Cryocoolers: Part 2: Applications


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Vibrations at the room temperature mounting flange will be transmitted to the vacuum shroud and anything else the shroud is physically contacting.

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This is important if the cryocooler is rigidly mounted on other instruments that are sensitive to vibrations. An example would be a cryocooler installed in the evacuated sample compartment of an FTIR spectrometer. Any vibrations from this room temperature mounting flange would be transmitted through the vacuum shroud to the spectrometer.

These vibrations may cause damage or misalignment to the delicate optics inside the spectrometer. When a cryocooler is mounted on a delicate instrument such as the FTIR spectrometer, a PT system would be a better choice than a GM system because the vibration level at the room temperature mounting flange of a PT system is significantly lower than in a GM system. The sample being cooled is usually mounted onto the 2nd stage. Any vibrations from the 2nd stage will be transmitted directly to the sample. GM cold heads have vibrations at the 2nd stage on the order of 20 microns along the axis of the cold head.

Unlike GM cold heads, PT cold heads do not have any internal moving parts to add to vibration levels of the cold head.

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However, expansion of the compressed Helium gas inside each stage of the cold head does contribute to the vibration level at the 2nd stage. This is true for PT and GM cold heads. The valve motor on GM cold heads is always bolted to this room temperature mounting flange. As the motor turns, the valve is opened and closed. This same motor also drives the pistons displacers inside the 1st and 2nd stage cylinders.

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The vibrations from this valve motor will contribute to the overall level of vibrations of the cold head. The movement of the displacers may also contribute to the vibrations at this flange on a GM cold head. A PT system would be a better choice if: a Vibrations at the sample must be less than 20 microns. Patent 5 issued July 23, , describes an improved regenerator for the low temperature stage e. An object of the present invention is to reduce the cost and to improve the reliability, efficiency and increase the cooling power of a cryocooler at both the low and high temperature ranges or stages, for example, from about 10K up to K, more generally from approximately K to approximately 10K.

Another object of the present invention is to utilize ductile magnetic rare earth lanthanide based solid solution alloys, which can be easily fabricated into tough, non-brittle, corrosion resistant spherical powders, or thin sheets, or thin wires, or porous monolithic forms such as cartridges , as the regenerator material.

Another object of the present invention is to provide a cryocooler with a regenerator having significantly higher heat capacity than the aforementioned previously used regenerator materials and combinations thereof, such as bronze, stainless steel, and lead. To reach temperatures below K, the invention envisions using two or more of the regenerator components in a particular embodiment.

A solid state solution alloy is a random statistical mixture of two or more metals and occasionally a metal matrix [solvent] with an interstitial element solute. H, B, C, N, 0] which occupy the crystalline sites of a solid.

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This is in contrast to an intermetallic compound in which the component atoms two or more occupy specific lattice sites in the crystal in an ordered arrangement. Thermodynamically, a solid solution alloy belongs to the same phase region as the solvent in contrast to an intermetallic compound, which is a different phase from that of both the solvent and solute. The regenerator bed can include other metals such as bronze, stainless steel, lead, etc. The magnetic regenerator is advantageous in that it can be tailored to improve cooling power and efficiency of the cryocooler in the above temperature ranges or stages of operation from approximately K to approximately 10K.

Moreover, since the regenerator rare earth metals and their solid solution alloys with other rare earth metals, non-rare earth metals and interstitial elements are relatively ductile as compared, for example, to brittle intermetallic compounds, the regenerator layers or particulates will not attrite or comminute and pulverize in use of the regenerator. Further, the rare earth metals and their solid solution alloys can be readily fabricated into wires, sheets, or spheres or porous monolithic form for use as regenerator components.

The advantage of the materials embodied in this invention, is that they can be easily and economically fabricated into a form which allows the design engineer to chose from spherical particles, wire mesh, flat plates, jelly rolls, porous monolithic forms, etc.

Cryocoolers

Furthermore, since these materials are tough, they will not deform as the soft lead spheres do or comminute or decrepitate and pulverize as the brittle intermetallic compounds do under the cyclic high pressure gas flows used in present day cryocoolers. Furthermore, the embodied materials are oxidation resistant and do not become fine oxide powders when exposed to air as does Nd metal spheres or foil, which are used as regenerator materials in cryocoolers operating at 10K or less.

The foregoing and other objects, features and advantages of the present invention will become apparent from the following more detailed description taken with the following drawings. Figure 2a is a composite graph of the volumetric heat capacities from 3. Figure 2b is a composite graph of the volumetric heat capacities below k of these same rare earth metals and lead, stainless steel, and bronze. Figure 3a, 3b, and 3c are graphs showing the effect of inclusion of interstitial elements in certain rare earth metal solid solution alloys on their heat capacity over the temperature ranges set forth.

Figure 4a, 4b, and 4c are composite graphs showing the volumetric heat capacities of certain rare earth metals and solid solution alloys and stainless steel and bronze over the temperature ranges set forth. Figure 5a and 5b are graphs of volumetric heat capacities of the listed rare earth metal solid solution alloys and Pb from 0 to K, Figure 5a, and from 0 to 50K, Figure 5b. Figure 5b also shows the volumetric heat capacity of Er 3 Ni an intermetallic compound. Figure 6, 7, 8, 9, 10, 11a, lib, 12a, 12b, and 12c are graphs of volumetric heat capacities of the listed rare earth metal solid solution alloys and Pb over the temperature ranges set forth.

Figure 14a, 14b, 15a, 15b, 16, and 17 are graphs illustrating the influence of Pr additions on the volumetric heat capacity of Er. Figure 17 also shows the volumetric heat capacity of the Nd 6 oEr 0 solid solution alloy.

Figure 18a is a graph of transition temperatures versus Pr concentration of Er-Pr solid solution alloys, while 18b is a graph of the maximum value of the volumetric heat capacity at the corresponding transition temperatures versus Pr concentration of Er-Pr solid solution alloys. Figure 20a and 20b are graphs of volumetric heat capacities of the listed Er-Nd solid solution alloys, Pb and ErNi an intermetallic compound over the temperature ranges set forth, showing the effect of Nd content on the volumetric heat capacity of Er.

Figure 21a and 21b are graphs of volumetric heat capacities of the listed rare earth metal solid solution alloys and stainless steel, Pb, and bronze over the temperature ranges set forth, showing the effect of Nd and Pr content on Er and Dy volumetric heat capacities.


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Figure 23, 24, 25, and 26 are graphs of volumetric heat capacities of the listed rare earth metal solid solution alloys and stainless steel, bronze and Pb over the temperature ranges set forth. Refer to Figure 2a, it is seen that near the magnetic ordering temperatures, the pure rare earth metals have higher volumetric heat capacities than those of bronze and stainless steel. By appropriate solid solution alloying various combinations of the heavy lanthanide metals Gd, Tb, Dy, Ho, Tm, and Er with each other, one would be able to fill m the gaps temperature ranges where the volumetric heat capacity of bronze is larger than that of any of the pure metals i.

This the filling of the gaps by proper alloying is discussed below. The volumetric heat capacities of these rare earth metals below K are shown m more detail m Figure 2b along with lead, stainless steel and bronze. It is seen that below 60K, where lead has a higher heat capacity than either stainless steel or bronze, four of the rare earth metals are better than lead for a significant temperature range: for Tb K, for Dy K, for Ho K and Er K and the same as lead from K, and K , note: the volumetric heat capacity of lead m the upper temperature regions above K are not shown m Figure 2a, but the heat capacities of the rare earth metals are indeed greater than that of lead for the quoted temperature ranges.

Thus for the temperatures below 80K, Er is the best candidate cryocooler regenerator material, when compared to those m common use to reach 10K, i. Finally, we note that the heat capacities very near the magnetic ordering temperatures are quite large over narrow temperature range for Tb at K and for Dy at K, and extremely large at the first order transitions of Dy at 91K and of Er at 19K.

One can improve their cryocooler regenerator capabilities by alloying with the interstitial elements, such as H, B, C, N, and 0, which smear out the first order phase transition by straining the crystal lattice and lower the large heat capacity values at the transition temperature and broaden the transition over a wider temperature range, see Figures 3a, b, and c. Three typical examples showing the effect of adding interstitial alloying agents on the volumetric heat capacities are shown in Figures 3a, b, and c.

Figure 3a shows that adding oxygen, carbon and nitrogen to essentially pure Gd to form a quaternary solid solution alloy Gd Gd is Figures 3b and 3c illustrate the large effect interstitial alloying agents have on the first order transitions of Dy 91K, Figure 3b and Er 19K, Figure 3c. In both cases the extremely large values of the volumetric heat capacity are greatly reduced, and the peaks are both broader and shifted to slightly higher temperatures.

The high temperature second order peaks.

Furthermore, the spin-slip magnetic transitions in Er at 22 and 50K are essentially eliminated by alloying. Figure 3b. These changes are beneficial in that they improve the regenerator properties of the pure rare earth elements by eliminating or reducing the high heat capacity spikes and shifting the entropy associated with the peaks over a wider, and thus more useful, temperature range. N 0 3 C 0 2 are available from a variety of commercial companies m the US and m other countries, e.

The Er alloys described below were made using the latter alloy, all had similar 0, N, and C levels, which do not change significantly when the starting alloy is alloyed with other elements, especially other rare earth metals since they have similar impurity levels. Since present day cryocoolers have three distinct temperature spans, either as a single stage cooler with a layered regenerator bed, or as a two and even three stage cooler which may or may not have layered regenerator beds m the various stages, we will discuss the utilization of these magnetic rare earth alloys m two of the three temperature regimes: high temperature K , intermediate temperature.

The two temperature regimes of interest are the high and intermediate temperature ones. For high temperature regime, the mam concern was to develop intra rare earth solid solution alloys which will primarily fill m the gaps valleys between the high heat capacity peaks shown m Figure 2a over the 60 to K temperature range; m particular, temperatures between 60 and 90K, 90 and K, and K, and K, and and K. Some examples of solid solution alloys which can be used to fill-m the gaps.

Other solid solution alloys, which would work above K, are shown in Figure 4a are: 1 Gd 25 Tb 75 with a maximum in the heat capacity at K; 2 Gd 50 Tb 5 o with a heat capacity maximum at K; and 3 Gd 75 Tb 25 with a maximum at K. The first alloy would cover the gap between and K, the second eliminates the gap between and K, while the third alloy would be effective between and K. In Figure 4b we show in more detail the overlapping heat capacities of Ho The latter has two advantages over the former, the volumetric heat capacity of Dy 80 Nd 2 o is larger above K and it costs significantly less than the Ho Their heat capacities are in general less than those of the other three materials Ho q3.

Thus by forming a layered rare earth lanthanide regenerator bed, one can expect a significant increase in the regenerator performance over either the bronze or stainless steel regenerator from 60 to K. We now turn our attention to the intermediate temperature region, and as seen in Figure 2b, pure erbium has the highest volumetric heat capacity between 18 and 70K of any known material. Based on this fact a number of Er-based solid solution alloys have been studied with the idea of replacing lead as the intermediate temperature regenerator material.

Except for the Er 95 La 5 solid solution alloy the general trend is to lower the 85K transition and to wipe out the 20K transition of Er 96 8 0, C, N with no substantial improvement of the volumetric heat capacity at temperatures above 15K, see Figure 5a. The Er 95 La 5 behaves much differently, the La addition significantly lowers the upper transition and shifts the lower peak from 20 to 35K, giving rise to a significant increase m the heat capacity between 30 and 40K over the Er 9b 8 0, C, N. The influence of increasing the carbon content and also the addition of boron to the Er 96 8 0, C, N base solid solution alloy is shown m Figure 7.

Both the addition of boron and extra carbon raises the 82K transition of the Er 9b 8 0 2 7 C 0 2 N 0 3 by 1 and 2K, respectively. The influence of the addition of the light lanthanide solid solution alloying elements, La, Ce, Pr and Nd, are discussed next. The addition of 5at.

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Cryocoolers: Part 2: Applications Cryocoolers: Part 2: Applications
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