Electrical and morphological characterization of platinum thin-films with various adhesion layers for high temperature applications

In this paper we report on the morphological and electrical properties of platinum (Pt) thin-films with Titanium (Ti) and, alternatively, Titanium dioxide (TiO₂) as adhesion layers for high temperature applications. All films were sputter deposited on silicon substrates and afterwards annealed in air up to 800 °C. The results show that Ti diffuses into Pt grain boundaries forming oxide precipitates (TiO_x) in the Pt grain boundaries. The resistivity of Pt/Ti thin-films increased continuously with annealing temperature up to 500 °C and decreases again continuously above 500 °C. In contrast, TiO₂ demonstrates a dense stable oxide layer after annealing. Pt/TiO₂ thin-films show a continuous decrease in the sheet resistance with increasing the annealing temperature. Accordingly, TiO₂ thin-film is the preferable adhesive layer for Pt over Ti thin-films for high temperature applications.

     

Structural, optical and electrical properties of CdO/Cd(OH)2 thin films grown by the SILAR method

Successive Ionic Layer Adsorption and Reaction (SILAR) was used to form Cd(OH)₂ thin films from aqueous cadmium–ammonia complex on glass substrates at room temperature and the thermal annealing effect on thin films was studied. The as-deposited films were annealed at 200, 300 and 400 °C for 1 h in an oxygen atmosphere for conversion from Cd(OH)₂ to CdO and change in the structural, optical and electrical properties of the films and the effect of the light on the electrical properties of the films were investigated. The structural and surface morphological properties of the films were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that Cd(OH)₂ phase is converted into the cubic CdO films by annealing. The band gap energy values of films decreased from 3.59 to 2.13 eV through increasing annealing temperature. It was found that the current increased with increasing light intensity and CdO films were more conductive than the as-deposited films.     

Influence of Cs doping in spray deposited SnO2 thin films for LPG sensors

Detection of low concentrations of petroleum gas was achieved using transparent conducting SnO₂ thin films doped with 0–4 wt.% caesium (Cs), deposited by spray pyrolysis technique. The electrical resistance change of the films was evaluated in the presence of LPG upon doping with different concentrations of Cs at different working temperatures in the range 250–400 °C. The investigations showed that the tin oxide thin film doped with 2% Cs with a mean grain size of 18 nm at a deposition temperature of 325 °C showed the maximum sensor response (93.4%). At a deposition temperature of 285 °C, the film doped with 3% Cs with a mean grain size of 20 nm showed a high response of 90.0% consistently. The structural properties of Cs-doped SnO₂ were studied by means of X-ray diffraction (XRD); the preferential orientation of the thin films was found to be along the (3 0 1) directions. The crystallite sizes of the films determined from XRD are found to vary between 15 and 60 nm. The electrical investigations revealed that Cs-doped SnO₂ thin film conductivity in a petroleum gas ambience and subsequently the sensor response depended on the dopant concentration and the deposition temperature of the film. The sensors showed a rapid response at an operating temperature of 345 °C. The long-term stability of the sensors is also reported.     

Pulsed laser deposited Y-doped BaZrO3 thin films for high temperature humidity sensors

Pulsed laser deposited (PLD) Y-doped BaZrO₃ thin films (BaZr_1-xY_xO_3-y/2, x = 0.2, y > 0), were investigated as to their viability for reliable humidity microsensors with long-term stability at high operating temperatures (T > 500 °C) as required for in situ point of source emissions control as used in power plant combustion processes. Defect chemistry based models and initial experimental results in recent humidity sensor literature [1] and [2]. indicate that bulk Y-doped BaZrO₃ could be suitable for use in highly selective, high temperature compatible humidity sensors. In order to accomplish faster response and leverage low cost batch microfabrication technologies we have developed thin film deposition processes, characterized layer properties, fabricated and tested high temperature humidity micro sensors using these thin films. Previously published results on sputtering Y-doped BaZrO₃ thin films have confirmed the principle validity of our approach [3]. However, the difficulty in controlling the stoichiometry of the films and their electrical properties as well as mud flat cracking of the films occurring either at films thicker than 400 nm or at annealing temperature above 800 °C have rendered sputtering a difficult process for the fabrication of reproducible and reliable thin film high temperature humidity microsensors, leading to the evaluation of PLD as alternative deposition method for these films.X-ray Photoelectron Spectroscopy (XPS) data was collected from as deposited samples at the sample surface as well as after 4 min of Ar⁺ etching. PLD samples were close to the desired stoichiometry. X-ray diffraction (XRD) spectra from all as deposited BaZrO₃:Y films show that the material is polycrystalline when deposited at substrate temperatures of 800 °C. AFM results revealed that PLD samples have a particle size between 32 nm and 72 nm and root mean square (RMS) roughness between 0.2 nm and 1.2 nm. The film conductivity increases as a function of temperature (from 200 °C to 650 °C) and upon exposure to a humid atmosphere, supporting our hypothesis of a proton conduction based conduction and sensing mechanism. Humidity measurements are presented for 200–500 nm thick films from 500 °C to 650 °C at vapor pressures of between 0.05 and 0.5 atm, with 0.03–2% error in repeatability and 1.2–15.7% error in hysteresis during cycling for over 2 h. Sensitivities of up to 7.5 atm⁻¹ for 200 nm thick PLD samples at 0.058 atm partial pressure of water were measured.     

Studies on the gas sensing behaviour of nanosized CuNb2O6 towards ammonia, hydrogen and liquefied petroleum gas

Appreciable changes in resistance of polycrystalline nanosized CuNb₂O₆ upon exposure to reducing gases like hydrogen, liquefied petroleum gas (LPG) and ammonia in ambient atmosphere recognize the material as a gas sensor. Nanosized CuNb₂O₆ synthesized by thermal decomposition of an aqueous precursor solution containing copper nitrate, niobium tartrate and tri-ethanol amine (TEA), followed by calcination at 700 °C for 2 h, has been characterized using X-ray diffraction (XRD) study, transmission electron microscopy (TEM), field-emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) analysis and Brunauer–Emmett–Teller (BET) surface area measurement. The synthesized CuNb₂O₆ exhibits monoclinic structure with crystallite size of 25 nm, average particle size of 25–40 nm and specific surface area of 55 m² g⁻¹.     

Sensing properties of chemically sprayed TiO2 thin films using Ni, Ir, and Rh as catalysts

The purpose of this work is double, to analyze the influence of (i) the addition of different catalysts and (ii) the implementation of different procedures to introduce them in the titanium dioxide (TiO₂) thin films, in order to improve the film sensitivity for detecting oxygen. For reaching these objectives, TiO₂ thin films were deposited on alumina substrates by the ultrasonic spray pyrolysis (USP) technique employing titanium(IV) oxide acetylacetonate (TiO(acac)₂) as a titanium precursor, and pure methanol as a solvent. Iridium, nickel, and rhodium were the three catalysts used, which were incorporated by impregnation and USP onto the TiO₂ thin films surface. The electrical characterization, consisting in surface resistance measurements of the films, in a mixture of oxygen and zero-grade air, was performed using interdigitated gold electrodes contacted on the alumina substrates. From these, the film response or resistance change of the TiO₂ films was estimated. Single TiO₂ thin films measured at 400 °C displayed a response of the order of 0.02 and 0.18 to oxygen of 1000 and 10,000 ppm, respectively, whereas TiO₂ thin films using impregnated rhodium proved to have the highest response to O₂, with a value in the order of 2.5 to a concentration of 1000 ppm of O₂ diluted in zero-grade air at an optimal operating temperature of 250 °C.     

Improved field emission characteristics of screen-printed CNT-FED cathode by interfusing Fe/Ni nano-grains

Carbon nanotube (CNT) cathode with and without interfusing nano-metal particles was prepared using screen-printing technology. For the good electric conductivity of metal, the turn-on electric field of the Fe/Ni and CNT composite film (Fe/CNT film) decreases to 1.42 V/μm comparing with the usual CNT film of 2.45 V/μm, and the emission current increases from 60 μA to 440 μA at an applied electric field of 2.3 V/μm. Furthermore, the field enhancement factor β increases from 1721 to 3242. By characterizing the prepared samples via X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), it is found that carbide Fe₃C phase is formed in Fe/CNT film, and the metal particles are filled in the interspaces of CNTs. It is evaluated that benefiting from good electrical conductivity and chemical inertness of metal carbide, Fe/CNT film achieves high emission characteristics and emission uniformity.     

Characterisation of high aspect ratio non-conductive ceramic microstructures made by spark erosion

The spark erosion process is widely used for micro structuring. Its possibility to structure materials independent of their material properties like high hardness or melting temperature enables to address a large material diversity. However the process requires a minimal electrical conductivity of 0.1 Scm⁻¹. Nevertheless recent research has shown that the usage of an assisting electrode makes a processing of non-conductive materials possible. Thus even ceramics like Al₂O₃ or ZrO₂ can be processed. These materials are becoming more and more interesting for industrial application and the field of miniaturisation due to their outstanding material characteristics like high hardness, bending strength, melting temperature and chemical inertness. In this study a new lacquer based assisting electrode is used to erode bars in zirconia samples. For this purpose a modular tool concept is established. Bars with aspect ratios of more than 80 are generated. The achieved bar heights are 1.5 mm and the smallest bar width is 8 μm. Furthermore a characterisation of the sidewall angles showed mean values between 0.4° and 2.2° depending on the bar height and width.     

Thermoelectric properties of SbNCa3 and BiNCa3 for thermoelectric devices and alternative energy applications

Thermoelectric properties of two antiperovskites SbNCa₃ and BiNCa₃ are calculated using first principles calculations. High values of Seebeck coefficients are observed for these materials. Electrical and thermal conductivities are also calculated. Increase in thermal conductivity and decrease in electrical conductivity are found with increasing temperature. The maximum values of thermal conductivity are 92×10¹⁴  W/m K s and 88×10¹⁴  W/m K s for SbNCa₃ and BiNCa₃ respectively at a temperature of 900 K. The peak values of 5×10²⁰/Ω m s and 5.2×10²⁰/Ω m s are achieved for n-type SbNCa₃ and BiNCa₃ respectively at a temperature of 300 K. Figure of merit is achieved for these materials at room temperature which shows that these materials can be useful for thermoelectric devices and alternative energy sources.     

Thermodynamic properties of sodium hexatitanate (Na2Ti6O13) at high temperature (298.15–1573 K)

Sodium hexatitanate (Na₂Ti₆O₁₃) was reported as an anode side material for Sodium ion batteries owing to low material cost, high energy efficiency, good thermal stability and long cycle life. Therefore, studies pertaining to the thermodynamic properties of Na₂Ti₆O₁₃ are indispensable for improving its service performance. However, a significant number of literature reviews concerning thermodynamic properties indicated that heat capacity of Na₂Ti₆O₁₃ at high temperatures should be confirmed. In this study, the 99.5% purity of Na₂Ti₆O₁₃ sample was successfully prepared via solid-state reaction using TiO₂ and Na₂CO₃ as initial materials. Heat capacity of the as-synthesized samples in the temperature range of 573–1523 K was measured using a multi-high temperature calorimeter 96 line. Heat capacity, Cp, from 298.15 to 1573 K was modeled as a polynomial formula with a prediction error of 3%: Cp = 474.08143 + 0.06286T-8.04068 × 10⁶ T⁻² (J⋅mol⁻¹⋅K⁻¹). In combination with the low-temperature data, heat capacity of Na₂Ti₆O₁₃ from 0 to 1573 K was given in present study. Values of changes in enthalpy, Gibbs free energy and entropy in the temperature range of 298.15–1573 K were calculated based on the temperature dependence of heat capacity.     

Investigation on the structural and spectral characteristics of deposited FBG stacks at elevated temperature

A composite nano-crystalline structured thin film was realized by depositing mixed Al₂O₃ and MgO coating material using physical vapor deposition approach and then annealing at high temperature. The film thus fabricated retains a high transmission even after annealing at 1500 °C. The grain size of less than 100 nm was measured by atomic force microscopy and the composite nano-crystalline structure of spinel and corundum was confirmed by the X-ray diffraction pattern analysis. Based on this, Bragg grating stacks were fabricated by depositing alternating quarter-wave layers of Al₂O₃ and Al₂O₃/MgO at the end of a sapphire crystal fiber at first and then the layers of NiO and Al₂O₃/MgO on the surface of a sapphire slice. The performance of the grating stacks at high temperature or after high temperature annealing was measured. It was found that the reflection peak measured from the grating stacks can survive a high temperature up to 1050 °C but will disappear after annealing at temperature of 1100 °C or above. A conclusion of inter-diffusion between layers of stack was obtained to explain the phenomenon of reflection peak disappearing after annealing at high temperature.     

Solid-state potentiometric SO2 sensor combining NASICON with V2O5-doped TiO2 electrode

A compact tubular sensor based on NASICON (sodium super ionic conductor) and V₂O₅-doped TiO₂ sensing electrode was designed for the detection of SO₂. In order to reduce the size of the sensor, a thick-film of NASICON was formed on the outer surface of a small Al₂O₃ tube; furthermore, a thin layer of V₂O₅-doped TiO₂ with nanometer size was attached on the NASICON as a sensing electrode. This paper investigated the influence of V₂O₅ doping and sintering temperature on the characteristics of the sensor. The sensor attached with 5 wt% V₂O₅-doped TiO₂ sintered at 600 °C exhibited excellent sensing properties to 1–50 ppm SO₂ in air at 200–400 °C. The EMF value of the sensor was almost proportional to the logarithm of SO₂ concentration and the sensitivity (slope) was −78 mV/decade at 300 °C. It was also seen that the sensor showed a good selectivity to SO₂ against NO, NO₂, CH₄, CO, NH₃ and CO₂. Moreover, the sensor had speedy response kinetics to SO₂ too, the 90% response time to 50 ppm SO₂ was 10 s, and the recovery time was 35 s. On the basis of XPS analysis for the SO₂-adsorbed sensing electrode, a sensing mechanism involving the mixed potential at the sensing electrode was proposed.     

Microdroplet formation of water and nanofluids in heat-induced microfluidic T-junction

This paper reports experimental investigations on the droplet formation and size manipulation of deionized water (DIW) and nanofluids in a microfluidic T-junction at different temperatures. Investigations of the effect of microchannel depths on the droplet formation process showed that the smaller the depth of the channel the larger the increase of droplet size with temperature. Sample nanofluids were prepared by dispersing 0.1 volume percentage of titanium dioxide (TiO₂) nanoparticles of 15 nm and 10 nm × 40 nm in DIW for their droplet formation experiments. The heater temperature also affects the droplet formation process. Present results demonstrate that nanofluids exhibit different characteristics in droplet formation with the temperature. Addition of spherical-shaped TiO₂ (15 nm) nanoparticles in DIW results in much smaller droplet size compared to the cylindrical-shaped TiO₂ (10 nm × 40 nm) nanoparticles. Besides changing the interfacial properties of based fluid, nanoparticles can influence the droplet formation of nanofluids by introducing interfacial slip at the interface. Other than nanofluid with cylindrical-shaped nanoparticles, the droplet size was found to increase with increasing temperature.     

Hydrothermal synthesis of hollow ZnSnO3 microspheres and sensing properties toward butane

Hollow ZnSnO₃ microspheres were successfully prepared by hydrothermal method at 160 °C for 12 h. The prepared material was characterized by field emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and X-ray diffraction measurements (XRD). The average diameter of the hollow ZnSnO₃ microspheres was in the range of 400-600 nm. Compared with solid ZnSnO₃ microspheres structure, the hollow ZnSnO₃ microspheres showed better response (S) to butane. To 500 ppm butane, the sensor response (S) of the hollow ZnSnO₃ microspheres was 5.79 at the optimum operating temperature of 380 °C, and the response and recovery time were 0.3 s and 0.65 s, respectively. The sensitivities of sensors based on this material were linear with the concentration of butane in the range of 100-1000 ppm.     

Synthesis and characterisation of nanosized TiO2-ZrO2 binary system prepared by an aqueous sol-gel process: Physical and sensing properties

Nanostructured TiO₂-ZrO₂ thin films and powders were prepared by a straightforward aqueous particulate sol-gel route. Titanium (IV) isopropoxide and zirconium (IV) acetate hydrate were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the specific surface area. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy revealed that the powder were crystallised at the low temperature of 500 °C, containing anatase-TiO₂ and tetragonal-ZrO₂ phases. Furthermore, it was found that ZrO₂ retarded the anatase-to-rutile transformation up to 900 °C. The activation energies for crystallite growth of TiO₂ and ZrO₂ components in the binary system were calculated 10.16 and 3.12 kJ/mol, respectively. Transmission electron microscope (TEM) image showed that one of the smallest crystallite sizes was obtained for TiO₂-ZrO₂ binary mixed oxide, being 5 nm at 500 °C. Field emission scanning electron microscope (FESEM) analysis revealed that the deposited thin films had nanostructured morphology with the average grain size of 20 nm at 500 °C and 36 nm at 900 °C. Thin films produced under optimised conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO₂ gases at low operating temperature of 150 °C, resulted in an increase of thermal stability of sensing films as well as a decrease in the power consumption. Furthermore, calibration curves revealed that TiO₂-ZrO₂ sensor follows the power law, S = A[gas]^B (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and it has excellent capability for the detection of low gas concentrations.     

H2 sensing properties of diode-type gas sensors fabricated with Ti- and/or Nb-based materials

A thermally oxidized TiO₂ or Nb₂O₅ film equipped with a top Pd film electrode and a bottom Ti or Nb plate electrode (Pd/MO(n)/M, MO: oxide film, M: metal plate, n: annealing temperature (°C)) has been investigated as a diode-type H₂ sensor under air or N₂ atmosphere. Pd/TiO₂(n)/Ti sensors showed relatively poor H₂ sensing properties in air, in comparison with Pd/anodic-TiO₂(n)/Ti sensors constructed with an anodized TiO₂ film equipped with a top Pd film electrode and a bottom Ti plate electrode, which were reported in our previous studies. On the other hand, Pd/Nb₂O₅(n)/Nb sensors showed relatively larger H₂ response with fast response and recovery speeds than Pd/TiO₂(n)/Ti sensors in air under high forward bias conditions. A Pd/Nb₂O₅(450)/Ti sensor, which was fabricated by radio-frequency magnetron sputtering of Nb metal on a Ti substrate followed by thermal oxidation at 450 °C, showed the largest H₂ response and relatively fast response and recovery speeds in air, among the sensors tested. In addition, H₂ response of the Pd/Nb₂O₅(450)/Ti sensor in air was much lower than that in N₂, but the logarithm of H₂ response was almost proportional to the logarithm of H₂ concentration in a wide range of H₂ concentration (10–8000 ppm) in air, and the H₂ sensitivity in air was much higher than that in N₂.     

Experimental investigation of phase equilibrium in the Al–Nb–Hf ternary system at 600 °C and 400 °C

The phase equilibria in the Al–Nb–Hf ternary system at 600 °C and 400 °C were experimentally investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and electron probe microanalysis (EPMA/WDS). In each isothermal sections, 13 three-phase regions were measured. And their phase region boundaries were precisely determined. Among the actually measured binary compounds, only Al₃Nb, AlNb₂ and AlNb₃ have a large range of solid solubility. In addition, two stable ternary compounds τ1-Al₁₁Nb₄Hf₅ and τ2-Al₂NbHf₂, which had certain solid solubility, were newly discovered. On the basis of the experimental results and reasonable inference, the isothermal sections of ternary system at 600 °C and 400 °C were constructed.     

Synthesis and enhanced gas sensing properties of crystalline CeO2/TiO2 core/shell nanorods

Crystalline CeO₂/TiO₂ core/shell nanorods were fabricated by a hydrothermal method and a subsequent annealing process under the hydrogen and air atmosphere. The thickness of the outer shell composed of crystal TiO₂ nanoparticles can be tuned in the range of 5-11 nm. The crystal core/shell nanorods exhibited enhanced gas-sensing properties to ethanol vapor in terms of sensor response and selectivity. The calculated sensor response based on the change of the heterojunction barrier formed at the interface between CeO₂ and TiO₂ is agreed with the experimental results, and thus the change of the heterojunction barrier at different gas atmosphere can be used to explain the enhanced ethanol sensing properties.     

A computational study on the cisplatin drug interaction with boron nitride nanocluster

We explored the reactivity, and electronic sensitivity of the synthesized B₁₂N₁₂ nanocluster to cisplatin (CP) anticancer drug by means of density functional theory calculations. It is predicted that the drug prefers to be adsorbed simultaneously from its hydrogen and halogen atoms on a BN bond of the BN nanocluster with adsorption energy about −14.9 kcal/mol. The electronic properties of B₁₂N₁₂ nanocluster are predicted to be sensitive to the CP drug and it benefits from a short recovery time about 81 ms at room temperature. After the adsorption of the CP drug, the conduction level of BN nanocluster meaningfully stabilizes and the valence level shifts to higher energies. As a result, the HOMO-LUMO energy gap significantly decreases. So, the BN nanocluster converts to a semiconductor with higher electrical conductivity after the adsorption process. The increase of electrical conductivity can produce an electrical signal which helps to detect the CP drug. Also, UV–vis calculations indicate that after the adsorption of the CP drug a strong peak appears in the visible region which helps to detect the drug.     

Preparation of TiO2 coated silicate micro-spheres for enhancing the light diffusion property of polycarbonate composites

The TiO₂ coated silicate micro-spheres (SMS) core–shell particles (SMS@TiO₂) were synthesized using the sol–gel reaction followed by calcination. The SMS@TiO₂ particles were used to enhance the light diffusion property of polycarbonate (PC) composites. Our experimental analysis shows that the TiO₂ was coated on the SMS particles and the optimum parameters of the reaction were 4:1 of the Si/Ti molar ratio and 500 °C of the calcination temperature. The UV–Vis spectra indicate that SMS@TiO₂ can absorb or hinder the UV light, which may prolong the service life of PC light diffusion materials. Compared to that of PC composites physically mixed with SMS + TiO₂, the haze of the PC/SMS@TiO₂ composites was little affected, while the transmittance was obviously enhanced, which can be increased from 55.5% for PC/TiO₂/SMS to 70.3% for PC/SMS@TiO₂ with only 0.6 wt% filler loading.