Docent Vlasova I, Kuryanov K.

Donetsk National University of Economics and Trade named after

M. Tugan-Baranovsky

Fabrication of nonmetallic PTCs

For nonmetallic and nonmagnetic pulse tube coolers, besides using nylon mesh, it is of great importance to avoid the presence of metals in all components of the cold finger. These components include vacuum chamber, pulse tube, regenerator tube, connecting flanges as well as heat exchangers.

The tubes of pulse tube and regenerator are the most important, because of the combined requirements of considerable tensile strength, and of a fine structure in texture in order to minimize the diffusion of high-pressure helium gas. This is a really critical demand for nonmetallic materials due to their intrinsic molecular structures. In the present research, the regenerator tube is made of a special glassfilled epoxy resin. The mechanical properties of this material were also tested. A typical cylinder fabricated by the special glassfilled epoxy resin with a wall thickness of 2 mm, and 4.5 MPa of helium filling pressure shows the same diffusion of helium as one made of stainless steel. For the coaxial configuration of the PTC, the requirements for the pulse tube can be quite easily fulfilled, since there is only a small load on the tube walls; both Nylon1010 and Teflon are suitable for the pulse tube.

For the hot end and cold end flange, we have used a special ceramic, which is non-magnetic, and electrically insulating. Its mechanical properties and the diffusion of helium through the wall meet the requirements of long-term operation. Moreover, it is easily machinable and can be fabricated into the required complicated shapes. The flange at the hot end is incorporated into the system and serves as hot heat exchanger.

The connection of tubes to both the cold end and the flange at the hot end are realized by a special synthetic epoxy resin adhesive, Cryo-3 Glue. The adhesive can be used at a temperature region between 4 and 333 K for various metallic and non-metallic materials with different expansion coefficients. The lower the temperature, the higher is the adhesive strength. During the experiments, no diffusion of helium through the connections into the vacuum chamber was observed. For the cold end connection, a screw thread is used to enhance the strength, as shown in Fig.1.

Fig. 1. The structure at the cold end.

The completely nonmetallic pulse tube cryocooler is fabricated as illustrated in Fig. 2.

Fig. 2. Photo of cold head with Cryo-3 Glue connection.

Experimental result of nonmetallic PTC

Experiments have been performed by use of a Leybold Polar SC7 linear compressor with charge pressures of 2.0–3.0 MPa. The hot end of the PTC is cooled by a fan, which is very simple and convenient, making the cooling system superior in contrast to the work of the former. The proper phase shift between mass flow and pressure is accomplished by means of double-inlet in combination with an inertance tube. As with the metallic PTC, the volume of the buffer tank is 750 cm³. There is a hole in the cold with a diameter of 1.6 mm and a length of 15 mm, where the Pt-100 temperature sensor was located. The resistance of the Pt-100 is measured by a four wire method.

Fig. 3 shows the typical variation of the lowest temperature Tcmin with compressor input power. Tcmin changes only a little when the input power is higher than 60 W, since then the hot end of the PTC is already quite hot. With the compressor efficiency of about 60%, the Joule heat loss in the ceramic at hot end of the pulse tube has a significant effect on the cooling performance. The hotter warm end of the non-metallic cold head compared to the metallic cold head, is a result of the low heat conductance of the ceramic parts. In former studies of a nonmetallic, water cooling of the hot end was used to minimize this effect, since otherwise the minimum temperature increased by about 10 K. Therefore, a compressor with higher efficiency, which means less Joule heat loss, would be better for this kind of nonmetallic cooler.

Fig. 3. No-load temperature versus input power for the nonmetallic PTC.

The cooling capacity for this nonmetallic PTC is displayed in Fig.  4. With 60 W input, we obtain a cooling power of 0.1 W at 80 K, while it is 0.14 W at 80 K with 70 W input. The slope of the load line is about 0.022 W K⁻¹ at 70 W input. For comparison, the cooling capacity of the metallic PTC with nylon mesh has much larger slope of about 0.033 W K⁻¹ at 75 W input, as shown in Fig. 4. This means that the poor heat conduction of the ceramic parts at the cold end has a negative effect with respect to the cooling capacity.

Fig. 4. Cooling power of the nonmetallic PTC versus refrigeration temperature at different input power.

The above experiments show that the nonmetallic PTC can operate well at 80 K. Certainly, for higher cooling requirements, the performance should be further improved.

Conclusions

This work is a further investigation on a nonmetallic and nonmagnetic pulse tube cryocooler. Theoretical analysis shows that nylon mesh as regenerator matrix should be as suitable as stainless steel mesh. However, the experiments by use of a metallic PTC show that the performance of with nylon matrix is much worse compared with that employing a stainless steel matrix. The high flow friction factor of the Nylon mesh can be one of the important reasons for this finding. Another important reason is the shrinking of the nylon mesh upon cooling which deteriorate refrigeration a lot, although it can be suppressed partially. Nevertheless, the metallic PTC with nylon mesh achieved a no-load temperature below 60 K. The completely nonmetallic coaxial PTC was fabricated by use of a regenerator tube consisting of glassfilled epoxy resin, a pulse tube consisting of Teflon, the cold end and hot end flange made of ceramic material, and heat exchangers from nylon mesh. At 70 W input power, a lowest temperature of 74 K and a cooling capacity of 0.14 W at 80 K have been achieved. The present job make an obvious progress in contrast to the former investigation, where the no-load temperature is 76.8 K. The cooling condition is also simplified dramatically, replacing the former extremely strict 5 °C water cooling by the simple fan cooling.

Although this nonmetallic PTC can work at temperatures near 80 K, the performance is considerably degraded compared with that of the metallic PTC version. Besides the nonmetallic regenerator matrix, there are still many problems with the nonmetallic PTC that must be solved. With further improvements, a nonmetallic PTC with the present configuration can be an alternative candidate for SQUID cooling.