Patent Grant
8844298
1. Field of the Disclosure
This disclosure relates to low temperature cooling devices. Particularly, this disclosure relates to a gas exchange housing for low stress and strain mounting of a sample while reducing vibrations transferred to, and providing optical access to, the sample.
2. Description of the Related Art
Cryogen-free refrigerators, also called cryo-coolers or cryostats, utilize a closed-cycle circulating refrigerant (often helium gas) to extract heat from a cold finger at cryogenic temperatures and pump it away to a heat exchanger. The cold finger is a metallic heat sink which is actively cooled by the refrigerant. The cold finger can be temperature controlled and serves as a mounting point for an object. The main goal of attaching the object to the cold finger is that the object will be cooled. Objects to be cooled can include semiconductor devices, detectors, mechanisms, material samples, or any other objects that require fixed, cryogenic temperature operation.
While direct contact coupling of the object can cool the object (i.e., cooling is where heat is transferred from the object to the cold finger), in some applications, direct contact alone does not provide adequate performance, because the mechanical operation of the cryo-cooler couples energy in the form of vibrations, acoustic noise, or other into the object via the thermal contacts of the object. In these applications, it is necessary to delicately mount the object such that it is cooled but isolated, or not disturbed, by cryo-cooling the apparatus.
In testing sensitive objects as mounted in open cycle cryostats, during sensitive experiments, problems are typically not observed. As is shown in
When moving from an open cycle system to a closed cycle system, one major change is seen. In an open cycle system, there is no force on the representative cold finger. However, in a closed cycle system a mechanical noise/force is present. If one uses mounts created for an open cycle cryostat the forces are coupled directly into the sample as shown in
In a conventional closed-cycle cryo-cooler, the sample is mounted to a cold finger in a vacuum, where good thermal contact is required. The thermal contact is ideally provided by a physical mount with substantial contact area and large thermal conduction with the sample. Good thermal contact is usually achieved with firm pressure and intermediary grease or other filling-material. Indium can be used to attach the object and the metal cold finger. However, a stiff contact from firm pressure can too easily couple vibrations or induce undesired stresses in sensitive materials.
The main problem in a closed-cycle cryo-cooler is that mechanical work is required to produce cooling power rather than obtaining cooling from a liquid source. This mechanical power produces the expansion of compressed gas at the cold finger, and thus typically transfers vibrations to the sample mount. This repeated rhythmic hammering, although at low frequency usually 1-2 Hz, drives higher frequency resonances inside the cryostat. High frequency, high intensity vibrations can also be created from the gas flow through the head across sharp corners or other imperfections in the piping, thereby creating noise that will interfere with a sensitive experiment. High intensity, high frequency vibrations couple into a sample through the physical contact of a mount. These high frequency vibrations produce a very unstable mounting platform for sample mounts. The best conventional sample holders in closed cycle cryostats coupled the least amount of vibrations into a crystalline sample, but still had inadequate performance.
To overcome the above mentioned problems, this disclosure identifies a sample mounting apparatus for a cryo-cooler comprising a housing having an outer wall surface for connecting to the cryo-cooler, and an inner wall surface; wherein the housing is sealable to contain an inert gas for thermal heat transfer; and a delicate mount attached to the inner wall surface of the housing for supporting the sample and substantially preventing vibrations from being transferred to the sample from the cryo-cooler.
Also disclosed is a method of reducing vibrations in a sample for use in a cryo-cooler comprising the steps of: a) mounting a sample in a sample mounting apparatus, wherein the sample mounting apparatus comprises a housing having an outer wall surface for connecting to a cryo-cooler, and an inner wall surface, and a delicate mount attached to the inner wall surface of the housing for supporting the sample and substantially preventing vibrations from being transferred to the sample from the cryo-cooler, b) sealing the walls of the housing by applying a sealant such that the walls do not allow gas to enter or leave the housing, c) evacuating the housing of gas via a gas inlet tube, d) adding an inert gas via the gas inlet tube, e) sealing the housing by closing the gas inlet tube, and f) attaching the outer wall of the housing to a cryo-cooler.
A block diagram representing the proposed solution is shown in
Additional advantages and other features of the present disclosure will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the disclosure. The advantages of the disclosure may be realized and obtained as particularly pointed out in the appended claims.
As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
Advantageous aspects of the described mounting system are good thermal contact through convection and conduction, de-coupling of external vibrations from the sample, preventing cracking of delicate samples from uncompensated thermal contraction, and avoiding temporary or permanent distortions of the sample from applied strains.
As shown in
The housing 10 comprises a main body 10a and a removable end plate 15, and the endplate 15 is sealed to the main body 10a with a sealant that can create an air-tight seal. Examples of sealants are indium, epoxy, and solder. The inert gas 30 sealed inside the housing 10 may be any inert gas suitable for heat transfer in low temperature applications. In certain embodiments, the inert gas is helium. Alternatively, nitrogen is used.
As also shown in
Alternatively, as shown in
In another embodiment of the present disclosure, such as shown in
Further, the housing optionally further comprises recesses in the external housing for inserting at least one magnet such that a variable homogeneous or inhomogeneous magnetic field is applied to the sample. For example, as shown in
A method of using the sample mounting apparatus for reducing vibrations in a sample for use in a cryo-cooler comprises the steps of mounting a sample in a sample mounting apparatus, such as the sample mounting apparatus described above in
Further details of particular embodiments of the present disclosure are discussed below.
In one embodiment of the present disclosure, a sample mounting apparatus 100, also called a gas exchange box (GXB), is described. As shown in
According to one embodiment of the present disclosure as shown in
Sealing the GXB 100 can be accomplished by methods including solder, indium, and epoxy. They each have their advantages and drawbacks. For example, solder can be hard to apply to large perimeter seals due to the elevated working temperature. Indium can be difficult to work with due to continuity requirements needed for an effective pressed indium seal. Epoxy should only be used in very thin layers, otherwise shear driven cracking or fatigue of the base material may occur due to differential thermal expansion or contraction. In certain embodiments, the GXB uses a mix of all three sealing options in different locations.
In other embodiments of the present disclosure, copper is used as a base material for the housing 10. Advantages of using copper include that copper is readily solderable, has high thermal conductivity, and is fairly readily machineable.
The GXB 100 is alternatively made of other materials such as aluminum, steel, brass, plastic, molded epoxy, or composite materials having sufficient thermal conductivity.
The filling and sealing of the system occur in the housing. As shown in
Conventional housings have been sealed using a single fill/drain tube 90 soldered in place into the fill and drain port 65, which allows an access point that can be used to fill the GXB 100 with helium, then vacuumed out, and repeated until the purity of the atmosphere inside the GXB 100 is at a suitable level. In this manner, the purity of helium inside the GXB 100 increases in a geometric progression of rarefaction on each evacuation, multiplied by a reasonable number of repetitions. Using a copper fill/drain tube 90, such as that shown in
Alternatively, another style of fill/drain tube can be used. Before adding the components that cannot be heated, an intermediate section of brass tubing is soldered to the housing. Then a copper fill/drain tube is attached and soldered to the brass section. The low thermal conductivity of the brass will allow for a solder joint to be created at the interface between the brass tube and copper fill/drain line without coupling excessive heat. This allows for crimping of a copper based tube. Another option is the use of a copper-nickel tube which has low thermal conductivity and material properties similar to that of copper.
Another aspect of the GXB 100 for use in cryogenic temperatures below 4 K is the use of helium as an exchange gas which allows for a good thermal cryogenic contact without significant vibrational coupling. The use of an exchange gas like helium for wide area thermal equilibration also allows for a very stable sample temperature. The main requirements are that the gas not leak from the GXB 100. If a leak occurs, the vacuum that is insulating the cryo-cooler may be compromised, resulting in potential warm up of the whole system, and loss of cooling at the sample.
Thermal oscillations of a cold finger without any helium gas can be approximately 300 mK with a frequency of about 1-2 Hz depending on the type of cryo-cooler. These oscillations can interfere with sensitive measurements or experiments. Through the use of GXB 100, these oscillations have been shown to be reduced to 1 mK levels.
When using a GXB 100, thermal drift is observed in the system due to changing environmental conditions or other unseen changes. This thermal drift is best described as oscillations in milli-Kelvin on a time scale of 10 minutes to several hours.
Another aspect of the GXB design is isolation of the sample from external closed-cycle cryocooler vibrations by using a GXB 100 that houses a carefully mounted sample 40 convectively cooled by an exchange gas 30. The mounts can be designed and altered for alternative samples that vary in sizes and constraints.
As shown in
When using epoxy, all bonding surfaces should cleaned thoroughly. The mating metal surface should be etched using some form of etching bath (such as sodium persulfate). All mating surfaces should be cleaned with an acetone wipe followed by a methanol wipe. This is to ensure no residual oils are left on the surface, and the combination of a clean etched surface will increase the strength of the epoxy bond, especially for use in challenging cryogenic environments.
When assembling the GXB 100, a shaft 21, (such as shown in
In other embodiments, a sample is supported with a point contact with the solid housing, such as a pointed jewel bearing. The bearings act as the point contact with the housing. This mounting style provides very little stress placed on the sample, and high frequency vibrations are dampened due to the movable jewel bearing interface. This type of bearing uses a pointed shaft usually made of stainless steel that contacts a cut in a conventional sapphire jewel called a Vee jewel. An example is shown in
All of the above mounting styles result in an amount of movement while at the same time supporting the sample. This movement is what allows some vibrational isolation from the cold finger. If the sample 40 is physically hard mounted to the cold finger, it would not be isolated. The easily implemented shaft 21 in the above examples could be replaced with some form of support like a sling setup, hammock or other apparatus that will support an object.
With the use of a GXB 100, a sample mount can be designed as if it were to be used in an open-cycle cryostat, which was a design for a sample mounted delicately in a helium environment. This mounting procedure allows for a very small force contact with the sample or just enough to keep the sample in position.
Another style of sample mounting on a shaft is shown in
Mounting of a window onto a GXB allows for optical access to a cooled sample or device without birefringence created by strain from differing coefficient of thermal expansion (CTE). Various methods have been developed to allow for optical access, including seals made from epoxy, solder, or indium.
According to one embodiment, window mounting is performed with a thin mounting flange 81 that is machined into the window seat to relieve stresses induced from the housing. This rim is machined in to create a ledge for a window is sit in, thus reducing the stress and strain that occur on the window 80 while thermally cycling between about 300 K to about 4 K (see
Windows that have been metalized at the sealing locations with deposited metal films, as shown in
When sealing a window 80 in place, care is taken to make sure the epoxy layer 85 is substantially even around the whole perimeter of the window 80. One method of accomplishing this is to epoxy the window 80 in place and twist the window 80 in the mount before the epoxy cures. This will allow the epoxy to flow into all crevices and fill any imperfections therefore creating an even layer.
When using epoxy, all components should be cleaned thoroughly. The mating metal surface should be etched using some form of etching bath (such as sodium persulfate). All mating surfaces should be cleaned with an acetone wipe followed by a methanol wipe. This is to ensure no residual oils are left on the surface, and the combination of a clean etched surface will increase the strength of the epoxy bond.
An interchangeable window mount 110 as shown in
In certain embodiments, stiffer crystalline window materials such as sapphire or Y3Al5O12 (YAG) are advantageous for several reasons. Fused silica windows are too fragile and prone to cracking over multiple thermal cycles, particularly in larger windows, and nominally non-birefringent BK7 glass windows became substantially birefringent under stress induced from compression forces from the mounts. Therefore, stiffer sapphire or YAG windows are used to overcome the birefringence effects and for their superior mechanical properties.
If applications where it is important that optical back reflections be minimized, the windows may be anti-reflection coated or wedged. Wedged windows reflect the optical beam in another direction, while anti-reflection coated windows allow the majority of the optical power to pass through with minimal reflected power.
The sample mount can, and in some cases is intended to, work with certain cryo-coolers, such as disclosed in U.S. Provisional Patent Application No. 61/136,138, filed Aug. 14, 2008, entitled “Apparatuses and Methods for Improving Vibration Isolation, Thermal Dampening, Optical Access in Cryogenic Refrigerators”.
The items described in that previous filing achieve low vibration by detailing another innovation which dampens vibration to any sample mount. It has been shown that the combination of the two items offers better decoupling of vibrations to the sample than the GXB 100 by itself, or the low vibration module by itself. The combination provides low displacement of the GXB 100 while still providing good thermal transfer, and also may reduce some of the forces transferred from the cold finger to the GXB 100. It is anticipated that while the low vibration module may be used by itself, and the GXB 100 may be used by itself, the most benefit for mounting delicate objects will be provided by these items being used together as shown in
The present disclosure can be practiced by employing conventional materials, methodology and equipment. Accordingly, the details of such materials, equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific materials, structures, chemicals, processes, etc., in order to provide a thorough understanding of the disclosure. However, it should be recognized that the present disclosure can be practiced without resorting to the details specifically set forth. In other instances, well known processing structures have not been described in detail, in order not to unnecessarily obscure the present disclosure.
Only a few examples of the present disclosure are shown and described herein. It is to be understood that the disclosure is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concepts as expressed herein.
This application is based upon and claims the benefit of priority from Provisional U.S. Patent Application 61/193,325 filed on Nov. 18, 2008, the entire contents of which are incorporated by reference herein.
This invention was made with government support under Grant No. DASG60-03-C-0075, awarded by the US ARMY SPACE & MISSILE DEFENSE COMMAND. The Government has certain rights in the invention.
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