The understanding of superconductivity has progressed in four stages. During the first period following the discovery of disappearance of electrical resistivity of mercury by Kamerlingh Onnes, attempts were made to understand superconductivity based only on zero resistance. The second period was initiated by the discovery of the Meissner effect. Based on new knowledge of the magnetic property of zero resistivity, phenomological theories were developed and new aspects of superconductivity were uncovered, culminating in the establishment of the BCS theory. The rediscovery of a second type of superconductivity together with flux quantization initiated the third period where fluxons play an active role. The fourth period, which is still in progress, was initiated by the discovery of oxides with dramatically high critical temperatures. All these activities were initiated by Kamerlingh Onnes' brilliant insight leading to the discovery of superconductivity.
A modified two-fluid model is applied to study flow and heat transfer of superfluid helium in a micro channel with a diameter as small as that of a superleak in a fountain effect pump. Variable properties of superfluid helium and energy dissipations due to the two-fluid mutual friction and the friction at the channel wall are taken into consideration. It is found that the normalfluid component flow is not trivial even in a channel of a diameter of micro meters, and that there exists an optimum diameter which attains the maximum mass flow rate. The flow of superfluid helium through a channel with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature and pressure, as well as the internal-convection mechanism is found to be the causes of the unique flows.
This paper describes the results of an experimental study of closed cryogenic two-phase thermosyphon with air as the working fluid. The thermosyphon, which was 270mm long with an inner diameter of 8mm, was operated over a wide pressure range, from near the triple point to the critical point of air. The axial temperature distributions within the thermosyphon were measured as a function of heat transfer rate under the conditions of various operating vapor pressure. The thermal resistance and the maximum heat transfer rate were obtained from the measurement. The present experimental data for air are compared with the data of thermosyphon with nitrogen as the working fluid.
High-temperature superconductor (HTS) materials are very promising for high-frequency passive devices because of their extremely low loss at the microwave region. A coplanar waveguide (CPW) is one of the simplest planar transmission lines and is especially well suited to HTS films because only one side of the substrate needs to be coated before patterning. Furthermore, since the size of the CPW device can be reduced in scale without changing its characteristic impedance, it is easy to miniaturize the HTS bandpass filter (BPF) and make it compatible with the monolithic microwave integrated circuits (MMICs). In this paper, HTS Y-Ba-Cu-O (YBCO) films were prepared on MgO polycrystalline substrates (50×25×1mm3) by the sol-gel process. Because this process requires no high vacuum, no special equipment and no large target, a YBCO film is easy to fabricate, at a very low cost, on a large substrate. The temperature dependences of the surface resistance (Rs) at different positions of the YBCO film around 9.9GHz were observed. The value of average Rs was 2.9mW at 30K. The large uniform YBCO film was prepared by the sol-gel process. CPW 2-pole band-pass filters (BPFs) (center frequency is around 2GHz) with attenuation poles are prepared by using YBCO sol-gel film. The electrode configuration of YBCO BPFs was patterned by the wet-etching technique with a photo mask. Microwave characteristics of our YBCO BPF with attenuation poles at both sides of the passband are better than that of Cu BPF. To realize a sharper cutoff and a narrow bandwidth BPFs, 3-pole BPF with attenuation poles were also prepared. It has an insertion loss of 2.41dB at the center frequency 1.98GHz, and the fractional bandwidth is 2.43%. These characteristics are better than those of Cu BPF (insertion loss is 5.505dB at 1.93GHz, and the fractional bandwidth is 2.84%). In our results, the possibility of realizing superconducting coplanar filters with attenuation poles is revealed.