We have reviewed the humidity sensors based on ceramic materials. We first discuss the operating principle of ceramic humidity sensors. This is followed by a section on the relationship between the conduction mechanism and microstructure characteristics of the sensing elements of ceramic humidity sensors. This part of the review is also focused on the methods for optimization of the microstructure of ceramic porous elements. The next section summarizes the information on the materials used for the ceramic humidity sensors fabrication and effect of dopants or hybrid compositions on the sensing ceramic-based materials. Then we analyze state-of-the-art techniques for producing ceramic sensors. The key research and technological challenges in the field are discussed at the end of the review. The review is based on 424 references published during from 1998–2013.

Recent trends of ceramic humidity sensors development: A review

This paper demonstrates a layer-by-layer self-assembled polyaniline (PANI)/graphene oxide (GO) film on quartz crystal microbalance (QCM) for humidity sensing. The morphological and compositional properties of PANI/GO composite films were examined by means of SEM, XRD and FT-IR. The humidity sensing properties of the PANI/GO nanocomposite-based QCM sensor were investigated by exposing to a broad range of 0–97%RH. The experimental results showed that the presented PANI/GO nanocomposite sensor has a high sensitivity, a good repeatability and fast response/recovery characteristics, which surpasses the previous reported counterparts. Moreover, the underlying humidity sensing mechanism of the PANI/GO-based QCM sensor was discussed by using Langmuir adsorption model. This work highlights the unique advantage of layer-by-layer self-assembled route for QCM sensor fabrication.

Facile fabrication of high-performance QCM humidity sensor based on layer-by-layer self-assembled polyaniline/graphene oxide nanocomposite film

We present a comprehensive review of the properties of the epitaxial 4H silicon carbide polytype (4H–SiC). Particular emphasis is placed on those aspects of this material related to room, high-temperature and harsh environment ionizing radiation detector operation. A review of the characterization methods and electrical contacting issues and how these are related to detector performance is presented. The most recent data on charge transport parameters across the Schottky barrier and how these are related to radiation spectrometer performance are presented. Experimental results on pixel detectors having equivalent noise energies of 144 eV FWHM (7.8 electrons rms) and 196 eV FWHM at +27 °C and +100 °C, respectively, are reported. Results of studying the radiation resistance of 4H–SiC are analysed. The data on the ionization energies, capture cross section, deep-level centre concentrations and their plausible structures formed in SiC as a result of irradiation with various particles are reviewed. The emphasis is placed on the study of the 1 MeV neutron irradiation, since these thermal particles seem to play the main role in the detector degradation. An accurate electrical characterization of the induced deep-level centres by means of PICTS technique has allowed one to identify which play the main role in the detector degradation.

https://iopscience.iop.org/article/10.1088/0957-0233/19/10/102001/pdf

Silicon carbide and its use as a radiation detector material

Synthesis and luminescence properties of ZnS and metal (Mn, Cu)-doped-ZnS ceramic powder

Graphene oxide thin film coated quartz crystal microbalance for humidity detection

ZnO nanoparticles have been synthesized by the sol–gel method with approximately 10 nm diameter and the humidity adsorption and desorption kinetics of ZnO nanoparticles were investigated by quartz crystal microbalance (QCM) technique. The morphology and crystal structure of the ZnO nanoparticles have been characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The roughness of the surface has been investigated using atomic force microscope (AFM). The dynamic Langmuir adsorption model was used to determine the kinetic parameters such as adsorption and desorption rates and Gibbs free energy under relative humidity between 45% and 88%. The relative sensitivity of the ZnO nanoparticles-based humidity sensor was determined by electrical resistance measurements. Our reproducible experimental results show that ZnO nanoparticles have a great potential for humidity sensing applications at room temperature operations.

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Humidity sensing properties of ZnO nanoparticles synthesized by sol–gel process

An experimental arrangement is described by which emission coefficient behavior associated with deep‐level traps in semi‐insulating GaAs can be surveyed, from the photoconductive transient behavior following a trap‐filling light pulse. This method, often referred to as PITS (for photoinduced transient spectroscopy) has analogies with the transient capacitance spectroscopy usable for conductive material, but some significant differences also. The necessary experimental precautions which can maximize the probability of getting reliable data are described, and illustrated by PITS results obtained with bulk semi‐insulating samples of Cr‐doped GaAs. Those results include data for five kinds of trap with activation energies from 0.3 to 0.8 eV, of which some are readily identifiable and others are not. The method described in this paper relies on analog measurement of the change in sample voltage during a specified time interval of the photoconductive decay.

Transient photoconductivity measurements in semi‐insulating GaAs. I. An analog approach

The humidity-sensing properties of ZnO nanowires synthesized by carbothermal catalyst-free vapor solid (VS) technique were studied. The morphology and the crystal structure were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The humidity adsorption and desorption kinetics of the synthesized ZnO nanowires were investigated via quartz crystal microbalance (QCM) measurements. The observed positive frequency shift of ZnO nanowires when loaded on the QCM crystal under varying relative humidity conditions can be explained in terms of visco-elastic variations in their mechanical stiffness.

/pdf/humidity-sensing-properties-of-a-zno-nanowire-film-as-4lnixw6e6b.pdf

Humidity-sensing properties of a ZnO nanowire film as measured with a QCM

We report the experimental studies of hot-electron energy and momentum relaxation in the steady state in GaN/AlGaN HEMT structures with a high two-dimensional electron density of n = 1.5×1013 cm-2. From the LO-phonon-scattering-limited component of the mobility we obtain for the LO phonon the energy of ω~90 meV and the momentum relaxation time of τm~4 fs. Drift velocity versus electric field characteristics obtained from the pulsed I-V measurements show that, at TL = 77 K, the drift velocity saturates at vd = 1.0×107 cm s-1 at electric fields in excess of E~7.5 kV cm-1, and at TL = 300 K it saturates at vd~5×106 cm s-1, at an electric field of around E~10 kV cm-1. Electron temperature as a function of applied electric field is obtained by comparing the measured electric field dependence of the mobility µE at a fixed lattice temperature, with the lattice temperature dependence of the mobility at a fixed low electric field. The electron energy loss rate is then determined from the electron temperature dependence of the power loss using the power balance equations. The effect of hot-phonon production on the observed momentum and energy relaxation of hot electrons is discussed within the framework of a theoretical model, which was originally developed for III-V material systems and has been adapted for a two-dimensional electron gas in GaN, and in which phonon drift is neglected.

https://iopscience.iop.org/article/10.1088/0953-8984/14/13/305/pdf

Energy and momentum relaxation of hot electrons in GaN/AlGaN

Zinc sulfide (ZnS) nanostructures were synthesized by vapor–liquid–solid (VLS) method which is based on thermal evaporation. The morphology, chemical composition and crystal structure of ZnS nanostructures were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses. The results of these studies revealed that wurtzite ZnS nanowires with diameters in range of 50–400 nm are obtained. In order to investigate the humidity sensing capability, quartz crystal microbalance (QCM) and electrical resistance measurement techniques were carried out at different relative humidity (RH) conditions between 33% and 100% RH at room temperature. QCM results show that the oscillating frequency of ZnS nanowires loaded on QCM crystal decreases in range of 33–84% RH, but increases at 90% and 100% RH. The sensitivity of ZnS nanowires-based sensor ( R air / R RH ) increases over 1000 times from 33% to 100% RH. These experimental results show that ZnS nanowires have a great potential for humidity sensing applications at room temperature.

/pdf/investigation-of-humidity-sensing-properties-of-zns-3leohzshts.pdf

M. C. Arikan

Papers

Humidity sensing properties of ZnO nanoparticles synthesized by sol–gel process

Transient photoconductivity measurements in semi‐insulating GaAs. I. An analog approach

Humidity-sensing properties of a ZnO nanowire film as measured with a QCM

Energy and momentum relaxation of hot electrons in GaN/AlGaN

Investigation of humidity sensing properties of ZnS nanowires synthesized by vapor liquid solid (VLS) technique