Method for evaluating and enhancing the cooling capability of AlN ceramic as a potential thermal guide in cryocooler cooled SQUID systems
ESHRAGHI Mohamad Javad, 笹田 一郎 （九大）； KIM Jin-mok, LEE Yong-ho （KRISS）
javad4*asem.kyushu-u.ac.jp
Abstract:We examined the thermal conductivity of AlN ceramic as a potential material for the thermal guide in a cryocooler cooled SQUID systems. Use of conductive materials such as a copper plate as SQUID holder increases the noise floor due to the thermal magnetic noise [1].
For examining the thermal conductivity of AlN， an AlN slab was prepared with dimensions of 3 mm in thickness， 12 cm in length and 4 cm in width. A thick copper plate of L-shape was used to fix an AlN slab to the second cold head of our two-stage pulse tube cryocooler. Apiezon N grease was used between AlN and copper holder. The temperature of the L-shaped copper holder， which was attached to the cold head， was measured with a Pt-Co temperature sensor. The temperature of the free end of the AlN slab was measured with a germanium temperature sensor which was attached to the AlN slab with bolt and Apiezon N grease. Output from both of the temperature sensors were acquired automatically with a DAQ board. We observed that the temperature of the free end of the AlN hardly fell below 7 K even one hour after the cold head had reached 3.75 K. So we conclude that this material by itself cannot cool LTs SQUID sensors to operate.
For enhancing the cooling capability of the sample we covered one side of a sample with 0.1 mm diameter， 12 cm long coated copper wires by placing them in parallel with each other. Because adjacent copper wires do not have electrical contact， electrons in the wires could fluctuate only one dimensionally， hence， based on the Ref. [1] the thermal magnetic noise may be much lower than that observed with a thick copper plate used as a SQUID holder. The temperature of the free end of the AlN sample with the copper wire decoration has successfully reached 4 K about 1 hour after the cold head had reached 3.75 K. Therefore we hope that this material with the aforementioned improvement could be used as a SQUID holder.
[1] T.Varpula， T.Poutanen， Magnetic field fluctuation arising from thermal motion of electric charge in conductors， J. Appl. Phys. （June 1984） 55（11）， 4015-4021