Multipurpose cryostat for low temperature electron irradiation of semi-conductors

A multipurpose cryostat is described which is designed for low temperature 1 MeV-electron irradiation and isochronal and isothermal recovery experiments in the temperature range from 2–350 K. It is suitable for simultaneous measurement of electrical resistivity, magnetoresistance, Hall effect, Seebeck effect, Nernst-Ettingshausen effect and spectroscopic investigation.     

Induction methods used in low temperature physics

A study has been made of induction bridges used in low temperature physics.In Part 1 the design of a mutual inductance bridge of the Hartshorn type is discussed. This design is based on a critical analysis of impurity effects of the different parts of the Hartshorn bridge. With this equipment frequencies up to 0.5 MHz can be used. Two methods have been developed to examine the secondary signal. In one of these use has been made of AD conversion techniques. In the other one, the secondary signal, produced by a superconducting sample, which is generally distorted, is analysed by using a Fourier expansion.In Part 2 equipment is described which enables us to measure the phase and amplitude of the harmonics of the output signal of the bridge. For synchronous detection a reference signal of the same frequency of the harmonic of interest is required. This reference signal is generated from the input signal of the bridge by means of a digital frequency multiplier with programmable multiplication factor N.In Part 3 some experimental results, showing the possibilities of the equipment, on some superconductors are presented.     

Phonon physics and low temperature detectors of dark matter

We discuss a number of topics in phonon physics that are relevant to the design of low temperature phonon-based dark matter detectors. These topics include the generation of primæval phonons by the nuclear recoil, the processes by which these phonons decay into lower energy phonons, and the ballistic propagation of these excitations to the sensors on the crystal surface.     

Variable temperature FIR-optical cryostat with unique features

This paper describes a convenient cryostat for optical and far infrared transmission and photoconductivity studies at temperatures in the range 5 K to 300 K. Up to four samples can be loaded in a cell and inserted into the cryostat from above without disturbing large metal seals or optical tails. Thermal contact between the heat sink and the cell is made automatically during the cooling process. The thermal link between the heat sink and liquid cryogen reservoir is varied by means of a modified thermosyphon. Even samples which are poor thermal conductors can be controlled in temperature within ± 10 mK by an exchange gas in the sample cell.     

The temperature of liquid nitrogen in cryostat dewars

Measurements of the temperature distribution of liquid nitrogen contained in 15 and 7.5 cm diameter dewars show that the bulk of the liquid is at an approximately constant temperature .15 to .3 K above the surface temperature. About half the liquid in the dewar is in a superheated state and this state, allthough easily disturbed from equilibrium by thermal and mechanical perturbations, occurs in an unclean vessel and with liquid nitrogen likely to contain impurities. The usefulness of this phenomenon is demonstrated by showing that an immersed copper waveguide was within a volume (50 cm long by 15 cm diameter) of liquid nitrogen that was isothermal to ±10 mK.     

A variable temperature,tensile testing cryostat

This paper describes a gas spray cryostat for tensile tests, which can also be used with testing instruments where both the crosshead on the top and the hydraulic piston on the bottom are moving simultaneously. A twin tube is used for the heat exchanger together with a ‘Thermocoax’ heater. This provides a short cooling and heating time during temperature change tests. The connexion between sample and load cell is frictionless, without the commonly used seals, and therefore very small forces can be resolved. In order to avoid unwanted deformation of the specimen due to the thermal expansion of the connexion rod, the specimen length is measured and controlled directly.     

Simple dipstick cryostat with versatile temperature controller for high thermal stability

A simple dipstick type vapour cryostat and a versatile electronic temperature controller which uses a proportional, integral and differential type of control are described. They have been used together for the calibration of thermometers in the liquid helium temperature range and for the measurement of the transition temperature of superconductors. Temperature stability of 2 mK (typical) is achieved easily at a working temperature of 7 K for a period exceeding an hour. The use of the system is quite general and can readily cover the temperature range up to 300 K with a suitable choice of temperature sensor and heat input.     

A cryostat for measuring specific heats of absorbed gases at low temperatures

A cryostat was designed and built for specific heat measurements of adsorbed gases on low thermal conductivity surfaces at low temperatures, without the use of a thermal switch. The calorimeter consists of five copper disks assembled with long ribbons of exfoliated graphite wound between them. The thermal time constant between the calorimeter and the cryostat's shield is about five hours at 4 K and the calorimeter's time constant is better than two minutes.     

A He3 cryostat with charcoal sorption pump for easy control of temperature

We have built the He³ cryostat with a charcoal sorption pump placed inside a small dewar vessel turned upside down and inserted in liquid He⁴·bath. The device is capable of providing any desired temperature within the range from 0.32 K to 3.20 K in a short time and holding it constant for a few hours if the external conditions remain unchanged.     

1.3 An evaporation cryostat for electrical and optical measurements on ultrathin metal film under ultra high vacuum conditions

An evaporation cryostat for electrical and optical measurements on ultrathin metal films under ultra high vacuum conditions is discussed. The main feature of this cryostat is that the electrical and optical measurements can be carried out simultaneously, or individually in situ or after deposition of the films in the temperature region 1–77 K. Some physical properties of ultra-thin films, which give the motivation for the construction, are also discussed.     

Optical interferometry at ultra low temperatures

We have developed optical, interferometric methods for investigations of interfaces at ultra low temperatures. In our scheme conventional optical windows are avoided: laser illumination (He-Ne) is guided into the cryostat via a single-mode optical fiber and images are taken using a CCD sensor mounted inside the 4-K vacuum can. A real-time video camera has been successfully used in investigations of superfluid³He down to 0.6 mK whereas a slow-scan camera has been employed for optimal contrast in low-intensity imaging of liquid/solid interfaces (reflection coefficient 10^–6). The investigated topics include (1) superfluid³He surface in rotation and during rapid deceleration, (2) hydrodynamics of thin superfluid³He layers, (3) superfluid/solid interface in⁴He, and (4) wetting of superfluid⁴He by normal³He in phase separated mixtures. A vertical resolution of 10 nm and even below has been achieved in these studies.     

极低频信号的一种离散频谱校正新方法

对于振动工程中常见的极低频、极短时、极高频(接近奈奎斯特频率)等极端频率信号,常用的离散频谱分析与校正方法存在较大误差.对极端频率信号的典型情形进行了分析,针对极端频率信号中的极低频信号,提出了一种计及负频率成分干涉影响的离散频谱校正新方法.该方法基于Blackman窗,利用局部谱峰附近的三条谱线,建立包含正负频率贡献的离散频谱校正模型,通过对模型的求解获得频率、幅值和相位校正公式.采用频段内扫描的方式对频谱校正公式进行了仿真验证,结果表明所提方法有效降低了负频率成分的干涉影响,对极低频信号的频率、幅值和相位校正有较高的精度.     

Structural analysis of aerosol spray pyrolysis ZnO films exhibiting ultra low ozone detection limits at room temperature

ZnO films grown by a home-made Aerosol Spray Pyrolysis (ASP) system at 350 °C have shown a characteristic columnar structure with an inhomogeneous layer structure that extends all the way to the surface. Surface morphology studies revealed grain sizes of the order of 80 nm and a highly porous structure which proved catalytic for the performance as gas sensing element. Conductometric ozone response tests have shown that the films of such a structure may be very effective in detecting ultra low ozone concentrations. The lowest ozone limit detected was 16 ppb at room temperature opening up the road for ultra sensitive inexpensive metal oxide ozone detectors.