Polycrystalline Al2O3 Substrates Research Articles

Near-Field Passive Wireless Sensor for High-Temperature Metal Corrosion Monitoring.

This work focuses on the fabrication and evaluation of a passive wireless sensor for the monitoring of the temperature and corrosion of a metal material at high temperatures. An inductor-capacitor (LC) resonator sensor was fabricated through the screen printing of Ag-based inks on dense polycrystalline Al2O3 substrates. The LC design was modeled using the ANSYS HFSS modeling package, with the LC passive wireless sensors operating at frequencies from 70 to 100 MHz. The wireless response of the LC was interrogated and received by a radio frequency signal generator and spectrum analyzer at temperatures from 50 to 800 °C in real time. The corrosion kinetics of the Cu 110 was characterized through thermogravimetric (TGA) analysis and microscopy images, and the oxide thickness growth was then correlated to the wireless sensor signal under isothermal conditions at 800 °C. The results showed that the wireless signal was consistent with the corrosion kinetics and temperature, indicating that these two characteristics can be further deconvoluted in the future. In addition, the sensor also showed a magnitude- and frequency-dependent response to crack/spallation events in the oxide corrosion layer, permitting the in situ wireless identification of these catastrophic events on the metal surface at high temperatures.

Formation of Ti2AlC-based thin films via magnetron sputtering and annealing under vacuum

High-purity Ti, Al and C targets were used to fabricate Ti2AlC-based thin films on industrial polycrystalline Al2O3 substrates via magnetron sputtering under argon, followed by vacuum annealing. The phase compositions of the Ti2AlC-based thin films were analysed, and scanning electron microscopy was performed to investigate their structures. Grain size and morphology depended on the deposition pattern. A material composed of ∼97.1% Ti2AlC and ∼2.9% TiC was obtained by sputtering Al for thirty minutes, then sputtering four alternating layers of Ti and C for ten minutes each. The thin film was subsequently annealed under vacuum at 900°C for 60 minutes. The samples contained lamellar hexagonal plate-shaped grains structures. The plate-shaped grains ranged from 200 nm to 400 nm in length depending on the sputtering mode. The long sides of the lamellar grains were predominantly oriented perpendicular to the alumina substrates. Regardless of the sputtering conditions, the coherent scattering region of Ti2AlC was ∼64 nm in size after annealing at 900°C. Changes in the size of the coherent scattering region of the TiC phase were non-linear and ranged from 9 to 60 nm.

Noble Metal/Mn3O4 Nanocomposite Systems:from Design to Applications As Ultra-Sensitive Gas Sensors for Nitrogen Mustard Simulant

The efficient detection of chemical warfare agents (CWAs) is of paramount importance in the development of reliable sensing devices for safety applications. In particular, the increased threats of chemical attacks by terrorist organizations have stimulated a significant interest in the early detection of CWAs. Due to the high toxicity and necessity of special infrastructures for storage/manipulation of CWAs, the activities aimed at their detection are carried out on mimicking compounds compatible with lab test security level. In this context, the recognition of di(propylene glycol) monomethyl ether (DPGME), a simulant of the vesicant nitrogen mustard, is of key importance. Among the various devices adopted for CWA sensing, chemoresistive gas sensors based on nanostructured metal oxides offer various advantages, encompassing low cost, limited power consumption, and good stability/sensitivity. To date, DPGME detection has been performed only in a few cases by the use of SnO2-based systems, and the development of highly efficient sensors for its selective monitoring is of primary importance for next-generation devices.Among metal oxides, p-type Mn3O4, a mixed-valence and environmentally friendly multifunctional system, is an attractive candidate for CWAs detection thanks to the tunable Mn redox chemistry and the inherent catalytic properties. On the other hand, gas sensors based on pure p-type materials like Mn3O4 typically feature responses lower than n-type ones, as well as modest sensitivity/selectivity. To circumvent these drawbacks, a viable alternative is the functionalization with noble metals particles, like Ag and Au, that can act as catalytic promoters of the involved chemical reactions at the nanoscale and/or favor the formation of metal/oxide Schottky junctions, improving charge carrier separation.Based on preliminary results on the development of pure Mn3O4 nanomaterials as gas sensors [1,2], we have reported for the first time on the gas-sensing performances of nanocomposite systems based on Mn3O4-M (M = Ag, Au) in DPGME detection [3]. The target materials were obtained by the initial chemical vapor deposition (CVD) of Mn3O4 on polycrystalline Al2O3 substrates, followed by silver or gold radio frequency (RF) sputtering, with the aim of obtaining a high dispersion of metal nanoparticles.The results yielded by a multi-technique characterization highlighted the formation of phase-pure Mn3O4 nanostructures eventually functionalized with Ag or Au and an intimate contact between metal oxide matrix and nanoparticles. These features enabled us to enhance the system sensitivity in the detection not only of standard volatile organic compounds (acetone and ethanol), but also of DPGME, down to the ppb scale. Moreover, the possibility of discriminating between DPGME, on one side, and acetonitrile (CH3CN) or dimethyl methyl phosphonate (DMMP), simulants of HCN (blood agent gas) and Sarin nerve gas, respectively, demonstrates the system selectivity toward the target warfare gas simulant. To shed light on the origin of enhanced selectivity due to functionalization, density functional theory (DFT) based calculations were performed. The results, providing a molecular insight into the interactions between the target systems and DPGME, enabled us to elucidate a dual-site activation mechanism based on the binding of DPGME to both manganese oxide and gold nanoparticles, accounting for the higher functional performances registered for Au-Mn3O4 sensors.As a whole, this study represents an important step forward in the fabrication of highly efficient sensors based on dual metal oxide-metal NP composites. The obtained results pave the way to mastering next-generation sensing devices for real world applications, which will be pursued by additional investigation of the system long-term stability and humidity influence on the resulting functional performances.

Manganese Oxide Nanoarchitectures as Chemoresistive Gas Sensors to Monitor Fruit Ripening.

The efficient detection of low-concentration ethylene is a challenging issue of key importance for food quality control end-uses. Herein, we report on the fabrication of MnO₂-based nanoarchitectures by a two-step plasma-assisted process, consisting in the initial chemical vapor deposition of MnO₂ (host) on polycrystalline Al₂O₃ substrates and the subsequent functionalization with Ag and Au-based nanoparticles (guest) by sputtering processes. The resulting composites, characterized by a high Ag/Au dispersion and an effective host-guest contact, were tested for the first time as chemoresistive gas sensors for ethylene recognition at low temperatures. The high sensitivity and promising responses, enhanced by metal particle introduction, candidate the target systems as attractive platforms for the eventual monitoring of vegetables/fruits ripening and ageing.

Structural study of thick hexaferrite films

We present details of the microstructural properties of Y- (Ba2Mg2Fe12O22, Ba0.5Sr1.5Zn2Fe12O22, Ba0.5Sr1.5Zn2Al0.08Fe11.92O22) and Z-type (Sr3Co2Fe24O41) hexaferrite thick films deposited by screen printing and drop casting on unpolished polycrystalline Al2O3 substrates. The hexaferrites thick films obtained by drop casting exhibited considerable roughness and porousness compared with those obtained by screen printing. We found that the powders’ morphology significantly affects the microstructure of the thick films formed by screen printing. The microstructural analysis of the thick films shows that their microstructure differs from that of the powders. Further, during the annealing process the grains in the thick films grow and form hexagonal particles, the latter having the largest size, best shape and being best observed in the case of the Ba2Mg2Fe12O22 film.

Wetting behaviors of molten melt drops on polycrystalline Al2O3 substrates in high magnetic fields

We measured the contact angles of Al and Sn drops on polycrystalline Al2O3 substrates in various high magnetic fields at different temperatures. The contact angles of both Al and Sn drops on the Al2O3 substrates decreased under high magnetic fields. These decreases strongly depend on the temperature, magnetic flux density, magnetic properties of the metal drops, and the reactivity of the metal drops with Al2O3. Our results reveal that the wetting behavior of molten metal drops on ceramics can be modified by a high magnetic field.

Increase of Operating Temperature of Magnetic Field Sensors Based on La–Sr–Mn–O Films With Mn Excess

The results of Mn excess influence on the magnetoresistive properties of nanostructured La1– x Sr x MnyO3±δ films grown on polycrystalline Al2O3 substrates by pulsed-injection metal-organic chemical vapor deposition technique are presented. The electrical transport properties and magnetoresistance of these films were investigated in the temperature range of 77–363 K in pulsed magnetic fields up to 20 T. The results were analyzed from the perspective of using these films for magnetic field sensors operating at high temperatures. It was obtained that the increase of Mn/(La + Sr) ratio from 1.05 to 1.15 leads to the increase of metal–insulator transition temperature from 240 to 270 K. This behavior was associated with the presence of (La + Sr) vacancies, which act as self-dopants. At temperatures higher than 340 K, the highest magnetoresistance values were obtained for films with 15% Mn excess. These results allowed us to produce pulsed magnetic field sensors operating at ambient temperatures up to 90 °C.

Electronic Transport and Magnetic Properties of Co/SiO2 Magnetic Nanocomposites

Electric, thermoelectric, and magnetic properties of the magnetic nanocomposite (MNC) Co/SiO2 consisting of nanometer‐size Co nanoparticles embedded in SiO2 dielectric matrix have been studied. MNC is obtained in the form of (0.8–2.3) μm thick films with Co concentration 31.7–65.4 at.% by the electron beam physical vapor deposition method on polycrystalline Al2O3 substrates. The temperature dependence of the resistivity follows the Mott law. The negative magnetothermoelectric power discovered in MNC Co/SiO2 can be explained by hopping conductivity through the magnetic localization centers in the SiO2 matrix or chemical interaction of Co with Si and O2, which results in a mixture of CoSi (ferromagnetic) and CoO (antiferromagnetic) phases. As the cobalt concentration increases, the particle size increases and the interparticle distances decrease. This is confirmed by the magnetic behavior of the sample with high Co concentration, which is dominated by large Co nanoparticles and strong interparticle magnetic interactions.

Impact of lanthanum and boron on the growth, thermomechanical properties and oxidation resistance of Ti–Al–N thin films

State-of-the-art machining applications require new developments in the field of protective coatings, to guarantee for excellent dimensional accuracy and effective processes. Recently, we showed an enormous enhancement in deposition rate, mechanical strength, and thermal stability of Ti–Al–N when using Ce-alloyed Ti–Al composite targets. Within this study, we further follow the concept of alloying lanthanides to nitride-based coatings, by sputtering Ti0.5Al0.5 composite targets alloyed with 2 mol% LaB6 in mixed Ar/N2 atmospheres. This allowed us to produce coatings that outperform not only Ti–Al–N (significantly) but also Ce-alloyed Ti–Al–N coatings.Compared with coatings prepared from non-alloyed Ti0.5Al0.5 composite targets (Ti0.43Al0.57N), we achieved an impressive increase in deposition rate from 77 to 139 nm/min. Furthermore, we observed an increase in hardness from 34 to 40 GPa (on polished polycrystalline Al2O3 substrates) in the as-deposited state, accompanied by a delayed spinodal decomposition, and an additional TiB2 precipitation, allowing for still 40 GPa after annealing at 1100 °C. Also the oxidation resistance could be significantly improved through the addition of LaB6. After exposure to ambient air at 850 °C for 10 h or 950 °C for 1 h, the oxide scale thickness is only 500 nm for the LaB6 alloyed coatings, whereas the Ti0.43Al0.57N counterpart was already fully oxidized after 10 h at 850 °C or 1 h at 950 °C.

Rotational magnetic anisotropy in polycrystalline FePt films fabricated by solid-state synthesis

It is shown that annealing 550°C of the Fe/Pt bilayer films fabricated by layer-by-layer deposition onto polycrystalline Al2O3 substrates in an atomic ratio of Fe: Pt = 71: 29 leads to the formation of the inplane rotational magnetic anisotropy with the easy axis that can be aligned by magnetic field in any direction in the film plane. The increase in the Pt content in the investigated film system to 50 at % leads to an increase in the anisotropy constant to 6.5 × 106 erg/cm3, which exceeds the value in the Fe71Pt29 sample by a factor of 6. In addition to the rotational anisotropy, the Fe50Pt50 films are characterized by the perpendicular rotational magnetic anisotropy, which is higher than the film shape anisotropy. Therefore, the easy axis in these samples can be aligned by magnetic field in any spatial direction. According to the X-ray and magnetic measurement data, the synthesized samples have a polycrystalline structure consisting of two ordered phases: magnetically hard L10–FePt and magnetically soft L12–Fe3Pt. This gives us grounds to suggest that the rotational magnetic anisotropy originates from the epitaxial intergrowth and exchange coupling of these two phases.

Nanostructured La–Sr–Mn–Co–O Films for Room-Temperature Pulsed Magnetic Field Sensors

The results of Co substitution for Mn in nanostructured La1- x SrxMn1- y Co y O3 films ( $x = 0.2$ ) grown on polycrystalline Al2O3 substrates by pulsed-injection metal-organic chemical vapor deposition technique, are presented. The Co content y in the films was changed as follows: 0; 0.023; 0.053; 0.078. The resistivity ( $\rho )$ dependences on temperature in zero magnetic field was investigated in the temperature range 5–300 K. It was found that the increase in Co content up to 0.078 results in the decrease of metal-insulator transition temperature and increase of the resistivity. The results of nanostructured films are compared with the epitaxial films grown on LaAlO3 substrate keeping the same technological conditions. The magnetoresistance (MR) and MR anisotropy (MRA) were investigated at room temperature (290 K) in permanent magnetic fields up to 2.35 T, and pulsed magnetic fields up to 20 T. It was found that the highest MR values are obtained for the intermediate Co content: $y = 0.053$ , while the MRA decreased with the increase of Co content. The results are discussed within the double-exchange interaction model for the perovskite oxides. The possibility to apply the manganite-cobaltite films for the development of room temperature B-scalar magnetic field sensors is considered.

Preparation and Characterization of Ca3Co4O9 Thin Films on Polycrystalline Al2O3 Substrates by Chemical Solution Deposition

Ca3Co4O9 thin films have been first prepared on polycrystalline Al2O3 substrates using chemical solution deposition method by multiple annealing processing. It is observed that the derived thin films are c-axis oriented although the substrates are polycrystalline Al2O3 substrates, suggesting the self-assembled c-axis orientation. The annealing temperature effects on the properties are investigated and discussed. The best performances are attributed to the 850 °C-annealed sample, whose resistivity, Seebeck coefficient and power factor at 300 K are 7.4 mΩ cm, 117 μV/K and 0.18 mW/m K−2 respectively, which is even better than those of the thin films deposited on single crystal substrates. The results will provide an effective route to optimize the properties of Ca3Co4O9 thin films using chemical solution deposition by multiple annealing processing even the substrates are polycrystalline.

Al2O3–Al2O3 and Al2O3–Ti Solder Joints—Influence of Ceramic Metallization and Thermal Pretreatment on Joint Properties

In this work, Al2O3–Al2O3 and Al2O3–Ti solder joints intended for space applications were realized by applying metallic multilayers both on Ti and on polycrystalline Al2O3 substrates to ensure wetting during the soldering process. The eutectic Au‐12Ge (wt%) alloy with a melting point of 361°C was used as solder material and two different multilayer systems on the ceramic (Cr‐Ni/Ti‐W/Au, Ti/Pt/Au) were investigated. The influence of different thermal pretreatments of the Al2O3 plates on the multilayer properties, the interface reactions during soldering and the mechanical properties were studied using Glow‐Discharge Optical Emission Spectroscopy (GDOES), X‐ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and a shear test device. It is shown that joints with a good quality can be produced at a soldering temperature of 400°C, whereas their properties and fracture behavior are strongly dependent on the metallization layer properties and the thermal pretreatments.

Structural and Electrical Properties of Mg–Si Thin Films Fabricated by Radio-Frequency Magnetron Sputtering Deposition

ABSTRACTMg–Si thin films were fabricated on glass, Si(100), Si(111), and polycrystalline Al2O3 substrates by radio-frequency (RF) magnetron sputtering deposition using an elemental composite target composed of Si chips on a Mg disk. The effect of deposition conditions such as the composition ratio of Mg/Si in the target area, substrate temperature, and the type of substrate on the thin film deposition was investigated. By controlling the deposition conditions, pure-phase Mg2Si polycrystalline films were successfully fabricated at room temperature. The crystalline orientation of the films was strongly influenced by the Mg/Si elemental composition ratio in the targets as well as by the surface roughness and porosity of the substrate. The electron concentration and mobility of nondoped Mg2Si films were 2.2 × 1016 cm-3 and 2.0 cm2/Vs, respectively. The electron concentration of Mg2Si films was drastically increased by impurity doping with Al and Bi.

Oxidation Behaviour of Ti<SUB>2</SUB> AlN Films Composed Mainly of Nanolaminated MAX Phase

In this paper, we reported the oxidation behaviour of Ti2AIN films on polycrystalline Al2O3 substrates. The Ti2AIN films composed mainly of nanolaminated MAX phase was obtained by first depositing Ti-Al-N films using reactive sputtering of two elemental Ti and Al targets in Ar/N2 atmosphere and subsequent vacuum annealing at 800 degrees C for 1 h. The Ti2AIN films exhibited excellent oxidation resistance and thermal stability at 600-900 degrees C in air. Very low mass gain was observed. At low temperature (600 degrees C), no oxide crystals were observed on film surface. Blade-like Theta-Al2O3 fine crystals formed on film surfaces at 700-800 degrees C. At high temperature (900 degrees C), firstly Theta-Al2O3 formed on film surface and then transformed into alpha-Al2O3. At 700-900 degrees C, a continuous Al2O3 layer formed on Ti2AIN films surface, acting as diffusion barrier preventing further oxidation attack. The mechanism of the excellent oxidation resistance of Ti2AIN films was discussed based on the experimental results.

High-temperature thermoelectric properties of nanostructured Ca3Co4O9 thin films

We prepared nanostructured Ca3Co4O9 (CCO) thin films by promoting localized epitaxial growth on polycrystalline Al2O3 substrates. The thermoelectric properties of the CCO films were studied in the temperature range 300 to 1023 K. We confirmed that localized epitaxial growth occurred on the seed grains that dominate the (006) plane. The nanostructured CCO thin films were found to have a maximum Seebeck coefficient of 206 μV/K and a power factor (at 920 K) of 0.514 mW/mK2. Moreover, the presence of nanostructure was found to reduce the thermal conductivity, and thus, should enhance the overall performance of CCO films in thermoelectric devices.

Preparation of highly (100)-oriented CeO2 films on polycrystalline Al2O3 substrates by laser chemical vapor deposition

Abstract CeO2 films were prepared on Al2O3 substrates by laser chemical vapor deposition at different laser power (PL) up to 182 W. The (100)-oriented CeO2 films were prepared at PL = 101–167 W (Tdep = 792–945 K). The texture coefficient (TC) of (200) reflection had a maximum of 6.7 at PL = 113 W (Tdep = 836 K). The (100)-oriented CeO2 films consisted of granular grains and showed a columnar cross section. The deposition rates (Rdep) of (100)-oriented CeO2 films showed a maximum of 43 μm h−1 at PL = 152 W (Tdep = 912 K).

(100)-oriented CeO2 films on polycrystalline Al2O3 substrates prepared by laser chemical vapor deposition

(100)-oriented CeO2 films on polycrystalline Al2O3 substrates prepared by laser chemical vapor deposition

Thermal stability of SrFeO3/Al2O3 thin films: Transmission electron microscopy study and conductometric sensing response

The SrFeO3/Al2O3 thin film system has been studied using transmission electron microscopy (TEM). The thin films of SrFeO3 were grown by pulsed laser deposition onto single crystal and sintered polycrystalline Al2O3 substrates at room temperature (RT) and 700 °C and subjected to annealing for various periods of time at 700–1000 °C. TEM characterization showed that the morphology of the film varied with changes of deposition temperature. Films deposited at RT featured a columnar structure and those deposited at 700 °C showed layers with crystalline grains. The interfacial structures of the films remained unchanged below 700 °C. Interfacial reactions were observed following annealing at 850 °C for 5 h. The phase transformation at the interface was characterized for the film annealed at 1000 °C for 5 h, for which the principal phases were identified as SrAl2−xFexO4 and SrFe12−yAlyO19. Evaluation for thin film conductometric sensing applications indicated that the untreated films deposited at 700 °C onto both single crystal and sintered Al2O3 substrates exhibited a p-type gas sensor response to oxygen at 500 °C.

In-situ growth of superconducting MgB2 thin films by HPCVD method at low temperature

An improved hybrid physical chemical vapor deposition (HPCVD) method for the preparation of magnesium diboride (MgB2) films is proposed. In this experiment, the temperature for the evaporation of magnesium is set to be ∼700°C, while the substrate is kept below 500°C. Inside the vacuum chamber, double heaters control the temperature for the evaporation of Mg and of the substrate independently. The deposition of MgB2 films on a polycrystalline Al2O3 substrates is obtained by the reaction between evaporated magnesium atoms and boron atoms that decompose from diborane near the heaters. The superconducting MgB2 films show a transition temperature at 38.5K and a zero-resistance temperature at 38K. The critical current density of the films is 105A/cm2.