Ceramic electrolytes and electrochemical sensors

The electrochemical method involving solid electrolytes has been known as a selective and an accurate way of sensing chemical species in the environment and even in liquid metal for some time. The most successful among the electrochemical sensors are the emission control sensor (-sensor) for the automobile engine and the oxygen sensor used in steelmaking, both made of stabilized zirconia. This article presents an overview of basic principles of various types of electrochemical sensors including active (potentiometric) and passive (amperometric) sensors. Recent advances in oxygen (O₂), carbon dioxide (CO₂) and hydrogen (H₂) sensors are also presented.     

Development of a selective gas sensor utilizing a perm-selective zeolite membrane

Reported here is a novel sensor that utilizes a zeolite film to selectively limit gas exposure of the sensing surface. A unique amperometric sensor design based on a non-porous mixed conducting sensing electrode enables the formation of a continuous zeolite film covering the entire sensor surface. The sensor was tested in a variety of oxygen containing gases. The sensor without a zeolite film responded strongly to both oxygen and carbon dioxide at a bias of 1.8 V. In contrast, the sensor coated with a zeolite film showed a discernable, but diminished response to oxygen, and a more marked drop in response to CO₂ indicating that the diffusion of oxygen through the zeolite film is preferential to that of CO₂. The response of the zeolite coated sensor to a mixture of oxygen and carbon dioxide gases is attributed primarily to the oxygen content. Expanding this concept using a variety of different zeolite structures covering an array of sensors, complete analyses of complex gaseous mixtures could be performed in a very small device.     

Chemical Fluid Deposition: A Hybrid Technique for Low‐Temperature Metallization

Chemical fluid deposition (CFD) is a novel approach to metal deposition that involves the chemical reduction of organometallic compounds in supercritical carbon dioxide to yield high purity films at low temperature. Since supercritical CO₂ can exhibit densities that approach those of a liquid solvent while retaining the transport properties of a gas, CFD is essentially a hybrid technique that uniquely blends the advantages of chemical vapor deposition (CVD) and electroless plating. Here, we describe the deposition of high‐purity films of Pt, Pd, Au, and Rh onto inorganic and polymer substrates by the reduction of appropriate precursors in CO₂ at 60 °C.     

Synthesis and characterisation of cerium sulphide over optical fiber and its gas sensing application

In this study, a cladding-modified optical fiber-based gas sensor is proposed for detection of carbon dioxide gas. Cerium disulphide (CeS₂) is synthesised and coated by chemical bath deposition (CBD) route over the region in which cladding is removed. This synthesised cerium disulphide is characterised by x-ray diffraction (XRD), photoluminescence spectrum (PL), ultra violet-visible absorption spectrum, ultra violet -visible reflectance and transmittance spectrum, scanning electron microscope images (SEM) and energy dispersive x-ray spectrum (EDX). The response of the sensor is recorded for different concentrations of test gases (nitrogen dioxide and carbon dioxide) and the sensor's selectivity is compared and analysed. It is observed that cerium disulphide is more sensitive to carbon dioxide compared to nitrogen dioxide.     

On glass formation for a Na2O·4TeO2 melt: Effect of melting temperature, time, and raw material

The effect of melting temperature, time, and the type of raw material, NaNO₃ or Na₂CO₃, as a source for Na₂O on the glass formation for a Na₂O·4TeO₂(NT₄) melt was investigated. Melting with NaNO₃ at 750°C for a short time (15 min) produced a glass that is slightly more chemically durable and more resistant to crystallization than glasses melted at a higher temperature (800°C), or for a longer time (60 min), or using Na₂CO₃. A thin surface layer (<1.5 nm) that contains some nitrogen and a higher concentration of bridging oxygen is suspected to be the reason for the higher chemical durability and higher resistance to crystallization for this glass. However, melting at 800°C for 60 min produced a glass, whose properties were independent of the type of raw material, NaNO₃ or Na₂CO₃, used.     

Fabrication, characterisation and application of (8 mol% Y2O3) ZrO2 thin film on Na3Zr2Si2PO12 substrate for sensing CO2 gas

A planar type CO₂ gas sensor employing (8 mol% Y₂O₃) ZrO₂ (YSZ) thin film on Na₃Zr₂Si₂PO₁₂ (Nasicon) substrate with Na₂CO₃ as an auxiliary electrode has been fabricated and tested in laboratory environment between 700–900 K. The YSZ thin film was fabricated on Nasicon and alumina substrate using radio frequency (RF) magnetron sputtering. The film was examined using SEM and X-ray diffraction (XRD) after treating the Nasicon-YSZ bi-layer structure at 1300 K for 2 h. The results indicate that a crack free YSZ film was produced on Nasicon surface that was well bonded to the substrate. The conductivity of sputtered YSZ thin film measured by ac-impedance spectroscopy has been found to be higher than that of YSZ pellet by approximately half an order of magniture. The bi-electrolyte planar sensor displays rapid response (t₉₅ 200 s) to CO₂ compared to the tube type sensor (t₉₅ 700 s) and the measured open circuit voltage of the electrochemical CO₂ sensor has been found to be Nernstian at all temperatures.     

Utilization of critical fluids in processing semiconductors and their related materials

The use of carbon dioxide in its various states: supercritical (SC-CO₂), liquid (L-CO₂) or pellet form (snow-CO₂) for processing and cleaning in semiconductor fabrication and related electronic devices is assessed in this review. An understanding of the fundamental mechanisms responsible for carbon dioxide-based processing, as in surface cleaning, is lacking. Although carbon dioxide is an excellent solvent for removing non-polar contaminants from a variety of surfaces, other CO₂-based cleaning and surface modification processes are based on mechanical or morphological-induced changes in the interfacial region. The extremely low surface tension of CO₂ is a favorable property in terms of its rapid and complete removal from the substrate after a treatment has been affected, and this characteristic of CO₂ also accounts for its negligible effect on the morphology of the substrate, as utilized in the microelectronic industry. Applications of critical fluids in integrated circuit manufacturing operations, such as wafer cleaning, film deposition, photoresist stripping, drying, and particulate removal are noted.     

Size-controllable synthesis of calcium carbonate nanoparticles using aqueous foam films as templates

A new method for the synthesis of calcium carbonate nanoparticles using two types of foam which are separately stabilized by surfactants with opposite change has been described here. In detail, one type of foam is formed by the aqueous solution of CaCl₂ which contains the anionic surfactant alkyl polyoxyethylene alcohol sulfate sodium (AES) and the other is formed by the aqueous solution of Na₂CO₃ and the cationic surfactant alkyl polyoxyethylene quaternary ammonium chloride (AEAC). Two types of foam contact each other in a specially designed apparatus after draining completely, then Ca²⁺ and CO₃²⁻ entrapped by the surfactant layers at the thin borders between the foam bubbles react and result in the generation of CaCO₃ nanoparticles. TEM determination showed that perfect mono-dispersed spherical nanoparticles were obtained and the size of particles can be conveniently controlled by changing the concentration of CaCl₂ and Na₂CO₃. Also, the mechanism leading to the synthesis of CaCO₃ nanoparticles was discussed in detail.     

Experimental and modelling studies of carbon dioxide adsorption by porous biomass derived activated carbon

The goal of the study was to produce a low-cost activated carbon from agricultural residues via single stage carbon dioxide (CO₂) activation and to investigate its applicability in capturing CO₂ flue gas. The performance of the activated carbon was characterized in terms of the chemical composition, surface morphology as well as textural characteristics. The adsorption capacity was investigated at three temperatures of 25, 50 and 100 °C for different types of adsorbate, such as purified carbon dioxide and binary mixture of carbon dioxide and nitrogen. The purified CO₂ adsorption study showed that the greatest adsorption capacity of the optimized activated carbon of 1.79 mmol g^?1 was obtained at the lowest operating temperature. In addition, the adsorption study proved that the adsorption capacity for binary mixtures was lower due to the reduction in partial pressure. The experimental values of the purified CO₂ adsorption were modelled by the Lagergren pseudo-first-order model, pseudo-second-order model, and intra-particle diffusion model. Based on the analysis, it inferred that the adsorption of CO₂ followed the pseudo-second-order model with regression coefficient value higher than 0.995. In addition, the adsorption study was governed by both film diffusion and intra-particle diffusion. The activation energy that was lesser than 25 kJ mol^?1 implied that physical adsorption (physisorption) occurred.     

Suzuki–Miyaura reaction and solventfree oxidation of benzyl alcohol by Pd/nitrogen-doped CNTs catalyst

Suzuki–Miyaura C–C coupling reactions were investigated with Pd/nitrogen-doped carbon nanotubes (Pd/N-CNTs) as a catalyst. Also, the same catalyst was examined for the solventfree oxidation of benzyl alcohol to benzaldehyde. Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized from 1-ferrocenylmethyl(2-methylimidazole) and benzophenone via a chemical vapour deposition technique. Acetonitrile was used as a solvent and source of both carbon and nitrogen constituents of N-CNTs. Pd nanoparticles (Pd NPs) were successfully dispersed on N-CNTs via a metal organic chemical vapour deposition method. SEM, TEM, XRD, elemental analysis and ICP-OES measurements were used to characterize the nanomaterials. From the TEM analysis, it was observed that Pd NPs were spherical and with particle sizes ranging from 3 to 8 nm. For Suzuki C–C coupling reactions, phenylboronic acid, aryl halide, Pd/N-CNTs catalyst and a base (NaOAc, K₂PO₄, K₂CO₃, NaOH, Et₃N and Na₂CO₃) were used. The optimized experiments indicate that K₂CO₃, as the base, and ethanol/water (1:1 v/v, 10 mL) mixture, as a solvent, are the best reaction conditions. The solventfree oxidation reactions of benzyl alcohol were also done with Pd/N-CNTs catalyst and benzyl alcohol as a substrate. In both sets of reactions, C–C coupling and oxidation, the increase in pyrrolic nitrogen species was found to be responsible for higher catalytic activities of Pd/N-CNT catalysts, and this was attributed to the ease of Pd NP dispersion on N-CNTs, relative to pristine CNTs. Also, the higher catalytic activity of Pd/N-CNTs could be ascribed not only to the smaller Pd NP size or surface area, but to also the surface properties and the nature of the support when compared with the undoped counterpart, Pd/CNTs.     

Characterization of zinc carbonate hydroxides synthesized by precipitation from zinc acetate and potassium carbonate solutions

Addition of potassium carbonate solution to zinc acetate solution at room temperature causes the precipitation of a white solid, whose composition and structure depend on the initial concentrations of the reagents. Hydrozincite, Zn₅(CO₃)₂(OH)₆, forms when the K₂CO₃ concentration is low and a new K-containing zinc carbonate hydroxide phase forms when the K₂CO₃ concentration is high. The chemical formula of the new phase has been determined to be Zn(CO₃)_0.61(OH)_0.780.233K₂CO₃ by TGA, CHNO and AA analysis. The new phase is insoluble in water and extensive water washing of the new phase does not change its composition or structure, suggesting the new phase is a single phase compound salt. Exposure of the new phase to a stream of humidified CO₂ causes disproportionation to separate phases of ZnCO₃ and K₂CO₃. Pure ZnCO₃ was synthesized for comparison by a new procedure under atmospheric conditions instead of the more common hydrothermal synthesis of ZnCO₃.     

Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapor deposition at low temperature

Vertically-aligned carbon nanotubes(CNTs) with multi-walled structure were successfully grown on a Fe-deposited Si substrate at low temperature below 330°C by using the microwave plasma chemical vapor deposition of methane and carbon dioxide gas mixture. This is apparently different from the conventional reaction in gas mixtures of hydrogen and methane, hydrogen and acetylene, and hydrogen and benzene ... etc. High quality carbon nanotubes were grown at lower temperature with CO₂ and CH₄ gas mixture than those used by the previous. After deposition, the microstructure morphology of carbon nanotubes was observed with scanning electron microscope and high-resolution transmission electron microscope. The characteristics of carbon nanotubes were analyzed by laser Raman spectroscopy. The results showed the variation of the flow rate ratio of CH₄/CO₂ from 28.5 sccm/30 sccm to 30/30 sccm and the DC bias voltage from –150 V to –200 V, at 300 W microwave power, 1.3–2.0 kPa range of total gas pressure, and substrate temperatures between 300°C and 350°C. Vertically aligned carbon nanotubes with the diameter of about 15 nm and multi-walled structure were illustrated by SEM and HRTEM. However, the highest yield of carbon nanotubes of about 50% was obtained at low temperature below 330°C by MPCVD for the CH₄/CO₂ gas mixture with properly controlled parameters.     

Design of high sorbent pectin/CaCO3 composites tuned by pectin characteristics and carbonate source

Five pectin samples – which differ by the methylation degree and/or amide content – were used to prepare inorganic/organic composites by CaCO₃ mineralization from supersaturated solutions. The pectin chemical structure and concentration could control the composite superstructure by induction or orientation of crystal growth. The inorganic materials may also control CaCO₃ polymorphism and morphologies and therefore different carbonate sources, such as Na₂CO₃, diethylcarbonate or ammonium carbonates, were used as modulators for crystal growth. The morphology of the new CaCO₃/pectin composites was investigated by SEM and the polymorphs content by X-ray diffraction, as compared to bare CaCO₃ samples prepared in similar conditions. The composites were tested as sorbents for Cu(II) and Ni(II) ions.     

Fabrication and characterization of nanostructured indium tin oxide film and its application as humidity and gas sensors

This paper reports the synthesis and characterization of nanocrystalline indium tin oxide (ITO) and its application as humidity and gas sensors. The structure and crystallite size of the synthesized powder were determined by X-ray diffraction. The minimum crystallite size was found 5 nm by Debye–Scherrer equation and confirmed by transmission electron microscopy image. Optical characterizations of ITO were studied using UV–visible absorption spectroscopy and Fourier transforms infrared spectroscopy. Thermal analysis was carried out by differential scanning calorimetry. Further, the ITO thin film was fabricated using sol–gel spin coating method. The surface morphology of the fabricated film was investigated using scanning electron microscopy images. For the study of humidity sensing, the thin film of ITO was exposed with humidity in a controlled humidity chamber. The variations in resistance of the film with relative humidity were observed. The average sensitivity of the humidity sensor was found 0.70 MΩ/%RH. In addition, we have also investigated the carbon dioxide (CO₂) and liquefied petroleum gas sensing behaviour of the fabricated film. Maximum sensitivity of the film was ~17 towards CO₂. Its response and recovery times were ~5 and 7 min respectively. Sensor based on CO₂ is 97 % reproducible after 3 months of its fabrication. Better sensitivity, small response time and good reproducibility recognized that the fabricated sensor is challenging for the detection of carbon dioxide.     

Preparation and characterization of carbon molecular sieves from chestnut shell by chemical vapor deposition

In this study, carbon molecular sieves (CMS) were produced from chestnut shell by chemical activation process followed by chemical vapor deposition (CVD) of methane. The influences of deposition temperature (800–900?°C), time (15–60?min) and flow rate of CH₄ (100–300?mL/min) on pore development of carbon molecular sieve were investigated. The produced CMSs were characterized by several techniques such as N₂ adsorption, CO₂ adsorption, CH₄ adsorption, elemental analysis, FTIR analysis and SEM analysis. The textural analysis of the CMS samples showed the successful deposition of methane on pores of the produced activated carbon derived from chestnut shell to yield a microporous CMS with a narrow pore size distribution. The deposition temperature, time and flow rate of CH₄ were shown to strongly affect the pore structure of the CMS. The maximum CO₂ adsorption capacity (525.7?mg/g) was obtained at a deposition temperature of 850?°C, time of 30?min, and CH₄ flow rate of 100?mL/min.     

Study on the chemical stability of Y-doped BaCeO3 − δ and BaZrO3 − δ films deposited by aerosol deposition

Barium cerate (BaCeO₃) has high proton conductivity but rather poor chemical stability in CO₂-containing atmospheres. Barium zirconate (BaZrO₃), in contrast, is a rather stable material, but exhibits poor sinterability. In the present work, powders of Y-doped BaCeO₃ and BaZrO₃ were synthesized via the solid solution reaction method, and dense ceramic membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ were prepared by the aerosol deposition method at room temperature. Aerosol deposition method is a technique that enables the fabrication of ceramic films at room temperature with a high deposition rate as well as strong adhesion to the substrate. The powders and aerosol-deposited membranes were characterized by X-ray diffraction, particle size analysis, scanning electron microscopy, and X-ray elemental mapping. The chemical stability of powders and aerosol-deposited membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ against water and carbon dioxide has been investigated, and it was found that BaZr_0.85Y_0.15O₃ materials showed a better chemical compatibility.     

Solid-state electrochemical CO2 gas sensors based on sodium ionic conductors

The sensing characteristics of a solid-state electrochemical CO₂ gas sensor, expressed as PtO₂, Na₂O Na ionic conductor Na₂CO₃CO₂, O₂Pt were investigated in terms of a two-electron electrochemical reaction. The number of electrons for the cell reaction was higher than 2 and approached 2 with an increase in the operating temperature up to about 500 °C. The introduction of water vapour induced a lowering of the e.m.f. and a prolongation of the response time. The formation of sodium oxides in the Na₂CO₃ layer was considered as a possible cause of these water effects. The sensing characteristics recovered completely after the water vapour was cutoff. The e.m.f. reduction due to water sorption was depressed by using a densified Na₃Zr₂Si₂PO₁₂. A densified Na₃Zr₂Si₂PO₁₂-based electrolyte is preferable for use as a gas sensor with a fast response and high stability for detection of CO₂ in air.     

Reaction of Np(VI) with H<Subscript>2</Subscript>O<Subscript>2</Subscript> in carbonate solutions

The kinetics and stoichiometry of the reaction of Np(VI) with H₂O₂ in carbonate solutions were studied by spectrophotometry. In the range 1–0.02 M Na₂CO₃, the reaction 2Np(VI) + H₂O₂ = 2Np(V) + O₂ occurs, as Δ[Np(VI)]/Δ[H₂O₂] ≈ 2. In Na₂CO₃ + NaHCO₃ solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H₂O₂ follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H₂O₂ concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na₂CO₃ solution containing 1 mM H₂O₂, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H₂O₂] = 1 mM, k passes through a minimum at [Na₂CO₃] = 0.5–0.1 M. The activation energy in a 0.5 M Na₂CO₃ solution is 48 kJ mol⁻¹.     

Reaction of ozone with Np(V) and Np(IV) in carbonate solutions

A spectrophotometric study showed that ozone in concentrated carbonate solutions forms complexes with CO ₃ ^2? ions, which inhibits the ozone decomposition. Free ozone oxidizes Np(V) at high rate. The bound ozone reacts with Np(V) at moderate rate. Np(IV) reacts with O₃ slowly, with Np(VI) formed in NaHCO₃ solution and only Np(V) formed in Na₂CO₃ solution.     

Development of accelerated processing techniques for cement-bonded wood particleboard

The potential for substantial acceleration of the curing process of cement-bonded wood particleboard by injection of diluted carbon dioxide gas was investigated experimentally. Milled particles of softwood and hardwood were used as reinforcement in cement boards processed under pressure. Injection of pure as well as diluted carbon dioxide gas greatly reduced (by two orders of magnitude) the press time required to yield dimensionally stable products of sufficient strength for initial handling. Diluted carbon dioxide gas (about 25% CO₂ concentration in air) yielded immediate flexural attributes (upon press release) which were competitive against those obtained with pure carbon dioxide gas. It is hypothesized that dilution of CO₂ gas lowers the excess rate of reactions, thereby controlling the rise in temperature and allowing for more thorough penetration (and reaction) of CO₂ gas. These benefits seem to compensate for the reduced rate of reactions of diluted CO₂ gas. Competitive immediate performance characteristics are thus achieved with diluted CO₂ gas with in spite of the lower CO₂ gas consumption (and thus lower processing costs).