均逐步降低;能隙则呈增加趋势,即CH2O参与化学反应的能力会受到电场的影响;红外与拉曼光谱在电场考察范围内均出现红移现象,且光谱强度还会受到电场的影响,即可利用电场对分子的红外及拉曼光谱进行调控,从而便于对相关谱图信息的捕捉;CH2O分子中的1C-2H键的束缚情况会受不同电场的影响,且1C-2H键解离势垒与电场具有极高相关性,研究结果为电场降解甲醛提供了重要的理论依据。     

Photoacoustic detection of ferro and paramagnetic resonance

A commercial ESR spectrometer was modified to detect the photoacoustic signal of modulated magnetic resonance absorption, operating at room temperature and at atmospheric pressure. Both magnetically ordered materials (YIG and FeBO₃) and paramagnetic substances were investigated. The measurements include various phenomena of ferromagnetic resonance: uniform precession, magnetostatic modes and spin-wave instabilities. Paramagnetic resonance was observed in the concentrated system MnF₂ and in the diluted system ZnS : Mn (1%), with well resolved hyperfine splitting.