Basics of Closed Cycle Cryostats for Optical Spectroscopy
Optical, electrical and magnetic property testing of materials can be done at low temperature using a Closed Cycle Refrigerator (CCR). Liquid Helium has become increasingly expensive over the past few years making the CCR’s or “Dry Cryostats” a more popular choice. The new high performance CCR’s are easy to set up and sub 3K performance can be achieved with the flip of a switch.
Experimental Set Up
A typical Closed Cycle Cryostat setup for an optical experiment consists of the following parts:
Cold head, compressor, hoses, vacuum shroud, radiation shield, sample holder, windows, experimental wiring, temperature controller, temperature sensors, vacuum system (vacuum pump, hose, filters, valves, gauge, etc).
The cryocooler has two main components: the cold head and the compressor. In addition, flexible high pressure helium hoses and power cables are supplied to connect the two. This is the heart of the system and is available in a range of sizes and cooling powers to suit the experiment, sample size, temperature range, sensitivity to vibrations etc. The basic cooler, the model CS202A will provide a temperature range of 9K to 350K.
Lower temperature down to 2.8K, and high temperature, up to 800K, are available. The basic cooler has vibrations in the range of 10 - 15 µm at 2 or 2.4 Hz. This can easily be reduced to 3 - 5 nm using a Ultra Low Vibration Interface, Model DMX-20, available as an option and retrofitable in some cases.
Vacuum shrouds for optical experiments are constructed from Stainless Steel or Aluminum. Stainless is more durable and less susceptible to adsorbing water vapor making it more high vacuum compatible which results in a cleaner sample environment. Aluminum is more cost effective. These shrouds are available for a wide variety of sample configurations. The standard shroud, series DMX-1 has a sample space of 1.4 inch diameter by 1.5 inches long.
A typical vacuum shroud will be mounted on the cryocooler using double O-Rings for vacuum seal. This allows the user to rotate the shroud (window with respect to the sample) without losing vacuum. The size of the optical block depends on the size of the sample space required by the sample holder (sample space inside the radiation shield). The vacuum shroud can be modified or customized for commonly available spectrometers, including a mounting arrangement to seal the spectrometer sample compartment when it is nitrogen purged.
Constructed from high purity copper (nickel plated for durability and low emissivity) or aluminum. ARS typically plates the OFHC (Oxygen Free High Conductivity) copper radiation shield with nickel or gold; this gives the surface a low thermal emissivity for better thermal performance. Nickel plating is more durable than gold and has about the same emissivity. Gold plating, without a nickel base, will diffuse into the copper over time, leaving the surface tarnished and blotchy, deteriorating the surface emissivity and its effectiveness.
Sample holders are designed for sample size (large or small), thin film, bulk, or liquid samples. Sample holders can also have electrical pins for easy wiring to the sample when performing electro-optical research. Sample holders are constructed of OFHC copper for high thermal conductivity and nickel plated.
Good window port design allows a maximum optical cone of light to the sample. This is defined by the f/# which is explained in fig-4. Depending on the type of experiment and the wavelength of the optical beams the window material can be for Visible, UV or IR experiments. Common materials are Quartz, Sapphire, CsI, KBr, ZnSe. Several materials are used for IR or UV experiments. Transmission curves (Fig-5) are available for common windows used for Optical Spectroscopy.
Windows for the vacuum shroud are at room temperature and sealed with an O-Ring (Fig-6).
The thickness of the window is a function of the window diameter and the material.
The larger the window, the thicker it has to be for mechanical strength to cope with the vacuum forces. Thin windows are advisable for collecting weak optical signals. The windows are mounted using a stress free mounting arrangement for minimal birefringence.
Coated windows for low reflection, brewster windows and wedge windows are available for specific experiments.
Cold windows for the radiation shield are advisable when the minimum temperature at the sample is critical. Cold windows will minimize the radiant heat through the optical hole of the shield falling on the sample. The sample temperature will be 1K lower with 4 cold windows instead of holes in the radiation shield. The drawback is the added thickness of the window material in the beam path. Cold windows will cut off IR wavelength so it is common in the UV and Vis region.
When doing transmission spectroscopy, the sample is generally mounted on the window installed on the sample holder. Indium gaskets ensure cold window temperature. The goal of this design is to have the lowest sample temperature capability which means good thermal conductivity of the window material. Use sapphire or quartz window if possible, as they have the highest thermal conductivity.
Getting to the lowest possible sample temperature and measuring it correctly will depend on several factors, including:
Proper sample holder mounting to the cryocooler:
This is a solid to solid surface conductive cooling. The typical area in actual contact varies as the normal pressure but expect in normal cases to have only a 10% (A. M. Khounsary, APS. ANL) surface contact. Using soft material between the two surfaces will help a lot. Indium is a good choice where the system is not UHV or the sample will not be heated above room temperature. If the system is UHV or has a high temperature interface, use annealed silver (annealed for softness so it can flow).
Sample mounting to sample holder:
Good heat transfer between the sample holder and the sample is crucial for good performance. Conductive heat transfer between two surfaces is a function of pressure between the two surfaces. As the sample is generally a delicate material, high forces are not possible. The best available option is to have a grease or epoxy glue the sample to the holder.
The advantage of grease is that it is easy to remove with alcohol or acetone. Recommended greases are CryconTM grease which is a copper loaded (Vacuum compatible) grease for good thermal conductivity or Apiezon grease. Both should be tested for optical background before they are used in the final experiment.
It is also advisable to install the sensor on the sample to see what the actual temperature of the sample is. Information on how long it takes the temperature to level out and the “ΔT” with respect to the control sensor are useful points to understand. This is even more important when user plans to control the temperature with a fast ramp.
Other materials which should be considered for sample mounting are varnish (IMI-7031 thinned with ethanol) and epoxy (Stycast).
If all this fails, use a clamp to hold the sample but thermal characterization is even more important when the sample is dry mounted. Spring washers and clips are available with most ARS sample holders.
Other factors to consider:
- Material of the sample holder window
- Material of the sample (thermal conductivity and emissivity of the sample)
- Accuracy of the temperature sensor
- Proper installation of the sensors