CO gas sensing properties in Pd-added ZnO sensors

Unadded and 0.5 mol% Pd-added ZnO bulk and thin films were prepared by sintering and sputtering, respectively, and their CO gas sensing properties were investigated. The effects of Pd addition, sensing temperature (100–500 °C), and humidity on the CO gas response were discussed. In the bulk sensors, Pd-addition lowered the temperature for the maximum CO gas response (sensitivity) from 400 to 300 °C, whereas the thin film sensors (unadded and Pd-added) exhibited maximum gas response at 200 °C. The Pd-addition enhanced the CO gas response in thin film sensors, and it was also effective for reducing the interference from humidity in both bulk and thin film sensors.     

Ferroelectric phase transition and electrical properties of Ba(Bi0.5Ta0.5)O3

A lead free polycrystalline material Ba(Bi_0.5Ta_0.5)O₃ was prepared by a standard high-temperature solid-state technique (calcination temperature?=?1180 °C and sintering temperature?=?1200 °C) using high-purity ingredients. The room temperature X-rays diffraction analysis of the material has confirmed its formation in the monoclinic crystal system. The study of microstructure using scanning electron microscopy (SEM) shows that the compound has well-defined grains uniformly distributed throughout the surface of the sample. Detailed studies of dielectric and impedance properties of the material were carried out in a wide frequency range (1 kHz ?1 MHz) at different temperatures (30 °C to 490 °C). Dielectric study shows that the material has ferroelectric properties with diffuse-phase transition around 315 °C. Complex impedance spectroscopic analysis establishes some correlation between the microstructure and electrical properties of the material. The frequency dependence of ac conductivity follows the Jonscher’s power law. The dc conductivity, calculated from the ac conductivity spectrum, shows the negative temperature coefficient of resistance behavior similar to that of a semiconductor. The temperature dependent pre-exponential factor shows peak, and frequency exponent possesses a minimum at transition temperature.     

Impurity behaviors of nitrogen in W–C–N thin diffusion barriers for Cu metallization schemes

To investigate the effect of nitrogen impurities in the tungsten–carbon thin films, the electrical and structural properties of W–C–N thin films deposited with rf magnetron sputtering method were measured. Interface characteristics of W–C–N/Si were studied with resistivity and crystal structure as a function of nitrogen impurity concentrations of as-deposited and annealed state for various annealing temperature. We also investigate the interface of Cu/W–C–N/Si for various nitrogen concentration by using XRD pattern and Nomarski microscope. Our experimental results indicate that nitrogen impurity provides stuffing effect for preventing the interdiffusion between Cu and Si interface after annealing up to 800°C for 30 min, because W–C–N thin films serve as a good diffusion barrier and this may be due to the role of nitrogen and carbon inside the W–C–N film not as bonded state but impurities     

High temperature electrical characterization of nano-crystalline BaTiO3 synthesized using Ba(NO3)2 and TiO2

The reaction of Ba(NO₃)₂ with TiO₂ was studied by thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques up to 1000°C and in nitrogen atmosphere. It was found that the formation of BaTiO₃ takes place above 600°C. BaTiO₃ powder was prepared by calcination of Ba(NO₃)₂ and TiO₂ precursor mixture at 800°C for 8 h. X-ray diffraction analysis of the synthesized BaTiO₃ confirmed the formation of tetragonal phase. Average crystallite size was found to be 44 nm, For the electrical and morphological characterization pellets of the obtained powder were sintered at 1000 °C for 12 h. Scanning electron micrograph (SEM) exhibits spherical and rod shaped grains. The dielectric constant, dissipation factor, complex plane impedance and ac conductivity of the sintered pellet has been measured in the temperature range of 40–600°C and frequency range of 100 Hz–2 MHz. DC conductivity of the sample was obtained from the impedance data. The conductivities (both ac and dc) and relaxation time (τ) exhibit two regions of temperature dependence, namely region I, which represents (280–450°C) and region II, which governs (450-600°C). Conduction and relaxation in both the temperature regions are explained in terms of hopping of electrons and doubly ionized oxygen vacancies (V_O^??).     

Microwave dielectric loss of thermally stressed MgTiO3 via TEM observation

The influence of cooling rate on the dielectric loss of MgTiO3 ceramics at microwave frequencies via thermal stress and TEM was investigated. The specimens were cooled down with 1, 5, 30 °C/min and air-quenching from the sintering temperature of 1,350 °C. As the cooling rate increased, Q·f value decreased due to an increase of the crystallographic strain. The line defects such as dislocations increased with an increase of cooling rate except the specimen cooled down at 1 °C/min, which showed no dislocation. This result revealed that the line defects contribute to the deterioration of dielectric losses.     

Dielectric,impedance and modulus spectroscopy of BaBi2Nb2O9

Barium Bismuth Niobate (BaBi₂Nb₂O₉) has been synthesized by solid state reaction method. The X-ray diffraction study confirms the formation of compound. Morphological analysis has been carried out from the scanning electron microscopy images and the elemental analysis from the energy dispersive spectroscopy profiles. Investigation of dielectric and ferroelectric properties of the sample was done by varying the temperature from 25 °C - 500 °C in a frequency range of 1 kHz- 1 MHz. At 100 kHz, the phase transition was observed at 214.02°C. Further, this ferroelectric bi-layered perovskite exhibits an interesting relaxor behavior with a strong dispersion of the dielectric permittivity. A detailed study on the impedance spectroscopy over a wide range of temperature and frequency exhibits the contribution of grain ad grain boundary on different electrical parameters. From modulus spectroscopy, the presence of non-Debye type of relaxation in the material has been manifested. The complex modulus plots support the negative temperature coefficient of resistance (NTCR) type behavior of the material.     

Electrical properties of Bi4Ti3O12 textured by screen printing

The focus of this work is to explore the electrical properties of bismuth titanate, Bi₄Ti₃O₁₂, textured through the process of screen printing. Textured BTO samples were produced using the templated grain growth technique and the electrical properties were measured both within and normal to the texture plane. The relative permittivity and polarization were determined as a function of electric field, temperature, and frequency. The electrical properties improved dramatically (P_r?=?25 ??C/cm², ??_r(??)?=?1800 at 1 MHz) compared to a randomly oriented sample (P_r?=?10 ??C/cm², ??_r(??)?=?850 at 1 MHz) when measured within the texture plane. A corresponding reduction of electrical properties normal to the texture plane was observed (P_r?=?2 ??C/cm², ??_r(??)?=?300 at 1 MHz). The electrical properties of bismuth titanate textured by screen printing compare favorably to other texture-inducing techniques such as tape casting and hot forging.     

NO2 adsorption behaviour on LaFeO3 electrodes of YSZ-based non-nernstian electrochemical sensors

X-ray photoelectron spectroscopy (XPS) was used to examine the NO₂ adsorption behaviour on the LaFeO₃ and Pt electrodes of planar yttria stabilized zirconia non-Nernstian gas sensors. The electrochemical sensors were exposed to the same gas atmosphere containing 1000 ppm NO₂ at 650°C. XPS of the as-prepared sensors and sensors after exposure to NO₂ revealed bonded nitrogen peaks on the surface of the semiconducting oxide but no nitrogen peaks on the Pt electrode. Therefore, NO₂ adsorption on a LaFeO₃ electrode plays an important role in the NO₂ detection mechanism.     

Effects of different morphologies of ZnO films on hydrogen sensing properties

This paper describes the characteristics of chemiresistor hydrogen (H₂) sensors with different ZnO film structures in which ZnO dense films, nanoparticles (NPs), and nanorods (NRs) were prepared by RF magnetron sputtering, the sol–gel method, and the hydrothermal method, respectively. These were decorated with a Pt NP catalyst to investigate the performance of devices comprised of these structures. The effects of the ZnO morphology and operating temperature on the gas sensing behavior of the sensor are reported in detail. The various ZnO film morphologies, which contributed significantly to differences between sensors, play a very important role in enhancement of the supported Pt catalyst area and initial oxygen absorption on the ZnO surface. ZnO dense films prepared by sputtering showed the fastest response with a 13.5 % resistance variation at 1,000 ppm H₂ because gas adsorption occurred only on the film surface. The sensor with ZnO NRs showed a slower response, but the highest change in resistance of 65.5 % occurred at 1,000 ppm H₂ at room temperature. H₂ sensing performance of the chemiresistor sensors was improved due to the Pt catalyst, which was more efficient in dissociating H₂ gas molecules even at low temperature. The best chemiresistor sensor was fabricated using ZnO NRs and had a response time of approximately 10 s, a 27 s recovery time, and an 81.5 % change in resistance at 200 °C.     

High-temperature piezoelectric crystals and devices

Conventional piezoelectric materials such as quartz are widely used as high precision transducers and sensors based on bulk acoustic waves. However, their operation temperature is limited by the intrinsic materials properties to about 500°C. High-temperature applications are feasible by applying materials that retain their piezoelectric properties up to higher temperatures. Here, langasite (La₃Ga₅SiO₁₄) and compounds of the langasite family are the most promising candidates, since they are shown to exhibit bulk acoustic waves up to at least 1400°C. The mass sensitivity of langasite resonators at elevated temperatures is about as high as that of quartz at room temperature. Factors limiting potential use of those crystals include excessive conductive and viscous losses, deviations from stoichiometry and chemical instability. Therefore, the objective of this work is to identify the related microscopic mechanisms, to correlate electromechanical properties and defect chemistry and to improve the stability of the materials by e.g. appropriate dopants. Further application examples such as resonant gas sensors are given to demonstrate the capabilities of high-temperature stable piezoelectric materials. The electromechanical properties of langasite are determined and described by a one-dimensional physical model. Key properties relevant for stable operation of resonators are found to be shear modulus, density, electrical conductivity and effective viscosity. In order to quantify their impact on frequency and damping, a generalized Sauerbrey equation is given. Mass and charge transport in single crystalline langasite are correlated with langasite??s defect chemistry and electromechanical properties. First of all, the dominant charge carriers are identified. Undoped langasite shows predominant ionic conduction at elevated temperatures. As long as the atmosphere is nearly hydrogen-free, the transport is governed by oxygen movement. A dominant role of hydrogen is observed in hydrogenous atmospheres since the diffusion coefficient of hydrogen is orders of magnitude higher than that of oxygen. The loss in langasite is found to be governed up to about 650°C by viscoelastic damping related to the above mentioned movement of oxygen ions. Donor doping is shown to lower the loss contribution. Above 650°C the impact of the conductivity related loss becomes pronounced. Here, lowering the conductivity results generally in decreased losses. The evaluation of langasite??s applicability is focused on mapping the regimes of gas insensitive operation. The most relevant feature with respect to frequency fluctuations of resonator devices is the formation of oxygen vacancies. In nominally hydrogen free atmospheres the calculated frequency shift becomes pronounced below oxygen partial pressures of 10^???17, 10^???24 and 10^???36 bar at 1000, 800 and 600°C, respectively. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen containing atmospheres applied here, langasite can be regarded as a stable resonator material above 10^???13 bar and 10^???20 bar at 800 and 600°C, respectively. The incorporation of OH-groups determines the frequency shift.     

Micromachined piezoelectric structures for high-temperature sensors

The availability of large size langasite (La₃Ga₅SiO₁₄) wafers enables micromachining of this high-temperature stable piezoelectric material which has been shown to exhibit bulk oscillations at temperatures of up to at least 1400 °C. In particular, the realization of miniaturized resonant sensors becomes feasible. Such resonators are operated far above their dielectric relaxation frequency leading to relatively low losses. Our specific research is focused on the development of monolithic structures to overcome problems originating from thermal stress. The concept includes the local doping of langasite by niobium, strontium and praseodymium. Their chemical diffusion coefficients were determined and found to be fairly small. Field enhanced diffusion results in an increased doping depth and concentration as demonstrated for niobium. The effect is governed by local heating of the sample. Optimized process conditions lead potentially to a pronounced drift of the dopants. Strontium doping increases the conductivity of langasite by three to four orders in magnitude and enables the formation of monolithic electrodes for high-temperature resonators as demonstrated by operation of such devices at temperatures as high as 800 °C. Micromachined functional structures including planar and biconvex membranes, as well as cantilevers, are prepared and demonstrated to be operational up to 930 °C. The analysis of their resonance behavior shows high resonator quality factors, e.g. 190 for a 60 MHz bulk acoustic resonator at the above-mentioned temperature.     

Electric properties of monophase polycrystalline sinters SiC, B4C, TiC and their composites as non-inductive volume resistors

Monophase polycrystalline SiC, B₄C, TiC and their SiC-TiC, SiC-B₄C composite sinters with a theoretical density of 97% are characterized by good mechanical and thermal durability as well as a wide range of electrical conductivity values. SiC, which has semiconductor conductivity and negative TCR, was combined with TiC, which has metallic conductivity and positive TCR. Produced in this way resistive elements, within a temperature range from 293 K to 348 K, exhibit a TCR close to zero, and an impedance independent of frequency within a range from 100 Hz to 1 MHz. The combination SiC with 40 wt% of B₄C has been produced resistive elements, which are resistive to oxidation. This combination has also completly resistive character, within a range of 100 Hz to 1 MHz. Most of the investigated materials are suitable for high temperature, noninductive volume resistors.     

Deep level transient spectroscopy study of the effect of Mn and Bi doping on trap formation in ZnO

Deep Level Transient Spectroscopy (DLTS) characterisation of sintered polycrystalline ZnO, and ZnO doped with Mn, and with Mn plus Bi, has been carried out to investigate the effect of these additions on the formation and activation of electron trap states in ZnO used for varistor applications. Samples were produced using a conventional solid state sintering technique, and sintered at 1100°C and 1200°C, quenching the Bi-free samples from the sintering temperature to preserve high temperature defect distribution and slow cooling the Bi-containing samples to develop varistor behaviour. The two well-known bulk ZnO traps, L1 (0.18 eV below the conduction band edge) and L2 (0.29 eV below the conduction band edge), were observed in both the undoped and doped samples. Detailed characterisation of the L1 and L2 traps indicated that they are due to point defects, since their energy was independent of the length of the fill pulse and the fill bias. The introduction of both 1% Mn and (1% Mn?+?2% Bi) caused several additional electron traps, some of which have not been reported previously, to appear deeper in the band gap with energies depending on composition and firing cycle,. The DLTS peaks associated with these additional traps were very broad and had activation energies that varied with fill pulse length: characteristics that indicate they are associated with extended defects.     

Preparation and electrical properties of sintered oxide composed of MnFeNiO4 with a cubic spinel structure

The preparation and electrical properties of a sintered body consisting of cubic spinel oxide MnFeNiO₄ were investigated. A sintered body with cubic spinel and NaCl-type oxides prepared at 1400 °C in Ar could be converted to one consisting of monophase cubic spinel oxide by oxidation at 1000 °C for more than 48 h in air. The electrical conductivity of a sintered body consisting of monophase cubic spinel oxide was confirmed to increase exponentially with increasing temperature, indicating that the oxide has intrinsic negative temperature coefficient (NTC) thermistor characteristics. The electrical conduction of the oxide was concluded to be controlled by the small polaron hopping mechanism. Changes in electrical conductivity, mobility, and charge carrier jump frequency in temperature dependence at 400 °C are assumed to be related to the variation in cation distribution accompanying the disproportionation of Mn ions.     

Piezoelectric properties and domain structure of the orthorhombic (K,Na,Li)(Nb,Ta,Sb)O3 single crystal

ABSTRACT

In this work, (K,Na,Li)(Nb,Ta,Sb)O₃ (KNLNTS) crystal is in the orthorhombic phase at room temperature. The orthorhombic-tetragonal phase transition temperature is 50.0°C, and the Curie temperature T_C of the tetragonal-cubic phase transition temperature is 253.8°C. The crystal is poled, the defects were “pinned” in specific position, and has positive effect in domain wall motions and better ferroelectric property (P_r is 6.49 µC/cm², E_c is 6.66 kV/cm). The domain configrations of the crystal were studied by means of a polarizing light microscopy (PLM), with poling along [100]_C direction.     

Electrochemical properties of doped ceria electrolyte under reducing atmosphere: Bulk and grain boundary

The electrochemical behavior of a symmetrical cell, Pt/Ce_0.8Sm_0.2O_1.9?δ/Pt, under reducing conditions and wide temperature range (250 – 600 °C) is detailed. In terms of the charge carriers transport through the electrolyte microstructure, AC impedance spectroscopy has been applied to address useful concerns about the transport properties over electrolytic and mixed conduction regimes. The impedance spectra at lower temperature and oxygen partial pressure show the electrochemical response of separated bulk and grain boundary contributions. The increase in the electronic conductivity from 250 to 400 °C shows that the electrochemical reduction Ce⁴⁺/Ce³⁺ is as kinetic as thermodynamically favorable in the experimental conditions. In a typical Nyquist plot of an impedance diagram, until temperatures as low as 400 °C, the high and low frequency arcs can be accessed and the influence of reducing atmosphere over both the components is presented. The apparent activation energy for the electronic process (ΔE) extracted from the total conductivity is 2.54 eV. Distinguished bulk (2.34 eV) and grain boundary (2.63 eV) activation energies point the latter as an energetic barrier in the redox reaction. The oxygen partial pressure dependence of individual capacitances suggests storage of electrical charge along grain boundaries which can potentially behave as a chemical capacitor.     

Effects of rapid thermal annealing on surface acoustic wave ultraviolet sensors using ZnO nanorods grown on AlN/Si structures

In this study, we demonstrate a high sensitivity of surface acoustic wave (SAW) ultraviolet (UV) sensor based on ZnO nanorods (NRs) grown on an aluminum nitride (AlN)/silicon (Si) layered structure. The one-dimensional ZnO NRs act as a high-UV sensing material due to their large surface-to-volume ratio. The fabrication of SAW UV sensor is entirely compatible with micro/nano electromechanical (M/NEMS) process with conventional lithography and synthesized ZnO NRs by hydrothermal method at low temperature. The rapid thermal annealing (RTA) process effectively improved the optical properties of ZnO NRs and the sensitivity of the SAW UV sensors. The resulting SAW UV sensors responded to various UV light intensities, and the RTA-processed samples showed high sensitivity. The SAW UV sensor after RTA treatment at 600 °C showed the highest sensitivity with a 130 kHz frequency shift at a UV light intensity of at 0.6 mW/cm², a 5-fold increase in sensitivity compare with as-grown sample.     

Temperature-dependence of electrical properties for the ceramic composites based on potassium polytitanates of different chemical composition

The samples of ceramic materials based on potassium polytitanate (PPT) characterized with various TiO₂/K₂O molar ratio, are produced by calcination at 900 °C and investigated. AC conductivity (σ_ac) of the obtained ceramics is measured at different temperatures between 200 and 800 °C in frequency range of 0.1 Hz–1 MHz. The method of combined impedance and modulus spectroscopy is used to analyze the obtained results. The activation energies of DC conductivity, bulk and grain-boundary conductivity as well as relaxation frequency for studied composites are estimated. Using the correlated barrier hopping (CBH) model, the energies of potential barrier between neighboring defect sites for all kinds of investigated materials are presented. The bulk and grain boundary parameters of the produced ceramic materials based on potassium polytitanates are calculated. The mechanism of different vacancies formation in the investigated ceramic system is discussed. The influence of precursor chemical composition on electrical properties for the ceramic composites based on potassium polytitanates is studied.     

Influence of Temperature on the Microstructure and Electrical Properties of BBT Thin Films

The BBT films were prepared by a spin-coating process from the polymeric precursor method (Pechini process). In order to study the influence of the temperature on the BBT microstructure and electrical properties, the films were deposited on platinum coated silicon substrates and annealed from 700@C to 800°C for 2 hours in oxygen atmosphere. The crystallinity of the films was examined by X-ray diffraction while the surface morphology was analysed by atomic force microscope. The dielectric properties and dissipation factor of BaBi 2 Ta 2 O 9 films at 1 MHz were observed. The polarization-electric field hysteresis loops revealed the ferroelectric characteristics of BaBi 2 Ta 2 O 9 thin films.     

Formation of a KNbO<Subscript>3</Subscript> single crystal using solvothermally synthesized K<Subscript>2-m</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6-m/2</Subscript> pyrochlore phase

A K_2-mNb₂O_6-m/2 single crystal with a pyrochlore phase formed when the Nb₂O₅?+?x mol% KOH specimens with 0.6?≤?x?≤?1.2 were solvothermally heated at 230 °C for 24 h. They have an octahedral shape with a size of 100 μm, and the composition of this single crystal is close to K_1.3Nb₂O_5.65. The single-crystal KNbO₃ formed when the single-crystal K_2-mNb₂O_6-m/2 was annealed at a temperature between 600 °C and 800 °C with K₂CO₃ powders. When annealing was conducted at 600 °C (or with a small amount of K₂CO₃), the KNbO₃ single crystal has a rhombohedral structure that is stable at low temperatures (< ? 10 °C). The formation of the rhombohedral KNbO₃ structure can be explained by the presence of the K⁺ vacancies in the specimen. The KNbO₃ single crystal with an orthorhombic structure formed when the K_2-mNb₂O_6-m/2 single crystal was annealed at 800 °C with 20 wt% of K₂CO₃.