R. N. Karekar

Grain size effects on H2 gas sensitivity of thick film resistor using SnO2 nanoparticles

The effect of ppm level H2 on the d.c. resistance of SnO2 thick film planar resistor with SnO2 particle size variation (∼ 20 to 50 nm) is reported. SnO2 nanoparticles were synthesized using the sol-gel method. The films were prepared using standard screen printing technology. The SnO2 crystallite size, D, is varied in the range of 20–50 nm and sensitivity for H2 is found to increase steeply as D decreases.

Formulation and characterization of ZnO:Sb thick-film gas sensors

ZnO:Sb based screen printed thick-film H2, CO and CH4 gas sensors were fabricated. Films were realized on an alumina substrate by mixing ZnO with lead borosilicate glass frits as a binder and Sb as a catalyst. It was observed that the ZnO:Sb based sensors were more sensitive to these gases than the pure ZnO sensors. The conductance of the sensor increased linearly with the square root of the gas concentration. The dependence of the sensitivity on the amount of the additive (Sb) is discussed. The sensors have a response time of 10 s. Material characterization was carried out by using bulk X-ray diffraction (XRD) and surface morphology by scanning electron microscopy (SEM).

Characterization of SnO2-based H2 gas sensors fabricated by different deposition techniques

Thick and thin films of SnO2 are extensively used for resistive gas sensors. Characteristics are compared for films produced by four different techniques – chemical vapour deposition (CVD), spray pyrolysis, vacuum evaporation and screen printing. The films are characterized for H2 sensing only, using a static measurement system to investigate their temperature selectivity. The samples are tested at a concentration of 300 p.p.m. H2 gas, a typical value for comparison. No selective peak is observed for CVD and spray deposited samples, and the selective peak for vacuum evaporated samples has a low sensitivity. But the selective peak for screen printed samples has a sensitivity of 55% (δR/Rair). And what is more, the screen printed samples have a repeatable response.

A microwave microstrip ring resonator as a moisture sensor for biomaterials: application to wheat grains

A miniaturized, non-destructive sensor employing a microwave microstrip ring resonator (MRR, GHz, mean diameter 3.69 mm) was developed for estimating the moisture content of a single wheat ( Triticum aestivum L) grain. A single wheat grain with a known amount of moisture was placed on the MRR at two different orientations ( and ) with respect to the feedline. The resonance frequency , bandwidth (B) and quality factor of the MRR were calibrated against the moisture content. The measurements were made with a scalar network analyser. The sensor was studied in the normal useful moisture range of 11-32% (on a wet-weight basis), the actual moisture values being obtained by an oven-drying method. The orientation was more sensitive to moisture than was the orientation. The total changes in for a 21% change in moisture content for and 90 orientations were 235 and 150 MHz, respectively. The errors in moisture estimation with for and were % and %, respectively. The corresponding values with B and for orientation were % and %, respectively. The proposed sensor is more sensitive than a reported waveguide resonator and is easy to operate, for the microstrip offers an open structure, thereby facilitating easy loading and unloading of the samples.

Humidity sensor using planar optical waveguides with claddings of various oxide materials

A planar optical waveguide, fabricated using an ion-exchange process in soda glass (Na+ exchanged by K+), is studied as a humidity sensor with screen printed claddings of BaTiO3, SnO2, Al2O3, ZrO2, V2O5 and ZnO as well as sintered ZnO, with cladding thicknesses between 20 and 25 μm. A He-Ne laser beam (0.1 mm dia., λ = 632.8 nm, 0.5 mW) is coupled to the guide using prism film coupling and a silicon photovoltaic detector is used to measure the intensity of transmission. The lengths of the claddings used, which give the maximum TM-polarization because of the relative attenuation, are between 3 and 5 mm. The relative humidity (RH) is varied from 3 to 98%. BaTiO3 offers maximum sensitivity as compared to the other oxides used. SnO2 exhibits a response time of 3 s and a recovery time of 10 min, the fastest amongst all. The maximum hysteresis of 1% is observed, except for V2O5 cladding (8%). The sensitivity curves show 3 regions, which are attributed to different possible dominant processes.

Effect of film thickness and curing temperature on the sensitivity of ZnO:Sb thick-film hydrogen sensor

This paper reports the effect of film thickness and curing temperature on the sensitivity of the screen-printed thick-film ZnO (incorporated with 7 wt%Sb) H2, CO and CH4 sensors. The sensitivity of the sensors increases up to 60 μm and decreases for higher film thickness. Increase in the curing temperature of the films, decreases the sensitivity of the sensors because of the increase in the average grain size, as observed by scanning electron microscopy.

Microwave microstrip ring resonator as a paper moisture sensor: study with different grammage

This paper reports use of a nondestructive, miniaturized, microwave microstrip ring resonator for moisture sensing application in papers. Advantages offered by this sensor over the prevailing sensors are room-temperature operation, response time in milliseconds, measurements unaffected by dusty environment and ionic conductivity of samples. Samples of eleven types were tested, with grammage (grams per square metre) and thickness varying from 21 to 70 g m−2 and 24 to 80 µm respectively. The wet basis moisture normalization (with respect to instantaneous moisture content in the sample) was established to remove scatter and to bring out clearly grammage dependence, thus avoiding error in moisture prediction due to density variations, using only scalar measurements. A single equation for fr variations is realized in terms of grammage and normalized percentage moisture (Mww) which is valid for all types of tested paper. A model of the wet paper is suggested mainly based on water–dry paper interaction, also considering parameters like thickness and surface roughness, to explain trends of the sensitivity curves. The estimated %Mww shows an error of ±0.9% in the estimated value as compared to the actual value.

Miniaturized nondestructive microwave sensor for chickpea moisture measurement

A miniaturized microstrip ring resonator (MRR) 1 in.×1 in. resonating at fro=10.27 GHz was used as a nondestructive moisture sensor for chickpea kernels (Cicer arietinum L.) for ease in loading and unloading. The change in the resonant frequency (Δfr) of the MRR is a measure of the amount of moisture in the overlaid kernel. The percentage of moisture (M) was varied from 0% (dry) to ∼50% (fully soaked) calculated on a wet weight basis. Δfr increased with M, although not linearly. Three regions were observed in the sensitivity curve. The first region extended from 0%–12%, the central region from 12%–43%, and the saturation region from 43%–50% in moisture content. In the central region the observed Δfr was 574 MHz, whereas in the first and third regions it was 44 and 55 MHz, respectively. The regions in the sensitivity curves indicate different dominant phenomena. A small scatter was observed in the first region, which increased with the increasing percent of moisture content.

Microwave ring resonator as a novel bio-material moisture sensor

Abstract A novel miniature microwave microstrip sensor for the study of moisture in bio-materials (paper and leaves) is suggested. The leaf-drying process (time-variation) exhibits the leaf-specific nature. The paper-moisture estimation using f r and Q r variations are the two complementary moisture sensing methods. The Q r variations are useful to sense low moisture level, less than 10% M ww , with sensitivity of the order of 300% M ww −1 , with an error of 1.57% M ww in the estimated moisture. The f r variations are useful to sense moisture in the wider range 0–40% M ww , with sensitivity of the order of 25 MHz % M ww −1 , with an error of 2.2% M ww in the estimated moisture. The trend of f r variations with percentage M ww for both types of bio-materials, could be divided broadly in two regions with an additional region shown by the high moisture containing leaves. A preliminary explanation of the related dominant phenomena is attempted.

An optical fiber weighing sensor based on bending

The bending of plastic optical fiber (POF) is used to develop a weighing sensor by fully gluing POF onto a strip of spring steel used as a clamped beam for sensing, keeping the glued fiber either on the top or bottom side of the beam. Force, in the form of weight, is applied to the beam in two ways: by attaching a pan (i) at one of the ends of the beam or (ii) at the center of the beam, with proper clamping. The known weight is added in the pan for calibration purposes. The output light intensity of the fiber, measured as voltage, changes practically linearly with increasing weight up to a certain limit due to the macrobending of POF. It is found that though elongative bending decreases the output intensity, compressive bending gives a reverse effect. The response time for sensing is found to be approximately 5–7 s with larger recovery time (≤1 min). No noticeable hysteresis is observed. The thickness and length of the beam are varied and optimized. The sensor specifications can be tailored by this to some extent. A minimum weight of 5 g and a maximum of ~1900 g could be measured in a linear range for the beam-length used.