Research on High-Resolution Miniaturized MEMS Accelerometer Interface ASIC.

High-precision microelectromechanical system (MEMS) accelerometers have wide application in the military and civil fields. The closed-loop microaccelerometer interface circuit with switched capacitor topology has a high signal-to-noise ratio, wide bandwidth, good linearity, and easy implementation in complementary metal oxide semiconductor (CMOS) process. Aiming at the urgent need for high-precision MEMS accelerometers in geophones, we carried out relevant research on high-performance closed-loop application specific integrated circuit (ASIC) chips. According to the characteristics of the performance parameters and output signal of MEMS accelerometers used in geophones, a high-precision closed-loop interface ASIC chip based on electrostatic time-multiplexing feedback technology and proportion integration differentiation (PID) feedback control technology was designed and implemented. The interface circuit consisted of a low-noise charge-sensitive amplifier (CSA), a sampling and holding circuit, and a PID feedback circuit. We analyzed and optimized the noise characteristics of the interface circuit and used a capacitance compensation array method to eliminate misalignment of the sensitive element. The correlated double sampling (CDS) technology was used to eliminate low-frequency noise and offset of the interface circuit. The layout design and engineering batch chip were fabricated by a standard 0.35 µm CMOS process. The active area of the chip was 3.2 mm × 3 mm. We tested the performance of the accelerometer system with the following conditions: power dissipation of 7.7 mW with a 5 V power supply and noise density less than 0.5 µg/Hz1/2. The accelerometers had a sensitivity of 1.2 V/g and an input range of ±1.2 g. The nonlinearity was 0.15%, and the bias instability was about 50 µg.

Potentiometric NO2 Sensors Based on Thin Stabilized Zirconia Electrolytes and Asymmetric (La0.8Sr0.2)0.95MnO3 Electrodes.

Here we report on a new architecture for potentiometric NO2 sensors that features thin 8YSZ electrolytes sandwiched between two porous (La0.8Sr0.2)0.95MnO3 (LSM95) layers-one thick and the other thin-fabricated by the tape casting and co-firing techniques. Measurements of their sensing characteristics show that reducing the porosity of the supporting LSM95 reference electrodes can increase the response voltages. In the meanwhile, thin LSM95 layers perform better than Pt as the sensing electrode since the former can provide higher response voltages and better linear relationship between the sensitivities and the NO2 concentrations over 40-1000 ppm. The best linear coefficient can be as high as 0.99 with a sensitivity value of 52 mV/decade as obtained at 500 °C. Analysis of the sensing mechanism suggests that the gas phase reactions within the porous LSM95 layers are critically important in determining the response voltages.

Efficient Electron Transfer through Interfacial Water Molecules across Two-Dimensional MoO3 for Humidity Sensing.

Resistive humidity sensors are required in flexible and integrated devices. Two-dimensional MoO3 offers a large interface area, enabling the modulation of its electrical properties over a wide range. In this study, 2D MoO3 was synthesized via liquid-phase exfoliation for humidity-sensing tests. In terms of high sensitivity, negligible hysteresis, linearity, and stability, the humidity-sensing performance of MoO3 is superior to those of other materials. The sensitivity reaches 9794 Ω/RH at 25 °C. The sensing mechanism of MoO3 was investigated by using impedance spectra and voltage-current scans under different humidity levels. The results indicate that the resistance change of MoO3 due to humidity originates from the interfacial conductance. Interfacial H2O adsorption induces efficient conducting paths via hydrogen bonding, decreases the potential barrier for electron transfer, and supplies additional electron states to the valence bands. In this study, electronic humidity sensing was investigated in depth, and a new perspective was proposed for electronic humidity sensing.

Influence of crystallite size on cation conductivity in faujasitic zeolites.

The influence of particle size on the ionic conductivity of ceramic materials is an active area of research, and novel effects are observed as particles approach the nanoscale in size. Zeolites are crystalline aluminosilicates with ion-exchangeable cations that are responsible for ionic conductivity at high temperatures. In this paper, we present systematic results for the first time of ionic conductivity in alkali metal ion-exchanged faujasitic zeolites with morphologies ranging from a zeolite membrane, micrometer-sized, submicrometer, and nanoparticles of zeolite. Using impedance spectroscopy in the range of 10 MHz to 0.1 Hz, we have obtained the activation energy (E(act)) of cation motion with these various morphologies in the temperature range of 525-625 °C. Overall, the E(act) decreases with Si/Al ratio. Surface modification of the zeolite particles was carried out with a silylating agent, which upon high temperature calcination should lead to the formation of a monolayer Si-O-Si film on the particle surface. This surface modification had minimal influence on the E(act) of micrometer-sized zeolites. However, E(act) increased rapidly as the zeolite particle approached the nanoscale. These observations led us to propose that, for the high-temperature, low-frequency (10(4)-10(5) Hz), long-range ionic conduction in zeolites, cation hopping across grain boundaries is relevant to ion transport, especially as the size of the crystallite approaches the nanoscale. Intergrain boundaries are more defective in the nanosized zeolite and contribute to the higher E(act).

Exploitation of unique properties of zeolites in the development of gas sensors.

The unique properties of microporous zeolites, including ion-exchange properties, adsorption, molecular sieving, catalysis, conductivity have been exploited in improving the performance of gas sensors. Zeolites have been employed as physical and chemical filters to improve the sensitivity and selectivity of gas sensors. In addition, direct interaction of gas molecules with the extraframework cations in the nanoconfined space of zeolites has been explored as a basis for developing new impedance-type gas/vapor sensors. In this review, we summarize how these properties of zeolites have been used to develop new sensing paradigms. There is a considerable breadth of transduction processes that have been used for zeolite incorporated sensors, including frequency measurements, optical and the entire gamut of electrochemical measurements. It is clear from the published literature that zeolites provide a route to enhance sensor performance, and it is expected that commercial manifestation of some of the approaches discussed here will take place. The future of zeolite-based sensors will continue to exploit its unique properties and use of other microporous frameworks, including metal organic frameworks. Zeolite composites with electronic materials, including metals will lead to new paradigms in sensing. Use of nano-sized zeolite crystals and zeolite membranes will enhance sensor properties and make possible new routes of miniaturized sensors.