Thin-film calorimetric H2O2 gas sensor for the validation of germicidal effectivity in aseptic filling processes

In common aseptic filling processes, hydrogen peroxide vapour is a predominantly applied antimicrobial for the inactivation of microorganisms in packages. During this process, the germicidal effectivity of the antimicrobial treatment depends especially on the H₂O₂ concentration of the gas mixture. For the detection of H₂O₂ in aseptic filling processes, a novel thin-film calorimetric gas sensor based on a differential set-up of a catalytically activated and a passivated temperature sensing element has been realised in the present work. The sensor device contains two meander-shaped platinum resistances as temperature sensing elements; both have been passivated with spin-coated perfluoralkoxy. As catalytically active materials for the calorimetric gas sensor, palladium, platinum black and manganese oxide particles have been studied in the developed experimental set-up, wherein MnO₂ has shown the highest sensitivity of 0.57 °C/% (v/v) towards H₂O₂. Afterwards, the characteristic of the sensor device with MnO₂ particles as catalyst has been examined at various H₂O₂ concentrations and additionally, the influence of gas temperature and gas flow rate on the sensor signal has been validated in the experimental set-up.     

Self-heated hydrogen gas sensors based on Pt-coated W18O49 nanowire networks with high sensitivity, good selectivity and low power consumption

Self-heating effect can be applied to develop gas sensors avoiding use of external heaters. We report here a prototype self-heated hydrogen gas sensor based on Pt-coated W₁₈O₄₉ nanowire networks. Such a sensor has been shown to have promising performance, e.g., high sensitivity (able to detect down to 50 ppm of H₂), good selectivity (poor response to ethanol, CH₄, CO, and C₃H₈), and low power consumption of 30-60 mW at 6 V compatible to a portable device. Coaxial cable model and percolation theory are applied to explain the gas sensing effect of the Pt-coated W₁₈O₄₉ nanowire networks.     

Determination of the composition of NO2 and NO mixture by thin film sensor and back-propagation network

In this work, vacuum deposited thin films of lead phthalocyanine (PbPc) were employed as gas sensor to detect mixtures of NO₂ and NO. Data collected from sensor responses were used to train a two-stage back-propagation network (BPN) for the determination of the gas mixture composition. The success rate for the two-stage BPN in identifying categories of gas mixture approaches 100%. The maximum error for the two-stage BPN in predicting NO concentration in a gas mixture with fixed NO₂ concentration is 14.7%.     

Calculation and correlation of the Na2SO4–NaCl–H2O2–H2O system at the temperature 288.15 K using the extended Pitzer model

The extended Pitzer model has been used for calculation and correlation of the ternary NaCl–H₂O₂–H₂O, Na₂SO₄–H₂O₂–H₂O and the quaternary Na₂SO₄–NaCl–H₂O₂–H₂O systems at the temperature of 288.15 K. The necessary thermodynamic functions (binary and ternary parameters of interaction and thermodynamic solubility products) have been derived from a least-squares optimization procedure with couples activity coefficient with solubility data and the theoretical solubility isotherms has been plotted. Good agreement with experimental solubility for ternary and quaternary mixtures indicates that the model can be successfully used to predict the component solubility of the electrolyte–nonelectrolytes–water systems containing peroxide. In addition, the Pitzer interaction parameters and the thermodynamic solubility product of 4Na₂SO₄⋅2H₂O₂⋅NaCl and Na₂SO₄⋅0.5H₂O₂⋅H₂O at 288.15 K are obtained.     

Application of the hoch-arpshofen model to ternary,quaternary, and larger systems

The Hoch-Arpshofen model is derived for ternary, quaternary and larger systems. The advantages of the model are the effect of a binary system in a ternary which is included in the basic derivation of the model, and that no special interpolation formulas are needed. Also, the maximum number of constants needed to describe a binary system is four: two for H_m, the enthalpy of mixing, and two for S_m^ex, the excess entropy of mixing.     

Flexible H2 sensor fabricated by layer-by-layer self-assembly of thin films of polypyrrole and modified in situ with Pt nanoparticles

A novel flexible H₂ gas sensor was fabricated by the layer-by-layer (LBL) self-assembly of a polypyrrole (PPy) thin film on a polyester (PET) substrate. A Pt-based complex was self-assembled in situ on the as-prepared PPy thin film, which was reduced to form a Pt-PPy thin film. Microstructural observations revealed that Pt nanoparticles formed on the surface of the PPy film. The sensitivity of the PPy thin film was improved by the Pt nanoparticles, providing catalytically active sites for H₂ gas molecules. The interfering gas NH₃ affected the limit of detection (LOD) of a targeted H₂ gas in a real-world binary gas mixture. A plausible H₂ gas sensing mechanism involves catalytic effects of Pt particles and the formation of charge carriers in the PPy thin film. The flexible H₂ gas sensor exhibited a strong sensitivity that was greater than that of sensors that were made of Pd-MWCNTs at room temperature.     

The effect of gas‐dynamic factors on selective carbon‐nanotube synthesis by injection CVD method for field‐emission cathodes

Abstract— Reversible selective growth of carbon‐nanotube (CNT) arrays on Si/SiO₂ topologies was investigated for field‐emission‐display applications. The method used was that of high‐temperature pyrolysis of fluid hydrocarbon (p‐xylene [C₈H₁₀]) in a mixture with volatile catalyst (ferrocene [Fe(C₅H₅)₂]) using Ar as the gas carrier. The synthesized CNT arrays were analyzed by SEM, TEM, Raman, and TGA analyses. Reversible CNT growth both on Si and SiO₂ surfaces was found to be sensitive to the gas‐carrier flow rate and the catalyst/hydrocarbon solution injection rate into the synthesis zone. This phenomenon can be explained by inverse domination of active sites on Si and SiO₂ surfaces at different flow rates of gas mixture, causing different types of catalyst precipitation followed by subsequent CNT growth. In principle, the possibility of growing CNTs using the proposed technology will allow the creation of precise geometries of field‐emission cathodes excluding the step of catalyst localization.     

Water pre-adsorption effect on room temperature SnO2 nanobelt ethanol sensitivity in oxygen-deficient conditions

The water adsorption effects on room temperature ethanol sensitivity of a SnO₂ single crystal nanobelt was studied. SnO₂ nanobelt showed better electrical conductivity in air than in vacuum. The current increased linearly with relative humidity in air and with water concentration in vacuum. In the presence of water vapor, the SnO₂ nanobelt electrical response was quite different, depending on the H₂O and C₂H₅OH molecules adsorption sequence. The current response to ethanol gas increased substantially when water was pre-adsorbed. However, no change was found without water pre-adsorbtion. This interesting behavior is ascribed to competition between the H₂O and C₂H₅OH molecules trying to adsorb on oxygen sites at the tin oxide surface. Dissociated water acts as the surface conduction channel resulting in better conductivity, while ethanol is physisorbed without water pre-adsorption. Based on this sensing mechanism, SnO₂ nanobelt can be used as a highly efficient ethanol detector in humid air.     

Improved hydrogen-sensing performance of a Pd/GaN Schottky diode with a surface plasma treatment approach

The improved hydrogen-sensing performance of a Pd/GaN Schottky diode with a simple surface treatment is demonstrated. The studied device with an inductively coupled-plasma (ICP)-treatment shows both the good sensitivity and fast response. A high hydrogen detection sensing response of 2.05 × 10⁵, under exposing to a 10,000 ppm H₂/air gas at room temperature, is obtained. It is found that, due to the increased surface roughness, more hydrogen atoms are adsorbed on the active layer which leads to the substantial increase of current change. In addition, the studied device shows a stable and widespread reverse voltage operating regime (−0.3 to −3 V) and a fast response about of 2.9 s. Therefore, this simple surface treatment approach gives the promise for hydrogen sensing applications.     

A basic-2 program for the “psychrometric problem” of ammonia,in neutral gas absorption refrigeration units

The ‘psychrometric problem’ for small neutral gas absorption refrigeration units concerns among others the determination of the mass fraction of the NH₃/H₂ gas mixture, when the ‘dry’ and ‘wet bulb’ temperatures are known. By the present work, the values of the mass fraction for various combinations of ‘dry’ and ‘wet bulb’ temperatures are computed and the relevant program codes are given. The pressure range compiles the steps of 17.5 bar, 20 bar, 22.5 bar, 25 bar, and 27.5 bar, i.e. the usual operating conditions of these units.     

Solubility prediction for the nonelectrolyte urea–hydrogen peroxide–water system and thermodynamic solubility product calculation for CO(NH2)2 ⋅ H2O2 using the modified Pitzer model

The Pitzer interaction model, which has been applied successfully to the thermodynamic simulation of electrolyte solutions and electrolyte–nonelectrolyte solutions, was extended to nonelectrolyte–nonelectrolyte solution system. In the present work, the modified Pitzer model was used for calculation and correlation of the ternary CO(NH₂)₂–H₂O₂–H₂O system at 283.15 K. The value of the Pitzer interaction parameters for the ternary system and the thermodynamic solubility product of CO(NH₂)₂⋅H₂O₂ were determined using a least-square optimization procedure with coupling activity coefficient and solubility data. The predicted isothermal solubilities agree well with the result obtained from the experiment. The results indicated that the modified Pitzer model could be successfully used to predict the component solubility of the nonelectrolyte–nonelectrolyte system.     

A basic-2 program to compute the enthalpy/mass fraction diagram of the real NH3/H2 gas mixture at 25 bar,for neutral gas absorption refrigeration units (technical note)

The thermodynamic properties of the NH₃/H₂ gas mixture, namely the enthalpy, the specific volume of the mixture and the NH₃ ‘partial pressure’ can be presented in an enthalpy/mass fraction diagram as a function of the state conditions (temperature, mass fraction, total pressure) to facilitate the calculation of the evaporation of NH₃ taking place in small absorption refrigeration units. A method and a computer program have been developed giving the desired quantities, starting from the corresponding data of the real components H₂ and NH₃ and using the method of the second virial coefficients. The relevant results are valid for a total pressure of 25 bar and temperatures between −40°C to +50°C.     

Temperature dependent H2S and Cl2 sensing selectivity of Cr2O3 thin films

The conductometric gas sensing characteristics of Cr₂O₃ thin films - prepared by electron-beam deposition of Cr films on quartz substrate followed by oxygen annealing - have been investigated for a host of gases (CH₄, CO, NO₂, Cl₂, NH₃ and H₂S) as a function of operating temperature (between 30 and 300 °C) and gas concentration (1-30 ppm). We demonstrate that these films are highly selective to H₂S at an operating temperature of 100 °C, while at 220 °C the films become selective to Cl₂. This result has been explained on the basis of depletion of chemisorbed oxygen from the surface of films due to temperature and/or interaction with Cl₂/H₂S, which is supported experimentally by carrying out the work function measurements using Kelvin probe method. The temperature dependent selectivity of Cr₂O₃ thin films provides a flexibility to use same film for the sensing of Cl₂ as well as H₂S.     

Thermodynamic study of the KMgAlClSO4H2O system at the temperature 298.15 K

The Pitzer ion-interaction model has been used for thermodynamic simulation of the binary AlCl₃H₂O, Al₂(SO4)₃H2O, ternary KClAlCl₃H₂O, K₂SO₄Al₂(SO₄)₃H₂O, MgCl₂AlCl₃H₂O, MgSO₄Al₂(SO₄)₃H₂O, and the quaternary KClMgCl₂AlCl₃H₂O systems at T=298.15 K. The optimum values of the binary parameters of ionic interactions for aluminum solutions have been calculated using activity data up to saturation of solutions. The ternary parameters have been chosen on the basis of the compositions of saturated ternary solutions taking into account the unsymmetrical mixing terms. Good agreement between experimentally determined and calculated solubilities has been found. Important thermodynamic characteristics (thermodynamic solubility product, standard molar Gibbs energy of formation) of the solid phases (simple and double salts) crystallizing in the systems under consideration are determined.     

Characteristics of a Pd–oxide–In0.49Ga0.51P high electron mobility transistor (HEMT)-based hydrogen sensor

An interesting hydrogen sensor based on a high electron mobility transistor (HEMT) device with a Pd–oxide–In_0.49Ga_0.51P gate structure is fabricated and demonstrated. The hydrogen sensing characteristics including hydrogen detection sensitivity and transient responses of the studied device under different hydrogen concentrations and temperature are measured and studied. The hydrogen detection sensitivity is related to a change in the contact potential at the Pd/insulator interface. The kinetic and thermodynamic properties of hydrogen adsorption are also studied. Experimentally, good hydrogen detection sensitivities, large magnitude of current variations (3.96 mA in 9970 ppm H₂/air gas at room temperature) and shorter absorption response time (22 s in 9970 ppm H₂/air gas at room temperature) are obtained for a 1.4 μm × 100 μm gate dimension device. Therefore, the studied device provides a promise for high-performance solid-state hydrogen sensor, integrated circuit (IC) and micro electro-mechanical system (MEMS) applications.     

Thermodynamics of formation of ammonium,sodium and potassium alums and chromium alums

The Pitzer ion-interaction model has been used for thermodynamic simulation of the binary Cr₂(SO4)₃H2O, KCr(SO4)₂H2O, and ternary (NH₄)₂SO₄Al₂(SO₄)₃H₂O, Na₂SO₄Al₂(SO₄)₃H₂O, K₂SO₄Al₂(SO₄)₃H₂O, (NH₄)₂SO₄Cr₂(SO₄)₃H₂O, Na₂SO₄Cr₂(SO₄)₃H₂O, and K₂SO₄Cr₂(SO₄)₃H₂O systems at T=298.15 K. The optimum values of the binary parameters of ionic interactions for chromium solutions have been calculated using osmotic data for unsaturated binary solutions. The mixing parameters have been chosen on the basis of the compositions of saturated ternary solutions and data on the binary solubility of the alums and chromium alums in water. Good agreement between experimentally determined and calculated solubilities has been found. Important thermodynamic characteristics (thermodynamic solubility product, standard molar Gibbs energy of formation) of the solid phases (simple and double salts) crystallizing in the systems under consideration are determined.     

Nano particulate SnO2 based resistive films as a hydrogen and acetone vapour sensor

SnCl₂ (solution) was spin coated on soda lime glass and Al₂O₃ substrate to obtain nano-particulate tin oxide film, directly by sintering at 550 °C for 40 minutes (min). The surface morphology and crystal structure of the tin oxide films were analyzed using atomic force microscopy (AFM) and X-ray diffraction (XRD). The size of SnO₂ nanostructure was determined from UV-vis and found to be ?3 nm. These films were tested for sensing H₂ concentration of 0.1-1000 ppm at optimized operating temperature of 265 °C. The results showed that sensitivity (R_air/R_gas per ppm) goes on increasing with decreasing concentration of test gas, giving concentration dependent changes. Special studies carried out at low concentration levels (0.1-1 and 1-10 ppm) of H₂, give high sensitivity (200 × 10⁻³/ppm) for lowest concentration (0.1-1 ppm) of H₂. The selectivity for H₂ against relative humidity (RH), CO₂, CO and LPG gases is also good. The sensor, at operating temperature of 200 °C, is showing nearly zero response to 300 ppm of H₂, and offering response to acetone vapour of 11 ppm. Selectivity for acetone against RH% and CO₂ was also studied. These sensors can be used as H₂ sensor at an operating temperature of 265 °C, and as an acetone sensor at the operating temperature of 200 °C.     

A single ZnO tetrapod-based sensor

Transferable ZnO tetrapods were grown by an aqueous solution method. An individual ZnO tetrapod-based sensor was fabricated by in situ lift-out technique and its ultraviolet (UV) and gas sensing properties were investigated. This single tetrapod-based device responds to the UV light rapidly and showed a recovery time of about 23 s. The sensitivity of a single ZnO tetrapod sensor to oxygen concentration was also investigated. We found that when UV illumination is switched off, the oxygen chemisorption process will dominate and assists photoconductivity relaxation. Thus relaxation dynamics is strongly affected by the ambient O₂ partial pressure as described.We also studied the response of ZnO tetrapod-based sensor in various gas environments, such as 100 ppm H₂, CO, i-butane, CH₄, CO₂, and SO₂ at room temperature. It is noted that ZnO tetrapod sensor is much more sensitive to H₂, i-butane and CO. It is demonstrated that a ZnO tetrapod exposed to both UV light and hydrogen can provide a unique integrated multiterminal architecture for novel electronic device configurations.