「問題導向學習」教案之品質改善：香港大學醫學院一年級生之經驗與回饋

To improve the quality of paper cases used for problem-based learning in the medical curriculum of the University of Hong Kong, Year 1 students were asked at the end of the year to fill in a questionnaire which addresses specific issues related to problem based learning. In addition, for each of the 6 new cases of PBL which were introduced into the first year, tutorials groups were asked to rate which of the learning objectives as specified in the cased were covered under the 4 themes: Biology of Health and Disease (Knowledge Component); Doctors and Patients (Clinical and communication Skills); Medicine and the Community (Societal and Population Medicine) and Professional Development (Medical Ethics and Law). Over eighty percent of the students found the PBL process to facilitate self directed learning and contributed to their learning. Over forty percent found there was unequal representation of the 4 themes with the issue of Professional Development to be the most inadequately covered. Various factors can be identified for the under representation and inadequate coverage of professional development issues. Possible solutions may include case modifications to ensure a fairer balance of 4 themes, alternative formats of enhancing the discussion of professional development issues, tutor training and coaching the students to address professional development issues

Detection and Characterization of Surface Cracks Using Leaky Rayleigh Waves

A number of ceramics such as silicon nitride, and zirconia are being considered for high temperature structural applications. The primary problem with these ceramics is their wide fracture strength variability. In consequence, non-destructive evaluation techniques are required to ensure their reliable use. The brittle nature of ceramics inhibits the strain energy release at flaws by plastic deformation. As a result, critical flaw size in these materials is small. For example, flaws in the size range of 20–100 µm are considered as “critical” in silicon nitride for engine applications. Surface cracks are particularly important since they are the major source of failure in ceramics (1). These cracks are generated during machining operations and usually consist of arrays of semi-elliptical cracks with random inclination to the surface, but a preferred alignment parallel to the direction of motion of the abrading particles (2).</p

Study Site Standard Operating Procedure SOP: P8 - Recruitment of subjects

Recently, improvements in instrumentation have provided the option of gas- or air-coupled ultrasonic testing as a realistic alternative to immersion or contact testing. In this paper we present theoretical and experimental results on resonant sound transmission methods mediated by ultrasonic coupling directly through ambient air. Although these methods are not new [1], they have only recently been considered for MHz applications. Advances in transducer technology [2–5] have further improved signal to noise ratios

Analytical calculation of collapse voltage of CMUT membrane

Because the collapse voltage determines the operating point of the capacitive micromachined ultrasonic transducer (CMUT), it is crucial to calculate and control this parameter. One approach uses parallel plate approximation, where a parallel plate motion models the average membrane displacement. This usually yields calculated collapse voltage 25 percent higher than the actual collapse voltage. More accurate calculation involves finite element method (FEM) analysis. However, depending on the required accuracy, the computation time may require many hours

Influence of the electrode size and location on the performance of a CMUT

The collapse voltage of micromachined capacitive ultrasonic transducers (CMUT) depends on the size, thickness, type, and position of the metal electrode within the membrane. This paper reports the result of a finite element study of this effect. The program (ANSYS 5.7) is used to model a circular membrane on top of a Si substrate covered by a Si3N4 insulation layer. We find that the collapse voltage increases in proportion to the metal thickness for constant membrane thickness. The collapse voltage of a membrane with a thin metal electrode decreases as the metal plate moves closer to the bottom of the membrane; whereas, for electrodes with larger metal thickness, the collapse voltage has a peak intermediate value. Decreasing the outer radius of the metal plate results in an asymptotic increase of the collapse voltage. For a finite metal thickness, an initial decrease in the collapse voltage is seen as the outer radius decreases. The collapse voltages of half-metallized and full-metallized structures are almost equal for typical metal plate thickness. The asymptotic increase of the collapse voltage is seen for ring shaped metal plates as the inner radius is varied from the center to the outer radius. In summary, we find that the influence of the metal electrode on the collapse voltage is a very important parameter in determining optimum performance of a CMUT

A new regime for operating capacitive micromachined ultrasonic transducers

We report on a new operation regime for capacitive micromachined ultrasonic transducers (cMUTs). Traditionally, cMUTs are operated at a bias voltage lower than the collapse voltage of their membranes. In the new proposed operation regime, first the cMUT is biased past the collapse voltage. Second, the bias voltage applied to the collapsed membrane is reduced without releasing the membrane. Third, the cMUT is excited with an ac signal at the bias point, keeping the total applied voltage between the collapse and snapback voltages. In this operation regime, the center of the membrane is always in contact with the substrate. Our finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency (k(T)(2)) higher than a cMUT operating in the conventional regime below its collapse voltage. This paper compares the simulation results of the coupling efficiencies of cMUTs operating in conventional and new operation regimes

Calculation and measurement of electromechanical coupling coefficient of capacitive micromachined ultrasonic transducers

The electromechanical coupling coefficient is an important figure of merit of ultrasonic transducers. The transducer bandwidth is determined by the electromechanical coupling efficiency. The coupling coefficient is, by definition, the ratio of delivered mechanical energy to the stored total energy in the transducer. In this paper, we present the calculation and measurement of coupling coefficient for capacitive micromachined ultrasonic transducers (CMUTs). The finite element method (FEM) is used for our calculations, and the FEM results are compared with the analytical results obtained with parallel plate approximation. The effect of series and parallel capacitances in the CMUT also is investigated. The FEM calculations of the CMUT indicate that the electromechanical coupling coefficient is independent of any series capacitance that may exist in the structure. The series capacitance, however, alters the collapse voltage of the membrane. The parallel parasitic capacitance that may exist in a CMUT or is external to the transducer reduces the coupling coefficient at a given bias voltage. At the collapse, regardless of the parasitics, the coupling coefficient reaches unity. Our experimental measurements confirm a coupling coefficient of 0.85 before collapse, and measurements are in agreement with theory

Dynamic analysis of CMUTs in different regimes of operation

This paper reports on dynamic analysis of an immersed single capacitive micromachined ultrasonic transducer (CMUT) cell transmitting. A water loaded 24 mum circular silicon membrane of a transducer was modeled. The calculated collapse and snapback voltages were 80 V and 50 V, respectively. The resonance frequency, output pressure and nonlinearity of the CMUT in three regimes of operation were determined. These regimes were: a) the conventional regime in which the membrane does not make contact with the substrate, b) the collapsed regime in which the center of the membrane is in constant contact with the substrate, and c) the collapse-snapback regime in which the membrane intermittently makes contact with the substrate and releases. The average membrane displacement was compared as the CMUT was operated in these regimes. A displacement of 70 A in the collapsed regime and 39 Angstrom in conventional regime operation were predicted when a 5 V pulse was applied to the CMUT cell biased at 70 V. The CMUT showed a 2(nd) harmonic at -16 dB and -26 dB in conventional and collapsed regimes of operation, respectively. Collapse-snapback operation provided increased output pressure at the expense of a 3(rd) harmonic at -10 dB. Our simulations predicted that the average output pressure at the membrane could be 90 kPa/V with collapse-snapback operation compared to 4 kPaN with conventional operation

Dynamic FEM analysis of multiple cMUT cells in immersion

This paper reports on the accurate modeling of immersion capacitive micromachined ultrasonic transducers (cMUTs) using the time-domain, nonlinear finite element package, LS-DYNA, developed by Livermore Software Technology Corporation (LSTC). A capacitive micromachined ultrasonic transducer consists of many cMUT cells. In this paper, a square membrane was used as the unit cell to cover the transducer area by periodic replication on the surface. The silicon membrane, silicon oxide post and insulation layer were modeled, and the contact region was defined on the membrane and the substrate surfaces. The 3-D finite element model also included a 500 mu m-thick substrate and the acoustic fluid medium, to take into account two main sources of coupling in cMUTs: Scholte wave propagating at the solid-fluid interface and Lamb wave propagating in the substrate. A highly efficient perfectly matched layer (PAIL) absorbing boundary condition was designed for the acoustic medium to truncate the computational domain. The cMUT was biased in-collapse or out-of-collapse with an applied potential difference between the membrane and substrate electrodes: a rectangular pulse excitation was then used for the conventional, collapsed or collapse-snapback operations of the cMUT. Collapsed operation of the cMUT generated six times greater acoustic output pressure (641 kPa) than the conventional operation (107 kPa) at both the same bias voltage (83 V) and the pulse amplitude (+5 V). The vacuum backing and impedance-matched backing were compared to determine the influence of wave reflections from the bottom of the substrate in the collapsed operation. The dynamic FEN,I results were compared to the experimental results for conventional and collapse-snapback operations by applying step voltages on biased cMUT membranes. The acoustic output pressure measurements of the cMUT were performed with a hydrophone. The hydrophone calibration data was used to find the sensed pressure. Taking the attenuation and diffraction losses into account, the pressure on the cMUT surface was extracted. The cMUT generated 348 kPa and 1040 kPa in the conventional and collapse-snapback operations, respectively, and good agreement was observed with the dynamic FEM results