Highly Sensitive Humidity Sensor Using Pd/Porous GaAs Schottky Contact

This paper investigates the suitability of porous GaAs as a semiconductor material for sensing humidity. The authors have developed two types of sensors based on Pd/porous GaAs and Pd/GaAs Schottky contacts for humidity measurements. It was found that the porosity on GaAs wafer promoted the sensing properties of the contact used as highly sensitive humidity sensor toward different amounts of relative humidity operated at room temperature. On the contrary, the Pd/GaAs sample operated at room temperature exhibited negligible sensitivity to relative humidity. The advantages of using porous GaAs for Schottky humidity sensor are the following: high sensitivity, low response time, and insignificant dependence on temperature. Current-voltage (I-V) characteristics of the Pd/porous GaAs Schottky humidity sensor exhibited a saturation current value of 8.5times10^-10 A under dry condition (5% relative humidity). This was increased to 7.0times10^-9 A when submitted to a relative humidity of 25%. The saturation current was further increased considerably to 3.0times10^-7 A as the relative humidity was increased to 95%. This is more than two orders of magnitude increase in saturation current compared to dry condition. A parameter called humidity sensitivity was defined using the current value at a fixed forward voltage of 0.2 V to present the sensitivity of the sensor. Response times are reported to discuss the adsorption and desorption characteristics of the device. Pd/porous GaAs sensor operated at room temperature showed a fast response time of 2 s and a sensitivity value of 93.5% in the presence of 25% relative humidity. Furthermore, the influence of increase in relative humidity as well as heating effects on the responsivity of the sensor is described. Scanning electron microscopy analysis of the Pd/porous GaAs sample exhibited highly porous structures     

Poly(ether ether ketone) grafted with sulfoalkylamine for highly sensitive humidity sensor with small hysteresis

Here, a series of novel poly(ether ether ketone) containing sulfoalkylamine pendant groups (SNPEEK-x) were synthesized by grafting copolymerization reaction. The chemical structure of SNPEEK-x was identified by ¹H NMR and the morphology of all films was measured by SAXS and TEM. The impedance varied for five orders of magnitude (from 10⁷ to 10² Ω) as the relative humidity increasing from 11 to 97% RH. At the same time, all samples exhibited a fast response time of less than?3 s, outstanding repeatability and good long-term durability against high humidity. Especially, SNPEEK-x sensors displayed an extremely small humidity hysteresis in the impedance versus RH relationship during absorption and desorption processes. These essential properties of humidity sensors such as the impedance at various humidity, sensitivity, linearity and humidity hysteresis revealed that SNPEEK-x are suitable to be used as humidity sensors.     

Transparent,stretchable, and rapid-response humidity sensor for body-attachable wearable electronics

Stretchable and conformal humidity sensors that can be attached to the human body for continuously monitoring the humidity of the environment around the human body or the moisture level of the human skin can play an important role in electronic skin and personal healthcare applications.However,most stretchable humidity sensors are based on the geometric engineering of non-stretchable components and only a few detailed studies are available on stretchable humidity sensors under applied mechanical deformations.In this paper,we propose a transparent,stretchable humidity sensor with a simple fabrication process,having intrinsically stretchable components that provide high stretchability,sensitivity,and stability along with fast response and relaxation time.Composed of reduced graphene oxide-polyurethane composites and an elastomeric conductive electrode,this device exhibits impressive response and relaxation time as fast as 3.5 and 7 s,respectively.The responsivity and the response and relaxation time of the device in the presence of humidity remain almost unchanged under stretching up to a strain of 60％ and after 10,000 stretching cycles at a 40％ strain.Further,these stretchable humidity sensors can be easily and conformally attached to a finger for monitoring the humidity levels of the environment around the human body,wet objects,or human skin.     

Capacitive Humidity Sensors With High Sensitivity and Subsecond Response Times

Capacitive-based humidity sensors were fabricated using coplanar interdigitated electrodes coated with nanostructured TiO₂ thin films produced by glancing angle deposition. In this letter, we show that increased sensitivity (nF/%RH) is obtained by decreasing the electrode periodicity or by increasing the planar area of the electrodes, or both. The devices were sensitive over a wide range of relative humidity levels (<1% to >92%) and exhibited extremely fast, subsecond response times. Typical adsorption and desorption response times were measured to be <220 and >400 ms, respectively     

The switch-type humidity sensing properties of polyacrylic acid and its copolymers

In this paper, switch-type humidity sensors based on PAA (polyacrylic acid) and its copolymers were examined. The humidity sensing properties of these polymers, such as hysteresis, response time, water resistivity and long-term stability were investigated. The factors which influence the humidity sensors' inflection point were investigated experimentally as well. The results show that, the inflection point of the humidity sensors varies with the content of crosslinking agent and the sample composition. In particular, humidity sensors based on Sample E and B show good property and stability.     

Hydrophobic ionic liquid-in-polymer composites for ultrafast,linear response and highly sensitive humidity sensing

Traditional ionic liquids are sensitive to humidity but with long response time and nonlinear response.Pure liquid-state ionic liquids are usually hard for dehydration which have ultralong response time for humidity sensing.The immobilization of ionic liquids provide a possible way for high performance humidity sensing.Hydrophobic materials and structures also promised faster response in humidity sensing,because of easier desorption of water.In this work,we prepared flexible humidity sensitive composites based on hydrophobic ionic liquid and polymer.The combination of hydrophobic ionic liquid with hydrophobic polymer realized linear response,high sensitivity with low hysteresis to humidity.By adjusting the ratio of ionic liquid,not only the impedance but also the hydrophobicity of composite could be modulated,which had a significant influence on the humidity sensing performance.The morphology and microstructure of the material also affected its interaction with water molecules.Due to the diverse processing methods of polymer,highly transparent film fabricated by spinning-coating and nanofibrous membrane fabricated by electrospinning could be prepared and exhibited different response time,which could be used for different application scenarios.Especially,the fibrous membrane made with electrospinning method showed an ultrafast response and could distinguish up to 120 Hz humidity change,due to its fibrous structure with high specific surface area.The humidity sensors with ultrafast,linear response and high sensitivity showed potential applications in human respiratory monitoring and flexible non-contact switch.To better show the multifunction of ionic liquid-polymer composite,as a proof of concept,we fabricated an integrated humidity sensitive color change device by utilizing lower ionic liquid content composite for sensing in the humidity sensing module and higher ionic liquid content composite as the electrolyte in the electrochromic module.     

Flexible Smart Noncontact Control Systems with Ultrasensitive Humidity Sensors

The development of noncontact humidity sensors with high sensitivity, rapid response, and a facile fabrication process is urgently desired for advanced noncontact human–machine interaction (HMI) applications. Here, a flexible and transparent humidity sensor based on MoO₃ nanosheets is developed with a low‐cost and easily manufactured process. The designed humidity sensor exhibits ultrahigh sensitivity, fast response, great stability, and high selectivity, exceeding the state‐of‐the‐art humidity sensors. Furthermore, a wearable moisture analysis system is assembled for real‐time monitoring of ambient humidity and human breathing states. Benefiting from the sensitive and rapid response to fingertip humidity, the sensors are successfully applied to both a smart noncontact multistage switch and a novel flexible transparent noncontact screen for smart mobile devices, demonstrating the potential of the MoO₃ nanosheets‐based humidity sensors in future HMI systems.     

Comparison of three humidity sensors for a pulmonary functiondiagnosis microsystem

Three capacitive humidity sensors developed for a portable clinical application are presented and compared. The first structures consist of interdigitated electrodes covered by a polyimide sensitive layer. The second structures have the same geometry but include a benzocyclobutene sensitive layer and a heating. resistor. The third structure has been developed with a new geometry, with the electrodes being stacked. Humidity measurement results are presented, in particular sensor response time in absorption that must be very small (less than 500 ms). The influence of the heating on the response time is described. In conclusion, the three sensors are compared and the most suitable structure for our application is indicated. Although polyimide has been widely used for realization of capacitive humidity sensors, we demonstrate here that it is not the best sensitive material for our application     

Synthesis of BaCO3 nanowires and their humidity sensitive property

Barium carbonate (BaCO3) nanowires have been synthesized for the first time by using the composite hydroxide mediated (CHM) method. The products are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Humidity sensors based on BaCO3 nanowires have been fabricated. The response to humidity in static and dynamic measurement proves the ultrasensitive property of the sensors. The resistance changes from 386 M(omega) to 7.1 M(omega) as the relative humidity (RH) increases from 20% to 95%. The response and recovery time of the resistance is 16 s and 56 s versus the changes of relative humidity from 25% to 85%. These results indicate promising applications of BaCO3 nanowires in a highly sensitive environmental monitoring and humidity control electronic device.     

Multiscale Humidity Visualization by Environmentally Sensitive Fluorescent Molecular Rotors

Building humidity sensors possessing the features of diverse‐configuration compatibility, and capability of measurement of spatial and temporal humidity gradients is of great interest for highly integrated electronics and wearable monitoring systems. Herein, a visual sensing approach based on fluorescent imaging is presented, by assembling aggregation‐induced‐emission (AIE)‐active molecular rotors into a moisture‐captured network; the resulting AIE humidity sensors are compatible with diverse applications, having tunable geometries and desirable architectures. The invisible information of relative humidity (RH) is transformed into different fluorescence colors that enable direct observation by the naked eyes based on the twisted intramolecular charge‐transfer effect of the AIE‐active molecular rotors. The resulting AIE humidity sensors show excellent performance in terms of good sensitivity, precise quantitative measurement, high spatial–temporal resolution, and fast response/recovery time. Their multiscale applications, such as regional environmental RH detection, internal humidity mapping, and sensitive human‐body humidity sensing are demonstrated. The proposed humidity visualization strategy may provide a new insight to develop humidity sensors for various applications.     

基于 AAO/PTFE 复合薄膜封装的湿度传感器应用

目的研究多孔阳极氧化铝/聚四氟乙烯(AAO/PTFE)复合薄膜应用于湿度传感器封装窗口的水蒸气输运特性和抗污染能力。方法通过旋涂法在双通阳极氧化铝薄膜上制备PTFE微孔膜,将该复合薄膜用作湿度传感器的封装窗口,在定温变湿的环境和油漆喷涂房中分析相对湿度测量的静态误差和动态响应,研究AAO/PTFE复合薄膜对传感器测量的影响和保护功能,并将其与PTFE和PVDF滤膜进行对比。结果定温变湿环境下静态和动态的测量结果表明,AAO/PTFE复合薄膜对湿度传感器造成的测量误差最大可达15%,且误差曲线可以分为30%~50%的低相对湿度区间、50%~80%的中相对湿度区间和大于80%的高相对湿度区间。静态测量的差异是由不同相对湿度区间内试验箱中空气流动速率不同造成的,而动态响应的差异主要源于不同区间内管壳内、外的相对湿度差不同。结论 PTFE薄膜的涂覆不仅保留了AAO滤膜优异的水蒸气输运性能,还有效提高了其表面抗污染能力。     

Polymer nanofibers embedded with aligned gold nanorods: a new platform for plasmonic studies and optical sensing

This paper reports the fabrication and characterization of polymer nanofibers embedded with gold nanorods in uniaxial alignment for applications in optical waveguiding and sensing. Using a waveguiding approach, we demonstrated highly efficient excitation of localized surface plasmon resonance in the embedded gold nanorods with a photon-to-plasmon-conversion efficiency as high as 70% for a single nanorod at its longitudinal resonance wavelength. On the basis of waveguiding polymer nanofibers embedded with gold nanorods, we further demonstrated compact optical humidity sensors with a response time of 110 ms and an operation optical power as low as 500 pW.     

高分子电阻型湿敏材料的老化机理

本文以聚苯乙烯磺酸钠为湿敏材料制备了高分子电阻型湿度传感器,研究了其在高温高湿环境下,施加交流电激励后的老化行为。考察了通电电压、通电时间等对传感器响应特性的影响,并对其老化机理进行了探讨。提出通电老化后,湿敏材料在不同湿度下的电阻变化决定于高温高湿环境造成的溶解效应和施加电压后引起的通道效应和离子破坏效应。研究表明,在较高湿度环境下(87~93%RH),施加800mv的电压可加速湿敏材料的老化过程,使湿度传感器响应信号较快达到稳定,从而有望改善湿度传感器的稳定性。     

过滤器材料对陶瓷湿敏元件稳定性的影响

本工作研究了用金属丝网作骨架浸渍能透水蒸汽的高分子材料的溶液，干后形成既能透水蒸汽、又能阻挡烟尘和有害气体透过的复合材料，用这种材料作过滤器封装湿敏元件，在最佳情况下，可使其漂移值减小到仅用铜丝网过滤器时的十分之一，而对湿滞和响应时间无明显的不良影响。     

ZrO2-TiO2 ceramic humidity sensors

Ceramics based on 0.5ZrO₂-0.5TiO₂ were evaluated as humidity sensors. The variation of phase change, microstructure, and conductivity with relative humidity were investigated for base ceramics doped with additives such as CrO_1.5, FeO_1.5 and MgO. It was found that FeO_1.5 enhanced the formation of ZrTiO₄. The addition of MgO and FeO_1.5 increased conductivity and its humidity sensitivity. Open porosity was not affected much by the addition of the three dopants. Nitrogen treatment of sintered ceramics at 900 °C was found to increase conductivities and humidity sensitivity of the ceramics.     

露点温度传感器发展趋势综述

介绍了目前露点温度传感器领域的研究现状,阐述了光学式、谐振式、电学式、热学式、重量式、化学式露点温度传感器的原理及构造,指出光学式露点温度传感器测量精度极高,其中冷镜式露点仪可作为湿度计量标准;谐振式露点温度传感器具有体积小、成本低、响应时间短、灵敏度高、可靠性好的特点;电学式露点温度传感器灵敏度高、功耗小,便于实现小型化、集成化;重量法是准确度最高的湿度绝对测量方法;化学法常用来测量低湿环境下的有机混合气体。探讨了露点温度传感器在环境监测、工业制造、医疗诊断等领域的应用情况,指出未来露点温度传感器将会向高精度、高稳定性、高响应的方向发展,且应用范围将进一步拓展,以满足极端环境下的测量需求。     

The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors

This review paper discusses the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors. The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics (absolute magnitude and selectivity of sensor response (S), response time (τ_res), recovery time (τ_rec), and temporal stability) of the gas sensor have been analyzed. A comparison of standard polycrystalline sensors and sensors based on one-dimension structures was conducted. It was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors. For example, it was shown that the decrease of thickness, grain size and degree of texture is the best way to decrease time constants of metal oxide sensors. However, it was concluded that there is not universal decision for simultaneous optimization all gas-sensing characteristics. We have to search for a compromise between various engineering approaches because adjusting one design feature may improve one performance metric but considerably degrade another.     

Humidity-induced magnetization and magnetic pole inversion in a cyano-bridged metal assembly

In general, magnetic properties of bulk magnetic materials are independent of the humidity of the environment. To obtain a magnetic material that has humidity-sensitive characteristics, water vapour must penetrate the lattice and act on spin sites. Nanoporous materials composed of metal-assembled complexes may be expected to display some humidity response because materials in this category can show functionalities such as gas storage and molecular recognition. Here, we demonstrate humidity-induced reversible variations in the magnetic properties of cyano-bridged cobalt(II)-manganese(II)-chromium(III) metal assemblies. The observed magnetic humidity response is due to adsorption and desorption of a ligand water molecule on the cobalt ion, which changes cobalt (II) between a 6- and 4-fold coordination geometry and switches the magnetic interaction between ferromagnetic coupling and antiferromagnetic coupling.     

New properties of SAW gas sensing

Novel attractive properties of SAW gas sensing are theoretically predicted and experimentally verified. The response upon gas exposure of SAW-based gas sensors can be increased, decreased, reversed, cancelled, speeded up, aged down, and selected for a given sensitive layer, simply by changing the substrate material and orientation. When utilized as a tool for analytical chemistry, the steady-state and kinetic properties of adsorption, desorption, and diffusion, together with other related processes, can be simply deduced from pure acoustic measurements. These new properties are shown to be produced by the change of the components of the elliptic polarization of the wave, varying with the propagation direction and the substrate material. Experimental results, obtained for quartz substrates coated with polycrystalline palladium and palladium-nickel films exposed to H(2), CO, N(2)O, and different concentrations of relative humidity in air are presented as an example.     

Formation of Uniform Water Microdroplets on Wrinkled Graphene for Ultrafast Humidity Sensing

Portable humidity sensors with ultrafast responses fabricated in wearable devices have promising application prospects in disease diagnostics, health status monitoring, and personal healthcare data collecting. However, prolonged exposures to high‐humidity environments usually cause device degradation or failure due to excessive water adsorbed on the sensor surface. In the present work, a graphene film based humidity sensor with a hydrophobic surface and uniformly distributed ring‐like wrinkles is designed and fabricated that exhibits excellent performance in breath sensing. The wrinkled morphology of the graphene sensor is able to effectively prevent the aggregation of water microdroplets and thus maximize the evaporation rate. The as‐fabricated sensor responds to and recovers from humidity in 12.5 ms, the fastest response of humidity sensors reported so far, yet in a very stable manner. The sensor is fabricated into a mask and successfully applied to monitoring sudden changes in respiratory rate and depth, such as breathing disorder or arrest, as well as subtle changes in humidity level caused by talking, cough and skin evaporation. The sensor can potentially enable long‐term daily monitoring of breath and skin evaporation with its ultrafast response and high sensitivity, as well as excellent stability in high‐humidity environments.