Adverse effect of substrate surface impurities on O2 sensing properties of TiO2 gas sensor operating at high temperature

It is well known that surface state of substrates greatly influences the sensing characteristics of gas sensors. In this work, alumina (Al₂O₃) substrates with different distribution of surface impurity concentrations were utilized to prepare TiO₂ based lambda sensors. Several methods, including XPS, XRD and SEM were employed to exam the substrate surface impurity contents, as well as the structure of the sensing films and the interface morphology between the substrates and sensing films. The results indicated that the gas sensors prepared on alumina substrates containing higher Ca impurities displayed lower oxygen sensitivities and longer response times at high operating temperatures above 600 °C. Therefore, it was concluded that Ca impurities on alumina substrates had to be limited to lower possible levels when fabricating TiO₂ lambda sensors.     

Investigation on the anti-reduction mechanism of Ti4+ in high dielectric constant system Ca0.9La0.067TiO3 by doping with Al2O3

Ca_0.9La_0.067TiO₃ (abbreviated as CLT) ceramics doped with different amount of Al₂O₃ were prepared via the solid state reaction method. The anti-reduction mechanism of Ti⁴⁺ in CLT ceramics was carefully investigated. X-ray diffraction (XRD) was used to analyze the phase composition and lattice structure. Meanwhile, the Rietveld method was taken to calculate the lattice parameters. X-ray photoelectron spectroscopy (XPS) was employed to study the valence variation of Ti ions in CLT ceramics without and with Al₂O₃. The results showed that Al³⁺ substituted for Ti⁴⁺ to form solid solution and the solid solubility limit of Al³⁺ is near 1.11 mol%. Furthermore, the reduction of Ti⁴⁺ in CLT ceramics was restrained by acceptor doping process and the Q × f values of CLT ceramics were improved significantly. The CLT ceramic doped with 1.11 mol% Al₂O₃ exhibited good microwave dielectric properties: ε_r = 141, Q × f = 6848 GHz, τ_f = 576 ppm/°C.     

Facile modified synthesis and intermediate temperature electrical properties of Sn0.95Al0.05P2O7/KSn2(PO4)3 composite electrolyte

In this study, Sn_0.95Al_0.05P₂O₇ and a novel dense Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite electrolytes were synthesized. The structural characterization of X–ray diffraction (XRD) and microstructual properties of scanning electron microscopy (SEM) were carried out. The XRD results indicated that an in-situ reaction between Sn_0.95Al_0.05P₂O₇ and inorganic melt salt take place to form the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite. The intermediate temperature electrical properties were determined by using impedance spectroscopy, oxygen concentration cell and hydrogen concentration discharge cell. Finally, the H₂/O₂ fuel cell using the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ as electrolyte membrane was constructed and the obtained maximum power output densities were 67.7 mW cm^?2 and 142.1 mW cm^?2 at 650 °C and 700 °C, respectively.     

Microstructure and elastic modulus of electrospun Al2O3-SiO2-B2O3 composite nanofibers with mullite-type structure prepared at elevated temperatures

In the present work, Al₂O₃-SiO₂-B₂O₃ composite nanofibers with mullite-type structure were prepared using electrospinning technique. The microstructure and elastic modulus of the composite nanofibers obtained at elevated temperatures were studied. The results showed that Al₄B₂O₉ phase formed at 900 °C and then transformed to Al₁₈B₄O₃₃ at 1100 °C. Mullite was also detected in the nanofibers prepared at 1100 °C. Amorphous SiO₂ existed in all samples even the calcination temperature reached up to 1400 °C. The continuous and uniform structure of the composite nanofibers was kept after calcining at different temperatures, while rougher surface was evident due to the growth of the grain caused by the elevated temperature. An increase of elastic modulus of the samples from 9.47 ± 1.91 GPa to 27.30 ± 2.61 GPa was observed when calcination temperatures increased from 800 °C to 1400 °C.     

Enhancing copper infiltration into alumina using spark plasma sintering to achieve high performance Al2O3/Cu composites

Al₂O₃/Cu (with 30 wt% of Cu) composites were prepared using a combined liquid infiltration and spark plasma sintering (SPS) method using pre-processed composite powders. Crystalline structures, morphology and physical/mechanical properties of the sintered composites were studied and compared with those obtained from similar composites prepared using a standard liquid infiltration process without any external pressure. Results showed that densities of the Al₂O₃/Cu composites prepared without applying pressure were quite low. Whereas the composites sintered using the SPS (with a high pressure during sintering in 10 min) showed dense structures, and Cu phases were homogenously infiltrated and dispersed with a network from inside the Al₂O₃ skeleton structures. Fracture toughness of Al₂O₃/Cu composites prepared without using external pressure (with a sintering time of 1.5 h) was 4.2 MPa m^1/2, whereas that using the SPS process was 6.5 MPa m^1/2. These toughness readings were increased by 18% and 82%, respectively, compared with that of pure alumina. Hardness, density and electrical resistivity of the samples prepared without pressure were 693 HV, 82.5% and 0.01 Ω m, whereas those using the SPS process were 842 HV, 99.1%, 0.002 Ω m, respectively. The enhancement in these properties using the SPS process are mainly due to the efficient pressurized infiltration of Cu phases into the network of Al₂O₃ skeleton structures, and also due to high intensity discharge plasma which produces fully densified composites in a short time.     

Fabrication and performance of dielectric tape based on CaO-B2O3-SiO2 glass/Al2O3 for LTCC applications

A non-aqueous tape-casting process for fabricating CaO-B₂O₃-SiO₂ glass/Al₂O₃ dielectric tape for LTCC applications was investigated. An isopropanol/ethanol/xylene ternary solvent-based slurry was developed by using castor oil, poly(vinyl butyral), and dibutyl phthalate as dispersant, binder, and plasticizer, respectively. The effects of dispersant concentration, binder content, plasticizer/binder ratio, and solid loading, on the properties of the casting slurry and resultant tape were systematically investigated. The results showed that the optimal values for the dispersant and binder contents, plasticizer/binder ratio, and solid loading were 2.0 wt%, 7.5 wt%, 0.6, and 62 wt%, respectively. The resultant flexible and uniform, 120-μm-thick CaO-B₂O₃-SiO₂ glass/Al₂O₃ tape had a density of 1.90 g/cm^?3, tensile strength of 1.66 MPa, and average surface roughness of 310 nm. Laminated tapes sintered at 875 °C for 15 min exhibited excellent properties: relative density of 97.3%, ε_r of 7.98, tan δ of 1.3 × 10^?3 (10 MHz), flexural strength of 205 MPa, and thermal expansion coefficient of 5.47 ppm/°C. The material demonstrated good chemical compatibility with Ag electrodes, indicating a significant potential in LTCC applications.     

Fabrication of porous Al2O3-based ceramics using ball-shaped powders by preceramic polymer process in N2 atmosphere

Porous Al₂O₃-based ceramics were successfully fabricated using ball-shaped powders by preceramic polymer process in N₂ atmosphere. These results showed that the amorphous Si-O-C ceramics were formed on the surface of ball-shaped Al₂O₃ particles by the pyrolysis of the silicone resin during sintering in N₂ atmosphere, which played a role in connecting the Al₂O₃ particles by forming the sintering necks. When the sintering temperatures increased from 1100 °C to 1600 °C, the formed Si-O-C ceramics still existed in the amorphous state and had no crystallization. Interestingly, the amorphous β-SiC formed at 1300 °C and its amount gradually increased with further increasing temperatures. The linear shrinkage rate of the samples varied from 0.49% to 0.73% and the weight loss rate increased from 2.01% to 10.77%. The apparent porosity remarkably varied with the range of 24.9% and 34.5%, as the bulk-density varied from 2.66 to 2.47 g/cm³. The bending strength gradually increased from 9.36 to 22.51 MPa with increasing temperatures from 1100 °C to 1500 °C, however, the bending strength remarkably decreased at 1600 °C, which was attributed to the comprehensive function of the high porosity, broken Al₂O₃ particles and weak connection between Al₂O₃ particles in the samples.     

Synthesis and oxidation behavior of (Mo0.97Nb0.03)(Si0.97Al0.03)2 ceramic

(Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ ceramic was synthesized by self-propagating high-temperature synthesis using commercial elemental powders, and then dense monolithic ceramic was prepared via vacuum hot pressing. The oxidation behavior of the bulk ceramic was investigated at 500 °C and 1200 °C. At 500 °C, due to the preferential oxidation of Nb-rich phase and the formation of uniform oxidation layer, the oxidation rate of (Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ is lower than pure MoSi₂. At 1200 °C, (Mo_0.97Nb_0.03)(Si_0.97Al_0.03)₂ shows better oxidation resistance than MoSi₂, owing to the uniform complex oxide layer with SiO₂, Al₂O₃ and Nb₂O₅ formed on the surface of the prepared ceramic.     

Co2O3 substitution effects on the structure and microwave dielectric properties of low-firing (Zn0.9Mg0.1)TiO3 ceramics

Low-firing (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ (x = 0.02–0.10) (ZMC_xT) microwave dielectric ceramics with high temperature stability were synthesized via conventional solid-state reaction. The influences of Co₂O₃ substitution on the phase composition, microstructure and microwave dielectric properties of ZMC_xT ceramics were discussed. Rietveld refinement results show the coexistence of ZnTiO₃ and ZnB₂O₄ phases at x = 0.02–0.10. (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ ceramic with x = 0.06 (ZMC_0.06T) obtains the best combination microwave dielectric properties of: ε_r = 21.58, Q × f = 53,948 GHz, τ_f = ? 54.38 ppm/°C. For expanding its application in LTCC field, 3 wt% ZnO-B₂O₃-SiO₂ (ZBS) and 9 wt% TiO₂ was added into ZMC_0.06T ceramic, great microwave dielectric properties were achieved at 900 °C for 4 h: ε_r = 26.03, Q × f = 34,830 GHz, τ_f = ? 4 ppm/°C, making the composite ceramic a promising candidate for LTCC industry.     

Durable superhydrophobic Zn/ZnO/TiO2 surfaces on Ti6Al4V substrate with self-cleaning property and switchable wettability

Durable superhydrophobic (SHP) Zn/ZnO/TiO₂ surfaces with dendritic structures on Ti₆Al₄V substrate were obtained by chemical etching, electrodeposition and following annealing process. The resultant coatings electrodeposited at ?1.5 V for 10 min and annealed at 190 °C for 60 min showed fine superhydrophobicity with a water contact angle of 160° and a rolling angle less than 1°, showing excellent rolling-off and self-cleaning properties. The morphology, chemical components and growth mechanism of samples were investigated by scanning electron microscopy (SEM), X-ray diffraction pattern (XRD), Energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Surface tribological properties were characterized by a universal mechanical tester (UMT). The as-prepared Zn/ZnO/TiO₂ surface still kept excellent SHP stability after exposure to the air, buried in soil and cold storage at 5 °C in the fridge for one year, as well as excellent repellence to some daily-used liquids such as coke, coffee, red wine, milk and tea. The surface can be reversibly switched between superhydrophobicity and superhydrophilicity by alternating UV illumination and dark storage or heating, which offer possibilities to widen future applications.     

Structural,photocatalytic, biological and catalytic properties of SnO2/TiO2 nanoparticles

TiO₂ and SnO₂/TiO₂ nanoparticles with different SnO₂ contents (0–20 wt%) were synthesized via surfactant-assisted sol-gel method using a cationic surfactant (cetyltrimethylammonium bromide, CTAB). The effects of SnO₂ content on the structural, optical, and catalytic activity of TiO₂ have been studied by X-ray diffraction (XRD), Transmission electron microscope (TEM), Scanning electron microscope (SEM), Fourier transformer infrared (FTIR) and UV–vis diffuse reflection spectroscopy (DRS). The total surface acidity of the prepared samples was measured by nonaqueous titration of n-butylamine in acetonitrile and the types of Brönsted and Lewis acid sites were distinguish using FTIR spectra of chemisorbed pyridine. XRD patterns analysis indicates that the crystallite size reduced remarkably and the transformation of anatase-to-rutile phase accelerated greatly with increasing the SnO₂ content. TEM images exhibit a spherical shape with an average particle size varying in the range 10–24 nm and high-resolution TEM images (HRTEM) show lattice fringes with interplanar spacing 0.35 nm and 0.32 nm which corresponding to anatase and rutile phases, respectively. SEM images show the amount of SnO₂ on the TiO₂ surface increases with increasing the SnO₂ content and the particles of SnO₂ were aggregated on TiO₂ surface with increasing SnO₂ content to 20% wt. The catalytic activity was tested by various applications: Photodegradation of Methylene Blue (MB) and Rhodamine B (RhB) under UV–vis irradiations and synthesis of xanthene (14-phenyl-14H-dibenzo [a,j] xanthene). Antibacterial and antioxidant activities were also studied. The antibacterial property test was carried out via agar disc diffusion method, and the results indicated that the prepared catalysts showed moderate antibacterial activity.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Effects of TiO2 addition on dielectric and energy storage properties of BaO-K2O-Nb2O5-SiO2 glass ceramics

In this work, 25.6BaO-6.4K₂O-32Nb₂O₅-36SiO₂-xTiO₂ (0 ≤ x ≤10 mol%) (BKNST) glass ceramics were synthesized by conventional melts and controllable crystallization method. The effects of different TiO₂ addition on the phase composition, dielectric and energy storage properties of BKNS glass ceramics were systematically evaluated. With the TiO₂ concentration increasing, a growing content of Ba₂TiO₄ phase was observed in the glass ceramics. The microstructures appeared to be homogenous and uniform with very low porosity through the addition of TiO₂, for which the maximal breakdown strength of 2112 kV/cm and the corresponding energy storage density of 9.48 J/cm³ were obtained with x = 7.5. The extremely low dielectric loss of less than 1‰ (25 °C, 100 kHz) and the obviously improved microstructure contributed to the increased breakdown strength. In addition, the discharge power density of the glass-ceramic capacitor (x = 7.5) was investigated using the RLC charge-discharge circuit and a relatively high value of 16 MW/cm³ at 300 kV/cm was obtained.     

Study on energy consumption of Al2O3 coating prepared by cathode plasma electrolytic deposition

Al₂O₃ coatings with large specific surface were prepared on cast nickel-based superalloy K418 by cathode plasma electrolytic deposition (CPED) in aqueous solutions at different concentrations. The significance of energy consumption and a simple calculation method during CPED were proposed, and the influence of electrolyte concentration on current density-voltage curve, energy consumption, and microstructure of coatings were studied. It was found that increasing the concentration of electrolyte can effectively reduce the current density at the initial stage while prolonging the deposition time and stepping up the energy consumption of whole coating preparation. The morphology observation results showed that the pore size of Al₂O₃ coatings enlarges with the increase of the concentration, and the optimum electrolyte concentration is 0.5–1 mol L^?1. Under this condition, the new method of oxidation pretreatment at 950 ℃ on samples for 30 min can efficiently decrease the current density during the early stage of preparation, which is beneficial to the deposition of complex-shaped samples with large size.     

Highly sensitive acetone-sensing properties of Pt-decorated CuFe2O4 nanotubes prepared by electrospinning

Pristine and Pt-decorated copper ferrite nanotubes (Pt-CuFe₂O₄ NTs, 0.1%, 0.5%, and 1.0%, mole percent) were prepared by a simple electrospinning method. The samples were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. Their gas-sensing properties were evaluated by a commercial CGS-4TPs system. Microscopic images showed that all samples consisted of well-defined nanotubes with diameter of 70?100 nm. Gas-sensing measurements revealed that the Pt-CuFe₂O₄ NTs had an improved acetone-sensing properties compared with pristine CuFe₂O₄ NTs. In particular, 0.5% Pt-CuFe₂O₄ NT-based sensor showed a high response (16.5 at 100 ppm), good selectivity, and long-term stability for acetone at 300 °C. In addition, more Pt dopants would have a greater effect on promoting the sensing properties of the CuFe₂O₄ NTs at high acetone concentrations. A gas-sensing enhancement mechanism of Pt-CuFe₂O₄ NT-based sensors was proposed, according to the catalytic oxidation process of acetone molecules, which could be due to the kinetic competition between Pt dopants and CuFe₂O₄ NTs.     

Temperature–stable dielectrics based on Cu–doped Bi2Mg2/3Nb4/3O7 pyrochlore ceramics for LTCC

Temperature–stable dielectrics based on Cu–doped Bi₂Mg_2/3Nb_4/3O₇ pyrochlore ceramics were prepared by conventional solid–state reaction. Microstructure analysis indicates that all of the specimen maintain the cubic pyrochlore phase, a fluorite–like phase of Bi₃NbO₇ and a Bi₅Nb₃O₁₅ formed for Cu doping. The dielectric constant is dominated by densification of samples and secondary phases, while the dielectric loss is related by the secondary phases, grain boundaries, and leakage current characteristics. The (1-x)BMN - xCuO(x = 0.1 mol%) ceramic sintered at 925 °C shows excellent dielectric properties with dielectric constant of ~184.06, dielectric loss of ~0.0017 and near zero τ_ε (?20 ppm/°C) is obtained at sintering temperature of 925 °C, which could be a promising candidate for LTCC.     

Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Investigation on a postmortem resin-bonded Al-Si-Al2O3 sliding gate with functional gradient feature

A resin-bonded Al-Si-Al₂O₃ sliding gate was designed on the basis of sintered alumina containing both Al and Si fine powders, and the sliding gate achieved good application results in the practical process of steel pouring. Moreover, the postmortem sliding gate was characterized and analysed by X-ray diffraction, scanning electron microscope, and energy dispersive X-ray spectroscopy. The results show that the postmortem sliding gate presents a functional gradient feature with a reinforcement zone–transition zone–plastic zone phase distribution, in which the phases in the reinforcement zone from 0 mm to 5 mm are Al₂O₃, Al₄O₄C, SiC, and Al₄C₃; i.e., the Al, Si, and carbon in the composite totally converted into non-oxide phases. Further, phases in the transition zone from 5 mm to 10 mm are Al₂O₃, SiC, and Al₄C₃, whereas phases in the plastic zone from 10 mm to 15 mm are Al₂O₃, SiC, Al₄C₃, Al and Si. The formation mechanism of the grade distribution of phases in the postmortem sliding gate is described as follows. During operation, Al and Si reacted with C so that Al₄C₃ and SiC formed in situ; then, Al₄C₃ further reacted with Al₂O₃, whereby Al₄O₄C was formed as the reinforcement phase at the 0–5-mm zone with a high temperature. As the temperature decreased from the hot face to interior, a part of the free Al and Si remained in the form of plastic phases.     

Giant electrocaloric effect of free-standing Pb0.85La0.1(Zr0.65Ti0.35)O3 thick films fabricated by the self-lift-off screen printing method

Free-standing Pb_0.85La_0.1(Zr_0.65Ti_0.35)O₃ (PLZT) ceramic thick films have been prepared via a facile and low-cost self-separating screen printing method for electrocaloric cooling, and the relation among the fabrication processes, phase composition, microstructure, dielectric characteristics, ferroelectric properties and electrocaloric effect (ECE) has been systematically investigated. Compared to the conventional ceramic thick films supported by substrates, the free-standing feature enables the EC cooling of the free-standing PLZT thick films to be fully used for cooling down different thermal loads rather than be futilely absorbed by the substrates. Furthermore, without the mechanical restriction of the substrates, the free-standing PLZT thick films can freely shrink during the high-temperature densification process, leading to their high density and favorable microstructures. Additionally, by introducing an adequate amount of excess PbO, the pyrochlore phase can be removed from the samples to yield high-purity perovskite PLZTs. With the comprehensive improvement in phase composition, microstructure and the elimination of mechanical strain between the active materials and substrates, the free-standing PLZT thick films exhibited an optimized ECE including changes of temperature and entropy of 1.95 °C and 2.09 J kg^?1 K^?1, which are almost 3 times that of the samples deposited on the Al₂O₃ substrates without excess PbO. This work would contribute to the development of ferroelectric ceramics, especially thick films, for practical EC cooling.     

Enhanced photocatalytic activity of porous single crystal TiO2/CNT composites by annealing process

To improve the photocatalytic performance of anatase TiO₂ (a-TiO₂), it is necessary to simultaneously increase its crystallinity and surface area. Our approach to achieve the desired morphology is to develop a porous single crystal that can be transformed from its mesocrystal form via annealing. We synthesized a-TiO₂ mesocrystals onto multiwalled CNTs using a facile one-pot chemical approach, and investigated the effect of the annealing temperature (200–600 °C) on the crystallinity, morphology, chemical bonding state, and photocatalytic performance of the TiO₂/CNT composites. The as-grown sample and sample annealed at 200 °C consisted of spindle-like a-TiO₂ mesocrystals. As the annealing temperature increased to 400 °C, the morphology of a-TiO₂ changed from mesocrystals into porous single crystals and the surface area enlarged due to the thermo-decomposition of organic residues between the subunits. The chemical bonding (Ti–O–C) between TiO₂ and CNT was also strengthened with increasing annealing temperature. On the other hand, the TiO₂ was separated from the CNT at 600 °C because of the large difference in the thermal expansion coefficients. The photocatalytic performance of the TiO₂/CNT composites was the highest at 400 °C due to the increased crystallinity, removal of the by-products, and strengthened Ti–O–C bonds, resulting in an increase in the photocatalytic active sites and efficient charge separation.