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An integrated sensor with three elements (zinc oxide, tin oxide and tungsten oxide) was fabricated by thick film techniques in order to develop a smell sensor. Using this sensor and pattern recognition method, the possibility of identifying 15 chemical compounds which belong to the alcohol, ester, ketone, benzene and hydrocarbon group was examined. The following results were obtained: All 15 compounds have different patterns, so they can be individually identified; compounds which have the same functional groups have similar patterns; and, when gas sensitivity of three elements is displayed in a three‐dimensional space, the compounds with the same functional group form a specific closed space. This indicates that the sensor can identify functional groups of chemical compounds.

This review aims to give an overview about zinc oxide (ZnO) based gas sensors and the role of doping in enhancing the gas sensing properties. Gas sensors based on ZnO thin film are preferred for sensing applications because of their modifiable surface morphology, very large surface-to-volume ratio and superior stability due to better crystallinity. The gas detection mechanism involves surface reaction, in which the adsorption of gas molecules on the ZnO thin film affects its conductivity and reduces its electrical properties. One way to enhance the gas sensing properties is by doping ZnO with other elements. A few of the common and previously used dopants include tin (Sn), nickel (Ni) and gallium (Ga).

In this brief review, previous works on doped-ZnO formaldehyde sensing devices are presented and discussed.

Most devices provided good sensing performance with low detection limits. The reported operating temperatures were within the range of 200̊C –400̊C. The performance of the gas sensors can be improved by modifying their nanostructures and/or adding dopants.

As of yet, a specific review on formaldehyde gas sensors based on ZnO metal semiconductors has not been done.

The University of Southampton has been active in the area of thick‐film sensors since their initial conception through to the present. Recent research at the university has concerned the use of thick‐film sensor arrays for the discrimination of chemical species in both gaseous and dissolved form. In addition, the detection of many physical parameters is now being addressed through the use of arrays of sensing elements with a view to improving on factors such as noise immunity, environmental cross‐sensitivity and long‐term accuracy. In the area of chemical sensing, extensive use has been made of thick‐film technology to allow low‐cost arrays of chemical sensors to be fabricated. The lack of specificity exhibited by the individual sensing elements has been demonstrably overcome through the use of signal processing techniques applied to the outputs of the array of sensors. Thick‐film chemical sensor research currently under way at Southampton includes a UK DTI/SERC funded LINK project concerning dissolved species monitoring for water quality assessment. Additionally, gas sensor arrays for the detection of toxic and flammable gases are being explored as part of a well established ongoing research programme. The use of thick‐film technology for the fabrication of physical sensors has been extensively documented. Current research at the University of Southampton includes an industrially sponsored project involving the use of thick‐film strain sensing resistors in the design of an accelerometer. The use of Z‐axis piezoresistivity and an array approach to solving noise and drift problems is seen as a significant novelty in this work.

In the surface mount industry, microelectronic devices are reflow soldered to printed circuit boards with the benefit of mildly activated rosin (RMA) based fluxes. The residues from these fluxes, when not properly cleaned from the component boards, have been cited for decreased circuit life due to corrosion of the solder joints and loss of insulating resistance. Post‐solder cleaning operations with CFC (chlorofluorocarbon) solvents have been deemed environmentally harmful. Hence, there is a great need in the surface mount community for a no‐clean or fluxless solder reflow process. The BOC Group has developed a novel, proprietary process, by which circuit boards and their components are attached with a solder paste under a reactive fluxing atmosphere. The post‐solder residue is non‐corrosive and so minimal that it does not require a post‐solder cleaning operation. The solder joints exhibit good wetting, excellent joint strength and are essentially void‐free. Assembled circuits processed in this way meet all the criteria for ionic cleanliness and surface insulation resistance without post‐solder cleaning.

Detection of toxic gases in an industrial environment is a growing problem. Laboratory analysis of samples involves delay, and the most suitable equipment would be continuously operating personal monitors. At present this can rarely be achieved, but research on metal oxide sensors points to the possibility of a number of applications in the near future.

The sequential electrochemical reduction analysis (SERA) method was evaluated in production and demonstrated to provide a non‐destructive, objective measure of the solderability of production printed wiring boards. Approximately 1000 boards were analysed just before wave soldering and the SERA parameters were correlated with the soldering defect occurrence rates. The data show that PWB solderability is determined primarily by the nature of the surface tin oxide, as reflected in the corresponding SERA plateau voltage, rather than by the oxide thickness. By proper choice of the value of the plateau voltage used as the criterion for solderability, the most advantageous trade‐off between the costs of rejected boards and the rework of defects can be made.

The detection of H₂ concentrations in concentrations undetectable by the conventional detection method of surface acoustic wave (SAW) sensors based on frequency shift, by correlating analyte presence with Fourier spectra components.

Fast Fourier Transform (FFT) and autocorrelation analysis of phase noise in a SnO2-coated SAW sensor was performed. Fourier spectra were obtained by FFT from the SAW sensor resonance frequency instability, in the absence of analyte, and for H₂ concentrations between 0.08 and 0.4 per cent.

All analyte concentrations are below the sensor limit of detection, which is 0.8 per cent for H₂. Although these analyte concentrations caused no significant change in the resonance frequency of the SAW resonator, the FFT spectra presented several modifications, namely, the appearance of a new peak and the decrease of randomness. The authors consider that the effect is because of the chaotic behavior of the temporal dependence of the SAW resonance frequency. This explanation is substantiated by the decrease observed in the SAW oscillator autocorrelation function, which is an indication for a chaotic behavior.

As chaotic systems are extremely sensitive to perturbation, measurement methods based on chaos diagnosis could potentially greatly improve the SAW detection.

Fourier spectra components were correlated with analyte presence in concentrations undetectable by the conventional SAW detection method based on frequency shift.

The purpose of this work is to explore the forms of intermetallic phase compounds (IMPCs) in Pt/In/Au and Pt/In/Ag joints by using isothermal solidification. This lead-free technique leads to formation of IMPCs having high-temperature stable joints for platinum-based micro-heater gas sensor fabricated on low temperature co-fired ceramic (LTCC) substrate.

Proposed task is to make an interconnection for Pt micro-heater electrode pad to the silver and gold thick-films printed on LTCC substrate. Both Pt/In/Au and Pt/In/Ag configured joints with different interactive areas prepared at 190 and 220°C to study temperature and contact surface area effects on ultimate tensile strength of the joints, for a 20 s reaction time, at 0.2 MPa applied pressure. Those delaminated joint interfaces studied under SEM, EDAX and XRD.

IMPCs identified through material analysis using diffraction analysis of XRD data are InPt3, AgIn2, AgPt, AgPt3, Au9In4 and other stoichiometric compounds. The interactive surface area between thick-films and temperature increment shows improvement in the formations of IMPCs and mechanical stability of joints. These IMPCs-based joints have improved the mechanical stability to the joints to sustain even at high operating temperatures. Elemental mapping of the weak joint contact interface shows unwanted oxide formations also reported. Physical inter-locking followed by the diffusion phenomenon on the silver substrate strengthen the interconnection has been noticed.

Inert gas environment creation inside the chamber to isolate the lead-free joint placed between heating stamp pads to avoid oxide formations at the interface while cooling which adds up to the cost of manufacturing. Most of the oxides at a joint-interface increase minute to moderate resistance with respect to the level of oxides took place. These oxides contributed heat certainly damage the micro-heater based gas sensors while functioning.

These isothermal solidification-based lead-free solder joints formation replace the existing lead-based packaging techniques. These lead-free interconnections on ceramic or LTCC substrate are reliable and durable, especially those designed to work for heavy-duty engines, even at severe environment conditions.

Platinum micro-heater-based gas sensors handles over a wide-range of temperatures about 300 to 500°C. The specific temperature level of different oxide films (SnO2) on the micro-heater is capable of detecting various specific gases. This feature of platinum based gas sensor demands durable and mechanically stable joints for continuous monitoring.

The purpose of this paper is to synthesize SnO₂–ZnO hollow nanofibers, study their sensing properties and introduce an attractive candidate for formaldehyde detection in practice.

Pure and SnO₂–ZnO hollow nanofibers were synthesized by electrospinning method and characterized via X-ray diffraction, field-emission scanning electron microscopy and Fourier transform infrared spectroscopy. The formaldehyde-sensing properties were investigated.

The optimum performance was obtained at 260°C by the 14 at.% SnO₂–ZnO hollow nanofiber sensor. The sensor could detect formaldehyde down to 0.1 ppm with rapid response–recovery time (4-6 s and 7-9 s, respectively), high sensitivity, good selectivity and stability. The relationship between the sensor’s sensitivity and formaldehyde concentration suggests that the adsorbed oxygen species on the sensor’s surface is O2−. The prominent sensing properties are attributed to the one dimensional hollow nanofiber structures and the promoting effects of SnO₂.

The sensor fabricated from 14 at.% SnO₂–ZnO fibers exhibits excellent formaldehyde-sensing characteristics. It can be used for formaldehyde detection in practice.

The electrospinning method is a very simple and convenient method for fabricating hollow nanofibers and the sensing material is of low cost.

To the best of the authors’ knowledge, studies on formaldehyde sensing of SnO₂–ZnO hollow nanofibers have not been reported before.

Low temperature co-fired ceramics (LTCC) technology-based micro-hotplates are of immense interest owing to their ruggedness, high temperature stability and reliability. The purpose of this paper is to study the role of thermal mass of LTCC-based micro-hotplates on the power consumption and temperature for gas-sensing applications.

The LTCC micro-hotplates with different thicknesses are designed and fabricated. The role of thermal mass on power consumption and temperature of these hotplates are simulated and experimentally studied. Also, a comparison study on the performance of LTCC and alumina-based hotplates of equivalent thickness is done. A thick film-sensing layer of tin oxide is coated on LTCC micro-hotplate and demonstrated for the sensing of commercial liquefied petroleum gas.

It is found from both simulation and experimental studies that the power consumption of LTCC hotplates was decreasing with the decrease in thermal mass to attain the same temperature. Also, the LTCC hotplates are less power-consuming than alumina-based one, owing to their superior thermal characteristics (low thermal conductivity, 3.3 W/ [m-K]).

This study will be beneficial for designing hotplates based on LTCC technology with low power consumption and better stability for gas-sensing applications.