Humidity sensing characteristics of hydrotungstite thin films

Thin films of the hydrated phase of tungsten oxide, hydrotungstite (H₂WO₄·H₂O), have been grown on glass substrates using a dip-coating technique. The b-axis oriented films have been characterized by X-ray diffraction and scanning electron microscopy. The electrical conductivity of the films is observed to vary with humidity and selectively show high sensitivity to moisture at room temperature. In order to understand the mechanism of sensing, the films were examined by X-ray diffraction at elevated temperatures and in controlled atmospheres. Based on these observations and on conductivity measurements, a novel sensing mechanism based on protonic conduction within the surface layers adsorbed onto the hydrotungstite film is proposed.     

Humidity sensing properties of nanostructured- bilayered potassium tantalate: Titania films

Nanostructured-bilayered potassium tantalate (KT): titania (TiO₂) films have been prepared using a sol-gel spin-coating process and the humidity sensing properties of the films are investigated. The films possess the grain size of nanometer order and nanoporous structure. The bilayered TiO₂/KT and KT/TiO₂ films exhibit a better linearity with narrower hysteresis loop in the resistance variation for relative humidity than single layered KT and TiO₂ films. The bilayered TiO₂/KT and KT/TiO₂ films show over three orders of change in the resistance during the relative humidity variation from 20 to 90%, exhibiting a good exponential relationship. The humidity sensing properties in the nanostructured-bilayered KT:TiO₂ films are discussed.     

Humidity sensing behaviour of polyaniline/magnesium chromate (MgCrO<Subscript><Emphasis Type="Bold">4</Emphasis></Subscript>) composite

‘In situ’ polymerization of polyaniline (PANI) was carried out in the presence of magnesium chromate (MgCrO ₄ ) to synthesize PANI/ceramic (MgCrO ₄ ) composite. These prepared composites were characterized by XRD, FTIR and SEM, which confirm the presence of MgCrO ₄ in polyaniline matrix. The temperature dependent conductivity measurement shows the thermally activated exponential behaviour of PANI / MgCrO ₄ composites. The decrease in electrical resistance was observed when the polymer composites were exposed to the broad range of relative humidity (ranging between 20 and 95% RH). This decrease is due to increase in surface electrical conductivity resulting from moisture absorption and due to capillary condensation of water causing change in conductivity within the sensing materials. PANI / MgCrO ₄ composites are found to be sensitive to low humidity ranging from 20 to 50 % RH.     

Human‐Finger Electronics Based on Opposing Humidity‐Resistance Responses in Carbon Nanofilms

Carbon nanomaterials have excellent humidity sensing properties. Here, it is demonstrated that multiwalled carbon‐nanotube (MWCNT)‐ and reduced‐graphene‐oxide (rGO)‐based conductive films have opposite humidity/electrical resistance responses: MWCNTs increase their electrical resistance (positive response) and rGOs decrease their electrical resistance (negative response). The authors propose a new phenomenology that describes a “net”‐like model for MWCNT films and a “scale”‐like model for rGO films to explain these behaviors based on contributions from junction resistances (at interparticle junctions) and intrinsic resistances (of the particles). This phenomenology is accordingly validated via a series of experiments, which complement more classical models based on proton conductivity. To explore the practical applications of the converse humidity/resistance responses, a humidity‐insensitive MWCNT/rGO hybrid conductive films is developed, which has the potential to greatly improve the stability of carbon‐based electrical device to humidity. The authors further investigate the application of such films to human‐finger electronics by fabricating transparent flexible devices consisting of a polyethylene terephthalate substrate equipped with an MWCNT/rGO pattern for gesture recognition, and MWCNT/rGO/MWCNT or rGO/MWCNT/rGO patterns for 3D noncontact sensing, which will be complementary to existing 3D touch technology.     

金属氧化物半导体气敏传感器稳定性研究进展

金属氧化物半导体(MOS)气敏传感器的稳定性是气敏器件实用化进程中最具有挑战性的因素。影响器件稳定性的主要因素包括颗粒尺寸、颈部宽度、微裂纹、电极、湿度和温度的变化。颗粒的长大和颗粒间颈部尺寸的变化降低了耗尽层对总电阻变化的贡献;微裂纹的加剧使器件的电阻发生漂移,并且为水蒸气、氧气和待测气体扩散到敏感膜内部提供便捷的通道;电极的退化影响电极与气敏材料之间的接触电阻Rc;温度和湿度的改变使气体的吸附、脱附、反应活性和电子迁移率等都发生变化,因而器件的稳定性得不到保证。在机理分析的基础上,分别介绍了提高器件稳定性的方法。     

Nanocrystalline spinel NixCu0.8?xZn0.2Fe2O4: a novel material for humidity sensing

The humidity sensing property of the ferrite systems Ni _x Cu_0.8−x Zn_0.2Fe₂O₄ with 0.0 ≤ x ≤ 0.8 (x = 0, 0.2, 0.4, 0.6, 0.8) was studied using the standard ceramic technique. The spinel structure of the compounds was confirmed by X-ray diffraction studies, BET study and surface morphology by scanning electron microscopy. The compounds were subjected to dc electrical conductivity studies at room temperature. The resistance measurements as a function of relative humidity (RH) in the range of 5–98% were done and the humidity sensing factors (S _f = R _5%/R _95%) were calculated. The composite NiCuZn-3 (x = 0.4) possessed the highest humidity sensing factor of 3051.9 ± 500, whilst CuZn–1 (x = 0) possessed the lowest humidity sensing factor of 45.3 ± 12. The other compounds possessed lower humidity sensing factors of 116.7 ± 35, 783.4 ± 160 and 416.2 ± 65 for x = 0.2, 0.6 and 0.8, respectively.     

Practical Aspects of Aerosol Characterization in an Environment of Controlled Temperature and Relative Humidity

Abstract

In the fields of environmental and pharmaceutical aerosols it is of importance to create atmospheres of controlled temperature and relative humidity for the investigation of airborne particulate behavior. Close control of humidity, especially where relative humidity sensing is used, also requires close temperature control because of the dependence of relative humidity on temperature. Unique designs of apparatus have been employed, in many cases, without considering the implications of the level of control required to obtain statistically acceptable data. This paper describes a novel apparatus, contrasts it with those in the literature and states specifically the level of control which can be achieved practically and how this compares with theoretical expectations.     

Dependence of the empirical model parameters of the surface resistance of a high-temperature superconducting film on the parameters of the technological process

The dependence of the surface resistance on the substrate heater temperature has been investigated for the purpose of optimizing the conditions for preparing YBa₂Cu₃O_7−δ films. It is shown that the resistance R _sur is highly sensitive to the accuracy of maintaining the substrate holder temperature to minimize the surface resistance of the films and maximize the parameter γ, which determines the temperature-dependent curves σ(t) and λ_L(t). Pis’ma Zh. Tekh. Fiz. 23, 79–84 (August 12, 1997)     

Nitrogen‐Doped Single Graphene Fiber with Platinum Water Dissociation Catalyst for Wearable Humidity Sensor

Humidity sensors are essential components in wearable electronics for monitoring of environmental condition and physical state. In this work, a unique humidity sensing layer composed of nitrogen‐doped reduced graphene oxide (nRGO) fiber on colorless polyimide film is proposed. Ultralong graphene oxide (GO) fibers are synthesized by solution assembly of large GO sheets assisted by lyotropic liquid crystal behavior. Chemical modification by nitrogen‐doping is carried out under thermal annealing in H₂(4%)/N₂(96%) ambient to obtain highly conductive nRGO fiber. Very small (≈2 nm) Pt nanoparticles are tightly anchored on the surface of the nRGO fiber as water dissociation catalysts by an optical sintering process. As a result, nRGO fiber can effectively detect wide humidity levels in the range of 6.1–66.4% relative humidity (RH). Furthermore, a 1.36‐fold higher sensitivity (4.51%) at 66.4% RH is achieved using a Pt functionalized nRGO fiber (i.e., Pt‐nRGO fiber) compared with the sensitivity (3.53% at 66.4% RH) of pure nRGO fiber. Real‐time and portable humidity sensing characteristics are successfully demonstrated toward exhaled breath using Pt‐nRGO fiber integrated on a portable sensing module. The Pt‐nRGO fiber with high sensitivity and wide range of humidity detection levels offers a new sensing platform for wearable humidity sensors.     

Electrical and optical properties of ZnO–WO<Subscript>3</Subscript> nanocomposite and its application as a solid-state humidity sensor

This study reports the humidity sensing characteristics of ZnO–WO₃ nanocomposite. Pellet samples of 0–5 weight% ZnO in WO₃ were sintered from 300 to 600 ^° C. When exposed to humidity, the resistance of the sensing samples was found to decrease with increase in relative humidity (RH). Five percent ZnO-doped WO₃ showed maximum sensitivity of 20.95 M Ω/%RH in 15–95% RH range. Sensor parameters like reproducibility, aging, hysteresis, response and recovery times were also studied. Sensing mechanism is discussed in terms of sintering temperature, composition and crystallite size of the sensing element. It was observed that sensing mechanism is strongly based on annealing temperature and percentage of doping. The sensing samples have also been investigated by X-ray diffraction, scanning electron microscope (SEM) and Raman spectroscopy. The crystalline size of the sample was identified by powder X-Ray Diffraction data. The SEM analysis was used to study the surface morphology. The structure, phase and the degree of crystallinity of the materials were examined by Raman spectroscopy.     

Effect of relative humidity on the optical and waveguide properties of thin chitosan films

We have measured the optical characteristics of thin films based on various ion forms of chitosan and studied their mutual correlation. Using the spectral ellipsometry and mode spectroscopy techniques, it has been established that the optical characteristics of chitosan depend on the relative humidity (RH) of the ambient medium. All samples are characterized by threshold RH values, at which the optical characteristics exhibit a change. The obtained data allow chitosan to be classified as a promising sensor material with the optical properties controlled by relative humidity of the ambient medium.     

Effect of crystallization on humidity sensing properties of sol-gel derived nanocrystalline TiO2 thin films

In the present work, we describe the effect of crystallization on humidity sensing properties of nanocrystalline TiO₂ thin films prepared by sol-gel techniques. Here, we report an enhancement in the relative humidity (RH) sensitivity just after the crystallization at 375 °C, which is attributed to increased surface activity near crystallization and lower crystallite size. After crystallization, the RH sensitivity was found to decrease with increasing grain size. The complex impedance of the sensor, measured using impedance spectroscopy, fits well with an equivalent circuit consisting of inter-granular resistance and capacitance in parallel. It was found that with the change in RH, only resistance changes significantly, when compared with the capacitance.     

Biopolymer-based nanocomposites: effect of lignin acetylation in cellulose triacetate films

Abstract

We have prepared all-biopolymer nanocomposite films using lignin as a filler and cellulose triacetate (CTA) as a polymer matrix, and characterized them by several analytical methods. Three types of lignin were tested: organosolv, hydrolytic and kraft, with or without acetylation. They were used in the form of nanoparticles incorporated at 1 wt% in CTA. Self-supported films were prepared by vapor-induced phase separation at controlled temperature (35–55 °C) and relative humidity (10–70%). The efficiency of acetylation of each type of lignin was studied and discussed, as well as its effects on film structure, homogeneity and mechanical properties. The obtained results are explained in terms of intermolecular filler-matrix interaction at the nanometer scale, for which the highest mechanical resistance was reached using hydrolytic lignin in the nanocomposite.     

Plasma deposited thin films suitable as moisture sensors

Plasma polymerized thin films from a mixture of hexamethyldisiloxane (HMDSO) and ammonia (NH₃) deposited directly onto a microdielectrometer chip have been evaluated as moisture sensors. Microdielectrometry was chosen as the measuring technique because of its fast response and real time detection capability. The sheet resistance of the plasma polymerized film was found to decrease by eight orders of magnitude with an increase in relative humidity from 0 to 92%. The moisture effect on the sheet resistance was also found to be reversible. The composition and structure of the films deposited from various mixtures of HMDSO/NH₃ have been elucidated by Electron Spectroscopy for Chemical Analysis (ESCA) and infrared spectroscopy.     

Growth and characterization of Hg1-xCdxTe epitaxial films by isothermal vapour phase epitaxy (ISOVPE)

Growth of Hg_1-xCd_xTe epitaxial films by a new technique called asymmetric vapour phase epitaxy (ASVPE) has been carried out on CdTe and CZT substrates. The critical problems faced in normal vapour phase epitaxy technique like poor surface morphology, composition gradient and dislocation multiplication have been successfully solved. The epitaxial films have been electrically characterized by using the Hall effect and capacitance-voltage (C-V) measurements.     

Book Review

The topic of this paper is the evaluation of the dark stability of different types of photographic reversal colour film.

Agfa, Fuji, Ilford and Kodak films have been subjected to accelerated ageing in the dark. The relative humidity was fixed at 50% and temperatures were selected in a range from 57°C to 92°C. Deterioration was monitored by optical density measurement of the least stable dye on neutral grey or colour sensitometric strips. Extrapolation according to the Arrhenius Law allowed us to evaluate the stability of the films at 24°C.     

Ageing and field effect studies on discontinuous silver films at near liquid nitrogen temperatures

The post deposition resistance changes in discontinuous silver films deposited in a vacuum of 2 × 10^–6 torr on glass substrates maintained at near liquid nitrogen temperatures have been studied. Reduced agglomeration rates in comparison with films studied at room temperature were obtained, supporting the thermally assisted mobility coalescence model explaining the post deposition resistance increase. The non-linearI-V characteristics of one of the films followed by observations of resistance changes before and after field effect measurements on the other films have been explained as arising due to field-induced structural changes. The investigations of the variation of film resistance with temperature revealed a transition temperature. A fall in resistance with increasing temperature below the transition temperature has been explained by an increase in the number of thermally charged islands. The increase in resistance with temperature above the transition temperature is due to an increase in the thermally assisted mobility coalescence.     

Thin Film Inspection with Millimeter-Wave Reflectometer

Abstract

Inspection of dielectric thin films (25.4–508 μm) placed directly above a conducting plate is performed with a W-band millimeter-wave network analyzer. The signal launched by a small antenna placed near the sample surface is picked up by a separate receiver. The phase of the received signal can be related to the thickness and permittivity of the thin films under inspection. While the transmitter and receiver are at larger distances from the dielectric surface, the system operates as a conventional reflectometer. But when they both are close to the surface, guided waves will be excited and even may become dominant over the direct reflection. The significance of the guided-wave contribution is controlled by the locations of the guided-wave poles in the complex k _ρ -plane. In this paper we employ a modal approach that previously was developed for geophysical remote sensing to study the effects of the guided waves, and to outline the limits of the conventional reflectometric method for evaluating thin films.     

Development and characterization of a mucoadhesive sublingual formulation for pain control: extemporaneous oxycodone films in personalized therapy

Objective: The aim of this work was the development of mucoadhesive sublingual films, prepared using a casting method, for the administration of oxycodone.

Materials and methods: A solvent casting method was employed to prepare the mucoadhesive films. A calibrated pipette was used to deposit single aliquots of different polymeric solutions on a polystyrene plate lid. Among the various tested polymers, hydroxypropylcellulose at low and medium molecular weight (HPC) and pectin at two different degrees of esterification (PC) were chosen for preparing solutions with good casting properties, capable of producing films suitable for mucosal application.

Results and discussion: The obtained films showed excellent drug content uniformity and stability and rapid drug release, which, at 8?min, ranged from 60% to 80%. All films presented satisfactory mucoadhesive and mechanical properties, also confirmed by a test on healthy volunteers, who did not experience irritation or mucosa damages. Pectin films based on pectin at lower degrees of esterification have been further evaluated to study the influence of two different amounts of drug on the physicochemical properties of the formulation. A slight reduction in elasticity has been observed in films containing a higher drug dose; nevertheless, the formulation maintained satisfactory flexibility and resistance to elongation.

Conclusions: HPC and PC sublingual films, obtained by a simple casting method, could be proposed to realize personalized hospital pharmacy preparations on a small scale.