Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

https://www.mdpi.com/1424-8220/10/6/5469/pdf

Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

In the recent past a great deal of research efforts were directed toward the development of miniaturized gas-sensing devices, particularly for toxic gas detection and for pollution monitoring. Though various techniques are available for gas detection, solid state metal oxides offer a wide spectrum of materials and their sensitivities for different gaseous species, making it a better choice over other options. In this article a critical parameter analysis of different metal oxides that are known to be sensitive to various gaseous species are thoroughly examined. This includes phase of the oxide, sensing gaseous species, operating temperature range, and physical form of the material for the development of integrated gas sensors. The oxides that are covered in this study include oxides of aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, indium, iron, manganese, molybdenum, nickel, niobium, ruthenium, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, and the mixed or...

Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review

Gas sensors operate by a variety of fundamentally different mechanisms1,2,3,4,5,6,7,8,9,10,11,12,13,14. Ionization sensors13,14 work by fingerprinting the ionization characteristics of distinct gases, but they are limited by their huge, bulky architecture, high power consumption and risky high-voltage operation. Here we report the fabrication and successful testing of ionization microsensors featuring the electrical breakdown of a range of gases and gas mixtures at carbon nanotube tips. The sharp tips of nanotubes generate very high electric fields at relatively low voltages, lowering breakdown voltages several-fold in comparison to traditional electrodes, and thereby enabling compact, battery-powered and safe operation of such sensors. The sensors show good sensitivity and selectivity, and are unaffected by extraneous factors such as temperature, humidity, and gas flow. As such, the devices offer several practical advantages over previously reported nanotube sensor systems15,16,17. The simple, low-cost, sensors described here could be deployed for a variety of applications, such as environmental monitoring, sensing in chemical processing plants, and gas detection for counter-terrorism.

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Miniaturized gas ionization sensors using carbon nanotubes

Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

/pdf/humidity-sensors-principle-mechanism-and-fabrication-q8mq7scozc.pdf

Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

Considerable scientific and technological efforts have been devoted to develop neuroprostheses and hybrid bionic systems that link the human nervous system with electronic or robotic prostheses, with the main aim of restoring motor and sensory functions in disabled patients. A number of neuroprostheses use interfaces with peripheral nerves or muscles for neuromuscular stimulation and signal recording. Herein, we provide a critical overview of the peripheral interfaces available and trace their use from research to clinical application in controlling artificial and robotic prostheses. The first section reviews the different types of non-invasive and invasive electrodes, which include surface and muscular electrodes that can record EMG signals from and stimulate the underlying or implanted muscles. Extraneural electrodes, such as cuff and epineurial electrodes, provide simultaneous interface with many axons in the nerve, whereas intrafascicular, penetrating, and regenerative electrodes may contact small groups of axons within a nerve fascicle. Biological, technological, and material science issues are also reviewed relative to the problems of electrode design and tissue injury. The last section reviews different strate- gies for the use of information recorded from peripheral interfaces and the current state of control neuroprostheses and hybrid bionic systems.

A critical review of interfaces with the peripheral nervous system for the control of neuroprostheses and hybrid bionic systems

Based on our experience of developing thick-film MnWO4 humidity sensors, a thin-film humidity sensor with nano-sized MnWO4 grains has been fabricated using the sol–gel technique. The thin-film sensor shows smaller humidity sensitivity than that compared to a thick-film sensor. However, it exhibits a fast response to humidity change and also has a very low temperature coefficient within the temperature range of 20–60°C. The absence of capillary pores in the well defined thin-films makes water condensation, and hence electrolytic conduction, impossible. The physically adsorbed multi-layered water molecules on the surface of the thin sensing film play a dominating role for the humidity sensing mechanism. Studies on thin-film sensors based on MnO and WO3 indicate that the W+6 sites in the hubnerite material contribute strongly to the humidity sensing mechanism.

Comparative study on micromorphology and humidity sensitive properties of thin-film and thick-film humidity sensors based on semiconducting MnWO4

The paper describes the results of studies on the fabrication and characterisation of a thick-film humidity sensor based on the semiconducting metal oxide MnWO 4 . The sensor element possesses a novel ‘sandwich’-configuration with a 40 μm porous MnWO 4 ceramic layer sandwiched by two 10 μm polarity-reversed, interdigitated metal films. Instead of traditional glass frits, LiCl powders are used as adhesion promoters for sintering the sensor paste. With this method, MnWO 4 powders with an average particle size of 3.0 μm are sintered at the standard thick-film firing temperature of 850°C. The sintered ceramic layer exhibits a porous structure. The novel electrode arrangement combines the advantages of humidity sensors in the form of a parallel capacitor with those in the form of an interdigital capacitor, permitting a high sensitivity and a fast response. The influence of temperature on the sensor characteristics has been compensated for by integrating a thick-film NTC resistor. The humidity sensor shows no cross-sensitivity to organic vapour. The organic contamination on the sensor surface can be burned out by heating the sensor element at about 400°C with the refresh heater printed on the back side of the substrate.

A novel thick-film ceramic humidity sensor

A low-cost CO2 sensor in thick film technology has been developed using BaTiO3 and various semiconducting oxides. The sensing principle of this new CO2 sensor is based on changes in conductance of the metal oxides in the presence of carbon dioxide. In this report, effects of preparation parameters and effects of various additives on the sensitivity of the CO2 sensor are described, and measurements of the sensor resistance as a function of CO2 concentration are presented. The sensor is applicable over a wide range of CO2 concentrations. Humidity changes have only minor effects on the sensor response.

A novel thick film conductive type CO2 sensor

A key-application for microsystems in life-science are active microimplants for restoring and substituting lost or impaired biological functions in humans. The development of functional micro devices that fit in the human eye and that take over lost biological functions to restore vision is a challenge that several international research groups have engaged in. This paper describes the joint multi-team efforts of German scientists to develop a microelectromechnical implant to stimulate retinal structures for regaining lost visional functions. Different types of active eye implants have been developed for stimulation in the epi-retinal and sub-retinal space. Microelectrode arrays for electrical stimulation have been integrated into flexible and rigid substrates. Innovative microtechniques have been developed for obtaining biocompatible flexible microstructures, interconnects, and hybrid assemblies to fit in the eye ball. Biocompatibility of material and devices was investigated in all design phases utilizing in vitro cell and tissue culture test. Devices at various stages of development have been evaluated in acute and chronic animal models. Passive stimulator electrode arrays (without electronic components) remained in animals for more than six months after implantation Furthermore, animal experiments have shown that active devices equipped with silicon chips could be inserted and housed into the eye. Fully functional chronic devices are presently tested in animals. Experiments on humans are planned in the near future.

Retina implant—a bioMEMS challenge

We present a new process for fabricating CO2 gas sensors. The process was to introduce heat-treatment in vacuum into fired sensing layers. The powders used for paste materials were BaTiO3 and additives, which were all microcrystalline in the range of 2–5 μm. Thick films were fired in air and subsequently heat-treated in vacuum. Their impedance was measured during vacuum-treatment. When the paste materials contained the additive of LaCl3, changes of impedance were observed. A low sensor resistance was achieved by adding 10 wt.% LaCl3 or higher amount to an equimolar mixture of BaTiO3 and CuO. When added LaCl3 amounted to 10 wt.%, a good sensitivity was obtained. Conventionally fired and vacuum-treated layers showed a different response to a change in partial oxygen pressure. Sensor surfaces were analysed using scanning electron spectroscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). No grain size effects were observed. A change of the chemistry in the sensing material was measured. The change indicated that LaOCl, being thermally transformed from LaCl3 during firing, is further oxidised to LaxOy during vacuum-treatment. The analysis gives a possible explanation of the changes in sensor resistance and sensitivity induced by the vacuum process.

Jörg-Uwe Meyer

Papers

Comparative study on micromorphology and humidity sensitive properties of thin-film and thick-film humidity sensors based on semiconducting MnWO4

A novel thick-film ceramic humidity sensor

A novel thick film conductive type CO2 sensor

Retina implant—a bioMEMS challenge

A new process for fabricating CO2-sensing layers based on BaTiO3 and additives