Monolithic Electrodes Research Articles

Defect chemistry based design of monolithic langasite structures for high temperature sensors

High temperature stable piezoelectric crystals open a wide range of new sensor applications due to the ability to monitor environmentally dependent mechanical properties of sensor films deposited onto piezoelectric resonators. An understanding of the defect chemistry of piezoelectric single crystals like langasite (La3Ga5SiO14) aids in establishing their application limits and enables the creation of monolithic structures by appropriate doping. Key defect chemical properties of langasite are outlined with respect to dopants which increase the electrical conductivity or lower the electromechanical losses. We focus here on electronic and ionic transport in heavily Sr-doped langasite since spatially localized doping can be used to create monolithic electrodes. The total electrical conductivity (impedance measurements), the transport of oxygen and dopants (diffusion runs using stable tracers and subsequent secondary ion mass spectrometry) as well as the electromechanical properties (network analysis) are correlated. Strontium doped areas exhibit an increased conductivity by about 4 orders of magnitude, largely governed by oxygen ions. For demonstration of the concept, langasite bulk acoustic wave resonators with Sr-doped monolithic electrodes are operated up to 800°C. The resonator quality factors are found to be sufficiently high to use such resonators as high sensitivity sensors.

Highly porous activated glassy carbon film sandwich structure for electrochemical energy storage in ultracapacitor applications: Study of the porous film structure and gradient

Glassy carbon plates were thermochemically gas phase oxidized to obtain monolithic sandwichlike electrode assemblies with high surface area porous films for electrochemical energy storage applications. Film thicknesses were varied by variation of oxidation parameters time, temperature, and oxygen concentration and measured with electron microscopy. The mass density of the porous carbon film material was estimated by fitting a geometrical model to experimental gravimetric data. Optical Raman spectroscopy line scans suggest that the porosity has a gradient between the surface and the film/bulk interface, which is supported by pore-size distribution data obtained from small-angle x-ray scattering (SAXS) on slightly oxidized and fully oxidized samples. Detailed inspection of the power law behavior of SAXS data suggests that the internal surface area of well-oxidized glassy carbon (GC) is compact and extends over the entire probed volume and thus has optimal pore connectivity. This effect goes along with pore enlargement and a relative decrease of internal surface area per volume. Slightly oxidized carbon has no pore space with a compact, high connectivity internal surface area. The corresponding SAXS power law and the x-ray density suggest that this high volumetric surface area must be interpreted as a result of surface roughness, rather than true geometric or volumetric surface area. In consequence, is this surface area of limited use for electrochemical energy storage?

Diffusion in High-Temperature Piezoelectric Single Crystals

The high-temperature stability of the single crystalline piezoelec-tric material langasite and associated transport phenomena such as the diffusion of the constituents gallium and oxygen are investi-gated. Small structures remain their shape upon annealing at 1350°C for 24 h. The transport phenomena are in accordance with the defect model of langasite. In particular, oxygen is found to be the dominant ionic charge carrier. A newly developed concept based on monolithic structures in langasite is realized by local dop-ing with praseodymium, strontium and niobium. Those elements either increase the conductivity or modify the rate of wet chemical etching. In order to demonstrate the feasibility of monolithic elec-trodes, the operation of locally Sr-doped bulk acoustic resonators is presented. Further, the correlation of electromechanical properties and transport processes is investigated using langasite bulk acous-tic resonators. Doping by niobium lowers e. g. the total loss at high temperatures.

Localized Electrochemistry on a 10 μm Spot on a Monolith Large Electrode: An Avenue for Electrochemical Microarray Analysis

By measurement of the modulation of ions under an ac field at the electrode/electrolyte interface layer (EIL) at a spatial modulation of 0.0001 nm, a quantitative measurement of local redox current on a 10 microm spot of a working electrode is achieved at a sensitivity of 0.02 nA/spot. The spot is defined by the probe laser beam. We show the principle for two redox ions, [Fe(CN)(6)](3-) and [Ru(NH(3))(6)](3+), on a 4 mm(2) Au electrode. The redox peaks from the optical and the electrochemical (cyclic voltammetry (CV)) responses, which are concomitantly measured, are within 0.005 V. The setup is identical to the conventional three-electrode measurement, and the laser beam does not perturb the electrochemical process. We demonstrate high sensitivity to "read" binding between two single-stranded DNA (ssDNA) on a 10 microm spot for DNA microarray analysis where the signal is "blind" to nonspecific binding. Potentially, quantitative combinatorial electrochemistry can be performed on a monolith electrode by scanning the laser beam on the electrode.

Force Characterization and Rigidity Analysis of a Monolithic Cochlear Prosthesis Actuator

Cochlear implants continue to be used in the treatment of profound deafness. Because of the tonotopic nature of the cochlea, more controlled insertion is perhaps the most important factor affecting device performance. The implant stiffness, and therefore the scala tympani (ST) wall contact force, contributes to insertion difficulties. Attempts to correlate the implant carrier structural properties and the intracochlear contact forces during insertion are limited. Researchers in the Michigan Center for Wireless Integrated Microsystems are developing perimodiolar-shaped silicon and parylene-based thin film cochlear electrode arrays and backing devices for a more controllable implantation. We report a method developed for measuring the thin film actuated electrode array rigidity to quantify the ST and modiolus wall contact forces during and after insertion. The method used a pneumatically actuated polyethylene terephthalate (PET) monolithic electrode actuator using pressurized air (0–200 kPa) for actuation. The prototype actuators consisted of PET tubes with an ID of 365 μm and a wall thickness of 58 μm. Force calculations using cantilever beam bending theory were performed to estimate the tube bending forces as a function of internal pressure and therefore variable structural stiffness. Based on estimations, a method was developed to measure such small forces avoiding the use of commercially available, relatively insensitive load cells. A fixture was fabricated incorporating two brass microcantilevers (reference and deflection arms) sensitive to sub-mN forces applied by the actuator on the deflection arm of the cantilevers. Microcantilver deflection data, captured by an interferometric microscope, was used to calculate the actuator force and eventually the reaction force acting on the actuator. The implant actuation forces ranged from 0–0.76 mN over an actuation pressure range of 0–140 kPa, from nearly straight to the relaxed perimodiolar post-implantation shape. For estimating the implant rigidity (EI), the actuator stiffness and the actuation pressure was correlated. The actuator stiffness at different actuation pressures was obtained both theoretically (using beam bending theory and PET tube structural properties) and experimentally (using the derived unconstrained actuator deflections at measured actuator forces). The theoretical and experimental stiffness values ranged from 3.6E-08 to 5.34E-07 N/μm and 2.5E-08 to 7.8E-06 N/μm respectively over the working pressure range. The calculated rigidity constant (EI) of the best prototype insertion tool from the experimental stiffness measurement was 6.71E06 Nμm2. The insertion tool-ST wall contact forces were calculated, using the estimated rigidity, in a hypothetical insertion situation. Force calculations assumed that the implant is equipped with actuator deflection feedback sensors and the actuator's stiffness remains constant over its entire length for a given operating pressure. A contact force of 1.19 mN was found acting on the cochlear ST wall when the insertion tool hits the wall and deflects by 200 μm at an actuation pressure of 140 kPa.

Corrosion behaviour of carbon fibres/magnesium metal matrix composite and electrochemical response of its constituents

The corrosion behaviour of carbon fibres reinforced magnesium metal matrix composite (MMC) was investigated with emphasis on the galvanic corrosion arisen between the magnesium matrix alloy and the carbon fibres. The susceptibility of carbon/magnesium interface to degradation was also ascertained. The corrosion behaviour was studied in both neutral and alkaline aqueous solutions containing different concentrations of NaCl, using electrochemical techniques, hydrogen evolution test, optical microscopy and scanning electron microscopy (SEM) coupled with EDX and WDX capabilities. Two matrix alloys were used, AS41 and AS41(0.5% Ca). The latter monolithic alloy had a significantly higher corrosion resistance marked by Ca-addition, but the presence of carbon fibres in MMCs invalidated the beneficial effect of Ca. Carbon, as an electrically conductive element, is electrochemically unstable despite its almost chemical stability in aqueous chloride environments. Therefore, the polarisation diagrams of MMCs were demonstrated in terms of the electrochemical activity of carbon fibres. This necessitated performing the electrochemical polarisation tests, individually, on both Mg MMC, monolithic magnesium matrix alloy and carbon fibres electrode.

Doped monolithic langasite structures for high temperature MEMS

Doping of langasite is performed in order to create monolithic structure components such as electrodes. In order to control the process, the diffusion coefficients of praseodymium, strontium and niobium are determined. An increase of the doping depth by application of an electric field is demonstrated. Strontium doped areas exhibit an increased conductivity by about 4 orders of magnitude. A langasite bulk acoustic wave resonator with Sr-doped areas as monolithic electrodes is operated up to 800 °C. For demonstration of the functionality and quality of langasite microstructures at elevated temperatures, planar and biconvex langasite membranes are prepared and characterized.

Monolithic Capillary-Based Ion-Selective Electrodes

Poly(styrene-co-divinylbenzene)-based monolithic capillaries of an inner diameter of 200 mum and a length of 2-5 mm have been used to construct Ca2+-, Ag+-, and Na+-selective electrodes. The membranes consist of a solution of ionophore and ion exchanger in bis(2-ethylhexyl) sebacate or 2-nitrophenyl octyl ether, which are used as plasticizers in conventional PVC-based membranes. With capillaries of low porosity, the potentiometric responses down to 10(-8)-10(-9) M solutions do not depend on the composition of the internal solution, which indicates a strong suppression of transmembrane ion fluxes. Thus, no tedious optimization of the inner solution is required with monolith ISEs. The lower detection limits of Ag+- and Ca2+-ISEs are comparable to the best ones obtained earlier with optimized inner solutions. Additionally, a monolithic Na+-selective ISE has been obtained exhibiting a lower detection limit of 3 x 10(-8) M Na+. With monolithic capillaries of higher porosity and fused-silica GC capillaries, the transmembrane flux effects are noticeable but still significantly smaller than with conventional PVC membranes.

Synthesis and Rate Performance of Monolithic Macroporous Carbon Electrodes for Lithium-Ion Secondary Batteries

Three-dimensionally ordered macroporous (3DOM) materials are composed of well-interconnected pore and wall structures with wall thicknesses of a few tens of nanometers. These characteristics can be applied to enhance the rate performance of lithium-ion secondary batteries. 3DOM monoliths of hard carbon have been synthesized via a resorcinol-formaldehyde sol–gel process using poly(methyl methacrylate) colloidal-crystal templates, and the rate performance of 3DOM carbon electrodes for lithium-ion secondary batteries has been evaluated. The advantages of monolithic 3DOM carbon electrodes are: 1) solid-state diffusion lengths for lithium ions of the order of a few tens of nanometers, 2) a large number of active sites for charge-transfer reactions because of the material's high surface area, 3) reasonable electrical conductivity of 3DOM carbon due to a well-interconnected wall structure, 4) high ionic conductivity of the electrolyte within the 3DOM carbon matrix, and 5) no need for a binder and/or a conducting agent. These factors lead to significantly improved rate performance compared to a similar but non-templated carbon electrode and compared to an electrode prepared from spherical carbon with binder. To increase the energy density of 3DOM carbon, tin oxide nanoparticles have been coated on the surface of 3DOM carbon by thermal decomposition of tin sulfate, because the specific capacity of tin oxide is larger than that of carbon. The initial specific capacity of SnO2-coated 3DOM carbon increases compared to that of 3DOM carbon, resulting in a higher energy density of the modified 3DOM carbon. However, the specific capacity decreases as cycling proceeds, apparently because lithium–tin alloy nanoparticles were detached from the carbon support by volume changes during charge–discharge processes. The rate performance of SnO2-coated 3DOM carbon is improved compared to 3DOM carbon.

Carbon Aerogels for Electrochemical Double Layer Capacitors

Carbon aerogels are prepared here via pyrolysis of resorcinol-formaldehyde aerogels. Their open porous and electrically conductive structure renders carbon aerogels suitable for the application in supercapacitors. Different types of electrodes can be derived from the sol-gel-precursors of carbon aerogels: Monolithic fibre-reinforced electrodes and polymer-carbon compounds. Both carbon fibre reinforced and polymeric bound aerogel electrodes based on polytetrafluoroethylene (PTFE) have been investigated in this work with respect to their electrical conductivity, surface area and capacitive performance. The capacitance of both electrode types is above 65 F/cm3 in aqueous electrolytes and this meets the demands of supercapacitor electrodes.

Carbon cloth-reinforced and activated aerogel films for supercapacitors

The incorporation of carbon cloth into resorcinol formaldehyde (RF) aerogel precursors allows the manufacture of flexible, mechanically stable monolithic carbon aerogel electrodes with thicknesses in the 100 μm range. These were used to build button cell supercapacitors (diameter: 22 mm, thickness: 6 mm) with a capacitance of about 11 F and a serial resistance of about 170 m Ω. These cells were characterized by cyclic voltammetry (CV) and impedance spectroscopy (IS). A fit function for CV data was developed and confirmed with IS. Self discharge properties and long-term stability of these cells were investigated and compared to cells made with Maxsorb carbon black binder electrodes and activated Kynol fiber cloth electrodes. In these tests the carbon aerogel showed significantly better results than the commercial materials. Activation tests on carbon aerogel electrodes were performed with promising results.

Influence of Ti interfacial layers on the electrical and microstructural properties of SOL-gel prepared PZT films

Abstract Sol-gel derived lead zirconate titanate (PZT) thin films were deposited onto monolithic Pt bottom electrodes which used Ti interlayers for Pt adhesion on thermally oxidized 〈111〉 Si. The rf-sputter deposited Pt/Ti films were annealed at 400° – 600°C for 15 minutes. Atomic Force Microscopy (AFM) was used to analyze the Pt surface features. Transmission Electron Microscopy (TEM), transmission electron diffraction, scanning transmission electron microscopy (STEM), and scanning electron microscopy (SEM) were used to investigate the microstructure of PZT films crystallized onto these electrodes. RT-66A testing was conducted to evaluate the electrode performance of the Pt/PZT/Pt structure devices. It was found that the lower temperature bottom electrode anneal created a more dense, smaller grained nucleated PZT film.

Sol-Gel-Derived Ceramic-Carbon Composite Electrodes: Introduction and Scope of Applications

A new class of composite electrodes made of sol-gel derived carbon-silica materials is introduced. Modifed porous composite carbon-silica electrodes can exhibit hydrophobic or hydrophilic surface characteristics and can serve as an indicator (inert) electrode, as a potentiometric (selective or reference) electrode, and in amperometric sensing and biosensing. The composite (carbon) ceramic electrodes (CCP) are rigid, porous, easily modified chemically and have a renewable external surface. The electrodes otter higher stability than carbon paste electrodes, and they are moreamenable to chemical modification than monolithic and (organic) composite carbon electrodes

Silicon charge electrode array for ink jet printing

A monolithic charge electrode array suitable for use in a multiple channel ink jet printing apparatus has been fabricated by anisotropic etching of trapezoidal slots through a

Carbon dioxide electric-discharge laser with preionization of the active volume by auxiliary spark discharges

Spark discharges on both sides of the main gap were used for the preionization of the gas in a 0.4-liter pulsed CO2 laser. The gap was formed by two monolithic metal electrodes. An output energy of 9 J was obtained at a pressure of 600 mm Hg for a CO2:N2:He =1:1:2 mixture.