Acoustic Resonances in Helium Fluids Excited by?Quartz Tuning Forks

Ordinary quartz tuning fork resonators, operated at about 30 or 200 kHz frequency, couple to acoustic first and second sound resonances in helium fluids under certain conditions. We have studied acoustic resonances in supercritical ⁴He, normal and superfluid ⁴He, and in isotopic mixtures of helium. Suggestive temperature, pressure, and concentration dependences are given. Furthermore, we propose a thermometric reference point device based on second sound resonances in helium mixtures, and indicate possible differences in the nature of second sound resonances in superfluid ⁴He and helium mixtures.     

Quartz Tuning Forks as Cryogenic Vacuum Gauges

We report measurements of the mechanical \(Q\) of a 32.7 kHz quartz tuning fork as a function of pressure for helium and argon at T  \(=\)  300 K and for helium in the temperature range 7.0–0.7 K. In the low pressure ballistic regime, the damping due to the surrounding gas is inversely proportional to \(P\) , while for higher pressures, a hydrodynamic treatment accounts for most of the variation of \(Q\) with \(P\) . We have combined the ballistic and hydrodynamic models together with calculations of the thermal transpiration correction to correlate the tuning fork \(Q\) at low temperature with the pressure measured with a room temperature pressure gauge. The fork was found to be useful as an in situ pressure gauge for pressures above \(\sim \) 0.1 mTorr. A dissipation peak and frequency drop associated with the superfluid transition in the adsorbed helium film is also observed for \(T<1.4\)  K.     

Acoustic Emission by Quartz Tuning Forks and Other Oscillating Structures in Cryogenic 4He Fluids

We report on experimental investigations of acoustic emission by quartz tuning forks resonating at frequencies 32 kHz, 38 kHz, 77 kHz and 100 kHz immersed in cold gaseous ⁴He and its normal and superfluid liquid phases. Frequency dependence of the observed low-drive-linewidth at 350 mK together with the temperature and pressure dependences (1.3 K < T < 4.2 K, 0 < p < 25 bar) of the observed damping of the high frequency (77 and 100 kHz) resonators measured in normal liquid ⁴He and its superfluid phase provide strong and direct evidence of the importance of sound emission by these tuning forks. Three analytical models of acoustic emission by vibrating tuning forks are developed and compared with the experimental results. We also discuss the importance of sound emission for experiments with the commonly used 32 kHz tuning forks as well as other oscillating structures??spheres, wires, grids and various micromachined sensors. We compare the relative importance of dissipative losses due to laminar viscous/ballistic drag and acoustic emission in liquid and superfluid ⁴He.     

Quartz Tuning Forks and Acoustic Phenomena: Application to Superfluid Helium

Immersed mechanical resonators are well suited for probing the properties of fluids, since the surrounding environment influences the resonant characteristics of such oscillators in several ways. Quartz tuning forks have gained much popularity in recent years as the resonators of choice for studies of liquid helium. They have many superior properties when compared to other oscillating bodies conventionally used for this purpose, such as vibrating wires. However, the intricate geometry of a tuning fork represents a challenge for analyzing their behavior in a fluid environment—analytical approaches do not carry very far. In this article the characteristics of immersed quartz tuning fork resonators are studied by numerical simulations. We account for the compressibility of the medium, that is acoustic phenomena, and neglect viscosity, with the aim to realistically model the oscillator response in superfluid helium. The significance of different tuning fork shapes is studied. Acoustic emission in infinite medium and acoustic resonances in confined volumes are investigated. The results can aid in choosing a quartz tuning fork with suitable properties for experiments, as well as interpreting measured data.     

Studies on Helium Liquids by Vibrating Wires and Quartz Tuning Forks

We present results of low-temperature experiments on dilute mixtures of ³He in ⁴He and on pure ³He, obtained by means of two kinds of mechanical oscillators immersed in the liquid sample: vibrating wires and quartz tuning forks. The helium sample was cooled either by adiabatic demagnetization of an immersed copper nuclear stage or by adiabatic melting of ⁴He in superfluid ³He. The measured effect of the surrounding fluid on the mechanical resonance of the oscillators is compared with existing theories. We also discuss resonances of second sound and the state of supersaturation, both observed by a tuning fork in helium mixtures.     

基于石英音叉探针的原子力显微镜测头开发

基于石英音叉探针开发了一种自感应原子力显微镜(AFM)测头。该测头通过自身输出电信号检测悬臂振幅变化,无需外部光学检测部件,易于集成。针对测头设计了微弱电流提取及寄生电容补偿电路。利用商业化锁相放大器实现了对测头幅度信号的获取。在此基础上对测头在调幅模式下的力-距离曲线和分辨力进行了测试。利用锁相放大器内置的PID模块实现了调幅模式下对样品表面形貌的测量。实验证明,该测头灵敏度为0.624 mV/nm,分辨力优于2 nm。     

量热探针在热等离子射流测量中的应用

为了测量等离子射流的温度和速度,设计了量热探针测量系统,采用补偿式的量热法来测量热等离子体 的比焓得到热等离子体的温度;也可作为水冷皮托管来测量射流的动压头确定射流流速。系统采用Φ0.8 mm探 针实际测量最高温度达8515K。根据等离子喷枪的输入功率和热效率估算出喷枪出口处射流的温度和速度,与探 针的测量结果比较吻合。     

Wetting of Spherical Surfaces by Quantum Fluids

We study adsorption of atoms and fluids on spherical surfaces and present resugts for helium on weak to moderately strong adsorbers such as Cs, Li and Mg. We investigate the growth of a helium film as a function of the number of atoms, by means of a finite range density functional theory. We identify a crossover between nonadsorption and weak adsorption as the sphere radius increases. Path Integral Monte Carlo simulations provide semiquantitative support to these predictions.     

Axial Phase of Quantum Fluids in Nanotubes

We explore the equations of state and other properties of various quantum fluids (³He, ⁴He, their mixtures, and H₂) confined within individual carbon nanotubes. Above a threshold number of particles, N _a, the fluid density near the axis begins to grow above a negligibly small value. The properties of this axial fluid phase are sensitive to the tube size and hence to the transverse compression in the case of a bundle of nanotubes. We consider He at zero temperature and H₂ at low temperatures.     

用于音叉式原子力显微镜的探针制备

建立了一种电化学腐蚀法钨探针制备系统,通过对电化学腐蚀过程中微弱电流信号的实时检测,实现对腐蚀电路快速通断的自动控制,断电时间0.3 ms。该系统制备出了具有与理想轮廓一致的指数形探针。分析了电解电压、电解液浓度与腐蚀电流的关系,得出腐蚀最佳参数。将石英音叉与钨探针粘接构成原子力显微镜测头,完成了调幅模式下的力曲线测试。实验结果表明:制作的钨探针满足原子力显微镜测头性能要求,音叉信号稳定,能识别10 nm的运动变化,为研制高性能音叉式原子力显微镜提供了实验技术基础。     

Propagation of a Turbulent Fronts in Quantum Fluids

This paper is part of our series of studies on the dynamics of inhomogeneous vortex tangles. We consider here the motion of turbulent fronts. There are a number of experiments in which the authors observed the development of superfluid turbulence in quantum fluids in the form of propagating fronts. There are also experiments in which the authors observed the appearance of the moving structures (“plugs”), regions with the high vortex line density in the counterflowing superfluid helium. These phenomena have both scientific and applied interest. For example, the possibility of spontaneous formation of “plugs” can radically affect heat transfer in channels with superfluid helium, which is used as a refrigerant for cooling superconducting devices. There are a number of hypotheses for how the fronts and “plugs” appear and what their dynamics are, in particular, how to evaluate the velocity of propagations. In the present work we elaborate the approach of the combustion-like propagation of the superfluid turbulent domain. A key assumption is that propagation of the front occurs in diffusion manner of diffusion with the source of the vortices behind the front, just like the combustion process. Additionally, there is a drift motion of the vortex tangles due to polarization of the vortex loops. Interplay of these effects determines the speed of the turbulent front propagation. We performed a comparison with the experimental data.     

Frequency Characteristics of a Quartz Tuning Fork Immersed in He II

The effect of dissipation on frequency characteristics of tuning forks was measured, the dissipation being induced by acoustic radiation of different wavelengths, excited by tuning forks. The tuning forks have been immersed in the superfluid helium. The fork resonance frequencies 32, 77 and 99 kHz have been measured at T=370 mK in the pressure range between SVP and 24.9 atm. Most of the tuning forks have been studied in a commercial can. It is found that at wavelength λ>0.6 cm the frequency dependence is determined by the relationship between density and pressure. It is established that a decrease in wavelength enhances influence of the acoustic radiation on the fork oscillation frequency. In the case where the sound wavelength is equal to the can internal diameter an acoustic resonance occurs. The frequency reaches values higher than the fork frequency in vacuum. Further reduction of the sound wavelength leads to the situation when the resonant frequency is similar to the frequency at long wavelengths.     

The Frequency Dependence of the Added Mass of Quartz Tuning Fork Immersed in He II

We measured the dependences of the resonance frequency of tuning forks immersed in liquid helium at \(T = 0.365\hbox { K}\) in the pressure interval from saturated vapor pressure to 24.8 atm. The quartz tuning forks have been studied with different resonance frequencies of 6.65, 8.46, 12.1, 25.0 and 33.6 kHz in vacuum. The measurements were taken in the laminar flow regime. The experimental data allow us to determine the added mass of a quartz tuning fork in He II. It was found that the added mass per unit length of the prong fork is frequency dependent. Some possible qualitative explanations for such dependence are proposed. In addition, we observed, at \(T = 0.365\hbox { K}\), the changes in added mass with pressure according to the pressure dependence of He II density.     

Calculation of the Checking Interval of Quartz Thermometers Used under Different Conditions

A method of correcting the checking interval of quartz thermometers based on the statistical results of checks is developed. The method enables the features of the use of thermal converters for measuring temperature differences to be taken into account.     

A Microscopic Description of Quantum Fluids Adsorbed in Carbon Nanotubes

Apart form their possible technological applications, the adsorption of quantum liquids inside carbon nanotubes allows for an experimental realization of quasi-one dimensional systems. We present a comparison between energetic and structural properties of ⁴ He and H ₂ inside a (5,5) nanotube and in a pure 1D arrangement based in diffusion Monte Carlo calculations.     

The Damping of a Quartz Tuning Fork in Superfluid 3He-B at Low Temperatures

We have measured the damping on a quartz tuning fork in the B-phase of superfluid ³He at low temperatures, below 0.3T _c. We present extensive measurements of the velocity dependence and temperature dependence of the damping force. At the lowest temperatures the damping is dominated by intrinsic dissipation at low velocities. Above some critical velocity an extra temperature independent damping mechanism quickly dominates. At higher temperatures there is additional damping from thermal quasiparticle excitations. The thermal damping mechanism is found to be the same as that for a vibrating wire resonator; Andreev scattering of thermal quasiparticles from the superfluid back-flow leads to a very large damping force. At low velocities the thermal damping force varies linearly with velocity, but tends towards a constant at higher velocities. The thermal damping fits very well to a simple model developed for vibrating wire resonators. This is somewhat surprising, since the quasiparticle trajectories through the superfluid flow around the fork prongs are more complicated due to the relatively high frequency of motion. We also discuss the damping mechanism above the critical velocity and compare the behaviour with other vibrating structures in superfluid ³He-B and in superfluid ⁴He at low temperatures. In superfluid ⁴He the high velocity response is usually dominated by vortex production (quantum turbulence), however in superfluid ³He the response may either be dominated by pair-breaking or by vortex production. In both cases the critical velocity in superfluid ³He-B is much smaller and the high velocity drag coefficient is much larger, compared to equivalent measurements in superfluid ⁴He.     

用于坐标测量机上测量与评定形位误差的软件包

为强化坐标测量机的功能，本文建立了评定直线、平面、圆、圆柱、圆锥、球等六项基本要素形状误差的、符合国家标准的数学模型，并用ＴＵＲＢＯＣ语言编制了计算程序．它不仅可直接用于评定位置时确定基准的理想要素，而且稍加扩展又可用来评定位置误差．本软件包可ＣＡＤ／ＣＡＭ系统联网，从而也极大地提高了ＣＡＤ／ＣＡＭ系统的效率．     

利用VB实现计量工作的微机管理

本文介绍了如何利用VB建立计量工作管理系统，并简述了该系统的设计思想，结构特点和主要功能。