Huijuan Dong

Rapid formation of Ni3Sn4 joints for die attachment of SiC-based high temperature power devices using ultrasound-induced transient liquid phase bonding process

High melting point Ni3Sn4 joints for the die attachment of SiC-based high temperature power devices was successfully achieved using an ultrasound-induced transient liquid phase (TLP) bonding process within a remarkably short bonding time of 8s. The formed intermetallic joints, which are completely composed of the refined equiaxial Ni3Sn4 grains with the average diameter of 2μm, perform the average shear strength of 26.7MPa. The sonochemical effects of ultrasonic waves dominate the mechanism and kinetics of the rapid formation of Ni3Sn4 joints.

An improved phase-locked loop method for automatic resonance frequency tracing based on static capacitance broadband compensation for a high-power ultrasonic transducer

The phase-locked loop (PLL) method is widely used for automatic resonance frequency tracing (ARFT) of high-power ultrasonic transducers, which are usually vibrating systems with high mechanical quality factor (Qm). However, a heavily-loaded transducer usually has a low Qm because the load has a large mechanical loss. In this paper, a series of theoretical analyses is carried out to detail why the traditional PLL method could cause serious frequency tracing problems, including loss of lock, antiresonance frequency tracing, and large tracing errors. The authors propose an improved ARFT method based on static capacitance broadband compensation (SCBC), which is able to address these problems. Experiments using a generator based on the novel method were carried out using crude oil as the transducer load. The results obtained have demonstrated the effectiveness of the novel method, compared with the conventional PLL method, in terms of improved tracing accuracy (±9 Hz) and immunity to antiresonance frequency tracing and loss of lock.

Dependence of Brillouin frequency shift on radial and axial strain in silica optical fibers

Brillouin frequency shift (BFS) in a single-mode optical fiber has been measured as a function of both radial and axial strain via the Brillouin optical time domain analysis technique. The effects of the two kinds of strain on the BFS are decoupled by making fiber pretensioned and relaxed while applying pressure. Linear relations have been found between the BFS and both kinds of strain. The radial strain coefficient C(vε(r)) is found to be 0.029 MHz/με, and the axial strain coefficient C(vε(a)) is 0.053 MHz/με. The result may give impetus to some potential applications of the optical fibers, such as a distributed pressure sensor based on Brillouin scattering.

Study on a new kind of surface sticking strain sensor with sensitivity enhanced based on FBG

Fiber Bragg Grating sensors are widely used in the field of optical sensing system for the Structure Health Monitoring (SHM) during the last 10 years. It can be used for monitoring the strain, temperature pressure and some other characters of the structure. In this paper a new kind of surface sticking strain sensor with sensitivity enhanced has been studied and developed. The structure of the sensors is presented and the Ansys software has been used to optimize the structure. A novel theory about how to enhance the sensitivity or widen the scale of the strain sensors based on FBG is introduced too. Based on the theory two types of sensor have been developed. One is the long-scale sensor that is able to monitor the big strain such as crack, the other is the high-precision sensor that is able to monitor the micro strain as O.5,&mgr;&Vegr; , and we conclude the sensitivity enhancing coefficient. As the sensitivity coefficient of bare fiber is 1 .2pm/&mgr;&Vegr; ,the theoretical enhancing sensitivity coefficient of long-scale sensor is K=3.3811 and the theoretical enhancing sensitivity coefficient of high-precision sensor is K=O.58. Then groups of experiments were carried out to validate the theory. The experimental sensitivity coefficient of long-scale sensor is O.35pm/&Vegr; with the coefficient of enhancing sensitivity is K=3.4286. The experimental sensitivity coefficient of high-precision sensor is 1.96pm/&mgr;&Vegr; with the coefficient of enhancing sensitivity is K=0.61 . Comparing the experimental results with the theory results, it proves that the two types of strain sensors can really reflect the strain status and the damage information of the structure, and it is stable and reliable for the practical project.

Pressure Sensitization of Brillouin Frequency Shift in Optical Fibers With Double-Layer Polymer Coatings

This paper performs a theoretical and experimental analysis of the pressure sensitivity of Brillouin frequency shift (PSoBFS) in silica optical fibers with double-layer polymer coatings. It is predicted numerically that the coated fibers have enhanced (ideally over 20 times) PSoBFS as compared with a bare fiber. In addition, the enhancement becomes obvious while the outer coating has a lower Youngs modulus and Poissons ratio, as well as larger thickness. Three fibers with different outer coatings are experimented via the Brillouin optical time domain analysis technique. The experimental results agree with the theoretical prediction. This quantitative evaluation can be guidable for designing BFS-based pressure sensors with coated fibers.

Homogeneous (Cu, Ni) 6 Sn 5 intermetallic compound joints rapidly formed in asymmetrical Ni/Sn/Cu system using ultrasound-induced transient liquid phase soldering process

Homogeneous (Cu, Ni)6Sn5 intermetallic compound (IMC) joints were rapidly formed in asymmetrical Ni/Sn/Cu system by an ultrasound-induced transient liquid phase (TLP) soldering process. In the traditional TLP soldering process, the intermetallic joints formed in Ni/Sn/Cu system consisted of major (Cu, Ni)6Sn5 and minor Cu3Sn IMCs, and the grain morphology of (Cu, Ni)6Sn5 IMCs subsequently exhibited fine rounded, needlelike and coarse rounded shapes from the Ni side to the Cu side, which was highly in accordance with the Ni concentration gradient across the joints. However, in the ultrasound-induced TLP soldering process, the intermetallic joints formed in Ni/Sn/Cu system only consisted of the (Cu, Ni)6Sn5 IMCs which exhibited an uniform grain morphology of rounded shape with a remarkably narrowed Ni concentration gradient. The ultrasound-induced homogeneous intermetallic joints exhibited higher shear strength (61.6 MPa) than the traditional heterogeneous intermetallic joints (49.8 MPa).

Design and development of a multi-hole broadband-based ultrasonic transducer.

In order to improve the efficiency of ultrasonic energy transformed from electricity for an ultrasonic transducer array, a novel 1/2 wavelength multi-hole broadband-based transducer was designed, developed and evaluated. The low equivalent mass of the transducer is realized in this work through drilling holes on the output end of the horn. In comparison with a traditional transducer, the developed transducer has demonstrated a lower mechanical quality coefficient and a wider broadband. As a result, an ultrasound treatment system for crude oil has been developed based on the new transducer design. Furthermore, we have demonstrated the effectiveness of the ultrasound treatment system on viscosity reduction of crude oil and paraffin.

Measurement of a piezoelectric transducer's mechanical resonant frequency based on residual vibration signals

The paper presents a novel measurement method for a piezoelectric transducer mechanical resonant frequency. A transducer natural frequency is obtained through analyzing residual vibration signals spectrum based on the Morlet wavelet transform, and subsequently the transducer mechanical resonant frequency is calculated against the relationship of the natural frequency and the mechanical resonant frequency, i.e. a novel scheme being termed as twice exciting/sampling in this work. Experiments carried on a transducer (the mechanical resonant frequency was 19.72 kHz) have shown that the accuracy was up to 0.18% under the condition of the exciting frequency being set manually within 15∼29 kHz. The constructed measurement system has advantages of simple structure and low cost.

Research on the Temperature Compensation Technology of Quasi-Distributed Fiber Bragg Grating

Optical fiber sensor technology is more and more widely applied in the health monitoring testing of the major projects and infrastructure, because its sensing elements possess characteristics like the small size, high durability, absolute measurement and distributed monitoring. With the development of the optical fiber communication technology, several different kinds of passive devices appear continuously. Fiber Bragg Grating (FBG for short) develops continuingly and speedily in the field of optical fiber communication and sensing technology, owing to its good properties, such as low insertion loss, wavelength absolute coding, being independence on polarization, the flexible adjustment of wavelength and bandwidth and easily connect to fiber. But Fiber Bragg Grating is sensitive about two parameters — the changing temperature and the outside strain. Therefore, cross sensitivity becomes the hot issue. This paper will introduce the temperature compensation technology of the strain monitoring system for a quasi-distributed Fiber Bragg Grating.Copyright

A Novel Method of Stabilizing the Vibration Velocity at the End of Piezoelectric Transducers

The vibration velocity at the end of a piezoelectric transducer is difficult to be stabilized. This paper theoretically analyzes the relationship of its admittance and frequency, and the main reasons of over voltage and over current caused by the traditional current-based stabilizing vibration velocity method. In light of the above, the authors present a novel method, which takes advantages over the traditional one in terms of the time and the amplitude of the overshoot voltage or current. Accordingly, the transducer and its generator can be effectively protected.