This paper reports on a normally open piezoelectrically actuated microvalve for high flow modulation at cryogenic temperatures. One application envisioned is to control the flow of a cryogen for distributed cooling with a high degree of temperature stability and a small thermal gradient. The valve consists of a micromachined die fabricated from a silicon-on-insulator wafer, a glass wafer, a commercially available piezoelectric stack actuator and Macor TM ceramic encapsulation that has overall dimensions of 1 × 1 × 1 cm 3. A perimeter augmentation scheme for the valve seat has been implemented to provide high flow modulation. In tests performed at room temperature the flow was modulated from 980 mL min −1 with the valve fully open (0 V), to 0 mL min −1 with a 60 V actuation voltage, at an inlet gauge pressure of 55 kPa. This range is orders of magnitude higher flow than the modulation capability of similarly sized piezoelectric microvalves. At the cryogenic temperature of 80 K, the valve successfully modulated gas flow from 350 mL min −1 down to 20 mL min −1 with an inlet pressure of 104 kPa higher than the atmosphere. The operation of this valve has been validated at elevated temperatures as well, up to 380 K. The valve has a response time of less than 1 ms and has operational bandwidth up to 820 kHz.
We present results of a microwave surface impedance study of the heavy fermion superconductor UBe 13 . We clearly observe an absorption peak whose frequency-and temperature-dependence scales with the BCS gap function '(T). Resonant absorption into a collective mode, with energy approximately proportional to the superconducting gap, is proposed as a possible explanation. A oneparameter fit to the data provides a simple relation between '(T) and the collective mode energy.PACS numbers: 74.70.Tx, 74.25.Nf.The superconducting states in heavy fermion compounds [1] such as UPt 3 and UBe 13 exhibit behaviors that differ markedly from those predicted by the theory of Bardeen, Cooper, and Schrieffer (BCS). Quantities such as the specific heat and ultrasonic attenuation, for example, are enhanced at low temperatures, displaying a power-law dependence on T instead of the usual exponential form. This suggests the existence of nodes in the superconducting energy gap. Even more surprising was the discovery of multiple superconducting phases in UPt 3 . Such behavior has been explained, at least qualitatively, within the framework of Ginzburg-Landau theory, assuming a multi-component order parameter. Much of the work on heavy fermion superconductivity, especially from a theoretical point of view, has been strongly influenced by the paradigm provided by superfluid 3 He. In the superfluid phases of 3 He, quasiparticle pairs form in states with relative orbital angular momentum quantum number l = 1 (p-wave), as opposed to the BCS l = 0 (s-wave) state. Consequently, the order parameter possesses a large number of degrees of freedom. This in turn gives rise to multiple superfluid phases and a rich spectrum of order parameter collective modes [2,3]. The observation and classification of collective modes, notably by means of ultrasound absorption experiments, was instrumental in determining the symmetries of the order parameters corresponding to each phase. With mounting evidence that the heavy fermion superconductors might be characterized by unconventional 3 He-like order parameters, it was natural to wonder whether they too could support collective oscillations. A number of theoretical investigations [4][5][6] of unconventional charged superfluids, assuming order parameters of various symmetries, have predicted mode frequencies (measured relative to the energy gap) similar to those found in 3 He. However, the extent to which damping, due in part to the presence of impurities (a complication not encountered in 3 He work), should limit the observation of collective modes is a difficult theoretical problem that has not been adequately addressed. High frequency (~2 GHz) longitudinal ultrasound measurements [7] of UBe 13 revealed a sharp attenuation peak just below the superconducting transition temperature T c . This was initially interpreted as the
The pressure dependence of the resonance frequency of several resonant ultrasound spectroscopy modes in a sample of fused silica has been measured at UCLA in atmospheres of air, helium, and argon near ambient temperature. For both compressional and torsional modes, the radiation resistance is linearly dependent upon pressure and increases with the molecular mass of the surrounding gas. The effects are larger for breathing modes than for torsional modes. They also increase with the molecular mass of the gas. A radiation impedance model is presented which explains some of these data qualitatively and quantitatively.
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