Bath cryostats

Our laboratories are currently equipped with three UHV Omicron microscopes, a variable-temperature scanning tunneling microscope (STM), a room-temperature atomic force microscope (AFM)/STM, and a low-temperature liquid helium bath cryostat STM, all of which are currently driven by Omicron Scala software and electronics. 

In the case of bath cryostats - in the most simple case a cold trap filled with LNj (liquid nitrogen) - the pumping surface is cooled by direct contact with a liquefied gas. On a surface cooled with LNj (T = 77 K) HjO and COj are able to condense. On a surface cooled to = 10 K all gases except He and Ne may be pumped by way of condensation. A surface cooled w/ith liquid helium (T = 4.2 K) is capable of condensing all gases except helium. 

If T limits not given, this is just a function of the cooling bath, cryostat being used. 

The saturation system consists in two glass-made saturators in a thermostated bath (cryostat). Both saturators are half-filled with the liquid VOC. Saturation is performed by bubbling helium through the liquid. The vapour pressure of the VOC in helium depends on the saturation temperature (7],). The cryostat can operate between -50°C and + 50°C. The temperature of the thermostated bath (Jc) is measured by a thermocouple with an accuracy of 0.3°C. The temperature of the liquid VOC (Js) in the second saturator is measured with an accuracy of 0.1°C by a PtlOO. A 0-2 bar pressure transducer is used to measure the total... 

The MB ion-exchanged faujasite sample (200 mg) is filled in a glass cuvette (Vitro Dynamics), evacuated at room temperature to 0.01 Pa for 1 h, sealed in vacuo and positioned in a liquid helium bath cryostat (Cryovac) at reduced pressure. 

Freezing mixtures or low-temperature bath (cryostats), cooled with solid COg or liquid nitrogen, are used for reaching temperatures below the ice point. 

A commercial microscope objective with a numerical aperture of NA = 0.85 and a magnification of 60 was adapted to the limited space inside the bath cryostat. A metal plate with a large pinhole of 50 to 150 pm in diameter held the sample. The... 

The Cryostat in Industrial Research (1) 274 A Circulating Air-bath Cryostat and Its Use in Dynamic Adsorption (2) 209 A Cryostat for Liquid Nitrogen Cooling of Rocket-bome Photomultiplier Tubes (3) 226... 

A He-bath cryostat equipped with a superconducting magnet can be utilized for Mossbauer investigations of materials in an applied magnetic field. Magnetic fields up to 10 T are very common today. 

Most instrument manufacturers will make temperature control units for use at around a few tens of degrees from ambient, with either thermoelectric (Peltier) temperature control, or more simply a cell holder which allows water, or other liquid, circulation from an external temperamre controlled bath. Cryostats are also available for precise very low temperature control, easily down to 77 K using liquid nitrogen cooling, less easily down to 4 K using liquid helium, and even to lower temperatures if required. A hot air blower, such as a hair dryer, is a convenient way to raise the temperature of a sample up to a few tens of degrees above ambient for the occasional experiment and a thermocouple or thermistor a convenient way to measure sample, or cell holder, temperature. 

The schematic of a standard on-top bath cryostat [8] is shown in Figure 3.5.3. Radiation shields and a fN2 tank surround the inner fHe tank. At the bottom of the IHe tank, the microscope is suspended by springs and radiation cooled by two concentric radiation shields that are connected to the f He and fN2 tanks, respectively. For fast conductive cooling, the microscope is pressed against the base plate of the f He reservoir. After cooling down, the microscope is released from the base plate for better vibration isolation [8j. The helium consumption of such a cryostat is about 0.11 h [12]. 

Flow cryostats can be operated at various temperatures up to room temperature [9]. These cryostats allow for a faster cooling down than bath cryostats. However, the lowest temperature is usually a few degrees higher than that achieved with a bath cryostat and also the helium consumption rate is considerably higher, namely about 1.3 lh [12]. 

Figure 3.5.3 Typicai design of an on-top bath cryostat used for LT-STM. From Ref [8].

Heyde, M., Thielsch, G., Rust, H.P., and Freund, H.J. (2005) A reverse pendulum bath cryostat design suitable for low temperature scanning probe microscopy. Meas. Sci. Technol., 16, 859-864. 

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