Temperature Wireless Measurement - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 15, NO. 2, FEBRUARY 2015

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An Embedded Passive Resonant Sensor Using Frequency Diversity Technology for HighTemperature Wireless Measurement Chen Li, Qiulin Tan, Wendong Zhang, Senior Member, IEEE, Chenyang Xue, and Jijun Xiong

Abstract— This paper presents an embedded wireless passive temperature sensor for measurements in high-temperature applications, such as compressors and turbine engines. The performance of the sensor was improved by optimizing its performance parameters. A high-temperature-resistant material was used, and an embedded structure design was introduced to enable the sensor to operate in high-temperature environments. A series LC resonant circuit containing a fixed inductance coil and variable capacitance that varies with temperature was embedded in an alumina ceramic substrate using high-temperature cofired ceramic technology. The temperature in the high-temperature environment was detected wirelessly via the frequency diversity of the sensor. Furthermore, the experimental results showed that the sensor can measure temperatures ranging from room temperature to 1000 °C, and the average sensitivity of the sensor is ∼2 KHz/°C. Index Terms— Wireless measurement, embedded LC resonant sensor, HTCC technology, passive temperature sensor.

I. I NTRODUCTION N ORDER to prevent the disastrous effects of hightemperature environments and to monitor temperature parameters in production processes, temperature sensors are increasingly used in many applications such as jet engines, high-speed shafts, disc brakes, aircraft engines, and space shuttles [1]–[4]. The application of temperature sensors can increase the safety and reliability of production processes. Currently, conventional sensors require a battery power supply for acquiring signals in monitoring systems, thus resulting in increased complexity. Further, the measurement system cannot be easily modified and maintained [5]. Wireless temperature measurement technology is suitable for temperature measurement in harsh environments.

I

Manuscript received June 7, 2014; revised September 19, 2014; accepted September 22, 2014. Date of publication September 26, 2014; date of current version November 26, 2014. This work was supported in part by the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, in part by the Shanxi Scholarship Council of China under Grant 2013-077, in part by the National Natural Science Foundation of China under Grant 61335008, in part by the National Science Fund for Distinguished Young Scholars under Grant 51225504, and in part by the Graduate Students Outstanding Innovation Project in Shanxi Province under Grant 20143020. The associate editor coordinating the review of this paper and approving it for publication was Dr. M. R. Yuce. The authors are with the Science and Technology on Electronic Test and Measurement Laboratory, Key Laboratory of Instrumentation Science and Dynamic Measurement, Ministry of Education, North University of China, Taiyuan 030051, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2014.2360392

Therefore, the development of wireless sensing technology has been attracting significant interest. Wireless passive temperature sensors can not only satisfy some strict requirements but also have many incomparable advantages, especially in some special environments such as rotating parts and sealing parts. Typical commercial wireless passive temperature sensors include high-temperature optical sensors, surface acoustic wave (SAW) sensors,and RF-powered LC sensors. High-temperature optical sensors are typically small, light, and inexpensive, and the operating temperature can reach up to 450 °C. However, the accuracy, sensitivity, and test range of the sensors are limited, and they cannot be easily employed in hostile environments such as those with radiation and electromagnetic interference [6]–[8]. SAW sensors are used not only for measuring temperature but also for chemical, pressure, humidity, force, and electric measurement by monitoring phase variations. This measurement technology can be applied in some harsh environments. However, the speed of sound depends on the environment, geometry, and temperature, which usually vary in other harsh environmental conditions. Therefore, SAW sensors have limited applications [9]–[11]. Wireless passive sensors based on the LC resonance principle have many advantages such as simple structure and ease of implementation and detection. Further, RF-powered LC wireless passive temperature sensors have been widely studied by researchers in recent years. In 2008, Wang et al. of Puerto Rico University proposed an LC resonant sensor that can wirelessly detect temperature. The sensor contains discrete inductance and capacitance components, which cannot be easily miniaturized, and it can work only in harsh environments up to 235 °C [5]. In 2014, Tan et al. designed a harshenvironment-oriented wireless passive temperature sensor using low-temperature cofired ceramic (LTCC) technology. The sensor can wirelessly detect temperature parameters in harsh environments. However, the sensor can only operate up to 700 °C [12]. In this study, an embedded wireless passive temperature sensor based on frequency diversity technology and hightemperature cofired ceramic (HTCC) technology is proposed. The temperature signal is transmitted to an antenna by mutual inductance coupling, and the temperature information can then be acquired via data processing, as shown in Fig. 1. The proposed wireless passive sensor can sense temperatures in harsh environments.

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Fig. 3. Fig. 1.

Cross-sectional view of the coil sensor.

Proposed wireless temperature measurement system.

The sensor is designed to have an electrical LC resonant circuit with a corresponding resonance frequency represented as: fs =

1 , √ 2π L S C S

(1)

And the quality factor of the sensor is represented as: 1 Ls QS = , (2) Rs C s

Fig. 2.

Structural diagram of temperature testing system.

The rest of the paper is organized as follows. Section II presents the measurement principle and model analysis of the sensor and the optimal design parameters of the proposed sensor. In Section III, the fabrication of the proposed sensor having the embedded structure, in which the inductance and capacitance were integrated onto a ceramic substrate using HTCC technology, is discussed. The physical and chemical stability of the structural and electrical materials ensure that the sensor can operate in high-temperature environments. In Section IV, the high-temperature performance of the proposed sensor is investigated, and the conclusions are presented in Section V. II. P RINCIPLE OF M EASUREMENT AND M ODEL A NALYSIS A. Temperature Measurement Principle In order to test the wireless capability of the LC resonant sensor, the coupling coil method is employed, in which the resonance frequency of the sensor is obtained via the mutual inductance coupling between the test antenna coil and the sensor. A schematic diagram of the sensor, which shows an LC circuit that is powered by a reader antenna formed by a constant inductance coil (plane spiral inductance) and a variable capacitance, is shown in Fig. 2. The formulas in Fig. 2 indicate that the dielectric constant of the ceramic substrate varies if the temperature on the sensor is altered. The variation in the dielectric constant affects the capacitance of the capacitor and is then translated into a resonant-frequency shift of the sensor. Finally, the detector antenna detects the temperature signal via wireless coupling between the inductance coils of the antenna and sensor. The impedance characteristics viewed from the test antenna can be detected by the impedance analyzer, as shown in Fig. 2.

Where RS , L S , and CS denote the capacitance, inductance, and resistance of the sensor, respectively. Further, we can determine the variation in the sensor resonance frequency with temperature to analyze the variations in the measured temperature parameters. The input impedance (Z p ) can be expressed as follows [13], [14]: ⎡ ⎤ f ⎢ ⎥ V1 fs ⎢ ⎥ Zp = = j 2π f L p ⎢1 + k 2 ⎥ 2 ⎣ I1 f 1 f ⎦ 1− + j fs Q s fs = F( f s ), (3) where V1 and I1 are the voltage and current of the testing antenna, respectively; L p is the inductance of the antenna, f is the excitation frequency, and k is the coupling coefficient between the inductance coils of the antenna and the sensor. B. Sensor Model Analysis The cross section of the sensor model consisting of three sections is shown in Fig. 3. The electrical components of the sensor are embedded in the ceramic substrate and enclosed by a stack of diaphragms, which prevent the electrical components from exposure to the external environment. The capacitor is electrically connected to the square-spiral-type inductor. The capacitor plates and inductor are electrically connected with a metalized via. These components form a passive LC resonator with a resonant frequency f 0 , which is dependent on temperature. In order to obtain more inductance coils in the square portion of the ceramic substrate to improve the coupling strength between the sensor and the antenna, the inductance coil is designed to be a square. The design of the planar spiral inductor is shown in Fig. 4. The square planar spiral inductance can be expressed as [15] L=

μ0 n 2 davg c1 c2 (ln( ) + c3 ρ + c4 ρ 2 ), 2 ρ

(4)

LI et al.: EMBEDDED PASSIVE RESONANT SENSOR USING FREQUENCY DIVERSITY TECHNOLOGY

Fig. 6.

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Schematic diagram of the capacitance plate. TABLE I

G EOMETRICAL PARAMETERS OF THE I NDUCTOR AND C APACITOR

Fig. 4.

Fig. 5.

Schematic diagram of the planar spiral inductor.

Quality factor versus the ratio of the inner and outer diameters.

where davg = ((din + dout )/2) denote the average diameter of the inductance coil. Further, μ0 is the permeability of vacuum, ρ = (dout − din )/(dout + din ) is the ratio of the inner and outer diameters of the inductance coil, and c1 , c2 , c3 , and c4 are the fit parameters, which depend on the shape of the planar spiral inductors. The quality factor Q of the planar spiral inductor is one of the key parameters affecting the sensing performance of the sensor. For a given number of inductance coils of a specific material, the width, and height, the maximum Q can be obtained by increasing the ratio of the inner and outer diameters of the inductance coil. Q can expressed as [16] Q=

1+α 1+α w0 μdhw(1 − α) (ln + 0.2235 + 0.726), 4πρr s 1−α 1−α (5)

where w and μ denote the angular frequency of the sensor and the relative permeability, respectively; and ρr is the resistivity of the material. Further, d, h, α, and s are the coil diameter, the height of the inductance coil, the ratio of the inner and outer diameters of the inductance coils, and the spacing between adjacent inductance coils, respectively. The simulated values of Q for the inductance coil as a function of α are shown in Fig. 5. From Fig. 5, we can see that Q of the inductance coil is a maximum when α is approximately 0.3. Considering the resonance-frequency requirement, the fabrication process, and the size limitations of the sensor, the outside and inside diameters of the sensor are designed to be 31 mm and 9 mm, respectively, and the quality factor of the sensor is approximately 78. Further, the line width

and line spacing of the inductance are set to be 300 μm and 300 μm, respectively, and the number of the inductance coils is 12. Fig. 6 shows a schematic diagram of the capacitance plate. The sensitive capacitor varies with temperature and consists of an alumina ceramic dielectric sandwiched between two metal plates. The dielectric constant of the alumina material varies as the temperature increases, thus resulting in a variation in the capacitive element of the sensor. The relationship between the dielectric constant and the temperature and capacitance of the sensor can be expressed as follows: ε0 a 2 εr (T ), (6) d where εr is the relative dielectric constant of the alumina ceramic material, which is a function of temperature; and ε0 is the permittivity of free space. Further A is the ratio of the plate area to the plate spacing of the capacitance: C S (T ) =

a2 . (7) d The resonance frequency of the sensor varies gradually with A. If A is designed to be too small, the sensor resonance frequency will increase, and the parasitic inductance and capacitance cannot be ignored in the testing process. To improve the detection sensitivity, the sensor can be designed to achieve a higher A. As mentioned above, the structural parameters of the sensor including the capacitance plate radius, the inductance coil, and the height of the sensitive membrane were designed to improve the performance of the sensor. The specific geometrical parameters of the capacitor and inductor are listed in Table I. A = ε0

III. FABRICATION Alumina is a good insulator and chemically stable; therefore, it can be used in high-temperature environments. The sintering temperature of alumina is between 1600°C and 1700°C. The sintering temperature of conventional

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Fig. 8.

Fig. 7.

Temperature process control for curing.

Temperature-sensor fabrication process.

thick-film conductive metals such as silver, gold, platinum, and copper is low; thus, they are not suitable as sintering conductive materials for the sensor. Therefore, the conductive paste, made with metals such as Wolfram (W) and Molybdenum (Mo), used for cofiring with the alumina ceramic must be have a high melting point. Tungsten matches well with the alumina ceramic, and the shrinkage of W is consistent with that of alumina. Further, W exhibits good electrical performance after sintering; however, it oxidizes in oxygen environments when the sintering temperature is 300 °C and can only be used in an inert-gas environment. In order to solve these problems, an embedded structure design, in which the capacitance and inductance are embedded in the ceramic substrate, was proposed. This ensures that the electrical properties of the element based on W are not affected by the external environment. The inductance and capacitance were integrated onto the ceramic substrate using HTCC technology, which involves processes such as cutting, drilling, via filling, capacitor electrode and inductor screenprinting, stacking, isostatic lamination, and high-temperature firing, to obtain the LC resonant circuit. Alumina ceramic tapes and compatible tungsten pastes were used for sensor fabrication. A part of the fabrication process is shown in Fig. 7. 1) Drilling, Filling, and Screening: The alumina cast tapes were cut into 6-inch squares and placed in a drying oven for approximately 30 min for pretreatment. The alignment hole and via of the sensor were created using an Nd:YAG micromachining laser system (Rofin Standard). The alignment hole was used to accurately laminate the cast tapes, and the via was used to establish a metal connection between the ceramic cast layers. After the drilling process, the via was filled with W paste to provide a metal interconnection between the inductance and capacitance of the ceramic tape layers. After the filling process, the inductance coil and capacitor plates were silk-screen printed using the implemented screening printing system. Because the tapes and pattern coincided at a fixed position on the screen-printing plate, they were aligned. The metal plates (acting as the capacitor plate) and the

Fig. 9.

Sensor and SEM image of the sensor sample surface.

surrounding spiral metal wires (acting as the planar inductor) were then screen-printed onto the tapes according to the previous design. After screen-printing, the ceramic tapes were placed in a drying oven at 100 °C in air for approximately 5 min for thermal treatment. 2) Lamination: All ceramic tape layers were stacked together according to the design requirement, as shown in Fig.7 (2). The ceramic tape layers were laminated at a pressure of 21 MPa for 15 min. The ceramic tape layers were then tightly bonded to form a complete ceramic substrate that cannot be separated into individual layers. 3) Cofiring: After the lamination process, the ceramic substrate was placed in a drying oven at 70 °C for approximately 10 min for thermal treatment. Thereafter, the ceramic substrate was sintered in an inert-gas environment at a peak temperature of 1630 °C for 2 h for a total firing time of approximately 27 h for ceramic substrate curing. The specific sintering cure curve is shown in Fig. 8. The heating rate was adjusted to be 1 °C/min between room temperature and 380 °C, which ensured the complete removal of the ceramic internal solvent, binder, and residual carbon, and W was not oxidized in the process. The heating rate was set to be 2 °C/min between 380 °C and 600 °C to ensure proper combination of W and the ceramic. The temperature was then increased up to 1630 °C and maintained for approximately 2 h to ensure the completion of the curing reaction of W and the ceramic substrate. The final fabricated sensor element and an SEM image of the sintered sample are shown in Fig. 9. As can be observed, the pore number of the sintered sample surface is at a minimum after high-temperature sintering, and the ceramic forms a dense alumina ceramic.

LI et al.: EMBEDDED PASSIVE RESONANT SENSOR USING FREQUENCY DIVERSITY TECHNOLOGY

Fig. 10.

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High-temperature measurement system setup.

Fig. 12.

Fig. 11. Overlay plots of (a) impedance phase versus temperature and (b) impedance magnitude versus temperature.

IV. R ESULTS AND D ISCUSSION To investigate the high-temperature characteristics of the proposed sensor, a temperature measurement system was built, as shown in Fig. 10. The measurement system comprises an E4991A Agilent impedance analyzer, a computer, and a high-temperature sintering furnace. The temperature of the sintering furnace was accurately controlled with the computer within a range from room temperature to 1200 °C. During the measurement procedure, the proposed sensor testing is conducted using a copper spiral coils as the reader antenna, where the outer diameter of the inductor coil is 35 mm, and the number of turns is 6. The antenna and sensor are separated by 1 cm through the heat insulation device, which was placed in the high-temperature sintering furnace. Then the sensor works in high-temperature environments for signal collection and the reader antenna is connected to the impedance analyzer E4991A for signal reading. A set of experimental tests were conducted by varying the temperature of the high-temperature sintering furnace. Fig. 11 shows the impedance characteristic curves of the testing antenna at room temperature and the testing antenna coupled with the sensor for different temperatures. We can see that the measured impedance phase is approximately 90°

Resonance frequency versus temperature.

when the sensor is not coupled with the sensor because the test antenna is an inductance coil with a very small resistance, and the impedance phase of the pure inductance is 90°. When the sensor is close to the testing antenna, and the frequency signal is approximately equal to the resonant frequency of the sensor, the impedance varies abruptly. This is because inductor coupling occurs between the testing antenna and the sensor, and the resonance frequency of the LC resonant sensor is approximately 27.6 MHz. According to Reinhard, the lowest point of the phase angle is not the resonance frequency of the sensor but nearly the resonance frequency, and the deviation is very small [17]. Therefore, we can use the lowest point to characterize the resonance frequency of the sensor. The resonance frequency of the sensor shifts toward low frequencies as the temperature increases. When the temperature is 1000 °C, the resonance frequency of the sensor changes to 24.46 MHz because the dielectric constant of the alumina material varies as the temperature increases, thus resulting in the variation in the capacitive element of the sensor. Finally, the variation in the capacitance is translated into a variation in the resonance frequency of the sensor. The curves of sensor resonance frequency versus temperature corresponding to numerous repetitive high-temperature tests are shown in Fig. 12. We can see that the resonant frequency of the sensor decreases as the temperature increases, and the repeatability error of the sensor is only approximately 3%. Using a 2nd degree polynomial fitting, we can obtain the relationship between the resonance frequency of the sensor and the temperature in harsh environments, which is expressed as F(MHz) = −1.825T 2 − 227.65T + 26.6817. Therefore, the sensor sensitivity is approximately 2 KHz/°C between room temperature and 1000 °C. And, after a set of high temperature experiments on the high-temperature measurement system setup, we can observe the minimum temperature variation detectable with the sensor is 0.5°C, and the overall temperature accuracy obtained from the sensor was approximately 1°C. V. C ONCLUSION This paper presents an embedded wireless passive LC resonant temperature sensor for high-temperature environments. In order to improve the performance of the sensor,

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the capacitance and inductance of the sensor were optimized. The sensor was fabricated with a high-temperature-resistant material, and an embedded structure design was introduced to ensure that the sensor can be employed in high-temperature environments. A planar spiral inductor and two capacitor plates were integrated into an alumina ceramic substrate using HTCC technology, and the inductor and capacitor are in series. The wireless temperature measurement ability of the fabricated sensor was tested using frequency diversity technology, and the limitations of high-temperature measurements were demonstrably overcome. Further, the experimental results show that the sensor can be used within a temperature range from room temperature to 1000 °C, and the sensitivity of the sensor is approximately 2 kHz/°C. R EFERENCES [1] C. Mandel, M. Schussler, and R. Jakoby, “A wireless passive strain sensor,” in Proc. IEEE Sensors Conf., Oct. 2011, pp. 207–210. [2] Y. Zhang, G. R. Pickrell, B. Qi, A. Safaai-Jazi, and A. Wang, “Singlecrystal sapphire-based optical high-temperature sensor for harsh environments,” Opt. Eng., vol. 43, no. 1, pp. 157–164, 2004. [3] D. J. Young, J. Du, C. A. Zorman, and W. H. Ko, “High-temperature single-crystal 3C-SiC capacitive pressure sensor,” IEEE Sensors J., vol. 4, no. 4, pp. 464–470, Aug. 2004. [4] O. J. Gregory and T. You, “Ceramic temperature sensors for harsh environments,” IEEE Sensors J., vol. 5, no. 5, pp. 833–838, Oct. 2005. [5] Y. Wang, Y. Jia, Q. Chen, and Y. Wang, “A passive wireless temperature sensor for harsh environment applications,” Sensors, vol. 8, no. 12, pp. 7982–7995, Oct. 2008. [6] S. Tao, J. C. Fanguy, and T. V. S. Sarma, “A fiber-optic sensor for monitoring trace ammonia in high-temperature gas samples with a CuCl2 -doped porous silica optical fiber as a transducer,” IEEE Sensors J., vol. 8, no. 12, pp. 2000–2007, Dec. 2008. [7] N. A. Riza and M. A. Arain, “Cryogenic temperature measurement using silicon carbide-based wireless optical sensor,” IEEE Photon. Technol. Lett., vol. 18, no. 24, pp. 2599–2601, Dec. 15, 2006. [8] A. B. Murphy, “Laser-scattering temperature measurements of a freeburning arc in nitrogen,” J. Phys. D, Appl. Phys., vol. 27, no. 7, pp. 1492–1498, 1994. [9] P. Zheng, T.-L. Chin, D. Greve, I. Oppenheim, V. Malone, and L. Cao, “High-temperature langasite SAW oxygen sensor,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 58, no. 8, pp. 1538–1540, Aug. 2011. [10] J. H. Hines, “Review of recent passive wireless saw sensor and sensortag activity,” in Proc. IEEE 4th Annu. Conf., Montreal, QC, Canada, Jun. 2011, pp. 1–2. [11] D. Chernenko, M. Zhovnir, B. Tsyganok, and O. Oliinyk, “Wireless passive pressure sensor using frequency coded SAW structures,” in Proc. 35th Int. Spring Seminar Electron. Technol., May 2012, pp. 424–428. [12] Q. Tan at al., “A harsh environment-oriented wireless passive temperature sensor realized by LTCC technology,” Sensors, vol. 14, no. 3, pp. 4154–4166, Mar. 2014. [13] P.-J. Chen, D. C. Rodger, S. Saati, M. S. Humayun, and Y.-C. Tai, “Microfabricated implantable parylene-based wireless passive intraocular pressure sensors,” J. Microelectromech. Syst., vol. 17, no. 6, pp. 1342–1351, Dec. 2008. [14] P.-J. Chen, S. Saati, R. Varma, M. S. Humayun, and Y.-C. Tai, “Wireless intraocular pressure sensing using microfabricated minimally invasive flexible-coiled LC sensor implant,” J. Microelectromech. Syst., vol. 19, no. 4, pp. 721–734, Aug. 2010. [15] S. S. Mohan, M. del Mar Hershenson, S. P. Boyd, and T. H. Lee, “Simple accurate expressions for planar spiral inductances,” IEEE J. Solid-State Circuits, vol. 34, no. 10, pp. 1419–1427, Oct. 1999. [16] J. Zhai, T. V. How, and B. Hon, “Design and modelling of a passive wireless pressure sensor,” CIRP Ann.-Manuf. Technol., vol. 59, pp. 187–190, Mar. 2010. [17] R. Nopper, R. Niekrawietz, and L. Reindl, “Wireless readout of passive LC sensors,” IEEE Instrum. Meas. Mag., vol. 59, no. 9, pp. 2450–2457, Sep. 2010.

Chen Li was born in 1987. He received the B.S. and M.S. degrees in dynamic testing technology and intelligent instruments from the North University of China, Taiyuan, China, in 2011 and 2014, respectively, where he is currently pursuing the Ph.D. degree in measurement technology and instrument. His current research interest is the research of the preparation technology of high-temperature sensor.

Qiulin Tan was born in Hunan, China. He received the B.S., M.S., and Ph.D. degrees from the North University of China (NUC), Taiyuan, China. He is currently with the National Key Laboratory, NUC, and the National Ministry of Education Key Laboratory. His research interest is in microsystem integration technology and microelectromechanical systems technology research.

Wendong Zhang (SM’95) was born in 1962. He received the M.S. degree and the Ph.D. degree in electrical engineering in 1986 and 1995, respectively. He is an IEEE Senior Member. He is currently a Professor with the North University of China, Taiyuan, China. His research interests include electrical measurement technology, and microsystems design and testing. From 1996 to 1998, he was a Post-Doctoral Researcher with Tsinghua University, Beijing, China. From 1998 to 2001, he conducted researches at the University of California at Berkeley, Berkeley, CA, USA, and the National Measurement Laboratory, Japan. In 2006, he conducted researches at École Normale Supérieure, Paris, France. His recent research interests lie in the fields of dynamic measurement, intelligent instruments, and micro and nano electromechanical systems.

Chenyang Xue received the master’s and Ph.D. degrees from the National Technical University of Athens, Athens, Greece, in 1997 and 2003, respectively. Recently, he conducted projects sponsored by State 863 Nature Funds and National Nature Funds. He is currently the Vice Director of the National Key Laboratory for Electric Measurement Technology, North University of China, Taiyuan, China. His research focuses on the accelerometers, hydrophone, and acoustic sensor.

Jijun Xiong was born in 1971. He received the B.S. and M.S. degrees from the North University of China, Taiyuan, China, in 1993 and 1998, respectively, and the Ph.D. degree from Tsinghua University, Beijing, China, in 2003. He is currently a Professor and Doctoral Tutor. He is devoted to the fundamental theory and instrumentation research on dynamic mechanical parameters measurement for more than 20 years, mainly focused on the scale effect in silicon microstructures based on the typical electromechanical conversion principle, the evolution law in the nanometer scale, and the applicable boundary scope under thermal environment.