Guoliang Sun

Solid Potentiometric

A solid potentiometric CO₂ sensor has been developed using Li₃PO₄ as the electrolyte, Li₂TiO₃/TiO₂ as the reference electrode, and Li₂CO₃ as the sensing electrode. The Li₃PO₄ film was deposited on Al₂O₃ substrate by resistance heating evaporation. Two Pt electrodes with designed patterns were formed on the electrolyte film by micro fabrication technologies. The reference electrode and the sensing electrode were prepared by thick-film technology to cover part of the two Pt electrodes. Various properties were tested: sensitivity under different conditions, response and recovery properties, cross-interference performance, and stability. The values of ΔEMF/dec are 22 mV/dec, 45 mV/dec, and 78 mV/dec in accordance with the operating temperatures: 300°C, 400°C, and 500°C. At 500°C, the response time and recovery time are 30 s and 40 s, respectively, which is better than those of 400°C. When the thickness of Li₃PO₄ film is greater than 650 nm, the EMF output of the sensor changes little with change of the film thickness. From the results, it can be concluded that the optimal working temperature for the sensor is 500°C, and the optimal thickness of Li₃PO₄ is about 650 nm.

{\rm CO}_{2}

A potentiometric CO2 gas sensor based on Li3PO4 film with the thickness of 0.8 μm prepared by thermal evaporation method was developed. Au thin film with the thickness of 400 nm deposited by sputtering method was used as the metal electrodes of the sensor. Li2CO3 and Li2TiO3 with 10 mol. % TiO2 were used as the sensing and reference electrodes by screen printing the materials on the Au thin film electrodes, respectively. Response characteristics of the sensor to CO2 in the range of 250 ppm to 5000 ppm at different working temperatures were investigated. The electromotive force (EMF) values of the sensor were linearly dependent on logarithm of CO2 partial pressure at the temperatures between 420 °C and 530 °C. Dependence of response and recovery time, initial EMF, ΔEMF/dec on working temperature was presented. It can be found that the response time and recovery time reduced with the enhancement of working temperature and gradually reached to the limit when the temperature is above 500 °C. However, the maxi...

Sensor Using

A thin-film type potentiometric sensor has been prepared by the implementation of electro-beam evaporation, rf magnetron sputtering methods, and micromachining processes. Sn film was deposited on n-Si/SiO(2) (400 nm) substrate. A deposited LaF(3) film was applied as solid electrolyte and sputtered Pt film was used as the sensing electrode. The patterns of the Pt and LaF(3) were realized by the micromachining processes. The LaF(3) film was characterized by scanning electron microscopy and energy dispersive x ray. Saturated aqueous solutions were used to achieve controlled humidity environments. When the sensor was exposed to humidity environments, the electromotive force (EMF) of the sensor was examined. It was found that the sensor varies with the relative humidity (RH). The stable response curve was presented and non-Nernst behavior between the average EMF values and RH may be shown.

{\rm Li}_{3}{\rm PO}_{4}

Electromotive force (EMF) transient curves presented that the sensor showed good repeatable response in the humidity environments using ambient atmosphere as the carrier gas at different temperatures. The 90% response time and recovery time were within 40 s and 50 s, respectively. The sensor also presented stable response characteristics in 75.1% RH and 83.6% RH humidity environments using N(2), 5% O(2), and 50% O(2) as the carrier gases, respectively. The EMF always increased with the partial pressure of oxygen in certain relative humidity. However, the ΔEMF was decreased with the increase of O(2) content in the carrier gas under the condition of the variation of relative humidity from 75.1% to 83.6%. These phenomena revealed that the sensor was sensitive to water vapor without oxygen in the sample gas and too much water vapor had adverse effect on the response to oxygen. Non-Nernst behavior of the sensor was discussed in detail.

Film as the Electrolyte

Planar-type potentiometric CO₂ gas sensors using thermal evaporated Li₃PO₄ thin film as the solid electrolyte were fabricated. Al₂O₃ plates with rough and smooth surfaces were used as the substrates of the sensors. X-ray diffraction analysis, atomic force microscope and scanning electron microscope were used to characterize the Li₃PO₄ films. The sensing properties were investigated in the range of 500~5000 ppm CO₂ concentrations at 480 °C. Both the rough substrate sensor (rsensor) and the smooth substrate sensor (s-sensor) showed a good Nernst behavior. The output EMF of s-sensor showed a more stable signal than the r-senor. Response and recovery times of the r-sensor were 40 s and 75 s, and for the s-sensor they were 35 s and 60 s. The ΔEMF/decade values obtained from the r-sensor and s-sensor were 45 mV/decade and 55 mV/decade, respectively. It can be found that the Nernsts slop of the s-sensor was closer to the theoretically value. The results revealed that the substrate surface roughness may influence the characteristics of Li₃PO₄ film and the response properties of the sensors to CO₂.

Response characteristics of a potentiometric CO2 gas sensor based on Li3PO4 solid electrolyte using Au film as the electrodes

A solid-state gas sensor with the structure of Pt micro grid (sensing electrode)/ LaF3 (solid electrolyte)/ Sn, SnF2 (reference electrode) was developed for detecting oxygen at room temperature. In the oxygen sensor, Pt micro grid with the thickness of 60nm was fabricated on one side of the solid electrolyte (LaF3) wafer by lift-off process, which was used as a novel structure of sensing electrode for the purpose of changing the diffusion process of gas species and investigating the relationship between the sensing electrode dimensions and properties of the sensor. In order to form SnF2 layer on the other side of the LaF3 wafer, Sn film about 2µm thick was deposited on LaF3 wafer using sputtering technique first. Then the sensor cell was electrolysed at 0.2µA for 6 min in air at room temperature. The sensing mechanism was investigated in detail and it was found that the electromotive force(EMF) of the sensor depends on the partial pressure of O2. The sensing properties of the sensor were also tested.

Humidity response properties of a potentiometric sensor using LaF3 thin film as the solid electrolyte

LaF3 is attractive because of its possible application at room temperature, in addition to other advantages and it has been investigated for measuring various types of gases. A solid-state gas sensor with the structure of (working electrode) SO2,Pt/LaF3/Sn,SnF2 (reference electrode) was developed for detecting sulfur dioxide at room temperature. With numerical simulation, a series of working electrodes with different line widths were designed. Typical lift-off process was applied to produce the micro pattern of Pt working electrode layer on one side of the solid electrolyte (LaF3) wafer. The sensor cell was electrolysed in order to form SnF2 layer with about 3nm thickness at the interface between the Sn film and the LaF3 wafer. The EMF of the sensor was found to vary logarithmically with a change in sulfur dioxide partial pressure, following Nernsts equation. Experiments show the working electrode with different line width somewhat influences on performance of the gas sensor. The sensing mechanism of the sulfur dioxide sensor was also investigated.